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

Nanosecond Laser System StretcherComp- ressor BBO DIY GRENOUILLE EO Transposition 1000x Amplification (NCOPCPA) Measurement Coulomb field GaP ¼λ plate & polariser Beam dump Beam dump Pulse Evolution time amplitude nanosecond probe “transposed pulse” stretched amplified recompressed 532nm 10mJ 10ns ~800nm 5ns 1mJ 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

Physics of EO Transposition Coulomb spectrum shifted to optical region Coulomb pulse temporally replicated 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 a phase change which is detected via polarization measurements Intensity ν few mm tens μm λt 800nm Coulomb field Optical field ~50fscirca 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 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 s tandard and robust optical diagnostic. Retrieves spectral intensity and phase from spectrally resolved autocorrelation. SpectrumSpectral 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 “Carrier” frequencyCan’t measure What we want to know Can be retrieved!

David Walsh, University of Dundee. CLIC Workshop 2014 Investigation into EO Transposition Δν ~44GHz Δ 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 Input pulse characteristics Optical probe lengthΔt ~ 10ps Optical probe energyS ~ 28nJ THz field strength maxE ~ 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 Pulse energy of ~15nJ is predicted 1μJ required for the commercial single-shot FROG, “Grenouille” Example: Pulse energy 1mJ Pulse duration 10ns “Typical” nanosecond pulse laser as probe Coulomb field for target CLIC bunch parameters (CDR) Bunch length 44μm Bunch charge 0.6pC PropertyFactor of improvement x36 ÷100 2 ÷2 2 x186 2 Overallx31 Extrapolation to CLIC parameters

David Walsh, University of Dundee. CLIC Workshop 2014 Parametric Optical Amplification Heavily attenuated 800nm, 50fs pulse 20mm BBO θ = α = nm, ~300MW/cm 2 optic axis θ α Photodiode or Spectrometer 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 = 5.6x10 6 fs 2 Calculations indicate nanosecond level jitter has negligible effect PulsePropertiesPeak Power 532nm Pump 10mJ, 10ns Gaussian temporal profile 1x10 6 W 800nm EO Transpostion Signal Amplified signal energy > 1 μJ, ~50 fs 20x10 6 W As above but stretched GVD = 5.6x10 6 fs 2 > 1 μJ, ~310 ps3.2x10 3 W >Pump! Not possible! Conjugate Stretcher and Compressor designs 1m 0.5m All gratings G=1200 lines/mm Θ deviation ~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 1.5mm 150μm Early 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 processes is 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 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 % 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 Resolution EO transposition scheme is now limited by 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 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!

David Walsh, University of Dundee. CLIC Workshop 2014 Compositing Spectral Data Theoretical response functions 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 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 Dundee) Engineer working system towards being a “turn-key” measurement Expand to multiple bunch monitor – Micro bunch evolution within macro bunch

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EOT System Layout David Walsh, University of Dundee. CLIC Workshop 2014