基于TDLAS技术的水汽低温吸收光谱参数测量

精确的气体光谱参数对气体浓度、温度等的光谱精确反演测量具有十分重要的意义,针对当前主流光谱数据库(例如HITRAN)中数据与实际数值存在相当误差的问题,自主研制了一套基于静态冷却技术的低温光谱实验平台,用于精确测量低温下的气体吸收光谱参数.运用该低温光谱实验平台,采用可调谐二极管激光吸收光谱(TDLAS)技术测量了温度为230—340 K、压强为10—1000 Pa时7240—7246 cm~(-1)波段的纯水汽振转跃迁光谱.采用Voigt线型多峰拟合方法,获得了5条水汽振转跃迁谱在不同温度、不同压强下的积分吸光度值及洛伦兹展宽值,运用线性拟合的方法得到这5条吸收线的自展宽半峰全宽系数及参考温度下的线强值.运用不确定度传递公式,计算得到实验结果的不确定度,与HITRAN2012数据库中的线参数进行对比,所测的5条吸收线中实验结果与数据库值最大相差10.96%,且实验结果的不确定度为1.11%—2.98%(置信概率p=95%,包含因子k=2),小于HITRAN2012数据库值的不确定度.

Measuring spectral parameters of water vapor at low temperature based on tunable diode laser absorption spectroscopy

Accurate and reliable spectral line parameters of gas are very important for measuring gas concentration and temperature.The mainstream spectrum database (e.g.HITRAN) includes the values from theoretical computation based on different models,which have some inevitable deviations from the corresponding actual values.To address this problem,we develop a low-temperature spectral experimental platform for simulating low temperature and low pressure environment so as to accurately measure gas absorption spectral parameters.The spectral experimental platform uses the static cooling technology combined with the Dewar insulation system to maintain the quartz cell at a constant temperature.Through adjusting the electric heating and liquid helium refrigeration,we can achieve temperature change and stability.Temperature of the low temperature absorption cell can be adjusted in a range of 100-350 K with a precision lower than 0.3 K and the temperature gradient in the cell is lower than 0.01 K/cm.The length of quartz cell is 100 cm,and a reflector can be used to increase optical path for absorption.The window diameter is 76 mm,and the spectral resolution is better than 0.001 cm^-1.We use a tunable diode laser spectrometer to measure absorption spectra of pure water vapor with the platform at different temperatures (230-340 K) and different pressures (10-1000 Pa).Voigt profile is the leastsquares fit to the measured spectra by using a multi-spectrum fitting routine.A filter is used to reduce electronic noise of detector signal.As spectral lines in the band of 7240-7246 cm^-1 are often used in low temperature wind tunnel flow field measurements,a distributed feedback (DFB) diode laser with a wavelength of 1381 nm is used in the experiment, and five water vapor lines are selected and measured.Firstly,from the linear fitting of line area and the full width at half maximum of collisional broadening (or pressure broadening) we obtain line strengths and self-broadening half-width coefficients at different temperatures.Secondly,from nonlinear fitting of line strengths and self-broadening half-width coefficients at different temperatures we obtain the values of line strengths and self-broadening half-width coefficients at the reference temperature (296 K).In the end,comparison between our experimental results and HITRAN2012 database values shows that the maximum discrepancy between the HITRAN database and the experimental result is 10.96%.A transparent uncertainty analysis is given for the measurement values.Uncertainties of our measured line strengths are in a 1.11%-2.98% range (95% confidence level,k=2),which is smaller than those of HITRAN2012 database values (uncertainties are in a range of 5%-10%).The accurate spectral parameters are obtained experimentally,and of great significance for improving the spectrum measurement accuracy of water vapor in low temperature environment in the future.