Polarization control in a He-Ne laser using birefringence feedback

The polarization dynamics of laser subjected to weak optical feedback from birefringence external cavity are studied theoretically and experimentally. It is found that polarization flipping with hysteresis is induced by birefringence feedback, and the intensities of two eigenstates are both modulated by external cavity length. The variations of hysteresis loop and duty ratios of two eigenstates in one period of intensity modulation with phase differences of birefringence element in external cavity are observed. When the phase difference is ?/2, the two eigenstates will equally alternatively oscillate, and the width of hysteresis loop is the smallest.