Narrowband, single line, 1 μs XeF laserW. D. Kimura, Dean R. Guyer, J. F. Seamans, and D. H. Ford Citation: Applied Physics Letters 51, 1063 (1987); doi: 10.1063/1.98792 View online: http://dx.doi.org/10.1063/1.98792 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/51/14?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A kinetics scheme for the XeF laser Appl. Phys. Lett. 33, 148 (1978); 10.1063/1.90287 1μs laser pulses from XeF Appl. Phys. Lett. 30, 160 (1977); 10.1063/1.89317 Picosecond XeF amplified laser pulses Appl. Phys. Lett. 30, 146 (1977); 10.1063/1.89311 Active mode locking of the XeF laser Appl. Phys. Lett. 29, 424 (1976); 10.1063/1.89105 354nm laser action on XeF Appl. Phys. Lett. 27, 435 (1975); 10.1063/1.88521

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Narrowband, single line, 1 I-ts XeF laser w. D. Kimura, Dean R. Guyer, J. F. Seamans, and D. H. Ford Spectra Technology. Inc., 2755 Northup Way, Belleuue, Washington 98004

(Received 16 June 1987; accepted for pUblication 10 August 1987)

A narrowband (5 GHz) electron beam pumped XeF osciHator/amplifier, lasing at 353.2 nm over a I-fls pulse duration, has been demonstrated. Line selection, bandwidth narrowing, and near-ditfraction-Hmited output are achieved by using an echelle grating and an intracavity solid etalon in a stable oscillator configuration. The amplifier features an off-axis design and yields output energies;::: 0.8 J. The oscillator and amplifier are located in the same gas chamber and are transversely pumped by the same electron beam. A Fabry-Perot interferometer together with a streak camera is used to verify narrowband operation over the entire pulse duration.

Achieving narrowband emission from electron beam (e­

beam) pumped excimer lasers typically has involved using an injection-locking or injection seeding scheme l

-4 in which the narrowband output from a second laser is injected into the e-beam pumped laser. Besides adding more complexity and expense to the laser system, the injection laser and the optics necessary to introduce the injection beam into the e­beam pumped laser can reduce the overall reliability of the system.

A better approach would be to use enly passive elements (i.e., dispersive optics) to achieve narrowband operation. These are less complex, less costly, and are inherently reli­able. Another advantage is that the Hne narrowing will theo­retically work for arbitrarily long pulse lengths. Such a tech­nique has been demonstrated in discharge-excited excimer lasers,5-7 but little work has been done in e-beam pumped lasers. This letter describes an e-beam pumped XcF Im;er system whose narrowband, single Hne, near-diffraction-lim­ited output is achieved using only dispersive elements.

To obtain a good output beam quality, an unstable reso­nator design is usually selected. I

,3.4 An unstable resonator also provides efficient extraction of energy from the active volume. However, it is difficult to find appropriate positions within an unstable resonator for dispersive optics (e.g., grat­ings and etalons). This is because generally, at any location within the unstable resonator, the forward and backward traveling waves will have different divergences. And, al­though it is possible to devise positions for etalons in unsta­ble resonators (e.g., within the Fresnel core), such schemes are not always suitable for the grating that is needed for wavelength selection.

To produce good beam quality with dispersive optics while still generating reasonable output energies, the Jaser system in this letter addresses the problem by employing a stable dispersive resonator to produce narrowband, near-dif­fraction-limited output, and then amplifying the osciHator output using an off-axis amplifier housed within the same <'­

beam pumped gas chamber. The experiments are performed on the Tahoma laser

facility8,'J; the e-beam was upgraded to 375 kV and the mag­netic guide field increased to 1.8 kG, With this upgrade, the e-beam deposition is ;::::250 kW /cm3

, as measured by a pres­sure rise technique, Usually only a 3,8-cm-diam volume near

the foil (0.13 mm aluminized Kapton) of the 14.6-cm-dium gas chamber is used during laser experiments on this device. However, for the experiments di.scussed here, the window system on the gas chamber was modified to include a pair of windows on each side of the gas chamber. This permitted a convenient means for incorporating an oscillator and ampli­fier within the same gas chamber. The gain length is 70 em; the windows are antireflection CAR) coated on both sur­faces, The gas mixture for these experiments is 0.5% Xe with NF3 as the halogen fuel in a neon buffer at 3.5 amagat and 294K.

