Abstract:
An optical disc includes a first substrate having a first central opening, a first signal layer formed on one of the surfaces of the first substrate, a second substrate having a second central opening, a second signal layer formed on one of the surfaces of the second substrate, and a hub having a central longitudinal axis. The first substrate and the first signal layer are designed such that a first central longitudinal axis of the first opening substantially coaxially aligns with a first central longitudinal axis of the first signal layer, and the second substrate and the second signal layer are designed such that a second central longitudinal axis of the second opening substantially coaxially aligns with a second central longitudinal axis of the second signal layer. The first substrate, the second substrate and the hub are bonded together with the result of substantially coaxially alignment of the central longitudinal axis of the first signal layer, the second signal layer and the hub.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims the benefit of the filing date of Provisional Patent Application Serial No. 60/312,121, filed on Aug. 14, 2001, and entitled “Method of Coaxially Aligning the Signal Center Axis with the Hub Center Axis of an Optical Disc”, and Provisional Patent Application Serial No. 60/314,473, filed on Aug. 23, 2001, and entitled “Dual Signal Layer and Thin Substrate Optical Disc and Related Methods.” 
         FIELD OF THE INVENTION  
         [0002]    This invention relates generally to optical discs, and in particular, a method of coaxially aligning the signal center axis of the optical disc with the hub center axis of the optical disc, and the resulting optical disc.  
         BACKGROUND OF THE INVENTION  
         [0003]    An optical disc typically consists of a signal layer formed on a disc-shaped substrate having a central opening. The signal layer spirals around the disc-shaped substrate about a center longitudinal axis. In addition, the optical disc typically includes a generally cylindrical and metallic hub situated within the central opening of the disc-shaped substrate. The hub also has a corresponding center longitudinal axis. When the optical disc is properly inserted into a reader, the hub is coaxially mounted to the spindle motor of the reader (i.e. the hub center is substantially concentric with the center of rotation of the motor). In order for the optical disc reader to read the signal layer properly, the signal center axis should be substantially coaxial with the center of rotation of the spindle motor. Therefore, it follows that the signal center axis should be substantially coaxial with the hub center axis.  
           [0004]    [0004]FIG. 1 illustrates a cross-sectional view of a prior art double-sided optical disc  100 . The optical disc  100  consists of a disc-shaped substrate  102  having a central opening  104 . An upper signal layer  106   a  is formed on the upper surface of the substrate  102  and a lower signal layer  106   b  is formed on the lower surface of the substrate  102 . The upper signal layer  106   a  has an associated signal center longitudinal axis C SA  and the lower signal layer  106   b  has an associated signal center longitudinal axis C SB . The optical disc  100  further consists of an upper hub  108   a  and a lower hub  108   b . The upper and lower hubs  108   a - b  consists of respective cylindrical portions  110   a - b  that extend coaxially within the central opening  104  of the substrate  102  and respective lip portions  112   a - b  that mount on the upper and lower surfaces of the substrate  102 , respectively. The upper hub  108   a  has an associated center longitudinal axis C HA  and the lower hub  108   b  has an associated center longitudinal axis C HB .  
           [0005]    As previously discussed, in order for the optical disc reader to properly read the signal layers  106   a - b  of the optical disc  100 , the signal center longitudinal axes C SA  and C SB  should be substantially coaxial with the hub center longitudinal axes C HA  and C HB , respectively. However, in the prior art optical disc  100 , the signal center longitudinal axes C SA  and C SB  do not necessarily coincide with the center longitudinal axis of the disc-shaped substrate  102  or with each other. In addition, the hub center longitudinal axes C HA  and C HB  do not necessarily coincide with the center longitudinal axes of the disc-shaped substrate  102  or with each other. Thus, in order to align the signal center longitudinal axes C SA  and C SB  respectively to the hub center longitudinal axes C HA  and C HB , lots of trial and error and/or specialized equipment are required. This is typically difficult to accomplish, time-consuming, expensive, and complicates the manufacturing of optical discs.  
           [0006]    Thus, there is a need for an improved method of aligning the signal center longitudinal axis with the hub center longitudinal axis of the optical disc.  
