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Generation and amplification of ultrashort light pulses using excimer lasers

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1986 Sov. J. Quantum Electron. 16 1315

(http://iopscience.iop.org/0049-1748/16/10/A09)

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Generation and amplification of ultrashort light pulses using excimer lasersS. A. Akhmanov, A. M. Val'shin, V. M. Gordienko, M. S. Dzhidzhoev, S. V. Krayushkin,I. A. Kudinov, V. T. Platonenko, V. K. Popov, and V. D. Taranukhin

M. V. Lomonosov State University, Moscow

(Submitted March 28, 1986)Kvantovaya Elektron. (Moscow) 13, 1992-1998 (October 1986)

An investigation is made of the characteristic features of the amplification of ultrashortradiation pulses in excimer media. An analysis is made of the dependence of the saturationenergy on the width of the spectrum of the amplified ultrashort pulse. A laser apparatus isdescribed which makes it possible to generate high-power mutually synchronized ultrashortpulses of adjustable duration in the infrared, visible, and ultraviolet regions of the spectrum.The dependences of the gain in an excimer amplifier on the energy density and wavelength ofthe input signal were investigated experimentally. Ultraviolet radiation pulses (λ = 308 nm)were obtained having a duration of 5 psec and an energy of 15 mJ, corresponding to a power of3GW.

INTRODUCTION

One of the promising applications of excimer lasers in-volves their use to generate ultrashort radiation pulses in theultraviolet region. Several different schemes for picosecondexcimer laser systems have already been realized, making itpossible to generate ultraviolet radiation with powers ex-ceeding 1 GW at wavelengths of 308 nm (Refs. 1-3, XeCllaser), 248 nm (Ref. 4, KrFlaser), and 193 nm (Ref. 5, ArFlaser). Their detailed analysis is given in Ref. 6. Our ap-proach to constructing such systems is reflected in Ref. 3 andconsists in using a passively mode-locked solid-state laser asthe master oscillator. This system is the most efficient forgenerating high-power spectrally limited very short pulsesand makes it possible to use the technique of optical pulseshortening and to realize the ultimate possibilities of excimersystems determined by the gain band width.

Our aim was to develop further the method described inRef. 3 for generating spectrally limited picosecond and sub-picosecond ultraviolet radiation pulses. We adopted a newsolution to the problem of synchronizing a passively mode-locked solid-state laser with an electric-discharge excimerlaser amplifier. The principal attention was concentrated oninvestigating the possibilities for generating very shortpulses at the output of an excimer amplifier and to realize thefull band width, reaching ~ 100 cm" 1 , for its gain.

1. AMPLIFICATION OF SHORT PULSES IN EXCIMERLASERS

In analyzing the amplification of short pulses the fun-damental questions are those of the possible specific outputenergy, the contrast ratio of the amplified signal, and thechange in the spectral composition and shape of the pulseduring amplification.

The characteristic features of excimer amplifiers are as-sociated with the large stimulated emission cross section(i7~ 10" 1 6 cm2), the large spectral band width of the gain(Avc~100 c m " 1 ) , and the complexity of its structure.There is a high probability of the appearance of parasiticlasing in these amplifiers (due to reflection from the elec-

trodes, discharge chamber walls, optical components, etc.)and superradiance plays an important role. In the first place,this can cause an uncontrolled output of energy from theamplifier active medium and, secondly, it prevents attain-ment of the required contrast ratio of the amplified radiationpulse.

If conditions are realized which completely eliminatethe influence of parasitic lasing, the energy Est stored in theactive transition and the gain per pass Κ are limited by super-radiance. The superradiance power in a solid angle ΔΩ,along the axis of the discharge gap is

xPsp ΔΩ (e°NL - 1 )/4naNL, (1)

where Psp =Est/rsp is the spontaneous (unamplified) radi-ation power in a solid angle of 4π; r s p is the radiative life-time; σ is the cross section of the stimulated transition; Ν isthe population difference; L is the length of the active medi-um.

Let the time required for the formation of a populationdifference be less than r s p by a factor of A. Then superra-diance will limit the increase in the inversion on reaching thelevel PAn <^APsp. It then follows from Eq. (1) that

(2)

Putting Δζ) = 2S/L2 (in two directions), whereS1 is the dis-charge cross section, Est ~fw)NLS, and the gain isΚ — exp(aNL), we obtain from expression (2) the maxi-mum values Ε ™χ and KmeLX ·.

