Abstract:
A reliable galvano-mirror employed for laser machining is disclosed. The mirror is far reduced its weight for getting a higher rotation. On the rear surface of the mirror, a lightweight rib-structure, which is formed from a mirror support beam centered across the surface and some ribs extending from the beam, holds the mirror. Against a distortion occurred in rotating at high speed, the mirror is provided with a high rigidity. And the mirror is structured integrally with the motor shaft holder. Besides, a slit is formed between the rib disposed close to the holder and the support beam centered across the rear surface of the mirror. Providing slit enables to minimize a local distortion caused by a stress from fastening screws to attach the mirror to the motor shaft.

Description:
This is a divisional application of Ser. No. 09/691,250 filed Oct. 19, 2000, U.S. Pat. No. 6,556,331. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a rapidly rotatable optical mirror, and an optical scanner and a laser machining apparatus employing the improved mirror. 
     BACKGROUND OF THE INVENTION 
     The prior art optical mirror will be described hereinafter with reference to the accompanying drawings. 
     FIGS. 12A and 12B show the structure of conventional lightweight optical mirror  120 . 
     Optical mirror  120 , as shown in FIG. 12A, comprises reflecting surface  121  and holder  128  which a motor shaft (not shown) is attached thereto. Holder  128  further comprises semi-circular cross section groove  122  for the motor shaft, and screw holes  123 . 
     FIG. 12B shows the structure of the mirror seen from the rear surface. As shown in FIG. 2B, reflecting surface  121  has on its rear surface: 
     (1) mirror support beam  124  extending from holder  128 ; 
     (2) plural strengthening ribs  125  extending from the both sides of beam  124  toward the rim of the rear surface of reflecting surface  121 ; and 
     (3) peripheral ribs  126  that are disposed close to holder  128  and extended along the rim of the rear surface of reflecting surface  121 . 
     The optical mirror structured above operates in a manner, which will be described hereinafter. Optical mirror  120  (FIGS. 12A,  12 B) is attached directly to the rotary shaft of the motor (not shown), and used for a galvanometer scanner in which the rotation angle of the motor defines a reflecting direction of light. 
     Laser beam and illumination light are reflected by mirror surface  121 . The shape and area of the reflected light depend on the shape of incident light and the rotation angle of the optical mirror. 
     To attach optical mirror  120  to the motor shaft, the motor shaft is fitted in semi-circular groove  122  and held with a retaining ring (not shown) having also a semi-circular groove, then secured by screws at screw holes  123 . Diameters both of groove  122  and the groove of the retaining ring are generally sized to be a few micrometers bigger than that of the motor shaft. However, the perimeter of the motor shaft measures bigger than the perimeter of roughly semi-cylindrical shape formed from facing each semi-circular portion of groove  122  and the retaining ring&#39;s groove. Therefore, fastening the screws to secure the optical mirror to the motor shaft inconveniently applies a stress to screw holes  123  vertically with respect to the reflecting surface  121 . 
     Optical mirror  120  is required to keep enough rigidity against a distortion occurred between reflecting surface  121  and holder  128  while the motor is rotating. For keeping enough rigidity, mirror support beam  124 , plural ribs  125 , and peripheral ribs  126  close to holder  128  are formed on the rear surface of reflecting surface  121 . In addition, as shown in FIGS. 12A and 12B, reflecting surface  121  of optical mirror  120  and holder  128  are formed in one piece. 
     Mirror support beam  124  functions as an absorber of the vibrations created in the axial direction of the motor shaft while the motor is rotating. Further, ribs  125  and  126  make a large contribution to minimize the fluttering of the mirror when rotating. 
     With the structure described above, however, a distortion occurs locally in the mirror surface when fastening the screws. When an optical scanner with such optical mirror is used for controlling the traveling path of laser beam in laser machining, flaws have often been detected outside the machined main hole in a workpiece. 
     FIG. 13 shows a distortion in the mirror surface when the motor shaft is attached and secured by screws to the conventional optical mirror, indicating distorted area by the curves. 
     It is apparent from FIG. 13 that the distortion which occurs at the screw holes disposed on the both sides of groove  122  is, through the peripheral ribs disposed on the rim of the mirror surface, carried to the mirror surface near the holder. 
