Abstract:
An optical pick-up apparatus according to the invention optically performs at least one of selective recording of information onto a plurality of recording layers of a plurality of optical recording mediums arranged in layers, and selective reproduction of information from the plurality of recording layers. Switching in optical circuits which are each adapted to guide a light beam to a desired recording layer is performed using a movable mirror, a galvano mirror or a biaxial movable mirror, to thereby perform selective recording of information onto the plurality of recording layers of optical disks or selective reproduction of information from the plurality of recording layers.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an optical pick-up apparatus for optically performing at least one of selective recording of information onto a plurality of recording layers employed in an optical recording medium, and selective reproduction of information from the plurality of recording layers. 
     Optical pick-up apparatuses as above are known from, for example, Japanese Patent Application KOKAI Publications Nos. 5-189797 and 2-206037. When an optical pick-up apparatus disclosed in Japanese Patent Application KOKAI Publication No. 5-189797 optically performs at least one of recording of information onto a desired recording layer, and reproduction of information from the desired recording layer, optical beam emission means itself is inclined or moved parallel in order to radiate the desired recording layer with a light beam. Further, when an optical pick-up apparatus disclosed in Japanese Patent Application KOKAI Publication No. 2-206037 optically performs at least one of recording of information onto a desired recording layer, and reproduction of information from the desired recording layer, a beam splitter or a shielding plate is used in order to radiate the desired recording layer with a light beam, and light reflected from the desired r recording layer is guided by another beam splitter or a prism to a reflected-light detecting unit corresponding to the desired recording layer. 
     In the first-mentioned conventional optical pick-up apparatus, the structure for inclining or parallel-moving the optical beam emission means itself is very large and complicated. Accordingly, the optical pick-up apparatus is complicated in structure, and its entire body is inevitably large and heavy. 
     Also, in the second-mentioned conventional optical pick-up apparatus, the structure for combining the beam splitter, the shielding plate and the prism is very large and complicated. As a result, the optical pick-up apparatus is complicated in structure, which increases its manufacturing cost. 
     This invention has been derived from these circumstances, and an object of this invention is to provide an optical pick-up apparatus which h is small in scale, simple in structure and low in cost, which optically performs at least one of selective recording of information onto a plurality of recording layers employed in an optical recording medium, and selective reproduction of information from the plurality of recording layers. 
     BRIEF SUMMARY OF THE INVENTION 
     In order to achieve the above-described object of the present invention, a first optical pick-up apparatus, according to the invention, which optically performs at least one of selective recording of information onto a plurality of recording layers of an optical recording medium, and selective reproduction of information from the plurality of recording layers, performs switching in optical circuits which are each adapted to guide a light beam to a desired recording layer, using a movable mirror, to thereby perform selective recording of information onto the plurality of recording layers of the optical recording medium or selective reproduction of information from the plurality of recording layers. 
     In order to achieve the above-described object of the present invention, a second optical pick-up apparatus, according to the invention, which optically performs at least one of selective recording of information onto a plurality of recording layers of a plurality of optical recording mediums arranged in layers, and selective reproduction of information from the plurality of recording layers, performs switching in optical circuits which are each adapted to guide a light beam to a desired recording layer, using a movable mirror which is movable in a direction of alignment of the optical recording mediums, to thereby perform selective recording of information onto the plurality of recording layers of the optical recording mediums or selective reproduction of information from the plurality of recording layers. 
     In order to achieve the above-described object of the present invention, a third optical pick-up apparatus, according to the invention, which optically performs at least one of selective recording of information onto a plurality of recording layers of an optical recording medium, and selective reproduction of information from the plurality of recording layers, performs switching in optical circuits which are each adapted to guide a light beam to a desired recording layer, and guiding of the light beam to a desired recording track on the desired recording layer, using galvano mirrors, to thereby perform selective recording of information onto the plurality of recording layers or selective reproduction of information from the plurality of recording layers. 
     The galvano mirrors have a simple structure and can be controlled with high accuracy. 
     In order to achieve the above-described object of the present invention, in a fourth optical pick-up apparatus, according to the invention, which optically performs at least one of selective recording of information onto a plurality of recording layers of a plurality of optical recording mediums arranged in layers, and selective reproduction of information from the plurality of recording layers, a plurality of reflection mirrors corresponding to the plurality of recording layers of the plurality of optical recording mediums are provided such that they are separated from each other in a direction of alignment of the plurality of optical recording mediums and in a direction parallel to the surfaces of the optical recording mediums; switching in optical circuits each adapted to guide a light beam to that one of the reflection mirrors arranged in the direction parallel to the surfaces, which corresponds to a desired recording layer, is performed using a movable mirror to thereby perform, via the plurality of reflection mirrors, selective recording of information onto the plurality of recording layers of the optical recording mediums or selective reproduction of information from the plurality of recording layers. The corresponding reflection mirror included in the plurality of reflection mirrors arranged in the direction parallel to the surfaces is adapted, if necessary, to further guide the light beam to that one of the reflection mirrors separated in the direction of alignment, which corresponds to a desired recording layer. 
     In order to achieve the above-described object of the present invention, in a fifth optical pick-up apparatus, according to the invention, which optically performs at least one of selective recording of information onto a plurality of recording layers provided on optical recording mediums and separated from each other in a direction perpendicular to the surfaces of the optical recording mediums, and selective reproduction of information from the plurality of recording layers, a light beam is entered in a direction parallel to the surfaces of the optical recording mediums for performing selective recording of information onto the plurality of recording layers separated in the direction perpendicular to the surfaces or selective reproduction of information from the plurality of recording layers. 
     Making a light beam to be entered in a direction parallel to the surfaces of the optical recording mediums as described above enables reduction of the size of the optical pick-up apparatus in a direction perpendicular to the surfaces of the optical recording mediums. 
     In order to achieve the above-described object of the present invention, a sixth optical pick-up apparatus, according to the invention, which optically performs at least one of selective recording of information onto a plurality of recording layers of a plurality of optical recording mediums arranged in layers, and selective reproduction of information from the plurality of recording layers, performs switching in optical circuits which are each adapted to guide a light beam to a desired recording layer, using a biaxial movable mirror, to thereby perform selective recording of information onto the plurality of recording layers of the optical recording mediums or selective reproduction of information from the plurality of recording layers. 
     The use of the biaxial movable mirror not only enables execution of at least one of selective recording of information onto a plurality of recording layers of a single optical recording medium, and selective reproduction of information from the plurality of recording layers, but also enables simplification of the structure of an optical circuit which can perform at least one of selective recording of information onto a plurality of recording layers of a plurality of optical recording mediums arranged in layers, and selective reproduction of information from the plurality of recording layers. 
     In order to achieve the above-described object of the present invention, a seventh optical pick-up apparatus, according to the invention, which optically performs at least one of selective recording of information onto a plurality of recording layers of a plurality of optical recording mediums arranged in layers, and selective reproduction of information from the plurality of recording layers, performs switching in optical circuits which are each adapted to:guide a light beam to a desired recording layer, using a biaxial movable mirror which can be inclined in a direction of alignment of the plurality of optical recording mediums and in a direction parallel to the surfaces of the optical recording mediums to thereby perform selective recording of information onto the plurality of recording layers of the optical recording mediums or selective reproduction of information from the plurality of recording layers. 
     The use of the biaxial movable mirror which can be inclined in both the direction of alignment and the direction parallel to the surfaces, not only enables execution of at least one of selective recording of information onto a plurality of recording layers of a single optical recording medium and selective reproduction of information from the plurality of recording layers, but also permits the structure of an optical circuit to be made compact, which circuit can perform at least one of selective recording of information onto a plurality of recording layers of a plurality of optical recording mediums arranged in layers, and selective reproduction of information from the plurality of recording layers. 
