Patent Publication Number: US-2016238839-A1

Title: Laser processing device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of and claims the benefit of priority under 35 U.S.C. §120 from U.S. Ser. No. 14/401,572 filed Nov. 17, 2014, the entire contents of which is incorporated herein by reference. U.S. Ser. No. 14/401,572 is a National Stage of PCT/JP13/065214 filed May 31, 2013, which was not published under PCT Article 21(2) in English and claims the benefit of priority from Japanese Patent Application No. 2012-135988 filed Jun. 15, 2012. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a laser processing device for processing an object to be processed by condensing and radiating laser light emitted from a laser oscillator to the object to be processed. 
     BACKGROUND ART 
     Hitherto, a laser processing device for processing an object to be processed by using laser light emitted from a laser oscillator has been well known (see, for example, Patent Literature 1). 
       FIG. 7  is a block diagram schematically illustrating an optical path configuration of a related-art laser processing device described in Patent Literature 1. 
     In  FIG. 7 , laser light L generated from a laser oscillator  1  is transmitted to a processing lens (not shown) in a processing head  4  via a transmission optical system to be condensed and radiated to an object to be processed (not shown) placed on a processing table  2 . 
     The processing table  2  and the processing head  4  include moving means  5  capable of moving each of the processing table  2  and the processing head  4  in at least one axial direction. The moving means  5  can move a relative position between the laser light L and the object to be processed in a desired direction and can locate the relative position at a desired position. 
     In this case, the moving means  5  is configured to move the processing table  2  in an X axis direction and to move the processing head  4  in a Y axis direction. 
     The transmission optical system for the laser light L includes a reflective beam expander mechanism  106  that the laser light L from the laser oscillator  1  enters, and a reflection mirror  8  for introducing the laser light L emitted from the reflective beam expander mechanism  106  into the processing head  4 . 
     The reflective beam expander mechanism  106  includes a reflection mirror  68  that the laser light L from the laser oscillator  1  enters, a spherical convex mirror  63  that the laser light L reflected by the reflection mirror  68  enters, and a spherical concave mirror  65  that the laser light L reflected by the spherical convex mirror  63  enters. 
     The reflective beam expander mechanism  106  increases a beam diameter of the laser light L by a desired scaling factor irrespective of a divergence angle of the laser light L generated from the laser oscillator  1 , and maintains an appropriate condensed light diameter at a processing point on the processing table  2 . 
     It is known that, generally, astigmatism in accordance with an incident angle occurs in light reflected by a spherical mirror such as the spherical convex mirror  63  or the spherical concave mirror  65 . In particular, when astigmatism occurs in the laser light L in a laser processing device, the light condensation ability is reduced and the beam shape becomes anisotropic at the processing point. 
     In this way, in a laser processing device of a type in which the processing head  4  moves, the reflective beam expander mechanism  106  for magnifying and collimating the laser light L is provided in the optical path in order to maintain an appropriate condensed light diameter at the processing point of the object to be processed. When a spherical mirror is used in the reflective beam expander mechanism  106 , in order to inhibit astigmatism, it is necessary to restrict the incident angle with respect to the spherical mirror to an acute angle. 
     Therefore, when a spherical mirror is used in the transmission optical system of the laser processing device, in order to avoid lowering of processing quality and occurrence of anisotropy in processing due to astigmatism, it is necessary to restrict the incident angle of the laser light L with respect to the spherical mirror to an acute angle so that the astigmatism does not adversely affect the processing quality. 
     It is known that, generally, when the incident angle with respect to the spherical mirror is set to be an acute angle (desirably 15° or less), lowering of the processing quality due to astigmatism is negligible. 
     Therefore, in  FIG. 7  (Patent Literature 1), the reflection mirror  68  in the reflective beam expander mechanism  106  restricts incident angles of the laser light L with respect to the spherical mirrors (spherical convex mirror  63  and spherical concave mirror  65 ) to acute angles, respectively. 
     However, when the reflection mirror  68  for restricting the incident angles with respect to the spherical mirrors is provided, the optical path is complicated, and further, in a strict sense, the astigmatism cannot be inhibited. Further, through absorption of the laser light by optical elements in the complicated optical path, the thermal lens effect is produced, and thus increase in the number of the optical elements is a factor of processing instability. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] JP 05-305473 A 
     SUMMARY OF INVENTION 
     Technical Problems 
     In the related-art laser processing device, when the reflective beam expander mechanism including the spherical mirror is used as the transmission optical system, the reflection mirror for restricting the incident angle with respect to the spherical mirror is provided as in Patent Literature 1, but there are problems in that the optical path configuration is complicated and, in addition, that astigmatism cannot be satisfactorily inhibited. 
