OPTICAL SCANNING UNIT COMPRISING A FIRST AND A SECOND LENS UNIT

Abstract: Optical scanning device for scanning an information plane (1a) of a disc-shaped optical recording medium (1) having an outer surface (1b) extending parallel to the information plane. The scanning device comprises a radiation source (7) for supplying a scanning beam (7a), and an optical scanning unit which comprises a lens system for focusing the scanning beam to a scanning spot (7b) on the information plane. The lens system has an optical axis (9) and comprises a first lens unit (9a) with a first lens (9a1) having an entrance side (11) and an exit side (13), and a second lens unit (9b) with a second lens (9b1) situated opposite the exit side of the first lens. The scanning unit also comprises a drive means for displacing the first lens along the optical axis and an adjusting means for adjusting the second lens with respect to the recording medium. To inhibit spherical and/or comatic and/or astigmatic aberrations during scanning, the adjusting means comprises a rotatably driven rotation element (15) which forms part of the second lens unit, is journaled in a stationary element (17) and has a disc-shaped surface (15a) provided with a pattern of grooves for cooperation with the outer surface of the recording medium.

Description: Optical scanning unit comprising a first and a second lens unit. The invention relates to an optical scanning device for scanning an information plane of a disc-shaped optical recording medium having at least an outer surface extending parallel to the information plane, said scanning device comprising a radiation source for supplying a scanning beam, and an optical scanning unit comprising a lens system for focusing the scanning beam to a scanning spot on the information plane, said lens system having an optical axis and comprising a first lens unit with a first lens having an entrance side for the scanning beam and an exit side situated opposite thereto, and a second lens unit with a second lens situated opposite the exit side of the first lens, said scanning unit also comprising a drive means for displacing the first lens along the optical axis and an adjusting means for adjusting the second lens with respect to the recording medium. A scanning device of this type is known from International Symposium on optical memory and optical data storage, conference edition July 8-12, 1996 (ISOM 96); OFA 2-1/345 - OFA 2-3/347; A. 0.8 Numerical Aperture Two Element Objective Lens for the Optical Disc; Kenji Yamamoto et al. The known device is intended for scanning a magneto- optical disc and is provided with a first holder having an objective lens and a transparent plate, and a second holder having a solid immersion lens (SIL). The magneto-optical disc has an information layer and a transparent layer, which information layer is read via the transparent layer. For focusing and tracking, a focusing actuator and a tracking actuator are provided for driving the first holder. A SIL actuator is present for displacing the second holder with the solid immersion lens in the focal direction in dependence upon a change of capacitance of a capacitor formed between the two holders. This has for its object to inhibit spherical aberration due to thickness deviations of the transparent layer. In this connection, reference is also made to JP-A 0821774. In addition to thickness variations, an optical disc located on a rotating table may be curved, for example bent through due to the fact that the disc is only supported in its center during scanning. Consequently, the optical disc has a certain tilt with respect to the scanning beam, which results in coma and astigmatism during scanning. Such optical aberrations are not prevented by the known measures. It is an object of the invention to provide a scanning device of the type described in the opening paragraph, with which the occurrence of spherical and or comatic and or astigmatic aberration can be inhibited during scanning. To this end, the scanning device according to the invention is characterized in that the adjusting means comprises a rotatably driven rotation element which forms part of the second lens unit and is journaled in a stationary element, said rotation element having a discshaped surface provided with a pattern of grooves for co-operation with the outer surface of the recording medium. During scanning, the groove pattern, generally consisting of spiral grooves, ensures that a pressure in the air present between the rotation element and the recording medium is generated when the rotation element is rotating. The adjusting means used in the scanning device according to the invention is actually a self-adjusting lens device which ensures during scanning that the second lens, also referred to as solid immersion lens (SIL), is instantaneously present at a correct position and in a correct parallel orientation with respect to the recording medium. The second lens is preferably a known partly spherical or aspherical lens with a plane lens surface remote from the first lens and hence facing the recording medium. The distance between the second lens and the recording medium is determined by, or is a function of, the thickness of the air gap between the outer face of the record carrier facing the lens system and the parallel extending disc-shaped surface of the rotating rotation element. The dimension, notably the height, of the air gap is, inter alia, dependent on the implementation of the groove pattern and the number of revolutions of the rotation element. In addition to the above-mentioned advantage, the adjusting means as