Research On CO₂ Detection Based On TDLAS And Quartz Tuning Fork Enhanced Photothermal Spectroscopy

Gas detection technology is closely related to our lives.In the field of biomedical science,human exhaled gases contain a variety of trace gases,and the detection of some characteristic trace gases can be used to predict diseases.In the field of industrial production and security,gas detection technology can be applied to trace explosive detection,air pollution monitoring and so on.Carbon dioxide（CO₂）is a common type of gas,whose concentration of too high or too low indirectly affects our all walks of life,research and development of gas detection technology with high measurement accuracy and good stability has a great help for people’s daily life,social production.Aiming at the problems of large equipment and inconvenient operation in some current gas detection technologies,this paper has done the following research:1.Detection of CO₂ gas based on tunable diode laser absorption spectroscopy（TDLAS）.A fiber coupled distributed feedback laser（DFB）with a central wavelength of 1580.51 nm was selected,and a stainless steel Herriot cell with an effective optical path of 15 m was designed to increase the interaction distance of the laser and the gas to be measured.Different concentrations of CO₂ gas were detected by wavelength modulation and harmonic detection technology.The experimental results show that with the increase of CO₂ concentration,the amplitude of 2f signal increase gradually,and the two parameters have a good correlation.In order to further study the stability and limit detection sensitivity of the TDLAS system,continuous measurement of CO₂ with a concentration of 2.5%was carried out within 1 hour,and the stability analysis of the system was measured according to the collected 9000 sets of2f signal amplitude.The results show that the TDLAS technology has a good stability.The Allan deviation was calculated based on the collected dates.The results show that when the integration time increased to 155 s,the Allan deviation reaches the minimum value,and the measurement accuracy of TDLAS system for CO₂is about 292 ppm.2.Using quartz tuning fork instead of photodetector,laser induced thermoelastic spectroscopy（LITES）based on quartz tuning fork was studied.Different from the traditional LITES system,tuning fork coated with nano-ZnO was studied for the first time.CO₂ was selected as the target analyte for performance verification,and its 2f signal was detected.Then,the stability of the system was verified by experiments.During the 3-hour test period,continuous measurement records were made on CO₂ with a concentration of 6%,and 8000data points from 2f signals were extracted for analysis.The amplitude varied between0.235-0.275 V,the mean value was 0.251 V,and the standard deviation was calculated as0.0063 V.It shows that the experimental system has excellent stability.When the integration time reaches 110 s,Allan deviation obtains the minimum value,and the optimal measurement accuracy of this LITES system reaches 132 ppm.Nanometer ZnO coating on the tuning fork has the merit of easy preparation and low cost.Compared with the bare tuning fork,the coated quartz tuning fork increases the 2f signal amplitude and signal-to-noise ratio by 1.4times and 1.24 times,respectively.Therefore the LITES system is a promising online gas detection technology.Finally,this paper explores the application of nano-ZnO in water vapor measurement,prepared a silicon nanowire/zinc oxide/reduced graphene（Si NW/ZnO/r GO）schottky structure sensor,and proves its application prospect in human respiration monitoring.