Abstract:
An optical disk drive having an optical disk that permits transmission of a laser beam, an optical disk holding mechanism configured to rotatably support the optical disk, a first laser beam irradiation mechanism disposed on one side of the optical disk, causing the first laser beam to be vertically incident on one surface of the optical disk, and a first evanescent optical system disposed on the other side of the optical disk. The first evanescent optical system is configured to receive the first laser beam which emanates from the laser beam irradiation mechanism and passes through the optical disk, and radiate the first evanescent wave to the other surface of the optical disk having at least one recording surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an optical disk drive. More particularly, this invention relates to an optical disk drive for recording and reproducing information by the use of evanescent light. 
     2. Description of the Related Art 
     Innovative development and improvement have been made from various aspects in technologies for achieving a high recording density in the information recording field. 
     One of the high density recording technologies that has been proposed is a method that uses a lens capable of generating evanescent waves. This lens is referred to as a “solid immersion lens”. In a solid immersion lens, rays of light undergo total internal reflection and generate an evanescent wave. The evanescent wave generated by the solid immersion lens forms a very fine optical spot. Therefore, the solid immersion lens makes it possible to improve the information recording density on a recording medium such as an optical disk. 
     As illustrated in FIG. 11 of the accompanying drawings, an optical disc drive using the solid immersion lens comprises a fixed optical system  53  including a laser beam source  68 , rise-up mirrors  56  for causing parallel rays of light to be incident vertically on a disk  51 , and an evanescent optical system  60  for causing the parallel rays of light incident from above to generate evanescent waves and irradiating a very fine optical spot at a lower portion. Recording and reproduction of information to and from the disk  51  is executed by means of the very fine optical spot irradiated from the evanescent optical system  60 . 
     Here, the term “rise-up mirror  56 ” represents all those optical components which are equipped with a plane for reflecting the rays of light, and is inclusive of a beam splitter. The evanescent optical system  60  is an optical element containing the solid immersion lens  65  described above. 
     The need to reduce the thickness of various devices related to personal computers has become greater in recent years as personal computers have been rendered “mobile”. 
     In the construction depicted in FIG. 11, however, both evanescent optical system  60  and rise-up mirror  56  are disposed on the disk plane side for recording or reproducing the information. In other words, a space corresponding to the height of at least these two optical elements is necessary on one of the surface sides of the disk. 
     When an optical disk drive can record or reproduce information on at least two disk surfaces on the basis of the construction shown in FIG. 11, the space corresponding to the height of the two optical elements must be secured on each disk surface with the result that the total thickness of the optical disk drive increases. 
     In view of the problem described above, the present invention is directed to provide an optical disk drive that can restrict the increase of the thickness of the apparatus particularly when the information is recorded or reproduced on at least two disk surfaces. 
     SUMMARY OF THE INVENTION 
     To accomplish the object described above, the present invention provides an optical disk drive as described below. 
     In a first aspect of the invention, an optical disk drive comprises an optical disk which permits transmission of a laser beam, an optical disk holding mechanism configured to rotatably support the optical disk, a first laser beam irradiation mechanism disposed on one side of the optical disk, configured to cause the first laser beam to be incident vertically on one surface of the optical disk, and a first evanescent optical system disposed on the other side of the optical disk, configured to receive a first laser beam which emanates from the laser beam irradiation mechanism and passes through the optical disk, and configured to radiate the first evanescent wave to the other surface of the optical disk having at least one recording surface. 
     Embodiments of this aspect of the present invention may have any of the following features: 
     (1) A track pitch of a signal train recorded to the optical disk may be smaller than the wavelength of the first laser beam. 
     (2) The first laser beam irradiation mechanism may have a first mirror disposed at a position opposing the first evanescent optical system. 
     (3) The first mirror may be a 45° mirror. 
     (4) The first mirror may be a prism. 
     (5) The first evanescent optical system may include a first solid immersion lens. 
     (6) The first solid immersion lens may include a bottom surface to which the first laser beam is incident and which radiates the first evanescent wave, and a reflecting surface which reflects the first laser beam incident from the lens bottom surface and converges the first laser beam to the lens bottom surface. 
     (7) The distance between the lens bottom surface of the first solid immersion leans and the surface of the optical disk opposing the lens bottom surface may be within the attenuation distance of the evanescent wave. 
