Patent Publication Number: US-6212153-B1

Title: High NA solid catadioptric focusing device having a flat kinoform phase profile

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of the following U.S. provisional patent application Ser. No. 60/091,788, filed on Jul. 6, 1998, titled “High NA Solid Catadioptric Focusing device Having a Flat Kinoform Phase Profile”, assigned to the same assignee as the present application, and incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to optical devices, and it particularly relates to a high numerical aperture (NA) catadioptric focusing device having a flat kinoform phase profile, for use in data storage systems such as optical and magneto-optical (MO) disk drives. 
     2. Description of Related Art 
     In a MO storage system, a thin film read/write head includes an optical assembly for directing and focusing an optical beam, such as a laser beam, and an electromagnetic coil that generates a magnetic field for defining the magnetic domains in a spinning data storage medium or disk. The head is secured to a rotary actuator magnet and a voice coil assembly by a suspension and an actuator arm positioned over a surface of the disk. In operation, a lift force is generated by the aerodynamic interaction between the head and the disk. The lift force is opposed by equal and opposite spring forces applied by the suspension such that a predetermined flying height is maintained over a full radial stroke of the rotary actuator assembly above the surface of the disk. 
     A significant concern with the design of the MO head is to increase the recording or areal density of the disk. One attempt to achieve objective has been to reduce the spot size of the light beam on the disk. The diameter of the spot size is generally inversely proportional to the numerical aperture (NA) of an objective lens forming part of the optical assembly, and proportional to the wavelength of the optical beam. As a result, the objective lens is selected to have a large NA. However, the NA in objective lenses can be 1 if the focusing spot were in air, thus limiting the spot size. Another attempt to reduce the spot size and to increase the recording areal density has been to use solid immersion lenses (SILs) with near field recording, as exemplified by the following references: 
     U.S. Pat. No. 5,125,750, titled “Optical Recording System Employing a Solid Immersion Lens”. 
     U.S. Pat. No. 5,497,359, titled “Optical Disk Data Storage System With Radiation-Transparent Air-Bearing Slider”. 
     Yet another attempt at improving the recording head performance proposes the use of near-field optics, as illustrated by the following reference: 
     U.S. Pat. No. 5,689,480, titled “Magneto-Optic Recording System Employing Near Field Optics”. 
     A catadioptric SIL system is described in the following references, and employs the SIL concept as part of the near-field optics: 
     Lee, C. W., et al., “Feasibility Study on Near Field Optical Memory Using A Catadioptric Optical System”, Optical Data Storage, Technical Digest Series, Volume 8, pages 137-139, May 10-13, 1998; and 
     “Parallel Processing”, 42 Optics and Photonics News, pages 42-45, June 1998. 
     While this catadioptric SIL system can present certain advantages over conventional SILs, it does not appear to capture the entire incident, collimated beam. This represents a waste of valuable energy that could otherwise increase the evanescent optical field. 
     Other concerns related to the manufacture of MO heads are the extreme difficulty and high costs associated with the mass production of these heads, particularly where optical and electromagnetic components are assembled to a slider body, and aligned for optimal performance. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is to satisfy the long felt, and still unsatisfied need for a near-field optical or MO disk data storage system that uses a catadioptric focusing device or lens with a high numerical aperture (NA), which does not introduce significant spot aberration on the disk. 
     Another aspect of the present invention is to provide a focusing device that has generally flat surfaces that act as reference surfaces and facilitate its manufacture and its assembly to the head. 
     The focusing device includes an incident surface, a bottom reflective surface, a focal pedestal, and a body. The incident surface is generally flat and is comprised of a central diffractive, optically transmissive facet or surface and a peripheral facet or surface comprised of a kinoform phase profile. In a data writing or reading mode, the incident optical beam, such as a laser beam impinges upon the central facet, and is diffracted thereby. The incident laser beam can be collimated, convergent or divergent. 
     The laser beam passes through the transparent body, and impinges upon the bottom reflective surface. The laser beam is then reflected by the bottom reflective surface, through the body, unto the kinoform phase profile. The laser beam is reflected and refracted by the peripheral kinoform phase profile as a focused beam, through the body, and is focused as a focal point. The focal point is preferably located at, or in close proximity to a pedestal edge, along a central axis, in very close proximity to the disk. This will allow the focused optical beam to propagate toward, or penetrate the disk through evanescent wave coupling, for enabling the transduction of data to and from the disk. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention and the manner of attaining them, will become apparent, and the invention itself will be understood by reference to the following description and the accompanying drawings, wherein: 
     FIG. 1 is a fragmentary perspective view of a data storage system utilizing a read/write head according to the invention; 
     FIG. 2 is a perspective view of a head gimbal assembly comprised of a suspension, and a slider to which the read/write head of FIG. 1 is secured, for use in a head stack assembly; 
     FIG. 3 is an enlarged, side elevational view of a catadioptric focusing device or lens forming part of the read/write head of FIGS. 1 and 2, and made according to the present invention; 
     FIG. 4 is an enlarged, side elevational view of another catadioptric focusing device forming part of the read/write head of FIGS. 1 and 2, and made according to the present invention; 
     FIG. 5 is a top plan view of the catadioptric focusing devices of FIGS. 3 and 4; 
     FIG. 6 is a bottom plan elevational view of the catadioptric focusing devices of FIGS. 3 and 4; and 
     FIG. 7 is an enlarged, side elevational view of yet another catadioptric focusing device forming part of the read/write head of FIGS. 1 and 2, and made according to the present invention. 
    
