Electro-Optic Monitor of the Bunch Longitudinal Profile
David Walsh, University of Dundee
Steven Jamison, Allan Gillespie, Mateusz Tyrk, Rui Pan, Thibaut Lefevre
David Walsh, University of Dundee. CLIC Workshop 2014
Outline of Talk
• Project Aims• EO Transposition System• Design Progress
– System parameters determination• Bunch-probe misalignment effects• Next Steps
David Walsh, University of Dundee. CLIC Workshop 2014
CLIC Requirement
CLIC project targets• Non-invasive• Single shot• Diagnostic target resolution ~20fs rms (Bunches ~150fs rms)
EO diagnostics: (encoding of Coulomb field into a laser intensity)Advantages• Scales well with high beam energy
– Particle methods get impractical (size, beam dumps)• Non-destructive
– Bunches can still be used– Live feedback
Challenges to be met• Unreliability, maintenance and cost of suitable ultrashort pulse laser systems• Temporal resolution We aim to improve on the
resolution and the robustness of EO diagnostics
NanosecondLaser System
Stretcher Comp-ressor
BBO DIYGRENOUILLE
EO Transposition
1000x Amplification(NCOPCPA)
Measurement
Coulomb field
GaP
¼λ plate& polariser Beam
dump
Beamdump
Pulse Evolution
time
ampl
itude
nano
seco
nd p
robe
“tran
spos
ed p
ulse
”
stretc
hed
ampl
ified
reco
mpr
esse
d
532nm10mJ10ns
~800nm5ns1mJ
1. Nanosecond laser brings reliability
2. Full spectral amplitude and phase measured via FROG technique
3. Coulomb field (bunch profile) calculated via time-reversed propagation of pulse
EO Transposition System
David Walsh, University of Dundee. CLIC Workshop 2014
David Walsh, University of Dundee. CLIC Workshop 2014
Physics of EO Transposition
Coulomb spectrum shifted to optical region
Coulomb pulse temporallyreplicated in optical pulse
envelope optical field S.P. Jamison Opt. Lett. v31 no.11 p1753
This is not true for short bunches!
Standard DescriptionPockels effect induces a phase change which is detected via polarization measurements
Inte
nsity
Inte
nsity
Inte
nsity
ν
few mm
tens μm
λt 800nm
Coulomb field Optical field
~50fs circa 20nm
this is already a potentially useful diagnostic!
Consider a single-frequency probe and short Coulomb field “pulse”
More Rigorous Description – nonlinear frequency mixing
David Walsh, University of Dundee. CLIC Workshop 2014
Characterisation of Transposed PulseConsiderations: * needs to be single shot
* autocorrelation not unambiguous – no shorter reference pulse available* low pulse energy
Solution: Grenouille (frequency resolved optical gating), a standard and robust optical diagnostic. Retrieves spectral intensity and phase from spectrally resolved autocorrelation.
=
Spectrum Spectral Phase
• The most sensitive “auto gating” measurement• Self-gating avoids timing issues (no need for a fs
laser)• Requires minimum pulse energy of > 10 nJ• Commercial systems offer > 1 μJ
𝐸 (𝑡 )=𝑅𝑒 (√ 𝐼 (𝑡 )𝑒𝑖 (𝜔𝑜 𝑡−𝜙 (𝑡 )) )“Carrier” frequency Can’t measure
What we want to know
<-Fourier->
Can be retrieved!
David Walsh, University of Dundee. CLIC Workshop 2014
Investigation into EO Transposition
Δν ~44GHzΔ t ~10ps FWHM
Femtosecond laser-based test bed
Femtosecond laser pulse spectrally filtered to produce narrow bandwidth probe
Auston switch THz source mimics Coulomb field.Well-characterised spectral and temporal profile.
• Verification of EO Transposition• Investigation of measurement thresholds / signal-to-noise ratios• Necessary for defining and verifying system parameters
David Walsh, University of Dundee. CLIC Workshop 2014
Frequency Offset (THz)-3 -2 -1 0 1 2 3
Rel
ativ
e In
tens
ity
100
101
102
103
104
105
106
107
108
109
FFT(TDS)THz off THz on
Time (ps)0 5 10 15 20
E-F
ield
(kV
/m) TDS
Input pulse characteristicsOptical probe length Δt ~ 10psOptical probe energy S ~ 28nJTHz field strength max E ~ 132kV/m
Total energy ~470pJ
Leaking probe
Output characteristics (4mm ZnTe)
| |2
Measurement of Transposed Spectrum
Upconversion of spectrum verified
David Walsh, University of Dundee. CLIC Workshop 2014
Scaling factors 𝑬𝒏𝒆𝒓𝒈𝒚 𝒖𝒑𝒄𝒐𝒏𝒗∝𝑷𝒐𝒘𝒆𝒓 𝒑𝒓𝒐𝒃𝒆× ( 𝑬𝒇𝒊𝒆𝒍𝒅× 𝒍×𝒓 )𝟐
Pulse energy of ~15nJ is predicted
1μJ required for the commercial single-shot FROG, “Grenouille”
is the EO crystal length, is the nonlinear coefficient
Example:Pulse energy 1mJPulse duration 10ns
“Typical” nanosecond pulselaser as probe
Coulomb field for target CLICbunch parameters (CDR)
Bunch length 44μmBunch charge 0.6pC
Property Factor of improvement
x36÷1002
÷22
x1862
Overall x31
24.5 MV/m
100 kW
Extrapolation to CLIC parameters
David Walsh, University of Dundee. CLIC Workshop 2014
Parametric Optical Amplification
Heavily attenuated800nm, 50fs pulse
20mm BBOθ = 23.81α = 2.25
532nm, ~300MW/cm2
optic axisθ
α
Photodiode orSpectrometer
Frequency (THz)300 320 340 360 380 400
Eff
icie
ncy
0.0
0.2
0.4
0.6
0.8
1.0
Phas
e-1pi
0pi
1pi
Rel
ativ
e Sp
ectr
al In
tens
ity
0
1
2
Efficiency Phase ChangeUnamplified Amplified
• Routinely used to produce “single-cycle” optical pulses, and amplification of CEP stabilised pulses has been demonstrated
• For phase matched and/or low conversion conditions phase is preserved• Small Phase and Amplitude distortions calculated (and so can be removed)• Bandwidth very broad >50THz• Stand-ins for pump and signal – picosecond laser system and Ti:Sapphire laser
Pulse spectrum maintained
Gain of >1000x verified
Further tests awaiting Grenouille
Pump derived from 50ps pulse laser
David Walsh, University of Dundee. CLIC Workshop 2014
Stretcher and Compressor Design
GVD = 5.6x106 fs2
Calculations indicate nanosecondlevel jitter has negligible effect
Pulse Properties Peak Power
532nm Pump10mJ, 10ns
Gaussian temporal profile
1x106 W
800nm EO TranspostionSignal
Amplified signal energy > 1 μJ, ~50 fs 20x106 W
As above but stretchedGVD = 5.6x106 fs2 > 1 μJ, ~310 ps 3.2x103 W
>Pump! Not possible!