     According to an amount of distortion measured by an interferometer, in the optical mirror made of a material containing beryllium for weight reduction, the Peak-Valley (P-V) value of the precision of the mirror surface measures no less than 4 μm. This amount of distortion is compatible to the optical path difference of approximately one-half of the wavelength (approx. 10 μm) of a carbon dioxide laser having relatively long wavelength. Generally, {fraction (1/20)}th of the wavelength of laser is defined to be optically aberration-free value (that is, approx. 0.5 μm for a carbon dioxide laser.) The P-V value in FIG. 13, however, shows as much as about 10 times the aberration-free value for the carbon dioxide laser. 
     Referring to FIG. 14, now will be described a two-dimensional optical scanner using the conventional optical mirror. 
     The conventional two-dimensional scanner, as shown in FIG. 14, comprises two sets of galvano-mirrors  140 A,  140 B and position control unit  148 . In FIG. 14, galvano-mirror  140 A further comprises motor  143 A having motor shaft  142 A, and optical mirror  141 A attached to motor shaft  142 A. Motor  143 A contains a position sensor (not shown) for position control. An output signal from the position sensor is fed into position control unit  148  for adjusting the position of the optical mirror. The explanation for galvano-mirror  140 B will be omitted because the mirror has the same structure as mirror  140 A described above. Hereinafter, depending on the parts constituting mirror  140 A or  140 B, either letter “A” or “B” is appended to the corresponding parts number. 
     Optical mirror  141 A of galvano-mirror  140 A, as shown in FIG. 14, horizontally rotates about motor shaft  142 A, while mirror  141 B of galvano-mirror  140 B vertically rotates about motor shaft  142 B. 
     The optical scanner structured above operates in a manner, which will be described hereinafter. Optical mirror  141 A reflects laser beam  145  shown in FIG. 14 to direct an intended position on optical mirror  141 B. In response to the reflection, the position sensor, which is built in motor  143 A of galvano-mirror  140 A, detects the orientation of mirror  141 A. Getting the signal back from the position sensor, position control unit  148  adjusts the reflecting direction. 
     Similarly, in response to the light incident on mirror  140 B, the position sensor, which is built in motor  143 B, detects the orientation of mirror  141 B. Getting the signal back from the position sensor, position control unit  148  adjusts the reflecting direction. 
     However, with the two-dimensional scanner employing mirrors  141 A and  141 B that have the conventional structure, the aimed surface cannot be radiated with the laser beam reflected from mirrors  141 A and  141 B due to a bad distortion. 
     FIG. 15 shows an optical system of the laser machining apparatus equipped with the optical scanner illustrated in FIG.  14 . In FIG. 15, the conventional laser machining apparatus comprises: 
     a) laser oscillator  151  that produces a laser beam; 
     b) collimator  152  collimating the output laser beam from laser oscillator  151 ; 
     c) mask changer  153  masking the collimated laser beam; 
     d) reflecting mirror  154  reflecting the laser beam passed through mask changer  153 ; 
     e) two-dimensional optical scanner  155  scanning the incident laser beam through reflecting mirror  154 ; 
     f) scanning lens  156  projecting the incident laser beam through optical scanner  155 ; and 
     g) two-dimensional machining table  158  for mounting workpiece  157  to be machined with the projected laser beam. (Workpiece  157  is an object to be machined on machining table  158 .) 
     The laser machining apparatus structured above operates in a manner, which will be described hereinafter. Laser oscillator  151  produces laser beam. After changed the beam diameter by Collimator  152 , the laser beam is irradiated over the mask placed on mask changer  153 . A portion of the laser beam, which passes through the mask, is launched into optical scanner  155  for controlling the scanning direction. Then scanning lens  156  projects the shape of the mask on workpiece  157  sitting on the two-dimensional machining table. Workpiece  157  is machined according to the projected mask shape. 
