Open waveguide excimer laser

A gas laser of the excimer type in which an open waveguide is employed, in which the excimer medium is moved to and through the open waveguide in a direction transverse to the optical axis. A discharge cell is defined by the upper and lower slabs of the open waveguide, and if desired, a microwave source is provided which supplies microwave energy to the discharge cell volume to excite the excimer medium and to initiate discharge.

REFERENCE TO CO-PENDING APPLICATIONS 
This application relates to the application of John L. Remo and Gerhard 
Schaefer, Ser. No. 279,674, filed Dec. 5, 1988, entitled Gas Laser With 
Discharge in 2-Dimensional Waveguide, and assigned to the same assignee as 
the present application. 
BACKGROUND OF THE INVENTION 
This invention relates to lasers and in particular to gas lasers of the 
excimer type. 
A gas laser is operated by creating a discharge within a gaseous medium 
within an optical resonator. The discharge causes an over population of 
upper energy levels within the medium, and subsequent transition to lower 
energy levels releases light at discrete frequencies within the resonator 
to maintain a lasing action. In a typical gas laser, a hollow tube 
confines the discharge between the mirrors of the optical resonator. 
Gas lasers often use hollow optical waveguides to excite the laser medium 
as well as to remove heat while still maintaining a good beam quality. 
Alternate reflection of the optical rays from opposing surfaces as the 
optical rays propagate down the waveguide enhances beam quality. Moreover, 
losses due to reflection rapidly increase at high angles of incidence. 
Thus the waveguide selectively discriminates against the oscillation of 
high-divergence optical modes. This further improves beam quality. 
However, the hollow waveguide retains heat and uses mainly the conductivity 
of the walls to remove heat by conduction. This results in a decrease of 
the extraction efficiency of the laser. 
Such devices also operate at gas pressures below one atmosphere and at 
relatively low excitation levels. At higher gas pressures and high RF 
power levels, inhomogeneities and instabilities develop in the gas 
discharge which limits the utility of the laser. The use of metallic 
electrodes in contact with the laser gas is undesirable with corrosive gas 
mixtures, especially gas mixtures of the type used with excimer lasers. 
In addition, microwave excitation of the gas medium within a hollow 
waveguide has been employed, in which the microwave excitation has been 
along the direction of the optical axis of the hollow waveguide and 
generally occupies the entire volume of the hollow waveguide. When used 
with a gaseous medium which occupies the entire volume, or a substantial 
portion of the volume of the hollow waveguide, it is difficult to adjust 
and maintain the position of the maximum amplitude of the standing E-field 
wave that is created. This decreases the efficiency that would otherwise 
be achievable. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the invention to provide a more efficient gas medium 
laser, particularly of the excimer type. 
It is a further object of the invention to provide a gas medium laser, 
particularly of the excimer type, which employs an open waveguide. 
According to a feature of the invention a gas laser, particularly of the 
excimer type, has an open waveguide, in which a discharge cell is 
provided, where the length of the discharge cell as measured along the 
direction of the optical axis of the open waveguide is greater than the 
width of the aforesaid cell. 
According to another feature of the invention gas flow means serve to move 
gas to and through the open waveguide in the region of the discharge cell. 
According to a still further feature of the invention a microwave 
excitation source serves to pump the laser medium and the microwave source 
delivers microwave energy to the region of the discharge cell. 
According to yet another feature of the invention suitable means serve for 
directing microwave energy to and through the aforesaid discharge cell to 
excite the laser medium in the region of the discharge cell. 
In one embodiment, the E-field is zero at the walls along the shorter side 
of the microwave chamber at the lowest order TE-mode. 
In accordance with the instant invention, an excimer laser is provided in 
which an open waveguide is employed. The open waveguide has top and bottom 
slabs which define the waveguide. The slabs are preferably made of a 
suitable dielectric material which is either chemically neutral with 
respect to the excimer gas medium employed, or suitable for use without 
substantial degradation of the slab or its dielectric properties. Examples 
of gas mixtures employed in excimer lasers are Xenon, Xenon-Chloride, 
Krypton-Fluoride and Argon-Fluoride, all of which are corrosive and all of 
which will destroy most metals in a short time. Dielectric materials 
useful as slab material to form the open waveguide upper and lower 
surfaces are well known in the art. However, examples of such materials 
are fused quartz for XeCl and other chloride gases, and alumina for 
flouride based gases. 
In accordance with another aspect of the invention, a discharge cell is 
contained within a section of the open waveguide assembly. The discharge 
cell comprises the upper and lower slabs of the waveguide, a gas flow 
chamber for moving the gas medium through the space between the upper and 
lower waveguides in a direction transverse to the optical axis, and a 
microwave chamber which intersects and is positioned so that the 
microwaves generated by a microwave generator are directed to and through 
the open waveguide in a direction which is both transverse to the optical 
axis, and at right angles to the direction of gas flow. 
