用于多程放大系统光束反转器的等离子体电极电光开关

第15卷　第2期强激光与粒子束Vol.15,No.2 2003年2月HIGH POWER LASER AND PAR TICL E B EAMS Feb.,2003　Article ID :　100124322(2003)022*******PEPC electro 2optical switch used in beamreverser of multipass amplif ier ΞZHAN G Xiong 2jun ,　ZHEN G Jian 2gang ,　ZHEN G Kui 2xing ,　DON G Yun ,L U Jing 2ping ,　PEN G Zhi 2tao ,　J IN Feng ,　WEI Xiao 2feng(Research Center of L aser Fusion ,CA EP ,P.O.Box 9192988,Mianyang 621900,Chi na ) Abstract :　A compact Plasma Electrode Pockels Cell (PEPC )electro 2optical switch will be used in beam revers 2er of multipass amplification system to control the output beam and restrain the self 2oscillation produced by the multi 2pass amplifier.In this paper ,the performance of the compact PEPC electron 2optical switch and its application in mul 2tipass amplification system are described in detail.Measurements of the o ptical performance show static transmittanceof 94%,switching efficiency of 98.9%.The experimental result in multipass amplifier indicates that the PEPC elec 2tron 2optical switch works well. K ey w ords :　PEPC ;　Electro 2optical switch ;　Beam reverser ;　Multipass amplifier ;　Self 2oscillation C LC number :　TN520 Document code :　A Compared to the traditional MOPA system ,Multipass amplification system can output the same energy with lower request to the energy output of front end.And it has high performance 2cost ratio.So it becomes the best choice to design the new generation large lasers [1,2].However ,high gain may bring self 2oscillation from amplifier.Self 2oscillation will cause the reduced gain of the amplifier and worse optical beam quality.More seriously it will damage the optic components in the system.So self 2oscillation must be restrained.Fig.1　Optical path setup of L 2turn beam reverser In American National Ignition Facil 2ity (N IF )baseline architecture ,a full 2aperture Pockels cell [3,4]and polarizerswitch the beam out of the amplifier cavi 2ty after four pass.The Pockels cell andthe polarizer also can isolate the self 2oscil 2lation and the beam back from the target.In contrast ,the French Megajoule laserdesign does not use a full 2aperture Pock 2els cell and polarizer to switch the beamout of the cavity.Instead ,they use a rel 2atively small set of optics ,called a revers 2er ,located in the center section of the transport spatial filter to steer the beam from pass 2to pass 3.The reverser has two versions ,called L 2turn and U 2turn [5].We also use the L 2turn beam reverser to achieve four 2pass amplifica 2tion.Figure 1illustrates the reverser used in our multipass amplifier.After the pick 2off mirror (BM1),a second mirror (BM3)directs the beam through a collimating lens (BM2)to the cavity mirror (BM7),which is oriented to reflect pass 2along pass 3.The isolation system ,a Pockels cell (BM5)between crossed polarizer blocks forward and backward transmission except during a 80ns window to allow passage of the shot pulse.The Pockels cell we used in the multipass amplifier system is a plasma electrode Pockels cell.The PEPC and the crossed polarizer con 2ΞR eceived d ate :2002208203; R evised d ate :2002212202Found ation item :Supported by National 863ProjectBiography :ZHAN G Xiong 2jun (19732),male ,Master of Science ;Current research direction :Solid Laser Technology ;E 2mail :stone -zxj @163.com 。

sist an optical switch ,which restrains the self 2oscillation produced by the multipass amplifier and the reflected beam.1　Plasma electrode Pockels cell electro 2optical switch In any Pockels cell ,polarization rotation of a transmission laser beam occurs when an electric field is applied to an electro 2optical crystal ,inducing birefringence.In a conventional ring electrode Pockels cell ,the electric field is applied to a cylindrical rod of crystal via metal rings on the end faces of the rod.To obtain sufficient field unifor 2mity across the active aperture ,the length of the rod must be one or two times the diameter.When its aperture gets larger ,it will bring more absorption loss and nonlinear effects.Therefore the Pockels cell 2based optical switch used in the multipass amplifier does not use traditional ring or thin 2film electrodes.It uses a plasma electrode Pock 2els cell [6]with a KD 3P crystal as the electro 2optical medium.Discharge plasma is formed on each side of the elec 2tro 2optical crystal and high voltage is applied across the crystal through the plasma electrodes to initiate optical switching.A PEPC is a complex opto 2mechanical 2electric system including the cell body ,plasma pulsers ,switch pulser ,vacuum and gas feedsubsystems.Fig.2　Photo of compactPEPC Fig.3　G as discharge photos.