Laser2

http://vcs.abdn.ac.uk/ENGINEERING/lasers/lasers.html

LASERS

Objectives • • • •

To outline the behaviour and Nature of Light To describe the processes of Absorption and Emission of Light To discuss the processes of Amplification of Light and The Laser To describe some of the common Types of Laser and their application

Table of Contents 1

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The Nature of Light o Waves o Photons Absorption and Emission of Light o Gaseous Light Sources o Emission from Solids Amplification of Light o Population Inversion o Optical Resonant Cavity o Longitudinal and Transverse Cavity Modes o Processes of Laser Action o Properties of Laser Light Types of Laser o Gas Lasers o Solid State Lasers o Semiconductor Lasers and Light Emitting Diodes Atomic Structure o Excitation & De-excitation of Atoms o Emission of Light from an Atom Behaviour of Solids o Conduction in Metals Semiconductors o Intrinsic Semiconductors o Extrinsic Semiconductors The pn Junction o Forward Potential Difference o Reverse Potential Difference

The Nature of Light 2

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The invention of the LASER is one of the most significant developments in science and engineering To enable us to understand and appreciate the operation of this unique device requires o an understanding of the behaviour and the properties of light itself Light is the medium by which we carry information through an optical system o our immediate task is to ask ourselves  what is light?  what are its origins? Light is not the easiest of natural phenomena to describe o for many centuries, scientists have debated, and argued over, the nature of light o we will not take part in this debate but will try to present a description of light which will satisfy our working needs To understand the operation of the LASER and other light sources, we need to appreciate the unique character of the light emitted from gases and solids o all radiating bodies when viewed by the naked eye appear to possess a characteristic colour:  sunlight is white  a piece of hot iron may be orange-red  a sodium street lamp is yellow o If the light from any of these sources is passed through a prism it spreads out in a series of component colours known as a SPECTRUM  sunlight appears as a continuous band of colours ranging from red through to violet  a piece of iron also shows a continuum from dull red to orange  a sodium lamp displays a series of bright, narrow lines o whether the spectral distribution is a CONTINUOUS SPECTRUM or in DISCRETE SPECTRAL LINES depends on  the nature of the source  the temperature of the source To completely describe the properties of light requires us to adopt two different models of behaviour: o the ELECTROMAGNETIC WAVE model o the PHOTON model

Waves

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The propagation of light through space can be described in terms of a travelling wave motion The wave is composed of a combination of mutually perpendicular electric and magnetic fields o the direction of propagation of the wave is at right angles to both field directions:  this is known as an ELECTROMAGNETIC WAVE o the wave carries energy with it

The electromagnetic wave •

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Light, radio waves, microwaves, X-rays and gamma rays are all electromagnetic waves o they differ only in their characteristic wavelength or frequency  the whole range of electromagnetic waves is encompassed in the ELECTROMAGNETIC SPECTRUM LIGHT is defined as the range between o 200 nm, in the short ULTRA-VIOLET, and 20 µm, in the far infra-red o VISIBLE LIGHT stretches from a wavelength of about 400 nm at the violet end of the spectrum to around 700 nm at the red end The theory of ELECTROMAGNETIC RADIATION which describes the above phenomena is one of the cornerstones of electrical engineering o first proposed in mathematical form by the great Scottish scientist James Clerk Maxwell, in the late 19th century To completely and formally describe the propagation of light through free space and the energy transferred by it requires that o we treat electromagnetic radiation as a VECTOR wave o and specify it, in three-dimensional co-ordinates, in terms of its magnitude and direction For most purposes, we may represent a travelling light wave as a onedimensional, scalar wave o provided that we remember it has a direction of propagation o we usually only describe such a wave in terms of the electric field  since the magnetic field is always at right angles we can extract it if necessary 

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The electromagnetic spectrum

One-dimensional representation of a lightwave

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The frequency ( ) and wavelength ( ) of the wave are related to the velocity of propagation of light in free space, by c=

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The wave is useful when describing phenomena like DIFFRACTION, INTERFERENCE and POLARISATION

Photons •

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Although light may be described as a travelling wave propagating through space o we can also discuss its behaviour in terms of the amount of energy imparted in an interaction with some other medium In this case, we can imagine a beam of light to be composed of o a stream of small lumps or QUANTA of energy, known as PHOTONS o each photon carries with it a precisely defined amount of energy  this energy depends only on its wavelength or frequency The energy of a single photon is given, in terms of its frequency, f, or wavelength,

, as,

Wph = hf = hc/ where h is Planck's constant and c is the velocity of propagation of the photon in free space Planck's constant, h = 663x10-36 J s velocity of light in free space, c = 300x106 m/s this equation shows how the energy and frequency of a photon are linked  

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Even although a photon can be thought of as a particle of energy o it still has a fundamental wavelength associated with it o this wavelength is equivalent to that of the propagating wave as described by the wave model The photon description of light is useful when the light source contains only a few photons The concept of a photon is due, principally to the work of three famous men o Max Planck, Neils Bohr and Albert Einstein, in the period from around 1900 to 1920 o by invoking the idea of light being emitted in tiny pulses of energy they were able to explain phenomena like BLACK-BODY RADIATION, EMISSION OF LIGHT FROM ATOMS and the PHOTOELECTRIC EFFECT

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Problem: Energy of a Photon To illustrate the use of the above equations, calculate the energy of a photon of 600 nm wavelength. This photon would be in the red part of the spectrum. Solution •

The energy of a single photon is, Wph = hc/ = (663x10-36 J s x 300x106 m s-1) / (600x10-9 m) = 332 x 10-21 J

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This is a tiny amount of energy! o We can put this into more manageable numbers by using the concept of the electronvolt. o An electronvolt (eV) is defined as the energy obtained by an electron when it is accelerated through a potential difference of 1 volt. Welectron = qV = 160x10-21 C x 1 V = 160x10-21J 1 eV.

