JP4851204B2 - Optical pickup and optical information processing apparatus - Google Patents

Description

The present invention is made in PMMA, to an optical pickup and an optical information processing apparatus Ru using the optical element changes were small optical function even ambient humidity is changed.

  In recent years, optical pickups and optical information processing apparatuses have been developed that can record and / or reproduce high-density information using a light beam from a semiconductor laser having a shorter wavelength. For example, a system using a blue-violet semiconductor laser having an oscillation wavelength λ of about 405 nm as a light source and an objective lens having a numerical aperture (hereinafter referred to as NA) of 0.85 has been developed. For example, when NA = 0.85, λ = 405 nm, information of about 25 GB can be recorded with respect to the recording capacity of 4.7 GB of the current DVD (NA = 0.6, λ = 660 nm).

  Further, even with an optical pickup capable of recording and / or reproducing such high-density information, it is necessary to ensure the recording and / or reproducing of information even with respect to CDs and DVDs that have been supplied in large quantities. is there. For recording and reproducing using one optical pickup in common with three different recording media, for example, using a single diffractive surface, a next-generation DVD using a light beam with a wavelength λ1 and a conventional light beam using a wavelength λ2 A DVD and a CD with a light beam having a wavelength of λ3 that are compatible with each other have been proposed. Therefore, a diffraction structure having wavelength selectivity is also provided (see Patent Documents 1 and 2).

Furthermore, it is desirable that the diffractive element as described above is made of resin for weight reduction. It can be said that it is suitable for mass production because a uniform shaped product can be quickly produced by molding using a mold.
JP 2005-11481 A JP 2005-158217 A

  However, such an optical element made of a resin material is more likely to be deformed by changes in temperature and humidity than glass, and fluctuations in aberration due to deformation due to changes in the temperature and humidity of this resin material are short wavelength. It becomes remarkable in increasing the NA of the objective lens and the objective lens. Therefore, the fluctuation of the environmental humidity, which has not been a problem with the conventional optical pickup optical system, is an amount that cannot be ignored because the wavefront deterioration increases with the fluctuation of the aberration when the wavelength of the light source is shortened and the NA of the objective lens is increased. There was a problem.

  The present invention is directed to solving the problems of the prior art, and uses an optical element that realizes a resin-made diffractive optical element with reduced wavefront accuracy with reduced deformation even when environmental humidity occurs. An object is to provide an optical pickup and an optical information processing apparatus.

In order to achieve the above object, an optical pickup according to claim 1 of the present invention performs one or more of information recording, reproduction, and erasing on two types of optical recording media having different substrate thicknesses. The first and second light sources having different wavelengths, the objective lens optimized with the first light source wavelength for condensing the light beam on the recording surface of the first optical recording medium, and the second light source And an optical element having a function of correcting aberrations that occur when the luminous flux passes through the substrate of the second optical recording medium.
The optical element is made of PMMA, has a plane perpendicular to the optical axis, has a circular diffraction area coaxial with the optical axis within the effective beam diameter through which the light beam passes, and an outer flat portion. Are divided into two regions.
The diffractive region has one or more diffractive structures whose cross-section is uneven on the surface in the vertical direction within the effective diameter of the light beam having the first light source wavelength, and the light beam having the first light source wavelength. Is transmitted as it is, and the light beam having the second light source wavelength is diffracted so as to correct the spherical aberration caused by the difference in the substrate thickness and the wavelength of the optical recording medium.
Then, the “shape change in the flat portion” due to the moisture absorption of the PMMA is made uniform along the circumferential direction of the concentric circle, or “a circle that is coaxial with the optical axis” or “a polygon shape that approximates the circle” The “transmission wavefront aberration of the first light source wavelength” within the effective beam diameter of the first light source wavelength is 0.02λrms or less.
In this way, by changing the outer shape of the optical element to “a circular shape coaxial with the optical axis or a polygonal shape approximating the circular shape”, the “shape change such as undulation” due to the “influence of moisture absorption” of the PMMA constituting the optical element. Is made uniform along the circumferential direction of the concentric circles.

In addition, the optical pickup according to claims 2 and 3 can correct spherical aberration by reducing the size and weight by making the cross section of the diffraction structure of the optical element rectangular, and by making the cross section of the diffraction structure stepped. Even if the environmental humidity changes, it is possible to obtain an optical element with good wavefront accuracy in which deformation such as swell is reduced.

As described above, the optical pickup according to claim 1 to 3, substrate thickness different two kinds of optical recording medium for recording information, reproducing, an optical pickup for any one or more of the erase light Science Occurs with two different substrate thicknesses and different light sources by correcting the aberration that occurs when the light beam from the second light source passes through the substrate of the second optical recording medium by the diffracted light from the diffractive structure of the element. Therefore, it is possible to provide a highly accurate optical pickup that is small and lightweight.

