Coherent light generators – Particular active media – Gas
Reexamination Certificate
1998-08-28
2001-02-27
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular active media
Gas
C372S057000, C372S098000
Reexamination Certificate
active
06195378
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to tangential fans, particularly tangential fans for producing gas flow in a gas laser chamber, and more particularly tangential fans for producing gas flow in excimer lasers and other pulse discharge lasers.
BACKGROUND
Transversely excited (TE) pulsed gas lasers commonly include a tangential fan to recirculate lasing gas inside a laser chamber.
FIGS. 1
a
and
1
b
are cross-sectional end and side views respectively showing the inner structure of a laser chamber
100
in a conventional TE excimer laser (see Akins et al., U.S. Pat. No. 4,959,840, issued Sep. 25, 1990, and incorporated herein by reference in its entirety). A laser enclosure
102
provides isolation between a laser chamber interior
105
and the exterior
110
. Typically enclosure
102
is formed by a pair of half enclosure members
112
and
114
(see
FIG. 1
a
), which are coupled together and sealed using an o-ring seal
116
, extending along a perimeter of enclosure
102
. Laser chamber interior
105
is filled to a predetermined pressure with a lasing gas
108
. A pulsed gas discharge is generated in a discharge region
122
by a high voltage pulse applied between a cathode assembly
118
and an anode assembly
120
. The pulsed gas discharge typically produces excited argon fluoride or krypton fluoride molecules, which generate laser pulse output energy. The pulse output energy propagates from discharge region
122
through an optical output window assembly
162
(see
FIG. 1
b
). Cathode assembly
118
and anode assembly
120
, defining discharge region
122
, extend parallel to one another along the length of laser chamber
100
.
Recirculation of lasing gas
108
is provided by a tangential fan
140
, which rotates about an axis
142
and includes a plurality of substantially parallel straight blade members
144
extending along the length of laser chamber
100
between hub members
146
. A typical rotation rate for current tangential fans is of the order of approximately 3800 revolutions per minute (rpm). As shown by arrows in
FIG. 1
a
, the flow of gas
108
is upward through tangential fan
140
and transversely across discharge region
122
as directed by a vane member
152
. Lasing gas
108
that has flowed through discharge region
122
becomes dissociated and heated considerably by the pulsed gas discharge. A gas-to-liquid heat exchanger
158
(not shown in
FIG. 1
b
) extending along the length of laser chamber
100
is positioned in the gas recirculation path to cool the heated gas. Other vane members, e.g. vane members
160
, direct the flow of gas
108
through heat exchanger
158
and elsewhere along the gas recirculation path. Recirculation cools and recombines lasing gas
108
, thereby allowing repetitively pulsed laser operation without replacing lasing gas
108
.
There are a variety of current issues relating to laser chamber
100
and its associated components, including, among other things, those described below.
The present tangential fan is difficult and expensive to fabricate. Blade members
144
and hub members
146
are individually stamped and formed from aluminum or another suitable alloy, such as an aluminum/bronze alloy, then dip brazed together to form tangential fan assembly
140
, using a braze material typically containing approximately 13 percent silicon by weight. This is a tedious and labor-intensive process. Because the brazed fan assembly has poor mechanical rigidity, post-machining can cause damage and warpage and is thus difficult or impractical. Therefore it is difficult to achieve precision alignment and critical tolerances. The brazed tangential fan assembly
140
is typically coated with electroless nickel.
Since lasing gas
108
is recirculated and reused, it is important to maintain cleanliness and to prevent contamination of the gas environment within laser chamber interior
105
, in order to maximize the pulse energy performance, stability, and working life of lasing gas
108
.
Undesirable vibrations in the rotating fan assembly adversely affect bearing life. Reduction of these vibrations will reduce bearing wear and allow the possibility of increasing the fan rotation speed for increased gas flow velocity. Particularly, adverse vibrations are associated with the low present natural vibrational frequency of the rotordynamic assembly, including the fan, bearings, shafts, and rotor. This low natural frequency is largely attributable to low first and subsequent bending mode frequencies of the fan, due to poor mechanical stiffness.
An aerodynamic buffeting effect has been observed, which, among other things, transmits vibrations to the fan bearings, causing bearing wear and premature failure. Measurements of the frequency of these vibrations suggest that they are caused by gas pressure fluctuations generated each time a fan blade member
144
passes in close proximity to the edge of anode assembly
120
. Of importance, the clearance between fan blade members
144
and the proximate edge of anode assembly
120
is particularly close, in order to minimize reverse flow leakage and maximize gas flow efficiency. Previous attempts to reduce aerodynamic buffeting by reshaping the anode assembly have resulted in an undesirable reduction in gas flow velocity by approximately ten or more percent.
Many applications require a substantially constant laser pulse output energy. However, strong and undesirable fluctuations in pulse output energy have been observed. These fluctuations have been found to be particularly severe at high laser pulse repetition rates.
Accordingly, it would be desirable to fabricate a tangential fan assembly economically, such that the finished fan assembly has improved mechanically rigidity against vibrations. Additionally, it would be desirable to minimize or eliminate potential contaminants from the laser chamber. Further, it would be desirable to minimize or eliminate vibrations arising from aerodynamic buffeting, and to minimize or eliminate pulse output energy fluctuations in a TE pulsed gas laser, particularly at high laser pulse repetition rates.
SUMMARY
A gas laser apparatus includes a tangential fan, configured to recirculate a lasing gas mixture. Generally, in accordance with the invention, a blade member of the fan varies in circumferential position between a first end flange and a second end flange in a continuous fashion, wherein blade members are twisted in a substantially helical fashion about the rotation axis of the fan.
In some configurations, circumferentially adjacent blade members are spaced evenly circumferentially relative to one another. Some versions have an odd integral number of blade members around the circumference. In some versions the circumferential number of blade members is constant longitudinally between the end flanges, whereas in other versions the circumferential number of blade members is variable between the end flanges. In some configurations, the circumferential position of blade members varies monotonically between the two end flanges. In other configurations, the variation reverses direction circumferentially one or more times between the two end flanges.
The tangential fan can operate in the chamber of a transverse-excited excimer laser, more particularly a krypton fluoride or argon fluoride excimer laser, or of a fluorine (F
2
) molecular gas laser.
Blade members extend longitudinally between and adjacent the outside circumference of the end flanges. Typically, the blade members are stiffened by one or more transverse substantially annular hub members, parallel with and spaced between the end flanges.
A tangential fan in accordance with the invention can be made using a conventional method of brazing together individually stamped and formed blade members and hub members. Finishing processes typically include post-machining, electropolishing, and electroless nickel coating.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings.
REFERENCES:
patent: 495984
Arroyo Teresa M.
Cymer Inc.
Inzirillo Gioacchino
Ogonowsky, Esq. Brian D.
LandOfFree
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