Optical Fibers

Topics:• Introduction

• Definition

• Parts of Optical Fibre

• Types

Step Index

Graded Index Fibres

• Parameters of Optical Fibres

Acceptance Angle

Acceptance Cone

Numerical Aperture (NA)

Topics:• Normalized Frequency

• Number of Modes

• Attenuation in Optical Fibres

• Dispersion

1. Intermodal Dispersion

2. Intramodal Dispersion

• Applications

1. Optical fibres in Communication

2. Sensors

Cont…….

Introduction:

• Optical fibers arelong, thin strands ofvery pure glassusually 125 µm indiameter. They arearranged in bundlescalled optical cablesand used to transmitlight signals over longdistances.

What is optical fiber

• Optical fibers are very fine fibers of glass. They consist ofa glass core, roughly fifty micrometres in diameter,surrounded by a glass "optical cladding" giving anoutside diameter of about 125 micrometres. They makeuse of total internal reflection to confine light within thecore of the fiber.

Parts of Optical Fiber

• Core – thin glass center of the fiber wherelight travels.

• Cladding – outer optical materialsurrounding the core.

• Buffer Coating – plastic

coating that protects

the fiber.

Optical Fiber

Transmission of Light Through Optical Fibers

• Total Internal Reflection

Reflection & refraction

μ2< μ 1

μ 1

1 1

1

2

2

Snell’s law

μ 2< μ 1

μ 1

1= c

c

Critical angle

1

2sin c

μ 2< μ 1

μ 11 > c

Total internal reflection

Total Internal Reflection

• The angle of the light is always greater than the critical angle.

• Cladding does not absorb any light from the core.

• The extent that the signal degrades depends upon the purity of the glass and the wavelength of the transmitted light.

Fiber Technology

μclad

μclad

μcore

THE OPTICAL FIBER

CORE(Higher index)

Dia ~ 5 – 50 µm

CLADDING(Lower index)Dia ~ 125 µm

AIR or JACKET

PROPAGATION THROUGH AN OPTICAL FIBER

fast

t tfast slow

input output

Types of optical fibres

Based on their transmission properties and the

structure they are of two types

1. Single Mode Fibre

2. Multimode Fibre

(a) Step Index Fibres

(b) Graded Index Fibres

Single (Mono) Mode :

This is called so because the refractive index of the fibre ‘step’ up as we move from the cladding to the core and this type of fibre allows single mode to propagate at a time due to very small diameter of its core.

In this fibre, the refractive indices of the cladding and the core remain constant

In this fibre, the size of its core (diameter) is typically around 9-10 μm.

μ clad

μcore

μ clad

Multimode fibre :

This is called so because it allow more than one mode to propagate. Over more than 100 modes can propagate through multimode fibres at a time. The size of its core is typically around 50 μm or 62.5 μm.

Step Index Fibres

Graded Index Fibres

Two Types

Multimode Step Index Fibres:

No of propagating modes α Core diameter/ Wavelength

Typically the core diameter is 50 μm to 100 μm and Numerical Aperture (NA) varies from 0.20 to 0.29 respectively

Due to higher value of NA , and larger core size in this case, fibre connections and launching of light is very easy

Due to several modes, the effect of dispersion gets increased, i.e. the modes arrive at the fibre end slightly different times and so spreading of pulses takes place.

μclad

μ clad

μ core

Multimode Graded Index Fibres:

In this fibre, the refractive index of the core decreases with increasing radial distance from the fibre axis. The value of the refractive index is highest at the centre of the core and decreases to a value at the edge of the core that equal the refractive index of the cladding.

By this type of fibre design, the dispersion of the modes is compensated. Also, light wave follow sinusoidal paths along the fibre.

μclad

μcore

μclad

The profile of the refractive index is nearly parabolic that results in continual refocusing of the ray in the core, and minimizing the model dispersion.

Standard graded index fibres typically have a core diameter of 50 μm or 62.5 μm and the cladding diameter of 125 μm.

Advantage of Graded Index Fibre over Step Index Fibre

Decrease in the modal dispersion

Advantage of Single Mode Fibre over Multi Mode Fibre

Lower signal loss and a higher information capacity or bandwidth than multimode fibres as the signal loss depends on the operational wavelength.

These fibres are capable of transferring higher amount of data due to low fibre dispersion.

Single mode fibres are known as low loss fibres

Comparison

refractiveindex

SMSingle-Mode

Fiber types Cont…….

MM-SIMulti-ModeStep Index

MM-GIMulti-ModeGraded Index

Types of Fibersst

ep-i

nd

exm

ult

imo

de μclad

μclad

μcore

μclad

μclad

μcore

μclad

μclad

μcore

step

-in

dex

sin

glem

od

eG

RIN

Cont…….

