Optical disk recording/reproducing apparatus recording/reproducing information to/from optical disks according to different standards

Abstract: An optical disc reproducing apparatus includes a semiconductor laser (1), an objective lens (6) converging a laser beam from the semiconductor laser on a recording surface (7a) of an optical disc (13), a polarization plane rotating unit (4) selectively rotating the plane of polarization of the laser beam according to the thickness of a substrate of the optical disc, and a polarizing filter (5) selectively shading a peripheral portion of the laser beam having the rotated plane of polarization. In one embodiment, a numerical aperture changing unit includes a non-patterned twisted nematic liquid crystal, and the polarizing filter includes a polarizing film (51) having a polarization characteristics at its peripheral portion.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to optical disc recording/reproducing apparatuses, and more particularly, to an optical disc recording/reproducing apparatus capable of recording/reproducing information to/from optical discs having different substrate thicknesses or recording densities. Description of the Background Art An optical disc reproducing apparatus has been recently provided which reads information recorded on an optical disc having a thickness of approximately 1.2 mm such as a CD (Compact Disc) and a CD-ROM with a semiconductor laser. In such an optical disc reproducing apparatus, focus servo control and tracking servo control are carried out with respect to an objective lens for pickup. A laser beam is directed to a pit train on a recording surface, whereby a signal such as sound, video, and data is reproduced. In order to record a long motion picture on such an optical disc, a technique of making the recording density high has been progressed in recent years. For example, an SD (Super Density) standard is proposed which records information for approximately 5 Gbytes on one side of an optical disc having the same diameter as that of a CD-ROM (12 cm). According to the SD standard, the thickness of the optical disc is approximately 0.6 mm. Information for approximately 10 Gbytes can be recorded on one optical disa including two SD specified disc substrates laminated with their signal surfaces therebetween. On the other hand, an MMCD (Multimedia Compact Disc) standard using a one-layered structure is proposed which records information for approximately 3.7 Gbytes on one side of an optical disc having the same diameter as that of'the CD-ROM (12 cm). According to the MMCD standard, the thickness of an optical disc is approximately 1.2 mm. Information for approximately 7.4 Gbytes can be recorded on one side of an optical disc according to the MMCD standard using a two-layered structure. An objective lens for pickup is designed in consideration of the thickness of a substrate of an optical disc to be read and the wavelength of a semiconductor laser to be used. Therefore, an optical disc having the substrate thickness different from the design cannot be read, since the spot of a laser beam is not converged on a recording surface of the optical disc. For example, an objective lens designed to be adapted to an optical disc having a substrate of 1.2 mm in thickness cannot converge the spot of a laser beam on a recording surface of an optical disc having a substrate of 0.6 mm in thickness, and cannot reproduce information recorded on such an optical disc. Tanaka et al. discloses in Japanese Patent Laying-Open No. 5-303766 an optical head including an aspherical optical element in order to correct aberration caused by the difference in substrate thickness of an optical disc. This optical element may have a function of changing the numerical aperture (NA) of an objective lens. An objective lens is generally displaced in a direction (tracking direction) perpendicular to the optical axis of a laser beam by tracking control. However, the aperture disclosed by Tanaka et al. is fixed to the optical axis of the laser beam, irrespective of tracking control. Therefore, if the objective lens is displaced in a similar tracking range to that in the case where there is no aperture provided, the deformation of the spot of the laser beam directed to a recording surface increases according to the amount of shift of the optical axis of the objective lens with respect to that of the laser beam. This is because the diameter of the laser beam reduced by the aperture causes a great deformation of the spot of the laser beam, as if the amount of displacement of the objective lens is relatively increased. Such a beam spot deforms not only in the track direction but also in the tracking direction perpendicular thereto. The deformation of the beam spot in the track direction causes deterioration of jitter. The deformation of the beam spot in the tracking direction causes crosstalk noise. Therefore, an optical disc having a substrate of approximately 1.2 mm in thickness cannot be read stably. Further, an optical disc according to the MMCD standard cannot be read. In the future, coexistence of an optical disc having the current density and a substrate thickness of approximately 1.2 mm (CD, CD-ROM), an optical disc according to the MMCD standard having a high density and a substrate thickness of approximately 1.2 mm, and an optical disc according to the SD standard having a high density and a substrate thickness of approximately 0.6 mm is expected. The optical discs according to the MMCD and SD standards are referred to as a digital video disc (DVD), in order to be differentiated from the CD and the CD-ROM. The MMCD and SD are temporary names. They may be changed in the future. In the present application, the MMCD and SD standards are used as determining physical characteristics of an optical disc such as substrate thickness and recording density. US-A-5 281 797 discloses an optical disc recording/reproducing apparatus as set out in the preamble of claim 1. In this prior art apparatus a half-wave plate for changing the direction of polarization is included in an optical disc head which can be controlled for tracking the pit track of an optical disc. Including a half-wave plate or other heavy object for changing the polarization direction into a trackable optical disc head has the disadvantage that it increases the mass which has to be controlled for the tracking movement. EP-A-0 731 457 (prior art under Art. 54(3) EPC) discloses an optical pick-up apparatus capable of reading data from different types of discs and having a numerical aperture control unit including polarizing plane changing means mounted to the displacement unit which carries the object lens. In this case, too, the polarizing plane changing means must follow the tracking movement of the objective lens. EP-A-0 742 554 and EP-A-0 747 893 (both prior art under Art. 54(3) EPC) disclose optical disc reading apparatus having aperture control means for changing the numerical aperture depending