Gas laser oscillator

Abstract: A gas laser oscillator comprises an ion laser tube (10) provided with a capillary tube (40) and an anode (30) and a cathode (20) as discharging electrodes disposed at its ends, plasma focusing electromagnets (50) to generate the magnetic field for plasma focusing disposed around the ion laser tube (10), and a pair of optical resonance mirrors (60,61) disposed at iespective ends of the ion laser tube (10). The magnetic field strength at the cathode end face (70) is set to . 100 gauss or less,and the diameter of the lines (100) of magnetic flux at the cathode end face (70) is made to be smaller than the diameter of the capillary tube (40) at the capillary tube end face (90). Cathode deterioration and sputtering at the end of the capillary tube are both reduced, prolonging the life of the ion laser tube.

Description: This invention relates to a gas laser oscillator, and particularly relates to a laser oscillator designed to prolong the service life of the ion laser tube. A conventional gas laser oscillator comprises an ion laser tube provided with a capillary tube and a cathode and an anode as discharging electrodes, electromagnets for plasma focusing arranged around the ion laser tube, and a pair of mirrors for optical resonance at the two ends of the ion laser tube. Application of voltage from the power supply across the cathode and the anode causes plasma discharge. The direct current voltage has been already applied to the electromagnets for plasma focusing. The magnetic field given by the plasma focusing magnets works so that the plasma generated between the cathode and the anode is focused. When the ion laser tube discharges, the mirrors at the two ends cause optical resonance and the laser beam is obtained. Since conventional gas laser oscillators as described above are designed considering the magnetic field strength at the capillary tube in the ion laser tube only, the cathode end face also has a considerably high magnetic field strength. Usually, the capillary tube is subjected to the magnetic field of about 0.1 tesla (1,000 gauss) and the magnetic field strength near the cathode end face is about 0.05 to 0.08 tesla (500 to 800 gauss). For this reason, plasma tends to concentrate to a certain limited part on the cathode, which deteriorates the cathode resulting in shorter service life of the ion laser tube. Besides, since the diameter of the lines of the magnetic flux covering the cathode end face becomes larger than the diameter of the capillary tube at the end of the capillary tube, the plasma does not sufficiently focus at the end of the capillary tube. This causes plasma crushing at the capillary tube end involving extremely high temperature, resulting in evaporation at the end face of the capillary tube (sputtering). As a result, substances separated from the end face of the capillary tube due to evaporation make the Brewster windows on the cathode side and the anode side dirty and confine the argon gas (gas laser medium in the ion laser tube) leading to gas consumption, which reduces the service life of the ion laser tube. The present invention is defined in claim 1 below to which reference should now be made. Advantageous features are set forth in the appendant claims. The invention will now be described by way of example with reference to the drawings, in which: Fig. 1 is a schematic diagram to show the configuration of a gas laser oscillator according to an embodiment of the present invention; Fig. 2 is a partial enlarged view of the gas laser oscillator as shown in Fig. 1; Fig. 3 is an explanatory view to illustrate the lines of magnetic flux at the end face of the cathode; and Fig. 4 is a schematic diagram to show the configuration of a gas laser oscillator according to another embodiment of the present invention. Referring to the attached figures, preferred embodiments of a gas laser oscillator according to the present invention will be described below. Fig. 1 is a diagram to show the outline configuration of a gas laser oscillator according to an embodiment of the present invention, and Fig. 2 is a partial enlarged view of that gas laser oscillator. In Figs. 1 and 2, a gas laser oscillator according to the present invention comprises an ion laser tube 10, an electromagnet for plasma focusing 50 and a pair of optical resonance mirrors 60 and 61. The ion laser tube 10 contains gas laser medium such as argon gas, and consists of a capillary tube 40 provided with a through hole having a diameter of D at the center, and a cathode 20 and an anode 30 as discharging electrodes disposed at the ends of the capillary tube 40. The capillary tube 40 is made of, for example, ceramic or other similar materials. The electromagnet for plasma focusing 50 is disposed so as to surround the ion laser tube 10. The optical resonance mirror 60 is of total reflection type and the optical resonance mirror 61 on the side of the anode 30 is a transmission type mirror which transmits several tens of percents of the light. Application of voltage from a power supply (not shown) across the cathode 20 and the anode 30 of the ion laser tube 10 causes discharge, which generates plasma. The plasma focusing electromagnet 50 is placed under the direct current voltage in advance so that it causes