|
We report on continuously tunable operation of a diode pumped lasers based on Tm-doped materials, emitting
in the 1.8 - 2.μ1 m spectral band. In our study we compare results obtained with three various single crystals
doped by Tm3+ ions: Yttrium Aluminum perovskite YAP (YAlO3), Gadolinium orthovanadate GdVO4, and
Yttrium Lithium Fluoride YLF (YLiF4). Following samples were available: the 3mm 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 2.7mm long a-cut crystal block of Tm:GdVO4 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 793nm was used for Tm:YAP and Tm:YLF; the second operating at wavelength 802nm
was used for Tm:GdVO4. In both cases, the continuous power up to 20W 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, 80mm long, hemispherical laser cavity. The curved (radius 150mm) 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 1.5mm 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 2.8W in
this set-up was obtained. The laser could be tuned from 1865nm up to 2036nm with a maximum at 1985 nm.
Laser based on Tm:YLF crystal was tunable from 1835nm up to 2010nm with a maximum at 1928 nm (3.0W
was reached). Using the Tm:GdVO4 tunable operation with greater that 1W output at 1920nm and 130nm
tuning range (1842-1972 nm) was demonstrated. The overall reached tuning range of over 200nm 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.
|
