GB1573275A

GB1573275A - Lasers

Info

Publication number: GB1573275A
Application number: GB1733677A
Authority: GB
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1977-01-13
Filing date: 1977-04-26
Publication date: 1980-08-20
Also published as: JPS6130438B2; CA1080838A; FR2377720B1; JPS5388597A; FR2377720A1

Description

(54) LASERS (71) We, XEROX CORPORATION, a corporation organised under the laws of the State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to laser discharge tubes employed in lasers such as may be used for optical data processing systems.

The use of lasers in optical data processing systems such as facsimile devices, digital printers is known. A single laser which provides light of single wavelength may be generally utilized for scanning information on a document, the reflected radiation flux being electrically transferred to a storage device or utilized to reproduce the information as a copy of the original document. A scanning laser is generally utilized to reproduce the document information (or for printing purposes only). Typically, a heliumneon laser which generates red laser light when energized has been utilized in many scanning/ reproducing applications. For example, Xerox Corporation, Stamford Connecticut, recently introduced a facsimile device, the Xerox Telecopier 200 ("Xerox" and "Telecopier" are registered trademarks of Xerox Corporation) which records on plain paper.The transceiver employs a lowenergy helium-neon laser and uses the xerographic principle to receive and print messages on ordinary, unsensitized paper.

Basically, when the transceiver is in the transmit mode, the laser provides a small stable beam of light to raster scan the original document. The reflected light is detected by a photosensor which translates the white and black of the document to electrical logic levels which may be transmitted by a phone line to a remote transceiver set to the receive mode. The receiver transceiver directs the laser beam onto a xerographic drum and by electrically modulating the laser with "1" and "0" logic levels in synchronism with the transmitter produces a copy of the original.

However, it would be desirable if a single laser could be provided to produce simultaneous laser radiation of more than one wavelength to allow the accurate reproductions of documents which contain information in other than black and white form, i.e. multicolored documents.

Although lasers have been produced in the prior art which are capable of producing multiline emissions simultaneously, such as an argon laser, these lasers are generally expensive and large in size, making them impractical for use in commercial systems, such as the Telecopier 200 transceiver described hereinabove.

An article in the Proceedings of the IEEE, He-Ne-Cd Laser with Two Color Output, S. A.

Ahmed et al, November, 1969, pages 20842085, describes, inter alia, a helium-neoncadmium laser which produces simultaneous lasing at 4416A and 6328A. However, the laser discharge essentially occurs through a single discharge tube of a single diameter making optimum adjustment of the blue (441 6A) and red (6328A) laser light extremely difficult.

Therefore, a simplified and relatively inexpensive laser which can produce a multiline emissions simultaneously and which can be adapted for commercial utilization would satisfy an apparent need in optical data processing technology.

The present invention provides a laser discharge tube for producing an output laser beam having a plurality of wavelengths comprising: a tube envelope enclosing first and second active lasing media, said tube envelope including end members hermetically sealing said envelope, a first discharge tube supported within said envelope, a second discharge tube supported within said envelope and coaxially aligned with said first discharge tube, a first electrode positioned adjacent one end of said first discharge tube, a second electrode positioned in operative relationship with one end of said second discharge tube, and wherein, when an electrical potential is applied, in use, between said first and second electrodes sufficient to maintain a discharge current between said electrodes through said first and second discharge tubes, said first active lasing medium produces a first laser beam of at least a first wavelength and said second active lasing medium produces a second laser beam comparing at least a second wavelength, whereby said first and second laser beams are combined into a laser output beam having a plurality of wavelengths.

In a first embodiment of the invention, a unitary positive column laser comprises two sections, the first section comprising a positive column helium-cadmium laser, the second section comprising a positive column heliumneon laser, the first and second sections being coaxially aligned and having different inner diameters. Simultaneous excitation of each section provides optimum excitation for red laser light, produced by the helium-neon section, and blue laser light, produced by the helium-neon-cadmium section. The present system also allows separate cadmium vapor pressure control by separately controlling the vaporization temperature of the cadmium and also allows confinement of the cadmium vapor whereby the vapor does not contaminate one of the optical windows which confines the active laser medium.By proper selection of the optical cavity parameters, simultaneous red and blue laser oscillations can be obtained for application in any optical data processing system that requires a red and blue laser radiation source. In a second embodiment, the helium-cadmium section is replaced by with a helium-selenium section whereby the tandem laser system is capable of producing multiline laser radiation in the red, blue and green colors.

Preferred features of the present invention will be described with reference to the accompanying drawings, given by way of example, wherein: Figure 1 shows a first embodiment of the present invention which simultaneously generates red and blue laser light, and Figure 2 shows a second embodiment of the present invention which simultaneously generates red, green and blue (white) laser light.