     In order to achieve the above-described object of the present invention, in an eighth optical pick-up apparatus, according to the invention, which optically performs at least one of selective recording of information onto a plurality of recording layers of a plurality of optical recording mediums arranged in layers, and selective reproduction of information from the plurality of recording layers, a plurality of reflection mirrors corresponding to the plurality of recording layers of the plurality of optical recording mediums are provided such that they are separated from each other in a direction of alignment of the plurality of optical recording mediums and in a direction parallel to the surfaces of the optical recording mediums. Switching in optical circuits which are each adapted to guide a light beam to that one of the reflection mirrors arranged in the direction parallel to the surfaces, which corresponds to a desired recording layer, is performed using a movable mirror to thereby perform, via the plurality of reflection mirrors, selective recording of information onto the plurality of recording layers of the optical recording mediums or selective reproduction of information from the plurality of recording layers; and the corresponding reflection mirror included in the plurality of reflection mirrors arranged in the direction parallel to the surfaces is adapted, if necessary, to further guide the light beam to that one of the reflection mirrors separated in the direction of alignment, which corresponds to a desired recording layer. 
     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 presently preferred embodiments of the present invention and, together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention in which: 
     FIG. 1 is a schematic longitudinal sectional view illustrating an optical pick-up apparatus according to a first embodiment of the invention; 
     FIG. 2 is a schematic horizontal sectional view illustrating the optical pick-up apparatus of FIG. 1; 
     FIG. 3A is an enlarged perspective view illustrating the optical circuit of an optical head incorporated in the optical pick-up apparatus of FIG. 1; 
     FIG. 3B is a schematic enlarged view of a quartered detector appearing in FIG. 3A; 
     FIG. 4 is a schematic enlarged view showing a galvano mirror, a kind of movable mirror, employed in the optical pick-up apparatus of FIG. 1; 
     FIG. 5A is a schematic longitudinal sectional view illustrating an optical pick-up apparatus according to a second embodiment of the invention; 
     FIG. 5B is a schematic horizontal sectional view illustrating the optical pick-up apparatus of FIG. 5A; 
     FIG. 5C is an enlarged perspective view illustrating the optical circuit of an optical head incorporated in the optical pick-up apparatus of FIG. 5A; 
     FIG. 6A is a schematic longitudinal sectional view illustrating an optical pick-up apparatus according to a third embodiment of the invention; 
     FIG. 6B is a schematic horizontal sectional view illustrating the optical pick-up apparatus of FIG. 6A; 
     FIG. 7A is a schematic longitudinal sectional view illustrating an optical pick-up apparatus according to a fourth embodiment of the invention; 
     FIG. 7B is a schematic horizontal sectional view illustrating the optical pick-up apparatus of FIG. 7A; 
     FIG. 8A is a schematic longitudinal sectional view illustrating an optical pick-up apparatus according to a modification of the fourth embodiment of the invention; 
     FIG. 8B is a schematic horizontal sectional view illustrating the optical pick-up apparatus of FIG. 8A; 
     FIG. 9A is a schematic longitudinal sectional view illustrating an optical pick-up apparatus according to a fifth embodiment of the invention; 
     FIG. 9B is a schematic horizontal sectional view illustrating the optical pick-up apparatus of FIG. 9A; 
     FIG. 9C is a more roughly schematic horizontal sectional view different from the horizontal sectional view of FIG. 9B, showing the optical pick-up apparatus of FIG. 9A; 
     FIG. 10A is a schematic front view illustrating a biaxial galvano mirror, a kind of biaxial movable mirror, employed in the optical pick-up apparatus of the fifth embodiment shown in FIGS. 9A to  9 C; 
     FIG. 10B is a schematic longitudinal sectional view of the biaxial galvano mirror of FIG. 10A; and 
     FIG. 11 is a schematic longitudinal sectional view illustrating an optical pick-up apparatus according to a sixth embodiment of the invention. 
     Optical pick-up apparatuses according to: various embodiments of the invention will be described in detail with reference to the accompanying drawings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     Referring first to FIGS. 1 to  4  included in the accompanying drawings, an optical pick-up apparatus according to a first embodiment of the invention will be described. 
     FIG. 1 is a schematic longitudinal sectional view illustrating an optical pick-up apparatus  10  according to the first embodiment of the invention; and. FIG. 2 is a schematic horizontal sectional view illustrating the optical pick-up apparatus  10  of FIG.  1 . 
     The optical pick-up apparatus  10  is disposed adjacent to a plurality of optical disks  12 , which are arranged concentric with each other in the form of layers. The optical disks  12  can be rotated at a predetermined speed in a predetermined direction by means of driving means (not shown). Each of the optical disks  12  has recording layers  12   a  and  12   b  respectively provided on both surfaces thereof which are directed opposite to each other along the axis-of-rotation A of the disks. 
     The optical pick-up apparatus  10  can swing within a predetermined range about an axis-of-rotation  15   a  which extends parallel to the axis-of-rotation A of the optical disks  12  in the vicinity of the edges of the optical disks  12 . The apparatus  10  has a main body housing  15  having arm members  14   a ,  14   b  and  14   c  corresponding to gaps defined between adjacent optical disks  12 . 
     When the main body housing  15  of the optical pick-up apparatus is swung about the axis-of-rotation  15   a  by publicly-known swinging means, extended end portions of the arm members  14   a ,  14   b  and  14   c  are swung along the surfaces of the optical disks  12 . At this time, the extended end portions of the arm members  14   a ,  14   b  and  14   c  are swung between those portions of the optical disks  12  which are in the vicinity of the axis-of-rotation A of the optical disks  12 , and edge portions of the optical disks  12 , and between an operative position in which each of the extended end portions is opposed to the recording layers  12   a  and  12   b  of the optical disks  12 , and an operation stop position in which the extended end portions are placed outside the edges of the optical disks  12 . 
     Galvano mirrors  16   a ,  16   b  and  16   c  each having an axis-of-rotation B parallel to the axis-of-rotation A of the optical disks  12  are arranged in the extended end portions of the arm members  14   a ,  14   b  and  14   c . In each of the extended end portions of the arm members  14   a ,  14   b  and  14   c , a pair of stationary mirrors  18  and  18   b  are provided at that portion of the extended end portion which is closer to the proximal end of the arm member  14   a ,  14   b  or  14   c  than the corresponding:galvano mirror  16   a ,  16   b  or  16   c.    
     The pair of stationary mirrors  18   a  and  18   b  are separated from each other in a direction along the recording layers  12   a  and  12   b  of each optical disks  12  and along recording tracks (not shown) which circumferentially extend in each of the recording layers  12   a  and  12   b , and are inclined in different directions to the recording layers  12   a  and  12   b.    
     Objective lenses  20   a  and  20   b  are provided in each of the extended end portions of the arm members  14   a ,  14   b  and  14   c  between a corresponding pair of stationary mirrors  18   a ,  18   b  and the recording layers  12   a  and  12   b  of a corresponding pair of adjacent optical disks  12  opposed to the stationary mirrors. 
     The galvano mirror  16   a ,  16   b  or  16   c , the pair of stationary mirrors  18   a  and  18   b  and the objective lenses  20   a  and  20   b  in one of the extended end portions of the arm members  14   a ,  14   b  and  14   c  are aligned with corresponding elements in another extended end portion along the axis-of-rotation A of the optical disks  12 . 
     Further, as is shown in FIG. 3A which is an enlarged view of an optical circuit provided in each of the extended end portions of the arm members  14   a ,  14   b  and  14   c , each of the extended end portions of the arm members  14   a ,  14   b  and  14   c  also contains a pair of quartered detectors  22   a  and  22   b  between the pair of stationary mirrors  18   a  and  18   b  and the pair of objective lenses  20   a  and  20   b.    
