JP2006236549A - Optical head apparatus, wide band phase plate, and polarization diffraction element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical head apparatus used for compatibility with three wavelengths and to provide a wide band phase plate and a polarization diffraction element used for the optical head apparatus. <P>SOLUTION: In the optical head apparatus provided with semiconductor lasers 401, 402 and 403, the polarization diffraction element 411, the wide band phase plate 412, and photo detectors 404, 405 and 406, the semiconductor lasers 401, 402 and 403 emit luminous fluxes of linearly polarized beams having a 410 nm wavelength band, a 660 nm wavelength band and a 780 nm wavelength band, respectively, the polarization diffraction element 411 transmits a linearly polarized beam in a prescribed polarization direction straight and diffracts a linearly polarized beam in the direction orthogonal to the prescribed polarization direction and the wide band phase plate 412 generates phase differences of nearly (2m<SB>a</SB>-1)λ/4, nearly (2m<SB>b</SB>-1)λ/4, and nearly m<SB>c</SB>λ (λ denotes wavelength and m<SB>a</SB>, m<SB>b</SB>and m<SB>c</SB>each denotes a natural number) to the beams having the 410 nm wavelength band, the 660 nm wavelength band and the 780 nm wavelength band, respectively. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical head device equipped with a broadband phase plate, a broadband phase plate, and a polarization diffraction element.

  In an optical head device that records optical information on an optical recording medium such as an optical disk and a magneto-optical disk (hereinafter referred to as an optical disk) and reproduces information recorded on the optical disk, the recording density is increased. The wavelength of laser light to be used is being shortened. However, even in an optical head device that reads and writes a high-density disk using a short wavelength laser beam such as HD DVD and Blu-ray, optical disks such as DVD-R / RW and CD-R / RW that have been widely used are also recorded.・ It is necessary to be able to reproduce, and in addition to the short wavelength light source, a red light source and a near-infrared light source are installed together so that all of these optical disks can be read and written. A playback method has been proposed.

  In order to achieve high light utilization efficiency for any wavelength used in each optical disk, high transmittance is required in the forward path from the light source to the optical disk, and high diffraction efficiency is required in the return path from the optical disk to the photodetector. An optical head device using a polarization optical system having a polarization diffraction element is conceivable. An example of the configuration of an optical head device using such a polarization optical system is shown in FIG.

  In FIG. 9, linearly polarized light beams from the semiconductor lasers 11, 12, and 13 in the emission wavelength bands of 410 nm, 660 nm, and 780 nm pass straight through the polarization diffraction elements 17, 18 and 19 for the respective wavelengths, and the polarization diffractions thereof. It is converted into circularly polarized light by the quarter phase plates 20, 21 and 22 integrated with the element. Thereafter, the light is combined by a combining prism 23 that reflects light in the wavelength band of 660 nm and transmits light in the wavelength band of 780 nm, and a combining prism 24 that transmits light in the band of 660 nm and 780 nm and reflects light in the wavelength band of 410 nm. The collimated lens 25 converts the light into parallel light, which is condensed and irradiated on the information recording surface of the optical disk 28 by the objective lens 26 common to the three wavelengths held by the actuator 27.

  The three-wavelength return light beam reflected by the information recording surface of the optical disk 28 and including the information of the pits formed on the information recording surface becomes circularly polarized light in the reverse direction with the rotation direction of the circularly polarized light reversed. Traveled in the direction of the light and again transmitted through the quarter phase plates 20, 21, and 22, and converted into linearly polarized light having a polarization direction orthogonal to the polarization direction of the light beam emitted from the light source, by the polarization diffraction elements 17, 18, and 19 The information diffracted and guided to the photodetectors 14, 15 and 16 corresponding to the respective wavelengths is read out.

  In the conventional optical head device as shown in FIG. 9, since it is necessary to arrange an optical element such as a polarization diffraction element or a quarter phase plate for each wavelength, the number of parts increases and the volume of the apparatus increases. Furthermore, there is a problem that assembly adjustment takes time. Therefore, a configuration in which three semiconductor lasers are arranged close to each other and a configuration using a semiconductor laser that can oscillate a plurality of wavelengths have been proposed.

As a broadband phase plate that can be used in common for the three wavelength bands of 410 nm, 660 nm, and 780 nm required for the optical head device in such a configuration, each wavelength can be obtained by stacking two layers exhibiting birefringence. Patent Document 1 discloses a broadband phase plate that generates a phase difference corresponding to a quarter wavelength with respect to a band. However, a polarization diffraction element that selectively diffracts all three wavelengths by polarization is problematic in that it is difficult to achieve good diffraction efficiency for all wavelength bands. Further, since an optical disk (CD) using light in the 780 nm band has a large protective layer thickness, the use of the polarizing optical system as described above tends to cause fluctuations in recording / reproducing characteristics due to the birefringence deviation of the protective layer. There is also.
Japanese Patent Application Laid-Open No. 2004-219977

  The present invention provides an optical head device used for three-wavelength compatibility, a broadband phase plate and a polarization diffraction element used for such an optical head device, and an object thereof is to solve the above-described problems.

The present invention provides an optical head device including a light source, a polarization diffraction element, a broadband phase plate, and a photodetector, and the light source emits linearly polarized light beams having wavelengths of 410 nm, 660 nm, and 780 nm. The polarization diffractive element is a polarization diffractive element that linearly transmits linearly polarized light in a predetermined polarization direction and diffracts linearly polarized light orthogonal to the predetermined polarization direction, and the broadband phase plate has a wavelength Approximately (2m _a −1) · λ / 4 for light in the 410 nm band, approximately (2m _b −1) · λ / 4 for light in the wavelength 660 nm band, approximately m _c for light in the wavelength 780 nm band A broadband phase plate that generates a phase difference of λ (λ is a wavelength, m _a , m _b , and _mc are natural numbers), and is emitted from the light source and incident on the polarization diffraction element at a wavelength range of 410 nm and 660 nm Obi The polarized light beam is transmitted straight through the polarization diffraction element, converted into substantially circularly polarized light by the broadband phase plate, irradiated to the optical recording medium, reflected by the information recording surface of the optical recording medium, and reversely circularly polarized light. The returned light beam is converted into substantially linearly polarized light having a polarization direction orthogonal to the light beam emitted from the light source by the broadband phase plate, is diffracted by the polarization diffraction element, and is guided to a photodetector. A linearly polarized light beam having a wavelength of 780 nm that is emitted and incident on the polarization diffraction element is transmitted straight through the polarization diffraction element and transmitted through the broadband phase plate without substantially changing the polarization state. The reflected light beam reflected on the information recording surface of the optical recording medium is transmitted without substantially changing the polarization state by the broadband phase plate, and transmitted straight by the polarization diffraction element. It is to provide an optical head device which is characterized in that is guided to the photodetector.

When the phase difference generated by the broadband phase plate becomes higher, the wavelength dependency of the phase difference generated by the broadband phase plate increases in each wavelength band. For example, when the wavelength of the light beam emitted from the light source fluctuates due to a temperature change In addition, the phase difference generated by the broadband phase plate may be greatly deviated from a desired value. Therefore, the phase difference is preferably low order. That is, m _a , m _b , and m _c are preferably natural numbers of 7 or less, and more preferably natural numbers of 6 or less.

  The phase difference generated by the broadband phase plate is approximately 9λ / 4 for light having a wavelength of 410 nm, approximately 5λ / 4 for light having a wavelength of 660 nm, and approximately λ for light having a wavelength of 780 nm, or It is preferable that the wavelength is approximately 11λ / 4 for light having a wavelength of 410 nm, approximately 5λ / 4 for light having a wavelength of 660 nm, and approximately λ for light having a wavelength of 780 nm.

  Here, the “wavelength 410 nm band” refers to a wavelength using the broadband phase plate of the present invention from the wavelength range of 390 to 440 nm, that is, one wavelength selected according to the wavelength of the light source to be used, “wavelength 660 nm band”, The “wavelength 780 nm band” refers to one wavelength that is similarly selected from a wavelength range of 640 to 680 nm and 760 to 800 nm, respectively.

Further, “the phase difference is approximately 9λ / 4” means that the phase difference expressed in angle is within a range of less than an angle ± 5 degrees corresponding to 9λ / 4. Similarly, “substantially 5λ / 4”, “substantially λ”, and “substantially 11λ / 4” mean that the angle is within a range of less than ± 5 degrees with respect to each angle. Here, the phase difference [degree] displayed in angle is the phase difference [nm] displayed in length,
There is a relationship of phase difference [degree] = (phase difference [nm] / wavelength [nm]) × 360 degrees. For example, a λ plate is 360 degrees when expressed in angle.

  The polarization diffraction element in the optical head device is a polarization diffraction element including a first diffraction grating and a second diffraction grating laminated, and the first diffraction grating has a wavelength of 410 nm incident from the light source. , 660 nm band and 780 nm band linearly polarized light beams are linearly transmitted, linearly polarized light of 410 nm wavelength having a polarization direction orthogonal to the linearly polarized light from the light source is diffracted, and linearly polarized light of 660 nm band is transmitted straight. The second diffraction grating linearly transmits linearly polarized light beams having wavelengths of 410 nm, 660 nm, and 780 nm incident from the light source, and has a polarization direction orthogonal to the linearly polarized light from the light source. A diffraction grating that linearly transmits linearly polarized light with a wavelength of 410 nm and diffracts linearly polarized light with a wavelength of 660 nm is preferable.

  Further, the first diffraction grating is made of an optically anisotropic material, and a blazed diffraction grating in which ridges extending in one direction having a sawtooth cross section are formed periodically and parallel to each other, or optical A quasi-blazed diffraction grating made of an anisotropic material and having ridges extending in one direction and having a sawtooth-shaped cross section approximating a desired sawtooth shape of a step shape of 8 or more steps, formed periodically and parallel to each other The second diffraction grating is preferably made of an optically anisotropic material, and the ridges extending in one direction having a rectangular cross section are periodically and parallel to each other. .

  The polarization diffraction element is preferably a polarization diffraction element that generates a focus error signal and / or a tracking signal.

