GB2132407A

GB2132407A - A laser cathode

Info

Publication number: GB2132407A
Application number: GB08334345A
Authority: GB
Inventors: Gordon Spencer Norvell
Original assignee: Litton Systems Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1982-12-27
Filing date: 1983-12-23
Publication date: 1984-07-04
Also published as: FR2595877A1; GB2132407B; GB2189341A; CA1255380A; IT8349566A0; DE3346232A1; JPS62205676A; GB8609664D0; IL70499A0; IT1197763B; IL70499A; JPS59132692A; FR2538610A1; GB8334345D0; GB2189341B; FR2538610B1; DE3607388A1

Abstract

An improved laser cathode 22 includes a generally hemispherical hollow shell 28 fabricated of material of preselected thermal character. The shell includes, at its inner surface, a covering layer 30 of material which includes aluminum. A thermal seal is created between a laser body 12 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. <IMAGE>

Description

SPECIFICATION A laser cathode The present invention pertains to laser cathodes.

A laser cathode serves to supply electrons for the lasing process. Often such a cathode is of generally dome-like configuration having an aluminum surface and is situated near the end of a channel within a laser body containing appropriate gases such as helium and neon. In operation, it is maintained at a negative potential, bombarded by positively-charged helium and neon ions and combine with the electrons supplied to the oxidised surface of the cathode by reason of its negative potential to produce uncharged gas molecules.

A conventional laser 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 the resulting laser output frequency. 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 thermal coefficient, including various glass ceramics such as those known by the trademarks "Zerodur" and "Cer-Vit". The cathode, on the other hand, must include metal 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 recognised 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-tometal 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 States Patent Number 4,273,282 for "Glass-or-Ceramic-to-Metal Seals".

While a cathode of aluminum or aluminum alloy will provide electrons to the lasing process, the degree of expansion it experiences under thermal stress, while not degrading to the shortterm operation of the laser, effects its long-term integrity. The large disparity in thermal expansion coefficients between the aluminum or other metal cathode and the glass ceramic laser body introduces substantial stresses into such 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 a cathode and laser body when cycled, for example, between 55 Centigrade and 1250 Centigrade.

While attractive sealing processes exist for bonding glass to glass, such as field-assisted bonding, these cannot be utilized to seal metal to glass. Thus, the aluminum-to-glass seal, commonly including indium, is limited by indium's melting temperature of 1 560 Centigrade.

According to one aspect of the invention, there is provided a laser cathode comprising a hollow shell of a first material, said shell including at its inner surface a cathode layer of a second, laser cathode, material comprising aluminum, so that the first material can be of a preselected thermal character suitable for use in a laser.

Thus, the first material may be vitreous and/or have a coefficient of linear expansion less than 11 x 1 O-6/0C to correspond (within 2 x 10-6/OC) to that of a typical laser body.

The shell may be generally hemispherical.

According to a second aspect, there is provided a laser comprising a laser body and a cathode sealed to the body and including a, preferably generally hemispherical, hollow shell fabricated of material of preselected thermal character, the shell of the cathode including an inner cathode layer comprising aluminum.

According to a further aspect of the invention, there is provided a method of producing a laser including creating a thermal seal between a laser body of low thermal coefficient material and a cathode, comprising the steps of fabricating a shell of said cathode of low thermal coefficient material and sealing said cathode shell to said body.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing, in which the single figure is a cross-sectional view of a laser 1 0.

The laser 10 includes a laser body 12, preferably formed of a cramic glass such as Cer Vit or Zerodur. A lasing cavity 14 resides within the laser body 1 2 having highly polished mirrors 16 and 18 at its opposite ends. An anode 20 and a cathode 22 communicate with upright bores 24 and 26 that feed the lasing cavity 1 4.

The cathode 22 is generally hemispherical, comprising an outer shell 28 of glass, quartz (or 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 glass blowing and moulding techniques. 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.

