JP2011060405A - Optical head - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical head which receives three kinds of light different in wavelength, depending on a layout of a light receiving area of a single photodetector. <P>SOLUTION: A wavelength selection diffraction element 10a is disposed only in a return optical path of light of a wavelength λ<SB>1</SB>in a wavelength band of 405 nm, light of a wavelength λ<SB>2</SB>in a wavelength band of 660nm, and light of a wavelength λ<SB>3</SB>in a wavelength band of785 nm, all of which are reflected in the same way on an optical disk. The wavelength selection diffraction element 10a has: a wavelength plate 12 which converts the light of wavelength λ<SB>1</SB>and the light of wavelength λ<SB>2</SB>into perpendicular linear polarized light, a first diffraction grating 13 to diffract the light of wavelength λ<SB>1</SB>but to allow the light of wavelength λ<SB>2</SB>to pass without diffracting, and a second diffraction grating 15 to allow the light of wavelength λ<SB>2</SB>to pass without diffracting while diffracting the light of wavelength λ<SB>1.</SB>The optical head, which is allowed to design each wavelength light receiving area of the photodetector with high degree of freedom, is obtained by using 0th diffraction efficiency high on the light of wavelength λ<SB>3</SB>and independently adjusting diffraction efficiency and a diffraction angle of each wavelength light. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention provides an optical recording medium such as a CD, a DVD, a magneto-optical disk, and a high-density optical recording medium (hereinafter referred to as “BD”) such as “Blu-ray” (registered trademark: BD) as an optical system that handles optical storage. The present invention relates to an optical head device for recording and / or reproducing information (hereinafter referred to as “recording / reproducing”) on an optical disc.

  In recent years, optical head devices for recording / reproducing on / from each optical disk using light of different wavelengths such as CD, DVD, and BD have been developed. There is a demand for reduction in the number of parts and miniaturization by using optical parts in common. Regarding a semiconductor laser as a light source, a so-called two-wavelength laser, in which a semiconductor element that oscillates two or three wavelengths is contained in one package by using a technique for integrally forming semiconductor lasers having a plurality of types of oscillation wavelengths, 3 Development of a semiconductor laser called a wavelength laser has been studied.

  In addition to this, in order to reduce the number of parts and the size of the optical head device, the spherical aberration is reduced with respect to each optical disc corresponding to the light of each wavelength for two or three lights having different wavelengths. An objective lens for condensing light has also been developed. Further, with respect to the photodetector that detects the signal light reflected by the information recording surface of each optical disc, the wavelength differs from the optical head device having the light receiving optical system and the photodetector independent of each wavelength of light. An optical head device having a light receiving optical system common to two or three lights and detecting optical information of a plurality of wavelengths with one photodetector has been studied and developed.

  The photodetector has a plurality of light receiving areas and is arranged so that light reflected from each optical disk reaches each light receiving area. Then, a reproduction RF signal, a focus error signal, a tracking error signal, and the like are generated by calculating a signal (light amount) of the light reaching the light receiving area. Here, as an optical head device in which a plurality of lights having different wavelengths reflected from the information recording surface of each optical disc are received by a single photodetector, for example, light reflected from a BD and HD-DVD / DVD / CD There has been reported an optical head device that receives more reflected light in different light receiving areas (sensor patterns) (Patent Document 1).

  The optical head device disclosed in Patent Document 1 includes a polarizing diffraction element between an anamorphic lens (cylindrical lens) disposed only in the optical path of light reflected by an optical disc and a photodetector. The polarizing diffractive element diffracts the light reflected by the BD when the light reflected by the BD and the light reflected by the HD-DVD / DVD / CD are incident as lights having polarization directions orthogonal to each other. The light reflected by the HD-DVD / DVD / CD is transmitted and adjusted so that each light reaches the light receiving area for BD and the light receiving area for HD-DVD / DVD / CD in one photodetector. Has been.

JP 2008-47200 A

  However, the optical head device of Patent Document 1 has a limitation that each light that reaches different light receiving areas must be incident on the polarizing diffraction element in a polarization state orthogonal to each other. Furthermore, the conventional polarizing diffractive element can be incident on the polarizing diffractive element as S-polarized light and P-polarized light orthogonal to each other to split the light into two directions for each polarization, For example, signal processing differs depending on each optical disc, and it cannot be applied to an optical head device using a photodetector having a light receiving area independently for light of three wavelengths. Further, when two lights having different wavelengths reach the same light receiving area, there is a restriction that the polarizing diffraction element must be transmitted in a straight line, that is, arranged in the extending direction of the 0th-order diffracted light.

  The present invention has been made to solve such a problem of the prior art. Signal light reflected from an optical disk corresponding to each of three lights having different wavelengths is supplied to each light of one photodetector. Provided is an optical head device that uses a wavelength selective diffraction element in order to reach a corresponding light receiving area, has a high degree of freedom in layout of the light receiving area in the photodetector, and can be recorded / reproduced with downsizing. For the purpose.

The present invention includes a light source that emits light having a wavelength λ ₁ that is a 405 nm wavelength band, light having a wavelength λ ₂ that is a 660 nm wavelength band, light having a wavelength λ ₃ that is a 785 nm wavelength band, and light having the wavelength λ ₁ . An optical head device comprising: an objective lens that condenses the light of the wavelength λ _{2 and} the light of the wavelength λ ₃ onto an optical disc; and a beam splitter that deflects the light reflected from the optical disc to a photodetector. beam splitter and has the wavelength lambda ₁ of the light, wavelength-selective diffraction element in an optical path in which the wavelength lambda ₂ of the light and the wavelength lambda ₃ of the light is common between the photodetector, the wavelength selection diffractive element Includes at least a wavelength plate, a first diffraction grating, and a second diffraction grating in this order from the beam splitter side, and the wavelength plate transmits the light having the wavelength λ ₁ and the light having the wavelength λ _{2 to be} transmitted. , Orthogonal to each other A first linear polarized light and the light of second linearly polarized light, the first diffraction grating of the wavelength lambda ₁ of light and the wavelength lambda ₂ of the light, and to reflect without diffracting either, other The second diffraction grating transmits the light diffracted by the first diffraction grating out of the light of the wavelength λ _{1 and} the light of the wavelength λ ₂ without diffracting the light, An optical head device for diffracting light is provided.

The wavelength plate is made of a birefringent material having optical axes aligned in the thickness direction, has a retardation value substantially equal to an integral multiple of λ ₁ with respect to the light of the wavelength λ ₁ , and the wavelength λ ₂ providing the optical head device having a substantially equal retardation values on an odd multiple of lambda _2/2 to light.

In addition, the first diffraction grating has periodic unevenness by a first birefringent material layer made of a birefringent material and a first isotropic material layer made of an isotropic material, When the ordinary refractive index of the first birefringent material layer is n _o1 , the extraordinary refractive index is n _e1 (n _o1 ≠ n _e1 ), and the refractive index of the isotropic material is n _s1 , the n _s1 provides approximately equal above optical head device to the _{n o1} or the _{n e1.}

In addition, the second diffraction grating has periodic unevenness by a second birefringent material layer made of a birefringent material and a second isotropic material layer made of an isotropic material, The ordinary refractive index of the second birefringent material layer is n _o2 , the extraordinary refractive index is n _e2 (n _o2 ≠ n _e2 ), and the refractive index of the second isotropic material layer is n _s2 . The n _s2 provides the optical head device as described above, which is substantially equal to the no ₂ or the _{ne 2} .

The second diffraction grating has periodic irregularities due to the second isotropic material layer made of an isotropic material on the plane of the transparent substrate, and the second isotropic material layer is refracted. Provided is the above optical head device in which (n _s2 −1) × d _A is substantially equal to an integral multiple of the wavelength λ ₁ or an integral multiple of the wavelength λ ₂ where the rate is n _s2 and the height is d _A.

  Further, the optical head device is provided in which the first diffraction grating and the second diffraction grating have a rectangular cross-sectional shape.

Furthermore, the wavelength selective diffractive element provides the above-described optical head device in which the light having the wavelength λ _{3 that} is transmitted in a straight line is 70% or more when the amount of incident light having the wavelength λ ₃ is 100%.

