Modulation formats transmitting many bits per sampling point can be generated driving optical modulators with electronic analog-to-digital converters. One example is the so-called 64-QAM, based on quadrature amplitude modulation involving both the phase and the amplitude of the optical signal with 64 distinct values per sampling point, corresponding to 6 bits. This straightforward approach does not work well at higher data rates, as electronics reaches its speed and linearity limits.
The way to overcome this limit is to combine several optical signals, each of which is modulated with simpler formats, into the more complex modulation format; the combination needs to be coherent to obtain the correct amplitude and phase of the desired optical modulation. The combination of optical signals can be realized either through linear superposition or nonlinear optical interactions; Chitgarha and co-authors show in this paper how the nonlinear combination of optical signals derived from the lines of optical frequency combs yields high-data-rate advanced modulation formats.
In this technique, three lines of an optical comb (derived from a mode-locked laser) are modulated separately, then injected into a periodically-poled Lithum Niobate waveguide (PPLN) together with three unmodulated comb lines chosen so that each unmodulated line is located symmetrically to one of the signal-carrying lines with respect to the PPLN quasi-phase matching wavelength ω0. In this way the sum-frequency generation nonlinear process generates three mutually coherent waves at the frequency 2ω0; in addition to the optical modulation, an appropriate overall phase and amplitude must be impressed over each of the signal-carrying comb lines so that the combination results in the desired advanced modulation format. For example, if each of the three comb lines carries a 4-QAM (a QAM with 4 distinct values) modulation they can be combined into a 64-QAM modulation using the amplitudes c1 = 1, c2 = 0.5 and c3 = 0.25 respectively and the same phase for each comb line. A continuous wave pump must also be injected in the PPLN to shift back the frequency of the obtained signal to the C-band of optical communications through difference frequency generation. In this way, the three signal-carrying comb lines are combined into a single advanced-modulation signal using two cascaded nonlinear optical processes in a single step.
The authors show constellation diagrams and bit error rates, assessing the performance of this technique.
I believe this is a successful step toward the examination of alternatives for communication at higher bit rates, above 100 Gbit/s per channel.
You must log in to add comments.