fibre optical sources & detectors

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 repeaters which are needed for periodical periodically ly amplifying and reshaping the signal when the link spans long distances.

The receiv receivers ers consis consists ts of photo photo detect detector, or, pulse pulse ampli amplifie fierr 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

Govt.Poly.Washim. ≅

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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 generally contain contain several several cylindrica cylindricall 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. Ligh Lightt emit emitti ting ng diod diodee (LED (LED)) 2. Inje Inject ctio ion n LAS LASER ER diode diode (ILD (ILD))

Light Emitting Diode (LED): It is simpl imply y a P-N junc juncttion ion diod diode. e. It is usua usuallly made ade from 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 char charge ge carr carrie iers rs are are inje inject cted ed acros acrosss the the P-N P-N junct junctio ion, n, thes thesee mino minori rity ty carr carrie iers rs 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. Govt.Poly.Washim. ≅

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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 o ptic 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. type. They are are : 1. The The incre ncreas asee in cur current rent dens densit ity y gene generrates ates a more more bri brilliant iant light ight spot spot.. (Higher directivity). 2. The smaller smaller emittin emitting g area makes it it easier easier to couple couple its emitted emitted light light into a fiber. fiber. Govt.Poly.Washim. ≅

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3. The small small effective effective area area has smaller smaller capacit capacitance ance which which allows allows the planer planer hetero 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. beam.

It consists consists of two differe different nt alloy layers layers having having differ different ent band gap and

refractive index on each side of the active region which is the source of incoherent light light source. source.

The constru constructi ction on of edge emitting emitting bouble bouble hetero hetero junctio junction n 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 angle per unit area area of the emitti emitting ng surface surface.. It is measur measured ed in watts/c watts/cm2. m2. High 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 current pulse and the onset of optical optical emission. emission. This time time delay is the factor, 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 emission of radiation. radiation.

LASERs LASERs are constructed constructed from many different different materials materials,,

including including gases, liquids liquids and solids. Although, 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 LASER emit emitss light light a of signal signal freque frequenc ncy. y.

Ther Theref efor oree LED LED is calle called d non

monochromatic source and LASER is monochromatic source.

The inject injection ion LASER LASER diode diode (ILD) is simil similar ar 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 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 emit light. When one polished end is gold plated, plated, the other end will emit light. 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. Phot Photon on abso absorp rpti tion on.. 2. Spot Spoten eneou eouss emis emissi sion on.. 3. Stim Stimul ulat ated ed emis emissi sion on..

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. energy. The open circle circle represen represents ts the initi initial al state of electr electron on & the heavy dot represents the final final state. According to Plank’s law, a transition transition between these two stats involves the absorption or emission of photon energy, hv12 = E2 – E1.

Normally Normally the system system is in the ground ground state. When photon 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 occurs without without any external external stimulatio stimulation n and is called spontaneous emission. 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 emitted photon is in phase with the the incident photon, and the resultant emission is known as stimulated emission.

In a thermal equilibrium, the density of excited electrons is very small. small. Most of photons photons incident incident on the system will therefore therefore be absorbed, absorbed, so that stimulated stimulated emission emission is essentially essentially negligibl negligible. e.

Stimulate Stimulated d emission will exceed exceed

absorption only if the population of the excited state is greater than that of the ground state. state.

This This conditi condition on is known known as popula populatio tion n inversio inversion. n.

Since Since this this is not an

equilib equilibriu rium m conditi condition, on, populat population ion invers inversion ion is achiev achieved ed by variou variouss ‘Pump ‘Pumping ing”” techni techniques ques..

In semico semicondu nducto ctorr laser, laser, populati population on invers inversion ion is accomp accompli lishe shed d 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 highly directio directional nal pattern pattern (i.e.LASE (i.e.LASER R emits light light which which is concentrat concentrated ed in very narrow region in one direction), it is easier to couple their light into an optical optical fiber. This reduces reduces the coupling losses losses and allows smaller smaller fibers to be used. 2. The optical optical power output output from from ILD is greate greaterr that for an LED. The output output optical optical power from ILD is 1 to 100 mW where as the output optical power from LED is 1 to 10 mW. 3. ILD ILD can can be used used at high higher er bit bit rate ratess (>20 (>200m 0mb/ b/s) s) and and for for long longer er dist distan ance ce communication than LED. 4. ILD ILD gener generat atee mono monoch chro roma mati ticc light light,, whic which h reduc reduces es chro chroma mati ticc or wave wavele leng ngth th dispersion.

Disadvantages Disadvantages of ILD : 1. ILDs ILDs are are highl highly y expens expensive ive than than LED. LED. 2. Becaus Becausee ILD operates operates at higher higher powers, powers, they have have much shorter shorter life life time than LEDs. 3. ILDs ILDs are more more temp temper erat atur uree depen dependa dant nt than LEDs. LEDs.

