Gain measurements in the KrCI excimer laser

E. Armandillo, A. Luches, V. Nassisi, and Maria R. Perrone

The single-pass/double-pass amplified spontaneous emission technique has been used to measure the netgain at 222.4 nm in a discharge-pumped UV-preionized KrCl laser. The net gain has been measured as a

function of pump rate and total pressure in He- and Ne-based gas mixtures. A maximum net gain of 0.15

cm- 1 has been obtained in a 0.09% HC1/10.1% Kr/1.5% He/88.3% Ne mixture at a total pressure of 355 kPa

and at a specific power loading of -21 MW/cc. The data presented are compared with previous net gain

measurements performed using the passive absorption cell method.

The short-wavelength KrCl excimer laser (222 nm)has many potential applications in areas such as pho-tochemistryl and annealing of semiconductor devices.2

In recent experiments 3 4 the KrCl laser was excited inUV-preionized transverse discharges. It was demon-strated that output energy levels similar to those ob-tained from KrF and XeCl can also be achieved withKrCl, by supplying to the lasing medium specific elec-trical powers -1 order of magnitude higher than thoserequired for KrF and XeCl.

The optical gain is an essential parameter for thecharacterization of any laser system. In this paper wereport the measurements carried out to determine thegain characteristics of a' discharge-pumped UV-preionized KrCl laser using the single-pass/double-passamplified spontaneous emission (ASE) technique.5 6

The net gain was measured as a function of pump rateand total pressure in the He- and Ne-based gas mixtureswhich were previously found to give the optimum out-put energy from this laser system.7 The KrCl spectralgain profile was also measured at the optimum excita-tion conditions. Finally, the data presented in thispaper are compared with previous net gain measure-ments performed with a simpler method.8

The layout of the experimental apparatus used for thegain measurements is shown in Fig. 1. The KrCl laserexcitation was obtained by an UV-preionized dischargedescribed previously 3 7 ; it will be only briefly outlinedhere. The discharge configuration is a capacitor

E. Armandillo is with ENEA-CRE, Laboratorio Tecnologie Spe-

ciali, Casaccia, Roma, Italy; the other authors are with UniversitA degliStudi, Dipartimento di Fisica, 73100 Lecce, Italy.

Received 7 July 1984.0003-6935/85/010018-04$02.00/0.© 1985 Optical Society of America.

transfer type made up of a primary capacitance of 60 nFand a secondary capacitance of 35 nF internal to the gaschamber. The profiled electrodes (solid cathode andplanar mesh anode) separated by 2 cm define a dis-charge length L = 35 cm. The arc sources, which pro-duce the UV-preionization, are situated behind theanode and govern the discharge width, which is 1 cmwide. The UV-grade silica windows of the gas chamberwere angled at 4° to the chamber axis to prevent gaindepletion caused by optical feedback.

The spontaneous emission from the excited KrClmolecules was monitored with a fast photomultipliertube (RCA 1P28) after spectrally filtering the radiationwith a monochromator. The output signals were dis-played on a Hewlett-Packard 1727A storage oscillo-scope.

The ASE technique involves a comparison of thefluorescence yield of various lengths of the active me-dium. In this paper the effective gain length was dou-bled by placing a mirror at one end of the gain region tomeasure single- and double-pass intensities. The netgain coefficient go in the excited laser mixture was cal-culated by

1go= -ln[(IDp/ISp - WT1L

(1)

where Isp and IDP are peak values of the single- anddouble-pass ASE intensities, R is the reflectivity of themirror used to measure IDP, and T is the transmissionof the cell window in front of the mirror.

Equation (1) assumes that Isp and IDP come from thesame volume and that this volume is uniformly excited.It is assumed also that the fluorescence radiation,coming from a small solid angle, is observed at a dis-tance large compared with the length of the excitedvolume. To meet this last condition a cell-detectorseparation of 500 cm was used. To minimize errors dueto geometric effects, two 2-mm apertures were locatedalong the optical path to the detector.

18 APPLIED OPTICS / Vol. 24, No. 1 / 1 January 1985

WINDOWSAPERTURES TRANSMISSION T

500 c. L-35cmMIRRORREFLECTIVITY R

Fig. 1. Experimental layout of the apparatus used for gain mea-surements: MC, monochromator; PM, photomultiplier; OS,

oscilloscope.

The single- and double-pass ASE intensities weremeasured on subsequent shots since the signals wereeasily reproducible from shot to shot. The error in thederivate values of go has been estimated to be <5% in-cluding an estimate of the precision of reading the dataand the shot-to-shot deviations.