Conjugate Stretcher and Compressor designs1m
1m
0.5m
All gratingsG=1200 lines/mmΘdeviation~15°
OK, and will not distort
David Walsh, University of Dundee. CLIC Workshop 2014
Testing Summary
• Commercial Q-Switched laser system parameters have been confirmed
• Laser has been sourced and ordered• Ancillary optics currently being assembled• Aim to test full system this year
That’s not all…
David Walsh, University of Dundee. CLIC Workshop 2014
Alignment Issues
1.5mm150μmEarly measurements of spectra often asymmetric and weak/unobservable
Adjustment of the THz alignment could modify the observed spectral sidebands!
Understanding this effect is crucial to correctly performing any EO measurement!
50cm
David Walsh, University of Dundee. CLIC Workshop 2014
Non-collinear Phase Matching
A natural consequence of considering nonlinear processesis that phase matching must be considered!
Same form as derived in NLO literature
Polarisation field set up by probe and THz (Coulomb) field:Expand fields into envelope and carrier:
Then solve paraxial wave equation using Gaussian transverse profiles:
=
David Walsh, University of Dundee. CLIC Workshop 2014
Predicted Effect of MisalignmentPhase matching efficiencies calculated in MatlabCode iterates through THz frequencies and calculates the efficiency for a range of upconversion directions
2
Consequences• Spectrally varying beam
propagation angle• Beams “walk off” one another at
distance or in the focus of a lens (e.g. fiber coupling)
David Walsh, University of Dundee. CLIC Workshop 2014
Experimental Confirmation of Predictions
David Walsh, University of Dundee. CLIC Workshop 2014
• 7% difference in slopes systematic error (n(THz), focussing optics)• Confirmed predictions of model• Enabled us to produce rule-of-thumb guides
Experimental Confirmation of Predictions
David Walsh, University of Dundee. CLIC Workshop 2014
Phasematching Summary
• We now have a proper understanding of the issue• Correct management of the optical beam is an essential part of any EO
system• Findings have wider-ranging implications and we aim to publish
conclusions soon• This could have been the cause of some difficulties with EO systems in the
past!
Just a little more…
David Walsh, University of Dundee. CLIC Workshop 2014
Temporal ResolutionEO transposition scheme is now limitedby materials:phase matching, absorption, stability
A key property of the EO Transposition scheme may be exploited• EOT system retrieves the spectral amplitude and phase• At frequencies away from absorptions, etc., the spectrum should still be faithfully
retrieved• Potential to run two “tried and tested” crystals with complementary response
functions side-by-side to record FULL spectral information!
Collaborative effort with MAPS groupat the University of Dundee ondevelopment of novel EO materials• Potential to produce an enhancement of nonlinear processes through metallic
nanoparticles• THz field-induced second harmonic enhancement being investigated• Attempts to characterise second harmonic from silver nanoparticles began in
Dundee yesterday!
David Walsh, University of Dundee. CLIC Workshop 2014
Compositing Spectral DataTheoretical response functions
Frequency (THz)0 5 10 15 20 25
Addi
tiona
l Pha
se (r
adia
ns)
-6
-4
-2
0
2
4
6ZnTeGaP
Frequency (THz)0 5 10 15 20 25
Nor
mal
ised
Effi
cien
cy
0.0
0.2
0.4
0.6
0.8
1.0
1.2ZnTeGaP
Use GaPUse GaPor ZnTe Use ZnTe Compositing Methodology
1. Capture two sets of data using both crystals
2. Align retrieved amplitude and relative phase where data overlaps (0 – 4 THz)
3. Patch e.g. use ZnTe spectrum with GaP data patching 4 - 7.5 THz region
Initial numerical simulations very promising! But not ready for dissemination.
David Walsh, University of Dundee. CLIC Workshop 2014
Next Steps• Completion and evaluation of full EO Transposition system
– Using lab based lasers– Pursuing options to test at an accelerator
• Ramping up efforts to improve bandwidth– Multi crystal– Enhance nonlinear effects (MAPS @ Dundee)
• Engineer working system towards being a “turn-key” measurement
• Expand to multiple bunch monitor– Micro bunch evolution within macro bunch
END
EOT System Layout
David Walsh, University of Dundee. CLIC Workshop 2014
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