CA1153093A - Optical noise suppression and power enhancement in positive column lasers using magnetic fields - Google Patents

Abstract

OPTICAL NOISE SUPPRESSION
AND POWER ENHANCEMENT IN POSITIVE
COLUMN LASERS USING MAGNETIC FIELDS

ABSTRACT OF THE DISCLOSURE

A technique for simultaneously improving the optical noise and output power performances of positive column lasers, and in particular metal vapor lasers such as helium-cadmium lasers. A magnetic device is positioned either in contact with or in the immediate proximity of the laser tube envelope. The magnetic device is capable of producing a magnetic field transverse to the optical axis of the laser tube in the vicinity of the cathode discharge glow or at the discontinuity between the positive column discharge glow and the cathode discharge glow.

Description

~ 53~3 BACKGROUND OF THE INVENTION
Uncontrolled optical power level fluctuations (noise) in the output beam of the positive column helium-cadmium lasers has been known to be severe for many years and has been reported in the literature since about 1969.
~luctuations as large as 50 to 75 percent of the average output level at frequencies ranging from a few hertz to many megahertz are common. Many different types of optical noise occur in these lasers, originating from many different internal mechanisms.
The use of helium-cadmium lasers in a raster output scanner (ROS) printing system, such as the ~erox 9700 Electronic Printing System, requires that the optical output fluctuations from these lasers be maintained within specified bounds in order to produce printed pages free from random unwanted background spots. Typically, the laser output must be maintained within +10% and -20% of its average output value for fluctuations occurring over a broad frequency range (lOHz to 100MHz typical).
Optical noise levels of helium-cadmium lasers can be controlled by adjustment of the discharge current, helium fill pressure, and cadmium vapor pressure. Other factors which influence the noise performance of these lasers includes geometrical factors in the vicinity of the discharge cathode and cathode end of the discharge capillary. Adjustment of the operating con-ditions of the laser (current, helium pressure, and cadmium pressure) may improve the noise performance of lasers within acceptable ranges for operation in ROS printers. However, it has been determined that a major factor in the manufacturing yield losses of lasers was due to an inability to consistently produce lasers satisfying the noise specifications.
It also has been determined that the type of noise which is of

-2 ~1 f~ ~ 5'~ 3 predominant concern for ROS printers is associated with striation wave phenomenon in the discharge capillary bore and originated or was controlled by phenomenon in the vicinity of the discharge cathode or cathode end of the capillary bore. What is desired therefore is to provide a simple inexpensive and reliable means for improving the optical noise and output power performances of positive column gas lasers.
SUMMAR~ OF THE PRESENT INVENTION
The present invention provides a technique for simultaneously improving the optical noise and output power performances of positive column lasers, and in particular, metal vapor lasers such as helium-cadmium lasers. Magnetic means are positioned either in contact with or in the immed-iate proximity of the laser tube envelope. The magnetic means is capable of producing a magnetic field transverse to the optical axis of the laser tube, in the vicinity of the cathode discharge glow or at the discon~inuity between the positive column discharge glow and the cathode discharge glow.
An aspect of the invention is as follows:
In a positive column helium-cadmium gas laser w~erein there is a cathode discharge glow, a positive column discharge glow and a discontinuit~ therebetween which com-prises: an envelope; an active non-flowing gaseous medium disposed within said envelope, said gaseous medium being supplied from a relatively steady reservoir; at least one anode and one cathode disposed within said envelope; laser mirror means disposed at opposite ends of said gas laser envelope along the optical axis thereof for confining the active gaseous medium within said envelope and for forming an optical resonant cavity; one of said laser mirror means being capable of allowing a portion of the laser radiation generated within said envelope to be emitted therefrom;
a capillary tube positioned within said envelope for the positive column discharge glow; and means for applying a potential between said anode and cathode to provide a discharge in said active gaseous medium which results in

-3-..

