optical electronics and laser instrumentation unit 1

8/14/2019 Optical Electronics and Laser Instrumentation Unit 1

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Optical Fibers & Modulators

Introduction to Optical Fibres:[5]

Since optical frequencies are extremely large (~ 1015 Hz) as compared to the conventional radio waves

(~ 106 Hz) and microwaves (~ 1010 Hz), a light beam acting as a carrier wave is capable of carrying far

more information than radio waves and microwaves.

In future the demand for flow of information traffic will be so high that only a light wave will be able

to cope with it.

Soon after the discovery of laser, some experiments on propagation of information carrying light

waves through open atmosphere were carried out, but it was realized that because of the vagaries of

the terrestrial atmosphere, eg. Rain, fog, etc, in order to have an efficient and dependable

communication system, one would require a guiding medium in which the information carrying light

waves could be transmitted.

This guiding medium is the optical fiber which is a hair-thin structure and guides the light beam from

one place to another.

In addition to the capability of carrying a huge amount of information, fibers fabricated with recently

developed technology are characterized by extremely low losses (~ 0.2 dB/km) as a consequence of

which the distance between two consecutive repeaters could be as large as 250 km. In comparison,

copper cables used today require repeaters every few kilometers.

In addition to long distance communications systems, optical fibers are also being extensively used for

Local area networks (LANs).

Modern Light wave communication:

1960 Ruby Laser

1962 Semiconductor laser (GaAlAs)

1966 Fiber technology

1970 First fibre produced with loss below 20dB/km

1980 Technology advancements brought Fibre loss < 0.5dB/km


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Conditions for Reflection & Refraction:(Basics Review)Laws of Reflection:

i.The incident ray, the normal and the reflected ray, all lie in one plane.

ii.The angle of incidence (i) and the angle of reflection (r) are equal

Laws of Refraction:

First Law:The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

2nd Law:(Snells law)

The ratioof the sine of angle of incidenceto the sine of the angle of reflectionfor any two give media

is constantfor a light of given colour.

i) 1Sini = 2Sinrii) The refracted wave should move towards the normal, if the light wave is incident from

the lighter medium to a denser medium.

iii) The refracted light wave should move away from the normal, if the light wave travels

from the optically denser to optically lighter medium.

Total Internal Reflection:

A ray of light traveling in a medium of higher refractive index and directed towards one of lower

refractive index (higher lower), passes into that medium(lower) only if the angle of incidence is not

too large.

If the angle of incidence is increased, a limit is reached when the ray does not enter the second

medium at all but is totally

reflected at the interface.

C = angle of critical incidence

This angle of incidence for

which the corresponding angle

of refraction is 90o is called

the angle of critical incidence

for the two media.

The phenomenon of total

internal reflectionwill take

place if the two conditions are

satisfied:i.Light should travel from denser medium to rarer medium.

ii.The angle of incidence should be greater than the critical angle of the medium.

By Snells law 1Sini = 2Sinr

When i = c, then 1Sinc = 2 Sin 90 therefore, Sin c = 2/1

where 1 & 2 are the refractive indices of denser and rarer medium respectively.


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(Reference: Optical Fibre Systems: Technology, Design and Applications.. Charles K.Kao)

Characteristics of Optical Fibre Systems:[4]

The basic optical fibre system is illustrated in Fig. It consists of a transmitter which transforms an

electrical signal to be transmitted into an optical signal, a receiver which converts the optical signal

back to the original electrical form, and a fibre transmission line which conducts the optical signal

from the transmitter to the receiver.

Three components are involved: the light source, the photodetector, and the optical fibre transmission

line.

The optical light source generates the optical energy which serves as the information carrier, similar

to a radio-wave source supplying electromagnetic energy at radio-wave lengths. The optical photodetector detects the optical energy and converts it into an electrical form. The optical fiber

transmission line is the equivalent of a pair of copper wires and functions as the conductor of optical

energy.

The major characteristics of optical fibre wave guides that distinguish optical fibre from other

systems can be separated into three categories: physical, optical and special characteristics.

a) characteristics of Optical Fibre Wave guides:

Optical fiber wave guides are thread like structures made from dielectric materials in a glassy

form. A typical fiber has a diameter of 125m (about the thickness of human hair). Lengths of

several kilometers are common, and much longer lengths are possible.

Optical fiber wave guides for operation in the 0.5 to 1.6m wave length region are generally made

with inorganic oxide glasses with a high silica content. Such wave guides have a specific gravity of

about 2.3. (The volume of a 125m diameter fiber 1km in length is about 12cm2 and weighs about

28g. It is very small in size and light in weight).

