Patent Publication Number: US-7225452-B2

Title: Optical head

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 10/205,627, filed Jul. 26, 2002 now U.S. Pat. No. 6,968,563, and to which priority is claimed under 35 U.S.C. §121. This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2001-228125, filed Jul. 27, 2001; and No. 2002-146167, filed May 21, 2002, the entire contents of both of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical head irradiating an optical disk with a light beam to record or reproduce information, particularly an optical head comprising a relay lens to correct spherical aberrations, and to an optical disk apparatus comprising such an optical head. 
     2. Description of the Related Art 
     In recent years, in the field of recording or reproduction using, for example, an optical disk as a recording medium, many attempts have been made to an optical disk recording and reproducing apparatus that carries out recording on and reproduction from a small-sized high-capacity optical disk in order to deal with high-definition still images or moving pictures. A technical method of increasing capacity is to reduce the diameter of a beam spot by reducing the wavelength of a laser beam emitted by an optical pickup apparatus and increasing the numerical aperture (NA) of an objective lens. 
     The optical disk is generally irradiated with a beam via a transparent optical disk substrate (a cover layer) that covers an information record surface of the disk. With increasing NA, coma is more likely to be caused by a variation in angle between the objective lens and the disk substrate. The causes of the variation in angle include a warp of the optical disk, inclination of a spindle motor that rotates the optical disk, and inclination of an objective lens driving mechanism mounted in the optical head. It is difficult to precisely adjust the angle to accommodate the increased NA, while allowing mass production of this optical disk. Coma resulting from the inclination occurs when a beam passes through the disk substrate, and can thus be lessened by reducing the thickness of the substrate to effect inclination. Accordingly, for an optical disk system using an objective lens with a large NA, an optical disk with a thinner substrate must be used to avoid inclination errors. 
     On the other hand, the objective lens is designed to form a beam spot with few spherical aberrations on the information record surface of the optical disk when the substrate of a specific optical thickness is used. Consequently, a spherical aberration may occur if the thickness of the substrate deviates from a designed optical thickness. Further, if as in a two-layer disk, two information record surfaces are irradiated with a laser beam from the same direction (compared to a double-sided disk on which recording and reproduction are carried out from different directions), the optical thickness of the transparent layer substrate always differs between the two layers. A spherical aberration caused by an error in the substrate thickness substantially increases consistently with NA. For a lens with a large NA such as 0.85, it is difficult to neglect the effects of an error in the optical thickness of an optical disk manufactured by a normal method. 
     The term “optical thickness” as used herein refers to a value determined by the thickness and refractive index of an optical disk substrate through which light is transmitted. Optical disk substrates having different thicknesses have an equal optical thickness if beam spots obtained by transmitting light through these substrates have an equal level of spherical aberration. Even if the substrate consists of a plurality of layers, its optical thickness is determined by the thicknesses and refractive indices of these layers. 
     Jpn. Pat. Appln. KOKAI Publication No. 5-151609 discloses various methods of correcting spherical aberrations caused by a variation in thickness of the optical disk substrate. One of these methods uses what is called a relay lens composed of a convex lens and a concave lens. The relay lens composed of the convex and concave lenses is arranged in front of a position where light emitted by a laser diode is incident on the objective lens. A beam incident on the optical disk through the objective lens has its spherical aberrations varied by varying the position of either the convex or concave lens in the direction of an optical axis. This cancels spherical aberrations that may occur in the optical disk to generate a beam spot with few aberrations on the optical disk. This publication uses a VCM (Voice Coil Motor) as a driving mechanism for the objective lens but does not disclose its structure or a control method therefor. 
     Jpn. Pat. Appln. KOKAI Publication No. 5-266511 discloses another method somewhat in detail. The method disclosed in this publication uses as relay lens driving means a method of attaching a rack to a relay lens to be moved so that a pinion is rotated to move the lens. This publication also discloses a method of using a measuring instrument provided in a recording and reproducing apparatus to measure a substrate thickness (cover layer thickness), or reading the thickness data recorded in the disk, and setting the position of the relay lens in accordance with the thickness. 
     Jpn. Pat. Appln. KOKAI Publication No. 2001-28147 also discloses another method somewhat in detail. In this example, the interval between a first and second lenses of the relay lens can be changed using voice coil motors. These voice coil motors are designed so as to vary the interval between the lenses linearly with input currents. To obtain a lens interval suitable for recording in or reproduction from the first or second layer, currents having the same magnitude and flowing in the opposite directions may be allowed to flow through the respective coils. 
     However, the above described conventional techniques have a plurality of problems. 
     Jpn. Pat. Appln. KOKAI Publication No. 5-151609 does not provide a detailed description of a driving mechanism using a VCM, as described previously. However, a lens moving mechanism to correct spherical aberrations cannot exhibit sufficient performance with a general simple VCM. Therefore, the technique disclosed in this publication does not allow a practical optical head to be manufactured. 
     Jpn. Pat. Appln. KOKAI Publication No. 5-266511 discloses an example in which the rack and pinion are used as a relay lens movement device. An optical disk apparatus with an optical head mounted therein is used in a portable computer, a music reproducing apparatus, or the like, and must thus be very small in size. However, with the configuration according to this publication, which has a large number of components including power transmission mechanisms such as a rotary motor and the rack and pinion which convert rotational movement into linear movement, it is difficult to provide a small-sized optical head. 
     The lens moving mechanism to correct spherical aberrations must move the lens only in the direction of the optical axis. If the lens is inclined or its optical axis deviates from its correct position as the lens is moved, an aberration may occur to hinder the appropriate beam spot from being obtained. Furthermore, if the optical axis deviates from its correct position, the direction of a beam traveling from the relay lens to the objective lens may change. Then, disadvantageously, the position of the beam spot may change. If the position of the beam spot shifts in the radial direction of the disk, then a tracking servo mechanism of an optical drive device moves an objective lens actuator in a tracking direction to prevent the beam spot from deviating from the proper track. However, of course, the acceleration and movement of the beam spot have upper limits. Accordingly, the inclination of the relay lens and the positional deviation of the optical shift must be minimized. None of the conventional techniques deal with this point. Jpn. Pat. Appln. KOKAI Publication No. 5-266511 also fails to describe a mechanism that provides such a function. 
     Actually, it is expected to be difficult to design a lens moving mechanism using a rack and a pinion, so as to prevent these problems. For example, a typical rack and pinion mechanism involves backlash and is thus readily affected by vibration or a shock. An optical disk apparatus includes vibration inducing components such as a spindle motor rotating the disk and an objective lens actuator even if it is of an installed type instead of a portable type. Further, equipment such as computer in which an optical disk apparatus is mounted often includes a source of vibration such as a fan. With backlash, the apparatus is prone to be affected such vibration. That is, it is difficult to control the position of the relay lens using the rack and pinion mechanism while information is being recorded on or reproduced from the optical disk. 
