SYSTEM COMPRISING OPTICAL DISCS AND A SCANNING DEVICE

Abstract: A system comprises optical discs of a first type (100) and optical discs of a second type (110) having a transparent substrate (112) with a thickness greater than that of the transparent substrate (102) of the optical discs of the first type. The system further comprises a scanning device which is capable of scanning optical discs of both types and which comprises an actuator (15) and an objective (2) for focusing a radiation beam passing through the substrate of an optical disc so as to form a scanning spot (S) on an information surface (104; 114). The actuator comprises a first actuator section (16) and a second actuator section (6) including the objective, which second actuator section is movable relative to the first actuator section. The system further comprises compensation means for compensating for the difference in thickness (t2-t1) between substrates of optical discs of said types, so that both during scanning of an optical disc of the first type and during scanning of an optical disc of the second type the second actuator section is movable starting from the same reference position for the purpose of focusing.

Description: System comprising optical discs and a scanning device. The invention relates to a system comprising optical discs, each of the optical discs having an optically transparent substrate and an optically scannable information layer present on said substrate, and a scanning device for scanning the optical discs, which scanning device comprises a turntable having a supporting surface for carrying an optical disc and a scanning unit comprising an actuator and an objective having an optical axis for focusing a radiation beam passing through the substrate of an optical disc situated on the turntable so as to form a scanning spot on the information surface, which actuator comprises a first actuator section and a second actuator section including the objective, which second actuator section, for the purpose of focusing, is movable relative to the first actuator section in directions parallel to the optical axis. Such a system is generally known in the form of the standardized "Compact Disc" system. This known system comprises optical discs - compact discs - having an information layer with a spiral track comprising a succession of pits and ridges. The information is stored in digital form in the track of a compact disc, the pits and ridges representing series of bits. The digital information on the compact disc is protected by a transparent substrate having a nominal thickness of 1.2 mm. The "Compact Disc" system further includes players with a scanning device for the optical scanning of a compact disc. Such a scanning device is disclosed in, for example, EP-A 0 464 912 (PHN 13.382). The scanning device comprises a turntable for supporting, centering and rotating a compact disc and further comprises an electro-optical scanning unit with an objective and an actuator for moving the lens. Optical scanning is effected by means of a solid-state laser source, the beam of laser light being focused on the information layer of the compact disc via the objective, the substrate being disposed between the objective and the information layer. Since the digital information on the compact disc is shielded by the substrate possible flaws and dust particles are not situated in the focal plane of the light beam which scans the disc, as a result of which they have comparatively little influence. In order to achieve a correct focusing of the light beam the actuator comprises a stationary actuator section and a movable actuator section and it comprises a drive assembly of a coil and a permanent magnet to move the objective in the direction of its optical axis. During focusing the movable actuator section performs translatory movements about a center position. A focus servo system provides the drive of the coil. In the course of time various types of optical discs, based on the "Compact Disc" standard, have been developed, such as CD-ROM, CD-I, CD-R, which all have a substrate of a thickness in conformity with the value specified in the standard. Recently, a digital video disc (DVD) has been proposed, which comprises two transparent substrates between which information layers are disposed. The substrates of said video disc each have a nominal thickness of 0.6 mm. Owing to the deviating substrate thickness this type of optical disc cannot readily be scanned by means of the scanning device of the known system. It is an object of the invention to modify the known system so as to obtain a system in which optical discs having substantial differences in substrate thickness can be scanned by the same scanning device of the system. The system in accordance with the invention comprises (1) optical discs of a first type and optical discs of a second type, each of said optical discs having an optically transparent substrate and an optically scannable information layer present on said substrate, the substrate of an optical disc of the first type, at least in an area situated opposite the information layer, having a first substrate thickness (t,) and the substrate of an optical disc of the second type, at least in an area situated opposite the information layer, having a second substrate thickness (tj) greater than the first substrate thickness, (2) a scanning device which is capable of scanning optical discs of both types and which comprises a turntable having a supporting surface for carrying an optical disc of each of said types and a scanning unit comprising an actuator and an objective having an optical axis for focusing a radiation beam passing