(4)

The estimate for ATmax involves the assumption thatthere is no feedback and it can differ markedly from the realvalue. Despite this, E™x is estimated to a good accuracywith formula (3) since ATmax is usually high ( ~ 103-104) andenters into formula (3) within the logarithm term. Thus, the

1315 Sov.J. Quantum Electron. 16(10), Oct. 1986 0049-1748/86/101315-05S04.10 © 1987 American Institute of Physics 1315

maximum energy which can be stored in the active transitionis proportional to the discharge cross section and is only avery weak function of its length.

The larger fraction of the energy stored in the mediumcan be removed by a short radiation pulse (τρ <δ~ι, where δis the collisional width) under conditions when the inputpulse possesses a sufficiently high energy (such that the sat-uration energy level is attained in a distance short comparedwith the total length of the amplifier) and a sufficiently widespectrum Δν.

The gain band of excimer molecules has a complexstructure. Roughly speaking, it can be represented as an as-sembly of overlapping electronic-vibrational-rotationalbands, it being sufficient for the spectrum of the injectedradiation to cover one of these bands in order for almost allthe excited molecules to take part. This is due to the fact thatin an excited electronic state the vibrational quantum energyis sufficiently high so that most of the molecules are in thelower vibrational level from which transitions are allowedsimultaneously to several vibrational levels of the lower elec-tronic term. The Ρ and R branches of each of the vibrational-rotational bands are practically superimposed on each other(this is due to the smallness of and considerable relative dif-ference between the rotational constants of molecules in theΒ and X states). The width Δνυ of a vibrational-rotationalband and the separation of the neighboring bands are of asimilar magnitude.

Assuming the saturation energy EMt to mean the energyof a pulse which reduces the gain by a factor of e, and assum-ing the gain to be incoherent, it can be shown that in the caseof the gain band structure described above:

E „ s Av/Av. ( 5 )

The energy Eex which can be removed from the amplifier inthe strong saturation regime is then related to 2ist by thefollowing expression:

π F in K-F Αν/Αν, (6)Ecll=EMlnK-Est 1 + Δ ν / Δ ν /

The estimates given by expressions (5) and (6) are valid ifΔν does not exceed the width of the gain band.

An approximate description of the incoherent amplifi-cation of a short pulse in an excimer medium can be given byusing the Frantz-Nodvik formula:

EOM = £ Μ , In U+exp (aL) lexp(Ein/EM ) - l l } , (7)where Ein is the energy of the input pulse and EM is definedby expression (5). This expression clearly loses its meaningif the pulse duration is so greatly reduced during its amplifi-cation that the spectral width and Esal begin to change.

We investigated numerically the amplification of spec-trally limited pulses having durations in the rangeΔν~' < rp <δ~ι. The system of equations employed, takinginto account the real structure of the energy spectrum of theXeCl molecule, was justified in Ref. 7. A study was made ofthe amplification of Gaussian pulses 5-30 psec duration indifferent spectral regions of theB(v = 0) — X(v = 2) transi-tion of the XeCl molecule for an active medium with a tem-perature of Τ = 300 Κ at a pressure Ρ = 3 atm.

20 JO t, psec

FIG. 1. Evolution of the shape of an ultrashort pulse during amplificationin an XeCl amplifier.

The results of the numerical experiments showed thatwhen picosecond pulses are amplified in an XeCl amplifier(despite the complex structure of the gain spectrum), coher-ent effects can be manifested. In particular, there is a partialsplitting of the amplified pulse into subpulses and a shorten-ing of its leading edge (see Fig. 1). This process is accompa-nied by definite oscillations in the population differences ofboth the individual rotational transitions and the entire vi-brational band.

A substantial reduction in the duration of the amplifiedpulses can consequently occur. For example, in the case of aninput pulse having an energy density of 20 /zJ/cm2 and aduration of 15 psec for the real gain lengths (L=0.5 m withass0.2 cm""1), the calculations show the duration to be re-duced to 6 psec, while for a 5-psec long input pulse the reduc-tion is to 2.5 psec.

The attainment of a high contrast ratio of an amplifiedradiation pulse requires that the energy Es of the amplifiedinput pulse should greatly exceed the noise energy En. Letthe energy of the amplified pulse be close to Eu. Then inaccordance with expression (1) the energy of the amplifiednoise signal will be

£•„ xΑΩ Κ4n \nK>

(8)

where r is the time for which the amplification exists and

(9)in In ΚΔΩ Κ ·

It is readily seen that the realization of a high contrastratio requires a reduction in AQ, i.e., spatial filtering of theradiation must take place. There are no technical difficultiesin realizing a divergence for which expression (9) exceedsunity. Assuming that the ideal filtering is provided, i.e.,ΔΩ = λ 2/S, and the maximum specific output energy isachieved (we assume that Κ = ATmM), while Tc^rsp, we esti-mate the maximum contrast ratio as