  In the optical head device of the present invention, the broadband phase plate is preferably laminated and integrated with the polarization diffraction element. With this configuration, a further effect can be obtained in reducing the size of the components constituting the optical head device and reducing the number of components, and the optical head device can be further reduced in size.

The present invention also includes a layer exhibiting birefringence and one or more transparent substrates, and is substantially (2 m _a −1) · λ / 4 for light having a wavelength of 410 nm and light having a wavelength of 660 nm. In contrast, a phase difference of approximately (2 m _b −1) · λ / 4 is generated, and approximately m _c · λ (λ is a wavelength, m _a , m _b , and _mc are natural numbers) for light having a wavelength of 780 nm. A broadband phase plate characterized by generating a phase difference of (2) is provided.

  In this case, the phase difference given by the broadband phase plate is approximately 9λ / 4 for light of a wavelength of 410 nm, approximately 5λ / 4 for light of a wavelength of 660 nm, and approximately λ for light of a wavelength of 780 nm. Alternatively, it is preferably approximately 11λ / 4 for light of a wavelength of 410 nm, approximately 5λ / 4 for light of a wavelength of 660 nm, and approximately λ for light of a wavelength of 780 nm.

  The birefringent layer preferably has a layer made of one birefringent material, and in that case, it preferably consists of a polymer liquid crystal. Alternatively, the birefringent layer preferably has a layer made of two birefringent materials, and in that case, it preferably consists of a polymer liquid crystal or stretched polycarbonate.

  The present invention is also a polarization diffraction element in which linearly polarized light having wavelengths of 410 nm, 660 nm, and 780 nm is used as incident light, and the first diffraction grating and the second diffraction grating are stacked and provided. The first diffraction grating linearly transmits linearly polarized light having a wavelength of 410 nm, 660 nm, and 780 nm in a predetermined polarization direction incident on the polarization diffraction element, and has a wavelength of a polarization direction orthogonal to the predetermined polarization direction. A diffraction grating that diffracts linearly polarized light in a 410 nm band and linearly transmits linearly polarized light in a wavelength of 660 nm, wherein the second diffraction grating has wavelengths of 410 nm, 660 nm, and 780 nm in the predetermined polarization direction. Linearly polarized light is transmitted in a straight line, linearly polarized light having a wavelength of 410 nm in the polarization direction orthogonal to the predetermined polarization direction is transmitted in a straight line, and linearly polarized light in a wavelength of 660 nm is transmitted. Providing a polarizing diffraction element which is a diffraction grating to diffract. As a result, the optical path can be switched according to the wavelength and polarization direction of the incident light, and a light beam necessary for detecting the focus error signal and / or the tracking signal can be generated. In addition, the use efficiency of light having a short wavelength can be increased, and a polarization diffraction element suitable for use in an optical head device used for three-wavelength compatibility can be realized.

  In this case, the first diffraction grating is made of an optically anisotropic material, and a blazed diffraction grating in which ridges extending in one direction having a sawtooth-shaped cross section are formed periodically and parallel to each other, or Pseudo blazed diffraction made of optically anisotropic material, with ridges extending in one direction and having a sawtooth cross section approximating the desired sawtooth shape to a step shape of 8 or more steps, formed periodically and parallel to each other The grating is a diffraction grating in which the second diffraction grating is made of an optically anisotropic material, and ridges extending in one direction having a rectangular cross section are formed periodically and parallel to each other. preferable.

  Furthermore, the product of the diffraction efficiency of the first diffraction grating and the transmittance of the second diffraction grating in the wavelength 410 nm band for linearly polarized light having a polarization direction orthogonal to the predetermined polarization direction is a wavelength 660 nm band. Is preferably larger than the product of the transmittance of the first diffraction grating and the diffraction efficiency of the second diffraction grating.

  The present invention also provides a laminated optical element in which the broadband phase plate is laminated and integrated with a polarization diffraction element that linearly transmits or diffracts incident linearly polarized light in accordance with the polarization direction. The laminated optical element of this configuration changes the linearly polarized light of the wavelength 410 nm band and the 660 nm band incident from the polarization diffraction element side into circularly polarized light and transmits it linearly, and polarizes the linearly polarized light of the wavelength 780 nm band. Polarization direction perpendicular to the predetermined polarization direction is circularly polarized in the direction opposite to the circularly polarized light in the wavelength 410 nm band and the 660 nm band, which is transmitted through straight without changing the state substantially and incident from the broadband phase plate side. The linearly polarized light having a wavelength of 780 nm in the predetermined polarization direction is transmitted in a straight line without substantially changing the polarization state. The effect of reducing the size and the number of parts can be obtained, and the optical head device can be further reduced in size.

  Further, it is preferable to further stack a diffraction grating that diffracts linearly polarized light having a wavelength of 780 nm. As a result, the optical head device can be further miniaturized.

  In the optical head device of the present invention, good recording / reproducing characteristics are realized for three wavelength bands of 410 nm, 660 nm, and 780 nm. In addition, the number of parts is reduced, the configuration is simplified, assembly adjustment is easy, and a configuration that is reduced in size and cost is realized. With respect to the 780 nm wavelength band, sufficiently high light utilization efficiency can be obtained, and deterioration in reproduction / recording characteristics due to deviation of the protective layer of the optical disk can be suppressed.

  The broadband phase plate of the present invention is preferably used in an optical head device that performs recording / reproduction using the three wavelength bands. When the broadband phase plate according to the present invention is used, good recording / reproduction characteristics are realized in the three wavelength bands in the optical head device, the number of parts is reduced, the configuration is simplified, and assembly adjustment is easy. Become.

  The polarization diffraction element of the present invention is preferably used in an optical head device that performs recording / reproduction using the three wavelength bands. When the broadband phase plate according to the present invention is used, good recording / reproduction characteristics are realized in the three wavelength bands in the optical head device, the number of parts is reduced, the configuration is simplified, and assembly adjustment is easy. Become.

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

(First embodiment)
FIG. 1 is a block diagram showing a first configuration example of the optical head device according to the first embodiment of the present invention. This configuration is an example of the configuration of the optical head device of the present invention, and the present invention is not limited to this.

  This optical head device includes semiconductor lasers 401, 402, and 403 that emit linearly polarized light beams having wavelengths of 405 nm, 660 nm, and 780 nm, respectively, as light sources having a wavelength of 410 nm, a wavelength of 660 nm, and a wavelength of 780 nm.

  The linearly polarized light beam emitted from the semiconductor laser 401 that emits a light beam with a wavelength of 405 nm is transmitted through the collimating lens 410 and the combining prism 408 that reflects the light with a wavelength of 405 nm and transmits the light with a wavelength of 660 nm and 780 nm. . Next, the light passes through the polarization diffraction element 411 held by the actuator 414, is converted into circularly polarized light by the broadband phase plate 412, and is condensed and irradiated on the information recording surface of the optical disk 415 by the objective lens 413 similarly held by the actuator 414. The The return light beam reflected from the optical disk 415 including the information of the pits formed on the information recording surface travels in the opposite direction on each path. That is, the return light beam is converted into linearly polarized light having a polarization direction orthogonal to the light beam emitted from the light source by the broadband phase plate 412, diffracted by the polarization diffraction element 411, transmitted through the collimator lens 410, and reflected by the combining prism 408. After that, the light is incident on the photodetector 404 and the information recorded on the optical disk 415 is read out.

  For a wavelength of 660 nm, a linearly polarized light beam emitted from the semiconductor laser 402 that emits a light beam having a wavelength of 660 nm is reflected by a combining prism 409 that reflects light having a wavelength of 660 nm and transmits light having a wavelength of 780 nm. After passing through a combining prism 408 that reflects light of 405 nm and transmits light of wavelength 660 nm and wavelength 780 nm, it passes through the collimating lens 410. Next, the light passes through the polarization diffraction element 411 held by the actuator 414, is converted into circularly polarized light by the broadband phase plate 412, and is condensed and irradiated onto the information recording surface of the optical disk 415 by the objective lens 413. The return light beam reflected from the optical disk 415 including the information of the pits formed on the information recording surface travels in the opposite direction on each path. That is, the return light beam is converted into linearly polarized light having a polarization direction orthogonal to the light beam emitted from the light source by the broadband phase plate 412, diffracted by the polarization diffraction element 411, transmitted through the collimator lens 410, and then transmitted through the combining prism 408. Then, after being reflected by the multiplexing prism 409, it enters the photodetector 405, and information recorded on the optical disk 415 is read out.

  When the wavelength is 780 nm, the thickness of the protective layer of the disk is large. Therefore, when the polarizing optical system as described above is used, the recording / reproduction characteristics fluctuate due to the birefringence deviation of the protective layer. Therefore, an optical system using a non-polarizing diffraction element formed of glass or the like is used. The linearly polarized light beam emitted from the semiconductor laser 403 passes through the non-polarized diffraction grating 407, reflects the light with a wavelength of 660 nm, and transmits the light with a wavelength of 780 nm, reflects the light with a wavelength of 405 nm, reflects the light with a wavelength of 660 nm, and After passing through the combining prism 408 that transmits light having a wavelength of 780 nm, the light passes through the collimator lens 410 and the polarization diffraction element 411 held by the actuator 414. Next, the return light beam that is transmitted through the broadband phase plate 412, condensed and irradiated on the information recording surface of the optical disk 415 by the objective lens 413 that is also held by the actuator 414, and reflected from the optical disk 415 travels backward on the same path. Then, the light enters the broadband phase plate 412. Since the light beam in the 780 nm wavelength band does not change the polarization state by the broadband phase plate, the return light beam is also transmitted through the polarization diffraction element 411 without being diffracted. The transmitted return light beam is transmitted through the collimating lens 410, then transmitted through the combining prism 408 and the combining prism 409, and the first-order diffracted light diffracted by the non-polarization diffraction element 407 is detected by the photodetector 406, and the optical disk 415 The information recorded in is read out.

  In the optical head device of the first configuration of the present invention, linearly polarized light is converted into circularly polarized light and circularly polarized light is converted into linearly polarized light for light beams in the wavelength bands of 410 nm and 660 nm, respectively. On the other hand, a broadband phase plate that does not substantially change the polarization state, and a polarization diffraction element that linearly transmits linearly polarized light in the same polarization direction as that of the light beam from the light source and diffracts linearly polarized light in the orthogonal polarization direction. With these, good recording / reproducing characteristics can be obtained in all three wavelength bands. In addition, an optical head device having good characteristics can be realized with a small number of components and a miniaturized configuration.

