SPIE Proceedings [SPIE SPIE Proceedings - (Sunday 12 February 2012)] - Visible and UV gas lasers...

Visible and UV gas lasers with high-current radiative discharges excitation

A.S.Kamrukov, N.P.Kozlov, Yu.S.Protasov

Moscow State Technical University, 107005, Moscow, USSR

ABSTRACT

It is a brief review of studies of lasing and energy-spectral characteristics of gas lasers utilizing - the

excimer-like mercury halide vapor HgX2 (X =Cl, Br, I), inert gas halogenides - XeF (B-X), (C-A) andmolecules of the stable halogens (12, Br2), optically pumped by wide-band UV-VUV radiation of high-currentradiative discharges.

1. INTRODUCTION

Advances of radiative-plasmodynamic processes of short-wavelength radiation laser-matter interactionresearch, aimed at solving some problems of applied physics and high technology - stimulate R&D of gasphase visible and ultraviolet lasers featuring high lasing energy and average power ratings. A wide range ofattractive laser-active mediums in such lasers includes dihalogenides of the metal MeF2 (M =Hg, Cd, Zn,...,iT =Cl, Br, I), homonuclear molecules of halogens (e.g. 12, Br2, Cl2), exciplex compaunds of oxides andhalogenides of noble gases (XeO, XeF, KrF,...); they make it possible to produce high power lasing over theentire spectral band (Fig.1).

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Fig.1 The main schemes of gas lasers with optical pumping

In contrast to electronic (electron-beam, electrical-discharge) excitation, the method of wide-band opticalpumping of gas lasers is rather perspective - physically and technologically'. Inspite of the methods of

SPIE Vol. 1397 Eighth International Symposium on Gas Flow and Chemical Lasers (1990) / 137

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formation of population inversion differences for these gas laser-active mediums, wide-band noncoherentpumping source requirements which govern efficient lasing are very similar - and are aimed mainly atprovidmg a powerful UV-VUV (in the spectral range hv -4-2O eV) radiation continuum with brightnesstemperatures 25-40 kK.

When high-power short-wave pumping sources are designed a number of principal problems shall betackled: a)input of energy into the large volumes of sufficiently dense (ne' 1O181O2O cm3) plasma havingtemperatures of several (3-10) eV and plasma heating; b)escape of plasma short-wave radiation; c)controllingemission spectrum variation.

2. HIGH-POWER UV-VUV PLASMA SOURCES FOR OPTICALLY PUMPED GAS LASERS

2.1. Plasma radiation sources with ohmic heating

Among the short wavelength (open-type) radiative discharges with ohmic heating - high current surfacedischarges - surface discharges with different methods of ignitions (sliding, barrier, laser spark) - arepowerful pulse-periodic sources of high brightness UV-VUV radiation for gas lasers optical excitation. Weproposed and studied a new type of high current surface discharge, namely, surface discharge with linearstabilized streamer (spark) channel2, having (in typical scheme) an additional system of distributed electrode-capacitor elements in the gap for initiation of extended spark discharges over the dielectric surface. This typeof surface discharges does not require the use of "hard' electrical circuits (i.e. circuits having a high valueof U0/L0) and makes it possible to form long discharge channels (.�0.5 m) with relatively low operating(5-25 kV) and ignition (—25 kV) voltage; simultaneously, the conditions are satisfied for the spatialstabilization of streamer channel and this makes it possible to obtain radiating (with TVBUrV4O kK) plasmasof an any given geometry and, in particular, strictly linear plasma columns.

2.2. Plasmodynamic sources of high-power UV-VUV radiation

High-current plasmodynamic discharges of various type3 differ from discharges characterized by ohmicheat evolution because plasma is heated by shock-wave thermalization of the kinetic energy of a high-velocityplasma flow decelerated in a dense gas, plasma, solids or magnetic field (as at a target).

So, (in particular, one can eliminate the main constraint in ohmic heating):- the plasma resistance is not depend explicitly on the temperature of the heated (radiating)

plasma;- the plasma heated generally outside the circuit part, so the heating rate can differ substantially

from the energy input rate to the discharge which is used in generation of high-power lightpulses with short pulse lengths and steep leading edges.

