Optical disc readout apparatus

Abstract: An optical disc readout apparatus is disclosed. The apparatus has an optical system which guides a laser beam onto a signal recording surface of a loaded disc through an objective and guides a reflected laser beam onto a photo detector. In the present invention, an effective numerical aperture of the objective is adjusted according to the distance between a substrate surface of the loaded disc and the signal recording surface, and the laser spot with a diameter suitable to a recording density of the loaded disc is focused on the signal recording surface thereof. Therefore, the data recorded in a plurality of optical discs with different types (=different thickness or different recording density) can preferably be read out. Other aspects of the apparatus include control of the amplification of the readout or tracking signals according to the disk, and the use of two light sources with orthogonal polarizations.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention generally relates to optical disc readout apparatus, and more particularly to an optical disc readout system which can read information from both an optical disc with a thick substrate and an optical disc with a thin substrate. The optical disc with thick substrate refers to an optical disc of approximately 1.2 mm thick, for example, and the optical disc with thin board refers to an optical disc of approximately 0.6 mm thick, for example. The thickness of the substrate is defined in this specification as a distance from the surface of the substrate to the information-bearing surface. Accordingly, an optical disc of approximately 1.2 mm thick composed of two optical discs of approximately 0.6 mm thick each, adhered to each other back to back, is also referred to as an optical disc of approximately 0.6 mm thick in this specification. Description of the Related Art: As a readout type optical disc for being recorded multimedia data, CD-ROM, etc. have been put to practical use. A CD-ROM is a medium having a diameter of 12 cm, a thickness of 1.2 mm, a track pitch of 1.6 µm and a recording capacity of 540 Mbyte on one side. A digital video disc (DVD) which can record video data by utilizing compression technologies, such as MPEG type compression technology, is under development. However, when video data of MPEG-2 Standards is tried to be recorded into a current CD-ROM at a data transfer rate of 4 Mbps, as little video data as only approximately 20 minutes can be recorded. This means that the current CD-ROM is not sufficient in capacity for recording a movie of about 120 minutes. To solve this problem, a technology for increasing the recording density of optical discs several times as much as that of current optical discs is under development. In this specification, the recording density equivalent to that of the CD-ROM is referred to as "standard density." For example, an SD of 12 cm in diameter, the diameter of which being the same as that of the CD-ROM, which can record a data of approximately 5 Gbyte on one side has been proposed. The SD has a track pitch of approximately 0.73 µm and the shortest pit length of approximately 0.4 µm, and has employed an efficient modulation mode. The thickness of the SD is 0.6 mm. When two pieces of such SDs are stuck back to back to each other, the combined piece can record as much data as approximately 10 Gbyte, which is equivalent to 240 minutes in movie data. An HDMCD of 12 cm in diameter, the diameter of which being the same as that of the CD-ROM, which can record a data of approximately 3.7 Gbyte on one side has been contrived. The HDMCD has a track pitch of approximately 0.84 µm and the shortest pit length of approximately 0.45 µm. The thickness of the HDMCD is 0.6 mm. As a prior art related to the present invention, there have been technologies disclosed in the Japanese Unexamined Patent Publication No. 7-57271. In the Japanese Unexamined Patent Publication No. 6-215406, an optical pickup is disclosed which can focus the respective beam spots upon the respective information-bearing surfaces of two different types of optical discs having different substrate thicknesses. In the Japanese Unexamined Patent Publication No. 5-303766, an optical pickup is disclosed which corrects the coma caused by the difference in substrate thickness of the optical discs without changing the focal length by advancing an aspherical optical element having no refracting power into a collimated light beam or retreating the optical element therefrom according to the thickness of the loaded optical disc. In this arrangement, the pickup can focus the respective beam spots upon the respective information-bearing surfaces of the respective optical discs having different board thicknesses. In the Japanese Unexamined Patent Publication No. 6-259804, an apparatus is disclosed which is equipped with a laser diode for reproducing the standard density CD with a thick board and a laser diode for recording and reproducing the high density optical disc with a