Abstract:
An interference-type optical pickup, an optical information detection method, and an optical information recording and reproducing apparatus, which allow easy adjustment of a path difference between two lights, have a high signal amplification effect and are suitable for size reduction of an optical system, are provided. A signal light reflected from an optical disk and a reference light branched from the same light source and guided into detectors without being projected onto the optical disk are caused to interfere with each other on detectors. Detector outputs in four interference states in which phase relationships between the reference light and the signal light are different from each other by degrees are simultaneously obtained and calculated. Accordingly, a readout signal RF that is always stable and amplified with high quality is obtained even when an optical path changes due to disk undulations. The four detector outputs are simultaneously obtained by use of four quadrant photo detectors, and are calculated, to thereby also obtain a servo signal.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    U.S. patent application Ser. No. 12/419,421 is a co-pending application of this application, the content of which is incorporated herein by cross-reference. 
     
    
     CLAIM OF PRIORITY 
       [0002]    The present application claims priority from Japanese patent application JP 2008-208702 filed on Aug. 13, 2008, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to an optical information detection method, and relates to an improvement in the S/N ratio of a readout signal in an optical disk device, for example. 
         [0005]    2. Background Art 
         [0006]    Optical disks have almost reached their limit in terms of the resolution limit of the optical system with Blu-ray Disc (BD) by use of a blue-violet semiconductor laser and a high NA objective lens having been commercialized. A multilayer structure is expected to be dominant in the optical disks in the future to further increase the capacity. In a multilayer optical disk, the reflectance from a specific layer has to be kept small since the intensities of lights detected from respective layers are required to be substantially the same as each other. Meanwhile, in the optical disk, a transfer rate has also been continuously increased along with an increase in the capacity since it is necessary to increase the copying speed for videos or the like. If the transfer rate continues to be increased, a sufficient S/N ratio of a readout signal cannot be ensured. Thus, it is necessary to improve the S/N ratio of a detected signal to achieve the multilayer structure and the higher rate at the same time. 
         [0007]    A technique regarding the improvement in the S/N ratio of the readout signal of the optical disk is described in JP Patent Publication (Kokai) Nos. 05-342678A (1993) and 06-223433A (1994), for example. Both of JP Patent Publication (Kokai) Nos. 05-342678A (1993) and 06-223433A (1994) relate to the improvement in the S/N ratio of the readout signal of a magneto-optical disk, and intend to amplify the amplitude of a faint signal by branching a light from a semiconductor laser before the light is projected onto an optical disk, combining a light which is not projected onto the optical disk with a reflected light from the optical disk to cause interference therebetween, and thereby increasing the intensity of the light which is not projected onto the optical disk. In differential detection between a transmitted light and a reflected light of a polarizing beam splitter that has been conventionally used in signal detection for the magneto-optical disk, the detection is performed basically by causing original incident polarization components and polarization components perpendicular to an incident polarization direction produced by polarization rotation by the magneto-optical disk to interfere with each other, and amplifying the perpendicular polarization components with the incident polarization. Accordingly, when the original incident polarization components are increased, the signal can be increased. However, in order to prevent data from being deleted or overwritten, the intensity of the light incident on the optical disk needs to be suppressed to a certain level or less. On the other hand, in the conventional techniques described above, a light caused to interfere with a signal light is separated from the signal light in advance, and the light is caused to interfere with the signal light without focusing the light onto the disk. Thus, the intensity of the light caused to interfere with the signal light for signal amplification can be increased regardless of the light intensity on the disk surface. Accordingly, in principle, as the light intensity is stronger within the allowable range of the intensity, a higher S/N ratio can be obtained in comparison with the noise of an amplifier for converting a photocurrent from a photo detector into a voltage, and the shot noise occurring in the photo detector. 
         [0008]    In JP Patent Publication (Kokai) No. 05-342678A (1993), an interference intensity is detected by causing two lights to interfere with each other. The optical path length of the light which is not reflected from the disk and caused to interfere with the signal light is made variable, thereby ensuring the amplitude of an interference signal. In JP Patent Publication (Kokai) No. 06-223433A (1994), the differential detection is also performed in addition to the interference intensity detection. The intensity component of each light which does not contribute to the signal is thereby canceled, and the noise component of the light is canceled, thereby improving the S/N ratio. A non polarizing beam splitter is used for the differential detection in this case. 
       SUMMARY OF THE INVENTION 
       [0009]    Each of the interferometer optical systems used in the aforementioned conventional techniques is a Mach-Zehnder type optical system, which has many optical parts and is thus not suitable for size reduction of the optical system. The Mach-Zehnder type interferometer optical system is an interferometer which separately includes means for dividing a light into a signal light and a reference light first, and means for combining again the signal light after any modulation is applied to the signal light as a signal with the reference light again to cause interference therebetween. On the other hand, a Twyman-Green or Michelson type interferometer optical system returns again the signal light and the reference light to the means for dividing the light first to cause interference therebetween. The reason why the Mach-Zehnder type optical system is used in the aforementioned conventional examples is not described in detail in JP Patent Publication (Kokai) Nos. 05-342678A (1993) and 06-223433A (1994). However, the reason is assumed to be that, since the signal light of the magneto-optical disk is produced by polarization rotation, a half wave plate (λ/2 plate (λ: wavelength)) whose rotation can be adjusted to adjust the polarization direction of the light for interference needs to be arranged in an optical path where the interference occurs such that the light is transmitted not in both directions but only in one direction. Furthermore, as another problem, a method of adjusting a path difference between the two lights is not specifically described therein, and the techniques are difficult to put into practical use. As a solution to the problem, it is described in JP Patent Publication (Kokai) No. 06-223433A (1994) that a reference mirror for obtaining the light for interference is arranged on the disk at a position spaced apart from a recording film. However, this solution is to propose a new disk standard, and not to improve the S/N ratio in the existing disk. 
