The birefringent filter is designed based on the interference principle of polarized light in birefringent crystals. Generally, quartz crystal material is used to process the quartz crystal into a parallel flat plate. The optical axis of the crystal is at a certain angle to the surface, and it is placed at the Brewster angle in the laser path. By rotating around its surface normal as the axis, tuning of the oscillating laser inside the cavity can be achieved. The Lyot filter consists of a series of birefringent crystal plates and polarizing plates. The birefringence axis of each crystal is rotated 45 ° relative to the axis of the polarizer, and the thickness of each crystal is half of the previous one. This can result in a very small transmission bandwidth and a large transmission peak period.