Abstract:
An optical information processing apparatus and an optical information processing method are provided. The optical information processing method includes steps of: irradiating a reference beam on an optical information recording medium having a plurality of recording regions which are disposed in a track direction to be adjacent to each other so as to enclose a specific recording region and first and second recording regions adjacent to the specific recording region; separating a readout beam reproduced from the specific recording region from the first and second neighborhood readout beams reproduced from the first and second recording regions by using the reference beam; and detecting intensities of the first and second neighborhood readout beams and comparing the intensities of the first and second neighborhood readout beams to calculate an optical information reproducing error.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    Embodiment of the invention relates to an optical information processing apparatus and an optical information processing method and, more particularly, to an optical information processing apparatus and an optical information processing method capable of detecting and controlling a tracking error of a reference beam and a servo error of a recording medium by using readout beams reproduced from neighborhood tracks in reproduction of optical information. 
         [0003]    2. Related Art 
         [0004]    Recently, as the rapid development of IT and multimedia industries, large-sized storage apparatuses are required to store a large sized image data or moving picture as well as test data. Accordingly, much attention has been paid on a holographic optical information processing apparatus having a large storage capacity and a high processing speed. 
         [0005]    The holographic optical information processing apparatus is a page-oriented memory which records and reproduces image information in units of an image page. The holographic optical information processing apparatus uses a parallel signal processing scheme to transmit data at a high transmission speed in comparison with CD or DVD. In addition, the holographic optical information processing apparatus uses a multiplexing scheme to record the image information on the same position so that the storage density can be greatly increased. 
         [0006]    In general, the holographic optical information processing apparatus uses a disk-shaped optical information recording medium having tracks. An optical pickup device moving along the tracks of the optical information recording medium records data in recording regions of the tracks and reproduces the data from the recording regions of the tracks. 
         [0007]    In order to increases a reliability of the holographic optical information processing apparatus, it is necessary to minimize a bit error rate (BER), that is, an error data in the reproduction of data. 
         [0008]    In order to minimize the BER, the tracking of the reference beam needs to be performed accurately along the center of the track. In addition, an angle servo of the reference beam needs to be performed accurately on the recording medium. 
         [0009]    An example of a method of detecting the tracking error is disclosed in US Patent Application Publication No. 2005/0030875, titled “Optical Information Recording Apparatus And Optical Information Reproducing Apparatus” by H. Horimai. In the Patent Document, a light source for reproducing data and a light source for obtaining a servo signal are separately provided. In addition, a beam reflected from pits recorded in advance on an optical information recording medium is detected by a photodiode or the like, and a tracking error signal is obtained by using the beam. 
         [0010]    An example of a method of detecting the angle servo error is disclosed in US Patent Application Publication No. 2005-0030876, titled “Optical Information Recording Apparatus And Optical Information Reproducing Apparatus” filed by Optware Corporation in Japan. In the Patent Document, a separate laser source of the angle servo control is used. 
         [0011]    In the reproduction of the optical information, the tracking error and the angle servo error need to be accurately detected. 
         [0012]    A sensitivity to the tracking error signal obtained based on the intensities of the beams reproduced or reflected from the optical information recording medium may be changed according to diffraction efficiencies of the recording regions or the neighborhood influence. 
         [0013]    In addition, if the tracking error signal does not have symmetry with respect to the origin (the origin corresponding to a case where there is no tracking error), complicated calculation and calibration processes need to be performed to determine the tracking error. 
         [0014]    In addition, since such a separate laser source is used to perform the angle servo control, the optical system and apparatus for servo control are complicated. Since the laser source for servo control uses a wavelength different from that of the optical information processing apparatus, constructions of the optical information recording medium or the optical information processing apparatus become more complicated. 
       SUMMARY 
       [0015]    Embodiment of the invention provides an apparatus and a method capable of accurately detecting a tracking error and an angle servo error and controlling and the tracking and the angle servo with a simple construction. 
         [0016]    Embodiment of the invention also provides an optical information processing apparatus and an optical information processing method capable of detecting and controlling a tracking error by using readout beams reproduced from neighborhood track. 
         [0017]    Embodiment of the invention also provides an optical information processing apparatus and an optical information processing method capable of detecting and controlling an angle servo error by using readout beams reproduced from neighborhood track. 
         [0018]    According to an aspect of Embodiment of the invention, there is provided an optical information processing method comprising steps of: irradiating a reference beam on an optical information recording medium having a plurality of recording regions which are disposed in a track direction to be adjacent to each other so as to enclose a specific recording region and first and second recording regions adjacent to the specific recording region; separating a readout beam reproduced from the specific recording region from the first and second neighborhood readout beams reproduced from the first and second recording regions by using the reference beam; and detecting intensities of the first and second neighborhood readout beams and comparing the intensities of the first and second neighborhood readout beams to calculate an optical information reproducing error. 
         [0019]    According to another aspect of Embodiment of the invention, there is provided an optical information processing method comprising steps of: irradiating a reference beam on an optical information recording medium having a plurality of recording regions which are disposed in a track direction to be adjacent to each other so as to enclose a specific recording region and first and second recording regions adjacent to the specific recording region; separating a readout beam reproduced from the specific recording region from the first and second neighborhood readout beams reproduced from the first and second recording regions by using the reference beam; and detecting intensities of the first and second neighborhood readout beams and comparing the intensities of the first and second neighborhood readout beams to calculate a tracking error. 
         [0020]    According to still another aspect of Embodiment of the invention, there is provided an optical information processing method comprising steps of: detecting a first neighborhood readout beam reproduced from a first track of an optical information recording medium and a second neighborhood readout beam reproduced from a second track thereof; and calculating as a tracking error a sum of an error of intensity of the first neighborhood readout beam and an error of intensity of the second neighborhood readout beam divided by a sum of the intensity of the first neighborhood readout beam and the intensity of the second neighborhood readout beam. 