through the substrate of an optical disc situated on the turntable so as to form a scanning spot on the information surface, which actuator comprises a first actuator section and a second actuator section including the objective, which second actuator section is movable relative to the first actuator section, and (3) compensation means for compensating for the difference in thickness ( between substrates of optical discs of said types, so that both during scanning of an optical disc of the first type and during scanning of an optical disc of the second type the second actuator section is movable both in a first direction at least substantially parallel to the optical axis and in a second direction opposite to said first direction, starting from the same reference position for the purpose of focusing. The compensation means ensure that during scanning of an optical disc, regardless of its type, the focus of the radiation beam is always situated in the plane of the information surface of the optical disc to be scanned, without any special requirements having to be imposed on the actuator. The movable actuator section has only one neutral position or center position, which corresponds to the above-mentioned reference position, relative to which only a limited travel is required for a correct focusing. It is to be noted that, in principle, it is possible to make the travel of the movable actuator section in a focusing direction so large that optical discs having different nominal thicknesses can be read without compensation means. In such an actuator the movable actuator section in fact has two reference positions relative to which focusing movements are to be performed. A disadvantage of such a design is that it requires a more powerful and therefore usually more bulky and more expensive drive system than needed in the scanning device in the system in accordance with the invention, in which the movable actuator section has one fixed reference position. Moreover, such a design requires additional energy for at least performing an extra travel to cover the distance between the two reference positions. Moreover, in such a design, in which the movable actuator section should be capable of covering comparatively large distances, problems may arise with respect to the linearity of the movements to be performed. An embodiment of the system in accordance with the invention is characterized in that the compensation means form part of the optical discs of one of the said types. In this case, the scanning device in the system in accordance with the invention dies not require any provisions to enable both types of discs to be scanned. The undesired optical effect caused by the difference in substrate thickness is eliminated by provisions on the discs themselves. An embodiment of the system in accordance with the invention is characterized in that the compensation means form part of the optical discs of the first type, the compensation means of each of said discs having on a substrate side remote from the information layer a contact surface for cooperation with the supporting surface of the turntable, the distance between said substrate side and the contact surface being related to the difference in thickness ( tj). The term substrate is to be understood to mean at least that part of the substrate which faces the information surface. An embodiment of the system in accordance with the invention is characterized in that the distance between the substrate and the contact surface is at least substantially equal to , where NA is the numerical aperture of the objective, nj is the refractive index of the material of the first substrate and n is the refractive index of the material of the second substrate. An embodiment of the system in accordance with the invention is characterized in that the compensation means of each optical disc of the first type comprise a rim which surrounds the center of the disc and which is situated outside the area of the information layer, which rim comprises me contact surface. This embodiment is practical, reliable and simple to realize. The rim is preferably integral with the substrate, is preferably made of the same material as the substrate, and is preferably formed simultaneously with the substrate. An embodiment of the system in accordance with the invention is characterized in that the thickness (t,) of the substrate of optical discs of the first type is at least substantially 0.6 mm and the thickness ( of the substrate of optical discs of the second type is at least substantially 1.2 mm, the first substrate being of a material having a refractive index which is at least substantially equal to the refractive index of the material of the second substrate and the distance between the substrate and the contact surface being at least substantially 0.33 mm. The refractive indices are at least substantially 1.57. The numerical aperture of the objective of the system is or is approximately 0.6. An embodiment of the system in accordance with the invention is characterized in that the compensation means form part of the scanning device. This embodiment provides a practical solution to the problem outlined hereinbefore, if the prevailing standard specifications do not allow adaptation of the optical discs of the system. An embodiment of the system in accordance with the invention is characterized in that the compensation means comprise a positioning device for moving the actuator and/or the turntable relative to a frame of the scanning device in a direction at least substantially parallel to the optical axis of the objective over a positioning distance which is related to the difference in thickness ( ti)- An embodiment of the system in accordance with the invention is characterized in that the compensation means comprise a positioning unit for moving the supporting surface of the turntable relative to other parts of the turntable in a direction at least substantially parallel to the optical axis of the objective over a positioning distance which is related to the difference in thickness (t^t,). In the two last-mentioned embodiments a desired movement can be obtained by the use of mechanical means which are known per se. Generally, an automatically operating positioning device is chosen, which is actuated upon receipt of a detection signal relating to the type of optical disc present in the scanning device. An embodiment of the system in accordance with the invention is characterized in that the positioning distance is at least substantially equal to t 2 t, sf -N ? , where NA is the numerical aperture of the objective, n, is the refractive index of the material of the first substrate and n 2 is the refractive index of the material of the second substrate. The invention further relates to an optical disc suitable for use in the system in accordance with the invention. The optical disc in accordance with the invention is an optical disc of the first type as defined in Claim 1 in combination with any one of the Claims 2, 3, 4, 5 and 6. The invention further relates to a scanning device suitable for use in the system in accordance with the invention. The scanning device in accordance with the invention is defined in Claim 1 in combination with any one of the Claims 7, 8, 9 and 10. The scanning device in accordance with the invention particularly comprises a positioning device for moving the entire actuator, i.e. both actuator sections, and/or the turntable relative to a frame of the scanning device in a direction parallel to the optical axis of the objective of the scanning device over a positioning distance which is related to the difference in substrate thickness of the discs to be scanned. Alternatively, the scanning device may comprise a positioning unit for moving the supporting surface of the turntable relative to the other parts of the turntable. In both cases the positioning distance is preferably equal to or substantially equal to , where NA is the numerical aperture of the objective, n, is the refractive index of the material of the first substrate and n 2 is the refractive index of the material of the second substrate. The invention will now be described in more detail, by way of example, with reference to the drawings, in which Figure 1 is a perspective view showing a scanning device suitable for use in the system in accordance with the invention, Figure 2 shows the scanning unit used in the scanning device of Figure 1 in a sectional view taken on the line II-II in Figure 1, Figure 3A is a diagrammatic sectional view taken on the line IIIA-IIIA in Fig. 1 and showing an embodiment of the system in accordance with the invention when an optical disc of a first type is present, Figure 3B shows the embodiment of Figure 3A in a sectional view taken on the line IIIB-IIIB in Figure 3A, Figure 4A shows the embodiment of Figure 3A in a diagrammatic sectional view taken on the line IIIA-IIIA in Fig. 1 , the optical disc of the first type having been replaced by an optical disc of a second type, Figure 4B shows the embodiment of Figure 4A in a sectional view taken on the line IVB-IVB in Figure 4A, Figure 5A is a perspective view showing an example of a positioning unit for positioning a supporting surface of a turntable, which unit is suitable for use in the system in accordance with the invention and is shown in a first position, Figure 5B is a radial view showing the positioning unit of Figure 5A in the first position, Figure 6A is a perspective view showing the positioning unit of Figure 5A in a second position, Figure 6B is a radial view showing the positioning unit of Figure 6A in the second position, Figure 7 shows diagrammatically an example of a positioning device for positioning a turntable, which device is suitable for use in the system in accordance with the invention, Figure 8 shows diagrammatically an example of a positioning device for positioning an actuator, which device is suitable for use in the system in accordance with the invention and occupies a first position, and Figure 8B shows the positioning device of Figure 8A in a second position. Figure 1 shows a scanning device suitable for use in the system in accordance with the invention. This scanning device comprises a frame 1 , which carries an electrically drivable turntable 3 for supporting and centering optical discs, the turntable 3 being rotatable about an axis of rotation 3 and having a supporting surface 3b. The scanning device further comprises mechanical guide means for translating a slide 5 radially with respect to the turntable 3 (as indicated by an arrow T). The guide means comprise a guide rod 7 secured to the frame 1 and a guide wall 9 adapted to cooperate respectively with sliding bushes 1 1 and sliding surfaces 13 of the slide 5. An electric motor, not shown and supported in the frame, serves for driving the slide 5 by means or not by means of a transmission mechanism. The slide 5 carries a scanning unit with an actuator 15, as well as a solid-state laser arranged adjacent the actuator 15. The actuator 15, which is shown in sectional view in Figure 2, comprises a movable section 6 with an optical axis 2a, which extends parallel to the axis of rotation 3a, a focusing coil 12 with a coil axis 12a, and two tracking coils 14a, 14b. A pivotable or flexible suspension 22 connects the movable section 6 to a stationary section 