Femtosecond laser inscribed high temperature fiber Bragg grating sensors

摘要

Silica-based fiber Bragg gratings (FBG) sensors are versatile devices that are typically fabricated using UV laser exposure. Their applicability is restricted to temperatures < 600℃ because of the erasure of the UV generated grating structure at higher temperature. FBGs made with femtosecond IR lasers and phase masks in standard single mode fiber can have high long-term thermal stability at 1000℃. Above 1000℃ however, the silica undergoes structural transformations that limit the functionality of the fiber. The most successful optical fiber used for high temperature sensor applications is the single crystal sapphire fiber, which has a glass transition temperature of 2030℃. Here we present our work on retro-reflective FBGs fabricated in single mode silica and multimode sapphire fiber. For sapphire fiber Bragg gratings (SFBG), no degradation of the grating strength at high temperature was observed when tested up to 1500℃. The SFBGs have discrete resonant wavelengths that could be used potentially as distributed optical sensor arrays up to 2000℃. To produce a single mode response, the multimode SFBGs were probed using tapered single mode fibers producing single and low order mode reflection/transmission responses. The taper coupling improved the spectral resolution of the sapphire FBG as compared to its multimode responses.