Sub-10-ns single-frequency linearly polarized 2055-nm thulium-doped distributed-feedback fiber laser inscribed by femtosecond-laser direct writing

The 2.05-µm band has gained significant interest for high-precision CO 2 remote sensing and mid-infrared generation via nonlinear frequency conversion, both of which demand single-frequency, linearly polarized nanosecond pulsed laser sources with high peak power and low noise. Here, we present a compact all-fiber source at 2055nm based on a distributed-feedback (DFB) silica thulium-doped fiber laser (TDFL). Femtosecond-laser direct-writing method has been employed to inscribe the DFB cavity in a commercial non-polarization-maintaining silica thulium-doped fiber (TDF). The DFB TDFL oscillator, pumped by a 1.55-µm gain-switching scheme, delivers stable single-longitudinal-mode pulses with a maximum average power of 120 mW, a pulse duration of 5.6 ns, and a polarization extinction ratio (PER) exceeding 18 dB. The output exhibits a near-Fourier-transform-limited linewidth of ∼100 MHz and maintains a Gaussian temporal profile across the entire operating range. When seeded into a single-stage master oscillator power amplifier (MOPA) based on a 10-µm-core silica double-clad-thulium-doped fiber (DCTDF), the average output power has been boosted to 0.62 W, corresponding to a peak power of 2.2 kW, while preserving a PER above 20 dB. Owing to the sub-10-ns pulse duration being shorter than the acoustic phonon lifetime in silica, the amplifier is free from stimulated Brillouin scattering (SBS) at the maximum output power. Furthermore, detailed analysis indicates the peak power of such a 2055-nm single-frequency TDFL MOPA with a 5.6 ns pulse width can be scaled up to ∼7.8 kW before SBS reaches the threshold. On the other hand, modulation-instability-induced four-wave mixing is expected to be the primary obstacle for further power scaling.