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Stimulated Brillouin laser based carrier recovery in high Q microcavity for coherent detection

Qin Wen, JINHUI QIN, Yong Geng, Guangwei Deng, Qiang Zhou, Heng Zhou, and Kun Qiu

DOI: 10.1364/OL.395270 Received 16 Apr 2020; Accepted 03 Jun 2020; Posted 03 Jun 2020  View: PDF

Abstract: We demonstrate all-optical carrier recovery exploring stimulated Brillouin laser in high Q whispering-gallery-mode microcavity, to achieve coherent data detection without requiring an independent local oscillator laser. Ultra-high optical signal to noise ratio better than 70 dB is achieved for the recovered carrier, thanks to that the generated stimulated Brillouin laser counter propagates with the incoming data signal and experiences high SBS efficiency. High frequency stability is obtained between the recovered carrier tone and the original data signal, enabling high performance coherent detection without the needing of electrical frequency drift compensation. Our study offers a low-complicity, high energy efficiency, and high robust carrier recovery solution.

Multi-line FLEET using interference masks

Yibin Zhang, Garrett Marshall, Steve Beresh, Daniel Richardson, and Katya Casper

DOI: 10.1364/OL.392779 Received 13 Mar 2020; Accepted 03 Jun 2020; Posted 04 Jun 2020  View: PDF

Abstract: A simple linear configuration for multi-line FLEET velocimetry is used for the first time to image an over-expanded unsteady supersonic jet. The FLEET lines are spaced 0.5-1.0 mm apart and up to five lines are simultaneously used to visualize the flowfield. These lines are created using multiple-slit interference masks, despite the mask blocking 25-30% of the 10 mJ incident beam. Maps of mean single-component velocity in the direction along the principal flow axis, and turbulence intensity in that same direction, are created using multi-line FLEET, and computed velocities agree well with that obtained from single-line (traditional) FLEET. Compared to traditional FLEET, multi-line FLEET offers increased simultaneous spatial coverage and the ability to produce spatial correlations in the streamwise direction. This FLEET permutation is especially well-suited for short-duration test facilities.

On the importance of frequency-dependent beam parameters for vacuum acceleration with few-cycle radially-polarized laser beams

Spencer Jolly

DOI: 10.1364/OL.394493 Received 06 Apr 2020; Accepted 03 Jun 2020; Posted 04 Jun 2020  View: PDF

Abstract: Tightly-focused, ultrashort radially-polarized laser beams have a large longitudinal field, which provides a strong motivation for direct particle acceleration and manipulation in vacuum. The broadband nature of these beams means that chromatic properties of propagation and focusing are important to consider. We show via single particle simulations that using the correct frequency-dependent beam parameters is imperative, especially as the pulse duration decreases to the few-cycle regime. The results with different spatio-spectral amplitude profiles show both a drastic increase or decrease of the final accelerated electron energy depending on the shape, motivating both proper characterization and potentially a route to optimization.

Subchannel-Drop and Add Operation by Using Silicon Photonic All-Optical Orthogonal Frequency Division Multiplexing Demultiplexers

Hiroyuki Uenohara and Kohei Shimosawa

DOI: 10.1364/OL.395689 Received 21 Apr 2020; Accepted 03 Jun 2020; Posted 04 Jun 2020  View: PDF

Abstract: This study investigates the subchannel-drop and add operation of a silicon photonic all-optical orthogonal frequency division multiplexing demultiplexer (O-OFDM-DEMUX) circuit. The proposed device consists of two O-OFDM-DEMUX circuits with four subchannels at a frequency spacing of 10 GHz and four 2×2 Mach–Zehnder interferometer (MZI)-type optical switches for drop and through operations, with another four 2×2 MZI-type optical switches for add and through operations. A subchannel on–off ratio of over 20 dB was achieved. In addition, a new subchannel was successfully added to the target subchannel, although the shape of the transmissivity spectrum was slightly degraded through the detuning of the spectrum.

Photoacoustic-ultrasonic dual mode microscopy with local speed-of-sound estimation

Chao Tao, Wentian Chen, Nghia Nguyen, XiaoJun Liu, and Richard Prager

DOI: 10.1364/OL.396246 Received 29 Apr 2020; Accepted 03 Jun 2020; Posted 04 Jun 2020  View: PDF

Abstract: Synthetic aperture imaging and virtual point detection have been exploited to extend the depth-of-view of photoacoustic microscopy. The approach is commonly based on a constant assumed sound speed, which reduces image quality. We propose a new self-adaptive technique to estimate the speed of sound that can be integrated with these methods. It is accomplished through linear regression between the time-of-flight detected at individual virtual detectors and their horizontal distances on the focal plane. The imaging results show our proposed method can effectively improve the lateral resolution, imaging intensity and spatial precision for inhomogeneous tissue.

Optofluidic waveguide bending by thermal diffusion for visible light control

Yunfeng Zuo, Hailiang Liu, and Yi Yang

DOI: 10.1364/OL.395932 Received 23 Apr 2020; Accepted 02 Jun 2020; Posted 04 Jun 2020  View: PDF

Abstract: Optofluidics has inspired many promising optical devices. Among them, waveguide bending is an important element for guiding light. Here, we demonstrated the thermal-diffusion liquids, acted as a natural transformation optical material in an annular structure. Compared with conventional step-index waveguide bending, this thermal one enables real-time tunable visible light bends by extreme angles, with nearly no power loss and intensity distribution. This unique light bending is because that gradient refractive index profiles caused by thermal diffusion meet the requirements by transformation optics. The work provides new insight into thermo-optic properties of liquids as natural technology to realize optofluidic gradient-index designs, and have potential for tunable optical systems.

2.05 µm chirped pulse amplification system at a 1 kHz repetition rate – 2.4 ps pulses with 17 GW peak power

Lorenz von Grafenstein, Martin Bock, Dennis Ueberschaer, Azize Koç, Uwe Griebner, and Thomas Elsaesser

DOI: 10.1364/OL.395496 Received 16 Apr 2020; Accepted 01 Jun 2020; Posted 04 Jun 2020  View: PDF

Abstract: Ho:YLF chirped pulse amplification (CPA) is implemented with a high-gain regenerative amplifier (RA) and a two-stage booster amplifier. We demonstrate the generation of 52.5 mJ pulses with a duration of 2.4 ps at a 1 kHz repetition rate. A peak power of 17 GW is achieved for the 2050 nm pulses. The CPA displays a remarkably high stability with a pulse-to-pulse rms as low as 0. %. The RA operates without any signs of bifurcation and delivers 12 mJ pulses. Seeding the booster amplifier with the RA output scales the pulse energy linearly up into the 50-60 mJ range. The amplifier system is operated at room temperature and shows a high extraction efficiency of 20.3% with respect to the optical pump power.

100-kHz krypton planar laser-induced fluorescence imaging

Stephen Grib, Paul Hsu, Naibo Jiang, Josef Felver, Stephen Schumaker, Campbell Carter, and Sukesh Roy

DOI: 10.1364/OL.395389 Received 16 Apr 2020; Accepted 01 Jun 2020; Posted 03 Jun 2020  View: PDF

Abstract: Krypton planar laser-induced fluorescence (Kr-PLIF) was demonstrated at a repetition rate of 100 kHz. To achieve this increased rate, a custom injection-seeded optical parametric oscillator was built to efficiently convert the 355-nm output of a high-energy, high-repetition-rate nanosecond burst-mode laser to 212.56 nm, to excite Kr from the ground to the 5p[1/2]0 electronic state. Successful tracking of flow-structures and mixture-fraction were demonstrated using detection speeds one hundred times greater than previously attained with a femtosecond laser source. The increase in repetition rate makes time-resolved Kr-PLIF relevant for high-speed flows in particular.

Subwavelength engineering for Brillouin gain optimization in silicon optomechanical waveguides

Jianhao Zhang, Omar ORTIZ, Xavier Le Roux, Eric Cassan, Laurent Vivien, Delphine Marris-Morini, Norberto Lanzillotti-Kimura, and Carlos Alonso-Ramos

DOI: 10.1364/OL.397081 Received 07 May 2020; Accepted 01 Jun 2020; Posted 01 Jun 2020  View: PDF

Abstract: Brillouin optomechanics has recently emerged as a promising tool to implement new functionalities in silicon photonics, including high-performance opto-RF processing and non-reciprocal light propagation. One key challenge in this field is to maximize the photon-phonon interaction and the phonon lifetime, simultaneously. Here, we propose a new strategy that exploits subwavelength engineering of the photonic and phononic modes in silicon membrane waveguides to maximize the Brillouin gain. By properly designing the dimensions of the subwavelength periodic structuration, we tightly confine near-infrared photons and GHz phonons, minimizing leakage losses and maximizing the Brillouin coupling. Our theoretical analysis predicts a high mechanical quality factor of up to 700 and a remarkable Brillouin gain yielding 3500 (W∙m)^(-1) for minimum feature size of 50 nm, compatible with electron-beam lithography. We believe that the proposed waveguide with subwavelength nanostructuration holds great potential for the engineering of Brillouin optomechanical interactions in silicon.

Self-amplifying memory based on multiple cascading four-wave mixing via recoil-induced resonance

Jose Tabosa, Jesús Pavón López, and Alvaro de Melo

DOI: 10.1364/OL.394302 Received 02 Apr 2020; Accepted 01 Jun 2020; Posted 02 Jun 2020  View: PDF

Abstract: We report on a new type of optical memory which allows for the amplification of the optical signal carrying the stored information during its reading process. The memory mechanism is demonstrated in an ensemble of cold cesium atoms and is based on the multiple parametric four-wave mixing exploring the external atomic degrees of freedom via recoil-induced resonances. We have particularly demonstrated the storage of light carrying orbital angular momentum with a four-fold amplifying factor for the retrieved signal during the reading process. Memory lifetimes of the order of hundreds of microseconds have been measured and possible applications for this self-amplifying memory are discussed.

An optical patching scheme for optical convolutional neural networks based on wavelength-division multiplexing and optical delay lines

Shaofu Xu, Jing Wang, and Weiwen Zou

DOI: 10.1364/OL.397344 Received 11 May 2020; Accepted 01 Jun 2020; Posted 01 Jun 2020  View: PDF

Abstract: Recent progress on optical neural networks heralds a new future for efficient deep learning accelerators, and novel architectures of optical convolutional neural networks provide potential solutions to the widely adopted convolutional models. In so-far optical convolutional neural networks, the data patching (a necessary process in the convolutional layer) is mostly executed with electronics, resulting in a demand for large input modulator arrays. Here, we experimentally demonstrate an optical patching scheme to release the burden of electronic data processing and to cut down the scale of input modulator array for optical convolutional neural networks. Optical delay lines replace electronics to execute data processing, which can reduce the scale of input modulator array. The adoption of wavelength-division multiplexing enables a single group of optical delay lines to simultaneously process multiple input data, reducing the system complexity. The optical patching scheme provides a new solution to the problem of data input, which is challenging and concerned in the field of optical neural networks.

On a universal solution to the transport-of-intensity equation

Jialin Zhang, Qian Chen, Jiasong Sun, Long Tian, and Chao Zuo

DOI: 10.1364/OL.391823 Received 16 Mar 2020; Accepted 31 May 2020; Posted 01 Jun 2020  View: PDF

Abstract: Transport-of-intensity equation (TIE) is one of the most well-known approaches for phase retrieval and quantitative phase imaging.It directly recovers the quantitative phase distribution of an optical field by through-focus intensity measurements in a noninterferometic, deterministric manner. Nevertheless, the accuracy and validity of state-of-the-art TIE solvers depend on restrictive preknowledge or assumptions, including appropriate boundary conditions, a well-defined closed region, and quasi-uniform in-focus intensity distribution, which, however, cannot be strictly satisfied simultaneously under practical experimental conditions. In this Letter, we propose a universal solution to TIE with the advantages of high accuracy, convergence guarantee, applicability to arbitrarily-shaped regions, and simplified implementation and computation. With the “maximum intensity assumption", we firstly simplified TIE as a standard Poisson equation to get an initial guess of the solution. Then the initial solution is further refined iteratively by solving the same Poisson equation, and thus, the instability associated with the division by zero/small intensity values and large intensity variations can be effectively bypassed. Simulations and experiments with arbitrary phase, arbitrary aperture shapes, and nonuniform intensity distributions verify the effectiveness and universality of the proposed method.

Wavelength-scanning second harmonic generation for determining absolute charge density at aqueous interfaces

Laetitia Dalstein, Jung-Ren Huang, and Yu-Chieh Wen

DOI: 10.1364/OL.396002 Received 23 Apr 2020; Accepted 31 May 2020; Posted 02 Jun 2020  View: PDF

Abstract: We develop hereby a new optical scheme based on second harmonic generation (SHG) at multiple wavelengths for unequivocal separation of the second-order and the electric-field-induced third-order nonlinear optical contributions from aqueous interfaces. The third-order SHG originating from the field-induced reorientation order of water molecules in the electrical double layer offers an optical label-free and assumption-free probe to the surface charge density and surface potential in the absolute scales. We verify this wavelength-scanning SHG scheme both theoretically and experimentally, and show that the approach is applicable to water interfaces with bulk ionic strength below 500 µM and has the detection sensitivity for surface charge density of ~10-⁴ C/m².

Tunable Multi-Resonance Using Complementary Circular Metamaterial

Fengwei Zhan and Yu-Sheng Lin

DOI: 10.1364/OL.394137 Received 31 Mar 2020; Accepted 31 May 2020; Posted 02 Jun 2020  View: PDF

Abstract: We present a design of tunable infrared (IR) resonator by using complementary circular metamaterial (CCM). CCM is composed of concentric rings. It exhibits superior characteristics of narrow multi-resonance generated by the coupling between two adjacent concentric rings in the IR wavelength range. It can be realized an effective modulation of reflection spectra by changing the height of each concentric ring. By slightly elevating the concentric rings, the corresponding resonances can be switched between on and off states and the resonances become more sensitive to surrounding refraction index. The figure-of-merit (FOM) is 10.91 for CCM exposed on the surrounding environment with different refraction index. The correlation coefficient is 0.998. The proposed CCM design provides the potential application in refraction index sensor and exhibits the possibility for future multichannel IR switch, environmental sensor, bandpass filter, wavelength-division multiplexing and so on.

ScVO₄:Bi³+ thermographic phosphor particles for fluid temperature imaging with sub-°C precision

Christopher Abram, Irin Panjikkaran, Simon Ogugua, and Benoit Fond

DOI: 10.1364/OL.392088 Received 18 Mar 2020; Accepted 30 May 2020; Posted 01 Jun 2020  View: PDF

Abstract: We synthesised and characterised ScVO₄:Bi³+ thermographic phosphor particles and demonstrated their use for temperature imaging in a near-ambient temperature liquid flow using a single laser/camera luminescence lifetime dual-frame ratio-based method. Owing to a high temperature sensitivity of up to 6 %/°C, the single-shot single-pixel temperature precision at a 400 µm spatial resolution is better than ±0.4 °C (1σ) across the 20 to 60 °C range, representing a factor >5 improvement compared to previous works using thermographic phosphors. The measurement duration is on the order of the luminescence lifetime (2 µs), which is applicable in both gas and liquid flows. The simplicity and precision of this thermometry method and ease with which it can be combined with simultaneous velocimetry will be of interest to the fluid mechanics and thermal science metrology communities.

Three-dimensional imaging with reflection synthetic confocal microscopy

Md Rasedujjaman, Kévin Affannoukoué, Nicolas Garcia-Seyda, Philippe Robert, Hugues Giovannini, Patrick Chaumet, Olivier Théodoly, Marie-Pierre Valignat, Kamal Belkebir, Anne Sentenac, and Guillaume Maire

DOI: 10.1364/OL.397364 Received 12 May 2020; Accepted 30 May 2020; Posted 01 Jun 2020  View: PDF

Abstract: Biomedical imaging lacks label free microscopy techniques able to reconstruct in 3D and with high resolution the contour of biological cells in solution, as required for the fast diagnosis of numerous diseases. Inspired by computational OCT techniques, we present a tomographic diffractive microscope in reflection geometry used as a synthetic confocal microscope, compatible with this goal and validated with the 3D reconstruction of a human effector T lymphocyte.

Ghost polarimetry with unpolarized pseudo-thermal light

Sergey Magnitskiy, Dmitry Agapov, and Anatoly Chirkin

DOI: 10.1364/OL.387234 Received 06 Jan 2020; Accepted 30 May 2020; Posted 01 Jun 2020  View: PDF

Abstract: We present the first experimental implementation of the concept of ghost polarimetry, that is, obtaining polarization images of objects using the ghost image method. The developed technique uses unpolarized spatially incoherent light with Gaussian statistics. Experimental results are presented that demonstrate the effectiveness of this version of polarimetry for obtaining ghost images of objects exhibiting the properties of linear dichroism. Image restoration of objects was carried out using our previously proposed theory, based on the Jones matrix calculus.

Doping-induced plateau of strong electromagnetic confinement in the momentum-space

Rushin Contractor, wanwoo noh, Quynh Levan, and Boubacar Kante

DOI: 10.1364/OL.395625 Received 20 Apr 2020; Accepted 29 May 2020; Posted 29 May 2020  View: PDF

Abstract: In this letter, we present a design strategy for the realization of electrically powered bound states in continuum (BIC) lasers. Despite a growing attention of the optics community for BICs, practical uses of BICs in an active device is still unestablished. A large index contrast and out-of-plane symmetries which aid the formation of bound states in continuum are not trivial to achieve using conventional approaches for semiconductor laser design. Here, we propose a doping scheme to circumvent this issue. We also show that the introduction of material absorption due to carriers deteriorates the quality factor of BIC modes and show that a suitable compromise between electrical conductivity and optical loss can be achieved.

