Using a cat's eye cavity to improve displacement self-sensing laser

A displacement self-sensing laser with a resolution of λ/8 was invented by us a few years ago. This simple laser device mainly consists of a half-intracavity He-Ne laser with a birefringence element in the cavity, outputting a beam with two orthogonally polarized components. One of its cavity mirrors can move along the optical axis of the laser to change the cavity length. When this mirror is pushed (or pulled) by an item to be measured, the polarization of the laser beam varies once every λ/8 variation of the cavity length. Though the displacement self-sensing laser has many outstanding functions, its measuring range was badly limited by inevitable misalignment resulted from sway of the cavity mirror in motion. The limited range prevents the sensor from going into actual application. This paper gives a solution to the problem, using a cat's eye retroreflector to replace the flat cavity mirror of the displacement self-sensing laser to construct an anti-misalignment cavity-cat's eye cavity (CEC). The misalignment loss in the CEC is theoretically analyzed and compared with that in a flat-concave cavity, and then the experiment to test the optical stability of CEC is carried out. Furthermore, the performance of the displacement self-sensing laser based on the CEC is also investigated. Results indicate that the misalignment tolerance of the CEC is much higher than that of the flat-concave cavity, i.e. the cavity stability is greatly improved. The measuring range of the displacement self-sensing laser can be up to 12 mm with a resolution of 79.097 nm and linearity better than 0.005%.