Trace gas detection technology plays an important role in applications such as greenhouse gas detection, industrial hazardous gas monitoring and medical breath gas analysis. Conventional methods such as gas chromatography, semiconductor, electrochemical sensor and contact combustion are widely employed for trace gas detection. However, these methods have one or more disadvantages, such as low sensitivity, low selectivity, frequent calibration requirement, system complexity and high cost. Recently, optical methods based on absorption spectroscopy have been used for trace gas detection, such as cavity ring-down spectroscopy, Fourier transform infrared spectroscopy, differential absorption spectroscopy, tunable diode laser absorption spectroscopy, non-dispersive infrared gas sensing technology, and photoacoustic spectroscopy (PAS).

Distinguished from other optical detection methods, PAS is an absorption spectroscopy technique without background noise, with high sensitivity, high selectivity, fast response time and so on. In addition, the structure of photoacoustic system is relatively simple and does not require a complex optical path calibration process, thus PAS has become an important technique for trace gas detection. Acoustic sensors are very important in PAS gas detection systems and directly affect the sensitivity of the photoacoustic system. Capacitive microphones are commonly used as acoustic sensors, which have the advantages of mature technology and low price. However, the capacitive microphones as electronic devices are inevitably affected by the electromagnetic interference and high temperature environments. Optical acoustic sensors with no electronics, featuring high sensitivity, high signal-to-noise ratio, wide frequency band response, and wide dynamic range, can break through the limitations of traditional capacitive sensors.

In recent years, all-optical PAS gas detection technology, which integrates the optical fiber sensing technology and PAS technology, has been rapidly developed. In the all-optical PAS system, the photoacoustic signal is detected by the optical acoustic sensor, so it has the characteristics of anti-electromagnetic interference and can greatly reduce the size of the photoacoustic system. Currently, the optical acoustic sensors are based on three main types of optical intensity attenuation principle, fiber grating principle, and interferometer principle. In particular, the interferometric all-optical PAS gas detection system based on the optical acoustic sensor of interference principle, featuring high signal-to-noise ratio and high sensitivity, has become a research hotspot in recent years, and a series of important research results have been achieved.

This paper reviews the research progress of the interferometric all-optical PAS gas sensing technology, and focuses on the all-optical PAS gas sensing technology based on Michelson interference principle and Fabry-Perot (F-P) interference principle.

There are mainly four types of interferometer-based acoustic sensors, namely Mach-Zender interferometer (MZI), Sagnac interferometer (SI), Michelson interferometer (MI) and F-P interferometer (FPI). The optical acoustic sensors based on MI and FPI principles can help to improve the sensitivity of acoustic wave detection due to their reflective interferometric structure, therefore this paper focuses on the application of MI and FPI based interferometric all-optical PAS in the field of gas detection.

In MI-based all-optical PAS, we first introduce the MI-based all-optical quartz-enhanced photoacoustic spectroscopy (QEPAS) technology (Fig. 1). This technology solves the problem that the traditional QEPAS is vulnerable to weak anti-electromagnetic interference and difficult to adapt to the detection of trace gases in harsh environments. Then, we present the MI-based principle of cantilever enhanced photoacoustic spectroscopy (CEPAS) technique (Figs. 2-4). However, the MI-based all-optical PAS gas detection system is susceptible to environmental vibration, which makes the system difficult to work in a wide range of applications in industrial environments.

The FPI-based all-optical PAS can be better applied to trace gas detection in industrial environments. The all-optical PAS technique based on the diaphragm-based FPI have been demonstrated (Figs. 5-8). The highly sensitive FPI-based all-optical PAS technique combined with QEPAS (Fig. 9) and resonant CEPAS (Fig. 11) techniques to achieve highly sensitive detection of trace gases are described in the second part. In the PAS gas detection technology, besides the pursuit of high sensitivity, the miniaturization of the sensing probe is also an important research topic. Therefore, the FPI-based miniaturized all-optical PAS technology is introduced in the third part. In particular, a fiber-tip all-optical photoacoustic gas sensing probe is introduced (Fig. 15) as well as a miniaturized gas sensing probe (Fig. 16) for simultaneous detection of multiple gases, which provide a new solution for long-range measurement of single or multiple gases in confined spaces. As the development of gas sensing technology has fully entered the practical stage, the applications of FPI-based all-optical PAS for environmental gas monitoring (Fig. 17 and Fig. 18), transformer fault monitoring (Figs. 19-21), and medical respiratory analysis (Fig. 22 and Fig. 23) are highlighted in the last part.

The interferometric all-optical PAS trace gas detection technology has broad application prospects in industrial production, environmental gas monitoring, and medical respiratory analysis. The future interferometric all-optical PAS trace gas detection technology will be developed towards ultra-high sensitivity, high stability, anti-interference ability, low cost, miniaturization, etc.