  Mutual phase coherence is the most crucial factor in asynchronous optical
sampling system, and its enhancement is commonly achieved by phase-locking both
femtosecond lasers to a shared narrow-linewidth cavity-stabilized laser. Here
we report such a system with a low residual optical phase jitter at a level of
0.04 rad in a Fourier frequency band from 1 Hz to 5 MHz around 1.55 {\mu}m,
corresponding to a timing jitter of 30 as. The residual phase jitter reaches 1
rad at a Fourier frequency between 0.06 Hz and 0.1 Hz, from which the
phase-coherence time is inferred at least 10 s. The multi-heterodyne beats
experimentally reveal a hardware-limited phase coherence time of ~8.15 s
throughout the direct lasing spectral band.
