Precision measurement of refractive index of air based on laser synthetic wavelength interferometry with Edlén equation estimation

A novel method for the precision measurement of refractive index of air (nair) based on the combining of the laser synthetic wavelength interferometry with the Edlén equation estimation is proposed. First, a nair_e is calculated from the modified Edlén equation according to environmental parameters measured by low precision sensors with an uncertainty of 10−6. Second, a unique integral fringe number N corresponding to nair is determined based on the calculated nair_e. Then, a fractional fringe ε corresponding to nair with high accuracy can be obtained according to the principle of fringe subdivision of laser synthetic wavelength interferometry. Finally, high accurate measurement of nair is achieved according to the determined fringes N and ε. The merit of the proposed method is that it not only solves the problem of the measurement accuracy of nair being limited by the accuracies of environmental sensors, but also avoids adopting complicated vacuum pumping to measure the integral fringe N in the method of conventional laser interferometry. To verify the feasibility of the proposed method, comparison experiments with Edlén equations in short time and in long time were performed. Experimental results show that the measurement accuracy of nair is better than 2.5 × 10−8 in short time tests and 6.2 × 10−8 in long time tests.