Abstract:
An optical disc storage system employs a read/write pick-up head assembly in which the optical path between the disc and the read/write light source, usually a laser diode, includes both a conventional objective lens formed of glass or plastic, with a fixed focus, and a liquid crystal lens which is electrically tunable to vary its refractive index and focal distance. The optical signal reflected from the disc is passed through this hybrid pick-up head assembly and demodulated to detect errors in the focus of the pick-up head and the tracking, and to adjust the focus by modifying the electrical signals applied to the LCD lens, and move the pick-up head in the plane of the disc to address the appropriate track.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of U.S. Provisional Patent Application Ser. No. 61/041,393 filed Apr. 1, 2008, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to optical memories and more particularly to read and/or write pick-up head assemblies employing combinations of conventional objective lenses in series with electrically tunable liquid crystal lenses for purposes of focus and track correction. 
       BACKGROUND OF THE INVENTION 
       [0003]    The technique of using a laser beam to read data recorded on an optical medium such as a CD, DVD or Blu-ray disc is well known in the art. The laser beam is focused on the track on the surface of the optical disc through an objective lens located in an optical pick-up head, and a photodetector is then used to transform the light reflected from the optical disc to regenerated signals so that the data recorded on the optical disc may be retrieved. During the data reading process, a tracking signal, focusing signal and the like have to be retrieved from the reflected light. The tracking signal and the focusing signal are used to control an actuator to move the objective lens toward and away from the disc, in a focusing direction. By way of example, U.S. Pat. No. 6,839,307 discloses a servo system of this type. 
         [0004]    The movement of the pick-up head toward and away from the disc, during readout, inherently requires some time and the mechanical movement of the actuator of the pick-up head will degrade the system reliability of the optical disc. 
       SUMMARY OF THE INVENTION 
       [0005]    Accordingly, the present invention is directed toward a system wherein the need for motion of the pick-up head toward and away from the disc in order to maintain appropriate focus and tracking, and the necessary actuator to produce this motion, are eliminated, and an essentially solid state pick-up head is produced. This pick-up head is inherently capable of more accurate focusing and tracking because of the elimination of the time required for motion of the pick-up head. Moreover, the elimination of motion eliminates the wear which inherently degrades the system reliability over time. 
         [0006]    Broadly, the system of the present invention employs the combination of a fixed focus objective lens combined with an electrically tunable liquid crystal lens. The structure, and possibly the refractive index of the liquid crystal, will be varied as a function of the voltage applied, which may be derived by processing the reflected signal to detect focus and tracking errors, with these signals used to feed the LC lens in the same way as the servo signal mechanically drives the lens toward and away from the disc in prior art systems. The liquid crystal lens may be of any of a variety of known types, including lenses in which the applied voltage physically shapes the lens, systems in which a plurality of liquid crystal droplets form the lens and the lens may be tuned by applying voltage to the droplets, as well as others. 
         [0007]    In one embodiment of the invention, which will be subsequently described in detail, the axis of the laser beam between the pick-up head and the disc can be varied by applying different voltages to different segments in the same electrode layer of the liquid crystal lens. 
         [0008]    The liquid crystal lens structure could employ a single LC layer, or double LC layers, with orthogonal orientation to each other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Other advantages, applications and objects of the present invention will be made apparent by the following detailed description of preferred embodiments of the invention. The description makes reference to the accompanying drawings in which: 
           [0010]      FIG. 1  is a diagram illustrating the hybrid objective lens of the present invention consisting of a conventional fixed focus objective lens which passes laser light through an electrically tunable LC lens; 
           [0011]      FIG. 2  is a block diagram showing the structure of an optical disc pick-up head assembly formed in accordance with the present invention in operating relationship to an optical disc; 
           [0012]      FIG. 3  is a schematic diagram of a typical electrically tunable LC lens structure; 
           [0013]      FIG. 4  is a schematic diagram of an alternative form of electrically tunable LC lens with a circular void in the center of the ITO layer; 
           [0014]      FIG. 5  is a perspective view of the ITO layers employed in the electrically tunable LC lens of  FIG. 4 ; 
           [0015]      FIG. 6  is a schematic diagram from a perspective view of another form of electrically tunable LC lens, illustrating only the ITO layers, with the upper ITO layer employing a central point electrode; 
           [0016]      FIG. 7  is a schematic diagram of another electrically tunable lens ITO layer employing a plurality of segments which allows the axis of the laser beam to be shifted; 
           [0017]      FIG. 8  is a schematic side view of an electrically tunable, axially steerable LC lens employing a center point ITO layer with a plurality of segments which may be used to steer the beam; 
           [0018]      FIG. 9  is a schematic diagram showing the movement of an optical disc pick-up assembly relative to a disc; and 
           [0019]      FIG. 10  is a schematic diagram of the various components of the pick-up head assembly and reflected beam processing elements to control a hybrid LC lens pick-up, all shown relative to a disc. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Referring to  FIG. 1 , which illustrates a basic structure of the hybrid objective lens module, generally indicated at  10 , in schematic form, the hybrid module simply comprises the combination of a conventional, fixed focus convex lens  20  and an electrically tunable LC lens  30  disposed in the beam outputted by a laser light source  40  to interrogate an optical disc (not shown). The fixed focus lens  20  serves as the objective to focus the light from the laser source  40  onto the disc. The lens could be either spherical or aspherical and made of glass or plastic. The lens structure could be in the form of a fixed single focal length lens, or a multiple lens structure with more than one lens surface curvature. 