on the type of disc. These prior art apparatus do not use polarization plane changing means. It is an object of the invention to provide an optical disc recording/reproducing apparatus which can record/reproduce information to/from optical discs having different standards from each other, for example digital video discs and compact discs, stably with one optical pickup device. According to the present invention the aforementioned object is achieved with an optical disc recording/reproducing apparatus as defined in claim 1. The dependent claims define further advantageous features of the apparatus according to the invention. The features and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram schematically showing the structure of an optical disc reproducing apparatus according to a first embodiment of the present invention. Fig. 2 is a schematic diagram showing the optical structure of an optical pickup device in Fig. 1. Fig. 3 is a cross-sectional view showing the structure of a polarizing filter in Fig. 2. Fig. 4 is a front view of the polarizing filter shown in Fig. 3. Fig. 5 is a schematic diagram for describing operation when a digital video disc is read using the optical pickup device in Fig. 2. Fig. 6 is a schematic diagram for describing operation when a compact disc is read using the optical pickup device of Fig. 2. Fig. 7 is a graph showing the relationship between spherical aberration and numerical aperture. Fig. 8 is a concept diagram of an objective lens having a function of correcting aberration. Figs. 9A to 9G are front views showing other examples of the polarizing filter of Fig. 4. Figs. 10A to 10D are front views showing further examples of the polarizing filter of Fig. 4. Fig. 11A is a front view for describing polarizing glass, and Fig. 11B is a front view of the polarizing glass which can be used instead of the polarizing filter of Fig. 2. Fig. 12 is a schematic diagram showing an optical system of an optical pickup device in an optical disc reproducing apparatus according to a second embodiment of the present invention. Fig. 13 is a schematic diagram showing an optical system of an optical pickup device in an optical disc reproducing apparatus according to a third embodiment of the present invention. Fig. 14 is a diagram for describing operation when a digital video disc is read using the optical pickup device of Fig. 13. Fig. 15 is a diagram for describing operation when a compact disc is read using the optical pickup device of Fig. 13. Fig. 16 is a schematic diagram showing an optical system of an optical pickup device in an optical disc reproducing apparatus according to a fourth embodiment of the present invention. Fig. 17 is a schematic diagram showing an optical system of an optical pickup device in an optical disc reproducing apparatus according to an fifth embodiment of the present invention. Fig. 18 is a block diagram showing a signal processing system in an optical disc reproducing apparatus according to a sixth embodiment of the prepent invention. Fig. 19 is a block diagram showing the schematic structure of the optical disc reproducing apparatus according to the sixth embodiment of the present invention. Fig. 20 is a front view showing the structure of a TN type liquid crystal used in an optical pickup device in Fig. 21. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described hereinafter in detail with reference to the drawings. In the drawings, the same reference characters denote the same or corresponding portions. First Embodiment Table 1 shows rated values and reproduction conditions of two kinds of optical discs which can be read in the optical reproducing apparatus according to the first embodiment. DVD (SD) CD (Standard) Rated value Substrate thickness 0.6 mm (0.55 ∼ 0.65 mm) 1.2 mm (1.1 ∼ 1.3 mm) Shortest pit length 0.4 µm (0.3 ∼ 0.5 µm) 0.83 µm (0.80 ∼ 0.90 µm) Track pitch 0.74 µm (0.73 ∼ 0.75 µm) 1.6 µm (1.5 ∼ 1.7 µm) Reproduction condition Wavelength 635 nm (620 ∼ 650 nm) Effective numerical aperture 0.60 (0.55 ∼ 0.65) 0.35 (0.30 ∼ 0.40) As is clear from this table, the optical disc reproducing apparatus according to the first embodiment can read both a digital video disc according to the SD standard of a high density and a thin substrate and a compact disc (including a CD-ROM) of a standard density and a standard substrate thickness. The digital video disc according to the SD standard has a substrate thickness of 0.6 (tolerance range 0.55-0.65) mm, the shortest pit length of 0.4 (tolerance range 0.3-0.5) µm, and a track pitch of 0.74 (tolerance range 0.73-0.75) µm. On the other hand, the compact disc has a substrate thickness of 1.2 (tolerance range 1.1-1.3) mm, the shortest pit length of 0.83 (tolerance range 0.80-0.90) µm, and a track pitch of 1.6 (tolerance range 1.5-1.7) µm. This optical disc reproducing apparatus reads both the digital video disc and the compact disc using a single laser wavelength of 635 (tolerance range 585-685, preferably 620-650) nm. Instead of the laser wavelength of 635 nm, a laser wavelength of 650 (tolerance range 600-700, preferably 635-665) nm may be used. In this embodiment, an objective lens is designed to be adapted to the digital video disc. The numerical aperture of the objective lens is set to 0.60 (tolerance range 0.55-0.65) for the digital video disc. The luminous flux of a laser beam is diaphragmed in reading of the compact disc so that the effective numerical aperture of the objective lens is changed to 0.35 (tolerance range 0.30-0.40) for the compact disc. Referring to Fig. 1, an optical disc reproducing apparatus 14 of the first embodiment includes an optical pickup device 15 picking up information recorded on an optical disc 13, a determining circuit 16 determining the thickness of a substrate 7 of optical disc 13, a reproduction signal processing circuit 17 processing a reproduction signal from optical pickup device 15, and a tracking control circuit 18 carrying out tracking control for an actuator (not shown) in optical pickup device 15 in response to a tracking error signal from optical pickup device 15. As shown in Fig. 2, optical pickup device 15 includes a semiconductor laser 1 generating a laser beam having a wavelength of 635 (tolerance ± 15) nm, a half prism 2, a collimator lens 3, a polarization plane rotating unit 4, a polarizing filter 5, an objective lens 6 focusing the laser beam to optical disc 13, a lens holder 19 holding objective lens 6 and polarizing filter 5 together, a condenser lens 8, and a photodetector 9. Therefore, the laser beam from semiconductor laser 1 is reflected by half prism 2, made parallel by collimator lens 3, and enters objective lens 6 through polarization plane rotating unit 4 and polarizing filter 5. Objective lens 6 together with polarizing filter 5 fixed thereto are displaced in the tracking direction by tracking control circuit 18 in Fig. 1, and in its optical axis direction (focus direction) by a focus control mechanism (not shown). The laser beam through