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Cardiac Pace-Maker
GENERAL OPTICAL FIBER COMMUNICATION
SYSTEM
The major elements of general fiber optic communication system are
shown in fig. A. The fiber optic system can be described in one sentence as a
transmission system employing a light emitting source, turned on off very rapidly by
electrical impulses, whose emission are sent through an optical fiber to light sensitive
receiver to convert the changing light intensities back into electrical impulses.
The information source provides an electrical signal to a transmitter.
The main function of the transmitter section is to convert an electrical signal into
optical signal. The transmitter consist of a light source and its drive circuitry. The
light source may be either semiconductor LASER or light emitting diode (LED)
depending on application and requirement of optical fiber communication system.
The transmission medium is optical fiber cable. The cable offers mechanical strength
and environmental protection to the optical fiber contained inside. The cable may
also contain copper wires for powering repeaters which are needed for periodically
amplifying and reshaping the signal when the link spans long distances.
The receivers consists of photo detector, pulse amplifier and the
signal restoring circuitry. The main function of the receiver is to convert optical
signal into electrical signal. Photo diodes (p-n, p-i-n or avalanche photo diode) and in
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some instances photo transistor and photo conductors are utilised for the detection of
the optical signal and optical electrical to conversion.
Additional components includes optical connectors, splices, couplers
or beam splitters etc. The connectors and splices are required for joining fiber pieces
together to achieve ling distance communication. The optical couplers are required to
coupled light source to fiber at transmitter side and from fiber to photo-detector at
receiver side. Optical amplifier the optical signal without changing it into electrical
form.
The optical fiber generally contain several cylindrical hair thin glass
fibers each of which is independent communication channel.
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LIGHT SOURCES
Essentially, there are two devices commonly used to generate light for
fiber optic communication system.
1. Light emitting diode (LED)
2. Injection LASER diode (ILD)
Light Emitting Diode (LED):
It is simply a P-N junction diode. It is usually made from
semiconductor material such as aluminium gallium arsenide (AIGaAs) or gallium
arsenide phosphide (GaAsP). LED emits light by spontaneous emission, light is
emitted as result of the recombination of electrons and holes. When LED is forward
biased, minority charge carriers are injected across the P-N junction, these minority
charge carriers are injected across the P-N junction, these minority carriers
recombine with the majority carriers and give up energy in the form of light. This
process is same in the conventional diode expect that the process is radiative, a
photon is produced. A photon is a quantum of electromagnetic wave energy. The
energy gap of the material used to construct the LED determined whether the light
emitted by it is visible or invisible and of what colour.
The simplest LED structure are homojunction, epitaxially grown or
signal diffused devices.
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Epitaxially Grown LED
Epitaxially grown LEDs are generally constructed of silicon doped
gallium arsenide. This is shown in fig. B. A typical wavelength of light emitted from
this construction is 940 nm, and a typical output power approximately 3m W at
100 mA of forward current.
Planer Diffused Homojunction LED
Planer diffused homojunction LED is shown in fig. C. The typical out
put power from this structure is 500 micro watts at wavelength of 900nm. The
primary disadvantage of homo junction LED is the non directionality of their light
emission, which makes them a poor choice as a light source for fiber optic system.
Planer Hetrojunction LED
The planer hetero junction LED is quite similar to the epitaxially
grown LED except that the geometry is designed such that the forward current is
concentrated from six layer of semiconductor materials as shown in fig.D.
The planer hetero junction LED has several advantages over the homo
junction type. They are :
1. The increase in current density generates a more brilliant light spot.
(Higher directivity).
2. The smaller emitting area makes it easier to couple its emitted light into a fiber.
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3. The small effective area has smaller capacitance which allows the planer hetero
junction LED to be used at higher speeds.
The radiant light power emitted from the LED is a linear as function
of the forward current passing through the LED.
Edge Emitting Double Hetrojunction LED
Edge emitting double hetero junction LED gives highly directive light
beam. It consists of two different alloy layers having different band gap and
refractive index on each side of the active region which is the source of incoherent
light source. The construction of edge emitting bouble hetero junction LED is
shown in fig.E.