  Hereinafter, the broadband phase plate of the present invention will be described first.

  The broadband phase plate of the present invention used in such an optical head device usually includes a birefringent layer and at least one transparent substrate. The broadband phase plate of the present invention may be configured such that the birefringent layer is held on one transparent substrate, or the birefringent layer is sandwiched between two transparent substrates. There may be. If the transparent substrate is transparent to the light of the three wavelength bands, various materials such as a resin plate and a resin film can be used, but if an optically isotropic material such as glass or quartz glass is used, This is preferable because the transmitted light is not affected by birefringence.

  As the birefringent layer used in the broadband phase plate of the present invention, a layer formed by orienting a birefringent organic material on a substrate or an organic material such as polycarbonate is stretched to orient molecules and impart birefringence. The polymer film can be used. An example of the birefringent organic material formed on the substrate while being oriented is liquid crystal. The liquid crystal is preferable because the dispersion coefficient can be approximated to a desired value by selecting one having various properties from various liquid crystal compounds or by preparing a mixed composition obtained by mixing them. Further, the thickness of the liquid crystal layer can be easily adjusted only by changing the diameter of the spacer sandwiched between the substrates sandwiching the liquid crystal layer when the liquid crystal layer is formed, which is preferable from the viewpoint that the retardation value can be easily controlled. As the liquid crystal, a low molecular liquid crystal can be used, and a polymer liquid crystal formed by polymerizing and curing a polymerizable liquid crystal precursor by means of photopolymerization or the like may be used. When polymer liquid crystal is used, the orientation direction is physically fixed and the mechanical strength of the broadband phase plate is increased, so that the reliability is improved, and the element is formed into a drum shape due to thermal expansion / contraction due to temperature change. This is preferable because the aberration caused by deformation is reduced.

As the birefringent material, in addition to the liquid crystal, an inorganic material film such as a TiO ₂ film formed by providing a birefringence on the substrate by a film forming method such as anisotropic vapor deposition, or a crystal processed to a desired thickness A sheet of an inorganic material having birefringence such as LiNbO ₃ can also be used.

  In general, retardation by a birefringent material is wavelength-dependent. When A, B, and C are dispersion coefficients specific to the material and the thickness of the birefringent material is dnm, the wavelength expressed by the equation (1) from Cauchy's equation. Is expressed by the function R (λ).

R (λ) = (A + B / λ + C / λ ² ) · d (1)
In the equation (1), if the phase differences at wavelengths of 405 nm, 660 nm and 780 nm are set to 9λ / 4, 5λ / 4, and λ, respectively, the conditional expressions at each wavelength are as follows: (2a) become.

R (405 nm) = 911 nm (2a)
R (660 nm) = 825 nm (2b)
R (780 nm) = 780 nm (2c)
When the dispersion coefficients A, B, and C are obtained from (2a), (2b), and (2c),
A = 363.7 / d, B = 436100 / d, C = −86840000 / d
(3)
Is obtained. That is, when a broadband phase plate is formed by forming a single layer having a thickness of dnm using a birefringent material having such a dispersion coefficient as the birefringent layer, the desired phase difference is obtained at the three wavelengths. This is preferable.

  In addition, the birefringent layer may have a configuration in which a plurality of layers exhibiting birefringence are laminated, and such a configuration is preferable because the range of selection of applicable birefringent materials is widened. The number of stacked layers is preferably two layers for simplification of the manufacturing process, but three or more layers can be stacked and used. The broadband phase plate of the present invention configured by laminating two layers exhibiting birefringence can be designed as follows.

When the birefringent layer has a laminated structure in which the first and second birefringent layers exhibiting birefringence are laminated, the phase difference generated when light is incident on the laminated birefringent layer is the Stokes- It is expressed by a determinant like equation (4) using parameters.

で表わされるように水平直線偏光である。式（４）においてθ_１，２は、第１および第２の複屈折層のそれぞれの進相軸と透明基板面内の基準方向とのなす角度であり、Δ_１，２は第１および第２の複屈折層のそれぞれで生じる位相差である。また、［Ｔ_θ１，２］は座標軸を角度θ_１，２だけ回転する旋光子行列で、［Ｃ_Δ１，２］は位相をΔ_１，２だけシフトさせる移相子行列で、それぞれ式（６）、式（７）のように表される。

Here, the polarization state of the incident light is expressed by equation (5):

It is horizontal linearly polarized light as expressed by In Equation (4), θ 1 and ₂ are angles formed by the fast axes of the first and second birefringent layers and the reference direction in the transparent substrate surface, and Δ 1 and ₂ are the first and second 2 is a phase difference generated in each of the two birefringent layers. [T _θ1,2 ] is an _optical rotator matrix that rotates the coordinate axis by an angle θ _1,2 , and [C _Δ1,2 ] is a phase shifter matrix that shifts the phase by Δ _1,2. ) And Expression (7).

Here, the shift amount delta _{1, 2} phases then retardation values of the angle [radian] Display at each wavelength is expressed by _{Δ 1,2 = Δn · d 1,2 ·} 2π / λ becomes equation.

If this determinant is solved under the condition in which the retardation value is set to a desired phase difference, the configuration of a broadband wave plate by lamination having a desired phase difference at each wavelength, that is, two layers made of a birefringent material Angles θ ₁ , θ ₂ and thicknesses d ₁ , d ₂ formed by the fast axis of each birefringent layer and the reference direction are obtained. However, this determinant cannot be solved analytically and only an approximate solution can be obtained. Furthermore, since the dispersion coefficient is specific to the material, in general, it is not possible to obtain a solution for a laminated structure that gives a desired phase difference at a wavelength of three or more wavelengths.

The present inventor has a phase difference at each wavelength in the wavelength 410nm band _{(2m a -1) · λ /} 4, at wavelength 660nm band _{(2m b -1) · λ /} 4, and the wavelength 780nm band _{m c} · lambda (lambda is the _{wavelength, m a, m b, m} c is a natural number) when placed with, as a first aspect, the stretching polycarbonate (dispersion coefficient as the birefringent material a = 0.01052, B = 0, C = Using 564.7), it has been found that a good approximate solution can be obtained for a laminated structure in which a phase difference of 9λ / 4 at a wavelength of 410 nm, 5λ / 4 at a wavelength of 660 nm, and λ at a wavelength of 780 nm is obtained. That is, if the first layer has a thickness of 75 μm and the phase advance axis is 12 degrees, and the second layer has a thickness of 66 μm and the phase advance axis is 65 degrees, the respective wavelengths are A phase difference of less than ± 5 degrees is obtained from the desired phase difference.

  As another aspect, when a birefringent material having dispersion coefficients of A = 0.1011, B = 0, and C = 9230 is used, 11λ / 4 in the wavelength 410 nm band, 5λ / 4 in the wavelength 660 nm band, and 780 nm wavelength It was found that a good approximate solution can be obtained for the phase difference of λ in the band. That is, if the first layer has a thickness of 1.7 μm and the phase advance axis is 8 degrees, and the second layer has a thickness of 7 μm and the phase advance axis is 147 degrees, A phase difference of less than ± 5 degrees is obtained from the desired phase difference at a wavelength of. Examples of the material having such a dispersion include polymer liquid crystals having a predetermined composition.

  With such a configuration, the broadband phase plate of the present invention generates the respective phase differences with respect to the linearly polarized light in the wavelength direction of 410 nm and 660 nm in the reference direction and the direction orthogonal to the reference direction, and the linearly polarized light is circularly converted. Circular polarized light is converted into linearly polarized light and transmitted through polarized light. In addition, light in the 780 nm wavelength band is transmitted without substantially changing the polarization state.

  If the difference between the phase difference generated by the broadband phase plate and the desired value is large, the linearly polarized light incident on the broadband phase plate is not converted into circularly polarized light and is emitted as elliptically polarized light, which is preferable because the signal strength of reproduction / recording is weakened. Absent. In addition, when the apparent optical axis of the phase plate is a direction where the angle formed with the reference direction in the broadband phase plate surface is 45 degrees, the apparent optical axis is a predetermined angle with respect to the polarization direction of the linearly polarized light incident thereon. If it is inclined from the angle, the desired phase difference cannot be obtained, and the emitted light becomes elliptically polarized light, which is not preferable.

  When the ellipticity is calculated with respect to the apparent optical axis when linearly polarized light having a polarization direction parallel or orthogonal to the reference direction is incident, the difference between the phase difference generated by the broadband phase plate and a desired value is obtained. When it is less than ± 5 degrees, the apparent optical axis is 0.92 or more when there is no inclination with respect to the polarization direction of the linearly polarized light that is incident, and the apparent optical axis is within ± 1 degree. A good ellipticity value of 0.91 or more can be obtained even when tilted. That is, if the difference between the phase difference generated by the broadband phase plate and the desired value is less than ± 5 degrees, a good ellipticity of 0.90 or more can be obtained even if the optical axis tilts to ± 1 degrees or less. It is done. If the difference between the phase difference of the broadband phase plate and the desired value is ± 4 degrees or less, the ellipticity is 0.91 or more even if there is an apparent optical axis inclination of ± 2 degrees or less, and 0.90 or more A good ellipticity can be obtained.

  Next, the polarization diffraction element of the present invention will be described.

  FIG. 4A is a diagram showing a conceptual cross-sectional structure of the polarization diffraction element 541 of the present invention. 4B and 4C are diagrams for explaining the operation of the polarization diffraction element 541. FIG. The polarization diffraction element 541 includes a first diffraction grating 551 and a second diffraction grating 552 stacked, and has a wavelength of 410 nm sandwiched between the quartz glass substrates 302 and 303 and the quartz glass substrates 302 and 303. Diffractive ridges 542 for the diffraction grating, ridges 543 for the diffraction grating for the wavelength 660 nm band, and filler 544. The substrates 302 and 303 are not limited to quartz glass as long as the materials have high transmittance in the wavelength band in which the polarization diffraction element 541 is used, and substrates made of other materials can also be used.

