Optical-Optical Synchronization Between Two Independent Femtosecond Yb-Fiber Lasers With 10-20 Instability in 105 s

Optical-optical synchronization between independent mode-locked lasers with attosecond timing precision is essential for arbitrary electric-field waveform generation, sub-cycle optical pulse synthesis, optical frequency transfer as well as next-generation photon-science facilities, e.g., X-ray free-electron lasers. Long-term stable operation with low timing drift is highly desired for all above applications. Here, we present a five-day uninterrupted timing synchronization between two independent femtosecond Yb-fiber lasers via balanced optical correlation method. The out-of-loop residual timing drift over the entire time frame reaches 733 as rms, corresponding to 1.36 x 10(-20) instability at 1.31 x 10(5) s. To the best of our knowledge, it is the first characterization of 10(5) s instability for subfemtosecond optical-optical synchronization based on mode-locked lasers.