We have found that, by employing a cathode shell 28 of material having a coefficient of thermal expansion that closely matches (e.g. within 2 x 10-6/OC) that of the laser body 12, the stresses exerted upon a seal 32 that secures the cathode to the laser body are greatly reduced and the life of the laser thus enhanced. For example, the coefficient of linear expansion of the shell will preferably be less than 11 x 1 0-6/0C and particularly from 8 to 10 x 1 O-6/0C, i.e. the normal range for glass and other vitreous materials.We have 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; at the same time, as long as the layer 30 is sufficiently thick to render the cathode 22 opaque, the performance of the cathode is fully adequate and equivalent to that of a cathode solely of aluminum or aluminum alloy.

The seal 32 may be formed of a number of materials and conventional processes include, but are not limited to, those that form an indium seal in conjunction with heat and/or pressure. In addition, due to the fact that both the laser body 12 and the shell 28 of the cathode 22 are formed of non-metallic, glass-like (vitreous) materials, the seal 32 can be formed in a field-assisted bonding process, such as that known as a Mallory process.

In such a process, the glass cathode and laser body are heated to a temperature of 3000 to 4000 Centigrade while a potential voltage is applied between the cathode and the laser body. As the assembly is heated, its electrical conductivity increases, allowing electrical current to flow through the cathode-laser body interface. The current causes the diffusion of aluminum atoms from the layer 30 into the glass. As a result, a strong permanent bond is formed that is not subject to certain failure modes that characterise conventional glass-to-metal bonds including, for example, those deriving from the melting temperature of indium.

Thus it is seen that improved methods and apparatus have been brought to the laser fabrication art by the present invention. By employing the teachings of this invention, one may provide laser apparatus of increased durability for use in thermal environments that would otherwise severely degrade performance capability. Further, by employing the teachings of the invention, one may employ advantageous bonding processes not applicable to the prior art in achieving the aforesaid results.

Claims

1. A laser cathode comprising a hollow shell of a first material, said shell including at its inner surface a cathode layer of a second, laser cathode, material comprising aluminum, so that the first material can be of a preselected thermal character suitable for use in a laser.

2. A cathode as defined in claim 1 , wherein said first material is vitreous.

3. A cathode as defined in claim 1 , wherein said first material comprises glass.

4. A cathode as defined in claim 1, wherein said first material comprises quartz.

5. A cathode as defined in claim 1, wherein said first material comprises a preselected glass ceramic.

6. A cathode as defined in any one of the preceding claims, wherein said second material is aluminum.

7. A cathode as defined in any one of claims 1 to 5, wherein said layer comprises an aluminum alloy.

8. A cathode as defined in any one of the preceding claims, wherein a portion of said shell is generally hemispherical.

9. A cathode as defined in any one of the preceding claims, wherein the layer is sufficiently thick as to be opaque.

10. A laser cathode substantially as hereinbefore described with reference to the accompanying drawing.

11. A laser comprising: (a) a laser body; (b) a laser cathode sealed to said body, and being according to any one of the preceding claims.

12. A laser as defined in claim 11, wherein said laser cathode is field assist sealed to said laser body.

1 3. A laser as defined in claim 12, wherein said body and said shell are both vitreous.

1 4. A laser substantially as hereinbefore defined with reference to the accompanying drawing.

1 5. A method of producing a laser including creating a thermal seal between a laser body of low thermal coefficient material and a cathode, comprising the steps of fabricating a shell of said cathode of low thermal coefficient material and sealing said cathode shell to said body.

16. A method as defined in claim 15, wherein the shell of said cathode comprises glass.

17. A method as defined in claim 15, wherein the shell of said cathode comprises quartz.

18. A method as defined in claim 15, wherein the shell of said cathode comprises a preselected glass ceramic.

19. A method as defined in claim 15, 1 6, 17 or 18, including the additional step of forming a cathode layer comprising aluminum at the interior of said cathode.

20. A method as defined in claim 19, wherein said layer is aluminum.

21. A method as defined in claim 19, wherein said layer is an alloy of aluminum.

22. A method as defined in any one of claims 1 5 to 21, wherein the cathode is pressure sealed to the laser body.

23. A method as defined in any one of claims 1 5 to 21 , wherein the cathode is field-assist sealed to the laser body.

24. A method of producing a laser substantially as hereinbefore described with reference to the accompanying drawing.