Abstract:
A method of using laser light to access a magneto-optical disc utilizes first and second objective lenses, the second objective lens being mounted in a light channel of a slider having a primary planar surface that flies over the surface of the magneto-optical disc. The laser light is directed through the first and second objective lenses that focus the laser light onto the surface of the magneto-optical disc. Focus of the laser light is maintained solely by moving the first objective lens in an axial direction with respect to the magneto-optical disc, the first objective lens being mounted to an actuator.

Description:
This application is a division of Ser. No. 09/026,798 filed Feb. 20, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention is in the field of disc drive mechanisms for reading data from and writing data to data storage discs. More particularly, the present invention is in the field of magneto-optical (MO) disc drives. 
     BACKGROUND OF THE INVENTION 
     Typical magneto-optical (MO) disc drives record data by locally heating a portion of the disc. MO discs, or media, include a recording layer of a magnetic material. The coercivity of the heated portion of the media is lowered when it is heated by the laser beam. This allows the magnetic polarity in that area to be reversed by an applied magnetic field. In such disc drives, data is read from media by illuminating areas of the storage media with a linearly polarized laser beam. The Kerr rotation effect causes the plane of polarization of the illuminating beam to be rotated. The direction of rotation depends on the magnetic polarity in the illuminated area of the storage media. When the disc is read, the polarization rotation is determined with a pair of optical detectors and a polarization beam splitter to produce an output data signal. Limitations of MO disc drives include data access time and density with which data can be stored. 
     FIG. 1 is a diagram of one prior MO recording system typically used with 130 mm (mm) diameter MO media. System  100  is an example of a “substrate incident” recording system. In substrate incident systems, laser light is incident on a thick substrate layer, travels through the substrate layer and is focused on a recording layer below the substrate layer. System  100  objective lens  102  for focusing a collimated beam of light on disc  116 . Disc  116  is an example of a typical two-sided MO disc. MO disc  116  includes substrate layers  104  and  114  forming outside layers on opposing sides of disc  116 . Substrate layers of  104  and  114  are made of materials such as plastic polycarbonate and are approximately 1.2 mm thick. Recording layer  106  is below substrate layer  104 , and recording layer  112  is below substrate layer  114 . Recording layers  106  and  112  can be made out of any one of a number of well-known materials, such as Tb—Fe—Co, a rare-earth transition-metal alloy. The laser light beam passing through objective lens  102  penetrates substrate layer  104  as shown and is incident on a focal point on the surface of recording layer  106 . 
     System  100  has several disadvantages. One of the disadvantages of system  100  is that it is necessary to apply energy to the recording layer to erase data prior to writing new data. This is because a large, stationary magnetic coil (not shown) having a large inductance is situated on the opposite side of disc  116  from objective lens  102  to assist in the writing process. Because the coil is held at a relatively great distance form the media surface and has a relatively large inductance, the magnetic field cannot be reversed at high frequencies. Therefore, it is necessary to erase data before writing new data. The necessity of erasing before rewriting slows the process of writing data to disc  116 . 
     Another disadvantage of system  100  is that the density of data stored on disc  116  is relatively low. A further disadvantage of system  100  is that only one side of disc  116  can be accessed at one time. This is because the relatively large coil occupies the space on the side of the disc opposite the objective lens. This space cannot therefore be used for another lens and actuator. In order to a different side of disc  116 , disc  116  must be removed, turned over, and reinserted into system  100 . Disc  116 , however, provides good data security because relatively thick substrate layers  104  and  114  allow disc  116  to be handled without danger of data loss or difficulty in reading data because of contamination. 
     FIG. 2 is a diagram of another prior MO recording system  200 . Collimated light beam  202  passes through objective lens  204  to disc  216 . Disc  216  includes substrate layer  206  that is typically 0.6-1.2 mm thick. Disc  216  further includes recording layer  208  between substrate layer  206  and protective layer  210 . In system  200 , the large, stationary coil of system  100  is replaced by a relatively small coil in flying magnetic recording head  214 . Flying height  212  is maintained by an air bearing created when disc  216  passes under flying magnetic recording head  214 . For writing to disc  216 , a magnetic field created by magnetic recording head  214  is used in conjunction with collimated light  202  which passes through objective lens  204 . The smaller coil of magnetic recording head  214  has less inductance than the large, stationary coil of system  100 . The reduced inductance allows direct overwrite of data on disc  216  by switching the magnetic field. 
