JP2012009096A - Wavelength selection wavelength plate, wavelength selection diffraction element and optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength selection wavelength plate on which three lights of linear polarization of different wavelength ranges are made incident and which emits linear polarization of two wavelength ranges almost perpendicularly to a linear direction of the another wavelength range.SOLUTION: Two wavelength plates with an optical axis twisted in a thickness direction are overlapped, a pretwist angle being an angle formed between the direction of incident linear polarization and optical axes of the respective wavelength plates of a light incident side, and a twist angle being an angle that is further twisted are adjusted to thereby emit lights of desired three wavelength ranges while making a polarization direction of light of one wavelength range almost perpendicular to the polarization direction of the lights of the two other wavelength ranges among the lights of desired three wavelength ranges, and a value that is high and stable in a predetermined wavelength range can be obtained about use efficiency η obtained by an azimuthal error ΔΨ and ellipticity κ being indexes of the orthogonality of the lights.

Description

  The present invention relates to a wavelength selection wave plate that selectively phase-modulates a plurality of lights having different wavelengths, a wavelength selection diffraction element that selectively transmits or diffracts these lights, and an optical system that handles optical storage. Information recording and / or reproduction (hereinafter referred to as “optical disc”) and optical recording media such as magneto-optical discs and high-density optical recording media (hereinafter referred to as “optical discs”) such as “Blu-ray” (registered trademark: BD). The present invention relates to an optical head device or the like that performs recording / reproduction.

  In an optical head device corresponding to a plurality of optical discs such as BD, DVD, CD, etc., semiconductor laser light sources having different wavelengths are used for optical discs of different standards, and light sources are provided on the information recording surface of the optical disc by objective lenses having different numerical apertures NA. The emitted light from the light is condensed, and the reflected light is branched by a beam splitter and received by a photodetector, whereby it is converted into an electric signal to record / reproduce information. Specifically, a semiconductor laser that emits light in a wavelength band of 405 nm corresponding to a wavelength range of 395 nm to 420 nm in BD, a semiconductor laser that emits light in a wavelength band of 660 nm corresponding to a wavelength range of 640 nm to 680 nm in DVD, and In the case of CD, a semiconductor laser that emits light having a wavelength band of 785 nm corresponding to a wavelength range of 765 nm to 805 nm is used.

  Here, when the optical head device is configured using components that are individually arranged in space for each of the BD, DVD, and CD optical discs, the optical head device is increased in size and weight, and the number of components of the optical component is increased. There is a problem of increasing. In particular, when an optical head device capable of recording / reproducing BD, DVD, and CD is mounted on a thin notebook personal computer or the like, further miniaturization and weight reduction are required. Therefore, in order to achieve miniaturization and weight reduction without increasing the number of optical components, the optical paths of light in each wavelength band for BD, DVD, and CD, in particular, three optical paths are used in common. It is effective to provide an optical component that obtains desired optical characteristics for the band light. Specifically, the light emitted from the light source corresponding to these three types of wavelength bands (hereinafter referred to as “three wavelengths”) can be combined and shared, and an optical component can be provided in the optical path. Alternatively, a photodetector that receives the signal light reflected from the information recording surface of the optical disc can be shared with the light of three wavelengths.

  A two-wavelength semiconductor laser in which a semiconductor laser that emits light for DVD and a semiconductor laser that emits light for CD is already integrated in an optical head device that records and reproduces both DVD and CD Is commercialized. Further, a photodetector that receives signal light reflected by a DVD and signal light reflected by a CD in a common single package is used. Further, as a light source, a three-wavelength semiconductor laser in which a BD semiconductor laser is integrated with a two-wavelength semiconductor laser including a DVD and a CD has been developed.

  Further, in the recording / reproducing of the optical head device, a tracking servo signal detection function is required in order to cause the laser light emitted from a light source such as a semiconductor laser to be traced on the track of the information recording surface of the optical disk. In other words, a diffractive element is arranged in the forward optical path from the semiconductor laser to the optical disk, and tracking servo is performed using three beams of light that are branched into straightly transmitted light (0th order diffracted light) that passes straight through the diffractive element and ± 1st order diffracted light. In many cases, a three-beam system is used as a signal. In addition, a diffraction element (also referred to as a “hologram diffraction element”) that is divided into a plurality of regions having different grating patterns with respect to the incident light beam is disposed in the return optical path from the optical disk to the photodetector. A hologram method is also used in which a tracking servo signal is generated by branching into a plurality of diffracted lights.

  In this case, in the three-beam method, light is emitted from the light sources of the semiconductor lasers for BD, DVD, and CD. For example, a diffraction element is arranged in an optical path in which these three wavelengths of light are shared, and information on each optical disc It is desirable to generate ± first-order diffracted light that conforms to the recording surface standard. In this case, as the diffraction element, the diffraction grating pattern such as the grating pitch and the longitudinal direction of the grating is adjusted, for example, ± first-order diffracted light having a desired diffraction efficiency is generated only for light of a specific wavelength, It is conceivable to use a wavelength-selective diffractive element that does not generate ± first-order diffracted light with respect to light having a wavelength of.

  Similarly, in the hologram method, it is conceivable to arrange a hologram diffraction element in an optical path in which three wavelengths of light are shared. For example, a hologram diffraction element has a plurality of regions having different diffraction directions and diffraction angles with respect to an incident light beam, and diffracted light of a plurality of wavelengths is common to a light receiving surface included in a single package photodetector The diffraction grating pattern is adjusted so that it can be incident on the light source. For example, it can be considered that a desired diffracted light is generated only for light of a specific wavelength and used as a wavelength-selective diffractive element that does not generate diffracted light for light of other wavelengths. Thereby, the effect of reducing the number of light receiving surfaces of the photodetector or reducing the area of the light receiving surface can be expected.

  The optical head device is designed so that the three-beam method and the hologram method are properly used according to the type of the optical disk to be recorded / reproduced, or the tracking servo signal is detected by the three-beam method. The focus servo signal may be detected by a hologram method. For example, a 3-beam method may be used when recording / reproducing DVDs and CDs, and a hologram method may be used when recording / reproducing BDs. In any case, the diffractive element arranged in the optical path common to the light of these three wavelengths should be diffracted originally by making it a wavelength selective diffractive element that only diffracts light of a specific wavelength. Since light loss and stray light generation are reduced with respect to light with no wavelength, stable recording / reproduction can be realized. Further, in the return path, the number of light receiving surfaces and the area of the light receiving surfaces of the photodetector can be reduced by arranging them as hologram diffraction elements in an optical path in which these three wavelengths of light are common, so that downsizing can be expected.

  As described above, a diffractive optical element in which a plurality of wave plates and diffraction gratings are alternately stacked has been reported as a wavelength selective diffractive element on which light of three wavelengths is incident (Patent Document 1). The wave plate included in the diffractive optical element, for example, functions as a half-wave plate for light of one specific wavelength band among the light of the three wavelengths incident as TM polarized light. It has a phase difference that functions as a full wave plate with respect to light in other wavelength bands. Then, only the light of one specific wavelength band rotates the polarization direction by 90 ° to make TE polarized light, diffracts only TE polarized light, and does not diffract TM polarized light. It has a lattice.

JP 2005-353225 A

  The wave plate used in the diffractive optical element of Patent Document 1 describes an example in which three wavelengths are a combination of 400 nm, 650 nm, and 780 nm, and a wavelength of an object whose polarization direction is rotated by 90 ° is 650 nm. At this time, the wave plate has a phase difference of 2π × 4 for 400 nm light, π × 4.92 for 650 nm light, and 2π × 2.05 phase difference for 780 nm light. It has been adjusted to generate. However, such a wave plate is realized by adjusting the thickness of a material having birefringence in order to generate a specific phase difference. In this case, the wavelength of incident light has a specific value. On the other hand, if it fluctuates, there is a problem that the phase difference fluctuates greatly. That is, the light that should originally be emitted from the wave plate as TE-polarized light also includes a TM-polarized light component, and the light that should be emitted as TM-polarized light also includes a TE-polarized light component, and There is a problem that these polarization components are not stable with respect to fluctuations in wavelength. As a result, for example, the amount of TE-polarized light incident on the diffraction grating is not stable, so that the diffraction efficiency of the diffractive optical element is not stable.

  Further, the diffractive optical element of Patent Document 1 includes two or more combinations of one wavelength plate and one diffraction grating when diffracting light of any two wavelengths among light of three wavelengths, for example. There must be. Further, in this case, since the light of three wavelengths is transmitted through the two wave plates, the fluctuation with respect to the desired phase difference is greater due to fluctuations in the incident wavelength, and it is difficult to obtain stable characteristics. There was also a problem of becoming.

  The present invention has been made in view of the above. In particular, when light of three wavelengths having different wavelengths is incident, the polarization direction of the light of one wavelength and the polarization directions of the other two lights are orthogonal to each other. Providing a wavelength selective wave plate and combining a polarization diffractive element that controls transmission or diffraction by a polarization component, does not diffract light of at least one wavelength, and is desired for light of at least one wavelength It is another object of the present invention to provide a wavelength selective diffraction element that can obtain diffraction efficiency, has a small wavelength dependency of diffraction efficiency, and has high light utilization efficiency, and a display device such as an optical head device and a laser projector using the same.

The present invention relates to linearly polarized light having at least one wavelength among linearly polarized light incident at _three wavelengths λ ₁ , wavelength λ ₂ , and wavelength λ ₃ (λ ₁ <λ ₂ <λ ₃ ) having predetermined different bands. In the wavelength selection wavelength plate that changes a polarization state, the wavelength selection wavelength plate includes a first wavelength plate and a second wavelength plate in order from a light incident side, and the first wavelength plate and the second wavelength plate are provided. The wave plate is formed by twisting the major axis direction of the liquid crystal molecules with respect to the thickness direction, and the light having the wavelength λ ₁ , the light having the wavelength λ _{2 and} the light having the wavelength λ ₃ incident on the first wave plate. An angle formed by a major axis direction of liquid crystal molecules on the side of linearly polarized light that is incident on the first wave plate with reference to the direction of the linearly polarized light that is incident in the same direction; Direction of the linearly polarized light incident on the wave plate and the first wave plate of the first wave plate A combination with an angle formed by the major axis direction of the liquid crystal molecules on the long plate side, or an angle formed by a minor axis direction of the liquid crystal molecules on the linearly polarized light side incident on the first wavelength plate, the first wavelength. A combination of the direction of the linearly polarized light incident on the plate and the angle formed by the minor axis direction of the liquid crystal molecules on the first wave plate side of the second wave plate is a pretwist angle α ₁ [°]. , Α ₂ [°], and the twist angle twisted in the thickness direction of the liquid crystal molecules of the first wave plate is β ₁ [°], and twist twisted in the thickness direction of the liquid crystal molecules of the second wave plate When the angle is β ₂ [°], the ellipticity of the light of the wavelength λ ₁ , the light of the wavelength λ _{2 and} the light of the wavelength λ ₃ emitted from the wavelength selection wavelength plate is 0.5 or less. And when further giving a first direction and a second direction orthogonal to the first direction, The angle formed by the polarization direction of the component with the largest vibration of light of any one wavelength with respect to one direction is in the range of −26 to 26 [°], and all the components of the light emitted at this wavelength Among them, the ratio of the light component in the first direction is 80% or more, and the angle formed by the polarization direction of the component having the largest vibration of the light of the other two wavelengths with respect to the second direction is −26. In the range of ˜26 [°], the α ₁ , the ratio of the light components in the second direction out of the total components of the light emitted at these wavelengths is 80% or more. the alpha _2, wherein the beta _1, wherein the beta _2, providing the first wavelength selective wave plate retardation value Rd ₂ retardation values Rd ₁ and the second wave plate wave plate is set in.

Further, the wavelength λ ₁ is a 405 nm wavelength band that ranges from 395 to 420 nm, the wavelength λ ₂ is a 660 nm wavelength band that ranges from 640 to 680 nm, and the wavelength λ ₃ is a range from 765 to 805 nm. Provided is the above-described wavelength selective wave plate having a wavelength band of 785 nm.

The wavelength λ ₁ is a 450 nm wavelength band that is in a range of 420 to 480 nm, the wavelength λ ₂ is a 533 nm wavelength band that is in a range of 500 to 560 nm, and the wavelength λ ₃ is in a range of 610 to 670 nm. Provided is the above-described wavelength selective wave plate having a wavelength band of 645 nm.

