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Semester II 2009/10 Dr Mohammad Faiz Liew Abdullah
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MKE 1083 Advanced Optical Communication
Dr Mohammad Faiz Liew Abdullah
Lecture : Fiber Optics Links and Network
Department of CommunicationFaculty of Electrical and Electronic Engineering
University Tun Hussein Onn Malaysia
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PON
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Passive Optical Network(PON) Topologies
BUS
RING
STAR
No O/E conversionPassive optical couplers
Folded Bus, Tree and Mesh Networks also exist
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Linear bus topology
,
10 log ( 1) 2 ( 2) 2o C thru TAP iL N
P N L NL N L L NLP
= + + + +
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Attenuators
Singlemode Variable Attenuator Repeatable, variable attenuation from 2 to 40dB
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Attenuators - contd.
iBandpass 1310/1550nmiFC, SC, ST, and D4 stylesiWavelength independentiPolarization insensitiveiLow modal noise
In line attenuatorDual window
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Optical CouplersiOptic couplers either split optical signals into multiple paths or
combine multiple signals on one path. iThe number of input (N)/ output (M) ports, (i.e.s N x M size)
characterizes a coupler. iFused couplers can be made in any configuration, but they
commonly use multiples of two (2 x 2, 4 x 4, 8 x 8, etc.).
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Coupler
iUses Splitter: (50:50) Taps: (90:10) or (95:05) Combinersi An important issue:
two output differ /2 in phasei Applications:
Optical Switches, Mach Zehnder Interferometers, Optical amplifiers, passive star couplers, ...
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Directional Coupler
A directional coupler forms the basis of many distribution network.
1 2
34
We assume that power P1 is incident on port 1 of the coupler.
This power will divide between ports 2 and 3 according to he desired splitting ratio.
Ideally, no power will reach port 4, the isolated port.
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Directional Coupler - cont
A directional coupler forms the basis of many distribution network.
1 2
34
Throughput loss
Tap loss1
210log10 P
PLTHP =
1
310log10 P
PLTAP =
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Directional Coupler - cont
A directional coupler forms the basis of many distribution network.
1 2
34
Directionality/ Cross talk
Excess loss
Coupling Ratio Splitting Ratio (in dB)
1
410log10 P
PLD =
32
110log10 PP
PLE +=
32
310log10 PP
PCR +=
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Coupler working principle
Put the cores close enough together to get a coupling effectPut the cores close enough together to get a coupling effect
All now depends on the length of the coupling sectionAll now depends on the length of the coupling section
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Common commercial devicesCommon commercial devices
Planar Waveguide CouplerPlanar Waveguide Coupler
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( )( )CzPPCzPP
202
201
sin
cos
==
Ccoupling coefficient
222 fiber coupler2 fiber coupler
PP44
PP00 PP11
PP33 PP22Z
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Coupler - Integrated Waveguide Directional Coupler
P2 = P0 sin2 kz P1 = P0 - P2 = P0 cos2 kz
k = coupling coefficient = (m + 1)/2
P0
P1
P2
P3
zP4
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Throughput port
Tap port
Coupling procedureCoupling procedure
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3dB coupler
3dB 3dB couplercoupler
Coupler - symbol
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Star Couplers
iOptical couplers with more than four ports. iTypes of star couplers:
transmission star couplerthe light at any of the input port is split equally through all output ports.
reflection star coupler
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Star coupler
PP11
PPNN
PPii
11
ii
NN
(P1+P2++PN)/N(P(P11+P+P22+++P+PNN)/N)/N
Star Coupler: N input are mixed and made Star Coupler: N input are mixed and made available on 8 outputsavailable on 8 outputs
Reflective Coupler: input can be on any fiber and Reflective Coupler: input can be on any fiber and output is split equally among all fibersoutput is split equally among all fibers
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Reflection Star Couplers
The light arriving at port A and is reflected back to all
ports. A directional coupler
separates the transmitted and received signals.
