Physics 1st Year (Unit 1)

                     

Syllabus (Theory)

Unit-1 Lasers and Fibre Optics Contact Hours:15

LASER Introduction, Spontaneous and Stimulated emission of radiation, Relation b/w Einstein’s A and B coefficients, Population inversion & types of pumping, Main components of a Laser, Construction & working of Ruby Laser and its applications, Construction & working of Helium-Neon laser and its applications.   

FIBRE OPTICS Fundamental ideas about optical fibre, Types of fibres, Acceptance angle and cone, Numerical aperture, Propagation mechanism and communication in optical fibres, Attenuation and losses

Topics for self-study: Sensors based on fibres and lasers


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CHAPTER 1

Lecture 1.1 Introduction of LASER

LASER

LASER stands for ‘Light Amplification by Stimulated Emission of Radiation’

Why is it important to study Laser?

The answer is……..

·        It offers a high-speed way to process various materials and hence is being used on a large scale in various industries.

·        Innovative technologies are driving the growth of industrial lasers. Manufacturers are introducing technologically advanced industrial lasers for various applications.

·        The global industrial laser market is highly dynamic, and growing at a fast pace. 

·        The increasing innovations taking place in fiber lasers play an important role in the growth of the global industrial laser market

Laser is a very intense, concentrated, highly parallel and monochromatic beam of light. Coherence is a very important property of laser. It is important to know the difference between incoherent and coherent light.

Incoherent Light: The light emitted from the Sun or other ordinary light sources such as tungsten filament and fluorescent tube lights is spread over a wide range of frequencies.              

For. eg. Sunlight is spread over Infra Red, Visible light and Ultra Violet spectrum. So, the amount of energy available at a particular frequency is very less and hence less intense. Such light is irregular and mixed of different frequencies, directions and duration, and is incoherent. Incoherent light is due to spontaneous and random emission of photons by the atoms in excited state. These photons will not be in phase with each other.

Coherent Light: Coherent light is uniform in frequency, amplitude, continuity and constant initial phase difference. Coherent beam of light is obtained due to stimulated emission of photons from the atoms jumping from meta-stable state to lower energy state.

The difference between non-monochromatic incoherent light, monochromatic incoherent light and monochromatic coherent light can be understood from the following diagram.

Characteristics of Laser Light which makes it dynamic:

1. Laser light is highly directional.

A laser beam departs from strict parallelism only because of diffraction effects. Light from other sources can be made into an approximately parallel beam by a lens or a mirror, but the beam divergence is much greater than for laser light.

2. Laser light is highly coherent.

Laser can maintain its coherence up-to kilo-meter. The corresponding coherence length for light from a tungsten filament lamp or a gas discharge tube is typically considerably less than 1 m.

3. Laser light is highly monochromatic.

Tungsten light, spread over a continuous spectrum, gives us no basis for comparison. The light from selected lines in a gas discharge tube, however, can have wavelengths in the visible region that are precise to about 1 part in 106. The sharpness of laser light can easily be thousand times greater, or 1 part in 109.

4. Laser light can be sharply focused. Flux densities for focused laser light of 1015 W cm-2 are readily achieved.

5. Brightness: The primary characteristic of laser radiation is that lasers have a higher brightness than any other light source. Brightness is defined as the power emitted per unit area per unit solid angle.

Various Atomic Interactions related to LASER:

a) Induced Absorption:

The transition of atoms from ground state to higher energy state by absorbing photons of suitable energy or frequency is called induced or stimulated absorption. These atoms absorb the supplied energy and go to the excited or higher energy state. If E1 and E2 are energies of ground state (lower energy) and excited state (higher energy) respectively, then the absorption process can be expressed as

A+ hν= A*

where, A can be represented as ground state of atom and A* can be represented as excited state of atom. Here                                hν=E2-E1

(b)Spontaneous emission

            The process by which excited electrons emit photons while falling to the ground level or lower energy level without any external agent is called spontaneous emission.

 Fig. 5. Spontaneous emission process[3]

The spontaneous emission process can be expressed as

A* = A+ hν

where, A can be represented as ground state of atom and A* can be represented as excited state of atom. Here                                hν=E2-E1

(c) Stimulated emission

         The process by which electrons in the excited state are stimulated to emit photons while falling to the ground state or lower energy state is called stimulated emission using external agent.

Fig. 6. Stimulated emission process[3]

The stimulated emission process can be expressed as

A* + hν = A+ 2hν

where, A can be represented as ground state of atom and A* can be represented as excited state of atom. Here                                hν=E2-E1

Features of Spontaneous Emission of Light

An excited atom can stay in the higher energy state only for the time of 10-8 s. After this time, it returns back to the lower energy state by emitting a photon of energy hν = E1 – E0. This emission is called ‘spontaneous emission’.

During spontaneous emission, photons are emitted randomly and hence they will not be in phase with each other. Therefore, the beam of light emitted is incoherent. Thus the important features of stimulated emission are

1. The emitted photon can move in any random direction. Thus light is not unidirectional.

2. There will be no phase relationship between the photons emitted from various atoms resulting in incoherent light.

3. The radiation is less intense because of spreading of light in random directions.

4. The emitted radiation consists of different frequencies or wavelengths.

Features of Stimulated Emission of Light

1. Since an atom in the excited state is induced to emit photon, it results in emission of two photons which are identical in terms of frequency, phase and direction, state of polarization.

2. The time of emission of photons is same hence the photons are in phase with each other resulting in coherent beam.

3. The radiation is highly intense, monochromatic, unidirectional and coherent.

4. The important feature of this process is that photon multiplication takes place.

Summary

1.This topic realizes the need to study laser.

2.The difference between conventional and laser light.

3.Three quantum mechanical processes when light interacts with medium.


Lecture 1.2 Einstein’s Coefficients

LASER

LASER stands for ‘Light Amplification by Stimulated Emission of Radiation’

Rate Equations of Absorption, Spontaneous Emission and Stimulated Emission

(a) Absorption

            The probability of occurrence of absorption process from state 1 to state 2 is proportional to the energy density u(ν) of the radiation

                                                          P12=B12u(ν)

Where, B12 is proportionality constant and is known as Einstein coefficient of absorption of radiation

Figure 1 Mechanism of absorption process

The rate of stimulated Absorption of photons is given by

                               Rab = N1P12 = N1B12u(ν)

Where Nis the population of ground state i.e. number of atoms per unit volume occupying the ground state.

(b) Spontaneous Emission

            An excited atom can stay in the higher energy state only for the time of 10-8 s. After this time, it returns back to the lower energy state by emitting a photon.

The probability of occurrence of spontaneous emission transition from state 2 to state 1 depends only on the properties of states 2 and 1 and is given by

                                                          P21=A21

Where, A21 is proportionality constant and is known as Einstein coefficient of absorption of radiation. Its reciprocal gives the life time of the excited state.

Figure 2 Mechanism of spontaneous emission process

 The rate of spontaneous emission of photons is given by

                               Rsp = N2P21 = N2A21

Where, Nis the population of excited state i.e. number of atoms per unit volume occupying the excited state.

a) Stimulated emission

            When photon of suitable frequency is made to fall on an excited atom in the higher energy state, the atom falls back to the ground state by emitting a photon of energy which is in phase with the stimulating (incident) photon.

The probability of occurrence of stimulated emission process from state 2 to state 1 is proportional to the energy density u(ν) of the radiation

                                                          P21=B21u(ν)

Where, B21 is proportionality constant and is known as Einstein coefficient of stimulated emission of radiation

Figure 3 Mechanism of stimulated absorption process

The rate of stimulated emission of photons is given by

                               Rst = N2P21 = N2B21u(ν)

Where, Nis the population of excited state i.e. number of atoms per unit volume occupying the excited state.

