Mar 13, 2016 - (Submitted February 13,1984). Kvantovaya Elektron. (Moscow) 11, 1750-1756 (September 1984). An experimental investigation was made of ...
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Efficiency of an optically pumped XeF laser
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1984 Sov. J. Quantum Electron. 14 1174 (http://iopscience.iop.org/0049-1748/14/9/A05) View the table of contents for this issue, or go to the journal homepage for more
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A comparative analysis was made of the influence of the composition of working mixtures in an iodine photodissociation laser on the SES threshold and mixtures ensuring a high directionality of laser radiation were identified. An experimental method was proposed for the determination of the SES parameters from measurements of the optical inhomogeneities appearing in a medium as a result of SES. 'V. S. Zuev, V. N. Netemin, and O. Yu. Nosach, Kvantovaya Elektron. (Moscow) 6, 875 (1979) [Sov. J. Quantum Electron. 9, 522 (1979)]. N. G. Basov, V. S. Zuev, O. Yu. Nosach, and E. P. Orlov, Kvantovaya Elektron. (Moscow) 7, 2614 (1980) [Sov. J. Quantum Electron. 10, 1527 (1980)]. 3 V. S. Zuev and E. P. Orlov, Preprint No. 145 [in Russian], Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow (1981); Kvantovaya Elektron. (Moscow) 8, 1968 (1981) [Sov. J. Quantum Electron. 11, 1191(1981)]. 4 N. G. Basov, V. S. Zuev, K. S. Korol'kov, O. Yu. Nosach, and E. P. Orlov, Izv. Akad. Nauk SSSR Ser. Fiz. 46, 1534 (1982). 5 V. S. Zuev and E. P. Orlov, Preprint No. 158 [in Russian], Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow (1981); Kvantovaya Elektron. (Moscow) 8, 1978 (1981) [Sov. J. Quantum Electron. 11, 1197(1981)]. 6 V. S. Zuev, O. Yu. Nosach, and E. P. Orlov, Kvantovaya Elektron. (Moscow) 8, 2699 (1981) [Sov. J. Quantum Electron. 11, 1644 (1981)]. 7 V. V. Likhansku and A. P. Napartovich, Kvantovaya Elektron. (Moscow) 8, 637 (1981) [Sov. J. Quantum Electron. 11, 384 (1981)]. "I. M. Bel'dyugin, L. A. Vasil'ev, M. G. Galushkin, A. M. Seregin, and N. V. Cheburkin, Kvantovaya Elektron. (Moscow) 10, 843 (1983) [Sov. J. Quantum Electron. 13, 523 (1983)]. 9 M. V. Bunkina, N. A. Kirichenko, and B. S. Luk'yanchuk, Kvantovaya 2
Elektron. (Moscow) 10, 1623 (1983) [Sov. J. Quantum Electron. 13,1068 (1983)]. B. L. Borovich, V. S. Zuev, V. A. Katulin, V. Yu. Nosach, O. Yu. Nosach, A. V. Startsev, and Yu. Yu. Stoflov, Kvantovaya Elektron. (Moscow) 2, 1282 (1975) [Sov. J. Quantum Electron. 5, 695 (1975)]. " B . L. Borovich, V. S. Zuev, V. A. Katulin, L. D. Mikheev, F. A. Nikolaev, O. Yu. Nosach, and V. B. Rozanov, High-Current Light-Emitting Discharges and Optically Pumped Gas Lasers [in Russian], VINITI, Moscow (1978), pp. 218, 219. 12 M. E. Riley, IEEE J. Quantum Electron. QE-19, 1209 (1983). 13 V. S. Starunov and I. L. Fabelinskii, Usp. Fiz. Nauk 98,441 (1969) [Sov. Phys. Usp. 12, 463 (1970)]. 14 S. A. Akhmanov, Yu. E. D'yakov, and A. S. Chirkin, Introduction to Statistical Radiophysics and Optics [in Russian], Nauka, Moscow (1981), p. 589. 15 B. V. Alekhin, V. V. Borovkov, B. V. Lazhintsev, V. A. Nor-Arevyan, L. V. Sukhanov, and V. A. Ustinenko, Kvantovaya Elektron. (Moscow) 6, 1948 (1979) [Sov. J. Quantum Electron. 9, 1148 (1979)]. 16 V. S. Zuev, K. S. Korol'kov, O. Yu. Nosach, and E. P. Orlov, Kvantovaya Elektron. (Moscow) 7, 2604 (1980) [Sov. J. Quantum Electron. 10, 1521 (1980). Π Α. Ν. Tikhonov and A. A. Samarskii, Equations of Mathematical Physics, Pergamon Press, Oxford (1964). 18 M. Abramowitz and I. S. Stegun (eds.), Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables, Dover, New York (1964). " Ν . Μ. Kroll, J. Appl. Phys. 36, 34 (1965). 20 I . S. Gradshteyn and I. M. Ryzhik (eds.), Table of Integrals, Series, and Products, Academic Press, New York (1965). 21 I . K. Kikoin (ed.), Tables of Physical Quantities [in Russian], Atomizdat, Moscow (1976), pp. 57, 76, 125, 141, 634. 22 A. I. Poltev, Construction and Design of High-Voltage Sulfur Hexafluoride Apparatus [in Russian], Energiya, Leningrad (1979), p. 8. 10
Translated by A. Tybulewicz
Efficiency of an optically pumped XeF laser V. S. Zuev, L. D. Mikheev, and D. B. Stavrovskii P. N. Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow
(Submitted February 13,1984) Kvantovaya Elektron. (Moscow) 11, 1750-1756 (September 1984) An experimental investigation was made of the characteristics of a photodissociation laser operating on the B-X (353 nm) and C-A (485 nm) transitions of the excimer XeF. The laser was pumped with vacuum ultraviolet radiation from an open high-current discharge. The energy in a laser pulse and the average specific output energy reached values of respectively 28 J and 18 J/ liter at 353 nm and 14.5 Jand 10J/literat485nm. Measurements were made of the instantaneous lasing efficiency, defined as the ratio of the laser output power to the electrical input power from the pump source. At the maximum of a laser pulse the efficiency reached 0.8% at 353 nm and 1 % at 485 nm (corrected for the geometrical utilization factor of the pump source radiation). It was established that the pump source efficiency was 7.5-8.5% and that the quantum efficiency of the formation of XeF(2?) averaged over the pump band was 85 + 5%. Ways of raising the laser efficiency to ~ 2 % were considered. 1. INTRODUCTION In Refs. 1 and 2 lasing was reported on the B-X (λ = 350 nm) and C-A (Azz4&0 nm) transitions of the excimer XeF, produced by the photolysis of XeF 2 with vacuum ultraviolet radiation from a high-current electric discharge. Construction of similar lasers, pumped by λ ~ 170 nm radiation from Xe 2 excited by an electron beam, was reported in 1174
Sov. J. Quantum Electron. 14 (9), Sept. 1984
Refs. 3 and 4. The results of a more detailed investigation of optically pumped XeF lasers are given in Refs. 5-10. The question of the efficiency of an optically pumped XeF laser was discussed in Refs. 7 and 11. It was shown11 that the efficiency of conversion of the pump energy, absorbed in the active medium, into laser emission energy obtained from the C-A transition is 10-30%, depending on the quantum efficiency of the formation of XeF(5) by photolysis
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of XeF 2 . Values for the quantum efficiency of the formation of XeF(5) in the 158 nm region (corresponding to the maximum of the strongest XeF 2 absorption band) obtained by different research teams differ by roughly a factor of three and are 0.3 ± 0.09 (Ref. 12) and 0.9 + 0.1 or - 0.2 (Ref. 13). The efficiency of a laser operating on the C-A transition achieved experimentally in the work of Ref. 7 was 0.17%, in terms of the energy carried by the electron beam. In the same study it was found that the efficiency can, in principle, be raised to 1-2%. In the present paper we shall report the results of experiments directed at clarifying the values, attainable in principle, of the internal and technical efficiencies for a photodissociation XeF laser pumped with vacuum ultraviolet radiation from an electric discharge. 2. FEATURES OF EXPERIMENTAL PROCEDURE AND CONDITIONS The investigations were performed using the apparatus described in Ref. 5. Only the construction of the laser cell was changed. The cylindrical cell used in the present experiments was made of Teflon and had an internal diameter of 70 mm. Nickel-plated metallic subassemblies were placed at the ends of the Teflon tube: they carried windows, optical resonator mirrors, and vacuum valves used to evacuate the cell and to fill it with the required gas mixture. The electrodes to which a high-voltage pulse was applied were positioned in the same region. Two 75-cm long cylindrical discharges served as the source of vacuum ultraviolet radiation. These were excited symmetrically relative to the axis of the cell at a distance of 54 mm from each other. A 0.05-mm diameter tungsten wire was employed to trigger them. The energy coupled into the discharges was stored in a 30-//F capacitor bank, charged to a voltage of 50 kV. About 85% of this energy was coupled into the discharge in ΙΟμβεα The optical resonator mirrors were mounted within the cell at a distance of 130 cm from each other. The laser exit aperture had the form of a 54-mm diameter circle with its center on the axis of the cell. The laser cell was evacuated to a pressure below 1 mTorr and filled with an XeF 2 -N 2 -Ar gas mixture. The mixture was prepared in a special container, subjected to prolonged passivation with fluorine, and was fed into the discharge chamber 5-10 min before the initiation of a discharge. The reduction in the XeF 2 concentration in the discharge chamber at the moment when the discharge was initiated, due to chemical reactions taking place at the walls of the chamber, did not exceed 20%. The values of the XeF 2 concentration given below were obtained at the moment of discharge initiation. The XeF 2 concentration was maintained at a level of ~ 3 Χ 10 17 c m " 3 in a series of experiments by heating the discharge chamber and the container in which the mixture was prepared to 40 °C, since the saturated vapor pressure of XeF 2 at room temperature (20-25 °C) was only 3-4.5 Torr (Ref. 14). Photolysis of the XeF 2 took place in the bleaching-wave regime. The photons causing dissociation of the XeF 2 were absorbed in a cylindrical layer whose thickness was between 1175
