A method of accelerating the response of thermopile detectors is proposed for optical power measurements. The physical model of the detectors is established, and the theoretical response is derived. The response is not completely described by a single exponential function. The difference between the conventional latency compensation method assuming a single exponential leads to overshoot and deviation from the equilibrium state. A new method with second-order differential operations is proposed, which is more appropriate for the non -exponential responses. In simulations, the second-order method theoretically shows 67% improvement in the measuring time. Experiments based on fabricated copper-substrate detectors show 43%, 65%, and 56% reduction in the measurement time compared with the conventional method, under the deviation ranges of +/- 1%, +/- 0.5%, and +/- 0.3%, respectively. Since the method is based on the adiabatic mechanism that generally exists in the light absorbing process, it is compatible with various types of calorimetric detectors.

In this article, we propose an electrical substitution and laser calibration technique for reproduction of terahertz radiant power. With this technique, the equivalence of terahertz absorption and electrical heating in the absorber is corrected and accurate terahertz radiant power is reproduced. A two-cavity absolute radiometer was developed for accurate reproduction of terahertz radiant power working at room temperature. The absolute radiometer has two absorptive cavities coated with different coatings. One cavity was coated with 3M Velvet-coating and used for the absolute reproduction of laser radiant power, and the other was coated with a mixture of silicon carbide particles and 3M Velvet-coating and used for the reproduction of terahertz radiant power. With the comparison of the two cavities under stable laser irradiance, the equivalence between terahertz absorption and electrical heating is calibrated, and the precision of absolute reproduction for terahertz power is improved. The reproduced power using this absolute radiometer agreed excellently with the terahertz power standard at the National Institute of Metrology, which had been used in the first international terahertz power comparison.

Frequency is one of the key parameters of terahertz radiation, and accurate and traceable measurement of terahertz frequency is crucial for research and applications of terahertz technology. In order to achieve accurate measurement and traceability of terahertz laser wavelength, we have developed a terahertz interferometer to measure the wavelength and frequency of terahertz radiation sources. We constructed Fabry-Perot and Michelson interferometers to measure the wavelength and frequency of terahertz radiation sources, including backward wave oscillator terahertz sources and terahertz frequency multipliers. We propose using high-resistivity silicon plates as terahertz beam splitters, which not only have a simple structure, but also enable wide-band wavelength and frequency measurements. Using a single silicon plate as the beam splitter, we achieved ideal measurement results across a wide frequency range from 90 GHz to 800 GHz. By performing Fourier transforms on the measured terahertz interference patterns, we obtained terahertz frequency information. We calibrated the two terahertz interferometers at frequencies of 100 GHz and 300 GHz using terahertz frequency combs, enabling accurate measurement and correction of the radiation wavelength from the terahertz sources. This allowed traceability of the interferometer measurement results to the International System of Units (SI). We also performed uncertainty analysis on the measurements. Terahertz interferometry provides a simple and convenient approach to measuring terahertz wavelengths, and will find wide applications in terahertz metrology.

Terahertz technology has been widely used in Chemical component recognition, high-speed communication, and security check imaging. Terahertz metrology plays an important role in all of these applications. The progress of terahertz spectrometers calibration and terahertz radiometry at the National Institute of Metrology, China, is introduced in this paper. A technique for calibration of terahertz time-domain spectrometers is introduced. The terahertz timedomain spectrometer is online calibrated with the terahertz echo pulse, and the measurement repeatability is improved. A high absorbance coating fabricated and a terahertz radiometer is developed. Broadband terahertz radiation is highly absorbed and accurate measured with this terahertz radiometer. A portable terahertz power is fabricated for measurement of terahertz sources and calibration of terahertz power meters.