Relation Between Einstein’s Coefficients

Let N1 and N2 be the number of atoms at any instant at any instant in the state 1 and 2, respectively. The probability of absorption for number of atoms from state 1 to 2 per unit

time i.e. rate is given by

                                                       Rab = N1P12 = N1B12u(ν)…….(1)

The total probability of transition for number of atoms from state 2 to 1, either by spontaneous or by stimulated emission per unit time i.e rate of emissions is given by

                               Rsp + Rst = N2A21 + N2B21u(ν)……(2)

In thermal equilibrium at temperature T, the absorption and emission probabilities are equal Thus equating (1) and (2)

                                        Rab = Rsp + Rst

                        N1B12u(ν) = N2A21 + N2B21u(ν)……..(3)

              After rearranging the terms, we obtain the expression for photon density as

                               u(ν) = N2A21/( N1B12– N2B21)….(4)

Divide R.H.S of above equation by N2 B12 in numerator and denominator

                                      u(ν) = A21/ B12 ( N1/N2 – B21/B12)….(5)

According to Boltzmann’s law, the distribution of atoms among the energy states E1 and E2 at the thermal equilibrium at temperature T is given

                                                    where, E=E2-E1=hν

Making these substitutions in (5) we obtain

                         u(ν) = A21/ B12 ( ehv/KT – B21/B12)…….(6)

Comparing equation number (6) with the Planck’s radiation law for black body radiations as given in equation (7) 

                       u(ν) = 8πhν3/c3 ( ehv/KT – 1)……………(7)

We obtain

                              A21/ B12= 8πhν3/c3………………..(8)

And                       B21/B12=1 or B12=B21………..(9)

Equations (8) and (9) are relations between Einstein’s coefficients

Significance

  1. It signifies thatprobability of absorption is same as that of stimulated emission
  2. It signifies that B21/A21is inversely proportional to frequency of the resonant radiation. Therefore,higher the frequency smaller is the value of B21. That is, it iscomparatively difficult to obtain the stimulated emission of higherfrequencies.

Lecture 1.3 Conditions for amplification of light

LASER

Conditions for Amplification of Light

We Know from the probability of three processes, the three rate equations are

The ratio of stimulated emission and absorption yields

And ratio of stimulated emission and spontaneous emission yields

This implies u(v) should be high and A21 should be small for stimulated emission to dominate.

Hence there are three conditions which must be satisfied to achieve stimulated emission and hence laser action.

1.     N2>>N1, higher state should be more populated

2.     A21 should be small which means higher level must have longer lifetime.

3.     u(v) should be high.

1. The first condition requires a state which is not preferred under thermal equilibrium as under conditions of thermal equilibrium ground state is more populated. Hence we need to achieve inverted state.

To emit photons which are coherent (in same phase), the number of atoms in the higher energy state must be greater than that in the ground state (lower energy). The process of making population of atoms in the higher energy state more than that in the lower energy state is known as population inversion’.

The method by which a population inversion is achieved is called pumping’. In this process atoms are raised to an excited state by injecting into system photon of frequency different from the stimulating frequency. Population inversion can be understood with the help of 3-energy level atomic systems.

Fig. 1. Mechanism of transitions in three level laser system[2]

Definition of Population Inversion

The state where the number of atoms present in the excited (or higher) state is greater than the number of atoms present in the ground state (or lower) state is called population inversion.

Or

The state with population present in the excited (or higher) state greater than the population present in the ground state (or lower) state is called population inversion.

Let us consider two level energy system of energies E1 and E2 as shown in figure. Let N1 and N1 be the populations (means number of atoms per unit volume) of energy levels E1 and E2.

According to Boltzmann’s distribution the population of an energy level E, at temperature T is given by

N = N0e(-E/KT)

Where N0 is the population of the lower level or ground state and k is the Boltzmann’s constant. From the above relation, the population of energy levels E1 and E2 are

N1= N0e(-E1/kT)

N2= N0e(-E2/kT)

At ordinary conditions N1 > N2, i.e., the population in the ground or lower state is always greater than the population in the excited or higher states. The stage of making, population of higher energy level greater than the population of lower energy level is called population inversion i.e., N2 > N1.

Fig.2. The population of states under normal condition and inverted condition[3]

Consider a three energy level system with energies E1, E2 and E3 of populations N1, N2, and N3. At normal conditions, E1 < E2 < E3 and N1 > N2 > N3. In the ground state E1 ,the life time of atom is more and the life time of atom in the excited state E3 is10–8 seconds. But in the intermediate state Ethe atom has more life time(10–3 seconds) than the excited state. So it is called meta-stable state.

Fig.3. The population of states under normal condition and inverted condition[3]

When a suitable energy is supplied to the system, atoms get excited into E3. After their lifetime the atoms are transit to E2. Due to more lifetime of an atom in state E2 , the atoms stay for longer time than compared with the state E3. Due to the accumulation of atoms in E2, the population inversion is established in between the E2 and E1 states.

2. The second condition requires the stay of atoms in the higher state for a longer life time. The atoms by pumping reach excited state. They stay there only for 10-8 seconds. Hence return to ground state via spontaneous emission. This problem is resolved by introducing meta-stable states in the system having longer life time of 10-3s. From excited state, atoms fall to meta-stable state where they stay for quite a longer time (10-3 seconds). Within this longer time more number of atoms get collected in the meta-stable state which is large than that at lower energy level. Thus population inversion is achieved. In atomic systems such as chromium, neon, etc, meta-stable states exist.

3. The third condition of high photon density is maintained using mirrors of optical cavity resonator. An optical cavity resonator consists of two plane polished mirrors with one mirror 99% polished and other semi-polished. These are used to reflect the photons until resonance occurs in the system.

Summary

1.Conditions of amplification of light are identified.

2.Concept of population inversion.


Lecture 1.4 Components of Laser

Components of LASER

Every LASER consists of three basic components. These are:

1.     Lasing material or active medium.

2.     External energy source.

3.     Optical resonator.

Fig. 1. Components of laser[3]

The Active Medium:

·        It is material in which the laser action is made to take place. It may be solid, liquid or gas. The very important characteristic requirement for the medium is that inversion should be possible in it. Many lasers are named after the material used.

·        For example: Ruby Laser and He-Ne laser

·        Only few atoms are responsible for laser action and are known as active centers while the rest of material is host.

Different Types of lasers

On the basis of active medium used in the laser systems, lasers are classified into several types

1.     Solid lasers :Ruby laser, Nd;YAG laser, Nd;Glass

2.     Liquid lasers : Europium Chelate laser, SeOCl2

3.     Gas lasers : CO2,He-Ne, Argon-Ion Laser

4.     Dye lasers :Rhodamine 6G

5.     Semiconductor lasers :InP, GaAs.

The Pump:

·        It is an external source which supplies energy to obtain population inversion. The pump can be optical, electrical or thermal. In Ruby Laser, we use optical pumping and in He – Ne Laser, we use electric discharge pumping.

·        The energy supplied by the pump excites the atoms to higher energy levels and through spontaneous emission , atoms fall to meta-stable states

·        The lifetime of the meta-stable energy state, in which population inversion occurs must be very large as compared to the normal life time of the excited atom in any other energy state.

