Joint stimulated resonant backscattering of incoherent beams in Nd:YAG amplifier

Abstract

In our experiments a beam from a cw mul- tilongitudinal mode Nd:YAG-laser (at l .06 pm) was chopped to 30 ps pulse duration and split into two cross-polarized beams that were launched into two or three flash-lamp pumped Nd:YAG-rods. The optical paths of the split waves were different, so that the waves were mutually incoherent in the NdYAG amplifi- ers. These two beams co-propagated in the laser crystal. In the amplifiers we used NdYAG-crystals produced by method of horizontal-directed crystallization and having about a 1% concen- tration of the Nd-ions. The total logarithmic gain of the amplifiers for a weak signal ranged from 6 to 11. We measured the power of the backscat- tered beams at the resonant frequency with the same polarizations as those of each initial wave (Fig. 1). The SS effect of one beam depended on the presence ofthe other. The backscattered power in the presence of two initial beams was much more than that of one beam (Fig. 2). As we measured, the increase of the SS signals (with different polarizations) for two cross- polarized waves was not caused by depolariza- tion in the NdYAG amplifier (curve 3 in Fig. 2). The case when one initial wave was much less than the other was considered. We ob- served that the weak beam, with negligible SS, scattered on the grating induced by the other strong cross-polarized wave and its scattering wave. In fact, we tested the scattering grating, induced by the strong initial wave and the scat- tering wave, using the weak signal. These facts showed that the simultaneous scattering was realized by joint scattering grat- ing. Each of the two pairs (the initial waves E:fl, E:, and the backscattering waves E:, E:, in Fig. 1) induced their own grating of population. The separate changes ofpopulation were sum- marized to give the joint scattering grating. So,