Laser apparatus

Abstract: According to the present invention there is provided a laser apparatus capable of efficient oscillation with an excitation power source (3) with comparatively low frequency, wherein discharge (DC) is obliquely generated within the rectangular section of a discharge space (4). There is also provided a laser apparatus with a pair of preliminary discharge excitation electrodes (112,222) which can readily initiate discharge. There is further provided a laser apparatus capable of efficient oscillation with an excitation power source (3) with comparatively low frequency, wherein provided is a pair of discharge excitation electrodes (1,2) located in the major side direction of discharge space (4) whose length is more than three times as long as that of minor side direction thereof. There is further provided a laser apparatus with a pair of discharge excitation electrodes (1,2) whose dimension is smaller than that of a pair of dielectric plates (10,20), which apparatus can prevent undesirable discharge. There is also provided a laser apparatus with more than three discharge spaces (4a,4b,4c) at least one of which is given a lower electric field having the apparatus oscillate efficiently. There is further provided a laser apparatus with an improved oscillation efficiency wherein furnished are dielectric double pipes (105,205) creating a substantially large discharge space (4).

Description: This invention relates to a gas laser apparatus which is improved in oscillation efficiency. Fig. 42 is a perspective view showing a conventional carbon dioxide laser apparatus of the wave guide type disclosed, for example, in R. Nowack et al., "High Power CO 2 Wave Guide Laser of the 1kW Category", SPIE (Society of Photooptical Instrumentation Engineers), Vol. 1276, Proceedings, "CO 2 Lasers and Application" (1990), pp. 18 - 28, Fig. 1. Referring to Fig. 42, reference numerals 1 and 2 each denote a discharge exciting metal electrode. Reference numeral 3 denotes an excitation power source (radio frequency power source here) connected to the metal electrode 1. Reference numerals 10 and 20 denote dielectric plates made of, for example, ceramics. The dielectric plates 10, 20 are opposite to each other and held in close contact with the metal electrodes 1 and 2, respectively. Reference numeral 4 denotes a discharge space (filled with mixture gas of CO 2 -He-N 2 which serves as a laser medium) defined between the dielectric plates 10 and 20. Reference numerals 5 and 6 denote arrow marks indicating the directions of coming in and going out of electrode cooling water to and from the electrodes, respectively. Reference numeral 7 denotes a total reflection mirror (resonator mirror). Reference 8 denotes output coupler (resonator mirror). Reference numeral 9 denotes a laser beam, and reference characters 21a and 21b denote inlet and outlet ports, respectively, for electrode cooling water provided at the electrode 1 (similar inlet and outlet ports (not shown) for electrode cooling water are provided also at the electrode 2). Subsequently, the operation of the apparatus will be described. When the metal electrode 1 is connected to the RF power source 3 and the other metal electrode 2 is connected to the ground, the RF discharge for exciting the laser is caused in the discharge space 4 filled with the mixture gas described above. Thus, the discharge energy is converted into light energy by an optical resonator constituted of the total reflection mirror 7 and the output coupler 8 and is outputted as a laser beam 9 from the output coupler 8. In a carbon dioxide gas laser, since the energy level at a low level of the laser is low, as the temperature of the gas rises, the low level concentration increases and the laser oscillation efficiency drops. Consequently, the cooling capacity of the laser gas makes a great factor which determines the laser oscillation efficiency. The ratio w/d between the major side (length w) and the minor side (gap length d) of a section of the rectangular discharge space 4 is called aspect ratio, and from the point of view of cooling of the gas serving as a laser medium, it is deduced that, when the aspect ratio is equal, the cooling capacity is similar. In particular, when the same power is thrown in, if the aspect ratio is equal, then the temperature of the gas is equal. Accordingly, in order to throw in a high power and cool the gas sufficiently to raise the laser oscillation efficiency, the aspect ratio should be set to a high value. In addition, for laser oscillation for which a high power density is required, the minor side d should be set to a small value. The cooling capacity for gas with respect to the length d of the minor side of the section of the rectangular discharge space 4 is shown in Fig. 43. In Fig. 43, a solid line indicates a power density at which the temperature of the gas is 250 °C where the composition of the gas is He-N 2 -CO 2 = 80-10-10 (%; rate in volume, molar fraction). It can be seen from Fig. 43 that the cooling capacity for the gas rises as the minor side d decreases. On the other hand, when the minor side (gap length) d is set to be short, the loss a in the propagation process of the laser light increases. The propagation loss a of the EH nm mode in the rectangular wave guide can be represented by the following formula. a = u nm 2 2π λ 2 (d/2) 2 1 2 (v 2 +1) v 2 -1 v = √ε/ε 0 wherein λ represents the laser wavelength; ε and ε 0 represent the permitivity with respect to the laser wavelength and the dielectric constant in vacuum (0.8854 x 10 -11 CV -1 m -1 ); and u nm represents the coefficient with respect to the order of each mode. Fig. 44 shows the result obtained by calculating, from the above formulae, the relationship between the gap length d and the propagation loss a where Al 2 O 3 (alumina) is used for a dielectric material and a wavelength (10.6 µm) of a carbon dioxide laser is used as a laser wavelength. As a result, the propagation loss a increases in proportion to the gap length d -3 . The normal wave guide type carbon dioxide gas laser apparatus is often used in the range of 1.5 ≦ d ≦ 2.5 (mm) in consideration of the cooling capacity of gas and the propagation loss of light. Due to the high output, when the length of the dielectric is long, the propagation loss naturally increases. Therefore, it is necessary to increase the gap length d to provide a higher output. Fig. 45 shows a result of an examination of the influence of the power source frequency of the RF power source 3 upon the output of the carbon dioxide gas laser in the condition of the gap length d = 2 mm. It is confirmed that, as the power source frequency increases, the laser output increases dramatically. The reason is given below. Fig. 46 shows a result of calculation of the electric field distribution in the direction of the gap d varying the frequency of the power source in the condition of the gas pressure of 80 Torr (1 Torr = 133 Pa) . In Fig. 46, reference character Z denotes a distance in the electric field direction, and Z = 0 represents the center of the gap while Z = 1.0 (mm) represents a boundary to a dielectric plate. As apparent from Fig. 46, it can be confirmed that, as the power source frequency increases, the region in which the electric field is high decreases while the low electric field region which is suitable for laser oscillation increases. Accordingly, if the power source frequency is raised, the low electric field region increases and the excitation efficiency of the laser rises as seen in Fig. 46. This variation of the electric field distribution can be explained from a discharge maintaining mechanism. The discharge maintaining mechanism is roughly explained from the relationship between the travel time t e of electrons through the gap d and the half period t s of the power source. In particular, in such a case that electrons drifting toward the anode collide with the anode (electrode), since the number of electrons and loss of energy are high, the electric field must provide the energy which compensates for the loss. Accordingly, the high electric field region becomes wide. This corresponds to the case wherein the half period t s of the power source is longer than the gap travel time t e of electrons. On the contrary when the variation of the electric field (half period t s of the power source) is shorter than the travel time t e of electrons, the polarity of the electrode is reversed (to the negative) before electrons drifting toward the anode arrive at the anode, and consequently, the electrons are urged back and will not collide with the electrode wall. Accordingly, in this instance, the loss in the number of electrons and the energy loss are small and the high electric field region may be made narrow. Although it may be different depending upon conditions, in the conditions calculated in connection with Fig. 45, since the drifting speed of electrons is almost 10 7 cm/s, the gap travel time t e is 0.2 cm (2 mm)/10 7 cm/s = 2 x 10 -8 sec. The critical frequency at which