used in the scanning device according to the invention has the advantage that the lens system can be brought and/or maintained in a focal position also in the presence of a stationary, i.e. non-rotating recording medium. The pattern of grooves or spiral grooves may be a known pattern, for example of a type as is used in axial dynamic spiral groove bearings, the grooves ensuring a pressure build-up between the grooved surface and, extending opposite this surface, the outer surface of the recording medium. When using the scanning device according to the invention, the measures applied render it possible to accurately follow, during scanning, the recording medium's outer surface facing the lens system, so that aberrations due to tilt of the medium can be limited to a minimum. This is particularly important when using radiation having a relatively short wavelength and/or when using a lens system having a relatively large numerical aperture. For obtaining larger information densities, the trend is to make use of radiation having an increasingly smaller wavelength and/or an increasingly larger numerical aperture. It is to be noted that EP-A 0 727 777 discloses a scanning device m which a pick-up unit with an objective lens and a semi-spheπcal lens secured m a sliding body which is supported by a leaf spπng are used When an optical disc is being scanned, the sliding body is in sliding contact with the optical disc surface facing the objective lens A drawback of this known scanning device is that the sliding action of the sliding body can be easily disturbed by dirt or dust particles on the surface facing the objective lens A further drawback is that positioning and oπentation of the objective lens can only take place when the optical disc rotates An embodiment of the optical scanning device according to the invention is characterized as defined m claim 2 The beaπng used requires a small space and can be easιl> integrated in the construction constituted by the stationary element and the rotation element The beaπng may compπse both a radial spiral groove beaπng section and an axial spiral groove beanng section, so that the rotation element is journaled by means of beaπng grooves both m the radial and the axial direction Duπng rotation of the rotation element, an axial beaπng gap having a certain ngidity, dependent on the rotational speed of the rotation element, is formed between the stationary element and the rotation element in the absence of the recording medium In the presence of the recording medium, also an air gap is formed between the rotation element and the recording medium, as descπbed hereinbefore In this way, a perfect balance can be realized between the rotation element and the recording medium, on the one hand, and the rotation element and the stationary element, on the other hand, withm which balance the outer surface of the medium can be accurately followed by the second lens A practical embodiment of the optical scanning device according to the invention is characteπzed as defined m claim 3 The electπc rotation motor used is preferably a dπve unit integrated in the combination constituted by the stationary element and the rotation element An embodiment of the optical scanning device according to the invention is characteπzed as defined in claim 4 In this embodiment, it is achieved that the position and oπentation of the second lens is directly coupled to the position and oπentation of the rotation element, so that an accurate tracking of the outer surface of the recording medium is easily ensured duπng use An embodiment of the optical scanning device according to the invention is characteπzed as defined in claim 5 The second lens is adjusted through adjustment of the stationary element Such a construction may be notably preferred if very stπct requirements are imposed on the central arrangement of one lens with respect to the other An embodiment of the optical scanning device according to the invention is characterized as defined in claim 6. An advantage of this embodiment is that the second lens unit is movable along with the first lens unit. Consequently, the second lens, viewed in the focusing direction, actually has a two-step adjusting facility, in which the drive means ensures a coarse adjustment and the adjusting means ensures a fine adjustment. An embodiment of the optical scanning device according to the invention is characterized as defined in claim 7. In this embodiment, a bearing, particularly a spiral groove bearing, is present between the rotation element and the stationary element, which bearing allows tilts of the rotation element with respect to the stationary element about axes which are transverse to the optical axis. An embodiment of the optical scanning device according to the invention is characterized as defined in claim 8. In this embodiment, tilts which are required for tracking the recording medium are made possible by the elastic means which is present and comprises, for example, one or more resilient elements. An embodiment of the optical scanning device according to the invention is characterized as defined in claim 9. This embodiment may be favorable if no large movements of the second lens unit in the focal direction are required. The mass of the movable part of the drive means may be smaller in this embodiment than in other embodiments. The invention also relates to an optical scanning unit suitable for use in the optical scanning device according to the invention. The scanning unit known from said ISOM 96 publication comprises a lens system for focusing the scanning beam to a scanning spot on an information