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A light travels in straight lines as we know it is highly impossible to make it follow a curved path to glimmer around corners. In 1970’s there was a great improvement and faster growing in fibre optics communications. The main idea of light in communication system is evolved from simple signal fires and lamps. Claude chappe has a first modern attempt in optical telegraph built in the 1790s.The first problem was alleviated with the advent of semiconductor age, the semiconductor laser invented by Theodore maiman in 1962 LED(light emitted diode) is improved in optical fibre which solved the problem o0f passing light through air. The communications through optical fibre was proposed in 1966 by Charles Kao and Charles Hockham of the standard telecommunication laboratory in England. In the past few decades many technologies are introduced among them optical fibre communication is the best communication system.
The main components of optical fibre are core, cladding and coating. The main duty of core is, it carries a light from transmitter to receiver.
- It is a smallest part in optical fibre communication system.
- It is very delicate optical fibre cable
- Generally it is made up of plastic or glass.
- The core made up with glass is mixed with pure silicon dioxide other impurities like germanium or phosphorous.
- These impurities are added to improve the refractive index at certain conditions
- The range of glass core is from 3.7µ to 200µ.
- 1.48 is the refractive index of core
- The core made with plastic is larger than glass.
- It is the protective and surrounded layer of core.
- It provides the low reflective index.
- In glass cladding the core and cladding is made with same material
- Different quantity of impurities is added to both core and cladding in order to make a difference in refractive index among them is about 1%.
- 1.46 is the refractive index of cladding
- 125µ and 140µ are the standard cladding diameters
- This is the main protective layer for the entire optical fibre.
- It protects the optical fibre from shocks and other external damages.
- Coating has an outside diameter of either 250µ or 500µ.
- Coating is colour less but to identify the coating in some applications it is coloured.
2) Advantages of Fiber optics:
- The band width of optical fibre is very high when compared with other communications.
- To increase band width in fibre optics is very easy.
- Data transmission is very fast in fibre optics.
- We can transmit data to longer distances without any noise.
- It is very difficult to tap the information because it is much secured.
3) Disadvantages of Fiber optics:
- Installation of fibre optics is very cost.
- When we face any problem with fibre optics we require special test equipment.
- Communication with fiber optic cable is more cost when compared with different broad band connection costs.
- In rural areas fiber optic communications are very less, in these days this is one of the main disadvantage of fiber optic.
4) Fibre Optic Losses:
Due to bending and breaking of optical fibre cables losses are occurred in fibre optics. Mainly there are two types.
- Intrinsic losses.
- Extrinsic losses.
4.1) Intrinsic losses:
A slight variation from one fibre to another fibre even manufactured with in specified tolerances. Losses are occurred by these variations. In Intrinsic losses there are four types of losses. They are
- NA (Numerical Aperture) mismatch losses.
- Core diameters mismatch losses.
- Concentric mismatch losses.
- Elliptical mismatch losses.
- Cladding diameter mismatch loss.
1) NA (Numerical Aperture) mismatches losses:
If there are two optical fibres are repaired we are connecting them with splices during this connection mismatches are occurred because the cone of acceptance in the receiving fiber cannot gather the complete light emitted by the transmitting fiber. This means the light is not travelling completely. This mismatch loss is known as numerical aperture mismatch loss.
2) Core diameter mismatch losses:
The difference between the core diameters is the reason of this loss. When the transmitting core greater than the receiving core or transmitting core is lesser than the receiving core light is not completely travelling. This type of mismatch is called as Core diameter mismatch losses.
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3) Concentric mismatch losses:
In ideal conditions the core the core and cladding are concentric, that means a single geometric centre is shared between them. The fibre core is likely to be offset by a slight amount from the cladding centre. In fiber cores when the transmitting and receiving are non concentric. They will not meet exactly and the light coming from the transmitting fiber is lost.
4) Elliptical mismatch losses:
If the fiber cores not the perfectly circular and fiber cores and cladding are not perfectly concentric this types of losses are occurred. The transmitting optical fiber core is not match with receiver core.
5) Cladding diameter mismatch loss.
If the diameter of the cladding are not same on the both fiber optics. This type of mismatch is occurred. This means the transmitter light is not completely sending to receiver core.
4.2) Extrinsic losses:
Generally in an ideal optical fibre the cores are centred on each other they are placed at 90 degrees angle to their faces. The ends should be in firm contact. Any miss arrangement in these conditions can cause some loses in the signal. There are three types of extrinsic losses.
- Lateral displacement.
- End separation.
- Angular misalignment.
1) Lateral displacement:
If the centres of core of two optical fibers are do not match this loss may occur. If the displacement increases less light from the transmitting fiber makes its way into the receiving fibre. A little amount of displacement is acceptable in larger fibers because the majority of the core s surface area is still in contact. In smaller fibers a slight offset can place the centre of the transmitting core entirely outside of the receiver core.
2) End Separation:
The end separation loss is due to Fresnel reflection, it takes place when the light passes from fiber refractive index into the air and vice versa. In refractive index each and every change causes an amount of reflected light and therefore the loss is occurred.
3) Angular Misalignment:
The optical signal will suffer from these losses when the fibers meet an angle. The solution for this loss is to arrange the fibre end properly that the both ends are in the same line during splicing.
4.3) Major causes of losses:
- Absorption loss.
- Scattering loss.
- Linear scattering losses.
- Non Linear scattering losses.
- Coupler losses.
- Insertion losses.
- Reflection losses.
- Impurity losses.
- Macro bending and micro bending losses.
- Packing fraction loss.
Absorption loss :
Absorption loss is occurred by the impurities in the fiber it self such as water and metals.
Material absorption losses: Material absorption losses are occurred by absorption of photons within the fiber these losses represent a fundamental minimum to the attainable loss.
