GB2187881A

GB2187881A - Silent discharge laser

Info

Publication number: GB2187881A
Application number: GB08605768A
Authority: GB
Inventors: Akira Eqawa; Hakaru Mizoguchi; Ryoichi Nodomi; Jun Ebihara; Kiyonori Matsuno
Original assignee: Komatsu Ltd
Current assignee: Komatsu Ltd
Priority date: 1986-03-11
Filing date: 1986-03-08
Publication date: 1987-09-16
Also published as: GB8605768D0; DE3607948A1

Abstract

A silent discharge laser system in which two reactors 21, 22 are connected respectively in parallel and in series with two discharge electrodes 2,3 between which silent discharge takes place. The values of the inductance of reactors 21, 22 are chosen so as to maximise the power factor. The laser gas 5 can be circulated by blowers 8 through the discharge region 4. <IMAGE>

Description

SPECIFICATION Silent Discharge Laser System Background of the Invention Field of the Invention The present invention relates to a silent discharge laser system in which silent discharge is used as a laser exciting source.

Description of the Prior Art Figs. 6(a) and 6(b) show an arrangement of a conventional silent discharge laser system, in which when an output of an A.C. power source 1 having a frequency of fis applied between a pair of discharge electrodes 2 and 3 coated with dielectric material, silent discharge takes place in a discharge region 4 between the electrodes 2 and 3 and acts to excite a laser gas 5 in the region 4. As a result, oscillation occurs between a totally reflective mirror 6 and a partiallytransmissive mirror 7, and part of the oscillated light is outputted as a laser beam from the partiaily transmissive mirror 7.Under this condition, the laser gas is circulated by blowers 8 through the discharge region 4 at a velocity of between several tens and several hundreds m/s, cooled by a heat exchanger 9 and again supplied into the discharge region 4.

Fig. 7 shows an electrical equivalent circuit of the laser system of Figs. 6 in a silent discharge mode, in which silent discharge takes place between the electrodes 2 and 3. In the drawing, reference numerals 10 and 11 are electrostatic capacitance of the dielectric material coated on the electrodes 2 and 3 respectively, numeral 12 is an electrostatic capacitance between the electrodes 2 and 3, and numeral 13 is an equivalent resistance of the silent discharge section. In an equivalent circuit shown in Fig. 8, the capacitance 10 and 11 in Fig. 7 are combined into a single capacitance 14.

By using these equivalent circuits of Figs. 7 and 8 which are devised by the inventor of the present invention, the discharge characteristics of the laser gas for the power source frequency higher than 10 KHz can be well explained. Fig. 9 are graphs showing relationships between the discharge power Wd and applied voltage Va and between the discharge power Wd and power factor cos(p of the discharge section with respect to the above conventional laser system.

Small circles "o" represent measured values and solid lines represents values calculated according to the above equivalent circuits. From Fig. 9, it will be appreciated appropriateness of these equivalent circuits.

Such prior art laser system has been defective in that the power factor cos of the discharge section is small as shown in Fig. 9 and thus the power efficiency is low. In an attempt to improve the power factor, there have been two proposals, that is: (1) A reactor is connected in series with the electrodes 2 and 3 and the inductance value of the reactor is set such that series resonance occurs in a circuit of the electrode capacitance 14 shown in Fig. 8 and the reactor (Japanese PatentAppln. Laid-Open Publication No. 79784/1983).

(2) A reactor is connected in parallel with the electrodes 2 and 3 and the inductance value of the reactor is set such that parallel resonance occurs in a circuit of the electrode capacitance 12 shown in Fig. 8 and the reactor (Japanese Patent Appln. Laid-Open Publication No. 48980/1983). However, as a result of analysis with use of the equivalent circuit of Fig. 8, it has been found that the electrode-to-electrode capacitance 12 or 14 cannot be completely compensated for, thus the power factor cannot be improved greatly. It has been confirmed in our experiments that the above methods cannot improve the power factor greatly.

Summary of the Invention It is an object of the present invention to provide a silent discharge laser system having a high power efficiency by eliminating the above defect of incomplete correction of power factor in a discharge section.

