All-fiber polarization-maintained figure-9 laser with frequency locking at a repetition rate of 247  MHz

We report on an all-fiber polarization-maintained (PM) erbium-based nonlinear amplifying loop mirror mode-locked laser (figure-9). Due to a hybrid optical element, the laser can be operated in a simple and compact configuration with a high repetition rate reaching up to 247 MHz. By balancing the length of the Er-doped active fiber and the PM-1550 passive fiber, the cavity dispersion was designed to be near zero to negative. The central wavelength was centered at 1567 nm with a 3 dB bandwidth of 26.3 nm. The pulse duration was measured to be 245 fs. Further, the fundamental repetition frequency was locked to a stable radio reference. The Allan deviation for frequency stability was calculated to be 3.146×10^-12 at 1 s' gate time. To our knowledge, this is the highest repetition rate for all-fiber figure-9 structure mode-locked lasers.