These and other features of the invention are more particularly set forth 
in the claims. Other objects and advantages of the invention will become 
apparent from the following detailed description of the preferred 
embodiment of the invention when read in light of the following drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, an open waveguide 10 includes an upper slab 12 and a lower slab 
14. The open waveguide is open along sides 16 and 18, and its optical axis 
is denoted by the numeral 20. Conventional reflecting mirrors 22, 24 
appear at either end of the open waveguide along the optical axis for 
reasons and for purposes well known to the art. 
In this preferred embodiment, a discharge cell 26 is formed by the upper 
and lower slabs 12 and 14, and by the intersection of a gas flow chamber 
28 with the upper and lower slabs. For best results, the gas flow chamber 
is rectangular in shape with the longer side 30 thereof parallel to the 
surface of the slabs 12 and 14. 
Gas flow is provided to and through the discharge cell to continually 
change the gas medium within the discharge cell during operation of the 
laser. Gas flow is preferably provided in a direction transverse to the 
optical axis 20 so that the flow of gas is to and through the open 
waveguide only in the small region defined by the discharge cell. The 
remainder of the open waveguide receives no gas flow, and is employed to 
remove high divergence optical wavelengths to thus improve the quality of 
the laser beam, and to increase the extraction efficiency of the laser. 
The gas flow chamber appears more specifically shown in FIG. 2. The gas 
flow chamber 30 comprises a pair of side walls 32 and 34 connected by a 
pair of open braces 36 and 38. The gas flow chamber may be made of any 
conventional material which will contain the typical corrosive gas medium 
employed in an excimer laser. The gas medium is pumped through the gas 
chamber 30 to and through the discharge cell by conventional pumping means 
(not shown), and the gas medium moves through a plurality of openings 40 
provided in both side walls 32 and 34 of the gas chamber, thus permitting 
the excimer gas medium to move to and through the discharge cell. 
Conventional means for collecting the used gas medium are also employed 
(not shown). 
Side walls 36, 38 may be solid instead of braces, provided that the solid 
side walls have one of more openings therein to permit the laser beam to 
exit the discharge cell in an unobstructed manner. 
The use of microwaves to excite the laser gas medium is known. However, 
microwave excitation, typically used to initiate lasing action by the art, 
has provided the microwave energy to the typical hollow waveguide 
assemblies now commonly used in a direction parallel to the optical axis. 
The hollow waveguide itself has been employed as the cavity or chamber 
through which the microwaves travel. Losses are incurred along the hollow 
waveguide, excitation is difficult to achieve, the construction of the 
hollow waveguide is more complex than would otherwise be the case, and 
servicing and adjustment in the field are made more difficult. It is also 
more difficult to adjust and control the position of the maximum amplitude 
of the E-field in the hollow waveguide, and more difficult to isolate the 
volume of the gas plasma on which the excitation energy acts since it 
typically occupies a large volume of the hollow waveguide. 
The open waveguide structure of the present invention, in which a discharge 
cell is defined, permits the microwave excitation pulse to be concentrated 
within the discharge cell, which is a relatively small volume containing 
the gas medium. Accordingly, a microwave chamber 42 is provided which 
intersects the volume of the discharge cell. A conventional source of 
microwaves 44 is connected to the microwave chamber and a conventional 
impedance matching circuit is also employed (not shown). See FIG. 3. 
The microwave chamber is of the same shape and internal cross section 
dimensions of length and width as that of the discharge cell, 
substantially as shown in FIG. 1. The microwave excitation source is 
positioned to deliver microwave pulses normal to the surface of the upper 
and lower slabs 12 and 14, which then pass through the discharge cell 
volume, substantially as shown. For best results, the microwave discharge 
chamber 42 extends through the discharge cell 26 and exits the other side 
of thereof, thus providing further microwave channeling after the pulse 
exits the discharge chamber 26. 
Having a substantial portion of the microwave chamber outside the discharge 
cell 26 and above and below the open waveguide structure provides for 
easier assembly, and in particular allows easy and simple adjustment of 
the point at which the maximum amplitude of the E-field standing wave 
occurs. To adjust the point at which the E-field standing wave maximum 
amplitude occurs, a short 50 is provided downstream at a selected point 
after the microwaves have passed through the discharge cell. The short 
forces the E-field to go to zero at the location of the short 50. 
To adjust the standing wave, the short 50 is made movable along the 
microwave chamber. Conventional means, such as a slit in the side of the 
microwave chamber allow access to the short connection and its adjustment 
along the microwave chamber. 
Placing the microwave chamber such that the direction of microwave travel 
is normal to the slabs 12 and 14, makes the E-field of the microwave 
source parallel to the discharge walls of the discharge cell. In addition, 
for best results, the optical axis is parallel to the longer side of the 
microwave chamber and discharge cell. In this instance, the short is best 
positioned along the smaller dimension of the microwave chamber walls. The 
source of microwaves can be provided either by a X-band source or an 
anode-cathode system outside the microwave chamber, or any other suitable 
source. 
