  Frequency combs with mode spacing in the range of 10 to 20 gigahertz (GHz)
are critical for increasingly important applications such as astronomical
spectrograph calibration, high-speed dual-comb spectroscopy, and low-noise
microwave generation. While electro-optic modulators and microresonators can
provide narrowband comb sources at this repetition rate, a significant
remaining challenge is a means to produce pulses with sufficient peak power to
initiate nonlinear supercontinuum generation spanning hundreds of terahertz
(THz) as required for self-referencing in these applications. Here, we provide
a simple, robust, and universal solution to this problem using off-the-shelf
polarization-maintaining (PM) amplification and nonlinear fiber components.
This fiber-integrated approach for nonlinear temporal compression and
supercontinuum generation is demonstrated with a resonant electro-optic
frequency comb at 1550 nm. We show how to readily achieve pulses shorter than
60 fs at a repetition rate of 20 GHz and with peak powers in excess of 2 kW.
The same technique can be applied to picosecond pulses at 10 GHz to demonstrate
temporal compression by a factor of 9x yielding 50 fs pulses with peak power of
5.5 kW. These compressed pulses enable flat supercontinuum generation spanning
more than 600 nm after propagation through multi-segment dispersion-tailored
anomalous-dispersion highly nonlinear fiber (HNLF) or tantala waveguides. The
same 10 GHz source can readily achieve an octave-spanning spectrum for
self-referencing in dispersion-engineered silicon nitride waveguides. This
simple all-fiber approach to nonlinear spectral broadening fills a critical gap
for transforming any narrowband 10 to 20 GHz frequency comb into a broadband
spectrum for a wide range of applications that benefit from the high pulse rate
and require access to the individual comb modes.
