Patent Document

This is a continuation application of reissue application Ser. No.  09 / 460 , 221  filed Dec.  13 ,  1999 , which is a continuation application of reissue application Ser. No.  08 / 396 , 981  filed Mar.  1 ,  1995  which issued as RE  36 , 445  on Dec.  14 ,  1999 , which was a reissue of U.S. Pat. No.  5 , 235 , 581  issued Aug.  10 ,  1993 . The following are related continuation reissue applications: application Ser. No.  09 / 420 , 603  filed Oct.  19 ,  1999 , application Ser. No.  09 / 609 , 699  filed Nov.  22 ,  1999 , application Ser. No.  09 / 609 , 529  filed Nov.  22 ,  1999 , application Ser. No.  09 / 460 , 222  filed Dec.  13 ,  1999 , and application Ser. No.  09 / 460 , 223  filed Dec.  13 ,  1999 . 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an optical disc apparatus which can record, reproduce, or erase information signals onto/from both of an optical disc having a recording density similar to that of a conventional CD (compact disc) and an optical disc having a recording density higher than the above recording density. 
     2. Description of the Prior Art 
     In recent years, in addition to an optical disc apparatus only for reproduction such as a CD player or the like, an optical disc apparatus which can record and reproduce an information signal is actively being developed. 
     Ordinarily, the recording and reproduction of an information signal onto/from an optical disc are executed by converging a beam which is radiated from a semiconductor laser or the like onto a recording layer of the optical disc by a lens. The recording layer here denotes a pit layer in the case of a CD and is a layer in which a deformation, a change in optical constant, a formation of a magnetic domain, or the like is performed by a converged laser beam in the case of a recordable optical disc. To raise a recording density of the optical disc, it is necessary to reduce a spot diameter D of the converged beam. There is the following relation among the spot diameter D, a numerical aperture NA of the lens, and a wavelength λ of the laser beam. 
             D   ∝     λ   NA             (   1   )             
 
     The above equation (1) denotes that the beam spot diameter D decreases by using a lens of a large NA. That is, by increasing NA, the high density recording can be executed. 
     When NA of the lens increases, however, an aberration of the converged beam due to an inclination error of the disc called a tilt increases. Particularly, a coma aberration increases. There is the following relation among a wave front aberration W c  of the coma, a tilt angle α, and NA when using a thickness d and a refractive index n of the disc substrate. 
               W   c     ∝           n   2     -   1       2   ⁢     n   3         ·   d   ·   α   ·       (   NA   )     3               (   2   )             
 
     The above equation (2) denotes that in the case of using a lens of NA which is larger than that of the conventional lens, even if a tilt angle is identical, the coma aberration increases. It will be understood from the equation (2), however, that there is an effect to suppress the coma aberration by setting the thickness d of the disc substrate to be thin. In the optical disc for the high density recording, therefore, it is preferable that the thickness of the disk substrate is thinner than that of the conventional optical disc, so that an optical head using an objective lens corresponding to the thin disc substrate is needed. 
     On the other hand, even in the optical disc apparatus corresponding to the high density recording, it is preferable that the conventional optical disc of a thick substrate can be also reproduced so that a great amount of conventional software resources can be utilized. 
     However, the optical head which has been designed for a thin substrate cannot be used for an optical disc of a thick substrate. The reasons will now be described hereinbelow. The objective lens for an optical disc has been designed so as to set off a spherical aberration which occurs when the converged beam passes in the disc substrate. Since such an aberration is corrected in accordance with the thickness of the disc substrate, the aberration correction is not accurately performed for the converged beam which passes through the disc substrate having a thickness different from the design value. The above point will now be explained with reference to the drawing.  FIGS. 18A and 18B  are schematic side elevational views for explaining a situation of the occurrence of the aberration due to the disc substrates having different thicknesses.  FIG. 18A  is a diagram in the case of using an objective lens which has been designed for a thin disc substrate and shows traces of lights in a state in which a beam has been converged through the disc substrate having the same thickness as the design value. In the diagram, a broken line indicates the surface of a recording layer and all of the lights emitted from the objective lens are converged to one point  0  on the recording layer surface.  FIG. 18B  is a diagram in the case of using an objective lens which has been designed for the same thin disc substrate as that of FIG.  18 A and shows traces of lights in a state in which the beam has been converged through the disc substrate having a thickness which is thicker than the design value. In  FIG. 18B , the lights emitted from the outermost peripheral portion of the objective lens are converged to a point O′ on the recording layer surface. However, the light locating near the optical axis is converged at the front side. Such a phenomenon is called a spherical aberration. Whenever the term “aberration” is used hereinafter, it means spherical aberration. When such an aberration occurs, the objective lens cannot converge the light beam until what is called a diffraction limit. Therefore, in the case of using the objective lens whose aberration has been corrected for a thin disc substrate, an information signal cannot be recorded, reproduced, or erased onto/from an optical disc having a thick disc substrate. Similarly, in the case of using the objective lens whose aberration has been corrected for a thick disc substrate, an information signal cannot be recorded, reproduced, or erased onto/from an optical disc having a thin disc substrate. 
     SUMMARY OF THE INVENTION 
     In consideration of the above drawbacks, it is an object of the invention to provide an optical disc apparatus which can record, reproduce, or erase information signals onto/from a plurality of optical discs in which thicknesses of disc substrates are different. 
     To accomplish the above object, according to the invention, there is provided an optical disc apparatus for recording, reproducing, or erasing information signals onto/from an optical disc by converging light fluxes onto a recording layer through a transparent disc substrate, comprising: N converging means whose aberrations have been corrected for N (N≧2) disc substrates having different thicknesses, respectively; disc discriminating means for discriminating the thickness of the disc substrate of the optical disc which has been loaded and for generating a discrimination signal according to the result of the discrimination; and control means for selecting one of the converging means in which the occurrence of the aberration due to the disc substrate is smallest in accordance with the discrimination signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a constructional diagram of an optical disc apparatus according to the first embodiment of the invention; 
         FIG. 2  is a plan view showing an arrangement of a main section of the optical disc apparatus in the first embodiment; 
         FIGS. 3A and 3B  are perspective views of cartridges of optical discs in the first embodiment; 
         FIGS. 4A and 4B  are schematic diagrams showing cross sections of the optical discs and states of convergences by objective lenses in the first embodiment; 
         FIG. 5  is a constructional diagram of an optical disc apparatus according to the second embodiment of the invention; 
         FIG. 6  is a detailed constructional diagram of optical heads in the second embodiment; 
         FIG. 7  is a constructional diagram of an optical disc apparatus according to the third embodiment of the invention; 
         FIG. 8  is a detailed constructional diagram of an optical head in the third embodiment; 
         FIGS. 9A and 9B  are a detailed constructional diagram of an optical head of an optical disc apparatus according to the fourth embodiment of the invention; 
         FIG. 10  is a cross sectional view showing optical discs whose substrate thicknesses are different and converging states by convergence grating couplers which have been designed in correspondence to the optical discs according to the fifth embodiment, respectively; 
         FIG. 11  is a schematic perspective view showing a construction of an optical head of an optical disc apparatus in the fifth embodiment; 
         FIG. 12  is a schematic perspective view showing a construction of an optical head of an optical disc apparatus according to the sixth embodiment of the invention; 
         FIG. 13  is a schematic perspective view showing a construction of an optical head of an optical disc apparatus according to the seventh embodiment of the invention; 
         FIG. 14  is a block diagram showing a construction of the optical disc apparatus in the seventh embodiment; 
         FIG. 15  is a block diagram showing a construction of an optical disc apparatus according to the eighth embodiment of the invention; 
         FIG. 16  is a schematic enlarged perspective view showing a convergence grating coupler of an optical head, an SAW transducer, and a portion in which surface acoustic waves have been formed in the eighth embodiment; 
         FIG. 17  is a characteristic diagram for explaining the principle of the tracking control in the eighth embodiment; and 
         FIGS. 18A and 18B  are schematic side elevational views showing occurring situations of aberrations due to disc substrates having different thicknesses. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of the invention will be described hereinbelow on the assumption that thicknesses of disc substrates are set to two kinds of thicknesses. 
