Observations of instabilities in two-mode He-Ne laser radiation by means of interference of polarized beams

We studied instabilities in two-mode He-Ne laser generation during half an hour after laser switching-on. We used Wollaston prism, polarizer, collecting lens and microscope in our observations. The pattern observed was a mix of two interference patterns created by the two laser modes. Instability was shown up as alternating appearance of two contrary patterns. Pattern replacement was realized through total pattern disappearance. External stationary magnetic field action appeared to stimulate increase of interference pattern visibility but retained evolution character. Bringing a sufficient delay in one of the interfering beams resulted in qualitative change of interference character. The pattern did not disappear and had a high visibility. Instability was revealed as fringe drift in direction depending on where delay line was put: on the path of first or second beam. All the features of evolution were explained on basis of interference equations considering orthogonality of linear polarizations of laser modes and change in mode amplitudes and frequencies.