Ther Therma mall stab stabil ilis isat atio ion n is

essential for ILD ILD where LED does not require such thermal stabilization. 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 very light lightly ly doped doped (alm (almos ostt pure pure or intr intrin insi sic) c) laye layerr of n-ty n-type pe 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. material. The intrinsic material is made thick enough so that most of the photons that enter the device are absorbed absorbed by this layer. The PIN photo diode operates operates just the opposite opposite of an LED. Most of the photons are absorbed by electrons in the valence band of the intrinsic material. material. When the photon are absorbed, absorbed, they add sufficien sufficientt energy to generate generate carriers in the depletion region and allow the current to flow through the device.


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The The PIN PIN phot photo o diod diodes es has has res respon ponsivi sivitty of the the orde orderr of 0.5 0.5 microa microampe mpere/ re/mic microw rowatt att..

It has rise time about 1 ns and a very very good freque frequency ncy

response response upto 1 GHz. GHz. The required required bias bias voltage voltage lies lies between between 5 to 10 volts. 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. p-layer. This causes a high electric electric field intensity intensity to be developed across the i-p-n i-p-n junction. junction. The high reversed reversed biased field intensity intensity causes impact ionization ionization to occur near breakdown voltage voltage of the junction. junction. During impact ionization, the carrier can gain sufficient energy to ionize other bound electrons. electrons. These ionized ionized carriers carriers,, in turn turn causes causes more more ionizat ionization ion to occur. The process continues like an avalanche and is, effectively, equivalent to an internal gain or carrier multiplication. multiplication. APDs are more sensitive sensitive than PIN diodes and require require the less additiona additionall amplification amplification.. The responsivit responsivity y and rise time of APDs are of the order of 15 micro micro ampere/micro ampere/micro watt watt and 2 ns respectively. respectively. The disadvantage 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 The perf perfor orma mance nce of photo photo detec detecto torr is alwa always ys char charac acte teri rise sed d by reponsivity. It specifies specifies the photo detector is is always characterised by reponsivity. It specifies the photo current generated per unit optical power. It is given by,

R – IP/Po

µ

A/µ 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 pho to diode.

iii)

Dark Current : Dark current is the current flowing through the photo diode without

light input. Govt.Poly.Washim. ≅

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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. Metrop Metropoli olitan tan teleph telephone one exchang exchanges. es. 2. Long Long haul comm commerc ercial ial trun trunkin king g system systems. s. 3. Under Under sea sea tran transmi smissi ssion on sys system tems. s. 4. Loca Locall Area Area netw networ ork k (LAN (LAN)) like like intr intrab abui uild ldin ing g comm communi unicat catio ions ns,, comput computer er networking, cable T.V. etc. 5. Comm Communi unicat catio ion n and cont contro roll in haza hazard rdou ouss situ situat atio ions ns such such as coal coal mine mines, s, fuel fuel mines etc. 6. Rail Railwa way y comm communi unicat catio ion. n. 7. On board board communicati communication on in aeroplane, aeroplane, shop, shop, train train etc. etc. 8. Milita Military ry applica applicatio tions ns includ including ing long long distan distance ce commun communica icatio tion, n, tracti tractical cal field field application, missile guidance systems, night vision system etc. 9. As sensors sensors having having very very high high sensitivi sensitivity ty and large large dynamic dynamic range. range. 10. Optica Opticall fiber fiber have their use in medica medicall applic applicati ation on for obtainin obtaining g cold cold light 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 optical fiber illuminat illuminates es whatever is at the other end. This can make a decorative lamp, lamp, a flexible illuminator 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 The Fibe Fiberr Opti Optica call Syst System em has has seve severa rall adva advant ntag ages es over over othe other r communication system so this is the most efficient and accurate system in point to point and hard reach places.

REFERENCES 1. Comm Communi unicat catio ion n Syst System em 2. Advance Advance Commun Communica icati tion on Syste System m -

Vrinda Pub Publication.


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Electrical Form Transmitter Input Signal

Drive Circuit

Light Source

Electrical Optical splice

Fiber flylead

Connector

Optical Fiber Electrical signal Optical signal

Repeater

Optical coupler or Beam splitter

Optical Receiver Electronics To other equipment Optical Transmitter

Receiver Fiber flylead Electrical Optical Amplifier


Photodetector

Signal restorer

Signal Output

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Figure: Major elements of an optical fibre transmission link. The basic components and the transmitter, transmitter, cable, receiver. receiver. Additional elements elements include include fibre fibre and cable splices, repeaters, beam splitters and optical amplifiers.

Fig. A.


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fibre optical sources & detectors

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