Abstract:
An optical storage method and apparatus having enhanced resolution uses a Fabry-Perot cavity to narrow a beam used for reading data stored on media. The method and apparatus achieve an enhanced resolution due to the reduction of beam size and the increased slope of the beam profile in a beam used to illuminate physical changes in the media corresponding to data encoded in the media. The Fabry-Perot cavity may be included in a media for use with standard optical storage devices or may be external to the media, as part of an optical storage head for use with standard media.

Description:
RELATED APPLICATIONS 
     This application is related to pending U.S. patent application Ser. No. 09/789,913 entitled “SYSTEM OF BEAM NARROWING FOR RESOLUTION ENHANCEMENT AND METHOD THEREFOR” filed on Feb. 21, 2001, the specification of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to optical storage systems and media, and more specifically, to an optical storage system incorporating a Fabry-Perot cavity to control illumination characteristics at a media surface. 
     2. Description of the Related Art 
     Optical media stores information via physical artifacts or discontinuities on a surface of interest. Digital data may be encoded by a series of pits in a reflective mask attached to a supportive plastic structure such as present-day compact disc (CD) media. 
     Data is read from the media by measuring the distribution of the field reflected by data-bearing features on the surface of interest. Present-day high resolution optical readers measure diffraction caused by data-bearing surface features by combining light reflected from an artifact on a rotating disc with light that was reflected from an adjacent artifact. A data signal is extracted by determining the distortion of the diffraction field and is detected by sampling one or more points within the field using detectors, thereby detecting the phase within the reflected beam as well as its amplitude. 
     Measuring phase and amplitude provides an improvement over amplitude-only systems, and using a diffracted field detection system permits detection of data-bearing features having very small height variations. However, the limitation on data density is the spatial resolution limitation set by the size of the focused beam on the surface of interest. 
     Therefore, it would be desirable to provide a method and apparatus having an enhanced spatial resolution for reading data from standard media by illuminating the media with a narrowed beam. It would further be desirable to provide an improved media having enhanced spatial resolution via a narrowed beam. 
     SUMMARY OF THE INVENTION 
     The foregoing objectives are achieved in an optical storage method and apparatus having enhanced resolution. A media storage for encoding data includes a first reflective surface having physical artifacts corresponding to encoded data and a second partially reflective surface positioned parallel to the first reflective surface and at a tuned optical distance from the first reflective surface, such that at a predetermined illumination wavelength, a beam transmitted through the second partially reflective surface and illuminating the first reflective surface has a minimum radius spot size at the first reflective surface. 
     As an alternative preferred embodiment, an optical storage system includes an optical illumination/collection subsystem for producing a beam to illuminate a media storage surface and collect the field reflected by it and a partially reflective surface positioned parallel to the media storage surface between the optical illumination system and the media storage surface at a tuned optical distance from the media storage surface. The positioning of the partially reflective surface produces a beam having a minimum radius spot size at the media storage surface. 
    
    
     The foregoing and other objects, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration depicting an optical storage system in accordance with a first preferred embodiment of the invention. 
     FIG. 2 is an illustration depicting components of an optical storage system in accordance with a second preferred embodiment of the invention. 
     FIG. 3 is a graph depicting the narrowed beam illuminating the surface of interest in the optical storage systems of FIG.  1  and FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference now to the figures, and particularly to FIG. 1, an optical storage system in accordance with a first preferred embodiment of the invention is depicted. In the first preferred embodiment, a novel media is used to increase resolution within an optical storage system. While the description is made generally with reference to optical retrieval, the techniques of the present invention may apply equally to recording systems that alter recordable media by illuminating the media with a narrowed beam. The present invention may also apply to a system in which light is transmitted through media rather than reflected from a fully reflective surface of interest. 
     An illumination subsystem  16  is provided to illuminate a surface of interest  13  within optical media  10 . A direct beam  17  is reflected by the surface of interest  13  which is moving in the plane of the figure as noted, so that the physical deviations of surface of interest  13  (from that of an perfectly planar surface) modulate the intensity and phase distribution of the reflected beam  18  which enters a detector  15 . While optical media  10  is depicted as a form of media having physical discontinuities in the height of surface of interest  13 , the present invention applies also to media with a constant-plane surface of interest having variable reflectivity, and surfaces having other height variation profiles. For example, the present invention applies to recordable media using inks that are rendered transparent in the recording process, producing a storage medium having an effective surface of interest with varying reflectivity. 
     Within optical media  10 , a partially reflective surface  12  is created by depositing an optical coating, a thin-film layer, a layer with differing index of refraction, or other means that will be apparent to those skilled in the art of media fabrication. The partially reflective surface  12  may be a bragg grating formed by layers of differing refractive index and spaced at one-half wavelength distance, or may be a refractive change in the material introduced by doping the material with another material to produce a layer having a differing refractive index from other layers within optical media  10 . 