         SUMMARY OF THE INVENTION  
         [0007]    An optical disc according to an embodiment of the invention includes a first substrate having a first central opening, a first signal layer formed on one of the surfaces of the first substrate, a second substrate having a second central opening, a second signal layer formed on one of the surfaces of the second substrate, and a hub having a central longitudinal axis. The first substrate and the first signal layer are designed such that a first central longitudinal axis of the first opening substantially coaxially aligns with a first central longitudinal axis of the first signal layer, and the second substrate and the second signal layer are designed such that a second central longitudinal axis of the second opening substantially coaxially aligns with a second central longitudinal axis of the second signal layer. The first substrate, the second substrate and the hub are bonded together with the result of substantially coaxially alignment of the central longitudinal axis of the first signal layer, the second signal layer and the hub.  
           [0008]    A thickness of the first substrate and a thickness of the second substrate can be substantially same.  
           [0009]    A thickness of the first substrate can also be less than a thickness of the second substrate. For example, the thickness of the first substrate is between 0.05 mm and 0.2 mm, and the thickness of the second substrate is greater than 0.3 mm.  
           [0010]    The hub may comprise a magnetic material or a magnetic sensitive material.  
           [0011]    The signal layer may comprise a recordable material such as a phase change material. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    FIGS.  1  illustrates a cross-sectional view of a prior art optical disc;  
         [0013]    [0013]FIG. 2A illustrates a cross-sectional view of an exploded exemplary optical disc in accordance with embodiment 1 of the invention;  
         [0014]    [0014]FIG. 2B illustrates a cross-sectional view of an assembled exemplary optical disc in accordance with embodiment 1 of the invention;  
         [0015]    [0015]FIG. 3A illustrates a cross-sectional view of an exploded exemplary optical disc in accordance with embodiment 2 of the invention;  
         [0016]    [0016]FIG. 3B illustrates a cross-sectional view of an assembled exemplary optical disc in accordance with embodiment 2 of the invention.  
         [0017]    [0017]FIG. 4A illustrates a cross-sectional view of an exploded exemplary optical disc in accordance with embodiment 3 of the invention;  
         [0018]    [0018]FIG. 4B illustrates a cross-sectional view of an assembled exemplary optical disc in accordance with embodiment 3 of the invention;  
         [0019]    [0019]FIG. 5A illustrates a cross-sectional view of an exploded exemplary optical disc in accordance with embodiment 4 of the invention; and  
         [0020]    [0020]FIG. 5B illustrates a cross-sectional view of an assembled exemplary optical disc in accordance with embodiment 4 of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    (Embodiment 1)  
         [0022]    [0022]FIG. 2A illustrates a cross-sectional view of an exploded exemplary optical disc  200  in accordance with embodiment 1 of the invention. The optical disc  200  comprises an upper disc-shaped substrate  202   a  with an upper central opening  204   a . The upper substrate  202   a  comprises an upper annular protrusion  206   a  extending above the top surface of the upper substrate  202   a , and defining the boundary of the upper central opening  204   a . In addition, the upper substrate  202   a  comprises an annular notch  208   a  located at the bottom surface and coaxial with the central opening  204   a  of the upper substrate  202   a . The upper central opening  204   a  has a central longitudinal axis C OA .  
         [0023]    The optical disc  200  comprises a lower disc-shaped substrate  202   b  with a lower central opening  204   b . The lower substrate  202   b  comprises a lower annular protrusion  206   b  extending below the lower surface of the lower substrate  202   b , and defining the boundary of the lower central opening  204   b . In addition, the lower substrate  204   b  comprises an annular notch  208   b  located at the top surface and coaxial with the central opening  204   b  of the lower substrate  202   b . The lower central opening  202   b  has a central longitudinal axis C OB .  
         [0024]    The optical disc  200  of the invention further comprises an upper signal layer  210   a  formed on the upper surface of the upper substrate  202   a , and a lower signal layer  210   b  formed on the lower surface of the lower substrate  202   b . The upper signal layer  210   a  has a central longitudinal axis C SA , and the lower signal layer  210   b  has a central longitudinal axis C SB . Additionally, the optical disc  200  comprises a cylindrical hub  212  having an annular protrusion  214  extending outwardly from the outer cylindrical wall of the hub  212 . The annular protrusion  214  is centrally located along the cylindrical wall of the hub  212 . The cylindrical hub  212  has a central longitudinal axis C H .  
