To start it from starting please read our latest Blog, “ Is OTDR useful to maintain fibre network or not “.
To be continued……………….
OTDR depends on two Optical Phenomena: Rayleigh Scattering and Fresnel Reflection
What is OTDR and Principle of OTDR
Rayleigh Scattering - Principle of OTDR
It is intrinsic to the fibre material itself and present along the length that gives us attenuation loss, db/km.
When electromagnetic pulse propagates through air medium, than the movement of electrons inside the medium, generates the electric field inside it. So when photons are allowed to transmit through these molecules, some photons are absorbed but then retransmitted to multiple directions by the air molecules. This is called as Rayleigh scattering.
The strength of scattering depends on the wavelength of light wave and the size of particle responsible for scattering. As misconception that runs among people that scattering happens due to collision of light with molecules, but that scattering does not occur due to collision rather it is the result of the electromagnetic interaction between photons and the particles of the medium.
Live example of Rayleigh scattering is the appearance of the sky is blue in colour. It is due to the Rayleigh scattering of sunlight by the molecules present in atmosphere.
As the sun light(white) propagates, it strikes the molecules of the atmosphere. It results in the redistribution of molecular charges due to electromagnetic field of incident light rays. It causes the vibration of the molecules and the charges start oscillating with the radiation frequency.
So during the interaction of incident molecule and incident light rays, some part of light molecules are absorbed within molecules of the atmosphere. Later these absorbed molecules are re-radiated in different direction of atmosphere that causes scattering of light and called as Rayleigh scattering.
What is OTDR and Principle of OTDR
Rayleigh scattering law is given by:
The intensity of scattered light is inversely proportional to the fourth power of the wavelength.
As per the equation, the shorter wavelength light scatters more than the longer wavelength due to inverse relationship between the wavelength and Intensity of scattered light.
The Rayleigh scattering across the atmosphere is the reason for the colour of sky as blue. Wavelength of blue colour is less than the red colour, hence the scattering of blue light is more than the scattering of red colour. That is why visibility of sky is blue in colour.
Most of the people comes with the question that white light comprises of seven colours and violet light and Indigo light has shorter wavelength than the blue light, still we only see sky as blue in colour not violet or Indigo in colour. Why?
So our colour visibility depends on the structure of eye as human eye retina is composed of basically 3 types of colour cones or rods. These are red, green and blue. And these rods have the tendency to respond to these light more efficiently than any other colour. By constructing various stimulations, the vision system of a person allows it to recognize various colours.
Although the spectrum of the light emitted by the sun is not uniform for all wavelength and is highly absorbed by the atmosphere. Thereby producing a vision of less violet and Indigo in the sky. Even human eyes are not that much sensitive to violet colour.
So, the red and green cones in our retina do not recognize other colours as these are not much scattered. However, blue cones show better stimulation towards the colour near that particular wavelength. Also due to the shorter wavelength, blue colour possesses better scattering and our eyes see the light blue colour of the sky.
What is OTDR and Principle of OTDR
Rayleigh scattering of pulse of light in Optical fibre
As per the composition of optical fibre, homogeneities, splices, bubbles in splices and airgaps between two end of different fibres, defects in fibre at the time of fabrication causes scattering of transmitted light through the length of optical fibre cable. As we know that glass fibre is a composition of random connection of molecules.
Due to this reason, some regions in the structure may contain either high or low molecular density. This leads to the variation in the refractive index of the material at different points inside the fibre.
The variation in the refractive index leads to Rayleigh scattering of the transmitted light.
Basically, the light rather being absorbed is radiated in different directions thus termed as a scattering of light.
What is OTDR and Principle of OTDR
Fresnel Reflection - Principle of OTDR
When a light beam (e.g. a laser beam) reaches an interface between two different transparent media, it is partly transmitted into the other medium and partly reflected back into the original medium.
Complete transmission without any reflection would require impedance matching. Essentially all optical materials (except for some photonic metamaterials) have the relative permeability μ = 1, and in that case the impedance depends only on the refractive index. Therefore, the optical reflectivity at such an interface depends only on the refractive indices of the materials, and it vanishes if those indices are identical.
Quantitatively, the reflectivity and transmissivity at such an interface can be calculated with Fresnel equations for an arbitrary angle of incidence. The reflections themselves are called Fresnel reflections.
What is OTDR and Principle of OTDR
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