Measurement of small intracavity phase anisotropy in a laser based on optical feedback

We demonstrate that the frequency difference between two eigenstates of one laser mode in polarization flipping varies with the small intracavity phase anisotropy of a laser. The relationship between the frequency difference and the magnitude of the intracavity phase anisotropy of the laser is experimentally and theoretically shown. When the strength of the optical feedback is weak, the frequency difference is independent of the frequency shift of the laser caused by the optical feedback. A direct and simple method to measure the small intracavity phase anisotropy in a laser is presented.