       FIG. 1  is a constructional digram of an optical disc apparatus in the first embodiment of the invention.  FIG. 2  is a plan view showing an arrangement of a main section of the embodiment  FIGS. 3A and 3B  are perspective views of cartridges of optical discs in the embodiment. 
     In  FIGS. 1  to  3 A and  3 B, reference numeral  1  denotes a first or second optical disc. Thicknesses of disc substrates of both of the first and second optical discs are different. Reference numeral  2  denotes a cartridge which encloses the optical disc  1  and protects. The cartridge  1  is made of plastics or the like. Reference numeral  3  denotes a first optical head and  5  indicates a second optical head. Each of the optical heads is constructed by a converging optical system comprising: an object lens; a semiconductor laser; a photo detector; a beam splitter; and the like (all of the above components are not shown). Each of the optical head detects an information signal, a focusing error signal, and a tracking error signal which have been recorded on the optical disc  1  on the basis of an intensity or an intensity distribution of the reflected lights of a laser beam irradiated onto the optical disc  1  and generates a photo detection signal to the outside. An information signal is recorded onto or erased from the optical disc  1  by modulating an intensity of the laser beam. Both of the optical heads have bases to hold the above optical devices and actuators. A reproduction information signal, a focusing error signal, and a tracking error signal which are generated from the photo detector of the first optical head  3  are expressed by S 1 , F 1 , and T 1 , respectively. Similar signals which are generated from the photo detector of the second optical disc  5  are also expressed by S 2 , F 2 , and T S , respectively. Reference numeral  4  denotes a first linear motor which is arranged below the optical disc  1  and moves the first optical head  3  in the radial direction of the disc in parallel with the disc surface. Reference numeral  6  denotes a second linear motor which is arranged below the optical disc  1  so as to face the first linear motor  4  and moves the second optical head  5  in a manner similar to the first optical head  3 . 
     As shown in  FIG. 2 , the second linear motor  6  is extended until the further outside of the outermost peripheral portion of the optical disc  1 . Therefore, when the second optical head  5  moves to the outermost side, the optical head  5  is projected from the lower surface of the optical disc. Reference numeral  7  denotes a discrimination hole formed on the surface of the cartridge  2 . 
     The cartridge in the embodiment will now be described with reference to  FIGS. 3A and 3B . The discrimination hole  7  is closed in the case where the optical disc  1  enclosed in the cartridge is the first optical disc shown in FIG.  3 A and is open in the case where it is the second optical disc shown in FIG.  3 B. Reference numeral  23  denotes a slide shutter. Since the optical disc apparatus of the embodiment has two optical heads, two slide shutters are provided. When the cartridges are removed from the optical disc apparatus, the slide shutters are closed to protect the internal discs from dusts. 
     Reference numeral  8  denotes a light emitting diode (hereinafter, abbreviated to an LED) which is arranged so as to be located over the discrimination hole  7  when the cartridge  2  has been loaded into the optical disc apparatus of the embodiment. Reference numeral  9  denotes a photo diode arranged at a position so as to face the LED  8  through the cartridge  2 . The photo diode  9  generates a detection signal to a system controller  22 , which will be explained hereinlater. Reference numeral  10  denotes a first selector for selecting either one of the first group of photo detection signals (S 1 , F 1 , and T 1 ) of the first optical head  3  and the second group of photo detection signals (S 2 , F 2 , and T 2 ) of the second optical head  5  and for outputting the selected signals. Reference numeral  11  denotes a tracking control circuit which receives the tracking error signal T 1  or T 2  from among the output signals of the first selector  10  and generates a tracking actuator driving signal to a second selector  12 , which will be explained hereinlater. Further, reference numeral  12  denotes the second selector to select either an actuator of the first optical head  3  or an actuator of the second optical head  5  as a destination to which the tracking actuator driving signal is supplied. Reference numeral  13  denotes a focusing control circuit which receives the focusing error signal F 1  or F 2  from among the signals which are generated from the first selector  10  and generates a focusing actuator driving signal to a third selector  14 , which will be explained hereinlater. Further, reference numeral  14  denotes the third selector for selecting either the actuator of the first optical head  3  or the actuator of the second optical head  5  as a destination to which the focusing actuator driving signal is supplied. Reference numeral  15  denotes a linear motor control circuit which supplies a driving signal to a fourth selector  16 , which will be explained hereinlater, by a control signal, which is supplied from the system controller  22 , which will be described hereinlater. Reference numeral  16  denotes the fourth selector to select either the first linear motor  4  or the second linear motor  6  as an output destination of the driving signal supplied from the linear motor control circuit  15 . Reference numeral  17  denotes a spindle control circuit which receives the reproduction information signal S 1  or S 2  and generates a control current to a spindle motor  18 , which will be explained hereinlater. Reference numeral  18  denotes the spindle motor to rotate the optical disc  1 . Reference numeral  18  denotes a signal processing circuit for executing signal processes such as demodulation and/or decoding or the like to the input signal S 1  or S 2 , for converting into an audio signal or the like, or for outputting the information signal to be recorded onto the optical disc  1  to an LD driving circuit  20 , which will be explained hereinlater. Reference numeral  20  denotes the LD driving circuit to supply a driving current for allowing laser beams to be emitted from the semiconductor lasers of both of the optical heads to a fifth selector  21 , which will be explained hereinlater. Reference numeral  21  denotes the fifth selector to switch the output destination of the above driving current to either one of the first and second optical heads  3  and  5  in accordance with a control signal from the system controller  22 , which will be explained hereinlater. The system controller  22  generates the driving current for allowing the LED  8  to emit the light and receives the detection signal from the photo diode  9  and generates control signals to the first to fifth selectors  10 ,  12 ,  14 ,  16  and  21 . The system controller  22  also generates control signals to the linear motor control circuit  15  and the signal processing circuit  19 . In  FIG. 1 , switching terminals in the first to fifth selectors  10 ,  12 ,  14 ,  16  and  21  on the first head side are designated by “A” and those on the second head side are designated by “B” in the diagram. 
     The first optical disc is a CD or an optical disc having a recording density similar to that of the CD and a thickness of the disc substrate is set to d 1  as shown in FIG.  4 A. For instance, assuming that the optical disc shown in  FIG. 4A  is a CD, d 1 =1.2 mm. The second optical disc is an optical disc which can execute a recording at a higher density than that of the first optical disc and a thickness of the disc substrate assumes d 2  as shown in FIG.  4 B. To reduce the aberration of the converged spot due to a tilt error, the thickness d 2  of the second optical disc is designed so as to be smaller than d 1 . For example, d 2 =0.3 mm. 
     The first optical head  3  has, for instance, a semiconductor laser of a wavelength of 780 nm and an objective lens of an NA of 0.45, so that the laser beam can be converged to a spot diameter φ of about 2.1 μm. In addition, an optical design of the objective lens has been made so as to correct the aberration by the disc substrate of the thickness d 1 . The second optical head  5  has, for instance, the same semiconductor laser as that of the first optical head and an objective lens of an NA of 0.8, so that the laser beam can be converged until a spot diameter of 1.2 μm. Moreover, an optical design of the objective lens has been made so as to correct the aberration by the disc substrate of the thickness d 2 . 
     In  FIGS. 4A and 4B , respective focal distances FD and working distances WD are shown.  
     The operation of the optical disc apparatus of the embodiment constructed as mentioned above will now be described hereinbelow. 
     First, the case where the cartridge  2  enclosing the second optical disc has been loaded into the optical disc apparatus of the embodiment will be described. When the cartridge  2  is loaded, the LED  8  emits the light and the photo diode  9  detects the presence or absence of a transmission light which passes through the discrimination hole  7 . As already described in conjunction with  FIGS. 3A and 3B , since the discrimination hole  7  is open in the case of the second optical disc, the photo diode  9  detects the transmission light and generates a detection signal to the controller  22 . The controller  22  determines that the disc in the loaded cartridge  2  is the second optical disc, so that the controller  22  supplies control signals to the first to fifth selectors  10 ,  12 ,  14 ,  16 , and  21  so as to select the terminals B on the second optical head side. Thus, the semiconductor laser of the second optical head  5  is selected as an output destination of the driving current which is supplied from the LD driving circuit  20 . The photo detector of the second optical head  5  is selected as an input destination of the tracking control circuit  11 , focusing control circuit  13 , spindle control circuit  17 , and signal processing circuit  19 . The actuator of the second optical head  5  is selected as an output destination of the actuator driving signals of the tracking control circuit  11  and focusing control circuit  13 . The second linear motor  6  is selected as an output destination of the driving current of the linear motor control circuit  15 . 
