Novel optical scheme based on cryogenic radiometer for successive calibration
Y-type calibration optical path of cryogenic radiometer (Not including the Brewster window)
High accuracy of radiometric calibration is a prerequisite and guarantee for quantitative remote sensing. The traditional calibration method based on radiation source could not meet the requirement for further application of remote sensing. Cryogenic radiometer is internationally recognized as a primary standard of light radiation with the highest accuracy, thus the radiation standard transfer chain which could be traced to cryogenic radiometer laid the foundation for quantitative remote sensing.
It has been demonstrated that the reflectance and the scattering effect of Brewster window plays a significant role in characterizing the absolute power of laser. As to the successive calibration in wide spectrum, the wavelength of the output beam from the tunable laser varies as the polarization state changes, thus it is essential to accurately measure the polarization state and carry out repeated adjusting in different wavelengths. However, the workload of this traditional experimental scheme is too large, and meanwhile the uncertainty increases due to the low repeatability of the optical path.
Researchers led by Prof. Xiaobin Zheng, from Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences designed and made use of a novel calibration optical path with the standard transfer detector which was placed inside the vacuum unit. This scheme theoretically eliminated the uncertainty caused by the window transmittance. It is reported in Chinese Optics Letters Vol.13, No.5, 2015.
In this study, a fine instrument with automatic reset technology was used to increase the accuracy of the cryogenic radiometer and standard transfer detector switching into the optical path, therefore, the uncertainty caused by spatial non-uniformity decreased. Two calibration optical paths, covering visible and infrared spectrum respectively, have been established, in which beam shaping, spatial filtering power stabling, and stray light suppression technology were put into use.
The novel optical scheme based on cryogenic radiometer for successive calibration in broad spectrum was introduced, which could efficiently reduce the window transmittance of cryogenic radiometer and therefore be used as a new calibration technique in this spectrum. At the meantime, the feasibility of cryogenic radiometer was improved. With the uncertainty being better than 10-4, this research result becomes a powerful support for the quantitative traceability of radiation standard.
Dr. Weiwei Pang, the first author of this article, believes that the improvement of the accuracy of the radiation standard and the standard transfer system, reduction of the standard transfer chain and the final enhancement of application level of radiation calibration for remote sensor would be urgent subjects worldwide.
As a matter of fact, the successive calibration experiments in ultraviolet-intermediate infrared band has not been carried out, especially those in infrared spectrum. In the next step, the research team would focus on the calibration process in the above band. Currently, a tunable laser has been equipped in their lab which covers the wavelength 350 nm~4.7 μm. Through the comparison of the cryogenic radiometer and synchrotron radiation source, the validation of the two methods to each other would be accomplished, which loads the foundation for a new technology of high precision radiation standard and transfer detector in ultraviolet band.
真空传递定标光提高低温辐射计测量的准确度
图片说明：低温辐射计Y型定标光路系统（不含布儒斯特窗口）
高精度的辐射定标是遥感定量化的前提和保障。基于辐射源的星载传感器定标的精度已难以满足目前遥感应用需求。低温辐射计是目前国际公认精度最高的光辐射功率基准，建立可以溯源至低温辐射计的辐射标准传递链路是目前辐射度学和遥感定量化的基础。
研究表明，布儒斯特窗口的反射和内部散射是低温辐射计测量激光的绝对功率最大的不确定度因素。对于宽谱段连续定标，调谐激光器出射波长的变化伴随着偏振状态的变化，需要重复地进行不同波长的光束偏振状态精密测试和调整，这些调整将带来巨大的工作量，同时严重影响测量光路的可重复性，增大了传递探测器定标不确定度。
中国科学院安徽光学精密机械研究所的郑小兵研究员课题组设计和应用新型的真空仓内传递探测器定标光路，从原理上消除窗口透过率测量这一最大不确定性因素。相关实验结果发表在Chinese Optics Letters 2015年第5期上。
实验采用自动精密复位装置，提高低温辐射计和传递探测器切入光束的位置重复准确性，降低空间非均匀性引入的不确定度。搭建了可见和红外两个谱段的独立定标光路，优化了光束整形、空间滤波、功率稳定和杂散光抑制等中间光路。
这一宽谱段连续定标的低温辐射计新型光路设计方案，可有效解决低温辐射计的窗口透过率评价技术难题，为准确和高效率地实现宽谱段定标提供了一种新型的测量技术手段，提升了低温辐射计应用的可行性。将为建立和应用不确定度优于10-4量级的新一代辐射基准
该论文第一作者庞伟伟博士认为，提高和改进辐射基准与标准传递系统的精度，有效缩短标准传递的链路，最终提高遥感传感器等工程应用辐射定标精度的水平，是迫切需要解决的问题。
鉴于紫外~中红外谱段连续光谱定标、特别是在红外波段相对于低温辐射计的连续谱段定标研究工作尚未开展，未来这支科研团队将进行该谱段的连续光谱定标实验。目前，该实验室已配置波长覆盖范围约350 nm~4.7 μm的可调谐激光器。通过开展低温辐射计和同步辐射源之间的比对实验研究，实现两种不同工作体制辐射基准的相互验证，为紫外谱段高精度辐射计基准和传递探测器探索新的技术途径。