Abstract:
An interference type optical head and an optical disk device that can easily adjust an optical path length difference of a couple of lights, ensure higher signal amplification effect, and are suitable for reduction in size are provided in order to improve a regeneration signal quality with amplification of signal in the case where reflectivity of each layer must be lowered and relative noise for the signal increases because read speed is high in a multilayer optical disk. In view of essentially improving an S/N ratio of the regeneration signal in high-speed rotation of a multilayer disk, a plurality of interference phases are generated and an optical system for differential calculation has been reduced in size with an angular selective polarization conversion element in the optical disk device for amplifying the signal with interference of the light not radiated to the disk with the reflected light from the disk. Moreover, adjustment of reflection mirror angle of the reference light is no longer required by using a corner cube prism as the reference light reflection mirror and highly accurate signal detection can also be realized with a simplified structure.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese application JP 2007-043124 filed on Feb. 23, 2007, JP 2007-130002 filed on May 16, 2007 and JP 2007-250375 filed on September 27, the contents of which are hereby incorporated by reference into this application. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to an optical head corresponding to high S/N of a regeneration signal and an optical disk device mounting the relevant optical head. 
       BACKGROUND OF THE INVENTION 
       [0003]    An optical disk as an optical recording medium has almost reached the limit in its resolution of an optical system as a product of Blue-Ray system utilizing a blue-color laser diode and an objective lens corresponding to high NA and is thought to be required to introduce in near future a multilayer structure of recording layer of an optical disk. In such a multilayer optical disk, it is required to detect almost identical light power from each layer and therefore reflectivity of the irradiation light of a particular layer cannot help thinking of being lowered. However, the optical disk is further required to attain high-speed dubbing of video contents in combination with improvement in recording capacity. In addition, since high transfer rate of data is also still improved, it may be assumed, if the present situation is continued, that it will become impossible to acquire sufficient S/N ratio of the reproduction signal. Accordingly, realization of high S/N ratio of a detection signal is essentially required to simultaneously conduct realization in further improvement in multilayer structure and high-speed operation in future. 
         [0004]    A technology for high S/N ratio of regeneration signal of an optical disk is disclosed, for example, in the patent document 1 (JP-A No. 1993-342678) and the patent document 2 (JP-A No. 1994-223433) or the like. These patent documents relate to a high S/N ratio of regeneration signal of a magneto-optical disk, aiming at increasing a light power of the light not irradiating an optical disk in order to amplify amplitude of a very weak signal, by branching the light from a laser diode before radiation to an optical disk for multiplexing with a reflected light from the optical click. In differential detection between a transmitted light and a reflected light of a polarization beam splitter that has been used in the related art for signal detection of the magneto-optical disk, detection is substantially conducted through amplification of an orthogonally polarized element with incident polarization with interference of an original incident polarization element and a polarized element orthogonally crossing the incident polarization direction generated with polarization rotation due to the magneto-optical disk. Accordingly, a signal can be amplified by increasing the original incident polarization element, but an incident light intensity of optical disk must be controlled to intensity lower than a certain predetermined level in order to prevent that data is erased or overwritten. Meanwhile, in the related art explained above, a light to be interfered with a signal light is previously separated therefrom, and this signal is interfered with the signal light without condensation to the disk in view of intensifying the light to be interfered for signal amplification without relationship with light intensity at the front surface of disk. Therefore, in the principle, the more intensified light intensity is, the more the S/N ratio can be increased in comparison with noise of an amplifier for voltage conversion of a photo-electric current from an optical detector and shot noise of the optical detector. 
         [0005]    In the patent document 1 (JP-A No. 1993-3426778), a couple of lights are interfered with each other to detect interference intensity. In this case, an optical path length of non-reflected light of disk to be interfered is set variably, aiming at acquisition of interference signal amplitude. In the patent document 2 (JP-A No. 1994-223433), differential detection is also conducted in addition to detection of interference intensity. Therefore, high S/N ratio is obtained by cancelling intensity element of each light not making contribution to signal and also cancelling noise element of these lights. In this case, a non-polarizing beam splitter is used for differential detection. 
       SUMMARY OF THE INVENTION 
       [0006]    An optical system of an interferometer used in the related arts explained above is a Mach-Zender type optical system and this system is not suitable for reduction in size of the optical system because a large number of optical components are used. In the optical system of the Mach-Zender type interferometer, a unit for dividing a light into a signal light and a reference light if different from a unit for multiplexing the signal and the reference light. Meanwhile, in an optical system of a Twyman-Green or a Michaelson type interferometer, interference is made by returning again the signal light and reference light to the unit for dividing first the light. Although a reason why the Mach-Zender type optical system is used is not explained in detail in the related arts explained above, it may be assumed that a λ/2 (half-wave) plate (λ: wavelength) that may be rotated for adjustment is required to arrange for transmitting the light only in one way, not in both ways, of the optical path for interference in order to adjust polarization direction of the lights to be interfered, because a signal light of magneto-optical disk is generated by polarization rotation. Moreover, in the related arts, it may be thought a certain difficulty lies for practical use, because a method for adjusting an optical path length difference between a couple of lights is not explained. The patent document 2 explains, in regard to this problem, that a mirror to reflect the reference light is arranged on a disk in separation from recording film. However, it means that a disk based on a new standardization is proposed and is not related to attain a high S/N ratio of the disk in the related art. 
