Quartz Enhanced Photoacoustic Spectroscopy Based Trace Gas Detection For Aerospace Life Support System

High sensitive gas sensor is widely used in aerospace life support system.But the usually used electrochemistry sensors and semiconductor sensors have the drawbacks of poor detection sensitivity and stability and short lifetime.Compared with other laser gas sensing techniques,gas sensor based on quartz-enhanced photoacoustic spectroscopy(QEPAS)method has the advantageous of high sensitivity,fast response,low power consumption and volume,strong adaptability to the environment,and wide applicable occasions.But up to now,the potential of QEPAS gas sensing has not been fully explored.Acetylene(C2H2)is a toxic and harmful gas.It has great significance in many fields such as aerospace life support system,the detection of fault gases in transformers and environmental monitoring.Due to this fact,we devoted to do some effort in theoretical and experimental fields for high sensitive C2H2 detection based on QEPAS method.In this paper,the development of QEPAS at home and abroad was introduced firstly.The detection of C2H2 was also summarized.The basic principle and theory of QEPAS technology was elaborated.From Lambert Beer's law,this paper introduced the basic principle of gas absorption,and the odd and even harmonic wave were calculated.Furthermore the optimization of the laser wavelength modulation depth has carried out in theoretical calculation.The QEPAS signal level S and thermal noise strength Vrms were introduced.From that we got the ways to enhance sensor signal amplitude.Finally,the method for QEPAS sensor signal enhancement was introduced.These theoretical researches have guiding functions for the achievement of high sensitivity QEPAS technology in the subsequent chapters of this thesis.In the experimental section,a 1530.37 nm(6534.37 cm-1)absorption line was selected as the target line for C2H2 detection using the HITRAN 2012 database.A distributed feedback diode laser with butterfly package and single-mode fiber coupled output was used as light source.A quartz tuning fork(QTF)with a smaller resonance frequency f0 will result in a longer effective integration time,which increases the QEPAS signal.In this paper,a QTF with f0 of 30.72 k Hz was adopted as acoustic wave transducer,and a certified mixture of 2000 ppm C2H2:N2 was used.In addition,a series of parameters of the system were optimized,and the minimum detection limit(MDL)was 2.7 ppm.The normalized noise equivalent absorption coefficient(NNEA)was 4.75×10-8 cm-1W/Hz1/2.Lastly,an EDFA amplified distributed feedback diode laser was used as the laser excitation source to construct a C2H2 QEPAS sensor to further improve the performance of C2H2 detection.The EDFA offers an adjustable output power from 50 m W to 1500 m W at the same wavelength as the seed laser.For the C2H2 sensor system operating at atmospheric pressure,a minimum detection limit of 33.2 ppb and a normalized noise equivalent absorption coefficient(NNEA)of 3.54×10-8 cm-1W/?Hz were achieved.Compared with the system without the erbium doped fiber amplifier,the detection limit had increased nearly two orders of magnitude.These findings suggest that using EDFA for power amplification of diode laser is an effective way to improve QEPAS sensor performance.