We suggest and examine the physical and technical aspects of using plasmodynamic methods to generatehigh-power UV-VUV radiation in vacuum4 and gases5 implemented by means of an erosion-typemagnetoplasma compressor6 (MPC) as a generator of hypersonic dense plasma flows. A plasma flowtraveling at velocities v1-4O-6O km/s is formed in MPCs by electromagnetic forces generated as a result ofinteraction of discharge currents with intrinsic magnetic fields. Chemical composition of the electric-dischargeplasma is governed by the products of erosion of the electrodes and insulators separating them in MPCs.From the technical point of view, MPCs as a directed kinetic energy generators with high specific energycontent (v2/2�.MJ/g) of the generated plasma hypersonic flow and high kinetic efficiency (i--O.6-O.8) arefairly simple and reliable devices which can operate in a wide range of compositions and initial pressures ofthe gases (p 1O21O2 atm) and are capable of high output energies (1O-1O J); they can be also used in thepulse-periodic regime at high repetition frequencies (f.1-1OO Hz) governed primarily by the power andoperational characteristics of the power sources and current switches (low induction capacitor banks, explosivemagnetocumulative7 and magnetohydrodynamic8 generators).

Experimental investigation and numerical analysis of high-current plasmodynamic MPC-discharges ("plasmafocus-type9 in vacuum and gases, R-constristed'°, discharges with axis constriction", localized'2 andcumulative'3), their spectral-brightness and energy characteristics showed the feasibility of constructing high-brightness (Tbr'>4O kK) sources of wide-band VUV radiation (open or lamp type) for optical pumping.The main radiating MPC configurations and their spectral-energy characteristics are show in Fig.2.

138 / SPIE Vol. 1397 Eighth International Symposium on Gas Flow and ChemicalLasers (1990)


Fig.2 Energy-spectral characteristics and spectral efficiency ofthe main plasmodynamic radiative configurations.

So, the powerful VUV light sources open type design on the basis of MPC discharges:in vacuum - have the brightness temperatures Tbr>S eV in the spectral range hv�.25 eV and radiation

power PrVaUdV�.O•1 GW/cm2. Emitting surface area cm2) can be increased for requiredby using multichannel MPC discharges geometry;

in gases - required spectral and brightness characteristics (TBUrV�.40 kK) in the range X 100 nm, spatialand geometric parameters are achieved by the choosing gasdynamic structure when take intoaccount the effect of the turbulent modification'4 the thermodynamic and optical properties ofthe transition region between the plasma flow and the unperturbed gas facilitating the escapeof UV and VUV photons from the the hot discharge zone into the unperturbated gas, (ofnesessary chemical composition and pressure level).

The plasmodynamic high power UV-UVV light sources of flashlamp-type on the basis of cumulativeMPC-discharges (when broad-band UV-VUV radiation is generated by means of cumulation of hypersoniccounterdirected plasma flows formed by opposite MPCs in vacuum (or dense gas) in an optical tube) havethe high brightness temperature of 30 kK in the UV and VUV spectral regions and a luminous bodyinsulated by transparent walls15 (providing under conditions of formation of a gas layer optically dense forthe hard VUV radiation).

In the case of localized MPC discharges a new scheme of a flashlamp-type emitter was suggested inwhich heating of an electric discharge plasma is implemented as a result of shock-wave thermalization of thedirectional kinetic energy of a high speed plasma flow upon its deceleration in the dense gaseous mediums(before optical target) which simultaneously plays the part of an gas filter for the hard component of theemission spectrum of the emitting plasma flow. Radiation flux densities in the short-wave UV region (180-250 nm), which correspond to the black body radiation with the temperature of the level of 40 kK havebeen achieved for the first time for the quartz light sources'6.


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3 GAS LASER MODULES WITH HIGH CURRENT SOURCES OF UV-VUV RADIATION

The above-mentioned features of high current light sources for optical pumping have been utilized indesign of gas laser modules; structurally, the laser module is a cylindrical thermostatic (50-350 CC) chambermeasuring 300 to 1500 mm in length and 120 to 500 mm in diameter, having set of optical pumping sourcesinstalled in its diameter plane (or along its axis), quartz windows and resonators, connected to a systems ofdifferential pumping and mixing.

