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.