High neon pressure longitudinal copper vapour laser

Volume 46, number 3,4

OPTICS COMMUNICATIONS

1 July 1983

HIGH NEON PRESSURE LONGITUDINAL COPPER VAPOUR LASER O.R. MARASOV and St. STOILOV Research Institute for Optics, P.O. Box 39, BG 1324, Sofia, Bulgaria Received 5 April 1983

The study of a longitudinal copper vapour laser at neon pressure up to 300 kPa is reported. At high neon pressure lower copper vapour concentrations are preferred. It is shown that the self-inductance of the discharge circuit causes a considerable additional power consumption from a direct current high voltage power supply. This additional power is of the order of the power stored in the self-inductance of the discharge circuit and reduces the overall laser efficiency.

1. Introduction Copper vapour laser (CVL) energy characteristics depend on many parameters. Some of these parameters are defined by the choice of circuit and discharge tube constructions, such as commutator switch, discharge tube (DT) self-inductance and commutator circuit self-inductance, etc. But most of the CVLs parameters are chosen to provide the optimum lasing conditions in the discharge plasma, such as copper vapour concentration, buffer gas type and pressure, electric field in the discharge region, excitation pulses repetition rate, duration and amplitude of the current pulses, etc. In recent publications the self-inductances of DT and the commutator circuit are discussed only in relation to their influence on the current pulse leading edge and/or on the time delay between current and voltage pulses. In this paper we show that discharge circuit self-inductance causes considerable additional power consumption from a direct current high voltage power supply (DC HV PS) and thus decreases the overall laser efficiency. The highest output power of CVLs is obtained at low neon pressure [ 1 - 3 ] . However, in many applications as recording and reconstruction of dynamic holograms [4], laser projection microscopy [5], wavefront reversal [6], efficient dye laser pumping [7], medicine, etc., an output power of a few marts is required. Experiments show that such output power 0 030-4018/83/0000-0000]$ 03.00 © 1983 North-Holland

may be generated at high neon pressure [1,3,8]. High pressure CVI.s have some advantages. They provide an increase of the DT lifetime in sealed-off constructions due to reduced copper diffusion. The thyratron lifetime is also increased because of the improved resistance matching between the commutator and DT. In previous papers is shown that longitudinal CVLs lase at neon pressure up to ~1 arm. But the neon pressure output dependencies reported in various papers are different [ 1,2,3,8], as it is difficult to account for the influence of all laser parameters on CVLs energy characteristics. In this paper it is shown that copper vapour concentration (Ncu) influences considerably the neon pressure output power dependencies. The operating neon pressure in a longitudinal CVLs was extended to 300 kPa (~3 atm).

2. Experimental setup The experiments reported here have been carried out on a copper vapour laser described in detail in ref. [3]. The DT was a discharge heated device with estimated self-inductance LDT ~ 0.3/all. The discharge channel was an alumina tube with internal diameter of 24 mm and length of 60 cm. The optical cavity was formed by a flat quartz plate and a concave dielectric mirror with a radius of 5 m and reflectivity over 99.7% on both laser lines. The electric circuitry of the laser is 221

Volume 46, number 3,4 . . . . . . . . . . . . . .

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1 July 1983

OPTICS COMMUNICATIONS

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Fig. 1. Electric circuitry of copper vapour laser. T, thyratron switch; C, discharge capacitor; L 1, charged inductance; LC , discharge circuit self-inductance; C 1, peaking capacitor; LDT, discharge tube self-inductance; DT, discharge tube. shown on fig. 1. LDT is the DT self-inductance and L C is the commutator circuit self-inductance. As will be shown later, these inductances influence the laser efficiency. High amplitude current pulses were formed by a low inductance capacitor discharge via a thyratron switch. The capacitor voltage was received in the dioderesonance charging circuit and is twice the DC PS high voltage. The voltage and current pulses were monitored with a Tektronix P6015 and P6303 voltage and current probes and a 456B oscilloscope. The laser output power was measured with a Rk 5100 power meter. The temperature in the discharge channel was deduced from measurements of the infrared radiation from the channel wall. The measured temperature value in the temperature range 1 4 0 0 - 1 7 0 0 ° C was reproduced with an accuracy better than 1.5%.

