Recording and reproducing optical disk device

Abstract: Magneto-optical disk recording and reproducing devices and actuators and distance detectors for use therewith are disclosed. The magneto-­ optical disk device comprises, in addition to the usual elements, a bimorph type actuator 9, 71 or an electromagnetic actuator 108-115 for driving the magnetic head 8 of the magneto-optical disk device. The bimorph actuator may have a metallic shim 74 whose width varies with respect to the longitudinal distance inversely proportional to the normal function of the first or the second free natural vibration mode. Alternatively, the bimorph type actuator may have a recess 9d formed on a bimorph element 9a to expose a piezoelectric ceramics which outputs a voltage corresponding to the acceleration of the vibration of the actuator; the driver circuit 96 of the actuator reduces the driving voltage when the frequency of the voltage generated at the recess comes into the neighborhood of a higher resonance frequency of the actuator. The optical distance detector comprises a light-emitting diode 14, 16, 31 and a pair of photodiodes 15, 17, 30 which detect the amount of light incident thereon after being reflected at a surface of an object the distance to which is measured. The geometric parameters of the photodiodes are selected in such a manner that the output of a first photodiode decreases as the measured distance increases while the output of a second photodiode increases as the measured distance increases. Thus, the normalized differential output of the two photodiodes, from which the measured distance is determined, varies substantially linearly with respect to the measured distance. In preferred forms, the photodetectors have stepped concentric configurations so that small distances can be measured accurately and efficiently.

Description: BACKGROUND OF THE INVENTION This invention relates to recording and reproducing optical disk devices, and more particularly to magneto-optical disk devices which utilize a magnetic material as the recording medium. The recording and reproducing optical disk devices of the above type effects recording on the disk as follows: the recording spot on the disk is heated by means of a light (laser) beam to raise the temperature thereat; in addition, a varying exterior magnetic field, namely the biasing magnetic field, is generated by a magnetic head to reverse the direction of the magnetization of the magnetic material on the disk. On the other hand, during the reproducing operation, a light beam of less energy is irradiated on the recording surface to read out the above direction of the magnetization on the disk by means of a magneto-optical effect, such as the magnetic Kerr effect or Faraday effect. This type of optical disk device is finding increasing applications as exterior memory devices of computers, audio disks, etc. Fig. 1 shows an example of the recording and reproducing optical disk device of the above type. A disk 1 is rotated by a disk driver motor 2. The light beam 4 emitted from the optical head 3 forms a converged light spot 5 on the surface of the disk 4; the diameter of the light spot 5 is generally from about 1.0 to 1.5 micrometers. The optical head 3 comprises following elements: a light source for emitting the light beam; optical elements for conducting to the surface of the disk 1 the light beam emitted from the light source; optical elements for conducting the light reflected from the disk surface; a photosensor element (a light-sensitive detector) for converting the reflected light into an electrical signal upon receiving the reflected light via the above optical elements for conducting the reflected light; and optical sensors for detecting the focusing and tracking errors of the light spot 5 on the recording surface of the disk 1. The objective lens of the optical head 3 which forms the light spot 5 on the disk 1 is driven in the focusing direction perpendicular to the recording surface of the disk 1 and in the direction perpendicular to the track on the recording surface, such that the focusing and the tracking errors are reduced. The optical head 3 is supported on the base 6, which is supported on the frame of the device via the bearings 7a and 7b to be driven in the radial direction A. Further, on the base 6 is disposed a magnetic head 8 for generating an exterior magnetic field from the side opposite to the side at which the optical head 3 is situated. The recording on the disk 1 is effected by modulating this exterior magnetic field generated by the magnetic head 8. As shown in detail in Fig. 2 (a) through (c), the distance (designated by H₁, through H₃, respectively) between the recording surface of the disk 1 and the magnetic head 8 varies as the disk 1 rotates. The strength of the magnetic field generated by the magnetic head 8 is generally from about 100 to 500 Oe at the recording surface; as illustrated in Fig. 3, the magnetic field strength B at the recording surface of the disk 1 (plotted along the ordinate) decreases as the distance H between the surface of the disk 1 and the magnetic head 8 (plotted along the abscissa) increases. The distance H between the recording surface of the disk 1 and the magnetic head 9 varies as the disk 1 rotates, due, for example, to an undulating motion of the recording surface of the disk 1. The above conventional recording and reproducing optical disk device therefore has the following problems. First the distance H between the magnetic head 8 and the recording surface of the disk 1 cannot be set as small as is desired. if the contact of the head 8 with the disk 1 is to be avoided: thus, the efficiency of the magnetic head 8 in producing a magnetic field at the recording surface of the disk 1 is reduced. Second, the variation of the distance H between the head 8 and the recording surface of the disk 1 results in a variation in the recording characteristics. Third. if the above problems are to be avoided, an extremely high mechanical precision is required of the device. GB-A-2 083 677 discloses a magneto-optical disk device in which magnetic head means are provided with apparatus for adjusting the position of the magnetic head. It has however, been found that it would be desirable to provide an improved arrangment for maintaining the position of the magnetic head relative to a disk. SUMMARY OF THE INVENTION It is a primary object of this invention therefore to provide an optical disk recording and reproducing device in which the distance between the magnetic head and the disk is maintained at a constant target value thereof. In order to maintain the distance between the disk and a recording and reproducing head of the optical disk device, it is necessary to use an actuator. Thus, it is an additional object of this invention to provide a bimorph type actuator which is suited to be used for controlling the distance between the head and the disk of an optical disk recording and reproducing device. Further, it is essential to detect the distance between the head and the disk for the proper control thereof. Thus, it is still another object of this invention to provide an optical distance detector which is suited to measure the distance between the head and the disk with accuracy. According to the invention there is provided an optical disk device as set out in Claim 1 or Claim 11. Preferred features of the invention are set out in Claims 2 to 10 and 12 to 20. The invention also provides a bimorph actuator as set out in Claim 9. The optical distance detector according to this invention comprises a light source, such as a light-emitting diode, and a first and second optical detectors, such as photodiodes. The light emitted from the light source is reflected by a surface of an object to be received by the two optical detectors, which outputs signals corresponding to the amount of light incident thereon. The light-receiving surfaces of the two optical detectors are coplanar and substantially perpendicular to the optical axis of the light source; and the light-receiving surface of the second optical detector is situated farther away from the optical axis of the light source than the light-receiving surface of the first optical detector. The geometric parameters of the light-receiving surfaces of the two optical detectors are selected in such a manner that in the measurement range of the distance, the output of the first decreases as the measured distance increases while that of the second increases as the measured distance increases. Thus, the normalized differential output of the two optical detectors, i.e., the ratio of the difference between the outputs of the first and second optical detectors to the sum thereof, is substantially in linear relationship with the measured distance. In a preferred form, the light-receiving surface of the two optical detectors are disposed concentrically around the optical axis of the light source. It is also preferred that the light-receiving surfaces of the two optical detectors are stepped forwards from the light-emitting surface of the light source in the direction of the optical axis of the light source toward the reflective surface of the object, so that separation between the light-emitting surface of the light source and the light-receiving surface of the first optical detector in the direction perpendicular to the optical axis of the light source may be made as small as is desired. BRIEF DESCRIPTION OF THE DRAWINGS The novel features which are believed to be characteristic of our invention is set forth with particularly in the appended claims. Our invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood from the detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic side elevational view of a conventional optical disk recording and reproducing device; Fig. 2 shows enlarged views of a recording portion of the device of Fig. 1 in operation, in three states in which the distance between the magnetic head is small (Fig. 2 (a)), intermediate (Fig. 2 (b)), and great (Fig. 2 (c)); Fig. 3 shows a relation between the distance between the magnetic head and the disk and the magnetic field strength generated by the magnetic head at the recording spot of the disk; Fig. 4 is a view similar to that of Fig. 1, but showing an optical disk device according to this invention having a bimorph type actuator for the magnetic head thereof; Fig. 5 is a prespective view of the bimorph type actuator of the optical disk device of Fig. 4; Fig. 6 is an enlarged schematic sectional view of a recording portion of the optical disk recording and reproducing device of Fig. 4 in operation; Fig. 7 is a prespective view of an optical distance detector assembly of an optical disk device according to this invention; Figs. 8 and 9 are side elevational views of an optical distance detector of Fig. 7 in operation; Fig. 10 shows a relation between the variation of the magnetic head distance and the variation in the magnetic field strength generated at the recording spot; Fig. 11 is a view simialar to that of Fig. 9, but showing another embodiment of an optical distance detector of an optical disk recording and reproducing device according to this invention; Fig. 12 is a side elevational view of an optical distance detector of Fig. 11 in operation; Fig. 13 shows the relations between the location of the two parts of the light-sensitive element of the optical distance detector of Fig. 11 and the intensity of light incident thereon, in the two cases of great and small measured distance; Fig. 14 shows the relation between the measured distance and the differential output of the two parts of the light-sensitive element of the optical distance detector of Fig. 11; Fig. 15 shows an axial section of a typical optical distance detector; Fig. 16 shows the relation between the measured distance and the intensity of light incident on the photodetector of the optical distance detector of Fig. 15; Fig. 17 shows a basic geometry of light-emitting diode and the photodetectors of an optical distance detector suitable for use in an embodiment of this invention; Fig. 18 shows the relations between the location of the photodetectors of Fig. 17 and the intensity of light incident thereon, in the two cases of great and small measured distance; Fig. 19 shows the relations of the measured distance and the intensity of light incident on the first and the second photodetector of Fig. 17, respectively; Fig. 20 shows the relations of the measured distance and the outputs of the photodetectors of Fig. 17 in the measurement range thereof; Fig. 21 is a schematic circuit diagram for computing a normalized differenctial output of the two photodetectors of the optical distance detector of Fig. 17. Fig. 22 shows the relation between the measured distance and the normalized differential output computed by the circuit of Fig. 21; Figs. 23 and 24 are plan views of the concentric arrangements of photodetectors of an optical distance detector suitable for use in an embodiment of this invention; Fig. 25 shows a stepped configuration of the photodetectors of an optical distance detector suitable for use in an embodiment of this invention, wherein Fig.25 (a) shows a side elevational view thereof and Fig.25 (b) shows a plan view thereof; Fig. 26 is a plan view of the substrate of the optical distance detector of Fig. 25; Fig. 27 is a side elevational view of the detector of Fig. 26 in operation; Fig. 28 shows a portion of an optical disk device in which an optical distance detector similar to that of Fig. 26 is installed, wherein Fig. 28 (a) is a plan view thereof while Fig. 28 (b) is a side elevational sectional view thereof; Fig. 29 shows a stepped concentric configuration of the photodetectors of an optical distance detector suitable for use