Patent Publication Number: US-2007115767-A1

Title: Optical pickup device

Description:
This application claims priority under 35 U.S.C. Section 119 of Japanese Patent Application No. 2005-334708 filed Nov. 18, 2005 and Japanese Patent Application No. 2005-341551 filed Nov. 28, 2005.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to optical pickup devices, and in particular, to an optical pickup device suited for use in suppressing spherical aberration.  
      2. Description of the Related Art  
      An objective lens of higher numerical aperture is recently being used with higher density of an optical disk. However, aberration tends to easily occur in a laser light due to error in substrate thickness etc. of the optical disk when such an objective lens of higher numerical aperture is used. In such a case, a spherical aberration correcting means thus becomes necessary in the optical pickup device.  
      Japanese Laid-Open Patent Publication No. 10-269611 (patent document 1) discloses use of a liquid crystal panel for a spherical aberration correcting element. Furthermore, Japanese Laid-Open Patent Publication No. 2000-40249 (patent document 2) and Japanese Laid-Open Patent Publication No. 10-289465 (patent document 3) disclose use of the liquid crystal panel for an astigmatism correcting element and a coma aberration correcting element.  
      According to the invention described in patent document 1, a wavefront state of the laser light is corrected so as to suppress the spherical aberration by a phase correcting effect of the liquid crystal panel. However, if optical axes are misaligned between the liquid crystal panel and the objective lens due to lens shift of the objective lens, attachment error etc., the aberration consequently occurs in the laser light. In this case, a configuration in which the liquid crystal panel is attached to an objective lens actuator to integrally displace the liquid crystal panel and the objective lens may be used so as to suppress the optical axes between the liquid crystal panel and the objective lens from being misaligned. However, the objective lens actuator becomes larger, and drive response or dynamic response of the objective lens is adversely affected. Furthermore, when attachment error of the liquid crystal panel with respect to the objective lens actuator occurs, the misalignment of the optical axes between the objective lens and the liquid crystal panel becomes fixed, and consequently, the aberration originating from the misalignment of the optical axes occurs on a steady basis irrespective of the shifted position of the objective lens.  
     SUMMARY OF THE INVENTION  
      The present invention, in view of solving the above problem, aims to provide an optical pickup device capable of effectively suppressing aberration from occurring in a laser light when a lens shift and the like occur to an objective lens.  
      An optical pickup device according to one aspect of the present invention includes a laser light source; an objective lens for converging a laser light exited from the laser light source onto a recording medium; and a phase correcting element, interposed between the laser light source and the objective lens, for providing a spherical aberration correcting effect only to one part of the laser light within an effective diameter of the objective lens.  
      In this aspect, the phase correcting element may be configured to provide the spherical aberration correcting effect to the laser light within a range of a constant distance from the center of the effective diameter.  
      According to this aspect, the spherical aberration correcting effect is not provided to all the laser light within the range of the effective diameter, but the spherical aberration correcting effect is provided only to one part thereof. Thus, even if optical axes are misaligned between the spherical aberration correcting element and the objective lens, aberration caused therefrom can be suppressed. This advantage will be verified in more detail in the following embodiment.  
      Since the present invention does not provide the spherical aberration correcting effect to all the laser light within the range of the effective diameter, but provides the spherical aberration correcting effect only to one part thereof, a means for providing another optical effect may be arranged in a region not used in the spherical aberration correcting effect out of the range of the effective diameter. For example, a means for providing an astigmatism correcting effect may be arranged in the relevant region. Therefore, the correction of the spherical aberration and the correction of the astigmatism are simultaneously achieved with one phase correcting element. If the phase correcting element is configured using liquid crystals, the correcting effect for the spherical aberration and the correcting effect for the astigmatism are provided by simply adjusting the electrode pattern as appropriate. Thus, the configuration of the phase correcting element can be simplified.  