  The first and second diffraction gratings 551 and 552 are configured to diffract or transmit linearly polarized light having a wavelength of 410 nm, a wavelength of 660 nm, and a wavelength of 780 nm, respectively, according to the wavelength and direction of polarization of light. Are laminated so that the polarization directions of the linearly polarized light to be diffracted or transmitted coincide with each other. Hereinafter, the polarization direction of linearly polarized light transmitted by these diffraction gratings is referred to as first polarization, and the polarization direction of linearly polarized light that is diffracted by these diffraction gratings and is orthogonal to the first polarization is referred to as second polarization.

  The order of stacking the first and second diffraction gratings is not limited. Further, as shown in the cross-sectional view of FIG. 4, the diffraction grating surfaces may be laminated to face each other, or the diffraction grating surfaces may be laminated in the same direction.

  Since the diffraction grating designed to diffract light of a certain wavelength also partially diffracts light of other nearby wavelengths, the first diffraction grating 551 that diffracts the second polarized light with a wavelength of 410 nm has the wavelength It also has a diffractive action on the second polarized light in the 660 nm band, and partially diffracts and transmits a part of the incident second polarized light in the 660 nm band. Similarly, the second diffraction grating 552 that diffracts the second polarized light having a wavelength of 660 nm has a diffraction effect on the second polarized light having a wavelength of 410 nm, and the incident second polarized light having a wavelength of 410 nm. Is partially diffracted and partially transmitted. The first and second diffraction gratings 551 and 552 transmit the first polarized light having a wavelength of 780 nm.

  Here, the use efficiency of light in the wavelength 410 nm band of the polarization diffraction element 541 is the product of the diffraction efficiency of the first diffraction grating 551 and the transmittance of the second diffraction grating 552 for the second polarized light in the wavelength 410 nm band, The utilization efficiency of light in the wavelength 660 nm band is the product of the diffraction efficiency of the second diffraction grating 552 and the transmittance of the first diffraction grating 551 for the second polarized light in the wavelength 660 nm band. At this time, it is preferable that the polarization diffraction element 541 has higher utilization efficiency of light having a wavelength of 410 nm than light utilization efficiency of light having a wavelength of 660 nm. With this configuration, when the polarization diffraction element 541 of the present invention is used in an optical head device, the read / write characteristics of the optical recording medium using light in the wavelength of 660 nm band and 780 nm band are maintained, and the optical recording medium is used. Light with a wavelength of 410 nm, which has low reflectance and detection sensitivity with a photodetector, can be used effectively.

  In such a polarization diffraction element 541, as the first diffraction grating 551, a blazed diffraction grating in which convex portions having a sawtooth-shaped cross section extending in one direction are formed periodically and in parallel with each other is used. As described above, each of the diffraction gratings 552 having a rectangular cross section extending in one direction and having a rectangular cross section formed periodically and in parallel with each other is used as described above. This is realized by adopting a configuration in which the directions of the first polarized light and the second polarized light coincide with each other.

The first diffraction grating 551 has n ₁ and n ₂ , respectively, that indicate the refractive indexes of the optically anisotropic material forming the ridges having a sawtooth cross-section with respect to linearly polarized light in directions orthogonal to each other within the diffraction grating surface. When (n ₁ ≠ n ₂ ), it is preferable to make n ₁ or n ₂ substantially equal to the refractive index of the filler 544. The polarization direction showing the same refractive index as that of the filler 544 is the polarization direction of the first polarization.

  Such a blazed diffraction grating having a convex portion having a sawtooth cross section can be formed by, for example, dry etching using a gray mask after forming a layer made of an optically anisotropic material. Further, the pseudo blazed diffraction grating, in which the cross-sectional shape of the ridge portion is replaced with a sawtooth shape, and the desired sawtooth shape is approximated by a multi-stepped shape having the same step width and the same step width; You can also Such a pseudo blazed diffraction grating can be formed by using a plurality of masks whose apertures are changed. The stepped step is a value obtained by dividing the diffracted wavelength, that is, 405 nm by Δn and the number of steps. Here, Δn is the difference between the refractive index of the optically anisotropic material constituting the convex portion of the sawtooth cross section with respect to the second polarized light having a wavelength of 410 nm and the refractive index of the filler, and the number of steps is 8 or more. It is said.

  With this configuration, the first polarized light having substantially the same refractive index of the convex portion of the sawtooth section and the filler 544 is transmitted as it is, and the wavelength of 410 nm is applied to the second polarized light having different refractive indexes. A diffraction grating that is diffracted in the band and partially diffracted and partially transmitted in the wavelength 660 nm band is obtained.

The second diffraction grating 552 is a diffraction grating in which the ridges of the rectangular cross section are extended in one direction, the ridges of the rectangular cross section are formed of an optically anisotropic material, and in the direction perpendicular to each other in the diffraction grating surface When the refractive index of the optically anisotropic material with respect to the linearly polarized light is n ₃ and n ₄ (n ₃ ≠ n ₄ ), n ₃ or n ₄ is substantially equal to the refractive index of the filler 544. It is preferable to do. The polarization direction showing the same refractive index as that of the filler 544 is the polarization direction of the first polarization.

  Such a sawtooth cross section can be formed by dry etching using a striped mask after forming a layer made of an optically anisotropic material. The height of the convex portion of the rectangular cross section is a value obtained by dividing the wavelength of diffraction, that is, 660 nm by Δn and 2. Here, Δn is the difference between the refractive index of the optically anisotropic material constituting the convex section of the rectangular cross section with respect to the second polarized light in the wavelength 660 nm band and the refractive index of the filler.

  By adopting this configuration, the first polarized light having substantially the same refractive index of the ridges having a rectangular cross section and the filler 544 is transmitted as it is, and the second polarized light having a different refractive index is used as a wavelength. A diffraction grating can be obtained that is partially diffracted and partially transmitted in the 410 nm band and diffracted in the wavelength 660 nm band.

  As the optically anisotropic material of the first and second diffraction gratings, when a polymer liquid crystal obtained by polymerizing and curing liquid crystal is used, the refractive index can be adjusted by selecting the material, and the desired refractive index can be adjusted by aligning the substrate. The direction showing the refractive index can be controlled, and the grating can be easily processed, which is preferable.

  A diffraction grating surface using a polymer liquid crystal is prepared by injecting a polymerizable liquid crystal into a cell in which two substrates are arranged to face each other with a gap determined so as to obtain the desired level difference. A polymer liquid crystal layer is formed by polymerizing and curing by thermal polymerization, and then one substrate is removed and the polymer liquid crystal layer is processed by photolithography and etching. By controlling the direction in which the liquid crystal molecules are aligned by forming an alignment film on the surface of the two substrates in contact with the liquid crystal and rubbing in one direction, the desired direction for the first polarization and the second polarization is obtained. An optically anisotropic material layer having a refractive index of 1 is obtained.

  The direction in which the ridges of the first and second diffraction gratings extend can be arbitrarily determined regardless of the directions of the first and second polarizations. That is, it is determined so that the return light is guided to a desired light receiving surface of the photodetector when incorporated in the optical head device.

  Further, the first and second diffraction gratings are diffraction gratings having a plurality of regions in the diffraction grating surface, and the direction in which the protruding strips extending in one direction in each region are periodically parallel to each other is extended. With the configuration in which the predetermined directions are different from each other, it is possible to generate a light beam necessary for detecting the focus error signal and / or the tracking signal.

  Hereinafter, a laminated optical element in which the broadband phase plate of the present invention and another optical element are laminated and integrated will be described.

  When the broadband phase plate of the present invention is laminated and integrated with other optical elements to form a laminated optical element, the number of parts constituting the optical head device can be further reduced, and adjustment is facilitated. In that case, it is more preferable that the birefringent layer is held on a single transparent substrate because the thickness of the laminated optical element can be reduced and the weight can be reduced. As another element to be laminated and integrated, a polarization diffraction element that linearly transmits or diffracts a light beam depending on the polarization direction of the incident linearly polarized light beam is preferably exemplified. FIG. 5 shows a schematic cross-sectional view of an example of a laminated optical element in which a broadband phase plate and a polarization diffraction element are laminated and integrated. In this case, the broadband phase plate 305 of the present invention is configured such that a birefringent layer 304 is held on a single quartz glass substrate 301. The polarization diffraction element is formed by combining a quartz glass substrate 302 on which a 410 nm wavelength band polarization diffraction element 306 is formed and a quartz glass substrate 303 on which a 660 nm wavelength band polarization diffraction element 307 is formed. The two surfaces are laminated so that the two substrates are filled with a filler 308 made of an isotropic transparent material. That is, the laminated optical element transmits linearly polarized light in a predetermined polarization direction in the wavelength 410 nm band and the 660 nm band, that is, the first polarized light, the polarization diffraction elements 306 and 307 pass straight through, and the broadband phase plate 305 converts it into substantially circular polarized light. In addition, the linearly polarized light in the polarization direction orthogonal to the predetermined polarization direction, that is, the second polarized light is sandwiched between the broadband phase plate 305 and the quartz substrates 302 and 303 so that the polarization diffraction elements 306 and 307 diffract the light. The polarization diffraction elements 306 and 307 are manufactured by stacking and integrating the directions.

  Hereinafter, the operation of the laminated optical element 540 in which the broadband phase plate of the present invention and the polarization diffraction element of the present invention are laminated and integrated will be described with reference to FIGS. 6B and 6C.

  The laminated optical element 540 includes a broadband phase plate 305 sandwiched between the quartz glass substrates 301 and 302 and a polarization sandwiched between the quartz glass substrates 302 and 303 as in the conceptual cross-sectional structure illustrated in FIG. A diffractive element 541 is laminated. The quartz glass substrate 302 is commonly used. With this configuration, the laminated optical element 540 converts the first polarized light with wavelengths of 410 nm, 660 nm, and 780 nm incident from the polarization diffraction grating 541 side into circularly polarized light, circularly polarized light, and first polarized light, respectively, and transmits them straight. The circularly polarized light in the reverse direction of the wavelength 410 nm band and the 660 nm band incident from the broadband phase plate 305 side of the laminated optical element 540 is converted into the second polarized light, and is diffracted and emitted. Further, the first polarized light having a wavelength of 780 nm that is incident from the broadband phase plate 305 side is transmitted in a straight line without substantially changing the polarization state.