     System  200  still possesses the disadvantage of relatively low storage densities, however. In addition, disc  216  is a one-sided, rather than a two-sided disc, reducing overall storage capacity. 
     System  200  also has the disadvantage of requiring mechanical coupling of light on one side of disc  216  and magnetic recording head  214  on the other side of disc  216 . Typically, this coupling is accomplished by mechanical linkages that pass from objective lens  202  to magnetic recording head  214  around the edge of disc  216 . The mechanical linkages cannot be allowed to interfere with the movement of objective lens  202  (during focussing) or with disc  216 . 
     FIG. 3 is a diagram of prior MO recording system  300 . System  300  is an example of an “air incident” design in which a lens is held very close to the media and laser light is incident on very thin protective layer  309  that is over recording layer  308  of disc  318 . System  300  employs flying magnetic recording head  316 , and a two-piece objective lens comprised of lens  314  and lens  312 . Prior art systems similar to system  300  use other lens designs, for example, three-piece objective lens designs. Lens  314  is held extremely close to disc  318 . Collimated light beam  302  passes through lens  312  and lens  314 . Lens  312  and lens  314  are integrated with slider  304  and magnetic recording head  316 . Flying height  306  for system  300  is typically less than the wavelength of the laser light used in reading from and writing to MO disc  318 . 
     Disc  318  has an MO recording layer  308  over substrate layer  310 . Because in system  300 , flying objective lens  314  is in close proximity to disc  318 , the need for a focus actuator is eliminated. As is known, focus actuators are mechanisms that adjust the height of an objective lens over a disc during read and write operations. In the case of system  300 , the height of flying objective lens, and thus the focus of flying objective lens  314 , is determined by the air bearing created between slider  304  and recording layer  308  during flight. 
     By maintaining the spacing between flying objective lens  314  and recording layer  308  at less than the wavelength of the laser light used, laser light can be focused in the near field mode of operation. As is known, the near field mode of operation uses the phenomenon of evanescent coupling, which requires that the objective lens be held very close to the recording layer. The use of evanescent coupling to perform recording allows a smaller spot size, and therefore, greater recording densities and better data throughput. 
     System  300  has several disadvantages. For example, the surface of layer  309  and the surface of lens  314  closest to the disc can be contaminated, causing permanent damage to data and to the disc drive system. 
     Another disadvantage of system  300  stems from the fact that because there is one objective lens and no focus actuator, the flying height must be tightly controlled. Variations in the flying height and thickness of protective layer  309  (if there is a protective layer; it is possible to have none) over the recording layer must be controlled within the depth of focus tolerance of the flying lens. Generally, the tolerance of flying height  306  and protective layer  309  thickness is a percentage of the nominal thickness. Therefore, in order to reduce the tolerance, the nominal thickness of protective layer  309  must be reduced. For example, the depth of focus tolerance is generally plus or minus 0.5 micron. A typical tolerance in applying protective layer  309  is ten percent of the thickness of the protective layer. Therefore, flying height  306  and the thickness of protective layer  309  together must be very small for the thickness variation of protective layer  309  to be less than 0.5 micron. 
     In the case of a near field system such as system  300 , the flying height (the distance between the bottom surface of flying lens  314  and the surface of recording layer  308 ) must be less than the wavelength of the laser light. The wavelength of the laser light is typically 700 nanometers. Therefore, the thickness of a protective layer on recording layer  308  would have to be on the order of 25 nanometers. This is extremely thin and would not protect data on recording layer  308  from manual handling in a removable disc application, or from corrosion or contamination during shelf life. Even with the protection of a cartridge that covers disc  318 , some contamination from particles in the atmosphere or from humidity or corrosive gases is inevitable over time. 
     Conventional disc drives all share similar disadvantages related to access of data on a storage disc. Current disc drives, even those designed to access two-sided media, are limited to accessing one side of the media at a time. It has not been possible, previously, to simultaneously and independently access both sides of a two-sided disc. One of the reasons for this is that reading/writing head mechanisms on either side of the disc are constructed to move together or not at all. Current disc drives therefore have limited data access speeds. This disadvantage is shared by previous MO drives and drives using other technologies, such as those used in computer hard disc drives. 
