Boscolo, Ilario

Cialdi, Simone

Gatti, Giancarlo

Ghigo, Andrea

Mattioli, Mario

Musumeci, Pietro

Petrarca, Massimo

Vicario, Carlo

CERN Document Server

EU contract number RII3-CT-2003-506395 CARE-Conf-06-088-PHIN  PHIN     PRODUCTION OF TEMPORALLY FLAT TOP UV LASER PULSES FOR SPARC PHOTOINJECTOR  M. Petrarca, P. Musumeci, M. C. Mattioli, Univ. “La Sapienza” INFN sez. Rome 1, Rome, Italy  C. Vicario, G. Gatti, A. Ghigo, INFN-LNF, Frascati, Rome, Italy  S. Cialdi, I. Boscolo, INFN sez. Milan, Milan, Italy  Abstract  In the SPARC photoinjector, the amplified Ti:Sa laser system is conceived to produce an UV flat top pulse profile required to reduce the beam emittance by minimizing the non-linear space charge effects in the photoelectrons pulse. Beam dynamic simulations indicate that the optimalpulse distribution must be flat top in space and timewith 10 ps FWHM duration, 1 ps of rise and fall time and a limited ripple on the plateau. In a previous work [1] it was demonstrated the possibility to use a programmable dispersive acousto-optics (AO) filter to achieve pulse profile close to the optimal one. In this paper we report the characterization of the effects of harmonics conversion on the pulse temporal profile. A technique to overcome the harmonics conversion distortions on the laser pulses at the fundamental wavelength in order to obtain the target pulse profile is explained too. Measurements and simulations in the temporal and spectral domain at the fundamental laser wavelength and at the second and third harmonics are presented in order to validate our work. It is also described a time diagnostic device for the UV pulses.  Contribution to the EPAC 2006, Edinburgh, Scotland     Work supported by the European Community-Research Infrastructure Activity under the FP6 “Structuring the European Research Area” programme (CARE, contract number RII3-CT-2003-506395) PRODUCTION OF TEMPORALLY FLAT TOP UV LASER PULSES FORSPARC PHOTOINJECTOR∗M. Petrarca† , P. Musumeci, M. C. Mattioli, Univ. “La Sapienza” INFN sez. Rome 1, Rome, ItalyC. Vicario, G. Gatti, A. Ghigo, INFN-LNF, Frascati, Rome, ItalyS. Cialdi, I. Boscolo, INFN sez. Milan, Milan, ItalyAbstractIn the SPARC photoinjector, the amplified Ti:Sa lasersystem is conceived to produce an UV flat top pulse pro-file required to reduce the beam emittance by minimizingthe non-linear space charge effects in the photoelectronspulse. Beam dynamic simulations indicate that the opti-mal pulse distribution must be flat top in space and timewith 10 ps FWHM duration, 1 ps of rise and fall time anda limited ripple on the plateau. In a previous work [1] itwas demonstrated the possibility to use a programmabledispersive acousto-optics (AO) filter to achieve pulse pro-file close to the optimal one. In this paper we report thecharacterization of the effects of harmonics conversion onthe pulse temporal profile. A technique to overcome theharmonics conversion distortions on the laser pulses at thefundamental wavelength in order to obtain the target pulseprofile is explained too. Measurements and simulations inthe temporal and spectral domain at the fundamental laserwavelength and at the second and third harmonics are pre-sented in order to validate our work. It is also described atime diagnostic device for the UV pulses.INTRODUCTIONThe SPARC project (Sorgente Pulsata Autoamplificatadi Radiazione Coerente) is a 150 MeV advanced pho-toinjector designed to drive a SASE-FEL in the visible(∼ 300 − 500 nm) [4]. The photoinjector, that is beingdeveloped at LNF, is conceived to explore the emittancecorrection technique and high current production with thepreservation of the transverse emittance in order to obtainhigh brightness electron beam. This is an important re-quirement for the good development of the SPARC SASEFEL and in particular for its future upgrade to reach the softx-ray. The accelerator consists of a Ti:Sa laser to illuminatea metal photocathode, an rf-gun and 3 SLAC s-band accel-erating sections. As it has been demonstrated [3, 4], thegeneration of high energy laser pulses with a few picosec-onds time width, center wavelength at 266 nm, flat toptemporal profile with fast rise and fall time, is required tostart the photo-emission process from metal cathodes in or-der to achieve electron bunches characterized by low trans-verse emittance∼ 1 mm−mrad and high current. In orderto manipulate the Gaussian natural shape of the usual laser∗Supported by: EU Commission in the FP6 program, CARE contractNo. RII3-CT-2003-506395 and EUROFEL contract No. SSA - 011935.