  We investigate the high-frequency timing jitter spectral density of
mode-locked fiber lasers in different mode-locked regimes. Quantum
noise-limited timing jitter spectra of mode-locked-regime-switchable Yb fiber
lasers are measured up to the Nyquist frequency with sub-100 as resolution. The
integrated rms timing jitter of soliton, stretched-pulse, and self-similar Yb
fiber lasers is measured to be 1.8 fs, 1.1 fs, and 2.9 fs, respectively, when
integrated from 10 kHz to 40 MHz. The distinct behavior of jitter spectral
density related to pulse formation mechanisms is revealed experimentally for
the first time.
