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We present a single-frequency thulium-doped fiber laser (TDFL) with a narrow linewidth of 20 kHz. Stable single-longitudinal-mode (SLM) lasing operation at 1957 nm is achieved using a segment of un-pumped polarization-maintaining thulium-doped fiber (PM-TDF) as an ultra-narrow bandwidth filter. A high optical signal-to-noise ratio (OSNR) of over 60 dB is obtained and a high power of over 400 mW is achieved with a high slope-efficiency (~45.8%) thulium-doped fiber amplifier (TDFA).

A high peak-power and narrow-linewidth all-fiber Raman pulsed laser operating around 1.65 µm is introduced. A 1541 nm laser seed is modulated into pulse trains, which will be used as the Raman pump laser, by driving a reflective semiconductor optical amplifier (RSOA) with a continuous periodic square-wave voltage. A homemade high peak-power 1541 nm pulsed laser is employed to modulate and amplify a 1653.7 nm distributed feedback laser (DFB) seed synchronously in a segment of the 52-meter-long highly germania-doped fiber (HGDF). The repetition-rate and the pulse-width of the 1653.7 nm pulsed laser are 100 kHz and 31 ns, respectively. The peak power is estimated to be as high as about 30.85 W, and a 3-dB linewidth as narrow as less than 0.08 nm is achieved when the average power of 1541 nm pump is 3.1 W. The wavelength of Raman pulsed laser can be tuned from 1652.0 nm to 1654.0 nm continuously with an optical signal-to-noise ratio (OSNR) of more than 35 dB.