COAXIAL CO2 LASER UTILIZING HIGH-FREQUENCY EXCITATION

Abstract: The coaxial carbon dioxide laser comprises a pair of electrodes (1,2) a laser pipe (3) containing a carbon dioxide laser gas flowing in the axial direction, a high-frequency power source (4), a totally reflective mirror (5) and an output coupling mirror (6). The electrodes are formed by depositing two helical conductors of the same pitch onto the outer surface of the laser pipe. By applying a high-frequency voltage, a circularly symmetrical laser light (b) of TEMOO mode can be obtained.

Description: The present invention relates to a coaxial type CO₂ laser which is adapted to be excited upon application of high frequency. A high frequency discharge excited CO₂ laser may have the following features: i) metallic electrodes and carbon dioxide gas (CO₂) are not in contact with each other, so that no chemical reaction takes place therebetween; ii) being of capacitive ballast type, the laser has a high energy efficiency; and iii) since transverse electric discharge requires relatively low voltage, employment of a power source composed of a solid state device will suffice, so that the size of the equipment can be reduced. For these reasons, a high frequency discharge excited CO₂ laser can comprise an oscillator which is i) small in size, and ii) high in efficiency. However, transverse electric discharge is generally used in the case of high frequency discharge excited lasers and the resulting gain distribution over the cross section of the laser tube does not have circular symmetry. Accordingly, a high frequency discharge excited laser is used in combination with a transverse gas flow whose gain distribution similarly is not circularly symmetrical, wherefore a good beam mode is not to be expected. For this reason, a DC discharge excited coaxial type CO₂ laser is used exclusively in applications where good beam mode is a requirement. However, in respects other than beam mode, the DC discharge excited coaxial type laser is inferior to a high frequency discharge excited type laser. There have already been disclosed in the prior art high frequency discharge excited coaxial type CO₂ lasers wherein two helical electrical conductors are affixed to an outer periphery of the laser tube. Thus see US-A-3427567, JP-A-56-138976 and JP-A-60-3170, the first of which discloses a laser according to the precharacterising part of the attached claim 1. It is an object of the present invention to provide a high frequency discharge excited coaxial type CO₂ laser, which at least partially overcomes the disadvantages of the prior art, and is equipped with both the advantages of a high frequency discharge excited type laser and those of a coaxial type laser, and therefore is able to produce a good beam mode. According to the present invention there is provided a high frequency discharge excited coaxial type CO₂ laser comprising: a laser tube composed of a dielectric material, in which a CO₂ laser gas is passed in an axial direction through the tube; two helical electrical conductors affixed to an outer periphery of the laser tube, having the same pitch as each other and being films of conductive material; and a high frequency power source operable to apply a high frequency voltage between the two helical electrical conductors, characterised in that said conductors each have a thickness which is small enough to prevent them peeling off the laser tube despite any difference in thermal characteristics between the conductors on one hand and the laser tube on the other hand, and in that the thickness D of the laser tube is given by the expression D = Vεω/2I, wherein when the laser is operational V is the discharge sustaining voltage, ε is the dielectric constant of the laser tube, ω is the frequency of the high frequency power source, and I is the current density. In embodiments of the present invention, the two helical electrical conductors are affixed to the periphery of the laser tube composed of a dielectric material, and the high frequency voltage is applied between the two helical electrical conductors by means of the high frequency power source to cause electrical discharge between the two helical electrical conductors in the laser tube. The CO₂ gas stream flowing in the laser tube possesses circular symmetry with respect to the axis of the laser tube, and the electric discharge occurs in a manner that it traces a helical locus gradually proceeding along, and rotating about the axis of, the laser tube because of the provision of the helical electrical conductors. Hence the mode of the electrical discharge becomes, as a whole, circularly symmetrical and it is possible to obtain the TEMoo mode which is proper or appropriate to the coaxial type laser, without sacrificing the advantages of the excited high frequency discharge such as no chemical reaction taking place between the electrodes and the CO₂ gas, high energy efficiency, and possible utilization of a power source composed of a solid state device. As a result a CO₂ laser for cutting can be made smaller in size and higher in efficiency. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view of part of a high frequency discharge excited coaxial type laser according to an embodiment of the present invention; and Fig. 2 is a cross-sectional view taken along line X - X of Fig. 1. Figs. 1 and 2 show part of a high frequency discharge excited coaxial type laser according to an embodiment of the present invention, wherein reference numeral 3 designates a laser tube in the form of a dielectric pipe having a circular cross section as shown in Fig. 2. The CO₂ laser gas is passed axially in the laser tube 3. Applied helically on the periphery of the laser tube are two stripes of conductive paint such as aquadag®, which form helical filmy electric conductors 1, 2 having the same pitch and a phase difference of 180 o ; a high frequency voltage from a high frequency power source 4 being applied between the two conductors 1, 2. Reference numerals 5 and 6 refer to a total reflection mirror and an output coupling mirror, respectively. In the laser thus constructed, the CO₂ laser gas is passed in the laser tube 3 in the direction indicated by an arrow a in Fig. 1, and the high frequency voltage is applied between the two helical conductors 1 and 2 by the high frequency power source 4, whereby in the laser tube 3 electric discharge occurs between the diametrically opposite faces of the conductors 1 and 2 which oppose each other across the dielectric layer of the laser tube 3. This electric discharge develops in a manner that it traces a helical locus gradually proceeding along and rotating around the axis of the tube, because of the helical arrangement of the two conductors 1, 2. Therefore, the electric discharge occurs, as a whole, in circular symmetry with respect to the axis of the tube. The CO₂ gas stream, the distribution of which affects the gain distribution and hence the electric discharge mode formation, flows along the axis of the laser tube 3 whose cross section is circular so that, cross-sectionally viewed, the gas stream roughly becomes circularly symmetrical. Consequently, the laser gain becomes circularly symmetrical to a great extent and therefore the laser beam b , which is emitted from the laser tube 3 via the output coupling mirror 6, assumes a circular mode, and as a result, the TEMoo mode proper or appropriate to a coaxial type laser is obtained. In view of dielectric breakdown strength, appropriate dielectric materials for the laser tube 3 include barium titanate, quartz, alumina, Mylar® and polyimide. A ceramic tube made of these materials can constitute an advantageous laser tube 3. Let D be the thickness of the laser tube 3, V the discharge sustaining voltage, ε the dielectric constant, ω the frequency of the high frequency power source, and I the current density. Then the voltage drop through the two dielectric layers of the laser tube 3 is given by Id/εω. In order to secure stable electric discharge, the value Id/εω should be equal to the discharge sustaining voltage V of the laser plasma, and hence the appropriate thickness D is given as follows: D = Vεω/2I Therefore, by using a dielectric tube of larger dielectric constant ω and/or increasing the frequency of the high frequency power source, it becomes possible to make the thickness D of the dielectric tube greater, so that if the dielectric tube is to be made of a ceramic no pinholes or the like are formed in the course of manufacturing, whereby stable discharge is obtained and the dielectric tube by itself can provide the laser tube with its required mechanical strength and hence the conductors 1,2 can be made thinner. Thus, if the conductors 1, 2 are formed of thin film, the conductors 1, 2 do not peel off the dielectric body 3, even if the conductors 1, 2 differ substantially from the dielectric body 3 in thermal characteristics such as thermal expansion coefficient, so that it is possible to select a material for the dielectric body 3 purely from the viewpoint of electrical properties and regardless of the thermal properties. Incidentally, in the above embodiment, the filmy conductors 1, 2 are formed by applying a conductive paint, but the filmy conductors 1, 2 may be formed of copper or gold applied by fused metal injection. Or methods like plating and evaporation may be used.