50 NEON

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Fig. 2. Discharge channel normalised temperature and DC current of the DC PS high voltage. UDC = 5.5 kV, f L = 10.2 kHz, C = 3.3 nF, C1 = 0.47 nF. tion pulses. This energy was introduced once again in the electric circuit when the current amplitude began to fall down. Because of the high electron concentration in the plasma during the second half of the current

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In the experiments the commutator had a considerable self-inductance L C ~ 1.5 #H. The commutator switch consisted of two TGI 1 1000/25 thyratrons connected in parallel [3]. When the neon pressure (PNe) increased at constant DC PS.high voltage, the direct current (IDc) of DC HV PS decreased (fig. 2). This reduced the consumed DC power PDC = UDCIDc (fig- 3a). In this connection we should note that the DC power at low neon pressure significantly overrates the power of the discharge capacitor PC = 2 U 2 c C f • The self-inductance L = L C + L D , which is always present in the discharge circuit causes the difference presented on fig. 3a. The high peak amplitude current (/max) pulses used in CVLs store in L a good deal of energy from the leading edge of excita-

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Volume 46, number 3,4

OPTICS COMMUNICATIONS

pulse the DT resistance was very low and the electric energy stored in the self-inductance could not dissipate fully. A substantial part of this energy charged the capacitor C to a voltage opposite to the DC HV PS potential. In order to compensate this opposite voltage, an additional DC power PL = 0.5 L12axfWasneeded. With the increase Of PNe the pulse current maximum receded. The power stored in the self-inductance was reduced too, i.e. at constant DC HV the difference between the power stored in the discharge capacitor and the power consumed from DC HV PS would be reduced. We should note that Smilanski et al. [9] have also observed a recharging of the discharge capacitor which they eliminated by adding a clamping diode in the circuit. During the experiments we observed that at increasing PNe the channel temperature also increased. The discharge channel temperature normalized at the maximum temperature as a function of neon pressure is shown on fig. 2. This variation of the channel temperature may be explained as follows. First, when PNe increases, the active DT resistance also increases and more active power is introduced in DT. And second, the selfinductance of the discharge circuit reduces the real power introduced into DT with about 2P L in comparison with DC HV power. The DT power reduction is due to power take-off from the leading edge of the current pulses and DC HV power consumption to compensate the opposite voltage of the discharge capacitor. When PNe increases, the real introduced power in DT increases too, because of the lower losses in the discharge circuit self-inductance (as noted above). We also observed a considerable difference between PC and PI)C (fig. 3b) at higher values of the PS voltage. In this case the experiments were carried out at constant discharge channel temperature. Because of the current amplitude rise with the increasing of the UDC the results on fig. 3b may be also explained by the existence of self-inductance in the discharge circuit. The difference between PC and PDC with the charge of neon pressure and DC high voltage shows that we must be careful when determining the CVL efficiency, because the discharge circuit self-inductance is always present. In the literature the efficiency is defined in several different ways. Walter [10] defines the CVL efficiency in relation to the power in the discharge capacitor and the power introduced into DT up to the end of the laser pulse. In ref. [11] efficiency was defined in relation

1 July 1983

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to the power in DT up to the end of the excitation pulse. Isaev [ 12] determined laser efficiency relative to the DC power consumption of DC HV PS. However the usual method for efficiency determination is in relation to the power in the discharge capacitor 0?C) or in relation to the DC power consumption of DC HV PS (~/DC)" Our measurements presented in fig. 4 show that these two efficiencies may differ considerably because of the discharge circuit self-inductance influence. In our view r/DC is more representative for the overall laser efficiency. The effects discussed here concerning discharge channel temperature variation when changing PNe and differences between PC and PDC in low neon pressure and/or high DC voltage should be always taken into account in CVL research and design. These effects would probably appear particularly in large bore, high average power CVI_s which as a rule operate at low neon pressure, high current and voltage pulses [13]. These effects may be essential also in other lasers with high pulse current and high repetition rate. 223

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OPTICS COMMUNICATIONS

when Ncu is reduced lasing of CVL at up to 300 kPa (about 3 atm) neon pressure is possible. This is the highest working neon pressure, as far as we know, for longitudinal CVLs up to now. The results in fig. 5 may be explained assuming that a complete copper vapour ionization takes place by the end o f the excitation pulses [14]. When Ncu is reduced the electron concentration at the end of excitation pulses is also reduced. In this way the decrease of Ncu makes the restoration time of the discharge region shorter and allows operation at high neon pressure.