         [0010]    The present invention has been made in view of the aforementioned problems, and it is an object of the present invention to provide an interference-type optical pickup, an optical information detection method, and an optical information recording and reproducing apparatus, which allow easy adjustment of a path difference between two lights, which have a high signal amplification effect, and which are suitable for size reduction of an optical system. 
         [0011]    In the present invention, a light beam emitted from a light source is divided into a first light beam and a second light beam. The first light beam is focused onto an optical information recording medium. A signal light reflected from the optical information recording medium is guided to four detectors. The second light beam is guided to the four detectors as a reference light without focusing the second light beam onto the optical information recording medium. The signal light and the reference light are caused to optically interfere with each other on the four detectors such that phase relationships between the signal light and the reference light are different from each other. Outputs from the four detectors are calculated, thereby obtaining a readout signal. 
         [0012]    To be more specific, the phase relationships between the signal light and the reference light are different from each other by 180 degrees on the first and second detectors, by 180 degrees on the third and fourth detectors, and by 90 degrees on the first and third detectors. Accordingly, out of a phase relationship of 360 degrees, four phase states shifted from each other by 90 degrees can be detected at the same time. Since the readout signal sinusoidally varies in accordance with a change of 360 degrees in the light phase state, a signal state in any given phase state can be reproduced based on an operation by observing four signals in phase states shifted from each other by 90 degrees. That is, stable reproduction and detection in any given phase state can be achieved. 
         [0013]    As the above operation, the square of a differential signal between the first detector and the second detector is added to the square of a differential signal between the third detector and the fourth detector. The set of the first and second detectors and the set of the third and fourth detectors are out of phase with each other by 90 degrees. Thus, when the differential output of the former is a sine, the differential output of the latter is a cosine. Accordingly, by calculating the square sum of the both signals, a constant maximum output signal can be obtained at all times. 
         [0014]    As another operation method, the readout signal is obtained by adding a square-root operation to the above operation. By performing an operation as described above, a readout signal that is proportional to the optical output of the light source can be obtained. 
         [0015]    As another operation method, quadrant photo detectors are employed as the first, second, third and fourth detectors. The square of a differential signal between each area of the first detector and each area of the second detector is added to the square of a differential signal between each area of the third detector and each area of the fourth detector. By calculating the square sum of the two differential signals in each of the areas, a constant output signal can be obtained at all times. The output signals respectively obtained from the areas are further added, thereby obtaining an RF signal. Furthermore, a differential operation is performed on the output signals respectively obtained from the areas by employing an astigmatic method in the detection of a focus error signal by adding astigmatism to a combined light obtained by combining the signal light and the reference light, and by employing a push-pull method in the detection of a tracking error signal. A servo signal such as a focus error signal and a tracking error signal can be thereby obtained. 
         [0016]    The plurality of photo detectors as detection means may be formed on the same substrate. The optical system is thereby prevented from increasing in size. Accordingly, signal amplification can be stably performed, and the optical system can be formed into a small size. 
         [0017]    Means for detecting the focus error of the signal light on the optical information recording medium as a signal is further provided. Based on the focus error signal, means for focusing the signal light is controlled to compensate for the focus error, and a path difference between the signal light and the reference light is adjusted to be within the coherence length of the light source by providing means for adjusting the optical path length of the reference light in the optical path of the reference light. Accordingly, even when an objective lens is driven in an optical axis direction by focus control and the optical path length from the means for dividing the light into the signal light and the reference light to the optical information recording medium is thereby changed to be longer than the coherence length of the light source, a signal amplification effect can be maintained by maintaining coherence between the signal light and the reference light. 
         [0018]    With the present invention, an interference-type optical pickup, an optical information detection method, and an optical information recording and reproducing apparatus, which allow easy adjustment of a path difference between two lights, which have a high signal amplification effect, and which are suitable for size reduction of an optical system, can be provided. Accordingly, in a case where the reflectance of each layer needs to be kept small such as a multilayer optical disk, or in a case where a reproduction speed is fast and noise is thus increased relative to the signal, the readout signal can be improved in quality by signal amplification. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic view illustrating a configuration example of an optical information recording and reproducing apparatus according to the present invention. 
           [0020]      FIG. 2  is a view illustrating a configuration example of an optical pickup which achieves an optical information detection method according to the present invention. 
           [0021]      FIG. 3  is a view illustrating the polarization directions of a signal light and a reference light and the polarization direction of a detected light. 
           [0022]      FIG. 4  is a schematic view illustrating a detector in an optical pickup according to the present invention. 
           [0023]      FIG. 5A  is a block diagram illustrating a configuration example of a signal operation circuit according to the present invention. 
           [0024]      FIG. 5B  is a block diagram illustrating a configuration example of a signal operation circuit according to the present invention. 
           [0025]      FIG. 6A  is a block diagram illustrating a configuration example of a signal operation circuit according to the present invention. 
           [0026]      FIG. 6B  is a block diagram illustrating a configuration example of a signal operation circuit according to the present invention. 
           [0027]      FIG. 7  is a view illustrating a configuration example of an optical pickup which achieves an optical information detection method according to the present invention. 
           [0028]      FIG. 8  is a block diagram illustrating a configuration example of a signal operation circuit according to the present invention. 