Since glass is a very strong and durable material, properly made and protected fiber is strong and

durable. It is also highly flexible and can negotiate bends as small as a few millimeters.

b) Optical Characteristics of Optical Fibers:The optical transmission characteristics of optical fibers are expressed in terms of attenuation and

bandwidth.


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Attenuation: The attenuation causes optical energy to be dissipated along the length of the

waveguide during transmission and reduces the available energy at the destination. Since the

transmitter power output and the detector sensitivity are fixed for a given signal, the fibre

attenuation sets the limit to the maximum distance that can be covered by the fiber guide.

Bandwidth or pulse dispersion:The bandwidth sets limit to the top frequency response in analogtransmission, while the pulse dispersion sets the limit to the maximum pulse rate in digital

transmission.

Fiber cables offer constant attenuation for any operating bandwidth, in contrast with copper cables,

where the attenuation of the cable increases as (bandwidth)1/2. It is a factor which simplifies the

system design.

c) Special Features of Optical Waveguides:

The optical fiber has wide temperature range of working (-55 to 125oC).

Optical fibers are not affected by electromagnetic interference. (fiber dimension is far less than the

wavelength of electromagnetic waves even in microwave range. Therefore it will not pickup theelectromagnetic radiation).

As radiation from the fibre is at optical wavelengths, it will not cause noise by their presence in the

electronic circuits.


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Propagation of light waves in an Optical Fibre:

a)Mechanism:If light enters at one end of a fibre in proper conditions, most of it is propagated down

the length of the fibre and comes out from the other end of the fibre. There may be some loss due to

a small fraction leakage through the side-walls of the fibre. This type of a fibre is called light-guide

or sometimes light-pipe.

The reason of the light beam confining inside the fibre is the Total internal reflection. The lightwhich enters at one end of a fibre at a slight angle to the axis of the fibre, follows a zig-zag path

due to series of reflections down the length of the fibre.

b)Conditons:Total internal reflection in the walls of the fibre can occur if and only if, the following

two conditionsare satisfied:

i)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).

ii)The light should be incident at an angle of (between the path of the ray and normal to the fibre

wall) which will be greater than the critical angle C.SinC = 2 /1

Basic structure of an Optical Fibre:

The central core of an optical fibre consists of a glass core with a certain refractive index 1 and

totally enclosed by a glass cladding, having refractive index 2, (1> 2).

The longitudinal cross-section of the fibre is as shown in the

figure. Any light wave, which travels along the core and meets

the cladding at the critical angle of incidence C will be totally

reflected. This reflected ray will then meet the opposite

surface of the cladding, again at the critical angle C and so is

again totally reflected. Therefore, the light wave is propagated

along the fibre core by a series of total internal reflections

from the core-cladding interface. This is a sort of step index

fibre, as there is clearly a sudden change of refractive index at

the junction of the core and the cladding.

Different Types of Fibers:

Based on refractive index profile, we have two types of fibers:

(i) Step index fiber, (ii) Graded index fiber.

Fiber Modes:

Based on the number of modes propagating through the fiber, there are multimode fiber and singlemode fiber.

Mode is the mathematical concept of describing the nature of propagation of electromagnetic waves in

a wave guide. Mode means the nature of electromagnetic field pattern (or) configuration along the

light path inside the fiber.

Step-Index Multi-mode:Light energy emanating from any practical point source, will


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have several paths with different angles of incidence at the boundary-layer. It may also contain

different colours with different wavelengths. Then it is called Step-index Multimodepropagation.

As shown in the figure, any light wave which is meeting the core-cladding interface at or above the

critical value of C will also be totally reflected and hence will propagate along the core. However, the

light wave with angle below C will pass into and be absorbedby the cladding.

Transit-time dispersion:

Various light waves traveling along the core, will have propagation paths of different lengths. Hence

they will take different times to reach a given destination. This results in a distortion called Transit-

time dispersion. As a result of this distortion, the variations of successive pulses of light may overlap

into each other, and thereby cause distortion of the information being carried.

Stepped index Monomode Fiber:The transit time dispersion problems can be solved by making the

core very thin, so that the core diameter is of the same order as the

wave length of the light wave to be propagated. The resultant

propagation is a single light wave as shown in the figure below. Thistype of fibre is called a Stepped-index Monomodefibre. This has a

very high capacity and very large bandwidth.

In order to get single mode the diameter of the core must satisfy the

relation d < 0.766 /NA

Chief characteristics of Stepped index monomode fiber are:

1.Very small core diameter

2.Low numerical aperture

3.Low attenuation4.Very high bandwidth

Disadvantage of monomode fibre: The use of very thin fibre creates mechanical difficulties in the

manufacture, handling and splicing the fibres. Hence this type of fibre is very expensive. This type of

fibre is used as under-sea cables where the expense is justified by the high return of earned income.


