Abstract:
An optical information storage device for directing an incident light beam onto a recording medium having a recording surface composed of lands and grooves as tracks and detecting a reproduced signal from a reflected light beam from the recording medium. The optical information storage device includes a phase plate provided in an optical path of the reflected light beam so as to be tiltable between a first position where the phase plate gives to the reflected light beam a first phase compensation amount required for detection of signals from the lands and a second position where the phase plate gives to the reflected light beam a second phase compensation amount required for detection of signals from the grooves; and a drive mechanism for tilting the phase plate. Examples of the drive mechanism include a solenoid, DC motor, and voice coil motor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an optical information storage device, and more particularly to an optical information recording and reproducing device for recording optical signals on both lands and grooves formed on a recording medium and reproducing the recorded optical signals from the recording medium. 
     2. Description of the Related Art 
     An optical disk has received attention as a memory medium that becomes a core in the recent rapid development of multimedia, and it is usually accommodated in a cartridge case to be provided as an optical disk cartridge for practical use. The optical disk cartridge is loaded into an optical disk drive to perform reading/writing of data (information) from/into the optical disk by means of an optical pickup (optical head). 
     A recent optical disk drive intended to realize size reduction is composed of a fixed optical assembly including a laser diode module, a polarization beam splitter for reflecting and transmitting a laser beam, and a photodetector for receiving reflected light from an optical disk, and a movable optical assembly including a carriage and an optical head having an objective lens and a beam raising mirror mounted on the carriage. The carriage is movable in the radial direction of the optical disk along a pair of rails by means of a voice coil motor. 
     A write-power laser beam emitted from the laser diode module of the fixed optical assembly is first collimated by a collimator lens, next transmitted by the polarization beam splitter, next reflected by the beam raising mirror of the optical head, and finally focused on the optical disk by the objective lens, thereby writing data onto the optical disk. On the other hand, data reading is performed by directing a read-power laser beam onto the optical disk. Reflected light from the optical disk is first collimated by the objective lens, next reflected by the polarization beam splitter, and finally detected by the photodetector, thereby converting the detected optical signal into an electrical signal. 
     A plurality of grooves are formed on a substrate of the optical disk in a concentric or spiral fashion to guide a laser beam to be directed onto the substrate. A flat portion defined between any adjacent ones of the grooves is called a land. In a general optical disk in the prior art, either the lands or the grooves are used as recording tracks on which information is recorded. However, a recent important technical subject to be considered is to increase a recording density by using both the lands and the grooves as the recording tracks to thereby decrease a track pitch. In this respect, various methods for realizing this subject have already been proposed. 
     In a magneto-optical disk drive as a kind of optical disk drive, a magneto-optical signal recorded on a magneto-optical disk is reproduced by directing a read-power laser beam onto the magneto-optical disk and differentially detecting a P-polarized light component and an S-polarized light component of reflected light from the magneto-optical disk by a method well known in the art. In this manner, the magneto-optical signal must be optimally reproduced by differentially detecting the P-polarized light component and the S-polarized light component of the reflected light. However, individual magneto-optical disk drives have differences in characteristics of their optical components, causing a phase difference between the P-polarized light component and the S-polarized light component of the reflected light in each magneto-optical disk drive. Further, a difference in kind between recording media also causes a similar phase difference. 
     FIG. 1 is a graph showing the relation between phase difference and carrier-to-noise ratio (CNR) in a 640-MB (megabytes) magneto-optical disk and in a 1.3-GB (gigabytes) magneto-optical disk. As apparent from FIG. 1, a phase difference giving a maximum value of the CNR is present in each of the 640-MB magneto-optical disk and the 1.3-GB magneto-optical disk. While the graph of FIG. 1 further shows that the CNR in the 1.3-GB magneto-optical disk higher in recording density is more insensitive to the phase difference, the 1.3-GB magneto-optical disk has a problem that the magneto-optical signal (MO signal) is largely undulated. 
     FIG. 2 is a graph showing the relation between phase difference and MO undulation/MO amplitude in a 640-MB magneto-optical disk and in a 1.3-GB magneto-optical disk. The MO undulation means that the envelope of an MO signal in one revolution of the disk is undulated. Such MO undulation is shown in FIG.  3 . The MO undulation causes a deterioration in jitter in cutting an MO signal at a certain slice level. As apparent from FIG. 2, the MO undulation in the 1.3-GB magneto-optical disk steeply changes with a change in phase difference. Accordingly, the phase difference must be adjusted to obtain an optimum reproduced signal quality. 
