Abstract:
The present invention provides an improved optical reading and recording system utilizing optical system employing an aspherical solid immersion lens (SIL). The improved optical system has an aspherical SIL between an objective lens and the recording media. The aspherical SIL reduces the focused electromagnetic radiation beam spot size by providing an increased numeric aperture (NA) of the optical system. The aspherical surface of the SIL provide greater manufacturing and operating tolerances between the objective lens and the aspherical SIL. Greater manufacturing tolerances provide an easier to manufacture optical system. Increased ease of manufacture reduces the cost of optical systems.

Description:
This invention relates generally to an optical recording system and more particularly to an optical recording system employing a solid immersion lens having an aspherical surface that tends to increase the optical system tolerance. 
     BACKGROUND 
     Optical data storage systems are of great commercial and academic interest due to their potential for very high data density storage. Unlike magnetic recording, where data density may be limited by particle size, the density of optical recording is limited by the size of the laser beam spot. Any technological improvement that tends to decreases the size of the laser beam spot may be used in a optical recording system to increase the data density. 
     According to the diffraction theory, when a laser is focused by a lens, the minimum laser beam spot size is the diffraction limit of the lens. The diffraction limit of the lens is proportional to the laser wavelength and inversely proportional to the numerical aperture (NA) of the lens. Decreasing the wavelength of the laser theoretically will decrease the diffraction limit and thus tend to result in a smaller laser beam spot size. 
     Another prior art method of reducing the laser spot size is to increase the NA of the lens. As will be appreciated, solid immersion lenses (SILs) can possess a very high NA. An optical system employing SIL could greatly increase NA and therefore greatly increase optical recording density. An optical system employing SIL also includes an objective lens for focusing the laser beam on the SIL. 
     SILs have a curved surface and a flat surface. The curved surface of the SILs are typically partial-sphere shapes such as a partial hemisphere less than half a sphere; or, a hemisphere exactly one half of a sphere; or, a truncated sphere more than a hemisphere, also known as a super-hemisphere. U.S. Pat. No. 5,881,042, by Knight (Knight) is one example utilizing spherical SILs. Knight is hereby incorporated by reference herein for all purposes. 
     In an SIL with a spherical curved surface, the assembly tolerance is severely limited due to rapidly increasing aberration when the lens position deviates from ideal position, especially in the optical axis direction. To control these tolerances, the objective lens and the SIL are rigidly mounted in an assembly which assures the distance between the objective lens and the spherical SIL remains constant. This results in a reading and recording head that is difficult and expensive to manufacture and heavier than ideal. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved optical reading and recording system employing an aspherical solid immersion lens (SIL). The improved optical system has an aspherical SIL between an objective lens and the recording media. The aspherical SIL reduces the focused electromagnetic radiation beam spot size by providing an increased numeric aperture (NA) of the optical system. The aspherical surface of the SIL reduces aberrations thereby providing greater manufacturing and operating tolerances. Greater manufacturing tolerances provide an easier to manufacture optical system. Increased ease of manufacture reduces the cost of optical systems. 
     According to the present invention, the objective lens and the aspherical SIL are no longer required to be rigidly mounted in one assembly as in the prior art. Utilizing an aspherical SIL allows the objective lens to be mounted in the arm and the aspherical SIL to be mounted in a separate, lighter weight, air bearing slider operating very near the recording media. A lighter weight, air bearing slider requires a reduced air bearing to operate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A illustrates an optical recording system having of an objective lens and a solid immersion lens with an aspherical surface. 
     FIG. 1B illustrates an optical recording system consisting of an objective lens and a super-solid immersion lens with an aspherical surface. 
     FIG. 2A illustrates the relative position of the objective lens and a solid immersion lens. 
     FIG. 2B illustrates the relative position of the objective lens and a super-solid immersion lens. 
     FIG. 3A illustrates the relationship between wavefront aberration and position deviation for a solid immersion lens. 
     FIG. 3B illustrates the relationship between wavefront aberration and position deviation for a super-solid immersion lens. 
     FIG. 4 illustrates an optical head in which objective lens and solid immersion lens or super-solid immersion lens are mounted together on a slider. 