Referring now to the figure, a first embodiment of the laser discharge tube (assembly 10) of the present invention is illustrated. Laser assembly 10 comprises an outer tube envelope 12, made of Pyrex (Trade Mark) glass for example, and an inner capillary discharge tube 14. Capillary discharge tube 14 comprises two coaxially aligned positive column sections 16 and 18, each made of glass, section 16 including a flared end portion 20 which, with an annular wall 19 supporting section 18, forms a reservoir 15 for the active laser medium, typically a metal, being utilized. Capillary discharge tube 18 coaxially extends through wall 19 into the reservoir 15, as shown. A cathode electrode 21 is disposed in a side arm 23 of tube envelope 12.

The outer tube envelope 12 has a typical diameter of 45 millimeters and a length of 60 centimeters (from mirror to mirror) whereas discharge tube section 16 has a typical length (including flared portion 20) of 20 centimeters, an inner diameter (non-flared portion) of 4 millimeters and an outer diameter (non-flared portion) of 7 millimeters. Discharge tube section 18 has a typical length of 20 centimeters and inner diameter of 1 millimeter and an outer diameter of 7 millimeters. The ends of the tube envelope 12 are sealed by end mirror assemblies 32 and 34 as shown. The mirror assembly 32 comprises a metal flange 36 sealed to the tube envelope and an apertured metal flange 38 joined thereto. A fully reflecting mirror 40 may be sealed to metal flange 38 by standard techniques or by the technique described in British Specification No. 1 539,912.

Mirror 40 typically comprises a glass substrate upon which is coated a reflecting layer comprising a plurality dielectric layers, the reflecting layer facing inward (within the tube envelope). End mirror assembly 34 comprises a metal flange 42 sealed to the tube envelope and an apertured metal flange 44 joined thereto.

A partially transmissive mirror 46 is sealed to apertured flange 44 by standard techniques or in a manner as described in the aforementioned specification. Mirror 46 comprises a glass substrate upon which is coated a partially transmissive layer of dielectric material, the transmissive layer being positioned within tube envelope 12. As will be set forth hereinafter, mirrors 40 and 46 are appropriately coated with layers of dielectric material such that only a laser beam 50 of a desired wavelength is transmitted by mirror 46, beam 50 being utilized by external apparatus such as for the scanning purposes as set forth hereinabove.

Anode pin 52 is inserted into the space within envelope 12 as shown and are glass sealed to the envelope using standard glass sealing techniques.

Variable voltage source 54 is connected between anode pin 52 and cathode 21 as shown.

In a first embodiment, a helium-neon gas mixture is filled within envelope 12 by standard techniques well known in the art, to a predetermined helium-neon total pressure. In the first embodiment, a few grams, typically 10 grams, of cadmium metal 70 is placed within flared portion 22 of envelope 12. A heater 72 is provided to vaporize the cadmium to a preselected pressure as described hereinbelow.

Although laser assembly 10 functions as a single, unitary device, for purposes of explanation, both sections of the tube will be described separately. The left hand section of the assembly operates as a positive column helium-neon laser tube would operate. That is, the helium-neon gas is introduced into envelope 12 at a preselected pressure (helium at 3.0 Torr and neon at 0.3 Torr) and a discharge is initiated between anode pin 52 and cathode 21, by maintaining a voltage of approximately 1Kv therebetween by adjusting source 54 and adjustable ballast resistor 55. Ballast resistor 55 functions limit the laser tube discharge current.The electrical discharge (from anode 52, through tubes 16 and 18 and to cathode 21), excites the helium atoms to a metastable state which, due to inelastic collisions of the second kind, transfers energy to the neon atoms which are elevated to the population inversion state. The neon atoms, in falling to a lower energy state, emit a laser light of a frequency corresponding to the two different energy levels as is well known in the art. For helium-neon lasers, a red light of a wavelength of 6328A is generated.

A solid cadmium charge 70 is deposited in the reservoir 15 prior to laser tube operation.

Heater 72 is energized and the cadmium metal is vaporized, the preferred vapor pressure being attained by controlling the cadmium temperature. In particular, the cadmium temperature is maintained at approximately 2800C by appropriate control of heater 72. Also confined within the section is gaseous helium and neon at a pressure as set forth hereinabove with reference to the left hand section. When a discharge is initiated between electrode 21 and anode pin 52 via voltage source 54 and ballast resistor 55, it excites the helium atoms to a metastable excited state from which energy is imparted to the vaporized cadmium atoms. This causes the cadmium atoms to ionize to an excited state required for lasing action.The ionized cadmium atoms are then transported along the length of the discharge confining bore tube 18 to cathode 21 via the process of cataphoresis in a manner well known in the art. When the excited ionized cadmium atoms return towards a lower energy state, laser radiation at 441 6A (blue) is produced. The vapor cadmium condenses in regions 74 due to the cooler tube operation thereat.

