Gas laser device

Abstract: A gas laser device includes a high frequency power source (19) for generating high frequency output voltage and a pair of electrodes (12,13) positioned sandwiching discharge gap (14) through which laser gas circulates. The pair of the electrodes (12,13) are connected to the high frequency power source (19) for applying the high frequency output voltage across the electrodes (12,13) to achieve discharge in the discharge gap (14) thereby to generate laser light (20). Each of the electrodes (12,13) includes an electrode part (12a,13a), a capacity coupling control part (12e,12f,13c,13f) provided on either side of the electrode part (12a,13a) and a dielectric shell (12b,13b). The dielectric shell (12b,13b) surrounds portions of the electrode part (12a,13a) and the capacity coupling control part (12e,12f,13e,13f) facing the discharge gap (14), thereby the discharge occurs between the dielectric shells (12b,13b) of a pair of the electrodes (12,13). The dielectric constant of the capacity coupling control part (12e,12f,13e,13f) is lower than that of the dielectric shell (12b,13b).

Description: Field of the Invention The present invention relates to a gas laser device which generates or amplifies laser light by applying a high frequency voltage across electrodes to achieve a discharge, and more particularly to a gas laser device designed to make the power density between the electrodes uniform by using a dielectric. Description of the Related Art Figure 18 shows an outline of the configuration of a conventional cross-flow gas laser device. In Figure 18, a pair of dielectric electrodes 1 are placed face to face sandwiching the discharge gap 2 through which the laser gas circulates in the direction indicated by the arrow A. The dielectric electrodes 1 are made by covering an electrode part 3, which has a shape elongated in the direction orthogonal to the laser gas flow (the direction orthogonal to the surface of the page), with a dielectric shell 4. If a high-frequency voltage from a high-frequency power source 5 is applied across the electrode parts 3 in a cross-flow gas laser device which has been made in this way, then the laser gas flowing through the discharge gap 2 is stimulated and laser light is generated in the direction perpendicular to the surface of the page by providing a resonator in a direction perpendicular to the surface of the page, though this is not depicted here. In this case, the presence of the dielectric shell 4 around the electrode part 3 makes the power density in the discharge gap 2 uniform, allows a homogenous discharge and gives a stable laser output. The frequency of the high-frequency voltage applied across the electrode parts 3 needs to be raised to at least 700 kHz if the laser oscillation is to be made more efficient. However, there are problems in that, if the output frequency of the high-frequency power source 5 is raised, the discharge is needlessly dispersed to either side of the electrode part 3 due to capacity coupling via the dielectric shell 4 which covers the electrode part 3, and, as a result, it is difficult to improve the laser oscillation efficiency sufficiently even though the output frequency of the high-frequency power source 5 has been raised. Further, in some cases, there are also problems that the laser oscillation efficiency will be reduced completely counter to what was initially intended. EP-A- 0 309 826 discloses a high frequency excited high power gas laser comprising a discharge channel having dielectric walls, the inner sides of which are provided with electrodes in the form of metal cores shielded from the discharge by a dielectric. SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to provide a gas laser device which is able to improve the laser oscillation efficiency by using a dielectric electrode. These and other objects of this invention as defined in claim 1 can be achieved by providing a gas laser device which includes a high frequency power source for generating high frequency output voltage and a pair of electrodes positioned sandwiching discharge gap through which laser gas circulates. The pair of the electrodes are connected to the high frequency power source for applying the high frequency output voltage across the electrodes to achieve discharge in the discharge gap thereby to generate laser light. Each of the electrodes includes an electrode part, a capacity coupling as leakage control port provided on either side of the electrode part and a dielectric shell. The dielectric shell surrounds portions of the electrode part and the capacity coupling control part facing the discharge gap, thereby the discharge occurs between the dielectric shells of a pair of the electrodes. The dielectric constant of the capacity coupling control part is lower than that of the dielectric shell. When a high-frequency voltage is applied across the electrode parts in a pair of dielectric electrodes, laser light is generated by the stimulation of the laser gas circulating in the discharge gap between these dielectric electrodes. In this case the power density in the discharge gap is made uniform because there are dielectric shells around the electrode parts. Further, because there are capacity coupling control parts with lower dielectric constants than the shells on the either side of the electrode parts, the discharge is controlled from dispersing to either side of the electrode parts even when a high-frequency voltage is applied to the electrode parts, with the result that the laser oscillation efficiency is improved. BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Figure 1 is a view showing a cross sectional structure of a cross-flow gas laser device according to a first embodiment of this invention; Figure 2 is an oblique view showing a pair of electrodes partially sectioned in the embodiment of Figure 1; Figure 3 is a diagram showing a result of the calculation of the distribution of the discharge current during the discharge operation in the embodiment of Figure 1; Figure 4 is a diagram showing the variation of the calculated leak current I1 with the width W, the spaces d1 and d2; Figure 5 is a diagram showing the relation of the calculated leak current I1 with the width W, the spaces d1 and d2; Figure 6 is an oblique view showing a pair of electrodes partially sectioned in a second embodiment of this invention; Figure 7 is an oblique view showing