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The Amplification of Ultra-short Laser PulsesFrancois Salin Center for Intense Lasersand Applications(CELIA)Universit Bordeaux I,Francewww.celia.u-bordeaux.frsalincelia.u-bordeaux.frGilles Darpentigny(CELIA),Vincent Bagnoud(LLE)Antoine Courjaud,Clemens Honninger,Eric Mottay(Amplitude Systemes),Luc Vigroux(Amplitude Technologies)and some additional stuff from Dan Mittleman,RicePulse compressorttSolid state amplifiertDispersive delay linetShort pulse oscillatorMostof this lecture courtesy ofPulse energy vs.Repetition rateRep rate(pps)Pulse energy(J)10910610310010-310-910-610010-3OscillatorCavity-dumped oscillatorRegARegenRegen+multipassRegen+multi-multi-pass1 W average powerA tWhat are the goals in ultrashort pulse amplification?Ipeak=EIncrease the energy(E),Decrease the duration(t),Decrease the area of the focus(A).Maximum intensity on targetNeeded to start the experimentNeeded to get useful resultsPave=ErSignal is proportional to the number of photons on the detector per integration time.Maximum average power at the detectorPulseenergyRep ratePulseenergyBeam areaPulse lengthIssues in Ultrafast Amplification and Their SolutionsPulse length discrepancies:Multi-pass amplifiers and regenerative amplifiers(“Regens”).Damage:Chirped-Pulse Amplification(CPA)Gain saturation:Frantz-Nodvick EquationGain narrowing:Birefringent filtersThermal effects:cold and wavefront correctionSatellite pulses,Contrast,and Amplified Spontaneous Emission:Pockels cellsSystems cost lots of money:Earn more moneyPockels cellpolarizergainpumpinput/outputCavity DumpingBefore we consider amplification,recall that the intracavity pulse energy is 50 times the output pulse energy.So we have more pulse energy.How can we get at it?What if we instead used two high reflectors,let the pulse energy build up,and then switch out the pulse?This is the opposite of Q-switching:it involves switching from minimum to maximum loss,and its called“Cavity Dumping.”E=Toutput EintracavityTransmission of output coupler:2%R=100%R=98%EintracavityECavity dumping:the Pockels cellA Pockels cell is a device that can switch a pulse(in and)out of a resonator.Its used in Q-switches and cavity dumpers.A voltage(a few kV)can turn a crystal into a half-or quarter-wave plate.VIf V=0,the pulse polarization doesnt change.If V=Vp,the pulse polarization switches to its orthogonal state.Abruptly switching a Pockels cell allows us to extract a pulse from a cavity.This allows us to achieve 100 times the pulse energy at 1/100 the repetition rate(i.e.,100 nJ at 1 MHz).Pockels cell(voltage may be transverse or longitudinal)PolarizerAmplification of Laser Pulses,in GeneralVery simply,a powerful laser pulse at one color pumps an amplifier medium,creating an inversion,which amplifies another pulse.Nanosecond-pulse laser amplifiers pumped by other ns lasers are commonplace.Laser oscillatorAmplifier mediumPumpEnergy levelsJpump(lpump/lL)lLlpumpSingle-pass Amplification MathAssume a saturable gain medium and J is the fluence(energy/area).Assume all the pump energy is stored in the amplifier,but saturation effects will occur.At low intensity,the gain is linear:At high intensity,the gain“saturates”and hence is constant:Intermediate case interpolates between the two:Amplifier mediumpumpJinJpumpJoutJsatlLlpumpJstoJsto=stored pump fluence=Jpump(lpump/lL)Jsat=saturation fluence(material dependent)Single-pass Amplification Mathwhere the small signal gain per pass is given by:This differential equation can be integrated to yield the Frantz-Nodvick equation for the output of a saturated amplifier:Frantz-Nodvick equationG0 exp(g0L)exp(JstoJsat)Higher pumping(Jsto)means higher efficiency and higher saturation and so lower gain.So you can have high gain or high extraction efficiency.But not both.11,21,41,61,822,22,42,600,20,40,60,81012345GainExtractionefficiency(Jout/Jsto)Jsto/JsatJout/JinAnother problem with amplifying ultrashort laser pulsesAnother issue is that the ultrashort pulse is so much shorter than the(ns or ms)pump pulse that supplies the energy for amplification.So should the ultrashort pulse arrive early or late?Early:Late:Pump energy arrives too late and is wasted.timepumppumptimeEnergy