the time t e corresponds to one half period of the RF power source 3 is 100 MHz. Accordingly, when the frequency of the RF power source is lower than 100 MHz, the high electric field region becomes wide as shown in Fig. 46 and the lower excitation efficiency drops as shown in Fig. 45. By the way, the conventional carbon dioxide gas laser apparatus shown in Fig. 42 employs a hybrid resonator in order to generate a laser beam of high convergency from the rectangular discharge space 4. In particular, the hybrid resonator operates, in the direction of the minor side d of the rectangular discharge space 4, as a wave guide resonator in which laser light propagates while being reflected by the dielectric plates 10 and 20, and operates, in the direction of the major side, as an unstable resonator (a resonator of the type in which light is not enclosed completely). In the case where a wave guide is employed resonator, if the distance (L wm ) between an end of a wave guide (dielectric plates 10 and 20) and a resonator mirror (reflecting mirror 7 and output coupler 8) is set to a great value, then the rate at which light escapes from the resonator becomes high, and consequently, the output efficiency of a laser beam drops. It is known that the loss by escapement of light increases in proportion to (L wm ) 3/2 . Thus, for example, in the conditions of the wave length of 10.6 µm (CO 2 laser) and the gap length of d = 2 mm, in order to suppress the loss of light low, it is necessary to set L wm to a small value of 10 mm or so. Accordingly, when the applied voltage is raised in order to increase the discharge power, not only the discharge occurs in the main discharge space 4, but also the discharge 41 toward the output coupler 8 occurs as seen in Fig. 47. In this instance, if the discharge occurs toward the output coupler 8, then the energy thrown in to the main discharge space 4 decreases and the laser excitation efficiency drops as seen from Fig. 48. (In Fig. 48), a point Ps denotes a discharge start power to the mirror.) Further, if corners of the dielectric plates 10 and 20 are present in the proximity of end portions of the metal electrodes 1 and 2, then when the applied voltage rises, the electric field strengths at the corners of the dielectric plates 10 and 20 become high as seen in Fig. 49, and the discharge 42 is liable to be concentrated also at locations around the corners. Since the conventional laser apparatus is constructed as described above, in the case where the length of the dielectric is desired to be longer in order to obtain a high output wave guide type laser, it is required to set the gap length d to be short in view of cooling whereas it is required to set the gap length d to be long in view of propagation loss of light. This is exactly the contradict requirement, which is impossible to realize. Further, if the permitivity ε with respect to the laser wavelength is set to be small from the above-described formulae (1) and (2), the propagation loss a is expected to be reduced. Actually, however, a material having a low permitivity is difficult to be sintered, often making it impossible to manufacture. As will be described later, the dielectric used in this system is not only required to have a nature as a wave guide path surface but also to have a function as a capacitor for discharge such as withstand voltage. For this reason, materials satisfied with these conditions have been extremely restricted. Further, in the case of the conventional CO 2 laser apparatus, the optimal frequency in the laser excitation is in the vicinity of 150 MHz. However, since this frequency is limited for use thereof under the Japanese Radio-wave Law, there remains a great problem in case of providing a general-purpose apparatus. Moreover, such an RF power source is expensive, and matching between the RF power source and a laser load is difficult. There are many problems as described. Consequently, the conventional gas laser apparatus further has a problem that, if the applied voltage is raised in order to increase the discharge power, then not only the discharge occurs in the main discharge space but also the discharge 41 toward the resonator mirror and the discharge 42 concentrated at the corners of the dielectric plates are caused, which deteriorates the stability of the laser apparatus. From document EP-A-0 392 485 there is known a laser apparatus with microwave excitatiun. A discharge space represents a laser housing in form of a ridge waveguide and the laser housing is preferably dimensioned such that its critical wavelength λ k is less than or equal to the microwave wavelength λ 0 . In an unpulsed operation mode, this prior art laser apparatus generates a homogenous plasma discharge over the entire discharge space. An advantageous further homogenizing of the plasma discharge may be achieved by an additionally applied magnetic field. The present invention has been made to eliminate such problems as described above, and it is a principal object of the present invention to a laser apparatus which can excite the laser readily at a high efficiency even in a frequency region approved, for example, by the radio wave law of Japan or in a low frequency region in which a countermeasure against radio wave leakage is easy. It is another object of the present invention to provide a laser apparatus which can start the discharge smoothly. It is a further object of the present invention to provide a stabilized laser apparatus which can suppress the discharge to an optical resonator mirror even upon application of a high voltage and concentration of the discharge at a corner portion of a dielectric. It is a still another object of the present invention to provide a laser apparatus which can minimize the light absorbing effect in a non-excited and non-cooled space so that a laser can be outputted at a high efficiency even in a high output region. It is another object of the present invention to provide a laser apparatus wherein laser light exciting spaces are formed and means is provided to solidly turn back laser beams such as a prism, a holding mirror and so forth, whereby when the laser light exciting spaces are connected in series, the equal effects are obtained and the apparatus can be miniaturized and provided at less cost. It is another object of the present invention to provide a high-output laser apparatus which can minimize the propagation loss of light while being satisfied with the conditions for discharge. According to the present invention, there is provided a laser apparatus comprising a discharge space between dielectric plates being in close contact with metal electrodes and forming a waveguide whose gap is in the minor side of a rectangular excitation section for radio-frequency stimulated laser excitation, the ratio between major side and minor side of said rectangular section is 3 or more, and outputting a laser beam in the direction intersecting perpendicularly to said rectangular section of said discharge space, characterized in that means are provided such that the discharge is generated obliquely within said rectangular section of the discharge space. Alternatively, according to the present invention, there is provided a laser apparatus comprising a discharge space between dielectric plates being in close contact with metal electrodes and forming a waveguide whose gap is in the minor side of a rectangular excitation section for radio-frequency stimulated laser excitation, the ratio between major side and minor side of said rectangular section is 3 or more, and outputting a laser beam in the direction intersecting perpendicularly to said rectangular section of said discharge space,characterized in that means are provided such that the discharge is generated obliquely with respect to the direction of an optical axis intersecting perpendicularly to said rectangular section within said discharge space. With the laser apparatus according to the present invention, since the discharge is caused in oblique directions within the rectangular section of the discharge space, the effective length of the gap can be set to be long, and for this reason, a proportion of a high electric field region for maintenance of discharge reduces, and an average electric-field intensity lowers to increase a laser excitation efficiency. Fig. 1 is a schematic sectional view showing a gas laser apparatus according to an embodiment of the present invention; Figs. 2 and 3 are graphs each illustrating a relationship between the discharge power and the laser output of the gas laser apparatus of Fig. 1; Fig. 4 is a schematic sectional view showing a gas laser apparatus according to another embodiment of the present invention; Fig. 5 is a schematic sectional view showing a gas laser apparatus according to a further embodiment of the present invention; Fig. 6 is a schematic sectional view showing a gas laser apparatus according to a still further embodiment of the present invention; Fig. 7 is a perspective view showing a gas laser apparatus according to