one of a pair of electrodes partially sectioned in a third embodiment of this invention; Figure 8 is an oblique view showing a pair of electrodes partially sectioned in a fourth embodiment of this invention; Figure 9 is a longitudinally sectioned side view of the central part of one of the electrodes of the embodiment shown in Figure 8; Figure 10 is an oblique view showing a pair of electrodes partially sectioned in a fifth embodiment of this invention; Figure 11 is an oblique view showing a pair of electrodes partially sectioned in a sixth embodiment of this invention; Figure 12 is a view showing a cross sectional structure of a cross-flow gas laser device according to a seventh and an eighth embodiment of this invention; Figure 13 is an oblique view showing a pair of electrodes partially sectioned in the embodiment shown in Figure 12; Figure 14 is a view showing details of a cross section of one of the electrodes shown in Figure 13; Figure 15 is a longitudinally sectioned side view of the central part of one of the electrodes of a ninth embodiment of the invention; Figure 16 is a view showing a cross sectional structure of a cross-flow gas laser device according to a tenth embodiment of this invention; Figure 17 is a view showing a lateral cross section of one of the electrodes (only right said portion thereof is shown) and the distribution of the discharge current during discharge operation obtained by the calculation in the embodiment of Figure 16; and Figure 18 is a cross sectioned side view of a pair of electrodes of a conventional cross-flow gas laser device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, the embodiments of this invention will be described below. A first embodiment of the present invention is described below with reference to Figure 1 and Figure 2. Figure 1 shows the cross sectional structure of a so-called cross-flow gas laser device in which the gas flow is orthogonal to the optical axis of the output. In Figure 1, a first dielectric electrode 12 is placed in the middle of the top of a chamber 11, which has a rectangular cross section, so as to face the inside of the chamber 11, and in this case the bottom surface of the first dielectric electrode 12 is arranged so as to form a planar shape in the same plane as the inside surface of the chamber 11. A second dielectric electrode 13, which forms a pair with the first dielectric electrode 12, is placed in the center inside the chamber 11, and it is arranged in such a way that there is a discharge gap 14 between the two dielectric electrodes 12 and 13 when it has been so placed. In this case, the second dielectric electrode 13 is arranged so as to be supported by support members 15 and 16, its top surface is parallel with the bottom surface of the first dielectric electrode 12 and it has a planar shape in the same plane as the support members 15 and 16. A blower 17 for circulating the laser gas and a heat exchanger 18 for cooling the laser gas are placed to the bottom on the inside of the chamber 11. The laser gas is sealed at a pressure of about 65 torr (1 Torr = 133.32 Pascal) inside the chamber 11, is circulated in the direction of the arrow 8 by means of the blower 17, and is cooled by the heat exchanger 18 after it has flowed through the discharge gap 14. The actual configuration of the first dielectric electrode 12 and second dielectric electrode 13 will be described with reference to Figure 2. Only the first dielectric electrode 12 is explained below because they have the same configuration, and the explanation of the second dielectric electrode 13 is omitted by giving identical references to parts which are identical to the first dielectric electrode 12. To elaborate, the first dielectric electrode 12 is made up of an electrode part 12a with a rectangular cross section formed with a shape elongated in the direction orthogonal to the flow of the laser gas mentioned above, and a rectangular container-shaped dielectric (e.g. ceramic) shell 12b provided so as to envelop the electrode part 12a. In this case, three spaces extending in a direction orthogonal to the laser gas flow are formed in an adjacent state in the shell 12b by dividing the inside with partition walls 12c and 12d. The electrode part 12a is provided in the central space so that the left and right spaces are constructed as capacity coupling control parts 12e and 12f with dielectric constants lower than that of the shell 12b. Moreover, the spaces forming the capacity coupling control parts 12e, 12f, 13e and 13f are configured so that the dielectric constants in these areas are extremely low by connecting these with the atmosphere, sealing an insulating gas on the inside or connecting the inside to the laser gas atmosphere. Further, as shown in Figure 1, when a high-frequency voltage from a high-frequency power source 19 is applied across the electrode parts 12a and 13a of the first dielectric electrode 12 and the second dielectric electrode 13, the laser gas flowing through the discharge gap 14 is stimulated and the laser light 20 is generated in a direction orthogonal to the surface of the page. In this case, because there are the dielectric shells 12b and 13b around the electrode parts 12a and 13a, the power density in the discharge gap 14 is made uniform and a homogenous discharge can therefore be formed and a stable laser output obtained. In this case, because a configuration has been adopted in which there are the capacity coupling control parts 12e, 12f and 13e and 13f with extremely low dielectric constants on either side of the electrode parts 12a and 13a, unnecessary dispersion of the discharge to either side of the electrode parts 12a and 13b is controlled even under circumstances where the high-frequency voltage has been applied as described above. As a result, unnecessary discharge losses can be controlled and therefore inconveniences do not arise even when the output frequency of the high-frequency power source 19 has been raised substantially, and the laser oscillation efficiency can thereby be improved. There are several reasons why the discharge in the discharge gap 14 may be unstable, but it is clear that one of these is the partial rise in temperature of the laser gaps, which has a major bearing on the instability of the discharge. This is known as thermal instability and stems from the fact that the electron density is subject to widely discrepant variations as