Self-confined hollow cathode laser

A radiation emission device characterized by a cylindrical cathode enclosed by an elongated envelope having two end sections is disclosed. A pair of anodes, one of which is located along each end section, serves to provide electrical energy to excite material inside the envelope, and to further provide a cataphoretic effect to prevent the excited material from drifting into contact with radiation transmission windows located at the terminus of each end section.

BACKGROUND OF THE INVENTION 
This invention relates to radiation emission devices in general, and in 
particular to radiation emission devices of the type which are frequently 
referred to as lasers. Lasers are generally characterized by an elongated 
envelope containing a material which can be raised from an initial energy 
state to a so-called excited energy state. The particular means used to 
excite the material in the envelope may vary. Thus, depending on the type 
of laser used, optical, electrical or chemical excitation means may be 
employed. 
After excitation, radiation may be emitted spontaneously as the excited 
material returns to a more stable energy level, and/or through stimulated 
emission. In either case, the wavelength of the radiation so emitted is a 
function of the quantum drop in the energy level of the excited material. 
This, in turn, depends upon the inherent characteristics of the material 
itself. 
The radiation, which propagates at a constant wavelength, generally leaves 
the envelope via radiation transmission means disposed at both ends 
thereof. The radiation transmission means are typically translucent 
windows which are often, but not necessarily, inclined at an angle which 
optimizes a particular polarization of light. This inclination is usually 
referred to as Brewster's angle, and the windows so inclined are often 
characterized as Brewster's windows. 
Lasers of the type described typically include reflection means such as 
concave mirrors located a predetermined distance beyond each translucent 
window. The mirrors are aligned such that the radiation emitted from a 
translucent window is reflected back into the envelope to stimulate the 
emission of a substantially increased amount of radiation which then 
passes through the opposite window. This increased radiation is likewise 
reflected back into the envelope by the other mirror, thereby increasing 
the emitted radiation even more. As the radiation is continuously 
reflected back and forth through the envelope, greater and greater amounts 
of radiation are produced. It is in this manner that the energy used to 
initially stimulate the emission of radiation is "amplified" by the laser 
device. Of course, in order to enable the amplified radiation to escape 
therefrom, at least one of the mirrors are generally made only partially 
reflective. 
Many different materials may be used to effect radiation emission, 
including certain members of the class of materials known as metals. 
Because the metals used in this type of laser must generally be 
transformed from a normally solid or liquid state, to a gaseous state in 
order to effect excitation, such lasers are frequently referred to as 
metal vapor lasers. It is thus clear that in metal vapor lasers, 
excitation means must be provided which first vaporize the metal and then 
raise the vaporized metal from an initial energy state to an excited 
energy state. 
Though metal vapor lasers of the type described have been used to emit 
radiation, it is well known that they can be subject to certain drawbacks. 
In particular, the vaporized metal tends to condense on the translucent 
windows located at the ends of the elongated envelope, thereby rendering 
the windows relatively opaque, and hence less capable of transmitting 
radiation. In the past, attempts to remedy this problem have included the 
use of cataphoretic means for establishing an electric field within the 
laser envelope. The electric field is typically arranged to accelerate the 
vaporized metal ion away from the region nearest the translucent windows, 
thereby confining the vaporized metal to the more central portions of the 
envelope. 
Metal vapor lasers have heretofore required relatively complicated, 
cumbersome, and inefficient apparatus to accomplish both excitation and 
confinement of the metal. It is therefore an object of this invention to 
provide an improved laser configuration which achieves these results more 
economically and more effectively. It is another object of this invention 
to provide an improved metal vapor laser having combination excitation and 
cataphoretic means which serve to excite the metal and then confine it 
within the laser envelope to reduce condensation on the translucent 
windows. Other objects, features and advantages of the invention, as 
summarized below, will be apparent upon reading the following detailed 
description in conjunction with the accompanying drawings. 
SUMMARY OF THE INVENTION 
In accordance with the foregoing objects, this invention pertains to a 
device for promoting the emission of radiation. The device comprises a 
first electrode, and an envelope, substantially surrounding the first 
electrode, having a pair of end sections each terminating in radiation 
transmission means. The device further comprises combination excitation 
and cataphoretic means, disposed in advance of the radiation transmission 
means, to cause at least some of the material enclosed within the envelope 
to be raised from an initial state to an excited state, and to further 
urge some of the material away from the nearest of the radiation 
transmission means.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT 
Referring now to the figures, a device for stimulating the emission of 
radiation is shown in the form of a laser 10. Laser 10 is comprised of an 
elongated envelope 20, preferably fabricated from glass. Initially, 
envelope 20 includes a passage for inserting gaseous material therein, 
though upon insertion, the envelope is ordinarily hermetically sealed. 
Envelope 20 includes a pair of end sections 21, 22, each terminating in 
corresponding radiation transmission means which are referred to 
hereinafter as windows 23, 24. If it is desired to optimize a particular 
polarization of light, windows 23, 24 may be inclined at Brewster's angle 
as indicated in the figures. 
