Nanosecond pulsed single-frequency two-stage holmium-doped fiber MOPA at 2054 nm and 2090 nm

A single-frequency polarization-maintaining holmium-doped fiber master oscillator power amplifier operating at signal wavelengths of 2054 nm and 2090 nm is presented. The two-stage setup delivers up to 240 W peak power and6.7 µJ pulse energy for a pulse width of 30.2 ns at a repetition rate of 100 kHz. The first amplifier stage is designed by simulation, tailored for high gain at the signal wavelength range, favoring amplification at 2090 nm. The design is discussed, and the measured values are compared with the simulation. The second stage is investigated regarding the efficiency for co- and counter-pumping. Stimulated Brillouin scattering was found to be the limiting factor for pulse peak power scaling in the second stage. The measured output pulse shapes are discussed and compared to pulse shapes derived with the Frantz-Nodvik model.