超快光学 第19章 放大课件

1、超快光学 第19章 放大,The Amplification of Ultra- short Laser Pulses,Francois Salin Center for Intense Lasers and Applications (CELIA) Universit Bordeaux I, France -bordeaux.fr Gilles Darpentigny (CELIA), Vincent Bagnoud (LLE) Antoine Courjaud, Clemens Honninger, Eric Mottay (Amplitude Systemes), Luc Vigroux

2、 (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,Most of this lecture courtesy of,超快光学 第19章 放大,Pulse energy vs. Repetition rate,Rep rate (pps),Pulse energy (J),Oscillator,Cavity-du

3、mped oscillator,RegA,Regen,Regen + multipass,Regen + multi-multi-pass,1 W average power,超快光学 第19章 放大,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

4、 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,Pulse energy,Rep rate,Pulse energy,Beam area,Pulse length,超快光学 第19章 放大,Issues in Ultrafast Amplification and Their Solut

5、ions,Pulse length discrepancies: Multi-pass amplifiers and regenerative amplifiers (“Regens”). Damage: Chirped-Pulse Amplification (CPA) Gain saturation: Frantz-Nodvick Equation Gain narrowing: Birefringent filters Thermal effects: cold and wavefront correction Satellite pulses, Contrast, and Amplif

6、ied Spontaneous Emission: Pockels cells Systems cost lots of money: Earn more money,超快光学 第19章 放大,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

7、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.”,超快光学 第19章 放大,Cavity dumping: the Pockels cell,A Pockels cell is a device that can switch a puls

8、e (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 cel

9、l 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,超快光学 第19章 放大,Amplification of Laser Pulses, in General,Very simply, a powerful l

10、aser 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,超快光学 第19章 放大,Single-pass Amplification

11、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 betwee

12、n the two:,Jsto= stored pump fluence = Jpump (lpump/lL) Jsat= saturation fluence (material dependent),超快光学 第19章 放大,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 sa

13、turated amplifier:,超快光学 第19章 放大,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

14、,Jout/Jin,超快光学 第19章 放大,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 arrive

15、s 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.,超快光学 第19章 放大,So we need many passes.,All ultrashort-pulse amplifiers are multi-pass.,This approach achieves much greater efficiency.,The ultrashort pulse

16、returns many times to eventually extract most of the energy.,超快光学 第19章 放大,Two main amplification methods,Multi-pass amplifier,Regenerative amplifier,超快光学 第19章 放大,Another multi-pass amplifier,A Pockels cell (PC) and a pair of polarizers are used to inject a single pulse into the amplifier.,超快光学 第19章

17、放大,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.,超快光学 第19章 放大,Pulse intensities inside an am

18、plifier 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?,超快光学 第19章 放大,Chirped-Pulse Amplification,Chirped

19、-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,Dispersi

20、ve delay line,t,Short pulse oscillator,超快光学 第19章 放大,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

21、 can compress the pulse.,Pulse stretcher,超快光学 第19章 放大,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:,超快光学 第19章 放大,Alexandrite,Ti:sapphire,Excimers,Nd:Glass,Dyes,Direct Amplification,Fluence (J/cm2),Pulse Du

22、ration (fs),CPA vs. Direct Amplification,CPA achieves the fluence of long pulses but at a shorter pulse length!,超快光学 第19章 放大,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 pro

23、vides 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!,超快光学 第19章 放大,Regenerative chirped-pulse amplification with a kHz pulsed pump,Wavelength: 800 nm (Repetition rates of 1 to 50 kHz) High Energy: 2

24、 mJ at 1 kHz Picosecond: 80 ps, 0.7 mJ at 1 kHz Short Pulse: 0.7 mJ at 1 kHz,Spectra Physics regen: the “Spitfire”,超快光学 第19章 放大,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

25、 (150W ave power!),超快光学 第19章 放大,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.,超快光学 第19章 放大,CPA is the

26、 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 efficiency Gain narrowing Thermal aberrations Contrast

27、ratio Damage threshold vs extraction efficiency,超快光学 第19章 放大,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 th

28、e amplifier at t = 0 ASE shares the gain and the excited population with the pulse.,Amplification reduces the contrast by a factor of up to 10.,超快光学 第19章 放大,Gain Narrowing (and ASE),On each pass through an amplifier, the pulse spectrum gets multiplied by the gain spectrum, which narrows the output s

29、pectrumand 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),超快光学 第19章 放大,Gain narrowing example,Ti:sapphire gain cross section,10-fs sech2 pulse in,Normalized spectral intensity,Cro

30、ss section (*10-19 cm2),Wavelength (nm),65-nm FWHM,32-nm FWHM,Factor of 2 loss in bandwidth for 107 gain Most Terawatt systems have 1010 small signal gain,longer pulse out,超快光学 第19章 放大,Beating gain narrowing,Introduce some loss at the gain peak to offset the high gain there.,Gain and loss,Spectrum:

31、before and after,超快光学 第19章 放大,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:sapphire Sub-200 fs in Nd:glass Sub-100 fs in Yb:XX,超快光学 第19章 放大,Intensity (arb. units),Wavelength (nm),Very broad spectra can be cr

32、eated this way.,A 100-nm bandwidth at 800 nm can support a 10-fs pulse.,超快光学 第19章 放大,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 aberrat

33、ions 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,超快光学 第19章 放大,Low temperature minimizes lensing.,Calculations

34、 for kHz systems Cryogenic cooling results in almost no focal power,In sapphire, conductivity increases and dn/dT decreases as T decreases.,Murnane, Kapteyn, and coworkers,超快光学 第19章 放大,Static Wave-front Correction,2.5 times improvement in peak intensity has been achieved,CUOS,超快光学 第19章 放大,Dynamic Co

35、rrection of Spatial Distortion,50 mm diameter 37 actuators,CUOS,超快光学 第19章 放大,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 i

36、n 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/cm2 so at 1021 W/cm2 we need a 1010 contrast ratio!,超快光学 第19章 放大,Major sources of poor contrast,Nanosecond

37、scale: pre-pulses from oscillator pre-pulses from amplifier ASE from amplifier,Picosecond scale: reflections in the amplifier spectral phase or amplitude distortions,超快光学 第19章 放大,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.,超快光学 第19章 放大,Typical 3rd-order autocorrelation,Amplified pulses have pre- and post-pulses.,超快光学 第19章 放