Fiber Optic Communications 143.332 Communication Systems ... · PDF fileFiber Optic...

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Fiber Optic Communications 143.332 Communication Systems

Chandratilak De Silva, LIYANAGE ([email protected]) Page 1

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A fiber-optic system is similar to the copper wire system in many respects. The difference is that fiber-optics use light pulses to transmit information down fiber lines instead of using electronic pulses to transmit information down copper lines. The basic elements of any point-to-point communication system are a transmitter that generates the signal (LEDs or LASERs), a transmission medium that carries the signal (Fiber-Optic Waveguide) and a receiver that detects the signal and converts it into a useful form (photo detector).



THE OPTICAL SYSTEM AND SPECTRUM

What we call ‘light’ is only a small part of the spectrum of electromagnetic radiation.



ADVANTAGES OF FOC

1) Enormous potential bandwidth.

2) Very high frequency carrier wave. (1014 Hz).

3) Low Loss (attenuation as low as 0.2 dB/km for glass)

4) Repeaters can be eliminated => low cost and reliability

5) Secure; cannot be trapped without affecting signal.

6) Electrically neutral; • no shorts / ground loop required. • applicable in dangerous environments.

7) Tough but light weight, • Expensive but tiny.



FIBRE OPTIC CABLES

What is a fiber optic cable? A fiber optic cable is a cylindrical pipe. It may be made out of glass or plastic or a combination of glass and plastic. It is fabricated in such a way that this pipe can guide light from one end of it to the other.

A fiber optic cable is composed of two concentric layers termed the core and the cladding. The core and cladding have different indices of refraction with the core having n1 and the cladding n2. Light is piped through the core. A fiber optic cable has an additional coating around the cladding called the jacket. The jacket usually consists of one or more layers of polymer. Its role is to protect the core and cladding from shocks that might affect their optical or physical properties. It acts as a shock absorber.



SNELL’S LAW

n1 sinφ1 = n2 sinφ2 n1 cosθ1 = n2 cosθ2

Snell’s law indicates that refraction can’t take place when the angle of incidence is too large. (Let φ1=600, n1=1.5 and n2=1.0. Can you calculate φ2?)



TOTAL INTERNAL REFLECTION



LIGHT GUIDING



NUMERICAL APERTURE (NA) CALCULATION

NA = (n12 – n2

2)1/2



OPTICAL FIBER TYPES

Fiber optic cable can be one of two types, multi-mode or single-mode. These provide different performance with respect to both attenuation and time dispersion.



OPTICAL FIBER TYPES

Fiber optic cable that exhibits multi-mode propagation with a step index profile is characterized as having higher attenuation and more time dispersion than the other propagation candidates have.

However, it is also the least costly and hence most widely used.



ATTENUATION

When compared with other candidates for the Transmission Medium commonly employed today, Optical Fibre has comparison when it comes to attenuation, interference and bandwidth.

Here frequency refers to the data rate. The attenuation of the fiber optic cables is much less. What is more their dependence upon frequency is even flat over quite a large range. You need not be concerned with the change in attenuation every time you decide to tweak the data rate.



ATTENUATION

Attenuation is principally caused by two physical effects, absorption and scattering. Absorption removes signal energy in the interaction between the propagating light (photons) and molecules in the core. Scattering redirects light out of the core to the cladding.

The three principal windows of operation, when propagating through a cable, are indicated. These correspond to wavelength regions where attenuation is low and matched to the ability of a Transmitter to generate light efficiently and a Receiver to carry out detection. The

'OH' symbols indicate that at these particular wavelengths the presence of Hydroxyl radicals in the cable material cause a bump up in attenuation. These radicals result from the presence of water.



