Abstract:
Provided is a method of controlling tracking of an information recording medium including an area for storing a holographic image and an area for storing servo spots formed discretely and at a predetermined interval, which is achieved by obtaining an RF-SUM signal by adding all of detection signals of a quadrant photodetector for detecting the servo spot, monitoring whether the RF-SUM signal exceeds a predetermined level, and performing tracking control in a section where the RF-SUM signal exceeds the predetermined level. Also provided is a related apparatus.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This application claims the benefit of Korean Patent Application No. 10-2004-0107162, filed on Dec. 16, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     Apparatuses and methods consistent with the present invention relate to reproducing holographic information stored on a recording medium, and more particularly, to a method and apparatus for tracking control of a holographic information recording medium and a method and apparatus for capturing an image from the holographic information recording medium.  
         [0004]     2. Description of the Related Art  
         [0005]     A holographic recording method is used to record information on an optical recording medium using a hologram at an ultrahigh density. According to the holographic recording method, an interference pattern is generated in the optical recording medium by allowing a signal beam containing image information to interfere with a particular reference beam. That is, the interference pattern is recorded on the optical recording medium so that the image information is recorded. To reproduce information from the recorded interference pattern, a reproduction reference beam similar to the beam used for recording is emitted onto the interference pattern recorded on the optical recording medium. This emission causes diffraction by the interference pattern so that the image information is reproduced. In a volume holography, high density information recording is possible as a hologram is recorded to be overlapped on the volume of the optical recording medium by changing a physical property of the reference beam.  
         [0006]      FIG. 1  shows the format of storing a holographic image on an information recording medium according to a conventional technology. Referring to  FIG. 1 , an interference pattern obtained by interference between a signal beam having information and a reference beam is recorded along a particular track on a holographic information recording medium. An image reproduced from the interference pattern includes a holographic data image in units of pages and servo spots. In a reproduction apparatus, a photodetector detects the position of servo spots to detect an image capture time point and perform tracking. Also, the servo spot is used to correct the position of the reference beam.  
         [0007]     When the holographic image is read out from a disk, the read images may be defocused, shifted, rotated, or distorted, which is due to disk wobbling, the decenter of the disk from a rotation shaft, or the deformation of the disk. These defects cause errors to signals measured by a detector or deteriorate quality of the signals.  
         [0008]     Unlike a typical compact disc in which each data is formed into a single pit, each image can contain several hundreds of thousands of pixels in a disk containing data as holographic images. Typically, an array having several hundreds of thousands of detection devices is provided to measure a single holographic image. All pixels not only for a single holographic image but also for all holographic images of the disk need to be accurately focused and located at accurate positions in relation to a detection array. However, it is difficult to accurately arrange numerous pixels in the holographic image.  
       SUMMARY OF THE INVENTION  
       [0009]     To address the above and/or other problems, the present invention provides a tracking control method and apparatus and an image capture method and apparatus for accurately reproducing two dimensional information from an information recording medium containing a holographic image, through tracking control and a shuttering signal.  
         [0010]     According to an aspect of the present invention, a method of controlling tracking of an information recording medium including an area for storing a holographic image and an area for storing servo spots formed discretely and at a predetermined interval comprises obtaining an RF-SUM signal by adding all of detection signals of a quadrant photodetector for detecting the servo spot, monitoring whether the RF-SUM signal exceeds a predetermined level, and performing tracking control in a section where the RF-SUM signal exceeds the predetermined level.  
         [0011]     The method further comprises turning off the tracking control when the RF-SUM signal is not more than the predetermined level.  
         [0012]     According to another aspect of the present invention, a method of controlling tracking of an information recording medium including an area for storing a holographic image and an area for storing servo spots formed discretely and at a predetermined interval comprises obtaining a shuttering signal by subtracting a sum of values of signals of third and fourth photodetection devices from a sum of values of signals of first and second photodetection devices, in a direction in which the information recording medium rotates, from detection signals of a quadrant photodetector for detecting the servo spot, monitoring whether the shuttering signal belongs to a predetermined section, and performing tracking control in a section where the shuttering signal belongs to the predetermined section.  