1316 Sov. J. Quantum Electron. 16 (10), Oct. 1986 Akhmanove/a/. 1316

(10)

2. APPARATUS

A schematic diagram of the laser apparatus is given inFig. 2. A solid-state laser utilizing a single crystal ofYAlO3:Nd3+ was used as the master oscillator. It was pas-sively mode locked and the resonator Q factor was electroni-cally controlled (using a system which is described in Ref.8). This electronic control made it possible to do the follow-ing: 1) to form an electric synchronization pulse preceding atrain of picosecond pulses by a time which was continuouslycontrollable over the range 0-1500 nsec with an accuracy of± 5 nsec; 2) to shorten the laser output from 35 to 15 psec;

3) to improve the stability of the output pulses as regards theenergy of the second harmonic from + 50 to + 6%. Theduration of a picosecond pulse was measured by the stan-dard method of noncollinear second harmonic generation.The duration was strongly dependent on the time delay forlasing to develop set by the g-factor control system. In theexperiments described below, this time (~500 nsec) wasdetermined by the time to trigger the excimer amplifiers, andthe pulse duration was about 20 psec.

The pulse energy after the first amplifier stage was ofthe order of 1 mJ. This radiation was divided into two chan-nels. About 50% of the radiation, after conversion into thesecond harmonic (A = 532 nm) in a KDP crystal, was usedfor the synchronous pumping of an SI60 dye laser. The re-maining fraction of the radiation, after amplification andconversion to the second harmonic, was used for longitudi-nal pumping of an SI60 dye-laser amplifier.

The dye laser, tuned to a wavelength of 616 nm, genera-ted spectrally limited pulses with τρ = 6 + 2 psec. The dura-tion was measured by the autocorrelation method of noncol-liner second harmonic generation with averaging over 50realizations for each point. The spectral width was deter-mined using a Fabry-Perot interferometer.

These pulses were amplified in the dye amplifier( ί ι τ η ρ = 5 ) to E~\0 μ] and converted to the second har-monic (Λ. = 308 nm) in a 6-mm thick ADP crystal, the con-version efficiency being about 15%. The picosecond ultra-violet "seed" pulses obtained in this way were applied to theinput of the first XeCl amplifier.

This amplifier took the form of an electric-dischargeexcimer laser with automatic spark preionization over a di-

FIG. 3. Electrical circuit of the excimer amplifier: C, and C2 are storagecapacitors; R are charging resistors; 1) electrodes; 2) preionization capa-citors; 3) glass tubes; 4) peaking capacitors; 5) spark gap; 6) triggering.

electric surface (glass tubes). Figure 3 shows the electricalcircuit schematically. An important circuit element was aspecially developed three-electrode spark gap with distor-tion of the electric field. Such a low-inductance spark gapprovided rapid switching-on of the laser (—150 nsec) and atemporal instability of ± 3 nsec. The dimensions of the dis-charge region were 6 X 20 X 500 mm. For an active mixturecomposition HCl:Xe:He = 1:10:1000 at a pressure of 3 atm,the laser pulse energy with a resonator formed by a nontrans-mitting aluminum mirror and a quartz substrate was 100 mJfor τρ = 10 nsec. Because of the short population inversionlifetime in the first excimer module (~ 12 nsec), this modulealso acted as a device for isolating a single picosecond pulsefrom 5-6 pulses of the input pulse train.

After the first amplifier stage the radiation passedthrough a spatial filter which suppressed the parasitic back-ground of nanosecond superradiance. The filter was formedby two quartz lenses (with focal lengths of F = 25 and 40cm) and a 200-// diameter Teflon aperture stop. Lenses ofrelatively long focal lengths were employed because airbreakdown was observed when focusing an amplified picose-cond pulse with an F = 10 cm lens. When a lens with F< 20cm was used the spatial structure of the beam beyond the

FIG. 2. Schematic diagram of the apparatus: I) solid-state picose-cond master oscillator; II) synchronously pumped dye laser; III)excimer amplifier; IV) dye amplifier; 1) cell with saturable absorberand nontransmitting resonator mirror; 2) electrooptic switch; 3) la-ser oscillator active element; 4) dispersive prism assembly; 5) exitmirror; 6) dye amplifier cell; 7) solid-state amplifier; 6) dye amplifi-er cell; 7) solid-state amplifier; 8) frequency doubler; 9) filters; 10)cell with dye and exit mirror.