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Therefore, Wph = 332x10-21 J / 160x10-21 J eV-1 = 2.07 eV

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Visible photons range in energy from 1.74 eV (700 nm) to 3.34 eV (400 nm)

Absorption and Emission of Light 7

Prerequisites: This section requires familiarity with the concepts and models of ATOMIC STRUCTURE

ATOMIC STRUCTURE •

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All atoms consist of a small, massive nucleus surrounded by smaller, lighter electrons o the nucleus consists of protons and neutrons o the electrons are negatively charged (-q), protons are positively charged (+q), neutrons are neutral (zero charge)  Fundamental unit of electric charge: q = 1.6x10-19 C The number of protons and electrons in an atom are equal; the net atomic charge is zero, therefore the atom is electrically neutral On the basis of the QUANTUM MODEL of the atom o electrons are held in stable ORBITS (ENERGY SHELLS) around nucleus by a balance between two opposing forces: o the force of electrostatic attraction pulls the negative electron towards the positive nucleus  known as the COULOMB ELECTROSTATIC FORCE o the force due to the mass-acceleration of the electron acts radially outwards from the nucleus  known as the CENTRIPETAL FORCE  this is the force which acts along a string when a stone is spun around your head o this results in a BINDING ENERGY which holds electrons in orbit around the nucleus The bound electrons do not possess any value of energy, but can only possess specific discrete energies according to the allowed orbits o the energy is said to be QUANTISED and the permitted values of energy are known as energy levels o the further the electron is from the nucleus the less tightly bound it will be to the nucleus.

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The orbital (quantum) model of a many-electron atom •

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Each atom has an infinity of possible orbits (energy shells) o the electrons are confined to orbit in these discrete shells o not all shells will contain an electron o the number of electrons in a given shell is restricted to specific values The outermost occupied shell is known as the VALENCE SHELL o the valence electrons are very important  they determine material properties o the maximum number of valence electrons is 8  an atom with a valence of 8 is a stable atom (inert or unreactive)  an atom with a valence of 1 is highly reactive

The processes of emission and absorption •

If energy is absorbed by an atom, the electrons are excited into vacant energy shells o the absorbed energy is equal in magnitude to the difference in energy between shells o o

W = |Wu-Wl|

ABSORBED ENERGY

where Wl is the energy of the initial (lower) state and Wu is the energy of the final (higher) state

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Absorption and emission of light between atomic energy levels •

The excited atom can SPONTANEOUSLY (randomly) de-excite to a lower level if a vacant site permits o the average length of time an atom stays in excited state is tens of nanoseconds o The corresponding emission of energy is given by, o

W = |Wu-Wl |

o o

EMITTED ENERGY

if absorbed or emitted as light, the quantum is known as a PHOTON.

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The energy of each photon is given by

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Wph =

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But the photon energy has also been given as

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Wph = h

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We clearly see that the energy of a photon is defined by o the energy emitted when an electron drops from an excited energy shell to one of lower energy o this energy is, in turn, related to and defines the frequency (or wavelength) of the emitted photon The energy diagram for a hydrogen atom shows some of the possible absorption and emission wavelengths

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W = |Wu - Wl|

= hc/

PHOTON ENERGY

PHOTON ENERGY

Problem: Emission Wavelength Calculate the wavelength of emission when an atom de-excites from an energy state at 12.09 eV to one at 10.2 eV above the ground state. Use the energy diagram of hydrogen to assist in your calculation.

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Energy diagram of hydrogen showing some optical transitions Solution •

The emission energy is W = Wu - Wl = 12.09 eV - 10.2 eV = 1.89 eV

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The photon energy is Wph =

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W = 1.89 eV

The wavelength of emission is thus = hc/Wph = 6.63x10-34 J s x 3x108 m s-1/(1.89 eV x 1.6x10-19 J eV-1) = 657.7x10-9 m = 657.7 nm

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Gaseous Light Sources • •

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The situation described above relates to that of a single isolated atom When vast numbers of atoms combine to form a low density gas o all the atoms exist in different states of excitation o the behaviour of the whole gas is characterised by the behaviour of the individual atoms  thus the energy structure of a gas is the same as that of the individual atom o many more electrons are now involved If gas is excited by addition of energy (e.g. thermal) o all the atoms will be in different states of excitation.  some will be in the ground state  some will be in excited states For a gas in thermal equilibrium with its surroundings at a temperature T o the numbers of atoms per unit volume, the POPULATION DENSITY, in a particular energy state depends solely on  the temperature of the gas, and  the difference in energy between the excited level and a known lower level The population density of atoms, Nu, in an excited state, Wu, in relation to those, Nl, in a lower energy state Wl o is given by the Boltzmann relationship, as, o o

Nu/Nl = exp[-(Wu-Wl)/kT] = exp[BOLTZMANN RELATIONSHIP

W/kT]

Boltzmann distributions of atoms in a gas in thermal equilibrium •

At a given temperature, the atoms will be distributed amongst all available levels:

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the ground state will contain most of the atoms the higher levels will contain relatively few atoms Raising the temperature of the gas pushes more atoms into the higher levels of excitation o they subsequently de-excite by spontaneous release of photons back to the lower levels o the population ratio of any pair of states at a particular temperature is governed by the Boltzmann equation at that temperature The processes we have just described form the basis of light emission from THERMAL LIGHT SOURCES A thermally excited atomic gas will emit light across all the energy transitions available to it o the light emitted is composed of many discrete, separate wavelengths or spectral lines  governed by the distribution of atoms amongst the available energy levels  giving rise to the CHARACTERISTIC SPECTRUM for that element o o

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CHARACTERISTIC SPECTRUM

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A train of incoherent photons •

The emitted light is said to be INCOHERENT in time and space. o the light is composed of many different wavelengths o the light is emitted in random directions o the light is emitted with different amplitudes o there is no phase correspondence between any of the emitted photons

Light emission from solids •

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When atoms are closer together, as in a dense gas like the sun or a solid such as a piece of hot iron o the energy structures associated with individual atoms influence one another This is the collective behaviour of many interacting atoms of different species, o rather than the characteristic behaviour of individual atoms of a particular element Instead of individual, discrete energy levels as for the isolated atom, o we find that the individual energy levels merge into broad BANDS of closely spaced energy levels o in between these bands are ENERGY GAPS, which atoms cannot possess As for a gas, at a given temperature o the lowest available energy levels are filled with electrons  these are known as the VALENCE BANDS

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above the valence bands are a range of primarily empty bands  these are the CONDUCTION BANDS Excitation raises atoms from the heavily populated valence bands into the relatively empty conduction bands Emission of a photon occurs when an electron in a conduction band relaxes to a valence band o with the release of the corresponding amount of energy Because emission occurs between BANDS of closely spaced energy levels, rather than the well-defined levels described earlier o the characteristic spectrum is a broadband continuum in which the individual transitions are indistinct  like that of the sun The overall spectral distribution of energy radiated from such a source o is given by the BLACK-BODY relationship o which relates the radiant energy emitted by a hot body to its temperature o

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The black-body spectrum •

In essence, the hotter the body is the more energy it will emit and the longer will be its wavelength of peak emission

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as a piece of iron is heated up it will begin to glow a dull red, changing to orange-red and yellow as it gets hotter such materials form the basis of arc discharge sources or incandescent lamps

Amplification of Light: The Laser Prerequisite: To understand the processes of amplification of light requires familiarity with THE NATURE OF LIGHT and ABSORPTION AND EMISSION OF LIGHT Stimulated emission • •

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Lasers are unique sources of light o they do not behave like conventional thermal light sources For atomic systems in thermal equilibrium with their surroundings, emission of light is the result of two main processes: o ABSORPTION of energy o SPONTANEOUS EMISSION of energy A third mechanism also exists: o although not a dominant process in thermal systems at room temperatures, it is crucial to the formation of LASER action o this process is known as STIMULATED EMISSION

STIMULATED EMISSION •

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In stimulated emission, atoms in an upper energy level can be triggered or stimulated in phase by an incoming photon of a specific energy o the incident photon must have an energy corresponding to the energy difference between the upper and lower states o the incident photon is not absorbed by the atom  it actually vibrates the pair of energy levels with whom its energy coincides  the atom de-excites with the consequent release of photons of the same energy as the incident photon The stimulated photons have unique properties: 16

the emitted photon is in phase with the incident photon the emitted photon has the same wavelength as the incident photon the emitted photon travels in same direction as incident photon The key to the above behaviour lies in the likelihood, or PROBABILITY, that an optical transition will actually occur: o not all transitions, upwards or downwards, occur with equal probability  some are more likely than others to absorb or emit a photon For spontaneous emission of a photon, the probability of occurrence is o inversely related to the average length of time that an atom can reside in the upper level of the transition before it relaxes  known as the SPONTANEOUS LIFETIME o typically, the spontaneous lifetime is some tens of nanoseconds o the shorter the spontaneous lifetime, the greater is the probability that spontaneous emission will occur For some pairs of energy levels in certain materials o the spontaneous lifetime can be of the order of microseconds to a few milliseconds  we call this a METASTABLE STATE o the likelihood that a spontaneous transition will take place between these levels is relatively low As the likelihood of spontaneous emission decreases the conditions which favour stimulated emission are enhanced o if an atom is excited into a metastable state it can stay there long enough for a photon of the correct frequency to arrive o this will stimulate the emission of a second photon  we have one photon in and two out  what we have done is add photons to the incoming beam by promoting stimulated emission at the expense of spontaneous emission  we have amplified light! The emitted photons all possess the same wavelength and vibrate in phase with the incident photons The above behaviour forms the basis of laser action and gives us the origin of the name LASER o Light Amplification by Stimulated Emission of Radiation o o o

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Population inversion •

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The above mechanism on its own is not enough to ensure laser action o as we have described it above, we have only created two photons from one o to be really called an amplifier we need to produce millions of photons We need a mechanism by which we can o add more and more atoms to the upper metastable state o hold them long enough to store energy 17

