OPTICAL HEAD

Abstract: In an optical head, a laser beam L emitted from a semiconductor laser (l) is collimated by a collimator lens (2) and is incident on a beam splitter (3). The laser beam passing through the beam splitter (3) is and a quarter wave plate (7) converged by an objective lens (8) onto an optical disk (9) and reflected by the opti­ cal disk (9). The reflected laser beam passing through the objective lens (8) and the quarter wave plate (7) is returned to the beam splitter (3) and is reflected from a polarizing surface (6) of the splitter (3). The reflected laser beam is guided to a beam emerging sur­ face (5A) of the prism (5), the surface (5A) being defined by a flat surface (l0) and a semicylindrical concave surface (ll). The laser beam is separated at the surfaces (l0, ll) and converged by a projection lens (l2) onto a photodetector (l3).

Description: The present invention relates to an optical head comprising the features of the pre-characterizing portion of claim 1, for applying a focused light beam to an information memory medium, thereby recording or retrieving information optically. Apparatuses and information memory media have recently been developed which are used to record or retrieve information optically. The memory media include compact disks for DAD, video disks, optical cards, or other retrieval-only memory media. The media further include those information memory media for document files, computer output memories, etc., which are used for recording and retrieval only, or which permit recording, retrieval, and erasure of information. These optical information recording/ retrieving apparatuses incorporate an optical head, which applies a light beam to an information memory medium, thereby optically recording on or retrieving information from the medium. In this optical head, a laser beam emitted from a semiconductor laser (light source) is focused on the memory medium by an objective lens, and the reflected beam from the medium passes again through the objective lens, and is then detected by a photodetector. A detection signal is processed and converted into an information signal. In an optical head of this type, the light transmitted through the objective lens must be focused exactly on a record film of the information memory medium, and a tracking guide of the medium must be traced by the focused laser beam. To attain this, the optical head is provided with a focusing servo loop to detect and maintain a focusing state of the objective lens, and a tracking servo loop for detecting the tracking guide, and continually directing the objective lens toward a desired track. The focusing state of the objective lens may be detected by the knife-edge method, as stated in U.S. Pat. Nos. 4,5l7,666; 4,546,460; 4,585,933 and 4,52l,680 (by Ando). The tracking guide may be detected by a method such that a diffraction component diffracted by the tracking guide is detected, as stated also in the aforesaid U.S. patents. According to optical heads disclosed in these patents, a light shielding plate or a so-called knife edge is located on the optical path. The knife edge must be positioned with high accuracy, and use of it increases the number of components that must be assembled. Thus, the assembly and adjustment require a lot of time, and the resulting optical system is inevitably large-sized. To cope with these problems, improved optical systems have been proposed in U.S. Pat. Appl. Ser. No. 682,922, filed on Dec. l8, l984, and U.S. Pat. Appl. Ser. No. 862,829, filed on May l3, l986 (both by Ando). In the systems disclosed in these applications, a prism is used as a beam splitter, which has a pair of beam-emerging surfaces, relatively inclined and bonded to each other on a boundary line. In these systems, one of the beam-emerging surfaces serves as a knife edge. A light beam emerging from the other emerging surface is directed to a photodetector for focusing detection, while a light beam emerging from the first emerging surface is directed to a photodetector for tracking-guide detection, located within a plane different from that of the focusing-detection photodetector. Signals from the photodetectors are processed into focusing and tracking signal. Not provided with any light shielding plate, the proposed systems are advantageous in that their components are relatively few, and can be assembled and adjusted wih ease. Since the photodetectors for focusing detection and tracking-guide detection are arranged separately, however, the prior art optical systems are still complicated in structure, bulky, and heavy in weight. Document JP-A-60 115 032, discloses an optical head with a laser beam sent through an objective lens on a projection lens. This objective lens consists of two adhering spherical lenses, whereof one is cut partially. The laser beam is divided into two parts by this objective lens at its two surfaces formed by the cutting of the one spherical lens. One part of the laser beam is irradiated to a first photodetector to detect a tracking shift, while the second part of the beam is irradiated to a second photodetector to detect an out-of-focus-state. Document JP-A-60 038 742, discloses an error signal detecting system with a laser beam reflected by an optical disk and again reflected by polarizing/ reflecting surface of a polarized beam splitter. Subsequently, the laser beam is sent to a prism. The lower half of the beam goes straight - i.e. without passing through the prism - to 2-split areas of a first photodetector in order to obtain a focus error signal, while the upper half of the beam is passing the prism and is polarized into two directions to be converted into focused beams and irradiated to 2-split areas of another photodetector in order to obtain a tracking error signal. In this prior art error signal detecting system the single laser beam is separated into one laser beam not passing through the prism and a second laser beam passing through the prism and being split into two second light beam segments. Document EP-A-0 099 123 discloses an optical head in which the detection beams are separated by means of an inclined surface and transmitted through two surfaces where the one is flat and the other is convex. The object of the present invention is to improve the above-described optical head, to be simple in construction, requiring only a relatively small number of components, thus permitting reduction in size and weight, and to be less influencable by alterations of the temperature. According to the present invention, there is provided an optical system for use with an optical memory, which comprises the features as claimed in claim 1. Preferred embodiments of the invention are claimed in the dependent claims. 