         [0013]     The predetermined section includes a section from a time point indicating a maximum value of the shuttering signal to a time point indicating a minimum value of the shuttering signal.  
         [0014]     The method further comprises turning off the tracking control when the shuttering signal is out of the predetermined section.  
         [0015]     According to another aspect of the present invention, a method of capturing a holographic image from an information recording medium including an area for storing a holographic image and an area for storing servo spots formed discretely and at a predetermined interval comprises obtaining a shuttering signal by subtracting a sum of values of signals of third and fourth photodetection devices from a sum of values of signals of first and second photodetection devices, in a direction in which the information recording medium rotates, from detection signals of a quadrant photodetector for detecting the servo spot, monitoring a time point when the shuttering signal reaches a zero level, and outputting a shuttering control signal instructing to capture the image stored in the information recording medium at a time point when the shuttering signal reaches a zero level.  
         [0016]     The monitoring of a time point comprises monitoring a time point when the shuttering signal reaches a zero level in a section where the shuttering signal changes from a (+) value to a (−) value.  
         [0017]     The first and second photodetection devices are two photodetection devices that come earlier in a direction in which the information recording medium rotates while the third and fourth photodetection devices are two photodetection devices that come later in a direction in which the information recording medium rotates.  
         [0018]     According to another aspect of the present invention, an apparatus for controlling tracking of an information recording medium including an area for storing a holographic image and an area for storing servo spots formed discretely and at a predetermined interval comprises a photodetection unit obtaining an RF-SUM signal by adding all detection signals of quadrant photodetection devices for detecting the servo spot, and a servo control unit monitoring whether the RF-SUM signal exceeds a predetermined level and performing tracking control during a section wherein the RF-SUM signal exceeds the predetermined level.  
         [0019]     According to another aspect of the present invention, an apparatus for controlling tracking of an information recording medium including an area for storing a holographic image and an area for storing servo spots formed discretely and at a predetermined interval comprises a photodetection unit obtaining a shuttering signal by subtracting a sum of values of signals of third and fourth photodetection devices from a sum of values of signals of first and second photodetection devices, in a direction in which the information recording medium rotates, from detection signals of a quadrant photodetector for detecting the servo spot, and a servo control unit monitoring whether the shuttering signal belongs to a predetermined section and performing tracking control during a period when the shuttering signal belongs to the predetermined section.  
         [0020]     According to another aspect of the present invention, an apparatus for capturing a holographic image from an information recording medium including an area for storing a holographic image and an area for storing servo spots formed discretely and at a predetermined interval comprises a photodetection unit obtaining a shuttering signal by subtracting a sum of values of signals of third and fourth photodetection devices from a sum of values of signals of first and second photodetection devices, in a direction in which the information recording medium rotates, from detection signals of a quadrant photodetector for detecting the servo spot, and a shuttering control unit monitoring whether the shuttering signal reaches a zero level and instructing to capture the image stored in the information recording medium at a time point when the shuttering signal reaches the zero level. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0022]      FIG. 1  is a view illustrating the format of storing a holographic image on an information recording medium according to a conventional technology;  
         [0023]      FIG. 2  is a block diagram of an apparatus for reproducing a holographic image from an information recording medium according to an embodiment of the present invention;  
         [0024]      FIG. 3  is a block diagram of a photodetection unit shown in  FIG. 2 ;  
         [0025]      FIG. 4  is a view showing a waveform of an RF-SUM signal shown in  FIG. 3 ;  
         [0026]      FIG. 5  is a view showing a waveform of a tracking error signal shown in  FIG. 3 ;  
         [0027]      FIG. 6  is a view showing a waveform of a shuttering signal shown in  FIG. 3 ;  
         [0028]      FIG. 7A  is a view illustrating an example of a servo control unit of  FIG. 2 ;  