    
     Similar numerals in the drawings refer to similar or identical elements. It should be understood that the sizes of the different components in the figures might not be in exact proportion, and are shown for visual clarity and for the purpose of explanation. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a disk drive  10  comprised of a head stack assembly  12  and a stack of spaced apart magnetic data storage disks or media  14  that are rotatable about a common shaft  15 . The head stack assembly  12  is rotatable about an actuator axis  16  in the direction of the arrow C. The head stack assembly  12  includes a number of actuator arms, only three of which  18 A,  18 B,  18 C are illustrated, which extend into spacings between the disks  14 . 
     The head stack assembly  12  further includes an E-shaped block  19  and a magnetic rotor  20  attached to the block  19  in a position diametrically opposite to the actuator arms  18 A,  18 B,  18 C. The rotor  20  cooperates with a stator (not shown) for rotating in an arc about the actuator axis  16 . Energizing a coil of the rotor  20  with a direct current in one polarity or the reverse polarity causes the head stack assembly  12 , including the actuator arms  18 A,  18 B,  18 C, to rotate about the actuator axis  16  in a direction substantially radial to the disks  14 . 
     A head gimbal assembly (HGA)  28  is secured to each of the actuator arms, for instance  18 A. With reference to FIG. 2, the HGA  28  is comprised of a suspension  33  and a read/write head  35 . The suspension  33  includes a resilient load beam  36  and a flexure  40  to which the head  35  is secured. 
     The head  35  is formed of a slider (or slider body)  47  secured to the free end of the load beam  36  by means of the flexure  40 , and a catadioptric focusing device or lens  50  retained by the slider  47 . The head  35  further includes an optical beam delivery means, such as a waveguide or a fiber  48 . A stationary or a micro-machined dynamic mirror  49  with wires  49 W, can be secured to a trailing edge  55  of the slider  47  at a  45  degree angle relative to the optical beam emanating from the fiber  48 , so as to reflect the optical beam onto the focusing device  50 , in order to transduce data to and from a storage medium  14  (FIG.  3 ). 
     The slider  47  can be a conventional slider or any other suitable slider. In the present illustration, the slider  47  includes a fiber channel for receiving the optical fiber  48 . Though the fiber channel is illustrated as being centrally located along a generally central axis of the slider  47 , it should be understood that the location of the fiber channel can be offset relative to the central axis. In a design where the optical beam is delivered through free space, for example when a fiber is not used, the optical beam can be transmitted through the fiber channel or a waveguide formed within the fiber channel. 
     The details of the focusing device  50  will now be described with reference to FIGS. 3,  5  and  6 . The focusing device  50  includes an incident surface  100 , a bottom reflective surface  105 , a focal pedestal  110 , and a body  115 . The incident surface  100  is generally flat and is comprised of a central diffractive, optically transmissive surface or central facet  130  and a peripheral reflector (or facet)  132  comprised of a diffractive or kinoform phase profile  133 . The body  115  is optically transparent, and the incident surface  100  is formed on a first side of the body  115 . The bottom reflective surface  105  is formed on a second side of the body  105 , such that the first of second sides are preferably, but not necessarily, oppositely disposed. The pedestal  110  is formed on the same side as the reflective surface  105 . 
     In a data writing mode, an incident optical beam, such as a laser beam  135  impinges upon the central facet  130 , and is diffracted thereby. The incident laser beam  135  can be collimated, convergent or divergent. The laser beam  135  passes through the transparent body  115 , and impinges upon the bottom reflective surface  105 . The laser beam  135  is then reflected by the bottom reflective surface  105 , through the body  115 , onto the peripheral reflector  132 . The laser beam  135  is refracted by the kinoform phase profile  133  as a focused beam  135 A, through the body  115 , and is further focused to a focal point  162  located within or on the surface of the pedestal  110  at, or in close proximity to an edge or surface of the pedestal  110  that defines a focal plane  163 . In a preferred embodiment, the focal point  162  is located at the central axis P, in very close proximity to the disk  14 , such that a localized evanescent field or light  170  interacts with disk  14 , for enabling data to be transduced to and from the disk  14 . A coil or coil assembly  64  is formed around the pedestal  110  and secured to the body  115 , for generating a desired write magnetic field. Wire traces  64 T (FIG. 3) connect the coil assembly  64  and contact pads  64 A (FIGS. 3,  6 ). 
     The focused beam  135 A defines an angle of incidence θ with the central plane P. It should be clear that the angle of incidence θ is greater than the angle of incidence θ′ had the optical beam  135  not undergone the sequence of reflections and diffractions as explained herein. Consequently, the NA of the focusing device  50  exceeds that of a conventional objective lens, as supported by the following equation: 
     
