The Supercontinuum Laser Source: The Ultimate White Light

452 Pages·2016·12.978 MB·English

by  Robert R. Alfano (auth.)

Robert R. Alfano
The
Supercontinuum
Laser Source
The Ultimate White Light
Third Edition
The Supercontinuum Laser Source

Robert R. Alfano
The Supercontinuum
Laser Source
The Ultimate White Light
Third Edition
With 259 Illustrations
RobertR.Alfano
DepartmentofPhysics
NewYork,NY,USA
ISBN978-1-4939-3324-2 ISBN978-1-4939-3326-6 (eBook)
DOI10.1007/978-1-4939-3326-6
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Preface to the Third Edition
The “white-light continuum” called “Supercontinuum” (SC) is generated using
ultrafast laser pulses propagating in solids, liquids, gases, and various designs of
opticalfibers(singlemode,multimode,microstructured,etc.).TheSClightsource
has become the ultimate white-light source, spread worldwide, and has advanced
ultrafast laser spectroscopy, condensed matter, biology, physics, and chemistry.
Specificapplicationsincludehigh-precisionopticalfrequencyandtimemetrology
(developmentofextremelyaccurateclocksandfrequencysynthesis),high-capacity
encoding anddecodingofinformationtoterabits/seccommunication (wavelength
divisionmultiplexing), imaging (optical coherence tomography), light pulse com-
pression, ultrafast laser pulse generation (attosecond pulses), and extremely accu-
rateclockdevelopment.Ithasalsobeentheenablingtechnologyintheresearchthat
ledtotwoNobelprizes.
SC generation starts with a physical effect known as self-phase modulation
(SPM).SPMisanonlinearopticaleffectwherethelasermodulatesitsownphase
as light propagates through the material. Afterwards, a number of other nonlinear
and linear effects further broaden the spectra often extending over hundreds of
nanometers.TheSClightsourcesarereadilyavailableandhavebeencommercial-
izedbymajoropticscompanies.
The SC has become one of the important light sources used for research in
optical and photonic sciences. This third edition of The Supercontinuum Laser
Source book reviews the progress achieved on the experimental and theoretical
understanding of the ultrafast nonlinear and linear processes responsible for the
supercontinuum generation andrelated applicationsoccurringover 46 years since
itsdiscoverybyRobertR.AlfanoandStanleyShapiroin1969.
Thereisgreatneedforasequelparttothesecondeditionofthebookcovering
therecentworldsurgeofresearchactivityonthesupercontinuumphenomenaand
the numerous technological applications that have occurred over the past 8 years.
Since the time the second edition was printed in 2006, over 2,860 articles
were published in scientific magazines out of over 3,580 published since the SC
discovery. See the dynamics of SC publications by year in the Figure below
v
vi PrefacetotheThirdEdition
(2015—January to June). These publications were cited over 69,000 times. This
testifiesenormousinterestSCexperiencesinrecentyears.
400
300
s
n
o
ti
a
c 200
bli
u
p
C
S
100
0
1985 1995 2005 2015
Year
Thevoidafterthesecondeditionwillpartlybecoveredinthisnewrejuvenated
edition, keeping key articles from the first and second editions and covering new
breakthroughs to understand the supercontinuum and its new diverse applications
includingSCgenerationandpropagationinall-normaldispersionfibers,attosecond
lightpulsegeneration,anddeepUVgeneration.
The supercontinuum is the generation of intense ultrafast broadband “white-
light”pulseswithspectra spanningfromtheultraviolettothenear-infraredwave-
lengthsthatarisesfromthenonlinearinteractionandpropagationofultrafastpulses
focused into a transparent material. The supercontinuum can be generated in
different states of matter—condensed matter (liquids and solid), gases, and struc-
turedmedia.Thesupercontinuumisoneofthemostdramaticandeleganteffectsin
opticalphysics.Theconversionofonecolortowhitelightisastartlingresult.This
isamulticoloredlightwithmanyofthesamedesirablepropertiesasconventional
laser light: intense, collimated, and coherent. The supercontinuum has a beam
divergence as good as of the input laser pulse. Moreover, the coherence lengthof
the supercontinuum is comparable with that of an incoherent white-light source
fromalightbulb.Theinterferencepatternmeasuredforthesupercontinuumfroma
pair of filaments in water shows a constant phase relationship between the
supercontinuumproducedbyeachfilament.Thereisaconstantphaserelationship
between the pump laser pulse and its supercontinuum. The white-light
supercontinuum is an ideal tunable ultrafast white-light laser source.