         [0021]    According to further still another aspect of Embodiment of the invention, there is provided an optical information processing method comprising steps of: irradiating a reference beam on an optical information recording medium having a plurality of recording regions which are disposed in a track direction to be adjacent to each other so as to enclose a specific recording region and first and second recording regions adjacent to the specific recording region; separating a readout beam reproduced from the specific recording region from the first and second neighborhood readout beams reproduced from the first and second recording regions by using the reference beam; and detecting intensities of the first and second neighborhood readout beams and comparing the intensities of the first and second neighborhood readout beams to calculate an angle servo error. 
         [0022]    According to yet another aspect of Embodiment of the invention, there is provided an optical information processing apparatus comprising: a light source which emits a beam; a polarized beam divider which divides the beam into a reference beam and a signal beam; an optical system which irradiates the reference beam on an optical information recording medium having a plurality of recording regions which are disposed in a track direction to be adjacent to each other so as to enclose a specific recording region and first and second recording regions adjacent to the specific recording region; a neighborhood beam separator which separates a readout beam reproduced from the specific recording region from the first and second neighborhood readout beams reproduced from the first and second recording regions by using the reference beam; an optical information detector which detects optical information from the readout beams; first and second photo detectors which detect the first and second neighborhood readout beams reproduced from the first and second recording regions, respectively; and a signal processing unit which receives intensities of the readout beams detected by the first and second photo detectors and calculates an optical information reproducing error. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The above and other features and advantages of Embodiment of the invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
           [0024]      FIG. 1  is a view illustrating a construction of an optical information processing apparatus according to an embodiment of Embodiment of the invention; 
           [0025]      FIG. 2  is a view illustrating a signal processing unit of the optical information processing apparatus according to the embodiment of Embodiment of the invention; 
           [0026]      FIG. 3  is a schematic view illustrating an example of tracks on an optical information recording medium; 
           [0027]      FIG. 4  is a schematic view illustrating readout beams detected by a photo detector in order to detect a tracking error; 
           [0028]      FIG. 5  is a schematic view illustrating readout beams detected by the photo detector in a case where there is a tracking error; 
           [0029]      FIG. 6  is a graph illustrating an intensity function of readout beams in a case where there is a tracking error; 
           [0030]      FIGS. 7 to 9  are views illustrating intensities of readout beams detected by photodiodes of the photo detector; 
           [0031]      FIGS. 10 to 12  are graphs illustrating tracking errors; 
           [0032]      FIGS. 13 to 15  are graphs illustrating tracking errors according to a change in efficiency; 
           [0033]      FIGS. 16 to 18  are graphs illustrating tracking errors according to a change in efficiency; 
           [0034]      FIG. 19  is a schematic view illustrating an example of tracks on an optical information recording medium; 
           [0035]      FIG. 20  is a schematic view illustrating readout beams detected by a photo detector in order to detect a servo error; 
           [0036]      FIG. 21  illustrates graphs of intensities of neighborhood readout beams detected by the photo detector in order to detect a servo error; 
           [0037]      FIG. 22  is a graph illustrating a compensation for time delay of a readout beam in a case where a servo error is detected; and 
           [0038]      FIG. 23  is a schematic view illustrating an optical information recording reproducing apparatus according to another embodiment of Embodiment of the invention 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0039]    Hereinafter, exemplary embodiments of Embodiment of the invention are described in detail with reference to the accompanying drawings. Like reference numerals denote like elements. 
         [0040]    Referring to  FIG. 1 , an optical information processing apparatus  100  includes an optical system  110 , a neighborhood beam separator  150 , an optical information detector  160 , a photo detector  170 , and a signal processing unit  180 . 
         [0041]    The optical system  110  includes a light source  112  and a multiplexer  114 . A reference beam generated by the light source  112  is irradiated on an optical information recording medium  200  through the multiplexer  114 . The multiplexer  114  performs angular multiplexing by adjusting an incidence angle of the reference beam on the optical information recording medium  200 . The multiplexer  114  may be constructed with a rotating mirror such as a Galvano mirror. 
         [0042]    The optical information recording medium  200  includes recording regions where optical information is recorded (or to be recorded). When the reference beam is irradiated on the recording regions, the reference beams are diffracted by interference patterns formed on the recording regions, so that readout beams are generated. The optical information recording medium  200  has a shape of disk and includes a plurality of circular tracks. Each of the tracks includes a plurality of the recording regions. 
         [0043]    Referring to  FIG. 3 , a track L 2 , a track L, a current track, a track R, and a track R 2  are sequentially disposed from the center of the optical information recording medium  200  in the outward direction. 
         [0044]    The current track denotes a track including a recording region Rc of which current data is to be reproduced. The current track is subject to tracking through current tracking error detection. The current track may be any one of the tracks on the optical information recording medium  200 . 
         [0045]    Neighborhood tracks are disposed at left and right sides of the current track. The track L (the first neighborhood track) and the track L 2  are sequentially disposed at the left side of the current track. The track R (the second neighborhood track) and the track R 2  are sequentially disposed at the right side of the current track. Here, the left side denotes the one side of the current track, and the right side denotes the other side of the current track. However, the left and right sides are not limited to the sides shown in  FIG. 3 . 
         [0046]    As described above, each of the tracks includes the recording region Rc, R L , R L2 , R R , and R R2 . In  FIG. 3 , one of recording region Rc is shown, and the recording regions R L , R L2 , R R , and R R2  are also similarly shown. However, actually, a plurality of recording regions Rc are disposed along the current track, and a plurality of the recording regions R L , R L2 , R R , and R R2  are also similarly disposed. The interference patterns of the reference beam and signal beams are recorded in the recording regions Rc, R L , R L2 , R R , and R R2 . When the reference beam is irradiated on the recording regions Rc, R L , R L2 , R R , and R R2 , the reference beam is diffracted by the interference patterns, so that the readout beams as same as the signal beams used at the time of recording are generated. 