     In the VCM disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-28147, the opposite sides of the second lens are supported by plate springs. It is difficult for such a center impeller support structure to establish linear moving directions. Thus, this structure is available only for very short strokes. Accordingly, a sufficient spherical aberration correcting capability is not ensured unless the focal lengths of the first and second lenses are short. In this case, however, the lenses may have to be aspherical, or the ranges of possible inclination of the lens or of permissible errors in eccentricity may be narrowed. As a result, the apparatus cannot be easily assembled. Furthermore, such a support structure is not rigid enough to support the lenses so as to prevent its optical axis from being inclined. Consequently, the lenses may be rotated by a possible vibration or shock, thereby increasing optical aberrations. Further, since the lens interval is controlled on the basis of the amount of current conducted, the effects of a vibration or shock cannot be eliminated. As a result, the lenses may also be moved in the direction of the optical axis by the vibration or the like. This publication cites Jpn. Pat. Appln. KOKAI Publication No. 10-188301 as a conventional example. According to Jpn. Pat. Appln. KOKAI Publication No. 10-188301, spherical aberrations are corrected by varying the interval between two objective lenses using a VCM. With this method, however, after the appropriate current required to correct the interval has been determined, it is maintained to retain the interval. Accordingly, the effects of disturbances cannot be eliminated. 
     As described above, in a conventional optical head having lenses mounted therein and which can be moved to compensate for spherical aberrations, the proper lens positions cannot be retained. This results in many optical aberrations and makes it difficult to stably control the optical head. 
     Further, the optical head has an objective lens driving device mounted therein as a lens moving mechanism. Suppression of the positional deviation of the optical axis is not considered for the objective lens driving device because the objective lenses are originally designed to shift the optical axes from a beam for a tracking operation. Furthermore, an objective lens actuator is designed mainly to allow a beam spot to dynamically follow movement of the disk such as decentering or side runout thereof so as to follow frequency components of as low as several kHz. However, a relay lens driving mechanism is designed mainly to correct a spherical aberration caused by a difference between layers of a multilayered optical disk as well as a difference in optical thickness between the transparent substrates of disks; the spherical aberration and thickness difference are not varied by rotation of the disk. A variation in optical thickness of the transparent substrate on the disk surface, which may have to be followed, is small. Consequently, the relay lens driving mechanism mainly perform DC operations and requires a control band of only about several hundred Hz at the most. Thus, the conventional moving devices determine the lens positions using a fixed current or use a mechanism such as a rack and pinion which is not capable of fast reciprocation. Consequently, it is unreasonable to use the objective lens driving apparatus as a relay lens driving mechanism. 
     Further, with the above described conventional techniques, the relay lens is coaxially arranged on the optical axis of the objective lens, thereby disadvantageously increasing the thickness of the optical head. An optical disk drive with an optical head mounted therein must be thin if it is used as a storage device for a computer, notably a portable computer. Thus, the optical head must not be thick. Obviously, the above described conventional optical heads are thicker than an optical head without any general relay lenses by an amount equal to the thickness of the relay lens. Moreover, even if a mirror is interposed between the objective lens and the relay lens to bend the optical axes through 90°, the driving mechanism used in the conventional examples makes it difficult to reduce the thickness of the optical head. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is provided to solve the above problems. It is an object of the present invention to provide an optical head comprising a relay lens driving mechanism which has a relay lens used to correct spherical aberrations, which precisely moves the relay lens, and which is unlikely to be affected by a vibration or shock. 
     To accomplish this object, according to a first aspect of the invention, an optical head comprises a lens to correct spherical aberrations; a lens holder holding the lens; a first parallel plate spring supporting the lens holder at one end thereof; an intermediate member attached to the other end of the first parallel plate spring; a second parallel plate spring arranged to extend from the intermediate member to the lens holder to support the intermediate member at one end thereof; and a fixing member fixeing the other end of the second parallel plate spring. 
     This configuration allows a relay lens to be moved only in the direction of an optical axis thereof as the lens holder is moved. Consequently, the optical axis of the lens is prevented from deviating from its correct position. 
     Further, according to another aspect of the invention, an optical head comprises a lens to correct spherical aberrations; a lens holder holding the lens; a guide rail extending parallel with an optical axis of the lens; a rail receiving section provided in the lens holder to receive the guide rail; a coil secured to the lens holder and having a winding axis extending perpendicularly to the guide rail; a permanent magnet provided opposite the coil; and a pressing section pressing the rail receiving section against the guide rail. 
     The winding axis for the coil is perpendicular to the optical axis, thereby enabling the optical head apparatus to be miniaturized. The lens is supported by the guide rail, thereby increasing supporting rigidity. The rail receiving section is pressed against the guide rail by the pressing section, thereby preventing the lens from inclined by a shock or vibration. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
         FIG. 1  is a perspective view showing the structure of a main part of an optical head according to a first embodiment of the present invention; 
         FIG. 2  is another perspective view showing the structure of the main part of the optical head according to the first embodiment of the present invention; 
         FIG. 3  is a perspective view showing the positional relationship between the optical head and an optical disk in  FIG. 1 ; 
         FIG. 4  is a schematic diagram showing the configuration of the optical head in  FIG. 1 ; 
         FIG. 5  is a schematic diagram showing the configuration and operation of the optical head in  FIG. 1 ; 
         FIG. 6  is a perspective view showing a relay lens driving mechanism according to the first embodiment of the present invention; 
         FIG. 7  is another perspective view showing the relay lens driving mechanism according to the first embodiment of the present invention; 
         FIG. 8  is a perspective view showing plate springs used in the relay lens driving mechanism according to the first embodiment of the present invention; 
         FIG. 9  is a perspective view useful in describing a position detecting device according to the first embodiment of the present invention; 
         FIG. 10  is a perspective view useful in describing a baffle according to the first embodiment of the present invention; 
         FIG. 11  is a perspective view showing a variation of the optical head according to the first embodiment of the present invention; 
         FIG. 12  is another perspective view showing the variation of the optical head according to the first embodiment of the present invention; 
         FIG. 13  is a perspective view showing a variation of a lens supporting mechanism used in the relay lens used in the relay lens driving mechanism according to the first embodiment of the present invention; 
         FIG. 14  is a perspective view of the lens supporting mechanism shown in  FIG. 13  as partially omitted; 
         FIG. 15  is a diagram showing the structure of a main part of an optical head according to a second embodiment of the present invention; 
         FIG. 16  is a perspective view showing the positional relationship between the optical head and an optical disk in  FIG. 15 ; 
         FIG. 17  is another perspective view showing the positional relationship between the optical head and optical disk in  FIG. 15 ; 
         FIG. 18  is a schematic diagram showing the configuration and operation of the optical head in  FIG. 15 ; 
         FIG. 19  is a perspective view partially showing the configuration of a relay lens driving mechanism according to the second embodiment of the present invention; 
         FIG. 20  is another perspective view partially showing the configuration of the relay lens driving mechanism according to the second embodiment of the present invention; 
         FIG. 21  is a perspective view showing the configuration of the relay lens driving mechanism according to the second embodiment of the present invention; 
         FIG. 22  is a perspective view showing the configuration of a movable part of the relay lens driving mechanism according to the second embodiment of the present invention; 
         FIG. 23  is a plan view showing the configuration of the movable part of the relay lens driving mechanism according to the second embodiment of the present invention; and 
         FIG. 24  is a diagram schematically showing the configuration of a variation of the optical head according to the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described below with reference to the drawings. 
