Slab laser with enhanced lifetime

A CO.sub.2 slab laser is disclosed having a pair of spaced apart electrodes defining a rectangular discharge region. RF energy is fed through the electrodes to excite the CO.sub.2 gas. A pair of mirrors are located adjacent the electrodes to define the resonant cavity. A recombinant surface is placed between the ends of the electrodes and the mirrors to quench oxidizing species generated by the discharge before they reach the mirrors. In this manner, the degradation of the mirrors is reduced so that the high power performance of the laser can be maintained. The recombinant surfaces can be defined by forming extension regions at the end of the electrodes between which the discharge is minimized. Alternatively, a mirror shield having a beam transmitting aperture can be used to quench the oxidizing species.

TECHNICAL FIELD 
The subject invention relates to an approach for preventing mirror 
degradation in a slab gas laser thereby maintaining maximum output power 
over a longer period of time. 
BACKGROUND OF THE INVENTION 
There has been considerable recent interest in developing CO.sub.2 slab 
lasers. These lasers have been shown to generate high output powers in an 
efficient manner. Prior art references discussing such slab lasers include 
"Radio-frequency excited Stripline CO and CO.sub.2 lasers," Gabai, 
Hertzberg and Yatsiv, Abstract presented at Conference on Lasers and 
Electro-optics, June 1984; U.S. Pat. No. 4,719,639, issued Jan. 12, 1988 
to Tulip; U.S. Pat. No. 4,939,738 issued Jul. 3, 1990 to Opower and U.S. 
Pat. No. 5,048,048 issued Sep. 10, 1991 to Nishimae. 
The assignee herein has developed a CO.sub.2 slab laser which is described 
in a copending patent application Ser. No. 07/596,788, filed Oct. 12, 1990 
now U.S. Pat. 5,38 herein by reference. The laser described therein 
includes a pair of spaced electrodes configured to define a rectangular 
discharge region. The lasing gas is excited by passing an RF current 
through the electrodes. A pair of mirrors are mounted at the ends of the 
electrodes to define the resonator. 
In the preferred embodiment of the latter laser, the spacing between the 
electrodes is selected so that light is guided between the surfaces of the 
electrodes. In the wider dimension, the light propagates in free space and 
is confined by the resonator mirrors. As described in the above cited 
application, to maximize performance, the resonator mirrors are spherical 
and selected to define a stable resonant cavity along the waveguide axis 
(the axis extending between the electrodes) and an unstable resonator 
perpendicular thereto (the free space axis). In addition, the spacing 
between the end of the electrodes and the mirrors is selected so that the 
radius of curvature of the wavefront of the laser beam in the waveguide 
axis at the mirror location matches the radius of curvature of the mirrors 
selected for the unstable resonator. Since the mirrors are spaced from the 
end of the electrodes, the gas discharge tends to extend out to the very 
ends of the electrodes. 
When this laser has been life tested at powers exceeding 150 Watts, the 
output power has begun to diminish after only a few hundred hours. This 
decrease in output power has been traced to the deterioration of the 
mirrors. As previously disclosed in the above cited application, mirror 
degradation at lower powers had been addressed by adding a very thin 
coating of thorium fluoride to the top surface of the mirror. Thorium 
fluoride is less reactive and helped to reduce the level of degradation of 
the mirrors. However, at higher powers, this protective overcoating has 
proved insufficient and the mirrors are still the primary lifetime 
limiting component of the laser. 
An analysis of mirrors which have shown degradation at higher powers 
indicates that the problem arises due to an initial oxidation of the 
mirror coating. The oxidized layer has a much higher absorption loss than 
the coating in its initial state. This added loss leads to a power decline 
in the laser. 
Oxidizing species are known to be generated in the gas discharge between 
the two electrodes. The species include oxygen atoms, ozone, excited 
oxygen molecules and possibly ions of either carbon monoxide, carbon 
dioxide or oxygen. Some of these species will escape the confines of the 
slab and begin diffusing away from the discharge region and towards the 
mirrors. Since the discharge region extends to the end of the electrodes, 
the possibility of the oxidizing species migrating to the mirrors is 
increased. 
Accordingly, it is an object of the subject invention to reduce the effects 
of the discharge on the resonator mirrors. 
It is a further object of the subject invention to control the amount of 
oxidizing species reaching the resonator mirrors. 
It is another object of the subject invention to enhance the lifetime and 
improve the performance of a slab laser. 