© 2006, VDV Works LLC

Fiber Types Cont……

PROPAGATION THROUGH AN OPTICAL FIBER

By grading the index profile in the core, the pulse broadening was reduced, but it was suggested in 1980 that one could make single-mode fibers which will allow only “one path” through the fiber, thereby removing the pulse broadening

μclad

μclad

μcore

t t

input output

GRADIENT-INDEX OPTICAL FIBER

μclad

μclad

μcore

Longer path is now located in lower index region; the larger time takenis compensated by faster travel leading to less pulse broadening

t tfast slow

input output

Acceptance angle

• acceptance angle: In fiber optics, half the vertex angle of that cone within which optical power may be coupled into bound modes of an optical fiber.

• Note 1: The axis of the cone is collinear with the fiber axis, the vertex of the cone is on the fiber end-face, and the base of the cone faces the optical power source.

• Note 2: The acceptance angle is measured with respect to the fiber axis.

• Note 3: Rays entering an optical fiber at angles greater than the acceptance angle are coupled into unbound modes.

Acceptance angle Cont………

1

2sin cCritical angle:

Multimode fiber

μ10

c

n0

n0 μ2

Numerical ApertureMultimode fiber

μ1

μ2

0

c

n0

μ 0

1for sinsin 0

2

2

2

1000NA

Numerical aperture:

1

21

2

1

2

2

2

1

21

2

: if

222

2

2

1NA

61.0 max,0NAHere Δ is the relative refractive index differenceOr fractional refractive index

Acceptance Cone

θ0

θr

AcceptanceCone

o

A B

Ls

θr

d

2

2

2

1

1

0

2

2

2

10

sin

sin

Acceptance angle θ0:

The cone associated with the angle 2θ0 is called the acceptance cone

Parameters of Optical fibres

Acceptance angle =2

2

2

1

1

0

2

2

2

10

sin

sin

Acceptance Cone = 2θ0

Numerical Aperture (NA) = Sinθ0 =2

2

2

1

Skip Distance Ls= 1sin

2

0

1

i

d

Number of reflections Nr = 1

sin

1

2

0

1

i

d

Numerical Problems

Activity 1: The refractive indices for core and cladding for a step index fibre are 1.52 and 1.41 respectivelyCaculate (1) Critical angle (2) Numerical Aperture (3) The maximum incidence angle

Hints: Given Here =μ1 = μcore = 1.52, and μ2 = μclad = 1.41

Critical angle θc = sin-1 (μ2/ μ1) Ans = 68.060

2

2

2

1Numerical Aperture Ans = 0.568

Maximum incidence angle (θ0)= 2

2

2

1

1sin

Ans θ0 = 34.60

Numerical ProblemsActivity 2: A light ray enters from air to a fibre. The refractive Index of air is 1.0. The fibre has refractive index of core is equal to 1.5 and that of cladding is 1.48. Find the critical Angle, the fractional refractive index, the acceptance angle andNumerical aperture.

Hints: Given Here =μ0 = μair = 1.0; μ1 = μcore = 1.5, and μ2 = μclad

= 1.48Critical angle θc = sin-1 (μ2/ μ1) Ans: θc = 80.630

2

2

2

1Numerical Aperture

Ans = 0.568

Acceptance angle (θ0) = 2

2

2

1

1sin Ans θ0 = 14.130

Fractional Refractive index1

21 = 1.33% of light

Numerical ProblemsActivity 3: Calculate the refractive indices of the core and cladding material of fibre from the following data:NA = 0.22, Δμr = 0.012 and

Hints:

2

2

2

1

Numerical Aperture

Ans : μ1= 1.424 = μcore and μ2= 0.988 μ1 = 1.41 = μcladding

Fractional Refractive index

1

21

core

claddingcore

r

Allowed Modes and Normalized Frequency2

2

1NA

dmm

Maximum Number of modes that propagate successively in the fibre

dmm

Hence, number of possible modes will be larger for higher ratio d/λ

For multimode fibres mm > 2

For single mode fibres mm < 2 NA

d 2OR

As is evident the parameter mm decides the number of possible modes since this parameter depends on core diameter d and the numerical aperture NA. Therefore, the number of allowed modes would be different for fibres of different core diameters.

2

2

2

1

dNA

dn

Normalized Frequency

2

2

nmm

Therefore, the number of modes mm in terms of normalized frequency is

NAa

nna

V22 2

2

2

1

V-parameter

V number: determines how many modes a fiber supports

Single-mode fiber:

NAdd

V22 2

2

2

1

405.2V

Numerical Problems

Activity 4: An optical fibre operating at 1.50 μm has a smallValue of core diameter 5.0 μm and fractional refractive indexdifference of 0.0075. Calculate the normalized frequency and acceptance angle, given μ2 = 1.4.

