How FBG Fiber-Optic Sensors Work

Fiber Bragg Grating (FBG) sensors leverage the unique optical properties of specially inscribed fibre to deliver highly accurate, robust measurements of strain, temperature, pressure and other physical parameters. Below is a step-by-step overview of their operation:

1. Fabrication of the Grating
A short segment of single-mode optical fibre is inscribed with a periodic modulation of the core’s refractive index. This is typically achieved by exposing the fibre core to ultraviolet light through a phase mask (or by femtosecond-laser inscription) to produce a permanent, periodic refractive-index change. FBGS+2ssie.dei.unipd.it+2
This periodic structure is known as the “grating”.

2. Bragg Wavelength Reflection
When broadband light is sent through the fibre, virtually all wavelengths pass through except for one specific wavelength — the so-called Bragg wavelength (λ_B) — which is strongly reflected by the grating. All other wavelengths continue onward with minimal attenuation. hbkworld.com+1
The Bragg wavelength is defined by the relation:

λB=2 neff Λ\lambda_B = 2 \, n_{\text{eff}} \, \LambdaλB​=2neff​Λ

where neffn_{\text{eff}}neff​ is the effective refractive index of the fibre core and Λ\LambdaΛ is the grating period. Wikipedia

3. Sensitivity to Environmental Changes
Because the grating’s period (Λ\LambdaΛ) and the refractive index (neffn_{\text{eff}}neff​) respond to external influences (such as mechanical strain, temperature changes, pressure or vibration), any deformation or thermal variation will shift the Bragg wavelength. In practice:

• If the fibre is stretched (strain), Λ\LambdaΛ increases and/or neffn_{\text{eff}}neff​ changes → λ_B shifts.

• If temperature changes, both the refractive index and the grating period may vary → λ_B shifts.
  By measuring the shift ΔλB\Delta\lambda_BΔλB​, one can determine the magnitude of the applied stimulus.

4. Interrogation and Multiplexing
A light-source and an “interrogator” unit monitor the reflected wavelength from the FBG. Because each grating has a unique Bragg wavelength (or can be spatially separated), multiple FBG sensors can be placed along one fibre (multiplexed) to monitor many points with a single fibre link.
Moreover, the signal is wavelength-encoded rather than relying solely on intensity, making the measurement inherently more stable and less susceptible to signal loss or drift. bdt.semi.ac.cn

5. Key Advantages for Structural and Industrial Monitoring

• High sensitivity and precision in strain, temperature, pressure measurement.

• Immunity to electromagnetic interference (EMI) because measurement is via optical wavelengths rather than electrical signals.

• Ability to embed or integrate into structures (composites, bridges, pipelines) due to the small size of the fibre and grating.

• Long multipoint monitoring along a single fibre, enabling distributed or quasi-distributed sensing over large areas.

• Robust performance in harsh environments (high temperature, corrosive, high-voltage) since the sensing medium is optical fibre.

6. Considerations and Compensation
Because the FBG sensor responds to both strain and temperature, careful calibration and sometimes compensation are required if only one parameter is of interest (for example strain alone). Techniques often involve using a second FBG measuring temperature only, or packaging the sensor in a mechanical assembly that decouples one effect.
Also, the interrogator system and fibre link design (connectors, loss, reflections) must be engineered appropriately to maintain accuracy and reliability.