超快光学 第19章 放大.ppt

The Amplification of Ultra-short Laser Pulses,Francois Salin Center for Intense Lasersand Applications(CELIA)Universit Bordeaux I,Francesalincelia.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,Rice,Pulse compressor,t,t,Solid state amplifier,t,Dispersive delay line,t,Short pulse oscillator,Mostof this lecture courtesy of,Pulse energy vs.Repetition rate,Rep rate(pps),Pulse energy(J),Oscillator,Cavity-dumped oscillator,RegA,Regen,Regen+multipass,Regen+multi-multi-pass,1 W average power,A t,What are the goals in ultrashort pulse amplification?,Ipeak=,E,Increase the energy(E),Decrease the duration(t),Decrease the area of the focus(A).,Maximum intensity on target,Needed to start the experiment,Needed to get useful results,Pave=E r,Signal is proportional to the number of photons on the detector per integration time.,Maximum average power at the detector,Pulseenergy,Rep rate,Pulseenergy,Beam area,Pulse length,Issues in Ultrafast Amplification and Their Solutions,Pulse 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 money,Cavity Dumping,Before 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.”,Cavity dumping:the Pockels cell,A 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.,V,If 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),Polarizer,Amplification of Laser Pulses,in General,Very 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 oscillator,Amplifier medium,Pump,Energy levels,Jpump(lpump/lL),lL,lpump,Single-pass Amplification Math,Assume 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:,Jsto=stored pump fluence=Jpump(lpump/lL)Jsat=saturation fluence(material dependent),Single-pass Amplification Math,where 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 equation,G,0,exp(,g,0,L),exp(,J,sto,J,sat,),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.,Gain,Extraction efficiency(Jout/Jsto),J,sto,/J,sat,Jout/Jin,Another problem with amplifying ultrashort laser pulses,Another 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.,time,pump,pump,time,Energy 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.,The ultrashort pulse returns many times to eventually extract most of the energy.,Two main amplification methods,Multi-pass amplifier,Regenerative amplifier,Another multi-pass amplifier,A Pockels cell(PC)and a pair of polarizers are used to inject a single pulse into the amplifier.,Regenerative amplifier geometries,This is used for 10-20-Hz repetition rates.It has a larger spot size in the Ti:sapphire rod.,The 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 Amplification,Chirped-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 1983,CPA is THE big development.,Pulse compressor,t,t,Solid state amplifier,t,Dispersive delay line,t,Short pulse oscillator,Stretching and compressing ultrashort pulses,Okay,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 stretcher,A pulse stretcher,This device stretches an 18-fs pulse to 600 psa factor of 30,000!A ray trace of the various wavelengths in the stretcher:,Alexandrite,Ti:sapphire,Excimers,Nd:Glass,Dyes,Direct Amplification,Fluence(J/cm2),Pulse Duration(fs),CPA vs.Direct Amplification,CPA achieves the fluence of long pulses but at a shorter pulse length!,Regenerative Chirped-Pulse Amplification at 100 kHz rep rates with a cw pump,Coherent RegA amplifier,A 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 kHz,Spectra Physics regen:the“Spitfire”,Pump laser for ultrafast amplifiers,Coherent“Corona”,high power,Q-switched green laser in a compact and more reliable diode-pumped package,15 mJ(ns)at a 10 kHz rep rate(150W ave power!),Extracted energy,Beam diameter,Pump power 100 W,Average Power,Rep rate,Average power for high-power Ti:Sapphire regens,These 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 efficiency,Amplified 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 example,Ti:sapphire gain cross section,10-fs sech2 pulse in,Normalized spectral intensity,Cross section(*10-19 cm2),Wavelength(nm),65-nm FWHM,32-nm FWHM,Factor of 2 loss in bandwidth for 107 gainMost Terawatt systems have 1010 small signal gain,longer pulse out,Beating gain narrowing,Introduce some loss at the gain peak to offset the high gain there.,Gain and loss,Spectrum:before and after,Gain-Narrowing Conclusion,Gain 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:XX,Intensity(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=,E,A T,We 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 aberrations,Thermal Effects in Amplifiers,Low temperature minimizes lensing.,Calculations for kHz systemsCryogenic cooling results in almost no focal power,In sapphire,conductivity increases and dn/dT decreases as T decreases.,Murnane,Kapteyn,and coworkers,Static Wave-front Correction,2.5 times improvement in peak intensity has been achieved,CUOS,Dynamic Correction of Spatial Distortion,50 mm diameter37 actuators,CUOS,Contrast ratio,Why 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 amplifier,Picosecond scale:reflections in the amplifierspectral phase or amplitude distortions,0-1-2-3-4-5-6-7-8-9-10,Front,Back,time,Spectral phase aberrations,Pre-pulses,ASE,0,ps,10 ns,ns,FWHM,Amplified pulses often have poor contrast.,Log(Energy),Pre-pulses do the most damage,messing up a medium beforehand.,Typical 3rd-order autocorrelation,Amplified 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.,V,Time,Voltage,fewns,Switching high voltage twice in a few ns is quite difficult,requiring avalanche transistors,microwave triodes,or other high-speed electronics.,amplifier,oscillator,stretcher,compressor,Pockels cells,10-2-10-3,10 ns,Pockels cells suppress pre-and post-pulses.,Unfortunately,Pockels cells arent perfect.They leak 1%.,Contrast improvement recipes,A 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(2 ns 10-90%)collimated beamsTemperature drift is also a problem in Pockels cells.Also:Good pump synchronization gives a factor 3-10,Multiple-stage multi-pass amplifiers,1 kHz Multi-pass system at the University of Colorado(Murnane and Kapteyn),0.2 TW,4 mJ,20 fs pulse length,Closed loop cryogenic cooling 100K,5 x 1J,20 Hz Nd:YAG lasers,10 fs oscillator,stretcher,100 Hz Regenerative amplifier,200 mJ,30 fs,100 Hz,High energy,high contrast 100-Hz system at CELIA,Pump beam,Ultrashort pulse-near field,Ultrashort pulse-far field,Amplified-pulse beam shapes,A 1-Joule Apparatus,Multi-Joule Systems,Nd:GlassEnergy 20 JoulesPulsewidth 40 TWRepetition rates every hour,Terawatt Laser SystemTi:sapphireEnergy 1 JoulePulsewidth 10 TWRepetition rates to 1 kHz,You can buy these lasers!,Lawrence Livermore National Labs High-Power Amplifiers,Laser Specifications,Even Higher Intensities!,National Ignition Facility(under construction),192 shaped pulses10.4 kJ per beam in UV(done)21 kJ per beam in IR(done)1.8 MJ total energy(planned)Pulses 0.2 to 25 ns in length,What to do with such high intensities,