Beam wavefront control of a thermal inertia laserfor inertial confinement fusion application

Dai Wanjun,1,* Hu Dongxia,1 Zhou Wei,1 Zhao Junpu,1 Jing Feng,1 Yang Zeping,2

Zhang Kun,1 Jiang Xuejun,1 Deng Wu,1 Zhao Runchang,1 Peng Zhitao,1

and Feng Bin1

1Research Center of Laser Fusion, CAEP, P.O. Box 919-988, Mianyang 621900, China2Institutes of Optics and Electronics, P.O. Box 350, Chengdu 610209, China

*Corresponding author: dwj8wy@163.com

Received 2 March 2009; revised 31 May 2009; accepted 8 June 2009;posted 9 June 2009 (Doc. ID 108236); published 22 June 2009

A novel scheme to correct aberration of each beam from the front-end to the target point in a thermalinertia laser (TIL) is presented. Each beam contains a deformable mirror (DM) with an aperture of70mm× 70mm at the injection of the main amplifier and a Hartman–Shack (HS) sensor in a parameterdiagnostic unit (PDU). A temporary HS sensor for measuring the static aberration of each beamwith 1Hzsource is placed at the target point. The sensor will be removed from the target point during the mainsingle shot, so we transfer the results measured at the target point to the sensors in the PDU. Dynamicaberration can also be measured by the HS sensor in the PDU during the single shot. In this way, we neednot calibrate the aberration of the PDU, and aberration of each beam can be corrected by the DMwith theHS sensor in the PDU. We demonstrate that with this scheme the divergence angle of the TIL pulses canbe improved from 100 to less than 60 μrad with a focal length of 2200mm and beam size of290mm× 290mm, which meets the requirement of a TIL. © 2009 Optical Society of America

OCIS codes: 140.0140, 140.3300, 140.6810.

1. Introduction

The thermal inertia laser (TIL) is the prototype of theSG-III laser facility with eight beams [1,2]. Theoutput energy is 10kJ of ultraviolet light. The TILconsists of six systems including a front-end (FD),preamplifier (PAM), main amplifier (MA), finaloptics assembly (FOA), beam control and parameterdiagnostic unit (PDU), and the integrated computercontrol system (ICCS). A fiber laser is split into eightand injected into the front-end. After passingthrough the preamplifier, the beams are injected intothe MA system. For the purpose of improving beamquality and energy conversion efficiency, combinedmultisegment amplifiers and aU-turn beam reverserwith a small aperture Pockels cell are applied in theTIL, which is different from the National Ignition

Facility (NIF). At NIF, each beam uses a large aper-ture Pockels cell without a beam reverser in the MAsystem. After propagating through themultipassMAsystem [3], the beams are subsequently transportedto the FOA, where the frequency of each beam isconverted from 1 to 3ω (1.053 μm to 0.351μm wave-length), and are focused onto the target. The opticalchain layout is shown in Fig. 1.

Beam aberration from the PAM to the target pointis mainly induced by two parts in the laser facility[4,5]: (i) aberration in the MA system, which ismainly generated by residual errors of variousoptical elements and thermally induced aberrationin neodymium glass; (ii) aberration in the target sys-tem, which is induced by surface errors of largetransport mirrors (TMs) and the frequency convertor(FC). With these aberrations, the focusability of high-energy beams decrease with a large divergence angleand low concentration of energy at the focal plane. AtNIF, each beam uses a large-aperture DM of

0003-6935/09/193691-04$15.00/0© 2009 Optical Society of America

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Fig. 1. (Color online) Schematic drawing of the multipass TIL design.

Fig. 2. (Color online) (a) Static beamwavefrontmeasured by anHS sensor at the target point. (b) Process of transferring entire static beamwavefront from the target to the PDU. (c) Transferred result in the PDU. (d) Difference between the results at the target point and the PDU.

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400mm× 400mm to correct aberrations in the MAsystem. In a TIL, beam quality of the MA systemcan be greatly improved with the U-turn beam rever-ser. To correct the aberration remaining in the TIL,an adaptive optics (AO) system is applied in eachbeam [6,7]. A DM with an aperture of 70mm ×70mm is placed at the injection of main amplifierwith 45 drivers, and a Hartman–Shack sensor(HS) is placed in the MA system PDU with 22 × 22arrays; aberrations of the PDU can be calibrated on-line with a fiber laser injected in a pinhole of spatialfilter 2(SF2) [8], as shown in Fig. 1.

