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A powerful transversely excited multigas laser system

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1990 Meas. Sci. Technol. 1 1188

(http://iopscience.iop.org/0957-0233/1/11/011)

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Meas. Sci. Technol. 1 (1990) 1188-1192. Printed in the UK

A powerful transversely excited multigas laser system

F Encinas Sanz and J M Guerra Perez Facultad de Ciencias Fisicas, Departmento de Optica, Universidad Complutense, Ciudad Universitaria 28040 Madrid, Spain

Received 12 April 1990, accepted for publication 4 July 1990

Abstract. A compact multigas capacitive transfer transversely excited (TE) gas laser is described. The laser produces a high-intensity beam ( ~ 2 0 MW cm-') operating with CO, mixtures at atmospheric pressure. A pulse of 20.5 mJ is obtained in the second positive (uv) N, band and 5 mJ in the first positive (IR) band. The laser is suitable to work with other gases and is useful as a pumping source for dye lasers.

1. Introduction

Tunable laser sources are valuable apparatus useful in many kinds of experiment. Dye lasers covering the near IR to the near uv (1100-310 nm) are the most popular tunable lasers, but such types of laser covering the mid and far IR and uv do not exist.

Capacitive transfer TE gas lasers are powerful and versatile laser sources. With minor replacements such kinds of laser are capable of working with many different gases, lasing in several uv and IR wavelengths.

In this article we describe one capacitive transfer TE powerful gas laser. The laser behaviour is studied work- ing with CO2 mixtures and pure N,. It is well known that a TE laser working with CO2 mixtures is also capable of using many other gases (N20, CO, etc) lasing in sev- eral IR wavelengths. On the other hand, if the laser works with pure N2 it could lase with excimeric mixtures if a suitable pre-ionization is added, and could lase in some visible transitions with Ne for instance. Usually the change in the lasing gas is accomplished by changing the optics, and perhaps the electrodes or the peaking capaci- tor. With a suitable mechanical design such changes may be easily performed.

2. Experimental set-up

Figure 1 shows an illustration of the experimental set- up. The gas envelope of the laser is a PVC structure in which the coupling inductance is reduced by screwing directly onto the metallic plates supporting the electrodes (both electrodes are easily exchangeable). The discharge was produced in the space between two specially profiled electrodes having a useful length of 65 cm. The spark gap SG and the storage capacitor C, are directly mounted onto the support plate of the upper electrode. The closely coupled (peaking) capacitor C, consists of two flat plates made of 2.4" thick fibre glass epoxy laminate faced with copper foil. They are connected to the electrodes by a thin copper sheet covering their entire length. In the same way the storage capacitor is connected to the earthed sheet of both capacitor plates. Symmetry and low coupling inductance were the two main guide lines used in the mechanical construction. A parallel arrange- ment with a maximum of four 50 n F low stray inductance commercial capacitors was used for storage. They were charged to a maximum of 30 kV.

To measure the temporal evolution of the electrical parameters, a current probe having a 10 ns risetime and

C COPPER PLATES - c, == c* f"'

DIELECTRIC

- -

Figure 1. Experimental set-up showing a cross sectional view of the electrodes (CO, laser) and a circuit scheme. C,, storage capacitor; C,, peaking capacitor; V,, charging potential; CP, current probe; SG, spark gap. Important parameters of the CO, laser chamber: A = 6 c m , B = 8 c m , C = D = 2 c m .

0957-0233/90/111188+05 $03.50 @ 1990 IOP Publishing Ltd

Powerful TE multigas laser system

a voltage probe with a 1 ns risetime were used. The CO2 laser pulseform was measured with a rapid photon drag detector and the N 2 laser with a low risetime photodiode. The pulse energy was measured by a thermopile with calibration traceability to the National Bureau of Stan- dards. The pulses were registered in realtime by a 500 MHz bandwidth transient programmable digitizer and 50 R line matched. All the electronic measurement devices and light detectors were placed in a Faraday cage.

3. Laser operation with CO, mixtures

The resonator consisted of a concave (10 m radius), gold- coated silicon full reflector and a partially transmitting output coupler. Two different plane output couplers were used: ZnSe either uncoated or 60% R/AR coated. The output energy was only slightly lower with the uncoated ZnSe coupler.