† massimo.petrarca@roma1.infn.itpulses and transform it into a uniform profile as requiredby SPARC, the following Ti:Sa laser chain has been cho-sen [4, 5]. A Ti:Sa oscillator generates 100 fs pulses witha repetition rate (r.r.) of 79.3 MHz. An acousto-optic pro-grammable dispersive filter called “DAZZLER” [6], usedto modify the spectral amplitude and phase function isplaced between the oscillator and the amplifier to obtain thetarget temporal profile. The amplifier is based on the wellknown “chirp pulse amplification” (CPA) process [7] and itis composed by a regenerative and a multipass amplifier. Itdelivers pulses at λ = 800 nm with energy of ∼ 50 mJand r.r. of 10 Hz. The pulses get into the harmonic conver-sion box (HGCB) where UV pulses with an energy up to∼ 4 mJ are produced. The HGCB is characterized by twotype-I BBO (Beta Barium Borate) crystals of 0.5 mm and0.3 mm used to produce respectively the second harmonicgeneration signal (SHG) and the third harmonic generationsignal.At the end of the laser chain there is a grating stretcherused to broaden temporally the pulses up to (8 − 12) ps.In this paper we present some pulse shape measurementsgoing through the evolution from the fundamental towardsthe third harmonic generation and we show how to operateto obtain the target pulse shape. At the end we show howthe UV pulse shape is retrieved.PULSE SHAPINGIn the previous work [8], we showed that by using theDAZZLER to introduce a proper spectral phase and am-plitude modulation in the initial IR pulses, it is possible tomanipulate their natural Gaussian shape to obtain the flattop profile with the right features required from SPARC.The obtained pulses profile is no more maintained as soonas the pulses go into the SHG and THG crystals to pro-duce UV light. The reasons for these are fully explainedin [9, 10] and briefly reported in the following for the sim-plest case of Type-I SH generation. In the situation of ”nondepletion” condition [11] and perfect phase-matching, theSH signal is given by:I˜2 ∝ sinc2 (βωd) d2∣∣∣[A˜1 ⊗ A˜1]∣∣∣2(1)where: β is a factor that takes into account the group ve-locities of the pulses inside the crystal, d is the crystallength and the dispersion factor is neglected due to the shortcrystal length (500 μm). Being the signal proportional tothe convolution product of the power spectrum A˜1(ω), itTHPCH153 Proceedings of EPAC 2006, Edinburgh, Scotland3152 07 Accelerator TechnologyT22 LasersFigure 1: From the left to the right: measured IR, BLUEand UV spectra are reported in solid line; simulated spec-tra for second and third harmonic, (dashed line) are alsoshown. The chirp increases from the top to the bottom.is clear that it is not possible to obtain a SH rectangularspectrum starting by just a first harmonic rectangular one,in fact the convolution product will yield a triangle. Onthe contrary by introducing a chirp into the fundamentalsignal, the initial rectangular shape can be preserved. Infact, considering A1(ω) = S(ω)eiφ(ω) with S(ω) real andφ(ω) = αω2/2, where the α parameter is the chirp strengthintroduced by the DAZZLER, we obtain:A˜1 ⊗ A˜1(ω) =∫S(ω−Ω)S(Ω)ei(φ(ω−Ω)+φ(Ω))dΩ (2)Since in the phase factor, φ(ω − Ω) + φ(Ω) is a parabolicfunction centered at Ω = ω/2, the dominant contributioncomes for Ω close to ω/2 when enough chirp is applied.Thus we can write:∫S(ω−Ω)S(Ω)ei(φ(ω−Ω)+φ(Ω))dΩ ∼ S2(ω/2)e2iφ(ω/2)(3)Therefore in the fundamental beam, the leading behav-ior of the SH spectral intensity is given by the square ofthe fundamental with twice the