Solid-state Raman frequency converters for CO 2 -DIAL systems at 1.6 μm

Measurement of the three-dimensional distribution of atmospheric trace gases, especially CO 2 , 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 CO 2 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 LiNH 2 C 6 H 4 SO 3 •H 2 O, Ba(NO 3 ) 2 , Li 2 SO 4 •H 2 O, Y(HCOO) 3 •2H 2 O, β-BBO and diamond. Our experiments with Ba(NO 3 ) 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(NO 3 ) 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.