a yet further embodiment of the present invention; Fig. 8 is a schematic sectional view showing a gas laser apparatus according to a yet further embodiment of the present invention; Fig. 9 is a schematic illustration of a gas laser apparatus according to a preliminary embodiment ; Fig. 10 is a graph illustrating a relationship between the discharge power and the laser output of the gas laser apparatus of Fig. 9; Fig. 11 is a schematic illustration of a gas laser apparatus according to a preliminary embodiment; Fig. 12 is a schematic illustration of a gas laser apparatus according to a yet further embodiment of the present invention; Fig. 13 is a schematic illustration of a gas laser apparatus according to a yet further embodiment of the present invention; Fig. 14 is a schematic sectional view showing a gas laser apparatus according to a yet further embodiment of the present invention; Fig. 15 is a circuit diagram showing an electrically equivalent circuit to the gas laser apparatus of Fig. 14; Fig. 16 is a graph illustrating the thrown in power characteristic of the gas laser apparatus of Fig. 14; Fig. 17 is a schematic illustration of a gas laser apparatus according to a yet further embodiment of the present invention; Fig. 18 is a side elevational view of a gas laser apparatus according to a yet further embodiment of the present invention; Fig. 19 is a perspective view showing a gas laser apparatus according to a yet further embodiment of the present invention; Fig. 20A is a plan view showing a gas laser apparatus according to a yet further embodiment of the present invention, and FIG. 20B is a sectional view taken along line A-A of Fig. 20A; Fig. 21A is a plan view showing a gas laser apparatus according to a yet further embodiment of the present invention, and FIG. 21B is a sectional view taken along line A-A of Fig. 21A; Fig. 22 is a perspective view of a laser apparatus according to an Embodiment 17 of the present invention; Fig. 23 is a graphic representation showing the laser oscillation characteristics of the laser apparatus shown in Fig. 22; Fig. 24 is a graphic representation showing the laser oscillation characteristics ofa conventional laser apparatus in comparison with the laser apparatus shown in Fig. 22 ; Fig. 25 is a graphic representation showing the relationship between the coefficient of light absorption of carbon dioxide and the gas temperature in the laser apparatus; Fig. 26 is a perspective view of a laser apparatus according to an Embodiment 18 of the present invention; Fig. 27 is a perspective view of a laser apparatus according to an Embodiment 19 of the present invention; Fig. 28 is a perspective view of a laser apparatus according to an Embodiment 20 of the present invention; Fig. 29 is a view showing a laser beam light path of the laser apparatus shown in Fig. 28 ; Fig. 30 is a sectional view of a laser apparatus according to an Embodiment 21 of the present invention; Fig. 31 is a view showing a laser beam light path of the laser apparatus shown in Fig. 30; Fig. 32 is a sectional view of the laser apparatus according to an Embodiment 22 of the present invention; Fig. 33 is a perspective view of the laser apparatus shown in Fig. 32; Fig. 34 is a view showing a laser beam light path shown in Fig. 33; Fig. 35 is a view showing the arrangement of a rectangular discharge space of the laser apparatus according to an Embodiment 23 of the present invention; Fig. 36 is a view showing a laser beam light path shown in Fig. 35; Fig. 37 is a sectional view showing essential parts of a waveguide path type CO 2 laser apparatus according to an Embodiment 24 of the present invention; Fig. 38 is a sectional view showing essential parts of a waveguide path type CO 2 laser apparatus according to an Embodiment 25 of the present invention; Fig. 39 is a sectional view showing essential parts of a waveguide path type CO 2 laser apparatus according to an Embodiment 26 of the present invention; Fig. 40 is a sectional view showing essential parts of a waveguide path type CO 2 laser apparatus according to an Embodiment 27 of the present invention; Fig. 41 is a sectional view showing essential parts of a waveguide path type CO 2 laser apparatus according to an Embodiment 28 of the present invention; Fig. 42 is a perspective view showing a conventional waveguide path type CO 2 laser apparatus; Fig. 43 is a graphic representation showing the relationship between the discharge space and the coefficient of laser excitation in the CO 2 laser apparatus shown in Fig. 42; Fig. 44 is a graphic representation showing the gap length and the cooling capacity in the CO 2 laser apparatus shown in Fig. 42; Fig. 45 is a graphic representation showing the relationship between the frequency of the excitation power source and the efficiency of the laser excitation in the laser apparatus shown in Fig. 42; Fig. 46 is a graphic representation showing the relationship between the frequency of the excitation power source and the electric-field distribution in the laser apparatus shown in Fig. 42; Fig. 47 is an explanatory view of the discharge toward a resonator mirror in the laser apparatus shown in Fig. 42; Fig. 48 is a view showing the oscillation characteristics at the time of occurrence of the discharge toward the resonator mirror in the laser apparatus shown in Fig. 42; and Fig. 49 is an explanatory view of the discharge occurring in the end of a dielectric in the laser apparatus shown in Fig. 42. In the following, embodiments of the present invention will be described with reference to Figs. 1 to 21. In those figures, like elements (members) to those described hereinabove with reference to Figs. 22 to 28 are denoted by like reference numerals and characters, and detailed description thereof is omitted herein. Embodiment 1. Fig. 1 is a schematic sectional view showing a gas laser apparatus according to the Embodiment 1 of the present invention. The present gas laser apparatus has a rectangular section wherein the ratio between the lengths of the major side and the minor side is equal to or higher than 3, and extracts a laser beam in the direction perpendicular to the section. Also in the present embodiment, cooling water flows through the electrodes 1 and 2 in order to cool the laser medium. While the basic construction of the present gas laser apparatus is similar to the conventional gas laser apparatus shown in Fig. 42, in the present embodiment, the dielectric plates 10 and 20 are provided with a thickness distribution in order to provide the electrostatic capacities of the electrodes with a distribution to generate the discharge in oblique directions in the section. Subsequently, operation of the present gas laser apparatus will be described. When an ac high voltage is applied between the metal electrodes 1 and 2, the discharge is generated in the discharge space 4. In this instance, since discharge energy is poured in in proportion to the electrostatic capacity, if a suitable distribution is provided to the thicknesses of the dielectric plates 10 and 20, then such discharge in oblique directions as indicated by reference character DC in Fig. 1 is caused. According to an experiment of the inventors, it was confirmed that, when a thickness distribution is to be provided to a same dielectric material, the discharge in oblique directions is caused principally if the ratio in thickness between a thinner portion and a thicker portion is set to 1:3 or more. According to this embodiment, since an equivalent discharge gap length in oblique direction can be set arbitrarily by selecting the pitch (reference p) at thinner portions of the dielectric plates 10 and 20, optimization can be achieved by the power source frequency. Figs. 2 and 3 illustrate results of an examination of the power source frequency dependency of the laser output at p = 5 mm and 15 mm, respectively, in the condition of the gap length of 2 mm. Under the condition of p = 5 mm of Fig. 2, the excitation efficiencies at the power source frequencies of 150 MHz and 13.56 MHz are substantially equal to each other. Further, under the condition of p = 15 mm of Fig. 3, also the excitation efficiency at 100 kHz is almost equal, and enhancement in laser excitation efficiency in a low frequency region is confirmed. Accordingly, it can be recognized that, if the equivalent discharge gap length is taken long by the oblique discharge provided by the present embodiment, even where a power source of a lower frequency is used, laser oscillation of a high efficiency is possible. Embodiment 2. It was confirmed that, if the metal electrodes 1 and 2 are molded with dielectric members 101 and 201 having a low dielectric constant as shown in Fig. 4, then it is possible to apply a higher voltage between them and an increase in output power is realized. Embodiment 3. Meanwhile, even if different dielectrics 102 and 202 are laminated on the dielectric plates 10 and 20, respectively, to form stepped dielectric layers as shown in Fig. 5, similar effects to those of the Embodiment 1 can be obtained. In