太赫兹时域光谱品质影响因素及改进研究
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摘要
太赫兹时域光谱技术是进行太赫兹波段下光谱研究的有效工具，它的出现解决了太赫兹波段下无法产生宽带辐射源的难题，使得光谱学上存在的太赫兹断层得以填补。随着这项技术的发展，对太赫兹波段下物质特性的研究也逐步拓展到以生物、化学、医药、安检为代表的各个领域。太赫兹时域光谱技术实用化已经成为当前太赫兹技术的一个主要发展方向之一。本论文围绕这一技术，首先探讨研究了作为衡量仪器性能的太赫兹时域光谱仪主要参数指标的规范定义问题。在此基础上，以定义的标准参数为信号品质衡量标准，通过实验，系统地研究分析了太赫兹信号产生、探测、传输和处理过程中的影响因素，以及这些影响因素对最终信号品质带来的影响关系。利用分析得到的最优配置参数对太赫兹信号的采集处理环节进行了改进，极大提高了硬件利用率和有效信息的提取率。最后，用改进后的系统对三聚氰胺粉末进行测试，得到了已往太赫兹时域光谱实验没有报道过的新的吸收谱线。
课题的研究内容和成果可以归纳为以下7个方面：
1.讨论了目前对太赫兹脉冲时域光谱技术的三个主要性能参数定义的规范问题；为频谱分辨精度的评估提出了新方法；对于频谱范围，提出了以噪声限为基准和频谱最大值为基准两种定义原则。对于信噪比参数，提出了标准信噪比的概念，可以将不同的系统性能进行统一量化评价。是太赫兹仪器标准化研究十分必要的工作
2.采用电流瞬冲模型，分析了用飞秒脉冲激光激发光电导天线产生和探测太赫兹时域脉冲信号环节存在的影响信号品质的因素，即环境常数、光电导天线结构参数、飞秒脉冲激光的功率和激发天线两级间偏置电压，并对其中飞秒激光功率和偏置电压这两个重要的因素进行了进一步的实验研究。通过实验，分析找到了针对于天津大学太赫兹中心现有的太赫兹时域光谱系统，在飞秒脉冲激光功率值和激发天线两级间偏置电压值的最优参数配置，大大优化了该系统的性能。
3.针对以相关器模型为基础的锁相放大技术原理，分析了太赫兹时域信号传输环节存在的影响信号品质的因素，即斩波器的频率与放置位置和锁相放大器的低通时间常数，并进行了进一步实验研究。以此，从实验结果中找到了针对于天津大学太赫兹中心现有的太赫兹时域光谱系统，在斩波器的频率与放置位置和锁相放大器的低通时间常数方面的最优参数配置。
4.分析比较了2个不同实验室的太赫兹光谱系统的控制、信号的采集、数据处理的计算机软件P1和P2的流程和数据处理形式，发现了两种不同流程的优缺点。如软件P1相对于P2流程，P1流程的优点是硬件利用率高，缺点是单数据点采集时间长。我们结合了软件P1和软件程序P2的优点，编写了新的系统软件，完成了对流程和数据保存格式的优化改进，并通过实验验证了其合理性。
5.以较优配置参数自行设计了CD552锁相放大电路，代替了原有的以商业化锁相放大器SR830为核心的太赫兹信号采集环节。该技术已获得了技术专利，大大提高了系统硬件的集成化、小型化的水平。对于上位机控制和数据处理程序也进行了进一步改进。
6.兼顾不同层次使用者应用的需要，对系统控制和处理程序的进行了改进，完成了分页面显示的设计；增加的光谱特性计算程序；使现有的太赫兹光谱软件更具实用性，显示界面更为人性化。
7.应用改进后的太赫兹时域光谱系统，对三聚氰胺粉末样品进行了光谱探测实验。在太赫兹波段得到了2.00、2.25、2.60THz处三个特征吸收峰。其中，前两个吸收峰为其他太赫兹研究者已报道过的，但2.60THz处的第三个峰，在我们之前是未见报道过的。2.60THz处的特征峰，我们用量子化学振动转动谱软件模拟证明了，三聚氰胺确实存在这一本质特征。同时，理论模拟结果也预示出现有的实际太赫兹光谱技术的局限性，即太赫兹光谱，对于三聚氰胺样品其光谱的分辨精度只能达到0.05THz。在该精度下只有较强的物质特征峰方能被探测到。因此，也为提高太赫兹时域光谱仪的性能，提出的更高的科学与技术的要求。
Terahertz time-domain spectroscopy (THz-TDS) is a powerful tool toinvestigation spectroscopic properties of matters at THz frequencies. The invention ofTHz-TDS not only delivers a wide band radiation source in the THz range, but alsofills up the “THz Gap” in spectroscopy. With the development of THz-TDS, studies ofsubstance characteristics have spread into various of fields, such as biology, chemistry,medicine, and security. So the realistic applications of THz-TDS technology havebecome one of the main development trends. By focusing on this task in my PhDwork, several issues on standardized definition of the main parameters to measure thesignal quality of THz-TDS have been pursued for the first time. Based on thestandardized parameters defined here, factors appeared in the THz signal generation,detection, transmission and data acquisition have been studied and their influencerules to the signal qualities have been explored systematically according to theexperiments. By the optimized parameters acquired in former experiments, the THzsignal acquiring and processing parts have been improved to increase the hardwareresource utilization ratio and the effective information extraction ratio. At last, themelamine powder has been characterized using the improved THz-TDS system. Anew absorption peak never been reported has been identified.
