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Connection-Oriented Networks 1 Chapter 8: Optical Fibers and Components TOPICS – WDM optical networks – Light transmitted through an optical fiber – Types of optical fibers – Impairments – Components: Lasers, optical amplifiers, couplers, OXCs

Connection-Oriented Networks1 Chapter 8: Optical Fibers and Components TOPICS –WDM optical networks –Light transmitted through an optical fiber –Types

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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 9

Electric fields

Cladding

Cladding

Core

1

A

B

2

Connection-Oriented Networks 10

Electric field amplitudes for

various fiber modes

Cladding

Cladding

Core

m=0 m=1 m=2

Connection-Oriented Networks 11

Propagation of modes

Cladding

Cladding

Cladding

a) step-index fiber

b) Graded-index fiber

Cladding

Connection-Oriented Networks 12

Single-mode fiber

Cladding

Cladding

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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.

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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.

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• 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

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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

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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 39

Amplification and Regeneration

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