The field of micro-endoscopy might be on the verge of a small revolution thanks to the possibility of using lensless multimode fibers as a reliable medium for delivering tightly focused and scanned laser light at the distal end of the fiber. This seemingly impossible task was demonstrated in the recently published Biomedical Optics Express article by Papadopoulos et. al. where the authors implemented the principles of optical phase conjugation to achieve digital scanning through a multimode fiber.

Optical phase conjugation techniques, also called time reversal, use the principle of exactly reversing the propagation direction and phase of a light beam. Due to the deterministic nature of light scattering and refraction this allows for the reversed light to return to its initial location even after traveling through static turbid media. Thanks to recent advances in optoelectronics it is nowadays possible to create digital phase conjugation (DCP) using a camera and a Spatial Light Modulator, instead of relying on conventional phase conjugation created in nonlinear crystals. Moreover DPC allows for “recording” and “replaying” of the optically conjugate fields. This feature of DPC resulted in the creation of new imaging schemes including one presented in this manuscript, namely imaging through a multimode fiber.

The key feature allowing operation of the proposed endoscopic scanning fluorescent microscope includes recording the phase lookup table for all the different points along the regular grid by scanning the calibration beam around the fiber facet. This lookup table is later used to scan the endoscopic imaging beam at the distal end of the fiber. The application of this recording and scanning principle allows for the construction and evaluation of an ultra-thin rigid endoscope (450µm diameter) using a 220µm diameter, 0.53 NA fiber. The system’s performance was modeled and validated for resolution and fluorescence collection efficiency as a function of working distance and distance from the optical axis. Additionally, comparison with lens-based optical systems of the same numerical aperture showed very similar performance. Finally, the authors successfully applied the system for imaging of fluorescently-stained neuronal cells.

In summary, the presented ultrathin rigid lensless endoscope offers several advantages including small size with relatively large field of view as well as the application of a passive element for illumination allowing for minimally invasive high-resolution fluorescent imaging. As predicted by authors, the possible applications of this system include high-resolution endoscopic imaging through direct tissue penetration with focus on in-vivo diagnosis based on cellular phenotype.

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