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Submitted on 1 Jan 1980
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OPERATING PARAMETERS OF A MINIATURE,
HIGH-REPETITION RATE RARE-GAS HALIDE
LASER
R. Sze
To cite this version:
JOURNAL DE PHYSIQUE ColZoque 6'9, suppZ&ment au nO1l, Tome 41, novembre 1980, page C9-479
R.C. Sze
University o f California, Los AZamos S c i e n t i f i c Laboratory, P. 0. Box 1663, Los AZamos, N. M. 87545 U. S. A.
Abstract.- Performance characteristics of a high-repetition rate rare-gas halide minilaser are repor- ted. We obtained energy per pulse at the 1 kHz repetition frequency greater than 1 mJ per pulse for KrF and 0.5 mJ per pulse for XeC1. We observed net small signal gains greater than 0.3 cm-I in XeCl and measured a wall plug efficiency as high as 0.25%.
The operating characteristics of a miniature laser operating at kilohertz repetition frequency using rare-gas halide laser mixtures is de- scribed. Energies per pulse at the kilohertz repetition frequency are greater than 1 mJ per pulse with KrF and 0.5 mJ per pulse with XeC1. The discharge volume is less than 1 cc and has volume dimensions of 10 cm x 0.4 cm (electrode separation) x 0.2 cm (electrode width). The Blumlein circuit is made of two dielectric strip lines of 5 x 10 inch dimensions. The capacitance of each line was varied from as low as 0.9 nF to a s high as 1.37 nF. Typical energy storage was 0.5 J per pulse and typical wall plug efficiencies
obtained were of the order of 0.25%. The Blumlein is switched with an EGG 1102 Thyratron coaxially attached in a low inductance configuration. Volt- age risetimes faster than 0.67 kV/ns were obtained with charging voltages as high as 22 kV on the Blumlein. Preionization of the active volume is accomplished through a corona discharge behind one of the electrodes (Fig. I ) , and full inductive charging of the Blumlein is through a saturating inductor that gives 500 ps of hold-off time to allow the thyratron to recover.
Gas flow is transverse to the discharge via holes drilled in the walls of the electrodes as indicated in Fig. 1. Figure 2 gives the flow rate
Saturating /
Fig.
t
a) Inductive charging electrical schematic of Blumlein device, b) transverse gas flow pattern of optical cavity.
C9-480 JOURNAL DE PHYSIQUE
and discharge volume clearances per second for one and two bellows pumps as a function of the static pressure at the laser head. In Fig. 3, average power and energy-per-pulse data is given for the different flow rates in KrF. One observes that the present crude flow patterns limit energy output at the high repetition rates even at 3.6 clearances per pulse for 1 kHz repetition frequen- cy. However, no degradation in the lasing beam quality is observed even at the high repetition rates.
Kr F gas mix
4 0 0
x one bellows pump
o two bellows pumps
I 1 I I
I
400 800 1200 1600 2000 Static pressure ( t o r n )
Fig. 2. Flow rate and clearance time data for KrF ( 0 . a F2/5% Kr/89.9% He) gas mix as a function of laser cavity pressure. Data for one double bellows pump and two double bellows pumps are presented. We fins that the average power increased with increasing pressure, as shown in the data for KrF and XeCl in Figs. 4 and 5, respectively. An analysis using Rigrod's steady-state oscillator theory3 ' gives very high losses (ao 1 0.36 cm-l) during the lasing time and results in net gains (go-a,) as high as 0.3 em-'. The net gain data is given as a functio~ of pulse repetition frequen-
o 2 0 0 0 clearonces/s 1 6 6 0 t o r r stat~c pressure
1
x 3 6 0 0 clearances/$ I 6 0 0 t o r r static pressure , ,,-0- - - 0 --
L 0. w 0.4 0.3 0.3 K r F w 0 2 100 200 3 0 0 600 1000 Pulse Repetition Rate(Hz)
Fig. 3. Average power and energy per pulse data are presented for different dis- char e volume clearance rates for KrF ( 0 . g P,/% Kr/% Ne/89.8% He) gas mix at different pulse repetition fre- quencies.
cy in Fig. 6. The initial rise in the gain as a function of the pulse repetition frequency is be- cause the voltage on the Blumlein is substantially lower at the lower pulse repetition rates. This is due to the current leaking back toward the power supply through the charging diode, re- sulting in lower voltages on the Blumlein. At the high repetition rates the drop in gain is attri- buted to insufficient clearance of the used gas due to the crude flow patterns and insufficient flow rate. Figure 7 gives the saturation intensity as a function of flow rate. The rise in saturation intensity as a function of pulse repetition rate is the result of electron quenching due to increasing discharge loading as a function of pulse repetition frequency.
