OPTICAL SYSTEM FOR DETECTING A POSITION OF AN OBJECTIVE LENS

Abstract: In an optical system, a laser beam emitted from a laser unit (2) is convered by a collimater lens (6) into a parallel light beam, and converged to a light reflecting surface (16) of an optical disk (14) having a tracking guide by an objective lens (12). The objective lens (12) is suspended in a lens barrel assembly (22) by a spring member so as to be moved by voice coil along the optical axis of the objective lens (12) for focus and by linear motor in a direction perpendicular to the optical axis for tracking control. As a result that the lens barrel assembly (22) is moved along the optical disk, the objective lens (12) is transferred to the direction perpendicu­ lar to the optical axis of the system. Then, part of the light beam transferred from the objective lens (12) is directed to any one of the first and second photosensitive sections (66-1, 66-2) of the photodetecor (64). As a result that the linear motor is driven in response to the difference of the signals from the first and second photosensitive sections (66-1, 66-2), the vibration of the objective lens (12) can be stopped.

Description: The present invention relates to a system for recording and reproducing or recording, reproducing and erasing information on an information recording medium such as an optical disk using a light beam and, more particularly, to an optical system for detecting a position of an objective lens in an optical head when an optical head for focusing a light beam on a light reflecting surface of an optical disk is skipped on the optical disk to trace a desired tracking guide using a light beam, thereby preventing the objective lens from vibrating. In an information recording medium such as an optical disk, information is recorded on a light reflecting surface of a tracking guide extending concentrically or spirally along the circumference of the disk by changing the condition of the light reflecting surface, for example forming pits on the light reflecting space. This recording information includes preinformation such as a tracking address and a sector address recorded in advance and data information such as an image to be recorded later, and the data information is recorded or reproduced while reading out the preinformation. Further, an objective lens is in general suspended by a leaf spring movably to the optical axial direction and to the direction perpendicular to the optical axis for the control of the focus and the tracking. When information is reproduced or recorded from a desired tracking guide by skipping from a predetermined position in such an optical head, the optical head is moved at a relatively high speed along the radial direction of the optical disk, for example, by a linear actuator. However, there arises a problem that, when the optical head is moved at a high speed, an objective lens vibrates in the head and, even if the lens is located at a position of a desired tracking guide, the lens continues vibrating so that information cannot be immediately recorded or reproduced. Prior art document EP-A-0 098 076 discloses a beam access apparatus for an optical disk system similar to that described in the precharacterizing part of claim 1. This apparatus includes an objective lens whose position is detected in a tracking direction with respect to an optical base in an optical head. In a head access mode, the objective lens is positioned and fixed at a prescribed position in the optical head to suppress damping vibrations to enable the head to be accessed at a high speed. In a lens access mode and a track follow mode subsequent to the head access mode, the controlling position for the optical head 1 is such that the objective lens is arranged at a prescribed position in the optical head to enable a track to be followed precisely by a light spot and track jumping of the light spot to be reliably carried out by driving the objective lens. This known beam access apparatus uses a photodetector which has two photosensitive regions for tracking servo control. This photodetector can indeed detect a vibration of the objective lens. However, the tracking servo control loop is closed in this known apparatus upon detection of a vibration involved. It is an object of the present invention to provide an optical system for optically detecting a position of an objective lens in an optical head to stop a vibration of the objective lens when the objective lens vibrates in the optical head. To solve this object the present invention provides an optical system as stated in claim 1. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram showing an embodiment of an optical system for recording and reproducing information according to the present invention; Fig. 2 is a sectional view showing a lens barrel for supporting an objective lens; Figs. 3A to 3C and Figs. 4A to 4C are views showing orbits of a light beam in an optical system for detecting a position of an objective lens; and Fig. 5 is a schematic view of another embodiment of an optical system for detecting a position of an objective lens according to the present invention. Fig. 1 shows an embodiment of an optical system for recording and reproducing information according to the present invention. In this system, a laser beam emitted from a laser unit 2 is collimated by a collimator lens 6 into a parallel light beam. The parallel light beam is reflected by a polarization beam splitter 8, and transmitted through a λ/4 plate 10 into an objective lens 12. This parallel light beam is converged by the lens 12 toward a light reflecting layer 16 of an optical disk 14. The disk 14 is placed on a turntable 18 rotated by a drive motor 20, and rotated together with the turntable 18. When the lens 12 is in a just-in-focusing state, the minimum beam spot corresponding to the beam waist of the converted light beam is formed on the light reflecting surface. When the lens 12 is in a defocusing state, a beam spot larger than the minimum beam spot corresponding to the beam waist of the converted light beam is formed on the light reflecting surface. A tracking guide is helically or concentrically formed on the light reflecting surface of the optical disk 14, preinformation such as a tracking address and a sector address is formed in advance as prepits on the tracking guide, and data information such as an image and so on is written as pits by the laser beam in the just-in-focusing state on the disk 14. The objective lens 12 is supported, as shown in Fig. 2, by a lens barrel assembly 22 so that the objective len can be finely moved in an optical axial direction of the lens 12 and in a direction perpendicular to the axial direction and to a direction perpendicular to the optical axis and that the tracking guide is extended. This assembly 22 has an outer cylinder 24 and an inner cylinder 26, and the inner cylinder 26 is capable of being transferred to the direction of the optical axis in a way that the end faces thereof are suspended by leaf spring members 28, 30 to the end faces of the outer cylinder 24. The cylinder 26 has a cup-shaped part extending from the lower end of the cylinder 26, and a voice coil 32 for a focus control is wound on the cup-shaped part of the cylinder 26. A permanent magnet 34 and yokes 36, 38 are formed on the inner surface of the cylinder 24. Thus, when the voice coil 32 is supplied with a current, the cylinder 28 moves along the optical axis depending upon the direction of the current. A leaf spring 40 is secured to the lower end of the cylinder 26, a lens holder 42 formed of a permanent magnet member is secured to the upper end of the spring 40, and the lens 12 is secured into the holder 42. A core frame (not shown) is provided on the outer periphery of the inner cylinder 26, and a coil 44 as a linear motor for a tracking control is wound on the core frame (not shown) opposite to the holder 42. Therefore, when the coil 44 is supplied with a current, the holder 42 is moved in a direction perpendicular to the optical axis, i.e., to the plane of the drawings, depending upon the direction of the current. The laser beam reflected from the light reflecting layer 16 of the disk 14 is introduced again through the lens 12 and the λ/4 plate 10 to the splitter 8. When the laser beam reciprocates to pass through the plate 10, the polarized surface of the laser beam rotates. Thus, the laser beam returned to the splitter 8 is directed through the splitter 8 toward a half mirror 46. The laser beam is divided into two by the mirror 46, and directed toward projection lenses 48, 50. One laser beam is directed to the lens 48 disposed between the mirror 46 and the lens 48. A part of the one laser beam is interrupted by a knife edge 52 inserted into a direction perpendicular to the optical axis, and the remaining part is converged by the lens 48 toward a photodetector 54 for a focus control. The photodetecting surface of the photodetector 54 is disposed substantially on an image forming plane on which an image of a beam spot is formed when the objective lens is in a just-in-focusing state. Therefore, the parallel laser beam is incident to the lens 48 and the converged laser beam is directed by the lens 48 toward the region between photosensitive regions 54-1 and 54-2, and signals of equal level are generated from the regions 54-1 and 54-2, when the objective lens 48 is in just-in-focusing state. On the other hand, the diverged or converted laser beam is incident to the lens 48 when in a defocusing state. The laser beam is directed by the lens 48 toward one of the regions 54-1, 54-2, and