this instance, if the dielectrics 102 and 202 are formed from a material having a lower dielectric constant than that of the dielectric plates 10 and 20, then it is possible to reduce the thicknesses of the dielectrics 102 and 202 to raise the effect of heat conduction for cooling the mixture gas. Embodiment 4. Meanwhile, if the neutral point of the power source is grounded so that voltages having the opposite polarities to each other (voltages displaced by 180° from each other in phase) are applied to the metal electrodes 1 and 2, then the insulation distances from the electrodes 1 and 2 to the ground can be designed short, and a compact laser oscillator can be realized. Embodiment 5. While the effects of the oblique discharge in the section of the discharge space 4 perpendicular to the optical axis are described in the Embodiment 1, similar effects to those of the Embodiment 1 can be obtained even if oblique discharge occurs in the direction of the optical axis as shown in Fig. 7. Embodiment 6. While the cases wherein the discharge is caused in oblique directions by a distribution of the electrostatic capacity are described in the embodiments described above, if, for example, as shown in Fig. 8, the metal electrodes 1 and 2 which are opposed to each other and another pair of metal electrodes 11 and 22 which are opposed to each other are short-circuited by way of the dielectric members 10 and 20 while a high voltage is applied between the adjacent metal electrodes 1 and 11 and the adjacent metal electrodes 2 and 22, then the discharge is caused between obliquely opposed electrodes having different potentials (in short, between the electrodes 1 and 22 and between the electrodes 2 and 11) and between adjacent electrodes (in short, between the electrodes 1 and 11 and between the electrodes 2 and 22). Consequently, similar effects to those described above are obtained. In short, if the discharge can be caused in oblique directions with respect to the dielectric members 10 and 20, then laser excitation of a high efficiency is possible even when a power source of a low frequency is employed. Embodiment 7 (preliminary embodiment not exemplifying the present invention) Fig. 9 is a schematic view showing a gas laser apparatus according to the preliminary embodiment referred to as Embodiment 7. The basic construction of the present gas laser apparatus is similar to that shown in Fig. 42. However, in the present exemplifying embodiment, the discharge electrodes 1 and 2 are disposed in the direction of the major side of the section of the discharge space 4 as shown in Fig. 9, and the discharge is caused in the direction of the major side of the discharge space 4. Since the ratio between the major side and the minor side (i.e., the aspect ratio) of the discharge space 4 is set equal to or higher than 3 similarly as in the Embodiment 1, the gap length can be increased by three or more times only by changing the direction of the discharge from the direction of the minor side to the direction of the major side. Consequently, the gap travel time t e of electrons described hereinabove is increased, and the electric field strength is put into a condition optimum for laser oscillation and laser oscillation of a high efficiency is realized even where the RF power source 3 has a low frequency with a comparatively long power source period t s . A result of a similar oscillation experiment which was conducted under similar conditions to those of Fig. 45 using the gas laser apparatus of the present preliminary embodiment is shown in Fig. 10. It can be seen from Fig. 10 that the laser oscillation efficiency in a low frequency region is improved. If the aspect ratio is further increased, then the laser oscillation efficiency will rely upon the power source frequency little more. Embodiment 8 (preliminary embodiment not exemplifying the present invention). In the present preliminary embodiment, the construction of the preliminary Embodiment 7 described above is modified such that, as shown in Fig. 11, discharge electrodes 111 and 221 are surrounded by dielectric members 103 and 203 of glass or the like, respectively. Also with this construction, similar effects to those of the preliminary Embodiment 7 can be exhibited (common with the Embodiment 1 in that a pair of dielectric members are disposed between the discharge electrodes). Embodiment 9. In the present embodiment, the construction of the preliminary Embodiment 7 described above is modified such that, as shown in Fig. 12, a dc power source 31 is employed to excite the laser by the dc glow discharge. The dc power source 31 is connected to a cathode pin electrode 112 and an anode 222. Also with the present construction, similar effects to those of the Embodiment 7 can be exhibited. Embodiment 10. The present embodiment has such construction which is a combination of the constructions of the Embodiments 7 and 9, and as shown in Fig. 13, in the present embodiment, the discharge for supplying energy lower than one half the power in the direction of the major side by the dc power source 31 is caused in the direction of the minor side by the RF power source 3. By this construction, the discharge in the direction of the major side is facilitated (so as to allow the discharge to be caused without applying an over-voltage). Further, the discharge can be stabilized. It is to be noted that, while a CO 2 laser is taken as an example in the embodiments described above, the present invention can be applied to such other gas lasers as a CO laser which are required to assure excitation by low energy electrons similarly to a CO 2 laser. Embodiment 11. Fig. 14 is a sectional view showing a gas laser apparatus according to the Embodiment 11 of the present invention. Referring to Fig. 14, reference numerals 1 and 2 each denote a metal electrode, and 13 and 23 each denote a metal angular pipe (through which cooling water circulates). A plurality of metal electrodes 1 and 2 are connected to the dielectric members 10 and 20, respectively, and are connected to the ac power source 3. The metal electrodes 1 and 2 are disposed in an alternate relationship on the opposite sides of the discharge space 4. Further, a metal pipe 13 or 23 in the electrically floating condition is disposed between each adjacent ones of metal electrodes 1 or 2. The metal electrodes 1, 2 and the metal angular pipes 13, 23 are cooled individually, and the laser gas in the discharge space 4 is cooled by way of the dielectric plates 10 and 20. Further, dielectric materials 15 and 25 are molded to cover over the entire electrodes 1, 2 and metal angular pipes 13, 23 to prevent the creeping discharge. Subsequently, operation will be described. If an ac high voltage is applied from the RF power source 3 between the metal electrodes 1 and 2, then the predischarge 44 is caused in the direction of the minor side of the discharge space 4. Then, if the voltage is further raised, the main discharge 45 is caused in oblique directions. This phenomenon will be described by way of an equivalent circuit of an electrode system shown in Fig. 15. The main discharge (plasma resistance R 1 ) 45 is, on one hand, connected to the power source 3 by way of an electrostatic capacity C 1 of the dielectric plate 10 (from the feed electrode 1) and, on the other hand, connected to the power source 3 by way of an electrostatic capacity C 1 of the other dielectric plate 20 (from the feed electrode 2). Meanwhile, the preliminary discharge (plasma resistance R 2 ) 44 is connected, when it is connected to the feed electrode 1 by way of the metal angular pipe 13, to the feed electrode 1 by way the electrostatic capacity C 1 of the dielectric plate 10, the metal angular pipe 13 and an electrostatic capacity C 2 of the dielectric material 15. On the other hand, when the preliminary discharge 44 is connected to the metal angular pipe 23 by way of the feed electrode 2, it is connected to the feed electrode 2 by way of the electrostatic capacity C 1 of the dielectric plate 20. Due to these connection configurations, the preliminary discharge 44 is caused between the metal angular pipe 13 and the feed electrode 2 and similarly between the feed electrode 1 and the metal angular pipe 13. In the present embodiment, the preliminary discharge starts at a very low voltage because the equivalent gap length in the direction of the minor side is short. Then, the discharge field is put into a weakly ionized condition by ultraviolet rays or charged particles generated by the preliminary discharge 44, and the main discharge 45 is fired at a comparatively low voltage. A difference in thrown-in power characteristics arising from whether the preliminary discharge is present or absent is shown in Fig. 16. Referring to Fig. 16, a solid line EX. II shows a thrown-in power characteristic when the preliminary discharge 44 is present, and a broken line shows another thrown-in power characteristic when the preliminary discharge 44 is absent. When there is no preliminary discharge, the main discharge 