Optical mirror, optical scanner and laser machining apparatus using the same

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: The present invention relates to an optical mirror rotatable rapidly, and an optical scanner and a laser machining apparatus employing the improved mirror. The prior art optical mirror will be described hereinafter with reference to the accompanying drawings. Figs. 10A and 10B show the structure of conventional lightweight optical mirror 120. Optical mirror 120, as shown in Fig. 10A, 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. 10B shows the structure of the mirror seen from the rear surface. As shown in Fig. 10B, 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. 10A, 10B) 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'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 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. 10A and 10B, 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. Besides, ribs 125 and 126 make a large contribute to minimize the fluttering of the mirror when rotating. With the structure described above, however, a distortion is occurred locally in the mirror surface when fastening the screws up. 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. 11 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, 1/20th 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. 11, however, shows as much as about 10 times the aberration-free value for the carbon dioxide laser. Referring to Fig. 12, now will be described a two-dimensional optical scanner using the conventional optical mirror. The conventional two-dimensional scanner, as shown in Fig. 12, comprises two sets of galvano-mirrors 140A, 140B and position control unit 148. In Fig. 12, galvano-mirror 140A further comprises motor 143A having motor shaft 142A, and optical mirror 141A attached to motor shaft 142A. Motor 143A 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 140B will be omitted because the mirror has the same structure as mirror 140A described above. Hereinafter, depending on the parts constituting mirror 140A or 140B, either letter "A" or "B" is appended to the corresponding parts number. Optical mirror 141A of galvano-mirror 140A, as shown in Fig. 12, horizontally rotates about motor shaft 142A, while mirror 141B of galvano-mirror 140B vertically rotates about motor shaft 142B. The optical scanner structured above operates in a manner, which will be described hereinafter. Optical mirror 141A reflects laser beam 145 shown in Fig. 14 to direct an intended position on optical mirror 141B. In response to the reflection, the position sensor, which is built in motor 143A of galvano-mirror 140A, detects the orientation of mirror 141A. 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 140B, the position sensor, which is built in motor 143B, detects the orientation of mirror 141B. Getting the signal back from the position sensor, position control unit 148 adjusts the reflecting direction. However, with the two-dimensional scanner employing mirrors 141A and 141B that have the conventional structure, the aimed surface cannot be radiated with the laser beam reflected from mirrors 141A and 141B due to a badly distortion. Fig. 13 shows an optical system of the laser machining apparatus equipped with the optical scanner illustrated in Fig. 12. In Fig. 13, the conventional laser machining apparatus comprises: a) laser oscillator 151 producing 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. 14 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. 14 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. 13 and, the data measured by an interferometer, which indicates the distortion of the mirror. Fig. 14 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. 14, main beam 161A for machining maintains its diameter's shape being circular (i.e., symmetric.) However, for example, at peripheral spot 162 of the scan area, main beam 162A for machining has no longer the symmetry in its shape. Furthermore, some beams with asymmetric beam diameter, for example, 162B, 162C, and 162D, are observed outside the main beam 162A. 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 161A 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. An optical mirror including supporting ribs on its rear surface and being attachable to a rotatable shaft is known, for instance, from prior art document EP-A-0 459 010. Prior art document US-4 655 543 describes an optical mirror achieving a lower distortion of the mirror surface by strain decoupling between the shaft and the mirror. It is the object of the present invention to provide an optical mirror which is attachable to a rotatable shaft in a way causing a very low distortion of the mirror surface. The solution a according to the invention is characterized by the features of claim 1. Preferred embodiments are specified by the features of the dependent claims. 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 of a configured two-dimensional optical scanner. Fig. 8 shows a schematic view of a laser machining apparatus employing the optical scanner. Fig. 9 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. 10A shows a perspective view of a conventional optical mirror, seen from the front surface of the mirror. Fig. 10B shows a perspective view of the conventional optical mirror shown in Fig. 10A, seen from the rear surface of the mirror. Fig. 11 illustrates a distortion occurred in the conventional mirror shown in Figs. 10A and 10B when the motor shaft is attached to the mirror and secured with screws. Fig. 12 shows a perspective view of a configured conventional two-dimensional optical scanner. Fig. 13 shows a schematic view of a conventional laser machining apparatus employing the conventional optical scanner. Fig. 14 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. 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. 10A. 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 semi-circular 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'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. 10A. 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 semi-circular 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'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. 10A and 10B. 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 embodiment relates to a two-dimensional optical scanner equipped with the optical mirror described earlier in 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. 7, two sets of galvano-mirrors 90A, 90B and position control unit 98. The optical mirror described in the first or the second preferred embodiment is used for optical mirror 91A and 91B for galvano-mirrors 90A and 90B. As the structure relating to the motor and motor shaft is the same as the conventional type shown in Fig. 12, the explanation will be omitted. Optical mirror 91A of galvano-mirror 90A, as shown in Fig. 7, rapidly rotates about motor shaft 92A in a horizontal direction, while mirror 91B of mirror 90B rapidly rotates about motor shaft 92B (not shown) in a vertical direction. The optical scanner structured above operates in a manner, which will be described hereinafter. Optical mirror 91A reflects laser beam 95 shown in Fig. 9 to direct an intended position on optical mirror 91B. In response to the reflection, the position sensor, which is built in motor 93A of galvano-mirrar 90A, detects the orientation of mirror 91A. 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 90B, the position sensor, which is built in motor 93B, detects the orientation of mirror 91B. 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 91A and 91B, that are described earlier in the first or the second preferred embodiment. Laser beam is reflected by mirrors 91A and 91B 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. Third Preferred Embodiment The embodiment relates to a laser machining apparatus equipped with the two-dimensional optical scanner described in the third embodiment. Fig. 8 shows an optical system of the laser machining apparatus employing the optical scanner shown in Fig. 7. The scanner described in the second preferred embodiment is used for two-dimensional scanner 105 (Fig. 8), 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. 12, 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 laser beam. After changed 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 155 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. 9 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. 9, each main beam for machining maintains its diameter'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'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. 8, 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. 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 the reflecting surface. With such improved structure, the distortion, which is occurred in the reflecting surface due to fastening screws, is locally 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.