         [0007]    In the related arts explained above, it is required, for amplification of signal, to adjust optical path length difference between signal light and reference light up to the accuracy of a fraction of wavelength in order to attain the maximum interference intensity. In actual, however, it is extremely difficult to always adjust it in this accuracy without radiation of the reference light to a disk. 
         [0008]    In view of addressing the problems explained above, an object of the present invention is to provide an interference type optical head and an optical disk device that can easily adjust an optical path length difference between a couple of lights, ensures high signal amplification effect and is suitable for reduction in size of an optical system. 
         [0009]    The present invention has employed the following unit to achieve the object explained above. 
         [0010]    According to an aspect of the present invention, an optical head is basically constituted with a light source such as a laser diode, a first dividing unit such as a polarization prism for dividing the light from this light source to a first light flux and a second light flux, a collecting unit such as an objective lens for collecting the first light flux and then radiating to an optical recording medium, a reflection mirror for reflecting the second light flux as the reference light without collection to the optical recording medium, a second dividing unit for dividing the light that has been superimposed and interfered by guiding again the signal light reflected from the optical recording medium and the reference light to the first dividing unit and for varying phase relationship of the signal light and reference light included in respective divided lights, and a plurality of detecting unit for detecting divided lights. Optical detectors as the plurality of detecting unit are characteristically formed on the same substrate. Accordingly, enlargement of optical system can be prevented, a signal can be amplified stably, and an optical system can be constituted small in size. 
         [0011]    Moreover, the reflection mirror is replaced with a corner cube. The corner cube is an element having the property that the reflected light is essentially returned in the same direction as the incident light because of symmetrical layout of reflected light path even if the light is incident in any incident angle. It is made by cutting a cubic body in the plane perpendicular to a diagonal coupling opposite vertexes of the cubic body. An incident light is applied from the direction of the cutting plane. When the signal light and reference light interferes, if the reference light is tilted, many interference fringes are generated with such interference and interference intensity is lowered through averaging process. However, in the corner cube, since the reflected light is never tilted even when the corner cube is tilted because of the property explained above, fall of such interference intensity can be prevented. However, an optical axis must be adjusted to a vertex of the corner cube for realizing matching between optical axes of incident light and reflected light. When an incident light is applied to a vertex or an edge line, scattered lights are generated due to the influence of chamfered region and chipping. Therefore, an optical axis is usually arranged with deviation from vertex and ridge line. However, in this embodiment, scattering of light is suppressed by setting a width of vertex or ridge line as narrower as possible for a diameter of light flux for elimination of displacement of the optical axis. Accordingly, adjustment may become easier and higher effect of amplification of signal can be maintained. 
         [0012]    According to another aspect of the present invention, the second dividing unit is a key of the small-sized structure of the optical system. This dividing unit is constituted with a third non-polarizing dividing unit, a selective polarization conversion element and a polarization splitting element formed on the same substrate not operating at least one of a couple of lights divided with the third dividing unit and converting at least another light into the circular polarization light. 
         [0013]    According to still another aspect of the present invention, the optical system can be constituted smaller to eliminate influence of mutual displacement by not individually arranging the third dividing unit, selective polarization conversion element, and polarization splitting element, but integrally joining these elements. 
         [0014]    Moreover, according to another aspect of the present invention, the selective polarization conversion element can be reduced in size by constituting the same element with an anisotropic optical material having an optical axis in the optical axis direction. As a method not using anisotropic optical material, for example, it is possible to give a phase difference to the polarized element at respective positions not only by varying the travelling directions of lights divided from the non-polarized light with the third dividing unit but also by separately arranging the polarization conversion elements in the optical axis direction up to the position perfectly separated in space so that the separated lights are inputted respectively at different positions to the polarization conversion element. However, in this case, the optical system becomes large in size because a certain distance is required to perfectly separate the divided lights. Accordingly, in the present invention, a uniaxial anisotropic optical material having the optical axis in the optical axis direction of incident light is used. In the case where such material is formed in the shape of a plate to constitute the polarization conversion element, the incident light vertical to the element does not generate any phase difference irrespective of the polarizing direction. However, in this case, the inclined incident light generates a phase difference determined by anisotropy of refraction index, incident angle, and thickness of element between the polarization (P-polarization) where an electric field vibrates within a plane including incident light axis and element normal and the polarization (S-polarization) where the electric field vibrates in the direction perpendicular to the same plane. Therefore, only the lights of the particular incident angles can be selectively circularly polarized, even if these lights are not spatially separated, by designing parameters to provide a phase difference of 90°. Thereby, it is no longer required to separately provide the third dividing unit for non-polarization in the optical axis direction and the polarization conversion element in view of realizing reduction in size of the optical system. 
         [0015]    According to another aspect of the present invention, a unit is also provided to detect defocus of signal light on the optical recording medium as a signal, defocus is compensated by controlling a unit for collecting and radiating a first light flux with a focus error signal, and the corner cube can be moved in the optical axis direction in order to adjust an optical path length difference between the signal light and the reference light within interference distance (coherence length) of a light source with the focus error signal. Therefore, the objective lens is driven in the optical axis direction with focus control and even when optical path length up to the optical recording medium from the first dividing unit varies in the degree of coherence length of light source or more, signal amplifying effect can be maintained by sustaining coherence between the signal light and the reference light. 
