Optical pickup

Abstract: An optical pickup includes a light source (21) for radiating a light beam, a diffraction element (22) for separating a light beam radiated from the light source into at least three beams, namely a main beam and two side beams, an objective lens (25) for converging the light beams separated by the diffraction element onto a signal recording surface of an optical recording medium (11), a light receiving unit (24) having a four-segment first light receiving portion (71) for receiving the main beam reflected by the recording surface of the optical recording medium and second and third light receiving portions (72, 73) arranged on respective sides of the first light receiving portion (71) for receiving the side beams reflected by the recording surface of the optical recording medium, and a calculation unit (26) for generating one tracking signal based on respective outputs of the first light receiving portion and for generating another tracking signal based on outputs of the second and third light receiving portions. In preferred emodiments of the apparatus, a disc discrimination unit sends a detection output of the optical disc both to the changeover switch selecting the desired tracking error signal and to an aperture ratio variable control unit.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an optical pickup for recording on and/or reproducing from an optical recording medium, to a reproducing apparatus and to a recording apparatus for an optical recording medium. More particularly, it relates to an apparatus which is capable of recording on and/or reproducing from plural sorts of optical discs having different values of track pitch, by one and the same apparatus. Description of the related Art Heretofore, in an apparatus for reproducing from an optical disc, such as a compact disc, a three-beam method has been in use as a system for detecting tracking error signals. This system uses a diffraction grating to split a light beam radiated by a semiconductor laser element into three beams, namely a main beam and two side beams. The main beam is radiated onto a recording track of an optical disc, while the side beams are radiated onto respective positions of the disc which are offset by one-fourth of a track to either side of the recording track. The light beams radiated onto the optical disc are reflected by the recording surface of the recording medium so as to be received by a photodetector. This photodetector is made up of a first photodetector portion for receiving a main beam and second and third photodetector portions for receiving the side beams. The tracking error signal is detected by finding the difference between the signals received by the second and third photodetector portions. Recently, a particular type of optical disc has been investigated in which data may be recorded at a high density for recording data of high precision, such as still pictures or moving pictures. With such an optical disc, it may be contemplated to set the track pitch to approximately 0.8 µm instead of to the value of 1.6 µm which is conventionally used, or to form a recording layer of a narrower track pitch as multiple layers. The optical disc, recorded at a high density, is not limited to a replay-only type, but may be a rewritable optical disc, such as a phase change type disc. A disc having a guide groove may also be envisaged as such a rewritable optical disc. However, with the optical disc for high density recording, it is difficult to detect tracking error signals by the above-mentioned three-beam system. That is, since the track pitch is of a narrow width, registration of the side spots of the three spots radiated on the recording surface of the optical disc becomes difficult. Also, if the high-density recording layer is formed as multiple layers, a problem is caused in that an offset is produced in the tracking error signal due to leakage of the reflected light from the layer other than the layer being recorded or reproduced. In addition, if the rewritable optical disc is a phase change type disc, and recorded and unrecorded portions are produced on the disc, noise is produced with the three-beam method due to differences in reflectance in the recorded and unrecorded portions, thus making it difficult to detect correct tracking error signals. Various published documents describe disc playing/recording apparatus using the three-beam method together with three light-receiving elements. Amongst these documents, US patent 4,773,052 describes a dual mode optical disk player/recorder in which there is a first, four-part, light-receiving element, and second and third light-receiving elements. A tracking error signal is generated based on the difference between the signals output by the second and third light-receiving elements. In the US-A-4773052 system, disks having different track pitch are accommodated by altering the distance between the disk and the light emitting/receiving elements. EP-A-0 392 775 describes such disc playing/recording apparatus including three, two-part light receiving elements. Two tracking error signals are generated based, respectively, on the difference between signals output by the central light-receiving element and signals output by each of the side light-receiving elements. No means are contemplated for enabling discs of different track pitch to be read by a common apparatus. EP-A-0 459 420 describes such disc playing/recording apparatus in which n pairs of light beams are used during reading/writing of the disc, in association with 2n light-receiving elements. Tracking error signals are generated based on the difference between the signals output from two non-neighbouring light-receiving elements, and these error signals are combined to give a single output. No means are contemplated for enabling discs of different track pitch to be read by a common apparatus. EP-A-0 704 841, which is mentioned with respect to A.54 (3) EPC, describes