the laser gas density is subject to partial variations as the temperature rises. Therefore, when partial stagnation occur in the laser gas flow in the discharage gap 14, the laser gas temperature is locally raised by the discharge input in this area, the electron density is subject to large variations due to the subsequent variation in the laser gas density and the discharge in the discharge gap 14 becomes unstable and, on occasion, so-called filamentation occurs. In the above embodiment, a configuration has been adopted in which the bottom surface of the first dielectric electrode 12 and the top surface of the second dielectric electrode 13, which is to say the surfaces facing the discharge gap 14 in the shells 12b and 13b, have a planar shape following the flow of laser gas in this discharge gap 14. Therefore the laser gas flows smoothly in the discharge gap 14, there is no risk that partial stagnation of laser gas flow will be induced, with the result that discharge stability is improved and that the laser oscillation efficiency can also be improved in this respect too. Generally, as for the calculation of the electric field, only the static electric field can be obtained just before starting discharge. In order to obtain the dimension of the effective capacity coupling control part, we have made it possible to calculate the distribution of the discharge current during the discharge operation. Typical examples of the calculation result are shown in Figure 3. In Figure 3, the cross sectional structure of the right half portion of the first dielectric electrode 12 is shown, and 12a is the electrode part, 12f is the capacity coupling control part and 12h is dielectric of the shell 12b. In Figure 3, the distribution of the discharge current I is shown such that the length of the segment extending from the surface of the dielectric 12h facing the discharge gap represents the magnitude of the discharge current I. Figure 3(a) shows the case where the capacity coupling control part is not provided. It is understood that the segment representing the current distribution exists not only below the electrode part 12a but also leaks out considerably in the lateral direction thereof. Figure 3(b) and 3(c) show the cases where the capacity coupling control parts 12f with a rectangular and a circular cross section are provided, respectively. In Figure 3(b), the corner portions of the capacity coupling control part 12f is made to have a curved surface with a appropriate curvature due to the problem of the strength of the structure of the part 12f. In Figure 3(b) and 3(c), as a result of providing the capacity coupling control part 12f in the electrode, it is understood that the current distribution at the surface facing the discharge gap just under the capacity coupling control part 12f is remarkably reduced. The dimensions of the effective capacity coupling control part 12f can be obtained by the electric field calculation method newly developed by us. The parameters of the dimensions are defined as follows: d1 is a space between the capacity coupling control part 12f and the surface of the dielectric 12h facing the discharge gap, W is a width of the capacity coupling control part 12f and d2 is a space between the capacity coupling control part 12f and the opposite surface facing the discharge gap. For the evaluation of the current control, a current I1 is used which is a current at a location 15mm away from the electrode part 12a. The current I1 is expressed as the ratio to the current at the center of the electrode part 12a. The leak current within a location which is away from the electrode part 12a by the length comparable to the radius of the actually used laser beam can be converted to the laser beam. The value of 15mm is employed because the leak current at the above-described location is permitted. Figure 4 shows the variation of the calculated leak current I1 with the width W, the spaces d1 and d2. In the case without the capacity coupling control part, the leak current I1 is about 16% where the width W is zero. The values of the width W, the spaces d1 and d2 are calculated where the leak current I1 is below 10% which is the value permitted for the practical use. The calculation result is shown in Figure 5. An approximate equation expressing the graph of Figure 4 is obtained and it is the following equation (1). d1 - 11 ≦ -(25 - 2 × d2)/W Accordingly, the capacity coupling control part 12f is to be provided with the dimensions satisfying the equation (1). In this calculation, it is supposed that aluminum oxide which is general ceramic is employed as the dielectric and the dielectric constant ε1 of the aluminum oxide is 10. And it is supposed that the capacity coupling control part 12f is filled with a gas and the dielectric constant ε2 of the gas is 1. In the case where other material is used, it is necessary to multiply the right side of the equation (1) by the factor ε2/ε1×10, approximately. Moreover, the equation (1) is obtained in the case where the space d between the electrode part 12a and the surface of the dielectric 12h facing the discharge gap is 5mm. In the case where the space d(mm) is not 5mm, it is necessary to multiply the right side of the equation (1) by d/5. Moreover, the laser light 20 produced as described above is put in a resonant state using a reflecting mirror which is not depicted. When the width of the discharge gap 14 needs to be set large, such as when the optical path in the resonator for the laser light 20 is formed the shape of the letter U or when it is formed in the shape of the letter z, it may be arranged as in Figure 6 which shows a second embodiment of the present invention. Namely, the width dimensions of the electrode parts 12a and 13a of the dielectric electrodes 12 and 13 may be arranged so as to obtain a state conforming with the optical path of the laser light 20 in the resonator. Further, when the width of the discharge gap 14 needs to be set large in this way, then, as in Figure 7 which shows a third embodiment of the present invention, a configuration may be adopted in which the electrode parts 12a and 13a has been divided in two corresponding to the optical path of the laser light 20. In this case a configuration may be adopted in which reinforcing partition walls 12g (only depicted on the side of the first dielectric electrode 12) are also provided between the divided electrode parts. Figure 