Photoacoustic spectroscopy (PAS) is a powerful and non-destructive optical analysis technique that can be used to quantitatively analyze the composition of gases, liquids, and solids. With the continuous innovation of modern laser techniques, various new laser light sources have emerged that play an important role in promoting the development of photoacoustic spectroscopy based on laser light sources. Moreover, novel signal processing algorithms and detection strategies have been reported. In theory, PAS is essentially established with wavelength independence, high resolution, and high sensitivity. These unique characteristics make them widely used in environmental science, solid-state physics, industrial process control, biomedicine, and other fields. However, the thermal noise caused by the absorption of the incident laser by the windows or inner wall of the photoacoustic (PA) cell (particularly when the high-power laser source is used as the signal excitation light source), and the electrical noise of electronic devices (such as acoustic signal detectors) are still key technical issues that limit the detection sensitivity of PA-spectroscopy-based gas sensors. To resolve the background noise problem in these technical issues, a differential resonance photoacoustic gas detection method that fully utilizes the phase-dependent characteristics of the PA resonance cavity is proposed.
Considering the problem of noise limitation in PAS system sensitivity, resonance enhancement detection strategies are usually adopted to achieve effective suppression of the background noise of the PA system. Typically, cylindrical resonant PA cells, Helmholtz resonators, spherical resonators, and quartz tuning forks are used. In the field of signal processing algorithms, differential detection is an effective method for eliminating background noise interference, improving signal quality, and improving the spectral signal-to-noise ratio (SNR), and has good application value in various signal processing. Therefore, in this study, a high-sensitivity PA gas detection technique is developed by combining resonance PAS characteristics based on the differential detection principle. To demonstrate the proposed technique, a cylindrical PA cell with double resonant cavities is designed, and relevant theoretical and experimental studies are conducted for sensitive sensing gas detection. A differential-resonance PAS gas sensor system is integrated by using a near-infrared diode laser near 1391.6 nm and a double PA cell. To further improve the detection sensitivity, a wavelength modulation spectroscopy second-harmonic (WMS-2F) detection method is employed. Moreover, the Allan variance analysis algorithm is used to evaluate the system sensitivity and stability.
To evaluate the gas-sensing technique, ambient water vapor (H2O) is analyzed. The potential crosstalk effect between the double-resonance cavities is investigated using theoretical simulations (Fig.1) and experimentally confirmed. Differential detection is applied for measuring background noise and H2O PA spectral signals. The calculated results indicate that the standard deviations of the background noise can be improved by approximately 1.9 times (Fig.4) by utilizing the phase-dependent characteristics of the two resonance cavities (Fig.5), and the PA spectral signal amplitude can also be significantly enhanced (Fig.7). Moreover, a detection limit of ~3.0×10-6 is obtained for ambient H2O concentration measurements under the optimal averaging time of 115 s without using differential detection (Fig.8). After using the differential algorithm, the system stability is further improved, the optimal stability time is increased to more than 200 s, and the corresponding detection sensitivity is improved to 2.0×10-6 (Fig.8).
This study proposes a high-sensitivity gas detection technique based on resonant PAS with differential detection principle. Allan variance analysis indicates that high-sensitivity detection of several 10-6 level H2O concentrations can be achieved using a low-power near-infrared (NIR) diode laser. Compared to the traditional single-channel PA detection mode, the results prove that the proposed differential resonant PAS detection technique can effectively improve the system stability and detection sensitivity, and the optimal signal average time can be doubled.
