All-Polarization-Maintaining, Ultra-Compact Tm-Doped Fiber Laser Designed for Mid-Infrared Comb

Developing compact and stable 2 $\mu \text{m}$ ultrafast laser sources is important for generating mid-inferred frequency combs. Here, we report on a simple, compact and reliable thulium-doped fiber laser based on figure-of-9 mode-locking. Self-starting and stable mode-locked operation is achieved with 0.30 nJ pulse energy and 128 fs pulse width under 121.3 MHz repetition rate. The generated pulses centered at 1990 nm have a 3 dB bandwidth of 39.4 nm. The laser has been assembled into a $197\times 78\times 20$ mm ³ fiber box under temperature-control. The repetition rate was phase locked over 20 h with a fluctuation standard deviation of 192.5 $\mu $ Hz. Such compact and highly-reliable mid-inferred laser could be used beyond laboratory and promote numerous critical applications including dual-comb spectroscopy, ranging and frequency calibration.