Research Of New Quartz-enhanced Photoacoustic Spectroscopy Technique

With the development of the modern civilization,human beings have changed their lives in the unprecedented speed and have simultaneously changed the composition of the atmosphere which can affect human health and alter climate patterns.The rising incidence of El Nino,dust storms and haze which impact most of the southeast China is a performance of the deteriorating ecological environment.For achieving harmony between human being and nature,the significance of environment protection was emphasized in “Outline of the Thirteen Five-Year Plan”.Therefore,it’s important to develop the high sensitive and high selective gas sensors to monitor the atmospheric pollutant.In addition,trace gas detection is significant in physics,chemistry,as well as in the life sciences applications.The development of robust and compact gas sensors with high sensitivity,high selectivity,fast response as well as uninterrupted operation is of great research significance and have great application prospect.Photoacoustic spectroscopy has been a research hotspot over optical spectroscopy for long time as this gas detection technique provides many advantages such as high selectivity and sensitivity,long service time as well as low maintenance charge.Quartz-enhanced photoacoustic spectroscopy(QEPAS)is a modification of conventional PAS,in which a quartz tuning fork acts as an acoustic wave transducer to detect the sound signal generated by the trace gas absorbing the excitation laser beam.The QEPAS technique combines the main characteristics of photoacoustic spectroscopy,such as free background and independence of excitation wavelength,with the benefit of a quartz tuning fork(QTF),for instance,high quality factor,low sensitivity to surrounding noise,narrow spectral bandwidth and low cost.QEPAS has attracted broad attention due to above-mentioned advantages and QEPAS-based sensors have been demonstrated for the detection of numerous inorganic and organic trace gases.The QEPAS is a good option to develop the portable gas sensor.The motivation of the thesis is theoretically and experimentally to research on high-performance gas sensor by use of QEPAS technique and to make QEPAS into practical application.The dissertation focuses on the optimization of the acoustic micro-resonator(AmR),the application of the custom QTF,simultaneous dual-gas detection,application of fiber amplifier for high sensitive trace gas detection as well as fast and continuous trace gas monitoring based on the beat frequency effect of the QTF.The main contents of this thesis are listed as follows:1.The new detection approaches to detect QTF frequency response curve were developed.After the QTF was excited by the modulated laser or acoustic wave,a detection light beam was added to the QTF to convert the amplitude of vibration of the QTF into the variation of the detection light intensity.The information of the QTF response to the different frequency can be obtained by demodulating the signals from the detection.As there is no electronic component at the measurement position,the approach can be used to obtain the QTF frequency response curve for the system calibration in the high temperature,high moisture and strong magnetic field circumstances.2.The impact of AmR positions with respect to quartz tuning fork on s ignal amplitude,Q-factor and signal-to-noise ratio(SNR)of the QEPAS was investigated.The results shown that using the variable yt,the vertical distance between the QTF opening and the top of the outer tube-wall,the optimized AmR position for the highest signal amplitude is fixed at yt = 1.3 mm,while the optimized AmR position for the fast response time is fixed at yt = 0.2 mm.In addition,there is a flat SNR peak,0.06 mm < yt <1.36 mm,in which the system can obtain a high SNR.The reported results are extremely useful in the assembly and design of the QEPAS spectrophone.3.Based on the results mentioned in the second part,the double acoustic micro-resonator quartz-enhanced photoacoustic spectroscopy(double Am R QEPAS)was developed and experimentally investigated.As there is a strong acoustic coupling between the QTF and the two AmRs,a very low Q factor is obtained.The response time which is 23 times faster than that of the bare QTF is obtained due to the low Q factor.4.Based on the fact that the QEPAS detection sensitivity scales linearly with excitation laser power,the higher detection sensitivity was obtained via combining the QEPAS with the fimber amplifier.With a laser power of 1,402 mW,the sensor