The Study Of Trace Ethylene Detection System In Mid-infrared Region

Ethylene as a gas phytohormone plays an important role in the growth and development of plants.It is commonly used to ripen fruits.In the medical field,it can be used to analyze oxidative stress in human beings by detecting the ethylene content in the exhaled gases.At the same time,ethylene is one of the major pollutants that promote the formation of tropospheric ozone in urban areas.It is mainly derived from automobile exhaust and industrial emissions.Therefore,the detection of trace gases in ethylene is of great significance in the fields of atmospheric chemistry,medical diagnosis,and plant biology.This paper focuses on the theoretical and experimental research on the output spectrum technology of off-axis integral cavity.By adjusting the incident angle of the laser in the integrated cavity output spectroscopy technology,an off-axis incident mode is used,thereby exciting a large number of high-order modes,increasing the cavity mode density,basically eliminating the cavity mode structure,suppressing the cavity mode noise,and improving the system signal-to-noise ratio.Compared with the near-infrared band,ethylene exhibits stronger absorption characteristics in the mid-infrared.In this paper,a set of off-axis integrated cavity output spectrum?OA-ICOS?device based on an external cavity quantum cascade laser?EC-QCL?is reported to measure 1 ppm standard ethylene gas.The wavelength range of the laser can achieve 962-1067cm^-1 and the line width is less than 10MHz.The emitted light is coupled into the resonant cavity composed of two high-reflectance lenses?>99.99%?through the optical isolator and is transmitted through the resonant cavity.The signal is received by the photodetector via the focusing lens.According to the HITRAN 2012database,simulations of ethylene interference from other gases in a typical air component were given.The measurement of ethylene concentration at 999.09 cm^-1 was used.At the same time,the effect of different pressure on the absorption spectrum of ethylene was simulated.The wavelength modulation technique was used to simulate the influence of the second harmonic?2f?of different spectral lines and different pressures,modulation coefficients and concentrations on the signal.In the experiment,a 100-ppm standard ethylene gas was measured using a 1.2 m-long cell consisting of two window plates of ZnSe.The effect of Mode-Hop on the detection of second harmonic signals at different laser frequencies was analyzed.The feasibility of this system for trace ethylene detection was verified,which laid the foundation for the next experimental work.