16 of the actuator 15, which stationary section is secured to the slide 5 and comprises a magnetic circuit with a permanent magnet 18. The scanning unit occupies such a position relative to the slide 5 that its longitudinal axis 1 is oriented transversely to the plane defined by the axis of rotation 3a and the optical axis 2a, as a result of which the objective 2 is movable both in the focusing direction (indicated by an arrow F) and in a tracking direction (indicated by an arrow T). In operation, for the purpose of roughly following an information track disposed in the plane of an information surface of an optical disc situated on the turntable, the slide is moved along a radial path, indicated by the arrow T, with respect to the axis of rotation 3a of the turntable 3. As the information track may be slightly off-centered on the optical disc lying on the turntable and the turntable may also exhibit small radial deviations, allowance is made for small radial excursions of the information track during rotation. The deviations which may occur in operation between the position of a scanning spot formed on the information surface of the disc by the objective and the position of the track portion to be scanned are reduced by means of a radial tracking system which includes the scanning unit 15, the objective 2 performing small high-frequency tracking movements in accordance with the arrow T. The system in accordance with the invention includes optical discs of a first type, which discs in the present example have two optically transparent substrates between which one or two optically scannable are disposed. Figures 3A and 3B show an example of such a disc, which disc is referred to as a DVD. The disc bears the reference numeral 100, the substrates bear the numerals 102a and 102b, and the information layer or layers bear the numeral 104. The substrates 102a and 102b each have a substrate thickness ti of 0.6 mm. The system in accordance with the invention further includes optical discs of a second type, in the present example compact discs (CDs). An example of a such a disc is shown in Figures 4A and 4B. The compact disc 110 shown therein has an optically transparent substrate 112 with a substrate thickness t 2 of 1.2 mm, on which an information layer 114 is disposed. In operation the information layer of each type of disc is scanned by means of a scanning spot formed by a radiation beam traversing the substrate carrying the information layer. In the present example the substrates are made from a polycarbonate. The embodiment of the system in accordance with the invention shown in Figures 3A, 3B, 4A, 4B may comprise a scanning device as shown in Figures 1 and 2 or a variant thereof. For the sake of simplicity corresponding parts in Figures 1 , 2 and Figures 3 A, 3B, 4 A, 4B bear the same reference numerals. The movable actuator section 6 has a center position O relative to which in operation small focusing movements in accordance with the arrow F in Figure 1 are possible in order to keep a scanning spot S formed by the objective 2 accurately in the plane of the information surface 104 or 114. The difference in thickness ( -i) between the substrates 112 and 102a is compensated for by the provision of a compensation means in the form of a rim 106a which surrounds a center hole of the optical disc 100. The rim 106a has a contact surface 108a, which engages with the supporting surface 3b of the turntable 3. In the present example the disc 100 also has a rim 106b similar to the rim 106a and having a contact surface 108b. The thickness t 100 of the rim 106a and the rim 106b, respectively, corresponds to , where n, and n 2 are the refractive indices of the material of the substrate 102a/ 102b and 112, respectively, and where NA is the numerical aperture of the objective. In the present example n, = n 2 = 1.57 and NA is 0.6. The thickness t 100 is consequently 0.33 mm. The positioning unit 200 shown in Figures 5A, 5B, 6A and 6B is suitable as a compensation means in the system in accordance with the invention, if optical discs having mutually different substrate thicknesses are used, without the discs themselves of either type having compensation means. The positioning unit 200 takes the form of an element having a central portion 201a with radially projecting limbs 201b and arranged on a turntable 203 of a scanning device, which turntable is rotatable about an axis of rotation 203a. The limbs 201b, which are resilient in a direction parallel to the axis of rotation 203a, have end portions 204. The positioning unit 200 is rotatable relative to the turntable 203 about the axis of rotation 203a between a first position, as is shown in Figures 5 A and 5B, and a second position, as is shown in Figures 6A and 6B. In the first position the end portions 204 are disposed on a supporting surface 203b of the turntable 203, the end portions 204 together defining a supporting surface 204b for supporting an optical disc of the first type. In the second position the end portions 204 engage in recesses 203c of the turntable 203 and the supporting surface 203b of the turntable forms a supporting surface for supporting an optical disc of the second type. The positioning unit 200 can be actuated by hand, electrically or electro- mechanically. It is obvious that instead of the positioning unit shown other positioning constructions can be used. The end portions 204 have a thickness ^ which is related to the thickness difference between the thickness ^ of the substrate of the optical disc of the second type and die thickness t j of the thinner substrate of the optical disc of the first type. As a matter of fact, the thickness , where NA