plane, which lens system has an optical axis, a first lens unit with a first lens having an entrance side for the scanning beam and a facing exit side, and a second lens unit with a second lens located opposite the exit side of the first lens, which scanning unit also comprises a drive means for displacing the first lens along the optical axis, and an adjusting means for adjusting the second lens with respect to the recording medium. It is an object of the invention to improve the known scanning unit in such a way that it can be used for inhibiting spherical abeπation and/or coma and/or astigmatism when it is used in a scanning device. To this end, the scanning unit according to the invention is characterized in that the adjusting means compπses a rotatably driven rotation element which forms pan of the second lens unit and is journaled in a stationary element, which rotation element has a discshaped surface provided with a pattern of spiral grooves for co-operation with an outer surface of a recording medium duπng operation. Embodiments of the scanning unit according to the invention may have one or more of the characteristic features as defined in claims 2 through 9. The invention also relates to an optical player as defined in claim 11. The displacement device mentioned in claim 11 may comprise a known slide or swivel arm. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows diagrammatically an optical player with an embodiment of the optical scanning device according to the invention, Fig. 2 shows diagrammatically a part of a first embodiment of the scanning unit according to the invention, used in the optical scanning device, Fig. 3 shows diagrammatically a larger part of the first embodiment, Fig. 4 shows diagrammatically a part of a second embodiment of the scanning unit according to the invention, Fig. 5 shows diagrammatically a part of a third embodiment, Fig. 6 shows diagrammatically a part of a fourth embodiment of the scanning unit according to the invention, Fig. 7 shows diagrammatically a part of a fifth embodiment of the scanning unit according to the invention, Fig. 8 is a pian view of an embodiment, shown diagrammatically, of an element of the scanning unit according to the invention, Fig. 9 is a plan view of a variant of the element shown in Fig. 8. The scanning device shown in Fig. 1 is intended for writing and/or reading a disc-shaped information or recording medium 1, particularly an optical disc, in which information can be stored or has been stored in an information plane la. Information may be stored in the form of a large number of optically, magneto-optically or magnetically readable small areas (not shown) which are arranged in concentric or quasi-concentric tracks jointly constituting a spiral track. The information plane la has a protective coating (not shown) on one side, the upper side in the situation shown, and is supported on the other side by a transparent substrate or transparent layer lb having an outer surface lc extending parallel to the information plane. The scanmng device compπses an electπc dπve unit 4, particularly an electπc motor, for rotatably dπvmg the recording medium 1 about a rotation axis 5 For fixing the recording medium 1, the dπve unit 4 may be provided with a rotary table or a dπve spmdle 5a with a clamping member 5b The scanning device further compπses a scanmng unit and a radiation source 7, particularly a laser source, for generating a scanning beam 7a The scanning unit compπses a lens system for focusing the radiation beam 7a to a scanning spot 7b on the information plane la of the recording medium duπng scanning of this recording medium The lens system has an optical axis 9 and compnses a first lens unit 9a, with a first lens 9a 1, and a second lens unit 9b with a second lens 9b 1 The first lens 9a 1, also referred to as objective lens, has an entrance side 11 for the scanning beam and a facing exit side 13 opposite which the second lens 9b 1, also referred to as SIL, is situated The scanning unit also compπses a dπve means, particularly an electromagnetic actuator, for displacing the first lens 9al along the optical axis 9, and an adjusting means for adjusting the second lens 9b 1 with respect to the recording medium 1 The adjusting means compπses a rotatably dπven rotation element 15 which forms part of the second lens unit 9b, is journaled in a stationary element 17 and has a disc-shaped surface 15a with a pattern of grooves, the surface 15a cooperating, duπng scanning and with a rotating rotation element 15, with the outer surface lc of the rotating or non-rotatmg recording medium 1 , generating a pressure in the air present between the two surfaces 15a and lc Fig 8 shows an embodiment of the element 15 m a plan view A rotationally symmetπcal part of the surface 15a has regularly distπbuted shallow grooves 15al which extend spirally in such a way that, upon rotation of the rotation element 15 with respect to a recording medium 1 which is present, the air between the rotation element 15 and the recording medium 1 is compressed Fig 9 shows an implementation of the surface 15a which two patterns with grooves 15al and 15a2 instead of one groove pattern are present In the presence of a recording medium, air is compressed in the grooves in this implementation, which results in an air gap between the rotation element and the recording medium The scanning device shown m Fig 1 forms part of an optical player which is also provided with a displacement device, m this example a slide 19 for radially or approximately radially displacing the scanning unit fixed thereon with respect to the rotation