       FIGS. 1 and 2  are perspective views showing the structure of a main part of an optical head according to a first embodiment of the present invention.  FIG. 3  is a perspective view showing the positional relationship between the optical head and an optical disk in  FIGS. 1 and 2 . 
     In  FIG. 3 , an optical disk  100  is a reproduction-only disk or recordable disk such as a phase-changing disk and a photomagnetic disk. In  FIGS. 1 and 2 , a light beam emitted by a light source  1  that irradiates an optical disk  100  with the light beam is collimated by a collimator lens  2 . The beam is then incident on a beam splitter  4  and subsequently passes through a quarter wavelength plate  5 . (A beam shaping prism that changes the shape of the light beam may be interposed between the collimator lens  2  and the beam splitter  4 .) Then, the light beam propagates through a relay lens system in a relay lens driving mechanism  7   a  according to this embodiment. The beam then has its orientation changed by 90° by a mirror  6  and then impinges on an objective lens  8 . The relay lens driving mechanism  7   a  moves a relay lens in the direction of its optical axis, the relay lens correcting spherical aberrations that may occur if there is an error in thickness of a cover layer of a disk. In this case, the objective lens  8  is composed of a combination of two lenses and has a high NA of about 0.85. An objective lens driving device  3  supports the objective lens  8  so that the lens  8  can be moved in the direction of its optical axis and in a radial direction of the disk. The light emitted from the objective lens  8  passes through the cover layer of the optical disk  100  and forms a beam spot on a desired reflection surface. 
     Reflected light from the disk  100  passes through the objective lens  8  and is then reflected by the mirror  6 . The light then propagates through the relay lens system and the quarter wavelength plate  5  and impinges on the beam splitter  4 . Then, the light is reflected by the beam splitter  4  and condensed by a convex lens  10 . Then, the light beam passes through a focus error signal generating element  11  and is applied to a photodetector  12 . The focus error signal generating element  11  may be based on any method that can realize a focus error signal. For example, for an astigmatism method, the focus error signal generating element  11  is a cylindrical lens. An output from the photodetector  12  is input to an arithmetic circuit  13  as shown in  FIG. 4 . The arithmetic circuit  13  outputs an information reproduction signal, a focus error signal, and a tracking error signal. The focus error signal and the tracking error signal have their phases compensated for by a phase compensation circuit  14 . On the basis of a signal resulting from the phase compensation, actuator drivers  15  and  16  in  FIG. 5  conduct currents through coils  17  and  18  of the objective lens driving device  3 . This controls the positions of the objective lens  8  in the direction of the optical axis and in the radial direction. 
     The relay lens system  7  shown in  FIG. 5  is used to correct a spherical aberration that may be caused by an error in the thickness of a cover layer of the disk  100 . At least either a lens  32  or a lens  33  (in this embodiment,  33 ) is moved in the direction of its optical axis to cause a spherical aberration in the relay lens system  7  so as to correct a spherical aberration caused by an error in the thickness of the cover layer of the disk  100 . This operation can be controlled using the method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 10-18830. However, this embodiment is provided with a position detecting device  22   a  that detects the position of the relay lens  33 . In this case, the position detecting device  22   a  is a photointerrupter composed of a light emitting diode  34   a  and a photodetector  35   a . The position of the relay lens  33  can be detected by allowing a baffle  23   a  operating integrally with the relay lens  33  to vary the quantity of light traveling from the light emitting diode  34   a  into the photodetector  35   a , depending on the position. Then, an output from the photodetector  35   a  is input to a lens position control circuit  19   a . The lens position control circuit  19   a  outputs a signal to a drive circuit  20   a  so as to place the relay lens at a specified position. The drive circuit  20   a  conducts a current through a coil  21   a  of the relay lens driving mechanism  7   a . Thus, in the present apparatus, position control is executed on the basis of feedback control. 
     A number of methods are available for determining the position of the relay lens  33 . For example, the following method can be used. A CPU  24  provided with A/D and D/A causes the relay lens position control circuit  19   a  to gradually moves the position of the relay lens  33  to the extent that a spherical aberration in the expected substrate thickness can be corrected. Then, a position of the lens  33  at which a reproduction information signal has the largest amplitude is detected. Subsequently, the corresponding value is continuously output to the relay lens position control circuit  19   a . In this manner, position control is executed on the basis of feedback control instead of open control as in the prior art. Accordingly, the position of the relay lens is not affected by a vibration or shock, thereby precisely achieving detection of the maximum value of the reproduction information signal and the subsequent retention of the lens position. Consequently, the error in the thickness of cover layer of the disk can be compensated and a larger-capacity optical disk apparatus is provided. 
     Next, the relay lens driving mechanism  7   a , mounted in the optical head of this embodiment, will be described in detail. In this case, of the two lenses constituting the relay lens system  7 , the lens  32  is fixed, whereas the lens  33  is movable. As shown in  FIG. 6 , the lens  32  is mounted in a base  42   a . A lens holder  27   a  has the coil  21   a  mounted thereon in a direction in which the coil is wound around an optical axis of the lens  33 . The lens holder  27   a  is supported by two parallel plate springs  25   a  and  25   b . While the plate springs are not flexed, their normals are parallel with the optical axis of the lens  33 . As shown in  FIG. 8 , the plate springs  25   a  and  25   b  are fixed to the lens holder  27   a  in their areas  25 - 1 . Areas  25 - 2  of the plate springs  25   a  and  25   b  are joined together by an intermediate joining member  28 . Areas  25 - 3   a  and  25 - 3   b  are fixed by blocks  37   a ,  37   b ,  45   a , and  45   b  so as not move relative to the base  42   a . Consequently, in the plate springs  25   a  and  25   b , it is areas  25 - 4   a  and  25 - 4   b  that actually function as plate springs. The plate springs  25   a  and  25   b  have a uniform thickness. The respective areas have widths W 48 , W 49   a , and W 49   b  having the following relationship:
 
 W 48/2 =W 49 a=W 49 b 
 
Further, the lengths L 46 , L 47   a , and L 47   b  of the respective areas are equal.
