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 minutes. Multistage phase-locked loops help to preserve this ultrahigh timing resolution throughout the entire delay range between pulses (10 ns). We also demonstrate that the same level of synchronization can be achieved with two lasers at different repetition frequencies.

• Received 30 November 2000

DOI:https://doi.org/10.1103/PhysRevA.64.021802