45 is not fired until after a high voltage (over-voltage) is applied once, and high energy is poured into the discharge field suddenly. However, when a preliminary ionizing mechanism is added to cause the preliminary discharge 44 as in the present embodiment, the main discharge 45 is fired smoothly, and an uncontrollable region (region until discharge is reached) which appears in conventional apparatus can be eliminated. By the way, since the preliminary discharge 44 is the discharge wherein the equivalent gap length is small, there is a drawback that, if the high power is thrown-in for the preliminary discharge 44 is increased, then as described hereinabove in connection with the prior art, the laser excitation efficiency decreases at a low power source frequency below 100 MHz. The inventors confirmed that, if the power to be thrown in for the preliminary discharge 44 is set to a value equal to or lower than 10 % of the power to be thrown in for the main discharge 45, then the role of the preliminary discharge can be exhibited sufficiently without deteriorating the excitation efficiency of the laser by (the preliminary discharge) + (main discharge). In this instance, as seen from Fig. 15, in both of the preliminary discharge 44 and the main discharge 45, the discharge energy (power) is poured in proportion to the electrostatic capacities C 1 and C 1 + C 2 , respectively, of the dielectric members corresponding to the discharge. Accordingly, the power to be thrown in for the preliminary discharge 44 can be set equal to or lower than 10 % of the electric power to be thrown in for the main discharge 45 by setting the electrostatic capacity of the dielectric members corresponding to the preliminary discharge 44 to a value equal to or lower than 10 % of the electrostatic capacity for the main discharge 45. Embodiment 12. While, in the Embodiment 11 described above, the main discharge (oblique discharge) 45 and the preliminary discharge 44 are described as being caused in the plane perpendicular to the optical axis of the rectangular discharge space 4, similar effects to those of the Embodiment 11 can be obtained even if the main discharge 45 and the preliminary discharge 44 are caused in the direction of the optical axis as in the present Embodiment 12 shown in Fig. 17. However, in Fig. 17, only the main discharge 45 is shown while the preliminary discharge 44 is omitted. Also the dielectric molded elements are omitted. Embodiment 13. Further, while, in the Embodiments 11 and 12, power is fed for the preliminary discharge 44 and the main discharge 45 from the common power source 3, similar effects to those of the Embodiments 11 and 12 can be obtained even if energy is supplied separately for the preliminary discharge 44 and the main discharge 45 from separate power sources 3 and 32 as in the Embodiment 13 shown in Fig. 18. Embodiment 14. Fig. 19 is a perspective view showing a gas laser apparatus according to the Embodiment 14 of the present invention. The gas laser apparatus of the present embodiment is different from the conventional gas laser apparatus shown in Fig. 42 in that the lengthwise dimension and the widthwise dimension of the metal electrodes 1 and 2 are set shorter than the lengthwise dimension and the widthwise dimension, respectively, of the dielectric plates 10 and 20 by 5 mm or more. In the present embodiment, since the lengthwise dimension and the widthwise dimension of the metal electrodes 1 and 2 are smaller than the lengthwise dimension and the widthwise dimension, respectively, of the dielectric plates 10 and 20 by 5 mm or more, also when the discharge power is increased (when the applied voltage is raised), concentration of the discharge 42 by concentration of the electric field at the corner portions of the dielectric plates 10 and 20 (refer to Fig. 49) can be prevented. Meanwhile, also the occurrence of the discharge 41 toward the output coupler 8 shown in Fig. 47) depends much upon the applied voltage. In the following, the discharge 41 will be described. In the Embodiment 14, where the gap length is represented by d, the distance between the dielectric plates 10, 20 and the output coupler 8 by L wm , the difference in length between the metal electrodes 1, 2 and the dielectric plates 10, 20 by L, the discharge starting voltage by V* and the applied voltage peak value by Vo p , the design standard of the distance (L wm + L) between the output coupler 8 and the metal electrodes 1, 2 is set in accordance with the following equation (3): L + L Wm ≧ (Vo p /V*)d (Vo p is an adjustable variable, and V and d are fixed values.) It was proved experimentally by the inventors that, under the conditions wherein the equation (3) is satisfied, no discharge 41 toward the output coupler 8 is caused. Further, the discharge power W d is obtained from the power source frequency f and the dielectric electrostatic capacity C of the discharging section in accordance with the following equation (4): W d = πfCV*(Vo p 2 - V *2 ) 1/2 (C and f are fixed values.) Accordingly, as apparent from the equations (3) and (4), when a high discharge power W d is to be thrown in, Vo p , that is, (L wm + L), should be set to a high value. Consequently, if the distance (L wm + L) between the output coupler 8 and the metal electrodes 1, 2 is set to a large value, then even if the applied voltage is raised, occurrence of the discharge 41 toward the optical resonator mirror 5 can be prevented. Embodiment 15. Further, if the construction of the Embodiment 14 described above is modified such that the width W of the metal electrodes 1 and 2 is set substantially equal to the width of a laser beam passing between them which is determined by the optical resonator as in the present embodiment shown in Figs. 20A and 20B, then the discharge energy can be converted into light energy without a loss. Embodiment 16. In the present embodiment, making use of the fact that the width of the metal electrodes 1 and 2 is smaller than the width W' of the dielectric plates 10 and 20, a pair of spacers 16 and 17 are disposed between the dielectric plates 10 and 20 between which the metal electrodes 1 and 2 are not present as shown in Figs. 21A and 21B so that the distance between the surfaces (light reflecting faces) of the dielectric plates 10 and 20 is kept fixed. Consequently, the gap distance between the dielectric plates 10 and 20 can be kept fixed without having an influence on the discharge of a laser beam. It is to be noted that, if the spacers 16 and 17 are constituted from an incombustible material against a laser beam, then a laser beam which may be produced other than in the direction of the regular optical axis by diffracted light of a laser beam or mis-alignment of the resonator causes no undesirable effect. Further, since the spacers 16 and 17 are formed from an insulating substance such as ceramics, they do not have an influence on the discharge at all and the anticipated objects can be achieved. Furthermore, if the spacers 16 and 17 are constituted from a metal, they can be produced at a low cost. In this instance, the somewhat pinched discharge is observed at the spacer portions. However, according to the inventors, it was confirmed that the discharge does not have an influence on the laser characteristic. Embodiment 17. Fig. 22 is a perspective view of a laser apparatus according to Embodiment 17 of the present invention. In this figure, reference numeral 70 designates a laser beam output window as the atmosphere shield window arranged on the side of the output coupler 8 for removing a laser beam. This output window 70 is installed away from the output coupler 8. Accordingly, a portion between the output coupler 8 and the output window 70 forms a non-discharge portion (a non-excitation portion) through which a laser beam 9 passes. In this non-discharge portion is arranged a cooling duct 711 as a laser gas cooling means. The cooling duct 711 has a rectangular opening in the central portion thereof. The length of major side and minor side of the rectangular opening is set to three times or less of the major side and the minor side of the laser beam which passes through the cooling duct 711. Further, in the Embodiment 17, dimensions of the lengths of metal electrodes 1 and 2 are set to be shorter than dimensions of the length of dielectric plates 10 and 20. Both lengthwise ends of the dielectric plates 10 and 20 are extended toward the reflection mirrors 7 and 8 from both lengthwise ends of the metal electrodes 1 and 2. Accordingly, a portion between the metal electrodes 1, 2 and the reflection mirrors 7, 8 also forms a non-discharge portion (a non-excitation portion). In these non-discharge portions are arranged, as laser gas cooling means, cooling pipes 712, 713 and 714, 715 having a cooling function which are electrically floated or grounded. Generally, since the output coupler 8 and the output window 70 are arranged away from each other, the laser beam can be naturally propagated to increase a beam diameter at the position of the output