The history of the laser

316 Pages·2005·9.481 MB·English

by  Mario Bertolotti

THE HISTORY OF THE LASER
Mario Bertolotti
University of Rome ‘La Sapienza’
Translated from Storia del laser by M Bertolotti
Bollati Boringhieri 1999
Copyright: # 1999 Bollati Boringhieri Editore Torino
Institute of Physics Publishing
Bristol and Philadelphia
Copyright © 2005 IOP Publishing Ltd.
#IOPPublishingLtd2005
Allrightsreserved.Nopartofthispublicationmaybereproduced,storedinaretrievalsystemor
transmittedinanyformorbyanymeans,electronic,mechanical, photocopying,recordingor
otherwise, without the prior permission of the publisher. Multiple copying is permitted in
accordance with the terms of licences issued by the Copyright Licensing Agency under the
termsofitsagreementwithUniversitiesUK(UUK).
BritishLibraryCataloguing-in-PublicationData
AcataloguerecordforthisbookisavailablefromtheBritishLibrary.
ISBN0750309113
LibraryofCongressCataloging-in-PublicationDataareavailable
CommissioningEditor:TomSpicer
ProductionEditor:SimonLaurenson
ProductionControl:SarahPlentyandLeahFielding
CoverDesign:Fre´de´riqueSwist
Marketing:NicolaNewey,LouiseHighamandBenThomas
PublishedbyInstituteofPhysicsPublishing,whollyownedbyTheInstituteofPhysics,London
InstituteofPhysicsPublishing,DiracHouse,TempleBack,BristolBS16BE,UK
USOffice:InstituteofPhysicsPublishing, Suite929,The PublicLedgerBuilding,150South
IndependenceMallWest,Philadelphia,PA19106,USA
TypesetbyAcademicþTechnical,Bristol
IndexbyIndexingSpecialists(UK)Ltd,Hove,EastSussex
PrintedintheUKbyMPGBooksLtd,Bodmin,Cornwall
Copyright © 2005 IOP Publishing Ltd.
CONTENTS
Prefacevii
Introduction1
1.Waveandcorpusculartheoriesoflight13
2.Spectroscopy31
3.Blackbodyradiation48
4.TheRutherford–BohrAtom64
5.Einstein82
6.Einsteinandlight:thephotoelectriceffectandstimulated
emission101
7.Microwaves115
8.Spectroscopy:ActII138
9.Magneticresonance154
10.Themaser176
11.Theproposalforanopticalmaser207
12.Themisfortune(orfortune?)ofGordonGould218
13.Andfinally—thelaser!226
14.Asolutioninsearchofaproblemormanyproblemswiththe
samesolution?Applicationsoflasers262
Bibliography297
Index 299
v
Copyright © 2005 IOP Publishing Ltd.
PREFACE
Itisamazinghowhumanimaginationanticipatedtheinventionofthelaser.
H G Wells in his famous novel ‘The War of the Worlds’ (1898) describes a
death ray, and in the Flash Gordon comics (1950) light-ray guns are
widely used;weapons which would be nowidentified as high-powerlasers.
The word laser is by now well known to the layman, who is literally
surrounded by applications of laser light, in the fields of medicine (surgical
and diagnostic procedures), telecommunications (fibre-optic telephone
links, compact-disk information storage and retriving, holograms) and
technology (laser drilling of materials, geodesic measurements, newspaper
printing).
Laserscomeindifferentshapes,sizesandprices,andgounderdifferent
namessuchasruby(thefirsttooperate),helium–neon,argon,semiconductor
laser and others. Notwithstanding their popularity, very few people really
know what a laser is and how it works. In this book, I will try to explain
in the clearest possible way (although it will prove impossible to avoid
some technical considerations) how people managed to build the first
lasers and the principles by which they operate (together with the masers,
their counterpartin the micro-waverange).
At this stage, it suffices to say that the laser is a light source with
peculiar characteristics, drastically different from those of conventional
sources such as a candle or a light bulb. In fact, laser light consists of a
single colour (not a mixture of colours like white light) and is radiated in a
single direction (not in all directions, as in a light bulb), which enables us
to collect it with a lens and focus it in a region of very small dimensions.
The spectral purity and directionality of laser light dramatically improves
theefficiency ofthis procedure, making it possible toconcentratea sizeable
amountofpowerinasmallregionfordifferentoperations,likethemeltingor
cutting ofa metal.
