OPTICAL PICK-UP DEVICE

Abstract: An optical pick-up device comprises a light beam source (1), an object lens system (4) for causing a light beam emitted from the light beam source (1) to impinge upon a record medium (D) and receiving a reflected light beam from the record medium (D), an intermediate lens system (20) composed of a single lens element for causing the light beam emitted from the light beam source (1) to be slightly converged and enter into the object lens system (4) and receiving the reflected light beam having passed through the object lens system (4), a beam splitter (2) for separating the reflected light beam having passed through both the object lens system (4) and the intermediate lens system (20) from the light beam emitted from the light beam source (1) to enter into the intermediate lens system (20), and a photodetector (6) for detecting the reflected light beam having passed through the beam splitter (2). A spherical aberration brought forth by the intermediate lens system (20) on the light beam passing through the intermediate lens system (20) and the object lens system (4) to the record medium (D) is cancelled at the object lens system (4).

Description: The present invention relates to optical pick-up device for reading information signals from record media, and more particularly, to an optical pick-up device operative to cause a light beam to impinge upon a record medium through an object lens system and guide a reflected light beam coming from the record medium through the object lens system to a photodetector, so as to obtain from the photodetector an output signal representing an information read from the record medium. In an optical disc player for optically reproducing an information signal recorded on a disc-shaped record medium, it is necessary to provide an optical pick-up device for reading the information signal from a record track formed on the disc-shaped record medium. An example of an optical pick-up device (cf. GB-A-2 052 132) is constituted, as schematically shown in Fig. 1, to form an optical unit containing a semiconductor laser 1, an object lens system 4, a photodetector 6 and other optical elements, and mounted on the optical disc player to be movable along a direction of a radius of a disc D which is loaded on the optical disc player and provided with, for example, a spiral record track formed thereon. In the optical pick-up shown in Fig. 1, a laser light beam from the semiconductor laser 1 passes through a beam splitter 2 without being deflected thereat and then enters into a collimating lens system 3 to be collimated thereby. The laser light beam having passed through the collimating lens system 3 is focused by the object lens system 4 to impinge upon the disc D so as to be modulated in intensity and reflected at the spiral record track on the disc D to be a reflected laser light beam. The reflected laser light beam from the disc D passes through the object lens system 4 and the collimating lens system 3 to the beam splitter 2 and is deflected at the beam splitter 2 so as to pass to the photodetector 6 through a light receiving lens system (concave lens system) 5. The photodetector 6 detects the reflected laser light beam from the disc D to produce detection output signals varying in response to variations in the reflected light beam. The detection output signals obtained from the photodetector 6 are supplied to a signal processing circuit (not shown in the drawings) in which a reproduced information signal, focusing control signal and tracking control signal are produced. The focusing control signal and tracking control signals are supplied to a lens driving unit 7 for focus control and a lens driving unit 8 for tracking control both disposed in relation to the object lens system 4, respectively, so that a focus servo control and a tracking servo control are performed. In a previously proposed optical pick-up device, the collimating lens system 3 by which the laser light beam from the semiconductor laser 1 is collimated to enter into the object lens system 4 is composed of a set of concave lens element 3a and convex lens element 3b bonded together, so that the spherical aberration of the laser light beam passing through the collimating lens system 3 is diminished. Further, Fig. 2 shows schematically another example of a previously proposed optical pick-up device employed in the optical disc player. The optical pick-up device shown in Fig. 2 corresponds to a modification of the optical pick-up device shown in Fig. 1 wherein a collimating lens system 10 is provided in place of the collimating lens system 3 shown in Fig. 1. In the optical pick-up device shown in Fig. 2, the collimating lens system 10 is composed of a single convex lens element and therefore may be easily produced at reduced cost. In the case of the previously proposed optical pick-up device shown in Fig. 1 in which the collimating lens system 3 composed of a combination of the concave and convex lens elements 3a and 3b is employed so that the spherical aberration of the laser light beam passing through the collimating lens system 3 is diminished, however, skillful works taking much time are required for polishing up four different lens surfaces including a pair of opposite outer surface of the concave lens element 3a and another pair of opposite outer surface of the convex lens element 3b, processing a peripheral portion of each of the concave lens element 3a and the convex lens element 3b, positioning both of the concave lens element 3a and the convex lens element 3b precisely in a predetermined mutual relation, and bonding the concave lens element 3a and the convex lens element 3b together precisely, in production of the collimating lens system 3. Therefore, there is a disadvantage that production cost