Also, ordinary light on a transparent substrate refractive index n _o, is uneven consist birefringent material layer having birefringence to be extraordinary refractive index n _e is formed, the in the recess of the birefringent material layer n _o or providing said n _e equal optical material polarization diffraction element formed, at least one, wavelength selective diffraction element provided within the wavelength selective wave plate described above.

  Moreover, the cross section of the unevenness | corrugation of the said birefringent material layer provides said wavelength selective diffraction element which is a Fresnel lens shape.

  Furthermore, at least one light source that emits light of three different wavelengths, an objective lens that condenses the light emitted from the light source on the optical recording medium, and light that detects light reflected from the optical recording medium Transmits light from the light source toward the optical recording medium and reflects light from the optical recording medium toward the optical detector or reflects light from the light source toward the optical recording medium. And a polarization beam splitter that transmits light from the optical recording medium to the photodetector, and an optical head device comprising the optical head device in the optical path between the light source and the optical detector, and the optical detector. An optical head device in which the above-described wavelength selective diffraction element is disposed in the optical path between the two is provided.

According to the present invention, with respect to linearly polarized light that is incident at the wavelength λ ₁ , the wavelength λ _2, and the wavelength λ ₃ , which are at least three different wavelength bands, one of these three wavelengths and the other two It is possible to provide a wavelength selection wavelength plate that emits light in polarization states in which light in one wavelength band is orthogonal to each other. In addition, it is possible to provide a wavelength selective diffraction element in which polarization diffraction elements having different diffraction efficiencies depending on the polarization direction of light emitted from the wavelength selective wavelength plate are disposed together with the wavelength selective wavelength plate. Furthermore, stable optical characteristics can be obtained by applying to a display device such as an optical head device or a laser projector using the wavelength selective wave plate and the wavelength selective diffraction element.

The schematic diagram which shows the example of the optical function of a wavelength selection wavelength plate. The cross-sectional schematic diagram of a wavelength selection wavelength plate and a wavelength selection diffraction element, and the plane schematic diagram which shows the relationship between the polarization direction of incident linearly polarized light, and an optical axis. The graph which shows distribution of utilization efficiency (eta) by the azimuth angle error (DELTA) Ψ and ellipticity (kappa) of the light radiate | emitted from a wavelength selection wavelength plate. 6 is a graph showing the wavelength (Λ) dependence of the polarization transmittance of the wavelength selection wavelength plate according to design example 1; The graph which shows the wavelength ((LAMBDA)) dependence of the polarization transmittance of the wavelength selection wavelength plate which concerns on the design example 2. FIG. 10 is a graph showing the wavelength (Λ) dependence of the polarization transmittance of the wavelength selection wavelength plate according to design example 3. 10 is a graph showing the wavelength (Λ) dependence of the polarization transmittance of the wavelength selection wavelength plate according to design example 4. The graph which shows the wavelength ((LAMBDA)) dependence of the polarization transmittance of the wavelength selection wavelength plate which concerns on the design example 5. FIG. The graph which shows the wavelength ((LAMBDA)) dependence of the polarization transmittance of the wavelength selection wavelength plate which concerns on the design example 6. FIG. The graph which shows the change of the wavelength ((LAMBDA)) dependence of polarization | polarized-light transmittance when the parameter regarding the pre twist angle of the wavelength selection wavelength plate which concerns on the design example 1 is changed. The graph which shows the change of the wavelength ((LAMBDA)) dependence of polarization | polarized-light transmittance at the time of changing the parameter regarding the pre twist angle of the wavelength selection wavelength plate which concerns on the design example 2. FIG. 14 is a graph showing a change in wavelength (Λ) dependence of polarization transmittance when a parameter related to a pre-twist angle of a wavelength selection wavelength plate according to design example 3 is changed. The graph which shows the change of the wavelength ((LAMBDA)) dependence of polarization | polarized-light transmittance at the time of changing the parameter regarding the pre twist angle of the wavelength selection wavelength plate which concerns on the design example 4. FIG. 1 is a schematic diagram of a first embodiment relating to an optical head device. FIG. An explanatory view of a lens action of a wavelength selection diffraction element used for a 1st embodiment concerning an optical head device, and a section schematic diagram of a wavelength selection diffraction element. Schematic diagram of a second embodiment according to the optical head device. Sectional schematic drawing of the wavelength selective diffraction element used in the second embodiment of the optical head device. FIG. 10 is a schematic diagram of a third embodiment according to the optical head device. FIG. 6 is a schematic cross-sectional view of a wavelength selective diffraction element used in a third embodiment according to the optical head device. The schematic diagram of embodiment which concerns on the optical system for display apparatuses. The cross-sectional schematic diagram of the wavelength selection diffraction element based on an Example, and the plane schematic diagram which shows the relationship between the polarization direction of the linearly polarized light which injects, and an optical axis. 6 is a graph showing the wavelength dependence of the light use efficiency in the wavelength selective wavelength plate according to the wavelength selective diffraction element of Example 1. FIG. 10 is a graph showing the wavelength dependence of the light use efficiency in the wavelength selective wavelength plate according to the wavelength selective diffraction element of Example 2. 10 is a graph showing the wavelength dependence of the light use efficiency in the wavelength selective wavelength plate according to the wavelength selective diffraction element of Example 3. 10 is a graph showing the wavelength dependence of the light use efficiency in the wavelength selective wavelength plate according to the wavelength selective diffraction element of Example 4. 10 is a graph showing the wavelength dependence of the light use efficiency in the wavelength selective wavelength plate according to the wavelength selective diffraction element of Example 5. 10 is a graph showing the wavelength dependence of the light use efficiency in the wavelength selective wavelength plate according to the wavelength selective diffraction element of Example 6.

FIG. 1 shows the output of light with respect to light having a wavelength λ ₁ , light having a wavelength λ ₂ , and light having a wavelength λ ₃ (λ ₁ <λ ₂ <λ ₃ ) incident on the wavelength selection wavelength plate according to the present invention. It is a schematic diagram which shows a polarization state. Here, the light of each wavelength is linearly polarized light having the same polarization direction, that is, in FIG. 1, the linearly polarized light in the X direction travels in the Z direction. In FIG. 1A, the wavelength λ ₁ does not change the polarization direction, and the wavelength λ ₂ and the wavelength λ ₃ are emitted as linearly polarized light orthogonal to the wavelength λ _1. A selective wave plate 1 is shown. Further, the combination of the wavelength of the light that does not change the polarization state and the wavelength of the light that changes the polarization state to the orthogonal linear polarization is not limited to this, for example, as in the wavelength selection wavelength plate 2 in FIG. a light optical and wavelength lambda ₂ wavelength lambda _1, the light of wavelength lambda _3, or a combination of, as a wavelength selective wave plate 3 in FIG. 1 (c), and the wavelength lambda ₂ light, the wavelength It may be a combination of light of λ ₁ and light of wavelength λ ₃ .

  FIG. 1 illustrates an example in which the polarization direction of incident linearly polarized light and the direction of polarization of emitted linearly polarized light match at least one of the three wavelengths of light. However, it is not limited to this. As will be described later, one of the polarization directions viewed from the XY plane does not necessarily coincide with the X direction and the Y direction among the light emitted from the wavelength selection wavelength plate. Of the three lights with different wavelengths emitted in the -Y plane, the light of two wavelengths and the light of the other wavelength may be in a relationship orthogonal to each other.

  In this way, the wavelength selection wavelength plate having a function of emitting light of two wavelengths out of the incident light of three wavelengths and the light of the other wavelength orthogonal to each other depends on the polarization direction as described later. It can be used in combination with a polarization diffraction element having a different diffraction efficiency, a combination with a polarization beam splitter, or an optical system having a different function for each wavelength of incident light. In the wavelength selective wave plates 1, 2 and 3 shown in FIG. 1, the optical axis direction of the birefringent material corresponding to the slow axis or the fast axis is parallel to the plane of the wavelength selective wave plate and the thickness direction is It is composed of a plurality of wave plates made of a birefringent material layer twisted around an axis. And, by setting the retardation value of each wave plate, the twist angle that is the angle at which the optical axis is twisted in the thickness direction, etc. within a predetermined range, the desired characteristics can be obtained, and the wavelength of the incident light It is possible to obtain a stable optical characteristic that does not greatly change the optical characteristic with respect to the fluctuation, that is, has a low wavelength dependency.

  Hereinafter, a specific configuration of the wavelength selection wavelength plate will be described. In particular, in the following, a wavelength-selective diffractive element including a wavelength-selective wave plate and a polarization diffractive element having a characteristic in which diffraction efficiency varies depending on the polarization direction, as a composite optical element that uses the function of the wavelength-selective wave plate. explain.

(Embodiments of wavelength selective wave plate and wavelength selective diffraction element)
FIG. 2A is a schematic cross-sectional view showing the basic configuration of the wavelength selective diffraction element according to the present embodiment. The wavelength selective diffraction element 20 has a configuration including a wavelength selective wave plate 23 and a polarization diffraction element 12. The wavelength selection wave plate 23 includes a first wave plate 21 and a second wave plate 22, and light having a wavelength λ ₁ , light having a wavelength λ ₂ , and light having a wavelength λ ₃ (λ ₁ <λ ₂ < λ ₃ ) is incident and has a function of generating a desired phase difference for each wavelength of incident light as described later. The polarization diffraction element 12 is formed by forming a diffraction grating 14 on a transparent substrate 13 with a birefringent material layer 14 a having a periodic pitch, and forming a transparent material 15 in a concave portion of the diffraction grating 14. The convex portion of the diffraction grating 14 corresponds to the birefringent material layer 14a, and the concave portion indicates a portion between the birefringent material layers 14a. The transparent material 15 may be formed at least in the concave portion, but may be formed in a portion covering the convex portion as shown in FIG. 2A as long as it is an isotropic material.

  The wavelength selection wavelength plate 23 has a structure in which the optical axis of the birefringent material corresponding to the slow axis or the fast axis is parallel to the plane of the wavelength selection wavelength plate 23 and twisted about the thickness direction. Two or more plates are stacked. Specifically, as will be described later, parameters such as the thickness of each wave plate, the angle between the polarization direction of incident light and the optical axis (pre-twist angle), and the twisting angle of the optical axis are set and desired parameters are set. Optical characteristics can be obtained. In addition, the 1st wavelength plate 21 and the 2nd wavelength plate 22 which comprise the wavelength selection wavelength plate 23 are comprised from a liquid crystal or a polymer liquid crystal.

Since the wavelength selection wave plate 23 is configured by twisting the optical axis in the thickness direction as described above, an angle at which the optical axis is twisted in the thickness direction (hereinafter, referred to as “twist angle”) is obtained. Therefore, it can be an effective design parameter. Therefore, by adjusting the twist angle, the wavelength selection wavelength plate 23 for light having different wavelengths, for example, light of wavelength λ ₁ , light of wavelength λ ₂ and light of wavelength λ ₃ (λ ₁ <λ ₂ <λ ₃ ). It is possible to adjust the polarization state of the light emitted from the light source and reduce the wavelength dependency. This configuration, for example, there is no significant change in the polarization state of the light even in the light of the wavelength near the wavelength lambda ₁ emitted, be emitted in a stable polarization state to variations in the wavelength of the incident light it can. Note that the twist angle may be adjusted in the range of greater than 0 ° to about 360 °.

FIGS. 2B and 2C are schematic plan views showing the first wave plate 21 and the second wave plate 22, respectively, and are configured by twisting the optical axis in the thickness direction. Indicates. FIG. 2B shows the optical direction of the light incident surface of the first wavelength plate 21 with the polarization direction 24 of the linearly polarized light incident on the first wavelength plate 21 as the X-axis direction. The angle formed by the shaft 25 is a pre-twist angle α ₁ [°], and the angle formed by the optical shaft 25 and the optical axis 26 of the light emitting surface is a twist angle β ₁ [°]. The concept of the sign of the twist angle beta ₁ is positive in the counterclockwise direction relative to the side of the optical axis 25 of light is incident (+), minus clockwise - the (). Further, the optical axes forming a twist angle beta ₁ is is either slow axis or fast axis, particularly if there is no description is assumed to be similarly slow axis.

