The performance of the Beamlet laser with 1D SSD implemented is investigated. Simulations indicate that the critical issue for laser performance is the amount of additional divergence owing to SSD in comparison to the size of the spatial filter pinholes. At the current {plus_minus}200 {mu}rad pinholes used on Beamlet, simulations indicate that the levels of SSD divergence anticipated for the National Ignition Facility (NIF) results in a very slight degradation to the near field beam quality. Experiments performed with the Beamlet front end show no degradation to the near field beam with up to 100 {mu}rad of SSD divergence. Measurements of the smoothing of a far field speckle pattern generated by a phase plate show the expected improvement in contrast with increasing amounts of SSD divergence

Henesian, M.

Moran, B.

Rothenberg, J.E.

Van Wonterghem, B.

UNT Digital Library

January 8, 1997This is a preprint of a paper intended for publication in a journal or proceedings.  Sincechanges may be made before publication, this preprint is made available with theunderstanding that it will not be cited or reproduced without the permission of theauthor.LawrenceLivermoreNationalLaboratoryUCRL-JC-124506PREPRINTThis paper was prepared for submittal to the2nd Annual International Conference onSolid-State Lasers for Application to ICFParis, FranceOctober 22-25, 1996J. E. Rothenberg, B. Moran,M. Henesian, and B. Van WonterghemPerformance of Smoothing by SpectralDispersion (SSD) on BeamletDISCLAIMERThis document was prepared as an account of work sponsored by an agency ofthe United States Government.  Neither the United States Government nor theUniversity of California nor any of their employees, makes any warranty, expressor implied, or assumes any legal liability or responsibility for the accuracy,completeness, or usefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privately owned rights.Reference herein to any specific commercial product, process, or service by tradename, trademark, manufacturer, or otherwise, does not necessarily constitute orimply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of California.  The views and opinions of authorsexpressed herein do not necessarily state or reflect those of the United StatesGovernment or the University of California, and shall not be used for advertisingor product endorsement purposes.Performance of Smoothing by Spectral Dispersion (SSD) on BeamletJoshua E. Rothenberg, Bryan Moran, Mark Henesian, and Bruno Van WonterghemLawrence Livermore National Laboratory, L-439P. O. Box 808, Livermore, CA  94551Phone: (510) 423-8613, FAX: (510) 422-5537email:  JR1 @ LLNL.GOVABSTRACTThe performance of the Beamlet laser with 1D SSD implemented is investigated.  Simulationsindicate that the critical issue for laser performance is the amount of additional divergenceowing to SSD in comparison to the size of the spatial filter pinholes.  At the current ±200 µradpinholes used on Beamlet, simulations indicate that the levels of SSD divergence anticipatedfor the National Ignition Facility (NIF) results in a very slight degradation to the near fieldbeam quality.  Experiments performed with the Beamlet front end show no degradation to thenear field beam with up to 100 µrad of SSD divergence.  Measurements of the smoothing of afar field speckle pattern generated by a phase plate show the expected improvement incontrast with increasing amounts of SSD divergence.Keywords:  Beam smoothing, smoothing by spectral dispersion, inertial confinement fusion.1. INTRODUCTIONOptimal performance of the inertial confinement fusion (ICF) target requires that the driverlaser beam be spatially conditioned such that the illumination on target is sufficiently uniform.   For indirect drive the enhancement of laser plasma instabilities by illumination nonuniformityunderlies the requirement to smooth the target illumination.  For direct drive, a muchsmoother beam is required because the illumination nonuniformity is directly imprinted ontothe target surface and is subsequently amplified by hydrodynamic instabilities.  A number ofapproaches have been suggested to achieve the desired level of uniformity for ICF.1-5  Allthese approaches make use of target illumination which is comprised of a time varying specklepattern.  Over a characteristic integration time set by target physics, the addition of statisticallyindependent speckle patterns blurs out the instantaneous spatial variations of intensity on thetarget.  Assuming Gaussian statistics, the RMS variation of the integrated fluence normalizedto the average is reduced from 1.0 (i.e. 100% for a static speckle pattern) to ~ 1 / N , where Nis the effective number of independent speckle patterns which are integrated.2The SSD method4 is particularly well suited to ICF with glass lasers because the pure phasemodulation used preserves the near field beam quality, and thereby allows for efficientextraction of energy from the amplifiers.  