         [0025]    [0025]FIG. 2B illustrates a cross-sectional view of the assembled exemplary optical disc  200  in accordance with embodiment 1 of the invention. Assembled, the lower surface of the upper substrate  202   a  is attached to the upper surface of the lower substrate  202   b  using an adhesive  218 . The attachment of the upper substrate  202   a  to the lower substrate  202   b  forms an annular groove  216  by the mating of the upper annular notch  208   a  to the lower annular notch  208   b . The annular protrusion  214  of the hub  212  registers within the annular groove  216 . The length of the annular protrusion  214  of the hub  212  is smaller than the depth of the annular groove  216  so that the outer cylindrical wall of the hub  212  are flushed with the walls of the central openings  204   a - b.    
         [0026]    The following explains the method of aligning the central longitudinal axes C SA  and C SB  of the upper and lower signal layers  210   a - b  to the central longitudinal axis C H  of the hub  212  in accordance with the invention. The upper substrate  202   a  and the upper signal layer  210   a  are designed such that the central longitudinal axis C SA  of the upper signal layer  210   a  coaxially aligns with the central longitudinal axis C OA  of the upper central opening  204   a  of the upper substrate  202   a . Also, the lower substrate  202   b  and the lower signal layer  210   b  are designed such that the central longitudinal axis C SB  of the lower signal layer  210   b  coaxially aligns with the central longitudinal axis C OB  of the lower central opening  204   b  of the lower substrate  202   b.    
         [0027]    The hub  212 , having its outer cylindrical wall flushed with the walls of the upper and lower central openings  204   a - b , has a central longitudinal axis C H  that is coaxially aligned with the central longitudinal axes C OA  and C OB  of the upper and lower central openings  204   a - b . Since the central longitudinal axes C SA  and C SB  of the upper and lower signal layers  210   a - b  coaxially align with the central longitudinal axes C OA  and C OB  of the upper and lower central openings  204   a - b , it follows that the central longitudinal axes C SA  and C SB  of the upper and lower signal layers  210   a - b  are coaxially aligned with the central longitudinal axis C H  of the hub  212 . This condition allows for proper reading of the signal layers  210   a - b  by an optical disc reader.  
         [0028]    (Embodiment 2)  
         [0029]    FIGS.  3 A-B illustrate respective cross-sectional views of an exploded and assembled exemplary optical disc  300  in accordance with embodiment 2 of the invention. The optical disc  300  is the same as the optical disc  200 , except that protective layers  316   a - b  cover respectively the upper and lower signal layers  310   a - b.    
         [0030]    (Embodiment 3)  
         [0031]    [0031]FIG. 4A illustrates a cross-sectional view of an exploded exemplary optical disc  400  in accordance with embodiment 3 of the invention. The optical disc  400  comprises an upper disc-shaped substrate  402   a  with an upper central opening  404   a . In the exemplary embodiment, the thickness of the upper substrate  402   a  is greater than approximately 0.3 mm. The upper central opening  404   a  has a central longitudinal axis C OA . The upper substrate  402   a  further comprises an upper signal layer  410   a  formed on the lower surface of the upper substrate  402   a . The upper signal layer  410   a  spirals around a central longitudinal axis C SA .  
         [0032]    The optical disc  400  further comprises a lower disc-shaped substrate  402   b  with a lower central opening  404   b . In the exemplary embodiment, the thickness of the lower substrate  402   b  is approximately 0.05 to 0.2 mm. The lower central opening  402   b  has a central longitudinal axis C OB . The lower substrate  402   b  further comprises a lower signal layer  410   b  formed on the upper surface of the lower substrate  402   b . The lower signal layer  410   b  spirals around a central longitudinal axis C SB .  
