Otical Switch Doc

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OPTICAL SWITCHING

Gandhiji Institute of Science & Technology Page 1

CHAPTER -1

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 solutionto 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 andlower 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.

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.


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CHAPTER-2

OPTICAL FIBERS

A fiber consists of a glass core and a surrounding layer called the cladding. The core andcladding 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.

Opticalfiber typically include a transparent core surrounded by a

transparent cladding material with a lowerindex of refraction. Light is kept in the core by total

internal reflection. This causes the fiber to act as a waveguide. Fibers that support many

propagation paths ortransverse modes are calledmulti-mode fibers (MMF), while those that only

support a single mode are called single-mode fibers (SMF). Multi-mode fibers generally have a

wider core diameter, and are used for short-distance communication links and for applications

where high power must be transmitted. Single-mode fibers are used for most communication

links longer than 1,050 meters (3,440 ft).

Optical fiber can be used as a medium for telecommunication and computer

networkingbecause it is flexible and can be bundled as cables. It is especially advantageous for

long-distance communications, because light propagates through the fiber with little attenuation

compared to electrical cables. This allows long distances to be spanned with few repeaters.

Additionally, the per-channel light signals propagating in the fiber have been modulated at rates

as high as 111 gigabits per secondby NTT, although 10 or 40 Gbit/s is typical in deployed

systems. Each fiber can carry many independent channels, each using a different wavelength of

light (wavelength-division multiplexing (WDM)).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

rareeartherbium. 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
http://en.wikipedia.org/wiki/Cladding_(fiber_optics)http://en.wikipedia.org/wiki/Index_of_refractionhttp://en.wikipedia.org/wiki/Index_of_refractionhttp://en.wikipedia.org/wiki/Index_of_refractionhttp://en.wikipedia.org/wiki/Total_internal_reflectionhttp://en.wikipedia.org/wiki/Total_internal_reflectionhttp://en.wikipedia.org/wiki/Waveguide_(optics)http://en.wikipedia.org/wiki/Transverse_modehttp://en.wikipedia.org/wiki/Multi-mode_fiberhttp://en.wikipedia.org/wiki/Single-mode_fiberhttp://en.wikipedia.org/wiki/Computer_networkhttp://en.wikipedia.org/wiki/Computer_networkhttp://en.wikipedia.org/wiki/Optical_communications_repeaterhttp://en.wikipedia.org/wiki/Gigabit_per_secondhttp://en.wikipedia.org/wiki/Nippon_Telegraph_and_Telephonehttp://en.wikipedia.org/wiki/Wavelength-division_multiplexinghttp://en.wikipedia.org/wiki/Wavelength-division_multiplexinghttp://en.wikipedia.org/wiki/Nippon_Telegraph_and_Telephonehttp://en.wikipedia.org/wiki/Gigabit_per_secondhttp://en.wikipedia.org/wiki/Optical_communications_repeaterhttp://en.wikipedia.org/wiki/Computer_networkhttp://en.wikipedia.org/wiki/Computer_networkhttp://en.wikipedia.org/wiki/Single-mode_fiberhttp://en.wikipedia.org/wiki/Multi-mode_fiberhttp://en.wikipedia.org/wiki/Transverse_modehttp://en.wikipedia.org/wiki/Waveguide_(optics)http://en.wikipedia.org/wiki/Total_internal_reflectionhttp://en.wikipedia.org/wiki/Total_internal_reflectionhttp://en.wikipedia.org/wiki/Index_of_refractionhttp://en.wikipedia.org/wiki/Cladding_(fiber_optics)


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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 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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CHAPTER -3

SWITCHING IN OPTICAL FIBERS

3.1.ELECTRONIC SWITCHING

Most current networks employ electronic processing and use the optical fibre only as

transmission medium.Switching and processing of data are performed by converting an

opticalsignal back to electronic form.

Electronic switches provide a high degree of flexibility in terms of switching and routing

functions.The speed of electronics,however,is unable to match the high bandwidth of an optical

fiber(give that fibre has a potential bandwidth of approximately 50Tb/s- nearly four orders of

magnitude higher than peak electronic data rates).

An electronic conversion at an intermediate node in the network introduces extra delay.

Electronic equipment is strongly dependent on the data rate and protocol (any system upgrade

results in the addition/replacement of electronic switching equipment).


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3.2.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 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 OPTICAL

Fig.3.1 Intelligent O-E-O Switch

OPTICAL

ELECTRONIC SWITCH

FABRIC

E/O O/E

LEGEND

Optical Signal

Electrical Signal

E/O Electrical to Optical Signal


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The word applies on several levels. In commercial terms (such as "the telecom optical

switch market size") it refers to any piece ofcircuit switching equipment between fibers. The

majority of installed systems in this category actually use electronic switching between

fibertransponders. Systems that perform this function by routing light beams are often referred

to as "photonic" switches, independent of how the light itself is switched. Away from telecom,

an optical switch is the unit that actually switches light between fibers, and a photonic switch is

one that does this by exploiting nonlinear material properties to steer light (i.e., to switch

wavelengths or signals within a given fiber).

Hence a certain portion of the optical switch market is made up of photonic switches.

These will contain within them an optical switch, which will, in some cases, be a photonic

switch.

An optical switch may operate by mechanical means, such as physically shifting an

optical fiber to drive one or more alternative fibers, or by electro-optic effects, magneto-optic

effects, or other methods. Slow optical switches, such as those using moving fibers, may be used

for alternate routing of an optical switch transmissionpath, such as routing around a fault. Fast

optical switches, such as those using electro-optic or magneto-optic effects, may be used to

perform logic operations; also included in this category are semiconductoroptical amplifiers,

which are optoelectronicdevices that can be used as optical switches and be integrated with

discrete or integrated microelectronic circuits.

