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Enhancements in the output energy of a KrF oscillatorA. W. McCown and J. A. B. Godard Citation: Journal of Applied Physics 64, 2879 (1988); doi: 10.1063/1.341603 View online: http://dx.doi.org/10.1063/1.341603 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/64/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electron beam pumped KrF lasers for fusion energy Phys. Plasmas 10, 2142 (2003); 10.1063/1.1564082 Modeling the energy deposition in the aurora KrF laser amplifier chain AIP Conf. Proc. 191, 119 (1989); 10.1063/1.38698 Enhancement of the specific output energy of an electronbeam pumped KrF laser by using Ne as the main buffergas Appl. Phys. Lett. 45, 356 (1984); 10.1063/1.95268 Compact coaxially excited high energy density KrF laser J. Appl. Phys. 55, 1410 (1984); 10.1063/1.333232 Electrically triggered multimodule KrF laser system with narrowlinewidth output Rev. Sci. Instrum. 54, 845 (1983); 10.1063/1.1137489
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Enhancements in the output energy of a Krf oscillator A. W. McCown and J. A. B. Godard Laser Physics and Applications Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
(Received 7 December 1987; accepted for publication 31 May 1988)
Enhancements of up to 45% in the output energy of a KrF excimer laser have been observed following optical pumping of the resonator with as little as 1 mJ of 248-nm radiation from an external source. Polarization of the input beam leads to enhancement and partial polarization ofthe output pulse for time delays between the 2 pulses of up to 100 ns. Enhancement is not seen in seedin~ experimen~s perfor~ed on a KrF amplifier, i.ndicating that the oscillator energy enhancement IS due to optical seedmg of the resonator cavity, resulting in a shorter amount of time required for the laser to reach threshold.
I. INTRODUCTiON
Several experiments were recently conducted at the University of Illinois l which demonstrated that enhancements in the output energy of an XeCl (30B-nm) oscillator resulted from optically pumping the laser cavity with a small amount of pulsed 308-nm radiation. Similar results were also obtained when the XeCl1aser was injected with 193-, 248-, and 351-nm radiation,z although smaller enhancements were observed. In addition, the time history of the enhancement, i.e., the energy enhancement as a function of time delay between the pump pulse and the XeCl laser pulse, was markedly different at the various wavelengths. The enhancement that resulted from 30g-nm pumping was attributed to the removal of a molecular species which absorbs at 308 nm, and cavity seeding, both of which shorten the time required for the laser to reach threshold, The significance of cavity seeding is that it eliminates the time necessary for the cavity fiux to build up from the spontaneous emission "noise." Thus, seeding can only take place when the seed and output wavelengths are identical.
This paper reports the results of experiments in which a KrF oscillator (and in separate experiments, an amplifier) was optically pumped with 248-nm radiation at different times before the output pulse exited from the laser. In order to study the effect of cavity seeding, the input pulse was variably polarized and attenuated using beamsplitters, and the output polarization was ex.amined. The results for the KrF oscillator injected with 248-nm radiation are similar to the Xeel data of Ref. L A peak enhancement of 45% occurred at a time delay between the input and output pulses of 40-50 ns. However, the output pulse was partially polarized, even for time delays in excess of 100 ns, proving that cavity seeding was taking place even at long time delays. In addition, no enhancement was observed in the extracted output of a KrF amplifier that was injected with 248-nm pulses, which were as large as 100 mJ. If a photochemical process was taking place, an enhancement in the extracted energy output would be observed. These results indicate that cavity seeding is the primary cause of the energy enhancement seen in KrF oscillators.
II. EXPERIMENTAL SETUP
In these investigations, two types of experiments were performed: oscillator-oscillator and oscillator-amplifier.