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 perspective view showing an optical head according to an embodiment of the present invention; Fig. 2 schematically shows an arrangement of an optical system shown in Fig. 1; Fig. 3 is a partial enlarged view of the optical system of Fig. 1; Figs. 4, 5 and 6 are plan views showing a light receiving surface of a photodetector shown in Fig. 1 and light beam spots formed thereon; Fig. 7 schematically shows an arrangement of an optical system of an optical head according to another embodiment of the invention; Fig. 8 is a plan view of a projection lens shown in Fig. 7; and Fig. 9 is a perspective view showing part of an optical system of an optical head according to still another embodiment of the invention. As shown in Figs. l and 2, divergent laser beam L is emitted from semiconductor laser l, converted into a parallel beam by collimator lens 2, and projected on beam splitter 3. Splitter 3 includes two prisms 4 and 5 and polarizing surface 6 between the prisms. Incidence surface 4A of prism 4 is inclined, relative to the optical axis of lens 2. Thus, as laser beam L, emitted from laser l, passes through surface 4A, its cross-section is changed from an elliptic shape to into a circular one. Incident on beam splitter 3, laser beam L then passes through the splitter, and is reflected by reflecting surface 20A of total-reflection prism 20, thus changing its path. Light beam L, reflected from the surface 20A, passes through l/4 λ plate 7, to be guided to objective lens 8, and is then converged on information memory medium (hereinafter referred to simply as disk or optical disk) 9, such as an optical disk, optical card, etc., by lens 8. As is generally known, optical disk 9 is formed with a record film, as a light-reflecting film. Projection or recess tracking guide 9A is formed concentrically or spirally on the surface of the record film. Information is stored in guide 9A, in the form of changes of conditions, e.g., changes of reflection rate. Semiconductor laser l is driven by a driver circuit (not shown). In a reproducing mode, laser beam L with a predetermined luminous intensity is emitted from laser l. In a recording mode, the emitted laser beam is intensity-modulated in accordance with the information to be recorded, so that the state or conditions of the record film are changed. In an erase mode, emitted laser beam L has a predetermined intensity higher than that in the reproducing mode, so that the state or conditions of the record film are changed in a like manner. Laser beam L converged on optical disk 9 is reflected by the record film, to be returned to objective lens 8. When lens 8 is kept focused on disk 9, that is, when the beam waist of the light beam converged by lens 8 is formed on disk 9, the beam returned to lens 8 is converted into a parallel light beam by lens 8. The parallel beam is returned to beam splitter 3 through l/4 λ plate 7. As laser beam L reciprocates through plate 7, its plane of polarization is rotated 90°. Accordingly, beam L is reflected by polarizing surface 6, without passing through it again, to be directed to laser light-emerging surface 5A of prism 5 of beam splitter 3. Laser light-emerging surface 5A is defined by first and second light beam-emerging surfaces l0 and ll. In this embodiment, as shown in Fig. 3, first surface l0 is a flat surface, while second surface ll is an astigmatic surface or a semicylindrical concave-lens surface, which is inclined, relative to surface l0. According to the present invention, surface l0 need not always be a flat surface, surface ll may be a spherical surface instead of cylindrical surface, and it is necessary only that surfaces l0 and ll have different radii of curvature. Each of these radii of curvature may be a complex radius, as well as a single one. Thus, surfaces l0 and ll may each be defined by a nonspherical surface. Moreover, surface 5A of prism 5 may be formed into flat and laser light-emitting surface 5B, which may be defined by a flat first surface and a second surface or astigmatic surface, in the form of a semicylindrical convex-lens surface inclined relatively to the flat surface. The laser beam reflected by polarizing surface 6 is transmitted through flat surface l0 and concave-lens surface ll to be split in two directions. Thereafter, the split beams are converged and projected on photodetector l3, having one light-receiving surface, by common projection lens l2. The light-receiving surface of photodetector l3 is substantially located so that the converging point or waist of laser beam L, transmitted through surface l0, is formed on or close to the light-receiving surface when objective lens 8 is in focus. Photodetector l3 has, on its light-receiving surface, a pair of photosensitive regions l4 for defocusing detection and another pair of photosensitive regions l5 for tracking-error detection. Regions l4 and l5 are separated from one another by photosensitive boundary regions. When lens 8 is in a focused