I
Av1600
ldoo
d m
A00
'
I
Static Pressure ( t o r r s )
XeCl(O.2%HCI/ 3°/~Xe/50/oNe/91.80/oHe) 0.6-No. clearances/shot r3.6 at 1 k H z
1200 1400 1600 1800 2000 2 2 0 0 Static Pressure (torrs)
Fig. 4. Average power vs static pressure for Fig. 5. Average Power vs static pressure for different pulse repetition rates at a dis- different pulse repetition rates at a charge volume clearance rate of 3600/s, discharge volume clearance rate of KrF lasing with (0.2%
F2/5%
Ne/89.8% He) 3600/s, XeCl lasing with (0.2% HC1/gas mix. Blumlein capacitances are 1.4 3% Xe/5% Ne/91.8% He) gas mix.
and 1.5 nF. Charging voltage is 12 kV. Blumlein capacitances are 1.4 and 1.5 nF. Charging voltage is 12 kV.
XeC1(.2°/oHC1/30/oXe/50/oNe/91 8°/oHe) Charg~ng Voltage= I0 kv
Pressure = 2 0 0 0 torrs
Repet~tlon Rate (HZ1
Fig. 6. The results of the net small' signal gain (go
-
a ) vs pulse repetition rate. The (---) daghed curve is a projection of thenet gain in the absence of flow limitation
effects. Data for .XeCl.
'
-XeC1(.2% HCI/3%Xe/5%Ne/9l 8%He) Charglng Voltage = l O k v
Pressure = 2 0 0 0 torrs
JOURNAL DE PHYSIQUE SUMMARY voltage
.
fluorescence (a voltage lasing ( 4 0 % R ) (b) voltage lasing (98%R)Fig. 8. Temporal behavior of XeCl lasing with Brewster angle windows and 44-cm cavity separation. (a) voltage and fluorescence, ( b ) voltage and lasing with 40%-R out- put coupler, and (c) lasing with 98%-R output coupler.
capacitance of the Blumlein is higher than that used in the gain measurements, the gain and loss coefficients, the ability to create a long pulse via manipulation of the cavity Q, and the estimated 3-11.5 time it takes the laser to reach saturation create the following picture of the temporal devel- opment of the laser. We note that the laser does not turn on until the fluorescence has peaked. This means that absorptions or discharge index disturbances are so severe that lasing cannot proceed until the energy disposition is over. Subsequently, lasing commences with the medium showing substantial losses. We interpret the post energy deposition and lasing period
i~
be
'&as-
acterized by large, but exponentially decaying absorptions. Subsequent to this time there is very little absorption in the medium as charac- t e r i ~ e d ~ b y the ability to lengthen the laser pulse width by manipulation of the cavity Q.
The operation of a miniature high-repetition rate rare-gas halide laser is described. The tech- niques of a saturating inducator and corona pre- ionization is employed which allows full charging of a thyratron triggered Blumlein circuit using only passive elements and which eliminates the necessity of delayed double-pulsing between the preionization discharge and the main discharge. Laser performance is observed to increase with increasing pressure, charging voltage and flow rate and to decrease minimally with decreasing capacitance on the Blumlein. At 1 kHz pulse repetition frequency over 1 mJ per pulse at KrF and 0.5 mJ per pulse a t XeCl wavelengths are obtained. The net small signal gains were meas- ured to be as high as 0.3 cm-' in XeCl. The wall-plug efficiencies are in the region of 0.25 to 0.12 percent.
RESUME
On decrit le fonctionnement d'un laser minia- ture de taux de repetition haute de halogenes du gaz rare. On emploie les mkthodes d'une induction de saturation et de preionisation 5 corona qui permettent le chargement complet d'un circuit de Blumlein d6clench6 par thyratron en utilisant seulement des 616ments passifs qui Climinent la nkcessitk de l'impulsion double retard6e entre la
d8cbare;e
de
pr6ionisation et d6charge princi- pale.Blumlein. On obtient une frgquence de rkp6tition des impulsions de 1 kHz sur 1 mJ par impulsion 2 KrF et 0.5 mJ par impulsion aux longueur dlonde de XeCl. On a mesure des gains globaux de signaux faibles qui sont aussi eleves que 0.3 cm-' dans XeC1. Les efficacitks de prises de courant sont dans la region de 0.25 i 0.12 pour cent.
References
1. Robert C . Sze and Peter B. Scott, "Mini- lasers for High-Repetition-Rate Rare-Gas Halide Oscillators,
"
1979 LASL Optics Confer- ence, Los Alamos, NM, May 1979, published in Los Alamos Conference on Optics '79, SPIE Vol. 190, pp. 305-310, Ed. Don Liebenberg (1979); and IEEE/OSA CLEA conferince, Washington, DC,
May 1979, (post-deadline paper).2. Robert C. Sze, "High-Repetition Rate Rare- Gas Halide Minilasers
