Abstract:
A method of compensating for output variations in laser power output over time in a system using a laser to record visible marks on an optical disc, the method comprising (a) applying a test light beam from the laser to the optical disc to determine a test value of an optical parameter, (b) determining a difference between the test value and a predetermined reference value for the optical parameter, and (c) adjusting the laser power output to compensate for the difference.

Description:
BACKGROUND  
       [0001]     Recently, apparatus and methods have been developed with the ability to generate an image on media using a low power laser. Such lasers are employed to read and write data on the data side of optical storage discs, such as CDs, DVDs and the like. Typically, various types of data are written on a data side of a disc by a laser beam while the disc is rotating. Data may be recorded by making marks on the recording media, indicative of a zero or one. Various data writing strategies have been employed, for the purposes of creating desirably shaped marks precisely positioned and having sharp edges to enable detection.  
         [0002]     The side of a data disc opposite the data side is often used for handwriting or affixing or marking a label with descriptions and illustrations corresponding to 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.), disclosing the use of laser sensitive materials on a disc label that react chemically with the application of light and heat and result in changes in color and shading on the label.  
         [0003]     In making marks on either side of a disc using an optical disc drive, the laser power output may be reduced with time, through dust accumulation or contamination build-up on optics associated with the laser. Effective laser power output may also be diminished over time due to attrition of laser output or front end sensors because of aging. Consequently, initial laser power output that was calibrated and set in the factory with the front sense diode voltage may be reduced significantly during usage. Some attrition of laser power output is not sensed by the front sense diode, since it measures power leaving the laser prior to the laser beam being processed by the optics. In addition, the outputs from photo-detectors used for sensing the reflections of laser power output may be skewed or distorted due to temperature variations.  
         [0004]     In marking on the label side of the disc, using the technology described in the Anderson application, the chemistry on the label is optimized to absorb as much light as possible at the wavelength of the writing laser beam. As a result, there is very little reflectivity of light at the frequency of the writing laser, and the change in reflectivity between written and unwritten areas may be less than the noise level of the detection circuit. Accordingly, it is difficult to calibrate optical power on the label recording medium, because of the low contrast between unwritten and written medium.  
         [0005]     One current method for calibrating laser power output is to measure the power required to correctly mark a medium using a marking procedure for the medium. Then the power measurement method is replicated in the drive factory, and each drive is tuned to the power specified by the medium for good marking. This method relies on having effective correlation between these two power measurements, which is sometimes not the case. Moreover, this approach does not involve measuring marks made in the field to determine whether effective optical density is being achieved.  
       SUMMARY OF THE INVENTION  
       [0006]     A method of compensating for variations in laser power output over time in a system using a laser to record visible marks on an optical disc, the method comprising (a) applying a test light beam from the laser to the optical disc to determine a test value of an optical parameter, (b) determining a difference between the test value and a predetermined reference value for the optical parameter, and (c) adjusting the laser power output to compensate for the difference. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a schematic diagram of a laser writing and detection system according to an embodiment of the present invention;  
         [0008]      FIG. 2  is a partial schematic view of the diagram shown in  FIG. 1 ;  
         [0009]      FIG. 3  is a planar view of an optical disc used in connection with an embodiment of the present invention;  
         [0010]      FIG. 4  is a schematic diagram of analog to digital converter circuitry used in connection with an embodiment of the present invention;  
         [0011]      FIG. 5  is a flow diagram of a method for preparing reference data used in connection with an embodiment of the present invention;  
         [0012]      FIG. 6  is a flow diagram of a method of adjusting laser power output in connection with an embodiment of the present invention;  
         [0013]      FIG. 7  is a flow diagram of a method of adjusting laser power output using a front end diode parameter; and  
         [0014]      FIG. 8  is a flow diagram of a method of adjusting a front sense diode parameter using a correction factor. 
     
    
     DETAILED DESCRIPTION  
       [0015]     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.  
         [0016]     In one embodiment of the present invention, a laser is used for providing images on a label side of an optical disc opposite to a data side of the disc. The laser is initially calibrated during production to provide an image of suitable optical density. During use, the laser beam output is sensed by a front sense diode and feedback is provided to adjust the laser power output in order to provide suitable images. However, the optics elements downstream from the laser may become dirty from dust or material ablation, thereby reducing the amount of light that reaches a recording medium. This embodiment of the present invention provides for methods and apparatus to measure the amount of light reaching the recording medium and to adjust the laser as needed to maintain suitable optical density for the label images.  