     The present invention has been made in order to solve the above-mentioned problems, and an object of the present invention is to obtain a laser processing device capable of satisfactorily restricting a beam divergence angle and radiating laser light without aberration and having a desired beam diameter to an object to be processed by using a reflective beam expander mechanism whose optical path configuration is not particularly complicated. 
     Solution to Problems 
     According to one embodiment of the present invention, there is provided a laser processing device, including: a laser oscillator for emitting laser light; a processing table for placing an object to be processed; a transmission optical system for transmitting the laser light emitted from the laser oscillator to the processing table; a processing head for condensing and radiating the laser light transmitted via the transmission optical system to the object to be processed; and moving means for changing a relative position between the object to be processed and the laser light to be radiated to the object to be processed, in which the transmission optical system includes: a reflective beam expander mechanism for collimating and magnifying the laser light from the laser oscillator; and a variable curvature spherical mirror, in which the reflective beam expander mechanism includes a spherical mirror and a mirror having different curvatures in two orthogonal axes, and in which the variable curvature spherical mirror is placed between the spherical mirror and the mirror having different curvatures in two orthogonal axes. 
     Advantageous Effects of Invention 
     According to one embodiment of the present invention, in the reflective beam expander mechanism constructing the transmission optical system, by using the mirror having different curvatures in two orthogonal axes, the beam divergence angle can be satisfactorily restricted and the laser light without aberration and having a desired beam diameter can be radiated to the object to be processed, without using the transmission optical system having a particularly complicated structure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram schematically illustrating an optical path configuration of a laser processing device according to a first embodiment of the present invention. 
         FIG. 2  is a block diagram schematically illustrating an optical path configuration of a laser processing device according to a second embodiment of the present invention. 
         FIG. 3  is a block diagram schematically illustrating an optical path configuration of a laser processing device according to a third embodiment of the present invention. 
         FIG. 4  is a block diagram schematically illustrating a principal part of a laser processing device according to a fourth embodiment of the present invention. 
         FIG. 5  is a block diagram schematically illustrating a principal part of a laser processing device according to a fifth embodiment of the present invention. 
         FIG. 6  is a block diagram schematically illustrating a principal part of a laser processing device according to a sixth embodiment of the present invention. 
         FIG. 7  is a block diagram schematically illustrating an optical path configuration of a related-art laser processing device. 
         FIG. 8  is a structural view schematically illustrating a mirror adjusting mechanism according to a seventh embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       FIG. 1  is a block diagram schematically illustrating an optical path configuration of a laser processing device according to a first embodiment of the present invention. 
     In  FIG. 1 , the laser processing device according to the first embodiment of the present invention includes a laser oscillator  1  that emits laser light L, a processing table  2  on which an object to be processed (not shown) is placed, a transmission optical system including a reflective beam expander mechanism  6  and a reflection mirror  8 , and a processing head  4  that radiates the laser light L that has passed through the transmission optical system to the object to be processed. 
     The laser light L emitted from the laser oscillator  1  is collimated and magnified by the reflective beam expander mechanism  6  provided in the transmission optical system, and then is introduced into the processing head  4  by the reflection mirror  8 . After that, the laser light L is condensed by a processing lens (not shown) in the processing head  4 , and then is radiated to the object to be processed on the processing table  2 . 
     Moving means  5  is provided to the processing table  2  and the processing head  4 . The moving means  5  horizontally moves the processing table  2  and the processing head  4  in ranges from positions indicated by the solid lines to positions  2 ′ and  4 ′ indicated by the dotted lines, respectively. 
     The moving means  5  moves the processing table  2  in an X axis (dotted arrow) direction and moves the processing head  4  in a Y axis (dotted arrow) direction under the control of control means (not shown), thereby changing a relative position between the laser light L and the object to be processed to enable processing at a desired position to be processed. 
     Note that, in  FIG. 1 , in order to adjust the relative position between the laser light L and the object to be processed, the moving means  5  for changing the relative position between the processing table  2  and the processing head  4  is used, but moving means for driving only the processing head  4  may also be used. 