Source: Australian Photonics CRC
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Fibre Star CouplerCombines power from N inputs and divided them between M outputs
NN
CR 1010 10110 loglog =
=Coupling ratio
= Ni iout
ineP
PL
,
log1010Excess loss
1
N
1
N
P1
PN
Power at any one output ).......(, Nio PPPnP ++= 211
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Star Coupler - 8 X 8
12345678
1, 2, ... 8
1, 2, ... 8
No of 3 dB coupler NNN dBc 23 2 log=
N/2
N2log
Star couplers are optical couplers with more than four ports
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Star Coupler - 8 X 8 - contd.
i If a fraction of power traversing each 3 dB coupler = Fp, where 0< Fp < 1.
Then, power lost within the coupler = 1- Fp.
Excess loss )(log log Npe FL 21010=
NN
CR 1010 10110 loglog =
=Coupling ratio(splitting loss)
Total loss = splitting loss + excess loss
NFLT 10103223110 log)log.( =
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Example:
Consider a commercially available 32x32 single mode coupler made from a cascade of 3dB fused fiber 2x2 couplers, where 5% of the power is lost in each element. Find the total loss experienced by a signal as it passes through the coupler.
Solution:
Total Loss ( )10 1 3.322log0.95 log3216.2dB
= =
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port1port1port2(50%)port2(50%)port3(50%)port3(50%)
But light entering on port2 will exit on port1 But light entering on port2 will exit on port1 attenuated attenuated by 50%(3dB)by 50%(3dB)! Thus if we try to combine two input ! Thus if we try to combine two input signals by using a Ysignals by using a Y--junction, the signals are combined junction, the signals are combined but each signal but each signal will lose half of its powerwill lose half of its power!!
Y coupler (splitter)
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Y- Couplers
1 X 8 coupler
Y-junctions are 1 x 2 couplers and are a key element in networking.
IiI1
I2
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Wavelength Selective Coupling/Splitting
The period of the shift is different for the two The period of the shift is different for the two different wavelengths. Each coupler/splitter must be different wavelengths. Each coupler/splitter must be designed for the particular wavelengths to be used. designed for the particular wavelengths to be used.
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Types of couplers
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Coupler - Characteristics
Design class No. of CR Le Isolationport (dB) directivity
(-dB)
2 x 2 2 0.1-0.5 0.07-1.0 40 to 55Single mode
2 x 2 2 0.5 1-2 35 to 40Multimode
N x N 3-32 0.33-0.03 0.5-8.0Star
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Example of commercial coupler
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Example
A 2x2 biconical tapered fiber has an input optical power level of Po=200W. The output powers at the other three ports are P1=90W, P2=85W and P3=6.3nW. Calculate the splitting/coupling ratio, excess loss, insertion loss and crosstalk.
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Solution:
2
1 2
85100% 100% 48.6%90 85
PP P
= = + + 0
1 2
20010log 10log 0.5890 85
P dBP P
= = + +
1
20010log 10log 3.4790
oP dBP
= =
2
20010log 10log 3.7285
oP dBP
= = 3
3
0
6.3 1010log 10log 45200
P dBP
= =
Splitting Ratio =
Excess loss =
Insertion loss(port 0 to port 1)=
Insertion loss(port 0 to port 2)=
Crosstalk =
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Splitters
i The simplest couplers are fiber optic splitters. i They possess at least three ports but may have more than 32 for more
complex devices. i Popular splitting ratios include 50%-50%, 90%-10%, 95%-5% and 99%-1%;
however, almost any custom value can be achieved. i Excess loss: assures that the total output is never as high as the input. It
hinders the performance. All couplers and splitters share this parameter. i They are symmetrical. For instance, if the same coupler injected 50 W into
the 10% output leg, only 5 W would reach the common port.
OutputOutput
Input
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9.4 Switches
a two-position switch a bypass switch
Fiber optic switches reroute the optic signals. Switches are useful in distribution networks, measuring equipment, and experiments.
switches
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1
2
3
FIBERS
GRIN LENSES SLIDING
PRISM
Sliding-prism, two-position switch
Two-position switch
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Bypass switch
1
2 3
4
BYPASS STATE
1
2 3
4
BRANCH STATE
In the bypass state, ports 1 and 4 are coupled; ports2 and 3 are isolated.In the branch state, ports 1 and 4 are isolated; ports2 and 3 are coupled
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MEMS Switches
iMEMS----Micro-Electro-Mechanical System MEMS Mirrors
The MEMS optical switch requires the capability to produce arrays of tiny movable mirrors to deflect light beams in a desired manner.