         [0017]     The laser may be calibrated by directing the laser beam to a mirror band on the optical disc. The reflected light is sensed and compared to a known reflectivity value for the band. Data is generated representative of the difference between the reflected light and the known reflectivity value. The laser power output is then increased as needed in response to the difference data in order to maintain a required level of laser light reaching the optical disc.  
         [0000]     Laser System  
         [0018]      FIGS. 1 and 2  show a data storage device  100  according to an embodiment of the invention, in which a laser  130  is used for reading from and/or writing to an optical disc  102 . More specifically, the data storage device  100  is used for reading and/or writing to an optically writable data side  104  of the optical disc  102 . After the data writing operation is completed, the optical disc is turned over so that the label side  106  of optical disc may be written upon, for the purpose of labeling and otherwise marking the optical disc  102 . An optical pickup unit (OPU)  110  provides the energy beam, lenses and other devices necessary to provide energy beams for reading and writing the medium, as explained below.  
         [0019]     The storage device  100  includes a spindle motor  112  and a spindle  114  for rotating the optical disc  102 . Specifically, the optical disc  102  is situated on the spindle  114 , which is rotated or moved by the spindle motor  112  to a position specified by an input control signal  116 . A controller  118  provides control signal  116  to the spindle motor  112 , and may be comprised of control hardware and/or software.  
         [0020]     The storage device  100  further includes a sled motor  120  and a sled rail  122  for moving a sled (not shown) that supports the OPU  110 . Specifically, the OPU  110  is situated on the sled that is moved on the sled rail  122  by the sled motor  120  to a given position specified by a sled control signal  124  coming from controller  118 .  
         [0021]     The OPU  110  writes and reads label data on label side  106  of optical disc  102 . In one embodiment, the optical disc  102  is rotated by spindle  114  as the label data is written on optical disc  102 . The sled causes the OPU  110  to move radially so that the label data is written in radial increments that may or may not constitute tracks on optical disc  102 . The manner of moving the spindle  114  and sled to cause the OPU  110  to write label data may vary in different embodiments of the invention. Examples of this label writing operation are given in copending U.S. patent application Ser. No. 10/898392, filed Jul. 23, 2004, entitled “Track Labeling On a Medium,” and copending U.S. patent application Ser. No. 10/867436, filed Jun. 14, 2004, entitled “Detecting Radius Of Optically Writable Label Side of Optical Disc At Which Markings Have Not Been Written.” 
         [0022]     Again referring to  FIG. 1 , controller  118  generates laser control signals  127  to a laser diode driver  128  to drive the OPU  110 . A tracking voice coil  160  is connected to a fine adjustment mechanism  162  to make small adjustments to the tracking operation of the sled and sled motor  120 . Concentric tracks, such as are used on the data side  104  of optical disc  102  may not be needed on the label side  106 . Instead, the sled may move in small increments, such as about  150  micron steps, to provide the desired image labeling. A focus actuator  164  is associated with lens  142  to adjust focus of lens  142  as desired. A tracking control signal  166  is directed from controller  118  to the tracking voice coil  160 . A focus control signal  170  is directed from the controller  118  to the focus actuator  164 .  
         [0023]     Next, looking at  FIG. 2 , the OPU  110  is shown in greater detail. Laser  130  is disposed to generate a light beam  132  that is directed to lens  134 , changing the diverging beam to a substantially collimated beam. The light beam  132  then passes to a dichroic mirror  136  that causes a portion of the light beam to be reflected and pass through a polarizing beam splitter  140 . The remainder of light beam  132  passes through dichroic mirror  136  to a front sense diode  138  that senses the strength of light beam  132 . After passing through polarizing beam splitter  140 , light beam  132  is directed through a quarter wave plate  141  and a focusing lens  142  to write on label surface  106  of optical disc  102 .  
         [0024]     The quarter wave plate  141  takes linearly polarized light from the laser (P polarized) and changes it into circularly polarized light. The circularly polarized light is reflected from the disc  102  and is changed back into linearly polarized light (S polarized) by the quarter wave plate  141 . 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  140 . The quarter wave plate  141  permits the beam splitter  140  to operate as is described below.  