window 70. In Fig. 22, the output coupler 8 and the output window 70 are merely arranged away from each other, Fig. 24 shows the characteristics of laser oscillation of a conventional laser apparatus having no laser gas cooling means. As will be apparent from Fig. 24, a laser output along with a discharge electric power linearly extends in a low output region. However, a saturation phenomenon of output is once observed at a position where laser output is approximately 500 W, and it has been confirmed that as an input increases, a laser output again increases. It has been found that an inclination (efficiency) of the laser output after saturation is lower than that prior to saturation. It has become clear that the intensity of light at which output is saturated largely depends upon the concentration of carbon dioxide, and the lower the concentration of carbon dioxide, the output saturation is harder to occur. It has been found as the result of the detailed study that the aforesaid phenomenon is the saturation phenomenon of the output due to the absorption of the laser light of carbon dioxide gas in the non-excitation and non-cooling space. That is, the saturation phenomenon of the output results from the effect of light absorption in the space which is not excited (in which reversal distribution is not formed) and which is not cooled, for example, in the space between the discharge space 4 and the reflection mirrors 7, 8 or between the output coupler 8 and the output window 70. Now, the relationship between the coefficient of light absorption of the carbon dioxide gas and the gas temperature is shown in Fig. 25. As will be apparent from Fig. 25, it is understood that as the gas temperature rises, the coefficient of light absorption increases, and finally a saturation occurs. That is, the process is repeated such that in the non-excitation space, the carbon dioxide gas absorbs the energy of the laser beam to induce a rise of gas temperature and the absorption amount of the energy increases. In this process, even if the input increases, the absorption amount of light increases, and therefore the laser output does not increase, and when the gas temperature reaches approximately 600 K, the coefficient of light absorption stops to increase. As described above, the conventional laser apparatus has problems in that the effect of light absorption increases in the non-excitation space and non-cooling space particularly at the time of high laser output increases, and by this effect of light absorption, the saturation phenomenon of laser output occurs or the oscillation efficiency lowers. However, in this Emoodiment 17, the laser gas cooling means (the cooling duct 711 and the cooling pipes 712, 713, 714 and 715) is provided in the non-discharge portions in the passage of the laser beam (between the output coupler 8 and the output window 70, and between the metal electrodes 1, 2 and the reflection mirrors 7, 8), whereby in the non-discharge portion (non-excitation space) where no reversal distribution occurs, heat caused by light absorption is present but the cooling thereof is sufficiently carried out so that the effect of light absorption can be minimized. Thereby, the rise of gas temperature is small. Accordingly, the coefficient of light absorption is suppressed to a low value, and the laser output with a high efficiency becomes possible. Fig. 23 is a view showing the characteristics of laser oscillation according to Embodiment 17. As will be apparent from this figure, according to Embodiment 17, the saturation phenomenon of the laser output and the lowering of the oscillation efficiency as seen in Fig. 24 are not at all observed, and the laser apparatus with high efficiency is obtained. Fig. 26 is a perspective view of a laser apparatus according to Embodiment 18 of the present invention. In the Embodiment 18, a small blower or the like is installed as a gas flow generating means so that as indicated by the arrow in the non-excitation portion, gas flows 150, 160 are forcibly generated, and the temperature rise of the laser gas at the non-excitation portion is suppressed by the gas flows 150, 160. This also provides the effect similar to that of the Embodiment 17. Embodiment 19. Fig. 27 is a perspective view of a laser apparatus according to Embodiment 19 of the present invention. In this Embodiment 19, side plates 51 and 52 for preventing a leakage of laser gas are provided on both sides of a discharge space 4 formed between dielectric plates 10 and 20. Gas flow passages 51a and 52a (only 51a is shown) are provided in the midst of the side plates 51 and 52. A laser gas is supplied from the gas flow passages 51a and 52a into the discharge space 4 as indicated by arrows 170 and 180, and gas is always exhausted by a vacuum pump (not shown) so that gas pressure in the discharge space 4 is constant. With the construction as described above, a gas flow occurs from the central portion of the discharge space 4 toward the reflection mirrors 7 and 8, and the similar effect is obtained. In this case, in the discharge space 4, the carbon dioxide gas is subjected to electron collision and dissociated into carbon monoxide and oxygen as given by the following formula: CO 2 + e → CO + O 2 / 2 For this reason, there is a most generous effect of obtaining an ideal situation wherein in the discharge space, the concentration of carbon dioxide gas is high, and in the non-excitation space, the concentration of carbon dioxide gas is low. From a view point that a low temperature gas flows into the non-excitation portion, the flow of the laser gas (arrows 170, 180) in Fig. 27 may be reversed. That is, it is significant to provide an arrangement wherein a laser gas is introduced from the non-excitation portion into the discharge space 4 and a laser gas is discharged from the central portion of the electrode (gas flow passages 51a, 52a of the side plates 51, 52). While in the Embodiment 19, the case has been described in which the dissociation phenomenon of the carbon dioxide caused by discharge is utilized to provide a difference in the concentration of carbon dioxide between the excitation space and the non-excitation space, it is to be noted in short that if the concentration of carbon dioxide in the non-excitation space is suppressed to be low, a similar effect is obtained. Embodiment 20. Fig. 28 is a perspective view of a waveguide path type CO 2 laser apparatus according to Embodiment 20 of the present invention. In this Embodiment 20, a pair of flat-plate like metal electrodes 1, 2 opposed to each other and dielectric plates 10, 20 plated in close contact with the opposed surfaces of the metal electrodes 1, 2 to form a discharge space 4 having a rectangular section therebetween constitute one unit, and such four units U1, U2, U3 and U4 are arranged to have a polygonal shape in section (a square shape in section). With such a construction as described, each of four discharge spaces 4a to 4d formed in the four units U1 to U4 is in the state arranged in a polygonal shape (a square shape in section). In the Embodiment 20, the one metal electrode 1 of each of the units U1 to U4 is connected to an excitation power source (RF power source) 3, and the discharge spaces 4a to 4d are filled with a mixture gas similarly to prior art. Further, in the Embodiment 20, reflection mirrors 7, 8 arranged opposedly on both lengthwise end openings of the discharge spaces 4a to 4d are comprised of a series of folded mirror plate portions 71, 72 and 81, 82 folded at a suitable angle through folded lines 7a, 8a, their folding directions being arranged to be different by 90°. More detailedly, in Fig. 29 which shows a beam light path of a laser beam of the waveguide path type CO 2 laser apparatus shown in Fig. 28, L1 and L2 represent two linear symmetrical axes which pass a center point of a space section which is surrounded by the discharge spaces 4a, 4b, 4c and 4d to form a beam light path. The reflection mirrors 7 and 8 are arranged so that one out of the linear symmetrical axes L1 and L2 accords with the folding lines 7a, 8a of the reflection mirrors 7, 8, and the directions of the folding lines 7a, 8a are different by 90°. The linear symmetrical lines L1 and L2 termed herein mean that the space section is symmetrical to left and right with respect to a certain line. For example, in Fig. 29, in the case where the dotted lines are arranged in section of the discharge spaces 4a, 4b, 4c and 4d, when the discharge space section is folded at the portion of the linear symmetric axis L1 or L2, the folded discharge space sections are exactly overlapped to provide an accorded line. In the reflection mirrors 7 and 8 folded as described above, a rectangular laser beam emitting port 83 as shown in Fig. 28 is provided on the reflection mirror 8 on the laser beam removing side. In Fig. 29, reference numeral 9a designates a laser beam which is present in the discharge spaces 4a, 4b, 4c and 4d. In Fig. 28, reference numeral 39 designates a laser beam which is emitted from the laser