         [0047]    If the reference beam is irradiated on recording regions RC of the current track, the original data are sufficiently reproduced. However, according to Embodiment of the invention, the reference beam is irradiated on the neighborhood tracks as well as the current track. More specifically, the reference beam is irradiated on the current track and the neighborhood tracks, that is, the track L and the track R. Therefore, the recording region RC of the current track and the recording regions RL and RR of the tracks L and R are irradiated simultaneously. 
         [0048]    However, the reference beam needs to completely enclose the recording region RL of the track L or the recording region RR of the track R. The reference beam may be irradiated so as to be wider than the recording region RC of the current track and overlap with the neighborhood tracks. As an example, the reference beam may be irradiated so as to partially overlap with the track L or the track R. As another example, the reference beam may be irradiated so as to overlap with the tracks L and L 2  or the tracks R and R 2 . 
         [0049]    Since the size of the irradiated reference beam is wider than the recording region Rc of the current track, the readout beam reproduced by the reference beam includes a readout beam reproduced from the recording region Rc and readout beams reproduced from the neighborhood tracks. Namely, when the reference beam is irradiated so as to overlap with the tracks L and R, the readout beam includes the readout beam reproduced from the recording region Rc of the current track, the readout beam reproduced from the recording regions RL of the track L, and the readout beam reproduced from the recording regions R R  of the track R. Hereinafter, the readout beam reproduced from the recording regions Rc of the current track is referred to as a main readout beam, the readout beams reproduced from the recording regions of the neighborhood tracks are referred to as neighborhood readout beams. 
         [0050]    For the convenience of description, the recording region Rc of the current track, the recording region R L  of the track L, and the recording region R R  of the track R are aligned in a straight line perpendicular to the direction of track. However, the recording region Rc of the current track, the recording region R L  of the track L, and the recording region R R  of the track R may not be aligned in the straight line (see  FIG. 19 ). 
         [0051]    Returning to  FIG. 1 , the readout beams reproduced by the reference beam pass through a first lens  122  to be irradiated on a polarized beam splitter  124 . The polarized beam splitter  124  passes a P polarization beam and reflects an S polarization beam. The P and S polarization beams are perpendicular to each other. Here, the P and S polarizations do not denote absolute polarization states. 
         [0052]    The P polarization readout beam divided by the polarized beam splitter  124  passes through a wavelength plate (a quarter wave plate)  126  to be changed into a circularly polarized beam. A second lens  128  focuses the readout beam on a neighborhood beam separator  150 . 
         [0053]    The neighborhood beam separator  150  has an aperture  155  to pass the main readout beam reproduced from the recording region Rc of the current track. The neighborhood beam separator  150  is a kind of a Nyquist filter. 
         [0054]    The main readout beam passing through the aperture  155  passes through a third lens  165  to be detected by an optical information detector  160 . The optical information detector  160  may be constructed with a pixel array such as CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor). The optical information detector  160  detects optical information contained in the main readout beam. Accordingly, the optical information detector  160  can detect a data page of the recording regions Rc of the current track. 
         [0055]    A reflecting layer  156  is formed on a region around the aperture  155  in the neighborhood beam separator  150  so as to reflect the readout beam. If there is no tracking error, only the main readout beam passes through the aperture  155 , and the neighborhood readout beam is reflected by the reflecting layer  156 . The readout beam that cannot pass through the aperture  155  is reflected by the reflecting layer  156 , so that the readout beam is irradiated on the second lens  128 . 
         [0056]    The reflected readout beam passing through the second lens  128  passes through the wavelength plate  128  to be changed from the circularly polarized beam to the S polarization beam. The S polarization readout beam is reflected by the polarized beam splitter  124  to be directed to a fourth lens  129 . The readout beam passing through the fourth lens  129  is irradiated on the photo detector  170 . 
         [0057]    The photo detector  170  detects an intensity of the readout beam. The photo detector  170  may be constructed with photodiodes or a photodiode array. A signal processing unit  180  receives information on the intensity of the readout beam detected by the photo detector  170  to calculate the tracking error. 
         [0058]    As shown in  FIG. 3 , the signal processing unit  180  calculates the tracking error and an angle servo error based on the intensity of the neighborhood readout beam reproduced at the time of reproduction of the optical information. The signal processing unit  180  includes a power meter  181  which measures the intensity of the readout beam detected by the photo detector  170 , an anti-aliasing filter  182  which adjusts a signal of the power meter  181 , and a digital signal processor (DSP)  183  which determines the adjusted signal of the anti-aliasing filter  182  to generate a control signal for controlling the multiplexer  114  and a servo controller  190 . 
         [0059]    The digital signal processor  183  includes an AD converter  184  which converts the signal measured by the power meter  181  to a digital signal, a low pass filter  185  which filters the digital signal of the AD converter  184  to remove an unnecessary signal, an error calculator  186  which analyzes the signal of the AD converter  184  to calculate the tracking error and the angle servo error, and a controller  187  which controls the multiplexer  186  based on the calculates the servo state of the reference beam. 
         [0060]    Referring to  FIG. 4 , the readout beams passing through the fourth lens  129  are detected in a form of spots by the photo detector  170 . Here, S L2 , S L , SC, S R , and S R2  denote spots of the readout beams reproduced from the recording regions R L2 , R L , Rc, R R , and R R2  of the tracks L 2  and L, the current track, and the tracks R and R 2 , respectively. 
         [0061]    The photo detector  170  includes a first photo detector  172  and a second photo detector  174 . The first photo detector  172  detects the spot S L , and the second photo detector  174  detects the spot S R . 
         [0062]    The first photo detector  172  is partitioned into two photodiodes A and B. The first photo detector  172  is partitioned in a rotation direction (or a track direction) of the optical information recording medium  200 . A position error of the spot S L  can be determined based on the intensity error of between the intensities P A  and P B  detected by the photodiodes A and B. Namely, if there is no tracking error, the intensity error P A −P B  is set to zero. Accordingly, the position error of the spot S L  can be determined based on the sign and magnitude of the intensity error P A −P B . 
         [0063]    The second photo detector  174  is partitioned into two photodiodes C and D. A position error of the spot SR can be determined based on the intensity error of between the intensities P C  and P D  detected by the photodiodes C and D. 
         [0064]    Although the photo detector  170  is constructed with the first and second photo detectors  172  and  174 , it is not limited thereto, but it may be constructed with one photo detector. In addition, instead of the photodiodes, the photo detector  170  may be constructed with other optical detecting devices. 
         [0065]    Now, the tracking error detection is described in detail. 
         [0066]    The signal processing unit  189  calculates a tracking error TE based on the intensities of the readout beams detected by the photo detector  170  by using Equation 1. 
         [0000]    
       