           [0029]      FIG. 9  is a view illustrating a configuration example of an optical pickup which achieves an optical information detection method according to the present invention. 
           [0030]      FIG. 10  is a view illustrating a polarization convertor and retarder according to the present invention. 
           [0031]      FIG. 11  is a view illustrating a polarization convertor and retarder according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    In the following, embodiments of the present invention will be described with reference to the drawings. 
       [Entire Configuration of Optical Information Recording and Reproducing Apparatus] 
       [0033]      FIG. 1  illustrates an example of the entire configuration of an optical information recording and reproducing apparatus for achieving an optical signal detection method according to the present invention. 
         [0034]    An optical information recording and reproducing apparatus  1  includes an optical pickup  101  and a rotary motor  102 . An optical information recording medium  103  is fixed to a rotary shaft of the rotary motor  102  and is thereby rotatably provided. The optical pickup  101  functions to record and/or reproduce digital information by projecting a light onto the optical information recording medium  103 . A reproduction light detected by the optical pickup  101  is current/voltage (IV) converted, and input to a signal processing circuit  105 . A readout signal and a servo signal are produced by the signal processing circuit  105 , and transmitted to a controller  104 . 
         [0035]    The controller  104  controls a servo control circuit  106 , an access control circuit  107 , and a position control system  108  based on the servo signal. The position control system  108  controls the rotation of the optical information recording medium  103  by the rotary motor  102 . The access control circuit  107  controls the position of the optical pickup  101 . The servo control circuit  106  controls the positions of an objective lens and reference light reflecting means of the optical pickup  101  described below. Accordingly, an optical spot  110  is positioned at any given position of the optical information recording medium  103 . The controller  104  controls a laser driver  109  depending on whether reproduction or recording is performed, thereby allowing a laser included in the optical pickup  101  described below to emit a light of appropriate power/waveform. 
       [Configuration (1) of Optical System of Optical Pickup] 
       [0036]      FIG. 2  illustrates a configuration example of the optical system of the optical pickup  101  in the optical information recording and reproducing apparatus  1 . 
         [0037]    A light emitted from a semiconductor laser  201  that is mounted on the optical pickup  101  is collimated into a collimated light by a collimator lens  202 , and transmitted through a first half wave plate  203 . The polarization direction thereof is thereby rotated by 45 degrees. The light whose polarization is rotated is split into two linearly polarized lights perpendicular to each other by a first polarizing beam splitter  204 . One of the polarized lights (the reproduction light) is reflected, and transmitted through a first quarter wave plate  207  to be converted into a circularly polarized light. The circularly polarized light is collected by an objective lens  208 , and focused onto an optical disk  209 . A reflected light  205  (referred to as signal light below) from the optical disk  209  is converted back into a collimated light by the objective lens  208 , and the collimated light is converted back into a linearly polarized light by the first quarter wave plate  207 . Since the rotational direction of the circularly polarized light is inverted by the reflection at the disk surface, the direction of the linearly polarized light is perpendicular to that of the original light. Thus, the signal light  205  is transmitted through the first polarizing beam splitter  204 , and travels toward a beam splitter  212 . On the other hand, a light  206  (referred to as reference light below) in a polarization direction transmitted through the first polarizing beam splitter  204  first is transmitted through a second quarter wave plate  210  to be converted into a circularly polarized light. The circularly polarized light is reflected by reference light reflecting means  211 , and converted into a linearly polarized light that is perpendicular to the original reference light by the second quarter wave plate  210  in a similar manner to the signal light  205 . Thus, the linearly polarized light is then reflected by the first polarizing beam splitter  204 , and combined with the signal light  205  to travel toward the beam splitter  212 . The signal light  205  and the reference light  206  are combined with the polarization directions being perpendicular to each other. 
         [0038]    The combined light is partially reflected by a servo beam splitter  213 , and guided to a photo detector  216  with astigmatism being imparted thereto by a collective lens  214  and a cylindrical lens  215 . A servo signal processing circuit  217  outputs a focus error signal (FES) and a tracking error signal (TES) from the output signal of the photo detector  216 . The focus error signal is fed back to a focus drive terminal of a two-dimensional actuator  228  on which the objective lens  208  is mounted, so that a focus position is closed-loop controlled. The same signal is further fed back to a one-dimensional actuator  229  on which the reference light reflecting means  211  is mounted, so that the reference light reflecting means  211  is driven in conjunction with the objective lens  208 . Accordingly, a path difference between the signal light  205  reflected from the optical disk  209  and the reference light  206  reflected from the reference light reflecting means  211  can be maintained at substantially 0. Since the coherence length of a normal semiconductor laser is several tens μm, the adjustment accuracy of the path difference is only required to be within the range. The tracking error signal is fed back to a tracking drive terminal of the two-dimensional actuator  228  on which the objective lens  208  is mounted, so that closed-loop control is performed. 
         [0039]    One of the combined lights transmitted through the servo beam splitter  213  is transmitted through the beam splitter  212  that is a half mirror. The polarization direction thereof is rotated by 45 degrees by a second half wave plate  218 . The light is collected by a lens  220 , and split into linearly polarized lights perpendicular to each other by a polarizing beam splitter  222 . The linearly polarized lights are detected by a first photo detector  224  (PD 1 ) and a second photo detector  225  (PD 2 ).  FIG. 3  illustrates the relationship among polarization components P and S of the lights detected by the two photo detectors PD 1  and PD 2 , the polarization direction (E sig ) of the signal light, and the polarization direction (E ref ) of the reference light. The photo detector PD 1  detects the P-polarized light, that is, the projection component of E sig  and E ref  in a P polarization direction. The photo detector PD 2  detects the S-polarized light, that is, the projection component of E sig  and E ref  in an S polarization direction. In the projection component in the S polarization direction, the sign of E ref  appears inverted in  FIG. 3 . Signals detected by the photo detectors PD 1  and PD 2  are respectively expressed by the following expressions. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           I 
                           