For a modest uniformity requirement (indirect drive,RMS 15 - 25 %) it is thought that the 1D SSD method is sufficient, and for a more uniformbeam (direct drive, RMS < 10%) 2D SSD is required.  A schematic of the 1D SSD technique isshown in Fig. 1.  In the usual implementation, pure sinusoidal phase modulation (FM) isimposed on the seed beam in the front end.  A diffraction grating or other dispersive elementthen induces angular divergence on the seed beam.  Each FM sideband is shifted off axis inproportion to its frequency shift, and thus the 1D SSD beam would focus to a series of shiftedspots in a line in the far field (hence, 1D SSD).  With the presence of a random phase plate,2this series of far field focal spots becomes a series of shifted speckle patterns.  To obtainoptimal smoothing these speckle patterns must be shifted by at least the half width of aspeckle, which, expressed as a far field angle is λ / D, where D  is the final beam width.  ForBeamlet or NIF, this minimum shift is ~3 µrad.  Thus if N  sidebands are imposed by themodulator, the required beam divergence in the main cavity will be ~3· N  µrad.  This increaseddivergence provides an essential limitation for 1D SSD, i.e. increased clipping from the spatialfilter pinholes, which leads to increased near field modulation and therefore reduced amplifierextraction.  Based on our current understanding of the target requirements, it is anticipatedthat the 1D SSD divergence required for indirect drive is ~ 25 µrad, and for direct drive 2DSSD is required with as much as 50 µrad.  In this paper we describe ongoing experiments andnumerical simulations which address the issue of performance on Beamlet with 1D SSD.PhasePlateDFrequencyFM↑∆θ= λ/DTargetPlaneFigure 1: Schematic of the arrangement used for the 1D SSD method.  Without a phaseplate, each sideband of the modulator would focus to a spot shifted by aminimum angle ( λ / D).  The phase plate then generates a series of specklepatterns, each shifted by this angle.31. NUMERICAL SIMULATIONSFigure 2 shows the far field distribution of an ideal clean beam as a function of increasingmodulation depth β , where the grating dispersion is chosen to provide "critical dispersion",i.e. where the adjacent sidebands of the modulator are separated by exactly λ / D.  Thissituation is also referred to as having a single "color cycle", since in this case a single cycle ofthe FM appears skewed across the beam.  For a modulation depth of β , the number of FMsidebands is ~2β , and thus the induced divergence as shown is ~6 β  µrad (throughout thispaper the far field angles discussed are scaled to the beam size in the main cavity ( D ≅  35 cm)so that the diffraction limited angular spacing λ / D ≅  3 µrad).  The pinholes in the Beamletmain cavity are set at ±200 µrad, and thus it appears as if there will be no interaction of theclean beam even at the largest beam divergence shown (~ 60 µrad).  However, note that thissimple calculation does not include any nonlinear effects nor aberrations of the opticalelements.Far field angle (µrad)β = 5  β = 10β = 0   β = 2.5-100       -50     0         50   100 -100       -50     0         50   100Figure 2: Integrated far field fluence of an ideal clean beam with critically dispersed (onecolor cycle) SSD and the indicated modulation depth (β ).4The more realistic situation including nonlinearities and estimated optical aberrations ismodeled in Fig. 3 using PROP92.  In this calculation, the integrated far field fluence is shownat the transport spatial filter plane (before the pinhole) for a 12 kJ, 3 ns pulse and ±200 µradmain cavity spatial filter pinholes.  In this figure the characteristic FM spectrum of Fig. 2 issuperimposed upon a noise background.  The cutoff effect of the main cavity pinholes is veryclearly manifested in the zero bandwidth case by a one decade drop in the noise floor at ±200µrad.  As the SSD divergence is increased this drop-off in the noise floor at ±200 µraddiminishes and is totally absent at 100 µrad of SSD divergence.  Also one sees that the noisefloor at the pinhole edge is ~ 10-4 of the peak without SSD and increases to ~ 10-3 of the peakas the SSD divergence in increased.  It is clear here that as the SSD divergence increases, the farfield energy is shifted towards the pinhole edge, and the modulation will increase in the nearfield owing to Gibbs related phenomena and nonlinear (Bespalov - Talanov) growth.SSD,25 µradSSD,50 µradSSD,100 µradNoBandwidth-200         -100         0     100  200Far field angle (µrad)-200         -100         0     100  20010010-110-210-310-410-510010-110-210-310-410-510010-110-210-310-410-510010-110-210-310-410-5Figure 3: PROP92 calculations showing the integrated far field intensity at the transportspatial filter pinhole plane, for a 12 kJ, 3ns pulse with ±200 µrad pinholes, andwith the indicated amount of SSD divergence.5Figure 4 shows the calculated near field intensity for the above case (12 kJ, 3ns pulse with ±200µrad pinholes) after the transport spatial filter focusing lens and at the end of the pulse, wherethe B-integral effects are severe.  