Abstract:
A method and apparatus are provided for visually marking a recording medium by inducing a phase change in a light-sensitive surface by heat absorption from laser radiation. The method includes applying the laser radiation to a desired portion of the light-sensitive surface according to a first power profile for a first period sufficient to substantially obtain a color change at the desired portion, and applying the laser radiation to the desired portion of the light-sensitive surface according to a second power profile for a second period sufficient to complete the color change at the desired portion.

Description:
BACKGROUND 
   Recently, apparatus and methods have been developed with the ability to generate an image on media using a low power laser, such as a laser of the type that is employed to read and write data on the data side of optical storage discs. Optical discs, such as recordable CDs, DVDs and the like typically have various types of data written on a data side of a disc by a laser beam while the disc is rotating. Various data writing strategies have been employed, for the purposes of creating desirably shaped marks precisely positioned and having sharp edges to enable detection during a subsequent read operation. In some cases, data writing strategies have included laser beams using modulated power, including laser pulses. 
   The other side of such a disc is often used for hand-writing or affixing a label with descriptions and/or illustrations descriptive of the recorded data. Recently, apparatus and methods have been developed with the ability to generate a label on the non-data side of an optical disc using the same laser that was employed to read and write digital or electronic data on the data side of the disc. See U.S. Patent Application Publication No. 2003/0108708 (Anderson, et al.), using laser sensitive materials on a disc label that react chemically upon the application of light and heat, resulting in changes in color and shading on the label. In such instances, labels may be generated on discs by applying continuous laser power to a label surface. However, this approach may result in undesirable ablation of the recording medium and power inefficiencies in the apparatus. 
   SUMMARY OF THE INVENTION 
   In one embodiment of the present invention, a method is provided for visually marking a recording medium by inducing a phase change in a light-sensitive surface by heat absorption from laser radiation. The method comprises applying the laser radiation to a desired portion of the light-sensitive surface according to a first power profile for a first period sufficient to substantially obtain a color change at the desired portion, and applying the laser radiation to the desired portion of the light-sensitive surface according to a second power profile for a second period sufficient to complete the color change at the desired portion. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a power versus velocity graph for a recording medium on which optically visible marks are formed by an image recording device according to an embodiment of the present invention; 
       FIG. 2  is a temperature versus time graph according to the embodiment represented in  FIG. 1 ; 
       FIG. 3  is an optically visible mark timing graph according to the embodiment represented in  FIG. 1 ; 
       FIG. 4  is a combined temperature versus time graph and optically visible mark timing graph for a first velocity according to the embodiment represented in  FIG. 1 ; 
       FIG. 5  is a combined temperature versus time graph and optically visible mark timing graph for a second velocity according to the embodiment represented in  FIG. 1 ; 
       FIG. 6  is a combined temperature versus time graph and optically visible mark timing graph for a varying power level according to another embodiment of the present invention; 
       FIGS. 7A and 7B  are power versus time graphs according to the embodiment shown in  FIG. 6 . 
       FIG. 8  is a schematic diagram of a laser writing and detection system according to an embodiment of the present invention; and 
       FIG. 9  is a partial schematic view of the diagram shown in  FIG. 1 ; 
   

   DETAILED DESCRIPTION 
   Reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features illustrated herein, and additional applications of the principles of the inventions as illustrated herein, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention. 
   In one field of electronic digital data recording technology, a laser is used to write and read digital data on a data side of a rotating disc medium, such as a compact disc (CD), digital versatile disc (DVD) or other such recording media, that is used in a recording device. The present invention involves writing or marking optically visible marks on a light sensitive medium, such as a label side of the recording medium opposite the data side, to form text, graphics, images, or a combination thereof that could be used to describe or represent the data on the recording medium. This imaging technology can be widely used to apply markings to any medium with light sensitive coating. In one embodiment the technology can be used to apply markings to one side of a recording medium that enables the formation of images. This medium may include two or more chemicals that can be liquefied by the application of heat. A laser may be used to apply heat to the chemicals on the label side, such as the same laser that is used to write and read data on the data side of the electronic media. As the chemicals are activated they begin to interact, forming visible marks on the media. By varying the application of the laser energy as it scans across the light-sensitive surface, the marks create images on or in the medium in any desired manner. 