     Further, in a magneto-optical disk drive for recording information on both the lands and the grooves of a recording medium, the width of each track is smaller than the diameter of a beam spot to be formed on the recording medium, so that the track covered by the beam spot is largely influenced by crosstalk from the adjacent track. Thus, such a land/groove recording method has a problem such that an undesirable light component reflected from any adjacent groove or land is increased to cause an associated phase difference, and a resultant change in polarization state of reproduced light. As a result, information cannot be well reproduced from the magneto-optical recording medium. 
     FIG. 4 is a graph showing the relation between phase difference and CNR in performing land reproduction and groove reproduction. As apparent from FIG. 4, the CNR changes with a change in phase difference in each of land reading and groove reading, and an optimum phase difference giving a maximum CNR is present in each case. Accordingly, it is necessary to perform phase compensation of polarized light components of reproduced light in each of land reading and groove reading, thereby obtaining an optimum phase difference between the P-polarized light component and the S-polarized light component. For example, Japanese Patent Laid-open Nos. 9-282730, 9-282733, and 10-134444 disclose techniques for switching a phase difference between polarized light components of reproduced light between in land reproduction and in groove reproduction. However, each technique employs a complex optical system. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an optical information storage device which can reproduce information well by providing different phase compensation amounts for land reading and groove reading with a relatively simple and inexpensive optical system. 
     In accordance with an aspect of the present invention, there is provided an optical information storage device for directing an incident light beam onto a recording medium having a recording surface composed of lands and grooves as tracks and detecting a reproduced signal from a reflected light beam from said recording medium, comprising a phase plate provided in an optical path of said reflected light beam so as to be tiltable between a first position where said phase plate gives to said reflected light beam a first phase compensation amount required for detection of signals from said lands and a second position where said phase plate gives to said reflected light beam a second phase compensation amount required for detection of signals from said grooves; and drive means for tilting said phase plate. 
     Preferably, the optical information storage device further comprises control means for supplying a first control signal to said drive means to tilt said phase plate to said first position when said lands are selected as said tracks, and for supplying a second control signal to said drive means to tilt said phase plate to said second position when said grooves are selected as said tracks. 
     The phase plate is fixed to a housing. The housing is pivotably moved by the drive means. Examples of the drive means include a solenoid, reversible DC motor, and voice coil motor. A first stopper and a second stopper are provided to stop the phase plate at the first position and the second position, respectively. Preferably, the first stopper and the second stopper are adjustable. 
     In the case of adopting a reversible DC motor as the drive means, a third position between the first position and the second position can be detected by the combination of a magnet and a Hall element. In the case of adopting a voice coil motor as the drive means, the phase plate can be stopped at an arbitrary position between the first position and the second position, so that a desired phase difference can be easily realized. Accordingly, even in the case that the adjustment is not carried out in assembling the device, an arbitrary phase difference can be realized after assembling the device. 