decays and is wasted.In both cases,pump pulse energy is wasted,and amplification is poor.So we need many passes.All ultrashort-pulse amplifiers are multi-pass.This approach achieves much greater efficiency.timepumpThe ultrashort pulse returns many times to eventually extract most of the energy.Two main amplification methodsMulti-pass amplifierpumpinputoutputgainPockels cellpolarizergainpumpinput/outputRegenerative amplifierAnother multi-pass amplifierA Pockels cell(PC)and a pair of polarizers are used to inject a single pulse into the amplifier.Regenerative amplifier geometriesThis is used for 10-20-Hz repetition rates.It has a larger spot size in the Ti:sapphire rod.Pockels cellFaraday rotatorthin-film polarizerPockels cellThe Ti:Sapphire rod is 20-mm long and doped for 90%absorption.This design is often used for kHz-repetition-rate amplifiers.Pulse intensities inside an amplifier can become so high that damage(or at least small-scale self-focusing)occurs.Solution:Expand the beam and use large amplifier media.Okay,we did that.But thats still not enough.Solution:Expand the pulse in time,too.Okay,so what next?Chirped-Pulse AmplificationChirped-pulse amplification in-volves stretching the pulse before amplifying it,and then compressing it later.We can stretch the pulse by a factor of 10,000,amplify it,and then recompress it!G.Mourou and coworkers 1983CPA is THE big development.Pulse compressorttSolid state amplifiertDispersive delay linetShort pulse oscillatorStretching and compressing ultrashort pulsesdf2ffdgratinggratingOkay,this looks just like a“zero-dispersion stretcher”used in pulse shaping.But when d f,its a dispersive stretcher and can stretch fs pulses by a factor of 10,000!With the opposite sign of d-f,we can compress the pulse.Pulse stretcherA pulse stretcherThis device stretches an 18-fs pulse to 600 psa factor of 30,000!A ray trace of the various wavelengths in the stretcher:Pulse stretcher characteristics:Inputpulsewidth:18fsOutputpulseduration:600psBandwidthpassed:105nmPulseenergyout:0.5nJAlexandriteTi:sapphireExcimers0,00010,0010,010,11101001101001000104105106Nd:GlassDyesDirectAmplificationFluence(J/cm2)Pulse Duration(fs)CPA vs.Direct AmplificationCPA vs.Direct AmplificationCPA achieves the fluence of long pulses but at a shorter pulse length!Regenerative Chirped-Pulse Amplification at 100 kHz rep rates with a cw pumpCoherent RegA amplifierA fs oscillator requires only 5 W of green laser power.An Argon laser provides up to 50 W.Use the rest to pump an amplifier.Today,we use an intracavity-doubled Nd:YLF pump laser(10W).Microjoules at 250 kHz repetition rates!Regenerative chirped-pulse amplification with a kHz pulsed pump Wavelength:800 nm(Repetition rates of 1 to 50 kHz)High Energy:2 mJ at 1 kHz Picosecond:80 ps,0.7 mJ at 1 kHzShort Pulse:0.7 mJ at 1 kHzSpectra Physics regen:the“Spitfire”Pump laser for ultrafast amplifiersCoherent“Corona”high power,Q-switched green laser in a compact and more reliable diode-pumped package15 mJ(ns)at a 10 kHz rep rate(150W ave power!)1 kHz10 kHz100 kHz20 mJ1.8 mJ0.2 mJ20 W18 W20 W3 mm1 mm250 mExtracted energyBeam diameterPump power 100 WAverage PowerRep rateAverage power for high-power Ti:Sapphire regensThese average powers are high.And this pump power is also.If you want sub-100fs pulses,however,the energies will be less.CPA is the basis of thousands of systems.Its available commercially in numerous forms.It works!But there are some issues,especially if you try to push for really high energies:Amplified spontaneous emission(ASE)Gain saturation:gain vs.extraction efficiencyGain narrowingThermal aberrationsContrast ratioDamage threshold vs extraction efficiencyAmplified Spontaneous Emission(ASE)Fluorescence from the gain medium is amplified before(and after)the ultrashort pulse arrives.This yields a 10-30 ns background with low peak power but large energy.Depends on the noise present in the amplifier at t=0ASE shares the gain and the excited population with the pulse.Amplification reduces the contrast by a factor of up to 10.Gain Narrowing(and ASE)On each pass through an amplifier,the pulse spectrum gets multiplied by the gain spectrum,which narrows the output spectrumand lengthens the pulse!As a result,the