spectral width and the SHspectrum changes its shape from triangular to rectangu-lar. Moreover, being the SH spectrum proportional to thesquare of the initial one, the increasing of the ripple in thehigher harmonic is explained too. The above considera-tions can be extended to the third harmonic generation. Theresults of this discussion is that it is necessary to introducea large enough chirp factor on the 800nm pulse in orderto produce a UV rectangular spectrum. The amount of thischirp has to be determined accurately because a large valuecan reduce too much the conversion efficiency. These con-siderations are in good agreement with the experimentaldata reported in Fig. (1).Figure 2: Comparison between the spectral, solid line, andtemporal profile, dashed line, in two cases.SPECTRUM TRANSFERRED IN TIMEIn the previous section we demonstrate that by using theDazzler to introduce proper spectral phase and amplitudemodulation in the fundamental beam, it is possible to pro-duce flat top UV spectra. Now we show that it is possible tohave the same produced spectra profiles but in the time do-main. In this way, once the correct flat top spectrum profileis obtained for the TH signal, the desired temporal shape isalmost obtained too [10]. This is achieved by introducingenough temporal linear chirp into the TH signal by send-ing the pulse trough the UV stretcher. In Fig. (2) two ex-amples are shown: we compare the spectral profile, solidline, and the time profile, dashed line. In both cases weobtained a good superposition between the spectrum andthe temporal profile. These two profiles, have been pro-duced by introducing with the Dazzler a proper phase andamplitude modulations in the fundamental. In Fig. (2), thespectral x-axis has been converted in the time coordinateusing the relation Δt = a · Δω (in our case a = 0.3 ps2)between angular frequencies and time delay which is in-troduced by the stretcher and that will be discussed lateron. In these two cases we have a strong pulse stretching,in fact the UV pulse length before the stretcher is about300 fs while the final pulse length is about 10 ps. Thetemporal profile has been traced by a UV-IR cross correla-tor that has been built, studied and characterized [12, 13]and briefly described later on. For completeness we reporthere the relation between the time profile and its spectrum.Let us consider a pulse with complex amplitude A(ω) anda quadratic phase function given by exp [i · aω 2/2]; the aparameter takes into account the chirp introduced by thestretcher. The time intensity profile, given by the squaremodulus of the inverse Fourier transform, turns out to be:I(t) =∣∣∣∣∫A(ω)ei(aω22 −ωt)dω∣∣∣∣2∼ I(ω = t/a) (4)where I(ω) = |A(ω)|2 is the power spectrum. In fact,since the phase function is a parabola centered at ω = t/a,when the stretched pulse has a temporal width much longerthan the input one, the leading contribution to the integralProceedings of EPAC 2006, Edinburgh, Scotland THPCH15307 Accelerator TechnologyT22 Lasers3153comes from the angular frequencies near the minimum ofthe phase. Therefore the spectral profile can be used as atime profile diagnostic. Moreover the temporal coordinatecan be obtained by the relation Δt = a ·Δω .TIME DIAGNOSTICTo measure the UV time pulse length a cross-correlatordevice based on the difference frequency generation (DFG)process is made using a BBO crystal by mixing a part of theIR pulses at frequency ω with the UV pulses at frequency3ω [12, 13]. Since the IR pulses of intensity IIR have atemporal width much shorter ∼ 100fs (thus are called ref-erence pulses) than the UV pulses∼ 10ps of intensity IUV ,the signal produced at frequency 3ω − ω is proportional tothe cross correlation function of the UV pulses:I(3ω − ω, τ) ∝∫IUV (3ω, t)IIR(ω, t− τ)dt (5)The device is capable to measure ∼ 100fs UV full widthhalf maximum pulses. A complete and more detailed de-scription of the device and of the physics behind it, is re-ported in [12, 13] where it is analyzed and discussed thepresence of the spatial chirp in the UV