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UNIT -3

OPTICAL FIBRES

Introduction:

Optical fibre is a very thin and flexible medium used for transmission oflight , having a cylindrical shape. It consists of three sections

1.The core. 2.Cladding . 3.Jacket

Core is the innermost section and is made of glass or plastic. This is the actual fibre andhas the remarkable property of conducting an optical beam

It is surrounded by its own cladding , a glass or plastic coating , that has optical

properties which are very different from those of core.

The outer section is called the Jacket made of plastic or polymer and other materials andis provided for protection against moisture, abrasion, crushing and other environmental dangers.

The core acts like a continuous layer of two parallel mirrors. A signal is first encodedinto a light beam, which is then passed in between the two boundaries and propagated as a resultof multiple internal reflections.

Advantages of optical fibre over wire or radio system & this is why telecommunicationindustries have introduced the fibre optic system :

1. Attenuation is less than the coaxial cable , so transmission within wide range ofdistance is possible without repeaters etc.

2. Smaller size and lighter weight so they occupy much less space.

3. Electromagnetic isolation :- Electromagnetic waves generated from electricaldisturbances or electrical noises do not interfere with light signals. So the system isnot vulnerable to interference, impulse noise or cross talk.

4. No physical electrical connection is required between the sender and receiver.

5. Fibre is much more reliable, because it can better withstand environmental conditionsuch as pollution , radiation and salt producer or corrosion.

6. No cross talk in optical fibre hence transmission is more secure & private as it is verydifficult to tap into a fibre

7. Greater bandwidth : B.W is higher than that of an equivalent wire transmission line.

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8. Higher data rate

Propagation or Transmission of light waves in a optical fibre :

The light waves propagate down the length of the fibre from one end to the other end ifproper condition are maintained . this is called light-guide & the mechanisim of the propagationis Total Internal Reflection of light waves by the inside surface of the fibre,

Fibre obeys laws of reflection & refraction light waves . The light which enters at oneend of a fibre at a slight angle to the axis of the fibre, follows a zig-zag path due to series ofreflection down the length of the fibre.

Condition for Total Internal Reflection:

1.The glass at around the centre of the fibre should have higher refractive index ( 1)than that of the material (cladding ) surrounding the fibre (2).

2.The light should be incident at an angle of ( b/w the path of the ray and normal to thefibre wall ) which will be greater than the critical angle (c)

Sin c = 2/1 ----- (1)

Figure 1.1

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Now consider the condition of reflection , refraction and total internal reflection.

a) In reflection angle of incidence is equal to the angle of reflection .

b) In refraction

(i) 1 sini = 2 sinr.

(ii) The refracted wave should move towards the normal , if the light wave isincident from the optically lighter medium to an optically denser mediumand the refracted light wave should move away from the normal , if thelight wave travels from the optically denser to lighter medium

c) The condition for total internal reflection is sinc = 2/1.

The angle of incidence at which total reflection first occurs is called the criticalangle c for the two medium.Light waves incident at angles greater than c will alsobe totally reflected .

Basic structure of an optical fibre & Propagation of light wave through it :

Figure 1.2 light wave propogation along a glass fibre core

Figure 1.2 shows a longitudinal cross section of a fibre. Any light wave, which travelsalong the core and meets the cladding at the critical angle of incidence c will be totallyreflected . This reflected ray will then meet the opposite surface of the cladding , again at thecritical angle c and so is again totally reflected .

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Therefore the light wave is propagated along the fibre core by a series of total internalreflections from the core cladding interface. This is a sort of step index fibre ,as there is clearlya sudden change of refractive index at the junction of the core and the cladding .

In figure 1.2 path of only one height beam is shown which is possible only from a very

tiny point source .But practically point source will have several paths with different angles ofincidence and contain different colures with different frequencies this is called step-index (SI)multimode operation.

Figure 1.3 Stepped index multimode operation

Thus the various light waves, travelling along the core , will have propagation paths ofdifferent lengths .

Hence they will take different times to reach a given destination .Thus a distortion isproduced and is called transient-time dispersion.

As a result of this distortion, the variation of successive pulses of light may overlap intoeach other ,and thereby cause distortion of the information being carried.

However this defect can be minimized by making the core diameter of the same order asthe wavelength of the light wave propagated.

The resultant propagation is a single light wave , as shown in fig (1.4).This type of fibreis called a stepped index mono mode fibre .

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Figure 1.4 step index mono mode propagation M1 > M2

This has very high capacity and large bandwidth

Graded index fibre:

Figure 1.5 Graded Index Multimode Propagation

It is a type of optical fibre whose refractive index is a function of the radial distance fromthe fibres axis.

This fibre does not depend on interfacial reflection it uses the index gradient to refocusthe rays within the core . It generally exhibits higher bandwidth than step ` index multimodefibres.

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As shown in the figure the individual waves being gradually refracted in the graded indexcore , instead of being reflected by the cladding.

Thus waves travelling at different incident angles will travel different distances from thehorizontal central axis , before being reflected back to recross the central axis.

It is obvious that light waves with large angle of incidence travel more paths than thosewith smaller angles . But we know that the decrease of refractive index allows higher velocity ofpropagation.

