Gas laser oscillator

In a high velocity gas laser oscillator of radio frequency or microwave exciting type, both end parts of the discharge tube are provided with gas flow rectifiers for making an inward flowing whirlwind at the discharge region of the discharge tube by blowing the discharge gas through plural slits located in a plane disposed at a predetermined angle with respect to a plane including axis of the discharge tube, wherein microwave power is applied to the discharge tube so as to define an annular discharge region in the discharge tube.

FIELD OF THE INVENTION AND RELATED ART STATEMENT 
1. Field of the Invention 
The present invention relates to a gas laser oscillator. More particularly, 
the present invention relates to a gas laser oscillator wherein its 
optical axis coincides with the axis of a discharge tube and which is 
particularly suitable for a high power and good beam mode. 
2. Description of the Related Art 
In recent years, high power gas laser oscillators with large output have 
become widely used, and are being used for working materials of high 
hardness and high melting temperature such as metal and ceramic. As one 
example of the conventional gas laser oscillator system, a high velocity 
axial flow CO.sub.2 gas laser oscillator is shown in FIG. 7. The laser 
oscillator of FIG. 7 comprises a discharge tube 1, such as glass or the 
like dielectric material, a reflector 6 disposed at one end of the 
discharge tube 1, a partial reflector 7 disposed at the other end of the 
discharge tube 1, two pairs of electrodes 2, 3 and 2', 3', located outside 
and at respective end parts of the discharge tube 1, a radio frequency or 
microwave oscillator 4 connected to feed radio frequency power or 
microwave power across the electrodes 2, 3 and 2', 3', a gas circulation 
system 10, which comprises a gas circulation blower 13 and a first heat 
exchanger 11 disposed in a part of the circulation pipe which is between 
the central part 101 of the discharge tube 1, the blower 13 and a second 
heat exchanger 12 disposed in the gas circulation pipe immediately down 
stream of the blower 13. 
In the above-mentioned configuration, the operation is as follows. 
Radio frequency power or microwave power is fed from the oscillator 4 
across the electrodes 2, 3 and 2', 3' to apply an electromagnetic field of 
radio frequency or microwave in a direction perpendicular to the optical 
axis of the laser oscillator, so that glow discharges are generated in 
discharge areas between each respective pair of electrodes 2, 3 and 2', 
3'. The laser gas passing through the respective discharge area 5, 5' are 
excited by obtaining energy from the discharge, and the laser gas becomes 
to a resonation state in an optical resonator formed by the reflector 6 
and the partial reflector 7. Accordingly, the laser oscillation is 
generated in the optical resonator formed in the discharge tube 1, and the 
generated laser beam 8 is output through the partial reflector 7. 
As a method for taking out a high power in the conventional laser 
oscillator as shown in FIG. 7, there has been a way of adopting a large 
inner diameter of the discharge tube 1, to obtain a laser beam 8 of a 
large diameter. For the conventional radio frequency power source, its 
frequency is mainly 27.12 MHz or below in accordance with the regulation. 
State of discharge (namely the scope of discharge) in the discharge 
regions 5 and 5', which is obtained by applying the radio frequency 
electric field across the electrodes 2, 3 and 2', 3' in a direction 
perpendicular to the optical axis, varies depending on the conditions of 
the inner diameter of the discharge tube 1 or thickness of the discharge 
tube 1 or length and curvature diameter of the electrodes. When the inner 
diameter of the discharge tube 1 is increased, the discharges are 
concentrated in a narrow region 5a defined as narrow regions which are 
between the two opposing electrodes 2, 3 and 2', 3', respectively, in the 
discharge regions 5, 5'see FIG. 8. Furthermore, there is as another 
conventional electrode configuration, which has a pair of ring-shaped 
electrodes (not shown), whereto radio frequency or microwave power is to 
be applied at respective ends of the discharge tube or a microwave power 
of 500 MHz or above is applied through a wave guide in a manner that its 
microwave electric field is perpendicular to the optical axis of the 
discharge tube to obtain discharging. However the above-mentioned prior 
arts devices do not give satisfactory results. For instance, FIG. 9 shows 
discharge state under such microwave application at the discharge region. 
As shown in FIG. 9, as a result of skin effect of the microwave, the 
discharge is made only in the ring shaped region 5b which is only adjacent 
to the inner wall of the discharge tube 1. 