such disc playing/recording apparatus in which there is a first, four-part, light-receiving element, and second and third, two-part, light-receiving elements. A tracking error signal is generated based on a combination of signals from all of these light-receiving elements, or based on the signals from the first light-receiving element only, or based on a combination of signals output by the second and third light-receiving elements. However, in EP-A-0 704 841 there is no discussion of any mechanism for selection of the tracking error signal to be used, and disks having different track pitch are accommodated by altering the separation between the beam spots. OBJECT AND SUMMARY OF THE INVENTION In view of the above-depicted state of the art, it is an object of the present invention to provide an apparatus capable of performing selective recording and/or reproduction on or from plural sorts of optical discs, such as optical discs with different track pitches. An optical pickup according to the present invention comprises: a light source for radiating a light beam; an element for separating a light beam radiated from said light source into at least three beams, namely a main beam and two side beams; an objective lens for converging the light beams separated by said element on a signal recording surface of an optical recording medium; and a light receiving unit having an at least two-segment first light receiving portion for receiving said main beam reflected by the recording surface of the optical recording medium and second and third light receiving portions arranged on both sides of said first light receiving portion for receiving the side beams reflected by the recording surface of the optical recording medium; error calculation means for generating a first tracking signal based on outputs of said second and third light receiving portions; and means for discriminating the sorts of the optical discs; characterised in that: the light beam is separated into at least three beams using a diffraction element; the calculation means is adapted for generating a second tracking error signal based on signals comprising respective outputs of said first light receiving portion; and there is provided switching means for selecting for output, based on a signal from said discrimination means, one of the tracking error signals calculated by said calculation means. A tracking servo system in an optical disc recording and/or reproducing apparatus according to the present invention includes the above-mentioned optical pickup and means for driving an objective lens based on the tracking error signal selected by said switching means. The discrimination unit discriminates the type of disc from among at least two sorts of optical discs having different track pitches, and the signal processing unit is responsive to the results of the discrimination to switch the calculation operations for obtaining a tracking error signal from a detection signal of the photodetector unit, so that at least two sorts of optical disc can be reproduced by simplified adjustment operations. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 illustrates schematically the structure of an optical disc recording and/or reproducing apparatus embodying the present invention. Fig.2 illustrates schematically the structure of an optical pickup embodying the present invention. Figs.3 and 4 illustrate a bi-axial mechanism of an objective lens, that may be used in the optical pickup of Fig. 2. Fig.5 illustrates a first embodiment of a tracking servo system according to the present invention. Figs.6A and 6B illustrate the state of radiation of three beams on signal pits of an optical disc. Fig.7 illustrates a second embodiment of a tracking servo system according to the present invention. Fig.8 illustrates a third embodiment of a tracking servo system according to the present invention. Fig.9 illustrates schematically the structure of an optical disc-recording and/or reproducing apparatus having an aperture ratio varying unit according to an embodiment of the present invention. Figs.10A and 10B are perspective views showing a light shielding ring used as the aperture ratio varying unit of Fig.9 and a movement mechanism for the light shielding ring. Figs. 11A, 11B show the manner in which the aperture ratio is varied by the light shielding ring shown in Figs.10A and 10B. Fig. 12 is a graph showing the relationship between the aperture ratio of the objective lens and the spatial frequency. Fig.13 is a perspective view showing a light shielding plate used as the aperture ratio varying unit of Fig.9 and a movement mechanism for the light shielding plate. Fig.14 illustrates an objective lens and a movement mechanism for the objective lens, in which the objective lens has lens portions having different aperture ratios and used as the aperture ratio varying unit of Fig.9. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, preferred embodiments of an optical pickup device according to the present invention will be explained in detail. Fig.1 shows, schematically, an optical disc reproducing apparatus according to the present invention. An optical disc reproducing apparatus 10 is a so-called compatible optical disc reproducing apparatus capable of reading out and reproducing information signals both from an optical disc having a track pitch of 1.6 µm and a substrate thickness of 1.2 mm, such as a compact disc, and from a double-layer optical disc 11 having a track pitch of approximately 0.8 µm and having two information signal layers 11a, 11b oriented in the same read-out direction. An optical pickup device 13 radiates a laser light beam to these optical discs having different track pitches and different substrate thicknesses, for reproducing information signals from tracks formed on the information signal layers. Referring to Fig.2, the optical pickup device 13 includes a light source 21 radiating a laser beam, such as a laser diode, and