SUMMARY OF THE INVENTION 
In accordance with these and other objects, the subject invention addresses 
the problem of mirror degradation in slab laser. The slab laser includes a 
pair of planar electrodes which are spaced apart to define a rectangular 
discharge region. A lasing gas is excited by passing an RF current through 
the electrodes. A pair of mirror are located near the ends of the 
electrodes to define the resonator. 
In accordance with the subject invention, a recombinant surface is provided 
between the ends of the electrodes and the mirrors. The recombinant 
surface functions to quench the oxidizing species prior to reaching the 
mirrors. In this manner, the rate of oxidation of the mirrors is reduced 
so that absorption loss is minimized and the maximum power output can be 
maintained over a longer period of time. 
Several examples of the type of reaction which can take place at the 
recombinant surface are indicated below. In these examples, the 
recombinant surface is noted as R. Two of the possible recombinant 
reactions of oxygen atoms, O, involve the formation of oxygen molecules, 
O.sub.2, and carbon dioxide molecules, CO.sub.2, as indicated below: 
EQU O+O+R.fwdarw.O.sub.2 +R+energy 
EQU CO+O+R.fwdarw.CO.sub.2 +R+energy 
A possible reaction of an excited oxygen molecule, O.sub.2 *, leads to the 
formation of an oxygen molecule in the ground state, O.sub.2, as 
indicated: 
EQU O.sub.2 *+R.fwdarw.O.sub.2 +R+energy (3) 
A possible ion reaction involving an oxygen ion, O.sub.2, leads again to an 
oxygen molecule in the ground state as one of the products: 
EQU O.sub.2.sup.- +R.fwdarw.O.sub.2 +R+energy (4) 
In all of these reactions the critical role of the recombinant surface is 
to carry away the energy change of the reaction and thus make possible the 
elimination of the reactive species. In the case of ionic reactions, it 
can also conduct away the charge carried by the ionic reactant. 
The recombinant surface can be configured in a variety of ways. In one 
approach, a region at the end of the electrodes is provided where no 
discharge is present. Since there is no discharge in this region, the 
surfaces can function to recombine the oxidizing species. The rate of 
recombination at this surface is significantly greater than the 
deactivation rate which occurs simply through gas to gas collisions. The 
recombinant surface at the end of the electrodes can be configured by 
forming a recessed shelf directly into the electrodes themselves. 
Alternatively, ceramic extension members can be affixed to the ends of the 
electrodes. 
Another approach for achieving this goal is to provide the recombinant 
surface in the form of a mirror shield. In the preferred embodiment, the 
mirror shield has a planar configuration and includes an aperture for 
transmitting the beam. The shield can be formed from either metal or 
ceramic. The shield can be supported from either the mirror mount or the 
end of an electrode. 
Further objects and advantages of the subject invention will become 
apparent from the following detailed description taken in conjunction with 
the drawings in which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning to FIG. 1, there is illustrated a slab laser 10 of the type found 
in the prior art. Additional details concerning a preferred embodiment of 
a CO.sub.2 slab laser can be found in the above cited copending 
application Ser. No. 07/596,788. Briefly, laser 10 includes a pair of 
elongated, planar electrodes 12 and 14 formed from a conductive metal such 
as aluminum. The electrodes are supported in spaced apart relationship to 
define a discharge region 16 having a rectangular cross section. A pair of 
mirrors 18 are disposed adjacent the ends of the electrodes to define the 
resonant cavity. 
As described in the above cited application, the mirrors and electrodes are 
located within a sealed housing (not shown). A lasing gas, for example 
CO.sub.2, is sealed within the housing. The gas is excited by supplying RF 
power from a generator 20 to the electrodes. An impedance matching network 
22 is used to match the RF generator to the excited discharge. 
In the preferred embodiment of the subject laser, the spacing between the 
electrodes 12, 14 is selected so that light will be reflected or guided 
off of the surfaces in the axis extending between the electrodes. The 
spacing is preferably on the order of 1.5 to 2.5 mm. The width of the 
electrodes is selected so that the light in the axis perpendicular to the 
waveguide axis will travel in free space. Applicants have tested 
electrodes of various widths ranging from 20 mm to 120 mm. The propagation 
of the beam will be controlled by the mirror 18 in the free space axis. 
