Bireflectance thin film in a high beat frequency laser

This paper investigates a kind of bireflectance thin film on the window plate of a 633 nm HeNe laser. The film is coated on the substrate with the application of external load on it. The load on the substrate is removed after the coating has been accomplished, then the strain on the substrate will be transferred to the multilayer coatings. Due to photoelastic effect, the multilayer film becomes an anisotropic film. Selecting appropriate film structure and suitable center wavelength, a high phase dispersion with nearly constant reflectivity around the working wavelength will be obtained. For normal incidence, a sufficient phase shift difference between the two orthogonal polarization states of the reflected wave will be produced. As a result, a dual-frequency laser with a beat frequency of 5 MHz can be carried out by using this kind of multilayer film. The theory of designing such a film is emphasized and an example of bireflectance thin film is proposed.