a magnetic field which focuses the plasma generated between the cathode 20 and the anode 30. When the ion laser tube 10 performs plasma discharge, the optical resonance mirrors 60 and 61 cause optical resonance so as to generate the laser beam. In this embodiment, the magnetic field generated by the plasma focusing electromagnet 50 is designed to be not more than 0.01 tesla (100 gauss) at the end face 70 of the cathode 20, because .we have found that a magnetic field not exceeding 0.01 tesla (100 gauss) prevents plasma from being concentrated to a limited part on the cathode end face 70, resulting in much longer service life of the cathode 20. Specifically, we have found from experiments that the service life of the cathode 20 when the magnetic field at the cathode end face 70 is 0.01 tesla (100 gauss) is twice the value when the magnetic field is 0.04 tesla (400 gauss) (substantially the same magnetic field strength for conventional gas laser oscillators). In addition, the gas laser oscillator of this embodiment is designed to have the lines of magnetic flux 100 which cover the cathode end face 70 smaller at the end face 90 of the capillary tube 40 than the diameter D of the capillary tube at the end face 90 of the capillary tube 40 as shown in Fig. 2. Fig. 3 shows the lines of magnetic flux 100 between the capillary tube end face 90 and the cathode end face 70. The ordinate axis is for the magnetic field strength and the abscissa axis is for the position of the cathode end face 70 and the capillary tube end face 90 in the ion laser tube 10. In the figure, Bc represents the magnetic field at the cathode end face 70 (tesla/gauss), rc is the radius of the lines of magnetic flux which cover the cathode end face 70 (ie the radius of the cathode end face itself) when the magnetic field is Bc (mm), B is the magnetic field at the capillary tube end face 90 (tesla/gauss) and r is the radius of the lines of magnetic flux at the capillary tube end face 90 when the magnetic field is B of the lines of magnetic flux which cover the cathode end face. Under these conditions, Bc × π × rc 2 = B × π × r 2 and the radius r of the lines of magnetic flux at the capillary tube end face 90 can be determined based on the above equation: r = rc Bc/B In this embodiment, the magnetic field of the plasma focusing electromagnet 50 is adjusted so that the magnetic field Bc at the cathode end face 70 does not exceed 0.01 tesla (100 gauss) and that the diameter (=2r) of the lines of magnetic flux which cover the cathode end face determined by the magnetic field strength B at the capillary tube end face 90 is smaller than the diameter D of the capillary tube. The magnetic field of the plasma focusing electromagnet 50 can be adjusted by changing the direct current applied thereto or by changing the winding density of the coil in the plasma focusing electromagnet 50. The magnetic field can be determined from experiments as in the conventional practices. There are several methods to keep the magnetic field strength at the end face 70 of the cathode 20 at 0.01 tesla (100 gauss) or less. In addition to the method where the magnetic field strength caused by the plasma focusing electromagnet 50 is changed, the following methods may be used. Noting that the magnetic field strength in the ion laser tube 10 depends on the position in the ion laser tube 10 as shown in Fig. 3, the cathode end face 70 may be moved to a position where the magnetic field becomes 0.01 tesla (100 gauss) or less while keeping the magnetic field strength caused by the plasma focusing electromagnet 50 as it is. Specifically, the cathode end face 70 is moved so as to increase the distance from the plasma focusing electromagnet 50, and then the distance between the plasma focusing electromagnet 50 and the cathode end face 70 is changed until the magnetic field Bc at the cathode end face 70 becomes 0.01 tesla (100 gauss) or less. Alternatively, the winding start point in the coil of a plasma focusing electromagnet 50 is adjusted until the magnetic field Bc at the cathode end face 70 becomes 0.01 tesla (100 gauss) or less. Then, the diameter of the lines of magnetic flux (=2r) covering the cathode end face determined by the magnetic field strength B at the capillary tube end face 90 is made to be smaller than the diameter D of the capillary tube. Specifically, since the radius r of the lines of magnetic flux at the capillary tube end face 90 varies as shown in Fig. 3 according to the above equation (1), the position of the capillary tube end face 90 is adjusted to a place where the diameter of the lines of magnetic flux 2r becomes smaller than the diameter D of the capillary tube 40. Alternatively, by increasing the winding density of the coil in the plasma focusing electromagnet 50 near the capillary tube end face 90, the strength of the magnetic field B at the capillary tube end face 90 may be enhanced until the diameter of the lines of magnetic flux (=2r) becomes smaller than the diameter D of the capillary tube. With the above setting, electrons generated at the cathode 20 are pulled toward the lines of magnetic flux 100 at the cathode end face 70, and led to the inside of the capillary tube 40. This reduces sputtering at the capillary tube end face 90 and thereby prolongs the