in an embodiment of the invention, wherein Fig.29 (a) is a plan view thereof while Fig.29 (b) is a side elevational view thereof; Fig. 30 shows the top and bottom views of the spacer of the distance detector of Fig. 29; Fig. 31 shows the bottom view of the spacer of Fig. 30 on which the light-emitting diode is mounted; Figs. 32 and 33 show the optical distance detector of Fig. 29 in operation, in the two cases of great and small measured distance, respectively (Figs. 32 (a) and 33 (a)), together with the intensity of light incident thereon (Figs. 32 (b) and 33 (b)); Fig. 34 shows a stepped configuration of the photodetectors of an optical distance detector similar to that shown in Fig. 30, wherein Fig. 34 (a) is a plan view thereof while Fig. 34 (b) is a side elevational sectional view thereof; Fig. 35 (a) and (b) show an end view and a plan view of a spacer member of the optical distance detector of Fig. 34, respectively; Fig. 36 is a plan view of the substrate of the optical distance detector of Fig. 34; Fig. 37 is a side elevational sectional view of another stepped concentric configuration of the photodetectors of an optical distance detector suitable for use in an embodiment of the invention; Fig. 38 is an exploded perspective view of the configuration of the photodetectors of Fig. 37; Fig. 39 is a circuit diagram showing the circuit for driving the light-emitting diode and the photodetectors of the optical distance detector of Fig. 37; Fig. 40 is a perspective view of a typical bimorph type actuator for driving the optical head of an optical disk recording and reproducing device; Fig. 41 is a side view of a piezoelectric bimorph element of the actuator of Fig. 40 in operation; Fig. 42 is a graph showing the frequency characteristics of the gain of the actuator of Fig. 40; Fig. 43 is a perspective view of a typical bimorph type actuator for driving the magnetic head of an optical disk recording and reproducing device, showing together therewith a driver circuit thereof; Fig. 44 is a graph showing the frequency characteritics of the gain of the actuator of Fig. 43; Fig. 45 is a perspective view of a bimorph type actuator for driving the optical head of an optical disk recording and reproducing device. which actuator having a shim of special geometry effective for suppressing vibrations modes at higher resonance frequencies; Fig. 46 is a plan view of the actuator of Fig. 45; Fig. 47 is a side view of the actuator of Fig. 45 in operation; Fig. 48 is a graph showing the frequency characteristics of the gain of the actuator of Fig. 46; Fig. 49 is a plan view of another bimorph type actuator for driving the optical head of an optical disk device, which actuator having a shim of special geometry effective for suppressing vibrations modes at resonance frequencies other than the second resonance frequency; Fig. 50 is a graph showing the frequency characteristics of the gain of the actuator of Fig. 49; Fig. 51 is a perspective view of another bimorph type actuator according to this invention. having a shim of special geometry similar to that of the shim of the actuator of Fig. 45; Fig. 52 is a perspective view of another bimorph type actuator according to this invention for driving a magnetic head chip of a magnetic tape recording and reproducing device. which actuator having a shim of special geometry similar to that of the shim of the actuator of Fig. 45; Fig. 53 is a perspective view of a further bimorph type actuator according to this invention for driving the magnetic head of an optical disk recording and reproducing device, which actuator having a shim of special geometry similar to that of the shim of the actuator of Fig. 45; Fig. 54 is a perspective view similar to that of Fig. 43, but showing an actuator according to this invention wherein the bimorph element has a recess for outputting an acceleration signal of the vibration of the actuator; Fig. 55 is a block diagram of the driver circuit of the actuator of fig. 54; Fig. 56 is a graph showing the frequency characteristics of the gain of the actuator of Fig. 54; Fig. 57 is a perspective view of another bimorph type actuator having a recess on one of the parallel plate-shaped bimorph elements according to this invention, which recess is similar to that of the actuator of Fig. 54; Fig. 58 is an axial sectional view of an electromagnetic actuator for driving the magnetic head of an optical disk recording and reproducing device according to this invention; Fig. 59 is an exploded perspective view of the actuator of Fig. 58, showing the parts of the actuator upside down with respect to the attitudes thereof shown in Fig. 58; Fig. 60 is a view similar to that of Fig. 58, but showing another embodiment of an electromagnetic actuator according to this invention; and Fig. 61 is a view similar to that of Fig 58, but showing still another embodiment of an electromagnetic actuator according to this invention. In the drawings, like reference numerals, except where stated otherwise, represent like or corresponding parts or portions. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, optical disk devices and portions thereof characteristic of this invention are described under respective headings. Optical Disk Device with a Bimorph Type Actuator Referring now to Figs. 4 through 6 of the drawings, a first embodiment of a recording and reproducing optical (more precisely magneto-optical) disk device according to this invention is described. This embodiment of optical disk device is characterized by a bimorph type actuator that controls the distance between the magnetic head and the disk. Under this heading, only the overall structure of the device is described. As shown in Fig. 4, this optical disk device is similar to that described above in reference to Fig. 1, except for the following difference: the device comprises a bimorph type actuator 9 mounted at one end thereof to a supporting base 6, and the magnetic head 8 is mounted to the lower surface of the free end portion of a bimorph type actuator 9, as shown in detail in Fig. 5. The actuator 9 consists of a pair of piezoelectric plate-shaped elements 9a and 9b which are coupled together via an electrically conductive plate such that a voltage applied thereacross causes one to expand and the other to contract; thus, the actuator 9 is bent and deflected in the direction B perpendicular to the recording surface 1a of the disk 1 in proportion to the voltage applied thereto. Further details of the structures of bimorph type actuators which are characteristic of this invention are described in detail under the heading "Bimorph Type Actuators" below. The objective lens 3a in the optical head 3 is driven in the focusing direction C by means of a lens actuator 3b, as shown explicitly in Fig. 6. As well known in the art, the actuator 3b is driven in response to an output signal of an optical distance detector (described below) so that the distance J between the lens 3a and the recording surface 1a on the disk 1 is maintained constant at a predetermined magnitude. The same voltage applied to the actuator 3 is applied to the bimorph type actuator 9 to move the free end thereof in direction B, so that the distance H between the magnetic head 8 and the recording surface 1a of the disk 1 is maintained constant at a predetermined magnitude. By the way, as is well known in the art. optical disk 1 comprises a layer of magnetic material 1b on which the recording of information is effected, and a substrate 1a transparent to the light (laser) beam on which the layer of magnetic material 1b is deposited by a method such as the vacuum deposition or sputtering. Optical Distance Detectors Under this heading, a few optical distance detectors for use with magneto-optical disk recording and reproducing device are described. Further optical distance detectors which are suited to be used with the magneto-optical disk recording and reproducing devices are described below under the next heading under which detailed explanation of the princple of their operation is given. These optical distance detectors are not in themselves part of the present invention, but can be employed in embodiments of the present invention. Optical distance detectors as described in the following two sections form the basis for divisional European Patent Application No. 93201147.1. First, referring to Figs. 7 through 9 of the drawings, an optical distance detector of an optical disk recording and reproducing device according to this invention is described. As shown in Fig. 7, a plate-shaped support member 13 of a L-shaped cross section is mounted at one end thereof to the two surface of the supporting base 6 of the optical disk device. Near the free end of the support member 13 are mounted a pair of optical distance detectors: a first detector consists of a light-emitting element (i.e. light-emitting diode) 14 and a partitioned light-sensitive element 15 consisting of a pair of light-sensitive detectors (i.e. photodetectors such as photodiodes) 15a and 15b; a second detector consists of a light emitting-element 16 and a partitioned light sensitive element 17 consisting of a pair of light-sensitive detectors 17a and 17b. The width of the support member 13 is substantially greater than that of the plate-shaped actuator 9 situated thereunder; the first detector consisting of elements 14 and 15 is situated above the surface of the disk 1, while the second detector consisting of elements 16 and 17 is situated above the actuator 9. Hence, as shown in Fig. 9, the light emitted from element 14 of the first detector is reflected on the surface of the disk 1 and received by the element 15; the first detector thus detects the distance between it and the surface of the disk 1. On the other hand, the light emitted from element 16 of the second detector is reflected on the upper surface of the actuator 9 and received by the element 17; the second detector thus detects the distance between it and the upper surface of the actuator 9. The photosensitve elements 15 and 17 consists of two photodetectors, as shown in Figs. 8 and 9; the details of the operation of the above first and second optical distance detectors will become clear from the descriptions below under this and the next heading. The distance H between the magnetic head 8 and the surface of the disk 1 can be easily calculated from the distances determined by the above first and second distance detectors. As shown in Fig. 10, the variation ΔH in the distance H between the magnetic head 8 and the disk 1 result in a variation ΔB in the magnetic field strength at the recording surface of the disk 1. However, if the actuator 9 is driven in direction B in response to the distance H determined by the distance detector of Figs. 7 through 9, the variation ΔH in the distance, and hence the variation in the recording characteristics, can be effectively reduced. Referring now to Figs. 11 through 14 of the drawings, another optical distance detector of an optical disk recording and reproducing device and the principle of operation thereof are described. As shown in Fig. 12, the disk 1 of the recording and reproducing device according to this embodiment is situated above the bimorph type actuator 9 carrying the magnetic head 8 thereon, optical head (not shown) being situated above the disk 1. The optical distance comprises a light-emitting element 18 and a partitioned light-sensitive element 19 consisting of a pair of light-sensitive detectors 19a and 19b, which are disposed on the upper surface of the plate-shaped actuator 9 beside the magnetic head 8, as best shown in Fig. 11. As a result, the light emitted from the element 18 is reflected by the lower surface of the disk 1 and received by the element 19, as shown in Fig. 12. The actuator 9 is applied with a voltage corresponding to the output of the distance detector, to be bent in the direction B perpendicular to the recording surface of the disk 1, so that the distance between the disk 1 and the magnetic head 8 is maintained constant. Next, referring to Figs. 13 and 14, let us describe the principle of operation of the optical distance detector comprising the light-emitting element 18 and the partitioned light-sensitive element 19. Fig.13 (a) and (b) show the relationship between the distance D along the line E-E in Fig. 11 (along which the section shown in Fig 12 is taken) from the center of the element 18, and the intensity of the reflected light incident on a point on line E-E at a distance D from the center of the element 18. Fig. 13(c) shows the distance D in alignment with the distance D taken along the abscissas of Fig. 13 (a) and (b). Fig. 13 (a) shows the relation in a case where the distance F between the lower surface of the disk 1 and the upper surfaces of the elements 18 and 19 is relatively great. In case, the intensity of reflected light incident on a point on line E-E at distance D from the center of the element 18 decreases slowly as the distance D increases; thus, the difference in the amounts of light incident on the detectors 19a and 19b, respectively, is small. On the other hand, when the disk 1 becomes near the surfaces of the elements 18 and 19, the intensity of reflected light incident on line E-E becomes markedly peaked at the center of the element 18 and decreases rapidly with the increase in the distance D, as shown in Fig. 13 (b); thus, in such case, the difference between the amounts of light incident on the cells 19a and 19b, respectively, becomes greater. The distance F between the disk and the surfaces of the elements 18 and 19 can be determined from the magnitude of the difference between the output levels of the light-sensitive detectors 19a and 19b. Fig. 14 shows the relation between the distance F (plotted along the abscissa) and