Experimental Studies Of The Stimulated Raman Scattering In H 2 - Ar Mixtures

Experimental Studies of the Stimulated Raman Scattering in H2 -Ar MixturesA.Luches, V.Nassisi and M.R.PerroneDipartimento di Fisica, Universita di Lecce, 73100 Lecce, ItalyAbstractStimulated Raman scattering experiments have been performed in H2 and H2 -Ar mixtures with an UV- preionizeddischarge laser (308 nm) as the pump. The energy conversion efficiency from the pump laser beam to Stokes be-ams has been investigated as a function of Ar concentration and pumping energy. It has been found that in theH2 -Ar mixture with 50% of Ar, the energy conversion efficiency to the first Stokes was more than 80% higherthan that obtained in H2 at the same total pressure (40 atm) and pump energy (60 mJ).IntroductionAt present there is a great interest in improving the performance of excimer lasers. Because of their highefficiency and short wavelength, excimer lasers are the most important drivers for high power laser - matterinteraction studies. Frequency shifting is widely investigated to improve the beam quality and to extend therange of application.Stimulated Raman scattering (SRS) in compressed molecular gases is one of the most efficient methods forshifting the output of high -power excimer lasers into the blue -green spectral region1 -3. which is of greatinterest for signal transmission through water. Hydrogen is expecially suited for this purpose, since it pre-sents 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 spontaneouslyscattered Stokes light is amplified by SRS at the expense of the pump field. As the first Stokes (S1) beamintensity becomes sufficiently high, it stimulates a second Stokes (S2) beam, and so on this process may rep-eat to produce higher Stokes beams5. Clearly, these last processes limit the energy conversion efficiencyfrom the pump laser beam to Sl, since they drain energy from the Si beam. Moreover, four wave mixing proces-ses of the pump beam and S1 can also produce S2 and higher order Stokes beams and so they limit the energyconversion efficiency to S1 still more. The four wave processes, like all parametric processes, have no thre-shold but require phase matching6. Then, this process can be minimized by the use of high pressure gas toincrease its linear dispersion, and also by utilizing a low -angle pumping geometry to reduce the angles whichlead 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 pri-mary goal was to investigate how the SRS and the four wave mixing processes in H2 are affected by the presence