(a )pre 2ionization ,(b )glow discharge Limited by the space of the reverser ,the Pockels cell is designed compactly.Figure 2shows the photograph of the constructed PEPC.To minimize the Pockels cell ’s dimension ,the body is fabricated from K9glass.And it was designed to be a whole configuration ,in contrast to the traditional separate sandwich structure [7,8].The KD 3P crystal aperture is 80mm ×80mm.And the shell external diameter is only 150mm.It is very difficult to get uni 2form glow discharge plasma electrode in such limited space.The plasma electrode is formed by gas discharge be 2tween a pair of anode and cathode.The anode is designed as simple bar of stainless steel ,and the cathode is de 2signed as many sorts of structure for the cathode is the key of getting uniform glow discharge.Furthermore ,we add the simmer discharge before the high current main discharge to minimize the jitter of the discharge time delay corresponding to the trigger pulse.Thus the uniform glow discharge is obtained by a special hollow cathode.Figure 3shows the photographs of gas discharge got by the CCD camera.Figure 3(a )is the photograph of the simmer discharge ,and figure 3(b )is the high current glow discharge photograph.It shows that the uniform glow dis 2charge fills the whole aperture of the crystal. Before used in the multipass amplifier ,the features of a compact PEPC are diagnosed in our laboratory.Figure 4illustrates the optical setup for electro 2optical switch performance diagnostics.The laser diode is used to aligning .A Q 2switch ,YA G pulsed laser is used as the optical source .The beam dimensionreachesFig.4　Optical setup for electro 2optical switch performance out 2line diagnostics151No.2 Zhang Xiongjun ,et al :PEPC electro 2optical switch used in beam reverser of multi pass amplifierthe full available aperture of the PEPC through a beam expander and a beam spatial filter.Photodiode 1is a refer 2ence detector to receive a portion of beam.P1and P2is a pair of crossed polarizer.And photodiode 2receives the beam that passes through the polarizer 2.So we can get the full aperture performance of the PEPC with a KD 3P crystal.The static transmission efficiency is 94%;switching efficiency is 98.9%;and half 2wave retardation volt 2age is 7.6kV.2　Application in multipass amplif ier When the compact PEPC electron 2optical switch is used in multipass amplifier system ,three aspects should be verified.The first is synchronization.The second is the isolated self 2oscillation effect of the electro 2optical switch ,namely the restraining capability to the self 2oscillation.The third is the infection to the multipass amplifier near field and the damage threshold of the switch crystal.Among these three issues ,the key is the restraining capability to the self 2oscillation.Fig.5　Synchronization relation of PEPC and multipass amplification system Figure 5illustrates the synchronization relation of PEPC and multipass amplification system.The core of the synchronization is the oscillator with the electro 2optical switch ,namely the time correlation of the Q 2switch pulse in the front 2end oscillator and the switch pulse of PEPC.When optical pulse reaches the electro 2optical switch ,the electro 2optical crystal must has been charged to half wave voltage.To protect synchronizer of the PEPC pulsers from damage by the high voltage discharge of the flash lamps through the earth 2line ,inductor isolators were con 2nected to the trigger cables before they were connected to the synchronizer.Figure 6shows the time -delay enve 2lope of the switch pulse.We can see that the jitter of the switch pulse corresponding to the trigger pulse is less