Thus all waves will reach a given print along the fibre at virtually the same time .As aresult the transient time dispersion is greatly reduced. This type of light wave propagation isreferred to as graded index multimode propagation.

Acceptance angle(0 ) : -

It is the maximum angle at which a light ray may be incident upon a fibre coreand accepted for transmission. Acceptance angle is a property of the fibre. It is dependent on therefractive indices of the core and cladding material.

It is given by 0=sin 1 (1-2)/0

If fibre surrounding medium is air , i.e., M0 =1

Acceptance angle 0=sin 1 (1-2) -------- (1)

Numerical Aperture :- It is the number which defines the light gathering capability of a fibre .It

is equal to the sine of maximum acceptance angle .

Sine of the maximum acceptance angle

i.e., N.A=sin0(max) = (1-2)/0

For fibre surrounded by air 0= 1 then N.A = 1-2-------(2)

Generally 1is only a few percentage greater than 2

N.A = (1+2) (1-2)

= 2 1 (1-2)

=2 1(1-2)/ 1

= 1 2 --------3 ; where 1+2 = 2 1 & = (1-2)/ 1----4

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is the fractional distance b/w the core and cladding refractive indices.

In optical fibre communication two bands are presently extensively used

(i) 800 nm to 900 nm

(ii) 1200 nm to 1400 nm. where the fibres have low losses .

Problems :

1. Compute the N.A , critical angle and the acceptance angle of an optical fibre from thefollowing data . 1 (core) = 1.55 & 2(cladding )=1.50

Sol) N.A = 1(2 ) , where = (1-2)/ 1

= 1.55-1.50/1.55

= 0.03226= 1.55(2*0.03226)

=0.394

Acceptance angle = sin N.A = 23.2

Critical angle = sin ( 2/1)

2. Calculate the refractive indices of the core and cladding material of a fibre from thefollowing data. N.A= 0.22 , =0.012

Sol) = (1-2)/ 1=0.012

N.A = 1(2) or 1=N.A/ (2) = 1.42

therefore 0.012=(1.42-2)/1.42

2=1.40

Applications of fibres :-

(1) When the transmission medium has a very large bandwidth , a single modefibre is used . eg:- under sea cable s/m.

(2) When the system B.W requirements are b/w 200MHZ & 2GHZ , a gradedindex multimode fibre is best choice.

eg: In intra-city trunks b/w telephone central offices.

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(3) When the s/m B.W requirements are lower , a step index multimode fibrewould be better

eg: Data links

Fibre Fabrication :

Fibre fabrication techniques :

(1) External chemical vapour deposition of spoot (external CVD)

(2) Internal chemical vapour deposition of glass (internal CVD)

(3) External chemical vapour deposition of spoot (plasma CVD)

(4) Multi-element glass

(5) Phasil s/mChemical vapour deposition :- It is a technique used to build i fibre perform bydeposition of vapour glass constituents .

In C.V.D process pure silica is taken as a base material and a small amount fdopants like GeO2 , B2O3 and P2O5 are mixed to produce the required change inrefractive index and then arranged in cylindrical perform. This preform has therequired refractive index variation , but the cross-sectional area of the cylindricalperform is many times that of the finished fibre.

A standard perform is normally of one metre length and of 0.02 metre diameter .

A finished fibre with 125m cladding dia has a diameter about 1/16oth of that ofthe standard perform .We can get a continous fibre of few kilometers from such astandard perform.

1.External Chemical Vapour Deposition (External C.V.D) :

The Process used to produce core as well as cladding material is of very pure form (ultrapure). The process compose of operating continuously is called a batch process .This process iscalled external chemical vapour deposition, outside vapour phase oxidation (ovpo) or outside

vapour deposition (ovd) process.

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Process details: flame hydrolysis process deposits the glass of required composition

The deposition may be either in the form of a powder of soot, layer by layer uniformly, overthe length of a mendral (shaft holding in lathe)

After the deposition has been completed the material is sintered. The consolated tube is thanthermally collapsed by heating to a high temperature and solid preform rd is coated.

This process can produce high quality multimode(S I & G R I W) fibre.

But this process is not suitable for fabrication of single mode fibre

CHARECTERISTICS1. No dimensional quenches

2. Typical numerical operator is 0.2

3. Very large preform can be produce.

4. Have adequate strength.

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5. Loss is very small.

6. Rate of deposition is 1 to 2 grms/min.

2. Internal chemical vapour deposition.

By this method w can process the fibre core and cladding material is very very pure stage,within a suitable tube.

The glass of the chemical deposition is deposited on the inside of the glass tube which is rotatingis a glass lathe.

The deposition material is now flushed by a travelling oxyhydrogen torch which moves along thetube and makes a transparent glassy fibre..

As the torch repeatedly treasures the length, typically 30 to 100 layers are deposited layer bylayer on the inside of the glass tub .Refractive index can changed layer by layer by changing thedopart concentration. For creating graded-index-profile. This technique has a very precisecontrol over the profile. The tube closes after the completion of deposition. The tube is thermallycollapsed into a solid preform..