     Therefore, the second optical head  5  irradiates the laser beam and converges onto an information track on the optical disc  1  without an aberration. Simultaneously, the second optical head  5  detects the reflected lights from the disc and generates the information signal S 2 , focusing error signal F 2 , and tracking error signal T 2 . Those signals are supplied through the first selector  10  to the respective circuits. That is, the signal S 2  is supplied to the spindle control circuit  17  and signal processing circuit  19 . The signal F 2  is supplied to the focusing control circuit  13 . The signal T 2  is supplied to the tracking control circuit  11 . The tracking control circuit  11  produces the tracking actuator driving signal in accordance with the signal T 2  and supplies to the actuator of the second optical head  5  through the second selector  12 , thereby eliminating the tracking error. In a manner similar to the above, the focusing control circuit  13  also produces the focusing actuator driving signal in accordance with the signal F 2  and supplies to the actuator of the second optical head  5  through the third selector  14 , thereby eliminating the focusing error. The linear motor control circuit  15  generates the driving current to the linear motor  6  in response to the control signal from the system controller  22 , thereby moving the second optical head  5  in the inner or outer rim direction of the optical disc  1 . The spindle control circuit  17  extracts a clock component from the information signal S 2  and controls the spindle motor  18 , thereby rotating the optical disc  1  at a constant linear velocity (CLV) or a constant angular velocity (CAV) or the like. The signal processing circuit  19  executes signal processes such as demodulation, decoding, and the like to the information signal S 2  in the reproducing mode and generates to the outside as audio or video signals or the like. On the other hand, the signal processing circuit  19  executes signal processes such as encoding, modulation, and the like to the audio or video signals or the like which have been supplied from the outside in the recording mode and generates to the LD driving circuit  20  as a recording signal. Until the cartridge  2  is loaded, the second optical head  5  records or reproduces the information signal onto/from the second optical disc  1 . 
     On the other hand, in the case where the cartridge  2  enclosing the first optical disc has been loaded into the optical disc apparatus of the embodiment, since the discrimination hole  7  is closed, the photo diode  9  doesn&#39;t detect the transmission light. Therefore, the system controller  22  determines that the disc in the cartridge  2  is the foregoing first optical disc. Thus, the controller  22  generates control signals to the first to fifth selectors  10 ,  12 ,  14 ,  16 , and  21  so as to select the terminals A on the first optical head side. Therefore, the semiconductor laser of the first optical head  3  is selected as an output destination of the driving current which is supplied from the LD driving circuit  20 . The photo detector of the first optical head  3  is selected as an input destination of the tracking control circuit  11 , focusing control circuit  13 , spindle control circuit  17 , and signal processing circuit  19 . The actuator of the first optical head  3  is selected as an output destination of the actuator driving signals of the tracking control circuit  11  and focusing control circuit  13 . The first linear motor  4  is selected as an output destination of the driving current of the linear motor control circuit  17 . Therefore, the first optical head  3  irradiates the laser beam and converges onto the information track on the optical disc  1  without an aberration. Simultaneously, the reflected lights from the disc are detected and generated as the information signal S 1 , focusing error signal F 1 , and tracking error signal T 1 . The above signals are supplied through the first selectors  10  to the respective circuits That is, the signal S 1  is supplied to the spindle control circuit  17  and signal processing circuit  19 . The signal F 1  is supplied to the focusing control circuit  13 . The signal T 1  is supplied to the tracking error detecting circuit  11 . The subsequent operations are similar to those in the case of the second optical disc mentioned above. 
     In the case where the objective lens of the second optical head is a lens of a high NA and a short operating distance, it is necessary to set an interval between the second optical head  5  and the surface of the optical disc  1  to be fairly narrower than that in the case of the first optical head  5 . Therefore, while the first optical disc is loaded, the controller  22  controls the second linear motor  6 , thereby moving the second optical head  5  to the outside of the disc surface as shown in FIG.  2 . Due to this, it is possible to prevent that the objective lens of the second optical head  5  collides with the optical disc  1  due to a surface oscillation. 
     As described above, according to the embodiment, by providing the first optical head  3  and second optical head  5  having convergence optical systems corresponding to the thicknesses of the disc substrate of the first and second optical discs, the signal can be preferably recorded, reproduced, or erased by the optical head suitable for the thickness of each disc substrate. Moreover, by providing the discrimination hole  7  formed on the cartridge  2  and the disc discriminating means comprising the LED  8  to detect the opening/closure of the discrimination hole  7  and the photo diode  9 , merely by loading the cartridge  2 , each of the optical heads can be automatically accurately selected. 
       FIG. 5  shows a constructional diagram of an optical disc apparatus according to the second embodiment of the invention. In the diagram, the second embodiment differs from the foregoing first embodiment with respect to a point that a third optical head  30  is arranged in place of the first and second optical heads  3  and  5  and a point that the second, third, and fourth selectors  12 ,  14 , and  16  are omitted. That is, the tracking actuator driving current which is generated from the tracking control circuit  11  and the focusing actuator driving current which is generated from the focusing control circuit  13  are directly supplied to the third optical head  30 . The driving current which is generated from the linear motor control circuit  15  is directly supplied to the first linear motor  4 . 
     Further,  FIG. 6  shows a detailed constructional diagram of the third optical head  30  in the second embodiment of the invention. In the diagram, reference numeral  1  denotes the first or second optical disc;  32  a first semiconductor laser as a light source;  33  a first collimating lens for converting a laser beam from the first semiconductor laser  32  into a parallel beam;  34  first beam splitter to divide the beam into two beams;  35  a first mirror to change the direction of the beam;  36  a first objective lens to converge the beam onto the optical disc  1 ;  37  a first detecting lens to converge the reflected lights which have been divided by the beam splitter  34 ; and  38  a first photo detector to obtain an information reproduction signal, a focusing error signal, and a tracking error signal from the converged reflected lights. The above components  32  to  38  construct a first converging optical system  31 . 
     Reference numeral  39  denotes a lens holder to hold the first objective lens  36  and a second objective lens  46 , which will be explained hereinlater. Reference numeral  40  denotes an actuator to support the lens holder  39 . The driving currents are supplied from the tracking control circuit  11  and focusing control circuit  13  to the actuator  40 . 
     Reference numeral  42  denotes a second semiconductor laser as a light source;  43  a second collimating lens to convert a laser beam from the second semiconductor laser  42  into a parallel beam;  44  a second beam splitter to divide the beam into two beams;  45  a second mirror to change the direction of the beam;  46  the second objective lens to converge the beam onto the optical disc  1 ;  47  a second detecting lens to converge the reflected lights which have been divided by the second beam splitter  44 ; and  48  a second photo detector to obtain the information reproduction signal, focusing error signal, and tracking error signal from the converged reflected lights. The above components  42  to  48  construct a second converging optical system  41 . The above first converging optical system  31 , the second converging optical system  41 , and the actuator  40  are mounted on a same base member (not shown) and construct the third optical head  30 . The base member is ordinarily made of aluminum or the like and is attached to the first linear motor  4 . 
     In a manner similar to the case which has been described in the first embodiment of the invention, for instance, the first objective lens  36  is designed so that NA=0.45 and the aberration by the disc substrate of a thickness d 1  is corrected. The second objective lens  46  is designed so that, for instance, NA=0.8 and the aberration by the disc substrate of a thickness d 2  is corrected. 