     (8) The optical disk drive may further comprise a second laser beam irradiation mechanism disposed adjacent to the first evanescent optical system, configured to cause the second laser beam to be vertically incident to the other surface of the optical disk, and a second evanescent optical system disposed adjacent to the first laser beam irradiation mechanism, configured to receive the second laser beam incident from the second laser beam irradiation mechanism, passing through the optical disk, and configured to radiate the second evanescent wave to one surface of the optical disk. 
     (9) The optical disk holding mechanism may hold two or more optical disks. 
     In a second aspect of the invention, an optical disk drive comprises two optical disks which transmit laser beams, disposed at upper and lower positions, a first laser beam irradiation mechanism disposed on the upper surface side of the optical disk at the upper position, and used for recording or reproducing information on the lower surface of the optical disk at the lower position, a first evanescent optical system configured to receive the first laser beam irradiated from the first laser beam irradiation mechanism and pass through the two optical disks, and configured to radiate the first evanescent beam to the lower surface of the optical disk at the lower position, a second laser beam irradiation mechanism disposed on the lower surface side of the optical disk at the lower position, and used for recording or reproducing information on the upper surface of the optical disk held at the lower position, a second evanescent optical system configured to receive the second laser beam irradiated from the second laser beam irradiation mechanism, passing through the optical disk at the lower position, and configured to radiate the second evanescent beam to the upper surface of the optical disk held at the lower position, a third laser beam irradiation mechanism disposed on the upper surface side of the optical disk at the upper position, and used for recording or reproducing information on the lower surface of the optical disk at the upper position, a third evanescent optical system configured to receive the third laser beam irradiated from the third laser beam irradiation mechanism, passing through the optical disk at the upper position, and configured to radiate the third evanescent beam to the lower surface of the optical disk at the upper position, a fourth laser beam irradiation mechanism disposed on the lower surface side of the optical disk at the lower position, and used for recording or reproducing information on the upper surface of the optical disk at the upper position, and a fourth evanescent optical system configured to receive the fourth laser beam irradiated from the fourth laser beam irradiation mechanism, passing through the two optical disks, and configured to radiate the fourth evanescent beams to the upper surface of the optical disk at the upper position. 
     Embodiments of this aspect of the present invention may have any of the following features: 
     (1) The first to fourth evanescent optical systems may include first to fourth solid immersion lenses. 
     (2) The first to fourth solid immersion lenses may include a lens bottom surface to which the first to fourth laser beams incident and which radiates the first to fourth evanescent waves, and reflecting surfaces for reflecting the first to fourth laser beams incident from the lens bottom surfaces and converging them on the lens bottom surfaces. 
     In the present invention, the two optical elements, i.e., the parallel beam irradiation mechanism and the evanescent optical system, can be separated from each other so as to interpose the optical disk not having an optical reflecting layer between them. Therefore, the present invention can improve freedom of disposition of these optical elements and will be able to restrict the thickness of the optical disk drive. Particularly when both surfaces of the optical disk are used for the recording surface or when a plurality of optical disks are used simultaneously the present invention provides a remarkable effect for restricting the thickness of the optical disk drive. 
    
    
     The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from the following description and from the following drawings in which like numerals refer to like parts. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side view showing an optical disk drive according to the first embodiment of the present invention; 
     FIG. 2 is a schematic plan view showing the optical disk drive according to the first embodiment of the present invention; 
     FIG. 3 is a schematic front view showing the optical disk drive according to the first embodiment of the present invention; 
     FIG. 4 is a schematic view showing an optical system of the optical disk drive according to the first embodiment of the present invention; 
     FIG. 5 is a schematic structural view showing an example of an evanescent optical system in the optical disk drive according to the first embodiment of the present invention; 
     FIG. 6 is a schematic structural view showing another example of the evanescent optical system in the optical disk drive according to the first embodiment of the present invention; 
     FIG. 7 is a schematic structural view showing still another example of the evanescent optical system in the optical disk drive according to the first embodiment of the present invention; 
     FIG. 8 is a schematic side view showing an optical disk drive according to the second embodiment of the present invention; 
     FIG. 9 is a schematic front view showing an optical disk drive according to the third embodiment of the present invention; 
     FIG. 10 is a schematic front view showing an optical disk drive according to the fourth embodiment of the present invention; and 
     FIG. 11 is a schematic view showing an optical system of an optical disk drive according to the prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention are explained hereinafter with reference to the accompanying drawings. 
     FIG. 1 is a schematic side view showing an optical disk drive according to the first embodiment of the present invention. FIG. 2 is a schematic plan view of the optical disk drive shown in FIG.  1 . FIG. 3 is a schematic front view of the optical disk drive shown in FIG.  1 . 