objective lens 6 is converged to be directed to a recording surface 7a of optical disc 13 through substrate 7 made of polycarbonate. The laser beam reflected by recording surface 7a enters condenser lens 8 through substrate 7, objective lens 6, polarizing filter 5, polarization plane rotating unit 4, collimator lens 3, and half prism 2. The laser beam passing through condenser lens 8 is converged to be directed to photodetector 9. In response to the directed laser beam, photodetector 9 generates a reproduction signal and a tracking error signal. In this embodiment, objective lens 6 is preferably designed to make small aberration due to difference in thickness of substrate 7. Polarization plane rotating unit 4 includes a TN (Twisted Nematic) type liquid crystal 44, and two transparent electrode plates 45 sandwiching liquid crystal 44. Since no voltage is applied to transparent electrode plates 45 in reading of the digital video disc, polarization plane rotating unit 4 rotates the plane of polarization of the laser beam by 90°. On the other hand, since a predetermined voltage is applied to transparent electrode plates 45 in reading of the compact disc, polarization plane rotating unit 4 does not rotate the plane of polarization of the laser beam. Therefore, the entering laser beam directly passes through polarization plane rotating unit 4. As shown in Fig. 3, polarizing filter 5 includes a doughnut-shaped polarizing film 51, two glass plates 52 sandwiching film 51, and a filter 53 having no polarization characteristics laminated on the surface of glass plate 52 on the side of objective lens 6. Polarizing film 51 shades a laser beam having the plane of polarization perpendicular to the surface of paper on which the figure is drawn (hereinafter referred to as a "paper surface"). Therefore, polarizing film 51 transmits a laser beam having the plane of polarization parallel to the paper surface with transmittance of approximately 70 to 90 %. When the laser beam having the plane of polarization parallel to the paper surface is directed, if filter 53 is not provided, there could be a difference in transmittance between the central portion and the peripheral portion of polarizing filter 5. In order to avoid this, filter 53 has transmittance of approximately 70 to 90 %, and equalizes transmittance of polarizing filter 5 in the entire surface thereof when the laser beam having the plane of polarization parallel to the paper surface is directed. As far as it is transparent and superior in optical characteristics, any material can be used for glass plate 52. Polycarbonate and a resin such as PMMA, for example, may be used instead of glass. Since polarizing filter 5 is fixed to objective lens 6, the lighter polarizing filter 5 is, the more stably tracking control and focus control of objective lens 6 may be carried out. When a polarizing element having a small difference in transmittance between its central portion and its peripheral portion is used, filter 53 is not required. The polarization characteristics of polarizing filter 5 are shown in Fig. 4. A peripheral portion 5a of polarizing filter 5 transmits only a laser beam polarizing in the longitudinal direction of the figure. A central portion (transparent aperture) 5b of polarizing filter 5 transmits a laser beam polarizing in any direction. When the numerical aperture of objective lens 6 is 0.6 (tolerance ± 0.05) and the effective diameter of objective lens 6 is 4 mm, the diameter of central portion 5b of polarizing filter 5 is set to 2.3 (tolerance ± 0.2) mm so that the effective numerical aperture of objective lens 6 becomes 0.35 (tolerance ± 0.05). When the effective diameter of objective lens 6 is other than 4 mm, the diameter of central portion 5b of polarizing filter 5 may be set so that the effective numerical aperture of objective lens 6 is 0.35. Operation of the first embodiment will now be described. First, determining circuit 16 in Fig. 1 determines the thickness of substrate 7 of optical disc 13 to be read, and applies a determination signal indicating the determined thickness to optical pickup device 15. In response to the determination signal, optical pickup device 15 is adapted so that the laser beam is converged on recording surface 7a of optical disc 13. In reading of the digital video disc according to the SD standard, no voltage is applied to transparent electrode plates 45 of polarization plane rotating unit 4. Therefore, TN type liquid crystal 44 rotates the plane of polarization of the laser beam by 90° as shown in Fig. 5, so that a laser beam 20 having the plane of polarization perpendicular to the paper surface from collimator lens 3 is entirely changed into a laser beam 38 having the plane of polarization parallel to the paper surface. Since the central portion of polarizing filter 5 does not have the polarization characteristic, laser beam 38 passes through the central portion. On the other hand, although the peripheral portion of polarizing filter 5 has the polarization characteristic, the direction of polarization matches that of laser beam 21. Therefore, laser beam 38 passes through both the peripheral portion and the central portion similarly. The entire laser beam 38 enters objective lens 6 without being shaded by polarizing filter 5. The laser beam is converged on a recording surface 70a of a digital video disc 70 by objective lens 6. The diameter of the beam spot formed on recording surface 70a is 0.9 (tolerance ± 0.1) µm. In reading of the compact disc, a predetermined voltage is applied to transparent electrode plates 45 of polarization plane rotating unit 4. Therefore, TN type liquid crystal 44 transmits laser beam 20 directly without rotating its plane of polarization, as shown in Fig. 6. As a result, laser beam 20 from polarization plane rotating unit 4 has the plane of polarization perpendicular to the paper surface, which is the same as that of laser beam 20 from collimator lens 3. Since the central portion of polarizing filter 5 does not have the polarization characteristic, laser beam 20 passes through the central portion. Since the peripheral portion of polarizing filter 5 has the polarization characteristics in the direction perpendicular to laser beam 20, however, laser beam 20 is shaded at the peripheral portion. Therefore, laser beam 20 enters objective lens 6 only through the central portion of polarizing filter 5. This laser beam 20 is converged on a recording surface 71a of a compact disc by objective lens 6. The diameter of the beam spot formed on recording surface 71a is 1.5 (tolerance ± 0.1) µm. As described above, in the first embodiment, objective lens 6 is designed to be adapted to the digital video disc according to the SD standard having a substrate 70 of 0.6 mm in thickness. Therefore, this objective lens. 6 has a numerical aperture of 0.6 (tolerance ± 0.05). Generally, as shown in Fig. 7, spherical aberration is proportionate to the biquadratic of the numerical aperture. This is because there is a