Pumping mechanisms (or techniques) of population inversion

A system in which population inversion is achieved is called as an active system. The method of raising the particles from lower energy state to higher energy state is called pumping. (Or the process of achieving of population inversion is called pumping). This can be done by number of ways. The most commonly used pumping methods are

1.     Optical pumping

2.     Electrical discharge pumping

3.     Chemical pumping

4.     Injection current pumping

Optical pumping

Optical pumping is used in solid laser. Xenon flash tubes are used for optical pumping. Since these materials have very broad band absorption, sufficient amount of energy is absorbed from the emission band of flash lamp and population inversion is created.    Examples of optically pumped lasers are ruby, Nd: YAG Laser

(Neodymium: Yttrium Aluminum Garnet), Nd: Glass Laser

Electrical discharge pumping

Electrical discharge pumping is used in gas lasers. Since gas lasers have very narrow absorption band pumping them any flash lamp is not possible. Examples of Electrical discharge pumped lasers are He-Ne laser, CO2 laser, argon-ion laser, etc

Chemical pumping

Chemical reaction may also result in excitation and hence creation of population inversion in few systems. Examples of such systems are HF and DF lasers.

Injection current pumping

In semiconductors, injection of current through the junction results in creates of population inversion among the minority charge carriers. Examples of such systems are InP and GaAs.

                                       In short, these techniques are summarized as

·        Optical Pumping:- Atoms are excited by external energy source.

·        Electrical Pumping:- Atoms are excited by electric field.

·        Chemical Reaction Pumping:- Energy is given by chemical process.

·        Direct Conversion:- Electrical energy directly converted into light.

The Resonator:

·        It consists of a pair of plane or spherical mirrors having common principal axis. The reflection coefficient of one of the mirrors is very near to 1 and that of the other is kept less than 1. The resonator is basically a feed-back device, that directs the photons back and forth through the laser medium and in the process, the number of photons is multiplied due to stimulated emission.

·        It also plays the role of direction selectivity. Only those photons are amplified in the active medium which are emitted along a preferred direction while all others are either absorbed or lost.

·        The system will emit laser light only when optical gain exceeds the optical losses in the system.

PRINCIPLE OF LASER SYSTEM

An atomic system having one or two meta-stable states is chosen. Normally, the number of atoms in the lower energy state is greater than that in the meta stable state. This population is inverted by a technique known as pumping. The atoms are made to fall from meta-stable state to lower energy state and photons are emitted by stimulated emission. The photons are reflected back and forth in the active medium to excite the other atoms. Thus a large number of photons are emitted simultaneously which possess the same energy, phase and direction. This process is called ‘amplification of light’.

To produce laser beam, the following two conditions must be fulfilled:

·        The meta-stable state should all the time have larger number of atoms than the number of atoms in lower energy state.

·        The photons emitted due to stimulated emission should stimulate other atoms to multiply the photons in the active medium.

TYPES OF LASER SYSTEM

Classification in number of ways:

1.     According to the state of laser medium: Gas, Liquid and Solid Laser.

2.     According to the type of pumping: Flash light, Chemical Action, and Electric Discharge Lasers

3.     According to the nature of output: Pulsed (P) or Continuous Wave (CW) Lasers

4.     Classification on the basis of Spectral region of the light: Ultra-Violet, Visible or Infra-Red Lasers.

THREE LEVEL LASER

Fig. 2. Three level laser[4]

·        Initially excited to a short-lived high-energy state .

·        Then quickly decay to the intermediate meta-stable level.

·        Population inversion is created between lower ground state and a higher-energy meta-stable state.

·        Example: RUBY LASER is three level laser.

FOUR LEVEL LASER

Fig. 3. Four level laser[4]

·        Laser transition takes place between the third and second excited states.

·        Rapid depopulation of the lower laser level.

·        Example: Helium-Neon laser is four level laser system

Summary

1.Various components of laser and their working is discussed

2.Types of laser based on medium, pumping technique , pumping scheme is explained’


Lecture 1.5 Ruby Laser

RUBY LASER

Important Points to remember

Ø To produce laser from solid, Ruby crystal is used.

Ø Ruby is an aluminum oxide crystal (Al2O3) in which some of the aluminum atoms have been replaced with Cr3+ chromium atoms (0.05% by weight).

Ø It was the first type of laser invented, and was first operated by Maiman in Research Laboratories on 1960.

Ø Chromium gives ruby its characteristic pink or red color by absorbing green and blue light.

Ø For a ruby laser, a crystal of ruby is formed into a cylinder. The ruby laser is used as a pulsed laser, producing red light at 6943 Å.

Fig. 1. Components of Ruby laser[1]

Principle Features:

·        Type : Doped Insulator Laser

·        Active Medium : Ruby crystal (Al2O3) doped with Cr3+ ions

·        Active Center : Chromium ions

·        Pumping Method Optical Pumping

·        Pumping Source : Xenon Flash Pump

·        Optical Resonator Ends of rods silver coated partially and totally reflecting

·        Nature of Output: Pulsed

·        Wavelength Emitted: 6943 A0

Construction:                       

Fig. 2. Experimental set up of Ruby laser[1]

·        In ruby laser 4 cm length and 5 mm diameter rod is generally used.

·        Both the ends of the rods are highly polished and made strictly parallel. The ends are silvered in such a way, one becomes partially reflected and the other end fully reflected.

·        The ruby rod is surrounded by xenon flash tube, which provides the pumping light to excite the chromium ions in to upper energy levels.

·        Xenon flash tube emits thousands joules of energy in few milli seconds, but only a part of that energy is utilized by the chromium ions while the rest energy heats up the apparatus.

·        A cooling arrangement is provided to keep the experimental set up at normal temperatures

Working:

Fig. 3. Energy level diagram of Ruby laser[3]

Ø Ruby crystal is surrounded by xenon tube. Ruby crystal is fully silvered at one side and partially silvered at the other end.

Ø A strong beam of blue green light is made to fall up on crystal from xenon tube and this light is absorbed by the crystal.

Ø Because of this, many electrons from ground state or normal state are raised to the excited state or higher state and electron falls to meta-stable state.

Ø During this transition photon is not emitted but excess energy of the electrons absorbed in crystal lattice.

Ø As electron drops to meta-stable state they remain there for certain time ~ 10-6 sec.

Ø Thus the incident blue green light from tube increases the number of electron in meta-stable state and then the population inversion can be achieved.

Ø If a light of different frequency is allowed to fall on this material, the electrons move back and forth between silvered ends of the crystal.

Ø While moving through they get stimulated and excited electrons radiate energy.

Ø Thus released photon has the same frequency as that of incident photon and is also in exactly same phase.

Ø When the intensity of light beam is increased the same process is repeated.

Ø Finally extremely intensified beam of light energies from the semi silvered side of the crystal.

Ø This way it is possible to get extremely intensified and coherent beam of light from the crystal. This beam is nothing but higher energetic beam – ie. LASER beam.

LASER OUTPUT & APPLICATIONS:

1.     Three level laser

2.     Solid laser

3.     Output wavelength-  694.3 nm

Applications:-

Ø Ruby lasers have declined in use with the discovery of better lasing media. However, they are still used in a number of applications where short pulses of red light are required. Holography around the world produce holographic portraits with ruby lasers, in sizes up to a meter squared.

Ø Many non-destructive testing labs use ruby lasers to create holograms of large objects such as aircraft to look for weaknesses in the lining.

Ø Ruby lasers are used extensively in tattoo and hair removal.

Ø Ruby lasers can be used for measurement of plasma properties such as electron density and temperature.

Ø Ruby lasers are used to remove the melanin of the skin.

Drawbacks of Ruby Laser…

• The laser requires high pumping power because the laser transition terminates at the ground state and more than half of ground state atoms must be pumped to higher state to achieve population inversion.

• The efficiency of ruby laser is very low because only green component of the pumping light is used while the rest of components are left unused.

• The laser output is not continues but occurs in the form of pulses of microseconds duration.

