Fiber Bragg Grating Sensor structure, working, advantages, disadvantages

This page describes structure, working operation, advantages and disadvantages of Fiber Bragg Grating Sensor. It mentions Fiber Bragg Grating (FBG) Sensor advantages or benefits and Fiber Bragg Grating Sensor disadvantages or drawbacks.

Introduction:
Fiber optic sensor works by modulation of any one or more properties of the light wave such as intensity, phase, polarization and frequency. Optical fiber consists of core, cladding and buffer coating. Clading reflects light back into the core and ensures light transmission through the core with minimal loss. This is possible due to high refractive index in the core relative to cladding, which causes total internal reflection of light. One of the commonly used fiber optic sensor is Fiber Bragg Grating sensor.

What is Fiber Bragg Grating?

Fiber Bragg Grating reflects wavelength of light which shifts in response to variations in temperature and/or strain. The refractive index is changed permanently according to exposed light intensity. The periodic variation of refractive index is known as fiber bragg grating.

When broadband light is fed to FBG, reflections from each of the segment of alternating refractive index interfere constructively for specific light wavelength. This wavelength is called as "Bragg Wavelength (λ_b)". Due to this, FBG reflects specific frequency (or wavelength) of light and allows all others for transmission 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.
➨ λ_b = 2 * n_eff * Λ
Where,
n_eff = Effective Refractive Index of core
Λ = Spacing between gratings (i.e. grating period)

Fiber Bragg Grating can be used for strain and/or temperature sensing. Let us understand Fiber Bragg Grating sensor working.

Fiber Bragg Grating Sensor working operation

The figure above depicts schematic of active type of FBG sensor system. Here FBG acts as main sensing unit. FBG is realized within a short section of a single mode mode fiber, by imprinting the periodic modulation of fiber core refractive index n_o.
➨ n_eff = n_o + δ_n*[ 1 + cos ((2*π*x)/Λ) ]
Where,
n_eff = Effective Refractive index of guided mode in fiber
x = fiber core axis
Λ = Period of refractive index modulation
δ_n = Amplitude of photo-induced index excursion

The change of wavelength of an FBG (Fiber Bragg Grating) due to strain and temperature can be expressed as follows.
➨ Δλ_B = { [ (1 - P_e)ε + (α + ((dn_eff/dT)/n_eff)*ΔT ]}*λ_B
Where,
P_e = Effective photo-elastic constant
α = Thermal expansion coefficient of fiber

• FBG (Fiber Bragg Grating) responds to both strain and temperature, it is required to account both effects and distinguish between the two.
• For sensing temperature, FBG sensor must remain unstrained. Hence changes in reflected wavelength due to temperature can be primarily described by changes in refractive index of fiber.
• For strain measurement, temperature effects need to be compensated on FBG. This is implemented by installing FBG temperature in close proximity to FBG strain sensor. Substraction of FBG temperature sensor wavelength shift from strain sensor wavelength shift removes any effects of temperature on the wavelength data. Hence 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 behaviour of ultrahigh quality optical telescope, precision tools, robotic surgical instruments etc.
• To measure nanometer level deformations in above structures which leads to 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 Fiber Bragg Grating Sensor:
➨It offers direct absolute measurement compare to conventional electric and alternative fiber sensors.
➨It offers unique wavelength multiplexing capability for installation of optical data bus network.
➨It is inexpensive.
➨It exhibits linear response in measurement of strain, pressure and temperature.
➨Fiber Bragg grating sensors mounting is similar to conventional gages. Moreover they are available in variety of form factors and mounting options.
➨FBG sensors are nonconductive, electrically passive and immune to EMI induced noise.
➨When used with 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 size, weight and complexity of measurement system.

Drawbacks or disadvantages of Fiber Bragg Grating (FBG) Sensor

Following are the drawbacks or disadvantages of Fiber Bragg Grating (FBG) Sensor:
➨It is thermal 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.
➨Refer advantages and disadvantages of fiber optic pressure sensor >> and fiber optic temperature sensor >> for more specific information.

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