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CHARACTERISTICS OF LED
i) Radiance (Brightness) :
Radiance is defined as the optical power radiated into a unit solid
angle per unit area of the emitting surface. It is measured in watts/cm2. High
radiance is necessary to couple sufficiently high optical power levels into a fiber.
ii) Response Time :
The emission response time is the time delay between the application
of current pulse and the onset of optical emission. This time delay is the factor,
limiting the bandwidth with which the source can be modulated directly by varying
the injected current.
iii) Quantum Efficiency :
The quantum efficiency is related to the fration of injected electron
hole pairs that recombine radiatively.
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INJECTION LASER DIODE
The word LASER is an acronym for light amplification by stimulated
emission of radiation. LASERs are constructed from many different materials,
including gases, liquids and solids. Although, the type of LASER used most often
for fiber optic communication is the semiconductor LASER.LED emits the light
having combinations of various wavelengths (ultimately various frequencies) where
as LASER emits light a of signal frequency. Therefore LED is called non
monochromatic source and LASER is monochromatic source.
The injection LASER diode (ILD) is similar to the LED. In fact
below a certain threshold current an ILD acts as LED. Above the thershold current,
an ILD oscillates; Lasing action occures. The construction of an ILD is similar to
that of LED except that the ends are highly polished. The mirror like end surfaces
traps the photons in the active region, as they reflect back & forth, stimulate free
electrons to recombine with the holes. The two larger sides are deliberately
roughened in the cutting process to discourage the light emission.
When this heterojuction diode is forward blased with a DC voltage,
both ends of the LASER chip emit light. When one polished end is gold plated, the
other end will emit light. The construction of ILD is shown in fig.F.
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The three key transition processes involved in LASER action are :
1. Photon absorption.
2. Spoteneous emission.
3. Stimulated emission.
These three processes are represented by the simple two energy level
diagrams in fig. G. Where E1 is the ground state energy and E2 is excited state
energy. The open circle represents the initial state of electron & the heavy dot
represents the final state. According to Plank’s law, a transition between these two
stats involves the absorption or emission of photon energy, hv12 = E2 – E1.
Normally the system is in the ground state. When photon of energy
hv12 impinges on the system an electron in state E1 can absorb the photon energy and
be excited to state E2 as shown in fig. H. Since this is an unstable state, the electron
will shortly return to the ground state, there by emitting a photon of energy.
hv12 = E2 – E1. This occurs without any external stimulation and is
called spontaneous emission. As shown in fig. 1
The electron can also be induced to make a downward transition from
the excited level to the ground level by external stimulation, as shown in fig. J. if a
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photon of energy hv12. This emitted photon is in phase with the incident photon, and
the resultant emission is known as stimulated emission.
In a thermal equilibrium, the density of excited electrons is very
small. Most of photons incident on the system will therefore be absorbed, so that
stimulated emission is essentially negligible. Stimulated emission will exceed
absorption only if the population of the excited state is greater than that of the ground
state. This condition is known as population inversion. Since this is not an
equilibrium condition, population inversion is achieved by various ‘Pumping”
techniques. In semiconductor laser, population inversion is accomplished by
injecting electrons into the material at the device contacts to fill the lower energy
states of the conduction band.
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ADVANTAGES OF INJECTION LASER DIODE
(ILD)
1. But to highly directional pattern (i.e.LASER emits light which is concentrated in
very narrow region in one direction), it is easilier to couple their light into an
optical fiber. This reduces the coupling losses and allows smaller fibers to be
used.
2. The optical power output from ILD is greater that for an LED.The output optical
power from ILD is 1 to 100 mW where as the output optical power from LED is
1 to 10 mW.
3. ILD can be used at higher bit rates (>200mb/s) and for longer distance
communication than LED.
4. ILD generate monochromatic light, which reduces chromatic or wavelength
dispersion.
Disadvantages of ILD :
1. ILDs are highly expensive than LED.
2. Because ILD operates at higher powers, they have much shorter life time than
LEDs.
3. ILDs are more temperature dependant than LEDs. Thermal stabilisation is
essential for ILD where LED does not require such thermal stabilization. Due to
this LASER has complex drive circuitry than LED.
