Silica distributed-feedback fiber laser beyond 2.1 µm

High-performance 2.1-2.2 µm single-frequency nanosecond-pulsed lasers are highly demanded for the remote sensing applications in the highly transparent 2.1-2.2 µm atmospheric window and for the biological applications for detecting characteristic C-H bondings. In this work, for what we believe to be the first time, we report a silica-based distributed feedback (DFB) fiber laser working beyond 2.1 µm. Using phase-mask-assisted femtosecond-laser inscription method, an off-center ultra-short cavity composed of distributed feedback fiber Bragg gratings was inscribed in a 23-mm-long non-polarization-maintaining (non-PM) heavily silica thulium-doped fiber (TDF). A 1.55-µm nanosecond-pulsed fiber laser with pulse width of 50-100 ns and repetition rate of 20-50 kHz was employed as the pump source. Through the inband resonant pumping scheme, gain-switched nanosecond pulsed laser output at a 2112 nm was obtained at the backward output port of the DFB thulium-doped fiber laser. To the best of our knowledge, this is so-far the longest laser wavelength from a silica-based DFB fiber laser. Single longitudinal mode operation was confirmed by a scanning Fabry-Perot interferometer (SFPI). At the absorbed pump power of 650 mW (pulse width: 100 ns, repetition rate: 50 kHz), the measured maximum average power and slope efficiency was 22.5 mW and 4.2%, respectively, while the gain-switched pulse width decreased to 15.2 ns. The corresponding pulse energy and peak power were then deduced to be 450 nJ and 29.5 W, respectively. Additionally, the polarization extinction ratio (PER) from the non-PM-TDF-based DFB laser was measured to be >18 dB, attributed to the orientation of the grating plane array of the FBGs fabricated by the fs-laser inscription method. Finally, we expect that under the pump of a multi-watt, 1.55-µm ns-pulsed fiber laser, an ultrashort gain-switched DFB TDFL incorporating low-loss (≤0.1 dB) FBGs can deliver an output with a pulse duration shorter than 20 ns and a peak power on the order of 100 W across the entire 2100-2200 nm wavelength range.