Experimental Studies Of The Stimulated Raman Scattering In H2 - Ar Mixtures

Stimulated Raman scattering experiments have been performed in H2 and H2 -Ar mixtures with an UVpreionized discharge laser (308 nm) as the pump. The energy conversion efficiency from the pump laser beam to Stokes beams has been investigated as a function of Ar concentration and pumping energy. It has been found that in the H2 -Ar mixture with 50% of Ar, the energy conversion efficiency to the first Stokes was more than 80% higher than that obtained in H2 at the same total pressure (40 atm) and pump energy (60 mJ). Introduction At present there is a great interest in improving the performance of excimer lasers. Because of their high efficiency and short wavelength, excimer lasers are the most important drivers for high power laser matter interaction studies. Frequency shifting is widely investigated to improve the beam quality and to extend the range of application. Stimulated Raman scattering (SRS) in compressed molecular gases is one of the most efficient methods for shifting the output of high -power excimer lasers into the blue -green spectral region1 -3. which is of great interest for signal transmission through water. Hydrogen is expecially suited for this purpose, since it presents both the largest Raman shift (4155 cm-1) and the largest gain4. When an intense laser beam is focused into a cell of a compressed molecular gas, the weak spontaneously scattered Stokes light is amplified by SRS at the expense of the pump field. As the first Stokes (S1) beam intensity becomes sufficiently high, it stimulates a second Stokes (S2) beam, and so on this process may repeat to produce higher Stokes beams5. Clearly, these last processes limit the energy conversion efficiency from the pump laser beam to Sl, since they drain energy from the Si beam. Moreover, four wave mixing processes of the pump beam and S1 can also produce S2 and higher order Stokes beams and so they limit the energy conversion efficiency to S1 still more. The four wave processes, like all parametric processes, have no threshold but require phase matching6. Then, this process can be minimized by the use of high pressure gas to increase its linear dispersion, and also by utilizing a low -angle pumping geometry to reduce the angles which lead to phase matching1-3. In the present work a XeC1 excimer laser (308 nm) was used to study SRS in H2 and H2-Ar mixtures. Our primary goal was to investigate how the SRS and the four wave mixing processes in H2 are affected by the presence of Ar. Then the energy conversion efficiency from the pump laser beam to S1 and higher Stokes was studied as a function of pump energy and Ar concentration. Experimental Apparatus The experimental apparatus is schematically shown in Fig. 1. The XeCl laser beam used in this experiment was obtained by means of a UVpreionized discharge previously described7 . The discharge configuration is of the capacitor transfer type, made up of a primary capacitance of 60 nF and a secondary capacitance of 35 nF, internal to the gas chamber to minimize the inductance of the circuit. The profiled electrodes (solid cathode and planar mesh anode) are separated by a 2 cm gap and define a discharge length of 35 cm. The external optical cavity consisted of a positive branch, confocal unstable resonator with a magnification of 20. The output coupler was an aluminum scraper mirror set at 45° to the resonator axis. The laser pulse length was 10 ns (full width at half maximum), and its spectral width 61,p was 50 cm-1. The laser beam was focused into the center of a 50 cm Raman cell with a 1 -m lens, in order to have a low angle pumping geometry. The laser beam diameter at the lens focus was found to be 0.95 mm from aperture transmission measurements. The beam was then recollimated by the lens L2 and separated into the various frequency components by a prism. The energy measurements were made at each wavelength with a pyroelectric detector. Results and Discussion The experimental results are summarized in Figs. 2-4. Figure 2 (first curve on the left) and Figure 3 460 /SPIE Vol 701 ECOOSA '86 (Florence 1986) e timulated Hainan Scat ering in H2 T