  The laminated optical element 540 may have a configuration in which diffraction gratings that diffract light having a wavelength of 780 nm band are further laminated. With this configuration, the optical head device can be further reduced in size.

(Second Embodiment)
FIG. 7 is a block diagram showing a second configuration example of the optical head apparatus according to the second embodiment of the present invention. In FIG. 7, an optical head device 500 includes a first light source 511 that emits a linearly polarized light beam having a wavelength of 410 nm, a second light source 528 that emits a linearly polarized light beam having a wavelength of 660 nm, and a first light source. 511 for combining the light beam emitted from the light source 511 and the light beam emitted from the second light source 528, a first collimator 512 for converting the combined light beam into a parallel light beam, and a first collimator 512. A first portion for monitoring the light quantity of the light beam directed to the optical recording medium 600 by receiving the light beam reflected by the polarization beam splitter 513 and receiving the light beam reflected by the polarization beam splitter 513. The photodetector 514, the first photodetector 516 that receives the return light of the light beam having a wavelength of 410 nm reflected by the optical recording medium 600, and the return on the first photodetector 516. A first condensing lens 515 that collects light, a third light source 521 that emits a linearly polarized light beam having a wavelength of 780 nm, and a beam conversion that converts the light beam emitted by the third light source 521 into a plurality of beams. A diffraction grating 522, a beam splitter 523 that transmits a part of the incident light beam and reflects the remainder, a second collimator 524 that converts the light beam reflected by the beam splitter 523 into a parallel light beam, and a light beam that has passed through the beam splitter 523 The second monitor photodetector 525 that monitors the amount of the light flux that travels toward the optical recording medium 600 and the second photodetector 527 that receives the return light having a wavelength of 780 nm reflected by the optical recording medium 600. A second condenser lens 526 that condenses the return light having a wavelength of 780 nm on the second photodetector 527 and a light beam that has passed through the polarization beam splitter 513. Multiplexing means 531 for combining the light beams transmitted through the second collimator 524, a laminated optical element 540 in which a polarization diffraction element and a broadband phase plate are laminated, and a light beam emitted from the laminated optical element 540 as an optical recording medium 600 And an objective lens 532 for condensing the light. Here, the polarization diffraction element and the broadband phase plate are the same as those in the first embodiment.

  The first light source 511, the second light source 528, and the third light source 521 are semiconductor lasers or the like that emit linearly polarized light beams having wavelengths of 405 nm, 660 nm, and 780 nm, respectively. The beam conversion diffraction grating 522 is configured to convert linearly polarized light having a wavelength of 780 nm into a plurality of beams such as three beams. The collimators 512 and 524 convert incident light into parallel light fluxes. The polarization beam splitter 513 reflects or transmits incident light according to the polarization direction.

  The monitor photodetectors 514 and 525 and the photodetectors 516 and 527 output an electrical signal corresponding to the amount of incident light. The condensing lenses 515 and 526 condense incident light on the light receiving surfaces of the photodetectors 516 and 527, respectively. The multiplexing means 531 multiplexes the incident linearly polarized light having a wavelength of 410 nm, the linearly polarized light having a wavelength of 660 nm, and the linearly polarized light having a wavelength of 780 nm. The objective lens 532 condenses incident light on the optical recording medium 600.

  The operation of the optical head device 500 according to the second embodiment of the present invention will be described below.

  In this optical head device 500, the laminated optical element 540 is formed by laminating a polarization diffraction element and a broadband phase plate, and placing a polarization diffraction element on the light source side and a broadband phase plate on the optical recording medium side. Has been.

  Of the linearly polarized light in the wavelength range of 410 nm and 660 nm emitted from the first light source 511 and the second light source 528, the component transmitted through the polarization beam splitter 513 is incident on the laminated optical element 540 as the first polarized light. Then, the second polarization component is reflected by the polarization beam splitter 513. Similarly, linearly polarized light having a wavelength of 780 nm band emitted from the third light source 521 is configured to enter the laminated optical element 540 as the first polarized light.

  The linearly polarized light having a wavelength of 410 nm emitted from the first light source 511 is transmitted through the multiplexing unit 529, is transmitted through the first collimator 512, is converted into a parallel beam, and is incident on the polarization beam splitter 513. The linearly polarized light having a wavelength of 410 nm incident on the polarizing beam splitter 513 is distributed into a first polarized light that passes through the polarizing beam splitter 513 and a second polarized light that is reflected by the polarizing beam splitter 513.

  The second polarized light having a wavelength of 410 nm reflected by the polarizing beam splitter 513 is incident on the first monitor photodetector 514, and the first monitor photodetector 514 detects the amount of light emitted from the first light source 511. . The first polarized light having a wavelength of 410 nm transmitted through the polarizing beam splitter 513 is transmitted through the combining means 531, first incident on the polarization diffraction element of the laminated optical element 540, transmitted straight, and converted into circularly polarized light by the broadband phase plate. Go straight ahead and exit.

  The circularly polarized light having a wavelength of 410 nm emitted from the laminated optical element 540 is condensed on the optical recording medium 600 by the objective lens 532, reflected by the optical recording medium 600, and becomes reversely circularly polarized return light. The return light having a wavelength of 410 nm is transmitted through the objective lens 532, first incident on the broadband phase plate of the laminated optical element 540, converted into the second polarized light, and diffracted at a predetermined angle by the polarization diffraction element for detecting a focus error signal. Are generated. Next, the light is transmitted through the multiplexing unit 531, reflected by the polarization beam splitter 513, collected on the light receiving surface on the first photodetector 516 by the first condenser lens 515, and collected by the first photodetector 516. Information is read.

  The linearly polarized light of the wavelength 660 nm band emitted from the second light source 528 reflects the multiplexing means 529, and the first collimator 512, the polarization beam splitter 513, and the multiplexing means 531 are reflected in the same manner as in the case of the wavelength 410nm band. Then, the light enters the laminated optical element 540 as the first polarized light. The second polarized light having a wavelength of 660 nm reflected by the polarization beam splitter 513 is incident on the first monitor photodetector 514 and the amount of light emitted from the second light source 528 is detected, as in the case of the wavelength of 410 nm. . As in the case of the wavelength 410 nm band, the first polarized light having a wavelength of 660 nm that is incident on the laminated optical element 540 passes through the polarization diffraction element and is converted into circularly polarized light by the broadband phase plate, and the laminated optical element 540 is circularly polarized. Go straight ahead and exit.

  The circularly polarized light having a wavelength of 660 nm band emitted from the laminated optical element 540 is condensed on the optical recording medium 600 by the objective lens 532, reflected by the optical recording medium 600, and becomes reversely circularly polarized return light. The return light of the wavelength 660 nm band is transmitted through the objective lens 532, converted into the second polarized light by the broadband phase plate of the laminated optical element 540, and diffracted by a predetermined angle by the polarization diffraction element, as in the case of the wavelength 410nm band. Thus, a light beam for detecting a focus error signal is generated. Next, the light is condensed on the light receiving surface on the first photodetector 516 through the multiplexing means 531, the polarization beam splitter 513, and the first condenser lens 515, and information is read out by the first photodetector 516. .

  Further, the linearly polarized light having a wavelength of 780 nm emitted from the third light source 521 is converted into, for example, three beams by the beam conversion diffraction grating 522 and is incident on the beam splitter 523, and a part of the linearly polarized light is transmitted through the beam splitter 523. , The amount of light emitted from the third light source 521 is monitored. The linearly polarized light reflected by the beam splitter 523 is reflected by the combining means 531 and enters the laminated optical element 540 as the first polarized light, and passes straight through the laminated optical element 540 without substantially changing the polarization state. .

  The first polarized light having a wavelength of 780 nm emitted from the laminated optical element 540 is condensed on the optical recording medium 600 by the objective lens 532, reflected by the optical recording medium 600, and becomes return light of the first polarized light. The return light passes through the objective lens 532, travels straight through the laminated optical element 540 again, is reflected by the multiplexing means 531, passes through the beam splitter 523, and is transmitted through the second condenser lens 526 to the second light. The light is collected on the light receiving surface on the detector 527. Information is read by the second photodetector 527.

(Third embodiment)
FIG. 8 is a block diagram showing a third configuration example of the optical head apparatus according to the third embodiment of the present invention. In FIG. 8, an optical head device 700 includes a light source 711 that emits linearly polarized light beams having wavelengths of 410 nm, 660 nm, and 780 nm, and a laminated optical element 540 in which a polarization diffraction element and a broadband phase plate are laminated. A three-wavelength laminated optical element 720 further laminated with a diffraction grating 721 for a wavelength of 780 nm that allows a light beam in the 780 nm band to pass straight through and partially diffract, a collimator 712, an objective lens 713, and a photodetector 714. I have. Here, the polarization diffraction element, the broadband phase plate, and the laminated optical element in which they are laminated are the same as those in the first embodiment.

  Here, it is assumed that the light beams in the wavelength range of 410 nm and 660 nm emitted from the light source 711 are linearly polarized light and the polarization direction is both the first polarized light. The light beam in the wavelength band of 780 nm may be either circularly polarized light or elliptically polarized light as long as it is coherent, but the first polarized light described above is preferable because the use efficiency of light in the wavelength band of 780 nm can be increased.

  The diffraction grating 721 for the wavelength of 780 nm band does not need to be a polarization type. For example, convex portions made of a transparent material extending in one direction are formed periodically and parallel to each other on one surface of the transparent substrate. An unpolarized diffraction element can be used. Further, the diffraction grating 721 for a wavelength of 780 nm band has, on one surface of at least one transparent substrate, ridges made of an optical multilayer film extending in one direction are formed periodically and parallel to each other. It is more preferable to use a diffraction grating having a structure filled and covered with a transparent filling material so as to fill the space portion of the concave portion between the convex portions and cover the uppermost surface of the convex portion. This is because the light in the wavelength band of 780 nm can be selectively diffracted and partially transmitted, and the light in the wavelengths of 410 nm and 660 nm can be transmitted without being substantially diffracted. .