thin board and can focus the laser beam output from the laser diode selected according to the optical disc upon the information-bearing surface through a common optical system. This prior art is acknowledged in the preamble of claim 1. In order to read information from the high density optical disc, the spot diameter of the laser beam focused upon the information-bearing surface must be reduced to approximately 0.9 µm. In order to reduce the spot diameter to such extent, the wavelength of the laser beams must be shortened or the numerical aperture NA of the objective must be increased. However, if the numerical aperture NA of the objective lens is increased, the coma increases in proportion to the third power of the numerical aperture NA of the objective. For this reason, if the laser beam which is fall upon the board surface of the optical disc inclines to the normal to the board surface, there is a problem that the coma increases and consequently the reproduced signals are degraded. The inclination of the optical disc, which makes it impossible for the laser beam to perpendicularly fall upon the board surface, is caused by the warp of the optical disc or otherwise. On the other hand, as the coma is also proportional to the thickness of the optical disc board, it is possible to control the coma due to the inclination of the optical disc board by reducing the thickness of the optical disc board. Based on this principle, in order to reduce the spot diameter of the laser beam to approximately 0.9 µm by increasing the numerical aperture NA of the objective while controlling the coma due to the inclination of the optical disc board, how to reduce the thickness of the optical disc board has been examined. The objective of the optical pickup is designed considering the thickness of the optical disc and the wavelength of the laser beam. Therefore, if the substrate thickness of the optical disc in which information is recorded or from which information is read is different from the substrate thickness supposed to be in the objective design process, wave aberration is caused. As a result, the laser beam fails to focus upon the information-bearing surface of the optical disc, and the information recording or information reproducing in or from the optical disc is impossible. For example, when an optical pickup mounted with a objective designed for an optical disc of 0.6 mm in board thickness is used, it is impossible to focus the laser beam upon the information-bearing surface of an optical disc of 1.2 mm in substrate thickness, and therefore, it is impossible to record the information in the optical disc of 1.2 mm thick or read the information recorded in the optical disc of 1.2 mm thick. This means, for two different types of optical discs with different substrate thicknesses, an optical pickup mounted with two objectives suitable to the respective different types of optical discs must be prepared. SUMMARY OF THE INVENTION It is presumed that a disc of standard density type with a thickness of 1.2 mm (CD, CD-ROM), a disc of high density type with a thickness of 1.2 mm (HDMCD), and a disc of high density type with a thickness of 0.6 mm (SD) will continuously exist in the future. The present invention has an object to provide an apparatus which can record data on optical discs with two or more different thicknesses, and the present invention has an object to provide an apparatus which can read out data from optical discs with two or more different thicknesses. The invention is set out in claim 1. In an optical disc readout apparatus which irradiates a laser beam onto the signal recording surface of an optical disc through an objective, guides the laser beam reflected from the signal recording surface into a photodetector, and thereby reads out the signal, the object of the present invention is fulfilled by, for example, constructing such optical disc readout apparatus by comprising a means for adjusting the effective numerical aperture of the objective through the adjustment of the optical system guiding the laser beam outputt from the laser source into the objective according to the thickness of the optical disc. In this construction, when the effective numerical aperture of the objective has been adjusted according to the thickness of the substrate of the loaded optical disc, the laser spot with a diameter suitable to the recording density of the loaded optical disc is focused on the signal recording surface of the loaded optical disc. The adjustment described above can be realized by, for example, interposing a stopping down means within the beam path between the laser source and the objective. The adjustment described above can also be realized by, for example, so constructing the optical disc readout apparatus that two different types of laser sources having mutually orthogonal directions of polarization are switched from one to the other in operation, and a polarization selection means which stops down the diameter of the beam path for one polarization but does not