Quartz‐Tuning‐Fork‐Filtered Heterodyne Interferometric Phase‐Dispersion Gas Sensing Based on Dual‐Channel Hilbert Phase Extraction

ABSTRACT A novel heterodyne Mach–Zehnder phase‐dispersion gas sensing method by combining quartz‐tuning‐fork (QTF) narrow‐band filtering with dual‐channel Hilbert phase extraction was proposed in this paper for the first time. The laser was split into a gas‐interacting signal arm carrying refractive‐index‐induced phase information and an acousto‐optic‐modulator (AOM)‐frequency‐shifted reference arm. After recombination, the two beams generated a photodetected heterodyne interference signal carrying the refractive‐index‐induced phase information. A 32.7 kHz QTF was used as a narrow‐band physical filter to suppress DC background and low‐frequency drift, concentrating the response near resonance. For phase readout, the AOM reference and QTF output were synchronously acquired to construct Hilbert analytic signals. Multiplying the detection‐channel analytic signal by the complex conjugate of the reference‐channel signal yielded a reference‐compensated instantaneous phase difference, converting absolute phase readout into relative phase readout and reducing offsets from system restart, acquisition‐trigger variations, and reference phase‐zero drift. Using C 2 H 2 as the target gas, the system achieves stable 2000 s measurement at 1%. Allan analysis shows 8.5 ppm sensitivity at 1 s and an optimized detection limit of 793 ppb at 53 s. This method integrates physical filtering, reference compensation, and digital phase readout for robust gas sensing.