an objective lens 25 for focusing the laser beam on the information signal layers of plural sorts of optical discs having different track pitches, of which only the double-layer optical disc 11 is shown in Fig.2. The double-layer optical disc 11 shown in Fig.2 has a first information signal layer 11a and a second information signal layer 11b. The optical pickup device 13 also includes a photodetector 24 for receiving the light reflected from the optical disc for converting it into electrical signals, and a disc discrimination unit 27 for discriminating the sort of the optical disc. The optical pickup device 13 further includes a detection signal processor 26 responsive to the optical disc sort discriminated by the disc discrimination unit 27 to switch the calculation operations for calculating tracking error signals from the detected signals of the photodetector 24 for producing tracking error signals, in addition to a focusing error signal and main playback signals. The tracking error signals and the focusing error signal, obtained by the detection signal processor 26 of the optical pickup device 13, are supplied to a servo circuit 16 of Fig.1. The servo circuit 16 manages tracking control and servo control responsive to these signals. Specifically, a focusing driving signal is applied in the optical pickup unit 13 to a bi-axial mechanism 20 holding the objective lens 25, for driving the objective lens 25 in a direction into and out of contact with, for example, the double-layer optical disc 11, for managing focusing control. Also, a tracking driving signal is applied to the bi-axial mechanism 20 for driving the objective lens 25 radially of, for example, the double-layer optical disc 11, for managing focusing control. A thread driving signal is generated by extracting low-frequency components of the tracking error signal for driving a thread mechanism for moving the optical pickup device 13 in its entirety radially of, for example, the double-layer optical disc 11. The main playback signal, obtained by the detection signal processor 26, is processed with demodulation for EFM and CIRC decoding into playback digital data which is then converted by a D/A converter 14 into an analog signal which is outputted at an output terminal 15. The servo circuit 16 controls rotation of a spindle motor 18 based on clock signals obtained from the main playback signals. The detailed structure and operation of the optical pickup device 13 will now be explained. Referring to Fig.2, a diffused laser beam, radiated by the optical source 21, is diffracted by a diffraction grating 22 and thereby separated into three beams, namely a 0 order beam and ± 1 order beams. The laser beams, diffracted by the diffraction grating 22, are reflected by a beam splitter 23 and collimated by a collimator lens 19 so as to enter the objective lens 25. The objective lens 25 is tracking- and focusing-controlled by the bi-axial mechanism 20 for converging the laser beams onto information signal layers of the optical disc, such as the first information signal recording layer 11a and the second information signal layers 11b of the double-layer optical disc 11, for forming three spots. The three reflected laser beams from the first information signal recording layer 11a and the second information signal layers 11b of the double-layer optical disc 11 reach a light receiving surface of the photodetector 24 via objective lens 25 and beam splitter 23. The bi-axial mechanism 20 is of an axial sliding type as shown in Figs.3 and 4, wherein a movable part 30 is constituted by a bobbin 30A formed of a nonmagnetic material. At a mid position of the bobbin 30A is formed an axially fitted tubular bearing 31. On the outer peripheral surface of the bobbin 30A is wound a focusing coil 32 for forming a ring about the bearing 31. The focusing coil 32 is used for moving the movable part 30 in the focusing direction, that is in a direction perpendicular to the disc surface. On the surface of the focusing coil 32 are formed two sets of tracking coils 33A, 33B in tight contact with the focusing coil 32 for moving the movable part 30 in the tracking direction, that is along the radius of the disc. These tracking coils 33A, 33B are wound about an axis extending perpendicular to the winding axis of the focusing coil 32 so that four rings are formed on the outer peripheral surface of the bobbin 30A. The mid portion of the bi-axial mechanism 20 is traversed by a supporting shaft 39. A stepped hole 43 is formed parallel to the center axis of the supporting shaft 39 in the bobbin 30A at an offset position with respect to the supporting shaft 39. Within this hole 43 is mounted a lens barrel 35 within which is secured the objective lens 25. The movable part 30, thus constructed, has the supporting shaft 39 set upright at a mid portion of a stationary yoke 38 of a magnetic material, and guided and introduced into a center hole of the bearing 31, so that the movable part 30 is supported for sliding along and for rotation about the supporting shaft 39. On the lower surface of the stationary yoke 38 is secured an annular permanent magnet 40 in intimate contact therewith about the supporting shaft 39 as center. On the lower end face of the permanent magnet 40 is secured a first yoke 42 having a lug 41. On the stationary yoke 38 is protuberantly formed a second yoke 44 for facing the lug 41 of the first yoke 42 on the inner side of the bobbin 30A. The stationary yoke 38, permanent magnet 40, the first yoke 42 and the second yoke 44 make up a magnetic circuit. The focusing coil 32 and the tracking coils 33A, 33B are arranged in a magnetic gap defined between the first yoke 42 and the second yoke 44. The stationary yoke 38 has the hole 43 larger in diameter than the outer diameter of the lens barrel 35 held by the bobbin 30A. Into this hole 43 is guided and introduced an upper end of the lens barrel 35. Figs.5 and 6 show a first embodiment of tracking servo