8 and Figure 9 show a fourth embodiment of the present invention and only those parts which differ from the first embodiment are discussed in the explanation below. In Figure 8, a first dielectric electrode 21 and a second dielectric electrode 22 placed face to face sandwiching the discharge gap 14 have the same shape and have shells 21a and 22a shaped like a rectangular container formed with an elongated shape in the direction orthogonal to the laser gas flow. These shells 21a and 22a are made of a dielectric such as a ceramic, and three spaces are formed in each in an adjacent state extending in a direction orthogonal to the laser gas flow by dividing the insides of each with partition walls 21b, 21c and 22b and 22c. In this case, the spaces in the middle are arranged as electrode housing chambers 21e and 22e intended to house electrode parts 21d and 22d with circular cross sections, and the left and right spaces are arranged as capacity coupling control parts 21f, 21g and 22f and 22g with lower dielectric constants than the shells 21a and 22a respectively. Moreover, the arrangement is such that the dielectric constant is extremely low by connecting these capacity coupling control parts 21f and 21g and 22f and 22g with the atmosphere, sealing an insulating gas on the inside or connecting the inside to the laser gas atmosphere. Water C, for example, is made to flow as a cooling medium inside the electrode housing chambers 21e and 22e. An explanation is given below of the actual configuration of the electrode housing chamber 21e on the side of the first dielectric electrode 21, and the parts connected thereto, with reference to Figure 9. Moreover, the configuration of the second dielectric electrode 22 is not explained as it is basically the same as the first dielectric electrode 21, and, when it proves necessary to mention the second dielectric electrode 22, the same references will be cited. Figure 9 is a longitudinally sectioned side view of the central part of the first dielectric electrode 21 shown in Figure 5. To elaborate, in Figure 9, openigns 23 and 24 are formed facing the electrode housing chamber 21e in each of the upper surface regions towards both edges of the shell 21a. The electrode housing chamber 21e is fixed by inserting resin sealing members 25 and 26 via these openings 23 and 24. In this case, the opening 23 positioned on one edge (the right edge in the figure) of the shell 21a is formed with a relatively long dimension between the edges, thereby forming an adjacent air chamber 27 via the sealing member 25 and electrode housing chamber 21e in the sheel 21a. Moreover, this air chamber 27 is connected to the atmosphere. Further, an outflow port 25a connected to the inside of the electrode housing chamber 21e is formed in the sealing member 25, and an inflow port 26a similarly connected to the inside of the electrode housing chamber 21e is formed in the sealing member 26. The water C (refer to Figure 5) flows, via the inflow port 26a and outflow port 25a, inside the electrode housing chamber 21e. The electrode part 21d is provided with a space between the inside wall of the electrode housing chamber 21e by being supported via a plurality of silicon rubber spacers 28 placed at intervals inside the electrode housing chamber 21e, so that the water C can flow through the area around it. A terminal 21h, which forms a thin rod, is formed projecting integrally at one end of the electrode part 21d, and this terminal 21h passes through the sealing member 25 in a watertight manner and projects into the air chamber 27. Further, a resin terminal mount 29 is attached to the end of the shell 21a, and the terminal 21h is connected, via a lead wire 30, to a relay terminal 29a on the terminal mount 29. Moreover, the relay terminal 29a is connected to the high-frequency power source. Because the embodiment constructed in this way has capacity coupling control parts 21f, 21g and 22f and 22g with extremely low dielectric constants on either side of the electrode parts 21d and 22d, the laser oscillation efficiency can be improved in the same way as in the first embodiment mentioned above. Moreover, in this embodiment, there is the water C around the electrode parts 21d and 22d, water generally has a relatively high dielectric constant of about 80 and so it is convenient that this embodiment can be expected to have the effect of making the power density uniform. In this embodiment, rises in the temperature of the electrode parts 21d and 22d are controlled by the water C flowing through the inside of the electrode housing chambers 21e and 22e. So it is possible to prevent distortion occurring between the electrode parts 21d and 22d and the shells 21a and 22a in the dielectric electrodes 21 and 22, due to the different thermal expansion coefficients of the two. Consequently, there is no risk that cracking or the like will be induced in the part leading out of the electrode housing chambers 21e and 22e of the electrode parts 21d and 22d (the sealing member 25 in this embodiment), and water leaking or unnecessary discharge, caused by cracking in this part, is not induced. Further, in the present embodiment, an arrangement is adopted in which the air chambers 27 are formed adjacent to the electrode housing chambers 21e and 22e via the sealing member 25 in the shells 21a and 22a, and the terminal 21h of the electrode parts 21d and 22d projects inside the air chamber 27. Therefore the static ground capacitance of the terminal 21h part can be reduced and abnormal discharge in the terminal 21h, which is inevitably of a sharp shape whereby electric field concentrating readily occurs, can be prevented from occurring from the outset. Moreover, in this case, the terminal 21h projecting inside the air chamber 27 is connected to the lead wire 30 for the power source inside the air chamber 27, and therefore the static ground capacity of the lead wire 30 can be reduced and abnormal discharge in this part can be prevented. Figure 10 shows a fifth embodiment of the present invention, and this is described below referring only to those parts which are different to the first embodiment discussed previously. In Figure 10, the first dielectric electrode 31 and the second dielectric electrode 32 positioned face to face sandwiching the discharge gap 14 have the same shape and have shells 31a and 32a shaped like a rectangular container formed with an