  The present invention optimally processes an optical signal reflected from the information recording surface of each optical disc and realizes miniaturization in an optical head device that records and reproduces each optical disc using a plurality of light beams having different wavelengths. An optical head device having an effect that can be achieved can be provided.

Schematic diagram showing a configuration example of an optical head device Sectional schematic diagram of the wavelength selective diffraction element according to the first embodiment Sectional schematic diagram of wavelength selective diffraction element according to the second embodiment Cross-sectional schematic diagram of another wavelength selective diffraction element Schematic diagram showing the relationship between the polarization direction of incident light and the optical axis (slow axis) of the wave plate

(First embodiment)
FIG. 1 is a schematic diagram of a compatible optical head device 100 that uses light of three different wavelengths and performs recording and reproduction of optical discs of respective standards. Each optical component (element) constituting the optical head device 100 will be described while following the optical path of light emitted from the light sources 101a and 101b. In the optical head device 100, the light source 101a is a semiconductor laser or the like that emits light in the 405 nm wavelength band, and the light source 101b is a hybrid type or monolithic that emits both light in the 660 nm wavelength band and light in the 785 nm wavelength band. Type semiconductor laser. Note that the wavelength band of 405 nm is 385 nm to 430 nm, the wavelength band of 660 nm is 630 nm to 690 nm, and the wavelength band of 785 nm is 760 nm to 810 nm.

  In the optical head device 100, the light in the 405 nm wavelength band emitted in the X direction from the light source 101a such as a semiconductor laser is diffracted by the grating element 102a into three beams, and passes through the dichroic prism 103 and the polarization beam splitter 104. The collimator lens 105 becomes parallel light. Then, it is deflected in the Z direction by the dichroic prism 106, passes through the quarter-wave plate 108a, and is condensed on the information recording surface of the optical disc 110a by the objective lens 109a. The optical path from the light source to the optical disk is defined as “outward path”, and the optical path from the optical disk to the optical detector (reflected) is defined as “return path”.

  The dichroic prism 103 has a function of transmitting light in the 405 nm wavelength band and reflecting light in the 660 nm wavelength band and light in the 785 nm wavelength band. On the other hand, the dichroic prism 106 has a function of reflecting light in the 405 nm wavelength band and transmitting light in the 660 nm wavelength band and light in the 785 nm wavelength band. The objective lens 109a is condensing with respect to light in the 405 nm wavelength band for BD, and the objective lens 109b is suitable for either light in the 660 nm wavelength band for DVD and light in the 785 nm wavelength band for CD. Also has a light collecting property.

  The light in the 660 nm wavelength band emitted from the light source 101 b is diffracted by the grating element 102 b into three beams, reflected by the dichroic prism 103, and transmitted through the polarization beam splitter 104. In addition, light in the 785 nm wavelength band emitted from the light source 101b is diffracted by the grating element 102b into three beams, reflected by the dichroic prism 103, and transmitted through the polarization beam splitter 104. The light in the 660 nm wavelength band and the light in the 785 nm wavelength band transmitted through the polarization beam splitter 104 are reflected by the mirror 107, pass through the quarter wavelength plate 108b, and are condensed on the information recording surface of the optical disk 110b by the objective lens 109b. . Here, the optical disc 110a corresponds to a BD, and the optical disc 110b corresponds to a DVD or a CD.

  The grating elements 102a and 102b generate 0th order diffracted light (straight forward transmitted light) and ± 1st order diffracted light and perform recording / reproduction using the 3-beam method. However, the optical head device 100 provides ± 1st order diffracted light. In the case of performing recording / reproduction using the one-beam method that does not generate the grating, the grating element 102a or 102b may not be arranged. Also, for example, when using the 1-beam method for light in the 660 nm wavelength band for DVD and the 3-beam method for light in the 785 nm wavelength band for CD, the grating 102b diffracts only light in one wavelength band. Alternatively, a wavelength selective grating may be used.

  The light of the return path in the 405 nm wavelength band reflected from the optical disk 110a is transmitted through the objective lens 109a and is linearly polarized (for example, orthogonal to the light of the forward polarized light (for example, S-polarized light) when transmitted through the quarter-wave plate 108a. P-polarized light is reflected by the dichroic prism 106, passes through the collimator lens 105, and is reflected by the polarization beam splitter 104. Then, it passes through the cylindrical lens 111, passes through the wavelength selective diffraction element 10 described later, and reaches the photodetector 112. In addition, the light of the return path in the 660 nm wavelength band and the 785 nm wavelength band reflected from the optical disk 110b is transmitted through the objective lens 109b and is transmitted through the quarter wavelength plate 108b as the linearly polarized light in the forward path (for example, S-polarized light). The light becomes orthogonally linearly polarized light (for example, P-polarized light), is reflected by the reflection mirror 107, passes through the dichroic prism 106 and the collimator lens 105, and is reflected by the polarization beam splitter 104. Then, it passes through the cylindrical lens 111, passes through the wavelength selective diffraction element 10 described later, and reaches the photodetector 112. In FIG. 1, for convenience, the locus of light in the 405 nm wavelength band for BD is indicated by a solid line, the trajectory of light in the 660 nm wavelength band for DVD and the 785 nm wavelength band for CD is indicated by a dotted line, and the optical axis is indicated by a one-dot chain line. Show.

  Next, the wavelength selective diffraction element 10 used in the optical head device 100 will be specifically described. FIGS. 2A and 2B are schematic cross-sectional views showing configurations of wavelength selective diffraction elements 10a and 10b corresponding to the wavelength selective diffraction element 10 of FIG. The grating 13 and the second diffraction grating 15 have in this order. The wave plate 12 is provided on one surface of the transparent substrate 11a and has a function of modulating the phases of incident light of three wavelengths to desired values. The wave plate 12 is composed of a layer made of a birefringent material with optical axes aligned in the thickness direction, or a layer made of a birefringent material with optical axes twisted in the thickness direction. Further, the wave plate 12 may be formed by stacking a plurality of these layers made of a birefringent material so that the directions of the optical axes are different. As the transparent substrate, various materials such as a resin plate and a resin film can be used as long as they are transparent to incident light. However, when an optically isotropic material such as glass or quartz glass is used, transmitted light is used. This is preferable because it does not affect birefringence.

The wave plate 12 may be made of, for example, a polymer liquid crystal obtained by polymerizing liquid crystal, or an organic material such as polycarbonate, polyolefin, PVA or the like that has been stretched to induce birefringence. Also, quartz, LiNbO ₃ , LiTaO _3. Single crystals having optical anisotropy such as KDP may be used. In the case of a polymer liquid crystal, an alignment film (not shown) may be provided. For example, in the case of a polymer liquid crystal in which the optical axis is twisted in the thickness direction, the alignment film sandwiching the polymer liquid crystal is used. The orientation directions are different. In addition, the wave plate 12 is configured to modulate light of each wavelength so as to modulate light of three different wavelengths, which is incident in the same polarization direction, into linearly polarized light orthogonal to at least two linearly polarized light. The phase difference is adjusted. Specific optical characteristics of the wave plate 12 will be described later.

  The first diffraction grating 13 is made of a birefringent material on the transparent substrate 11b and has a convex portion corresponding to the first birefringent material layer 13a, and an isotropic material made of the first isotropic material layer 14. And a concave portion corresponding to the above has a diffraction grating structure periodically arranged. The first isotropic material layers 14 may be formed so as to cover the first birefringent material layers 13a, but may be alternately arranged at the same height as the first birefringent material layers 13a.

In the first diffraction grating 13, the first birefringent material layer 13a has a refractive index anisotropy Δn ₁ (=) represented by an absolute value of a difference between the ordinary light refractive index n _o1 and the extraordinary light refractive index n _{e1. | n e1 -n o1 |> 0} ) has a refractive index _{n s1} of the first isotropic material layer _14, a combination of substantially equal material and either _{n e1} or _{n o1.} For the first isotropic material layer 14, a UV curable transparent adhesive or the like can be used. In this way, for _example, in the case of _{n s1 ≒ n o1 (n s1} ≠ n e1), of the light incident on the first diffraction grating 13, the fast axis of the first birefringent material layer 13a The light in the direction has substantially the same refractive index and therefore passes straight through the first diffraction grating 13, while the light in the slow axis direction does not match the refractive index and is at least partially diffracted. Further, the materials constituting the first birefringent material layer 13a and the first isotropic material layer 14 take into consideration the wavelength dispersion characteristics of the refractive index.