difference in optical path between a laser beam reaching recording surface 70a through the center of objective lens 6 and a laser beam reaching recording surface 70a through the outermost periphery of objective lens 6, as shown in Fig. 8. Therefore, objective lens 6 is designed so as to minimize such spherical aberration. More specifically, the central portion of objective lens 6 is designed so as to minimize aberration in reading of an optical disc of a little more than 0.6 mm in thickness. Since aberration of the central portion of objective lens 6 in reading of an optical disc of 0.6 mm in thickness becomes a little greater in this case, it is desired that the peripheral portion of objective lens 6 is designed to reduce the aberration. In reading of a compact disc having a substrate 71 of 1.2 mm in thickness using objective lens 6 designed for a digital video disc, the effective numerical aperture of objective lens 6 is 0.35 (tolerance ± 0.05), since polarizing filter 5 shades the laser beam to the peripheral portion of objective lens 6 as described above. Since the laser beam does not enter the peripheral portion of objective lens 6 designed for a digital video disc, objective lens 6 can converge the laser beam on recording surface 71a of the compact disc. Therefore, in reading of the digital video disc, objective lens 6 converges the laser beam incident on its entire portion on recording surface 70a of the digital video disc. In reading of the compact disc, objective lens 6 converges only the laser beam incident on the central portion on recording surface 71a of the compact disc. As described above, according to the first embodiment, since polarizing filter 5 selectively shades the peripheral portion of the laser beam, the effective numerical aperture of objective lens 6 becomes smaller. As a result, this optical disc reproducing apparatus can read not only the digital video disc but also the compact disc. Although the optical disc reproducing apparatus according to the first embodiment includes a reflecting mirror for changing the optical path of the laser beam by 90° between collimator lens 3 and polarizing filter 5, this mirror is not shown. This reflecting mirror makes the optical pickup device thin by changing the optical path as described above. Although the effective numerical aperture of objective lens 6 changes according to the thickness of substrate 7 in the first embodiment, the effective numerical aperture may change according to the recording density instead of the thickness of substrate 7. In the first embodiment, polarization plane rotating unit 4 is positioned between collimator lens 3 and polarizing filter 5. However, polarization plane rotating unit 4 may be positioned between semiconductor laser 1 and half prism 2, or between half prism 2 and collimator lens 3. Further, although polarizing filter 5 has a circular transparent aperture at its central portion as shown in Fig. 4, polarizing filter 5 may have a polygonal transparent aperture such as a triangle and an octagon instead of such a circular transparent aperture as shown in Figs. 9A to 9G. When polarizing filter 5 is fixed to objective lens 6 as in the above described first embodiment, the optical axis of objective lens 6 always matches the center of the transparent aperture of polarizing filter 5. Therefore, displacement of objective lens 6 in the tracking direction does not cause the luminous flux of the laser beam to change. However, polarizing filter 5 may be provided independently apart from objective lens 6. Polarizing filter 5 is not necessarily fixed to objective lens 6. In this case, objective lens 6 is desirably fixed to the optical axis of the laser beam. If satisfactory reproduction.cannot be carried out, the transparent aperture may be extended in the movement direction of objective lens 6. Since objective lens 6 and polarizing filter 5 are displaced in the tracking direction by tracking control circuit 18, the transparent aperture may be longer from side to side as shown in Figs. 10A to 10D, for example. In the rectangular transparent aperture shown in Fig. 10A, the shorter side is 2.3 mm, and the longer side is 2.5 to 3.3 mm. Further, the transparent aperture may be in asymmetry. Since a CD player carrying out reproduction using the transparent aperture reproduces information from inward to outward of a disc, the transparent aperture is elongated in the outward tracking direction of the disc. Further, a hologram element having the polarization characteristics or polarizing glass shown in Figs. 11A and 11B may be used instead of polarizing filter 5. Further, instead of polarizing filter 5, an optical thin film having the polarization characteristics may be formed on the surface of an optical component such as a reflecting mirror positioned between polarization plane rotating unit 4 and objective lens 6. Polarizing glass is manufactured by orienting silver compounds in glass in a predetermined direction and reducing the surface to deposit silver as shown in Fig. 11A. The reduced silver film has the polarization characteristic. Therefore, in the polarizing glass used instead of polarizing filter 5, silver only at peripheral portion 5a is deposited, and silver of central portion 5b is not deposited, as shown in Fig. 11B. Therefore, peripheral portion 5a of the polarizing glass has the polarization characteristic, while central portion 5b does not have the polarization characteristic. Since silver is used for the polarizing glass, the laser beam having the plane of polarization the same as the polarization characteristics of peripheral portion 5a can pass through peripheral portion 5a by 100 %. Therefore, it is not necessary to provide filter 53 for decreasing transmittance at the central portion as shown in Fig. 3. Even if the luminous flux of the laser beam is diaphragmed, a sufficient quantity of light can be obtained. It is desirable to use silver as a material for providing the polarizing glass with the polarization characteristic. However, any other metal material may be used as far as it provides the polarization characteristic. In the above described first embodiment, TN type liquid crystal 44 is used for electrically rotating the plane of polarization. However, an STN (Super Twisted Nematic) type liquid crystal or a ferroelectric liquid crystal may be used instead. The ferroelectric liquid crystal rotates the plane of polarization of the laser beam by 45° in response to application of a positive voltage in a short time, and maintains such a state. On the other hand, in response to application of a negative voltage in a short time, the ferroelectric liquid crystal rotates the plane of polarization of the laser beam by 45° in the direction opposite to that in application of the positive voltage, and maintains such a state. Therefore, by applying the positive voltage in reading of the digital video disc and the negative voltage in reading of the compact disc, for example, the ferroelectric liquid crystal can rotate