Patent Publication Number: US-2005128895-A1

Title: Optical head and optical disc apparatus equiped with optical head

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a divisional of U.S. application Ser. No. 09/695,284, filed Oct. 25, 2000, which claims the benefit of priority from Japanese Patent Application No. 11-304021, filed Oct. 26, 1999, and Japanese Patent Application No. 11-309717, filed Oct. 29, 1999, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates to an optical head for recording data in a recording medium of an optical disc by using a laser beam or for reproducing the data recorded in an optical disc and an optical disc apparatus equipped with the particular optical head.  
      An optical head is constructed such that, when data are recorded, a recording surface of an optical disc is irradiated with a laser beam having a plurality of predetermined light intensities so as to change the structure of the recording surface of the optical disc, thereby forming recording marks having a plurality of different reflectances so as to record the data.  
      The recording method includes, for example, a phase change recording method in which the phase of the recording surface of an optical disc is changed so as to change the level of the reflected light in a plurality of graduations, and a chromatic change type recording method in which a photosensitive chromatic is arranged on the recording surface of the optical disc and the level of the reflected light is made binary by the color development of the chromatic.  
      The construction of the known optical head and the recording-reproducing principle thereof will now be described with reference to  FIGS. 10A and 10B .  FIG. 10A  shows the state of a known optical head as viewed in a planar direction, i.e., in a direction perpendicular to the recording surface of the optical disc, and  FIG. 10B  shows the state of the known optical head as viewed in a direction parallel to the recording surface of the optical disc.  
      As shown in  FIGS. 10A and 10B , an optical head  900  includes a base  901 . Arranged within the base  901  is a laser diode  91  emitting a laser beam of a predetermined wavelength. The laser beam emitted from the laser diode  91  is reflected from the recording surface of an optical disc and the reflected light is received by a light detector  92 . The laser diode  91  and the light detector  92  collectively form an IOU (Integrated Optical Unit)  90 . Also arranged within the base  901  are a beam splitter  93  arranged on the optical path of the laser beam emitted from the laser diode  91  of the IOU  90 , a collimator  95  for converting the laser beam passing through the beam splitter  93  into a parallel light, a mirror block  96  for reflecting the laser beam collimated by the collimator  95  in a direction of right angles, and an objective lens  97  for collecting the laser beam reflected from the mirror block  96  on a predetermined position of a recording surface of the optical disc. Also arranged is a lens  94  for collecting the laser beam reflected in a direction of right angles from the laser beam running toward the collimator  95  on a light receiving surface of the light detector  92 . It follows that an optical path for irradiating an optical disc in a predetermined position with a laser beam is formed between the laser diode  91  (IOU) and the objective lens  97 .  
      It should be noted that a laser driving circuit member  98  for driving the laser diode  91  of the IOU  90  and the light detector  92  are fixed to the outer wall of the base  901 .  
      Data are recorded by the optical head  900  as follows.  
      Specifically, the laser diode  91  is operated on the basis of the driving signal generated from the laser driving circuit member  98 . The laser beam emitted from the laser diode  91  is incident on the beam splitter  93 . A predetermined proportion of the laser beam incident on the beam splitter  93  passes through the beam splitter  93  so as to be guided to the mirror block  96 . The beam guided to the mirror block  96  is reflected toward the objective lens  97  so as to be collected on a predetermined position on the recording surface of an optical disc D. On the other hand, that portion of the laser beam which is incident on the beam splitter  93  and is not reflected toward the mirror block  96  is reflected toward the monitor light detector  92 .  
      The monitor light detector  92  is arranged to receive a part of the laser beam emitted from the laser diode in order to detect the light intensity of the laser beam irradiating the optical disc. The system of detecting the light intensity of the laser beam running forward toward the optical disc, not for detecting the monitor laser light inherent in the laser diode emitted rearward of the laser diode (light source), is called a front monitor system, which is a construction inherent in the recording optical head  900  in which it is necessary to supervise strictly the intensity of the laser beam in recording a digital signal in an optical disc. To be more specific, since the rise and fall of a recording laser beam for recording a digital signal are required to form an ideal rectangular wave, the light amount of the laser beam emitted from the laser diode is detected and fed back so as to have the output of the laser diode kept controlled.  
      The optical head  900  shown in  FIGS. 10A and 10B  is used in a relatively large optical disc apparatus and, thus, the optical head  900  itself is large. Therefore, the laser driving circuit member  98  and the monitor light detector  92  can be mounted to the outer wall of the base  901 .  
      However, in accordance with requirement for miniaturization of the optical disc apparatus, the optical head  900  is also miniaturized. As a result, the size of the part that can be mounted to the housing member corresponding to the base  901  is limited.  
      Particularly, when it comes to the optical disc apparatus used in a notebook type personal computer, the thickness of the optical disc apparatus is required to be further decreased, making it necessary to limit the height of the optical head  900  to a very small range. As a result, the mounting layout in mounting the peripheral parts of the optical head is greatly restricted.  
      Also, since the optical disc of a DVD type has been put to a practical use, it is desired that the recording of data on optical discs such as a CD-R and a CD-DW capable of reproducing a CD disc for music or capable of recording data with a CD type optical disc be performed in a single optical disc in an optical disc apparatus capable of reproducing a DVD type optical disc. Under the circumstances, the optical head is required to be further miniaturized and to be made thinner.  
     BRIEF SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a miniaturized optical head capable of overcoming the restriction in the mounting layout of parts and suitable for mounting in a thin optical disc apparatus.  
      Another object of the present invention is to provide an optical head, which can be housed in a thin optical disc apparatus, the optical head being capable of reproducing a DVD disc and a CD disc, and capable of recording data in optical discs in which data can be recorded such as a CD-R disc and a CD-RW disc.  
      According to a first aspect of the present invention, there is provided an optical head device comprising: 
          a light source for emitting a light beam having a predetermined wavelength;     a laser driving circuit member for driving the light source for allowing the light source to emit the light beam;     a monitor light detecting part for detecting the light amount of the light beam emitted from the light source;     an objective lens for collecting the light beam on a predetermined position of the optical disc;     a light receiving element for receiving the light beam reflected from the optical disc and for converting the received light beam into an electric signal;     a base having an open portion and holding an optical member arranged in the open portion for guiding the light beam in a manner to form an optical path of the light beam from the light source to the objective lens;     a holding member for holding the monitor light detecting part within the open portion of the base in parallel to the optical path and in a manner not to interfere with the light beam;     a guide member for guiding the optical head in the radial direction of the optical disc; and     a disk motor for rotating the optical disk by predetermined speed.        

      According to a second aspect of the present invention, there is provided an optical head used in an optical disc apparatus in which an optical disc is irradiated with a light beam for recording data in the optical disc or for reproducing data from the optical disc, at least one of a circuit member and an element for reproducing or recording data being housed in the optical head, comprising: 
          a light source for emitting a light beam having a predetermined wavelength;     an objective lens for collecting the light beam for irradiating the optical disc with the light beam;     a driving mechanism for moving the objective lens in a predetermined direction for at least one of a focusing and a tracking to the optical disc;     a light receiving element for receiving the light beam reflected from the optical disc and for converting the receiving light beam into an electric signal;     a base having an open portion and holding an optical member arranged in the open portion for guiding the light beam in a manner to form an optical path of the light beam from the light source to the objective lens;     a light source driving circuit member which comprises the light source driving circuit for driving the light source;     a signal processing circuit member which comprises the signal processing circuit for processing the electric signal from the light receiving element;     a driving mechanism driving circuit for driving the driving mechanism; and     a holding member for holding at least one of the driving circuit member, the light source driving circuit member, the signal processing circuit member, and the driving mechanism driving circuit member within the open portion of the base in parallel manner to the optical path within the base and in a manner not to interfere with the optical path within the base.        

