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, subcycle 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 $1.36\times 10^{{\rm -20}}$ instability at $1.31\times 10^{{\rm 5}}{\rm{\,s}}$ . 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.