Laser cathode production process

A process for producing a laser cathode by vacuum coating provides well-controlled, slow layer growth in the presence of the laser gas at a total pressure of up to 5.times.10.sup.-4 mbar. Such process includes laser gas atoms in the cathode layer. Without interruption of the vacuum, the coated cathode is moved into a (second) position and laser gas is introduced under slight pressue against the cathode coating by means of a nozzle. At the same time, a number of discharge cycles are initiated via the electrically conductive base of the cathode basic element and an electrode, arranged concentrically with respect to the nozzle, having the effect of premature sputtering-off to cause a levelling of the current density and a further inclusion of inert gas atoms in the Al coating. After a number of discharge cycles, a small amount of oxygen may be added to form an Al.sub.2 O.sub.3 passivation layer before flooding of the recipient. The process substantially increases the operating reliability and long-term stability of laser amplifiers and oscillators.

BACKGROUND 
1. Field of the Invention 
This invention relates to a laser cathode production process. More 
particularly, this invention pertains to a process for producing a laser 
cathode of the type that includes a substantially spherical glass or 
glass-ceramic base of predetermined thermal properties with an aluminum or 
aluminum alloy interior coating. 
2. Description of the Prior Art 
Laser cathodes of the type that include a basic element of glass or 
glass-ceramic of domed shape and semispherical dome curvature are 
described in West German patent application No. DE-A1-33 46 232. Such 
cathodes are most commonly employed in laser amplifiers and oscillators. 
In operation, a negative potential is applied to the laser cathode and it 
is bombarded by the positively charged ions (e.g. He.sup.+ and Ne.sup.+) 
that combine with the electrons supplied to the (oxidized) inner surface 
of the cathode as a result of the negative potential. As a result, 
uncharged gas molecules are again formed. The impacts of the ions with the 
cathode's surface produces surface sputtering processes and absorption of 
light inert gases (e.g., He and Ne). Such processes reduce the service 
life of the cathode and, of course, that of the entire laser arrangement 
as such sputtering effects continually contaminate the laser gases. 
In prior coating processes, the cathode became naturally oxidized in an 
unreproducible way when the coating unit was opened after coating. 
Attempts were made to enhance cathode service life by post-oxidation of 
the applied Al layer (e.g., by introduction of pure oxygen and an 
oxygen/inert gas mixture). Such efforts have realized only limited 
success. The mirrors of the laser arrangement are adversely affected by 
such oxygen charging processes. 
SUMMARY AND OBJECTS OF THE INVENTION 
It is, therefore, an object of the present invention to provide a laser 
cathode production process that substantially increases the operating 
service life of laser arrangements equipped therewith. 
It is a further object of the invention to attain the preceding object 
without requiring subsequent oxygen treatment and thereby avoid 
accompanying undesirable secondary effects. 
The foregoing objects are accomplished by the invention that provides a 
process for producing a laser cathode of the type that includes a 
substantially semispherical basic element of glass or glass-ceramic 
material with a vacuum-deposited aluminum or aluminum-alloy interior 
coating. The process is begun by placing the cleaned basic element into 
the recipient of a vacuum unit and into an electrically conductive base 
that has an opening aligned with the interior of the cathode basic 
element. 
The pressure in the recipient is reduced so that the oxygen partial 
pressure is less than 2.times.10.sup.-9 mbar and the residual gas pressure 
does not exceed 2.times.10.sup.-7 mbar. The recipient is charged with at 
least one laser gas under slight pressure at the same time the aluminum or 
aluminum alloy coating is applied in a coating process that is 
controllable with regard to the layer growth rate, such coating covering a 
portion of the base to provide an electrically conductive base. The coated 
basic element is then brought into position over a gas inlet device in the 
recipient without interrupting the vacuum so that a laser gas mixture is 
admitted to the recipient up to a total pressure of a few mbar. Thereafter 
an electrical discharge is repeatedly struck to the coating contacted to 
the outside via the conductive base with an initiating electrode 
projecting into such recipient. 
The preceding and other features of this invention will become further 
apparent from the detailed description that follows. This description is 
accompanied by a set of drawing figures. Numerals of the drawing figures 
correspond to those of the written description, like numerals referring to 
like features throughout.