 
     As shown in  FIG. 7 , a permanent magnet  26   a  is placed opposite the plate springs of the relay lens driving coil  21   a  so as to be slightly separate from the coil  21   a . The permanent magnet  26   a  is mounted on a yoke  43   a  consisting of a ferromagnetic material such as a steel plate. The yoke  43   a  is attached to the base  42   a . Accordingly, the permanent magnet  26   a  is fixed to the base  42   a . The permanent magnet  26   a  is magnetized so that magnetic fluxes emitted by the permanent magnet cross the coil  21   a  as shown by the arrows in  FIG. 7 . 
     Description will be given of the operation of the relay lens driving mechanism configured as described above. In  FIG. 7 , when a current is conducted through the coil  21   a , it passes through a magnetic field generated by the permanent magnet  26   a . Thus, Lorentz force is generated in the coil  21   a . Then, force is exerted on the lens holder  27   a  in the direction of the optical axis of the lens  33 . Since the lens holder  27   a  is supported by the plate springs  25   a  and  25   b , the Lorentz force flexes the plate springs  25   a  and  25   b  to move the lens holder  27   a  in the direction of the optical axis. In this case, the plate springs  25   a  and  25   b  are assembled together so as to always lie parallel with each other. Accordingly, even when the lens holder  27   a  is moved in the direction of the optical axis, the lens  33  is not inclined. Further, the rigidity of the lens holder  27   a  in a vertical direction (Z direction) is high because of the high in-plane rigidity of the plate springs, which support the lens holder. Consequently, the lens  33  is prevented from moving in the vertical direction. Furthermore, the parallel plate spring arrangement has its length reduced in a direction (in this case, a Y direction) perpendicular to that in which the springs are flexed. A value for this reduction cannot be calculated using a simple formula for material dynamics. However, even when the plate springs are flexed, their length remains unchanged. Thus, reasonably, the decrease in length in the Y direction is understood on the basis of the knowledge of general material dynamics. This amount of contraction can be calculated taking structural non-linearity into consideration. That is, when the lens holder  27   a  is displaced in an X direction to bend the plate springs  25   a  and  25   b , two Y direction distances are reduced: a Y direction distance from the junction between the plate springs and the lens holder  27   a  to the intermediate joining member  28  and a Y direction distance from the intermediate joining member  28  to the fixing members  37   a  and  37   b.    
     The dimensions of the plate springs  25   a  and  25   b  are as described above. Thus, in  FIG. 8 , the flexural rigidity of each of the areas  25 - 4   a  and  25 - 4   b  in the X direction is half of that of the area  25 - 5  in the X direction. The lens holder  27   a  is supported by one end of the area  25 - 5  of each plate spring. The other end of the area  25 - 5  of the plate spring is joined to the intermediate joining member  28 . Furthermore, the intermediate joining member  28  is supported at the same position in the Y direction by the areas  25 - 4   a  and  25 - 4   b  of the plate springs. Thus, when the lens holder  27   a  is displaced by, for example, a distance x 1  in the X direction, the intermediate joining member  28  is displaced by a distance x 1 /2. This reduces both the distance from the junction between the plate springs and the lens holder  27   a  to the intermediate joining member  28  and the distance from the intermediate joining member  28  to the fixing members  37   a  and  37   b , by the same distance y 1 . That is, the displacement of the intermediate joining member  28  in the Y direction relative to the fixing members  37   a  and  37   b  is as large as the displacement of the lens holder  27   a  in the Y direction relative to the intermediate joining member  28 , but is directionally opposite thereto. Consequently, although the intermediate joining member  28  moves the distance y 1  in the Y direction, the lens holder  27   a  is not displaced in the Y direction. 
     In the relay lens driving mechanism configures as described above, even when an attempt is made to move the lens  33 , it is displaced only in the direction of its optical axis. Thus, spherical aberrations can be appropriately corrected. Further, the lens is supported by the two parallel plate springs, so that the lens  33  is rigid in a direction in which it may fall down. As a result, the lens is not inclined even with a shock or vibration. 
     Preferably, the plate springs with reduced rigidity in the X direction require a smaller current required to displace and hold the lens  33  in the direction of its optical axis, thereby reducing power consumption. However, reduced rigidity in the directions other than the moving direction may result in inclination or the like. Thus, the dimensions of the plate springs  25   a  and  25   b  must be determined considering the balance of the rigidities of the respective parts. 
     Further, in this embodiment, the force generated in the coil  21   a  concentrates on the left side thereof in  FIG. 7 . In contrast, the center of gravity of a movable part lies to the right of the area in which the force is generated. Thus, the force generation point deviates from the center of gravity of the movable part, so that moment force around a Z axis is generated in the movable part. Accordingly, intense vibration occurs and cannot be easily controlled if the frequency of an AC component of driving force is equal or close to the resonant frequency of the movable part around the Z axis. The resonant frequency can be set in a kHz order using a typical design method and material. As described previously, a frequency of about several hundred Hz is sufficient for a control band of the relay lens driving mechanism and is sufficiently lower than the frequency of rotation vibration around a Z axis. Accordingly, this resonance around the Z axis creates no problems. Further, the vibration can be suppressed to some degree using damping members, described later. If the control band is composed of high frequencies, the center of gravity must coincide with the point of action of force. However, the control band of the relay lens driving mechanism is composed of low frequencies, thus enabling design in which the center of gravity deviates from the point of action of force. 
     As described above, in this configuration, the center of gravity deviates from the point of action of force. Accordingly, the relay lens driving mechanism  7   a  in this embodiment can be constructed to have a small height in the Z direction. That is, the height in the Z direction is determined by only the diameter of the lens  33  and the thickness of the coil  21   a . Consequently, the height in the Z direction can be reduced by avoiding arranging the permanent magnet and other elements in the Z direction. Furthermore, only one magnet is provided and this embodiment thus uses the minimum required arrangements to generate Lorentz force. Hence, advantageously, the number of parts required is small and manufacture costs are low. If the center of gravity and the point of action of force are arranged closer to each other, permanent magnets must be arranged substantially symmetrically with respect to the objective lens. However, a support section composed of the plate springs  25   a  and  25   b  and others is provided in the right of  FIG. 7 . Accordingly, the permanent magnets must be arranged above and below the coil  21   a , thereby making it impossible to reduce the thickness of the apparatus. 