These mirror can be constructed in two ways:
MIRROR
HINGE
SUBSTRATE SUBSTRATE
HINGE
MIRROR
As thin-film mirrors As bulk mirrors
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MEMS Switches
MIRROR
HINGE
SUBSTRATE SUBSTRATE
HINGE
MIRROR
As thin-film mirrors As bulk mirrors
iThe mirror movement can be controlled in a number of ways: electrostatic, electromagnetic, piezoelectric, or thermal.
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MEMS Switches
MEMS switches are constructed in 2D and 3D In 2D MEMS switches, the light travels in a plane defined as the plane
of the mirror array.
INPUT FIBERS
COLLIMATORS
MIRROR DOWNOUTPUT FIBERS
MIRROR UP
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MEMS Switches
iThe 3D MEMS switch is a bit more complicated to construct than the 2D switch. The switch insertion loss and the switching time are two
important property. The application of the MEMS switches-----OXC (Optical
Cross Connect)
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I/O Fibers
Imaging LensesReflector
MEMS 2-axis Tilt Mirrors
MEMS arraysMEMS arrays
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1N MEMS Switch11N MEMS SwitchN MEMS Switch
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Fiber Bragg Gratings (FBG)
FBG is a periodic refractive index variation (Period ) written along the fibre (single-mode) core using high power UV radiation.
All the wavelengths satisfying the condition 0 = 2 neff are reflected
If the optical period is 0 / 2, the grating reflects wavelength 0selectively. Useful in filtering communication channels in or out.
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Laser BeamLaser Beam
--11 +1+100
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lengthlength
Period Period
= effn2Selected Selected wavelengthwavelengthcore indexcore index
Grating Grating periodperiod
Fiber Bragg GratingsFiber gratings are a periodic variation in the refractive index of the core as measured along its axis.
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))00 ++++== 22cos(cos( ++ zzAAnnNN nneffeffcorecoreFiber gratingFiber grating
0nN core =FiberFiber
Fiber Bragg Gratings
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Applications of the Bragg grating
Filter for WDM systemWavelength-selective mirrors for fiber lasersWavelength stabilization of laser diodesStrain and temperature measurements in
composite fiber optic sensors.Dispersion compensationGain stabilization and equalization in erbium-
doped fiber amplifiersFixed filterTunable filters
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Filter for WDM system
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wavelength
For a given grating period a particular wavelength (frequency) of light is reflected. In this case yellow light will be reflectedIf the grating spacing is changed (e.g. reduced due to compression of the fibre or a drop in temperature} the wavelength of the reflected light changes. In this case it becomes higher and reflects blue light
In practice the colour shifts will be much finer than those illustated
Optical fibre
Grating pattern etched into body of fibre
Detector
http://www.co2sink.org/ppt/fbganimation.ppt
Fiber Bragg Gratings (FBG)
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Optical Isolator
i laser diodes are particularly sensitive to light energy reflected back from the rest of the system. The reflected light increases the noise in the emitted beam, degrading system performance.iAn optical isolator will ensure a low level of return to the laser
diode. It is a one-way transmission line. It will allow propagation in only one direction along the fiber.iInsertion loss:
Low loss (0.2 to 2 dB) in forward direction
High loss in reverse direction:20 to 40 dB single stage, 40 to 80 dB dual stage)
iReturn loss: More than 60 dB without connectors
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IsolatorsIsolators
Isolator/coupler hybridsIsolator/coupler hybrids
Example Isolator
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Optical Circulators
iBased on optical crystal technology similar to isolators Insertion loss 0.3 to 1.5 dB, isolation 20 to 40 dB
iTypical configuration: 3 port device Port 1 -> Port 2 Port 2 -> Port 3 Port 3 -> Port 1
Agilent Tech. LW Div.
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Optical Add Drop
2
3
1, 2, 3, 4,1, 3, 4,
2 2
1, 2, 3, 4,1
3 port circulator
FBGcoupler
Dropped w
avelength
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Operation
iAn input signal at port 1 is sent on out at port 2. iAll wavelength except 2 pass through the FBG.
iSince 2 satisfies the FBG condition, it gets reflected, enters port 2 of the circulator, and exits at port 3 as a dropped wavelength.iNew information can be transmitted using 2 to the
network by coupler.
