Self-Pumped Phase Conjugation in a Supersonically Flowing Medium

摘要

This program investigated the feasibility of producing degenerate two wave mixing gain in a noninverted resonant medium with local response. According to theory, two wave mixing gain should be possible if the resulting index grating is shifted in space with respect to the intensity grating produced by the two crossed beams. A pulsed molecular beam valve operated at 100 C was used to produce the supersonically cooled I2 jet. To achieve the moving grating effect, the pump and probe beams were to be crossed in a plane containing the flow axis, with each beam nearly perpendicular to that axis. The pulsed beam valve produced a broad forward peaked distribution of molecules. The velocity distribution at any given point, however, was mainly along the direction of the line joining that point to the nozzle orifice. This was the evidenced by the fluorescence pattern along the laser beam path, which consisted of sharp filaments across the laser beam corresponding to various hyperfine components of a single rotational transition Doppler tuned into resonance with the laser beam at various angles of inclination. Gain in the planned experiment would be observable only within a narrow range of detuning (obtained by crossing the pump and probe beams), and therefore only a small solid angle of the I2 jet is useful. Various nozzles were tried with the pulsed valve device. Under the best circumstances, an integrated absorption of only about 10% was measured. With the required angular selection, the expected gain would be on the order of .01%. Since the stability of the laser was such that even after signal averaging a noise of the order of 0.1% remained, it precluded our attempt to demonstrate gain, self pumped, supersonic flow.