Claims: A high frequency discharge excited coaxial type CO₂ laser comprising: a laser tube (3) composed of a dielectric material, in which a CO₂ laser gas is passed in an axial direction through the tube (3); two helical electrical conductors (1,2) affixed to an outer periphery of the laser tube (3), having the same pitch as each other and being films of conductive material; and a high frequency power source (4) operable to apply a high frequency voltage between the two helical electrical conductors (1,2), characterised in that said conductors (1,2) each have a thickness which is small enough to prevent them peeling off the laser tube (3) despite any difference in thermal characteristics between the conductors (1,2) on one hand and the laser tube (3) on the other hand, and in that the thickness D of the laser tube (3) is given by the expression D = Vεω/2I, wherein when the laser is operational V is the discharge sustaining voltage, ε is the dielectric constant of the laser tube (3), ω is the frequency of the high frequency power source (4), and I is the current density. A high frequency discharge excited coaxial type CO₂ laser as claimed in claim 1, wherein said two helical electrical conductors (1,2) are arranged such that a phase difference of 180 0 exists therebetween. A high frequency discharge excited coaxial type CO₂ laser as claimed in claim 1 or 2, wherein said laser tube (3) is circular in cross section. A high frequency discharge excited coaxial type CO₂ laser as claimed in any preceding claim, wherein said electrical conductors (1,2) are conductive paint applied as films to the outer periphery of said laser tube (3). A high frequency discharge excited coaxial type CO₂ laser as claimed in any preceding claim, wherein the two conductors (1,2) have diametrically opposite faces opposing each other, and wherein electric discharge occurs between the diametrically opposite faces when the laser is in use. A high frequency discharge excited coaxial type CO₂ laser as claimed in any preceding claim, wherein said dielectric material comprises either barium titanate, quartz, alumina, or polyimide. A high frequency discharge excited coaxial type CO₂ laser as claimed in any preceding claim, wherein said laser tube (3) has a thickness which is large enough to prevent pin holes from being formed during manufacturing and to provide said laser tube (3) with sufficient mechanical strength.

Inventors: KARUBE, NORIO
Applicants: FANUC LTD.
Application year: 1987
Main classification: H01S3
IPC codes: H01S3