First birefringent material layer 13a may be formed by a birefringent material, and the like larger refractive index anisotropy [Delta] n ₁ to use a polymer liquid crystal obtained by curing the liquid crystal monomer preferably. Further, the polymer liquid crystal can be freely set by making the longitudinal direction of the unevenness of the first diffraction grating 13 different from the slow axis corresponding to the alignment direction of the liquid crystal. Therefore, in the wavelength selective diffraction element 10a of FIG. 2A, the longitudinal direction of the first diffraction grating 13 is the Y-axis direction, but the light receiving area layout of the photodetector that transmits or diffracts the light is used. Accordingly, the longitudinal direction in the XY plane can be freely set. Further, in the effective region where light enters, the first diffraction grating 13 is not limited to a region where the longitudinal direction of the diffraction grating is only one direction, but is composed of a plurality of regions having different longitudinal directions, and enters each region. The light may be diffracted in different directions.

  Next, the second diffraction grating 15 will be described. Similarly to the first diffraction grating 13, the second diffraction grating 15 is made of a birefringent material on the transparent substrate 11 c and a convex portion corresponding to the second birefringent material layer 15 a and an isotropic material. The concave portion corresponding to the second isotropic material layer 16 has a diffraction grating structure that is periodically arranged. The second isotropic material layers 16 may be formed so as to cover the first birefringent material layers 15a, but may be alternately arranged at the same height as the second birefringent material layers 15a.

In the second diffraction grating 15, the second birefringent material layer 15 a has a refractive index anisotropy Δn ₂ (=) represented by an absolute value of a difference between the ordinary light refractive index n _o2 and the extraordinary light refractive index n _e2. | N _e2 −n _o2 |> 0), and the refractive index n _{s2 of} the second isotropic material layer 16 is a combination of materials substantially equal to either _{ne 2} or no ₂ . In this way, for _example, in the case of _{n s2 ≒ n o2 (n s2} ≠ n e2), of the light incident on the second diffraction grating 15, the fast axis of the second birefringent material layer 15a The light in the direction has substantially the same refractive index and therefore passes straight through the second diffraction grating 15, while the light in the slow axis direction does not have the same refractive index and is at least partially diffracted. The materials constituting the second birefringent material layer 15a and the second isotropic material layer 16 take into account the wavelength dispersion characteristics of the refractive index.

The second birefringent material layer 15a is also preferably made of a polymer liquid crystal because the refractive index anisotropy Δn ₂ is large. In the case of n _{s1 ≈no 1} and n _{s2 ≈no} 2, the liquid crystal alignment direction of the first birefringent material layer 13a is orthogonal to the liquid crystal alignment direction of the second birefringent material layer 15a. . For example, in FIG. 2A, the orientation direction of the first birefringent material layer 13a is the Y-axis direction, and the orientation direction of the second birefringent material layer 15a is the X-axis direction. The combination of the orientation directions of the first birefringent material layer 13a and the second birefringent material layer 15a is not limited to this, and the first birefringence is obtained when n _s1 ≈n _e1 and n _s2 ≈ne _2. The orientation direction of the conductive material layer 13a may be the X-axis direction, and the orientation direction of the second birefringent material layer 15a may be the Y-axis direction.

Furthermore, for example, in the case of _{n s1} ≒ _{n o1} and _{n s2} ≒ _{n e2,} or the alignment direction of the first birefringent material layer 13a and the second birefringent material layer 15a is both the X-axis direction or, Either may be in the Y-axis direction. That is, of the incident linearly polarized light in the X-axis direction and linearly polarized light in the Y-axis direction, the first diffraction grating 13 transmits one linearly polarized light and diffracts the other linearly polarized light. The second diffraction grating 15 is configured to diffract one linearly polarized light and transmit the other linearly polarized light. In other words, the linearly polarized light in the X-axis direction is diffracted only in one of the first diffraction grating 13 and the second diffraction grating 15, and the linearly polarized light in the Y-axis direction is diffracted only in the other. Further, the longitudinal direction of the second diffraction grating 15 is not limited to the Y-axis direction as in the case of the first diffraction grating 13, and is composed of a plurality of regions having different longitudinal directions and is incident on each region. Each light may be diffracted in different directions. Further, the region is not limited to the one divided by the longitudinal direction of the diffraction grating. For example, even if the longitudinal direction of the diffraction grating is the same, there are a plurality of regions having different grating pitches, or the longitudinal direction of the diffraction grating. Even if the grating pitch is the same, there may be a plurality of regions having different diffraction grating heights.

  The wavelength selective diffractive element is not limited to the rectangular cross-sectional shape of the diffraction grating as in the wavelength selective diffractive element 10a in FIG. 2A, but the first in the wavelength selective diffractive element 10b illustrated in FIG. The cross-sectional shapes of both the diffraction grating 13 and the second diffraction grating 15 may be a blazed shape or a pseudo-blazed shape approximating the blazed shape to a staircase shape, one of which is a (pseudo) blaze shape and the other is a rectangular shape. It may be. Further, when the cross-sectional shapes of both the first diffraction grating 13 and the second diffraction grating 15 are (pseudo) blazed shapes, the inclination in the oblique direction in one cross-section may be different. The wavelength selective diffractive element 10b is an example in which the first birefringent material layer 13b and the second birefringent material layer 15b are blazed, and the other numbers are the same as those of the wavelength selective diffractive element 10a. Avoid duplicating explanations. In this case, the incident light can be diffracted in one direction with high diffraction efficiency. Further, the cross-sectional shapes of the first diffraction grating 13 and the second diffraction grating 15 may be a combination of a rectangular shape and a (pseudo) blaze shape. Furthermore, when the first diffraction grating 13 and the second diffraction grating 15 are composed of a plurality of regions having different longitudinal directions of the diffraction grating, the first diffraction grating 13 and the second diffraction grating 15 have a diffraction grating region whose cross-sectional shape is rectangular, ) You may have the combination with the area | region of the diffraction grating used as a blaze | braze shape.

Next, an optical action for three lights having different wavelengths incident on the wavelength selective diffraction element will be described. FIG. 2 (c), three wavelength light traveling in the Z-axis direction in the wavelength selective diffraction element 10a, i.e., the wavelength lambda ₁ of the light, the wavelength lambda ₂ of the light, the wavelength lambda ₃ of both light Y axis direction FIG. 2 schematically shows a state when the light is incident as linearly polarized light. The wavelength λ ₁ is a 405 nm wavelength band, the wavelength λ ₂ is a 660 nm wavelength band, and the wavelength λ ₃ is a 785 nm wavelength band.

The birefringent material and the thickness of the wave plate 12 are adjusted so that the wave plate 12 is a so-called λ plate that has a retardation value substantially equal to an integral multiple of λ ₁ with respect to light having the wavelength λ ₁ . The retardation value Rd (λ _m ) for light of wavelength λ _m is (m is an integer of 1 to 3), and the wave plate 12 is composed of a single-layer birefringent material whose optical axes are aligned in the thickness direction. Is expressed by the product of the refractive index anisotropy Δn (λ _m ) of the birefringent material with respect to light of wavelength λ _m and the thickness d of the wave plate 12. That is, in this case, Rd (λ ₁ ) ≈pλ ₁ (an integer of p ≧ 1). Further, the wavelength plate 12, in addition to the properties that make lambda plate with respect to the wavelength lambda ₁ of the light becomes substantially equal to the retardation value to an odd multiple of lambda _2/2 with respect to the wavelength lambda ₂ of light, so-called 1/2 It is adjusted to be a wave plate. That, and _{Rd (λ 2) ≒ (2q} -1) λ 2/2 (q ≧ 1 integer). As a result, the light of wavelength λ ₁ that passes through the wave plate 12 remains unchanged as linearly polarized light in the Y-axis direction, and the light of wavelength λ ₂ becomes linearly polarized light in the X-axis direction.

