detectoThe different types of photodetectors mainly include a photodiode, MSM photodetector, phototransistor, photoconductive detector, phototubes & Photomultipliers.r11.ppt

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Photodetector : Circuit, Working, Types & Its Applications
The photodetector is an essential component in an optical receiver that converts the incoming optical signal into an electrical signal. Semiconductor photodetectors are usually called photodiodes because these are the main types of photodetectors utilized in optical communication systems due to their quick detection speed, high detection efficiency & small size. At present, photodetectors are widely used in industrial electronics, electronic communications, medicine & healthcare, analytical equipment, automotive & transport, and many more. These are also known as photosensors and sensors of light. So, this article discusses an overview of a photodetector – working with applications.



What is Photodetector?
A photodetector definition is; an optoelectronic device that is used to detect the incident light or optical power to convert it into an electrical signal is known as a photodetector. Usually, this o/p signal is proportional to the incident optical power. These sensors are absolutely needed for different scientific implementations like process control, fiber optic communication systems, safety, environmental sensing & also in defense applications. Examples of photodetectors are phototransistors and photodiodes.

Photodetector
Photodetector
How Photodetector Works?
Photodetector simply works by detecting light or other electromagnetic radiation or devices may by receiving the transmitted optical signals. Photodetectors that use semiconductors operate on the electron-hole pair creation upon the light irradiation principle.


Once a semiconductor material is illuminated through photons that have high or equivalent energies to its bandgap, then absorbed photons encourage valence band electrons to move into the conduction band, so leaving behind holes within the valence band. The electrons in the conduction band perform as free electrons (holes) that can disperse under the power of an intrinsic or externally applied electric field.

The photo-generated electron-hole pairs because of optical absorption may recombine & re-emit light unless subjected to an electric field-mediated separation to give an increase to a photocurrent, which is a fraction of the photo-generated free charge carriers received at the electrodes of the photodetector arrangement. The photocurrent magnitude at a specified wavelength is directly proportional to the intensity of incident light.

Properties
The properties of photodetectors are discussed below.

PCBWay
Spectral Response – It is the photodetector’s response as a photon frequency function.

Quantum Efficiency – The number of charge carriers generated for each photon

Responsivity – It is the output current separated by the total power of light dropping on the detector.

Noise

1. What Are Laser Diodes?
Datadoesnotcomeintheformofopticalsignals.Instead,datais
electrical.
Youneedtoconvertthoseelectricalsignalsintolight,whichis
wherethelaserdiodecomesintoplay.
Youpumpelectricalcurrentdirectlyintothelaserdiode;this
currentstimulationinthesemiconductormaterialinthediode
resultsinthegenerationofanemittedphoton.
Theseemittedphotonsalonewouldn’tdomuch,butthrough
sustainedelectricalcurrent,youcancreateastreamofphotons,
allwiththesamephaseandwavelength.
1

2. 2
Fig.4.17Thethreekeytransitionprocessesinvolvedinlaseraction.
Theopencirclerepresentstheinitialstateoftheelectronandthe
heavydotrepresentsthefinalstate;incidentphotonsareshownon
theleftofeachdiagramandemittedphotonsareshownontheright.

3. Fabry-Perot resonator cavity for a laser diode
3

4. Fig.4.18Fabry-Perotresonatorcavityforalaserdiode.The
cleavedcrystalendsfunctionaspartiallyreflectingmirrors.The
unusedend(therearfacet)canbecoatedwithadielectric
reflectortoreduceopticallossinthecavity.Notethatthelight
beamemergingfromthelaserformsaverticalellipse,even
thoughthelasingspotattheactive-areafacetisahorizontal
ellipse.
4

5. Stimulated emission
Anatomintheexcitedstateneednotwaitforspontaneous
emissiontooccur.
Aphotonofenergyhv=E2–E1caninducetheexcitedatomto
makeadownwardtransitionreleasingtheenergyintheformofa
photon.
Thus,theinteractionofaphotonwithanexcitedatomtriggers
theexcitedatomtodroptothelowerenergystategivingupa
photon.
Thephenomenonofforcedemissionofphotonsiscalledinduced
emissionorstimulatedemission.Thisprocessmaybe
representedas
A* + hv= A + hv (3)
5