beam and it ispointed out the importance of the crystal angle alignment.In fact changing the crystal phase matching angle, the tem-poral shape of the harmonic signal generated as well as itsefficiency changes. The changes of the temporal shape dueto the crystal angle mismatching have been observed. Toovercome this problem the following procedure has beenadopted: we first generated a two peaks spectrum profileand then we aligned the crystal angle in order to have thesame shape in the time domain. This is possible once againbecause in our case the spectrum shape is transferred in thetime domain.CONCLUSIONIn this paper we have presented a technique to pro-duce the desired UV flat top laser pulses required from theSPARC project to achieve high brightness electron beamthrough the photoemission process form e metal photocath-ode. The whole technique is based on the Dazzler. Usingthis device it is possible to introduce the proper spectralphase and amplitude modulation in the initial IR pulses inorder to manipulate their spectral shape and to obtain thetarget UV spectrum profile. A way to eliminate the distor-tions introduced by the SH and TH crystals is explained.This is done by introducing trough the Dazzler a chirp fac-tor in the fundamental beam. We also showed that the UVspectrum shape can be transferred to time profile by intro-ducing a large enough chirp factor in the UV beam. So thatonce the desired UV spectrum is achieved, the temporalshape is obtained too. As it is shown in Fig. (2), the desiredrise and fall time of the pulses are longer (∼ 2.5 ps) thanthe required one < 1 ps. This will be the subject of furtherstudies.A time diagnostic device necessary to study the time profileof UV pulses has been presented: the UV-IR cross correla-tor.We acknowledge the supports of the EU Commissionin the FP6 program, CARE contract No. RII3-CT-2003-506395 and EUROFEL contract No. SSA - 011935.REFERENCES[1] H. Loos et al. ”Temporal E-Beam Shaping in an S-BandAccelerator”, Proc. Particle Accelerator Conference, p.642,2005, Knoxville, TN, USA.[2] M. Ferrario, M. Boscolo, V. Fusco, C. Vaccarezza, C. Roncis-valle, J. B. Rosenzweig, L. Serafini, in ICFA Workshop “Thephysics and applications of high brigthness electron beams”(Chia Laguna, Sardinia, Italy, July 2002)[3] J. Yang, F. Sakai, T. Yanagida, M. Yorozu, Y. Okada, K.Takasago, A. Endo, A. Yada, M. Washio, J. Appl. Phys. 92,1608 (2002)[4] Eds: L. Palumbo, J. Rosenzweig. Autori: D. Alesini etal. (INFN/LNF), F. Alessandria et al. (INFN/Milano), D.Levi, M. Mattioli, G. Medici (INFN/Roma1), L. Catani etal. (INFN/Roma2), G. Dattoli et al. (ENEA/FIS), S. Reiche,J. Rosenzweig, G. Travish (UCLA), D. Dowell et al. (SLAC),Technical Design Report for the SPARC Advanced Photo-Injector, Preprint INFN National Laboratories of Frascati,12/01/2004[5] C, Vicario et al., Commissioning of the laser system forSPARC photoinjector Proceedings of EPAC 2006 (Edim-burgh, Scotland)[6] F. Verluise, V. Launde, J. P. Huignard, P Tournois, A. Migus,j. Opt. Soc. Am. B, 17, 138 (2000)[7] P. Maine, D. Strickland, P. Bado, M. Pessot, G. Mourou,IEEE J. Quantum Electronics , 24, 398, (1988).[8] C. Vicario, M. Petrarca, A. Ghigo, Laser temporal pulseshaping experiment for SPARC photoinjector, Proceedings ofEPAC 2004 (Lucerne, Switzerland)[9] S.Cialdi, F. Castelli, I. Boscolo, Appl. Phys. B, 3, (2006),383-389.[10] C. Vicario, M. Petrarca, S. Cialdi High power third har-monic flat pulse laser generation Accepted for publication byOptics Letters[11] A. Yariv, Quantum Electronics, (Wiley, New York, 1989,3rd ed.).[12] M.Petrarca, C. Vicario, S. Cialdi et al. UV-IR Cross Corre-lator, Report INFN-SPARC/LS-06/002 (2006).[13] M. Petrarca, C. Vicario, S. Cialdi, in preparationTHPCH153 Proceedings of EPAC 2006, Edinburgh, Scotland3154 07 Accelerator TechnologyT22 Lasers

Production of Temporally Flat Top UV Laser Pulses for SPARC Photoinjector

Production of Temporally Flat Top UV Laser Pulses for SPARC Photoinjector

Abstract

Similar works

Full text

Available Versions

CERN Document Server