         [0029]      FIG. 7B  is a waveform diagram for controlling tracking according to the servo control unit shown in  FIG. 7A ;  
         [0030]      FIG. 8A  is a view illustrating another example of a servo control unit of  FIG. 2 ;  
         [0031]      FIG. 8B  is a waveform diagram for controlling tracking according to the servo control unit shown in  FIG. 8A ;  
         [0032]      FIG. 9A  is a view illustrating an example of a shuttering control unit of  FIG. 2 ;  
         [0033]      FIG. 9B  is a waveform diagram for controlling shuttering according to the shuttering control unit shown in  FIG. 9A ;  
         [0034]      FIG. 10  is a flow chart for explaining a method of controlling tracking according to an embodiment of the present invention;  
         [0035]      FIG. 11  is a flow chart for explaining a method of controlling tracking according to another embodiment of the present invention; and  
         [0036]      FIG. 12  is a flow chart for explaining a method of controlling shuttering according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]      FIG. 2  is a block diagram of an apparatus for reproducing a holographic image from an information recording medium according to an embodiment of the present invention. Referring to  FIG. 2 , a reproduction apparatus  200  includes a laser source  210 , an optical system  220 , a Galvano mirror  230 , a mirror driving unit  240 , a spindle motor  250 , an image capture unit  260 , a photodetection unit  270 , and a signal processing unit  280 .  
         [0038]     The laser source  210  emits a laser beam. The laser beam passes through the optical system  220  and is reflected by the Galvano mirror  230 . Then, the laser beam passes through an information recording medium  290  and is projected onto the image capture unit  260  and the photodetection unit  270 .  
         [0039]     The photodetection unit  270  detects a servo spot and generates a tracking error signal, a shuttering signal, and an RF-SUM signal and provides the generated signals to the signal processing unit  280 . The signal processing unit  280  receives the tracking error signal, the shuttering signal, and the RF-SUM signal provided from the photodetection unit  270  and outputs a signal for servo control and a signal for shuttering control to the mirror driving unit  240  and the image capture unit  260 .  
         [0040]     The signal processing unit  280  includes a shuttering control unit  281  and a servo control unit  282 . The shuttering control unit  281  receives a shuttering signal from the photodetection unit  270 , detects a predetermined time point, and provides a shuttering control signal to the image capture unit  260  at the time point. The servo control unit  282  receives the shuttering signal, the tracking error signal, and the RF-SUM signal from the photodetection unit  270 , detects a predetermined section in the shuttering signal or RF-SUM signal, and corrects a tracking error signal using the result of the detection, thus performing tracking control. The shuttering control unit  281  and the servo control unit  282  according to the present embodiment are described below in detail.  
         [0041]     The mirror driving unit  240  receives a tracking control signal to control the position of the Galvano mirror  230 . The image capture unit  260  receives the shuttering control signal from the signal processing unit. When the shuttering control signal is received, the image capture unit  260  captures an image from the image recording medium  290  and outputs captured image data to a PC  205 .  
         [0042]      FIG. 3  is a block diagram illustrating a detailed structure of the photodetection unit shown in  FIG. 2 . Referring to  FIG. 3 , the photodetection unit  270  includes a first photodetection device  301 , a second photodetection device  302 , a third photodetection device  303 , a fourth photodetection device  304 , five adders  305 ,  306 ,  307 ,  308 , and  310 , and two subtracters  309  and  311 .  
         [0043]     The photodetection unit  270  converts the amount of a spot projected onto each photodetection device of a quadrant photodetector to an electrical signal, combines signals of the respective photodetection devices, and generates the tracking error signal, the shuttering signal, and the RF-SUM signal for servo.  
         [0044]     In the following description, A, B, C, and D denote signals detected by the first through fourth photodetection devices  301 - 304 , respectively.  
         [0045]     The tracking error signal is expressed as (A+D)-(B+C) and is obtained as the subtracter  309  receives a signal (A+D) output by the adder  305  and a signal (B+C) output by the adder  306  and subtracts the signal (B+C) from the signal (A+D).  
         [0046]     The RF-SUM signal is expressed as (A+B)+(C+D) and is obtained as the adder  310  receives a signal (A+B) output by the adder  307  and a signal (C+D) output by the adder  308  and adds the signal (A+B) to the signal (C+D).  