     The optical pick-up apparatus  10  includes a light beam emitting/receiving unit  24  of a publicly-known structure in the main body housing  15  in the vicinity of the axis-of-rotation  15   a . In this embodiment, the light beam emitting/receiving unit  24  has a laser diode  24   a  as a light source. A laser beam emitted from the laser diode  24   a  is guided to a beam splitter formed of a prism assembly  24   c  via a collimate lens  24   b , where it is split into two beams. One of the two beams is guided to a photodetector  24   d  for detecting the intensity of light, while the other is guided downward parallel to the axis-of-rotation  15   a.    
     The optical pick-up apparatus  10  includes a movable mirror  26  with inclined-angle detecting means  25  located below the light beam emitting/receiving unit  24  in a position corresponding to the lowest arm member  14   c , and also includes stationary mirrors  28   a  and  28   b  located at proximal ends of the arm members  14   a  and  14   b  respectively, which are located above the lowest arm member  14   c . The mirror surfaces of the stationary mirrors  28   a  and  28   b  are directed obliquely downward. 
     In this embodiment, the movable mirror  26  consists of a galvano mirror having an axis-of-rotation  26   a  parallel to the recording layers  12   a  and  12   b  of the optical disks  12 . When the movable mirror  26  swings about the axis-of-rotation  26   a  along the axis-of-rotation A of the optical disks  12  to thereby incline, it selectively reflects a laser beam guided downward from the light beam emitting/receiving unit  24 , to one of the galvano mirror  16   c  located in the extended end portion of the lowest arm member  14   c , and the stationary mirrors  28   a  and  28   b  located at the proximal ends of the arm members  14   a  and  14   b  located above the lowest arm member  14   c.    
     The laser beam having entered the stationary mirror  28   a  or  28   b  of the upper arm member  14   a  or  14   b  is reflected by the mirror  28   a  or  28   b  to the galvano mirror  16   a  or  16   b  located in the extended end portion of the arm members  14   a  or  14   b  located above. 
     When as described above, the laser beam is selectively guided to any of the galvano mirrors  16   a ,  16   b  and  16   c  in the extended end portions of the arm members  14   a ,  14   b  and  14   c , the galvano mirror  16   a ,  16   b  or  16   c  selectively swings about the axis-of-rotation B parallel to the axis-of-rotation A of the optical disks  12 , thereby reflecting it to one of a corresponding pair of the inclined stationary mirrors  18   a  and  18   b . The stationary mirror  18   a  or  18   b  which the laser beam has entered reflects it to a corresponding objective lens  20   a  or  20   b  via a corresponding quartered detector  22   a  or  22   b . The objective lens  20   a  or  20   b  which the laser beam has entered guides the laser beam to a corresponding recording layer  12   a  or  12   b  of a corresponding optical disk  12  to record information onto the corresponding recording layer  12   a  or  12   b , or to reproduce information from the corresponding recording layer  12   a  or  12   b.    
     In each of the arm members  14   a ,  14   b  and  14   c , it is preferable that the optical path length from the light beam emitting/receiving unit  24  to the recording layer  12   a  or  12   b  via the corresponding objective lens  20   a  or  20   b , and the arrangement of the optical elements along the optical paths should be set so that laser beams guided to the recording layers  12   a  and  12   b  via the respective objective lenses  12   a  and  12   b  can have the same optical characteristics, and so that reflected laser beams returned from the recording layers  12   a  and  12   b  to the light beam emitting/receiving unit  24  can have the same optical characteristics. 
     A laser beam reflected from the recording layer  12   a  or  12   b  is guided to the light beam emitting/receiving unit  24 , passing through the same optical path as above but in the opposite direction. The reflected laser beam having entered the light beam emitting/receiving unit  24  is split by the beam splitter of the prism assembly  24   c . Part of the split, reflected laser beam is guided to a photodetector  24   e  for servo, while the other of the split, reflected laser beam is divided into two deflected beam components by a Wollaston prism  24   f  and then enters, via a lens  24   g , a photodetector  24   h  for detecting a reflected light signal. 
     As clearly shown in FIG. 3B, each of the quartered detectors  22   a  and  22   b  comprises publicly-known four photodetectors  30   a ,  30   b ,  30   c  and  30   d  combined annular, four adders  32   a ,  32   b ,  32   c  and  32   d  each connected to a corresponding pair of adjacent photodetectors  30   a ,  30   b ,  30   c  and  30   d , and a pair of differential amplifiers  34   a  and  34   b  each connected to a corresponding pair of the adders  32   a ,  32   b ,  32   c  and  32   d.    
     Each of the quartered detectors  22   a  and  22   b  detects that a laser beam, which should be emitted, after passing through a central aperture surrounded by the four photodetectors  30   a ,  30   b ,  30   c  and  30   d , from a corresponding objective lens  20   a  or  20   b  onto a desired recording track included in multiple recording tracks (not shown) which extend circumferentially on a corresponding recording layer  12   a  or  12   b  of a corresponding optical disk  12 , deviates in a radial tracking direction C of the optical disk  12  or in a tangential direction D parallel to the circumferential direction. 
     After detecting deviation in the tangential direction D, each of the four quartered detectors  22   a  and  22   b  controls, together with the inclined-angle detecting means  25  of the movable mirror  26 , the inclined angle of the movable mirror  26  and also the inclined angle of the galvano mirror  16   a ,  16   b ,  16   c  or  16   d , so that the laser beam will always follow the desired track of the recording layer  12   a  or  12   b . After detecting deviation in the tracking direction C, each of the four quartered detectors  22   a  and  22   b  controls, together with the inclined-angle detecting means  25  of the movable mirror  26 , the inclined angle of the movable mirror  26  and also the inclined angle of the galvano mirror  16   a ,  16   b ,  16   c  or  16   d , so that information can be correctly recorded onto a desired track using the laser beam, or can be correctly reproduced from a desired track. 
     The quartered detectors  22   a  and  22   b  detect, on the basis of the sum of the outputs of the adders  32   a  and  32   b , whether the laser beam enters the combination of the stationary mirror  18   a  and the objective lens  20   a , or the combination of the fixed lens  18   b  and the objective lens  20   b , and accordingly detect whether the laser beam enters the recording layer  12   a  or  12   b  which corresponds to the objective lens  20   a  or  20   b.    
     The detection results of the quartered detectors  22   a  and  22   b  are input to driving means (not shown) for driving the movable mirror  26 . On the basis of the detection results, the driving means not shown prevents deviation in the tracking direction C, i.e. in the radial direction of each optical disk  12 , of the laser beam to be converged via the objective lens  20   a  or  20   b  onto a desired track included in multiple recording tracks (not shown) which extend circumferential on the recording layer  12   a  or  12   b  of the disk  12 , or prevents deviation of the laser beam in the tangential direction D, i.e. in the circumferential direction of the disk. Thus, defective recording and reproduction of information can be avoided. 
     FIG. 4 roughly shows the structure of a galvano mirror  16   a  as a representative of the galvano mirrors  16   a ,  16   b  and  16   c  located in the extended end sections of the arm members  14   a ,  14   b  and  14   c  and the movable mirror  26 . The other galvano mirrors  16   b  and  16   c  have the same structure and size as the galvano mirror  16   a , while the movable mirror  26  has the same structure as the galvano mirror  16   a  although they have different sizes. 
     The galvano mirror  16   a  comprises a mirror support member  34 , an outer frame  36  which surrounds the mirror support member  34 , and a pair of bridge members  38  which coaxially bridge the mirror support member  34  and the outer frame  36  and constitutes the axis-of-rotation B. The mirror support member  34 , the outer frame  36  and the pair of bridge members  38  are formed integral as one body. 
     A damping material  39  is adhered to a connecting portion of the outer frame  36  and each of the bridge members  38 . A coil  40  is wound on the mirror support member  34 , while a pair of magnets  42  are fixed on the outer frame  36  in a direction perpendicular to the direction in which the pair of bridge members  38  extend such that they are opposed to the mirror support member  34 . 
