JP5081773B2 - Optical pickup, objective lens, spherical aberration correction element, and optical information recording / reproducing apparatus - Google Patents

Description

  The present invention relates to an optical pickup having a function of recording or reproducing an information signal on an optical information recording medium.

  As a large-capacity optical disk following the DVD, a BD (Blu-ray Disc) having a capacity of 23 to 27 GB in a single layer and a capacity of about 50 GB in a single layer using a blue-violet laser and a recording / reproducing apparatus for the same have been commercialized. In this BD double-layer disc, there is a difference in cover layer thickness of about 25 μm between the two information recording surfaces, and the numerical aperture (NA) of the objective lens used is about 0.85, which is higher than that of DVD. Therefore, it is essential for the recording / reproducing apparatus to be provided with spherical aberration correcting means for focusing the light beam on each information recording surface.

  An example of this spherical aberration correction means is disclosed in Patent Document 1, and “when a collimating lens unit is provided between the light source and the objective lens and the thickness of the cover layer deviates from the reference value and spherical aberration occurs, the collimating lens is moved back and forth. To cancel the generated spherical aberration. "

  On the other hand, technical development of a BD multilayer disc having three or more information recording surfaces, for example, four information recording surfaces, is in progress (Non-Patent Document 1).

JP 2004-103087 A OPTRONICS (2007) NO.6, p126-p127

However, the BD multi-layer disc having information recording surfaces such as three surfaces and four surfaces naturally has the closest information recording surface as viewed from the surface on the light incident side as compared with the BD double layer disc having two information recording surfaces. The difference in the cover layer thickness on the farthest information recording surface becomes large. Therefore, it is expected that the spherical aberration correction is insufficient on each information recording surface of the BD multilayer disc only by means of moving the collimating lens back and forth as described in Patent Document 1, and the light spot in the focused state is expected to be insufficient. There is concern that it is difficult to maintain good quality.
SUMMARY OF THE INVENTION In view of the above, the present invention provides an optical pickup capable of maintaining good light spot quality in a focused state on each information recording surface of a BD multilayer disc having three or more information recording surfaces. With the goal.

  The object described above can be realized by the configuration and means described in the claims of the present invention as an example.

  According to the present invention, an optical pickup capable of maintaining good quality of a light spot in a focused state on each information recording surface of a BD multilayer disc having three or more information recording surfaces, and capable of good recording and reproduction. Can be provided.

  Embodiments of the optical pickup of the present invention will be described below with reference to the drawings.

  FIG. 1A shows an information recording medium 104 (BD multilayer disc) having three information recording surfaces 101, 102, and 103 indicated by dotted lines and a BD optical pickup, and FIG. It is the figure which expanded and showed.

  First, the optical pickup for BD of this embodiment will be described with reference to FIG. A BD laser light source 105 emits a linearly polarized divergent beam having a wavelength of 405 nm, which is converted into a substantially parallel light beam by a BD collimator lens 109 via a polarizing beam splitter 106, a light beam multi-splitting element 107, and a BD auxiliary lens 108. The The BD collimator lens 109 moves in the direction of the optical axis 111 indicated by an arrow 110 by a spherical aberration correction mechanism (not shown) using, for example, a stepping motor or a piezoelectric element. The spherical aberration correction mechanism (not shown) is provided with a position detection sensor (not shown) that detects a position such as an initial position of the BD collimator lens 109.

  The light beam multi-dividing element 107 is an element in which a polarizing grating (not shown) and a quarter wavelength plate are bonded and integrated, and the polarizing grating (not shown) receives a linearly polarized light beam in a predetermined direction. It has a function of diffracting and transmitting a linearly polarized light beam in a direction orthogonal to the predetermined direction. That is, the beam multi-splitting element 107 transmits a light beam that passes from the left to the right of the paper and diffracts a light beam that passes from the right to the left of the paper. The light beam emitted from the polarization beam splitter 106 is transmitted through the polarizing grating (not shown) of the light beam multi-splitting element 107 without being diffracted, and is circularly polarized by the quarter wavelength plate (not shown). Converted. The light beam emitted from the BD collimator lens 109 is vertically bent by the BD rising mirror 112 and then condensed by the BD objective lens 113, and the three information recording surfaces 101, 102, 103 of the information recording medium 104 are collected. The target information recording surface is irradiated. Reference numeral 114 denotes a circular aperture provided to limit the light beam incident on the BD objective lens 113 to a desired diameter. The BD objective lens 113 is a single lens made of glass or resin.

  The light beam reflected by the information recording surface is incident on the light beam multi-splitting element 107 through the BD objective lens 113, the BD rising mirror 112, the BD collimator lens 109, and the BD auxiliary lens 108. This incident light beam is converted from circularly polarized light to linearly polarized light in the direction orthogonal to the forward path (optical path from the BD laser light source 105 to the BD objective lens 113) by the above-described (not shown) quarter-wave plate, and by the polarizing grating Divided into a plurality of light beams. The plurality of light beams reach the light receiving unit 116 of the BD photodetector 115 through the polarization beam splitter 106. In this embodiment, as a servo signal detection method, a knife edge method is used for a focus error signal (hereinafter referred to as FES), and a push-pull (hereinafter referred to as PP) method is used for a tracking error signal (hereinafter referred to as TES). The position of the focused spot irradiated on the information recording medium 104 is controlled. The knife edge method and the PP method are well-known techniques, and thus the description thereof is omitted here.