         [0025]     During generation of reference data for calibration, laser  130  again generates light beam  132  that passes through lens  134 , acting to change the light beam  132  to a substantially collimated beam. Beam  132  passes through dichroic mirror  136  and polarizing beam splitter  140  to impinge upon an image written earlier on label surface  106  by beam  132 . The resulting reflected beam from the label surface  106  of optical disc  102  passes back through lens  142  and quarter wave plate  141  to polarizing beam splitter  140 . A portion of the reflected beam  153  is reflected to be intercepted by four photo-detectors  156 . A SUM signal  158  is generated by the photo-detectors  156  and sent to a processor  159 , shown in  FIG. 1 . This SUM signal corresponds to the reflected light from the optical disc surface  106 .  
         [0026]     In a label writing operation, the laser  130  may emit a light beam having a wavelength of about 780 nanometers in order to effectively interact with chemicals on the medium and cause image marking to take place. The focus actuator  164  and tracking voice coil  160  are used in writing data on the data side  104  of optical disc  102 . When writing to the label side  106 , in some situations, tracking voice coil  160  may not be used, since there may not be any tracks to follow.  
         [0027]     The laser diode driver  128  may utilize a digitally enabled current source set to a selected current value by an external resistor. The four photo-detectors  156  represent four quadrant sensors used to generate the SUM signal  158 . The four photo-detectors  156  transmit this signal to the processor  159 . In turn, the processor  159  may cause the controller  118  to generate control signals  127  to the laser diode driver  128 , to thereby adjust the power level of laser  130 .  
         [0000]     Mirror Band Reflection  
         [0028]      FIG. 3  shows an optical disc  200  including a disc surface  202  having a mirror ring or band  206  of known reflectivity near the center of the disc. Mirror band  206  is adjacent to a ring spoke area  208  having spoke edges  210  that may be used for calibration. Alternately, a solid mirror band may be used.  
         [0029]     The mirror band or area may be provided as part of the standard disk manufacturing process by leaving a band of substrate flat and un-embossed with pits or grooves. Also, a mirror band can be provided by masking a ring or other area of the normal metallic disc under-surface at the time that recording medium is coated on the disc. Alternately, a special metallic or other mirror surface may be applied to the recording medium to form a mirror band or area.  
         [0030]     As discussed with respect to  FIG. 2 , the light beam  132  from the detection laser  130  is directed to the optical disc  102 , corresponding to optical disc  200  in  FIG. 3 . During calibration, the light beam  132  strikes the mirror band  206  of the optical disc shown in  FIG. 3  and is reflected back to the OPU  110 . The reflected light beam  153  is directed to photo-detector  156  from the mirror area and generates the corresponding SUM signal  158 .  
         [0031]     With reference now to  FIG. 4 , using processor  159 , the SUM signal  158  may be passed through a threshold detector  230  comprising an analog to digital converter  232  having the SUM signal  158  as a positive input and a reference signal  234  as the negative input. The output signal  236  may be used to adjust the power level of laser  130  by comparing signal  236  to calibration data stored in the processor  159  in a manner discussed below.  
         [0000]     Initial Preparation of Calibration Data for Laser Power Output  
         [0032]     Referring to  FIG. 5 , a process  300  is shown according to an embodiment of the present invention for preparing reference data for a laser  130  and storing the reference data at the time that the data storage device  100  is manufactured. At step  302 , the laser  130  may be initially calibrated at the time of manufacture by directing a laser light beam  132  through the optics to the mirror band  206 . At step  304 , the reflected light beam  153  is directed to the four photo-detectors  156  resulting in the SUM signal  158  being produced. Next, at step  306 , the light beam  132  from the output of the laser  130  may also be detected by the front sense diode  138  before the light beam  132  passes through certain elements of the optics. The front sense diode  138  generates a laser output signal  148  indicative of the level of power being emitted from laser  130  before the light beam  132  passes through all of the optics. Next, at step  308 , the SUM signal  158  and the laser output signal  148  are measured in the factory for different selected power levels of the laser  130 . At step  310 , reference ratios of the SUM signal to the laser output signal for each selected power level are determined. At step  312 , the reference ratios are stored in processor  159 .  
         [0000]     Adjustments to Laser Power Output Level During Use  
         [0033]     During use of the laser  130 , dust and ablation materials may accumulate on the optics elements, thereby reducing the amount of light that is passed through the optics elements to the optical disc label surface  106 . Thus, over time the SUM signal  158  may decrease. In contrast, the light beam  132  that is detected by the front sense diode  138  will remain the same, except for small intensity decreases in laser output over a long period of time. Accordingly, the ratio of the SUM signal  158  over the laser output signal  148  will decrease over time. The power output of laser  130  will need to be increased to compensate for the partial loss of light as a laser beam passes through the optics elements.  