     Furthermore, the joining member  28  is arranged midway between the plate springs  25   a  and  25   b . This may cause resonance in the X direction. Resonance may also occur which has a mode in which the movable part rotates around a Y axis. The rotation vibration around the Y axis is designed to be symmetric with respect to the Z direction. Accordingly, this vibration is in principle not induced by excitation but may be caused by a manufacture error. Damping members can be used to suppress these resonant vibrations to the level at which they create no problems. For example, a laminate structure may be provided in which each plate spring is sandwiched between damping members. In this embodiment, vibration is suppressed by applying gel materials  36   a  and  36   b  to the neighborhoods of the intermediate-joining-member- 28 -side roots of the areas between the area  25 - 5  of the plate spring and the areas  25 - 4   a  and  25 - 4   b  thereof, respectively. 
     Now, a position sensor in this embodiment will be described in detail.  FIG. 9  is a perspective view showing the arrangement of the position detecting device  22   a .  FIG. 10  shows the shape and arrangement of a baffle. In this embodiment, the position detecting device  22   a  is used as a position sensor as described previously. Element incorporating sections  22   a   1  and  22   a   2  of the position detecting device  22   a  incorporate a light emitting element  34   a  and a light receiving element  35   a . When no obstacle is present in a slit  22   a   3 , light generated by the light emitting element  34   a  such as a light emitting diode is incident on the light receiving element  35   a  such as a photodiode. The light receiving element  35   a  provides a signal depending on the quantity of light received. Accordingly, when the baffle  23   a  is moved into or out from the slit  22   a   3 , its position can be detected. The baffle  23   a  is attached to the lens holder  27   a , thereby enabling the position of the lens holder  27   a  to be detected. If the position detecting device is small in size, it often provides small-sized effective beams and thus has a position detection range smaller than the range within which the lens holder  27   a  is to be positioned. Thus, in this embodiment, an oblique notch  23   a   1  is formed in the baffle  23   a  to reduce a variation in baffling effect caused by movement of the baffle  23   a . As a result, the position detection range is sufficiently enlarged compared to the range within which the lens holder  27   a  is movable. Consequently, the control circuit can place the lens at any position on the basis of feedback control. Thus, the lens is not substantially affected by a vibration or shock as described previously. 
     This embodiment has such a structure that light from the light emitting element  34   a  is unlikely to be irregularly reflected by certain parts before impinging on the light receiving element  35   a  of the position detecting device  22   a  and that extraneous light is unlikely to fall on the light receiving element  35   a . That is, position detecting device supporting pillars  41   a  and  41   b  are present in the Z direction as shown in  FIG. 5 , and the position detecting device itself and the lens holder  27   a  are present in the X direction. The position detecting device itself is also present in the Y direction. Accordingly, the slit  22   a   3  is surrounded by the baffling components to prevent unwanted light from being incident on the light receiving element  35   a . Therefore, stable position detection can be achieved. 
     Further, the position detecting device  22   a  is arranged at a side of the mirror  6  as shown in  FIG. 2 . Although optical elements can be arranged at the opposite side the relay lens driving mechanism (in this embodiment, the optical elements  11  and  12  are arranged), no optical elements can be arranged in the area in which the position detecting device  22   a  is located. When the position detecting device  22   a  is arranged in this area, the space can be more efficiently used to provide a small-sized optical head. 
     The position sensor may be replaced with another non-contact type. For example, the position sensor may be composed of a magnet and a Hall element. 
     Now, a variation of the optical head according to the first element will be described.  FIGS. 11 and 12  are perspective views schematically showing this variation. The configuration of this optical head is substantially the same as that of the first embodiment. Thus, parts with the same functions are denoted by the same reference numerals, and description thereof is omitted. 
     In this variation, compared to the first embodiment, the mirror  6  is oriented so that light traveling in a direction parallel to a tangent line of the disk is incident on the objective lens  8 . Further, a relay lens driving mechanism  7   a ′ is constructed to drive the lens  32 , located closer to the objective lens  8 . Even in this case, when the position detecting device  22   a  is arranged closer to the outer circumference of the disk than the mirror  6 , the space can be more efficiently used to provide a small-sized optical head. That is, the optical head of the present invention can be provided regardless of the spherical aberration correcting lens to be moved or the orientation of the mirror  6 . 
     Next, a variation of a plate spring arrangement of the relay lens driving mechanism which can be used for the optical head of the present invention will be shown with reference to  FIGS. 13 and 14 . This arrangement has functions equivalent to those of the lens supporting mechanism composed of the plate springs according to the first embodiment. 
     A lens  64  is fixed to a lens holder  65 . The lens holder  65  has a coil  66  mounted thereon in a direction in which the coil is wound around an optical axis of the lens  64 . A permanent magnet (not shown) is provided so that when a current is conducted through the coil  66 , force is generated in the vertical direction of the figures. The lens holder  65  is supported by two parallel plate springs  60   a  and  60   b . The other ends of the plate springs  60   a  and  60   b  retain an appropriate interval between the plate springs via a spacer  63   a . Furthermore, another set of two parallel plate springs  61   a  and  61   b  are connected to the lens holder via spacers  63   b  and  63   c . In this case, the plate springs  60   a  and  60   b  and the plate springs  61   a  and  61   b  are attached to the spacers  63   a ,  63   b , and  63   c  on the same side thereof. The other ends of the plate springs  61   a  and  61   b  are fixed to support beams  66   a  and  66   b . Further, the support beams  66   a  and  66   b  are connected together using spacers  67   a  and  67   b . Some or all of the support beams  66   a  and  66   b  and the spacers  67   a  and  67   b  are fixed to the base (not shown). 
     As described above, those portions of the four plate springs other than their fixed portions which function as plate springs all have the same length, thickness, and width. Here, it is assumed that a current is conducted through the coil  66  to generate force in the X direction of the figures. At this time, the spacers  63   a ,  63   b , and  63   c  are displaced by a distance ΔX 1  in the X direction and by a distance ΔY 1  in a −Y direction. This is because the spacers are forced to move in the −Y direction when displaced by the distance ΔX 1  because the length of the plate springs in the Y direction does not substantially change. Similarly, the plate springs  60   a  and  60   b  are flexed by a distance ΔX 2  in the X direction and by a distance ΔY 2  in the Y direction. However, since the four plate springs have the same shape as described previously, the distance ΔX 1  equals the distance ΔX 2 , whereas the distance ΔY 1  equals the distance ΔY 2 . The spacer supporting the plate springs  61   a  and  61   b  is moved the distance ΔY 1  in the −Y direction. Consequently, even when the plate springs  60   a  and  60   b  is contracted by the distance ΔY 2 , i.e. the distance ΔY 1  in the Y direction, the lens  64  is not moved in the Y direction but is only moved in the X direction by a distance ΔX 1 +ΔX 2 =2ΔX 1 . That is, the optical axis of the lens  64  is prevented from deviating from the correct position thereof in spite of movement in the X direction. In this embodiment, the intervals between the four plate springs are equal, but they need not be equal. Further, the four plate springs need not necessarily have the same shape. Slight deviation of the optical axis may be permitted depending on optical conditions. Thus, the dimensions of the plate springs may be set to meet such conditions. That is, the lens supporting mechanism used for the optical head of this embodiment and composed of the plate springs has only to be such that two sets of plate springs are connected together in series and that the directions of the plate springs are reversed where they are connected together. In this case, the set of parallel plate springs may be constituted by two or more parallel plate springs operating in parallel. 
