Coherent light generators – Particular active media – Gas
Reexamination Certificate
2000-10-19
2004-06-29
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Gas
C372S082000, C372S086000, C372S087000
Reexamination Certificate
active
06757315
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an gas laser pumped by an electrical gas discharge, and particularly to a corona-type preionization device and technique for generating a stable pulsed gas discharge for pumping of an active medium of an excimer or molecular fluorine gas discharge laser.
2. Discussion of the Related Art
UV-preionization of an electrical discharge in a pulsed gas laser is typically realized by means of an array of spark gaps or by another source of UV-radiation (surface, barrier or corona gas discharges), disposed in the vicinity of at least one of the solid electrodes of the main discharge of the laser. Early on in the development of excimer lasers (e.g., KrF, ArF, XeCl, XeF, etc.), conventional pulsed electrical gas discharges typically used for pumping the active media exhibited a high degree of instability. The development of discharge instabilities causes the glow discharge, a precondition for laser emission, to have a short phase (e.g., having a typical duration from 10-100 ns) and to thus be terminated more quickly than is preferred. The desired way of generating a high quality gas discharge for use in excimer and molecular lasers, including the molecular fluorine (F
2
) laser, is to provide an intense, yet uniform preionization of the gas volume before the main gas discharge occurs.
One way of providing this preionization is by photo-ionizing the laser gas with UV-light emitted from an auxiliary gas discharge before the main gas discharge is switched on. Some known methods of preionizing high pressure gas lasers include x-ray, spark and corona-gap preionization. See R. S. Taylor and K. E. Leopold, Pre-preionization of a Long Optical Pulse Magnetic-Spiker Sustainer XeCl Laser, Rev. Sci. Instum. 65 (12), (December 1994). The spark method involves the use of spark gaps (ordinary or stabilized by a dielectric surface; see, e.g., U.S. patent application Ser. No. 09/532,276 which is assigned to the same assignee as the present application and is hereby incorporated by reference). The corona-gap method involves the use of pulsed corona-like discharges near a dielectric surface. Spark gap preionizers produce a periodic series of preionized volumes of laser gas along the elongated discharge area of the laser, resulting in some nonuniformity of the discharge. Thus, corona-type preionization is preferred in the present invention.
Areas of focus for design improvement of corona-gap preionizers include the geometry of the dielectric body, and the arrangement of the preionization electrodes. See U.S. Pat. No. 4,718,072 to Marchetti et al. (showing a grounded internal preionization electrode surrounded by a dielectric having a positive potential applied to its outer surface through contact with the positively biased main electrode); European Patent Application (published) EP 0 532 751 A1 (showing an internal preionization electrode surrounded by a dielectric buried in one of the main electrodes); U.S. Pat. No. 4,953,174 to Eldridge et al. (showing the dielectric surrounding an internal preionization electrode abutting with a main discharge electrode); see also R. Marchetti et al., A New Type of Corona-Discharge Preionization Source for Gas Lasers, J. Appl. Phys. 56 (11), (Dec. 1, 1984); U.S. Pat. No. 4,380,079 to Cohn et al.
Reconfiguration of external, electrical circuits is another area where corona-gap pre-ionizer design improvement efforts have been focused. See Taylor et al., citation above; U.S. Pat. No. 5,247,531 to Muller-Horsche (showing an excitation of preionization electrodes affected by the same high voltage source as the main discharge electrodes including a time delay inductance between them), U.S. Pat. No. 5,247,534 to Muller-Horsche (including flow bodies configured to facilitate laser gas flow and formed of material exhibiting secondary x-ray emission characteristics) and U.S. Pat. No. 5,247,535 to Muller-Horsche (disclosing electron emission from a heated cathode, wherein x-rays emitted as the electrons impinge upon a separate anode serve to preionize the laser gas volume).
U.S. Pat. No. 5,337,330 to Larson, hereinafter referred to as the '330 patent, describes the conventional corona-like preionization arrangement generally shown in
FIG. 1
a
. See also U.S. Pat. No. 5,247,391 to Gormley, and U.S. Pat. No. 4,953,174 to Eldridge et al. A discharge chamber having the preionization arrangement of
FIG. 1
a
includes a high voltage main electrode
1
and a grounded main electrode
2
. Each preionization unit includes one internal preionization electrode
3
a
located on one side of main discharge region
5
between the main discharge electrodes
1
,
2
. Each preionization unit includes a dielectric tube
3
b
of generally cylindrical shape surrounding the internal preionization electrode
3
a
. A preionization discharge (ultraviolet emission)
4
from the preionization electrodes
3
a
&
6
and dielectric tubes
3
b
causes a preionization of the volume of the main gas discharge. A pair of external preionization electrodes
6
of the preionization units comprise metal plates and are each directly connected to the nearby main discharge electrode
1
(e.g., the cathode at high potential).
FIG. 1
b
shows a conventional preionization unit setup wherein only one internal corona-discharge preionization electrode
3
a
is employed. See U.S. Pat. No. 4,240,044 to Fahlen et al.
In the case of the preionization unit of either of
FIG. 1
a
or
1
b
, energy stored in the dielectric tubes
3
b
during a preionization phase, will also be absorbed into the main discharge
5
. However, that added energy typically will not increase the laser output due to a high wave impedance of the dielectric tubes
3
b
. The tubes
3
b
act much like a charged transmission line in that this wave impedance is typically much higher than the impedance of the main gas discharge. The high wave impedance is caused by a distributed inductivity of each whole dielectric tube
3
b
(as a transmission line) and a concentrated inductivity at the point of electrical connection of the tubes
3
b
with the internal corona discharge electrodes
3
a.
The residual energy produces high voltage electrical oscillations between the capacitance of the dielectric tubes
3
b
of the preionization units and the main gas discharge volume. These high voltage oscillations are undesirable because they significantly reduce the ability of the dielectric tubes
3
b
of the preionization unit to resist direct high voltage breakdown and over-flashing near the open ends of the dielectric tubes
3
b
. Moreover, these oscillations deteriorate the quality of the main gas discharge
5
and thus hinder the operation of the laser, particularly during operation at a high repetition rate. Furthermore, the oscillations cause additional wear to the main gas discharge electrodes
1
,
2
and the internal corona discharge electrodes
3
a
, and also cause contamination and a reduced lifetime of the laser system.
FIG. 1
c
shows one technique described in the '330 patent for alleviating the high-voltage breakdown and over-flashing problems caused by these oscillations. That technique involves providing a preionization tube
7
with bushings
8
at opposite ends made from an identical material as the tube
7
and integral with the tube
7
. The tube
7
with the opposed bushings
8
is described as being machined from a single integral piece of material. U.S. Pat. Nos. 5,818,865 and 5,991,324 describe furtherances of the design described in the '330 patent. The manufacturing of the tubes described in the '330, '865 and '324 patents undesirably involves complexity and cost. Moreover, the high voltage oscillations continue to degrade the quality of the discharge and produce undesirable wear to the main gas discharge electrodes and the internal corona discharge electrodes, and also cause contamination and a reduced lifetime of the laser system, as discussed above.
FIGS. 1
d
and
1
e
illustrate another technique which is described at U.S. patent applicatio
Berger Vadim
Bragin Igor
Stamm Uwe
Tassy-Julien Ivan
Ip Paul
Lambda Physik AG
Rodriguez Armando
Stallman & Pollock LLP
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