The gas mixture of helium and neon fills the entire laser tube structure. The cadmium vapor is distributed in capillary tube 18 near cathode 21 by cataphoretic pumping. Cataphoresis also confines the cadmium to the portion of the capillary tube 18 near cathode 21. Thus, the helium-neon gas mixture is the only active medium in capillary tube 16 adjacent anode 52 while the gas and vapor mixture of helium, neon and cadmium is the active medium in the portion of capillary tube 18 nearest cathode 21.

It should be noted that section 16, in addition to providing path for the helium-neon discharge, also provides for the cataphoretic confinement of the cadmium vapor atoms which may diffuse towards mirror 40. This prevents cadmium vapor from condensing on the reflecting surface of mirror 40. The discharge current, in essence, forms a continuous filament passing through both capillary tubes 16 and 18. The inner diameter of capillary tube 16 adjacent anode 52 is chosen to optimize the 6328A output resulting from the excitation reaction of the discharge on the helium-neon gas, the diameter (in millimeters) being approximately equal to io/ 10 wherein io is the discharge current in milliamps (ma). Typically, io is selected to be 40 ma providing an inner diameter of 4 millimeters.The inner diameter of the capillary tube near cathode 21 is chosen to optimize the 441 6A output resulting from the helium-neoncadmium discharge and is equal to io/40. For an io of 40 ma, the diameter is therefor 1 millimeter.

The use of two coaxial discharge tubes having different inner diameters selected to optimize the output therefrom allows the output laser beam 50 to be correspondingly optimized for external utilization.

In summary, after the discharge is initiated and the cadmium vaporized, voltage source 54 causes lasing action to occur. The left hand section in essence provides red laser light, the right hand section in essence providing the blue laser light component of light beam 50 notwithstanding the presence of helium-neon gas therein (helium-cadmium interaction predominates over the helium-neon interaction).

The dielectric coatings on integral mirrors 40 and 46 are selected so that only red and blue light (combined in one beam) is transmittes by mirror 46 as beam 50. Typical dielectric coatings include alternate layers of titanium and silicon dioxide, each layer having a predetermined thickness.

It should be noted that the temperature applied to the cadmium charge 70 determines its vapor pressure (and thus the intensity of the light produced) the cadmium pressure essentially not being affected by the helium-neon pressure.

Although not shown in Figure 1, the simultaneous generation of red and blue light as a single beam 50 can be dispersed into its separate component colors by using a prism or appropriate light filters.

Referring now to Figure 2, a second embodiment of the present invention is illustrated. The embodiment shown is substantially identical to that shown in Figure 1, the only difference being a charge of selenium metal 80 is utilized in the right hand section instead of cadmium.

Selenium vapor, when maintained at a predeter mined temperature of approximately 2700C by heater 72 interacts with the helium-neon gas wherein the helium ions and/or metastable atoms causes the selenium atoms to be positively ionized and excited to a higher energy state.

When the excited selenium atoms return to its initial or ground state, multiline emissions, including blue and green laser light is produced.

The blue and green laser light emissions include the following wavelengths: 4604A (blue), 4976A (blue-green,5069A (green), 5 176A (green), and 5306A (green). The confinement of the selenium vapor is identical to the process described with reference to Figure 1 hereinabove and the inner diameters of capillary discharge tube sections 16 and 18 are substantially identical to the helium-cadmium embodiment described with reference to Figure 1. The laser mirrors, in this embodiment,. are coated for broadband reflectance (from about 4800A to about 6500A).

In operation, after vaporizing the selenium to the appropriate vapor pressure and initiating the discharge in both sections, voltage source 54 and ballast resistor 55 act to maintain the discharge for lasing action, red, blue and green (white) light being simultaneously transmitted as beam 50. As set forth hereinabove with reference to Figure 1, the separate color components of beam 50 can be obtained if desired by utilizing a prism to disperse each color component or by providing appropriate color filters.

The laser assembly described with reference to Figures 1 and 2 hereinabove provides a multiline laser source for many applications, such as laser scanning as described hereinabove.

The laser structure described hereinabove is simple, compact and low cost, the low cost feature arising from the fact that only one set of laser mirrors, one power supply and one unit unitary structure are necessary. Further, no alignment fixturing is required to ensure coaxiality of the multiple color laser beams.