     The direction and angle of the mirror support member  34  which swings about the axis-of-rotation B can be controlled by controlling the direction and intensity of a current flowing through the coil  40 . 
     In the above-described optical pick-up apparatus  10  of the first embodiment, a laser beam emitted from the light emitting/receiving unit  24  is selectively guided, by the movable mirror  26  opposed to the light beam emitting/receiving unit  24 , to an optical path directed to the galvano mirror  16   a  in one of the extended end portions of the arm members  14   a ,  14   b  and  14   c . The galvano mirror  16   a  in each of the extended end portions of the arm members  14   a ,  14   b  and  14   c  directs the incident laser beam to that one of the objective lenses  20   a  and  20   b  which corresponds to and is opposed to a desired one of the recording layers  12   a  and  12   b , and then to the desired recording layer. 
     Second Embodiment 
     FIG. 5A is a schematic longitudinal sectional view illustrating an optical pick-up apparatus  50  according to a second embodiment of the invention; and FIG. 5B is a schematic plan view illustrating the optical pick-up apparatus  50  of FIG. 5A according to the second embodiment. 
     In the second embodiment, structural elements similar to those employed in the first embodiment described with reference to FIGS. 1 to  4  are denoted by corresponding reference numerals, and no detailed description will be given thereof. 
     In this embodiment, a laser beam is downwardly emitted from the light beam emitting/receiving unit  24  and passes through a relay lens  51  in a manner parallel to the axis-of-rotation  15   a  of the main body housing  15 . This laser beam is reflected, by a galvano mirror  52  located corresponding to the lowest arm member  14   c , to one of two pairs of stationary mirrors  54   a ,  54   b ,  56   a  and  56   b  located corresponding to the proximal ends of the arm members  14   a  and  14   b  above the lowest arm member  14   c , or to a single stationary mirror  58  located corresponding to the proximal end of the lowest arm member  14   c , or horizontally to the proximal end of the lowest arm member  14   c.    
     The two pairs of stationary mirrors  54   a ,  54   b ,  56   a  and  56   b  and the single stationary mirror  58  are directed obliquely downward. 
     The galvano mirror  52  can swing about an axis-of-rotation  52   a  parallel to the recording layers  12   a  and  12   b  of each of a plurality of optical disks  12 , and along the axis-of-rotation A of the optical disks  12 . The stationary mirrors  54   a  and  54   b  (or the stationary mirrors  56   a  and  56   b ) located corresponding to the proximal ends of the arm member  14   a  (or  14   b ) above the lowest arm member  14   c  are separated from each other by a predetermined distance along the axis-of-rotation A. The stationary mirror  58  located corresponding to the proximal end of the lowest arm member  14   c  is upwardly separated, by a predetermined distance along the axis-of-rotation A, from the optical path of the laser beam horizontally reflected by the galvano mirror  52  into the lowest arm member  14   c.    
     In each of the extended end portions of the arm members  14   a ,  14   b  and  14   c , there are provided a stationary mirror  60  inclined sideways, and a prism  62  of a triangular cross section having a single ridge opposed to the stationary mirror  60 . The ridge of the prism  62  extends parallel to the recording layers  12   a  and  12   b  on the opposite surfaces of each optical disk  12 . 
     FIG. 5C is an enlarged view roughly showing the optical system located in each of the extended end portions of the arm members  14   a ,  14   b  and  14   c.    
     The laser beam, which is reflected from the galvano mirror  52  to one of the two pairs of stationary mirrors  54   a ,  54   b ,  56   a  and  56   b  located corresponding to the proximal ends of the arm members  14   a  and  14   b  above the lowest arm member  14   c , or to the single stationary mirror  58  located corresponding to the proximal end of the lowest arm member  14   c , or horizontally to the proximal end of the lowest arm member  14   c , enters the inclined stationary mirror  60  located in the extended end portion of a corresponding one of the arm members  14   a ,  14   b  and  14   c , via a corresponding imaging lens  64 . 
     There are two optical paths through which the laser beam is guided from the galvano mirror  52  into each of the arm members  14   a ,  14   b  and  14   c . A laser beams which passes through the upper optical path, is reflected from the inclined stationary mirror  60  to the upper inclined reflection surface of the prism  62 , where it is upwardly guided, via the quartered detector  22   a  and the objective lens  20   a , to the recording layer  12   b  on the lower surface of the optical disk  12  which is opposed to the lens  20   a . A laser beam, which passes through the lower optical path, is reflected from the inclined stationary mirror  60  to the lower inclined reflection surface of the prism  62 , where it is downwardly guided, via the quartered detector  22   b  and the objective lens  20   b , to the recording layer  12   a  on the upper surface of the optical disk  12  which is opposed to the lens  20   b.    
     In the above-described optical pick-up apparatus  50  of the second embodiment, a laser beam emitted from the light emitting/receiving unit  24  is selectively guided, by the movable mirror  52  opposed to the light beam emitting/receiving unit  24 , to one of the two optical paths directed to the inclined stationary mirror  60  and the prism  62  in one of the extended end portions of the arm members  14   a ,  14   b  and  14   c . The prism  62  directs the laser beam having its path selected, to that one of the objective lenses  20   a  and  20   b  which corresponds to and is opposed to a selected one of the recording layers  12   a  and  12   b , and then to the selected recording layer  12   a  or  12   b.    
     In this embodiment, the two upper and lower optical paths, which extend from the galvano mirror  52  opposed to the light beam emitting/receiving unit  24 , to the inclined stationary mirror  60  via the proximal end of each of arm members  14   a ,  14   b  and  14   c , have their directions vertically changed by the prism  62 . 
     In the second embodiment, the shape of the housing main body  15  when seen in plan view, in particular, the shape of each of the arm members  14   a ,  14   b  and  14   c  when seen in plan view can be made smaller in a width direction perpendicular to the longitudinal direction, than in the above-described first embodiment where the vertical swinging of a laser beam in the extended end portion of each of the arm members  14   a ,  14   b  and  14   c  is performed using the galvano mirror  16   a  and a pair of inclined stationary mirrors  18  and  18   b  horizontally separated from each other, which are incorporated in the arm member  14   a ,  14   b  or  14   c.    
     It is preferable also in this embodiment that the optical path length from the light beam emitting/receiving unit  24  to the recording layer  12   a  or  12   b  via the corresponding objective lens  20   a  or  20   b , and the arrangement of the optical elements along the optical paths should be set so that laser beams guided to the recording layers  12   a  and  12   b  via the respective objective lenses  12   a  and  12   b  can have the same optical characteristics, and so that reflected laser beams returned from the recording layers  12   a  and  12   b  to the light beam emitting/receiving unit  24  can have the same optical characteristics. 
     Further, also in this embodiment, the inclined stationary mirror  60 , the prism  62 , the pair of quartered detectors  22   a  and  22   b  and the objective lenses  20   a  and  20   b  in one of the extended end portions of the arm members  14   a ,  14   b  and  14   c  are aligned with corresponding optical elements in another extended end portion along the axis-of-rotation A of the optical disks  12 . 
     Third Embodiment 
     FIG. 6A is a schematic longitudinal sectional view illustrating an optical pick-up apparatus  70  according to a third embodiment of the invention; and FIG. 6B is a schematic plan view illustrating the optical pick-up apparatus  70  of FIG. 6A according to the third embodiment. 
     In the third embodiment, structural elements similar to those employed in the first embodiment described with reference to FIGS. 1 to  4  are denoted by corresponding reference numerals, and no detailed description will be given thereof. 