axis 5, the slide 19 being movable along a straight guidance member 21 In this example, the scanning beam 7a emitted by the radiation source 7 duπng scanning is colhmated by a colhmator lens 8b after reflection by a reflective surface of a transparent plane-parallel plate 8a and subsequently focused to the scanning spot 7b by said lens system. The information layer le reflects the radiation beam to a reflected radiation beam which is converged by the lenses which are present and reaches a detection system 8c via the plate 8a. The detection system 8c supplies a detection signal during read-out, which detection signal corresponds to the information stored in the information layer la. A servosystem is provided for accurately positioning the lens 9al with respect to the recording medium 1. The scanning unit partly shown in Figs. 2 and 3 has a lens system for focusing a scanning beam to a scanning spot on an information plane 101a of a medium 101, particularly an information or recording medium. The lens system has an optical axis 109 and is provided with a first lens unit 109a, with a first lens 109al, and a second lens unit 109b with a second lens 109b 1. The scanning unit further comprises a known electromagnetic actuator 110 for displacing the first lens along the optical axis 109, and an adjusting means for adjusting the second lens 109bl with respect to the recording medium 101. The adjusting means forms part of the second lens unit 109b and comprises a rotation element 115 having a disc-shaped surface 115a provided with a pattern of grooves 115b and facing the medium 101. The rotation element 115 is rotatably journaled with respect to a cylindrical stationary element 117 for which a radial and axial spiral groove bearing 118a, 118b is used in this example. Such a bearing is known per se and used, for example, as a bearing for a head drum in a video recorder as shown in JP-A 1-8505. To drive the rotation element 115, an electric rotation motor is provided which has a stator section comprising the stationary element 117 and a rotor section comprising the rotation element 115. The rotor section comprises, for example a permanent disc-shaped or cylindrical magnet, and the stator section comprises a set of drive coils in order that both sections of the rotation motor can magnetically co-operate over an air gap 120. During rotation of the rotation element 115. hence when the rotation motor is energized, the spiral groove bearing which is present ensures that pressure, is built up in the medium, for example in the form of air,between the stationary and rotary bearing sections, which not only results in a perfect bearing but also in a force on the second lens unit 109b, which force is directed towards the recording medium. Simultaneously, pressure is built up in the air between the surface 115 provided with grooves 115b and the outer surface 101b of the recording medium 101, which exerts a force on the second lens unit 109b, which force is directed away from the recording medium 101. When the scanning unit is suitably dimensioned, both oppositely directed forces ensure a stable position of the second lens 109bl at a defined distance d from the recording medium 101. so that an accurate tracking of the outer surface 101b is ensured during scanning. In order that the required movements of the lens unit 109b during tracking of the outer surface 101b are made possible, the bearing in this example is dimensioned in such a way that the rotation element 115 can perform tilted movements with respect to the stationary element 117 secured to the first lens unit 109a through a rigid connection 122. The scanning device according to the invention, partly shown in Fig. 4, is intended for scanning a disc-shaped optical recording medium 201 having an information plane 201a and an outer surface 201b extending parallel thereto. The scanning device comprises a lens system with an optical axis 209, provided with a first lens unit 209a, with a first lens 209al, and a second lens unit 209b with a second lens 209b 1. The scanning unit also comprises an electromagnetic drive means 210 for displacing the first lens unit 209a with the first lens 209al along the optical axis. Moreover, the scanning unit comprises an adjusting means for adjusting the second lens unit 209b with the second lens 209b 1 with respect to the recording medium 201. The adjusting means comprises a rotatably driven rotation element 215 which is axially and radially journaled in a stationary element 217. The rotation element 215 forms part of the second lens unit 209b, supports the second lens 209b 1 and has a disc-shaped surface 215a with a pattern of spiral grooves for co-operation with the outer surface 201b of the medium 201. The stationary element 217 is secured to a movable part 210a of the drive means 210 by way of an elastic means, in this example three leaf springs 224. This renders the combination constituted by the rotation element 215 and the stationary element 217 tiltable about axes directed transversely to the optical axis 209 during tracking of the recording medium 201. The embodiments shown in Figs. 5, 6 and 7 largely resemble the embodiments already described. The description of the embodiments shown in Figs. 5, 6 and 7 is therefore mainly limited to details deviating from the scanning units shown in Figs. 1, 2 and 3. The scanning unit shown in Fig. 5 has a first lens unit 309a and a second lens unit 309b with a second lens 309b 1. The second lens unit 309b comprises a stationary element 317 which is secured in a displaceable and tiltable manner to the second lens unit 309a via an elastic