         [0047]     The shuttering signal is expressed as (C+D)-(A+B) and is obtained as the subtracter  311  receives a signal (C+D) output by the adder  308  and a signal (A+B) output by the adder  307  and subtracts the signal (A+B) from the signal (C+D).  
         [0048]      FIG. 4  shows a waveform of the RF-SUM signal shown in  FIG. 3 . A unit (a) of  FIG. 4  shows positions  410 ,  420 , and  430  of a servo spot passing across the photodetection devices  301 ,  302 ,  303 , and  304 . A unit (b) of  FIG. 4  shows a waveform of the RF-SUM signal output as the servo spot passes across the photodetection devices.  
         [0049]     Referring to the units (a) and (b) of  FIG. 4 , when the servo spot passes the position  410  halfway overlapping each of the photodetection devices  301  and  302 , the signal RF-SUM has a value of a point on a curve indicated by the position  410  which is equivalent to a sum of an amount of the signal detected by the photodetection device  301  and an amount of the signal detected by the photodetection device  302 .  
         [0050]     When the servo spot passes the position  420  that is just the center of the photodetection devices  301 ,  302 ,  303 , and  304 , referring to the unit (b) of  FIG. 4 , the signal RF-SUM has value of a point on a curve indicated by the position  420  which is equivalent to a sum of the amounts of signals detected by the photodetection devices  301 - 304 . At this time, the RF-SUM value indicates the maximum value.  
         [0051]     When the servo spot passes the position  430  halfway overlapping each of the photodetection devices  303  and  304 , referring to the unit (b) of  FIG. 4 , the signal RF-SUM has a value of a point on a curve indicated by the position  430  which is equivalent to a sum of an amount of the signal detected by the photodetection device  303  and an amount of the signal detected by the photodetection device  304 .  
         [0052]      FIG. 5  shows a waveform of a tracking error signal shown in  FIG. 3 . A unit (a) of  FIG. 5  shows positions  510 ,  520 , and  530  of a servo spot passing across the photodetection devices  301 ,  302 ,  303 , and  304 . A unit (b) of  FIG. 5  shows a waveform of the tracking error signal output as the servo spot passes across the photodetection devices.  
         [0053]     Referring to the units (a) and (b) of  FIG. 5 , when the servo spot passes the position  510  halfway overlapping each of the photodetection devices  301  and  304 , the tracking error signal has a value of a point on a curve indicated by the position  510 . Since the tracking error signal is not detected from the photodetection devices  302  and  303 , the value of the tracking error signal is equivalent to a sum of an amount of the signal detected by the photodetection device  301  and an amount of the signal detected by the photodetection device  304  which indicates the maximum value.  
         [0054]     When the servo spot passes the position  520  that is just the center of the photodetection devices  301 ,  302 ,  303 , and  304 , referring to the unit (b) of  FIG. 5 , the tracking error signal has a value of a point on a curve indicated by the position  520 . Since an amount of the signals detected by the photodetection devices  301  and  302  and an amount of the signals detected by the photodetection devices  303  and  304  are almost the same, the tracking error signal has a value close to 0.  
         [0055]     When the servo spot passes the position  530  halfway overlapping each of the photodetection devices  302  and  303 , referring to the unit (b) of  FIG. 5 , the tracking error signal has a value of a point on a curve indicated by the position  530 . Since the signal is not detected from the photodetection devices  301  and  304 , the tracking error signal has a value equivalent to the negative sum of an amount of the signal detected by the photodetection device  302  and an amount of the signal detected by the photodetection device  303  which indicates the minimum value.  
         [0056]      FIG. 6  shows a waveform of a shuttering signal shown in  FIG. 3 . A unit (a) of  FIG. 6  shows positions  610 ,  620 , and  630  of a servo spot passing across the photodetection devices  301 ,  302 ,  303 , and  304 . A unit (b) of  FIG. 6  shows a waveform of the shuttering signal output as the servo spot passes across the photodetection devices.  