ATTENUATION

Mode Material Index of Refraction Profile λ microns Size (microns) Atten. dB/km Bandwidth MHz/kmMulti-mode Glass Step 800 62.5/125 5.0 6 Multi-mode Glass Step 850 62.5/125 4.0 6 Multi-mode Glass Graded 850 62.5/125 3.3 200 Multi-mode Glass Graded 850 50/125 2.7 600 Multi-mode Glass Graded 1300 62.5/125 0.9 800 Multi-mode Glass Graded 1300 50/125 0.7 1500 Multi-mode Glass Graded 850 85/125 2.8 200 Multi-mode Glass Graded 1300 85/125 0.7 400 Multi-mode Glass Graded 1550 85/125 0.4 500 Multi-mode Glass Graded 850 100/140 3.5 300 Multi-mode Glass Graded 1300 100/140 1.5 500 Multi-mode Glass Graded 1550 100/140 0.9 500 Multi-mode Plastic Step 650 485/500 240 5 @ 680 Multi-mode Plastic Step 650 735/750 230 5 @ 680 Multi-mode Plastic Step 650 980/1000 220 5 @ 680 Multi-mode PCS Step 790 200/350 10 20

Single-mode Glass Step 650 3.7/80 or 125 10 600 Single-mode Glass Step 850 5/80 or 125 2.3 1000 Single-mode Glass Step 1300 9.3/125 0.5 * Single-mode Glass Step 1550 8.1/125 0.2 *

* Too high to measure accurately. Effectively infinite. (As of [4])



NUMBER OF MODES (Nm) FOR A STEP INDEX FIBER

Nm is for a step index fiber is given by,

2NAdiameter core5.0

××=

λπ

mN

eg: NA = 0.29 (a typical value) D = 100µm λ = 850nm 1000>>mN



NUMBER OF MODES (Nm) FOR A STEP INDEX FIBER (cont…)

Each mode has its own characteristic velocity through a step index optical fiber. This cause pulses to spread out as they travel along the fiber. This is called modal dispersion. The more modes the fiber transmits, the more pulse spread out it has. The basic requirement for a single mode fiber is that the core be small enough to resist transmission to a single mode.



NUMBER OF MODES (Nm) FOR A GRADED INDEX FIBER

( ) ( ) arnnn

ararnrn

≥=∆−≈∆−

≤≤

∆−=

for121

0for21)(

2121

1

21

1

α

where r = radial distance from the fiber axis, a = the core radius, n1 = refractive index of core axis n2 = refractive index of cladding. α = the shape of the index profile

1

2121

22

21

2 nnn

nnn −≈−=∆



NUMBER OF MODES (Nm) FOR A GRADED INDEX FIBER (cont…)

For ( ) 1nrn →∞→α (step index)

[ ] ( )

>≤−≈−=arara

rNAnrnrNAfor0for1)0()()(

212

22 α

where axial NA is defined as:

[ ] ( ) ∆≈−=−= 2)0()0( 12

122

21

212

22 nnnnnNA

The number of modes for this case is given by,

∆+

= 21

222

nkaNm αα where, λ

π2=k .



ELECTROMAGNETIC WAVE THEORY

To analyze optical waveguides it is required to consider Maxwell’s equations that give the relationship between electrical and magnetic fields.



CIRCULAR WAVEGUIDES

TE (Transverse Electric) Mode

The lower cutoff frequency (or wavelength) for a particular TE mode in circular waveguide is determined by the following equation:

c,mnmn

2 (m)prπλ =

′ ’

where p'mn is

m p'm1 p'm2 p'm3 0 3.832 7.016 10.174 1 1.841 5.331 8.536 2 3.054 6.706 9.970



TM (Transverse Magnetic) Mode

The lower cutoff frequency (or wavelength) for a particular TM mode in circular waveguide is determined by the following equation:

c,mnmn

2 (m)prπλ =

’

where pmn is

m pm1 pm2 pm3 0 2.405 5.520 8.654 1 3.832 7.016 10.174 2 5.135 8.417 11.620

However detailed analysis of this will not be carried out here.



CIRCULAR WAVEGUIDE MODES



LEAKY MODES

Some modes can propagate short distances in the optical fiber. Hence there are guided and unguided modes with respect to a given optical fiber.

Modes that are just beyond the threshold for propagating in a multimode fiber can travel for short distances in the fiber cladding.

The difference between the highest-order modes guided in a multimode fiber and the lowest order modes that are not guided is quite small. Hence slight changes in conditions may allow light in a normally guided mode to leak out of the core.

Slight bends of a multimode fiber are enough to allow escape of these leaky modes. Likewise some light in the cladding mode may be recaptured due to the bends.