      When the phase correcting element is configured using the liquid crystals, the phase correcting element includes a first electrode; a second electrode arranged facing the first electrode; a first orientation film arranged on a surface facing the second electrode of the first electrode; a second orientation film arranged on a surface facing the first electrode of the second electrode; and a liquid crystal layer filled between the first orientation film and the second orientation film; wherein the first electrode has an electrode pattern for providing the spherical aberration correcting effect to the laser light within a constant distance from the center of the effective diameter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention, together with the above and other objects and novel features thereof, may best be understood by reference to the following description of the embodiment together with the accompanying drawings in which:  
       FIG. 1  is a view showing an optical system of an optical pickup device according to one embodiment of the present invention;  
       FIGS. 2A and 2B  are views showing configurations and electrode patterns of a phase correcting element according to the embodiment of the present invention;  
       FIGS. 3A and 3B  are views showing configurations and electrode patterns of a phase correcting element according to the conventional art (comparative example);  
       FIGS. 4A and 4B  are views showing verification results by the electrode pattern according to the conventional art (comparative example) and the electrode pattern according to the embodiment of the present invention;  
       FIGS. 5A and 5B  are views showing variants of the electrode pattern according the embodiment of the present invention;  
       FIGS. 6A  to  6 D are views explaining an astigmatism correcting effect according to the embodiment of the present invention;  
       FIGS. 7A and 7B  are views showing variants of the electrode pattern according to the embodiment of the present invention; and  
       FIGS. 8A and 8B  are views showing further variants of the electrode pattern according to the embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
      The embodiments of the present invention will now be described. In the present embodiment the present invention applies to an optical pickup device used in a next generation DVD (Digital Versatile Disk) having a substrate thickness of 0.6 mm.  
       FIG. 1  shows an optical system of the optical pickup device according to the present embodiment. In the figure, a circuit configuration (reproduction circuit  201 , servo circuit  202 , and liquid crystal drive circuit  203 ) for drive controlling the optical pickup device is shown for the sake of convenience.  
      As shown in the figure, the optical pickup device includes a semiconductor laser  11 , a polarizing beam splitter (polarizing BS)  12 , a collimator lens  13 , a phase correcting element  14 , a mirror  15 , a λ/4 plate  16 , an objective lens  17 , an objective lens actuator  18 , a detection lens  19 , and a light detector  20 .  
      The semiconductor laser  11  exits the laser light of blue wavelength (407 nm in the present embodiment). The polarizing BS  12  transmits substantially all the laser light entered from the semiconductor lens  11 , and substantially reflects all the laser light entered from the collimator lens  13 . The collimator lens  13  converts the laser light from the polarizing BS  12  to parallel light. The phase correcting element  14  adjusts the wavefront state of the laser light from the collimator lens  13 . The details of the phase correcting element  14  will be hereinafter described.  
      The mirror  15  raises the laser light from the phase correcting element  14  so as to be directed towards the objective lens  17 . The λ/4 plate  16  converts the laser light from the mirror  15  to a circularly polarized light, and converts the laser light from the objective lens  17  to a linear polarized light that is orthogonal to the plane of polarization of the laser light from the mirror  15 . The objective lens  17  converges the laser light from the λ/4 plate  16  onto the disk. The objective lens actuator  18  drives the objective lens  17  in the focus direction and in the tracking direction according to the drive signal from the servo circuit  202 .  
      The detection lens  19  introduces astigmatism to the laser light from the polarizing BS  12  so as to allow the generation of a focus error signal based on the astigmatism method. The light detector  20  outputs a detection signal based on the laser light converged by the detection lens  19 . A sensor pattern is provided to the light detector  20  to generate a reproduction RF signal, a tracking error signal and the focus error signal.  
      The reproduction circuit  201  generates the reproduction RF signal based on the detection signal input from the light detector  20  and further demodulates the reproduction RF signal to generate the reproduction data. The servo circuit  202  generates the tracking error signal and the focus error signal based on the detection signal input from the light detector  20 , and further generates the tracking servo signal and the focus servo signal based on the tracking error signal and the focus error signal, and outputs the signals to the objective lens actuator  18 . The liquid crystal drive circuit  203  generates a signal for driving the phase correcting element  14  based on the detection signal input from the light detector  20  and outputs the relevant signal to the phase correcting element  14 . The liquid crystal driving circuit  203  generates the servo signal that converges the reproduction RF signal to a satisfactory state, and outputs the servo signal to the phase correcting element  14 .  
      The configuration of the phase correcting element  14  will now be described with reference to  FIGS. 2A and 2B .  
       FIG. 2A  is a side cross sectional view in a case where the phase correcting element  14  is cut along the laser light transmitting direction. As shown in the figure, the phase correcting element  14  is configured by glass substrates  141  and  142 ; electrode layers  143  and  144 ; an orientation film  145 ; a liquid crystal layer  146 ; and a seal material  147 .  
