Low Noise Frequency Combs for Higher-Order QAM Formats through Cross Phase Modulation of Modelocked Laser Pulses

An active, harmonically modelocked fiber laser with phase unstable output pulses from its long cavity is shown transformable into a low noise frequency comb suited for phase noise sensitive communications. Cross phase modulation (XPM) of its output pulses with a CW probe laser in a highly nonlinear fiber (HNLF) produces a frequency comb with noise at its minimum for low timing jitter pulses. Experiments with 2 ps pulses at 40 GHz repetition show spectral lines obtained from within a bandwidth of 30 lines achieve low bit error rate (BER) after their individual modulation with 96 Gb/s DP 64-QAM signals, reaching below the limit of hard-decision forward error correction (FEC), and close to that of a narrow linewidth CW laser. Its suitability as a seed for even broader comb generation is demonstrated by addition of a second stage HNLF, enabling a larger bandwidth of nearly 50-comb lines to reach a BER below the FEC limit for the same signal. The benefit of XPM for improving the phase noise of a modelocked semiconductor laser frequency comb is also shown, with limitation from larger pulse timing jitter. The results highlight the capability for enabling new applications of otherwise unusable pulse sources.