Optical head unit

Abstract: An optical head unit includes a motor (31, 31A) including a stator (51) and a rotor (32) which is rotatably provided on the stator, where at least the rotor has a generally circular or polygonal surface, and an optical system provided on the surface of the rotor of the motor. The optical system includes a light emitting part (60) for emitting light, a focus actuator (34), an objective lens (33) provided on the focus actuator, and a light receiving part (64, 65). The light emitted from the light emitting part is output via the objective lens, and light received via the objective lens is received by the light receiving part.

Description: BACKGROUND OF THE INVENTION The present invention generally relates to optical head units, and more particularly to an optical head unit which is suited for use in an optical disk unit. In optical disk units, there are demands to increase the access speed and to reduce the thickness of the optical disk unit. For this reason, it is desirable that the optical head unit has a structure which is unaffected by vibration and is suited for making the high-speed access to information stored in a disk. In addition, it is desirable that the optical head unit has a thin structure. FIG.1 shows a first example of a conventional optical head unit. An optical head unit of this type is disclosed in EP-A-203816. An optical head unit 1 shown in FIG.1 includes a light emitting part 2 and a light receiving part 3 which are fixedly provided on a substrate. An arm 8 has a built-in parallel prism 5, and an objective lens 6 and a focus actuator 7 are provided at a tip end of the arm 8. This arm 8 is rotated by a motor 4. A laser beam 9 emitted from the light emitting part 2 is directed towards the parallel prism 5 by a mirror 10, as indicated by 9a. An access to a predetermined track on a magneto-optic disk 11 is made when the motor 4 is driven and the arm 8 is rotated by the motor 4. On the other hand, FIG.2 shows a second example of a conventional optical head unit. An optical head unit 20 shown in FIG.2 is proposed in a Japanese Laid-Open Patent Application No.61-261830, for example. In FIG.2, an arm 23 is fixed on a shaft 22 of a motor 21. An objective lens 24, a focus actuator 25, a light emitting part 26 and a light receiving part 27 are built into the arm 23. An access to a predetermined track on a disk 28 is made when the motor 21 is driven and the arm 23 is rotated by the motor 21. From document EP-A-0074131 a similar optical head unit with the features of the preamble of claim 1 is known. This optical head unit comprises a motor, an optical system including a semiconductor lens as a light emitting part for emitting light, a focus actuator, an objective lens, and a photodetector as a light receiving part, wherein the light emitted from the semiconductor lens is output via the objective lens and the light received via the objective lens is received by the photodetector. However, the conventional optical head units suffer from the following problems. First, the arms 8 and 23 of the optical head units 1 and 20 both have a relatively long and narrow shape. For this reason, the arms 8 and 23 lack rigidity and easily vibrate, thereby making a high-speed access difficult. Second, it is difficult to make the optical head unit 1 thin because the laser beam 9 travels along a three-dimensional optical path. Third, it is also difficult to make the optical head unit 20 thin because the arm 23 is fixed on the shaft 22 which projects upwardly from the motor 21. SUMMARY OF THE INVENTION The invention is defined in the appended claims. Accordingly, it is a general object of the present invention to provide a novel and useful optical head unit in which the problems described above are eliminated. Another and more specific object of the present invention is to provide an optical head unit comprising a motor including a lower disk shaped stator and an upper disk shapped rotor which is rotatably provided on the stator, where at least the rotor has a generally circular or polygonal surface, and an optical system provided on the upper surface of the rotor of the motor. The optical system includes a light emitting part for emitting light, a focus actuator, an objective lens provided on the focus actuator, and a light receiving part, the light emitted from the light emitting part is output via the objective lens and light received via the objective lens is received by the light receiving part. According to the optical head unit of the present invention, it is possible to make a high-speed access to an optical disk , and the optical head unit is strong against vibration even though the optical head unit has a thin structure. Still another object of the present invention is to provide an optical head unit of the type described above wherein the objective lens and the focus actuator are provided along one diameter of the rotor on one side relative to a center of rotation of the rotor, and the light emitting part and the light receiving part are generally provided on the other side relative to the center of rotation of the rotor. According to the optical head unit of the present invention, the rotational balance of the rotor becomes extremely good. A further object of the present invention is to provide an optical head unit of the type described above wherein the light emitted from the light emitting part travels approximately above the center of rotation of the rotor before reaching the objective lens. According to the optical head unit of the present invention, an optimum rotational balance of the rotor is realized. Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a perspective view showing a first example of a conventional optical head unit; FIG.2 is a perspective view showing a second example of a conventional optical head unit; FIGS.3, 4 and 5 respectively are a perspective view, a cross sectional view and a plan view showing a first embodiment of an optical head unit according to the present invention; FIG.6 is a cross sectional view taken along a line VI-VI in FIG.5; FIG.7 is a cross sectional view taken along a line VII-VII in FIG.5; FIG.8 is a cross sectional view taken along a line VIII-VIII in FIG.6; FIG.9 is a bottom view showing a bottom of a rotor of the first embodiment; FIG.10 is a plan view showing a cartridge type magneto-optic disk unit applied with the first embodiment; FIG.11 is a front view showing the magneto-optic disk unit shown in FIG.10; FIG.12 is a side view showing the magneto-optic disk unit shown in FIG.10; and FIG.13 is a cross sectional view showing a second embodiment of the optical head unit according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS A description will be given of a first embodiment of an optical head unit according to the present invention. FIGS.3, 4 and 5 respectively show a perspective view, a cross sectional view and a plan view of the first embodiment. FIG.6 shows a cross section taken along a line VI-VI in FIG.5, FIG.7 shows a cross section taken along a line VII-VII in FIG.5, and FIG.8 shows a cross section taken along a line VIII-VIII in FIG.6. In an optical head unit 50, a rotary voice coil motor 31 for tracking has a flat structure and includes a lower disk shaped stator 51 and an upper disk shaped rotor 32. The center of the rotor 32 is supported on bearings 52 so that the rotor 32 is rotatable with respect to the stator 51. The rotor 32 is arranged adjacent to the stator 51. Flat driving coils 53 are adhered on the top surface of the stator 51, while a ring-shaped magnet 54 is fixed on the bottom surface of the rotor 32. The magnet 54 is magnetized in a peripheral direction. The driving coils 53 and the magnet 54 are also shown in FIGS.8 and FIG.9 which shows a bottom view of the rotor 32. Three driving coils 53 are provided in this embodiment, but an odd number of driving coils 53 more than three may be provided in correspondence with the number of S and N poles provided in the ring-shaped magnet 54. Accordingly, the rotary voice coil motor 31 is a moving magnet type, and the focus actuator 34, the light emitting part 35 and the light receiving part 36 are virtually unaffected by undesirable effects of heat which is generated from the driving coils 53. Parts forming an optical system are mounted on the upper surface of the rotor 32 in a predetermined arrangement. As shown particularly in FIGS.3, 5 and 6, an objective lens 33, a mirror 55 and a focus actuator 34 are provided on the rotor 32 along one diameter 40 on one side relative to a center of rotation 41 of the rotor 32. The focus actuator 32 includes a base part 56 which is provided in a vicinity of the center of rotation 41, a leaf spring 57, driving coils 58 for focusing, and a magnetic circuit 59. A base part of the leaf spring 57 is fixed to the base part 56 and the leaf spring 57 extends in an outer peripheral direction of the rotor 32 along the diameter 40. The objective lens 33 is mounted on a tip end of the leaf spring 57. The driving coils 58 are respectively provided on both sides of the tip end of the leaf spring 57. The magnetic circuit 59 is fixed on the rotor to confront the driving coils 58. A semiconductor laser 60 which forms a light emitting part, a collimator lens 61, a diffraction grating 62 and a beam splitter 63 are generally provided on the rotor 32 along the diameter 40 on the other side relative to the center of rotation 41. Photodiodes 64 and 65 which form a light receiving part, a polarization beam splitter 66, a cylindrical lens 67, a convex lens 68, a 1/2 wave plate 69 and a photodiode 70 are also provided on the rotor 32. Because the rotary voice coil motor 31 of the optical head unit 50 has a flat structure and the parts of the optical system are fixed on the rotor 32 in a plane, a height h1 of the optical head unit 50 shown in FIG.6 is 15 mm or less. FIGS.10, 11 and 12 respectively show a plan view, a front view and a side view of a 3.5-inch disk cartridge type magneto-optic disk unit 80 which is applied with the first embodiment, that is, the optical head unit 50. As shown in FIGS.10 through 12, the stator 51 of the optical head unit 50 is fixed on a bottom plate 82 of a main body 81 of the magneto-optic disk unit 80. For example, a width of the magneto-optic disk unit 80 along a direction X in FIG.10 is 101.6 mm, and a length of the magneto-optic disk unit 80 along a direction Y is 146 mm. A disk cartridge 91 accommodates a 3.5-inch magneto-optic disk 90. This disk cartridge 91 is inserted into