Research On Ultra - High Temperature Sensing Based On Thermal Reproduction Fiber Bragg Grating

Fiber Bragg gratings (FBGs) have been widely used as fiber-optic communication and sensors due to their intrinsic advantages. However, the periodic modulation of refractive index (grating pitch) induced by UV light exposure can be significantly erased at the temperature higher than 350℃, which limits the FBG’s applications under high temperature. Regenerated grating (RG) takes the advantage of a simple thermal process and has successfully achieved an operation temperature up to 1000℃ through tailoring the fiber core dopants.In this thesis, grating regeneration was studied and the main work is as followed:We present a sensitivity-improved fiber-optic strain sensor using an etched and regenerated fiber Bragg grating (ER-FBG) suitable for a large range of temperature measurements. The process of chemical etching (from 125μm to 60μm) provides regenerated gratings (at a temperature of 680℃) with a stronger reflective intensity (from 43.7%to 69.8%), together with an improved and linear strain sensitivity (from 0.9 pm/με to 4.5 pm/με) over a large temperature range (from room temperature to 800 ℃), making it a useful strain sensor for high temperature environments.A pair of regenerated gratings (RGs) with Bragg wavelengths of 1547 nm and 1304 nm in a single piece of single mode fiber has been fabricated for simultaneous monitoring of temperature and strain in ultra-high temperature (UHT) environment. The proposed structure exhibits the deviation of 28.3με and 4.1℃ at the ranges of 0-1000με and 25-900℃, respectively.In this chapter, a pair of regenerated gratings has been used for simultaneous measurement of strain and ultra-high temperatures up to 800 ℃. Two regenerated gratings with different temperature sensitivities have been produced in germanosilicate and boron-codoped germanosilicate fibers, respectively, meanwhile, one of them is chemical etched to reduce the fiber diameter for achievement the larger strain sensitivity. The proposed structure has RMS in measured strain and temperature of 11.9 and 4.6℃ at the ranges of 0-1000με and 20-800℃ respectively.The feasibility of thermal regeneration of chirped fiber Bragg gratings inscribed in commercial single-mode fibers (SMF) using a 248 nm excimer laser has been experimentally demonstrated. Regenerated chirped gratings (RCGs) with a strong and symmetrical reflection profile can be achieved by 80 minutes of thermal treatment at temperature of 800℃. Experimental temperature tests show that the RCGs provide a stable and reproducible spectral response (both in bandwidth and in reflective intensity) over the temperature range from 25℃ to 1000℃, indicating their good potentials for fiber laser and sensing under high temperature environment.