service life of the ion laser tube 10. In the above embodiment, the diameter D of the capillary tube 40 is kept constant while the position of the capillary tube end face 90 is moved or the diameter (=2r) of the lines of magnetic flux is adjusted corresponding to the diameter D. Fig. 4 shows another embodiment where the sputtering at the capillary tube end face 70 is suppressed without changing the position of the capillary tube end face 90 or the diameter of the lines of magnetic flux. A gas laser oscillator according to this embodiment, without changing the position of the capillary tube end face 90 or the magnetic field strength, forms a taper 45 as shown in the figure on the side of the capillary tube end face 90 of the capillary tube 40, with setting the diameter of the taper 45 to be larger than the diameter of the lines of magnetic flux. In this case, too, electrons caused by the cathode 20 are pulled toward the lines of magnetic flux and led to the capillary tube 40. Therefore, the sputtering at the capillary end face 90 is suppressed. As learned from the above description, limitation of the magnetic field strength at the cathode end face 70 to a value not more than 0.01 tesla (100 gauss) eliminates concentration of discharge plasma to a limited part of the cathode end face 70, which results in lower deterioration of the cathode 20. In addition, since the diameter of the lines of magnetic flux 100 which cover the cathode end face 70 is designed to be smaller at the capillary tube end face than the capillary tube diameter at the capillary tube end face 90, electrons generated at the cathode 20 are pulled by the magnetic field toward the lines of magnetic flux 100 at the cathode end face 70. This realizes sufficient focusing of plasma at the capillary tube end face 90 with reducing sputtering at the capillary tube end face 90, resulting in prolonged service life of the ion laser tube 10. Various modifications can be made to the above embodiments. For example, the magnetic field strength at the cathode end face 70 is set at 0.01 tesla (100 gauss) or less and at the same time the diameter of the lines of the magnetic flux 100 which cover the cathode end face 70 is set at the capillary tube end face 90 smaller than the diameter of the capillary tube at the capillary tube end face 90 in the above embodiment, but the service life of the ion laser tube 10 can be prolonged just by setting the magnetic field strength at the cathode end face 70 to be 100 gauss or less, or just by setting the diameter of the lines of magnetic flux 100 at the capillary tube end face 90 to be smaller than the diameter of the capillary tube at the capillary tube end face 90. It is intended to cover in the appended claims all such modifications as fall within the true scope of the present invention.

Claims: A gas laser oscillator comprising: an ion laser tube (10) provided with a capillary tube (40), and an anode (30) and a cathode (20) as discharging electrodes disposed at its two ends, plasma focusing electromagnets (50) to generate the magnetic field for plasma focusing disposed around said ion laser tube (10), and a pair of optical resonance mirrors (60,61) disposed at respective ends of said ion laser tube (10), characterized in that said magnetic field is such that it pulls the electrons generated at the cathode (20) toward the lines of magnetic flux at the cathode end face (70) and leads them to the inside of the capillary tube (40) and in that said magnetic field strength is set at 0.01 tesla (100 gauss) or less at said cathode end face (70). A gas laser oscillator according to claim 1, wherein the magnetic field of said plasma focusing electromagnets (50) is adjusted so that the magnetic field strength at said cathode end face (70) is 0.01 tesla (100 gauss) or less. A gas laser oscillator according to claim 1, wherein said cathode (20) is disposed at such a position that the magnetic field strength at the cathode end face (70) becomes 0.01 tesla (100 gauss) or less. A gas laser oscillator according to claim 1, 2 or 3, wherein the diameter of the lines (100) of magnetic flux which cover said cathode end face (70) is made to be smaller at the end face of the capillary tube than the diameter of the capillary tube (40) at the end face of the capillary tube (40). A gas laser oscillator according to claim 4, wherein said plasma focusing electromagnets (50) are disposed so that the diameter of the lines (100) of magnetic flux at said capillary tube end face (90) becomes smaller than the diameter of the capillary tube at the end face of the capillary tube (40). A gas laser oscillator according to claim 4, wherein the positions of the cathode (20) and the capillary tube (40) are determined so that the diameter of the lines (100) of magnetic flux at said capillary tube end face (90) becomes smaller than the diameter of the capillary tube (40) at the capillary tube end face (90). A gas laser oscillator according to claim 4, 5 or 6, wherein a taper (45) having a diameter larger than that of the lines (100) of magnetic flux at said capillary tube end face (90) is formed near the end face of said capillary tube (40).

Inventors: KASAHARA SHINJI, KASAHARA, SHINJI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: H01S3
IPC codes: H01S3