A Seminar Report

On

OPTICAL SWITCHES

BY

TALAWARA MANJUNATH

USN 1PI07EC115

in partial fulfillment for the seventh semester Seminar

of

BACHELOR OF ENGINEERING IN

ELECTRONICS AND COMMUNICATION ENGINEERING

Under the supervision of

Mr. UMESH KUMAR

August 2010 – December 2010

DEPARTMENT OF ELECTRONICS AND COMMUNICATION

ENGINEERING
PES INSTITUTE OF TECHNOLOGY

100 FEET RING ROAD, BANASHANKARI III STAGE

Bengaluru-560085
 PES Institute of Technology,

100 Feet Ring Road, BSK 3rd Stage, Bangalore-560085

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

CERTIFICATE

This is to certify that TALAWARA MANJUNATH,1PI07EC115 has

satisfactorily completed a seminar on OPTICAL SWITCHES
prescribed by PES Institute of Technology (Autonomous Institute
under VTU) in the year 2010 (August-December ).

Signature of guide Signature of HOD

Mr. Kogali Nagaraj Prof.K.Raghunath

Name of the Student: TALAWARA MANJUNATH

University Seat Number: 1PI07EC115

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.
CONTENTS

1. INTRODUCTION 1
2. OPTICAL FIBERS 2
3. OPTICAL SWITCHES 4
 MEMS 5
 Thermo-optical waveguide 9
 Bubble Switch 11
 Liquid Crystal Switch 12
 Nonlinear Optical Switch 14
4. CONCLUSION 16
5. BIBLIOGRAPY 17
INTRODUCTION

Explosive information demand in the internet world is creating 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 are
capable 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.

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 interface of 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 by stretches 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 are jammed 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 techno speak.
 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 lower speed electronic signals. Finally, switched
signals would have to be reconverted to photons and re aggregated 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.

OPTICAL SWITCHES
Optical switches will switch a wavelength or an entire fiber form one pathway to
another, leaving the data-carrying packets in a signal untouched. An electronic
signal from electronic processor will set the switch 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 the gradual withering away of Asynchronous Transfer
Mode (ATM), another phone company standard for packaging information.

Optical Switches
1) 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.

Working:
Software 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.
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 few milliseconds,
fast enough for the most demanding switching applications. The above switch
offered more than 10 terabits per second of total switching capacity, with each
of the channels supporting 320 GB per second – 128 times faster than current
electronic switches. Eventually such switches might support the pet bit
(quadrillion-bit) systems that are looming on the horizon.
2. Size
Each mirror in one MEMS switch is half a millimeter in diameter, about the
size 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. The entire 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 provide a
300-fold reduction in power consumption over electronic switches.
4. Economical
Standard silicon circuit manufacturing processors make the
technology cost 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 larger switches.
6. Stability
Silicon microns afford greater stability than if the mirrors
were fabricated from metal.
7. Accurate
Use of silicon fabrication technology results in stiffer mirrors that are less
prone to drifting out of alignment and which are robust, long lived and scalable
to large number of devices on wafer. Superior Software control algorithms let
the individual elements manipulated precisely.
8. Well-matched to optics application
The technology is also well matched to optics applications – because easily
accommodates the need to expand or reconfigure the number of pathway
through the switch.
 Principle of MEMS optical switch operation

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 labeled
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
waveguidebut ends upon the waveguide labeled 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

The general principle of thermo-optical switching elements

BUBBLE SWITCH
Construction and Working
The switch consist of a silica waveguide with arrays of intersecting light pipes
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 move into 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

LIQUID CRYSTAL SWITCH
Even more people are familiar with the liquid crystal displays found in digital
watches and some forms of computer output devices than are familiar with
inkjet printers. Liquid crystals can also be used as a basis for optical switches
as well. When an electrical field is applied to the liquid crystal, the molecules
line up and so can become opaque. The liquid crystal switches rely on a change
in the polarization of optical signals with the application of electrical voltage
to make a switching element. Because the liquid crystal molecules are so long
and thin, they will let only light of a particular orientation pass through the
liquid crystal. Liquid crystal switching elements are built with two active
components, the cell and the displacer. The liquid crystal cell is formed by
placing the liquid crystals between two plates of glass. The glass is coated with
an oxide material that conducts electricity and is also transparent. The glass
plate form the electrodes of the cell portion of the switching element. The main
function of the cell is to reorient the polarized light entering the cell as
required. The displacer is a composite crystal that directs the polarized light
leaving the cell. Light polarized in one direction is directed to one output
waveguide by the displacer, while light polarized at a 90 degree angle is
directed to a second output waveguide.

Working
The upper portion of the figure shows the path of a light wave when no voltage
is applied to the cell. Input light of arbitrary polarization lines up with the
default polarization orientation of the liquid crystals inside the cell. The
displacer also has a default orientation and the light emerges as shown 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 crystals in the cell and those
in the displacer both change their orientation under the influence of the
voltage. The polarized light now takes the second output path.
 The general structure of the liquid crystal switching element

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 optical
phenomena 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 of refraction 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. The pulse 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.

Nonlinear optical switching

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.
BIBLIOGRAPHY
1. Rajiv Kumar, “Optical Switching”, Telecommunications, Nov-
Dec 2002.
2. Walter Goralski, Optical Networking and WDM, Tata Mc
Grawhill edition.
3. Rajiv Ramaswami, Kumar N Sivarajan – Optical networks. A
practical perspective.