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 allow the production of great numbers of stimulated photons Because of the existence of the longer lifetime states described earlier it is possible to create a situation o where the rate at which atoms are PUMPED into one of these states exceeds the rate at which they leave o a large number of atoms can be excited into, and held in, the upper state leaving an almost empty state below them o atoms can stay in this metastable state without de-exciting while the population is being built up  this is known as a POPULATION INVERSION  we have stored atoms in this upper energy state A population inversion arises when more atoms are in a higher state of excitation than the one below o this situation is in violation of the conditions relating to thermal equilibrium In practice, it is not possible to create a working laser based on absorption and emission between only two energy levels as described above For any pair of levels o the rate at which the upper level is populated by absorption equals that at which atoms leave by stimulated emission o the best we can hope for in a two-level system is an equality of populations in the upper and lower levels  population inversion cannot be achieved o laser systems utilising three or four energy levels are needed In a three level system: o atoms are pumped into the highest of the three levels  called the pump level o spontaneous de-excitation occurs from the pump level to the metastable level  which lies between the pump level and ground and serves as the upper level of the laser transition o laser emission occurs between the metastable level and the ground state

Population inversion in a three-level laser •

An improvement on this behaviour is obtained with a four-level structure 18

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where the laser transition takes place between the third and second excited states we need depopulation of the lower laser level to be rapid  to ensure that the upper level is always full and the lower level always empty

A four level laser structure

Optical Resonant Cavity •

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We have shown that to obtain light amplification from a given medium we need o to establish a population inversion between a chosen pair of energy levels o promote stimulated emission at the expense of spontaneous emission. Even yet this is not enough to sustain laser action o if we do not confine this system in a special way  it would radiate spontaneously in so many different directions that we would not be able to sustain stimulated emission In practice, photons need to be confined in the system to allow the number of photons created by stimulated emission to exceed all other mechanisms This can be achieved by bounding the laser medium between two mirrors  this forms an OPTICAL RESONANT CAVITY o one mirror is totally reflecting and the other partially reflecting The reflectivity of the partial reflector is normally between 10 to 90 %

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this is necessary to ensure that some laser light can escape and provide useful optical power The cavity provides a method of OPTICAL FEEDBACK such that o the stimulated beam is made to pass backwards and forwards several times  it stimulates further emission as it goes o

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Stimulated photons can bounce back and forward along the cavity, creating more stimulated emission as they go o any photons which do not travel along the optical axis are lost o any photons which are not of the correct frequency are lost

The laser system •

When the number of photons produced by stimulated emission exceeds that produced randomly: o the system gain exceeds the losses o laser action can proceed

Longitudinal Cavity Modes • •

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In an optical resonant cavity only specific LONGITUDINAL MODES OF OSCILLATION can be supported An oscillation mode in a resonant cavity can be likened to o the establishment of standing waves on a stretched string pinned at both ends o only those modes corresponding to multiples of half a wavelength can be supported  all other modes will die away This is also true in a laser cavity o known as the LONGITUDINAL or AXIAL MODES of the cavity

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Because of the laser wavelength in relation to the length of the cavity o many million modes are theoretically possible o but only those modes which are close to the laser frequency can be supported Energy will build up in these allowed modes until o the gain of the system exceeds the accumulative effect of the losses Laser action takes place when o the gain provided by stimulated emission from the upper laser level just exceeds any losses due to spontaneous emission, off axis photons and absorption and scattering at the mirrors Laser light is emitted in a highly pure, directional beam along the optic axis of the cavity o the spectral linewidth of laser lines is typically around 1 to 500 GHz

Transverse Cavity Modes • •

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In the above discussion we have tacitly ignored any width to the cavity Of course the cavity will have a finite width and will support TRANSVERSE modes arising from waves travelling off-axis along the cavity o these modes influence the spatial profile of the beam o defined in terms of the Transverse ElectroMagnetic wave distribution across the cavity, TEM modes. The FUNDAMENTAL mode is the TEM 00 mode and corresponds to a smooth distribution of light across the output of the laser The transverse modes are a function of the cavity width

Transverse modes in a laser cavity

The processes of LASER Action •

Consider a three-level atomic structure consisting of levels p, u and l o the energy of the upper pumping level is denoted by Wp

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the upper level of the laser transition by, Wu the lower (ground) level by, Wl Initially, the system is in thermal equilibrium with its surroundings o the populations, Np, Nu and Nl, of the respective levels are given in accordance with the Boltzmann relationship  Np is much less than Nu, which is, in turn, much less than Nl Supplying pump energy to the system, equivalent in magnitude to Wp o will raise atoms in the ground state, l, to the upper pump level, p De-excitation from the pump level to the upper laser level can occur o by spontaneous emission o or by non-radiative processes such as collisional de-excitation If the rate at which the upper laser level is fed from the pump level exceeds that of spontaneous de-excitation from the laser level o a population inversion can be established between u and l, such that Nu becomes much greater than Nl Ideally the rate at which the upper laser level is fed from the pumping level should be rapid o with consequent depopulation to the ground level taking place relatively slowly The LASER is now primed for action Initially while the inversion is building up o photons may be spontaneously emitted between the laser levels o most of these photons will be emitted in random directions and will ultimately pass out of the laser cavity o a few, though, may travel along the axis of the laser system  these photons can stimulate the emission of more photons between the laser levels o as they pass back and forward along the optical cavity the amount of stimulated emission will build up  as will the population inversion o at some point the number of photons being produced by stimulated emission will exceed all other mechanisms  at this point a cascade of photons will be emitted through the output mirror in a short sharp burst The emitted photons all have same wavelength, phase, amplitude and direction We have just described the operation of a pulsed laser o If we can continually re-establish the population inversion o then we can produce a continuous wave LASER beam In reality it is light OSCILLATION we are dealing with, but who wants to back a LOSER? o o