     The operation of the optical disc apparatus of the embodiment constructed as mentioned above will now be described hereinbelow. First, the case of the second optical disc will be explained. When the cartridge  2  is loaded into the optical disc apparatus of the embodiment, the LED  8  emits the light and the photo diode  9  detects the presence or absence of the transmission light which passes through the discrimination hole  7 . If the transmission light has been detected, the system controller  22  determines that the disc in the loaded cartridge  2  is the second optical disc, so that the controller  22  selects the second converging optical system  41  of the third optical head  30 . That is, the system controller  22  generates control signals to the first and fifth selectors  10  and  21  so as to make the second semiconductor laser  42  and the second photo detector  48  operative. The laser beam emitted from the second semiconductor laser  42  is converted into the parallel beam by the second collimating lens  43 . The parallel beam is converged onto the second optical disc  1  by the second objective lens  46  through the second beam splitter  44  and the second mirror  45 . The laser beam reflected by the disc is again converted into the parallel beam by the second objective lens  46  and is separated from the optical axis by the second beam splitter  44  through the second mirror  45  and is converged onto the second photo detector  48  by the second detecting lens  47 . 
     The second photo detector  48  produces the information signal, focusing error signal, and tracking error signal from the converged reflected light from the disc and supplies to the first selector  10 . The actuator  40  finely moves the lens holder  39  in the tracking and focusing directions by the driving currents from the tracking control circuit  11  and the focusing control circuit  13 , thereby accurately converging the laser beam onto an information track on the optical disc  1 . 
     Since the operations of the first linear motor  4 , first selector  10 , tracking control circuit  11 , focusing control circuit  13 , linear motor control circuit  15 , spindle control circuit  17 , spindle motor  18 , signal processing circuit  19 , LD driving circuit  20 , fifth selector  21 , and system controller  22  are substantially the same as the optical disc apparatus of the first embodiment mentioned above, their descriptions are omitted here. 
     On the other hand, in the case of the first optical disc, since the photo diode  9  doesn&#39;t detect the transmission light, the system controller  22  decides that the disc in the loaded cartridge  2  is the first optical disc mentioned above, so that the controller  22  selects the first converging optical system  31  of the third optical head  30 . That is, the system controller  22  generates control signals to the first and fifth selectors  10  and  21  so as to make the first semiconductor laser  32  and the first photo detector  38  operative. The operation of the first converging optical system  31  is substantially the same as that of the second converging optical system  41  mentioned above. Until the cartridge  2  is unloaded, the recording, reproduction, or erasure of the information signal onto/from the first optical disc is executed by the first converging optical system  31 . 
     As described above, according to the second embodiment, in addition to the effect by the first embodiment, since the first converging optical system  31  and the second converging optical system  41  are mounted on the same base and construct the third optical head  30 , the linear motor can be commonly used as a single part and the number of parts can be reduced. 
       FIG. 7  shows a constructional diagram of an optical disc apparatus according to the third embodiment of the invention. In the diagram, the third embodiment differs from the second embodiment with respect to a point that a fourth optical head  50  is arranged in place of the third optical head  30  and a point that the first selector  10  and the fifth selector  21  are omitted. That is, the photo detection signal which is generated from the fourth optical head  50  is directly supplied to the tracking control circuit  11 , focusing control circuit  13 , spindle control circuit  17 , and signal processing circuit  19 . The driving current which is generated from the LD driving circuit  20  is directly supplied to the fourth optical head  50 . 
     Further,  FIG. 8  shows a detailed constructional diagram of the fourth optical head  50  in the third embodiment of the invention. In the diagram, reference numeral  1  denotes the first or second optical disc;  32  the first semiconductor laser as a light source;  33  the first collimating lens to convert the laser beam from the first semiconductor laser  32  into the parallel beam;  34  the first beam splitter to divide the laser beam into two beams; and  35  the first mirror to change the direction of the beam. The first objective lens  36  and the second objective lens  46  are the same as those mentioned in the second embodiment of the invention. The first objective lens  36  is arranged between the first mirror  35  and the optical disc  1 . The second objective lens  46  is arranged between the first beam splitter  34  and the optical disc  1 . Moreover, as mentioned in the foregoing embodiment, the aberration of the first objective lens  36  has been corrected in correspondence to the first optical disc having the disc substrate of the thickness d 1 . The aberration of the second objective lens  46  has been corrected in correspondence to the second optical disc having the disc substrate of the thickness d 2 . Reference numeral  51  denotes a first shutter arranged between the first objective lens  36  and the first mirror  35  and on the optical path which passes through the first objective lens  36 . Reference numeral  52  denotes a second shutter which is arranged between the second objective lens  46  and the first beam splitter  34  and on the optical path which passes through the second objective lens  46 . The first and second shutters  51  and  52  open or close the optical paths by control signals from the system controllers  22 , respectively. Reference numeral  37  denotes the first detecting lens to converge the reflected lights which have been divided by the beam splitter  34 . Reference numeral  38  denotes the first photo detector to obtain the information reproduction signal, focusing error signal, and tracking error signal from the converged reflected light. 
     Among the foregoing component elements, the first objective lens  36  constructs the first converging optical system mentioned in the second embodiment of the invention together with the first semiconductor laser  32 , the first collimating lens  33 , the first beam splitter  34 , and the first mirror  35 . The second objective lens  46  constructs the second converging optical system together with the first semiconductor laser  32 , the first collimating lens  33 , the first beam splitter  34 , and the first mirror  35  which are commonly used for the first converging optical system. The first converging optical system is mounted onto a common base (not shown) together with the first and second shutters  5  and  52 , thereby constructing the fourth optical head  50 . Since the lens holder  39  and the actuator  40  have the same construction as those in the third optical head  30  in the second embodiment, their descriptions are omitted here. The fourth optical head  50  is attached to the first linear motor  4 . 
     The operation of the optical disc apparatus in the embodiment with the above construction will now be described hereinbelow. The kind of optical disc is detected in a manner similar to the above. When the system controller  22  determines that the disc in the loaded cartridge  2  is the second optical disc, the controller  22  generates control signals to the first and second shutters  51  and  52  of the fourth optical head  50 . When the control signals are supplied, the first shutter  51  is closed and the second shutter  52  is open. In the above state, the laser beam emitted from the first semiconductor laser  32  is converted into the parallel beam by the first collimating lens  33  and is divided into the transmission light and the reflected light by the first beam splitter  34 . The transmission light is shut out by the first shutter  51  via the first mirror  35 . Consequently, only the reflected light passes through the second shutter  52  and is converged onto the optical disc  1  by the second objective lens  46 . The light reflected by the optical disc  1  is again converted into the parallel light by the second objective lens  46  and passes through the second shutter  52  and is reflected and separated by the first beam splitter  34  and is converged onto the first photo detector  38  by the first detecting lens  37 . The first photo detector  38  generates the focusing error signal and tracking error signal from the converged reflected light of the disc and reproduces the information signal on the disc. The above operations are executed until the cartridge  2  is unloaded. 
     Since the operations of the first linear motor  4 , tracking control circuit  11 , focusing control circuit  13 , linear motor control circuit  15 , spindle control circuit  17 , spindle motor  18 , signal processing circuit  19 , LD driving circuit  20 , and system controller  22  are the same as those in the optical disc apparatus of the foregoing first embodiment, their descriptions are omitted here. 
     On the other hand, if the system controller  22  determines that the disc in the loaded cartridge  2  is the first optical disc, the first shutter  51  is opened and the second shutter  52  is closed. In the above state, in the transmission light and the reflected light by the first beam splitter  34 , the reflected light is shut out by the second shutter  52  and only the transmission light passes through the first shutter  51  and is converged onto the optical disc  1  by the first objective lens  36 . The other operations are executed in a manner similar to those in the case of the second optical disc. 
     As mentioned above, according to the third embodiment, in addition to the effects by the second embodiment, since the first and second shutters  51  and  52  are provided as light flux selecting means, the semiconductor laser, collimating lens, beam splitter, detecting lens, and photo detector in each of the converging optical system can be commonly used and the size and weight of the optical head can be reduced. Therefore, assuming that a driving force of the linear motor is the same, the improvement of the performance such as reduction of the seeking time and the like can be realized. 
