Single-shot electro-optic sampling (EOS) is a powerful characterization tool for monitoring the shape of electron bunches, and coherent synchrotron radiation pulses. For reaching high acquisition rates, an efficient possibility consists to associate classic EOS systems with the so-called photonic time-stretch technique [1]. We present recent results obtained at SOLEIL and ANKA using this strategy. In particular, we show how a high sensitivity variant of photonic time stretch [2] EOS enabled to monitor the CSR pulses emitted by short electron bunches at SOLEIL [3]. We could thus confirm in a very direct way the theories predicting an interplay between two physical processes. Below a critical bunch charge, we observe a train of identical THz pulses stemming from the shortness of the electron bunches. Above this threshold, CSR emission is dominated by drifting structures appearing through spontaneous self-organization. We also consider the association of time-stretch and EOS for recording electron bunch near fields at high repetition rate. We present preliminary results obtained at ANKA, aiming at recording the electron bunch shape evolution during the microbunching instability

Bielawski, S.

Blomley, E.

Brubach, J.-B.

Bründermann, E.

Evain, C.

Funkner, S.

Hiller, N.

Labat, M.

Le Parquier, M.

Manceron, L.

Müller, A.-S.

Nasse, M. J.

Niehues, G.

Roussel, E.

Roy, P.

Schuh, M.

Schönfeldt, P.

Steinmann, J. L.

Szwaj, C.

Tordeux, M.-A.

Walter, S.