DETAILED DESCRIPTION 
Turning now to the drawings, FIG. 1 is a sectional diagrammatic 
representation of the coating process of the invention. A substantially 
semispherical basic element 1 of glass or glass-ceramic material having 
thermal properties that are particularly suitable for laser amplifier or 
oscillator operation with an internal diameter of, for example, 12.5 mm, 
is first cleaned by an appropriate process (e.g. ultrasonic, ozone or 
manual cleaning) and placed onto a mount or base 2 in the recipient of a 
vacuum unit (not shown). A through-opening 9 of the base 2 is aligned with 
the interior of the basic element 1. A mask cover 3 is provided on the 
underside with an opening 10, the diameter of which is greater than that 
of the through-opening 9. 
As indicated diagrammatically by 5, the inside of the basic element 1 is 
coated from below by electron beam vaporization or like known method, such 
as sputtering or thermal vaporization of Al or suitable Al alloy material. 
The arrangement of the basic element 1, the base 2 and the cover 3 and the 
sizes and alignments of the openings 9 and 10 are selected so that, apart 
from the inner surface of the basic element 1, a partial area of the base 
2 is also Al-coated. Continuous electrical connection is thereby 
established between the Al coating 4 of the laser cathode and that of the 
base 2. The base 2 necessarily consists of an electrically conductive 
material. The significance of such continuous electrical connection will 
become apparent below. 
According to the process of the invention, the pressure within the 
recipient is first reduced so that the oxygen partial pressure becomes, 
for example, less than 2.times.10.sup.-9 mbar and the residual gas 
pressure does not exceed 2.times.10.sup.-7 mbar. Thereafter, the recipient 
is flooded with at least one laser gas or gas mixture (e.g. He/Ne) under 
slight pressure to increase the total pressure to, for example, 
5.times.10.sup.-4 mbar. Then the Al or Al-alloy coating is applied at a 
low growth rate. During the slow layer growth atoms of a subsequent 
operating gas (e.g. inert gases such as argon, helium or neon) may be 
incorporated into the coating which cannot later diffuse into the layer as 
such layer is already saturated. 
The coating process used must be precisely controllable providing a coating 
deposition rate of below 1 nm/sec. Such slow growth ensures that a 
relatively high inert gas fraction is incorporated into the Al layer. The 
high gas background pressure achieves a high degree of layer uniformity 
over the inner surface of the basic element 1, and thus of the cathode, as 
well as a good edge coverage regardless of the coating method chosen. 
Total layer thickness should be about 150 nm (thinnest point). 
Measurements of the deposition rate and layer thickness may be performed 
in the region of the cathode by a measuring device that includes 
oscillating crystals. In the case of thermal vaporization, layer thickness 
may be simply determined from the mass of the material vaporized. 
After coating the base 2, the coated cathode 1, 4, and a cover 6 having a 
reduced aperture are brought into position over a nozzle 7 without 
interruption of the vacuum. Evacuation is carried out during repositioning 
to return to the residual gas pressure before coating to obtain an oxygen 
partial pressure which is as low as possible. Subsequently, a laser gas 
mixture is admitted into the recipient through the nozzle 7 until overall 
pressure of a few mbar is reached in the cathode and its surroundings. The 
nozzle 7 is formed of an electrically non-conductive material. An 
electrode 8 coated, for example, with precious metal is contained in 
concentric arrangement within the nozzle 7. 
An electrical discharge is struck by direct- or alternating-current voltage 
from outside through the current path electrode 8, the inert gas mixture, 
the Al coating, the base 2 and the electrical lead-throughs on the vacuum 
unit (not shown). Such electrical charge further charges the cathode layer 
4, the surface of which is not yet oxidized, with atoms of the inert gas 
and impresses the current necessary for subsequent operation. 
After repeated evacuation and striking cycles, sputtering-off of the Al 
atoms occurs at points of high current density that accompany the 
electrical discharge to smooth the surface until the operating current 
density becomes homogeneous. After a number of striking cycles, a small 
amount of oxygen may be added to the inert gas to occasion slow oxidation 
of the Al surface to Al.sub.2 O.sub.3. This operation is carried out 
carefully so that the impressed current is retained and the atoms of the 
inert gases included in the conductive layer remain to the maximum extent 
possible. The Al.sub.2 O.sub.3 layer formed on the surface provides a 
diffusion barrier and is distinguished by a substantially smaller 
sputtering-off rate than Al or an Al alloy. 
After attaining an adequate degree of oxygen passivation, it is desirable 
to perform additional striking cycles under conditions expected to occur 
during subsequent cathode use to ensure reliable striking. That concludes 
the production process. Cathode storage outside the vacuum unit should 
occur only in an inert gas atmosphere. 
Thus it is seen that the present invention provides a laser cathode 
production process that substantially increases the operating service life 
of a laser arrangement equipped therewith. The process does not require 
the subsequent oxygen treatments of the prior art that are known to 
produce undesirable secondary effects. 
While this invention has been described with reference to a 
presently-preferred embodiment, it is not so limited in scope. Rather, the 
scope of this invention is limited only insofar as defined by the 
following set of claims and includes all equivalents thereof.