WHAT WE CLAIM IS: 1. A laser discharge tube for producing an output laser beam having a plurality of wavelengths comprising: a tube envelope enclosing first and second active lasing media, said tube envelope including end members hermetically sealing said envelope, a first discharge tube supported within said envelope, a second discharge tube supported within said envelope and coaxially aligned with said first discharge tube, a first electrode positioned adjacent one end of said first discharge tube, a second electrode positioned in operative relationship with one end of said second discharge tube, and wherein when an electrical potential is applied, in use, between said first and second electrodes sufficient to maintain a discharge current between said electrodes through said first and second discharge tubes, said first active lasing medium produces a first laser beam of at least a first wavelength and said second active lasing medium produces a second laser beam comprising at least a second wavelength, whereby said first and second laser beams are combined into a laser output beam having a plurality of wavelengths.

2. A laser discharge tube according to claim 1 wherein the inner diameter of said first discharge tube is different from the inner diameter of said second discharge tube.

3. A laser discharge tube according to claim 2 wherein the inner diameters of said first and second discharge tubes are selected whereby to optimize, in use, the output of said first and second laser beams.

4. A laser discharge tube according to any of claims 1 to 3 wherein said first active lasing medium comprises a gas and said second active lasing medium comprises, in use, a vaporized metal.

5. A laser discharge tube according to claim 4 wherein the other end of said first discharge tube is enlarged to form a reservoir portion for said metal, the other end of said second discharge tube extending into said reservoir portion.

6. A laser discharge tube according to claim 5 further including heater means operatively associated with said reservoir portion for vaporizing said metal.

7. A laser discharge tube according to any of claims 4 to 6 wherein said gas comprises a mixture of helium and neon and said metal comprises cadmium, said first laser comprising red laser light and said second laser beam comprising blue laser light.

8. A laser discharge tube according to any of claims 4 to 6 wherein said gas comprises a mixture of helium and neon gas and said metal comprises selenium, said first laser beam comprising red laser light and said second laser beam comprising blue and green laser light.

9. A laser discharge tube according to any of claims 1 to 8 wherein said end members comprise optical mirrors sealed to an apertured flange member, said apertured flange member being affixed to the ends of said tube envelope.

10. A laser discharge tube according to claim 9 wherein one of said optical mirrors reflects said first and second laser beams and the other of said optical mirrors partially transmits said first and second laser beams as said combined laser output beam.

11. A laser discharge tube according to any of claims 1 to 11 in combination with a means for applying said electrical potential to said first and second electrodes.

12. A combination as claimed in claim 11 wherein said applying means comprises an adjustable voltage source.

13. A laser discharge tube substantially as herein described with reference to Figure 1 or 2 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. heater 72 interacts with the helium-neon gas wherein the helium ions and/or metastable atoms causes the selenium atoms to be positively ionized and excited to a higher energy state. When the excited selenium atoms return to its initial or ground state, multiline emissions, including blue and green laser light is produced. The blue and green laser light emissions include the following wavelengths: 4604A (blue), 4976A (blue-green,5069A (green), 5 176A (green), and 5306A (green). The confinement of the selenium vapor is identical to the process described with reference to Figure 1 hereinabove and the inner diameters of capillary discharge tube sections 16 and 18 are substantially identical to the helium-cadmium embodiment described with reference to Figure 1. The laser mirrors, in this embodiment,. are coated for broadband reflectance (from about 4800A to about 6500A). In operation, after vaporizing the selenium to the appropriate vapor pressure and initiating the discharge in both sections, voltage source 54 and ballast resistor 55 act to maintain the discharge for lasing action, red, blue and green (white) light being simultaneously transmitted as beam 50. As set forth hereinabove with reference to Figure 1, the separate color components of beam 50 can be obtained if desired by utilizing a prism to disperse each color component or by providing appropriate color filters. The laser assembly described with reference to Figures 1 and 2 hereinabove provides a multiline laser source for many applications, such as laser scanning as described hereinabove. The laser structure described hereinabove is simple, compact and low cost, the low cost feature arising from the fact that only one set of laser mirrors, one power supply and one unit unitary structure are necessary. Further, no alignment fixturing is required to ensure coaxiality of the multiple color laser beams. WHAT WE CLAIM IS:

1. A laser discharge tube for producing an output laser beam having a plurality of wavelengths comprising: a tube envelope enclosing first and second active lasing media, said tube envelope including end members hermetically sealing said envelope, a first discharge tube supported within said envelope, a second discharge tube supported within said envelope and coaxially aligned with said first discharge tube, a first electrode positioned adjacent one end of said first discharge tube, a second electrode positioned in operative relationship with one end of said second discharge tube, and wherein when an electrical potential is applied, in use, between said first and second electrodes sufficient to maintain a discharge current between said electrodes through said first and second discharge tubes, said first active lasing medium produces a first laser beam of at least a first wavelength and said second active lasing medium produces a second laser beam comprising at least a second wavelength, whereby said first and second laser beams are combined into a laser output beam having a plurality of wavelengths.