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The processes of laser action

Properties of laser light Monochromaticity •

Laser light is concentrated in a narrow range of wavelengths o lasers produce the purest (most MONOCHROMATIC) light available

Coherence

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All the emitted photons bear a constant phase relationship with each other in both time and phase o the light is said to be COHERENT

A train of coherent photons Beam divergence •

All photons travel in the same direction o the light is contained in a very narrow pencil  almost COLLIMATED o laser light is low in divergence (usually)

High irradiance •

Because all the light is concentrated into a narrow spatial band o light possesses high radiant power per unit area (i.e. high irradiance)

Problem: Irradiance of a Laser Estimate the irradiance of a 1 mW laser beam with a diameter of 1 mm Solution: •

The irradiance (power per unit area incident on a surface) is E=

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/A = 1x10-3 W/(

(1x10-3 m)2/4) = 1273 W/m2

We should note that the irradiance of sunlight at the earth is about 1400 W/m2!

TYPES OF LASER 24

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Many hundreds of different lasers are now available, o only a few types are in regular use in engineering We will try to give a broad insight of the operation and properties of such lasers We should bear in mind that optimum laser design is more complex than outlined here o we are only attempting to get a feel for what is involved o for example, in all our analyses so far we have assumed an optical cavity bounded by parallel reflectors  in practice many different mirror arrangements involving combinations of curved and plane are adopted

Typical properties for the lasers discussed here are listed in the table below. LASER & LED PARAMETERS Type

HeNe Argon Ruby Ruby Nd-YAG GaAlAs LED FreeQQrunning switched switched SemiSemiGas Gas solid solid solid conductor conductor state state state

Power or Energy

5 mW 1.5 W

Wavelength

632.8 514.5 nm nm

Pulse duration Divergence (full angle) Linewidth

cw

cw

1J

50 mJ

10 mW

694.3 694.3 nm 1064 nm 820 nm nm 350 µs 30 ns 10 ns cw (FR) (QS) (QS)

1 mrad 1 mrad 5 mrad 5 mrad 1.5 1 GHz GHz

250 mJ

5 mrad

330 330 GHz 180 GHz GHz

Spontaneous 100 ns 3 ms lifetime Refractive 1 1 1.5 index 0.8 Beam diam 1 mm 10 mm mm active area diam Threshold Current Forward voltage drop Max Forward current Max power dissipation

20 mW 880 nm cw

20°

40°

4 nm

50 nm

3 ms

550 µs

1 ns

1.5

1.82

3.6

5 mm

5 mm 200 µm

200 µm

80 mA 2V

1.5 V

200 mA

100 mA

220 mW

200

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Approx Cost £800 £25000 £35000 £15000 £35000

£200

£0.40

Practical Lasers •

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The basic requirements of any laser are similar, they all comprise: o an ACTIVE MEDIUM with a suitable set of energy levels to support laser action o a source of PUMPING ENERGY in order to establish a population inversion o an OPTICAL CAVITY to introduce optical feedback and so maintain the gain of the system above all losses Lasers are usually classified in terms of their active (lasing) medium. o For example, some of the more important classes of interest to engineers are:  Solid state lasers  Gas lasers  Semiconductor lasers

Gas Lasers The Helium-Neon (HeNe) LASER • • • • • •

The second working LASER system to be demonstrated. The first gas LASER to be produced. The first LASER to produce a continuous output beam The active laser medium is a gaseous mixture of He & Ne atoms, in a roughly 10:1 proportion The gas is enclosed in a cylindrical quartz DISCHARGE tube o the tube is sealed at each end by a mirror to form the optical cavity Pumping is via an electrical discharge passed through the gas o a GLOW DISCHARGE is created in the gas between the electrodes. o a pulse of about 10 kV is applied across the electrodes to start the discharge o an electric current is induced through the gas o a steady current of 3 to 10 mA (dc) is sufficient to keep the discharge established

A HeNe laser 26

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The lighter He atoms are excited by collisions with electrons in the discharge o the He atoms collide with the heavier Ne atoms and transfer their energy to them o Ne atoms are excited by the collisions into their metastable state where population inversion builds up Randomly emitted photons trigger de-excitation from the metastable level to the ground state. Laser light is emitted o wavelength of 633 nm (red) o powers in the range 0.5 to 50 mW o beam divergence about 1 mrad

PHOTO SHOWS HeNe LASER BEAM

The Argon-ion Laser • •

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Unlike the HeNe laser, the active medium in the argon laser is a plasma of excited IONS An electric discharge is created in a narrow tube of gaseous argon o argon atoms are first ionised and then excited  by multiple collisions with electrons into their upper energy levels Because of the existence of a set of closely-spaced upper levels o several laser transitions occur simultaneously in the blue-green region of the spectrum o the strongest being at 514 and 488 nm Due to the high energy required to ionise and excite the argon atoms o very high current densities are needed, of the order of 1 A mm-2 A magnetic field surrounds the laser tube to help constrict the gas discharge and keep the current density high o the longitudinal field, increases the electron density in the plasma by constraining the electrons to move in a helical path around the field lines o this prevents loss of electrons to the walls. The discharge tube is normally made of a material with a low thermal conductivity such as berrylium oxide (BeO), graphite or a metal-ceramic tube construction

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to keep running temperatures low, metal discs are inserted inside the tube to act as heat exchangers.