beam emitting port 83. The operation will be described hereinbelow. In Fig. 29, the laser beam 9a is reflected downwardly in the direction of the arrow from a black point P of an upper folded mirror plate portion 81 in output coupler 8 and moves forward. The laser beam 9a is turned back and reflected in the direction of one folded mirror plate portion 71 of the other reflection mirror 7 at a lower folded mirror plate portion 82. Then, the laser beam is turned back and reflected in the lateral direction from the folded mirror plate portion 71 toward the other folded mirror plate portion 72 continuous thereto, and thereafter the laser beam is turned back and reflected toward the lower folded mirror plate portion 82 of the one reflection mirror 8 at the folded mirror plate portion 72. In the lower folded mirror plate portion 82, the light beam is turned back and reflected toward the upper folded mirror plate portion 81. In the lower folded mirror plate portion 81, the laser beam is turned back and reflected in the direction of the one folded mirror plate portion 72 (left side in the figure) of the other reflection mirror 7. In this folded mirror plate portion 72, the laser beam is turned back and reflected in the direction of the other folded mirror plate portion 71 (right side in the figure) and thereafter turned back and reflected to as to be returned to the black point P from the folded mirror plate portion 71. That is, in the output coupler 8, the incident laser beam 9a from the other reflection mirror 7 is turned back and reflected vertically, and in the other reflection mirror 7, the incident laser beam 9a from the output coupler 8 is turned back and reflected in the lateral direction. Accordingly, the light path of the laser beam 9a which is turned back at the one reflection mirror 7 and is again reflected in the direction of the output coupler 8 is not reversely returned along one and the same light path unlike the light path of the laser beam before being turned back and reflected as previously mentioned. The laser beam 9a returned to the one reflection mirror 8 is at a position separately from the black point P (the folded mirror plate portion 81 having the black point P is the folded mirror plate portion 82), and the laser beam 9a is again turned back upwardly (the folded mirror plate portion 81) from that position, and turned back and reflected in the direction of the folded mirror plate portion 72 of the other reflection mirror 7. The laser beam 9a reached the folded mirror plate portion 72 is turned back and reflected in the direction of the folded mirror plate portion 71, and then the laser beam is folded back and reflected in the direction of the folded mirror plate portion 81 of the other reflection mirror at the folded mirror plate portion 71, and returns to the black point P. As described above, the laser beam 9a is continuous as a single beam, and four beam paths which reciprocate between the reflection mirrors 7 and 8 are formed by the discharge spaces 4a to 4d each having a rectangular section whereby the laser beam 9a is to be amplified. In short, in the waveguide path type CO 2 laser apparatus according to the Embodiment 20, the rectangular sections of the four discharge spaces 4a to 4d are arranged in a square shape, and the turn-back reflection mirrors 7 and 8 are used, whereby the laser beam 9a can be amplified within the respective discharge spaces 4a to 4d, and the thus amplified laser beam 9a can be handled solidly as one laser beam. Thus, the apparatus can be miniaturized, and the ceramic plate as a material for dielectrics 10 and 20 can be divided every unit U1 to U4 for use, thus reducing the cost. Embodiment 21. Fig. 30 is a sectional view showing a waveguide path type CO 2 laser apparatus according to Embodiment 20 of the present invention. Fig. 31 is a perspective view showing a beam path of a laser beam shown in Fig. 30. In the Embodiment 20, four units U1 to U4 are arranged so that the rectangular sections of the respective discharge spaces 4a to 4d are in the square shape, whereas in the Embodiment 31, five units U1 to U5 are used, and rectangular sections of the discharge spaces 4a to 4e are arranged in a pentagon shape. The operation of the Embodiment 21 is similar to that of the Embodiment 20 but the beam path of the laser beam 9a is a beam path shown at the arrow in Fig. 31 unlike the case of the Embodiment 20. The operation and effects are similar to those of the case of the Embodiment 20. Embodiment 22. Fig. 32 is a sectional view showing a waveguide path type CO 2 laser apparatus according to Embodiment 22 of the present invention. Fig. 33 is a perspective view showing a beam path of a laser beam shown in Fig. 34. In the Embodiment 22, six units U1 to U6 are used, and rectangular sections of the discharge spaces 4a to 4f are arranged in a hexagon shape. The beam path of the laser beam 9a is a beam path shown at the arrow in Fig. 34, and the similar operation and effects are obtained. That is, the Embodiments 20, 21 and 22 are characterized in that a plurality of discharge spaces each having a rectangular section are arranged in a polygon shape in section, the laser beam 9a is turned back and reflected by the turn-back type reflection mirrors 7 and 8 to reciprocate it plural times, and the laser beam to be turned back and reflected passes through all the discharge spaces and are amplified whereby it can be handled as a single beam path. Accordingly, the discharge spaces are suffice to be the number which can be arranged in a polygon shape and the number is not limited. Embodiment 23. Fig. 35 is a sectional view showing essential parts of a waveguide path type CO 2 laser apparatus according to Embodiment 23 of the present invention. Fig. 36 is a perspective view showing a beam path of a laser beam shown in Fig. 35. In this Embodiment 23, rectangular sections of four discharge spaces 4a to 4d are arranged at equal intervals about a point of intersection of two linear symmetrical axes L1 and L2 intersecting perpendicularly to each other. With this construction, there is obtained a beam path of a laser beam 9a indicated at the arrow in Fig. 36. Accordingly, the operation and effects similar to the case of the Embodiment 20 are obtained. The linear symmetrical axes L1 and L2 mean the fact similar to that mentioned in connection with the Embodiment 20 (Fig. 29). Embodiment 24. Fig. 37 is a sectional view showing essential parts of a waveguide path type CO 2 laser apparatus according to Embodiment 24 of the present invention. In the Embodiment 23, the rectangular sections of the four discharge spaces 4a to 4d are arranged at equal intervals in the radial direction, whereas in this Embodiment 24, rectangular sections of five discharge spaces 4a to 4e are radially arranged at equal intervals. A beam path of a laser beam obtained in this case is similar to that shown in Fig. 31. Accordingly, the similar operation and effects are obtained. Embodiment 25. Fig. 38 is a sectional view showing essential parts of a waveguide path type CO 2 laser apparatus according to Embodiment 25 of the present invention. In this Embodiment 25, rectangular sections of six discharge spaces 4a to 4f are radially arranged at equal intervals. Accordingly, also in this case, the similar operation and effects are obtained. While in the aforementioned Embodiments 23 to 25, the case has been described in which four to six discharge spaces are radially arranged, it is to be noted that the number of the discharge spaces is not limited but rectangular sections of a plurality of discharge spaces may be radially arranged so that at least two linear symmetrical axes or more may be obtained. Embodiment 26. Fig. 39 is a sectional view showing essential parts of a waveguide path type CO 2 laser apparatus according to Embodiment 26 of the present invention. The fundamental configuration of Embodiment 26 is similar to that of prior art example (Fig. 42). In this Embodiment 26, dielectric plates 10 and 20 are formed from two kinds of dielectric layers 611, 612 and 621, 622 which are different in dielectric constant from each other. The two kinds of dielectric layers 611, 612 and 621, 622 may be connected by electric insulative adhesive layers or may be connected by conductive materials. In this case, on the side of metal electrodes 1 and 2, there are provided high dielectric-constant layers 611, 621 formed of a high dielectric-constant material (dielectric constant ε H ), and on the side of the waveguide path (discharge side), there are provided low dielectric-constant layers 611, 622 formed of a low dielectric-constant material (dielectric constant: ε L ). A thickness (tH) of the high dielectric-constant layers 611, 621 is set to be thicker (tH > tL) than a thickness (tL) of the low dielectric-constant layers 612, 622. The operation of the Embodiment 26 is basically similar to the case of the conventional apparatus shown in Fig. 42. When an alternating voltage is applied to the metal electrodes 1 and 2, laser excitation is