elongated shape in the direction orthogonal to the laser gas flow. These shells 31a and 32a are made of a dielectric such as a ceramic, and pipe-shaped electrode parts 31c and 32c are also provided extending in a direction orthogonal to the laser gas flow in the facing inner walls 31b and 32b. Moreover, the electrode parts 31c and 32c are fixed to the inner walls 31b and 32b by brazing for example. The spaces formed around the electrode parts 31c and 32c in the shells 31a and 32a are connected with the atmosphere so that they function as capacity coupling control parts 31d and 32d with extremely low dielectric constants. Further, the water acting as the cooling medium flows inside the electrode parts 31c and 32c. The embodiment arranged in this way also has a configuration with capacity coupling control parts 31d and 32d with extremely low dielectric constants around the electrode parts 31c and 32c, and therefore the laser oscillation efficiency can be improved in the same way as in the first embodiment mentioned above. The temperature rise in the electrode parts 31c and 32c can be controlled by the water flowing thorough the inside so that there is no risk that unwanted distortion will be induced similarly to the fourth embodiment mentioned above. In this embodiment, in particular, the electrode parts 31c and 32c can also be used as the conduit for the water which is the cooling medium and the overall configuration can be simplified. Moreover, in the fifth embodiment mentioned above, when there is a risk that the operation of also fixing the electrode parts 31c and 32c will be difficult, a configuration may be adopted in which the shells 31a and 32a are respectively divided into container-shaped main units 31e and 32e and lids 31f and 32f as in Figure 11 which shows a sixth embodiment of the present invention. Additionally, the present invention is not limited to the various embodiments given above, and, for example, an arrangement using a cooling medium other than water may be used, and various modifications may be used provided they do no depart from the purport of the invention. Other embodiments of the present invention are described with reference to the figures. Figure 12 shows a seventh embodiment of the present invention, it shows the cross sectional configuration of a cross-flow gas laser device in which the gas flow is orthogonal to the output optical axis. In Figure 12, a first dielectric electrode 37 is placed inside of a chamber 36 in the middle of the top of the chamber 36 which has a rectangular cross section. A second dielectric electrode 38 forming a pair with the first dielectric electrode 37 is placed in the middle in the chamber 36, and the two dielectric electrodes 37 and 38 are arranged to sandwich the discharge gap 39 when the dielectric electrode 38 has been placed in this state. In this case, the top surface of the second dielectric electrode 38 is formed to be parallel with the bottom surface of the first dielectric electrode 37. A blower 40 for circulating laser gas and a heat exchanger 41 for cooling this laser gas are placed to the bottom inside the chamber 36. The laser gas is circulated in the direction of the arrow B by the blower 40 and is cooled by the heat exchanger 41 after it has flowed through the discharge region. Actual configuration of the first dielectric electrode 37 and second dielectric electrode 38 are described with reference to Figure 13 and Figure 14. But because they have the same configuration, only the first dielectric electrode 37 is described below and the description of the second dielectric electrode 38 is omitted by giving identical references to parts which are identical to the first dielectric electrode 37. To elaborate, the first dielectric electrode 37 is made up of an electrode part 37a with a rectangular cross section formed in a shape elongated in the direction orthogonal to the flow of the said laser gas, and a shell 37b made of a ceramic which is a dielectric provided so as to envelop this electrode part 37a except for its top surface. In this case, the shell 37b is formed in a state adjacent to the electrode part 37a by means of partition walls 37c and 37d, and by providing the electrode part 37a in the center, the left and right spaces are made as capacity coupling control parts 37e and 37f with lower dielectric constants than the shell 37b. Figure 14 shows a cross section of the first dielectric electrode 37 in greater detail. The electrode part 37a comprises a metal electrode 42, a conductive rubber 43 which is a conductive substance, and an insulating rubber 44 which is an insulating substance. Water 45 acting as a cooling medium flows in the metal electrode 42. The conductive rubber 43 is used to keep the shell 37b and the metal electrode 42 hermetically attached. The insulating rubber 44 plays the role of mitigating the electric field of the metal electrode 42. Here, although the water 45 acting as the cooling medium flows in the metal electrode 42, a cooling part through which a cooling medium flows may be placed in contact with the metal electrode or close to the metal electrode. Moreover, the metal electrode 42 may be directly bonded to the shell 37b without using the conductive rubber 43. Further, in Figure 14, the relationship between the thickness ΔX1 of the dielectric below the electrode part 37a and the thickness ΔX2 of the dielectric below the spaces 37e and 37f satisfies the equation ΔX1 ≧ ΔX2. The operation of the embodiment is described below. As shown in Figure 12, when a high frequency voltage from a high frequency power source 46 is applied between the electrode parts 37a and 38a of first dielectric electrode 37 and second dielectric electrode 38, the laser gas flowing through the discharge gap 39 is stimulated and laser light is produced in a direction perpendicular to the surface of the page by a resonator placed in a direction perpendicular to the page, which is not depicted. Here, there are dielectric shells 37b and 38b around the electrode parts 37a and 38a, and therefore a uniform high frequency discharge is achieved in the discharge gap 39 thereby achieving a stabilised laser output. In this case, there are capacity coupling control parts 37e and 37f, and 38e and 38f with extremely low dielectric constants on either side of the electrode parts 37a and 38a, and the thickness ΔX1 of the dielectric below the electrode parts 37a is thicker