The optical information processing apparatus according to claim 4 is an optical information processing apparatus that performs one or more of information recording, reproduction, and erasing by irradiating a recording surface of an optical recording medium with a light beam . By including the optical pickup according to any one of the items, it is possible to correct wavefront aberration generated by two different substrate thicknesses and light sources, and to provide a highly accurate optical information processing apparatus.

According to the present invention, it is possible to correct spherical aberration by reducing the size and weight of the aberration correction element, and uniformizing the shape change due to moisture absorption of the resin (PMMA) of the aberration correction element, and responding to changes in environmental humidity. An optical element with improved wavefront accuracy with reduced deformation and the like, an optical pickup using the optical element, and an optical information processing apparatus can be provided.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing a schematic configuration of an optical pickup according to Embodiment 1 of the present invention. As shown in FIG. 1, a compatible optical pickup that records or reproduces information on optical storage media having different substrate thicknesses with different effective pupil radii using a single objective lens 106, using a plurality of light sources having different wavelengths. It is.

In this example , the case where information is recorded on or reproduced from an optical recording medium having two different substrate thicknesses is taken as an example. The substrate thicknesses of the first and second optical recording media 107 and 137 are 0.1 mm and 0.6 mm, respectively, and the first and second numerical apertures are NA 0.85 and NA 0.65, respectively. The light source wavelengths are 405 nm (blue-violet region) and 660 nm (red region), respectively.

As shown in FIG. 1, the optical pickup in this example has a semiconductor laser 101, a collimator lens 102, a prism 104, a quarter wavelength plate 105, an objective lens 106, and a polarization beam splitter 103 with respect to the first optical recording medium 107. , Detection lens 108, aperture limiting element 109, aberration correction element 501, and light receiving element 110.

  The center wavelength of the semiconductor laser 101 as the first light source is 405 nm, the numerical aperture NA of the objective lens 106 is 0.85, and the substrate thickness of the first optical recording medium 107 is 0.1 mm. The light emitted from the semiconductor laser 101 is made into substantially parallel light by the collimator lens 102. The light beam that has passed through the collimator lens 102 enters the polarization beam splitter 103 and is deflected by the prism 104. Information is recorded and reproduced by being focused on the first optical recording medium 107 via the quarter-wave plate 105, the aperture limiting element 109, the aberration correcting element 501, and the objective lens 106.

  Reflected light from the first optical recording medium 107 passes through the objective lens 106 and the quarter-wave plate 105, is then separated from the incident light by the polarization beam splitter 103, and deflected, and is detected by the detection lens 108. 110, the reproduction signal, the focus error signal, and the track error signal are detected.

  Similarly, the light beam emitted from the semiconductor laser 130 a having a center wavelength of 660 nm with respect to the second optical recording medium 137 is deflected by the prism 104 through the divergence angle conversion lens 132 and the wavelength selective beam splitter 133. Then, the light is condensed on the second optical recording medium 137 through the quarter-wave plate 105, the aperture limiting element 109, the aberration correction element 501, and the objective lens 106. The substrate thickness of the second optical recording medium 137 is 0.6 mm, and the numerical aperture NA of the objective lens is 0.65. For switching the numerical aperture NA, a wavelength-selective aperture limiting element 109 is used.

  Reflected light from the second optical recording medium 137 passes through the objective lens 106 and the quarter-wave plate 105 and is then deflected by the wavelength-selective beam splitter 133 and separated from incident light by the hologram element 130b and received. Guided on the element 130c, a reproduction signal, a focus error signal, and a track error signal are detected.

  Here, the objective lens 106 is designed to record and reproduce the first optical recording medium 107 having a thickness of 0.1 mm with high accuracy. The design wavelength is 405 nm, and it is designed so that the wavefront aberration becomes 0.01 λrms or less sufficiently at 405 nm.

  The objective lens 106 has an aspherical shape on both sides, and in a rectangular coordinate system with the vertex of the surface as the origin and the optical axis direction as the X axis, r is the paraxial radius of curvature, κ is the number of cones, A, B, When C, D, E, F, G, H, J,. ,

で表される。各面および各領域の面データを（表１）に示す。

It is represented by The surface data of each surface and each region is shown in (Table 1).

対物レンズ１０６において、ガラス部の硝材は住田光学製のＫＶＣ８１であり、樹脂部の硝材は、波長４０５ｎｍの屈折率はｎ＝１．５６３、波長６６０ｎｍの屈折率はｎ＝１．５３６、波長７８５ｎｍの屈折率はｎ＝１．５３１である。また、対物レンズの有効瞳半径は２．１５ｍｍである。

In the objective lens 106, the glass material of the glass part is KVC81 manufactured by Sumita Optical Co., Ltd., and the glass material of the resin part has a refractive index of n = 1.563 at a wavelength of 405 nm, n = 1.536 at a wavelength of 660 nm, and a wavelength of 785 nm. The refractive index of n is 1.531. The effective pupil radius of the objective lens is 2.15 mm.

2 is a cross-sectional view for explaining the aberration correcting element in this example , FIG. 3 is a top view, and FIG. 4 is an enlarged view of a four-step staircase diffraction structure.