      Further, according to a third aspect of the present invention, there is provided an optical head, comprising: 
          an objective lens for irradiating an optical disc with a light beam and for receiving the light beam reflected from the optical disc;     a first light source for emitting a first light beam having a first wavelength;     a second light source for emitting a second light beam having a second wavelength;     an optical path synthesizing-separating element for allowing the first beam and the second beam to be incident on the objective lens and for separating beams reflected from the objective lens, the reflected beams corresponding to the first and second light beams, respectively;     first and second light detectors for detecting the first and second reflected beams, respectively, reflected from the optical path synthesizing-separating element; and     a converging optical system arranged between the second light source and the optical path synthesizing-separating element for diminishing the diverging angle of the diverging light flux emitted from the second light source so as to guide the light flux to the optical path synthesizing-separating element.        

      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 invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.  
       FIG. 1A  is a plan view showing an optical head according to one embodiment of the present invention, as viewed in a direction parallel to a recording surface of an optical disc;  
       FIG. 1B  is a side view showing the optical head shown in  FIG. 1A , as viewed in a direction perpendicular to the recording surface of the optical disc;  
       FIG. 2  schematically shows the outer appearance of a base forming the optical head body shown in  FIGS. 1A and 1B ;  
       FIG. 3  schematically shows the inner state of the base shown in  FIG. 2  as viewed from the rear side;  
       FIG. 4  shows in a dismantled fashion the base and the optical head arranged within the base, which are shown in  FIGS. 2 and 3 ;  
       FIG. 5  schematically shows an embodiment of the present invention where a laser driving circuit member is set on the base as viewed from the rear side, which is shown in  FIG. 3 ;  
       FIG. 6  is a side view of the base shown in FIGS.  2  to  5 ;  
       FIG. 7  is a plan view schematically showing an embodiment of the present invention where base shown in FIGS.  2  to  6  is incorporated in a motor base of an optical disc apparatus;  
       FIG. 8  schematically explains an embodiment of the present invention where the motor base shown in  FIG. 7  is assembled with the optical disc apparatus;  
       FIG. 9A  is a plan view showing an embodiment of the present invention where an optical head differing from the optical head shown in  FIGS. 1A and 13  is observed in a direction parallel to the recording surface of the optical disc;  
       FIG. 9B  is a side view showing an embodiment of the present invention where the optical head shown in  FIG. 9A  is observed in a direction perpendicular to the recording surface of the optical disc;  
       FIG. 10A  is a plan view showing an embodiment of the present invention where the optical head applied to a known large optical disc apparatus is observed in a direction parallel to the recording surface of the optical disc;  
       FIG. 10B  is a side view showing an embodiment of the present invention where the optical head shown in  FIG. 10A  is observed in a direction perpendicular to the recording surface of the optical disc;  
       FIG. 11  schematically explains the developed state of the main constituents which are extracted from the optical head shown in  FIGS. 1A, 1B ,  2  to  6 ,  9 A and  9 B;  
       FIG. 12  schematically illustrates the configuration of the main constituents of another optical head, which can be applied to the optical head shown in  FIGS. 1A, 1B ,  2  to  6 ,  9 A and  9 B;  
       FIG. 13A  is a graph showing the relationships between the optical magnification and the light utilization and between the optical magnification and the power of the laser beam emitted from the objective lens in the case of using the optical head shown in  FIG. 11  or  12 ;  
       FIG. 13B  is a graph showing the Gaussian distribution of the laser beam irradiating the optical disc in the case of using the optical head shown in  FIG. 11  or  12 ;  
       FIG. 14  is a graph showing the direction relative to the light source of the lens differing from the objective lens included in the optical head shown in  FIG. 11  or  12  and explaining the degree of influence of the wave front aberration in the case where a lens shift is imparted to the objective lens;  
       FIGS. 15A  to  15 D schematically explain the inclination of the lens differing from the objective lens included in the optical head shown in  FIG. 11  or  12  and the state of the ghost light caused by 0 order light of the laser beam or ±1 order light of the laser beam;  
       FIG. 15E  is a graph for explaining the magnitude of each of the ghost light beams caused by the conditions shown in  FIGS. 15A  to  15 D;  
       FIGS. 16A  to  16 C schematically explain the information recording surfaces of various optical discs in which data can be recorded and from which data can be reproduced by the optical head shown in  FIGS. 1A, 1B ,  9 A and  9 B; and  
       FIG. 17  is a block diagram schematically showing the signal processing system for obtaining a reproducing signal, a tracking signal and a focus signal from the reflected laser beam reflected from the optical disc obtained by using the optical head shown in  FIGS. 1A, 1B ,  9 A,  9 B,  11  or  12 .  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The construction of an optical head according to one embodiment of the present invention will now be described with reference to the accompanying drawings.  
       FIGS. 1A and 1B  collectively show an optical head according to one embodiment of the present invention. Specifically,  FIG. 1A  is a plan view of the optical head as viewed in a direction parallel to the recording surface of the optical disc, and  FIG. 1B  is a side view showing the state as viewed in a direction perpendicular to the recording surface of the optical disc.  
      As shown in  FIGS. 1A and 1B , an optical head  100  includes a base  101 , an IOU (Integrated Optical Unit)  10  arranged in a predetermined position within the base  101 , a laser diode  11  arranged within the IOU  10  to act as a first light source and emitting a laser beam (light beam) of a first wavelength, and a light detector  12 . The laser beam emitted from the laser diode  11  is reflected from a recording medium of an optical disc D, and the reflected laser beam is received by the light detector  12 . Upon receipt of the reflected laser beam, the light detector  12  produces an electric signal conforming with the intensity of the reflected laser beam. The base  101  includes an open portion  102  in which is formed an optical path for transmitting the laser beam of the first wavelength emitted from the laser diode  11  of the IOU  10  to the objective lens  13  for collecting the laser beam on a recording surface of the optical disc D.  
      Arranged within the open portion  102  along the optical path extending from the laser diode  11  within the IOU  10  to the objective lens  13  are a beam splitter  14 , a collimator  15  for converting the laser beam passing through the beam splitter  14  into a parallel light, and a mirror block  16  for reflecting the laser beam collimated by the collimator  15  in substantially right angles toward the objective lens  13 . The laser beam of the first wavelength emitted from the laser diode  11  has a wavelength of, for example, 780 nm and can be used for reproducing data from a known CD type optical disc and for recording data in a CD-R disc or a CD-RW disc.  
      On the other hand, a second IOU  20  for DVD is arranged in a predetermined position within the open portion  102  of the base  101  in a direction substantially perpendicular to the line joining the laser diode  11  of the IOU  10 , the light detector  12  and the beam splitter  14 . The second IOU  20  has a laser diode  21  emitting a laser beam having a second wavelength, which can be applied to an optical disc of a DVD type, and a light detector  22  formed integrally with the laser diode  21 . The laser beam of the second wavelength emitted from the laser diode  21  is reflected from the optical disc and the reflected second laser beam is received by the light detector  22 .  
