Abstract: A nanosecond-long laser pulse beam external reshaping optical path was designed using a series of multiple optical ring-cavities. In gas laser spontaneous Raman scattering experiments, it avoids effectively the strong laser induced gas dissociation, optical quertz window components damage and the ignition of the flammable gas, so improves substantially the signal-to-noise ratio of the weak signal generated by the Raman effect. The results of numerical study and optimization design showed that the delay time of the cavity 1 equals to the full-width-half-maximum(FWHM) of the original laser beam, and the ratios between the delay times of the subsequent cavities should be the nultiples of 2, the splitting ratio of the beam splitter is 30%~50%. Using 2 optical ring-cavities, the original laser beam with pulse width(FWHM) 6.5 ns, energy 400 mJ was widened to 35 ns(FWHM) with a peak power reduction of 20% to less than 0.02 GW. The concentration measurements were performed in the gas mixture pool based on the quantification by the laser Raman spectrum.This technique can be further used in the quantitative measurement of gas mixture concentration and temperature field of the optical engine.

Key words: power machinery and engineering, spontaneous Raman spectrum, Nd:YAG laser, laser beam pulse reshaping