基于光强与吸收率非线性同步拟合的吸收光谱测量方法

针对高压环境吸收谱线加宽以及波分复用技术合波透射信号分析测试难题,提出利用非线性拟合方法对激光吸收光谱测量中激光强度与吸收光谱进行耦合求解.建立激光强度非线性变化与多谱线吸收拟合函数关系,解决了特殊环境下无法获取光谱基线的难题,实现了波分复用过程合波后光谱信号的分离与诊断.通过仿真验证该方法的可行性,分析计算了激光器特性和特征谱线位置等因素对拟合结果的影响.搭建实验台实现了1—10 atm变压力环境下6330—6337 cm~(-1)波段CO_2吸收光谱叠加信号的诊断分析,对气液两相脉冲爆轰过程中7185.6 cm~(-1)与7444.35 cm~(-1)波段波分复用光谱信号进行测试与拟合,无需分光设备实现了耦合光路分离和温度计算,研究结果对激光吸收光谱技术在高压环境以及燃烧环境下波分复用技术的发展具有重要意义.

Laser intensity and absorbance measurements by tunable diode laser absorption spectroscopy based on non-line fitting algorithm

A novel approach to using tunable diode laser absorption spectroscopy (TDLAS) is developed for measuring the laser intensity and absorbance of gas with highly broadened and congested spectra by wavelength division multiplex (WDM) technology. Direct absorption spectroscopy with non-linear algorithm is utilized, because this fitting method offers benefits in dealing with blended spectral features according to the relationship between transmitted laser intensity and absorbance by Beer law. Compared with traditional TDLAS sensing with WDM, this approach has some advantages of transmissions demultiplexing without additional optic gratings and detectors. Following the published theory, the absorbance and transmitted laser intensity are incorporated into an improved non-linear fitting model. A solution to a simulation of CO₂ blended spectrum at a pressure of 5 atm is exploited to demonstrate the ability to recover the absorption in a high pressure environment, inferring the optimal combination of parameters in the model. The influences of these nonideal laser effects, such as nonlinear and linear coefficients, are investigated by the multiplexed transmission simulations at rovibrational transitions of H₂O near 7444 cm^-1 and 7185 cm^-1. Errors in absorbance fitting is larger when nonlinear or linear coefficients of two lasersbecome closer. The satisfied results can be obtained when linear coefficients ratio is limited whitin a range from 0.05 to 0.67. In addition, the essential transition spacing in multiplexed transmissions, larger than the full width of transitions, is considered to be able to improve the fitting accuracy. This approach is validated in a static absorption cell over a pressure range from 1 to 10 atm at room temperature to demonstrate the ability to measure the blended CO₂ spectrum from 63307 cm^-1 to 6337 cm^-1 by a single DFB laser. The sensor method resolves laser intensity with a nonlinear coefficient of 1.4×10^-4 and recovers absorbance with a root mean square (RMS) precision of 3.2%, which demonstrates the applicability of this sensor to high-pressure gas sensing systems. Another approach to validating the gas temperature and measuring H₂O by WDM is presented in a gas-liquid two phase pulsed detonation engine running with a filling fraction of 100%. Two fiber coupled lasers, respectively, near 7185.6 cm^-1 and 7444.35 cm^-1 are scanned at 20 kHz to achieve a temporal resolution of 50 μs for monitoring detonation exhaust. A fixed spectrum interval (about 0.7 cm^-1) of transitions in multiplexed transmission is created through temperature adjustment in DFB laser to provide more independent absorption information. Recovered linear coefficients of 0.18 and 0.46 in two DFB lasers are in good agreement with the results from the simulations. An instantaneous temperature measurement of 1183 K in the exhaust 7.45 ms after detonation wave provides the confirmation of the ability of this method to infer the temperature and H₂O time histories in the whole detonation process. In conclusion, the novel approach based on TDLAS has tremendous potential applications in high pressure combustion diagnosis and WDM spectrum analysis.