      The laser beam emitted from the second laser diode within the IOU  20  for DVD has a wavelength of, for example, 650 nm. The laser beam is reflected by the beam splitter  14  toward the collimator  15  while gradually diverging along the optical path within the open portion  102  and, then, collimated by the collimator  15  so as to be guided to the mirror block  16 . It follows that both the laser beam having a wavelength of 780 nm, which is emitted from the laser diode  11  of the IOU  10  for CD described previously, and the laser beam having a wavelength of 650 nm, which is emitted from the laser diode  21  of the IOU  20  for DVD, are incident on the objective lens  13 .  
      As described above, the optical path within the open portion  102  is formed by the beam splitter  14 , the collimator  15  and the mirror block  16 .  
      A monitor light detector  17 , which receives a part of the laser beam emitted from the laser diode  11  of the IOU  10  for CD for detecting the light amount of the laser beam irradiating the optical disc, is arranged within the open portion  102  of the base  101 .  
      As apparent from  FIG. 1B , the monitor light detector  17  is fixed to a ceiling portion positioned closer to the optical disc than the beam splitter  14  within the open portion  102  (optical path). On the other hand, a laser driving circuit member  18  for driving the laser diode  11  within the IOU  10  for CD is held in a floor portion positioned farther from the optical disc than the beam splitter  14  within the open portion (optical path)  102 , as apparent from  FIG. 1B .  
      The principle of data reproduction from a DVD type disc and a CD type disc by the optical head  100  shown in  FIGS. 1A and 1B  will now be described.  
      The optical head shown in  FIGS. 1A and 1B  is an interchangeable head capable of reproducing data from optical discs (DVD, CD) differing from each other in standards.  
      The format of the CD type is capable of application to a CD-R and a CD-RW capable of recording. Where CD-R or CD-RW is mounted, the laser beam for CD permits recording data by changing the structure of the recording surface to cause the reflected light to have two different intensities.  
      The principle of reproducing a DVD disc will now be described first.  
      The laser beam emitted from the laser diode  212  included in the IOU  20  for DVD is reflected by the beam splitter  14  having a dichroic film capable of reflecting the laser beam having the second wavelength (650 nm) formed thereon so as to be guided to the collimator  15 . The laser beam guided to the mirror block  16  is reflected by the mirror surface of the mirror block  16  toward the objective lens  13  so as to be imparted with predetermined converging properties by the objective lens  13  and, then, collected on a predetermined position (pit column) on the recording surface of the optical disc D, which is a DVD type disc.  
      If the optical disc D conforms with DVD standards, the laser beam collected on the recording surface is reflected by the recording surface of the optical disc D so as to be brought back to the objective lens  13 . The laser beam is then converted into a parallel light by the objective lens  13  and, then, further reflected by the mirror block  16  so as to be brought back to the beam splitter  14 . The reflected laser beam that is brought back to the beam splitter  14  is reflected toward the IOU  20  for DVD by the function of the dichroic film of the beam splitter  14 .  
      The reflected laser beam guided to the IOU  20  for DVD is received by the light detector  22  of the IOU  20  for DVD so as to be converted into an electric signal. Then, the electric is processed in a signal processing circuit shown in, for example,  FIG. 17 , so as to be converted into an RF signal (reproduction signal), a focus error signal and a tracking error signal. These converted signals are output. Incidentally, the RF signal is output from the optical head so as to be reproduced as a data signal by a digital signal processing circuit (not shown) within the optical disc apparatus. On the other hand, the focus error signal and the tracking error signal are utilized for the known focus control for aligning the distance between the position of the objective lens  13  and the recording surface of the optical disc D with the focus point of the objective lens  13  and for the known tracking control for aligning the center of the laser beam passing through a predetermined position of the objective lens  13  so as to be collected on the recording surface of the optical disc D with the center of the pit column formed on the recording surface.  
      The principle of reproducing the data signal from a CD type disc or recording the data signal in a CD-R disc or a CD-RW disc will now be described.  
      Specifically, the laser beam emitted from the laser diode  11  included in the IOU  10  for CD is transmitted through the beam splitter  14  provided with a dichroic film capable of transmitting the laser beam having the first wavelength (780 nm) and, then, converted into a substantially parallel light by the collimator  15  so as to be guided to the mirror surface of the mirror block  16 .  
      The laser beam guided to the mirror block  16  is reflected toward the objective lens  13  so as to be imparted with predetermined converging properties by the objective lens  13  and, thus, to irradiate the recording surface of the optical disc D. Therefore, if the optical disc D conforms with the CD standards, the irradiating laser beam is reflected by the recording surface of the optical disc D so as to be brought back to the objective lens  13 . Then, the light is converted into a substantially parallel reflected laser beam so as to be brought back to the mirror block  16 .  
      The reflected laser beam guided to the mirror block  16  passes through the beam splitter  14  so as to be incident on the light detector  12  included in the IOU  10  for CD.  
      The reflected laser beam guided to the light detector  12  included in the IOU  10  for CD is converted into an electric signal by the light detector  12  and, then, processed by the signal processing circuit shown in, for example,  FIG. 17 , so as to generate predetermined electric signals capable of forming an RF signal (reproduction signal), a focus error signal and a tracking error signal.  
      The above description covers the case where the optical disc D is a known CD type optical disc. The case of recording data in an optical disc conforming with the CD-R or CD-RW standards is equal to the above case in that the laser beam having the first wavelength (780 nm), which is emitted from the laser diode  11  included in the IOU  10  for CD, is transmitted through the beam splitter  14 , the collimator  15  and the mirror block  16  so as to irradiate the optical disc D. Where the optical disc D is a CD-R disc, the intensity of a laser beam of a recording power having the maximum light intensity higher than that of the laser beam irradiating the optical disc D of CD type is modulated by the driving circuit member  18  in accordance with the data to be recorded and, then, the chromatic film on the recording surface of the optical disc D is irradiated with the laser beam having the maximum light intensity modulated by the driving circuit member  18 . As a result, changes are generated in the chromatic film so as to record the data. Similarly, in the case of the CD-RW disc, phase changes are generated in that portion of the recording film which is irradiated with the laser beam of the recording power so as to record the data.  
      As described above, the optical head  100  shown in  FIGS. 1A and 1B  is capable of coping with discs of different formats, and, thus, is configured so that a plurality of optical systems such as IOU are housed in a single base  101 . Therefore, when the head is miniaturized, the layout of the parts housed within the optical head  100  is restricted.  
      In the optical head  100 , the open portion  102  in which the optical path is arranged is increased with an increase in the optical path. The open portion is a relatively large space within the optical head  100 , and components other than the optical components forming the optical path are not disposed in the open portion.  
      Under the circumstances, in the optical head  100  shown in  FIGS. 1A and 1B , attention is paid to the space in which is arranged the optical path of the laser beam ranging between the laser diode  11  and the objective lens  13 , and members for controlling the data recording in and data reproduction from the optical disc D are arranged appropriately in the open portion  102 , in which the optical path is arranged, of the optical head  100 .  
      To be more specific, the main members arranged within the open portion  102  are the laser driving circuit member  18  for driving the laser diode  11  and the monitor light detector  17 .  
      The laser driving circuit member  18  and the monitor light detector  17  are arranged within the open portion  102  substantially in parallel to the optical path of the laser beam so as not to intercept the optical path of the laser beam. Incidentally, in the optical head shown in  FIGS. 1A and 1B , the laser driving circuit member  18  is arranged farther from the optical disc D than the beam splitter  14 , which is arranged in substantially the center of the open portion  102 . On the other hand, the monitor light detector  17  is arranged closer to the optical disc D than the beam splitter  14 , which is arranged in substantially the center of the open portion  102 .  