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Graded Index multimode propagation:As shown in the figure below, the individual waves are gradually refracted in the graded-index core,

instead of being reflected by the cladding. Thus waves traveling at different incident angles will travel

different distances from the horizontal central axis. It is obvious that light waves with large angle of

incidence travel more paths than those with smaller angles. But decrease in refractive index allows a

higher velocity of propagation. Thus

all waves will reach a given point alongthe fibre at virtually the same time.

As a resulttransit time dispersionis

greatly reduced. This type of wave

propagation is referred to asgraded

index multimode propagation.

Graded index fibers have intermediate bandwidth and capacity. It is a less expensive method of

overcoming transit time dispersion. This fibre has a property of gradually changing refractive index

(increasing from the outside of the fibre core to the centre of it).

The variation of refractive index of the core of the graded index fibre with radius measured from thecentre of the core, is given by (x) = 1 [1 - 2 (x/a)

p ]2

where 1 = refractive index at the centre of the core. p = index profile.

Plastic Fibres:Other thanglass cored fibersPlastic fibers (plastic core) have been manufactured from a polymer

preform drawing into a fibre. The losses associated with these fibers are usually in the hundreds of

decibels. They operate at low temperature. We can use plastic fibers up to a maximum of 125oC, while

glass fibers can be used right up to a maximum temperature of 1000oC.

However, plastic fibre cables have an inherent potential for many present and future applications. It is

an ideal medium for sensor, process control, and short distance communications.

PCS-Fibre:

A fibre, having glass core and plastic cladding, is called plastic clad silica or PCS fibre. The

characteristics of such a fibre are the following:

1.It has high NA

2.Large core diameter

3.High attenuation

4.Low bandwidth

The advantage of larger coreis thegreater coupled power. The high value of NA permits the use ofless expensive surface emitting LEDs.

Other than high attenuation and low band width, there are some major problems with Plastic fibers:

Plastic fibers have very poor mechanical strength

They have low maximum operating temperature.


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S.No Step Index Fiber Graded Index Fiber

1 The refractive index of the core is

uniform throughout and undergoes an

abrupt (or) step change at the cladding

boundary

The refractive indexof the core is made to

vary in the parabolic manner such that the

maximum refractive index is present at the

centre of the core.

2 The diameterof the core is about 50 ~

200min the case of multimode fiberand

10 min the case of single mode fiber.

The diameter of the core is about 50 m in

the case of multimode fiber.

3 The light rays propagating through it are

in the form of meridional rayswhich will

cross the fiber axis during every

reflection at the core-cladding boundary

and are propagating in a zig-zag manner.

The light rays propagating through it are in

the form of skew rays (or) helical rays

which will not cross the fiber axis at any

time and are propagating around the fiber

axis in a helical (or) spiral manner.

4 Signal distortion is more in multimode

step index fiber since the rays reflected

at high angles or the higher order modes

travel a greater distance than the rays

reflected at low angles or the lower order

modes, to reach the exit end of the fiber.

So high angle rays arrive later than the

low angle rays.

Hence the signal pulses are broadened out

(dispersion) and distortion takes place.

But this distortion does not take place insingle mode step index fiber.

Signal distortion is very low because of self

focusing effect. Here the light rays travel

at different speeds in different paths of the

fiber because the refractive index varies

throughout the fiber. As a result light rays

near the outer edge travel faster than the

light rays near the centre of the core.

In effect, light rays are continuously

refocused as they travel down the fiber and

almost all the rays reach the exit end of the

fiber at the same time due to the helicalpath of the light propogation.


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S.No Single Mode Fiber Multimode Fiber

1 In single mode fiber only one mode (HE11mode) can propagate through the fiber.

Multimode fiber allows a large number of

paths or modes for the light rays traveling

through it.

2 The single mode fiber has smaller core

diameter and the difference between the

refractive indices of the core and the

cladding is very small.

Generally in multimode fiber the core

diameter and the relative refractive index

difference is larger than the single mode

fiber.

3 In practice there is no dispersion (i.e. no

degradation of signal during traveling

through the fiber.)

Even though there is self focusing effect

there is signal degradation due to multimode

dispersion and material dispersion.

4 Since the information transmission

capacity in optical fiber is inversely

proportional to dispersion, the single

mode fibers are more suitable for long

distance communication.

Due to large dispersion and attenuation of

the signal the multimode fibers are less

suitable for long distance communication.

5 Launching of light into single mode fibers

and jointing of two fibers are very

difficult.