Single mode, multimode and graded index fibre can be fabricated

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CHARECTERISTICS>

1. Dimensional restrictions outer dia to core dia ratio should be 3:2.

2. The typical value of the NA of the fibre produced is 0.22.

3. Rate of deposition is 0.5gm/minute

4. Adequate strength

5. Prefom size limited. To 20km of 125 um outer dia fibre

3. External chemical vapour deposition of glass.

This process of fiber fabrication produces synthetic silica on industrial scale

But the addition of deport requires the control of the vaporization characteristics. Fluorine ndoped silica can be produced by this process. As shown in this fig flame torch is replaced byplasma torch..

CHARECTERISTICS

1. No dimensional restriction

2. NA = 0.2

3. Deposition rate = 1.0gm/min

4. Loss : scatter loss limited

Multi-element glass : a the mullti-element glasses are manufactured from very pure basicoxides and carbonates.

In this method, raw material are purified by nut chemical process. The require

chemicals are mix is a clean environment and fixed in pure silica crucibles.The desired glasses are drawn from the crucible for fibre formation.

It is basically a double crucible arrangement.

This process is used for large volume continuous fiber-fabrication and cost advantage forlarge scale production.

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CHARECTERISTICS

1. Range of NA is 0.2 to 0.6

2. No dimensional restrictions

3. High rate of production

4. Possible composition: NA-B-Si,Pb-Si,Th-B-Si,Na-ca-Si,k-B-SI,Ge-B-Si.

6. Phasil S/m : It is the vicor glass process based fabrication process. Number of stagesinvolved in the process are lengthy, but efficient and a large no of preforms at a time isparallel.

In the material stage,vicor rods are leached as result only a honey comb silica structure isleft.

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In the next stage, the required pure do pant molecules r tufted into the rod by a solution

treatment..A seond partical leached process forms the clad layer .The fiber preform is then formedby dying and sitering the structure.

Profile cored can also be obtained by selecting stuffing and leaching

CHARECTERISTICS>

1. No dimensional restrictions

2. High rate of production

3. Typical NA is 0.2(optimum)

4. Loss due to absorption or scattering in a large range of 5 to 10db/km

COMPARISON

Name of the technique Performance Expense

1. Internal C.V.D Best Expensive

2. External C.V.D Cord less ,,

3. Plasma ,, ,, moderate moderate cost andvolume

4. Phasil s/m moderate low cost large volume

5. Multi element glass moderate low cost large vol ofproduction

Fiber drawing and coating:

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Fiber drawing process involves the drawing of the fiber from the melts of the core and claddingglasses with in a concentric crucible with central orifices .this is called as double crucibletechnique.

Another fiber technique consists of heating the fiber perform top and elongating the molten glass

at the tip to form the fiber.

Fiber drawing:

Fig shown is used to draw fibers from performs the perform is attached to a precision feed. Thefeed moves into the furnace at the desired speed.

The design of the drawing process is such that the process will produce fiber melts as littlevariance in diameter as possible.

Coating:

A coating by means of a coating solution is applied to the fibre, after it has been drawn andmeasured.

The coating is a buffer used to protect the fiber from abrasion and interaction with environment.

The appropriate fiber coating materials are kynal,epoxy,silicon and uv curved resin.

Slightly tapered solid nozzle and flexible nozzle, made from plaster is used to coat the fiber,without damaging the surface of the fiber.

Important parameters for fiber drawing & coating processes:

a) Size of the perform.

b) Temperature of drawing.

c) Heating method ex: co2 laser, o2-h2 flame.

d) Coating material ex: uv epoxy.

e) Drawing rate:

A precise core dia is required to build fiber compatible with precision connectors designedfor low connector loss.

A laser micrometer is normally used in this fiber drawing process for accurate measurement &continuous monitoring of the diameter during pulling.

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The relation b/w fiber dia df, preform dia dp, fiber pulling velocity vp and

Df = dp(vp/vf)

Dp = preform dia

Vp = fiber pulling velocity

Vf = feeding velocity of the preform

Methods of fabricating fibers..

1. Double crucible method

2. Rod in tube method

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Double cruciblemethod:

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With conventional technique very high purity glass rods with different refractiveindices are prepared.

These rods consists of soda-boron-silicate(Na2o-B2O3-SiO2) with differentcomposition for core and cladding

To draw cladded core fiber from theses glass rods , two cylindrical crucibles are

arranged concentrically

The rods are fed slowly into heated crucibles where they are melted filling up thecrucibles with glass melt.

The height of melt must be constant

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The crucible temperature is maintain at 850c to 1100c depending on the choice ofglass composition

At the base the crucible tappers down to concentric nozzles where the glass are pulledto fiber

The pulling speed is find by the rotating drum on to which the filter is wound

ADVANTAGES.

1. Fiber is continuously fabricated in fabrication stem.

2. Fiber of unlimited length can be manufactured.

3. Low cost.

4. Capable of fabricating graded index fibers.

5. B.W of 400 to 900 mHz can be obtained

Rod In Tube method

Here a glass rod, with a higher refractive index is placed in a glass tube of lower refractive

index and made of a thermally compatible material.

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