stop down for the other polarization is provided. In this construction, the laser sources are switched from one to the other according to the determination whether an optical disc with the standard thickness is loaded or an optical disc with a thinner thickness is loaded. The standard thickness is a thickness ranging, for example, from 1.15 mm to 1.25 mm, and the thinner thickness is a thickness ranging, for example, from 0.55 mm to 0.65 mm. The adjustment described above can also be realized by, for example, selectively reading out either the reflected beam of the one polarized beam with the beam diameter thereof stopped down or the reflected beam of the other polarized beam transmitted with the beam diameter thereof not stopped down. This can specifically be constructed by, for example, providing the first photodetector which detects the reflected beam of the one polarized beam with the beam diameter thereof stopped down and the second photodetector which detects the reflected beam of the other polarized beam transmitted with the beam diameter thereof not stopped down, and sending the detection signal of either photodetector to the readout signal processing part. The object of the present invention is fulfilled also by, for example, adjusting the degree of amplification of the high frequency component of the readout signal or the degree of gain-up of the readout signal according to the thickness of the loaded optical disc. The object of the present invention is also fulfilled by, for example, using an objective so designed that the inner circumferential part is longer in focal length than the outer circumferential part, and adjusting the effective numerical aperture of such objective so that the laser spot can be focused onto the signal recording surfaces of the respective optical discs of different types with different thicknesses. For example, when an optical disc of the standard thickness is loaded, the laser beam that have passed through the inner circumferential part of the objective are focused onto the signal recording surface, and when an optical disc of the thinner thickness is loaded, the laser beam that have passed through the outer circumferential part of the objective are focused onto the signal recording surface. In the latter case, if the inner circumferential part of the objective is shaded, the laser spot is further stopped down due to the optical super-resolution phenomenon. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1(a) is a schematic illustrating the optical system of the first embodiment, and FIG. 1(b) is a descriptive view of a paralyzation selection filter for use in the first embodiment; FIG. 2 is a graph showing the relations between the spot diameter and the effective numerical aperture when an objective with a numerical aperture of 0.6 adjusted to an SD with a thickness of 0.6 mm is used, the effective numerical aperture is adjusted by inserting a diaphragm into the beam path, and laser beam is condensed to a signal recording surface of a disc with a thickness of 1.2 mm; FIG. 3 is a schematic illustrating the optical system of the second embodiment; FIG. 4 is a schematic illustrating the optical system of the third embodiment; FIG. 5 is a block diagram illustrating a signal processing part for use when a HDMSC and an SD are made compatible with each other; and FIG. 6 is a view illustrating an important part of the fourth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT (1) First Embodiment: FIG. 1 illustrates the first embodiment according to the present invention. In this figure, a semiconductor laser 11 and a semiconductor laser 12 output laser beams with the directions of polarization orthogonal to each other. The thin full line indicates the laser beam from the semiconductor laser 11, and the thin broken line indicates the laser beam from the semiconductor laser 12. The semiconductor laser 11 is mounted for an optical disc 51 with a thickness of 0.6 mm, and is designed to output a laser beam with a wavelength of 635 ± 15 nm (or 650±15 nm). Here, ±15 nm is a tolerance. The semiconductor laser 12 is mounted for an optical disc 52 with a thickness of 1.2 mm, and is designed to output a laser beam with a wavelength of 635 ± 15 nm (or 650± 15 nm). The optical disc 51 with a thickness of 0.6 mm is a high-density SD, and the optical disc 52 with a thickness of 1.2 mm is a high-density HDMCD or a standard-density CD. The numerical aperture of an objective 24 is so designed as to be 0.6 for the optical disc 51 with a thickness of 0.6 mm. When the optical disc 51 with a thickness of 0.6 mm is loaded, the semiconductor laser 11 is turned ON, and the semiconductor laser 12 is turned OFF. The laser beam with a wavelength of 635 nm (P polarized beam) output from the semiconductor laser 11 is passed through polarized beam splitter 21, refracted by a collimator lens 22 to be a collimated beam, passed through a half mirror 23, passed through a polarizing filter 31 without being stopped down, condensed by the objective 24 to focus