     In accordance with another aspect of the present invention, there is provided an optical information storage device comprising a housing having a base; an optical recording medium rotatably accommodated in said housing and having a recording surface composed of lands and grooves as tracks; a light source mounted on said base; an optical head having an objective lens for focusing an incident light beam emitted from said light source onto said recording surface of said optical recording medium; a photodetector mounted on said base for detecting a reproduced signal from a reflected light beam from said optical recording medium; a phase plate provided in an optical path of said reflected light beam so as to be tiltable between a first position where said phase plate gives to said reflected light beam a first phase compensation amount required for detection of signals from said lands and a second position where said phase plate gives to said reflected light beam a second phase compensation amount required for detection of signals from said grooves; and drive means for tilting said phase plate. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the relation between phase difference and CNR in a 640-MB magneto-optical disk and in a 1.3-GB magneto-optical disk; 
     FIG. 2 is a graph showing the relation between phase difference and MO undulation/MO amplitude in a 640-MB magneto-optical disk and in a 1.3-GB magneto-optical disk; 
     FIG. 3 is an illustration of MO undulation; 
     FIG. 4 is a graph showing the relation between phase difference and CNR in performing land track reading and groove track reading; 
     FIG. 5 is an upper perspective view of a magneto-optical disk drive including a phase compensation unit according to the present invention; 
     FIG. 6 is a lower perspective view of the magneto-optical disk drive; 
     FIG. 7 is a plan view showing a condition where a magneto-optical disk cartridge is slightly inserted in the magneto-optical disk drive; 
     FIG. 8 is a plan view of an optical system in the magneto-optical disk drive; 
     FIG. 9 is a right side view of FIG. 8; 
     FIG. 10 is a front elevation of FIG. 8; 
     FIG. 11 is an illustration of the principle of phase compensation in the present invention; 
     FIG. 12 is a graph showing an example of the relation between phase plate tilt angle and phase difference; 
     FIG. 13 is a graph showing another example of the relation between phase plate tilt angle and phase difference; 
     FIG. 14 is an elevational view of a phase compensation unit according to a first preferred embodiment of the present invention; 
     FIG. 15 is a right side view of the phase compensation unit shown in FIG. 14; 
     FIG. 16 is a perspective view of the phase compensation unit shown in FIG. 14; 
     FIG. 17 is an exploded perspective view of the phase compensation unit shown in FIG. 14; 
     FIG. 18 is a view similar to FIG. 14, showing a modification of the first preferred embodiment; 
     FIG. 19 is an elevational view of a phase compensation unit according to a second preferred embodiment of the present invention; 
     FIG. 20 is a right side view of the phase compensation unit shown in FIG. 19; 
     FIG. 21 is a perspective view of the phase compensation unit shown in FIG. 19; 
     FIG. 22 is an exploded perspective view of the phase compensation unit shown in FIG. 19; 
     FIG. 23 is a view similar to FIG. 19, showing a modification of the second preferred embodiment; 
     FIG. 24 is an elevational view of a phase compensation unit according to a third preferred embodiment of the present invention; 
     FIG. 25 is a right side view of the phase compensation unit shown in FIG. 24; 
     FIG. 26 is a perspective view of the phase compensation unit shown in FIG. 24; 
     FIG. 27 is an exploded perspective view of the phase compensation unit shown in FIG. 24; 
     FIG. 28 is a view similar to FIG. 24, showing a modification of the third preferred embodiment; and 
     FIG. 29 is a right side view of the phase compensation unit shown in FIG.  28 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 5, there is shown a perspective view of a magneto-optical disk drive  10  including a phase compensation mechanism according to the present invention, as viewed from the upper side. FIG. 6 is a perspective view of the magneto-optical disk drive  10  as viewed from the lower side. The magneto-optical disk drive  10  accepts a magneto-optical disk cartridge  14  having a cartridge case and a magneto-optical disk rotatably accommodated in the cartridge case, and performs reading/writing of information from/into the magneto-optical disk of the magneto-optical disk cartridge  14 . 
     As will be hereinafter described in detail, the magneto-optical disk drive  10  includes a load/eject mechanism for the magneto-optical disk cartridge  14 , a spindle motor for rotating the magneto-optical disk, a bias magnetic field generating mechanism, a positioner, an optical head, and a fixed optical unit. The magneto-optical disk drive  10  further has an insert opening  12  for accepting the magneto-optical disk cartridge  14 . 
     FIG. 7 is a plan view showing a condition where the magneto-optical disk cartridge  14  is slightly inserted in the magneto-optical disk drive  10  from the insert opening  12 . Reference numeral  22  denotes a drive base of the magneto-optical disk drive  10 . A cartridge holder  24  for holding the magneto-optical disk cartridge  14  inserted into the magneto-optical disk drive  10  is mounted on the drive base  22 . The cartridge holder  24  is formed with a guide groove  26 . The guide groove  26  is composed of a first portion obliquely extending from one end of the insert opening  12  (FIG. 5) laterally inward of the cartridge holder  24  and a second portion extending from an inward end of the first portion to the rear end of the cartridge holder  24  in parallel to the longitudinal direction of the magneto-optical disk drive  10 . A first slider  28  and a second slider  30  are slidably engaged with the guide groove  26 . The second slider  30  is connected to the first slider  28  by a spring (not shown), so that when the first slider  28  is moved inward of the cartridge holder  24  along the guide groove  26 , the second slider  30  is moved through this spring to the rear end of the cartridge holder  24  along the guide groove  26 . 