Claims: A rotatable optical mirror (10, 30, 60) attachable to a motor shaft, the rotatable optical mirror (10, 30, 60) comprising: a reflecting surface (11, 31, 61) having a rim and a rear surface; a holder (18, 38, 68) by which the optical mirror (10, 30, 60) is attachable to the motor shaft; a support beam (14, 34, 64) disposed on the rear surface and extended from the holder (18, 38, 68); and a plurality of first ribs (15, 35, 65) extended from the support beam (14, 34, 64) towards the rim; characterized in that peripheral ribs (16, 36, 66) are disposed close to the holder (18, 38, 68) and extended along the rim, and that there are provided slits (17, 37, 67) between the holder (18, 38, 68) and the peripheral ribs (16, 36, 66). The rotatable mirror according to claim 1, wherein the peripheral ribs (16, 36, 66) are disposed along a periphery of the rear surface. The rotatable mirror according to claims 1 or 2, wherein the peripheral ribs (16, 36, 66) are spaced at least 1 mm from the support beam (14, 34, 64). An optical scanner comprising an optical mirror (91A, 91 B) according to any of claims 1 to 3 and a motor (92A, 92B) having a motor shaft. A laser machining apparatus comprising an optical scanner (155) according to claim 4, the optical scanner (155) being arranged on an optical path of a laser beam from a laser oscillator (101).

Inventors: KARASAKI HIDEHIKO, KAWAZOE KIMIKO, KARASAKI, HIDEHIKO, KAWAZOE, KIMIKO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 2000
Main classification: G02B7
IPC codes: G02B7, B23K26, G02B26, G02B5