LASER OSZILLATOR

Abstract: The laser oscillator establishes r-f discharge by applying an r-f voltage to a number of discharge regions via a dielectric member. The laser oscillator has r-f power sources (26a,26b) and further has current detectors (CT2a,CT2b), arranged to detect currents flowing through discharge tubes (1) and mutual current flowing between the neighbouring discharge tubes. The currents of the current detectors (CT2a,CT2b) are used as a feedback currents to prevent the combination current from getting out of control. Also provided is a device to prevent interference in the discharge regions.

Description: The present invention relates to a laser oscillator device for producing a high-power laser beam for cutting workpieces of metal or the like, and more particularly to a laser oscillator device capable of stable high-frequency discharge pumping. High-frequency discharge pumped axial-flow CO₂ lasers are in the process of being developed because of their advantages of high-power output and stable oscillation. One example of such laser is disclosed in Japanese Patent Application No. 61-243212, of which EP-A-0287677 is an equivalent. A conventional axial-flow-type high-frequency discharge pumped laser oscillator device is illustrated in FIG. 10 of the accompanying drawings. The illustrated laser oscillator device includes a discharge tube 1 comprising four tube segments. However, the discharge tube 1 may have a desired number of tube segments dependent on the output to be produced. A total reflection mirror 2 and an output coupling mirror 3 are accurately positioned at the ends of the discharge tube 1. Denoted at 4 is an outgoing laser beam. The segments of the discharge tube 1 have gas inlet and outlet ports connected to a single roots blower 7. Cooling units 5, 6 serve to cool a laser gas heated by the discharge and the roots blower 7. The laser gas flows in the discharge tube 1 and gas delivery tubes in the directions of the arrows. Electrodes 8a, 8b - 11a, 11b are connected to respective high-frequency power supplies 12, 13, 14, 15. The gas flows in the discharge tube 1 at a speed of about 100 m/second. An electric discharge is produced in the discharge tube 1 by a high-frequency voltage applied by the high-frequency power supplies 12 - 15 to generate laser oscillation. The principles of a conventional high-frequency power supply are illustrated in Fig. 11 of the accompanying drawings. Denoted at 16 is a DC power supply, and 17 a high-frequency (RF) power supply. DC output from the DC power supply 16 is applied to the high-frequency power supply 17. The high-frequency power supply 17 comprises a branch circuit composed of four FETs 18 - 21, a boost transformer 22, and an impedance matching circuit 23. The output terminals of the high-frequency power supply 17 are coupled to the discharge tube 1 through electrodes 8al, 8a2. A CT coil 24 serves to detect a current flowing through the discharge tube 1. A current feedback line 25 feeds back the detected current to control the current supplied to the discharge tube 1 to be constant. The high-frequency discharge pumping laser has the following many advantages over the ordinary DC discharge pumping laser: (i) Since it produces an electrodeless discharge, there is no problem of electrode material deterioration. (ii) No ballast resistor is required. (iii) There is no cathode fall, and high energy efficiency is obtained. (iv) The laser can be operated at a low voltage and hence is safe for the operator. (v) The dissociation ratio of CO₂ is low, and so is the running cost. (vi) The laser has excellent pulse characteristics. (vii) The laser device can be reduced in size. (viii) The phenomenon of electron capture can be utilised by sufficiently increasing the frequency to obtain high efficiency. (ix) The freedom of selection of discharge tube materials is high. However, the current and voltage of the high frequency discharge pumping laser tend to oscillate before and after a discharge is started, based on plasma coupling Oscillation at about 1 - 10 Hz occurs when the discharge current in the discharge tube 1 is controlled by the feedback system shown in FIG. 11. As a result, various alarming conditions for the DC power supply 16 tend to take place, and the FETs 18 - 21 may be destroyed. It is an object of the present invention to provide a high-frequency oscillator device which will solve the aforesaid problems and which will not produce alarming conditions for a DC power supply and also not destroy components such as FETs. According to a first aspect of the present invention, there is provided a laser oscillator device for applying high-frequency voltages to a plurality of discharge regions through a dielectric to produce a high-frequency discharge for laser pumping, the device comprising: high-frequency power supplies for converting a DC power supply into high-frequency power supply outputs; and, current detectors for detecting currents flowing through the discharge regions and