Comparison of tunable lasers based on diode pumped Tm-doped crystals

摘要

We report on continuously tunable operation of a diode pumped lasers based on Tm-doped materials, emitting in the 1.8 - 2.1 μm spectral band. In our study we compare results obtained with three various single crystals doped by Tm~(3+) ions: Yttrium Aluminum perovskite YAP (YAlO_3), Gadolinium orthovanadate GdVO_4, and Yttrium Lithium Fluoride YLF (YLiF_4). Following samples were available: the 3 mm long a-cut crystal rod of Tm:YAP with 4% at. Tm/Y (diameter 3 mm); the 8mm long b-cut crystal rod of Tm:YLF with 3.5 % at. Tm/Y (diameter 3 mm); the 2.7 mm long a-cut crystal block of Tm:GdVO_4 with 2 % at. Tm/Gd (crystal face 5×3 mm). For active medium pumping, the laser diode radiation was used. Because the tested samples differs significantly in absorption spectra, two fibre-coupled (core diameter 400 μm) temperature-tuned laser diodes were used: first operating at wavelength 793 nm was used for Tm:YAP and Tm:YLF; the second operating at wavelength 802 nm was used for Tm:GdVO_4. In both cases, the continuous power up to 20 W was available for pumping. The diode radiation was focused into the active crystal by two achromatic doublet lenses with the focal length f= 75 mm. The measured radius of pumping beam focus inside the crystal was 260 μm. The longitudinally diode pumped crystals were tested in linear, 80 mm long, hemispherical laser cavity. The curved (radius 150 mm) output coupler reflectivity was ~ 97 % in range from 1.8 up to 2.1 μm. The pumping flat mirror had maximal reflectivity in this range and it had high transmission around 0.8 μm. A 1.5 mm thick birefringent plate made from quartz (Lyot filter) inserted under a Brewster's angle was used as a tuning element. This plate was placed inside the resonator between the crystal and the output coupler. Using Tm:YAP crystal, the maximal output power of 2.8 W in this set-up was obtained. The laser could be tuned from 1865 nm up to 2036 nm with a maximum at 1985 nm. Laser based on Tm:YLF crystal was tunable from 1835 nm up to 2010 nm with a maximum at 1928 nm (3.0 W was reached). Using the Tm:GdVO_4 tunable operation with greater that 1W output at 1920 nm and 130 nm tuning range (1842 — 1972 nm) was demonstrated. The overall reached tuning range of over 200 nm covers many important atmospheric absorption lines and contains also the local absorption peak of liquid water, making them attractive for applications such as high resolution spectroscopy, atmospheric remote sensing, laser radar, and laser microsurgery.