Electro-Optic Monitor of the Bunch Longitudinal Profile

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


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



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


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!


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


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


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


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


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


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…


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


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:

=


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)


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

Experimental Confirmation of Predictions


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…


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!


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.


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

EOT System Layout


Electro-Optic Monitor of the Bunch Longitudinal Profile

Documents

Transcript of Electro-Optic Monitor of the Bunch Longitudinal Profile