Abstract

Brillouin induced four-wave mixing can be used to phase conjugate weak signals with reflection coefficients of up to 10⁶.¹ This high reflectivity results from an absolute instability that produces exponential temporal growth in both the signal and phase-conjugate beams. This occurs when the pump beams have exceeded a certain critical intensity.² Theoretical predictions of critical intensity values have been found to agree well with experimental results.³ The effects of varying the signal frequency can also be predicted by theory. We report on experiments designed to test these predictions. The pump and signal beams were produced from two separately injection seeded Nd:YAG Q-switched lasers, with pulse durations of 40 ns. The relative frequencies of the lasers were precisely controlled by feedback. The four-wave mixing medium was carbon disulphide. The second pump beam was produced by reflecting the first from an SBS cell. Different liquids in this cell were used to vary the phase mismatch. We measured the signal amplification vs signal frequency for three different phase mismatch values. The results were found to agree well with theory. We carried out a heteorodyne measurement of the phase conjugate frequency, and found that it remained constant as the signal frequency was varied; this is also predicted by theory.

© 1991 Optical Society of America