FIG. 10 shows relations of discharge states, small signal gain 
distributions and beam shape for three states of dischargings, namely (A) 
discharge of FIG. 8, (B) discharge of FIG. 9 and (C) discharge in the 
whole area in case of DC. As shown in FIG. 10, the small signal gain 
distribution and beam shape i.e. burn pattern are influenced by the state 
of the discharge. 
The output laser beam 8 (FIG. 7) is converged and used for laser working, 
etc., and an even discharge shown by (C) in FIG. 10, which has the small 
signal gain distribution is almost in Gauss mode, is most desirable. 
However, in actual working, the mode shape is liable to become in non-even 
shape as shown in the discharge state of (A) or (B). When cutting or the 
like working is made by using such uneven mode beam, there arises 
undesirable problems of directivity in the cutting work in case of the 
discharge state (A) or poor convergence of laser beam in the case of 
discharge state (B). 
In the known DC discharge of the laser, the discharge generally expands in 
the whole discharge area easily and there is no need of making a 
particular endeavor. However the DC discharge gas laser has used less 
recently, because its electrodes consumption requires periodical 
maintenances. Therefore recently, the radio frequency or microwave laser 
oscillator have become widely used. In such radio frequency or microwave 
laser oscillation, however it is generally difficult to expand the 
discharge area into the whole space of the section of the discharge tube. 
Accordingly, there are such trials proposed as shown in FIG. 11 wherein 
plural electrode pairs are combined so as to obtain more even discharge 
area in the discharge tube 1, and as shown in FIG. 12 wherein a small 
diameter discharge tube having a small diameter 2t which is only as large 
as twice of thickness t of the discharge area in discharge tube so as to 
eliminate the non-discharge area. However, such conventional 
configurations still have the respective problems of complication of 
configuration and operation and shorter life of the discharge tube due to 
concentration of discharge in small diameter area. 
Another prior art DE P8706780.8, which is a priority base for U.S. Pat. No. 
4,780,881, shows a configuration of blowing a discharging gas from a 
single large diameter gas blowing pipe which is disposed in tangential 
relation to the circular section of the discharge tube. However, this 
prior art does not suggest elimination of the non-discharge area in the 
circular cross-section of the discharge tube. 
Furthermore, as shown in the Japanese published patent application Hei 
3-123089 (Tokkai Sho 3-123089), by generating a turbulence of discharge 
gas to be put into the discharge tube, an increase of radio frequency 
power was achieved. However, this turbulence did not give good mode of 
laser beam probably because of inner wall ridges. 
Furthermore, in the DC discharge type gas laser oscillator, there has been 
a proposal to flow the discharge gas in a helical pattern to prevent 
undesirable generation of discharge arc region, as disclosed in Horiuchi 
U.S. Pat. No. 4,672,621. However, the above-mentioned Horluchi Patent did 
not propose improvement of the laser beam mode in the radio frequency or 
microwave discharge. 
In summary, in the radio frequency or microwave high power gas laser 
oscillator of the prior art, the non-discharge area on the cross-sectional 
plane of the discharge tube 1, particularly at the central part of the 
cross-section, can not be eliminated. 
The above-mentioned prior art requires particular designs of electrode 
shape (diameter width and length and gap to the discharge tube) or 
additional adjustment of electrode shape and radio frequency or microwave 
oscillator to minimize unevenness of the discharge due to deviation of the 
electrode disposition and electrode size from the design. Such adjustment 
requires a long time and skilled labor. Furthermore, decreasing the size 
of the discharge tube results in poor output power. 
OBJECT AND SUMMARY OF THE INVENTION 
The present invention purposes to solve the above-mentioned problems, and 
aims to provide a high power gas laser oscillator with good output beam 
mode suitable for working by means of an even discharging for any desired 
diameter of discharge tube, only by using simple-shaped electrode. 
According to the configuration of the present invention, by providing the 
gas in the discharge tube with a velocity vector having a component in a 
radial direction on a transverse cross-section plane perpendicular to axis 
of the discharge tube, satisfactory output beam mode of laser is 
obtainable without need of spreading the discharge in cross section. The 
improvement of the beam mode of the laser in accordance with the present 
invention is prominent as the lasing oscillation frequency becomes higher. 