the magneto-optic disk unit 80 in a direction 92 shown in FIG.10 and is lowered in a direction 93 shown in FIG.11 to be placed on a disk motor 94. The optical head unit 50 is arranged at a position such that the objective lens 33 confronts the magneto-optic disk 90 which is exposed through a shutter (not shown) of the disk cartridge 91 which opens when the disk cartridge 91 is lowered and placed on the disk motor 94. Next, a description will be given of the operation of the optical head unit 50. A laser beam 42 emitted from the semiconductor laser 60 travels by way of the collimator lens 61, the diffraction grating 62 and the beam splitter 63, and travels immediately above the center of rotation 41 to reach the mirror 55. The mirror 55 directs the laser beam 100 to the objective lens 33, and the objective lens 33 converges the laser beam 100 on the magneto-optic disk 90. This laser beam 100 which is output via the objective lens 33 is approximately perpendicular to the top surface of the rotor 32. A laser beam 100 which is reflected from the magneto-optic disk 90 reaches the beam splitter 63 by way of the objective lens 33 and the mirror 55, and a predetermined component of the laser beam 100 is reflected. A reflected laser beam 101 from the beam splitter 63 passes through the 1/2 wave plate 69 so that the plane of polarization is rotated by 45°, and then reaches the deflection beam splitter 66 by way of the convex lens 68 and the cylindrical lens 67. Hence, the reflected laser beam 101 is polarized and split into two directions 103 and 104 to be respectively received by the photodiodes 64 and 65. Outputs of the photodiodes 64 and 65 are differentially amplified so as to reproduce the information recorded on the magneto-optic disk 90. The output of the semiconductor laser 60 is monitored by the photodiode 70. When making an access to the magneto-optic disk 90, a control current is applied to the driving coils 53 so as to drive the rotary voice coil motor 31. As a result, the rotor 32 rotates in a direction 110 or 111 shown in FIG.5, and the objective lens 33 is moved between positions P1 and P2 in FIG.10. The rotor 32 is considerably rigid in the peripheral direction. For this reason, even when the rotor 32 rotates at a high speed, the rotor 32 will not bend or vibrate. Therefore, the position of the objective lens 33 is accurately determined by the rotary position of the rotor 32, and it is unnecessary to provide an actuator for controlling the position of the objective lens 33 in the tracking direction. In addition, the parts of the optical system are uniformly distributed on both sides of the rotor 32 relative to the center of rotation 41. Hence, the rotor 32 is evenly balanced in the rotating direction of the rotor 32, thereby making it possible to smoothly rotate the rotor 32 at a high speed. For the above described reasons, the access to the magneto-optic disk 90 can be made at a high speed using the optical head unit 50, when compared to the conventional optical head units. On the other hand, the parts of the optical system are arranged so that the laser beam 42 emitted from the semiconductor laser 60 travels immediately above the center of rotation 41. In other words, in the plan view shown in FIG.5, the laser beam 42 passes approximately the center of rotation 41. Accordingly, the balance of the rotor 32 in the rotating direction is set to an optimum balance in this embodiment, thereby making it possible to more smoothly rotate the rotor 32 at the high speed and enable the high-speed access to the magneto-optic disk 90. Next, a description will be given of a second embodiment of the optical head unit according to the present invention, by referring to FIG.13. FIG.13 shows a cross section of the second embodiment. In FIG.13, those parts which are the same as those corresponding parts in FIG.4 are designated by the same reference numerals, and a description thereof will be omitted. In an optical head unit 30A shown in FIG.13, the flat driving coils 53 of a motor 31A are adhered on the bottom surface of the rotor 32, while the ring-shaped magnet 54 is fixed on the top surface of the stator 51. Otherwise, the structure of this embodiment is the same as that of the first embodiment described above. In the described embodiments, the present invention is applied to the magneto-optic disk unit 80 which operates with the magneto-optic disk 90. However, the present invention is applicable to any type of optical disk unit which operates with an optical disk. In addition, the shape of the rotor 32 and the stator 51 is not limited to the disk shaped as in the case of the described embodiments. For example, the rotor 32 and/or the stator 51 may have a polygonal shape or a generally circular shape. Further, it is sufficient that at least the rotor 32 has the generally circular shape. Moreover, the motor which is employed in the present invention is not limited to the rotary voice coil motor 31, and any flat type motor may be employed. Preferably, the rotor 32 and the stator 51 are made of magnetizable materials so that the magnetic field is restricted to the vicinity of the rotary voice coil motor 31. Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention as defined in the appended claims.