More specifically, the diffraction grating 721 for the wavelength of 780 nm band can be formed as follows. First, on a transparent substrate such as quartz glass or glass, a low refractive index material layer having a refractive index n _L and a high refractive index n _H having a sufficiently small optical absorption at a wavelength used by a vacuum deposition method, a sputtering method, or the like. An optical multilayer film in which a refractive index material layer is laminated with a predetermined film thickness is formed. As the material of each layer, inorganic materials such as SiO ₂ , SiON, ZrO ₂ , Ta ₂ O ₅ , Nb ₂ O ₅ , TiO ₂ , Al ₂ O ₃ , MgF ₂ , and organic materials can be used. Preferred examples of the refractive index material layer include SiO ₂ and examples of the high refractive index material layer include Ta ₂ O ₅ .

As for the film thickness configuration of each layer of the optical multilayer film, the sum of the optical film thicknesses (n _L d _L + n _H d _H ) is (2m + 1) / 2 times the reflection band wavelength λ _r (where m is 0 or a positive integer) A set of two layers of a low-refractive material layer having a thickness d _{L and} a high-refractive index material layer having a thickness d _H , each having a determined thickness, are alternately stacked, and The average refractive index of the laminated optical multilayer film, that is, the value obtained by dividing the total optical film thickness of each layer by the total film thickness is substantially equal to the refractive index n _M of the filler, which will be described later, and the wavelengths of 410 nm and 660 nm. The film configuration that is made equal to each other is determined as the basic configuration. Here, the reflection band wavelength λ _r is a wavelength determined to transmit light in the three wavelength bands of the 410 nm band, the 660 nm band, and the 780 nm band, and is 720 nm.

  Next, this optical multilayer film was processed by photolithography and dry etching, and the strip-shaped convex stripes extending in one direction and the optical multilayer film were removed to expose the substrate surface. It is assumed that the recesses are in a lattice shape formed in parallel and periodically. Further, a layer made of a filler having a refractive index of nM is formed so as to fill the concave portion and cover the periphery of the convex portion so as to have a smooth surface, whereby a diffraction grating for a wavelength of 780 nm band is obtained. The diffraction grating for the wavelength of 780 nm band obtained in this manner allows light in the wavelength of 780 nm band to pass straight through and partly diffracts, and allows light in the wavelengths of 410 nm and 660 nm to pass through without being diffracted.

At this time, it is preferable that the above-mentioned optical multilayer film further finely adjust the total number of layers and the film thickness of each layer from the basic configuration, and the filler has sufficient optical absorption for the wavelength used. Small materials are preferably used, and oxides such as SiO ₂ and SiON, oxynitrides, organic materials such as acrylic resins and epoxy resins, fluorine-containing aromatic polymer materials, and birefringent materials such as liquid crystals are used. be able to.

  In the configuration of this optical head device 700, the three-wavelength laminated optical element 720 includes a laminated optical element 540 in which a polarization diffraction element and a broadband phase plate are laminated, and a diffraction grating 721 for a wavelength of 780 nm band. With this configuration, the optical head device can be further reduced in size. At this time, in the three-wavelength laminated optical element 720, the first and second polarizing diffraction gratings are placed on the light source side from the broadband phase plate layer, and the order of lamination and the direction of the diffraction grating surface are arbitrary. . Further, the order and direction in which the diffraction grating 721 for a wavelength of 780 nm band is laminated in the laminated optical element for three wavelengths is arbitrary.

  The operation of the optical head device 700 according to the second embodiment of the present invention will be described below. The linearly polarized light of the first polarized light having a wavelength of 410 nm and 660 nm emitted from the light source 711 is converted into circularly polarized light through the three-wavelength laminated optical element 720, and is transmitted through the collimator 712 to be converted into a parallel light beam. The light is condensed on the optical recording medium 600 by the lens 713. The return light in the wavelength range of 410 nm and 660 nm reflected by the optical recording medium 600 is transmitted through the objective lens 713 and the collimator 712, and is converted into the second polarized light by the three-wavelength laminated optical element 720 and at a predetermined angle. The light beam is diffracted to generate a focus error signal detection light, which is incident on the light receiving surface on the photodetector 714, and information is read by the photodetector 714.

  The light beam having a wavelength of 780 nm band emitted from the light source 711 passes through the laminated optical element 540 constituting the three-wavelength laminated optical element 720 substantially straightly and goes straight except for a part of the diffraction grating 721 for the wavelength 780 nm band that is diffracted. To Penetrate. The light beam in the wavelength band of 780 nm that has passed through the three-wavelength laminated optical element 720 is transmitted through the collimator 712 to be converted into a parallel light beam, collected on the optical recording medium 600 by the objective lens 713, and reflected by the optical recording medium 600. Return light. The return light in the wavelength band of 780 nm passes through the objective lens 713 and the collimator 712, and is diffracted at a predetermined angle by the action of the diffraction grating 721 for wavelength band of 780 nm that constitutes the three-wavelength laminated optical element 720. The light is incident on the upper light receiving surface, and information is read out by the photodetector 714.

  In the above description, a configuration is described in which a monitor unit including a beam splitter and a photodetector for detecting the amount of light emitted from the light source 711 is not provided. However, such a monitor unit is provided in a predetermined optical path. It is good also as a structure.

  Hereinafter, the present invention will be described more specifically with examples, but the present invention is not limited to the following description. Examples 1 to 3 and Examples 5 to 9 are examples, and Example 4 is a comparative example.

[Example 1]
First, a broadband phase plate composed of one layer of birefringent material made of polymer liquid crystal will be described with reference to the schematic cross-sectional view of FIG.

Polyimide films 102 and 104 were applied to quartz glass substrates 101 and 105 having a refractive index of 1.47 at a wavelength of 405 nm, and the surfaces were rubbed with nonwoven fabric in the directions of arrows 106 and 107 to form a horizontal alignment film. As shown by arrows 106 and 107 in FIG. 1B, the rubbing is performed by setting one side of the substrate as a reference direction, and an angle with the reference direction is 45 degrees. It was done in the same direction. Thereafter, a seal (not shown) in which a spacer was mixed in the periphery was applied, and both the substrates were arranged so that the distance was constant at 12.8 μm to form an empty cell. Next, a photopolymerizable liquid crystal compound prepared by mixing the liquid crystal compounds (1), (2), (3) and (4) shown in [Chemical Formula 1] at a molar ratio of 1: 1: 1: 1 was prepared, and this was used as a seal. After injecting into the empty cell from the provided injection hole and sealing the injection hole, the liquid crystal compound was photopolymerized by irradiating with ultraviolet light to form a polymer liquid crystal layer 103 having a thickness of 12.8 μm.

The formed polymer liquid crystal layer 103 has dispersion coefficients A = 0.05052, B = 9.824, and C = −615.9 that are sufficiently close to the above-described equation (3), which is an ideal solution of the dispersion coefficient. Further, wavelength 405 nm, 660 nm, respectively ordinary refractive index _{n o} of 785 nm 1.566,1.537,1.532, the extraordinary refractive index _{n e} were respectively 1.637,1.601,1.594 . When a linearly polarized light whose polarization direction is parallel to the reference direction is incident on the broadband phase plate of this example and the generated phase difference is obtained, 808 degrees with respect to light having wavelengths of 405 nm, 660 nm, and 785 nm, 447 degrees and 364 degrees. Further, when the ellipticity for each wavelength is obtained with the direction forming an angle of 45 degrees with the reference direction in the phase plate surface as the optical axis of the phase plate, 0.97, 0.95, and 0.03 are good values. there were.

[Example 2]
Next, a broadband phase plate having a configuration in which two stretched and oriented stretched polycarbonate layers are laminated will be described with reference to FIG.

  A quartz glass substrate 201 having a refractive index of 1.47 at a wavelength of 405 nm and a first polycarbonate layer 203 having a thickness of 75 μm as a first birefringent layer with one side of the glass substrate as a reference direction is shown in FIG. As shown by the arrow 205, the angle was set so that the angle between the fast axis direction and the reference direction was 12 degrees. Next, a second polycarbonate layer 204 having a thickness of 66 μm as a second birefringent layer is formed on the first polycarbonate layer 203 as shown by an arrow 206 in FIG. The first polycarbonate layer 203 was bonded so that the angle formed with the reference direction was 65 degrees and the angle formed with the fast axis direction of the first polycarbonate layer 203 was 53 degrees, and the quartz glass substrate 202 was bonded and laminated.

  In the same manner as in Example 1, the phase differences with respect to light having wavelengths of 405 nm, 660 nm, and 785 nm obtained by making linearly polarized light whose polarization direction is parallel to the reference direction incident on the broadband phase plate of this example are 805 degrees and 445 respectively. The ellipticity was a good value of 0.91, 0.91, and 0.04, respectively.

[Example 3]
Next, a broadband phase plate having a structure in which two polymer liquid crystal layers are laminated will be described. In the same manner as in Example 1, a horizontal alignment film made of a polyimide film whose surface was rubbed was formed on a quartz glass substrate and sealed to form two sets of empty cells having a substrate interval of 1.7 μm and 7 μm, respectively. The rubbing is performed in a direction in which the angle formed with one side of the substrate as a reference direction when the substrate is opposed to an empty cell with a substrate interval of 1.7 μm is 8 degrees, and for an empty cell with a substrate interval of 7 μm. It was given so as to be in the direction of the degree. Next, a photopolymerizable liquid crystal compound prepared by mixing the liquid crystal compounds (5), (6), (7) and (8) shown in [Chemical Formula 2] at a mass ratio of 1: 1: 1: 1 was prepared. Similarly, it injected into each cell and photopolymerized, and formed the polymer liquid crystal layer. The polymer liquid crystal layer thus formed has dispersion coefficients of A = 0.1011, B = 0, and C = 9230, and the ordinary refractive index no at wavelengths of 405 nm, 660 nm, and 785 nm is 1.608, .557, 1.551, and extraordinary refractive index ne were 1.765, 1.679, 1.667, respectively.