     In this embodiment, a laser beam is emitted from the light beam emitting/receiving unit  24  in a substantially horizontal direction along the recording layers  12   a  and  12   b  of the optical disks  12 . The horizontally emitted laser beam is directed to a galvano mirror  72  located at the same level as the proximal end of the arm member  14   b  which is the middle one of the arm members  14   a ,  14   b  and  14   c . The galvano mirror  72  has an axis-of-rotation  72   a  extending parallel to the axis-of-rotation A of the optical disks  12 , and is adapted to horizontally swing a laser beam from the light beam emitting/receiving unit  24 . 
     The galvano mirror  72  selectively creates one of two optical paths which are directed to the extend end portion of the middle arm member  14   b  and horizontally separated from each other by a predetermined distance. One of the two optical paths directly linearly extends from the galvano mirror  72  to the extended end portion, while the other linearly extends to the extended end portion after being bent by a stationary mirror  73 . When the laser beam from the galvano mirror  72  is guided through any one of the two optical paths, it passes through an imaging lens  74  and then reaches a corresponding one of a pair of inclined stationary mirrors  18   a  and  18   b , which are horizontally located in the extended end portion of the arm member  14   b  with a predetermined distance therebetween. The pair of inclined stationary mirrors  18   a  and  18   b  incline in opposite directions. The corresponding one of the pair of inclined stationary mirrors  18   a  and  18   b  directs the laser beam to a corresponding one of the recording layers  12   a  and  12   b  of corresponding ones of the optical disks  12  via a corresponding one of the quartered detectors  22   a  and  22   b  and a corresponding one of the objective lenses  20   a  and  20   b.    
     As shown in FIG. 6B, the galvano mirror  72  also selectively creates one of two optical paths which are directed to a pair of first stationary mirrors  76   a  and  76   b  located on one side of the proximal end of the middle arm member  14   b  in the horizontal direction. The pair of first stationary mirrors  76   a  and  76   b  are inclined upward for the upper arm member  14   a.    
     A pair of second stationary mirrors  78   a  and  78   b  are arranged above the pair of first stationary mirrors  76   a  and  76   b  at the same level as the upper arm member  14   a . The pair of second stationary mirrors  78   a  and  78   b  are inclined toward the extended end portion of the upper arm member  14   a.    
     The laser beam reflected from the galvano mirror  72  and having entered one of the pair of first stationary mirrors  76   a  and  76   b  is guided by the one of the first stationary mirrors  76   a  and  76   b  to a corresponding one of the pair of second stationary mirrors  78   a  and  78   b  located above. The corresponding one of the pair of second stationary mirrors  78   a  and  78   b  directs the laser beam, via an imaging lens  74 , to a corresponding one of a pair of inclined stationary mirrors  18   a  and  18   b  located in the extended end portion of the upper arm member  14   a  and horizontally separated from each other by a predetermined distance. The pair of inclined stationary mirrors  18   a  and  18   b  are inclined in opposite directions, and the corresponding one of the mirrors  18   a  and  18   b  directs the incident laser beam to a corresponding one of the recording layers  12   a  and  12   b  of corresponding ones of the optical disks  12  via a corresponding one of quartered detectors  22   a  and  22   b  and a corresponding one of objective lenses  20   a  and  20   b.    
     As is shown in FIG. 6B, the galvano mirror  72  further selectively creates one of two optical paths which are directed to a pair of third stationary mirrors  80   a  and  80   b  located on the other side of the proximal end of the middle arm member  14   b  in the horizontal direction. The pair of third stationary mirrors  80   a  and  80   b  are inclined downward for the lower arm member  14   c.    
     A pair of fourth stationary mirrors  82   a  and  82   b  are arranged below the pair of third stationary mirrors  80   a  and  80   b  at the same level as the lower arm member  14   c . The pair of fourth stationary mirrors  82   a  and  82   b  are inclined toward the extended end portion of the lower arm member  14   c.    
     The laser beam reflected from the galvano mirror  72  and having entered one of the pair of third stationary mirrors  80   a  and  80   b  is guided by the one of the third stationary mirrors  80   a  and  80   b  to a corresponding one of the pair of fourth stationary mirrors  82   a  and  82   b  located below. The corresponding one of the pair of fourth stationary mirrors  82   a  and  82   b  directs the laser beam, via an imaging lens  74 , to a corresponding one of a pair of inclined stationary mirrors  18   a  and  18   b  located in the extended end portion of the lower arm member  14   c  and horizontally separated from each other by a predetermined distance. The pair of inclined stationary mirrors  18   a  and  18   b  are inclined in opposite directions, and the corresponding one of the mirrors  18   a  and  18   b  directs the incident laser beam to a corresponding one of the recording layers  12   a  and  12   b  of corresponding ones of the optical disks  12  via a corresponding one of quartered detectors  22   a  and  22   b  and a corresponding one of objective lenses  20   a  and  20   b.    
     As described above, in this embodiment, a laser beam is guided by the single galvano mirror  72  to a desired one of the recording layers  12   a  and  12   b  of the optical disks  12  via a desired one of the objective lenses  20   a  and  20   b  of a desired one of the arm members  14   a ,  14   b  and  14   c , as a result of only changing the swing angle of the single galvano mirror  72  and using the functions of the aforementioned plural stationary mirrors for the arm members  14   a ,  14   b  and  14   c . Thus, information can be recorded onto or reproduced from a desired recording layer using a laser beam reflected from the single galvano mirror  72 . 
     It is preferable also in this embodiment that the optical path length from the light beam emitting/receiving unit  24  to the recording layer  12   a  or  12   b  via the corresponding objective lens  20   a  or  20   b  included in each of the arm members  14   a ,  14   b  and  14   c , and the arrangement of the optical elements along the optical paths should be set so that laser beams guided to the recording layers  12   a  and  12   b  via the respective objective lenses  12   a  and  12   b  can have the same optical characteristics, and so that reflected laser beams returned from the recording layers  12   a  and  12   b  to the. light beam emitting/receiving unit  24  can have the same optical characteristics. 
     Further, also in this embodiment, the pair of inclined stationary mirrors  18   a  and  18   b , the pair of quartered detectors  22   a  and  22   b  and the objective lenses  20   a  and  20   b  in one of the extended end portions of the arm members  14   a ,  14   b  and  14   c  are aligned with corresponding optical elements in another extended end portion along the axis-of-rotation A of the optical disks  12 . 
     Fourth Embodiment 
     FIG. 7A is a schematic longitudinal sectional view illustrating an optical pick-up apparatus  80  according to a fourth embodiment of the invention; and FIG. 7B is a schematic plan view illustrating the optical pick-up apparatus  80  of FIG. 7A according to the fourth embodiment. 
     In the fourth embodiment, structural elements similar to those employed in the first embodiment described with reference to FIGS. 1 to  4  are denoted by corresponding reference numerals, and no detailed description will be given thereof. 
     Also in this embodiment, a laser beam is emitted from the light beam emitting/receiving unit  24  in a substantially horizontal direction along the recording layers  12   a  and  12   b  of the optical disks  12 . The horizontally emitted laser beam is directed to a movable mirror  92  located at the same level as the proximal end of the arm member  14   c  which is the lowest one of the arm members  14   a ,  14   b  and  14   c . The movable mirror  92  has an axis-of-rotation  92   a  extending parallel to the horizontal direction, and is adapted to swing a laser beam from the light beam emitting/receiving unit  24  in the vertical direction along the axis-of-rotation A of the optical disks  12 . 
     A plurality of stationary mirrors  94  directed horizontal are provided corresponding to the proximal ends of the arm members  14   a ,  14   b  and  14   c . The movable mirror  92  selectively directs the laser beam to one of the stationary mirrors  94 . 