     The optical disk drive  1  according to the first embodiment of the present invention includes, as shown in FIG.  1  through FIG. 3, an optical disk  20  that does not have an optical reflecting layer (or in other words, transmits a laser beam) and an optical disk holding mechanism  2  for rotatably holding the optical disk  20 . The optical disk holding mechanism  2  includes a support portion  2   a  for supporting the optical disk  20  and a spindle motor  2   m  for rotating the support portion  2   a . The spindle motor  2   m  is fixed to a frame portion if of the main body  30 . 
     A first parallel beam irradiation mechanism (laser beam irradiation mechanism)  6   a  for allowing parallel laser beams (laser beams) to be incident vertically on the surface of the optical disk  20  is disposed on one side (for example, the lower surface) of the optical disk  20  as depicted in FIG.  1 . 
     It will be assumed hereby that the track pitch of the signal train recorded to the optical disk  20  is smaller than the wavelength of the parallel laser beam the first parallel beam irradiation mechanism  6   a  irradiates. It may be assumed also that both surfaces of the optical disk  20  may be utilized as the recording surface in this embodiment. 
     A first evanescent optical system  10   a  is provided on the other side of the optical disk  20  (for example here, the upper surface). This first evanescent optical system  10   a  receives the parallel laser beams that are incident from the first parallel beam irradiation mechanism  6   a  and pass through the optical disk  20 , and emits the evanescent waves to the upper surface of the optical disk  20 . 
     The first parallel beam irradiation mechanism  6   a  of this embodiment includes a first beam source portion  26   a  and a first rise-up mirror  16   a  disposed at a position opposing the first evanescent optical system  10   a , for changing the direction of the parallel laser beams from the first beam source portion  26   a . The first rise-up mirror  16   a  may comprise a so-called “45° mirror”. 
     As shown in FIG. 4, the first beam source portion  26   a  may comprise a laser beam source  41 , a collimater lens  42 , a prism  43  and a beam splitter  44 . Optical signal detectors  48   a  to  48   d  are disposed in an optical path branching from the beam splitter  44  through another beam splitter  45 , a  ½ wavelength plate 46 and a polarizing beam splitter 47.    
     This embodiment assumes that both surfaces of the optical disk  20  can be utilized as the recording surface, as previously described. Therefore, a second parallel beam irradiation mechanism  6   b  for allowing the laser beam to be incident vertically may be disposed on the other side (for example here, the upper surface side) of the optical disk  20 , adjacent to the first evanescent optical system  10   a , as shown in FIGS. 2 and 3. 
     A second evanescent optical system lob may be disposed on one of the surface sides (for example here, the lower surface side) of the optical disk  20  adjacent to the first parallel beam irradiation mechanism  6   a . This second evanescent optical system lob receives the parallel laser beams that are incident from the second parallel beam irradiation mechanism  6   b  and pass through the optical disk  20 , and emits the evanescent waves to the lower surface of the optical disk  20 . 
     The second parallel beam irradiation mechanism  6   b  of this embodiment includes a second beam source portion  26   b  and a second rise-up mirror  16   b  disposed at a position opposing the second evanescent optical system lob for changing the direction of the parallel laser beam from the second beam source portion  26   b , in the same way as the first parallel beam irradiation mechanism  6   a . The second beam source portion  26   b  has the same construction as that of the first optical source portion  26   a  shown in FIG.  4 . The second rise-up mirror  16   b , too, may comprise the so-called “45° mirror”. 
     As shown in FIGS. 1 and 2, the first and second beam source portions  26   a  and  26   b  are provided in swing arms  4   a  and  4   b , respectively, which extend in a disk radial direction. A beam outgoing surface is disposed at the distal end portion of each swing arms  4   a  and  4   b , and a proximal end portion of each of them is supported by the support main body portion  30 . 
     The support main body portion  30  is rotatably put on the device main body frame portion if through a bearing  35 . The support main body portion  30  has a driving coil  37  that drives rotary movement round the bearing  35 . In this case, the beam source portions  26   a  and  26   b  may be provided in a fixed fashion in the device main body frame portion if instead of the swing arms  4   a  and  4   b.    
     On the other hand, distal end portions of the swing arms  4   a  and  4   b  that extend in the arm shape in parallel with each other in different levels support the first and second rise-up mirrors  16   a  and  16   b , respectively. The support main body portion  30  supports the proximal end portion of each swing arm  4   a ,  4   b.    
