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Wir schaffen Wissen – heute für morgen Paul Scherrer Institut Carlo Vicario Laser pulse shaping for high brightness electron source SLAC 03/09/2010 Introduction • The control of the laser pulse geometry is required in order to minimize the space charge non linear forces and implement effective emittance compensation schemes • Laser optimal 3D distribution beer can or uniform filled ellipsoid • Other qualifying laser parameters: – high energy, at the proper wavelength – limited jitters and fluctuations – Reliability – repetition rate • The beer can shape is obtained with • Flat top time shape • Pulse duration 10° RF resolution (rise time) better than 1° RF, ripple • Uniform hard-edge circular spot • Combined spatial and temporal shaping for ellipsoidal shape SLAC 03/09/2010 Seite 2 Outline • Incomplete overview of the laser pulse shaping approaches •Generation of flat top time pulse • Programmable frequency domain IR shapers – Liquid crystal mask spatial light modulator – Acusto Optic Programamble Dispersive Filter DAZZLER • Multi-stage pulse shapers – IR filters + UV shaping – UV dazzler •Time domain flat top – Un-coherent pulse stacking – Coherent pulse stacking •Laser transverse shaping •Strategy for the generation of uniformly filled ellipsoidal laser •Conclusions SLAC 03/09/2010 Seite 3 Frequency domain filters • The electro-optical devices can control the laser pulse at sub-ns resolution • Spectral amplitude and phase manipulation for time shaping at sub-ps resolution • • Large bandwidth source is desirable to high fidelity control the pulse profile: Ti: Sa. This systems are limited by repetition rate. Spectral amplitude and/or phase modulation through programmable filter Oscillator SLAC 03/09/2010 IR shaper CPA Harmonics Seite 4 Liquid crystal mask spatial light modulator Programmable liquid crystal (LC) array placed in the Fourier plane of a 4f system. Here, focused spectral components can be selectively attenuated or phase shifted when a external voltage is applied to the liquid crystal. Alignment difficult and limits for tunability Using LC SLM phase modulation driven by a genetic algorithm Yang et al. significantly improved the emittance. UV meas streak camera A. M. Weiner, Rev. Sci. Instrum. 71, 1929 (2000). Yang et al., J. Appl. Phys 92, 1608 (2002) SLAC 03/09/2010 03.09.2010 Seite 5 Acusto optic dipersive programmable filter: DAZZLER •A co-propagating acoustic generates a transient Bragg grating in a birefringent crystal, to diffract selectively the optical wavelengths. Pros Cons Programmable Efficiency < 50% Alignment λ= 650-1650 nm Tunability Eth=0.1GW/cm2 Phase & amplitude Max duration 10 ps relaibility Rep rate 1 KHz Dazzler by Fastlite Half --waveplate Dazzler Dazzler no ΔT=10ps rt=1ps TeO2 out λ =800 nm in ne C. Vicario et al proc EPAC 2004 SLAC 03/09/2010 03.09.2010 P. Tournois et al., Opt: Comm:, 140, (1997), 245 Seite 6 Amplification and UV conversion •The IR pulse shapers should pre-compensate the distortions experienced by the flat pulse in the amplifier and the harmonic conversion. •Amplifier distortions: red-shift, gain narrowing and high order phase can be effectively compensated •Harmonics distortions: –High efficiency implies small bandwidth –The spectrum can be strongly modified especially for short IR pulse. –Tilting of the non linear crystal determines asymmetry in the harmonic spectra. –The harmonic generation increase the ripples and worse the rise time. ω 2ω 3ω Dazzler UV pulse rt=2.1 ps Opt. Lett, 31,2006, 2885 SLAC 03/09/2010 7 Seite 7 Two stages time shaping: implemented at SPARC The long rise time could be improved by properly clipping the tails of the UV spectrum. This was accomplished by the UV stretcher in order to have allow spectral masking and variable pulse length. Pulse shaping IR DAZZLER and UV stretcher GAUSSIAN FLAT TOP Also starting from a gaussian spectrum (no IR shaper) and a proper cut is possible to achieve a flat top like profile in the stretcher spectrum time Simulations Appl. Opt. 46, 22 (2007) 4959 SLAC 03/09/2010 8 UV pulse shaping for FERMI MAIN FEATURES: • Fourier-domain shaping based on transmission diffraction gratings • 2- element deformable mirror (DM) with multilayer coating • Amplitude band-pass filtering by knife edges • The version shown below starts from a short UV pulse and includes variable stretching, a lower loss version (in use now) starts with a long UV pulse and does not include the stretching IN G2 G1 ∆∆ f OUT ∆∆ DM M1 L L f