Mueller matrix ellipsometer using dual continuously rotating anisotropic mirrors

Alexander Ruder, Brandon Wright, DARIN PEEV, Rene Feder, Ufuk Kilic, Matthew Hilfiker, Eva Schubert, Craig Herzinger, and Mathias Schubert

DOI: 10.1364/OL.398060 Received 19 May 2020; Accepted 29 May 2020; Posted 01 Jun 2020  View: PDF

Abstract: We demonstrate calibration and operation of a single wavelength (660 nm) Mueller matrix ellipsometer in normal transmission configuration using dual continuously rotating anisotropic mirrors. The mirrors contain highly spatially coherent nanostructure slanted columnar titanium thin films deposited onto optically thick gold layers on glass substrates. Upon rotation around the mirror normal axis, sufficient modulation of the Stokes parameters of light reflected at oblique angle of incidence is achieved. Thereby, the mirrors can be used as polarization state generator and polarization state analyzer in a generalized ellipsometry instrument. A Fourier expansion approach is found sufficient to render and calibrate the effects of the mirror rotations onto the polarized light train within the ellipsometer. The Mueller matrix elements of a set of anisotropic samples consisting of a linear polarizer and a linear retarder are measured and compared with model data and very good agreement is observed.

Deep learning assisted Shack-Hartmann wavefront sensor for direct wavefront detection

Lejia Hu, Shuwen Hu, Wei Gong, and Ke Si

DOI: 10.1364/OL.395579 Received 21 Apr 2020; Accepted 29 May 2020; Posted 03 Jun 2020  View: PDF

Abstract: Conventional Shack-Hartmann wavefront sensor (SHWS) requires wavefront slope measurement of every micro-lens for wavefront reconstruction. In this letter, we applied deep learning on SHWS to directly predict the wavefront distributions without wavefront slope measurements. The results show that our method could provide lower root mean square wavefront error and higher detection speed. The performance of the proposed method is also evaluated on challenging wavefronts while the conventional approaches perform insufficiently. This work provides a new approach to perform direct wavefront detection in SHWS-based applications.

Dispersion control in a near-infrared subwavelength resonator with a tailored hyperbolic metamaterial

Evgenij Travkin, Thomas Kiel, Sergey Sadofev, Sascha Kalusniak, Kurt Busch, and Oliver Benson

DOI: 10.1364/OL.397088 Received 07 May 2020; Accepted 29 May 2020; Posted 03 Jun 2020  View: PDF

Abstract: We demonstrate experimentally and computationally an intricate cavity size dependence of the anomalous near-infrared mode spectrum of an ordinary optical resonator that is combined with a ZnO:Ga-based hyperbolic metamaterial (HMM). Specifically, we reveal the existence of a resonance in subwavelength-sized cavities and demonstrate the control over the first-order cavity mode dispersion. We elaborate that these effects arise due to the HMM combining the mode dispersions of purely metallic and purely dielectric cavity cores into a distinct intermediate regime. By tailoring the HMM fill factor this unique dispersion of a subwavelength resonator can be freely tuned between these two limiting cases.

Low-Aberration High-Speed-Compatible Optical DelayLine

Xavier Audier, Wei-Wen Chen, and Marcus Cicerone

DOI: 10.1364/OL.397314 Received 12 May 2020; Accepted 29 May 2020; Posted 03 Jun 2020  View: PDF

Abstract: We describe a simple approach to dispersion-free optical delay line design that provides very low aberration over an extended delay range. In this approach we minimize aberrations by directing non-axial beam displacements along a line of symmetry built into the apparatus. We show improved performance and significant reduction of wavefront aberrations by comparing simulation and experimental results with a similar delay line that lacks this line of symmetry. The new design facilitates transform-limited recovery of spectral resolution in Fourier transform coherent anti-Stokes Raman scattering, and accordingly we demonstrate 3.5 $cm^-1$ spectral resolution with a 10 ps delay scan range.

Measurement of the full complex degree of spatial coherence using Fresnel diffraction from a phase step

Helia Hooshmand, Masoomeh Dashtdar, and Khosrow Hassani

DOI: 10.1364/OL.394107 Received 31 Mar 2020; Accepted 29 May 2020; Posted 03 Jun 2020  View: PDF

Abstract: A filed-portable, single-shot and very simple method is presented for measuring the full complex degree of coherence (CDC) of a quasi-monochromatic field using the Fresnel diffraction from a phase step. To validate the proposed technique, the CDC of the light emitted from an incoherent source with variable size is investigated. The results are in excellent agreement with theoretical predictions of Van Cittert-Zernike theorem. This technique is also applicable to characterize the coherence properties of sources in other spectral regions, e.g. x-rays.

Experimental Demonstration of Mid-IR Absorption Enhancement in Single Layer CVD Graphene

Abedin Nematpour, Nicola Lisi, Rosa Chierchia, and Maria Luisa Grilli

DOI: 10.1364/OL.397286 Received 12 May 2020; Accepted 29 May 2020; Posted 04 Jun 2020  View: PDF

Abstract: Mid-IR absorption of single layer graphene was simulated and experimentally demonstrated by embedding CVD single layer graphene inside a Fabry-Perot (FP) filter made by alternating quarter wave Si and SiO2 layers fabricated by radiofrequency sputtering. The absorption from the graphene layer was modelled by using COMSOL Multiphysics in four different configurations, depending on its position inside the filter, a Fabry-Perot made of two asymmetric dielectric mirrors separated by a cavity. In the first three configurations, graphene was inserted at the center of the optical cavity and inside the top or bottom dielectric mirrors forming the FP. The fourth configuration involves two layers of graphene, each positioned inside one of the dielectric mirrors. The calculated electric field distribution inside the FP shows two symmetric maxima just above and below the cavity, i.e. inside the mirrors, while the electric field at the center of the cavity is negligible. For the experimental demonstration the graphene geometry corresponding to the maximum electric field intensity was chosen and, between two equivalent alternatives, the one with the easiest fabrication procedure was selected. Results demonstrates a maximum experimental absorption of 50 % at 4342 nm for single layer graphene when inserted in the top mirror of the Fabry-Perot, in excellent agreement with the simulated value of 53%.

Lasing of N₂+ induced by filamentation in air as a probe for femtosecond coherent anti-Stokes Raman scattering

Xiaodong Zhao, Stefan Nolte, and Roland Ackermann

DOI: 10.1364/OL.391989 Received 04 Mar 2020; Accepted 29 May 2020; Posted 29 May 2020  View: PDF

Abstract: We investigated ultrashort pulse filamentation and lasing action of N₂+ for pump-probe experiments in gases. Using femtosecond coherent anti-Stokes Raman scattering, the white-light supercontinuum of the filament was used to excite ro-vibrational Raman transitions in air, CO₂ and CH₄. We show that the lasing pulse of N₂+ may act as probe for these excited levels by detecting the corresponding CARS signals. This feature may be applied to remote sensing applications, as the temporal and spatial alignment of the probe beam and the filament is intrinsically provided.

Tm3+-doped fluorotellurite glass microsphere resonator laser at 2.3 μm

Pengfei Wang, yating yi, Xin Wang, Angzhen Li, Shijie Jia, Ya-Xian Fan, Gilberto Brambilla, Shunbin Wang, and Haiyan Zhao

DOI: 10.1364/OL.396843 Received 04 May 2020; Accepted 28 May 2020; Posted 29 May 2020  View: PDF

Abstract: In this letter, we report lasing at 2.3 μm in Tm3+ single-doped and Tm3+/Ho3+ co-doped fluorotellurite glass microsphere resonators. By employing a 793 nm diode laser as a pump and exploiting whispering gallery mode microresonators (WGMR), dual-wavelength lasing at 1.9 and 2.3 μm, and triple-wavelength lasing at 1.9, 2.07 and 2.3 μm are achieved in Tm3+-doped, and in Tm3+/Ho3+ co-doped microspheres, respectively. The introduction of Ho3+ ions significantly reduces the lasing threshold of Tm3+ at 2.3 μm because of energy transfer.

Ultra-compact silicon mode-order converters based on dielectric slots

yaotian zhao, Xuhan Guo, Yong Zhang, Jinlong Xiang, kangnian wang, Hongwei Wang, and Yikai Su

DOI: 10.1364/OL.391748 Received 02 Mar 2020; Accepted 28 May 2020; Posted 29 May 2020  View: PDF

Abstract: Ultra-compact mode-order converters with dielectric slots are demonstrated on a silicon-on-insulator platform. We propose a mode converter that converts the TE0 mode into the TE1 mode with an ultra-small footprint of only 0.8×1.2 μm². The measured insertion loss is less than 1.2 dB from 1520 nm to 1570 nm. To reduce the insertion loss, we further optimize the structure and design two mode converters that convert the TE0 mode into the TE1 mode and the TE2 mode with footprints of 0.88×2.3 μm² and 1.4×2.4 μm² respectively. Their measured insertion losses are both less than 0.5 dB. Additionally, this method is cascadable and scalable for high-order mode conversion.

Time-resolved multimode heterodyne detection fordissecting coherent states of matter

Filippo Glerean, Giacomo Jarc, alexandre Marciniak, Francesca Giusti, Giorgia Sparapassi, Angela Montanaro, Enrico Maria Rigoni, Jonathan Tollerud, and Daniele Fausti

DOI: 10.1364/OL.394661 Received 27 Apr 2020; Accepted 28 May 2020; Posted 29 May 2020  View: PDF

Abstract: Unveiling and controlling the time evolution of the momentumand position of low energy excitations suchas phonons, magnons and electronic excitation is thekey to attain coherently driven new functionalities ofmaterials. Here we report the implementation of femtosecondtime and frequency resolved multimode heterodynedetection and show that it allows for the independentmeasurement of the time evolution of theposition and momentum of the atoms in coherent vibrationalstates in α-quartz. The time dependence ofthe probe field quadratures reveals that their amplitudeis maximally changed when the atoms have maximummomentum while their phase encodes a different informationand evolves proportionally to the instantaneousatomic positon. We stress that this methodology,providing the mean to map both momentum and positionin one optical observable, may be of relevance forboth quantum information technologies and time domainstudies on complex materials.

A high linearity silicon modulator capable of actively compensating the input distortion

qiang zhang, hui Yu, Penghui Xia, Zhilei Fu, Xiaofei Wang, and Jianyi Yang

DOI: 10.1364/OL.391715 Received 28 Feb 2020; Accepted 28 May 2020; Posted 29 May 2020  View: PDF

Abstract: In this paper, we demonstrate an ultra-high linearity silicon carrier-depletion-based modulator by integrating a dual-parallel Mach-Zehnder modulator (DP-MZM) with a 1×2 thermo-optical switch. The operation principle is to manipulate power distributions of RF and optical signals among the two-sub MZMs, so their third-order nonlinearities can cancel each other. Spurious-free dynamic ranges (SFDR) for the third-order intermodulation distortion (IMD3) are measured to be 1 /120 dB·Hz6/7 at 1/10 GHz, which represents a record-high linearity achieved with silicon-based modulators. As a contrast, SFDRs of a reference single MZM are 113/108 dB·Hz4/5 at the same frequencies. Furthermore, we firstly demonstrate that this device is able to actively compensate nonlinear distortions of RF driving signals in the optical domain. Carrier-to-distortion ratios (CDRs) of deliberately distorted two-tone signals are improved from 40/50 dB to 45/72 dB after the modulation.

Video-rate lensless endoscope with self-calibrationusing wavefront shaping

Elias Scharf, Jakob Dremel, Robert Kuschmierz, and Juergen Czarske

DOI: 10.1364/OL.394873 Received 10 Apr 2020; Accepted 28 May 2020; Posted 28 May 2020  View: PDF

Abstract: Lensless fiber endoscopes are of great importance for keyhole imaging. Coherent fiber bundles (CFB) can be used in endoscopes as remote phased arrays to capture images. One challenge is to image at high speed while correcting aberrations induced by the CFB. We propose the combination of digital optical phase conjugation, using a spatial light modulator, with fast scanning, for which a 2D-galvo-scanner and an adaptive lens are employed.We achieve the transmission of laser and image scanning through the CFB, largely without aberration. Video-rate imaging at 20 Hz with sub-cellular resolution is demonstrated. The sub-millimeter-diameter scanning endoscope has a great potential in biomedicine, for manipulation, e.g. in optogenetics, as well as in imaging.

Generation of above-TW 1.5-cycle visible pulses at 1 kHz by post-compression in a hollow fiber

Tamas Nagy, Martin Kretschmar, Mark Vrakking, and Arnaud Rouzée

DOI: 10.1364/OL.395830 Received 22 Apr 2020; Accepted 28 May 2020; Posted 28 May 2020  View: PDF

Abstract: We report on the generation of 6.1 mJ, 3.8 fs pulses by the compression of a kHz Ti:sapphire laser in a large-aperture long hollow fiber. In order to find optimal conditions for spectral broadening at high pulse energies, we explore different parameter ranges where ionization or the Kerr effect dominates. After identifying the optimum parameter settings, large spectral broadening at high waveguide transmission is obtained. The intense 1.5-cycle pulses are used for high-harmonic generation in argon and neon.

Compact diaphragm-based optical accelerometers with μg/√Hz resolution

Behrad Habib Afshar and Michel Digonnet

DOI: 10.1364/OL.395737 Received 28 Apr 2020; Accepted 28 May 2020; Posted 29 May 2020  View: PDF

Abstract: A compact fiber accelerometer that meets the resolution requirement for aircraft navigation is reported. It detects extremely weak acceleration-induced vibrations of a spring-loaded diaphragm using a two-wave interferometer with a π/2 biasing step micro-fabricated on the diaphragm. A single-mode fiber provides a laser beam that interrogates the interferometer. This sensor has a measured flat-band sensitivity with a bandwidth of 10.7 kHz, and a resolution limited by thermo-mechanical noise of 13 µg/√Hz at 100 Torr, and 712 ng/√Hz at 20 mTorr.

An ultra-low NA step-index large mode area Yb-doped fiber with a germanium doped cladding for high power pulse amplification

Raghuraman Sidharthan, Di Lin, Kang Jie Lim, Huizi Li, Huiting Serene Lim, CHEN-JIAN CHANG, Yue Seng, Song-Liang Chua, Yong-min Jung, David Richardson, and Seongwoo Yoo

DOI: 10.1364/OL.396157 Received 05 May 2020; Accepted 28 May 2020; Posted 29 May 2020  View: PDF

Abstract: High concentration rare earth doped, large mode area (LMA) step-index fibers which feature a very high cladding absorption per unit length at the pump wavelength, high efficiency and excellent beam quality are ideal for high power pulsed fiber lasers/amplifiers where large effective mode areas and short device lengths are crucial in order to reduce detrimental nonlinear effects associated with high peak power operation. In this paper, we propose a new way to realize such fibers simply by employing a germanium (Ge)-doped cladding rather than a pure silica cladding to offset the high refractive index associated with using a high concentration of ytterbium (Yb) in the core. This approach allows us to separate the two inter-linked fiber design parameters of pump absorption and numerical aperture (NA). Using a conventional modified chemical vapor deposition (MCVD) process combined with solution doping, a low NA (0.04), LMA (475 µm²) silica core fiber is fabricated with a cladding absorption value of >20 dB/m, which is the highest value among LMA step-index fibers with NA < 0.06 so far reported. The fabricated Yb-doped fiber was tested in a high power picosecond amplifier system and enabled the generation of 190 ps laser pulses with a 101 µJ pulse energy and 0.5 MW peak power at an average power of 150 W.

Accurate 4Pi single-molecule localization using an experimental PSF model

Yiming Li, Elena Buglakova, Yongdeng Zhang, Jervis Thevathasan, Joerg Bewersdorf, and Jonas Ries

DOI: 10.1364/OL.397754 Received 14 May 2020; Accepted 27 May 2020; Posted 01 Jun 2020  View: PDF

Abstract: Interferometric single-molecule localization microscopy (iPALM, 4Pi-SMS) uses multiphase interferometry to localize single fluorophores and achieves nanometer isotropic resolution in 3D. The current data analysis workflow, however, fails to reach the theoretical resolution limit due to the suboptimal localization algorithm. Here, we develop a method to fit an experimentally derived point spread function (PSF) model to the interference 4Pi-PSF. As the interference phase is not fixed with respect to the shape of the PSF, we decoupled the phase term in the model from the 3D position of the PSF. The fitter can reliably infer the interference period even without introducing astigmatism, reducing the complexity of the microscope. Using a spline-interpolated experimental PSF model and by fitting all phase images globally, we show on simulated data that we can achieve the theoretical limit of 3D resolution, the Cramér-Rao lower bound (CRLB), also for the 4Pi microscope.

Backward terahertz difference frequency generation via modal phase-matching in a planar LiNbO3 waveguide

Brett Carnio and Abdulhakem Elezzabi

DOI: 10.1364/OL.393283 Received 20 Mar 2020; Accepted 27 May 2020; Posted 02 Jun 2020  View: PDF

Abstract: The backward difference frequency generation process is used to produce narrowband THz radiation via modal phase-matching in a SiO2-LiNbO3-air planar waveguide. The TM0 pump mode, TE0 signal mode, and TE0 or TE2 idler modes are selected to satisfy the backward difference frequency generation phase-matching condition, thus allowing narrowband (i.e. <100 GHz linewidth) THz radiation generation in the spectral range of 2.4-3.2 THz. To date, this is the first investigation of THz radiation generation in a waveguide via the modal phase-matched backward difference frequency generation process.

Femtosecond-written volume Bragg gratings in fluoride glasses

Lauris Talbot, Daniel Richter, Maximilian Heck, Stefan Nolte, and Martin Bernier

DOI: 10.1364/OL.396022 Received 29 Apr 2020; Accepted 27 May 2020; Posted 28 May 2020  View: PDF

Abstract: We report on what we believe are the first volume Bragg gratings written inside bulk multicomponent fluoride glasses. The gratings inscribed with tightly focused IR-femtosecond pulses in combination with the phase-mask technique exhibit refractive index modulations of up to 5 x 10^(-4) with reflectivities up to 90% at a wavelength near 2.8 µm. Such highly compact and narrow band filters could have a significant impact on numerous high-end applications from UV to the mid-IR.

Generation of 0.7 mJ multicycle 15 THz radiation by phase-matched optical rectification in lithium niobate

Dogeun Jang, Jae Hee Sung, Seong Ku Lee, Chul Kang, and Ki-Yong Kim

DOI: 10.1364/OL.393913 Received 30 Mar 2020; Accepted 27 May 2020; Posted 27 May 2020  View: PDF

Abstract: We demonstrate efficient multicycle terahertz pulse generation at 14.6 THz from large-area lithium niobate crystals by using high-energy (up to 2 J) femtosecond Ti:sapphire laser pulses. Such terahertz radiation is produced by phase-matched optical rectification in lithium niobate. Experimentally, we achieve maximal terahertz energy of 0.71 mJ with conversion efficiency of ~0.04%. We also find that the effective interaction length for optimal terahertz conversion is fundamentally limited by terahertz absorption, laser pulse stretching by material dispersion, and terahertz-induced nonlinear cascading effects on the driving laser pulse.