Intrinsic absorption losses: Interaction with more than one component with glass leads to the intrinsic absorption loss.
I couples energy guided to radiation modes which causes the energy losses from the fiber. If there is a core diameter irregularity in fiber access direction also a reason of scattering losses.
Linear Scattering loss:
The quantity of light power is transferred from wave is directly proportional to the power in the wave it also causes by inhomogeneties in the glass when the size of it smaller than wave length.
Non linear scattering loss:
If the electric field with in the fiber has high values then it leads to the presence of non linear scattering ,it also causes when significant power is scattered in all the directions.
The fiber optics coupler are active or passive devices the coupling loss in optical fiber is defined as,
= output power =inputpower
Connection losses increased by fiber to fiber connection due to the following sources of intrinsic and extrinsic. Mainly we have four types coupling losses
Insertion losses are combination of coupling loss and additional fibre losses. If joints of fiber can increased the attenuation of fibre this is done in multimode operation. Fiber joints can leads to the second order mode in single mode fiber.
Light waves of reflection and transmission occur because frequency do not match the natural resonant frequencies of vibration of object.
The first source of impurities material in glass fibre is metallic ions, the loss due to this reduces the contribution below 1DB/KM.
Macro and microbending losses:
These loss may occur due to sharp bend in fibre, to produce high losses a short length of optical fibre is to be bend, as tight as the fibre optic the losses are worst. The major problem in macro losses is in the hands of the installer.The losses in micro bends is same as the macro bands but it just differs in the size and cause. The radius is equal or less then the diameter the outer layer will shrink and get shorter when the fibre is too cold, fibre optic cables are available with a range of temperatures from C to C.
Packing fractin losses:
Single emitter sometimes uses a bundle of fibres, if claddings are in contacts many fibres are packed together. Large area source can match a large bundle in order to eliminate area mismatch loss. Small sources can emit less like than the larger once , in single fibre larger one has more power to couple into a bundle than into a single fibre.
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Fibre optic as a sensor:
Sensor provide link to interface between the electronic units and physical world the sensor can detect physical and chemical qualities such as temperature, pressure, vibration, flow , acceleration, proximity,and chemical concentrations. A basic sensor is made upof a light source(laser or led), a length of fibre and optical detector. Fibre optic itself acts as a sensor by varying he intensity of light these measurements are done. Only the source and the detector is required in the sensors hence it is very simpler.
Based on the performance characteristics there are four different types of sensors they are
1. Extrinsic sensors.
2. Intrinsic sensors.
3. Fibre bragg grating (FBG sensors).
4. Long period grating sensor.
In extrinsic sensors the outside part of the fibre undergoes to the sensing effect. The fibre acts as a collection system in light delivery. For example a chemical sensor utilizes a sensitive material on the tip, light is passé through the fibre and reflected back. In the chemical solution as the concentration changes the tip properties may change and the reflection of the light also changes which gives the measure of chemical concentration.
In intrinsic sensor the changes takes place within the fibre. The change is outside the fibre and the fibre remains unchanged when the intrinsic sensor is in contract to extrinsic sensors. For example when a fibre with ruff surfaces is placed between two plates, the fibre is pressed by the plates when the pressure increases the attenuation of the fibre increases due to this.
Bend and micro bend sensor:
Bent in a optical fibre leads to a portion of propagating light beam along the bend is incident at angles must be smaller than the critical angle by which attenuation, this can be used for sensing measure load and stream are found by this mechanism, lose of power occurs if any load lead to a bending of fibre. This measure gives the distribution of strain and load with the use of lost power. A series of random bends and small bends along the fibre is known a micro bending. It acts as a coupling between cladding and core modes in a single mode fibre and between multimode fibre.
Fibre bragg grating(FBG Sensor):
TO MEASURE TEMPERATURE AND STRAIN:
To measure sensing mechanical strain, temperature and acceleration we use FBG sensors. Parameters that changes any of these results in a change in reflected wavelength, these changes when measured, sensing or external perturbations can be done.
Long period grating sensor:
Periodic perturbations along the length of the fibre with periods greater than hundred micro meters which includes coupling between the light propagating in core and cladding is long period grating. Cladding code influence the power transmitter through the fibre used to find the refractive index when there is any change in the medium around the fibre.
Interferometric sensors: #
In this the light is transmitted through to fibres. one of them (reference arm) isolated from environment and its properties are constant. And another fibre (measure arm) is exposed to parameter during sensing. The face of the light is changed by the parameters. The interferometric sensors has the greatest sensitivity and it has highest performance capabilities.
The Interferometric sensor. ( John F.R, 1997, pp-543).
ADVANTAGES OF FIBRE OPTICS SENSORS:
- It allows an access to normally inaccessible areas of interest.
- It is an non-electrical.
- Due to small size and less weight of the sensors it effective in cost.
- It has high sensitivity.
- It has high reliability.
- It is very easy to install.
Transmitter converts electrical signal into light signal. It has two functions
- Light emitter
- It works as a soirce of light coupled into optical cable.
- It modulates the light to represent the binary data.
Light emitting diode:
The transmitter are directly modulate when the drive current passed through the LED is varied. The power is directly proportional to the current flowing in the LED. According to the applications the drive currents is measured. The drive current is switched on and off in digital applications. And the current is varied in Analog application. The LED transmitter is packed with the receiver since the space is reduced and simplifies the circuit designing which reduces the cost.
Characteristics of the LED:
- Recommended operating conditions.
- Electrical characteristics
- Optical characteristics
- Data rate
Recommended operating conditions:
- It describe the temperature and voltage ranges that device can operate in without damage.
- Without any fluctuations the maximum and minimum operating temperatures can be measured.
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