CA1255381A - High performance laser and method of making same - Google Patents

Abstract

ABSTRACT OF THE INVENTION
Improved electrodes for a laser include hollow, shell-like members formed of material, such as glass, whose ther-mal coefficient closely matches that of the laser body.
Thin film metallic layers coat the interiors of the elec-trodes assuring device performance and superior thermal qualities. Field assist bonding of the electrodes to the laser body produces an assembly of increased performance quality that is readily amenable to advantageous manufactur-ing processes.

Description

~2~;iS~

HIGH PERFORMANCE LASER AND METHOD OF MAKING SAME

',:
'~' Field of the Invention :~ - The present invention pertains to improvements in the laser arts. ~ore particularly, this invention relates to an ~ .
. improved laser including an improved electrode and method of attachment thereof to a laser body.
~, .::

.

. :. -~25~3~1 Description of the Prior Art In the lasing process, the laser electrodes, anode and cathode, interact to provide a flow of current through the lasing gases, exciting these gases to the higher energy states required for lasing action. Often the electrodes are situated near the ends of channels within a laser body containing appropriate gases such as helium and neon.
The cathode i5 commonly of a generally dome-like metallic configuration whereas a functional metallic anode may take a variety of ~orms, including dome or disk shaped, although its operation is less sensitive to shape than the cathode. In operation, the cathode is maintained at a negative potential bombarded by positively-charged helium and neon ions, to act as an electron emitter while the anode maintained at a pGsitive potential serves as an electron collector.
A conventional laser application, such as a ring laser gyroscope includes highly polished mirrors situated at opposed ends of the laser body. When such a laser is employed as an element of an instrumentation system only a relatively small amount of variation in the distance between the mirrors is tolerable as this distance is critical to resulting laser output. The maintenance of a preselected distance, within tolerance, poses a difficult technical problem when the laser is operated in a relatively extreme thermal environment. To combat this problem, the laser body is commonly ,~

MLS/lcm .

,` :.

;3~3~l 3 fabricated of material of extremely low thermal coefficient, including various glass ceramics such as those known by the trademarks "Zerodur" and "Cer-Vit". The cathode and anode, on the other hand, include a metal conductor to provide a flow of current through the lasing gases.

Currently, metal or metal alloy laser electrodes are produced by a number of recognized methods including stamp-ing and machining. Such methods require extensive cleaning and preparation of the internal surfaces. Additionally, in some applications the electrodes must be sealed to the laser body. Thus, a glass-to-metal seal is commonly effected in ~ accordance with the differing compositions of the electrodes ; and the laser body. Indium is commonly employed as a seal-ing agent. Such an indium seal is disclosed in United States Patent Number 4,273,282 of Norvell, et al. ~or "Glass-or-Ceramic-to-Metal Seals".
':
While electrodes of metal or metal alloy will provide the necessary electrical contact from the exterior to the i interior of the laser and hence provide a means for passing the current for the lasing process, the degree of expansion :
they experience under thermal stress, while not degrading to ; the short-term operation of the laser, effects its long-term integrity. The large disparity in thermal expansion coefficients between the metal electrode and the glass ceramic laser body introduces substantial stresses into such :- .
- , - .

:
-~L2~ 4 a system. The mismatch in the coefficients of thermal expansion of aluminum and Zerodur, for example, limits the life expectancy of a seal between such an electrode and laser body when cycled, for example, between -55 degrees Centigrade and 125 degrees Centigrade. Thus, the aluminum-to glass seal, commonly including indium, is limited by indium's melting temperature of 156 degrees Centigrade.

The stress introduced into a thermally stressed system including glass ceramic laser body joined to a metallic electrode may result in distortion of the laser body by a small amount. This distortion or bending may severely de-grade the performance characteristics of the laser in such applications as, for instance, the ring laser gyroscope. In addition to the physical distortion r the relative movement and cold flow of the indium sealant at the laser body-e~lec-trode interface will lead to eventual seal failure. Al-though "hard" glass seals exist, they are unsuitable in light of the stress-caused differential thermal expansions.
Such stresses can actually rupture the glass laser body.
:~;

The foregoing problems interact to limit the effective-~`ness and appropriate methods of manufacture of lasers for applications, such as ring laser gyroscopes, wherein freedom from contaminants is essential for optimum production qual-ity and instrument performance. In the manufacture of such :; '' ", ~ .r . . -~ . ~ ' '' . ".,; . " '' , 53~

a precision apparatus, heat is commonly utili~ed to liberate volatile materiAls (such as water, alcohols and plastics).