Active terahertz time differentiator using piezoelectric micromachined ultrasonic transducer array

Fatemeh Amirkhan, Alexandre Robichaud, xavier ropagnol, Mathieu Gratuze, Tsuneyuki Ozaki, Frédéric Nabki, and Francois Blanchard

DOI: 10.1364/OL.393917 Received 30 Mar 2020; Accepted 27 May 2020; Posted 27 May 2020  View: PDF

Abstract: The rapid growth of information technology is closely linked to our ability to modulate and demodulate a signal, whether in the frequency or in the time domain. This essential step allows the multiplexing of information as well as its portability to different media. For electromagnetic radiations in the terahertz (THz) frequency range, this critical element is part of an active research area with enormous potential for the future of telecommunications and ultra-fast sensing. Recent demonstrations of THz modulation involve active semiconductor metamaterial surfaces or use a grating-based micromirror for frequency offset tuning. However, a wideband and active differentiator in the THz frequency band is yet to be demonstrated. Here, we propose a simple method to differentiate a THz pulse by inducing tiny phase changes on the THz beam path using a piezoelectric micromachined ultrasonic transducers array. We precisely demonstrate that the THz signal detected after the piezoelectric device is proportional to the first-order derivative of the THz pulse. The proposed technique will be able to support a wide range of THz applications, from edge detection for imaging to peak detection schemes for telecommunication systems, in addition to improving the sensitivity of THz spectroscopic methods. Furthermore, as it is a matrix modulator in which each transducer is electronically controllable, the development of new spatiotemporal computational information retrieval in the THz frequency range is highly anticipated.

Coupler-Induced Phase Matching of ResonantHyperparametric Scattering.

Anatoliy Savchenkov, Andrey Matsko, and Skip Williams

DOI: 10.1364/OL.394996 Received 15 Apr 2020; Accepted 27 May 2020; Posted 28 May 2020  View: PDF

Abstract: We show that an evanescent field coupler can break the symmetry of a high quality factor monolithic ring microcavity enabling generation of strongly nondegenerate frequency harmonics involving a few mode families that are orthogonal in an unperturbed microcavity. Using this property we explain observed experimentally generation of broadband frequency combs in magnesium fluorite whispering gallery mode resonators pumped in the strong normal dispersion regions, either at 780 nm or 493 nm.

Enhanced Optical Edge Detection based on Pancharatnam-Berry Flat Lens with Large Focal Length

Ting Li, Yang Yang, Xinyang Liu, Yan Wu, Yuan Zhou, Siji Huang, Xiaochun Li, and Huihui Huang

DOI: 10.1364/OL.395879 Received 27 Apr 2020; Accepted 27 May 2020; Posted 28 May 2020  View: PDF

Abstract: In this paper, we employ photoaligned liquid crystals to record flat lens pattern through a simple exposure method to produce a large focal length Pancharatnam-Berry (PB) lens with a film thickness of 2μm, without using expensive equipment. This PB lens with focal length of 240.6m (at 532nm) can transmit up to 97% across visible wavelengths and maintain high diffraction efficiency (>90%). An improved two-dimensional optical edge detection design based on this lens is proposed. We experimentally demonstrate the integrity and high-efficiency of edge information which may play an important role in the application of image processing and high-contrast microscopy.

Compact three-dimensional computational imaging using dynamic virtual camera

Anhu Li, Xingsheng Liu, and Zhao Zusheng

DOI: 10.1364/OL.395983 Received 22 Apr 2020; Accepted 27 May 2020; Posted 29 May 2020  View: PDF

Abstract: We present a three-dimensional computational imaging architecture based on the imaging principle of a dynamic virtual camera, which enables the spatial reconstruction using a single camera and a compact wedge prism device. By rotating the prism for camera boresight adjustment, the proposed system can capture an object from different viewpoints. Each image captured appears to be recorded directly with one virtual camera moving in a certain path, facilitating the computational process for stereo matching and profile reconstruction. The experimental results have demonstrated that our architecture allows a compact and flexible system to achieve the three-dimensional imaging performance competitive to conventional stereovision.

Observation of doubly-periodic solutions of the nonlinear Schrödinger equation in optical fibers

Guillaume Vanderhaegen, Pascal Szriftgiser, Alexandre Kudlinski, Corentin Naveau, Matteo Conforti, Stefano Trillo, Nail Akhmediev, and Arnaud Mussot

DOI: 10.1364/OL.394604 Received 08 Apr 2020; Accepted 26 May 2020; Posted 27 May 2020  View: PDF

Abstract: We report the first experimental observation of doubly-periodic first-order solutions of the nonlinear Schrödinger equation in optical fibers. We confirm, experimentally, the existence of A and B type solutions. This is done by using the initial conditions that consists of a strong pump and two weak sidebands. The evolution of power and phase of the main spectral components is recorded using sophisticated heterodyne time-domain reflectometry. Another important part of our experiment is active loss compensation. With this setup, we reach a good agreement between theory and experiment.

Time-Gated Luminescence Imaging for Background Free in vivo Tracking of Single Circulating Tumor Cells

Xu Cao, Cuiping Yao, Shudong Jiang, Jason Gunn, Austin Van Namen, Petr Brůža, and Brian Pogue

DOI: 10.1364/OL.391350 Received 26 Feb 2020; Accepted 26 May 2020; Posted 28 May 2020  View: PDF

Abstract: Fluorescence imaging is severely limited by background and autofluorescence of tissues for in vivo detection of circulating tumor cells (CTCs). Time-gated luminescence (TGL) imaging in combination with luminescent probes that possess hundreds of microsecond emission lifetimes can be used to effectively suppress this background, which has predominantly nanosecond lifetimes. This work demonstrated the feasibility of TGL imaging using luminescent probes for in vivo real time imaging and tracking of single CTCs circulating freely in the blood vessels with higher accuracy given by substantially higher signal to background noise. The luminescent probe used in this work was a commercial Eu3+ chelate (EuC) nanospheres with super long lifetime of near 800 μs, which enabled TGL imaging to achieve background free detection with ~5 times higher signal to noise ratio (SNR) versus steady state. Phantom and in vivo mouse studies indicated that EuC labeled tumor cells moving in medium or bloodstream at the speed of 1~2 mm/s could be captured in real time.

Nonlinear time-lens with improved power efficiency through a discrete multilevel pump

Manuel Fernández, Luis Romero Cortes, Saikrishna Konatham, Benjamin Crockett, Laureano Bulus Rossini, Pablo Costanzo Caso, and Jose Azana

DOI: 10.1364/OL.396342 Received 29 Apr 2020; Accepted 26 May 2020; Posted 26 May 2020  View: PDF

Abstract: We report a novel all-optical discrete multilevel time-lens (DM-TL) design based on cross-phase modulation (XPM). In this approach, the pump is synthesized such as the quadratic phase modulation is applied to the probe in constant-level time-bins with a maximum phase excursion of 2π. As a result, a considerable reduction in the required pump power is achieved incomparison to the conventional approach based on a parabolic pump. To illustrate the concept, the proposed DM-TL is here applied to the energy-preserving conversion of a continuous-wave (CW) signal into a train ofpulses according to the theory of temporal Talbot array illuminators. We demonstrate CW-to-pulse conversion gains up to 12 at repetition rates exceeding 16 GHz, with a power saving with respect to the conventional parabolic TL that is more significant for increasing conversion gains.

Localized vortex beams in anisotropic Lieb lattices

Cristian Mejía Cortés, Jorge Castillo, and Mario Molina

DOI: 10.1364/OL.397222 Received 12 May 2020; Accepted 26 May 2020; Posted 26 May 2020  View: PDF

Abstract: We address the issue of nonlinear modes in a twodimensionalwaveguide array, spatially distributed inthe Lieb lattice geometry, and modeled by a saturablenonlinear Schrödinger equation. In particular, we analyzethe existence and stability of vortex-type solutionsfinding localized patterns with symmetric and asymmetricprofiles, ranging from topological charge S = 1to S = 3. By taking into account the presence ofanisotropy, which is inherent to experimental realizationof waveguide arrays, we identify different stabilitybehaviors according to their topological charge. Ourfindings might give insight on experimental feasibilityto observe these kind of vortex states.

Hypersensitive H2 Sensor Based on Polymer Planar Bragg Gratings Coated with Pt-Loaded WO3-SiO2

Stefan Kefer, Jixiang Dai, Minghong Yang, Bernhard Schmauss, and Ralf Hellmann

DOI: 10.1364/OL.395341 Received 16 Apr 2020; Accepted 26 May 2020; Posted 26 May 2020  View: PDF

Abstract: This letter demonstrates a novel hydrogen sensor based on a polymer planar Bragg grating coated with Pt-loaded WO3-SiO2. The reflected Bragg signal shows a distinct peak splitting correlated to substrate anisotropies originating from the injection molding process. Especially at low H2 concentrations, both sensing peaks exhibit an outstanding response to the heat generated by the exothermic reaction between hydrogen molecules and coating. Thereby, a hydrogen volume ratio of 50 ppm leads to a Bragg wavelength shift of -37 pm which yields an outstandingly low detection limit of only 10 ppm H2 in air. Thus, functionalized polymer planar Bragg gratings are eminently suitable for H2 leak detection applications.

Ultracompact optical fiber acoustic sensors based on fiber-top spirally-suspended optomechanical microresonator

Mian Yao, Yangxi Zhang, Xia Ouyang, A. Ping Zhang, Hwa Yaw Tam, and Ping Kong Wai

DOI: 10.1364/OL.393900 Received 27 Mar 2020; Accepted 25 May 2020; Posted 26 May 2020  View: PDF

Abstract: Acoustic wave sensors with high sensitivity and small size are highly desired for a wide variety of important and emerging applications such as photoacoustic gas sensing and bio-imaging. Here, we present an ultracompact optical fiber acoustic sensor based on an optomechanical resonator that is directly in-situ printed on the end face of a standard single-mode optical fiber by using an optical 3D μ-printing technology. The fiber-top optomechanical microresonator is composed of a microscale suspended polymer micro-disk that forms a Fabry-Pérot (FP) interferometric cavity together with the optical fiber end-face and acts as the acoustic wave-sensitive micromechanical resonator, simultaneously. The microbeams for suspending the micro-disk are devised with a spiral structure to overcome the small-size imposed low deflection amplitude so as to improve its sensitivity to acoustic waves. The sensor with high sensitivity of 118.3 mV/Pa and low noise equivalent acoustic signal level of 0.328 μPa/Hz½ at audio frequency is experimentally demonstrated. Moreover, with a resonance amplification mechanism, the sensitivity can be enhanced by 40.1 times when the frequency of the acoustic wave matches with the natural resonance frequency of the optomechanical resonator. Such an ultra-small fiber-tip acoustic sensor has not only miniaturization-induced broad bandwidth but also structure-enhanced ultrahigh sensitivity and thus is very promising in various acoustic wave-based sensing, imaging and testing applications.

Sub-MHz linewidth 780.24 nm distributed feedback laser for ⁸⁷Rb applications

Eugenio Di Gaetano, Scott Watson, Euan McBrearty, Marc Sorel, and Douglas Paul

DOI: 10.1364/OL.394185 Received 02 Apr 2020; Accepted 25 May 2020; Posted 26 May 2020  View: PDF

Abstract: A distributed feedback GaAs-based semiconductor laser with a laterally-coupled grating is demonstrated at a wavelength of 780.24 nm with up to 60 mW power. A mode expander and aluminium free active layers have been used to reduce the linewidth to 612 kHz whilst maintaining high output power. The laser demonstrates over 40 dB side-mode suppression ratio with > 0.3 nm of tuning suitable for atom cooling experiments with the D2 ⁸⁷Rb atomic transition. This laser has substantial potential to be integrated into miniaturised cold atom systems.

Adaptive optics approach to surface-enhanced Raman scattering

Mariia Shutova, Alexander Sinyukov, Blake Birmingham, Zhenrong Zhang, and Alexei Sokolov

DOI: 10.1364/OL.394548 Received 06 Apr 2020; Accepted 25 May 2020; Posted 26 May 2020  View: PDF

Abstract: Surface-enhanced Raman scattering (SERS) spectroscopy is a popular technique for detecting chemicals in small quantities. Rough metallic surfaces with nanofeatures are ones of the most widespread and commercially successful substrates for efficient SERS measurements. A rough metallic surface creates a high-density random distribution of so-called ‘hot spots’ with local optical field enhancement causing Raman signal to increase. In this letter we revisit the classic SERS experiment [Surf. Sci. \textbf{158}, 229(1985)] with rough metallic surfaces covered by a thin layer of copper phthalocyanine molecules. As a modification to the classic configuration, we apply an adaptive wavefront correction of a laser beam profile. As a result we demonstrate an increase in brightness of local SERS hot spots and redistribution of Raman signal over the substrate area. We hypothesize that the improvement is due to optimal coupling of the shaped laser beam to the random plasmonic nanoantenna configurations. We show that the proposed adaptive-SERS modification is independent of the exact structure of the surface roughness and topography, works with many rough surfaces, and gives brighter Raman hot spots in comparison with conventional SERS measurements. We prove that the adaptive-SERS is a powerful instrument for improving SERS sensitivity.

Single-frequency Q-switched Er:YAG laser with high frequency and energy stability via Pound-Drever-Hall locking method

Chaoyong Chen, Qing Wang, Shuai Huang, Xu Zhang, Kaixin Wang, gao mingwei, and Chunqing Gao

DOI: 10.1364/OL.396501 Received 01 May 2020; Accepted 25 May 2020; Posted 26 May 2020  View: PDF

Abstract: We present an injection-seeding Q-switched 1645 nm Er:YAG ceramic laser with the high frequency stability and energy stability by combining injection-seeding technique and Pound-Drever-Hall technique. Energy of 10.31 mJ single-frequency pulses with the optimal frequency stability (525kHz) and relative energy stability (0.52%) at a high pulse repetition rate of 1kHz are obtained. To the best of our knowledge, this is the highest frequency stability and energy stability so far in the high PRF, large pulse energy single-frequency Q-switched Er-doped solid laser. This single frequency laser with high stability provides an excellent light source for coherent lidar system.

Large-scale multiplexed weak reflector array fabricated with femtosecond laser for fiber-optic quasi-distributed acoustic sensing system

Mengshi Wu, Chi Li, Xinyu Fan, Changrui Liao, and Zuyuan He

DOI: 10.1364/OL.395725 Received 20 Apr 2020; Accepted 25 May 2020; Posted 26 May 2020  View: PDF

Abstract: In this letter, we propose a large-scale multiplexed weak reflector array fabrication method by using femtosecond laser with relatively high reflectivity and low transmission loss. This kind of weak reflector array can be used in quasi-distributed acoustic sensing system as the sensing fiber instead of single mode fiber (SMF) to achieve an ultra-high signal-to-noise ratio (SNR). An automated fabrication system is designed, and a reflector geometric structure with high reflectivity and low scattering loss is designed based on this system. As a prototype to demonstrate the performance, one thousand weak reflectors are written on the SMF with an interval of 10 m, a reflectivity around -42 dB and a transmission loss of 0.34 dB/km. Comparing with the method of using SMF as the sensing fiber, at least 15.8 dB enhancement to the SNR can be achieved by using the reflector array as the sensing fiber.

Plug-and-play adaptive optics for commercial laser scanning fluorescence microscopes based on a refractive device

Paolo Pozzi, Martino Quintavalla, aaron wong, gerard borst, Stefano Bonora, and Michel Verhaegen

DOI: 10.1364/OL.396998 Received 06 May 2020; Accepted 25 May 2020; Posted 26 May 2020  View: PDF

Abstract: In this paper we present a solution for simple implementation of adaptive optics in any existing laser scanning fluorescence microscope. Adaptive optics is implemented by the introduction of a multi-actuator adaptive lens between the microscope body and the objective lens. Correction is performed with a sensorless method by optimizing the quality of the images presented on screen by the microscope software. We present the results acquired on both a commercial linear excitation confocal microscope and a custom made multiphoton excitation microscope.

Wave-Vector Encoded Nonlinear Endo-Microscopy

Guan-Yu Zhuo, Po-Lin Tsai, HSIEN-YI Wang, and Ming-Che Chan

DOI: 10.1364/OL.395622 Received 21 Apr 2020; Accepted 25 May 2020; Posted 28 May 2020  View: PDF

Abstract: Based on a rigid square fiber for wave-vector delivery, we present a novel Wave-vector Encoded Nonlinear-optical Endo-microscopy (WENE). WENE overcomes three tangled issues, including femtosecond pulse broadening induced signal degradation, complexity of packaging miniaturized scanners in the distal end, and pixel-like images, which cannot be fully addressed by current distal-scanning nonlinear endo-microscopy (NE) or fiber-bundle-based proximal scanning NE. Due to the advantages of its simplicity in overall configuration and package in the distal end, the capability of addressing the issue of pulse broadening, and offering continuous wave-vector delivery, the demonstrated WENE shows great promises for future basic research on biomedical processes and minimally invasive utilization for clinical diagnosis.

Single mode surface-emitting DFB lasers with a large-area oxidized aperture based on surface grating

can liu, Pengfei Zhang, Minwen Xiang, xiang ma, Chun Jiang, Bao Tang, Qiaoyin Lu, and Weihua Guo

DOI: 10.1364/OL.394744 Received 09 Apr 2020; Accepted 25 May 2020; Posted 27 May 2020  View: PDF

Abstract: We propose and experimentally demonstrate an 850 nm single-mode surface-emitting distributed feedback (DFB) laser based on surface grating. The laser comprises a second-order grating section sandwiched by two first-order grating sections. The second-order grating provides not only surface emission but also phase shift to ensure the Bragg wavelength of the gratings to be the lasing wavelength. A high side-mode suppression ratio (SMSR) of 47 dB has been achieved. By employing a large-area rectangle-shaped oxidized aperture, a low threshold current of 1.8 mA and a low differential resistance of 59 Ω are successfully achieved in a horizontal-cavity surface-emitting laser.

