Optical resonator for laser oscillating apparatus

An optical resonator for a laser oscillating apparatus has an arrangement of folding mirrors facing one another across the lasing gas medium that radiates the laser beam. The arrangement includes a pair of reflecting surfaces that are approximately orthogonal to one another and effectively face the lasing gas medium as a whole. Also, the laser beam that is incident from the lasing gas medium is caused to be reflected successively from both reflecting surfaces to be emitted in the direction of the lasing gas medium.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical resonator for laser oscillating 
apparatus, and in more detail, to an optical resonator whose optical axis 
remains substantially unchanged even when the light path of the laser beam 
is distorted due to temperature change or the like in the lasing gas 
medium, and moreover, permits to obtain a polarized laser beam. 
In recent years, various attempts have been given in order to obtain a 
laser oscillating apparatus which is small in size and has a high output 
power. 
As one such oscillating apparatus, there exists one in which the optical 
resonator is composed of a primary mirror, an output mirror, and an 
appropriate number of folding mirrors. 
In this device it is attempted to reduce the length of the resonating 
cavity by multiply folding a laser beam that travels back and forth 
between a primary mirror and an output mirror by means of the folding 
mirrors that are placed to intervene the beam path, so as to let the laser 
beam pass through the lasing gas medium for many times. 
An exmaple of such a structure is, for example, to place a relatively large 
front folding mirror at one end of the lasing gas medium, to arrange a 
somewhat small primary mirror and a semi-transmissive output mirror in the 
vicinity of the front folding mirror, and to arrange a rear folding mirror 
at the other end of the lasing gas medium so as to oppose the front 
folding mirror and others. Such as optical resonator is disclosed in 
Applicant's co-pending application Ser. No. 741,756, filed June 6, 1985, 
"for Gas Laser Having Thermally Stable Optical Mount." 
In this optical resonator, a ray of radiation which is reflected by the 
primary mirror on one end of the lasing gas medium passes through the 
lasing gas medium to reach the rear folding mirror on the other side. 
After being reflected from the rear folding mirror, the ray reaches the 
front folding mirror on the first end via again the lasing gas medium. 
Thereafter, the ray is reflected many times between the front folding 
mirror and the rear folding mirror, and eventually reaches the output 
mirror on the first end from the rear folding mirror on the other end of 
the lasing gas medium. 
According to the optical resonator, there are formed multiple light paths 
between the front folding mirror and the rear folding mirror, to give an 
effective light path which is several times the length of the optical 
resonator. This then permits to realize a laser oscillating apparatus of 
small size and high output power. 
Now, in a laser oscillating apparatus of the above kind, there was a 
problem that when a nonuniformity in the spatial distribution of 
temperature in the lasing gas medium is produced due to supply of energy 
by a pumping drive, the light path of the laser beam is distorted, 
resulting in a situation in which it is not possible to output a 
predetermined laser beam. 
Furthermore, when a polarized beam is desired for use, for example, in high 
precision laser processings, there was a problem in that it is required to 
insert a special light polarizing means in the optical resonator. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an optical resonator for 
laser oscillating apparatus which is capable of suppressing the deviation 
in the optical axis to a minimum even when there is caused a distortion in 
the optical path in the optical resonator by the generation of a deviation 
in the spatial distribution of temperature in the lasing gas medium. 
Another object of the present invention is to provide an optical resonator 
for laser oscillating apparatus which is capable of producing a polarized 
laser beam without specifically inserting light polarizing means in the 
optical resonator. 
Still another object of the present invention is to provide an optical 
resonator for laser oscillating apparatus which is capable of suppressing 
the deviation of the optical axis to a minimum, when the laser oscillating 
apparatus is chosen to be a gas laser oscillating apparatus of three 
orthogonal axis type and the optical path is distorted due to large 
temperature difference in the direction of the gas flow. 
In order to attain the above objects, there is employed in the present 
invention orthogonal type folding mirrors in which a pair of reflecting 
surfaces that are approximately mutually orthogonal are arranged to face 
the lasing gas medium, as a part of the folding mirror for the optical 
resonator, the laser beam incident from the direction of the lasing gas 
medium is caused to be reflected successively from both reflecting 
surfaces, and the reflected beam is arranged to be sent out in the 
direction of the lasing gas medium.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
As shown in FIG. 1, the laser oscillating unit 1 is constructed by storing 
the laser oscillating elements in a cylindrical laser housing 3. The laser 
oscillating device comprises the oscillating unit 1 and a control unit 
that controls the oscillating unit 1. On the right-side surface of the 
laser housing 3 there are connected numerous connecting members such as 
the power source lines 5, a gas supply tube 7 for supplying a mixed gas of 
CO.sub.2, He, N.sub.2, and others, and the cooling water pipes 9 for 
internal cooling. On the left-side surface of the laser housing 3 there is 
formed an output window 11 for the laser beam LB. 
