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OPTICAL NETWORKS
Building Blocks
A. Gençata
İTÜ, Dept. Computer Engineering
2005
Introduction
ƒ An introduction to WDM devices.
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optical fiber
optical couplers
optical receivers
optical filters
optical amplifiers
optical switches.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Optical Fiber
ƒ Excellent physical medium for high-speed networking
ƒ Two low-attenuation windows:
ƒ Centered approx. 1310 nm, a window of 200 nm
ƒ Centered approx. 1550 nm, a window of 200 nm
ƒ Combined, they provide a theoretical upper bound of 50
THz.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Optical Fiber
ƒ By using low-attenuation windows for data transmission,
the signal loss can be made very small.
ƒ This reduces the number of amplifiers needed.
ƒ 1550 nm window is preferred for long-haul wide-area
networks.
ƒ Standards set for industry: ITU grid.
ƒ Optical signals have been sent over 80 km without
amplification.
ƒ Low error rates: Typically operate at BERs of less than
10-11.
ƒ Fiber is also, flexible, light, reliable in corrosive
environment.
ƒ Immune to electromagnetic interference, and does not
cause interference.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Optical Transmission in Fiber
ƒ Fiber is a thin filament of glass which acts as a
waveguide.
Waveguide: A physical medium which allows the propagation of
electromagnetic waves like light.
ƒ Due to total internal reflection, light propagates
the length of the fiber with little loss.
ƒ Light speed in vacuum: c = 3·108 m/s.
ƒ Light can also travel through any transparent
material, but the speed is slower.
ƒ The ratio of the speed of light in vacuum to that
in a material is the material’s refractive index n.
nmat = c / cmat
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Optical Transmission in Fiber
ƒ When light travels from material of a given
refractive index to a material of a different
refractive index,
ƒ the angle at which the light is transmitted in the
second material depends on:
ƒ the refractive indices and
ƒ the angle at which light strikes the interface.
ƒ Snell’s Law:
nasin Θa = nbsin Θb
ƒ If na > nb and Θa is greater than some critical value,
the rays are reflected back into material a from its
boundary with material b.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Optical Transmission in Fiber
ƒ Fiber consists of a core completely surrounded
by a cladding.
ƒ Core and cladding consist of glass of different
refractive indices.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Critical Angle
ƒ Typical delay of light in optical fiber is 5 µs/km
ƒ Single-mode
vs. multimode
fiber
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Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Attenuation
ƒ Attenuation in optical fiber leads to a reduction of the
signal power as the signal propagates over some
distance.
ƒ When determining the maximum distance that a signal
can propagate for a given transmitter power and receiver
sensitivity, one must consider attenuation.
ƒ Receiver sensitivity is the minimum power required by a
receiver to detect the signal.
ƒ Let P(L) be the power of the optical pulse at distance L
km from the transmitter, and
ƒ A be the attenuation constant of the fiber (in dB/km).
ƒ Attenuation is characterized by:
P(L) = 10− A.L / 10 P(0)
where P(0) is the power at the transmitter.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Dispersion
ƒ Dispersion is the widening of a pulse duration as it
travels through a fiber.
ƒ As a pulse widens, it can broaden enough to interfere
with neighboring pulses (bits) on the fiber, leading to
inter-symbol interference.
ƒ Dispersion thus limits the bit spacing and the maximum
transmission rate on a fiber-optic channel.
ƒ Several kinds:
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intermodal dispersion
chromatic dispersion
material, waveguide, profile dispersions
polarization mode dispersion
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Nonlinearities in Fiber
ƒ Nonlinear effects in fiber may potentially have a
significant impact on the performance of WDM optical
communication systems.
ƒ Nonlinearities in fiber may lead to attenuation, distortion,
and cross-channel interference.
ƒ In a WDM system, these effects place constraints on the
spacing between adjacent wavelength channels, limit the
maximum power on any channel, and may also limit the
maximum bit rate.
