Frequency Correction in Terahertz Absorption Spectroscopy
WU Bin1, 2, SHI Xue-shun1, 2, SUN Qing3, DENG Yu-qiang3, WANG Heng-fei2, CHEN Kun-feng2
1. Science and Technology on Electronic Test & Measurement Laboratory, Qingdao 266555, China 2. The 41st Institute of China Electronics Technology Group Corporation, Qingdao 266555, China 3. Optics Division, National Institute of Metrology, Beijing 100013, China
摘要： 在用太赫兹时域光谱(THz-TDS)系统测量样品THz吸收光谱的过程中，电光采样系统的采样误差往往导致测得的THz吸收频率与真实值间存在偏差。针对此问题，利用一氧化碳(CO)分子THz吸收峰的分布特性研究了对THz-TDS系统测量数据进行修正、从而提高THz吸收频率测量精度的方法。首先通过实验测得了2.0×105 Pa 压强下CO气体的一系列等间隔分布的THz吸收峰，然后把测得的吸收峰峰位与JPL标准数据库中CO分子的吸收频率进行对比，得到了实验数据的误差值。通过分析误差值随吸收频率的分布规律，发现二者间呈正线性相关，在此基础上拟合得到了实验数据的误差修正模型。用所建立的模型对实验数据进行修正后，最大误差值为3.36 GHz，较修正前降低了两个数量级，表明根据CO分子的THz吸收峰可有效修正THz-TDS系统的测量误差，从而提高THz吸收光谱的精度。本研究对材料分析识别以及分子光谱标准数据库的建立具有重要意义。
关键词：THz吸收光谱;一氧化碳;电光采样系统
Abstract：Measurement errors in frequency domain always appear when testing samples’ terahertz (THz) absorption spectrum using terahertz time-domain spectroscopy (THz-TDS) system，which is supposed to be attributed to the sampling accuracy of the high speed electro-optic sampling system. In order to make the measurement have a high accuracy, the method of error correction was studied in the present article. Carbon monoxide in gas phase was employed as our standard sample, and its absorption spectrum at the pressure of 2.0×105 Pa was measured experimentally. Comparing the obtained absorption frequencies with the corresponding standard data in JPL database, we got the error values, and their distribution law shows that the values have a linear correlation with the standard absorption frequency. Based on this, the error correction model was built. Using the model to correct the experimental data, the result shows the maximum error after correction is reduced to 3.36 GHz, which is two orders of magnitude lower than the error before correction. This states that the model can be used to correct the error of the THz spectrum caused by high speed electro-optic sampling system. At last, the authors draw a conclusion that the THz-TDS system is supposed to be corrected by the terahertz spectrum of carbon monoxide before measurement, in this way, the terahertz spectrum of sample can have a high accuracy. The study contributes to the material identification and the construction of molecular spectroscopy database in THz region.
Key words：THz absorption spectroscopy;Carbon monoxide;Electro-optic sampling system
[1] Jassimn H, Abdullah I, Shaban A. European Journal of Scientific Research, 2012, 74: 487. [2] Palka N, Trzcinski T, Szustakowski M. Acta Physica Polonica A, 2012, 122: 946. [3] MA Feng-ying, CHI Quan, SU Jian-po, et al(马凤英, 池 泉, 苏建坡, 等). Spectroscopy and Spectral Analysis(光谱学与光谱分析), 2012, 32(3): 610. [4] Hosako I, Fukunaga K. J. Infrared Milli. Terahz. Waves, 2011, 32: 722. [5] ZHANG Cun-lin, ZHANG Yan, ZHAO Guo-zhong, et al(张存林, 张 岩, 赵国忠, 等). Terahertz Sensing and Imaging(太赫兹感测与成像). Beijing: National Defense Industry Press(北京：国防工业出版社), 2008. 65. [6] Naftaly M, Dudley R, Fletcher J, et al. J. Opt. Soc. Am. B, 2009, 26: 1357. [7] Klatt G, Gebs R, Janke C, et al. Opt. Express, 2009, 17: 22847. [8] Foltynowicz R J, Allman R E. Terahertz Time Domain Spectroscopy of Atmospheric Water Vapor From 0.4 to 2.7 THz. United States: Sandia National Laboratories, 2005. [9] Xin X, Altan H, Saint A, et al. J. Appl. Phys., 2006, 100: 094905-1. [10] Rothmana L S, Jacquemart D, Barbe A, et al. J. Quant. Spectrosc. Radiat. Transf., 2005, 96: 139. [11] Varberg T D, Evenson K M. IEEE Trans. Instrum. Meas., 1993, 42: 412. [12] Western C M. Pgopher, a Program for Simulating Rotational Structures. University of Bristol. http://pgopher.chm.bris.ac.uk.
