CA1255380A - Laser cathode - Google Patents

Abstract

IMPROVED LASER CATHODE
ABSTRACT OF THE DISCLOSURE
An improved laser cathode includes a generally hemispherical hollow shell fabricated of material of preselected thermal character. The shell includes, at its inner surface, a covering layer of material which includes aluminum. A thermal seal is created between a laser body of low thermal coefficient material and the cathode by one of a number of conventional sealing processes. By substantially matching the thermal coefficient of the shell of the cathode to that of the laser body the overall laser structure is rendered less vulnerable to thermal cycling.

Description

i38~3 The present invention pertain~ to improvement3 in the laser art~ More particularly, this invention relates to an lmproved la~er including an improved cathode and method of attachment thereof to a la~er body.

DESCRIPTION OF ~HE PRIOR ART

A la~er cathode ~erve~ to ~upply electrons for the la~ing proce~s. Often such cathode i~ of generally dome-like configuration having an aluminum ~urface and ~ituated near the end of a channel within a la~er body containing appropriate ga~es such Q~ helium and neon. In operation, it is main-tained at a negative potential, bombarded by po~itively-charged helium and neon ion~
that combine with the electron~ supplied to the oxidi~ed ~urface of the cathode by rea~on of it~ negative potential to produce uncharged ga~ molecule3.

A conventlonal la~er application9 euch a~ a ring laser gyroscope include~ highly polished mirrors situated at opposed end~
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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 fabricated of material of extremely low tharmal coefficient, including various glass ceramics such as those known by the trademarks "Zerodur" and "Cer-Vit". The cathode, on the other hand, includes a metal conductor to function as a source of ; electrons. As mentioned above, aluminum has often been utilized for the laser cathode.
Currently, aluminum or aluminum alloy laser cathodes are produced by a number of recognized methods including stamping and machining. Such methods require extensive cleaning and preparation of the internal surface of the ~; cathode. Additionally, in some applications the cathode must be sealed to the laser body. Thus a glass-to-metal seal is commonly effected in accordance with the differing compositions of the cathode and the laser body. Indium is commonly employed as a sealing agent. Such an indium seal is disclosed in United ~; 20 States Patent Number 4,273,282 of Norvell, et al. for "Glass-or-Ceramic-to-Metal Seals".
While a cathode of aluminum or aluminum alloy will provide the necessary electrical contact from the exterior to the interior of the laser and hence provide a means for passing the current for the lasing process, the degree of expansion it experiences under thermal stress, while not degrading to the short-term operation of the laser, aEfects its long-term MLS/l~ 2 .~ .

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integrity The large disparity in thermal expansion coef~icient3 between the aluminum or other metal cathode and the gla3s ceramic laser body introduce3 ~ubstantial ~tre~3e3 into 3uch a ey3tem. The mi~match in the coefficients of thermal expan3ion of aluminum and Zerodur, for e~ample, limit~ the life e~pectancy of a 3eal between ~uch a cathode and laser body when cycled, for example, between -55 Centigrade and 125 Centigrade. The aluminum-to-gla~3 ~eal, commonly including indium, i8 limited by indium'3 melting temperature of 156 Centigrade.
The stres3 introduced into a thermally 3-tre~sed system including gla3s ceramic laser body joined to a metallic cathode may re~ult in di~tortion of the lacer body by a small amount. Thi3 di~tortion or bending may 3everely degrade the performance characteristic~ of the la3er in ~uch application3 a3, for in~-tance, the ring la~er eyroscope. In addition to the phy3ical di3tortion, the relative movement and cold flow of the indium sealant at the la3er body-cathode interface will lead to eventual ~eal failure.
Although 'hard' gla3~ eals exi3t, they are un~uitable in light of the stres~-cau~ed differential thermal expansions. Such stre~sec can actually rupture the glas3 la~er body, The foregoing problems interact to limit the effectivenea~ and appropriate method~ of manufacture of laRer~ for - application~, ~uch as ring la~er gyroscopes, wherein freedom from ~; ~ contaminants is essential for optimum production quality and ~ . .
~ l in~trument performance. In the manufacture of ~uch a precision ; ~ apparatus, heat is commonly utilized to liberate volatile materials uch a~ water, alcohols and pla3tic3).
Upon a~sembly of the ring laser gyro apparatus, including la~er body, mirrors and electrodes, the in3trument is placed upon a kh/!

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fill ~tand and the assembly baked to liberate unde~ired cont~minant~. Thi~ baking proce~, and -the resultant purity of the laser, are limited in effectiveness by the 156 degree Centigrade melting point of the indium seal. (Otherwise, the assembly could be baked at an approxlmately 100 degree Centigrade higher temperature, limited by the capacity of the mirror~ of -the assembly). Thu~, in addition to the harmful effects of mismatching of thermal stresses, the conventional laser assembly that includes a metallic cathode and ceramic dielectric laser body of mi~matched thermal expansion coefficient joined by an indium seal i~ limited in effectiveness of opera-tion and ease of manufacture.
SUMMA~Y OF THE INVENTIO~
The present invention overcomes the afore~aid ~hortcomings of the prior art by providing, in a laser of the type including a dielectric body of preselected thermal expansion characteristic material and a cathode fixed thereto, the improvement being the cathode comprising a preselected dielectric ~ material having a thermal expansion characteristic that clo~ely ; matches the la~er body, such cathode being field assist bonded to the dielectric laser body.
In a further aspect, the invention provide~ an improved method for manufacturing a ring laser gyro~cope. Ihe improved method comprises the steps of fabricating a laser cathode in part of preselected dielectric material having a thermal expansion characteristic that closely matche~ the laser body. Thereafter, the cathode is field a~sist bonded to the laser body. Finally, the ~ .
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The foregoing and additional advantages and features of the invention will become apparent from the detailed description which follows. In the detailed description, reference is made to numerals indicating features of the invention in accompanying figures, like numerals referring to like features throughout.