                             PD 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                         = 
                           
                          
                         
                           
                              
                             
                               
                                 
                                   1 
                                   2 
                                 
                                  
                                 
                                   E 
                                   sig 
                                 
                               
                               + 
                               
                                 
                                   1 
                                   2 
                                 
                                  
                                 
                                   E 
                                   ref 
                                 
                               
                             
                              
                           
                           2 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               1 
                               4 
                             
                              
                             
                               
                                  
                                 
                                   E 
                                   sig 
                                 
                                  
                               
                               2 
                             
                           
                           + 
                           
                             
                               1 
                               4 
                             
                              
                             
                               
                                  
                                 
                                   E 
                                   ref 
                                 
                                  
                               
                               2 
                             
                           
                           + 
                           
                             
                               1 
                               2 
                             
                              
                             
                                
                               
                                 E 
                                 sig 
                               
                                
                             
                              
                             
                                
                               
                                 E 
                                 ref 
                               
                                
                             
                              
                             
                               cos 
                                
                               
                                 ( 
                                 
                                   
                                     ϕ 
                                     sig 
                                   
                                   - 
                                   
                                     ϕ 
                                     ref 
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           I 
                           
                             PD 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                         = 
                           
                          
                         
                           
                              
                             
                               
                                 
                                   1 
                                   2 
                                 
                                  
                                 
                                   E 
                                   sig 
                                 
                               
                               - 
                               
                                 
                                   1 
                                   2 
                                 
                                  
                                 
                                   E 
                                   ref 
                                 
                               
                             
                              
                           
                           2 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               1 
                               4 
                             
                              
                             
                               
                                  
                                 
                                   E 
                                   sig 
                                 
                                  
                               
                               2 
                             
                           
                           + 
                           
                             
                               1 
                               4 
                             
                              
                             
                               
                                  
                                 
                                   E 
                                   ref 
                                 
                                  
                               
                               2 
                             
                           
                           - 
                           
                             
                               1 
                               2 
                             
                              
                             
                                
                               
                                 E 
                                 sig 
                               
                                
                             
                              
                             
                                
                               
                                 E 
                                 ref 
                               
                                
                             
                              
                             
                               cos 
                                
                               
                                 ( 
                                 
                                   
                                     ϕ 
                                     sig 
                                   
                                   - 
                                   
                                     ϕ 
                                     ref 
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0040]    In the above expressions, the absolute value is squared since the detected signal is light energy. It is assumed that E sig  and E ref  are completely coherent for the sake of simplicity. 
         [0041]    The other of the combined lights transmitted through the servo beam splitter  213  is reflected by the beam splitter  212  that is a half mirror, and converted into a circularly polarized light by a third quarter wave plate  219  that is arranged by being rotated at 45 degrees relative to the polarization directions of the signal light  205  and the reference light  206 . Since the original polarization directions of the signal light  205  and the reference light  206  are different from each other by 90 degrees, the light is converted into a circularly polarized light in an opposite rotational direction. The circularly polarized light is collected by a lens  221 , and split into linearly polarized lights perpendicular to each other by a polarizing beam splitter  223 . The linearly polarized lights are detected by a third photo detector  226  (PD 3 ) and a fourth photo detector  227  (PD 4 ).  FIG. 3  similarly illustrates the relationship among polarization components P and S of the lights detected by the two photo detectors PD 3  and PD 4 , the polarization direction (E sig ) of the signal light, and the polarization direction (E ref ) of the reference light. The relationship is different from that of the PD 1  and PD 2  in that E sig  and E ref  have a phase difference of 90 degrees. Signals detected by the photo detectors PD 3  and PD 4  are respectively expressed by the following expressions. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           I 
                           
                             PD 
                              
                             
                                 
                             
                              
                             3 
                           
                         
                         = 
                           
                          
                         
                           
                              
                             
                               
                                 
                                   1 
                                   2 
                                 
                                  
                                 
                                   exp 
                                    
                                   
                                     ( 
                                     
                                       
                                         - 
                                          
                                       
                                        
                                       
                                         π 
                                         4 
                                       
                                     
                                     ) 
                                   
                                 
                                  
                                 
                                   E 
                                   sig 
                                 
                               
                               + 
                               
                                 
                                   1 
                                   2 
                                 
                                  
                                 
                                   exp 
                                    
                                   
                                     ( 
                                     
                                       
                                         + 
                                          
                                       
                                        
                                       
                                         π 
                                         4 
                                       
                                     
                                     ) 
                                   
                                 
                                  
                                 
                                   E 
                                   ref 
                                 
                               
                             
                              
                           
                           2 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               1 
                               4 
                             
                              
                             
                               
                                  
                                 
                                   E 
                                   sig 
                                 
                                  
                               
                               2 
                             
                           
                           + 
                           
                             
                               1 
                               4 
                             
                              
                             
                               
                                  
                                 
                                   E 
                                   ref 
                                 
                                  
                               
                               2 
                             
                           
                           + 
                           
                             
                               1 
                               2 
                             
                              
                             
                                
                               
                                 E 
                                 sig 
                               
                                
                             
                              
                             
                                
                               
                                 E 
                                 ref 
                               
                                
                             
                              
                             
                               sin 
                                
                               
                                 ( 
                                 
                                   
                                     ϕ 
                                     sig 
                                   
                                   - 
                                   
                                     ϕ 
                                     ref 
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           I 
                           
                             PD 
                              
                             
                                 
                             
                              
                             4 
                           
                         
                         = 
                           
                          
                         
                           
                              
                             
                               
