Spatial resolution of optical recording systems is improved by use of a hemispherical Solid Immersion Lens (SIL) placed in close proximity to the recording surface. The wavelength (.lambda.) of the light in the SIL (.lambda.s) is reduced by the index of refraction (Ns) of the SIL. In other words: EQU .lambda.s=.lambda./Ns (1)
In a diffraction-limited system consisting of only an objective lens (OL), having a numeric aperture (NA) (where NA=sin(ThetaMax) where ThetaMax is the maximum angle of incidence of the light relative to the optical system axis), the focal spot has a full width at half maximum amplitude (FWHM) given by: EQU FWHM=0.6*.lambda./NA (2)
An optical path arrangement in accordance with equation (2) is illustrated in FIG. 1. In this figure, an optical head 10 is positioned closely above a relatively moving optical storage medium, such as a rotating optical disk 12. One example of an optical head including an objective lens formed as part of an air bearing slider is described in commonly assigned U.S. Pat. No. 5,105,408 to Lee et al., entitled: "Optical Head with Flying Lens", the disclosure thereof being incorporated herein by reference. In the present FIG. 1 example 10, a collimated light beam 14 from e.g. a laser light emission source (not shown in FIG. 1) is converged to a focal point at the media 12 by a conventional objective lens 16. With the FIG. 1 optical system 10, the resultant FWHM light intensity distribution (0.6*.lambda./NA) is shown as graph 20 of FIG. 1A (drawn in alignment with an optical axis .alpha. of the FIG. 1 optical system 10).
By placing the center of a hemispherical SIL 30 in alignment with the optical axis .alpha. and at the focal point of the FIG. 1 system 10, the effective wavelength can be reduced by 1/Ns. Therefore, EQU FWHMsil=0.6.lambda.s/NA=0.6*.lambda./NA*Ns (3)
Therefore the spot size is reduced by a factor of 1/Ns and the potential storage capacity is increased by Ns.sup.2, as shown in FIGS. 2 and 2A. In FIG. 2 a hemispherical flat surface 32 of the SIL 30 forms a part of an air bearing surface of the optical head, enabling the SIL 30 to be placed at a flying height very close to the surface of the optical disk 12 as is known with conventional flying heads used in magnetic hard disk drives.
The spot size of the FIG. 2 type of system can be further reduced by using a Super-Sphere SIL (SSIL) 40. The exemplary SSIL 40 is intermediately between a hemisphere and a complete sphere, as shown in FIG. 3. The SSIL 40 has a flat surface 42 that is located at a focal distance (Ds) from the center of the sphere having a radius (R). Accordingly, focal distance is given by: EQU Ds=R/Ns (4)
If the focal point of the objective lens (OL) 16 is set at a distance R*Ns in back of the SSIL center (when the SIL is not present), then the light rays will converge to a point that is at the back surface of the SSIL 40 (equation 4). This geometry also surprisingly achieves an aberration free focus in that all rays converge to this point. One advantage of this focal arrangement is that the steeper angle of the most extreme rays in the SSIL (Thetamaxsil) results in a larger effective SSIL numeric aperture (NAssil=sin(Thetamaxsil)). Therefore, the FWHM is further reduced:
FWHMssil=0.6*.lambda./Ns*NAssil (5)
Since the improvement in NAssil involves very complex algebra, the interested reader is referred for further explanation to an article by T. Suzuki et al., entitled: "Solid Immersion Lens Near Field Optical Approach for High Density Optical Recording", IEEE Trans. on Magnetics, Vol. 34, No. 2, March 1998, pp. 399-403.
Improvements in FWHM offered by the SIL and the SSIL systems come with certain limitations and drawbacks. For example, the back surface of the high index SIL must be maintained in close proximity to the disk surface (e.g. 3 microinches). Also, the SIL must be formed as an accurate sphere, and the objective lens must be accurately aligned to one of the two focal points discussed above. Further, with the SSIL approach, these requirements are even more stringent than they are with the SIL approach. Therefore, practical implementation and use of the SSIL has heretofore been deferred for future development and refinement.
Present SIL systems require laborious and complex optical path alignment techniques and steps for aligning the objective lens and the SIL along an optical axis as well as the separate manufacture of each lens. One example of this prior approach is found in U.S. Pat. No. 5,729,393 to Lee et al., entitled: "Optical Flying Head with Solid Immersion Lens Having Raised Central Surface Facing Medium". The Lee et al. '393 patent describes an optical near field recording system in which an objective lens and a SIL are optically aligned and mounted in optical alignment to an air bearing slider. The objective lens is separate from, and apparently not in physical direct contact with, the SIL. A bottom surface of the SIL is contoured to present a closest point to the rotating recording and playback optical disk medium. Features of the SIL and slider recede adjacently away from the closest point and function at least in part as an air bearing, so that perturbations in the flying attitude of the slider do not affect optical transmission between the closest point of the bottom surface of the SIL and the storage medium.
U.S. Pat. No. 5,497,359 to Mamin et al., entitled: "Optical Disk Data Storage System with Radiation-Transparent Air-Bearing Slider", discloses an aspheric SIL that does not require a separate objective lens. The disclosed aspheric SIL was made of a single material, as by injection molding or by diamond micro-machining. Injection molding implies use of a plastic material which would necessarily further imply a low index of refraction (e.g. 1.5). Such materials and approaches would limit resolution. Precision micro-machining of complex aspheric surfaces is not presently viable for a reproduceable, low cost manufacturing technique for volume production.
Therefore, a hitherto unsolved need has remained for a compact, yet readily manufacturable, solid immersion lens system which integrally includes an objective lens in a manner overcoming limitations and drawbacks of the prior designs and methods.