Upon as3embly of the ring laser gyro apparatus, including la~er body, mirror~ and electrodes, the instrument i~
placed upon a fill stand and the assembly baked to liberate unde~ired contaminants, This baking process, and the re~ultant purity of the laser, are limited in effectiveness by the 156 degree Centrigrade mel~ing point of the indium seal. (Otherwi~e, the a~sembly could be baked at an approximately 100 degree Centigrade higher temperature, limited by the capacity of the mirrors of the assembly). Thus, in addition to the harmful effects of mismatching of thermal stresses, the conventional laser ~: assembly that include~ a metallic electrode and ceramic dielectric Z la~er body of miamatched thermal expansion coefficient joined by Z~ an indium seal is limited in effectivene~s of operation and ease ~ of manufacture.
j :~

-~ SUMMARY OF THE INVENTION

The present invention overcomes the aforesaid ~; shortcomings of the prior art by providing in a la~er of the type 1ncluding a dielectric body of preselected thermal expansion characteri~tic material and at lea~t one electrode fixed thereto by the method of field-assist bonding, the improvement being the electrode comprising a preselected dielectric material having a i~ ' ,. ' ~ ~ - 5 -: j ,~ .
kh/jj f 1 , . i .
, . ...... .

. . - .
... .

.. ... ..
Z ` . , : ~ ~

3~

thermal expansion characteriotic that closely matches the laser body and having a metallic coating.

In a further aopect, the invention provides an improved method for manufacturing a ring laser gyroscope. The improved method comprise~ the step~ of fabricating a la~er electrode in part of preselected dielectric material having a thermal expansion characteristic that clo~ely matches the laser body. Thereafter, the electrode i~ field as~ist bonded to the laser body. Finally, the body is baked, with the electrode fixed thereto, at a temperature in e~ce~e of 156 degrees Centigrade.

The foregoing and additional advantage~ and features of the invention will become apparent from the detailed de~cription which follow~. In the detailed deecription, reference i~ made to numeral~ indicating features of the invention in accompanying :`
figuree, like numerals referring to like features throughout.

~` BRIEF DESCRIPTION OF THE DRA~ING
: ~':
~ The Figure is a cro3s-sectional view of a la~er in ;~ accordance with the invention.
:~'''; ' ., .
, .
~:~

:::
::
~ k / ~

q~.
' ;,: ~ ~ ' :

,: : ':
.. :

~ 7 DETAII.ED DESCRIPTION

Turning now to the Figure, there is shown a side sec-tional view of a laser 10 in accordance with the invention.
The laser 10 includes a laser body 12, prefexably formed of a ceramic glass such as Cer-Vit or Zerodur. A lasing cavity 14 resides within the laser body 12 having highly polished mirrors 16, 18 at its opposite ends. An anode 20 and a cathode 22 communicate ~lth upright bores 24 and 26 that féed the lasing cavity 14.

A
The cathode 22 is generally-hemispherical, comprising an outer shell 28 of glass, fused silica or glass-ceramic that includes a thin film layer 30 of aluminum or an alloy of aluminum at its interior. The shell 28 may be fabricated by any number of methods well-known in the glass and quartz forming arts including giass blowing and molding techniques.
Additionally, the shell 28 can be machined from a glass ce-ramic such as Zerodur, Cer-Vit or the doped glass known by the trademark "ULE". Appropriate techniques for coating the interior surface of the shell 28 to the form layer 30 in-.
clude vacuum deposition, sputter coating an ion plating ofaluminum or aluminum alloys. A cross-sectional view of the anode 20 would dlsclose a substantially identical configu-ration therefor. In the instance of the anode, copper or copper alloy may form the thin film layer. Many other met-als are suitable including nickel, chromium, iron, titanium, tungsten, aluminum and gold.

~ ~ . .................................. .
' " :.

';' ' .:' ~ 8 The inventor has found that, by employing electrodes including a shell having a coefficient of thermal expansion that closely matches that of the laser body 12, the stresses exerted upon the seals that secure the electrodes to the laser body are greatly reduced both the performance and the life of the laser are thus enhanced. He has further found that suitable metallic thin film layers do not possess suf-ficient mass to impose significant stresses upon the seal;
thus, as long as the metallic layer is sufficiently thick to render the electrode uniformly conductive, the performance of the electrode is fully adequate and equivalent to that of an electrode solely of metal or metal alloy.

