基于光线追迹法的低温辐射计光吸收腔设计

当前，工作在液氦温度的低温辐射计可以有效规避电路系统中非自发加热带来的误差，是国际上精度最高的光功率计量设备。理想低温辐射计在工作过程中，其核心器件-吸收腔对相同的热功率与电功率应当表现出相同的温升。然而对于实际情况，由于吸收腔涂层中复杂的光-物质相互作用，系统的光-电加热路径难以重合，黑体腔热传导分布的梯度差异导致误差的产生。当前国际上对光电不等效性产生的影响仍缺乏直观清晰的认知。在此，利用蒙特卡洛光线追迹方法，文中对低温辐射计吸收腔辐照度的空间分布进行了仿真。计算表明:当吸收腔斜底角控制在60°，涂层吸收率达到0.95时，系统在激光进入的第一次与第二次反射中分别吸收了98%与1.9%的能量，比例约为51.2∶1。通过在吸收腔斜底板和下侧面同时布置加热器，可实现光加热、电加热路径的耦合。进一步地，通过分别计算单加热器与双加热器布置下系统温度随时间的变化，文中证明了加热路径的不同将引入约为0.005%的光电不等效性。

Design of the absorption cavity in the cryogenic radiometer based on the ray-tracing method

Based on the equivalent electrical and optical heating process, radiometers are employed for the metrology of irradiation powers. Working at the liquid helium temperature, the cryogenic radiometer is designed to reduce the nonspontaneous heating by the electric components in the system and is thus currently the most accurate irradiation power metrology measurement facility. During the calibration process of an ideal cryogenic radiometer, the core device-absorption cavity should demonstrate an equivalent temperature increase for the same optical and electrical heating power. However, practical heating routines lack equivalency due to the divergence in the temperature gradient from the complicated optical-matter interactions in black coatings. Herein, utilizing the ray-tracing method, we investigate the tilting angle-dependent spatial optical field distribution in the absorption cavity. With the inclined base angle of the absorption chamber controlled at 60° and the absorption rate of the coating reaching 0.95, the energy of the laser is absorbed in the first and second reflection processes of 98% and 1.9%, respectively, with a ratio of 51.2∶1. The coincidence of the optical and electrical heating paths could thus be realized by placing heaters simultaneously on the inclined bottom plate and the lower side of the absorption cavity. Furthermore, by calculating the time-dependent system temperature with a single inclined bottom heater and calculating the double heater arrangement, an optical-electrical nonequivalency induced by the different heating paths of approximately 0.005% is indicated. Our method constructs an equivalent heating routine for optical and electronic sources, indicating a nonequivalency of 0.005% induced by the different arrangements of heaters. Multiheaters applied with delicate power are recommended to optimize the temperature discrepancy.