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Connection-Oriented Networks 1
Chapter 8:Optical Fibers and
ComponentsTOPICS
– WDM optical networks– Light transmitted through an optical fiber
– Types of optical fibers– Impairments– Components: Lasers, optical amplifiers, couplers, OXCs
Connection-Oriented Networks 2
WDM optical networks
A point-to-point connection
In-lineamplification
optical
fiber
opticalfiberW
1
…
Tx
Wavelengthmultiplexer
Poweramplifie
r
W
1
…
Tx
Rx
Rx
Wavelengthdemultiplexer
Pre-amplifie
r
Connection-Oriented Networks 3
An example of an optical network
Mesh network
Ring 1
Ring 2
Ring 4
Ring 3
Connection-Oriented Networks 4
How light is transmitted through an optical fiber
Waves and electrical fields
Source
Electricfield
Wave
Connection-Oriented Networks 5
Core
Cladding
Cladding
Radial distance
Refractive index
n1
n2
Core
Cladding
Radial distance
n2
n1
Cladding
Core
Refractive index
a) Step-index fiber b) graded-index fiber
Core and cladding
An optical fiber
Connection-Oriented Networks 6
Refraction and reflection of a light ray
Incident rayReflected ray
Refracted ray
r
f
n2
n1
Connection-Oriented Networks 7
Angle of launching a ray into the fiber
Core
Cladding
Cladding
rl
Cladding
Cladding
Core
Opticaltransmitter
Cladding
Cladding
Core
Connection-Oriented Networks 8
Multi-mode and single-mode fibers
• Core/diameter of a multi-mode fiber:– 50/125 m, – 62.5/125 m, – 100/140 m
• Core/diameter of single-mode fiber– 9 or 10 / 125 m
Connection-Oriented Networks 10
Electric field amplitudes for
various fiber modes
Cladding
Cladding
Core
m=0 m=1 m=2
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Propagation of modes
Cladding
Cladding
Cladding
a) step-index fiber
b) Graded-index fiber
Cladding
Connection-Oriented Networks 13
Impairments
•The transmission of light through an optical fiber is subjected to optical effects, known as impairments.
•There are:–linear impairments, and–non-linear impairments.
Connection-Oriented Networks 14
Linear impairments
• These impairments are called linear because their effect is proportional to the length of the fiber.
• Attenuation:– Attenuation is the decrease of the optical power along the length of the fiber.
• Dispersion – Dispersion is the distortion of the shape of a pulse.
Connection-Oriented Networks 15
Attenuation
800 1000 1200 1400 1600 1800
0.5
1.0
1.5
2.0
2.5
Wavelength, nm
Attenuation, dB
Connection-Oriented Networks 16
Attenuation in Fiber
• Attenuation– P(L) = 10-AL/10P(0)
•Where P(0) optical power at transmitter,•P(L) power at distance L Km, and•A = attenuation constant of the fiber
• Received Power must be greater or equal to– receiver sensitivity Pr
– Lmax = 10/A log10(P(0)/P(r))
Connection-Oriented Networks 17
Dispersion
•Dispersion is due to a number of reasons, such as –modal dispersion, –chromatic dispersion, –polarization mode dispersion.
Connection-Oriented Networks 18
Modal dispersion
• In multi-mode fibers some modes travel a longer distance to get to the end of the fiber than others
• In view of this, the modes have different delays, which causes a spreading of the output pulse
Powe
r Powe
r
TimeTime
Powe
r
Time
Connection-Oriented Networks 19
Chromatic dispersion
• It is due to the fact that the refractive index of silica is frequency dependent. In view of this, different frequencies travel at different speeds, and as a result they experience different delays.
• These delays cause spreading in the duration of the output pulse.
Connection-Oriented Networks 20
• Chromatic dispersion can be corrected using a dispersion compensating fiber. The length of this fiber is proportional to the dispersion of the transmission fiber. Approximately, a spool of 15 km of dispersion compensating fiber is placed for every 80 km of transmission fiber.