TRANSMITTERS IN OPTICAL FIBER SYSTEMS

TThhee TTrraannssmmiitttteerr ccoommppoonneenntt ooff aann OOFF ssyysstteemm sseerrvveess ttwwoo ffuunnccttiioonnss.. FFiirrsstt,, iitt mmuusstt bbee aa ssoouurrccee ooff tthhee lliigghhtt ccoouupplleedd iinnttoo tthhee ffiibbeerr ooppttiicc ccaabbllee.. SSeeccoonnddllyy,, iitt mmuusstt mmoodduullaattee tthhiiss lliigghhtt ssoo aass ttoo rreepprreesseenntt tthhee bbiinnaarryy ddaattaa tthhaatt iitt iiss rreecceeiivviinngg ffrroomm tthhee SSoouurrccee.. WWiitthh tthhee ffiirrsstt ooff tthheessee ffuunnccttiioonnss iitt iiss mmeerreellyy aa lliigghhtt eemmiitttteerr oorr aa ssoouurrccee ooff lliigghhtt.. WWiitthh tthhee sseeccoonndd ooff tthheessee ffuunnccttiioonnss iitt iiss aa vvaallvvee,, ggeenneerraallllyy ooppeerraattiinngg bbyy vvaarryyiinngg tthhee iinntteennssiittyy ooff tthhee lliigghhtt tthhaatt iitt iiss eemmiittttiinngg aanndd ccoouupplliinngg iinnttoo tthhee ffiibbeerr.. TThhee TTrraannssmmiitttteerr ccaann bbee tthhoouugghhtt ooff aass EElleeccttrroo--OOppttiiccaall ((EEOO)) ttrraannssdduucceerr.. AAnn LLEEDD oorr aa LLDD ((LLaasseerr DDiiooddee)) ggeenneerraatteess aann ooppttiiccaall bbeeaamm wwiitthh ssuucchh ddiimmeennssiioonnss tthhaatt iitt ccaann bbee ccoouupplleedd iinnttoo aa ffiibbeerr ooppttiicc ccaabbllee.. HHoowweevveerr,, tthhee LLDD pprroodduucceess aann oouuttppuutt bbeeaamm wwiitthh mmuucchh lleessss ssppaattiiaall wwiiddtthh tthhaann aann LLEEDD.. TThhiiss ggiivveess iitt ggrreeaatteerr ccoouupplliinngg eeffffiicciieennccyy.. EEaacchh ccaann bbee mmoodduullaatteedd wwiitthh aa ddiiggiittaall eelleeccttrriiccaall ssiiggnnaall..



TRANSMITTERS IN OPTICAL FIBER SYSTEMS

TTwwoo mmeetthhooddss ffoorr mmoodduullaattiinngg LLEEDDss oorr LLDDss aarree sshhoowwnn aabboovvee.. MMoorree oonn LLEEDD aanndd LLDD wwiillll bbee ssttuuddiieedd sseeppaarraatteellyy..



RECEIVERS IN OPTICAL FIBER SYSTEMS

TThhee RReecceeiivveerr ccoommppoonneenntt ooff ooff aann OOFF ssyysstteemm sseerrvveess ttwwoo ffuunnccttiioonnss.. FFiirrsstt,, iitt mmuusstt sseennssee oorr ddeetteecctt tthhee lliigghhtt ccoouupplleedd oouutt ooff tthhee ffiibbeerr ooppttiicc ccaabbllee tthheenn ccoonnvveerrtt tthhee lliigghhtt iinnttoo aann eelleeccttrriiccaall ssiiggnnaall.. SSeeccoonnddllyy,, iitt mmuusstt ddeemmoodduullaattee tthhiiss lliigghhtt ttoo ddeetteerrmmiinnee tthhee iiddeennttiittyy ooff tthhee bbiinnaarryy ddaattaa tthhaatt iitt rreepprreesseennttss.. IInn ttoottaall,, iitt mmuusstt ddeetteecctt lliigghhtt aanndd tthheenn mmeeaassuurree tthhee rreelleevvaanntt IInnffoorrmmaattiioonn bbeeaarriinngg lliigghhtt wwaavvee ppaarraammeetteerrss iinn tthhee ffiibbeerr ooppttiicc ddaattaa lliinnkk ccoonntteexxtt iinntteennssiittyy iinn oorrddeerr ttoo rreettrriieevvee tthhee SSoouurrccee''ss bbiinnaarryy ddaattaa.. TThhee vveerryy hheeaarrtt ooff tthhee RReecceeiivveerr iiss tthhee mmeeaannss ffoorr sseennssiinngg tthhee lliigghhtt oouuttppuutt ooff tthhee ffiibbeerr ooppttiicc ccaabbllee.. LLiigghhtt iiss ddeetteecctteedd aanndd tthheenn ccoonnvveerrtteedd ttoo aann eelleeccttrriiccaall ssiiggnnaall..




