Stability of Polarization Modes in a Quasi-Isotropic Laser.

The stability of the polarization modes of a quasi -isotropic He Ne laser operating at 3.39 mu m is examined. The laser operates in a single longitudinal, single transverse mode but the state of polarization varies as a function of tuning. The variation in the polarization modes of the laser may be a continuous function of tuning or may involve sudden jumps in the polarization state depending on the experimental control parameters. These sudden jumps, or polarization "flips", are found to be experimentally controllable using polarized optical feedback. A simple model is presented that describes the quasi-isotropic laser using a set of coupled nonlinear dynamical equations. This model emphasizes the importance of considering the combined effects of the passive cavity, the active medium, and the polarized feedback when determining the polarization modes of the laser. The steady state polarization modes of the laser are stationary solutions to the equations. The polarization flips are found to be associated with instabilities in these stationary solutions. Periodic solutions to the model may be useful in describing Faraday and Zeeman lasers. We perform experiments to test this model extensively over a wide range of control parameters. The important control parameters are the length of the laser (frequency), tube current (gain), and the amplitude, phase and azimuthal orientation of the linearly polarized feedback. In the many cases examined, the measurements and the calculations are in substantial agreement. There are no adjustable parameters used in the computations. The major source of the small discrepancies between theory and experiment is attributed to the neglect of terms in the calculation of the material polarization beyond third order in the electrical field strength.