  The aberration correction element 501 is a compatible element for correcting the spherical aberration generated by the difference in the substrate thickness of the optical recording medium and the difference in the wavelength of the light beam emitted from the semiconductor laser 130a having a center wavelength of 660 nm.

  As shown in FIG. 3, the aberration correction element 501 has a circular central diffraction region 502a in which a diffraction structure is formed, and a flat portion 502b in the peripheral region within the effective beam diameter 502c having a wavelength of 405 nm. The diffraction region 502a transmits a light beam having a wavelength of 405 nm as it is, and diffracts the light beam having a wavelength of 660 nm so as to correct spherical aberration caused by the difference in the substrate thickness and the wavelength of the optical recording medium.

  Therefore, a groove is formed in the diffraction region 502a, and the diffraction region 502a corresponds to the effective diameter of a light beam having a wavelength of 660 nm. The effective pupil radius of a light beam having a wavelength of 660 nm is 1.74 mm. On the other hand, the flat portion 502b in the peripheral region transmits a light beam having a wavelength of 405 nm and a wavelength of 660 nm as it is.

  The cross section of the diffraction region 502a of the aberration correction element 501 is composed of a plurality of annular projections formed concentrically as shown in FIG. 2, and each annular projection has four steps. Here, as shown in FIG. 4, the number of stages includes the lowest stage. Further, the pitch of the ring-shaped convex portions is gradually narrowed from the inside toward the outside so that the diffractive structure has a lens effect. The minimum value of the pitch 502d is 11 μm, and the number of ring zones is 68. The number of ring zones is the number of one period (pitch 502d) of the diffractive structure.

Further, the groove depth and height of each step of the diffraction structure of the aberration correction element 501 will be described with reference to FIG. In a diffractive optical system, not all the incident light energy is converted into outgoing light, but is converted only with an efficiency called diffraction efficiency. When the serrated kinoform shape as shown by the dotted line in FIG. 4 is blazed at a certain wavelength, the diffraction efficiency at that wavelength is theoretically 100% in the case of thin approximation. In this example , of the two wavelengths, a transmitted light is used for a light beam having a wavelength of 405 nm, and a diffracted light is used for a light beam having a wavelength of 660 nm. In addition, it becomes easier to produce an ideal kinoform shape by using a staircase shape. The diffractive structure may have a sawtooth kinoform shape as shown by a dotted line in FIG.

  In FIG. 4, assuming that the groove depth of the sawtooth diffractive structure is D, the phase difference between the groove depths D that gives the maximum diffraction efficiency of the zero-order light and the first-order light is as shown in (Table 2).

波長４０５ｎｍに対しては、溝深さＤの位相差が４λ，８λ，１２λ・・・となるように、波長６６０ｎｍに対しては、溝深さＤの位相差が３λ，７λ，１１λ・・・または１λ，５λ，９λ・・・となるような溝深さＤを設定すれば良い。すなわち、波長４０５ｎｍに対しては、１段の高さＨによる位相差が１λである。１段の高さが波長の整数倍に設定して０次光の回折効率を最大にしている。

For the wavelength of 405 nm, the phase difference of the groove depth D is 4λ, 8λ, 12λ,..., And for the wavelength of 660 nm, the phase difference of the groove depth D is 3λ, 7λ, 11λ,. Or a groove depth D such that 1λ, 5λ, 9λ... That is, for a wavelength of 405 nm, the phase difference due to the height H of one step is 1λ. The height of one step is set to an integral multiple of the wavelength to maximize the diffraction efficiency of the 0th order light.

Further, polymethyl methacrylate (hereinafter referred to as PMMA) is used as the material of the aberration correction element 501. PMMA is one of the most widely used resins for optical components because of its high transparency and weather resistance, and its strength that is particularly suitable for injection molding.

FIG. 5 shows the dispersion characteristics of PMMA. FIG. 6 is a diagram showing the relationship between the transmittance and the groove depth in FIG. 6 in which the diffraction efficiency of the zero-order light with a wavelength of 405 nm and the first-order light with a wavelength of 660 nm is calculated from the dispersion characteristics of FIG. When the groove depth D of the staircase-shaped diffractive structure is around 4.8 μm, the efficiency of the zero-order light with a wavelength of 405 nm and the primary light with a wavelength of 660 nm is high, which are 100% and 70%, respectively. Therefore, in this reference example , the groove depth D of the diffractive structure of the aberration correction element 501 was set to 4.8 μm.

Further, the height of the flat portion 502b shown in FIG. 2 is set so as to be an integral multiple of the wavelength with respect to the lowest level of the groove. In this reference example , the thickness was set to 3.2 μm corresponding to a phase difference twice the wavelength. The light beam that has passed through the region of the flat portion 502b is outside the effective diameter for the light beam having a wavelength of 660 nm, and thus becomes unnecessary light for spot formation. Therefore, as shown in FIG. 1, a dielectric multilayer film that blocks the light beam having a wavelength of 660 nm and transmits only the light beam having a wavelength of 405 nm is used by the wavelength-selective aperture limiting element 109.

