We present an amplifier system for 2 µm ultrafast laser pulses for potential material processing applications. The amplifier gain material is a 0.75 at.% doped Ho:YAG slab crystal measuring 10 mm x 1.5 mm x 55 mm. The pump source is an in-house developed continuous wave Tm:YLF slab laser which produces a maximum output power of 340 W, centred at the 1908 nm Ho:YAG absorption peak. The pump beam full widths were 0.2 mm by 5.3 mm in the slab. The seed for the experiment was a mode-locked Tm:LuScO3 laser that produced 200 fs pulses (~23.6 nm spectral bandwidth) centred at 2094 nm. The spectral peak of the seed laser was chosen so as to spectrally overlap both the 2090 and 2097 nm emission peaks of Ho:YAG. The pulse repetition frequency of the seed laser was 115 MHz, and the average power as measured after an optical isolator was ~57 mW. In the initial experiment the seed was focused into the slab using a spherical doublet lens pair to a beam diameter of 0.2 mm. The measured single pass gain was ~10 (up to 0.54 W) when pumped with 280 W. The effective pump power (disregarding transmitted pump light) in the gain volume used for amplification was estimated to be 8.3 W. The spectral bandwidth of the output signal was measured at several output powers and shown to converge to ~11.8 nm. Based on these results and in-house simulations we will implement a pre-amplifier and scale 2 µm ultrashort pulses to >100 W average power at MHz PRFs.
We present our efforts to power-scale the output from a single crystal, linear cavity Zinc Germanium Phosphide (ZnGeP2) optical parametric oscillator (OPO). Our initial doubly-resonant OPO produced a maximum total output power of 1.1 W while double-pass pumped with a 7 W 150 ns pulsed Ho:YLF laser. Next, we developed a pump source with higher power (38 W) and shorter pulse lengths (65 ns). This resulted in the demonstration of a maximum 5 W output from the OPO. However, the higher pump power could no longer pass through an optical isolator (limited damage threshold) and single pass pumping had to be used to prevent feedback to the pump laser. In addition, a longer folded OPO cavity had to be used to rid the cavity of the transmitted pump light using available optics. Overall, this resulted in a slightly less efficient optical to optical conversion (14 % vs. 15 %). To improve efficiency the OPO cavity was subsequently optimised in terms of pump and cavity mode sizes and output coupler mirror reflectivity (50 % vs. 80 % at 3 to 5 μm). The output coupler was also specified for high transmission of the pump allowing a short cavity length of only 20 mm with a ZnGeP2 crystal length of 15 mm. Operating near degeneracy, a total output power of 14 W was achieved when single-pass pumped with 48.8 W (conversion efficiency of 29 %).
High-power lasers at 2 µm are required for optical countermeasure applications, but can also be used for processing of materials where the mid-infrared laser wavelength provides an advantage. These applications can benefit from power scaling the 2 µm output, with the requirement to maintain a compact footprint. In particular, a Tm-doped slab laser design can be very compact, as an alternative to high power Tm:fiber lasers at 1.9 µm. It can be used directly for modulated continuous-wave output or for pumping Ho-doped lasers and amplifiers that emit at 2.1 µm.
We have directly compared Tm:YLF and Tm:LLF slab crystals (1.5 mm x 11 mm x 20 mm), in an otherwise identical diode end-pumped laser configuration, to evaluate the power scaling to 150 W of these two related materials. We will present the analysis of the thermal lens behaviour of that could not be fully supressed for Tm:LLF in the slab architecture when pumped at 450 W of incident pump power from the high-brightness 793 nm laser diode stack (Lasertel T6 Diode). Further power scaling to the 300 W output power level of Tm:YLF in a dual-end-pumped slab laser configuration will be presented, in which parasitic internal lasing has been supressed through careful consideration of the slab geometry.
The improved Tm:YLF laser will be used to pump a Ho:YAG slab (1.5 mm x 10 mm x 55 mm) to amplify seed pulses from a nanosecond Q-switched oscillator. A spatially and temporally resolved model has been developed to determine the optimal pump configuration and crystal dimensions to amplify seed pulses from 7 W average power at 10 kHz repetition rate, to upwards of 150 W at 2.1 µm. The model is based on rate equations and determines the distribution of thermal load throughout the crystal, permitting accurate prediction of saturation- and thermal-induced aberrations in the amplifier.
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