Trace Gas Sensing Based On Light-Induced Thermoelastic Mechanism In Quartz

Quarzt-enhanced photoacoustic spectroscopy（QEPAS）trace gas sensing technology has the advantages of high sensitivity,good selectivity,and real-time online measurement,etc.It has an important value in atmospheric environment monitoring,electric power safety detection,medical pathological diagnosis,combustion field measurement and so on.In traditional QEPAS sensing research,near-infrared diode laser with low output power and block optical lens are always utilized in such sensor system,which makes it difficult to meet the requriements of high detection sensitivity and adapt to practical application.Besides,due to the limited performance of quartz tuning fork（QTF）,QEPAS sensor would be severely restricted in complex and harsh environments sensing such as high temperature,strong oxidation,strong corrosive and other gas detection.Therefore,this thesis carried out research on high sensitive trace gas sensing technology based on QTF to achieve the detection limit down to ppb(10^-9)and high performance to realize the online real-time detection of trace gases in environment sensing application.In order to improve the performance of QEPAS sensor and further reduce the detection limit,this thesis proposed a QEPAS sensor based on a high-power excitation laser source,which used an erbium-ytterbium double-doped fiber amplifier to amplify the power of the seed excitation laser in QEPAS sensor system.The amplified laser power is as high as 1200 m W,which made the sensor obtained an excellent detection limit of NH₃ gas with ppb level and HCN gas with ppt(10^-12)level.Besides,to improve the sensor’s stability,simplify the system configuration and suppress the optical noise caused by high-power laser.This thesis has carried out the research on the integration of the photoacoustic detection unit and the whole sensor.Using the 3D printing technique combined with the gradient index lens to integrate the discrete optical elements and the photoacoustic detection gas chamber of the traditional optical platform,a photoacoustic detection unit with a weight of 5g and a QEPAS sensor with a weight of 3.2 kg was obtained.The online continuous monitoring of CO gas which released from the simulated fire combustion was realized.In the above long-term actual measurement,the silver coating on the surface of the QTF in the QEPAS sensor system is prone to oxidation and corrosion.Therefore,this thesis proposed a quartz based light-induced thermoelastic spectroscopy（LITES）trace gas sensing technology.Different from the principle of light-excited acoustic wave detection in QEPAS,LITES uses a QTF to detect the changed light intensity which was absorbed by the target gas molecule.In LITES system,the QTF does not need to be placed in the detected gas environment,so it is a non-contact sensing technique that meets the actual application requirements in complex and harsh conditions.The physical mechanism proposed in this paper is different from the existing literature reports.Therefore,this thesis conducted the theroretical and experimental research on the physical mechanism of LITES,and fully demonstrated the correctness of the light-induced thermoelastic mechanism in LITES.QTF is the core device in LITES sensor system.Therefore,the performance enhancement characteristics of the LITES sensor and the sensing characteristics of the light-induced thermoelastic effect of the QTF in the mid-infrared band were investigated in this thesis.To increase the light absorptivity and improve the light-thermo-elastic-electric conversion efficiency in the piezoelectric quartz crystal.This thesis proposed to reduce the inherent energy loss in QTF,and to increase the optical absorption path length.Besides,we also designed an asymmetric gold plating scheme for the QTF.In the CO gas sensing research,the detection limit of ppb level was achieved by using the mid-infrared 2.3μm diode laser and 4.6μm quantum cascade laser.It is revealed that using the principle of light-induced thermoelastic mechanism,QTF is expected to become a photodetector device with full band coverage.