       FIGS. 9A and 9B  show a detailed constructional diagram of an optical head of an optical disc apparatus in the fourth embodiment of the invention. In  FIG. 9A , since the optical disc  1 , first semiconductor laser  32 , first collimating lens  33 , first beam splitter  34 , second objective lens  46 , first detecting lens  37 , and first photo detector  38  are constructed in a manner similar to those in the foregoing fourth optical head  50 , their descriptions are omitted here. Reference numeral  56  denotes a lens holder to hold the second objective lens  46 ;  57  an actuator to which the lens holder  56  is attached;  54  a wave front correcting lens attached to a slider  55 , which will be explained hereinlater, so that the optical axis is in parallel with the optical axis of the second objective lens  46 ; and  55  the slider which supports the wave front correcting lens  54  and is arranged so as to transverse in the plane which is perpendicular to the light flux between the first beam splitter  34  and the second objective lens  46 , thereby enabling the wave front correcting lens  54  to be moved in such a plane. Moreover, such a movable range is set to a position (shown by P 1  in the diagram) where the wave front correcting lens  54  is perfectly deviated out of the light flux or a position (shown by P 2  in the diagram) where the optical axis of the slider  55  coincides with the optical axis of the second objective lens  46 . The above-mentioned component elements are attached to a base (not shown) and construct a fifth optical head  53 . 
       FIG. 9B  is a plan view when the wave front correcting lens  54  and the slider  55  are seen from the direction of the optical axis. In the diagram, the lens  54  is movable in the directions shown by arrows. The wave front correcting lens  54  has been designed in a manner such that a synthetic optical system with the second objective lens  46  is identical to the foregoing first objective lens. That is, the lens  54  has been designed so as to correct the aberration by the disc substrate of the first optical disc. In the fifth optical head  53 , the second objective lens  46  constructs the second converging optical system mentioned in the second embodiment of the invention together with the first semiconductor laser  32 , first collimating lens  33 , and first beam splitter  34  and can be also regarded such that they construct the first converging optical system by adding the wave front correcting lens  54  to the second converging optical system. 
     Since a whole construction of the optical disc apparatus in the fourth embodiment is substantially the same as that of the optical disc apparatus of the third embodiment shown in  FIG. 7  mentioned above, its description is omitted here. 
     The operation of the optical disc apparatus in the fourth embodiment with the above construction will now be described hereinbelow with respect to only the fifth optical head  53 . The kind of optical disc is detected in a manner similar to the above. If the system controller  22  determines that the disc in the loaded cartridge  2  is the second optical disc, the controller  22  generates a control signal to the slider  55 . When the control signal is supplied, the slider  55  moves the wave front correcting lens  54  to the position P 1 . The laser beam emitted from the first semiconductor laser  32  is converted into the parallel light by the first collimating lens  33  and is reflected by the first beam splitter  34  and is converged onto the optical disc  1  by the second objective lens  46 . The light reflected by the optical disc  1  is again converted into the parallel light by the second objective lens  46 . The parallel light passes through the first beam splitter  34  and is converged onto the first photo detector  38  by the first detecting lens  37 . The first photo detector  38  generates a photo detection signal in a manner similar to the above. The above operations are executed until the optical disc  1  is unloaded. 
     On the other hand, if the system controller  22  decides that the disc in the loaded cartridge  2  is the first optical disc, the slider  55  moves the wave front correcting lens  54  to the position P 2 . Thus, the laser beam emitted from the first semiconductor laser  32  passes through the wave front correcting lens  54  and the second objective lens  46  and is converged onto an information track on the optical disc  1  without an aberration. Thus, the operations similar to those in the case of the second optical disc are executed. 
     As mentioned above, according to the embodiment, in addition to the effects by the second embodiment, since the wave front correcting lens  54  serving as an aberration correcting means is held by the slider  55  and is movably arranged, the objective lens can be commonly used and a total mass which must be moved by the actuator  57  can be reduced. Thus, a burden to the driving force of the actuator can be reduced and a low electric power consumption can be accomplished. 
     The optical head  50  in the fourth embodiment has the second objective lens  46  corresponding to the optical disc having the disc substrate to the thickness d 2  and, further, corrects the aberration to the optical disc having the disc substrate of the thickness d 1  by the wave front correcting lens  54 . However, an opposite construction can be also used. Namely, the above effect is also derived by a construction such that the first objective lens  36  corresponding to the optical disc of the disc substrate of the thickness d 1  is used in place of the second objective lens  46  and a wave front correcting lens which has been designed so as to correct the aberration due to the disc substrate of the thickness d 2  is provided. 
     Although the above three embodiments have been described with respect to the case where there are two kinds of thicknesses of the disc substrates, the invention, can be also applied to the case of three or more kinds of thicknesses of the disc substrates. In such a case, the number of optical elements such as objective lenses and the like is increased in accordance with the number of kinds of thicknesses of the disc substrates. With respect to the discriminating means of the optical disc, three or more kinds of optical discs can be discriminated by, for instance, checking a plurality of discrimination holes which are formed in the cartridge. For example, by forming n discriminating holes, 2 n  kinds of optical discs can be discriminated. 
     Further, although the discrimination hole  7  formed on the cartridge  2 , the LED  8 , and the photo diode  9  have been used as disc discriminating means, paints of different reflectances can be also coated onto the surface of the cartridge  2  in place of the discrimination hole or a mechanical switch or the like can be also used in place of the LED and the photo diode. 
     Further, a difference between thicknesses of the disc substrates can be also directly discriminated by a reflected laser beam from the discs without using the cartridge. For example, in the case of the converging optical system corresponding to the thin disc substrate, a tracking error signal cannot be ordinarily obtained from an optical disc of a thick disc substrate due to a spherical aberration of the converging beam. Consequently, two optical discs having different thicknesses can be discriminated by checking the presence or absence of the tracking error signal. In such a case, there is an excellent effect such that the apparatus is simplified because there is no need to use the detecting means such as LED and photo diode and the like. 
     The optical head in each of the above-described optical disc apparatuses has been constructed by a conventional optical system using the objective lens made of a quartz glass or the like. An optical head in each of optical disc apparatuses of embodiments, which will be explained hereinlater, differs from the above optical head and is constructed by forming an optical system onto a thin film waveguide. 
       FIG. 10  is a block diagram showing a construction of an optical disc apparatus according to the fifth embodiment of the invention. Further,  FIG. 11  is a schematic perspective view showing a construction of an optical head of the optical disc apparatus in the fifth embodiment of the invention. Since a construction shown in  FIG. 10  is substantially the same as that of the optical disc apparatus in the second embodiment of the invention shown in  FIG. 5  except that a sixth optical head  60  is used, its description is omitted here. The sixth optical head  60  shown in  FIG. 11  will now be described in detail hereinbelow. 
     In  FIG. 11 , reference numeral  1  denotes the same optical disc as that described in the foregoing embodiments. Reference numeral  200  denotes an information track formed on the optical disc  1 . Reference numeral  61  denotes a substrate formed by LiNbO 3  or the like. The substrate  61  is attached to a head base through a focusing actuator and a tracking actuator and constructs the sixth optical head  60  together with them. Since the focusing actuator, tracking actuator, and head base which have conventionally been well known can be used as those components, their detailed description and the drawings are omitted here. Reference numeral  62  denotes an optical waveguide formed on the substrate  51  by Ti diffusion or the like;  63  a first semiconductor laser coupled to an edge surface of the optical waveguide  62 ; and  64  a first waveguide lens arranged on an optical path of the waveguide light which has been emitted from the first semiconductor laser  63  and entered the optical waveguide  61 . For instance, a Fresnel lens formed by an electron beam lithography can be used as a lens  64 . Reference numeral  65  denotes a first converging grating coupler formed on the optical path of the parallel waveguide light. The coupler  65  emits the waveguide light to a position out of the optical waveguide  62  and converges onto the optical disc  1 . The first converging grating coupler  65  is a grating having a chirp (irregular period) by a curve formed on the waveguide by electron beam direct drawing or the like. Reference numeral  66  denotes a first beam splitter which is arranged between the first waveguide lens  64  and the first converging grating coupler  65  and separates the waveguide light which has been returned into the optical waveguide  62  through the first converging grating coupler  65  after it had been reflected by the optical disc  1 . Reference numeral  67  denotes a first waveguide converging lens which is arranged on the optical path of the return waveguide light which has been separated by the first beam splitter  66  and converges the return light. Reference numeral  68  denotes a first photo detector which is coupled to the side surface of the optical waveguide  62  and detects the return waveguide light which has been converged by the first waveguide converging lens  67 . 
