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Explore current and evolving laser pulse shaping methods for transversal and longitudinal shaping, from Pi Shapers to spatial light modulators. Discover the possibilities for precise pulse shaping and the challenges faced in achieving optimal results.
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T. Quast, Helmholtz-Zentrum Berlin Available and Future Laser Pulse Shaping Technology (Reality and Future Directions for Spatio-Temporal Laser Pulse Shaping) Bild von 3d Ellipsoid courtesy of T. Rao courtesy of L. Hein Future Light Sources Workshop 2012, JLab
Laser Pulse Shaping – why do we need it ? Space Charge.. best solution: Space charge forces are linear for 3d ellipsoid: Space charge force of a Gaussian distribution 2nd best solution: „laser world is gaussian“ Transversal flattop + Longitudinal flattop „beercan“
„very ambitious“ „advanced“ „easy“ Introduction into Laser Shaping • Three „stages“ of Laser Pulse Shaping + Transversal Longitudinal ≠ Spatio-Temporal (3d)
Spatial Shaping • transversal Shaping – every one does it.. • Gauss to flattop – the simple way: • Transport laser to vincinity of gun • Overilluminated iris cuts out only the inner „flat“ part • Iris is imaged onto the cathode principle • High position Stability (iris located near cathode) • Robust and simple setup • Spot size and laser power can be varied easily (but not independent) • Any pattern can be imaged onto cathode pro • what a waste of laser power ! • Problems from laser transmission beamline (spot size / shape stability) are transfomed into intensity fluctuations con Has to be done carefully
Transversal Flattop – Pi Shaper • asheric optics – Pi shaper • - aspheric refractive (reflective) elements • High transmission (90%) • very sensitive to input laser parameters • tilt, decenter, size (few mrad, 10ths of µm) • TEM00 mode required (difficult with UV) Use both:„mild“ shaper and pinhole T=70% w=1.1w0 w=w0 w=0.9w0 fromG.Klemz, I.Will, Proc. FEL06 alternative: deformable mirror and genetic algorithm
Longitudinal shaping – different methods 1.Direct space to time (with grating and mask) Works well but transmission 10E-2 2.DAZZLER - (Acousto Optic Programmable Dispersive Filter) - only up to 100kHz Rep.rate (this rules out many of the existing bunch patterns) - shaping up to a few ps (restricion from possible crystal length)
Longitudinal shaping II • Pulse Stacking … (large variety) 3.Spatial light modulator (SLM) Practically only for fs pulses 4. Pulse stacking with polarizer (pol. beam splitters) Courtesy of S. Schreiber, DESY • difficult geometric alignment • intensity variations due to imperfect polarizers
Longitudinal shaping III Birefringent crystals – reduced complexity with Linear setup from H. Tomizawa, RadPhysChem 80, 10 (2010)
Longitudinal shaping IV • 3 stage stacker w. birefringent crystals 3x YVO4 d=(24,12,6mm) T= 62%; 532 nm from: A.K. Sharma et al. PRSTAB 12, 033501 (2009)
Longitudinal shaping V • High precision pulse shaper (MBI) Taken from: Will, Klemz, Optics Express 16 (2008) , 4922-14935 Theory for N = 10 crystals: 1024 components aranged in 11 groups
temperature controlled birefringent crystal motorized rotationstage Shaped ouputpulses Gaussianinputpulses Laser pulse shape measured: OSS signal (UV) FWHM = 25 ps Will, Klemz, Optics Express 16 (2008) , 4922-14935 edge10-90~ 2 ps edge10-90~ 2.2 ps birefringent shaper, 13 crystals shaper for high resolution • 13 crystal pulse shaper for high resolution
Pulse shaping • different possible pulse shapes Gaussian: Flat-top: FWHM ~7 ps FWHM ~ 2 ps FWHM ~ 24 ps FWHM ~ 19 ps FWHM ~ 11 ps FWHM ~ 17 ps • Feedback with optical sampling system (OSS): • dynamic range of streak camera not sufficient • scanning of 100 subsequent pulses (~0.2ps res.) • shaping is done after oscillator in IR • sampling for feedback signal in the UV Simulated pulse-stacker without feedback FWHM ~ 24 ps FWHM ~ 24 ps courtesy of I. Will, MBI
Self evolving • Self evolving beam - space charge force • start with a parabolic (or half sphere) Laser intensity profile • automatic evolution into a uniform ellipsoidal (3D) beam • Easy - no longitudinal laser shaping - only a short (100fs) pulse (clipped gaussian) needed pro: • cannot put high charge in it • short pulse may damage cathode • only fast response photocathode material => metal • requires high accelerating gradient con: O. J. Luiten et al., Phys. Rev. Lett. 93, 094802 (2004)
spatial shaper ZnSe lens achromatic lens camera DAZZLER 3d pulse shaping • Use chromatic aberration of a dispersive lens • Refractive index n is a function of frequency (dispersion) • Focal length of focussing lens changes with frequency • Parabolic frequency change by giving the pulse a cubic phase Courtesy of Yuelin Li
3d pulse shaping • A first proof of principle experiment • In principle it is working • quality suffers from AOPDF limitations • No pinholes in transport ! (changing size) • no dispersion in transport ! • only IR so far (conversion ??) From Y. Li et al. PRSTAB 12, 020702 (2009)
3d pulse shaping (alternative) • Stacking a 3d-ellipsoid… con • Alignment ? • Coherence and diffraction ? • Slice number limited pro • No dependance on nonlinear effects Not been demonstrated yet Z.He et al. Proc of PAC2011, TUP200
…back to Reality… • Survey on longitudinal shaping
…reality II… • Survey (cont.)
pulse shaping • Summary • transversal pulse shaping has to be done carefully • with advanced pulse shaping the beam transport becomes an issue (dont mess it up..) • advanced and controlled pulse stacking setup for flat top laser pulses work nicely • first steps into 3d ellipsoidal shaping are done – further exploration needed • Remarks • Overall performance of a gun largely depends on careful technical implementation of the Ph. Cath Laser • Stable laser parameters improve the overall performance • Put more emphasis on laser diagnostic and feedback • More complicated shaping schemes, utilizing nonlinear effects causes unwanted coupling of parameters • Less degrees of freedom • Potential source of instability • Is it worth it ?
Z-polarization gun - laser induced shottky effect z-polarization (interesting concept) • workfunction is lowered of the intense laser field • Requires very moderate laser parameters: 2.6µJ, 100fs, ~800nm • Focussing down to 20µm results in 21 pC „If the Schottky-effect-induced Z-field is large enough, we expect that electrons will make oscillations with the Z-field frequency on the outermost surface of the metal cathode and will be extracted with the external electric field of the RF cavity“ H. Tomizawa, Proc FEL2010