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