the plane of polarization of the laser beam by 90°. Using such a ferroelectric liquid crystal shortens a voltage application time for rotating the plane of polarization, resulting in reduction of power consumption. In the above described first embodiment, half prism 2 is used. However, a polarizing beam splitter may be used instead of half prism 2, and a quarter-wave plate may be inserted between polarizing filter 5 and objective lehs 6. According to such a structure, the use efficiency of the laser beam is improved. In the above described embodiment, the laser beam of 635 nm in wavelength is used. However, a laser beam of 650 (tolerance ± 15) nm in wavelength may be used. In this case, although the spot diameter of the laser beam increases by approximately 0.1 µm, satisfactory reproduction can be carried out. If the tolerance of wavelength 635 nm is ± 50 nm, or if the tolerance of wavelength 650 nm is ± 50 nm, satisfactorily reproduction can be carried out. In the above described first embodiment, objective lens 6 has a numerical aperture of 0.6 so as to be adapted to the digital video disc according to the SD standard. However, if objective lens 6 has a numerical aperture of 0.52 so as to be adapted to the substrate of 1.2 mm in thickness, both the compact disc and the digital video disc according to the MMCD standard can be read. According to the MMCD standard, an optical disc has a substrate of 1.2 (tolerance ± 0.05) mm in thickness and a high recording density. In this case, the compact disc can be read even if the effective numerical aperture of objective lens 6 is not 0.35. However, if the effective numerical aperture of objective lens 6 is 0.35, coma generated by the tilt or warp of the substrate is decreased, allowing more favorable reproduction. In the above described first embodiment, polarizing film 51 transmitting only the laser beam having the plane of polarization parallel to the paper surface is used. However, when the laser beam having the plane of polarization parallel to the paper surface is incident on polarization plane rotating unit 4, a polarizing film transmitting only the laser beam having the plane of polarization perpendicular to the paper surface may be used instead. Substitution, modification, and the like of the above described components can also be applied to embodiments to be described later. Second Embodiment Referring to Fig. 14, in the optical disc reproducing apparatus according to the second embodiment, polarizing filter 5 in Fig. 2 is laminated on the surface of objective lens 6, whereby polarizing filter 5 is fixed to objective lens 6. The second embodiment is made more compact than the first embodiment. An optical thin film having the polarization characteristics may be formed on the surface of objective lens 6 instead of polarizing filter 5. Third Embodiment Referring to Fig. 13, the optical disc reproducing apparatus according to the third embodiment includes, unlike that of the first embodiment, an objective lens 37 designed to be adapted to a substrate of 0.8 mm in thickness, and a spherical aberration correcting plate 10 of an aspherical shape inserted between collimator lens 3 and polarization plane rotating unit 4. This optical disc reproducing apparatus can reproduce information recorded on the digital video disc according to the SD standard, the compact disc, and the digital video disc according to the MMCD standard. Spherical aberration correcting plate 10 corrects spherical aberration of objective lens 37 according to the type of optical disc to be read. Spherical aberration correcting plate 10 is formed of a hologram element, a diffraction grating and the like, for example. In reading of the digital video disc according to the SD standard, a spherical aberration correcting plate 100 for a thin substrate is inserted, and no voltage is applied to transparent electrode plates 45 of polarization plane rotating unit 4, as shown in Fig. 14. Since polarization plane rotating unit 4 rotates the plane of polarization of laser beam 20 by 90° similarly in the first embodiment, the entire laser beam 38 passing through polarization plane rotating unit 4 is transmitted through polarizing filter 5. Since objective lens 37 in the seventh embodiment is designed to be adapted to a substrate of 0.8 mm in thickness as described above, spherical aberration would be produced without spherical aberration correcting plate 100. However, since spherical aberration correcting plate 100 is inserted in the seventh embodiment, objective lens 37 converges laser beam 38 on recording surface 70a of the digital video disc according to the SD standard without spherical aberration being produced. On the other hand, in reading of the compact disc or the digital video disc according to the MMCD standard, a spherical aberration correcting plate 101 for a substrate of a standard thickness is inserted, and a predetermined voltage is applied to transparent electrode plates 45 of polarization plane rotating unit 4, as shown in Fig. 15. Therefore, similar to the case of the first embodiment, polarization plane rotating unit 4 transmits laser beam 20 without rotating the plane of polarization. Polarizing filter 5 shades the peripheral portion of laser beam 20, and transmits only the central portion of laser beam 20. Since spherical aberration correcting plate 101 for a substrate of a standard thickness is inserted in this embodiment, objective lens 37 converges laser beam 20 on recording surface 71a of the compact disc or the digital video disc according to the MMCD standard without spherical aberration being produced. In the third embodiment, polarization plane rotating unit 4 is provided between spherical aberration correcting plate 10 and polarizing filter 5, and spherical aberration correcting plate 10 is provided between collimator lens 3 and polarization plane rotating unit 4. However, polarization plane rotating unit 4 and spherical aberration correcting plate 10 may be provided between semiconductor laser 1 and half prism 2 or between collimator lens 3 and polarizing filter 5. Spherical aberration correcting plate 10 may be provided between half prism 2 and collimator lens 3 or between collimator . lens 3 and polarizing filter 5. More specifically, polarization plane rotating unit 4 and spherical aberration correcting plate 10 may be positioned anywhere as far as they are closer to semiconductor laser 1 than polarizing filter 5. Polarization plane rotating unit 4 and spherical aberration correcting plate 10 may be positioned oppositely to the case of Fig. 13. Although objective lens 37 is designed to be adapted to substrate 7 of 0.8 mm in thickness in the seventh embodiment, objective lens 37 may be designed to be adapted to a substrate of other than 0.8 mm in thickness, for example, 0.6 to 1.2 mm in thickness. When the objective lens is designed to be approximately adapted to a substrate of 0.6 mm in thickness, the digital video disc according to the SD standard having substrate 70 