According to an embodiment of the invention, the microwave chamber extends 
through the slabs 12 and 14 and provides the walls of the discharge cell, 
instead of the gas flow chamber. In this instance, the microwave discharge 
chamber is provided with a grid like structure in the region of the 
discharge cell to permit the gas from the gas flow chamber to flow to and 
through the discharge cell. 
According to an embodiment of the invention, the microwave source has a 
rise time of from about 10 ns to about three of four time 10 ns. Also for 
best results, the location of the short is adjusted so that the maximum 
amplitude of the voltage standing wave occurs at the position of the 
discharge cell. 
The use of an open waveguide structure permits the construction of an easy 
to maintain and relatively simple excimer laser. In addition, good 
discrimination against high incidence beam angles to obtain a more 
coherent beam at the output ends of the waveguide is also obtained. Still 
further, the replacement of parts in use is simplified, along with 
servicing of the laser, when an open waveguide structure is employed. 
The use of an open waveguide structure is particularly well suited to the 
use of a microwave source for the initial excitation of the excimer gas 
medium. It is in addition well suited to the use of means for flowing gas 
through the waveguide, which aids in the removal of heat, and continually 
refreshes the gas medium, thus decreasing the waiting time between pulses. 
The use of a discharge cell in connection with excimer lasers provides 
particular benefits. First, the discharge cell is a defined area, which 
can be made rather small in comparison to the overall length of the open 
waveguide, thus confining the initiation of lasing action to a small 
region of the waveguide. 
In a typical laser, such as one that uses a hollow waveguide, the entire 
volume of the hollow waveguide is employed to move gas medium to and 
through the laser. More gas is required, its extraction efficiency is low, 
it is difficult to build and repair, and it is difficult to service and 
adjust in the field. In addition, microwave pumping is more difficult to 
achieve and more difficult to control and adjust to obtain peak 
efficiency. 
In using the discharge cell of the present invention, the open waveguide 
structure permits a small section of the open waveguide, as measured along 
the optical axis, to be employed for the purpose of concentrating the 
desired components to obtain effective and efficient lasing action. The 
gas flow chamber is a hollow chamber which, for best results, is inserted 
so that the gas flow is between the upper and lower slab in a direction 
transverse to the optical axis, and parallel to the surfaces of the upper 
and lower slabs. The gas flow chamber is hollow and is providing with an 
opening along the optical axis on two sides thereof to permit the laser 
beam to exit the chamber and to travel along the optical axis of the 
waveguide. 
In one embodiment of the invention the gas flow chamber is a complete unit 
in and of itself and fits within the volume defined by the upper and lower 
slabs, or alternatively, the upper and lower slabs define the upper and 
lower regions of the gas flow chamber in the area bounded by the upper and 
lower open waveguide slabs. In this manner, the "active" section of the 
open waveguide is confined to relatively small section of the waveguide. 
The remainder of the waveguide located outside the discharge cell area is 
employed as intended, namely as a conduit for the beam, its enhancement, 
the removal of heat, and the selective control and removal of high 
divergence optical wavelengths, thus increasing the output power of the 
beam. 
In this invention, excitation means, comprising a microwave energy source, 
are positioned to deliver microwave energy to pump the laser medium in the 
region of the discharge cell. In one embodiment, the excitation means 
includes a microwave chamber or cavity which has an internal cross section 
of substantially the same dimensions as the discharge cell. Preferably, 
the microwave chamber or cavity is positioned to deliver microwave energy 
and to irradiate the entire cross section of the flowing gas medium, thus 
confining the action of the microwave energy to the small volume of the 
discharge cell. 
In one embodiment, the microwave chamber is positioned so that it 
intersects the upper and lower slabs of the open waveguide assembly. In 
this manner, the microwave pumping energy can pass directly through the 
discharge cell and excite the gas medium flowing therein. And, because a 
substantial section of the microwave chamber is positioned external to the 
discharge cell, and external to the open waveguide structure, it is 
relatively easy to build, to adjust, and to service in the field. 
In one embodiment, the length of the discharge cell, defined as the 
distance measure along the optical axis, is greater than the width, herein 
defined as the distance measured along the surface of the open waveguide 
slabs transverse to the optical axis. A conventional E-field is supplied, 
and a short is placed in the microwave chamber and is positioned on the 
downstream side of the open waveguide with reference to the direction of 
propagation of the microwaves. The short achieves a high overvoltage for 
the initiation of gas discharge and enhances the effectiveness of the 
excimer laser. The location of the short along the microwave chamber is 
made adjustable so that the maximum amplitude of the voltage standing wave 
occurs at the position of the discharge cell. 
Other and further modifications may be made to the invention as described 
hereinabove, and to the embodiment shown and described herein. It is 
intended to cover all such modifications which fall within the spirit and 
scope of the invention as defined in the claims appended hereto.