Abstract

The effect of a reverse wave suppressor (RWS) mirror on the performance of a cw HF unstable ring laser has been measured in terms of forward/reverse wave power ratio (p_F/p_R) and forward wave beam quality. It was found that a well-aligned RWS mirror gave the best suppression (p_F/p_R ≃ 100) and diffraction-limited beam quality. Resonator performance degraded rapidly with misalignment of the RWS mirror: a 200-μrad tilt reduced p_F/p_R to 2.5 and the beam quality by a factor of 2. The forward wave near-field pattern showed distinct fringes as the RWS mirror was misaligned, suggesting that a higher-order mode was causing the beam quality degradation. A novel technique was developed to reduce the alignment sensitivity of the RWS mirror. By use of a distorted relay mirror, an aberrated (≃2λ) reverse wave was fed back into the ring. The aberrated reverse wave feedback reduced the sensitivity of resonator performance to RWS mirror tilt. Diffraction-limited beam quality and good suppression (p_F/p_R ≃ 20) were observed over a tilt range of 600 μrad. It is postulated that the aberrated RWS mirror provides sufficient feedback to suppress the reverse wave over a wide angular range yet not enough to produce higher-order transverse modes in the forward wave.

© 1985 Optical Society of America