What is a temperature-sensitive fiber Bragg grating

Temperature Fiber Bragg Gratings (FBGs) measure temperature by detecting shifts in the Bragg wavelength caused by changes in the fiber's refractive index and thermal expansion.

Operating Principle

A 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, is determined by the effective refractive index of the fiber core and the grating period. When the temperature changes, the refractive index and the grating period both vary due to the thermo-optic effect and thermal expansion, causing a measurable shift in the Bragg wavelength . This shift is directly proportional to the temperature change, allowing precise temperature measurement.

Temperature Sensitivity

Standard silica FBGs, such as those made from SMF-28 fiber, typically exhibit a thermal sensitivity of around 12.5 pm/°C, which remains nearly constant regardless of fiber geometry . The sensitivity is governed by the intrinsic properties of silica, including its thermal expansion coefficient and thermo-optic coefficient (approximately 8.3 × 10⁻⁶ °C⁻¹), . This makes FBGs highly stable for temperature sensing, even when other modifications, such as tapering, are applied to enhance strain sensitivity.

Advantages

FBG temperature sensors offer several benefits over conventional electrical sensors:

• Electromagnetic immunity, making them suitable for high-voltage or RF environments .
• Compact size and the ability to be embedded in materials like concrete or composites .
• High precision and repeatability in temperature measurement .
• Versatility in mounting, including gluing to steel surfaces or embedding in protective ceramic or stainless steel casings for harsh environments .

Applications

FBG temperature sensors are widely used in:

• Power stations and oil & gas facilities for monitoring high-temperature processes .
• Railroads and infrastructure monitoring, where electrical sensors may fail .
• Structural health monitoring, often combined with strain sensors for thermal compensation .
• Aerospace and industrial systems, where compact, robust, and multiplexed sensing is required .

Summary

Temperature FBGs are optical fiber-based sensors that detect temperature changes through shifts in the Bragg wavelength caused by thermal effects. They provide high precision, electromagnetic immunity, and adaptability for harsh or complex environments, making them a preferred choice in industrial, civil, and energy applications .