which may be used in optical disc recording and/or reproducing apparatus for recording and/or reproducing data from plural discs having different track pitches. With the present first embodiment of the optical disc recording and/or reproducing apparatus, it is possible to reproduce an optical disc having a substrate thickness of 1.2 mm and a track pitch of 1.6 µm, an optical disc having a substrate thickness of 1.2 mm and a track pitch of 0.8 µm and an optical disc having a substrate thickness of 0.6 mm and a track pitch of 0.8 µm. It is in addition possible to record a rewritable phase-change type optical disc having a substrate thickness of 0.6 mm and a track pitch of approximately 0.8 µm. Referring to Fig.5, the optical disc recording and/or reproducing apparatus of the first embodiment has a light receiver 24 for receiving the light beam radiated from the light source 21 and reflected from the signal surface of the optical disc 11 via the objective lens 25, collimator lens 19 and the beam splitter 23 after irradiation of the optical disc 11 via the diffraction grating 22, beam splitter 23, collimator lens 19 and the objective lens 25, and a signal detector 26 for generating two sorts of tracking error signals based on a light volume detection signal from the receiver 24. The optical disc recording and/or reproducing apparatus also includes a disc discrimination unit 27 for discriminating the sorts of the optical discs recorded or reproduced, a changeover switch 28 for selecting between the tracking error signals generated by the signal detector 26, and an objective lens driving unit 29 for driving the objective lens 25 based on the selected tracking error signal. The light receiver 24 has first to third light receivers 51 to 53 for receiving the light beams split by the diffraction grating 22 into three portions and reflected by the optical disc 11. The first light receiver 51 receives the main beam (0-order light beam) of the three split light beams and is divided into at least two areas of A1 and B1. The second and third light receivers 52, 53 receive two side beams (±1 order light beams) of the three split light beams and each is divided into two areas, respectively E1, G1, and F1, H1. Of the outputs of the receivers 51 to 53, the outputs E1, G1 of the receiver 52 and the outputs F1, H1 of the receiver 53 are summed respectively at adders 54 and 58, outputs of which are supplied to a comparator 61 for generating a first tracking error signal. The outputs E1, G1 of the receiver 52 are supplied to a comparator 55, while the outputs F1, H1 of the receiver 53 are supplied to a comparator 57. A difference output of the comparator 55 and a difference output of the comparator 57 fed via a variable gain amplifier 59 are summed together and the resulting sum output is supplied via a further variable gain amplifier 60 to a comparator 62. A difference output of the comparator 62 gives a second tracking error signal. Figs.6A and 6B illustrate the state of irradiation of the discs of different track pitches with the three split light spots. Fig.6A illustrates an example of an optical disc having a track pitch of approximately 0.84 µm, in which side beams are illuminated at positions which are offset by one-half the track pitch with respect to the main beam. Fig.6B shows an example of an optical disc having a track pitch of 1.6 µm in which side beams are illuminated at positions which are offset by one-fourth the track pitch with respect to the main beam. With the optical disc of Fig.6A, since the track pitch is one-half that of the optical disc of Fig.6B, offset due to optical axis deviation of the objective lens 25 or offset due to tilt of the optical disc are produced frequently. If an optical disc is judged by the disc discrimination unit 27 to be an optical disc having a track pitch of 1.6 µm, a first tracking error signal, which is a differential output of the comparator 61, is selected. On the other hand, if an optical disc is judged by the disc discrimination unit 27 to be an optical disc having a track pitch of 0.8 µm, a second tracking error signal, which is a differential output of the comparator 62, is selected. By detecting the tracking error signals as described above, a three-beam method is applied to reproduction of a compact disc, while a differential push-pull method, removing unneeded offset, is applied to reproduction of a high recording density replay-only disc or a recording/reproducing disc. Thus, a compatible optical disc recording and/or reproducing apparatus may be realized by employing a common optical system and by simply switching the calculation operations. Fig.7 shows a second embodiment of tracking servo for use in apparatus for recording and/or reproducing plural discs having different track pitches. The optical disc recording and/or reproducing apparatus in the present second embodiment is similar to that of the first embodiment except for the structures of the light receiver 24 and the signal detector 26. The light receiver 24 has first to third light receivers 71 to 73 for receiving the three light beams split by the diffraction grating 22 and reflected by the optical disc 11. The first light receiver 71 receives the main beam (0-order light beam) of the three split light beams and is divided into four areas, A2, B2, C2 and D2. The second and third light receivers 72, 73 receive two side beams (±1 order light beams) of the three split light beams and have portions E2, F2 for receiving the ±1 order side beams of the three split light beams. Of the outputs of the light receivers 71 to 73, the outputs of the light receivers 72 and 73, that is the outputs of the areas E2 and F2, are supplied to a comparator 74 where a difference output E-F, that is the first tracking error signal, is produced. Of the outputs of the first light receiver 71, the outputs of the areas A2 and C2 are summed by an adder 75, while the outputs of the areas B2 and D2 are summed by