         [0016]    Accordingly, it is possible to provide an interference type optical head and an optical disk device that can easily adjust an optical path length difference of a couple of lights, ensures higher signal amplifying effect and is suitable for reduction in size of the optical system. Therefore, regeneration signal quality can be improved through amplification of signal in the cases where reflectivity of each layer of a multilayer optical disk must be lowered, and relative noise for signal increases because a read speed is high. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  shows a structure of a basic embodiment of the present invention; 
           [0018]      FIG. 2  is an explanatory diagram of a polarization phase conversion separation element; 
           [0019]      FIG. 3  is an explanatory diagram of an RF signal light receiving part and a calculation circuit; 
           [0020]      FIG. 4A  and  FIG. 4B  are explanatory diagrams of interference phase difference in the polarization state in the linear polarization side and the circular polarization side. 
           [0021]      FIG. 5  shows an example of an incident angle vs. phase difference characteristic of an angular selective polarization conversion element; 
           [0022]      FIG. 6  is an explanatory diagram of reflected light lines of a corner cube; 
           [0023]      FIG. 7A ,  FIG. 7B , and  FIG. 7C  show polarization rotations by the corner cube; 
           [0024]      FIG. 8  shows an embodiment for conducting tracking detection by a differential push-pull method; 
           [0025]      FIG. 9  shows a structure for detecting an RF signal, a focus error signal, and a tracking error signal in each interference phase difference; 
           [0026]      FIG. 10  shows a circuit structure for signal amplification due to differential detection; 
           [0027]      FIG. 11  shows an embodiment where the corner cube is mounted to an actuator with an objective lens; 
           [0028]      FIG. 12  shows an embodiment where a couple of sheets of polarizing grating are used in place of an angular selective polarization conversion element; 
           [0029]      FIG. 13  shows an embodiment where a half-wave plate  103  is replaced with the polarization conversion element  1300  in the embodiment of claim  11 ; 
           [0030]      FIG. 14  shows polarization state and intensity ratio of signal light and reference light when the polarization conversion element  1300  in claim  13  is used; 
           [0031]      FIG. 15  shows a structure of a liquid crystal element when the liquid crystal element is used as the polarization conversion element; 
           [0032]      FIG. 16  is a side view of  FIG. 15 ; 
           [0033]      FIG. 17  shows an embodiment where a polarization phase compensation element  122  is replaced with a polarization compensation element  1701  in the embodiment of  FIG. 1 ; and 
           [0034]      FIG. 18  shows an embodiment where the polarization phase compensation element  122  is replaced with the polarization compensation element  1701  in the embodiment of  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]    The preferred embodiments of the present invention will be explained below with reference to the accompanying drawings. 
         [0036]      FIG. 1  is a basic embodiment of the present invention. The light from a laser diode  101  is inputted, with a collimating lens, to a polarization prism  104  with transmission through a half-wave plate  103  as the collimated light. The polarization prism  104  has a function to transmit in almost 100% the P-polarization incident to the separating plane and to reflect in almost 100% the S-polarization. In this case, it is possible that a part of the light is reflected by the polarization prism  104  as the S-polarization and a part of the light is transmitted as the P-polarization. The reflected light is converted to a circular polarization through a quarter-wave plate  105  and is then collected to a recording film on the optical disk  108  with an objective lens  107  mounted to a two-dimensional actuator  106 . The reflected light from the optical disk returns the same optical path as the collimated light converted with the objective lens  107  and enters the polarization prism  104  as the linear polarization that is rotated by 90° in the polarization direction from the original incident light with the quarter-wave plate  105 . Thereby, since polarization is rotated, the reflected light from the optical disk  108  passes through a polarization prism  104  as the P-polarization and enters the polarization prism  113 . Meanwhile, the P-polarization having passed the polarization prism  104  in the light from the laser diode  101  enters a corner cube prism  112  mounted to a one dimensional actuator  111  which can move in the optical axis direction. As will be explained later, since polarization and phase are disturbed by reflection at the corner cube, a compensation element  122  is inserted against such disturbance. This compensation element also has a role to change polarization of the returned light to the S-polarization. The returned light of S-polarization returns the same optical path with the same optical axis to enter the polarization prism  104 . Thereby, since polarization is rotated, the reflected light from the corner cube prism  112  is reflected by the polarization prism  104  and enters a polarization prism  113  superimposing on the reflected light from the optical disk  108 . However, the reflected light from the optical disk and the reflected light from the corner cube prism  112  are mutually in the linear polarizations orthogonally crossing with each other. Unlike the polarization prism  104 , the polarization prism  113  has a function to transmit a part of the P-polarization and reflects in almost 100% the S-polarization. Therefore, the reflected light from the corner cube prism  112  is reflected in almost 100%, while the reflected light from the disk partially passes through the polarization prism  113  and a part of this reflected light is reflected. The reflected light enters a polarization phase conversion splitting element  114  and is divided to four lights of different phases with interference by a couple of lights while the reflected light from the optical disk  108  and the reflected light from the corner cube prism  112  are superimposed thereon and the four split lights are individually detected in four light receiving parts provided on a 4-split light detector  116  with a collecting lens  115 . In the figure, four divided lights are simplified so that the lights are separated and collected in the two light fluxes. However, actually, the four light fluxes are collected. An RF calculation circuit  120  outputs a reproduced RF signal (RFS) from the detected signal. On the other hand, the reflected light from the optical disk  108  having passed the polarization prism  113  is given astigmatism with a cylindrical lens  118  and is then collected to a 4-split light detector  119  and a servo signal calculation circuit  121  outputs, from the output signal thereof, focus error signal (FES) and a tracking error signal (TES). The focus error signal is fed back to a focus actuating terminal of the two-dimensional actuator  106  mounting the objective lens  107  and thereby a focus position is closed-loop controlled. Moreover, the focus error signal is also fed back to the one dimensional actuator  111  mounting the corner cube prism  112  and the corner cube prism  112  is also driven in conjunction with the objective lens  107 . Accordingly, optical path length difference between the signal light reflected from the optical disk  108  and the reference light reflected from the corner cube prism  112  can be maintained to almost zero. Since coherence length of an ordinary laser diode is about several tens of μm, it is enough when adjustment accuracy of optical path length difference is within this range. The tracking error signal is fed back to a tracking actuating terminal of the two dimensional actuator mounting the objective lens  107  and is then closed-loop controlled. 