The layout for the former case is illustrated in Fig. 1. Two excimer lasers [Lambda Physik EMG-20lE (20t) and EMG-101MSC (101)], separated by 1.5 m, were arranged so that their beams were counterpropagating. Each laser was filled with a KrF gas mixture (0.2% Fry, 5% Kr balance He' research grade rare gases). When fired separat~ly, the laser~ yielded a maximum of 750 mJ (from the 201) and 335 mJ (from the 101, both unpolarized) pulses. Each laser resonator consisted of a flat dielectric full reflector (R ~ 99.5 % ) and a flat MgF2 output coupler (92% transmission). The laser pulse widths were 29- and 2S-ns full width at half maximum (FWHM) for the 201 and 101, respectively. Both lasers were fired at 10 Hz with a firing jitter of ± 2 ns.
Alignment and aperturing of the laser beams were accomplished by passing them through two rectangular apertures (2.45 XO.95 cm2
) which kept laser electrodes from being irradiated. The lasers were slightly misaligned to prevent the formation of one large laser cavity. To further isolate the lasers, an attenuator, composed of 10 optosil fiats with a transmission of 54% was placed in the beam path at a 3° angle of incidence to the laser beams. This angle was chosen in order to minimize polarization of the beams and to prevent beam walk off. Two suprasil quartz beamsplitters were likewise set at 3" and reflected approximately 8% of the incident beam onto Gentec ED-200 pyroelectric energy detectors, which monitored the energies of the pulses coming from the two lasers, The detectors were interfaced to statistical readout units, which calculated the average of 100 shots and the standard deviation.
Timing between the pulses was controlled with a trigger generator and a digital delay generatoL A vacuum photodiode monitored stray light which had scattered off of the attenuator and allowed the temporal separation of the pulses to be calculated from scope traces. At the start of each experiment, the initial output energy of each laser was measured with a Gentec ED-500 energy detector. Three hundred shots were averaged. Only 34% of the 101 output beam was transmitted through the experimental apparatus to the entrance of the 201 while 25% of the 201 beam arrived at the 101 inpuL Data acquisition consisted of recording the averaged energy output of the probed laser (the laser which was fired later in time) as a function of the time delay, 1M, between the peaks of the two pulses. Although a fraction of the energy of the injected pulse was included in the probed laser energy measurement, its value was detennined and subtracted out.
2879 J. AppL Phys. 64 (6), 15 September 1988 0021-8979/88/182879·06$02.40 @ 1988 American Institute 01 Physics 2879
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EMG-101 EMG-201
KrF OR ArF ,l.J, ATTENUATOR , I / ~ __ J ENERGY
KrF LASER
ENERGY DETECTOR
VACUUM PHOTODIODE
OSCILLOSCOPE
DETECTOR
FIG. !. Diagram of the experimental apparatus. The dashed lines represent a 45" beamsplitter and energy detector which were used in polarization studies. In this orientation, the 101 is the probed lasec
The enhancement was then calculated by taking the difference in the output energy of the probed laser with and without the firing of the seed laser and dividing by the unenhanced output energy.
The input beam was polarized after being transmitted through two 20% and/or 50% dielectric beam splitters (not shown in Fig. 1) which preferentially reflected the S over the P polarization. The reflectors were oriented at an angle of 45° to the beam axis in such a way as to eliminate beam walkof[ Transmission through a 45° suprasil beam splitter (shown dashed in Fig. 1) further polarized the beam. This flat was inserted to allow the polarization of the output beam to be monitored by comparing the energy measured from this beamsplitter to that from the 3° fiat, which is insensitive to polarization. As the output pulse became Ppolarized, less energy was reflected by the 45° fiat as compared to the 3° flat, Approximate reflectivities versus polarization for the optics used in these experiments are given in Table 1.
III,RESULTS Enhancements in the output energy of the 201 for var
ious time delays between the peaks of the laser pulses are illustrated in Fig. 2. Each data point is the average of 300 shots and the uncertainties shown are the standard deviations from the mean. Both laser beams were randomly polarized, and the output energy was determined from reflections off the 3° quartz flat. Injecting the 201 cavity with 50 mJ of 248-nm radiation at a time delay of 45 ns caused the 201 energy output to increase from 580 to 77S mJ; an enhancement of 34% (not including the 50-mJ injected energy). Enhancement continued for time delays in excess of 100 ns.