     The reflective beam expander mechanism  6  includes at least one mirror having different curvatures in two orthogonal axes. 
     In  FIG. 1 , the reflective beam expander mechanism  6  includes a spherical convex mirror  63  that reflects the laser light L from the laser oscillator  1  and a concave mirror  62  having different curvatures in two orthogonal axes. 
     The two orthogonal axes of the concave mirror  62  have curvatures different from each other, and the concave mirror  62  further reflects the laser light L reflected by the spherical convex mirror  63  to cause the laser light L to enter the reflection mirror  8  on the processing table  2  side. 
     Note that, in this case, the concave mirror  62  having different curvatures in two orthogonal axes and the spherical convex mirror  63  are used in the reflective beam expander mechanism  6 , but a convex mirror having different curvatures in two orthogonal axes and a spherical concave mirror may also be used. 
     Further, the arrangement order of the concave mirror  62  and the spherical convex mirror  63  is not limited to that in the configuration illustrated in  FIG. 1 , and the arrangement order of the mirrors may be set in reverse order. 
     As the simplest reflective beam expander mechanism that collimates and increases the beam diameter of the laser light L, it is conceivable to use a spherical convex mirror and a spherical concave mirror, but, as described above, astigmatism in accordance with an incident angle occurs in light reflected by a spherical mirror, and processing quality is significantly lowered due to the anisotropic beam shape and degraded light condensation ability caused by the astigmatism. 
     On the other hand, in the first embodiment of the present invention, the concave mirror  62  having different curvatures in two orthogonal axes is used in the reflective beam expander mechanism  6 , and the curvatures of the two axes of the concave mirror  62  are designed so that aberration is not caused in the reflected light. 
     As a result, no restriction is imposed on the incident angle of the laser light L, and thus an optical path design that uses the concave mirror  62  as a reflection mirror is possible. Thus, the beam diameter can be magnified and collimated not only without restricting the incident angle on the spherical convex mirror  63  but also without causing astigmatism. 
     As described above, the laser processing device according to the first embodiment ( FIG. 1 ) of the present invention includes the laser oscillator  1  for emitting the laser light L, the processing table  2  on which the object to be processed is placed, the transmission optical system for transmitting the laser light L emitted from the laser oscillator  1  to the processing table  2 , the processing head  4  for condensing and radiating the laser light L transmitted via the transmission optical system to the object to be processed, and the moving means  5  for changing the relative position between the object to be processed and the laser light L to be radiated to the object to be processed. 
     The transmission optical system includes the reflective beam expander mechanism  6  for collimating and magnifying the laser light L from the laser oscillator  1 , and the reflective beam expander mechanism  6  includes a mirror having different curvatures in two orthogonal axes. 
     The reflective beam expander mechanism  6  includes the spherical convex mirror  63  and the concave mirror  62  having different curvatures in two orthogonal axes. Alternatively, the reflective beam expander mechanism  6  includes a spherical concave mirror and a convex mirror having different curvatures in two orthogonal axes. 
     By using a mirror designed so that two orthogonal axes thereof have curvatures different from each other so as to inhibit aberration when reflecting light in the reflective beam expander mechanism  6  for magnifying and collimating the laser light L in this way, a beam divergence angle can be satisfactorily restricted and the laser light L without aberration and having a desired beam diameter can be radiated to an object to be processed by using the reflective beam expander mechanism  6  whose optical path configuration is not particularly complicated. 
     Further, no restriction is imposed on the incident angle of the laser light L with respect to the mirror having different curvatures in two orthogonal axes, and thus the optical path design flexibility is enhanced and the optical system is simplified. 
     Further, the reflection mirror  68  for restricting the incident angle with respect to a spherical mirror as in the related-art device ( FIG. 7 ) can be eliminated and the number of optical elements in the transmission optical system can be reduced to simplify the optical structure. Thus, influence of the thermal lens effect of the optical elements is reduced, which enables stable processing over a long period of time. 
     Note that, in  FIG. 1 , in order to inhibit aberration during reflection, the incident angle of the laser light L with respect to the spherical convex mirror  63  is restricted to an acute angle. However, when the curvatures of the concave mirror  62  are designed so that astigmatism that occurs when the concave mirror  62  gives reflection and astigmatism of reflected light that occurs in accordance with the incident angle with respect to the spherical mirror  63  are canceled out, the incident angle of the laser light L with respect to the spherical convex mirror  63  is not restricted to an acute angle. 