The main achievements of this PhD work are listed in the following:
1. Issues on the definition of three main parameters in THz-TDS have beendiscussed. A method for evaluating the spectral resolution precision has beenintroduced. Two kinds of definition principles, which are using the noise level as thereference and using spectral maximum as the reference, have been proposed. A newconcept of standard signal-to-noise ratio has been defined to evaluate the performanceof different systems quantitatively.
2. The quality factors occurred during the generation and detection parts of THz-TDS have been analyzed according to the current surge model, some of whichhave been studied experimentally. The optimized parameters have been acquired fromthe experiments.
3. The quality factors occurred in the transmission part have been analyzedaccording to the principles of weak signal detection. The optimized parameters havebeen acquired from the experiments on the chopper and lock-in amplifier.
4. The advantages and disadvantages of program flow P1and P2have beencompared. According to the result, an optimum THz signal acquiring program flowand data format have been determined by combining the advantages together. Itsvalidity has been proved by experiment.
5. Using the optimum parameters, a new CD552lock-in amplifying circuit hasbeen designed to replace the former THz transmission part which uses the commerciallock-in amplifier SR830. The hardware resource is used more effectively.
6. The data acquiring and processing program in the host computer is alsoimproved. The multi-page structure design considering the demands of users atdifferent professional levels makes the interface more user-centered. The addedprogram for spectral characteristics calculation makes the software with highpracticability.
7. The melamine powder sample has been tested on the improved THz-TDSsystem. Three peak absorptions, which are at2.00,2.25and2.60THz, respectively, inthe THz range have been observed. The first two pesks have been reported by otherresearchers during their THz experiments while the3rd is not. The3rd spectrum isalso proved to be the essential characteristic of melamine with the help of the softwarefor vibrational and rotational spectra simulation of chemicals used in quantumchemistry. Also, the simulation result indicates that some spectral characters ofmelamine can not be distinguished under the0.05THz spectrum resolution precision,which set a still higher demand of THz-TDS system.
课题的研究内容和成果可以归纳为以下7个方面：
1.讨论了目前对太赫兹脉冲时域光谱技术的三个主要性能参数定义的规范问题；为频谱分辨精度的评估提出了新方法；对于频谱范围，提出了以噪声限为基准和频谱最大值为基准两种定义原则。对于信噪比参数，提出了标准信噪比的概念，可以将不同的系统性能进行统一量化评价。是太赫兹仪器标准化研究十分必要的工作
2.采用电流瞬冲模型，分析了用飞秒脉冲激光激发光电导天线产生和探测太赫兹时域脉冲信号环节存在的影响信号品质的因素，即环境常数、光电导天线结构参数、飞秒脉冲激光的功率和激发天线两级间偏置电压，并对其中飞秒激光功率和偏置电压这两个重要的因素进行了进一步的实验研究。通过实验，分析找到了针对于天津大学太赫兹中心现有的太赫兹时域光谱系统，在飞秒脉冲激光功率值和激发天线两级间偏置电压值的最优参数配置，大大优化了该系统的性能。
3.针对以相关器模型为基础的锁相放大技术原理，分析了太赫兹时域信号传输环节存在的影响信号品质的因素，即斩波器的频率与放置位置和锁相放大器的低通时间常数，并进行了进一步实验研究。以此，从实验结果中找到了针对于天津大学太赫兹中心现有的太赫兹时域光谱系统，在斩波器的频率与放置位置和锁相放大器的低通时间常数方面的最优参数配置。
4.分析比较了2个不同实验室的太赫兹光谱系统的控制、信号的采集、数据处理的计算机软件P1和P2的流程和数据处理形式，发现了两种不同流程的优缺点。如软件P1相对于P2流程，P1流程的优点是硬件利用率高，缺点是单数据点采集时间长。我们结合了软件P1和软件程序P2的优点，编写了新的系统软件，完成了对流程和数据保存格式的优化改进，并通过实验验证了其合理性。
5.以较优配置参数自行设计了CD552锁相放大电路，代替了原有的以商业化锁相放大器SR830为核心的太赫兹信号采集环节。该技术已获得了技术专利，大大提高了系统硬件的集成化、小型化的水平。对于上位机控制和数据处理程序也进行了进一步改进。
6.兼顾不同层次使用者应用的需要，对系统控制和处理程序的进行了改进，完成了分页面显示的设计；增加的光谱特性计算程序；使现有的太赫兹光谱软件更具实用性，显示界面更为人性化。
7.应用改进后的太赫兹时域光谱系统，对三聚氰胺粉末样品进行了光谱探测实验。在太赫兹波段得到了2.00、2.25、2.60THz处三个特征吸收峰。其中，前两个吸收峰为其他太赫兹研究者已报道过的，但2.60THz处的第三个峰，在我们之前是未见报道过的。2.60THz处的特征峰，我们用量子化学振动转动谱软件模拟证明了，三聚氰胺确实存在这一本质特征。同时，理论模拟结果也预示出现有的实际太赫兹光谱技术的局限性，即太赫兹光谱，对于三聚氰胺样品其光谱的分辨精度只能达到0.05THz。在该精度下只有较强的物质特征峰方能被探测到。因此，也为提高太赫兹时域光谱仪的性能，提出的更高的科学与技术的要求。
