Study Of Photoacoustic Spectroscopy Via A Hammer-shaped Quartz Tuning Fork

Trace gas detection with high precision,over a large dynamic range,and in real-time,is crucial in various gas sensing applications such as national defense safety monitoring,environmental detection,and smart medical diagnostic.While traditional non-optical gas concentration detection technologies,such as thermos catalysis,gas chromatography and electrochemistry,have been widely used in various applications due to their own significant characteristics,however,they suffer from certain limitations such as short lifespan,cross-interference,and long response time.In recent years,Quartz-enhanced photoacoustic spectroscopy（QEPAS）has rapidly developed as a laser gas sensing.In QEPAS,a quartz tuning fork is used as an acoustic transducer to detect photoacoustic signals that contain information of target gas concentration.It has the characteristics of wavelength-independent,strong anti-environmental noise interference ability,high sensitivity,as well as a positive correlation between sensitivity and optical power.The key issues with traditional commercial quartz tuning forks used in QEPAS is described:1)the high resonant frequency of commercial quartz tuning forks makes it impossible to achieve highly sensitive detection of low relaxation gases;2)the extremely narrow prong spacing of commercial quartz tuning forks prevents them from making full use of new light sources such as THz and LEDs sources to efficiently improve detection sensitivity.To address these challenges,this thesis proposes a solution that involves design of a new high-performance quartz tuning fork with a hammer-shaped.Finally,an acetylene QEPAS sensor is built based on the hammer-shaped quartz tuning fork,and the core parameters of the system are experimentally optimized,and the normalized noise equivalent absorption coefficient of 3.84×10^-9cm^-1 W/√Hz is obtained,which verifies the excellent performance of the self-made hammer-shaped quartz tuning fork in QEPAS.The research works in this thesis are as follows:1.On the basis of improving the theoretical model of traditional quartz tuning forks,the vibration model of special-shaped quartz tuning forks is constructed,and the theoretical optimal parameters of the special-shaped quartz tuning fork are calculated through theoretical simulation.The resonance frequency f₀ and the quality of Q of quartz tuning fork are the key indicators affecting the performance of QEPAS sensors.Based on the analysis of tuning fork theory model,a high quality factor and a low resonance frequency cannot be taken into account at the same time for the traditional standard form of quartz tuning fork.Therefore,based on the special-shaped quartz tuning fork model constructed,this thesis uses COMSOL software to design a hammer-shaped quartz tuning fork that is different from the traditional symmetrical quartz tuning fork.2.According to the theoretical results,a new hammer-shaped quartz tuning fork was prepared,and the core parameters such as resonance frequency and quality factor of the customized quartz tuning fork were determined.Through a large number of theoretical analysis and experimental studies,the geometric parameters of the hammer-shaped quartz tuning fork were determined,which include the height of the base,the thickness of the prong,the total length of the prong,the width of the prong at the top of the hammer-shaped part,the width of the prong below the hammer-shaped part,the length of the hammer-shaped part at the top.After the design and preparation of the hammer-shaped quartz tuning fork,the electrical parameters were experimentally determined,and the resonance frequency and quality factor of the hammer-shaped tuning fork were determined to be f₀=12,459.6 Hz and Q=15,540,respectively,which were basically consistent with the theoretical calculation values,and verified the correctness of the theoretical model and analysis calculation.3.The QEPAS system is built based on the hammer-shaped quartz tuning fork,and the experimental parameters such as the optimal excitation position of the quartz tuning fork and the optimal modulation depth of the system are optimized to achieve high-precision measurement of the ppb level of acetylene.The core component of QEPAS is the acoustic detection module,which contains an acoustic micro-resonator in addition to the quartz tuning fork,and the geometric parameters of the resonator are closely related to the tuning fork resonance frequency and geometric size,so it is necessary to prepare a matching acoustic micro-resonator for the hammer-shaped quartz tuning fork.After the resonator parameters have been determined experimentally,the position of the resonator between the prongs of the tuning fork needs to be experimentally optimized.In addition,the laser wavelength modulation depth is also optimized.On the basis of the optimization of the above parameters,the QEPAS sensor based on the hammer-shaped quartz tuning fork has a detection sensitivity of 282 ppb for acetylene when using a 1532 nm near-infrared light source as an excitation light source,and the normalized noise equivalent absorption coefficient is 3.84×10^-9cm^-1W/√Hz.Finally,a QEPAS system was built to detect carbon monoxide,and the detection performance of hammer-shaped quartz tuning fork and commercial quartz tuning fork for low relaxation rate gas was compared.It can be seen from the experimental results that the hammer-shaped quartz tuning fork improves the detection performance of low relaxation rate gas.