         [0037]      FIG. 2  is an explanatory diagram showing a structure and a function of a polarization phase conversion separation element. The polarization phase conversion splitting element  114  is constituted with a non-polarizing grating  203  as a non-polarizing splitting element, an angular selective polarization conversion element  204  as a selective polarization conversion element, and a polarization splitting grating  205  as a polarization splitting element. In  FIG. 1 , these elements are shown in the integrated state, but these are separated here for the convenience of explanation. The same function may be attained even in the case where these are separated or integrated. When the signal light and the reference light enter the polarizing grating  203  in the manner that the signal light polarization direction  201  is orthogonally crossing the reference light polarization direction  202 , a couple of lights are respectively split into a couple of light with different traveling directions without relation to the polarization direction. Such separation can be realized easily by blazing the non-polarizing grating  203 . One is a zero-order light travelling in straight and the other is a first order diffracted light diffracted in the predetermined diffraction angle. Next, when these lights enter the angular selective polarization conversion element  204 , any phase difference is not generated in the zero-order light traveling in straight but a phase difference is generated in the first order diffracted light inputted with a certain tilt and the signal light and the reference light are converted into circular polarizations in inversed rotating directions. Such conversion is possible when the optical axis  206  has uniaxial anisotropy for the plane of the angular selective polarization conversion element and the plane including diffracted light axis and the incident light axis has the angle of 45° respectively for the polarization directions of the signal light and the reference light. Thereby, both signal light and reference light in the angular selective polarization conversion element respectively have the P-polarization and S-polarization equally in the first order diffracted lights. Therefore, a phase difference for attaining circular polarization can be uniquely determined with anisotropy of refractive index (difference between refractive indices for polarization perpendicular and parallel to optical axis) and incident angle. Moreover, the emitted light of the angular selective polarization conversion element  204  is incident to the polarization splitting element  205 . As the polarization splitting grating, an element described in the Japanese Patent No. 3832243, for example, may be used. This element can easily be realized by forming a blazed grating with an anisotropic material such as liquid crystal, lithium niobate, and quartz, etc. Namely, such materials have different refractive indices depending on the polarization direction and therefore the polarization splitting grating element is enough when it is arranged to provide inverted phase distributions caused by the grating in accordance with a certain polarization direction and that orthogonally crossing thereto. Accordingly, it is possible to realize the polarization direction where the plus first order diffracted light and the minus first order diffracted light are crossing orthogonally. Otherwise, an element that can be formed by laminating anisotropic optical crystals such as the Wollaston prism may be used in place of the element explained above. As explained above, phase difference in the interference between the signal light and the reference light in the divided four lights can be set respectively to 0°, 90°, 180°, and 270°. 
         [0038]      FIG. 3  is a diagram showing an arrangement of the light-receiving part of the 4-split light detector  116  and an arrangement and a function of the RF signal calculation circuit  120  in  FIG. 1 . The 4-split light detector  116  includes four light-receiving parts  301 ,  302 ,  303 , and  304  for receiving four lights shown in  FIG. 2  and these receiving parts respectively receive the lights having interference intensity in the interference phase differences of 0°, 90°, 270°, and 180°. The respective outputs are applied to differential amplifiers  305 ,  306  for differential calculation. Thereafter, the RF signal can be detected with a root sum squares calculation circuit  307 . 
         [0039]      FIGS. 4A and 4B  are diagrams for explaining that phase difference by interference of four lights are generated in 0°, 180°, 90°, and 270° with the polarization phase conversion splitting element shown in  FIG. 2 . In these figures, Eref indicates an electric field vector of the reference light and Esig, an electric field vector of the signal light.  FIG. 4A  shows the state of polarization in the linear polarization side, while  FIG. 4B , the state of polarization in the circular polarization side in  FIG. 2 . Since the reference light and the signal light are orthogonally crossing in the polarization direction, a projected vector to each polarization separated by the polarization splitting grating is in the same direction in the side of PD 1  but in the inverted direction in the side of PD 2 . Accordingly, interference occurs between the reference light and the signal light when the phase difference is 0° in PD 1  and is 180° in PD 2 . Next, in  FIG. 4B , since both reference light and the signal light are respectively in the circular polarization in different rotating directions, projected vectors to the side of PD 3  and that to the side of PD 4  are not located at the end part of the line indicating the vector but are deviated to the intermediate location at the front ends of arrow marks. In this case, a phase difference becomes 90° and 270°, respectively. 
         [0040]    Moreover, these relations are indicated with numerical equations and amplification of the reproduced RF signal with the reference light will be explained with calculation shown in  FIG. 3 . Interference intensities of incident lights to PD 1 , PD 2 , PD 3 , and PD 4  may respectively be expressed with the following numerical equations. 
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                                
                               