In accordance with the present invention, the above object is attained by providing a silent discharge laser system in which silent discharge is used as an exciting source for laser oscillation and first and second reactions are connected respectively in series and in parallel with discharge electrodes.

Brief Description of the Drawings In the accompanying drawings: Fig. 1 schematically shows an embodiment of a silent discharge laser system in accordance with the present invention; Fig. 2 is an equivalent circuit of the laser system of Fig. 1; Fig. 3 and 4 are equivalent circuits of different embodiments with boosting transformer being provided in different positions respectively; Fig. 5 is an equivalent circuit of the boosting transformer; Fig. 6 is a prior art silent discharge laser system, in which (a) is a front view of the laser system and (b) is a side view thereof; Fig. 7 and 8 are equivalent circuits of the laser system of Fig. 6; and Fig. 9 is a graph showing discharge characteristics of the prior art laser system actually-measured and theoretically-calculated on the basis of the equivalent circuit of Fig. 8.

Detailed Description of Preferred Embodiments Fig. 1 shows an embodiment of a silent discharge laser system in accordance with the present invention, in which the same parts or members as those in Fig. 6 are denoted by the same reference numerals. In the laser system of the embodiment, reactors 21 and 22 are provided between an AC power source 1 and electrodes 2 and 3. More specifically, the reactor 21 is connected in parallel with the electrodes 2 and 3 while the reactor 22 is connected in series with the electrodes.

Fig. 2 is an equivalent circuit of the laser system of Fig. 1, wherein capacitance 12 is a capacitance between the electrodes 2 and 3 and capacitance 14 is a combined capacitance of dielectric materials coated on the electrodes 2 and 3 respectively. Further, capacitance 23 is a combined capacitance of stray capacitance of wiring lines connecting between the power source 1 and the electrodes 2 and 3, etc.

Assume that R is the value of an equivalent resistance 13 of silent discharge occurring between the electrodes 2 and 3, Cg, Cd and CO are the values of the capacitance 12, 14 and 23 respectively, Lc and Ld are the values of inductances of the reactors 21 and 22 respectively, Va and 1a are the effective values of output voltage and current of the power source 1 respectively, and to (=2nf) is the angular frequency of the power source 1. Then the equivalent impedance Za of the laser system when viewed from the side of the power source 1 is expressed as follows:

The inductance L0 and Ld as selected so that the impedance Za is real number are expressed by the following equations (2) and (3).

where,

Cd kd= (=constant) Cd+Cg in this case, the impedance Za is expressed as follows:

Hence, the effective current 1a is as follows:

A current flowing through the discharge section or resistance R has an effective value of 1d as follows:

Substituting L0 and Ld in Equation (3) for Equation (6), the current Ld is expressed by the following equation (7).

Thus, the power Wd supplied into the discharge section is:

Since the power factor is

by definition, it becomes as follows:

Thus, all power from the power source is supplied into the discharge section, realizing a laser system having a high power efficiency. The effective value Vd of the equivalent discharge voltage is expressed by the following equation (10): Vd=ldR=kdVa (From Equation (7)) (10) Since the value Vd is constant that is determined by the discharge conditions, the output voltage Va of the power source is also kept constant (Vd/Kd), it does not occur that the voltage Va is increased by the increase of the discharge power as shown in Fig. 9. As a result, it becomes unnecessary to limit the input discharge power due to the consideration of electrical insulation properties of the power source.Therefore, the power source can be made small in size.

The present invention is not a simple combination of the foregoing Japanese Patent Laid-Open Application Nos. 79784/1983 (which is referred to as prior art A, hereinafter) and 48980/1983 (which is referred to as prior art B, hereinafter), but is a novel idea in that the power factor of the discharge section can be completely corrected on the basis of the equivalent circuit of Fig. 8 which the inventor of the present invention has devised. More particularly, according to the analysis based on the equivalent circuit of Fig. 8, the resonance inductance values of the reactors connected in series (prior art A) and in parallel (prior art B) with the electrodes must be set respectively as follows.