         
           
             
               
                 
                   TE 
                   = 
                   
                     
                       
                         ( 
                         
                           
                             P 
                             A 
                           
                           - 
                           
                             P 
                             B 
                           
                         
                         ) 
                       
                       + 
                       
                         ( 
                         
                           
                             P 
                             C 
                           
                           - 
                           
                             P 
                             D 
                           
                         
                         ) 
                       
                     
                     
                       
                         P 
                         A 
                       
                       + 
                       
                         P 
                         B 
                       
                       + 
                       
                         P 
                         C 
                       
                       + 
                       
                         P 
                         D 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
         [0067]    According to Embodiment of the invention, the tracking error TE is obtained by using a difference between intensities of the neighborhood readout beams. The neighborhood readout beams include readout beams reproduced from two tracks which are different from each other. The tracking error is defined as a sum of the intensity error P A −P B  of the neighborhood readout beams reproduced from the track L and the intensity error P C −P D  of the neighborhood readout beams reproduced from the track R divided by a sum of the intensities P A +P B  of the neighborhood readout beams reproduced from the track L and the intensities P C +P D  of the neighborhood readout beams reproduced from the track R. 
         [0068]    When there is no tracking error, the spot Sc passes through the aperture  155  without reflection on the reflecting layer  156 , so that the spot Sc is not detected by the photo detector  170 . In addition, among the neighborhood readout beams, the spot S L  is detected by the first photo detector  172 , and the spot S R  is detected by the second photo detector  174 . In this case, P A −P B =0, and P C −P D =0, so that the tracking error TE=0 is obtained by using Equation 1. 
         [0069]    Referring to  FIG. 5 , if there is a tracking error, the reference beam is not accurately irradiated on the current track. Therefore, the main readout beam does not completely pass through the aperture  155 , and a portion of the main readout beam is reflected by the reflecting layer  156 . Due to the tracking error, the positions of the spots S L2 , S L , Sc, S R , and S R2  detected by the photo detector  170  are changed. Since P A −P B ≠0 and P C −P D ≠0, the tracking error TE≠0 is obtained by using Equation 1. Accordingly, the tracking error can be corrected by shifting the optical pickup device based on the obtained tracking error value TE. 
         [0070]    Now, Equation 1 for calculating the tracking error is described in detail. 
         [0071]    In order to accurately calculate the tracking error, a tracking error function has symmetry with respect to the origin. The origin corresponds to a case where there is no tracking error. The symmetry with respect to the origin denotes that the tracking errors of the left and right sides of the current track have the same shape. If there is no symmetry with respect to the origin, the shifting amounts of the optical pickup device tracking error in the left and right sides need to be separately determined according to the signs of the tracking errors, so that the control of tracking is complicated. 
         [0072]    In addition, for a suitable equation of tracking error, sensitivity of efficiency of the readout beam needed to be lowered. In addition, the tracking error needs be invulnerable to a variation of the intensity of the readout beam. 
         [0073]    Namely, in such a holographic optical information processing apparatus, since the readout beams are generated by diffraction of the reference beams on the interference patterns, the efficiencies of the readout beams detected by the photo detector  170  are not uniform according to the interference patterns. If the value of the tracking error is changed according to the efficiency of the readout beam, the tracking error needs to be re-adjusted according to the efficiency of the readout beam, so that the control of the tracking error is complicated. 
         [0074]    A function of intensity of the spot detected by the photo detector  170  is denoted by f(x). The functions of intensity of the spots S L2 , S L , Sc, S R , and S R2  are denoted by f L2 (x), f L (x), fc(x), f R (x), and f R2 (x) respectively. 
         [0075]    The functions f L2 (x), f L (x), fc(x), f R (x), and f R2 (x) of the spots can be assumed to be equal to each other because the readout beams pass through a fourth lens  129  which is a Fourier transformation lens. Namely, f(x)=f L2 (x)=f L (x)=fc(x)=f R (x)=f R2 (x) . In addition, due to the Fourier transformation, the function of intensity is an even function, that is, f(x)=f(−x). 
         [0076]      FIG. 6  illustrates a case where there is a tracking error that a spot is shifted by x0. In this case, the function of intensity is divided into the following sections defined by Equation 2. 
         [0000]        S   1 =∫ −a   −x     0     f ( x ) dx=∫   x     0     −a   f ( x ) dx    
         [0000]        S   2 =∫ −x     0     −x     0     +a   f ( x ) dx=∫   x     0     −a   x     o     f ( x ) dx    
         [0000]        S   3 =∫ a−x     0     f ( x ) dx=∫   −a   −a+x   f ( x ) dx    [Equation 2] 
         [0077]    Here, x0 denotes a shifting distance of a reproduced spot, and “a” denotes a radius of the spot. 
         [0078]    The shifting direction of the spot is determined according to a sign of the x0. In the embodiment, a positive sign (+) of the x0 denotes right sifting of the spot, and a negative sign (−) of the x0 denotes left sifting of the spot. Therefore, the shifting of the spots shown in  FIGS. 4 and 5  corresponds to the negative sign (−) of the x0. 
         [0079]    The intensities of the readout beams detected by the photodiodes A, B, C, and D of the first and second photo detectors  172  and  174  may be represented by two sections according to the sign of the x0 and the efficiencies of the spots. 
         [0080]    If the sign of the x0 is positive, the intensities of the readout beams detected by the photodiodes A, B, C, and D can be represented by Equation 3. 
         [0000]        P   A   + =η L2   S   1 +η L   S   3    
         [0000]        P   B   + =η L   S   2    
         [0000]        P   C   + =η C   S   1 +η R   S   3    
         [0000]        P   D   + =η R   S   2    [Equation 3] 