Comparing the case in which SSD is absent (left image), with50 µrad of SSD (right image) one sees quite a small degradation of the beam quality.  The RMScontrast of the intensity (relative to the average) increases from 12% with SSD absent to 14%with 50 µrad of SSD, and the peak to average intensity ratio increases from 1.7 to 1.8.However, if one increases the SSD divergence further to 100 µrad (which is beyond the levelthat is anticipated to be required for ICF on the NIF), then the RMS increases to 17% and thepeak to average intensity ratio increases to 2.1, which represents a significant degradation inthe near field beam quality.  From these calculations we see that the crucial parameter is thedivergence associated with the SSD method.  As this divergence becomes an appreciablefraction of the pass band of spatial pinhole, then the background level of far field intensitynear the pinhole edges increases and thus the near field beam quality degrades.  Theseconsiderations are very much dependent on the level of optical aberrations in the beam in theabsence of SSD.  If one can successfully reduce or compensate for these aberrations, then onewould expect the beam quality in the presence of a given amount of SSD divergence to beimproved as well.  Figure 4: PROP92 calculations showing the near field intensity after the spatial filterfocusing lens at the pulse end, for a 12 kJ, 3ns pulse with ±200 µrad pinholes.The left image is without SSD, has contrast of 12%, and peak to average intensityratio of 1.7.  The right image is with 50 µrad of SSD, has contrast of 14%, andpeak to average intensity ratio of 1.8.63. EXPERIMENTAL INVESTIGATIONFigure 5 shows the modifications which were made to Beamlet to allow for theimplementation of SSD.  The existing integrated optic phase modulator in the master oscillatorroom was used to impose varying amounts of FM on the beam.  After amplification by theregenerative amplifier and spatial shaping, the beam is usually directed by two turningmirrors towards the four pass preamplifier (path shown in bold).  Instead, one of these mirrors(now on a kinematic mount) is removed and the beam enters the SSD apparatus.  The gratingis oriented for exact Littrow diffraction, which allows the simple interchange of gratings ofvaried dispersion.  The diffracted beam is picked off by a 50% beam splitter and directed backinto the four pass preamplifier.  This arrangement although lossy (the throughput is ~20%), isconvenient in that it provides a simple means of maintaining the grating in the  exact Littrowconfiguration.BeamSplitterf/60, 2:1 Telescope in Air2.5 cm beam5 cmsquareinputbeamRelayPlanef = 3mf = 1.5m1200-1800 g/mmGrating(Relay Plane) ↓Existing BeamPathKinematicMirror↓To 4-pass preampFigure 5: Schematic of modifications made to Beamlet to accommodate 1D SSD.Removing the kinematic mirror allows the beam to traverse the SSD sectionbefore going to the four pass preamplifier.7These initial experiments were performed with the beam propagating only as far as the fourpass preamplifier.  Therefore, these results do not address the effects of the opticalnonlinearities in the main amplifier.  The high power performance will be addressed in thenear future in a series of full system shots.  The measured time integrated near fieldpreamplifier beam is shown in Fig 6 without SSD (left) and with 100 µrad of SSD (right).Although there are a number of artifacts in this image from the diagnostics arrangement(streaks of bubbles and various coherent interference fringes), one sees that there is noobservable degradation to the near field as a result of the SSD imposed.  In fact, if onecalculates the contrast and peak to average intensity over a sub-aperture of the beam one findsthat these measures actually improve with the application of SSD.  However this improvementin the beam is presumably just an artifact, a result of SSD bandwidth and divergence washingout of some of the coherent fringe structure.     Figure 6: Near field images of the Beamlet preamplifier output beam.  The left image iswithout SSD, has contrast of 11%, and peak to average intensity ratio of 1.38.  Theright image is with 100 µrad of SSD, has contrast of 6.1%, and peak to averageintensity ratio of 1.33.8The measured far field of the preamplifier output is shown in Fig. 7 for a varied amount ofSSD.  Without SSD present (upper left) the focal spot is nearly diffraction limited (FWHM ~ 3.5µrad).  As the SSD divergence is increased one sees this focal spot spread along a line.  Forcritical dispersion (lower left, Ncc = 1, i.e. a single color cycle), 8 GHz modulation frequency,and 60 GHz of total bandwidth, the FM sidebands are not quite resolved, and the SSDdivergence obtained of ~ 25 µrad is the level required for indirect drive ICF.  