     Second Embodiment 
     Note that, in the above-mentioned first embodiment ( FIG. 1 ), the reflective beam expander mechanism  6  including the concave mirror  62  having different curvatures in two orthogonal axes and the spherical convex mirror  63  is used, but, as illustrated in  FIG. 2 , a reflective beam expander mechanism  6 A including a convex mirror  61  having different curvatures in two orthogonal axes and the concave mirror  62  having different curvatures in two orthogonal axes may also be used. 
       FIG. 2  is a block diagram schematically illustrating an optical path configuration of a laser processing device according to a second embodiment of the present invention. The same components as those described above (see  FIG. 1 ) are denoted by the same reference symbols as those described above, and detailed description thereof is omitted herein. 
     In  FIG. 2 , the reflective beam expander mechanism  6 A includes the convex mirror  61  having different curvatures in two orthogonal axes and the concave mirror  62  having different curvatures in two orthogonal axes. 
     In the reflective beam expander mechanism  6  described above ( FIG. 1 ), only one mirror having different curvatures in two orthogonal axes is used, but, in the reflective beam expander mechanism  6 A according to the second embodiment ( FIG. 2 ) of the present invention, two mirrors each having different curvatures in two orthogonal axes (convex mirror  61  and concave mirror  62 ) are used. 
     By using two mirrors each having different curvatures in two orthogonal axes in the reflective beam expander mechanism  6 A in this way, no restriction is imposed on the incident angles of the laser light L with respect to the mirrors in the reflective beam expander mechanism  6 A. Thus, the optical path design flexibility is enhanced and the reflection mirror  68  for restricting the incident angle becomes unnecessary, which reduces the influence of the thermal lens effect to stabilize the processing precision. 
     Further, all the mirrors in the transmission optical system can be used as reflection mirrors, and thus the reflection mirror  8  on the processing table  2  side becomes unnecessary. Thus, the transmission optical system can be further simplified. 
     Further, the influence of the thermal lens effect of the optical elements is further reduced, which enables stable processing over a long period of time. 
     Third Embodiment 
     Note that, in the above-mentioned first and second embodiments ( FIG. 1  and  FIG. 2 ), the processing table  2  movable in the X axis direction is used, but an immovable processing table  2 A may also be used as illustrated in  FIG. 3 . 
       FIG. 3  is a block diagram schematically illustrating an optical path configuration of a laser processing device according to a third embodiment of the present invention. The same components as those described above (see  FIG. 1  and  FIG. 2 ) are denoted by the same reference symbols as those described above, and detailed description thereof is omitted herein. 
     In  FIG. 3 , moving means  5 A is provided to the processing head  4  on the processing table  2 A, for moving the processing head  4  in the X axis direction and in the Y axis direction in ranges from the position indicated by the solid lines to positions  4 ′ indicated by the dotted lines. 
     Further, in the transmission optical system for the laser light L, an optical path length fixing mechanism  7  is inserted between the reflective beam expander mechanism  6 A and the reflection mirror  8  on the processing table  2 A side. 
     In this case, the processing table  2 A is larger than the processing table  2  described above and has a processing region larger than that of the processing table  2 . It is not efficient to drive the processing table  2 A, and hence the moving means  5 A changes the relative position between the laser light L and an object to be processed only by driving the processing head  4 . 
     Note that, when the processing head  4  is moved in the Y axis direction, a reflection mirror  28  is also moved in a range from a position indicated by the solid lines to a position  28 ′ indicated by the dotted lines. 
     Further, under this state, an optical path length of the laser light L from the laser oscillator  1  to the processing head  4  greatly differs depending on a processing position on the processing table  2 A, which may cause an error in the condensed light diameter of a beam radiated to an object to be processed. Therefore, the optical path length fixing mechanism  7  for cancelling out fluctuations in optical path length to make compensation is provided. 
     The optical path length fixing mechanism  7  includes a mirror group  78  including a plurality of mirrors for causing a direction of travel of incident light and a direction of travel of output light to be opposite and in parallel to each other. 
