Performance test of FBG and bimetal for very low temperature thermometer use

In accelerator institutes which adapt superconducting acceleration scheme, several refrigerants, like liquid helium, are involved in its operation. For the accelerator's stable operation, it is necessary to control all refrigerant's flow condition by monitoring temperatures with thermometers. For this purpose, the thermometer must be possessed an ability to correctly measure the very low temperature. A new idea to construct the thermometer for very low temperature measurements was proposed and now its R and D activities are undergoing. Important constituents of the new thermometer are FBG which equipped gratings on the fiber core region and bimetal. So far, each component has been evaluated own mechanical or optical properties in various environment. In this report, several obtained results are described. (author)