Longitudinal mode separation tuning in 633 nm helium-neon lasers using induced cavity birefringence

The theory of laser cavity birefringence induced by insertion of two quarter-wave plates into the cavity is developed to show how the intermode frequency of a two-mode laser can be tuned through a free spectral range by relative rotation of the plates. Birefringence is demonstrated in a helium-neon laser at 633 nm using an open cavity having a tube with anti-reflection coated windows. The lock-in between modes when the intermodal frequency approaches zero was investigated and found to be smallest when the tube was placed between the plates which were aligned with axes parallel. Active frequency stabilization at a frequency separation of 50 MHz was demonstrated using the mode-balancing technique.