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 µm wavelength using a Quantum Cascade Laser (QCL). The method normalizes the second harmonic (R 2f ) component, dominated by laser-gas interaction and optical intensity, by the first harmonic (R 1f ) 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 4.85×10 −9 Wcm −1 Hz −1/2 for calibration-free R 2f /R 1f measurements is demonstrated.

Performance Comparison of 3-D Printed Photoacoustic Gas Sensors and a Commercial Quartz Enhanced Photoacoustic Spectrometer

“…This allows the same optical set-up to be used to characterise each sensor. In our previous work, the optical coupling ports were designed for use with a either a fibre coupled GRIN lens [6] or using 2 mm thick windows [13]. In this work, the optical coupling ports remained open We define the length as L, which defines the buffer length as L/2, and the resonator and optical coupling port diameter as d (parameters given in Table II).…”

“…The PAS cells used in this paper were also housed within a stainless steel vessel, but this was to allow for characterisation of the PAS cells without the need to gas seal each PAS cell using optical coupling via windows or GRIN lenses. In our previous work, we have shown that the 3D printed PAS cells can be easily manufactured with sealed windows [13] and with fibre collimators [6], and this ensures that gas samples can be taken within closed volumes of less than 0.1 cm 3 (30 mm x 2 mm cell).…”

Optical Gas-Cell Dynamic Adsorption in a Photoacoustic Spectroscopy-Based SOF2 and SO2F2 Gas Sensor

“…When light passes through gas, if the absorption spectrum of the gas overlaps with the optical wavelength, the output light intensity will decrease after the gas absorption. The decrescent is related to the gas absorption coefficient, the gas concentration, and the absorption optical path [ 13 ]: where λ is the wavelength of the light, I(λ) is the output light intensity, I 0 (λ) is the input light intensity, α(λ) is the absorption coefficient of the gas, C is the concentration of the target gas, and L is the absorption optical path.…”

Mid-Infrared Hollow-Core Fiber Based Flexible Longitudinal Photoacoustic Resonator for Photoacoustic Spectroscopy Gas Sensing

“…It could directly enhance the photoacoustic effect by increasing the optical power and optimizing the photoacoustic resonator [5]. Conventional H-type longitudinal photoacoustic resonators were generally used [6,7]. Various modified structures were also reported and applied in photoacoustic cells [8][9][10].…”