     When the magneto-optical disk cartridge  14  is inserted from the insert opening  12  into the magneto-optical disk drive  10 , the first slider  28  comes into abutment against an end portion  20   a  of a shutter opening arm  20  mounted to a shutter  18  of the magneto-optical disk cartridge  14 . During further insertion of the magneto-optical disk cartridge  14  into the magneto-optical disk drive  10 , the first slider  28  is moved along the guide groove  26  inward of the cartridge holder  24  to push the shutter opening arm  20 , thereby opening the shutter  18 . 
     Further mounted on the drive base  22  are a pair of magnetic circuits  34 , a pair of guide rails  36 , a fixed optical unit  38  having a semiconductor laser and a photodetector, and a spindle motor  40 . Reference numeral  42  denotes a carriage for carrying an optical head  44  having an objective lens. The carriage  42  is provided with a pair of coils  46  at opposite positions respectively corresponding to the pair of magnetic circuits  34 . The magnetic circuits  34  and the coils  46  constitute a voice coil motor (VCM). When a current is passed through the coils  46 , the carriage  42  is guided by the pair of guide rails  36  to move in the radial direction of a magneto-optical disk  16 . Reference numeral  48  denotes a bias magnetic field generating mechanism mounted on the cartridge holder  24  so as to cover a range of movement of the optical head  44 . 
     Referring to FIG. 8, there is shown a plan view of an optical system in the magneto-optical disk drive  10 . FIG. 9 is a right side view of FIG. 8, and FIG. 10 is a front elevation of FIG. 8. A laser beam emitted from a semiconductor laser  50  is converted into a parallel light beam by a collimator lens  52 , and the parallel light beam enters a polarization beam splitter  54 . The polarization beam splitter  54  has a transmitting characteristic and a reflecting characteristic as set in the following manner. For example, letting Tp and Ts denote the transmittances of a P-polarized light component and an S-polarized light component, respectively, and Rp and Rs denote the reflectances to a P-polarized light component and an S-polarized light component, respectively, the transmitting characteristic and the reflecting characteristic are set to satisfy the relations of Tp:Rp=80:20 and Ts:Rs=2:98. 
     A laser beam transmitted by the polarization beam splitter  54  according to the transmitting characteristic thereof is reflected by a beam raising mirror  56  in the optical head  44 , and then focused onto the magneto-optical disk  16  by an objective lens  58  in the optical head  44 . In writing information onto the magneto-optical disk  16 , a bias magnetic field having a fixed direction is applied to a laser directing position on the magneto-optical disk  16  by the bias magnetic field generating mechanism  48  (see FIG. 7) located opposite to the objective lens  58  with respect to the magneto-optical disk  16 . Accordingly, a recording mark is formed on the magneto-optical disk  16  by the laser beam focused thereon. 
     In reproducing the information recorded on the magneto-optical disk  16 , a laser beam having a power lower than that of the laser beam for recording is directed onto the magneto-optical disk  16 , and the orientation of a polarization plane of reflected light from the recording mark on the magneto-optical disk  16  is detected. The reflected light from the magneto-optical disk  16  is collimated by the objective lens  58 , next reflected by the beam raising mirror  56 , and next reflected by the polarization beam splitter  54  according to the reflecting characteristic thereof. The light beam from the polarization beam splitter  54  is passed through a phase compensation unit mounting space  60  for mounting a phase compensation unit (to be hereinafter described) which is characteristic of the present invention. Thereafter, the light beam is divided into two laser beams by a first beam splitter  62 . 
     The laser beam reflected by the first beam splitter  62  is passed through a Wollaston prism  64 , and next focused on a two-section photodetector  68  for detecting a magneto-optical signal (MO signal) by a condenser lens  66 . On the other hand, the laser beam transmitted by the first beam splitter  62  is passed through a condenser lens  70 , and next divided into two laser beams by a second beam splitter  72 . One of these two laser beams is introduced into a four-section photodetector  76  for detecting a focusing error, and the other laser beam is introduced into a two-section photodetector  78  for detecting a tracking error. In this preferred embodiment, the focusing error is measured by a knife-edge method, and the tracking error is measured by a push-pull method. Reference numeral  74  denotes a knife edge. 
