2.05 μm high-energy thulium-doped fibre amplifier based on backward pumping.

We demonstrate a thulium (Tm)-doped fibre amplifier using a Ho: GdVO solid-state laser as the master oscillator operating at 2048.53 nm. In experiment, a novel spatial coupling system was used, combining a single lens with a curved end cap to simplify the structure and reduce losses. The integrated fibre end cap design effectively protected the fibre core from damage caused by high-energy short-pulse seed lasers. The power amplifier (PA) stage adopted backward pumping to directly achieve pure laser output. In continuous wave (CW) mode, a maximum laser output power of 33.4 W was obtained under a maximum pump power of 151 W injection at 18 °C. The slope efficiency of the amplification stage was 19.5%. The output laser wavelength was 2048.57 nm, and the full width at half maximum (FWHM) was 0.14 nm. In Q-switched mode, a maximum single pulse energy of 3.86 mJ was obtained at a pulse repetition frequency (PRF) of 5 kHz with a pulse width of 22.1 ns, corresponding to a peak power of 174.7 kW and an average output power of 19.3 W. No nonlinear optical effects, such as stimulated Raman scattering (SRS) or stimulated Brillouin scattering (SBS) was observed throughout the experiment, and the fibre amplifier output remained stable. Ultimately, the maximum single pulse energy obtained by a 2.05 μm backward-pumped Tm-doped fibre amplifier (TDFA) at a PRF of 5 kHz and a pulse width of 22.1 ns was 3.86 mJ.