Q-switching of a 1.7 μm Thulium-doped fiber laser based on frequency-shifted feedback

We demonstrate a frequency-shifted feedback (FSF) Thulium-doped fiber laser (TDFL) operating at 1726 nm that generates stable Q-switched pulses in an all-fiber Fabry–Pérot (FP) cavity. The laser cavity consisted of a 1560-nm-pumped Thulium-doped fiber (TDF), a pair of fiber Bragg grating (FBG) mirrors, and an acousto-optic frequency shifter (AOFS). Q-switched operation was achieved without the use of a saturable absorber or conventional intracavity loss modulation; instead, pulse generation was governed by the combined effects of FSF and intracavity spectral filtering. At a pump power of 1.53 W, stable Q-switched pulses with a pulse duration of ∼2.1 μs and a repetition rate of ∼71.6 kHz were generated. The laser output was centered at 1726 nm with a narrow spectral linewidth of ∼0.08 nm, and the pulse energy reached ∼269 nJ. Long-term temporal stability of the laser output was experimentally verified. Replacing the AOFS with a variable optical attenuator under comparable loss conditions resulted only in continuous-wave (CW) output. These results demonstrate the feasibility of FSF-based Q-switching as an attractive alternative to conventional Q-switching schemes for implementing pulsed TDFLs in the 1.7 μm spectral region.