     Technology exists to make multiple disc drives appear to a client device as a single drive. Redundant arrays of independent drives (RAIDs) divide incoming data into multiple streams which are written to multiple drives simultaneously. RAID drives can be used to increase throughput by dividing a single incoming data stream and writing portions of it to multiple drives simultaneously. RAIDs can also be used to achieve data redundancy by sending different copies of the same data to multiple drives simultaneously. Although access speed can be increased by using RAIDs, RAIDs are expensive and complex because they are merely devices containing duplicate conventional disc drives, each of which has all the limitations previously discussed with respect to current disc drives. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an MO drive that enables higher density data storage on one-sided or two-sided media that is coated to enable handling of the media without risk of data loss. 
     It is another object of the invention to provide an MO drive that enables direct overwrite of data without initial erasure. 
     It is a further object of the invention to provide an MO drive that accesses both sides of a two-sided disc simultaneously and independently. 
     A method and apparatus for magneto-optical storage and access of data is described. The apparatus comprises a flying magnetic head, wherein the flying magnetic head comprises: a slider that flies over a magneto-optical disc during read and write operations performed on a magneto-optical disc; a magnetic coil fixedly attached to the slider, the magnetic coil defining a channel through the slider, the magnetic coil and the slider forming a smooth, planar surface parallel to a surface of the disc during read and write operations; and a first objective lens fixedly attached in the channel such that a surface of the first objective lens is parallel to the smooth, planar surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a prior art magneto-optical (MO) data storage and retrieval system. 
     FIG. 2 is a diagram of a prior art MO data storage and retrieval system. 
     FIG. 3 is a diagram of a prior art MO data storage and retrieval system. 
     FIG. 4A is a diagram of an MO data storage and retrieval system according to one embodiment of the present invention. 
     FIG. 4B is a diagram of a MO data storage and retrieval system according to another embodiment of the present invention. 
     FIG. 5 is a top view of a MO drive according to one embodiment. 
     FIG. 6 is a partial side view of the MO drive of FIG.  5 . 
     FIG. 7 is a partial end view of the MO drive of FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     The present invention includes a magneto-optical (MO) disc drive and an MO disc. Far field recording is performed with a first objective lens that focuses collimated laser light that then passes through a second objective lens that is fixed in a light channel in a flying magnetic head. The flying magnetic head flies above the MO disc during data access operations. Higher numerical aperture is achieved over prior techniques that use protected media. Consequently, increased data storage densities are achieved over conventional storage techniques that use protected media. In one embodiment, the MO disc (media) includes two recording layers covered with protective coating layers that guard data from damage even during manual handling of the MO disc. In one embodiment, a flying magnetic head with an integrated objective lens is situated on each side of the MO disc. An objective lens is also situated on each side of the MO disc above the flying magnetic heads. Magnetic heads and objective lenses on respective sides of the MO disc are independently actuated to allow simultaneous and completely independent access to both sides of the MO disc. 
     FIG. 4A is a diagram of an MO data storage and retrieval system  400  according to one embodiment of the present invention. In FIG. 4A, elements that are similarly numbered except for an “a” or “b” are identical, or functionally equivalent. For example, objective lens  414   a  is identical, or functionally equivalent to, objective lens  414   b . Therefore, functionally equivalent elements will be described with reference to one of the similarly numbered elements. 
     Objective lens  414   a  focuses collimated laser light beam  402   a . Flying heights  422   a  and  422   b  are each 0.05 to 5.0 microns above respective surfaces of MO disc  420  depending upon the specific embodiment. Magnetic recording heads  418   a  and  418   b  produce magnetic fields with relatively low inductance, allowing direct overwriting of data upon switching the magnetic field. The preferred embodiment uses magnetic field modulation for reading and writing operations, which is a known technique. Higher storage densities can be accomplished by modulating magnetic fields produced by magnetic recording heads  418   a  and  418   b  during write and read processes. 
     Magnetic recording head  418   a  includes light channel  416   a  which is a hole through the center of magnetic recording head  418   a . Objective lens  424   a,  in this embodiment, is a solid immersion lens (SIL) that is fixed in light channel  416   a  as shown. Lens  424   a  is a flying objective lens in parallel to MO disc  420 . In this embodiment, lens  424   a  is recessed in magnetic recording head  418   a . Because lens  424   a  is recessed, it is protected from contamination that could damage lens  424   a  and degrade performance of system  400 . In the embodiments of FIG. 4B, lens  424   a  is coplanar with the surface of magnetic recording head  418   a . In cases where lens  424   a  flies a distance above disc  420  which is less than the wavelength of the laser light, the embodiment of FIG. 4B achieves near field recording. 
