Note: Descriptions are shown in the official language in which they were submitted.
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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~; of the la3er body. When ~uch a laser is employed a9 an element of
an in3trumentatlon system only a relatively small amount of
variation in the distance between the mirrors is tolerable as this
dlstance is critical to resulting
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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
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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
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~ 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
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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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~- ` body is baked, with the cathode fixed thereto, at a temperature in
excess of 156 degrees Centigrade.
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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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relatively low melting point of the indium seal permits 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
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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
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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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