~53~3 stimulated emission gain for continuous wave generation of laser radiation; the improvement comprising:
magnetic circuit means positioned relative to said envelope in the vicinity of said cathode discharge glow or at said discontinuity between said positive column discharge glow and said cathode discharge glow for producing a magnetic field having an intensity in the range from about 250 gauss to about 800 gauss and having a component transverse to the optical axis of the laser envelope, said magnetic circuit means comprising two poles of magnetic material facing each other across a gap, said laser envelope being positioned in said gap and between said poles, said gap being selected to provide a highly directional magnetic field with the lines of force being directed from one pole face to the other, said magnetic field intensity being of sufficient strength to break down any double sheath space charges within the envelope; thereby improving the optical noise and output power performances of said laser.
It is an object of an aspect of the present invention to provide a technique for simultaneously improving the output power and optical noise performance of positive-column lasers.
It is an object of an aspect of the present invention to provide a simple and inexpensive means of improving the performance of internal mirror helium-cadmium lasers with respect to its optical noise characteristics.
It is an object of an aspect of the present invention to provide a technique for simultaneousl~ improving the output power and optical noise performance of positive-column laser tubes wherein a magnetic means of a specified fieldstrength is placed in contact with or in the immediate proximity - 3a -~LS~ 3 of the laser tube, the field being in the vicinity of the cathode discharge glow or at the discontinuity between the positive column discharge glow and ~he cathode discharge glow.

BRIEF DESCRIPTION OF T~E DR~WINGS
For a better understanding of t,he invention as well as other objects and further features thereof, reference is made to the following description which is to be read in con-junction with the following drawings wherein:
Figure 1 is a cross-sectional view of a metal vapor laser discharge tube and illustrates the placement of the magnetic means relative thereto in accordance with the teach-ings of the present invention, and Figures 2(a)-2(c) illustrates alternate arrangements for the magnetic means to reduce laser optical noise.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to Figure 1, a metal vapor laser tube 10 with which the present invention may be utilized is shown.
The details of the laser tube 10 are disclosed in U.S. Patent No. 4,233,568 issued November 11, 1980 to Xerox Corporation.
Howèver, for the purposes of illustrating to the reader an environment in which the present invention is utilized, por-tions of the laser tube structure will be described in detail hereinafter. The tube comprises an outer glass envelope 12 with glass main discharge capillary tube 14 having a bore 16 supported therewithin. A large volume metal-containing reservoir 18 is formed by glass baffle 20 hermetically sealed to the inner wall of envelope 12 and the disc shaped base portion 22 of cataphoretic confinement capillary tube 24.
Baffle 20 also serves to support the discharge capillary tube 14 within envelope 12.

~3~93 The reservoir volume, typically 20 cm3, contains a metal charge 25. In the laser illustrated, the metal charge is cadmium. Glass capillary tube 24, adjacent to and coaxial with discharge capillary tube 14, provides cataphoretic confinement of the cadmium vapor in reservoir 18 and primary protection of anode end mirror 30, described in more detail hereinafter.
An anode pin electrode 32, preferably made of Kovar, a conventional iron cobalt-nickel alloy, is positioned within laser discharge tube 10 and adjacent the end of glass capillary tube 24. A diffusion confinement capillary tube 34, preferably made of metal such as Kovar*, is formed on the other side of anode pin 32 and is coaxially aligned with tube 24. Tube 34, in conjunction with tube 24, mimimizes the amount of cadmium vapor in the region of mirror 30. The tube 34 is supported by a disk shaped metal flange member 35, downwardly extending portions, or tabs, 37 thereof being attached to flange 40. The flared end portion 38 of tube 12 is also sealed to metal flange member 40 by standard glass to metal fusing techniques. An apertured flange member 42 is welded to flange member 40 and the high temperature spherical resonator mirror 30, comprising a glass substrate and a plurality of dielectric reflecting layers is hard sealed to the laser tube body by the techniques described, ~5 for example, in aforementioned U.S. Patent No. 4,233,568.
Ar extended adjustment flange 43 is sealed to flange 42 and contains a plurality of adjusting screws 45 to allow mirror 30 to be adjusted for alignment purposes if necessary.
An arrangement which may be utilized for this adjustment is disclosed in U.S. Patent No. 4,149,779 issued April 17, 1979 to Xerox Corporation.
A resistive heater 50 comprising a plurality of coils 51 is wrapped * trade marks B

