fibre optical sources & detectors

8/14/2019 fibre optical sources & detectors

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Fiber Optical Sources & Detectors

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 threshold current,

an ILD oscillates; Lasing action occurs. 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 heterojunction diode is forward biased 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 Planks 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 easier 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 (Avalanche 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

doped 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 responsivity 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 doped 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 detector.

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 communication fields such as :

1. Metropolitan telephone 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 Fiber Optical Sources and detectors, it is

cleared somewhat basic about Fiber Optical Sources & Detectors.

The Fiber Optical System has several advantages over other

communication system so this is the most efficient and accurate system in point to

point and hard reach places.

REFERENCES

1. Communication System

2. Advance Communication System

- Vrinda Publication.

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Electrical Form

Transmitter ElectricalInput Fiber Optical splice

Signal Drive Light flyleadCircuit Source

Connector

Optical

FiberElectrical signal Repeater Optical coupler or Optical signal Beam splitter

OpticalReceiver

Electronics

To other equipment

OpticalTransmitter

Receiver

Fiberflylead

ElectricalOptical Photo- Signal Signal

Amplifier detector restorer Output

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Figure: Major elements of an optical fibre transmission link. The basic components

and the transmitter, cable, receiver. Additional elements include fibre and cable

splices, repeaters, beam splitters and optical amplifiers.

Fig. A.

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