The gas laser oscillator in accordance with the present invention 
comprises: 
a discharge tube, 
a gas circulation system connected to the discharge tube for circulating a 
gas in the discharge tube, 
means for applying microwave power to the discharge tube to excite the gas, 
a reflector provided on one end of the discharge tube, 
a partial reflector provided on the end of the discharge tube, and means 
for providing the gas with a velocity vector having a component in radial 
direction on a transverse cross-section plane which is perpendicular to 
axis of the discharge tube, wherein an annular discharge region is defined 
in the discharge tube. 
According to the above-mentioned configuration, by means of the member for 
providing the gas with a velocity vector having a component in radial 
direction on a transverse cross-section plane, the laser gas in the 
discharge region is evenly blended in a manner to eliminate the 
non-discharge region in a central part on the cross-section, and thereby 
the small signal gain distribution in the discharge region is made even. 
Therefore, the discharging in the discharge region in the discharge tube 
is prevented from undesirable concentration of discharge, and undesirable 
uneven distribution of the laser energy as in the conventional gas laser 
oscillator is eliminated. Therefore, the limit of the injection power is 
drastically increased and hence laser beam of a high power can be stably 
emitted. 
While the novel features of the invention are set forth particularly in the 
appended claims, the invention, both as to organization and content, will 
be better understood and appreciated, along with other objects and 
features thereof, from the following detailed description taken in 
conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereafter preferred embodiments of the present invention will be described 
with reference to FIG. 1 through FIG. 6. 
&lt;&lt;FIRST EMBODIMENT&gt;&gt; 
FIG. 1 shows a first preferred embodiment of the present invention wherein 
the corresponding parts and components are designated with the same 
reference numerals as the prior art example of FIG. 7. 
As a first preferred embodiment of the gas laser oscillator, a high 
velocity axial flow CO.sub.2 gas laser oscillator is shown in FIG. 1. The 
laser oscillator of FIG. 1 comprises a discharge tube 1 such as of glass, 
ceramic, quarts or similar heat resistive dielectric material, a reflector 
6 disposed at one end of the discharge tube 1, a partial reflector 7 
disposed at the other end of the discharge tube 1, two pairs of electrodes 
2, 3 and 2', 3', outside and at respective end parts of the discharge tube 
1, a radio frequency or microwave oscillator 4 connected to feed radio 
frequency power or microwave power across the electrodes 2,3 and 2', 3', 
and a gas circulation system 10 for providing a high velocity gas flow in 
the discharge tube 1. Further, the oscillator of the present invention has 
a pair of gas flow-rectifiers 14, which features this radio frequency- or 
microwave-excited gas laser oscillator, is provided in respective end 
chambers 105 connected between respective ends of the discharge tube 1 and 
gas feeding ports of the gas circulation system 10. 
Detailed configuration will be elucidated below. The gas circulation system 
comprises gas circulation pipes, a blower 13, a first heat exchanger 11 
disposed in a part of the circulation pipe which is between the central 
part 101 of the discharge tube 1 and the blower 13 and a second heat 
exchanger 12 disposed in the circulation pipe immediately down stream of 
the blower 13. 
Each of the above-mentioned gas flow rectifier 14 is disposed in the end 
chamber 105 in a manner as shown in FIG. 3(c). That is, the end chamber 
105 is a cup-shaped enclosure made of the same material as that of the 
discharge tube 1 in a manner continuous with the end of the discharge tube 
1 but with a larger diameter than the discharge tube 1. The gas flow 
rectifier 14 is a pipe of metal or glass as shown in FIG. 3(a), with 
several slits 142, whose cutting plane is directed to have a predetermined 
angle on cross-section against a plane including the axis of the discharge 
tube 1 and the edge of the slit 142, as shown in FIG. 3(b). The distal end 
of the gas flow rectifier 14 is hermetically connected to the inner wall 
(i.e., mirror face) of the mirror 6, and the inner end (which is opposite 
to the distal end), whereat the slits 142 ends, is hermetically connected 
by welding to the end of the discharge tube 1, or at the connection part 
of a flange 106 of the end chamber 105 to the end of the tube 1, via a 
known glass-metal welding layer 141. The other end chamber 105 connected 
to the partial mirror 7 is configured substantially in the same way as the 
above-mentioned, except that the partial mirror 7 is connecting to its 
distal end instead of the mirror 6. 
In the above-mentioned configuration, the typical operation is as follows. 