       
           NA=n.  sin θ,  
       
     
     where n is the index of refraction of the lens body  115 . According to the present invention, it is now possible to select the lens body  115  of a material with a high index of refraction n, in order to increase NA. 
     The peripheral kinoform phase profile  133  is formed of a pattern of refractive profiles i.e.,  200 ,  201 ,  202 . While only three refractive profiles are illustrated, it should be understood that a greater number of refractive profiles can be selected. The pattern of refractive profiles  200 ,  201 ,  202  is coated with a reflective surface  210 . In another embodiment, the peripheral kinoform phase profile  133  can be made of an appropriate diffractive grating or an appropriate lens structure such as a Fresnel lens. 
     The focal pedestal  110  can be formed integrally with lens body  115 , and extends below the bottom reflective surface  105 . 
     With particular reference to FIGS. 5 and 6, the focusing device  50  is generally cylindrically shaped with a circular cross-section, and is formed within a substrate  225 . The transmissive surface  130  (FIG. 5) is concentric relative to, and is disposed within the reflective surface  210 . The central facet  130  can simulate holographic or virtual flat, spherical, conical or other suitable diffractive surfaces  233  (shown in dashed lines in FIG.  3 ), while retaining its generally flat configuration. The reflective surface  210  is ring shaped. In an alternative design, the kinoform phase profile can simulate an aspherical refractive or diffractive surface  234  (shown in dashed lines in FIG.  3 ), while retaining its generally flat configuration. 
     The pedestal  110  can be generally conically shaped (with an edge  111  shown in dashed line in FIG.  3 ), cylindrically shaped (as shown in FIG.  4 ), or it can have a trapezoidal (or another suitable) cross-section, and is co-axially and concentrically disposed relative to the bottom reflective surface  105 . In an alternative embodiment, the central facet  130  includes an alignment ring  237  (shown in dashed lines in FIG.  5 ), that assists in the alignment of the optical focusing device  50  during assembly to the slider body  47 . 
     As explained herein the optical focusing device  50  can be made using molding, etching, or other suitable manufacturing techniques. The flatness of the incident surface  100  helps facilitate wafer processing techniques to be used to mass assemble a lens wafer in which a plurality of optical focusing devices  50  are formed, to a slider wafer in which a plurality of sliders  47  are formed. 
     Using the present focusing device  50 , it is possible to reduce the spot size on the disk  14  to less than 0.3 microns. The focusing device  50  can be made of any suitable transparent material, including but not limited to glass, crystal, plastic, or a combination thereof. 
     FIG. 4 illustrates another catadioptric focusing device  400  according to the present invention. The focusing device  400  is generally similar in function and design to the focusing device  50 , and has its incident surface  100 A comprised of a peripheral kinoform phase profile  133 A. The peripheral kinoform phase profile  133 A is formed of a reflective surface  210  that coats a pattern of concentric binary refractive profiles i.e.,  420 ,  421 ,  422 . The resolution of the refractive profiles  420 ,  421 ,  422  can vary, for example increased, in order to obtain a more precise control over the diffraction of the laser beam  135 A. 
     FIG. 7 illustrates another focusing device  450  according to the present invention. The focusing device  450  is generally similar in function and design to the focusing devices  50  and  400 , and has its incident surface  100 B comprised of a peripheral kinoform phase profile  133 B. The peripheral kinoform phase profile  133 B is formed of a reflective surface  210  that coats a pattern of concentric binary refractive profiles i.e.,  200 ,  201 ,  202  or  420 ,  421 ,  422 . Whereas in the focusing devices  50  and  400 , the incident surfaces  100 A,  100 B are formed integrally with the lens body  115 , the incident surface  100 B can be formed of a separate plate  100 P which is secured to the lens body  115  along a generally flat surface  455  (shown in a dashed line). 
     Another optional distinction between the focusing device  450  of FIG.  7  and the focusing devices  50  and  400  of FIGS. 3 and 4, respectively, is that the focal pedestal  110  can be made of a separate plate that is secured to the lens body  115  along a central, non-reflective surface  463  of the bottom of the lens body  115 . 
     Though exemplary dimensions of the focusing device  50  and peripheral reflector  132  are shown for illustration purpose, it should be clear that other patterns can be selected. It should also be understood that the geometry, compositions, and dimensions of the elements described herein may be modified within the scope of the invention and are not intended to be the exclusive; rather, they can be modified within the scope of the invention. Other modifications can be made when implementing the invention for a particular environment. The use of the focusing device is not limited to data storage devices, as it can be used in various other optical applications, including but not limited to high resolution microscopy, surface inspection, and medical imaging.