the differential output of the partioned light-sensitive element 19, namely the difference in the output of the light-sensitive detectors 19a and 19b (plotted along the ordinate), wherein the target level of distance F is marked by a dotted line. The distance F between the elements 18 and 19 of the distance detector and the disk 1 varies around the target distance thereof in a region in which the differential output decreases monotonously (i.e. substantially linearly) as the distance F increases. Thus, in its variation region, the distance F corresponds to the differential output by a one-to-one relationship. As a result, the distance F, and hence the distance between the magnetic head 8 and the recording surface of the disk 1, are determined uniquely by the differential output of the element 19. Further Optical Distanct Detectors In the following, further optical distance detectors are described. These optical distance detectors comprise, as a light source, a light-emitting diode, and, as a light-sensitive element, a pair of photodetectors (photodiodes) which detects the amounts of light incident thereon after being reflected by a specular surface of an object; they are all characterized in their utilization of the normalized differential output (described under the sub-heading (b) below); most of them are characterized in their special geometry of the photodetectors. They are especially suited to be used as a distance detector in a magneto-optical disk recording and reproducing device, although they may be used for other purposes as well. Before embarking on the description thereof, however, the structure and the principle of operation of a typical optical distance detector are reviewed. (a) Typical Structure of Optical Distance Detector Fig. 15 shows the structure of a typical optical distance detector. The distance detector comprises the following elements: a light-emitting diode 21 disposed on a base plate 22; a photodetector (photodiode) 23 disposed on the same base plate 22; a converging lens 24 including two convex lens portions having distinct optical axes, which convex lens portions converging the emitted and the reflected light, respectively; and a window pane 25 closing the open end of the housing 26 accomodating the diode 21, the photodetector 23, and the converging lens 24. The operation of the distance detector of Fig. 15 is as follows: The light emitted from the diode 21 is converged by the lens 24 and is radiated from the window pane 25 of the housing 26, to be reflected by a surface of the object 27. such as the recording surface of an optical disk, the distance from which to the distance detector is measured. (The optical axis of the light from the light-emitting diode 21 to the surface of the object 27 is shown by a dot and dash line 28). The light reflected at the surface of the object 27 enters into housing 26 via the window pane 25 and is converged by the lens 24 to be received by the photodetector 23. (The optical axis of the light from the surface of the object 27 to the photodetector 23 is shown by a dot and dash line 29.) The light-emitting diode 21 is driven at a constant output level; hence, the amount of light emitted from the diode 21 is constant. Thus, the amount of light incident on the element 23 is determined by the distance between the object 27 and the distance detector, and the reflectance and the geometric form of the surface of the object 27. In the case where the reflectance and the geometric form of the object 27 do not vary, the amount of light incident on the photodetector 23 is determined solely by the distance between the photodetector and the object 27. Fig. 16 shows the relation between these two factors: the distance between the distance detector and the reflective surface of the object, and the intensity of light incident on the photodetector. As shown in the figure, the intensity of light (plotted along the ordinate) is peaked at the distance of about 4.5 mm and falls off thereabove as the distance becomes greater. Thus, as long as the distance to be measured is within a range in which the relation between the measured distance and the intensity of light incident on the photodetector is linear (or more precisely monotonous), the output of the photodetector 23 (which is proportional to the intensity of light incident thereon) corresponds to the measured distance by a one-to-one relationship. Thus, the distance to be measured can be determined uniquely from the output level of the photodetector 23. The typical optical distance detector as described above, however, has the following disadvantage. Namely, the output of the element 23 depends not only on the distance but also on the reflectance of the surface of the measured object 27. Thus, when the reflectance of the object 27 varies, the distance detector is incapable of determining the distance. As a result, the application of this optical distance detector is limited to the case where the reflectances of the measured objects do not vary from one object to another. The optical distance detectors having partitioned light-sensitve elements described above are improved in this respect; however, they still leave much to be desired. Hence, in the following further embodiments of the optical distance detector according to this invention, which aims at solving the above problem, are described. The optical distance detectors described in the following are also suited to detect extremely small distances and may be installed with advantage in an optical recording and reproducing disk device for determining the distance between the disk and the optical or magnetic head thereof. (b) Fundamenal Structure and Organization. Referring now to Figs. 17 through 22 of the drawings, a distance detector is described which may be employed in an embodiment of the invention. The description of the fundamental structure and organization under this sub-heading (b) applies to the detectors described below under sub-headings (c) through (e), except where it is stated otherwise or it is apparent from the description that the fundamental structure and organization do not apply to that particular detector. As shown in Fig. 17, the optical distance detector comprises a light-emitting diode 31 consitituting the light source, and first and second photodetectors (photodiodes) 30a and 30b. According to this arrangement the light-emitting diode 31 is disposed on the same plane on which the photodetectors 30a and 30b are disposed, which plane is referred to as the detector plane hereinafter. The second photodetector 30b, which lies farther away from the light-emitting diode 31 than the first photodetector 30a, has a light-receiving area greater than that of the first photodetector 30a for a reason which will become clear below. The operation of the distance detector of Fig. 17 is as follows. The light-emitting diode 31 is energized at a constant output level. Thus, the rectangualr upper surface of the diode 31, which forms substantially a completely diffusive light-emitting surface, emits light in all directions almost in equal intensity. Let us suppose that a specular reflective surface of an object, the distance between which and the detector plane is to be measured, lies above and parallel to the detector plane. Then the light emitted from the diode 31 is reflected by the specular reflective surface of the object and received by the first and second photodetectors 30a and 30b. Fig. 18 shows the relationship between the position on the line j-j in Fig. 17 and the intensity of light incident thereon. Namely, along the abscissas of Fig. 18 (a) and (b) is plotted the distance D from the center of the light-emitting surface of the diode 31 along the line j-j in Fig. 17, which distance D is shown in alignment therewith at the bottom row (c) of the same figure; along the ordinates of Fig. 18 (a) and (b) is plotted the intensity of light which is incident on a point on the line j-j at distance D from the center of the diode 31. Fig. 18 (a) shows the relationship in a case where the distance F between the surface of the object and the detector plane is relatively small. On the other hand, Fig. 18 (b) shows the relationship in a case where the distance F is relatively great. As shown in Fig. 18 (a), when the distance F is small, the intensity of incident light has a marked peak at the center of the light-emitting diode 31 and falls off rapidly as the distance F increases. On the other hand, when the distance F is great, the peak of the incident light at the center of the diode 31 becomes less marked, the distribution of the intensity of incident light spreading over the distance D with a slow decreasing rate. Fig. 18 shows only two representative cases (a) and (b). However, it will be apparent from the figure that as the distance F becomes smaller, the distribution of the intensity of light becomes more and more concentrated at the center (i.e. the point where distance D is equal to zero) and the height of the peak becomes increasingly higher. Thus, if the intensity of light incident on a fixed point at a constant distance D from the center of the diode 31 is observed, it will first increase and the decrease as the distance F increases. Namely, the relationship between the distance F and the intensity of light on a fixed point has the form similar to that shown in Fig. 17, in which the measured distance plotted along the abscissa corresponds to the distance F. The intensity of light incident on a fixed point increases as the distance F increases until it reaches a peak or maximum; thereafter it decreases as the distance F increases. The relation between the distance F and the intensity of incident light, however, is different from one point to another. Namely, as the distance D becomes greater, the peak of the intensity of light is reached at an increasingly larger value of the distance F and the height of the peak becomes lower. Fig. 19 shows the relation between the above distance F (plotted along the ordinate) and the intensity of light (plotted along the ordinate) incident on the upper surface of the photodetectors 30a and 30b, wherein the curves PD1 and PD2 show the intensities of light incident on photodetectors 30a and 30b, respectively. Since the first photodetector 30a is situated nearer to the light source (i.e. the light-emitting diode 31), the intensity of light incident on it has a peak when the distance F is relatively small, and falls off thereafter as the distance F increases, as shown by the curve PD1. The second photodetector 30b, on the other hand. is situated farther away from the light source; thus, the intensity of light incident on it has a lower peak which is reached when the distance F is relatively great, as shown by the curve PD2. The parameters of the distance detector of Fig. 17, such as the separations g₁ and g₂ between the diode 31 and the photodetector 30a and between the photodetectors 30a and 30b, respectively, or the widths w₁ and w₂ of the photodetectors 30a and 30b, are selected in such a way that the variation range of the distance F to be measured by the distance detector lies between the peaks of the incident light intensity curves PD1 and PD2, as shown by a double-headed arrow in Fig. 19. Thus, within the measured distance range, the intensity of light incident on the first detector 30a decreases as the distance F increases, while the intensity of light incident on the second detector 30b increases as the distance F increases. It is a feature of this arrangement that the geometric parameters of the first and second photodetectors are selected in such a manner that the variation range of the measured distance F is contained in an interval between the peaks of the intensity of light incident on the first and second photodetectors 30a and 30b. Fig. 20 shows the relationship between the above distance F and the output levels of the photodetectors 30a and 30b within the variation range of the measured distance F. Since the light-receiving surface of the second photodetector 30b has an area greater than that of the first photodetector 30a to compensate for the decline of the intensity of incident light, the level of the output PD2 of the second photodetector 30b is substantially the same as that of the output PD1 of the first photodetector The output PD1 of the first photodetector 30a nearer to the light source decreases substantially linearly (i.e. monotonously) in the variation range of the measured distance F, while the output PD2 of the second photodetector 30b farther away from the light source increases substantially linearly in the variation range of the measured distance F. The distance F between the surface of the object and the detector plane is determined from the normalized differential output of the first and second photodetectors 30a and 30b, as described in the following. Namely, by means of an electric circuit schematicaly shown in Fig. 21, the difference x and the sum y of the outputs PD1 and PD2 of the first and second photodetectors 30a and 30b are computed: x = PD1 - PD2, y = PD1 + PD2. Then, the differential output x of the two photodetectors 30a and 30b is normalized by taking the ratio of it with regard to y: x/y = (PD1 - PD2) / (PD1 + PD2). As noted above, the output PD1 decreases monotonously as the distance F increases in the variation range of the measured distance, while the output PD2 increases monotonously as the distance F increases in the same range. Thus, the value of the differential output x decreases substantially linearly as the distance F increases. Further, this differential output x is normalized with respect to the sum y of the two outputs PD1 and PD2 by taking the ratio x/y. Thanks to the normalization with respect to the sum y of the two outputs PD1 and