Shock wave dissipating laser chamber

Details Shock wave dissipating laser chamber Shock wave dissipating laser chamber

1998-10-09

2001-04-03

Scott, Jr., Leon (Department: 2881)

Coherent light generators

Particular operating compensation means

C372S092000, C372S058000, C372S104000

Reexamination Certificate

active

06212211

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a laser chamber, such as that used with a pulsed energy laser, and in particular the present invention relates to laser chambers having shock wave dissipation properties.
BACKGROUND
Pulsed laser systems, such as excimer lasers, are well known.
FIG. 1
is a side view of a laser chamber
10
used in a pulsed laser system. Laser chamber
10
includes an electrode structure
12
, a blower
14
, windows
16
,
18
, a laser beam
20
. Between electrode structure
12
is the laser discharge region
24
.
FIG. 2
is a front view of laser chamber
10
. As shown in
FIG. 2
, laser chamber
10
additionally includes heat exchanger
26
, a pre-ionizer
28
, baffles
30
and a current return
32
, which is used to connect the lower of electrodes
12
to ground.
As well known by those skilled in the art, a pulsed laser system, such as an excimer laser, produces high energy, high frequency pulses in a gas that is between electrodes
12
in laser chamber
10
. The gas, which may contain krypton and fluorine, is maintained at high pressure, for example 3 atm. Pre-ionizer
28
first floods the gas within discharge area
24
with free electrons (10
6
to 10
7
per cm
3
). Once the gas within discharge area
24
is conditioned with a sufficiently increased electron density, electrodes
12
produce a high energy discharge, which may be for example 15-50 kV. The lasing action from the high energy discharge occurs within 100 nsec from the time of discharge.
The high energy discharge in discharge area
24
produces a large amount of local heating and pressure disturbances in the gas. The thermal and pressure disturbances change the index of refraction of the gas, which has a deleterious effect on the energy efficiency of the laser system. The high energy discharge of the gas does not affect the lasing action from the pulse that caused it because the lasing action occurs within a short amount of time after the high energy discharge, approximately 100 nsec. However, subsequent high energy discharges, which occur at a frequency of approximately 1 KHz, will be produced in the highly disturbed, thermally energetic gas unless the gas is circulated within laser chamber
10
. Thus, blower
14
is used to circulate the gas within laser chamber
10
. Heat exchanger
26
is placed in the path of the gas flow to cool the gas as it circulates. Typically, the gas in laser chamber
10
is circulated at a speed of 20-30 meters per second through discharge region
24
, however, this speed is dictated by the frequency of the pulsed laser system.
It is desirable for the circulating gas within laser chamber
10
to be as uniform and as stable as possible, i.e., thermally, optically, and kinetically stable, because a stable gas maximizes the energy output of the laser system. One cause of disturbance in the gas is shock waves generated from the high energy discharge from electrodes
12
. Shock waves from the high energy discharge are reflected by the walls of laser chamber
10
, as well as from heat exchanger
26
and other components, back into discharge area
24
where the shock waves interfere with the energy output of the pulsed laser system.
Another cause of disturbance in the gas that is circulating within laser chamber
10
is heat exchanger
26
. Although heat exchanger
26
is necessary to cool the thermally excited gas, heat exchanger
26
acts as a choke to the gas flow within laser chamber
10
. Consequently, blower
14
is required to overcome the impedance of heat exchanger
26
. Further, the position and configuration of heat exchanger
26
disturbs the uniformity of the circulating gas. Fins (not shown) on heat exchanger
26
are conventionally used to assist in heat exchange. However, fins, which are typically one inch high and 0.1 inch apart, further impede the flow of the circulating gas.
In addition, laser chamber
10
fails to circulate the entire volume of gas. The flow of the gas in laser chamber
10
is illustrated by arrows, as shown in FIG.
2
. Baffles
30
are used in conjunction with blower
14
to guide the gas flow around laser chamber
10
, nevertheless, there are typically dead areas within laser chamber
10
where the gas fails to circulate properly. For instance, laser chamber
10
, as shown in
FIG. 2
, has a dead area in the center of laser chamber
10
where the gas circulates in a small area, i.e., an eddy, and thus fails to circulate throughout laser chamber
10
.
SUMMARY
A laser chamber in accordance with an embodiment of the present invention redirects the shock waves away from the discharge area and into other areas of the laser chamber where the shock waves can be dissipated. In conventional systems, the walls of the laser chamber, the heat exchanger and/or other components within the laser chamber provide surfaces for shock waves to be deflected back into the discharge region, thereby disturbing the energy stability of subsequent pulses. Thus, a laser chamber that redirects the shock waves away from the discharge area to be dissipated elsewhere advantageously maintains stability of the gas within the discharge area during pulsing.
One embodiment of the laser chamber has an electrode structure that defines a laser discharge area, a blower that circulates gas within the laser chamber and a heat exchanger with a surface area that defines a passage for the gas circulating within the laser chamber. The circuitous path defined by the heat exchanger allows shock waves to be directed away from the discharge region and dissipated in other areas of the laser chamber. Further, the additional surface area of the heat exchanger efficiently cools the thermally excited gas. In some embodiments, the heat exchanger is curved to create an inner surface area defining a space, and an outer surface. A protrusion from the side wall of the laser chamber extends into the space defined by the heat exchanger thereby lengthening the passage for the circulating gas.
In another embodiment, the working volume of the laser chamber is increased through the addition of ancillary chambers. The ancillary chambers are fluidically coupled to the laser chamber and are positioned such that shock waves generated by high energy discharges of the electrodes propagate directly into the openings of the ancillary chambers. The shock waves may then dissipate within the ancillary chambers rather than being reflected back to the laser discharge area. Flow guides, such as blowers or flow vanes, may be included within the ancillary chambers. The flow guides within the ancillary chambers generate a circulation of gas within the ancillary chambers that supports the circulating gas within the laser chamber at the openings of the ancillary chambers. The flow guides (or other baffles) within the ancillary chambers also act to trap the shock waves within the ancillary chamber, allowing the shock waves to dissipate through the action of multiple reflections within the ancillary chambers. Thus, the gas flow within the laser chamber remains stable and uniform.


REFERENCES:
patent: 5924975 (1999-07-01), Goldowsky
patent: 5978405 (1999-11-01), Juhasz et al.

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