     The one of the stationary mirrors  94  directs the laser beam to a corresponding one of galvano mirrors  16   a ,  16   b  and  16   c  located in the extended end portions of the arm members  14   a ,  14   b  and  14   c , respectively. Each of the galvano mirrors  16   a ,  16   b  and  16   c  has an axis-of-rotation B parallel to the axis-of-rotation A of the optical disks  12 . Each of the galvano mirrors  16   a ,  16   b  and  16   c  selectively swings about its axis-of-rotation B to thereby reflect the laser beam to one of a pair of stationary mirrors  18   a  and  18   b  inclined in opposite directions. That one of the stationary mirrors  18   a  and  18   b  which has received the laser beam reflects it to a corresponding one of objective lenses  20   a  and  20   b  via a corresponding one of quartered detectors  22   a  and  22   b . The one of the objective lenses  20   a  and  20   b  which the laser beam enters guides it to a corresponding one of the recording layers  12   a  and  12   b  of corresponding ones of the optical disks  12 , thereby recording information onto the corresponding recording layer  12   a  or  12   b  or reproducing information from the corresponding recording layer  12   a  or  12   b.    
     It is preferable that the optical path length from the light beam emitting/receiving unit  24  to the recording layer  12   a  or  12   b  via the corresponding objective lens  20   a  or  20   b  included in each of the arm members  14   a ,  14   b  and  14   c , and the arrangement of the optical elements along the optical paths should be set so that laser beams guided to the recording layers  12   a  and  12   b  via the respective objective lenses  12   a  and  12   b  can have the same optical characteristics, and so that reflected laser beams returned from the recording layers  12   a  and  12   b  to the light beam emitting/receiving unit  24  can have the same optical characteristics. 
     In the above-described optical pick-up apparatus  80  of the fourth embodiment, a laser beam emitted from the light emitting/receiving unit  24  is selectively guided, by the movable mirror  92  opposed to the light beam emitting/receiving unit  24 , to the optical path directed to the galvano mirror  16   a  in the extended end portion of one of the arm members  14   a ,  14   b  and  14   c . The galvano mirror  16   a  in the extended end portion of the one of the arm members  14   a ,  14   b  and  14   c  directs the incident laser beam to a corresponding one of the pair of the objective lenses  20   a  and  20   b , and hence to a corresponding one of the recording layers  12   a  and  12   b  opposed to the objective lenses  20   a  and  20   b , respectively. 
     Further, also in this embodiment, the pair of inclined stationary mirrors  18   a  and  18   b , the pair of quartered detectors  22   a  and  22   b  and the objective lenses  20   a  and  20   b  in one of the extended end portions of the arm members  14   a ,  14   b  and  14   c  are aligned with corresponding optical elements in another extended end portion along the axis-of-rotation A of the optical disks  12 . 
     Modification of the Fourth Embodiment 
     FIG. 8A is a schematic longitudinal sectional view illustrating an optical pick-up apparatus  90  according to a modification of the fourth embodiment of the invention; and FIG. 8B is a schematic plan view illustrating the optical pick-up apparatus  90  of FIG. 8A according to the modification of the fourth embodiment. 
     In the modification of the fourth embodiment, structural elements similar to those employed in the second embodiment described with reference to FIGS. 5A to  5 C are denoted by corresponding reference numerals, and no detailed description will be given thereof. 
     Also in this modification, a laser beam is emitted from the light beam emitting/receiving unit  24  in a substantially horizontal direction along the recording layers  12   a  and  12   b  of the optical disks  12 . This beam is directed to a movable mirror  92  located at the same level as the proximal end of the arm member  14   c  which is the lowest one of the arm members  14   a ,  14   b  and  14   c . The movable mirror  92  has an axis-of-rotation  92   a  extending parallel to the horizontal direction, and is adapted to swing a laser beam from the light beam emitting/receiving unit  24  in the vertical direction along the axis-of-rotation A of the optical disks  12 . In this modification, the movable mirror  92  is a galvano mirror. 
     Plural pairs of stationary mirrors  100   a  and  100   b  are provided corresponding to the proximal ends of the arm members  14   a ,  14   b  and  14   c . Each pair of stationary mirrors  100   a  and  100   b  are arranged vertically adjacent to each other along the axis-of-rotation A of the optical disks  12 , and are directed horizontal. The movable mirror  92  selectively directs a laser beam from the light beam emitting/receiving unit  24 , to one of the plural pairs of stationary mirrors  100   a  and  100   b.    
     Each of the pair of stationary mirrors  100   a  and  100   b  reflects the laser beam from the movable mirror  92 , to an inclined stationary mirror  60  located in each of the extended end portions of the arm members  14   a ,  14   b  and  14   c , via a corresponding one of imaging lenses  64  provided for each of the arm members. 
     Each of the arm members  14   a ,  14   b  and  14   c  has two optical paths vertically arranged and extending from the movable mirror  92  through the pair of stationary mirrors  100   a  and  100   b , respectively. A laser beam, which passes through the upper optical path, is reflected first by the inclined stationary mirror  60  then by the upper inclined reflection surface of a prism  62 , and is then directed to the recording layer  12   b  provided on the lower surface of the optical disks  12 , via the quartered detector  22   a  (see FIG. 5C) and the objective lens  20   a  which are located above the prism  62 . A laser beam, which passes through the lower optical path, it is reflected first by the inclined stationary mirror  60  then by the lower inclined reflection surface of the prism  62 , and is then directed to the recording layer  12   a  provided on the upper surface of the optical disks  12 , via the quartered detector  22   b  (see FIG. 5C) and the objective lens  20   b  which are located below the prism  62 . The structure of the optical circuit in the extended end portion of each of the arm members  14   a ,  14   b  and  14   c  is completely the same as that of the optical circuit employed in the extended end portion of each of the arm members  14   a ,  14   b  and  14   c  of the optical pick-up apparatus  50  according to the second embodiment, which was described with reference to FIGS. 5A,  5 B and SC. 
     In the above-described optical pick-up apparatus  90  according to the modification of the fourth embodiment, the laser beam emitted from the light beam emitting/receiving unit  24  is selectively guided, by the movable mirror  90  opposed to the light beam emitting/receiving unit  24 , to one of the two optical paths which are directed, via the pair of stationary mirrors  100   a  and  100   b , respectively, to the stationary mirror  60  and the prism  62  located in the extended end portion of one of the arm members  14   a ,  14   b  and  14   c . Further, the prism  62  directs the laser beam having passed through one of the two optical paths, to a corresponding one of the pair of the objective lenses  20   a  and  20   b , and then to a corresponding one of the recording layers  12   a  and  12   b  opposed to the pair of objective lenses  20   a  and  20   b , respectively. 
     It is preferable also in this modification that the optical path length from the light beam emitting/receiving unit  24  to the recording layer  12   a  or  12   b  via the corresponding objective lens  20   a  or  20   b  included in each of the arm members  14   a ,  14   b  and  14   c , and the arrangement of the optical elements along the optical paths should be set so that laser beams guided to the recording layers  12   a  and  12   b  via the respective objective lenses  12   a  and  12   b  can have the same optical characteristics, and so that reflected laser beams returned from the recording layers  12   a  and  12   b  to the light beam emitting/receiving unit  24  can have the same optical characteristics. 
     Further, also in this modification, the inclined stationary mirror  60 , the prism  62 , the pair of quartered detectors  22   a  and  22   b  and the objective lenses  20   a  and  20   b  in one of the extended end portions of the arm members  14   a ,  14   b  and  14   c  are aligned with corresponding optical elements in another extended end portion along the axis-of-rotation A of the optical disks  12 . 
     Fifth Embodiment 
     FIG. 9A is a schematic longitudinal sectional view illustrating an optical pick-up apparatus  110  according to a fifth embodiment of the invention; FIG. 9B is a schematic horizontal sectional view illustrating the optical pick-up apparatus  110  of FIG. 9A; and:FIG. 9C is another schematic horizontal sectional view of the optical pick-up apparatus  110  of FIG.  9 A. 
     In the fifth embodiment, structural elements similar to those employed in the first embodiment described with reference to FIGS. 1 to  4  are denoted by corresponding reference numerals, and no detailed description will be given thereof. 