1317 Sov. J. Quantum Electron. 16(10), Oct. 1986 Akhmanovefa/. 1317

focus was strongly distorted because of the nonlinear pro-cesses in the beam constriction (ionization, self-focusing,etc.). All this provided indirect evidence of the low diver-gence of the amplified radiation.

The lenses of the spatial filter also served as optical ele-ments for matching the exit aperture of the first excimeramplifier to the entry aperture of the second.

The second excimer laser took the form of an electric-discharge XeCl laser similar to that described in Ref. 9, withimproved preionization and a slightly modified dischargegeometry. The dimensions of the discharge region were12 X 30 X 700 mm. The laser operated under the same condi-tions as those in the first module. In the lasing regime itcould produce an energy of 0.5 J in a rp = 25 nsec pulse.

Experiments were performed on amplifying a pulsewhose spectrum was broadened to such an extent that it cov-ered two vibrational-rotational bands. This was done in or-der to clarify the dependence, given by Eq. (6), of the energywhich can be extracted from the amplifier by a short pulse inthe deep saturation regime on the spectral width Δν of theinput signal. The dye laser radiation, before being frequencydoubled, was transmitted by a 5-m long single-mode fiber-optic waveguide. The radiation spectrum was then broad-ened to approximately 100 cm" \ The pulse with the broad-ened spectrum was amplified to an energy of about 50 μί in atwo-channel dye amplifier which was transversely pumpedusing an excimer XeCl laser. The frequency doubling tookplace in a 2-mm thick KDP crystal corresponding to an an-gular phase matching spectral width of 40 c m " 1 at a wave-length of 616 nm. The second-harmonic conversion effi-ciency was increased by shortening the pulse with thebroadened spectrum by means of a pair of 1200 mm""1 dif-fraction gratings. The ultraviolet "seed" pulses obtained hadthe duration of τρ =0.5 psec, the spectral width of Δν~80cm"1, ζηάΕ~2μ3.

3. EXPERIMENTAL RESULTS

An investigation was made of the dependence of theXeCl laser gain on the input signal energy density. The re-sults of the measurements are given in Fig. 4. The small-signal gain was a ==0.19 cm" 1 . In the saturation regime ofthe excimer amplifier, the gain of the pulses with the broadspectrum (80 c m " ' ) was approximately twice that of thepulses with the narrow spectrum (8 c m " 1 ) for the sameinput signal energy density.

The maximum energy of the picosecond ultraviolet ra-diation pulse amplified in the first stage was EoM ~ 1 mJ witha power contrast ratio of about 2000 for Em =* 1.5 μ]. Thesaturation energy density estimated from formula (7), g%at

= £ 8 a t /Sb (where Sb is the beam cross section), was about1.1 mJ/cm2 in good agreement with the results of Ref. 1.

One can estimate from Eqs. (3), (4), and (6) the maxi-mum energy which can be extracted from an amplifier in thestrong saturation regime, for a given discharge geometry,taking into account that the width of the spectrum of theinjected radiation is Δν=* 8 cm" ' , while the width of a singlevibrational-rotational transition in the XeCl molecule is

'w-s

in, mJ/cm2

FIG. 4. Dependence of the gain per pass in an excimer XeCl laser on theinput signal energy density.

Δν,,=22 cm" 1 . We obtain, from Eqs. (3) and (4), thatA"mM - 2 0 000 and E™* = 7 mJ. From Eq. (6) we find themaximum energy from the first amplifier to be E™x~ 1.9mJ. It can be seen that the value obtained is in good agree-ment with the experimental results.

The spectral dependence of the small-signal gain in anexcited HCl-Xe-He mixture was also investigated. This isgiven in Fig. 5 and it is in agreement with the correspondingdependence taken from Ref. 1.

The same measurements were made for the second am-plifier module. The results obtained agree with those de-scribed above. The energy of a picosecond pulse at the out-put of the second excimer amplifier is EOM ~ 15 mJ, with anenergy contrast ratio of 2 and a power contrast ratio of about4000. Since, in this case, saturation was achieved in a dis-tance which was small compared with the total amplifierlength, the output energy can be estimated from the formula^out = $b &,MaL> where Sb ~2 cm2. It can be seen that thetheoretical estimate of EOM ~ 16 mJ is in good agreementwith the experimental value.

Thus, although we have not yet made direct measure-ments of the duration of the ultraviolet pulses, all the indi-rect data provides evidence that in our system there is effi-cient amplification of picosecond and subpicosecond pulses(see also the numerical experiment reported in Ref. 7). Thecharacteristics of the laser system are given in Table I. Allthe experimental results were obtained using a computer-based data acquisition and processing system with an Elek-tronika DZ-28 microcomputer.

mo •3010 Μ7Λ MS λ, nm

FIG. 5. Dependence of the gain of an excimer amplifier on the wavelengthof the input signal.