  In the BD optical system, a BD objective lens 113 having a numerical aperture of 0.85, which is higher than that of DVD, is used in order to reduce the focused spot on the information recording surface. However, the spherical aberration caused by the cover layer thickness error on the information recording surface or the difference in the cover layer thickness between the information recording surfaces increases in proportion to the fourth power of the numerical aperture of the objective lens. In the optical system, the means for correcting the spherical aberration is essential. In this embodiment, from the viewpoint of miniaturization, a collimating lens system is constituted by two lenses of the BD auxiliary lens 108 and the BD collimating lens 109, and the BD collimating lens 109 is formed by the spherical aberration correction mechanism (not shown). By moving in the optical axis direction, the light beam incident on the BD objective lens 113 is converted from parallel light into weakly divergent or weakly convergent light, and correction is performed so as to cancel the generated spherical aberration.

  Depending on the space where the optical pickup can be mounted, the collimating lens system is a single collimating lens, or a collimating lens is followed by a beam expander consisting of a concave lens and a convex lens that expands incident parallel light and emits parallel light. The spherical aberration correction means may be used. In addition, a liquid crystal lens including a liquid crystal layer and a diffractive lens may be used as the spherical aberration correction unit.

  Also, a diffraction grating for forming three spots (not shown) is provided between the BD laser light source 105 and the polarizing beam splitter 106, the light beam multi-dividing element 107 is changed to a quarter wavelength plate, and the polarizing beam splitter 106 and the BD A BD optical pickup may be configured by providing a cylindrical lens (not shown) between the photodetectors 115. In this case, as a servo signal detection method, the astigmatism method is used for the focus error signal, and the differential push-pull (DPP) method is used for the tracking error signal, and the position of the focused spot irradiated on the information recording medium 104 is determined. Control.

  Next, the information recording medium 104 (BD multilayer disc) will be described with reference to FIG. The information recording medium 104 has 101 (L2 layer), 102 (L1 layer), and 103 (L0 layer) indicated by dotted lines in order from the surface 117 when viewed from the light incident side of the light beam condensed by the BD objective lens 113. It has three information recording surfaces. 118 shown with a broken line shows the intermediate surface of said 101 (L2 layer) and 103 (L0 layer). In this figure, the distance from the surface 117 to 101 (L2 layer) is c, the distance to 102 (L1 layer) is d, the distance to 103 (L0 layer) is e, and the distance to the intermediate plane 118 is B. To do. In this embodiment, for example, c = 60 μm, d = 75 μm, e = 100 μm, and B = (60 + 100) / 2 = 80 μm.

  The BD objective lens 113 will be further described with reference to FIG. FIG. 2A shows a state in which the parallel light 201 is incident on the BD objective lens 113 and is focused, and the focal point 202 is formed on the information recording medium 104. The focal point 202 is located at a distance T from the surface 117 on the light incident side to the broken line 203, and at this position, the wavefront aberration of the light spot is minimized or the spot diameter is minimized. Hereinafter, the distance T will be referred to as a reference cover layer thickness of the objective lens. The broken line 203 indicates the position of the reference cover layer of the objective lens.

  FIG. 2B shows a ray diagram in the state of FIG. In this embodiment, when the wavelength of the BD objective lens 113 is 405 nm, the numerical aperture (NA) is 0.85, the effective light beam diameter φD is about 2.4 mm, and a certain reference cover layer thickness T is set, the first surface The shape of the BD objective lens 113 is determined by performing optimization calculation for the curvature radius of 204 and the second surface 205, the conic constant, and the (even order) aspherical constant. The other parameters, the refractive index and the axial center thickness, are the same.

For comparison with FIG. 1, FIG. 3 shows an information recording medium 301 (BD double-layer disc) having two information recording surfaces 303 and 304 indicated by dotted lines. The information recording medium 301 has two information recording surfaces 303 (L1 layer) and 304 (L0 layer) indicated by dotted lines in order from the surface 302 when viewed from the light incident side of the light beam condensed by the BD objective lens 113. have. 305 indicated by a broken line indicates an intermediate surface between the 303 (L1 layer) and the 304 (L0 layer). In this figure, the distance from the surface 302 to 303 (L1 layer) is represented as a, the distance from 304 (L0 layer) to b, and the distance to the intermediate surface 305 as A. In the case of a BD dual-layer disc, the nominal values of a and b are determined by the standard as a = 75 μm and b = 100 μm, respectively.
A = (75 + 100) /2=87.5 μm.

  FIG. 4 shows the BD objective lens 113 in which the reference cover layer thickness T of the BD objective lens 113 described with reference to FIG. An example is shown in which the wavefront aberration of the condensed spot on the information recording surface 103 (L0 layer), 102 (L1 layer), and 101 (L2 layer) is calculated.

  4A shows a ray diagram in the system of the BD auxiliary lens 108, the BD collimating lens 109, and the reference cover layer 203. FIG. The BD collimator lens 109 is at the reference position, and shows a state in which the parallel light 402 is emitted from the emission surface 401 and is incident on the BD objective lens 113.

  The BD auxiliary lens 108 is fixed. When the BD collimating lens 109 moves in the + direction of the arrow 110 along the optical axis, that is, in the direction approaching the BD objective lens 113, convergent light is emitted from the emission surface 401. On the other hand, when the BD collimator lens 109 moves in the negative direction of the arrow 110, that is, in a direction away from the BD objective lens 113, divergent light is emitted from the emission surface 401. In this embodiment, the combined focal length of the BD auxiliary lens 108 and the BD collimating lens 109 is about 17 mm, and the focal length of the BD collimating lens 109 is about 10 mm.