• The defects due to crystalline imperfections are also present in this laser.

Summary

1.Principle features of Ruby Laser

2.Construction, working applications of Ruby laser

3.Drawbacks of Ruby laser


Lecture 1.6 He-Ne Laser

He-Ne LASER

First gas laser developed by Ali Javan and his coworkers in 1961.

Principle Features:

·        It is a gas laser

·        It uses four level pumping scheme

·        It uses electric discharge pumping technique

·        Output wavelength 632.8 nm

·        Continuous wave laser

Construction:

Fig. 1. Components of He-Ne laser[1]

He-Ne Laser: An atomic laser consisting of

·        Active Medium; a mixture of 10 parts of He to 1 parts of Ne where Ne-atoms are active centers have energy levels suitable for laser transitions

·        He-atoms help efficient excitation of Ne-atoms

·        Discharge tube of about 30 cm long, 1.5 cm in diameter, filled with a mixture of He & Ne gases in 10:1 ratio.

·        Electrodes connected to High Voltage (10 kV) to produce discharge in gas.

·        Glass mirrors at the end of discharge tube acting as mirrors of optical cavity resonator

·        Glass windows at Brewster’s angle to obtain polarized light

Working:

Fig. 2. He-Ne laser experimental set up[1]

·        High Voltage of 10kV applied across the gas- ionizes the gas

·        Electrons & ions accelerated towards anode and cathode

·        Electrons being smaller in mass acquire higher velocity

·        Electrons transfer K.E. to He atoms through inelastic collisions

·        He atoms excited to meta-stable levels

·        He transfer energy to Ne-atom through collisions –Resonant transfer of energy. Possible in He-Ne atoms

·        Ne-atoms being heavy, could not be pumped up efficiently without He-atoms

·        Role of He-atoms is to excite Ne-atoms and cause Population Inversion

·        Probability to transfer energy from He-Ne is more; 10 He per 1 Ne atoms.

·        Reverse probability i.e. Ne-He is extremely small

Fig. 3. Energy level diagram of He-Ne laser[2]

·        5s & 4s levels in Ne ; Meta-stable States Accumulation of atoms

·        Population inversion between 5s and 4p, 3p levels & 4s and 3p levels

·        Lasing takes place and light is produced as shown in diagram

·        Ne-atoms in level 3p decays rapidly to 3s

·        3s to ground transition induced by collisions with walls of discharge tube.

·        Discharge tube made as narrow as possible to enhance probability of atomic collisions with walls.

·        3s level is more likely to be populated by the electric discharge itself

·        An increase in population at 3s causes decrease in P.I. Lasing ceases.

·        Current in discharge tube maintained at low level

Applications:

1.     Major applications as Red light at 632.8 nm:

2.     Laboratories

3.     Laser Printing

4.     Bar Code Reader Scanners

5.     Surveying

6.     Alignment

Summary

1.Principle features of first gas laser He-Ne Laser

2.Construction , working and applications of He-Ne Laser


Lecture 1.7 Applications of Laser-Holography

HOLOGRAPHY

Producing 3-D image of an object is known as holography

·        Two step process

·        Recording of hologram

·        Reconstruction of image

INTRODUCTION:

·        Holography from the Greek, whole+ write) is the science of producing holograms”

·        It is an advanced form of photography that allows an image to be recorded in three dimensions

·        “Holography is a method of producing a visual three-dimensional (3-D) image of an object by means of light wave patterns recorded on a photographic plate.”

·        Dennis Gabor…Father of holography

HOLOGRAM:

·        Photograph of an interference pattern which, when suitably illuminated by laser light, produces a three-dimensional image.

·        If you look at these holograms from different angles, you see objects from different perspectives, just like you would if you were looking at a real object

·        It shows Divisible property i.e If you cut one in half, each half contains whole views of the entire holographic image

·         

DIFFERENCE BETWEEN HOLOGRAPHY AND PHOTOGRAPHY

Table 1: Difference between photography and holography

CONSTRUCTION OF HOLOGRAM:

To create a hologram, a laser beam (coherent light) is split in two beams :

Reference beam: that stays undisturbed

Object beam: strikes the object and then bounces onto the plate

Working of holography is divided into two phases:

1.     Recording

2.     Reconstruction

RECORDING OF HOLOGRAM

Fig. 3. Recording of hologram[3]

·        Laser light as a source of light is used

·        Beam divider device that use mirrors and prisms to split laser beam into two beams

               1. Object beam

               2. Reference beam

·        Mirrors…Direct the beams of light to correct locations

·        It is based upon interference

·        Holographic Film…to record image very high resolution

RECONSTRUCTION OF IMAGE:

·        When a hologram is later illuminated with coherent light of the same frequency that created it, a three-dimensional image of the subject appears.

·        Hologram acts as diffraction grating and diffracts the laser in different directions.

·        The real image is formed where rays actually meet while a 3-D virtual image is formed behind the hologram.

·        Can be seen through naked eye

·        Image floats in free space

 Summary

Holography-recording and reconstruction of image


Lecture 1.8 Applications of Laser

APPLICATIONS OF LASER

·        Communication: Modulated laser beams are being used for transmitting messages. Due to high degree of coherence, the loss of transmitted energy is comparatively much less.

·        Surgery: Laser beam has been used successfully for bloodless surgery.For Example: It can be used to weld the detected retinas. The Laser beam can be used for drilling the teeth, removal of tumors, removal of infected cell etc. It can further be used fro preventing the tooth decay by depositing hard materials on the surface of the tooth.

Fig.2. Laser in surgery[5]

·        Industry: Laser can be focus into very fine beam, resulting in raising of temperature to about 1000 K and can be used for drilling holes and fusing and melting of metals

·        Measurement of Long Distances: During Apollo flight no 11, on July 20, 1969, Armstrong and Aldrin planted a previously designed array of triple prisms on the moon.The laser beam sent from the earth was reflected from these prisms and was received on the earth. It enables us to determine the distance of the moon from the earth with an error within 6 m. Later, experiments lowered the error to within 30 cm.

·        Nuclear Fusion: Laser beam can be used to induce the nuclear fusion. By concentrating the laser beam to a very narrow spot, temperature may rise to about 108 K and nuclear fusion can occur at this temperature.

·        Scientific Research: Used in Michelson Morley Experiment. This experiment was conducted to test ether drift.

·        Lasers in military: A laser beam can be bounced off a target such as enemy air plane or ship, to determine its distance and speed. Laser can serve as a war weapon. High energy lasers are being employed to destroy enemy air crafts and missiles.

OTHER APPLICATIONS of LASER:

·        3-D Photography

·        Bio-medical Applications

·        Compact disk

·        Grocery store

·        Holographic Interfero-metry

·        Holographic Scanners

CHAPTER 2

Lecture 2.1 Introduction to Fiber Optics

  • Brief Introduction
  • Optical fiber construction
  • Critical angle & Total Internal Reflection

Brief Introduction

An optical fiber is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to transmit light between the two ends of the fiber and find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths than wire cables. Fibers are used instead of metal wires because signals travel along them with lesser amounts of loss; in addition, fibers are also immune to electromagnetic interference, a problem from which metal wires suffer excessively.

The uses of optical fibre are given bellow:

1. Communication – Telephone transmission method uses fibre-optic cables. Optical fibres transmit energy in the form of light pulses. The technology is similar to that of the coaxial cable, except that the optical fibres can handle tens of thousands of conversations simultaneously.

2. Medical uses – Optical fibres are well suited for medical use. They can be made in extremely thin, flexible strands for insertion into the blood vessels, lungs, and other hollow parts of the body. Optical fibres are used in a number of instruments that enable doctors to view internal body parts without having to perform surgery.

