Abstract

We have studied, experimentally, stimulated rotational Raman amplification in H₂ using picosecond pulses. The transient amplification of the first Stokes was measured as a function of the pump laser energy, the pressure of the H₂ Raman medium, and of the relative timing of the pump and seed Stokes pulses. Both the seed Stokes beam at 549 nm and the pump beam at 532 nm were spatially filtered, collimated, and then spatially overlapped before being focused with a 1-m focal length lens into the center of a 1-m length Raman amplifier cell. H₂ pressure was varied from 50 to 250 psi. We investigated configurations in which both beams were circularly polarized but in opposite directions and in which both beams had parallel linear polarization. The latter configuration was observed to have a substantially lower gain coefficient than expected relative to the case with circularly polarized beams. Stokes amplifications of up to a few times 10⁹ were observed with the form exp(kIⁿ), where I is the laser intensity and n is a number close to 1. This is in contrast to a value of n = 0.5, which would be expected from theory in the high gain limit.¹ The results of the rotational Stokes amplification follow closely the behavior of the vibrational Stokes (683-nm) amplification, work done previously in our laboratory. The Stokes amplification is maximum when the Stokes seed pulse arrives ahead of the pump pulse by about half of a pulse duration which, for these experiments, was 30 ps. As the pump pulse is advanced, bringing it into temporal overlap with the seed Stokes pulse, the gain falls off rapidly. The reduction in gain is more gradual as the pump pulse is delayed relative to the seed Stokes pulse from the optimal timing. The amplification at optimal timing is 100 times the amplification seen when both pulses are coincident in time.

© 1986 Optical Society of America