     In FIG.  3  and FIG. 4 suspension arms  8   a  and  8   b  protrude in a direction crossing the swing arms  4   a  and  4   b  and are connected to the distal end portions of the swing arms  4   a  and  4   b , respectively. These suspension arms  8   a  and  8   b  support the evanescent optical system  10   a  and the second evanescent optical system  10   b , respectively. 
     Each evanescent optical system  10   a ,  10   b  has its surface for receiving the parallel laser beams and the surface for irradiating the evanescent waves that are positioned on the same side. Structural examples are shown in FIGS. 5 to  7 . 
     Each evanescent optical system  10   a ,  10   b  shown in FIGS.  5  to  7  has solid immersion lens  15   a ,  15   b  and  15   c . Each of these immersion lenses  15   a  to  15   c  has a flat bottom surface  16   a , a convex bottom surface  16   b , a concave bottom surface  16   c  and an upper surface side reflecting surface (to be described later)  17   a  to  17   c  that is shaped into an appropriate shape. 
     The parallel laser beams incident from the bottom surface ( 16   a  to  16   c ) side pass through the lens  15   a  to  15   c , and are reflected by the upper surface side reflecting surface  17   a  to  17   c , are converged to the lens bottom surface  16   a  to  16   c  and generate the evanescent waves on the lens bottom surface  16   a  to  16   c . The evanescent waves are irradiated from the lens bottom surfaces  16   a  to  16   c  on the disk  20 . 
     In other words, each reflecting surface  17   a  to  17   c  on the upper surface side is shaped into a shape so that the parallel laser beams emanated from the lens bottom surface  16   a  to  16   c  and reflected by the reflecting surface  17   a  to  17   c  can form the image on the lens bottom surface  16   a  to  16   c.    
     Each lens bottom surface  16   a  to  16   c  is positioned in such a manner that the distance relative to the optical disk  20  falls within the attenuation distance of the evanescent beam. 
     When a magneto-optic disk is used for the optical disk, a magnetic coil  18   a  to  18   c  may be disposed in the proximity of the solid immersion lens  15   a  to  15   c  to generate a required magnetic field as shown in FIGS. 5 to  7 . 
     Next, the mode of operation of this embodiment having the construction described above will be explained. 
     The optical disk  20  not having the optical reflecting layer maybe fitted to the support portion  2   a  of the optical disk holding mechanism  2  as shown in FIGS. 1 to  3 . 
     When the optical disk  20  is fitted and removed (when possible), the evanescent optical system  10   a ,  10   b  and the parallel beam irradiation mechanism  6   a ,  6   b  move back from the periphery of the optical disk holding mechanism  2  according to the rotation of the support main body portion  30 . As a result, the optical disk  20  can be fitted and removed easily. 
     The optical disk  20  fitted to the support portion  2   a  may be rotated by the operation of the spindle motor  2   m . On the other hand, the rotation of the support main body portion  30  positions the first evanescent optical system  10   a  with the second rise-up mirror  16   b  and the second evanescent optical system  10   b  with the first rise-up mirror  16   a  to the desired positions in such a manner as to interpose the optical disk  20  between them (refer to FIG.  3 ). 
     The first beam source portion  26   a  of the first parallel beam irradiation mechanism  6   a  irradiates the parallel laser beams from the beam outgoing surface. The first rise-up mirror  16   a  changes the direction of the parallel laser beams to the orthogonal direction, and the parallel beams are incident vertically on the lower surface of the optical disk  20 . 
     The track pitch of the signal train recorded to the optical disk  20  is smaller than the wavelength of the parallel laser beams as previously described. Therefore, the parallel laser beams are restricted from being diffracted by the tracks of the optical disk  20 . For, only the beams of the 0-order are allowed to pass because the angle of refraction of the ±1-order light exceeds 90°. 
     The parallel laser beams incident on the lower surface of the optical disk  20  pass through the optical disk  20  and are then incident into the first evanescent optical system  10   a . The first evanescent optical system  10   a  irradiates the evanescent waves to the upper surface of the optical disk  20  by means of the laser beams it receives. 
     Explanation will be given in further detail. The parallel laser beams incident from the lens bottom surface  16   a  to  16   c  of the solid immersion lens  15   a  to  15   c  (refer to FIGS. 5 to  7 ) pass through the lens  15   a  to  15   c , are reflected by the reflecting surface  17   a  to  17   c  on the upper surface side, are converged to the lens bottom surface  16   a  to  16   c , and generate the evanescent waves on the lens bottom surface  16   a  to  16   c . The evanescent waves are irradiated from the lens bottom surface  16   a  to  16   c.    