     Similarly, reference numeral  69  denotes a second semiconductor laser coupled to the edge surface of the optical waveguide  62 ;  70  a second waveguide lens arranged on the optical path of the waveguide light which has been emitted from the second semiconductor laser  69  and entered the optical waveguide  61 ; and  71  a second converging grating coupler formed on the optical path of the parallel waveguide light. The coupler  71  emits the waveguide light to a position out of the optical waveguide  62  and converges onto the optical disc  1 . Reference numeral  72  denotes a second beam splitter which is arranged between the second waveguide lens  70  and the second converging grating coupler  71  and separates the waveguide light which has been returned into the optical waveguide  62  through the second converging grating coupler  71  after it had been reflected by the optical disc  1 . Reference numeral  73  denotes a second waveguide converging lens which is arranged on the optical path of the return waveguide light which has been separated by the second beam splitter  72  and converges the return waveguide light. Reference numeral  74  denotes a second photo detector which is coupled to the side surface of the optical waveguide  62  and detects the return waveguide light converged by the second waveguide converging lens  73 . 
     A curve chirp grating of the first converging grating coupler  65  has been designed in a manner such that, for instance, NA=0.45 and the emission light can be converged until a diffraction limit and the aberration due to the disc substrate of the thickness d 1  can be corrected. The second converging grating coupler  71  has been designed in a manner such that, for example, NA=0.8 and the aberration due to the disc substrate of the thickness d 2  can be corrected. 
     The first and second beam splitters  66  and  72  are attached at positions which are deviated so that the reflected light of each beam splitter does not enter the other beam splitter as a stray light. 
     Such an optical waveguide and a waveguide type device have been described in detail in, for example, Nishihara, Haruna, and Saihara, “Optical Integrated Circuit”, Ohm Co., Ltd., 1985, or the like. In the invention, both of the above well-known optical waveguide and waveguide type device can be used in the optical waveguide  62  or the like. 
     The operation of the optical head in the fifth embodiment with the above construction will now be described hereinbelow. 
     If the optical disc  1  is the first optical disc, the driving current is supplied to the first semiconductor laser  63 . Then, the laser  63  emits a laser beam from one edge surface of the optical waveguide  62 . The laser beam propagates as a waveguide light. The waveguide light is converted into the parallel light by the first waveguide lens  64 . The parallel light transmits the first beam splitter  66  and subsequently enters the first converging grating coupler  65 . The coupler  65  extracts the parallel light out of the optical waveguide  62  and converges onto the information track  200  on the first optical disc  1 . The reflected light from the disc surface again enters the optical waveguide  62  through the first converging grating coupler  65  and propagates as a return waveguide light in the opposite direction. Further, the return waveguide light is reflected in the direction of the first waveguide converging lens  67  in the first beam splitter  66 . The lens  67  converges the return waveguide light onto the first photo detector  68 . The first photo detector  68  detects the information signal and the servo signals such as focusing error signal, tracking error signal, and the like which have been recorded on the first optical disc  1  on the basis of an intensity and an intensity distribution of the return waveguide light and generates to the outside. By modulating the driving current which is supplied to the first semiconductor laser  63 , the sixth optical head  60  emits the intensity modulated laser beam, thereby recording or erasing the information signal onto/from the first optical disc  1 . 
     On the other hand, if the optical disc  1  is the second optical disc, the operations similar to those in the case of the foregoing first optical disc are executed by the second semiconductor laser  69 , second waveguide lens  70 , second converging grating coupler  71 , second beam splitter  72 , second waveguide converging lens  73 , and second photo detector  74 . 
     The substrate  61  is supported from the head base by a focusing actuator and a tracking actuator. The position of the substrate  61  itself is controlled by the foregoing servo signals so that the laser beam is accurately irradiated onto the information track  200  on the disc. 
     According to the fifth embodiment as mentioned above, by providing the first converging grating coupler  65  which is formed on the optical waveguide  62  and corresponds to the thickness of the disc substrate of the first optical disc and the second converging grating coupler  71  which is formed on the optical waveguide  62  and corresponds to the thickness of the disc substrate of the second optical disc, a desired one of the couplers  65  and  71  can be independently used in accordance with the kind of disc, so that the aberration of the converged spot can be corrected in accordance with the thickness of the disc substrate and the signal can be preferably recorded, reproduced, or erased. Moreover, since the optical waveguide device having the converging grating couplers is used, the size and weight of the optical head can be reduced. 
     Although the thickness of the disc substrate has been set into two kinds of thicknesses in the fifth embodiment, the invention can be also applied to three or more kinds of thicknesses of disc substrates. In such a case, the number of component elements on the substrate  61  is increased in accordance with the number of thicknesses. 
     In the sixth optical head  60 , either one of the semiconductor lasers has been allowed to emit the light. However, it is also possible to allow both of the semiconductor lasers to simultaneously emit the lights. In such a case, by designing two converging grating couplers for the optical disc having the same substrate thickness, two tracks on the optical disc  1  can be simultaneously reproduced or recorded. Thus, there is an excellent effect such that the reproducing or recording transfer speed can be doubled. 
       FIG. 12  is a schematic perspective view showing a construction of an optical head of an optical disc apparatus according to the sixth embodiment of the invention. 
     In the diagram, the sixth embodiment has substantially the same construction as that of the sixth optical head  60  shown in  FIG. 11  except a third beam splitter  81  and a waveguide mirror  82  and the same parts and components as those shown in  FIG. 11  are designated by the same reference numerals. That is, an optical head of the sixth embodiment, namely, a seventh optical head  80  is constructed in the following manner. In place of the second semiconductor laser  69  and the second waveguide lens  70  in the sixth optical head  60  shown in  FIG. 11 , the third bean splitter  81  is arranged on the optical path between the waveguide lens  64  and the beam splitter  66 . In the two waveguide lights divided by the third beam splitter  81 , the waveguide mirror  82  is arranged in the direction of the waveguide divided in the direction different from the direction of the first beam splitter  66  and the position of the mirror  82  is set to a position where the waveguide light reflected by the waveguide mirror  82  passes through the second beam splitter  72 . 
     The operation of the seventh optical head  80  with the above construction will now be described hereinbelow. 
     The driving current is supplied to the first semiconductor laser  63 . The laser  63  emits a laser beam from one edge surface of the optical waveguide  62 . The laser beam propagates as a waveguide light. The waveguide light is converted into the parallel light by the first waveguide lens  64  and is divided into the transmission light and the reflected light by the third beam splitter  81 . The transmission light is transferred to the first converging grating coupler  65  through the first beam splitter  66 . The reflected light is reflected by the waveguide mirror  82  and enters the second converging grating coupler  71  through the second beam splitter  72 . The subsequent operations are executed in a manner similar to those of the sixth optical heat  60  in the fifth embodiment of the invention. 
     According to the sixth embodiment as mentioned above, in addition to the effects by the foregoing fifth embodiment, by dividing the waveguide light emitted from one semiconductor laser into two lights by the third beam splitter  81  and guiding to the respective converging grating couplers, the number of semiconductor lasers which are used can be reduced. 
     Although the sixth embodiment has been described on the assumption that the number of thicknesses of the disc substrates is set to two kinds of thicknesses, the invention can be also applied to three or more kinds of thicknesses of the disc substrates. Now, assuming that the number of kinds of thicknesses of the disc substrates is equal to N, it is sufficient to use N converging grating couplers and (N−1) beam splitters for dividing the waveguide light emitted from the semiconductor laser. To equalize all of light quantities of the laser beams which are converged onto the discs, it is preferable to design the beam splitters so as to set division ratios of the light quantities of the beam splitters as follows.
 
1:N−1
 
1:N−2
 
1:N−3
 
.
 
.
 
.
 
1:1
 
       FIG. 13  is a schematic perspective view showing a construction of an optical head of an optical disc apparatus according to the seventh embodiment of the invention.  FIG. 14  is a block diagram showing a construction of the optical disc apparatus. 