2880 J. Appl. Phys., VoL 64, No.6, 15 September 1988
A sman secondary enhancement occurred at later times (150 < tlt < 200 ns), which is discussed below.
In order to determine the extent to which cavity seeding took place, the injected pulse was variably polarized and attenuated and the experiment was repeated using both the 3· and the 45° beamsplitters with separate energy detectors. Since a 45' quartz flat reflects 16.9% of Spolarized light and 1.7% of the Ppolarization, while a 3° flat reflects 7.7% regardless of polarization, a comparison of the energy detected using the separate flats determines the output polarization. If Eo is the output energy of the laser and Ep and Es are the total P and S polarized output energies, respectively, then the energy detected at 45°, Ed' is given by
Ed = 0.169E, + O.017b~. (1)
Rewriting ( 1) and taking into account the fact that Eo = Es + Ep ' one obtains
1;, = 1.11- (Ed /0.152Eo)' (2)
where 1;" the fraction of the output photons which are P
TABLE 1. Reflectivities of Sand P polarization for various optics.
Optic
3" quartz flat 4S quartz fiat
20% beamsplitter' 50% beamsplitter'
" Manufacturer's specifications.
s
0.077 0.169 0.32 0.67
P
0.077 0.017 0.095 0.29
A. W. McCown and J. A. B. Godard 2880
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!-Z 40 UJ :::i I.IJ (,) Z 30 ..: ::t: Z W
20 >-
" a: w z 10 W
I-Z UJ (,) a: w II. 80 100 120
TIME DELAY, At (ns)
FIG. 2. Enhancement in the output energy of the 201 laser for different time delays. A 50-mI seed pulse from the 101 was injected into the 201, whose unperturbed output was 580 mI. Each data point is the average 0000 laser shots with the standard deviation shown.
polarized, is just Ep I Eo. For the case of unpolarized light,./;, equals 0.5 as expected. The 3° beamsplitter, which is polarization independent, is used to determine Eo.
Figure 3 displays the fraction of the output energy which is P polarized,i;" for several cases of input polarization. The polarization of the seed beam was varied by transmission through various optics, as given in Table n. The energy contained in the injected pulse also decreased, from 110 mJ for the 0.54 P polarized case to 13 mJ for the case where the input beam was almosttotaHy P polarized (0.92) . As seen in the figure, the output beam is partially polarized by the injected pulse in an cases and at time delays in excess of 100 ns.
The effects of injected energy and input beam polarization on the output were differentiated by examining the enhancement as a function of time delay for different seed pul.se energies while monitoring the output with the 3° beamsplittef. The results are illustrated in Fig. 4. The 101 laser was seeded with either 80-, 8-, or 0.7 -mJ pulses from the 201, and because of the choice of beamsplitter, the measurements were not affected by polarization. Comparing Figs. 2 and 4, it is seen that the peak enhancements are approximately
1.0
Input 'P' Polarization , ... Unpoiarl.ed
-r:; • 0.54 &. 0.e6 ::10 1:1.._ 0,8 0 0.68 --::1<11
O'l:! r:;!11
:8~ u 0.6 01-""c., lL-
0.4 0 25 50 75 100 125
Time Delay, t.t (ns)
FIG. 3. A set of five curves demonstrating the effect of an increasingly P polarized input beam on the 201 output pulse polarization, which is calculated using a 45' beamsplitter that preferentially reflects S polarized light. The injected energies were 110, 80, 40, and 13 mJ and decreased as the seed became more P polarized.