of 0.6 mm in thickness can be read stably. When the objective lens is designed to be approximately adapted to a substrate of 1.2 mm in thickness, the compact disc or the digital video disc according to the MMCD standard having substrate 71 of 1.2 mm in thickness can be read stably. When the objective lens is designed to be adapted to substrate 70 of 0.6 mm in thickness, spherical aberration correcting plate 100 for a thin substrate in Fig. 14 does not have to be inserted. When the objective lens is designed to be adapted to substrate 71 of 1.2 mm in thickness, spherical aberration correcting plate 101 for a substrate of a standard thickness in Fig. 15 does not have to be inserted. Fourth Embodiment The optical disc reproducing apparatus according to the fourth embodiment includes half mirror plate 48 instead of half prism 2, as shown in Fig. 16. Non-patterned polarization plane rotating unit 4 is inserted between half mirror plate 48 and diffraction grating 47. Since polarization plane rotating unit 4 is not patterned unlike in the ninth embodiment, polarizing filter 5 is patterned. In Fig. 34, the reflecting mirror is not shown. Fifth Embodiment In the optical disc reproducing apparatus according to the fifth embodiment, polarization plane rotating unit 4 and diffraction grating 47 are positioned oppositely to those of Fig. 16, as shown in Fig. 17. As in this embodiment, polarization plane rotating unit 4 may be inserted between diffraction grating 47 and semiconductor laser 1. Sixth Embodiment In the optical disc reproducing apparatus capable of reading the digital video disc according to the MMCD standard among those of the above embodiments, reproduction signal processing circuit 17 of Fig. 1 preferably includes an amplifier 90a for MMCD and an amplifier 90b for SD or CD. Further, tracking control circuit 18 in Fig. 1 preferably includes an amplifier 91a for MMCD and an amplifier 91b for SD or CD. In the sixth embodiment, the reproduction signal generated by the photodetector is amplified by amplifier 90a or 90b, and the amplified reproduction signal is applied to a reproduction signal processing portion 90c. The tracking error signal simultaneously generated by the photodetector is amplified by amplifier 91a or 91b, and the amplified tracking error signal is applied to a tracking control portion 91c. Amplifier 90a for MMCD has a larger gain than amplifier 90b for SD or CD. Amplifier 90a has a large gain particularly in a high frequency component of the reproduction signal. Amplifier 90a for MMCD is activated in reading of the digital video disc according to the MMCD standard, and amplifier 90b for SD or CD is activated in reading of the digital video disc according to the SD standard or the compact disc. Amplifier 91a for MMCD has a larger gain than amplifier 91b for SD or CD. Amplifier 91a for MMCD is activated in reading of the digital video disc according to the MMCD standard, and amplifier 91b for SD or CD is activated in reading of the digital video disc according to the SD standard or the compact disc. Therefore, in reading of the digital video disc according to the MMCD standard, the reproduction signal is amplified by amplifier 90a for MMCD more significantly than in reading of the digital video disc according to the SD standard or the compact disc, to be applied to reproduction signal processing portion 90c. On the other hand, the tracking error signal is amplified by amplifier 91a for MMCD more significantly in reading of the digital video disc according to the MMCD standard than in reading of the digital video disc according to the SD standard or the compact disc, to be applied to tracking control portion 91c. As a result, jitter and noise of the reproduction signal and the tracking error signal obtained from the digital video disc according to the MMCD standard can be reduced. Similar to the reproduction signal and the tracking error signal, a focus error signal is also preferably amplified more significantly in reading of the digital video disc according to the MMCD standard than in reading of the digital video disc according to the SD standard or the compact disc. Seventh Embodiment In the above embodiments, a laser beam having a wavelength of 585 to 685 nm or 600 to 700 nm is used. However, there is no limitation on the wavelength of the laser beam. For example, a laser beam having a short wavelength may be used. A laser beam having a wavelength in the range of 350 to 700 nm can be used. The effective numerical aperture of the objective lens is not limited to ones shown in the above embodiments. The effective numerical aperture can be set in the range of 0.20 to 0.65. Although the thickness of the substrate of the optical disc to be read is 1.2 mm and 0.6 mm in the above embodiments, the thickness is not limited thereto. The following table 2 shows rated values and reproduction conditions of each of optical discs when the digital video disc according to the SD standard, the compact disc, and a high density digital video disc are read using a blue laser (wavelength: 350-450 nm, typical wavelength: 415-445 nm). In the high density digital video disc, the thickness of the substrate is 0.6 (tolerance ± 0.05) mm, the pit length is 0.25 (tolerance ± 0.05) µm, the pit depth is 72 (tolerance ± 10) nm, and the track pitch is 0.50 (tolerance ± 0.08) µm. Therefore, the high density digital video disc has a recording density higher than the digital video disc according to the SD standard. As is clear from the table 2, in order to read the above described three kinds of optical discs with a blue laser, the effective numerical aperture of the objective lens is set to 0.20-0.30 in reading of the compact disc, 0.36-0.46 in reading of the digital video disc according to the SD standard, and 0.55-0.65 in reading of the high density digital video disc. The objective lens is preferably designed to be adapted to the high density digital video disc, with the numerical aperture of 0.55-0.65. In this case, the effective numerical aperture of the objective lens can be changed to 0.36-0.46 or 0.20-0.30. Alternatively, the objective lens is preferably designed to be adapted to the digital video disc according to the SD standard, having the numerical aperture of 0.36-0.60. In this case, the effective numerical aperture of the objective lens can be changed to 0.20-030. The effective numerical aperture of the objective lens can be changed with the method indicated in the above embodiments. Therefore, although the digital video disc according to the SD standard, the compact disc, and the high density digital video disc can be read with the blue laser, the blue laser is suitable for reading of the digital video disc according to the SD standard and the high density digital video disc. The following table 3 shows rated values and reproduction conditions of respective optical discs when the above described three kinds of optical discs are read with a green laser (wavelength: 450-550 nm, typical wavelength: 517-547 