':
The seal 32 is preferably formed in a field-assisted :
bonding process, such as that known as a Mallory process.
In such a process, the glass electrode and laser body are heated to a temperature of 300 to 400 degrees Centigrade while a voltage potential is applied between the electrode and the laser body. As the assembly is heated, its electri-cal conductivity increases, allowing electrical currént to ~;
~ flow through the electrode-laser body interface. The cur-~:.
rent causes diffusion of the metal from the thin film layer ~ into the glass. As a result, a strong, permanent bond is ;`~ formed that is not subject to certain failure modes that characterlze conventional glass-to-metal bonds including, 'ff :; :

, . :. ': .'` ~ . ':
', ' ~
~: ' , , . :., '' : ,, ,. ~ ': ' :':
: ` ' .

. . .
~ .. .. ` :

.' ,.

~ 9 for example, those deriving from the melting temperature of indium.

The closely matched thermal characteristics of the laser body 12, anode 20 and cathode 22 permit the use of field assisted bonding processes. Such processes result in bonds of greatly enhanced strength (thousands of p.s.i. as contrasted with indium seal strength in the hundreds of p.s.i.). As previously mentioned, the very strength of such bond can permit the transmission of destructive thermal stresses between a laser body and an electrode of differing thermal character.

When the closely matched laser body and electrode are joined ky a field assist bonding process, the resultant as-sembly in the instance of a ring laser gyroscope, is amenable to highly advantageous manufacturing processes that improve the quality and performance of the resultant instru-ment dramatically. The removal of the constraints due to thermal expansion mismatch and the relatively low melting point of the indium seal permits the assembly (including electrodes fused thereto) to be baked, in a low pressure en-vironment, at a temperature approximately 100 Centigrade de-grees higher than that of the melting point of indium. (In the instance of a ring laser gyroscope, bakeout of the in-strument on the fill stand would thus be limited by the mir-rors of the assembly to approximately 250 degrees Centigrade ~ ' , , . ~

, , ~ :, :.' ''' ' :' '.

~ ii3~

as opposed to the indium melting point of approximately 150 ~ degrees Centigrade).

:
A highly desirable result of the increased bakeout tem-perature is its effect upon the vacuum environment. A 100 degree Centigrade increase in bakeout temperature increases material vapor pressures by more than two decades, a greater-than-one-hundred-fold increase. Since the cleaning of the assembly is a function of the differential between ~; vapor pressure and that of the surrounding environment, it ; follows that one hundred times less pumping time is required to attain a given level of cleanliness. ~s a result, the manufacture of a laser in accordance with the invention is less expensive and its performance quality and useful life-time are increased.
: : -Thus, it is seen that improved methods and apparatus ~- have been brought to the laser fabrication art by the pre-sent invention. By employing the teachings of this inven~
tion, one may provide laser apparatus of increased durabil-ity for use in thermal environments that would otherwise severely degrade performance capability. Further, by em-ploying the teachings of the invention, one may employ ad-vantageous bonding processes not applicable to the prior art in achieving the aforesaid results.
;
:j ,:

i ~: ;: :, ' ~ : ' '. ' ~ ~

.: . : .::
:. .: ~.
~:: : .

Such bonding processes, in conjunction with the config-uration of the laser electrodes, provide a laser assembl~ of increased quality at decreased costs of manufacture.

~.
: :

`; :
~ ~ ,.: ., ~ , .
:'~: ., `

, :::

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a laser of the type including a dielectric body of preselected thermal expansion characteristic material and at least one electrode-fixed thereto by the method of field-assist bonding, the improvement comprising:
a) said at least one electrode comprising preselected dielectric material having a thermal expansion characteristic that closely matches said body; and b) said at least one electrode having a metallic coating.

2. A laser as defined in Claim 1 wherein:
a) said electrode comprises a hollow, substantially-hemispherical shape; and b) said metallic coating is located at the interior of said substantially-hemispherical shape.

3. A laser as defined in Claim 2 wherein said at least one electrode comprises both a cathode and an least one anode.

4. In a method for manufacturing a ring laser gyroscope including the steps of preparing a laser body of preselected thermal characteristic material and fixing at least one electrode thereto, the improvement comprising the steps of:
a) fabricating said electrode in part of preselected dielectric material having a thermal expansion characteristic that closely matches said body; then b) field-assist bonding said electrode to said body;
and then c) baking said body with said electrode fixed thereto at a temperature in excess of 156 degrees Centigrade.

5. A method as defined in Claim 4 including the step of coating said electrode with metal.