     FIG. 5 illustrates an optical head in which objective lens is mounted on an arm and a solid immersion lens or super-solid immersion lens is mounted on a slider. 
     FIG. 6A illustrates the solid immersion lens with a portion of the flat surface removed, leaving the centrally located portion. 
     FIG. 6B illustrates the super-solid immersion lens with a portion of the flat surface removed, leaving the centrally located portion. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A illustrates an optical system including an aspherical solid immersion tens (SIL) in accordance with one embodiment of the present invention. FIG. 1B illustrates an optical system including an aspherical super-solid immersion lens (SSIL) in accordance with one embodiment of the present invention. FIGS. 1A and 1B are described simultaneously as appropriate. 
     As will be appreciated by those skilled in the art, an aspherical solid immersion lens (SIL) is a solid lens with a curved, non-spherical surface and a flat surface. In comparison, a spherical SIL is a solid lens with a curved surface with a constant radius and a flat surface. A hemispherical SIL is a spherical SIL where the flat surface is exactly two radii in width. A super-hemispherical SIL resembles a truncated sphere greater than a hemispherical SIL, where the flat surface is less than two radii in width. Those skilled in the art will also understand the practical implications of the following theoretical discussion 
     With reference to FIGS.  1 (A) and  1 (B) respectively, electromagnetic radiation from an electromagnetic radiation source such as a light source or a laser is focused by an objective lens  1 ,  2  and a solid immersion lens  3 ,  6  onto a media  9 ,  10 . Reflected and diffracted electromagnetic radiation from the media  9 ,  10  is detected by electromagnetic radiation detectors (not shown) such as a photo-detector. When information is being written on the media  9 ,  10 , the electromagnetic radiation source is modulated at high power. The modulated, high power electromagnetic radiation irradiates the media  9 ,  10 . When the information is being read from the media  9 ,  10 , a low power continuous wave electromagnetic radiation illuminates the media  9 ,  10 . The reflected and diffracted electromagnetic radiation from the media  9 ,  10  is detected by an electromagnetic radiation detector. 
     As one skilled in the art would appreciate, the media  9 ,  10  whether rewritable, recordable, or read only optical media may be read utilizing the present invention. Utilizing the present invention to write or record information on the media  9 ,  10 , depends upon the specific type of media  9 ,  10 . The present invention can also be utilized in combination with other well known components such as an electromagnetic writing coil. 
     In accordance with one embodiment of the present invention, the SIL has an aspherical surface  4 ,  7  facing the objective lens  1 ,  2  and a flat surface  5 ,  8  facing the media  9 ,  10 . The distance  11 ,  12  between the top surface of the media  9 ,  10  and the flat surface of the SIL  5 ,  8  is preferably less than one hundred nanometers. Constituent material  13 ,  14  is located between the flat surface of the SIL  5 ,  8  and the media  9 ,  10 . The constituent material  13 ,  14  preferably shares an equivalent index of refraction n with the SIL and are considered as part of SIL. 
     FIG. 2A illustrates the relative positions of the objective lens  1  and the solid immersion lens  3 , in accordance with one embodiment of the present invention. The light is forwarded on a focal point  20  by objective lens  1 , the focal point  20  of the objective lens  1  being at the curvature center of the pole part of SIL  3 . 
     FIG. 2B illustrates the relative positions of the objective lens  2  and the super-solid immersion lens  6 , in accordance with one embodiment of the present invention. The light is focused on a focal point  21  by objective lens  2 , with the optical system utilizing an objective lens  2  and a super-solid immersion lens  6 . Focal point  21  of objective lens  2  is at (n−1/n)r below the flat surface  8  of super-solid immersion lens  6 . 
     A suitable formula for expressing an aspherical surface is:        z   =         cx   2       1   +       1   -       (     1   +   k     )          c   2          x   2               +     Ax   4     +     Bx   6     +     Cx   8     +       Dx   o   10     .                              
     In this equation, “z” is the sag of the surface parallel to the y axis, “c” is the curvature at the pole of the surface, “k” is the conic coefficient. “A,” “B,” “C,” “D” are the 4th, 6th, 8th, and 10th order deformation coefficients, respectively. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Aspherical surface parameters for solid immersion lens 
               
             
          
           
               
                 Lens 
                   
                   
                   
                   
                   
               
               
                 No. 
                 K 
                 A 
                 B 
                 C 
                 D 
               
               
                   
               
             
          
           