Sov. J. Quantum Electron. 14 (9), Sept. 1984
a few millimeters and roughly 1 cm, depending on the XeF 2 17 concentration which was varied in the range (l-3)Xl0 3 c m " . The velocity of this layer was a few kilometers per second and exceeded that of the discharge boundary (~ 1 km/sec). The mechanism of the bleaching wave and the conditions for its appearance in the case of the photolysis of XeF 2 were discussed in Ref. 11. 3. FACTORS DETERMINING THE EFFICIENCY OF THE INVESTIGATED LASER Before discussing the efficiency we must mention an important feature of the laser, namely that the population inversion for the B-X and C-A transitions is proportional to the pump power, since the characteristic times of all the processes leading to the formation and decay of the population inversion are much shorter than the duration of a pump pulse and no buildup of inversion occurs. In turn, the pump power is proportional to the electrical power coupled into the discharge (this will be discussed in more detail below). The operation of an XeF laser pumped with vacuum ultraviolet radiation from a high-current discharge can therefore be characterized by an instantaneous lasing efficiency a: nm). The FEU-69 photomultiplier recorded photons from a volume element of the discharge chamber bounded by the surface of a truncated cone with a small angle at its apex. The axis of the cone was parallel to the discharge axis and separated from it by a distance of 16 mm. The spatial and temporal resolutions of the measuring channel were respectively 2 mm and 0.2 μβεα The time dependence of the luminescence power was displayed on the screen of an S8-2 oscilloscope. Complete dissociation of the XeF 2 in the investigated volume occurred during the observation time. Absolute calibration of the measuring channel was performed using an EV-45 standard light source having a brightness temperature of 39 kK (Ref. 15). The oscillograms obtained made it possible to follow the evolution of the unsaturated gain g 0 for the C-A transition, since the luminescence power and g0 are related by the expression (6)
go = aCAwxCA, 18
2
where aCA = 9X 10~ c m " (Ref. 6) is the gain cross section; TCA = 1 0 0 + 1 0 nsec (Ref. 13) is the time for a spontaneous radiative transition from the C state to the A state. It was established that for the XeF 2 :N 2 :Ar = 1.1 Χ 10 17 :10 19 :3.2χ 1019 c m " 3 mixture, employed in the laser experiments, the maximum value of g0 in the bleaching wave at t = 3/isec w a s g j " = (6 ± 1)X 10" 3 c m " 1 . The number of photons n0 emitted due to the C-A transition (per unit volume of the medium during the time for complete dissociation of the XeF2) was determined by integrating a pulse of the luminescence power. An XeF 2 -F 2 mixture was employed in the experiments in which the dependence of n 0 on iV0 (the initial concentration of XeF2) was studied. The nitrogen concentration was maintained constant at a value of 2.5 Χ 1019 cm" 3 , while the XeF 2 concentration was varied from 1.1 XlO 1 6 to 8.8 XlO 1 6 c m " 3 . The value of n0 then varied from (3.9 + 0.6) Χ10 1 5 to (6.4 + 1) Χ 1015 c m " 3 . A comparison was made between the experimental and theoretical dependences no(No). The latter were obtained by analyzing the kinetics of formation and decay of XeF(C). Let us give the main results of a comparison of the theory and experiment. We were able to explain the experimental results only by following the assumption made by the authors of Ref. 6 that the fluorine formed by the photolysis of XeF 2 is a strong quenching agent of XeF(C). The rate constant for the quenching reaction, given in Ref. 6 as k ρ = (1.1 + 0.4) χ 10" 9 cmVsec, enables the observed dependence no(No)t0 be explained and the average quantum effiZuevefa/.
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ciency of the formation of XeF(2?) over the pump band, φΒ, to be determined. The value obtained, namely φΒ =0.85 ± 0.05, is found to be close to the result of Ref. 13 (φ™ =: 1), χ rather than to that of Ref. 12 (