     In this preferred embodiment, a phase compensation unit to be hereinafter described is inserted in the phase compensation unit mounting space  60  to perform phase compensation for the laser beam reflected on the magneto-optical disk  16 . In the case of a commercially available 3.5-inch magneto-optical disk drive, a cubic space having a side of about 13 mm may be used as the phase compensation unit mounting space  60  in consideration of a circuit space  80  for accommodating a printed circuit board. 
     FIG. 11 illustrates the principle of phase compensation in the present invention. When a laser beam is directed to a phase plate  82  such as a half-wave plate, a phase difference Δ is generated between a Z-axis component (S-polarized light component) of the laser beam and an X-axis component (P-polarized light component) of the laser beam. The phase difference Δ is a function of the thickness d of the phase plate  82  and the refractive indices n z  and n x  of the phase plate  82  in the Z-axis and X-axis directions, and it is expressed as follows: 
     
       
         Δ= 2π(   n   z   −n   x ) d/λ   
       
     
     By tilting the phase plate  82 , the thickness d along the optical path of the laser beam in the phase plate  82  is changed, resulting in a change in the phase difference Δ. This principle is applied to all the preferred embodiments to be hereinafter described. 
     FIG. 12 is a graph showing an example of the relation between the tilt angle θ of a phase plate and the phase difference Δ as obtained by calculation. In the case that quartz is adopted as the phase plate, a phase difference ranging from 0° to +180° can be obtained by changing the tilt angle of the phase plate from 0° to +25°. In each preferred embodiment to be hereinafter described, the tilt angle of the phase plate is assumed to be changed between 0° and +32°. In the case that lithium niobate (LiNbO 3 ) is adopted as the phase plate, a phase difference ranging from 0° to −180° can be obtained by changing the tilt angle of the phase plate from 0° to +10°. FIG. 13 is a graph showing another example of the relation between the tilt angle of a phase plate and the phase difference. In this example, the phase plate is formed of quartz. By using this phase plate, a phase difference ranging from −20° to +180° can be obtained by changing the tilt angle of the phase plate from 0° to +30°. 
     Referring to FIG. 14, there is shown an elevational view of a phase compensation unit  84 A according to a first preferred embodiment of the present invention. FIG.  15  is a right side view of the phase compensation unit  84 A; FIG. 16 is a perspective view of the phase compensation unit  84 A; and FIG. 17 is an exploded perspective view of the phase compensation unit  84 A. The phase compensation unit  84 A is mounted in the phase compensation unit mounting space  60  shown in FIG.  8 . Referring mainly to FIG. 17, a phase plate  86  formed of a uniaxial crystal of quartz is fitted with a hole  88   a  of a housing  88  formed of resin, and is fixed to the housing  88  by an adhesive. A shaft  94  is press-fitted with a through hole  88   b  of the housing  88 , and the housing  88  is pivotably mounted through the shaft  94  to a frame  90  formed of iron. Two C-rings  96  are engaged with the shaft  94  at its opposite ends to prevent axial movement of the shaft  94  relative to the frame  90 . 
     Reference numeral  98  generally denotes an actuator for tilting the phase plate  86 . The actuator  98  is composed of a solenoid  98   a  including a magnetic circuit, a T-shaped plunger  98   b  operatively connected to the solenoid  98   a,  and a coil spring  98   c  mounted on the T-shaped plunger  98   b  for normally biasing the plunger  98   b  in its projecting direction. The plunger  98   b  is slidably engaged at its front end with a U-shaped recess  88   c  cut in the housing  88 . The frame  90  has a hole  92  for allowing pass of the laser beam reflected on the disk  16 , and a pair of tapped holes  90   a  and  90   b.  A pair of adjustable screws  100  and  102  are threadedly engaged with the tapped holes  90   a  and  90   b  of the frame  90 , respectively. The screws  100  and  102  function as stoppers for the housing  88  in the inoperative and operative conditions of the solenoid  98   a,  respectively. 
     In the inoperative condition of the solenoid  98   a,  the plunger  98   b  is expanded (projected from the solenoid  98   a ) by the biasing force of the coil spring  98   c,  and an upper end portion of the housing  88  is in abutment against the screw  100 . In this condition, the phase plate  86  is substantially perpendicular to an optical path of the reflected light beam. When a DC voltage (+5V) is applied to the solenoid  98   a,  the plunger  98   b  is contracted (pulled into the solenoid  98   a ) against the biasing force of the coil spring  98   c,  so that the housing  88  is rotated clockwise as viewed in FIG. 14 about the axis of the shaft  94  until a lower end portion of the housing  88  abuts against the screw  102 . 
