1.Key Laboratory of Atmospheric Optics, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China 2.Science Island Branch of Graduate School, University of Science and Technology of China, Hefei 230026, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 41205021) and the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 2015264).
Received Date:30 August 2018
Accepted Date:06 November 2018
Available Online:12 March 2019
Published Online:20 March 2019
Abstract: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. Keywords:laser heterodyne/ instrument line shape function/ radio frequency filter/ transmittance spectrum
4.实验结果利用3.53 ${\text{μm}}$激光外差光谱仪测量了合肥地区2018年4月10日11:30—13:00约1.5 h内的透过率光谱, 积分时间分别为10 ms和100 ms时测得的其中一组透过率谱如图3所示. 图 3 不同积分时间测量的透过率谱 Figure3. Measured transmittance spectra with different integral time
从测量结果可明显看出, 积分时间为100 ms时, 虽然测量结果具有较高的信噪比, 水汽的吸收谱线未被明显平滑, 但甲烷的吸收谱线平滑较为严重. 主要是由于水汽含量的75%都集中在距离近地面4 km以下的大气中, 吸收线型主要为线宽较宽的Lorentz线型; 而甲烷的浓度在整层大气中的分布较为均匀, 因此线宽较窄的Voigt和Gauss线型对整层的吸收结果也有较大影响. 利用实验获得的不同积分时间情况下的透过率谱数据, 结合合肥地区的大气温、湿、压模式和美国标准大气成分模式对水汽和甲烷柱浓度进行了最小二乘拟合反演[21], 并将ILS函数耦合进反演过程. 在反演水汽和甲烷柱浓度时, 分别使用RF滤波函数和本文的ILS函数, 并对使用两种函数的反演结果和残差进行比较分析. 其中一组反演的透过率结果和残差比较如图4所示. 图 4 (a)积分时间为10 ms透过率拟合结果及残差; (b)积分时间为100 ms透过率拟合结果及残差 Figure4. (a) Fitting results of transmittance and residuals with $\tau$ = 10 ms; (b) fitting results of transmittance and residuals with $\tau$ = 100 ms.
由反演结果可以看出, 积分时间为10 ms时, 系统的光谱分辨率很高, 使用RF滤波函数和本文ILS函数得到的反演结果无明显差别. 而积分时间为100 ms时, 使用本文的ILS函数反演时, 甲烷吸收峰值处的残差值减小至3.75 × 10–4, 残差比使用RF滤波函数时减小约100%, 而水汽吸收峰值处残差无明显变化. 此外, 残差平方和是反映反演结果准确性的重要指标之一, 因此对11:30—13:00时间段内测量的数据使用不同的ILS函数进行反演, 残差平方和的结果如图5所示. 图 5 透过率残差平方和的变化 Figure5. Variation of sum of squared residual of transmittance
由反演结果的残差可看出积分时间为100 ms时, 使用本文线型函数进行反演的残差平方和平均值为1.282, 比使用RF滤波函数反演时减小了0.244, 减小约16%. 水汽和甲烷的柱浓度反演结果如图6所示. 图 6 不同ILS函数反演出的水汽和甲烷柱浓度变化 Figure6. Variations of water vapor and methane column density inversed with different ILS function