Universal self-similar asymptotic behavior of optical bump spreading in random medium atop incoherent background: Reply to comment

Chunhao Liang, Xiaofei Li, Sergey Ponomarenko, Zhiheng Xu, Fei Wang, and Yangjian Cai

DOI: 10.1364/OL.398282 Received 21 May 2020; Accepted 25 May 2020; Posted 02 Jun 2020  View: PDF

Abstract: In his comment [1] Charnotskii claims that the cross-spectral densities of recently studied in [2] partially coherent beams atop a statistical background do not satisfy the non-negative definiteness requirement. We argue that Charnotskii’s claim stems from his misunderstanding of the non-negative definiteness concept as applied to the model of Ref. [2].

A new strategy for optical thermometry based on temperature dependent charge-transfer to Eu3+ 4f-4f excitation intensity ratio in Sr3Lu(VO4)3:Eu3+ and CaWO4:Nd3+

Qianyang Chang, Xianju Zhou, Xiao Zhou, Lingni Chen, Guotao Xiang, Sha Jiang, Li Li, Yanhong Li, and Xiao Tang

DOI: 10.1364/OL.396456 Received 04 May 2020; Accepted 24 May 2020; Posted 01 Jun 2020  View: PDF

Abstract: The optical thermometry has been developed as a promising temperature-sensing technique. We propose a new strategy of fluorescence intensity ratio (FIR) based on abnormal thermal quenching effect. In the phosphors of Sr3Lu(VO4)3:Eu3+ and CaWO4:Nd3+, the f-f emission intensity of the doped lanthanide ions declines with raising temperature upon the excitation of the charge transfer band (CTB) of the host. The abnormal thermal quenching is caused by the thermally-activated absorption. The opposite change tendency of M-O (M=V5+ or W6+) CTB and Ln3+ (Ln=Eu3+ or Nd3+) f-f transitions has been observed in the temperature-dependent excitation spectra and employed as the thermometric probe in ratiometric luminescent thermometry. The strategy applies to the FIR technique in lanthanide singly doped phosphors and eliminating the limitation of thermal-coupled levels. It opens up new possibilities of ratiometric optical thermometry. In addition, the derived maximum relative sensitivity (SR) is larger than the value obtained via thermal-coupled levels in the same sample. It illustrates that the optical thermometry based on abnormal thermal quenching might be a feasible and effective method.

Low-RF-Loss and Large-Rejection Reconfigurable Brillouin-Based RF Photonic Bandpass Filter

Matthew Garrett, Yang Liu, Pan Ma, Duk-Yong Choi, Steve Madden, and Benjamin Eggleton

DOI: 10.1364/OL.395477 Received 16 Apr 2020; Accepted 24 May 2020; Posted 26 May 2020  View: PDF

Abstract: We present a high-performance radio frequency (RF) photonic bandpass filter enabled by combining on-chip Brillouin scattering with a suppressed carrier phase modulation scheme. We achieve a low RF loss of 5 dB and a large stopband rejection of more than 40 dB, which represents a significant improvement of 20 dB to the RF passband gain and 31 dB to the RF rejection ratio over traditional modulation schemes under the same optical power consumption. We further demonstrate filter reconfigurability including multiple passbands, wide frequency (1-20 GHz) and bandwidth tunability (30-350 MHz) without compromising the RF performance.

All-optical logic gates using dielectric-loaded waveguides with quasi-rhombus metasurfaces

Chaonan Yao, Amer Kotb, Bin Wang, Subhash Singh, and Chunlei Guo

DOI: 10.1364/OL.396978 Received 05 May 2020; Accepted 23 May 2020; Posted 01 Jun 2020  View: PDF

Abstract: Nanostructure and nanoantenna-based all-optical (AO) devices have attracted significant research interests in recent years due to their small size, high information capacity, ultrafast processing, low power consumption, and overall practicality. Here, in this paper, we propose a novel metasurface having quasi-rhombus shaped antennas to modulate optical modes in a dielectric-loaded waveguide for the realization of a complete family of logic gates including NOT, AND, OR, XOR, NAND, NOR, and XNOR. These logic operations are realized using destructive and constructive interferences between the input optical signals. The high contrast ratios of about 33.39, 27.69, and 33.11 dB are achieved for the NAND, NOR, and XNOR logic gates, respectively with the speed as high as 108 Gb/s.

Efficient Cherenkov-type optical-to-terahertz converter with terahertz beam combining

Michael Bakunov, Evgeny Efimenko, Sergey Gorelov, Nikita Abramovsky, and Sergey Bodrov

DOI: 10.1364/OL.391871 Received 02 Mar 2020; Accepted 23 May 2020; Posted 26 May 2020  View: PDF

Abstract: A nonlinear structure for efficient Cherenkov-type terahertz emission from ultrashort laser pulses is proposed, modelled and experimentally demonstrated. The structure comprises a thin (a few tens of microns thick) layer of lithium niobate sandwiched between two silicon prisms. A focused to a line ultrashort laser pulse propagates in the lithium niobate layer and generates Cherenkov wedge of terahertz radiation in the prisms. The radiation experiences total internal reflection in the prisms and emerges into free space as two adjacent terahertz beams collinear to the pump laser beam. The structure can generate a cm-wide terahertz beam with high transverse uniformity and flat frequency spectrum. The optical-to-terahertz conversion efficiency as high as 0.35% have been experimentally obtained with 10-μJ laser pulses. It can be further enhanced by reducing the thickness of the lithium niobate layer.

All-fiber third-order orbital angular momentum mode generation employ an asymmetric long-period fiber grating

XiaoDong He, Jiajing Tu, xiaowen wu, She Gao, Lei Shen, Chenglong Hao, Yuanhua Feng, Liu Wei ping, and Zhaohui Li

DOI: 10.1364/OL.394333 Received 03 Apr 2020; Accepted 23 May 2020; Posted 26 May 2020  View: PDF

Abstract: The third-order orbital angular momentum (OAM±3) guided mode generation is demonstrated for the first time by employing an asymmetric long-period fiber grating (AS-LPFG). The proposed AS-LPFG is modeled by coupled local-mode theory which is extended to the coupling of core modes and is fabricated by multi-cycle scanning ablation with increasing power in a six-mode fiber. The experiments demonstrate that one fabricated AS-LPFG can convert the LP01 mode to the third-azimuthal-order (3AO, LP31 or OAM±3) guided mode with efficiency of ~99.8%. The model and the method presented, in principle, can be used to generate any other high-order modes.

High-Speed Fiber-optic Scanning Nonlinear Endomicroscopy for imaging Neuron Dynamics in vivo

Hyeon-Cheol Park, Honghua Guan, Ang Li, Yuanlei Yue, Ming-Jun Li, Hui Lu, and Xingde Li

DOI: 10.1364/OL.396023 Received 24 Apr 2020; Accepted 23 May 2020; Posted 26 May 2020  View: PDF

Abstract: Fiber-optically based two-photon fluorescence endomicroscopy is emerging as an enabling technology for in vivo histological imaging of internal organs and functional neuronal imaging on freely-behaving animals. However, high-speed imaging remains challenging due to the expense of miniaturization and lack of suited fast beam scanners. For many applications, higher imaging speed is highly desired especially for monitoring functional dynamics such as transient dendritic responses in neuroscience. This work reports the development of a fast fiber-optic scanning endoscope with an imaging speed higher than 26 frames/sec. In vivo neural dynamics imaging with the high-speed endomicroscope was performed on a freely-behaving mouse over the primary motor cortex that expressed GCaMP6m. The results demonstrate its capability of real-time monitoring of transient neuronal dynamics with very fine temporal resolution.

Cross-Phase Modulation Instability in PM ANDi Fiber-Based Supercontinuum Generation

Etienne Genier, AMAR GHOSH, Swetha Sampath Bobba, Patrick Bowen, Peter Moselund, Ole Bang, John Dudley, and Thibaut Sylvestre

DOI: 10.1364/OL.397106 Received 07 May 2020; Accepted 22 May 2020; Posted 26 May 2020  View: PDF

Abstract: We demonstrate broadband supercontinuum generation in an all-normal dispersion polarization-maintaining photonic crystal fiber and we report the observation of a cross-phase modulation instability sideband that is generated outside of the supercontinuum bandwidth. We demonstrate this sideband is polarized on the slow axis and can be suppressed by pumping on the fiber's fast axis. We theoretically confirm and model this nonlinear process using phase-matching conditions and numerical simulations, obtaining good agreement with the measured data.

Fast beam steering with an optical phased array

David Gozzard, Lyle Roberts, James Spollard, Paul Sibley, and Daniel Shaddock

DOI: 10.1364/OL.393007 Received 17 Mar 2020; Accepted 22 May 2020; Posted 26 May 2020  View: PDF

Abstract: Optical phased arrays (OPAs) are devices that use the coherence of light to control the interference pattern in the far field, enabling them to steer a laser beam with no moving parts. As such, OPAs have potential applications in laser communications, target acquisition and tracking, metrology, and directed energy. Here we present a control architecture for an actively phase locked OPA capable of steering a laser beam at speeds limited by the actuation bandwidth of electro-optic modulators. The system achieved an output phase stability of λ/770 and steering speeds up to 1 MHz. The digital control architecture can be extended to GHz steering speeds, is readily scalable to hundreds of emitters, and is compatible with high-power arrays.

Optical design of connecting electrodes for tandem organic light-emitting diodes

Man Jiaxiu, Shou-Jie He, Chang-Sheng Shi, Han-Nan Yang, Deng-Ke Wang, and Zheng Lu

DOI: 10.1364/OL.388303 Received 15 Jan 2020; Accepted 22 May 2020; Posted 26 May 2020  View: PDF

Abstract: Connecting electrode plays a crucial role to assist charge injection into the adjacent electroluminescent units in tandem organic light-emitting diodes (OLEDs). In this study, we demonstrate that Mg:Ag alloy is an effective connecting electrode for bottom- and top-emitting tandem OLEDs. Optical cavity design and simulation are also conducted to predict the luminance efficiency of tandem OLEDs. It is found that the theoretical luminance efficiency of tandem OLED is close to but no higher than twofold enhancement over the luminance efficiency of single OLED optimized to the first resonance mode, which is theoretically higher than high order resonance modes. It is also found that the optical properties of Mg:Ag connecting electrodes, while have relatively small influence on weak microcavity bottom-emitting tandem OLEDs, have large influence on strong microcavity top-emitting tandem OLEDs.

Mid-infrared frequency combs at 10 GHz

Abijith Kowligy, David Carlson, Dan Hickstein, Henry Timmers, Alexander Lind, Peter Schunemann, Scott Papp, and Scott Diddams

DOI: 10.1364/OL.391651 Received 28 Feb 2020; Accepted 22 May 2020; Posted 26 May 2020  View: PDF

Abstract: We demonstrate mid-infrared (MIR) frequency combs at 10 GHz repetition rate via intra-pulse difference-frequency generation (DFG) in quasi-phase-matched nonlinear media. Few-cycle pump pulses (≤15 fs, 80 pJ) from a near-infrared (NIR) electro-optic frequency comb are provided via nonlinear soliton-like compression in photonic-chip silicon-nitride waveguides. Subsequent intra-pulse DFG in periodically-poled lithium niobate waveguides yields MIR frequency combs in the 3.1--4.8 μm region, while orientation-patterned gallium phosphide provides coverage across 7--11 μm. Cascaded second-order nonlinearities simultaneously provide access to the carrier-envelope-offset frequency of the pump source via in-line f--2f nonlinear interferometry. Our experiments provide a straightforward method to generate high-repetition rate frequency combs suitable for condensed phase spectroscopy and applications such as laser heterodyne radiometry.

All-fiber self-mixing laser Doppler velocimetry with much less than 0.1pW optical feedback based on adjustable gain

Yuanyang Zhao, Desheng Zhu, Youze Chen, Yourui Tu, Tiezhu Bi, Yunkun Zhao, Benli Yu, and Liang Lu

DOI: 10.1364/OL.397819 Received 18 May 2020; Accepted 22 May 2020; Posted 27 May 2020  View: PDF

Abstract: The all-fiber self-mixing laser Doppler velocimetry with the adjustable gain is experimentally investigated based on distributed Bragg reflector fiber laser. In the measurement system, the modulation gain of the injected light in the laser cavity is adjusted by a pair of fiber coupled AOMs in the external cavity, which can change the intensity and frequency of the self-mixing modulation effect. Simultaneously, the minimum feedback intensity from the target to the laser for successful measurements is 0.063 pW. Thus the all-fiber laser velocimetry can adapt to the detection of ultra-weak optical feedback and wide-range velocity measurements in various complex scenes.

Dynamic beam splitter employing an all-dielectric metasurface based on elastic substrate

Hasan Kocer, Yilmaz Durna, Hamza Kurt, and Ekmel Ozbay

DOI: 10.1364/OL.392872 Received 16 Mar 2020; Accepted 21 May 2020; Posted 27 May 2020  View: PDF

Abstract: Beam splitters are an indispensable part of optical measurements and applications. We propose a dynamic beam splitter incorporating all-dielectric metasurface in an elastic substrate under external mechanical stimulus of stretching. The optical behavior at 720 nm wavelength shows that it can be changed from a pure optical-diode like behavior to a dynamic beam splitter. Even though the structure is designed to run at 720 nm, the approach is scalable to any wavelength. Various cases including wavelength and polarization dependencies are thoroughly investigated to demonstrate the principles of operating conditions of two different regimes of the designed metasurface.

Extremely low light scattering in nanostructured glasses formed by simultaneous nucleation and diffusion-limited growth of particles: modelling

Mikhail Shepilov

DOI: 10.1364/OL.386605 Received 06 Jan 2020; Accepted 21 May 2020; Posted 26 May 2020  View: PDF

Abstract: A structural model proposed earlier is used to simulate light scattering in nanostructured glass formed by diffusional growth of simultaneously nucleated particles with small volume fraction. Computer simulation of the structure allows one to calculate partial structure factors for polydisperse particles and to estimate light scattering intensity in the interference approximation. Interference effects lead to a significant decrease in scattered intensity compared with independent scattering in a range of long wavelengths. In some spectral range, the dependence of scattered intensity on the inverse wavelength is described by power function with the exponent p = 7.1 which is appreciably greater than the Rayleigh value 4.

A tunable UV spectrometer for Doppler broadening thermometry of mercury

Cecilia Clivati, Stefania Gravina, Antonio Castrillo, Giovanni Costanzo, Filippo Levi, and Livio Gianfrani

DOI: 10.1364/OL.393793 Received 02 Apr 2020; Accepted 21 May 2020; Posted 26 May 2020  View: PDF

Abstract: We realised a tunable UV laser source at 253 nm for Doppler-broadening thermometry on the ¹S₀-³P₁ intercombination line in mercury vapors. Our setup is based on the two-stage second harmonic generation of a 1015 nm diode laser in a fiber-coupled periodically-poled lithium niobate waveguide crystal and a beta-barium borate crystal in enhancement cavity, and we exploit injection locking of a 507 nm diode laser to boost the available optical power after the first duplication. The realized source has 1E-4 relative intensity stability, Gaussian shape and over 10 GHz mode-hop-free tunable range. These features are necessary for retrieving the thermodynamic temperature of the atomic sample from the absorption profile with 1E-6 accuracy. This will make Doppler broadening thermometry a viable technique for the practical realization of the kelvin in the new International System of Units.

Multi-foci metalens for terahertz polarization detection

Ruoxing Wang, Jin Han, Jianlong Liu, Hao Tian, Weimin Sun, Li Li, and Xianzhong Chen

DOI: 10.1364/OL.395580 Received 20 Apr 2020; Accepted 21 May 2020; Posted 26 May 2020  View: PDF

Abstract: We propose a reflective terahertz (THz) metalens with four focal points for polarization detection of THz beams. The metalens is composed of Z-shaped resonators with spatially variant orientations, a reflective gold layer and a dielectric spacer between them. The polarization states of the focal points include left circular polarization, right circular polarization, incident polarization state and a polarization state whose major axis is rotated π/4 in comparison with that of the incident polarization. The handedness, the ellipticity and the major axis of the polarization state can be determined based on the light intensities of the focal points. The uniqueness of the designed device renders this technique very attractive for applications in compact THz polarization detection, communications and information processing.

Multi optical bottles from the second order chirpedsymmetric Airy vortex beams

chuangjie xu, WU YO, and Dongmei Deng

DOI: 10.1364/OL.388569 Received 17 Jan 2020; Accepted 21 May 2020; Posted 21 May 2020  View: PDF

Abstract: In this letter, we introduce a new structure of multi optical bottles formed by the second order chirped symmetric Airy vortex beams (CSAVBs). The numbers and the locations of the optical bottles embedded in the CSAVBs depend on the numbers and the locations of the vortices. Besides, the lengths and the sizes of the optical bottles could be adjusted by changing the chirp parameter and the topological charges of the vortices, respectively. We believe that our results can diversify the optical tweezers system for multi particle manipulation.

Thermal accumulation at kilohertz repetition rates inside fused silica under ultrafast laser irradiation

Xiao Jia and Xin Zhao

DOI: 10.1364/OL.396360 Received 29 Apr 2020; Accepted 21 May 2020; Posted 21 May 2020  View: PDF

Abstract: Thermal accumulation effect has been proved to reduce ablation threshold and improve ablation rate during multi-pulse ultrafast laser ablation. It was widely believed that this effect cannot be triggered until laser repetition rate is raised to MHz range. In this letter, for the first time, we experimentally discover strong thermal accumulation in fused silica at kilohertz repetition rates and its significant contribution to enhance ablation rate. It is found that the threshold repetition rates to trigger thermal accumulation are intrinsically determined by material thermal diffusivity and insensitive to ambient conditions. We observe two-fold enhancement of ablation rate and clearly discriminate the contribution from thermal and non-thermal accumulation effects by 35% and 50%~70%, respectively. A multi-physics model is developed to assist the understanding of the process. This study promotes the fundamental understanding of thermal/non-thermal accumulation effects and opens the door to low-repetition-rate thermal accumulation for low thermal diffusivity materials.