3. Simple uses – The simplest application of optical fibres is the transmission of light to locations otherwise hard to reach. Also, bundles of several thousand very thin fibres assembled precisely side by side and optically polished at their ends, can be used to transmit images.

Optical fiber construction

A typical optical fiber comprises three main components: the core, which carries the light; the cladding, which surrounds the core with a lower refractive index and contains the light; and the coating, which protects the fragile fiber within [1].

Core

The core, which carries the light, is the smallest part of the optical fiber. The optical fiber core is usually made of glass, although some are made of plastic. The glass used in the core is extremely pure silicon dioxide (SiO2), a material so clear that you could look through 5 miles of it as though you were looking through a household window. In the manufacturing process, dopants such as germania, phosphorous pentoxide, or alumina are used to raise the refractive index under controlled conditions.

Lasers and Fibre Optics

Figure 1 Basic structure and parts of optical fibre [2]

Optical fiber cores are manufactured in different diameters for different applications. Typical glass cores range from as small as 3.7μm up to 200μm. Core sizes commonly used in telecommunications are 9μm, 50μm and 62.5μm. Plastic optical fiber cores can be much larger than glass. A popular plastic core size is 980υm.

Cladding

The cladding is surrounding the core and providing the lower refractive index to make the optical fiber work. When glass cladding is used, the cladding and the core are manufactured together from the same silicon dioxide-based material in a permanently fused state. The manufacturing process adds different amounts of dopants to the core and the cladding to maintain a difference in refractive indexes between them of about 1%. A typical core may have a refractive index of 1.49 at 1300nm while the cladding may have a refractive index of 1.47. These numbers, however, are wavelength dependent. The core of the same fiber will have a different refractive index at a different wavelength. Like the core, the cladding is manufactured in standard diameters. The two most commonly used diameters are 125μm and 140μm. The 125μm cladding typically supports core sizes of 9μm, 50μm, 62.5μm and 85μm. The 140μm cladding typically has a 100μm core.

Coating

The coating is the protective layer of the optical fiber. The coating absorbs the shocks, nicks, scrapes, and even moisture that could damage the cladding. Without the coating, the optical fiber is very fragile. A single microscopic nick in the cladding could cause the optical fiber to break when it’s bent. Coating is essential for all-glass fibers, and they are not sold without it. The coating is solely protective. It does not contribute to the light-carrying ability of the optical fiber in any way. The outside diameter of the coating is typically either 250μm or 500μm. Generally the coating is colorless. In some applications, however, the coating is colored, so that individual optical fibers in a group of optical fibers can be identified. The coating found on an optical fiber is selected for a specific type of performance or environment. Once of the most common types of coating is acrylate. This coating is typically applied in two layers. The primary coating is applied directly on the cladding. This coating is soft and provides a cushion for the optical fiber when it is bent. The secondary coating is harder than the primary coating and provides a hard outer surface. Acrylate, however, is limited in temperature performance. A typical acrylates may perform at temperatures up to 125º C. Silicon, carbon, and polyimide are coatings that may be found on optical fibers that are used in harsh environments such as those associated with avionics, aerospace, and space. They may also be used on optical fibers designed for mining, or oil and gas drilling.

Standards

While many combinations of core and cladding sizes are possible, standards are necessary to ensure that connectors and equipment can be matched properly. This is especially important when dealing with components as small as those used in fiber optics, where even slight misalignments can render the entire system useless [3].

So what are optical fibers made of then?

Turns out they can be made of just glass, glass plus polymers, or just polymers (“plastic optical fibers” -POF-). The most basic optical fiber consist of:

An inner cylinder with high refractive index, called the core.

A middle cylinder with a lower refractive index, called the cladding.

An outer protective polymer layer (usually polyurethane or PVC) called the jacket.

For glass optical fibers, the diameter of the core ranges between 10-600 microns, the cladding thickness is between 125-630 microns, and that of the jacket varies between 250-1040 microns. For POF all diameters range between 750-2000 microns. As can be seen, one of the main differences between glass and plastic optical fibers is their diameter. This makes POF easier to handle.

The material used for currently commercialized fibers (core and cladding) include pure glass (SiO2), plastic, or a combination of both. The use of one or the other material will be determined by such factors as quality and economics.

Plastic optical fibers (POF) have the advantage of being made of cheaper materials than glass and to operate in the visible range of the spectrum. However, they show a high loss, and for that reason their applications are confined to short distance transmission. In spite of this, POF is widely used for medical and industrial instruments, and currently research is carried out about using POF as a replacement of copper wiring for data transmission in automobiles. If you use silica glass for the core, it must be high purity in order to allow the light to be transmitted along the core with minimal loss [4].

I- Glass-Glass Fiber

  • Glass as silica (SiO2) with refractive Index (RI) of 1.458 at 850nm
  • Addition of GeO2 and P2O5 increases RI.
  • Addition of B2O3 and fluorine decreases RI.

I- Glass-Glass Fiber

  • Combinations can be—
  • GeO2 – SiO2 core, SiO2 Cladding.
  • P2O5 – SiO2 core, SiO2 Cladding.
  • SiO2 core, B2O3 -SiO2 Cladding.
  • GeO2 – B2O3 – SiO2 core, B2O3 -SiO2 Cladding.

II- Plastic clad Glass Fiber

  • Glass core and plastic cladding.
  • Higher losses.
  • Short distance (several hundred meters).
  • Reduced cost.
  • Core Silicon resin RI = 1.405 at 850nm
  • Clad is Teflon PEP (Perfluoronated ethylene propylene) with RI = 1.338.
  • Large NA with large RI difference.
  • Core dia of 150 to 600µm.
  • LED as source.

III- Plastic Fiber

  • Very short distance (100m max).
  • High attenuation.
  • Low cost, tough, durable and inexpensive.
  • Core dia of 110 to 1400µm.
  • LED as source.
  • Polystyrene core (1.6), methyl methacrylate clad (1.49). NA = 0.6.
  • Polymethyle methacrylate core(1.49), its co-polymer(1.40), NA = 0.5

Remember!

  • Germanium dioxide and phosphorus pentaoxide increase the refractive index of glass
  • Boron oxide decreases the refractive index of glass.
  • These oxides are known as dopants.
  • Changing composition of the mixture during the process influences refractive index profile of the preform [5].

Transmission of light through optical fibres or Principle of optical fibre

Total internal reflection is a phenomenon that happens when a propagating wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other on the other side of the boundary and the incident angle is greater than the critical angle, the wave cannot pass through and is entirely reflected. The critical angle is the angle of incidence above which the total internal reflection occurs. This is particularly common as an optical phenomenon, where light waves are involved, but it occurs with many types of waves are involved, but it occurs with many types of waves, such as electromagnetic waves in general or sound waves.

Critical angle

  • The angle at which total internal reflection occurs is called the critical angle of incidence.
  • At any angle of incidence (θ1) greater than the critical angle, light is totally reflected back to the glass medium.
  • For n1 > n2, the angle of refraction q2 is always greater than the angle of incidence θ1.
  • When the angle of refraction θ2 is 90o the refracted ray emerges parallel to the interface between the media.
Description: 9edf3cfe0edcf2ff1932e7f13644b158.png

Figure 3 when the angle of incidence is equal to the critical angle, the angle of refraction is equal to 90° [6]

Refractive index

  • The index of refraction of a material is the ratio of the speed of light in a vacuum to the speed of light in the material

                                           n = c/v

The factor n is the index of refraction (or refractive index) of the medium.

For air and gases n ~ 1. At optic frequencies, the refractive index of water is 1.33.