1318 Sov. J. Quantum Electron. 16(10), Oct. 1986 Akhmanovefa/. 1318

TABLE I. Characteristics of laser system.

λ, μ

1,0640,5320,616

r, psec

20156

E,mS

20,50,01

?,MW

10030

2

λ, μ

0,6160.3080.308

τ, psec

0.56

0,5

£,mJ

0.0515

0.1

P,MW

1002500

200

CONCLUSIONS

A multipurpose laser system, consisting of a number ofsynchronized lasers, was developed and built by us. It cangenerate high-power ultrashort pulses with specified spec-tral and temporal characteristics in the ultraviolet part of thespectrum. The energy which can be obtained from an ex-cimer amplifier in the saturation regime increases when ul-trashort pulses with a wider spectrum are amplified. Thesystem can generate subpicosecond pulses at the wavelengthof308nm.

The authors are grateful to A. A. Podshivalov for hishelp with the experiments.

'P. B. Corkum and R. S. Taylor, IEEE J. Quantum Electron. QE-18, 1962(1982).

2S. Szatmari and F. P. Schafer, Opt. Commun. 48, 279 (1983).3S. A. Akhmanov, A. M. Val'shin, V. M. Gordienko, M. S. Dzhidzhoev, S.V. Krayushkin, V. T. Platonenko, and V. K. Popov, Kvantovaya Elek-tron. (Moscow) 11, 1897 (1984) [Sov. J. Quantum Electron. 14, 1278(1984)].

"S. Szatmari and F. P. Schafer, Appl. Phys. Β 33, 95 (1984).5 H. Egger, T. S. Luk, K. Boyer, D. F. Muller, H. Pummer, T. Srinivasan,and C. K. Rhodes, Appl. Phys. Lett. 41, 1032 (1982).

6V. K. Popov, Usp. Fiz. Nauk 147, 587 (1985) [Sov. Phys. Usp. 28, 1031(1985)].

7A. E. Korenchenko, V. T. Platonenko, and V. D. Taranukhin, Abstractsof Twelfth All-Union Conf. on Coherent and Nonlinear Optics [in Rus-sian], State University, Moscow (1985), p. 754.

8A. M. Val'shin, V. M. Gordienko, S. V. Krayushkin, V. T. Platonenko,and V. K. Popov, Kvantovaya Elektron. (Moscow) 13, 1713 (1986)[Sov. J. Quantum Electron. 16, 1125 (1986) ].

9M. S. Dzhidzhoev, V. T. Platonenko, and V. K. Popov, Vestn. Mosk.Univ. Ser. 3 25, No. 5, 113 (1984).

Translated by A. N. Dellis

Numerical and experimental investigations of the energy capabilities of achemical OD(OH)-CO2 laser

A. S. Bashkin, N. M. Gamzatov, Yu. P. Podmar'kov, Ο. Ε. Porodinkov, andA. N. Oraevskii

P. N. Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow

(Submitted July 16, 1985)Kvantovaya Elektron. (Moscow) 13, 1999-2008 (October 1986)

A calculation model is developed of a pulsed chemical OD-CO2 laser. Comprehensiveexperimental tests are reported: they show that the model describes satisfactorily the main timeand energy characteristics of a laser. Numerical optimization of the composition of the mixtureis carried out in respect of the main components on the assumption of instantaneous generationof D atoms and a study is made of the influence of temperature on the energetics of the laser.The approximation of instantaneous mixing of the fluxes is used to show that a cw chemicalOD-CO, laser should be characterized by high values of the specific output energy ( — 150 J/g) and of the chemical efficiency ( ~ 10% ).

1. INTRODUCTION

An important advantage of a chemical laser based onthe transfer of vibrational energy from excited hydroxylmolecules to carbon dioxide molecules is the nontoxicity ofthe final products of the reactions that occur in the activemedium. The source of vibrationally excited hydroxyl mole-cules is the highly exothermal reaction1·2

9)+O2, — ΔΗ=78 kcal/mole. (1)Investigations of the distribution of the vibrational-rota-tional energy in OH (υ) molecules have shown that ~90%of the thermal effect of the reaction (1) is used in vibrationalexcitation of the OH molecules and that the upper vibration-al levels with υ = 7-9 are populated first.3·4 At low pressuresοΐρ0~0Α Torr a complete inversion is established between

1319 Sov. J. Quantum Electron. 16(10), Oct. 1986 0049-1748/86/101319-07S04.10 © 1987 American Institute of Physics 1319