     Because MO disc  420  includes coating layer  408   a  over recording layer  414   a , the system of FIG. 4A records data using a far field technique, rather than a near field technique. The distance between lens  424  and MO disc  420  does, however, reduce parallelism or tilt concerns caused by relative attitudes of the surfaces of MO disc  420  and lens  424   a . Therefore, many mechanical tolerances of the drive mechanism and disc are eliminated and a higher numerical aperture (NA) is achieved. NAs greater than 0.85 are achieved, resulting in higher data storage densities than were possible with the approximately 0.55 NA previously achievable with far field MO techniques. Slider  404   a  is integrated with magnetic recording head  418   a  and enables magnetic recording head  418   a  to fly above MO disc  420  on an air bearing when disc  420  is spinning. The embodiment of FIG. 4A records data on MO disc  420  using a far field technique. In far field recording techniques, the focal distance (the distance from a focal point on a recording layer of MO media to an objective lens) is greater than the wavelength of incident light of collimated light beam  402   a . In this embodiment, the focal distance is relatively large, and thus lens  424   a  is not in contact with, or in close proximity to, the recording layer. 
     In the embodiments of FIGS. 4A and 4B, actuators controlling objective lens  414   a  and magnetic head  418   a  are completely independent from separate actuators controlling objective lens  414   b  and magnetic head  418   b . Therefore simultaneous and independent access of both sides of MO disc  420  takes place. 
     For example, recording layer  410   a  is written to at the same time recording layer  410   b  is read from by respective head assemblies. Actuators are known in the art and are not shown for clarity. Objective lens  414   a  and objective lens  424   a , in these embodiments, are flexibly coupled so that they are commonly actuated. Specifically, lens  424   a  moves axially as determined by the characteristics of disc  420  because lens  424   a  rides on an air bearing. Lens  414   a  moves in response to lens  424   a  so that focus can be maintained regardless of physical variations in the surface of disc  420 . 
     A known flexure and gimbal suspension assembly is used to suspend each slider  404  and magnetic head  418 . Suspension assemblies are not shown in FIGS. 4A and 4B for clarity. In a particular embodiment, a suspension assembly with magnetic head is loaded by spinning the disc and then engaging, or loading, the suspension assembly and magnetic head. An air bearing is formed between the slider  404  and the spinning disc and the slider surface comes in contact with the disc surface. In other embodiments, the slider rests on the disc surface when the disc is not moving. In these embodiments, the slider rests on the disc surface until the disc reaches a certain revolutionary speed, after which an air bearing is formed and the slider is separated from the disc surface. 
     MO disc  420 , in this embodiment, has spiral grooves in both recording layers. The spiral grooves on opposite recording layers spiral in opposite directions so that the spinning disc can be accessed simultaneously from both sides. Other embodiments use a two-sided MO disc with concentric grooves. Spiral groves are preferable when data to be stored and accessed is of a sequential nature. Concentric grooves are preferable when data to be stored and retrieved is of a less sequential and more “random” nature. The embodiments shown include an MO disc with a 130 mm diameter form factor. Other embodiments use different MO discs, for example discs having 80 mm, 90 mm, or 120 mm diameter form factors. 
     MO disc  420  includes a central substrate layer  412 . On either side of substrate layer  412  are recording layers  410   a  and  410   b , respectively. Coating layer  408   a  forms one surface of MO disc  420  and covers recording layer  410   a . Coating layer  408   b  forms the opposite surface of MO disc  420  and covers recording layer  410   b.    
     FIG. 5 is a top view of disc drive  700  according to one embodiment of the invention. In this embodiment, Disc drive  700  includes two optical pickup/front end electronics assemblies  716   a  and  716   b . Assemblies  716 , in this embodiment, are moved back and forth over respective sides of MO disc  710  by a linear actuator. Other embodiments could use other actuators, for example, rotary actuators. 
     Optical pickup/front end electronics assembly  716   a  is situated over one side of MO disc  710 , and assembly  716   b  is situated over the opposite side of MO disc  710 . Each of the assemblies  716  are integrated optics assemblies. As is known, integrated optics assemblies include, in one unit, a focus actuator, a tracking actuator, a coarse actuator, optical components, and front end electronics. In this embodiment, optical components include objective lenses such as lenses  414  and  424  of FIGS. 4A and 4B. In the embodiment shown in FIG. 5, integrated optics are chosen, in part, for ease of assembly. Extreme precision is required to align the optical components of the mechanism. When integrated optics are used, alignment can be performed on an assembly, such as assembly  716   a , on a separate station before assembling the entire disc drive. This makes assembly faster and less expensive. 