BRIEF DESCRIPTION OF THE D~RAWING
The Figure is a cross-sectional view of a laser in accordance with the invention.

DETAILED DESCRIPTION
Turning now to the Figure, there is shown a side sectional view of a laser 10 in accordance with the invention.
The laser 10 includes a laser body 12, preferably 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 with upright bores 24 and 26 that feed the lasing cavity 14.
The cathode 22 is generally-hemispherical comprising an outer shell 28 of glass, fused silica or glass-ceramic that includes a thin ~ilm layer 30 of aluminum or an alloy of aluminum at its interior. The shell 28 may be ~abricated by any number of methods well-known in the glass and quartz forming arts including glass blowing and moldiny techniques.

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Additionally, the shell 28 can be machined from a glass ceramic 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 form layer 30 include vacuum deposition, sputter coating and ion plating of aluminum or aluminum alloys.
The inventor has found that, by employing a cathode shell 28 of material having a coefficient of thermal expansion that closely matches that of the laser body 12, the stresses exerted upon a seal 32 that secures the cathode to the laser body are greatly reduced both the performance and the life of the laser are thus enhanced. ~e has further found that a thin film layer 30 of aluminum or aluminum alloy does not possess sufficient mass to impose significant stresses upon the seal;
thus, as long as the metallic layer 30 is sufficient thick to render the cathode 22 uniformly conduction, the performance of ; the cathode is fully adequate and equivalent to that of a cathode solely of aluminum or aluminum alloy.

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The seal 32 is preferably formed in a field-a~sisted bonding process, ~uch as that known as a Mallory proces~, In such a proce~s, the glass cathode and laser body are heated to a temperature of 300 to 400 Centigrade while a potential voltage i9 applied between the cathode and the laser body. As the assembly is heated, its electrical conductivity increase3, allowing electrical current to flow through the cathode-laser body interface. The current cau~ec the diffusion of aluminum atoms from the layer 30 into the glacs. As a result, a strong permanent bond i~ formed that ie not subject to certain failure mode~ that characteri~e conventional glass-to-metal bond~ including, for example, those deriving from the melting temperature of indium.
The closely matched thermal characteri~tics of the laser body 12, and cathode 22 permit the use of field assi~ted bonding processes. Such processes reault in bonds of greatly enhanced strength (thousands of p.a.i. as contrasted with indium seal strength in the hundred~ of p.s.i.). As previou31y mentioned, the very strength of auch bond can permit the transmis~ion of destructive thermal stres~e~ between a laser body and a cathode of differing thermal character.
When the closely matched laser body and cathode are joined by a fiald a~sist bonding process, the resultant as~embly, in the in~tance of a ring la~er gyroscope, is amenable to highly advantageous manufacturing proce~ses that improve the quality and performance of the resultant instrument dramatically. The removal of the constraints due to thermal expansion mismatch and the ~ ~:
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assembly (including elec-trodes fueed there-to) to be baked, in a low pres~ure enviromnent, at a temperature approximately 100 Centigrade degrees higher than that of the melting point of indium. (In the ~ame in~tance of a ring laser gyroscope, bakeout of the instrument on the fill stand would thus be limited by the mirrors of the as~embly to appro~imately 250 degrees Centigrade as opposed to the indium melting point of appro~imately 150 degrees Centigrade).
A highly de~irable result of the increased bakeout temperature i~ its effect upon the vacuum environment. A 100 degree Centigrade increase in bakeout temperature increa~es material vapor pres~ures 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 pres3ure and that of the surrounding environment, it follows that one hundred times less pumping time is required to attain a given level of cleanliness.
As a result, the manufacture of a laser in accordance with the invention is less expensive and its performance quality and useful lifetime are increased.
Thus it is seen that improved methods and apparatus have been brought to the la~er fabrication art by the present invention~ By employing the teachings of this invention, one may provide laser apparatus of increaeed durability for use in thermal environments that would otherwise severely degrade performance :
capability. Further, by employing the teachings of the invention, ;~ one may employ advantageou~ bonding proceeses not applicable to the prior art in achieving the afore3aid resulte. Such bonding kh/~!i , ~ .

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proce~e~, in conjunction with the configuration of the laser cathode, provide~ a la~er ae~embly of increa~ed quality at decreased cost~ of manufacture. Field a~ t bonding of the cathode to the la~er body produce~ an assembly of increaoed performance quality that i~ readily amenable to advantageous manufacturing proce~e~.

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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 a cathode fixed thereto, the improvement comprising:
a) said cathode comprising preselected dielectric material having a thermal expansion characteristic that closely matched said body; and b) said cathode is field-assist bonded to said dielectric body.

2. A laser as defined in Claim 1 wherein said electrode further includes a metallic coating.

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

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 a cathode thereto, the improvement comprising the steps of:
a) fabricating said cathode in part of preselected dielectric material having a thermal expansion characteristic that closely matches said body then b) field-assist bonding said cathode to said body; and then c) baking said body with said cathode 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 cathode with metal.