6. ThenumberofstimulatedtransitionsNstoccurringinthe
materialduringtimeDtmaybewrittenasNst=B21N2QDt,
whereB21representstheprobabilityofastimulatedemissionas
depictedintheabovefigure.
6

7. Unit 4
Optical Detector

8. Introduction
Adetector’sfunctionistoconvertthereceivedoptical
signalintoanelectricalsignal,whichisthenamplified
beforefurtherprocessing.
8
I
Light

9. a)Highsensitivityattheoperatingwavelength.
b)Highfidelity.Toreproducethereceivedsignal
waveformwithfidelity(Example:foranalog
transmissiontheresponseofthephotodetectormust
belinearwithregardtotheopticalsignalovera
widerange).
c)Largeelectricalresponsetothereceivedoptical
signal.Thephotodetectorshouldproducea
maximumelectricalsignalforagivenamountof
opticalpower.
9
Requirements:

10. 10
d)Shortresponsetime.(pn-µsec,PIN/APD-nsec)
e)Minimumnoise.
f)Stability.
g)Smallsize
h)Lowbiasvoltage.
i)Highreliability
j)Lowcost

11. 11

12. 12

13. 13

14. 14

15. 15
Representationofapinphotodiodecircuitwithanapplied
reversebias.Anincidentopticalpowerleveldecays
exponentiallyinsidethedevice.

16. 16

17. 17
absorbed

18. Thisdeviceisreversebiasedandtheelectricfield
developacrossthep-njunctionsweepsmobile
carriers(holesandelectrons)totheirrespective
majoritysides(pandn).
Aphotonincidentinornearthedepletionregionof
thisdevicewhichhasanenergygreaterthanorequal
tothebandgapenergyE
gofthefabricatingmaterial
(i.e.hf>E
g)willexciteanelectronfromthevalence
bandintotheconductionband.
Thisprocessleavesanemptyholeinthevalanceband
andisknownasthephotogenerationofanelectron-
hole(carrier)pair.
18

19. 19
Carrierspairssogeneratednearthejunctionare
separatedandswept(drift)undertheinfluenceofthe
electricfieldtoproduceadisplacementcurrentinthe
externalcircuitinexcessofanyreverseleakagecurrent
(Fig5.1(a)).
Photogenerationandtheseparationofacarrierpairin
thedepletionregionofthisreversebiasedp-njunction
isillustratedinFig.5.1(b).

20. 20

21. 21

22. Optical Detection Principles
22
Operationofthep-nphotodiode:(a)thestructureofthereversebiasedp-njunction
illustratingcarrierdriftinthedepletionregion;(b)theenergybanddiagramofthereverse
biasedp-njunctionshowingphotogenerationandthesubsequentseparationofanelectron-
holepair.
Fig. 5.1
(a)
(b)

23. 23

24. 24
Absorptionoutsidedepletionregion–diffusioncurrent-
reducesspeed.
Absorptioninsidedepletionregion–driftcurrent–fastdue
tothelargeelectricalfield.
Thedepletionregionmustbesufficientlythicktoallowalarge
fractionoftheincidentlighttobeabsorbedinordertoachieve
maximumcarrierpairgeneration.(PN1to3µm,PIN20to50µm).
However,sincelongcarrierdrifttimesinthedepletionregionrestrict
thespeedofoperationofthephotodiodeitisnecessarytolimitits
width.

25. 25
Photodetector Characteristics
1. Responsivity
Responsivity -ratio of current output to light input
varies with wavelength
theoretical maximum resposivity: 1.05A/W at 1300nm
typical responsivity: 0.8 -0.9 A/W at 1300nm
formula for theoretical maximum responsivity (quantum efficiency = 100%)
where:
R = theoretical maximum responitivity in Amps/Watt
= quantum efficiency
= wavelength in nanometers1240

R
R = ηeλ
hc
e=1.6e-19, h=6.63e-34, c=3e81
AW
P
I
R
o
p

26. 26
Quantum Efficiency -
ratio of primary
electron-hole pairs
created by incident
photons to the
photons incident
on the diode material
Figure 5.2 Typical Spectral Response of
Various Detector Materials
(Illustration courtesy of Force, Inc.)
2. Quantum Efficiencyp
r
r
e
 
number of electrons collected
Number of incident photons
where r
pis the incident photon rate (photon per second) and r
e
is the corresponding electron rate (electrons per second)

27. 27
3. Capacitance of a detector
dependent upon the active area
of the device and the reverse
voltage across the device.
A smaller active diameter
makes it harder to align the
fiber to the detector.
Also, only the center should
be illuminated
photodiode response is slow
at the edges
edge jitter
Figure 6.2 Capacitance versus Reverse
Voltage
(Illustration courtesy of Force, Inc.)

