3.FO Couplers, Switches, Isolators

8/14/2019 3.FO Couplers, Switches, Isolators

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Fiber Optics Components


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Introduction In an optical network there are many

situations where it is necessary to combine

signals and/or to split them multiple ways. Fiber optic couplers either split optical

signals into multiple paths or combinemultiple signals on one path.

One way of classification of components canbe as Passive components and activecomponents


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Couplers work on incident light andsplit at other end and is called as

coupling ratio which can beexpressed as in % or in db.

At the receiver, a detector absorbs

the signal flow.


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Multiple receivers, connected in a series,would receive no signal past the firstreceiver which would absorb the entire

signal. Thus, multiple parallel optical output ports

must divide the signal between the ports,reducing its magnitude.

The number of input and output ports,expressed as an N x M configuration,characterizes a coupler.

The letter N represents the number ofinput fibers, and M represents the numberof output fibers.


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Splitters The simplest couplers are fiber optic splitters.

These devices possess at least three ports but

may have more than 32 for more complexdevices.

Following figure illustrates a simple 3-portdevice, also called a tee coupler.


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Splitters contd.., One fiber is called the common fiber,

while the other two fibers may be calledinput or output ports.

The coupler manufacturer determinesthe ratio of the distribution of lightbetween the two output legs.

Popular splitting ratios include 50%-

50%, 90%-10%, 95%-5% and 99%-1%;however, almost any custom value canbe achieved. (These values aresometimes specified in dB values.)


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For example, using a 90%-10% splitterwith a 50 W light source, the outputswould equal 45 W and 5 W.

There could be couplers which candivide power in equal amounts called as50-50 couplers or 3db couplers.

A coupler with uneven split ratios are

called as taps. Splitters, taps, star couplers and WDM

(wave length division multiplexers) allcome under passive couplers.


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Directional Couplers The four-port directional coupler, also denoted

2 x 2 coupler, is the simplest coupler.

The light arrives for instance at port A and issplit between port C and D.

In the most common case, 50% of the lightpower will go in C and D.


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Fabrication of Optical

Couplers Optical couplers are made by fusing and tapering two

fibres together so that the cores are close enough to eachother for optical coupling to occur.

In practice,

twist two fibres together

heat them in a furnace: the fibres are softened andthey fuse.


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Beam splitter A beam splitter is an optical device that

splits a beam of light in two.


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Fused Taper Couplers To make these you put two regular single-

mode fibres into direct contact, heat thesection and then apply pulling pressure to theends of the fibres.

The fibres are drawn out and become thinner(both the cladding and the cores).


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In the process the fibres fusetogether.

In FBTs also the coupling ratio canbe any thing like 10-90, 20-80 etc.,


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Star Couplers A star coupler is just a multi-way coupler where

each input signal is made available on everyoutput fibre.

There are two types of star couplers as Transmission star Reflective star

In transmission star light at one input port is splitequally among all output ports.


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Star Couplers contd.., For instance in the figure shown input at port

A and is split equally through ports G, H, I,J,Kand L

In reflective star, input on any one fiber is splitamong all the fibers.

For example, here in this figure input on port Asplits the signal on all ports A,B,C,D,E,F


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Planar Couplers Couplers can also be

formed by creating awave guide onmaterials like silica.

Such couplers arecalled planar couplerswhich are custom buildtype.

These couplers offer

low optical losses,compact design,ruggedness andreproducibility.


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Y-junctions or 1 x 2Couplers

Y-junctions are 1 x 2 couplers and are a keyelement in networking.

Ideally the incoming light is split equallybetween the two arms.

Y-junctions can be concatenated to create 1 x4 couplers or 1 x 8 couplers


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Specifications of Coupler

Coupling Ratio Excess Loss

Insertion Loss Return Loss Directivity

Coupling Ratio:

It indicates the percentage division of opticalpower between the output ports by means ofthe splitting ratio or coupling ratio.


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Excess Loss

Excess loss is a measure of practical manufactureversus theory.

It is the additional loss of a device over and above

the loss required by theory. Excess Loss for a 2x2 coupler with input ports as 1

and 2 is

Excess Loss = input-(output1 + output2) Excess loss is generally around 0.05 db for small

couplers. Planar couplers exhibit more excess losses with

star couplers having the maximum loss value.


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Insertion Loss

It is the amount of signal lost in the total transit throughthe coupler.

It is also referred to as the loss for a particular port.Return Loss

Most of the times couplers reflect a part of the signal backdown the input fibre. (due to the bending in the device)

These losses are called return loss


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Directivity

The amount of signal lost in the

adjacent end is called directivity. This is also called near end rejection


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Switches

Switches are also a key elements in opticalnetworks.

Switches act as routers and redirect the

optical signals in a specific direction. Networks that require protection switching

(switching between redundant paths), wherekey attributes must operate reliably after a longperiod in one position, system monitoring, anddiagnosis commonly feature these devices.


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Below figure gives some commonswitch configurations


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Mechanical Switchescontd..,

This causes them to be relatively slow withswitching times in the 10-100 ms range.

They can achieve excellent reliability, insertion

loss, and isolation. Usually, mechanical optical switches collimate

the optical beam from each input and outputfiber and move these collimated beams aroundinside the device.

This allows for low optical loss, and allowsdistance between the input and output fiberwithout deleterious effects.


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Electro Optic Switches

Electro optic switches are typically semiconductor-based, and their operation depends on the change ofrefractive index with electric field.

Depending on the voltage applied light is switched toeither of the outputs.

Basically They convert photons from the input side toelectrons internally in order to do the switching andthen convert back to photons on the output side.


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Photonic Switches

Photonic is a term coined fordevices that work using photons,

analogous to the electronic devicesworking with electrons.

Switching speeds are very high

(order of nano seconds)


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Isolators

When a light is launched in to the fiber lightcan be reflected back due to the impuritiespresent in the fiber.

This back reflection results in excess noise,signal drop out at high speeds which may leadto failure of laser device.

To prevent back reflection of light from fiber to

the light source, isolators are used.


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Isolators is like one way valve for light allowingbeam in one direction but not in oppositedirection.

Circulator: Passive three-port devices that couple light

from Port 1 to 2 and Port 2 to 3 and have highisolation in other directions


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Attenuator in optical systems, is apassive device for reducing the

amplitude of a signal withoutappreciably distorting thewaveform.


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3.FO Couplers, Switches, Isolators

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