  Next, one substrate of the liquid crystal cell sandwiching the polymer liquid crystal layer is released and peeled off, and the polymer liquid crystal layer is overlapped so that the reference direction is coincident and the angle formed by the rubbing directions is 139 degrees. A broadband phase plate having a configuration in which two polymer liquid crystal layers of this example were laminated was obtained by bonding using a transparent adhesive in the three wavelength bands.

  In the same manner as in Example 1, when the phase difference of the emitted light is obtained by making linearly polarized light having a polarization direction parallel to the reference direction incident on the broadband phase plate of this example, light having wavelengths of 405 nm, 660 nm, and 785 nm is obtained. On the other hand, they were 990 degrees, 452 degrees, and 356 degrees, respectively, and the ellipticities were 0.99, 0.97, and 0.03, respectively.

[Example 4]
A broadband polycarbonate phase plate was formed by adhering one stretched polycarbonate layer stretched and oriented in the same manner as in Example 2 to a quartz glass substrate so that the angle between the fast axis direction and the reference direction was 45 degrees. The thickness of the polycarbonate layer stretched so that the phase differences generated with respect to light having wavelengths of 405 nm, 660 nm, and 785 nm were 810 degrees, 450 degrees, and 360 degrees was 70 μm. When the phase difference generated by the broadband phase plate of this example was measured in the same manner as in Example 1, it was 869 degrees, 451 degrees, and 367 degrees for light of wavelengths 405 nm, 660 nm, and 785 nm, respectively. The desired phase difference was not obtained.

[Example 5]
An optical head device having the configuration shown in FIG. 1 was constructed by incorporating the broadband phase plate of Example 1. In this optical head device, the linearly polarized light having wavelengths of 405 nm, 660 nm, and 780 nm emitted from the semiconductor lasers 401, 402, and 403 reflects the light having the wavelength of 660nm and transmits the light having the wavelength of 780nm, and the light having the wavelength of 405nm. The light is reflected by a light combining prism 408 that reflects light and transmits light having wavelengths of 660 nm and 780 nm, passes through a collimating lens 410, a polarization diffraction element 411 held by an actuator 414, the broadband phase plate 412 of Example 1, and an objective lens 413. The information recording surface of the optical disk 415 is condensed and irradiated. The return light beam reflected from the optical disk 415 including the information of the pits formed on the information recording surface travels in the opposite direction on each path.

  Here, the light sources of the three wavelengths are aligned so that the polarization direction of the linearly polarized light of the emitted light beam is not diffracted by the polarization diffraction element 411. Therefore, the light beams having wavelengths of 405 nm and 660 nm The light is transmitted straight without being diffracted by the element 411, enters the broadband phase plate 412 of Example 1, and generates phase differences of 9λ / 4 and 5λ / 4, respectively, is converted into circularly polarized light, and is irradiated onto the optical disk. In the return path, the return light beam reflected by the information recording surface and converted into the reverse circularly polarized light is converted into linearly polarized light orthogonal to the light source by the broadband phase plate 412 of Example 1, and is diffracted by the polarization diffraction element 411. The light is guided to the photodetectors 404 and 405 having the respective wavelengths through the collimating lens 410 and the combining prisms 408 and 409, and the recorded information is read out.

  For the wavelength of 780 nm, the linearly polarized laser beam emitted from the semiconductor laser 403 passes through the non-polarized diffraction grating 407 and then passes through the combining prisms 409 and 408, and is held by the collimator lens 410 and the actuator 414. The optical disc 415 is condensed and irradiated through the polarization diffraction element 411, the broadband phase plate 412 of Example 1 and the objective lens 413. Since the polarization state of the light beam having the wavelength of 780 nm cannot be changed by the broadband phase plate 412 of Example 1, the return light beam having the wavelength of 780 nm is transmitted straight through the polarization diffraction element 411 without being diffracted even in the return path, and the collimating lens 410 and the combining prism. First-order diffracted light that passes through 408 and 409 and is diffracted by the non-polarized diffraction grating 407 is guided to the photodetector 406, and information recorded on the optical disk 415 is read out.

  As described above, in the optical head device of this example using the broadband phase plate of Example 1, writing and reading are performed on the optical disc with good recording / reproducing characteristics using light beams with wavelengths of 405 nm, 660 nm, and 780 nm. I was able to do.

[Example 6, Example 7]
An optical head device having the same configuration as in Example 5 was prepared and evaluated except that the broadband phase plates of Examples 2 and 3 were used, respectively. As a result, in the same manner as in Example 5, it was possible to write to and read from the optical disc with good recording / reproducing characteristics using light beams with wavelengths of 405 nm, 660 nm, and 780 nm.

[Example 8]
Polarization diffraction element 541 showing a schematic cross-sectional view of FIG. 4 (a), when the polymerization, the extraordinary refractive index _{n e} with ordinary refractive index _{n o} with respect to the wavelength 405nm of light 1.541 is 1.588, wavelength 660nm of the ordinary refractive index _{n o} is the extraordinary refractive index _{n e} at 1.517 with respect to light of acrylic 1.556 photopolymerizable polymer liquid crystal, and, with respect to the refractive index of the wavelength of 405nm light 1 .541, using a 1.519 filler for light having a wavelength of 660 nm, and forming as follows. First, an alignment film is formed on one surface of the quartz glass substrate 303, and an alignment process is performed by a rubbing method. In this example, the rubbing direction is the longitudinal direction of the diffraction grating.

  Next, an epoxy resin adhesive is used as a sealing material (not shown), and the epoxy resin adhesive is printed on the outer periphery of the optically effective area of the substrate surface on which the alignment film of the quartz glass substrate 303 is formed. Similarly, an alignment film is formed on one substrate surface of a predetermined substrate (not shown) (hereinafter referred to as a replacement substrate), subjected to alignment treatment by a rubbing method, and the alignment treatment surface is opposed to the quartz glass substrate 303. The replacement substrate is faced and thermocompression bonded, a seal is formed, and a cell is manufactured. The rubbing direction of the replacement substrate was made parallel to each other when the substrates were opposed to each other.

Next, the above-described photopolymerizable polymer liquid crystal is injected into this cell by vacuum injection, and UV exposure is performed to solidify the polymer liquid crystal. Next, the replacement substrate is removed, and the polymer liquid crystal is etched using the photolithography technique and the etching technique, and the pseudo-blazed diffraction grating surface having the same step width and the same step difference between the steps is formed in 8 steps. Form. Here, the pseudo-blazed diffraction grating surface has a pitch p of 20 μm, and a step difference of each step is a value obtained by dividing the wavelength 405 nm diffracted by the first diffraction grating by Δn and the number of steps. Δn is the difference 0.047 between the refractive index 1.541 of the filler and the polymer liquid crystal of the extraordinary refractive index _n e 1.588, the number of steps is "8" above.

  Similarly, a diffraction grating surface made of a polymer liquid crystal and having a rectangular cross section is formed on one surface of the quartz glass substrate 302. Here, the diffraction grating having a rectangular cross section has a pitch p of 33 μm, and a level difference of 660 nm that is diffracted by the second diffraction grating is divided by Δn and 2.

  Finally, the quartz glass substrate 303 and the quartz glass substrate 302 are disposed so that the diffraction grating forming surfaces of the respective substrates face each other and the rubbing directions of the respective substrates are parallel to each other, and between the substrates and each diffraction grating. The first diffraction grating 551 having a sawtooth cross section for a wavelength of 410 nm and a second diffraction grating 552 having a rectangular cross section for a wavelength of 660 nm are stacked so as to fill the concave portion of the surface. The polarization diffraction element 541 is formed.

  Finally, the polarizing diffraction element 541 and the broadband phase plate 305 are laminated to obtain the laminated optical element 540 of Example 8.

  An optical head device having the same configuration was produced except that the broadband phase plate and the polarization diffraction element in the optical head device having the configuration of Example 5 were replaced with the laminated optical element of this example, and evaluation similar to that in Example 5 was performed. . As a result, it was possible to write to and read from the optical disk with good recording / reproducing characteristics using light beams having wavelengths of 405 nm, 660 nm, and 780 nm.

[Example 9]
A non-polarized diffraction grating 721 for a wavelength of 780 nm band is further added to the laminated optical element 540 manufactured in the same manner as in Example 8 except that the pitches of the first and second diffraction gratings are 1.0 μm and 1.7 μm, respectively. By laminating, the three-wavelength laminated optical element 720 of Example 9 is obtained. This non-polarization diffraction element 721 forms an optical multilayer film on one surface of one transparent substrate, processes the optical multilayer film by photolithography and dry etching, has a rectangular cross-sectional shape, and extends in one direction. The ridges and the recesses from which the substrate surface is exposed by removing the optical multilayer film form a diffraction grating surface that is formed in parallel and periodically, and another sheet is formed with the grating formation surface inside. A cell is formed facing the transparent substrate and filled with a filler.

A fluorine-containing aromatic polymer is used as the filler. The optical multilayer film uses a SiO ₂ film as a low-refractive index material layer and a Ta ₂ O ₅ film as a high-refractive index material layer, respectively. Each film thickness was determined so that the _two layers of SiO ₂ film and Ta ₂ O ₅ film were alternately stacked, with a total of 32 layers and a total film thickness of 3.85 μm. And formed by a vacuum deposition method. The average refractive index of this optical multilayer film is made substantially equal to the refractive index of the fluorinated aromatic polymer used as the filler, that is, 1.58 in the 410 nm wavelength band and 1.54 in the 660 nm wavelength band. The grating pitch of the grating is 2 μm and the duty is 0.5.

[Example 10]
Using the three-wavelength laminated optical element of Example 9, the optical head device shown in FIG. Good recording / reproducing characteristics can be obtained by writing to and reading from an optical disc using light beams having wavelengths of 405 nm, 660 nm, and 780 nm as light source light.

  According to the configuration of the optical head device of the present invention, it is possible to record / reproduce each optical disc using light having wavelengths of 410 nm, 660 nm and 780 nm with good characteristics. Further, when the broadband phase plate of the present invention is used, an optical head device capable of stably recording and / or reproducing with respect to the optical disk using the light of the three wavelength bands is realized.

