Microresonator frequency combs harness the nonlinear Kerr effect in anintegrated optical cavity to generate a multitude of phase-locked frequencylines. The line spacing can reach values in the order of 100 GHz, making it anattractive multi-wavelength light source for applications in fiber-opticcommunications. Depending on the dispersion of the microresonator, differentphysical dynamics have been observed. A recently discovered comb statecorresponds to the formation of mode-locked dark pulses in a normal-dispersionmicrocavity. Such dark-pulse combs are particularly compelling for advancedcoherent communications since they display unusually high power conversionefficiency. Here, we report the first coherent transmission experiments using64-quadrature amplitude modulation encoded onto the frequency lines of adark-pulse comb. The high conversion efficiency of the comb enables transmittedoptical signal-to-noise ratios above 33 dB while maintaining a laser pump powerlevel compatible with state-of-the-art hybrid silicon lasers.
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