         [0033]    Additionally, the optical disc  400  comprises a hub  412  having an upper cylindrical portion  412   a  and a lower cylindrical portion  412   b . In the exemplary embodiment, the diameter of the outer walls of the lower cylindrical portion  412   b  is greater than the diameter of the outer walls of the upper cylindrical portion  412   a . The hub further includes a thru-opening  412   c  that extends longitudinally and coaxially through the upper and lower cylindrical portions  412   a  and  412   b  of the hub  412 . The central longitudinal axis of the hub  412  can be represented as C H .  
         [0034]    [0034]FIG. 4B illustrates a cross-sectional view of the assembled exemplary optical disc  400  in accordance with the invention. Assembled, the lower surface of the upper substrate  402   a  is attached to the upper surface of the lower substrate  402   b  using an adhesive  414 . Also assembled, the upper cylindrical portion  412   a  of the hub  412  extends coaxially within the opening  404   a  of the upper substrate  402   a . Additionally, the lower cylindrical portion  412   b  of the hub  412  extends coaxially within and below the opening  404   b  of the lower substrate  402   b.    
         [0035]    The following explains the method of aligning the central longitudinal axes C SA  and C SB  of the upper and lower signal layers  410   a - b  to the central longitudinal axis C H  of the hub  412  in accordance with the invention. The upper substrate  402   a  and the upper signal layer  410   a  are designed such that the central longitudinal axis C SA  of the upper signal layer  410   a  substantially coaxially aligns with the central longitudinal axis C OA  of the upper central opening  404   a  of the upper substrate  402   a . Also, the lower substrate  402   b  and the lower signal layer  410   b  are designed such that the central longitudinal axis C SB  of the lower signal layer  410   b  substantially coaxially aligns with the central longitudinal axis C OB  of the lower central opening  404   b  of the lower substrate  402   b . These substantially coaxial relations C SA =C OA  and C SB =C OB  can be easily obtained by regular molding of substrates, similar to current CD, DVD, and MD molding processes.  
         [0036]    The hub  412 , having its upper and lower cylindrical portions  412   a - b  flushed with the walls of the upper and lower central openings  404   a - b , has a central longitudinal axis C H  that is substantially coaxially aligned with the central longitudinal axes C OA  and C OB  of the upper and lower central openings  404   a - b . Since the central longitudinal axes C SA  and C SB  of the upper and lower signal layers  410   a - b  substantially coaxially align with the central longitudinal axes C OA  and C OB  of the upper and lower central openings  404   a - b , it follows that the central longitudinal axes C SA  and C SB  of the upper and lower signal layers  410   a - b  are substantially coaxially aligned with the central longitudinal axis C H  of the hub  412 . This condition allows for proper reading of the signal layers  410   a - b  by an optical disc reader.  
         [0037]    (Embodiment 4)  
         [0038]    [0038]FIG. 5A illustrates a cross-sectional view of an exploded exemplary optical disc  500  in accordance with embodiment 4 of the invention. The optical disc  500  comprises an upper disc-shaped substrate  502   a  with an upper central opening  502   a  having an upper portion  504   a - 1  and a lower portion  504   a - 2 . In the exemplary embodiment, the diameter of the upper portion  504   a - 1  is less than the diameter of the lower portion  504   a - 2  of the upper central opening  504   a . The upper central opening  502   a  has a central longitudinal axis C OA . Also in the exemplary embodiment, the thickness of the upper substrate  502   a  is greater than approximately 0.3 mm. The upper substrate  502   a  further comprises an upper signal layer  510   a  formed on the lower surface of the upper substrate  502   a . The upper signal layer  510   a  spirals around a central longitudinal axis C SA .  
         [0039]    The optical disc  500  further comprises a lower disc-shaped substrate  502   b  with a lower central opening  504   b . In the exemplary embodiment, the thickness of the lower substrate  502   b  is approximately 0.05 to 0.2 mm. The lower central opening  502   b  has a central longitudinal axis C OB . The lower substrate  502   b  includes an annular protrusion  508  that extends below the lower surface of the lower substrate  502   b  and defines a lower portion of the central opening  504   b . In addition, the lower substrate  502   b  further comprises a lower signal layer  510   b  formed on the upper surface of the lower substrate  502   b . The lower signal layer  510   b  spirals around a central longitudinal axis C SB .  