FIG. 4B shows the information recording surfaces 103 (L0 layer), 102 (L1 layer), and 101 (L2 layer) in a state where the BD collimating lens 109 is at the reference position and spherical aberration correction is not performed.
The example which calculated the wavefront aberration of the light spot in FIG. Here, the reference cover layer thickness T of the BD objective lens 113 was set to 92.5 μm, 87.5 μm, 83.75 μm, 80 μm, and 75 μm. When the cover layer thickness of the information recording surface matches the reference cover layer thickness of the BD objective lens 113, the wavefront aberration is almost zero and is minimized. However, when the BD objective lens 113 deviates from the reference cover layer thickness, the wavefront aberration (spherical aberration) suddenly increases, and the L0 layer (cover layer thickness 100 μm) and L1 layer (cover layer thickness 75 μm). , A large spherical aberration (mostly a third order component) of about 0.25 to 0.3 λ rms occurs between the L2 layers (cover layer thickness 60 μm).

  FIG. 4 (c) calculates the wavefront aberration remaining in the light spot after moving the BD collimating lens 109 from the reference position in the direction of the arrow 110 and correcting the spherical aberration generated in FIG. 4 (b). An example is shown. The wavefront aberration of the light spot between the L0 layer, the L1 layer, and the L2 layer is greatly improved to about 0.017 λrms or less (residual spherical aberration is a higher-order component of the fifth or higher order). When the reference cover layer thickness T of the BD objective lens 113 is in the range of 80 μm <T <87.5 μm, the residual wavefront aberration is suppressed on the average on the information recording surfaces of the L0 layer, L1 layer, and L2 layer ( As indicated by a two-dot chain line 403, the maximum value is 0.013λ rms). In particular, when T = (80 + 87.5) /2=83.75 μm, the residual wavefront aberration is best suppressed on average on each information recording surface of the L0 layer, the L1 layer, and the L2 layer (two-dot chain line) As shown by 404, it is understood that the maximum is less than 0.01λ rms).

  The above 87.5 μm corresponds to A described in FIG. 3, and the above 80 μm corresponds to B described in FIG. From the description so far, the spherical aberration can be suppressed on average when the condition B <T <A between the L0 layer, the L1 layer, and the L2 layer, and the average is best when the condition T = (A + B) / 2. The effect that spherical aberration can be suppressed is obtained.

  FIG. 5 shows an example in which the spot diagram of the light spot after the spherical aberration correction shown in FIG. Similarly to FIG. 4C, when the reference cover layer thickness T of the BD objective lens 113 is in the range of 80 μm <T <87.5 μm in the L0 layer, the L1 layer, and the L2 layer, the light spot is averaged. In addition, when T = (80 + 87.5) /2=83.75 μm, the effect that the light spot is narrowed down on average can be obtained.

  FIG. 6 shows an example in which the amount of movement of the BD collimator lens 109 necessary for correcting the spherical aberration shown in FIG. 4C is calculated. Here, the amount of movement 0 indicates a state in which the BD collimator lens 109 is at the reference position described with reference to FIG. 4A, that is, a state in which parallel light is emitted from the emission surface 401 of the BD collimator lens 109. Depending on the reference cover layer thickness of the BD objective lens 113, the required amount of movement of the BD collimator lens 109 on each of the information recording surfaces of the L0 layer, the L1 layer, and the L2 layer is different, but the reference cover layer thickness of any BD objective lens 113 In this case, when the BD collimator lens 109 is moved by 1 mm in the optical axis direction, the spherical aberration caused by the cover layer thickness error of about 40 μm can be corrected. In order to prevent an increase in the size of the spherical aberration correction mechanism (not shown), it is necessary to prevent an extreme difference between the + and − directions of the movement amount of the BD collimator lens 109. In this respect, the conditions of the reference cover layer thickness T of the BD objective lens 113 described with reference to FIG. 4, that is, 80 μm <T <87.5 μm and T = (80 + 87.5) /2=83.75 μm are desirable. It's a condition.

  7A, when the BD collimator lens 109 is at the reference position as shown in FIG. 7A, the distance from the exit surface 401 to the aperture 114 of the BD objective lens 113 is set to 10 mm. In this example, the intensity of the light beam emitted from the BD objective lens 113 is calculated while the intensity of the light beam emitted from the BD is constant.

  FIG. 7B shows information on the L0 layer, the L1 layer, and the L2 layer when the BD collimator lens 109 is moved in the direction of the arrow 110 and the spherical aberration correction described with reference to FIGS. 4 and 6 is performed. An example in which the emission intensity from the BD objective lens 113 on the recording surface is calculated is shown. The reference cover layer thickness T of the BD objective lens 113 is the same setting as in FIG. In this figure, in the state where the parallel light 402 emitted from the emission surface 401 of the BD collimator lens 109 is incident on the BD objective lens 113, the value normalized by the emission intensity from the BD objective lens 113 is output from the objective lens on the vertical axis of the graph. Strength [standardized value] was used. As a matter of course, in a state where parallel light is incident on the BD objective lens 113, the output intensity from the BD objective lens 113 is the same regardless of the reference cover layer thickness of any of the BD objective lenses 113 described above.

