Background:

Fourier transform photoacoustic spectroscopy is a spectral analysis method that integrates the broadband advantages of Fourier transform spectroscopy with the high sensitivity, background-free nature, and wavelength-independent characteristics of the photoacoustic effect. Due to its high sensitivity and versatility, Fourier transform photoacoustic spectroscopy has become a crucial tool for both qualitative and quantitative gas analysis, significantly enhancing scientific research and practical applications. In recent years, Fourier transform photoacoustic spectroscopy has garnered extensive attention from the academic community, resulting in numerous in-depth studies. However, most current research efforts have been concentrated on combining Fourier transform spectroscopy with non-resonant photoacoustic detection modules, with relatively limited exploration of its integration with resonant photoacoustic detection modules. Furthermore, research on broadband measurement, simultaneous multi-component gas detection, and full-spectrum analysis remains underdeveloped.

Innovation:

In light of the current research landscape, Professor Dong Lei's team at Shanxi University has developed an innovative differential resonant photoacoustic cell-based Fourier transform photoacoustic spectroscopy for background-free gas detection. This method successfully achieves high-sensitivity gas detection across the 1-10 μm range. The technology demonstrates significant potential to address the challenges associated with full-spectrum detection, making it particularly relevant for advancing research in the terahertz domain. The findings from this study have been published in Chinese Optics Letters, Volume 22, Issue 10, 2024 (X. Liu et al., "Differential photoacoustic cell-based Fourier transform photoacoustic spectroscopy for background-free gas detection"), and have been highlighted as the cover article for this issue. Professors Dong Lei and Hongpeng Wu from the Institute of Laser Spectroscopy at Shanxi University are the corresponding authors, with doctoral student Xiaoli Liu serving as the first author.

This cover image illustrates the application of Fourier transform photoacoustic spectroscopy utilizing a resonant photoacoustic cell in broadband gas sensing. In this process, a broadband light source passes through a Michelson interferometer before interacting with the target gas in a resonant photoacoustic cell, where it generates acoustic signals. The resonant photoacoustic cell establishes a standing wave field, which enhances and amplifies these acoustic signals. By capturing and processing these signals, this technique showcases the exceptional broadband detection capabilities of Fourier transform photoacoustic spectroscopy.

The experimental setup for Fourier transform photoacoustic spectroscopy based on a resonant photoacoustic cell is illustrated in Figure 1(a). This system employs a differential resonant photoacoustic cell and introduces amplitude modulation at the resonance frequency of the cell, enabling uniform modulation of all frequency components from the broadband light source. Following amplitude modulation by the modulator and phase modulation by the Michelson interferometer, the broadband light source is shaped and directed into the photoacoustic cell, where it interacts with the target gas. The resulting photoacoustic signals, generated by the target gas upon absorbing the broadband light, are detected differentially using two microphones and subsequently sent to a lock-in amplifier for 1f demodulation. The demodulated signals undergo Fourier transform processing to extract the broadband absorption characteristics of the target gas. The acoustic field distribution within the dual-channel differential resonant photoacoustic cell is depicted in Figure 1(b). When the photoacoustic cell operates at its resonance frequency, the gas is compressed in one chamber while expanding in another, exhibiting a piston-like motion. This leads to pressure oscillations in the two acoustic resonant cavities that are out of phase with one another, resulting in an acoustic field with equal amplitude but opposite phases. In this configuration, background noise from the window, external environmental disturbances, flow noise, and other noise components generated during gas passage through the photoacoustic cell remain in phase. By subtracting the signals collected by the microphones in the dual-channel resonant cavities, the amplitude of the photoacoustic signals can be effectively amplified while significantly suppressing noise interference.

Figure 1(a) Diagram of the experimental setup, (b) The acoustic field distribution within the differential resonant photoacoustic cell

Figure 2 presents the results of single-detection for methane and acetylene gases, showcasing the complete absorption spectra for both gases within the spectral range of the light source. This demonstrates the system's detection capabilities in the 1-10 μm range. These findings are significant for promoting simultaneous multi-gas detection and full-spectrum analysis.

Figure 2(a) The absorption spectra for methane gas, (b) The absorption spectra for acetylene gas

Summary and outlook：

In summary, this study has developed a resonant photoacoustic cell-based Fourier transform photoacoustic spectroscopy for background-free gas detection. The first author of the paper, Xiaoli Liu, a doctoral student at Shanxi University, states that this technology holds promise for achieving full-spectrum detection, which is particularly significant for terahertz range detection. In the future, the team plans to integrate the supercontinuum spectral response characteristics of quartz tuning fork spectroscopy with Fourier transform spectroscopy to further explore its potential applications in related fields.