of the collimating lens system 3 is increased. While, in the case of the optical pick-up device shown in Fig. 2 in which the collimating lens system 10 consists of the single convex lens element so as to be easily produced at reduced production cost is employed, there is a defect that it causes a relatively large spherical aberration of the laser light beam passing through the collimating lens system 10 and therefore the laser light beam collimated by the collimating lens system 10 is apt to have a large wave front aberration. For example, Fig. 3 shows a relationship between the length (L) of an optical path from a principal plane of the single convex lens element constituting the collimating lens system 10 to a light emitting point in a semiconductor laser 1 and the wave front aberration (Ab) of the laser light beam at a spot formed on the disc D by the laser light beam, wherein Lf represents the focal distance of the single convex lens element. It is understood from this relationship that when the single convex lens element is so located that the principal plane of the single convex lens element is distant by the focal distance Lf from the light emitting point of the semiconductor laser 1 along the optical path from the principal plane of the single convex lens element to the light emitting point in the semiconductor laser 1, that is, the length (L) of the optical path is selected to be identical with the focal distance Lf, the wave front aberration (Ab) is considerably large. In the case where the wave front aberration (Ab) of the laser light beam at the spot formed on the disc D by the laser light beam is considerably large as described above, the optical pick-up device employing the collimating lens system 10 comes to be deteriorated in its optical characteristic, such as an information reading performance. Accordingly, it is an object of the present invention to provide an optical pick-up device employing an intermediate lens system disposed on an optical path between a light beam source and an object lens system for causing a light beam from the light beam source to enter into the object lens system, which avoids the aforementioned problems encountered with the prior art. Another object of the present invention is to provide an improved optical pick-up device employing an intermediate lens system which is disposed on an optical path between a light beam source and an object lens system for causing a light beam from the light beam source to enter into the object lens system so that the light beam from the light beam source is focused properly by the object lens system to impinge upon a record medium, and by which the wave front aberration of the light beam passing through the object lens system to the record medium is sufficiently diminished so as not to deteriorate an optical characteristic, such as an information reading performance, of the device. A further object of the present invention is to provide an improved optical pick-up device employing an intermediate lens system which is disposed on an optical path between a light beam source and an object lens system for causing a light beam from the light beam source to enter into the object lens system so that the light beam from the light beam source is focused properly by the object lens system to impinge upon a record medium, and can be easily produced at reduced cost, and by which the wave front aberration of the light beam passing through the object lens system to the record medium is sufficiently diminished. According to the present invention, there is provided and optical pick-up device comprising a light beam source, object lens means for focusing a light beam from said light beam source to cause the same to impinge on a record medium and receiving a reflected light beam coming from the record medium, intermediate lens means for causing the light beam emitted from said light beam source to enter into said object lens means and for receiving the reflected light beam having passed through said object lens means, said intermdiate lens means comprising a single lens element, beam splitting means for separating the reflected light beam having passed through both of said object lens means and said intermediate lens means from the light beam emitted from said light beam source to enter into said intermediate lens means, and photodetecting means for detecting the reflected light beam having passed through said beam splitting means, characterized in that said intermediate lens means comprising a single plano-convex element is located along the optical path and situated at a distance slightly longer than the focal distance of said intermediate lens element from the light emitting point so as to converge the light beam emitted from said light beam source slightly as compared with a parallel light beam, so that a spherical aberration brought forth by said intermediate lens means on the light beam passing through said intermediate lens means and said object lens means is cancelled at the object lens means. In the optical pick-up device thus constituted in accordance with the present invention, the beam splitter and the intermediate lens system composed of the single lens element are provided on the light path between the light beam source formed with a semiconductor laser or the like and the object lens system, and the light beam from the light beam source passes through the beam splitter to the intermediate lens system and is focused by the intermediate lens system to be slightly converged as compared with a parallel light beam and to enter into the object lens system. The light beam having passed through the intermediate lens system is focused sufficiently by the object lens system to impinge upon the record medium. In such an arrangement, the light beam entering