The effect of temperature on fiber Bragg gratings

Fiber Bragg gratings (FBGs) are sensitive to temperature, with their Bragg wavelength shifting due to both thermal expansion and the thermo-optic effect of the fiber material.Mechanisms of Temperature Sensitivity

FBGs reflect light at a specific Bragg wavelength, which depends on the effective refractive index of the fiber core and the grating period. Temperature changes affect both factors:

• Thermo-optic effect: The refractive index of silica changes with temperature, causing a shift in the Bragg wavelength.
• Thermal expansion: The physical length of the grating expands or contracts with temperature, also altering the reflected wavelength. In standard single-mode fibers like SMF-28, the thermal sensitivity is typically around 12.5 pm/°C, largely independent of geometric modifications such as tapering .

Nonlinear Temperature Response

The temperature response of FBGs is not strictly linear. Over certain temperature ranges (e.g., 70–80 °C), the Bragg wavelength exhibits quadratic behavior due to the temperature dependence of the thermo-optic coefficient of silica . This nonlinearity should be considered in high-precision sensing applications.

High-Temperature Considerations

At elevated temperatures (600–1200 °C), FBGs can experience long-term wavelength drift, which affects their reliability for continuous monitoring. This drift is caused by physical changes in the fiber material over time, although the exact mechanisms are still under investigation . High-temperature-resistant FBGs are designed to mitigate these effects, but long-term stability remains a key consideration in extreme environments.

Practical Implications

• FBGs can be used as temperature sensors or in strain sensing where temperature compensation is required.
• For accurate measurements, both the linear and nonlinear temperature effects must be accounted for.
• In high-temperature applications, specialized fibers or coatings may be necessary to reduce drift and maintain sensor accuracy. In summary, temperature directly affects the Bragg wavelength of FBGs through intrinsic material properties and thermal expansion, with sensitivity typically around 12.5 pm/°C, but nonlinearities and long-term drift must be considered for precise or high-temperature applications .