28. 28
Response Time
Time needed for the photodiode to
respond to optical input and
produce an external current
Dependent on
photodiode capacitance
load resistance
design of photodiode
Measured between 10% and 90%
of amplitude
90%
10%
Vout
Time
Rise
Time
Fall
Time

29. 29
Response Time
Approximate -3 dB frequency formula:
where:
R = Impedance that the detector operates into
C = Capacitance of the detector
Rise or fall time formula:
Formula for and f
-3dBRC
f
dB
2
1
3
 RC2.2 dBf
3
35.0



30. Example 6.1 If the absorption coefficient of In 0.53Ga0.47As
is 0.8 mm–1 at 1550 nm, what is the penetration depth at which
P(x)/Pin = l/e = 0.368?
30

31. Example 6.2 A high-speed In0.53Ga0.47As pin photodetector is
made with a depletion layer thickness of 0.15 mm. What percent
of incident photons are absorbed in this photodetector at 1310 nm
if the absorption coefficient is 1.5 mm–1 at this wavelength?
31

32. 32
Problems
Calculate the theoretical maximum responsivity of a detector at 1550nm.
Calculate the theoretical maximum responsivity of a detector at 820nm.
Calculate the -3dB frequency and rise time of a detector with a capacitance of 0.5pF
operating into an impedance of 50W.
Calculate the responsively of a detector with quantum efficiency of 10% at
800 nm.
Ans: 6.45 A/W
A detector operating at 800 nm produces an output current of 80 A for an
incident light beam of power 800 W. Calculate the quantum efficiency and
responsivity of the detector.
Ans: 0.1 A/W , 15.5%
Answers: 1.25 Amps/Watt, 0.661 Amps/Watt, 6.4 GHz

33. Avalanche Photodiode : Working & Its Applications
Thediodewhichusestheavalanchemethodtoprovideextra
performanceascomparedtootherdiodesisknownasavalanche
photodiode.
33
Thesediodesareusedtochangethesignalsfromopticalto
electrical.Thesediodescanbeoperatedinhighreversebias.The
avalanchephotodiodesymbolissimilartotheZenerdiode.
Symbol of APD

34. Avalanche Photodiode Construction
34

35. TheconstructionofboththePINphotodiodeandAvalanche
photodiodeissimilar.
Thisdiodeincludestwoheavilydoped&twolightlydoped
regions.
Here,heavilydopedregionsareP+&N+whereaslightlydoped
regionsareI&P.
35

36. Semiconductor diodes can be classified
into two categories
1.With internal gain
2.Without internal gain
Semiconductor photodiodes without
internal gain generate a single electron
hole pair per absorbed photon.
36
Semiconductor Photodiodes

37. 37
It is therefore important that the photons are absorbed in the
depletion region.
Thus it is made as long as possible by decreasing the doping in
the n type material.
The depletion region width in a p-n photodiode is normally 1-
3µm and is optimized for the efficient detection of light at a
given wavelength.

38. 38
Typical output characteristics for the reverse-biased p-n
photodiode is illustrate in Fig 5.4.
The different operating conditions may be noted moving from no
light input to a high light level.

39. 39
Figure 5.4

40. Inordertoallowoperationatlongerwavelengthswherethelight
penetratesmoredeeplyintothesemiconductormaterialawider
depletionregionisnecessary.
Toachievedthisthen-typematerialisdopedsolightlythatitcan
beconsideredintrinsic,andtomakealowresistancecontacta
highlydopedn-type(n
+
)layerisadded.
Thiscreatesap-i-n(orPIN)structureasmaybeseeninFig.5.4
wherealmostalltheabsorptiontakesplaceinthedepletion
region.
40
b) p-i-n Photodiode

41. Figure 5.4
41
p-i-n photodiode showing combined absorption and depletion region.

42. 42
Germanium p-i-n photodiodes which span the entire wavelength
range of interest are also commercially available, but the dark
current is relatively high.
Dark current arises from surface leakage currents as well as generation-
recombination currents in the depletion region in the absence of illumination.