than the thickness ΔX2 of the dielectric below the spaces 37e and 37f. Therefore the discharge is controlled from being unnecessarily dispersed to either side of the electrode parts 37a and 38a even when a high frequency voltage is applied as described above. In this case the condition satisfying the equation ΔX1 ≧ ΔX2 suppress the unnecessary dispersion more efficiently. As a result, because losses caused by unnecessary discharge are controlled, the output frequency of the high frequency power source 46 can be raised to at least 700 kHz, thereby allowing an improvement in the laser oscillation efficiency. Further, because the opposite side of the discharge surfaces of the electrode parts 37a and 38a are not surrounded by dielectrics, the stresses which concentrate in this region due to discharge arcing can be eliminated, and the reliability of the dielectric electrode is improved. The discharge gap side is surrounded by a dielectric, but the side opposite the discharge surface side is not surrounded by a dielectric so that there is the advantage that the production of the electrode parts 37a and 38a is made easier. A cooling path thorough which water 45 acting as a cooling medium flows is provided inside the metal electrode 42 so that the rise in temperature of the electrode parts 37a and 38a is controlled, and therefore distortion can be prevented from occurring between the electrode parts 37a and 38a and the shells 37b and 38b in the dielectric electrodes 37 and 38 due to the different thermal expansion coefficients of the two. The spaces forming the capacity coupling control parts 37e, 37f, 38e and 38f are connected with the stmosphere sealing an insulating gas on the inside or connecting the inside to the laser gas atmosphere, or are put into a very low pressure vacuum state where the discharge is not liable to arcing so that it is also possible to reduce the dielectric constant in this part. The dielectric constant of the spaces forming the capacity coupling control parts 37e, 37f, 38e and 38f are low. This means that dielectric breakdown can be avoided in the capacity coupling control parts and discharge dispersion can be controlled. An eighth embodiment according to the present invention is explained below with reference to Figures 12, 13 and 14. In this embodiment, the blower 40 circulates the laser gas with a gas pressure P and a heat exchanger 41 cools this laser gas. The insulating gas nitrogen is sealed at a gas pressure Pi in the spaces forming the capacity linking control parts 37e, 37f, 38e and 38f. The relationship between the gas pressure P of the laser gas and the gas pressure Pi of the insulating gas satisfies the equation Pi ≧ P. According to this embodiment, by sealing the insulating gas in the space the dielectric constant in this part can be reduced, thereby allowing a suitable reduction in the capacity coupling via the space and allowing the discharge dispersion to be controlled. Further, the relationship between the pressure Pi of the insulating gas and the gas pressure P in the laser device satisfies the condition Pi ≧ P, and therefore dielectric breakdown can be avoided in the capacity coupling control part in the space and the discharge dispersion is more effectively controlled without abnormal discharge in the space. Further, air, laser gas and the like may be used in addition to nitrogen as the insulation gas. If laser gas is used as the insulating gas, there will be no problems with laser oscillation even if the insulating gas leaks from the space. Next, a ninth the embodiment of this invention is described with reference to the drawings. A cross sectional configuration of a cross-flow gas laser device according to this embodiment is the same as the seventh embodiment shown in Figure 12. The explanation is curtailed by appending the same references to parts which are identical with the seventh embodiment. Figure 15 is a longitudinally sectional side view of the central part of Figure 14 and shows a detailed longitudinal cross section of the first dielectric electrode 37 of the gas laser device shown in Figure 12. The first dielectric electrode 37 is provided with a lead wire part 47 at approximately central position in the longitudinal direction thereof, a water inflow port 48a for supplying water 45 as a cooling medium to the metal electrode 42 and a longitudinal capacity coupling control part 49a. The lead wire part 47 is provided for applying the high frequency voltage from the high frequency power source 46 to the metal electrode 42 covered with the insulating rubber 44. The dielectric constant of the longitudinal capacity coupling control part 49a is lower than that of the shell 37b. A lid 50a is fixed at the terminal position of the shell 37b in the longitudinal direction to make the pressure in the capacity coupling control parts 37e and 37f to be constant. Inside of the lid 50a, a gas feeding hole 51a is provided to supply gas from outside. Though not dipicted in Figure 15, the left side portion from the lead wire part 47 of the first dielectric electrode 37 is constructed as the same as the right side portion thereof. A water outflow port 48b, a longitudinal capacity coupling control part 49b, a lid 50b and a gas feeding hole 51b are provided in the left side portion of the first dielectric electrode 37a. The operation and advantages of the left side portion of the electrode 37a is the same as those of the right side portion thereof, so that the latter will be described below. In this embodiment, at the right side of the insulating rubber 44 the longitudinal capacity coupling control part 49a is provided which is filled with the laser gas. As the dielectric constant of the laser gas is 1, the discharge is controlled from being dispersed in the longitudinal direction in the same manner as in the lateral direction in the seventh embodiment. Also, the capacitance between the terminal portion of the metal electrode 42 in the longitudinal direction and the laser chamber can be reduced. This means that the floating capacitance is reduced and the high frequency output power can be supplied to the discharge part. Insulation distance at the high frequency region can be reduced. Namely, the insulation distance at the high frequency region is Σ di / εi , where di is an actual length of material i, and εi is a dielectric constant of the material i. For simplicity, if there are three materials between