      The glass substrate  141  has a square plate shape of a constant thickness. The electrode layers  143  and  144  are made of an electrically conductive material allowing the transmission of the laser light, and the periphery thereof is circular. The orientation films  145 ,  145  are arranged on the side surfaces of the liquid crystal layer  146  of the electrode layers  143  and  144 . The liquid crystal layer  146  is formed by filling the liquid crystals between the orientation films  145 ,  145 . In the liquid crystal layer  146 , the orientation direction of the liquid crystal molecules is changed by applying potential via the electrode layers  143  and  144 . The seal material  147  is arranged to prevent leakage of the liquid crystals.  
      The electrode layer  144  has a uniform film shape continuous across the entire surface. The electrode layer  143  is formed with an electrode pattern as shown in  FIG. 2B . In other words, a circular electrode El and three ring shaped electrodes E 12 , E 13 , E 14  are arranged concentrically on the electrode layer  143 .  
      When different potentials are applied to the electrodes E 11  to E 14  while maintaining a constant potential at the electrode layer  144  (e.g., earth potential), the orientation direction of the liquid crystal molecules between the electrodes E 11  to E 14  and the electrode layer  144  changes according to the applied potential. The index of refraction of the liquid crystal layer  146  thus changes at the position of the electrodes E 11  to E 14 , and the phase of the laser light that passes the position of the electrodes E 11  to E 14  changes. As a result, the wavefront state of the laser light after passing the liquid crystal layer  146  changes according to the state of change in the relevant phase. Therefore, the wavefront state of the laser light can be adjusted by controlling the potential to be applied to the electrodes E 11  to E 14 .  
      The electrode pattern according to the present embodiment has only two ring shaped electrodes E 12  and E 13  arranged on the inner circumferential part, and only one continuous uniform ring shaped electrode  14  arranged on the outer side thereof, as shown in  FIG. 2B . Therefore, the laser light has a uniform phase according to the potential to be applied to the electrode E 14  in the region between the inner side of the beam incident diameter (corresponds to effective diameter of objective lens  17 ) and the electrode E 13 .  
       FIGS. 3A and 3B  show the configuration example of the phase correcting element described in patent document 1. As shown in  FIG. 3B , a circular electrode E 21  and three ring shaped electrodes E 22 , E 23 , E 24  are concentrically arranged on the inner circumferential side, and three ring shaped electrodes E 26  are further arranged on a slightly inner side of the beam incident diameter on the electrode layer  143  of the phase correcting element. A ring shaped electrode E 27  is further arranged on the outer side. Thus, the ring shaped electrode is also formed on the slightly inner side of the beam incident diameter in the phase correcting element described in patent document 1, different from the phase correcting element according to the present embodiment.  
      Verification  
      The inventors of the present invention compared and verified the occurrence state of the wavefront aberration at the beam converging position in a case where the phase correcting element of  FIG. 2  according to the present embodiment is used and in a case where the phase correcting element of  FIG. 3  according to the conventional art is used. The following description is based thereon.  
       FIGS. 4A and 4B  show the verification results (simulation). The conditions for the present verification are as follows. 
      Numerical aperture of objective lens: 0.65     Focal distance of objective lens: 2.3 mm     Substrate thickness of disk: 0.585 mm (error with respect to reference thickness=0.015 mm)     Wavelength of used laser: 407 nm    
      (a- 1 ), (a- 2 ), and (a- 3 ) of  FIG. 4A  are simulation results (conventional example) when the pattern of the electrode layer  143  is configured as in  FIG. 3B , and (b- 1 ), (b- 2 ), and (b- 3 ) of  FIG. 4B  are simulation results (embodiment) when the pattern of the electrode layer  143  is configured as in  FIG. 2B .  
      (a- 1 ) of  FIG. 4A  and (b- 1 ) of  FIG. 4B  show the relationship of the wavefront before correction (wavefront in a case where wavefront correction is not performed at the phase correcting element); the wavefront after correction (wavefront in a case where wavefront correction is performed at the phase correcting element); and the liquid crystal phase (distribution of phase introduced to the laser light by phase correcting element) with respect to when the misalignment (lens shift of the objective lens with respect to the optical axis of the phase correcting element) does not occur in the optical axes between the phase correcting element and the objective lens is not produced. (a- 2 ) of  FIG. 4A  and (b- 2 ) of  FIG. 4B  show the relationship of the wavefront before correction, the wavefront after correction, and the liquid crystal phase with respect to when misalignment (lens shift) of the optical axes occurs by 0.5 mm between the phase correcting element and the objective lens. In these figures, the horizontal axis indicates the distance in the radial direction from the optical axis of the objective lens when setting ½ of the effective diameter of the objective lens to 1, and the vertical axis indicates the distribution state of the wavefront and the phase in a standardized manner.  