onto the signal recording surface 51a of the optical disc 51 with a thickness of 0.6 mm. The diameter of the laser spot at this time is 0.9 µm. The laser light reflected by the signal recording surface 51a is refracted by the objective 24to be a collimated beam, passed through the polarizing filter 31 without being stopped down, reflected and bent by 90° by the half mirror 23, condensed by a condenser lens 25 to be focused onto a photodetector 40, and detected as a signal corresponding to the recording data thereby. When the optical disc 52 with a thickness of 1.2 mm is loaded, the semiconductor laser 12 is turned ON, and the semiconductor laser 11 is turned OFF. The laser beam with a wavelength of 635 nm output from the semiconductor laser 12 (S polarized beam) is reflected by the polarized beam splitter 21, collimated by the collimator lens 22, passed through the half mirror 23, cut at the circumferential part by the polarizing filter 31 and passed through the center part thereof (stopped down by the polarizing filter 31), condensed by the objective 24 to be focused onto the signal recording surface 52a of the optical disc 52 with a thickness of 1.2 mm. The diameter of the laser spot at this time is 1.3 µm. That is, by being stopped down by the polarizing filter 31, the effective numerical aperture of the objective 24 is now roughly 0.4. The polarizing filter 31 is a polarization selection filter which allows the P polarized beam to pass therethrough without being stopped down (double headed arrows in FIG. 1(b)) but allows the S polarized beam to be stopped down to be smaller in diameter (black dots in FIG. 1(b)). When the numerical aperture of the objective 24 is 0.6 and the focal length f thereof is 3.3 mm, and when the effective diameter of the laser beam (the diameter of the outer circle in FIG. 1(b)) is 3.96 mm, the diameter equivalent to a numerical aperture of 0.4 after being stopped down by the polarizing filter 31 (the diameter of the inner circle in FIG. 1(b)) is 2.64 mm. The laser beam reflected by the signal recording surface 52a is collimated by the objective 24, passed through the polarizing filter 31 without being stopped down (for the reason that the laser beam have already been stopped down), reflected and bent by 90° by the half mirror 23, condensed by a condenser lens 25 to focus onto a photodetector 40, and detected as a signal corresponding to the recording data thereby. In the construction described above, the polarizing filter 31 is used. Instead of the polarizing filter 31, however, a polarization selection hologram may be used. The polarization selection hologram is an optical element for allowing only the center part of one polarized beam to pass therethrough by diffusing the circumferential part. Also in the construction described above, by disposing the polarizing filter 31 immediately before the objective 24 so that both the objective 24 and the polarizing filter 31 can integrally shift even at the time of tracking servo control to achieve good condensation characteristics or readout characteristics. If some degradation is allowable in the readout characteristics, however, the polarizing filter 31 may be provided as a fixed aperture. In this case, the polarizing filter 31 may be disposed in any position between the semiconductor laser 12 and the objective 24. Furthermore, if the polarizing filter 31 is provided in such a part that is off the beam path of the semiconductor laser 11 (i.e., between the semiconductor laser 12 and the polarized beam splitter 21), an ordinary stop may be used instead of the polarizing filter. (2) Spot Diameter to Effective NA: Next, referring to FIG. 2, description will be given to the relations between the effective numerical aperture (NA) and the laser spot diameter. FIG. 2 illustrates the relations between the effective NA and the laser spot diameter on an actual case where laser beam was condensed onto a signal recording surface of an optical disc of 1.2 mm thick by using an objective of NA 0.6 which is designed for a 0.6 mm thick disc, and an aperture was set within the beam path and the effective NA was varied by adjusting the aperture. Here, the black dots indicate a case where laser beam of 635 nm in wavelength was used, and the white dots indicate a case where laser beam of 780 nm in wavelength was used. In an area where the effective NA is smaller than 0.4, the laser spot diameter is roughly proportional to λ/NA. In an area where the NA is larger than 0.4, however, as a phenomenon of "when an optical disc having a thickness different from the regular thickness is read out, the spherical aberration increases in proportion to the fourth power of the numerical aperture" is effective, the larger the numerical aperture is, the harder the stopping down of the laser spot diameter is. Here, the regular thickness is a thickness which is designed to match to a mounted objective. For this reason, as illustrated in FIG. 2, when the effective numerical aperture is around 0.4 (0.35 - 0.4), the laser spot