Diffractometry based vortex beams fractional topological charge measurement

S. Mohammad Ali Hosseini Saber, Ehsan Ahadi Akhlaghi, and Ahad Saber

DOI: 10.1364/OL.395440 Received 12 May 2020; Accepted 20 May 2020; Posted 20 May 2020  View: PDF

Abstract: In this letter, we investigate the Fresnel diffraction of vortex beams from a phase plate and propose a novel method to determine the fractional part of the topological charge of vortex beams. When a vortex beam with a fractional topological charge illuminates the edge region of a transparent plate, the visibility of the diffraction pattern on two sides of the beam will be different. Rotation of the phase plate changes the visibility on the left and right sides of the beam, periodically. By measuring three consecutive angles of the minimum visibilities the fractional part of the topological charge is obtained. The proposed method is verified experimentally and is shown to be independent of the phase plate and vortex beam parameters. The precision of the method is obtained better than 0.01.

Enhancing the Kramers-Kronig Receiver via Dispersion-Based Spatial Diversity

Lior Blech, Cristian Antonelli, Antonio Mecozzi, Yonina Eldar, and Mark Shtaif

DOI: 10.1364/OL.393514 Received 24 Mar 2020; Accepted 20 May 2020; Posted 20 May 2020  View: PDF

Abstract: We report a scheme for reconstructing the complex envelope of an optical signal from two decorrelated measurements of its intensity. The decorrelation is achieved by splitting the received optical signal intotwo copies, and by dispersing one of the copies prior to photo detection. The reconstructed complex-valued signal is obtained by means of an iterative algorithm that requires only a few tens of iterations. The startingpoint of the search procedure is produced by Kramers-Kronig (KK) reconstruction. With this procedure the continuous-wave tone that accompanies the received signal is reduced by 5 dB to 6 dB compared to the requirement of a KK receiver alone.

Utilizing adaptive optics to mitigate intra-modal-group power coupling of graded-index few-mode fiber in a 200-Gbit/s mode-division-multiplexed link

Runzhou Zhang, Hao Song, Haoqian Song, Zhe Zhao, Giovanni Milione, Kai Pang, Jing Du, Long Li, Kaiheng Zou, Huibin Zhou, Cong Liu, karapet Manukyan, Nanzhe Hu, Ahmed Almaiman, Jeffery Stone, Ming-Jun Li, Brittany Lynn, Robert Boyd, Moshe Tur, and Alan Willner

DOI: 10.1364/OL.394307 Received 03 Apr 2020; Accepted 20 May 2020; Posted 20 May 2020  View: PDF

Abstract: We experimentally demonstrate the utilization of adaptive optics (AO) to mitigate intra-group power coupling among linearly-polarized (LP) modes in a graded-index few-mode fiber (GI FMF). Generally in this fiber, the coupling between degenerate modes inside a modal group tends to be stronger than between modes belonging to different groups. In our approach, the coupling inside the LP11 group can be represented by a combination of orbital-angular-momentum (OAM) modes, such that reducing power coupling in OAM set tends to indicate the capability to reduce the coupling inside the LP11 group. We employ two output OAM modes l=+1 and l=-1 as resultant linear combinations of degenerate LP11a and LP11b modes inside the LP11 group of a ~0.6-km GI FMF. The power coupling is mitigated by shaping the amplitude and phase of the distorted OAM modes. Each OAM mode carries an independent 20-, 40-, or 100-Gbit/s quadrature-phase-shift-keying (QPSK) data stream. We measure the transmission matrix (TM) in the OAM basis within LP11 group, which is a subset of the full LP TM of the FMF-based system. An inverse TM is subsequently implemented before the receiver by a spatial light modulator (SLM) to mitigate the intra-modal-group power coupling. With AO mitigation, the experimental results for l=+1 and l=-1 modes show, respectively, that: (i) intra-modal-group crosstalk is reduced by > 5.8 dB and > 5.6 dB, (ii) near-error-free bit-error-rate (BER) performance is achieved with a penalty of ~ 0.6 dB and ~ 3.8 dB, respectively.

All-fiber Mamyshev oscillator enabled by chirped fiber Bragg gratings

Vincent Boulanger, Michel Olivier, Félix Guilbert-Savary, Francois Trepanier, Martin Bernier, and Michel Piche

DOI: 10.1364/OL.396218 Received 24 Apr 2020; Accepted 20 May 2020; Posted 20 May 2020  View: PDF

Abstract: We present a linear cavity self-polarizing Mamyshev oscillator at 1550 nm fully made of polarization-maintaining fibers. The cavity filters are chirped fiber Bragg gratings with a gaussian reflectivity profile which allow for a greater reflectivity, larger bandwidth and dispersion control. This mode-locked fiber laser architecture shows an unprecedented simplicity while delivering 21.3 nJ pulses compressed to 108 fs with a competitive 22.3 % pump conversion efficiency. Mode locking is initiated with an external saturable absorber mirror. Numerical simulations show how nonlinearity can be managed with highly dispersive filters inside a Mamyshev oscillator.

How good is your metalens? Experimental verification of metalens performance criterion

Jacob Engelberg, Talia Wildes, Chen Zhou, Noa Mazurski, Jonathan Bar-David, Anders Kristensen, and Uriel Levy

DOI: 10.1364/OL.394680 Received 07 Apr 2020; Accepted 20 May 2020; Posted 26 May 2020  View: PDF

Abstract: A metric for evaluation of overall metalens performance is presented. It is applied to determination of optimal operating spectral range of a metalens, both theoretically and experimentally. This metric is quite general and can be applied to the design and evaluation of future metalenses, particularly achromatic metalenses.

In-system optimization of hologram for high-stability parallel laser processing

HONGHAO ZHANG, Satoshi Hasegawa, Hidetomo Takahashi, Haruyoshi Toyoda, and Yoshio Hayasaki

DOI: 10.1364/OL.392578 Received 20 Mar 2020; Accepted 19 May 2020; Posted 20 May 2020  View: PDF

Abstract: A method for optimizing a computer-generated hologram (CGH) for high-stability laser processing is proposed. The CGH is optimized during laser processing; therefore, unpredicted dynamic changes in the laser processing system, in addition to its static imperfections, are automatically compensated for by exploiting the rewritable capability of the spatial light modulator. Consequently, the short-term and long-term stability are improved, which will contribute to the realization of high-speed, high-precision laser processing. A CGH that generated 36 parallel beams was continuously optimized, and the maximum uniformity reached 0.98, which is higher than reported in previous research. This is the first demonstration of gradual improvement of parallel laser processing with in-process optimization of the CGH. Furthermore, it was also demonstrated that the performance of the laser processing system against unexpected disturbances was improved.

Super-resolved multispectral lensless microscopy via angle-tilted, wavelength-multiplexed ptychographic modulation

Pengming Song, Ruihai Wang, Jiakai Zhu, Tianbo Wang, Zichao Bian, Zibang Zhang, Kazunori Hoshino, Michael Murphy, Shaowei Jiang, Chengfei Guo, and Guoan Zheng

DOI: 10.1364/OL.394923 Received 10 Apr 2020; Accepted 19 May 2020; Posted 20 May 2020  View: PDF

Abstract: We report an angle-tilted, wavelength-multiplexed ptychographic modulation approach for multispectral lensless on-chip microscopy. In this approach, we illuminate the specimen with lights at 5 wavelengths simultaneously. A prism is added at the illumination path for spectral dispersion. Lightwaves at different wavelengths, thus, hit the specimen at slightly different incident angles, breaking the ambiguities in mixed state ptychographic reconstruction. At the detection path, we place a thin diffuser in-between the specimen and the monochromatic image sensor for encoding the spectral information into 2D intensity measurements. By scanning the sample to different x-y positions, we acquire a sequence of monochromatic images for reconstructing the 5 complex object profiles at the 5 wavelengths. An up-sampling procedure is integrated into the recovery process to bypass the resolution limit imposed by the imager pixel size. We demonstrate a half-pitch resolution of 0.55 µm using an image sensor with 1.85-µm pixel size. We also demonstrate quantitative and high-quality multispectral reconstructions of stained tissue sections for digital pathology applications.

K-means clustering of coherent Raman spectra from extracellular vesicles visualized by label-free multiphoton imaging

Yi Sun, Ethan Chen, Jalen Thomas, Yuan Liu, Haohua Tu, and Stephen Boppart

DOI: 10.1364/OL.395838 Received 23 Apr 2020; Accepted 19 May 2020; Posted 19 May 2020  View: PDF

Abstract: Extracellular vesicles (EVs) have emerged as potential biomarkers in cancer research and for clinical diagnosis. Little is known, however, about their spatial distributions in tissue and the different sub-populations that may exist. Here we report the use of label-free nonlinear optical imaging techniques to provide spatially-resolved chemical information of EVs within untreated tissues. A multimodal nonlinear optical imaging system incorporating multiphoton autofluorescence and hyperspectral coherent anti-Stokes Raman scattering (CARS) imaging was built to visualize the spatial tissue distribution and probe the spectra of EVs. K-means clustering is performed on the CARS spectra from EVs in rat mammary tissues and human breast tumor tissue to reveal both the spatial distribution of EV clusters and their different chemical signatures. Correlations are identified between EV clusters and metabolic information.

Widefield localization fluorescence microscopy for transcranial imaging of cortical perfusion with capillary resolution

Zhenyue Chen, Quanyu Zhou, Justine ROBIN, and Daniel Razansky

DOI: 10.1364/OL.396123 Received 23 Apr 2020; Accepted 18 May 2020; Posted 20 May 2020  View: PDF

Abstract: Imaging of cerebral vasculature is impeded with the existing fluorescence microscopy methods due to intense light scattering in living tissues and the need for highly invasive craniotomy procedures to resolve structures on a capillary scale. We propose a widefield localization fluorescence microscopy technique for high-resolution transcranial imaging and quantitative assessment of cortical perfusion in mice. The method is based on tracking single fluorescent microparticles sparsely distributed in the blood stream using a simple CMOS camera and a continuous wave laser source. We demonstrate quantitative transcranial in vivo mapping of the blood flow velocity and direction at capillary level resolution (5µm) across the entire cortex. The new technique opens a new high resolution transcranial window into the brain function in health and disease.

Diffractive Optical Elements in Single Crystal Diamond

Thibault Wildi, Marcell Kiss, and Niels Quack

DOI: 10.1364/OL.393679 Received 25 Mar 2020; Accepted 18 May 2020; Posted 18 May 2020  View: PDF

Abstract: We demonstrate the design, fabrication and experimental characterization of near-field binary phase transmission Diffractive Optical Elements (DOEs) in Single Crystal Diamond. Top-hat and arbitrary pattern DOE beam shapers were numerically optimized using an Iterative Fourier Transform Algorithm (IFTA). Commercially available Single Crystal Diamond plates (3 mm x 3 mm x 0.3 mm) were patterned using hardmask deposition (α-Si), e-beam lithography and O₂ plasma based diamond reactive ion etching. The resulting binary phase relief patterns were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Experimental characterization of the Single Crystal Diamond DOEs in transmission at λ = 532 nm confirms excellent uniformity of the resulting top-hat beam profile as required in copper welding applications.

Perfectly vertical surface grating couplers using subwavelength engineering for increased feature sizes

Mohsen Kamandar Dezfouli, Yuri Grinberg, Daniele Melati, Pavel Cheben, Jens Schmid, Alejandro Sánchez-Postigo, Alejandro Ortega-Moñux, J. Gonzalo Wangüemert-Pérez, Ross Cheriton, Siegfried Janz, and Dan-Xia Xu

DOI: 10.1364/OL.395292 Received 15 Apr 2020; Accepted 18 May 2020; Posted 18 May 2020  View: PDF

Abstract: We present perfectly vertical grating couplers for the 220 nm SOI platform incorporating subwavelength metamaterials to increase the minimum feature sizes and achieve broadband low back-reflection. Our study reveals devices with high coupling efficiencies are distributed over a wide region of the design space with varied back-reflections, while still maintaining minimum feature sizes larger than 100 nm and even 130 nm. Using 3D-FDTD simulations, we demonstrate devices with broadband low back-reflection of less than -20 dB over more than 100 nm bandwidth centered around the C-band. A coupling efficiency of 72% and 67% are achieved for a minimum feature size of 106 nm and 130 nm, respectively. These gratings are also more fabrication tolerant compared to similar designs not using metamaterials.

Light beams with volume superoscillations

Thomas Zacharias and Alon Bahabad

DOI: 10.1364/OL.394270 Received 02 Apr 2020; Accepted 18 May 2020; Posted 19 May 2020  View: PDF

Abstract: Using a superposition of shifted Bessel beams with different longitudinal wave vectors and orbital angular momenta we realize an optical beam having simultaneous axial, angular, and radial focusing narrower than the Fourier limit. Our findings can be useful for optical particle manipulation and high-resolution microscopy.

Increasing the enhancement factor for DMD based wavefront shaping

Kibum Nam and Jung-Hoon Park

DOI: 10.1364/OL.394752 Received 13 Apr 2020; Accepted 18 May 2020; Posted 19 May 2020  View: PDF

Abstract: Focusing through scattering media is a subject of great interest due to its direct impact in the field of biomedical optics. However, the greatest barrier currently limiting direct applications is the fact that most scattering media that we wish to deliver light through are dynamic. To focus or deliver light through dynamic scattering media, using a digital micromirror device (DMD) has been demonstrated to be a potential solution as it enables fast modulation speeds. However, since a DMD is a binary amplitude modulator, the large number of controlled modes needed to acquire adequate focus enhancement has limited optimal usage. Here, we demonstrate a novel scheme to use the ‘thrown-away’ components of light to effectively use a binary amplitude DMD as a binary phase modulator thereby increasing the correction efficiency by a factor of two. Our concept can be applied to any iterative optimization algorithm and can speed up the iterative optimization process by increasing the enhancement factor, rather than the measurement or modulation speeds.

When is retardance autocalibration of microgrid-based full Stokes imagers possible and useful?

Benjamin Le Teurnier, Li Xiaobo, Matthieu Boffety, Haofeng Hu, and Francois Goudail

DOI: 10.1364/OL.396574 Received 01 May 2020; Accepted 18 May 2020; Posted 26 May 2020  View: PDF

Abstract: Full Stokes polarimetric images can be obtained from two acquistions with a microgrid polarization camera equipped with a retarder. When the retardance is imperfectly known, it can be calibrated from the measurements, but this requires three image acquisitions and may cause a divergence of estimation variance at low signal to noise ratio. We determine closed-form equations allowing to decide in which experimental conditions autocalibration is possible and useful, and to quantify the performance gain obtained in practice. These results are validated byreal-world experiments.

Indium Iodide Single Crystal – Breakthrough Material for Infrared Acousto-Optics

Dmitriy Porokhovnichenko, Evgeniy Dyakonov, Sergey Kuznetsov, Valery Voronov, Pavel Fedorov, Ksenia Zaramenskikh, Akhmedali Gasanov, Liya Zhukova, Alexandr Korsakov, and Dmitrii Salimgareev

DOI: 10.1364/OL.393737 Received 25 Mar 2020; Accepted 17 May 2020; Posted 18 May 2020  View: PDF

Abstract: Measurements of the refractive indices and the full set of longitudinal acousto-optical figure of merit coefficients in an orthorhombic single crystal InI were carried out. The high acousto-optic characteristics (M₂ up to 1100 × 10^(-15) s³/kg) and strong optical anisotropy (Δn = 0.47) make it possible to design various types of high-performance acousto-optic devices of the middle and far infrared spectral range. In combination with a wide transparency range (0.62-51 μm), the obtained characteristics make the crystal extremely promising in comparison with most of existing analogues.

Doubling the acquisition rate by spatial multiplexing of holograms in coherent sparse coded aperture correlation holography

Nathaniel Hai and Joseph Rosen

DOI: 10.1364/OL.394475 Received 06 Apr 2020; Accepted 17 May 2020; Posted 18 May 2020  View: PDF

Abstract: Optical, spatial or temporal, multiplexing is a well-known approach to optimize the performance of imaging systems. Following the recent discovery about the capability to record a coherent hologram in an interferenceless working mode, we propose a motionless method to spatially multiplex more than one hologram in a single camera exposure. Using the rather simple multiplexing framework based on coded aperture correlation holography, we effectively increase the acquisition rate of dynamic scenes and the holographic data compression by twofold. Quantitative or qualitative phase microscopy, and acquisition of a bipolar hologram from a single camera shot, experimentally confirm the applicability of the suggested technique.

Mode dynamics in high power Yb-Raman fiber amplifier

hanwei zhang, Hu Xiao, Xiaolin Wang, Pu Zhou, and Xiaojun Xu

DOI: 10.1364/OL.393879 Received 31 Mar 2020; Accepted 17 May 2020; Posted 19 May 2020  View: PDF

Abstract: Yb-Raman fiber amplifier (YRFA) is a compact setup that can be applied to achieve high-power narrow linewidth or special wavelength lasers. In this letter, we realized a high power YRFA with seed wavelengths of 1090 nm and 1150 nm, tandem-pumped by 1018 nm fiber laser. The dynamic of mode interaction has been carefully studied. Beam cleanup effect in the large mode area, step-index fiber has been observed for the first time, when the pump power ranges from 800 W to 1700 W. A model taking into account of the Raman mode interaction is proposed to explain this phenomenon, which is agree well with the experiments. Mode instability (MI) effect is also observed in the amplifier and the threshold is about 2 kW that is lower than the conventional Yb-doped fiber amplifier. Stimulated Raman scattering is attributed to the reason of the onset of MI. Finally, 1338 W 1150 nm laser is achieved by this YRFA, which is the highest power report at this wavelength as we know.

Ultrawide bandgap AlN metasurfaces for ultraviolet focusing and routing

Zelin Hu, Linyun Long, Rongqiao Wan, Chen Zhang, Lei Zhang, jianchang Yan, Huigao Duan, and Liancheng Wang

DOI: 10.1364/OL.395909 Received 27 Apr 2020; Accepted 16 May 2020; Posted 19 May 2020  View: PDF

Abstract: All-dielectric metasurface offers a promising way to control the amplitude, polarization, and phase of light. However, ultraviolet (UV) component metasurfaces are rarely reported due to significant absorption loss for most dielectric materials and the required smaller foot print or feature size. Here we demonstrate broadband UV focusing and routing in both transmission and reflection modes in simulations by adopting aluminum nitride (AlN) with ultrawide bandgap and a waveplate metasurface structure. As for experiments, the on-axis, off-axis focusing characteristics in transmission mode have been investigated at representative UVA (375nm) wavelength for the first time. Furthermore, we fabricated UV transmission router for mono-wavelength, guiding UV light to the designated different spatial positions of the same or different focal planes. Our work is meaningful for the development of UV photonics components and devices and would facilitate the integration and miniaturization of UV nanophotonics.

