Explanation of the mechanism for acousto-optically induced unidirectional operation of a ring laser

A mechanism is proposed for the nonreciprocal behavior of a traveling-wave acousto-optic Q switch, whose use can provide an effective way to enforce unidirectional and hence single-frequency operation of a ring laser. Simple energy and momentum conservation considerations show that the Bragg condition is satisfied at different angles of incidence for the two counterpropagating beams, which leads to a difference in their diffraction losses. The loss difference, measured directly for a diode-pumped Nd:YAG ring laser, yielded values in close agreement with calculated values. Implications of these results for both single-frequency cw and Q-switched operation of diode-pumped solid-state ring lasers are discussed.