1 is a block diagram showing a first configuration example of an optical head device according to a first embodiment of the present invention. Schematic sectional view and plan view of a broadband phase plate comprising one layer of the birefringent material of the present invention Schematic sectional view and plan view of a broadband phase plate comprising two layers of the birefringent material of the present invention Schematic sectional view of the polarization diffraction element 541 of the present invention, and a diagram for explaining the operation Schematic sectional view of an element in which the broadband phase plate of the present invention and a polarization diffraction element are laminated and integrated The figure for demonstrating the notional sectional structure of the laminated optical element 540 of this invention, and the effect | action of the laminated optical element 540 The block diagram which shows the 2nd structural example of the optical head apparatus based on 2nd Embodiment of this invention. Block diagram showing a third configuration example of the optical head apparatus according to the third embodiment of the present invention. Block diagram showing the configuration of a conventional optical head device

Explanation of symbols

11, 12, 13 Semiconductor laser 14, 15, 16 Photo detector 17, 18, 19 Polarization diffraction element 20, 21, 22 1/4 phase plate 23, 24 Combined prism 25 Collimator lens 26 Objective lens 27 Actuator 28 Optical disk 101 , 105 Quartz glass substrate 102, 104 Alignment film (polyimide film)
103 Polymer liquid crystal layer 106, 107 Rubbing direction 201, 202 Quartz glass substrate 203, 204 Polycarbonate layer 205, 206 Fast axis direction of polycarbonate layer 301, 302, 303 Quartz glass substrate 304 Birefringent layer 305 Broadband phase plate 306 410 nm Wavelength Polarization diffraction element for band 307 Polarization diffraction element for wavelength band of 660 nm 308 Filler 401, 402, 403 Semiconductor laser 404, 405, 406 Photodetector 407 Unpolarized diffraction grating 408, 409 Combined prism 410 Collimate lens 411 Polarization diffraction element 412 Broadband phase plate 413 Objective lens 414 Actuator 415 Optical disk 500, 700 Optical head device 511, 521, 528, 711 Light source 512, 524, 712 Collimator 513 Polarized beam sp 515, 526 Condensing lens 516, 527, 714 Photo detector 522 Beam conversion diffraction grating 523 Beam splitter 529, 531 Multiplexing means 532, 713 Objective lens 540 Laminated optical element 541 Polarized light Diffraction element 542 Saw-shaped section of first diffraction grating 543 Projection section of second diffraction grating rectangular section 544 Filler 551 First diffraction grating 552 Second diffraction grating 600 Optical recording medium 720 3 Wavelength laminated optical element 721 Diffraction grating for wavelength 780 nm band

Claims (19)

In an optical head device comprising a light source, a polarization diffraction element, a broadband phase plate, and a photodetector,
The light source is a light source that emits a linearly polarized light beam having a wavelength of 410 nm, 660 nm, and 780 nm,
The polarization diffractive element is a polarization diffractive element that linearly transmits linearly polarized light in a predetermined polarization direction and diffracts linearly polarized light orthogonal to the predetermined polarization direction,
The broadband phase plate has a wavelength of about (2 m _a −1) · λ / 4 for light of a wavelength of 410 nm and a wavelength of about (2 m _b −1) · λ / 4 for light of a wavelength of 660 nm, and a wavelength of 780 nm. A broadband phase plate that generates a phase difference of approximately m _c · λ (λ is a wavelength, m _a , m _b , and m _c are natural numbers) with respect to light,
A linearly polarized light beam having a wavelength of 410 nm and 660 nm that is emitted from the light source and incident on the polarization diffraction element is transmitted straight through the polarization diffraction element, and is converted into substantially circular polarization by the broadband phase plate, thereby being an optical recording medium. Irradiated to
The return light beam reflected by the information recording surface of the optical recording medium and turned into substantially circularly polarized light in the reverse direction is converted into substantially linearly polarized light in the polarization direction orthogonal to the light beam emitted from the light source by the broadband phase plate, and the polarized light Diffracted by the diffraction element and guided to the photodetector,
A linearly polarized light beam having a wavelength of 780 nm that is emitted from the light source and incident on the polarization diffraction element is transmitted straight through the polarization diffraction element, and transmitted through the broadband phase plate without substantially changing the polarization state. Irradiated to an optical recording medium,
The return light beam reflected by the information recording surface of the optical recording medium is transmitted by the broadband phase plate without substantially changing the polarization state, is transmitted straight by the polarization diffraction element, and is guided to the photodetector. An optical head device.   The phase difference generated by the broadband phase plate is approximately 9λ / 4 for light having a wavelength of 410 nm, approximately 5λ / 4 for light having a wavelength of 660 nm, and approximately λ for light having a wavelength of 780 nm. Item 4. The optical head device according to Item 1.   The phase difference generated by the broadband phase plate is approximately 11λ / 4 for light having a wavelength of 410 nm, approximately 5λ / 4 for light having a wavelength of 660 nm, and approximately λ for light having a wavelength of 780 nm. Item 4. The optical head device according to Item 1. The polarization diffraction element is a polarization diffraction element comprising a first diffraction grating and a second diffraction grating laminated,
The first diffraction grating is:
Linearly transmitting a linearly polarized light beam having a wavelength of 410 nm, 660 nm, and 780 nm that is incident from the light source;
A diffraction grating that diffracts linearly polarized light having a wavelength of 410 nm and having a polarization direction orthogonal to the linearly polarized light from the light source, and linearly transmits linearly polarized light having a wavelength of 660 nm,
The second diffraction grating is
Linearly transmitting a linearly polarized light beam having a wavelength of 410 nm, 660 nm, and 780 nm that is incident from the light source;
4. A diffraction grating that linearly transmits linearly polarized light with a wavelength of 410 nm and has a polarization direction orthogonal to the linearly polarized light from the light source and diffracts linearly polarized light with a wavelength of 660 nm. The optical head device according to any one of the above. The first diffraction grating is made of an optically anisotropic material, and a blazed diffraction grating in which ridges extending in one direction having a sawtooth-shaped cross section are formed periodically and parallel to each other, or optical anisotropy A quasi-blazed diffraction grating made of a material and having ridges extending in one direction having a sawtooth cross section approximating a desired sawtooth shape with a step shape of 8 or more steps, formed periodically and parallel to each other,
5. The optical head according to claim 4, wherein the second diffraction grating is a diffraction grating made of an optically anisotropic material and having ridges extending in one direction having a rectangular cross section formed periodically and parallel to each other. apparatus.   6. The optical head device according to claim 4, wherein the polarization diffraction element generates a focus error signal and / or a tracking signal.   The optical head device according to claim 1, wherein the broadband optical plate is a laminated optical element in which the polarization diffraction element is laminated and integrated. And a layer and one or more transparent substrate exhibits birefringence, approximately _{(2m a} -1) to the light flux of wavelength 410nm band · lambda / 4, substantially for light having a wavelength of 660nm band _(2m b -1 ) · Λ / 4, a broadband phase characterized by generating phase differences of approximately _mc · λ (λ is a wavelength, m _a , m _b , and _mc are natural numbers) for light in a wavelength band of 780 nm. Board.   The phase difference generated by the broadband phase plate is approximately 9λ / 4 for light having a wavelength of 410 nm, approximately 5λ / 4 for light having a wavelength of 660 nm, and approximately λ for light having a wavelength of 780 nm. Item 9. The broadband phase plate according to Item 8.   The phase difference generated by the broadband phase plate is approximately 11λ / 4 for light having a wavelength of 410 nm, approximately 5λ / 4 for light having a wavelength of 660 nm, and approximately λ for light having a wavelength of 780 nm. Item 9. The broadband phase plate according to Item 8.   The broadband phase plate according to claim 8, 9 or 10, wherein the birefringent layer has a layer made of one birefringent material.   The broadband phase plate according to claim 8, wherein the birefringent layer has a layer made of two birefringent materials.   The broadband phase plate according to claim 11 or 12, wherein the birefringent material is a polymer liquid crystal.   The broadband phase plate according to claim 12, wherein the birefringent material is stretched polycarbonate. A polarization diffraction element in which a first diffraction grating and a second diffraction grating are stacked and provided,
The first diffraction grating is
Linearly polarized light having a wavelength of 410 nm, 660 nm and 780 nm in a predetermined polarization direction incident on the polarization diffraction element is transmitted in a straight line;
A diffraction grating that diffracts linearly polarized light having a wavelength of 410 nm in a polarization direction orthogonal to the predetermined polarization direction and linearly transmits linearly polarized light having a wavelength of 660 nm,
The second diffraction grating is
Linearly polarized light having a wavelength of 410 nm, 660 nm, and 780 nm in the predetermined polarization direction,
A polarization diffraction element characterized by being a diffraction grating that linearly transmits linearly polarized light having a wavelength of 410 nm in a polarization direction orthogonal to the predetermined polarization direction and diffracts linearly polarized light having a wavelength of 660 nm. The first diffraction grating is made of an optically anisotropic material, and a blazed diffraction grating in which ridges extending in one direction having a sawtooth-shaped cross section are formed periodically and parallel to each other, or optical anisotropy A quasi-blazed diffraction grating made of a material and having ridges extending in one direction having a sawtooth cross section approximating a desired sawtooth shape with a step shape of 8 or more steps, formed periodically and parallel to each other,
The polarization diffraction according to claim 15, wherein the second diffraction grating is a diffraction grating made of an optically anisotropic material and having ridges extending in one direction having a rectangular cross section formed periodically and parallel to each other. element.   For linearly polarized light having a polarization direction orthogonal to the predetermined polarization direction, the product of the diffraction efficiency of the first diffraction grating and the transmittance of the second diffraction grating in the wavelength 410 nm band is the product in the wavelength 660 nm band. The polarization diffraction element according to claim 16, wherein the polarization diffraction element is larger than a product of transmittance of the first diffraction grating and diffraction efficiency of the second diffraction grating.   A laminated optical element in which the broadband phase plate according to claim 8 and the polarization diffraction element according to claim 15 are laminated and integrated.   A laminated optical element for three wavelengths, wherein the laminated optical element according to claim 17 or 18 and a diffraction grating that diffracts linearly polarized light having a wavelength of 780 nm band are laminated.

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