Abstract:
The invention discloses a system and method for adjusting the servo controls by monitoring the spot status projected upon the photo detector of an optical pickup head. By utilizing a beam splitting device, the present invention splits the reflected laser beam which projects to photo detector originally into two reflected laser beams, in which a first reflected laser beam still projects to the photo detector for generating the servo control signals, and a second reflected laser beam projects to an image sensor for obtaining the beam profile of reflected laser beam as the servo control still works normally. By reference to analyze the obtained beam profile, the characteristics of the optical pickup head system would be more realized and be helpful for adjusting the optical servo control system.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a servo control system of an optical disc drive. More particularly, the invention relates to a method and a system for adjusting the servo control by monitoring the spot status on the photo detector.  
         [0003]     2. Description of the Prior Art  
         [0004]     An ordinary optical disc drive reads out the data stored on a disc by detecting the reflected laser beam via a pickup head.  FIG. 1  is a structure diagram of the pickup head of a conventional optical disc drive. Light source  100 , for example a laser diode, emits a laser beam with a fixed wavelength accurately onto a recording layer  140  of an optical disc by propagating through a polarized beam splitter (PBS)  110 , a relay lens set  120 , and an objective lens  130 . The reflected laser beam from the optical disc is guided to form a spot properly on a photo detector  160  by propagating through the objective lens  130 , the relay lens set  120 , the polarized beam splitter (PBS)  110 , and a focusing lens  150 . The photo detector  160  detects the spot projected on the detecting plane of the photo detector  160  and transforms the optical signals into electronic signals via a photoelectric conversion for generating servo control signals, thereby controlling the operations of the pickup head. Besides, the above-mentioned electronic signals are also transmitted to a signal processor (not shown in the drawing) for further extraction of the stored data.  
         [0005]     The operations of the pickup head is mainly controlled by a servo control system of an optical disc drive. The servo control system is capable of controlling objective lens  130  by a focusing actuator to make the laser beam properly focus on the optical disc  140  (focusing servo), and moving the pickup head parallel to the disc plane by a sled motor and a tracking actuator to make the laser spot exactly project on the center of a track (Track-Following servo).  
         [0006]     Referring to  FIG. 2A-2E , the common photo detector has a detecting plane that is divided into four quadrants: A, B, C, and D. Each quadrant detects the intensity of light projected on it and further generates electronic signals respectively. By reference to a signal processor, the combinations of the four electronic signals generated by the four quadrants are processed to regenerate the stored data on the disc. Besides, by comparing the spot status on the detecting plane, the status of focusing and the track-following can be determined, and the servo control system can further controls the pickup head. As shown in  FIG. 2A , the spot  200  illustrates the spot on the photo detector  160  with accurate focusing and track-following. If a spot  210  on the photo detector  160  is in the shape of an ellipse with a B-D alignment (as shown in  FIG. 2B ), it means the objective lens  130  in  FIG. 1  is too close to the optical disc; on contrast, if a spot  220  is in the shape of an ellipse with an A-C alignment (as shown in  FIG. 2C ), it means the objective lens  130  is too far from the optical disc. The two cases mentioned above show that the pickup head does not have a proper focusing on the disc. As for the spot  230  shown in  FIG. 2D , it means the pickup head deviates and does not exactly follow the track.  
         [0007]     Since the reflected laser beam provides both the information for servo control and the information of the stored data, improper track-following and focusing can directly influence the accuracy of the servo control and data extraction. Conventional photo detectors can only detect the spot intensity by dividing the detecting plane into four quadrant regions, and it is unlikely to have the detail beam profile of the reflected laser beam by conventional photo detector. However, sometimes the detail beam profile of the reflected laser beam could contain very critical information for servo control. Referring to  FIG. 2E , the spot  240  shapes the same to the spot  200  which is with exact focusing and track-following. But the distribution of light intensity of spot  240  is not as uniform as spot  200 . The uneven distribution of light intensity may be due to aberration from inclination of discs, inclination of objective lens  130 , or inclination of other lens sets and thus for example results in the higher intensity in the A and B quadrants. The conventional four-quadrant photo detector can not differentiate the spot  240  with the spot  230 , therefore it could not find the aberration phenomenon or the inclined optical disc. In this case, if the photo detector could perform a more accurate detection about the reflected laser beam, it might help the servo control system to have a better control against the aberration or the inclination of lens and discs. An instinctive solution is to utilize an image sensor having a higher image resolution instead of the four-quadrant photo detector. However, the bandwidth requirement for reading the stored data on the optical disc is about tens of MHz while most image sensors, such as a 80×80 pixels charged coupled device (CCD) sensor or a CMOS sensor, are not practicable because their transmission bandwidth is too low (usually about tens of kHz).  
         [0008]     Accordingly, the current pickup head could not simultaneously provide the sufficient information to servo control system and detect the detailed beam profile of the reflected laser beam. Even adopting the image sensors with high image resolution instead of the conventional four-quadrant photo detector, the bandwidth thereof does not satisfy the demand for high-speed transmission of an optical disc drive. Hence, the invention provides a new system for adjusting the servo control of an optical disc drive, which allows the pickup head to detect the beam profile of the reflected laser beam while performing normal servo controls.  
