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Electro-Optic BunchProfile Monitors
DA Walsh, SP Jamison, WA Gillespie, MA Tyrk, R Pan, T Lefevre
Electro Optic EncodingCoulomb field of relativistic bunch
probe laser
non-linear crystal
Standard Description• Coulomb field flattens ,and represents
charge distribution• Pockels effect induces polarization
ellipticity• Technique borrowed from THz electo-optic
sampling where (tprobe << tTHz)
Spectral Decoding o Chirped optical input o Spectral readouto Use time-wavelength relationship
~1ps
Temporal Decodingo Long pulse + ultrashort pulse gateo Spatial readout (cross-correlator crystal)o Use time-space relationship
~150fs
Benefits of EO techniques
Electro optic techniques• Scale well with high beam energy
– Particle methods get impractical (size, beam dumps)
• Non-destructive– Bunches can still be used– Live feedback
CLIC targets• Bunches will be ~150fs rms• Diagnostic target ~20fs rms
We aim to improve on the resolution and robustness of EO diagnostics
Physics of EO encoding
More Rigorous Description – nonlinear frequency mixing
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 Description• Pockels effect induces polarization ellipticity
which is detected as leakage through crossed polarisers
• Theory borrowed from THz electo-optic sampling where (tprobe << tTHz)
• For short pulses extra frequencies are generated, confusing the retrieval processes
c (2)( ;w wthz,wopt)
wopt + wcoulomb
Generation of Optical Sidebands
wcoulomb
woptwopt - wcoulomb
wopt
EO c
ryst
al Sum over allvalues ofwcoulomb
Inte
nsity
Inte
nsity
Inte
nsity
ν
few mm
tens μm
λt 800nm
Coulomb field Optical field
~50fs circa 20nm
Direct measurement• Direct measurements of wcoulomb (CTR) tricky – bandwidth of
many octaves!• A rather complex system is being developed to perform this
single shot – difficult to calibrate• Long wavelength components don’t propagate
Spectrum is related to the time profile via a Fourier transform this is already a potentially useful diagnostic!
Optical up-conversion measurement• Relative bandwidth has shrunk (few %)• Long wavelength components shifted
to the optical and easy to propagate• Single shot spectrum easily obtained
Consider single frequency probe and coulomb fields
Consider a single frequency probe and short coulomb field “pulse”
Development and Benchmarking
Δν <50GHz(Δ t >9ps)
Femtosecond laser based test bed
Femtosecond laser pulse spectrally filtered to produce narrow bandwidth probe
Auston switch THz source mimics coulomb field.
Field strengths up to 1 MV/cm.
Has a well characterised spectral and temporal profile.
Investigation of measurement thresholds / signal to noise ratios
Required to define system requirements
Early test results
Testing has revealed complications re. beam crossing-angle tolerance.May have implications for all electro optic sampling systems!Thorough investigation is underway.
1.5mm150μm
System installed at Daresbury to attempt single shot CTR spectrum measurement
Successful observation of sidebandsTHz pulse measured via EO sampling
ZnTe
ProbeSum Freq.
THz Diff Freq. Detection
Complete CharacterisationProblem: Unknown phaseRetrieval of temporal profile via spectrum alone still requires assumptions regarding the spectral phase. Many pulse profiles fit a given spectrum.
Solution: Frequency Resolved Optical Gating (FROG)This is a standard and robust method for characterising laser pulses.Unambiguously retrieves spectral intensity and spectral phase from the spectrogram
=
Spectrum Spectral Phase
Need to know:
Baltuska, Pshenichnikov, and Weirsma, J. Quant. Electron., 35, 459 (1999).
𝐼 (𝜔 , 𝑡 )∝|∫ 𝐸 (𝑡 )𝐸 (𝑡−𝜏 )𝑒− 𝑖𝜔𝑡 𝑑𝑡|2
Considerations:Self-gating avoids timing issues (no need for a fs laser)Single shot measurement possible but requires minimum pulse energy of > 10 nJ
4.5 fspulse
Non-collinear Chirped Pulse Amplification
Problem: Up-conversion is relatively weak – our calculations suggest energies of a few nJ.Signal needs amplifying without loss of information.
