Print - Chapter 4 _ Optical Receivers _ Optical Fibre Communication

8/2/2019 Print - Chapter 4 _ Optical Receivers _ Optical Fibre Communication

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Chapter 4 : Optical Receivers IOptical FibreCommunication

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Q. 1. What do you mean by Photo detection?Ans. Photodetection. It is the processwhereby optical poweris detected and then converted to electrical power. Photodetector devices perform photo detection. The performance ofan optical detector can be determined by its ability to detectsmallest optical power possible and generate a maximumelectric power at the output with an absolute minimum degreeof distortion. Optical detection must also exhibit acomparatively wide Bandwidth and sharp response toaccommodate high bit rate criteria for selecting particularphotodiode is its ability to interface with optical cables, alongoperating life and cost highly sensitivity, fast response, lownoise, high reliability.

Q. 2. Define quantum efficiency and responsively of aphoto detector. How does the responsively depend onthe quantum efficiency of the device and wavelength ofthe incident radiation.Ans. Quantum efficiency: Quantum efficiency isdefined as theratio of number of electrons collected to the number ofincident photons. It isalso defined as the fraction of incidentphotons which are absorbed by photo detector and generateelectrons which are collected at detector terminals.

Rp = Incident photon rate (photons/secs)Re = electron rate (electrons/ sec)

. Rthen Quantum efficiency, Y / = Rep

If

All the incident photons are not absorbed to generate electronhole pairs therefore quantum efficiency is generally less thanone. Itdepends on the absorption coefficient of thesemiconductor material usedwithin the photo detector.Responsivity.Responsivity represents the sensitivity of a photo detector.The function of photo detector is to convert the optical signal


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Chapter 4 : Optical Receivers IOptical Fibre Communicationinto electrical signal. When the incident on semiconductormaterial hasan energy greater than band gap energy then anelectron-hole pair is generated each time a photon is absorbedby semiconductor. More photons that strike the photodetector, more charge carriers will be produced i.e. greaterwill be the photo current I."i.e. photo current is directlyproportional to incident optical power Pm.I e x p.p III

Ip = R Pinwhere R is constant called responsivity of photo detector in (A/W).Relationship of responsivity. with quantum efficiency.

. RQuantum efficiency '1 = = . - - - ! . . . .RpAs the energy of photon E = hv then

Pi nincident photon rate.1), = = . - ' ; ; ; ; where Pin = = incident optical power.IpElectron rate R e = q

.& _Quantum efficiency" = = Rp.

But

Ip'" " q'1 Pilllt v

'1 - Ip hvqX P ill2 _ ' 1 QP;II hv

R '" ' q qhI)C

V ~ ).

R = = '1qA = "the 1.2 4From above equation responsivity is directly proportional toquantum efficiency at particular wavelength. An idealresponsivity versus wavelength quantum efficiency is shown infig. for a silicon photo diode having unit 4.1.


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Chapter 4 : Optical Receivers IOptical Fibre Communication

l O . 8 8b.s;.;;1 0.665iO.44

G o>a : : 0.220.5 1.0 AeINavelength (jJm)

Flg.4.1

Responsivity of photo detector increases with wavelengthbecause more photons are present for same optical power. Forphotons having energy have less than band gap energy E, thelinear dependence of responsivity on wavelength does notcontinue because photon energy becomes too small togenerate electrons. The quantum efficiency drops to zero.

Q. 3. What do you mean by the long wavelength cut offpoint for photodetector.

Ans. For intrinsic, absorption of photons to take place, theenergy of incident photon should be equal to or greater thanband gap energy Eof the photo detector materialhv ~ Egh eT i;! Egh e- . < ! ! AEg

h eA


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Chapter 4 : Optical Receivers IOptical Fibre CommunicationLighti iregion

Fig. shows basic structure of a photodiode.

E Field

TAbsorptionRegionJ : . .