is the numerical aperture of the objective used in the scanning device, n, is the refractive index of the material of the substrate of the optical disc of the first type and n 2 is the refractive index of the material of the substrate of the optical disc of the second type. In the embodiment of the system in accordance with the invention shown partly in Figure 7 the compensation means comprise a positioning device 300 for moving a turntable 303 relative to a frame 311 of the scanning device in a direction parallel to the optical axis of an objective present in the scanning device of the system. The turntable, which is rotatable about an axis 393a and which has a supporting surface 303b, is supported in an element 313 which is connected to the frame 311 by means of a flexible suspension device 313. In the present example the element 313 is supported by a cylindrical element 315, which is eccentrically rotatable about a spindle 317. The turntable 303 can be moved over a distance t 303 by rotating the element 300. It is obvious that instead of the positioning device shown it is also possible to use other positioning constructions. As already described in the present document, the system in accordance with the invention includes optical discs of a first type having a substrate thickness t, and optical discs of a second type having a greater substrate thickness t^. In the situation shown the turntable 303 is in a position (P2) for cooperation with a disc of the second type; after a movement over the distance t 303 the turntable occupies a position (Pl) for cooperation with a disc of the first type. In both situations the movable section of the actuator used in the scanning device of the system can be moved from the same reference position in order to perform focusing movements. The distance t 303 is equal to s/ϊ- r ' , where NA is the numerical aperture of the objective used in the scanning device, n, is the refractive index of the material of the substrate of the optical disc of the first type and n 2 is the refractive index of the material of the substrate of the optical disc of the second type. In the present example the refractive indices n, and n 2 are 1.57 and the numerical aperture NA is 0.6. The positioning distance t 303 is 0.33 mm. The scanning device in accordance with the invention shown partly in Figures 8A and 8B is suitable for scanning optical discs of a f first type, i.e. discs having a substrate thickness t l s and optical discs of a second type, i.e. discs having a substrate thickness t^, tj being greater than t,. The scanning device has been provided with a frame and a turntable in a manner as shown in Figure 1. Figures 8A and 8B only show the level of the supporting surface 3b of the turntable. The scanning device further comprises a slide 405 having a scanning unit with an actuator 415, the slide 405 corresponding to the slide 5 in Figure 1 and the actuator 415 being a variant of the actuator 15 shown in Figure 1. The actuator 415 comprises a stationary actuator section 416 secured to a sliding element 404 and a movable actuator section 406 coupled to the actuator section 416 via a flexible suspension 422. The movable actuator section 406 carries an objective 402 with an optical axis 402a, the objective being movable in focusing directions and tracking directions in a manner similar to that illustrated in Figure 1. The sliding element 404, which preferably forms part of an electrical sliding device, is movable relative to the slide 405 in directions x transverse to the optical axis 402a for the purpose of moving the actuator 415 parallel to the optical axis 402a over a distance t 41J . The movement of the actuator 415 is derived from the sliding movement of the sliding element 404, a guide member 420 of the actuator 415 cooperating with guide surfaces 422 of the sliding element 404, and a sliding surface 424 of the actuator 415 cooperating with a straight-line guide 426 of the frame. In Figure 8A the actuator 415 occupies a position relative to the turntable in which optical discs of the first type can be scanned. In Figure 8B the actuator 415 occupies a position relative to the turntable in which optical discs of the second type can be scanned. In both situations the movable section 416 can be moved from the same reference position in order to perform focusing movements parallel to the optical axis 402a, so that in both situations the movable section 416 has the same travel or amplitude. The distance t 415 is equal to γ/l -NA ; , where NA is the numerical aperture of ΪVTNA 2 the objective, n, is the refractive index of the transparent substrate of discs of the first type and n 2 is the refractive index of the transparent substrate of discs of the second type. It is to be noted that the invention is not limited to the embodiments disclosed herein. Many variants to the embodiments shown are possible within the scope of the invention. It is essential, however, that there are compensation means in order to compensate for the difference in thickness ( between transparent substrates of various optical discs which can be scanned by means of the scanning device in accordance with the invention, so that for scanning discs with a thinner substrate the movable section of the actuator need not perform larger movements in the focusing directions than necessary for scanning discs with a thicker substrate. In other words, when the compensation means as proposed are used an actuator which is suitable for scanning an optical disc having a substrate of a given thickness is automatically suitable for scanning a disc having a thinner substrate if scanning is effected through the substrate in both cases.