         [0081]    Here, η L2 , η L , ηc, η R , and η R2  denote the efficiencies of the spots S L2 , S L , Sc, S R , and S R2 , respectively. The superscript “+” denotes the sign of the x0. 
         [0082]    If the sign of the x0 is negative, the intensities of the readout beams detected by the photodiodes A, B, C, and D can be represented by Equation 4. 
         [0000]        P   A   − =η L   S   2    
         [0000]        P   B   − =η L   S   3+η   C   S   1    
         [0000]        P   C   − =η R   S   1    
         [0000]        P   D   − =η R   S   3 +η R2   S   1    [Equation 4] 
         [0083]    If the reference beam is irradiated on the current track, the track L, and track R, the spots S L2  and S R2  are not nearly detected by the photo detector  179 . Accordingly, η L2 ≈0, and η R2 ≈0. When the tracking errors are denoted by TE 1 =P A −P B  and TE 2  =P C −P D , Equations 3 and 4 can be represented by Equation 5. 
         [0000]        TE   1   + =η L   S   3 −η L   S   2    
         [0000]        TE   1   − =η L   S   2 −η L   S   3 −η C   S   1    
         [0000]        TE   2   + =η C   S   1 +η R   S   3 −η R   S   2    
         [0000]        TE   2   − =η R   S   2 −η R   S   3    [Equation 5] 
         [0084]    In Equation 5, the tracking errors do not have symmetry due to η C S 1 . 
         [0085]    In order to obtain symmetry, a new tracking error TE 3  is defined by Equation 6. 
         [0000]        TE   3   =TE   1   +TE   2 =( P   A   −P   B )+( P   C   −P   D )   [Equation 6] 
         [0086]    The tracking error TE 3  of Equation 6 can be represented by Equation 7 by using Equation 4. 
         [0000]        TE   3   + =(η L   S   3 +η C   S   1 +η R   3 )−(η L   S   2 +η R   S   2 ) 
         [0000]        TE   3   − =(η L   S   2 +η R   S   2 )−(η L   S   3 +η C   S   1 +η R    S   3    [Equation 7] 
         [0087]    Namely, the absolute values of the TE 3  are equal to each other irrespective of the signs thereof. Therefore, the tracking error TE 3  has symmetry. 
         [0088]    Now, the tracking error according to the efficiency of the readout beam is described. 
         [0089]    The tracking error TE 3  of Equation 6 is not invulnerable to a variation of the intensity of the readout beam. Namely, the efficiencies η L2 , η L , η R , and η R2  of the spots may be represented by Equation 8 by using the efficiency ηc of the spot Sc. 
         [0000]      η L2 =k L2 η C    
         [0000]      η L =k L η C    
         [0000]      η R =k R η C    
         [0000]      η R2 =k R2 η C    [Equation 8] 
         [0090]    Here, k L2 , k L , k R , and k R2  are constants. By substituting Equation 8 to Equation 7, the tracking error TE 3  can be represented by Equation 9. 
         [0000]        TE   3   + =η C {( k   L   +k   R )( S   3   −S   2 )+ S   1 }  [Equation 9] 
         [0091]    If there is a variation of the intensities of the readout beams, the efficiencies of the readout beams are changed, so that the constants k L2 , k L , k R , and k R2  are also changed. Namely, k L =k L +Δk L , k R =k R +Δk R . By substituting the changed constants to Equation 9, the tracking error TE 3  can be represented by Equation 10. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             TE 
                             3 
                             + 
                           
                           ′ 
                         
                         = 
                         
                           
                             TE 
                             3 
                             + 
                           
                            
                           
                             | 
                             
                               k 
                               = 
                               
                                 k 
                                 + 
                                 
                                   Δ 
                                    
                                   
                                       
                                   
                                    
                                   k 
                                 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             η 
                             C 
                           
                            
                           
                             { 
                             
                               
                                 
                                   ( 
                                   
                                     
                                       k 
                                       L 
                                     
                                     + 
                                     
                                       k 
                                       R 
                                     
                                     + 
                                     
                                       Δ 
                                        
                                       
                                           
                                       
                                        
                                       
                                         k 
                                         LR 
                                       
                                     
                                   
                                   ) 
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       S 
                                       3 
                                     
                                     - 
                                     
                                       S 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                               + 
                               
                                 S 
                                 1 
                               
                             
                             } 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             TE 
                             3 
                             + 
                           
                           + 
                           
                             
                               η 
                               C 
                             
                              
                             Δ 
                              
                             
                                 
                             
                              
                             
                               
                                 k 
                                 LR 
                               
                                
                               
                                 ( 
                                 
                                   
                                     S 
                                     3 
                                   
                                   - 
                                   
                                     S 
                                     2 
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             TE 
                             3 
                             + 
                           