• Dispersion compensating fiber introduces attenuation of about 0.5 dB/km.
Connection-Oriented Networks 21
Polarization mode dispersion (PMD)
• It is due to the fact that the core of the fiber is not perfectly round.
• In an ideal circularly symmetric fiber the light gets polarized and it travels along two polarization planes which have the same speed.
• When the core of the fiber is not round, the light traveling along the two planes may travel at different speeds.
• This difference in speed will cause the pulse to break.
Connection-Oriented Networks 22
Non-linear impairments
• They are due to the dependency of the refractive index on the intensity of the applied electrical field. The most important non-linear effects in this category are: self-phase modulation and four-wave mixing.
• Another category of non-linear impairments includes the stimulated Raman scattering and stimulated Brillouin scattering.
Connection-Oriented Networks 23
Types of fibers
• Multi-mode fibers: They are used in LANs and more recently in 1 Gigabit Ethernet and 10 Gigabit Ethernet.
• Single-mode fiber is used for long-distance telephony, CATV, and packet-switched networks.
• Plastic optical fibers (POF)
Connection-Oriented Networks 24
Single-mode fibers:• Standard single-mode fiber (SSMF): Most of the installed fiber falls in this category. It was designed to support early long-haul transmission systems, and it has zero dispersion at 1310 nm.
• Non-zero dispersion fiber (NZDF): This fiber has zero dispersion near 1450 nm.
Connection-Oriented Networks 25
• Negative dispersion fiber (NDF): This type of fiber has a negative dispersion in the region 1300 to 1600 nm.
• Low water peak fiber (LWPF): The peak in the attenuation curve at 1385 nm is known as the water peak. With this new type of fiber this peak is eliminated, which allows the use of this region.
Connection-Oriented Networks 26
Plastic optical fibers (POF) • Single-mode and multi-mode fibers have a high cost and they require a skilled technician to install them.
• POFs on the other hand, are very low-cost and they can be easily installed by an untrained person.
• The core has a very large diameter, and it is about 96% of the diameter of the cladding.
• Plastic optic fibers find use in digital home appliance interfaces, home networks, and cars
Connection-Oriented Networks 27
Components
• Lasers• Photo-detectors and optical receivers
• Optical amplifiers• The 2x2 coupler• Optical cross connects (OXC)
Connection-Oriented Networks 28
Light amplification by stimulated emission of
radiation (Laser)• A laser is a device that produces a very strong and concentrated beam.
• It consists of an energy source which is applied to a lasing material, a substance that emits light in all directions and it can be of gas, solid, or semiconducting material.
• The light produced by the lasing material is enhanced using a device such as the Fabry-Perot resonator cavity.
Connection-Oriented Networks 29
Fabry-Perot resonator cavity. It consists of two partially reflecting parallel flat mirrors, known as facets, which create an optical feedback that causes the cavity to oscillate.Light hits the right facet and part of it leaves the cavity through the right facet and part of it is reflected.
Left facet Right facet
Connection-Oriented Networks 30
• Since there are many resonant wavelengths, the resulting output consists of many wavelengths spread over a few nm, with a gap between two adjacent wavelengths of 100 to 200 GHz.
• A single wavelength can be selected by using a filtering mechanism that selects the desired wavelength and provides loss to the other wavelengths.
Connection-Oriented Networks 31
Tunable lasers
• Tunable lasers are important to optical networks
• Also, it is more convenient to manufacture and stock tunable lasers, than make different lasers for specific wavelengths.
• Several different types of tunable lasers exist, varying from slow tunability to fast tunability.
Connection-Oriented Networks 32
Modulation
• Modulation is the addition of information on a light stream
• This can be realized using the on-off keying (OOK) scheme, whereby the light stream is turned on or off depending whether we want to modulate a 1 or a 0.
