Strain-free FBG sensors for accurate temperature measurements from cryogenic temperatures to 300 degrees C

Abstract

Fiber Bragg gratings (FBGs) are well-known optical sensors, which have been widely used to perform temperature and strain measurements. Due to the cross-sensitivities of FBGs to both temperature and axial strain changes, using these fiber sensors for high-accuracy temperature measurements remained questionable. This paper presents an FBG sensor packaging technique that produces strain-free, multiplexable fiber temperature sensors. Using a precision CO2 laser heating process, a low-loss and mechanically robust fiber taper is formed near the FBG sensor, which relieves potential axial strain influence on FBG's temperature measurements. FBG sensors with tapered junctions were housed in a two-hole PEEK tube. The entire structure is then inserted into a thicker hollow PEEK tubing and welded in place. This design protects the fiber sensor from mechanical breakage and isolates it from external stress. This paper reports highly accurate temperature measurements from 77k to 567k. It presents a viable approach to developing multiplexable temperature sensors for cryogenic environment applications.