     In the above embodiment, the relay lens is composed of two lenses, but each of these lenses may be formed by sticking a plurality of lenses together. That is, if one or more lenses provided separately from the objective lens are moved in the direction of their optical axes to compensate for a spherical aberration resulting from a difference between the optical thickness of the substrate of the optical disk and an optical thickness set when the objective lens is designed, the optical head of the present invention is particularly effective if the lenses can be moved in the directions of their optical axes and the range within which the lenses can be inclined is very small. 
     Then, the features of the optical head according to the first embodiment will be described below in brief. 
     (1) An optical head according to the present invention comprises a lens to correct spherical aberrations, a lens holder which holds the lens, a first parallel plate springs which supports the lens holder at one end thereof, an intermediate member attached to the other end of the first parallel plate spring, a second parallel plate spring arranged to extend from the intermediate member to the lens holder to support the intermediate member at one end thereof, and a fixing member which fixes the other end of the second parallel plate spring. Thus, the lens is moved only in the direction of the optical axis passing through the lens, thereby preventing the positional deviation of the optical axis. 
     (2) In addition to the arrangements of (1), the optical head of the present invention comprises a coil mounted on the lens holder in a direction in which the coil is wound around the optical axis of the lens, and a permanent magnet provided opposite one end of the first parallel plate spring via the lens holder the coil so as to maintain a predetermined gap between the permanent magnet and the coil. That is, this configuration enables smooth electromagnetic driving compared to the use of a motor and gears. 
     (3) In addition to the arrangements of (1), the optical head of the present invention comprises a position sensor that detects a position of the lens holder. This enables the position of the lens holder to be detected and controlled, thereby avoiding the effects of a vibration or shock. 
     (4) In the optical head of the present invention configured as described in (3), the position sensor is composed of a light emitting element, a light receiving element, and a baffle provided in the lens holder. That is, the position sensor is of a non-contact type, so that no force is exerted on a movable part such as the lens holder. This prevents a forced variation in position of the lens, thereby preventing an increase in number of optical aberrations which increase is caused by the positional deviation of the spherical aberration correcting lens. 
     (5) In the optical head of the present invention configured as described in (4), a periphery of the tip portion of the baffle (baffling section) is not perpendicular to a direction in which the baffle is moved. If the light emitting and receiving elements are small in size, it provides small-sized effective beams. Consequently, the position detection range of the position sensor cannot cover the range within which the spherical aberration correcting lens is movable. However, the detection range can be enlarged by obliquely cutting the baffle. This serves to provide a small-sized optical head. 
     Further, as shown in  FIG. 11 , the optical head is configured so that the optical axis of the relay lens is perpendicular to the objective lens of the optical head and to the direction in which the optical head is moved (that is, the radial direction of the disk). Alternatively, the optical head is configured so that perpendiculars of plate springs supporting the relay lens are substantially perpendicular to the moving direction of the optical head and to the objective lens. This prevents the relay lens from being moved by inertia force effected on the relay lens as the optical head is moved in the radial direction of the optical head. Thus, the position of the lens can be retained even if the optical head fails in a seeking operation and collides against a stopper. Further, if the direction of the optical axis of the relay lens coincides with the moving direction of the optical head, when the optical head is moved in the radial direction, holding force must be generated which is as strong as the resulting inertia force and is directionally opposite thereto. However, the optical head configured as described above requires no such holding force and consumes less power. 
     Furthermore, the optical head is configured so that the arrangement and shape of the plate springs are symmetric with respect to a plane extending through the center of gravity of the movable part perpendicularly to the optical axis and to a plane containing the optical axis and the direction in which the plate springs are supported. If the center of gravity of the movable part deviates from the center of the interval between the two plate springs, then moment force is generated as the optical head is moved, resulting in inclination. However, the plate springs have the weakest torsional rigidity around their supporting direction. Accordingly, the moment force can be avoided to reduce possible inclination while the optical head is being moved. 
     Now, a second embodiment of the invention will be described. 
       FIG. 15  is a perspective view showing the structure of a main part of an optical head according to the second embodiment.  FIGS. 16 and 17  are perspective views showing the positional relationship between an optical disk and the optical head. In  FIG. 15 , a light beam emitted by the light source  1  is collimated by the collimator lens  2 . The beam is then incident on the beam splitter  4  and subsequently passes through the quarter wavelength plate  5 . Then, the light beam propagates through the relay lens system in a relay lens driving mechanism  7   b  according to this embodiment. The beam then has its orientation changed by 90° by the mirror  6  and then impinges the optical disk  100  via the objective lens  8 . Reflected light from the disk  100  passes through the objective lens  8  and is then reflected by the mirror  6 . The light then propagates through the relay lens system and the quarter wavelength plate  5  and impinges on the beam splitter  4 . Then, the light is reflected by the beam splitter  4  and condensed by the convex lens  10 . Then, the light beam passes through the focus error signal generating element  11  and is applied to the photodetector  12 . 
     An output from the photodetector  12  is input to the arithmetic circuit  13  as described previously to obtain an information reproduction signal, a focus error signal, and a tracking error signal. The focus error signal and the tracking error signal have their phases compensated for by the phase compensation circuit  14 . On the basis of a phase compensation signal from the phase compensation circuit  14 , the actuator drivers  15  and  16  conduct currents through the coils  17  and  18  of the objective lens driving device  3 , as shown in  FIG. 18 . This controls the positions of the objective lens  8  in the direction of the optical axis and in the radial direction. 
     The relay lens system  7  is used to correct a spherical aberration that may be caused by an error in the thickness of the cover layer of the disk  100 . At least either the lens  32  or lens  33  (in this embodiment,  32 ) is moved in the direction of its optical axis to cause a spherical aberration in the relay lens system in the relay lens driving mechanism  7   b  so as to correct a spherical aberration caused by an error in the thickness of the cover layer of the disk  100 . In this case, a position detecting device  22   b  is provided to detect the position of the relay lens  32 . 
     The position detecting device  22   b  is a reflection photosensor composed of a light emitting diode  34   a  and a photodetector  35   b . Light from the light emitting element  34   b  is reflected by a reflecting plate  23   b  (preferably a white plate, which diffusively reflects light) that moves together with the relay lens  32 . The light is then incident on the photodetector  35   b . The amount (area) of that part of the reflecting plate  23   b  which is opposite to the position detecting device  22   b  varies with the position of the relay lens  32 . This correspondingly varies the quantity of light reflected or light incident on the photodetector  35   b . Consequently, the position of the relay lens  32  can be detected. Then, an output from the photodetector  35   b  is input to a lens position control circuit  19   b . The lens position control circuit  19   b  outputs a correction signal to a drive circuit  20   b  so as to place the relay lens at a specified position input by the CPU  24 . The drive circuit  20   b  conducts a current corresponding to the correction signal, through a coil  21   b  of the relay lens driving mechanism  7   b . Thus, also in this embodiment, position control is executed on the basis of feedback control. 
