PULSE LASER DEVICE

Abstract: In a pulse laser exciting circuit according to this invention, a diode (11) is provided in parallel with a capacitor (4) for generating pulses so as to conduct in the inverse polarity direction of an applied voltage and it prevents that the voltage of inverse polarity generated across the capacitor (4) is applied to the capacitor (4) by conducting the voltage of inverse polarity. The energy due to the voltage of inverse polarity is consumed through the diode (11). Therefore, since neither arc nor streamer is generated between the main discharge electrodes, there is an effect that a pulse laser whose life times of the electrodes are long and which can oscillate at a high repetition rate is obtained.

Description: The present invention relates to a pulse laser apparatus. FIG.1 is an excitation circuit of a conventional excimer laser shown in, for example, Denki Gakkai (Institute of Electrical Engineers), Japan, April 1986, Technical Report, Part II, "State of the art of short wavelength laser". In this figure, numerals 1 and 2 are a pair of main discharge electrodes which are facing to each other; numeral 3 is a peaking capacitor attached in parallel with the main discharge electrodes 1 and 2; numeral 4 is a pulse generation capacitor, and one electrode thereof is connected to the main discharge electrode 1. Numeral 5 is a switch connected across the other end of the pulse generation capacitor 4 and the main discharge electrode 2; and in this prior art, it is comprised of a thyratron. Numeral (6) is a charging reactor, and numeral 7 is a charging terminal. Next, the operation is elucidated. A positive high voltage is applied to the charging terminal 7, and the pulse generation capacitor 4 is charged up through the charging reactor 6. Graphs which illustrate temporal variations of voltages appearing on both ends of the pulse generation capacitor 4 and of the peaking capacitor 3, after closing the switch 5 at t=t₀, are shown in FIG.2. Electric charge stored in the pulse generation capacitor 4 during a period of t₀ ≦ t ≦ t₁ is transferred to the peaking capacitor 3, and at t=t₁ a discharge starts between the main electrodes 1 and 2. In excimer lasers, although a preionization discharge prior to the main discharge is required, electrodes and its associating circuit for this are omitted in the elucidation in FIG.1 and other figures for the present patent. During a period of t₁ ≦ t ≦ t₂, energy is injected into a main discharge taking place across the main discharge electrodes 1 and 2 from the peaking capacitor 3, thereby the laser oscillates. In a laser such as an excimer laser in which its discharging resistance is small (for example, 0.2 Ω), voltage appearing across both ends of the peaking capacitor 3 becomes an oscillatory waveform, and hence a reverse polarity voltage appears (FIG.2, t=t₂). At a time point when the oscillation almost terminates, a voltage (denoted by Vr in FIG.2(a)) which is reverse in polarity with respect to that at a time of charging up appears across both ends of the pulse generation capacitor 4. Another example of the conventional pulse laser excitation circuit is shown in FIG.3. In this example, electric charge of a pulse generation capacitor 4 is transferred to a peaking capacitor 3 through a saturable reactor 8. This circuit is referred to as an MPC circuit, in which, in order to reduce the loss in a thyratron switch 5, another pulse generation capacitor 9 and a current suppression reactor 10 are provided. The working waveform of this circuit is shown in FIG.4, wherein similar to the conventional example previously described, the current oscillates, after the discharge started across main electrodes 1 and 2 at t=t₁ and finally a voltage Vr which is reverse in the polarity with respect to that at the beginning appears across both ends of the pulse generation capacitor 4. Since the conventional pulse laser excitation circuit is constituted as has been described above, a reverse polarity voltage appears across the both ends of the pulse generation capacitor; and this energy (=1/2cVr²) is dissipated as an arc or a streamer at the main discharge electrodes considerably later (for example, an order of 1 µs later) after the main discharge appeared. This is a so-called after-current, which has the problem that it does not contribute to the laser generation, but damages the main discharge electrodes and shortens the lifetime of the electrodes. There is also the problem that, owing to a flow of after-currents, a high-repetition rate oscillation becomes impossible. The object of the present invention is to overcome the problems such as described above, and to provide a pulse laser apparatus with an excitation circuit which is capable of elongating the lifetime of the electrodes as well as performing a high-repetition rate oscillation. A pulse laser apparatus is provided in accordance with the present invention as defined