All-optical Fiber Quartz Enhanced Photoacoustic Spectroscopy Based On M-Z Interferometer

Trace gas detection plays an important role in diverse fields,such as environmental atmospheric monitoring,medical diagnostics,combustion studies and industrial process controls.Quartz enhanced photoacoustic spectroscopy(QEPAS),one of trace gas detection methods,is attracted wide attention due to superior sensitivity,small size,immunity to ambient acoustic noise and low cost.The conventional QEPAS,which was first reported in 2002,employs a quartz crystal as a sharply resonant acoustic transducer to detect acoustic oscillation induced in an absorbing gas by modulated optical radiation.Then the deformation of the QTF prong caused by the weak acoustic wave,is converted into separation of electrical charges by means of piezoelectric effect,and the concentration information of the target gas is achieved by demodulating the modulated electrical charges.However,the local electric read-out method based on piezoelectric effect is not practical for operation in some extremely harsh environments,such as high temperature or strong electromagnetic interference environments.Moreover,the signal generated from the QTF electrodes is not suitable for long distance transmission.Therefore we present a miniaturized photoacoustic spectrometer based on optical fiber interferometer.The other laser is added as the probe light except the excitation light source in a conventional QEPAS setup,and the vibration signal of the QTF prong is converted into the interferometric signal by means of the optical fiber Mach-Zehnder(M-Z)interferometer.Firstly,according to the basic theory of QEPAS,we investigate theoretically the vibration equation of the QTF,the generation and detection of the photoacoustic signal.And trace CO2 detection using QEPAS is performed.Because of the QEPAS defect,a novel approach combining optical coherence technology with QEPAS is presented.According to the theoretical analysis,the interferometric read-out signal is feasible.However,due to the drifting of phase and polarization state,the interferometer is too unstable to achieve accurate signal.To improve the stability of the M-Z interferometer,an active homodyne-phase-compensation technique is used to lock phase at quadrature state.Secondly,on the basis of the technique,a photoacoustic signal system based on M-Z interferometer for all-optical readout is designed.The phase of M-Z interferometer is controlled using the active homodyne phase compensation technique.More specifically,the error signal from subtracting the two output signals of the M-Z interferometer is sent to a proportion integration differentiation(PID).The output signal from the PID controls a home-made PZT phase modulator to lock the phase of M-Z interferometer at quadrature state.The instability issue of the optical fiber sensor is eventually addressed.Finally,all-optical fiber quartz enhanced photoacoustic spectroscopy system based on M-Z interferometer is designed.To assess the performance of the device,we conduct a series of measurements for H2O:N2 mixture.The results show that a sensitivity of the interference read-out method is comparable to the sensitivity of the conventional piezoelectric read-out method.In order to verify the new system capacity of immunity from electromagnetic interference,the simulative electromagnetic interference is added near the QTF.A clear signal from all-optical fiber quartz enhanced photoacoustic spectroscopy system is observed when the conventional QEPAS system stops working.