In addition, the wavelength selection wavelength plate 23 has a range that α ₁ and α ₂ can take as −180 [°] <α ₁ ≦ + 180 [°] and −180 [°] <α ₂ ≦ + 180 [°], respectively. At this time, even if the values of α ₁ and α ₂ are values (angles) added or subtracted by 180 ° from the original set value, there is no change in the optical characteristics due to the original set value. . That is, 0 [°] <α ₁ ≦ + 360 [°], 0 [°] <α ₂ ≦ + 360 [°] may be used.

Further, even when the signs of α ₁ , α ₂ , β ₁ and β ₂ are changed with respect to the original set values, there is no change in the optical characteristics due to the original set values. Further, if the values of α ₁ and α ₂ are respectively values (angles) obtained by adding or subtracting 90 [°] to the original set values, the optical axes have been described as slow axes. Since this relationship corresponds to expressing the relationship as fast axes, there is no change in the optical characteristics due to the original set value. That is, 90 [°] <α ₁ ≦ + 270 [°] and 90 [°] <α ₂ ≦ + 270 [°] may be used.

FIG. 2C shows the optical direction of the surface of the second wavelength plate 22 on which light is incident with reference to the polarization direction 24 of linearly polarized light incident on the first wavelength plate 21 as the X-axis direction. The angle formed by the shaft 27 is a pre-twist angle α ₂ [°], and the angle formed by the optical shaft 27 and the optical axis 28 of the light emitting surface is a twist angle β ₂ [°]. The concept of the sign of the twist angle beta ₂ is the same as the twist angle beta _1.

In this way, when the first wave plate 21 and the second wave plate 22 are, for example, wave plates having a twist type liquid crystal in which the major axis direction of the liquid crystal molecules is twisted in the thickness direction, the twist angle β ₁ and β ₂ can also be given as design parameters. Therefore, the wavelength selection wavelength plate according to the present embodiment can obtain optical characteristics with less wavelength dependency and can reduce the overall thickness of the wavelength selection wavelength plate. A decrease in rate can also be suppressed.

Next, optical characteristics obtained by the wavelength selection wave plate 23 will be described. The wavelength selection wave plate 23 is configured so that light of the remaining one wavelength is substantially orthogonal to light of any two wavelengths among the incident light of wavelength λ ₁ , light of wavelength λ ₂ , and light of wavelength λ _3. To be set. Then, when the first linear polarization direction and the second linear polarization direction orthogonal to each other in a plane orthogonal to the optical axis of the light emitted from the wavelength selection wavelength plate 23 are given, for example, two specific wavelengths Ideally, the light is only the light component in the first linear polarization direction, and the remaining light of one wavelength is only the light component in the second linear polarization direction. That is, the angle of deviation from the desired azimuth angle Ψ of the light transmitted through the wavelength selection wavelength plate 23, that is, the azimuth angle error ΔΨ is 0 [°], the polarization state is linearly polarized light, and the ellipticity Ideally, κ is zero.

  In such ideal transmitted light, when light of a component with a specific polarization direction is used, there is no light of a component with a polarization direction orthogonal thereto, so there is no component that loses the amount of light. The utilization efficiency of light used for transmission or diffraction is 100%. That is, the utilization efficiency η can be defined with the component in the desired linear polarization direction as 100% of all the light transmitted through the wavelength selection wave plate.

  In the wavelength selection wavelength plate 23 of the present invention, it is preferable that the light use efficiency η is 80% or more because the light component lost as an optical system is small, and the use efficiency η is more preferably 90% or more. And more preferably 95% or more. FIG. 3 shows the distribution of the utilization efficiency η at this time, with respect to the light transmitted through the wavelength selection wavelength plate 23, given the azimuth error ΔΨ [°] on the horizontal axis and the ellipticity κ on the vertical axis. In the case where the light is transmitted as elliptically polarized light, the major axis direction of the ellipse is used as the azimuth angle of the transmitted light. In FIG. 3, the line which connected the conditions used as the combination of utilization efficiency (eta) = 80%, 90%, and 95% is shown. Thus, the region where the utilization efficiency η is 80% or more is a region (including ΔΨ = 0) inside the line indicated as η = 80%. A region where the utilization efficiency η is 90% or more and 95% or more is also indicated by the same concept. Therefore, in order to obtain the desired utilization efficiency η, ΔΨ and κ of the light transmitted through the wavelength selection wavelength plate 23 can be set based on the combination shown in FIG.

  From FIG. 3, for example, a first direction and a second direction that are orthogonal to each other in a plane orthogonal to the optical axis of the light emitted from the wavelength selection wavelength plate 23 are provided, and the wavelength selection wavelength among the three wavelengths of incident light When one light emitted from the plate 23 is based on the first direction, if ΔΨ is in the range of −26 to 26 [°], the utilization efficiency η has a solution of 80% or more. Even if the ellipticity κ is a large value, the utilization efficiency η is 80% or more if the absolute value of ΔΨ is a small value. On the other hand, when the other two wavelengths of light are based on the second direction, if ΔΨ is in the range of −26 to 26 [°], the utilization efficiency η has a solution of 80% or more. As described above, one of the three wavelengths of light has a utilization efficiency η of 80% or more with respect to the first direction, and the other two have a utilization efficiency η of 80% or more with respect to the second direction. The conditions of each wave plate constituting the wavelength selection wave plate 23 having the following characteristics are set.

Next, consider generalizing the relationship between the wavelength of incident light, the thickness of the first wave plate 21, the thickness of the second wave plate 22, and the intrinsic wavelength dispersion characteristics of the birefringent material. . The materials of the first wave plate 21 and the second wave plate 22 will be described as the same material, but generally the product of the thickness d and the refractive index anisotropy Δn, including cases where the materials are different. Can be used as a parameter. That is, assuming that the retardation value of the first wave plate 21 is Rd ₁ and the retardation value of the second wave plate 22 is Rd ₂ , the Rd ₁ and Rd ₂ are adjusted according to the design. Note that Rd ₁ and Rd ₂ have wavelength dependency because the refractive index anisotropy Δn has wavelength dependency, and both Rd ₁ (λ) and Rd ₂ (λ) are functions as a function of wavelength λ. It can also be expressed.

Here, an arbitrary wavelength lambda, the specific wavelength is lambda _A, ordinary refractive index n _o of the wave plate for light having a wavelength lambda _(lambda), the extraordinary refractive index and n _{e (λ),} the wavelength lambda _A of the ordinary refractive index of the wave plate for light _{n o (λ a),} when the extraordinary refractive index and _{n e (λ a),} the refractive index anisotropy [Delta] n of each wavelength _{(λ), Δn (λ a} ) Is
_{Δn (λ) = | n e} (λ) -n o (λ) | ··· (1a)
Δn (λ _A ) = | n _e (λ _A ) −n _o (λ _A ) | (1b)
It becomes.

Further, using the refractive index anisotropy Δn (λ) and Δn (λ _A ) of each wavelength, a parameter Λ representing the wavelength dependence of light incident on the wavelength selection wavelength plate 23 is expressed as follows:
Λ = {Δn (λ _A ) · λ} / {Δn (λ) · λ _A } (2)
Give in.

Next, the first wave plate 21 having a thickness of d _1, when light of a specific wavelength lambda _A incident, the thickness relative to the wavelength lambda _A d ₁ and _{(lambda A),} the thickness d ₂ When light having a specific wavelength λ _A is incident on the second wave plate 22, the thickness based on the wavelength λ _A is d ₂ (λ _A ).
d ₁ (λ _A ) = Δn (λ _A ) · d ₁ / λ _A (3a)
d ₂ (λ _A ) = Δn (λ _A ) · d ₂ / λ _A (3b)
It becomes. Here, it is assumed that both the first wave plate 21 and the second wave plate 22 have the same material, that is, the same refractive index anisotropy.

The above formula (3a), using equation (3b), a first wave plate having a thickness of d _1, when light of a specific wavelength lambda _A incident, the thickness relative to the wavelength lambda _A d ₁ and _{(lambda a),} the second wavelength plate having a thickness of d _2, when the light of a specific wavelength lambda _a incident, the thickness relative to the wavelength lambda _a and d _{2 (lambda a),} further Table 1 shows design examples 1 to 6 when pre-twist angles α ₁ and α ₂ and twist angles β ₁ and β ₂ are given.

In this design, the first wave plate 21 and the second wave plate 22 are respectively provided with thicknesses d ₁ and d ₂ , pre-twist angles α ₁ and α ₂ , twist angles β ₁ and β ₂ (β ₁ ≠ 0, By giving (β ₂ ≠ 0), the controllability of the polarization transmittance Tp with respect to the parameter Λ representing the wavelength dependence of the light incident on the wavelength selection wave plate defined by the equation (2) is improved. Here, the polarization transmittance Tp represents the light intensity of linearly polarized light in the 0 ° direction incident on the wavelength selection wavelength plate 23 as 100%. That is, the wavelength selection wavelength plate 23 improves the controllability of the band where the polarization transmittance Tp is near 100% and the band where the polarization transmittance Tp is near 0%, and is selectively a half-wave plate for light in one wavelength band. And has a function of selectively transmitting light in the other wavelength band without changing the polarization state (λ plate).

  4 to 9 show the characteristics of polarization transmittance Tp [%] with respect to Λ under the conditions of design examples 1 to 6, respectively. Note that Λ on the horizontal axis is a parameter representing wavelength dependence based on the equation (2).

  FIG. 4 showing the characteristics based on design example 1 can cope with a case where the combination of wavelengths of a plurality of incident light is, for example, a wavelength band near Λ = 2.0 and a wavelength band near Λ = 1.0. , Has become a preferred design.

As described above, in the design example 1, the pretwist angles α ₁ and α ₂ are 78 ° and −3 °, respectively, and the wavelength selection wavelength plate 23 is emitted for the light in the respective wavelength bands as shown in FIG. An example in which the lights are orthogonal to each other is shown. Further, when the light emitted from the wavelength selection wavelength plate 23 is orthogonal to each other in any combination of wavelength bands, the pre-twist angles α ₁ ′ and α ₂ ′ are set based on α ₁ and α ₂ of the design example 1,
α ₁ ′ = α ₁ + k ₁ · θ (4a)
α ₂ ′ = α ₂ + k ₂ · θ (4b)
And the twist angles β ₁ ′ and β ₂ ′ with reference to β ₁ and β ₂ in Design Example 1,
β ₁ ′ = β ₁ + k ₃ · θ (4c)
β ₂ ′ = β ₂ + k ₄ · θ (4d)
And thicknesses d ₁ ′ and d ₂ ′ with reference to d ₁ and d ₂ of design example 1,
d ₁ ′ = d ₁ · λ + k ₅ · θ (4e)
d ₂ ′ = d ₂ · λ + k ₆ · θ (4f)
(K _{1 to} k ₆ are coefficients, θ is an angle [°]), and may be adjusted based on this.

FIG. 10 shows k ₁ = −0.78, k ₂ = 0.14, k ₃ = −0.32, k ₄ = −0.28, k ₅ = 0.008, and k ₆ = 0.002. This figure shows the polarization transmittance Tp [%] with respect to Λ when the angle is fixed and θ = −4, 0, 4, 8, 12 [°]. By designing in this way, the interval between the wavelength band close to Tp = 100% and the wavelength band close to Tp = 0% can be controlled. Further, from FIG. 10, for example, in the wavelength band near Λ = 2.0, if the value of θ is decreased, Tp in the wavelength band increases. Therefore, the value of θ may be set according to the target specification. . Further, the possible range of θ is about −4 to 12 [°].

  Next, referring to FIG. 5 showing the characteristics based on design example 2, the combination of wavelengths of a plurality of incident light is, for example, a wavelength band near Λ = 1.5 and a wavelength band near Λ = 1.0. It is a preferable design that can cope with. The design example 2 is a more preferable design when the wavelength bands desired to function selectively are closer to each other and the design example 1 cannot cope with them.

Thus, in the design example 2, when the pretwist angles α ₁ and α ₂ are −17 ° and −14 °, respectively, the wavelength selection wavelength plate 23 is emitted for the light in the respective wavelength bands as shown in FIG. The example in which the light beams are orthogonal to each other has been shown. However, when the light beams emitted from the wavelength selection wavelength plate 23 are orthogonal to each other in any combination of wavelength bands, the pre-twist is based on α ₁ and α ₂ in the design example 2. The angles α ₁ ′ and α ₂ ′ are the above formulas (4a) and (4b), and the twist angles β ₁ ′ and β ₂ ′ are set as the above formulas (4c) using β ₁ and β ₂ of the design example 3 as a reference. ) And the equation (4d).

