Abstract:
An optical data storage system includes an optical disc and an armature adjacent the disc. An optical transducer mounted on the arm provides an optical beam output directed parallel to the plane of the disc. An optical actuator mounted on the arm and positioned in the optical beam adjusts the direction of the optical beam and a mirror is positioned to direct the optical beam in a direction perpendicular to the disc and toward the disc. An optical head is positioned at a distal end of the arm and is adapted to couple the optical beam to the optical disc. An arm actuator is coupled to the arm to move the arm relative to the disc and provide coarse positioning of the optical head relative to tracks on the disc. The optical actuator provides fine adjustment of the optical beam between tracks on the disc which are proximate the optical head due to the coarse adjustment by the arm actuator.

Description:
This application claims priority benefits from U.S. provisional patent application 60/123,493 entitled “FIXED OPTICS MODULE FOR OPTICAL DATA STORAGE” filed on Mar. 9, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to optical systems. In particular, the present invention relates to optical systems in data storage devices. 
     Optical data storage systems access data by focusing a laser beam or other light source onto a data surface of a medium and analyzing the light reflected from or transmitted through the medium. In general, data is stored in optical storage systems in the form of marks carried on the surface of the medium which are detected using a reflected laser light. 
     Compact discs, which are widely used to store computer programs, music and video, are one type of optical data storage system. Typically, compact discs are permanently recorded during manufacture by stamping the surface of the compact disc. Another type of optical system is a write once read many (WORM) system in which a user may permanently write information onto a blank disc. Other types of systems are erasable, such as phase change and magneto-optic (M-O) systems. Phase change systems detect data by sensing a change in reflectivity. M-O systems read data by measuring the rotation of the incident light polarization due to the magnetic state of the storage medium. 
     In many prior art optical disc storage systems, the optical head is carried on a mechanical actuator which moves the head across the disc surface. In some more recent data storage systems, the optical head is carried on a slider which flies over the surface of the disc and is located at the end of an actuator arm. A significant portion of the optics in such systems is not carried on the armature. Instead, much of the optics is spaced apart from the armature and light from the optics is directed to slider, for example by directing a beam toward the slider or by using an optical fiber. 
     SUMMARY OF THE INVENTION 
     An optical data storage system includes an optical disc and an armature adjacent the disc. An optical transducer mounted on the arm provides an optical beam output directed parallel to the plane of the disc. An optical actuator mounted on the arm and positioned in the optical beam adjusts the direction of the optical beam and a mirror is positioned to direct the optical beam in a direction perpendicular to the disc and toward the disc. An optical head is positioned at a distal end of the arm and is adapted to couple the optical beam to the optical disc. An arm actuator is coupled to the arm to move the arm relative to the disc and provide coarse positioning of the optical head relative to tracks on the disc. The optical actuator provides fine adjustment of the optical beam between tracks on the disc which are proximate the optical head due to the coarse adjustment by the arm actuator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified diagram of a disc drive storage system in accordance with the present invention. 
     FIG. 2 is a plan view of an armature in the disc storage system of FIG. 1 which includes a fixed optics module in accordance with the invention. 
     FIG. 3 is a perspective view showing the fixed optics module of FIG. 2 in greater detail. 
     FIG. 4 is a perspective view of the fixed optics module in accordance with another embodiment of the invention. 
     FIG. 5 is a diagram used to illustrate the effects of head skew. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a simplified block diagram of an optical disc drive storage system  10  in accordance with the present invention. Storage system  10  includes storage media  12  such as an optical disc carried on spindle  14  and rotated by spindle motor  16 . An actuator arm  20  carries a fixed optics module  22  thereon having an optical head  24  positioned at a distal tip for positioning over a surface of disc  12 . Armature  20  can be, for example, rotary or linearly actuated using actuator  26  and radially positioned selectively over tracks on disc  12 . A controller  30  controls positioning of the head  24  using actuator  26  and spindle motor  16 . Data can be read from and written to the surface of disc  12  by controller  30  using fixed optics module  22  as described below in more detail. 