providing feedback signals in accordance with the detected currents for controlling the output currents of the high-frequency power supplies; characterised by: current detectors for detecting mutual currents between adjacent discharge regions and for providing feedback signals in accordance with the detected mutual currents for controlling the output currents of the high-frequency power supplies. According to a second aspect of the present invention, there is provided a laser oscillator device for applying high-frequency voltages to a plurality of discharge regions through a dielectric to produce a high-frequency discharge for laser pumping, the device comprising: high-frequency power supplies for converting a DC power supply into high-frequency power supply outputs; and, current detectors for detecting currents flowing through the discharge regions and for providing feedback signals in accordance with the detected currents for controlling the output currents of the high-frequency power supplies; characterised by: a common signal source for driving the high-frequency power supplies, the high-frequency voltages being applied to the discharge regions in phase. According to a third aspect of the present invention, there is provided a laser oscillator device for applying high-frequency voltages to a plurality of discharge regions through a dielectric to produce a high-frequency discharge for laser pumping, the device comprising: high-frequency power supplies for converting a DC power supply into high-frequency power supply outputs; and, current detectors for detecting currents flowing through the discharge regions and for providing feedback signals in accordance with the detected currents for controlling the output currents of the high-frequency power supplies; characterised by: signal sources of different frequencies for driving the high-frequency power supplies; and, low-pass filters for removing high-frequency components from the feedback signals from the current detectors. According to a fourth aspect of the present invention, there is provided a laser oscillator device for applying high-frequency voltages to a plurality of discharge regions through a dielectric to produce a high-frequency discharge for laser pumping, the device comprising: current detectors for detecting currents flowing through the discharge regions and for providing feedback signals in accordance with the detected currents for controlling the output currents of high-frequency power supplies; characterised by: a current detector for detecting mutual currents between adjacent discharge regions and for providing feedback signals in accordance with the detected mutual current for controlling the output currents of the high-frequency power supplies; and in that: the high-frequency power supplies for converting a DC power supply into high-frequency power supply outputs are parallel-connected. According to a fifth aspect of the present invention, there is provided a laser oscillator device for applying high-frequency voltages to a plurality of discharge regions through a dielectric to produce a high-frequency discharge for laser pumping, the device comprising: high-frequency power supplies for converting a DC power supply into high-frequency power supply outputs; and, current detectors for detecting currents flowing through the discharge regions and for providing feedback signals in accordance with the detected currents for controlling the output currents of the high-frequency power supplies; characterised by: grounded auxiliary electrodes disposed on intermediate portions of the discharge regions. The aforesaid problems are caused by a mutual current between electrodes. The drawbacks will be described with reference to an arrangement having two electrodes for the sake of brevity. FIG. 2 shows a current flowing between electrodes. Denoted at 1 is a discharge tube. High-frequency voltages are applied to electrodes 8al, 8a2 and electrodes 8bl, 8b2 by high-frequency power supplies 26a, 26b, respectively. It is assumed here that the high-frequency power supply 26a has a frequency fl, a voltage el, and a phase 1, and an inverter 26b has a frequency f2, a voltage e2, and a phase 2. Rogowski coils are positioned as CTl, CT2, and CT3. Currents flowing through these three coils will be described. The following three discharge currents flow in the discharge tube 1: In-tube discharge current = Id Intertube coupling current = Is Power supply loss current = Ir The discharge current Id is given by: Id = e/Z where Z is the impedance between the discharge electrodes 8a, 8b. The intertube coupling current Is is a current flowing between the adjacent electrodes on the discharge tube. This current flows as a displacement current at a high frequency since ions remain in a downstream region of the gas. The current Is is difficult to control because the impedance z between the electrodes 8a1, 8b1 is abruptly reduced upon a transition from a glow discharge to an arc discharge due to unstable