ƒ Types:
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Nonlinear refraction
Stimulated Raman scattering
Stimulated Brillouin Scattering
Four-wave mixing
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Nonlinearities in Fiber
ƒ It is shown that, in a WDM system using channels
spaced 10 GHz apart and a transmitter power of 0.1 mW
per channel, a maximum of about 100 channels can be
obtained in the 1550-nm low-attenuation region.
ƒ The details of optical nonlinearities are very complex,
and beyond our scope.
ƒ However, it is important to understand that they are a
major limiting factor in the available number of channels
in a WDM system, especially those operating over
distances greater than 30 km.
ƒ The existence of these nonlinearities suggests that WDM
protocols which limit the number of nodes to the number
of channels do not scale well.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Optical Fiber Couplers
ƒ Coupler is a general term that covers all devices that combine light
into or split light out of a fiber.
ƒ It can be either active or passive device.
ƒ The splitting ratio, α, is the amount of power that goes to each
output.
ƒ For a two-port splitter, the most common splitting ratio is 50:50,
though splitters with any ratio can be manufactured.
ƒ Combiners are the reverse of splitters, and when turned around, a
combiner can be used as a splitter.
ƒ A 2 × 2 coupler is a 2×1 combiner followed immediately by a 1×2
splitter.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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A 16 x 16 Passive Star Coupler
ƒ The passive-star coupler (PSC) is a multi-port device in which light
coming into any input port is broadcast to every output port.
ƒ The PSC is attractive because the optical power that each output
receives Pout equals:
Pout = Pin / N
where Pin is the optical power introduced into the star by one node.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Optical Transmitter
ƒ Lasers are used as optical transmitter.
ƒ The transmitters used in WDM networks often
require the capability to tune to different
wavelengths.
ƒ Some primary characteristics of interest for
tunable lasers are:
ƒ the tuning range,
ƒ the tuning time,
ƒ whether the laser is continuously tunable (over its
tuning range) or discretely tunable (only to selected
wavelengths).
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Types of Transmitters
ƒ The table summarizes tuning range and time of
different types of lasers.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Optical Receivers
ƒ Optical filters transform the optical signal into electronic
signal.
ƒ There are different types of tunable optical filters.
ƒ These filters are characterized primarily by their tuning
range and tuning time.
ƒ The tuning range specifies the range of wavelengths
which can be accessed by a filter.
ƒ A wide tuning range allows systems to utilize a greater
number of channels.
ƒ The tuning time of a filter specifies the time required to
tune from one wavelength to another.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Tunable Filters
ƒ The table shows various types of filters and their
tuning range and time.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Alternate Receiving Devices
ƒ An alternative to tunable filters is to use fixed
filters or grating devices.
ƒ Grating devices typically filter out one or more
different wavelength signals from a fiber.
ƒ Such devices may be used to implement optical
multiplexers and demultiplexers or receiver
arrays.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Optical Amplifiers
ƒ All-optical amplification may differ from opto-electronic
amplification in that it may act only to boost the power of
a signal, not to restore the shape or timing of the signal.
This type of amplification is known as 1R (regeneration).
ƒ In 2R (regeneration and reshaping), the optical signal is
converted to an electronic signal which is then used to
directly modulate a laser.
ƒ The optical signals may be amplified by first converting
the information stream into an electronic data signal, and
then retransmitting the signal optically. Such
amplification is referred to as 3R (regeneration,
reshaping, and reclocking).
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Types of Amplifiers
ƒ Semiconductor laser amplifier
ƒ Fabry-Perot
ƒ Traveling-wave
ƒ Doped-fiber amplifier
ƒ EDFA
ƒ PDFFA
ƒ Raman amplifier
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Switching Elements
ƒ According to the signal carriers, there are
ƒ optical switching and
ƒ electronic switching.
ƒ In the switching granularity point of view, there are two
basic classes,
ƒ circuit switching corresponding to wavelength routing,
ƒ cell switching corresponding to optical packet switching and
optical burst switching.