  The aperture limiting element 109 described above may not be used. This is because in the case of the high NA objective lens 106, the light flux in the peripheral region with respect to the light flux other than the design wavelength of 405 nm has a large aberration generation amount and does not affect the spot formation.

Next, the outer shape of the aberration correction element in this example will be described in detail. As shown in FIG. 3, the outer shape of the aberration correction element 501 is the same circular shape as the boundary between the diffraction region 502a and the flat portion 502b. The circular shape includes a polygon approximated to a circle, and the same effect can be obtained even in an octagon as shown in FIG. 7, for example.

Resin such as PMMA used in this example has the advantage that it can be injection-molded, so that it is most widely used in optical parts and is easily mass-produced. However, it has a weak hygroscopicity. This influence not only fluctuates optical characteristics such as refractive index and transmittance, but also appears as shape deformation.

  FIG. 8 is a top view showing a conventional aberration correction element. The outer shape of the conventional aberration correction element 511 shown in FIG. 8 is a quadrangular shape, and is not circular, unlike the boundary between the diffraction region 512a and the flat portion 512b.

  9A to 9D are diagrams showing the transmitted wavefront shape of the conventional aberration correction element 511 shown in FIG. 8 having a wavelength of 405 nm. FIG. 9A is a diagram showing the wavefront shape as a result of measuring the transmitted wavefront within the effective beam diameter 512c. FIGS. 9B and 9C are diagrams showing the results of wavefront measurement with the wavefront shape of FIG. 9A divided into regions. FIG. 9B is a diagram showing the shape of the transmitted wavefront 512g in the diffraction region, and FIG. 9C is a diagram showing the shape of the transmitted wavefront 512f in the flat portion between the effective beam diameter 512c and the diffraction region 512a.

  When wavefront aberration is calculated based on the shape of each wavefront, the wavefront shape in the vicinity of the effective beam diameter 512c in FIG. 9A has a PV value of 0.5λ and a wavefront aberration of 0.1λrms, and the wavefront accuracy is very poor. . Usually, it is desirable that the wavefront aberration of the optical component is 0.02λrms or less. In addition, the wavefront aberration in the diffraction region 512a in FIG. 9B is 0.02λrms, which is good wavefront accuracy, whereas the wavefront aberration of the transmitted wavefront 512f in the flat portion in FIG. It can be seen that the reason why the wavefront accuracy within the effective ray diameter 512c is as large as 13λrms is in the flat portion.

  FIG. 9D is a plot of the wavefront shape of the flat portion along the circumferential direction. It is considered that the flat portion is wavy along the square shape of the aberration correction element 511 and is a factor of wavefront deterioration.

  The cause of this undulation is that the boundary between the diffraction region and the flat part has 512d and far 512e near the side of the outer shape of the rectangle, and there is a difference in the moisture absorption of PMMA between 512d and far 512e. This difference in moisture absorption becomes a difference in deformation, which is considered to cause undulation.

FIGS. 10A to 10D are diagrams showing the transmitted wavefront shape of the aberration correction element 501 of this example with a wavelength of 405 nm. FIG. 10A is a diagram showing the wavefront shape as a result of measuring the transmitted wavefront within the effective beam diameter 502c. FIGS. 10B and 10C are diagrams showing the results of wavefront measurement by dividing the wavefront shape of FIG. 10A into regions. FIG. 10B is a diagram showing the shape of the transmitted wavefront 502g in the diffraction region, and FIG. 10C is a diagram showing the shape of the transmitted wavefront 502f in the flat portion between the effective beam diameter 502c and the diffraction region 502a.

  FIG. 10D is a plot of the wavefront shape of the flat portion along the circumferential direction. When the wavefront aberration is calculated from the wavefront shape in the vicinity of the effective ray diameter 502c in FIG. 10A, the PV value is 0.1λ, the wavefront aberration is 0.02λrms, and the wavefront aberration in the diffraction region 502a in FIG. The wavefront aberration of the transmission wavefront 502f in the flat portion in FIG. 10C is 0.017λrms, and there is no waviness in the peripheral portion, and the wavefront accuracy is good.

  In this way, by making the outer shape of the aberration correction element 501 the same circular shape as the boundary between the diffraction region and the flat portion, the change in shape due to the moisture absorption of PMMA is made uniform, and the swell can be reduced. .

From the above, the aberration correction element 501 of the present example has the structure of the aberration correction element 501 with reduced warpage and undulation even when a hygroscopic resin is used as a material. Wavefront aberrations generated by different types of substrate thicknesses and light sources can be corrected, and a highly accurate optical pickup can be provided.

