Coherent light generators – Particular pumping means – Electrical
Reexamination Certificate
2000-09-13
2003-03-18
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular pumping means
Electrical
C372S038100, C372S038020, C372S038050
Reexamination Certificate
active
06535540
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a discharge device for pulsed laser which performs pulsed laser oscillation by preionizing across main discharge electrodes disposed in a laser medium and performing a main discharge to excite the laser medium.
2. Description of the Prior Art
TEA laser causes a uniform glow discharge across a pair of opposing main discharge electrodes to form an inverted population region necessary for laser oscillation.
To obtain the glow discharge spread to fill the entire main discharge gap, the TEA laser has to cause preionization before starting the main discharge to ionize the entire main discharge gap previously. Especially, an excimer laser has to ionize as many as possible immediately before the main discharge because the electrons in a negative gas have a short lifetime.
Currently, various types of methods using X rays, a spark discharge, a corona discharge or the like are used as a preionizing method. Among them, a method using the corona discharge is extensively used because it is relatively simple and easy and has less contamination of a laser gas.
FIG. 8
shows an equivalent circuit of a conventional capacity shift type magnetic pulse compression discharge device, namely of a discharge device for pulsed laser, which employs the corona discharge for the preionization.
FIG. 9
shows an example of voltage and electric current waveforms at respective points of the discharge device for pulsed laser shown in FIG.
8
.
In the discharge device for pulsed laser shown in
FIG. 8
, corona preionization capacitor (hereinafter called the preionization capacitor) Cpp and corona preionization electrode (hereinafter called the preionization electrode)
4
are disposed to preionize in main discharge gap
3
which is formed across a pair of main discharge electrodes
1
,
2
, and a laser medium in the main discharge gap
3
across the main discharge electrodes
1
,
2
is preionized by UV (ultraviolet) light produced by a corona discharge at the preionization electrode
4
.
The discharge device for pulsed laser shown in
FIG. 8
has two-stage magnetic pulse compression circuit utilizing a saturation phenomenon of three magnetic switches AL
0
to AL
2
each made of a saturable reactor.
In the discharge device for pulsed laser shown in
FIG. 8
, an electric charge is applied from high-voltage power source HV to capacitor C
0
through the magnetic switch AL
0
and coil L
1
.
Then, when pulse oscillation synchronizing signal (trigger signal) TR, which is turned on in synchronization with a repetition frequency of a pulsed laser oscillation, is input, main switch SW is turned on (at time t
0
shown in FIG.
9
). When the main switch SW is turned on, electric potential VSW of the main switch SW drops sharply to zero. When time integral (namely, time integral value of voltage VC
0
) S
0
of voltage difference “VC
0
−VSW” between the capacitor C
0
and the main switch SW as voltages at both ends of the magnetic switch AL
0
reaches a threshold value which is based on a set characteristic of the magnetic switch AL
0
, the magnetic switch AL
0
is saturated at time tl, and electric current pulse i
0
flows through a loop of the capacitor C
0
, the magnetic switch AL
0
, the main switch SW and capacitor C
1
.
Duration &tgr;
0
in which the electric current pulse i
0
starts to flow and becomes
0
(time t
2
shown in FIG.
9
), namely electric charge transfer time &tgr;
0
in which the electric charge is completely transferred from the capacitor C
0
to the capacitor C
1
, is based on respective capacitance of the inductance, the capacitor C
0
and the capacitor C
1
after the magnetic switch AL
0
is saturated with loses due to the main switch SW and the like disregarded.
Meanwhile, when time product S
1
of the voltage VC
1
of the capacitor C
1
reaches the threshold value which depends on the set characteristic of the magnetic switch AL
1
, the magnetic switch AL
1
is saturated and has a low inductance at time t
3
. Thus, electric current pulse i
1
flows through a loop of the capacitor C
1
, capacitor C
2
and the magnetic switch AL
1
. The electric current pulse i
1
becomes
0
at time t
4
after a lapse of predetermined transfer time &tgr;
1
which is determined by an inductance after the saturation of the magnetic switch AL
1
and the capacitance of the capacitors C
1
, C
2
.
When time product S
2
of voltage VC
2
of the capacitor C
2
reaches a threshold value which is based on a set characteristic of the magnetic switch AL
2
, the magnetic switch AL
2
is saturated at time t
5
. Thus, electric current pulse i
2
flows through a loop of the capacitor C
2
, peaking capacitor CP and the magnetic switch AL
2
. The flow of the electric current pulse i
2
rises voltage Vcp of the peaking capacitor Cp and voltage VCpp of the preionization capacitor Cpp.
Then, when the voltage of the preionization electrode
4
rises to a predetermined preionization start voltage through the preionization capacitor Cpp, a corona discharge is caused at the preionization electrode
4
to flow electric current i
3
, and the main discharge gap
3
is preionized.
Besides, the voltage VCp of the peaking capacitor Cp rises further with the progress of charging. And, when the voltage VCp reaches a predetermined main discharge start voltage, a laser gas between the main discharge electrodes
1
,
2
is undergone an electrical breakdown at time t
6
, and a main discharge is started across the main discharge electrodes
1
,
2
to flow electric current i
4
. The laser medium is excited by the main discharge caused across the main discharge electrodes
1
,
2
, and laser light is emitted in several nsec.
Then, the voltages of the peaking capacitor Cp and the preionization capacitor Cpp drop sharply owing to the main discharge and return to the states before the charging was started after a lapse of a predetermined period.
Such a discharge operation is repeated by the switching operation of the main switch SW which is synchronized with the trigger signal TR to perform pulsed laser oscillation at a predetermined repetition frequency (namely, a pulse oscillation frequency).
In such a case, because it is determined that the electric charge transfer circuit of each stage which is comprised of the magnetic switch and the capacitor has smaller inductance toward later stages, the pulse compression operation is performed so that the peak values of the electric current pulses i
0
to i
2
become high sequentially and the electrifying duration becomes shorter sequentially. As a result, an intense discharge is caused across the main discharge electrodes
1
,
2
in a short period of time.
In the magnetic pulse compression circuit, duration td (hereinafter called the emission delay time) from the time t
0
when the trigger signal TR is input and the main switch SW is turned on to the time t
6
when the laser light is actually emitted depends on electrifying widths &tgr;
0
, &tgr;
1
, &tgr;
2
of the electric current pulses i
0
,
01
, i
2
and saturation time &sgr;
0
, (&tgr;
0
+&sgr;
1
), (&tgr;
1
+&sgr;
2
) of the respective magnetic switches AL
0
to AL
2
.
The electrifying widths (electric charge transfer time) &tgr;
0
, &tgr;
1
, &tgr;
2
are based on the inductance of the magnetic switch and the capacitance of the capacitor included in the electric charge transfer circuits of the respective stage, and such values are greatly influenced by the atmosphere temperature of the magnetic pulse compression circuit.
Besides, variations &sgr;
0
, &sgr;
1
, &sgr;
2
in the saturation time are based on a time integral of the voltage applied to the respective magnetic switches AL
0
to AL
2
, so that they are greatly influenced by the voltage V
0
of the high-voltage power source HV.
A discharge device other than the discharge device for pulsed laser shown in
FIG. 8
is disclosed in Japanese Patent Application No. 9-271207 (Laid-Open Publication No. 11-112300) filed in Japan in the name of the applicant of this patent application.
Density Ne
0
of
Kawasuji Yasufumi
Matsunaga Takashi
Jr. Leon Scott
Komatsu Ltd.
Welsh & Katz Ltd.
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