     FIG. 16 shows the strength distribution of laser spots, comparing with each other the strength at some spots in the entire scan area. If there is any distortion in the optical mirror, the strength distribution of laser spots varies depending on the position of the scan area. FIG. 16 shows the state of the distribution schematically. The strength distribution of laser spots is obtained by the position-by-position calculation of the scan area, using the machining optical system shown in FIG. 15 and, the data measured by an interferometer, which indicates the distortion of the mirror. FIG. 16 shows the calculated strength distribution of laser spots, comparing the strength with each other at nine spots in the scan area. 
     At central spot  161  of the scan area, as shown in FIG. 16, main beam  161 A for machining maintains its diameter&#39;s shape being circular (i.e., symmetric.) However, for example, at peripheral spot  162  of the scan area, main beam  162 A for machining has no longer the symmetry in its shape. Furthermore, some beams with asymmetric beam diameter, for example,  162 B,  162 C, and  162 D, are observed outside the main beam  162 A. Each asymmetric beam has appreciable beam strength. The fact has an adversely affect in machining a workpiece made of resin with a relatively low work threshold. That is, at central spot  161  where main beam  161 A maintains its beam diameter being asymmetric, the machined hole on a workpiece maintains its shape being circular (i.e., symmetric.) However, for example, at peripheral spot  162 , the machined hole on the workpiece undergoes a distortion due to an asymmetric shape of beam diameter. Besides, some asymmetric beams existed outside the main beam make unwanted holes near the machined main hole in a workpiece. Such workpiece has been treated as a serious nonconforming piece due to the flaws near the machined main hole. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention addresses the problems above. This provides an optical mirror with a structure minimizing a distortion that occurs in the mirror surface due to the stress from fastening screws when the optical mirror is attached to the motor shaft. It is an object that a laser machining apparatus with the mirror offers a consistent machining quality throughout the scan area. 
     The optical mirror of the present invention comprises a reflecting surface having optical characteristics, a holder to attach the mirror to other member, and a plurality of ribs disposed on the rear of the reflecting surface. The mirror also has slits in the ribs peripherally disposed close to the holder. 
     In the optical mirror that is attached to the motor shaft and rotates, the mirror comprises: 
     (1) a reflecting surface having optical characteristics; 
     (2) a holder to attach the mirror to motor shaft; 
     (3) a mirror support beam centered across the rear surface; and 
     (4) ribs extending from the support beam toward the rim of the rear surface. 
     The mirror is also structured so that the motor shaft-to-be-attached surface, or attachment surface, of the holder is held almost vertically with respect to the reflecting surface, or in other words, such that the attachment surface is substantially perpendicular with respect to the reflection surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a perspective view seen from the rear surface of the optical mirror in accordance with a first preferred embodiment. 
     FIG. 2 illustrates a distortion occurred in the mirror shown in FIG.  1 . 
     FIG. 3 shows a perspective view seen from the rear surface of another optical mirror in accordance with the first preferred embodiment. 
     FIG. 4 illustrates a distortion occurred in the mirror shown in FIG.  3 . 
     FIG. 5 shows the relation between the distortion in the mirror surface and slit length “L” in the ribs disposed near the holder in accordance with the first preferred embodiment. 
     FIG. 6 shows a perspective view seen from the rear surface of roughly rectangular optical mirror in accordance with the first preferred embodiment. 
     FIG. 7 shows a perspective view seen from the rear surface of the optical mirror in accordance with a second preferred embodiment. 
     FIG. 8 illustrates a distortion occurred in the optical mirror shown in FIG.  2 . 
     FIG. 9 shows a perspective view of a configured two-dimensional optical scanner. 
     FIG. 10 shows a schematic view of a laser machining apparatus employing the optical scanner. 
     FIG. 11 shows the strength distribution of laser spots in the entire area that the laser machining apparatus shown in FIG. 10 can scan, comparing the spots with each other. 
     FIG. 12A shows a perspective view of a conventional optical mirror, seen from the front surface of the mirror. 
     FIG. 12B shows a perspective view of the conventional optical mirror shown in FIG. 12A, seen from the rear surface of the mirror. 
     FIG. 13 illustrates a distortion occurred in the conventional mirror shown in FIGS. 12A and 12B when the motor shaft is attached to the mirror and secured with screws. 
     FIG. 14 shows a perspective view of a configured conventional two-dimensional optical scanner. 