Solution: Non-collinear Chirped Pulse Amplification (NCPA)
~800nmfemtosecondsignal
Stretcher Compressor
Stretching factor103 or more to preventsaturation, damage, NL effects
Amplified pulse thenrecompressedBBO
Routinely used to produce “single-cycle” optical pulsesAmplification with robust nanosecond pulse lasersHigh gains of 107 or moreGain bandwidths >100nm (50THz)Preservation of phase of pulse is possible
Few ns, ~10mJ pump pulse @ 532nm Beams ~1.5mmdiameter
Gain >1000x(~300MW/cm2)
Envisaged Spectral Up-conversionCharacterisation System
(2) Amplification
Stretcher CompressorSingle Shot
FROGNL crystal
(3) Measure:=
(4) Calculate properties at NL crystal (to remove remaining spectral amplitude and any residual phase distortion)
(1) up-convert Coulomb field
50ps 60mJ1064nmNd:YAG(doubled)
SpectrallyfilteredTi:Sapphire
THz Source(Spectral intensity and phase distortions can be both modelled and measured)
(4) Calculate properties at NL crystal (to remove remaining spectral amplitude and any residual phase distortion)
Envisaged Spectral Up-conversionCharacterisation System
(2) Amplification
Stretcher CompressorSingle Shot
FROGNL crystal
(3) Measure:=
(1) up-convert Coulomb field
Commercial nanosecond Nd LaserIntegrated frequency conversion
(OPO)
In beam pipe
Way Forward & ChallengesContinue test program:
Characterising the amplification stage in the next few monthsValidate understanding of amplification and stretching/compressionThis will allow us to calculate required nanosecond laser parameters
Design, build and validate appropriate single-shot FROG apparatus(or buy off-the-shelf if pulse energy is sufficient)
In order to exceed bandwidths of ~8THz material issues need to be addressed:Common EO materials have phonon absorptions that distort/absorb signalInvestigate possibility of compositing pulse data from different materialsSpeculatively consider novel materials: poled polymers (robust?)
meta materials (possible?)
• Investigation of thin film ‘meta-materials’ (silver nanoparticles embedded in glass matrix) for novel EO bunch profile systems
• Experimental characterisation of those materials as novel EO detectors
• Observed dichroic effect resulting in separation of Surface Plasmon Resonance (SPR) band in first stages of experiments
• Second-harmonic generation to be investigated in next few months
Talisker picosecond laser system
Materials and Photonic Systems
CTF3 Spectral Decoding EO Diagnostic
P: Polarizer H: Half wave plate Q: Quarter wave plate : Mirror with actuators : Finger camera
Grating
Laser: Wavelength: 780 nm Duration: 100 fsRepetition: 37.4815 MHz Pulse energy: 4 nJCrystal: Thickness: 1mm Separation: 5-10 mm
System Schematics
Chamber 1Chamber 2
Califes requirements relaxed compared to CLIC – only need 1ps resolution
Spectral decoding provides simple and robust diagnostic system
Implementation of frequency doubled, amplified fibre laser to improve reliability
Laser ↔ e-bunch synchronization
74.9 MHz
37.5 MHz
Non-standard design, providing higher pulse energy
Laser parametersWavelength: 780 nm & 1560 nm Duration: <120 fsRepetition: 37.5 MHz @ 780 nm Pulse energy: 4 nJ @ 780 nm
Laser head and controller
Synchronization boxPD in RF 1
RF 2
Phase out PID inPiezo control
Motor control
Laser and synchronization system
--Streak camera measuring the arriving time of laser and beam--Phase shifter control laser arriving time for scanning
Summary
• We have conceived of a novel, high resolution Electro-Optic bunch length diagnostic
• Mechanisms to test and characterise this system are being put in place – results in next few months
• Ways to overcome material limitations are being investigated
• Spectral decoding at CALIFES to be commissioned within the next two months