Connection-Oriented Networks 33
WDM and dense WDM (DWDM)• WDM or dense WDM (DWDM) are terms used interchangeably.
• DWDM refers to the wavelength spacing proposed in the ITU-T G.692 standard in the 1550 nm window (which has the smallest amount of attenuation and it also lies in the band where the Erbium-doped fiber amplifier operates.)
• The ITU-T grid is not always followed, since there are many proprietary solutions.
Connection-Oriented Networks 34
The ITU-T DWDM grid
Channelcode
(nm) Channelcode
(nm) Channelcode
(nm) Channelcode
(nm)
18 1563.05 30 1553.33 42 1543.73 54 1534.25
19 1562.23 31 1552.53 43 1542.94 55 1533.47
20 1561.42 32 1551.72 44 1542.14 56 1532.68
21 1560.61 33 1590.12 45 1541.35 57 1531.90
22 1559.80 34 1550.12 46 1540.56 58 1531.12
23 1558.98 35 1549.32 47 1539.77 59 1530.33
24 1558.17 36 1548.52 48 1538.98 60 1529.55
25 1557.36 37 1547.72 49 1538.19 61 1528.77
26 1556.56 38 1546.92 50 1537.40 62 1527.99
27 1555.75 39 1546.12 51 1536.61
28 1554.94 40 1545.32 52 1535.82
29 1554.13 41 1544.53 53 1535.04
Connection-Oriented Networks 35
Photo-detectors and optical receivers
• The WDM optical signal is demultiplexed into the W different wavelengths, and each wavelength is directed to a receiver.
• Each receiver consists of a – photodetector, – an amplifier, and – signal-processing circuit.
Connection-Oriented Networks 36
Optical amplifiers
• The optical signal looses its power as it propagates through an optical fiber, and after some distance it becomes too weak to be detected.
• Optical amplification is used to restore the strength of the signal
Connection-Oriented Networks 37
Amplifiers: power amplifiers, in-line amplifiers, pre-amplifiers
In-lineamplification
optical
fiber
opticalfiberW
1
…
Tx
Wavelengthmultiplexer
Poweramplifie
r
W
1
…
Tx
Rx
Rx
Wavelengthdemultiplexer
Pre-amplifie
r
Connection-Oriented Networks 38
1R, 2R, 3R
• Prior to optical amplifiers, the optical signal was regenerated by first converting it into an electrical signal, then apply – 1R (re-amplification), or– 2R (re-amplification and re-shaping) or – 3R (re-amplification, re-shaping, and re-timing)
and then converting the regenerated signal
back into the optical domain.
Connection-Oriented Networks 40
The Erbium-doped fiber amplifier (EDFA)
Laser850 nm
Signal to be amplified1550 nm
Isolator
Coupler
Erbium-doped fiberIsolator
Connection-Oriented Networks 41
Two-stage EDFA
Signal to be
amplified1550 nm
Laser850 nm
Isolator
Coupler
Erbium-doped fiber
Laser850 nm
Coupler
Isolator
Connection-Oriented Networks 42
The 2x2 coupler
The 2x2 coupler is a basic device in optical networks, and it can be constructed in variety of different ways. A common construction is the fused-fiber coupler.
Couplingregion
Taperedregion
Taperedregion
Input 1
Output 1
Output 2
Fiber 1
Fiber 2Input 2
Connection-Oriented Networks 43
3-dB coupler
A 2x2 coupler is called a 3-dB coupler when the optical power of an input light applied to, say input 1 of fiber 1, is evenly divided between output 1 and output 2.
Connection-Oriented Networks 44
• If we only launch a light to the one of the two inputs of a 3-dB coupler, say input 1, then the coupler acts as a splitter.
• If we launch a light to input 1 and a light to input 2 of a 3-dB coupler, then the two lights will be coupled together and the resulting light will be evenly divided between outputs 1 and 2.
• In the above case, if we ignore output 2, the 3-dB coupler acts as a combiner.