PD2, the level of this normalized differential output x/y of the two photodetectors 30a and 30b is not affected by the variation in the reflectance of the specular surface of the object at which the light emitted from the diode 31 is reflected. Thus, the distance F can be determined by utilizing the relation between the distance F and the normalized differential output that is shown in Fig. 22. The normalized differential output x/y becomes equal to zero, i.e. has a zero crossing point N as shown in Fig. 22, when the outputs PD1 and PD2 of the first and second photodetectors become equal. If the distance of F at the zero-crossing point N in Fig. 22 is equal to the target distance of F, then the normalized differential output x/y itself may be used as the error signal of the distance F with respect to the target distance. Thus, it is preferred that the parameters of the diode 31 and photodetectors 30a and 30b, such as the separations g₁ and g₂ and the dimensions of photodetectors 30a and 30b w₁, l₁, w₂, and l₂, shown in Fig. 17, are selected in such a way that the distance of F at the above zero crossing point N becomes equal to the target distance of F. Since the normalized differential output x/y has a zero crossing point where the outputs PD1 and PD2 become equal, the zero crossing point N can be set substantially arbitrarily at any desired position by varying the above parameters of the diode 31 and the photodetectors 30a and 30b. For example, if the separation g₂ between the two photodetectors 30a and 30b is made smaller, the peak of the intensity of light incident on the second photodetector 30b, i.e. the peak of the curve PD2 and Fig. 19, moves toward left in the figure; hence, the output of the second photodetector 30b, shown by the curve PD2 in Fig. 20, increases more rapidly in the region in which the distance F is small. Thus, the zero crossing distance becomes smaller, i.e. the position of the zero crossing point N is moved toward left in Fig. 22. Conversely, if the separation g₂ is made greater, the zero crossing distance of F becomes greater. Of course, the position of the zero crossing point N, i.e. the zero crossing distance of F, can as well be changed by varying the ratio of the areas of the light-receiving surfaces of the two photodetectors 30a and 30b. It is noted in this connection that if the ratio of the two areas is equal to unity, i.e., if the two areas are equal to each other, the amount of light incident on the first photodetector 30a is always greater than that incident on the second photodetector 30b; as a result, the zero crossing point N is removed to infinity. On the other hand, when the area of the second photodetector 30b increases and the ratio of the area of the second photodetector 30b to that of the first photodetector 30a increases above unity, the zero crossing distance becomes smaller. Thus, in order to make the zero crossing distance equal to the target distance, it is necessary to select the ratio of the areas of the second photodetector 30b to the first 30a at a value greater than unity, i.e, to make the area of the second photodetector 30b greater than that of the first 30a. (c) Concentric Geometry of the Photodetectors. Fig. 23 shows another optical distance detector which is fundametally similar to the above detector shown in Fig. 17 but in which the geometry of the photodectors is adapted to the radially symmetric distribution of the light emitted from the light-emitting diode. Namely, the distribution of the light emitted from the light-emitting diode 31 is substantially rotationally symmetric with respect to the axis of the light-emitting diode 31 which pass through the center of the diode 31 at right angles with the surface thereof. Thus, for the purpose of enhancing the accuracy of measurement, the most efficient geometry of the light-receiving surfaces of the first and second photodetectors is that in which annular surfaces of the photodetectors are disposed around the light-emitting diode 31 in concentric relationship: in such geometry, the concentric annular surfaces of the two photodetectors receive the reflected light at locations at which the intensity of incident light is substantially equal and uniform (i.e. take a single value). Thus, the first and second photodetectors 30a and 30b of the embodiment shown in Fig. 23 have substantially the forms of annuli concentric with the disk-shaped light-emitting diode 31 disposed at the central portion thereof. Across the light-emitting diode 31 are attached a pair of electrodes 34b and 34c, which extends through the cut-out portions formed in the annular photodetectors 30a and 30b. An electrode 34a attached to the first photodetector 30a extends through the cut-out portion of the annular second photodetector 30b, to which an electrode 34d is attached. Thus, the cut-out portions of the annular photodetectors 30a and 30b of this embodiment serve for the purpose of leading out the electrodes 34a through 34d. Fig.24 shows still another photodetector having a fundamentally concentric geometry of the light-emitting diode and the two photodetectors. The first photodetector 30a has the form of a segment of a disk the center of which conincides with that of the light-emitting diode 31. the segment being cut off by a chord opposing a side of the rectangular light-emitting diode 31. The second photodetector 30b has the form of a segment of an annulus which is concentric with the segmental disk-shaped first photodetector 30a, the segment being cut from the annulus by a line obtained by producing the chord that bounds the segment of the first photodetector 30a. Thus, the inner and outer arcs that bound the segment of annulus of the second photodetector 30b are centered around the light-emitting diode 31. The efficiency of receiving the reflected light is reduced in this geometry of the photodetectors 30a and 30b, compared with the case of Fig. 23; however, in the case of the geometry of Fig. 24, the light-emitting diode 31 and the photodetectors 30a and 30b can be produced as separated chips which are mounted later on a substrate, which makes the production thereof easier. (d) Stepped Configuration of the Photodetectors In the case where the distance to be measured in small and the variation range of the measured distance is limited around a small distance, the areas of the first and second photodetectors, especially that of the first photodetector, must be made small, and the separations between the light-emitting diode and the first and second photodetectors must be reduced in order to attain enough precision in the measurement. In principle, these reductions of separations and surface areas of optical elements can be accomplished in a coplanar geometry discussed above. However, since these optical elements must be electrically isolated from each other, it is difficult to reduce the separations therebetween under a certain limit in the case of a coplanar geometry of the light-emitting diode and the photodetectors. Thus, Fig. 25 shows a verticaly stepped configuration of the photodetectors which is suited to measure a small distance; Fig. 25 (a) and (b) are a side elevational view and plan view of the light-emitting diode and the photodetectors, respectively. According to this embodiment, the photodetector chip 39 carrying the first and second photodetectors 30a and 30b on the upper surface thereof is mounted on a printed circuit board or substrate 35 via an electrically conductive plate-shaped spacer 37. The first light-emitting diode 31 is mounted directly on the substrate 35. As shown in Fig. 26, the circuit board 35 carries thereon a printed circuit comprising the following pattern of electrical interconnections: a pair of leads 35a and 35b to be coupled to the lower and upper (light-emitting) surface of the light-emitting diode 31, respectively; a lead 35c having an enlarged rectangular end portion that is to be coupled to the lower surface of the photodetector chip 39 which constitutes the cathode of the photodetectors 30a and 30b; and a pair of leads 35d and 35e to be coupled to the anodes (i.e. upper surfaces) of the photodetectors 30a and 30b, respectively. On the other hand, the photodetector chip 39 comprises on the upper surface thereof bonding pads 39a and 39b coupled to the anodes of the first and second photodetectors 30a and 30b, respectively. As shown in Fig. 25 (b), the following pairs are electrically coupled to each other by a bonding wire: the upper or light-emitting surface of the diode 31 is coupled to the lead 35b by the bonding wire 35f; the bonding pad 39a for the anode of the first photodetector 30a is coupled to the lead 35d by a bonding wire 35g; and the bonding pad 39b for the anode of the second photodetector 30b is coupled to the lead 35e by a bonding wire 35h. Other electrical connections are made by means of an electrically conductive adhesive material: the connection between the lower surface of the diode 31 and the lead 35a; the connection between the photodetector chip 39 and the electrically conductive spacer 37; and the connection between the spacer 37 and the lead 35c. In the above structure, the thickness of the plate-shaped spacer 37 is selected at a dimension greater than the thickness of the light-emitting diode 31; further, the side of the rectangular spacer 37 opposing the light-emitting diode 31 recedes from the side of the chip 39 thereabove which likewise opposes the diode 31; as a result, the separation between the opposing side surfaces of the spacer 37 and the diode 31 can be made as great as is desirable, as is apparent from Fig. 25 (a). Thus, even if there is some extrusion of electrically conductive adhesive material in the assembling process, there is no danger of an occurence of short circuit between the leads 35c for the cathode of the photodetectors and the lead 35a for the lower surface of the diode 31. Further, between the light-emitting surface of the diode 31 and the light receiving surfaces of the photodetector chip 19 is formed a vertical drop in the direction of the optical axis thereof, i.e., in the direction perpendicular to the surface of the diode 31 or photodetectors 30a and 30b. Since the thickness of the spacer 37 is greater than the thickness of the diode 31. the vertial drop is greater than the thickness of the photodetector chip 19. The method of operation of the photodetector configuration shown in Figs. 25 and 26 is as follows: As shown in Fig. 27, the light emitted from the diode 31 is reflected by the opposing specular surface of an object 27 and the reflected light is received by the photodetectors 30a and 30b. Since the spacer 37 has a thickness greater than that of the diode 31 (i.e. the vertical drop of the light-emitting surface with respect to the light-receiving surfaces of the photodetectors is greater than the thickness of the light-emitting diode 31), the separation between the light-emitting diode 31 and the photodetector chip 39 can be made as small as is desired, without incurring the danger of short circuit between the leads 35a and 35c or between the spacer 37 and the lead 35a. Thus, the light-emitting surface of the diode 31 and the light-receiving surface of the photodetector chip 39 (especially the light-receiving surface of the photodetector 30a) can be made sufficiently near to each other, in their horizontal positions. If necessary, the projections of the light-emitting and light-receiving surfaces to a horizontal plane (the projections being made on a horizontal plane in a direction parallel to their optical axes that are perpendicular to the light-emitting or light-receiving surface thereof) may partially overlap each other. As a result, the distance detector according to this arrangement is capable of measuring extremely small distances with accuracy. Fig. 28 shows an arrangement wherein an optical distance detector whose structure is substantially identical to that of Figs. 25 through 27 is installed in an magneto-optical disk device for the purpose of controlling the distance between its magnetic head 8 and the optical disk 1. Fig. 28 (a) shows the plan view of the magnetic head 8 and the optical distance detector. The optical distance detector includes a substrate 35 carrying the printed circuit comprising the leads 35a through 35e coupled to the electrodes of the light-emitting diode 31 and the photodetectors 30a and 30b thereon. Fig. 28 (b) shows a vertial cross section thereof along line k-k in Fig. 28 (a). As shown in Fig. 28 (b), the magnetic head 8 and the distance detector on the substrate 35 are mounted on a support member 38 which constitute an actuator for adjusting the separation g between the magnetic head 8 and the optical disk 1. The operation of the assembly shown in Fig. 28 is as follows. The actuator constituted by the member 28 is driven in response to the distance F between the light-receiving surface of the photodetector chip 39 and the optical disk 1. Thus, as discussed above, it is preferred that the geometric parameters of the diode 31 and the photodetectors 30a and 30b (such as the separations therebetween and the areas of the two photodetectors 30a and 30b) are selected in such a way that the following condition is satisfied. Namely, when the separation g is equal to the target value thereof, the normalized differential output x/y of the first and second photodetectors 30a and 30b vanishes. In this manner, the normalized differential output x/y can be used as the error signal in the control of the separation g to the target value thereof. By the way, the reason that the separation g between the magnetic head 8 and the disk 1 must be controlled by means of an actuator in an magneto-optical disk device is as follows. Commonly, a separation of a few micrometers is formed by