A technique is presented to amplify the voltage response of thermopile optical detectors. It is based on the parallel combination of multiple operational amplifiers. The background noise of the voltage response has been deceased, which significantly improves the repeatability of the measurement results. Thus, this technique enables the same detector to measure weaker optical power or irradiance at the same precision level. The corresponding amplifying circuit is designed and fabricated. For the same detector, the experimental results show that the standard deviation of the background noise of the combined n op-amps is about 1/n lower comparing with the conventional single op-amp scheme, which is consistent with the theoretical expectations. Furthermore, the lasers of 10 μW and 1μW were also measured by the specific detector and the amplifier circuit. For a 10 μW response, the measurement repeatability of the 8-combined op-amps is about 1.4%, which is better than the 3.3% of a single op-amp. For 1 μW laser, the measurement result of voltage response of the 8-combined op-amps can be precisely quantified; however, the result of the single op-amp is hardly distinguished. The presented technique based on multiple op-amps is practical and can be potential in many applications. We hope this technique could offer help for expanding the measurement ranges of thermopile optical detectors at weaker optical power and irradiance.

光辐射计量基标准是国家计量体系的重要组成部分,是相关量值的溯源源头.光辐射计量正在朝着量值复现量子化、计量基标准量值传递扁平化方向发展. 根据发展趋势,中国计量科学研究院牵头承担了国家重点研发计划"光辐射计量基标准研究"项目(项目编号:2016YFF0200300),在光辐射计量基标准体系中的波长、功率以及它们与时间、空间量组合的量值方面开展研究.项目研究了光辐射测量量子化技术、高稳定度激光波长复现技术、高温固定点黑体辐射技术、光腔衰荡法气体成份测量技术、太赫兹关键参数计量技术.

提出了一种太赫兹阵列探测器响应度校准溯源方法.首先,使用阵列探测器的每一像元对太赫兹辐照场中心进行逐一扫描,识别有效像元、死像元和过热像元,并对有效像元响应值进行归一化处理,获得阵列探测器的相对辐照度响应值.其次,使用中心像元对太赫兹辐照场进行扫描测量,扫描总面积大于太赫兹光斑尺寸,保证太赫兹功率被完整测量,获得中心像元与其他有效像元的太赫兹功率响应值.最后,用标准太赫兹功率计标定高莱功率计,扩展太赫兹功率校准量限,实现微瓦级太赫兹辐射功率测量溯源,用高莱功率计测得的太赫兹辐射源总功率对阵列探测器测得的积分响应值进行校准,得到阵列探测器辐照度响应绝对值.对探测器进行测量校准和不确定度分析,测得辐照度响应度的相对扩展不确定度为Urel=20％(k=2),测量结果可溯源至国家太赫兹辐射功率标准.

Abstract. A laser power scale transfer technique was developed based on shape detection; using this technique, detectors can be precisely centered on laser beams, thereby significantly reducing measurement errors introduced by detector nonuniformities and misalignments. A modified imaging scheme was designed and embedded in the scale transfer setup, and the effective resolution was proven to be better than 100  μm, compatible for incident lasers with diameters under 15 mm. For different types of detectors, absolute positioning was realized using objective algorithms with repeatability errors of 7 to 13  μm, which is an improvement of over 30 times the limitation of human-vision-based positioning. Correspondingly, the relative measurement error caused by detector non-uniformity for a typical calibration was reduced by a factor of 19–40 in the scale transfer process. The proposed technique thus has the potential for detector-based optical metrological applications that are influenced by geometric accuracy.

A broadband THz metamaterial absorber was designed based on the transmission line theory, and the inductive mesh structure was selected as the meta-surface. The absorber samples were fabricated by electrohydrodynamics(EHD)-based printing technology, which is a cost-effective and high-precision technology for flexible electronic device fabrication. The absorption in (98-353) GHz exceeded 90%, and the experimental data matched well with the theoretical and simulation results. It was also proven that the absorption spectra were insensitive to the linewidth and surface resistance of the inductive mesh structure, so that the design has high tolerance to possible fabrication error.

