Comparison of Different Types of Optical Fibres

OPTICAL FIBRE
An optical fibre is a thin strand of glass or plastic that serves as a transmission medium over which the information passes. To be more technical, an optical fibre
is a dielectric waveguide that operates at optical frequencies (10¹³ to 10¹⁶ Hz) and transfers information in the form of light by the mechanism of total internal reflection. The optical fibre consist of a central core surrounded by a cladding layer.

Types of Optical Fibres
The different types of optical fibres are,

• Step index and Graded index fibre based on refractive index profile
• Single mode and multimode fibre based on number of modes
• Glass, Plastic clad and All plastic fibres based on material

Comparison
A detailed comparison of different types of optical fibres is presented below:
1. Step Index and Graded Index Fibre
This classification is based on the refractive index profile of core and cladding. Refractive index or Index of Refraction is the fundamental optical parameter of a medium. The value of refractive index shows how a ray of light travels in a medium.

Step index fibre	Graded index fibre
It is a fibre in which the refractive index of the core is uniform throughout the length of the fibre .	The refractive index of the core is not constant.
The refractive index undergoes an abrupt change (like step function) at the cladding boundary Single mode Step Index Fibre Multimode Step Index Fibre	The core refractive index is made to vary as a function of radial distance from the center of the fibre. Multimode Graded Index Fibre
The RI profile is given by, Where, n1 -- RI of core;  n2 -- RI of cladding  and a – radius of core.	The RI profile is given by, Where, n1 -- RI of core;  n2 -- RI of cladding  and a – radius of core.
The ri of the cladding is slightly less than that of the core	The ri is maximum at the axis of the core and it decreases from core to cladding.
The diameter of the core is about 50 to 200µm in the case of multimode fibre and 10µm in case of single mode fibre	The diameter of the core is about 50µm in the case of multimode fibre
Ray of light is offered a path that is confined to the plane containing the fiber core axis when passed through the step index fiber. The rays are called MERIDIONAL RAYS. Meridonial Rays	The ray of light is offered a helical path to pass through the graded index fibre. The rays are named as SKEW RAYS. Skew Rays
The losses and pulse distortion are high	The losses and pulse distortion are reduced because of the self-focusing effect.
Information carrying capacity is less	The information carrying capacity is increased.

2. Single Mode and Multi Mode Fibres
Mode describes the path of propagation of light ray along the optical fibre. The pattern of light ray propagation classifies fibres into single mode and multi mode.

Single mode fibres	Multi mode fibres
It has only one path for transmission which means only a single mode can travel through it. Single mode Step Index Fibre	In multimode fibres, more than one mode can propagate through the fibre. Multimode Graded Index Fibre Multimode Step Index Fibre
The core diameter is narrow (2 to 10µm)	The core diameter is wide (>50µm)
The refractive index difference between core and cladding is very small.	The refractive index difference between core and cladding is large.
The single mode fibers are always step index as the core is narrow which cannot be made graded index.	It can be made by either step index or graded index fibre, since the core is wide.
Only LASER can be used as the optical source.	LED and LASER are used as optical source.
There is no dispersion because of no degradation of signals during its propagation through the fibre.	There is signal distortion due to multimode dispersion and material dispersion.
These are suitable for long distance communication because of its greater information carrying capacity.	Large dispersion and attenuation limits its application to short distance communication. These fibres are used in Local Area Network.
Launching of light into these fibres and joining two fibres are very difficult.	Launching of light into these fibres and joining two fibres are easy.
Fabrication is very difficult making it costlier.	Fabrication is comparatively easier and it is not costly.

3. Glass,Plastic Clad and All Plastic Fibres
An optical fibre is usually made of glass. It can also be made out of plastic inorder to add strength to it. Based on the material of fabrication of optical fibre, it is categorised into  Glass, Plastic clad and All plastic fibres.

Glass fibres	Plastic clad fibres	All plastic fibres
In glass fibres, both the core and cladding are made of glass consisting of either silica or a silicate.	Plastic clad fibres have a plastic cladding made of silicone rubber and a glass core made of silica.	All plastic fibres have plastic core and cladding.
Glass fibres resist deformation at temperatures as high as 1000˚C, thermal shock and are highly transparent.	These fibres exhibit lower radiation induced losses and have an improved performance.	The plastic core and cladding reduces the requirement of buffer jacket for fibre protection.
Losses are less.	Medium losses	High losses
Mechanical strength is poor.	Mechanical strength is medium	Large mechanical strength.
These are used for long distance communication because of low losses.	These are used for medium distance communication.	These fibres are used for very short-haul low cost links.

Also read Advantages and disadvantages of optical fibers