the metal electrode 42 (with the voltage V) and the laser chamber (with voltage zero), which are the insulating rubber 44, the longitudinal capacity coupling control part 49a and the laser gas, the voltage V0 of the edge of the first dielectric electrode 37 is shown as follows: V0 = d3/ε3 d1/ε1 + d2/ε2 + d3/ε3 x V where d1, d2, d3 and ε1, ε2, ε3 is the distance and the dielectric constant of the insulating rubber 44, the longitudinal capacity coupling control part 49a and the laser gas, respectively. As the distances d1 and d2 and the dielectric constants ε1 and ε3 are definite values, respectively, the distance d3 can be set to be smaller value when the dielectric constant ε2 of lower value is selected, because the voltage V0 should be smaller than the breakdown voltage V break . So, the insulation distance between the edge of the first dielectric electrode 37 and the laser chamber at the high frequency region can be reduced when the material with a lower dielectric constant in the longitudinal capacity coupling control part 49a is used. In this embodiment, the area of the insulating rubber 44 with a dielectric constant of 3.5 is reduced and the area of the laser gas with a dielectric constant of 1 is increased. Here, the dielectric constant of those is smaller than 10 which is the dielectric constant of the shell 37b. In this embodiment, the laser gas is filled in the longitudinal capacity coupling control part 49a. But insulation gas can be filled therein instead, as filled in the capacity coupling control part 37e in the seventh embodiment. Next, a tenth embodiment of this invention is described with reference to the Figures 16 and 17. A cross sectional configuration of a cross-flow gas laser device according to this embodiment is the same as the seventh embodiment shown in Figure 12 except the configuration of the electrodes. The explanation is curtailed by appending the same references to parts which are identical with the seventh embodiment. Figure 16 shows the cross sectional configuration of a cross-flow gas laser device according to this embodiment in which the gas flow is orthogonal to the output optical axis. In Figure 16, a first dielectric electrode 52 is placed inside of the chamber 36 in the middle of the top of the chamber 36. A second dielectric electrode 53 forming a pair with the first dielectric electrode 52 is placed in the middle in the chamber 36. The two dielectric electrodes 52 and 53 are arranged to sandwich the discharge gap 39. In this case, top surface of the second dielectric electrode 53 is formed to be parallel with the bottom surface of the first dielectric electrode 52. The blower 40 for circulating the laser gas and the heat exchanger 41 for cooling the laser gas are placed to the bottom inside the chamber 36. The first and second dielectric electrodes 52 and 53 are made up of electrode parts 52a and 53a and shells 52b and 53b, respectively. Because they have the same configuration, only the first dielectric electrode 52 is described below. Figure 17 shows a cross section of the first dielectric electrode 52 (only right half portion thereof is shown) and the distribution of the discharge current during discharge operation obtained by the calculation. The first dielectric electrode 52 is made up of the electrode part 52a and the shell 52b made of glass, ceramics and so on. The electrode part 52a is made up of a metal electrode 54, an insulator 55 made of, for example, insulating rubber covering the metal electrode 54 and a capacity coupling control part 56a not covered with dielectric. The metal electrode is arranged such that water 55 as a cooling medium flows in the metal electrode 54. The first dielectric electrode 52 has symmetrical configuration, so that a capacity coupling control part 56b (not dipicted) is provided at left side of the insulator 55. Hereinafter, only right portion of the first dielectric electrode 52 will be described. As the insulator 55 is in the atmosphere of the laser gas, it is designed that the space electric field E shown in the Figure is lower than the discharge electric field so as to avoid the breakdown at the capacity coupling control part 56a. For this purpose, the insulator 55 is provided with an electric field relaxation area to relax the space electric field. In this case, it is necessary that the curved surface is to be provided with a curvature larger than that of radius of 5mm. According to this embodiment, the distribution of the discharge current I is shown in Figure 17. The distribution is shown in the same manner as in Figure 3. It is supposed that the width W0 of the metal electrode 24 is 20 mm, the depth W1 of the insulator 55 is 10mm, the height of the insulator 55 is 20mm, the dielectic constant of the insulator 55 is 3.5 and the radius of curvature of the electric field relaxation area is 7mm. In this case, the leak discharge current I1 at a location 17mm apart from the right edge of the metal electrode 54 is about 3% of the maximum current value. The space electric field E is lower than the breakdown electric field in this case, so that the breakdown does not occur at the capacity coupling control part 56a. Taking the laser takeout efficiency into consideration, it is necessary that an extent of the discharge at the position where the laser light is took out is decided such that the leak current I1 at the position is lower than 10%. In the above-described case, the leak current I1 is about 10% when the width W1 is 10mm. In order to take out the laser light with high efficiency, it is necessary that the upstream edge where the laser light is took out is positioned at least on the downstream edge of the metal electrode 54. In the case that the laser beam with a beam radius of 20mm is took out, this condition corresponds to that the center of the laser beam is set 20mm downstream from the center of the discharge and the width W1 of the insulator 55 is 20mm. In this case, CO₂ molecules excited in the area upstream from the laser beam are carried to the laser beam area by the laser gas flow thereby a lot of loss does not occur. Accordingly, it is decided that the width W1 of the insulator 55 is not more than 20mm and the width W0+2×W1 is not more than 60mm. In this embodiment, the capacity coupling control part 56a is provided in the direction of the laser gas flow, the capacity coupling control part with the same configuration of this embodiment