Discharge excitation pulsed laser oscillation device

Abstract: A pulsed transversely excited gas laser device includes a pair of main electrodes (1,2) having main surfaces opposing each other across a main discharge gap (3) and running perpendicular to the optical axis of the laser. A corona discharge preionizer (10) for radiating ultraviolet radiation is substantially accommodated within a recess formed on the main surface of one of the main electrodes (1,2) at an upstream side of the foot of the main discharge 3. The corona discharge preionizer (10) is separated from the inner wall and bottom surface of the recess except at a surface portion thereof which is in contact with a projection extending sideways from a side wall of the recess next to the main discharge (3). The corona discharge generated over the preionizer (10) radiates ultraviolet radiation to preionize the laser gas.

Description: This invention relates to a discharge excitation pulsed laser oscillation device (pulsed transversely excited gas laser devices) according to the preamble of claim 1, and more particularly to the electrode structure of the discharge excitation pulsed laser oscillation devices. Fig. 2 is a schematic sectional view of a conventional discharge excitation pulsed laser oscillation device (pulsed transversely excited gas laser device), showing a section thereof perpendicular to the optical axis of the laser. The device is disclosed in: Marchetti and E. Penco, "A new type of corona-discharge photoionization source for gas lasers", Journal of Applied Physics 56(11), December 1984, pp. 3163 - 3168. In Fig. 2, a pair of main electrodes 1 and 2 opposing each other extend parallel to the optical axis of the laser. Across the main electrodes 1 and 2 is formed a main discharge 3. A laser gas flow 4 is supplied from a laser gas supply device (not shown), along a directional perpendicular to the optical axis of the laser, to the region between the main electrodes 1 and 2. A pair of preionizers 5 disposed on the upstream and the downstream side (with respect to the direction of the laser gas flow 4) of the main electrodes 1 and 2 preionize the laser gas between the main electrodes 1 and 2 such that the main discharge 3 is generated. Each preionizer 5 consists of: a hollow dielectric pipe 6, an auxiliary electrode 7 inserted into the bore of the dielectric pipe 6 and a conductor wire 8 which is attached over the outer surface of the dielectric pipe 6 and is kept at the same potential as the main electrode 2. Next, the operation of the discharge excitation pulsed laser oscillation device of Fig. 2 is described. First, the laser gas is supplied from a laser gas supply device to the region between the main electrodes 1 and 2. When a voltage is applied across the main electrode 2 and each of the auxiliary electrodes 7, the corona discharge 9 starts from the point at which the conductor wire 8 and the dielectric pipe 6 are in contact and covers the outer surface of the dielectric pipe 6. The ultraviolet radiation radiated from the corona discharge 9 preionizes the laser gas between the main electrodes 1 and 2. Next, upon application of a voltage across the main electrodes 1 and 2, the preionized laser gas begins to discharge, and the main discharge 3 is generated across the main electrode 1 and main electrode 2. The laser gas is excited by the main discharge 3, such that the laser commences to oscillate in a direction perpendicular to the surface of the drawing in Fig. 2. In the case of the discharge excitation pulsed laser oscillation device of Fig. 2, the discharge remnants, such as the ions and metastable gas molecules and atoms generated by the main discharge 3, and particles sputtered out from the electrodes, are carried downstream with the laser gas flow 4. By the time when the next main discharge 3 is started by the subsequent voltage pulse, the new laser gas is supplied to the region between the main electrodes 1 and 2 and the ultraviolet radiation is radiated thereto from both sides. Thus, the number of preionized electrons (photoelectrons) remains constant and the distribution thereof is symmetric for the upstream and the downstream side (with respect to the laser gas flow 4) in the region between the main electrodes 1 and 2. Thus, the main discharge 3 is maintained constant throughout the operation. Fig. 3 is a schematic sectional view of a conventional discharge excitation excimer laser oscillation device. The device is disclosed in: T. S. Fahlen, "HIGH-AVERAGE-POWER EXCIMER LASER", DOE/SF/90024--T2,1977, United States Energy Research and Development Administration. A tube 1a having an outer diameter of 1/4 inches is mounted on an end of a main electrode 1 opposing the other main electrode 2. The preionizer 5, including a hollow dielectric pipe 6 and an auxiliary electrode 7 which consists of a wire inserted into the bore of the dielectric pipe 6, is disposed at the downstream side (with respect to the laser gas flow 4) of the main electrode 1 such that the dielectric pipe 6 is partly in contact with the main electrode 1. The operation of the laser device of Fig 3 is similar to that of the laser device of Fig. 2. After the laser gas is supplied to the region between the main electrodes 1 and 2, a voltage is applied across the main electrode 1 and the auxiliary electrode 7, such that the corona discharge 9 is generated from the point where the main electrode 1 and the dielectric pipe 6 are in contact, and thence covers the whole dielectric pipe 6. The laser gas between the main electrodes 1 and 2 are preionized by the ultraviolet radiation radiated from the corona discharge 9. When a voltage is applied across the main electrodes 1 and 2, the preionized laser gas commences to discharge, thereby generating the main discharge 3. The laser gas is excited by the main discharge 