         [0034]      FIG. 6  shows a process  350 , referring to the elements in  FIG. 2 , according to an embodiment of the present invention for adjusting the power level of laser  130  during usage in the field, that is, in normal usage. At step  352 , test light beam  132  from laser  130  is directed through the optics to the mirror band  206  at a field power level. At step  354 , the reflected light beam  153  is directed to the photo-detector  156  to generate a SUM signal  158 . Next, at step  356 , the test light beam  132  from the output of the laser  130  may also be detected by the front sense diode  138  before the light passes through the optics. Front sense diode  138  generates a laser test output signal  148  indicative of the level of power being emitted from laser  130  before the light passes through the optics.  
         [0035]     Next, at step  358 , the amplitudes of the SUM signal  158  and the laser output signal  148  are measured. Since the light beam  132  passes through the optics elements twice before reaching the photo-detector  156 , the SUM signal  158  may need to be adjusted to compensate for attrition of the light passing twice through dirty optic elements. Then, a test ratio of SUM signal  158  over laser output signal  148  is determined, relative to the field power level of laser  130 . This test ratio may need to be further adjusted for the reflectivity differences between the reference medium and the present field medium. At step  360 , the field ratio is compared to the corresponding stored reference ratio of SUM signal to laser output signal for the same power level as the field power level. At step  362 , the field power level of laser  130  is then adjusted to minimize the difference between the field ratio and the corresponding reference ratio.  
         [0036]     It should be understood that various other methods may be used within the scope of the present invention to compare light beam outputs in the field to known light beam values stored in the processor and to adjust the power of the laser to compensate for a partial loss in light as the laser beam passes through the optics elements.  
         [0000]     High Contrast Coating  
         [0037]     As an alternative to the mirror band or ring  206 , shown in  FIG. 3 , a high-contrast coating may be added as a ring or other test area on the optical disc. This coating adds to the contrast between written and unwritten areas, so that marks may be more easily detected. Thus, the coating might take the place of the mirror in providing a band of known reflectivity that is useful in calibrating the laser.  
         [0038]     By way of review, the imaging materials on the disc react to the write laser operating at an infrared frequency having a wavelength of about 780 nanometers. The imaging coating on the label surface is a mixture of color-forming dye, activator and infrared antenna contained in a polymer matrix. The antenna absorbs the laser energy and converts it to heat. The heat causes the activator, dye and matrix to melt, allowing the activator to interact with the dye. The interaction results in a chemical change to the dye, resulting in an opacity change in the visible light spectra. But in the infrared frequencies, the opacity change is small and there is very little contrast between the written and unwritten materials.  
         [0039]     In this embodiment, a thin coating of material may be provided in a narrow ring on the inner diameter of the disc or at another area on the disc. The coating material may be very similar to or the same as the medium material, except that the dye released may be opaque at 780 nm and transparent at visible frequencies. The activator and polymer matrix might remain the same as the medium material. The new material would provide a higher contrast between the written and unwritten media that would improve the accuracy and repeatability of power calibration.  
         [0040]     As another alternative, the coating material may be compact disc read (CDR) or compact disc read/write (CDR/W) material. Alternately, the coating material may be DVD or DVD read/write (DVD/W) material. The disadvantages with these alternatives are that a correlation may need to be performed between the results obtained with the CDR/W or DVD/W material and the medium material, and the calibration may leave visible marks on the disc.  
         [0000]     Thermal Compensation for Temperature Variations  
         [0041]     In another embodiment of the present invention, an rf write laser having a beam with a wavelength of 780 nm is used to deliver energy and make marks on an optical disc. It was determined that, using only front-end calibration in the field using a monitor of the light beam emitted from a laser, there was a loss of power of as much as 30% over the life of the laser system.  
         [0042]     To reduce this loss of power, methods of direct and indirect optimal power calibration (OPC) were tested. Direct OPC refers to a process of marking a disc and then observing the reflectivity of the marks. Indirect OPC refers to the process wherein a mirror band is used with high reflectivity to provide a test area to test laser light beam power without directly sensing a mark. Field power ratios are compared to reference power ratios of the SUM signal and the laser output signal at different power levels.  