      As described above, in the optical head shown in  FIGS. 1A and 1B , the free space in which the optical path is formed is effectively utilized so as to overcome the restriction in the layout of the members arranged within the optical head  100 . As a result, the optical head  100  can be miniaturized while decreasing the thickness thereof. Incidentally, in the optical head  100  described above, the monitor light detector  17  and the laser driving circuit member  18  are arranged within the open portion  102 . It should be noted in this connection that the technical idea of the present invention is to arrange the members for controlling the data reproduction and recording within the open portion  102  so as not to intercept the laser beam. It follows that it is possible to arrange within the open portion  102  other members such as an actuator driver for changing the position of the objective lens  13  and a signal processing circuit.  
      Also, in the optical head  100  shown in  FIGS. 1A and 1B , the laser beam emitted from the laser diode  11  of the IOU  10  for CD is gradually diverged along the optical path. In other words, the laser beam has a small diameter on the emitting side near the light source. Therefore, relatively large members can be arranged if the monitor light detector  17 , the laser driving circuit member  18 , the actuator driver or the signal processing circuit are arranged in the space on the emitting side of the open portion  102  near the light source.  
      FIGS.  2  to  6  schematically show another embodiment of the optical head shown in  FIGS. 1A and 1B .  
      As shown in  FIG. 2 , in an optical head  200 , an objective lens  201  for collecting the laser beam emitted from the laser diode on a predetermined position on the recording surface of the optical disc D, a member held a monitor light detector  202  and an IOU  203  for CD are mounted to a base  216 , which is a main body portion of the optical head.  
      A plate-like (claw-like) guide bearing  302   b  arranged on one edge portion of the base  216  and a cylindrical guide bearing  303   b  arranged on the other edge of the base  216  are coupled with guide shafts (not shown) so as to hold the optical head  200  such that the optical head  200  is movable to an optional position in a radial direction of the optical disc D. The monitor light detector  202  is fixed to the base  216  by a screw  206   a  with a flexible printed circuit board  206  interposed there between. A holding plate  206   b  is mounted on the flexible printed circuit board  206  in order to suppress the rocking of the optical head  200  when the optical head  200  is moved along a guide shaft (not shown). The holding plate  206   b  is fixed together with the printed circuit board  206  to the base  216  by the screw  206   a.    
       FIG. 3  shows the inner structure of the optical head covering the state that the inner region of the base shown in  FIG. 2  is viewed from the rear side. As apparent from  FIG. 3 , an IOU  211  for DVD, an IOU  203  for CD, a coupling lens  215 , a beam splitter  208 , a collimator  210 , a mirror block  212 , and a yoke  213  constituting an actuator are arranged within an open portion  207  of the base  216 . Incidentally, screw holes  218   b  for receiving screws (not shown), which is for fixing a metal cover (not shown), are formed on the side of the IOU  211  for DVD of the base  216 .  
      Upon receipt of a driving signal given by an actuator driver (not shown), the actuator  213  drives the objective lens  201  in each of the focus direction and the track direction of the optical disc D.  
      A laser diode (not shown) included in the IOU  211  for DVD emits a laser beam Bm having a wavelength of 650 nm toward the optical disc D of DVD standards.  
      The emitted laser beam Bm, which is a diverging beam, is transmitted through the beam splitter  208 , collimated by the collimator  210  and, then, is incident on the mirror block  212 . The laser beam Bm guided to the mirror block  212  is reflected by the mirror block  212  toward an objective lens  201  shown in  FIG. 2 . Then, the laser beam BM is collected by the objective lens  201  on a predetermined position on the recording surface of the optical disc D.  
      The laser beam irradiating the optical disc D is reflected from the recording surface of the optical disc D so as to be incident again on the objective lens  201 . The reflected laser beam incident on the objective lens  201  passes through the mirror block  212 , the collimator  210  and the beam splitter  208  so as to be brought back to the IOU  211  for DVD.  
      A hologram element explained herein later with reference to  FIG. 11  or  12  is arranged within the IOU  211  for DVD. The reflected laser beam is guided to a light detector shown in  FIG. 11  or  12  by the diffraction effect produced by a pattern (not shown) formed in the hologram element. The reflected laser beam is converted into electric signals by the light detector, with the result that an RF signal, a focus error signal and a tracking error signal are generated from the light detector.  
      The laser diode included in the IOU  203  for CD emits a laser beam Bm having a wavelength of 780 nm to a disc of the CD standard. The disc of the CD standard includes the optical discs conforming with the CD-R or CD-RW standards.  
      The laser beam Bm emitted from the laser diode, which is a diverging beam, is imparted with a predetermined cross sectional beam diameter in the coupling lens  215  and, then, reflected by the beam splitter  208 . Further, the reflected laser beam is collimated by the collimator  210  and, then, guided to the mirror block  212 . The laser beam Bm is reflected by the mirror block  212  so as to be incident on the objective lens  201  shown in  FIG. 2 . The objective lens  201  permits a predetermined position on the recording surface of the optical disc D to be irradiated with the laser beam.  
      The laser beam irradiating the optical disc D is reflected from the recording surface of the optical disc D so as to be brought back to the objective lens  201  and, then, is guided to the IOU  203  for CD through the mirror block  212 , the collimator  210 , the beam splitter  208  and the coupling lens  215 .  
      A hologram element explained herein later with reference to  FIG. 11  or  12  is arranged within the IOU  203  for CD. The reflected laser beam is guided to a light detector shown in  FIG. 11  or  12  by the diffraction effect produced by a pattern (not shown) formed in the hologram element. The reflected laser beam is converted into electric signals by the light detector, with the result that an RF signal, a focus error signal and a tracking error signal are generated from the light detector.  
      The above description on the optical disc of the CD standard covers a CD disc exclusively for reproduction. The case where the set optical disc is a CD-R or CD-RW disc is equal to the above case in that the laser beam having the first wavelength of 780 nm, which is emitted from the laser diode  11  included in the IOU  10  for CD, passes through the beam splitter  14 , the collimator  15 , the mirror block  16  and the objective lens  13  so as to irradiate the optical disc D. Where the optical disc D is formed of a CD-R disc, the intensity of the laser beam of the recording power, which has the maximum light intensity higher than that of the laser beam irradiating the optical disc D of CD type, is modulated by the driving circuit member  18  in accordance with the data to be recorded. In this case, the chromatic film on the recording surface of the optical disc D is irradiated with the intensity-modulated laser beam so as to bring about changes in the chromatic film and, thus, to perform the data recording. Similarly, in the case of the CD-RW disc, phase changes are brought about in that portion of the recording film which is irradiated with the laser beam of the recording power so as to perform the data recording.  
       FIG. 4  is an oblique view showing in a dismantled fashion the optical head shown in  FIGS. 2 and 3 , the optical head being dismantled into the base, the top cover on the side of the optical disc and the bottom cover on the side remote from the optical disc.  
      The flexible printed circuit board  206  and the metal cover  218  are mounted to the base  216  of the optical head  200 . As described previously, the monitor light detector  202  is mounted to the flexible printed circuit board  206 .  