The gas in the discharge volume underwent lateral pre-ionization with ultraviolet radiation from two in- complete surface corona discharges (Ernst and Boer 1978, Hasson and von Bergmann 1976, Ernst 1982) ac- ross a glass dielectric plate 3 mm thick. With the usual configuration of this pre-ionization system (figure 2(a)) (Ernst and Boer 1978, Reits 1980, Witteman 1987), up to a certain threshold voltage depending on the pressure and composition of the gas, a normal uniform corona is observed. As the voltage is further increased the regimen of surface sparking is reached (an inhomogeneous com- plete surface discharge is formed). Ernst and Boer (1978) relate arcing across the glass plates with the voltage rise- time. In our case to inhibit arcing development in the surface pre-ionizing discharge we have modified the di- electric surface geometry. To attain higher potentials be- tween electrodes (allowing us to work with mixtures much richer in N2 and CO2 and hence to obtain higher peak power and pulse energy) with uniform corona be- haviour, we used the pre-ionizer geometries described in figures 2(b) and (c). The optimum value obtained for the peaking capacitor was C, = 8 nF.

The influence of the electrode profile in the gain dis- tributions was previously studied (Robinson 1976) for Chang profiles (Chang 1973) with different degrees of field homogeneity. We studied the effect of a compact Chang profile or an Ernst profile (Ernst 1984) on the laser characteristics. The Chang and Ernst profiles corre-

Cl a1

Figure 2. Pre-ionizer geometries used. The sparking threshold potential is greater in (b) and (c) than in (a) .

spond to the values k = 0.01 and k = 0.015, respectively. In the case of the compact Chang profile, a deviation of 5 x in the homogeneity is assumed between the centre (x = 0) and at a distance equal to the separation d = 2 cm between the electrodes (x = d) . On the other hand, in the Ernst profile the deviation is 1 x at the same distance d from the centre of the profile.

The increase in homogeneity in the Ernst profile is obtained without increasing the total width of the elec- trode (figure 3), a problem which would have required a deformation in the profile on the edges.

With these two types of electrode the energy profiles of the output radiation were measured perpendicularly to the electric discharge field (direction x) (figure 4, curves a, b). Measurement of the profiles was performed by moving in the said direction a slit which was parallel to the field lines (the width of the slit being 1 mm and its height equal to the separation between electrodes). Thus the effective volume of the discharge was found, its width being that measured at the mid-height of the profile ob- tained in each case. As can be expected (Ernst 1984), although both electrode profiles have the same total width, the Ernst profile gives a higher discharge volume. In these measurements we have also observed a gain switch peak smoothing from the centre to the discharge edges. Thus the energy and peak power profile were different (figure 4, curve c). The peak power profile is narrower than the energy profile. Without surpassing the charging potential of 30 kV, the optimum functioning conditions with both types of electrode are those found in table 1. For these measurement we used the pre-ionizer

ERNST

CHANG

Figure 3. Ernst and Chang profile. The broken curve represents the theoretical Ernst profile and the full curve is the real one truncated on the back surface.

- y.. \

C 70 20 x (")

Figure 4. The profiles of the output energy obtained with the Chang (a) and the Ernst (b) electrodes and the peak power profile with the Ernst electrodes (c). X direction is perpendicular to the electric discharge field.

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F Encinas Sanz and J M Guerra Perez

Table 1. Conditions of maximum output energy with both types of electrode. A, intensity profile width; V,, discharge volume; V,, charging voltage; C,, storage capacitor; €,, input energy; E,, output energy; W;, specific input energy; WO, specific output energy. In both cases C, = 3 nF.

A(”) V,(l) V.(kV) C,(nF) €,(J) .Eo(J) W(J I - ’ ) Wo(J I - ’ ) Mixture

Ernst profile (aluminium) 28 0.36 30 150 67.5 12 190 34 1:3:5 Chang profile (steel) 13 0.17 27 100 36.5 6.2 215 36 1:3:6

geometry shown in figure 2(c) which has the highest arc-

The results described in table 1 were obtained at the border of the plasma instabilities limit using high grade purity gases. The addition of uncontrolled small quantit- ies of propene (Scott and Smith 1984) significantly im- proved the plasma homogeneity. It is possible to add another 50 nF capacitor to the storage array C, attaining