ƒ As far as the transparency of signal is considered, there
are
ƒ opaque switching and
ƒ transparent switching.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Optical Cross-connect (OXC)
ƒ An optical cross-connect (OXC) switches optical signals
from input ports to output ports.
ƒ A basic cross-connect element is the 2 × 2 crosspoint
element.
ƒ It routes optical signals from two input ports to two output
ports and has two states: cross state and bar state.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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1024x1024 Clos Fabric
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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MEMS
ƒ Micro-electro mechanical systems.
ƒ Widely believed to be the most promising for large-scale optical
cross-connects.
ƒ Based on mirrors, membranes, and planar moving waveguides.
ƒ The two major approaches are 2-Dimensional and 3-Dimensional
approaches.
ƒ The 3-D Optical MEMS based on mirrors is popular because it is
suitable for compact, large-scale switching fabrics.
ƒ The optical signals passing through the optical fibers at the input port
are switched independently by the MEMS mirrors with two-axis tilt
control and then focused onto the optical fibers at the output ports.
ƒ In the switch, any connection between input and output fibers can be
accomplished by controlling the tilt angle of each mirror.
ƒ The 3D MEMS-based OOO switches is in sizes ranging from 256 x 256
to 1000 x 1000 bi-directional port machines.
ƒ Encouraging research show that 8000 x 8000 ports will be practical
within the foreseeable future.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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3D MEMS Optical Switch
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Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Wavelength Routing Devices
ƒ A wavelength-routing device can route signals arriving at
different input fibers (ports) of the device to different
output fibers (ports) based on the wavelengths of the
signals.
ƒ Wavelength routing is accomplished by:
ƒ demultiplexing the different wavelengths from each input port,
ƒ optionally switching each wavelength separately,
ƒ and then multiplexing signals at each output port.
ƒ The device can be either:
ƒ non-reconfigurable, in which case there is no switching stage
between the demultiplexers and the multiplexers, and the routes
for different signals arriving at any input port are fixed (referred to
as routers),
ƒ or reconfigurable, in which case the routing function of the switch
can be controlled electronically.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Non-reconfigurable Wavelength Router
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Wavelength Routing Switch
ƒ The WRS has P incoming fibers and P outgoing fibers.
ƒ On each incoming fiber, there are M wavelength channels.
ƒ Similar to the nonreconfigurable router, the wavelengths on
each incoming fiber are separated using a grating
demultiplexer.
ƒ The outputs of the demultiplexers are directed to an array of
M P×P optical switches between the demultiplexer and the
multiplexer stages.
ƒ All signals on a given wavelength are directed to the same
switch.
ƒ The switched signals are then directed to multiplexers
associated with the output ports.
ƒ Finally, information streams from multiple WDM channels are
multiplexed before launching them back onto an output fiber.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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PxP WRS
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Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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Wavelength Conversion
ƒ Consider the following network.
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Wavelength Conversion
ƒ Three lightpaths have been set up:
ƒ C to A on wavelength λ1,
ƒ C to B on λ2, and
ƒ D to E on λ1.
ƒ To establish a lightpath, we require that the
same wavelength be allocated on all the links in
the path.
ƒ This requirement is known as the wavelengthcontinuity constraint.
ƒ Can we establish a lightpath from A to B?
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
32
Wavelength Conversion
ƒ It is easy to eliminate the wavelength-continuity
constraint, if we were able to convert the data
arriving on one wavelength along a link into
another wavelength at an intermediate node and
forward it along the next link.
ƒ Such a technique is referred to as wavelength
conversion.
ƒ A single lightpath in such a wavelengthconvertible network can use a different
wavelength along each of the links in its path.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
33
Wavelength Converter
ƒ The function of a wavelength converter is to
convert data on an input wavelength onto a
different output wavelength among the N
wavelengths.
Optical Networks
Ayşegül Gençata, İTÜ, Dept. Computer Engineering
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