3, such that the laser oscillates in the direction perpendicular to the surface of the drawing. The preionizer 5 is disposed only at the downstream side of the main electrode 1. Thus this excimer laser device has a simplified structure. The above conventional laser devices have the following disadvantages. In the case of the device of Fig. 2, the preionizers 5 are disposed on both the upstream and downstream side (with respect to the laser gas flow 4) of the main electrodes 1 and 2. However, the ultraviolet radiation radiated from the preionizer 5 at the downstream side is absorbed by the discharge remnants generated by the main discharge 3. Thus, as the oscillation frequency increases, the center of the distribution of preionized electrons (photoelectrons) is shifted to the upstream side. Thus, as shown in Fig. 4, the central position of the laser beam is also shifted to the upstream side (with respect to the laser gas flow 4) as the oscillation frequency increases. On the other hand, in the case of the laser device of Fig. 3, the preionizer 5 is disposed only at the downstream side (with respect to the laser gas flow 4) of the main electrodes 1 and 2. The ultraviolet radiation radiated from the preionizer 5 is absorbed by the discharge remnants generated by the main discharge 3. Thus, as the oscillation frequency increases, the number of preionized electrons decreases, and the homogeneity of the main discharge 3 is deteriorated. As a result, the energy is not effectively deposited into the laser gas. Thus the laser oscillation efficiency deteriorates and the output power decreases. Thus, it is necessary to substitute the laser gas between the main electrodes 1 and 2 by a new volume of the laser gas by the time when the next main discharge 3 is generated. To accomplish this, a laser gas supply device of large capacity becomes necessary. A discharge excitation pulsed laser oscillation device having a main electrode provided with grooves or holes for mounting auxiliary electrodes is known from the JP-A-63-217 684 and the JP-A-63-133 583. A discharge excitation pulsed laser oscillation device wherein a corona electrode is formed by inserting a plurality of metallic wires in a cylindrical body formed of a dielectric in a flat and hollow form at prescribed intervals in the width direction of the cylindrical body, wherein said cylindrical body is inserted into a recess in a main electrode is known from the JP-A-01-080 081. EP-A-0 167 746 discloses a discharge excitation pulsed laser oscillation device having one of its main electrodes provided with a plurality of recesses containing auxiliary electrodes. SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a discharge excitation pulsed laser oscillation device by which the output power can be maintained even with a laser gas supply device of smaller capacity, and the central position of the output laser beam is fixed irrespective of the oscillation frequency. The above object is accomplished in accordance with the principle of this invention by a discharge excitation pulsed laser oscillation device of the initially defined type which has the characterizing features of claim 1. Preferred embodiments are listed in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS The features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The structure and method of operation of this invention itself, however, will be best understood from the following detailed description, taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic sectional view of a discharge excitation pulsed laser oscillation device according to an embodiment of this invention; Fig. 2 is a schematic sectional view of a conventional discharge excitation pulsed laser oscillation device (pulsed transversely excited gas laser device), showing a section thereof perpendicular to the optical axis of the laser; Fig. 3 is a schematic sectional view of a conventional discharge excitation excimer laser oscillation device; and Fig. 4 is a diagram showing the shift of the central position of the laser beam accompanying the increase of the laser oscillation frequency. In the drawings, like reference numerals represent like or corresponding parts or portions. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the accompanying drawings, the preferred embodiment of this invention is described. Fig. 1 is a schematic sectional view of a discharge excitation pulsed laser oscillation device according to an embodiment of this invention. In Fig. 1, the parts 1 through 9 correspond to those described above with reference to Figs. 2 and 3. A main electrode 1 opposes an other main electrode 2 to form a main discharge 3 therebetween. The laser gas flow 4 is supplied from a laser gas supply device (not shown) disposed to the right of the main electrodes 1 and 2 in Fig. 1. A preionizer 10 is substantially accommodated within a recess formed on the substantially flat upper surface near the location thereof where the foot of the main discharge 3 is formed. The recess is formed at the upstream side of the main discharge 3 with respect to the laser gas flow 4. The preionizer 10 includes a hollow dielectric pipe 6, and an auxiliary electrode 7 inserted within the dielectric pipe 6. The dielectric pipe 6 is separated by a gap from the walls of the recess of the main electrode 2, except at a portion which is in contact with a projection (shown at the upper left corner of the recess in Fig. 1) formed on a side wall of the recess next to the main discharge 3. The dielectric pipe 6 is made of an alumina ceramic consisting primarily of alumina. Next the operation of the laser device of Fig. 1 is described. After the laser gas is supplied to the region between the main electrodes 1 and 2, a voltage pulse is applied across the main electrode 2 and the auxiliary electrode 7. As a