Abstract
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–Pérot 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 × 10−9 for air and a single measurement only takes 10 ms, which is applicable for gas refractivity monitoring and compensating in real time.
Highlights
High-precision refractive index of an ambient gas is of great importance in the following three aspects
The 250-MHz-spaced comb modes are too densely spaced to be resolved by the VIPA-based spectrometer, so we first use a FFP cavity with a FSR of 5 GHz to filter out unwanted comb lines, generating a 2-dimensional (2D) array of well separated bright spots when imaged by the camera
The response times of these sensors still have influences on the discrepancy, since the proposed measurement apparatus stands on real-time measurement approach while the air refractive index derived from Ciddor’s equation lags behind real-time values
Summary
High-precision refractive index of an ambient gas is of great importance in the following three aspects. The wavelength resolution of the OSA is low (0.2 nm) and the spectral frequencies are inaccurate caused by frequency sampling nonlinearity, which contributes much to the measurement uncertainty of air refractive index. The spectral interferograms are obtained by a high-resolution spectrometer consisting of a VIPA (virtually imaged phased array), diffraction grating and near-infrared camera[32]. The 250-MHz-spaced comb modes are too densely spaced to be resolved by the VIPA-based spectrometer, so we first use a FFP (fiber Fabry–Pérot) cavity with a FSR (free spectral range) of 5 GHz to filter out unwanted comb lines, generating a 2-dimensional (2D) array of well separated bright spots when imaged by the camera. From the obtained spectrometer image where any individual spatial unit represents a comb mode frequency, a one-dimensional (1D) interferograms of respective interference intensity with frequency can be reconstructed by concatenating the spatial elements along a reasonable zigzag line
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