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LIGHT DETECTORS
There are two devices that are commonly used to detect light energy
in fiber optic communication receivers, PIN (Positive – intrinsic – Negative) diodes
and APD (Adalanche Photo Diodes).
1) PIN Photo Diode :
PIN photo diode is the most common device used as the light detector
in fiber optic communication system.
A very lightly doped (almost pure or intrinsic) layer of n-type
semiconductor material is sand witched between the junction of the two heavily
deped n and p- type contact areas as shown in fig. K. Light enters the device through
a very small windows and falls on the carriers void intrinsic material. The intrinsic
material is made thick enough so that most of the photons that enter the device are
absorbed by this layer. The PIN photo diode operates just the opposite of an LED.
Most of the photons are absorbed by electrons in the valence band of the intrinsic
material. When the photon are absorbed, they add sufficient energy to generate
carriers in the depletion region and allow the current to flow through the device.
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The PIN photo diodes has resposivity of the order of 0.5
microampere/microwatt. It has rise time about 1 ns and a very good frequency
response upto 1 GHz. The required bias voltage lies between 5 to 10 volts. The
disadvantages of this diode is its poor sensitivity and poor signal to noise ratio.
2) Avalanche Photo Diodes :
An APD is p-i-p-n structure as shown in fig. L. Light enters the diode
and is absorbed by the thin, heavily deped p-layer. This causes a high electric field
intensity to be developed across the i-p-n junction. The high reversed biased field
intensity causes impact ionization to occur near breakdown voltage of the junction.
During impact ionization, the carrier can gain sufficient energy to ionize other bound
electrons. These ionized carriers, in turn causes more ionization to occur. The
process continues like an avalanche and is, effectively, equivalent to an internal gain
or carrier multiplication. APDs are more sensitive than PIN diodes and require the
less additional amplification. The responsivity and rise time of APDs are of the
order of 15 micro ampere/micro watt and 2 ns respectively. The disadvantage of
APDs are relatively long transit times, additional internally generated noise due to
the avalanche multiplication factor and its temperature sensitiveness which requires
compensating networks.
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PARAMETERS OF PHOTO DETECTORS
i) Quantum efficiency (n) :
Quantum efficiency is defined as the ratio of number of electrons-hole
pairs generated to the number of incident photons on the surface of photo detetor.
ii) Responsivity (R):
The performance of photo detector is always characterised by
reponsivity. It specifies the photo detector is always characterised by reponsivity. It
specifies the photo current generated per unit optical power. It is given by,
R – IP/Po A/W
Where Po - Optical power incident on photo diode.
Ip - current flowing through the photo diode due to
optical power.
Responsivity is function of wavelength and material of photo diode.
iii) Dark Current :
Dark current is the current flowing through the photo diode without
light input.
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APPLICATION OF FIBER OPTIC
COMMUNICATION SYSTEM
The application of optical fiber communication systems extend in all
facts of communiction fields such as :
1. Metropolition telphone exchanges.
2. Long haul commercial trunking systems.
3. Under sea transmission systems.
4. Local Area network (LAN) like intrabuilding communications, computer
networking, cable T.V.etc.
5. Communication and control in hazardous situations such as coal mines, fuel
mines etc.
6. Railway communication.
7. On board communication in aeroplane, shop, train etc.
8. Military applications including long distance communication, tractical field
application, missile guidance systems, night vision system etc.
9. As sensors having very high sensitivity and large dynamic range.
10. Optical fiber have their use in medical application for obtaining cold light
illumination in the fields of opthalmology, gynaecology, ENT, general surgery
etc.
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11. A light source of one end of a bundle of optical fiber illuminates whatever is at
the other end. This can make a decorative lamp, a flexible illuminator for hard to
reach places illuminated signboards.
CONCLUSION
While studying, delivering and adopting the knowledge of advance
system or advance techniques we have to develop our basic knowledge and keep it
fresh.
As my seminar topic on Cardiac pace maker I have conculed Fiber
Optical Sources and detectors it is that the paienst who are suffering from heart
failure clerared somewhat basic about the Fiber Optical Sources and Detectors.
Problem can be overcome by given enteral or interal.
The Fiber Optical System has several advantages over other
communication system so this is the most efficient and accurate system in point to
like of palent is saved. Point and hard reach places.
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