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Multiplex & Demultiplex
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DWDM
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Baseband Transmission
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Baseband Transmission
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Example : Analog to Digital
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Data Rate & Bandwidth
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Transmission Impairments
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Modulation : Digital Data, Analog Signal
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Amplitude Modulation and ASK
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Frequency Modulation and FSK
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Phase Modulation and PSK
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Amplitude Shift Keying
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Frequency Shift Keying
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Phase Shift Keying
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Multiple Access Methods
iTDMA Time Division Multiple Access Done in the electrical domain
iSCMA Sub Carrier Multiple Access FDM done in the electrical domain
iCDMA Code Division Multiple Access Not very popular
iWDMA Wavelength Division Multiple Access (The most promising)
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Sub Carrier Multiplexing
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Single Mode Fiber
Baseband Data
Baseband-RFModulation
RF-Optical Modulation
Optical - RF Demodulation
Gain BPF
200 THz1.8 GHz
RF-Baseband Demodulation
Baseband Data
Receiving End
Transmitting End
A Closer Look.
Two different Modulations for each RF Carrier !
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Sub Carrier Multiplexing
iEach modulating RF carrier will look like a sub-carrieriUnmodulated optical signal is the main carrier iFrequency division multiplexed (FDM) multi channel systems
also called as SCM
Frequency
Unmodulated (main) carrier
Sub-carriers
f1
f2
f1
f2
f0
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Sub Carrier Multiplexing
iAbility to both analog and digitally modulated sub-carriersiEach RF carrier may carry voice, data, HD video or digital
audioiThey may be modulated on RF carriers using different
techniquesiMultiple digital signals are multiplexed onto one RF signal
and then sent at one optical wavelength. iMUX and DEMUX accomplished electronically not
optically. iLimited by BW of electrical and optical components. iCan be combined with other multiplexing schemes such as
SONET (Synchronous Optical Network) and DWDM to extend transmission capacity.
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TDMA
iSignals are multiplexed in timeiThis could be done in electrical domain (TDMA)
or optical domain (OTDMA)iHighly time synchronized transmitter/receiveriStable and precise clocksiMost widely used (SONET, GPON etc.)
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Time Division Multiplexing (TDM)
Individual channels are modulated at high data rates (Channels A-C, more would be used in an actual system). An Optical Pulse generator forms high-speed pulses at rates less than the period of the transmitted data. The bit period for these signals is compressed to T/N, multiplexed, and transmitted through optical fiber. A high-speed clock and regenerator demodulates the signals. All optical 3R regeneration processes (re-amplifying, re-shaping, and re-timing) can greatly extend the capability of this technique beyond 100 Gb/s). A demonstration of 1.28 Tb/s has been demonstrated (Nakazawa, et.al., Elect. Lett. 2000).
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Code Division Multiplexing (CDM)
Each channel transmits its data bits as a coded channel specific sequence over available BW, wavelength, and time slots.
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Space Division Multiplexing (SDM)
i The channel routing path is determined by different spatial positions (fiber locations). i High BW space switching matrix is formed.