English

KITopen

HIGH REPETITION-RATE ELECTRO-OPTIC SAMPLING: RECENTSTUDIES USING PHOTONIC TIME-STRETCHC. Evain, C. Szwaj, E. Roussel, M. Le Parquier, S. Bielawski∗PhLAM, Université Lille 1, FranceEléonore Roussel, J.-B. Brubach, L. Manceron, M.-A. Tordeux, M. Labat, P. Roy,Synchrotron SOLEIL, Gif-Sur-Yvette, FranceNicole Hiller, Paul Scherrer Institute, PSI (Switzerland)Edmund Blomley, Stefan Funkner, Erik Bründermann, Michael Johannes Nasse,Gudrun Niehues, Patrik Schönfeldt, Marcel Schuh, Johannes Leonard Steinmann, Sophie Walter,and Anke-Susanne MüllerKarlsruhe Institute of Technology (Germany)AbstractSingle-shot electro-optic sampling (EOS) is a powerfulcharacterization tool for monitoring the shape of electronbunches, and coherent synchrotron radiation pulses. Forreaching high acquisition rates, an efficient possibility con-sists in associating classic EOS systems with the so-calledphotonic time-stretch technique. We present several setupsthat may be used for adding the time-stretch functionality toexisting EOS systems, and focus on experimental tests madein two situations. At SOLEIL, we present a setup which isoptimized for high SNR recording of THz CSR pulses. AtKARA (Karlsruhe Research Accelerator), the storage ring ofthe test facility and synchrotron radiation source ANKA atKIT, we show how the time-stretch strategy can be tested us-ing almost no modification of an existing spectrally-encodedEOS system. Finally we present recent results on CSR andthe microbunching instability that have become accessibleusing photonic time stretch.INTRODUCTION: HIGH REPETITIONRATE EOSSingle-shot Electro-Optic Sampling [1] (EOS) is an effi-cient technique for monitoring electron bunch shapes [2–4]by recording the electric field in the near-field of the bunch,and is also capable of recording the Coherent SynchrotronRadiation emitted by the electrons [5]. The principle(Fig. 1a) consists of modulating a stretched laser pulse withthe electric pulse to be characterized. As a result, the infor-mation is imprinted in the spectrum of the laser pulse, andthe information can be retrieved by recording the spectrumshape. In classical spectral encoding EOS, the spectrum istypically recorded using a diffraction grating and a camera.Although this technique is particularly efficient, operationof EOS at high repetition rates (1 MHz or more) remined upto recently a largely open challenge. The main bottleneck inhigh-speed EOS was the readout part, as commercial linearcameras are typically limited to the 100 KHz range. Twoapproaches to this problem have been undertaken recently:(i) one direction has been to develop specific cameras that∗ serge.bielawski@univ-lille1.frFiber L2(>> L1) Photodetector+ oscilloscopeTimeseveral nanosecondsdispersion (Fiber L1)T1=several psEO crystal polarisationopticsfs laserTHz pulse or e-bunch near fieldTHz pulse or e-bunch near fielddispersion (Fiber L1)T1=several psEO crystal polarisationopticsfs laseroptical spectrum analyzer(grating + camera) wavelength(a)(b)time (ns)Figure 1: Principles of (a) usual spectrally-encoded EOS(b) photonic time-stretch EOS. In both cases a chirped laserpulse is modulated by the electric pulse under investigation,and the main difference concerns the readout. In classicalspectral encoding, the output spectrum is recorded using asingle-shot optical spectrum analyzer (usually composed of agrating and a camera). In photonic time-stretch, a long fiber(typically few kilometers-long) stretched the output signal,so that it can be recorded by a single pixel photodetector andan oscilloscope (typically with few GHz bandwidth).can operate at several Megahertz, the KALYPSO projectat Karlsruhe Institute of Technology [6, 7], (ii) a secondresearch direction has been devoted to an alternate type ofreadout: photonic time-stretch [8–10].In photonic time-stretch EOS [8–10], the output pulse(Fig. 1b) is dispersed in a long fiber (typically with fewkilometers length). As a result, the EOS signal appearsas slowed-down replica of the THz electric field, and canbe recorded using a single pixel commercial photodetectorand an oscilloscope (or acquisition board) with few GHzbandwidth. The acquisition rate can thus be pushed to thehundreds of MHz range, using commercial devices.Historically, the photonic time-stretch technique has beenintroduced by the B. Jalali team in 1999 [11, 12] for increas-ing the bandwidth of A/D digitizers in general. Variantsof the technique have also been widely used for recordingoptical spectra at hundreds of MHz rates [13] (a techniqueknown as Dispersive Fourier Transform, or DFT), as well ashigh repetition-rate imaging [14–16].6th International Beam Instrumentation Conference IBIC2017, Grand Rapids, MI, USA JACoW PublishingISBN: 978-3-95450-192-2 doi:10.18429/JACoW-IBIC2017-TU1AB24 Time-Resolved Diagnostics and SynchronizationTU1AB2121ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.We present here result using the photonic time-stretchapproach in the context of single-shot EOS. We presentresults at SOLEIL on free-propagating THz CSR pulses,and at KARA on near-field EOS of electron bunches.RESULTS AT SOLEIL: ANALYSIS OF THZCSR BURSTSWe first performed a test at SOLEIL in 2013 [8], where wecould record THz Coherent Synchrotron Radiation (CSR)pulses in single bunch mode, at 850 KHz repetition rate.This first experiment has been performed in conditions ofhigh electron bunch charge, in order to reduce as much aspossible the SNR requirement.Then we started to study the microbunching instability, bymonitoring the CSR pulses in more challenging situations.First we considered nominal-alpha regimes at less intensecurrent. Then we aimed at recording the CSR pulses onlow-alpha