nm). As is clear from the table 3, in order to read the three kinds of optical discs using the green laser, the effective numerical aperture of the objective lens is set to 0.25-0.35 in reading of the compact disc, 0.45-0.55 in reading of the digital video disc according to the SD standard, and 0.55-0.65 in reading of the high density digital video disc. The objective lens is preferably designed to be adapted to the high density digital video disc, having the numerical aperture of 0.55-0.65. In this case, the effective numerical aperture of the objective lens can be changed to 0.45-055 or 0.25-0.35. Alternatively, the objective lens is preferably designed to be adapted to the digital video disc according to the SD standard, having the numerical aperture of 0.45-0.60. In this case, the effective numerical aperture of the objective lens can be changed to 0.25-035. The effective numerical aperture of the objective lens can also be changed with the method indicated in the above embodiments. Although the green laser is suitable for reading of the digital video disc according to the SD standard and the compact disc, the green laser can also read the high density digital video disc. Only two kinds of discs, the digital video disc according to the SD standard and the compact disc, may be read with the blue laser. In this case, the numerical aperture of the objective lens may be set to 0.36-060, and the effective numerical aperture of the transparent aperture may be switched selectively in the range of 0.25-0.35. On the other hand, when only two kinds of discs, that is, the digital video disc according to the SD standard and the compact disc, are read with the green laser, the numerical aperture of the objective lens may be set to 0.45-060, and the effective numerical aperture of the transparent aperture may be switched selectively in the range of 0.20-030. Eighth Embodiment Although only reproduction of information on the optical disc was described in the above embodiments, the present invention can be applied to recording of information on the optical disc. By using a semiconductor laser having a wavelength of 680 (tolerance ± 15) nm, 650 (tolerance ± 50) nm, 635 (tolerance ± 50) nm, 500 (tolerance ± 50) nm, or 400 (tolerance ± 50) nm and a power of 30 mW, for example, information can be recorded on the digital video disc according to the SD standard, the compact disc, and the high density digital video disc. In this case, the effective numerical aperture of the objective lens is set so as to be adapted to the respective optical discs and the respective wavelengths. Nineth Embodiment In the above embodiments, the effective numerical aperture of the objective lens is changed according to the substrate thickness or the recording density of the optical disc. However, the effective numerical aperture of the objective lens may be changed in a multistage manner according to an error rate of the detected reproduction signal. As shown in Fig. 19, an optical disc reproducing apparatus 56 according to the sixteenth embodiment includes an optical pickup device 201, an amplifier 207 amplifying the reproduction signal from optical pickup device 201, a signal processing circuit 202 processing the amplified reproduction signal, an error rate calculating circuit 204 calculating an error rate produced in a predetermined period based on the output of an error detecting circuit 203 in signal processing circuit 202, a liquid crystal drive circuit 206 driving such a polarization plane rotating unit 57 as shown in Fig. 20 in optical pickup device 201, and a controller 205 controlling liquid crystal drive circuit 206 according to the error rate from error rate calculating circuit 204. Polarization plane rotating unit 57 has a plurality of transparent apertures 58 different in diameter as shown in Fig. 20. One of the plurality of transparent apertures 58 is selected. The plane of polarization of the laser beam passing through the selected transparent aperture is rotated. However, the plane of polarization of the laser beam passing through polarization plane rotating unit 57 excluding the selected transparent aperture 58 is not rotated. According to the eleventh embodiment, the diameter of transparent aperture 58 is changed according to the error rate. Therefore, the diameter of transparent aperture 58 can easily be set so that the error rate correlating with the amount of jitter is minimized.

Claims: An optical disc recording/reproducing apparatus for irradiating an optical disc with a laser beam to record/reproduce information to/from said optical disc, comprising: a laser (1, 27, 11, 12) for generating a laser beam to be directed to said optical disc (7); an objective lens (6) for focusing the laser beam from said laser to said optical disc; and means for changing the effective numerical aperture of the objective lens (6), said means comprising: polarization plane rotating means (4) for selectively rotating the planes of polarization of said laser beam; shading means (5) for selectively shading a peripheral portion of the laser beam depending on whether the laser beam has its plane of polarization rotated or not, said shading means comprises a polarizing filter (51) having a peripheral portion (5a) transmitting only laser light polarized in a specific direction and a central portion (5b) transmitting laser light polarized in any direction, characterized in that said polarization plane rotating means (4) comprises a liquid crystal (44) sandwiched between two transparent electrode plates (45), and said objective lens (6) together with the polarizing filter (5) fixed thereto is displaceable in the tracking direction by tracking control means (18). The apparatus according to claim 1, wherein said shading means (5) is provided on a surface of said objective lens (6). The apparatus according to claim 1, wherein said objective lens (6) is designed to reduce its aberration. The apparatus according to claim 1, wherein said liquid crystal is of a twisted nematic type, or of a super twisted nematic type, or has a ferroelectric characteristic. said polarization plane rotating means includes a Faraday cell (23). The apparatus according to claim 1, wherein said laser includes a first light emitting element (28, 31, 11) for generating said laser beam having the plane of polarization in a first direction, and a second light emitting element (29, 32, 12) for generating said laser beam having the plane of polarization in a second direction different from said first direction, wherein said polarization plane rotating means selectively activates said first and second light emitting elements. The apparatus according to claim 5, wherein said laser further includes a sub-mount (30), and said first and second light emitting elements are mounted on said sub-mount. The apparatus according to claim 5, wherein said laser further includes a semiconductor substrate (33), and said first and second light emitting elements are formed on said semiconductor