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OSI & Layer Model
This Course
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Types of Networks
iLocal Area Network (LAN) Interconnect users in a localized area: a building,
campus or enterprise
iMetropolitan Area Network (MAN)iWide Area Network (WAN)
National, Regional
iSpecial Networks Undersea, Intercontinental
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The Public Network
Long Haul Network
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Global Network Hierarchy
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Fiber in the Access End
Fiber increasingly reaches the user
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Network Terminologies
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Terms use in Fiber Optic Communication
Topology logical manner in which nodes linkedSwitching transfer of information from source to
destination via series of intermediate nodes; Circuit Switching Virtual circuit establishedPacket Switching Individual packets are directedSwitch is the intermediate node that stream the incoming
information to the appropriate outputRouting selection of such a suitable pathRouter translates the information from one network to
another when two different protocol networks are connected (say ATM to Ethernet)
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Optical Cross Connects
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Synchronous Optical Networks
iSONET is the TDM optical network standard for North America (It is called SDH in the rest of the world)iIt focus on the physical layer
iSTS-1, Synchronous Transport Signal consists of 810 bytes over 125 usi27 bytes carry overhead informationiRemaining 783 bytes: Synchronous
Payload Envelope
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SONET/SDH Bit Rates
STM-649953.28 OC-192
STM-324976.64 OC-96
STM-162488.32 OC-48
STM-81244.16 OC-24
STM-4622.08 OC-12
STM-1155.52 OC-3
-51.84OC-1
SDHBit Rate (Mbps)SONET
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Digital Transmission Hierarchy (T-Standards)
Additional framing bits stuffed at each level to achieve synchronization
Not possible to directly add/drop sub-channels
DS1
DS2
DS3
Predominant before optical era
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Basic STS-1 SONET frame
STS-1=(90*8bits/row)(9rows/frame)*125 /frame 51.84 Mb/ss =
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Basic STS-N SONET frame
STS-N signal has a bit rate equal to N times 51.84 Mb/sEx: STS-3 155.52 Mb/s
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ATM over SONET
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SONET Add Drop Multiplexers
ADM is a fully synchronous, byte oriented device, that can be used add/drop OC sub-channels within an OC-N signal
Ex: OC-3 and OC-12 signals can be individually added/dropped from an OC-48 carrier
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SONET/SDH Rings
iSONET/SDH are usually configured in ring architecture to create loop diversity by self healingi2 or 4 fiber between nodesiUnidirectional/bidirectional traffic flowiProtection via line switching (entire OC-N
channel is moved) or path switching (sub channel is moved)
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2-Fiber Unidirectional Path Switched Ring
Ex: Total capacity OC-12 may be divided to four OC-3 streams
Node 1-2OC-3
Node 2-4; OC-3
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2-Fiber UPSR
iRx compares the signals received via the primary and protection paths and picks the best oneiConstant protection
and automatic switching
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4-Fiber Bi-directional Line Switched Ring (BLSR)
Node 13; 1p, 2p 31; 7p, 8p Al
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BLSR Fiber Fault Reconfiguration
In case of failure, the secondary fibers between only the affected nodes (3 & 4) are used, the other links remain unaffected
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BLSR Node Fault Reconfiguration
If both primary and secondary are cut, still the connection is not lost, but both the primary and secondary fibers of the entire ring is occupied
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Generic SONET networkLarge National Backbone
City-wide
Local Area
Versatile SONET equipmentare available that support wide range of configurations, bit rates and protection schemes
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WDM Networks
iBroadcast and Select: employs passive optical stars or buses for local networks applications Single hop networks Multi hop networks
iWavelength Routing: employs advanced wavelength routing techniques Enable wavelength reuse Increases capacity
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Single hop broadcast and select WDM
i Each Tx transmits at a different fixed wavelengthi Each receiver receives all the wavelengths, but selects (decodes) only
the desired wavelengthi Multicast or broadcast services are supported
i Dynamic coordination (tunable filters) is required
Star Bus
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A Single-hop Multicast WDM Network
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Multi-hop Architecture
Four node broadcast and select multihop networkEach node transmits at fixed set of wavelengths and receive
fixed set of wavelengthsMultiple hops required depending on destinationEx. Node1 to Node2: N1N3 (1), N3N2 (6)No tunable filters required but throughput is less
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Data packet
In multihop networks, the source and destination information is embedded in the header
These packets may travel asynchronously (Ex. ATM)
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Shuffle Net
Shuffle Net is one of several possible topologies in multihop networks
N = (# of nodes) X (per node)
Max. # of hops = 2(#of-columns) 1
(-) Large # of s(-) High splitting loss A two column shuffle net
Ex: Max. 2 X 2 - 1= 3 hops
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Wavelength Routing
iThe limitation is overcome by: reuse, routing and conversioniAs long as the logical
paths between nodes do not overlap they can use the same
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12X12 Optical Cross-Connect (OXC)Architecture
This uses space switching
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Optical Cross Connects (OXC)
iWorks on the optical domainiCan route high capacity wavelengthsiSpace switches are controlled electronicallyiIncoming wavelengths are routed either to desired
output (ports 1-8) or dropped (9-12)iWhat happens when both incoming fibers have a
same wavelength? (contention)
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4X4 Optical cross-connect
Wavelength switches are electronically configuredWavelength conversion to avoid contention
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Optical Fiber Communication System Design
There are many factors that must be considered to ensure that enough light reaches the receiver. Without the right amount of light, the entire system will not operate properly.Basic system requirements as below:
transmission type:digital or analogperformance : BER for digital system
SNR for analog.transmission bandwidthspacing between the terminal equipment or intermediate repeatercostreliability
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Fiber Optic System Design- Step-by-Step
Select the most appropriate optical transmitter and receiver combination based upon the signal to be transmitted (Analog, Digital, Audio, Video, RS-232, RS-422, RS-485, etc.).