               
                 31a 
                 −0.073503 
                 −0.00403026 
                 0.226207 
                 −1.5356 
                 3.94669 
               
               
                 32a 
                 −0.081156 
                 −0.00542 
                 0.2659 
                 −1.81179 
                 4.65731 
               
               
                 33a 
                 −0.084389 
                 −0.0067065 
                 0.293426 
                 −2.0 
                 5.12858 
               
               
                 34a 
                 −0.081413 
                 −0.00195713 
                 0.210662 
                 −1.42661 
                 3.6929 
               
               
                 35a 
                 −0.077746 
                 0.000548846 
                 0.161355 
                 −1.07828 
                 2.79763 
               
               
                 36a 
                 −0.121386 
                 −0.00896402 
                 0.41 
                 −2.7984 
                 7.18388 
               
               
                   
               
             
          
         
       
     
     Above Table 1 lists the parameters of a selection of aspherical SILs  31   a - 36   a . The aberrations of aspherical SILs  31   a - 36   a  are illustrated in FIG.  3 A. Table 2 lists the parameters of a selection of aspherical SSILs  31   b - 34   b . The aberrations of aspherical SSILs  31   b - 34   b  are illustrated in FIG.  3 B. In these examples, the numerical aperture of the objective lens=0.4, refractive index (n) of the SIL and SSIL=1.83 and r=1 millimeter. 
     FIG.  3 A and FIG. 3B illustrate the tolerance advantages of various aspherical SILs over spherical SILs. Referring first to FIG. 3A, curves  31   a  through  36   a  represent the aberration for each of the corresponding aspherical SILs  31   a - 36   a  described in Table 1. Curve  37   a  represents the aberration for a spherical SIL which is not listed in Table 1. 
     In one example, if the critical wavefront aberration W is defined as 0.015 rms, then the corresponding tolerance for spherical SIL  37   a  is 0.0009 mm. Contrasting to the aspherical SILs  35   a ,  36   a  the corresponding tolerance are 0.0015 mm and 0.0017 mm respectively. The corresponding tolerances provided by aspherical SILs  31   a - 34   a  are also illustrated in FIG.  3 A. Aspherical SIL  36   a  provides nearly twice the tolerance over the spherical SIL  37   a , 0.0017 mm versus 0.0009 mm. One skilled in the art can appreciate the clear advantage of the increased tolerance provided by the aspherical SIL over the spherical SIL. 
     In another example, if the critical wavefront aberration W is defined as 0.010 rms, then the corresponding tolerance for spherical SIL  37   a  is 0.0005 mm, and aspherical SILs  31   a - 35   a  are suitable for use in such an application. SIL  6   a  is not suitable for use in this application because the aberration of SIL  6   a  is larger than 0.01 rms at dy=0. 
     Referring next to FIG. 3B, curves  31   b  through  34   b  represent the aberration for each of the corresponding aspherical super solid immersion lenses (SSILs)  31   b  through  34   b  described in Table 2. Curve  35   b  represents the aberration for a spherical SSIL, which is not listed in Table 2. 
     In one example, if the critical wavefront aberration W is defined as 0.015 rms, then the corresponding tolerance (dy) for spherical SSIL  35   b  is 0.0009 mm. Contrasting to the aspherical SSIL  33   b ,  34   b  the corresponding tolerance are 0.0016 mm and 0.0016 mm respectively. The corresponding tolerances provided by aspherical SSILs  1   b ,  2   b  are also illustrated in FIG.  3 B. Aspherical SSILs  33   b ,  34   b  provide substantially increased tolerance over the spherical SSIL  35   b , 0.0016 mm versus 0.0009 mm. One skilled in the art can appreciate the clear advantage of the increased tolerance provided by the aspherical SSIL over the spherical SSIL. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Aspherical surface parameters for super solid immersion lens 
               
             
          
           
               
                 Lens 
                   
                   
                   
                   
                   
               
               
                 No. 
                 K 
                 A 
                 B 
                 C 
                 D 
               
               
                   
               
             
          
           
               
                 31b 
                 −0.001361 
                 −0.00481637 
                 0.0392665 
                 −0.077322 
                 0.057546 
               
               
                 32b 
                 −0.001508 
                 −0.00618335 
                 0.0423884 
                 −0.08028 
                 0.589106 
               