                           + 
                           
                             Δ 
                              
                             
                                 
                             
                              
                             
                               TE 
                               3 
                               + 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     10 
                   
                   ] 
                 
               
             
           
         
       
     
         [0092]    Here, Δk LR =Δk L +Δk R , ΔTE +   3 =η C Δk LR (S 3 −S 2 ). From Equation 10, the relation |ΔTE 3   |=|ΔTE   3   + |=|ΔTE 3   − |=|η CΔk   LR (S 3 −S 2 )| can be obtained. It should be nodded that the tracking error TE 3 ′ has symmetry with respect to the origin. ΔTE 3  depends on Δk LR . Therefore, although the tracking error TE 3 ′ has symmetry with respect to the origin, the tracking error may be changed according to the efficiencies of the readout beams. 
         [0093]    By substituting Equation 8 to the tracking error TE of Equation 1, the tracking error can be represented by Equation 10. 
         [0000]    
       
         
           
             
               
                 
                   
                     TE 
                     + 
                   
                   = 
                   
                     
                       
                         
                           ( 
                           
                             
                               k 
                               L 
                             
                             + 
                             
                               k 
                               R 
                             
                           
                           ) 
                         
                          
                         
                           ( 
                           
                             
                               S 
                               3 
                             
                             - 
                             
                               S 
                               2 
                             
                           
                           ) 
                         
                       
                       + 
                       
                         S 
                         1 
                       
                     
                     
                       
                         
                           ( 
                           
                             
                               k 
                               L 
                             
                             + 
                             
                               k 
                               R 
                             
                           
                           ) 
                         
                          
                         
                           ( 
                           
                             
                               S 
                               3 
                             
                             + 
                             
                               S 
                               2 
                             
                           
                           ) 
                         
                       
                       + 
                       
                         S 
                         1 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     10 
                   
                   ] 
                 
               
             
           
         
       
     
         [0094]    The error of the tracking error according to the efficiency of the readout beam can be represented by Equation 11. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           
                             TE 
                             + 
                           
                         
                         = 
                           
                          
                         
                           
                             TE 
                             + 
                           
                            
                           
                             | 
                             
                               k 
                               = 
                               
                                 k 
                                 + 
                                 
                                   Δ 
                                    
                                   
                                       
                                   
                                    
                                   k 
                                 
                               
                             
                           
                            
                           
                             - 
                             
                               TE 
                               + 
                             
                           
                            
                           
                             | 
                             k 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               - 
                               2 
                             
                              
                             Δ 
                              
                             
                                 
                             
                              
                             
                               k 
                               LR 
                             
                              
                             
                               S 
                               1 
                             
                              
                             
                               S 
                               2 
                             
                           
                           
                             
                               
                                 
                                   { 
                                   
                                     
                                       
                                         ( 
                                         
                                           
                                             k 
                                             L 
                                           
                                           + 
                                           
                                             k 
                                             R 
                                           
                                           + 
                                           
                                             Δ 
                                              
                                             
                                                 
                                             
                                              
                                             
                                               k 
                                               LR 
                                             
                                           
                                         
                                         ) 
                                       
                                        
                                       
                                         ( 
                                         
                                           
                                             S 
                                             3 
                                           
                                           + 
                                           
                                             S 
                                             2 
                                           
                                         
                                         ) 
                                       
                                     
                                     + 
                                     
                                       S 
                                       1 
                                     
                                   
                                   } 
                                 
                               
                             
                             
                               
                                 
                                   { 
                                   
                                     
                                       
                                         ( 
                                         
                                           
                                             k 
                                             L 
                                           
                                           + 
                                           
                                             k 
                                             R 
                                           
                                         
                                         ) 
                                       
                                        
                                       
                                         ( 
                                         
                                           
                                             S 
                                             3 
                                           
                                           + 
                                           
                                             S 
                                             2 
                                           
                                         
                                         ) 
                                       
                                     
                                     + 
                                     
                                       S 
                                       1 
                                     
                                   
                                   } 
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     11 
                   
                   ] 
                 
               
             
           
         
       
     
         [0095]    Here, the following equations are satisfied. 
         [0000]      ( k   L   +k   R   +Δk   LR )( S   3   +S   2 )&gt;0 
         [0000]      S 1 &gt;0 
         [0000]      ( k   L   +k   R )( S   3   +S   2 )&gt;0 
         [0096]    Therefore, Equation 11 can be represented by Equation 12. 
         [0000]    
       
         
           
             
               
                 
                   
                      
                     
                       Δ 
                        
                       
                           
                       
                        
                       TE 
                     
                      
                   
                   &lt; 
                   
                      
                     
                       
                         2 
                          
                         Δ 
                          
                         
                             
                         
                          
                         
                           k 
                           LR 
                         
                          
                         
                           S 
                           1 
                         
                          
                         
                           S 
                           2 
                         
                       
                       
                         
                           
                             S 
                             1 
                           
                            
                           
                             ( 
                             
                               
                                 k 
                                 L 
                               
                               + 
                               
                                 k 
                                 R 
                               
                             
                             ) 
                           
                         
                          
                         
                           ( 
                           
                             
                               S 
                               3 
                             
                             + 
                             
                               S 
                               2 
                             
                           
                           ) 
                         
                       
                     
                      
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     12 
                   
                   ] 
                 
               
             
           
         
       
     
         [0097]    Here, 
         [0000]    
       
         
           
             
                
               
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     k 
                     LR 
                   
                 
                 
                   
                     k 
                     L 
                   
                   + 
                   
                     k 
                     R 
                   
                 
               
                
             
             &lt; 
             1. 
           
