Optical pickup apparatus

Abstract: An optical pickup apparatus for recording information signals on an optical recording medium (23) is disclosed. The apparatus includes a light source (1) radiating a diffused light flux, an object lens (9) on which a light flux radiated from said light source (1) is incident and which is adapted for converging the light flux on the optical recording medium (23), a light flux dividing device (6) arranged at a position on a light path between the light source (1) and the object lens (9) at which the light flux remains to be a diffused light flux, which light flux separating device is adapted for guiding the light flux converged on the optical recording medium (23) and reflected by the optical recording medium (23) out of the light path, a light detection unit (13) for detecting the light led out of the light path by the light flux dividing device (6), and a polarized light dependent light flux separating device (11) arranged at a position on a light path between the light flux separating device (6) and the light detection device (13) at which the reflected light flux remains to be a converging light flux. In this manner, recording characteristics of information signals may be improved and a simplified structure and a small size of the apparatus may be realized.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an optical pickup apparatus for writing information signals on or from a magneto-optical disc. More particularly, it relates to an optical pickup apparatus in which a light beam radiated from a light source is radiated via an object lens to an information recording medium and in which the return light beam reflected from the information recording medium is detected by light detection means for reading out the information signals recorded on the information recording medium. Description of the Prior art An information recording medium for writing information signals by optical means has so far been proposed. A magneto-optical disc, typical of this type of the information recording medium, is made up of a disc substrate, and a signal recording layer. The signal recording layer is formed of a material having its direction of magnetization inverted by heating it to a temperature higher than the Curie temperature and applying a corresponding magnetic field. The optical pickup apparatus is employed for writing and reading information signals on or from the information recording medium. Referring to Fig.2, the optical pickup apparatus is made up of a laser diode device 1, as a light source, and an object lens 9, adapted for converging the light flux radiated from the laser diode element 1 on the signal recording layer. The optical pickup apparatus is also provided with a photodetector 12 for detecting the light flux converged on and reflected from the signal recording layer. The laser diode device 1 is provided with a can-shaped package and radiates light by being supplied from plural terminals. The laser device 1 includes a semiconductor chip arranged in the package and radiates light from a terminal end of an active layer of the semiconductor chip. The laser diode device is a point-shaped light source. The light flux radiated from the laser diode device 1 is collimated by a collimator lens 7 before being incident on a polarized beam splitter 5. The object lens 9 converges the light flux on a signal recording surface which is a boundary surface between a signal recording layer 21 of a magneto-optical disc 23 and a disc substrate 20. On the signal recording surface, the information signals are written on a spirally extending substantially concentric recording tracks. The light flux converged on the signal recording surface is reflected by the signal recording surface so as to be re-incident as a reflected light flux on the object lens 9. The reflected light flux is collimated before being re-incident on the polarized beam splitter 5. The reflected light flux re-incident on the polarized beam splitter 5 is reflected for the most part by a semi-transmitting film 6 of the polarized beam splitter 5 so as to be incident on a three-beam Wollaston prism 11. The semi-transmitting film 6 of the polarized beam splitter 5 is so designed that, of the reflected light flux, about 95% of the S-polarized light is reflected by the semi-transmitting film 6 and about 80% of the P-polarized light component is transmitted through the semi-transmitting film 6. The reflected light flux incident on the three-beam Wollaston prism is transmitted through the prism 11 so as to be converged by a condenser lens 22 on a light receiving surface of the photodetector 12. The photodetector 12 has plural light receiving surfaces and is provided within a package. The photodetector 12 is designed to output the intensity of the fluxes of the light received by the light receiving surfaces as electrical signals. The electrical signals outputted from the photodetector 12 are signals which have detected changes in the light volume of the reflected light flux, changes in the polarized light or changes in the extent of the astigmatism. For writing information signals on the magneto-optical disc 23, the optical pickup device converges the flux of light radiated from the laser diode device 1 on the signal recording layer 21 for locally heating the signal recording layer 21. At this time, an external magnetic field is applied to the signal recording layer 21 by a magnetic device 17, as shown