Further, the wavelength plate 12 may be adjusted so as to have a desired retardation value preferentially for the light of wavelength λ _{1 and} the light of wavelength λ ₂ , but for example, for the light of wavelength λ ₃ Thus, the diffraction efficiency of the 0th-order diffracted light (straight forward transmitted light) of the wavelength selective diffraction element 10a can be increased by using elliptically polarized light having a large light intensity in the X-axis direction or Y-axis direction. Thus, with respect to the wavelength lambda ₃ of the light, the polarization of the first diffraction grating 13 in consideration of the diffraction characteristics of the second diffraction grating 15, passes through the wavelength plate 12 as high zero-order diffraction efficiency is obtained It is good to adjust the state. The wavelength plate 12, lambda plate with respect to the wavelength lambda ₁ of the light, although a half-wave plate for the wavelength lambda ₂ of light, a half-wave plate for the wavelength lambda ₁ of the light, wavelength A λ plate is used for the light of λ _2, the light of wavelength λ ₁ transmitted through the wave plate 12 becomes linearly polarized light in the X-axis direction, and the light of wavelength λ ₂ remains unchanged as linearly polarized light in the Y-axis direction. You may adjust so that it may not.

Next, referring to FIG. 2C, the optical action of the light transmitted through the wave plate 12 in the first diffraction grating 13 and the second diffraction grating 15 will be described. Light of the wavelength lambda ₁ of the light, the wavelength lambda ₂ of the light and the wavelength lambda ₃ are incident on the wavelength-selective diffraction element 10a while traveling in the Z-axis direction as light in the Y-axis direction of the linearly polarized light, respectively. As described above, the light of wavelength λ ₁ that has passed through the wave plate 12 remains linearly polarized light in the Y-axis direction, the light of wavelength λ _{2 is} the light of linearly polarized light in the X-axis direction, and the light of wavelength λ ₃ . Becomes elliptically polarized light and enters the first diffraction grating 13. In the first diffraction grating 13, since the liquid crystal of the first birefringent material layer 13a is aligned parallel to the Y-axis direction and n _{s1 ≈no1} , a part of the light of wavelength λ ₁ is diffracted. and, light of the wavelength lambda ₂ is straightly transmitted without being diffracted. Further, light of wavelength lambda _3, the components of the linearly polarized light in the X-axis direction of the light elliptically polarized light is straightly transmitted, part of the light component in the Y-axis direction of the linearly polarized light is diffracted.

On the other hand, in the second diffraction grating 15, the liquid crystal of the second birefringent material layer 15 a is aligned parallel to the X-axis direction and n _{s2 ≈no 2} , so that the light of wavelength λ ₁ is not diffracted. straight passes, part of the wavelength lambda ₂ of light is diffracted. Further, light of wavelength lambda _3, the components of the linearly polarized light in the Y-axis direction of the light elliptically polarized light is straightly transmitted, part of the light component in the X-axis direction of the linearly polarized light is diffracted.

Further, as a relation between the grating pitch and the diffraction angle of the diffraction grating, when light having a wavelength λ is incident on the diffraction grating having the grating pitch P perpendicular to the transparent substrate surface, the normal of the transparent substrate surface (= light traveling direction) The diffraction angle θ [°] of the Q-order diffracted light (Q = ± 1, ± 2,...) With respect to
θ = Sin ⁻¹ (Qλ / P)
Therefore, it is preferable to adjust the diffraction efficiency and diffraction angle of light of each wavelength in accordance with the specification of the photodetector 112 in the optical head device 100 and the position of each light receiving area.

Thus, the wavelength selective diffraction element 10a, the wavelength plate 12, out of the wavelength lambda ₁ of light and the wavelength lambda ₂ of the light, and to reflect not diffract either the first diffraction grating 13 for diffracting the other And the second diffraction grating 15 that allows the light diffracted by the first diffraction grating 13 to pass through without being diffracted and diffracts the other, so that each of the two wavelengths of light is independently desired. Can be diffracted at the diffraction efficiency and diffraction angle. Therefore, for example, even when the light receiving area for BD and the light receiving area for DVD are shared among the light receiving areas of the photodetector, the adjustment can be made by the configurations of the first diffraction grating 13 and the second diffraction grating 15.

The light of wavelength λ ₃ for CD is Q-order diffracted light (Q = ± 1, ± 2,...) Diffracted by the first diffraction grating 13 and the second diffraction grating 15 in the photodetector 112. In some cases, only the 0th-order diffracted light that passes straight through the two diffraction gratings is detected without being subjected to light detection. Therefore, in this case, it is preferable that the first diffraction grating 13 and the second diffraction grating 15 have high zero-order diffraction efficiency with respect to light having the wavelength λ ₃ , and the total amount of light that passes straight through these two diffraction gratings. The 0th-order diffraction efficiency may be 70% or more, and more preferably 80% or more. For example, when the optical head device 100 uses the three-beam method in which three beams are used in the forward optical path for the light of wavelength λ ₃ for CD, the wavelength selective diffraction element disposed only in the return optical path 10a makes it possible to reach the photodetector 112 without reducing the light utilization efficiency of the reciprocated three beams.

(Second Embodiment)
The second embodiment includes wavelength selective diffraction elements 20a and 20b different from the wavelength selective diffraction element 10a of the first embodiment as the wavelength selective diffraction element 10 used in the optical head device 100. FIGS. 3A and 3B are schematic cross-sectional views showing the configurations of the wavelength selective diffraction elements 20a and 20b, respectively. The wavelength plate 22, the first diffraction grating 23, and the second diffraction grating 25 are provided. It has been.

The wave plate 22 is provided on one surface of the transparent substrate 21a, and has the same function as the wave plate 12 of the first embodiment. The first diffraction grating 23 corresponds to the first isotropic material layer 24 made of an isotropic material and a convex portion corresponding to the birefringence material layer 23a on the transparent substrate 21b. The concave portion has a diffraction grating structure arranged periodically. The first isotropic material layers 24 may be formed so as to cover the birefringent material layers 23a, but may be alternately arranged at the same height as the birefringent material layers 23a. The first diffraction grating 23 has a function similar to that of the first diffraction grating 13 of the first embodiment. That is, one of the ordinary light refractive index n _o1 and the extraordinary light refractive index n _e1 of the birefringent material layer 23 a is a combination of materials substantially equal to the refractive index n _s1 of the isotropic material layer 24.

  Further, the wavelength selective diffraction element is not limited to the rectangular cross-sectional shape of the diffraction grating as in the wavelength selective diffraction element 20a in FIG. 3A, but as in the wavelength selective diffraction element 20b in FIG. The cross-sectional shape of the first diffraction grating 23 may be a (pseudo) blazed shape. The wavelength selective diffraction element 20b is an example in which the birefringent material layer 23b has a blazed shape, and other than that, the same number as the wavelength selective diffraction element 20a is attached.

  Next, the second diffraction grating 25 will be described. The second diffraction grating 25 is made of an isotropic material and has a diffraction grating structure in which convex portions corresponding to the second isotropic material layer 25a are periodically spaced apart. Note that the isotropic material that becomes the second isotropic material layer 25a may be the same material as the transparent substrate 21b. For example, the surface of the transparent substrate 21b is processed into a concavo-convex shape to obtain the second isotropic material layer 25a. The anisotropic material layer 25a may be obtained. The medium around the isotropic material layer 25a is air (refractive index = 1), which is different from the wavelength selective diffraction element 10a according to the first embodiment. Accordingly, since a difference in refractive index occurs between the isotropic material layer 25a and air, the second diffraction grating 25 is designed as follows.

If the refractive index of the second isotropic material layer 25a is n _s2 , light having a wavelength that is not diffracted by the second diffraction grating 25 is λ _A , and the height of the second isotropic material layer 25a is d _A. , (N _s2 −1) × d _A may be adjusted so as to be approximately equal to an integral multiple of λ _A. By doing so, the wavefront of the light of the wavelength lambda _A that passes through the second diffraction grating is straightly transmitted without being changed.