in claims 1 and 3. A diode is disposed in a manner that it is in parallel with a pulse generation capacitor and conducts current in a direction which is reverse with respect to an applied voltage. This diode makes a conduction of a reverse polarity voltage appearing on the pulse generation capacitor, thereby preventing an impression of a reverse polarity voltage on the pulse generation capacitor. FIG.1 is a configuration drawing of a conventional pulse laser apparatus, FIG.2 is a voltage waveform curve showing the action of FIG.1; FIG.3 is a configuration drawing of another conventional pulse laser apparatus; FIG.4 is a voltage waveform curve showing the action of FIG.3; FIG.5 is a configuration drawing of a pulse laser apparatus showing one embodiment of the present invention; FIG.6 is a voltage waveform curve showing the action of FIG.5; and FIG.7 through FIG.9 are constitutional drawing showing other embodiments of the present invention. In the following, one embodiment of the present invention is elucidated referring to the drawings. In FIG.5, numeral 11 is a diode; numeral 12 is a resistor; and a series-connected matter of the diode 11 and the resistor 12 is connected in parallel with a pulse generation capacitor 4. The polarity of the connection of the diode 11 is such that it is connected in a direction of non-conducting state with respect to the charging polarity from the charging terminal 7. Numeral 13 is a switch composed of a saturable reactor connected between a pulse generation capacitor 4 and a switch 5 consisting of a thyratron. In this case, in parallel with the main discharge electrodes 1 and 2, a series-connected matter of the pulse generation capacitor 4 and the switch is connected thereto. This switch consists of a series-connected matter of the thyratron 5 and the saturable reactor 13. Upon impression of a trigger to the thyratron switch 5 at t=t₀, after a time lapse determined by a product of voltage × time of the saturable reactor 13 (FIG.6, t=t₀′), electric charge begins to be transferred from the pulse generation capacitor 4 to the peaking capacitor 3. Voltage waveform is shown in FIG.2. At t=t₁, a main discharge starts between the main discharge electrodes 1 and 2, and at t=t₂, voltage of the peaking capacitor 3 reverses its polarity. Although a reverse voltage is going to occur on the pulse generation capacitor 4 after t=t₂, any reverse voltage does not appear on the pulse generation capacitor 4 since the diode 11 and the resistor 12 are connected in parallel to the pulse generation capacitor 4, and an excess energy produced as a result of oscillation after the discharge is dissipated by the resistor 12. As a result, an after-current flowing between the main discharge electrodes (1) and (2) disappears, and thus arc or streamer does not occur. In this embodiment, by connecting the saturable reactor 13, a merit that the response speed required for the diode 11 can be relaxed to a slow one. This is owing to the following reason. Since the saturable reactor 13 of FIG.5 is being saturated in a direction of a solid line arrow shown in the figure, once a reverse electric charge is stored in the pulse generation capacitor 4, this saturable reactor 13 falls in the blocked state and hence stops its successive oscillatory electric charge movement. Since it is possible to let the diode 11 flow the reverse electric charge slowly through the resistor 12 for a relatively long time interval (for example, 200 ns) during which the saturable reactor keeps this blockade, the response speed of the diode 11 is allowed to be slow, and the peak current can be made small. Therefore, even a low-cost diode can be used. Different from FIG.5, an embodiment of a case that a high-speed and large-current diode is used is shown in FIG.7. In this case, even without using a saturable reactor, the same effect as in the embodiment described above can be obtained. An embodiment of the present invention which is corresponding to the conventional example shown in FIG.3 is shown in FIG.8. A series-connected matter of a switch consisting of a saturable reactor 8 and pulse generation capacitor 4 are connected in parallel between the main electrodes 1 and 2. A series-connected matter of a diode 11 and a resistor 12 is connected in parallel with this pulse generation capacitor 4. Electric charge which is charged up in another pulse generation capacitor 9 is capacitance-transferred to the pulse generation capacitor 4 by a thyratron switch 5. For the polarity of this voltage which is initially charged up in the pulse generation capacitor 4, the diode 11 is connected in the polarity that it goes to the non-conducting state. In this case also, the same effect as in the embodiment described above is exhibited. In the embodiment described above, although a case of using a peaking capacitor 3 has been shown, even an embodiment without using the peaking capacitor 3 (for example, an embodiment