43. Semiconductor Photodiode with Internal Gain -
Avalanche Photodiode (APD)
TheAPDhasmoresophisticatedstructurethanthep-i-n
photodiodeinordertocreateanextremelyhighelectricfield
region.
Therefore,aswellasthedepletionregionwheremostofthe
photonsareabsorbedandtheprimarycarrierpairsgenerated
thereisahighfieldregioninwhichholesandelectronscan
acquiresufficientenergytoexcitenewelectron-holepairs.
Theprocessisknownasimpactionizationandisthe
phenomenonthatleadstoavalanchebreakdown.
43

44. Figure 5.5
44
(a) (b)
(a)Avalanchephotodiodeshowinghighelectricfield(gain)region.(b)Carrierpair
multiplicationinthegainregion.

45. 45
Itrequiresveryhighreversebiasvoltage(100-400V)inorder
thatthenewcarrierscreatedbyimpactionizationcanthemselves
produceadditionalcarriersbythesamemechanismasshownin
Fig.5.5(b).
Carriermultiplicationfactorsasgreatas10
5
maybeobtained
usingdefectfreematerialstoensureuniformityofcarrier
multiplicationovertheentirephotosensitivearea.

46. 46
Highreversevoltage.Thisaccelerateselectronsandholes
thereuponacquireshighenergy.Theystrikeneutralatoms
andgeneratesmorefreechargecarriers.Thesesecondary
chargesthenionizeothercarriers.
Primarygeneratedelectronsstrikebondedelectronsatthe
VBandexcitethemtotheCB.KnownasImpact
Ionization.
Themainadvantagecomparedtop-i-nphotodiodeisthe
multiplicationorgainfactor,M.

47. 47
OftenanasymmetricpulseshapeisobtainedfromtheAPD
whichresultsfromarelativelyfastrisetimeastheelectronsare
collectedandafalltimedictatedbythetransittimeoftheholes
travellingataslowerspeed.
Hence,althoughtheuseofsuitablematerialsandstructuresmay
givesrisetimesbetween150and200ps,falltimesofa1nsor
morearequitecommonwhichlimittheoverallresponseofthe
device.

48. Drawbacks of The Avalanche Photodiode
1.Fabrication difficulties due to their more complex structure and
hence increased cost.
2.The random nature of the gain mechanism which gives an
additional noise contribution.
3.The high bias voltages required (100-400 V).
4.The variation of the gain with temperature.
48

49. 49

50. Multiplication Factor
The multiplication factor M is a measure of the internal gain
provided by the APD.
It is define as:
where I is the total output current at the operating voltage and I
P
is the initial or primary photocurrent.
The gain M, increases with the reverse bias voltage, V
d.
where n=constant and V
BRis the breakdown voltage of the
detector which is usually around 20 to 500V.
50

51. Bandwidth: Maximum frequency or bit rate that a photodiode can
detect. Determined by the response time.
Theresponsetimelimitedbythreefactors.
1)Thetransittimeofthecarriersacrosstheabsorptionregion,
=d/V
sat
2)TheRCtimeconstantincurredbythejunctioncapacitance(C
j)
ofthediodeanditsload.C
j=A/d.isthepermittivityofthe
semiconductorandAistheactiveareaofthephotodiode.
3)Thetimetakenbythecarrierstoperformtheavalanche
multiplicationprocess(forAPD).
V
sat=saturationvelocity
51

52. Comparison between PIN and APD
52
Material StructureRisetime λ(µm) R(A/W) Dark Current
(nA)
Gain
Silicon PIN 0.5 0.3-1.1 0.5 1 1
Germanium PIN 0.1 0.5-1.8 0.7 200 1
InGaAs PIN 0.3 0.9-1.7 0.6 10 1
Silicon APD 0.5 0.4-1.0 75 15 150
Germanium APD 1.0 1.0-1.6 35 700 50
InGaAs APD 0.25 1.0-1.7 12 100 20

53. Selection Chart
53
CHOICES 0.6-0.8 µm 0.8-0.9 µm1.2-1.7 µm
SOURCE LED LED LED LED(1.3µm)
LASER VCSEL VCSEL LD
FIBER GLASS 3 dB/km <1 dB/km
MM GRIN MM GRIN MM GRIN
SMF SMF
PLASTIC 160 dB/km
SI SI
DETECTOR MATERIAL Si Si InGaAs
Ge Ge Ge
PIN PIN PIN PIN
APD APD
Q1: Compare the important properties between a PIN photodiode and an
Avalanche Photodiode (APD).

