FBGS (Fiber Bragg Grating Sensors)

 Fiber Optics has undergone remarkable advancement over the last three decades. Apart from being the fastest medium of carrying light signals for transfer of audio, video and data for medical and telecommunication purposes, optical fibers have seen a tremendous growth for sensing, control and instrumentation. FBGS Contrary to telecommunications wherein optical fibers are made immune to external mechanisms, fibers for sensing are made susceptible and sensitive to various physical phenomena.

Reasons for wide applications of optical fibers in variety of applications have been their unique attributes:

·         Low Loss, high bandwidth
·         Immunity to electromagnetic interference
·         Excellent resolution and range
·         Passive operation, intrinsically safe
·         Water and corrosion resistant
·         Rugged, small size and light weight
·         Can be Multiplexed in parallel or in series
·         Modest cost per channel
 
Some Important Terms used in Optical Fiber Propagation:
1.      Total Internal Reflection: When light travels from a rarer medium to a denser medium, it gets away from the normal while refracting. This means that there is a path change in light’s propagation. However, when we tilt the angle of incidence to such an extent that minimum amount of light is refracted, there comes an angle called critical angle. When the angle of incidence is equal to the critical angle, there will be no refraction and all light will be absorbed at the interface between the rarer and denser medium. At incident angles greater than the critical angle, the light is reflected back into the rarer medium. This phenomenon when the light is reflected back from the rarer medium-denser medium interface into the rarer medium is termed as Total Internal Reflection.

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2.      Polarization of light: Light, being a transverse wave, is a mixture of electric and magnetic components and that is why is also called an electromagnetic wave. Polarisation can be defined as the direction of the electric component of the light wave. When electric field restricted to a particular direction, light is said to be polarised and in the cases of random distribution of electric field, light is unpolarized.

3.      Phase: Light is a sinusoidal wave. Phase can be defined as the time at which a pulse of sine wave at a particular point. When two or more sign waves reach a point but have different portions of the pulses, they have a phase difference.

4.      Intensity: It is the luminous power that a ray of light has.

5.      Wavelength: There regular period after which a wave repeats it’s shape can be termed as wavelength.

 

OPTICAL SENSING
The heart of any optical fiber based sensing system is the fiber itself. Central portion is made up of Silica and is called “Core”. Surrounding the core is a region called cladding which is also made of silica; however, refractive index of cladding is less than that of core. This fulfils the condition for total internal reflection. Surrounding the cladding are the layers which provide mechanical strength to the fiber.. The glass fiber is coated with acrylate, polyimide or ORMOCER (organic modulated ceramic) to protect it especially against water and hydrogen.
 

FBGS1
In an optical fiber based sensing system, light is sent down the optical fiber and properties of the light are made to change in response to the physical phenomena. Properties of light which are made use of include Polarization, wavelength, phase and intensity.

 

FBGS

 

The principle of operation of a fiber sensor is that the transducer modulates one of the properties of light which gives results in change in the characteristics of the optical signal received at the detector. Sensing is done based on detecting a change in one or more of the light properties.

 

The fiber sensor can be intrinsic (modulation takes place directly in the fiber) or extrinsic (modulation is performed by some external transducer).

 

Various optical sensing technologies are:

 

 S. No
 Optical Sensing technology
Used for Measurements of
1
Fabry-Perot
Temperature, Pressure
2
Fiber Bragg grating (FBG)
Strain, Temperature, Pressure, Acceleration, Displacement
3
Rayleigh
Strain, Temperature
4
Brilliouin
Strain, Temperature
5
Raman
Temperature

 

 

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