       SUMMARY OF THE INVENTION  
       [0009]     As mentioned above, an objective of the invention is to provide an optical drive, which might detect the detailed beam profiles of the reflected laser beam as well as the servo control system, which operates normally. By analyzing the beam profile of a reflected laser beam, the characteristics of the pickup head and the servo control thereof might be further realized, thereby being helpful to the optical drive design.  
         [0010]     Besides, detecting the beam profile of the reflected laser beam could obtain a distinct pickup head status, for example the disc is inclined, thereby assisting the servo controls of an optical drive. Another objective of the invention is to provide a fine tunable optical drive that can fine-tune the servo controls of an optical drive or further provide other servo controls optionally.  
         [0011]     According to the objectives of the invention, a beam-splitting device is disposed on the optical path of the reflected laser for obtaining two reflected laser beams within a similar beam profile. One reflected laser beam projects to a four-quadrant photo detector for generating servo control signals and further being processed to extract the stored data on the disc. Another reflected laser beam projects to an image sensor for obtaining an identical beam profile with the beam profile of the reflected laser beam projected on four-quadrant photo detectors. In the next procedure, a beam profile analyzer analyzes the beam profile to finely tune the focusing control and the track-following control, or further provides additional servo controls, such as the tilt control, to the optical drive. Saying a beam-splitting device could simply be a beam splitter, a cubic beam splitter, or other practicable optical lens, and saying image sensor has higher image sensitivity, such as an 80×80 pixel CCD/CMOS camera. Besides, in the embodiments of the invention, a set of lenses disposed between the said beam-splitting device and the said image sensor may be applied to acquire a more distinct beam profile. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred embodiments with reference to the accompanying drawings in which:  
         [0013]      FIG. 1  is a structure diagram of the pickup head of a a convention optical disc drive;  
         [0014]      FIG. 2A  to  FIG. 2E  illustrates different spot status on the photo detector;  
         [0015]      FIG. 3A  is a block diagram illustrating a first embodiment of the system for adjusting the servo control according to the present invention;  
         [0016]      FIG. 3B  is a block diagram illustrating a second embodiment of the system for adjusting the servo control according to the present invention;  
         [0017]      FIG. 4  illustrates an embodiment of the monitoring system according to the present invention; and  
         [0018]      FIG. 5  is a flow chart illustrating a method for adjusting the servo control according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]      FIG. 3A  is a block diagram illustrating a first embodiment of the system for adjusting the servo control according to the present invention. For simplifying the drawing,  FIG. 3A  depicts only the optical path of the reflected laser beam without further explaining the emitting of the laser beam. In  FIG. 3A , solid arrows are used to show the electric signal path while the dotted arrows are used for optical paths. Basically, the conventional optical disc drive includes a photo detector  300 , a relay lens set  310 , and a signal processing unit  320 . Photo detector  300  contains a detecting plane divided into four quadrants A, B, C, and D for receiving the reflected laser beam from disc  370 , and transforming it into electronic signals  305  for the signal process unit  320 . The signal process unit  320  processes the electronic signals  305  to generate servo control signals  325 , which are further transmitted to a focusing actuator, sled motor, and tracking actuator (not shown in the drawing) for managing the track-following and the focusing operations of a pickup head. Besides, the signal processing unit  305  further transforms the electronic signals  305  into digital data signals  326  for succeeding decoding circuits, thereby reading out the stored data on the optical disc  370 . The relay lens set  310  represents the optical components disposed in the optical path from the optical disc  370  to the photo detector  300  such as various kinds of lens, reflect mirror, collimating lens, etc. The main purpose of relay lens set  310  is to make the reflected laser beam properly projecting on the photo detector  300 .  
         [0020]     With comparisons to a conventional optical drive, the optical disc drive  30  according to the first embodiment of the present invention further has a monitoring system  36  that contains a beam-splitting device  330 , an image sensor  340 , and a beam profile analyzer  350 . The beam-splitting device  330  splits the reflected laser beam into two reflected laser beams having the same beam profiles, in which a first reflected laser beam  332  still projects onto the photo detector  300  for generating the servo control signals and information for stored data, and a second reflected laser beam  334  projects onto an image sensor  340 . The same beam profiles means the spot on the photo detector  300  and the image sensor  340  are supposed to have similar intensity distribution and shapes though the total energy for each spot is not necessary equal to one another. The beam-splitting device  330  could be a beam splitter, a cubic beam splitter, or a polarized beam splitter which transmits part of incident light intensity to a photo detector  300  and reflects part of incident light intensity to the image sensor  340 . The transmission/reflection ratio of a beam-splitting device  300  could be tuned according to the coating on the beam splitting device  330 , and the intensity of the split two reflected laser beams  332 ,  334  is not necessary equal to one another.  
