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.