7.5 kW high-peak-power all-fiber single-frequency Tm/Tm-Ho combined fiber laser at 2.05 µm by balancing stimulated Brillouin scattering and parasitic oscillation

A 2.05 µm high peak-power all-fiber single-frequency pulsed polarization-maintaining (PM) laser amplifier structure is proposed. The main amplifier is comprised of the combination of Tm-doped fiber (TDF) and Tm-Ho co-doped fiber (THDF), and the stimulated Brillouin scattering (SBS) and parasitic oscillation thresholds can be effectively improved. By replacing the 1.7 m TDF with a 1.7 m TDF/THDF combination, the peak power is increased from 4.8 kW to 6.75 kW at a pulse repetition frequency (PRF) of 3 kHz and a short pulse width of 80 ns, which is 1.4 times higher than that of a TDF amplifier, limited by SBS. Meanwhile, the pulse energy is increased from 610 µJ to 760 µJ at a long pulse width of 210 ns, which is 1.25 times higher than that of a TDF amplifier, limited by parasitic oscillations. By optimizing the combined fiber length, a 7.5 kW laser output is achieved with a TDF/THDF length of 1.5 m. The linewidth of the single-frequency pulsed light is about 6.56 MHz at the pulse width of 80 ns, approaching the Fourier transform limit, and the beam quality factor M² is estimated to be about 1.13.