Cross-correlation traces of flat-top pulses with different duration 1.0 1.0 rms=4.5% 0.8 0.6 0.6 a.u. a.u. 0.8 rms=3.2% 0.4 0.4 0.2 0.2 0.0 0.0 0 2 4 6 8 TIME(ps) SLAC 03/09/2010 10 12 14 0 2 4 6 8 10 12 14 16 TIME(ps) Miltcho Danailov, FERMI Trieste 9 Laser system layout SwissFEL BS Ti:Sa oscillator- amplifier IR DAZZLER+ MAZZLER SHG CrossCorrelator THG Amplified Ti:SA laser 780-845nm DAZZLER and MAZZLER loop for wavelength tuning and transform limited pulse 25 fs @ 100Hz Cathode UV-Dazzler UV pulse stretcher: four prism setup capable of stretching to 3-10ps, without significant losses The UV pulse wavelength is spanned between 260-280 for best thermal emittance UV DAZZLER for temporal manipulation SLAC 03/09/2010 Seite 10 Wavelength tuning and thermal emittance Wavelength tuning Thermal emittance measurements εn/σx (mm.mrad / mm) 0.8 0.7 0.6 Cu Mo Nb Al Bronze εTh Formula (Φ0= 4.4 eV) εTh Formula (Φ0= 4.3 eV) εTh Formula (Φ0= 4.1eV) εTh Formula (Φ0= 3.9 eV) 0.5 0.4 0.3 4.3 SLAC 03/09/2010 4.4 4.5 4.6 4.7 4.8 Photon Energy (eV) Exp. Cond.: Q < 1 pC; 5.2 MeV σt,laser= 4 ps rms; DC field 25 MV/m 4.9 Seite 11 UV DAZZLER experiment at SwissFEL Commercial AODPF UV DAZZLER based on KDP crystal to work in the 250-410 nm range. Resolution 0.1 nm, rep.rate 1 KHz, maximum pulse length 5 ps with 7.5 cm crystal. Limit of the maximum input energy due to limited useful area in the crystal and to two photon absorption phenomenon. Pulse shaping on un-diffracted beam Time domain pulse shaping for alternative high efficiency approach Total transmitted energy SLAC 03/09/2010 19 % Diffracted energy 03.09.2010 Seite 12 Time domain pulse shaping: pulse stacking •The flat top distribution can be obtained by overlapping several delayed replica of a short pulse. •N Michelson-like interferometer cascaded for the generation of 2N replicas. •The rise time and the ripples depend on the input pulse, the final length by n° replicas •ps input required: laser alternative to the Ti:SA •Output interference between pulses is avoided by cross polarization and eventually chirped •Reduced losses •Alignment can be difficult OPTICAL SETUP Electron beam for different input length ……. Tomizawa, Quantum Electronics 37, 697 (2007) M. Y. Sheverdin, Proc. PAC07 LLNL SLAC 03/09/2010 Seite 13 Pulse stacking using bi-refringent crystal Use crystal birefringence to generate delayed replicas along the ordinary and extraordinary axes The time separation depend on the crystal length and on Δn t d = to − te = no L − ne L c +45° -45° +45° α-cut YVO4 for (for λ>400 nm) and α-cut BBO (λ>190 nm) Low losses for AR coated crystal. Poor flexibility BAZAROV et al. Phys. Rev. ST Accel. Beams 11, 040702 (2008) SLAC 03/09/2010 Seite 14 Coherent pulse stacking: DESY approach XFEL demands for flat top at MHz repetition rate drive laser Optical setup The pulse shaping is done by stacking of ps IR pulses that interfere at the output polarizer Pulse length depends on the number of crystals and the rise time and ripples depends on the input pulse length and the programmed phase Relative interpulse phase control is accomplished by tuning the crystals temperature with accuracy of 0.03 °C edges1.3-1.7 ps 10 crystals λ = 266 nm Crystal type YVO4 N° crystals 10-13 Crystal length 2.7 mm (Ne-No)/c 0.75 ps/mm Phase shift 2π / 67 C FWHM = 19.3 ps I. Will et al.; Optics Express 16 (2008) 14922 I. Will; NIM A 594 (2008) 119 SLAC 03/09/2010 Ingo Will, Guido Klemz, Max-Born-Institute Berlin, in cooperation with DESY Seite 15 Stacked pulse for different pulse length FWHM = 2.5 ps FWHM = 4 ps FWHM = 7 ps 2 5 ps 2 5 ps Input IR pulse 2 5 ps edges ~ 2ps edges ~ 3ps 2 4 ps ~ 2 3 ps 2 5 ps 2 5 ps edges ~ 6 ps ~ 19 ps UV shaped (13 crystals) 2 5 ps Low total transmission 10% but it can work with CsTe2 cathode Defined polarization output polarization for the harmonic generation Shaper suitable for high repetition rate Programmable through the temperature control Ingo Will, Guido Klemz, Max-Born-Institute Berlin, in cooperation with DESY SLAC 03/09/2010 Seite 16 Transverse beam shaper REFRACTIVE HOMOGEINEIZER An input UV Gaussian beam can be mapped into a top hat spot by means of a proper aspheric telescope Very limited losses, commercial available systems Limitation: •Sensitive to misalignment •Difficult optic fabrication •Perfect input mode is not always available •Beam transport difficult Hoffnagle et al, Appl. Opt 39, 6488 (2000) LCLS virtual cathode images OVERFILLED APERTURE A perture selects the most uniform part of the beam and which is imaged