     In this embodiment, a laser beam is emitted from the light beam emitting/receiving unit  24  via a relay lens  51  in a substantially horizontal direction along the recording layers  12   a  and  12   b  of the optical disks  12 . This laser beam is directed to a galvano mirror  112  located at the same level as the proximal end of the arm member  14   b  which is the middle one of the arm members  14   a ,  14   b  and  14   c . The galvano mirror  112  has a first axis-of-rotation  112   a  extending parallel to the axis-of-rotation A of the optical disks  12 , and is adapted to vertically and horizontally swing a laser beam incident thereon. 
     The galvano mirror  112  selectively creates one of two optical paths which are directed to the extend end portion of the middle arm member  14   b , when it swings about the first axis-of-rotation  112   a  by a predetermined distance. The two optical paths are horizontally separated from each other by a predetermined distance as shown in FIG.  9 C. 
     One of the two optical paths directly linearly extends from the galvano mirror  112  to the extended end portion, while the other linearly extends to the extended end portion after being bent by a stationary mirror  114 . When the laser beam from the galvano mirror  112  is guided through any one of the two optical paths, it passes through an imaging lens  74  and then reaches a corresponding one of a pair of inclined stationary mirrors  18   a  and  18   b , which are horizontally located in the extended end portion of the arm member  14   b  with a predetermined distance therebetween. The pair of inclined stationary mirrors  18   a  and  18   b  incline in opposite directions. The corresponding one of the pair of inclined stationary mirrors  18   a  and  18   b  directs the laser beam to a corresponding one of the recording layers  12   a  and  12   b  of corresponding ones of the optical disks  12  via a corresponding one of the quartered detectors  22   a  and  22   b  and a corresponding one of the objective lenses  20   a  and  20   b.    
     As is shown in FIGS. 9A and 9B, the galvano mirror  112  selectively directs the laser beam to one of a pair of stationary mirrors  116   a  and  116   b  which are horizontally located at the same level as the upper arm member  14   a  with a predetermined distance therebetween, when it swings about the first axis-of-rotation  112   a  and a second axis-of-rotation  112   b  by predetermined distances. The pair of stationary mirrors  116   a  and  116   b  are inclined toward the extended end portion of the upper arm member  14   a.    
     One of the pair of stationary mirrors  116   a  and  116   b  directs the laser beam via the imaging lens  74 , included in the upper arm member  14   a  to a corresponding one of a pair of inclined stationary mirrors  18   a  and  18   b  which are horizontally located with a predetermined distance therebetween in the extended end portion of the upper arm member  14   a . The pair of inclined stationary mirrors  18   a  and  18   b  incline in opposite directions. The corresponding one of the pair of inclined stationary mirrors  18   a  and  18   b  directs the laser beam to a corresponding one of the recording layers  12   a  and  12   b  of corresponding ones of the optical disks  12  via a corresponding one of the quartered detectors  22   a  and  22   b  and a corresponding one of the objective lenses  20   a  and  20   b.    
     Furthermore, as is shown in FIGS. 9A and 9B, the galvano mirror  112  selectively directs the laser beam to one of a pair of stationary mirrors  118   a  and  118   b  which are horizontally located at the same level as the lower arm member  14   c  with a predetermined distance therebetween, when it swings about the first axis-of-rotation  112   a  and the second axis-of-rotation  112   b  by predetermined distances. The pair of stationary mirrors  118   a  and  118   b  are inclined toward the extended end portion of the lower arm member  14   c.    
     One of the pair of stationary mirrors  118   a  and  118   b  directs the laser beam via the imaging lens  74  included in the lower arm member  14   c  to a corresponding one of a pair of inclined stationary mirrors  18   a  and  18   b  which are horizontally located with a predetermined distance therebetween in the extended end portion of the lower arm member  14   c . The pair of inclined stationary mirrors  18   a  and  18   b  incline in opposite directions. The corresponding one of the pair of inclined stationary mirrors  18   a  and  18   b  directs the laser beam to a corresponding one of the recording layers  12   a  and  12   b  of corresponding ones of the optical disks  12  via a corresponding one of the quartered detectors  22   a  and  22   b  and a corresponding one of the objective lenses  20   a  and  20   b.    
     As described above, in this embodiment, a laser beam is guided by the single galvano mirror  112  to a desired one of the recording layers  12   a  and  12   b  of the optical disks  12  via a desired one of the objective lenses  20   a  and  20   b  of a desired one of the arm members  14   a ,  14   b  and  14   c , as a result of only changing the swing angle of the single galvano mirror  112  about the first axis-of-rotation  112   a  and/or the second axis-of-rotation  112   b , and using the functions of the aforementioned plural stationary mirrors for the arm members  14   a ,  14   b  and  14   c . Thus, information can be recorded onto or reproduced from a desired recording layer using a laser beam reflected from the single galvano mirror  112 . 
     When the optical pick-up apparatus  110  of the fifth embodiment which uses the galvano mirror  112  with the first and second axes-of-rotations  112   a  and  112   b , as described above with reference to FIGS. 9A to  9 C, is compared with the optical pick-up apparatus  70  of the third embodiment shown in FIGS. 6A and 6B, which performs similar operations to the optical pick-up apparatus  110  of the fifth embodiment, using the galvano mirror  78  which has only one axis-of-rotation  72   a  extending along the axis-of-rotation A of the optical disks  12 , the number of stationary mirrors required in the former apparatus from the galvano mirror  112  to a desired one of the objective lenses  20   a  and  20   b  of a desired one of the arm members  14   a ,  14   b  and  14   c  is smaller, and accordingly the size of the housing when seen in plan view is also smaller than in the case of the latter apparatus. 
     It is preferable also in this embodiment that the optical path length from the light beam emitting/receiving unit  24  to the recording layer  12   a  or  12   b  via the corresponding objective lens  20   a  or  20   b  included in each of the arm members  14   a ,  14   b  and  14   c , and the arrangement of the optical elements along the optical paths should be set so that laser beams guided to the recording layers  12   a  and  12   b  via the respective. objective lenses  12   a  and  12   b  can have the same optical characteristics, and so that reflected laser beams returned from the recording layers  12   a  and  12   b  to the light beam emitting/receiving unit  24  can have the same optical characteristics. 
     Further, also in this embodiment, the pair of inclined stationary mirrors  18   a  and  18   b , the pair of quartered detectors  22   a  and  22   b  and the objective lenses  20   a  and  20   b  in one of the extended end portions of the arm members  14   a ,  14   b  and  14   c  are aligned with corresponding optical elements in another extended end portion along the axis-of-rotation A of the optical disks  12 . 
     FIG. 10A is an enlarged front view of the galvano mirror  112  which has the first and second axes-of-rotation  112   a  and  112   b ; and FIG. 10B is an enlarged longitudinal sectional view of the galvano Mirror  112  of FIG.  10 A. 
     The galvano mirror  112  comprises a cubic outer frame  112   c , a mirror support member  112   e  located in the central opening of the outer frame  112   c  and supported by an elastic member  112   d  like a bar spring fixed on the center of a wall which constitutes the bottom of the central opening, four magnets  112   f  fixed on the four sides of the inner peripheral surfaces of the outer frame  112   c , and four coils  114   g  provided on those four portions of the outer peripheral surfaces of the mirror support member  112   e  which are opposed to the four magnets  112   f . A damping material  114   h  is attached to the periphery of the elastic member  112   d  between the bottom wall of the central opening of the outer frame  112   c  and the bottom wall of the mirror support member  112   e.    
     The galvano mirror  112  constructed as above can be swung through a desired swing angle about the first axis-of-rotation  112   a  by controlling the intensity and direction of a current to be flown into a pair of coils  114   g  located in the direction of the second axis-of-rotation  112   b , while it can be swung through a desired swing angle about the second axis-of-rotation  112   b  by controlling the intensity and direction of a current to be flown into another pair of coils  114   g  located in the direction of the first axis-of-rotation  112   a.    