      A laser driving circuit  217  for driving the laser diode included in the IOU  203  for CD is mounted to the metal cover  218  with the flexible printed circuit board  218  interposed there between. Four openings  218   d  are formed in the metal cover  218 . One of these four openings  218   d  is coupled with a projecting fulcrum  218   c  formed on the base  216  so as to determine the relative position between the cover  218  and the base  216 . The metal cover  218  is fixed to the base  216  by inserting screws  218   a  through the remaining three openings  218   d  so as to be engaged with support portions  218   b  having internally threaded holes.  
       FIG. 5  shows the base shown in  FIG. 4  as viewed from the side of the top cover  206 . Incidentally, the laser driving circuit  217  cannot be seen originally because the metal cover  218  is fixed to the base  216  by the projecting fulcrum  218   c  and the three support portions  218   b . However, in order to show the positional relationship between the optical path of the optical system arranged within the open portion  207  and the laser driving circuit  217 , the laser driving circuit  217  is depicted to be seen through the metal cover  218  in  FIG. 5 .  
      As shown in  FIG. 5 , the laser driving circuit  217  arranged along the optical path and held in parallel to the optical path.  
       FIG. 6 , which is a side view showing the base shown in FIGS.  2  to  5 , shows the positional relationship among the laser driving circuit  217 , the monitor light detector  202  and the optical path of the laser beam Bm. As shown in  FIG. 6 , the laser beam Bm emitted from the laser diode included in the IOU  211  for DVD passes through the beam splitter  208 , is collimated by the collimator  210  and, then, guided to the mirror block  212 . Therefore, the laser driving circuit  217  and the monitor light detector  202  are arranged within the open portion  207  of the optical head  200  in upper and lower portions of the base  216  in a manner to have the beam splitter  208  sandwiched there between.  
      The laser driving circuit  217  is mounted to the metal cover  218  with a flexible printed circuit board interposed there between and is inserted into the open portion  207  so as to be held on the base  216 . In this case, the metal cover  218  having the laser driving circuit  217  mounted thereon is held by the support portions  218   b  and the projecting fulcrum  218   c  apart from the base  216  by the height of the clearance Cr.  
      As apparent from  FIG. 6 , the laser driving circuit  217  is exposed to the outer air through the clearance Cr, making it possible to obtain a cooling effect for cooling the laser driving circuit  217 .  
      On the other hand, the monitor light detector  202  is mounted to the flexible printed circuit board  206  and is fixed to the base  216 . Incidentally, as shown in  FIG. 6 , the laser driving circuit  217  and the monitor light detector  202  are arranged in parallel to the laser beam Bm and held so as not to interfere with the optical path of the laser beam Bm. In the optical head  200  shown in FIGS.  2  to  6 , the laser driving circuit  217  and the monitor light detector  202  are arranged in parallel along the optical path ranging between the IOU  211  for DVD and the collimator  210 . However, the laser driving circuit  217  and the monitor light detector  202  may be arranged anywhere desired as far as the laser driving circuit  217  and the monitor light detector  202  extend in parallel to the optical path of the laser beam ranging between the laser diode and the objective lens.  
      As described above, in the optical head shown in FIGS.  2  to  6 , the space for forming an optical path is effectively utilized so as to overcome the restriction of the layout relating to many members housed in the optical head  200  so as to provide an optical head that is miniaturized and has a decreased thickness.  
      Needless to say, it is possible to arrange, for example, an actuator driver for moving the objective lens  201  in an optional direction and a signal processing circuit in the space for forming the optical path within the base  216  in the optical head shown in FIGS.  2  to  6  as in the optical head described previously in conjunction with  FIGS. 1A and 1B .  
      As described above, in the optical head shown in FIGS.  2  to  6 , the monitor light detector  202  is arranged above the beam splitter  208  and the laser driving circuit  217  is arranged below the beam splitter  208 . It follows that the monitor light detector  202  and the laser driving circuit  217  do not interfere with the laser beam Bm in the upper and lower portions of the beam splitter  208 .  
       FIG. 7  schematically shows an optical head unit  300  (motor substrate for an optical disc apparatus) having the optical head  200  shown in FIGS.  2  to  6  attached thereto.  
      As shown in  FIG. 7 , guide shafts  302   a  and  303   a  movably supporting the optical head  200  in a radial direction of the optical disc D are mounted in predetermined positions of the optical head unit  300 . The guide shafts  302   a  and  303   a  are coupled with a claw-shaped guide bearing  302   b  and a cylindrical bearing  303   b  arranged on both edges of the base  206  of the optical head  200  so as to support the optical head  200  such that the optical head  200  can be moved to an optional position in the radial direction of the optical disc D.  
      The optical head unit  300  includes a turntable  301  for holding the optical disc D, a motor  303  producing a driving force for moving the optical head  200  in the radial direction of the optical disc D, and a pinion gear  304   a  for transmitting the driving force of the motor  303  to the optical head  200 .  
      A rack  304   b  in mesh with the pinion  304   a  is arranged on the optical head  200 . If the pinion  304   a  is rotated in a predetermined direction by the rotation of the motor  303 , the rack  304   b  is moved in an optional direction so as to move the optical head  200  in an optional direction in the radial direction of the optical disc D.  
       FIG. 8  schematically shows an optical disc apparatus equipped with a base  312  having the optical head unit  300  shown in  FIG. 7  incorporated therein.  
      As shown in  FIG. 8 , the base  312  can be withdrawn in an A-A′ direction. The optical disc D is disposed on the turntable  301 . Also, when data is reproduced from and recorded in the optical disc D, the turntable  312  is housed in the optical disc apparatus  310 . Where the housed table  312  is withdrawn, an eject button  313  mounted to a predetermined position of the optical disc apparatus  310  is depressed so as to cause a loading mechanism (not shown) to discharge the table  312  from the optical disc apparatus  310 .  
       FIGS. 9A and 9B  schematically show another embodiment of the optical head shown in FIGS.  2  to  6 , wherein  FIG. 9A  is a plan view as viewed in a direction parallel to the recording surface of the optical disc, and  FIG. 9B  is a side view showing the state as viewed in a direction perpendicular to the recording surface of the optical disc.  
      As shown in  FIGS. 9A and 9B , an optical head  110  includes a base  111 . The IOU  10  for CD, which is arranged in a predetermined position of the base  111 , includes a first light source of the laser diode  11  for emitting a laser beam (light beam) having the first wavelength and a light detector  12 . The laser beam emitted from the laser diode  11  is reflected from the recording medium of the optical disc. The reflected laser beam is received by the light detector  12  so as to produce an electric signal conforming with the intensity of the reflected laser beam. The base  111  also has an open portion  112  housing an optical path for transmitting the laser beam of the first wavelength emitted from the laser diode  11  of the IOU  10  to the objective lens  13  for collecting the laser beam on the recording surface of the optical disc D.  
      The laser beam emitted from the laser diode  11  of the IOU  10  for CD, which has a wavelength of, for example, 780 nm and is gradually diverged along the optical path within the open portion  113 , is guided by the collimator  15  and the mirror block  16 , which are the optical members substantially equal in construction to those described previously in conjunction with  FIGS. 1A and 1B , to the objective lens  13 . Predetermined converging properties are imparted to the laser beam by the objective lens  13 , and the laser beam is collected on a predetermined position on the recording surface of the optical disc D. The laser beam reflected from the recording surface of the optical disc D is brought back to the objective lens  13  so as to be collected on the light detector  12  within the IOU  10  through the mirror block  16  and the collimator  15 .  
