SRS conversion efficiency assessment of a single cell Raman gas mixture for DIAL ozone lidar

Abstract

A single Raman cell configuration useful for DIAL ozone lidar is designed and optimized. The conversion efficiency and flexibility of using a single Raman cell filled with a mixture of high pressure Raman active gases hydrogen (H2) and methane (CH4) have been examined and reported. The stimulated Raman scattering (SRS) conversion efficiency of Raman active gases with different total cell pressures and the volume mixing ratio excited with a focused, frequency quadrupled Nd:YAG laser with a maximum pulse energy of 25 mJ and a pulse duration of 10 ns at 100 Hz repetition rate are examined in detail. The gas combination of H2:CH4 emits a coaxial beam of two wavelengths, 288.4 nm (CH4) and 299.1 nm (H2), with a maximum total conversion efficiency of about 45%. The optimum volume mixing ratio for generating the required wavelength pair with almost equal energies is found to be 2:1 (H2:CH4) at a total cell pressure of 18 bar. The contribution of cascade Raman scattering (CRS) and four-wave mixing (FWM) to the higher order Stokes lines is examined. The laser attenuation due to soot formation under various mixing ratios in the cell is also presented.