         [0021]     Image sensor  340  detects the spot imaging upon the detecting plane thereof for converting the second reflected laser beam  334  into electronic signals, wherein the electronic signals will later output to the beam profile analyzer  350 . The image sensor  340  has a higher image resolution than a conventional photo detector  300 , for instance an 80×80 pixel CCD or CMOS sensor. A beam profile analyzer  350  analyzes the beam profiles of the second reflected laser beam  334  to adjust the servo control of an optical drive. In other words, the beam profile analyzer  350  processes the input electronic signals from the image sensor  320  to output a control signal  355  to a signal process unit  320  for adjusting the servo control signal  325 .  
         [0022]     For example, referring to  FIG. 2E , the spot  240  has larger light intensity over AB quadrants. As mentioned before, the spot  240  may be caused by an inclined disc or inclined objective lens, which is unrecognizable with a spot  230  ( FIG. 2D ) by a conventional four-quadrant photo detector. Assuming that the track-following control of an optical drive is determined according to the function of (A+B)−k(C+D), wherein k is fixed coefficient  1  in conventional optical drive, and A, B, C, and D represent the generated electronic signal of each detecting quadrant. In accordance with saying function, signal process unit  320  generates the track-following control signal to control the pickup head moving inner/outer of the disc.  
         [0023]     The combination of image sensor  340  and beam profile analyzer  350  detects the effects resulting from a inclined disc; further the beam profile analyzer  350  generates a control signal  355  to signal process unit  320  for adjusting the servo control  325 . The said control signal  355  may modify the coefficient k (i.e., change coefficient k from 1 to 1.3) to compensate for the tilt condition, or adjust the weighting of A, B, C, and D signals while combining them as digital data signal  326  for extracting data more accurately.  
         [0024]      FIG. 3B  is a block diagram illustrating a second embodiment of the system for adjusting the servo control according to the present invention. In this embodiment, the control signal is generated by beam profile analyzer for tuning the optical drive directly without being processed by the signal processing unit  320 ′. Contrary to the first embodiment, beam profile analyzer  350 ′ analyzes the beam profiles of a reflected laser beam  334 ′, and generates another servo control signal  385  by reference to another signal process unit that is built-in in the beam profile analyzer  350 ′ (not shown in the drawing). The said control signal  385  tunes the servo control of the optical drive, for instance, by controlling the rotating speed of an optical disc or the tilt angle of an objective lens. The output signal  305 ′ of the photo detector  300 ′ is still passed to the signal process unit  320 ′ (similar to the signal process unit  320  of  FIG. 3A ) for generating digital data signal  326 ′ and servo control signal  325 . Functions of other components, such as photo detector  300 ′, beam-splitting device  330 ′, or image sensor  340 ′ . . . etc., are the same to those in  FIG. 3A , which will not be described redundantly.  
         [0025]     Beside, some optical lenses may be added to the optical path from a beam-splitting device  330  to the image sensor  340  to make the image projected upon image sensor  340  being clearer.  FIG. 4  illustrates an embodiment of the monitoring system  36  according to the present invention. After being split by the beam-splitting device  430 , the reflected laser beam forms a spot imaged at the plane  42  has the same beam profile to that upon plane  44  on the pphoto detector  400 . The image sensor  440  could be moved from the original position of the plane  42  to the position as  440  shows, and a set of lens  410  which contains a convex lens, objective lens, or other optical lens, is added between the plane  42  and image sensor  440  to enlarge the spot imaging upon the detecting plane of image sensor  440 , thereby having the beam profile with enhanced details.  
         [0026]      FIG. 5  is a flow chart illustrating a method for adjusting the servo control according to the present invention. The method comprises the steps of: first, project parts of the reflected laser beam onto a high-resolution image sensor (step  510 ). Then the said image sensor detects the beam profile of the reflected laser beam and further transforms it into a first electronic signal (step  520 ). Next, input the first electronic signal to a beam profile analyzer for analyzing the beam profile of the reflected laser beam, and further provides a second electronic signal to the servo control signal generating unit (step  530 ). Afterwards, the servo control signal generating unit adjusts the servo control of an optical drive by reference to the second electronic signal (step  540 ).  
         [0027]     The said servo control signal generating unit is the signal process unit  320 ,  320 ′ appeared in  FIG. 3A, 3B , or might be integrated into the beam profile analyzer  350 ′. In addition, the ways for adjusting the servo control, for instance by modifying the gains of each quadrant detecting plane to adjust the servo control signal, are not limited to that described in the preferred embodiment of the invention.  
         [0028]     Hence, it concludes that the optical drive, which utilizes an image sensor having a higher resolution to obtain the beam profiles of a reflected laser beam benefits from achieving better servo control performance. Furthermore, the present invention doesn&#39;t discard the original servo control mechanism of the optical drive, but merely increases a beam-splitting device and an image sensor to obtain the detailed beam profile of the reflected laser beam. Therefore, it helps to improve the servo control performance of an optical drive without surging the costs of manufacturing.  
         [0029]     The above-mentioned are only the preferred embodiments of the present invention, not intended to limit the scope thereof. It will be appreciated and carried out by those professions skilled in the art. Thus, many modifications of the embodiments that can be made without departing from the spirit of the present invention should be covered by the following claims.