mode, i.e., the prefered mode for users of CSR.This led us to upgrade our time-stretch setup, with theaim to increase the sensitivity.We first upgraded our laser system by adding a home-made Ytterbium amplifier. This allowed us to test the EOSconfiguration know as near extinction. Data analysis re-vealed that the strategy was indeed efficient for increasingthe EOS signal. We also noticed that the SNRwas essentiallylimited by the noise of the optical source.In order to go further, and attempt approching shot-noiselimited detection, a main challenge was tomanage the opticalnoise of the laser source, and we thus searched a way forperforming EOS while canceling out the laser source noise.In a slightly different context, in Ref. [17], AhmedSavolainen and Hamm presented a trick that allows a near-extinction EOS setup to provide two complementary outputs.Balanced detection can thus be perfomed ar the same timeas near-extinction EOS, opening the way to a drastic noisereduction.In practice, this upgrade could be performed by adding aset of Brewster plate just after the EO crystal, and using thetwo ports of the output polarizer. This setup is representedin Fig. 2, and a detailed characterization of the EO setup canbe found in Ref. [9].In Fig. 3, we displayed a typical series of THz pulses dur-ing a CSR burst in nominal-alpha (i.e., long bunch) mode.5 bunches are simultaneously recorded in this experiment.These CSR pulses are produced by the microbunching in-stability, and are observed above a threshold for the bunchcharge. As expected from theory, below threshold, the aver-age shape of the electron bunch is unable to radiate coher-ently, as the emission is shielded by the vacuum chamber.The situation is different with short electron bunches.If their size is sufficiently small, they can radiate CSRTHz CSR pulsesYDFApulse pickerYb fiber laser(50 fs,1030 nm)2 km PMfiberQWPZnSeBrewsterplatesGaPOAPMPPMfiberPBSdelayVOAbalancedphotodetectorGaPorientation[001][-110]xPolarization states:linear horizontallinear verticalGHz oscilloscopereplica of the THzpulses (at ns scale)Top viewFigure 2: Photonic time-stretch setup designed for highsensitivity EOS. The EO modulation is performed in theGallium Phosphide crystal. Then the time-stretch is providedby the 2 Km-long fiber. As a result, we obtain a “replica” ofthe THz pulses, on the oscilloscope, that are “slowed down”in time, with a factor of the order of 150-200. See Ref. [9]for details.even below the threshold of the microbunching instability.Hence two regimes are expected in this case. Below themicrobunching instability threshold, we expect a CSR radi-ation due to the “shortness” of the electron bunch. Abovethreshold, we expect also an emission due to the appearanceof microstructures in the bunch. This scenario is displayedin Fig. 4, and studied in detail in Ref. [10].Time (ps)Stretched time (ns)-0.200.20.40.60.810 5 10 15 20 25 30 350 1 2 3 4 5 6 7turn n+2turn n+1turn nEOS signal (V)051015200 0.5 1|TF(EOS signal)|Frequency (THz)Figure 3: Series of CSR pulses obtained with long electronbunches (15 ps RMS), above the microbunching instabilitythreshold.PRELIMINARY RESULTS AT KARA:ELECTRON BUNCH NEAR-FIELDIn parallel, we started a joined PhLAM-ANKA/KARAcollaboration aiming at testing the possibility to perform pho-tonic time-stretch on a near-field electron-bunch EOS system.For this preliminary test, we did not specifically adapt the ex- Detectivity Enhancement using “Near Extinc-tion” Scheme with Balanced Detection Results: Long Bunch and Short Bunch Modes 6th International Beam Instrumentation Conference IBIC2017, Grand Rapids, MI, USA JACoW PublishingISBN: 978-3-95450-192-2 doi:10.18429/JACoW-IBIC2017-TU1AB2TU1AB2122ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.4 Time-Resolved Diagnostics and Synchronization(b)(a)0200400600800round-trips n-200200 5 10 15 20V avgEOS (t) (mV)time t (ps)(d)0 5 10 15 20time t (ps)(e)(c)0 5 10 15 20time t (ps)(f)Figure 4: Series of CSR pulses obtained with short electronbunches in the so-called low alphamode. (a): below, and (b),(c) above the microbunching instability threshold. (d), (e),(f): averages of the EOS signals. See Ref. [10] for details.isting EOS setup. Wemainly replaced the single-shot opticalspectrum analyzer by a 2 km HI1060 fiber, followed by anamplified InGaAs photodetector (Discovery SemiconductorDSC-R412).We optimized the dynamic range of the detection, byadded an Ytterbium-doped fiber preamlifier before thestretching fiber. In order to subtract the reference laser pulses(without EOS signal) we proceeded in an analog way. Wefed the two balanced detector inputs with successive laserpulses (with and without EOS signal). The stretch factorwas M=80, and we low-pass filtered the data at 5 GHz at theprocessing stage.A typical series of EOS signals (at each turn in the ring) isrepresented in Fig. 5. Although we did not work specificallyon the SNR optimization, we can already see dynamicalfeatures as a global oscillation at the synchrotron frequency,and even the appearance of microstructures (at the right),which may be attributed to the microbunching instability.Data analysis showed guidelines for further SNR improve-ment. The SNR is still limited by the available laser power.We thus expect improvements in the next experimental shifts,by optimizing the losses in the EOS setup. A second foreseenpossibility is a modification of the EOS setup for cancellingout the laser common noise, using balanced detection tech-nique of Ref. [9].CONCLUSION, FUTURE DIRECTIONSPhotonic time-stretch appears as a viable candidate forhigh-repetition-rate acquisition of single-shot EOS signals.The acquisition rate can in be extended in principle up tothe