As noted above, in the structure illustrated in FIG. 1, the mirror surfaces 
become significantly degraded over time when the laser is operated at high 
power. As the mirrors degraded, absorption increased and the output power 
diminished. The degradation of the mirrors appears to have been caused by 
oxidizing species which escape the discharge region and migrate to the 
mirrors. It has been found that the degradation can be substantially 
retarded by providing a recombination surface between the ends of the 
electrodes and the mirrors. 
FIG. 2 illustrates a first approach for providing a recombination surface. 
As illustrated therein, the ends of each of the electrodes 12, 14 is 
provided with a recessed shelf 30. In one experimental arrangement, the 
depth D of the recess was 5.0 mm so that the total spacing between the 
surfaces was 5 mm. The length L of the recess was 10 mm. The opposed 
recessed shelves 30 function to significantly increase the spacing between 
the electrodes in this region. Because of the spacing increase, the extent 
of the gas discharge 16 in this region is substantially reduced. 
Accordingly, the planar surfaces of the shelves can act as a recombinant 
surface for quenching the oxidizing species before they reach the mirrors. 
In this embodiment as well as the others to be discussed below, it is 
intended that regions at both ends of the electrodes be modified as shown 
in the Figures. 
As disclosed in the prior application, in order to maximize performance, it 
is desirable to set the spacing between the electrodes and the mirrors so 
that the radius of curvature of the wavefront of the laser beam in the 
waveguide axis at the mirror location matches the radius of curvature of 
the mirror selected for the unstable resonator. This approach assumes that 
the beam is confined in the waveguide dimension as it exits the electrode 
structure. In the embodiment shown in FIG. 2, the waveguide structure 
terminates at point P and the beam then begins to propagate in free space 
in the region of the recessed shelves where the separation between the 
electrodes is increased. For this reason, when designing the resonator 
dimensions, the "end" of the electrode should be considered based on the 
location of point P rather than the physical end of the electrode. 
FIG. 3 illustrates an alternate embodiment where the recombinant surface is 
defined by a nonconductive insulator added as extension member 36 to the 
ends of the electrodes 12 and 14. Preferably, the extension member 36 is 
formed from a ceramic material. Since the RF energy will not be carried by 
the member 36, the discharge 16 will essentially terminate at the end of 
the aluminum electrodes. Accordingly, the members 36 will provide a 
recombinant surface for quenching the oxidizing species prior to reaching 
the mirrors. 
In the embodiment illustrated in FIG. 3, the spacing between the ceramic 
members 36 is the same as the spacing between the electrodes 12, 14. 
Therefore, the waveguide action will be maintained throughout the region 
of the members 36. Thus, in order to maximize performance, the mirror 
spacing should be based on the total length of the electrodes plus the 
members 36. 
The embodiment of FIG. 4 represents a combination of the approaches shown 
in FIGS. 2 and 3. More specifically, a pair of ceramic extension members 
40 are connected to the ends of the electrodes 12, 14. In this case, the 
thickness T of the members 40 is less than the thickness of the electrodes 
so that the composite structure has dimensions similar to the structure in 
FIG. 2. The combination of the ceramic members with the recessed shelf 
configuration further minimizes the amount to which the discharge 16 will 
extend past the end of the conductive electrodes. Members 40 provide the 
recombinant surface for quenching the oxidizing species. Like the 
embodiment in FIG. 2, since the waveguide action terminates where the 
shelf structure begins, the mirror spacing should be determined from the 
actual end of the conductive electrode. The approach shown in FIG. 3 may 
be preferable to that shown in FIG. 4 because it might minimize waveguide 
losses. 
FIG. 5 illustrates another embodiment of the subject invention. In this 
embodiment, a ceramic cover plate 50 is attached to the opposed inner 
surface of each of the electrodes 12 and 14. The ceramic plates have a 
thickness on the order of 2 mm. Each of the cover plates extends beyond 
the ends of the electrodes a distance D.sub.2 of 10 mm. The surfaces of 
the plates extending beyond the electrodes defines the recombinant 
surfaces. Since the separation between the plates remains constant to the 
end, the plates act as the waveguide structure and the mirror spacing 
should be calculated from the end of the plates 50. 
FIGS. 6 through 10 illustrate recombinant surfaces oriented in a plane 
perpendicular to the axes of the resonant cavity. In FIG. 6, the 
recombinant surface is defined by a planar mirror shield 60. The mirror 
shield 60 has a central aperture 62 which is aligned with the mirror 18 
and transmits the laser beam. The shield can be formed from metal or 
ceramic. In either case, the shield functions to quench the oxidizing 
species before reaching the mirror. 