         
       
     
       As a result, the error ΔTE of the tracking error according to the efficiency of the readout beam can be represented by Equation 13. 
       [0098]    
       
         
           
             
               
                 
                   
                      
                     
                       Δ 
                        
                       
                           
                       
                        
                       TE 
                     
                      
                   
                   &lt; 
                   
                      
                     
                       
                         2 
                          
                         
                           S 
                           2 
                         
                       
                       
                         
                           S 
                           3 
                         
                         + 
                         
                           S 
                           2 
                         
                       
                     
                      
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     13 
                   
                   ] 
                 
               
             
           
         
       
     
         [0099]    Namely, since the upper bound of the error ΔTE is independent of the variation Δk of the efficiency η of the reproduced spot, the sensitivity to the efficiency of the readout beam is lowered. Therefore, if there is a variation of the intensity of the readout beam, the tracking error is not nearly influenced by the variation of the intensity of the readout beam. Namely, the tracking error is invulnerable to the variation of the intensity of the readout beam. 
         [0100]    Hereinafter, simulation of the detection of the tracking error according to Embodiment of the invention is described. 
         [0101]      FIGS. 7 to 9  are views illustrating the intensities of the readout beams detected by photodiodes A, B, C, and D of the photo detector  170 . The efficiencies of the spots are set to 1. Namely, η L2 =η L =η R =η R2 =1. 
         [0102]      FIG. 7  illustrates a case where there is no tracking error. The intensities of the readout beams detected by the photodiodes A and B of the first photo detector  172  are not nearly different from each other. Similarly, the intensities of the readout beams detected by the photodiodes C and D of the first photo detector  174  are not nearly different from each other. 
         [0103]      FIG. 8  illustrates a case where the sign of the x0 is negative. The detected spots are shifted toward the left side. In this case, a portion of the spot Sc of the main readout beam is detected by the photodiode B. 
         [0104]      FIG. 9  illustrates a case where the sign of the x0 is positive. The detected spots are shifted toward the right side. In this case, a portion of the spot Sc of the main readout beam is detected by the photodiode C. 
         [0105]      FIGS. 10 to 12  are graphs illustrating tracking errors.  FIGS. 10 ,  11 , and  12  illustrate TE 1 , TE 3  of Equation 7, and TE of Equation 1, respectively. 
         [0106]    Referring to  FIGS. 10 to 12 , it can be understood that, as x is changed, the TE 1  becomes asymmetric with respect to the origin, but the TE 3  and TE are maintained symmetric with respect to the origin. 
         [0107]      FIGS. 13 to 15  are graphs illustrating tracking errors according to a change in efficiency. Here, it is assumed that η L2 =1, η L  =0.4, η R =0.7, and η R2 =1.  FIGS. 13 ,  14 , and  15  illustrate TE 1 , TE 3  of Equation 7, and TE of Equation 1, respectively. 
         [0108]    Referring to  FIGS. 13 to 15 , it can be understood that, as x is changed, the TE 1  becomes asymmetric with respect to the origin. Although the TE 3  is maintained symmetric with respect to the origin, the magnitude thereof is greatly changed as shown in  FIG. 14 . On the country, the magnitude of the TE is not greatly changed shown in  FIG. 15 . Therefore, the tracking error is not sensitive to the efficiency of the readout beam. 
         [0109]      FIGS. 16 to 18  are graphs illustrating tracking errors according to a change in efficiency. Here, it is assumed that η L2 =1, η L =1.6, η R =0.8, and η R2 =1.  FIGS. 16 ,  17 , and  18  illustrate TE 1 , TE 3  of Equation 7, and TE of Equation 1, respectively. 
         [0110]    Referring to  FIGS. 16 to 18 , it can be understood that, the TE is maintained symmetric with respect to the origin, and the magnitude thereof is not greatly changed according to the change in efficiency of the readout beam similarly to the case shown in  FIGS. 13 to 15 . 
         [0111]    Now, detection of the angle servo error is described in detail. 
         [0112]    An ideal arrangement of the neighborhood readout beams and the photo detector  170  may be implemented as shown in  FIG. 3 . Namely, in  FIG. 3 , the optical information is ideally recorded on the recording regions Rc, R L , and R R  of the current track, the track L, and the track R in the recording medium. 
         [0113]    The signal processing unit  180  obtains an angle servo error AE defined by Equation 14 based on the intensities of the neighborhood readout beams detected by the photo detector  170 . 
         [0000]    
       
         
           
             
               
                 
                   AE 
                   = 
                   
                     
                       
                         ( 
                         
                           
                             P 
                             A 
                           
                           + 
                           
                             P 
                             B 
                           
                         
                         ) 
                       
                       - 
                       
                         ( 
                         
                           
                             P 
                             C 
                           
                           + 
                           
                             P 
                             D 
                           
                         
                         ) 
                       
                     
                     
                       
                         P 
                         A 
                       
                       + 
                       
                         P 
                         B 
                       
                       + 
                       
                         P 
                         C 
                       
                       + 
                       
                         P 
                         D 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     14 
                   
                   ] 
                 
               
             
           
         
       
     
         [0114]    According to Embodiment of the invention, the angle servo error AE is calculated from a difference between the intensities of the neighborhood readout beams. The neighborhood readout beams include neighborhood readout beams reproduced from two tracks that are different from each other. The servo tracking error AE is defined as a difference between the intensities P A +P B  of the neighborhood readout beams reproduced from the track L and the intensities P C +P D  of the neighborhood readout beams reproduced from the track R divided by a sum of the intensities P A +P B  of the neighborhood readout beams reproduced from the track L and the intensities P C +P D  of the neighborhood readout beams reproduced from the track R. 
         [0115]    In an actual recording medium, there are errors of the distances between the recording regions Rc, R L , and R R  formed on the recording medium and errors between the diameters of spots formed in the tracks. Therefore, as shown in  FIGS. 19 and 20 , the detected spots reproduced from the recording region Rc of the current track and the recording regions R L  and R R  or the tracks L and R are in the shifted state in the detection of the photo detector  170 . 
         [0116]      FIG. 21  illustrates graphs of the intensities of the neighborhood readout beams detected by the first and second photo detectors  172  and  174  in the states shown in  FIG. 20 . As shown in  FIG. 21 , it can be seen that the maximum values of the intensities of the neighborhood readout beams detected by the first and second photo detectors  172  and  174  occur with a predetermined time delay according to the states of the tracks. 
         [0117]    Accordingly, even in a normal angle servo state of the reference beam, an angle servo error may be detected according to the difference between the intensities detected by the first and second photo detectors  172  and  174 . 
         [0118]    Therefore, the angle servo error of the reference beam and the intensities of the neighborhood readout beams detected by the first and second photo detectors  172  and  174  needs to be calculated by taking into consideration the time delay according to the states of the tracks. Accordingly, the angle servo error AE can be represented by Equation 15. 
         [0000]    
       