Figure 2 shows the net gain measurements as afunction of the specific power loading supplied to thelasing medium, in gas mixtures using as buffer gas Ne(0.09% HCl/10.1% Kr/1.5% He/88.3% Ne) and He (0.09%HCl/10.1% Kr/89.8% He) at a total pressure of 355 kPa.From these gas mixtures the highest laser energies werepreviously obtained (3.1 and 2.1 J/liter, respectively).The specific power loading values are calculated as theratio of the energy stored in the secondary capacitanceto the energy deposition time (which is 25 and 30 nseclong in the Ne and He mixtures, respectively) to theactive volume (70 cc). The gain data of Fig. 2 are ob-tained at the ( = 222.4-nm) wavelength where themaximum gain occurs. From Fig. 2 in both mixes, alinear dependence of go with Wp, the specific powerloading, can be inferred. In the Ne-based mix the netgain reaches a peak value of go 0.15 cm-1 , very nearthat measured in XeCl and KrF lasers at specific powerloadings -1 order of magnitude smaller.9 10 These gainmeasurements confirm the previous observations thatthe KrCl laser can be recognized as a source of coherentUV radiation comparable with the XeCl and the KrFlasers, provided that specific electrical powers -1 orderof magnitude higher than those required for the previ-ously mentioned excimer lasers are supplied to the KrCllasing medium.

One observes from Fig. 2 that at equal specific powerloadings, higher gain values are measured in the Ne-based gas mix than in the He mixtures. This meansthat the more efficient operation of KrCl in the neonmixtures is not due to increased energy transfer into thedischarge. This may partially result from the hotterelectron energy distribution in neon mixtures as sug-gested in Ref. 11. In fact, the lowest excited state thatneeds to be accessed to produce KrCl upper laser statesis the Kr(3P1) resonance trapped level.12 Then it isessential to have a large number density of electronswith energies above 10.03 eV. Figure 13 in Ref. 11shows the electron density distribution curves for pureNe and He as a function of E/N, the ratio of the appliedelectric field to the gas density. From these curves welearn that, in the range of E/N observed in our experi-

0.14 -

0.12 V

0.10 -

Ne

29He

0.08 I-

I L I I 10 14 18 22 26 Wp(MW/cc)

Fig. 2. Net gain as a function of the specific power loading in He-(o) and Ne- () based mixtures at 355 kPa of total pressure and at

222.4 nm.

g.(cm-1)

0.14

0.12

0.10 _

Ne

03

He

0.08 _

0.06 H

100 200 300

Fig. 3. Net gain as a function of pressure in He- () and Ne- (0)based mixtures at 45-kV charging voltage and at 222.4 nm.

mental conditions, larger metastable and ionic forma-tion rates should occur in Ne-based than in He-basedlaser mixtures. However, the higher values of go mea-sured in the neon mix can also be due to a lower ab-sorption coefficient at 222 nm in such a gas mixture. Infact, Taylor et al.9 have measured the absorption coef-ficient in an UV-preionized XeCl laser and have foundthat in the Ne-based laser mix the level of absorptionis 10 times lower than for the He-based laser mix-ture.

Figure 3 gives the net gain coefficient as a functionof filling pressure at 45 kV of charging voltage in the He-and Ne-based laser mixtures. In both mixtures go in-creases montonically with filling pressure and thebenefits of using neon instead of helium become moreapparent at higher gas pressures.

The curves of Fig. 3 indicate that further increases ingo should be possible at gas pressures higher than 355kPa, which is the safe pressure limit of our device.However, quenching processes of the upper laser statelimit the growth rate of go. It was observed that the

1 January 1985 / Vol. 24, No. 1 / APPLIED OPTICS 19

PT(kPaj

0.0

go(c m-l)

0.141-

0.12 -

0.10 -

0.08 -

0.06I I

221 222 223 -(nmJ

Fig.4. Gain vs wavelength for a gas mix of 0.09% HC1,10.1% Kr,1.5%He, 88.3% Ne at 355-kPa filling pressure and 45-kV charging

voltage.

go(cmw)

0.14 -

0.12 -

0.10 _

10 14 18 22 Wp MW/cc)

Fig. 5. Comparison of the net gain values as a function of the specificpower loading measured with the ASE technique (0) and with theabsorbing cell method (A). The gas mix is 0.09% HCl, 10.1% Kr, 1.5%

He, 88.3% Ne at 355-kPa filling pressure.

fluorescence signal Isp, which was 25-nsec long (fullwidth at half-maximum) at 150 kPa narrows to 13 nsecat 355 kPa. The slower rise of go at higher gas pressurescan also be related to the lowering of E/N with in-creasing pressure.' 3"14

Measurements of the gain vs wavelength were madeusing the same experimental procedure but at differentwavelengths as selected by the monochromator. Figure4 shows the spectral gain profile of KrCl obtained in theNe buffer gas mix. This low resolution gain spectrumlooks quite homogeneous and shows a possible tuningrange of 1.3 nm.