FIG. 11 shows that k ₁ = 0.48, k ₂ = 0.57, k ₃ = −0.74, and k ₄ = −0.80 in the above equations (4a) to (4d). , Θ = −2, 2, 6, 10, 14 [°], the polarization transmittance Tp [%] with respect to Λ. By designing in this way, the interval between the wavelength band close to Tp = 100% and the wavelength band close to Tp = 0% can be controlled. Further, from FIG. 11, for example, when the value of θ is reduced in the wavelength band near Λ = 1.5, the polarization transmittance Tp in the wavelength band increases, so the value of θ is set according to the target specification. It is good to set. Further, the range that θ can take is about −2 to 14 [°]. Note that k ₅ = k ₆ = 0.

  Next, referring to FIG. 6 showing the characteristics based on design example 3, the combination of wavelengths of a plurality of incident light is, for example, a wavelength band near Λ = 1.0 and a wavelength band near Λ = 0.7. It is a preferable design that can cope with. The design example 3 is a more preferable design when the wavelength bands to be selectively functioned are closer to each other and the design example 2 cannot cope with them.

As described above, in the design example 3, when the pretwist angles α ₁ and α ₂ are 70 ° and −27 °, respectively, as shown in FIG. In the combination of arbitrary wavelength bands, when the light emitted from the wavelength selection wavelength plate 23 is orthogonal to each other, the pre-twist angle is based on α ₁ and α ₂ of the design example 3. α ₁ ′ and α ₂ ′ are the above formulas (4a) and (4b), and the twist angles β ₁ ′ and β ₂ ′ are set as the above formulas (4c) with β ₁ and β ₂ of design example 3 as a reference. The thickness d ₁ ′ may be adjusted as the above formula (4e) based on the formula (4d) and d _{1 of the} design example 3 as a reference.

FIG. 12 shows the above equations (4a) to (4e), where k ₁ = −0.32, k ₂ = 0.2, k ₃ = −0.98, k ₄ = −1.98, k ₅ = −0.014, respectively, and the polarization transmittance Tp [%] with respect to Λ when θ = −2, 2, 6, 10 [°] is changed. By designing in this way, the interval between the wavelength band close to Tp = 100% and the wavelength band close to Tp = 0% can be controlled. From FIG. 12, for example, in the wavelength band near Λ = 1.25, if the value of θ is increased, Tp in the wavelength band is decreased, and accordingly, it is necessary to use properly according to the intended specification. Further, the possible range of θ is about −2 to 10 [°]. Note that k ₆ = 0.

  Next, referring to FIG. 7 showing the characteristics based on the design example 4, the combination of wavelengths of a plurality of incident light is, for example, a wavelength band near Λ = 1.0 and a wavelength band near Λ = 0.55. The design is more preferable.

Thus, in the design example 4, the pre-twist angles α ₁ and α ₂ are −25 ° and 36 °, respectively, and the light emitted from the wavelength selection wavelength plate 23 with respect to the light in the respective wavelength bands as shown in FIG. However, when the light emitted from the wavelength selection wavelength plate 23 is orthogonal to each other in any combination of wavelength bands, the pre-twist angle is based on α ₁ and α ₂ in the design example 4. α ₁ ′ and α ₂ ′ are the above formulas (4a) and (4b), and the twist angles β ₁ ′ and β ₂ ′ are set as the above formulas (4c) with β ₁ and β ₂ of design example 4 as a reference. It is good to adjust as a formula (4d).

FIG. 13 shows that k ₁ = −0.62, k ₂ = −0.32, k ₃ = 1.64, and k ₄ = 0.32 in the above equations (4a) to (4e). , Θ = −5, 0, 5, 10, 15 [°], the polarization transmittance Tp [%] with respect to Λ. By designing in this way, the interval between the wavelength band close to Tp = 100% and the wavelength band close to Tp = 0% can be controlled. Further, the possible range of θ is about −5 to 15 [°]. Note that k ₅ = k ₆ = 0.

  Further, in design example 5 and design example 6, as shown in FIGS. 8 and 9, respectively, the interval between the wavelength band near Tp = 100% and the wavelength band near Tp = 0% was controlled to be smaller. This is a design example.

Next, W (λ) as a parameter depending on the wavelength λ,
W (λ) = Δn (λ) · d / λ (5)
Give as. The three wavelengths of interest are given as λ _a , λ _b , and λ _c , respectively, and the wavelength λ _a is a wavelength at which the polarized light of the other two wavelengths λ _b and λ _c and the outgoing light are orthogonal to each other. To do. Also, for example, using the equation (5), the difference ΔW _ab between the values obtained for the two wavelengths λ _a and λ _b is expressed as ΔW _ab = W (λ _a ) −W (λ _b ) To do. Note that W ₁ (λ _a ) corresponds to a parameter of the first wave plate 21 with respect to light of wavelength λ _a based on the above equation (5). The assumed combination of three wavelengths is a wavelength around 405 nm, 660 nm, 790 nm, or 460 nm, 530 nm, and 660 nm. As an optimization method, the optimization can be easily performed by repeatedly changing each parameter so that the polarization transmittance approaches a desired value.

Here, the magnitude relationship of the three wavelengths of interest is considered as λ _a <λ _b <λ _c . Then, the pre-twist angle alpha ₁ of the first wave plate _21, the pre-twist angle alpha ₂ of the second wave plate _22, the twist angle beta ₁ of the first wave plate _21, the twist angle of the second wave plate 22 beta _{2, W 1} (lambda _a) of the first wave plate 21 based on the above equation (5), the following results to calculate the condition of _{W 2} (lambda _a) of the second wave plate 22.

Here, when W ₁ (λ _b ) / W ₁ (λ _a ) ≧ 1.5, in a practical range,
α ₁ + β ₁ = 45 °,
α ₂ + β ₂ = 45 °,
α ₁ + β ₁ + α ₂ + β ₂ = 90 °,
α ₁ −α ₂ = 75 °,
α ₁ = 75 °,
W ₁ (λ _a ) / W ₂ (λ _a ) = 1.5,
The optimum value can be easily obtained by designing the wavelength selection wave plate 23 with the parameter as the center value.

As the range for changing the parameters of _{the, (α 1 + β 1)} Δ is changed with respect to the value of (α ₁ + β ₁₎ in the range of -10 ° ~ + 10 °, ( α 2 + β 2) the value of delta is varied (α ₂ + β ₂₎ relative to the, -10 ° ~ + 10 ° _{range, (α 1 + β 1 +} α 2 + β 2) Δ is changed with respect to _{(α 1 + β 1 + α} 2 + β 2 ) has the value range of -10 ° ~ + 10 °, (the value of α ₁ -α ₂₎ Δ is changed with respect to (α ₁ -α ₂₎ is in the range of -25 ° ~ + 25 °, the alpha ₁ The value of Δα ₁ to be changed is in the range of −10 ° to + 10 °, and Δ {W ₁ (λ _a ) / changed with respect to {W ₁ (λ _a ) / W ₂ (λ _a )}. The value of W ₂ (λ _a )} may be assumed to be in the range of −0.2 to +0.2.

In the case of W ₁ (λ _b ) / W ₁ (λ _a ) <1.5, in a practical range,
α ₁ + β ₁ = 45 °,
α ₂ + β ₂ = −135 °,
α ₁ + β ₁ + α ₂ + β ₂ = 90 °,
α ₁ −α ₂ = 10 °,
α ₁ = 75 °,
W ₁ (λ _a ) / W ₂ (λ _a ) = 2.5,
The optimum value can be easily obtained by designing the wavelength selection wave plate 23 with the parameter as the center value.

As the range for changing the parameters of _{the, (α 1 + β 1)} Δ is changed with respect to the value of (α ₁ + β ₁₎ in the range of -10 ° ~ + 10 °, ( α 2 + β 2) the value of delta is varied (α ₂ + β ₂₎ relative to the, -10 ° ~ + 10 ° _{range, (α 1 + β 1 +} α 2 + β 2) Δ is changed with respect to _{(α 1 + β 1 + α} 2 + β 2 ) has the value range of -10 ° ~ + 10 °, (the value of α ₁ -α ₂₎ Δ is changed with respect to (α ₁ -α ₂₎ is in the range of -10 ° ~ + 10 °, the alpha ₁ The value of Δα ₁ to be changed is in the range of −10 ° to + 10 °, and Δ {W ₁ (λ _a ) / changed with respect to {W ₁ (λ _a ) / W ₂ (λ _a )}. The value of W ₂ (λ _a )} may be assumed to be in the range of −0.2 to +0.2.

Next, the magnitude relationship between the three wavelengths of interest is considered as λ _b <λ _a <λ _c . Further, when W ₁ (λ _b ) / W ₁ (λ _a ) <1.5, in a practical range,
α ₁ + β ₁ = 30 ° or −75 °,
α ₂ + β ₂ = 50 °,
α ₁ + β ₁ + α ₂ + β ₂ = 90 °,
α ₁ −α ₂ = 90 °,
α ₁ = 70 °,
W ₁ (λ _a ) / W ₂ (λ _a ) = 3.0,
The optimum value can be easily obtained by designing the wavelength selection wave plate 23 with the parameter as the center value.

As the range for changing the parameters of _{the, (α 1 + β 1)} Δ is changed with respect to the value of (α ₁ + β ₁₎ in the range of -10 ° ~ + 10 °, ( α 2 + β 2) the value of delta is varied (α ₂ + β ₂₎ relative to the, -15 ° ~ + 15 ° _{range, (α 1 + β 1 +} α 2 + β 2) Δ is changed with respect to _{(α 1 + β 1 + α} 2 + β 2 ) has the value range of -15 ° ~ + 15 °, (the value of α ₁ -α ₂₎ Δ is changed with respect to (α ₁ -α ₂₎ is in the range of -15 ° ~ + 15 °, the alpha ₁ The value of Δα ₁ to be changed is in the range of −15 ° to + 15 °, and Δ {W ₁ (λ _a ) / to be changed with respect to {W ₁ (λ _a ) / W ₂ (λ _a )}. The value of W ₂ (λ _a )} may be assumed to be in the range of −0.2 to +0.2.

Next, the magnitude relationship between the three wavelengths of interest is considered as λ _b <λ _a <λ _c . Further, when W ₁ (λ _b ) / W ₁ (λ _a ) <1.5, in a practical range,
α ₁ + β ₁ = 30 ° or −75 °,
α ₂ + β ₂ = 50 °,
α ₁ + β ₁ + α ₂ + β ₂ = 90 °,
α ₁ −α ₂ = 90 °,
α ₁ = 70 °,
W ₁ (λ _a ) / W ₂ (λ _a ) = 3.0,
The optimum value can be easily obtained by designing the wavelength selection wave plate 23 with the parameter as the center value.

As the range for changing the parameters of _{the, (α 1 + β 1)} Δ is changed with respect to the value of (α ₁ + β ₁₎ in the range of -10 ° ~ + 10 °, ( α 2 + β 2) the value of delta is varied (α ₂ + β ₂₎ relative to the, -15 ° ~ + 15 ° _{range, (α 1 + β 1 +} α 2 + β 2) Δ is changed with respect to _{(α 1 + β 1 + α} 2 + β 2 ) has the value range of -15 ° ~ + 15 °, (the value of α ₁ -α ₂₎ Δ is changed with respect to (α ₁ -α ₂₎ is in the range of -15 ° ~ + 15 °, the alpha ₁ The value of Δα ₁ to be changed is in the range of −15 ° to + 15 °, and Δ {W ₁ (λ _a ) / to be changed with respect to {W ₁ (λ _a ) / W ₂ (λ _a )}. The value of W ₂ (λ _a )} may be assumed to be in the range of −0.2 to +0.2.

Next, the magnitude relationship between the three wavelengths of interest is considered as λ _b <λ _c <λ _a . Further, when W ₁ (λ _b ) / W ₁ (λ _a ) ≦ 0.5, in a practical range,
α ₁ + β ₁ = 45 ° or 0 °,
α ₂ + β ₂ = 45 °,
α ₁ + β ₁ + α ₂ + β ₂ = 90 °, or 45 °,
α ₁ −α ₂ = 75 °,
α ₁ = 75 °,
W ₁ (λ _a ) / W ₂ (λ _a ) = 1.5,
The optimum value can be easily obtained by designing the wavelength selection wave plate 23 with the parameter as the center value.