in Fig.2. The state of light flux convergence on the signal recording layer 21 and adjustment of the position of radiation of the lignt flux on the signal recording layer 21 may also be adjusted at this time based on the various information produced on the basis of electrical signals outputted from the photodetector 12. The information signals may be read by the optical pickup device from the magneto-optical disc 23 on the basis of the information derived from the photodetector device 12 while the light flux radiated by the laser diode element is converged and radiated on the signal recording layer 21. Meanwhile, in the above-described optical pickup device, the polarized beam splitter 5 is arranged at a point on a light path between the laser diode device 1 and the object lens 9 where the light flux becomes a parallel light flux. The three-beam Wollaston prism 11 is also so arranged that the parallel light flux reflected by the reflective film 6 of the polarize beam splitter 5 is incident on the prism 11. It is because the polarized beam splitter 5 arranged in the converging light flux or in the divergent (diffused) light flux tends to produce double refraction at the reflective film 6 of the polarized beam splitter 5. If double refraction should be produced at the reflective film 6, the so-called CN ratio is lowered to render it impossible to read out information signals from the magneto-optical disc 23 satisfactorily. The double refraction is produced because the semi-transmitting film 6 of the polarization beam splitter 5 has incident angle dependent incident light flux transmittance so that phase differences are produced at the semi-transmitting film 6 between the P-polarization component and the S-Polarization component of the converging and divergent light fluxes. In the read-only optical pickup apparatus, for reduce the size and the structure of the apparatus, the beam splitter is placed at a position on a light path of the reflected light flux from the optical disc at which the reflected light flux is the converging light flux. If the beam splitter is arranged in the converging light flux, the void space between the laser diode device and the collimator lens 7 may be utilized efficiently and the converging lens 22 may be eliminated to render it possible to reduce the size and the structure of the optical pickup apparatus. In other words, the fact that the polarized beam splitter 5 has to be provided in the collimated light flux is hindersome if a simpler structure and a smaller size of the optical pickup apparatus should be achieved. There is also disclosed in US Patent US-A-4,771,414 an optical pickup apparatus in which a three-beam Wollaston prism is used for introducing a light beam reflected from a signal recording surface of a magneto-optical disc into a photodetector. In the published Japanese patent application (publication number JP-A-3084745) an optical pickup apparatus is described, in which a beam splitter for separating the converged light being reflected from the surface of a magneto-optical disc to direct the reflected light onto light detecting means is provided. In order to reduce the amount of divergence and convergence of the light falling onto the beam splitter, however, a condenser lens is positioned between the light source of the optical pickup device and the beam splitter. In the European patent application EP-A-0 439 876 A1 an optical pickup apparatus is described in which the light beam emitted from a coherent light source and reflected from the reflective surface of a disc-shaped recording medium passes through a ¼-wavelength plate and falls as convergent beam onto a beam splitter having two beam separation surfaces. Dependent on the polarization part of the reflected convergent beam is reflected onto a first light detector and another part is reflected onto a second light detector. OBJECTS AND SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical pickup apparatus which renders it possible to record information signals on an information recording medium with satisfactory recording characteristics. It is another object of the present invention to provide an optical pickup apparatus which renders it possible to simplify its construction and to reduce its size. In accordance with the present invention, there is provided an optical pickup apparatus for recording information signals on an optical recording medium, as defined in claim 1. The above and other objects of the present invention will become apparent from the following description. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 2 is a schematic side elevational view showing a conventional optical pickup apparatus. Fig.1 is a schematic plan view showing the construction of an optical pickup apparatus according to the present invention. Fig.3 is a schematic plan view showing the construction of a modification of an optical pickup apparatus according to the present invention. Fig.4 is a schematic side elevational view showing the construction of the optical pickup apparatus shown in Fig. 3. Fig.5 is a plan view showing a light receiving surface of a photodetector of the optical pickup apparatus shown in Figs.1 to 4. Figs.6A to C are graphs showing characteristics of a polarized beam splitter constituting the optical pickup apparatus where Fig.6A shows the relation between the incident angle and the transmittance of the light flux of the P-polarized light, Fig.6B shows the relation