3.1. Excimer-like opticall pumped mercury dihalide lasers

Generation of stimulated radiation in mercury halide vapors due to an allowed ionic-covalent B-Xtransition in molecules (radicals) of mercury monohalides HgX formed from the original molecules of HgX2dihalides as a result of dissociation (here, X =Cl, Br, I). Minima of the potential curves of the B2 1/2 andX2 +

1/2 states of the HgX molecules are shifted by 0.04-0.06 nm, so that in accordance with the Franck-Condon principle the transition occur preferentially from lower vibrational (v= 0-3) levels of the B states toupper levels (v' 15-25) of the X state. The lower active levels are emptied by vibrational relaxation occuringat a high rate, which is responsible for the quasi-cw nature of lasing (t > > 20 nsec) subsystems.

Main features of mercury halogenides, by which they compare favorably with other blue-green laser-activemediums are based on large cross-sections of indiced transitions (i 1016 cm2) and regeneration of activemolecules which makes it posiible to develope high gains (a > 102 cm') and allows closed-circuit highfrequency pulse-periodic operation.3. 1. 1. Blue-green molecular HgBr/HgBr2 laser

Lasing was observed when HgX vapor was pumped with radiation from an open linearly stabilizedsurface discharge acting directly in the active mixtures. Experiments have been carried on installation17 withdischarge parameters: W0 =(O.95-2.85)1O J, U0 =25 kV, t/2 ' J15. The gases (SF6, CF4, N2) mixed with inertgases have been as buffer gases with PE'054 atm.

As regards output energy (—3 J), the optimum composition is a mixture (Ar:N2)- 1:1.5, p —2.5 atm andvapours at a concentration of (0.8 to 2)4O' cm3. Under these conditions detectable are 6 or 7 lines groupednear the wavelength X'5O2 nm and 504 nm. The maximum unsaturated gain and internal loss index havebeen —0.04 cm' and --O.OO2 cm' respectively. The total efficiency HgBr laser is 1L°3%, which is apresent-day maximum value for optically pumped visible gas lasers. According to measurements the totallaser efficiency can be 1.2 to 2%, which corresponds to instantaneous values of lasing efficiency, determinedin the experiments.

The specific laser energy output 15 J/L, long pulse lasing 4.1o6 5 are the best present-daycharacteristics for optical pumped visible gas lasers of this type.

3.1.2. Green-emitting mercury chloride laser

Interest to the laser is based on possibilities of attaining high energy/power characteristics and a highefficiency and frequency of lasing in the green region of the spectrum on a wide lasing wavelength tuningfrom 533 to 565 nm. Under mensioned conditions the lasing of HgCl laser was in the form of two lines? =558,559 nm which are in the green region of the spectrum and corresponed to allowed bound-boundtransitions between the lower (v= 0 and 1) levels of the electronically excited state B2 1/2 and the uppervibrational (v' 22 and 23) level of the ground state X2 1/2 of the HgCl2 molecules; lasing line width(-%- 1 nm) is determined by unresolved vibrational structure.

The laser energy output is optimal at an active medium absorption factor of 1.2 m1 (i.g. a repetitionfrequency f-O.1 Hz, W0= 8.5.102 J).

The output energy and the specific values of this energy (—2.1 J and —7.6 J/L) were much higher thanthe values obtaind for electric discharge HgCl lasers or for lasers optically pumped with narrow-bandradiation'8, the laser efficiency is respect to the stored energy was as high as -O.1%.3. 1.3. Blue-violet HgI/Hg12 laser

Under our conditions '—1 torr + N2:Ar =1.5:1, p 2.5 atm) the lasing was observed when theconcentration of Hg12 active molecules exceeded 1.2.1016 cm3. The maximum laser radiation energy recordedat this way was E0 O.5 J (W =2.851O J) which was much higher than the values obtained in electric

140 / SPIE Vol. 1397 Eighth International Symposium on Gas Flow and Chemical Lasers (1990)


discharge HgI laser'9. The optimum value of the concentration of the active molecules was in the range(78)1O16 cm3 and corresponded to a depth of penetration of the pump photons (X-225 nm) in the activemedium amounting to l-O.3 cm - i.e., HgI laser operated in the photodissociation wave regime (v'— 10 km/s,L3'5 j.ts).