discharge property. The current Is is varied by the difference in phase between the voltages e1, e2. The loss current Ir is a current drained from the power supply to ground and fed back to the circuit from the grounded point. Generally, the loss current Ir largely depends upon the temperature. The currents flowing through the respective CTs can be determined as follows: [Current I₁ of CT1] I₁ = Id + Is (when e1, e2 are in phase), I₁ = Id + 3Is (when e1, e2 are in opposite phase). Generally, [Current I₂ of CT2] I₂ = Id + Is (when e1, e2 are in phase), I₂ = Id (when e1, e2 are in opposite phase). Generally, [Current I₃ of CT3] I₃ = I₁ + Ir. These currents are plotted in FIG. 3. The graph of FIG. 3 has a horizontal axis representing the phase difference between the voltages e1, e2 and a vertical axis representing the values of the currents. The curves indicate the values of the currents I₁, I₂, I₃, respectively. The current I₃ is however indicated as a web-like curve having a certain width since the current varies with temperature. [Operation of the first aspect of the invention] In FIG. 3, the current I₂ is of a maximum value (Id + Is) when the voltages e1, e2 are in phase, and of a minimum value ( Id = I - Is ) when the voltages e1, e2 are in opposite phase (the phase difference is π). I indicates all the current supplied from the power supply. When the current Is is produced by an arc discharge, it provides a considerable portion of the entire current I as the impedance z between the discharge tubes is greatly reduced. As a consequence, the current from a high-frequency power supply 26a is abruptly lowered under feedback control, thus destroying the FETs 18 ∼ 21 shown in FIG. 11. The feedback current should be I₁ (CT1) or I₃ (CT3). Where I₁ (CT1) is used as the detected current, CT1 is on the high-voltage size. Therefore, the Rogowski coil used as CT1 must be capable of withstanding a high voltage. [Operation of the second aspect of the invention] Prior to the start of a discharge, the voltage oscillates, and after a discharge has started, the current oscillates. Since the oscillating voltage may well go beyond a prescribed voltage, it tends to produce various alarming conditions for the power supply, stop the device, and destroy the FETs or the like in the worst cases. The current oscillation makes the laser output unstable. This is caused by using any of the currents as the feedback current. An example which employs I₁ (CT1) will be described below. I₁ = Id(t) + Is{2 - sin2π(f1 - f2)t} Since the I₁ (CT1) is maintained at a constant level K when it is fed back, Id(t) becomes: Id(t) = K - Is{2 - sin2π(f1 - f2)t} and fluctuates at the period of (f1 - f2). Inasmuch as the quartz oscillator of a drive signal source used has a six-figure degree of accuracy, the period of (f1 - f2) is in the rage of 1 to 10 Hz. This also holds true for the oscillation before the discharge. Therefore, the oscillation is removed by applying the voltages to the electrodes in phase. [Operation of the third aspect of the invention] The oscillation is eliminated by removing an AC component from the feedback current signals with the low-pass filters by making the frequencies of the power supplies widely different from each other. [Operation of the fourth aspect of the invention] The oscillation is removed by driving the discharge tubes with the common parallel-connected power supplies. [Operation of the fifth aspect of the invention] Effects of the mutual current are removed by providing the electrode on the intermediate portion of the discharge tube. In the drawings: FIG. 1 is a circuit diagram of a first embodiment; FIG. 2 is a view showing currents between electrodes; FIG. 3 is a diagram showing currents of current detectors; FIG. 4 is a circuit diagram of a second embodiment; FIG. 5 is a circuit diagram of a third embodiment; FIG. 6 is a circuit diagram of a fourth embodiment; FIG. 7 is a circuit diagram of a first modification of the fourth embodiment; FIG. 8 is a circuit diagram of a second modification of the fourth embodiment; FIG. 9 is view showing a fifth embodiment; FIG. 10 is a view showing the principles of a conventional axial-flow RF discharge pumped laser oscillator device; and FIG. 11 is a view illustrating the principles of a conventional high-frequency power supply circuit for laser pumping. Best Mode for Carrying Out the Invention Embodiments of the present invention will hereinafter be described in specific detail with reference to the drawings. [First embodiment] FIG. 1 is a circuit diagram of a first embodiment of the present invention. Designated at 26a, 26b are high-frequency power supplies, 22a, 22b boosting transformers, 23a, 23b impedance matching circuits, 1 discharge tubes, and CT1a, CT1b, CT2a, CT2b, CT3a, CT3b current detectors (Rogowski coils). Since the circuit arrangement has two identical circuits on its righthand and lefthand sides, only the lefthand circuits will be described below. As described above in [Operation], the currents