optical switching minor project
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A Project Report on
OPTICAL SWITCHING AND
ADVANCEMENTS
Minor Project – Semester VII (ECS – 712)
Department of Electronics and Communication Engineering
Faculty of Engineering and Technology, Jamia Millia University
Submitted in the partial fulfilment of the requirements for the award of the degree of
Bachelor of Technology
by
MOHD YASIR KHAN
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ACKNOWLEDGEMENT
I extend my sincere gratitude towards Prof. Mainuddin,
our teacher of Wireless Communication for giving us his
invaluable knowledge and wonderful technical guidance.
I express my thanks to all the other faculty members of
Electronics & Communication Engineering Department,
Jamia Millia Islamia University for their kind cooperation
and guidance for preparing and presenting this project.
I also thank all my family members and friends for their
help and support.
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CONTENTS
ABSTRACT
INTRODUCTION
OPTICAL FIBERS
OPTICAL SWITCHES
MEMS
Thermo-Optic Switch
Bubble Switch
Liquid Crystal Switch
Nonlinear Optical Switch
ADVANCEMENTS IN OPTICAL SWITCHES
CONCLUSION
BIBLIOGRAPY
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ABSTRACT
Theoretically optical switches seem to be future proof
with features of scalability, flexibility, bit rate and protocol
independent coupled with lower infrastructure costs but a
network service provider must evaluate the pros and cons
and all possible options to select optimum combination of
electronic and photonic switches to meet the capacity and
traffic management requirements. This seminar presents
an overview on optical switches.
Optical switches including mems, bubble, Thermo-
optical, Liquid crystal and non-linear optical switches
have been discussed.
Finally all optical switching a technology that’s still in
its infancy but holds tremendous potential, since it
switches optical packets, is also with.
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INTRODUCTION
Explosive information demand in the internet world iscreating enormous needs for capacity expansion in next
generation telecommunication networks. It is expected
that the data-oriented network traffic will double every
year. Optical networks are widely regarded as the ultimate
solution to the bandwidth needs of future communication
systems. Optical fiber links deployed between nodes arecapable to carry terabits of information but the electronic
switching at the nodes limit the bandwidth of a network.
Optical switches at the nodes will overcome this
limitation. With their improved efficiency and lower costs,
Optical switches provide the key to both manage the new
capacity Dense Wavelength Division Multiplexing(DWDM) links as well as gain a competitive advantage for
provision of new band width hungry services. However, in
an optically switched network the challenge lies in
overcoming signal impairment and network related
parameters. Let us discuss the present status, advantages
and challenges and future trends in optical switches.
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OPTICAL FIBERS
A fiber consists of a glass core and a surrounding layer called the cladding.
The core and cladding have carefully chosen indices of refraction to ensure
that the photos propagating in the core are always reflected at the interfaceof the cladding. The only way the light can enter and escape is through the
ends of the fiber. A transmitter either alight emitting diode or a laser sends
electronic data that have been converted to photons over the fiber at a
wavelength of between 1,200 and 1,600 nanometers.
Today fibers are pure enough that a light signal can travel for about 80
kilometers without the need for amplification. But at some point the signal
still needs to be boosted. Electronics for amplitude signal were replaced bystretches of fiber infused with ions of the rare-earth erbium. When these
erbium-doped fibers were zapped by a pump laser, the excited ions could
revive a fading signal. They restore a signal without any optical to
electronic conversion and can do so for very high speed signals sending
tens of gigabits a second. Most importantly they can boost the power of
many wavelengths simultaneously.
Now to increase information rate, as many wavelengths as possible arejammed down a fiber, with a wavelength carrying as much data as possible.
The technology that does this has a name-dense wavelength division
multiplexing (DWDM ) – that is a paragon of technospeak.
Switches are needed to route the digital flow to its ultimate destination.
The enormous bit conduits will flounder if the light streams are routed
using conventional electronic switches, which require a multi-terabit signal
to be converted into hundreds of lowerspeed electronic signals. Finally,switched signals would have to be reconverted to photons and
reaggregated into light channels that are then sent out through a
designated output fiber.
The cost and complexity of electronic switching prompted to find a means
of redirecting either individual wavelengths or the entire light signal in a
fiber from one path way to another without the opto-electronic conversion.
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OPTICAL SWITCHES
Optical switches will switch a wavelength or an entire fiberform one
pathway to another, leaving the data-carrying packets in a signal
untouched. An electronic signal from electronic processor will set theswitch in the right position so that it directs an incoming fiber – or
wavelengths within that fiber- to a given output fiber. But none of the
wavelengths will be converted to electrons for processing.