     As described previously, the CPU  24 , provided with the A/D and D/A, causes the relay lens control circuit  19   b  to gradually move the position of the relay lens  32  to the extent that a spherical aberration in an expected substrate thickness can be corrected. Then, a position of the lens  32  at which a reproduction information signal has the largest amplitude is detected. Subsequently, the corresponding value is continuously output to the relay lens position control circuit  19   b . In this manner, position control is executed on the basis of feedback control instead of open control as in the prior art. Accordingly, the position of the relay lens is not affected by a vibration or shock, thereby precisely achieving detection of the maximum value of the reproduction information signal and the subsequent retention of the lens position. 
     Next, the relay lens driving mechanism  7   b , mounted in the optical head of this embodiment, will be described in detail. In this case, of the two lenses constituting the relay lens, the lens  32  is movable, whereas the lens  33  is fixed. 
     As shown in  FIG. 19 , the lens  33  is mounted in a base  42   b . The movable lens  32  is mounted in the lens holder  27   b  as shown in  FIG. 20 . The lens holder  27   b  has the coil  21   b  mounted thereon and wound around an winding axis (Y direction) perpendicular to the optical axis (X direction) of the lens  32 . The winding axis for the coil is perpendicular to the optical axis, thereby allowing the size of the apparatus to be reduced. 
     As shown in  FIG. 21 , a permanent magnet  26   b  is mounted on a yoke  43   b  consisting of a ferromagnetic material such as a steel plate. The yoke  43   b  is attached to a base  42   b . Accordingly, the permanent magnet  26   b  is fixed to the base  42   b . The permanent magnet  26   b  is slightly separate from the coil  21   b . The permanent magnet  26   b  is of a bipolar type magnetized in directions that are parallel with the winding axis for the coil  21   b  but opposite to each other around a substantially central position of the magnet in the direction of the optical axis of the lens  32 , as shown by the arrow in  FIG. 21 . This bipolar magnetized structure serves to increase efficiency, while reducing power consumption. Similar effects are produced by combining two unipolar magnets. 
     The coil  21   b  has two straight portions corresponding to the respective magnetic poles of the permanent magnet  26   b  and extending in the direction of the optical axis of the lens  32  and perpendicularly to the winding axis for the coil  21   b , respectively, as shown at  21   b   1  and  21   b   2  in  FIG. 20 . The lens holder  27   b  is supported by two parallel guide rails  50   a  and  50   b  parallel with the direction of the optical axis of the lens  32 . The guide rails  50   a  and  50   b  are supported by a base  42   b  and a sub-base  44  fixed to the base  42   b , as shown in  FIG. 21 . 
     The lens holder  27   b  is provided with a rail receiving section  49  as a bearing section which has a cavity  49   a , and the rail receiving section  49  is provided with coaxial holes  47   a  and  47   b , as shown in  FIG. 20 . A guide rail  50   a  penetrates the holes  47   a  and  47   b  while maintaining a small gap therein which is sufficient to allow the lens holder  27   b  to slide. The cavity  49   a  in the rail receiving section  49  is provided so that the holes  47   a  and  47   b , which contacts with the guide rail  50   a , are as far from each other as possible in the X direction of the rail receiving section  49 . This is effective in improving the accuracy of the orientation of the lens holder. 
     As shown in  FIG. 22 , the lens holder  27   b  is provided with a U-shaped notch  48  slidably penetrated by the guide rail  50   b . The holes  47   a  and  47   b  and the U-shaped notch  48  are preferably formed of material having a small friction coefficient such as that having a PPS-equivalent lubrication grade. The surfaces of the guide rails  59   a  and  50   b  may be coated with a lubricant such as grease. 
     Furthermore, the lens holder  27   b  has a small iron piece  29  (a shaded portion in the figure) set therein close to the center of the coil and composed of a ferromagnetic material such as a silicon steel plate. The permanent magnet generates force F in the direction of the wiring axis for the coil. The small iron piece  29  and the permanent magnet  26   b  operate as pressing means for pressing the rail receiving section  49  against the guide rail. 
       FIG. 23  is a plan view showing a movable part of the relay lens driving mechanism. The winding axis for the coil  21   b  lies at predetermined angle from a plane containing the centers of the guide rails  50   a  and  50   b . A line, which extends from the small iron piece  29  in a direction perpendicular to the plane containing the centers of the guide rails  50   a  and  59   b , crosses this plane at a point between the guide rails  50   a  and  50   b.    
     These components are arranged in the Z direction in the following order: 
     Holes  47   a  and  47   b &gt;center of driving force acting on the coil  21   b =center of the iron piece  29 ≧optical axis  32   a  of the lens  32 &gt;U-shaped notch  48   
     The above order causes the lens holder to be pressed against the guide rails  50   a  and  50   b  in the direction of the winding axis for the coil. This stabilizes a position A at which the guide rail  50   a  contacts with the holes  47   a  and  47   b  and a position B at which the guide rail  50   b  contacts with the U-shaped notch  48 . As a result, the position of the lens holder  27   b  is stabilized to suppress inclination and deviation of the optical axis of the lens  32 . Therefore, optical aberrations are restrained. 
     If light is projected on an XY plane, the distance between the guide rail  50   a  and the center of driving force generated in the coil can be reduced by positioning a central position  21   b   3  of the coil  21   b  closer to the guide rail  50   a  or arranging the winding axis for the coil  21   b  in the Y direction. This suppresses moment force that may be generated in the lens holder  27   b  to act on the guide rail  50   a . This moment force presses the holes  47   a  and  47   b  against the guide rail  50   a , thereby generating frictional force in the X direction. If the frictional force exceeds the driving force generated in the coil  21   b , the lens  32  cannot be moved. Then, what is called a “pinch” phenomenon may occur. The term “pinch” as used herein refers to inclination of the center axes of the holes  47   a  and  47   b  caused by the moment force or hard-pressing of the periphery of the hole  47   a  or  47   b  against the guide rail  50   a , with the result that the lens  32  cannot be moved. In this embodiment, smaller moment force serves to prevents the holes  47   a  and  47   b  from pinching the guide rail  50   a , thereby preventing the lens from becoming inoperative. 
