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Measurement of the three-dimensional distribution of atmospheric trace gases, especially CO2, is an important factor to
improve the accuracy of climate models and to understand the global effects of the greenhouse effect. This can be
achieved by differential absorption Lidar (DIAL).
The absorption spectrum of CO2 features several suitable absorption lines for a ground-based or air-borne DIAL system
working at wavelengths between 1.57 μm and 1.61 μm. An appropriate laser transmitter must emit laser pulses with
pulse energies of more than 10 mJ and pulse duration in the nanosecond range. For high spectral purity the bandwidth is
required to be less than 60 MHz.
OPOs and Er-doped solid-state lasers emit around 1.6 μm, but we describe here alternatively Nd:YAG and Nd:glass laser
systems with Raman converters. The use of stimulated Raman scattering in crystalline and ceramic materials is a
possibility to shift the wavelength of existing lasers depending on the size of the Raman shift. After the investigation of a
large number of Raman-active materials some of them could be identified as promising candidates for the conversion of
typical Nd:YAG emission wavelengths, including LiNH2C6H4SO3•H2O, Ba(NO3)2, Li2SO4•H2O, Y(HCOO)3•2H2O, β-BBO and diamond. Our experiments with Ba(NO3)2 showed that the choice of the material should not be restricted to
those with an adequate first order Stokes Raman line position, but also second or third order Raman shift should be
considered.
Development of Raman frequency converters for high pulse energies concentrates on linear and folded resonator designs
and seeded Raman amplifiers using the Raman material as a direct amplifier. With Ba(NO3)2 pulse energy up to 116 mJ
and 42 % quantum efficiency at the third Stokes wavelength with 1599 nm has been demonstrated. High power operation
at 5 W with compensation of thermal lensing was achieved.
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