THE LAYOUT OF A TYPICAL ARGON LASER IS SHOWN IN PHOTO (Photograph reproduced with permission from Lexel Laser, Inc. of Fremont, USA and Lamda Photometrics Limited of Harpenden, England) •

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Argon lasers o emit around 1 to 20 W of flux distributed amongst all the lasing wavelengths o as much as 5 or 6 W can be obtained at the most powerful of these wavelengths, the 514 nm line. Common uses of argon lasers are o holography, eye surgery, spectrochemistry, optical image processing, semiconductor processing and last, but not least in terms of numbers of lasers supplied, laser light shows

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ARGON LASER IN HOLOGRAPHY 1

ARGON LASER IN HOLOGRAPHY 2

AN ARGON LASER BEING USED FOR HOLOGRAPHY IS SHOWN IN VIDEO CLIP

The Carbon Dioxide Laser • • •

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The third gas laser we will discuss is fundamentally different from the other two. The important energy levels are provided not by the distribution of electrons but o by the wiggling and jiggling of the entire carbon dioxide molecule itself. The CO2 molecule can be pictured as a linear arrangement of O-C-O atoms which vibrate in relation to each other. o several different modes of vibration give rise to a set of energy levels with transitions far into the infra-red. The principal CO2 wavelength is 10.6 µm o this is in the far infrared region of the spectrum. Continuous power outputs up to 25 kW are obtainable o this laser is the favoured choice for materials-processing applications such as cutting, welding and annealing 29

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Unlike most other gas lasers, the CO2 has an appreciably high efficiency o typically 10 to 15 % To reach the high powers required from these lasers, cavity lengths can stretch to 2 or 3 meters or more

A CO2 LASER BEING USED FOR CUTTING STEEL IS SHOWN IN PHOTO

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Video 2a

2b

Video

(Laserobot video reproduced with permission from Robomatix Technologies Ltd (http://www.robomatix.co.il))

Video2c (With permission from Howden Laser Division)

A CO2 LASER BEING USED FOR CUTTING STEEL IS SHOWN IN VIDEOS

Solid State Lasers •

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Solid state lasers are characterised by having as their active medium, o a solid rod or slab of crystalline insulator  doped with a small amount of impurity. To help avoid confusion in terminology with the SEMICONDUCTOR laser o solid state lasers are sometimes now referred to as DOPED INSULATOR LASERS. It is the impurity constituent which provides the required energy structure to produce laser action o the crystalline lattice primarily acts as a host material but also influences the overall energy structure.

The Ruby Laser • •

The ruby laser takes its place in history by being the first working laser to be demonstrated Theodore Maiman, working at Hughes Labs. In the USA, showed the first working laser to the world in 1960 o the enthusiastic reader should get a hold of Maiman's original account of his work:  "Stimulated optical radiation in ruby masers", Nature, Vol. 187, p493, (1960) 31

•

•

The active medium is a cylindrical crystal of synthetic sapphire (Al2O3) o doped with roughly 0.05%, by weight, of chromium ions (Cr3+)  RUBY The ends of the rod are polished flat and parallel o HIGH standards of flatness and parallelism are demanded:  the flatness over the entire end face should vary by no more than a quarter of a wavelength ( /4) both surfaces should be parallel to within a few seconds of arc. The ruby is irradiated by a short pulse of light from a xenon-filled flashtube The ruby absorbs pumping energy in the blue-green region of the spectrum o excites the chromium ions to the upper level of the laser transition  emits its principal laser energy at a wavelength of 694.3 nm Crystal and flashtube are placed parallel to each other within a polished pumping chamber o ensures that as much light as possible is pumped into the rod Typical output energies range from a few millijoules to several hundred joules Beam divergence about 5 mrad Radiant emission occurs as a short pulse of energy over a duration of around 250 µs. Typical energies for ruby lasers range from a few millijoules to several hundred joules. The output pulse actually occurs in a series of relaxation spikes superimposed on the fluorescent background 

• •

•

• • • • •

A solid state laser

32

A COMMERCIAL RUBY LASER IS SHOWN IN PHOTO (Reproduced with permission from Lumonics Ltd)

A FOCUSED BEAM FROM A RUBY LASER PRODUCING ABLATION AT A METAL SURFACE IS SHOWN IN PHOTO

THE VIDEO SHOWS A RUBY LASER ABLATING A METAL SURFACE The Nd-YAG Laser • • •

Now supersedes the ruby as the most common doped insulator laser Host material is a crystal of yttrium-aluminium-garnate (Y3Al5O12), YAG o doped with 0.7% by weight of neodymium (Nd3+) ions Laser emission takes place at 1.064 µm (infra-red)

SEMICONDUCTOR LASERS AND LIGHT EMITTING DIODES Prerequisites: •

This section requires familiarity with the concepts of SEMICONDUCTORS and THE pn JUNCTION

33

Light Emitting Diodes (LED's • •

•

All pn junctions emit light on passage of forward biased current Si & Ge are not efficient producers of light o compound semiconductors, are better  e.g. GaAS, GaP, GaAlAs In a basic pn junction, free electrons in n-type diffuse into p-type under forward bias o in the p-region they meet a majority of holes & recombine  excess energy emitted as light

Light emitting diodes •

Typical emission wavelengths for LEDs o GaAs - 880 nm o GaP - 550 nm or 700 nm o GaAsP - 580 nm or 660 nm o Si - 1100 nm o Ge - 1810 nm