Connection-Oriented Networks 45
A banyan network of 3-dB couplers
1
2
3
6
4
5
7
8
128
128
128
128
128
128
128
128
Connection-Oriented Networks 46
Optical cross connects (OXCs)
1
W
1
W
1
W
1
W
Switch fabric
Fiber 1
Fiber N Fiber N
Fiber 1
……
CPUInput fibers
Output
fibers
Connection-Oriented Networks 47
OXC (cont’d)
• Optical cross-connects
OXC
IP routerTx Rx
Local Add Local Drop
Access Station
Wavelength Router
WDM link
GMPLS Plane
UNI
To & from other nodes
To & from other nodes
Connection-Oriented Networks 48
OXC: switching fabric
• Switching fabric
OXC
Input WL λ1to output 1
Output 1
2
3
MEMS: one mirror per output
4
Connection-Oriented Networks 49
OXC: switching fabric (cont’d)
OXC
Input WL λ1to output 4
Output 1
2
3
MEMS: one mirror per output
4
• Switching fabric
Connection-Oriented Networks 50
OXC functionality
• It switches optically all the incoming wavelengths of the input fibers to the outgoing wavelengths of the output fibers.
• For instance, it can switch the optical signal on incoming wavelength i of input fiber k to the outgoing wavelength i of output fiber m.
Connection-Oriented Networks 51
• Converters: If it is equipped with converters, it can switch the optical signal of the incoming wavelength i of input fiber k to another outgoing wavelength j of the output fiber m. This happens when the wavelength i of the output fiber m is in use. Converters typically have a limited range within they can convert a wavelength.
Connection-Oriented Networks 52
• Optical add/drop multiplexer (OADM):
An OXC can also be used as an OADM. That is, it can terminate the optical signal of a number of incoming wavelengths and insert new optical signals on the same wavelengths in an output port. The remaining incoming wavelengths are switched through as described above.
Connection-Oriented Networks 53
Transparent and Opaque Switches
Transparent switch:The incoming wavelengths are switched to the output fibers optically, without having to convert them to the electrical domain.
Opaque switch:The input optical signals are converted to electrical signals, from where the packets are extracted. Packets are switched using a packet switch, and then they are transmitted out of the switch in the optical domain.
Connection-Oriented Networks 54
Switch technologies
Several different technologies exist:– micro electronic mechanical systems (MEMS)
– semiconductor optical amplifiers (SOA) – micro-bubbles– holograms – Also, 2x2 directional coupler , such as the electro-optic switch, the thermo-optic switch, and the Mach-Zehnder interferometer, can be used to construct large OXC switch fabrics
Connection-Oriented Networks 55
2D MEMS switching fabric
Down
Actuator
Mirror
Up… …
… …
…
i
j
Input ports
Output ports
…… …
…
…
…
…
…
…
Connection-Oriented Networks 56
A 2D MEMS OADM
12W
Add wavelength
s
Terminate wavelength
s
12W
Add wavelengths
i12W 12W
… …
… …
…
… …
Drop wavelengths
…
…
…… …
… …
Logical design 2D MEMS implementation
…
Connection-Oriented Networks 57
3D MEMS switching fabric
MirrorInsid
e ring x
axis
y axis
Output wavelengths
Input wavelengths
MEMSarray
MEMS array
Connection-Oriented Networks 58
Semiconductor optical amplifier (SOA)
• A SOA is a pn-junction that acts as an amplifier and also as an on-off switch
p-type
n-type
Current
Optical signal
Connection-Oriented Networks 59
2x2 SOA switch• Wavelength1 is split into two optical signals, and each signal is directed to a different SOA. One SOA amplifies the optical signal and permits it to go through, and the other one stops it. As a result 1 may leave from either the upper or the lower output port.
• Switching time is currently about 100 psec.
Polymerwaveguides
PolymerwaveguidesSOAs
1
2