FIG. 7 is a plan view of a mirror shield 60 which has been tested in a 
laser of the type described in the above identified application. FIGS. 8 
and 9 illustrate the mirror shield 60 mounted within such a laser. In 
these Figures, the electrodes 12 and 14 are mounted within a sealed 
housing 82. The end of the housing is sealed with an end cap 84. This end 
cap includes a mirror mount 86 for supporting mirror 18. The end of the 
laser which is illustrated is also the output coupler so an output window 
90 is also shown. Further details of the assembly can be found in the 
above identified application. 
In accordance with the subject invention, the mirror shield 60 is mounted 
to the mirror mount 86 by a pair of mounting screws 94. In the 
experimental embodiment, the mirror shield was formed from aluminum and 
the screws 94 were formed from stainless steel. In this manner, an 
electrical connection was established between the shield 60 and the 
grounded mirror mount 86. This approach is preferred to maximize the 
attraction of any ionized oxidized species to the shield. 
Shield 60 is rectangular and has generally planar surfaces. The beam 
transmitting aperture 62 is elongated and had a height of 5.1 mm and a 
width of 50 mm. The shield includes holes 98 for receiving mounting screws 
94. The detents shown in phantom line in FIG. 7 are provided to 
accommodate other mirror mounting hardware. The shield 60 was used in a 
laser where the electrodes had a width of 44 mm. The laser beam 106 is 
edge coupled out of the resonator, past the end of mirror 18 and through 
aperture 62. 
FIG. 10 illustrates an alternate embodiment utilizing a mirror shield 110 
having an aperture 112 for transmitting the beam. In this embodiment, the 
shield 110 is mounted to the end of the electrode 14 rather than to the 
mirror mount as in FIGS. 8 and 9. The connection to the electrode can be 
achieved in any suitable manner. In the embodiments shown in FIGS. 6 
through 10, the mirror spacing should be determined as in the basic 
structure shown in FIG. 1 since the waveguide effect ends at the end of 
the electrodes. 
A few of the approaches discussed above have been fabricated and tested. 
More specifically, the approaches shown in FIGS. 1 and 6 have been shown 
to be effective in reducing the degradation of the mirrors. Based on this 
testing, it appears that the approach shown in FIG. 6 will achieve more 
consistent results. 
FIG. 11 is a graph of life test data on slab lasers having about a 65 cm 
overall length. For these life tests, the laser devices were excited by a 
pulsed radio-frequency power supply operating at 81 MHz. The peak power 
was 5500 watts with pulse width of 400 to 500 .mu.s at frequencies of 1000 
Hz. The gas laser mix was in a ratio of 3:1:1 for helium, carbon dioxide, 
and nitrogen plus the addition of 5% xenon to a total initial pressure of 
80 torr. The laser resonator consisted of two high reflecting mirrors with 
radii of curvature of 693 mm for the high reflecting end and 619 mm for 
the output end of the tube. The output mirror was cut shorter so that a 
4.7 mm beam is allowed to diffract around this edge in a negative branch 
unstable resonator. The mirrors were separated by 656 mm while the 
electrodes used had an overall length of 616 mm and a width of 44 mm. The 
electrodes were separated by 1.9 mm. Each mirror consisted of a silicon 
substrate with an enhanced gold coating. The top layer of the dielectric 
stack was approximately tenth wave thickness of thorium tetrafluoride. The 
dielectric stack of zinc sulfide and thorium tetrafluoride was used to 
enhance the mirror reflectivity at 10.6 .mu.m to &gt;99.5%. 
Curves 120 and 122 represent laser tubes formed in accordance with the 
prior art. As can be seen, as the tubes are run at increasingly higher 
powers, the output power tends to drop off at increasingly faster rates. 
Curve 124 is a plot of a tube with mirror shields of the type shown in 
FIGS. 8 and 9. As can be seen, this tube has been run for almost 900 hours 
at an average power in excess of 200 Watts with almost no appreciable drop 
in that power. Testing on this tube is continuing. This preliminary data 
indicates that the shields are functioning as recombinant surfaces for 
quenching the oxidizing species before they reach the mirror. 
While the subject invention has been described with reference to the 
preferred embodiments, various changes and modifications could be made 
therein, by one skilled in the art, without varying from the scope and 
spirit of the subject invention as defined by the appended claims.