  We demonstrate 14.3-attosecond timing jitter [integrated from 10 kHz to 94
MHz offset frequency] optical pulse trains from 188-MHz repetition-rate
mode-locked Yb-fiber lasers. In order to minimize the timing jitter, we shorten
the non-gain fiber length to shorten the pulsewidth and reduce excessive
higher-order nonlinearity and nonlinear chirp in the fiber laser. The measured
jitter spectrum is limited by the amplified spontaneous emission limited
quantum noise in the 100 kHz - 1 MHz offset frequency range, while it was
limited by the relative intensity noise-converted jitter in the lower offset
frequency range. This intrinsically low timing jitter enables
sub-100-attosecond synchronization between the two mode-locked Yb-fiber lasers
over the full Nyquist frequency with a modest 10-kHz locking bandwidth. The
demonstrated performance is the lowest timing jitter measured from any
free-running mode-locked fiber lasers, comparable to the performance of the
lowest-jitter Ti:sapphire solid-state lasers.