effected by the discharge occurred in the discharge space 4. Since the low dielectric-constant layers 612, 622 are formed on the waveguide path surface on which laser beam is reflected, the propagation loss of light can be minimized as will be understood from the above-described formula (1). Next, the electric characteristics will be mentioned. When a sine wave voltage is applied, an energy : discharge power Wd charged into the discharge space 4 is given by the following formula (5): Wd = fπCdV* V2op - V*2 wherein f represents the power source frequency, V* represents the discharge voltage, and Vop represents the crest value of the applied voltage. Cd represents the electrostatic capacity of the dielectrics 10 and 20. In the case where the dielectrics 10 and 20 are formed of a composite material (the electrostatic capacity: (C H ,C L ) as shown in Fig. 59, the electrostatic capacity is as follows: Ceff = C H C L C H +C L From the area S and the thickness t of the dielectric plates 10 and 20 and the specific permeability ε, the electrostatic capacity C is as follows: C = ε 0 ε S t So, if the thickness (t H ) of the high dielectric-constant layers 611 and 621 is set to be sufficiently thicker (t H > t L ) than the thickness (t L ) of the low dielectric constant layers 612 and 622, C L >> C H is given from the formula (7). It is understood from the formulae (6) and (8) that the effective electrostatic capacity Ceff substantially accords with the capacity of of the high dielectric-constant layers 611 and 621 (C tH = C tL ). That is, even if the low dielectric-constant layers 612, 622 are plated on the high dielectric-constant layers 611, 621, if its thickness increases, the electric characteristics remain unchanged, and the similar discharge characteristics are obtained. In the waveguide path type laser apparatus, the functions required by the dielectrics 10 and 20, that is, the electric performance related to the discharge and the optical performance related to the light propagation are shared with the dielectrics 10 and 20 layers. Therefore, the low dielectric-constant materials which are hard to be sintered that could not be used heretofore for the apparatus of this kind and the materials from which thick plates could not be fabricated due to the weak thermal distortion are rendered possible to use to extremely widen an allowance of materials to be used. Embodiment 27. In the aforementioned Embodiment 26, the case has been disclosed in which the low dielectric-constant layers 612, 622 are made to be sufficiently thin. In the case where the power source frequency to be used is sufficiently high, even an extremely low voltage a high power can be used as will be apparent from the aforementioned formula (5). In this case, not much electric characteristics are not required for the dielectric plates 10 and 20. In the case of such conditions as described, it is not necessary to make the low dielectric-constant layers 612, 622 sufficiently thinner than the high dielectric-constant layers 611, 621 but as shown in Fig. 40, the thickness of the high dielectric-constant layers 611, 621 may be made to be substantially equal to that of the low dielectric-constant layers 612, 622, or the relationship of the thickness therebetween may be reversed. Embodiment 28. In the Embodiments 26 and 27, the case has been described in which two dielectric materials are superimposed for use. However, as shown in Fig. 41, the low dielectric-constant layers 612, 622 may be formed on the surfaces of the high dielectric-constant layers 611, 621 by flame spraying. Of course, the low dielectric-constant materials may be coated by processes other than the flame spraying to obtain the similar effects. Embodiment 29. In the case of a carbon dioxide gas laser of wavelength 10.6 µm, preferable materials used for the low dielectric-constant layers 612, 622 are BeO or AIN. Embodiment 30. In the Embodiments 26 to 29, the CO 2 laser apparatus has been described, but of course it can be applied to the waveguide path type laser apparatus.

Claims: A laser apparatus comprising a discharge space (4) between dielectric plates (10, 20) being in close contact with metal electrodes (1, 2) and forming a waveguide whose gap is in the minor side of a rectangular excitation section for radio-frequency stimulated laser excitation, the ratio between major side and minor side of said rectangular section is 3 or more, and outputting a laser beam in the direction intersecting perpendicularly to said rectangular section of said discharge space (4), characterized in that means are provided such that the discharge (DC) is generated obliquely within said rectangular section of the discharge space. A laser apparatus comprising a discharge space (4) between dielectric plates (10, 20) being in close contact with metal electrodes (1, 2) and forming a waveguide whose gap is in the minor side of a rectangular excitation section for radio-frequency stimulated laser excitation, the ratio between major side and minor side of said rectangular section is 3 or more, and outputting a laser beam in the direction intersecting perpendicularly to said rectangular section of said discharge space (4), characterized in that means are provided such that the discharge (DC) is generated obliquely with respect to the direction of an optical axis intersecting perpendicularly to said rectangular section within said discharge space. A laser apparatus according to claim 1, wherein said electrode (1, 2) for discharge excitation is provided, and a distribution is provided for electrostatic capacity of said electrode (1, 2) to thereby realize said oblique discharge. A laser apparatus according to claim 1, wherein a hot flow is generated in the direction of the minor side of said rectangular section to cool gas, a preliminary discharge (44) is generated in the direction of the minor side within said rectangular section, and a main discharge (45) is generated obliquely. A laser apparatus according to claim 2, wherein a hot flow is generated in the direction of the minor side of said rectangular section to cool gas, a preliminary discharge (44) is generated in the direction of the minor side within the surface intersecting perpendicularly to said rectangular section, and a main discharge (45) is generated obliquely. A laser apparatus according to claim 4 or 5, wherein said preliminary discharge (44) and said main discharge (45) are effected by the energy supplied from one and the same power source (3), and energies charged into said preliminary discharge (44) and said main discharge (45) are set by an electrostatic capacity (C 1 (10), C 1 (20), C 2 ) for said preliminary discharge and an electrostatic capacity (C 1 (10), C 1 (20)) for said main discharge. A laser apparatus according to claim 6, wherein said electrostatic capacity (C 1 (10), C 1 (20), C 2 ) for said preliminary discharge (44) is less than 1/10 of said electrostatic capacity (C 1 (10), C 1 (20)) for said main discharge (45). A laser apparatus according to claim 6, wherein said main discharge (45) is caused between said electrodes (1, 2), and said preliminary discharge (44) is caused between conductive members (13, 23) connected to the electrodes (1,2) through dielectric materials (10, 20) to thereby set the magnitudes of the electrostatic capacity for the main discharge (45) and that for the preliminary discharge (44). A laser apparatus according to claim 1 or 2, wherein said discharge space (4) is constituted by opposed two or more flat plates (10, 20; 10, 10a, 10b, 20, 20a, 20b; 10, 10c, 20, 20c), said flat plates being used as waveguide paths for the laser beam. A laser apparatus according to claim 1 or 2 wherein the length and the width of said electrodes (1, 2) are set to be shorter by 5mm or more than the length and the width, respectively, of said dielectric plates (10, 20). A laser apparatus according to claim 10, wherein the width W of said electrode (1, 2) is set to be equal to the width of the laser beam determined by an optical resonator (7, 8). A laser apparatus according to claim 11, wherein a spacer (16, 17) securely held on the part extended from the electrode between said light reflection surfaces is formed of a noninflammable material with respect to the laser beam in order that the distance between said pair of light reflection surfaces disposed opposedly each other is kept constant. A laser apparatus accoraing to claim 1 or 2, wherein said electrodes (1, 2) for discharge excitation are molded with a flexible insulating material (101, 201).

Inventors: KUZUMOTO MASAKI, NISHIMAE JUN-ICHI, TAKENAKA YUSHI, YAMAMOTO TAKASHI, YOSHIZAWA KENJI, KUZUMOTO, MASAKI, NISHIMAE, JUN-ICHI, TAKENAKA, YUSHI, YAMAMOTO, TAKASHI, YOSHIZAWA, KENJI, Kuzumoto, Masaki, c/o Mitsubishi Denki K. K., Nishimae, Jun-ichi, c/o Mitsubishi Denki K. K., Takenaka, Yushi, c/o Mitsubishi Denki K. K., Yamamoto, Takashi, c/o Mitsubishi Denki K. K., YOSHIZAWA, KENJI, C/O MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1993
Main classification: H01S3
IPC codes: H01S3