54. Optical Receiver Design
54

55. Anopticalreceiversystemconvertsopticalenergyintoelectrical
signal,amplifythesignalandprocessit.Thereforetheimportant
blocksofopticalreceiverare:
Photodetector/Front-end
Amplifier/Linerchannel
Signalprocessingcircuitry/Datarecovery.
Noisegeneratedinreceivermustbecontrolledpreciselyasit
decidesthelowestsignallevelthatcanbedetectedand
processed.
Hencenoiseconsiderationisanimportantfactorinreceiver
design.Anotherimportantperformancecriteriaofoptical
receiverisaverageerrorprobability.
55
Optical Receiver Designpage 186

56. System Design Considerations
Unit 5
In optical system design major consideration
involves
-Transmission characteristics of fiber
(attenuation & dispersion).
-Information transfer capability of fiber.
-Terminal equipment & technology.
-Distance of transmission.
56

57. In long-haul communication applications repeaters are inserted at
regular intervals as shown in Fig. 6.2.1
57

58. 58
Repeater regenerates the original data before it is retransmitted as
a digital optical signal. The cost of system and complexity
increases because of installation of repeaters.
• An optical communication system should have following basic
required specifications –
a) Transmission type (Analog / digital).
b) System fidelity (SNR / BER)
c) Required transmission bandwidth
d) Acceptable repeater spacing
e) Cost of system
f) Reliability
g) Cost of maintenance.

59. 59
Multiplexing
•Multiplexingofseveralsignalsonasinglefiberincreasesinformation
transferrateofcommunicationlink.InTimeDivisionMultiplexing
(TDM)pulsesfrommultiplechannelsareinterleavedandtransmitted
sequentially,itenhancethebandwidthutilizationofasinglefiberlink.
•InFrequencyDivisionMultiplexing(FDM)theopticalchannel
bandwidthisdividedintovariousnon-overlappingfrequencybandsand
eachsignalisassignedoneofthesebandsoffrequencies.Bysuitable
filteringthecombinedFDMsignalcanberetrieved.
•Whennumberofopticalsourcesoperatingatdifferentwavelengths
aretobesentonsinglefiberlinkWavelengthDivision
Multiplexing(WDM)isused.Atreceiverend,theseparationor
extractionofopticalsignalisperformedbyopticalfilters(interference
filters,diffractionfiltersprismfilters).
•AnothertechniquecalledSpaceDivisionMultiplexing(SDM)used
toseparatefiberwithinfiberbundleforeachsignalchannel.SDM
providesbetteropticalisolationwhicheliminatescross-coupling
betweenchannelsbutitiscostly.

60. System Architecture
• From architecture point of view fiber optic communication
can be classified into three major categories.
1. Point –to –point links
2. Distributed networks
3. Local area networks.
60

61. Point-to-Point Links
Apoint-to-pointlinkcomprisesofonetransmitterandareceiver
system.
Thisisthesimplestformofopticalcommunicationlinkandit
setsthebasisforexaminingcomplexopticalcommunication
links.
Foranalyzingtheperformanceofanylinkfollowing
importantaspectsaretobeconsidered.
a)Distanceoftransmission
b)Channeldatarate
c)Bit-errorrate
•Allaboveparametersoftransmissionlinkareassociatedwith
thecharacteristicsofvariousdevicesemployedinthelink.
Importantcomponentsandtheircharacteristicsarelistedbelow.
61

62. 62
Whenthelinklengthextendsbetween20to100km,losses
associatedwithfibercableincreases.
Inordertocompensatethelossesopticalamplifierand
regeneratorsareusedoverthespanoffibercable.
Aregeneratorisareceiverandtransmitterpairwhichdetects
incomingopticalsignal,recoversthebitstreamelectricallyand
againconvertbackintoopticalformbymodulatinganoptical
source.
Anopticalamplifieramplifytheopticalbitstreamwithout
convertingitintoelectricalform.

