Tailored optical feedback for bandwidth scaling and spectral equalization of high repetition rate electro-optic frequency combs

Direct bandwidth scaling of high-repetition-rate electro-optic (EO) frequency combsis limited by the power handling capability of modulators used. Significant bandwidth can beachieved by using nonlinear spectral broadening of a single laser comb, but current techniques forthis have limited spectral flatness. Using dual or multiple laser combs enable enhanced nonlinearbroadening with equalized spectra. However, if the lasers are uncorrelated, coherence is notpreserved and only a multi-wavelength source is obtained. If the multiple lasers are derived froma single initial comb, coherence is preserved after spectral broadening. One way of achievingthis is by direct filtering and amplification of a few comb lines from an initial EO comb andusing it to coherently pump a cascade of nonlinear fibers. Alternatively, optical feedback can beused to feed a fraction of the entire output back. While the former technique requires additionalcomponents, the latter suffers from limited bandwidth scaling and degraded signal-to-noiseratio. We have discovered that by filtering the feedback to only consist of a few, suitable comblines, a coherent, equalized, bandwidth scaled comb is obtained. We demonstrate a 25 GHzrepetition-rate frequency comb in the C-band with∼27 dBm power and 100 usable sub-carriers.