  We demonstrate a novel, robust and compact fiber laser mode-locked by
nonlinear polarization evolution (NPE) in polarization-maintaining (PM) fibers.
The reflectivity of the artificial saturable absorber (SA) is analyzed to
explain the mode-locking mechanism in the laser cavity. Experimentally, three
linear laser schemes that feature repetition rates 94 MHz, 124 MHz and 133 MHz
are systematically investigated. When the pump power is 1100 mW, the 124-MHz
laser cavity delivers highly stable pulses with a single-pulse energy of 0.92
nJ. After the compression, the pulse duration obtained from the 124-MHz fiber
laser is 250 fs, while the corresponding transform-limited pulse duration is
124 fs. The highest fundamental repetition rate that could be achieved in our
experiment is 133 MHz, as mentioned above. The noise characterization has been
performed with different cavity lengths and therefore different net-cavity
dispersion. The 68-fs timing jitter and the 0.01% relative intensity noise
(RIN) of the 133-MHz fiber laser have been realized integrated from 1 kHz to 10
MHz. Furthermore, the root-mean-square (RMS) power fluctuation is 0.35% in 2
hours, which implies superior stability of the output power. Thus, this linear
fiber oscillator provides a competitive low-noise light source for optical
applications appropriate for complex environments.