diameter is the smallest (1.3 µm for a laser beam of 635 nm, 1.65 µm for a laser beam of 780 nm). According to the present invention, it is possible to read out the optical disc of 1.2 mm thick by using an objective designed for the optical disc of 0.6 mm thick. In other words, it is possible to interchange the standard-density CD of 1.2 mm thick and the high-density SD of 0.6 mm thick. The wavelength may be used 635 ±15 nm, 650±15 nm, 680±15 nm or 780±15 nm. Incidentally, the semiconductor laser 11 and the semiconductor laser 12 may be different in wavelength. (3) Interchange of HDMCD and SD: As described above, when the laser beam with a wavelength of 635 nm is used, it is possible to focus the laser spot of 1.3 µm in diameter onto the signal recording surface of the optical disc of 1.2 mm thick by using the objective designed for the optical disc of 0.6 mm thick. On the other hand, when the laser beam with a wavelength of 780 nm is used, it is possible to focus the laser spot of 1.65 µm in diameter on the signal recording surface of the optical disc of 1.2 mm thick by using the objective designed for the optical disc of 0.6 mm thick. In the currently-used standard-density CD of 1.2 mm thick, as the readout is possible with the laser spot of 1.6 µm in diameter, the readout is possible by stopping down the laser beam with a wavelength ranging from 620 nm to 800 nm to a range from 1.3 µm to 1.65 µm as described above. In reading out the high-density HDMCD of 1.2 mm thick, however, as the laser spot diameter of around 1.1 µm is required, the construction described above can not achieve good readout characteristics. Therefore, for interchanging the high-density HDMCD of 1.2 mm thick and SD of 0.6 mm thick, such circuitry as illustrated in FIG. 5 is employed. Specifically, in reading out the HDMCD, an amplifier 60a is selected, the degree of gain-up of the readout signal and the degree of amplification of high frequency component are increased than those of the SD, and this data is sent to a readout signal processing part 70, and on the other hand, an amplifier 61a is selected, the degree of gain-up of the tracking error signal is increased than that of the SD, and this data is sent to a tracking servo controller 71. Reversely, in reading out the SD, an amplifier 60b is selected, the degree of gain-up of the readout signal and the degree of amplification of high frequency component are decreased than those of the HDMCD, and this data is sent to a readout signal processing part 70, and on the other hand, an amplifier 61b is selected, the degree of gain-up of the tracking error signal is decreased than that of the HDMCD, and this data is sent to a tracking servo controller 71. By forming the circuit as descrived above, the readout signal with frequent jitter and large noise can be compensated for. Incidentally, this construction requires a wavelength ranging from 620 nm to 665 nm. (4) Second Embodiment: FIG. 3 illustrates the second embodiment. According to the second embodiment, as photodetectors, a photodetector 41 for detecting the reflected beam of the laser beam output from the semiconductor laser 11, and a photodetector 42 for detecting the reflected beam of the laser beam output from the semiconductor laser 12 are used. For this arrangement, a polarized beam splitter 26 is disposed ahead of the photodetectors 41 and 42 so that the reflected beam of the laser beam output from the semiconductor laser 11 are condensed onto the photodetector 41, and the laser beam output from the semiconductor laser 12 are condensed onto the photodetector 42. As two photodetectors are used in this way, an operation that the photodetector is positioned with respect to the first semiconductor laser (the semiconductor laser 11 or the semiconductor laser 12) and then the second semiconductor laser (the semiconductor laser 12 or the semiconductor laser 11) is positioned with respect to the photodetector is no longer necessary. As the other parts of the construction of the second embodiment are the same as those of the first embodiment, the same parts are referred to by the same reference numerals, and the description thereof will be omitted. (5) Third Embodiment: FIG. 4 illustrates the third embodiment. According to the third embodiment, as a semiconductor laser, a one-chip semiconductor laser 10 is used. The semiconductor laser 10 is composed of two semiconductor lasers which can output laser beam of TE mode and TM mode respectively with the directions of polarization orthogonal to each other mounted on one chip. The laser wavelength is 635 nm. As one chip semiconductor laser having TE mode and TM mode is used, and the laser beam of either TE mode or TM mode is selected according to the optical disc of 0.6 mm thick is loaded or 1.2 mm thick is loaded, the polarized beam splitter 21 illustrated in FIGS. 1 and 3 is no longer necessary. As the other parts of the construction of the third embodiment are the same as those of the first and second embodiments, the same parts are referred to by