     Further, the optical path length fixing mechanism  7  includes a moving mechanism  79  for translating the mirror group  78  in a range from a position indicated by the solid lines to a position  78 ′ indicated by the dotted lines. 
     The moving mechanism  79  adjusts, under the control of control means (not shown), the optical path length of the laser light L to be always at a predetermined value by moving the position of the mirror group  78  so as to cancel out change in optical path length caused by the movement of the processing head  4 . 
     Note that, in  FIG. 3 , the optical path length fixing mechanism  7  is used with respect to the processing table  2 A that changes the relative position between the laser light L and an object to be processed only by moving the processing head  4 , but the optical path length fixing mechanism  7  can also be applied to a configuration in which both the processing head  4  and the processing table  2  are moved as described above ( FIG. 1  and  FIG. 2 ). 
     Further, a case where the reflective beam expander mechanism  6 A according to the above-mentioned second embodiment ( FIG. 2 ) is used is described, but this embodiment is similarly applicable to a case where the reflective beam expander mechanism  6  according to the above-mentioned first embodiment ( FIG. 1 ) is used. 
     The laser light L emitted from the reflective beam expander mechanism  6 A is collimated, and thus the condensed light diameter of the laser light L radiated to an object to be processed on the processing table  2 A ideally does not change even when the optical path length changes. However, in a strict sense, it is impossible to completely restrict the divergence angle, and thus increase in condensed light diameter along with the increase in optical path length cannot be completely avoided. 
     On the other hand, by inserting the optical path length fixing mechanism  7  as illustrated in  FIG. 3 , even a laser processing device using the large processing table  2 A causing the optical path length to be larger can maintain a fixed condensed light diameter on the processing table  2 A, and the processing quality can be maintained. 
     As described above, the transmission optical system according to the third embodiment ( FIG. 3 ) of the present invention includes the optical path length fixing mechanism  7 , and the optical path length fixing mechanism  7  includes the mirror group  78  constructed by a plurality of mirrors and the moving mechanism  79  for translating the mirror group  78 . The plurality of mirrors constructing the mirror group  78  are placed so that the direction of travel of incident light to the mirror group  78  and the direction of travel of output light from the mirror group  78  are opposite and in parallel to each other. 
     Further, the moving mechanism  79  translates the mirror group  78  with respect to the directions of travel of the incident light and the output light so as to cancel out change in relative position between the laser light L to be radiated to an object to be processed and the object to be processed to maintain a fixed optical path length of the laser light L radiated to the object to be processed. 
     The optical path length fixing mechanism  7  is provided in this way, and hence the condensed light diameter of the laser light L radiated to an object to be processed can be maintained independently of the relative position between the laser light L and the object to be processed, and thus high quality processing can be maintained. 
     Fourth Embodiment 
     Note that, in the above-mentioned third embodiment ( FIG. 3 ), the reflective beam expander mechanism  6 A including the convex mirror  61  having different curvatures in two orthogonal axes and the concave mirror  62  having different curvatures in two orthogonal axes is used, but, as illustrated in  FIG. 4 , a reflective beam expander mechanism  6 B including the concave mirror  62  having different curvatures in two orthogonal axes, the spherical convex mirror  63 , a variable curvature spherical mirror  67 , and the reflection mirror  68  may also be used. 
       FIG. 4  is a block diagram schematically illustrating a principal part of a laser processing device according to a fourth embodiment of the present invention. The same components as those described above (see  FIG. 1  to  FIG. 3 ) are denoted by the same reference symbols as those described above, and detailed description thereof is omitted herein. 
     In  FIG. 4 , the reflective beam expander mechanism  6 B according to the fourth embodiment of the present invention includes, as a transmission optical system, the variable curvature spherical mirror  67  that reflects the laser light L from the laser oscillator  1 , the reflection mirror  68  that reflects the laser light L reflected by the variable curvature spherical mirror  67 , and the convex mirror  61  and the concave mirror  62  that reflect the laser light L reflected by the reflection mirror  68 . Two orthogonal axes of each of the convex mirror  61  and the concave mirror  62  have curvatures different from each other. 
     The concave mirror  62  having different curvatures in two orthogonal axes reflects and introduces, into the optical path length fixing mechanism  7 , the laser light L reflected by the convex mirror  63  having different curvatures in two orthogonal axes. 