Terahertz time-domain spectroscopy (THz-TDS) technology is widely used in many fields such as material composition identification, explosive detection, drug and drug composition analysis, and medical diagnosis. The traditional THzTDS uses a Ti; sapphire femtosecond laser as a light source, which is bulky and costly, and limits the large-scale application of THz-TDS. Using a fiber femtosecond laser combined with a fiber-coupled terahertz photoconductive antenna, the THz-TDS system can be designed to be very compact and flexible, while eliminating the need for a free-space optical path, greatly reducing the number of optical mounts The influence of external environment on the signal such as vibration has great application potential in industry and in the field. In this paper, a fiber-type THz-TDS system is designed and developed, and the three subsystems of optics, electricity and software are briefly introduced. The femtosecond pulse width is controlled by fiber dispersion management, so that the femtosecond pulse width of the terahertz photoconductive antenna is kept at about 50 fs, thereby eliminating the terahertz time domain pulse broadening caused by femtosecond pulse broadening. By precisely controlling the polarization state of the femtosecond laser, the polarization direction of the pump laser and the detection laser is kept parallel with the fast or slow axis of the polarization maintaining fiber, thereby eliminating the splitting phenomenon of the terahertz time domain pulse and obtaining the signal to noise ratio. Better than 12 000 single-pulse terahertz time domain waveforms. The variable angle optical path structure design enables easy switching of terahertz transmission spectrum measurement and reflection spectrum measurement, as well as the measurement of variable angle terahertz spectrum.

太赫兹作为新的技术手段在物质成分识别、高速通信、生物医学、安检成像和军事国防等领域具有重要的作用.太赫兹技术的各种应用都建立在对太赫兹本身及其与物质相互作用测量的基础上,因此准确的太赫兹参数测量及相关量值溯源是太赫兹应用的技术支撑和保障.介绍中国计量科学研究院在太赫兹辐射参数计量标准研究中形成的测量技术和溯源方法、研制的测量仪器和测量装置、制定的计量校准法规和建立的计量标准装置,对太赫兹辐射时域、频域、空域和强度等参数给出了量值溯源传递图并进行了测量不确定度分析.提出的太赫兹计量方法和标准装置可保障太赫兹技术研究和应用中的量值可靠,也可为其他太赫兹参数的测量提供参考.

Terahertz metrology plays an important role in terahertz applications. National Institute of Metrology carried out research on measurement standards for key parameters traceable measurement of terahertz radiation. The measurement technology and measurement standards for temporal, spectral, spatial and radiant parameters of terahertz are introduced, and Traceable measurement of terahertz spectrum, power and frequency in this paper.

Terahertz frequency comb, which is generated from femtosecond frequency comb, is applied to measure the spatial radiant power density of terahertz source. Two technologies, electro-optic sampling and photoconductive detection, are applied to generate terahertz frequency comb, and the spatial power density of a 100 GHz source is measured with both of these two techniques. The total radiant power is traceable to a standard terahertz radiometer, and the absolute power density of the terahertz source is obtained. The measurement results with both of the two techniques, electro-optic sampling with 800 nm femtosecond optical pulse in free space and photoconductive antenna detection with 1 550 nm femtosecond optical pulse in fiber, are analyzed and compared. Moreover, the spatial power density of the terahertz radiation at different distance away from the terahertz source are measured, and the involvement of the terahertz radiation in free space is experimentally studied.

A fiber-type tcrahcrtz (THz) time-domain spectrometer is designed by combining a fiber femtosecond laser with a fiber-coupled THz photoconductive antenna. The effect of laser polarization on the THz time-domain waveform, intensity, and spectral characteristics arc studied experimentally. Via the precise control and the optimization of the polarization state of the femtosecond laser, the time-domain pulse splitting is eliminated, and a single-peak THz time-domain pulse with a high signal-to-noise ratio is obtained.

