激光外差光谱仪的仪器线型函数研究

激光外差是一种基于相干探测原理的高灵敏度光谱检测技术,因其同时具有很高的光谱分辨能力,被广泛应用于诸多研究领域.在光谱测量过程中,仪器线型函数对吸收谱线的平滑作用,会对气体浓度的反演结果产生影响.为了获取激光外差光谱仪的仪器线型函数,基于激光外差原理和信号处理过程,对影响仪器线型函数的射频滤波带宽和积分时间等参数进行了分析,获得了仪器线型函数表达式.利用自行建立的激光外差光谱仪,多次测量了3.53μm波段内水汽、甲烷的吸收谱线,分别将射频滤波频域响应函数和本文获得的仪器线型函数耦合进水汽、甲烷柱浓度的反演.结果表明,射频滤波带宽为30 MHz、积分时间分别为10 ms和100 ms时,光谱仪的实际分辨率分别约为0.005 cm~(-1)和0.025 cm~(-1);使用仪器线型函数对积分时间为100 ms时测量的数据进行反演,透过率残差平方和与甲烷吸收峰值处的残差分别减小16%和100%,提高了气体浓度反演的准确度.

Instrument line shape function of laser heterodyne spectrometer

Laser heterodyne is a kind of technique based on coherent detection with high sensitivity and spectral resolution for spectrum measurements. For these reasons, it has been widely used in many research fields, such as trace gases' detection of earth's or terrestrial planets' atmosphere. However, when the laser heterodyne spectrometer is used for measuring the spectrum, the instrument line shape (ILS) function usually smooth the spectrum, which affects the inversion results of the gas column density. In previous researches, the radio frequency (RF) filter response function was usually used as the ILS, but recent studies indicated that the ILS without consideration of the influence of lock-in amplifier was not precise enough. In order to obtain the ILS function of the laser heterodyne spectrometer, the main factors which influence the ILS are analyzed, including the RF filter bandwidth, integral time and low-pass filter of lock-in amplifier, and the process is based on the principle of laser heterodyne technology and the flow of heterodyne signal processing. The presented ILS is the convolution of RF filter, wavelength variation in the integral time and the low-pass filter. In addition, for testing the effectiveness of the ILS in this paper, the laser heterodyne spectrometer which was built in our laboratory is used for the multiple measurement of the absorption of water vapor and methane in the band of 3.53 \${\text{μm}}$\ and the column densities are retrieved with different ILS. The experimental results show that the actual resolutions of the laser heterodyne spectrometer are about 0.005 cm^-1 and 0.025 cm^-1 when the integral times are set to be 10 ms and 100 ms respectively. Furthermore, the RF filter response function and the ILS function presented in the paper are respectively used in the procedure of water vapor and methane inversion. The results show that when the ILS function used for the retrieval, the sum of squared residual reduces about 16% and the residuals at the peak of methane absorption reduces almost 100% compared with the scenario when using the RF filter function. Above all, the comprehensive analysis of the laser heterodyne spectrometer in this paper indicates that the ILS function is more precise than pioneering studies and this work will be helpful for retrieving the precise profiles of trace gases.