                                 
                                   1 
                                   2 
                                 
                                  
                                 
                                   exp 
                                    
                                   
                                     ( 
                                     
                                       
                                         + 
                                          
                                       
                                        
                                       
                                         π 
                                         4 
                                       
                                     
                                     ) 
                                   
                                 
                                  
                                 
                                   E 
                                   sig 
                                 
                               
                               + 
                               
                                 
                                   1 
                                   2 
                                 
                                  
                                 
                                   exp 
                                    
                                   
                                     ( 
                                     
                                       
                                         - 
                                          
                                       
                                        
                                       
                                         π 
                                         4 
                                       
                                     
                                     ) 
                                   
                                 
                                  
                                 
                                   E 
                                   ref 
                                 
                               
                             
                              
                           
                           2 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               1 
                               4 
                             
                              
                             
                               
                                  
                                 
                                   E 
                                   sig 
                                 
                                  
                               
                               2 
                             
                           
                           + 
                           
                             
                               1 
                               4 
                             
                              
                             
                               
                                  
                                 
                                   E 
                                   ref 
                                 
                                  
                               
                               2 
                             
                           
                           - 
                           
                             
                               1 
                               2 
                             
                              
                             
                                
                               
                                 E 
                                 sig 
                               
                                
                             
                              
                             
                                
                               
                                 E 
                                 ref 
                               
                                
                             
                              
                             
