Discharge circuit for pulsed laser

Details Discharge circuit for pulsed laser Discharge circuit for pulsed laser

2000-09-08

2004-02-17

Eckert, George (Department: 2815)

Coherent light generators

Particular active media

Gas

C372S038020, C372S038070, C372S038050, C372S038100

Reexamination Certificate

active

06693938

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a discharge circuit 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. 10
shows an equivalent circuit of a conventional capacity shift type magnetic pulse compression discharge device, namely of a discharge circuit for pulsed laser, which employs the corona discharge for the preionization.
FIG. 11
shows an example of voltage and electric current waveforms at respective points of the discharge circuit for pulsed laser shown in FIG.
10
.
In the discharge circuit for pulsed laser shown in
FIG. 10
, 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 between a pair of main discharge electrodes
1
,
2
, and a laser medium in the main discharge gap between the main discharge electrodes
1
,
2
is preionized by UV (ultraviolet) light produced by a corona discharge at the preionization electrode
4
.
The discharge circuit for pulsed laser shown in
FIG. 10
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 circuit for pulsed laser shown in
FIG. 10
, 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 the moment t
0
shown in FIG.
11
). When the main switch SW is turned on, electric potential VSW of the main switch SW drops sharply to zero. When time integral (namely, a 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 the moment t
1
, 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 &dgr;
0
in which the electric current pulse i
0
starts to flow and becomes
0
(the moment t
2
shown in FIG.
11
), namely electric charge transfer time &dgr;
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 integral 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 the moment 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 the moment t
4
after a lapse of predetermined transfer time &dgr;
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 integral 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 the moment 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 the moment t
6
, and a main electrical discharge is started across the main discharge electrodes
1
,
2
to flow electric current i
4
. The laser medium is excited by the main electrical 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 an electrical 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 electrical discharge is caused across the main discharge electrodes
1
,
2
in a short period of time.
An electrical discharge circuit other than the discharge circuit for pulsed laser shown in
FIG. 10
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 electrons produced by the preionization and its spatial distribution have an influence upon the growth and stability of a glow discharge in the high-pressure laser gas. And, they are included in the factors which exert an influence upon the laser output power, the spatial intensity distribution of the laser beam, the pulse width and the like.
With the increase of the electron density Ne
0
, the generation and maintenance of the stable glow discharge are facilitated, and the laser output power obtained is also increased.
But, in the above conventional discharge circuit for pulsed laser, the main discharge (namely, the glow discharge) by the main discharge electrodes
1
,
2
is started in a state that the preionization in the main discharge gap by the corona discharge is not performed sufficiently, namely in a state before reaching the aforesaid electron density Ne
0
with that the stable glow discharge can be caused and maintained. Therefore, the stability and oscillation efficiency of

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