Depletion. ,__.~.,..._=_",...;_::-:: _!l:~gionN Diffusion Region. _ _ _ , r - - ' - - - - - - - - - - - - - - - - - - - - - - - - -

Fig. shows PN photodiode with depletion anddiffusion region.A reverse biased P-n junction consists of a region known asdepletion region which is devoid of free charge carriers. Thereis large electric field across depletion region which opposesflow of electrons from n side to p-side and holes from P to sideto n side. A reverse biased P-N junction diode has smallamount of reverse saturation current due to thermallygenerated electron-hole pairs. When P-N junction isilluminated with light on one side, electron hole pairs arecreated through absorption. The photo induced electrons inconduction band of P type will move across the junction to n-side and holes produced in valence band of N type will flowacross junction to P side. This process of diffusion and rapidcrossing of depletion region takes place so rapidly that there islittle possibility of recombination. The resulting flow of currentis proportional to incident optical power.

Current(~)

OpticalPower

t800\ 6 0 0400

Low lighllevel200

Dark currentz 10 20 30 40

Reverse b ias vcI tage (V){b)a)

Fig. 4.4

Fig. 4.4(a) Variation of Optical Power inside the photodiode. Asshown in Fig. 4.4(a) optical power decreases exponentially asthe incident light is absorbed inside the depletion region.Fig. 4.4( b ) shows the curves between the photo currentversus reverse biased voltage with light intensity as


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Chapter 4 : Optical Receivers IOptical Fibre CommunicationparameterThe bandwidth of a P-nphotodiode is limited by the transittime r;

whereTtr = = w /v dW = Width of depletion regionVd '" drift velocity

The width of depletion region depends on theconcentration of acceptor and donor impurities. Drift velocitydepends on the applied voltage.

Disadvantages. When the light strikes P-Njunctionphotodiode. Some of the photons enters into the n region.These electron-hole pairs are not affected by the field acrossthe junction and do not contribute to the photo current. Theconversion efficiency of P-Njunction photodiode is low but theyrespond very quickly to any change of light intensity.Detection process in P-njunction photo diode

Consider a reverse biased P-Njunction photodiode. Due toreverse biasing, a thick depletion layer develops on the eitherside of the junction. The large potential

fig.4.5barrier across the depletion layer prevent the majority carriersto cross the junction. Supposea photon of light is incident in ornear the depletion region. If the incident photon has energy hvequal to or greater than the bandgap energy Egof thesemiconductor material of the P-Njunction, the photon willexcite an electron from valence band to conduction band. This,processwill generate an "electron-hole" pair as shown in Fig.4.5. This is known as photogeneration. The photogeneratedelectron- hole pairs are separated in the depletion layer andare swept away by the electric field due to the applied reversebiased votlage. In order to achieve maximum carrier pairgeneration, the depletion region should be sufficiently thick sothat large fraction of the incident light can be absorbed.

Q. 5. Explain the principle of p-i-n photodlode.

Ans. p-i-n photodiode consists of p and n regions separated bya very lightly deped intrinic (i) region. The intrinic layer hasonly a very small amount of dopant and acts as a widedepletion layer. In normal operation, a sufficiently largereverse bias voltage isapplied across the device so that theintrinsic region is fully depleted of carriers. At longerwavelengths, light penetrates more deeply into thesemiconductor material. To operate at Iqnger wavelength, we


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Chapter 4 : Optical Receivers IOptical Fibre Communicationmust have a wider depletion region which is obtamed in P-i-nphoto diode.