FIG. 11 is a diagram illustrating a schematic configuration of an optical pickup reference example .
Here, components having the same functions corresponding to the components described in FIG. 1 showing the reference example are denoted by the same reference numerals. As shown in FIG. 11, a compatible optical pickup that has first, second, and third light sources having different wavelengths and records or reproduces three types of optical recording media using an aberration correction element 521. is there.

Since the configuration of the optical pickup for the first and second light sources in this reference example is the same as that of the above-described embodiment , the overlapping description is omitted. In the third light source in the infrared region, the light beam emitted from the semiconductor laser 140a having the center wavelength λ3 of 780 nm is deflected by the prism 104 via the divergence angle conversion lens 142 and the wavelength selective beam splitter 143. Then, the light is condensed on the third optical recording medium 147 through the quarter-wave plate 105, the aberration correction element 521, and the objective lens 106. The substrate thickness of the third optical recording medium 147 is 1.2 mm, and the numerical aperture NA of the objective lens 106 is 0.4. The reflected light from the third optical recording medium 147 passes through the objective lens 106, the aberration correction element 521, and the quarter wavelength plate 105, and then is separated from the incident light and deflected by the wavelength selective beam splitter 143, thereby being a hologram element. The light is guided onto the light receiving element 140c by 140b, and a reproduction signal, a focus error signal, and a track error signal are detected.

FIG. 12 is a cross-sectional view for explaining the aberration correction element in the present reference example , FIG. 13 is a bottom view, and FIG. 14 is an enlarged view of a two-step staircase diffraction structure.

  The aberration correction element 521 generates a light beam emitted from the semiconductor laser 130a having a center wavelength of 660 nm and a light beam emitted from the semiconductor laser 140a having a center wavelength of 780 nm due to a difference in substrate thickness of the optical recording medium and a difference in wavelength. It is a compatible element for correcting spherical aberration.

As shown in FIG. 12, the aberration correction element 521 has diffractive structures formed on the front surface and the back surface. The diffractive structure formed on the surface has the same structure as the aberration correction element 501 described in the reference example , and the top view is shown in FIG.

  On the surface of the aberration correction element 521, similarly to the aberration correction element 501, a circular central diffraction region 502 a in which a diffraction structure is formed within a light beam effective diameter 502 c having a wavelength of 405 nm, and a flat portion 502 b in the peripheral region are provided. Have. The diffraction region 502a transmits a light beam having a wavelength of 405 nm and a wavelength of 780 nm as it is, and diffracts the light beam having a wavelength of 660 nm so as to correct a spherical aberration caused by a difference in substrate thickness of the optical recording medium and a difference in wavelength.

  Therefore, a groove is formed in the diffraction region 502a, and the diffraction region 502a corresponds to the effective diameter of a light beam having a wavelength of 660 nm. The effective pupil radius of a light beam having a wavelength of 660 nm is 1.74 mm. On the other hand, the flat portion 502b in the peripheral region transmits a light beam having a wavelength of 405 nm, a wavelength of 660 nm, and a wavelength of 780 nm as it is.

  The structure of the back surface of the aberration correction element 521 is shown in FIG. Within the effective diameter 522c of the light beam having a wavelength of 405 nm, there is a circular central diffraction region 522a in which a diffraction structure is formed, and a flat portion 522b in the peripheral region. The diffraction region 522a transmits a light beam having a wavelength of 405 nm and a wavelength of 660 nm as it is, and diffracts the light beam having a wavelength of 780 nm so as to correct a spherical aberration caused by a difference in substrate thickness of the optical recording medium and a difference in wavelength.

  Therefore, a groove is formed in the diffraction region 522a, and the diffraction region 522a corresponds to the effective diameter of a light beam having a wavelength of 780 nm. The effective pupil radius of a light beam having a wavelength of 780 nm is 1.34 mm. On the other hand, the flat portion 522b in the peripheral region transmits a light beam having a wavelength of 405 nm, a wavelength of 660 nm, and a wavelength of 780 nm as it is.

  Similar to the aberration correction element 501, the surface of the aberration correction element 521 is composed of a plurality of annular projections formed concentrically, and each annular projection has four steps. The pitch of the ring-shaped convex portions is gradually narrowed from the inside to the outside so that this diffractive structure has a lens effect. The minimum pitch is 11 μm and the number of ring zones is 68.

  Similarly, the back surface of the aberration correction element 521 includes a plurality of annular projections formed concentrically, and each annular projection has two steps. The pitch of the ring-shaped convex portions is gradually narrowed from the inside to the outside so that this diffractive structure has a lens effect. The minimum pitch is 12 μm and the number of ring zones is 57.

Next, the groove depth on the surface of the aberration correction element 521 will be described. The groove depth is the same as that of the aberration correction element 501 of the reference example . FIG. 15 shows the result of scalar calculation of the diffraction efficiencies of 0th-order light with a wavelength of 405 nm, 1st-order light with a wavelength of 660 nm, and 0th-order light with a wavelength of 780 nm when PMMA is used. Similar to the reference example , when the groove depth D of the diffractive structure is around 4.8 μm, the efficiency of the 0th-order light having a wavelength of 405 nm, the 1st-order light having a wavelength of 660 nm, and the 0th-order light having a wavelength of 780 nm is high, and is 100% and 70%, respectively. % And 100%. The groove depth D of the diffractive structure on the surface of the aberration correction element 521 of Embodiment 1 was set to 4.8 μm.