  In the L0 layer and the L1 layer, although there is a slight difference in the reference cover layer thickness of any BD objective lens 113, the objective lens emission intensity is substantially equal. On the other hand, in the L2 layer, there is a difference in the output intensity of the objective lens depending on the reference cover layer thickness of the BD objective lens 113. As shown in FIG. 6, as the reference cover layer thickness T of the BD objective lens 113 decreases, the amount of movement of the BD collimator lens 109 necessary for spherical aberration correction increases and enters the BD objective lens 113. This is because there is a difference in the state of convergent light (light utilization efficiency is in a better direction). However, in the L0 layer, the L1 layer, and the L2 layer, the output intensity of the objective lens is 1 (two-dot chain line 701) to 1.07 (two-dot chain line 702) when the reference cover layer thickness T is 80 μm <T <87.5 μm. ), Particularly in the case of T = (80 + 87.5) /2=83.75 μm, it falls within the range of 1 (two-dot chain line 701) to 1.05 (two-dot chain line 703), which is practically hindered. It can be said that there is no level.

  If the change in the output intensity of the objective lens is extremely large in the L0 layer, the L1 layer, and the L2 layer, it is necessary to control the emission intensity from the BD laser light source 105 for each information recording surface, and the BD laser light source 105 is controlled. It will be complicated. However, in the case of the present embodiment, since the change in the output intensity of the objective lens is small on each information recording surface, the effect that the control of the BD laser light source 105 is simplified without being complicated is obtained.

  FIG. 8 shows that the reference cover layer thickness T of the BD objective lens 113 is fixed at T = (80 + 87.5) /2=83.75 μm, and the BD collimator lens 109 is at the reference position as shown in FIG. In this case, the distance L from the emission surface 401 to the aperture 114 of the BD objective lens 113 is changed, and the intensity of the light beam emitted from the BD laser light source 105 is made constant, so that the light beam emitted from the BD objective lens 113 is The example which calculated the intensity | strength is shown. FIG. 8B shows the result. In this figure, in the state where the parallel light 402 emitted from the emission surface 401 of the BD collimator lens 109 is incident on the BD objective lens 113, the value normalized by the emission intensity from the BD objective lens 113 is output from the objective lens on the vertical axis of the graph. Strength [standardized value] was used. As a matter of course, in a state where parallel light is incident on the BD objective lens 113, the intensity of emission from the BD objective lens 113 is equal regardless of the distance L.

  In the range of the arrow 804 where two curves out of the three curves of the curve 801 (L0 layer), the curve 802 (L1 layer), and the curve 803 (L2 layer) intersect, that is, 5 mm ≦ L ≦ 10 mm, the information recording surface The difference in the output intensity of the objective lens among the L0 layer, the L1 layer, and the L2 layer is small and stable within about 5%. Further, the output intensity of the objective lens is most stable when L = about 7 mm shown by the solid line 805. When the focal length of the BD objective lens 113 is expressed as f0, the range of the arrow 804 in which the difference in the objective lens output intensity is stable is expressed as about 3.6 ≦ (L / f0) ≦ about 7, and is most objective. A solid line 805 where the lens emission intensity is stable can be expressed as L / f0≈5. When the focal length of the BD collimator lens 109 is expressed as fc, fc = 10 mm. Therefore, the range of the arrow 804 in which the difference in the output intensity of the objective lens is stable is expressed as 0.5 ≦ (L / fc) ≦ 1. The solid line 805 where the output intensity of the objective lens is most stable can be expressed as L / fc≈0.7. As described above, by defining the condition of the reference cover layer thickness T of the BD objective lens 113 and the conditions L / f0 and L / fc of the spherical aberration correcting element, any of the L0 layer, the L1 layer, and the L2 layer is defined. Also on the information recording surface, the effect that the output intensity of the objective lens can be stabilized can be obtained.

In the present embodiment described above, the following symbols are used to summarize the obtained effects as follows.
A: Distance from the surface on the light incident side to the intermediate surface of the L1 layer and the L0 layer in the BD double layer disc. B: Layer L2 and L0 layer from the surface on the light incident side in the BD multilayer disc (three layers). L: Distance from the intermediate plane L: Distance from the exit surface to the aperture of the BD objective lens when the BD collimator lens is at the reference position f0: Focal length of the BD objective lens, fc: Focal length of the BD collimator lens For an information recording medium (BD multi-layer disc) having an information recording surface, from the viewpoint of spherical aberration correction, the condition of the reference cover layer thickness T of the BD objective lens 113 is sufficiently defined as B <T <A. By correcting the spherical aberration, the spherical aberration of the light spot on each information recording surface can be suppressed on the average, and by defining the condition T = (A + B) / 2, the spherical aberration can be corrected further and the average on the best each Effect that the spherical aberration of the light spot broadcast recording surface can be suppressed. Further, also from the viewpoint of the light spot after the spherical aberration correction, by defining the same conditions as those for the spherical aberration correction, it is possible to obtain an effect that the light spot can be narrowed on average on each information recording surface. Furthermore, also from the viewpoint of the objective lens output intensity, by defining the same conditions as the spherical aberration correction, it is possible to obtain an effect that the objective lens output intensity can be stabilized on each information recording surface. In addition, regarding the spherical aberration correction element, by defining a combination of the condition of the reference cover layer thickness T of the BD objective lens 113 and the conditions L / f0 and L / fc of the spherical aberration correction element, any information recording surface can be used. Also, the effect that the output intensity of the objective lens can be stabilized can be obtained.