  • Glass has many compositions, each with a slightly different n. An approximate refractive index of 1.5 is representative for the silica glasses used in fibers; more precise values for these glasses lie between ~1.45 and ~1.48.

  Snell’s Law

           Snell derived the relationship between the different angles of light as it passes from one  transparent medium to another. When light passes from one transparent material to another, it bends according to Snell’s law which is defined as:

n1sin(θ1) = n2sin(θ2)

where:-

n1 is the refractive index of the medium the light is leaving

θ1 is the incident angle between the light beam and the normal (normal is 90° to the interface between two materials)

n2 is the refractive index of the material the light is entering

θ2 is the refractive angle between the light ray and the normal

Description: Snell's Law

Figure 4 Snell’s Law [7]

Note:

For the case of θ1 = 0° (i.e., a ray perpendicular to the interface) the solution is θ2 = 0° regardless of the values of n1 and n2. That means a ray entering a medium perpendicular to the surface is never bent. The above is also valid for light going from a dense (higher n) to a less dense (lower n) material; the symmetry of Snell’s law shows that the same ray paths are applicable in opposite direction.

Description: Total Internal Reflection

Figure 5 Total internal reflection [7]

When a light ray crosses an interface into a medium with a higher refractive index, it bends towards the normal. Conversely, light traveling cross an interface from a higher refractive index medium to a lower refractive index medium will bend away from the normal. This has an interesting implication: at some angle, known as the critical angle θc, light traveling from a higher refractive index medium to a lower refractive index medium will be refracted at 90°; in other words, refracted along the interface. If the light hits the interface at any angle larger than this critical angle, it will not pass through to the second medium at all. Instead, all of it will be reflected back into the first medium, a process known as total internal reflection.


Lecture 2.2 Acceptance Angle

Acceptance Angle & acceptance cone

Acceptance angle

It is the maximum angle of a ray (against the fiber axis) hitting the fiber core which allows the incident light to be guided by the core.

Acceptance cone

In fiber optics, the cone within which optical power may be coupled into the bound modes of an optical fiber. Here, the acceptance cone is derived by rotating the acceptance angle about the fiber axis.

Expression of Acceptance angle

Applying Snell’s law for Air-Core media

n0sinθI = n1 sin θr ………….. (1)

 From the right angle triangle ABC

 θr +θ = 90

 θr = 90 –θ

n0 sinθ i = n1 sin (90 -θ )

 n0 sinθI = n1cosθ

 sinθi = (n1/n0)  cosθ ……………..(2)

when θ = critical angle (θc ) →θi =θm

sinθm= (n1/n0 ) cosθc …………………. (3)

According to law of refraction

n1 sin n2 sin r

here i θc= 90

sinθc = n2 /n1  sin 90 = n2/n1 

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Substitute equation (4) in (3)

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Which is the required expression for maximum Acceptance Angle in optical fibers.

Importance of acceptance angle or acceptance cone

  • The angle θ max is called the acceptance angle of the fibre. Acceptance angle is the maximum angle that a light ray can have relative to the axis of the fibre and propagate down the fibre. Thus, only those rays that are incident on the face of the fibre making angles less that θ0 will undergo repeated total internal reflections and reach the other end of the fibre. Hence, larger acceptance angles make it easier to launch light into fibre.
  • In three dimensions, the light rays contained within the cone having a full angle 2θmax are accepted and transmitted along the fibre. Therefore, the cone is called the acceptance cone. Light incident at an angle beyond θ max refracts through the cladding and corresponding optical energy is lost.

Summary

  1. It is the maximum angle of a ray (against the fiber axis) hitting the fiber core which allows the incident light to be guided by the core.
  2. Higher the acceptance angle, larger amount of light enters into optical fibre.
  3. Relation
  4. Acceptance angle totally depends upon refractive index of core and cladding.

Lecture 2.3 Numerical Aperture

  • Numerical Aperture
  • V-number or Normalized Frequency

Numerical Aperture

Definition: Numerical Aperture is the measure of the ability of an optical fiber to collect or confine the incident light ray inside it. It is among the most basic property of optical fiber.

Numerical aperture is abbreviated as NA and shows the efficiency with which light is collected inside the fiber in order to get propagated.

NA is related to acceptance angle. As acceptance angle is that max angle through which light enters the fiber. Hence the acceptance angle and numerical aperture are related to each other.

We know light through an optical fiber is propagated through total internal reflection. Or we can say multiple TIR takes place inside the optical fiber for the light ray to get transmitted from an end to another through an optical fiber.

Basically when the light is emitted from an optical source, then the fiber must be highly efficient so as to collect the maximal emitted radiation inside it.

Thus we can say that the light gathering efficiency of an optical fiber is the key characteristic while transmitting a signal through an optical fiber [1].

image

Thus

  • Numerical aperture determines the light gathering ability of the fibre.
  • It is a measure amount of light that can be accepted by a fibre.
  • NA is dependent only on the refractive indices of the core and cladding materials and does not depend on the physical dimensions of the fibre.
  • The value of NA ranges from 0.13 to 0.5. A large NA implies that a fibre will accept large amount of light from the source.

V-number or Normalized Frequency

  • No. of modes supported by optical fiber is obtained by cut-off condition known as normalized frequency or V-Number
  • V- number can be reduced either by reducing numerical aperture or by reducing diameter of fiber
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Number of Modes traveling in Fibre

The total number of mode traveling in a fibre depends on the V – Number and is related as:

Number of modes for Step Index Fibre Nm = V2/2

For Graded Index Fiber Nm = V2/4

  • Normalized frequency is a dimensionless parameter and simply called V-number or value of the fiber.
  • It combines in a very useful manner the information about three parameters, a, Δ and λ.
  • Limiting parameter for single and multimode propagation in optical fiber.

⇒ ≤ 2.405 for Single Mode operation (SMF)

V > 2.405, then the fibre is multimode fibre (MMF) [2].


Lecture 2.4 & 2.5 Types of Fibers

  • Modes
  • Single mode step index optical Fiber
  • Multimode step index optical Fiber
  • Graded index Fiber

Modes

When the light ray is guided through an optical fibre, it propagates in different types of modes. In simple terms, these modes can be visualized as the possible number of allowed paths of light in an optical fibre.

We can see that all the paths are zig-zag paths excepting the axial direction.

The rays having propagation angles between θ=0 and θ = θc will be in a position to undergo total internal reflections, all of them will not however propagate along the optical fibre.

Only a certain ray direction is allowed. As zigzag ray gets repeatedly reflect at the walls of the fibre, phase shift occurs. Consequently, the waves traveling along certain zig-zag paths will be in phase and undergo constructive interference, while the waves along certain other paths will be out of phase and diminish due to destructive interference. The light ray paths along which the waves are in phase inside the fibre are known as modes.

Types of modes:

  • In a fibre of fixed thickness, the modes that propagate at angles close to critical angle Өc (i.e. critical propagation angle θc) are higher order modes.
  • Modes that propagate with angles larger than the critical angle (i.e. lower than the critical propagation angle) are lower order modes as shown in figure 1
  • In case of lower order modes, the fields are concentrated near the centre of the fibre.
  • In case of higher order modes, the fields are distributed more towards the edge of the wave-guide and tend to send light energy into the cladding. This energy is lost ultimately.
  • The higher order modes have to traverse longer paths and hence take larger time than the lower order modes to cover a given length of the fibre.
  • Thus,the higher order modes arrive at the output end of the fibre later than the lower order modes.

Types of Optical Fibers

The types of optical fibers depend on the refractive index, materials used and mode of propagation of light.

The classification based on the mode of propagation of light is as follows:

Single Mode Fibers: Single mode fibre (SMF) has a smaller core diameter and can support only one mode of propagation i.e. zero order mode.