     Note that, the arrangement order of the variable curvature spherical mirror  67  and the reflection mirror  68  is not limited to that in the configuration illustrated in  FIG. 4 , and the arrangement order of the mirrors may be set in reverse order. 
     With the reflection mirror  68 , it is possible to restrict the incident angle of the laser light L with respect to the variable curvature spherical mirror  67  so as to inhibit astigmatism that occurs in the laser light L reflected by the variable curvature spherical mirror  67  to a range in which the processing quality is not influenced. 
     In the above-mentioned first to third embodiments ( FIG. 1  to  FIG. 3 ), the condensed light diameter of the laser light L at a processing point on an object to be processed is fixed, but by providing the variable curvature spherical mirror  67  in the reflective beam expander mechanism  6 B as illustrated in  FIG. 4 , the condensed light diameter of the laser light can be changed. 
     In general, in drilling processing such as piercing processing, by appropriately changing the condensed light diameter during the processing, processing at higher speed can be carried out compared with a case where the condensed light diameter is fixed. 
     Further, in processing a corner portion or the like, heat is liable to accumulate in the object to be processed and a cut surface may become rough, but by changing the condensed light diameter during the processing to change the range of irradiation of the laser light to the object to be processed, high quality and high precision processing can be carried out. 
     As described above, the transmission optical system according to the fourth embodiment ( FIG. 4 ) of the present invention includes the variable curvature spherical mirror  67  and can change the condensed light diameter of the laser light L radiated to an object to be processed, and hence, processing at higher speed and with higher quality can be realized. 
     Fifth Embodiment 
     Note that, in the above-mentioned fourth embodiment ( FIG. 4 ), the reflective beam expander mechanism  6 B including the reflection mirror  68  is used, but, as illustrated in  FIG. 5 , a reflective beam expander mechanism  6 C that does not require the reflection mirror  68  may also be used. 
       FIG. 5  is a block diagram schematically illustrating a principal part of a laser processing device according to a fifth embodiment of the present invention. The same components as those described above (see  FIG. 4 ) are denoted by the same reference symbols as those described above, and detailed description thereof is omitted herein. 
     In  FIG. 5 , the reflective beam expander mechanism  6 C according to the fifth embodiment of the present invention includes, as a transmission optical system, the spherical convex mirror  63  that reflects the laser light L from the laser oscillator  1 , the variable curvature spherical mirror  67  that reflects the laser light L reflected by the spherical convex mirror  63 , and the concave mirror  62  having different curvatures in two orthogonal axes. 
     The spherical convex mirror  63  and the variable curvature spherical mirror  67  are placed so as to be substantially opposed to each other so that the laser light L that has entered the corresponding mirror is emitted to an opposite direction. 
     The concave mirror  62  reflects the laser light L reflected by the variable curvature spherical mirror  67  to introduce the laser light L to the optical path length fixing mechanism  7  side. 
     The reflective beam expander mechanism  6 B described above ( FIG. 4 ) requires the reflection mirror  68  for restricting the incident angle with respect to the variable curvature spherical mirror  67  in order to inhibit astigmatism. The reflective beam expander mechanism  6 C illustrated in  FIG. 5  does not require the reflection mirror  68 , because the spherical convex mirror  63  is placed on an incident side of the variable curvature spherical mirror  67  and the variable curvature spherical mirror  67  and the spherical convex mirror  63  are placed so as to be opposed to each other. 
     Specifically, by using the reflective beam expander mechanism  6 C illustrated in  FIG. 5 , the reflection mirror  68  is unnecessary and the incident angles of the laser light L with respect to the variable curvature spherical mirror  67  and the spherical convex mirror  63 , respectively, can be restricted to a range in which astigmatism does not influence the processing quality. 
     Further, simplification of the transmission optical system reduces the influence of thermal lenses of the optical elements, and thus, stable processing can be realized. Further, because the transmission optical system is simplified, the optical path design flexibility can be enhanced. 
     As described above, the reflective beam expander mechanism  6 C according to the fifth embodiment ( FIG. 5 ) of the present invention includes the variable curvature spherical mirror  67  and the spherical convex mirror  63  (spherical mirror), and the variable curvature spherical mirror  67  is placed so as to be opposed to the spherical convex mirror  63  in the reflective beam expander mechanism  6 C. 