Terahertz time-domain spectroscopy (THz-TDS) is a powerful tool toinvestigation spectroscopic properties of matters at THz frequencies. The invention ofTHz-TDS not only delivers a wide band radiation source in the THz range, but alsofills up the “THz Gap” in spectroscopy. With the development of THz-TDS, studies ofsubstance characteristics have spread into various of fields, such as biology, chemistry,medicine, and security. So the realistic applications of THz-TDS technology havebecome one of the main development trends. By focusing on this task in my PhDwork, several issues on standardized definition of the main parameters to measure thesignal quality of THz-TDS have been pursued for the first time. Based on thestandardized parameters defined here, factors appeared in the THz signal generation,detection, transmission and data acquisition have been studied and their influencerules to the signal qualities have been explored systematically according to theexperiments. By the optimized parameters acquired in former experiments, the THzsignal acquiring and processing parts have been improved to increase the hardwareresource utilization ratio and the effective information extraction ratio. At last, themelamine powder has been characterized using the improved THz-TDS system. Anew absorption peak never been reported has been identified.
The main achievements of this PhD work are listed in the following:
1. Issues on the definition of three main parameters in THz-TDS have beendiscussed. A method for evaluating the spectral resolution precision has beenintroduced. Two kinds of definition principles, which are using the noise level as thereference and using spectral maximum as the reference, have been proposed. A newconcept of standard signal-to-noise ratio has been defined to evaluate the performanceof different systems quantitatively.
2. The quality factors occurred during the generation and detection parts of THz-TDS have been analyzed according to the current surge model, some of whichhave been studied experimentally. The optimized parameters have been acquired fromthe experiments.
3. The quality factors occurred in the transmission part have been analyzedaccording to the principles of weak signal detection. The optimized parameters havebeen acquired from the experiments on the chopper and lock-in amplifier.
4. The advantages and disadvantages of program flow P1and P2have beencompared. According to the result, an optimum THz signal acquiring program flowand data format have been determined by combining the advantages together. Itsvalidity has been proved by experiment.
5. Using the optimum parameters, a new CD552lock-in amplifying circuit hasbeen designed to replace the former THz transmission part which uses the commerciallock-in amplifier SR830. The hardware resource is used more effectively.
6. The data acquiring and processing program in the host computer is alsoimproved. The multi-page structure design considering the demands of users atdifferent professional levels makes the interface more user-centered. The addedprogram for spectral characteristics calculation makes the software with highpracticability.
7. The melamine powder sample has been tested on the improved THz-TDSsystem. Three peak absorptions, which are at2.00,2.25and2.60THz, respectively, inthe THz range have been observed. The first two pesks have been reported by otherresearchers during their THz experiments while the3rd is not. The3rd spectrum isalso proved to be the essential characteristic of melamine with the help of the softwarefor vibrational and rotational spectra simulation of chemicals used in quantumchemistry. Also, the simulation result indicates that some spectral characters ofmelamine can not be distinguished under the0.05THz spectrum resolution precision,which set a still higher demand of THz-TDS system.
引文
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