Claims: CLAIMS: 1. A system comprising optical discs of a first type and optical discs of a second type, each of said optical discs having an optically transparent substrate and an optically scannable information layer present on said substrate, the substrate of an optical disc of the first type, at least in an area situated opposite the information layer, having a first substrate thickness (t^ and the substrate of an optical disc of the second type, at least in an area situated opposite the information layer, having a second substrate thickness ? ) greater than the first substrate thickness, - a scanning device which is capable of scanning optical discs of both types and which comprises a turntable having a supporting surface for carrying an optical disc of each of said types and a scanning unit comprising an actuator and an objective having an optical axis for focusing a radiation beam passing through the substrate of an optical disc situated on the turntable so as to form a scanning spot on the information surface, which actuator comprises a first actuator section and a second actuator section including the objective, which second actuator section is movable relative to the first actuator section, and compensation means for compensating for the difference in thickness (t 2 - t j ) between substrates of optical discs of said types, so that both during scanning of an optical disc of the first type and during scanning of an optical disc of the second type the second actuator section is movable both in a first direction at least substantially parallel to the optical axis and in a second direction opposite to said first direction, starting from the same reference position for the purpose of focusing. 2. A system as claimed in Claim 1 , characterized in that the compensation means form part of the optical discs of one of the said types. 3. A system as claimed in Claim 1 or 2, characterized in that the compensation means form part of the optical discs of the first type, the compensation means of each of said discs having on a substrate side remote from the information layer a contact surface for cooperation with the supporting surface of the turntable, the distance between said substrate side and the contact surface being related to the difference in thickness (t t,). 4. A system as claimed in Claim 3, characterized in that the distance between the substrate and the contact surface is at least substantially equal to /l -NA 2 . where NA is the numerical aperture of the objective, nj is the refractive index of the material of the first substrate and n 2 is the refractive index of the material of the second substrate. 5. A system as claimed in Claim 3 or 4, characterized in that the compensation means of each optical disc of the first type comprise a rim which surrounds the center of the disc and which is situated outside the area of the information layer, which rim comprises the contact surface. 6. A system as claimed in Claim 3 or 4, characterized in that the thickness (t j ) of the substrate of optical discs of the first type is at least substantially 0.6 mm and the thickness (t^) of the substrate of optical discs of the second type is at least substantially 1.2 mm, the first substrate being of a material having a refractive index which is at least substantially equal to the refractive index of the material of the second substrate and the distance between the substrate and the contact surface being at least substantially 0.33 mm. 7. A system as claimed in Claim 1, characterized in that the compensation means form part of the scanning device. 8. A system as claimed in Claim 7, characterized in that the compensation means comprise a positioning device for moving the actuator and/or the turntable relative to a frame of the scanning device in a direction at least substantially parallel to the optical axis of the objective over a positioning distance which is related to the difference in thickness ( ). 9. A system as claimed in Claim 7, characterized in that the compensation means comprise a positioning unit for moving the supporting surface of the turntable relative to other parts of the turntable in a direction at least substantially parallel to the optical axis of the objective over a positioning distance which is related to the difference in thickness ( - 10. A system as claimed in Claim 8 or 9, characterized in that the positioning distance is at least substantially equal to 14 V 1 -NA : where NA is the numerical aperture of the objective, n, is the refractive index of the material of the first substrate and n 2 is the refractive index of the material of the second substrate. 11. An optical disc suitable for use in the system as claimed in any one of the Claims 1 to 10, which disc is an optical disc of the first type, as defined in Claim 1 in combination with any one of the Claims 2, 3, 4, 5 and 6. 12. A scanning device suitable for use in a system as claimed in any one of the Claims 1 to 10, which scanning device has been defined in Claim 1 and one of the Claims 7, 8, 9 and 10.

Inventors: HOMBURG FREDERICUS GERARDUS AN, HOMBURG, FREDERICUS, GERARDUS, ANTONIUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: G11B7
IPC codes: G11B7