Next, the optical action in the second diffraction grating 25 of the light transmitted through the wavelength selective diffraction element 20a will be described with reference to FIG. Wavelength lambda ₁ is 405nm wavelength range, the wavelength lambda ₂ is 660nm waveband, the wavelength lambda ₃ and 785nm wavelength range, and enters the wavelength selection diffraction element 20a, the wavelength lambda ₁ light, the wavelength lambda ₂ of the light and the wavelength lambda ₃ The light enters the wavelength selective diffraction element 20a while traveling in the Z-axis direction as linearly polarized light in the Y-axis direction. The state of light of each wavelength that passes through the wave plate 24 and the first diffraction grating 23 has the same effect as the wavelength selective diffraction element 10a of the first embodiment. That is, in the first diffraction grating 23, a portion of the Y-axis direction of the linearly polarized light of wavelength lambda ₁ is light light is diffracted, light of the wavelength lambda ₂ is light in the X axis direction of the linearly polarized light is substantially straight To Penetrate. In addition, in the light of wavelength λ ₃ that is elliptically polarized light, the linearly polarized light component in the X-axis direction passes straight through, and a part of the linearly polarized light component in the Y-axis direction is diffracted.

The second diffraction grating _{25, (n s2 -1) × d} A is approximately equal to an integral multiple of the wavelength lambda _1, preferably approximately equal to an even multiple of the wavelength lambda _1, more preferably 2 times the wavelength lambda ₁ N _s2 and d _A are set so as to be substantially equal. Here, the wavelength λ ₁ is a 405 nm wavelength band for BD, and when (n _s2 −1) × d _A ≈2λ ₁ , light having a wavelength λ ₃ corresponding to the 785 nm wavelength band for CD is also used. , (N _s2 −1) × d _A ≈λ ₃ . Therefore, in the second diffraction grating 25, by reducing the amount of the primary or higher order diffracted light with respect to wavelength lambda ₃ of the light, it is possible to increase the zero-order diffraction efficiency of straightly transmitted, the light utilization efficiency of the optical detector 112 It can also be raised.

On the other hand, as described above, when the second diffraction grating 25 is (n _s2 −1) × d _A ≈2λ ₁ , (n _s2 −1) × d _A ≠ mλ ₂ (m ≧ 1) integer), the second diffraction grating 25, a part of the wavelength lambda ₂ of light is diffracted. Thus, the wavelength selective diffraction element 20a, the wavelength plate 22, and diffracts the light of wavelength lambda _1, the first diffraction grating 23 which transmits light of the wavelength lambda _2, diffracted light having a wavelength lambda _2, the wavelength lambda _Since the second diffraction grating 25 that transmits _one light is included, the light of these two wavelengths can be diffracted independently at a desired diffraction efficiency and diffraction angle. The second diffraction grating 25 is part which is convex portion has a second isotropic material layer 25a, may be a birefringent material, in which case the incident light having a wavelength of lambda ₁ It is preferable to give d _A in consideration of the refractive index (normal light refractive index or extraordinary light refractive index) for linearly polarized light in the Y-axis direction.

In addition, the wavelength selective diffraction element according to the first and second embodiments receives light of wavelength λ ₁ , light of wavelength λ ₂ , and light of wavelength λ ₃ as linearly polarized light in the Y-axis direction. it is assumed that both may be one as a light in the X axis direction of the linearly polarized light, and the wavelength lambda ₁ of light and the wavelength lambda ₂ of light a linearly polarized light of the same polarization direction may be a light of linear polarization light of the wavelength lambda ₃ is orthogonal to it. Of the incident light of each wavelength, it is only necessary that the light of wavelength λ _{1 and} the light of wavelength λ ₂ can be diffracted independently by the two diffraction gratings.

  Further, as a different from the wavelength selective diffraction element according to the first embodiment and the second embodiment, a wavelength selective diffraction element comprising a wave plate and one diffraction grating can be used. FIG. 4A is a schematic cross-sectional view showing the wavelength selective diffraction element 30 constituted by a portion sandwiched between the transparent substrate 11a and the transparent substrate 11b in the wavelength selective diffraction element 10a according to the first embodiment. The parts constituting the wavelength selective diffraction element 30 are given the same numbers as the wavelength selective diffraction element 10a to avoid duplication of explanation.

FIG. 4B shows that light of wavelength λ ₁ , light of wavelength λ ₂ and light of wavelength λ ₃ are incident on the wavelength selective diffraction element 30 while traveling in the Z-axis direction as linearly polarized light in the Y-axis direction. It is a schematic diagram which shows an effect | action when it does. In this case, the (first) diffraction grating 13 diffracts the light having the wavelength λ ₁ and transmits the light having the wavelength λ ₂ without diffracting it. For example, when the optical head device 100 uses the three-beam method in which three beams are used in the forward optical path for the light having the wavelength λ ₂ for DVD and the light having the wavelength λ ₃ for CD, The wavelength selective diffraction element 30 disposed only on the light source can reach the photodetector 112 without lowering the light utilization efficiency of the reciprocated three beams.

Example 1
In this example, a specific design example and optical characteristics of the wavelength selective diffraction element 10a according to the first embodiment shown in FIG. 2A will be described.

  First, a quartz glass substrate is washed and dried as the transparent substrate 11a, and an antireflection film is formed on one surface of the quartz glass substrate using a vacuum deposition method. Next, a polyimide film formed by applying polyimide is rubbed on the other surface of the quartz glass substrate to form an alignment film. Then, a liquid crystal monomer having birefringence is uniformly applied on the alignment film and irradiated with UV light, whereby the thickness 9 of the polymer liquid crystal in which the major axis direction of the liquid crystal molecules is uniform and aligned in the thickness direction. A 1 μm wave plate 12 is produced. The normal light / abnormal light refractive index and retardation value with respect to light of each wavelength of the polymer liquid crystal of the wave plate 12 are as shown in Table 1.

  Next, the quartz glass substrate is washed and dried as the transparent substrate 11b, and a polyimide film formed by applying polyimide is rubbed on one surface of the quartz glass substrate to form an alignment film. Then, a liquid crystal monomer having birefringence is uniformly applied on the alignment film and irradiated with UV light, whereby the thickness 1 of the polymer liquid crystal in which the major axis direction of the liquid crystal molecules is uniform and aligned in the thickness direction. A 3 μm polymer liquid crystal film is prepared. Then, using a photolithography and etching technique, the cross-sectional shape of the diffraction grating has a rectangular irregularity so that the longitudinal direction of the diffraction grating and the alignment direction (slow axis direction) of the polymer liquid crystal coincide. Processing is performed to obtain the first diffraction grating 13 having the first birefringent material layer 13a. At this time, the pitch P of the first diffraction grating 13 is 1.6 μm, and the width of the first birefringent material layer 13 a with respect to the pitch P is 0.8 μm (Duty ratio: 0.5). The polymer liquid crystal used as the first birefringent material layer 13a is the same material as the polymer liquid crystal of the wave plate 12, and the ordinary / abnormal light refractive index for light of each wavelength is as shown in Table 1. is there.

  Next, the quartz glass substrate is washed and dried as the transparent substrate 11c, and an antireflection film is formed on one surface of the quartz glass substrate using a vacuum deposition method. Next, a polyimide film formed by applying polyimide is rubbed on the other surface of the quartz glass substrate to form an alignment film. Then, a liquid crystal monomer having birefringence is uniformly coated on the alignment film and irradiated with UV light, whereby the thickness 2 of the polymer liquid crystal in which the major axis direction of the liquid crystal molecules is uniform and aligned in the thickness direction. A polymer liquid crystal film of 8 μm is prepared. After that, it has periodic irregularities with a rectangular cross section using photolithography and etching technology so that the longitudinal direction of the diffraction grating and the direction orthogonal to the alignment direction of the polymer liquid crystal (the fast axis direction) coincide with each other. By processing into a diffraction grating shape, a second diffraction grating 15 having a second birefringent material layer 15a is obtained. At this time, the pitch P of the second diffraction grating 15 is 2.4 μm, and the width of the birefringent material layer 15 a with respect to the pitch P is 1.2 μm (Duty ratio: 0.5). The polymer liquid crystal used as the second birefringent material layer 15a is the same material as the polymer liquid crystal of the wave plate 12, and the ordinary light / abnormal light refractive index for light of each wavelength is as shown in Table 1. is there.

