Improving FBG sensor sensitivity at cryogenic temperature by metal coating

Commercially available fiber Bragg grating (FBG) sensors cannot be used for measuring cryogenic temperatures because they are made of silica the thermal expansion coefficient of which tends to zero when approaching 4 K. Because of the many advantages of fiber optic sensors with respect to conventional ones, in this paper it is shown how to circumvent such a limitation by applying a proper metal coating. This approach drastically increases temperature measuring capability of FBGs at cryogenic environments typically encountered in application involving liquid gases or in space. Various metals have been deposited by electrowinning on the external fiber surface previously treated with an aluminum precoating. Also, a special casting process has been developed. The explored temperature region was 4.2-40 K. The paper reports the characterization of FBG sensors coated with different metals and shows the validity of this new temperature sensor with respect to conventional ones.