STUDY OF CADENCE OPERATING OF XeCl EXCIMER LASER TRIGGERED BY PHOTOIONISATION

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STUDY OF CADENCE OPERATING OF XeCl EXCIMER LASER TRIGGERED BY

PHOTOIONISATION

D. Beaupere, B. Lacour

To cite this version:

D. Beaupere, B. Lacour. STUDY OF CADENCE OPERATING OF XeCl EXCIMER LASER TRIG- GERED BY PHOTOIONISATION. Journal de Physique Colloques, 1987, 48 (C7), pp.C7-47-C7-50.

�10.1051/jphyscol:1987706�. �jpa-00226911�

JOURNAL DE PHYSIQUE

Colloque C7, supplkment au n012, Tome 48, decembre 1987

STUDY OF CADENCE OPERATING OF XeCl EXCIMER LASER TRIGGERED BY PHOTOIONISATION

D. BEAUPERE and B. LACOUR

L a b o r a t o i r e s d e M a r c o u s s i s , Centre d e R e c h e r c h e s d e l a Compagnie G e n e r a l e d 1 E l e c t r i c i t e , D i v i s i o n O p t r o n i q u e ,

d e N o z a y ,

M a r c o u s s i s , F r a n c e

ABSTRACT

This work demonstrates the feasibility of hlgh repetition rate operation of photo triggered exclmer lasers.

Photo triggering of excimer lasers was put forward in 1982 at Laboratoires de Marcoussis and greatly increases the reliability of the electrical driving circuit : according to this proposal. the laser head is directly connected to the storage line and loaded to its deslgn voltage with a relatlveiy slow rise time puise. The triggering of a stable discharge Is then obtained by rapid lnitlation of the required electronic density In the interelectrode spacing using photoionisation.

The experimental set-up includes a laser head and a gas circulation loop to ensure active medium renewal.

A fiat nickel anode and a screen cathode form part of the laser head. its active volume is 30 cm3. A silica encapsulated electrode takes place under the cathode to ensure preionisation.

Electrical driving circuit includes two ail solid state magnetic switching devices.

Using a 2.5 bar gas mixture and for single shot operation the 'laser delivers a pulse of 40 ns duration with energy up to 50 mi. Best efficiency can reach 2 . 5 %.

For high repetition rate operation up to 400 Hz. 5 W mean power has been obtained.

I. INTRODUCTION

Many applications in various areas such as microiithography. photopolymerisation or material processing, requires UV lasers producing beam with average power extending from a few watts to several kilowatts. Excimers lasers have proved very appropriate to produce high energy pulses at medium pulse repetition frequency E l . 23

Since their discovery i n 1975 C31. discharge excited excimer lasers have almost always been operated through a combination of a gas preionisatlon followed by an electrical discharge triggered by a fast. high overvoltage. Practical implementation of such an excitation scheme requlres a fast. low inductance switch to be introduced between the energy storage line and the discharge. Photo triggering was put forward In 1982 at

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987706

C7-48 J O U R N A L DE _PHYSIQUE

Laboratoires de Marcoussis C43 and solved the problem raised by the fast electrical switch by getting rid of it C51.

Flgure 1 gives a pictorlal dlagram of a photo triggered iaser discharge during its various main phases on a few microseconde time scale.

a 1 1 2 1 3 1 4

1

02 u (0

-

3 I

a

0) L

C m

0 V )

b'

5 lo3 150 ,100 1 30 1 50 -100 1

I I

Time

(ns)

-

Figure 1 : Pictorial diagram of the iaser discharge.

Phase one is the loading of the storage line. directly connected to the iaser head. to its design voltage with a relatively slow rise time ⁽² 5 ps). The triggering of a stable discharge is then obtained during the second phase by rapid injection of UV or X-rays giving rise to tke required electronic density in the interelectrode spacing. An uniform energy deposit into the iaser medium during phase 3 and 4 requires fast rise time of the initial electronic density ( a few nanoseconds).