As the range for changing the parameters of _{the, (α 1 + β 1)} Δ is changed with respect to the value of (α ₁ + β ₁₎ in the range of -10 ° ~ + 10 °, ( α 2 + β 2) the value of delta is varied (α ₂ + β ₂₎ relative to the, -10 ° ~ + 10 ° _{range, (α 1 + β 1 +} α 2 + β 2) Δ is changed with respect to _{(α 1 + β 1 + α} 2 + β 2 ) has the value range of -10 ° ~ + 10 °, (the value of α ₁ -β ₁₎ Δ is changed with respect to (α ₁ -β ₁₎ is in the range of -25 ° ~ + 25 °, the alpha ₁ The value of Δα ₁ to be changed is in the range of −15 ° to + 15 °, and Δ {W ₁ (λ _a ) / to be changed with respect to {W ₁ (λ _a ) / W ₂ (λ _a )}. The value of W ₂ (λ _a )} may be assumed to be in the range of −0.2 to +0.2.

When W ₁ (λ _b ) / W ₁ (λ _a )> 0.5, in a practical range,
α ₁ + β ₁ = −120 °,
α ₂ + β ₂ = 60 °,
α ₁ + β ₁ + α ₂ + β ₂ = −60 °,
α ₁ −α ₂ = 130 °,
α ₁ = 20 °,
W ₁ (λ _a ) / W ₂ (λ _a ) = 1.5,
The optimum value can be easily obtained by designing the wavelength selection wave plate 23 with the parameter as the center value.

As the range for changing the parameters of _{the, (α 1 + β 1)} Δ is changed with respect to the value of (α ₁ + β ₁₎ in the range of -15 ° ~ + 15 °, ( α 2 + β 2) the value of delta is varied (α ₂ + β ₂₎ relative to the, -15 ° ~ + 15 ° _{range, (α 1 + β 1 +} α 2 + β 2) Δ is changed with respect to _{(α 1 + β 1 + α} 2 + β 2 ) has the value range of -15 ° ~ + 15 °, (the value of α ₁ -α ₂₎ Δ is changed with respect to (α ₁ -α ₂₎ is in the range of -15 ° ~ + 15 °, the alpha ₁ The value of Δα ₁ to be changed is in the range of −15 ° to + 15 °, and Δ {W ₁ (λ _a ) / to be changed with respect to {W ₁ (λ _a ) / W ₂ (λ _a )}. The value of W ₂ (λ _a )} may be assumed to be in the range of −0.2 to +0.2.

Next, the function of the polarization diffraction element 12 will be described. In FIG. 2A, as an example, light of wavelength λ ₁ and light of wavelength λ ₂ travel in the Z direction from the first wave plate 21 as linearly polarized light in the X direction, Consider a case where light of wavelength λ ₁ is incident on the polarization diffraction element 12 as light of linearly polarized light in the X direction, and light of wavelength λ ₂ and light of wavelength λ ₃ are linearly polarized light in the Y direction. Here, the polarization diffraction element 12 linearly transmits the incident light having the wavelength λ ₁ as the linearly polarized light in the X direction, and the wavelength selective wavelength plate 23 changes the incident light having the wavelength λ ₂ into the linearly polarized light in the Y direction. Has the function of diffracting.

In this case, for example, a birefringent material layer 14a that forms the diffraction grating 14, the ordinary refractive index n _o, has an extraordinary refractive index n _e, transparent material 15 is a isotropic refractive index n _s n _The birefringent material layer 14 a has a refractive index substantially equal to _e, and the direction of the extraordinary light refractive index of the birefringent material layer 14 a is the Y direction, which is the longitudinal direction of the diffraction grating 14. At this time, straight transmission because the difference in refractive index is not generated between the light of the wavelength lambda ₁ incident at the light in the X direction of the linear polarized light refractive index n _s and ordinary refractive index n _o, Y direction linearly polarized light The light having the wavelength λ _{2 and} the light having the wavelength λ ₃ incident at 1 have a refractive index difference between the refractive index n _s and the extraordinary light refractive index _ne, and are diffracted.

The polarization diffraction element 12 transmits linearly polarized light in the X direction and diffracts linearly polarized light in the Y direction. However, the transmission / diffracted polarization direction changes depending on the function of the wavelength selection wavelength plate 23. May be. For example, all the three wavelengths of light are incident on the wavelength selection wavelength plate 23 as linearly polarized light in the X direction, and the light of wavelength λ ₁ is emitted as linearly polarized light 25 ° from the X direction, and the wavelength λ ₂ And the light of wavelength λ ₃ are emitted as linearly polarized light of −65 °, and the longitudinal direction of the concave and convex portions of the diffraction grating 14 coincides with the direction of −65 °, for example, Similarly, light of wavelength λ ₁ is transmitted with high efficiency, and light of wavelength λ ₂ and light of wavelength λ ₃ are diffracted with high efficiency. The diffraction grating 14 shown in FIG. 2A schematically represents the diffraction grating structure, and the longitudinal direction of the unevenness is the Y direction. However, the longitudinal direction of the unevenness is set according to the function of the wavelength selection wavelength plate 23. An adjusted structure may be used.

The polarization diffraction element 12 may have various forms depending on the function required for the wavelength selective diffraction element 20. For example, as shown in FIG. 2A, the diffraction grating 14 has a rectangular cross section having a periodic pitch. In the case of having irregularities, ± first-order diffracted light, ± second-order diffracted light,... Can be generated by diffraction. Further, it is possible to prevent generation of 0th order diffracted light (straight forward transmitted light) by diffraction, or to generate a light amount ratio between 0th order diffracted light and ± 1st order diffracted light at a constant rate. For example, when the height of the birefringent material layer 14a is h, the refractive index anisotropy Δn (= | n _e −n _o |), and the retardation value of the birefringent material layer 14a, which is a product of h and Δn, By adjusting, it is possible to adjust the diffraction efficiency η _{0 of the 0th-} order diffracted light, the diffraction efficiency η _{± 1 of the ± 1st-} order diffracted light, and the like.

Moreover, although the cross section of the diffraction grating 14 was demonstrated as a rectangular thing, it is not restricted to this. For example, in order to increase the diffraction efficiency of only the + 1st order diffracted light, the cross section may be a blazed shape, or a pseudo blazed shape in which the blazed shape is approximated to a staircase shape. Further, it may have a Fresnel lens shape having concentric irregularities on the optical axis of the incident light. In this case, the light that travels by generating a lens function depending on the polarization direction of the incident light. The divergence state of can be changed. As the birefringent material layer 14a, liquid crystal, polymer liquid crystal, photonic crystal, lithium niobate (LiNbO ₃ ), crystal, or the like can be used.

The transparent material 15 may be any material as long as it is transparent to incident light, and an isotropic material having no birefringence can be used, but is not limited thereto. For example, when a birefringent material is used, the material used for the birefringent material layer 14a may be a material in which the refractive index in the optical axis direction matches the wavelength dispersion of the refractive index. Assuming that the difference in refractive index between the birefringent material layer 14a and the transparent material 15 is Δn _A for light having a wavelength λ incident on the polarization diffraction element 12 in a specific optical axis direction, these in the diffraction grating 14 It is preferable to set the optical path difference Δn _A · h of the light having the wavelength λ transmitted through the material of 0.1 to 0.1λ or less because the straight-line transmittance (= 0th-order diffraction efficiency: η ₀ ) becomes 80% or more. Furthermore, it is more preferable that Δn _A · h is 0.05λ or less because η ₀ becomes 90% or more.

  Further, if the polarization direction of light having a wavelength to be diffracted among the light incident on the polarization diffraction element 12 coincides with the polarization direction diffracted by the polarization diffraction element 12, the light can be diffracted with high diffraction efficiency. For example, if the Y direction corresponding to the longitudinal direction of the concave and convex portions of the diffraction grating 14 in the polarization diffraction element 12 is the polarization direction to be diffracted, the polarization direction of the light to be diffracted out of the light incident on the polarization diffraction element 12 is the Y direction. The reference is preferably within 10 ° (within a range of −10 ° to + 10 °), and more preferably within 5 ° (within a range of −5 ° to + 5 °).

  That is, this is related to the utilization efficiency η shown in FIG. 3, and if it is within 10 ° with respect to the Y direction, the utilization efficiency η is 80% or more even if the ellipticity κ is 0.45 or less, Further, if it is within 5 °, the utilization efficiency η is 80% or more even when the ellipticity is about 0.5 or more. Even if the angle (azimuth angle ΔΨ) is within 25 °, the utilization efficiency η is 80% or more if the ellipticity κ is within 0.15.

(First embodiment of optical head device)
The present embodiment relates to an optical head device in which the above-described wavelength selective diffraction element is disposed in an optical path in which three wavelengths of laser light are shared. Further, since the required function of the wavelength selective diffraction element and the structure based thereon are different depending on the configuration of the optical head device, the wavelength selective diffraction element will also be described in relation to the configuration of the optical head device.

  FIG. 14 is an example of a schematic diagram of a compatible optical head device 100 that uses three wavelengths of light and performs recording / reproduction of optical discs of respective standards. Light in the 405 nm wavelength band emitted in the X direction from a light source 101 a such as a semiconductor laser is diffracted by the grating element 102 a into three beams, and passes through the dichroic prisms 103 and 104 and the polarization beam splitter 105, and is collimated by the collimator lens 106. It becomes parallel light, deflected in the Z direction by the mirror 107, transmitted through the quarter-wave plate 108, condensed by the objective lens 109, and condensed on the information recording surface of the optical disk 110. The optical path from the light source to the optical disk is referred to as “outward path”, and the optical path from the optical disk to the optical detector (reflected) is referred to as “return path”.

  The dichroic prism 103 has a function of transmitting light in the 405 nm wavelength band and reflecting light in the 660 nm wavelength band. The dichroic prism 104 has a function of transmitting light in the 405 nm wavelength band and light in the 660 nm wavelength band and reflecting light in the 785 nm wavelength band. The objective lens 109 is compatible with these three wavelengths of light, and in particular, the light utilization efficiency is increased by making the light of the 785 nm wavelength band for CD enter while diverging.

  The light in the 660 nm wavelength band emitted from the light source 101b is diffracted by the grating element 102b into three beams, reflected by the dichroic prism 103, and transmitted through the dichroic prism 104 and the polarization beam splitter 105. The light in the 785 nm wavelength band emitted from the light source 101 c is diffracted by the grating element 102 c into three beams, is reflected by the dichroic prism 104, and is transmitted through the polarization beam splitter 105. The light in the 660 nm wavelength band and the light in the 785 nm wavelength band transmitted through the polarization beam splitter 105 reach the optical disc 110 in the same manner as the light in the 405 nm wavelength band.

  The light in the return path of each wavelength band reflected from the optical disk 110 is transmitted through the objective lens 109, becomes linearly polarized light orthogonal to the forward polarized light when transmitted through the quarter wavelength plate 108, is reflected by the reflection mirror 107, The light passes through the collimator lens 106 and is reflected by the polarization beam splitter 105. Then, it passes through the cylindrical lens 111, passes through a wavelength selective diffraction element 50 described later, and reaches the photodetector 112. Here, the wavelength selective diffraction element 50 causes a concave lens function to be generated only for light in the 785 nm wavelength band for the CD so that all the photodetectors 112 reach the same position. The focal length of light is adjusted. In FIG. 14, the trajectory of the light in the 405 nm wavelength band for BD and the light in the 660 nm wavelength band for DVD is shown by a solid line, and the trajectory of the light in the 785 nm wavelength band for CD is shown by a dotted line.

  Next, the wavelength selective diffraction element 50 will be specifically described. FIG. 15A is a schematic cross-sectional view showing the configuration of the wavelength selective diffraction element 50, which includes a wavelength selective wavelength plate 51 and a polarization diffraction element 52. As the wavelength selection wavelength plate 51, as described in the embodiment of the wavelength selection wavelength plate, it is possible to use a configuration in which two layers of wavelength plates having optical axes twisted in the thickness direction are stacked. The polarization diffraction element 52 is formed by forming a birefringent material layer 54 a on a transparent substrate 53, a diffraction grating 54 having a Fresnel lens cross section, and a transparent material 55 in a concave portion of the diffraction grating 54. The convex portion of the diffraction grating 54 corresponds to the birefringent material layer 54a, and the concave portion indicates a portion between the birefringent material layers 54a. The transparent material 55 only needs to be formed in at least the concave portion, but may be formed in a portion covering the convex portion as shown in FIG. 15A as long as it is an isotropic material.