means such as a leaf spring construction 324. The second lens 309bl is secured to the stationary element 317. The scanning unit also comprises an adjusting means in the form of a rotatably driven rotation element 315 which is journaled in the stationary element 317, and which has a disc-shaped surface 315a with a spiral groove pattern for co-operation with an outer surface 301b of a recording medium 301. The journaling of the rotation element 315 with respect to the stationary element is effected in this example by means of an aerodynamic spiral groove beaπng which compπses both axial beaπng sections and radial bearing sections The movements made by the second lens 309b 1 during adjustment are directly derived from the movements of the stationary element 317 in the construction shown. This has the advantage that the position of the second lens 309bl is less sensitive to fluctuations of the rotating rotation element 315 due to small radial deviations of the mass center from the geometrical axis. The second lens 309b 1 does not make a rotation. Fig. 6 shows an embodiment of a scanning device with a stationary element 417 which is rigidly secured to a stationary frame 418, for example, constituted by a stationary part of a drive means for displacing a first lens 409a along an optical axis 409. A rotation element 415, which is aerodynamically journaled in the stationary element 417, supports a second lens 409b. It is to be noted that the invention is not limited to the embodiments shown. Direct variations of the implementation of the adjusting means are possible within the scope of the invention. However, an essential point is that the adjusting means comprises a rotatably driven rotation element which is journaled in a stationary element and has a surface with a pattern of grooves for co-operation with an outer surface of a recording medium for building up pressure in air which is present between the two surfaces. Fig. 7 shows an embodiment of a scanning device with a stationary element 517 which is resiliently secured to a stationary frame 518. It is to be noted that the invention is not limited to the embodiments shown, For example, the pattern of grooves may be implemented in a different way than is shown. As regards the claims, it is to be noted that various characteristic features as defined in the dependent claims may occur in combination.

Claims: CLAIMS 1 An optical scanning device for scanning an information plane of a disc-shaped optical recording medium having at least an outer surface extending parallel to the information plane, said scanning device compnsmg a radiation source for supplying a scanning beam, and an optical scanning unit compnsmg a lens system for focusing the scanning beam to a scanning spot on the information plane, said lens system having an optical axis and compnsmg a first lens unit with a first lens having an entrance side for the scanning beam and an exit side situated opposite thereto, and a second lens unit with a second lens situated opposite the exit side of the first lens, said scanning unit also compnsmg a dnve means for displacing the first lens along the optical axis and an adjusting means for adjusting the second lens with respect to the recording medium, characteπzed m that the adjusting means compπses a rotatably dπven rotation element which forms part of the second lens unit and is journaled m a stationary element, said rotation element having a disc-shaped surface provided with a pattern of grooves for co-operation with the outer surface of the recording medium 2 An optical scanning device as claimed in claim 1 , characteπzed by the presence of a spiral groove beaπng for journalmg the rotation element with respect to the stationary element 3 An optical scanning device as claimed m claim 1 , charactenzed by the presence of an electnc rotation motor which has a stator section compnsmg the stationary element and a rotor section compnsmg the rotation element and being capable of magnetically co-operating with the stator section over a gap 4 An optical scanning device as claimed m claim 1, charactenzed m that the second lens forms part of the rotation element 5 An optical scanning device as claimed in claim 1 , characteπzed in that the second lens forms part of the stationary element 6. An optical scanning device as claimed in claim 1, characterized in that the stationary part is supported by a movable part of the drive means. 7. An optical scanning device as claimed in claims 4 and 6, characterized in that the stationary element is rigidly secured to the movable part, the rotation element being tiltably journaled with respect to the stationary element. 8. An optical scanning device as claimed in claim 6, characterized in that the stationary part is connected to the movable part by way of an elastic means. 9. An optical scanning device as claimed in claim 1 , characterized in that the stationary part is secured to a stationary frame of the scanning unit. 10. An optical scanning unit suitable for use in the optical scanning device as claimed in any one of the preceding claims and implemented as defined in one or more of claims 1 through 9. 11. An optical player comprising a drive unit for rotatably driving a disc-shaped optical recording medium about a rotation axis, comprising a scanning device as defined in one or more of claims 1 through 9, and provided with a displacement device for substantially radially displacing the scanning unit with respect to the rotation axis.

Inventors: ZIJP FERRY, ZIJP, FERRY
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1999
Main classification: G11B7
IPC codes: G11B7, G11B21