     Other embodiments use split optics. Split optics include a moving portion and a stationary portion. The moving portion travels over the disc and includes an objective lens, a mirror, a fine actuator, a coarse actuator, and a focus actuator. The fixed portion includes a laser diode, a detector, optical components, and front end electronics. 
     In this embodiment, a focus actuator and a fine actuator are coupled to a coarse actuator. The coarse actuator performs relatively large movements laterally across the surface of the disc. The focus actuator moves axially with respect to the disc for focusing the laser light. The fine actuator performs small lateral movements, or microsteps, for keeping the focused laser light on a track of the disc. In this embodiment, a magnetic head  418 , including an integrated objective lens  424 , is actuated commonly with a lens  414 . Lenses  414  and  424  are actuated by the focus actuator by the fine actuator. 
     Disc drive  700  includes carriage coil  702 , return magnetic path assembly  704 , and magnet  706 . Spindle motor  708  engages MO disc  710  as explained more fully below. In this embodiment, dimension  718  is approximately 200 mm, dimension  714  is approximately 140 mm, and  712  is approximately 130 mm. Other embodiments of disc drive  700  could operate with MO discs of varying form factors. For example, disc drives embodying the invention could be used with MO discs as described herein, but with diameter form factors such as 80 mm, 90 mm, or 120 mm. 
     Disc drive  700  is an embodiment that includes two optical pickup/front end electronics assemblies. Other embodiments include only one optical pickup/front end electronics assembly that accesses one side of MO disc  710 . These embodiments only read or write one side of a disc at one time. 
     FIG. 6 is a side view  800  of the disc drive of FIG.  5 . Disc drive  700  is partially enclosed by top cover  802 , bottom cover  804  and printed circuit board (PCB) assembly  808 . Objective lens  810  of assembly  716   a  is indicated. 
     Assembly  716   a  and  716   b  are identical, or functionally equivalent. Magnetic head and suspension  818  are indicated for assembly  716   b . In this embodiment, the magnetic head is designed as a magnetic field modulation head. Magnetic field modulation techniques are known in the art. Coarse carriage coil  814  is indicated for assembly  716 . Spindle motor  816  is shown engaged with disc  812 . Reference number  806  designates the spindle motor in the disengaged position. Spindle motor  708  is disengaged during insertion or removal of disc  812 . Spindle motor  708  moves up and engages with MO disc  710  after insertion of MO disc  710 . 
     FIG. 7 is a diagram of end view  900  of disc drive  700 . End view  900  references one magnetic head/suspension assembly  902 , and objective lens  904 . In this embodiment, magnetic head/suspension assembly  902  is mounted on the coarse actuator body and is not attached to focus actuator  904 . 
     Top cover  906 , bottom cover  912 , and PCB assemblies  908  are also shown. Optical pickup/front end electronics assemblies  916  are shown on either side of MO disc  910 . Carriage coil  914  for one assembly  916  is shown. In this embodiment, dimension  924 , the thickness of MO disc  910 , is 0.6-2.4 mm. In this embodiment, dimension  918  is 5 mm, dimension  922  is 10 mm, and dimension  920  is 41.3 mm. 
     The embodiments shown perform parallel processing of data or redundant processing of data in one disc drive. Optical pickup/front end electronics assemblies  716  of FIG. 5 are operated by independent actuators, and the incoming data stream is divided between the two assemblies to perform independent reading and/or writing to either side of MO disc  710 . A user can thus choose to increase throughput, or lower access time, by using parallel access. A user can alternately choose to access both sides of MO disc  710  redundantly in order to produce backup copies of data. When parallel access mode is chosen, client devices that perform command queuing can be serviced more quickly because commands in the queue can be smoothly executed even if they are not of the same type. For example, a write operation can be performed on one side of the MO disc  710  at the same time a read operation is performed on the opposite side of MO disc  710 . 
     The embodiments shown perform MO recording with increased data density, simultaneous, independent access to two data storage surfaces, and direct overwrite capability. Other embodiments include only one optical pickup/front end electronics assembly and therefore do not perform simultaneous, independent access to two data storage surfaces. 
     The invention has been described in terms of particular embodiments. For example, the embodiments shown include an MO disc of a particular form factor and a disc drive with integrated optics and linear actuators. One skilled in the art, however, may make modifications and alterations to the specific embodiments shown without departing from the spirit and scope of the invention as set forth in the following claims.