Dual-frequency laser using anisotropic bireflectance film

摘要

A kind of bireflectance thin film on the window plate of a 633nm He-Ne laser is presented in this paper. The film of non quarter-wave-stack 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 transfer to the multilayer film. Due to photoelastic effect, the multilayer film becomes an anisotropic film. Selecting appropriate film structure and suitable center wavelength, a high phase dispersion with a nearly constant reflectivity will be obtained around the working wavelength. For normal incidence, a phase shift difference between the two orthogonal polarization states of the reflected wave will produce. As a result, a dual-frequency laser with a beat frequency of 4-5 megahertz can be carried out by using this kind of bireflectance thin film. Based on this principle, A He-Ne dual-frequency laser equipped with bireflectance cavity mirror is described. The mode coupling is reduced by utilizing transverse Zeeman effect so that two linear and orthogonal polarization components with 5MHz beat frequency are generated. The effect of the magnetic field's direction on the dual-frequency as well as the polarization property of the laser are investigated by experiments. After stabilizing the frequency, the laser is calibrated with the iodine frequency stabilization laser at Chinese National Institute of Metrology (CNM). Experimental results indicate that the expanded uncertainty of wavelength in vacuum is 1x10~(-7) (k=3) with the frequency stabilization of 6.6xl0~(-10) (sampled in 1000s).