  FIG. 9A shows an information recording medium 905 (BD multilayer disc) having four information recording surfaces 901, 902, 903, and 904 indicated by dotted lines, and an BD optical pickup, and FIG. 9B shows information recording. FIG. 6 is an enlarged view of a medium 905. Since the BD optical pickup is the same as that of the first embodiment, the description thereof is omitted here.

  The information recording medium 905 (BD multi-layer disc) will be described with reference to FIG. The information recording medium 905 is seen from the light incident side of the light beam condensed by the BD objective lens 113, and is indicated by dotted lines 901 (L3 layer), 902 (L2 layer), and 903 (L1 layer) in order from the surface 906, respectively. , 904 (L0 layer) has four information recording surfaces. 907 indicated by a broken line indicates an intermediate surface between the above 901 (L3 layer) and 903 (L0 layer). In this figure, the distance from the surface 906 to 901 (L3 layer) is f, the distance to 902 (L2 layer) is g, the distance to 903 (L1 layer) is h, and the distance to 904 (L0 layer) is i. The distance to the intermediate plane 907 is denoted as B. In this embodiment, for example, f = 54 μm, g = 69 μm, h = 87 μm, i = 100 μm, and B = (54 + 100) / 2 = 77 μm.

  FIG. 10 shows a BD objective lens in which the reference cover layer thickness T of the BD objective lens 113 is changed as described with reference to FIG. 113 shows an example in which the wavefront aberration of the focused spot on each information recording surface 904 (L0 layer), 903 (L1 layer), 902 (L2 layer), and 901 (L3 layer) is calculated.

  Since the system of the BD auxiliary lens 108, the BD collimating lens 109, and the reference cover layer 203 is the same as that in FIG. 4A of the first embodiment, description thereof is omitted here.

  Here, the reference cover layer thickness T of the BD objective lens 113 was set to 92.5 μm, 87.5 μm, 82.25 μm, 77 μm, and 72 μm. As shown in FIG. 10A, when the cover layer thickness of the information recording surface coincides with each reference cover layer thickness of the BD objective lens 113, the wavefront aberration is almost zero and becomes the minimum. However, when the BD objective lens 113 deviates from each reference cover layer thickness, the wavefront aberration (spherical aberration) suddenly increases, and the L0 layer (cover layer thickness 100 μm) and the L1 layer (cover layer thickness 87 μm). A large spherical aberration (mostly a third order component) of about 0.25 to about 0.4λ rms occurs between the L2 layer (cover layer thickness 69 μm) and the L3 layer (cover layer thickness 54 μm).

  FIG. 10B shows the spherical aberration generated in FIG. 10A corrected by moving the BD collimating lens 109 from the reference position in the direction of the arrow 110 shown in FIG. 4A of the first embodiment. Later, an example of calculating the wavefront aberration remaining in the light spot will be described. The wavefront aberration of the light spot between the L0 layer, the L1 layer, the L2 layer, and the L2 layer is greatly improved to about 0.023 λ rms or less (residual spherical aberration is a higher order component of the fifth or higher order). When the reference cover layer thickness T of the BD objective lens 113 is in the range of 77 μm <T <87.5 μm, the residual wavefront aberration is averaged on the information recording surfaces of the L0 layer, L1 layer, L2 layer, and L3 layer. Is suppressed (maximum 0.015 λ rms as indicated by a two-dot chain line 1001). In particular, when T = (77 + 87.5) /2=82.25 μm, the residual wavefront aberration is best suppressed on average on each of the information recording surfaces of the L0 layer, L1 layer, L2 layer, and L3 layer (2 It can be seen that the maximum value is 0.011 λ rms) as indicated by a dashed line 1002.

The above 87.5 μm corresponds to A described in FIG. 3 of the first embodiment, and the above 77 μm corresponds to B described in FIG. 9B of the present embodiment. From the description so far, spherical aberration can be suppressed on average when the condition B <T <A, and most average when condition T = (A + B) / 2, between the L0 layer, the L1 layer, the L2 layer, and the L3 layer. The effect that spherical aberration can be suppressed well is obtained.
FIG. 11 shows an example in which the spot diagram of the light spot after the spherical aberration correction shown in FIG. 10B is calculated. Similarly to FIG. 10B, when the reference cover layer thickness T of the BD objective lens 113 is in the range of 77 μm <T <87.5 μm in the L0 layer, L1 layer, L2 layer, and L3 layer, light is averaged. When the spot is narrowed down and T = (77 + 87.5) /2=82.25 μm, the effect that the light spot is best narrowed on average is obtained.

FIG. 12 shows an example in which the amount of movement of the BD collimator lens 109 necessary for the spherical aberration correction shown in FIG. 10B is calculated. Here, the movement amount 0 indicates a state in which the BD collimator lens 109 is at the reference position described with reference to FIG. 4A of Embodiment 1, that is, a state in which parallel light is emitted from the emission surface 401 of the BD collimator lens 109. . Depending on the thickness of the reference cover layer of the BD objective lens 113, the amount of movement of the BD collimator lens 109 required on the information recording surfaces of the L0 layer, L1, L2, and L3 layers varies. Even in the cover layer thickness, when the BD collimating lens 109 moves 1 mm in the optical axis direction, the spherical aberration caused by the cover layer thickness error of about 40 μm can be corrected. In order to prevent an increase in the size of the spherical aberration correction mechanism (not shown), it is necessary to prevent an extreme difference between the + and − directions of the movement amount of the BD collimator lens 109. In this respect, the conditions of the reference cover layer thickness T of the BD objective lens 113 described with reference to FIG. 10, that is, 77 μm <T <87.5 μm and T = (77 + 87.5) /2=82.75 μm are desirable. FIG. 13 which can be referred to as the condition, as shown in FIG. 7A of the first embodiment, when the BD collimator lens 109 is at the reference position, the distance from the exit surface 401 to the aperture 114 of the BD objective lens 113 is 10 mm. In this example, the intensity of the light beam emitted from the BD objective lens 113 is calculated with the intensity of the light beam emitted from the BD laser light source 105 set constant.