Next, the shape of the Fresnel lens will be described. The Fresnel lens has a concentric blazed shape centered on the optical axis, and a circle formed by connecting the apexes of each blazed shape is a blazed ring. The blazed shape of the cross section may be a pseudo blazed shape that approximates a stepped shape. FIG. 15B is a diagram for explaining the shape of the Fresnel lens that generates the concave lens action, and the curve α in FIG. 15B indicates the difference in the amount of change in phase that the light receives after passing through the lens. Is shown. Here, when the function is a concave lens, the distribution of the difference in the amount of change in phase is expressed by Expression (6).
φ (r) = a ₁ r ² + a ₂ r ⁴ + a ₃ r ⁶ + a ₄ r ⁸ +... (6)
Here, r is the radial distance from the optical axis, a _i (i = 1, 2, 3, 4,...) Is a constant, and φ (r) is the amount of phase change at the distance r. This is the difference distribution.

Then, the light having a wavelength for which the concave lens function is desired to be generated, here the wavelength is 785 nm (= wavelength λ ₃ ), and the distribution does not change even if a phase difference that is an integral multiple of the wavelength λ ₃ is subtracted from the curve α. The Fresnel lens can be equivalently shaped like a curve β or a curve γ. The birefringent material layer 54a has a normal light refractive index n _o (λ ₃ ) and an extraordinary light refractive index n _e (λ ₃ ) with respect to light having a wavelength λ ₃ (= 785 nm), and a transparent material 55 with respect to light having a wavelength λ ₃ . refractive index _{n s (λ 3)} is to match the _{n o (λ 3).}

  Here, in the optical head device 100, the light in the return path of each wavelength band reflected from the optical disk 110 becomes linearly polarized light in the Y direction and enters the wavelength selective diffraction element 50, so that the 785 nm wavelength band for CD is used. The wavelength selective diffractive element 50 may be configured so as to function as a concave lens only for the light of.

FIG. 15C shows the state of light in the 785 nm wavelength band (wavelength λ ₃ ) that passes through the wavelength selective diffraction element 50. Here, the wavelength selection wavelength plate 51 transmits the light in the 405 nm wavelength band (wavelength λ ₁ ) and the light in the 660 nm wavelength band (wavelength λ ₂ ) without changing the polarization state, and in the 785 nm wavelength band (wavelength λ ₃ ). It shall have a function of making the polarization state of light orthogonal (azimuth angle Ψ ₃ = 90 °). The polarization diffraction element 52 includes a diffraction grating 54 having a Fresnel lens shape in which the slow axis direction of the birefringent material layer 54a is the X direction.

At this time, the light of wavelength λ _{1 and} the light of wavelength λ ₂ are transmitted as they are without changing the divergence state without causing the lens action in the polarization diffraction element 52, whereas the light of wavelength λ ₃ is transmitted as shown in FIG. ), The polarization diffraction element 52 functions as a concave lens. Therefore, the divergence state changes (expands), so that the focal length changes, and the light can be condensed on the photodetector 112 of the optical head device 100. In this case, the wavelength selection wave plate 51 may change the polarization state of the light with the wavelength λ _{1 and} the light with the wavelength λ ₂ (azimuth angles ψ ₁ , ψ ₂ ≠ 0). However, it may be perpendicular to the polarization state of the light having the wavelength λ ₃ , and the slow axis direction of the birefringent material layer 54 a may be parallel to the polarization direction of the wavelength λ ₃ .

The refractive index _{n s (λ 3)} of the transparent material 55 is not limited to the case to match the ordinary refractive index _{n o} of the birefringent material layer 54a (lambda _3), an extraordinary refractive index _{n e (λ 3)} to match the, fast axis of the birefringent material layer 54a may be set to be parallel to the polarization direction of the wavelength lambda _3. In this way, by arranging the wavelength selective diffraction element 50 in the optical path in which the light of these wavelengths is common in the optical head device 100, one photodetector is shared with respect to a plurality of (three or more) wavelengths of light. Therefore, the optical head device can be downsized.

(Second Embodiment According to Optical Head Device)
The form of the wavelength selective diffraction element for providing the optical head device with a function different from the above will be described. FIG. 16 is a schematic diagram of a compatible optical head device 200 that performs recording / reproduction of each standard optical disc using light of three wavelengths based on the present embodiment. The optical head device 200 differs greatly from the optical head device 100 in that a light source such as a semiconductor laser that emits light of three wavelengths is disposed as one light source 201.

  In the case of the optical head device 200, the wavelength selective diffraction element 60 and / or the wavelength selective diffraction element 70 having different functions can be arranged. First, light in the 405 nm wavelength band emitted in the X direction from the light source 201 is transmitted through the wavelength selective diffraction element 60 that selectively generates three beams according to the wavelength of incident light. Accordingly, in the optical head device 200, the 1-beam method can be used for recording / reproduction according to the standard of the optical disc, or the 3-beam method using 0th-order diffracted light and ± 1st-order diffracted light can be used. A specific configuration of the wavelength selective diffraction element 60 will be described later.

  The light in the 405 nm wavelength band that is transmitted straight through or diffracted through the wavelength selective diffraction element 60 is transmitted through the polarization beam splitter 202 and is converted into parallel light by the collimator lens 203. The light in the 405 nm wavelength band that has passed through the collimator lens 203 is reflected by the dichroic prism 204, deflected in the Z direction, transmitted through the quarter-wave plate 206a, and condensed by the objective lens 207a, and an optical disc such as a BD. The light is condensed on the information recording surface 208a. The dichroic prism 204 has a function of reflecting light in the 405 nm wavelength band and transmitting light in the 660 nm wavelength band and light in the 785 nm wavelength band.

  Next, the light in the wavelength band of 660 nm and the light in the wavelength band of 785 nm emitted from the light source 201 are transmitted straight through or transmitted through the wavelength selective diffraction element 60, transmitted through the polarization beam splitter 202, and converted into parallel light by the collimator lens 203. Then, the light passes through the dichroic prism 204, is reflected by the mirror 205, is deflected in the Z direction, passes through the quarter-wave plate 206b, is condensed by the objective lens 207b, and is collected on the information recording surface of the optical disk 208b such as DVD or CD. Condensate.

  The return light of each wavelength band reflected from the optical disk 208a or 208b becomes linearly polarized light orthogonal to the forward linearly polarized light when reciprocating the quarter-wave plate 206a or 206b, and travels to the polarization beam splitter 202. Follow the same light path. Then, the light is reflected by the polarization beam splitter 202, transmitted straight through or diffracted by the wavelength selective diffraction element 70, passes through the cylindrical lens 209, and reaches the photodetector 210.

  Next, the configuration of the wavelength selective diffraction element 60 will be specifically described. FIG. 17A is a schematic cross-sectional view showing the configuration of the wavelength selective diffraction element 60, which includes a wavelength selective wavelength plate 61, a polarization diffraction element 12, and a wavelength selective wavelength plate 62. As the wavelength selection wavelength plates 61 and 62, as described in the embodiment of the wavelength selection wavelength plate, a configuration in which wavelength plates having optical axes twisted in the thickness direction are stacked in two layers can be used.

For example, in the optical head device 200, a one-beam method is used for light in the 405 nm wavelength band, and a three-beam method for generating three beams by diffracting light in the 660 nm wavelength band and light in the 785 nm wavelength band is used. The wavelength selective diffraction element 60 is adapted to the above. At this time, it is assumed that linearly polarized light traveling in the X direction with the polarization direction in the Z direction is incident on the wavelength selective diffraction element 60 shown in FIG. The wavelength selection wavelength plate 61 transmits only light in the 405 nm wavelength band (wavelength λ ₁ ) as linearly polarized light in the Z direction, and transmits light in the 660 nm wavelength band (wavelength λ ₂ ) and 785 nm wavelength band (wavelength λ ₃ ). Light having a function of transmitting as linearly polarized light in the Y direction is used.

  Since the polarization diffraction element 12 has a function of generating diffracted light with respect to linearly polarized light in the Y direction and linearly transmitting light with linearly polarized light in the Z direction, light in the 660 nm wavelength band and light in the 785 nm wavelength band. 3 beams including diffracted light are generated. Then, by arranging the wavelength selection wavelength plate 62 having the same function as the wavelength selection wavelength plate 61, the light in the 660 nm wavelength band and the light in the 785 nm wavelength band transmitted through the polarization diffraction element 12 are again converted into linearly polarized light in the Z direction. Can be light. Further, the wavelength selection wave plate 62 transmits linearly polarized light in the Z direction in the 405 nm wavelength band without changing the polarization state. Accordingly, these lights transmitted through the wavelength selective diffraction element 60 have a function of diffracting only light of a specific wavelength.

  In this way, by aligning the polarization direction of the linearly polarized light of each wavelength band that has passed through the wavelength selective diffraction element 60, the forward light and the return path of each wavelength band incident on the polarization beam splitter 202 of the optical head device 200 are obtained. Since deflection separation can be performed with light, the optical head device can be miniaturized. Moreover, although the wavelength selection diffraction element 60 showed the example which diffracts the light of a 660 nm wavelength band, and the light of a 785 nm wavelength band, it is not restricted to the structure which has this function, It diffracts only with respect to the light of a 405 nm wavelength band. Alternatively, light in two wavelength bands may be selectively diffracted from light in three wavelength bands.

  Next, the wavelength selective diffraction element 70 will be described. The wavelength selective diffraction element 70 is disposed in an optical path that is common in the return path of light of these three wavelengths in the optical head device 200. There are two functions of the wavelength selective diffraction element 70 in the optical head device 200. One is demultiplexing that adjusts the traveling directions of the three lights incident on the wavelength selective diffraction element 70 to be different when the photodetector 210 is provided with a light receiving area for receiving light of each wavelength band. It gives a function.

  Another function is to correct the optical axis of light in each wavelength band generated on the configuration of the optical head device. For example, the light source 201 emits light of three wavelengths, but has a function of correcting a slight optical axis shift caused by the physically different emission points, whereby the signal light from the optical disk is detected by the photodetector 210. Can be obtained with high accuracy. The position of the wavelength selective diffraction element 70 is not limited to the optical path only in the return path, but may be in the optical path only in the forward path, but when used in the optical path only in the return path, the utilization efficiency of the light reaching the optical disks 208a and 208b is used. Can be increased. Further, when the wavelength selective diffraction element 70 is used in the optical path only for the return path, it may be disposed in the optical path between the cylindrical lens 209 and the photodetector 210.

  Next, a configuration example of the wavelength selective diffraction element 70 will be specifically described. FIG. 17B is a schematic cross-sectional view showing the configuration of the wavelength selective diffraction element 70, and has a configuration in which two wavelength selective diffraction elements are arranged so as to overlap each other. In the polarization diffraction element 72a, a birefringent material layer 74a having a periodic pitch and a blazed cross section are formed on a transparent substrate 73, and a transparent material 75 is formed in a concave portion of the diffraction grating 74. Thus, the wavelength selection wavelength plate 71a is formed on the polarization diffraction element 72a. Similarly, in the polarization diffraction element 72 b, the birefringent material layer 77 a has a periodic pitch and a blazed cross section is formed on the transparent substrate 76, and the diffraction grating 77 is transparent in the concave portion. The material 78 is formed, and the wavelength selection wavelength plate 71b is formed on the polarization diffraction element 72b. The cross-sectional shape of the diffraction grating may be a pseudo blazed shape that approximates the blazed shape to a staircase shape. Although the cross-sectional shape may be rectangular, a blazed shape or a pseudo-blazed shape is preferable because of the superiority of the light utilization efficiency for diffraction.

  As the wavelength selection wave plates 71a and 71b, as described in the above embodiment, a wave plate in which the optical axes are twisted in the thickness direction and laminated in two layers can be used. In FIG. 17B, the blazed shapes of the diffraction grating 74 and the diffraction grating 77 are different from each other. However, the blazed shapes are the same, for example, different diffraction angles with respect to light beams having different pitches to be diffracted. You may design as follows. Here, the slow axis direction of the diffraction gratings 72a and 72b is the Y direction, the transparent material 75 has a refractive index equivalent to the ordinary refractive index of the birefringent material layer 74a, and the transparent material 78 is a birefringent material layer. It has a refractive index equivalent to the ordinary light refractive index of 77a.