         
           
             
               
                 
                   AE 
                   = 
                   
                     
                       
                         
                           ( 
                           
                             
                               P 
                               A 
                             
                             + 
                             
                               P 
                               B 
                             
                           
                           ) 
                         
                         max 
                       
                       - 
                       
                         
                           ( 
                           
                             
                               P 
                               C 
                             
                             + 
                             
                               P 
                               D 
                             
                           
                           ) 
                         
                         max 
                       
                     
                     
                       
                         
                           ( 
                           
                             
                               P 
                               A 
                             
                             + 
                             
                               P 
                               B 
                             
                           
                           ) 
                         
                         max 
                       
                       + 
                       
                         
                           ( 
                           
                             
                               P 
                               C 
                             
                             + 
                             
                               P 
                               D 
                             
                           
                           ) 
                         
                         max 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     15 
                   
                   ] 
                 
               
             
           
         
       
     
         [0119]    Equation 15 needs to satisfy the following conditions. 
         [0000]    
       
         
           
             
               
                 ( 
                 
                   
                     P 
                     A 
                   
                   + 
                   
                     P 
                     B 
                   
                 
                 ) 
               
               max 
             
             = 
             
               
                 max 
                 
                   t 
                   ∈ 
                   T 
                 
               
                
               
                 ( 
                 
                   
                     P 
                     A 
                   
                   + 
                   
                     P 
                     B 
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             
               
                 ( 
                 
                   
                     P 
                     C 
                   
                   + 
                   
                     P 
                     D 
                   
                 
                 ) 
               
               max 
             
             = 
             
               
                 max 
                 
                   t 
                   ∈ 
                   T 
                 
               
                
               
                 ( 
                 
                   
                     P 
                     C 
                   
                   + 
                   
                     P 
                     D 
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             T 
             = 
             
               MAX 
                
               
                 { 
                 
                   
                     T 
                      
                     
                         
                     
                      
                     1 
                   
                   , 
                   
                     T 
                      
                     
                         
                     
                      
                     2 
                   
                 
                 } 
               
             
           
         
       
     
         [0120]    Here, T is a time interval taken when one recording region passes through one photo detector  172  or  174 . 
         [0121]    The time interval T of Equation 15 can be determined as a graph shown in  FIG. 22 . The time interval is set to be in a range of 85% to 95% of an average of the time interval detected by taking into consideration noise of intensity which is detected as a sum of time intervals according to a positively increasing slope and a negatively decreasing slope corresponding to changes in the intensities of the readout beams detected by the first and second photo detectors  172  and  174 . 
         [0122]    The multiplexer and the tilting state of the recording medium can be corrected according to the intensities of the neighborhood readout beams detected by the first and second photo detectors  172  and  174 . 
         [0123]    Embodiment of the invention is not limited to the aforementioned embodiment, but various modifications may be available. For example, instead of one light source, two or more light sources may be used to generate the reference beams. In this case, the reference beam of the one light source is irradiated on the recording region of the current track, and the reference beam of the other light source is irradiated on the recording region of the neighborhood track, so that the readout beams can be generated. 
         [0124]      FIG. 23  is a schematic view illustrating an optical information recording reproducing apparatus according to another embodiment of Embodiment of the invention. The same reference numerals as those of the embodiment shown in  FIG. 1  denote the same elements. 
         [0125]    Referring to  FIG. 23 , the optical information recording reproducing apparatus  300  includes an optical system  310 , a neighborhood beam separator  150 , an optical information detector  160 , a photo detector  170 , and a signal processing unit  180 . Except for the optical system  130 , the other elements are same as those of the embodiment shown in  FIG. 1 . Hereinafter, the optical system  310  is described, but description of the same elements is omitted. 
         [0126]    The optical system  310  includes a light source  312 , an beam splitter  313 , a multiplexer  314 , and a spatial optical modulator  318 . The beam generated by the light source  312  is divided into a reference beam and a signal beam by the beam splitter  313 . The reference beam is reflected by the multiplexer  314  to be irradiated on the optical information recording medium  150  at a predetermined incidence angle. 
         [0127]    The path of the signal beam is changed by a reflecting mirror  316  to propagate toward the spatial optical modulator  318 . The spatial optical modulator  140 ) optically modulates the input information or data to generate a two-dimensional data image or data page. The data page is projected on the signal beam, and the signal beam is irradiated on the optical information recording medium  200 . When the reference beam and the signal beam are irradiated, the interference pattern therebetween is recorded on the optical information recording medium  200 . 
         [0128]    In the reproduction of the data, the signal beam is blocked, and only the reference beam is irradiated, so that the readout beam is generated from the interference pattern. 
         [0129]    According to Embodiment of the invention, a tracking error can be detected and controlled by using readout beams reproduced from neighborhood tracks. 
         [0130]    In addition, according to Embodiment of the invention, an angle servo error can be detected and controlled by using readout beams reproduced from neighborhood tracks.