Optical coherent control of stimulated Brillouin scattering via four-wave mixing

Youhei Okawa and Kazuo Hotate

DOI: 10.1364/OL.390083 Received 10 Feb 2020; Accepted 16 May 2020; Posted 18 May 2020  View: PDF

Abstract: We propose and demonstrate a novel method for controlling stimulated Brillouin scattering in a phase-sensitive manner, using polarization-decoupled non-degenerate four-wave mixing. Our results indicate that Brillouin gain can be enhanced or eliminated in a polarization-maintaining fiber via acoustic wave interference by controlling the relative phase of orthogonally polarized lights. This method paves the way for the all-optical control of Brillouin interaction.

Distributed time delay sensing in random fiber grating array based on chirped pulse φ-OTDR

yuan wang, Stephen Mihailov, Xiaoyi Bao, Liang Chen, and Ping Lu

DOI: 10.1364/OL.392554 Received 13 Mar 2020; Accepted 16 May 2020; Posted 18 May 2020  View: PDF

Abstract: A high precision distributed time delay measurement in a chirped pulse phase optical time domain reflectometry (φ-OTDR) system based on random fiber grating array is proposed and demonstrated, in which temperature induced refractive index and fiber dimension changes associated time delay could be measured for distributed temperature sensing. The random fiber grating array includes many inscribed refractive index change locations at periods of sub-micron. When laser pulses are launched into the fiber grating, the backscattered light possesses many unique localized speckle patterns at different locations. Those patterns change with temperature, and hence the backscattering spectral response will change accordingly. By measuring the localized speckle pattern change due to the change of the temperature over the chirped pulse spectrum and performing cross correlation calculation, we can realize distributed temperature measurements in real time using a MHz bandwidth DFB laser. Unlike conventional φ-OTDR which measures distributed phase change along the fiber using an ultra-narrow linewidth laser, the distributed time delay presented in this work is measured along the random grating array. It is shown that the time resolved localized pattern trace is stable with very small intensity fluctuation thanks to the enhanced inhomogeneity and reflectivity. The minimum detectable temperature variation is 0.028 ℃ at meter order of magnitude spatial resolution. © 2019 Optical Society of America

Passive quadrature demodulation of polarimetric low-finesse fiber-optic Fabry-Perot interferometric sensors

Yupeng Zhu and Ming Han

DOI: 10.1364/OL.392931 Received 17 Mar 2020; Accepted 16 May 2020; Posted 18 May 2020  View: PDF

Abstract: We propose and demonstrate a passive quadrature demodulation method using polarized light and a fiber-optic sensor with a birefringent low-finesse Fabry-Perot (FP) cavity. With precisely controlled birefringence in the FP cavity, the fringes probed by light polarized along the two principal axes can have a quadrature phase shift and can be separated detected. We demonstrated the concept for ultrasound detection using a sensor with an FP cavity formed by two low-reflectivity chirped fiber Bragg gratings on a coiled polarization-maintaining (PM) fiber. By controlling the bending radius, the bending length, and the twist of the coil structure, an optimized total phase delay of 108° between the fringes of the two polarizations along the principal axes of the PM fiber. Using a linearly polarized laser source and a polarization beam splitter, the signals at the two polarizations were separated and measured by two photodetectors. Experimental results show that the sensor can detect ultrasonic signal when the sensor spectra experience environmental drifts using a laser at fixed wavelength.

Direct measurement of coupled spatiotemporal coherence of parametric down-conversion under negative group velocity dispersion

Paula Cutipa-Gimenez, Kirill Spasibko, and Maria Chekhova

DOI: 10.1364/OL.397700 Received 14 May 2020; Accepted 16 May 2020; Posted 26 May 2020  View: PDF

Abstract: We present a direct measurement of the spatiotemporal coherence of parametric down-conversion in the range of negative group-velocity dispersion. In this case, the frequency-angular spectra are ring-shaped and temporal coherence is coupled to spatial coherence. Correspondingly, the lack of coherence due to spatial displacement can be compensated with the introduction of time delay. We show a simple technique, based on a modified Mach-Zehnder interferometer, which allowed us to measure time coherence and near-field space coherence simultaneously, with a complete control over both variables. This technique will be also suitable for the measurement of second-order coherence, where the main applications are related to the two-photon spectroscopy.

Non-contact Reflection-mode Optical Absorption Spectroscopy Using Photoacoustic Remote Sensing

Kevan Bell and Parsin Haji Reza

DOI: 10.1364/OL.394637 Received 08 Apr 2020; Accepted 15 May 2020; Posted 18 May 2020  View: PDF

Abstract: A method of remote optical absorption spectroscopy is described which utilizes the photoacoustic remote sensing (PARS™) detection technique. A nanosecond tunable excitation source is used to excite thermo-elastic pressure-induced elasto-optic modulations within targets across a wide wavelength range from 210 nm to 680 nm providing optical absorption contrast. These modulations are read remotely as back-reflected intensity variations within a continuous-wave 1310 nm detection beam. The absorption spectra of several samples including dyes and biological macromolecules are captured with an 8 mm working distance in reflection-mode without the use of containment chambers or acoustic detection. This represents an initial investigation into the characteristics of this technique which may facilitate optical absorption measurement within previously inaccessible sample types due to their size or opacity.

A universal form of arrays with spectral singularities

Dmitry Zezyulin and Vladimir Konotop

DOI: 10.1364/OL.395788 Received 21 Apr 2020; Accepted 15 May 2020; Posted 18 May 2020  View: PDF

Abstract: An array of non-Hermitian optical waveguides can operate as a laser or as a coherent perfect absorber, which corresponds to a spectral singularity of the underlying discrete complex potential. We show that all lattice potentials with spectral singularities are characterized by the universal form of the gain-and-loss distribution. Using this result we systematically construct potentials characterized by several spectral singularities at arbitrary wavelengths, as well as potentials with second-order spectral singularities in their spectra. Higher-order spectral singularities demonstrate a greatly enhanced response to incident beams resulting in the excitation of high-intensity lasing modes.

Coherent MIMO radar network enabled by photonics with unprecedented resolution

Salvatore Maresca, Filippo Scotti, Giovanni Serafino, Leonardo Lembo, Antonio Malacarne, Fabio Falconi, Paolo Ghelfi, and Antonella Bogoni

DOI: 10.1364/OL.391296 Received 05 Mar 2020; Accepted 15 May 2020; Posted 19 May 2020  View: PDF

Abstract: The conventional concept of radar is based on stand-alone and independent apparatuses. Superior performance is possible exploiting distributed point of views (i.e. distributed radars) and centralized data fusion algorithms, but systems based only on RF technology are not able to guarantee the requested degree of coherence and high capacity links among radars. In the current distributed systems, radars act almost independently from each other. Thus, data fusion, which must be necessarily performed on locally pre-processed information, can only exploit a partial information content, harming the imaging capability of the distributed system. Here we present the first complete analysis and experiment of a distributed coherent multiple-input-multiple output radar system enabled by photonics which maximize the information content extracted by a centralized data fusion, providing unprecedented resolution capabilities. Stepping from previous achievements, where photonics has been demonstrated in single radars, and preliminary investigations on distributed systems, here photonics is used for providing coherence and high capacity links among radars. Both experiments and simulations have verified the superior cross range resolution introduced by the photonics-based coherent MIMO approach. The numerical analysis also demonstrated the benefits of coherent multi-band operation for side lobes reductions, i.e. false alarms reductions.

Superradiance Paradox in waveguide lattices

Stefano Longhi

DOI: 10.1364/OL.396368 Received 28 Apr 2020; Accepted 15 May 2020; Posted 15 May 2020  View: PDF

Abstract: Recently, it has been suggested that the collective radiative decay of two point-like quantum emitters coupled to a waveguide,separated by a distance comparable to thecoherence length of a spontaneously emitted photon, leads to an apparent "superradiance paradox" by which one cannot decide whether independent or collective emission occurs. The resolution of the paradox stems from the strong non-Markovian dynamics arising from the delayed field-mediated atom interaction. Here we suggest an integrated optics platform to emulate the superradiance paradox, based on photon escape dynamics in waveguide lattices. We also suggest that Markovian decay dynamics and independent photon emission can be restored by frequent (Zeno-like) observation of the system.

Shift of zero-field level-crossing resonance in the Cs D1 line and its use in vector magnetometry

D. Brazhnikov, Stepan Ignatovich, Irina Mesenzova, Alexey Novokreshchenov, and Andrei Goncharov

DOI: 10.1364/OL.396470 Received 30 Apr 2020; Accepted 15 May 2020; Posted 15 May 2020  View: PDF

Abstract: We examine the level-crossing resonance in a cesium vapor cell filled with a buffer gas under counterpropagating pump and probe light waves with orthogonal linear polarizations. An optical transition Fg=4 → Fe=3 is excited in the D1 line. Probe-wave transmission is analyzed versus a static magnetic field applied along the wave vectors. This configuration can provide ultrahigh-contrast electromagnetically induced absorption resonances. We report here a new magneto-optical effect in the resonance shift caused by a transverse magnetic field and discuss how it can be applied in vector magnetometry.

Quantum-inspired detection for Spectral Domain Optical Coherence Tomography

Sylwia Kolenderska, Frederique Vanholsbeeck, and Piotr Kolenderski

DOI: 10.1364/OL.393162 Received 20 Mar 2020; Accepted 15 May 2020; Posted 15 May 2020  View: PDF

Abstract: The intensity levels allowed by safety standards (ANSI or ICNIRP) limit the amount of light that can be used in a clinical setting to image highly scattering or absorptive tissues with Optical Coherence Tomography (OCT). To achieve high-sensitivity imaging at low intensity levels, we adapt a detection scheme - which is used in quantum optics for providing information about spectral correlations of photons - into a standard spectral domain OCT system. This detection scheme is based on the concept of Dispersive Fourier Transformation, where a fibre introduces a wavelength-dependent time delay measured by a single-pixel detector, usually a high-speed photoreceiver. Here, we use a fast Superconducting Single-Photon Detector (SSPD) as a single-pixel detector and obtain images of a glass stack and a slice of onion at the intensity levels of the order of 10 pW. We also provide a formula for a depth-dependent sensitivity fall-off in such a detection scheme which can be treated as a temporal equivalent of diffraction-grating-based spectrometers.

Self-similar propagation of optical pulses fibers with positive quartic dispersion

Antoine Runge, Tristram Alexander, Joseph Newton, Pranav Alavandi, Darren Hudson, Andrea Blanco-Redondo, and C. Martijn de Sterke

DOI: 10.1364/OL.393835 Received 27 Mar 2020; Accepted 15 May 2020; Posted 15 May 2020  View: PDF

Abstract: We study the propagation of ultrashort pulses in opticalfiber with gain and positive (or normal) quartic dispersion by self-similarity analysis of the modified nonlinear Schrödinger equation. We find an exact asymptotic solution, corresponding to a triangle-like, T^4/3 intensity profile, with a T^1/3 chirp, which is confirmed by numerical simulations. This solution follows different amplitude and width scaling compared to the conventional case with quadratic dispersion. We also suggest, and numerically investigate, a fiber laser consisting of components with positive quartic dispersion which emits quartic self-similar pulses

Combining nonlinear Fourier transform and neural network-based processing in optical communications

Oleksandr Kotlyar, Maryna Pankratova, Morteza Kamalian-Kopae, Anastasiia Vasylchenkova, Jaroslaw Prilepsky, and Sergei Turitsyn

DOI: 10.1364/OL.394115 Received 31 Mar 2020; Accepted 15 May 2020; Posted 15 May 2020  View: PDF

Abstract: We propose a method to improve the performance of the nonlinear Fourier transform-based optical transmission system by applying the neural network post-processing of the nonlinear spectrum at the receiver. We demonstrate through the numerical modelling about one order of magnitude bit error rate improvement and compare this method with machine learning processing based on the classification of the received symbols. The proposed approach also offers a way to improve numerical accuracy of the inverse NFT, therefore, it can find a range of applications beyond the optical communications.

Platinum substrate for surface plasmon microscopy at small angles

Hossein Hassani, Nikolaus Wolf, Xiaobo Yuan, Roger Wördenweber, and Andreas Offenhäusser

DOI: 10.1364/OL.396051 Received 24 Apr 2020; Accepted 15 May 2020; Posted 15 May 2020  View: PDF

Abstract: Platinum is reported as the main component of the substrate in surface plasmon microscopy (SPM) of metal−dielectric interface for small-angle measurements. In the absence of a narrow dip in the angular spectrum of platinum, the refractive index of the dielectric medium or the thickness of a deposited layer is proven deducible from the observed sharp peak, close to the critical angle. The sensitivities of RI- and thickness-measurements using platinum are compared with that of a gold surface plasmon resonance chip. Furthermore, the thickness of a structured layer of APTES on the platinum substrate is measured to be 0.7 nm, demonstrating high sensitivity of the technique.

Uniform colorization of metal surfaces through omnidirectional femtosecond laser-induced periodic surface structures

Taek Yong Hwang, Heedeuk Shin, Hai Joong lee, Hyo Soo Lee, Chunlei Guo, and Byounghwak Lee

DOI: 10.1364/OL.396375 Received 28 Apr 2020; Accepted 15 May 2020; Posted 15 May 2020  View: PDF

Abstract: Following femtosecond(fs) laser pulse irradiation, the formation of a new type of low spatial frequency laser-induced periodic surface structures(LSFLs), namely omnidirectional LSFLs(OD-LSFLs) with the periodic ordering of their orientations are investigated on Ni. Using a liquid crystal polymer patterned depolarizer, we periodically rotate the polarization of fs laser pulses across the laser spot and create OD-LSFLs by raster scanning fs laser pulses. It is also shown that the period of OD-LSFL orientation rotation can be controlled with the defocused distance, and OD-LSFLs can significantly expand the viewing angle of the structural colors in the azimuthal direction without noticeable color degradations.

Effective pulse reverse-engineering for strong field-matter interaction

Du Ran, Bin Zhang, Ye-Hong Chen, zhicheng shi, Yan Xia, Reuven Ianconescu, Jacob Scheuer, and Avraham Gover

DOI: 10.1364/OL.397053 Received 06 May 2020; Accepted 15 May 2020; Posted 20 May 2020  View: PDF

Abstract: We propose a scheme to control the evolution of a two-level quantum system in the strong coupling regime, based on the idea of reverse-engineering.A coherent control field is designed to drive the system along a user-predefined evolution trajectory without utilizing the rotating-wave approximation (RWA).As concrete examples, we show that complete population inversion, an equally weighted coherent superposition, and even oscillation-like dynamics can be achieved.Since there are no limitations on the coupling strength between the control field and matter, the scheme is attractive for applications such as accelerating desired system dynamics and fast quantum information processing.

Partially coherent Ince-Gaussian beams

Adad Yepiz, Benjamin Perez-Garcia, and Raul Hernandez-Aranda

DOI: 10.1364/OL.395591 Received 27 Apr 2020; Accepted 14 May 2020; Posted 14 May 2020  View: PDF

Abstract: We report on the study and generation of Ince-Gaussian beams in the spatially partially coherent regime. The inherent random fluctuations both in time and space of these partially coherent fields make their characterization difficult. Our results show that the cross-correlation function provides insight into the composition of the Ince-Gaussian beam, as well as into its spatial coherence structure and singularities. Our experimental findings are in excellent agreement with numerical simulations.

Multiscale approach and linear assignment problem in designing mirrors generating far-field irradiance distributions

Dmitry Bykov, Leonid Doskolovich, and Evgeni Bezus

DOI: 10.1364/OL.393895 Received 30 Mar 2020; Accepted 14 May 2020; Posted 14 May 2020  View: PDF

Abstract: We propose a multiscale approach for designing mirrors generating prescribed irradiance distributions in the far field. Our design method is based on calculating a ray mapping from a Monge–Kantorovich mass transportation problem and on reducing this problem to a linear assignment problem (LAP). The proposed multiscale formulation of the LAP allows one to efficiently calculate freeform mirror surfaces defined on meshes with the size up to at least 1000 x 1000. As an example requiring a mesh of this size, we design a mirror generating a grayscale image of A. Einstein on a zero background. The proposed multiscale approach is general and can be applied to a wide range of inverse problems of nonimaging optics.

Synchrotron based VUV excitation induced ultrahigh quality cool white light luminescence from Sm doped ZnO

Puneet Kaur, Rahul ., Kriti Kriti, Simranpreet Kaur, KANDASAMI Asokan, and davinder singh

DOI: 10.1364/OL.395393 Received 17 Apr 2020; Accepted 13 May 2020; Posted 14 May 2020  View: PDF

Abstract: We report that rare-earth (RE) ion, Sm, doped ZnO acts as white light emitting vacuum ultraviolet (VUV) phosphors and possess ultrahigh color rendering index (CRI) and color quality scale (CQS). The VUV-excited emission spectra measured from the synchrotron source reveals emergence of multi-colour emission bands in the visible-IR region and substantially depend on the concentration of Sm3+ ions. A mechanism is proposed to elucidate the origin behind the high energy band gap excitation of host charge carrier and subsequent energy transfer to the Sm3+ states leading to additional Green-Yellow-Orange emission bands of Sm3+(4G5/2→6HJ (J= 5/2, 7/2 and 9/2)). High quality cool white light (CCT 5600k) having CIE coordinates (0.33, 0.35) with CRI as high as 95.89 and CQS value of 94.49 is achieved for Zn0.985Sm0.015O under synchrotron VUV radiations. This study demonstrates that RE activated ZnO based phosphors are expected to be a promising candidate in solid state lighting as well as plasma display devices.