  • These fibers are used for long-distance transmission of signals.
  • A typical single mode fiber may have a core radius of 3 μm and a numerical aperture of 0.1 at a wavelength of 0.8 μm.

Advantages of Single Mode Fiber

  • Only one path is available.
  • Core diameter is small
  • No dispersion
  • Higher band width (1000 MHz)
  • Used for long haul communication
  • Fabrication is difficult and costly

Disadvantages of Single Mode Fiber

  • Difficult to couple light in and out of the tiny core
  • Highly directive light source (laser) is required
  • Interfacing modules are more expensive

Multimode Fibers: A multimode fibre has a larger core diameter and supports a number of modes i.e. higher order modes and lower order modes. These fibers are used for short-distance transmission of signals.

  • If more than one mode is transmitted through optical fiber, then it is said to be a multimode fiber.
  • The larger core radius of multimode fibers make it easier to launch optical power into the fiber and facilitates the end to end connection of similar powers.

Advantages of Multi- Mode Fiber

  • With a larger fiber core and good alignment tolerances
  • Inexpensive
  • Easy to couple light into Fiber
  • Multimode fiber cable also provides high speed and high bandwidth over short distances. 
  • Multimode fiber cable is often used in data and audio/video applications in LANs.

Disadvantages of Multi-Mode Fiber

  • Multimode fiber cable has high dispersion and attenuation rate
  • The quality of optical signals is reduced as the transmission distance is getting longer.

The classification based on the materials used is as follows:

  1. Plastic Optical Fibers: The polymethyl-methacrylate is used as a core material for the transmission of the light.
  2. Glass Fibers: It consists of extremely fine glass fibers.

The classification based on the refractive index is as follows:

  1. Step Index Fibers: It consists of a core surrounded by the cladding which has a single uniform index of refraction.
  2. Graded Index Fibers: The refractive index of the optical fiber decreases as the radial distance from the fiber axis increases.

Step index fiber

  • In a step index fiber, the refractive index changes in a step fashion, from the centre of the fiber, the core, to the outer shell, the cladding.
  • It is high in the core and lower in the cladding. The light in the fiber propagates by bouncing back and forth from core-cladding interface.
  • Step-index fibers have a uniform core with one index of refraction, and a uniform cladding with a smaller index of refraction. 
  •  The step index fibers propagate both single and multimode signals within the fiber core.
  •  The light rays propagating through it are in the form of meridinal rays which will cross the fiber core axis during every reflection at the core – cladding boundary and are propagating in a zig – zag manner.

Graded index fiber

  • In graded index fibers, the refractive index of the core varies gradually as a function of radial distance from the fiber center.
  • The refractive index of the core decreases as we move away from the center.
  • The refractive index of the core is made to vary in the form of parabolic manner such that the maximum refractive index is present at the center of the core.
  • Graded Index fiber is another type of optical fiber in which the refractive index of the core is non-uniform. This non-uniformity is present because the refractive index is higher at the axis of the core and continuously reduces with the radial movement away from the axis.

Three types of optical fibre

1 Single Mode step index fibre: 

Structure:

  • A single mode step index fibre has a very fine thin core of diameter of 8μm to 12 μm as shown in the figure 13.
  • It is usually made of germanium doped silicon.
  • The core is surrounded by a thick cladding of lower refractive index. The cladding is composed of silica lightly doped with phosphorous oxide.
  • The external diameter of the cladding is of the order of 125μm.

Propagation of light

  • SMF Light travels in SMF along a single path that is along the axis.
  • Obviously,it is the zero-order mode that is supported by a SMF, both Δ and N A are very small for single mode fibres.
  • This small value is obtained by reducing the fibre radius and by making Δ, the relative refractive index change, to be small.
  • The low N A means low acceptance angle.
  • Costly laser diodes are needed to launch light into SMF.

2 Multimode step index fibre

Structure:

  • A multimode step index fibre is very much similar to the single mode step index fibre except that its core is of larger diameter.
  • The core diameter is of the order of 50 to 100μm, which is very large compared to the wavelength of light.
  • The external diameter of cladding is about 150 to 250μm.

Propagation of light in MMF:

  • Multimode step index fibre allows finite number of guided modes.
  • We can also say that many zigzag paths of propagation are permitted in a MMF.
  • The path length along the axis of the fibre is shorter while the other zigzag paths are longer.
  • Because of this difference, the lower order modes reach the end of the fibre earlier while the high order modes reach after some time delay.

3 Graded index (GRIN) fibre

Structure 

  • The graded index fibre is a multimode fibre with a core consisting of concentric layers of different refractive indices.
  • Therefore,the refractive index of the core varies with distance from the fibre axis. Atypical structure and its index profile.
  • The size of the graded index fibre is about the same as the step index fibre.

 Propagation of light 

  • Fig 8 shows the ray diagram of refraction at the various high and low index interfaces within graded index fibre, giving an overall curved ray path.
  • The figure shows the light transmission in a graded index fibre. As light ray goes from a region of higher refractive index to a region of low refractive index,it is bent away from the normal the process continues till the condition for total internal reflection is met.
  • Then the ray travels back towards the core axis, again being continuously refracted see figure. The turning around may take place even before reaching the core–cladding interface.
  • Thus,continuous refraction is followed by total internal reflection and again continuous refraction towards the axis.
  • In the graded index fibre, rays making larger angles with the axis traverse longer path but they travel in a region of lower refractive index.
  • Consequently,all rays traveling through the fibre, irrespective of their modes of travel,will have almost the same optical path length and reach the output end of the fibre at the same time see figure.

Summary of different Fibers

C:\Users\DELL INSPIRON\Downloads\Capture.JPG (23).jpg

Figure 9 Summary of different Fibers [5]

Summary 

  1. Modes are different paths followed by light in optical fibre.
  2. There are three types of modes- higher order modes, lower order modes, zero order modes.
  3. On basis of modes, optical fibre divided into single mode and multimode.
  4. On basis of refractive index, optical fibre divided into step index and graded index.
  5. Graded index fiber has no time difference in propagation of time.

Lecture 2.6 Losses in Fibers

  • Dispersion in Optical Fiber
  • Intermodal and intramodal dispersion
  • Losses in optical Fiber (Attenuation loss)

Dispersion 

The dispersion cause that optical pulses to broaden as they travel along a fiber, the overlap between neighboring pulses, creating errors in the receiver output, resulting in the limitation of information-carrying capacity of a fiber.

In communication, dispersion is used to describe any process by which any electromagnetic signal propagating in a physical medium is degraded because the various wave characteristics (i.e., frequencies) of the signal have different propagation velocities within the physical medium.

Types of Dispersion 

Dispersion are of two types:

  •  Intra modal or Chromatic Dispersion
  •  Intermodal or Modal Dispersion

Intra modal or Chromatic Dispersion

Intramodal or Chromatic dispersion (CD) is caused by the fact that single mode glass fibers transmit light of different wavelengths at different speeds. The ratio of the speed of light in a medium to the speed in a vacuum defines the index of refraction or refractive index of the material.

Material Dispersion

  •  This is due to intrinsic properties of the material, glass.
  •  Glass is a dispersive medium. We can recall from our high school physics that glass has different refractive index for different colors.
  •  Different colors (wavelengths) have different velocity in glass.
  •  A type of dispersion that occurs in optical fiber due to the interaction of various wavelengths with the physical matter in the crystalline structure of the glass.
  •  The refractive index of the glass varies according to the wavelength of the optical signal.
  •  Material dispersion is the phenomena whereby materials cause a “bundle” of light to spread out as it propagates.