     FIG. 15 shows a schematic view of a conventional laser machining apparatus employing the conventional optical scanner. 
     FIG. 16 shows the strength distribution of laser spots in the entire area that the conventional laser machining apparatus can scan, comparing the spots with each other. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the accompanying drawings, now will be described the optical mirror of the present invention, the optical scanner employing the mirror, the laser machining apparatus employing the scanner. 
     First Preferred Embodiment 
     FIG. 1 shows a perspective view seen from the rear side of reflecting surface  11  of optical mirror  10  in accordance with the first preferred embodiment. Reflecting surface  11  of optical mirror  10  has the same construction as conventional optical mirror  120  shown in FIG.  12 A. Optical mirror  10  according to the embodiment, as shown in FIG. 1, comprises reflecting surface  11  and holder  18  to which a motor shaft (not shown) is attached. Holder  18  further comprises semi-circular groove  12  for the motor shaft, and screw holes  13 . 
     Reflecting surface  11 , as shown in FIG. 1, has on its rear surface: 
     (1) mirror support beam  14  extending from holder  18 ; 
     (2) plural strengthening ribs  15  extending from the both sides of beam  14  toward the rim of the rear surface of reflecting surface  11 ; and 
     (3) peripheral ribs  16  that are disposed close to holder  18  and extended along the rim of mirror  10 . In addition, slits  17  are formed between mirror support beam  14  and ribs  16  disposed near the holder. 
     Referring to the accompanying drawings, now will be described such structured mirror  10  of the embodiment. 
     To attach optical mirror  10  to the motor shaft, the motor shaft is fitted in semicircular groove  12  and held with a retaining ring (not shown) having also a semi-circular groove, then secured by screws at screw holes  13 . Diameters both of groove  12  and the groove of the retaining ring are generally sized to be a few micrometers bigger than that of the motor shaft. However, the perimeter of the motor shaft measures bigger than the perimeter of roughly semi-cylindrical shape formed from facing each semi-circular portion of groove  12  and the retaining ring&#39;s groove. Therefore, fastening the screws to secure the optical mirror to the motor shaft inconveniently applies a stress to screw holes  13  vertically with reflecting surface  11 . 
     Optical mirror  120  is required to keep enough rigidity against a distortion occurred between reflecting surface  11  and holder  18  while the motor is rotating. For keeping enough rigidity, mirror support beam  14 , plural ribs  15 , and peripheral ribs  16  close to holder  18  are formed on the rear surface of reflecting surface  11 . In addition, reflecting surface  11  and holder  18  are formed in one piece. 
     Mirror support beam  14  functions as an absorber of the vibrations created in the axial direction of the motor shaft while the motor is rotating. Besides, ribs  15  and  16  make a large contribute to minimize the fluttering of the mirror when rotating. 
     FIG. 2 shows a distortion occurred in reflecting surface  11  when the optical mirror of the embodiment is attached to the motor shaft. In the FIG. 2, the curves show the distortion based on the measurement results. Slits  17  formed in ribs  16  cut off a distortion that is occurred at screw holes  13  disposed on the both sides of semi-circular groove  12 . That is, slits  17  can minimize the propagation of the distortion to ribs  16  disposed along the rim of the rear surface of reflecting surface  11 . As a result, reflecting surface  11  has less distortion in the proximity to holder  18  than the conventional optical mirror shown in FIGS. 12A and 12B. Slits  17  formed in ribs  16  can minimize the stress from fastening screws to carry to reflecting surface  11  and thereby reduce the distortion in surface  11 . 
     FIG. 3 shows another structure according to the embodiment, seen from the rear side of reflecting surface  31  of optical mirror  30 . Reflecting surface  31  of optical mirror  30  has the same construction as conventional optical mirror  120  shown in FIG.  12 A. Optical mirror  30  of the embodiment, as shown in FIG. 3, comprises reflecting surface  31  and holder  38  to which a motor shaft (not shown) is attached. Holder  38  further comprises semi-circular groove  32  for the motor shaft, and screw holes  33 . 
     As shown in FIG. 3, reflecting surface  31  has on its rear surface: 
     (1) mirror support beam  34  extending from holder  38 ; and 
     (2) plural strengthening ribs  35  extending from the both sides of beam  34  toward the rim of the rear surface of reflecting surface  31 . 