     A construction of the optical head in  FIG. 13  will be first described in detail. 
     In the diagram, since the optical disc  1 , information track  200 , substrate  61 , optical waveguide  62 , first semiconductor laser  63  and first waveguide lens  64  are fundamentally identical to the component elements in the seventh optical head  80  shown in  FIG. 12 , their detailed descriptions are omitted here. Reference numeral  91  denotes an SAW (surface acoustic wave) transducer arranged on the optical waveguide  62  so that a surface acoustic wave generated by the SAW transducer crosses the optical path of the waveguide light emitted from the first waveguide lens  64 . The SAW transducer  91  is constructed by a cross finger electrode comprising a piezoelectric transducer of ZnO or the like. Reference numeral  92  indicates a surface acoustic wave generated by the SAW transducer  91 ;  96  a third converging grating coupler formed on the optical path of the waveguide light which has been diffracted by such a surface acoustic wave  92  and propagates in the first direction; and  97  a fourth converging grating coupler which is likewise formed on the optical path of the waveguide light propagating in the second direction. Each of the couplers  96  and  97  emits the waveguide light to a region out of the optical waveguide  62  and converges onto the optical disc  1 . Reference numeral  93  denotes a fourth beam splitter which is arranged between the first waveguide lens  64  and the progressing path of the surface acoustic wave  92  and reflects the waveguide light returned into the optical waveguide  62  through the third or fourth converging grating couplers  96  and  97  after it had been reflected by the optical disc  1 . Reference numeral  94  denotes a third waveguide converging lens which is arranged on the optical path of the return light reflected by the fourth beam splitter  93  and converges the return light and  95  indicates a third photo detector which is coupled to the side surface of the optical waveguide  62  and detects the return light converged by the third waveguide converging lens  94 . 
     The above SAW transducer has also been described in detail in the foregoing “Optical Integrated Circuit” or the like and both of the well-known optical waveguide and waveguide type device described in the above literature can be also obviously used. 
     The operation of the optical head in the seventh embodiment with the above construction will now be described hereinbelow. 
     The first semiconductor laser  63  emits a laser beam from one edge surface of the optical waveguide  62 . The laser beam propagates as a waveguide light. The waveguide light is converted into the parallel light by the first waveguide lens  64  and transmits through the fourth beam splitter  93 . After that, the light transverses the surface acoustic wave  92  generated from the SAW transducer  91 . At this time, the propagating direction of the parallel waveguide light is changed by an acoustic optical interaction with the surface acoustic wave  92 . Since a deflection angle at this time changes in accordance with a frequency of the surface acoustic wave  92 , the waveguide light can be propagated in any one of the directions of the third and fourth converging grating couplers  96  and  97  in accordance with frequencies of high-frequency voltages which are applied to the SAW transducer  91  from the outside (it is now assumed that the frequencies of the high-frequency frequency voltages are set to f 1  and f 2 , respectively). In the case of the first optical disc, therefore, the high-frequency voltage of the frequency f 1  is applied to the SAW transducer  91  from the outside, thereby allowing the parallel waveguide light to enter the third converging grating coupler  96 . The third converging grating coupler  96  extracts the parallel waveguide light to a region out of the optical waveguide  62  and converges onto the information track  200  on the first optical disc  1 . The reflected light from the disc surface again enters the optical waveguide  62  through the third grating coupler  96  and propagates as a return waveguide light in the opposite direction. The progressing direction of the waveguide light is changed by the surface acoustic wave  92  and, after that, the waveguide light is reflected in the direction of the third waveguide converging lens  94  by the fourth beam splitter  93 . The third waveguide converging lens  94  converges the return light to the third photo detector  95 . The third photo detector  95  detects the information signal and the servo signals such as focusing error signal, tracking error signal, and the like which have been recorded on the first optical disc  1  on the basis of an intensity and an intensity distribution of the return light and generates to the outside. By modulating the driving current which is supplied to the first semiconductor laser  63 , an eighth optical head  90  emits the intensity modulated laser beam, thereby recording or erasing the information signal onto/from the first optical disc  1 . 
     On the other hand, in the case of the second optical disc, the high-frequency voltage of the frequency f 2  is applied to the SAW transducer  91  from the outside, thereby allowing the parallel waveguide light to enter the fourth converging grating coupler  97 . The subsequent operations are executed in a manner similar to those in the case of the first optical disc. 
     The substrate  61  is supported from the head base by a focusing actuator and a tracking actuator (not shown). The position of the substrate  61  itself is controlled by the servo signals so that the laser beam is accurately irradiated onto the information track  200  on the disc. 
     An optical disc apparatus having the eighth optical head  90  mentioned above will now be described with reference to FIG.  14 . 
     In the diagram, the optical disc  1 , cartridge  2 , first linear motor  4 , discrimination hole  7 , LED  8 , photo diode  9 , tracking control circuit  11 , focusing control circuit  13 , linear motor control circuit  15 , spindle control circuit  17 , spindle motor  18 , signal processing circuit  19 , LD driving circuit  20  and system controller  22  are the same as those in the optical disc apparatus in the third embodiment according to the invention. Reference numeral  90  denotes the eighth optical head which is constructed by the waveguide substrate, focusing actuator, tracking actuator, head base and the like. Reference numeral  85  denotes a constant voltage generating circuit which receives a control signal from the system controller  22  and generates a predetermined voltage V i . Reference numeral  86  denotes a V/f converting circuit which receives the voltage V i  from the constant voltage generating circuit  85  and generates a high-frequency signal of a frequency f which is proportional to V i . The V/f converting circuit  86  generates a high-frequency signal of the frequency f 1  when the input voltage V i =V 1  and generates a high frequency signal of the frequency f 2  when V i =V 2 . Reference numeral  87  denotes an SAW driving circuit to apply a high-frequency voltage of the same frequency as the frequency f of the high-frequency signal supplied from the V/f converting circuit  86  to the SAW transducer  91  of the eighth optical head  90 . 
     The operation of the optical disc apparatus in the seventh embodiment with the above construction will now be described hereinbelow. 
     First, if the cartridge  2  enclosing the second optical disc has been loaded into the optical disc apparatus of the seventh embodiment, the system controller  22  determines that the disc in the loaded cartridge  2  is the second optical disc by the detection signal of the photo diode  9 , so that the controller  22  generates a control signal to the constant voltage generating circuit  85  so as to generate the voltage V 2 . The V/f converting circuit  86  converts the input voltage V 2  into the frequency f 2 , so that the SAW driving circuit  87  applies the high-frequency voltage of the frequency f 2  to the SAW transducer  91  of the eighth optical head  90 . Therefore, in the eighth optical head  90 , the laser beam is irradiated from the second converging grating coupler  97  and is converged without an aberration onto the information track  200  on the second optical disc having the disc substrate of the thickness d 2 . At the same time, the third photo detector  95  of the eighth optical head  90  detects a focusing error signal and a tracking error signal from the reflected light from the optical disc and supplies to the tracking control circuit  11  and the focusing control circuit  13 . Further, the information signal on the disc is supplied to the signal processing circuit  19  and the spindle control circuit  17 . 
     On the other hand, in the case of the first optical disc, the system controller  22  generates a control signal to the constant voltage generating circuit  85  so as to generate the voltage V 1 . The V/f converting circuit  86  converts the input voltage V 1  into the frequency f 1 , so that the SAW driving circuit  87  applies the high-frequency voltage of the frequency f 1  to the SAW transducer  91  of the eighth optical head  90 . Therefore, in the eighth optical head  90 , a laser beam is emitted from the first converging grating coupler  96  and is converged without an aberration onto the information track  200  on the first optical disc having the disc substrate of the thickness d 1 . The other operations are executed in a manner similar to those in the case of the foregoing second optical disc. 
     According to the embodiment as mentioned above, in addition to the effects of the above sixth embodiment, the number of semiconductor lasers which are necessary in the eighth optical head  90  is only one and each of the converging grating couplers does not simultaneously emit the laser beam, so that an emission power of the semiconductor laser can be efficiently taken out of the converging grating coupler. That is, the optical head having a transfer efficiency better than that of the seventh optical head  80  in the foregoing sixth embodiment can be provided. 
