An efficient continuous-wave and Q-switched single-pass two-stage Ho:YLF MOPA system

• The Ho:YLF laser operation at two wavelengths in the same configuration.
• The efficient two-stage Ho:YLF MOPA system is developed.
• High output power, slope and optical-to-optical efficiencies have been achieved.
• Above 0.84 MW pulse peak power at 2050 nm wavelength is recorded.
• An excellent beam quality of the Ho:YLF-MOPA laser system with M2 of below 1.1.

We report on the efficient operation of an Ho:YLF laser single-pass in-band pumped by a Tm-doped fiber laser. The research in a continuous-wave (CW) operation in an oscillator scheme was done for a crystal of 0.5 at% Ho dopant concentration and the length of 30 mm for the output coupler transmittances of TOC=10%, 20%, 30% and 40%. At room temperature, for the output coupling transmission of 20%, the maximum CW output power of 24.5 W for 82.5 W of incident pump power, corresponding to the slope efficiency of 35.4% and optical-to-optical conversion efficiency of 29.7% was achieved. The highest slope efficiency of 81.6% with respect to absorbed pump power was obtained. Carrying out the measurements of the laser spectrum, for the out-coupling transmittance of TOC=30%, we observed a very short time wavelength shift between 2051.5 and 2062.4 nm in an Ho:YLF laser operation. Trying to fully utilize the pump power unabsorbed by the active crystal in an oscillator stage, an amplifier system based on two additional Ho:YLF crystals was developed. For the output coupling transmission of 40% the slope efficiency increased from 31.5% in an oscillator scheme to 47.3% with respect to the incident pump power in a two-stage amplifier scheme with a beam quality parameter of M2 better than 1.1. For a Q-switched operation, for the maximum incident pump power and the pulse repetition frequency (PRF) of 1 kHz, pulse energies of 18.5 mJ with a 22 ns FWHM pulse width corresponding to 841 kW peak power in the amplifier system were recorded.