     Optical mirror  30  shown in FIG. 3 differs from optical mirror  10  shown in FIG. 1 in that ribs  16  are removed from its constitution. 
     Referring to FIG. 3, now will be described such structured mirror  30  of the embodiment. To attach optical mirror  30  to the motor shaft, the motor shaft is fitted in semicircular groove  32  and held with a retaining ring (not shown) having also a semi-circular groove, then secured by screws at screw holes  33 . Diameters both of groove  32  and the groove of the retaining ring are generally sized to be a few micrometers bigger than that of the motor shaft. However, the perimeter of the motor shaft measures bigger than the perimeter of roughly semi-cylindrical shape formed from facing each semi-circular portion of groove  32  and the retaining ring&#39;s groove. Therefore, fastening the screws to secure the optical mirror to the motor shaft inconveniently applies a stress to screw holes  33  vertically with reflecting surface  31 . 
     Optical mirror  30  is required to keep enough rigidity against a distortion occurred between reflecting surface  31  and holder  38  while the motor is rotating. For keeping enough rigidity, mirror support beam  34  and a plurality of ribs  35  are formed on the rear surface of reflecting surface  31 . In addition, reflecting surface  31  and holder  38  are formed in one piece. 
     Mirror support beam  34  functions as an absorber of the vibrations created in the axial direction of the motor shaft while the motor is rotating. Besides, ribs  35  make a large contribute to minimize the fluttering of the mirror when rotating. 
     FIG. 4 shows a distortion occurred in the reflecting surface of the optical mirror shown in FIG.  3 . In the FIG. 4, the curves show the distortion based on the measurement results. As shown in FIG. 4, the distortion occurred at screw holes  33 , which are disposed on the both sides of semi-circular groove  32 , is cut off its propagation to reflecting surface  31 . As a result, optical mirror  30  has less distortion in the proximity to holder  38  than the conventional optical mirror shown in FIGS. 12A and 12B. That is, optical mirror  30  shown in FIG. 1 can minimize the stress from fastening screws to carry to reflecting surface  31  and thereby reduce the distortion in surface  31 . 
     FIG. 5 shows the relation between the distortion in the mirror surface and slit length “L” in the ribs disposed near the holder. The distortion in the reflecting surface shown in FIG. 5 is measured, with the motor shaft attached to the optical mirror. The optical mirror made of a material containing beryllium is employed for the measurement. Generally, in terms of the required precision for the surface of the optical mirror described above, the acceptable distortion is at most 900 nm. It is equivalent to the optical path difference of at most one and a half wavelengths, when the light having its beam diameter greater than 25 mm is reflected and measured by an interferometer employing helium neon laser. 
     As shown in FIG. 5, if at least-1 mm slit (whose length is indicated by “L”) is formed in the rib disposed close to the holder, the amount of distortion in the surface can be suppressed within the acceptable value, i.e. at most 900 nm. As shown in FIG. 5, the slit whose length “L” is greater than 2 mm have no additional advantage to the P-V value for the surface precision of the reflecting surface. 
     Therefore, forming slits  17  (FIG. 1) in ribs  16  near the holder is a crucial determinant in terms of minimizing the distortion caused by fastening screws to carry on the reflecting surface. 
     The reflecting surfaces in FIGS. 1 and 3 are both shown in roughly circular. 
     FIG. 6 shows a perspective view of another optical mirror of the embodiment, seen from the rear surface of the roughly rectangular mirror. The reflecting surface, as shown in FIG. 6, has a roughly rectangular shape. 
     Reflecting surface  61  has on its rear surface: 
     (1) mirror support beam  64  extending from holder  68 ; 
     (2) plural strengthening ribs  65  extending from the both sides of beam  64  toward the rim of the rear surface of reflecting surface  61 ; 
     (3) peripheral ribs  66  that are disposed close to holder  68  and extended along the rim of mirror  60 . In addition, slits  67  are formed between mirror support beam  64  and ribs  66  disposed close to the holder. 
     Like the mirror shown in FIG. 1, the optical mirror shown in FIG. 6 is also effective. Also like the structure shown in FIG. 3, ribs  66  disposed close to the holder may be removed from the constitution. 