shown in FIG.9) can exhibit the same effects as in the embodiment described above. And, although the case that the series-connected matter of the diode 11 and the resistor 12 were connected in parallel with the pulse generation capacitor (4) has been shown in the above-mentioned embodiment, in a case where the internal resistance of the diode 11 is large, the same effect as in the above-mentioned embodiment can be exhibited even when the resistor 12 is omitted. And, in the above-mentioned embodiment, although a thyratron was shown as the switch 5, it is also possible to use a series-connected matter of semiconductor switch (thyristor, SIT transistor, FET, IGBT or the likes), or switches such as a spark gap or a rail switch can also be used. And, although it has been explained that the charging was done in the positive polarity from the charging terminal 7, the negative polarity can also be used. In either way, the same effect as in the above-mentioned embodiment can be exhibited. Hereupon, although, as for a laser, explanation was given on an excimer laser in the above-mentioned embodiment, the embodiment can be applicable for any pulse laser as far as its discharge resistance is small and the discharge current is oscillatory, the same effect as in the above-mentioned embodiment can be exhibited. Besides, in place of the capacitor in the above-mentioned embodiment, use of a pulse forming circuit, which is a distributed constant circuit, can exhibit the same effect. Moreover, in the above-mentioned embodiment, even with other charging circuit elements such as a low-resistance resistor for the charging reactor (6), the same effect can be expected. As has been described above, according to the present invention, by dissipating a reverse voltage energy occurring on the pulse generation capacitor through the diode connected in parallel, arc or streamer can be avoided. Hence a pulse laser is obtainable in which the lifetime of the main discharge electrodes becomes long and a high-repetition rate oscillation becomes possible.

Claims: A pulse laser apparatus comprising: a pair of main discharge electrodes (1, 2) opposed to one another; a peaking capacitor (3) connected in parallel with said pair of main discharge electrodes (1, 2); a pulse generation capacitor (4) connected in series with said pair of main discharge electrodes (1, 2); and a charging terminal (7) connected in series with said pulse generation capacitor (4) for charging an electric charge on said pulse generation capacitor (4) with a predetermined charging polarity, wherein a main discharge is produced between said pair of main discharge electrodes (1, 2) by transferring the electric charge on said pulse generation capacitor (4) onto said peaking capacitor (3); characterized in that: a diode (11) is provided in series with a resistor (12), both of which are connected in parallel across said pulse generation capacitor (4) in a direction of non-conduction with respect to said predetermined charging polarity, thereby to prevent a reverse polarity voltage from appearing on said pulse generation capacitor (4). A pulse laser apparatus in accordance with claim 1, further comprising a saturable reactor (8) connected in series with said pulse generation capacitor (4). A pulse laser apparatus comprising: a pair of main discharge electrodes (1, 2) opposed to one another; a first pulse generation capacitor (9) connected in series with said pair of main discharge electrodes (1, 2); a second pulse generation capacitor (4) connected in parallel with said pair of main discharge electrodes (1, 2); and a charging terminal (7) connected in series with said first pulse generation capacitor (9) for charging an electric charge on said first pulse generation capacitor (9), wherein a main discharge is produced between said pair of main discharge electrodes (1, 2) by transferring the electric charge on said first pulse generation capacitor (9) onto said second pulse generation capacitor (4), characterized in that: a diode (11) is provided in series with a resistor (12), both of which are connected in parallel across said second pulse generation capacitor (4) in a direction of non-conduction with respect to a direction of voltage that said second pulse generation capacitor (4) is to be initially charged, thereby to prevent a reverse polarity voltage from appearing on said pulse generation capacitor (4). A pulse laser apparatus in accordance with claim 3, further comprising a saturable reactor (8) connected between said first and second pulse generation capacitor (9, 4) and said pair of main discharge electrodes (1, 2).

Inventors: MINAMITANI YASUSHI MITSUBISHI, NAKATANI HAJIME MITSUBISHI DEN, SUGITATSU ATSUSHI MITSUBISHI D, MINAMITANI, YASUSHI MITSUBISHI DENKI K. K., NAKATANI, HAJIME MITSUBISHI DENKI K. K., SUGITATSU, ATSUSHI MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1991
Main classification: H01S3
IPC codes: H01S3