to the cathode. A spatial filter or a capillary can be used for clean the higher spatial frequencies. This method is intrinsically lossy but is the most simple and widespread for photoinjector application. SLAC 03/09/2010 Refractive beam shaper Overfilled aperture Seite 17 Deformable mirror / micro-lenses array Deformable mirrors are formed by2D array of movable elements. The device is used to compensate wave-front aberration. Flat top has been demonstrated using genetic algorithm to drive a 37 elements mirror. Reflectivity at 260 nm 70% Commercially available Complex optimization Micro lenses array to image different parts of the input beam over the same output spot. Potentially it is not influenced by the input dishomogeneities larger than the lenslets. Optimal results at short focal distance (20 cm). UV optics available. Never tested in a photoinjector due to the difficulties to transport the beam. SLAC 03/09/2010 Seite 18 3D shaping: uniformly filled ellipsoidal The most desired electron distribution is ellipsoidal shape • Fully linear phase space for complete emittance compensation can be achieved using ellipsoidal laser geometry • LCLS photoinjector 1 nC εprojected = 1.02 mm-mrad εprojected = 0.58 mm-mrad • Brightness C. Limborg ICFA 05 Erice Very challenging to combine spatial and temporal control the laser profile SLAC 03/09/2010 Seite 19 Ellipsoidal 3D electron beam shaping ELECTRON BEAM SHAPING •Using a pancake laser, the electron beam evolves to ideal ellipsoid (Serafini-Luiten) •Not big effort to generate 50 fs UV laser pulse •This scheme has been recently demonstrated up to 50 pC charge, distortion at higher charge Time RF deflector Musumeci et al., Phys. Rev. Lett. 100, 244801 (2008) Brigthness 1014 A/m2 SLAC 03/09/2010 Seite 20 Pulse stacking variable beam size The ellipsoidal shape can be approximated by pulse stacking different laser beam size michelson interferometer like Energy losses 75% Easy idea, bit complex implementation SLAC 03/09/2010 Seite 21 Deformable mirror and fiber bundle Fiber bundle and deformable mirror ha been used, to generate ellipsoidal like laser geometry Limitation: short focal length demands for transmissive cathode long tails in the time profile H Tomizawa SLAC 03/09/2010 Seite 22 Use chromatic aberrations • • • The chromatic focal shift from a lens can be used to generate the ellipsoidal shape Program ω(t) and the A(t) through a DAZZLER Top hat beam at the lens with IRIS Simulated laser Simulated emittance R Cylinder Ellipse Proof of principle at 800 nm EXP Cut view along t-r planes measurements SIM Li & Lewellen, PRL 100, 078401(2008) Li & al,PRSTAB 12, 020702 (2009) SLAC 03/09/2010 Simulations indicate the depression in the center is due to diffraction effect by iris Seite 23 Conclusions Spectral domain pulse shaping demonstrated to be able to produce flat top pulse • Energy losses are in general significant for UV shapers • Repetition rate limited to few KHz (due to laser and shapers) • Demonstrated rise time about 1 ps at 10 ps pulse length and ripples of few percent Pulse stacking for flat top profile • Good efficiency (except for coherent stacking) • High repetition rate is possible • Not very flexible output shape (birefringent crystal) fine control of interference Transverse homogeneity • Overfilling an aperture is still the most widespread approach • Deformable mirror has complex but still possible implementation Uniformly filled ellipsoidal laser • Variable size beam pulse stacking • Pancake evolution demonstrated for low charge • Deformable mirrors + fiber bundle long tail shape and require backward illuminated cathode • Lens aberrations scheme needs further exploration • Future schemes: 3 dimensional holography?? SLAC 03/09/2010 Seite 24 Acknowledgments to Ingo Will, Guido Klemz(MBI) Siggy Schreiber(Desy) Sasha Gilevich (LCLS) Alexander Trisorio, Christoph Hauri, Romain Ganter (PSI) Hiromitsu Tomizawa (Spring8) Miltcho Danailov (FERMI) SLAC 03/09/2010 Seite 25 Diode – RF gun combination 166 mm Cathode Pulsed Solenoid RF Cavity 3 mm Anode Hollow cathode Geometry: - DLC Coating - Exchangeable Cathode Insert (Al, Cu, Mo, Nb,…, FEA) DLC coated Hollow Cathode (> 150 MV/m) Quantum Efficiency Quantum Efficiency versus Material & Wavelength Exp. Cond.: 30 <Q < 200 pC; 5.2 MeV σt,laser= 4 ps rms; 25 MV/m Cu Mo Al Nb 10-5 QE reduces by a factor 2 over 20 nm 10-6 260 265 270 275 280 285 Laser Wavelength (nm) - Reproducibility of QE for same material (& preparation) = 50 % LCLS pulse shape SLAC 03/09/2010 Seite 28