The damage threshold of zero-order wave plate in air gap is higher, and the phase difference changes little with temperature, which is very stable. There are 1/2 wave plates which rotate the polarization direction and 1/4 wave plates which can convert linear polarization into circular polarization. When the product is fixed on the frame or installed on the bracket, it will not cause stress to the components. Both sides of the two wave plates are coated with anti-reflection films, which have high transmittance.