         [0021]    After passing through the conventional lens  20 , the beam from the source  40  passes through an electrically tunable LC lens  30 . The LC lens  30  may take any known form such as that shown in U.S. Pat. Nos. 4,572,616; 6,545,739; etc. Broadly, as described in more detail in connection with the subsequent figures, it consists of a liquid crystal layer disposed between a pair of electrodes so that the electric field experienced by the lens may be adjusted to produce changes in the focal length of the hybrid lens  10 . 
         [0022]    The focal length of the hybrid lens  10  is thus influenced by both the focal length of the conventional lens  20  and the current focal length of the LC lens  30 . In  FIG. 1 ,  60  indicates a particular focal length of the hybrid lens  10 . By modifying the voltage applied to the LC lens  30 , the focal point may be moved in the axial distance, for example to the focal point  70 . With one of the LC lenses having segmented electrodes, such as illustrated and described in connection with  FIG. 7 , the focal point may be moved transverse to the axial dimension, for example to point  80  in  FIG. 1 .  FIG. 1  illustrates the shift in the axial direction as D V  and the shift in the transverse direction as D H . 
         [0023]      FIG. 2  illustrates a block diagram of the structure of an optical disc pick-up assembly  200 , employing a hybrid lens structure of the type illustrated in  FIG. 1 . 
         [0024]    The assembly includes a light transmission module  210  which is preferably a laser diode. The wavelength of the laser diode  210  depends upon the type of optical disc  240  loaded into the system. For example a 780 nm laser diode is required for CDs, a 650 nm light source for DVDs and a 405 nm light source for Blu-ray discs (BD). In order to create an optical disc player that may play all of the available varieties of optical discs, a plurality of different laser diode sources may be provided. Other conventional optical components associated with the laser diode such as a collimating lens, diffraction gratings, a dichroic mirror and others will typically be provided in the light transmission module  210 . 
         [0025]    The light beam from the light transmission module  210  is directed at a light path handling module  220 . This module performs the wavelength purification, light beam splitting for transmission and receiving, retarding plate and other well known functions. The light from the unit  220  is passed to the objective lens module  230  of the type generally indicated at  10  in  FIG. 1 . Additionally, it usually contains a hologram diffraction filter to accommodate the different wavelengths with matching numerical aperture (NA) and focal depth for different reflection distances. For example, the NA value for a CD—0.45, DVD—0.60 and BD—0.85. The NA is defined by D/2f where D is the active diameter of the objective lens and f is the focal length of the objective lens. The reflection thicknesses for the different discs are CD—1.2 mm, DVD—0.6 mm and BD—0.1 mm. The light reflected from the optical disc  240  is captured by the light receiving module  250  which is a part of the pick-up head assembly  200 . This module is operative to receive the modulated light and demodulate it and provide it to a data processing unit (not shown). The unit includes conventional elements such as an optical sensor detecting lens, cylindrical lenses, etc. In order to detect the reading error from the disc, there will be a plurality of sensing segments on the light sensor unit. By detecting the position of focus of the light, the error data can be processed and appropriate correction actions taken. 