the same reference numerals, and the description thereof will be omitted. (6) Fourth Embodiment: FIG. 6 illustrates the fourth embodiment. According to the fourth embodiment, the objective 24 and polarizing filter 31 of the first through third embodiments are replaced by an objective 240 and a polarizing filter 310 respectively. As the other parts of the construction of the fourth embodiment are the same as those of the first through third embodiments, the illustration of the entire optical system will be omitted, and description will be given to the objective 240 and the polarizing filter 310. The objective 240 is designed by changing the focal length between the inner circumferential part and the outer circumferential part. Specifically, the objective 240 is designed with different curvatures between the inner circumferential part and the outer circumferential part so that the laser beam passing through the inner circumferential part of the objective 240 can be focused on the signal recording surface 52a of the optical disc of 1.2 mm thick as indicated by the chain line with one dot in FIG. 6(a) and the laser beam passing through the outer circumferential part of the objective 240 can focused on the signal recording surface 51a of the optical disc of 0.6 mm thick as indicated by the thin full line FIG. 6(a). The polarizing filter 310 is a filter for shading only the outer circumferential part of the S polarized beam in the same way as the polarizing filter 31 described above does. That is, the polarizing filter 310 allows both the outer circumferential part and inner circumferential part of the P polarized beam to transmit therethrough as indicated by the directional thin lines with arrows at both ends and directional broken lines with arrows at both ends in FIG. 6(c) and allows only the inner circumferential part of the S polarized beam to transmit therethrough as indicated by the black dots in FIG. 6(c). In reading out the HDMCD of 1.2 mm thick, the S polarized beam indicated by the black dots is output from the laser source, and the outer circumferential part thereof is shaded by the polarizing filter 310 and the inner circumferential part thereof is transmitted therethrough. As indicated by the chain line with one dot in FIG. 6(a), this S polarized beam is focused onto the signal recording surface 52a of the HDMCD of 1.2 mm thick by the inner circumferential part of the objective 240, and thereby the data is read out from the HDMCD. In reading out the SD of 0.6 mm thick, the P polarized beam is output from the laser source as indicated by the directional thin full lines with arrows at both ends and the directional broken lines with arrows at both ends in FIG. 6(c). The P polarized beam is not shaded by the polarizing filter 310 and both the outer circumferential part and inner circumferential part thereof are condensed by the objective 240. Among of all the laser beam parts, the laser beam part passing through the outer circumferential part of the objective 240 is focused onto the signal recording surface 5 1a of the SD of 0.6 mm thick as indicated by the directional thin full lines in FIG. 6(a), and thereby the data is read out from the SD. Here, the laser beam part passing through the outer circumferential part of the objective 240 have not yet been stopped down on the signal recording surface 51a, the adverse effect of the reflected beam part thereof are negligible. Unlike the polarizing filter 31 previously described, the polarizing filter 310 may be substituted by a filter which can shade the outer circumferential part of the S polarized beam and the inner circumferential part of the P polarized beam. In other words, a filter which can transmit only the outer circumferential part of the P polarized beam as indicated by the directional thin full lines with arrows at both ends in FIG. 6(c) and only the inner circumferential part of the S polarized beam as indicated by the black dots in FIG. 6(c) may be used for this construction. In such construction, in reading out the SD of 0.6 mm thick, an optical super-resolution is generated to the P polarized beam whose inner circumferential part is shaded by the polarizing filter 310 and whose outer circumferential part passes through the polarizing filter 310 and is focused onto a signal recording surface 51a by the objective 240. Therefore, there is an effect that the diameter of the laser spot focused on the signal recording surface 51a can further be stopped down. (7) Other Embodiments: In each embodiment described above, the effect of the present invention is achieved by reducing the effective numerical aperture by transmitting one polarized beam and stopping down the other polarized beam through the polarization selection optical element. In each embodiment described above, either a laser beam of 430 nm in wavelength or a laser beam of 532 nm in wavelength, the former is a blue laser and the latter is a green laser, is not refered to. However, the present invention is also realized by using each of them.