                               sin 
                                
                               
                                 ( 
                                 
                                   
                                     ϕ 
                                     sig 
                                   
                                   - 
                                   
                                     ϕ 
                                     ref 
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0042]    In the expressions, exp(±iπ/4) represents that E sig  and E ref  have a phase difference of ±45 degrees (a difference of 90 degrees) at the quarter wave plate. 
         [0043]    Thus, a component |E ref | 2  that is irrelevant to the information on the optical disk  209  is included in the signals detected by the respective detectors. Therefore, differential signals between the PD 1  and the PD 2 , and between the PD 3  and the PD 4  are respectively obtained as described below. 
         [0000]      Sig1 =I   PD1   −I   PD2   =|E   sig   ∥E   ref |cos(φ sig −φ ref )   (5) 
         [0000]      Sig2 =I   PD3   −I   PD4   =|E   sig   ∥E   ref |sin(φ sig −φ ref )   (6) 
         [0044]    Signals, each of which is a product of the signal light amplitude intensity and the reference light amplitude intensity, are thereby obtained, which means that when the intensity of the reference light is increased, a larger signal output can be obtained. That is, the intensity of the signal light can be amplified. 
         [0045]    The expressions (5) and (6) are accompanied by sin and cos, which represent a phase difference between the signal light and the reference light. However, since the reference light and the signal light pass through separate optical paths and the objective lens  208  tracks the disk by moving up and down by a focus servo in conjunction with the rotation of the disk, the optical path length of the signal light continuously changes. Thus, the phase terms of the expressions (5) and (6) are not fixed, and the signals obtained in this method greatly fluctuate. 
         [0046]    Accordingly, the present invention focuses on that the expression (5) is accompanied by cos and the expression (6) is accompanied by sin, a signal is obtained by calculating the square sum of the both signals by a signal operation circuit  230 . 
         [0000]        S =(Sig1) 2 +(Sig2) 2   =|E   sig | 2   |E   ref | 2    (7) 
         [0047]    By performing an operation as described above, a constant signal can be stably and reliably obtained even when the signal light and the reference light are changed in phase. By calculating the square sum as shown in the expression (7), the signal proportional to the signal light intensity |E sig | 2  is obtained as an output S, and thus, the same signal waveform as those of conventional CD, DVD and BD is obtained in an RF signal. The amplification factor thereof is |E ref | 2 , which means that the amplification factor can be increased by increasing the reference light intensity. An RF signal may be also obtained by calculating a square root after obtaining the square sum. When the square root is calculated, the output is proportional to the square root of the signal light intensity, and thus, the RF signal has the same signal waveform as that of a conventional magneto-optical disk. 
         [0048]    Although it is described above that the photo detectors PD 1 , PD 2 , PD 3  and PD 4  output the total light intensities of the received lights, quadrant photo detectors each having four areas A, B, C and D as shown in  FIG. 4  may be employed as the four photo detectors PD 1 , PD 2 , PD 3  and PD 4 , to separately output the detected light intensities as I PD1 (X), I PD2 (X), I PD3 (X), and I PD4 (X) as another operation. Note that X denotes one of the areas A, B, C and D. Differential signals are calculated from signals detected in the areas X (X denotes one of the areas A, B, C and D) of the PD 1  and the PD 2 , and of the PD 3  and the PD 4  as shown in the following expressions. 
         [0000]      Sig1′( X )= I   PD1 ( X )− I   PD2 ( X )=| E   sig ( X )∥ E   ref ( X )|cos(φ sig −φ ref )   (8) 
         [0000]      Sig2′( X )= I   PD3 ( X )− I   PD4 ( X )=| E   sig ( X )∥ E   ref ( X )|sin((φ sig −φ ref )   (9) 
         [0049]    Signals, each of which is a product of the signal light amplitude intensity and the reference light amplitude intensity, are thereby obtained. A signal is further obtained by calculating the square sum of the both signals by the signal operation circuit  230 . 
         [0000]        S ′( X )=(Sig1( X )) 2 +(Sig2( X )) 2   =|E   sig ( X )| 2   |E   ref ( X ) 2    (10) 
         [0050]    By performing an operation as described above, a constant signal can be stably and reliably obtained even when the signal light and the reference light are changed in phase in a similar manner to the expression (7). 
         [0051]      FIG. 5A  is a circuit block diagram illustrating a specific configuration example of the signal operation circuit  230 . Sig  1  and Sig  2  are digitalized by AD convertors  501  and  502 , squared by squaring circuits  503  and  504 , and added by an accumulator  505 . A digital signal S is thereby obtained. Alternatively, as shown in  FIG. 5B , the digital signal S may be obtained by performing a square-root operation by a square-root operation circuit  506  after the addition in the accumulator  505 . 
         [0052]      FIG. 6A  is a circuit block diagram illustrating another specific configuration example of the signal operation circuit  230  when the four quadrant photo detectors are used in the optical pickup  101 . Sig 1 ′(X) and Sig 2 ′(X) are respectively digitalized by AD convertors  601 , squared by squaring circuits  602 , and added by an accumulator  603 . A digital signal S′(X) is thereby obtained. Note that X denotes one of the areas A, B, C and D. Furthermore, the RF signal is obtained by the following operation by an accumulator  604 . 
         [0000]        RF=S ′( A )+ S ′( B )+ S ′( C )+ S ′( D )   (11) 
         [0053]    Alternatively, as shown in  FIG. 6B , the RF signal may be obtained by performing a square-root operation by a square-root operation circuit  605  after the operation in the expression (11). 
         [0054]    The reference light reflecting means  211  is realized by a reflection mirror, for example. In this case, it is necessary to adjust the tilt of the mirror such that the reflected light by the reference light reflecting means  211  is not inclined relative to the optical axis of the incident reference light  206 . To this end, the reference light reflecting means  211  may include a mechanism for adjusting the tilt of the wavefront of the reference light  206  by detecting the tilts of the reflection mirror and the wavefront of the reference light  206  and feeding the tilts back to the reference light reflecting means  211 . The reference light reflecting means  211  may be also realized by a collective lens and a reflection mirror. The reference light can be reflected in a direction opposite to the incident light by collecting the reference light  206  by the collective lens and placing the reflection mirror at a focus position thereof In this case, even when the reflection mirror is inclined, only the optical axis of the reference light is misaligned with respect to the signal light. Thus, the angle of the reflection mirror is adjusted more easily than in the case where the reference light reflecting means  211  is realized only by the reflection mirror. Alternatively, the reference light reflecting means  211  may be realized by a corner cube reflector. Since the corner cube reflector is an element which always returns the reflected light in the same direction as that of the incident light even when the light is incident at any incident angle, it is not necessary to adjust the tilt of the wavefront. 
         [0055]    The servo control circuit  106  in  FIG. 1  controls the focus of the objective lens  208  by the two-dimensional actuator  228  based on the servo signal, and at the same time, controls the position of the reference light reflecting means  211  via the one-dimensional actuator  229  in accordance with the change in the optical path length of the signal light along with the movement of the objective lens  208 , so that a difference in the optical path length between the reference light and the signal light is always 20 μm or less. The length of 20 μm is sufficiently smaller than a coherence length of 70 μm of the semiconductor laser  201  used in the present embodiment. Therefore, the reference light and the signal light are always maintained at a substantially completely coherent state. 