     Second Preferred Embodiment 
     The optical mirror according to the second embodiment of the present invention will be explained with reference to the accompanying drawings. 
     FIG. 7 shows the structure of optical mirror  70  seen from the rear side of reflecting surface  71 . Reflecting surface  71  of optical mirror  70  has the same construction as conventional optical mirror  120  shown in FIG.  12 A. Optical mirror  70  according to the embodiment, as shown in FIG. 7, comprises reflecting surface  71  and holder  78  to which a motor shaft (not shown) is attached. Holder  78  further comprises semi-circular groove  72  for the motor shaft, and screw holes  73 . Besides, the surface with semi-circular groove  72  that is attached to the motor shaft is formed so as to hold in a nearly vertical position relative to reflecting surface  71 . 
     Reflecting surface  71 , as shown in FIG. 7, has on its rear surface: 
     (1) mirror support beam  74  extending from holder  78 ; 
     (2) plural strengthening ribs  75  extending from the both sides of beam  74  toward the rim of the rear surface of reflecting surface  71 ; and 
     (3) peripheral ribs  76  that are disposed close to holder  78  and extended along the perimeter of mirror  70 . 
     Referring to the accompanying drawings, now will be described such structured mirror  70  of the embodiment. 
     To attach optical mirror  70  to the motor shaft, the motor shaft is fitted in semicircular groove  72  and held with a retaining ring (not shown) having also a semi-circular groove, then secured by screws at screw holes  73 . Diameters both of groove  72  and the groove of the retaining ring are generally sized to be a few micrometers bigger than that of the motor shaft. However, the perimeter of the motor shaft measures bigger than the perimeter of roughly semi-cylindrical shape formed from facing each semi-circular portion of groove  72  and the retaining ring&#39;s groove. Therefore, fastening the screws to secure the optical mirror to the motor shaft inconveniently applies a stress to screw holes  73  parallel with reflecting surface  71 . 
     Optical mirror  70  is required to keep enough rigidity against a distortion occurred between reflecting surface  71  and holder  78  while the motor is rotating. For keeping enough rigidity, mirror support beam  74  and plural ribs  75  are formed on the rear surface of reflecting surface  71 . In addition, reflecting surface  71  and holder  78  are formed in one piece. 
     Mirror support beam  74  functions as an absorber of the vibrations created in the axial direction of the motor shaft while the motor is rotating. Besides, ribs  75  make a large contribute to minimize the fluttering of the mirror when rotating. 
     The surface with semi-circular groove  72  that is attached to the motor shaft is formed so as to hold in a nearly vertical position relative to reflecting surface  71 . Due to the structure, a stress is generated in a direction parallel to reflecting surface  71  when fastening the screws at screw holes  73 . 
     FIG. 8 shows a distortion occurred in the reflecting surface of the optical mirror according to the embodiment. In the FIG. 8, the curves show the distortion based on the measurement results. It is apparent from FIG. 8 that the distortion occurred at screw holes  73  which are disposed on the both sides of semi-circular groove  72  is not carried to reflecting surface  71 . That is, such structured optical mirror of the embodiment can minimize or even eliminate the stress generated vertically with reflecting surface  71  and thereby reduce the distortion in surface  71 . 
     Although the reflecting surface of the embodiment is roughly circular shaped, roughly rectangular shaped surface, as shown in FIG. 6, is also effective. 
     Third Preferred Embodiment 
     The embodiment relates to a two-dimensional optical scanner equipped with the optical mirror described earlier in the two embodiments. Now will be described the optical scanner with reference to accompanying drawings. 
     The two-dimensional optical scanner of the embodiment comprises, as shown in FIG. 9, two sets of galvano-mirrors  90 A,  90 B and position control unit  98 . The optical mirror described in the first or the second preferred embodiment is used for optical mirror  91 A and  91 B for galvano-mirrors  90 A and  90 B. 
     As the structure relating to the motor and motor shaft is the same as the conventional type shown in FIG. 14, the explanation will be omitted. 