means of an air cushion formed between the magnetic head and the magnetic recording medium of a magnetic recording and reproducing device. However, the strong point of the magneto-optical disk device is its immunity to dust; thus, if this advantage of the magneto-optical disk device is to be preserved, the separation between the magnetic head and the disk must be controlled within a range of from a few tens of micrometers to a few hundreds of micrometers. Thus, in the case of magneto-optical disk device, the air floating method is of no avail. Consequently, it is necessary to detect the separation between the magnetic head and the disk by a distance detector and control it in response to the output to the distance detector. Further, the distance detector must be small-sized and light-weighted to be used with an magneto-optical disk recording and reproducing device. The optical distance dectectors according to the above arrangements satisfy these requirements. (e) Stepped Concentric Configurations of the Photodetectors As discussed above in connection with the configuration of Fig. 23, the radiation distribution of the light emitted from the light-emitting diode is substantially radially symmetric, i.e. rotationally symmetric with respect to the central axis of the light-emiting diode; hence, concentric configuration of the first and second photodetectors centered around the light-emitting diode is most efficient and accurate in determining the distance from the amounts of light incident thereon. However, various technical difficulties accompanies such configuration of the photodetectors mounted on the same chip or substrate on which the light-emitting diode is mounted. The difficulties become greater if the separations between the optical elements or areas of the photodetectors are to be made smaller for the purpose of determining small distances with enough precision. Thus. in the following, let us describe arrangements having stepped concentric configuration, whereby the electrical connections can be made easily and measurement of small distance can be made accurately and efficiently. Fig. 29 shows a first arrangement of the stepped concentric configuration of the photodetectors, wherein Fig. 29 (a) shows a plan view of the configuration while Fig. 29 (b) shows a vertical section thereof along line 1-1 in Fig. 29 (a). Annular first and second photodetectors 30a and 30b are carried on a disk-shaped photodetector chip 39 having a central circular through hole 39c extending vertically therethrough. The disk-shaped photodetector chip 39 is mounted on the upper surface of an electrically insulating rectangular spacer member 40 having a circular through hole 40d extending vertically therethrough in registry with the through hole 39c of the photodetector chip 39c. The diameter of the through hole 40d of the spacer 40 is greater than that of the through hole 39c of the photodetector chip 39. A rectangular light-emitting diode 31 is mounted at the upper light-emitting surface thereof to the lower surface of the spacer 40, in registry with its through hole 40d; since the rectangular diode 31 has sides longer than the diameter of the through hole 40d of the spacer 40, the through hole 40d is closed from below by the upper surface of the light-emitting diode 31. Electrical connections to and from the diode 31 and the photodetectors 30a and 30b are effected as follows. The spacer 40 has a printed circuit on both the upper and lower surfaces for making electrical interconnections. Namely, as shown in Fig. 30 (a), on the upper surface of the spacer 40 is carried a printed circuit which includes the following electrical connections: an electrode or lead 40c having a disk-shaped end portion with a central hole in registry with the through hole 40d of the spacer, which electrode 40c is to be coupled electrically, via an electrically conductive adhesive agent, to the cathode of the photodetectos 30a and 30b formed on the lower surface of the disk-shaped photodetector chip 39; and a pair of leads 40a and 40b to be electrically coupled to the anodes on the upper surfaces of the first and second photodetectors 30a and 30b, respectively, via bonding wires 41a and 41b, respectively. Further, as shown in Fig. 30 (b), on the lower surface of the spacer 40 is carried a printed circuit which includes the following electrical connections: a lead 40g having a rectangular end portion with a central hole in registry with the through hole 40d of the spacer 40, which lead 40g is to be coupled electrically to the upper or light-emitting surface of the diode 31 via an electrically conductive adhesive; and a lead 40f which is to be electrically coupled to the lower surface of the light-emitting diode 31 via a bonding wire 41c, as shown in Fig. 31. Figs. 32 and 33 show the assembly of Fig. 29 in operation: the light emitted from the diode 31 radiates through the through holes 40d and 39c of the spacer 40 and the photodetector chip 39 to be reflected by the lower surface of the object, i.e. an optical disk 1; the reflected by the disk 1 is received by the first and second photodetectors 30a and 30b, which are concentric with the light-emitting diode 31. Fig. 32 (a) shows a vertical section along a diameter of the disk-shaped photodetector chip 39; the view corresponds to the case where the distance between the disk 1 and the detector surface (i.e. the upper hand, Fig. 33 (a) shows the same section in the case where the distance is great. Figs. 32 (b) and 33 (b) show, in the cases of small and great measured distance F, respectively, the relation between the distance D (taken along the abscissa) from the center of the concentric photodetectors and the intensity of light (taken along the ordinate) incident on a point on the detector surface which is separated from its center by a distance D. Since the distribution of the intensity of light incident on the detector surface is rotationally symmetric with respect to the center of the photodetector chip 39, the intensities of light incident on the concentric annular light-receiving surfaces of the photodetectors 30a and 30b are substantially uniform over the whole area thereof. Thus, the configuration of the photodetectors according to this embodiment realises accurate and efficient measurement of the distance F between the surface of the chip 39 and the disk 1. Further, the light-emitting surface of the diode 31 and the light-receiving surface of the first photodetector 30a can be made horizontally (i.e. radially) so close to each other, that the projections of these light-emitting and light receiving surfaces to a horizontal plane along the direction of the optical axis are overlapped upon each other; thus, the measurement of extremely small distances is made possible. Referring now to Fig. 34 of the drawings, let us describe a second arrangement having a stepped concentric configuration of photodetectors in which the efficiency in forming the electrical connections is improved. Fig. 34 (a) and (b) are a plan view and a vertical sectional view thereof, respectively, Fig. 34 (b) showing a section along the line m-m in Fig. 34 (a). A rectangular photodetector chip 39 having a central through hole 39c carries on its upper surface annular first and second photodetectors 30a and 30b which are concentric with the central hole 39c. The second photodetector 30b has a radially extending cut-out portion through which the lead 39a coupled to the anode of the first photodetector 30a extends. The lower surface of the rectangular photodetector chip 39 which constitute the cathode of the photodetectors 30a and 30b is mounted on a printed circuit board or substrate 35 via an electrically conductive spacer member 43. As shown in Fig. 35, the spacer 43 has horizontally the form of U, the open end of which is directed downward in the figure. On the other hand, as shown in Fig. 36, the substrate 35 carries on its upper surface the following pattern of a printed circuit: a connection or lead 35c having a U-shaped end portion on which the spacer 43 is mounted via an electrically conductive adhesive; leads 35a and 35b coupled to the lower and the upper surface of the light-emitting diode 31, respectively, via an electrically conductive adhesive and a bonding wire 35f, respectively; and leads 35d and 35e to be coupled to the anode of the first and second photodetectors 30a and 30b, respectively, via bonding wires 35g and 35h, respectively. The light-emitting diode 31 is mounted on the end portion of the lead 35a on the substrate 35, to be horizontally in registry with the central hole 39c of the photodetector chip 39. The connection between the cathode (lower surface) of the chip 39 and the spacer 43 is effected by an electrically conductive adhesive. By the way, the spacer 43 is thick enough to allow the bonding wire 35f to be coupled to upper surface of the didoe 31 without making contact with the lower surface of the photodetector chip 39. The operation of the assembly of Fig. 34 is as follows. The light emitted from the diode 31 is radiated upward through the through hole 39c to be reflected by a specular surface of an object. Thus, its operation is substantially indentical to that of the first embodiment (shown in Fig. 29) having the stepped concentric photodetector configuration. However, in the case of this second arrangement the electrical connections are made by means of a printed circuit formed on a single surface of the substrate 35. Thus, the time and labor required in forming the pattern of printed circuit on the substrate 35 and in mounting the electrical components thereon are reduced. Referring now to Figs. 37 and 38 of the drawings, a third arrangement having a stepped concentric configuration of photodetectors is described. Fig. 37 shows a vertical section thereof, while Fig. 38 is an exploded perspective view thereof. A rectangular plate-shaped photodetector chip 39 carries on its upper surface annular first and second photodetectors 30a and 30b which are concentric with the central through hole 39c of the chip 39. The photodetector chip 39 is mounted on a substrate 35 via an electrically conductive spacer 45. The rectangular plate-shaped spacer 45 has a U-shaped notch 45a having a large enough dimension to receive a rectangular plate-shaped light-emitting diode 31 horizontally therein without contact. On the upper surface of the substrate 35 is formed a pattern of printed circuit comprising leads 35j and 35k. The light-emitting diode 31 is mounted on the end portion of the lead 35j via an electrically conductive adhesive material. The spacer 45 is mounted, via an electrically conductive adhesive material on the end portion of the lead 35k having a form corresponding thereto. The lower surface of the photodetector chip 39 consituting its cathode is connected via an electrically conductive adhesive material to the upper surface of the spacer 45 and the upper surface of the light-emitting diode 31. Thus, in the case of this arrangement the cathode of the photodetector chip 39 and the light-emitting surface of the diode 31 are electrically coupled to each other. Thus, the circuit for driving them must be organized with a special care. Fig. 39 shows an example of the drive circuit for the light-emitting diode 31 and the photodetectors (photodiodes) 30a and 30b. The diode 31 is coupled across a battery 54 via a resistor 55. On the other hand, the photodetectors 30a and 30b, whose cathodes are coupled to the positive terminal of the battery 54, are coupled at their anodes to the inverting input of the amplifiers 56a and 56b, respectively. The output PD1 and PD2 of the amplifiers 56a and 56b, which constitute the output of the photodetectors 30a and 30b, respectively, are fed back to the inverting input of the amplifiers 56a and 56b, respectively, by negative feedback resistors 57a and 57b, respectively. The non-inverting inputs of the amplifiers 56a and 56b are grounded. When the light emitted from the diode 31 radiates on the reverse-biased photodiodes 30a and 30b, a reverse current proportional to the amount of incident light flows through each one of the photodiodes 30a and 30b. Thus, the outputs PD1 and PD2 corresponds to the amount of light incident on the photodiodes 30a and 30b, respectively. Bimorph Type Actuators (a) Typical Structures and their Disadvantages. In the optical disk recording and reproducing device described above, a bimorph type actuator is used to control the distance between the magnetic head and the recording surface of the magnetic disk. Fig. 40 show a typical structure of the bimorph type actuator which is used for driving the object lens of the optical head. Thus, a bimorph type actuator 61, which consists of a pair of parallel plate-shaped bimorph elements 61a and 61b, is fixed at one end thereof to a support member 60, and, at another end thereof, coupled to an object lens holder 3c on which the objective lens 3a of the optical head is mounted. As shown in Fig. 41, each bimorph plate 61a or 61b consists of a pair of piezoelectric plate-shaped elements 62a and 62b and an elastic shim 63 held therebetween. The shim 63 is made of a metal such as copper and also functions as an electrode for the piezoelectric plates 62a and 62b. Leads 67 and 69 are coupled to the upper surface of the piezoelectric plates 62a and 62b, respectively; lead 68 is coupled to the electrically conductive shim 63. Thus, the piezoelectric plates 62a and 62b are polarized in the direction of their thickness when a driving voltage is applied thereacross via the electrodes 67 through 69. The parallel structure of the two bimorph plates 61a and 61b of the actuator 61 is aimed at minimizing the slanting of the lens 3a from the horizontal direction. The operation of the bimorph type actuator 61 