from the current detectors CT1a or CT3a are used as feedback currents for avoiding uncontrollable conditions caused by coupling currents between electrodes (8a1, 8a2 and 8b1, 8b2). Use of CT1a or CT3a has both advantages and disadvantages. Where CT1a is used, it is not affected by the power supply loss current Ir (current drained to ground), but has a high voltage. Where CT3a is used, it is affected by the power supply loss current Ir, but it may be a low-voltage current detector. [Second embodiment] FIG. 4 is a circuit diagram of a second embodiment. Those parts in FIG. 4 which are identical to those of the FIG. 1 are denoted by identical reference characters, and will not be described. In this embodiment, the problem of unstable current and voltage due to the phase difference between the electrodes can be solved by driving the high-frequency power supplies 26a, 26b with one signal source (the same clock) 33. The signal source 33 has one quartz oscillator for driving the high-frequency power supplies 26a, 26b. Where the number of the electrodes used is increased, feedback signal lines 34a, 34b become longer, and hence should be sufficiently protected against noise. [Third embodiment] FIG. 5 is a circuit diagram of a second embodiment. Those parts in FIG. 5 which are identical to those of the FIGS. 1 and 4 are denoted by identical reference characters, and will not be described. Denoted at 16 is a DC power supply, and 35 a low-pass filter. In this embodiment, the power supplies have different drive frequencies, respectively. For example, if f1 = 2 MHz, and f2 = 2.1 MHz, then the current I₁ (CT1) becomes: I₁ = Id + 2Is + AC (100 KHz) and hence is the sum of the DC component and the AC component. Where a low-pass filter having a cutoff frequency of 10 KHz is used, the AC component is removed leaving only the DC component, so that stable current control is made possible. With current detectors located at CT2, CT3, the currents become: I₂ = Id + Is/2 + AC(100 KHz) I₃ = Id + 2Is + Ir + AC(100 KHz). In this case, stable current control can be performed no matter where current detectors may be positioned. [Fourth embodiment] FIG. 6 is a circuit diagram of a fourth embodiment. Those parts in FIG. 6 which are identical to those of the FIG. 2 are denoted by identical reference characters, and will not be described. In this embodiment, a single high-frequency power supply 26 supplies electric power. The electrodes are interconnected by a lead 36. While the two electrodes are coupled, a suitable number of electrodes may be interconnected dependent on the capacity of the power supply and the output wattage thereof. FIG. 7 shows a circuit arrangement in which four high-frequency power supplies 26a ∼ 26d are connected in parallel. Those parts which are identical to those of FIGS. 4 and 6 are designated by identical reference characters, and third and fourth ones of the same parts are denoted by identical reference characters with suffixes c, d, respectively. To the power supplies, there are connected in series ballast inductances L1 - L4 which are connected in parallel. A capacitor C1 and an inductance L5 are also connected as a smoothing circuit. FIG. 8 illustrates a circuit arrangement in which four identical high-frequency power supplies are connected in parallel. Although not shown, each of the power supplies has therein a loss circuit for producing a loss ranging from 10 to 30 % so that even if any of the high-frequency power supplies is destroyed, the other high-frequency power supplies will not be affected. [Fifth embodiment] FIG. 9 is a circuit diagram of a fifth embodiment. Those parts in FIG. 9 which are identical to those of the FIG. 2 are denoted by identical reference characters, and will not be described. Those currents flowing in the lefthand circuit are identified by a suffix a, and those in the righthand circuit are identified by a suffix b. In this embodiment, a third electrode 37 is disposed intermediate between the electrodes 8a, 8b and connected to ground. The electrode 37 allows mutual currents Isa, Isb to flow to ground, thus preventing plasma coupling due to the mutual currents. This arrangement is electrically simple. However, since the current Is is greater than would be if the electrode 37 were not present and hence in order to prevent the current Is from being produced in the arc-discharge mode, it is necessary that the distances between the electrode 37 and the electrodes 8a, 8b be about twice the distance between the electrodes on the discharge tube. This is to avoid being subjected to a C ballast due to one of the discharge tube walls when the currents flow into the electrode 37. With the present invention, as described above, inasmuch as the device is not affected by mutual currents between the electrodes, various alarming conditions resulting from uncontrollable operation of the feedback loop and damage of semiconductor parts such as FETs are prevented from occurring, and stable current control can be performed.