TThhee ddeemmoodduullaattiioonn ddeecciissiioonn pprroocceessss iiss ccaarrrriieedd oouutt oonn tthhee rreessuullttiinngg eelleeccttrriiccaall ssiiggnnaall.. TThhee lliigghhtt ddeetteeccttiioonn iiss ccaarrrriieedd oouutt bbyy aa pphhoottooddiiooddee.. TThhiiss sseennsseess lliigghhtt aanndd ccoonnvveerrttss iitt iinnttoo aann eelleeccttrriiccaall ccuurrrreenntt.. HHoowweevveerr,, tthhee ooppttiiccaall ssiiggnnaall ffrroomm tthhee ffiibbeerr ooppttiicc ccaabbllee aanndd tthhee rreessuullttiinngg eelleeccttrriiccaall ccuurrrreenntt wwiillll hhaavvee ssmmaallll aammpplliittuuddeess.. CCoonnsseeqquueennttllyy,, tthhee pphhoottooddiiooddee cciirrccuuiittrryy mmuusstt bbee ffoolllloowweedd bbyy oonnee oorr mmoorree aammpplliiffiiccaattiioonn ssttaaggeess..




AA ccoommpplleettee RReecceeiivveerr mmuusstt hhaavvee hhiigghh ddeetteeccttaabbiilliittyy,, hhiigghh bbaannddwwiiddtthh aanndd llooww nnooiissee.. IItt mmuusstt hhaavvee hhiigghh ddeetteeccttaabbiilliittyy ssoo tthhaatt iitt ccaann ddeetteecctt llooww lleevveell ooppttiiccaall ssiiggnnaallss ccoommiinngg oouutt ooff tthhee ffiibbeerr ooppttiicc ccaabbllee.. TThhee hhiigghheerr tthhee sseennssiittiivviittyy,, tthhee mmoorree aatttteennuuaatteedd ssiiggnnaallss iitt ccaann ddeetteecctt.. IItt mmuusstt hhaavvee hhiigghh bbaannddwwiiddtthh oorr ffaasstt rriissee ttiimmee ssoo tthhaatt iitt ccaann rreessppoonndd ffaasstt eennoouugghh aanndd ddeemmoodduullaattee,, hhiigghh ssppeeeedd,, ddiiggiittaall ddaattaa.. IItt mmuusstt hhaavvee llooww nnooiissee ssoo tthhaatt iitt ddooeess nnoott ssiiggnniiffiiccaannttllyy iimmppaacctt tthhee BBEERR ooff tthhee lliinnkk aanndd ccoouunntteerr tthhee iinntteerrffeerreennccee rreessiissttaannccee ooff tthhee ffiibbeerr ooppttiicc ccaabbllee TTrraannssmmiissssiioonn MMeeddiiuumm..



WAVELENGTH DIVISION MULTIPLEXING (WDM)



MULTILAYER THINFILM REFLECTOR USED FOR WDM



REFERENCES

[1] Gerd Keiser, “Optical Fiber Communications”, McGraw-Hill. [2] Jeff Hecht, “Understanding Fiber Optics”, Prentice Hall. [3] “Fiber Optic Cable Tutorial”, hhttttpp::////wwwwww..aarrcceelleecctt..ccoomm//ffiibbeerrccaabbllee..hhttmm. [4] K S Schnelder, “Fiber Optic Data Communications for the Premises Environment”, hhttttpp::////wwwwww..tteelleebbyytteeuussaa..ccoomm//ffoopprriimmeerr//ffoocchh11..hhttmm.

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