     That is, since the plunger  98   b  is slidably engaged with the recess  88   c  of the housing  88 , the phase plate  86  fixed to the housing  88  is tilted in the opposite directions shown by a double-headed arrow  104  in FIG. 14 by switching on and off the applied voltage to the solenoid  98   a,  thereby obtaining a phase difference between a P-polarized light component and an S-polarized light component of the reflected light beam according to the tilt angle of the phase plate  86 . As mentioned above, the screws  100  and  102  are adjustably mounted on the frame  90 , and function as stoppers for the housing  88  in the inoperative and operative conditions of the solenoid  98   a,  respectively. By adjusting the feeds of the screws  100  and  102 , the tilt angle of the phase plate  86  can be arbitrarily changed within the stroke of the plunger  98   b.    
     By adjusting the tilt angle of the phase plate  86  with the screws  100  and  102  so as to provide optimum phase differences in reproducing information recorded on a land track and a groove track, it is possible to give optimum phase compensation amounts for land track reading and groove track reading to the reflected light beam by energizing and de-energizing the solenoid  98   a.  For example, the position where the housing  88  abuts against the screw  100  is used for land track reading, and the position where the housing  88  abuts against the screw  102  is used for groove track reading. 
     A control circuit  99  such as an MPU is connected to the solenoid  98   a  as shown in FIG.  14 . The control circuit  99  determines whether the track undergoing reproduction is a land or a groove according to the address on the magneto-optical disk  16  or the polarity of a tracking error signal, for example, and supplies to the solenoid  98   a  a control signal indicative of a land or a groove. Accordingly, the phase plate  86  can be tilted according to whether the track undergoing reproduction is a land or a groove, and an optimum phase compensation amount for each track reproduction can be given to the reflected light beam. 
     The screws  100  and  102  as stoppers may also be used for fine adjustment in assembling each of individual magneto-optical disk drives so as to provide optimum phase compensation amounts for land track reproduction and groove track reproduction in each magneto-optical disk drive. Further, in the case that the optimum phase compensation amounts for land track reproduction and groove track reproduction are equal to each other, the operative condition of the solenoid  98   a  where the phase plate  86  is tilted may be used as phase points for land track reading and groove track reading, and the inoperative condition of the solenoid  98   a  where the phase plate  86  is not tilted may be used as a point of zero phase difference. In this case, the present invention can be applied also to a conventional recording disk not requiring phase compensation, thus realizing higher downward compatibility. 
     Referring to FIG. 18, there is shown a modification  84 A′ of the phase compensation unit according to the first preferred embodiment of the present invention. In this modification, the phase plate  86  is mounted in a housing  88 ′ so as to be preliminarily tilted with respect to the housing  88 ′. In the inoperative condition of the solenoid  98   a,  the housing  88 ′ is tilted from its vertical position counterclockwise as viewed in FIG. 18 by the biasing force of the coil spring  98   c,  and an upper end portion of the housing  88 ′ abuts against a screw  100 ′ as a stopper. In this condition, the phase plate  86  is substantially perpendicular to the optical path of the reflected light beam. 
     When the solenoid  98   a  is excited, the housing  88 ′ is rotated clockwise as viewed in FIG. 18, and a lower end portion of the housing  88 ′ abuts against a screw  102 ′ as another stopper. By switching on and off the applied voltage to the solenoid  98   a,  the phase plate  86  is tilted in the opposite directions shown by a double-headed arrow  106 , thereby obtaining a phase difference corresponding to the tilt angle of the phase plate  86 . According to this modification, the phase plate  86  is preliminarily tilted with respect to the housing  88 ′, so that the clockwise and counterclockwise rotating angles of the housing  88 ′ from its vertical position can be made equal to each other. 