     Sixth Embodiment 
     FIG. 11 is a schematic longitudinal sectional view of an optical pick-up apparatus  120  according to a sixth embodiment of the invention. 
     In the sixth embodiment, structural elements similar to those employed in the first embodiment described with reference to FIGS. 1 to  4  are denoted by corresponding reference numerals, and no detailed description will be given thereof. 
     In this embodiment, a laser beam is downwardly emitted from the light beam emitting/receiving unit  24  along the axis-of-rotation A of the optical disks  12 . This laser beam is directed to a galvano mirror  122  located at the same level as the proximal end of the arm member  14   b  which is the middle one of the arm members  14   a ,  14   b  and  14   c . The galvano mirror  122  has an axis-of-rotation  122   a  extending along the axis-of-rotation A of the optical disks  12 , and is adapted to horizontally swing a laser beam incident thereon. 
     The galvano mirror  122  selectively creates one of two optical paths which are directed to the extend end portion of the middle arm member  14   b  and horizontally separated from each other by a predetermined distance. When the laser beam from the galvano mirror  122  is guided through any one of the two optical paths, it passes through an imaging lens  74  in the middle arm member  14   b  and then reaches a corresponding one of a pair of inclined stationary mirrors  18   a  and  18   b , which are horizontally located in the extended end portion of the arm member  14   b  with a predetermined distance therebetween. The pair of inclined stationary mirrors  18   a  and  18   b  incline in opposite directions. The laser beam is then guided by a corresponding one of the pair of inclined stationary mirrors  18   a  and  18   b  to a corresponding one of the recording layers  12   a  and  12   b  of corresponding ones of the optical disks  12  via a corresponding one of the quartered detectors  22   a  and  22   b  and a corresponding one of the objective lenses  20   a  and  20   b.    
     As shown in FIG. 11, the galvano mirror  122  also selectively creates one of two optical paths which are directed to a single first stationary mirror  124  located on one side of the proximal end of the middle arm member  14   b  in the horizontal direction. The single first stationary mirror  124  inclines upward for the upper arm member  14   a . A single second stationary mirror  126  is provided above the single first stationary mirror  124  as the same level as the upper arm member  14   a , and inclines toward the extended end portion of the upper arm member  14   a.    
     Any selected one of the optical paths from the galvano mirror  122  to the single first stationary mirror  124  further extends from the first stationary mirror  124  to the single second stationary mirror  126 . Any one of the two optical paths extends from the single second stationary mirror  126  via an imaging lens  74  in the upper arm member  14   a  to a corresponding one of a pair of inclined stationary mirrors  18   a  and  18   b  which are horizontally located with a predetermined distance therebetween in the extended end portion of the upper arm member  14   a . The pair of inclined stationary mirrors  18   a  and  18   b  incline in opposite directions. The corresponding one of the pair of inclined stationary mirrors  18   a  and  18   b  directs the laser beam to a corresponding one of the recording layers  12   a  and  12   b  of corresponding ones of the optical disks  12  via a corresponding one of the quartered detectors  22   a  and  22   b  and a corresponding one of the objective lenses  20   a  and  20   b.    
     As shown in FIG. 11, the galvano mirror  122  further selectively creates one of two optical paths which are directed to a single third stationary mirror  128  located on the other side of the proximal end of the middle arm member  14   b  in the horizontal direction. The single third stationary mirror  128  inclines downward for the lower arm member  14   c.    
     A single fourth stationary mirror  130  is provided below the single third stationary mirror  128  as the same level as the lower arm member  14   c , and inclines toward the extended end portion of the lower arm member  14   c.    
     Any selected one of the optical paths from the galvano mirror  122  to the single third stationary mirror  128  further extends from the third stationary mirror  128  to the single fourth stationary mirror  130 . Any one of the two optical paths extends from the single fourth stationary mirror  130  via an imaging lens  74  in the lower arm member  14   c  to a corresponding one of a pair of inclined stationary mirrors  18   a  and  18   b  which are horizontally located with a predetermined distance therebetween in the extended end portion of the lower arm member  14   c . The pair of inclined stationary mirrors  18   a  and  18   b  incline in opposite directions. The corresponding one of the pair of inclined stationary mirrors  18   a  and  18   b  directs the laser beam to a corresponding one of the recording layers  12   a  and  12   b  of corresponding ones of the optical disks  12  via a corresponding one of the quartered detectors  22   a  and  22   b  and a corresponding one of the objective lenses  20   a  and  20   b.    
     As described above, in this embodiment, a laser beam is guided by the single galvano mirror  122  to a desired one of the recording layers  12   a  and  12   b  of the optical disks  12  via a desired one of the objective lenses  20   a  and  20   b  of a desired one of the arm members  14   a ,  14   b  and  14   c , as a result of only changing the swing angle of the single galvano mirror  122 , and using the functions of the aforementioned plural stationary mirrors for the arm members  14   a ,  14   b  and  14   c . Thus, information can be recorded onto or reproduced from a desired recording layer using a laser beam reflected from the single galvano mirror  72 . 
     It is preferable also in this embodiment that the optical path length from the light beam emitting/receiving unit  24  to the recording layer  12   a  or  12   b  via the corresponding objective lens  20   a  or  20   b  included in each of the arm members  14   a ,  14   b  and  14   c , and the arrangement of the optical elements along the optical paths should be set so that laser beams guided to the recording layers  12   a  and  12   b  via the respective objective lenses  12   a  and  12   b  can have the same optical characteristics, and so that reflected laser beams returned from the recording layers  12   a  and  12   b  to the light beam emitting/receiving unit  24  can have the same optical characteristics. 
     Further, also in this embodiment, the pair of inclined stationary mirrors  18   a  and  18   b , the pair of quartered detectors  22   a  and  22   b  and the objective lenses  20   a  and  20   b  in one of the extended end portions of the arm members  14   a ,  14   b  and  14   c  are aligned with corresponding optical elements in another extended end portion along the axis-of-rotation A of the optical disks  12 . 
     Although the above-described optical pick-up apparatuses of the various embodiments record information onto or reproduce information from a recording layer of an optical disk such as a CD (Compact Disk) or a DVD (Digital Video Disk) as a result of radiating the recording layer of the optical disk with a light beam via an objective lens opposed to the recording layer, they can be used to record information onto or reproduce information from a recording layer of an optical disk by further converging a light beam from the objective lens using a solid immersion lens as known from Japanese Patent Application KOKAI Publication No. 5-189796, before radiating the recording layer of the optical disk with the light beam. 
     Such a technique of optically recording/reproducing information as using a combination of an objective lens and a solid immersion lens is known from, for example, “Nikkei Byte”, September 1997 or “Nikkei Electronics”, Sep. 22, 1997. The objective lens and the solid immersion lens are mounted on a slider which will float above the surface of the recording layer of the optical disk due to a wind occurring in accordance with the rotation of the optical disk, as in the case of the magnetic head of the conventional magnetic hard disk. The slider floats substantially 100 nm to 150 nm above the recording layer. The conventional optical recording/reproducing method in which the optical head is separated by 1 mm or more from the surface of the optical disk such as the CD or the DVD is referred to as “Far Field Recording”, while the optical recording/reproducing method using the aforementioned floating-type combination of the solid immersion lens and the objective lens is referred to as “Near Field Recording”. In the near field recording, a laser beam to be used to record or reproduce information can be made to have a spot size substantially one tenth that of a laser beam used in the far field recording. Accordingly, the recording density in the near field recording can be substantially ten times that in the far field recording. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the present invention in its broader aspects is not limited to the specific details, representative devices, and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept at defined by the appended claims and their equivalents.