Supercontinuumhasovertakenthestudyofothernonlinearopticaleffectssuchas
secondharmonicgeneration(SHG)andtwo-photonabsorptionforusefulnessina
numberofdiverseapplications.Thesupercontinuumfieldismoreactivethanever
in46yearsandisfindingtodaynewandnoveluses.
PrefacetotheThirdEdition vii
Variousprocessesareinvolvedinthesupercontinuumgeneration.Wheneveran
intenseultrashortlaserpulsepropagatesthroughamedium,itchangestherefractive
indexfromthedistortionoftheatomicandmolecularconfiguration,whichinturn
changesthephase,amplitude,andfrequencyoftheincidentpulse.Thephase and
amplitudechangecancauseafrequencysweepofthecarrierwavewithinthepulse
envelope andcanaltertheenvelopeandspatialdistribution(self-focusing).There
arevariousmechanismsresponsiblefortheindexofrefractionchangewithinten-
sity in material. The frequency broadening mechanisms are electronic cloud dis-
tortion, reorientation, librations, vibrational and molecular redistribution, to name
themajorones.Theoperationofthesemechanismsdependsonitsrelaxationtime
relevanttothelaserpulseduration.Therelaxationtimesassociatedwithelectronic
distributionisoftheorderofBohrorbittime~150as;reorientationtimeis~1ps;
rocking and libration response about the field is ~1 ps; vibrational dephasing is
~0.1ps;andmolecularmotionis~1ps.Mostofthesemechanismsareinvolvedin
thesupercontinuumgenerationwith100fstopslaserpulses.
Soonafterthesupercontinuumdiscoveryin1969,initiallyitfoundapplications
in time-resolved pump-supercontinuum probe absorption and excitation spectros-
copy to study the fundamental picosecond (10(cid:1)12 s) and femtosecond (10(cid:1)15 s)
processes that occur in biology, chemistry, and solid-state physics. Briefly, in
biology, the primaryevents inphotosynthesis andvision were explored;inchem-
istry,abetterunderstandingofthebasicchemicaldynamicalstepsinreactionsand
nonradiative processes in photoexcited chemical was achieved; and in solid-state
physics, the underlying kinetics of how elementary excitations behave and relax,
suchasopticalphonons,polaritons,excitons,carriers(electronsandholes)dynam-
ics among the intervalleys and intravalleys of semiconductors as well as spin
dynamics,wereunraveled.
With the advent of microstructure fibers, there has been a rebirth of the
supercontinuumfield inthe typeofapplications inwhich thesupercontinuumcan
playadecisiverole.Theseapplicationsincludefrequencyclocks,phase stabiliza-
tion and control, timing, optical coherence tomography (OCT), ultrashort pulse
compression, optical communication, broad-spectrum LIDAR, atmospheric sci-
ence,UVpulsegeneration,lightingcontrol,attosecond(10(cid:1)18s)pulsegeneration,
andcoherencecontrol.MostrecentlySCwasusedinsuper-resolutionmicroscopy
and has entered biomedical optics field. Most recently, the Supercontinuum was
coupledasalightsourceformicroscopes.
Over the past several years, supercontinuum generation in microstructure pho-
tonic crystal fibers by ultrashort pulse propagation has become a subject of great
interest worldwide. The main reasons are the low pulse energies required to
generate the supercontinuum; its coherences and high brightness makes the con-
tinuumanidealwhite-lightsourcefordiverseapplications;andtheeffectsofzero
dispersion and anomalous dispersion regions have resulted in higher-order solu-
tions generation, pulse compression, and an ultrabroadband continuum exceeding
1,000 nm and extending from the ultraviolet to the infrared spectral regions. The
development of all-normal dispersion fibers leads to the cleaner single
supercontinuumpulsegenerationforpump-probestudies.
viii PrefacetotheThirdEdition
In microstructural fibers, when pump wavelength lies in an anomalous disper-
sion region, it is the solitons that initiate the formation of the continuum. In a
normal dispersion region, self-phase modulation is the process that initiates the
continuumgeneration.Thecombinationoffour-wavemixingandRamanprocesses
extendsthespectralwidthofthecontinuum.Inthatregard,thepulsedurationofan
ultrafast laser determines the operational mechanisms—for 10 fs to 1,000 fs laser
pulses, self-phase modulation (SPM) and soliton generation dominate; and for
pulses >30 ps, stimulated Raman and four-wave mixing play a major role in
extending the spectra. Of course, the pump wavelength location relative to the
zerodispersionwavelengthandtheanomalousdispersionregionplaysaroleinthe
activemechanismandcoherenceregionofthesupercontinuum.Amajoradvancein
thesupercontinuumpulsegenerationoccurswithall-normaldispersionmicrostruc-
turalfibersviaSPM.Thesupercontinuumspectracanspanmorethanatwo-optical
octavebandwidthspreadfrom380nmto1,600nmusing200fspulseswithenergy
in the tens of nanojoules. The shorter wavelength UV generation is limited by
multiphoton absorption. SC generation with deeper UV spectra component up to
200 nm can be achieved with tapered fibers. The deep UV supercontinuum will
becomeusefulforproteinandgeneresearch.