As shown in FIG. 2, in the upper part of the inside of the laser housing 3, 
there are provided a pair of upper and lower discharge electrodes 15U and 
15D that have numerous pin-shaped electrodes 13 with a prescribed 
separation between them, along the longitudinal directions of the laser 
housing 3. Underneath these discharge electrodes 15U and 15D there is 
provided a blower 17 which is rotated in the counterclockwise direction, 
in FIG. 2 to generate a gas flow A in the counterclockwise direction in 
the laser housing 3. 
Between the blower 17 and the lasing space B that is formed by the pair of 
upper and lower discharge electrodes 15U and 15D, there are provided heat 
exchangers 19 and 21. Between the blower 17, the heat exchangers 19 and 
21, or the discharge electrodes 15U and 15D, there are appropriately 
provided partitioning plates for smoothing the gas flow A. In the interior 
of the laser housing 3, there are arranged a primary mirror 25, a front 
folding mirror 27, and an output mirror 29 (any of which is not shown in 
FIG. 2) and an orthogonal type rear folding mirror 23 on the front and the 
rear sides, respectively, with the lasing space B in between. 
As shown in FIG. 3 in detail, on the output side of the laser oscillating 
unit 1, there is arranged a front folding mirror 27 that consists of a 
relatively large plane mirror, and in addition, there are arranged a 
primary mirror 25 which consist of concave mirror totally reflective and 
an output mirror 29 that consists of a plane mirror with transmissivity of 
about 40% above and below, respectively, on the right-hand side of the 
front folding mirror 27. Further, on the rear side of the laser housing 3, 
there is arranged an orthogonal-type folding mirror 23, facing the front 
folding mirror 27. 
The orthogonal-type folding mirror 23 consists of two plane reflecting 
mirrors 31 and 33 that are supported orthogonal to each other by means of 
a suitable supporting means. The orthogonally situated reflecting surfaces 
are arranged directly opposing the front folding mirror 27, with the 
intersecting line of the surfaces perpendicular to the direction Y of the 
gas flow. However, the posture of direction opposition does not have to be 
very exact. 
With reference to the coordinate system formed by the X, Y and Z axes the 
primary mirror 25 and the output mirror 29 in the present example are 
arranged with an attitude that has a predetermined angle inclined with 
respect to the X-axis attitude of the front folding mirror 27. 
As shown in FIG. 3 to FIG. 5, the laser beam LB is, after reflected at a 
reflecting point i on the primary mirror 25, reflected successively at 
reflecting points h and h' on the rear folding mirror 23, and get to a 
reflecting point g on the front folding mirror 27. Thereafter, it arrives 
at a reflecting point a on the output mirror 29 through each of the 
reflecting points 
f'.fwdarw.f.fwdarw.e.fwdarw.d.fwdarw.d'.fwdarw.c.fwdarw.b'.fwdarw.b on the 
rear folding mirror 23 or on the front folding mirror 27. 
Therefore, in this optical resonator, the optical path that connects the 
reflecting point i on the primary mirror 25 and the output point a on the 
output mirror 29, is folded three-dimensionally for eight times between 
the front folding mirror 27 and the rear folding mirror 23. 
When discharge is started by the supply of a laser medium which is a 
mixture of CO.sub.2, N.sub.2, He, to the space between the discharge 
electrodes 15U and 15D, radiation is emanated from the mixed gas. Then, 
the radiation is amplified successively as it propagates along the optical 
path shown in FIG. 3, and a desired laser beam is output from the output 
window 11. 
In this example, the optical path is folded for eight times by the optical 
resonator, as mentioned in the above, it becomes possible to reduce the 
length of the optical resonator to an extremely small value. Moreover, the 
folded optical path is designed to be arranged three-dimensionally in the 
optical resonator, as shown in FIG. 3, the diameter of the cross-section 
of the optical resonator can be made extremely small. Hence, it becomes 
possible in this embodiment to realize a laser oscillating unit which is 
small in size for both of the longitudinal and the lateral directions, and 
yet is possible to output a large power. 
Moreover, the radiation which is emitted from the laser medium is reflected 
with an approximate reflection angle of 45.degree. by each of the first 
and second reflecting mirrors 31 and 33, so that it becomes possible to 
obtain a laser beam of a desired polarization. 
Now, as discharge proceeds in the optical resonator, there is generated a 
large amount of heat which will lead to a marked difference in the 
temperature on the upstream side and that on the downstream side of the 
blow of gas that constitutes the laser medium. (If the temperatures on the 
upstream and downstream sides of the gas flow are designated by T.sub.1 
and T.sub.2, respectively, there will hold a relation T.sub.1 &lt;T.sub.2.) 
Then, due, for example, to the changes in the refractive index of the 
medium, the optical path of the laser beam will be distorted toward lower 
temperature (namely, T.sub.1) side, as indicated in FIG. 6. In such a 
case, in the conventional optical resonator in which a pair of folding 
mirrors F.sub.1 and F.sub.2 are arranged on both sides of the laser 
oscillating region, the optical path will be distorted markedly in each 
time the laser beam LB is reflected from the folding mirror, as shown in 
FIG. 7, so that the alignment of the optical system will get out of order 
markedly. 