         [0057]     Referring to the units (a) and (b) of  FIG. 6 , when the servo spot passes the position  610  halfway overlapping each of the photodetection devices  303  and  304 , the shuttering signal has a value of a point on a curve indicated by the position  610 . Since the signal is not detected from the photodetection devices  301  and  302 , the value of the shuttering signal is equivalent to a sum of an amount of the signal detected by the photodetection device  303  and an amount of the signal detected by the photodetection device  304  which indicates the maximum value.  
         [0058]     When the servo spot passes the position  620  that is just the center of the photodetection devices  301 ,  302 ,  303 , and  304 , referring to the unit (b) of  FIG. 6 , the shuttering signal has a value of a point on a curve indicated by the position  620 . Since an amount of the signals detected by the photodetection devices  301  and  302  and an amount of the signals detected by the photodetection devices  303  and  304  are almost the same, the shuttering signal has a value close to 0.  
         [0059]     When the servo spot passes the position  630  halfway overlapping each of the photodetection devices  301  and  302 , referring to the unit (b) of  FIG. 6 , the shuttering signal has a value of a point on a curve indicated by the position  630 . Since the signals are not detected from the photodetection devices  303  and  304 , the value of the shuttering signal is equivalent to the negative sum of an amount of the signal detected by the photodetection device  301  and an amount of the signal detected by the photodetection device  302 , which indicates the minimum value.  
         [0060]      FIG. 7A  illustrates an example of the servo control unit of  FIG. 2 . Referring to  FIG. 7A , the servo control unit  282  includes an RF-SUM signal level monitoring unit  710  and a tracking error signal correction unit  720 . The RF-SUM signal level monitoring unit  710  receives the RF-SUM signal from the photodetection unit  270  and monitors whether the value of the received RF-SUM signal exceeds a predetermined level. When it is detected during monitoring that the value of the RF-SUM signal exceeds the predetermined level, the RF-SUM signal level monitoring unit  710  provides a signal to start tracking control to the tracking error signal correction unit  720 .  
         [0061]     The tracking error signal correction unit  720  receives the tracking error signal from the photodetection unit  270 . When a tracking control on signal is received from the RF-SUM signal level monitoring unit  710 , the tracking error signal correction unit  720  generates a tracking control signal to correct the tracking error signal and outputs the generated tracking control signal to the mirror driving unit  240 .  
         [0062]      FIG. 7B  is a waveform diagram for controlling tracking according to the example shown in  FIG. 7A . Lines (a), (b), and (c) denote the RF-SUM signal, the tracking error signal, and the tracking control signal, respectively.  
         [0063]     Referring to the line (a) of  FIG. 7B , the RF-SUM signal has a gradually changing positive value in a section where the servo spot passes the photodetection device, and a value of “0” in a section where the servo spot does not pass the photodetection device. Since the servo spot is recorded discretely, not continuously, referring to the line (a) of  FIG. 7B , on a track of the information recording medium, the RF-SUM signal has a predetermined value in a section where the servo spot and the photodetection device are overlapped and a value of “0” in the other section.  
         [0064]     The RF-SUM signal level monitoring unit  710  performs tracking control in a section where the servo spot passes the photodetection device, in particular, in sections  770  and  780  where a value of the RF-SUM signal is over a predetermined level. Referring to the line (a), when it is detected that a value of the RF-SUM signal exceeds a predetermined level at a time point  730 , the RF-SUM signal level monitoring unit  710  starts tracking control by turning on a signal for tracking control. When the tracking error signal indicates, for example, a (+) value, as indicated by the line (b), the tracking error signal correction unit  720  converts the tracking control signal to a (+) value to perform the tracking control as indicated by the line (c). When it is detected that the value of the RF-SUM signal is not more than the predetermined level at a time point  740 , the RF-SUM signal level monitoring unit  710  terminates the tracking control by turning off the signal for tracking control so that the tracking control is not performed in a section between the time points  740  and  750 . That is, when the RF-SUM signal is under a predetermined level, the tracking control is turned off and a level of a control signal at this time is maintained to be uniform so that the position of the Galvano mirror is fixed.  
