870 fs, 448 kHz pulses from an all-polarization-maintaining Yb-doped fiber laser with a nonlinear amplifying loop mirror.

We demonstrate a mode-locked long all-polarization-maintaining fiber laser with a nonlinear amplifying loop mirror. The fiber oscillator directly delivers 221 ps chirped pulses at the repetition rate of 448 kHz. The pulses can be further amplified up to 134 nJ and compressed down to 870 fs by a grating pair. This kind of laser is self-starting and stable long term, and it has potential application in high-power fiber amplification for industrial applications.