Dispersion Interferometry for Relative Atmospheric Pressure Measurement

Abstract

Second harmonic interferometry is a common method in fusion research to measure dispersive phase shifts, i.e. for line-average electron densities [1]. Using this technique, a contact-less, relative barometer is implemented by measuring the pressure-dependent dispersive phase shift in air. This change in phase is converted into a change in pressure via the Ciddor equation [2]. In order to minimize the footprint of the experimental setup, a single crystal dispersion interferometer (SCDI) [3] is improved upon by generating a reference beam which contains both the fundamental and second harmonic beam coaxially, thereby reducing the system complexity. The device is used to measure changes in the dispersion of air in a pressurized chamber between 101 to 105 Pa and compared against a piezoresistive pressure transceiver. The deviation between both sensors was found to be less than 150 Pa. [1] Drachev, V. P., et al. "Dispersion interferometer for controlled fusion devices." Rev. Sci. Instrum. 64(4) (1993) [2] Ciddor, Philip E. "Refractive index of air: new equations for the visible and near infrared." Appl. Opt. 35(9) (1996) [3] Lee, Dong-Geun, et al. "The new single crystal dispersion interferometer installed on KSTAR and its first measurement." Rev. Sci. Instrum. 92(3) (2021)