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

High-accuracy knowledge of gas refractivity is typically crucial for optical interferometry, precise optical systems, and calculable pressure standard development. Here, we demonstrate an absolute gas refractometer by spectral interferometry and a high-resolution spectrometer. The spectral interferometry relies on a comb with fiber Fabry-Perot filtering cavity, and a double-spaced vacuum cell. The spectrometer employs a virtually imaged phased array, diffraction grating and near-infrared camera to fully resolve the comb modes. Finally, by means of fast-Fourier-transform, the group refractivity can be derived from the spectrally resolved interferograms of the two beams propagating in the inside and outside of the vacuum cell. To confirm the feasibility and performance of the gas refractometer, the measurement of ambient air was conducted. The proposed scheme has a combined uncertainty of 1.3 x 10(-9) for air and a single measurement only takes 10 ms, which is applicable for gas refractivity monitoring and compensating in real time.