                                 E 
                                 sig 
                               
                                
                             
                              
                             
                                
                               
                                 E 
                                 ref 
                               
                                
                             
                              
                             
                               sin 
                                
                               
                                 ( 
                                 
                                   
                                     ϕ 
                                     sig 
                                   
                                   - 
                                   
                                     ϕ 
                                     ref 
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0041]    From these numerical equations, the output signals Sig 1  and Sig 2  of the differential amplifiers  305 ,  306  in  FIG. 3  may be expressed as follows. 
         [0000]        Sig 1 =I   PD1   −I   PD2   =|E   sig   ∥E   ref |cos(φ sig −φ ref )  (5) 
         [0000]        Sig 1 =I   PD3   −I   PD4   =|E   sig   ∥E   ref |sin(φ sig −φ ref )  (6) 
         [0042]    Therefore, the signal obtained by amplifying electric field amplitude of the regeneration signal with electric field amplitude of the reference signal can be detected as expressed with the following numerical equation. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         S 
                         = 
                         
                           
                             
                               
                                 ( 
                                 
                                   Sig 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                                 ) 
                               
                               2 
                             
                             + 
                             
                               
                                 ( 
                                 
                                   Sig 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     I 
                                     
                                       PD 
                                        
                                       
                                           
                                       
                                        
                                       1 
                                     
                                   
                                   - 
                                   
                                     I 
                                     
                                       PD 
                                        
                                       
                                           
                                       
                                        
                                       2 
                                     
                                   
                                 
                                 ) 
                               
                               2 
                             
                             + 
                             
                               
                                 ( 
                                 
                                   
                                     I 
                                     
                                       PD 
                                        
                                       
                                           
                                       
                                        
                                       3 
                                     
                                   
                                   - 
                                   
                                     I 
                                     
                                       PD 
                                        
                                       
                                           
                                       
                                        
                                       4 
                                     
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                              
                             
                               E 
                               sig 
                             
                              
                           
                            
                           
                              
                             
