Coherent light generators – Particular pumping means – Electrical
Reexamination Certificate
2000-09-08
2003-01-14
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular pumping means
Electrical
C372S088000
Reexamination Certificate
active
06507596
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gas laser apparatus, and more particularly to a gas laser apparatus with which a favorable beam profile can be obtained. It also relates to a gas laser apparatus with which the flow rate of the laser gas is increased and the output energy of the laser is more stable, which makes high-repetition operation possible.
2. Description of the Related Art
With an excimer laser, whole of a rare gas is pre-ionized in a main discharge space immediately prior to discharge excitation in order to obtain a uniform glow discharge throughout the rare gas in the main discharge space.
FIG. 4
is a schematic cross section of the overall structure of a conventional excimer laser apparatus featuring pre-ionization electrodes.
As shown in
FIG. 4
, an excimer laser mainly comprises at least one pair of main discharge electrodes
1
a
and
1
b
which constitute a main discharge space
3
by facing each other, a laser gas G that flows from outside the main discharge space
3
into the main discharge space
3
, is excited by discharge with the main discharge electrodes
1
a
and
1
b
, and flows from inside the main discharge space
3
to outside the main discharge space
3
, at least one pair of pre-ionization electrodes
2
a
and
2
b
provided on the gas inflow side and the gas outflow side of the main discharge space
3
so as to sandwich the main discharge space
3
for pre-ionizing the laser gas G by directing ultraviolet light from luminescent spots Ha and Hb located around the outer periphery toward the main discharge space
3
, a fan
40
for circulating the laser gas G, and a heat exchanger
41
for cooling the laser gas G flowing out of the main discharge space
3
.
The pre-ionization electrode
2
a
on the outflow side comprises a hollow cylindrical dielectric pipe Y
1
, a cylindrical internal electrode F
1
provided in the hollow center of the dielectric pipe Y
1
, and an external electrode (not shown) in contact with the outer periphery of the dielectric pipe Y
1
. Similarly, the pre-ionization electrode
2
b
on the inflow side comprises a hollow cylindrical dielectric pipe Y
2
disposed at the outer-periphery, a cylindrical internal electrode F
2
provided in the hollow center of the dielectric pipe Y
2
, and an external electrode (not shown) in contact with the outer periphery of the dielectric pipe Y
2
.
With the excimer laser shown in
FIG. 4
, the laser gas G is blown by the fan
40
in the direction L into the main discharge space
3
. After this, voltage is applied between the internal electrodes F
1
and F
2
and the external electrodes (not shown) of the pre-ionization electrodes
2
a
and
2
b
, ultraviolet light is directed toward the main discharge space
3
from luminescent spots Ha and Hb located on the outer periphery of the dielectric pipes Y
1
and Y
2
, and the laser gas G is pre-ionized. The pre-ionized laser gas G is excited by discharge with the main discharge electrodes
1
a
and
1
b
, and flows out of the main discharge space
3
in the direction R. As it flows in the direction R, the laser gas G is cooled by the heat exchanger
41
, after which it is again blown by the fan
40
in the direction L into the main discharge space
3
. Thus, with the excimer laser shown in
FIG. 4
, the laser gas G is circulated by the fan
40
, and pulse oscillation is performed at a high-repetition frequency.
The electron density of the laser gas G is different between the situations in which the gas flows into the main discharge space
3
and in which it flows out of the main discharge space
3
. This change in the electron density of the laser gas G inside the main discharge space
3
will be described through reference to
FIGS. 5
a
,
5
b
,
5
c
, and
5
d.
FIGS. 5
a
,
5
b
,
5
c
, and
5
d
are diagrams illustrating the transition in the electron density of the laser gas G within the main discharge space
3
.
First, as shown in
FIG. 5
a
, the laser gas G
1
is blown by the fan
40
in the direction L and into the main discharge space
3
. Voltage is then applied between the internal electrodes F
1
and F
2
and the external electrodes (not shown) of the pre-ionization electrodes
2
a
and
2
b
, ultraviolet light is directed toward the main discharge space
3
from luminescent spots Ha and Hb located on the outer periphery of the dielectric pipes Y
1
and Y
2
, and the laser gas G
1
is pre-ionized to an electron density of about 10
8
/cm
3
.
Next, the laser gas G
1
is excited by discharge with the main discharge electrodes
1
a
and
1
b
within the main discharge space
3
. The laser gas G
1
is ionized when subjected to discharge excitation, so the electron density rises, resulting in a laser gas G
2
with an electron density of about 10
14
/cm
3
(
FIG. 5
b
).
The electron density of the laser gas G
2
drops after discharge excitation, resulting in a laser gas G
3
with an electron density of about 10
11
/cm
3
. The electron density of the laser gas G
3
is higher than that of the laser gas G
1
. This laser gas G
3
flows out of the main discharge space
3
in the direction R (
FIG. 5
c
).
Next, the laser gas G
1
again flows in the direction L and into the main discharge space
3
. Meanwhile, the laser gas G
3
is blocked from flowing out by the pre-ionization electrode
2
a
located on the gas outflow side of the main discharge space
3
, and therefore remains for a time on the gas outflow side of the main discharge space
3
. The presence of the laser gas G
1
and the laser gas G
3
, which has a higher electron density than the laser gas G
1
, within the main discharge space
3
changes the distribution of the electron density of the laser gas within the main discharge space
3
. Voltage is then applied between the internal electrodes F
1
and F
2
and the external electrodes (not shown) of the pre-ionization electrodes
2
a
and
2
b
, ultraviolet light is directed toward the main discharge space
3
from luminescent spots Ha and Hb located on the outer periphery of the dielectric pipes Y
1
and Y
2
, and the laser gases G
1
and G
3
are pre-ionized (
FIG. 5
d
).
If the electron density of the laser gas here is high, the pre-ionization intensity of the laser gas will be raised, and the intensity of the laser light will also be higher.
Therefore, as shown in
FIG. 5
d
, if the laser gas G
3
, whose electron density is higher than that of the laser gas G
1
flowing into the main discharge space
3
, remains on the gas outflow side of the main discharge space
3
, the pre-ionization intensity of the laser gas G
3
will be stronger than the pre-ionization intensity of the laser gas G
1
, so the intensity of the laser light on the gas outflow side of the main discharge space
3
will be higher than the intensity of the laser light in the center of the main discharge space
3
.
FIG. 6
is a graph of beam profiles indicating the distribution of light intensity along the discharge width of the main discharge electrodes
1
a
and
1
b
. In
FIG. 6
, the center Xc of the discharge width corresponds to the center of the main discharge space
3
, the left side of the figure corresponds to the gas inflow side of the main discharge space
3
, and the right side corresponds to the gas outflow side of the main discharge space
3
.
As shown in
FIG. 6
, the original beam profile is the beam profile Pc, in which the center Xc of the discharge width is the maximum light intensity and which is symmetrical to the left and right with respect to the center Xc of the discharge width.
In the case of
FIG. 5
d
, however, the intensity of the laser light on the gas outflow side of the main discharge space
3
is stronger than the intensity of the laser light in the center of the main discharge space
3
, so the location of the maximum light intensity of the beam profile deviates from the center Xc of the discharge width to the location XR (the right side in the figure), as shown in FIG.
6
.
In other words, the beam profile in the case of
FIG. 5
d
is the beam profile PR, which is not symmetrical to th
Hori Tsukasa
Inoue Hirotoshi
Ishihara Takanobu
Matsunaga Takashi
Jr. Leon Scott
Komatsu Ltd.
Welsh & Katz Ltd.
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