Optical feedback characteristics in a helium neon laser with a birefringent internal cavity

The output characteristics of optical feedback in a helium–neon laser with a birefringent internal cavity are studiedsystematically in five different regions of the gain curve for the two orthogonally polarized modes. When the laseroperates in the two end regions of the laser gain curve, one of the two orthogonally polarized modes will be a leadingone in optical feedback. Strong mode competition can be observed. However, when the laser operates in the middleregion of the laser gain curve, the two modes can oscillate equally with optical feedback. Besides the intensity of thetwo polarized lights, the total light intensity is also studied at the same time. M-shaped optical feedback curves arefound. Particularly, when the average intensities of the two lights are comparable, the intensity modulation curve ofthe total light is doubled, which can be used to improve the resolution of an optical feedback system.