         [0040]    Additionally, the optical disc  500  comprises a hub  512  having an upper cylindrical portion  512   a , a lower cylindrical portion  512   b , and a middle cylindrical portion  512   c . In the exemplary embodiment, the diameter of the outer walls of the middle cylindrical portion  512   c  is greater than the diameters of the outer walls of the lower and upper cylindrical portion  512   a - b , which are substantially the same. The hub further includes a thru-opening  512   d  that extends longitudinally and coaxially through the upper, lower, and middle cylindrical portions  512   a - c  of the hub  512 . The central longitudinal axis of the hub  512  can be represented as C H .  
         [0041]    [0041]FIG. 5B illustrates a cross-sectional view of the assembled exemplary optical disc  300  in accordance with embodiment 4 of the invention. Assembled, the lower surface of the upper substrate  502   a  is attached to the upper surface of the lower substrate  502   b  using an adhesive  514 . Also assembled, the upper cylindrical portion  512   a  of the hub  512  extends coaxially within the upper portion  504   a - 1  of the upper central opening  504   a  of the upper substrate  502   a . The middle cylindrical portion  512   c  of the hub  512  extends coaxially within the lower portion  504   a - 2  of the upper central opening  504   a  of the upper substrate  502   a . Additionally, the lower cylindrical portion  512   b  of the hub  512  extends coaxially within the opening  504   b  of the lower substrate  502   b.    
         [0042]    The following explains the method of aligning the central longitudinal axes C SA  and C SB  of the upper and lower signal layers  510   a - b  to the central longitudinal axis C H  Of the hub  512  in accordance with the invention. The upper substrate  502   a  and the upper signal layer  510   a  are designed such that the central longitudinal axis C SA  of the upper signal layer  510   a  substantially coaxially aligns with the central longitudinal axis C OA  of the upper central opening  504   a  of the upper substrate  502   a . Also, the lower substrate  502   b  and the lower signal layer  510   b  are designed such that the central longitudinal axis C SB  of the lower signal layer  510   b  substantially coaxially aligns with the central longitudinal axis C OB  of the lower central opening  504   b  of the lower substrate  502   b . These substantially coaxial relations C SA =C OA  and C SB =C OB  can be easily obtained by regular molding of substrates, similar to current CD, DVD, and MD molding processes.  
         [0043]    The hub  512 , having its upper and middle cylindrical portions  512   a  and  512   c  flushed with the walls of the upper and lower portions  504   a - 1 - 2  of the upper central openings  504   a , has a central longitudinal axis C H  that is coaxially aligned with the central longitudinal axis C OA  of the upper central opening  504   a . Also, the hub  512 , having its lower cylindrical portion  512   b  flushed with the wall of the lower central opening  504   b , has its central longitudinal axis C H  coaxially aligned with the central longitudinal axis C OB  of the lower central opening  504   b . Since the central longitudinal axes C SA  and C SB  of the upper and lower signal layers  510   a - b  coaxially align with the central longitudinal axes C OA  and C OB  of the upper and lower central openings  504   a - b , it follows that the central longitudinal axes C SA  and C SB  of the upper and lower signal layers  510   a - b  are coaxially aligned with the central longitudinal axis C H  of the hub  512 . This condition allows for proper reading of the signal layers  510   a - b  by an optical disc reader.  
         [0044]    In the exemplary optical discs  200 ,  300 ,  400  and  500 , the substrates may be formed of a polycarbonate, the hub is formed of a magnetically-sensitive metal, the adhesive is formed of a bonding resin, such as a ultraviolet curing resin, the signal layers are formed of a reflective layer, such as a phase change material (Te—Ge—Sb), and the protective layer is formed of a ultraviolet curing resin with lower viscosity. The optical disc  200 ,  300 ,  400  or  500  can be a compact disc (CD), a digital versatile disc (DVD), a micro disc (MD), a Data Play disc, or other format. These discs can be formed by a molding process or by a stamping process. If a molding process is used, the discs can be removed from the molding fixture using an air ejection process.  
         [0045]    In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.