         [0043]     In addition, thermal effects account for an additional plus or minus 6% in variations of the resulting data. Accordingly, thermal compensation methods were used to adjust for power loss caused by temperature changes. Typically, a photo-detector is used for sensing a reflection of light from an image. The photo-detector output is likely to be temperature sensitive and may vary substantially with thermal changes. In accordance with an embodiment of the invention, adjustments may be made to the SUM signal to compensate for temperature changes due to thermal effects.  
         [0044]     If there is appropriate compensation for thermal effects, using indirect OPC, a repeatability level of plus or minus 1% was achieved. It is understood that factory calibration accuracy is about plus or minus 5%. Front monitor accuracy is about plus or minus 1%. Adding these last three numbers, the range of error is about plus or minus 8%, which is well below a specification criteria of 10%. Accordingly, the use of indirect OPC according to the present invention, provides reliable results if thermal compensation is used.  
         [0000]     Adjusting Laser Power Output to Calibrated Front Sense Diode.  
         [0045]     Referring back to  FIG. 2 , the light beam  132  is directed through lens  134  to a dichroic mirror  136  that causes a portion of the beam to be reflected and pass through a polarizing beam splitter  140  to eventually reach the surface  106  of the optical disc. The remainder of light beam  132  passes through dichroic mirror  136  to a front sense diode  138  that senses the strength of light beam  132 .  
         [0046]     There are a number of factors that cause variation in the measurement of a laser power output level that will produce a good mark on the disc surface. The front sense diode  138  has been determined to have a relatively stable current over time, making it a good candidate for establishing a bench mark for maintaining the quality of marks on the disc. By tying the power measured by the front sense diode directly to a desired or target mark quality, much of the measurement variation can be removed.  
         [0047]     Referring now to  FIG. 7 , a flow diagram is shown for a process  400  of adjusting laser power output using a reference parameter for the front sense diode. In the factory, or in the field, the front sense diode  138  output voltage or current may be set to correspond to a laser power output that yields a desired mark quality. This desired mark quality may correspond to a desired operating point that is well centered between ablation and fade limits. At that operating point, a reference value for the front sense diode  138 , such as current, is determined and stored in memory for further use. Then, as the system drives are writing on an optical disc, the current of the front sense diode  138  is maintained at the stored reference value.  
         [0048]     Using the above approach, at step  402 , the writing laser beam is directed to the front sense diode  138  to generate a test current in the diode. At step  404 , this test current is compared to the reference value for diode current stored in memory, and the difference between the test current and the reference value is determined. At step  406 , the laser power output is adjusted, usually by increasing the power as the laser ages, to minimize the difference. Thus, any variations in the power of laser  130  over time will cause a difference in the current of the front sense diode. Thus differences in laser power output over time will be measured against the reference standard of the front sense diode current. When the front sense diode  138  is operating at the required reference level, the laser write power should be at a level to produce a mark of desired quality.  
         [0000]     Adjusting Front Sense Diode with Correction Factor.  
         [0049]     A further adjustment, involving changes to the output of the front sense diode may further improve marking quality over time. In label writing over the life of a laser and laser drive, the power calibration may drift for a number of reasons, including contamination buildup on the optics. This drift may continue to degrade the power margin of a laser drive, which may eventually cause a laser drive to go out of media specification.  
         [0050]     During data side operation, the laser drive executes optimal power calibration (OPC) routines that adjust the write power for minimal jitter and maximum waveform symmetry. Although this power optimization is specific to the media being written, the overall trend of required write power across several discs may be determined, so as to assess how much the laser power output has drifted compared to a front sense diode parameter, such as current. This determination may be carried out by maintaining a running average of the front sense diode parameter compared to the actual optimal power determined using OPC. A correction factor may be determined from this relationship and then applied to the laser system during a process of writing to a label side or area. For label side printing, the front sense diode parameter would be adjusted by the correction factor to set the labeling laser power output.  
         [0051]     Referring to  FIG. 8 , a flow diagram is shown for a process  450  of adjusting the output of the front sense diode by a correction factor determined from writing data. At step  452 , a determination is made of changes in laser power output during writing data on an optical disc. At step  454 , a correction factor is derived from these changes. The correction factor may be determined by comparing a running average of front sense diode current versus actual optimal power determined via direct or indirect OPC, as mentioned above, or in some other manner. At step  456 , the current output or other output parameter of the front sense diode is adjusted by the correction factor to compensate for the laser power output drift previously determined during data writing.  
         [0052]     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.