      (a- 3 ) of  FIG. 4A  and (b- 3 ) of  FIG. 4B  are verification results showing the relationship between the amount of lens shift and the wavefront aberration. In addition to the total wavefront aberration (solid line), only the change in the tertiary spherical aberration is extracted and shown in (a- 3 ) and (b- 3 ).  
      In the verification, the potential that produces the liquid crystal phase shown in (a- 1 ) of  FIG. 4A  and (b- 1 ) of  FIG. 4B  is applied to the phase correcting element according to the conventional art and the phase correcting element according to the present embodiment via the electrodes E 21  to E 27  and the electrodes E 11  to E 14 , respectively, of the electrode layer  143 .  
      With reference to (a- 1 ) of  FIG. 4A  and (b- 1 ) of  FIG. 4B , if misalignment of the optical axes (lens shift) does not occur at the objective lens, the wavefront state of the laser light is corrected over substantially the entire range when the phase correcting element according to the conventional art is used, whereas a relatively large change is seen on the wavefront state of the circumferential part of the beam diameter when the phase correcting element according to the embodiment is used. In this case, the wavefront aberration at the beam converging position is 7.4 mλrms when the phase correcting element according to the conventional art is used, and is 23.0 mλrms when the phase correcting element according to the embodiment is used. Therefore, the phase correcting element according to the conventional art excels in the aberration correcting ability when the lens shift is not produced.  
      When the misalignment of the optical axes (lens shift) of 0.5 mm occurs at the objective lens, the change in the wavefront state in the beam diameter direction becomes larger if the phase correcting element according to the conventional art is used than when the phase correcting element according to the embodiment is used, as shown in (a- 2 ) of  FIG. 4A  and (b- 2 ) of  FIG. 4B . In this case, the wavefront aberration at the beam converging position drastically increases to 44.8 mλrms when the phase correcting element according to the conventional art is used, and is suppressed to 37.3 mλrms when the phase correcting element according to the embodiment is used. Therefore, the phase correcting element according to the embodiment excels in the aberration correcting ability when the lens shift is produced.  
      In comparing the aberration correcting ability of the phase correcting element according to the conventional art and the phase correcting element according to the embodiment with reference to (a- 3 ) of  FIG. 4A  and (b- 3 ) of  FIG. 4B , with regards to the total wavefront aberration, the aberration correcting ability of both phase correcting elements is about the same extent when the amount of lens shift is about 0.2 mm, and the phase correcting element of the present embodiment exhibits a more superior aberration correcting ability when the amount of lens shift is larger. In particular, with regards to the tertiary spherical aberration component based on the substrate thickness error etc., the aberration correcting ability of both phase correcting elements is about the same extent if the amount of lens shift is a little over 0.15 mm, and the phase correcting element of the present embodiment exhibits a more superior correcting ability when the amount of lens shift is larger.  
      Therefore, the wavefront aberration produced in time of lens shift is more effectively suppressed according to the present embodiment compared to the conventional art. Furthermore, the number of electrode patterns of the electrode layer is reduced, and the configuration of the phase correcting element is simplified according to the present embodiment, as apparent from comparing and referencing  FIGS. 2A and 2B , as well as  FIGS. 3A and 3B .  
      In the above embodiment, the continuous electrode E 14  is arranged on the outer side of the ring shaped electrode  13  as shown in  FIGS. 2A and 2B , but an electrode for correcting other aberrations may be arranged in the relevant region.  
       FIGS. 5A and 5B  are configuration examples in a case where the astigmatism correction electrodes E 31  to E 38  are arranged on the outer side of the ring shaped electrode E 13 . The correcting effect for the spherical aberration is not affected even if the astigmatism correction electrodes E 31  to E 38  are arranged on the outer side of the ring shaped electrode E 13  and the astigmatism correcting effect is simultaneously performed since the aberration function of astigmatism and the aberration function of the spherical aberration do not influence each other.  