than 20ns.And figure 7shows the typical temporal relation of switch pulse and optical pulse.The switch pulse is mea 2sured with a fast high 2voltage probe connected between two discharge anodes and the optical pulse is measured by a photodiode behind the reverser cavity mirror.The oscillograph illustrates that the electro 2optical switch synchro 2nizes with main laser pulse exactly. Before the PEPC electro 2optical switch was inserted into the beam reverser ,we had measured the optical pulse including flash lamp discharge light behind the amplifier cavity mirror with a photodiode.The waveform was shown in figure 8(a ).The waveform illustrates that the self 2oscillation produced by multipass amplifier is very strong.When the electro 2optical switch has been inserted into the beam reverser ,we got the optical pulse wave 2form shown in the figure 8(b ).In this graph ,the waveform is very clean ,and no self 2oscillation can be seen.At the same time ,we measured the isolation ratio of the electro 2optical switch with polarizerse in the L 2turn bean re 2verser.It reaches about 10000. Figure 9is the multipass amplification system near field image got by science CCD camera.Photograph (a )was obtained without the electro 2optical switch.Photograph (b )was got when the electro 2optical switch was251HIGH POWER LASER AND PARTICL E BEAMS Vol.15Fig.6　Time 2delay envelope of switchpulse Fig.7　Typical temporal relation of switch pulse and opticalpulseFig.8　Optical pulses waveform got behind amplifier cavity mirror (including flash discharge light )working.So the inserting electro 2optical switch in multipass amplifier brings no spatial modulation.From the two CCD images ,we can also find that the switching efficiency of the compact PEPC is uniform in the whole aperture.In the experiments ,the maximum energy output of the multipass amplifier is 750J.Here the average fluence on the electro 2optical crystal is about 2J /cm 2.Fig.9　Multipass amplifier near field image got by CCD3　Conclusion This article describes the compact PEPC electro 2optical switch constructed in National Key Laboratory of Laser Fusion and its application in the multipass amplifier.Through a specially designed hollow cathode and the simmer discharge added before the high current main discharge ,a uniform steady plasma electrode is obtained.Its feature parameters measured indicate that the compact PEPC electron 2optical switch has excellent switching perfor 2mance.And its application in the multipass amplifier shows that it can accurately synchronize with the main laser pulse and can effectively restrain the self 2oscillation produced by the multipass amplifier.R eference :351No.2 Zhang Xiongjun ,et al :PEPC electro 2optical switch used in beam reverser of multi pass amplifier451HIGH POWER LASER AND PARTICL E BEAMS Vol.15[1]　Peng H S,Jin F,Zhang X M,et al.Status of the SG2III solid state laser[A].SPIE[C].1998,3092:25.[2]　Van wonterghem B M,Murray J R,Campbell J H,et al.Performance of a prototype for a large aperture multipass Nd:glass laser for inertial con2finement fusion[J].A ppl Opt,1997,36:4832.[3]　Rhodes M A,Fochs S,Alger T.2×1prototype plasma2electrode Pockels cell for the National Ignition Facility[A].SPIE[C].1997,3047:203—206.[4]　Rhodes M A,Wood B,De Y oreo J J,et al.Performance of large2aperture optical switches for high energy inertial2confinement fusion laser[J].A p2pl Opt,1995,34:5312—5325.[5]　Vann C S,Seznec S,Laniesse F,et al.Testing a new multipass laser architecture on Beamlet[R].UCRL2L R210582129522,1995.[6]　G oldhar J,Henesian M rge2aperture electro2optical switches with plasma electrodes[J].Journal of Quant um Elect ronics,1986,22(7):1137—1147.[7]　Lu J P,LüC X,Zhang X J,et al.Study on PEPC electro2optical switch[J].High Power L aser and Particle Beams,1997,9(1):72—78.[8]　Lu J P,Zheng K X,Zhang X J,et al.Study on large aperture plasms electrode pockels cell electro2optical switch[J].High Power L aser and Parti2cle Beams,2000,12(s1):137—140.用于多程放大系统光束反转器的等离子体电极电光开关张雄军,　郑建刚,　郑奎兴,　董　云,　鲁敬平,　彭志涛,　景　峰,　魏晓峰(中国工程物理研究院激光聚变研究中心,四川绵阳621900) 摘　要:　等离子体电极电光开关将用在多程放大系统光束反转器中,作为输出控制和抑制主放大前系统内的自激振荡,由于光束反转器空间限制,等离子体电极电光开关采用一体化结构设计。