of Fig. 40 is as follows. When a voltage is applied across the piezoelectric plates 62a and 62b of each one of the bimorph plates 61a and 61b of the bimorph type actuator 61 via the leads 67 through 69 in such a way that the electrodes 67 and 69 have the same polarity while the electrode 68 has the opposite polarity, one of the piezoelectric plates 62a and 62b expands and the other contracts; for example, the upper plate 62a expands and the lower plate 62b contracts, as shown by horizontal arrows in Fig. 41. However, the elastic shim 63, to which the opposing surfaces of the piezoelectric plates 62a and 62b are fixedly secured, does not change its length; thus, the bimorph plates 61a and 61b are bent downward in the direction B1 as shown in Fig. 42. Conversely, when the polaritiy of the applied voltage is reversed, the upper piezoelectric plate 62a contracts and the lower plate 62b expands; as a result, the bimorph plates 61a and 61b, and hence the whole actuator 61, are deflected in the upward direction. The focusing error of the objective lens 3a is detected by an optical distance detector, for example, and the voltage applied to the bimorph plates 61a and 61b are controlled in accordance with the detected focusing error. Thus, the holder 3c and the object lens 3a mounted thereon are driven in the vertical direction B perpendicular to the recording surface of the optical disk in response to the detected focusing error, and the distance between the lens 3a and the recording surface of the optical disk is adjusted in such a manner that the focusing error is reduced to zero. The bimorph type actuator of Fig. 40 as described above, however, has the following disadvantages. As shown in Fig 42, the gain (taken along the ordinate) of the actuator, i.e., the amount of vertical deflection or displacement thereof in relation to the voltage applied thereacross, has high peaks, with regard to the frequency f (taken along the abscissa) of the voltage applied to the actuator, at relatively low second, third, and fourth resonance frequencies f₂, f₃, and f₄, in addition to the first or lowest resonance frequency f₁. Thus, if the frequency of the voltage applied across the bimorph plates 61a and 61b of the actuator 61 comes into the range where the higher resonance frequencies f₂, f₃, etc. lie, the amount of deflection of the actuator, or the amplitude of oscillation thereof becomes abruptly great at these higher resonance frequencies. As a result, the frequency range of the actuator 61 must be limited within a region which lies below and outside of the higher resonance frequencies. Further, the structure in which the bimorph plates 61a and 61b run parallel to each other makes the assembling process a time and labor consuming process. Further, the same structure limits the amount of vertical displacement of the lens 3a which can be achieved by the deflection of the actuator 61. On the other hand, Fig. 43 shows a typical structure of a bimorph type actuator for driving the magnetic head 8 of an optical disk recording and reproducing device. The actuator of Fig. 43 is similar to that described above in connection with an embodiment of an optical disk recording and reproducing device having a bimorph type actuator for driving its magnetic head. Namely, the actuator 9 of Fig. 43 comprises a pair of piezoelectric plate-shaped elements 9a and 9b provided with electrodes on their surfaces, and an electrically conductive plate 9c fixedly held therebetween. A pair of supporting plates 6a fixedly supports therebetween the oscillation center of the bimorph type actuator 9 from the upper and lower sides thereof. The magnetic head 8 is mounted to the free end of the actuator 9 to oppose the recording surface of the optical disk 1. As described above in reference to Figs. 7 through 9, etc., the distance between the magnetic head 8 and the surface of the disk 1 is detected by a distance detector (not shown) from which a distance signal Z is supplied to the driver circuit 96 which drives the actuator 9. The driver circuit 96 outputs a voltage V in response to the distance signal Z to a terminal 95 coupled to the electrode at the upper surface of the upper piezoelectric plate 9a and to the electrode at the lower surface of the lower piezoelectric plate 9b. The terminal 94 coupled to the electrically conductive plate 9c is grounded. Thus, a voltage V outputted from the driver circuit 96 which corresponds to the distance error between the magnetic head 8 and the recording surface of the disk 1 is applied across the piezoelectric plates 9a and 9b of the actuator 9. In response thereto, the actuator 9 is deflected in the direction B perpendicular to the recording surface of the disk 1 to maintain the distance between the head 8 and the disk 1 at a predetermined constant magnitude. Since the structure and operation of optical disk recording and reproducing device have already been described, further description of the device is deemed unnecessary. The distance signal Z varies at various frequencies; thus, the voltage V corresponding thereto comes to be an A.C. voltage having the same frequency as the distance signal Z; the actuator 9 is applied with this voltage V outputted from the driver circuit 96. Thus, the actuator of Fig. 43 has the same kind of disadvantage as the actuator of Fig. 40 with respect to the frequency characteristics of its gain (i.e. the amount of displacement or deflection of the actuator in relation to the voltage applied thereacross). Fig. 44 shows an example of the relation between the frequency f (taken along the abscissa) and the gain or amount of displacement of the actuator (taken along the ordinate), which relation is determined primarily by the dimension and mechanical properties of the actuator. As shown in the figure, the amount of displacement has high peaks at relatively low second and third resonance frequencies f₂ and f₃, besides the first or lowest resonance frequency f₁. Thus, if the frequency of the voltage applied across the actuator 9 comes into the range where the higher resonance frequencies f₂, f₃ lie, the amount of deflection of the actuator, or the amplitude of oscillation thereof, becomes undesirably great at these higher resonance frequencies. As a result, the frequency range of the actuator 9 must be limited within a region which lies below and outside of the higher resonance frequencies f₂ and f₃ which are relatively low. The above problems of the actuators of Figs. 40 and 43 are solved according to the principle of this invention in two ways: first, by a special geometry of the bimorph type actuator itself; and second, by providing a circuit for supressing the higher order resonance frequency components. In the following, embodiments of the bimorph type actuator which solve the above problems according to these two methods are described under each sub-headings. (b) Actuators having a Shim of Special Geometry. Under this sub-heading, embodiments of bimorph type actuators having an elastic shim member with a special geometry that is effective for suppressing vibrations of undesirable modes are described. In these embodiments, the width of the shim is varied in the longitudinal direction of the plate-shaped actuators according to the function which represents the form of a beam vibrating in a normal free lateral vibration mode, as explained in detail below. First, referring to Figs. 45 and 46, let us describe a first bimorph type actuator in which the shim held between the piezoelectric plate-shaped elements have a special geometry for suppressing vibrations of higher modes other than the first or lowest mode. The actuator 71 comprises a pair of rectangular plate-shaped piezoelectric element 72 and 73, and a plate-shaped elastic shim 74 held fixedly therebetween. The width of the shim 74 varies with respect to the longitudial distance x according to a special function as described in detail below. The piezoelectric plates 72 and 73 are provided at upper and lower surfaces thereof with electrodes and are thereby polarized in the direction of their thickness. The actuator 71 is fixedly supported at one end thereof by a support member 60 and is coupled at the free end thereof to the holder 3c carrying the objective lens 3a. The width b = b(x) of the shim 74 is related to the longitudinal distance x measured from the side the support member 60 in the following manner: Let Y(x) be normal function of the first or lowest mode of free natural lateral vibration of a beam which is supported at its two ends by a fixed and roller support, respectively. Then, the form of the sides of the shim 74 is defined in such manner that the width b = b(x) is inversely proportional to this normal function Y(x): b(x) ∝ 1/Y(x). The normal function Y(x) of the first or lowest mode of vibration may be defined as follows: The free natural vibration of the beam in the lateral direction (i.e. in a direction perpendicular to the longitudinal axis of the beam) may be expressed in the form: y(x,t) = Y(x)·sin(ωt), wherein y(x,t) represents the amount of deflection (i.e. lateral displacement) as a function of the longitudinal distance x and the time t, and sin(ωt) it a function of time t along. (ω is the angular frequency of the vibration.) The function Y(x) represents the form of the vibrating beam as a funtion of the distance x. When the beam is vibrating in its lowest or first natural free vibration mode, the function Y(x) representing the form of the beam in such vibration mode is referred to as the normal function of the first or lowest mode of vibration. As well known in the mechanics of vibrations of beams, the normal function Y(x) of the first or lowest mode of vibration of a beam which is supported at its two ends by a fixed and a roller support, respectively, is given by the following equation: Y(x) = (cos β1 - cosh β1)(cosh βx - cos βx) + (sin β1 + sinh β1)(sinh βx - sin βx), wherein 1 is the longitudinal length of the beam and the value of β1 is given by: β1 = 2.365. In operation, a voltage corresponding to the focusing error is applied across the piezoelectric plates 72 and 73 of the actuator 71; thus, the actuator 71 is deflected in the direction B to reduce the focusing error of the lens 3a. Since the shim 74 has the horizontal form as described above (i.e. its width b is related to the longitudinal distance x as defined above), and, further, the amount of bending of a beam at a lateral cross section thereof is proportional to the moment of inertia of the cross sectional area and hence is inversely proportional to its width provided that its thickness is constant, the actuator 71 is deflected in the vibration mode of a beam supported at its two ends by a fixed (i.e. built-in) and a roller support. As a result, as shown in Fig. 47, the objective lens holder 3c is translated in the direction B without being slanted from the horizontal direction. Further, as shown in Fig. 48, thanks to the special geometry of the shim 74, higher resonance peaks f₂, f₃, f₄ of the gain (the amplitude of deflection with respect to the driving voltage), which peaks correspond to natural free vibrations of higher modes, are suppressed. Hence, the actuator 71 can be used in a wide frequency range spreading over the higher resonance frequencies. In the above actuator the width b = b(x) of the shim 74 is related to the longitudinal distance x in such a manner that it is inversely proportional to the normal function of the first vibration mode. Fig. 49 shows a second actuator in which the the width b = b(x) of the shim 74 is related to the longitudinal distance x in such a manner that it is inversely proportional to the normal function of the second vibration mode. Namely, the width b = b(x) of the shim 74 of the actuator of Fig. 49 is given by the following equation: b(x) ∝ 1/Y(x), wherein Y(x) is the normal function of the second mode of free natural lateral vibration of a beam which is supported at its two ends by a fixed and a roller support, respectively. Thus, as shown in Fig. 50, the peaks f₁, f₃, and f₄, etc. of the gain of the actuator 71 other than the second resonance peak f₂ are suppressed; as a result, the so-called resonance frequency of the system f₀ can be set at a higher frequency. According to the above-described first and second actuators, the width b = b(x) of the shim has been set by the equation: b(x) ∝ 1/Y(x), wherein Y(x) is the normal function of the first or the second mode, respectively, of free natural lateral vibration of a beam which is supported at its two ends by a fixed and a roller support, respectively. However, Y(x) in the above equation may be the normal function of the third or the fourth mode, or a combination of normal functions of two (or more) vibration modes. Further, the function Y(x) used in the above equation in defining the width b of the shim as a function of the longitudinal distance x may be the normal function of a beam which is fixedly supported at one of its ends but free at the other, or which is free both at its two ends. Fig. 51 shows an actuator 71 whose shim 74 has the same geometry as that of the first embodiment but which is used to drive the holder 3c carrying the objective lens 3a in the direction C perpendicular to the optical axis of the lens 3a. Thus, the actuator 71 controlls the tracking error of the lens 3a. It will be easy for those skilled in the art to combine a pair of actuators, one for the focusing and the other for the tracking control, to obtain an actuator assembly for controlling the position of the lens 3a in the two directions. Fig. 52 shows another actuator 71 whose shim 74 has the same geometry as that of the first embodiment but which is used to drive a magnetic head chip 8a of a magnetic tape recording and reproducing device. Namely, a magnetic