signals of different levels are generated from the regions 54-1, 54-2. The signals generated from the regions 54-1, 54-2 are respectively amplified by preamplifiers 56, 58, and supplied to a differential amplifier 60. Thus, the focusing signal of zero level is generated from the amplifier 60 when the objective lens 48 is in the just-in-focusing state, and a defocusing signal of plus or minus level or plus level is generated from the amplifier 60 when the objective lens 48 is in the defocusing state. Since a voice coil driver 62 supplies an energizing current to the voice coil 32 for a focus control in response to the signal generated from the amplifier 60, the lens 12 is allowed to retain in the just-in-focusing state by the voice coil 32, or to transfer from the defocusing state to the just-in-focusing state. The other laser beam reflected from the mirror 46 is directed by the lens 50 to a photodetector 64 for a tracking control. A beam spot is formed by the laser beam projected to the photodetecting surface of the photodetector 64, and a diffraction image of the tracking guide is produced as a dark part in the beam spot. When the laser beam directed from the lens 12 toward the light reflecting surface 16 is suitably tracing the tracking guide, the diffraction image of the tracking guide is synmetrically produced in the beam. spot in respect to the center of the beam spot. On the other hand, when the laser beam erroneously traces the tracking guide, the diffraction image of the tracking guide is unsynmetrically produced in the beam spot, that is, the chart part is displaced from the center of the beam spot. Thus, light rays of equal intensity are incident on the photosensitive regions 64-1, 64-2 of the photodetector 64 for the tracking control, and signals of equal level are generated from the regions 64-1, 64-2. On the other hand, when the laser beam erroneously traces the tracking guide, light rays of different intensities are incident on the regions 64-1, 64-2 of the photodetectors 64, and signals of different levels are generated from the regions 64-1, 64-2. The signals generated from the regions 64-1, 64-2 are amplified by preamplifiers 67, 68 and supplied to a differential amplifier 70. Therefore, while the laser beam is suitably tracing the tracking guide, the tracking signal of zero level is generated from the amplifier 70, and while the laser beam erroneously traces the tracking guide, the tracking signal of plus or minus level is generated from the amplifier 70, and the signal is supplied through a switching circuit 74 to a linear motor driver 72. Since the driver 72 supplies the coil 44 with the energizing current in response to the signal generated from the amplifier 70, the lens 12 is allowed to retain at the position by the coil 44, or moved to a direction perpendicular to the optical axis so that the laser beam suitably traces the tracking guide. The signals amplified by the preamplifiers 67, 68 are added by an adder 76, the sum output is outputted as data information so that the data information is read out from the optical disk. When an optical head 84 is, for example, moved from a predetermined position such as a home position to reproduce or record information from a desired tracking guide, a linear actuator 82 is actuated by an actuating signal from the CPU 80, and the head 84 is moved by the actuator 82 radially of the head 14 at a relatively fast speed. Since the lens 12 is suspended indirectly by the spring members 28, 30 at this time as described above, the lens 12 is vibrated at the moving time of the head 84, i.e., displaced radially of the disk 14 in the assembly 22. Even if the lens 12 is displaced radially of the disk 14, the lens position or location is detected by the regions 66-1, 66-2 of the photodetector 64 as described below, and the lens 12 is so maintained as that the optical axis of the optical system always coincides with that of the lens 12. A system for detecting a parallel light beam returned through the lens 12 from the layer 16 in a just-in-focusing state will be described with reference to Fig. 2 for the simplicity of description. As apparent from Fig. 1, in the optical system as shown in Fig. 1, the diameter of the opening of the lens 12 is sufficiently smaller than that of the parallel light beam directly toward the layer 16, and the diameter of the beam is determined so that the lens 12 is capable of being transferred by the coil 44 in the beam. Thus, only the light beam passed through the lens 12 is directed toward the layer 16, and the parallel light beam emerged from the lens 12 has a diameter substantially equal to the diameter of the opening of the lens 12 as shown in Figs. 3A to 3C. As shown in Figs. 3A to 3C, the photodetector 64 is arranged on the region to which the beam is