                               E 
                               ref 
                             
                              
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0043]    Here, it can be understood by conducting such sum squares calculation that phase difference does not give any influence on the reference light and signal light obtained finally. Accordingly, adjustment of optical path length difference as small as a fraction of wavelength that has been conducted in the related art is no longer required in the present invention. Here, sum squares of a couple of differential signals may also be provided as outputs, which omits the root calculation explained above. In the case where the root calculation is not conducted, since a signal that is proportional to signal intensity can be obtained, the signal waveform that is identical to that in the CD, DVD, and Blue-ray disk in the related art can be obtained. Since an output when the root calculation is conducted is proportional to a root of the signal light intensity, this output has the signal waveform identical to that of a magneto-optical disk signal in the related art. 
         [0044]      FIG. 5  shows results of calculations of polarization phase difference for incident angles of light using lithium niobate as an example, as the angular selective polarization conversion element  204 . Here, setting for the calculation has been conducted as follows. Namely, extra-ordinary refractive index is 2.200, ordinary refractive index is 2.286, and thickness of element is 1 mm. Therefore, it is obvious that only the diffracted light can be selectively converted to the circular polarization light by adjusting a diffraction angle of the non-polarizing grating to result in the incident angle of about 4.5°. 
         [0045]      FIG. 6  is the explanatory diagram for explaining that the light incident to the reflection surface of the corner cube prism is reflected passing the same optical path. The incident light beam is reflected basically passing all planes of the adjacent three planes through reflection of three times. A solid line in  FIG. 6  indicates a light beam, a broken line indicates a projection to each plane of the light beam, and a dotted line indicates an auxiliary line showing the reflecting position. It is also apparent that projection of the light beam at each plane forms a part of the parallelogram and the lights are reflected in the same direction because of its symmetry. However, the incident light and the reflected light are parallel but show a certain positional deviation. In order to overcome this problem, it is required that the light is radiated by setting the optical axis toward the vertex and any displacement of light flux must be eliminated as a whole. In this case, it can also be thought that the light is scattered at the vertex or edge, but a sharp edge must be formed in view of mitigating influence of such scattering of light. Since three edge lines are appearing in the reflected light, images of six edges can be observed. 
         [0046]      FIG. 7A  is a front view of the corner cube. A thick line indicates an edge line of a plane and a dotted line, an auxiliary line for explanation. The light entering the corner cube prism returns after reflection of three times as explained above, respective reflections are total reflections, and a phase difference is generated depending on polarization of the incident light. As a result, the returning light is different from the incident light in its polarization.  FIGS. 7A ,  7 B, and  7 C respectively show the axes of the polarization direction of the light when the respective lights are reflected. The light entering from six regions (1) to (6) in  FIG. 7A  are reflected at the reflection plane in different sequence, these lights are returned respectively in different polarizations. In order to guide the returned light from the corner cube to the detector as the reference light, this returned light must be returned as the light of S-polarization not depending on the incident place. Moreover, the returned light from each region must be set in the identical phase with each other. These conditions can be achieved with the polarization phase compensation element  102 , existing only in the regions (2), (3), and (6) as shown in  FIG. 7 , that is constituted by sequentially arranging a phase plate  701  for giving an adequate phase difference to the S-polarization and the P-polarization, a quarter-wave plate  702  to which the incident light and the reflected light are applied to the entire part of the plate, and a half-wave plate  703  that varies the optical axis direction depending on each region. As an example, the setting values when a material of the corner cube is BK7 and wavelength of light is 405 nm are listed in the table  1 . A phase difference of the phase plate indicates a delay of the S-polarization for the P-polarization. The optical axis of the quarter-wave plate and the half-wave plate unit the direction of fast axis for the perpendicular direction when it is seen from the incident light direction (the counter-clockwise direction is the positive direction). 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Example of Setting Values of Polarization Phase Compensation 
               
               
                 Element 
               
             
          
           
               
                   
                   
                 Optical Axis of 
                 Optical Axis of 
               
               
                   
                 Phase Difference 
                 Quarter-wave 
                 Half-wave 
               
               
                 Region 
                 of Phase Plate 
                 Plate 
                 Plate 
               
               
                   
               
             
          
           
               
                 (1) 
                 — 
                 −17.72° 
                 28.64° 
               
               
                 (2) 
                 33.29° 
                 −17.72° 
                 −1.36° 
               
               
                 (3) 
                 33.29° 
                 −17.72° 
                 58.64° 
               
               
                 (4) 
                 — 
                 −17.72° 
                 58.64° 
               
               
                 (5) 
                 — 
                 −17.72° 
                 −1.36° 
               
               
                 (6) 
                 33.29° 
                 −17.72° 
                 28.64° 
               
               
                   
               
             
          
         
       
     