The spectrum of the stimulated radiation obtained from mercury iodide photodissociation lasercorresponded to the electronic B-X transition in the HgI molecule with the vibrational numbers v-v'= 1-17(X =444 nm), 0-15 (X =443 nm), and 1-16 (X =442 nm); the bulk of the radiation was concentrated in thegroup of lines at X =443 and 444 nm.

3.1.4. Multicolour visible radiation emitted by mercury halides vapor laserThe HgX2 molecules have several absorption bands in the ultraviolet range and this correspond to their

dissociation channel responsible for deformation of radicals in various electronic states. These electronicallyexcited radicals form in the B2 1/2 state only when the HgX2 molecules absorbed radiation in the so-coldb-bands with maxima located at the wavelengths of X-183,198 and 225 nm for HgC12, HgBr2, Hg12respectively. The half-widths of this bands are 51O cm'. Irradiation with longer wavelength ultravioletradiation creates HgX molecules in the ground X2 1/2 state. The wavelength dependenses of the absorptioncross-sections of molecules of the various mercury halides demonstrate that the active laser mediumcomposed of the three mercury dihalides mensioned early can have fairly wide absorption region ofx 175-240 nm when the concentration of molecules are suitably selected. This facilitates a more efficientutilization of the emission spectrum of a wide-band optical pumped source.

An active mixture consisted of mercury dihalide vapors at pressure of PrHX 140 torr and it containedbuffer gases (N2:Ar =1.5:1), p =2.5 Amagat.

Simultaneous lasing was observed for all the binary mixtures (HgC12-HgBr2, HgC12-Hg12, and HgBr2-Hg12)as well as for a ternary HgC12-HgBr2-Hg12 mixture. The space and time structures of the laser radiation fieldwere copmletely identical for different mercury halide mixtures.

Analysis of emission spectrum of three-color laser utilizing a mixture with equal energy yields of theradiation in the individual spectral components shows that in case of the HgC1 molecules emitted two linesat =558 and 559 nm in the green part of the spectrum, corresponding to transitions with the vibrationalnumbers vv" in the range 1-23 and 0-22, whereas the HgBr molecules emitted four lines in the blue-violetpart of the spectrum with maxima at =444.0 and 443.2 nm (1-17 and 0-15 transitions). The laser emissionlines coincided with the strongest lines in the spontaneous emission spectra of the HgX molecules.

When the partial composition of a mixture of mercury dihalides was altered, there was a change in theenergy distribution of the output radiation between the various spectral components, i.e., the "chromaticity" ofthe emission spectrum changed.

The energy measurements indicated that the total output energy E obtained from lasers, utilizing HgXmixtures was less than the maximum energy E1 of the lasers, utilizing any one of the halides -E-n2E,where n is a number of halides in the mixture.

The reduction of the output energy in the case of mercury dihalide mixtures was clearly due toenhancementof the role of the quenching of the B state of the monohalides (kq > 1010cm3/sec) when thetotal concentration of the HgX2 molecules in the active mixture was increased.

The total radiation energy of the lasers with binary mixtures of mercury dihalides was 0.5 0.1 J(depending on the mixture) and the maximum output energy obtained for a three-color HgC1-HgBr-HgI laserwas ---O.31 J, so that the efficiency of transformation of the lasing time was -O.1% and —O.O7%,respectively.

So, a combination of vapors of different halides could be used as the active medium and this provides anew opportunity for constructing a high-power photodissociation laser emitting 'white' light20.3.1.5. Low-temperature lasing of mercury monohalides

Mercury dihalides HgX2 have a low vapor pressure at room temperature. The medium must be heatedto --200 °C for efficient operation of Hg lasers and this creates some technological and operationaldifficulties that limit appreciably the scope for whitespread application of this lasers.

In order to reduce the operating temperature of HgX lasers (< 100°C) one can replase dihalides withmore volatile organically substituted mercury halides RHgX, where R is an organic radical. Trifluoromethilmercury bromide, chloride, iodine - CF3HgBr, CF3HgC1, CF3HgI - are potentially useful for this purpose.

SPIE Vol. 1397 Eighth International Symposium on Gas Flow and ChemicalLasers (1990) / 141


In the laser cell a vapor of trifluoromethil mercury halides was diluted with a buffer gas (N2:Ar 1.5:1)at a total pressure atm.