directed. As shown in Fig. 3A, the regions 64-1, 64-2 for detecting the signals are located in the optical path of the light beam when the optical axis of the lens 12 is aligned with that of the optical system, and the regions 66-1, 66-2 for detecting the position of the lens 12 are located out of the optical path. Therefore, as shown in Fig. 3A, when the optical axis of the lens 12 is aligned with that of the optical system, the light beam is incident only on the regions 64-1, 64-2 for detecting the signal, but not incident on the regions 66-1, 66-8 for detecting the position of the lens 12. Consequently, an output signal is not generated from the regions 66-1, 66-2. On the other hand, as shown in Figs. 3B or 3C, when the head 84 is moved radially of the disk 14 at a relatively high speed so that the lens 12 is vibrated, i.e., displaced radially of the disk 14, the light beam is incident on not only the regions 64-1, 64-2 but any one of the regions 66-1, 66-2 for detecting the position of the lens 12. As a result, an output signal is generated from any of the regions 66-1, 66-2. The differential signal of both represents the displacement of the optical axis of the optical system from that of the lens 12. As apparent from above, part of the regions 66-1, 66-2 for detecting the position of the lens 12 may be located in the optical path of the light beam when the optical axis of the lens 12 is aligned with that of the optical system. In this case, part of the light beam may be detected by the regions 66-1, 66-2. In the system shown in Fig. 1, the position of the lens 12 is detected by the regions 66-1, 66-2 in the same manner as described above. That is, when the optical axis of the lens 12 is aligned with that of the optical system as shown in Fig. 4A, the light beam is not incident on the regions 66-1, 66-2 for detecting the position of the lens 12. Thus, an output signal is not generated from the regions 66-1, 66-2. On the other hand, as shown in Figs. 4B or 4C, when the head 84 is displaced, the light beam is incident on any of the regions 66-1, 66-2 for detecting the position of the lens 12. Thus, an output signal is generated from any one of the regions 66-1, 66-2. This signal is amplified by preamplifiers 90, 92, and supplied to a differential amplifier 94. Therefore, a position signal of plus or minus level is generated from the amplifier 94, and supplied through a switching circuit 74 to the second voice coil driver 72. Since the driver 72 supplies the coil 44 with the energizing current in response to the signal generated from the amplifier 94, the lens 12 is moved so that the optical axis of the lens 12 is aligned with that of the optical system. In other words, the vibration of the lens 12 is stopped. In a system shown in Fig. 1, a switching signal is applied to the switching circuit 74 simultaneously when the actuating signal is generated from the actuator 82 from the CPU 80 in case the head 84 is moved, and the amplifier 94 for detecting the position of the lens 12 is connected through the switching circuit 74 to the linear motor driver 72. When the head 84 is stopped and the vibration of the lens 12 is stopped, an output signal is not generated from the regions 66-1, 66-2. Thus, an output signal is not generated from the amplifier 94. When the position signal of zero level is applied from the converter 78 to the CPU 80, the CPU 80 applies a switching signal to the switching circuit 74, the amplifier 94 is disconnected from the driver 72, and the amplifier 70 is connected through the switching circuit 74 to the driver 72. As a result, a tracking servo control is operated, and the laser beam suitably traces the tracking guide. According to the present invention as described above, the position of the objective lens in the optical head can be optically detected, and the vibration of the lens can be reliably stopped even when the lens is vibrated in the head. The present invention is not limited to the particular embodiments described above. Various other changes and modifications may be made within the scope of the present invention. For example, as shown in Fig. 5, prisms 96, 98 may be disposed out of the optical path of the light beam emerged from the objective lens 12 when the optical axis of the lens 12 is aligned with that of the optical system. In this embodiment, when the optical axis of the objective lens is aligned with that of the optical system, the light beam is incident on only the regions 66-1, 66-2 for detecting the position of the lens. On the other hand, when the head 84 is displaced, the light beam is partly incident on any of the prisms 96, 98, part of the light beam is deviated by any of the prisms 96, 98, and directed toward any of the regions 66-1, 66-2. As a result, the position of the lens 12 can be detected.