per unit area. This value is one of the higher reported in the literature: 2.6 Jcm-’ (Ernst and Boer 1978), 2.6 J cm-’ (Reits 1980), 2.4 J cm-’ (Vuong and Puzewicz

reaches 20 MW cm-2. On the other hand, small local where s 2: 65 x 2.5 cm2 is the discharge section, inhomogeneities on the electrode surfaces can play an i, ‘Y 3.7 kA (figure 6(b)) and Vd is the drift velocity for the important role in discharge degradation (Dyer 1978, corresponding value of E , / N (V, = 37 kV and N the gas Kline and Denes 1982). In fact, if matt Surface electrodes density at 1 atm) with C02:N,:He = 1 : 1 : 8 (Lowke et al are used to avoid oblique modes, the energy which can 1973) as the mixture used. With these values the electron be introduced per unit discharge volume decreases density is of the order of n,f ‘y 1013 cm-3. appreciably. On the other hand, if the time evolution V(t )

Figure 5 is a reproduction ofthe impression on glossY (figure 6(a)) of the potential or the field E ( t ) = V ( t ) / d is black Paper of an impact of the near field with the two measured up to the maximum V, at the instant t , and types of electrode. The particular spatial Structure of the this experimental result is used in Townsend’s equation: radiation emitted can be seen, which is typical of a multi- mode laser action.

at the moment of electrical breakdown. To do this we measure the potential V between the electrodes and the current i provided by the condenser C,. This current corresponds to the total circulating through C, verifying (figure 1)

the laser i,

where the current of the incomplete surface corona dis- charge (low-intensity dicharge) has been neglected (Ser- afetinides et al 1987). When the potential reaches the maximum V,, then d V/dt = 0 and therefore i, = 0, im- plying that at this instant t ,

ing surface voltage threshold. i = ip + i, (2)

an energy output of 16 J per pulse and 2.9 J cm-2 energy i, = i(t,) = iL(tm). (3) The corresponding electron density can be calculated from the relationship

i, = enef uds (4) 1982). The intensity in the centre of the transverse profile

We have made an estimation of the electronic density n,, 4 0 = eXp ( J , ( a ( E ) - P ( E ) ) U d ( E ) dt ) (5)

(1) dV

p p dt

plus what circulates through the principal discharge of

i = C -

Figure 6. Time synchronized voltage and current pulses. Mixture = 1 : 1 :8, C, = 100 nF, C, = 8 nF and V, = 30 kV (Ernst electrodes). Vertical scale: (a) 10 kV per division, (b) 4 kA per division. Horizontal scale: 40 ns per division.

Figure 5. Near-field impression on glossy black paper of an impact.

1190

a prediction of the electron density growth from the pre- ionization level neo can be made. Using a linear adjust- ment of the potential measured experimentally (figure 6(a)) along with the data for the mixture 1:1:8 (Lowke et al 1973) of the ionization coefficients cc(E), attachment /?(E) and drift velocity, from (5) it was found that electron density growth up to the maximum poten- tial was

25 = - - - h 20 z E - 1 5 ~

7 - - -

* 0

z - - -

s to: w - -

5 7 - -

From this we can estimate the pre-ionization level of our system which gives neo N lo6 ~ m - ~ . This level is above the minimum value estimated in literature to obtain a homogeneous volume discharge (nee 'v 104-105 cmP3) (Palmer 1974, Karnyushin 1978). From this analysis it can be concluded that even though the pre-ionization system based on incomplete surface discharges is sufficient and easily constructed, the pre-ionization levels which it supplies are clearly less than those reached in systems based on complete surface discharges (Suzuki et al 1980). As is well known, a high pre-ionization level allows the quantity of energy per unit of volume de- posited in the discharge to be increased, because it helps to minimize and delay the development of instabilities in the discharge leading to the glow-arc transition (Dyer 1978, Kline et a1 1976).

0

0 0

A IR a

'. A A

4. Laser operation with pure N,

25 - n

v

* l 5 P i o W Z U 5

0

i"!

The optical resonator consisted of a flat metallic full re- flector and an uncoated flat quartz output coupler. One electrode exhibits a cylindrical profile (1 cm diameter) and the other is flat. Both were rounded at the ends to avoid the generation of spark channels and the overall estimated length of the discharge is 65 cm. Brass, stainless steel and copper electrodes were tested without notice- able influence on laser performance.