     In a typical optical data storage system, a laser beam is focussed onto an optical disc. Because the area density of the disc varies inversely with the spot size squared, it is desirable to focus the beam to the smallest possible spot. In typical prior art optical disc storage systems, much of the optics was positioned off of the armature and the optical signal was coupled to the optical head either through a separate armature or through a fiber optic cable. Another, more complex system is shown in U.S. Pat. No. 4,794,586 issued Dec. 27, 1988 to Korth, entitled “ROTARY ACCESS ARM FOR OPTICAL DISCS” in which the laser and other optics is carried on the armature. However, the distal end of the armature is slid in an arcuate track which supports the armature and does not rely on a slider. The objective lens is positioned over tracks by actuating the armature using voice coils  6 A and  6 B. The prior art has also used a separate galvo mirror as a fine tracking actuator which is used to deflect the direction of the laser beam. The deflection of the beam is used to translate the beam across tracks on the disc for fine actuation. However, such systems have typically used optics which are separate from the armature. 
     The present invention provides an optical source such as a laser, beam-shaping and relay optics and magneto-optical detection optics for a magneto-optical disc drive in which the optical module is carried on the actuator armature. 
     FIG. 2 is a top plan view of armature  20  showing fixed optics module  22  and optical head  24  (not shown in FIG.  2 ). Optical head  24  can be carried, for example, on a slider  40  adapted to fly or be dragged over the surface of disc  12 . Fixed optics module  22  includes quadrant detector  42 , two-beam Wollaston prism  44 , beam splitter  45 , laser  46 , microlens  48 , front facet detector  50 , a first relay lens  52 , a rotatable mirror  54 , a second relay lens  56  and a Turning mirror/detector  58 . As illustrated in FIG. 2, all of the optical elements of fixed optics module  22  are carried on armature  20 . Thus, as armature  20  is actuated by actuator  26  of FIG. 1, all of the elements are moved. This is particularly advantageous because the alignment between the various elements are fixed and do not need to be adjusted due to the translation of the armature  20  or otherwise have their alignment changed which can cause distortion or loss of focus. Further, unlike prior art techniques, the optical beam does not need to be aligned with and directed from a separate armature onto the optical head. 
     FIG. 3 is a more detailed perspective view of fixed optics module  22  in accordance with the invention. Fixed optics module  22  includes laser  46  which comprises, for example a laser diode which generates an optical beam directed toward microlens  48 . The beam emerges from microlens  48  as a circularized, diverging beam that is approximately s-polarized. The beam next passes through a beam splitter which is a “leaky” beam splitter (R S =0.75, T S =0.25, R P =0 and T P =1) that transmits roughly 25% of the optical power to a laser power monitor provided by front facet detector  50 . Detector  50  is used to monitor the power output from laser  46  and provides an output to controller  30  which can adjust the beam strength or compensate for a weak beam. Approximately 75% of the optical beam from laser  46  is directed toward disc  12 . The beam passes through a first relay lens  52  that causes the light to converge. The beam next reflects off of rotatable mirror  54 , comes to a focus, and is collimated by second relay lens  56 . Rotatable mirror  54  provides an optical actuator and is rotated by controller  30  using an actuator (not shown) to cause fine translation between beta tracks  70  carried on the surface of disc  12 . The rotation of mirror  54  by controller  30  is in response to an error signal is described below. 
     The collimated beam next reflects off a Turning mirror/detector element  58  which reflects most of the light toward the objective lens  24  carried on slider  40  (shown in phantom in FIG.  3 ). Such a configuration is described in U.S. patent application Ser. No. 09/055,373, filed Apr. 6, 1998, which is incorporated herein by reference. A small portion of the outer part of the beam is used to generate a position signal such that controller  30  shown in FIG. 1 can monitor the position of the beam and responsively actuate rotatable mirror  54 . Such a detector is described in U.S. patent application Ser. No. 09/268,012, filed Mar. 15, 1999 and entitled “ASYNCHRONOUS ANALOG DEMODULATOR AND METHOD FOR A NULL-TYPE SERVO PATTERN” which is incorporated herein by reference. 