2. Method of Beam Aberration Measurement

Usually, AO systems are used to correct aberrationbefore the FOA to ensure high conversion efficiencyin a TIL. Residual error after correction is greatly re-duced with a peak-to-valley (PV) value of about 0:3λand a rms value of about 0:05λ. However, focal spotsmeasured by a CCD at the target point do not greatlychange after correction, and some beams even be-come worse than focal spots measured without cor-rection. This means that the surface errors oftransport mirrors and the FOA cannot be ignored.From above, aberration of each beam from injec-

tion to the target point should be corrected for thegoal of getting a good focal spot. At NIF, surface er-rors of large TMs and the FC are controlled in themanufacturing process of the optical elements andthrough optical adjustment. However, manufactureof large TMs with good optical quality is difficult withhigh cost and long time. In a TIL, a temporary HSsensor is adjusted offline and placed at the targetpoint. Static aberration can be directly measuredwith a PV value of 4:0λ and a rms value of 0:73λas shown in Fig. 2(a). Aberration of each beam canbe obtained by this way one by one.However, it will take too much time to correct

aberration of each beam with only one HS sensor

at a single shot. Each beam should first be adjusted,and then sampled with appropriate energy to the HSsensor, one by one. Here, a novel method is used totransfer the static aberration measured by the HSsensor at the target point to the one in the PDU ofeach beam. The process is shown in Figure. 2(b).First, static aberration is corrected by the DM withthe HS sensor at the target point. Second, the wave-front measured with the HS sensor in the PDU is setas a reference. Finally, static aberration can be ob-tained by the HS sensor in the PDU when an AO sys-tem is open loop. The result is shown in Fig. 2(c), witha PV value of about 3:43λ and a rms value of about0:729λ. Residual error measured by the HS sensor atthe target point when the static aberration is cor-rected by the DM with the HS sensor in the PDUis shown in Fig. 2(d) with a PV value of about 0:6λand a rms value of about 0:1λ. That means the trans-fer method is effective. Dynamic aberration can alsobe measured by the HS sensor in the PDU atsingle shot.

Here, aberration of the PDU need not to becalibrated online if it is stable. The stability ofaberration in the PDU and target system have beenmonitored for a long time. The change was small(PV < 0:5λ) during the routing operations. By thismethod, the difficulty of precise manufacture andfine adjustment of large TMs can be avoided. Inthe future, a large DM should be placed in frontof the TMs to improve the efficiency of frequencyconversion.

Fig. 3. Focal spot measured at the target point (a) before and (b) after correction.

Table 1. Analysis of Focal Spot Before and After Correction

Focal spot (encircled energy) 70.0% 80.0% 90.0% 95.0%Before correction (μrad) 56.2 65.2 78.3 91.5After correction (μrad) 32.9 41.5 50.9 59.2

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3. Effect with Aberration Correction

Aberration of each beam is corrected by a DMwith anHS sensor in the PDU. Figures 3(a) and 3(b) show thefocal spot measured at the target point with a CCD(1024 × 1024) beforeandafter correction, respectively.The beam aperture is 290mm× 290mm and focallength is 2200mm, which corresponds one diffractionlimit (DL), being 2:42 μrad of the divergence angle.Analysis of the focal spot is shown in Table 1. The di-vergence angle of 95% encircled energy changes from91.5 to 59:2 μrad.To compare the results with and without correction

of the entire beam aberration, an x-ray pinhole cam-era is used to record the images when the eight beamfocal spots are positioned on the target with 2 × 4pinhole arrays, with each pinhole of 100 μm, asshown in Figs. 4(a) and 4(b), respectively. The focalspots with correction improve remarkably than oneswithout correction. This will be good for experimentsusing a TIL.However, aberrations of some beams are too large

with low optical quality of mirrors after the MAsystem. To assure a beam passes through the spatialfilter hole freely, the ability of the AO system shouldbe limited. Some other control methods should beapplied in the future, such as a continuous phaseplate (CPP) or a large DM after the spatial filter.

4. Conclusion

Aberrations of eight beams were obtained by placinga temporary HS sensor at the target point. The re-sults were transferred to the HS sensor in thePDU for aberration correction of each beam. The ef-fectivity of this method is verified by comparing the

measured result between the HS sensor at the targetpoint and in the PDU. The divergence angle of theTIL pulses can be improved from 100 to less than60 μrad after correction. The effect with correctionof eight beams was perfect, which meets the experi-ment requirement of a TIL. By this means, require-ments of large TMs and final optic components canbe loosened some, which will cut cost and save time.

The authors thank Yang Zeping and Li Ende of theInstitute of Optics and Electronics for support in thisresearch.

References

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Fig. 4. Focal spots (a) without and (b) with correction.

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