     The information may be recorded or reproduced to or from the information recording track on the upper surface of the optical disk  20  as the evanescent waves irradiated to the upper surface of the optical disk  20  are utilized. 
     On the other hand, the second beam source portion  26   b  of the second parallel beam irradiation mechanism  6   b  irradiates the parallel laser beams from the beam outgoing surface. The second rise-up mirror  16   b  changes the direction of the parallel laser beams to a right angle, and the parallel laser beams are vertically incident on the upper surface of the optical disk  20 . 
     In this case, too, possible diffraction of the parallel laser beams by the tracks of the optical disk  20  can be avoided. For, only the beams of the 0-order pass since the angle of refraction of the ±1-order exceeds 90°. 
     The parallel laser beams incident on the upper surface side of the optical disk  20  pass through the optical disk  20  and are incident into the second evanescent optical system lob. The second evanescent optical system lob irradiates the evanescent waves to the lower surface of the optical disk  20  by means of the laser beams it receives. 
     The information may be recorded or reproduced to or from the information recording tracks on the lower surface of the optical disk  20  as the evanescent waves irradiated to the lower surface of the optical disk  20  are utilized. 
     As described above, this embodiment disposes separately the first parallel beam irradiation mechanism  6   a  together with the second evanescent optical system  10   b  from the second parallel beam irradiation mechanism  6   b  with the first evanescent optical system  10   a  in such a fashion as to interpose the optical disk  20  not having the optical reflecting layer inside. Therefore, this embodiment can remarkably reduce the thickness of the optical disk drive  1 . For, the thickness of the optical disk drive  1  required on one of the sides of the optical disk  20  is the thickness of either of the parallel beam irradiation mechanism  6   a ,  6   b  and the evanescent optical system  10   a ,  10   b  that has a greater thickness (generally, the parallel beam irradiation mechanism  6   a ,  6   b ). 
     In this embodiment, the tracking pitch of the signal train recorded to the optical disk  20  is smaller than the wavelength of the parallel laser beams. Therefore, this embodiment can avoid possible diffraction of the parallel laser beams by the tracks of the optical disk  20 . 
     Each parallel beam irradiation mechanism  6   a ,  6   b  of this embodiment includes a rise-up mirror  16   a ,  16   b  disposed at the position opposing the evanescent optical system  10   a ,  10   b . Therefore, this arrangement makes it easy to constitute and arrange the optical system. 
     Next, an optical disk drive according to the second embodiment of the present invention will be explained with reference to FIG.  8 . This drawing is a schematic side view of the optical disk drive according to the second embodiment. 
     The optical disk drive according to the second embodiment uses a rise-up mirror  36  comprising a prism in place of the rise-up mirror having the 45° mirror as shown in FIG.  1 . 
     The rest of the constructions are substantially the same as those of the optical disk drive  1  of the first embodiment shown in FIGS. 1 to  6 . In the second embodiment, like reference numerals are used to identify like constituent portions of the first embodiment shown in FIGS. 1 to  6 , and the explanation in detail of such portions is omitted. 
     In this embodiment, the rise-up mirror  36  comprising the prism may be constituted into a smaller thickness than the rise-up mirror comprising the 45° mirror. Therefore, this embodiment can further reduce the overall thickness of the optical disk drive. 
     An optical disk drive according to the third embodiment of the present invention will be explained with reference to FIG.  9 . The drawing is a schematic front view of the optical disk drive according to the third embodiment. 
     In the optical disk drive shown in FIG. 9, the optical disk holding mechanism  2  (not shown in FIG. 9) can hold two optical disks  20  and  21  in parallel with one another, and includes four parallel beam irradiation mechanism  6   c  to  6   f  and four evanescent optical systems  10   c  to  10   f  for forming pairs with the parallel irradiation mechanisms  6   c  to  6   f , respectively, so that information may be recorded or reproduced to or from the four disk surfaces. 
     As shown in FIG. 9, each of the four evanescent optical systems  10   c  to  10   f  may be disposed in such a fashion that the bottom surface of its solid immersion lens faces the corresponding one of the four recording surfaces and is positioned within the attenuation distance of evanescence from each disk surface. 
     As shown also in FIG. 9, the first parallel beam irradiation mechanism  6   c  used for recording or reproducing the information, on the lower surface of the optical disk  20  held at the lower position, may be disposed on the upper surface side of the optical disk  21  held at the upper position. The parallel laser beams irradiated from the first parallel beam irradiation mechanism  6   c  pass through the two optical disks  20  and  21  and are incident on the first evanescent optical system  10   c  for irradiating the evanescent beams on the lower surface of the optical disk  20 . 