     Referring to FIG. 19, there is shown an elevational view of a phase compensation unit  84 B according to a second preferred embodiment of the present invention. FIG. 20 is a right side view of the phase compensation unit  84 B; FIG. 21 is a perspective view of the phase compensation unit  84 B; and FIG. 22 is an exploded perspective view of the phase compensation unit  84 B. The phase compensation unit  84 B may be mounted in the phase compensation unit mounting space  60  shown in FIG.  8 . Referring mainly to FIG. 22, the phase plate  86  is fitted with a hole  108   a  of a housing  108  formed of resin, and is fixed to the housing  108  by an adhesive. The housing  108  has a through hole  108   b.  A shaft  112  of a reversible DC motor  110  is press-fitted with the through hole  108   b  of the housing  108 , thereby fixedly mounting the housing  108  on the shaft  112  of the motor  110 . By applying voltages of different polarities to the DC motor  110 , the housing  108  can be rotated about the axis of the shaft  112  in the opposite directions shown by a double-headed arrow  122  in FIG. 19, thereby obtaining a phase difference corresponding to the tilt angle of the phase plate  86 . 
     A pair of stoppers  114  and  116  are fitted with the shaft  112 , and are fixed to an end surface of the DC motor  110  by an adhesive or the like in such a manner that a given angle is defined between the stoppers  114  and  116 . By changing the polarity of the voltage applied to the DC motor  110 , the housing  108  can be made to abut against the stoppers  114  and  116 , thereby giving to the reflected light beam optimum phase compensation amounts for land track reproduction and groove track reproduction. Like the first preferred embodiment, the stoppers  114  and  116  may be adjusted in position to be fixed to the DC motor  110 , thereby realizing optimum phase compensation amounts desired by individual magneto-optical disk drives. 
     In FIG. 19, the reflected light beam propagates in the direction shown by an arrow  124 , and the phase compensation unit  84 B may be mounted in the phase compensation unit mounting space  60  shown in FIG. 8 so that the phase plate  86  is substantially perpendicular to the optical path along the direction  124  in the condition where the housing  108  abuts against the stopper  116 . Like the first preferred embodiment, a control circuit  99  such as an MPU is connected to the DC motor  110  as shown in FIG.  20 . The control circuit  99  determines whether the track undergoing reproduction is a land or a groove according to the address on the magneto-optical disk  16  or the polarity of a tracking error signal, for example, and supplies to the DC motor  110  a control signal indicative of a land or a groove. Accordingly, the phase plate  86  can be tilted according to whether the track undergoing reproduction is a land or a groove, and an optimum phase compensation amount for each track reproduction can be given to the reflected light beam. 
     Furthermore, a magnet  118  is mounted in the housing  108 , and a Hall element  120  is mounted on the DC motor  110  so as to face the magnet  118 . By detecting the rotative position of the magnet  118  with the Hall element  120  during rotation of the housing  108 , another phase point in addition to the two phase points for land track reading and groove track reading can be selected. By using one of these three phase points corresponding to the magnet  118 , the stopper  114 , and the stopper  116  as a point of zero phase difference, higher downward compatibility can be realized like the first preferred embodiment. 
     Referring to FIG. 23, there is shown a modification  84 B′ of the phase compensation unit according to the second preferred embodiment. In this modification, the phase plate  86  is mounted in a housing  108 ′ so as to be preliminarily tilted with respect to the housing  108 ′, and the clockwise and counterclockwise rotating angles of the housing  108 ′ from its vertical position are made equal to each other. When the housing  108 ′ abuts against the stopper  116 , the phase plate  86  is substantially perpendicular to the optical path of the reflected light beam as shown by an arrow  124  in FIG.  23 . By driving the DC motor  110  in the normal and reverse directions, the phase plate  86  can be rotated in the opposite directions shown by a double-headed arrow  126  in FIG. 23, thereby changing the tilt angle of the phase plate  86 . 
     Referring to FIG. 24, there is shown an elevational view of a phase compensation unit  84 C according to a third preferred embodiment of the present invention. FIG. 25 is a right side view of the phase compensation unit  84 C; FIG. 26 is a perspective view of the phase compensation unit  84 C; and FIG. 27 is an exploded perspective view of the phase compensation unit  84 C. The phase compensation unit  84 C may be mounted in the phase compensation unit mounting space  60  shown in FIG.  8 . Referring mainly to FIG. 27, the phase plate  86  is fitted with a hole  128   a  of a housing  128  formed of resin, and is fixed to the housing  128  by an adhesive. As shown in FIG. 24, the phase plate  86  is preliminarily tilted with respect to the housing  128 . 