Claims: A laser oscillator device for applying high-frequency voltages to a plurality of discharge regions through a dielectric to produce a high-frequency discharge for laser pumping, the device comprising: high-frequency power supplies (26a, 26b) for converting a DC power supply into high-frequency power supply outputs; and, current detectors (CT₁, CT₃) for detecting currents (Id) flowing through the discharge regions and providing feedback signals in accordance with the detected currents for controlling the output currents of the high-frequency power supplies; characterised by: current detectors (CT₁,CT₃) for detecting mutual currents (Is) between adjacent discharge regions and for providing feedback signals in accordance with the detected mutual currents (Is) for controlling the output currents of the high-frequency power supplies. A device according to claim 1, wherein the current detectors (CT₁) are disposed on high-voltage sides of the discharge regions. A device according to claim 1, wherein the current detectors (CT₃) are disposed between grounded points of the discharge regions and low-voltage sides of the high-frequency power supply outputs. A laser oscillator device for applying high-frequency voltages to a plurality of discharge regions through a dielectric to produce a high-frequency discharge for laser pumping, the device comprising: high-frequency power supplies (26a,26b) for converting a DC power supply into high-frequency power supply outputs; and, current detectors (CT₁,CT₂,CT₃) for detecting currents flowing through the discharge regions and for providing feedback signals in accordance with the detected currents for controlling the output currents of the high-frequency power supplies; characterised by: a common signal source (33) for driving the high-frequency power supplies, the high-frequency voltages being applied to the discharge regions in phase. A laser oscillator device for applying high-frequency voltages to a plurality of discharge regions through a dielectric to produce a high-frequency discharge for laser pumping, the device comprising: high-frequency power supplies (26) for converting a DC power supply into high-frequency power supply outputs; and, current detectors (CT₁,CT₂,CT₃) for detecting currents flowing through the discharge regions and for providing feedback signals in accordance with the detected currents for controlling the output currents of the high-frequency power supplies; characterised by: signal sources (16) of different frequencies for driving the high-frequency power supplies; and, low-pass filters (35) for removing high-frequency components from the feedback signals from the current detectors (CT₁,CT₂,CT₃). A laser oscillator device for applying high-frequency voltages to a plurality of discharge regions through a dielectric to produce a high-frequency discharge for laser pumping, the device comprising: current detectors (CT₁) for detecting currents (Id) flowing through the discharge regions and for providing feedback signals in accordance with the detected currents for controlling the output currents of high-frequency power supplies; characterised by: a current detector (CT₁) for detecting mutual currents (Is) between adjacent discharge regions and for providing feedback signals in accordance with the detected mutual current (Is) for controlling the output currents of the high-frequency power supplies; and in that: the high-frequency power supplies (26a-d) for converting a DC power supply into high-frequency power supply outputs are parallel-connected. A laser oscillator device for applying high-frequency voltages to a plurality of discharge regions through a dielectric to produce a high-frequency discharge for laser pumping, the device comprising: high-frequency power supplies (26a, 26b) for converting a DC power supply into high-frequency power supply outputs; and, current detectors (CT₁) for detecting currents flowing through the discharge regions and for providing feedback signals in accordance with the detected currents for controlling the output currents of the high-frequency power supplies; characterised by: grounded auxiliary electrodes (37) disposed on intermediate portions of the discharge regions.

Inventors: EGAWA, AKIRA FANUC MANSION HARIMOMI 3-303, IEHISA, NOBUAKI, KARUBE, NORIO, MANABE, MITSUO MIYAZAKI BLDG., 303, YAMAZAKI, ETSUO
Applicants: FANUC LTD.
Application year: 1988
Main classification: H01S3
IPC codes: H01S3