pulse lengthens,and it can be difficult to distinguish the ultrashort pulse from the longer Amplified Spontaneous Emission(ASE)Gain narrowing exampleTi:sapphire gain cross section10-fs sech2 pulse in00.20.40.60.8100.511.522.536507007508008509009501000Normalized spectral intensityCross section(*10-19 cm2)Wavelength(nm)65-nm FWHM32-nm FWHMFactor of 2 loss in bandwidth for 107 gainMost Terawatt systems have 1010 small signal gainlonger pulse outBeating gain narrowingBirefringent platePolarizerPolarizerEEEwith filterwithout filterSpectrumWavelength(nm)Gain&modulation650850900Wavelength(nanometers)70075080011.522.595020%GainmodulationBeforeAfterIntroduce some loss at the gain peak to offset the high gain there.Gain and lossSpectrum:before and afterGain-Narrowing ConclusionGain narrowing can be beaten.We can use up to half of the gain bandwidth for a 4-level system.Sub-20 fs in Ti:sapphireSub-200 fs in Nd:glassSub-100 fs in Yb:XX00.20.40.60.81750800850Intensity(arb.units)Wavelength(nm)Very broad spectra can be created this way.A 100-nm bandwidth at 800 nm can support a 10-fs pulse.Heat deposition causes lensing and small-scale self-focusing.These thermal aberrations increase the beam size and reduce the available intensity.Ipeak=EA TWe want a small focused spot size,but thermal aberrations increase the beam size,not to mention screwing it up,too.Now the average power matters.The repetition rate is crucial,and wed like it to be high,but high average power means more thermal aberrationsThermal Effects in AmplifiersLow temperature minimizes lensing.Calculations for kHz systemsCryogenic cooling results in almost no focal powerIn sapphire,conductivity increases and dn/dT decreases as T decreases.Murnane,Kapteyn,and coworkersStatic Wave-front Correction2.5timesimprovementinpeakintensityhasbeenachievedCUOSDynamic Correction of Spatial Distortion50 mm diameter37 actuatorsCUOSContrast ratioWhy does it take over 2 years between the first announcement of a new laser source and the first successful experiment using it?Because the pulse has leading and following satellite pulses that wreak havoc in any experiment.If a pulse of 1018 W/cm2 peak power has a“little”satellite pulse one millionth as strong,thats still 1 TW/cm2!This can do some serious damage!Ionization occurs at 1011 W/cm2so at 1021 W/cm2 we need a 1010 contrast ratio!Major sources of poor contrast Nanosecond scale:pre-pulses from oscillatorpre-pulses from amplifierASE from amplifierPicosecond scale:reflections in the amplifierspectral phase or amplitude distortions 0-1-2-3-4-5-6-7-8-9-10FrontBacktimeSpectral phase aberrationsPre-pulsesASE0ps10 nsnsFWHMAmplified pulses often have poor contrast.Log(Energy)Pre-pulses do the most damage,messing up a medium beforehand.Typical 3rd-order autocorrelationAmplified pulses have pre-and post-pulses.A Pockels cell“Pulse Picker”A Pockels cell can pick a pulse from a train and suppress satellites.To do so,we must switch the voltage from 0 to kV and back to 0,typically in a few ns.VTimeVoltagefewnsSwitching high voltage twice in a few ns is quite difficult,requiring avalanche transistors,microwave triodes,or other high-speed electronics.amplifieramplifieroscillatoroscillatorstretcherstretchercompressorcompressorPockels cellsPockels cells1010-2-2-10-310 ns10 nsPockels cells suppress pre-and post-pulses.Unfortunately,Pockels cells arent perfect.They leak 1%.Contrast improvement recipesA Pockels cell improves the contrast by a few 100 to 1000.We need at least 3 Pockels cells working in the best conditions:on axis(do not tilt Pockels cells)broadband high-contrast polarizers(not dielectric)fast rise time(20 JoulesPulsewidth 40 TWRepetition rates every hourTerawatt Laser SystemTi:sapphireEnergy 1 JoulePulsewidth 10 TWRepetition rates to 1 kHzYou can buy these lasers!Lawrence Livermore National Labs High-Power AmplifiersLaser SpecificationsUSP 1996USP(In Development)Janus 1996Total Energy1 Joule10 Joule6-10 JoulePulse Length100 fs30-100 fs600 fsPeak Intensity5.e19 W/cm25-10.e20 W/cm25.e18 W/cm2Wavelength800 nm,400 nm800 nm,400 nm1053 nm,526 nmPulse Contrast10-7 in 400 nm1.e-7 in 400 nm1.8 MJ total energy(planned)Pulses 0.2 to 25 ns in lengthWhat to do with such high intensities