Advanced resonator designs for Ho3+:YAG lasers and ZGP OPOs are presented. A segmented Ho3+:YAG crystal for power scaling was investigated. An improved power performance could not be shown up to 60W of output power, which is attributed to the low overall crystal temperature. Exploiting the Porro prism resonator concept for the first time, a 200-times increased alignment tolerance compared to a corresponding mirror resonator was achieved. The performance of a ZGP OPO could be improved by a negative lens in the resonator, which significantly enhances the beam quality.
We present a Ho3+:YAG laser source and use it to pump a linear ZGP OPO with a novel design intended to improve the mode matching properties of the resonator. Beam quality measurements are used to evaluate the performance of the novel design in comparison with a conventional linear resonator. Operated at 25 kHz repetition rate, the Ho3+:YAG laser delivers 2.2 mJ, 20 ns Q-switched pulses. This results in a pulse peak power of 108 kW while the average output power is 58W. In the optimal ZGP OPO configuration, 14.1W of signal and idler output power are achieved with a conversion efficiency of 49.8 % with respect to the absorbed pump power. A clearly improved beam quality of 2.1 and 3.3 (2.4 and 3.5) in the x- and y-axis of the signal (idler) beam compared to the conventional linear resonator is shown.
We report on laser resonators with a segmented and a homogeneously doped Ho3+:YAG crystal delivering over 60 W of output power with near-diffraction-limited beam quality. The resonators with both crystals exhibit high slope efficiencies around 67% and maximum pulse energies of 1.14 mJ and 1.04 mJ are measured for the homogeneously doped and segmented crystal, respectively, at a repetition rate of 50 kHz. Q-switched pulses with a pulse peak power of 108 kW are generated with the homogeneous crystal at a repetition rate of 25 kHz. In a slight redesign of the cavity, 1:24 mJ, 33 ns pulses with a pulse peak power of 38kW are measured.
We present a crossed-Porro prism resonator with a Ho3+:YAG crystal and investigate it with a focus on the alignment stability. Furthermore, we show a single-Porro-ended resonator optimized for Q-switched operation. Both resonators are compared to corresponding mirror resonators. In the crossed-Porro prism resonator, a maximum output power of 30.7 W is reached with a high slope efficiency of 67.4 %. By tilting each of the prism axes one by one and measuring the entailed drop in output power, the alignment sensitivity is determined. In comparison to a corresponding mirror resonator, it is improved by a factor of up to 200. With this design, 170 ns Q-switched pulses with an energy of 0.51 mJ are generated at a repetition rate of 50 kHz. In the single-Porroended resonator significantly shorter pulses with a duration of 55 ns and a maximum pulse energy of 0.8 mJ were achieved.
We report on an actively Q-switched high-pulse-energy Ho3+:YAG laser in-band pumped by a Tm3+-doped fiber laser, both operated at room temperature. The Ho3+:YAG active medium inside a plane-plane cavity is pumped using a commercial Tm3+-doped fiber laser at 1908 nm from one side. In continuous operation a maximum power of 20.1 W with a slope efficiency of 45.1%, central wavelength of 2090 nm and a beam quality factor M2 below 1.5 were achieved. Q-switched operation was achieved using a Brewster-cut acousto-optic modulator (AOM) based on crystalline Quartz. During Q-switching the incident power was kept stable at 47 W to obtain an M2 of 1.3 and a stable thermal lens inside the laser crystal. With the variation of the repetition frequency a lower limit of the quasi-continuous pulsed regime was investigated and measured to be approximately 3 kHz. The maximum pulse energy in Q-switching operation was achieved with a repetition frequency of 700 Hz leading to an energy of 15 mJ at 12.1 ns pulse width, corresponding to a peak power of 1.2 MW. The laser showed no sign for a loss of performance during many hours of testing. Using this laser as a pump source for a double resonant OPO, a maximum mid-infrared output power of 6.3 W could be achieved at a repetition frequency of 2 kHz, accompanied by a low threshold power of 1.6 W and a slope efficiency of 49.2%.
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