Cut-off Wavelength

Definition: the wavelength below which multiple modes of light can be propagated along a particular fiber, i.e., λ> = λ c, single mode, λ < λc, multi-mode

NAd

c405.2

2

Attenuation in optical Fiber

• When light travels along the fibre, there is a loss of optical power, which is called attenuation.

Definition:Attenuation: Ratio of optical input power (Pi) to the

optical output power (Po)

Optical Input power: The power transmitted into the fibrefrom an optical source

Optical output power: The power received at the fibre end

Fiber Attenuation Cont……..This relation defines the signal attenuation or absorption coefficient in terms of length L of the fibre:

0

10log10

P

P

L

i

Length L of the fibre is expressed in kilometersHere, the unit of Attenuation is decibels/kilometer i.e. dB/km.

The main causes of attenuation in optical fibre are:

(a)Absorption

(b)Scattering

(c)Bending losses

Each mechanism of loss is influenced by the properties of fibre

Material and fibre structure.

Fiber Attenuation Cont……..Absorption losses over a length L of fiber can be described by the usual exponential law for light intensity (or irradiance) I

LeII 0

Where I0 is the initial intensity or the irradiance of the light.

The attenuation profile is shown in figure which shows the amount ofAttenuation is also wavelength dependent.

In the figure, two absorption peaks at 1.25 μm and 1.4 μm are observed which are respectively due to thepeculiarities of the single mode fibre and the traces of water remaining in the fibre as an impurity.

For commercially available fibers

Loss 0.5 dB/km @ 1310 nm0.25 dB/km @ 1550 nm

Dispersion• A pulse of light sent into a fibre broaden in time as it

propagate through the fibre. This phenomenon is known as pulse dispersion.

• Intermodal Dispersion: Different rays take different times to propagate through a given length of the fibre. Or Different modes travel with different speeds

• Intarmodal dispersion: Any given source emits over a range of wavelength and because of the intrinsic property of the material of the fibre, different wavelength takes different amount of time to propagate along the same path.

Dispersion in Fiber Optics

• Dispersion occurs when photons from the same light pulse take slight different paths along the optical fiber. Because some paths will be longer or shorter than other paths the photons will arrive at different times thus smearing the shape of the pulse.

• Over long distances, one pulse may merge with another pulse. When this happens, the receiving device will not be able to distinguish between pulses.

The dispersive effects in a single mode fibre are much smaller than a multimode fibre. Due to dispersion, optical pulses in optical fibresspread and hence the signal spread over long distances.

Pulse Dispersion in Optical Fibre

In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency.Such medium is called a dispersive medium.

Intermodal Dispersion

Different rays take

different times to

propagate through a given

length of the fibre.

In the language of wave

optics, this is known as

intermodal dispersion

because it arises due to

the different modes

travelling with different

speeds.

Intramodal Dispersion

Any given light source emits

over a range of wavelength

and because of the intrinsic

property of the material of the

fibre, different wavelengths

takes different amounts of

time to propagate along the

same path. This is known as

material dispersion or

intramodel dispersion

There are several factors that cause dispersion in optical fibres.For Example:(1) In multimode fibres different axial speeds of differenttransverse modes cause intermodal dispersion that limits the performance of the fibre.(2) In singe mode fibres, though intermodal dispersion is eliminated, chromatic dispersion occures because of The slight variation in the index of the glass with the wavelength of the light.Dispersion limits the bandwidth of the fibre because thespreading optical pulses limit the rate that pulses canfollow one another on the fibre and still remain distinguishable at the receiver.

Cont……..

Chromatic DispersionFor example, when white light passes through a prism some of the wavelengths of light bend more because their refractive index is higher, i.e. they travel slower This is what gives us the "Spectrum" of white light. The "red' and "orange" light travel slowest and so are bent most while the "violet" and "blue" travel fastest and so are bent less. All the other colours lie in between

Figure -The Dispersion of White Light

Fibre Optics Communication

Massage Input

Modulator(Transmitter)

Optical Source

Optical Fibre

Optical detector

Demodulator(Receiver)Destination

Optical SourceLED or Laser

External Modulator

Optical

AmplifierOptical Detector

PIN Diode

Optical Fiber

Connector/Splice

Electrical Signal Electrical Signal

A TYPICAL OPTICAL COMMUNICATION SYSTEM

Processing electronics

Optical Fibre Sensors

Optical Fibre sensors are fibre based devices that are used for sensing some typical quantities like temperature of mechanical strain.