     Next, the optical beam enters the optical head, which may contain objective lens  24 , a Solid Immersion Lens (SIL), or additional optical elements. As used herein, optical head can include any or all such elements. After interacting with the disc surface, the reflected light returns through optical head  24  and retraces the path toward laser source  46 . The beam splitter  45  transmits 100% of the p-polarized light (which contains the data signal) and 25% of the s-polarized light. Finally, the beam is split into two beams by two-beam Wollaston prism  44  and is collected by a quadrant detector  52 . The polarization of the two beams exiting the Wollaston prism  44  are at +45° and −45° with respect to the split direction. The quadrant detector generates a magneto-optical data signal, a push-pull tracking error signal and a focus error signal. Operation of the quadrant detector is described in more detail in U.S. patent application Ser. No. 09/158,040, filed Sep. 21, 1998, entitled “APPARATUS AND METHOD FOR DIFFERENTIAL WAX-WANE FOCUSING AND PUSH-PULL TRACKING FOR MAGNETO-OPTICAL DATA STORAGE” which is incorporated herein by reference. 
     During operation, controller  30  moves slider  40  over the surface of disc  12  by actuating actuator  26 . Fine positioning of the optical beam is through the use of an optical actuator such as rotatable mirror  54 . As used herein, an optical actuator is any device which can change the direction of an optical beam. 
     The fixed optics module of the present invention is advantageous because the tracking functions are accomplished by moving parts that are not flying on the optical head. The rotatable mirror can be located on the arm near the pivot point and may therefore be much larger than the extremely small optical elements that fly on the optical head. Additionally, the fixed optics module can accommodate either near field or far field recording techniques, depending on the type of optical head implemented. The beam does not have to retain a Gaussian profile as it progresses through the system. The return beam in a near field recording technique, such as with a Solid Immersion Lens, is extremely non-Gaussian, and is poorly suited for use with prior art optical heads. The fixed optics module of the present invention uses free-space light delivery to the head and can accommodate near-field recording techniques. 
     FIG. 4 is a perspective diagram of fixed optics module  22  in accordance with another embodiment. Elements in FIG. 4 which are similar to elements in FIGS. 1-3 have retained their numbering for simplicity. In the embodiment of FIG. 4, light from the optics module  22  is directed onto an optical switch  80  instead of directly onto an optical head. Switch  80  can be used to direct the light into any one of a plurality of optical fibers  82  in a fiber bundle to deliver light to a disc surface in the disc drive  10 . The switch may consist of a lens and a fiber bundle placed at a focal point of the lens. As the mirror  54  in the fixed optics module  22  rotates, the beam that enters the switch changes angles and moves between fiber tips. 
     In contrast to conventional beam splitter designs, the beam splitter  45  of the present invention provides a flatter response with respect to wave length of the reflectivities of each polarization state and of the phase shift between the p and s polarization. The beam splitter also achieves this improved response using fewer layers in its coating. Such a beam splitter is available from Toyocom Communication Equipment Co. Ltd. of Tokyo, Japan. 
     FIG. 5 illustrates an additional advantage of the present invention over the prior art with the invention, the effects of head skew can be reduced. 
     One quantity that affects performance is head skew, where the head skew is defined as the angle between a data track at the head and the split between the two detector elements that are used to generate the push-pull tracking error signal. In the presence of large amounts of head skew, the push-pull tracking error signal is degraded. An example of head skew is shown in FIG.  5 . The circle  100  is the outline of the return beam, and the arcs  102  on the left and right sides of circle  100  subtend the region where the tracking error signal is generated. Rectangles  104  are the left and right halves of a split detector, which collects the light in the left and right halves of the beam, respectively. In the absence of head skew, the left and rights sides of the beam pattern are split symmetrically by the edge between the detector elements. 
     In one embodiment, the optical path along the armature is centered on the line between the actuator pivot point and the head. This design showed very little head skew (−3°) at the inner diameter of the disk, but significant head skew (+17°) at the outer diameter. The tracking performance at the outer diameter was unacceptably poor. 
     In a preferred embodiment, the optical path was moved away from the line that connects the pivot point and the head, and balanced the head skew between −10° at the inner diameter and +10° at the outer diameter. These head skew values showed acceptable tracking performance at both inner and outer diameters of the disk. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. As used herein, “optical transducer” means any element or groups of elements which are used to generate light and sense light. In the examples set forth herein, laser  46  and quadrant detector  42  provide an optical transducer. However, those skilled in the art will recognize that optical transducers can be formed by different or additional elements.