     Further, by arranging the fourth beam splitter  93  between the waveguide lens  64  and the SAW transducer  91 , the return lights from the two converging grating couplers can be detected by one photo detector. 
     Although the number of thicknesses of the disc substrates has been set to two kinds of thicknesses in the embodiment, the invention can be also obviously applied to three or more kinds of thicknesses of the disc substrates. In such a case, the number of converging grating couplers is increased in accordance with the number of kinds of thicknesses and the optical paths are switched by the SAW transducer  91  in accordance with the increased number of such couplers. 
     An optical disc apparatus in the eighth embodiment of the invention will now be described. 
       FIG. 15  is a block diagram showing a construction of the optical disc apparatus in the eighth embodiment. In the diagram, reference numeral  1  denotes the first or second optical disc;  2  the cartridge;  4  the linear motor;  7  the discrimination hole;  8  the LED;  9  the photo diode;  13  the focusing control circuit;  15  the linear motor control circuit;  17  the spindle control circuit;  18  the spindle motor;  19  the signal processing circuit;  20  the LD driving circuit;  22  the system controller;  85  the constant voltage generating circuit;  86  the V/f converting circuit; and  87  the SAW driving circuit. The above component elements are the same as those in the optical disc apparatus in the seventh embodiment of FIG.  14  and their detailed descriptions are omitted here. Reference numeral  90  denotes an optical head which is substantially the same as the eighth optical head  90  mentioned above except that the optical head in the eighth embodiment does not have a tracking actuator. Therefore, the optical head in  FIG. 15  is also referred to as an eighth optical head  90  hereinafter for convenience of explanation. Reference numeral  100  denotes a tracking error detecting circuit which receives a tracking error signal from the third photo detector  95  of the eighth optical head  90  and generates a tracking error voltage V TE  to an adder  101 , which will be explained hereinafter. Reference numeral  101  denotes the adder. The voltage V TE  which is generated from the tracking error detecting circuit  100  and the voltage V i  which is generated from the constant voltage generating circuit  85  are supplied to the adder  101 , so that the adder generates a voltage V 0  (V 0 =V TE +V i ) to the V/f converting circuit  86 . The V/f converting circuit  86 , SAW driving circuit  87 , tracking error detecting circuit  100 , and adder  101  construct a tracking control circuit  102 . That is, it is the inventive point of the eighth embodiment that the tracking control is executed by using the SAW transducer  91  of the eighth optical head  90 . 
     The principle of the tracking control of the eighth embodiment will now be described hereinbelow with reference to the drawings.  FIG. 16  is an enlarged schematic perspective view of the converging grating coupler, SAW transducer, and portion where a surface acoustic wave has been formed. The waveguide light which enters the converging grating coupler is oscillated between solid lines and broken lines in accordance with a microchange of the frequency of the surface acoustic wave. Such an oscillation angle is called a deflection angle (shown by θ). Therefore, the emission light from the converging grating coupler is also oscillated and the converged spot moves. Since the deflecting angle θ changes in almost proportional to the frequency of the surface acoustic wave, by changing the frequency in accordance with the tracking error amount, the converged spot can be accurately positioned onto the information track. 
     The operation of the optical disc apparatus of the embodiment constructed as shown in  FIG. 16  will now be described hereinbelow. First, if the disc in the loaded cartridge  2  is the second optical disc, the system controller  22  controls the constant voltage generating circuit  85  so as to set the output voltage V i  into V 2 . The output voltage V TE  of the tracking error detecting circuit  100  has been initialized to “0”. The adder  101  adds the voltages V i  and V TE  and generates the voltage V 0  (=V 2 ) to the V/f converting circuit  86 . The V/f converting circuit  86  changes a frequency f s  of an output signal in accordance with the input voltage V 0 . The optical disc apparatus has been designed in a manner such that the signal of a frequency f s (=f 1 ) is generated when V 0 =V 1  and the signal of a frequency f s (=f 2 ) is generated when V 0 =V 2  and the frequency f s  changes in proportion to the input voltage V 0 . Therefore, the V/f converting circuit  86  supplies a high-frequency signal of the frequency f 2  to the SAW driving circuit  87 . The SAW driving circuit  87  applies a high-frequency voltage of the frequency f 2  to the SAW transducer  91  of the eighth optical head  90 . In the eighth optical head  90 , consequently, the laser beam is emitted from the fourth converging grating coupler  97  and is converged without an aberration onto the information track on the second optical disc. At the same time, in the eighth optical head  90 , the reflected light from the disc is detected by the third photo detector  95 . A tracking error signal is supplied to the tracking error detecting circuit  100 . A focusing error signal is supplied to the focusing control circuit  13 . The information signal is supplied to the spindle control circuit  17  and the signal processing circuit  19 . The tracking error detecting circuit  100  produces the tracking error voltage V TE  in accordance with a track deviation amount of the converged spot on the information track  200  and supplies to the adder  101 . The adder  101  sends the output voltage V 0 =V 2 +V TE  to the V/f converting circuit  86  as mentioned above. In accordance with the output voltage V 0 , the output signal frequency f s  of the V/f converting circuit  86  is deviated from the frequency f 2  by a value corresponding to the tracking error (assumes d f ). As mentioned above, when the frequency of the driving voltage to the SAW transducer  91  changes, the emitting position of the light from the fourth converging grating coupler  97  changes and the position of the converged spot on the optical disc  1  changes for the track. Therefore, by setting a converting equation between V 0  and f s  of the V/f converting circuit  86  so as to allow the converged spot on the optical disc  1  to approach the track, the tracking error is eliminated. The other operations are executed in a manner similar to those of the optical disc apparatus in the seventh embodiment. 
     On the other hand, in the case of the first optical disc, the system controller  22  controls the constant voltage generating circuit  85 , thereby setting the output voltage V i  into V 1 . Thus, the V/f converting circuit  86  generates a high-frequency signal of the frequency f 1  to the SAW driving circuit  87  and the SAW driving circuit  87  applies a high-frequency voltage of the frequency f 1  to the SAW transducer  91  of the eighth optical head  90 . Consequently, in the eighth optical head  90 , the laser beam is emitted from the third converging grating coupler  96  and is converged without an aberration onto the information track  200  on the first optical disc. At the same time, the tracking error detecting circuit  100  supplies the tracking error voltage V TE  to the adder  101  from the input signal T 1 . The input voltage of the V/f converting circuit  86  is set to V 0 =V 1 +V TE  and the tracking error can be eliminated in a manner similar to the case of the second optical disc. 
       FIG. 17  is a graph showing the principle of the tracking control of the embodiment and shows the relations among the V 0  and f s  and the deflection angle of the waveguide light in the eighth optical head  90 . As shown in the graph, by varying the V 0  and f s  by only an amount which is proportional to the tracking error signal from V 1 , accordingly, f 1  as a center in the case of the first optical disc or by only an amount which is proportional to the tracking error signal from V 2 , accordingly, f 2  as a center in the case of the second optical disc, the oscillation angle of the waveguide light can be finely varied. Therefor, by varying the emitting positions of the light beams from the two converging grating couplers, the converged spot can be allowed to trace on the track. 
     According to the eight embodiment as mentioned above, in addition to the effects of the foregoing seventh embodiment, the change-over of the waveguide lights which enter the converging grating couplers and the tracking control can be executed by the SAW transducer  91 . Thus, the optical head can be simplified and the number of manufacturing steps can be reduced. 
     Since the surface acoustic wave  92  is located between the fourth beam splitter  93  and the two converging grating couplers, the return waveguide light from the optical disc  1  is not influenced by the tracking control on the optical path after the surface acoustic wave  92 . Therefore, the converging position on the third photo detector is not moved by the tracking control, so that a deterioration in photo detection signal can be prevented. 
     In the embodiment, although the SAW transducer has been used as both of the optical path switching means and the optical path deflecting means for tracking control, the SAW transducer can be also provided for the optical head only for the tracking control. For instance, it is also possible to form the SAW transducer for the sixth optical head  60  in the fifth embodiment or the seventh optical head  80  in the sixth embodiment and to execute the tracking control.

Technology Category: 3