state, laser beam L, transmitted through flat surface l0, is directed to the boundary region between regions l4, as shown in Fig. 4. When lens 8 is in defocusing state, beam L is directed to either of regions l4. As objective lens 8 moves along its optical axis, beam L is deflected in the direction of arrow I, so that a beam spot on the light-receiving surface of photodetector l3 is moved in the direction of arrow I, depending on the defocusing degree. Thus, the defocusing state is detected by the so-called knife-edge method. As laser beam L passes through concave cylindrical surface ll, the laser beam is expanded in a lateral direction. Since surface ll has no lens effect in the direction perpendicular to its base axis, the laser beam is elongated in the perpendicular direction, and is converged on photosensitive regions l5 for tracking-error detection. In this manner, an elongated laser beam spot is formed on regions l5, as shown in Fig. 4. A far-field pattern of the tracking guide is formed in the elongated spot, so that tracking errors can be detected by processing signals from photosensitive regions l5, that is, by the so-called push-pull method. Also, the recorded information can be detected by processing the signals from regions l5. According to the arrangement described above, photodetector l3 and projection lens l2 can each be one in number, so that the optical system is simple in construction, requiring only a relatively small number of components, and thus permitting reduction in size and weight. Further, the laser beams for defocusing detection and tracking-error detection can be divided by only the two surfaces, i.e., first surface l0 and second surface or cylindrical concave-lens surface ll. Therefore, the optical system can be assembled and adjusted with ease, and the optical head can be manufactured at relatively low costs. For the nonspherical curved surface, furthermore, semicylindrical concave-lens surface ll is preferred to a semicylindrical convex-lens surface, as shown in Fig. l. The reason is that the use of the concave surface leads to a substantial reduction of the influences of the laser beams, for defocusing detection and tracking-error detection, on each other. Referring now to Figs. 4, 5 and 6, this point will be explained. If a semicylindrical lens surface, whether concave or convex, is used as second surface ll, laser beam spots Lf and Lt are formed on photodetector l3, as shown in Fig. 4, when objective lens 8 is in focus. If a semicylindrical convex-lens surface is used as the second surface, and when lens 8 is in a defocusing state such that it is located closer to optical disk 9 than when in focus, on the other hand, beam spot Lt for tracking-error detection becomes narrower, and partially enters photosensitive regions l4 for defocusing detection, as shown in Fig. 5. As a result, the levels of photoelectric signals from regions l4, responsive to defocusing detection characteristics and the degree of defocusing, provided by beam spot Lf for defocusing detection, are changed substantially. The influence of laser beam Lt for tracking-error detection depends on the distance between beams Lt and Lf when lens 8 is in focus. This distance is limited within a certain range, only due to restrictions on the manufacture of the optical head, and it is hard to keep the distance constant. If a semicylindrical concave-lens surface is used as second surface ll, and when objective lens 8 is in the aforesaid defocusing state, laser beam Lt for tracking-error detection becomes wider than in the focusing state, so that less light enters photosensitive regions l4 for defocusing detection. Thus, the influence of beam Lt on the defocusing detection characteristics can be reduced. Figs. 7 and 8 shows a second embodiment of the present invention. In this embodiment, laser light-emitting surface 5A of prism 5 is formed as a single surface, instead of being defined by first and second light beam-emerging surfaces l0 and ll. Instead of surface 5A of prism 5, light emitting surface l6 of projection lens l2 is defined by flat surface l6A and concave-lens surface l6B, as shown in Fig. 8, whereby a laser beam is separated. Thus, projection lens l2, which is formed by grinding part of a flat portion of a plain convex lens, has unground flat surface l6A and ground astigmatic surface l6B or semicylindrical concave-lens surface l6B, inclined relatively to surface l6A. Fig. 9 shows a third embodiment of the present invention. In this embodiment, mirror l9, for use as a member for separating laser beam L, has flat mirror surface l9A and semicylindrical convex-mirror surface l9B, for reflecting beam L. The mirror is disposed between projection lens l2 and beam splitter 3. In the optical system shown in Figs. 7 and 8, like the one shown in Figs. l and 2, a laser beam emerging from flat surfaces l6 and l8 is directed to photosensitive regions for focusing detection, while a laser beam emerging from the concave surface is directed to photosensitive regions for tracking-guide detection. In the regions for focusing detection, the focus is detected in accordance with the position of a beam spot formed in the regions. In the regions for tracking-guide detection, the tracking guide is detected by a far-field pattern formed in a beam spot. The present invention is not limited to the optical systems described above, and may be applied also to a magneto-optical disk system, as stated in EP-A-0 201 917, filed on May l4, l984, by Ando. In the optical systems described above, the optical components are fixed to a bonding surface of base frame 22 of a lens barrel. The bonding surface is located parallel to direction I, in which the laser beam is deflected when objective lens 8 moves along its optical axis. Thus, even if the optical axis is dislocated in the optical head, by a change of ambient temperature, the