Remote Measurement of Refractive Index of Air Using Tandem Interferometer over Long Optical Fiber

The refractive index of air is accurately measured using a vacuum cell and a tandem low-coherence interferometer with a communication optical fiber of 30 km length with standard uncertainties of 0.20 ppm for the phase refractive index and 0.29 ppm for the group refractive index. Experimental results are compared with the value calculated from the equation for the refractive index of air using data obtained from measurements of the surrounding air, with a standard uncertainty of 0.22 ppm, and the difference between the average values is within 0.1 ppm. This method is applicable to the in situ high-precision measurement of optical length.