Research On The Cryogenic Refractive Index Measuring Technology For IR Materials

Nowadays, the cryogenic optical systems are widely used for IR detection,especially for weak IR signal. In the past, most of the cryogenic optical systemsare reflective. It is very difficult to design refractive cryogenic optical systemsbecause there is lack of cryogenic, infrared and refractive index data. Thereflective cryogenic optical systems have played a significant role in infrareddetection tasks for a long time. As the rapid development of IR detection, thereflective cryogenic optical systems cannot meet the need of tasks. It is the verytime for developing the refractive cryogenic optical systems. One of the keypoints is the cryogenic refractive index of IR materials. Many efforts have beendone to study the cryogenic refractive index of IR materials, but none is open topublic, so, to research on the cryogenic refractive index measuring technologyresearch is a very important foundation for the development of cryogenicrefractive optical system.Compared with the common refractive index measurement, the cryogenicindex measurement is more challenging. First, the cryogenic temperature isequal to120K, which needs a vacuum environment to maintain such a lowtemperature. Second, the sample material would endure a big change oftemperature, which may cause the deformation or movement of the sample.Third, it is a problem to keep the temperature of sample materials at the settled point. Fourth, it is a problem to rotate the sample prism at the vacuumenvironment.A sample chamber is designed to solve the problem of temperature detectingand setting of sample prism. The mathematical model of sample chamber isalso acquired. A FEM analysis model is used to get the temperature field andstress of sample chamber and sample prism at the cryogenic temperature. Theerror sources are to be analyzed. The final experiments’ results coincide withthe previous analysis. The cryogenic refractive index of fused Silica is veryclose to the NASA’s result, and the error is about1.6x10-5. It is clear that themethod meet the demand of cryogenic measurement.