Here, the demultiplexing function described as the first function of the wavelength selective diffraction element 70 will be described. The light in each wavelength band reflected by the polarization beam splitter 202 of the optical head device 200 is incident on the wavelength selective diffraction element 70 as linearly polarized light in the Y direction. At this time, the wavelength selection wavelength plate 71a transmits light in the 405 nm wavelength band (wavelength λ ₁ ) without changing the polarization state, and transmits light in the 660 nm wavelength band (wavelength λ ₂ ) and 785 nm wavelength band (wavelength λ ₃ ). Those having the function of making linearly polarized light in the X direction with respect to the light of the above are used.

  Of the light in each wavelength band transmitted through the wavelength selection wavelength plate 71a, only the light in the 405 nm wavelength band, which is linearly polarized light in the Y direction, is diffracted by the diffraction grating 74, and the light in the 660 nm wavelength band and the light in the 785 nm wavelength band are It goes straight through. Then, the wavelength selection wavelength plate 71b uses light in the 405 nm wavelength band as linearly polarized light in the X direction, light in the 785 nm wavelength band as linearly polarized light in the Y direction, and light in the 660 nm wavelength band does not change the polarization state. The one having a function of transmitting light as it is is used. Of the light in each wavelength band transmitted through the wavelength selection wavelength plate 71b, only the light in the 785 nm wavelength band, which is linearly polarized light in the Y direction, is diffracted by the diffraction grating 77, and the light in the 405 nm wavelength band and the light in the 660 nm wavelength band are diffracted. By allowing light to pass straight through, the traveling direction of light in each wavelength band can be separated. As a result, a light receiving area for light in each wavelength band can be provided in the photodetector 210 to detect an optical signal in each wavelength band.

  Moreover, although the wavelength selection diffraction element 70 demonstrated the function to demultiplex the light of each wavelength band, you may combine it. That is, the traveling directions (optical axes) of the light of the 405 nm wavelength band, the light of the 660 nm wavelength band, and the light of the 785 nm wavelength band incident on the wavelength selective diffraction element 70 are different from each other, but the optical axes of the emitted light are aligned and emitted. It can be made to function with the same configuration. In this case, for example, even when the light receiving area of the photodetector 210 is used in common with light of each wavelength band, the signal light from each optical disk can be accurately reached, and the photodetector 210 and the optical head device A reduction in size of 200 can be realized.

  In FIG. 17A and FIG. 17B, the wavelength selective diffraction elements 60 and 70 are shown to be diffracted along the XZ plane for the sake of explanation. What is necessary is just to determine the longitudinal direction of the unevenness | corrugation of a diffraction grating according to each azimuth angle with respect to the light of each wavelength band which a wavelength selection wavelength plate has. Therefore, for example, a configuration in which the longitudinal direction of the unevenness of the diffraction grating 74 of the wavelength selective diffraction element 70 and the longitudinal direction of the diffraction grating 77 are not parallel may be employed. The wavelength selective diffraction element 70 may be a hologram diffraction element having a structure in which the diffraction gratings 74 and 77 are divided into a plurality of regions, for example. In this case, the longitudinal direction and shape of the diffraction grating may be set for each divided region.

(Third embodiment of the optical head device)
FIG. 18 is a schematic view of a compatible optical head device 250 that records and reproduces optical discs of the respective standards using light of three different wavelengths based on the present embodiment. Use the same numbers for common parts to avoid duplication of explanation. In addition, the optical head device 250 is different from the optical head device 200 in that light having a wavelength band of 405 nm emitted from the light source 201 is not provided with a diffractive element that generates three beams in the forward optical path (wavelength selection). The 660 nm wavelength band light and the 785 nm wavelength band light are both diffracted by the wavelength selective diffraction element 80 disposed in the optical path of the return path using the one-beam method.

  Next, the configuration of the wavelength selective diffraction element 80 will be specifically described. FIG. 19 is a schematic cross-sectional view showing the configuration of the wavelength selective diffraction element 80, in which two wavelength selective diffraction elements are arranged so as to overlap each other. In the polarization diffraction element 82a, a birefringent material layer 84a has a periodic pitch and a blazed cross section is formed on a transparent substrate 83, and a transparent material 85 is formed in a concave portion of the diffraction grating 84. Thus, the wavelength selection wavelength plate 81 is formed on the polarization diffraction element 82a. Similarly, in the polarization diffraction element 82b, a diffraction grating 87 having a birefringent material layer 87a having a periodic pitch and a blazed cross section is formed on a transparent substrate 86, and transparent in a concave portion of the diffraction grating 87. A material 88 is formed. The cross-sectional shape of the diffraction grating may be a pseudo blazed shape that approximates the blazed shape to a staircase shape. Although the cross-sectional shape may be rectangular, a blazed shape or a pseudo-blazed shape is preferable because of the superiority of the light utilization efficiency for diffraction.

  As described in the embodiment of the wavelength selection wavelength plate, the wavelength selection wavelength plate 81 uses a structure in which wavelength plates having optical axes twisted in the thickness direction are stacked in two layers. In FIG. 19, the blazed shapes of the diffraction grating 84 and the diffraction grating 87 are different from each other. However, the blazed shapes are the same, for example, the pitches are different from each other, and the respective diffraction angles with respect to light of the wavelength to be diffracted are different. You may design it. Here, the slow axis direction of the polarization diffraction element 82a is the Y direction, and the transparent material 85 has a refractive index equivalent to the ordinary refractive index of the birefringent material layer 84a. Furthermore, the slow axis direction of the polarization diffraction element 82b is the X direction, and the transparent material 88 has a refractive index equivalent to the ordinary refractive index of the birefringent material layer 87a.

Here, the demultiplexing function of the wavelength selective diffraction element 80 will be described. The light of each wavelength reflected by the polarization beam splitter 202 of the optical head device 250 is incident on the wavelength selective diffraction element 80 as linearly polarized light in the Y direction. At this time, the wavelength selection wavelength plate 81 transmits the light in the 405 nm wavelength band (wavelength λ ₁ ) and the light in the 660 nm wavelength band (wavelength λ ₂ ) without changing the polarization state, and the 785 nm wavelength band (wavelength λ ₃ ) Light having a function of converting light into linearly polarized light in the X direction.

  Of the light in each wavelength band transmitted through the wavelength selection wavelength plate 81, light in the 405 nm wavelength band and light in the 660 nm wavelength band that are linearly polarized light in the Y direction are diffracted by the diffraction grating 84, and light in the 785 nm wavelength band is It goes straight through. Of the light in each wavelength band transmitted through the polarization diffraction element 82a, the diffraction grating 87 diffracts only the light in the 785 nm wavelength band that is linearly polarized light in the X direction, and the light in the 405 nm wavelength band and the light in the 660 nm wavelength band. Light is transmitted straight. At this time, for example, light in the 660 nm wavelength band and light in the 785 nm wavelength band can be diffracted and received in common in a light receiving area (not shown) of the photodetector 210.

Specifically the grating pitch of the diffraction grating 84 of the wavelength-selective diffraction element 80 in FIG. 19 and _{P D,} when the grating pitch of the diffraction grating 87 and _{P C, P C} is approximately equal to _{P D} × _{λ 3} / _{λ 2} If adjusted so that the diffraction directions of the light in the 660 nm wavelength band and the light in the 785 nm wavelength band are substantially the same, the light in these wavelength bands can be received in the same light receiving area. In addition, the combination of the wavelength bands that share the light receiving area is not limited to this, and may be light in the 405 nm wavelength band and light in the 660 nm wavelength band. It may be set so as to be shared.

  In FIG. 19, the wavelength selective diffractive element 80 is shown to diffract along the XZ plane for the sake of explanation. However, the present invention is not limited to this, and the light in each wavelength band possessed by each wavelength selective wavelength plate is used. What is necessary is just to determine the longitudinal direction of the unevenness | corrugation of a diffraction grating according to each azimuth with respect to. Therefore, for example, a configuration in which the longitudinal direction of the unevenness of the diffraction grating 84 of the wavelength selective diffraction element 80 and the longitudinal direction of the diffraction grating 87 are not parallel may be employed. The wavelength selective diffraction element 80 may be a hologram diffraction element having a structure in which the diffraction gratings 84 and 87 are divided into a plurality of regions, for example. In this case, the longitudinal direction and shape of the diffraction grating may be set for each divided region.

(Embodiment related to optical system for display device)
So far, the optical head device using three lights having different wavelengths as light of 405 nm wavelength band, light of 660 nm wavelength band, and light of 785 nm wavelength band has been described. A wavelength selective wave plate can be used in the optical system for the apparatus. FIG. 20 illustrates a light source 301R that emits light in a 645 nm wavelength band (610 to 670 nm) that is red, a light source 301G that emits light in a 533 nm wavelength band (500 to 560 nm) that is green, and a 450 nm wavelength band (that is blue). It is a schematic diagram of the optical system 300 for display apparatuses which handles the light source 301B which radiate | emits light (420-480 nm).

  In the optical system 300 for a display device, the light from these three light sources 301R, 301G, and 301B can be combined into an image. A configuration for combining these three wavelengths of light will be described. The red light emitted from the light source 301R travels in the X direction with linearly polarized light in the Z direction, and the green light emitted from the light source 301G travels in the Z direction with linearly polarized light in the Y direction. The polarization beam splitter 302 has a function of transmitting linearly polarized light in the Z direction and reflecting linearly polarized light in the Y direction. The polarizing beam splitter 302 multiplexes the red light and the green light, and these lights in the X direction. To deflect. The wavelength selection wave plate 303 transmits red light without changing the polarization state, and linearly polarized light orthogonal to the direction of linearly polarized light incident on green light, that is, linearly polarized light in the Z direction. Convert to

  The blue light emitted from the light source 301B travels in the Z direction with linearly polarized light in the Y direction. The polarization beam splitter 304 has a function of transmitting linearly polarized light in the Z direction and reflecting linearly polarized light in the Y direction, like the polarization beam splitter 302. Thus, the red light and the green light are transmitted in a straight line, and the blue light is reflected to multiplex these three lights. Of these three wavelengths of light emitted from the polarization beam splitter 304, the light is incident on the wavelength selection wavelength plate 305 in a state where the polarization directions of the red light and the green light are orthogonal to the polarization direction of the blue light.

  The wavelength selection wavelength plate 305 converts the light into linearly polarized light orthogonal to the direction of linearly polarized light incident on red light and green light, that is, converts it into linearly polarized light in the Y direction. Transmit without changing the polarization state. As a result, the light of these three colors incident on the collimating lens 306 becomes linearly polarized light in the same direction, that is, linearly polarized light in the Y direction. By aligning the direction of linearly polarized light in this way, controllability is improved in the optical system from the collimator lens 306 to the video display, such as easy deflection of the video signal of light.

This embodiment is based on the basic configuration of the wavelength selection wave plate 20. FIG. 21A shows a specific configuration of the wavelength selective diffraction element 400 based on this embodiment, and an example of the manufacturing method and design will be described. The polymer liquid crystal layer 403a serving as the first wave plate and the polymer liquid crystal layer 403b serving as the second wave plate have a configuration in which the twist angles β ₁ and β ₂ are not 0, respectively. It is prepared by the following procedure.

The transparent substrate 401a on which the alignment film 402a subjected to the horizontal alignment process is formed on one side and the transparent substrate with the alignment film which is not illustrated are adjusted to face each other by adjusting the angle formed by these alignment directions. Then, by inserting a liquid crystal monomer between two transparent substrates with an alignment film arranged at a predetermined uniform interval, a twisted liquid crystal monomer layer is formed between these transparent substrates. Next, a polymer liquid crystal layer 403a having a predetermined twist angle beta ₁ by polymerizing and curing the liquid crystal monomer. Further, a transparent substrate (not shown) facing the transparent substrate 401a is excluded.

Then, made of the same polymeric liquid crystal material by the same manufacturing method, it is formed on the polymer liquid crystal layer 403b and the transparent substrate 401b having a predetermined twist angle beta _2. Note that the first wave plate 21 and the second wave plate 22 of the wavelength selection wave plate 23 in FIG. 2A correspond to the polymer liquid crystal layers 403a and 403b in FIG. 21A, respectively. The obtained polymer liquid crystal layers 403a and 403b have a refractive index anisotropy Δn of about 0.092 for light with a wavelength of 405 nm, about 0.077 for light with a wavelength of 660 nm, and about 0.00 for light with a wavelength of 785 nm. 075.