Intheapplicationsmentionedabove,thelaserisbasicallyusedasavery
powerful light bulb. However, there are others (like optical communica-
tions),inwhichitsmostimportantcharacteristicsarethespectralbandwidth
andangularapertureoftheemittedbeam.Tounderstandthem,weneedto
consider what light is and how it is emitted, which in turn depends on the
emitter, the atom, a task which requires the introduction of some basic
concepts of quantum mechanics. We will discuss the different emission
mechanisms, that is spontaneous emission—the dominating process in all
Copyright © 2005 IOP Publishing Ltd.
naturalsources—andstimulatedemission—theprocessgoverninglaserlight
and responsible forits peculiar characteristics.
Inordertoexplainthedifferentphenomenaaccordingtoanhistorical
sequence, we will retrace the story of light and the first steps of quantum
mechanics. In so doing, we will appreciate that science is built gradually,
like a jigsaw, and that many of its ideas, too advanced with respect to their
historical context, are bound to remain unappreciated and unused, while
others may blossom simultaneously and independently in the minds of
many people, as if they were the unavoidable consequence of the preceding
ideas and the indispensable premiseof those tofollow.
Before beginning our story I wish to thank The American Institute of
Physics Emilio Segre` Visual Archives who provided the authorization to
publish the photographs. A special acknowledgment goes to my wife who
read the Italian text and suggested a number of changes which notably
improveditsclarity.FinallyIwishtothankTimRichardsonforconverting
my Anglo–Italian into English, and Tom Spicer and Leah Fielding of
Insituteof Physics Publishingfor their encouragement.
Copyright © 2005 IOP Publishing Ltd.
INTRODUCTION
The creation of the world, as described in the first book of Genesis, is not
actually at variance with the most recent cosmological theory of the Big
Bang, according to which the Universe started with a great explosion of
light.
Buthowdoesthelightoriginate?Achildwouldlookatinastonishment
andrespondthatlightcomesfromtheSunorfromanelectriclamporafire.
Certainly this would be correct. However, why does the Sun emit light and
similarly, though to a lesser extent, why does fire? For thousands of years
mankind did not ask, or rather linked light to philosophical and religious
concepts, putting emphasis principally on the problems connected with
vision, which in those early times were the most pertinent issues related to
light. In Greek mythology we find the Titan Epimetheus, who assumed the
task of giving to each animal of the Creation a particular characteristic to
protect itself and survive. He provided the tortoise with a hard shell, the
wasp with a sting, and so on, until when he came to the human race he
hadexhaustedallthepossibilitiesofnatureandwasunabletofindanything
for man. Plato writes that man stood ‘nude, barefoot, without a house and
unarmed’; Epimetheus asked for help from his brother Prometheus who
stolefirefromZeusandpresentedittomantohelpdevelopmankind,culture
and technologies. Full of rage and jealousy, Zeus punished Prometheus by
chaining him to the Caucasian mountains where every day an eagle tore at
his liver. To prevent mankind enjoying the gift, he then ordered Ephesus
to mould the first mortal woman, the beautiful Pandora, who married
Epimetheus and through curiosity opened a box, given to her protection,
full of all the evils of the world which spread and caused misfortune to all
mankind.
Inasimilarlyfantasticwaythenatureoflightwascleartotheancient
Egyptians,forwhomitoriginatedfromtheglanceofRah,theirSungod.A
priestin1300BCwrote‘WhenthegodRahopenshiseyesthereislight;when
hecloses hiseyes, night falls’.
It would be possible to quote many other examples showing that the
problem of the origin and nature of light was in ancient times considered
ina religious and fantastic frame.
Copyright © 2005 IOP Publishing Ltd.
The understanding oflight ofthe ancientGreeks
Pythagoras, in the 6th century BC when in Greece philosophy and science
were developing together, formulated a theory of light according to which
rectilinear visual rays leave the eye and touch objects, so exciting visual
sensation.
AccordingtoEmpedocles(circa483–423BC),Aphrodite,thegoddessof
love, forged our eyes with the four elements with which he thought every-
thing was made (soil, water, air and fire) and lit the fire, just like a man
using a lantern to light up his path in the dark. Vision occurred from the
eye to theobject: theeyes emitted their own light.
Plato (circa 428–427 to 348–347BC) assumed that the fire in the eye
emitted light and that this interior light, mixed with daylight, formed a
linkbetweenobjectsintheworldandthesoul,becomingthebridgethrough
whichthesmallest movements ofexternal objectsgeneratevisualsensation.
Accordingtothephilosophertwoformsoflight—oneinternalandtheother
external—mix and act as mediator between man and a dark and cavernous
external world.
