Long range water vapor DIAL-systems require efficient and rugged laser sources. The quasi-three-level transition
from R<sub>1</sub> to Z<sub>5</sub> in Nd:GSAG with 943nm wavelength is a promising candidate. An actively Q-switched Nd:GSAG
laser was established. Up to 31mJ output pulse energy and up to 26Hz repetition rate was achieved. Injection
seeding was used to obtain single frequency operation. The seed laser is a distributed feedback laser diode. Laser
frequency was stabilized by ramp-hold-fire method. By tuning the wavelength of the seed laser a 0.83nm tuning
range of the pulsed Nd:GSAG laser was obtained. Measured by Fabry-Perot interferometer the spectral line width
was approximately 50MHz.
A Nd:GSAG laser operated at the <sup>4</sup>F<sub>3/2</sub>
→<sup>4</sup>I<sub>9/2</sub> transition was tuned by a FPI-etalon achieving a tuning range of
1.5 nm with a center wavelength at 942.7 nm. Three water vapor absorption wavelengths with different absorption
strength as suitable for a water vapor LIDAR are within this tuning range and lasing could be achieved at all
three wavelength. Q-switched pulse energies up to 26mJ were obtained as required for long range detection.
Wavelength around 940 nm lasing can be obtained by quasi-three-level operation of Nd doped laser crystal.
Diode end pumping provides the necessary high pump intensity in the laser crystal. In order to get high efficiency of the
end pumped laser system, the overlap coefficient between pump beam and laser beam should be optimized. Thermal lens
coefficient is one of the most important parameters to design the laser cavity structure. The time dependent heat
conduction equation is solved numerically in order to study the thermal lens effect in pulsed pumping laser crystal.
Calculated results showed that the thermal lens coefficients change with different pump frequencies. Experiments are
done with Nd:YAG and Nd:GSAG laser rod. The thermal lens coefficient of Nd:YAG at pump frequency 50 Hz with
pump beam diameter 1.5 mm is 10.2 Wm, while the thermal lens coefficient of Nd:GSAG at pump frequency 50 Hz with
pump beam diameter 1.75 mm is 5.9 Wm.
An actively Q-switched Nd:GSAG laser with 942nm wavelength was frequency doubled in a critically type-I
phase-matched LBO. Maximum pulse energy of 8mJ with 300ns pulse duration at 471 nm was obtained with 19mJ
incident radiation at 10Hz. The corresponding conversion efficiency was 42%. The frequency doubling of a focused
Gaussian beam involves spatially dependent phase mismatching due to beam divergence. It decreases the conversion
efficiency and deteriorates the beam quality. According to the theoretical calculation, elliptical focusing was used to
improve the second harmonic beam quality and slightly increase the conversion efficiency.