into the object lens system is converged by the intermediate lens system so slightly as to be focused by the object lens system in substantially the same manner as the parallel light beam and to be subjected to cancellation of the spherical aberration,which is imposed thereon by the intermediate lens system, at the object lens system. Accordingly, the fact that the light beam is slightly converged before it enters into the object lens system to be focused thereby does not exert any substantially harmful influence upon an optical characteristic such us an information reading performance of the device, but causes the light beam impinging upon the recording medium to have a reduced spherical aberration. Then, the reflected light beam from the record medium passes through the object lens system and the intermediate lens system to the beam splitter, and is guided by the beam splitter to the photodetector so that a detection output signal representing an information read from the record medium is obtained from the photodetector. In such an optical pick-up device according to the present invention, the intermediate lens system is formed by the single lens element and therefore can be easily produced at reduced cost without requiring skillful works taking much time. Further, as a result of the fact that the light beam from the light beam source is focused by the intermediate lens system so as to be converged slightly as compared with the parallel light beam and enter into the object lens system, a wave front aberration of the light beam passing through the intermediate lens system and the object lens system to the record medium is sufficiently diminished and consequently the optical characteristic of the device is not deteriorated though the intermediate lens system consisting of the single lens element is used. The above and other objects, features and advantages of the present invention will become apparent from the following detailed description which is to be read in conjunction with the accompanying drawings. Fig. 1 is a schematic illustration showing an optical pick-up device proposed previously; Fig. 2 is a schematic illustration showing another optical pick-up device proposed previously; Fig. 3 is a characteristic diagram used for explaining the optical characteristic of the optical pick-up device shown in Fig. 2; Fig. 4 is a schematic illustration showing one embodiment of optical pick-up device according to the present invention; and Fig. 5 is a characteristic diagram used for explaining the optical characteristic of the embodiment shown in Fig. 4. The invention will now be described by way of example with reference to the accompanying drawings. Fig. 4 shows one embodiment of optical pick-up device according to the present invention. This embodiment is so constituted as to correspond substantially to a modification of the optical pick-up device shown in Fig. 1, wherein an intermediate lens system 20 is provided in place of the collimating lens system 3. Accordingly, in Fig. 4, elements and parts other than the intermediate lens system 20 and corresponding to those of Fig. 1 are marked with the same references and further description thereof will be omitted. In the embodiment shown in Fig. 4, the intermediate lens system 20 which is disposed on an optical path between a beam splitter 2 and an object lens system 4 for guiding a laser light beam emitted from a semiconductor laser 1 and having passed through the beam splitter 2 to the object lens system 4, is formed by a single plano-convex lens element. Further, the intermediate lens system 20 is so located that a principal plane of the single plano-convex lens element is distant by a distance Lx, which is slightly longer than the focal distance of the single plano-convex lens element, from a light emitting point of the semiconductor laser 1. Accordingly, the laser light beam emitted from the semiconductor laser 1 and having passed through the beam splitter 2 to the intermediate lens system 20 is focused by the intermediate lens system 20 so as to be converged slightly as compared with a parallel light beam as shown imaginarily with a dot and dash line in Fig. 4 to make, for example, a minute angle α in relation to the parallel light beam, and then is caused to enter into the object lens system 4. Further, the laser light beam is focused by the object lens system 4 to impinge upon a disc D on which a spiral record track is formed. In such a situation, the laser light beam entering into the object lens system 4 from the intermediate lens system 20 is converged by the intermediate lens system 20 so slightly as to be focused by the object lens system 4 in substantially the same manner as a parallel light beam. As described above, the laser light beam emitted from the semiconductor laser 1 is focused by the intermediate lens system 20, which is so located that a principal plane of the single plano-convex lens element constituting the intermediate lens system 20 is distant by the distance Lx, which is slightly longer than the focal distance of the single plano-convex lens element, from the light emitting point of the semiconductor laser 1, so as to be converged slightly to make the minute angle α in relation to the parallel light beam and then is caused to enter into the object lens system 4, and thereby, the spherical aberration imposed by the intermediate lens system 20 on the light beam passing through the intermediate lens system 20 and the object lens system 4 is cancelled at the object lens system 4. Consequently, the wave front aberration of the light beam impinging upon the disc D is sufficiently diminished in such a manner as shown in Fig. 5 through the intermediate lens system 20 which consists of the single plano-convex lens element is used. Fig. 5 shows a