  Nonlinear polarization evolution (NPE) is among the most advanced techniques
for obtaining ultrashort pulses with excellent optical performance. However, it
is challenging to design environmentally stable NPE fiber oscillators using
only polarization-maintaining (PM) fibers. Here, we use the same PM fiber and
non-reciprocal phase shifter to design two different devices, which are capable
of acting as effective NPE saturable absorbers (SAs) in two all-PM linear
cavity fiber lasers. These two laser setups differ in the position of the
non-reciprocal phase shifter, the presence of which is crucial for the proposed
fiber lasers to reduce their mode-locking thresholds and achieve high
repetition rates above 100 MHz. The mode-locking principle and pulse evolution
in the laser cavity are investigated theoretically. The first all-PM fiber
oscillator emits sub-200 fs stretched pulses with low peak powers. The second
oscillator, with a simpler architecture, directly delivers stretched pulses
with high peak powers, the spectral bandwidth greater than 30 nm, and the pulse
duration less than 90 fs. To the best of our knowledge, 79 fs achieved in this
design is the shortest pulse duration provided by PM fiber lasers using NPE
mode-lockers.