  The BD collimator lens 109 is moved in the direction of the arrow 110, and the spherical aberration correction described with reference to FIGS. 10 and 11 is performed, and the L0 layer, the L1 layer, the L2 layer, and the L3 layer on each information recording surface. The emission intensity from the BD objective lens 113 was calculated. The reference cover layer thickness T of the BD objective lens 113 is the same setting as in FIG. In this figure, as in Example 1, the value normalized by the emission intensity from the BD objective lens 113 in the state where the parallel light 402 emitted from the emission surface 401 of the BD collimator lens 109 is incident on the BD objective lens 113 is shown. It was set as the objective lens output intensity [normalized value] on the vertical axis of the graph. As a matter of course, in a state where parallel light is incident on the BD objective lens 113, the output intensity from the BD objective lens 113 is the same regardless of the reference cover layer thickness of any of the BD objective lenses 113 described above.

  In the L0 layer and the L1 layer, although there is a slight difference in the reference cover layer thickness of any BD objective lens 113, the objective lens emission intensity is substantially equal. On the other hand, in the L2 layer and the L3 layer, there is a difference in the objective lens output intensity depending on the reference cover layer thickness of the BD objective lens 113. As shown in FIG. 12, as the reference cover layer thickness T of the BD objective lens 113 becomes thinner, the amount of movement of the BD collimator lens 109 necessary for spherical aberration correction increases and enters the BD objective lens 113. This is because there is a difference in the state of convergent light (light utilization efficiency is in a better direction). However, in the L0 layer, the L1 layer, the L2 layer, and the L3 layer, when the reference cover layer thickness T is 77 μm <T <87.5 μm, the objective lens output intensity is 1 (two-dot chain line 1301) to about 1.085. In the range of (two-dot chain line 1302), in particular, when T = (77 + 87.5) /2=82.25 μm, it falls within the range of 1 (two-dot chain line 1301) to about 1.06 (two-dot chain line 1303). Therefore, it can be said that there is no problem in practical use.

  If the change in the output intensity of the objective lens is extremely large in the L0 layer, the L1 layer, and the L2 layer, it is necessary to control the emission intensity from the BD laser light source 105 for each information recording surface, and the BD laser light source 105 is controlled. It will be complicated. However, in the case of the present embodiment, since the change in the output intensity of the objective lens is small on each information recording surface, the effect that the control of the BD laser light source 105 is simplified without being complicated is obtained.

  FIG. 14 shows a case where the reference cover layer thickness T of the BD objective lens 113 is fixed to T = (77 + 87.5) /2=82.25 μm, and as shown in FIG. When 109 is at the reference position, the distance L from the emission surface 401 to the aperture 114 of the BD objective lens 113 is changed, and the intensity of the light beam emitted from the BD laser light source 105 is made constant to be emitted from the BD objective lens 113. In this figure showing an example of calculating the intensity of the light beam to be emitted, in the state where the parallel light 402 emitted from the emission surface 401 of the BD collimator lens 109 is incident on the BD objective lens 113 as in the first embodiment, the BD The value normalized by the output intensity from the objective lens 113 was defined as the objective lens output intensity [normalized value] on the vertical axis of the graph. As a matter of course, in a state where parallel light is incident on the BD objective lens 113, the intensity of emission from the BD objective lens 113 is equal regardless of the distance L.

  A range of an arrow 1405 where two curves out of four curves of a curve 1401 (L0 layer), a curve 1402 (L1 layer), a curve 1403 (L2 layer), and a curve 1404 (L3 layer) intersect, that is, 4 mm ≦ When L ≦ 10 mm, the difference in the output intensity of the objective lens among the information recording surfaces L0, L1, L2, and L3 is small and stable within about 7%. Furthermore, the objective lens output intensity is most stable when L = about 6.3 mm indicated by a solid line 1406. When the focal length of the BD objective lens 113 is expressed as f0, the range of the arrow 1405 where the difference in the objective lens output intensity is stable is expressed as about 2.8 ≦ (L / f0) ≦ about 7, and the most objective The solid line 1406 can be expressed as L / f0≈4.5 when the lens emission intensity is stabilized. When the focal length of the BD collimator lens 109 is expressed as fc, fc = 10 mm. Therefore, the range of the arrow 1405 in which the difference in the output intensity of the objective lens is stabilized is 0.4 ≦ (L / fc) ≦ 1. The solid line 1406 where the output intensity of the objective lens is most stable can be expressed as L / fc≈0.63. Thus, by defining the conditions of the reference cover layer thickness T of the BD objective lens 113 and the conditions L / f0 and L / fc of the spherical aberration correction element in combination, the L0 layer, L1 layer, L2 layer, L3 The effect that the output intensity of the objective lens can be stabilized is obtained on any information recording surface of the layer.

