An improved technique based on discrete Fourier transform for demodulating the plasma-induced phase shift in a dispersion interferometer

This article announces a technique for detecting plasma-induced phase shifts in a dispersion interferometer (DI) with artificial modulation of probing radiation, based on the use of a sliding discrete Fourier transform with periodic correction of the results of calculations and procedures for suppressing leaks in the frequency domain by window functions. Within the framework of this technology, the results of measurements of CO2 laser-based DI are formed with a maximum differential deviation of <2° at a rate of phase shift variation on 45° (about 1019 m-2) during the period of the modulating signal. As the rate of phase shift decreases, the magnitude of the differential deviation decreases proportionally, at a level commensurate with the measurement error caused by the noise component of the photodetector signal (∼1016 m-2). The feature of the considered technique is the improved accuracy at the cost of twice reduced the time resolution. The hardware implementation of the phase shift calculation with ADC sampling rate instead of modulating frequency is proposed.