The delicate beginnings of a transition towards a mechanical view of
vision started with Euclid, the great Alexandrian mathematician who lived
around 300BC and in his writings on optics provided a clever geometric
theory of vision. Euclid continued to believe that light came from the eye
but, at variance with the vague luminous and ethereal emanation assumed
byEmpedoclesandPlato,itbecamearectilinearlightraytowhichmathema-
tical deduction could be applied. In his extended mathematical studies the
philosopher gave geometrical form to visual rays and developed some of
the laws of geometrical optics as we know them today. He, and like him
Archimedes (circa 287–212BC) and Heron (3rd or 2nd century AD), joined
Pythagoras and his disciples. Instead Democritus (470–360BC) and the
atomists assumed that the illuminated objects emitted atoms, which consti-
tuted images of those same objects, and which, when collected by the eye,
generated vision.
The damage done by Aristotle
Lateron,Aristotle(384–322BC)definedlightas‘theactionofatransparent
body, in that it is transparent’ observing that a transparent body has the
‘power’ to transmit light, but does not become effectively transparent until
light hasgonethrough it and triggered its transparency.
If we observe the eyes of a cat at night, we notice they are bright and
that cats can easily walk in the dark; this fact convinced ancient people of
therealexistenceofafireintheeyesastoldbyEmpedoclesandPlato.How-
ever,apricklyquestionarose:ifasourceoflightexistsintheeye,whyisman
not able to see at night? Answers were many, but Aristotle cut discussion
short by insisting that dark air is opaque: only when a lamp is fired does it
Copyright © 2005 IOP Publishing Ltd.
become transparent because light activates its latent transparency, after
which man can see. We can still ask why the same reasoning did not apply
tothecatthatseeswithoutthelampfired.Inanycasealltheseconsiderations
did not answer the questions concerning the nature of light and how it is
produced.DuringtheMiddleAges,whenproblemsofnaturewerediscussed
on the basis of Aristotelian philosophy, according to which the ‘nature’ of
things consists of the reasons for their existence, that is in their ultimate
end,no progress was made to finda solution.
Saint Thomas Aquinas (1227–1274) declared that ‘the origin of our
knowledge is in the senses, even of those things that transcend sense’ and
‘metaphysics has received its name, that is beyond physics, because to us,
who naturally arrive at the knowledge of things immaterial by means of
things sensible, it offers itself by rightsas an object of study after physics’.
Aristotelismwasextensivelyadoptedin13th-centuryEurope,dominat-
ingforatleastfourcenturiessomuchsothatevenin1624theparliamentin
Parisdeclaredthat,undersentenceofdeath,nobodycouldsupportorteach
doctrines opposed tothose of Aristotle.
The scholars of the Middle Ages considered Aristotelism an encyclo-
paedic body of knowledge which could not be improved. They dropped
the view of Saint Thomas concerning the relationship between physics and
metaphysics affirming ‘it is not the province of physics to theorize on its
ownfactsandlawsortoundertakeareconstructionofcosmologyormeta-
physics...if a physical theory is inconsistent with received metaphysical
teaching,itcannotbeadmitted,becausemetaphysicsisthesupremenatural
science, not physics’. Accordingly they interpreted the external world by
applying only formal logic, extracting deductions from dark and sterile
principleswhichinrealityrepresentedthepetrifactionofflawedAristotelian
physics, an approach which brought nothing more than a prolix sophistry
which prevented scientific progress during theMiddleAges.
Although Aristotle may be considered one of the greatest philoso-
phers—one of the founders of logic—his teaching arrived at the moment
ofdeclineofthecreativeperiodofGreekthinking,andinsteadofstimulating
furtherintellectualactivityitwasacceptedasadogmaandhaltedanyother
philosophicalactivity.Twothousandyearslater,atthetimeofthearousalof
newphilosophical thinking,practicallyanyprogressinscience,inlogic and
inphilosophywasforcedtobeginwithanoppositiontoAristoteliantheories.
The rise ofmodern science
A necessary condition for the emergence of modern science was emancipa-
tion from the Thomist philosophy. The process was aided by a number of
circumstances. During the 15th century, various causes contributed to the
decline of the papacy which resulted in a very rapid political and cultural
change to society. Gunpowder strengthened central government at the
Copyright © 2005 IOP Publishing Ltd.
Figure1. (a)Ptolemaicmodelaccepteduptotheearly17thcentury:theEarthisinthecentre
andSunandplanetsrevolvearoundit.Planetsdescribesmallcircles(epicycles)whosecentres
move on large circles (deferents) with the Earth as the centre. (b) Copernican vision of the
solar system: the Sun is at the centre and the planets turn around it in concentric circular
orbits (from Burgel B H 1946 Dai mondi lontani, Einaudi, Torino, p 37 and Abetti G 1949
Storiadell’Astronomia,Vallecchi,Firenze).
expenseofthefeudalnoblesociety,andthenew—essentiallyclassic—culture
venerated Greece and Rome and condemned the Middle Ages.