   The present invention is concerned with controlling the amount of laser power delivered to the recording medium by the recording device so as to maintain the energy input of the recording medium between a minimum level needed to provide the desired chemical changes and a maximum level at which disintegration or mechanical ablation of the recording medium starts to occur. The amount of energy delivered to the recording medium is controlled by the level of power applied, the length of time that power is delivered to the recording medium and the speed at which the recording device is rotating the medium. In addition, there is a specific set of changes in properties of the recording medium which occur during the process of imaging that are temperature dependent, such as glass transition, viscosity, and molecular mobility changes. 
     FIG. 1  is a graph  10  showing power applied to a recording medium versus rotating velocity of the recording device. A maximum read power level  12  is shown, which is significantly below a minimum write power level  14  needed to write on the recording medium (not shown). The minimum write power line  14  is the minimum laser power needed to create an optically visible mark having acceptable optical density and fade tolerance. The maximum write power line  16  shown is the maximum continuous laser power above which a significant amount of ablation in the recording medium begins to occur. 
   Without subscribing to, or being limited by one particular theory or set of changes,  FIG. 2  is a graph  20  representing one possibility of chemical and physical changes to the recording medium and showing the temperature of the recording medium versus time with energy added at a constant rate. In a first time period  22 , the temperature  24  rises until the specific heat of the solid medium has been applied, at which point  26 , a phase change begins to occur with the solid changing to a glass or a liquid. During the next time period  28 , the temperature  30  of the recording medium remains a constant level, as all of the applied power is used to provide the latent heat of fusion, or glass transition, needed for the recording medium to fully change to a glass or a liquid. 
   At point  32 , the latent heat of fusion has been provided. As power is continually applied during a glass or a liquid specific heat period  34 , the temperature  36  of the glass or a liquid recording medium again begins to rise. Finally, at an ablation point  38 , a phase change from solid, glass or a liquid to vapor begins to occur. During the next period  40 , the temperature  42  is held steady as the latent heat of vaporization is provided to the recording medium. 
   During period  40 , undesirable ablation occurs, as the recording medium is vaporized. Accordingly, it is desirable to maintain the power applied to the recording medium for labeling purposes at a level, or in a profile or sequence, such the temperature of the recording medium does not substantially reach the ablation point  38 . However, it is also important during the writing period to keep the temperature of the recording medium at a sufficiently high level to continue the label writing process. This result can be achieved by combining power levels with timing periods and rotating speed of the recording medium in the recording device, as will be described in more detail below. 
     FIG. 3  shows an imaged mark timing graph  50 , depicting without quantification the time required to make an acceptable mark, according to the present invention. Time period  52  represents a time during which the recording medium (not shown) must be heated to a level at which liquefaction begins. A second longer time period  54  represents a time during which the marking materials become liquefied enough to mix together. Finally, a shorter time period  56  represents a time during which the mixed marking materials react sufficiently to form a desired optically visible mark. The purpose of the process shown in graph  50  is to form a mark having a certain predetermined contrast with the unmarked background of the label. Looking next at  FIG. 4 , a combined graph  60  is shown in which the graph  50  of  FIG. 3  has been superimposed over the time versus temperature graph  20  of  FIG. 2 . The time to heat  52  coincides with a combination of the periods  22  and  28  in graph  20 . This is the time during which the solid recording medium is heated to the point of that the liquefaction phase change has been completed. The time to mix  54  and the time to react  56  both occur during the specific heat period  34  of the glass or a liquid, prior to the time that the undesirable ablation point  38  is reached. The rotating speed of the recording device (not shown) is at a rate (referred to as 1×) such that the time graph  50  ends with the temperature of the recording medium at point  62  before the ablation point  38  is reached. 
   Accordingly, with the recording device rotating at speed 1×, if the application of power to the recording medium ceases at point  62 , no ablation will occur in the write process shown in graph  62 .  FIG. 5  shows a combined graph  70 , with the recording device (not shown) rotating at twice the speed (referred to as 2×) as the rotating speed of the recording device relative to the graph  60  shown in  FIG. 4 . Again, graph  70  is a combination of the time graph  50  shown in  FIG. 2  and the time versus temperature graph shown in  FIG. 3 . As before, the time to heat  52  coincides with the combination of periods  22  and  28  in  FIG. 2 . However, since the recording device is rotating at twice the speed of that shown in  FIG. 4 , the time to mix  54  and the time to react  56  combine to extend to a point  72  that is past ablation point  38  and into the vaporization time period  40 . Accordingly, with the recording device rotating at speed 2×, significant ablation of the recording medium will occur before the time to react has been completed. 