  The groove depth on the back surface of the aberration correction element 521 will be described. In FIG. 14, assuming that the groove depth of the sawtooth diffractive structure is D, the phase difference of the groove depth at which the maximum diffraction efficiency of the zero-order light and the first-order light is obtained is as shown in (Table 3).

波長４０５ｎｍと波長６６０ｎｍに対しては、溝深さＤの位相差が２λ，４λ，６λ・・・となるように、波長７８０ｎｍに対しては、溝深さＤの位相差が１λ，３λ，５λ・・となるような溝深さを設定すれば良い。すなわち波長４０５ｎｍ，６６０ｎｍに対しては、１段の高さを波長の整数倍に設定して０次光の回折効率を最大にしている。

For the wavelength 405 nm and the wavelength 660 nm, the phase difference of the groove depth D is 2λ, 4λ, 6λ..., And for the wavelength 780 nm, the phase difference of the groove depth D is 1λ, 3λ, What is necessary is just to set the groove depth which will be set to 5λ. That is, for the wavelengths 405 nm and 660 nm, the height of one step is set to an integral multiple of the wavelength to maximize the diffraction efficiency of the 0th order light.

FIG. 16 shows the scalar calculation of the diffraction efficiencies of the 0th-order light with a wavelength of 405 nm, the 0th-order light with a wavelength of 660 nm, and the 1st-order light with a wavelength of 780 nm when PMMA is used. When the groove depth D of the diffractive structure is around 4.0 μm, the efficiency of the 0th-order light with a wavelength of 405 nm, the 1st-order light with a wavelength of 660 nm, and the 0th-order light with a wavelength of 780 nm are high, and are 100%, 100%, and 40%, respectively. Become. The groove depth D of the diffractive structure of the aberration correction element 521 of Embodiment 1 was set to 4.0 μm.

Further, the height of the flat portion 502b on the surface of the aberration correction element 521 was set to 3.2 μm as in the reference example . The height of the flat portion 522b on the back surface of the aberration correction element 521 was set to 4 μm, similar to the groove depth D of the diffractive structure. A light beam having a wavelength of 660 nm and a wavelength of 405 nm transmitted through the flat portion 522b is transmitted light having the same phase as the diffraction region.

Next, the outer shape of the aberration correction element in the first embodiment will be described in detail. Here, although it is the back side of the aberration correction element 521, as shown in FIG. 13, the outer shape of the aberration correction element 521 is the same circular shape as the boundary between the diffraction region and the flat portion.

  FIG. 17 is a diagram showing a conventional aberration correction element 531. As shown in FIG. 17, the outer shape of the aberration correction element 531 is a quadrangle, and the boundary between the diffraction region 532a and the flat portion 532b is not circular. 18A to 18D are diagrams showing the transmitted wavefront shape of the conventional aberration correction element 531 shown in FIG. 17 having a wavelength of 405 nm. FIG. 18A is a diagram showing the wavefront shape as a result of measuring the transmitted wavefront within the effective beam diameter 532c. FIGS. 18B and 18C are diagrams showing the results of wavefront measurement with the wavefront shape of FIG. 18A divided into regions. 18B and 18C are diagrams showing the wavefront measurement results by dividing the result of FIG. 18A into regions. 18B is a diagram showing the shape of the transmitted wavefront 532g in the diffraction region, and FIG. 18C is a diagram showing the shape of the transmitted wavefront 532f in the flat portion between the effective beam diameter 532c and the diffraction region 532a.

  When wavefront aberration is calculated based on the shape of each wavefront, the wavefront shape in the vicinity of the effective beam diameter 532c in FIG. 18A has a PV value of 0.6λ and a wavefront aberration of 0.09λrms, which is very poor. . Usually, it is desirable that the wavefront aberration of the optical component is 0.02λrms or less. Further, the wavefront aberration in the diffraction region 532a in FIG. 18B has a good wavefront accuracy of 0.019 λrms, whereas the wavefront aberration of the transmitted wavefront 532f in the flat portion in FIG. 18C is 0.093 λrms. It can be seen that the flat part is the cause of the poor wavefront accuracy within the effective beam diameter 532c.

  FIG. 18D is a plot of the wavefront shape of the flat portion along the circumferential direction. It is considered that the flat portion is wavy along the square shape of the aberration correction element 531 and causes wavefront deterioration.

  The cause of this undulation is that the boundary between the diffraction region and the flat portion includes a portion 532d and a portion 532e far from the sides of the outer shape of the quadrangle, and there is a difference in the moisture absorption of PMMA between the portion 532d and the portion 532e far from the side. This difference in moisture absorption becomes a difference in deformation, which is considered to cause undulation.

