OBSERVATION OF RANDOM PI-PHASE JUMPS IN A RING LASER WITH BACKSCATTERING

We have observed jumps of plus or minus pi in the phase difference between the two counterpropagating modes of a HeNe ring laser, whenever the complex backscattering coefficients R//1R//2 satisfy the condition R//1 plus R//2 equals 0. The experiments made use of two retro-reflectors that allowed light to be reflected back into the ring laser cavity with controllable amplitudes and phases. The phase jumps are usually accompanied by oscillatory instabilities of the two mode intensities. The observed behavior is found to be consistent with computer solutions of the coupled equations of motion.