Waveguide Dispersion

This is due dispersive nature of the bound medium. In a bound medium like the optical fiber, the velocity is a function of frequency.

•     Waveguide dispersion is chromatic dispersion which arises from waveguide effects: the dispersive phase shifts for a wave in a waveguide differ from those which the wave would experience in a homogeneous medium.

Intermodal or modal Dispersion

  • In a multimode fiber different modes travel at different velocities.
  • Pulse broadening due to intermodal dispersion (sometimes referred to simply as modal or mode dispersion) results from the propagation delay differences between modes within a multimode fiber.
  • As the different modes which constitute a pulse in a multimode fiber travel along the channel at different group velocities, the pulse width at the output is dependent upon the transmission times of the slowest and fastest modes.
  • If a pulse is constituted from different modes then intermodal dispersion occurs.
  • Modal dispersion is greatest in multimode step index fibers.
  • The more modes the greater the modal dispersion.
  • Typical bandwidth of a step index fiber may be as low as 10 MHz over 1 km.

Attenuation in Optical Fibers

  • Attenuation limits the optical power which can reach the receiver, limiting the operating span of a system.
  • Once the power of an optical pulse is reduced to a point where the receiver is unable to detect the pulse, an error occurs. Attenuation is mainly a result of:
  • Light Absorption
  • Scattering of light
  • Bending losses
  • Attenuation is defined as the ratio of optical input power (Pin) to the optical output power (Pout).
  • The following equation defines signal attenuation as a unit of length :

Types of Attenuation

Absorption Loss:

Caused by the fiber itself or by impurities in the fiber, such as water and metals.

•     Scattering Loss:

Intrinsic loss mechanism caused by the interaction of photons with the glass itself.

•     Bending loss:

Loss induced by physical stress on the fiber.

Material Absorption Losses

Material absorption is caused by absorption of photons within the fiber.

– When a material is illuminated, photons can make the valence electrons of an atom transition to higher energy levels

– Photon is destroyed, and the radiant energy is transformed into electric potential energy. This energy can then

  • Be re-emitted (scattering)
  • Frees the electron (photoelectric effects) (not in fibers)
  • Dissipated to the rest of the material (transformed into heat)
  • In an optical fiber Material Absorption is the optical power that is effectively
  • Converted to heat dissipation within the fiber.

Two types of absorption exist:

– Intrinsic Absorption, caused by interaction with one or more of the components of the glass.

– Extrinsic Absorption, caused by impurities within the glass.

  1. Intrinsic Absorption is caused by basic fiber material properties. If an optical fiber is absolutely pure, with no imperfections or impurities, ten all absorption will be intrinsic. Intrinsic absorption in the ultraviolet region is caused by electronic absorption bands. Intrinsic Absorption occurs when a light particle (photon) interacts with an electron and excites it to a higher energy level.
  2. Extrinsic Absorption is caused by impurities caused by impurities introduced into the fiber material. The metal impurities such as iron, nickel and chromium are introduced into the fiber during fabrication. Extrinsic Absorption is caused by the electronic transition of these metal ions from one energy level to another energy level.

Fiber Bend Losses

•     Bending loss is classified according to the bend radius of curvature : 

1.        Microbend Loss        2.        Macrobend Loss

Microbend Loss are caused by small discontinuities or imperfections in the fiber. Uneven coating applications and improper cabling procedure increases micro bend loss. External forces are also a source of micro bends [3].

Macrobend Loss

Macrobend Losses are observed when a fiber bend’s radius of curvature is large compared to the fiber diameter. These bends are a great source of loss when the radius of curvature is less than several centimetre.

Linear Scattering Losses

  • Light scattering is a form of scattering in which light in the form of propagating energy is scattered.
  • Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media.
  • Deviations from the law of reflection due to irregularities on a surface are also usually considered to be a form of scattering.
  • When these irregularities are considered to be random and dense enough that their individual effects average out, this kind of scattered reflection is commonly referred to as diffuse reflection.

Advantages of optical fiber communication

1. Cheaper:

  • Optical fibres are made from silica (SiO2) which is one of the most abundant materials on the earth.
  • The overall cost of a fibre optic communication is lower than that of an equivalent cable communication system.

 2. Smaller in size, lighter in weight, flexible and strong:

  • The cross section of an optical fibre is about a few hundred microns. Hence, the fibres are less bulky.
  • Typically,a RG-19/U coaxial cable weights about 1100kg/km while a PCS fibre cable weights 6kg/km only. Optical fibres are quite flexible and strong.

 3. Not hazardous:

  • A wire communication link could accidentally short circuit high voltage lines and the sparking occurring thereby could ignite combustible gases in the area leading to a great damage.
  • Such accidents cannot occur with fibre links since fibre links are made of insulating materials.

4. Immune to EMI and RFI:

  • In optical fibre, information is carried by photons. Photons are electrically neutral and cannot be disturbed by high voltage fields, lightening, etc.
  • Therefore,fibres are immune to externally caused background noise generated through electromagnetic interference (EMI) and radio frequency interference (RFI).

5. No cross talk:

  • The light waves propagating along the optical fibre are completely trapped within the fibre and cannot leak out.
  • Further,light cannot couple into the fibre from sides. In view of these features,possibility of cross talk is minimized when optical fibre is used. Therefore,transmission is more secure and private.

 6. Wider bandwidth:

  • Optical fibres have ability to carry large amounts of information.
  • While a telephone cable composed of 900 pairs of wire can handle 10,000 calls, a 1mm optical fibre can transmit 50,000 calls.

7. Low loss per unit length:

  • The transmission loss per unit length of an optical fibre is about 4dB/km.Therefore, longer cable-runs between repeaters are feasible.
  • If copper cables are used, the repeaters are to be spaced at intervals of about 2km. In case of optical fibres, the interval can be as large as 100km and above.

Disadvantages of optical fiber communication

  • Specialist skills needed
  • Cost of installation
  • Cost of transmission equipment from electrical to optical signals
  • Optical fibers cannot carry electrical power

Summary 

  1. Dispersion losses in optical fiber
  2. Attenuation Losses in cables
  3. Advantages and disadvantages

Lecture 2.7 Advantages & Applications of Optical Fibers

  • Advantages of optical fiber communication
  • Disadvantages of optical fiber communication
  • Applications of optical fiber communication

Advantages of optical fiber communication

— Increased Bandwidth and Channel Capacity

— Low Signal Attenuation

— Immune to Noise

— No Cross talk

— Lower Bit Error Rates

— Signal Security

— Electrical Isolation

— Reduced Size and Weight of Cables

— Radiation Resistant and Environment Friendly

— Resistant to Temperature Variations etc.

Disadvantages of optical fiber communication

— Specialist skills needed

— Cost of installation

— Cost of transmission equipment from electrical to optical signals

— Optical fibers cannot carry electrical power

Applications of optical fiber communication

C:\Users\DELL INSPIRON\Downloads\Capture.JPG (29).jpg

Figure 1. Applications of fiber optic as sensor [3]

  • As fibers are very flexible, they are used in flexible digital cameras.
  • Fibers are used in mechanical imaging i.e. for inspection of mechanical welds in pipes and engines of rockets, space shuttles, airplanes.
  • Fibers are used in medical imaging such as endoscopes and laparoscopic.
  • Fibers can be used under sea communication.
  • Fibers are used in military applications such as aircrafts, ships, tanks etc.
  • Nuclear testing applications use optical fiber phase sensors and transducers
  • Fibers are used in public utility organizations like railways, TV transmission etc.
  • Fibers are used in LAN systems of offices, industrial plants and colleges etc.
  • Fibers are used in telecommunication such as voice telephones, video phones, telegraph services, message services and data networks [1-2].

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