head chip 8a is mounted to the lower surface of the actuator 71. The actuator 71 drives the magnetic head chip 8a according to a principle similar to the one described above; thus, the dislocation of the magnetic head chip 8a from the track on the magnetic tape is adjusted by the actuator 71, wherein the gain of the actuator is substantially leveled at the higher resonance frequencies. Fig. 53 shows embodiment of the invention in which an actuator 71 whose shim 74 has the same geometry as that of the first actuator above is used to drive a magnetic head of a magneto-optical recording and reproducing device. Namely, a magnetic head 8 is mounted to the lower surface of the actuator 71 to oppose the magneto-optical disk 1. A distance detector (not shown) detects the distance between the magnetic head 8 and the disk 1; in response to the detected distance, the actuator 71 drives the magnetic head 8 according to a principle similar to the one described above, so that the error of the distance may be reduced to zero. By the way, a pair of piezoelectric elements 72 and 73 are attached to both the upper and lower surfaces of the shim 74 to obtain the bimorph type actuator 71 in the case of the above embodiments; however, a bimorph type actuator according to this invention may also be obtained by attaching a single piezoelectric element to one of the surface of the shim having a geometry that is effective in suppressing the vibration at undesirable resonance frequencies according to this invention. The bimorph type actuators described under this sub-heading having a shim of special geometry effective for suppressing the vibration of undesirable modes have following advantages: first, they can be used in a wide frequency range extending over the higher resonance frequencies; second, they are capable of being deflected without being slanted at their free end, which makes them especially suited to be used as an actuator for driving the objective lens of an optical head, etc. Further, compared with the parallel paired structure of Fig. 40, they are simple in structure and can be assembled more easily; in addition, they allow a greater amount of deflection or displacement at their free end. (c) Actuators with Drive Voltage Modification Circuitry Under this sub-heading, actuators provided with a simple circuit which suppresses the driving voltage of the actuator when the vibration frequency of the actuator approaches undesirable resonance frequencies. Fig. 54 shows a first embodiment of such an actuator provided with drive voltage modification circuitry, which embodiment is similar to the actuator described under the sub-heating (a) in reference to Fig. 43. Thus, like reference numerals are used in both figures and the description of the actuator of Fig. 54 here is limited in the main to points which are different from the case of the actuator of Fig. 43. The upper piezoelectric plate-shaped element 9a of the bimorph type actuator 9 have a rectangular stepped portion or recess 9d formed at a corner of the upper surface thereof at the free oscillating end of the actuator 9. An electric lead coupled to the bottom surface of the recess 9d, at which the piezoelectric crystal of the element 9a is exposed, is coupled to a terminal 97; the terminal 97 in its turn is coupled to the actuator driving circuit 96 to supply a voltage C corresponding to the acceleration of the piezoelectric element 9a. As shown in Fig. 55, the drive circuit 96 comprises, in addition to the output circuit 96b, frequency detector circuit 96a for detecting the frequency f of the acceleration voltage C. The output circuit 96b outputs an actuator driving voltage V corresponding to the distance signal Z which, as remarked before, is outputted from a distance detector 99 which detects the distance between the magnetic head 8 and the recording surface of the disk 1; the output circuit 96b, however, modifies the level of the voltage V when the frequency f of the acceleration voltage C comes near or equal to the higher resonance frequencies (i.e. second and third resonance frequencies f₂ and f₃). Namely, when the frequency f of the acceleration voltage C comes near to or equal to the higher resonance frequencies, the output circuit 96b determines the magnitude of the factor by which the drive voltage V must be reduced, if the gain of the actuator is to be leveled at the frequency f near or equal to the higher resonance frequencies f₂ and f₃. In accordance with the determined reduction factor, the output circuit 96b reduces the level of the voltage V. Thus, the operation of the driver circuit 96 is identical unless the frequency of the vibration of the actuator 9 is outside of the neighborhoods of the higher resonance frequencies of the actuator 9. When, however, the frequency f of the acceleration voltage C, which frequency corresponds to the frequency of vibration of the actuator 9, comes near into the neighborhood of a higher resonance frequency, the output level of the voltage V determined on the basis of the distance signal Z is reduced by a factor determined on the basis of the frequency f. Thus, as shown in Fig. 56, the gain, i.e. the amount of displacement in the direction B of the actuator with respect to the level of the driving voltage V, is leveled at the higher resonance frequencies f₂ and f₃. Thus, actuator according to this embodiment can be used in a wide frequency range extending over the higher resonance frequencies. An advantage of the actuator according to this embodiment is this: the piezoelectric ceramics exposed at the bottom of the recess 9d functions as a acceleration sensor of the actuator 9 and directly outputs a acceleration signal voltage C; as a result, the frequency of the vibration of the actuator can be detected by a simple and inexpensive circuit organization. Fig. 57 shows another embodiment of an actuator 100 having a recess 100d for outputting an acceleration signal; the actuator 100 is used to support and drive the holder 3c of the objective lens 3a of a magneto-optical disk recording and reproducing device. Thus, for the purpose of avoiding the slanting of the lens 3a from the horizontal direction in the displacement or deflection in the vertical direction B, two plate-shaped parallel bimorph elements 100a and 100b fixedly supported at a vibration center thereof by supporting members 6a are coupled at the free end thereof to the holder 3c. The recess 100d formed on the upper bimorph element 100a is coupled to a drive circuit (not shown) as in the case of the actuator of Fig. 55. Optical Disk Device with an Electromagnetic Actuator In the above, optical disk devices having a bimorph type actuator have been described. Under this heading,optical disk recording and reproducing devices having an electromagnetic actuator for adjusting the position of its magnetic head are described. Since the optical disk devices described under this heading are similar in structure and operation to the optical disk devices described above, except for their electromagnetic actuators, descriptions hereinbelow are limited in the main to their electromagnetic actuators. Electromagnetic actuators are outside the scope of the present invention, but can be used in an embodiment of the invention. Fig. 58 shows a side elevational cross section of a first optical disk device having an electromagnetic actuator for driving the magnetic head. The device comprises an optical head 3 for radiating a light (laser) beam 4 on the disk 1 via an objective lens 3a and a reflection mirror 3d. The light beam 4 is converged at the spot 5 on the recording surface of the disk 1, on which recording of information is effected. Now, let us describe the structure of the electromagnetic actuator according to Fig. 58, which is shown in an exploded perspective view in Fig. 59 (wherein, it is noted. the parts are shown upside down with respect to their attitudes in Fig. 58). First, the immobile portions of the actuator comprises the following parts: a cylindrical yoke 108 fixedly mounted at the upper end surface thereof to the supporting base member 16 of the optical disk device, the yoke 108 having an annular recess 108a extending in its axial direction from the end surface thereof opposing the recording surface of the magneto-optical disk 1; an annular permanent magnet member 109 mounted to the radially inwards facing side surface of the recess 108a formed in the yoke 108, the annular magnet 109 being magnetized in the radial direction to have a south and a north pole at its inner and outer side surface, respectively; a flat annular magnetic shield 110 made of a magnetic material, substantially closing the annular opening of the annular recess 108a of the yoke 108. The movable parts which are moved with the magnetic head 8 of the optical disk device, on the other hand, have the following structure. A hollow cylindrical coil bobbin 113 carrying a coil 114 on the outer side surface thereof is coupled, via a pair of flat annular elastic support members 111 and 112, to the radially inwards facing side surface of the recess 108a of the yoke 108. To the end of the bobbin 113 opposing the recording surface of the disk 1 is mounted a substrate board 115 carrying the magnetic head 8 of the optical disk device. In addition to the magnetic head 8, the substrate board 115 carries on the surface opposing the recording surface of the optical disk 1, a light-emitting element 116 and a light-sensitive element 117 portioned into two detector parts, as shown in Fig. 59, which elements 116 and 117 together constitute an optical distance detector for detecting the distance between the magnetic head 8 and the recording surface of the disk 1. The operation of the actuator of Figs. 58 and 59 is as follows. When the recording is effected, the magnetic head 8 is energized to generate a biasing magnetic field at the recording spot of the disk 1 which is heated by the light beam 4 radiated from the optical head 3. The error of the distance between the magnetic head 8 and the recording surface of the disk 1 is detected by the optical distance detector constituted by the elements 116 and 117. Namely, the light emitted from the element 116 is reflected by the disk 1 to be received by the element 117. The distance error can be determined from the differential output of the two detector parts of the light-sensitive element 117. (For details of the operation of the optical distance detector, reference may be made to the description above concerning the optical distance detectors.) In response to the distance signal Z outputted from the element 117, the actuator driver circuit 114a supplies a current, the amount of which corresponds to the distance error, to the coil 114 carried on the bobbin 113. Thus, by means of the electromagnetic force acting between the coil 114 and the permanent magnet 109. the bobbin 113 is driven in its axial direction, to drive the magnetic head 8 carried on the substrate 115 in the direction B perpendicular to the recording surface of the disk 1. As a result, the error of the distance between the disk 1 and the magnetic head 8 is reduced. and the magnetic head 8 is maintained at a constant distance from the recording surface of the disk 1. By the way, the shield 110 prevents the magnetic flux from leaking to the magnetic head 8 from the magnetic circuit formed by the yoke 108 and the permanent magnet 109. Referring now to Fig. 60, a second optical disk device having an electromagnetic actuator is described. The device is identical to that shown in Figs. 58 and 59 (wherein like reference numerals represent like parts), except for the following points: The substate board 115 carrying the magnetic head 8 and the optical distance detector (not shown) is mounted to an end of the bobbin 113 via a plate-shaped support member 118; the member 118 is mounted to the end of the bobbin 113 opposing the disk 1. A balancing weight 119 is mounted to the end of the member 118 opposite to the end to which the substrate board 115 is mounted. Further, the bobbin 113 is coupled to the yoke 108 via a single elastic support member 112. Referring next to Fig. 61, a third optical disk device having an electromagnetic actuator is described. The device is identical to that shown in Figs. 59 and 60 (wherein like reference numerals represent like parts), except for the following points: The substate board 115 carrying the magnetic head 8 and the optical distance detector (not shown) is coupled to an end of an elastic support member 121 which in its turn is coupled at the other end thereof to a support member 120 fixed to the supporting base 6. Thus, the bobbin 113 carrying the coil 114 is supported by the members 120 and 121 via the substrate board 115 coupled to an end thereof. It will be apparent to those skilled in the art that many modifications may be made to the optical disk recording and reproducing device having an actuator according to this invention. For example, distance detector of the type utilizing the electrostatic capacity or the signal of the optical head for the detection of the distance may be used instead of the optical distance detectors. Further, the actuators can be used for moving the magnetic head to allow a sufficient separation between the head and the disk during the loading and unloading of the optical disk. In such case, the magnetic head is moved by the actuator in the direction away from the surface of the disk by a length of about 3 to 10 mm. While we have described and shown particular embodiments of our invention under each heading above, it will be understood that many modifications may be made without departing from the scope thereof, and we contemplate by the appended claims to cover any such modifications as fall within the true scope of our invention.