     This improves the processing precision as in the above description, and by placing the spherical convex mirror  63  and the variable curvature spherical mirror  67  so as to be opposed to each other, the reflection mirror  68  for restricting the incident angles of the laser light L with respect to the mirrors to acute angles, respectively, becomes unnecessary. Thus, the processing can be stabilized through simplification of the optical path and reduction in thermal lens effect. 
     Sixth Embodiment 
     Note that, in the above-mentioned fifth embodiment ( FIG. 5 ), the reflective beam expander mechanism  6 C including the concave mirror  62  having different curvatures in two orthogonal axes and the variable curvature spherical mirror  67  is used, but, as illustrated in  FIG. 6 , a reflective beam expander mechanism  6 D including a variable curvature mirror  64  having changeable curvatures in two orthogonal axes may also be used. 
       FIG. 6  is a block diagram schematically illustrating a principal part of a laser processing device according to a sixth embodiment of the present invention. The same components as those described above (see  FIG. 5 ) are denoted by the same reference symbols as those described above, and detailed description thereof is omitted herein. 
     In  FIG. 6 , the reflective beam expander mechanism  6 D according to the sixth embodiment of the present invention includes, as a transmission optical system, the spherical convex mirror  63  that reflects the laser light L from the laser oscillator  1  and the variable curvature mirror  64  having changeable curvatures in two orthogonal axes. 
     The variable curvature mirror  64  having changeable curvatures in two orthogonal axes has the function of both the concave mirror  62  having different curvatures in two orthogonal axes and the variable curvature spherical mirror  67  described above ( FIG. 5 ), and the variable curvature mirror  64  reflects the laser light L reflected by the spherical convex mirror  63  to introduce the laser light L to the optical path length fixing mechanism  7  side. 
     By using the reflective beam expander mechanism  6 D illustrated in  FIG. 6 , the transmission optical system is simplified and the number of the optical elements is reduced. Thus, the influence of thermal lenses can be further reduced, and in addition, stable processing can be realized. Further, the transmission optical system is simplified, and hence the optical path design flexibility can be enhanced. 
     As described above, the reflective beam expander mechanism  6 D according to the sixth embodiment ( FIG. 6 ) of the present invention includes the variable curvature mirror  64  having changeable curvatures in two orthogonal axes. With the variable curvature mirror  64 , both the mirror having different curvatures in two orthogonal axes and the variable curvature spherical mirror described above can be collected as a single optical element. 
     Therefore, the number of the optical elements can be reduced, and the stabilization of the processing precision can be realized through simplification of the optical path and reduction in thermal lens effect. 
     Seventh Embodiment 
     Note that, the convex mirror  61  having different curvatures in two orthogonal axes and the concave mirror  62  having different curvatures in two orthogonal axes of the above-mentioned first to sixth embodiments ( FIG. 1  to  FIG. 6 ) may be provided in a mirror adjusting mechanism  90  illustrated in  FIG. 8 . 
     In  FIG. 8 , the mirror adjusting mechanism  90  includes a mechanism that can move a fixed mirror in a horizontal direction and in a vertical direction and can rotate the fixed mirror within a mirror plane about a center of the mirror by using a horizontal direction adjustment screw  91 , a vertical direction adjustment screw  92 , and a rotational direction adjustment screw  93 . 
     The convex mirror  61  and the concave mirror  62  each having different curvatures in two orthogonal axes have lower precision curvatures around edges thereof due to a problem of processing precision of a spherical surface, and thus it is desired to radiate the laser light L to the vicinity of centers thereof. However, a pass line of the laser light L changes depending on thermal loads of the transmission optical system and the oscillator and change in surrounding environments such as temperature and humidity. 
     Further, when an incidence plane of the laser light L deviates from an axis along which the curvature is designed, a beam has a shape of an ellipsoid of revolution, and the processing quality is lowered. 
     By the mirror adjusting mechanism  90  illustrated in  FIG. 8 , even if the pass line of the laser light L changes, the laser light L is radiated to centers of the convex mirror  61  and the concave mirror  62  each having different curvatures in two orthogonal axes, and the laser light can be transmitted without distortion of the beam shape. Note that, in this embodiment, the mirror adjusting mechanism  90  is a mechanism that carries out adjustment with the horizontal direction adjustment screw  91 , the vertical direction adjustment screw  92 , and the rotational direction adjustment screw  93 , but a piezoelectric element may be used instead of a screw.