         [0065]     Next, in the section  780  where the servo spot passes the photodetection device, the tracking control is performed likewise. However, when the tracking error signal indicates, for example, a (−) value, as indicated by the line (b), the tracking error signal correction unit  720  converts the value of the tracking control signal to be (-) as indicated by the line (c) to perform the tracking control.  
         [0066]      FIG. 8A  illustrates another example of the servo control unit of  FIG. 2 . Referring to  FIG. 8A , the servo control unit includes a shuttering signal level monitoring unit  810  and a tracking error signal correction unit  820 . The shuttering signal level monitoring unit  810  receives a shuttering signal from the photodetection unit  270  and monitors whether a value of the received shuttering signal belongs to a predetermined section. When it is detected during monitoring that the value of the received shuttering signal belongs to the predetermined section, the shuttering signal level monitoring unit  810  provides a signal for starting tracking control to the tracking error signal correction unit  820 .  
         [0067]     The tracking error signal correction unit  820  receives a tracking error signal from the photodetection unit. When a tracking error on signal is received from the shuttering signal level monitoring unit  810 , the tracking error signal correction unit  820  generates a tracking control signal to correct the tracking error signal and outputs the generated tracking control signal to the mirror driving unit  240 .  
         [0068]      FIG. 8B  is a waveform diagram for controlling tracking according to the servo control unit shown in  FIG. 8A . Lines (a), (b), and (c) denote the shuttering signal, the tracking error signal, and the tracking control signal, respectively.  
         [0069]     Referring to the line (a) of  FIG. 8B , the shuttering signal has a shape similar to a sine wave changing from a (+) value to a (−) value in a section where the servo spot passes the photodetection device and a value of “0” in a section where the servo spot does not pass the photodetection device. Since the servo spot is recorded discretely, not continuously, on a track of the information recording medium, referring to the line (a), the shuttering signal has a predetermined value in a section where the servo spot and the photodetection device are overlapped and a value of “0” in the other section.  
         [0070]     The tracking error signal correction unit  820  performs tracking control in a section where the servo spot passes the photodetection device, in particular, in sections  870  and  880  where a value of the shuttering signal belongs to a predetermined section. Referring to the line (a), when it is detected that a value of the shuttering signal belongs to a predetermined section at a time point  830 , the shuttering signal level monitoring unit  810  starts tracking control by turning on a signal for tracking control. When the tracking error signal indicates, for example, a (+) value, as indicated by the line (b), the tracking error signal correction unit  820  converts the tracking control signal to a (+) value to perform the tracking control as indicated by the line (c). When it is detected that the value of the shuttering signal escapes from the predetermined section at a time point  840 , the shuttering signal level monitoring unit  810  terminates the tracking control by turning off the signal for tracking control so that the tracking control is not performed in a section between the time points  840  and  850 . That is, the tracking control is turned off in the section between the time points  840  and  850  and a level of a control signal at this time is maintained so that the position of the Galvano mirror is fixed.  
         [0071]     Next, in the section  880  where the servo spot passes the photodetection device, the tracking control is performed likewise. However, when the tracking error signal indicates, for example, a (−) value, as indicated by the line (b), the tracking error signal correction unit  820  converts the value of the tracking control signal to be (−) as indicated by the line (c) to perform the tracking control.  
         [0072]     In the line (a), the predetermined section of the shuttering signal where tracking control is performed is from a time point indicating the maximum value of the shuttering signal to a time point indicating a minimum value of the shuttering signal. The predetermined section for tracking control is not limited to the section from a time point indicating the maximum value of the shuttering signal to a time point indicating a minimum value of the shuttering signal, but can be any section in which the value of the shuttering signal changes.  