Claims: An optical system having a fixed optical axis, for focusing a light beam onto a light reflecting surface (16) of an information record carrier, comprising: a light source (2) for generating a light beam; collimating means (6) for collimating the light beam; an objective lens (12) having an optical axis, for converging the collimated light beam on the light reflecting surface (16) and transferring the light beam reflected from the light reflecting surface (16); supporting means (28, 30, 40) for supporting said objective lens (12) movably along the optical axis of the lens (12) and a predetermined direction perpendicular thereto, first moving means (32, 34, 42, 44) for finely moving said objective lens (12) along the optical axis of said objective lens (12) and the predetermined direction perpendicular to the optical axis; photodetecting means (64) having at least first and second photosensitive sections (64-1, 64-2) which are separately arranged, for detecting a part of the light beam transferred from said objective lens (12) to generate electric signals, and second moving means (72) for moving a head assembly (22) including said objective lens (12), said supporting means (28, 30, 40) and said first moving means (32, 34, 42, 44) in the predetermined direction; wherein: said photodetecting means (64) generate the electric signals, when said optical head assembly (22) is moved by the second moving means (72) and said objective lens (12) is vibrated over an allowable range along the predetermined direction, said first moving means (32, 34, 42, 44) maintaining the objective lens (12) in the allowable range in response to the electric signals from the photodetecting means (64), characterized in that third and fourth photosensitive sections (66-1, 66-2) are arranged out of an optical path, through which a light beam transferred from said objective lens (12) is passed when the optical axis of said objective lens (12) is aligned with that of said optical system, said third and fourth photosensitive sections (66-1, 662) detecting the position of the objective lens (12) when the objective lens (12) is displaced radially of said informaton record carrier (12). An optical system according to claim 1, characterized by further comprising: a projection lens (50) for converting a light beam transferred from said objective lens (12). An optical system according to claim 2, characterized in that said third and fourth photosensitive sections (66-1, 66-2) are arranged out of an optical path, through which a light beam converted by the projection lens (50) is passed when the optical axis of said objective lens (12) is aligned with that of said optical system. An optical system according to claim 1, characterized in that said first moving (32, 34, 42, 44) for finely moving said objective lens is energized in response to the difference of signals generated from the third and fourth photosensitive sections (66-1, 66-2). An optical system according to claim 1, characterized in that said first moving means (32, 34, 42, 44) for finely moving said objective lens (12) stops the movement of the objective lens (12) in response to the difference of signals generated from the third and fourth photosensitive sections (66-1, 66-2), when the head assembly (22) is caused to move. An optical system according to claim 1, characterized by further comprising: a tracking guide formed on said light reflecting surface (16), and means (64-1, 64-2) for detecting a light beam transferred from the objective lens (12) to generate a tracking signal, thereby energizing said first moving means (32, 34, 42, 44) for finely moving the objective lens (12) in response to the tracking signal. An optical system according to claim 6, characterized by further comprising: means (80, 74) for connecting said photodetecting means (66-1, 66-2) to said first moving means (32, 34, 42, 44) for finely moving the objective lens (12) and disconnecting said detecting means (64-1, 64-2) for generating a tracking signal when said head assembly (22) is moved, and disconnecting said first and second photosensitive sections (66-1, 66-2) from said first moving means (32, 34, 42, 44) for finely moving the objective lens (12) and connecting said detecting means (64-1, 64-2) for generating a tracking signal when the head assembly (2) is stopped and the optical axis of the objective lens (12) is aligned with that of said optical system.

Inventors: ANDO, HIDEO C/O PATENT DIVISION, DOI, AKIHIKO
Applicants: KABUSHIKI KAISHA TOSHIBA, TOSHIBA AUTOMATION EQUIPMENT ENGINEERING LIMITED
Application year: 1985
Main classification: G11B7
IPC codes: G11B7