can be also provided in the longitudinal direction of the metal electrode 54. In this case, the expansion in the longitudinal direction of the discharge can be controlled and the width of the metal electrode 54 in the longitudinal direction and capacity between the laser container and so on can be reduced. And, the insulation distance at the high frequency region can also be reduced. As is clear from the above explanation, the present invention is able to provide a gas laser in which at least one pair of dielectric electrodes, where a dielectric shell has been provided around the electrode parts to which a high frequency voltage is applied, is placed sandwiching the discharge gap, whereby the discharge is controlled from dispersing to either side of the electrode parts, even when a high frequency voltage is applied to the electrode parts, by adopting a configuration in which a capacity coupling control part with a lower dielectric constant than the shell is provided on either side of the electrode parts in the dielectric electrode. Therefore the laser oscillation efficiency can be improved even though a dielectric electrode is used. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims: A gas laser device, comprising: a high frequency power source (19) for generating a high frequency output voltage; and a pair of electrodes (12, 13) positioned for sandwiching a discharge gap through which laser gas circulates; said pair of electrodes being connected to said high frequency power source for applying said high frequency output voltage across said electrodes to achieve a discharge in said discharge gap thereby to generate laser light (20); each of said electrodes including an electrode part (12a, 13a) a capacity coupling control part (12e, 12f; 13e, 13f) provided on either side of said electrode part and a dielectric shell (12b); said dielectric shell surrounding portions of said electrode part and said capacity coupling control part facing said discharge gap, thereby said discharge occurring between said dielectric shells of said pair of electrodes; and the dielectric constant of said capacity coupling control part being lower than that of said dielectric shell. The gas laser device according to Claim 1, wherein: said dielectric shell includes a container made of dielectric with a rectangular cross section; said container includes a partition wall to divide an inside space of said container into a first space and a second space, both extending in a direction orthogonal to a flow of said laser gas; said electrode part it provided in said first space; and said capacity coupling control part is made up of said second space. The gas laser device according to Claim 2, wherein: said electrode part includes a metal electrode and cooling means for flowing cooling medium to cool said metal electrode in said first space. The gas laser device according to Claim 3, wherein: said container includes an air chamber at a terminal position in the longitudinal direction thereof; said air chamber is coupled to said first space through a sealing member; and an end of said metal electrode in said first space in passed through said sealing member to said air chamber and is connected to a lead wire which is connected to said high frequency power source. The gas laser device according to claim 1, wherein: said dielectric shell includes a container made of dielectric with a rectangular cross section; said electrode part includes a pipe-shaped metal electrode on an inside wall of said container facing said discharge gap; said capacity coupling control part is made up of a space formed around said electrode part in said container; and said electrode part includes cooling means for flowing cooling medium to cool said pipe shaped metal electrode inside said pipe-shaped metal electrode. The gas laser device according to Claim 1, wherein: said dielectric shell has a configuration in which a surface thereof facing said discharge gap has a planar shape following flow of said laser gas in said discharge gap. The gas laser device according to Claim 1, wherein: said electrode has a configuration which satisfies a following relation; d1 - 11 ≦ -(25 - 2 × d2)/W where W is width of said capacity coupling control part, d1 is a distance between said capacity coupling control part and a surface of said dielectric shell facing said discharge gap and d2 is a distance between said capacity coupling control part and an opposite surface of said dielectric shell facing said discharge gap. The gas laser device according to Claim 1, wherein: said capacity coupling control part is made up of pair of spaces, each of which is surrounded with said dielectric shell: and said electrode part is positioned between said pair of spaces, and a surface thereof facing said discharge gap is surrounded with said dielectric shell. The gas laser device according to Claim 8, wherein: said electrode part includes a metal electrode and cooling means for flowing cooling medium to cool said metal electrode. The gas laser device according to Claim 8, wherein: an insulating gas is filled in said pair of spaces or the inside thereof is connected to said laser gas atmosphere. The gas laser device according to Claim 10, wherein: a pressure Pi of said insulating gas is not less than a pressure P of said laser gas. The gas laser device according to Claim 1, wherein: said capacity coupling control part is provided on either side of said electrode part both in lateral direction and in longitudinal direction of said electrode part. The gas laser device according to Claim 1, wherein: said electrode includes a metal electrode surrounded by an insulator, said metal electrode faces said discharge gap through said dielectric shell; a pair of spaces are provided on either side of said electrode part, each of which faces said discharge gap through said dielectric shell; and said capacity coupling control part is made up of said spaces.

Inventors: MURATA TAKAAKI C O INTELLECTUA, SHINONAGA HIDEYUKI C O INTELLE, SUZUKI HIROKATSU C O INTELLECT, TAMAGAWA TOHRU C O INTELLECTUA, TERAI KIYOHISA C O INTELLECTUA, MURATA, TAKAAKI, C/O INTELLECTUAL PROPERTY DIV., SHINONAGA, HIDEYUKI, C/O INTELLECTUAL PROPERTY DIV, SUZUKI, HIROKATSU, C/O INTELLECTUAL PROPERTY DIV., TAMAGAWA, TOHRU, C/O INTELLECTUAL PROPERTY DIV., TERAI, KIYOHISA, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1993
Main classification: H01S3
IPC codes: H01S3