     Optical mirror  91 A of galvano-mirror  90 A, as shown in FIG. 9, rapidly rotates about motor shaft  92 A in a horizontal direction, while mirror  91 B of mirror  90 B rapidly rotates about motor shaft  92 B (not shown) in a vertical direction. 
     The optical scanner structured above operates in a manner, which will be described hereinafter. Optical mirror  91 A reflects laser beam  95  shown in FIG. 9 to direct an intended position on optical mirror  91 B. In response to the reflection, the position sensor, which is built in motor  93 A of galvano-mirror  90 A, detects the orientation of mirror  91 A. Getting the signal back from the position sensor, position control unit  98  adjusts the reflecting direction. 
     Similarly, in response to the light incident on mirror  90 B, the position sensor, which is built in motor  93 B, detects the orientation of mirror  91 B. Getting the signal back from the position sensor, position control unit  98  adjusts the reflecting direction. In this way, the two-dimensional scanner of the embodiment enables to properly guide light  95  incident from a fixed direction onto a desired point on the surface. 
     The scanner of the embodiment employs optical mirrors  91 A and  91 B, that are described earlier in the first or the second preferred embodiment. Laser beam is reflected by mirrors  91 A and  91 B then irradiated properly, with very little distortion, on an intended surface. 
     Although the reflecting surface of the optical mirror of the embodiment is roughly circular shaped, it is possible to employ a roughly rectangular shaped surface for the optical mirror, as shown in FIG. 6, for the same effect. Combination of these different shaped mirrors is also available: one optical mirror may have a roughly circular mirror, while the other may have a roughly rectangular one. 
     Fourth Preferred Embodiment 
     The embodiment relates to a laser machining apparatus equipped with the two-dimensional optical scanner described in the third embodiment. 
     FIG. 10 shows an optical system of the laser machining apparatus employing the optical scanner shown in FIG.  9 . The scanner described in the third preferred embodiment is used for two-dimensional scanner  105  (FIG.  10 ), which is employed for the laser machining apparatus of the embodiment. Scanner  105  has of course the optical mirror described in the first or the second embodiment. A laser oscillator and the rest of the structure but the optical mirror for scanner  105  are the same as those of the conventional type shown in FIG. 14, so that the description will be omitted. 
     The laser machining apparatus structured above operates in a manner, which will be described hereinafter. Laser oscillator  101  produces a laser beam. After changing the beam diameter by Collimator  102 , the laser beam is irradiated to the mask placed on mask changer  103 . A portion of the laser beam, which passes through the mask, is launched, via reflecting mirror  104 , into optical scanner  105  for controlling the scanning direction. Then scanning lens  106  projects the shape of the mask on workpiece  107  sitting on the two-dimensional machining table. Workpiece  107  is machined according to the projected mask shape. 
     FIG. 11 shows the strength distribution of laser spots, comparing with each other the strength at nine spots in the entire scan area. At all the spots in the center and the periphery, as shown in FIG. 11, each main beam for machining maintains its diameter&#39;s shape being circular, i.e. symmetric. In machining a workpiece made of resin with relatively low work threshold, each main beam can evenly machines each hole with its diameter&#39;s shape being circular, or symmetric. Unlike the machining with the conventional apparatus, machined workpieces have no flaws such as unwanted holes near the main machined hole. 
     Although the reflecting surface of the optical mirror for the laser machining apparatus of the embodiment is roughly circular shaped, it is possible to employ a roughly rectangular shaped surface for the optical mirror, as shown in FIG. 6, for the same effect. As shown in FIG. 10, combination of these different shaped mirrors is also available: one optical mirror may have a roughly circular mirror, while the other may have a roughly rectangular one. 
     With respect to the optical mirror according to an embodiment of the present invention, of the strengthening ribs disposed on the rear surface, the ribs close to the holder have slits adjacent to the support beam running across the rear side of the reflecting surface. Besides, in another embodiment, the holder for the optical mirror is structured such that the motor shaft-to-be-attached surface of the holder is arranged almost vertically with respect to the reflecting surface. With such improved structure, the distortion, which is occurred in the reflecting surface due to fastening screws, is local and negligible. In laser machining, the reflecting mirror of the present invention allows the workpiece to be free from flaws outside its main machined hole.