~53~3 around the tube envelope 12 adjacent reservoir section 18 and within insulating layex 53 for controlling the cadmium vapor pressure and is utilized in conjunction with a tube voltage detection circuit to detect the discharge voltage between anode 32 and cathode structure 52 for maintaining a substantially constant laser output independent of ambient environment temperatures. The ends of the heater coils are terminated in appropriate connectors to allow the heater 50 to be connected to a transformer. On the cathode end section of laser tube 10 is provided a diffusion confinement section 60 having an apertured glass baffle 62 and metal (preferably Kovar) diffusion confinement capillary tube 64 which allows only a vanishingly small amount of cadmium vapor from being in the vicinity of a cathode end mirror 66. A glass baffle 68 is provided ad~acent the cathode end of the discharge capillary 14 and effectively acts to separate the cadmium condensate 70 from the electric field at the cathode structure 52 to prevent the trapping, or gettering of helium gas by the cadmium which would otherwise occur. It should be noted that this anti-get-tering effect provided by baffle 68 allows the cadmium to con-dense anywhere on the inside wall of envelope 12 between baf-fles 62 and 68 and be removed from the gas mixture flowing from the bore 16 of main discharge capillary 14.
High temperature resonator mirror 66, comprising a glass substrate and a plurality of dielectric reflecting layers, is hard sealed to apertured metal flange member 100 utilizing the techniques described in the aforementioned U.S. Patent No.

4,233,568. The flared portion 102 of envelope 12 is sealed to a metal flange member 108 by standard glass to metal sealing techniques and the laser end mirror assembly, comprising mirror 66 and flange member 100, is inert gas welded to flange member iL153~3 108. An extended adjustment flange 110 is sealed to flange 100, and contains a plurality of adjusting screws 112 to allow mir-ror 66 to be adjusted. This type of arrangement is disclosed in aforementioned U~S. Patent No. 4,149,779. The joinin~ of laser end mirror assemblies (mirror 30 and flange 42 and mirror 66 and flange 100) hermetically seals the laser discharge tube 10 and provide the optical laser cavity required for lasing action.
An automatic helium pressure regulator 148 may be provided to actively control the helium pressure in the laser tube 10. A high pressure helium reservoir 150 comprises a metal outer member 152, a permeable membrane 154, preferahly formed of a permeable glass membrane such as Corning 7052 glass where-in the helium permeation rate therethrough is strongly temper-ature dependent, and high pressure helium (typically He3at 2 atmospheres) which is introduced into reservoir 150 via tubu-lation 153. Membrane 154 is fused to flange member 156 by stan-dard sealing techniques, metal member 152 also being sealed to flange member 156 as shown. An extension, or appendage 160, preferably comprising Corning 7052 glass and connected to the main tube envelope 12 via tube portion 161, is joined to flange 156 via flange 158 which is in turn sealed to the flared end 159 of appendage 160. ~ membrane heater 162 comprising a heater wire 163 surrounding a heat insulating support member 164 is supported adjacent membrane 154 as shown (it should be noted a source of heat may be applied directly to metal portion 152 in lieu of separate heater 162). When current is supplied to the heater wire 163, the heat generated thereby is sufficient to heat membrane 154 and cause the high pressure helium contained in the reservoir formed by the metal portion, or cover 152, and flange 156 to permeate through membrane 154 at a predetermined ~f~ 5~93 rate greater than the rate caused by ambient temperature or other heat effects generated within the laser tube 10 such as the heat generated by the tube discharge, conventional radi-ation and convection effects etc. The heater leads are con-nected to feed through pins 170 and 172 as shown which are in turn connected to an external control circuit. Three additional feed through pins 174,176 and 178 are provided. Coupled to pins 174 and 176 is thermistor 180 and coupled to feed through pins 176 ànd 178 is thermistor 182. In the embodiment shown, both thermistors are placed within the appendage 160 although the system would be operative if one of the thermistors was sealed to the outside wall of appendage 160 and connec~ed to the appropriate position in the electrical circuit. Suffice to say for the purposes of this disclosure is that the thermistors act in a manner to maintain the helium pressure within tube 10 sub-stantially constant at a predetermined value.
The anode mirror assembly comprises an apertured metal flange 42 having a fully reflecting mirror 30 sealed to metal flange 42. Mirror 30 typically comprises a glass substrate upon which is coated a substantially totally reflecting layer com-prising a plurality of dielectric layers, the reflecting layer facing inward (within the tube envelope). The cathode end mirror assembly comprises an apertured metal flange 100 having a par-tially transmissive mirror 66 sealed to apertured metal flange 100 in a manner as described in aforementioned U.S. Patent No.
4,233,568. Mirror 66 comprises a glass substrate upon which is coated a partially transmissive layer of dielectric material, ~he transmissive layer being positioned within tube envelope 12.
Mirrors 30 and 66 are appropriately coated with layers of di-electric material such that only a laser beam of a desired wave-length (i.e. approximately 4416A) is transmitted by mirror 30 ~1~3a~3 the beam being utilized by external apparatus such as for thescanning purposes as set forth hereinabove. Typical dielectric materials include SiO2, TiO2 among -8a-~`:

~L5;~ 3 others.
The improvement to the helium-cadmium laser tube described hereinabove is in that a relatively low cost solution to the inherent high optical noise characteristics in positive column helium-cadmium lasers is provided, the optical noise being reduced to low values which increases the utility of such lasers in raster output scanning applications, such as in the Xerox 9700 Electronic Printing System described hereinabove. In particular, the present invention consists of a magnet or a plurality of magnets placed either in con-~3 tact with or in the immediate proximity of the outer envelope 12~1aser tube 10. The magnets may be either a permanent magnet or an electromagnet.
The exact configuration of the magnetic material is not of particular significance but the ma8net must be capable of producing a magnetic field of ~t least 250 gauss in the vicinity of the cathode discharge glow (approximately at location 71) or at the discontinuity (approximately at location 69) between the positive column discharge glow and the cathode discharge glow due to the difference in the dimensions between the positive column discharge glow and the cathode discharge glow (area of positive ion/electron double sheath). These locations correspond to two sources of discharge tube optical noise which have been previously identified in the prior art. The exact positioning of the magnets for noise reduction could be determined by experimentation ~ necessary.
The prior art has suggested that the production of noise in the vicinity of the cathode is due to the oscillation of positive ions which are trapped in the potential minimum existing in front of a hot cathode. This explanation would be consistent with the ~ observation that the noise in the laser discharge is reduced by directing a magnetic field to the cathode glow. The magnetic field could be viewed as either modifying (reducing) the ~530~3 potential minimum or as imparting sufficient kinetic energy to the positive ions to allow them to escape the potential minimum. The use of an external magnet has been shown in this invention to be a particularly simple means of suppressing noise mechanisms attributabte to the existence of a cathode potential minimum.
The discontinuity in conditions at the interface of the narrow capillary and a larger diameter chamber has also been shown by the prior art ~ c~ t ~ci ~
to be c~duetive to the production of noise in a discharge tube. Specifically, the discharge adjusts itself to form a double sheath (finite layer of positive space charges)Jone sheath being the wall sheath which is well known in positive column discharge theory and the other being referred to as a constriction sheath. The constriction sheath consists of a space layer of positive charges which situates itself across the interface of two discharge chambers which have significantly different diameters. It has been suggested that this whole sheath may be set into motion, like a vibrating memhrane stretched across the opening, by the passage of high-velocity electrons through it. It is believed that, in the current invention,a transverse magnetic field set up by proper placement of the magnet(s) influences both the structure of the positive-ion sheath and the motion of the high energy electrons in such a way that this source of noise is effectively suppressed.
It should be noted that magnetic fields have been used in the past to cure various problems associated with the operation of positive column lasers. Solenoidal magnets, for example, are commonly used in positive column argon and krypton ion lasers to enhance optical gain by constricting the discharge.
Inhomogeneous magnetic fields transverse to the discharge axis are commonly used in positive column helium neon lasers to enhance 6328A output by suppressing , j ~S3(~93 the gain of the competing 3.39~6~ transition.
The present invention utilizes magnets primarily to suppress optical noise in positive-column helium-cadmium lasers. It is believed that the technique described could be utilized with other positive column type gas lasers although the noise mechanism (striation waves interacting with double sheaths) may in some cases not be necessarily operative in the same current and pressure ranges which the gas media require for efficient laser operation.
The specific application of the magnets provides a localized relatively high intensity magnetic field in the range from about 250 gauss to about 800 gauss, the preferred range being from about 400 to about 600 gauss, in the region of the laser tube near the cathode end of the discharge capillary bore 14 whereby the optical noise is suppressed as much as 10 to 50 times~ The minimum value of the field required for noise suppression may vary between 250 and 300 gauss whereas fields above 700 gauss tend to cause the laser to be noiser than at the lower range and may cause the laser to be noiser than without a magnet.
It has been observed that the output power of these lasers is enhanced from 10% to about 50% as the optical noise is suppressed. An arrangement of mag-nets 200 mounted adjacent to or on the surface of the outer tube envelope 12 is provided, the figure illustrating the use of a pair of magnets 202 and 204 on the cathode end of the laser tube 10 and positioned on one side of the tube 10. The two magnets are positioned on a magnetic circuit such that the mag-netic fields generated by each magnet are coupled and thereby enhanced, the field generated being substantially transverse to the optical ~resonator) axis and constant in time. Although the mechanism believed responsible for noise suppression depends on a transverse field, a field with lines of force that are curved may suffice provided that it has a strong enough transverse componen-t.