achieved a fast(1 s integration time)H2S detection sensitivity of 734 ppb.The detection limitcan be further decreased to 142 ppb w ith an integration time of～67 s.Our reported QEPAS sensor system obtained the lowest H2 S detection limit compared with other H2 S QEPAS platformsreported in the literature.5.Quartz-enhanced photoacoustic spectroscopy based on a beat frequency effect(BF-QEPAS)was developed and experimentally investigated.The beat frequency signal can be obtained when the piezoelectric signal generated by the QTF,which is excited by a acoustic pulse,is demodulated at its non-resonance frequency.The resonance frequency and Q-factor of the quartz tuning fork(QTF)as well as the trace gas concentration can be obtained simultaneously by detecting the beat frequency signal generated when the transient response signal of the QTF is demodulated at its non-resonance frequency.The detection sensitivty of the BF-QEPAS system for H2 O is one order of magnitude higher than that obtained by the conventional QEPAS technique,while the averaging time is three orders of magnitude shorter than that for conventional QEPAS.6.The custom QTFs with low resonance frequency and large spatial separation between the QTF prongs were design and realization.The larger spatial separation between the QTF prongs allows the QEPAS based sensors benefit from the power boosted by a fiber amplifier,while preventing the prongs and AmR to be illuminated by stray light.In addition,benefit from the low fundamental resonance frequency f0 as well as the overtone frequency f1,the frequency division multiplexing technique of a QTF was developed and experimentally demonstrated via using a custom QTF with f0=2,868 Hz and f1=17,741 Hz.The results show that the QTF can be operated in the combined vibration of the foundational flexural mode and the first overtone vibration mode.And the multi-gas can be detected simultaneously via demodulating the QTF piezoelectric signal at f0 and f1,respectively.The main innovative points of the thesis are as follows:1.The impact of acoustic micro-resonator positions with respect to quartz tuning fork on signal amplitude and response time was investigated.The results show that the highest signal amplitude and the lowest Q factor appear at the two absolute positions,respectively.Furthermore,a flat peak of the SNR curve was found,which is different from previously reported results.These results are helpful to design and assemble new spectrophones.2.QEPAS based on double acoustic microresonators is developed and experimentally investigated.The double AmRs provide two independent detection channels that allow optical signal addition or cancellation form different optical wavelengths and facilitate rapid multi-gas sensing measurements using time division multiplexing technique,thereby avoiding laser beam combination.Moreover,custom made QTFs with few kHz fundamental resonance frequencies allow the implementation of their 1st overtone flexural modes for QEPAS trace-gas detection.A dual-gas QEPAS sensing approach based on a QTF frequency division multiplexing(FDM)technique is developed.The QTF in a dual-gas QEPAS sensor is excited simultaneously in the fundamental and 1st overtone flexural modes by two independently modulated lasers.The FDM QEPAS sensor realizes a continuous real-time dual-gas monitoring with a simple setup and small sensor size compared with previous multi-gas QEPAS sensors.3.A power-boosted quartz-enhanced photoacoustic spectroscopy sensor is developed for sub-ppm H2 S trace-gas detection in the near-infrared spectral region.The sensor is based on o ff-beam QEPAS with an erbium-doped fiber amplified 1582 nm distributed feedback laser.The offset of the sensor floor noise caused by stray light and gas flow can be removed by an electrical modulation cancellation method,which lowers the noise to the theoretical thermal noise level.A custom made QTF with its two prongs spaced ~800 μm apart is employed instead of the standard QTF,which makes the system obtain a similar sensitivity,but with the advantages of easy optical alignment,simple installation,and long-term stability.4.Beat-frequency quartz-enhanced photoacoustic spectroscopy was developed.The resonance frequency and Q-factor of the QTF as well as the trace gas concentration can be obtained simultaneously by detecting the beat frequency signal generated when the transient response signal of the Quartz tuning fork is demodulated at its non-resonance frequency.Hence,BF-QEPAS avoids a calibration process and permits continuous monitoring of a targeted trace gas.