BiasV Oltage +

Electric Field

D e p le ti on R e g io n

xF I li i. 4 . 6

When an incident photon has an energy greater than orequal to the bandgap energy Eof the semiconductor material,the energy of the photon excites an electron from the valenceband to the conduction band. This processproduces electronhole pairs. The generated carriers are called photocarriers.Light is incident on depletion region so photo generatd carriersare generated in the depletion region. The high electric fielddeveloped across the depletjon region causesthe carriers toseparate and to be collected by the reverse biased voltage.This causesa current to flow in the external circuit which isreferred to as photocurrent.The performance of p-i-n photodiodes can be improved byusing a double heterostructure design. In this intrinsic layer issandwiched between the p-type and n-type layers of adifferent semiconductor whose band gap is chosen such thatlight is absorbed only in the middle i-layer. For light waveapplications a P-i-n photodiode using In GaAsfor intrinsic layerand In Pand 8 surrounding P-type and n-type layers are used.

n+-In P(Substrate)

Incident light

F/ir. 4.7 In ~ P-I.fl p hotodlocfe.

Q. 6. Explain the operation of Avalanche photodiodewith the help of necessary diagram.OR

Explain the principle and construction of reach throughAPD with a neat sketch.

Ans. Avalanche photo diodes amplify the signal during thedetection process. They usea similar principle to that of photo


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Chapter 4 : Optical Receivers IOptical Fibre Communicationmultiplier tubes. In APDSmultiplication takes place within thesemiconductor material. An internal amplification of between10 and 100 times takes place inAPDS.Avalanche photo diodes (APDs) internally multiply the primarysignal photocurrent before it enters the input circuitry of thefollowing amplifier which increases receiver sensitivity. Forcarrier multiplication to take place, the photogenerted carriersmust traverse a region where a very high electric field ispresent. Photogenerated electron or whole get energy fromhigh field region and ionizes the bound electrons in the valenceband. This carrier multiplication mechanism is known asimpact ionization. The newly generated carriers are againaccelerated by, high electric field and gain

_ Regiont M i ni m um F i el dRequired for impactionization

z

Fig. 4.8Enoughenergy to cause further impact ionization. Thisphenomenon is avalanche effect. Below diode breakdownvoltage, a finite number of carriers are created where asabove breakdown infinite no. of carriers is created.Silicon Reach through Avalanche photo diodeSilicon Reachthough avalanche photodiode is shown in figure4.8. It is composed of high resistivity Ptype materialdeposited as an epitaxial layer on p + (heavily deposed ptype) substrate. A p type diffusion or ion implant is then madein high resistivity material followed by the construction of a (n+) layer. This configuration is referred to as P + n pn+] reachthrough structure.

When a low reverse bias voltage isapplied acrossAPD, mostof the potential drop isacross pn+ junction. When we increasethe reverse bias, the width of depletion layer increases whichincreases the electric field across Pn+ for upto the pointneeded to causeavalanche breakdown. At this point, thedepletion layer just 'reaches through' to nearly intrinsic Hregion.

Photons passthrough the n+p junction and are absorbed inthe H layer. This absorption produces a free electron in the


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Chapter 4 : Optical Receivers IOptical Fibre Communicationconduction band and a hole in the valence band. The electricpotential across the st layer is sufficient to attract the electronstowards one contact and the holes towards the other. Thepotential gradient across the 1r layer is not sufficient for thecharge carriers to gain enough energy for multiplication totakeplace Around the Junction between the n+ and Players,the electric field is somtense that the charge carriers arestrongly accelerated and pick up energy, when these electronscollide with other atoms in the lattice they produce newelectron hole pairs. The newly released charge carriers arethemselves accelerated in opposite directions and collideagain.

Germanium Avalanche photodiodeGermanium avalanthe photo diodes are used to fabricate moresensitive and fast APDS.These are used over wavelengthrange of O.8.um -1.6.um .. They have same n+p structure as thatof Si APDS.GeAPDShave dark current which is very muchsensitive to temperature variations. They have relatively highabsorption coefficient at 1 .3.ur n due to which they have quitelow avalanche breakdown voltages. Ge APDstructures arefabricated to provide multiplication initiated by holes to reduceexcess noise factor in longer wavelength.

