Transcript evolution de la grenouille

Electro-Optic Monitor of the
Bunch Longitudinal Profile
David Walsh, University of Dundee
Steven Jamison, Allan Gillespie, Mateusz Tyrk,
Rui Pan, Thibaut Lefevre
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
David Walsh, University of Dundee. CLIC Workshop 2014
EO Transposition System
EO Transposition
~800nm
5ns
1mJ
Coulomb field
GaP
¼λ plate
& polariser
Stretcher
Beam
dump
BBO
Beam
dump
Measurement
Compressor
amplitude
532nm
Nanosecond 10mJ
Laser System 10ns
1000x Amplification
(NCOPCPA)
DIY
GRENOUILLE
Pulse
Evolution
time
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
David Walsh, University of Dundee. CLIC Workshop 2014
Physics of EO Transposition
Standard Description
Pockels effect induces a phase change which is
detected via polarization measurements
This is not true for short bunches!
More Rigorous Description – nonlinear frequency mixing
Coulomb spectrum shifted to
optical region
Coulomb pulse temporally
replicated in optical pulse
optical field
envelope
S.P. Jamison Opt. Lett. v31 no.11 p1753
Consider a single-frequency probe and short Coulomb field “pulse”
Optical field
~50fs
t
few mm
tens μm
Intensity
Intensity
Intensity
Coulomb field
ν
this is already a potentially useful diagnostic!
David Walsh, University of Dundee. CLIC Workshop 2014
circa 20nm
800nm
λ
Characterisation of Transposed Pulse
Considerations:
* 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.
What we
want to
know
𝐸 𝑡 = 𝑅𝑒
𝐼 𝑡 𝑒𝑖
“Carrier” frequency
𝜔𝑜 𝑡−𝜙 𝑡
<-Fourier->
Can’t measure
•
•
•
•
𝐸 𝜔 = 𝑆 𝜔 𝑒 −𝑖𝜑
𝜔
Spectrum Spectral Phase
Can be retrieved!
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
David Walsh, University of Dundee. CLIC Workshop 2014
Investigation into EO Transposition
•
•
•
Verification of EO Transposition
Investigation of measurement thresholds / signal-to-noise ratios
Necessary for defining and verifying system parameters
Femtosecond laser-based test bed
Auston switch THz source mimics
Coulomb field.
Well-characterised spectral and
temporal profile.
Δν ~44GHz
Δ t ~10ps FWHM
Femtosecond laser pulse spectrally filtered
to produce narrow bandwidth probe
David Walsh, University of Dundee. CLIC Workshop 2014
Measurement of Transposed Spectrum
Input pulse characteristics
Optical probe length
Δt ~ 10ps
Optical probe energy
S ~ 28nJ
THz field strength max
E ~ 132kV/m
Output characteristics (4mm ZnTe)
109
THz off
THz on
Relative Intensity
107
TDS
E-Field (kV/m)
|FFT(TDS) |2
108
106
0
5
10
Time (ps)
15
20
105
104
103
102
101
Total energy ~470pJ
100
-3
-2
-1
0
1
2
Frequency Offset (THz)
Leaking probe
Upconversion of spectrum verified
David Walsh, University of Dundee. CLIC Workshop 2014
3
Extrapolation to CLIC parameters
Scaling factors
𝑬𝒏𝒆𝒓𝒈𝒚𝒖𝒑𝒄𝒐𝒏𝒗 ∝ 𝑷𝒐𝒘𝒆𝒓𝒑𝒓𝒐𝒃𝒆 × 𝑬𝒇𝒊𝒆𝒍𝒅 × 𝒍 × 𝒓
𝟐
𝒍 is the EO crystal length, 𝒓 is the nonlinear coefficient
Example:
“Typical” nanosecond pulse
laser as probe
Pulse energy 1mJ
Pulse duration 10ns
𝑃𝑜𝑤𝑒𝑟𝑝𝑟𝑜𝑏𝑒 ~ 100 kW
Coulomb field for target CLIC
bunch parameters (CDR)
Bunch length 44μm
Bunch charge 0.6pC
𝐸𝑓𝑖𝑒𝑙𝑑~
Property
Factor of improvement
𝑃𝑜𝑤𝑒𝑟𝑝𝑟𝑜𝑏𝑒
x36
𝑙
÷1002
𝑟
÷22
𝐸𝑓𝑖𝑒𝑙𝑑
x1862
Overall
x31
2𝑄
4𝜋𝜀0 𝑅𝑙𝑏
= 24.5 MV/m
Pulse energy of ~15nJ is predicted
