A Calibration-Free Methodology for Resonantly Enhanced Photoacoustic Spectroscopy Using Quantum Cascade Lasers

Abstract

Photoacoustic spectroscopy (PAS) is a highly sensitive technique for trace gas sensing, which requires frequent re-calibration for changing environmental influences and input light power fluctuation. This is a major drawback against its deployment for on-site, long-term remote applications. To address this drawback here we present the theory and application of a Calibration-Free Wavelength Modulation Photoacoustic Spectroscopy (CF-WM-PAS) technique. It is applied for measurements of CH4 gas concentration in the mid-infrared at $8.6~\mu {m}$ wavelength using a Quantum Cascade Laser (QCL). The method normalizes the second harmonic ( ${R}_{2{f}}$ ) component, dominated by laser-gas interaction and optical intensity, by the first harmonic ( ${R}_{1{f}}$ ) component dominated by Residual Amplitude Modulation (RAM) DC offset, to isolate the output from changes in the gas matrix, optical intensity and electrical gain. This normalization technique removes influences from changes in the resonant frequency, gas concentration and incident optical power. It is confirmed using a ±1600 Hz change in modulation frequency around the resonance, a 1% to 10% change in gas concentration and an up to 78.3% attenuation in input light intensity with a custom built miniaturized 3D-printed sensor. A Normalized Noise Equivalent Absorption (NNEA) of $4.85\times 10^{-9}$ Wcm−1Hz−1/2 for calibration-free ${R}_{2{f}}/{R}_{1{f}}$ measurements is demonstrated.