result, the corona discharge 9 is generated from the point at which the main electrode 2 and the dielectric pipe 6 are in contact, and thence covers the whole outer surface of the dielectric pipe 6. An ultraviolet radiation is radiated from the corona discharge 9, such that the laser gas between the main electrodes 1 and 2 is preionized. Thereafter, when a voltage pulse is applied across the main electrodes 1 and 2, the preionized laser gas starts to discharge and the main discharge 3 is generated thereacross. The laser gas is excited by the main discharge 3, such that the laser oscillates in a direction perpendicular to the surface of the drawing (transversely excited gas laser). Discharge remnants generated by the main discharge 3 are carried away by the laser gas flow 4 to the downstream side by the time when the next discharge begins, and a new volume of the laser gas supplied from the laser gas supply device replaces the region between the main electrodes 1 and 2. By repeating the above operations, the laser oscillation is repeated. As described above, the dielectric pipe 6 is substantially embedded within the recess of the main electrode 2. Thus, the dielectric pipe 6 does not obstruct the laser gas flow 4, and the laser gas flow 4 is supplied at a high flow rate. Further, by selecting the gap width(separation) between the main electrode 2 and the dielectric pipe 6 at a value greater than the thickness of the dielectric pipe 6, the progression length of the corona discharge 9 can be increased, thereby augmenting the amount of preionized gas. As described above, in the case of the conventional laser oscillation device of Fig. 2, the central position of the laser beam is shifted accompanying the variation of the oscillation frequency (pulse repetition frequency). In the case of this embodiment, however, the central position of the laser beam remains fixed over the oscillation frequency (pulse repetition frequency) range of from 1 to 1000 Hz. Further, the laser gas flow 4 is not obstructed by the preionizer 10 and effectively carries away the remnants which absorb the ultraviolet radiation and deteriorates the efficiency of laser oscillation. Thus, a laser gas supply device having half the supply capacity as that of the laser device of Fig. 3 is sufficient for maintaining the same level of output laser power under identical conditions. The absorption of the ultraviolet radiation by the discharge remnants is negligible for the discharge excitation pulsed laser oscillation device according to this invention. Thus, irrespective of the laser oscillation frequency, the distribution and the amount of the preionized electrons (photoelectrons) are maintained constant. Thus, the homogeneity of the main discharge 3 is maintained throughout the operation and a stable laser oscillation is realized. According to this invention, a high level of the laser output power is maintained even at a high oscillation frequency. This is particularly manifest when the laser gas includes a gas having a high affinity to electrons, such as HCl and F₂, and especially for the KrF, ArF, and F₂ excimer lasers. Further, according to this invention, the central position of the laser beam is fixed. This is confirmed, not only for the excimer lasers, but also for TEA CO₂ laser. Furthermore, in the case of the above embodiment, the preionizer generated the corona discharge for preionizing the laser gas. However, the preionizer mad generate an arc discharge for the same purpose. Even then, the central position of the laser beam is not shifted with the variation of the oscillation frequency. However, a large amount of energy is deposited from the arc discharge to the laser gas at the upstream side, to raise the temperature of the laser gas. Thus, a laser gas supply device of a relatively large capacity becomes necessary for maintaining a high output power level.

Claims: A discharge excitation pulsed laser oscillation device comprising: a pair of main electrodes (1, 2) having main surfaces opposing each other across a main discharge gap (3), wherein one of said main electrodes (1, 2) has a recess formed on said main surface thereof; a laser gas supply means (4) for supplying a laser gas to said main discharge gap region (3) between said pair of main electrodes (1, 2); and a discharge preionizer means (10) completely accomodated within said recess formed on said one of said main electrodes (1, 2); characterized in that said discharge preionizer means (10) within said recess is exclusively situated on the upstream side of said main discharge gap region (3) with respect to the flow direction of said laser gas of a location where a foot of a main discharge across said main electrodes (1, 2) is formed. A discharge excitation pulsed laser oscillation device as claimed in claim 1, wherein said preionizer means (10) is a corona discharge preionizer or an arc discharge preionizer. A discharge excitation pulsed laser oscillation device as claimed in claim 2, wherein said preionizer means (10) comprises: a dielectric pipe (6) completely accomodated within said recess, and an auxiliary electrode (7) inserted in said dielectric pipe (6). A discharge excitation pulsed laser oscillation device as claimed in claim 3, wherein a side wall of said recess has a projection extending sideways therefrom to contact with an outer surface portion of said dielectric pipe (6), said dielectric pipe (6) being separated from said recess except at said outer surface portion in contact with said projection. A discharge excitation pulsed laser oscillation device as claimed in one of the preceding claims, wherein said main surfaces of said main electrodes (1, 2) are completely flat.

Inventors: NAKATANI HAJIME C O MITSUBISHI, NAKATANI, HAJIME, C/O MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H01S3
IPC codes: H01S3