FIGS. 19A to 19D are diagrams showing the transmitted wavefront shape of the wavelength 405 nm of the aberration correction element 521 of this reference example . FIG. 19A is a diagram showing a wavefront shape as a result of measuring the transmitted wavefront within the effective beam diameter 522c. FIGS. 19B and 19C are diagrams showing the results of wavefront measurement with the wavefront shape of FIG. 19A divided into regions. FIG. 19B is a diagram showing the shape of the transmitted wavefront 522g in the diffraction region, and FIG. 19C is a diagram showing the shape of the transmitted wavefront 522f in the flat portion between the effective beam diameter 522c and the diffraction region 522a.

  FIG. 19D is a plot of the wavefront shape of the flat portion along the circumferential direction. When the wavefront aberration is calculated from the wavefront shape in the vicinity of the effective ray diameter 522c in FIG. 19A, the PV value is 0.15λ, the wavefront aberration is 0.02λrms, and the wavefront aberration in the diffraction region 522a in FIG. The wavefront aberration of the transmitted wavefront 522f in the flat portion in FIG. 19C is 0.019λrms, and there is no waviness in the peripheral portion, and the wavefront accuracy is good.

  Thus, by making the outer shape of the aberration correction element 521 the same circular shape as the boundary between the diffraction region and the flat portion, the change in shape due to moisture absorption of PMMA is made uniform, and the swell can be reduced. .

From the above, the aberration correction element 521 of the first embodiment has the structure of the aberration correction element 521 with reduced warpage and undulation despite using PMMA having hygroscopicity as a material. With one objective lens, wavefront aberrations generated by three different substrate thicknesses and light sources can be corrected, and a highly accurate optical pickup can be provided.

FIG. 20 is a block diagram showing one form of an optical information processing apparatus according to Embodiment 2 of the present invention, and an optical recording medium using any one of the optical pickups described in the above reference example and Embodiment 1 . A device that performs one or more of information reproduction, recording, and erasing.

  As shown in FIG. 20, the optical information processing apparatus is roughly constituted by a spindle motor 10, a feed motor 12, an optical pickup 9, and the like, and these are controlled by a system controller 14 that controls the entire optical information processing apparatus. Then, the movement of the optical pickup 9 in the tracking direction is performed by a servo control circuit 13 including a feed motor 12. For example, when reproducing information on the optical recording medium 7, a control signal from the system controller 14 is supplied to the servo control circuit 13 and the modem circuit 11.

  The servo control circuit 13 rotates the spindle motor 10 at a set number of revolutions and drives and controls the feed motor 12. The modulation / demodulation circuit 11 is supplied with a focusing error signal detected by a photodetector (not shown) of the optical pickup 9, a tracking error signal, and position information of where the optical recording medium 7 is read out. The focusing error signal and tracking error signal are supplied to the servo control circuit 13 via the system controller 14.

  The servo control circuit 13 drives the focusing coil of the actuator by a focusing control signal, and drives the tracking coil of the actuator by a tracking control signal. The low frequency component of the tracking control signal is supplied to the servo control circuit 13 via the system controller 14 to drive the feed motor 12. As a result, feedback servo of focusing servo, tracking servo, and feed servo is performed.

  Further, the position information of where the optical recording medium 7 is read is processed by the modulation / demodulation circuit 11 and supplied to the spindle motor 10 as a spindle control signal, and controlled to a predetermined rotational speed corresponding to the reproduction position of the optical recording medium 7. It is driven and actual reproduction is started from here. The reproduction data processed and demodulated by the modem circuit 11 is transmitted to the outside through the external circuit 15.

  Next, when data is recorded, the same process as the reproduction is performed until the feedback servo of the focusing servo, tracking servo and feed servo is applied. A control signal indicating where the input data input through the external circuit 15 is recorded on the optical recording medium 7 is supplied from the system controller 14 to the servo control circuit 13 and the modulation / demodulation circuit 11.

  In the servo control circuit 13, the spindle motor 10 is controlled to a predetermined rotational speed, and the feed motor 12 is driven to move the optical pickup 9 to the information recording position of the optical recording medium 7. Also, the input signal input to the modulation / demodulation circuit 11 via the external circuit 15 is modulated based on the recording format and supplied to the optical pickup 9. In the optical pickup 9, the modulation of the emitted light and the emitted light power are controlled, and recording on the optical recording medium 7 is started.

  Further, the type of the optical recording medium 7 is discriminated by the reproduction data signal, for example, the liquid crystal driving circuit is controlled to control the liquid crystal optical rotator in the optical pickup, and the incident light is controlled according to the type of the optical recording medium 7 to be reproduced. Switch the polarization direction. Furthermore, as a method for determining the type of the optical recording medium 7, a tracking servo signal or a focus servo signal may be used.

  If an optical pickup using the aberration correction element of the present invention is provided in an optical pickup provided in a reproduction-only optical reproducing apparatus and an optical recording / reproducing apparatus capable of both recording and reproduction, optical recording media having different substrate thicknesses are provided. 7 information recording and reproduction quality can be improved.

