  Repetition-rate-scanned lasers are practical in accordion frequency comb
generation that serves as a variable gearbox connecting optical and radio wave
domains. Rapid and wide-range scanned repetition rate can benefit versatile
purposes, however scanning robustness remains unsecured that typically requires
complicated feedback loops. Recently, multiplexed lasers have been demonstrated
with the nature of common-noise rejection among simultaneously emitted combs.
Here, we propose a bidirectional polarization-multiplexed fiber laser that
delivers synchronized pulses with rapid-scanned and reference-free repetition
rates. Benefiting from the all polarization-maintaining fiber configuration,
the laser shows good robustness and inter-comb coherence. As rapid as 493.5
kHz/s scanning rate over 329-kHz scanning range of fundamental repetition rate
is realized. The 1-hour and 1-day maximal variations of difference frequency
are merely 0.52 Hz and 5.46 Hz. The capability to rebuilt steady state after
mode hopping is also demonstrated. These results provide a promising solution
for developing high-performance accordion-frequency laser sources.
