Stability of polarized modes in a quasi-isotropic laser

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Abstract
The polarization states of quasi-isotropic lasers are highly sensitive to residual cavity anisotropics and to weak (parasitic) anisotropic feedback. Internal anisotropics are usually constant, whereas the effective mirror anisotropy produced by feedback is strongly frequency (phase) dependent. In this paper we develop a model for such a laser and obtain analytic steady-state solutions for the case when both anisotropics are parallel. The linear stability analysis is also analytic. For the 3.39-μm He–Ne laser the theory explains the previously observed variations with frequency of the intensity of the laser, the regions of monostable linear polarization, and in the bistable region the inverse dependence of the width of the hysteresis loop on the low-signal net gain. In contrast to Lamb’s theory the calculations show that the polarization flip arises from an instability in the relative phase between the vector components of a mode.