1~ ~3~3 Alternate configurations for the magnets may be provided. For example, a magnet configuration having a magnetic circuit with two magnets on one side and one magnet on the other and a connecting metal (magnetic conductor) bridge may also be utilized. A magnetic circuit consists of a single piece of magnetic material, such as Alnico, (trade mark) or two pieces of magnetic material which are connected by a magnetically susceptible material, such as iron, shaped in a form in which the two poles face each other across an air gap. A highly directional magnetic field can be obtained with this arrange ment with the lines of force being directed from one pole face to the other.
Figures 2(a)-2(c) are simplified representations showing two magnets 206 and 208 at both sides of the tube envelope 12 and connected by magnetic circuit 210, the magnets being positioned at alternative positions adjacent the outer tube envelope 12 in proximity to the cathode end of the dis-charge capillary tube and functioning to suppress optical noise and enhancing output power as described herein.
While the invention has been described wlth reference to its preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equiva-lents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings.

Claims (4)

CLAIMS:

1. In a positive column helium-cadmium, gas laser wherein there is a cathode discharge glow, a positive column discharge glow and a discontinuity therebetween which com-prises: an envelope; an active non-flowing gaseous medium disposed within said envelope, said gaseous medium being supplied from a relatively steady reservoir; at least one anode and one cathode disposed within said envelope; laser mirror means disposed at opposite ends of said gas laser envelope along the optical axis thereof for confining the active gaseous medium within said envelope and for forming an optical resonant cavity; one of said laser mirror means being capable of allowing a portion of the laser radiation generated within said envelope to be emitted therefrom;
a capillary tube positioned within said envelope for the positive column discharge glow; and means for applying a potential between said anode and cathode to provide a discharge in said active gaseous medium which results in stimulated emission gain for continuous wave generation of laser radiation; the improvement comprising:
magnetic circuit means positioned relative to said envelope in the vicinity of said cathode discharge glow or at said discontinuity between said positive column discharge glow and said cathode discharge glow for producing a magnetic field having an intensity in the range from about 250 gauss to about 800 gauss and having a component transverse to the optical axis of the laser envelope, said magnetic circuit means comprising two poles of magnetic material facing each other across a gap, said laser envelope being positioned in said gap and between said poles, said gap being selected to provide a highly directional magnetic field with the lines of force being directed from one pole face to the other, said magnetic field intensity being of sufficient strength to break down any double sheath space charges within the envelope; thereby improving the optical noise and output power performances of said laser.

2. The gas laser as defined in Claim 1 wherein said magnetic circuit means comprises a single piece of magnetic material.

3. The gas laser as defined in Claim 1 wherein said magnetic circuit means comprises at least two pieces of magnetic material connected by a magnetically susceptible material.

4. The gas laser as defined in Claims l or 2 wherein said magnetic circuit means is positioned in the vicinity of said discharge glow from said cathode.