2881 J. Appl. Phys., Vol. 64, No.6, 15 September; 9S8
TABLE H. Injected pulse polarization for the curves of Fig. 3. BS signifies beamsplitter,
Optics
12-3' as 1·45', 12_3' as
1-45' as, 12-3' BS, 2-20% as 1_45' as. 12_3' BS. 2-50% as
1-45' ES, 12-3' BS, 2-20% as. 2-50% BS
Polarization
Unpolarized 0.46 S, 0,54 P 0.32 S, 0.68 P 0.14 S, 0.86 P
0.08 S, 0.92 P
equal, but as the seed energy decreases, the peak shifts to smaller time delays. Although not shown, the enhancement produced by the 8-mJ seed pulse peaked at 35 ns, while the 80- and 0.7 -m! pulses produced peaks at 40 and 25 ns, respectively. In addition, low-energy seeding resulted in enhancements that dropped off at smaller b.t's.
To determine whether the observed effect was wavelength sensitive, the 101 laser was fined with an argon fluoride gas mixture containing 0.34% Fz, 15.9% Ar, and 83.8% He to a total pressure of 2.2 bar. The laser produced 180 mJ of ArF radiation, of which 60 mJ was directed into the input of the 201 laser, and enhancement was recorded as a function of time delay. Figure :5 contains both the ArP seeding results and the late time KrF results mentioned earlier. In the ArF case, a slight suppression takes place for small time delays, and an enhancement begins at a time delay of 160 ns. This late time enhancement is duplicated in KrF seeding, indicating that a similar process is taking place. In both cases, the enhancement faUs to less than 20% of its peak value for time delays greater than 220 ns.
IV, KrF AMPLIFIER STUDIES
It is difficult to determine the exact cause of the energy enhancement in an oscillator-oscillator experiment. Al-
50
I-Z I.IJ :e 40 W
~ -< i 30 W
>-(!) a: w Z
20 W l-Z
~ a: W II.
120
T!ME DELAY, AI (ns)
FIG. 4. The effect ofinjected pulse energy on enhancement. The 101 laser was monitored for inputs of 80 and 0,7 mI, and it emitted 235 mJ of energy in the absence of an injected pUlse.
A. W, McCown and J. A. B. Godard 2381
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• ~i"1 = 193 nm
.. ~inj = 248 nm
]
-40~----~50~~~lO~O~--~~--~~----~250
TIME DELAY, llt (n5)
FIG. 5. Energy enhancement for seeding with 193-nm radiation, and the late time effect of 24S-nm seeding. The ArF laser produced lSO-mJ pulses, of which 60 mJ entered the 201 laser cavity. With no seed pulse, the 201 produced 715-mJ pulses. The 248-nm data were taken under the conditions of Fig. 2.
though the data of Fig. 3 demonstrate that cavity seeding takes place for 0 < f:.t < 125 ns, the enhancement may also be caused by a photochemical process. However, if one of the lasers is configured as an amplifier, an enhancement in energy output could only be caused by a photochemical process, since a 248-nm pulse is required to extract energy from the amplifier.
The experimental setup of Fig. 1 was, therefore, modified. The full reflector of the 201 laser was removed, and its MgFz windows were canted to reduce amplified spontaneous emission (ASE). An experiment was performed that
50 ns OPTICAL DELAY Lf..IE
INPUT ENERGY
FIG. 6. Amplifier extracted energy as a function of input energy. ASE has been subtracted out. The amplifier is nearly saturated for an input extracting pulse greater than 10 mI.
determined the amount of input energy required to saturate the amplifier. Figure 6 is a collection of data points representing energy extracted from the amplifier (ASE subtracted out) as a function of input energy. Optimum energy extraction was obtained when the injecting laser (101) was triggered 15 ns before the amplifier (201).
