Calibration-free heterodyne phase-sensitive dispersion spectroscopy: quantitative gas sensing and recovery of absorption spectra

Abstract

Heterodyne phase-sensitive dispersion spectroscopy (HPSDS) is a quantitative non-intrusive gas sensing technique based on the determination of the refractive index of the target gas in the vicinity of an absorption transition. Since the phase instead of the intensity of the probing laser light is targeted, the technique boasts the advantage of being normalization-free. It is thus largely immune to laser intensity fluctuations due to either system instability or ambient interferences. Previous HPSDS-based sensors typically require calibration using standard mixtures to establish a look-up table between the measured phase signal and gas concentrations, which is both cumbersome and problematic when there are significant compositional variations between the calibration standards and the target gas. In this work, we present a robust and generic technique that addresses this issue with a successful realization of fully calibration-free measurements. Spectral-fitting to the entire dispersion spectra with free variables related to transition linecenter, broadening width, and integrated absorbance were used to eliminate the effects of unknown spectral broadening coefficients. What we believe to be a novel analytical model was proposed to unify both direct injection-current dithering-based HPSDS that includes simultaneous frequency/intensity modulation, and the external electro-optic modulator (EOM) modulation-based HPSDS with a non-ideal linear response of EOM. The proposed technique was first validated via numerical experiment to determine the gas concentration and the recovery of the absorption profiles. Actual experiments were subsequently performed for the measurement of CH4 near 1.65 µm, N2O near 4.46 µm, and NO near 5.26 µm, collectively demonstrating the capability of the technique for both near- and mid-infrared lasers with diverse modulation characteristics. Further demonstrations were performed to measure NH3 concentrations at elevated temperatures through the fitting of the multiple dispersion spectra near 9.06 µm. The robust iterative spectral-fitting strategy and the measurement accuracies confirm the robustness of the proposed calibration-free (CF) HPSDS technique for quantitative gas sensing.