Optical switching may eventually make obsolete existing lightwave
technologies based on the ubiquitous SONET (Synchronous Optical
Network) communications standard, which relies on electronics for
conversion and processing of individual packets. In tandem with thegradual withering away of Asynchronous Transfer Mode (ATM), another
phone company standard for packaging information.
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“MEMS”
Introduction
Micro-electro Mechanical Systems or MEMS is a new process for device
fabrication, which builds “micromechines” that are finding increasing
acceptance in many industries ranging form telecommunications to
automotive, aerospace, consumer electronics and others.
In essence, MEMS are Mechanical Integrated circuits, using photo
lithographic and etching processes similar to those employed in making
large scale integrated circuits – devices that are deposited and patterned on
a silicon-wafer’s surface.
Construction
In MEMS, oxide layers are etched away to sculpt the device’s structural
elements. Instead of creating transistors, though, lithographic processes
built devices a few tens or hundreds of microns in dimension that move
when directed by an electrical signal. Silicon mirrors are manufactured by
self-assembly- a novel step that takes its name from the way amino-acids in
protein molecules fold themselves into three-dimensional shapers. In the
final stage of manufacture, tiny springs on the silicon surface release the
mirrors and a frame around each on lifts them and locks them in place,
positioning them high enough above the surface to allow for a range of
movement.
WorkingSoftware in the switch’s processor makes a decision about where an
incoming stream of photons should go. It sends a signal to an electrode on
the chip’s surface that generates an electric field that tilts the mirrors. The
wavelengths bounce off the input mirrors and get reflected off another
mirror onto output mirrors that direct the wavelength into another fiber.
Switches with 256 incoming fibers and same number of outgoing fibers
have been successfully tested and employed.
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Analogy
To understand the working of switch, consider a room with many windows
and a movable mirror inside. On manipulating the mirror, the sunlight
streams through a window could be reflected off the desired window.
Advantages :
1. Fast
No opto-electronic conversion, so the entire process lasts a fewmilliseconds, fast enough for the most demanding switching applications.The above switch offered more than 10 terabits per second of totalswitching capacity, with each of the channels supporting 320 GB per
second – 128 times faster than current electronic switches. Eventually suchswitches might support the petabit (quadrillion-bit) systems that arelooming on the horizon.
2. Size
Each mirror in one MEMS switch is half a millimeter in diameter, about thesize of the head of a pin. Mirrors rest one millimeter apart and all 256-mirrors are fabricated on a 2.5 centimeter-square piece of silicon. Theentire switch is about the size of a grape- fruit –32 times denser than an
electronic switch.
3. Power reduction
With no processing, or opto-electronic conversion, these switches providea 300-fold reduction in power consumption over electronic switches.
4. Economical
Standard silicon circuit manufacturing processors make the technologycost effective.
5. Larger Switches
The design of mirror-arrays uses one mirror for input and one for output.Coupled with the VLSI technique, they promote building of much largerswitches.
6. Stability
Silicon microns afford greater stability than if the mirrors were fabricatedfrom metal.
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7. Accurate
Use of silicon fabrication technology results in stiffer mirrors that are lessprone to drifting out of alignment and which are robust, long lived andscalable to large number of devices on wafer. Software control algorithmslet the individual elements manipulated precisely.
8. Well-matched to optics applicationThe technology is also well matched to optics applications – because easilyaccommodates the need to expand or reconfigure the number of pathwaythrough the switch.
Principle of MEMS optical switch operation
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“THERMO-OPTIC SWITCH”
The MEMS is not the only way to produce an optical switch architecture
that uses many small and inexpensive components to control the flow of
light from input to output. One interesting approach is to use what are
known as Thermo-optical waveguides. Waveguides can be built by the
some standard process used to make integrated circuits and so like “fibers
on a chip”. Waveguides have a core and cladding made of glass with
differing indices of refraction, just like normal fiber optic cables.
The basic Thermo-optical switching element has an input waveguide and
two possible output waveguides. In between there are two short, internal
waveguides that first split the input light and then couple the two internal
waveguides together again. The recombined light would proceed down the
“default” output waveguide. But thermo-optical effect makes it possible to
use this coupling of the light as a switching element.
Working
The general principle of thermo-optical switching element is shown in the
figure. An input light wave is split onto two separate waveguides. If no heat
is applied to the lower branch in the figure, the coupler will output the
waveform on to the waveguide labelled output#1 in the figure. The figure
shows the heating element activated, and a slightly different phase induced
into the waveform on the lower branch. So the output light wave does not
take the default waveguide but ends upon the waveguide labelled output#2
instead.