At this point, the experimental arrangement of Fig. 7 was employed. The attenuator and one of the 3· beamsplitters of Fig. 1 were removed from the beam path and replaced with two 80% reflectors. These formed the beginning and the end of a 50-ns optical delay line that was constructed with three normal incidence full reflectors (FRs) and two 45c FRs. The optical delay could be reduced to 35 ns by intercepting the beam before it reached the last normal incidence FR, as seen in Fig. 7. A 2X telescope located in the delay line was fashioned from two cylindrical lenses (7.5-and IS-cm focal lengths ) and served to collimate the beam,
, ____ MIRROR FOR 35 ns )0"-- DELAY UNE
-----------------------~~-------------- -----~ ~SLi;_----
-~~_=====tilf_: . N.. HR r*----~_"I
':
OSCILLATOR' x2 TELESCOPE ,.o~ SEED PUlSE
I EXTRACTING , PULSE ENERGY
DETECTOR
AMPLFER I 1-,,-1 ~. _~ __ _
1 _-------- It 11.1 _______ _____ ..,,_~~-+-----+\-_---.-I f r __ ----- u (
ENERGY DETECTOR
80% REFLECTORS
VACUUM PHOTOD10DE
OSCILLOSCOPE
FIG. 7. Modified experimental setup, which included a 50-ns delay line that could be shortened to 35 ns by moving the final mirror to the position shown by dashed lines. The laser on the right has been converted into an amplifier by removing its full reflector and tilting the cavity windows to reduce ASE.
2882 J. Appl. Phys., Vol. 64, No.6, 15 September 1988 A. W. McCown and J. A. B. Godard 2882
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while a slit, which was actually a hole burned through a Polaroid film, acted as a spatial fiiter. The portion of the 101 beam which passed through the 80% reflectors acted as the seed pulse, while the delayed (extracting) beam extracted energy from the 201. An energy detector determined the energies of these pulses as wen as the energy extracted from the amplifier.
In the configuration shown in Fig. 7 a seed pulse containing 7.2 mJ of energy passed through the 201 amplifier, followed 50 ns later by a B.S-mJ extracting pUlse. Five hundred shots were made with the seed pulse blocked, resuIting in an average amplifier output of575 ± 16mJ. When the seed pulse was allowed to pass through the amplifier, the difference in the averaged extracted energy was only 8.1 mJ. The experiment was repeated using the 35-ns delay line with similar results, and the roles ofthe two lasers were reversed. A l00-mJ seed pulse followed 50 ns later by a 20-mJ extracting pulse produced no enhancement in the extracted output of the lOllaser. This indicates that no measurable enhancement takes place for the seeding of a KrF amplifier by 248-nm radiation.
V. DISCUSSION
The experiments described in this paper have been performed in an attempt to determine the cause of the large energy enhancements observed when an excimer laser oscillator is injected with radiation at its own wavelength, and to explain the time history of the enhancement. Taken together, the results ofthese experiments indicate that the enhancement is a result of optical seeding of the laser cavity, which anows stimulated emission to be built up from a seeded field, rather than from the noise (ASE). If this is the case, one expects the peak in the enhancement to occur when the flux of the seed pulse peaks in the probed laser cavity at the same time as the laser's discharge excitation. 3 The osciUograms shown in Fig. 8 demonstrate the output pulse enhancement. Figure 8 (a) is a photodiode waveform taken of the 201 pulse with and without a 50-mJ seed pulse, while 8{b) shows enhancement of the 101 output pulse following a 80-mJ injected pulse. In both cases, the time delay was 50 ns and lasing began 6 ns earHer when seeding took place. In addition, the peak intensities were larger for the case of seeding. The earlier rise of the pulse indicates that the injected flux should peak inside the probed laser about 10 ns before the laser "turns on," or 20-30 ns before its intensity peaks. The injected intensity is a maximum throughout the laser cavity when the peak intensity has made one complete round trip (~1O ns) through the cavity. Since the lasers are separated by 150 em, the peak enhancement should occur at a time delay of 35-45 ns. This is shown to be the case in Fig. 4, although the cause of the peak shifting to smaner time delays as the seed energy decreases is not presently understood, and a complete understanding awaits further study.