In contrast to that, in the present embodiment, an orthogonal rear folding 
mirror 23 which has a first and a second reflecting mirrors 31 and 33 is 
provided on one end of the optical resonator. Thus, for example, if the 
optical path for a light that is incident upon the reflecting mirror 33 
follows a route which consists of the distorted optical paths P.sub.3 and 
P.sub.4 with respect to the distortion-free optical paths P.sub.1 and 
P.sub.2, then the optical path of the light that is reflected from the 
first reflecting mirror 31 will follow a distorted optical path P.sub.4 
that is parallel to P.sub.3. 
Here, the optical path P.sub.4 will be bent toward the lower temperature 
(namely, T.sub.1) side of the gas flow A, entirely similarly to the 
incident optical path P.sub.3, in accordance with the law of reversibility 
of propagation path of light, so that it will propagate parallel to the 
distortion-free optical path P.sub.2 when it arrives at the folding mirror 
27. Such a phenomenon of eliminating distortion will be identically 
operative also in the case of repetition of a plurality of reflections. 
Accordingly, in the optical resonator shown in the present example the 
ultimate deviation of the optical axis, for example, the deviation 
corresponding to the slight deviation .DELTA.Y in the optical paths 
P.sub.2 and P.sub.4 on the surface of the front folding mirror 27, will 
become a maximum. 
Hence, according to the present embodiment, even if the optical path of the 
laser beam is distorted due to generation of a deviation in the spatial 
distribution of temperature in the laser medium, it becomes possible to 
minimize the mismatch in the alignment of the optical system. 
FIG. 9 and FIG. 10 show a plan view and a front view, respectively, of a 
mirror supporting device for supporting the rear folding mirror 23. 
The mirror supporting device 35 comprises principally a substrate member 37 
and a rectangular isosceles member 39 (namely, 39A and 39B) that is 
mounted on the substrate member 37. 
The substrate member 37 consists of a U-shaped plate member. On the base 
section of the U-shaped plate member there is provided a through hole 41, 
and on the parallel arm section of the letter U there are fixed with bolts 
45 fixing units 43 for holding the substrate member 37 in laser housing, 
with its position freely adjustable. Fixed to the through hole 41, with 
its hoard section arranged in the interior of the letter U by using two 
nuts 47 for positioning, is a bolt 49. On the fixing unit 43 these are 
provided oblong apertures 51, and the position of the substrate member 37 
is adjustable in the direction of gas flow Y in the laser housing, by 
loosening or fastening the bolts, not shown, that are inserted through the 
oblong apertures 51. 
The rectangular isosceles member 39 (39A and 39B) is constructed by 
joining, with bolts 53, two mirror mounting plates 39A and 39B placed so 
as to include an angle of approximately 90.degree., or more precisely, to 
form an angle which is slightly greater than the desired intersecting 
angle. One end of the rectangular isosceles member 39 (39A and 39B) is 
joined with bolts 55 to the end sections of the arms of the letter U of 
the substrate member 37, while the other end is supported freely by 
pressing it against the head section of the bolt 49 on the base section of 
the letter U for the substrate member 37. In order to be supported 
elastically in suitable manner, the joining surface on both ends of the 
mirror mounting plate 39A are given chamferings as shown in the FIG. 10. 
On the joined surface side of the rectangular isosceles member on the 
mirror mounting plate 39B, there is provided a notch which permits a part 
of the first reflecting mirror 31 to show itself toward the rear side, as 
will be described later. 
On the mirror mounting plates 39A and 39B there are provided respectively 
mirror fixing plates 61 that are held by the bolts 59. The mirror mounting 
plates 39A and 39B hold the reflecting mirrors 31 and 33, respectively, 
parallel to the mounting plates 39A and 39B by the use of the fixing 
plates 61. 
The mirror supporting device 35 is fixed to the laser housing so as for the 
reflecting mirrors 31 and 33 to have attitude as shown in FIG. 2 and FIG. 
3. In the present example, the fixing units 43 are provided with oblong 
apertures 51, and the mirror supporting device 35 is arranged to be 
adjustable for its position along the direction of gas flow Y. 
Further, in the mirror supporting device 35 it is possible to give a fine 
adjustment to the intersecting angle between the first and second 
reflecting mirrors 31 and 33, in the neighborhood of a predetermined angle 
(for instance, 90.degree.), within the tolerable range of elastic 
distortion that occurs in the area of the bolts 53 and 55. The above 
adjustment can be accomplished by moving the head section of the bolt 49 
with respect to the substrate material 37, by varying the engaging 
positions of the nuts 47 and 47 that are engaged with the bolt 49. 
Namely, according to the mirror supporting device 35 of the present 
embodiment, when there arises a mismatch in the alignment of the primary 
mirror 25, the output mirror 29, the front folding mirror 27, and so on 
shown in FIG. 3, it is possible to adjust the mirror alignment by not only 
changing the angles of these mirrors themselves but also the intersecting 
angle between the reflecting mirrors 31 and 33 of the rear folding mirror 
23. These changes in the mirror angles make it possible to obtain a more 
appropriate alignment of the optical system. For example, when there 
occurs a mismatch only in the alignment of the primary mirror 25, from a 
predetermined angle, what is needed is a slight adjustment of the bolts 49 
in the mirror supporting device 35.