High-power stimulated emission from media utilizing CF3HgBr was observed at temperatures of the lasercell and the vaporizer above 88 °C. In this case, the coefficient of absorption of the active medium atX=257 nm varied in the range --O.95-1.75 m'. Radiation was only amplified in one (X=502 nm) of the twoknown lasing bands of the HgBr radical, for which up to four vibrational-rotational components weredistinguished in the spectral substructure.

Lasing of the HgCl molecule, formed from CF3HgC1, was obtained at a cell temperature T--95°C. Thecoefficient of absorption in the medium at X =247 nm was '1 m' and the laser pulse energy was --6 mJ.Two groups of lines, near X =558 and 552 nm, where recorded in the stimulated emission spectrum21. Forcomparison, we note that, in the case of an HgCl/HgCl2 photodissociation laser, lasing is generally observednear X=558 and 559 nm.

3.2. Visible and UY XeF laser with wide-band optical pumping

XeF lasing is one of the best illustration of efficiency application wide-band optical pumping concepts forgas laser technique. VUV optically pumped photodissociation XeF (B-X) lasers ensure smooth lasing wavetuning from 345 to 353 nm by changing buffer gas pressure and demonstrate the maximum output energy inthe blue-green spectrum range at the (C-A) transition.

An investigation have been performed of a XeF laser with an active length 1.7 m and an exit apertureof O.12x0.2 m. The built-in resonator L'2.8 m, T1=T2=30% (X=350 nm, B-X; T1=T2= 11%, X480 nm,C-A) was employed. The optical pumping source was a single channel sectiohalized surface low voltagedischarge with the input power density averaged over the half period dP/dx 41.6 MW/cm, t/2 =6 is, andbrightness temperature in VUV T 30 kK.

The maximum output energy due to the B-X transition was E =70 J and it was obtained from a mixtureof the Ar:N2 =3:1 composition containing an XeF2 vapor at p =2 torr and the total pressure of the mixturePz 1.4 atm. The specific output energy averaged over the lasing region (zone) was 4.2 J/l. The lasingenergy at the (C-A) transitions it was as high as 98 J with the average output energy from the activevolume releasing 90% of the total energy was 5.5 J/l, the technical efficiency reached O.17%2223.

3.3. Optically pumped UV molecular (I2j2) dimer lasers

Homonuclear halogen molecules are promissing candidates for high-power lasing in UV-VUV(X =157-344 nm).

3.3.1. 12 laser

To optically excite molecular iodine (A =200 10 nm) it's possible to use flashlamp-type light sources inthe microsecond range. A cumulative type plasmodynamic source24 was used to investigate the lasingcharacteristics. The mechanism of population inversion in the case of 12 laser is well known. The necessary12 vapor pressure was ensured in the thermostabilized laser cell at 70-80 °C. Under this conditions lasingoccures at two wavelength X 342.0 and 342.4 nm corresponding to transition with the vibrational numbers1-14 and 2-15. The maximum energy generation (EL'-J) has been obtained using a buffer gas(perfluoromethan) at a pressure of 1.5 atm and of '2 molecules p1 3 torr with lasing duration beingT 16 J.L5 corresponding to the pumping pulse.3.3.2. Br2 laser

The first achievement of lasing in optically pumped molecular bromide has been given recently25.A possible mechanism of photoexcitation and lasing of bromine vapor in the presence of a buffer gas

consists in the following. Absorption of pump radiation in the VUV spectral range (AXpump 150-170 nm)leads to the Br2 molecule being excited from the X' ground covalent state to the D' Coulomb state.The excited bromine molecules relax to the lower D3[f2g ionic state when they collide with the buffer gasparticles and emit in the 292 nm region due to the allowed D3H2g >A'3112 bound-bound electronictransition (the laser transition). Rapid depopulation of the lower active level (a weakly bound state) is dueto electron-vibrational relaxation and, possibly, to dissociation during collisions with atoms of the buffer gas.

In our experiments the Br2 vapor was optically pumped by thermal VUV radiation from multichannel

142 / SPIE Vol. 1397 Eighth International Symposium on Gas Flow and Chemical Lasers (1990)


( 10) plasmodynamic discharges of magnetoplasma compressors formed directly in the laser active medium.The maximum brightness temperature Tbr of a discharge in argon, at a pressure of PAr' 2-4 atm, reached

35-38 kK in the ultraviolet spectral range for light pulse duration of /2 =6-8 is, ('61O J), the expansionvelosity of the plasma in this range of argon pressure was 3.6-2.8 km/s.

