The bending radius of a fiber Bragg grating (FBG) sensor affects strain and Bragg wavelength, with typical practical limits around a few millimeters for standard single-mode fibers.

Bending Effects on FBG Sensors

Fiber Bragg gratings are sensitive to both strain and temperature, and bending introduces additional strain that can shift the Bragg wavelength. In single-mode fibers (SMF), the core is located along the neutral axis, so bending minimally affects the central core, making the FBG primarily sensitive to temperature rather than bending-induced strain . However, in multicore fibers (MCF), off-center cores experience tangential strain when the fiber is bent: cores on the outer side of the bend are stretched, while inner cores are compressed, which can influence the Bragg wavelength .

Typical Bending Radius

Simulation studies and experimental setups indicate that FBG fibers can tolerate bend radii down to approximately 3–5 mm without significant degradation of the grating or excessive strain . For example, an interpolated refractive index map of a bent fiber shows noticeable changes at a bend radius of 3.5 mm, demonstrating the sensitivity of the core and cladding to tight bends . Bending below this radius may induce microbending losses or permanent deformation, affecting both temperature and strain measurements.

Practical Considerations

• Sensor packaging: Proper encapsulation or embedding in materials can reduce the impact of bending on the FBG response .
• Calibration: When using FBGs for temperature sensing, the Bragg wavelength at zero strain and reference temperature must be known to separate bending effects from thermal effects .
• Multicore fibers: If using MCF for combined bending and temperature sensing, the bending radius directly affects off-center cores, which can be exploited for bending measurement but requires careful calibration to isolate temperature readings .

Summary

For temperature sensing with FBGs, maintaining a bend radius above ~3–5 mm is generally recommended to avoid strain-induced errors. Single-core fibers are less sensitive to bending, while multicore fibers can provide additional bending information but require careful consideration of core positions relative to the neutral axis. Proper packaging and calibration are essential to ensure accurate temperature measurements.