degree of defocusing can be prevented from being detected in a wrong manner. This point will now be described further in detail. Base frame 22 of the lens barrel is formed from aluminum, in order to lighten the optical head in weight, while beam splitter 3 is made of glass (e.g., BK-7). Therefore, even though the optical axis can be adjusted correctly at normal temperature, it will possibly be dislocated at a high temperature, due to thermal distortion, caused by the difference in the coefficient of thermal expansion between aluminum and glass. In this case, if splitter 3 is bonded to frame 22 in a manner such that the plane of bonding between the two members extends at right angles to moving direction I of beam spots on photodetector l3, in the defocusing state, direction II of the dislocation of the optical axis, caused by thermal distortion, if any, will be parallel to direction I. Therefore, the direction of the dislocation of the optical axis, attributable to the difference in the coefficient of thermal expansion between splitter 3 and frame 22, is made perpendicular to the moving direction of beam spots on photodetector l3, obtained when the objective lens is not focused on the optical disk. Thus, splitter 3, prisms 20 and 5, and l/4 λ plate 7, which are all made of glass, are bonded to one another as an integral structure. The resultant integral component is bonded to aluminum base frame 22 in a manner such that one lateral face of each of splitter 3 and prism 20 is opposed to frame 22. The plane of bonding between the component and frame 22 extends parallel to moving direction I of beam spots on photodetector l3, obtained when objective lens 8 is out of focus. According to the arrangement described above, if thermal distortion is caused between the integral component and base frame 22, the component rotates in direction II, and never in direction III. Accordingly, the dislocation of the optical axis, attributable to the difference in the coefficient of thermal expansion between the glass component and aluminum frame 22, occurs only in direction II, and not in direction III which is parallel to moving direction I of beam spots on photodetector l3, in the defocusing state. Thus, the optical axis can never be dislocated in direction III or I, so that the degree of defocusing cannot be detected in a wrong manner. In other words, defocusing can always be detected with high accuracy and reliability. Since direction II is perpendicular to direction I, moreover, wrong detection of defocusing degree cannot be entailed if the optical axis is dislocated in direction II.

Claims: An optical system for an optical memory, comprising: means (1, 2, 3, 7, 8) including a prism (5) for directing a light beam to the optical memory (9); means (10, 11, 19A, 19B) for separating the resultant light beam, transferred from the optical memory, into first and second light beams and including first and second light beam emerging surfaces (10, 11, 19A, 19B) from which the first and second light beams are emerged, with said first and second light beam emerging surfaces (10, 11, 19A, 19B) having different radii of curvature; and means (13) for detecting the first and second light beams; characterized in that said directing means are comprising means (22) for holding a beam splitter (3), and said holding means (22) are having a bonding surface to which the beam splitter (3) is bonded, said bonding surface being chosen such that a dislocation of the optical axis of said directing means due to different thermal expansion of said beam splitter (3) and frame (22) is perpendicular to a predetermined direction which is the direction of movement of said light beams over said detecting means (13) due to focus variations of the system. The optical system according to claim 1, characterized in that said detecting means (13) includes first and second pairs of photosensitive regions (14, 15), so that the first and second light beams are directed to the first and second pairs of photosensitive regions (14, 15), respectively. The optical system according to claim 2, characterized in that said directing means (1, 2, 3, 7, 8) has an optical axis, and includes an objective lens (8) movable along the optical axis, so that when the objective lens (8) is moved along the optical axis, the first light beam, emerging from the separating means (10, 11, 19A, 19B), is deflected in the predetermined direction, whereby a first light beam spot, formed on the detecting means (13) by the first light beam, is moved in the predetermined direction; and said first pair of photosensitive regions (14) are arranged in the predetermined direction. The optical system according to claim 3, characterized in that said optical memory (9) includes a tracking guide (9A), and said second pair of photosensitive regions (15) are arranged in a direction perpendicular to the predetermined direction. The optical system according to claim 1, characterized in that said beam splitter (3) is transferring the light beam to the objective lens (8), and directing the resultant light beam, received from the optical memory (9), to the separating means (10, 11, 19A, 19B). The optical system according to claim 1, characterized in that said separating means includes a mirror (19) formed with the first and second light beam-emerging surfaces (19A, 19B), whereby the resultant light beam is reflected. The optical system according to claim 1, characterized in that said first light beam-emerging surface (11, 16B) is a spherically concave surface, and said second light beam-emerging surface is a flat surface. The optical system according to claim 1, characterized in that said first light beam-emerging surface is a concave semicylindrical surface.

Inventors: ANDO, HIDEO C/O PATENT DIVISION, K.K. TOSHIBA
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: G11B7
IPC codes: G11B7