      A monitor light detector  17  for detecting the light amount of the laser beam emitted from the laser diode  11  of the IOU  10  for CD is arranged in a predetermined position, i.e., in the ceiling of the base  111  that can be fixed by a top cover that is not described in detail, of the open portion  112  formed within the base  111 . On the other hand, a laser driving circuit member  18  for driving the laser diode  11  of the IOU  10  for CD is fixed to a predetermined position, i.e., the floor portion of the base  111  that can be fixed by a base cover that is not described in detail, within the open portion  112  of the base  111 .  
      As described above, the laser driving circuit member  18  is arranged in the vicinity of the IOU  10  for CD and within the space formed between the inclined portion of the optical path on the emitting position of the diverging laser beam Bm and the floor portion of the base  111  on the side away from the optical disc D with the optical path interposed therebetween, as illustrated by the optical head shown in  FIGS. 9A and 9B .  
      As described above, in the optical head shown in  FIGS. 9A and 9B , the laser driving circuit  19  for driving the laser diode  11  within the IOU  10  is held in the space formed between the inclined portion of the base  111  and the floor portion of the base  110 . It follows that the space having the optical path formed therein can be utilized more effectively compared with the optical head described previously, making it possible to further miniaturize the optical head.  
      As described above, in the optical head shown in  FIGS. 9A and 9B , the space in which the optical path is formed is utilized efficiently so as to overcome the restriction of the layout*of the members housed in the optical head. It follows that it is possible to provide a miniaturized optical head having a small thickness.  
       FIG. 11  schematically shows in a dismantled fashion the main constituents of the optical head that can be utilized in any of the optical heads described with reference to  FIGS. 1A and 1B , FIGS.  2  to  6 , and  FIGS. 9A and 9B .  
      An optical head  400  shown in  FIG. 11  includes a first IOU  31  used for reproducing the data from a DVD disc and a second IOU  41  used for reproducing data from a CD disc.  
      The first IOU  31  has a first laser diode  32  (first light source) for emitting a laser beam having a first wavelength, a first light detector  33 , and a first hologram element  34 . The laser beam emitted from the first laser diode  32  passes through the first hologram element  34  and, then, through a wavelength selecting film  36  of a dichroic prism  35  with the diverging properties of the laser beam left unchanged. Further, the laser beam is collimated by a collimator  37 . The laser beam collimated by the collimator  37  passes through a dichroic filter  38  and predetermined converging properties are imparted to the laser beam by an objective lens  39  so as to be collected on a predetermined position on the recording surface of the optical disc D.  
      The laser beam reflected from the recording surface of the optical disc D passes through the objective lens  39 , the dichroic filter  38  and the collimator  37  so as to be brought back to the wavelength selecting film  36  of the dichroic prism  35 . The reflected laser beam that has been brought back to the wavelength selecting film  36  passes again through the wavelength selecting film  36  so as to be diffracted by the first hologram element  34  of the first IOU  31  and, thus, to be collected on a predetermined position on the first light detector  33 .  
      The second IOU  41  has a second laser diode  42  (second light source) emitting a laser beam having a second wavelength, a second light detector  43 , and a second hologram element  44 . The laser beam emitted from the second semiconductor laser  42  passes through the second hologram element  44 , and predetermined converging properties are imparted to the laser beam by a coupling lens  45  for converting the large diverging angle of the diverging light flux of the laser beam having the second wavelength to a small diverging angle. Further, the laser beam is guided to the wavelength selecting film  36  of the dichroic prism  35 . The laser beam having the second wavelength, which is guided to the wavelength selecting film  36  of the dichroic prism  35 , is reflected by the wavelength selecting film  36  so as to be incident on the collimator  37 . The diverging properties of the laser beam of the second wavelength incident on the collimator  37  are weakened by the collimator  37  so as to be guided to the dichroic filter  38 . Then, the laser beam passes through a predetermined region of the dichroic filter  38  so as to be incident on the objective lens  39 . Predetermined converging properties are imparted to the laser beam of the second wavelength incident on the objective lens  39  by the objective lens  39  so as to be collected on a predetermined position on the recording surface of the optical disc D.  
      The laser beam of the second wavelength, which is reflected from the recording surface of the optical disc D, passes through the objective lens  39 , the dichroic filter  38  and, then, the collimator  37  so as to be brought back to the wavelength selecting film  36  of the dichroic prism  35 . The reflected laser beam of the second wavelength, which has been brought back to the wavelength selecting film  36 , is reflected by the wavelength selecting film  36  so as to be incident on the coupling lens  45 . The reflected laser beam of the second wavelength incident on the coupling lens  45  is diffracted by the hologram element  44  of the second IOU  41  so as to be collected on the light receiving surface of the light detector  43 .  
      The dichroic prism  40  will now be described. Specifically, the dichroic prism  40  is a cube having two corner cube prisms bonded to each other, which is an optical path synthesizing-separating element having a wave selection film  41  formed on the bonded plane of the two prisms. The wavelength selecting film  41  transmits substantially completely the laser beam (first light) having a wavelength of about 650 nm and emitted from the laser diode  32  of the first IOU  31  toward the DVD disc and reflects substantially completely the laser beam (second light) having a wavelength of about 780 nm and emitted from the laser diode  42  of the second IOU  41  toward the CD disc.  
      The dichroic filter  38  will now be described. Specifically, a circular opening-like wavelength selecting region is formed in the center of the dichroic filter  38 . The laser beam having a wavelength of 650 nm, which is directed to a DVD disc, can be transmitted through the entire region of the dichroic filter  38 . On the other hand, the laser beam having a wavelength of 780 nm, which is directed to a CD disc, can be transmitted only inside the circular opening-like wavelength selecting region that is formed in the center of the dichroic filter  38 . It follows that the dichroic filter  38  provides a large opening with respect to the laser beam directed to the DVD disc, and provides a small opening with respect to the laser beam directed to the CD disc. In other words, the opening is narrowed with respect to the laser beam directed to the CD disc.  
      As described above, the dichroic filter  38  does not restrict the opening with respect to the laser beam having a wavelength of 650 nm and directed to the DVD disc and restricts the opening with respect to the laser beam having a wavelength of 780 m and directed to the CD disc such that the numerical aperture (NA) is not larger than 0.45.  
      The DVD disc system and the CD disc system are equal to each other in the operating principle of the reproduced signal system using signals generated from the light detectors  33 ,  43  of the first and second IOU  31 ,  41 , respectively. To be more specific, the primary diffracted laser beam obtained by diffracting the laser beam reflected from the optical disc D by the corresponding hologram element  34  or  44  is received by the light detector  33  or  43  having a 4-divided or 6-divided photo diode so as to be converted into electric signals in the light detector  33  or  43 . A reproduced signal, a focus error signal, a tracking error signal, etc. are obtained from the electric signals converted from the primary diffracted beam noted above.  
      In the optical head  400  shown in  FIG. 11 , the optical magnification is set at about 8, which can be regarded as an infinite system or a substantially infinite system, in the optical system for DVD in order to obtain the optimum reproducing characteristics in the DVD System. Therefore, the objective lens  39  and the collimator  37  are designed to satisfy this magnification. On the other hand, in the optical system for CD, the numerical aperture (NA) is set at 0.5 to meet the CD-R and CD-Rw standards, and the coupling lens  45  is arranged in addition to the objective lens  39  and the collimator  37  in order to increase the emitting power of the objective lens  39 . By using the coupling lens  45 , the optical magnification can be lowered while increasing the emitting power of the objective lens  39 . In other words, it is possible to collect the laser beam for CD on the recording surface of the CD disc by utilizing the infinite system of the magnification  8  for DVD.  