In the present embodiment described above, the following symbols are used to summarize the obtained effects as follows.
A: Distance between the L1 layer and the L0 layer when viewed from the surface on the light incident side in the BD double-layer disc. B: L3 layer and L0 layer as viewed from the surface on the light incident side in the BD multilayer disc (four layers). L: the distance from the exit surface to the aperture of the BD objective lens when the BD collimator lens is at the reference position f0: the focal length of the BD objective lens, fc: the focal length of the BD collimator lens For an information recording medium (BD multi-layer disc) having an information recording surface, from the viewpoint of spherical aberration correction, the condition of the reference cover layer thickness T of the BD objective lens 113 is sufficiently defined as B <T <A. By correcting the spherical aberration, the spherical aberration of the light spot on each information recording surface can be suppressed on the average, and by defining the condition T = (A + B) / 2, the spherical aberration can be corrected further and the average on the best each Effect that the spherical aberration of the light spot broadcast recording surface can be suppressed. Further, also from the viewpoint of the light spot after the spherical aberration correction, by defining the same conditions as those for the spherical aberration correction, it is possible to obtain an effect that the light spot can be narrowed on average on each information recording surface. Furthermore, also from the viewpoint of the objective lens output intensity, by defining the same conditions as the spherical aberration correction, it is possible to obtain an effect that the objective lens output intensity can be stabilized on each information recording surface. In addition, regarding the spherical aberration correction element, by defining a combination of the condition of the reference cover layer thickness T of the BD objective lens 113 and the conditions L / f0 and L / fc of the spherical aberration correction element, any information recording surface can be used. Also, the effect that the output intensity of the objective lens can be stabilized can be obtained.

  From the above, by applying the present invention, the quality of the light spot in the in-focus state can be kept good on each information recording surface of the BD multilayer disc having three or more information recording surfaces, and good recording, An effect is obtained that an optical pickup capable of reproduction and a recording / reproducing apparatus can be realized. Even if the present invention is applied to a BD double-layer disc having two information recording surfaces, the quality of the light spot in the focused state is not greatly deteriorated on each information recording surface. There is also an advantage of compatibility.

  Although the optical pickup for BD has been described in the first and second embodiments, the present invention is not limited to the BD, and for example, by adding a DVD / CD optical system using a DVD / CD compatible objective lens. It is possible to realize an optical pickup compatible with BD / DVD / CD. In the first and second embodiments, the objective lens has been described as a single lens dedicated to the BD. However, the objective lens is not limited to this. For example, the phase of the incident wavefront is directly below the single lens dedicated to the BD. It is also possible to realize an optical pickup compatible with BD / DVD / CD with a single objective lens by integrally providing a phase compensation plate having a function of compensating for spherical aberration by changing in the case of CD. In addition, it is possible to form an objective lens made of resin and provide a diffraction groove on the incident surface to obtain an objective lens compatible with BD / DVD and to realize an optical pickup compatible with BD / DVD.

  So far, an embodiment of an optical pickup corresponding to a BD multilayer disk having three or more information recording surfaces has been described. In this embodiment, an embodiment of an optical information recording / reproducing apparatus equipped with the optical pickup is described. This will be described with reference to FIG.

  FIG. 15 shows a schematic block diagram of an information recording / reproducing apparatus 1501 for recording and reproducing information. Reference numeral 1502 denotes an optical pickup according to the present invention, and a signal detected from the optical pickup 1502 is sent to a servo signal generation circuit 1503 and an information signal reproduction circuit 1504 in the signal processing circuit.

  The servo signal generation circuit 1503 generates a focus control signal, tracking control signal, and spherical aberration detection signal suitable for the optical disc 1505 from the signal detected by the optical pickup 1502, and passes through the objective lens drive control circuit 1506 based on these. An objective lens actuator (not shown) in the optical pickup 1502 is driven to control the position of the objective lens 1507. The servo signal generation circuit 1503 generates a spherical aberration detection signal from the optical pickup 1502, and a spherical aberration correction optical system (not shown) in the optical pickup 1502 passes through the spherical aberration correction drive circuit 1508 based on this signal. Drive the correction lens. The information signal reproduction circuit 1504 reproduces the information signal recorded on the optical disc medium 1505 from the signal detected from the optical pickup 1502 and outputs the information signal to the information signal output terminal 1509. Note that some of the signals obtained by the servo signal generation circuit 1503 and the information signal reproduction circuit 1504 are sent to the system control circuit 1510.

  A laser drive signal is sent from the system control circuit 1510 to drive the laser light source lighting circuit 1511 to control the light emission amount using a front monitor (not shown), and to the optical disk medium 1505 via the optical pickup 1502. Record the signal. Note that an access control circuit 1512 and a spindle motor drive circuit 1513 are connected to the system control circuit 1510, and access direction position control of the optical pickup 1502 and rotation control of the spindle motor 1514 of the optical disk 1505 are performed, respectively. When the user controls the information recording / reproducing apparatus 1501, the control is performed by the user instructing the user input processing circuit 1515 from the user input apparatus 1517. At this time, the processing status and the like of the information recording / reproducing apparatus are displayed by the display processing circuit 1516, and the processing status is displayed on the display device 1518.