between the incident angle and the reflectance of the light flux of the S-polarized light and Fig. 6C shows the relation between the incident angle and the phase difference of the light flux. Fig.7 is a schematic side elevational view showing a further modification of the optical pickup apparatus according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig.1, the optical pickup apparatus according to the present invention includes a laser diode 2, as a light source, and an object lens 9, for converging the light flux radiated by laser diode 2 on a magneto-optical disc as an information recording medium. The optical pickup apparatus also includes a photodetector 13 for detecting the light flux converged on and reflected back from the magneto-optical disc. The laser diode 2 is arranged in a can-shaped package 1 and fed with current via plural terminals 3 to emit the light. Laser diode 2 is arranged as a semiconductor chip having an active layer the end of which radiates the flux of light. The laser diode 2 is a substantially point-shaped light source. The light flux radiated from the laser diode 2 is separated into three parallel light fluxes having different proceeding directions by being transmitted through a diffraction grating 4 before being incident on a polarization beam splitter 5 operated as a light flux splitting device. About 63% of the light flux is transmitted through the semi-transmitting film 6 so as to be incident on the collimator lens 7. The light fluxes incident on the collimator lens 7 are collimated by the collimator lens 7. The collimated light flux is reflected by about 90° in its light path by reflection by a planar reflective mirror 8 before being incident on an object lens 9. The object lens 9 converges the light fluxes on the signal recording surface which is the boundary between the signal recording layer of the magneto-optical disc and the disc substrate. On the signal recording surface, the information signals are written along a spirally extending substantially concentric recording track. The light fluxes separated by the diffraction grating 4 are radiated on the signal recording surface so as to be astride a recording track for executing a tracking error detection system known as a so-called three-beam system. The positional deviation between the radiated position of the main central beam and the recording track may be detected by comparing the volumes of reflected light of both side beams by the signal recording surface. The light fluxes converged on the signal recording surface are reflected by the signal recording surface so as to be re-incident as reflected light fluxes on the object lens 9. The reflected light fluxes thus re-incident on the object lens 9 are substantially collimated so as to be re-incident on the collimator lens 7 via the planar reflective mirror 8. The reflected light fluxes re-incident on the collimator lens 7 are converged as converged light fluxes which are again incident on the polarized beam splitter 5. The reflected light flux, which is the converged light flux re-incident on the beam splitter 5, is substantially in the state of S-polarized light at the semi-transmitting film 6 of the polarized beam splitter 5. Referring to the graph of Fig.6A, as for the light flux incident on the polarized beam splitter 5 at an incident angle of 0°, that is at a right angle, and on the semi-transmitting film 6 at an angle of 45°, about 68% of the P-polarized light component is transmitted through the semi-transmitting film 6. Referring to the graph of Fig.6B, as for the light flux incident on the polarized beam splitter 5 at an incident angle of 0°, about 67% of the S-polarized component is reflected by the semi-transmitting film 6. In the graphs of Figs.6A to 6C, curves a , b , c , d and e indicate characteristics for the wavelengths of 770 nm, 780 nm, 790 nm, 800 nm and 810 nm of the incident light fluxes, respectively. The reflectance of the semi-transmitting film 6 has only low dependency on the direction of polarization, the reflectance has only low dependency on the angle of incidence of the light flux as shown in the graph of Fig. 6B. Consequently, even although the reflected light fluxes incident on the polarized beam splitter 5 are converging light fluxes, double refraction is not produced in the reflected light fluxes. That is, the phase difference between the P-polarized light component and the S-polarized light component at the semi-transmitting film 6 is not increased despite fluctuations in the incident angle, as shown in the graph of Fig.6C. The reflected light flux reflected by the semi-transmitting film 6 is guided outside of the light path between the laser diode 2 and the object lens 9 so as to be transmitted through a cylindrical lens 10 before being incident on a three-beam Wollaston prism 11 which is a polarization-dependent light flux separating device. The cylindrical lens 10 is a lens having a cylindrical surface producing astigmatism in the light flux transmitted therethrough. Although the cylindrical lens 10 may be planar on its one side and cylindrical on its other side, the abovementioned one side may be convex or concave depending on the position of the photodetector 13. The three-beam Wollaston prism 11 is formed by a pair of triangular prisms, each of which is a uniaxial crystal of quartz, rutile or calcite and which are bonded to one another to form a rectangular-shaped prism. The directions of