ALL OPTICAL SWITCH

OPTICAL OPTICAL

Fig.3.2 All Optical Switch

All-optical switches get their name from being able to carry light from their input to theiroutput ports in its native stateas pulses of light rather than changes in electrical voltage.

Alloptical switching is independent on data rate and data protocol.

OPTICAL SWITCH

FABRIC
http://en.wikipedia.org/wiki/Circuit_switchinghttp://en.wikipedia.org/wiki/Transpondershttp://en.wikipedia.org/wiki/Electro-optic_effecthttp://en.wikipedia.org/wiki/Magneto-optic_effecthttp://en.wikipedia.org/wiki/Magneto-optic_effecthttp://en.wikipedia.org/wiki/Routinghttp://en.wikipedia.org/wiki/Transmission_(telecommunications)http://en.wikipedia.org/wiki/Fault_(technology)http://en.wikipedia.org/wiki/Logic_operationhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Optical_amplifiershttp://en.wikipedia.org/wiki/Optoelectronichttp://en.wikipedia.org/wiki/Optoelectronichttp://en.wikipedia.org/wiki/Optical_amplifiershttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Logic_operationhttp://en.wikipedia.org/wiki/Fault_(technology)http://en.wikipedia.org/wiki/Transmission_(telecommunications)http://en.wikipedia.org/wiki/Routinghttp://en.wikipedia.org/wiki/Magneto-optic_effecthttp://en.wikipedia.org/wiki/Magneto-optic_effecthttp://en.wikipedia.org/wiki/Electro-optic_effecthttp://en.wikipedia.org/wiki/Transpondershttp://en.wikipedia.org/wiki/Circuit_switching


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3.2.2.MEMS OPTICAL SWITCHES

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-wafers surface.

MEMS can be considered a subcategory of optomechanical switches,however,because of

the fabrication process and miniature natures,they have different characteristics,performance and

reliability concerns.

MEMS use tiny reflective surfaces to redirect the light beams to a desired port by either

ricocheting the light off of neighbouring reflective surfaces to a port by steering the light beam

directly to a port.

Analog-type,or 3D,MEMS mirror arrays have reflecting surfaces that pivot about axes to

guide the light.

Digital-type,or 2D,MEMS have reflective surfaces that pop up and lay down to

redirect the light beam propagating parallel to the surface of substrate.

The reflecting surfaces actuators may be electrostatically-driven or electromagnetically-

driven with hinges or torsion bars that bend and straighten the miniature mirrors.

CONSTRUCTION

In MEMS, oxide layers are etched away to sculpt the devices 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.


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WORKING

Software in the switchs processor makes a decision about where an incoming stream of

photons should go. It sends a signal to an electrode on the chips 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 320GB per second

128 times faster than current electronic switches. Eventually such switches might support the

petabit(quadrillion-bit)system 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.


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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.

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.

Fig.3.3 Principle Of MEMS Optical Switch Operation


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3.2.2.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.

Planar lightwave circuit thermo-optical switches are usually polymerbased or silica on

silicon substrates.Electronic switches provide a high degree of flexibility in terms of terms of

switching and routing functions.

The operation of these devices is based on thermo optic effect.It consists in the variation

of the variation of the refractive index of a dielectic material,due to temperature variation of the

material ifself.

Thermo-optic switches are small in size but have a drawback of having driving-power

characteristics and issues of potical performance.There are two categories of thermo-optic

switches:1)Interferometric 2)Digital potical switches.

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 athway through the switch. branch. So the output light wave does not

take the default waveguidebut ends upon the waveguide labeled output#2 instead.


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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 thatpopulatephysics

laboratories, one reason the waveguide technology issometimes called a SILICON OPTICAL

BENCH.

Fig.3.4 The General Principle Of Thermo-Optical Switching Elements


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3.2.3.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 defaultwaveguide 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.

Fig.3.5 The General Principle Of The Bubble Optical Switch


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3.2.4.LIQUID CRYSTAL SWITCH

Even more people are familiar with the liquid crystal displays found in digital watches

liquid crystal, the molecules line up and so can become opaque.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

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.


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Fig.3.6 The General Structure Of The Liquid Crystal Switching Element


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3.2.5.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 incidentlight 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


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the unamplified pulse. Although non-linear switches have yet to reach commercial development,

the technology shows promise for the future.

Fig.3.7Nonlinear Optical Switching


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CHAPTER-4

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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CHAPTER-5

REFERENCES

TEXT BOOKS REFERED:

1.Rajiv Kumar, Optical Switching, Telecommunications, Nov-Dec 2002.2. Walter Goralski, Optical Networking and WDM, Tata McGrawhill edition.3. Rajiv Ramaswami, Kumar N SivarajanOptical networks. A practical perspective.4. Alberto Bononi,Optical Networking,springer publication-1999.5. Robeto Sabella,Optical Networks:Design and Modeling-1999.

6. Peng-Jun Wan,Multichannel Optical Networks.

7.Goralski,Optical Networking & Wdm-2001.

8.Robert C.Elsenpeter,Toby J.Velte,Optical Networking:A Beginners Guide-2002

9.Rudra Dutta,Ahmed E.Kamal,George N.Rouskas,Traffic Grooming for Optical Networks.

WEBSITES VIEWED:

www.alldatasheets.com www.howstuffworks.com www.google.com www. books.google.com www.wikipedia.com www.opticalnetworking.com www.ebooks.com
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