2 is a diagram illustrating an information recording medium having three information recording surfaces and a BD optical pickup in Embodiment 1. FIG. FIG. 3 is a diagram illustrating a BD objective lens in Example 1. 2 is a diagram illustrating an information recording medium having two information recording surfaces in Embodiment 1. FIG. In Example 1, the graph which shows the example which calculated the wavefront aberration of the condensing spot in each information recording surface L0 layer, L1 layer, and L2 layer. FIG. 3 is a diagram illustrating an example of calculating a spot diagram of a light spot after spherical aberration correction in the first embodiment. FIG. 3 is a diagram illustrating an example of calculating a moving amount of a BD collimating lens necessary for spherical aberration correction in the first embodiment. 6 is a graph showing an example of calculating the intensity of a light beam emitted from a BD objective lens in Example 1. FIG. In Example 1, the graph which shows the example which calculated the intensity | strength of the light beam radiate | emitted from a BD objective lens by changing the distance from the BD collimating-lens exit surface to the aperture of a BD objective lens. FIG. 6 is a diagram illustrating an information recording medium having four information recording surfaces and a BD optical pickup in the second embodiment. In Example 2, the graph which shows the example which calculated the wavefront aberration of the condensing spot in each information recording surface L0 layer, L1 layer, L2 layer, and L3 layer. In Example 2, the figure which shows the example which calculated the spot diagram of the light spot after spherical aberration correction. In Example 2, the graph which shows the example which calculated the movement amount of the BD collimating lens required for spherical aberration correction. In Example 2, the graph which shows the example which calculated the intensity | strength of the light beam radiate | emitted from a BD objective lens. In Example 2, the graph which shows the example which changed the distance from the BD collimating-lens exit surface to the aperture of a BD objective lens, and calculated the intensity | strength of the light beam radiate | emitted from a BD objective lens. In Example 4, it is a figure which shows the schematic block of the information recording / reproducing apparatus which records and reproduces | regenerates information.

Explanation of symbols

104 Information recording medium 105 having three information recording surfaces (L0 layer, L1 layer, L2 layer) BD laser light source 109 BD collimating lens 113 BD objective lens 118 Intermediate surface 203 between L2 layer and L0 layer 203 Reference cover layer of objective lens Information recording medium 305 having two information recording surfaces (L0 layer, L1 layer) 305 An intermediate surface 905 of L1 layer and L0 layer Four information recording surfaces (L0 layer, L1 layer, L2 layer, L3 layer) Information recording medium 907 having an intermediate surface between the L3 layer and the L0 layer

Claims (8)

A laser light source;
A spherical aberration correction element;
An objective lens capable of condensing the light beam emitted from the laser light source on each information recording surface of an information recording medium having a plurality of information recording surfaces;
When the information recording medium is viewed from the surface on the light incident side, the cover layer thickness at the intermediate surface of the information recording medium having two information recording surfaces is expressed as A,
When the information recording medium is viewed from the surface on the light incident side, the cover layer thickness at the intermediate surface of the information recording medium having three or more information recording surfaces is expressed as B,
A light spot focused on the information recording medium when the information recording medium is viewed from the surface on the light incident side in a state where a parallel light beam is incident on the objective lens with a reference cover layer thickness of the objective lens The reference cover layer thickness T of the objective lens is defined by the following relational expression when expressed as a cover layer thickness T that minimizes the total wavefront aberration or the spot diameter: pick up.
B <T <A 2. The optical pickup according to claim 1 , wherein the reference cover layer thickness T is defined by the following relational expression in the objective lens.
T = (A + B) / 2   3. The optical pickup according to claim 1, wherein the objective lens is formed of a single lens made of glass or resin having a numerical aperture of about 0.85. 4. The optical pickup according to claim 1, wherein the spherical aberration correction element is a collimating lens composed of one or two lenses, a beam expander composed of a concave lens and a convex lens, or a liquid crystal layer and a diffractive lens. An optical pickup comprising a liquid crystal lens comprising: 5. The optical pickup according to claim 1, wherein the spherical aberration correction element is a collimating lens, the focal length of the objective lens is f 0, the focal length of the collimating lens is fc, and the exit surface of the collimating lens When the distance from the objective lens to the aperture of the objective lens is expressed as L, the L, f0, and fc are defined by the following relational expression.
2.5 <(L / f0) <7.5
0.4 ≦ (L / fc) ≦ 1 The optical pickup according to any one of claims 1-5, wherein the laser light source, an optical pickup, which comprises emitting a blue-violet laser beam having a wavelength of 405nm band. An objective lens mounted on an optical pickup for recording and reproducing an information recording medium having a plurality of information recording surfaces,
When the information recording medium is viewed from the surface on the light incident side, the cover layer thickness at the intermediate surface of the information recording medium having two information recording surfaces is expressed as A,
When the information recording medium is viewed from the surface on the light incident side, the cover layer thickness at the intermediate surface of the information recording medium having three or more information recording surfaces is expressed as B,
The reference cover layer thickness of the objective lens is the light condensed on the information recording medium when the information recording medium is viewed from the light incident side surface in a state where a parallel light beam is incident on the objective lens. An objective lens characterized in that the reference cover layer thickness T is defined by the following relational expression when it is expressed as a cover layer thickness T at which the total wavefront aberration of the spot is minimum or the spot diameter is minimum.
B <T <A An optical pickup according to any one of claims 1 to 6, drives the actuator equipped with at least the objective lens, the optical information recording and reproducing apparatus characterized by comprising a circuit for controlling.

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