         [0026]    A typical LC lens structure  300  is schematically illustrated in  FIG. 3 . Layer  331  is usually transparent glass with a high transmission rate and solid in nature.  332  constitutes the electrode layer. It is transparent and indium-tin oxide is the material widely used because it is both electrically conductive and transparent. Element  333  is an alignment layer to assure that the LC modules of the LC core  334  are aligned in the desired orientation and direction. Typically it is formed of an organic material such as polyimide or nonorganic material such as SiO 2  or SiO X .  335  is the power supply that creates a potential difference across the two layers  332  in each half and causes the liquid crystal material  334  to vary in optical properties as the voltage between the two electrodes is changed. 
         [0027]      FIG. 4  is a diagram, similar to  FIG. 3 , illustrating a form of liquid crystal cell which has a central hole in the center of the ITO electrode layer  441 . Layer  442  is the glass substrate, layer  443  the alignment layer, and  445  the liquid crystal layer.  446  provides the electric field across the two electrodes  441  and  444  in each half The central hollow ITO electrode layer may take any one of several forms such as those disclosed in U.S. Patent Application Publication 2007/0139333. The central hole in the ITO layer shapes the electric field applied to the LC layer  445  to produce an appropriate shape to the liquid crystal layer. 
         [0028]      FIG. 5  is a perspective view of the two ITO electrode layers in the device of  FIG. 4 . When voltage is applied to the two ITO layers  441  and  444 , the electric field created is stronger along the inner edge of the circular hole in the electrode  441  and weaker toward the center and will force the LC molecules to form an equivalent convex lens effect. As the voltages vary the focal length of this will be changed. The total focal length of this objective lens module will be, for the example shown in  FIG. 1 , 1/f(t)=1/f(c)+1/f(1) where f(t) is the total focal length, f(c) is the focal length provided by the fixed focal length of the conventional lens, and (f)1 is the focal length contributed by the LC lens, its focal length depending on the voltage applied. 
         [0029]    In some different designs the conventional objective lens could have multiple focal lengths and consist of more than one curvature surface as disclosed in U.S. Patent Application 60/942,310 or U.S. patent application Ser. No. 11/850,248.  FIG. 6  discloses the two opposed ITO electrode layers of an LC lens wherein the upper layer  660  is powered by a central point electrode  665 . The bottom plain ITO layer is denominated  661 , and  670  is the power unit of the ITO segments. Again, this structure will force the LC molecules to form an equivalent lens effect. 
         [0030]      FIG. 7  discloses the ITO upper and lower layers  700  and  710  of an LC lens module in which the upper ITO layer  700  is divided into four segments, I, II, III and IV. Each of the segments is connected to a driver unit  750  by connections  701 ,  702 ,  703  and  704  respectively. By selectively energizing one or a combination of the segments of the upper electrode  700 , the laser beam may be optically steered in a direction transverse to the beam axis. 
         [0031]    The center point ITO construction of the type shown in  FIG. 6  can be combined with the plural segmented construction illustrated in  FIG. 7  to achieve light axis movement. The basic structure of this electrode design is disclosed in U.S. Patent Application 60/033,050. It consists of an upper module with an alignment layer  881 , an ITO layer  882  with a center point electrode, an insulation layer  883  consisting of a thin glass layer like SiO 2  or SiO X , a plain ITO layer  884  formed on a substrate  885 . 
         [0032]      FIG. 9  is a diagram showing the movement of a disc pick-up head assembly  200  relative to an optical disc  240 . The head is stationary in a plane transverse to the plane of the paper with the LC lens accommodating the necessary focus adjustment while an actuator must move the head  200  in a radial direction to locate the proper track. 
         [0033]      FIG. 10  illustrates the light path in the entire pick-up head assembly. In this schematic drawing, the optical disc  110  is interrogated with a laser beam derived from a light transmission module which is passed through a light path handling module  170  to an objective lens module  140 . The reflected beam from the disc passes through the objective lens module in the reverse direction and then through the light path handling module. It is then passed to a light receiving module  150  and then to a demodulator  180  which derives the intelligence on the disc and passes it to appropriate utilization circuitry and to an error tracking module  160 . This module performs a comparison algorithm on the reflected processed beam and derives an error message. The error message is compiled into a control signal and fed to a driver unit  170 . There are many different error detecting algorithms and methods disclosed in the prior art patents. The driver unit will modify the voltage on the liquid crystal module contained within the objective lens module  140  and/or apply different voltages at different ITO segments if a segmented ITO layer is utilized.