Thespanoveranoctave(i.e.,450–900nm)isimportantincontrollingthephase
of the carrier wave inside the pulse envelope of a mode-locked pulse train. Using
thefand2fwavesinthesupercontinuum,thecarrier-envelopeoffset(CEO)phase
can be detected using heterodyne beating between the high-frequency end of the
supercontinuum and the double low end frequency of the supercontinuum in an
interferometer. These phase-controlling effects are important for maintaining the
accuracyoffrequencycombsforclockingandtiminginmetrology,high-intensity
atomicstudies,andattosecondpulsegeneration.
The increasing worldwide demand for large-capacity optical communication
systems needs to incorporate both the wavelength and the time. The ultrabroad
bandwidth andultrashort pulsesof the supercontinuummay be the enabling tech-
nology to produce a cost-effective superdense wavelength division multiplexing
(>1,000 λ) and time multiplexing for the future Terabits/s to Pentabits/s commu-
nication systems and networks. The supercontinuum is an effective way to obtain
numerouswavelengthchannelsbecauseiteasilygeneratesmorethan1,000optical
longitudinal modes while maintaining their coherency. Next advance of SC is in
superresolutionmicroscopy.
Thisthirdeditionconsistsofkeychaptersfromsecondedition,revisedchapters,
andnewchapters.Themajorportionofthebookwillbefromthepasteditions.These
chapterslaydowntheunderstandingandfoundationofthebirthandevolutionofthe
supercontinuumfield.Theygooversalientexperimentalandtheoreticalconceptsof
theresearchworksuptodate.
SpecialthankstoDr.KestutisSutkusforhisassistanceintheproductionofthe
thirdedition.
NewYork,NY,USA RobertR.Alfano
Preface to the Second Edition
The“supercontinuum”(SC)hasbecomeoneofthehottesttopicstostudyinoptical
and photonic sciences since the first book on the supercontinuum was published,
entitled The Supercontinuum Laser Source, by Springer in 1989. That book, now
becoming Part I in this second edition, reviewed the progress achieved on the
experimental and theoretical understanding of the ultrafast nonlinear and linear
processes responsible for the supercontinuum generation and related applications
occurring over 20 years since its discovery by Robert R. Alfano and Stanley
Shapiroin1969.
There is a great need for a sequel part covering the recent worldwide surge of
researchactivityonthesupercontinuumphenomenaandthenumeroustechnolog-
icalapplicationsthathaveoccurredoverthepast15years.Thisvoidwillpartlybe
coveredinthisnewrejuvenatedsecondedition,calledPartII,byanoverviewofthe
recent advanceswith anupdated compendiumofreferencesonthe various break-
throughstounderstandthesupercontinuumanditsnewdiverseapplications.
The supercontinuum is the generation of intense ultrafast broadband “white-
light” pulses spanning the ultraviolet to the near infrared that arises from the
nonlinearinteractionandpropagationofultrafastpulsesfocusedintoatransparent
material. The supercontinuum can be generated in different states of matter—
condensed media (liquids and solids) and gases. The supercontinuum is one of
the most dramatic and elegant effects in optical physics. The conversion of one
colortowhite-lightisastartlingresult.Thisismulticoloredlightwithmanyofthe
same desirable properties as conventional laser light: intense, collimated, and
coherent.Thesupercontinuumhasabeamdivergence asgoodasthatoftheinput
pump laser pulse. Moreover, the coherence length of the supercontinuum is com-
parable with that of an incoherent white-light source from a light bulb. The
interference pattern measured for the supercontinuum from a pair of filaments
inwatershowsaconstantphaserelationshipbetweenthesupercontinuumproduced
by each filament. There is a constant phase relationship between the pump
laser pulse and its supercontinuum. The white-light supercontinuum is an ideal
tunableultrafastwhite-lightlasersource.Supercontinuumhasovertakenthestudy
ix