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Chapter 8: Optical Fibers and Components

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Amplification and Regeneration The Erbium-doped fiber amplifier (EDFA) Two-stage EDFA The 2x2 coupler The 2x2 coupler is a basic device in optical networks, ... – PowerPoint PPT presentation

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Title: Chapter 8: Optical Fibers and Components


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

2
WDM optical networks
?1
?1
Tx
Rx


optical fiber
optical fiber
?W
In-line amplification
Pre- amplifier
Power amplifier
?W
Tx
Rx
Wavelength multiplexer
Wavelength demultiplexer
  • A point-to-point connection

3
An example of an optical network
4
How light is transmitted through an optical fiber
Wave
Electric field
Source
  • Waves and electrical fields

5
An optical fiber
Cladding
Core
Cladding
Core and cladding
Cladding
Cladding
Core
Core
n1
n1
Refractive index
Refractive index
n2
n2
Radial distance
Radial distance
a) Step-index fiber b) graded-index fiber
6
Refraction and reflection of a light ray
Refracted ray
?f
n2
n1
??
?r
Incident ray
Reflected ray
7
Angle of launching a ray into the fiber
Cladding
Cladding
Core
Core
??
?r
?l
Cladding
Cladding
Cladding
Optical transmitter
Core
Cladding
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

9
Electric fields
Cladding
A
Core
2
1
Cladding
B
10
Electric field amplitudes for various fiber modes
Cladding
Core
Cladding
m0
m1
m2
11
Propagation of modes
Cladding
Cladding
a) step-index fiber
Cladding
Cladding
b) Graded-index fiber
12
Single-mode fiber
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.

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.

15
Attenuation
2.5
2.0
1.5
Attenuation, dB
1.0
0.5
800
1000
1200
1400
1600
1800
Wavelength, nm
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))

17
Dispersion
  • Dispersion is due to a number of reasons, such as
  • modal dispersion,
  • chromatic dispersion,
  • polarization mode dispersion.

18
Modal dispersion
Power
Power
Power
Time
Time
  • 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

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.

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.

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.

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.

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)

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.

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.

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

27
Components
  • Lasers
  • Photo-detectors and optical receivers
  • Optical amplifiers
  • The 2x2 coupler
  • Optical cross connects (OXC)

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.

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.

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.

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.

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.

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.

34
The ITU-T DWDM grid
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.

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

37
?1
?1
Tx
Rx


optical fiber
optical fiber
?W
Pre- amplifier
In-line amplification
Power amplifier
?W
Tx
Rx
Wavelength multiplexer
Wavelength demultiplexer
  • Amplifiers
  • power amplifiers,
  • in-line amplifiers,
  • pre-amplifiers

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.

39
Amplification and Regeneration
40
The Erbium-doped fiber amplifier (EDFA)
41
Two-stage EDFA
42
The 2x2 coupler
Fiber 1
Input 1
Output 1
Input 2
Output 2
Fiber 2
Coupling region
Tapered region
Tapered region
  • 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.

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.

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.

45
A banyan network of 3-dB couplers
?1??2????8
?1
?2
?1??2????8
?3
?1??2????8
?4
?1??2????8
?5
?1??2????8
?6
?1??2????8
?7
?1??2????8
?1??2????8
?8
46
Optical cross connects (OXCs)
47
OXC (contd)
  • Optical cross-connects

Wavelength Router
OXC
WDM link
To from other nodes
To from other nodes
GMPLS Plane
UNI
Access Station
IP router
Tx
Rx
Local Add
Local Drop
48
OXC switching fabric
  • Switching fabric

MEMS one mirror per output
Input WL ?1 to output 1
OXC
Output 1
2
3
4
49
OXC switching fabric (contd)
  • Switching fabric

MEMS one mirror per output
Input WL ?1 to output 4
OXC
Output 1
2
3
4
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.

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.

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.

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.

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

55
2D MEMS switching fabric
Input ports


Up






i
Down



Actuator
Mirror



Output ports
j
56
A 2D MEMS OADM
Drop wavelengths




?1??2????W
?1??2????W



?1??2????W
?1??2????W
i








Add wavelengths
Terminate wavelengths
Add wavelengths
Logical design
2D MEMS implementation
57
3D MEMS switching fabric
y axis
Mirror
Inside ring
x axis
58
Semiconductor optical amplifier (SOA)
  • A SOA is a pn-junction that acts as an amplifier
    and also as an on-off switch

Current
p-type
n-type
Optical signal
59
  • A 2x2 SOA switch
  • Wavelength ?1 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.
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