  An optical element according to the present invention, an optical pickup using the optical element, and an optical information processing apparatus make it possible to correct spherical aberration by reducing the size and weight of the aberration correction element, and to uniformize the shape change due to moisture absorption of the resin of the aberration correction element. In response to changes in environmental humidity, it is possible to obtain a wavefront precision element with reduced deformation such as swells and a device using the same, and to reduce deformation due to environmental humidity etc. This is useful for realizing the device.

The figure which showed schematic structure of the optical pick-up in Embodiment 1 of this invention. Sectional drawing for demonstrating the aberration correction element in this example Top view for explaining the aberration correction element in this example Enlarged view of the four-step staircase diffraction structure for explaining the aberration correction element in this example Diagram showing the dispersion characteristics of PMMA The figure which shows the relationship of the transmittance | permeability-groove depth (4 steps) which carried out the scalar calculation of the diffraction efficiency. The figure which shows the example of the external shape of the aberration correction element in this example Top view showing a conventional aberration correction element (A)-(d) is a figure for demonstrating the transmitted wave front of the conventional aberration correction element. (A)-(d) is a figure for demonstrating the transmitted wave front of the aberration correction element of this example . The figure which showed schematic structure of the optical pick-up in the reference example of this invention Sectional drawing for demonstrating the aberration correction element in this example Bottom view for explaining the aberration correction element in this example Enlarged view of a two-step staircase diffraction structure for explaining the aberration correction element in Example 1 The figure which shows the relationship of the transmittance | permeability-groove depth (4 steps) which carried out the scalar calculation of the diffraction efficiency. The figure which shows the relationship of the transmittance | permeability-groove depth (two steps) which carried out the scalar calculation of the diffraction efficiency Top view showing a conventional aberration correction element (A)-(d) is a figure for demonstrating the transmitted wave front of the conventional aberration correction element. (A)-(d) is a figure for demonstrating the transmitted wave front of the aberration correction element of a reference example . The block diagram which shows 1 form of the optical information processing apparatus in Embodiment 2 of this invention

Explanation of symbols

7 optical recording medium 9 optical pickup 10 spindle motor 11 modulation / demodulation circuit 12 feed motor 13 servo control circuit 14 system controller 15 external circuit 101, 130a, 140a semiconductor laser 102 collimator lens 103 polarization beam splitter 104 prism 105 quarter wave plate 106 objective Lens 107 First optical recording medium 108 Detection lens 109 Aperture limiting element 110, 130c, 140c Light receiving element 120 Movable part 130b, 140b Hologram element 132, 142 Divergence angle conversion lens
133, 143 Wavelength selective beam splitter 137 Second optical recording medium 147 Third optical recording medium 501, 511, 521, 531 Aberration correction elements 502a, 512a, 522a, 532a Diffraction areas 502b, 521b, 522b, 532b Flat part 502c, 512c, 522c, 532c Effective beam diameter 502d Pitch 512d, 532d Near 512e, 532e Far 512f, 522f, 532f Transmitted wavefront 512g, 522g, 532g in flat part Transmitted wavefront in diffraction region

Claims (4)

  In an optical pickup that performs one or more of information recording, reproduction, and erasing on two types of optical recording media having different substrate thicknesses,
  First and second light sources having different wavelengths;
  An objective lens optimized with the first light source wavelength for condensing a light beam on the recording surface of the first optical recording medium;
  An optical element having a function of correcting an aberration generated when the light beam from the second light source passes through the substrate of the second optical recording medium,
  The optical element is made of PMMA, has a plane perpendicular to the optical axis, has a circular diffraction area coaxial with the optical axis within the effective beam diameter through which the light beam passes, and an outer flat portion. Are divided into two areas,
  The diffractive region has one or more diffractive structures whose cross-section is uneven on the surface in the vertical direction within the effective diameter of the light beam having the first light source wavelength, and the light beam having the first light source wavelength. And diffracting the light beam having the second light source wavelength so as to correct the spherical aberration caused by the difference in the substrate thickness and the wavelength of the optical recording medium,
  A plate having a circular shape coaxial with the optical axis or a polygonal shape approximate to the circular shape so that the shape change in the flat portion due to moisture absorption of the PMMA is made uniform along the circumferential direction of the concentric circles. And
  An optical pickup, wherein a transmitted wavefront aberration of the first light source wavelength within a light beam effective diameter of the first light source wavelength is 0.02λrms or less. The optical pickup according to claim 1, wherein a cross section of the diffractive structure of the optical element is rectangular . The optical pickup according to claim 1, wherein a cross section of the diffractive structure of the optical element has a step shape . At least one of information recording, reproduction, and erasing is performed by irradiating the recording surface of the optical recording medium with a light beam.
In an optical information processing device,
An optical information processing apparatus comprising the optical pickup according to claim 1 .

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