Q. 7. What are the advantages and disadvantages ofavalanche photodiode?

Ans. Advantages of avalanche photodiode:(i) Avalanche photodiode are more sensitive to detection ofoptical signal.(ii) APDSamplify the signal during the detection of opticalsignal.Disadvantages of avalanche photodiode:(i) APDSrequire high bias voltages which are wavelengthdependent.(ii) APDShave random nature of gain mechanism which givesan additional noisecontribution.(iii) APDSare costly as compared to other photodiodes.(iv) APDSare difficult to fabricate.

Q. 8. Explain the effect of temperature on avalanchegain in APDS.

Ans. Avalanche gain inAPDSis temperature sensitive becauseelectrons and holes ionization rates depend on temperature.The effect of temperature on gain is more at high biasvoltages.To minimize the effect of temperature on avalanche gain, the


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Chapter 4 : Optical Receivers IOptical Fibre Communicationelectric field in multiplying region of p-n junction should bechanged. This isobtained by using compensation circuit thatadjusts the applied bias voltage on photo detector whentemperature changes. The multiplication factor for chargecarriers is given by

1M=---l - ( ~ rwhere, VB = breakdown voltage . .

n = parameter varies between 2.5 and 7V = Va - IMRM withVa = reverse bias voltage appliedto photo detector.1 M = Multiplied photocurrent~ . = Photodiode series resistance and detector load resistance.

Q. 9. Briefly explain the term multiplication factor w.r.t.APD?

Ans. Multiplication factor M isa measure of internal gainprovided by APD. It is defined as the ratio of total multipliedoutput current to the primary unmultiplied current.

. '.I'

M"IrI .. Total multiplied J"output current

. Ip = Primary unmultiplied currentwhere

Multiplication depends on physical and operationalcharacteristics of photo detector device. Operationalcharacteristics include the width of avalanche region, thestrength of electric field and type of semiconductor materialemployed.

Q. 10. What are the different noise mechanisms whichare responsible for current fluctuations In opticalreceivers,OR

Derive the expressions of signal to noise ratio in P-i-n.And APD receivers.

Ans.Receiver NoiseOptical Receivers convert incident optical power into electriccurrent through photodiode. The relationship . J " s R .P in assumesthat sucha conversion is noise free. But this is not the caseeven for perfect receiver. Two fundamental noise mechanisms


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Chapter 4 : Optical Receivers IOptical Fibre Communicationshot noise and thermal noise lead to fluctuations in currenteven when incident optical signal has constant power.Electrical noise induced by current fluctuations affects thereceiver performance.NoiseMechanismsThe photocurrent in photo diode depends of light power inputi .e . . Ip = R P .. Even if input light power is constant; the photocurrent does not remain constant as in reality it contains noisecomponents.1. Shot Noise. Suppose input power is constant which meansnumber of photons per unit of time is constant. But theactual number of photons arrived at a particular time isunknown and so it isa completely random variable.Hence, the no. of photo generated electrons at anyparticular instant isa random variable. The number ofelectrons producing photocurrent will also vary because oftheir random recombinations and absorptions even thoughthe average number of electrons is constant, the actualnumber of electrons will vary.

Deviation of actual is of electrons from the average number isknown as shot noise.The photodiode current generated in response to a constant opticalsignal isI(I) ... f y , + i, (t)where, I p = R P in Is average current

is(t) .. Current fluctuation related to shot noise. .The autocarrelation function of i ; (t) is're.lated to spectral density S s ( F ) by Wi.ener-Khinchin theorem. . -

110

< i.(I ) i.(t + r) >.. f-00 S s (F ) expo (2m ft) d f . ( 1)


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Chapter 4 : Optical Receivers I Optical Fibre Communicationwhere angle brackets denote an ensemble average over and fluctuations.-r:t'e s ~al density of mot noise isronstant and is given by S s . (F ) .. q lp where

5s(F) IS two sided spectral density +ve and -ve frequencies are induded in above" n(I). If only + ve frequencies are considered b y changing the lower limit of integrationto zeros, one sided spectral density becomes 2q~. .

Nosie variance is obtained by setting 1" ::: Oin equation (1),2 2u~ ..


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