FBG in coreless silica fiber for high temperature measurements

The paper demonstrates for the first time the possibility of measuring high temperatures up to 1200 °C using an FBG with central wavelength 985 nm formed in a coreless pure silica fiber using the femtosecond point-by-point FBG-writing technique. In our case, the length of the coreless fiber from the FBG to the connection with the multimode transport fiber was not less than 50 cm, and average temperature sensitivity was about 9 pm/°C. It is shown that such an FBG remains stable at temperatures up to 1150 °C for at least an hour. At 1200 °C, degradation of the reflectivity was observed. The use of a fully multimode interrogation scheme made it possible to significantly increase the intensity of reflected light and reduce the effect of mode interference on the obtained FBG spectrum, and the choice of a spectral range of up to 1100 nm makes it possible to use relatively inexpensive spectral devices based on silicon photodetectors.