     On the other hand, the second parallel beam irradiation mechanism  6   d  used for recording or reproducing the information to or from the upper surface of the optical disk  20  held at the lower position may be disposed on the lower surface side of the optical disk  20  held at the lower position. The parallel laser beams irradiated from the second parallel beam irradiation mechanism  6   d  pass through the optical disk  20  and are incident on the second evanescent optical system  10   d  for irradiating the evanescent beams to the upper surface of the optical disk  20 . 
     Next, the third parallel beam irradiation mechanism  6   e  used for recording or reproducing the information to or from the lower surface of the optical disk  21  held on the upper side may be disposed on the upper surface side of the optical disk  21  held at the upper position. The parallel beams irradiated from the third parallel beam irradiation mechanism  6   e  pass through the optical disk  21  and are incident into the third evanescent optical system  10   e  for irradiating the evanescent beams to the lower surface of the optical disk  21 . 
     The fourth parallel beam irradiation mechanism  6   f  used for recording or reproducing the information to or from the upper surface of the optical disk  21  held on the upper side may be disposed on the lower surface side of the optical disk  20  held on the lower side. The parallel laser beams irradiated from the fourth parallel beam irradiation mechanism  6   f  pass through the two optical disks  20  and  21  and are incident on the fourth evanescent optical system  10   f  for emitting the evanescent beams to the upper surface of the optical disk  21 . 
     The rest of the constructions are substantially the same as those of the optical disk drive  1  according to the second embodiment shown in FIG.  8 . In this embodiment, like reference numerals are used to identify like constituent portions as those of the second embodiment shown in FIG. 8, and the explanation in detail of such portions is omitted. 
     According to this embodiment, all the parallel beam irradiation mechanisms  6   c  to  6   f  may be disposed in bulk on the upper and lower sides of the optical disks  20  and  21  as a whole. Therefore, the gap between the optical disks  20  and  21  can be set to a small value and eventually, the overall thickness of the optical disk drive can be remarkably reduced. 
     Next, the optical disk drive according to a fourth embodiment will be explained with reference to FIG.  10 . 
     FIG. 10 is a schematic front view of the optical disk drive according to the fourth embodiment. 
     As shown in FIG. 10, the first parallel beam irradiation mechanism  6   c  used for recording or reproducing information to or from the lower surface of the optical disk  20  held on the lower side maybe disposed on the lower surface side of the optical disk  20  held on the lower side. A conventional-type first evanescent optical system  50   c  for emitting the evanescent beams to the lower surface of the optical disk  20  by means of the parallel laser beams irradiated from the first parallel beam irradiation mechanism  6   c  may be interposed between the first parallel beam irradiation mechanism  6   c  and the optical disk  20  held on the lower side. 
     The fourth parallel beam irradiation mechanism  6   f  used for recording or reproducing the information to or from the upper surface of the optical disk  21  held on the upper side may be disposed above the optical disk  21  held on the upper side. A conventional-type fourth evanescent optical system  50   f  for irradiating the evanescent beams to the upper surface of the optical disk  21  by means of the parallel laser beams irradiated from the fourth parallel beam irradiation mechanism  6   c  may be interposed between the fourth parallel beam irradiation mechanism  6   f  and the optical disk  21  held on the upper side. 
     The rest of the constructions are substantially the same as those of the optical disk drive according to the third embodiment that is shown in FIG.  9 . In the fourth embodiment, like reference numerals are used to identify like portions of the third embodiment shown in FIG. 9, and the explanation in detail is omitted. 
     In this embodiment, too, all the parallel beam irradiation mechanisms  6   c  to  6   f  may be disposed in bulk on the upper and lower sides of the optical disks  20  and  21  as a whole. Therefore, the overall thickness of the optical disk drive  1  can be reduced, though not to the extent of the optical disk drive according to the third embodiment. 
     As explained above in detail, the present invention may separately arrange the parallel beam irradiation mechanism and the evanescent optical system while interposing the optical disk not having the optical reflecting layer inside. Therefore, freedom of disposition of these optical elements can be improved, and the thickness of the optical disk drive can be restricted. 
     Particularly, when both surfaces of an optical disk are used as a recording surface or when a plurality of optical disks are used simultaneously as the operation object, the thickness of the optical disk drive may be remarkably restricted by the present invention.