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Fig. 5. Dependence of CVL output power on neon pressure at different copper vapour concentration (UDC = 7 kV, C = 2.2 nF,faL = 12.1 kHz, C 1 = 0.33 nF). oNCu = 7.7 X 1014 cm- ; <>NCu = 1.2 X 1015 cm-3; ~zNCu =2.4 × 10 Is cm-3;~NCu= 3.0X 1015cm -3.

In experiments for extending the operating neon pressure region (fig. 5) we changed the experimental conditions in order to limit the influence o f discharge circuit self-inductance on the laser energy characteristics. As a c o m m u t a t o r switch a EGG 1802 thyratron mounted in a coaxial construction with DT was employed. Thus the overall discharge circuit inductance was reduced to about 0.4--0.5/~H. For efficient lasing at high neon pressure higher electric field in the discharge region is needed. In order to limit the amplitude o f the excitation current pulses ~he discharge capacitor was reduced to C = 2.2 n F , and the repetition rate was increased to fL = 12 kHz. The reduction o f the discharge circuit self-inductance and current pulses maximum value resulted in a smaller difference (less than 15%) between PDC and PC in the investigated neon pressure region at constant UDC= 7 kV. The laser output dependence on the neon pressure at constant DC high voltage and for different copper vapour concentrations are shown in fig. 5. It is evident from the results in the figure that 224

It follows from the results presented in this paper that discharge circuit self-inductance in CVLs always increases the DC power consumption of DC HV PS in comparison with the power stored in the discharge capacitor. The differences in the values of these two powers is o f the order of the power stored in the discharge circuit self-inductance. This difference should always be taken into account when laser efficiency is determined. Our experiment shows that longitudinal CVLs can operate at up to 300 kPa (~3 atm). In order to produce higher laser output power at high neon pressure, lower copper vapour concentration is needed. This may be another reason for the reduction of copper vapour diffusion and increasing of sealed-off DT lifetime at high neon pressure.

Acknowledgements The authors acknowledge the technical help and assistance o f I. Draganov, G. Gopovski, M. Toshev, D. Popova and S. Serafimov during the experiments. We are grateful to Prof. I. Tomov and Prof. I. Klimovskii for helpful discussions.

References [1] V.A. Burmakin, A.N. Evtyunin and M.A. Lesnoi, Kvant. Electron. 6 (1979) 1589. [2] P.A. Bokhan, V.A. Gerasimov, V.I. Solomonov, V.I. Silantiev and V.B. Stcheglov, Effective gas-discharge metal vapour laser (Tomsk, 1978) p. 139.

Volume 46, number 3,4

OPTICS COMMUNICATIONS

[3] O.R. Marazov, M.A. Angelov, V.L. Zekov, S.S. Ivanov and V.T. Borisov, Electropromishlenost, No. 12 (1982) 528; O.R. Maxazov, I.D. Draganov and G.S. Gopovski, Bulgarian Journal of Physics (1983), to be published. [4] K.I. Zemskov, M.A. Kazaryan, V.G. Mokerov, G.G. Petrash and A.G. Petrova, Kvant. Electron. 5 (1978) 425. [5] K.I. Zemskov, M.A. Kazaryan, V.V. Savranskji and G.A. Shafeev, Kvant. Electron. 6 (1979) 2473. [6] F.V. Bunkin, V.V. Savransky and G.A. Shafeev, Kvant. Electron. 8 (1981) 2015. [7] Huang and K. Namba, Jap. J. Appl. Phys. 20 (1981) 2383. [8] I. Smilanski, G. Erez, A. Kerman and L.A. Levin, Optics Comm. 30 (1979) 70.

1 July 1983

[9] Sh. Gabay, I. Smilanski and Z. Karny, IEEE J. Quant. Electron. QE-18 (1982) 50. [ 10] W.T. Walter, N. Solimene, M. Pilteh and G. Gould, IEEE J. Quant. Electron. QE-2 (1966) 474. [ 11 ] Yu. Babeiko, L.A. Vasirev, A.V. Svixidov, A.V. Sokolov and L.V. Tatarintsev, Kvant. Electron. 6 (1979) 1102. [12] A.A. Isaev and G.Yu. Limerman, Kvant. Electron. 4 (1977) 1413. [13] R.S. Anderson, B.E. Warner, C. Larson, Sr. and R.E. Grove, IEEE J. Quant. Electron. QE-17 (1981) 50. [ 14] V.M. Batenin, I.I. Klimovskii, M.A. Lesnoi and A.A. Selezneva, Kvant. Electron. 7 (1980) 988.

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