  Using a transparent adhesive as the first isotropic material layer 14, bonding is performed so as to fill the space between the wave plate 12 and the first diffraction grating 13. At this time, the angle α formed by the alignment direction (slow axis direction) of the polymer liquid crystal of the wave plate 12 with reference to the alignment direction of the first birefringent material layer 13a when viewed from the transparent substrate 11a side is 45 [°. ] Or -45 [°]. The sign of the angle α is plus (+) clockwise with respect to the orientation direction of the first birefringent material layer 13a.

Then, a transparent adhesive is used as the second isotropic material layer 16 so that the space between the flat surface of the transparent substrate 11 b and the second diffraction grating 15 is filled. At this time, the first isotropic material layer 14 for obtaining the wavelength selective diffraction element 10a is formed by bonding so that the longitudinal direction of the second diffraction grating 15 coincides with the longitudinal direction of the first diffraction grating 13. Refractive indices (n _s1 , n) of light of each wavelength of the first isotropic material (adhesive) and the second isotropic material (adhesive) forming the second isotropic material layer 16 _s2 ) is as shown in Table 2.

  Linearly polarized light parallel to the longitudinal direction of the first diffraction grating 13 and the second diffraction grating 15 is made incident on the manufactured wavelength selective diffraction element 10a. At this time, as shown in FIG. 2C, incident light is linearly polarized light in the Y-axis direction corresponding to the longitudinal direction of the first diffraction grating 13 and the second diffraction grating 15 in the Z direction. Make it progress. FIG. 5 is a schematic diagram showing the relationship between the (linear) polarization direction of incident light and the optical axis (slow axis) of the wave plate 12, and the polarization direction 40 of light incident in the Y-axis direction. The angle α between the wavelength plate 12 and the alignment direction 12a of the polymer liquid crystal is shown. In this embodiment, α is 45 [°]. The incident light is light having a wavelength of 405 nm, light having a wavelength of 660 nm, and light having a wavelength of 785 nm.

When light having a wavelength of 405 nm is incident as linearly polarized light in the Y-axis direction, the light passes through the wave plate 12 as linearly polarized light in the Y-axis direction. The light having a wavelength of 405 nm transmitted through the wave plate 12 is diffracted because it is in the direction of the extraordinary refractive index of the polymer liquid crystal of the first diffraction grating 13. In this case, the diffraction efficiency is 81.9 [%] for the 0th order diffraction efficiency and 8.7 [%] for the ± 1st order diffraction efficiency. Next, light having a wavelength of 405 nm transmitted / diffracted through the first diffraction grating 13 is incident on the second diffraction grating 15, and is in the direction of the ordinary refractive index of the polymer liquid crystal of the second diffraction grating 15. Since the refractive index coincides with the refractive index of the second isotropic material, it transmits straight without being diffracted, so that the 0th-order diffraction efficiency η ₀ (405) is 81.8 [%], ± 1st-order diffraction efficiency η _{± 1} (405) becomes 8.7 [%].

Next, when light having a wavelength of 660 nm is incident as linearly polarized light in the Y-axis direction, the wave plate 12 acts as a half-wave plate, and is thus modulated into linearly polarized light in the X-axis direction. Specifically, if the amount of light before entering the wave plate 12 is 100 [%], the linearly polarized light component in the X-axis direction is 99.6 [%]. Of the light having a wavelength of 660 nm transmitted through the wave plate 12, the linearly polarized light in the X-axis direction is in the direction of the ordinary refractive index of the polymer liquid crystal of the first diffraction grating 13. The light travels straight without being diffracted and enters the second diffraction grating 15. Of the light having a wavelength of 660 nm incident on the second diffraction grating 15, the linearly polarized light in the X-axis direction is diffracted because it is in the direction of the extraordinary light refractive index of the polymer liquid crystal of the second diffraction grating 15. At this time, the zero-order diffraction efficiency for the linearly polarized light in the X-axis direction is 84.1 [%], and the ± first-order diffraction efficiency is 7.9 [%]. Here, since the component of linearly polarized light in the X-axis direction among the light having a wavelength of 660 nm before and after transmitting through the wave plate 12 is 99.6 [%], ± 1st-order diffraction efficiency η _{± 1} (660) = 7 .8 [%].

On the other hand, among the light having a wavelength of 660 nm transmitted through the wave plate 12, the linearly polarized light in the Y-axis direction is diffracted because it is in the direction of the extraordinary refractive index of the polymer liquid crystal of the first diffraction grating 13. At this time, the zero-order diffraction efficiency of the first diffraction grating 13 for the linearly polarized light in the Y-axis direction is 95.7 [%], and the ± first-order diffraction efficiency is 2.1 [%]. However, since the component of linearly polarized light in the Y-axis direction among the light having a wavelength of 660 nm before and after transmitting through the wave plate 12 is 0.4 [%], the light diffracted by the first diffraction grating 13 is almost 0. It is. Of the light having a wavelength of 660 nm, the linearly polarized light in the Y-axis direction that travels straight through the first diffraction grating 13 has a zero-order diffraction efficiency of 99.9 [%] in the second diffraction grating. As a result, when the 0th-order diffraction efficiency of the linearly polarized light in the X-axis direction and the 0th-order diffraction efficiency of the linearly polarized light in the Y-axis direction out of the light having a wavelength of 660 nm transmitted through the wave plate 12 is zero, Folding efficiency η ₀ (660) = 84.1 [%].

Next, light having a wavelength of 785 nm is incident as linearly polarized light in the Y-axis direction. Assuming that the amount of light before entering the wave plate 12 is 100%, the component of linearly polarized light in the X-axis direction is 94.2%. Of the light having a wavelength of 785 nm transmitted through the wave plate 12, the linearly polarized light in the X-axis direction is in the direction of the ordinary refractive index of the polymer liquid crystal of the first diffraction grating 13. The light passes through almost straight without being diffracted and enters the second diffraction grating 15. Of the light having a wavelength of 785 nm incident on the second diffraction grating 15, the linearly polarized light in the X-axis direction is diffracted because it is in the direction of the extraordinary refractive index of the polymer liquid crystal of the second diffraction grating 15. At this time, the zero-order diffraction efficiency of the second diffraction grating 15 for the linearly polarized light in the X-axis direction is 89.6 [%], and the ± first-order diffraction efficiency is 5.1 [%]. Here, the component of linearly polarized light in the X-axis direction among the light having a wavelength of 785 nm before and after transmitting through the wave plate 12 is 94.2 [%]. Note that ± first-order diffraction efficiency η _{± 1} (785) = 4.9 [%].

On the other hand, among the light with a wavelength of 785 nm transmitted through the wave plate 12, the linearly polarized light in the Y-axis direction is diffracted because it is in the direction of the extraordinary light refractive index of the polymer liquid crystal of the first diffraction grating 13. At this time, the 0th-order diffraction efficiency for linearly polarized light in the Y-axis direction is 97.2 [%], and the ± 1st-order diffraction efficiency is 1.4 [%]. However, since the component of linearly polarized light in the Y-axis direction out of the light having a wavelength of 785 nm before and after transmitting through the wave plate 12 is 5.8 [%], the light diffracted by the first diffraction grating 13 is almost 0. It is. Of the light with a wavelength of 785 nm, the linearly polarized light in the Y-axis direction that travels straight through the first diffraction grating 13 has a zero-order diffraction efficiency of 99.9 [%] in the second diffraction grating. As a result, of the light having a wavelength of 785 nm transmitted through the wave plate 12, the zeroth order diffraction efficiency of the linearly polarized light in the X-axis direction and the zeroth-order diffraction efficiency of the linearly polarized light in the Y-axis direction Folding efficiency η ₀ (785) = 90.0 [%]. Table 3 summarizes the zero-order diffraction efficiency and ± first-order diffraction efficiency at each wavelength in this example.