         [0056]      FIG. 2  is a view illustrating a light beam at the time of reproducing the information recorded on the optical disk  209 , wherein the light from the semiconductor laser  201  is split into two light beams of the signal light  205  and the reference light  206 . Since the reference light is unnecessary in recording, the light transmitted through the first half wave plate  203  may be converted into an S-polarized light, and all the S-polarized light may be reflected by the first polarizing beam splitter  204  to be used as the signal light. This is realized by changing the intensity ratio of the signal light and the reference light by rotating the first half wave plate at the time of recording and reproducing, for example. Alternatively, a liquid crystal wave plate in which the polarization direction of the incident light is switched when a voltage is applied may be used, or a method of inserting and removing the half wave plate may be also used. 
       [Configuration (2) of Optical System of Optical Pickup] 
       [0057]      FIG. 7  illustrates another configuration example of the optical system of the optical pickup  101  in the optical information recording and reproducing apparatus  1 . In the optical pickup  101  shown in  FIG. 2 , the RF signal and the servo signal (the focus error signal and the tracking error signal) are detected by the separate detection systems. However, the RF signal and the servo signal may be detected by the same detection system as shown in  FIG. 7 . The quadrant photo detectors as shown in  FIG. 4  are employed as photo detectors  701 ,  702 ,  703  and  704 . Astigmatism is imparted to the signal light by cylindrical lenses  705  and  706 . The signal light is then guided to the photo detectors  701 ,  702 ,  703  and  704 . A signal operation circuit  707  outputs an RF signal and a servo signal from the output signals thereof. An astigmatic method is used to detect the focus error signal and a push-pull method is used to detect the tracking error signal. 
         [0058]    A differential push-pull method may be also used to detect the tracking error signal by placing a grating in the optical path of the signal light  205 . In the differential push-pull method, the light incident on the optical disk  209  is divided into three beams by the grating. When a main spot on the disk is positioned on an information track, the rotation of the grating is adjusted such that two sub spots are positioned between the adjacent tracks. The reference light is also divided into three beams. The three beams are respectively caused to interfere with the corresponding beams of the signal light, thereby amplifying the tracking error signal by a differential operation. 
         [0059]      FIG. 8  is a circuit block diagram illustrating a specific example of the signal operation circuit  707 . Sig 1 ′(X) and Sig 2 ′(X) are respectively digitalized by AD convertors  801 , squared by squaring circuits  802 , and added by an accumulator  803 . A digital signal S′(X) is thereby obtained. Note that X denotes one of the areas A, B, C and D. Furthermore, the RF signal and the servo signal (the focus error signal and the tracking error signal) can be obtained by the following operation by an accumulator  804  and subtracters  805  and  806 . 
         [0000]        RF=S ′( A )+ S ′( B )+ S ′( C )+ S ′( D ) 
         [0000]        FES=S ′( A )+ S ′( C )− S ′( B )− S ′( D ) 
         [0000]        TES=S ′( A )+ S ′( B )− S ′( C )− S ′( D )   (12) 
         [0060]    Alternatively, the RF signal may be obtained by performing a square-root operation by a square-root operation circuit after the addition in the accumulator  804 . With the configuration, in a case of using a multilayer disk, the signal of a light from a layer to be detected can be selectively amplified against signal leakage from the adjacent layers (different layers from the layer to be detected, which are displaced in an optical axis direction from the light collecting point of the objective lens). Crosstalk can be thereby more effectively reduced. 
       [Configuration (3) of Optical Pickup Optical System] 
       [0061]      FIG. 9  illustrates another configuration example of the optical system of the optical pickup  101  in the optical information recording and reproducing apparatus  1 . In the optical pickup shown in  FIGS. 2 and 7 , the signals shown in the expressions (1), (2), (3) and (4) are detected by the separate photo detectors. In a present embodiment, however, the lights are received by a photo detector  904  in which the photo detectors are packaged into one detector, and a signal operation circuit  905  performs a signal operation. The signal light reflected by the optical disk  209  and the reference light reflected by the reference light reflecting means  211  enter a polarization convertor and retarder  901 , and divided into four lights having different phase differences due to the interference between the two lights. The four lights are respectively detected by four photo receiving sections provided on the quadrant photo detector  904  by a collective lens  902  and a cylindrical lens  903 . 
         [0062]      FIG. 10  is a view illustrating the configuration and function of the polarization convertor and retarder  901 . The polarization convertor and retarder  901  includes a non polarizing grating  1003 , a retarder which has angle selectivity  1004  which converts a polarization direction, and a polarization splitting grating  1005 . Although shown in an integrated manner in  FIG. 9 , the elements  1003  to  1005  are separately shown in  FIG. 10  for the convenience of description. There is no difference in the function whether the elements  1003  to  1005  are integrated or separated. 
         [0063]    When the signal light and the reference light enter the non polarizing grating  1003  such that a signal light polarization direction  1001  and a reference light polarization direction  1002  are perpendicular to each other, the two lights are respectively split into two lights in different traveling directions regardless of the polarization directions. This is easily achieved by brazing the non polarizing grating  1003 . One of the lights is a zero-order light which travels in a straight line, and the other of the lights is a first-order diffracted light which is diffracted at a predetermined diffraction angle. Subsequently, the lights enter the retarder which has angle selectivity  1004 . Here, no phase difference occurs in the zero-order light which travels in a straight line. In the first-order diffracted light which is incident at a tilt, however, there occurs a phase difference of 90 degrees between the signal light and the reference light. To this end, an optical axis  1006  needs to have uniaxial anisotropy perpendicular to the incident surface of the retarder which has angle selectivity, and the diffracting direction of the diffracted light of the non polarizing grating needs to correspond to the signal light polarization direction  1001  or the reference light polarization direction  1002 . The phase difference between the signal light component and the reference light component of the first-order diffracted light can be uniquely determined based on the thickness of the retarder which has angle selectivity  1004 , the refractive index anisotropy (a difference between a vertical refractive index and an in-plane refractive index), and the incident angle of the first-order diffracted light. 
         [0064]    The emission lights from the retarder which has angle selectivity  1004  further enter the polarization splitting grating  1005 . For example, an element disclosed in JP Patent No. 3832243 can be used as the polarization splitting grating. This is easily achieved by forming a blazed grating using an anisotropic material such as liquid crystal, lithium niobate, and crystal. That is, since the material has a different refractive index depending on the polarization direction, the polarization splitting grating may be arranged such that a phase distribution applied by the grating is inverted in accordance with a given polarization direction and a polarization direction perpendicular thereto. Accordingly, the first-order diffracted light and a minus first-order diffracted light can have polarization directions perpendicular to each other. Alternatively, an element formed by cementing anisotropic optical crystals such as a Wollaston prism may be used instead. The directions of polarization to be split in the present embodiment are a direction of 45 degrees relative to the signal light and the reference light and a direction perpendicular thereto. The phase difference in the interference between the signal light component and the reference light component in the four lights split as described above can be set to 0 degree, 90 degrees, 180 degrees and 270 degrees as shown in  FIG. 10 . 