The separation between electrodes was changed from 1 to 4 cm. The pulse energy shows an almost linear in- crease with the separation (figure 7).

No pre-ionization was included but in spite of such simplicity the pulse-to-pulse reproducibility was better than 10% in one single energy pulse measurement. Also, the beam quality is good with a cross section of 40 x 10 mm2 in the near field.

is described. The laser works in TEA mode with CO2 mixtures giving a high-energy pulse and high peak inten- sity output in the 10.6 pm wavelength.

With minor changes it may operate with pure N,, displaying very good performances in the uv and the IR positive systems. The pulse width ( N 13 ns) is good for pump dye lasers and is easily focused in a very narrow / ,

Powerful TE multigas laser system

uv

Working at a pressure of 6.65 kPa (50 Torr) the peak- ing capacitor was varied up to a maximum of 40 nF and the pulse energy grew steadily without reaching satu- ration (figure 8).

To obtain high pulse energy it is very important to have the resonator carefully aligned. In this case simul- taneous lasing is produced in the uv (337nm) and IR (several bands between 748 nm and 1498 nm) N, positive systems. The uv and IR outputs have similar pulse widths (figure 9). The IR pulse was isolated with a low pass filter. The maximum IR pulse energy is 5mJ and the corre- sponding peak power reached 0.4MW, which are the best values reported in the literature (Kunabenchi et a1 1986). In the uv pulse we attained 20.5 mJ and 1.5 MW when the pressure was 8 kPa (60 Torr).

5. Conclusions

1191

F Encinas Sanz and J M Guerra Perez

the level which can be obtained with pre-ionization sys- tems based on complete surface discharges.

References

Chang T Y 1973 Improved uniform-field electrode profiles for TEA laser and high voltage applications Rev. Sci. Instrum. 44 405-7

Dyer P E 1978 Field uniformity requirements in TEA CO2 lasers J . Phys. E; Sei. Instrum. 11 1099-101

Ernst G J 1982 A 10 cm aperture, high quality TEA CO2 laser Opt. Commun. 44 125-9

~ 1984 Uniform-field electrodes with minimum width (laser applications) Opt. Commun. 49 275

Ernst G J and Boer A G 1978 Construction and performance characteristics of a rapid discharge TEA COz laser Opt. Commun. 27 105-10

Hasson V and von Bergmann H M 1976 High-pressure glow discharges for nanosecond excitation of gas lasers and low inductance switching applications J . Phys. E; Sci. Instrum. 9 73-6

Influence of preionization conditions on the development of a homogeneous discharge in gases Sou. J . Quantum Electron 8 319-23

laser discharges Applied Atomic Collision Physics vol 3 (New York: Academic) pp 387-422

Karnyushin V N, Malov A N and Soloukhin R I 1978

Kline L E and Denes L J 1982 Pre-ionized self sustained

Kline L E, Denes L J and Pechersky M J 1976 Arc suppression in CO2 laser discharges Appl. Phys. Lett. 29 574-6

Kunabenchi R S, Gorbal M R and Savadatti M I 1986 Nitrogen lasers Progress in Quantum Electronics vol9 (Oxford: Pergammon) pp 259-329

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atmospheric-pressure glow discharges A p p l . Phys. Lett. 25 Palmer A J 1474 A physical model on the initiation of

138-40 Reits B J 1980 Parametric measurements on a doped CO2

Robinson A M 1976 Laser gain profiling with uniform-field

Scott S J and Smith A L S 1984 Efficacy of laser

TEA laser Opt. Commun. 33 75-9

electrodes J . Appl. Phys. 47 608-13

preionisation with a semiconductor source and propene adition J . Phys. E : Sci. Instrum. 17 1242-3

Serafetinides A A, Papadopoulos A D and Rickwood K R 1987 Investigation and comparison of preionisation processes in gas laser systems Opt. Commun. 63 264-8

Dependence of laser output on initial photoelectron density in TEA COz laser Appl. Phys. Lett. 36 26-8

Vuong N T and Puzewicz Z 1982 Pulsed TEA COz laser input energy density of 1.1 kJ/liter stabilized by a double preliminary discharge Sou. J . Quantum electron 12 92-3

Suzuki S, Ishibashi Y, Obara M and Fujioka T 1980

Witteman W J 1987 The CO2 laser (Berlin: Springer) pp 178-85

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