an adder 76. The phase differences of the outputs of the adders 75, 76 are compared by a phase comparator 77 to produce the second tracking error signal. If a disc is judged by the disc discrimination portion 27 to be an optical disc having a track pitch of 1.6 µm, the first tracking error signal, which is a difference output from the comparator 74, is selected. If a disc is judged to be an optical disc having a track pitch of 0.8 µm, the second tracking error signal, which is a difference output from the comparator 77, is selected. In the second embodiment, similarly to the first embodiment, a three-beam method is applied for reproducing, for example, a compact disc, while a phase difference based tracking error detection system, advantageous for removing the offset, is applied for recording or reproducing a high recording density optical disc. Fig.8 shows a third embodiment for a tracking servo for use in apparatus for recording and/or reproducing plural discs with different track pitches. The optical disc recording and/or reproducing apparatus in the present third embodiment is similar to that of the first and second embodiments except for the structures of the light receiver 24 and the signal detector 26. The light receiver 24 has first to third light receivers 81 to 83 for receiving the three light beams split by the diffraction grating 22 and reflected by the optical disc 11. The first light receiver 81 receives the main beam (0-order light beam) of the three split light beams and is divided into four areas, A3, B3, C3 and D3. The second and third light receivers 82, 83 receive two side beams (±1 order light beams) of the three split light beams and each of them is divided into two portions, respectively, E3, G3, and F3, H3. Of the outputs of the light receivers 81 to 83, the outputs E3 and G3 of the light receiver 82 and the outputs F3 and H3 of the light receiver 83 are summed by adders 84, 92, respectively. The outputs of the adders 84, 92 (E3 + G3, F3 + H3) are supplied to a comparator 95 for producing a first tracking error signal. Of the outputs of the first light receiver 81, the outputs A3, C3 are summed by an adder 86, while outputs B3 and D3 are summed by an adder 89. The phase differences of the outputs of the adders 86 and 89 are compared by a comparator 96 for producing a second tracking error signal. The outputs E3, G3 of the light receiver 82 are sent to a comparator 85, while the outputs F3, H3 of the light receiver 83 are supplied to a comparator 91. A difference output of the comparator 85 is summed to a difference output of the comparator 91 passed through a variable gain amplifier 93, and the resulting sum output is supplied via a variable gain amplifier 94 to a comparator 97. The outputs A3, D3 of the light receiver 81 are summed by an adder 88, while the outputs B3, C3 of the light receiver 81 are summed by an adder 87. Outputs of the adders 87 and 88 are supplied to a comparator 90, an output of which is sent to a comparator 97. With an output of the comparator 97, a third tracking error signal is detected. With the present third embodiment, the three-beam method is applied for reproducing a compact disc, for example, while the phase difference based tracking error signal detection method, advantageous for removing the offset, is applied to recording or reproduction of a high density optical disc. In addition, in the present embodiment, tracking error signal detection of the differential push-pull system is applied for recording or reproduction of, for example, a phase change type rewritable optical disc. Thus the same optical pickup may be used for recording or reproducing plural sorts of optical discs. Now, with the optical disc recording and/or reproducing apparatus of the present invention, an optical disc with a substrate thickness of 0.6 mm may be recorded or reproduced as a first optical disc with, for example, a track pitch of approximately 0.8 µm. For such recording or reproduction, a semiconductor laser radiating a laser beam with a wavelength of, for example, 635 nm, is used as the light source 21. The objective lens 25 has an aperture ratio of, for example, 0.52. Therefore, if a second type of optical disc, having a substrate thickness of 1.2 mm, such as a compact disc, is used, spherical aberration is generated due to errors in substrate thickness, so that correct reproduction of the recorded data cannot be achieved. Consequently, in preferred embodiments of the optical disc recording and/or reproducing apparatus according to the present invention, the disc discrimination unit 27 sends a detection output of the optical disc both to the changeover switch 28 selecting the desired tracking error signal and to an aperture ratio variable control unit 100, as shown in Fig.9. If fed with a detection output specifying the second type of optical disc, with a substrate thickness of 1.2 mm, the aperture ratio variable control unit 100 forms a corresponding motor driving pulse and routes the pulse to a stepping motor 102 of a variable ratio varying unit 101 shown in Fig.10a. This rotates the stepping motor 102 in a direction of moving a light shielding ring 103 into a light path of the laser beam; the rotary force of the stepping motor 102 is transmitted via a gear portion 105a meshing with a gear portion 104a of a rotary gear 104 to a ring slider 105. Thus the light shielding ring 103 is controlled to be moved over the objective lens 25 along with the ring slider 105, as shown in Fig.10B. The light shielding ring 103 thus shields, by its light shielding portion 103b, a portion of the laser beam radiated from the objective lens 25, for varying the aperture ratio of the objective lens 25 to 0.37 (corresponding to 70% of the aperture ratio of 0.52) for the second type of optical disc. The shielded portion of the laser beam is by the outer peripheral portion and corresponds to 30% of the entire laser beam. Thus, during reproduction of the second type