crystal axes of the triangular prisms are normal to the optical axes and cross-linked at an angle of 45° with one another. The reflected light fluxes incident on the three-beam Wollaston prism 11 are incident substantially at right angles to the outer lateral sides of the three-beam Wollaston prism 11 so as to be obliquely transmitted through the junction surface of the two triangular prisms. Consequently, the reflected light flux is refracted in different directions, depending on the direction of polarization, when transmitted though the junction surface of the triangular prism, so that the reflected light flux is separated into three light fluxes by the prism 11. That is, the reflected light flux incident on and transmitted through the three-beam Wollaston prism 11 undergoes astigmatism by the cylindrical lens 10, at the same time that it is separated into three light fluxes having different proceeding directions depending on the difference in the direction of polarization. The reflected light fluxes transmitted through the three-beam Wollaston prism 11 are converged on the photodetector 13. Referring to Fig.5, the photodetector 13 is composed of first to eighth light-receiving sections 13a, 13b, 13c, 13d, 13e, 13f, 13g and 13h, and is contained in a package 12. The photodetector 13 is adapted for converting the light intensity of light fluxes received by the light-receiving sections 13a to 13h into light detection signals as electrical signals which are outputted at plural terminals 14 associated with the light-receiving surfaces 13a to 13h. The light fluxes converged on the photodetector 13 are the light fluxes separated by the diffraction grating 4 and reflected so as to be separated by the three-beam Wollaston prism 11 in accordance with the direction of polarization. Light detection signals corresponding to the light intensities of these light fluxes may be produced from output signals of the photodetector 13. The first to fourth light-receiving surfaces 13a to 13d are arranged in adjacency to one another in the manner of radially dividing the central one of the reflected light fluxes, that is the sole light-receiving surface arranged on the optical axis of the main beam, into four equal parts with the point of intersection of the sole light-receiving surface with the optical axis as a center. These first to fourth light-receiving surfaces 13a, 13b, 13c and 13d are arranged so that the direction passing through the first and third light receiving surfaces 13a, 13c and the direction passing through the second and fourth light-receiving surfaces 13b, 13d are aligned with the direction of the astigmatism produced by the cylindrical lens 10. The fifth and sixth light-receiving surfaces 13e, 13f are sized to contain the first to fourth light-receiving surfaces 13a to 13d and are arranged on both sides of the first to fourth light-receiving surfaces 13a to 13d. These fifth and sixth light-receiving surfaces 13e and 13f on one hand and the first to fourth light-receiving surfaces 13a to 13d correspond to the points of light convergence of the light fluxes separated by the diffraction grating 4 and reflected by the half transmitting film 6. The seventh and eighth light-receiving surfaces 13g, 13h are sized to contain the first to fourth light-receiving surfaces 13a to 13d and are arranged on both sides of the first to fourth light-receiving surfaces 13a to 13d with the direction of interconnecting the light-receiving surfaces 13g, 13h being at right angles to the direction interconnecting the fifth and sixth light-receiving surfaces 13e, 13f. These seventh and eighth light-receiving surfaces 13g and 13h on one hand and the first to fourth light-receiving surfaces 13a to 13d on the other hand correspond to the points of light convergence of the light fluxes reflected by the half-transmitting film 6 and separated by the diffraction grating 4. With the present light detection means, changes in the light volume of the reflected light flux, changes in the state of polarization or changes in the degree of astigmatism, may be detected on the basis of the above-mentioned light detection signals. That is, with the above light detection signals, difference signals of light detection signals corresponding to the light fluxes separated by the three-beam Wollaston prism 11, that is G - H, where G is the light detection signal by the seventh light-receiving surface 13g and H is the light detection signal by the eighth light-receiving surface 13h, represents a readout signal of the information signals written on the magneto-optical disc. On the other hand, with the above light detection signals, difference signals of light detection signals corresponding to the light fluxes separated by the diffraction grating 4, that is E - F, where E is the light detection signal by the fifth light-receiving surface 13e and F is the light detection signal by the sixth light-receiving surface 13f, represents a so-called tracking error signal indicating the deviation of the main beam from the recording tack. Besides, the light flux received by the light-receiving surface undergoes astigmatism corresponding to defocusing of the light fluxes on the signal recording surface of the magneto-optical disc by being passed through the cylindrical lens 10. Consequently, the difference between the sum signals of the light detection signals by the light-receiving surfaces arrayed in the direction along which the astigmatism