         [0047]      FIG. 8  shows another embodiment of the present invention in which a differential push-pull method is used as a tracking error signal detection system. In this differential push-pull method, three light beams generated by a grating  801  are incident to a disk. When a main spot on the disk is arranged on a recorded track, rotation adjustment of the grating  801  is conducted to arrange a couple of sub-spots between the adjacent tracks. Here, three beams of the reference light are used and these lights are respectively interfered with corresponding signal lights and the tracking error signal is also amplified with differential calculation. Moreover, the focus error signal in the astigmatism method is also amplified with differential detection with 4-split detection of the zero-order light of the grating  801  keeping each interference phase difference. The amplified focus error signal is received with a light detector  802  formed in the shape of a package to conduct signal calculation. 
         [0048]      FIG. 9  corresponds to  FIG. 3  and shows a circuit structure for respectively detecting four RF signals (RFS 1 , RFS 2 , RFS 3 , RFS 4 ), focus error signals (FES 1 , FES 2 , FES 3 , FES 4 ) and tracking error signals (TES 1 , TES 2 , TES 3 , TES 4 ), using a 4-split optical detector  901  for main beam, 2-split optical detectors  901 ,  903  for sub-beams, a summing amplifier  904 , and differential amplifiers  905 ,  906  for the four interference lights having interference phase differences of 0°, 180°, 90°, and 270° of the signal light and the reference light. These differential amplifier circuits, etc. are built in a signal calculation circuit  803  of  FIG. 8 . 
         [0049]      FIG. 10  shows a circuit structure for respectively detecting the amplified signal from each interference phase difference shown in  FIG. 9  with the differential detection and the root sum squares calculation. This circuit structure can also obtain respectively the RF signal, focus error signal, and tracking error signal with the root sum square calculation circuit after obtaining the differential signals of 0° and 180°, 90° and 270° with a differential amplifier  1001 . The structure explained above is preferential in reduction of crosstalk, because the signal by the light from the layer to be detected can be selectively amplified for leakage signal from the layer defocused to a large extent in the case of the multilayer disk. 
         [0050]      FIG. 11  shows an optical system attained by improving the system shown in  FIG. 8 . In this optical system, the corner cube prism  112  is mounted to an objective lens actuator  1101  with the objective lens  107 . Thereby, if the objective lens  107  is driven in the optical axis direction following surface fluctuation of the optical disk  108  under the condition that a focus servo is in the ON state, the corner cube prism  112  is also driven simultaneously, bringing about a merit that the optical length difference of the signal light and the reference light does not basically change. In recent years, a so-called 2-lens actuator has been put into the practical use. In this actuator, an objective lens for BD and an objective lens assuring compatibility for both DVD and CD can be mounted together. The present embodiment can be realized easily by mounting the corner cube prism  112  to the position of one lens of such actuator. By mounting a lens compatible for three systems of BD/DVD/CD on the actuator with the corner cube prism, plural kinds of optical disks can be applied. 
         [0051]      FIG. 12  shows another embodiment different from the angular selective circular polarization plate shown in  FIG. 2 . Here, the non-polarizing grating  203  of  FIG. 2  is replaced with the polarizing grating  1201  and moreover the angular selective circular polarization plate  204  is replaced with the second polarizing grating  1202 . Respective optical axis directions  1203 ,  1204  are crossed orthogonally as shown in the figure. The subsequent polarizing grating  207  is arranged in the same layout as that of  FIG. 2 . Thereby, the light of linear polarization along the optical axis  1203  is partially diffracted with the polarizing grating  1201  and orthogonal polarization is not diffracted. Moreover, a part of the linear polarization light along the optical axis  1204  is diffracted with the second polarizing grating  1202  and orthogonal polarization is not diffracted. Accordingly, the lights diffracted with the first and second polarizing gratings are mutually polarized orthogonally and the phase of the grating arrangement is deviated only by ¼ of the period P of grating (90°) as shown in  FIG. 12 . Therefore, since the phases of diffracted lights are also deviated by 90°, the polarization state when a couple of diffracted lights are combined becomes circular polarization. Here, only one diffracted light is indicated and it can be easily realized by blazing the diffraction grating such as the step grating and sawtooth grating. Moreover, the polarization state of the light not diffracted may be maintained in the state same as the polarization state of the first incident light by setting the diffraction efficiency of the polarization grating  1201  equal to that of the polarization grating  1202  for orthogonal polarization. In these figures, elements are separated for the convenience of explanation but these elements can be integrated through lamination in the actual optical system. In the structure explained above, the non-polarizing grating is replaced with the polarizing grating, in comparison with the structure of  FIG. 2 , but manufacturing cost can be lowered a little, because the angular selective circular polarization plate formed of a comparatively expensive anisotropic optical crystal is replaced with the polarizing grating that can be formed easily by solidification of liquid crystal. In addition, it is required to enlarge an incident angle or to make thicker the element in order to generate phase difference of 90° in the angular selective circular polarization plate. However, in this embodiment, since a phase difference of the circular polarization can be realized with the desired grating pitch, it is preferential for reduction in size of the element. 