Claims: An optical disk device capable of recording and reproducing information on and from a magneto-optical disk (1) including a layer of magnetic material, comprising optical head means (3) for radiating a light beam on a recording surface (1a) of said magneto-optical disk (1); magnetic head means (8) for generating, in a recording process, a biasing magnetic field at a recording spot on the recording surface (1a) of the magneto-optical disk (1), which spot is being heated by a light beam (5) radiated from said optical head means (3), to effect a recording of information by reversing a direction of magnetization of said magnetic material heated by said light beam, wherein reproduction of information is effected by radiating a light beam (5) from said optical head means (3) on the magneto-optical disk (1) and receiving a light reflected by the recording surface of the magneto-optical disk (1) to read information out from the reflected light; and means for positioning the magnetic head means (8); characterised in that the positioning means comprise distance detector means (14,15,16,17;18,19) for detecting the distance between the magnetic head (8) and the recording surface (1a) of the magneto-optical disk (1); a bimorph type actuator (9,71) including a plate-shaped piezoelectric bimorph type element (9a,9b) on a free end of which said magnetic head means (8) is mounted to oppose the recording surface (1a) of the magneto-optical disk (1); and actuator driver means (96) for driving said bimorph type actuator (9) in response to the distance detected by said distance detector means (14,15,16,17;18,19) so that an error of said distance with respect to a target distance thereof is reduced. An optical disk device as claimed in claim 1, wherein said distance detector means (14,15,16,17;18,19) comprises an optical distance detector which includes a light-emitting element (14,16;18) and a light-sensitive element (15,17;19) which receives the light reflected from a surface of the disk (1). An optical disk device as claimed in claim 2, wherein said light-sensitive element (15,17) comprises two photodetectors (15,17). An optical disk device as claimed in claim 3, wherein said distance detector (14,15,16,17;18,19) determines the distance from a differential output of said two photodetectors (15,17). An optical disk device as claimed in claim 1, 2, 3 or 4 wherein said bimorph type actuator (9,71) includes a plate-shaped piezoelectric bimorph type element (72,73) which comprises: a plate-shaped piezoelectric ceramics (72,73); and a plate-shaped support member (74) of an electrically conductive metallic material fixedly coupled at a surface thereof to a surface of said piezoelectric ceramics (72,73), wherein said support member (74) has a width that varies with respect to a longitudinal distance from an end of a free portion of the actuator (71) such that vibration modes at undesirable resonance frequencies are suppressed. An optical disk device as claimed in claim 5, wherein the width of said plate-shaped support member (74) varies with respect to the longitudinal distance from an end of the free portion of the actuator (71) in an inversely proportional relation to a normal function of a first mode of free natural vibration of the actuator (71), so that vibration modes at higher resonance frequencies other than the first vibration mode are suppressed. An optical disk device as claimed in claim 5, wherein the width of said plate-shaped support member (74) varies with respect to the longitudinal distance from an end of the free portion of the actuator (71) in an inversely proportional relation to a normal function of a second mode of free natural vibration of the actuator (71), so that vibration modes other than the second vibration mode are suppressed. An optical disk device as claimed in any of claims 1 to 4, wherein said actuator (9) comprises a plate-shaped piezoelectric bimorph element (9a) on a surface of which a recess (9d) is formed to expose a piezoelectric ceramics at a bottom thereof, and said actuator driver means (96) is electrically coupled to said bottom of the recess on said piezoelectric bimorph element (9a) to receive a voltage corresponding to an acceleration of a vibrating end of said actuator (9), said actuator driver means (96) reducing a driving voltage of said actuator (9) on the basis of a level of said voltage corresponding to the acceleration of the vibrating end of the actuator (9). A bimorph type actuator (9;100) driving a magnetic head of an optical disk device as claimed in claim 1, comprising: a plate-shaped piezoelectric bimorph element (9a; 100a) having a recess (9d; 100d) formed on a surface thereof to expose a piezoelectric ceramics of said bimorph element(9a; 100a); frequency detector means (96a), electrically coupled to said recess (9d; 100d) of the bimorph element (9a; 100a), for detecting a frequency of a voltage generated at the exposed piezoelectric ceramics of the bimorph elements (9a; 100a); distance detector means (99) for detecting a distance between said magnetic head (8) of the optical disk device and the magneto-optical disk (1); actuator driver means (96), coupled to said distance detector means (99) and said frequency detector means 96a, for outputting to said piezoelectric bimorph element (9a;100a) an actuator driving voltage which corresponds to a distance detected by said distance detector means (99), wherein said actuator driver means (96) reduces a level of said actuator driving voltage when said frequency of the voltage generated at the exposed piezoelectric ceramics of the bimorph element (9a; 100a) falls into a neighborhood of one of higher resonance frequencies of the actuator (9; 100) other than the first resonance frequency, thereby leveling a gain of the actuator (9; 100) at such higher resonance frequencies. A bimorph type actuator (100) as claimed in claim 9, further comprising a second plate-shaped piezoelectric bimorph element (100b) extending parallel to said plate-shaped piezoelectric bimorph element (100a) having formed on a surface thereof a recess (100d) exposing a piezoelectric ceramic element at the bottom of the recess (100d). An optical disk device capable of recording and reproducing information on and from a magneto-optical disk (1) including a layer of magnetic material, comprising optical head means (3) for radiating a light beam (4) on a recording surface of said magneto-optical disk (1); magnetic head means (8) for generating, in a recording process, a biasing magnetic field at a recording spot (5) on the recording surface of the magneto-optical disk (1), which spot (5) is being heated by a light beam (4) radiated from said optical head means (3), to effect a recording of information by reversing a direction of magnetization of said magnetic material heated by said light beam (4), wherein reproduction of information is effected by radiating a light beam (4) from said optical head means (3) on the magneto-optical disk (1) and receiving a light reflected by the recording surface of the magneto-optical disk (1) to read recorded information out from the reflected light; and means for positioning the magnetic head means (8); characterised in that the positioning means comprise distance detector means (116, 117) for detecting the distance between the magnetic head (8) and the recording surface of the magneto-optical disk (1); an electromagnetic actuator (108, 109, 110, 111, 112, 113, 114, 115) including an electromagnetically driven movable member (113, 114, 115) on which said magnetic head means (8) is mounted to oppose the recording surface of the magneto-optical disk (1); and actuator driver means (14a) for driving said electromagnetic actuator in response to the distance detected by said distance detector means (116, 117) so that an error of said distance with respect to a target distance thereof is reduced. An optical disk device as claimed in claim 11, wherein said electromagnetic actuator comprises: a cylindrical yoke (108) coupled to a supporting base of the optical disk device, said yoke having an annular recess (108a) extending in an axial direction from an end surface thereof opposing a recording surface of the magneto-optical disk (1); an annular permanent magnet (109) mounted to a radially inwards facing side surface of said recess of the yoke (108) to oppose a radially outwards facing side surface of said recess across a predetermined annular gap; a hollow cylindrical bobbin (113) disposed in said annular gap between said permanent magnet (109) and the radially outwards facing side surface of the recess of the yoke (108); a coil (114) carried on an outer side surface of said bobbin (113); a flat annular magnetic shield member (110) substantially closing an annular opening of said recess of the yoke (108); and a substrate board member (115) secured to said bobbin (113) at an end thereof opposing a recording surface of the magneto-optical disk (1), the magnetic head (8) of the optical disk device being mounted to said substrate board member (115) to oppose the recording surface of the magneto-optical disk (1). An optical disk device as claimed in claim 12, wherein said bobbin (113) is elastically coupled to the radially inwards facing side surface of the recess of the yoke by means of an annular elastic member (111). An optical disk device as claimed in claim 13, wherein said substrate board member (115) is secured to said end of the bobbin (113) opposing the recording surface of the magneto-optical disk (1), through an intermediary of a plate-shaped support member (118) at one end of which the substrate board member (115) is mounted. An optical disk device as claimed in claim 12, wherein said substrate board member (115) is coupled to an end of an elastic support member (121) which in its turn is supported by a supporting base (120) of the optical disk device. An optical distance detector arranged to detect the distance between the magnetic heat (8) and the surface of the disk (1) usable an optical disk device as claimed in any of claims 1 to 8 or 11 to 15 comprising light source means (31) for radiating light towards said surface of the disk (1); first optical detector means (30a) for detecting the amount of light which is incident on a light-receiving surface thereof after being reflected by said surface of the disk (1), said light-receiving surface being substantially perpendicular to an optical axis of said light source means (31); second optical detector means (30b) for detecting the amount of light which is incident on a light-receiving surface thereof after being reflected by said surface of the disk (1), the light-receiving surface of the second optical detector means (30b) being situated on a plane on which said light-receiving surface of the first optical detector means (30a) lies, at a position farther away from said optical axis of the light source means (31) than the light-receiving surface of the first optical detector means (30a); subtractor means, coupled to outputs (PD1, PD2) of said first and second optical detector means (30a, 30b), for computing a difference between outputs of said first and second optical detector means (30a,30b); adder means, coupled to outputs (PD1, PD2) of said first and second optical detector means, for computing a sum of outputs of said first and second optical detector means (30a, 30b); and normalizer means, coupled to said subtractor means and said adder means, for computing a ratio of said difference between the outputs of the first and second optical detector means (30a, 30b) to said sum of the outputs of the first and second optical detector means (30a, 30b); wherein separations, in a radial direction perpendicular to the optical axis of the light source (31), between the light source means (31) and the first optical detector means (30a) and between the first and second optical detector means (30a, 30b), and areas of the first and second optical detectors (30a, 30b) are selected in such a manner that, in a measurement range of the measured distance between the disk (1) and the optical distance detector, the amount of light incident on the light-receiving surface of the first optical detector means (30a) decreases as the measured distance increases while the amount of light incident on the light-receiving surface of the second optical detector means (30b) increases as the measured distance increases so that said ratio, computed by said normalizer means, of the difference between the outputs of the first and second optical detector means (30a, 30b) to said sum of the outputs of the first and second optical detector means (30a, 30b), varies substantially linearly with respect to the measured distance. An optical distance detector as claimed in claim 16, wherein the outputs of said first and second optical detector means (30a, 30b) becomes equal to each other at a target value of said measured distance, so that said ratio, computed by said normalizer means, of the difference between the outputs of the first and second optical detector means (30a, 30b) to said sum of the outputs of the first and second optical detector means (30a, 30b), corresponds to a deviation of said measured distance from its target value. An optical distance detector as claimed in claim 16 or 17, wherein the light-receiving surfaces of said first and second optical detector means (30a, 30b) comprise surfaces disposed concentrically around the optical axis of the light source means (31). An optical distance detector as claimed in any one of claims 16 to 18, wherein the light-receiving surfaces of said first and second optical detector means (30a, 30b) are stepped forwards with respect to a light-emitting surface of the light source means (31), in the direction of the optical axis of the light source means (31) toward the reflective surface of the object. An optical distance detector as claimed in claim 19, wherein a height of the step between the light-receiving surfaces of the optical detector means (30a, 30b) and the light-emitting surface of the light source means (31) in the direction of the optical axis of the light source means is greater than a thickness of the first and second optical detector means (30a, 30b) in the direction of the optical axis of the light source means (31).

Inventors: KIME KENJIRO C O DENSHINSHOHIN, NAKAJIMA HAJIME C O OHYOKIKI K, NAKAMURA KEIJI DENSHINSHOHIN K, WATANABE ISAO C O OHYOKIKI KEN, KIME, KENJIRO C/O DENSHINSHOHIN KAIHATSU KENKYUSHO, NAKAJIMA, HAJIME C/O OHYOKIKI KENKYUSHO, NAKAMURA, KEIJI DENSHINSHOHIN KAIHATSU KENKYUSHO, WATANABE, ISAO C/O OHYOKIKI KENKYUSHO
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1989
Main classification: G11B11
IPC codes: G11B11, G01B11, G11B7