Semiconductor lasers or Laser Diodes • •

Formed from heavily doped pn-junctions o Based on modified light-emitting diode structure To achieve laser action, need to ensure high concentration of e-h pairs available for recombination o this is achieved by high doping concentrations across junction o long spontaneous lifetime materials enhance stimulated emission

34

Laser diodes •

• • •

• •

Laser diodes are constructed so that light emerges from ends rather than through the wide gap o narrow active layer contains holes across the whole length o ends are cleaved, polished and made flat & parallel o sides are roughened to trap light inside crystal Light which is spontaneously generated is reflected back & forth causing stimulated emission High current densities are needed to produce stimulated emission & population inversion Available across a wide range of wavelengths o 633, 770, 809 nm, 1.1, 1.3 µm depending on material/structure o AlGaInP, GaAlAs, InGaAsP - MQW, DH, DFB Powers range from a few mW's to several W's cw Pulsed powers up to 100's of Watts peak

ATOMIC STRUCTURE •

• •

All atoms consist of a small, massive nucleus surrounded by smaller, lighter electrons o the nucleus consists of protons and neutrons o the electrons are negatively charged (-q), protons are positively charged (+q), neutrons are neutral (zero charge)  Fundamental unit of electric charge: q = 1.6x10-19 C The number of protons and electrons in an atom are equal; the net atomic charge is zero, therefore the atom is electrically neutral On the basis of the QUANTUM MODEL of the atom o electrons are held in stable ORBITS (ENERGY SHELLS) around nucleus by a balance between two opposing forces: o the force of electrostatic attraction pulls the negative electron towards the positive nucleus  known as the COULOMB ELECTROSTATIC FORCE o the force due to the mass-acceleration of the electron acts radially outwards from the nucleus 35

known as the CENTRIPETAL FORCE  this is the force which acts along a string when a stone is spun around your head o this results in a BINDING ENERGY which holds electrons in orbit around the nucleus The bound electrons do not possess any value of energy, but can only possess specific discrete energies according to the allowed orbits o the energy is said to be QUANTISED and the permitted values of energy are known as energy levels o the further the electron is from the nucleus the less tightly bound it will be to the nucleus. 

•

The orbital (quantum) model of a many-electron atom •

•

Each atom has an infinity of possible orbits (energy shells) o the electrons are confined to orbit in these discrete shells o not all shells will contain an electron o the number of electrons in a given shell is restricted to specific values The outermost occupied shell is known as the VALENCE SHELL o the valence electrons are very important  they determine material properties o the maximum number of valence electrons is 8  an atom with a valence of 8 is a stable atom (inert or unreactive)  an atom with a valence of 1 is highly reactive

Excitation & De-excitation of Atoms •

If atoms absorb energy, electrons are excited into higher energy levels or shells o electrons can be excited into and out of empty or partially empty shells

36

•

Atoms can be further excited to higher energy states or can release this energy if electrons drop back to lower levels o the range of energies which the valence electrons can possess are shown on an energy level diagram

Absorption and emission of energy in a hydrogen atom •

•

•

•

•

The simplest atom of all is the hydrogen atom o it has only one electron o the energy diagram shows all the possible energy states from ground to fully ionised  the diagram shows the energy required to excite the electrons into higher levels When all electrons in an atom are in their lowest possible energy levels o the atom is said to be in its GROUND STATE  corresponding to an excitation energy of 0 eV o for the hydrogen atom its single electron is in the first energy shell (the K-shell) When atoms ABSORB energy, one or more of the outer electrons are EXCITED into higher energy levels or shells o the amount of energy absorbed is equal to the difference between the initial and final energy level  the atom is said to be EXCITED o to excite the hydrogen (the L-shell) atom into its first excited shell requires 10.2 eV When in an excited state, two things can happen: o the atom may be further excited into an even higher state o it may release some, or all, of its excess energy by dropping back to a lower state When an atom receives enough energy o an electron may be excited out of the influence of the atom altogether  the atom is said to be IONISED

37

o o o

for a hydrogen atom this will occur if it receives 13.6 eV of excitation energy when an atom loses an electron  the net charge is positive - we now have a POSITIVE ION when an atom gains an electron  the net charge is negative - we now have a NEGATIVE ION

Emission of Light from an Atom •

•

•

When atoms are excited and de-excited as described above o the energy is seen to be emitted or absorbed between any pair of energy levels in discrete amounts  this is known as a QUANTUM of energy o the emitted energy may manifest itself in several different forms:  heat, light, collisions, chemical energy etc Our interest is specifically with those energy TRANSITIONS which result in the emission of light o known as OPTICAL TRANSITIONS o when emitted as light the quantum of energy is known as a PHOTON The excited atom can SPONTANEOUSLY (randomly) de-excite to a lower level if a vacant site permits o The corresponding emission energy of each photon is given by,

• •

Wph =

W = |W2 - W1|

• •

But the photon energy has also been given as

•

Wph = h

= hc/

PHOTON ENERGY

PHOTON ENERGY

38

The energy level diagram for the hydrogen atom •

The energy -level diagram for hydrogen is shown o the diagram shows all the possible energy states from ground to fully ionised o some possible emission wavelengths are shown

BEHAVIOUR OF SOLIDS Energy Structure of Solids •

•

A solid is a 3-d array of vast numbers of atoms or ions linked in crystalline structure o valence electrons are far from nucleus o can be detached from the atom if enough energy supplied o free to move through crystal Properties depend on how tightly bound electrons are in crystal

Insulators • • •

electrons tightly bound to host ion need large amounts of energy to break free low numbers of free electrons