To characterize a wide-band and high-absorption terahertz radiometer, it is necessary to study the properties of the absorbing coating material. First, we simulated the absorption rates of common absorbing materials to find materials that have high absorption in terahertz range. Further, we mixed silicon carbide and 3M black lacquer to increase the absorption rate of the coating. In addition, we simulated the variation in the internal particle size of the coating. Finally, the mixed coating was prepared based on the simulation results, and the samples were measured using the terahertz time-domain spectrometer. The measurement results show that the spectral absorbance of the mixed coating is greater than 0.99, which is basically consistent with the simulation results.

A fiber-type terahertz time-domain spectrometer was designed by combining fiber femtosecond laser with fiber-coupled photoconductive antenna. The effects of laser polarization on the terahertz time-domain waveform, intensity and spectral characteristics are studied experimentally. By precise control and optimization of the polarization state of the femtosecond laser, the time-domain pulse splitting phenomenon is eliminated, and a single peak time-domain pulse with high signal-to-noise ratio is obtained.

A fiber-type terahertz time-domain spectrometer was developed by combining fiber femtosecond laser with fiber-coupled terahertz photoconductive antenna. And variable angle terahertz reflectance of high-resistance silicon wafer and composite absorbent material was measured using this spectrometer. The measurement results are in good agreement with the theoretical calculation results.

Broadband THz metamaterial absorbers have important applications in terahertz modulation, imaging, and communication, but current designs usually involve complex nesting or stacking resonant structures. This paper reports a single-resonant-structure and optically transparent broadband terahertz metamaterial absorber. The absorber demonstrates > 80% broadband absorption in a wide frequency range of 0.4~1.04 THz, or 88.9% of the central frequency. The thickness of the absorber is only 1/15 of the wavelength corresponding to the lowest absorption frequency. The absorption spectrum has been experimentally verified with a reflective terahertz time-domain spectrometer. Further study shows that the absorber is insensitive to the polarization angle, and the absorbance decreases very little for incident angles less than 30°. The absorption spectrum can be finely tuned by varying the structure parameters.

This paper proposes a method for identifying organic compounds by applying a differential principal component-analysis (PCA)-support-vector-machine (SVM) to the tcrahcrtz time-domain spectral data. First, the tcrahcrtz absorption spectrum is calculated according to the tcrahcrtz time-domain signal of the material sample; then, the features of the data in the frequency range of 0.2-2.5 THz arc extracted. During the feature extraction, an expansion -of-sample-size method based on differential data is proposed and combined with the PCA method to achieve the feature extraction. Finally, the SVM is used to establish a mathematical model for the corresponding relationship between the extracted features and the material category, and the unknown samples arc identified according to this model. The tcrahcrtz-spectral data of 15 organic compounds arc identified using the proposed method, and the correct recognition rate is 93.33%. The experimental results show that the correct recognition rate of organic compounds by the proposed method is the highest when compared with those by the linear-discriminant analysis method and the absorption peak frequency-amplitude method.

Terahertz (THz) devices, including the THz broadband absorbers, have attracted much attention in the recent decade. However, due to the micron-level precision requirement, the mainstream fabrication technology for THz devices still rely on complex and expensive photolithography or nanoimprinting techniques. Electrohydrodynamics (EHD) printing technology offers a novel, convenient, and cost-effective solution to this issue. A broadband THz absorber with over 90 absorption in the range of 98-353GHz was designed and fabricated by EHD printing technology, and the inductive fishnet grid structure was used for the metasurface. The equivalent circuit model of the absorber was established based on the transmission line theory and impedance matching theory. Simulation results matched well with the theoretical and experimental results, and further demonstrated that the absorbers were polarization insensitive. Furthermore, both simulation and experiments proved that the absorption spectrum was insensitive to the fishnet grid linewidth and surface resistance, i.e., the design has high tolerance to possible fabrication errors. Therefore, the EHD printing technology can be a promising fabrication method of THz absorbers for future mass production.