     The distance between partially reflective surface  12  and surface of interest  13  is carefully controlled so as to create a beam-narrowing effect via a tuned distance between surface of interest  13  and partially reflective layer for at least one of the distances corresponding to the deviations of surface of interest  13 . The positioning of partially reflective surface  12  with respect to surface of interest  13  is such that a condition relative to the resonance condition is maintained. In actuality, the position is a location close to anti-resonance that produces a beam of minimum radius spot size at the surface of interest, through interference of the direct and reflected beams within the Fabry-Perot cavity. At the illumination wavelength emitted from illumination system  16 , a predetermined number of wavelength fractions exist between partially reflective surface  12  and one of the data positions of surface of interest  13 . Surface variation of the data pattern should be held to less than a quarter of the illumination wavelength, as at variations greater than one quarter of the illumination wavelength the next resonance may interfere with the optical detection signal. 
     The positioning of surface of interest  13  and partially reflective surface  12  produces a beam-narrowing effect due to the resonant cavity created between partially reflective surface  12  and surface of interest  13 . This positioning at a tuned optical distance reduces the effective spot size of the beam, increasing the number of data bits that may be encoded without interference from light reflecting from adjacent data bits. Referring now to FIG. 3 the intensity profile  71  of direct beam  17  emitted from illumination subsystem  16  and an intensity profile  72  of the illumination at surface of interest  13  is depicted in graphical form. As shown by the figure, a reduction in spot size over the gaussian illumination of direct beam  17  of 40% may be achieved using the resonant cavity of the present invention. This cavity is known in the art as a Fabry-Perot cavity. In this application, the Fabry-Perot cavity produces a beam-narrowing effect as described in the above-incorporated patent application “SYSTEM OF BEAM NARROWING FOR RESOLUTION ENHANCEMENT AND METHOD THEREFOR”. 
     Referring now to FIG. 2, an optical storage system in accordance with a second preferred embodiment of the invention is depicted. In the second embodiment, a system is implemented that uses media that may be standard media or a new media devised to take advantage of the improved resolution of the present invention. In the second embodiment, a reference plate  22  having a partially reflective surface  29  is inserted between an illumination subsystem comprising a laser and a beam expander  26  producing a direct beam  27  for illuminating a surface of interest  23  within a standard optical storage media  20 . 
     Reference plate  22  is located such that partially reflective surface  29  is positioned at a distance from surface of interest  23  corresponding to a predetermined number of wavelengths of the light emitted by illumination subsystem  26 . The distance between partially reflective surface  29  and surface of interest  23  is the predetermined number of wavelengths for at least one of the distances corresponding to the deviations of surface of interest  23 . As described above for the first embodiment of the invention, this positioning of partially reflective surface  29  and surface of interest  23  creates a Fabry-Perot cavity between partially reflective surface  29  and surface of interest  23 . A detector including associated optics  25  is used to detect a reflected beam  28 , permitting detection of data variations in surface of interest  23  and providing control functions for positioning reference plate  22 . 
     Referring again to FIG. 3, the intensity profile improvement as applied to the second embodiment of the invention is described. The intensity profile  71  of direct beam  27  emitted from illumination subsystem  26  and an intensity profile  72  of the illumination at surface of interest  23  is depicted in graphical form. As shown by the figure, a reduction of 40% in the illumination spot size relative to the spot size produced by gaussian illumination  27  may be achieved using the resonant cavity of the present invention. 
     In contrast to the first embodiment of the invention wherein the tuned optical distance between surface of interest  13  and partially reflective surface  12  is fixed in the manufacture of storage media  10 , in the second embodiment of the invention, the tuned optical distance between partially reflective surface  29  and surface of interest  23  is generally greater and requires dynamic control. The control mechanism is based on the average reflectivity of the Fabry-Perot cavity and is a feedback control apparatus as may be readily implemented by one skilled in the art of positional control. 
     The control mechanism for controlling the position of reference plate  22  and thereby partially reflective surface  29  includes a detector and associated optics  25  coupled to a servo system  31 . Detector  25  may comprise a single detector for receiving data as well as controlling position or separate detectors may be used for data and positional control. Servo  31  has an output coupled to piezoelectric transducer  33  for positioning reference plate  22  with respect to surface of interest  23 . Detector  25  controls servo  31  to move piezoelectric transducer  33  until a nominal predetermined optical power level is detected by detector  25 . The servo system loop will then maintain the position of reference plate  22  at the tuned optical distance to produce a reduced illumination spot size at surface of interest  23 . 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.