Located along each end section 21, 22, in advance of respective windows 23, 
24 are a pair of terminals which serve as anodes 25, 26. Anodes 25, 26 
extend through envelope 20 and, as shown in FIG. 3, are connected to a 
source of electrical energy, which in this case, is a positive 300 volt 
supply. As explained in greater detail hereinafter, anodes 25, 26 are used 
in combination means for effecting the excitation of material inside 
envelope 20, as well as for establishing an electric field therein to 
accelerate the excited material away from the nearest one of windows 23, 
24. 
Disposed inside glass envelope 20 is a substantially cylindrical, hollow 
electrode, which in this embodiment serves as a cathode 11. To facilitate 
the distribution of gaseous material inside envelope 20, cathode 11 may 
include a number of perforations 13 along its axis. Preferably located 
midway between the ends of cathode 11 is a terminal 12 which extends 
through glass envelope 20 where it can be connected to a point of low 
potential such as ground as shown in FIG. 3. Though cathode 11 can be 
fabricated from any number of materials, molybdenum is preferred. 
Moreover, the approximate parameters for the components of laser 10 are as 
follows: diameter of cathode 11--1 cm., diameter of envelope 20--1-1/2cm., 
length of envelope 20--30 cm. It should be observed, however, that these 
parameters are given for exemplary purposes only, and should not be 
construed as limitative. 
In operation, a preselected doping material which can be excited from an 
initial energy state to an excited energy state for the purpose of 
effecting the emission of radiation, and a preselected host material, are 
inserted within the confines of envelope 20. The particular types and 
quantities of such material may vary, though in this exemplary embodiment, 
about 1-2 grams of cadmium metal are placed inside cathode 11 and are used 
for doping, while gaseous helium at a pressure of approximately 4-10 torrs 
is inserted within envelope 20 and serves as the host material. The 
cadmium which, as shown in FIG. 1, is initially in solid form, is 
identified herein by reference numeral 32, and the helium inserted within 
envelope 20, is represented herein by reference numeral 31. 
When a positive 300 volts are applied to anodes 25, 26, some of the helium 
atoms 31 become ionized while others are raised to certain excited states. 
The ionized helium and electrons define a conductive path between anodes 
25, 26 and cathode 11. As a result, cathode 11 is heated to a temperature 
of about 280.degree. C., thereby causing the cadmium placed therein to 
vaporize. 
The vaporized cadmium atoms invariably collide with the helium ions and/or 
excited state helium to create an energy exchange therebetween. This 
energy exchange raises the cadmium atoms from their initial energy state 
to an ionized excited energy state, characterized by a positive electronic 
charge. Simultaneously, the excited and/or ionized helium returns to its 
initial state. However, the continuous application of 300 volts at anodes 
25, 26 creates a constant supply of ionized and excited helium within 
envelope 20 to promote further energy exchanges with the vaporized cadmium 
atoms. 
The excited cadmium may return to its initial energy state through 
spontaneous and/or stimulated emission. In the process of returning to its 
initial energy state, radiation is emitted at a frequency which is 
dependent upon the internal properties of the excited material. Thus, for 
cadmium a characteristic red, green and blue light are emitted through 
windows 23, 24. 
Disposed beyond each of windows 23, 24 is a concave mirror 41, 42 shown in 
FIG. 3. In a manner well known in the art, mirrors 41, 42 reflect the 
radiation emitted through windows 23, 24 into envelope 20 to stimulate the 
emission of increased amounts of radiation. These increased amounts of 
radiation also pass through windows 23, 24 until they are again reflected 
back inside envelope 20 by mirrors 41, 42. Thus, as explained above, 
initially stimulated emission of radiation is "amplified" many times by 
laser 10. Of course, to allow the "amplified" energy to escape from the 
device, at least one of mirrors 41, 42 are made only partially reflective. 
As explained above, a positive voltage of 300 volts is applied to anodes 
25, 26. This positive voltage creates, within envelope 20, electric field 
gradients identified by reference numerals 43, 44 in FIG. 3. Electric 
field gradients 43, 44 are directed from cathode 11 towards anodes 25, 26, 
respectively. Consequently, the areas immediately in advance of windows 
23, 24 are more positive than the more central areas of envelope 20. Thus 
the cataphoretic effect tends to accelerate the positively charged, 
excited cadmium vapor away from the nearest of windows 23, 24 and toward 
cathode 11. As a result, the excited cadmium will generally be prevented 
from drifting too close to windows 23, 24 to condense thereon. Thus, all 
of the adverse effects that typically result from the condensation of 
vaporized metal onto windows 23, 24 are mitigated, if not completely 
eliminated. 
In view of the foregoing, it should be apparent that the configuration of 
laser 10 described above includes combination excitation and cataphoretic 
means which effectively cause the doping material to be excited from an 
initial energy state to an excited state, and further substantially 
prevent the excited doping material from drifting toward and condensing on 
windows 23, 24. This is achieved, in part, by providing no more than one 
pair of anodes 25, 26 in advance of windows 23, 24, and a more centrally 
located cathode 11. As a result, the effects of excitation and 
cataphoresis can be achieved in a relatively simple, economical and 
efficient laser device. 
Though the exemplary embodiment of the invention herein disclosed is 
preferred, it will be clear to those skilled in the art that numerous 
modifications and refinements can be made without departing from the true 
scope of the invention. Accordingly, all such modifications and 
refinements are intended to be covered by the appended claims.