      To be more specific, in the optical head  400  shown in  FIG. 11 , an ordinary convex lens is used as the coupling lens  45 , and the spherical aberration is canceled by making optimum the lens curvature, thickness, position and the position of the light source so as to achieve a beam spot quality in which the wave front aberration is about 0.02 λrms.  
       FIG. 13A  is a graph showing the relationships between the optical magnification and the light utilization and between the optical magnification and the power of the laser beam emitted from the objective lens in the case of using the optical head shown in  FIG. 11 . It is clearly seen that both the light utilization, graphed by curve “a,” and the power of the laser beam emitted from the objective lens, which is graphed by curve “b,” decrease with an increase in the optical magnification. Also, as apparent from the Gaussian distribution shown in  FIG. 13B , the light utilization is improved with a decrease in the optical magnification so as to increase the emitting power of the objective lens. If the optical magnification is made unduly small, however, the beam loading rate is lowered and other factors are increased. For example, the influence of the aberration in the optical path represented by the astigmatism is increased. As a result, the quality of the beam spot collected on the recording surface of the optical disc is lowered. When it comes to the transmittance of the laser beam, the number of optical members arranged in the optical path in the case of the DVD/CD common use is larger than that in the case of the optical system used exclusively for the CD, leading to reduction in the transmittance of light in the optical path. Under the circumstances, the optical magnification for the CD system is finally set at 4 in the optical head  400  shown in  FIG. 11 .  
      The relationship between the optical magnification and the light utilization shown in  FIG. 13A  is derived from the fact that the intensity distribution of the beam is as shown in  FIG. 13B  and has the relationship denoted by the formula shown in  FIG. 13B . It follows that the intensity is high and the light is utilized efficiently in the portion where the beam radius is small.  
      The ordinary convex lens (coupling lens)  45  is arranged such that the convex surface faces the objective lens on the side opposite to the light source. To be more specific, if the change in the beam spot wave front aberration λrms when a lens shift is imparted to the objective lens  39  is obtained on the basis that the total aberration in the case where the convex surface of the coupling lens  45  is positioned to face the objective lens  39  is represented by curve “a” shown in  FIG. 14 , the coma aberration in the case where the convex surface of the coupling lens  45  is positioned to face the objective lens  39  is represented by curve “b” shown in  FIG. 14 , the total aberration in the case where the convex surface of the coupling lens  45  is positioned to face the light source (laser diode)  31  is represented by curve “c” shown in  FIG. 14 , and the coma aberration in the case where the convex surface of the coupling lens  45  is positioned to face the light source (laser diode)  31  is represented by curve “d” shown in  FIG. 14 , the coma aberration can be suppressed even if a lens shift is imparted to the objective lens  39  in the case where the convex surface of the coupling lens  45  is positioned to face the objective lens  39 .  
      It should also be noted that the coupling lens  45  is arranged in a position at a predetermined angle inclined from the optical axis extending from the laser diode  42  toward the dichroic prism  35 . This is done in order to suppress the ghost light that the laser beam emitted from the second laser diode  42  is reflected on the surface of the coupling lens (ordinary convex lens)  45  and the reflected light is partly incident on the light detector  37  as the ghost light. To be more specific, the laser beam reflected from the surface of the coupling lens  45 , forms the ghost light in the case where the laser beam is not diffracted in the hologram element  44  so as to be incident on the light detector  33  as a zero (0) order diffracted light and in the case where the primary diffracted light is incident on the light detector  33 . It should be noted that the ghost light in each of these cases can be suppressed by inclining the coupling lens  45  by a predetermined angle, e.g., 4°, from the optical axis shown in  FIG. 15E . However, if the angle of inclination of the coupling lens  45  is excessively large, astigmatism is generated so as to lower the beam spot quality. Such being the situation, in the optical head  400  shown in  FIG. 11 , the coupling lens  45  is inclined from the optical axis by about 2°. Where the angle of inclination was set at 3°, the astigmatism was increased so as to aggravate the entire wave front aberration.  
       FIGS. 15A and 15B  exemplify the routes of the ghost light caused by the 0-order light and the primary light in the case where the inclination was set at 4°. On the other hand,  FIGS. 15C and 15D  exemplify the routes of the ghost light caused by the 0-order light and the primary light in the case where the inclination was set at 0°. Incidentally, the hologram element  44  of the second IOU  41  used in the optical head  400  shown in  FIG. 11  is made integral with the coupling lens  45  by an outer case  49 .  
      As described above, in the optical head shown in  FIG. 11 , the optical magnification of the DVD system is set at about 8 and the optical magnification of the CD system is set at about 4 independently in spite of the construction that the single objective lens  39  is used for forming the optical systems for both the DVD disc and the CD disc. As a result, the data can be reproduced from both the DVD disc and the CD disc. Also, it is possible to obtain an optical head having characteristics adapted for recording of data in and reproduction of data from CD-R and CD-RW, free from deterioration of the characteristics, and capable of suppressing the ghost light in also the case where a lens shift is imparted to the objective lens  39 .  
       FIG. 12  shows another embodiment of the optical head shown in  FIG. 11 . In the optical head shown in  FIG. 12 , an IOU is not used, and a laser diode and a light detector are used independently.  
      The optical head  500  shown in  FIG. 12  includes a first laser diode  551  for emitting a laser beam for DVD having a wavelength of 650 nm, a second laser diode  561  emitting a laser beam for CD having a wavelength of 780 nm, a flat plate beam splitter  552  reflecting the laser beam emitted from the first laser diode  551  toward an objective lens  575 , a wavelength selecting film  572 , a prism beam splitter  571  that permits transmitting the laser beam reflected from the flat plate beam splitter  552  and reflects the laser beam emitted from the second laser diode, a collimator  573 , and a dichroic filter  574 . Also, a coupling lens  562  for setting the optical magnification of the CD system at a predetermined value is arranged between the second laser diode  561  and the prism beam splitter  571 . On the other hand, the laser beam reflected from the optical disc D is incident on the flat plate beam splitter  552  through the objective lens  575  and a light detector  581  is arranged in a direction in which the reflected laser beam is transmitted through the flat plate beam splitter  552 .  
      The wavelength selecting film  572  of the prism beam splitter  571  permits transmitting substantially 100% of the laser beam for DVD having a wavelength of 650 nm, and also permits transmitting about 50% of the laser beam for CD having a wavelength of 780 nm. Naturally, about 50% of the laser beam for CD having a wavelength of 780 nm is reflected from the wavelength selecting film  572 . It should be noted that the flat plate beam splitter  552  is formed of a half mirror exhibiting a wavelength selecting capability. To be more specific, the flat plate beam splitter  552  permits transmitting 50% of the laser beam for DVD having a wavelength of 650 nm and reflecting the remaining 50% of the laser beam, and also permits transmitting 100% of the laser beam for CD having a wavelength of 780 nm. On the other hand, a circular opening-shaped wavelength selecting region is formed in the center of the dichroic filter  574 . The laser beam for the DVD disc having a wavelength of 650 nm can be transmitted through the entire region of the dichroic filter  574 . On the other hand, the laser beam for the CD disc having a wavelength of 780 nm is transmitted only inside the central circular opening-shaped wavelength selecting position of the dichroic filter  574 . It follows that the dichroic filter  574  provides a large opening relative to the laser beam for the DVD disc, and the opening is restricted for the laser beam for the CD disc. In other words, the dichroic filter  574  performs the function of diminishing the opening in respect of the laser beam for the CD disc.  