Claims: An optical head unit, comprising a motor (31, 31A); an optical system including a light emitting part (60) for emitting light, a focus actuator (34), an objective lens (33) provided on said focus actuator, and a light receiving part (64, 65), the light emitted from said light emitting part (60) being output via said objective lens (33) and the light received via said objective lens (33) being received by said light receiving part (64, 65), characterized in that said motor (31, 31A) includes a lower disk shaped stator (51) and an upper disk shaped rotor (32) which is rotatably provided on said stator; and said optical system is provided on the upper surface of said rotor of said motor (31, 31A). The optical head unit as claimed in claim 1, characterized in that the objective lens (33) and the focus actuator (34) are provided along one diameter of the rotor (32) on one side relative to a center of rotation (41) of the rotor, and the light emitting part (60) and the light receiving part (64, 65) are generally provided on the other side relative to the center of rotation of the rotor. The optical head unit as claimed in claim 2, characterized in that the light emitted from the light emitting part (60) travels approximately above the center of rotation (41) of the rotor (32) before reaching the objective lens (33). The optical head unit as claimed in any of claims 1 to 3, characterized in that said optical system further includes means (55) for outputting the light emitted from the light emitting part (60) via the objective lens (33) in a direction approximately perpendicular to the surface of the rotor (32). The optical head unit as claimed in any of claims 1 to 4, characterized in that said optical system further includes a beam splitter (63) which is provided between the light emitting part (60) and the objective lens (33), said beam splitter directing the light received via the objective lens to the light receiving part (64, 65). The optical head unit as claimed in claim 5, characterized in that said optical system further includes a polarization beam splitter (66) which is provided between the beam splitter (63) and the light receiving part (64, 65), said light receiving part including a pair of light receiving elements (64, 65) for receiving split lights via the polarization beam splitter. The optical head unit as claimed in any of claims 1 to 6, characterized in that the stator (51) and the rotor (32) of said motor (31, 31A) are made of a magnetizable material. The optical head unit as claimed in any of claims 1 to 7, characterized in that said motor (31) includes a ring-shaped magnet (54) provided on a bottom surface of the rotor (32), and N driving coils (53) provided on a top surface of the stator (51) at positions confronting the ring-shaped magnet, where N is an arbitrary odd number greater than or equal to three and corresponding to a number of S and N poles provided in the ring-shaped magnet. The optical head unit as claimed in any of claims 1 to 7, characterized in that said motor (31A) includes a ring-shaped magnet (54) provided on a top surface of the stator (51), and N driving coils (53) provided on a bottom surface of the rotor (32) at positions confronting the ring-shaped magnet, where N is an arbitrary odd number greater than or equal to three and corresponding to a number of S and N poles provided in the ring-shaped magnet. The optical head unit as claimed in any of claims 1 to 9, characterized in that said motor (31, 31A) is a voice coil motor. The optical head unit as claimed in any of claims 1 to 10, characterized in that the light output via the objective lens (33) is converged on an optical disk (90), and the light received via the objective lens and received by the light receiving part (64, 65) is light reflected from the optical disk. The optical head unit as claimed in any of claims 1 to 11, characterized in that the surface of said rotor (32) has a generally circular or polygonal shape.

Inventors: FUJIMAKI TOHRU, FUTAMATA AKIO, OZAWA YASUYUKI, FUJIMAKI, TOHRU, FUTAMATA, AKIO, OZAWA, YASUYUKI, FUJIMAKI, TOHRU, C/O FUJITSU LIMITED, Futamata, Akio, c/o Fujitsu Limited, Ozawa, Yasuyuki, c/o Fujitsu Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1991
Main classification: G11B7
IPC codes: G11B7, G11B11