FIBRE OPTICS

Conventional methods of long distance communication use radio waves (~ 106 Hz) and micro waves (~1010 Hz) as carrier waves. A light beam acting as carrier waves is capable of carrying far more information since optical frequencies are extremely large (~1015 Hz).
Soon after the discovery of laser, some preliminary experiments in propogation of information carrying light waves through the open atmosphere wave carried out, but it was realized that the unwanted elements such as rain, fog etc. leads to adverse effects. Thus in order to have an efficient and dependable communication system one would require a guiding medium in which the information carrying light waves could be transmitted. This resulted in the development of optical fibre which is an efficient guiding medium for laser light.
Basic principle-total internal reflection
The basic principle of optical fibre is multiple total internal reflection. When a ray of light travel from denser to a rarer medium, at an angle of incidence greater than critical angle qc, the ray is not reflected but it is reflected into the same denser medium. This property is called total internal reflection. Light signals are transmitted through optic fibres by multiple total internal reflection.

Fibre construction and fibre dimension
An optical fibre consists of a vary thin transparent cylindrical core having refractive index n1 surrounded by a cylindrical shell called cladding of slightly lower refractive index n2. The core cladding system is surrounded by plastic jackets.
Optical fibres are hair thin threads of glass or plastic. Plastic fibres have the advantage of more flexibility than glass fibres but attenuation is greater in plastic fibres, comparing with glass fibres.
Core diameter ranges from 5 to 600µm. Cladding diameter ranges from 125 to 750µm. The thickness of the jacket is around 100µm. totally optical fibre has an outer diameter ranging from 0.1mm to 1.5 mm.
Light propogation in optical fibres
Consider an optical fibre with n1 and n2 as the refractive indices of core and cladding material respectively. Consider a ray of light entering through one end making an angle q with the axis.

By Snell’s law, n1=sini/ sinq
Or sinq=sini/ n1 ------(1)
At core –cladding interface, refractive index µ=n1/n2
Total internal reflection takes place at core-cladding interface if the angle of incidence at core-cladding interface is equal or greater than critical angleÆc.
We have µ=n1/n2=1/sinÆc
sinÆc=n2/n1 -----(2)
For total internal reflection to take place,
Æ > Æc
or, sinÆ > sinÆc
i.e, sinÆ > n2/n2 (from equ(2))
But from figure, sinÆ =cosq
cosq > n2/n1
i.e, (1-sin2q)1/2 > n2/n1
1-sin2q > (n2/n1)2
sin2q < sinq =" sini/n1">Acceptance angle
The maximum angle of incidence at which light may enter the fibre in order to be propagated is, im= sin-1(n2-n22)1/2
This angle is called acceptance angle for the fibre.
Numerical aperture
Numerical aperture of an optical fibre is a measure of its gathering capacity and it is denoted as the sine of acceptance angle.
i.e, NA=sinim=(n2-n22)1/2
Step index and graded index fibre
If the optical fibre has a core of uniform constant refractive index n1 and a cladding of slightly lower refractive index n2, it is called a step index fibre. The cross sectional refractive index profile is as shown:

If the core of optic fibre has a non-uniform refractive index that decreases gradually from the centre towards the core-cladding boundary, it is called a graded index fibre. The cladding surrounding the core has a uniform refractive index, slightly lower than the refractive index of the core. The cross sectional refractive index profile is as shown:

Single mode and multimode fibre
In single mode fibres, only one mode of light rays are guided through the fibre. For this, a step index fibre with a small core diameter is used.
In multimode fibres, a number of modes of light rays guided through the fibres. For this, a step index fibre with a large core diameter or a graded index fibre is used.
Signal distortion and transmission losses
1) Attenuation
Absorption and scattering of light traveling through a fibre leads to decrease the strength of the signal which is referred as attenuation of the signal.
2) Intermodal dispersion
The intermodal dispersion occurs in multimode fibres where rays associate with various modes travel different distances through the fibre. As a result, the signal broadens and the output signal is no longer identical with the input signal. Signal broadening is less in graded index and step index single mode fibres.
3) Material dispersion or chromatic dispersion
If we use white light, all the colours of the input radiation are not reacting the other end at the same time since they travel with different velocities. This type of signal distortion is called material dispersion or chromatic dispersion.
4) Waveguide dispersion
In single mode fibres, 80% of signal travels through the core. The remaining 20% travels through the cladding, move faster than the signal through the core. The signal distortion occurring due to this is called waveguide dispersion. Waveguide dispersion is small in multimode fibres and can be ignored.
Light wave communication using optic fibre
A simple block diagram of fibre optic communication system is shown below:

Optical transmitter
A light emitting diode (LED) or a semiconductor laser can be used as optical source. Modulation modulates the input signal and optical signal and then transmitted through optical fibre cables to the receiver.
Optical receiver
A photodiode can be used as optical detector. The detected wave is demodulated to extract the signal.
Advantage of fibre optic communication
1) Wide band width.
2) Low attenuation and other transmission losses.
3) Small size and weight.
4) Safe from electrical interference caused by lightning, electric motors, fluorescent tube and other electrical noise sources.
5) Lack of cross talk between parallel fibres.
6) Easy installation and easy maintenance.
7) Flexible.
8) Temperature resistance.
9) Highly economical.
10) High degree of signal security.
11) Longer life span.

Fibre amplifier
Fibre loss in optical fibres is the main disadvantage of fibre optic communication system. To overcome this, optical amplifiers are used. There are 4 types of optical amplifiers:
Erbium doped fibre amplifier (EDFA)
Fibre Raman amplifier (FRA)
Semiconductor laser amplifier (SLA) and
Integrated optical amplifier (IOA)
Erbium doped fibre amplifier (EDFA)
Here Erbium doped silica fibres are used. When incident photon energy is incident on a doped fibre, Erbium ions in the medium are made to move to higher energy levels. The Erbium ions in the excited state return to the ground state either spontaneously or by stimulation. Erbium doped fibres have long metastable states leading to coherent amplification. A practical configuration of EDFA is as shown:

Advantages of EDFA
High gain
High output power
Low noise
Less gain variation
Wide bandwidth
Compatible to transmission fibre with minimum loss
Cross talk immunity and
Low power consumption.