     The housing  128  has a through hole  128   b,  and a coil  130  is embedded in a lower portion of the housing  128  below the through hole  128   b.  A shaft  132  is press-fitted with the through hole  128   b  of the housing  128 , and the housing  128  is pivotably mounted through the shaft  132  to a pair of magnetic frames  134  and  136  each formed of iron or the like. Two C-rings  138  are engaged with the shaft  132  at its opposite ends to prevent axial movement of the shaft  132  relative to the magnetic frames  134  and  136 . A permanent magnet  140  is bonded to the frame  134  so as to face the coil  130 . The frames  134  and  136  functioning as a yoke and the permanent magnet  140  form a magnetic circuit. This magnetic circuit and the coil  130  constitute a voice coil motor (VCM). By passing a current through the coil  130 , the housing  128  can be rotated about the axis of the shaft  132  in the opposite directions shown by a double-headed arrow  152  in FIG. 24 to thereby tilt the phase plate  86 . 
     A pair of stoppers  142  and  144  for making abutment against the housing  128  are rotatably supported to the frames  134  and  136  at their upper portions so as to extend therebetween. Two C-rings  146  are engaged with each of the stoppers  142  and  144  at its opposite ends to prevent axial movement of the stoppers  142  and  144  relative to the frames  134  and  136 . The stoppers  142  and  144  have the same structure such that each of the stoppers  142  and  144  is composed of a resin pole  150  and a shaft  148  press-fitted in the resin pole  150  in eccentric relationship with each other. A slot  148   a  is formed at one end of the shaft  148 . By fitting a flat-blade screwdriver into the slot  148   a  of the shaft  148  and rotating the stoppers  142  and  144 , the tilt angle of the phase plate  86  can be suitably adjusted to thereby obtain optimum phase compensation amounts for land track reading and groove track reading like the first preferred embodiment. When the housing  128  abuts against the stopper  144 , the phase plate  86  is substantially perpendicular to the optical path of the reflected light beam shown by an arrow  154  in FIG.  24 . 
     As shown in FIG. 25, a control circuit  99  such as an MPU is connected to the coil  130 . The control circuit  99  determines whether the track undergoing reproduction is a land or a groove according to the address on the magneto-optical disk  16  or the polarity of a tracking error signal, for example, and supplies to the coil  130  a control signal indicative of a land or a groove. Accordingly, the phase plate  86  can be tilted according to whether the track undergoing reproduction is a land or a groove, and an optimum phase compensation amount for each track reproduction can be given to the reflected light beam. 
     Like the first preferred embodiment, two phase points for land track reproduction and groove track reproduction can be set by the abutment of the housing  128  against the stoppers  142  and  144 . In addition, the phase plate  86  can be stopped at a desired tilt angle by controlling the current passed through the coil  130 , thus realizing a desired phase difference. Accordingly, an arbitrary phase difference can be realized after installing the phase compensation unit  84 C into the magneto-optical disk drive without the need for adjustment at the time of assembling the phase compensation unit  84 C. 
     FIG. 28 is an elevational view of a modification  84 C′ of the phase compensation unit according to the third preferred embodiment, and FIG. 29 is a right side view of FIG.  28 . In this modification, the phase plate  86  is mounted in a housing  128 ′ formed of resin so as not to be tilted with respect to the housing  128 ′. A pair of stoppers  156  and  158  for making abutment against the housing  128 ′ are rotatably supported to the frames  134  and  136  at their lower portions so as to extend therebetween. The phase compensation unit  84 C′ may be mounted in the phase compensation unit mounting space  60  shown in FIG. 8 so that when the housing  128 ′ abuts against the stopper  156 , the phase plate  86  is substantially perpendicular to the optical path of the reflected light beam shown by an arrow  160  in FIG.  28 . By passing a current through the coil  130 , the phase plate  86  can be tilted in the opposite directions shown by a double-headed arrow  162  in FIG.  28 . 
     In each of the first to third preferred embodiments mentioned above, there is provided a phase compensation unit having a phase plate, a switching mechanism, and an adjusting mechanism. Accordingly, the present invention is applicable not only to a 3.5-inch magneto-optical disk drive, but also to any other land/groove recording/reproducing device such as a 5-inch magneto-optical disk drive and a digital video disk drive (DVD). 
     According to the present invention as described above, optimum phase compensation amounts for land track reading and groove track reading can be provided by a relatively simple and inexpensive optical system, thereby improving the quality of a reproduced signal. 
     The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.