Study On An All-optical Demodulation Technology For Quartz Enhanced Photoacoustic Spectroscopy System

The development of trace gas detection technology has great significance in thefield of environmental monitoring, medical diagnostics and anti-explosion terrorist.With the development of science and technology, quartz enhanced photoacousticspectroscopy（QEPAS） has become a new direction of development of trace gasdetection technology. Traditional quartz enhanced photoacoustic spectroscopy detectionsystem used a laser and a quartz tuning fork as the core device to measure the content oftrace gases, used electrical demodulation system to processing the piezoelectric currentsignal which generated by the vibration of quartz tuning fork. However, the electricaldemodulation method is vulnerable to external electromagnetic interference, can’t beused in explosive, high-temperature and high-humidity and other harsh environments,and the essence of this demodulation system means it difficult to realize remotedetection. Focus on these disadvantages, all-optical demodulation for quartz enhancedphotoacoustic spectroscopy system for trace gases detection is proposed. A fiberFabry-Perot（F-P） sensor demodulation method instead of the traditional electrolyticdemodulation method to detect the vibration signal of quartz tuning fork, so as to realizethe detection of trace gases content.First of all, the principles and theory of quartz enhanced photoacousticspectroscopy is described. The principles of photoacoustic effects, the theory ofphotoacoustic spectroscopy are described in detail. The characteristics of the vibrationof quartz tuning fork are analysed. The relationship between the interference lightintensity of the low fidelity extrinsic type fiber F-P and the cavity length is built up. Toobtain a high sensitivity of the optical fiber F-P sensor, the relative position between thedetection fiber and quartz tuning fork is optimized. At last, the correlation detectionprinciple of digital phase-locked amplifier is described.Second, the quartz enhanced all-optical demodulation system is designed. Thesystem is divided into three parts. They are photoacoustic excitation module, opticalfiber F-P sensor demodulation module and signal processing module. The key point ofthe photoacoustic excitation module is to use fiber lead method instead of the traditionalfocus mode. The fiber led the laser to the two interdigital central of the quartz tuningfork to produce photoacoustic signal can reduce the loss of laser energy in an openenvironment. The focus point of fiber F-P sensor demodulation module is the choice of light source, photoelectric detector and the structural design of the fiber F-P sensor. Thesoftware and hardware design of digital signal processor-based digital lock-in amplifieris important of signal processing module.Finally, the quartz enhanced photoacoustic spectroscopy trace gas detection systemwas constructed. The feasibility and performance of the system are evaluated bydetection of water vapor in an open environment. The water vapor concentration isdetected in electrical demodulation mode and optical fiber F-P sensor demodulationmode in QEPAS system. The normalized noise absorption coefficient of7.15×10^-7cm^-1·W/Hz^1/2and2.80×10^-7cm^-1·W/Hz^1/2is achieved. So the detectionsensitivity of all-optical QEPAS system is2.6times than conventional electricaldemodulation mode QEPAS system. For improve the system sensitivity, the structure ofthe resonance tube is added in the all-optical QEPAS system, the system sensitivity is3times than that without resonance tube. Compare with the electrically demodulationsystem, the all-optical QEPAS system has strong anti-electromagnetic interferencecapability, suitable for inflammable and explosive, high temperature, high humidity andother harsh environments, and is convenience to realize remote detection,multi-attunement, group network detection.