63. Thespacingbetweentworepeateroropticalamplifieriscalledas
repeaterspacing(L).TherepeaterspacingLdependsonbitrate
B.Thebitrate-distanceproduct(BL)isameasureofsystem
performanceforpoint-to-pointlinks.
Two important analysis for deciding performance of any fiber
link are –
i) Link power budget / Power budget
TheLinkpowerbudgetanalysisisusedtodeterminewhetherthe
receiverhassufficientpowertoachievethedesiredsignal
quality.Thepoweratreceiveristhetransmittedpowerminus
linklosses.
ii) Rise time budget / Bandwidth budget
63

64. System Consideration
Before selecting suitable components, the operating wavelength
for the system is decided.
Theoperatingwavelengthselectiondependsonthedistanceand
attenuation.
Forshorterdistance,the800-900nmregionispreferredbutfor
longerdistance100or1550nmregionispreferredduetolower
attenuationsanddispersion.
64

65. The next step is selection of photodetector. While selecting a
photodetector following factors are considered –
i) Minimum optical power that must fall on photodetector to
satisfy BERat specified data rate.
ii) Complexity of circuit.
iii) Cost of design.
iv) Bias requirements.
65

66. Next step in system consideration is choosing a proper optical
source, important factors to consider are –
i) Signal dispersion.
ii) Data rate.
iii) Transmission distance.
iv) Cost.
v) Optical power coupling.
vi) Circuit complexity.
66

67. Thelastfactorinsystemconsiderationistoselectionofoptical
fiberbetweensinglemodeandmultimodefiberwithsteporgraded
indexfiber.
Fiber selection depends on type of optical source and tolerable
dispersion. Some important factors for selection of fiber are :
i) Numerical Aperture (NA), as NA increases, the fiber coupled
power increases also the dispersion.
ii)Attenuation characteristics.
iii)Environmental induced losses e.g. due to temperature
variation, moisture and dust etc
67

68. Link Power Budget
Foroptimisinglinkpowerbudgetanopticalpowerlossmodelis
tobestudiedasshowninFig.6.2.3.Letlcdenotesthelosses
occuratconnector.
Lspdenotes the losses occur at splices.
αfdenotes the losses occur in fiber.
68

69. Link Power Budget
Allthelossesfromsourcetodetectorcomprisesthetotalloss
(PT)inthesystem.
Linkpowermarginconsidersthelossesduetocomponent
agingandtemperaturefluctuations.Usuallyalinkmarginof6-8
dBisconsideredwhileestimatinglinkpowerbudget.
Totalopticalloss=Connectorloss+(SplicinglossXFiber
attenuation)+Systemmargin(Pm)
PT= 2lc+ αfL+ System margin (Pm)
where, L is transmission distance.
69

70. Example 6.2.1 : Design as optical fiber link for transmitting 15 Mb/sec of data
for a distance of 4 km with BER of 10^-9.
Solution :
Bandwidth x Length = 15 Mb/sec x 4 km = (60 Mb/sec) km
Selecting optical source : LED at 820 nm is suitable for short
distances. The LED generates –10 dBm optical power.
Selecting optical detector : PIN-FER optical detector is reliable
and has –50 dBm sensitivity.
Selection optical fiber : Step-index multimode fiber is selected.
The fiber has bandwidth length product of 100 (Mb/s) km.
70

71. Links power budget :
Assuming :
Splicing loss ls = 0.5 dB/slice
Connector loss lc= 1.5 dB
System link power margin Pm –8 dB
Fiberattenuation αf= 6 dB/km
Actual total loss = (2 x lc) + αfL+ Pm
PT = (2 x 1.5) + (6 x 4) + 8
PT = 35 dB
Maximum allowable system loss Pmaxis
Pmax= Optical source output power-optical receiver sensitivity
= -10 dBm–(-50 dBm)
= 40 dBm
Since actual losses in the system are less than the allowable loss,
hence the system is functional.
71