         [0052]      FIG. 13  shows an embodiment attained by replacing the half-wave plate  103  with the polarization conversion element  1300  in the embodiment shown in  FIG. 11 . Thereby, a splitting ratio of the signal light reflected by the polarization prism  104  and the reference light can be varied. Accordingly, in the case of recording to the optical disk  108 , the optical disk is irradiated with all light of the laser diode for effective recording by changing the splitting ratio. In addition, in the case of signal readout from the optical disk  108 , the signal can be amplified by increasing intensity ratio of the signal light and reference light. 
         [0053]      FIG. 14  shows polarization of the light after passing through the polarization conversion element  1300  in  FIG. 13  and intensity ratio of the signal light and the reference light. This figure shows that polarization can be changed to the signal readout state where intensity ratio of the reference light is enlarged from the state where the efficiency of the signal light is set to 100% by varying an applied voltage to a liquid crystal element. In order to change the splitting ratio, alternating voltage is applied to the liquid crystal. Here, it may be apparent that when a phase difference between the signal light and the reference light is set, for example, to 135° as an example of the reproducing operation, a ratio of the signal light and the reference light can be shared to 14.6%:85.4%. In this case, it is also apparent when it is assumed that intensity reflectivity of disk is 5%, of the intensity of the reference light incident to the detection optical system is 116 times higher than that of the signal light. Here, a signal gain becomes 10.8 times. 
         [0054]      FIG. 15  and  FIG. 16  respectively show a structure of element in the case where a liquid crystal element is used as the polarization conversion element in  FIG. 13 . The liquid crystal element has a structure that the liquid crystal is sandwiched with glass plates  1501 ,  1502  and these are sealed with a sealant  1506 . In this case, the glass substrates  1501  and  1502  are formed in different sizes so that transparent electrodes  1503 ,  1505  are exposed from the plane where the glass substrate  1501  is exposed. The transparent electrode  1503  is patterned to a side surface of liquid crystal of the glass substrate  1501  and the transparent electrode  1505  is formed as the electrode for electrical connection with the transparent electrode  1504  patterned to the side surface of liquid crystal of glass substrate  1502  via a conductive resin  1507 . Since an alternating voltage is applied to the liquid crystal with the transparent electrodes  1503 ,  1504 , a phase difference between a couple of orthogonally crossing linear polarizations that have been determined in direction with rubbing process of the liquid crystal can be varied. 
         [0055]      FIG. 16  is a side view of the liquid crystal element of  FIG. 15 . This figure indicates liquid crystal  1601  is sandwiched with the glass substrates  1501  and  1502 . 
         [0056]      FIG. 17  shows another embodiment modified from that of  FIG. 1  by replacing the polarization phase compensation element  122  with a polarization compensation element  1701 . The polarization compensation element  1701  is formed by eliminating the phase plate from the polarization phase compensation element  122  and is constituted with a quarter-wave plate  1702  and a half-wave plate  1703 . In this case, since phase of the reference light is different in the regions (1), (4), (5) and the regions (2), (3), (6) shown in  FIG. 7 , the order of interference signal is lowered. However, since phase difference among the regions is as small as 33.29 degrees, order of interference can be set up to about 95% in comparison with the case where the phase is identical in all regions and therefore sufficient interference signal can be obtained. In addition, since the polarization phase compensation element  122  is required to show that the divided regions of the phase plate and half-wave plate can be seen to be overlapped with the regions of the corner cube when they are viewed from the direction of optical axis of the incident light, the polarization phase compensation element  122  and corner cube prism showed be integrally mounted to the actuator  111 . If not, polarization of returned light is varied or the interference signal is modulated due to displacement generated when the actuator  111  is driven. However, in this embodiment, only the half-wave plate  1703  must be mounted to the actuator  111  and the quarter-wave plate  1702  can be arranged separately from the actuator because this quarter-wave plate  1702  does not include any divided region. Therefore, weight of a moving part to the actuator  111  can be suppressed and lowering of its characteristic can also be suppressed. 
         [0057]      FIG. 18  shows still another embodiment modified from that of  FIG. 11  by replacing the polarization phase compensation element  122  with the polarization compensation element  1701 . In this case, like the embodiment shown in  FIG. 17 , the quarter-wave plate  1702  of the polarization compensation element  1701  is not mounted to the actuator but the half-wave plate  1702  is mounted. Thereby, lowering of characteristic of the actuator can be suppressed. 
         [0058]    The present invention enables realizes stable detection of a reproduction signal of a high-speed optical disk of the large capacity and multilayer type with higher signal quality and can provide application thereof into a wide range of industrial field such as a large capacity video recorder, a hard disk data backup device, and a stored information archives device or the like.