Silent discharge type laser device

A silent discharge type laser device which comprises a detector for detecting the amount of electric charges passed between electrodes and controlling means for controlling an output current or voltage of a power supply so that a charge amount detected by the detector is equal to a target level, thereby keeping the passage charge amount always constant.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a laser device of a silent discharge type 
in which silent discharging is used to excite laser oscillation. 
2. Description of the Prior Art 
Referring to FIG. 4, there is shown a CO.sub.2 laser device in which silent 
discharging is used to promote laser oscillation. More specifically, FIGS. 
4 (a) and (b) are front and side views of the laser device showing its 
general arrangement wherein when an output of an AC power source 1 is 
applied between a pair of electrodes 2, 3 coated with dielectric material, 
silent discharge takes place in a discharging space 4 defined by the 
electrodes and acts to excite laser medium gas 5 trapped in the space 4. 
As a result, laser oscillation occurs in an optical resonator which 
comprises a totally-reflective mirror 6 and a semi-transparent (that is, a 
partially-reflective) mirror 7. Such laser oscillation is partially taken 
out as a laser beam through the semi-transparent mirror 7. 
The above-mentioned laser medium gas is passed through the discharging 
space 4 at a velocity of several tens to several hundreds m/s by blowers 
8, cooled by a heat exchanger 9 and sent again to the discharging space 4. 
Shown in FIG. 5 is an electrical equivalent circuit of the foregoing laser 
device in such a condition that silent discharge is occurring between the 
electrodes 2 and 3. In the drawing, reference numerals 10 and 11 represent 
the capacitances of the dielectric-coat layers of the electrodes 2 and 3 
respectively, 12 the capacitance of a gap between the electrodes 2 and 3, 
13 the equivalent resistance of silent discharge zone of the space 4, 14 a 
stray capacitance present in wiring lines and a chamber 15. FIG. 6 is an 
equivalent circuit when the capacitances 10 and 11 in FIG. 5 are combined 
to form a single capacitance 16. 
These equivalent circuits have been found by the inventors of the present 
invention as a result of their studies, and it has been found that such 
equivalent circuits are very useful in explaining the discharging 
characteristic of the laser device in the laser medium gas when the 
frequency of the power source 1 exceeds 10 KHz. 
More specifically, FIG. 7 shows a laser oscillation characteristic, that 
is, a relationship between a silent discharging power W.sub.d (input 
power) consumed by the equivalent resistance 13 and a laser output 
W.sub.r. It will be seen from FIG. 7 that a constant laser output can be 
obtained by controlling the discharge power W.sub.d to be constant. 
Assuming that the values of the capacitances 12, 14 and 16 are C.sub.g, 
C.sub.m and C.sub.d respectively and the value of the equivalent 
resistance 13 is R in FIG. 6, vector relationships between the current and 
voltage at various points in FIG. 6 are as shown in FIG. 8 wherein 
physical quantities with dots are in the form of complex while physical 
quantities without dots are in the form of absolute value (effective value 
when phase is not taken into consideration). 
From FIG. 8, the discharging power W.sub.d is expressed by: 
EQU W.sub.d =V.sub.d .multidot.I.sub.d. . . (1) 
where, V.sub.d is a terminal-to-terminal voltage across the equivalent 
resistance 13 and I.sub.d is a current flowing through the resistance 13. 
It will be appreciated from the equation (1) that V.sub.d is a constant, 
determined by the discharging conditions and thus when the current I.sub.d 
is made constant, the discharging power W.sub.d, i.e., the laser output 
can be made constant. 
Referring to FIGS. 9 and 10, there are shown other prior-art laser devices 
in which an output voltage V.sub.a of a power source 1 is controlled so 
that an output current I.sub.a of the power source 1 detected by 
respective current detectors 17 and 18 is equal to a set value, as 
proposed, for example, in Japanese Patent Laid-Open Publication No. 
147185/83. However, these laser devices are defective in that it is 
impossible to detect a variation in the current I.sub.d resulting from a 
variation in the stray impedance (based on the capacitance C.sub.m in FIG. 
6) present in lines wired between the power source 1 and the laser 
oscillation section or resulting from a variation in the capacitance 
C.sub.d, thus impeding the sufficient supression of variation of the 
discharging power W.sub.d. 
Shown in FIG. 11 in a vector representation is a relationship between the 
current and voltage at the various points of FIGS. 9 and 10 when the 
above-mentioned capacitance C.sub.d is varied. 
Generally, the silent discharge current I.sub.b shown in FIG. 6 contains, 
as shown in FIG. 12, such pulse current components that are considerably 
high in frequency than the power source 1 and that vary irregularly. In 
such a case, the foregoing prior art devices have had a problem that it is 
difficult to detect reliably the above-mentioned pulse current due to the 
frequency characteristics of the current detectors 17 and 18, resulting in 
a poor controllability. 
Accordingly, the present invention is directed to eliminating such problems 
in the prior art. To this end, in accordance with the present invention, 
there are provided a means for detecting the amount of electric charges 
passed between electrodes and a means for controlling an output voltage or 
current of a power source so that the detected value of the charge-amount 
detecting means is equal to a present value. The present invention can 
control its discharging power to be constant regardless of variations in 
the capacitances of dielectric material layers coated on the discharge 
electrodes or variations in the capacitances present in the wiring lines 
and chamber. Further, since the charge amount is accurately detected while 
not affected by such pulse current, the present invention can provide a 
high control accuracy.

DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 1 shows an embodiment of a silent discharge type laser device in 
accordance with the present invention and FIG. 2 is an equivalent circuit 
of the same laser device. In the drawings, the same elements as those 
shown in FIGS. 5 and 6 are denoted by the same reference numerals or 
symbols. 
In the illustrated embodiment, the output voltage Va or output current Ia 
of a power source 1 is so controlled that the integration of the current 
I.sub.b passed between the electrodes 2 and 3, that is, the charge amount 
Q.sub.b is equal to a target value. 
The above charge amount Q.sub.b can be detected by various means, but in 
the illustrated embodiment, the charge amount Q.sub.b is detected on the 
basis of the terminal-to-terminal voltage V.sub.b across a capacitor 200 
which is inserted in a path through which the current I.sub.b flows. It is 
further desirable that the capacitor 200 is disposed as close to the 
electrodes 3 and 4 as possible. 
The amount of charge accumulated in the capacitor 200 is represented by the 
integration of the current I.sub.b and the terminal-to-terminal voltage 
V.sub.b across the capacitor 200 is proportional to the such charge 
amount. Hence, the charge amount Q.sub.b can be detected on the basis of 
the terminal-to-terminal voltage V.sub.b. 
A power source control means 100 includes a setter 101 for setting a target 
value for the charge amount passed across the electrodes 2 and 3, a 
circuit 102 for receiving the terminal-to-terminal voltage V.sub.b to 
generate the absolute value (actually, which is an average of effective 
values or absolute values) of the voltage V.sub.b, a subtractor 103 for 
finding a difference between the output of the absolute value circuit 102 
and the output of the charge-amount setter 101, and a power control 
circuit 104 for receiving the difference and increasing or decreasing the 
output voltage V.sub.a or output current I.sub.a of the power source 1 so 
that the difference at 103 becomes zero. 
The output of the absolute value circuit 102 corresponds to the above 
charge amount Q.sub.b. Therefore, according to the present embodiment, the 
output voltage V.sub.a or output current I.sub.a is so controlled that the 
charge amount Q.sub.b passed through the discharge space 4 is kept always 
at a target value. 
Shown in FIG. 3 is a vector diagram showing a relationship between the 
current and voltage at various points in the equivalent circuit of FIG. 2. 
A relationship between the charge amount Q.sub.b and the discharging power 
W.sub.d is readily found from the vector diagram of FIG. 3. That is, 
EQU I.sub.b =I.sub.d +j.omega.C.sub.g .multidot.V.sub.d (2) 
EQU V.sub.b =I.sub.b /j.omega.C.sub.b (3) 
EQU W.sub.b =I.sub.d .multidot.V.sub.d =Iphd d.multidot.V.sub.d (4) 
EQU Q.sub.b =I.sub.b /j.omega.=C.sub.b .multidot.V.sub.b (5) 
Accordingly, the discharging power W.sub.d is expressed as follows: 
##EQU1## 
Since the quantities C.sub.g and V.sub.d in the equation (6) are constants 
determined by the discharging conditions, if the quantity Q.sub.b is 
constant then the discharging power W.sub.d becomes constant. 
Shown by dotted lines in FIG. 3 are the currents and voltages at the 
various points in the laser device of the foregoing embodiment when the 
dielectric layers coated on the discharge electrodes 2 and 3 are increased 
in the capacitance C.sub.d. In the illustrated embodiment, with the charge 
amount Q.sub.b controlled to be constant, the current I.sub.d can be made 
constant even when the capacitance C.sub.d is varied. Therefore, the 
discharging power W.sub.d is kept constant regardless of a variation in 
the capacitance C.sub.d . 
In the similar way, even when the capacitance C.sub.m is varied, the 
discharging power W.sub.d is kept constant. In this embodiment, further, 
the voltage V.sub.b shown in the equation (5) has such a waveform close to 
sinusoidal one as shown by a dotted line in FIG. 12, so that the charge 
amount Q.sub.b can be detected while not affected by any pulse current, 
whereby highly accurate control can be performed over the discharging 
power W.sub.d. 
As clear from the foregoing explanation, the power source control means 100 
and the power source 1 cooperate to function as a constant-power supply 
which controls the discharging power W.sub.d to be constant. 
How to detect the charge amount Q.sub.b is not restricted to that disclosed 
in the foregoing embodiment with use of the capacitor 200, but there may 
be employed various methods, for example, of detecting the current I.sub.b 
and converting it to a corresponding charge amount with use of an 
integrator, or of inserting a resistance or a current transformer instead 
of the capacitor 200 and integrating the terminal-to-terminal voltage 
across the resistance or the output voltage of the current transformer to 
detect a corresponding charge amount. 
In addition, although both of the discharge electrodes 3 and 4 have been 
coated with dielectric material in the foregoing embodiment, it goes 
without saying that the present invention can be applied to a silent 
discharge type laser device in which either one of the electrodes is 
coated with the dielectric.