relationship between the length (L₀) of the optical path from the principal plane of the single plano-convex lens element constituting the intermediate lens system 20 to the light emitting point in the semiconductor laser 1 and the wave front aberration (Ab) of the laser light beam at a spot formed on the disc D by the laser light beam, wherein Lf₀ represents the focal distance of the single plano-convex lens element. It is understood from the relationship shown in Fig. 5 that when the intermediate lens system 20 is so positioned that the principal plane of the single plano-convex lens element constituting the intermediate lens system 20 is distant by the distance Lx which is slightly longer than the focal distance Lf₀ from the light emitting point of the semiconductor laser 1 along the optical path from the principal plane of the single plano-convex lens element to the light emitting point in the semiconductor laser 1, that is, the length (L₀) of the optical path is selected to be identical with the distance Lx, the wave front aberration (Ab) of the laser light beam at the spot formed on the disc D by the laser light beam is substantially minimized. That is, when the intermediate lens system 20 is positioned in such a manner as described above, the wave front aberration (Ab) of the laser light beam which passes through the intermediate lens system 20 and the object lens system 4 to impinge upon the disc D is sufficiently diminished. While, under such a condition as mentioned above, the laser light beam entering into the object lens system 4 is converged by the intermediate lens system 20 so slightly as to be focused by the object lens system 4 in substantially the same manner as the parallel light beam, and therefore the fact that the laser light beam converged slightly enters into the object lens system 4 to be focused thereby does not exert any substantially harmful influence upon an optical characteristic such us an information reading performance of the embodiment shown in Fig. 4. The laser light beam impinging upon the disc D as described above is modulated in intensity and reflected at the spiral record track formed on the disc D to be a reflected laser light beam. The reflected laser light beam from the disc D passes through the object lens system 4 and the intermediate lens system 20 to the beam splitter 2 and is deflected at the beam splitter 2 so as to pass to a photodetector 6 through a light receiving lens system 5. Then, detection output signals obtained from the photodetector 6 are supplied to a signal processing circuit (not shown in the drawings) in which a reproduced information signal, focusing control signal and tracking control signal are produced. In this case also, the focus control signal and tracking control signal are supplied to a lens driving unit 7 for focus control and a lens driving unit 8 for tracking control both disposed in relation to the object lens system 4, respectively, so that a focus servo control and a tracking servo control are performed. It is to be understood that the intermediate lens system 20 is not limited to be constituted by such a single plano-convex lens element shown in Fig. 4 and may be constituted by a single biconvex lens element or any other single convex lens element shaped into a different figure.

Claims: An optical pick-up deivice comprising, a light beam source (1), object lens means (4) for focusing a light beam from said light beam source to cause the same to impinge on a record medium (D) and receiving a reflected light beam coming from the record medium, intermediate lens means (20) for causing the light beam emitted from said light beam source to enter into said object lens means and for receiving the reflected light beam having passed through said object lens means, said intermediate lens means comprising a single lens element beam splitting means (2) for separating the reflected light beam having passed through both of said object lens means and said intermediate lens means from the light beam emitted from said light beam source to enter into said intermediate lens means, and photodetecting means (6) for detecting the reflected light beam having passed through said beam splitting means, characterized in that said intermediate lens means (20) comprising a single plano-convex element is located along the optical path and situated at a distance slightly longer than the focal distance of said intermediate lens element from the light emitting point so as to converge the the light beam emitted from said light beam source (1) slightly as compared with a parallel light beam, so that a spherical aberration brought forth by said intermediate lens means (20) on the light beam passing through said intermediate lens means and said object lens means is cancelled at the object lens means. An optical pick-up device according to claim 1, wherein said intermediate lens means (20) is provided in an optical path between said light beam source (1) and said object lens means (4) to be so located that a principal plane of said single lens element is distant by a distance (Lx) slightly longer than the focal distance (Lf₀) of said single lens element from a light emitting point of said light beam source (1). An optical pick-up device according to claim 2, wherein said intermediate lens means (20) is so positioned that the light beam entering into said object lens means (4) is converged by said intermediate lens means so slightly as to be focused by said object lens means in substantially the same manner as a parallel light beam. An optical pick-up device according to claim 1, wherein said intermediate lens means comprises a single plano-convex lens element.

Inventors: NAKAMURA, AKIRA
Applicants: SONY CORPORATION
Application year: 1987
Main classification: G11B7
IPC codes: G11B7