Decisiveelementsfortherenewalofsciencewerethenewrelationship
between Earth and Sun as proposed by Nicolas Copernicus (1473–1543) in
1543, according to which the Earth revolves around the Sun and not vice-
versa, as was assumed since the times of Ptolemy (Egyptian astronomer,
mathematician and geographer, circa 100–178AD) (see figure 1) and, at the
beginning of the 17th century,the success of Kepler’s (1571–1630) theories.
Postulating three laws which rule the motion of planets around the Sun,
Kepler demonstrated the falsity of the Aristotelian principle according to
which celestial bodies are ofa different speciesfrom terrestrial ones.
KeplerwasborninthesmalltownofWeilinWurttemberg.Hewaseducated
tobecome aprotestantpastorbut,beinginfavourofCopernicus’ideas,was
forcedtogiveupthisaspiration.Hisprofessorofmathematicsandastronomy
recommended him for a teaching position in Graz, where he published in
1596hisfirstwork,MysteriumCosmographicum,inwhichheclearlyexpresses
his belief in a mathematical harmony of the Universe. Being a protestant, he
was exiled when the Archduke Ferdinand began the rigorous counter-
reformation, and took refuge in Prague on the invitation of the astronomer
TychoBrahe(1546–1601)withwhomhecollaborateduntilhisdeath.Heused
theexactastronomicalobservationsofTychotoobtainhislawsofplanetary
motion. After the death of the Emperor Rudolf II, he moved to Linz in an
efforttodefend,successfully,hismotherfromachargeofwitchcraft.Whenin
1619 the Archduke Ferdinand was raised to the imperial throne with the
name of Ferdinand II, the persecutions of protestants increased and in 1626
KeplerwasforcedtoleaveLinz.Aftertravellingwidely,hediedin1630while
Copyright © 2005 IOP Publishing Ltd.
journeying to Ratisbone to obtain justice from the Parliament. The Thirty
Years War removed thereafter any trace of his burial, which was outside the
city gates. Whilst not believing in them, Kepler, one of the architects of the
astronomical revolution,producedhoroscopesthroughouthislifetoincrease
hismeagrefinances.
Kepler was fascinated by the old Pythagorean idea—which favoured the
spherical form—and tried to find in the movements of planets the same
proportions that appear in musical harmonics and in the shapes of regular
polyhedra. In his vision, the planets were still living entities with an indivi-
dualsoul, like theEarth. The rejection of thisfantastic viewofthephysical
world, started by Galileo and ended by Newton, is barely alluded to by
Kepler, and is present only in his scientific method of treating problems, at
variance with the magic/symbolicattitude typical, forexample, ofalchemy.
ThecelestialbodieswiththeSunatthecentreare,forKepler,arealiza-
tion,althoughimperfect,ofasphericalimageoftheHolyTrinity.Alreadyin
Mysterium Cosmographicum he writes: ‘the image of the trine God is a
sphericalsurface,i.e.theFatheristhecentre,theSonistheexternalsurface
andtheHolySpiritisliketheraysthatfromthecentreirradiatetowardsthe
spherical surface’.
Fromhisexaminationofthemotionoftheplanetshededucedthatthey
revolve around the Sun, describing ellipses with the Sun at one of the foci
(first law), and that the line which joins the Sun to a planet covers equal
areas in equal times (second law). He then demonstrated that the orbits
are not casual but the maximum distance of a planet from the Sun is in
some ratio with the time employed to make a tour around the Sun itself
(third law; figure2).
Figure2. Kepler’slaws.(a)ThefirstlawstatesthatplanetsmoveonellipticorbitswiththeSun
atonefocus.Theperihelionandaphelionarethepointofminimumandmaximumdistancefrom
Sun,respectively.(b)ThesecondlawstatesthatthelinewhichconnectstheSuntotheplanet
covers equal area in equal times. Therefore, the two shaded areas are equal if to cover each
one the same time is employed, and the planet must have higher speed when travelling in
thesegmentnearertotheSunthanwhenittravelsinamoredistantsegment.Thethirdlaw
establishesthatthesquareofthetimeaplanetemploystomakeafulltriparoundtheSunis
proportionaltothecubeofthemajorsemi-axisofitsorbit.
Copyright © 2005 IOP Publishing Ltd.