  When the wavelength-selective diffraction element 10a having such optical characteristics is arranged at the position of the wavelength-selective diffraction element 10 of the optical head device 100 in FIG. 1, desired diffraction efficiency for light with a wavelength of 405 nm and light with a wavelength of 660 nm. Thus, light can be made to reach according to the pattern of each light receiving area of the photodetector 112, and high zero-order diffraction efficiency can be obtained for light with a wavelength of 785 nm. Therefore, by using the wavelength selective diffraction element, it is possible to realize an optical head device having high controllability in a light receiving optical system for recording / reproducing BD, DVD and CD.

(Example 2)
In this example, a specific design example and optical characteristics of the wavelength selective diffraction element 20a according to the second embodiment shown in FIG.

  First, a quartz glass substrate is washed and dried as the transparent substrate 21a, and an antireflection film is formed on one surface of the quartz glass substrate using a vacuum deposition method. Next, a polyimide film formed by applying polyimide is rubbed on the other surface of the quartz glass substrate to form an alignment film. Then, a liquid crystal monomer having birefringence is uniformly applied on the alignment film and irradiated with UV light, whereby the thickness 9 of the polymer liquid crystal in which the major axis direction of the liquid crystal molecules is uniform and aligned in the thickness direction. A 1 μm wave plate 22 is produced. In addition, the ordinary light / abnormal light refractive index and the retardation value with respect to light of each wavelength of the polymer liquid crystal of the wave plate 22 are as shown in Table 1 as in the example.

  Next, the quartz glass substrate is washed and dried as the transparent substrate 21b, and a silicon dioxide film having a thickness of 1.67 μm is formed on one surface of the quartz glass substrate by vacuum deposition. Thereafter, the second diffraction grating 25 having the (second) isotropic material layer 25a is obtained by processing into a diffraction grating shape having a periodic unevenness with a rectangular cross section using photolithography and etching techniques. . At this time, the pitch P of the second diffraction grating 25 is 14.0 μm, and the width of the (second) isotropic material layer 25 a with respect to the pitch P is 7.0 μm (Duty ratio: 0.5). To do. Thereafter, an antireflection film made of a multilayer film is formed on the surface of the quartz glass substrate processed with silicon dioxide into a diffraction grating shape by using a vacuum deposition method.

  Next, on the surface of the transparent substrate 21b opposite to the surface on which the diffraction grating made of silicon dioxide is formed, a polyimide film formed by applying polyimide is rubbed to form an alignment film. Then, a liquid crystal monomer having birefringence is uniformly applied on the alignment film and irradiated with UV light, whereby the thickness 1 of the polymer liquid crystal in which the major axis direction of the liquid crystal molecules is uniform and aligned in the thickness direction. A 3 μm polymer liquid crystal film is prepared. Then, using a photolithography and etching technique, the cross-sectional shape of the diffraction grating has a rectangular irregularity so that the longitudinal direction of the diffraction grating and the alignment direction (slow axis direction) of the polymer liquid crystal coincide. Processing is performed to obtain the first diffraction grating 23 having the first birefringent material layer 23a. At this time, the pitch P of the first diffraction grating 23 is 9.0 μm, and the width of the first birefringent material layer 13 a with respect to the pitch P is 4.5 μm (Duty ratio: 0.5). Further, the first diffraction grating 23 is processed so that the longitudinal direction thereof is parallel to the longitudinal direction of the second diffraction grating 25. The ordinary light / abnormal light refractive index with respect to light of each wavelength of the polymer liquid crystal serving as the first birefringent material layer 23a is as shown in Table 2.

Then, a transparent adhesive is used as the first isotropic material layer 24 and is bonded so as to fill the space between the wave plate 22 and the first diffraction grating 23 to obtain the wavelength selective diffraction element 10a. At this time, the angle α formed by the alignment direction (slow axis direction) of the polymer liquid crystal of the wave plate 22 with respect to the alignment direction of the first birefringent material layer 23a when viewed from the transparent substrate 21a side is 45 [°. Adhere so that As shown in FIG. 4, the sign of the angle α is positive (+) clockwise with respect to the orientation direction of the first birefringent material layer 23a. Each of the first isotropic material (adhesive) for forming the first isotropic material layer 24 and the second isotropic material (silicon dioxide) for forming the second diffraction grating 25 is used. Table 4 shows the refractive indices (n _s1 , n _s2 ) for light of a wavelength.

  Linearly polarized light parallel to the longitudinal direction of the first diffraction grating 23 and the second diffraction grating 25 is made incident on the manufactured wavelength selective diffraction element 20a. At this time, as shown in FIG. 3C, incident light is linearly polarized light in the Y-axis direction corresponding to the longitudinal direction of the first diffraction grating 23 and the second diffraction grating 25 in the Z direction. Make it progress. In this embodiment, α is 45 °, and incident light is light having a wavelength of 405 nm, light having a wavelength of 660 nm, and light having a wavelength of 785 nm.

When light having a wavelength of 405 nm is incident as linearly polarized light in the Y-axis direction, the light passes through the wave plate 22 with almost 100% linearly polarized light in the Y-axis direction. The light having a wavelength of 405 nm transmitted through the wave plate 22 is diffracted because it is in the direction of the extraordinary refractive index of the polymer liquid crystal of the first diffraction grating 23. In this case, the diffraction efficiency is 79.0 [%] for the 0th order diffraction efficiency and 8.7 [%] for the ± 1st order diffraction efficiency. Next, light having a wavelength of 405 nm transmitted / diffracted through the first diffraction grating 23 is incident on the second diffraction grating 25, and the second isotropic material layer 25 a constituting the second diffraction grating 25 and From the relationship between the refractive index difference from air (refractive index = 1) and the height (d _A ) of the second isotropic material layer 25a, the 0th-order diffraction efficiency is 88.8 [%]. Accordingly, assuming that the amount of light before entering the wave plate 22 is 100 [%], 0th-order diffraction efficiency η ₀ (405) = 70.1 [%], ± 1st-order diffraction efficiency η _{± 1} (405) = 7 .7 [%].

Next, when light having a wavelength of 660 nm is incident as linearly polarized light in the Y-axis direction, the wave plate 22 functions as a half-wave plate, and thus is modulated to linearly polarized light in the X-axis direction. Specifically, if the amount of light before entering the wave plate 12 is 100 [%], the linearly polarized light component in the X-axis direction is 99.6 [%]. Of the light having a wavelength of 660 nm transmitted through the wave plate 22, linearly polarized light in the X-axis direction is in the direction of the ordinary refractive index of the polymer liquid crystal of the first diffraction grating 23. The light travels straight without being diffracted and enters the second diffraction grating 25. The light having a wavelength of 660 nm incident on the second diffraction grating 25 has a difference in refractive index between the second isotropic material layer 25a constituting the second diffraction grating 25 and air (refractive index = 1), the second From the relationship of the height (d _A ) of the isotropic material layer 25a, the zero-order diffraction efficiency is 72.5 [%] and the ± first-order diffraction efficiency is 8.7 [%]. Here, the component of linearly polarized light in the X-axis direction that passes straight through the first diffraction grating 23 out of light having a wavelength of 660 nm before and after transmitting through the wave plate 22 is 99.6 [%], so ± 1 Next diffraction efficiency η _{± 1} (660) = 8.6 [%].

On the other hand, among the light having a wavelength of 660 nm transmitted through the wave plate 22, the linearly polarized light in the Y-axis direction is diffracted because it is in the direction of the extraordinary refractive index of the polymer liquid crystal of the first diffraction grating 23. At this time, the 0th-order diffraction efficiency for linearly polarized light in the Y-axis direction is 94.9 [%], and the ± 1st-order diffraction efficiency is 2.1 [%]. However, since the component of linearly polarized light in the Y-axis direction of light having a wavelength of 660 nm before and after transmitting through the wave plate 22 is 0.4 [%], the light diffracted by the first diffraction grating 23 is almost 0. It is. Of the light having a wavelength of 660 nm, the linearly polarized light in the Y-axis direction that has been transmitted straight through the first diffraction grating 23 has a zero-order diffraction efficiency of 71.9 [%] and ± 1 in the second diffraction grating. Since the second-order diffraction efficiency is 8.8 [%], out of the light having a wavelength of 660 nm transmitted through the wave plate 22, the zero-order diffraction efficiency of the linearly polarized light in the X-axis direction and the linearly-polarized light in the Y-axis direction. When the zero-order diffraction efficiencies are added up, the zero-order diffraction efficiency η ₀ (660) = 72.4 [%].