Launching of light into fiber and jointing of

two fibers are easy in these fibers.

6 Fabrication is very difficult and so the

fiber is so costly.

Fabrication is less difficult and so the fiber

is not costly.


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Acceptance Angle and Acceptance Cone of a Fibre:In a stepped index multimode fibre any light wave which travels along the core and meets the cladding

at the critical angle of incidence C will be totally reflected. The reflected ray will then meet the

opposite surface of the cladding, again at the critical angle C and so it will again totally reflected.

Any other light wave, which is meeting the core cladding interface at or above the critical value C will

also be totally reflected and hence will propagate along the core. However, any light wave entering thecore-cladding interface at an angle below C will pass into and be absorbed by the cladding.

If the external incident angle is O corresponding to the critical angle C at the core-cladding

interface of the fibre, as shown in the figure, the light will stay in the fibre.

O is the maximum external incident angle. Any light wave impinging on the core within this maximum

external incident angle O is coupled into the fibre and will propagate. This angle is called the

Acceptance angle.

Mathematical expression for Half-Acceptance angle O:

From the above figures a & b, applying law of reflection at A and B, we get

OSinO = 1Sin(90-C) ------ (1)

OSinO = 1CosC

1SinC = 2Sin(90) ---------- (2)

SinC = 2/1

OSinO = 1(1-sin2C) = 1(1-22/12)= (12-22)

SinO = (12-22)/O

Half Acceptance angle O = Sin-1(12-22)/O

If the surrounding fibre medium is air i.e. O = 1, then

Half Acceptance angle O = Sin-1(12-22)

Thus the light which travels within a cone defined by the acceptance angle is trapped and guided. This

cone is referred as Acceptance cone.


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Numerical Aperture:An important term associated with a fibre is the Numerical Aperture also called the Figure of merit

for optical cables.

NA = SinO (max) = (12-22)/O

If the fibre is surrounded by air (O = 1) then

NA = SinO = (12

-22

)

Generally 1 is only a small percentage greater than 2.

NA = (1+2)( 1 -2) ~ 21(1 -2)

NA = 212(1 -2)/1

NA = 1 2where 1 + 2 ~ 21 and = ( 1 - 2)/1

= Fractional difference between the core and cladding refractive indexes.

The figure shows the variation of NA with Acceptance angle.

It is observed that numerical aperturesfor the fibers used in short distance communication are in

the range of 0.4 to 0.5, whereas for long distance communications numerical apertures are in the range

of 0.1 to 0.3.

Though small numerical aperture is applied for long distance communications, the disadvantage of small

NA is it makes harder to launch power (signal) into the fiber.

Example 3.1:

Compute the NA and the Acceptance angle of an optical fibre from the following data.1 (core) = 1.55 and 2 (cladding) = 1.50.

Soln:

NA =1 2 where = (1 - 2)/1 = (1.55-1.5)/1.55 = 0.03226= 1.552x0.03226 = 0.394

Acceptance angle O = Sin-1(NA) = 23.2o

Example 3.2:

Compute the NA, acceptance angle, and the critical angle of the fibre having 1 (core refractive index)

= 1.50 and the refractive index of the cladding = 1.45.

Soln:

= (1 - 2)/1 = (1.5-1.45)/1.5 = 0.033

NA = 12 = 1.52x0.33 = 0.387

Acceptance angle O = Sin-1(NA) = 22.78o.

Critical angle C = Sin-1(2/1) = Sin

-1(1.45/1.5) = 75.2o


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Example 3.3:

Calculate the refractive indices of the core and cladding material of a fibre from the following data:

NA = 0.22; = 0.012

Soln:

= (1 - 2)/1 = 0.012

NA =1 2 ; or 1 = NA/2 = 1.42

0.012 = (1.42 - 2)/1.42 or 2 = 1.40

Other Latest types of Optical Fibers:

High Purity Silica Fibre (HPSUV) > Core: high purity silica; Cladding: Doped silica;

High Purity Silica (HPSIR) -> same as above but slightly different doping

Chalcogenide Fibre -> Core: As2S3; Cladding: AsxS1-x ,

Halide Fibre -> Core: Silver halide

Tapered Optical Fibres -> for getting max amount of power from poor quality laser spot.

Applications of Optical Fibers:1.When the transmission bandwidth requirement is very large a single mode fibre is used. Eg:

Undersea cable system

2.When the bandwidth requirements are between 200MHz and 2 GHz km; a graded index multimode

fibre would be the best choice. Eg: In intra-city trunks between telephone central offices.

3.When the system bandwidth requirements are lower, a step-index multimode fibre would be better.Eg: Data links.

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