Figure 1 is a schematic diagram of the oscillator/ ampli­fier system. The osciHator is positioned farthest frem the e­beam (;::::8 em from the foil). It consists of a double-pass, flat-fiat resonator (> 99% R rear mirror, 50% R output coupler), with an echelle grating serving as the middle re­flector. The echeUe grating (79 grooves/mm, 75°58' blaze angle) provides the advantage of high reflectivity while op­erating at a high di.spersion order (70 at 350 nm). The pri­mary purpose of the grating is to select one laser line (353.2 nrn) out of the three XeF normally lases on. The grating by itself also significantly reduces the linewidth of the oscillator output to;:::;: 15 GHz.

Additional line narrowing is achieved by utilizing an intracavity solid 6talon (2 or 5 mm thick). The free-spectral ranges for the etalons are 50 GHz (2 mrn) and 20 GHz (5 mm). The dalons are coated 40% R at 353 nm and have a reflectivity finesse of3.3 and a flatness finesse of ;:::;:25. A low reflectivity was chosen to mtnimizc insertion losses siIlce the etalon must be operated ofr' angle to the laser axis to avoid lasing olfthe etalon faces. The oscillator output decreases by

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FIG, L Schematic diagram of (he narrowband oscillator/amplifier system. The oscillator and amplifier arc pumped by the same electron beam.

1063 Appl. Phys. Lett. 51 (14), 5 October 1987 0003-6951/87 J 401063·03$01.00 (~, i 987 AmericaI"' Institute of Physics 1063 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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approximately a factor of 2 when the etalon is inserted. A pair of intracavity apertures controls the spatial

modes within the oscillator. Aperture sizes of 6 mm diame­ter yield the maximum oscillator output energy while stili maintaining a near-diuraction-limited output. The output of the oscillator is only weakly polarized with the grating and etalon; but with the inclusion of an intracavity air-gap polar­izer, the polarization extinction ratio of the oscillator output is> 100; 1. This polarizer is used during all the measurements discussed in this letter.

The narrowband output of the oscillator is :S 10 mJ. To increase the energy, the oscillator output is amplified by em­ploying a three-pass amplifier positioned near the e-beam foil (see Fig. 1). A convex reflector (3.3 m focal length) expands the beam within the amplifier to increase utilization of the pumped volume. A concave mirror (5.6 m focal length) partially recollimates the beam to ~ 1 cm diameter at the output. It should be emphasized that the focal lengths of the convex and concave mirrors were not optimized for this particular system. More efficient use of the amplifier pumped volume can be obtained with optimized mirrors which expand the beam larger than was done during this experiment. This should increase the total output energy of the amplifier.

The aperture within the amplifier serves the important purpose of preventing self-lasing of the amplifier. The aper­ture size is adjusted to be slightly larger in diameter than the laser beam propagati.ng inside the amplifier. Self-lasing [or amplified spontaneous emission (ASE) j of the amplifier must be controlled to avoid adding a broadband component to the amplified narrowband oscillator beam. A Cassegmin­ian-type amplifier was tested but, with the particular optics available at the time, self-lasing could not be prevented. An off-axis amplifier proved to be the best configuration for this experiment.

Figure 2 shows photodiode traces of the XeF* sidelight fluorescence and the amplifier output. This is for the case of 0.2% NF3 , a 5-mm etalon, and 6-mm apertures within the oscillator. The amplifier output energy is 0,8 J and the pulse length is 0:::: I fls [full width at half-maximum (FWHM)]. Except for a peak at the beginning, the output is relatively constant. Optimum performance is obtained at 0.2% NF"

TIME

FIG. 2. Photodiodc traces of the XeF* sidelight fluorescence (top trace) and the amplifier output (bottom Irae!:). The gas mixture is 99.3% Ne/ 0.5% Xe/O.2% NFl at 3.5 amagat and 2'14 K. Thc time scale is 200 ns/div.

