Terminology » Fiber Optic » Fiber Bragg Grating Sensor: Structure, Working, Advantages, Disadvantages

Fiber Bragg Grating Sensor: Structure, Working, Advantages, Disadvantages

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This page describes the structure, working operation, advantages, and disadvantages of a Fiber Bragg Grating (FBG) Sensor.

Introduction

Fiber optic sensors work by modulating one or more properties of the light wave, such as intensity, phase, polarization, and frequency. An optical fiber typically consists of a core, cladding, and buffer coating.

The cladding reflects light back into the core, ensuring light transmission through the core with minimal loss. This is possible due to a high refractive index in the core relative to the cladding, which causes total internal reflection of light.

One of the commonly used fiber optic sensors is the Fiber Bragg Grating sensor.

fiber optical cable

What is Fiber Bragg Grating?

A Fiber Bragg Grating (FBG) reflects a specific wavelength of light, which shifts in response to variations in temperature and/or strain. The refractive index is permanently changed according to the exposed light intensity. This periodic variation of the refractive index is known as a fiber bragg grating.

Fiber Bragg Grating sensor working

When broadband light is fed to an FBG, reflections from each segment of alternating refractive index interfere constructively for a specific light wavelength. This wavelength is called the “Bragg Wavelength (λb)”. Due to this, an FBG reflects a specific frequency (or wavelength) of light and allows all others to transmit to the other end. A fiber bragg grating can be used as an inline optical filter to block certain wavelengths. The fundamental principle behind its working operation is Fresnel reflection.

λb = 2 * neff * Λ

Where:

  • neff = Effective Refractive Index of core
  • Λ = Spacing between gratings (i.e., grating period)

Fiber Bragg Gratings can be used for strain and/or temperature sensing. Let’s understand how a Fiber Bragg Grating sensor works.

Fiber Bragg Grating Sensor Working Operation

Fiber Bragg Grating Sensor System

The figure above depicts a schematic of an active type of FBG sensor system. Here, the FBG acts as the main sensing unit.

An FBG is realized within a short section of a single-mode fiber by imprinting a periodic modulation of the fiber core refractive index (no).

neff = no + δn * [1 + cos((2*π*x)/Λ)]

Where:

  • neff = Effective Refractive index of guided mode in fiber
  • x = fiber core axis
  • Λ = Period of refractive index modulation
  • δn = Amplitude of photo-induced index excursion

The change in wavelength of an FBG due to strain and temperature can be expressed as follows:

*ΔλB = {[(1 - Pe)ε + (α + ((dneff/dT)/neff)ΔT]} * λB

Where:

  • Pe = Effective photo-elastic constant

  • α = Thermal expansion coefficient of fiber

  • Because an FBG responds to both strain and temperature, it’s required to account for both effects and distinguish between the two.

  • For sensing temperature, the FBG sensor must remain unstrained. Hence, changes in reflected wavelength due to temperature can be primarily described by changes in the refractive index of the fiber.

  • For strain measurement, temperature effects need to be compensated for on the FBG. This is implemented by installing an FBG temperature sensor in close proximity to the FBG strain sensor. Subtracting the FBG temperature sensor wavelength shift from the strain sensor wavelength shift removes any effects of temperature on the wavelength data. Hence, a temperature-compensated strain value can be achieved using this approach.

Applications of FBG sensors

Following are the applications of FBG (Fiber Bragg Grating) Sensors:

  • Strain or structural health monitoring of engineering structures such as bridges, footbridges, dams, skyscrapers, aircraft wings, etc.
  • To monitor the behavior of ultrahigh-quality optical telescopes, precision tools, robotic surgical instruments, etc.
  • To measure nanometer-level deformations in the above structures which lead to the initiation of cracks.
  • Measurement of strain as well as temperature.

Benefits or Advantages of Fiber Bragg Grating Sensor

Following are the benefits or advantages of a Fiber Bragg Grating Sensor:

  • It offers direct absolute measurement compared to conventional electric and alternative fiber sensors.
  • It offers unique wavelength multiplexing capability for the installation of an optical data bus network.
  • It is inexpensive.
  • It exhibits a linear response in the measurement of strain, pressure, and temperature.
  • Fiber Bragg grating sensors’ mounting is similar to conventional gages. Moreover, they are available in a variety of form factors and mounting options.
  • FBG sensors are nonconductive, electrically passive, and immune to EMI-induced noise.
  • When used with a high-power tunable laser, it can perform measurements over long distances with little or no loss in signal integrity.
  • Each optical channel can measure dozens of FBG sensors, unlike electrical sensing systems. This reduces the size, weight, and complexity of the measurement system.

Drawbacks or Disadvantages of Fiber Bragg Grating (FBG) Sensor

Following are the drawbacks or disadvantages of a Fiber Bragg Grating (FBG) Sensor:

  • It is thermally sensitive.
  • It is difficult to demodulate wavelength shift.
  • It is expensive to build and maintain.
  • It is difficult to discriminate wavelength shift due to temperature and strain separately.