hundreds of MHz range (i.e., the typical repetition rateof laser oscillators). Tests can be made on existing EOSsystems, with minimal modifications of the EOS part. Whenneeded, SNR can be significantly improved by a slight mod-ification of the EOS design for performing balanced detec-tion [9].Figure 5: Preliminary result: Time-stretch EOS recordingof the electron-bunch near-field at KARA, at each turn (i.e.,at 2.7 MHz acquisition rate). Left vertical scale (Fast time)corresponds to the input time, Right vertical time (Stretchedtime) corresponds to the time at oscilloscope input. Hori-zontal time scale corresponds to the number of round-trips.Future works concern the improvements of time-stretchEOS performances for specific applications. Detection offree-propagating THz pulses requires high-sensitivity. De-tection of electron bunch near-fields can required high dy-namic range, when small structures (i.e., due to the mi-crobunching instability) are the object of interest. Anotherimportant direction consists of performing time-stretch EOSat the 1550 nm wavelength, in order to take advantage of thewidely developed high-quality (and low-cost) componentsof the telecommunication market.Finally, another important question concerns the compari-son of the performances of camera-based spectrally-encodedEOS, with photonic time-stretch, in order to evaluate theirrespective best domains of application.ACKNOWLEDGEMENTSThe work has been supported by the BQR of Lille Uni-versity (2015). The work has also been supported bythe Ministry of Higher Education and Research, Nord-Pas de Calais Regional Council and European RegionalDevelopment Fund (ERDF) through the Contrat de Pro-jets État-Région (CPER photonics for society), and theLABEX CEMPI project (ANR-11- LABX-0007). Prepa-ration of the experiment used HPC resources from GENCITGCC/IDRIS (i2015057057, i2016057057).The work per-formed at KARA, KIT has been supported by the GermanFederal Ministry of Education and Research (Grant Nos.05K10VKC and 05K13VKA).REFERENCES[1] Jiang, Z. & Zhang, X.-C. Electro-optic measurement of THzfield pulses with a chirped optical beam. Appl. Phys. Letters72, 1945 (1998).[2] Wilke, I. et al. Single-shot electron-beam bunch length mea-surements. Phys. Rev. Lett. 88, 124801 (2002).[3] Steffen, B. et al. Electro-optic time profile monitors forfemtosecond electron bunches at the soft x-ray free-electronlaser flash. Physical Review Special Topics-Accelerators andBeams 12, 032802 (2009).[4] N. Hiller, A. Borysenko, E. Hertle, V. Judin, B. Kehrer, A.-S.Müller, M.J. Nasse, P. Schönfeldt, M. Schuh, N.J. Smale,J.L. Steinmann, B. Steffen, P. Peier, V. Schlott, "Single-shot6th International Beam Instrumentation Conference IBIC2017, Grand Rapids, MI, USA JACoW PublishingISBN: 978-3-95450-192-2 doi:10.18429/JACoW-IBIC2017-TU1AB24 Time-Resolved Diagnostics and SynchronizationTU1AB2123ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.Electro-optical Diagnostics at the ANKA Storage Ring" inProceedings of IBIC2014, Monterey, CA, USA, MOPD17,pp. 182-186 (2014).[5] Müller, F. et al. Electro-optical measurement of sub-ps struc-tures in low charge electron bunches. Phys. Rev. ST Accel.Beams 15, 070701 (2012).[6] L. Rota, M. Balzer, M. Caselle, M. Weber, N. Hiller, A. Moz-zanica, C. Gerth, B. Steffen, D. R. Makowski, A. Mielczarek,"KALYPSO: A Mfps Linear Array Detector For Visibleto NIR Radiation" in Proceedings of IBIC2016, Barcelona,Spain, WEPG46, pp. 740-743 (2016).[7] P. Schönfeldt, E. Blomley, E. Bründermann, M. Caselle, S.Funkner, N. Hiller, B. Kehrer, M. J. Nasse, G. Niehues, L.Rota, M. Schedler, M. Schuh, M. Weber, A.-S. Müller, "To-wards Near-Field Electro-optical Bunch Profile Monitoringin a Multi-bunch Environment" in Proceedings of IPAC2017,Copenhagen, Denmark, MOPAB055, pp. 227-230 (2017).[8] Roussel et al. Observing microscopic structures of a relativis-tic object using a time-stretch strategy. Scientific Reports 5,10330 (2015).[9] Szwaj, C. et al. High sensitivity photonic time-stretch electro-optic sampling of terahertz pulses. Review of Scientific In-struments 87, 103111 (2016).[10] Evain, C. et al. Direct observation of spatiotemporal dy-namics of short electron bunches in storage rings. PhysicalReview Letters 118, 054801 (2017).[11] Coppinger, F., Bhushan, A. & Jalali, B. Photonic time stretchand its application to analog-to-digital conversion. IEEETrans. on Microwave Theory and Techniques 47, 1309 (1999).[12] Mahjoubfar, A. et al. Time stretch and its applications. NaturePhotonics 11, 341–351 (2017).[13] Goda, K. & Jalali, B. Dispersive fourier transformation forfast continuous single-shot measurements. Nature Photonics7, 102 (2013).[14] Chen, C. L., Mahjoubfar, A. & Jalali, B. Optical data com-pression in time stretch imaging. PloS one 10, e0125106(2015).[15] Chen, C. L. et al. Deep learning in label-free cell classifica-tion. Scientific reports 6 (2016).[16] Mahjoubfar, A., Chen, C. L. & Jalali, B. Design of warpedstretch transform. Scientific reports 5 (2015).[17] Ahmed, S., Savolainen, J. & Hamm, P. Detectivity enhance-ment in thz electrooptical sampling. Review of ScientificInstruments 85, 013114 (2014).6th International Beam Instrumentation Conference IBIC2017, Grand Rapids, MI, USA JACoW PublishingISBN: 978-3-95450-192-2 doi:10.18429/JACoW-IBIC2017-TU1AB2TU1AB2124ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.4 Time-Resolved Diagnostics and Synchronization

High repetition-rate electro-optic sampling: Recent studies using photonic time-stretch

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High repetition-rate electro-optic sampling: Recent studies using photonic time-stretch

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