      In time of correcting the astigmatism, the same potential is applied to the electrodes diametrically opposite each other out of the electrodes E 31  to E 38 . For example, potential V1 is applied to the pair of E 31  and E 35  and to the pair of E 34  and E 38 ; and potential V2 different from the potential V1 is applied to the pair of E 32  and E 36  and to the pair of E 33  and E 37 , as shown in the upper part of  FIG. 6A . Thus, the phase distribution in which the peaks and valleys of the phase appear every 90 degrees in the beam circumferential direction can be produced at the phase correcting element, as shown in the lower part of  FIG. 6A . As a result, the astigmatism correcting effect is introduced to the laser light passing through the phase correcting element.  
      As shown in  FIGS. 6B, 6C ,  6 D, the direction of astigmatism in the beam circumferential direction can be changed by appropriately changing the electrodes to be applied with potential. If the electrodes are divided into eight segments in the circumferential direction as shown in  FIGS. 5A and 5B , the direction of astigmatism can be changed by 22.5 degrees.  
      The astigmatism correcting electrode may be further divided into two in the radial direction, as shown in  FIGS. 7A and 7B . In this case, the phase changes in the radial direction and a more precise introduction of the spherical aberration correcting effect and the astigmatism correcting effect can be performed.  
      In the above example, the electrode pattern for introducing the correcting effect for the spherical aberration or the astigmatism is arranged only on one of the electrode layers  143  out of the two electrode layers  143  and  144 , but the electrode pattern for correcting other aberrations may be arranged on the other electrode layer  144 .  
      For example, the phase distribution for providing the coma aberration correcting effect can be provided to the phase correcting element by controlling the application potential of the electrodes E 41  to E 45  if the electrode pattern as shown in  FIG. 8A  is formed on the electrode layer  144 . Since the aberration function of the coma aberration, and the aberration function of the astigmatism and the aberration function of the spherical aberration do not influence each other, the correcting effect for the spherical aberration and the correcting effect for the astigmatism are not influenced even if the coma aberration correcting electrodes E 41  to E 45  are arranged on the electrode layer  144  and the coma aberration correcting effect is simultaneously performed.  
      Furthermore, the electrode pattern of  FIG. 2B  may be applied as the electrode pattern of the electrode layer  143 , and the electrode pattern in which the correcting effects of the astigmatism and the coma aberration can be simultaneously performed, as shown in  FIG. 8B  may be applied as the electrode pattern of the electrode layer  144 . In this case, the astigmatism is corrected by controlling the application voltage to the electrodes E 31  to E 38 , and the coma aberration is corrected by controlling the application voltage to the electrodes E 41  to E 43 .  
      The embodiments of the present invention have been described, but the present invention is not limited thereto, and the embodiments may be modified in other various ways.  
      For example, an example of applying the present invention to the optical pickup device for next generation DVD has been described, in the above embodiment but the present invention may be applied to an optical pickup for DVD, and a compatible optical pickup device of the next generation DVD and the DVD. In the above embodiment, the phase correcting element  14  is arranged on the optical path from the semiconductor laser  11  to the objective lens  17  to correct the aberration on the optical disk, but a different aberration correcting element may be further arranged to correct the aberration on the light detector  20 .  
      In the embodiment, the liquid crystal phase in the range on the outer side than about 0.5 mm in the objective lens shift direction from the center in the horizontal axis is made constant with reference to (b- 1 ) of  FIG. 4B , but the starting position at where the liquid crystal phase becomes constant is not limited thereto, and for example, the liquid crystal phase may be made constant from the position on the outer side than 0.5 mm from the center. In this case, the width or the number of levels of the ring shaped electrode on the inner circumferential part is appropriately adjusted.  
      Furthermore, again referring to (b- 1 ) of  FIG. 4B , the liquid crystal phase of the range on the outer side than about 0.5 mm in the objective lens shift direction from the center in the horizontal axis is made constant in the embodiment, but for example, the liquid crystal phase may be slightly raised in the range on the outer side from about 0.5 mm from the center, and then made constant from where the liquid crystal phase on the outer side is raised, and still obtain the effects substantially similar to the verification shown in  FIG. 4B . In this case, the electrode for raising the liquid crystal phase is separately arranged.  
      In addition, various modifications may be appropriately made on the embodiment of the present invention within the scope of the technical concept described in the Claims.  
      It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.