         [0073]      FIG. 9A  illustrates an example of the shuttering control unit  281  of  FIG. 2 . Referring to  FIG. 9A , the shuttering control unit  281  includes a zero level detection unit  910 . The zero level detection unit  910  receives a shuttering signal from the photodetection unit  270  and monitors whether the value of the received shuttering signal is at a zero level. When the value of the shuttering signal is detected to be at the zero level during monitoring, the zero level detection unit  910  outputs a shuttering control signal to the image capture unit  260 .  
         [0074]      FIG. 9B  is a wave diagram for controlling shuttering according to the shuttering control unit  281  shown in  FIG. 9A . A unit (a) of  FIG. 9B  shows positions  920 ,  930 , and  940  of a servo spot passing across the photodetection devices  301 ,  302 ,  303 , and  304 . A unit (b) of  FIG. 9B  shows a waveform of the shuttering signal output as the servo spot passes across the photodetection devices.  
         [0075]     As described above with reference to  FIG. 8B , the shuttering signal has a shape similar to a sine wave changing from a (+) value to a (−) value in a section in which the servo spot passes across the photodetection devices and has a value “0” in a section where the servo stop does not pass the photodetection devices.  
         [0076]     Since it is preferable to obtain an image when the servo spot is located just at the center of the photodetection devices, the zero level detection unit  910  detects a time point where the value of the shuttering value becomes zero in a section where the servo spot passes across the photodetection devices, that is, the value of the shuttering signal changes from a (+) value to a (−) value, and outputs a shuttering control signal at the detected time point.  
         [0077]      FIG. 10  is a flow chart for explaining a method of controlling tracking according to an embodiment of the present invention. Referring to  FIG. 10 , when the photodetection unit  270  receives a signal projected from the information recording medium  290  and outputs an RF-SUM signal, the RF-SUM signal level monitoring unit  710  of the servo control unit  282  according to the present embodiment monitors whether the value of the RF-SUM signal exceeds a predetermined level ( 1010 ).  
         [0078]     When the RF-SUM signal is detected to exceed the predetermined level, the RF-SUM signal level monitoring unit  710  transmits a signal to the tracking error signal correction unit  720  to start tracking control. Then, the tracking error signal correction unit  720  corrects the tracking error signal ( 1020 ). The tracking error signal correction unit  720  turns off the tracking control signal when the RF-SUM signal is not more than the predetermined level ( 1030 ).  
         [0079]      FIG. 11  is a flow chart for explaining a method of controlling tracking according to another embodiment of the present invention. Referring to  FIG. 11 , when the photodetection unit  270  receives a signal projected from the information recording medium  290  and outputs a shuttering signal, the shuttering signal level monitoring unit  810  of the servo control unit  282  according to the present invention monitors whether the value of the shuttering signal exceeds a predetermined level ( 1110 ). The predetermined section is from a time point indicating the maximum value of the shuttering signal to a minimum value of the shuttering signal.  
         [0080]     When the shuttering signal is detected to belong to the predetermined section, the shuttering signal level monitoring unit  810  transmits a signal to the tracking error signal correction unit  820  to start tracking control. Then, the tracking error signal correction unit  820  corrects the tracking error signal ( 1120 ). The tracking error signal correction unit  820  turns off the tracking control signal when the shuttering signal is out of the predetermined section ( 1130 ).  
         [0081]      FIG. 12  is a flow chart for explaining a method of controlling shuttering according to an embodiment of the present invention. Referring to  FIG. 12 , when the photodetection unit  270  receives a signal projected from the information recording medium  290  and outputs a shuttering signal, the zero level detection unit  910  of the shuttering control unit  281  according to the present embodiment receives the shuttering signal ( 1210 ).  
         [0082]     The zero level detection unit  910  monitors whether the value of the shuttering signal reaches a zero point in a section where the value of the shuttering signal changes from a (+) value to a (−) value ( 1220 ). The zero level detection unit  910  outputs a shuttering control signal to the image capture unit  260  at a time point when the shuttering signal changes from a (+) value to a (−) value ( 1230 ).  
         [0083]     While this invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.  
         [0084]     As described above, according to the present invention, two dimensional information can be accurately reproduced from the image recording medium containing a holographic image, through the tracking control and the shuttering signal.