Determine the operating power available (AC, DC, etc.).
Determine the special modifications (if any) necessary(Impedances, bandwidths, connectors, fiber size, etc.).
Carry out system link power budget.
Carry out system rise time budget (I.e. bandwidth budget).
If it is discovered that the fiber bandwidth is inadequate for transmitting the requiredsignal over the necessary distance, then either select a different transmitter/receiver (wavelength) combination, or consider the use of a lower loss premium fiber
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Link Power Budget
Po = Receiver sensitivity (i.e. minimum power requirement)SM= System margin (to ensure that small variation the system operating
parameters do not result in an unacceptable decrease in system performance)
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Power Budget
i Power budget is a detail description of how the available power is used.i Transmitter output T dBmi Receiver sensitivity R dBmi Excess power T-R dBi Splicing attenuation A1 dBi Fiber loss A2 dBi Penalty for transmitter realization A3 dBi Penalty for receiver realization A4 dBi Temperate effect A5 dBi Jitter A6 dBi Safety margin A7 dBi Total loss TA dBi Excess power margin T-R-TA
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Link Power Budget - ExamplePower margin is normally provided in the analysis to allow for component aging, temperature fluctuations, and losses arising from components that might be added at future dates.
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Example:The following parameters are established for a long haul single mode optical system operating at a wavelength of 1.3um.
Mean power launched from the laser transmitter -3 dBmCabled fiber loss 0.4dB /kmSplice loss 0.1dB/kmConnector losses at the transmitter and receiver 1dB eachMean power required at the APD receiver:
When operating at 35Mbit/s (BER 10-9) -55dBmWhen operating at 44Mbit/s (BER 10-9) -44dBm
Required safety margin 7 dBEstimate:i The maximum possible link without repeaters when operating at 35Mbit/s. It may be
assumed that there is no-dispersion-equalization penalty at this rate.i The maximum possible link without repeaters when operating at 44Mbit/s and
assuming no-dispersion-equalization penalty ati The reduction in the maximum possible link without repeaters of (b) when there is a
dispersion equalization penalty of 1.5 dB. It may be assumed for the purposes of this estimate that the reduced link length has the 1.5dB penalty.
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Solution:
( )i o fc j cr aP P L M = + + +( ) ( )3 55 0.4 0.1 2 7odBm dBm L = + + +
0.5 52 9L = 43 860.5
L km= =
( ) ( )3 44 0.4 0.1 2 7odBm dBm L = + + +0.5 41 9L =
32 640.5
L km= =
( ) ( )3 44 0.4 0.1 2 7 1.5odBm dBm L = + + + +0.5 41 10.5L =
30.5 610.5
L km= =
(a)
(b)
(c)
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Rise Time Budget
iTotal system rise timeiTsystem = 1.1(Ts2 + Tn2 + Tc2 + TD2 )1/2iTs : Source 10-90% rise timeiTD : Detector 10-90% rise timeiTn : intermodal dispersioniTc : intramodal/chromatic dispersioniRZ: BT (max) = NRZ: BT (max) =
iFor analog system, maximum 3 dB bandwidth is:iBW (max) =
systemT35.0
systemT7.0
systemT35.0
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Example:
An optical fiber system is to be designed to operate over an 8km length without repeaters. The rise times of the chosen components are:
Source (LED) 8 nsFiber: intermodal 5 ns/kmIntramodal 1 ns/kmDetector(p-i-n) 6 ns
From system rise time considerations, estimate the maximum bit rate that may be achieved on the link when using an NRZ format.
Solution:Tsystem = 1.1(TS2 + Tn2 + TC2 + TD2 )1/2
= 1.1(82 + (8 x 5)2 + (8 x 1)2 + 62 )1/2= 46.2 ns
NRZ BT(max) = 9syst
0.7 0.7 15.2Mbit/sT 46.2 10
= =