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

Abstract

Direct bandwidth scaling of high-repetition-rate electro-optic (EO) frequency combs is limited by the power handling capability of modulators used. Significant bandwidth can be achieved by using nonlinear spectral broadening of a single laser comb, but current techniques for this have limited spectral flatness. Using dual or multiple laser combs enable enhanced nonlinear broadening with equalized spectra. However, if the lasers are uncorrelated, coherence is not preserved and only a multi-wavelength source is obtained. If the multiple lasers are derived from a single initial comb, coherence is preserved after spectral broadening. One way of achieving this is by direct filtering and amplification of a few comb lines from an initial EO comb and using it to coherently pump a cascade of nonlinear fibers. Alternatively, optical feedback can be used to feed a fraction of the entire output back. While the former technique requires additional components, the latter suffers from limited bandwidth scaling and degraded signal-to-noise ratio. We have discovered that by filtering the feedback to only consist of a few, suitable comb lines, a coherent, equalized, bandwidth scaled comb is obtained. We demonstrate a 25 GHz repetition-rate frequency comb in the C-band with ∼27 dBm power and 100 usable sub-carriers.