Fiber Bragg Grating Thermometry

Fiber Bragg Grating (FBG) thermometry uses optical fiber sensors to measure temperature by detecting shifts in the Bragg wavelength, offering high sensitivity and immunity to electromagnetic interference.Principle of OperationA Fiber Bragg Grating is a segment of optical fiber with a periodic variation in the refractive index of its core, which reflects specific wavelengths of light while transmitting others . The reflected wavelength, known as the Bragg wavelength, depends on the grating period and the effective refractive index of the fiber core. When the surrounding temperature changes, the fiber undergoes thermal expansion and the refractive index changes due to the thermo-optic effect, causing a measurable shift in the Bragg wavelength . This shift is used to determine the temperature with high precision.Temperature Response and AccuracyFBG thermometers typically exhibit a temperature-dependent wavelength shift of approximately 10 pm/K around room temperature . Studies have shown that polyimide-coated FBGs demonstrate a quadratic dependence on temperature over the range of 233 K to 393 K, with combined expanded uncertainties of about 500 mK (k=2) when calibrated against platinum resistance thermometers . The main sources of measurement uncertainty include peak center fitting and thermal hysteresis of the fiber coating.FabricationFBGs are fabricated using photosensitive optical fibers exposed to spatially varying ultraviolet light, often in a Michelson interferometer setup. The UV exposure induces photo-chemical changes in the fiber, creating the periodic refractive index modulation that forms the Bragg grating . The Bragg wavelength can be tailored during fabrication, allowing multiple FBG sensors to be multiplexed along a single fiber for distributed temperature sensing.ApplicationsFBG thermometry is used in a variety of fields due to its robustness against mechanical shock and electromagnetic interference:Industrial and nuclear environments: High-temperature resistant FBGs can operate in extreme conditions, such as liquid sodium coolant in fast reactors, withstanding temperatures above 600°C and radiation exposure .Medical procedures: FBG arrays embedded in optical fibers provide high-resolution thermometry during radiofrequency ablation, achieving 0.1°C accuracy and 1.2 mm spatial resolution for monitoring tissue temperature .Structural monitoring: FBGs are used for on-line monitoring of temperature and strain in critical infrastructure, leveraging their multiplexing capability and spectral encoding for precise measurements .AdvantagesFBG thermometers offer several benefits over traditional resistance thermometers:High sensitivity and accuracy with minimal drift over time.Immunity to electromagnetic interference, making them suitable for harsh environments.Multiplexing capability, allowing multiple sensors along a single fiber.Compact and lightweight, ideal for embedded or minimally invasive applications. FBG thermometry represents a modern photonic alternative to conventional thermometry, combining precision, robustness, and versatility for both industrial and biomedical applications .