     Furthermore, if light is projected on an XZ plane, frictional force generated between the guide rail  50   b  and the U-shaped notch  48  constitutes moment force acting in a direction opposite to that of force generated by the coil  21   b . This reduces the possibility of the pinch phenomenon. It should be appreciated that effective measures for preventing the pinch phenomenon include maximizing the distance of the holes  47   a  and  47   b  in the X direction or reducing the friction coefficient of the guide rails  50   a  and  50   b.    
     In this embodiment, the guide rail  50   a  slides through the holes  47   a  and  47   b . However, if the lens holder  27   b  and the guide rail  50   a  are secured to each other so that the guide rail  50   a  can slide along the base  42   a  and the guide rail  50   a  can slide through the hole in the sub-base  44 , frictional force can be reduced which results from moment force generated by the coil  21   b.    
     The operation of the relay lens driving mechanism configured as described above will be descried with reference to  FIGS. 21 and 22 . When a current is conducted through the coil  21   b , it passes through a magnetic field generated by the permanent magnet  26   b . Consequently, Lorentz force is generated in the coil  21   b . In particular, the force generated in the portions  21   b   1  and  20   b   2  of the coil is exerted on the lens holder  27   b  in the direction of the optical axis of the lens  32 . Since the lens holder  27   b  is slidably supported by the guide rails  50   a  and  50   b , the lens holder  27   b  is moved in the direction of the optical axis. In this case, the guide rails  50   a  and  50   b  are assembled parallel with the optical axis of the lens  32 , and force is exerted between the small iron piece  39  and the permanent magnet  26   b . Accordingly, even when the lens holder is moved in the direction of the optical axis, the lens  32  is not inclined or the optical axis thereof does not deviate from its correct position. As described previously, the small iron piece  29  and the permanent magnet  26   b  act as pressing means for pressing the rail receiving section  49  against the guide rail. 
     With the relay lens driving mechanism configured as described above, when the lens  32  is moved, it is displaced only in the direction of the optical axis to enable spherical aberrations to be appropriately corrected. Further, the lens is supported by the two guide rails, and force is exerted between the small iron piece  29  and the permanent magnet  26   b . This provides high supporting rigidity to prevent the lens from being inclined by a shock or vibration. 
     Further, as described previously, effective driving force is generated in the two straight portions  21   b   1  and  21   b   2  of the coil. Therefore, an optical head can be provided which allows an actuator to operate efficiently, which consumes less power, and which generates less heat. 
     The supporting rigidity in the X direction is provided by a magnetic spring effect attributed to force generated between the small iron piece  29  and the permanent magnet  26   b  in the direction in which the small iron piece  29  approaches the position at which the permanent magnet is divided into the two poles. 
     Now, the position detecting device  22   b  in this embodiment will be described in detail. In this embodiment, the position sensor is a reflection photosensor as described previously. A smaller photosensor must be used in order to reduce the size of the optical head. However, a small-sized reflection photosensor provides small-sized effective beams and thus has a position detection range smaller than the range within which the lens holder  27   b  is to be positioned. Thus, in this embodiment, edges of the baffle  23   b  which otherwise extend perpendicularly to the moving direction of the lens holder are inclined as shown at  23   b   1  in  FIG. 20 . This reduces a variation in area of the reflecting plate  23   b , located opposite the position sensor  22   b , the variation associated with movement of the lens holder  27   b . As a result, the position detection range is sufficiently enlarged compared to the range within which the lens holder  27   b  is movable. Consequently, the control circuit can place the lens at any position on the basis of feedback control. Thus, the lens is not substantially affected by a vibration or shock as described previously. Further, since the position sensor is of a non-contact type, no force is exerted on the movable part such as the lens holder. This hinders a forced variation in position of the spherical aberration correcting lens and prevents an increase in number of optical aberrations which increase is caused by the positional deviation of the spherical aberration correcting lens. Furthermore, the position sensor may be of another non-contact type composed of a magnet and a Hall element as in the first embodiment. 
     Next, the direction in which the optical head and the relay lens  32  are moved will be described. In this embodiment, as shown in  FIGS. 16 and 17 , the moving direction of the relay lens  32  coincides with the circumferential direction of the disk. The optical disk reads information from various radial positions thereon by moving in the radial direction. However, as the optical disk is moved, acceleration occurs to exert inertia force on the relay lens moving section. In this embodiment, the movable part of the relay lens driving mechanism  7   b  is moved in the circumferential direction of the disk instead of the radial direction thereof. This prevents the position of the movable part from being varied by the inertia force exerted thereon. Further, it becomes unnecessary to cause the coil  21   b  to generate holding force corresponding to the inertia force. As a result, power consumption can be reduced. 
     In order to move the optical head in the radial direction, high power must be supplied to an actuator that moves the optical head. Accordingly, if the power consumption of the relay lens driving mechanism  7   b  does not increase while the optical lens is moving, the peak power consumption of an optical disk drive apparatus or the like in which the optical head is mounted can be effectively reduced. The reduced power consumption contributes to miniaturizing a power circuit that supplies power. 
       FIG. 24  schematically shows the configuration of a variation of the optical head according to the second embodiment. A relay lens driving mechanism  7   b ′, shown in  FIG. 24 , is obtained by rotating the components of the relay lens driving mechanism  7   b  described previously, through 180°. That is, the relay lens driving mechanism is configured so that the lens holder  27   b  for the movable lens is located closer to the light source  1  than the fixed lens. Furthermore, instead of the lens  32 , the lens  33  is used as the movable lens. By thus moving one of the two relay lenses which is located further from the objective lens, a variation in quantity of light associated with movement of the lens is often reduced to provide good optical characteristics. 
     In the above embodiment, the relay lens is composed of two lenses, but each of these lenses may be formed by sticking a plurality of lenses together. That is, if one or more lenses provided separately from the objective lens are moved in the direction of their optical axes to compensate for a spherical aberration resulting from a difference between the optical thickness of the substrate of the optical disk and an optical thickness set when the objective lens is designed, the optical head of the present invention is particularly effective if the lenses can be moved only in the direction of the optical axis and the range within which the lenses can be inclined is very small. 
     Further, in the above embodiment, the magnet is magnetized to have two poles. However, equivalent effects are produced by combining two unipolar magnets. Furthermore, only one unipolar magnet may be used to generate force in only one of the straight portions  21   b   1  and  21   b   2  of the coil, through the power consumption may increase. Also in this case, the magnetic spring effect described previously is produced. 
     The present invention is not limited to the above embodiments. For implementations, many variations may be made to the embodiments without deviating from the spirits of the invention. Further, embodiments may be properly combined together, and the combinations may produce further effects. Furthermore, the above embodiments include various levels of inventions, and various inventions can be extracted by properly combining the plurality of disclosed configuration requirements. For example, if the effects described herein are still obtained after omitting some of the configuration requirements shown in the embodiments, then the configuration obtained by omitting the requirements can be extracted as an embodiment of the invention.