Comb Spacing Multiplication Enabled Widely Spaced Flexible Frequency Comb Generation

We experimentally demonstrate a new approach for the generation of widely spaced flexible optical frequency combs in a highly nonlinear fiber (HNLF) using relatively low-speed components. A cavity-less 10-GHz optical pulse source is passively multiplied to 50 GHz via the fractional temporal Talbot effect. The repetition rate multiplied pulse train then propagates in an HNLF together with a continuous-wave probe laser to achieve comb spacing multiplication through cross-phase modulation. Numerical study shows that the timing jitter of the initial 10-GHz optical pulse induces insignificant excess phase noise at high-frequency offset relative to the optical comb lines, which favors a low phase noise wideband frequency comb generation. The spectral linewidth of the central 17 comb lines are characterized, exhibiting well-preserved linewidth quality as the probe laser. Furthermore, the tunability of the generated comb, relevant from a practical perspective, is analyzed by means of conversion efficiency. Finally, the central 11 comb lines are individually modulated by Nyquist-shaped 32 GBaud 16 quadratic-amplitude modulation signals, demonstrating the applicability of the comb source in providing optical carriers for a coherent transmitter.