   The slope of the temperature rise in  FIG. 5  is steeper than the slope of the temperature rise in  FIG. 4  because the power being used with respect to the graph in  FIG. 5  is twice as much as the power being used at the rotating speed for the graph shown in  FIG. 4 . In any event, the important aspect being illustrated in  FIG. 5  is that, at higher speeds, the reaction time may run into the vaporization region, resulting in ablation, as shown in  FIG. 5 . 
   It may be desirable to increase the rotating speed to reduce the labeling time.  FIGS. 4 and 5  show that, with the application of constant power, there is a rotating speed limit between 1× and 2×, above which significant ablation occurs in the recording medium. Accordingly, it may be desirable to limit the level of power applied to the recording medium, such that the rotating speed of the recording device is not the controlling parameter in the process of recording images on the label side of the recording device. 
   Referring now to  FIG. 6 , a combined graph  80  is shown, again superimposing the time graph  50  of  FIG. 3  over the time versus temperature graph  20  of  FIG. 2 . As before, the time to heat  52  coincides with the specific heat time  22  and the latent heat of fusion time  28 , which together result in liquefaction of the recording medium. However, the amount of power provided after liquefaction is no longer constant but has been modified so that the temperature does not exceed a value indicated by point  82 . By modulating the power to hold the temperature at the value shown at  82 , the glass or a liquid recording medium is maintained without ablation during the time to mix  54 , the time to react  56  and beyond. That is, the period  34  of specific heat for the glass or a liquid recording medium extends indefinitely, because the ablation temperature point is never reached. Likewise, the power level maintains the temperature of the recording medium substantially constant, whether the rotating speed of the recording medium is at 1× or 2× or other speeds. 
   As discussed above, by changing the power level after liquefaction, and maintaining that level below the ablation point, no substantial ablation will occur. Although not shown in  FIG. 6 , this change might be achieved by reducing the average power level, or by making some other change in the average power level sufficient to keep the energy level of the recording medium below a second energy level at which substantial ablation or disintegration of the recording medium occurs. 
     FIGS. 7A and 7B  illustrate two ways in which the power may be modulated to achieve the result shown in  FIG. 6 . In  FIG. 7A , a time versus laser power graph  100  is shown, in which the laser power level is maintained at a high level  102  during the time period to heat  104  and part of the time period to mix  106 . Then the laser power is dropped to a lower level  108  for the remaining mixing time period and through the shorter time period to react  110 . Both levels  102  and  108  are sufficiently high to maintain a write on the medium. After the desired reaction time period  110  has ended, the laser power is dropped to a standby level or off  112 . 
   In  FIG. 7B , another time versus laser power graph  120  is shown, in which the laser power is maintained at a high level  122  during the time period to heat  124  and during part of the time period to mix  126 . Then the laser power level is delivered in a series of pulses  128  modulating from level  122  to a low or off level  130 . Again, after the desired reaction time period  130  is completed the laser power drops to the low or off level  132 . 
     FIGS. 8 and 9  show a data storage device  200  according to an embodiment of the invention, in which a laser is used for writing marks on an optical disc and for detection of the written marks to determine the results of the write operation. The data storage device  200  is used for reading from and/or writing to an optical disc  202 . More specifically, the data storage device  200  is used for reading and/or writing to an optically writable data side  204  of the optical disc  202 . After the data writing operation is completed, in one embodiment the optical disc is turned over so that the label side  206  of optical disc may be written upon, for the purpose of labeling and otherwise marking the optical disc  202 . In another embodiment, the data side and the label side may be the same side of the optical disc. An optical pickup unit (OPU)  210  provides the energy beam, lenses and other devices necessary to provide energy beams for reading and writing the medium, as explained below. 
   The storage device  200  includes a spindle motor  212  and a spindle  214  for rotating the optical disc  202 . Specifically, the optical disc  202  is situated on the spindle  214 , which is rotated or moved by the spindle motor  212  to a position specified by an input control signal  216 . A controller  218  provides control signal  216 , and is comprised of control hardware and/or software. 
