GB2189341A - Gas lasers - Google Patents

Abstract

Electrodes (20,22) for a laser include a hollow, shell-like, members formed of material, such as glass, whose thermal coefficient closely matches that of the laser body. Thin film metallic layers (30) coat the interiors of the electrodes 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 manufacturing processes. <IMAGE>

Description

GB 2 189 341 A SPECIFICATION and the glass ceramic laser body introduces

substantial stresses into such a system. The Lasers mismatch in the coefficients of thermal expansion of aluminum and Zerodur, for example, limits the life The present invention pertainsto improvements in 70 expectancy of a seal between such an electrode and the laser arts. More particularly, this invention laser body when cycled, for example, between -55 relates to an improved laser including an improved degrees Centigrade and 125 degrees Centigrade.

electrode and method of attachment thereof to a Thus, the aluminum-toglass seal, commonly laser body. including indium, is limited by indium's melting In the lasing process,the laser electrodes, anode 75 temperature of 156 degrees Centigrade.

and cathode, interaetto provide a flow of current The stress introduced into a thermally stressed through the lasing gases, exciting these gasesto the system including a glass ceramic laser bodyjoined higher energy states required for lasing action. Often to a metallic electrode may result in distortion of the the electrodes are situated nearthe ends of channels laser body by a small amount. This distortion or within a laser body containing appropriate gases 80 bending may severely degrade the performance such as helium and neon. characteristics of the laser in such applications as, The cathode is commonly of a generally dome-like for instance,the ring lasergyroscope. In addition to metallic configuration whereas a functional metallic the physical distortion,the relative movement and anode maytake a variety of forms, including dome or cold flow of the indium sealant atthe laser diskshaped, although its operation is less sensitive 85 body-electrode interfacewill lead to eventual seal to shapethan the cathode. In operation, the cathode failure. Although "hard" glass seals exist,they are is maintained at a negative potential and the anode is unsuitable in light of the stress-caused differential maintained at positively-charged helium and neon thermal expansions. Such stresses can actually ions,to act as an electron emitterwhile the anode rupturethe glass laser body.

serves as an electron collector. 90 The foregoing problems interact to limitthe A conventional laser application, such as a ring effectiveness and appropriate methods of laser gyroscope, includes highly polished mirrors manufacture of lasers for applications, such as ring situated at opposed ends of the laser body. When laser gyroscopes, wherein freedom from such a laser is employed as an element of an contaminants is essential for optimum production instrumentation system only a relatively small 95 quality and instrument performance. In the amount of variation in the distance between the manufacture of such a precision apparatus, heat is mirrors is tolerable asthis distance is critical to commonly utilized to liberate volatile materials (such resulting laseroutput. The maintenance of a as water, alcohols and plastics).

preselected distance, within tolerance, poses a Upon assembly of the ring laser gyro apparatus, difficuittechnical problem when the laser is operated 100 including laser body, mirrors and electrodes, the in a relatively extreme thermal environment. To instrument is placed upon a fill stand and the combat this problem, the laser body is commonly assembly baked to liberate undesired contaminants.

fabricated of material of extremely lowthermal This baking process, and the resultant purity of the coefficient, including various glass ceramics such as laser, are limited in effectiveness bythe 156 degree those known bythe trademarks "Zerodur" and 105 Centigrade melting point of the indium seal.

"Cer-Vit" (RTM). The cathode and anode, on the (Otherwise, the assembly could be baked at an other hand, include a metal conductorto provide a approximately 100 degree Centigrade higher flow of currentthrough the lasing gases. temperature, limited bythe capacity of the mirrors of Currently, metal or metal alloy laser electrodes are the assembly). Thus, in addition to the harmful produced by a number of recognized methods 110 effects of mismatching of thermal stresses,the including stamping and machining. Such methods conventional laser assembly that includes a metallic require extensive cleaning and preparation ofthe electrode and ceramic dielectric laser body of internal surfaces. Additionally, in some applications mismatched thermal expansion coefficient joined by the electrodes must be sealed to the laser body. an indium seal is limited in effectiveness of operation Thus, a glass-to-metal seal is commonly effected in 115 and ease of manufacture.

accordance with the differing compositions of the According to one aspect of the invention, there is electrodes and the laser body. Indium is commonly provided a laser of the type including a dielectric employed as a sealing agent. Such an indium seal is body and an anodefixed thereto, said anode disclosed in United States Patent Number4,273,282 comprising dielectric material having a thermal or Norvell, et al. for "Giass-or-Ceramic-to-Metal 120 expansion characteristic that closely matchesthat of Seals". said body and said anode isfield-assist bonded to