         [0065]      FIG. 11  is a view illustrating a different embodiment from the retarder which has angle selectivity shown in  FIG. 10 . A polarizing grating  1101  is provided instead of the non polarizing grating  1003  in  FIG. 10 . Furthermore, a second polarizing grating  1102  is provided instead of the retarder which has angle selectivity  1004 . Optical axis directions  1103  and  1104  thereof are arranged perpendicular to each other as shown in  FIG. 11 . 
         [0066]    The subsequent polarizing grating  1005  is arranged in the same manner as that shown in  FIG. 10 . In such a manner, a light having a linear polarization component along the optical axis  1103  is partially diffracted by the first polarizing grating  1101 . A light having a linear polarization component along the optical axis  1104  is partially diffracted by the second polarizing grating  1102 . Accordingly, the lights diffracted by the first and second polarizing gratings have polarization directions perpendicular to each other. Furthermore, the phase of the grating arrangement is shifted only by one fourth (90 degrees) of a grating period P as shown in  FIG. 11 . In such a manner, the phases of the diffracted lights are also shifted from each other by 90 degrees, so that a polarization state obtained by combining the two diffracted lights is a circular polarization state. Only one diffracted light is shown here, and this is easily realized by brazing the grating such as a step grating and a brazed grating. Moreover, by diffracting the lights having the polarization components perpendicular to each other by the first and second polarizing gratings at an equal light intensity ratio, the polarization state of the light which is not diffracted can be maintained in the same polarization state as that of the light which is incident first. Although the elements are separately shown in  FIG. 11  for the convenience of description, the elements can be cemented together and thereby integrated in an actual optical system. 
         [0067]    In the configuration, although the non polarizing grating is replaced with the polarizing grating, the retarder which has angle selectivity made of a relatively expensive anisotropic optical crystal is replaced with the polarizing grating which can be easily formed by solidification of liquid crystal or the like, and thus, the cost is relatively reduced in comparison with the configuration shown in  FIG. 10 . In order to generate the phase difference of 90 degrees at the retarder which has angle selectivity, it is necessary to increase the light beam incident angle, or the element thickness. In the present embodiment, however, the phase difference between the circularly polarized lights can be achieved by any given grating pitch. The element can be thereby effectively reduced in size. 
         [0068]    In the aforementioned embodiments, the example in which the light from a single light source is divided is described. However, first and second light beams emitted from two light sources having longer coherence lengths than a distance that a light travels in a vacuum within a time period corresponding to the data acquisition period of the readout signal may be also used instead of using the divided lights from the single light source. In this case, the wavelengths of the two light sources need to substantially match each other. Since the light interference state is almost constant during the data acquisition of the readout signal, the aforementioned effects of the present invention can be similarly obtained. 
         [0069]    The optical system in the present invention is not limited to the ones shown in  FIGS. 2 ,  7  and  9 . For example, as an optical element for obtaining signal outputs in four phase difference states different from each other by 90 degrees, a composite optical functional element such as a polarization control prism, a polarizing diffractive optical element, and a nanophotonic material may be employed in addition to the optical system using the half wave plates  203  and  218 , and the quarter wave plate  219 . In any case, by forming an optical system capable of obtaining signal outputs in at least four states in which the phase difference states between the signal light and the reference light are different from each other and performing a signal operation/selection, the effects of the present invention can be obtained. 
         [0070]    The effects of the present invention are not limited to the optical pickup for amplifying the readout signal of the optical disk. For example, the effects of the present invention can be also obtained when the light intensity of the signal light is amplified by causing the signal light to interfere with another light (the reference light) by use of a homodyne detection method such as signal amplification of a carrier wave in optical communication and amplification of a detected signal in optical measurement. 
       DESCRIPTION OF SYMBOLS 
       [0071]      1  Optical information recording and reproducing apparatus 
         [0072]      101  Optical pickup 
         [0073]      102  Rotary motor 
         [0074]      103  Optical information recording medium 
         [0075]      104  Controller 
         [0076]      105  Signal processing circuit 
         [0077]      106  Servo control circuit 
         [0078]      107  Access control circuit 
         [0079]      108  Position control system 
         [0080]      109  Laser driver 
         [0081]      110  Optical spot 
         [0082]      201  Semiconductor laser 
         [0083]      202  Collimator lens 
         [0084]      203  Half wave plate 
         [0085]      204  Polarizing beam splitter 
         [0086]      205  Signal light 
         [0087]      206  Reference light 
         [0088]      207  Quarter wave plate 
         [0089]      208  Objective lens 
         [0090]      209  Optical disk 
         [0091]      210  Quarter wave plate 
         [0092]      211  Reference light reflecting means 
         [0093]      212  Beam splitter 
         [0094]      213  Servo beam splitter 
         [0095]      214  Collective lens 
         [0096]      215  Cylindrical lens 
         [0097]      216  Photo detector 
         [0098]      217  Servo circuit 
         [0099]      218  Half wave plate 
         [0100]      219  Quarter wave plate 
         [0101]      220 ,  221  Lens 
         [0102]      222 ,  223  Polarizing beam splitter 
         [0103]      224  First photo detector 
         [0104]      225  Second photo detector 
         [0105]      226  Third photo detector 
         [0106]      227  Fourth photo detector 
         [0107]      228  Two-dimensional actuator 
         [0108]      229  One-dimensional actuator 
         [0109]      230  Signal operation circuit 
         [0110]      501 ,  502  AD convertor 
         [0111]      503 ,  504  Squaring circuit 
         [0112]      505  Accumulator 
         [0113]      506  Square-root operation circuit 
         [0114]      601  AD convertor 
         [0115]      602  Squaring circuit 
         [0116]      603 ,  604  Accumulator 
         [0117]      605  Square-root operation circuit 
         [0118]      701 ,  702 ,  703 ,  704  Photo detector 
         [0119]      705 ,  706  Cylindrical lens 
         [0120]      707  Signal operation circuit 
         [0121]      801  AD convertor 
         [0122]      802  Squaring circuit 
         [0123]      803 ,  804  Accumulator 
         [0124]      805 ,  806  Subtracter 
         [0125]      901  Polarization convertor and retarder 
         [0126]      902  Collective lens 
         [0127]      903  Cylindrical lens 
         [0128]      904  Photo detector 
         [0129]      905  Signal operation circuit 
         [0130]      1001  Signal light polarization direction 
         [0131]      1002  Reference light polarization direction 
         [0132]      1003  Non polarizing grating 
         [0133]      1004  Retarder which has angle selectivity 
         [0134]      1005  Polarization splitting grating 
         [0135]      1006 ,  1007  Optical axis 
         [0136]      1101 ,  1102  Polarizing grating 
         [0137]      1103 ,  1104  Optical axis