is produced and the sum signals of the light detection signals by the light-receiving surfaces arrayed in the direction normal to the direction along which the astigmatism is produced, that is (A + C) - (B + D), where A, B, C and D indicate the light detection signals by the first to fourth light-receiving surfaces 13a to 13d, respectively, represents a so-called focusing error signal indicating the degree of defocusing produced on the signal recording surface of the magneto-optical disc. In the above-described optical pickup apparatus, the relative position between the cylindrical lens 10 and the three-beam Wollaston prism 11 may be exchanged, as shown in Figs.3 and 4. In this case, the light fluxes reflected by the reflective film 6 of the polarized beam splitter 5 are transmitted through the three-beam Wollaston prism 11 and the cylindrical lens 10 in this sequence before being converged on the light-receiving surface of the photodetector 13. When writing information signals on the magneto-optical disc by the optical pickup apparatus, the light flux radiated by the laser diode 2 is converged on the signal recording surface which is the boundary surface between the disc substrate 20 and the signal recording layer 21 of the magneto-optical disc 23 for locally heating the signal recording surface 21, as shown in Fig.4. Simultaneously, an external magnetic field is applied by the magnetic head device 17 on the signal recording layer 21. The object lens 9 is driven so as to be moved depending on various control signals, such as focusing error signals or tracking error signals produced by the photodetector 13 for adjusting the state of convergence of the light fluxes on the signal recording layer 21 or the position of radiation of the light fluxes on the signal recording layer 21. The information signals may be read by the optical pickup apparatus from the magneto-optical disc 23 on the basis of signals obtained from photodetector 13 while the light flux radiated by the laser diode 2 is converged on the signal recording layer 21. The optical pickup apparatus according to the present invention is not limited to the above-described embodiments. Thus, as shown in Fig. 7, instead of the polarized light beam splitter 5 a planar plate 15 carrying a semi-transmitting film 16 thereon may be arranged between the object lens 9 and the photodetector 13 at an inclined position with respect to the light path. That is, in the present optical pickup apparatus, the light flux radiated from the laser diode 2 is first transmitted through the diffraction grating 4 and reflected by the semi-transmitting film 16 of the planar plate 15 before being incident on the object lens 9. The light flux is converged by the object lens 9 on the signal recording surface which is the boundary surface between the disc substrate 20 and the signal recording surface 21 of the magneto-optical disc 22. The light fluxes converged on the signal recording surface is reflected by the signal recording surface before being incident as a reflected light flux on the object lens 9. The reflected light flux re-incident on the object lens 9 is re-incident on the semi-transmitting film 16 in the form of a converged light flux. The reflected light flux re-incident on the semi-transmitting film 16 is partially transmitted through the semi-transmitting film 16 before being incident on the three-beam Wollaston prism 11. The reflected light flux transmitted through the three-beam Wollaston prism 11 while being converged is finally converged on the light-receiving surface of the photodetector 13. In the optical pickup apparatus, shown in Fig.7, the reflected light flux undergoes astigmatism when transmitted through the planar plate 15, so that there is no necessity of providing a cylindrical lens such as is provided in the optical pickup apparatus of the preceding embodiment. The object lens of the optical pickup apparatus has so-called finite system characteristics so that there is no necessity of providing a collimator lens such as that used in the optical pickup apparatus of the preceding embodiment. In the above-described optical pickup apparatus according to the present invention, the light flux led out of a light path between the light source radiating a divergent light flux and the object lens converging the light flux on the information signal recording medium by a light flux dividing device arranged at a position on the light path at which the light flux remains to be a divergent light flux, is a light flux converged on and reflected by the information signal recording medium, and is a converging light flux. Consequently, the reflected light flux is transmitted through the polarized light dependent light flux dividing device while being converged so as to be ultimately converged on a photodetector adapted for detecting the reflected light flux. Consequently, with the present optical pickup apparatus, the void space between the light source and the collimator lens collimating the light flux radiated from the light source may be utilized effectively. Besides, with the present optical pickup apparatus, it is unnecessary to provide a converging lens at the light flux incident side of the photodetector. Meanwhile, in the optical pickup apparatus according to the present invention, the light flux reflectance in the light flux separating device may be properly set for preventing double refraction in the light flux separating device.