These sensors are sometime used for sensing vibrations, pressure, acceleration or concentrations of chemical species

Principle: When a light beam is sent through a optical fibre, then its parameters either in the fibre or several fibreBraggs grating experience subtle change. Then the light reaches a detector arrangement measure these changes. The light may be changed in five of its optical properties i.e. intensity, phase, polarization, wavelength and spectral distribution.

Optical Fibre Sensors

(A) Intrinsic Sensors

Light Source

Light Detector

A

B

FibrePressure

Pressure

(A) Intrinsic Sensors (B) Extrinsic Sensors

•In this type of fibres, sensing medium is itself fibre•Measure the variation in Intensity of transmitted light signals •Useful in measuring the force being exerted between the two objects •If one apply the pressure then due to micro bending losses the light •intensity at the detector will decrease •If we remove the pressure the intensity will increase.

(B) Extrinsic Sensors

•The delivery of light and its collection is done by the fibre

•Used to measure vibration, rotation, displacement, velocity, acceleration, torque and twisting.

•A major benefit of these sensors is their ability to reach places which are otherwise inaccessible .

FOR EXAMPLE:

Useful to measure the temperature inside aircraft jet engine.

Used to measure the internal temperature of electrical transformer.

The amount of light launched into the return fibre will decrease as the distance between the two fibers is increased. However if length is decreased the light intensity collected by the receiver will decrease. This way these optical fibre sensors are capable of determining small shifts between objects.

Light Source

Light Detector

Feed Fibre Return Fibrel

(B) Extrinsic Sensors Cont……..

Advantages of optical fibres compared with Traditional metal communication lines:

1. Fibre optics cables can carry more data as their bandwidth is

greater than metal cables

2. Fibre optics cables are less susceptible (sensitive) then metal cables

to interference.

3. Fibre optic cables are much thinner and lighter than metal wires.

4. Through fibre optic cables the data can be transmitted digitally rather

than analogically.

5. Attenuation through fibre cables is very low in transmitting the data

over a long distance, so there is no need of repeaters (thing that

repeats)

An optical fiber (or fibre) is a glass or plastic fiber that carries light along its length.

Fiber optics

Applied science Engineering

Optical fibers are widely used as these

• permits transmission over longer distances

•and at higher bandwidths (data rates) than other forms of communications.

•Fibers are used instead of metal wires because signals travel along them with less loss, and they are also immune to electromagnetic interference.

Areas of Applications

• Carry plain old telephone service (POTS)

• For transmission of data

• Transmitting broadband signals

• In the biomedical industry

• Non-Communication Applications (sensors etc…)

• Telecommunications

• Local Area Networks

• Cable TV

• Optical Fiber Sensors

The advantages of fiber optic over wire cable

• Thinner

• Higher carrying capacity

• Less signal degradation

• Light signal

• Low power

• Flexible

• Less Expensive

• Digital signals

• Light weight

• Non-flammable

The terminating equipment is still costly as compared to copper

equipment.

1. IT has to be handled carefully.

2. Optical fiber is more expensive per meter than copper

Communication is not totally in optical domain, so repeated

electric –optical – electrical conversion is needed.

1. Tapping is not possible. Specialized equipment is needed to tap a

fiber.

1. Optical fiber splicing (joining by interweaving strands) is a

specialized technique and needs expertly trained manpower.

DISADVANTAGES OF OPTICAL FIBERS…

1. The splicing (joining by interweaving strands) and testing

equipments are very expensive as compared to copper

equipments.

2. Optical fiber can not be join together as easily as copper cable. It

requires training and expensive splicing (joining by interweaving

strands) and measurement equipment.

DISADVANTAGES OF OPTICAL FIBERS…

A Light Sources

LED (Light emitting diode) ILD (injection laser diode)

Optical source

+ ––

TRANSMITTER

FIBER

Performance

Modulation speedFiber-coupled power

–

Light Emitting Diode (LED)

Typical performance data

Power in MM-fiber: 100 W

Power in SM-fiber: 1 W

Direct Modulation Bandwidth: 100 MHz

+

Laser

Typical performance

Power (in fiber): 5-10 mWMax: 100-300 mWDirect Modulation Bandwidth: 1-10 GHz

• Telecommunications

• Internet Access

• Cable and Satellite Television

• Decorative Light Source

Characteristics

• Glass Core

• Glass Cladding

• Ultra Pure Ultra Transparent Glass

• Made Of Silicon Dioxide

• Low Attenuation

• Popular among industries

Towers

Buildings

Towers

Installation of Antennas

Thank YouThank You

Any Questions or Comments?