Radio frequency power of e.g. 13.56 MHz and 2 kV is applied from the 
oscillator 4 across the electrodes 2, 3 and 2', 3', thereby to generate an 
electromagnetic field of radio frequency wave in the direction 
perpendicular to the optical axis of the laser oscillator. Then glow 
discharges are generated in discharge areas 5, 5' between respective pair 
of electrodes 2,3 and 2', 3'. The laser gas passing through the respective 
discharge areas 5, 5' are excited by obtaining energy from the discharge, 
and the laser gas comes to a resonation state in an optical resonator 
formed by the reflector 6 and the partial reflector 7. Laser oscillation 
is generated in the optical resonator formed in the discharge tube 1, and 
the generated laser beam 8 is output through the partial reflector 7. 
The gas is blown by the blower 13 in a direction, as shown by arrows 9, 
namely from the blower 13, through a second heat exchanger which cools the 
temperature of the gas raised as a result of compression by the blower 13, 
pipe 10, the space in the end chamber 105, the slits 142 of the gas flow 
rectifier 14 inwards, inside space of the gas flow rectifier 14, discharge 
region 5 in the discharge tube 1, the central part 101, the first heat 
exchanger 11 which cools the temperature of the gas raised by the laser 
oscillation and back to the blower. 
As shown in FIG. 3(b), the gas flow rectifier 14 has plural axial direction 
slits 142, each of which has a cutting plane that is slanted in a manner 
so as to have a predetermined angle against a plane which is defined by 
the axis of the discharge tube 1 and the edge of the slit 142. As a 
result, the gas which flows through plural slits 142 inwards towards the 
cylindrical space inside the gas flow rectifier 142 produce an inwards 
motion as illustrated in FIG. 3(d). 
To discuss more to the detail, the gas flows inwards from the end chamber 
105 through the plural slits 142 of the gas flow rectifier 14 into the 
circular inside space therein, and makes an inwards blowing whirlwind as 
shown in FIG. 3(c), owing to the direction of the cutting plane of the 
slits 142 which is slanted with respect to the plane including the axis of 
the discharge tube 1 and the gas flow acquires a velocity vector having a 
component in a radial direction on the transverse cross-section plane 
which is perpendicular to the axis of the discharge tube. 
According to many experiments carried out by the inventors, the existence 
of the velocity vector having a component in a radial direction on the 
transverse cross-section plane makes satisfactory improvements in 
eliminating the non-discharge area, and hence in achieving the good laser 
oscillation mode. In other words, the inventors' experiments showed that a 
simple helical gas flow made by injecting gas flow from an inlet port 
disposed in a direction which is substantially-tangential to the circular 
inner wall of the discharge tube, which gives only a simple helical gas 
flow but does not give the radial direction component of velocity vector, 
did not give any satisfactory improvement of the laser oscillation mode. 
FIG. 4 is a schematic diagram showing profile of intensity of the gas 
velocity when using the gas flow rectifier, with regard to the diametric 
position along a diameter on a transverse cross-section plane in the 
embodiment shown by FIG. 1 through FIG. 3(d). As shown in FIG. 4, as a 
result of adopting the gas flow rectifier 14 of the above-mentioned 
configuration, a special pattern profile curve of the radial direction 
component of the velocity vector Vr was obtained apart from the case of 
exclusion of the gas flow rectifier 14. 
FIG. 5(a) and FIG. 5(b) show the characteristic relation between the radial 
direction component of velocity vector (m/sec.) and the small signal gain 
(in %m). FIG. 5(a) is a graph showing the relation between radical 
component of gas flow velocity vector and small signal gain value. FIG. 
5(b) is a sectional view showing point of measurement of the gas flow 
velocity. As shown in FIG. 5(b), the discharge area does not spread in the 
whole cross section of the discharge tube 1. In case the gas velocity has 
a component in radial direction in the cross section of FIG. 5(b), the 
small signal gain value distributes in the whole cross section. 
The gas flow velocity can be divided into a component of the axial 
direction and a component in the vertical direction with respect to the 
axis. Further, the component of the vertical direction (which is the 
component on a vertical cross-sectional plane which is perpendicular to 
the optical axis) at a point on the vertical cross-section can be divided 
into component of radial direction and component in tangential direction 
on a circle which has a center on the optical axis. And the inventors' 
experiments and FIG. 5 proved that the component of radial direction is 
influential to improvement in the small signal gain. 