39

low conductivity electric currents do not pass easily o

•

Metals •

•

Electrons very loosely bound to host ions o very easy to break free from ions o free to "wander" around crystal o large numbers high conductivity movement of electrons produces current in opposite direction

Semiconductors •

Electrons have moderate binding energies o at absolute zero, all electrons are tightly bound => insulator o at very high temps, material can conduct => conductor

Conduction in metals •

•

•

Free electrons in metal have a wide range of energies & velocities o they behave as a "cloud" or "sea" of electrons o individual electrons collide with ion cores as they drift through crystal o individual electrons may travel in many different directions There is no net flow of current o the flow in one direction is randomly balanced by the flow in another o the average velocity of the electrons is zero The electron cloud can be accelerated by applied external electric-field o this constitutes a potential difference being applied across the ends o the electron cloud moves in opposite direction to field with drift velocity vd o this constitutes an electric current in direction of field

Conduction of electrons in a metal •

For n electrons/unit volume, the current flow through the conductor is (in the direction of the electric field)

40

I = total charge/unit time = - nqAl /t = - nqAvd The current density, is J = I/A = - nqvd The force on each electron under influence of applied E-field F = -qE But, F = mea a = -qE/me = vd/ where

is average time between collisions

Thus vd = -q

E/me

Thus the current density J = nq2

E/me

Expressing conductivity as = nq2

/me

We have the current density as

J=

E

OHM's LAW

That the above equation is indeed a representation of Ohm's Law can be verified by substitution of E = V/ l,

= 1/

, and J = I/A

SEMICONDUCTORS • •

Typical semiconducting materials are silicon & germanium They are usually elements from Group IV of the periodic table o 4 valence electrons in outer shell o form 4-way covalent bonds with neighbours

41

Diagrammatic representation of a Si crystal • •

Resistivities of semiconductors lie in the range 0.01 to 10 m Semiconductors are classified into two categories which define their behaviour: o INTRINSIC semiconductors are nominally pure materials o EXTRINSIC semiconductors have a specific amount of impurity elements added

Intrinsic semiconductors • •

•

•

• •

Nominally pure material At low temps, down to absolute zero o the valence electrons tightly bound to ion cores  thus conductivity is low, resistivity is high  they behave like insulators As the temperature increases o the valence electrons gain energy (thermal) o the bonds between electrons and the host ion can be broken  electron freed from host ion o free electrons can move through crystal  thus the material conductivity increases Broken bond (absence of electron) o can be treated as a "HOLE" in the lattice structure  this is a vacant site into which an electron may be attracted o holes have similar properties to electrons but possess an effective positive charge o they move in same direction as applied field (opposite to electrons) Electrons & holes move through crystal under action of electric field The number of holes and electrons in an intrinsic semiconductor are equal o ni = pi ~ 1016 m-3 at RT

Extrinsic semiconductors •

Contain impurities which replace some of original atoms in lattice o these impurities alter the basic material properties

42

•

Group V impurities (e.g. P) o possess five valence electrons  each atom covalently tries to bond with five lattice atoms o leaves an excess electron for every impurity atom  known as n-TYPE semiconductor  electrons dominate the conduction process

Diagrammatic representation of n- and p-type semiconductors •

• •

Group III impurities (e.g. B) o possess three valence electrons o each atom covalently bonds with 3 host atoms  leaves unmade bond o constitutes an excess hole for every impurity atom  known as p-TYPE semiconductor  holes dominate conduction processes The total free electron density is sum of intrinsic and extrinsic electrons The total free hole density is sum of intrinsic and extrinsic holes

THE pn JUNCTION Equilibrium conditions •

•

•

A pn junction is fabricated from a single slice of semiconductor o one side doped p-type o other side doped n-type In trying to neutralise charges o free electrons in n-type diffuse across junction to p-type o free holes in p-type diffuse to n-type o electrons & holes close to junction recombine A DEPLETION REGION (free of mobile charge carriers) develops on either side of junction o fixed negative ions on p-side o fixed positive ions on n-side

43

these residual charges prevent further diffusion so that recombination between holes and electrons is inhibited A potential difference develops across the junction o equilibrium potential V0 o depletion region has high resistivity  because there are no mobile charge carriers The behaviour of the pn-junction may be altered on application of an external voltage across its ends o the potential may be applied in either  a forward potential difference  a reverse potential difference 

•

•

A pn junction semiconductor under equilibrium conditions

Forward potential difference •

If the pn junction is subjected to a forward external potential difference applied across it o the p region made positive with respect to. n-region  forward potential of V applied o the equilibrium condition is disturbed 44

•

•

Depletion region is very resistive compared to rest of material o any external potential dropped almost entirely across depletion region  the junction potential reduces to Vo-V  the width of the depletion region is also reduced o the electrons in n-type can fall down the potential barrier to p-type o the holes in p-type likewise fall down to the n-type  there is a net flow of current from p to n-types We can consider the MAJORITY CARRIERS in each side as being injected across the junction

A pn junction under forward potential conditions

Reverse potential difference • •

•

The p-type is made negative with respect to the n-type The external voltage adds to the internal voltage o the potential barrier is increased o the width of the depletion region increases  thus majority carriers are repelled further from junction  electrons in n-type and holes in p-type find it difficult to cross junction The only current present is that due to a few thermally-generated minority carriers on each side of junction o holes in n-type, electrons in p-type  produces a reverse leakage current from n to p

45

A pn junction under reverse potential conditions

46