The active medium consisted of Br2 vapor (p 3-20 torr) strongly diluted with an argon buffer gas(PA;' 1-4 atm), and was preparedby mixing the gases directly in the laser cell, which was passivated beforethe gases were admitted.

As a rule, the stimulated emission spectrum consisted of a large number (-'- 20) of partially overlappinglines, grouped near three basic wavelength: 291.5, 292.9, and 292.5 nm. Lasing occured due to transitionswhich had the maximum intensity in the spontaneous emission spectra of the bromine molecule. The relativeintensities of the lasing spectral components varied as a function of the composition of the medium and theresonator parameters.

The laser output energy increased monotonically with the pressure of the argon buffer gas up to itsmaximum value, for the present apparatus, of p'-4 atm. The range of optimal partial pressures of the activemolecules was POBIrt34O torr. Outside this range reductions were observed in both the laser output powerand the duration of lasing. The maximum laser output energy of EL- 1.1 recorded in the experiments, forall overall laser pulse duration of ILS , was obtained using an active mixture Br2:Ar =2.410'7:1.F1020cm3and a resonator with R1R2 --'O.93.

Despite the fact that there is quite a substantial reserve evidently for raising the output parameters ofthe investigated lasers, the results obtained serve to provide evidence that the energy possibilities of allmensioned laser active media are similar. From the technical point of view, the use of molecular bromine asthe working component of the active mixture of an optically pumped ultraviolet laser is preferable, since Br2is a more stable (it is replenished) and is also a less chemically aggressive and toxic substance than xenondifluoride and has quite a high saturated vapor pressure at room temperatures (psBart =200 torr).

Finally, we have to add that high potential possibilities of the radiative plasmodynamic discharges as apowerful optical pumping sources with universal energy-spectral characteristics have been stressed in a widespectral-energy range:- indeep VUV band - by design, theoretical and experimental research of the first photoionization-

recombination atomic xenon laser excited by ionising VUV radiation of themultichannel plasmodynamic MPC-discharge (open-type VUV source, T�.40 kKin the spectral range .1OO nm), acting in the active medium of the laser26;

- in UV band - by design and sucsessful experiments on wide-band optical excitation of new laserwith bleaching wave (utilizing an enter solutiOn of coumarine-6 with 1,4-diphenylbutadiene) excited by UV radiation of flashlamp type source based oncumulative MPC-discharge27.

. 4. CONCLUSION

Our lasing experiments in a wide spectral range, using new class of high-power radiative dischargesproved high potential, universal possibilities and efficiency of the optical pumping concepts for developmentgas laser techniques.

5. REFERENCES

1. A.S.Kamrukov, N.P.Kozlov, Yu.S.Protasov,in Plasma Accelerators and Ion Injectors [in Russian], Nauka,Moscow, pp.3-81,1984.

2. S.N.Bugrimov, A.S.Kamrukov, N.P.Kozlov et al., "High brightness pulse-periodic ultraviolet radiationsource utilizing a linearly stabilized surface discharge", Sov.J.Quantum Electron., Vol.16, No.1, p.44, 1986.

3. A.S.Kamrukov, N.P.Kozlov, Yu.S.Protasov, in The Radiative Plasmodynamics, Vol.1, [in Russian],Energoatomizdat, Moscow, pp.9-145, 1990.

4. A.S.Kamrukov, N.P.Kozlov, Yu.S.Protasov, "Dynamics and radiation of the open (vacuum) plasmo-dynamic discharges "plasma focus" type", Teplofiz.Vys.Temp., Vol.20, No.2, p.359, 1982 (SovJ.High Temp.).

5. A.S.Kamrukov, N.P.Kozlov, Yu.S.Protasov, S.G.Shushkovskii, "Bright thermal VUV sources based onplasmodynamic discharges in gases", Ibid., Vol.27, No.1, p.152, 1989.



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SPIE Proceedings [SPIE SPIE Proceedings - (Sunday 12 February 2012)] - Visible and UV gas lasers...

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