1064 AppL Phys. Lett., Vol. 5 i , No. 14, 5 October 1987

The linewidth of the oscillator/amplifier system is mea­sured with a Fabry-Perot interferometer. Figure 3 shows time-integrated fringe profiles produced by the interferome­ter of the laser system output when using a 5-mm etalon in the oscillator. The distance between the major peaks is 37 GHz; hence the widths of the peaks (FWHM) indicate a laser linewidth of 0:::: 5 GHz. For the 5-mm etalon, the band­width of the echel1e grating (;:::; 15-20 GHz) allows more than one etalon mode to lase, as evidenced by the small fringe peaks also seen in Fig. 30 Additional modes are not observed for the 2-mm etaloll case because its mode separation lies outside the grating bandwidth. The bandwidth of the laser output with the 2-mm eta10n is ::::::: 10 GRz. Therefore, for this particular experimental arrangement, a 3-4 mm 6talon would have allowed only one mode to lase with an estimated resultant linewidth of 7-8 GHz.

Critical to this demonstration is the determination that the oscillator output is narrowband over the entire pulse du­ration and that the amplifier does not increase the linewidth. This is verilled using a streak camera to measure the time dependence of the interferometer fringe widths. The output from the interferometer is apertured so that only a thin slice across the fringe pattern is imaged onto the streak camera. Figure 4 shows the streak photograph of the amplifier out­put given in Fig. 2. (The length of the streaks corresponds to the laser pulse duration shown in Fig. 2.) The photograph has been intentionally overexposed to reveal any weak broadband emission in between the fringes. There is no evi­dence of any significant broadband emission occurring dur­ing the l-J..l s pulse. (The weak fringe streaks seen in Fig. 4 are due to the secondary etalon mode referred to in Fig. 3.) Comparison of streak photographs of the oscillator and am­plifier outputs indicates little change of the linewidth as a result of amplification.

Earlier during this program attempts were made to nar­rowband a low magnification (iii = 1.3) unstable resonator on Tahoma using short pulse (z 15 ns) injection sceding

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J:-'IG. 3. Fabry-Perot intefcrometer output showing a typical time-integrat­ed linewidth measurement of tile amplifier output when using a 5-mm eta­Ion in the oscillator. The free-spectral rauge (FSR) between the major fringe peaks is 37 GHz which yields a bandwidth for the amplifier of :::::5 GHz. The smaller peaks are from a secolld etalon mode which partially lies within the bandwidth of the grating,

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TIME

FIG. 4. Streak camera photograph of the Fabry-Perot interferometer out­put showing a 5-GHz bandwidth for thc amplifier output given in Fig. 2. A thin slice of the interferometer fringe pattern is apertured onto the camera such. thai the time dependence of the fringe widths can be determined by the change in widths of the streaks on the photograph. The photograph has been intentionally overexposed in an attempt to reveal any low level broad­band emission in between the fringes.

from a Nd:Y AG pumped dye laser doubled into the Uv. Injection was made either through a hole in the rear mi.rror or through a partially reflecting (90% R) dot output coupler. Streak camera measurements of the interferometer output showed that the laser output did not remain. narrow­band, but gradually increased in linewidth during the I-ps

1065 Appl. Pl1ys. Lett., Vol. 51, No. 14, 5 October 1987

laser pulse. Attempts were also made to passively narrow­band the unstable resonator by using the grating and etalon to reflect light from a hole in the rear mirror back into the resonator. The streak camera measurements indicated that the output was not narrowband,

The authors wish to thank Dr. S. E. Moody for his he!p­ful discussions and to acknowledge the assistance of F. P. Braun during this experiment. This work was supported by the Air Force Weapons Laborator'j, contract F29601-85C-0062, as administered by TRW, Inc., through subcontract E63659DZ5S.

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