Advantages
Because they can be built on a common material substrate like silicon,waveguides tend to be small and inexpensive, and they can be
manufactured in large batches. The substrates, called wafers, can serve as
platforms to attach lasers and detectors that would enable transmission or
receipt of optical pulses that represent individual bits. Integration of
various components could lead to photonic integrated circuit, a
miniaturized version of the components that populate physics laboratories,
one reason the waveguide technology is sometimes called a SILICON
OPTICAL BENCH.
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The general principle of thermo-optical switching elements
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“BUBBLE SWITCH”
Construction and Working
The switch consist of a silica waveguide with arrays of intersecting lightpipes that from a mesh. A small hole sits at a point where these light pipes
intersect. It contains an index-matching fluid (one whose index of
refraction is the same as the silica). So if no bubble is present at the
junction, the light proceeds down the default waveguide path. If a bubble of
fluid is present at the junction, the light is shifted onto the second output
waveguide. The bubble act as a mirror that reflects the light wave to
another branch of the switching element An ink-jet printing head
underneath can blow a bubble into the hole, causing light to bend and moveinto another waveguide. But if no bubble is present, the light proceeds
Straight. That this switch works at all is a testament to the extraordinary
sophistication of the fluid technology behind printers.
The general principle of the bubble optical switch
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“LIQUID CRYSTAL SWITCH”
Even more people are familiar with the liquid crystal displays found indigital watches and some forms of computer output devices than are
familiar with inkjet printers. Liquid crystals can also be used as a basis foroptical switches as well. When an electrical field is applied to the liquidcrystal, the molecules line up and so can become opaque.
The liquid crystal switches rely on a change in the polarization of opticalsignals with the application of electrical voltage to make a switchingelement. Because the liquid crystal molecules are so long and thin, they willlet only light of a particular orientation pass through the liquid crystal.
Liquid crystal switching elements are built with two active components, thecell and the displacer. The liquid crystal cell is formed by placing the liquidcrystals between two plates of glass. The glass is coated with an oxidematerial that conducts electricity and is also transparent. The glass plateform the electrodes of the cell portion of the switching element. The mainfunction of the cell is to reorient the polarized light entering the cell asrequired. The displacer is a composite crystal that directs the polarizedlight leaving the cell. Light polarized in one direction is directed to oneoutput waveguide by the displacer, while light polarized at a 90 degreeangle is directed to a second output waveguide.
Working
The upper portion of the figure shows the path of a light wave when novoltage is applied to the cell. Input light of arbitrary polarization lines upwith the default polarization orientation of the liquid crystals inside thecell. The displacer also has a default orientation and the light emerges asshown in the figure. The lower portion of the figure shows the path of a
light wave when voltage is applied to the cell. Note that the liquid crystalsin the cell and those in the displacer both change their orientation underthe influence of the voltage. The polarized light now takes the secondoutput path.
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The general structure of the liquid crystal switching element
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“NON-LINEAR OPTICAL SWITCH”
Another type of optical switch takes advantage of the way of the refractive
index of glass changes as the intensity of light varies. Most of the opticalphenomena in everyday life are linear. If more light is shined on a mirror,
the surface reflects more of the incident light and the imaged room appears
brighter.
A non-linear optical effect, however, changes the material properties
through which the light travels. Mirror becomes transparent when more
light is shined on it. Glass optical fibers experience non-linear effects, some
of which can be used to design very fast switching elements, capable of
changing their state in a femtosecond (quadrillionth of a second time scale).Consider a non-linear optical loop mirror, a type of interferometer in which
two light beams interact.
In the mirror a fiber splitter divides an incoming beam. In one instance
each segment travels through the loop in opposite directions recombines
after completing the circle and exist on the same fiber on which it entered
the loop. In cases, though, after the two beams split, an additional beam is
send down one side of the loop but not the other. The intensity of light
produced by the interaction of the coincident beams changes the index ofrefraction in the fiber, which in turn changes the phase of the light. The
recombined signal with its altered phase, exits out a separate output fiber.
In general, non-linear optical switching requires the use of very short
optical pulses that contain sufficient power to elicit nonlinear effects from
the glass in the fiber. An optical amplifier incorporated into the switch,
however, can reduce the threshold at which these non-linear effects occur.
For the purpose of switching the intensity dependent phase change
induced by the silica fiber itself could be used as the non-linearity. Thepulse traversing the fiber loop clockwise is amplified by an EDFA shortly
after it leaves the directional coupler. This configuration is called Non-
linear Amplifying Loop Mirror (NALM). The amplified pulse has higher
intensity and undergoes a larger phase shift on traversing the loop
compared to the unamplified pulse. Although non-linear switches have yet
to reach commercial development, the technology shows promise for the
future.
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Nonlinear optical switching
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ADVANCEMENTS IN OPTICAL SWITCHES
Companies Producing Optical Switches
Many companies have chosen to take advantages of the potential that
optical switches hold. The market of optical switches is dominated by four
companies, namely Lucent, Nortel, Alcatel and Cisco, with the first two
holding 50% of the market share. Besides the companies mentioned above
and within the Technologies section, here are some other companies worth
noting.