In a previous paper8 the "line-centre" net gain of ourdischarge-excited KrCl laser was measured by a simplermethod. 8 "15 A passive absorption cell, containing a gas(NO2) which absorbs radiation at the wavelength of thelaser (222 nm), was inserted into the laser cavity. Byincreasing the pressure of the absorbing gas the laserwas reduced at threshold. The net gain coefficient g0was calculated using the cw threshold condition. Fig-ures 5 and 6 display the experimental net gain valuesobtained with both techniques in the same experimentalconditions. The agreement between the data is quitesatisfactory. Both figures show that the absorbing cellmethod gives net coefficients smaller than those mea-sured with the ASE technique. This can be assignedto gain depletion effects due to axial ASE at high spe-cific power loadings and partially to the gain reductioncaused by the cavity length, which is more than twicethe length of the active medium.

In conclusion, the ASE technique has been used toprobe the gain of a KrCl transverse discharge laser withUV preionization. The good agreement with previousmeasurements supports the reliability of the data pre-sented in this paper.

The authors wish to thank V. Nicolardi for valuabletechnical assistance. This work was supported byMinistero della Pubblica Istruzione.

g0(cm-

0.14

0.12 [-

0.10 1-

0.081-

0.06 F

0.0.

4I I I

100 200 300 P.(k Pay

Fig. 6. Comparison of the net gain values as a function of fillingpressure measured with the ASE technique (0) and with the ab-sorbing cell method (A). The gas mix is 0.09% HCl, 10.1% Kr, 1.5%

He, 88.3% Ne at 45-kV charging voltage.

References1. G. Hancock and H. Zacharias, "Laser-Induced Fluorescence from

CO(Al II)," Chem. Phys. Lett. 82, 402 (1981).2. E. D'Anna, G. Leggieri, A. Luches, M. R. Perrone, I. Catalano, and

G. Majni, "Metal/Silicon Reactions Using Pulsed Excimer andRuby Lasers," Proc. Int. Conf. Laser Processing and DiagnosticApplication in Electronics Materials, Linz (1984) (to be pub-lished).

3. E. Armandillo, A. Luches, V. Nassisi, and M. R. Perrone, "Im-proved Lasing Performance of KrCl Excimer Laser," Appl. Phys.Lett. 42, 860 (1983).

4. J. E. Andrew, P. E. Dyer, and P. J. Roebuck, "Improved EnergyOutput from Discharge Pumped ArF and KrCl Lasers," Opt.Commun. 49, 182 (1984).

5. C. V. Shank, A. Dienes, and W. T. Silfvast, "Single Pass GainExciplex 4-MU and Rhodamine 6G Dye Laser Amplifiers," Appl.Phys. Lett. 17, 307 (1970).

6. C. B. Edwards, F. O'Neill, and M. S. Shaw, "Absorption and GainMeasurements in the KrF Laser Medium at High Pump Rate,"Appl. Phys. Lett. 38, 843 (1981).

7. A. Luches, V. Nassisi, and M. R. Perrone, "Experimental Studiesof a KrCl Discharge Laser," in Optoelectronics in Engineering,W. Waidelich, Ed. (Springer, Berlin, 1984).

20 APPLIED OPTICS / Vol. 24, No. 1 / 1 January 1985

l

0.08 _-

8. E. Armandillo, A. Luches, V. Nassisi, and M. R. Perrone, "GainMeasurement in the KrCl Discharge Laser," to be published inOptics Comm.

9. R. S. Taylor, P. B. Corkum, S. Watanabe, K. E. Leopold, and A.J. Alcock," Time-Dependent Gain and Absorption in a 5J UVPreionized XeCl Laser," IEEE J. Quantum Electron. QE-19,416(1983).

10. S. Watanabe, A. J. Alcock, K. E. Leopold, and R. S. Taylor,"Spatially Resolved Gain Measurements in UV Preionized Ho-mogeneous Discharge XeCl and KrF Lasers," Appl. Phys. Lett.38, 3 (1981).

11. R. C. Sze, "Rare-Gas Halide Avalanche Discharge Lasers," IEEEJ. Quantum Electron. QE-15, 1338 (1979).

12. R. C. Sze and P. B. Scott, "Intense Lasing in Discharge ExcitedNoble-Gas Monochlorides," Appl. Phys. Lett. 33, 419 (1978).

13. R. C. Sze, "Improved Lasing Performance of XeCl Using Ar andNe Diluents," J. Appl. Phys. 50, 4596 (1979).

14. A. Luches, V. Nassisi, and M. R. Perrone, "Performance Studyof the KrCl Discharge Laser with Liquid Chlorine Donors," to bepublished in Optics Comm.

15. E. Armandillo, A. J. Kearsley, and C. E. Webb, "A SimpleTechnique for Measuring the Gain of RGH Lasers," J. Phys. E:Sci. Intrum. 15, 177 (1982).

Mviegsl Caenar

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