Passive all-optical synchronization for polarization-maintaining ultrafast fiber lasers

We have proposed and implemented for the first time to our best knowledge a passive and all-optical pulse synchronization for polarization-maintaining ultrafast fiber lasers. Specifically, the synchronization system was comprised of two independent Yb-doped and Er-doped mode-locked fiber lasers in a master-slave configuration. Master pulses were injected into the slave laser cavity consisting of a nonlinear amplifying loop mirror, which provided an effective fast intensity modulator due to the periodic introduction of nonreciprocal phase difference. As a result, robust and tight timing synchronization was achieved with a cavity mismatch tolerance of 800 $\mu$m and a relative timing jitter of 26 fs within 1-MHz bandwidth. In combination with all-polarization-maintaining structure of fiber lasers, long-term stable operation was demonstrated over 12 hours without the need of temperature stabilization and vibration isolation. The implemented synchronous laser system could find immediate applications such as pump-probe microscopy, two-color spectroscopy and nonlinear frequency mixing.