Research On The Method Of All-optical Quartz Enhance Photoacoustic Signal Detection

Quartz enhanced photoacoustic spectroscopy gas detection technology has playedan extremely important role in air pollution monitoring, fire alarm, explosives detectionand medical pathological diagnosis and other fields. But the system structure is morecomplex, and mainly uses the commercialized instruments that is bulky and expensive,it can not be used to the on-site detection. Therefore, Combining with circuit design andsoftware processing algorithms, the photoacoustic signal demodulation system wasproposed, in order to realize miniaturization, integration and cheapness.Firstly, based on the molecular theory of infrared spectroscopy and spectralbroadening mechanisms, this paper detailedly expounded the principle of photoacousticsignal. Meanwhile, using the wavelength modulation and the second harmonicgeneration theory，the characteristics of photoacoustic signal was analyzed. Then, therelationship between the intensity of photoacoustic signal and the gas concentration wasdeduced,and types and main sources of the strong background noise of the weakphotoacoustic signal was analyzed. Finally, according to the amplitude and frequencycharacteristics of the photoacoustic signal and noise, the principle of orthogonal vectorlock-in method for weak signal detection was reaserched.Secondly, the laser temperature control system was designed, which mainlyincluded temperature sampling and differential comparison, PID control, PWM wavepower drive and protection circuit, so that it ensured the excitation laser output stability.In order to satisfy the following data acquisition and signal processing requirements,eight order band-pass filter circuit and the pre-amplifier circuit was designed, and theconfiguration optimization of the key parameters was completed, as well as thesignal-to-noise ratio of control system was improved. Meanwhile, the signal processingprogram was compiled,which included signal acquisition and control, data storage,orthogonal vector lock-in and data fitting and so on. Finally, in order to improve thestability of optical fiber F-P cavity directly which directly affected the sensitivity andthe precision of the signal demodulation, the mechanical fixture was designed andmachined,which included quartz tuning fork and optical fiber head of detection laser.In order to make the output characteristics of the laser meet the demand of thesystem and ensure the stable output wavelength of the laser in the gas absorption linecenter, the laser characteristics experiment was completed.Then, the stability of the fiber optic Fabry-Perot cavity and quartz tuning fork resonance frequency were test, and theparameters of the experimental system were determined. Afterward,the characteristicsof filter circuit and amplifying circuit were tested, and the test results met the designrequirements. Finally, in the open environment, with the experimental temperature of24℃, relative humidity of65%, and the water content in air about2.03%,thephotoacoustic signal detection experiment for detection of water vapour was carried out.A normalized noise equivalent absorption coefficient5.53×10-7cm-1·W/Hz1/2wasachieved.With the achieved experimental results, the photoacoustic signal detection system,which was based on the combination of circuit design and software algorithm,wasverified to be correct and feasible. And it provided theoretical and experimental basisfor enhanced photoacoustic gas detection system from the laboratory to on-sitedetection.