At largedispersion (right, 2.5 and 4 color cycles) divergence of up to 100 µrad can be obtained, and onesees that the individual FM sidebands are resolved so that the far field is simply representativeof the FM spectrum.NoSSDβ = 3.8Ncc= 1∆ν =60 GHzβ = 3.8Ncc= 4∆ν =60 GHzβ = 6.7Ncc= 2.5∆ν =70 GHzFar Field Angle (µrad) Far Field Angle (µrad)Figure 7: Far field images and vertically integrated lineouts of the Beamlet preamplifieroutput beam with the indicated amounts of 1D SSD.  The modulation depth β ,the number of color cycles ( Ncc ), and the total bandwidth ( ∆ν ) are shown foreach shot.The smoothing performance of the preamplifier beam was investigated (Fig. 8) using a randombinary phase plate designed for 633 nm.  Because the binary elements in this phase plate donot produce a π phase shift at 1.05 µm, a coherent (unaberrated by the phase plate) focus isfound on axis in the focal spot as an artifact of this measurement.  This focal spike is muchbrighter than the surrounding speckle pattern and also causes saturation along a horizontalline owing to the electronic readout mechanism of the CCD.  Avoiding this saturated region,one can determine the contrast in the remainder of the speckle pattern.  The CCD pixel spacingcorresponds to a far field angular resolution of 0.9 µrad.  The required Nyquist samplingperiod is 1.5 µrad and thus adequate resolution should be present to completely resolve thespeckle structure.  Nevertheless, the measured contrast with no SSD present (Fig. 8 (a)), is only50%, which is significantly less than the expected 100% contrast for a static speckle pattern.9However, as the amount of SSD is increased in Fig. 8 (b) - (d), one sees a reduction in themeasured contrast of the smoothed patterns relative to the measured 50% contrast of the staticpattern which is consistent with the calculated values.  It is also interesting to note the periodicstructure along the horizontal direction in Fig. 8(d).  This is a result of the use of multiple (2.5)color cycles for this shot. Multiple color cycles cause coherences in the spatial structure of thebeam such that particular spatial frequencies are not smoothed.5(a)   (b)(c)   (d)Figure 8: Smoothing of far field images of the preamplifier beam focused through arandom phase plate with varying amounts of SSD.  The saturated foci andemanating horizontal stripes are an artifact of this experiment.  (a) static speckle,without SSD, and contrast = 50%; (b) modulation depth 1.4, total divergence 9µrad, 1 color cycle, and contrast = 26%; (c) modulation depth 3.8, total divergence25 µrad, 1 color cycle, and contrast = 20%; (d) modulation depth 6.7, totaldivergence 100 µrad, 2.5 color cycles, and contrast = 15%.  The white bar at thebottom of each image corresponds to a far field angle of 100 µrad.104.  CONCLUSIONSNumerical modeling of Beamlet using PROP92 indicates that the critical issue for laserperformance is the amount of additional divergence owing to SSD in comparison to the size ofthe spatial filter pinholes.  With the present Beamlet pinhole size (±200 µrad) the calculatednear field beam  degradation is found to be small for SSD divergence at or below the levelanticipated to be necessary for ICF on the NIF (~ 50 µrad).  Measurements on the Beamletpreamplifier output show no observable degradation of the near field beam owing to theadditional SSD divergence of up to 100 µrad.  Smoothing observed with the preamplifieroutput beam shows improvement in contrast consistent with the amount of SSD imposed.5. ACKNOWLEDGMENTThis work was performed under the auspices of the U. S. Department of Energy by LawrenceLivermore National Laboratory under Contract No. W-7405-Eng-48.6. REFERENCES1. R. H. Lehmberg and S. P. Obenschain, "Use of induced spatial incoherence for uniformillumination of laser fusion targets," Optics Comm. 46, 27-31 (1983) and R. H. Lehmbergand J. Goldhar, "Use of incoherence to produce smooth and controllable irradiationprofiles with KrF fusion lasers," Fusion Technology 11, 532-541 (1987).2. Y. Kato, K. Mima, N. Miyanaga, S. Arinaga, Y. Kitagawa, M. Naktsuka, and C.Yamanka, "Random phasing of high-power lasers for uniform target acceleration andplasma instability suppression," Phys. Rev. Lett. 53, 1057-1060 (1984).3. D. Véron, H. Ayral, C. Gouedard, D. Husson, J. Lauriou, O. Martin, B. Meyer, M.Rostaing, and C. Sauteret, "Optical spatial smoothing of ND-glass laser beam," OpticsComm. 65, 42-45 (1988).4. S. Skupsky, R. W. Short, T. Kessler, R. S. Craxton, S. Letzring, and J. M. Soures,"Improved laser-beam uniformity using the angular dispersion of frequency-modulatedlight," J. Appl. Phys. 66, 3456-3462 (1989).5. J. E. Rothenberg, "Improved Beam Smoothing with SSD using Generalized PhaseModulation" paper P35 in these proceedings.Technical Information Department  • Lawrence Livermore National LaboratoryUniversity of California • Livermore, California  94551

Performance of smoothing by spectral dispersion (SSD) on Beamlet

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Performance of smoothing by spectral dispersion (SSD) on Beamlet

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