Download Wir schaffen Wissen – heute für morgen Paul Scherrer Institut Carlo Vicario

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
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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
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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
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IR shaper
CPA
Harmonics
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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)
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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
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03.09.2010
P. Tournois et al., Opt: Comm:, 140, (1997), 245
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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
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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
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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)
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10
12
14
0
2
4
6
8
10
12
14
16
TIME(ps)
Miltcho Danailov, FERMI Trieste
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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
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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
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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
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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
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19
%
Diffracted energy
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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
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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)
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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
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Ingo Will, Guido Klemz, Max-Born-Institute Berlin, in cooperation with DESY
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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
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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.
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Refractive beam shaper
Overfilled aperture
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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.
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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
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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
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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
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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
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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)
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Simulations indicate the depression in the center is due
to diffraction effect by iris
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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??
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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)
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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
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