   The storage device  200  further includes a sled motor  220  and a sled rail  222  for moving a sled (not shown) that supports the OPU  210 . Specifically, the OPU  210  is situated on the sled that is moved on the sled rail  222  by the sled motor  220  to a given position specified by a sled control signal  224  coming from controller  218 . 
   The OPU  210  writes and reads label data on label side  206  of optical disc  202 . In one embodiment, the optical disc  202  is rotated by spindle  214  as the label data is written on optical disc  202 . The sled causes the OPU  210  to move radially so that the label data is written in radial increments that may or may not constitute tracks on optical disc  202 . The manner of moving the spindle  214  and sled to cause the OPU  210  to write label data is not a part of this invention and may vary in different applications of the invention. 
   Again referring to  FIG. 8 , controller  218  generates laser control signals  227  to a laser diode driver  228  to drive the OPU  210 . A tracking voice coil  260  is connected to a fine adjustment mechanism  262  to make small adjustments to the tracking operation of the sled and sled motor  220 . Concentric or spiral tracks, such as are typically used on the data side  204  of optical disc  202  may not be needed on the label side  206 . Instead, the sled may move in small increments, such as about 250 micron steps, to provide the desired image labeling. A focus actuator  264  is associated with lens  242  to adjust focus of lens  242  as desired. A tracking control signal  266  is directed from controller  218  to the tracking voice coil  260 . A focus control signal  270  is directed from the controller  218  to the focus actuator  264 . 
   Next, looking at  FIG. 9 , the OPU  210  is shown in greater detail. A laser  230  is disposed to generate a laser beam  232  that is directed to lens  234 , changing the diverging beam to a substantially collimated beam. The beam  232  then passes to a dichroic mirror  236  that causes a portion of the beam to be reflected and pass through the polarizing beam splitter  240 . The remainder of beam  232  passes through dichroic mirror  236  to a front sense diode  238  that senses the strength of beam  232 . After passing through polarizing beam splitter  240 , beam  232  is directed through a quarter wave plate  241  and a focusing lens  242  to write on label surface  206  of optical disc  202 . 
   The quarter wave plate  241  changes linearly polarized light from the laser (P polarized) into circularly polarized light. The circularly polarized light is reflected from the disc  202  and is changed back into linearly polarized light (S polarized) by the quarter wave plate  241 . The resulting light is at 90 degrees polarization to the original light from the laser. This allows the returned S polarized light to be reflected by the polarizing beam splitter  240 . The quarter wave plate  241  is required for the beam splitter  240  to operate. 
   During detection, laser  230  again generates beam  232  that passes through a lens  234 , acting to change the beam  232  to a substantially collimated beam. Beam  232  passes through dichroic mirror  236  polarizing beam splitter  240  to impinge upon the image written earlier on label surface  206  by focused beam  232 . The reflected beam  253  passes back through lens  242  and quarter wave plate  241  to polarizing beam splitter  240 . A portion of the reflected beam  253  is intercepted by four photo-detectors  256 . A SUM signal  258  is generated by photo-detectors  256  and sent to a processor  259 , shown in  FIG. 8 . This SUM signal corresponds to the reflected light from the disc surface. 
   In a label writing operation, laser  230  may be an rf diode laser emitting a light beam having a wavelength of, in one embodiment, about 780 nanometers in order to effectively interact with chemicals on the medium and cause image marking to take place. Different wavelength light beams may be emitted in other embodiments to effectively interact with different chemistries for making marks. The focus actuator  264  and tracking voice coil  260  are used in writing data on the data side  204  of optical disc  202 . 
   When writing to the label side  206 , the tracking voice coil  260  may not be used, since there may not be any tracks to follow. The laser diode driver  228  may utilize a digitally enabled current source set to a selected current value by an external resistor. The four photo-detectors  256  represent four quadrant sensors used to generate servo control signals that may drive focus and tracking control signals from controller  218 . 
   The laser diode driver  228  may utilize a digitally enabled current source set to a selected current value by an external resistor. The four photo-detectors  256  represent four quadrant sensors used to generate an output signal  258  to the processor  259 . In turn, the processor  259  may cause the controller  218  to generate control signals  227  to the laser diode driver  228 , to thereby adjust the power level of laser  230 . 
   It should be understood that the above-referenced arrangements are illustrative of the application for the principles of the present invention. It will be apparent to those of ordinary skill in the art that numerous modifications can be made without departing from the principles and concepts of the invention as set forth in the claims.