of optical disc, the light shielding ring 103 is controlled to be moved over the objective lens 25 so that a portion of the laser beam from the objective lens 25 is shielded as the laser beam is illuminated on the second optical disc, as shown in Fig.lla. This prohibits spherical aberration from being produced, during reproduction of the optical disc having a substrate of an increased thickness, due to errors in the substrate thicknesses. Specifically, if the second type of optical disc is reproduced using the objective lens 25 with the aperture ratio kept at 0.52, wavefront aberration of approximately 0.3 rmsλ is produced due to substrate thickness error of 0.6 mm, thus producing significant distortion in the spatial frequency characteristics, as indicated by circle marks O in the graph of Fig.12. Conversely, if the aperture ratio of the objective lens 25 is controlled to 0.37 by the light shielding ring 103, the wavefront aberration is decreased to approximately 0.07 rmsλ, thus eliminating the distortion in the spatial frequency characteristics, as shown by □ marks in the graph of Fig.12. Meanwhile, marks ⋄ indicate spatial frequency characteristics in a case where reproduction is performed using an optical system dedicated to the second type of optical disc. Comparison of the marks ⋄ and □ reveals that the two characteristics are similar to each other at approximately 1100/mm. If the aperture ratio of the objective lens 25 is controlled by the light shielding ring 103 to 0.37, the spherical aberration may be decreased to a fourth power of the aperture ratio, that is about 25%, as compared to the spherical aberration generated on reproducing the second type of optical disc with the aperture ratio of the objective lens 25 remaining unchanged at 0.52. Thus it becomes possible to sufficiently reproduce the second type of optical disc having the substrate thickness different from that of the first disc, using the optical system for the first type of optical disc. If fed with a detection output specifying a first type of optical disc, with a substrate thickness of 0.6 mm, the aperture ratio variable control unit 100 forms a corresponding motor driving pulse and routes the pulse to the stepping motor 102 of a variable ratio varying unit 101 shown in Fig.10A. This rotates the stepping motor 102 in a direction of moving the light shielding ring 103 out of the light path of the laser beam, so that the rotary force of the stepping motor 102 is transmitted via the gear 105a meshing with the gear portion 104a of the rotary gear 104 to the ring slider 105. Thus the light shielding ring 103 is moved away from the objective lens 25 along with the ring slider 105. Thus the laser beam from the objective lens 25 may be illuminated on the first type of optical disc, with the substrate thickness of 0.6 mm, without being shielded, as shown in Fig.llB. In this case, the wavelength of the laser beam is 635 nm, and the aperture ratio of the objective lens 25 is 0.52, so that the spatial frequency is equal to 1500/mm, as shown by marks × in Fig.12, and hence the first type of optical disc, having a small recording pit size, can be reproduced satisfactorily. It is seen from the above that, with the optical disc recording and/or reproducing apparatus according to preferred embodiments of the present invention, the light shielding ring 103, for shielding a portion of a laser beam from the objective lens 25, is provided in the optical system for the first type of optical disc, having the substrate thickness of 0.6 mm, and is used only for reproduction of the second type of optical disc, having the substrate thickness of 1.2 mm, for shielding a portion of the laser beam radiated from the objective lens 25 for variably controlling the aperture ratio of the objective lens 25 for conformity to the second type of optical disc. This enables reproduction of the two different sorts of optical discs having different substrate thicknesses. Since the two sorts of optical discs with different substrate thicknesses may be reproduced in this manner, the optical disc reproducing apparatus may be improved in universality of application. A second technique according to the present invention for varying the aperture ratio in the optical disc recording and/or reproduction apparatus according to the present invention is now explained. According to the previous technique, the aperture ratio of the objective lens 25 is variably controlled by the light shielding ring 103 and the ring slider 105. According to the present technique, a pair of light shielding plates 106, 109 as shown in Fig.13 are used for shielding a portion of the laser beam from the objective lens 25, using a pair of light shielding plates 106, 109 as shown in Fig.13, for variably controlling the aperture ratio of the objective lens 25. Except for the mechanism related with this construction, the optical disc recording and/or reproducing apparatus employing the present technique is similar in structure to the optical disc recording and/or reproducing apparatus employing the previous first technique. Thus detailed description of the remaining portions of the apparatus is omitted for clarity. The aperture ratio varying unit 101, provided in apparatus employing the present second technique, is made up of stepping motors 107, 110 for controlling the movement of the light shielding plates 106, 109, as shown in Fig.13. The light shielding plates 106, 109 are arranged on a straight line perpendicular to a laser beam radiated from the objective lens 25 so that one ends of the plates face each other. Part of the bottom surface portions consecutive to the facing ends of the light shielding plates 106, 109 is designed as light shielding portions 106b, 109b for shielding a portion of the laser beam radiated from the objective lens 25. The bottom sides of the light shielding plates 106, 109 are provided with rack gears 106a, 109a, respectively, so as not to contact with the