               
                 33b 
                 −0.000169 
                 −0.00319578 
                 0.010553 
                 −0.0148438 
                 0.00779814 
               
               
                 34b 
                 −0.001404 
                 −0.00760953 
                 0.0398503 
                 −0.0697267 
                 0.0488919 
               
               
                   
               
             
          
         
       
     
     FIG. 4 illustrates one embodiment in accordance with the present invention. Objective lens  41  and solid immersion lens  42  with aspherical surface are mounted together onto an air bearing slider  43  suspended by a suspension  44  from arm  45 . The flat surface  46  of the SIL  42  is substantially aligned with the lower surface  47  of the air bearing slider  43 . The large tolerance provided by the aspherical SIL, decreases assembly labor and time and therefore reduces the production costs. SIL  42  can be an aspherical solid immersion lens or an aspherical super-solid immersion lens. 
     FIG. 5 illustrates another embodiment in accordance with the present invention. The objective lens  51  is mounted on a fixed arm  55  and the SIL  52  is mounted on a separate, air bearing slider  53  suspended by suspension  54 . The flat surface  56  of the SIL  52  is substantially aligned with the lower surface  57  of the air bearing slider  53 . In this embodiment, the SIL  52  is capable of moving independent of the objective lens  51 . This embodiment is made possible by the improved features of the aspherical solid immersion lens. SIL  52  can be an aspherical solid immersion lens or an aspherical super-solid immersion lens. Separating SIL  52  from the objective lens  51  and mounting the SIL  52  on the air bearing slider  53  results in a lighter weight air bearing slider  53 . 
     FIG. 6A illustrates an aspherical solid immersion lens  61  in accordance with one embodiment of the present invention. The aspherical solid immersion lens  61  includes an aspherical surface  62  and a flat surface  63 . A portion of the flat surface  63  has been removed. A centrally located portion  64  remains. The centrally located portion  64  may be employed to serve as the lower section of aspherical solid immersion lens  61 . The centrally located portion  64  allows for the removal of part of the aspherical solid immersion lens  61 . As the refracted electromagnetic radiation does not extensively use the lower periphery of the aspherical solid immersion lens  61 , there is less need for the lower periphery of the aspherical solid immersion lens  61 . By removing the portion of the flat surface  63 , more room can be gained for the placement of an electromagnetic writing coil (not shown) and the weight of the aspherical solid immersion lens  61  can be reduced. 
     The centrally located portion  64  can be grown using known techniques onto aspherical solid immersion lens  61 . Alternatively, the area around centrally located portion  64  can be etched or mechanically removed. The centrally located portion  64  can be integral and constituted of the same material as aspherical solid immersion lens  61 . 
     FIG. 6B illustrates an aspherical super-solid immersion lens  71  in accordance with one embodiment of the present invention. The aspherical super-solid immersion lens  71  includes an aspherical surface  72  and a flat surface  73 . A portion of the flat surface  73  has been removed. A centrally located portion  74  remains. The centrally located portion  74  may be employed to serve as the lower section of aspherical super-solid immersion lens  71 . The centrally located portion  74  allows for the removal of part of the aspherical super-solid immersion lens  71 . As the refracted electromagnetic radiation does not extensively use the lower periphery of the aspherical super-solid immersion lens  71 , there is less need for the lower periphery of the aspherical super-solid immersion lens  71 . By removing the portion of the flat surface  73 , more room can be gained for the placement of an electromagnetic writing coil (not shown) and the weight of the aspherical super-solid immersion lens  71  can be reduced. 
     The centrally located portion  74  can be grown using known techniques onto aspherical super-solid immersion lens  71 . Altematively, the area around centrally located portion  74  can be etched or mechanically removed. The centrally located portion  74  can be integral and constituted of the same material as aspherical super-solid immersion lens  71 . 
     Data may be written to a magneto-optical disk by also employing a separate magnetic field. In another embodiment of the invention, a magnetic coil is employed to create a magnetic field for interaction with the optical recording medium. This coil can be co-axial with the SIL  61 ,  71 . In this embodiment, the strongest point of the vertical magnetic field can be nearly coincident with the focused beam on the recording media. Like the SIL  61 ,  71 , this coil can be integrated into the body of the air bearing slider  43 ,  53 . 
     While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.