While electrodes of metal or metal alloywill said dielectric body.

provide the necessary electrical contactfrom the According to a second aspect of the invention, exteriorto the interior of the laser and hence provide there is provided a laser of thetype including a a means for passing the currentforthe lasing 125 dielectric body of preselected thermal expansion process, the degree of expansion they experience characteristic material and at least one electrode underthermal stress, while not degrading to the fixed thereto, said at least one electrode comprising short-term operation of the laser, effects its preselected dielectric material having athermal long-term integrity. The large disparity in thermal expansion characteristic that closely matches that of expansion coefficients between the metal electrode 130 said body and carrying a metallic coating not 2 GB 2 189 341 A 2 containing a] uminum and said at least one electrode enhanced. For example, the coefficient of linear is field-assist bonded to said dielectric body. expansion of the shell will preferably be less than 14

According to either aspect, a preferred x 10PC and particularly from 8 to 10 X 10-610C, i.e.

embodiment comprises at least one electrode the normal range for glass and othervitreous comprising a shell of dielectric material having at its 70 materials. We have further found that suitable inner surface a deposited coating of metallic metallic thin film layers do not possess sufficient material, the dielectric material constituting a major mass to impose significant stresses upon the seal; portion of the mass of the electrode so as to make the thus, as long as the metallic layer is sufficiently thick electrode relatively insensitive to thermal cycling. to renderthe electrode u n Uorm lyconductive, the Preferablythe structure can be baked at over 1560C, 75 performance of the electrode is fully adequate and preferably upto 250'Cwithout creating adverse equivalentto that of an electrode solely of metal or stresses. metal alloy.

According to athird aspectofthe invention,there Theseal 32 in this embodiment is formed in a is provided a method for manufacturing a laser field-assisted bonding process,such asthatknown including the steps of preparing a laserbodyof 80 as a Mallory process. In such a process, the glass preselected thermal characteristic material, electrode and laser body are heated to a temperature fabricating an electrode in part of preselected of 300 to 400 degrees Centigrade while a voltage dielectric material having a thermal expansion potential is applied between the electrode and the characteristic that closely matches that of said body, laser body. As the assembly is heated, its electrical then field-assist bonding said electrode to said body 85 conductivity increases, allowing electrical current-to and then baking said bodywith said electrodefixed flowthrough the electrode-laser body interface. The thereto ata temperature in excess of 156 degrees current causes diffusion of the metal from thethin Centigrade. film layer into the glass. As a result, a strong, For a better understanding of the invention and to permanentbond is formed that is not subjectto show howthe same may be carried into effect, 90 certain failure modesthat characterise conventional reference will now be made, bywayof example,to glass-to-metal bonds including,for example,those the accompanying drawing, in which the single deriving from the melting temperature of indium.

figure is a cross-sectional view of a laser in The closely matched thermal characteristics of the accordance with the invention. laser body 12, anode 20 and cathode 22 permitthe Turning nowto the Figure, there is shown a side 95 use of field assisted bonding processes. Such sectional view of a laser 10 in accordance with the processes result in bonds of greatly enhanced invention. The laser 10 includes a laser body 12, strength (thousands of p.s.i. as contrasted with preferably formed of a ceramic glass such as Cer-Vit indium seal strength in the hundreds of p.s.i.). As or Zerodur. A lasing cavity 14 resides within the laser previously mentioned, the very strength of such body 12 having highly polished mirrors 16,18 at its 100 bond can permitthe transmission of destructive opposite ends. An anode 20 and a cathode 22 thermal stresses between a laser body and an communicate with upright bores 24 and 26 thatfeed electrode of differing thermal character.