Claims: An optical disc readout apparatus comprising: a first laser source (11) for emitting a first laser beam of plane-polarized light when a first type disc (51) having a first separation between the signal recording surface and the disc-surface is loaded; a second laser source (12) for emitting a second laser beam of plane-polarized light when a second type disc (52) having a second separation thicker than the first between the signal recording surface and the disc-surface is loaded, the polarization plane of the first laser beam being different from the polarization plane of the second laser beam; an optical system for guiding either the first or the second laser beam onto a signal recording surface of a loaded optical disc (51/52) and guiding a reflected laser beam therefrom onto a photo detector (40); and characterized by a selective partial-transmitting means (31,310) disposed in a position between a objective lens (24) being part of the apparatus and the laser sources (11, 12) for transmitting the first laser beam according to its polarization plane to focus onto the signal recording surface of the loaded optical disc and for transmitting only a center part of the second laser beam according to its polarization plane to focus onto the signal recording surface of the loaded optical disc. The optical disc readout apparatus according to claim 1, wherein the selective partial-transmitting means is a partial polarizing filter (31) which transmits the first laser beam without cutting off and transmits the second laser beam with cutting off its circumferential part. The optical disc readout apparatus according to claim 1, wherein the selective partial-transmitting means is a partial polarizing hologram which transmits the first laser beam without diffusing and transmits the second laser beam with diffusing its circumferential part. The optical disc readout apparatus according to claim 1, wherein the second type disc (52) has the CD standard separation between the signal recording surface and the disc-surface. The optical disc readout apparatus according to claim 1, wherein the first laser source (11) is a semiconductor laser and the second laser source (12) is an other semiconductor laser. The optical disc readout apparatus according to claim 1, wherein the first laser source and the second laser source are formed on a common semiconductor laser chip (10). The optical disc readout apparatus according to claim 1, wherein the objective lens has a first focal lengths in its center part (L1) and a second focal length different from the first focal length in its circumferential part (L2), and wherein the selective partial-transmitting means transmits the first laser beam to be incident to the objective lens (240) of at least its circumferential part (L2) and transmits the center part of the second laser beam to be incident to the objective lens (240) of its center part (L1).

Inventors: KAJIYAMA SEIJI, KANOU YASUYUKI, TSUCHIYA YOICHI, KAJIYAMA, SEIJI, KANOU, YASUYUKI, TSUCHIYA, YOICHI, Kajiyama, Seiji 1121-5, Kandake, Kanou, Yasuyuki 1646, Oki, Tsuchiya, Yoichi 819-4, Minamijyuku
Applicants: SANYO ELECTRIC CO, SANYO ELECTRIC CO. LTD
Application year: 1996
Main classification: G11B7
IPC codes: G11B7, G11B20