      In the optical head  500  of the particular construction, the laser beam for DVD, which is emitted from the laser diode  551  and has a wavelength of 650 nm, is reflected from the flat plate beam splitter  552  in about half the light amount so as to be incident on the prism beam splitter  571 . The laser beam having a wavelength of 650 nm, which is incident on the prism beam splitter  571 , is transmitted through the wavelength selecting film  572  so as to pass through the collimator  573  and the dichroic filter  574  and, thus, to be incident on the objective lens  575 .  
      The laser beam for the DVD disc having a wavelength of 650 nm, which is incident on the objective lens  575 , is imparted with predetermined converging properties by the objective lens  575  so as to be collected on a predetermined position on the recording surface of the optical disc D.  
      The laser beam having a wavelength of 650 nm, which is reflected from the optical disc D, passes through the objective lens  575 , the dichroic filter  574 , the collimator  573  and the prism beam splitter  571  in the order mentioned so as to be brought back to the flat plate beam splitter  552 . The laser beam reflected from the optical disc D, which is brought back to the flat plate beam splitter  552 , then passes through the flat plate beam splitter  552  so as to be incident on the light detector  581 .  
      On the other hand, the diverging angle of the laser beam for CD having a wavelength of 780 nm, which is emitted from the laser diode  561 , is limited to a predetermined angle by the coupling lens  562  so as to be incident on the prism beam splitter  571  equipped with the wavelength selecting film  572 . The laser beam for the CD disc having a wavelength of 780 nm, which is incident on the prism beam splitter  571 , is reflected by the wavelength selecting film  572  and the reflected laser beam passes through the collimator  573  and the dichroic filter  574  so as to be incident on the objective lens  575 .  
      The laser beam for the CD disc having a wavelength of 780 nm, which is incident on the objective lens  575 , is imparted with predetermined converging properties by the objective lens  575  so as to be collected on a predetermined position on the recording surface of the optical disc D.  
      The laser beam having a wavelength of 780 nm, which is reflected from the optical disc D, passes through the objective lens  575 , the dichroic filter  574  and the collimator  573  in the order mentioned so as to be brought back to the prism beam splitter  571 . The laser beam having a wavelength of 780 nm, which has been brought back to the prism beam splitter  571 , then passes through the prism beam splitter  571  and, further, through the flat plate beam splitter  552  so as to be incident on the light detector  581 .  
      It should be noted that the optical head shown in  FIG. 12  is equal to the optical head shown in  FIG. 11  in respect of the optical magnification for each of the DVD disc and the CD disc as well as in the role and arrangement of the coupling lens  562 .  
      It follows that it is possible to obtain an optimum beam spot in recording data in the CD-R disc or the CD-RW disc as well as in reproducing data from the DVD disc and the CD disc. Also, even in the case where a lens shift is imparted to the objective lens, it is possible to obtain an optical head low in deterioration of the characteristics and unlikely to be affected by the ghost light.  
      The optical head of the present invention is not limited to the embodiments shown in  FIGS. 1A and 1B , FIGS.  2  to  6 ,  FIGS. 9A and 9B ,  FIGS. 11 and 12 . For example, a refractive index portion type lens having a layer differing in the refractive index in the thickness direction or the radial direction or a diffraction type lens utilizing the diffraction can be used as the coupling lens.  
      Also, in any of the embodiments described above, the coupling lens is arranged in a position inclined from the optical axis of the laser beam emitted from a laser diode for CD. Needless to say, however, the coupling lens can be applied to any optical head. Incidentally, the optical system including the laser diode, in which the coupling lens is arranged, constitutes the recording system to the optical disc of the CD type.  
       FIG. 16A  shows the construction of the recording surface of a CD disc, and  FIG. 16B  shows the construction of the recording surface of a DVD-ROM. Further,  FIG. 16C  shows the construction of the recording surface of a DVD-RAM. As shown in these drawings, the individual optical discs D greatly differ from each other in the track pitch and in the shortest pitch length, making it necessary to use a light source capable of emitting laser beams differing from each other in the wavelength as described above.  
       FIG. 17  exemplifies the system of electric signals for processing the signals read by the optical head shown in each of  FIGS. 1A and 1B , FIGS.  2  to  6 ,  FIGS. 9A and 9B ,  FIGS. 11 and 12 .  
      As shown in  FIG. 17 , photo diodes  6 A,  6 B,  6 C,  6 D,  6 E and  6 F are mounted in a light detector  606 . The output of the photo diodes  6 A,  6 B,  6 C,  6 D,  6 E and  6 F are amplified by buffer amplifiers  623   a ,  623   b ,  623   c ,  623   d ,  623   e  and  623   f , respectively, and output signals A to F are obtained from these buffer amplifiers.  
      The individual signals A to F are processed such that a signal (A+B) is formed by an adder  631 , and a signal (C+D) is formed by another adder  632 . The signal (A+B) formed by the adder  631  and the signal (C+D) formed by the adder  632  are processed by an adder  633  to form a signal {(A+B)−(C+D)}. The signal {(A+B)−(C+D)} is used as a focus error signal.  
      An adder  634  forms a signal (A+C), and an adder  635  forms a signal (B+D). These signals (A+C) and (B+D) are supplied to a phase difference detector  631 . The output of the phase difference detector  631  is used as a tracking error signal of the DVD disc. On the other hand, a signal (E-F) obtained on the basis of the detection signal of the sub-beam is neglected by turning off a switch  642 .  
      The signals (A+C) and (B+D) are also supplied to an adder  636 . Incidentally, the adder  635  forms a signal (A+B+C+D), i.e., an HF signal. On the other hand, a signal (E-F) is formed from signals E and F by an adder  637 . The signal (E-F) is used as a tracking error signal in the CD disc. In other words, a switch  652  is turned on when the optical disc apparatus including the optical head is in a CD reproduction mode. The circuit construction can be modified in various fashions and is not limited to that shown in  FIG. 17 .  
      The signal processing circuit described above covers the case where the laser beam reflected from each of the CD disc and the DVD disc is detected by a single light detector. Needless to say, however, it is possible to add a signal processing circuit by mounting an element to each of the light detectors included in the optical head shown in  FIG. 11 . Alternatively, it is possible for the signal processing circuit of the construction shown in  FIG. 17  to be connected as it is to each of the light detectors. In this case, the switch is maintained at a constant state in accordance with the reproduced disc.  
      As described above, the present invention provides an optical head, which permits reproducing data from each of the DVD type disc and the CD type disc by using a single objective lens, which permits exhibiting appropriate characteristics in recording/reproducing data in and from CD-R or CD-RW, and which permits suppressing the deterioration of the characteristics and also permits suppressing the influence given by the ghost light even in the case where a lens shift is imparted to the objective lens.  
      The present invention also provides an optical head, which permits overcoming the restriction of the layout in mounting the parts and also permits mounting the optical disc in a thin optical disc apparatus.  
      Further, in the optical head of the present invention, many members, each has at least one of circuit members, are arranged on the side of the light source in which the diverging angle of the diverging laser beam is small so as to effectively utilize the space (open portion) within the optical head.  
      Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.