Tailored optical potentials for Cs atoms above waveguides with focusing dielectric nano-antenna

Angeleene Ang, Alexander Shalin, and Alina Karabchevsky

DOI: 10.1364/OL.394557 Received 14 Apr 2020; Accepted 13 May 2020; Posted 18 May 2020  View: PDF

Abstract: Tuning the near-field using all-dielectric nano-antennae offers a promising approach for trapping atoms, which could enable strong single-atom/photon coupling. Here we report the numerical study of an optical trapping of single Cs atom above a waveguide with silicon nano-antenna which produces a trapping potential for atoms in a chip-scale configuration. Using counter-propagating incident fields, bichromatically detuned from the atomic cesium D-lines, we numerically investigate the dependence of the optical potential on the nano-antenna geometry. We tailor the near-field potential landscape by tuning the evanescent field of the waveguide using a toroidal nano-antenna, a configuration that enables trapping of ultracold Cs atoms. Our research opens up a plethora of trapping atoms applications in a chip-scale manner from quantum computing, to quantum sensing to list a few.

Lattice induced plasmon hybridization in metamaterials

Subhajit Karmakar, Deepak Kumar, Ravi Varshney, and Dibakar Roy Chowdhury

DOI: 10.1364/OL.393702 Received 26 Mar 2020; Accepted 12 May 2020; Posted 14 May 2020  View: PDF

Abstract: We explore an inherent connection between two important concepts of physics – resonance hybridization and lattice effect in sub-wavelength periodic structures. Our study reveals that coupling with lattice mode is the prime deciding factor to determine the nature, position and line shape of the hybridized states. Modulating lattice modes can effectively control mode hybridization and convert resonance nature (either from electric to magnetic mode or dipole to Fano resonance and vice versa) without changing any other structural dimensions in subwavelength plasmonic metamaterials. Outcomes of this study can be exploited in designing linear and nonlinear photonic structures towards futuristic meta devices.

Polarization splitting directional coupler using tilted subwavelength gratings

José Luque-González, alaine herrero, Alejandro Ortega-Moñux, Marina Sánchez-Rodríguez, Aitor Velasco, Jens Schmid, Pavel Cheben, I. Molina-Fernández, and Robert Halir

DOI: 10.1364/OL.394696 Received 13 Apr 2020; Accepted 12 May 2020; Posted 14 May 2020  View: PDF

Abstract: On-chip polarization splitters are key elements for coherent optical communication systems and polarization diversity circuits. These devices are often implemented with directional couplers that are symmetric for one polarization and strongly asymmetric for the other polarization. To achieve this asymmetry, highly dissimilar waveguides are used in each coupler arm, often requiring additional material layers or etch steps. Here we demonstrate polarization splitting with a directional coupler composed of two virtually identical, fully etched subwavelength gratings, which only differ in the tilt angle of the silicon segments. Our device exhibits deep-UV compatible feature sizes, is only 14 µm long, and covers a 72 nm bandwidth with insertion losses below 1 dB and an extinction ratio in excess of 15 dB.

Stimulated Brillouin scattering in a tapered dual-core As₂Se₃-PMMA fiber for simultaneous temperature and strain sensing

Haiyang Wang, Song Gao, Chams Baker, yuan wang, Liang Chen, and Xiaoyi Bao

DOI: 10.1364/OL.391734 Received 09 Mar 2020; Accepted 12 May 2020; Posted 12 May 2020  View: PDF

Abstract: Chalcogenide fibers are currently being widely used in nonlinear optical signal processing as they exhibit ultrahigh nonlinearity. Here, we propose a sensor based on stimulated Brillouin scattering (SBS) for simultaneoustemperature and strain measurement in a dual-core tapered As₂Se₃-Polymethyl methacrylate (As₂Se₃-PMMA) fiber using a Brillouin optical time domain analysis (BOTDA) system. Different Brillouin frequency responses under temperature and strain variations and the separation of Brillouin frequency shifts in two principal polarization axes are demonstrated experimentally over a 50 cm-long tapered dual-core hybrid fiber. The measured temperature coefficients are -3.8272 MHz/ºC and -3.3302 MHz/ºC, and the strain coefficients are -0.06143 MHz/με and - 0.03463 MHz/με. Due to the different temperature and strain dependences of Brillouin frequency peaks in two polarizations, temperature and strain resolutions of 1 °C and33 με are realized respectively. Numerical simulations are also reported to account for the different Brillouin frequency shifts in two polarization axes.

Adaptive thermal stabilization of an integrated photonic spectrometer using parasitic interference fringes

Chu Teng, Chi Xiong, Eric Zhang, William Green, and Gerard Wysocki

DOI: 10.1364/OL.393725 Received 27 Mar 2020; Accepted 12 May 2020; Posted 12 May 2020  View: PDF

Abstract: Parasitic fringe drift from unwanted scatterings limits the long-term stability of waveguide-based optical spectrometers. Yet their spectral features provide relevant information that can be used to improve performance of the spectrometer. We show that fringe drift can be extracted and utilized to perform accurate thermal stabilization especially in the case of integrated waveguide sensors. In this letter, effective stabilization of a methane silicon photonic sensor is demonstrated, and significant reduction in fringe noise is clearly observed.

Extending epipolar geometry for real-time structured light illumination

Kai Liu, Kangkang Zhang, Jinghe Wei, Jianwen Song, Daniel Lau, Ce Zhu, and Bin Xu

DOI: 10.1364/OL.390212 Received 10 Feb 2020; Accepted 11 May 2020; Posted 12 May 2020  View: PDF

Abstract: Structured light illumination, scanning along both horizontal and vertical directions, achieves more robust accuracy. By introducing the constraint of epipolar geometry, we previously proposed real-time 3D reconstruction by means of look-up tables; however, we only knew these offline derived tables, i.e., parameters relating phases to 3D coordinates, were the combinations of the elements in calibration matrices of a camera and a projector, and suffered from long-time computation of the tables. In this Letter, by parameterizing the line perspectively mapping a 3D world coordinate into the camera and projector spaces, we propose to extend the epipolar analysis, i.e., introducing ideas of phase and optical poles, to geometrically address these parameters via analytic closed-form equations, with which we can (1) directly derive look-up tables in real time from the calibration matrices and (2) optimally reduce the number of tables from 11 to 5 to save much more memory space while further accelerate processing rate. Additionally, although we focus on structured light, stereo vision can also benefit from the extended epipolar analysis to (1) use tables to conduct 3D reconstruction in real time over matched correspondences, and (2) compute epipolar lines without using a fundamental matrix and further limit matching range on epipolar lines. Experiments show that with the same level of accuracy, we significantly reduce the time to compute the look-up tables from more than 20 minutes to 20 milliseconds while increasing our processing rate from approximately 320 to 492 fps.

Portable Broadband Photoacoustic Spectroscopy for Trace Gas Detection by Quantum Cascade Laser Arrays

CHIEN-SHENG LIAO, Romain Blanchard, Christian Pflügl, Masud Azimi, FRED HUETTIG, and Daryoosh Vakhshoori

DOI: 10.1364/OL.395202 Received 14 Apr 2020; Accepted 11 May 2020; Posted 12 May 2020  View: PDF

Abstract: We report a portable broadband photoacoustic spectroscopic system for trace gas detection using distributed feedback quantum cascade laser arrays. By sequentially firing 128 lasers, our system acquires a photoacoustic spectrum covering 565 cm-1 (935-1500 cm-1) with a normalized-noise-equivalent-absorption coefficient of 2.5×10-9 cm−1WHz−1/2. The firing sequence that determines when and which laser to be activated is programmable, which enables frequency-multiplexing excitation. For demonstration, 12 lasers are modulated simultaneously at distinct frequencies and a photoacoustic spectrum is acquired within 13 ms. The compactness (11” ×6.7” ×5.1”, 7.8 lbs) and low power consumption enable convenient installation for on-site monitoring.

Intravital confocal fluorescence lifetime imaging microscopy in the second near-infrared window

Wei Zheng, Jia Yu, Rongli Zhang, Yufeng Gao, Zonghai Sheng, Min Gu, Qinchao Sun, Jiuling Liao, Ting Wu, Zhanyi Lin, Peiheng Wu, Lin Kang, Hui Li, and Labao Zhang

DOI: 10.1364/OL.394684 Received 09 Apr 2020; Accepted 11 May 2020; Posted 12 May 2020  View: PDF

Abstract: Benefitted from suppressed light scattering and diminished autofluorescence of tissues in the second near-infrared (NIR-II) window (1000-1700 nm), NIR-II fluorescence microscopy has recently become an attractive technique for intravital imaging. However, currently available NIR-II imaging technologies are mainly confined to the morphological characterization of cells and tissues. In this paper, we present a NIR-II confocal fluorescence lifetime imaging microscopy to assess both morphological and biochemical information of live samples. A homemade superconducting single-photon detector (SSPD) was used to facilitate NIR-II fluorescence lifetime measurement. The SSPD has many advantages including, high sensitivity to NIR-II signals (detection efficiency: > 50%), fast temporal response (~109 ps), low timing jitter (~50 ps), and low dark count rate (<100 cps). We demostrate the feasibility of the developed microscopy system by comparing fluorescence lifetimes of range of NIR-II fluorophores and by performing multicolor three-dimensional fluorescence lifetime imaging of mouse ear in vivo. The results show that the biochemical properties of fluorophores probed by fluorescence lifetime provide complementary information to biomedical studies, significantly benefitting the diverse applications in life science.

Single-shot laser scattering technique refined for the real-time monitoring and sizing of individual nanoparticles and nanobubbles in bulk water

KOTA ANDO, Yoshiharu Uchimoto, and Takashi Nakajima

DOI: 10.1364/OL.394934 Received 10 Apr 2020; Accepted 11 May 2020; Posted 13 May 2020  View: PDF

Abstract: Understanding the growth dynamics and transport mechanism of nanoparticles/nanobubbles in a solution is an important issue in nanoscience and nanotechnology. Using a standard CMOS camera and a nanosecond laser at 532 nm, we demonstrate the far-field detection of polystyrene nanoparticles in bulk water. Conveniently, the sizes of individual nanoparticles are found to be reliably estimated from the brightness of scattering signals under the single laser pulses. Since the scattering efficiency of polystyrene nanoparticles is very similar to that of nanobubbles with the same size, our results clearly imply that the detection of nanobubbles in bulk solution is also possible. The developed technique is very simple and versatile with minimum experimental constraints, and can be applied for a variety of purposes.

3D-MSM AlN Deep Ultraviolet Detector

tao li, Linyun Long, Zelin Hu, Rongqiao Wan, Gong Xiaoliang, zhang lei, Yuan Yongbo, jianchang Yan, Wenhui Zhu, jinmin li, and Liancheng Wang

DOI: 10.1364/OL.394338 Received 07 Apr 2020; Accepted 10 May 2020; Posted 12 May 2020  View: PDF

Abstract: Conventional MSM UV detector has the disadvantage of limited adjustable structural parameters, finite electrical field and long carrier path. Here we demonstrate 3D-MSM structural AlN based deep UV detector, which was fabricated through simple trench etching and metal deposition, while flip bonding to the silicon substrate forms the FC-3DMSM device. 3D-MSM devices exhibits improved responsiveness and response speed, compared with conventional MSM device. Time-dependent photoresponse of all devices are also investigated. The enhanced performance of the 3D-MSM device should be attributed to the intensified electrical field from the 3D metal electrode configuration and the inhibition of the carrier vertical transport, which unambiguously increases the carrier collection efficiency, migration speed and thus the responsivity and speed. This work should advance the design and fabrication of AlN based deep UV detectors.

In vivo time-domain diffuse correlation spectroscopy above the water absorption peak

Lorenzo Colombo, Marco Pagliazzi, Sanathana Konugolu Venkata Sekar, Davide Contini, Turgut Durduran, and Antonio Pifferi

DOI: 10.1364/OL.392355 Received 09 Mar 2020; Accepted 09 May 2020; Posted 12 May 2020  View: PDF

Abstract: Time-domain diffuse correlation spectroscopy (TD-DCS) is a newly emerging optical technique which exploits pulsed, yet coherent light to non-invasively resolve the blood flow in depth. In this work, we have explored TD-DCS at longer wavelengths compared to those previously used in literature (i.e. 750-850 nm). The measurements were performed using a custom-made titanium-sapphire mode-locked laser, operating at 1000 nm, and an InGaAs photomultiplier as a detector. Tissue-mimicking phantoms and in vivo measurements during arterial arm cuff occlusion in n=4 adult volunteers were performed to demonstrate the proof-of-concept. We obtained good signal-to-noise ratio, following the hemodynamics continuously with a relatively fast (1 Hz) sampling rate. In all the experiments, the auto-correlation functions show a decay rate approximately 5-fold slower compared to shorter wavelengths. This work demonstrates the feasibility of in vivo TD-DCS in this spectral region and its potentiality for biomedical applications.

Polarization Mobius strips on elliptical paths in three-dimensional optical fields

Isaac Freund

DOI: 10.1364/OL.392331 Received 17 Mar 2020; Accepted 09 May 2020; Posted 11 May 2020  View: PDF

Abstract: Polarization Mobius strips are studied on ellipticalpaths surrounding lines of circular polarization in fully three-dimensional(i.e. non paraxial) optical fields. It is found that as the eccentricity,azimuthal orientation, or centering of the path changes, right-handed Mobius strips can change into left-handed ones, and vice versa, and that Mobius strips with one half twist can change into strips with three half twists, and vice versa. These transformations are shown to occur in a possibly unexpected way, not observed previously, that is universal for alltwo-component singularities.

Dual-periodically-poled lithium niobate microcavities supporting multiple coupled parametric processes

Li Zhang, zhenzhong Hao, qiang Luo, Ang Gao, ru zhang, chen yang, Feng Gao, Fang Bo, Guoquan Zhang, and Jingjun Xu

DOI: 10.1364/OL.393244 Received 23 Mar 2020; Accepted 09 May 2020; Posted 11 May 2020  View: PDF

Abstract: Periodically poled lithium niobate (PPLN) microcavities with additional reciprocal vectors attract attentions as a platform for efficient parametric nonlinear optical processes with wavelength and polarization flexibility. Here, we report the simultaneous realization of multiple coupled parametric processes in PPLN microdisk cavities with dual periods as a result of the significantly increased number of reciprocal vectors to fulfill quasi-phase matchings for a series of nonlinear processes. PPLN microdisks with up to 1.43×10⁵ quality factors and unit domain size of 90 nm in width were fabricated by using CMOS compatible microfabrication techniques and electrically poled with the help of piezoresponse force microscopy. The conversion efficiency of second harmonic signal was measured to be 51% /W. Our work paves the way towards efficientcascaded parametric effects including third- and fourth-harmonic generations.

Mitigating offset frequency drift in frequency combs using a customized power law dispersion

Nathan Henry, Jacob Khurgin, and David Burghoff

DOI: 10.1364/OL.393357 Received 24 Mar 2020; Accepted 09 May 2020; Posted 26 May 2020  View: PDF

Abstract: We introduce a new passive technique of mitigating the phase noise in optical frequency combs (FC) by reducing the drift of offset frequency. This can be achieved by customizing the dispersion to attain a power law dependence of the wavevector on frequency k(ω)~ωα, insuring a constant ratio between group and phase velocities. When this condition is maintained the offset frequency is passively stabilized and phase noise is reduced. Using quantum cascade laser (QCL) FCs as an example, we demonstrate, analytically and numerically, that the desired dispersion can be easily engineered by properly adjusting the thickness of the QCL active region and that stable offset frequency can be combined with low residual group dispersion.

Distributed sparse signal sensing based on compressive sensing OFDR

Shuai Qu, Zengguang Qin, Yanping Xu, Zhaojun Liu, zhenhua cong, Heng Wang, and Zhao Li

DOI: 10.1364/OL.391971 Received 04 Mar 2020; Accepted 08 May 2020; Posted 11 May 2020  View: PDF

Abstract: The maximum detectable vibration frequency response of OFDR system is limited by the tunable rate of the laser source. Unlike the uniform sampling with time-resolved method in conventional OFDR system, the sampling frequency is randomly modulated so that the vibration signal applied on the interrogation fiber is sampled by a multi-frequency sub-Nyquist sampling method and reconstructed by the compressive sensing technique. First, we give a full treatment to prove that the proposed method has the same performance with conventional method. Second in the further proof of concept experiment, for a conventional OFDR system, the measurable frequency of sparse signal can be achieved up to 200Hz with a tunable laser sweeping rate of 40nm/s. This method makes a significant step toward high-performance OFDR system with sparse signal detection, especially for evaluating the intrinsic frequency of the object structural condition. © 2020 Optical Society of America

High spectro-temporal purity single-photons from silicon micro-racetrack resonators using a dual-pulse configuration

Ben Burridge, Imad Faruque, John Rarity, and Jorge Barreto

DOI: 10.1364/OL.393077 Received 19 Mar 2020; Accepted 07 May 2020; Posted 12 May 2020  View: PDF

Abstract: Single-photons with high spectro-temporal purity are an essential resource for quantum photonic technologies. The highest reported purity up until now from a conventional silicon photonic device is 92% without any spectral filtering. We have experimentally generated and observed single-photons with 98.0 ± 0.3% spectro-tempral purity using a conventional micro racetrack resonator and an engineered dual pump pulse.

More than ten times enhancement of laser focused intensity through a re-entrant cone in the petawatt regime

Olimpia Budriga, Laura Ionel, Dragos Tatomirescu, and Kazuo Tanaka

DOI: 10.1364/OL.395316 Received 15 Apr 2020; Accepted 07 May 2020; Posted 14 May 2020  View: PDF

Abstract: A versatile method to enhance the laser pulse intensity by one order of magnitude from 8×10^20 W/cm^2 by using a single plastic micro-cone target is proposed. We found an increase of the initial laser pulse intensity by more than ten times for a micro-cone tip diameter of 5 µm by performing 2D Particle-in-Cell simulations.Numerical simulations of the spatio-temporal electromagnetic field distribution are used to replicate similar dependence of the maximum laser intensity to the cone tip diameter.

Grüneisen-relaxation photoacoustic microscopy at 1.7 μm and its application in lipid imaging

Jiawei Shi, Can Li, Huade Mao, Yuxuan Ren, Zhi-Chao Luo, Amir Rosenthal, and Kenneth Kin-Yip Wong

DOI: 10.1364/OL.393780 Received 27 Mar 2020; Accepted 07 May 2020; Posted 11 May 2020  View: PDF

Abstract: We report the first demonstration of Grüneisen relaxation photoacoustic microscopy (GR-PAM) of lipid-rich tissue imaging at the 1.7 μm band, implemented with a high-energy thulium-doped fiber laser (TDFL) and a fiber-based delay line. GR-PAM enhances the image contrast by intensifying the region of strong absorbers and suppressing out-of-focus signals. Using GR-PAM to image swine-adipose tissue at 1725 nm, an 8.26-fold contrast enhancement is achieved in comparison to conventional PAM. GR-PAM at the 1.7 μm band is expected to be a useful tool for label-free high-resolution imaging of lipid-rich tissue, such as atherosclerotic plaque and nerves.