(Reactor Inductance Value in Prior Art A) The reactor inductance value must be set so that resonance occurs in a circuit of the reactor and a capacitor having a capacitance corresponding to kd times of the capacitance 14 shown in Fig. 8 (refer to Equation (3)).

(Reactor inductance Value in Prior Art B) The reactor inductance value must be set so that resonance occurs in a circuit of the reactor and a capacitor having a capacitance corresponding to a series-combined value between the capacitor 14 and the electrode-to-electrode capacitor 12 shown in Fig. 8 (refer to Equation (2)).

However, actually, in these prior arts, the respective reactor inductance values are not set as mentioned above. This is because the equivalent circuits of these prior arts are different from the equivalent circuit of Fig.8.

In general, silent discharge requires a high voltage and therefore a boosting transformer is often connected between a power source and discharge electrodes. Fig. 3 shows another embodiment of the laser system of the invention in which a boosting transformer 24 is provided between the reactors 21 and 22 and the electrodes 2 and 3, whereas Fig. 4 shows yet another embodiment in which the boosting transformer 24 is provided between the power source and the reactors 21 and 22. In this way, even when the boosting transformer 24 is employed, the present invention can exhibit substantially the same power factor compensation effect as in the above embodiment.The embodiment of Fig. 3 is more effective to compensate the power factor than the embodiment of Fig. 4 because the inductances L0 and Ld in Fig. 3 are required to have respectively a value 1/(n2) (n is the turn ratio of the transformer 24) smaller than that in Fig.

4. The inductances L0 and Ld may include inductance of wiring connecting between the power source 1 and electrodes 2 and 3, or a leak inductance of the boosting transformer 24. If the boosting transformer 24 has a primary winding having an inductance L, and a secondary winding having an inductance L2 with a mutual inductance M, then the equivalerit circuit of the transformer 24 is expressed as shown in Fig. 5. Therefore, appropriately designing the transformer 24, the transformer 24 can also act as the reactors 21 and 22.

Further, the transformer 24 may perform part of the reactors 21 and 22.

In equation (8), kd and Va are constants that are determined by the discharge conditions. Accordingly, the discharge power is controlled by the equivalent resistance R in the above embodiment. It has been confirmed from our experiments that the resistance R is inversely proportional to the discharge current 1d that is: K R=- (K=Vd: constant) (11) Id Thus, the discharge power Wd is controlled by the discharge current 1d When power factor compensation is carried out using the aforementioned reactors 21 and 22, the power source 1 may preferably be of a constant-current type and in the above embodiments, the constant-current type of power source is used.

Although the electrodes 2 and 3 are both coated with dielectric material in the above embodiments, silent discharge is possible by coating only one of the electrodes with the dielectric material.

Claims

1. A silent discharge laser system comprising: a pair of electrodes at least one of which is coated with dielectric material, an AC power source having a predetermined frequency for causing silent discharge between said electrodes, said silent discharge being used as an exciting source for laser oscillation, and first and second reactors connected respectively in series and parallel with said electrodes.

2. A silent discharge laser system as set forth in claim 1, wherein an inductance value of said first reactor is determined such that series resonance occurs at said predetermined frequency in said first reactor and a capacitance having a value equal to

times of a capacitance Cd of said dielectric material coated on either one or both of said electrodes, and wherein an inductance value of said second reactor is determined such that parallel resonance occurs at said predetermined frequency in said second reactor and a capacitance, having a value equal to a parallel-combined capacitance between a series-combined capacitance of said capacitance Cd and an electrode-to-electrode capacitance Cg and a stray capacitance CO existing between said power source and said electrodes.

3. A silent discharge laser system as set forth in claim 1, further comprising a boosting transformer provided between said electrodes and said first and second reactors.

4. A silent discharge laser system as set forth in claim 1, further comprising a boosting transformer provided between said first and second reactors and said power source.

5. A silent discharge laser system as set forth in claim 3 or 4, wherein said boosting transformer also functions as said first and second reactors.

6. A silent discharge laser system as set forth in claim 1, wherein said power source is of a constant-current type.