Patent ID: 8665452

Claim:
A method for measuring the refractive index of air based on laser synthetic wavelength interferometry, the method comprising: using a dual-frequency laser to output orthogonal linear polarization lights with wavelengths λ 1 and λ 2 ; emitting the orthogonal linear polarization lights to a laser synthetic wavelength interferometer composed of a beamsplitter, a first polarizing beamsplitter, a second polarizing beamsplitter, a first corner-cube retroreflector, and a second corner-cube retroreflector; providing, in a measuring optical path of the interferometer, a quartz vacuum cavity of a length L and being disposed in parallel to a light propagation direction; providing a first detector and a second detector of photoelectric type for receiving interference signals of the wavelengths λ 1 and λ 2 , respectively, when moving the first corner-cube retroreflector of the interferometer; before starting to measure the refractive index of air, evacuating the quartz vacuum cavity, receiving interference signals of the wavelengths λ 1 and λ 2 , and moving the first corner-cube retroreflector to find a position where the phase difference between the interference signals of the wavelengths λ 1 and λ 2 is set to an original value of zero; then introducing air into the quartz vacuum cavity until the air inside the cavity is consistent with air outside and taking the interference signal of the wavelength λ 1 as a reference signal; using the first detector to directly detect an integer N of interference fringes of the wavelength λ 2 ; then receiving interference signals of the wavelengths λ 1 and λ 2 and moving the first corner-cube retroreflector such that the phase difference between the interference signals of the wavelengths λ 1 and λ 2 becomes zero again; according to principle of subdivision of laser synthetic wavelength interference fringes, measuring and obtaining a fraction ε of the interference fringes of the wavelength λ 2 resulting from variation of the refractive index of air as follows ɛ · λ 20 2 = λ 2 λ S · Δ ⁢ ⁢ L wherein ΔL is the displacement of the first corner-cube retroreflector, λ 20 is the vacuum wavelength, and λ s is a synthetic wavelength formed by λ 1 and λ 2 ; and finally, calculating the refractive index n of air according to the relation of the refractive index of air n to the length L of the vacuum cavity, and the number N and fraction ε of the interference fringes as follows: n - 1 = ( N + ɛ ) · λ 20 2 ⁢ L .