Femtosecond two-color source synchronized at 100-as-precision based on SPM-enabled spectral selection

Yi Hua, Gengji Zhou, Wei Liu, Ming Xin, Franz Kaertner, and Guoqing Chang

DOI: 10.1364/OL.391161 Received 27 Feb 2020; Accepted 07 May 2020; Posted 11 May 2020  View: PDF

Abstract: We demonstrate both numerically and experimentally that self-phase modulation enabled spectral selection generates wavelength tunable energetic pulses that are tightly synchronized to the excitation pulses. The synchronization quantified by relative timing jitter is at the 100-as precision level, at least 10 times lower than can be achieved by Raman soliton pulses derived from the same source laser. This ultrafast two-color source is suitable for many important applications that require tight pulse synchronization.

A carbon nanotube-synchronized dual-color fiber laser for coherent anti-Stokes Raman scattering microscopy

Yan Li, Kangjun Zhao, Bo Cao, Xiaosheng Xiao, and Changxi Yang

DOI: 10.1364/OL.393449 Received 23 Mar 2020; Accepted 07 May 2020; Posted 11 May 2020  View: PDF

Abstract: We present a passively-synchronized dual-color fiber laser at 1.03 and 1.53 μm for coherent anti-Stokes Raman scattering (CARS) microscopy. This fiber laser consists of both Yb- and Er-doped laser cavities, combined by a ~1.2-m common branch with a single-walled carbon nanotubes based saturable absorber. Stable passively-synchronized mode-locked state is obtained within a total cavity length mismatch of 50 μm. We demonstrate the capability of the synchronized dual-color fiber laser for CARS spectroscopy in both low- and high-wavenumber regions, with resolution of 6.6 cm¯¹. Furthermore, a CARS image in the field of view 150×60 μm² of polystyrene at Raman shift of ∼3041 cm¯¹ has also been achieved. The results show the feasibility of our passively-synchronized dual-color fiber laser to be employed as a stable and low-cost ultrafast laser source for CARS.

Lasing in the medium with properties of the Christiansen filter

Olga Burdukova, Vladimir Petukhov, and Yury Senatsky

DOI: 10.1364/OL.394276 Received 01 Apr 2020; Accepted 07 May 2020; Posted 08 May 2020  View: PDF

Abstract: Lasing in a dense slurry-like mixture, similar to the optical medium of the Christiansen filter, is reported. A cuvette with LiF crystal particles and an immersion liquid containing pyrromethene 567 dye was placed in a 2 plane mirrors resonator and pumped by pulses of the 2nd harmonic of the Nd:YAG laser. 20 ns pulses at 545–570 nm wavelengths with energies up to 0.6 mJ were obtained at the output of this slurry laser. The central part of the laser beam with a divergence of 6 mrad was usually accompanied by a ring structure of scattered radiation. The conditions of the generation development and formation of the output beam profile in a slurry laser as well as its possible applications are discussed.

Transmission filters forming orthogonal basis for spectral imaging purposes

Mika Flinkman, Toni Saasatamoinen, Pertti Pääkkönen, Joonas Lehtolahti, Pauli Fält, and Hannu Laamanen

DOI: 10.1364/OL.395795 Received 22 Apr 2020; Accepted 07 May 2020; Posted 08 May 2020  View: PDF

Abstract: Hyperspectral imaging has become common techniquein many different applications enabling accurate iden-tification of materials based on their optical proper-ties, however it requires complex and expensive tech-nical implementation. Less expensive way to producespectral data, spectral estimation, suffer from complexmathematics and limited accuracy.We introduce anovel method where spectral reflectance curves canbe reconstructed from the measured camera responseswithout complex mathematics. We have simulated themethod with seven non-negative broadband transmis-sion filters extracted from Munsell color data throughprincipal component analysis and used sensitivity andnoise levels characteristic for Retiga 4000DC 12-bitmonochrome camera. Method is sensitive to noise butproduces sufficient reproduction accuracy even with sixfilters.

Relativistic near-single-cycle optical vortex pulses from noble gas-filled multipass cells

Huabao Cao, Roland Nagymihály, and M. P. KALASHINIKOV1

DOI: 10.1364/OL.392394 Received 09 Mar 2020; Accepted 06 May 2020; Posted 07 May 2020  View: PDF

Abstract: We propose to obtain relativistic near-single-cycle optical vortices carrying orbital angular momentum through the post-compression of Laguerre-Gaussian pulses in gas-filled multipass cells. Our simulations revealed that 30 fs optical vortex pulses centered around 800 nm with pulse energy of millijoule level can be compressed to near-single-cycle duration with topological charges from 1 to 20 within an argon-filled cell with 5 passes. The spectral broadening preserves the topological charge of the input beam and the spatio-spectral couplings are also discussed. The energy of the vortex pulses could be scaled up by increasing the dimensions of the cell. The relativistic near-single-cycle vortices are of great interest for the generation of ultrashort helical electron bunches based on hybrid electron acceleration in underdense plasmas, and isolated relativistic extreme ultraviolet optical vortices from high-order harmonic generation in solid foils.

Single-shot structured-light-field three-dimensional imaging

Xiaoli Liu, Zewei Cai, Giancarlo Pedrini, Wolfgang Osten, and Xiang Peng

DOI: 10.1364/OL.393911 Received 27 Mar 2020; Accepted 06 May 2020; Posted 07 May 2020  View: PDF

Abstract: This letter reports an approach for single-shot three-dimensional imaging combining structured illumination and light-field imaging. The sinusoidal distribution of the radiance in the structured-light field can be processed and transformed for computing the angular variance of the local radiance difference. The angular variance across the depth range exhibits a single-peak distribution trend that can be used to obtain the unambiguous depth. The phase computation which generally requires the acquisition of multi-frame phase-shifting images is no longer mandatory, enabling single-shot structured-light-field three-dimensional imaging. The proposed approach was experimentally demonstrated through a dynamic scene.

High laser damage threshold liquid crystal optical switch based on gallium nitride transparent electrode

Zhibo Xing, Wei Fan, Dajie Huang, He Cheng, and Gang Xia

DOI: 10.1364/OL.390440 Received 14 Feb 2020; Accepted 06 May 2020; Posted 08 May 2020  View: PDF

Abstract: High laser damage threshold spatial light modulator are in urgent demand in high power laser fields. In this letter, Liquid crystal optical switches using Si-doped GaN and Mg-doped GaN as transparent electrode were fabricated. The influence of the conductive properties of GaN were analyzed. The transmission and absorption characteristics of GaN and its sapphire substrate were tested. The results show that the liquid crystal device based on gallium nitride can be expected to play an important role in the fields of visible and near-infrared laser region with high laser damage threshold more than 1J/cm2.

400G direct modulation using a DFB+R laser

Di Che, Yasuhiro Matsui, Xi Chen, Richard Schatz, and Patrick Iannone

DOI: 10.1364/OL.392873 Received 17 Mar 2020; Accepted 05 May 2020; Posted 12 May 2020  View: PDF

Abstract: We demonstrate a direct-modulation and direct-detection system with a back-to-back line rate of 411.6 (net bit rate of 337.5) Gb/s using a 65-GHz DFB+R laser. The O-band laser with a chirp parameter of 0.6 supports dispersion-tolerant transmissions up to 15 km without an optical amplifier.

Plasmonic nanojet: an experimental demonstration

Igor Minin, Oleg Minin, Igor Glinskiy, Rustam Khabibullin, Radu Malureanu, Andrei Lavrinenko, Dmitry Yakubovsky, Aleksey Arsenin, Valentyn Volkov, and Dmitry Ponomarev

DOI: 10.1364/OL.391861 Received 04 Mar 2020; Accepted 05 May 2020; Posted 07 May 2020  View: PDF

Abstract: We propose and study a microstructure based on a dielectric cuboid combined to a thin metal film that can act as an efficient plasmonic lens allowing focusing surface plasmons at the subwavelength scale. Using a numerical simulation of surface plasmon-polariton (SPP) field intensity distributions in the microstructure, we observe plasmonic nanojet (PJ) and high-intensity subwavelength spots at the telecommunication wavelength of 1530 nm. The fabricated microstructure was measured using amplitude and phase-resolved scattering scanning near-field optical microscopy. We show the first experimental realization and observation of the PJ effect for the SPP waves. Such novel and simple platform can provide new pathways for plasmonics, high-resolution imaging, biophotonics as well as optical data storage.

Violation and recovery of Babinet principle in dielectric-slab supported complementary metasurfaces

Simone Zanotto and Giorgio Biasiol

DOI: 10.1364/OL.391994 Received 09 Mar 2020; Accepted 05 May 2020; Posted 07 May 2020  View: PDF

Abstract: Symmetry principles and theorems are of crucial importance in optics. Indeed, from one side they allow to get direct insights into phenomena by eliminating unphysical interpretations; from the other side, they guide the designer of photonic components by narrowing down the parameter space of design variables. In this Letter we highlight that Babinet principle, or better Babinet theorem (i.e., the vectorial identity that relates the fields scattered by complementary screens), is broken down in a very common and technologically relevant situation: that of a patterned conducting screen placed on a dielectric slab. The symmetry property predicted by Babinet theorem is correctly recovered for pairs of geometrically complementary – but application-wise unrealistic – free-standing patterned screens. Our analysis merges experimental data with fully vectorial electromagnetic modeling, and provides also an alternative form of Babinet theorem that highlights a connection with the concept of electromagnetic duality.

Optimal polarimeter structures for estimating polarization degree, angle and ellipticity in the presence of additive noise

Francois Goudail and jun dai

DOI: 10.1364/OL.387934 Received 05 Sep 2019; Accepted 05 May 2020; Posted 11 May 2020  View: PDF

Abstract: In polarimetry, it is well known that measurement matrices based on spherical 2 designs optimize Stokes vector estimation in the presence of additive noise. We investigate the optimal matrices for estimation of the degree of polarization (DOP), the angle of polarization (AOP), and the ellipticity (EOP), which are non-linear functions of the Stokes vector. We demonstrate that spherical 2 designs also optimize DOP and EOP estimation, but not AOP estimation, for which optimal structures consist of linear analyzers forming a regular polygon on the equator of the Poincaré sphere.

Screening of the quantum dot Förster coupling at small distances

Chelsea Carlson, Stephen Hughes, and Andreas Knorr

DOI: 10.1364/OL.391466 Received 28 Feb 2020; Accepted 04 May 2020; Posted 04 May 2020  View: PDF

Abstract: We study the near-field energy transfer rates between two finite size quantum dot disks, generalizing the result of Förster coupling between two point dipoles. In particular, we derive analytical results for the envelope of the electronic wavefunction for model potentials at the boundaries of quantum dot disks and demonstrate how the Förster interaction is screened as the size of the dots becomes comparable to the dot-dot separation.

Transverse drag of slow light in moving atomic vapor

Yakov Solomons, Chitram Banerjee, Slava Smartsev, Jonathan Friedman, David Eger, Ofer Firstenberg, and Nir Davidson

DOI: 10.1364/OL.394389 Received 06 Apr 2020; Accepted 04 May 2020; Posted 14 May 2020  View: PDF

Abstract: The Fresnel-Fizeau effect of transverse drag, in which the trajectory of a light beam changes due to transverse motion of the optical medium, is usually extremely small and hard to detect. We observe transverse drag in a moving hot-vapor cell, utilizing slow light due to electromagnetically induced transparency (EIT). The drag effect is enhanced by a factor 4∙10^5, corresponding to the ratio between the light speed in vacuum and the group velocity under the EIT conditions. We study the contribution of the thermal atomic motion, which is much faster than the mean medium velocity, and identify the regime where its effect on the transverse drag is negligible.

Coherently enhanced third-harmonic generation in cascaded microfibers

Xiujuan Jiang, Zhennan Chen, Timothy Lee, and Gilberto Brambilla

DOI: 10.1364/OL.389998 Received 07 Feb 2020; Accepted 02 May 2020; Posted 04 May 2020  View: PDF

Abstract: A scheme using cascaded microfibers is proposed for efficient third harmonic (TH) generation. TH from the microfibers could overlap coherently by tuning the phase difference via the input pump power, yielding great output enhancement. By optimizing the system, conversion efficiency about 20% with silica microfibers is demonstrated analytically and numerically. Moreover, because the TH output features are dominated by behavior analogous to optical interference, the influence of random diameter deviation of each microfiber is reduced, and the TH generation process could be well designed and controlled.

Control of topology of two-dimensional solitons in a laser with saturable absorption by means of a coherent holding radiation

Nikolay Rosanov, Sergey Fedorov, and Nikolay Veretenov

DOI: 10.1364/OL.394727 Received 08 Apr 2020; Accepted 02 May 2020; Posted 07 May 2020  View: PDF

Abstract: We propose a simple method to control the topology of laser vortex solitons and their complexes in a wide-aperture laser with saturable absorption by means of weak coherent holding radiation. The holding radiation acting on initial “free” vortex solitons, induces appearance of new peripheral vortices and splitting of multiple central vortices, as well as reconfiguration of energy flow topology. A wide variety of these stable vortex structures makes the scheme promising for topologically protected information processing.

Deuterated Silicon Dioxide for HeterogeneousIntegration of Ultra Low-Loss Waveguides

Warren Jin, Demis John, Jared Bauters, Tony Bosch, Brian Thibeault, and John Bowers

DOI: 10.1364/OL.394121 Received 01 Apr 2020; Accepted 01 May 2020; Posted 01 May 2020  View: PDF

Abstract: Ultra low-loss waveguide fabrication typically requires high-temperature annealing beyond 1000 ⁰C to reduce hydrogen content in deposited dielectric films. However, realizing the full potential of ultra low loss will require integration of active materials that cannot tolerate high temperature. Uniting ultra low-loss waveguides with on-chip sources, modulators, and detectors will require low-temperature, low-loss dielectric to serve as passivation and spacer layers for complex fabrication processes. We report a 250 ⁰C deuterated silicon dioxide film for top cladding in ultra low-loss waveguides. Using multiple techniques, we measure propagation loss below 12 dB/m for the entire 1200 to 1650 nm range and top-cladding material absorption below 1 dB/m in the S, C, and L bands.

Optical density of states near planar ENZ materials

Claudio Silvestre Castro, Eugenio Mendez, Alexandre Vial, Gilles Lerondel, Yann Battie, aurelien Bruyant, and Rémi Vincent

DOI: 10.1364/OL.392017 Received 18 Mar 2020; Accepted 01 May 2020; Posted 04 May 2020  View: PDF

Abstract: We study the local density of optical states (LDOS) for lossy dielectric substrates whose electric permittivity has a vanishing real part. A criterion for evaluating the threshold height above (below) which radiative (non-radiative) processes dominate for a dipole emitter is established. We focus on the case of a vertical dipole above the Epsilon-near-zero (ENZ) substrate and show that, in the lossless case, complete LDOS cancellation originates from radiative modes in its near-field. We evaluate the performance of commercially available ENZ materials and quantify the limits of such cancellation effects with the intrinsic losses of the substrate.

Occlusion capable optical-see-through near-eye display using a single digital micromirror device

Yeon-Gyeong Ju, myeong-ho Choi, PENGYU Liu, Brandon Hellman, TED LEE, Yuzuru Takashima, and Jae-Hyeung Park

DOI: 10.1364/OL.393194 Received 19 Mar 2020; Accepted 01 May 2020; Posted 12 May 2020  View: PDF

Abstract: Occlusion of a real scene by displayed virtual images mitigates incorrect depth cues and enhances image visibility in augmented reality applications. In this Letter, we propose a novel optical scheme for the occlusion-capable optical-see-through near-eye display. The proposed scheme uses only a single spatial light modulator as both the real scene mask and virtual image display simultaneously. A polarization-based double-pass configuration is also combined, enabling a compact implementation. The proposed scheme is verified by optical experiments which demonstrate 60Hz RGB video display with 4-bit depth for each color channel and per-pixel dynamic occlusion of 90.6% maximum occlusion ratio.

Waveguide platform for quantum anticentrifugal force

Andrzej Gajewski, Daniel Gustaw, Roshidah Yusof, Norshamsuri Bin Ali Hasim, Karolina Slowik, and Piotr Kolenderski

DOI: 10.1364/OL.392216 Received 12 Mar 2020; Accepted 28 Apr 2020; Posted 05 May 2020  View: PDF

Abstract: This work is a proposition of an experimental platform to observe quantum fictitious anticentrifugal force. We present an analytical and numerical treatment of a rectangular toroidal dielectric waveguide. Solving the Helmholtz equation we obtain analytical solutions for transverse spatial modes and estimate their number as a function of system characteristics. On top of that, the analysis of the structure was extended onto a real material platform, a thin film lithium niobate on insulator rib waveguide. The framework presented here can be directly applied to analyze of the phenomenon of quantum anti-centrifugal force.

Boson band mapping: revealing ultrafast laser induced structural modifications in chalcogenide glass.

Thomas Gretzinger, Toney Fernandez, Simon Gross, Alexander Arriola, and Michael Withford

DOI: 10.1364/OL.393511 Received 24 Mar 2020; Accepted 21 Apr 2020; Posted 22 Apr 2020  View: PDF

Abstract: The formation of femtosecond laser direct-written waveguides in gallium lanthanum sulphide (GLS) chalcogenide glass with a peak index contrast of Δnmax = 0.0 and an average positive refractive index change of Δnwaveguide = 0.0049 is explained for the first time. Evidence of structural change and ion migration are presented using Raman spectroscopy and electron probe microanalysis (EPMA), respectively. Raman microscopy revealed a frequency shift and a change in full-width at half maximum(FWHM) variation of the symmetric vibration of the GaS4 tetrahedra. The boson band was successfully used to identify and understand the material densification profile in a high refractive index glass waveguide. EPMA provided evidence of ion migration due to sulphur, where the observation of an anion S2- migration causing material modification is reported for the first time. These results will enable optimisation of future mid-infrared and nonlinear integrated optical devices in GLS glass based on femtosecond laser written waveguides.

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