Measurement of air refractive index fluctuation based on interferometry with two different reference cavity lengths

Abstract

A measurement method based on interferometry with two different reference cavity lengths is presented and applied in air refractive index measurement in which the two cavity lengths and a laser wavelength are combined to generate two wavelength equivalents of cavity. Corresponding calculation equations are derived, and the optical path configuration is designed, which is inspired by the traditional synthetic wavelength method. Theoretical analyses indicate that the measurement uncertainty of the determined index of refraction is about 2.3×10(-8), which is mainly affected by the length precision of the long vacuum cavity and the ellipticity of polarization components of the dual-frequency laser, and the range of nonambiguity is 3.0×10(-5), which is decided by the length difference of the two cavities. Experiment results show that the accuracy of air refractive index measurement is better than 5.0×10(-8) when the laboratory conditions changes slowly. The merit of the presented method is that the classical refractometry can be also used without evacuation of the gas cavity during the experiment. Furthermore, the application of the traditional synthetic wavelength method may be extended by using the wavelength equivalents of cavity, any value of which can be easily acquired by changing cavity length rather than using actual wavelengths whose number is limited.

Revision to the humidity correction equation in the calculation formulae of the air refractive index based on a phase step interferometer with three frequency-stabilized lasers

“…The air parameters were measured and applied to the presented equation and Boensch's humidity correction equation. These two corresponding RIA calculation results were both compared with the directly measured results by a refractometer which can get real-time RIA based interferometry with two different reference cavity lengths that we built in previous work [12]. The comparison result is shown in figure 3, where the interval of every two experimental sampling points is about 5 min, and the environmental temperature varies from 22.2 °C to 23.2 °C during the comparison experiment process.…”

Measurement of air refractive index fluctuation based on a laser synthetic wavelength interferometer

“…Based on the principle of the quasi-synthetic wavelength method, Zhang et al presented a refractormeter that is capable of measuring the n air with an unambiguous range of 1.000 395 and uncertainty of 3.1 × 10 −8 at 633 nm absolutely [9]. Similarly, Chen et al realized the n air measurement with uncertainty better than 5.0 × 10 −8 by using two different reference cavity lengths [10]. Although the methods Zhang and Chen proposed adopt the pre-evacuated sealed cavities and avoid the evacuation of the air cavity during the experiment, the measurement unambiguous range is limited.…”

High-precision gas refractometer by comb-mode-resolved spectral interferometry

“…The second approach is called absolute measurement, which means it is needless to do successive phase measurement during the period of gas pumping or filling. Recently, some absolute methods for gas refractivity measurement have been reported by use of a Fabry–Pérot cavity 23 , two-color method 24 – 27 , trapezoidal cavity 28 , and synthetic pseudo-wavelength (SPW) method 29 , 30 . However, Fabry–Pérot cavity method also needs measurements under vacuum to obtain the ultimate value.…”