Active synchronization and carrier phase locking of two separate mode-locked femtosecond lasers

Two independent mode-locked femtosecond lasers are synchronized to an unprecedented precision. The rms timing jitter between the lasers is 4.3 fs observed within a 160 Hz bandwidth over tens of seconds, or 26 fs within a 50 kHz bandwidth. Novel multi-stage phase-locked loops help to preserve this ultrahigh timing resolution while setting on arbitrary delay between the two pulse trains (0-5 ns). Under such synchronization, phase locking between the carrier frequencies of the two femtosecond lasers has been achieved, It is also demonstrated that the same level of synchronization can be achieved with two lasers at different repetition frequencies.