substrate. The apparatus according to claim 1, wherein said polarization plane rotating means rotates said laser around its optical axis. The apparatus according to claim 1, wherein said shading means includes a circular or polygonal transparent aperture at its center. The apparatus according to claim 1, wherein said laser beam has a wavelength of 350 nm to 700 nm. The apparatus according to claim 1, wherein said objective lens has an effective numerical aperture of 0.20 to 0.65. The apparatus according to claim 1, wherein said objective lens is designed to be adapted to the thickness of a substrate of a digital video disc, and said shading means shades a peripheral portion of said laser beam in reading of a compact disc. The apparatus according to claim 12, wherein the thickness of the substrate of said digital video disc is 0.55 mm to 0.65 mm, and said compact disc has a substrate of 1.1 mm to 1.3 mm in thickness. The apparatus according to claim 13, wherein said laser beam has a wavelength of 350 nm to 450 nm, said objective lens has a numerical aperture of 0.55 to 0.65, and said shading means changes the effective numerical aperture of said objective lens to 0.36 to 0.46 or 0.20 to 0.30. The apparatus according to claim 13, wherein said laser beam has a wavelength of 350 nm to 450 nm, said objective lens has a numerical aperture of 0.36 to 0.60, and said shading means changes the effective numerical aperture of said objective lens to 0.20 to 0.30. The apparatus according to claim 13, wherein said laser beam has a wavelength of 415 nm to 445 nm. The apparatus according to claim 13, wherein said laser beam has a wavelength of 450 nm to 550 nm, said objective lens has a numerical aperture of 0.55 to 0.65, and said shading means changes the effective numerical aperture of said objective lens to 0.45 to 0.55 or 0.25 to 0.35. The apparatus according to claim 13, wherein said laser beam has a wavelength of 450 nm to 550 nm, said objective lens has a numerical aperture of 0.45 to 0.60, and said shading means changes the effective numerical aperture of said objective lens to 0.25 to 0.35. The apparatus according to claim 13, wherein said laser beam has a wavelength of 517 nm to 547 nm. The apparatus according to claim 13, wherein apparatus said laser beam has a wavelength of 585 nm to 685 nm, said objective lens has a numerical aperture of 0.55 to 0.65, and said shading means changes the effective numerical aperture of said objective lens to 0.30 to 0.40. The apparatus according to claim 20, wherein said laser beam has a wavelength of 620 nm to 650 nm. The apparatus according to claim 13, wherein said laser beam has a wavelength of 600 nm to 700 nm, said objective lens has a numerical aperture of 0.55 to 0.65, and said shading means changes the effective numerical aperture of said objective lens to 0.30 to 0.40. The apparatus according to claim 22, wherein said laser beam has a wavelength of 635 nm to 665 nm. The apparatus according to claim 13, further comprising: photodetecting means (9) for detecting light reflected from said optical disc to generate a reproduction signal; first amplifying means for amplifying the reproduction signal from said photodetecting means; and means for increasing the gain of said first amplifying means in reading of the digital video disc having a substrate of 1.15 mm to 1.25 mm in thickness. The apparatus according to claim 24, wherein said photodetecting means further generates a tracking error signal for controlling said objective lens to cause said laser beam to trace a track of said optical disc, said optical disc recording/reproducing apparatus further comprising: second amplifying means for amplifying the tracking error signal from said photodetecting means; and means for increasing the gain of said second amplifying means in reading of the digital video disc having a substrate of 1.15 mm to 1.25 mm in thickness. The apparatus according to claim 24, further comprising means for increasing the gain of said first amplifying means with respect to a high frequency component of said reproduction signal in reading of said digital video disc having the substrate of 1.15 mm to 1.25 mm in thickness. The apparatus according to claim 1, wherein said objective lens (6,37) is designed to be adapted to the thickness of a substrate of a first digital video disc having a first recording density, and said shading means does not shade said laser beam in reading of said first digital video disc and shades a peripheral portion of said laser beam in reading of a second digital video disc having a second recording density lower than said first recording density. The apparatus according to claim 27, wherein said first and second digital video discs each have a substrate of 0.55 mm to 0.65 mm in thickness. The apparatus according to claim 28, wherein said first digital video disc has a pit depth of 62 nm to 82 nm, a pit length of 0.20 µm to 0.30 µm, and a track pitch of 0.42 µm to 0.58 µm, and said second digital video disc has a pit depth of 95 nm to 115 nm, a pit length of 0.38 µm to 0.42 µm, and a track pitch of 0.69 µm to 0.79 µm. The apparatus according to claim 29, wherein said laser beam has a wavelength of 350 nm to 450 nm, said objective lens has a numerical aperture of 0.55 to 0.65, and said shading means changes the effective numerical aperture of said objective lens to 0.36 to 0.46. The apparatus according to claim 29, wherein said laser beam has a wavelength of 415 nm to 445 nm. The apparatus according to claim 28, wherein said first digital video disc'has a pit depth of 78 nm to 98 nm, a pit length of 0.20 µm to 0.30 µm, and a track pitch of 0.42 µm to 0.58 µm, and said second digital video disc has a pit depth of 95 nm to 115 nm, a pit length of 0.38 µm to 0.42 µm, and a track pitch of 0.69 µm to 0.79 µm. The apparatus according to claim 32, wherein said laser beam has a wavelength of 450 nm to 550 nm, said objective lens has a numerical aperture of 0.55 to 0.65, and said shading means changes the effective numerical aperture of said objective lens to 0.45 to 0.55. The apparatus according to claim 33, wherein said laser beam has a wavelength of 517 nm to 547 nm. An apparatus as claimed in claim 1, further comprising: determining means (16) for determining a standard of said optical disc to generate a determination signal indicating the determined standard; and numerical aperture changing means (4, 5, 24, 35, 36, 39, 40) for changing the effective numerical aperture of said objective lens in response to the determination signal from said determining means, wherein the standard determined by said determining means is the thickness of the substrate of said optical disc, or the recording density of said optical disc.

Inventors: ICHIURA SHUICHI, KAJIYAMA SEIJI, KANOU YASUYUKI, TSUCHIYA YOICHI, ICHIURA, SHUICHI, KAJIYAMA, SEIJI, KANOU, YASUYUKI, TSUCHIYA, YOICHI
Applicants: SANYO ELECTRIC CO, SANYO ELECTRIC CO., LTD.
Application year: 1996
Main classification: G11B7
IPC codes: G11B7, G11B19