Next, light having a wavelength of 785 nm is incident as linearly polarized light in the Y-axis direction. Assuming that the amount of light before entering the wave plate 22 is 100%, the component of linearly polarized light in the X-axis direction is 94.2%. Of the light having a wavelength of 785 nm transmitted through the wave plate 22, the linearly polarized light in the X-axis direction is in the direction of the ordinary refractive index of the polymer liquid crystal of the first diffraction grating 23. The light passes through almost straight without being diffracted and enters the second diffraction grating 25. The light having a wavelength of 785 nm incident on the second diffraction grating 25 has a difference in refractive index between the second isotropic material layer 25a constituting the second diffraction grating 25 and air (refractive index = 1), the second From the relationship of the height (d _A ) of the isotropic material layer 25a, the zero-order diffraction efficiency is 87.1 [%]. Here, the component of linearly polarized light in the X-axis direction that passes straight through the first diffraction grating 23 out of light having a wavelength of 785 nm before and after transmission through the wave plate 22 is 94.2 [%]. The folding efficiency is 82.0 [%].

On the other hand, among the light with a wavelength of 785 nm transmitted through the wave plate 22, the linearly polarized light in the Y-axis direction is diffracted because it is in the direction of the extraordinary refractive index of the polymer liquid crystal of the first diffraction grating 23. At this time, the 0th-order diffraction efficiency for linearly polarized light in the Y-axis direction is 96.6 [%], and the ± 1st-order diffraction efficiency is 1.4 [%]. However, since the component of linearly polarized light in the Y-axis direction out of light having a wavelength of 785 nm before and after transmission through the wave plate 22 is 5.8 [%], the light diffracted by the first diffraction grating 23 is almost 0. It is. Of the light with a wavelength of 785 nm, linearly polarized light in the Y-axis direction that has been transmitted straight through the first diffraction grating 23 has a 0th-order diffraction efficiency of 87.7 [%] in the second diffraction grating. Of the light having a wavelength of 785 nm transmitted through the wave plate 22, the zero-order diffraction efficiency of the zero-order diffraction efficiency of the linearly polarized light in the X-axis direction and the zero-order diffraction efficiency of the linearly-polarized light in the Y-axis direction is η ₀ (785) = 86.9 [%]. Note that ± first-order diffraction efficiency η _{± 1} (785) = 0.5 [%]. Table 5 summarizes the zero-order diffraction efficiency and ± first-order diffraction efficiency at each wavelength in this example.

  When the wavelength selective diffraction element 20a having such optical characteristics is arranged at the position of the wavelength selective diffraction element 10 of the optical head device 100 in FIG. 1, desired diffraction efficiency for light having a wavelength of 405 nm and light having a wavelength of 660 nm. Thus, light can be made to reach according to the pattern of each light receiving area of the photodetector 112, and high zero-order diffraction efficiency can be obtained for light with a wavelength of 785 nm. Therefore, by using the wavelength selective diffraction element, it is possible to realize an optical head device having high controllability in a light receiving optical system for recording / reproducing BD, DVD and CD.

  As described above, the present invention provides a wavelength-selective diffraction element that transmits / diffracts light with a desired diffraction efficiency for incident BD wavelength light, DVD wavelength light, and CD wavelength light, respectively. By using the optical head device, it is possible to provide an optical head device that has a high degree of freedom in the layout of the light receiving area of the photodetector and has an effect of realizing miniaturization.

10, 10a, 10b, 20a, 20b, 30 Wavelength selective diffraction element 11a, 11b, 11c, 21a, 21b Transparent substrate 12, 22 Wave plate 12a Direction of optical axis (slow axis) of wave plate 13, 23 First Diffraction gratings 13a, 23a, 23b, 25b Birefringent material layers 14, 24 (first) isotropic material layer 15, 25 Second diffraction grating 15b Isotropic material layer 25a (second) isotropic Material layer 40 Direction of linear polarization of incident light 100 Optical head device 101a, 101b Light source 102a, 102b Grating element 103, 106 Dichroic prism 104 Polarizing beam splitter 105 Collimator lens 107 Mirror 108a, 108b 1/4 wavelength plates 109a, 109b Objective Lens 110a, 110b Optical disk 111 Cylindrical lens 112 Optical inspection Vessel

Claims (7)

A light source that emits light of wavelength λ ₁ that is 405 nm wavelength band, light of wavelength λ ₂ that is 660 nm wavelength band, light of wavelength λ ₃ that is 785 nm wavelength band, light of wavelength λ ₁ , and wavelength λ _In an optical head device comprising: an objective lens that condenses the light of _{2 and} the light of the wavelength λ _{3 on} an optical disc; and a beam splitter that deflects the light reflected from the optical disc to a photodetector.
A wavelength-selective diffractive element in an optical path between the light of the wavelength λ ₁ , the light of the wavelength λ ₂ , and the light of the wavelength λ ₃ between the beam splitter and the photodetector;
The wavelength selective diffraction element has at least a wavelength plate, a first diffraction grating, and a second diffraction grating, respectively, from the beam splitter side.
The wave plate converts the light having the wavelength λ ₁ and the light having the wavelength λ _{2 to be} transmitted as first linearly polarized light and second linearly polarized light that are orthogonal to each other,
The first diffraction grating transmits _one of the light of wavelength λ _{1 and} the light of wavelength λ ₂ without diffracting, and diffracts the other,
The second diffraction grating transmits the light diffracted by the first diffraction grating out of the light having the wavelength λ _{1 and} the light having the wavelength λ ₂ without diffracting the light and diffracting the other light. Head device. The wave plate is made of a birefringent material having optical axes aligned in the thickness direction, has a retardation value substantially equal to an integral multiple of λ ₁ with respect to the light with the wavelength λ ₁ , and has the light with the wavelength λ ₂ the optical head device according to claim 1 having a substantially equal retardation values on an odd multiple of lambda _2/2 relative. The first diffraction grating has periodic unevenness by a first birefringent material layer made of a birefringent material and a first isotropic material layer made of an isotropic material,
When the ordinary refractive index of the first birefringent material layer is n _o1 , the extraordinary refractive index is n _e1 (n _o1 ≠ n _e1 ), and the refractive index of the isotropic material is n _s1 , the n _s1, the optical head apparatus according to claim 1 or claim 2 substantially equal to said _{n o1} or the _{n e1.} The second diffraction grating has periodic irregularities by a second birefringent material layer made of a birefringent material and a second isotropic material layer made of an isotropic material,
The ordinary refractive index of the second birefringent material layer is n _o2 , the extraordinary refractive index is n _e2 (n _o2 ≠ n _e2 ), and the refractive index of the second isotropic material layer is n _s2 . 4. The optical head device according to claim 3, wherein the n _s2 is substantially equal to the no ₂ or the _{ne 2} . The second diffraction grating has periodic irregularities due to a second isotropic material layer made of an isotropic material on the plane of the transparent substrate,
When the refractive index of the second isotropic material layer is n _s2 and the height is d _A , ( _{ns 2} −1) × d _A is approximately an integral multiple of the wavelength λ ₁ or an integral multiple of the wavelength λ _2. The optical head device according to claim 3 which is equal.   6. The optical head device according to claim 1, wherein cross-sectional shapes of the first diffraction grating and the second diffraction grating are rectangular. The wavelength selection diffraction element, when the amount of incident wavelength lambda ₃ of the light is 100%, the light according to any one of claims 1 to 6, light of the wavelength lambda ₃ to straightly transmitted is 70% or more Head device.

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