the lasing cavity 14. When the closely matched laser body and The cathode 22 is general ly-hemispherical, electrode arejoined by a field assist bonding comprising an outer shell 28 of glass, fused silica or 105 process, the resultant assembly in the instance of a glass-ceramic that includes a thin film layer 30 of ring laser gyroscope, is amenable to highly aluminum or an alloy of aluminum at its interior. The advantageous manufacturing processes that shell 28 may be fabricated by any number of improve the quality and performance of the resultant methods well-known in the glass and quartzforming instrument dramatically. the removal of the arts including glass blowing and molding 110 constraints dueto thermal expansion mismatch and techniques. Additionally, the shell 28 can be the relatively low melting point of the indium seal machined from a glass ceramic such as Zerodur, permits the assembly (including electrdesfused Cer-Vit orthe doped glass known bythe trademark thereto) to be baked, in a low pressure environment, "ULE". Appropriate tech n iques for coating the at a temperature approximately 100 Centigrade interior surface of the shell 28 to the form layer30 115 degrees higherthan that of the melting point of included vacuum deposition, sputter coating an ion indium. (in the instance of a ring laser gyroscope, plating of aluminum or aluminum alloys. A bakeout of the instrument on the fill stand would cross-sectional view of the anode 20 would disclose thus be limited by the mirrors of the assemblyto a substantially identical configuration therefor. In the approximately 250 degress Centigrade as opposed instance of the anode, copper or copper alloy may 120 to the indium melting point of approximately 150 form the thin film layer. Many other metals are degrees Centigrade).

suitable including nickel, chromium, iron, titanium, A highly desirable result of the increased bakeout tungsten, aluminum and gold. temperature is its effect upon thevacuum We havefound that, byemploying electrodes environment. A 100 degree Centigrade increase in including a shell having a coefficientof thermal 125 bakeout temperature increases material vapor expansion that closely matches (e.g. within 2 X pressures by morethan two decades, a 1 OPC) that of the laser body 12, the stresses greater-than-one-hundred- fold increase. Since the exerted upon the seals that secure the electrodes to cleaning of the assembly is a function of the the laser body are greatly reduced and both the differential between vapor pressure and that of the performance and the life of the laser are thus 130 surrounding environment, itfol lows that one 3 GB 2 189 341 A 3 hundredtimes less pumpingtime is requiredto 9. A method of manufacturing a ring laser attain a given level of cleanliness. As a result, the substantially as hereinbefore described with manufacture of a laser in accordance with the reference to the accompanying drawing.

Claims (8)

1. invention is less expensive and its performance 10. A laser manufactured

   bythe process of Claim quality and useful lifetime are increased. 70 7,8or9.

   Thus, it is seen that improved methods and 11. A laser substantially as herein before apparatus have been brought to the laser fabrication described with reference to the accompanying art bythe present invention. By employing the drawing.

   teachings of this invention, one may provide laser 12. A laseras defined in anyone of Claims 1 to 6 apparatus of increased durabilityfor use in thermal 75 and 10and 11 andwhich isa ring laser.

   environments that would otherwise severely 13. A method according to anyone of claims 7to degrade performance capability. Further, by 9wherein the laser is a ring laser.

   employing the teachings of the invention, one may employ advantageous bonding processes not applicable to the prior art in achieving the aforesaid results. Printed for Her Majesty's Stationery Office by Croydon Printing Company (1) K) Ltd,8187, D8991685.

   Such bonding processes, in conjunction with the Published by The Patent Office, 25 Southampton Buildings, London, WC2A l AY, configuration of the laser electrodes, provide a laser from which copies maybe obtained.

   assembly of increased quality at decreased costs of manufacture.

   CLAIMS 1. A laser of the type including a dielectric body and an anode fixed thereto, said anode comprising dielectric material having a thermal expansion characteristic that closely matches that of said body and said anode is field-assist bonded to said dielectric body.
2. 2. A laser as defined in Claim 1 wherein said anode further includes a metallic coating.
3. 3. A laser as defined in Claim 2 wherein said anode comprises a hollow, substantially-hemispherical shape and said metallic coating is located at the interior of said substantially-hemispherical shape.
4. 4. A laser of the type including a dielectric body of preselected thermal expansion characteristic material and at least one electrode fixed thereto, said at least one electrode comprising preselected dielectric material having a thermal expansion characteristic that closely matches that of said body and carrying a metallic coating not containing aluminum and said at least one electrode is field-assist bonded to said dielectric body.
5. 5. A laser as defined in Claim 4 wherein said electrode comprises a hollow substantially-hemispherical body of said dielectric material and said metallic coating is located atthe interior of said substantial ly-hemispherical body.
6. 6. A laser according to anyone of the preceding claims and comprising an anode and a cathode each of which is a prefabricated shell of dielectric material field-assist bonded to said body.
7. 7. A method for manufacturing a laser including the steps of preparing a laser body of preselected thermal characteristic material, fabricating an electrode in part of preselected dielectric material having a thermal expansion characteristic that closely matchesthat of said body, then field-assist bonding said electrodeto said body and then baking said bodywith said electrodefixed thereto at a temperature in excess of 156 degrees Centigrade.
8. 8. A method as defined in Claim 7 including the step of coating said electrode with metal.