Ciena Corporation developed one of the first DWDM systems, and produces
switches for both regional, metropolitan and long haul carrier networks. Itsrecent advances in its "MultiWave CoreStream" products has gained
industry attention by enabling optical systems to perform at 16 Tb/s.
The first all-optical switch was produced by Corvis Corportion, and was
called the "Corvis Optical Switch". The company has been able to gain other
technological breakthroughs such as the ability to transmit signals over
20,000 miles without the need to regenerate signals.
Trellis Photonics has come up with a "Intelligent Lambda Switch" which
uses hologram technology to reflect specific wavelengths of light, using an
array of tiny crystals. The technology is immensely scalable, with Trellis
proposing a to develop 3840 x 3840 port switch, with no moving parts and
a crystallite structure which is 95 percent efficient.
Future of the Technology
Most believes that internet traffic is currently overwhelmed by the
immense number of people who try to access it every day. A growing
demand for video conferencing and audio will also challenge the data
capacities and bandwidth of networks in the future. Optical Networks is the
clear solution, with experiments demonstrating that data rates of 400 Gb/s
can be achieved using DWDM technologies.
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The future of Optical Switching is bright for the industry, which is set to
grow to $6bn in a few years time. With the evolution of an AON, equipment
and managerial costs will be lowered and efficiency will be increased in the
network.
With the new GMPLS standard coming into place, companies are able to
truly design networks and systems that are able to fully utilize the power of
optical networks while consumers will be able to enjoy high data rates,
without bottlenecks.
However, until all-optical switches develop a level of intelligence matching
that of an electro-optical switch, both technologies will continue to be used
by all sectors of the industry.
The Fastest Optical Switch
To date, the speed of optical switches is limited by the properties of the
underlying materials, but not by the speed of light. Now scientists from
COPS at the University of Twente and the FOM-Institute Amolf in
Amsterdam in the Netherlands, and the Institute for Nanoscience and
Cryogenics (CEA/INAC) in Grenoble in France have managed to switch-onand switch-off a semiconductor optical cavity within a world-record short
time of less than 1 picosecond, or one millionth of a millionth second. The
results are published online on April 22 in the leading American journal
Applied Physics Letters, and are expected to yield ultrafast optical data
communication, tiny on-chip light sources and lasers, and perhaps even
switches for quantum bits of information.
In optics, cavities are widely used for their ability to store light in a volume
in space for a particular duration in time. A generic cavity consists of two
mirrors separated by a length of transparent material. In the cavity light
bounces back and forth between the mirrors. Since light is an
electromagnetic wave, it appears that only waves whose wavelength (or
color) matches the cavity length can exist in the cavity. This is the result of
constructive interference where crests and valleys of many waves coincide,
and therefore add up to a high intensity. As a result, the allowed waves
resonate to form a standing wave in the cavity, as shown in Figure 1.
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Light does not circulate indefinitely inside a cavity, since on every round-
trip a little bit of light energy leaks out. Thus light is stored during a cavity
storage time. The light waves that escape from the cavity to the right in
Figure 1 are selectively transmitted. All other colors of light are reflected
from the front of the cavity. Hence, Figure 1 illustrates the popular filter
function of a cavity, since from incident white light with many colorsonly one color is selectively transmitted to the right.
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CONCLUSION
Photonic packet – switched networks offer the potential of
realizing packet-switched networks with much higher
capacities than may be possible with electronic packet-
switched networks. However, significant advances in
technology are needed to make them practical, and there
are some significant roadblocks to overcome, such as he
lock of economical optical buffering and the difficulty of
propagating very high speed signals at tens and hundreds
of gigabits/second over any significant distances of optical
fiber. There is a need for compact soliton light sources. At
this time, fast optical switches have relatively high losses,
including polarization-dependent losses, and are not
amenable to integration, which is essential to realize large
switches. Temperature dependence of individual
components can also be a significant problem when
multiplexing, demultiplexing, or synchronizing signals at
such high bit rates.
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BIBLIOGRAPHY
Books: Rajiv Kumar, “Optical Switching”, Telecommunications, Nov-Dec
2002. Walter Goralski, “Optical Networking and WDM”, Tata Mc Grawhill
edition.
Rajiv Ramaswami, Kumar N Sivarajan – “Optical Networks”.
Web Pages:
www.lightwave.ee.columbia.edu
www.journals.elsvier.com/optical-switching-networking
www.technav.ieee.org/tag/1265/optical-switching
www.phy.duke.edu/research/photon/qelectron/proj/switch