1μJ required for the commercial
single-shot FROG, “Grenouille”
David Walsh, University of Dundee. CLIC Workshop 2014
Parametric Optical Amplification
•
•
•
•
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
Pump derived from 50ps pulse laser
α
1.0
Heavily attenuated
800nm, 50fs pulse
θ
optic axis
20mm BBO
θ = 23.81
α = 2.25
Phase
Photodiode or
Spectrometer
Efficiency
2
Efficiency
Phase Change
Unamplified
Amplified
1pi
0.8
1
0.6
0pi
0.4
Gain of >1000x verified
0.2
0.0
-1pi
300
0
320
340
360
Frequency (THz)
Further tests awaiting Grenouille
David Walsh, University of Dundee. CLIC Workshop 2014
380
400
Relative Spectral Intensity
•
Stretcher and Compressor Design
Pulse
Properties
Peak Power
532nm Pump
10mJ, 10ns
Gaussian temporal
profile
1x106 W
800nm EO Transpostion
Signal
Amplified signal energy
> 1 μJ, ~50 fs
20x106 W
>Pump! Not possible!
As above but stretched
GVD = 5.6x106 fs2
> 1 μJ, ~310 ps
3.2x103 W
OK, and will not distort
Conjugate Stretcher and Compressor designs
1m
0.5m
GVD = 5.6x106 fs2
1m
All gratings
G=1200 lines/mm
Θdeviation~15°
Calculations indicate nanosecond
level jitter has negligible effect
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
Early measurements of
spectra often asymmetric
and weak/unobservable
150μm
1.5mm
Adjustment of the THz alignment
could modify the observed spectral
sidebands!
50cm
Understanding this effect is crucial to correctly performing any EO measurement!
David Walsh, University of Dundee. CLIC Workshop 2014
Non-collinear Phase Matching
A natural consequence of considering nonlinear processes
is that phase matching must be considered!
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:
𝐸𝑓𝑓(𝜔3, 𝜃, 𝜑)=
Same form as derived in NLO literature
David Walsh, University of Dundee. CLIC Workshop 2014
Predicted Effect of Misalignment
Phase matching efficiencies calculated in Matlab
Code 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
Experimental Confirmation of Predictions
• 7% difference in slopes systematic error (n(THz), focussing optics)
• Confirmed predictions of model
• Enabled us to produce rule-of-thumb guides
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 Resolution
EO transposition scheme is now limited
by materials:
phase matching, absorption, stability
Collaborative effort with MAPS group
at the University of Dundee on
development 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!
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!
David Walsh, University of Dundee. CLIC Workshop 2014
Compositing Spectral Data
Theoretical response functions
Use GaP Use GaP
or ZnTe
Compositing Methodology
Use ZnTe
Normalised Efficiency
1.2
1.
ZnTe
GaP
1.0
0.8
2.
0.6
0.4
0.2
3.
0.0
Additional Phase (radians)
0
5
6
10
15
Frequency (THz)
20
25
ZnTe
GaP
4
Capture two sets of data using
both crystals
Align retrieved amplitude and
relative phase where data
overlaps (0 – 4 THz)
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.
2
0
-2
-4
-6
0
5
10
15
Frequency (THz)
20
25
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
David Walsh, University of Dundee. CLIC Workshop 2014
END
EOT System Layout
David Walsh, University of Dundee. CLIC Workshop 2014