72. Example6.2.2:Atransmitterhasanoutputpowerof0.1mW.Itisusedwithafiberhaving
NA=0.25,attenuationof6dB/kmandlength0.5km.Thelinkcontainstwoconnectorsof
2dBaverageloss.Thereceiverhasaminimumacceptablepower(sensitivity)of–35dBm.
Thedesignerhasalloweda4dBmargin.Calculatethelinkpowerbudget.
72
Source power Ps = 0.1 mW Ps = -10dBm
SinceNA = 0.25
Coupling loss = -10log (NA^2) = -10log (0.25^2) = 12 dB
Fiber loss = αf x L
lf = (6dB/km) (0.5km)
lf = 3 dB
Connector loss = 2x (2 dB)
lc = 4 dB
Design margin Pm = 4 dB
Actual output power Pout = Source power –(Σ Losses)
Pout = -10dBm –[12 dB + 3 + 4 + 4] so Pout = -33 dBm
Since receiver sensitivity given is –35 dBm.
Pmin = -35 dBm
As Pout > Pmin, the system will perform adequately over the
system operating life.

73. Rise Time Budget
Risetimegivesimportantinformationforinitialsystemdesign.
Rise-timebudgetanalysisdeterminesthedispersionlimitationof
anopticalfiberlink.
Total rise time of a fiber link is the root-sum-square of rise time
of each contributor to the pulse rise time degradation.
73

74. Four basic elements that contributes to the rise-time are,
Transmitter rise-time (ttx)
Group Velocity Dispersion (GVD) rise time (tGVD)
Modal dispersion rise time of fiber (tmod)
Receiver rise time (trx)
74

75. Wavelength Division Multiplexing (WDM)
Opticalsignalsofdifferentwavelength(1300-1600nm)can
propagatewithoutinterferingwitheachother.
Theschemeofcombininganumberofwavelengthsovera
singlefiberiscalledwavelengthdivisionmultiplexing(WDM).
Eachinputisgeneratedbyaseparateopticalsourcewitha
uniquewavelength.
Anopticalmultiplexercoupleslightfromindividualsourcesto
thetransmittingfiber.
Atthereceivingstation,anopticaldemultiplexerisrequiredto
separatethedifferentcarriersbeforephotodetectionofindividual
signals.Fig.7.1.1showssimpleSDMscheme
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76. 76
Topreventspurioussignalstoenterintoreceivingchannel,
thedemultiplexermusthavenarrowspectraloperationwith
sharpwavelengthcut-offs.Theacceptablelimitofcrosstalk
is–30dB.

77. Features of WDM
Important advantages or features of WDM are as mentioned
below –
1.Capacityupgrade:Sinceeachwavelengthsupports
independentdatarateinGbps.
2.Transparency:WDMcancarryfastasynchronous,slow
synchronous,synchronousanaloganddigitaldata.
3.Wavelengthrouting:Linkcapacityandflexibilitycanbe
increasedbyusingmultiplewavelength.
4.Wavelengthswitching:WDMcanaddordropmultiplexers,
crossconnectsandwavelengthconverters.
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78. Passive Components
ForimplementingWDMvariouspassiveandactivecomponents
arerequiredtocombine,distribute,isolateandtoamplifyoptical
poweratdifferentwavelength.
Passivecomponentsaremainlyusedtosplitorcombineoptical
signals.Thesecomponentsoperatesinopticaldomains.Passive
componentsdon’tneedexternalcontrolfortheiroperation.
Passivecomponentsarefabricatedbyusingopticalfibersby
planaropticalwaveguides.Commonlyrequiredpassive
componentsare–
1. N x N couplers
2. Power splitters
3. Power taps
4. Star couplers.
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79. 2 x 2 Fiber Coupler
79
A device with two inputs and two outputs is called as 2 x 2
coupler.
Fusedbiconicallytaperedtechniqueisusedtofabricate
multiportcouplers.
Theinputandoutputporthaslongtaperedsectionoflength‘L’.
Thetaperedsectiongraduallyreducedandfusedtogetherto
formcouplingregionoflength‘W’.

80. Inputopticalpower:P0.
Throughtputpower:P1.Coupledpower:P2.Crosstalk:P3.
Powerduetorefelction:P4.
Thegradualtaperedsectiondeterminesthereflectionofoptical
powertotheinputport,
hencethedeviceiscalledasdirectionalcoupler.
80