The results shown in Fig. 3 verify that photons from the seed laser are still present in the cavity of the probed laser up to lOOns after the seed laser is fired. An interesting observation is that.t;, is the greatest at time delays 000-80 ns, how-
2883 J. Appl. Phys., Vol. 64, No.6, 15 September Hl88
••••••• -••• '.' •••••••••••••• ~ ••••••••••••••••• ~ ....... ~.".; •••••••• O;' ....... :.:.:.: •••••••••••••••• :.:.;.:.:.; •••• .-••••• ~ ••• :.:.:.:.:.::;;.:.: ••••••• ;> ••• ";:'".:.:.:.:.~ ••• : ••••••••••••••• ,. •••••
ever, this may be due to reflections back through the optical system which reenter the probed laser having a larger P polarization than indicated. Although there are photochemical processes that retain a memory of the polarization of the incident photons, only cavity seeding can account for the matching of output and input polarizations as seen in these experiments. One can account for the enhancements seen at long time delays by considering the amount of time it takes for the intensity of the seed pulse to drop from its peak to 1 % cfits peak value. From Fig. 8, this can be from 40 to 50 ns. Therefore, the BUK of the seed pulse is at least 1 % of its peak value at discharge excitation inside the probed laser for a At of 80-90 ns. The peak enhancement is saturated for injected energies less than 1 mJ, as seen in Fig. 4, so that 1 % of the peak intensity should still show an effect. Since the photon lifetime of these resonators is 9-10 us, and taking into account fluorine absorption and the presence of two apertures inside the laser;~ one would expect an intensity of at least 1 W Icm2 to be present in the cavity for a Mof 110-120 ns and an input energy of 80 mJ. Bigio and Slatkine5 reported efficient injection locking of a KrF laser with an input intensity of 0.1 W /cm2
• Similar results have been obtained by others. !,6,7
If a photochemical process such as absorption or photoionization is to saturate at less than a millijoule, the corresponding cross section that describes the process must be extremely large. If
dN = _ 0'[ N. (3) dt Iiw'
where N is an absorbing species, (7 is the cross section which describes the absorption process, fuv is the photon energy (5-eV for KrF), and lis the injected intensity, one can integrate over the laser pulse length to find the fraction F of the species which absorbs
F = 1 - exp( - O'E lIiwA), (4)
where E is the energy of the injected pulse and A is the crosssectional area of the beam (2.33 cm2
). If saturation is arbi-
a)
b)
FIG. 8. KrF laser waveforms with and without injected pulses. The upper set of curves demonstrate the effect of seeding the 201 cavity while the lower set show the result of 101 seeding. In both cases, the larger pulse is the enhanced output while the lower curve is the unperturbed output. Injected energies were 50 and 80 mJ into the 201 and tol lasers, respectively, at II time delay of SOns.
A. W. McCown and J. A. 8. Godard 2883
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trarily defined to be the point at which 90% of the molecules have absorbed, then
exp( - aE IfwJA) = 0.1. (5)
Using the above parameters for the beam and the measured saturation energy of 0.5 mJ from Fig. 4, the required cross section for the hypothetical absorption process is _ 10- 14
crn2• This value for the cross section is only a lower limit, since the injected beam is attenuated and absorbed (mostly by F2 ) as it passes through the probed laser. However, even the lower limit is unrealistic to describe absorption by molecules which are inside the laser cavity.
The data displayed in Fig. 5 for ArF pumping of the KrF laser indicate that the enhancement seen for o < At < 120 ns is extremely wavelength dependent. Cavity seeding and photoassociation8
•9 satisfy this criterion, and the
latter was initially suspected as the cause of enhancements observed in the XeCI experiments, although it was later ruled out. The enhancements observed for time delays between 150 and 220 ns are comparable for ArF and KrF seeding, and are attributed to photoionization leading to a better preionization of the discharge. 10 This was the explanation given in Ref. 2 for enhancements in the XeCl output after ArF and KrF seeding. The KrF laser is apparently less sensitive to an improved preionization, since the enhancements are smaller than those observed in XeCl. The large cross section for electron attachment by F2 accounts for the rapid decrease in enhancement for time delays greater than 200 ns.