light shielding portions 106b, 109b, respectively. These rack gear portions 106a, 109a are designed to mesh with gear portions 108a, 111a of rotary gears 108, 111 provided on rotary shafts 107a, 110a of the stepping motors 107, 110, respectively. The above-described structure of the aperture ratio varying unit 101 is controlled to be driven by a motor driving pulse supplied from the aperture ratio varying controlling unit 100 responsive to a detection output of the disc discrimination unit 27. That is, if fed from the disc discrimination circuit 27 with a detection output specifying the reproduction of the second type of optical disc, having the substrate thickness of 1.2 mm, the aperture ratio variable control unit 100 generates motor driving pulses for rotating the stepping motors 107, 110 in a direction of reducing the gap delimited between the facing ends of the light shielding plates 106, 109. These motor driving pulses are supplied to the stepping motors 107, 110. This drives the stepping motors 107, 110 into rotation. The rotational force of the stepping motors is transmitted via the gear portions 108a, 111a of the gears 108, 111 to the rack gear portions 106a, 109a of the light shielding portions 106, 109. The light shielding plates 106, 109 are controlled to be moved for hiding part of the objective lens 25. The range of hiding of the objective lens 25 by the light shielding plates 106, 109 is set to a range which will give an aperture ratio of the objective lens 25 of 0.37 equal to the aperture ratio required for the second type of optical disc. By controlling the movement of the light shielding plates 106, 109 in this manner, part of the laser beam radiated from the objective lens 25 is shielded by the light shielding portions 106b, 109b of the light shielding plates 106, 109 for setting the aperture ratio of 0.37 for the objective lens 25. Thus, the second type of optical disc, with the substrate thickness of 1.2 mm, may be reproduced correctly, as with the first technique described above. Next, if fed from the disc discrimination circuit 27 with a detection output specifying the reproduction of the first type of optical disc, having the substrate thickness of 0.6 mm, the aperture ratio variable control unit 100 generates motor driving pulses for rotating the stepping motors 107, 110 in a direction of enlarging the gap delimited between the facing ends of the light shielding plates 106, 109. The motor driving pulses are supplied to the stepping motors 107, 110. This drives the stepping motors 107, 110 into rotation. The rotational force of the stepping motors is transmitted via the gear portions 108a, 111a of the gears 108, 111 to the rack gear portions 106a, 109a of the light shielding portions 106, 109. The light shielding plates 106, 109 are controlled to be moved to positions not shielding the laser beam radiated from the objective lens 25. This gives the aperture ratio of the objective lens 25 of 0.52 which is equal to the aperture ratio for the first type of optical disc. Thus the first type of optical disc may be reproduced correctly. A third technique for varying the aperture ratio in optical disc recording and/or reproduction apparatus according to the present invention is now explained. With the optical disc recording and/or reproducing apparatus employing the present third technique, an objective lens having two sorts of aperture ratio (as shown in Fig.14) is employed in place of the objective lens 25, and the aperture ratio variable control unit 101 and the objective lens portion having the aperture ratio corresponding to the substrate thickness of the optical disc being reproduced is used by switching. Since the optical disc recording and/or reproducing apparatus employing the present third technique is similar in structure to the optical disc reproducing apparatus employing the first and second techniques except the objective lens, the following description of the third technique concentrates on the objective lens, while explanation of the remaining portions is not made for clarity. That is, the above objective lens has a first light condensing portion 112 having an aperture ratio (0.52) for the first type of optical disc, having the substrate thickness of 0.6 mm, and has a second light condensing portion 113 having an aperture ratio (0.37) for the second type of optical disc, having the substrate thickness of 1.2 mm. The objective lens has a slider 114 for moving the objective lens on an optical path of the laser beam. The bottom portion of the slider 114 has a rack gear meshing with a rotating gear formed on the stepping motor. The force of rotation of the stepping motor is transmitted via the rotating gear and the rack gear to the slider 114 for controlling movement of the objective lens. If fed with a detection output from the disc discrimination unit 27 specifying the reproduction of the first type of optical disc, with the substrate thickness of 0.6 mm, the aperture ratio variable control unit 100 supplies a motor driving pulse to the stepping motor for controlling movement of the first light condensing portion 112 into the light path of the laser beam. This drives the stepping motor into rotation so that the first light condensing portion 112 of the objective lens is moved by the slider 114 on the light path of the laser beam. Since the first light condensing portion 112 has the aperture ratio of 0.52 for the first type of optical disc, the first type of optical disc may be correctly reproduced by controlling movement of the first light condensing portion 112 into the light path of the laser beam. If fed with a detection output from the disc discrimination unit 27 specifying the reproduction of the second type of optical disc, with the substrate thickness of 1.2 mm, the aperture ratio variable control unit 100 supplies the motor driving pulse to the stepping motor for controlling movement of the second light condensing portion 113 onto the light