Ultrashort pulse laser for high density plasma experiments

J Quanr Sperrrosr Radror Tram/er Vol 51. No I,2 PP 407-410 1994 Elsev~erSnence Lrd Pnnwd m Great Bnlam 0022473(93)E0048-W 00224073‘94 $6 00 + 0 00

Pergamon

ULTRASHORT

PULSE LASER FOR HIGH DENSITY PLASMA EXPERIMENTS

W E WHITE, F G PATTERSON, D F PRICE,and R Lawrence

LIvermore Natlonal Laboratory. L-251, P 0

SHEPHERD

Box 808. LIvermore

CA 94550,

U SA

Abstract-The

use of high Intensity ultrashort laser pulses In the study of hot. hgh density plasmas reqlures pulses of extreme temporal fidehty We have developed a system that provides contmuously vanable mdependent tumng of the lugher order frequency dependent phase of ultrashort laser pulses Numerical results are presented mdlcatmg that h&er order phase terms that lead to temporal wmgs can be compensated by a properly adJusted air spaced doublet lens wthm the pulse stretcher

Hugh Intensity ultrashort pulse laser systems are becommg quite common m the study of hot, high denstty plasma phystcs Relymg on the ultrashort pulse nature of these systems to lomze and heat a plasma m a time period short m comparison to plasma decompresslon times Imposes very stnct reqturements on the temporal fidehty of the laser pulses As the peak mtensltles of these systems contmue to Increase, the “prepulse” mtenstty must be mamtamed below the level at which a plasma would be created and have time to expand before bemg heated by the mam pulse Current laser systems’ are capable of producmg peak mtensltres m excess of IO’”W/cm’, requlnng an mtenslty contrast ratto of approxtmately SIXorders of magmtude Controlhng the shape of a laser pulse over this type of dynamic range becomes even more difficult consldenng that these laser systems are based on the techruque of Chu-ped Pulse Amphficatlon (CPA)* m whtch, for example, a 100 fsec laser pulse 1s temporally stretched to approx 400 psec This IS typically done by lmpartmg a frequency dependent phase or “chirp” In a dlsperslve stretcher’ as shown m Fig I Once stretched, the pulse IS amphtied to the desired energy and then recompressed m a parallel gratmg pulse compressor Ideally, the compressor would exactly remove the chu-p placed on the pulse m the stretcher and amphfiers, resultmg m a transform hmlted short pulse In reality, the phase cannot be exactly compensated and the compressed pulse contams residual frequency dependent phase components which hmlt the fidelity of the resulting pulses Expandmg the frequency dependent phase about the central optlcal frequency [Eq (I)] allows the hmttmg phase terms to be quanttfied cP(w)=cp,+~,(w--o,)+cpz(o

-O,)*+(p,(W -W,)‘+Cp,(ClJ-CZIJ,)~+

(1)

In this case, the (p2term represents the “lmear chirp” which IS Introduced to stretch the pulse. and removed to recompress the pulse The higher order phase terms cp, and (p4 are generally not completely removed m the compressor and are usually responsible for the llmltatlons m pulse fidehty In an effort to understand the sources of these phase aberrations, we have developed a numerlcal model of our chn-ped pulse amphficatton system that tracks the phase of each frequency component of the laser pulse as It propagates through a general CPA system Inmal mvestlgatlon of the sources of the phase aberrattons yielded three Important results First, chromauc aberration m the stretcher lens results m thtrd order phase error (cp,) Second, spherical aberration of the stretcher lens produces fourth order phase order error ((p4) Fmally, group velocity dlsperslon m the matenal between the stretcher and compressor introduces second order phase which must be compensated by changing the length of the compressor This mismatch between the stretcher and compressor results m slgntficant thtrd and fourth order phase aberration This final pomt mdlcates that a 407

W E WHITE et al

408

Diffractton Gratma

Fig

49Third order phase

adjustment

I Diagram of a pulse stretcher wth an air-spaced douhlet Ien> for higher order phase compensanon

perfectly matched stretcher and compressor would not necessarily eliminate the temporal wmgs that result from phase aberrations As a result of this analysis. we used our numerical model to design a stretcher that allows for independent adjustment of the second. third, and fourth order phase components Our design IS based on the now standard stretcher design (Fig I) as well as the result that chromatic and spherical aberration of the lens results m third and fourth order frequency dependent phase errors We begm with an air spaced doublet lens In the pulse stretcher corrected for chromatic and spherlcal aberration This results m a bery well matched stretcher and compressor as shown In Fig 2 Upon addition of dispersive material between the stretcher and compressor (IO” of SF8 In this case), the compressor IS lengthened to compensate for the second order phase resulting In higher order phase aberrations and temporal wings (Fig 3) The doublet lens can then be adjusted to compensate for the residual phase aberrations by changing the relative alignment of the lens elements (Fig 4) Changmg the separation of the lens elements Introduces spherical aberration and thus Introduces fourth order phase that can be adJusted to cancel any fourth order phase In the system Likewise. relative offset of the lens elements across the dlsperslve plane of the stretcher Introduces chromatic aberration, thus allowing adJustmen of the third order phase These adjustments are almost completely decoupled and result in monotonic adjustment of their respective phase terms Furthermore. these adjustments ha\e negllglble effect on fifth or higher order phase terms In conclusion. we have shown that a stretcher can be designed which allows compensation of the higher order phase terms that can compromise the pulse fidelity of high mtenslty ultrashort pulse laser systems Control of these phase-aberrations is necessary-m order to provide the level

!i!J:_ 780

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(nm)

-40 I

-2000

-1000 Tim: (fs)

Rg 2 Phase vs wavelength (a) and lntensq

\s time (b) for a properly adjusted stretcher and compressor

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Ultrashort pulse laser for high density plasma expenments

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Wavelength (nm)

_8_

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0 Time (fs)

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--

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FIN 3 Phase vs wavelength (a) and Intensltq vs time (b) for an unadjusled stretcher and compressor wtth IO SF8 m between Note odd and even temporal wmg structure resultmg from third and fourth order phase aberrahons

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Time (fs) Fig 4 Phase (a) and mtenslty (b) when air spaced doublet IS adJusted to compensate for the aberraltons mtroduced by IO’ of SF8 glass between the stretcher and compressor

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of temporal pulse fidehty that IS needed of hot, sohd density plasmas

If these laser systems are to be useful m the productton

WHITE et al

REFERENCES J J Mackhn and J F Young, Opr Lerf 16, 1001 (1991), A Sulhvan. H Hamster, H C Kapteyn, S Gordon, W E White H Nathel, R J Blair, and R W Falcone. Opt Letr 16, 1406 (1991). W E White. J R Hunter, L Van Woerkom. T Dltmlre, and M D Perry Opr Lerr 17, 1069 (199,) .! D Stnckland and G Mourou Opr Commun 56,219 (1985), P Mame. D Stnckland P Bado, M Pessot and G Mourou, /EEE J Quanr Elec 24, 398 (1988) 3 0 E Martmez IEEE J Quanr Elec QE-23, 59 (1987)

I J D Kmetec,