The lack of any enhancement in the amplifier output eliminates the possibility that the enhancement is caused by a dramatic change in the amount of absorbers or excitedstate molecules in the laser cavity. Since an extracting pulse is injected into the cavity, a seed pulse could only improve the output if it affected the chemistry inside the cavity. As there is no enhancement, any chemical effects are small. Therefore, the large enhancements which occur in oscillator-oscillator experiments are caused by optical seeding of the cavity. The conclusion that can be drawn from these results is that 40%-50% more energy is available for extraction from these lasers. This extra energy should also be extracted when the laser is run as an amplifier. An energy enhancement of 15% was observed when the 101 laser was configured as an amplifier, and injected with 60 mJ of 248-nm radiation. The laser delivered 325 inJ as an oscillator and 375 m] (not including the 60-mJ pulse) as an amplifier. AI-
2884 J. Appl. Phys., Vol. 64, No.6, 15 September 1988
though a 40% enhancement was not realized, it is expected that a longer injected pulse would be able to extract over the entire excitation pulse length of the amplifier.
VI. CONCLUSIONS
Energy enhancements of 265 mJ representing a 35% increase in energy output have been observed when a commercial KrF excimer laser was seeded with less than 1 mJ of 248-nm radiation. The intensity of the seed pulse is much larger than the spontaneous emission background~ even at large time delays, and allows the laser to reach threshold faster, thereby preventing the loss of energy in the form of ASE. These results imply that under normal operating conditions nearly one-third of the energy deposited in the gas mixture of a commercial excimer laser is radiated to the walls of the laser cavity. Polarization of the output beam after seeding the laser with a polarized input as wen as amplifier studies show that photochemical processes are not responsible for the observed enhancements, except at large time delays (6.t > 150 ns), where photoionization produces a small enhancement for both 248 and 193 seeding.
ACKNOWLEDGMENTS
The authors are grateful to Michael Case of Acton Research Corporation for providing the mirror reflectivities, to L. Henson, M. Blackwell, and D. Hatch for excellent technical assistance, and to G. Eden, D. Geohegan, F. Feiock, and R. Gibson for several valuable discussions and ideas. This work was supported by the Department of Energy under the Inertial Confinement Fusion program.
'D. B. Geohegan. A. W. McCown. and J. G. Eden, IEEEl. Quantum Electron. QI<:-22, 501 (1986).
1D. B. Geohegan, A. W. McCown, and J. G. Eden, J. Opt. Soc. Am. B 2, 925 (1985).
Jr. Goldhar, W. R. Rapoport, and J. R, Murray, IEEE J. Quantum Electron. QE-16, 235 (1980).
4The transmission of 248-nm radiation through the cavity containing 3.8 Torr of fluorine was measured to be 0.73, and each aperture was assumed to block 5% of the beam.
5!. J. Bigio and M. Slatkine, Opt. Lett. 6, 336 (1981). "D. P. Greene and J. G. Eden. Opt. Lett. 10, 59 (1985), 7W. Mtickenheim, K. HohIa, E. Albers, H.V. Bergmann, and D. Basting, in Excimer Lasers-l 983, edited by C. K. Rhodes, H. Egger, and H. Pummer (American Institute of Physics, New York, 1983), Vol. 100. pp. 80-9S.
"A. W. McCown and J. G. Eden, J. Chern. Phys. 81, 2933 (1984). 9G. Inoue, l. K. Ku. and D. W. Setser, J. Chern. Phys. 76, 733 (1982). lOR. S. Taylor. A. J. Alcock, and K. E. Leopold, Opt. Lett. 5, 216 (1980).
A. W. McCown and J. A. B. Godard 2864
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