path of the laser beam. This drives the stepping motor into rotation so that the second light condensing portion 113 of the objective lens is moved by the slider 114 into the light path of the laser beam. Since the second light condensing portion 113 has the aperture ratio of 0.37 adapted for the second type of optical disc, the second type of optical disc may be correctly reproduced by controlling movement of the second light condensing portion 113 into the light path of the laser beam. In the above embodiments, the aperture ratio is variably controlled for the first and second types of optical disc having different substrate thicknesses. However, since it is sufficient in the case of the light shielding plates 106, 109 shown in Fig.13 to variably control the laser beam light shielding range depending on the substrate thickness of the optical disc, it is possible to variably control three or more aperture ratios of the light shielding ranges of the objective lens for enabling reproduction of three or more types of optical disc having different substrate thicknesses. In the embodiment of Fig.14 of an objective lens having the light condensing portions 112, 113 with two different aperture ratios, light condensing portions with three or more different aperture ratios may be provided for enabling reproduction of three or more types of optical disc having different substrate thicknesses. In the above description of the third technique for varying aperture ratio, the light condensing portions 112, 113 are controlled to be moved by the slider 114. It is however possible to provide a rotary shaft between the light condensing portions 112, 113, using the shaft sliding type biaxial mechanism shown in Fig.3, and to control the objective lens to be rotated about the rotary shaft as the center of rotation for controlling movement of the light condensing portions 112, 113 on the light path of the laser beam.

Claims: An optical pickup comprising: a light source (21) for radiating a light beam; an element (22) for separating a light beam radiated from said light source into at least three beams, namely a main beam and two side beams; an objective lens (25) for converging the light beams separated by said element (22) on a signal recording surface of an optical recording medium (11); and a light receiving unit (24) having an at least two-segment first light receiving portion (51/71/81) for receiving said main beam reflected by the recording surface of the optical recording medium and second (52/72/82) and third (53/73/83) light receiving portions arranged on both sides of said first light receiving portion for receiving the side beams reflected by the recording surface of the optical recording medium; calculation means (24) for generating a first tracking error signal based on outputs of said second and third light receiving portions; and means (27) for discriminating the sorts of the optical discs; characterised in that: the light beam is separated into at least three beams using a diffraction element (22); the calculation means (24) is adapted for generating a second tracking error signal based on signals comprising respective outputs of said first light receiving portion (51/71/81); and there is provided switching means (28) for selecting for output, based on a signal from said discrimination means (27), one of the tracking error signals calculated by said calculation means (24). The optical pickup as claimed in claim 1, wherein said calculation means (24) calculates the second tracking error signal by performing a calculation which includes calculating phase differences between signals obtained on summing pairs of outputs of said first light receiving portion (71/81), said calculation means (24) calculating the first tracking error signal by calculating the differences of signals from said K second (72/82) and third (73/83) light receiving portions. The optical pickup as claimed in claim 1, wherein: said first light receiving portion of the light receiving unit is a four-segment light receiving portion (81) and the second and third light receiving portions (82/83) are each two-segment light receiving portions; and said calculation means (24) is adapted for generating said first tracking error signal based on differential outputs of said first, second and third light receiving portions and is adapted for generating a third tracking error signal by a differential output between an output of said second light receiving portion and an output of said third light receiving portion. The optical pickup as claimed in claim 1, 2 or 3, further comprising means for varying the aperture ratio of said optical lens. The optical pickup as claimed in any previous claim, wherein said objective lens has plural lens portions having different aperture ratios. A tracking servo system, for an optical disc recording and/or reproducing apparatus, the tracking servo system comprising: an optical pickup according to any one of claims 1 to 5, and means (29) for driving an objective lens based on the tracking error signal selected by said switching means (28). The tracking servo system as claimed in claim 6, wherein said calculation means (24) calculates the phase differences of signals produced on summing desired pairs selected from four outputs of said first light receiving portion (71/81) for producing the second tracking error signal, said calculation means (24) also calculating the differences of signals from the second (72/82) and third (73/83) light receivers for producing said first tracking error signal.

Inventors: OHSATO KIYOSHI, UEMURA KAMON, UTSUMI MASAMICHI, YAMAKAWA AKIO, OHSATO, KIYOSHI, UEMURA, KAMON, UTSUMI, MASAMICHI, YAMAKAWA, AKIO, Ohsato, Kiyoshi, c/o Sony Corporation, Uemura, Kamon, c/o Sony Corporation, Utsumi, Masamichi, c/o Sony Corporation, Yamakawa, Akio, c/o Sony Corporation
Applicants: SONY CORP, SONY CORPORATION
Application year: 1996
Main classification: G11B7
IPC codes: G11B19, G11B7