Abstract

Birefringence in dielectric multilayer reflectors, generally caused by anisotropic structural form or strain, results in different phase shifts on reflection, depending on the state of polarization of the incident light. This phenomenon is well known, but less well known are some of the problems it causes. We describe three such problems: Upon length-tuning a low-power He–Ne laser, we observe that adjacent longitudinal modes have orthogonal polarization states. As length changes, we see abrupt changes in polarization of all modes. This can be disastrous for polarization sensitive applications, and can be avoided by choosing a laser whose mirrors have birefringence. Resonances of a passive Fabry-Perot cavity can be split, with components separately excitable by orthogonally polarized light. We observe round-trip phase shift of ~0.08 rad. The single-frequency output of a He–Ne laser may be Zee-man-split by a longitudinal magnetic field to yield two circularly polarized components whose frequency separation passes through an extremum as the laser is tuned through line center.¹ To stabilize such a laser one obtains a heterodyne beat between the two Zeeman components. Unfortunately, not every laser exhibits this beat, and it is necessary to select a laser whose mirrors have very little birefringence.

© 1986 Optical Society of America