Claims: An optical pickup apparatus for recording information signals on an optical recording medium (23), comprising a light source (2) radiating a diverging light beam, an object lens (9) on which the light beam radiated from said light source (2) is incident and which is adapted for converging the light beam on said optical recording medium, a polarized beam splitter (5) arranged at a position along a first optical path between the light source (2) and the object lens (9) at which said polarized beam splitter (5) receives the diverging light beam from the light source (2) before said diverging light beam has propagated through the object lens (9), wherein said polarized beam splitter (5) deflects a reflected light beam that has converged on said optical recording medium (23) and been reflected by said optical recording medium (23) out of said first light path to a second light path, light detecting means (13) for detecting reflected light beam deflected to the second light path by the polarized beam splitter (5) a prism (11) for generating three light beams, said prism (11) optical element being arranged between the polarized beam splitter (5) and the light detecting means (13) at a converging point of said reflected light beam on the second light path, characterized in that said polarized light beam splitter (5) is adapted for transmitting about 68% of a P-polarized light component of a light flux incident on said polarized beam splitter (5) at a right angle thereto and on a multi-layer dielectric film (6) of said polarized beam splitter (5) at 45° thereto, while reflecting about 67% of an S-polarized light component of the incident light flux. The optical pickup apparatus according to claim 1, further comprising a lens (7) provided between said polarized beam splitter (5) and the object lens (9) for collimating the diverging light beam from said light source (2) into a parallel light beam. The optical pickup apparatus according to any one of claims 1 or 2 further comprising an optical element (10) provided between said polarized beam splitter (5) and said light detection means (13) for producing changes in the contour of a spot on said light detection means (13) with in-plane displacement of said optical recording medium (23). The optical pickup apparatus according to any one of claims 1 to 3, wherein said prism (11) is an optical element made up of two prisms of uniaxial optical crystals which are connected to each other with a multi-layer dielectric film in-between so that the optical axes of the prisms form an angle of 45° relative to each other. The optical pickup apparatus according to any one of claims 1 to 4, wherein said polarized beam splitter (5) is arranged at an inclination of 45° relative to the optical axis of the light beam extending from said object lens (9) to said photodetector (13). The optical pickup apparatus according to any one of claims 1 to 5, further comprising a cylindrical lens (10) arranged on a light path between said polarized beam splitter (5) and said light detection means (13) with its optical axis inclined by 45°. The optical pickup apparatus according to any one of claims 1 to 6, further comprising a deflecting device (8) arranged between the polarized beam splitter (5) and the object lens (9), for deflecting an optical path of each beam incident thereon from the polarized beam splitter (5) by an angle substantially equal to 90 degrees. The optical pickup apparatus according to any one of claims 1 to 6, characterized in that the three light beams generated by said prism (11) each having a different polarization in response to the reflected light beam from the polarized beam splitter (5).

Inventors: ANDO NOBUHIKO, ASOMA YOSHITO, HINENO SATOSHI, ANDO, NOBUHIKO, ASOMA, YOSHITO, HINENO, SATOSHI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: G11B11
IPC codes: G11B7, G11B11