Taking the example of CO.sub.2 gas laser oscillator, the relaxation period 
of natural discharge of CO.sub.2 molecules as the laser medium is about 2 
m sec.; and it is considered that in the laser resonator the CO.sub.2 
molecules instantly transit to the lower level owing to a discharge. 
However, the inventors found that, by diffusing the laser gas within the 
above-mentioned relaxation period to the places whereto the 
above-mentioned discharge is not yet expanded, it becomes possible to 
obtain an even distribution of discharge even in the places where 
discharges are not made. That is, by providing such a velocity vector 
component in radial direction as to enable a motion of the discharge gas 
for a distance in radial direction of about half of the inner diameter of 
the discharge tube 1 within the time period of from 1 m sec. to 2 m sec., 
which is the natural relaxation period of CO.sub.2, the small signal gain 
becomes very much even, regardless of distribution of discharge pattern in 
the discharge tube. And hence the mode of the output laser beam becomes 
even. 
Furthermore, by selecting the velocity vector Vr in the radial direction no 
larger than the below-mentioned value, the beam mode can be improved in 
the radio frequency or microwave lasing: 
EQU Vr.ltoreq.D/Trn, (1), 
where 
D: the inner diameter of the discharge tube, 
Trn: the natural relaxation period of excitation level of the laser medium. 
FIG. 2 shows a second preferred embodiment of the gas laser oscillator 
system of a high velocity axial flow CO.sub.2 gas laser oscillator. The 
gas laser oscillator of FIG. 2 is fundamentally similar to the gas laser 
oscillator of FIG. 1 except the way of excitement of the gas. 
&lt;&lt;SECOND EMBODIMENT&gt;&gt; 
The second embodiment of the gas laser oscillator is a high velocity axial 
flow CO.sub.2 gas laser excited by a microwave power. The laser oscillator 
of FIG. 2 is microwave excited gas laser oscillator; and it comprises a 
discharge tube 1 such as of glass, ceramic, quarts or the like heat 
resistive dielectric material, a reflector 6 disposed at one end of the 
discharge tube 1, a partial reflector 7 disposed at the other end of the 
discharge tube 1, a waveguide 15 and 15' coupled to, and in a manner to 
wrap, the respective end parts of the discharge tube 1, a microwave 
oscillator 4', connected to feed a microwave power to the discharge region 
5, 5', a gas circulation system 10 for providing a high velocity gas flow 
in the discharge tube 1, and a pair of gas flow-rectifiers 14' provided in 
respective end chambers 105 connected between respective ends of the 
discharge tube 1 and gas feeding ports of the gas circulation system 10. 
The gas circulation system comprises gas circulation pipes, a blower 13, a 
first heat exchanger 11 disposed in a part of the circulation pipe which 
is between the central part 101 of the discharge tube 1 and the blower 13 
and a second heat exchanger 12 disposed in the circulation pipe at the 
part immediately down stream side of the blower 13. 
Each gas flow rectifier 14 is disposed at the end chamber 105 in the same 
manner as shown in FIG. 1A. 
In the above-mentioned configuration of FIG. 2, the operation is as 
follows. 
Microwave power of e.g. 2.45 GHz or higher frequency from the power supply 
4' is applied to the discharge regions 5, 5', and then, glow discharges 
are generated in discharge areas 5, 5' in the waveguide part. The laser 
gas passing through the respective discharge area 5, 5' are excited by 
obtaining energy from the microwave, and the laser gas becomes to a 
resonation state in an optical resonator formed by the reflector 6 and the 
partial reflector 7. Laser oscillation is generated in the optical 
resonator formed in the discharge tube 1, and the generated laser beam 8 
is output through the partial reflector 7. 
Other operation, function and technical advantages, of course including the 
function of the gas flow rectifier 14, are similar to those of the first 
embodiment, and the descriptions of operation function and advantages on 
the first embodiment similarly apply with necessary modification. 
Although the present invention has been described in terms of the presently 
preferred embodiments, it is to be understood that such disclosure is not 
to be interpreted as limiting. Various alterations and modifications will 
no doubt become apparent to those skilled in the art after having read the 
above disclosure. Accordingly, it is intended that the appended claims be 
interpreted as covering all alterations and modifications as fall within 
the true spirit and scope of the invention.