This operating procedure eliminates the need for a complex and expensive component of poor reliability : the thyratron : it is replaced by an ail solid state magnetic switching devices including thyristors.

The goal of the present work. ( a part of the EUREKA EUROLASER program) is to demonstrate the feasibility of high repetition rate operation of photo triggered excimer lasers.

2. EXPERIMENTAL SET-UP

The experimental set up includes a laser head and a closed cycle circulation ioop to ensure active medium renewal by high speed transverse gas circuiation.

The iaser configuration includes a nickel anode and a stainless steel screen cathode.

Corona preionisation discharge is ensured through the screen cathode by a silica encapsulated electrode. The electrode gap is 1 cm and the discharge length is 30 cm.

The central part of the electrodes has a 1 cm fiat profile defining an active volume of 30 cms.

The optical cavity is formed by a five meter radius maximum reflecting fused quartz mirror and a fiat, 73 % transmitting window. located 40 cm apart.

The gas circulation ioop includes an axial fan and a heat exchanger provided with fiowlng water. Owing to the fan rotating speed the gas flow rate can reach 90 i / s ; the corresponding gas velocity in the iaser cavity is 30 m/s. A direct current motor brings power into the circulator through a ferro fluidic type rotating shaft seal.

The electrical driving clrcult Includes a pulse formlng line made of discrete capacitors directly connected to the electrodes and two all soild state magnetic switching devices.

The schematic diagrams of the two electrical circuits are shown in figures 2 and 3 :the llrst one insures tho charge 01 the PFL whlie the socond one provides a fast high voltage pulse (10 kV, 10 ns risetime) to the prelonlzlng electrode.

I

t o pulse forrnlng l l n e

Figure 2 : Schematic diagram of electrical circuit used to load the PFL

f 1

Figure 3

--,"-nm

³

Primary capacitors are 25 pF for main circuit (C1) and 0.44 pF for prelonizatlon ( C ' l ) . Charging voltage is less than 1 kV.

3. EXPERIMENTAL RESULTS

During preiimary experiments the iaser has been operated in a single shot mode. Using a 0.16 % HeCI, 0.8 % Xe. 99 % Ne gas mixture at P = 2.5 atm pressure a maximum output energy of 50 mj has been extracted with an electrical efficiency of 2.3 % in 40ns (FWHM) pulses. The charging voltage was 7 kV and the capacitor value C = 90 nF.

A peak efficiency of 2.5 % with an output energy of 32 mJ was obtained at V = 6.5 kV and with C = 60 nF. Figure 4 shows output energy and electricai efficiency of the laser as a function of the charging voitage.

The iaser has been then operated in the repetitive mode up to a PRF of 400 Hz and a maximum output average power of 5 W has been obtained with the electrical efficiency of 1 %.

Figure 5 shows laser mean power versus PRF during those experiments.

We attribute the decrease of the energy per pulse between single shot and PRF operation to poor flow quality in the iaser cavity : experiments are now underway in order to improve the laser head geometry.

JOURNAL DE PHYSIQUE

Figure 4 : Electrical efficiency and output energy of the XeCi laser as a function of charging voltage.

REPETITlOh FATE (142) Figure 5 : Laser average power versus PRF.

CONCLUSION

We have demonstrated the feasibllity of high PRF operation of a photo triggered XeCl excimer laser.

The use of ail solid state magnetic switching devices leading to simple and reliable electrical design makes photo triggering very attractive from an industrial point of view.

R E F E R E N C E S

[I 1 C. P. Wang and 0. L. Glbb IEEE J. Quantum Electron QE 15. 318 (1979)

C21 T. J. Fahlen

IEEE J. Quantum Electron QE 16. 1260 (1980)

C31 C. P. Wang, Rev. Sci. Instr. 47, 91 (1976)

141 0. de Wltte, 8 . Lacour. C. Vannler presented at Conference on Lasers and Nectro Optics paper WD6 Phoenlx Arlzona, Aprll 1982

C51 8. Lacour and C. Vannfer to be published i n J. of Appl. Phys.