Next, the alignment direction of the liquid crystal molecules of the polymer liquid crystal layer 403a and the alignment direction of the liquid crystal molecules of the polymer liquid crystal layer 403b are opposed to each other at an angle of 62 °, and are bonded using the transparent adhesive 404. FIG. 21B shows a slow axis 411 on the alignment film 402a side of the polymer liquid crystal layer 403a, and a slow axis 412 on the adhesive 404 side of the polymer liquid crystal layer 403a. FIG. 21C is a plan view showing the slow axis 413 on the adhesive 404 side of the polymer liquid crystal layer 403b and the slow axis 414 on the alignment film 402b side of the polymer liquid crystal layer 403a.

The light incident on the wavelength selective diffraction element 400 of this embodiment is a combination of a wavelength of 405 nm, a wavelength of 660 nm, and a wavelength of 785 nm, all incident in the polarization direction 410 of linearly polarized light shown in FIGS. 21 (b) and 21 (c). Let The conditions of the polymer liquid crystal layer 403a that is the first wave plate and the polymer liquid crystal layer 403b that is the second wave plate constituting the wavelength selection wave plate 407 are given as Examples 1 to 3, and the conditions are as follows. Table 2 is organized. The materials used are the same as those in Example 1, and the thicknesses d ₁ and d ₂ , pre-twist angles α ₁ and α _2, and twist angles β ₁ and β ₂ of the polymer liquid crystal layers 403a and 403b are respectively set. It has changed.

Here, in each wavelength selection wavelength plate 407 manufactured under the conditions of Table 2, the azimuth angle with respect to light with a wavelength of 405 nm is Ψ ₁ [°], the azimuth angle with respect to light with a wavelength of 660 nm is Ψ ₂ [°], and the wavelength is 785 nm. Table 3 summarizes the results of each example when the azimuth angle with respect to light is Ψ ₃ [°].

In the first embodiment, according to the conditions shown in Table 2, as shown in Table 3, the azimuth angle Ψ ₁ for light with a wavelength of 405 nm is 0 °, the azimuth angles Ψ for light with a wavelength of 660 nm and light with a wavelength of 785 nm ₂ and Ψ ₃ are 90 [°], and the angles formed by Ψ ₁ and Ψ ₂ and Ψ ₁ and Ψ ₃ can be made orthogonal.

  Next, a method for manufacturing the wavelength selective diffraction element 500 including the polarization diffraction element 408 will be described. A polymer liquid crystal layer having a thickness of about 1.2 μm in which polymer liquid crystals are aligned in a specific direction is formed on the transparent substrate 401c by the same manufacturing method as the polymer liquid crystal layers 403a and 403b. The refractive index anisotropy Δn of the obtained polymer liquid crystal layer is about 0.173 for light with a wavelength of 405 nm, about 0.147 for light with a wavelength of 660 nm, and about 0.144 for light with a wavelength of 785 nm.

  Next, a diffraction grating 405 having a convex portion height of about 1.2 μm is formed by a fine processing process of an organic material using photolithography and etching. Here, the unevenness is formed so that the longitudinal direction of the unevenness of the diffraction grating 405 is parallel to the orientation direction, that is, the slow axis of the diffraction grating 405.

Then, in accordance with the 0 [°] a slow axis as the orientation direction of the polarization diffraction element 408 (diffraction grating 405), substantially equal to uniform refraction and ordinary refractive index of the polymer liquid crystal to form a diffraction grating 405 (n _o) The wavelength selective diffraction element 400 including the polarizing diffraction element 408 is formed by filling the concave portion of the diffraction grating 405 using a transparent adhesive 406 having a rate of about 100%.

As shown in FIG. 21B, when a laser beam that is 405 nm linearly polarized light having a polarization direction 410 is incident from the wavelength selective wave plate 407 of the manufactured wavelength selective diffraction element 400, zero-order diffraction efficiency (= straight transmission) Ratio) η ₀ (405) is 0.5% or less, while it can be confirmed that ± 1st-order diffraction efficiency η _{± 1} (405) is 38% or more.

Similarly, when laser light that is linearly polarized light of 660 nm and 785 nm with the polarization direction 410 is incident, the 0th-order diffracted light rates η ₀ (660) and η ₀ (785) of the light of the respective wavelengths become 98% or more. Thus, the wavelength selective diffraction element 400 in which the diffraction efficiency changes depending on the wavelength of incident light can be obtained.

Similarly, the angles formed by Ψ ₁ and Ψ ₃ and Ψ ₂ and Ψ ₃ can be orthogonalized according to the conditions of Example 2 shown in Table 2, and Ψ ₁ and Ψ ₂ and , Ψ ₂ and Ψ ₃ can be orthogonally crossed. As a result, it is possible to obtain the wavelength selective diffraction element 400 that transmits light of one wavelength straightly with high efficiency and diffracts light of the other wavelength with high efficiency.

In addition, FIG. 22, FIG. 23, and FIG. 24 show transmission or diffraction utilization efficiency η for light in each wavelength band of the wavelength selective diffraction element manufactured under the conditions of Example 1, Example 2, and Example 3, respectively. As a result, the light utilization efficiencies η in the light of the 405 nm wavelength band, the light of the 660 nm wavelength band, and the light of the 785 nm wavelength band all showed high values. In particular, by providing appropriate twist angles β ₁ and β ₂ as conditions for reducing the wavelength dependence of the utilization efficiency η of incident light, it is possible to obtain a wavelength selective diffraction element having a high light utilization efficiency η. .

Also, as defined above, the azimuth angle for light with a wavelength of 405 nm is ψ ₁ [°], the azimuth angle for light with a wavelength of 660 nm is ψ ₂ [°], and the azimuth angle for light with a wavelength of 785 nm is ψ ₃ [°. The configuration in which two are 90 ° and the other one is 0 ° is shown in Table 4 as Examples 4 to 6. Further, Table 5 summarizes the results under the conditions.

  In the wavelength selection wavelength plates of Examples 4 to 6, the polarization direction of the emitted light is 0 ° or 90 ° with respect to the three-wavelength light incident as linearly polarized light having the same polarization direction. Especially when the polarization direction of the light emitted from the wavelength selection wave plate cannot be set freely so that the optical axis direction of the birefringent material layer of the polarization diffraction element coincides with the production of the wavelength selection diffraction element. Use is effective.

The fourth embodiment shows a configuration in which the azimuth angle Ψ ₁ is orthogonal to the azimuth angle Ψ ₂ and the azimuth angle Ψ _3, and FIG. 25A, FIG. 25B, and FIG. The light utilization efficiency η for light in the 405 nm wavelength band, 660 nm wavelength band, and 785 nm wavelength band is shown for the wavelength selective diffraction element 400 manufactured based on the conditions.

The fifth embodiment shows a configuration in which the azimuth angle Ψ ₂ is orthogonal to the azimuth angle Ψ ₁ and the azimuth angle Ψ _3, and FIG. 26A, FIG. 26B, and FIG. The light utilization efficiency η for light in the 405 nm wavelength band, 660 nm wavelength band, and 785 nm wavelength band is shown for the wavelength selective diffraction element 400 manufactured based on the conditions.

The sixth embodiment shows a configuration in which the azimuth angle Ψ ₃ is orthogonal to the azimuth angle Ψ ₁ and the azimuth angle Ψ _2, and FIG. 27A, FIG. 27B, and FIG. The light utilization efficiency η for light in the 405 nm wavelength band, 660 nm wavelength band, and 785 nm wavelength band is shown for the wavelength selective diffraction element 400 manufactured based on the conditions.

  As described above, the wavelength selection wavelength plate according to the present invention orthogonalizes one or two of the three wavelengths of light incident in the same linearly polarized light direction and the light of the remaining wavelengths and the wavelength thereof. It has small dependence and stable optical characteristics. The wavelength selective diffractive element using the wavelength selective wave plate according to the present invention can selectively diffract light of one or two wavelengths out of the three wavelengths, and an optical head using the three wavelengths of light. In the apparatus, it is possible to obtain a stable transmittance and diffraction efficiency including wavelength selectivity, small wavelength dependence, and further downsizing of the optical head apparatus can be realized.

1, 2, 3, 23, 51, 61, 62, 71a, 71b, 81, 303, 305, 407 Wavelength selection wavelength plate 12, 52, 72a, 72b, 82a, 82b, 408 Polarization diffraction element 13, 53, 73 , 76, 83, 86, 401a, 401b, 401c Transparent substrate 14, 54, 74, 77, 84, 87, 405 Diffraction gratings 14a, 54a, 74a, 77a, 84a, 87a Birefringent material layers 15, 55, 75 78, 85, 88 Transparent material 20, 50, 60, 70, 80, 400 Wavelength selective diffraction element 21 First wave plate 22 Second wave plate 24, 410 Polarization direction of incident linearly polarized light 25, 411 First Optical axes 26, 412 on the light incident side of the first wave plate Optical axes 27, 413 on the light emitting side of the first wave plate Optical axes 28, 414 on the light incident side of the second wave plate Light of the second wave plate Radiation-side optical axes 100, 200, 250 Optical head devices 101a, 101b, 101c, 201, 301R, 301G, 301B Light sources 102a, 102b, 102c Grating elements 103, 104, 204 Dichroic prisms 105, 202, 302, 304 Polarized beams Splitter 106, 203, 306 Collimator lens 107, 205 Mirror 108, 206a, 206b 1/4 wavelength plate 109, 207a, 207b Objective lens 110, 208a, 208b Optical disc 111, 209 Cylindrical lens 112, 210 Photo detector 300 For display device Optical system 402a, 402b Alignment film 403a, 403b Polymer liquid crystal layer 404 Adhesive

Claims (6)

The polarization state of the linearly polarized light having at least one wavelength among the linearly polarized light incident at the _three wavelengths λ ₁ , λ ₂ , and λ ₃ (λ ₁ <λ ₂ <λ ₃ ) having predetermined different bands is changed. In the wavelength selective wave plate,
The wavelength selection wave plate is provided with a first wave plate and a second wave plate in order from the light incident side,
In the first wave plate and the second wave plate, the major axis direction of the liquid crystal molecules is twisted with respect to the thickness direction,
The light of the wavelength λ ₁ incident on the first wave plate, the light of the wavelength λ _{2 and} the light of the wavelength λ ₃ are linearly polarized light in the same direction, and the direction of the incident linearly polarized light is used as a reference.
The angle formed by the major axis direction of the liquid crystal molecules on the side of the linearly polarized light incident on the first wave plate, the direction of the linearly polarized light incident on the first wave plate and the first of the second wave plate A combination with an angle formed by the major axis direction of the liquid crystal molecules on the wavelength plate side, or an angle formed by a minor axis direction of the liquid crystal molecules on the side of the linearly polarized light incident on the first wavelength plate, The combination of the direction of the linearly polarized light incident on the wave plate and the angle formed by the minor axis direction of the liquid crystal molecules on the first wave plate side of the second wave plate is a pretwist angle α ₁ [°. ], Α ₂ [°],
The twist angle twisted in the thickness direction of the liquid crystal molecules of the first wave plate is β ₁ [°], and the twist angle twisted in the thickness direction of the liquid crystal molecules of the second wave plate is β ₂ [°]. and when,
The ellipticity of the light of the wavelength λ ₁ , the light of the wavelength λ _{2 and} the light of the wavelength λ ₃ emitted from the wavelength selection wavelength plate is 0.5 or less,
Further, when the first direction and the second direction orthogonal to the first direction are given, the polarization direction of the component having the largest vibration of the light of any one wavelength with respect to the first direction is The formed angle is in a range of −26 to 26 [°], and the ratio of the light component in the first direction is 80% or more out of all components of light emitted at this wavelength, and the second direction. The angle formed by the polarization direction of the component with the largest vibration of the light of the other two wavelengths is in the range of −26 to 26 [°], and all the components of the light emitted at these wavelengths are Among them, the α ₁ , the α ₂ , the β ₁ , the β ₂ , the retardation value Rd ₁ of the first wave plate, and the ratio of the respective light components in the second direction are 80% or more. Wavelength selection for which retardation value Rd ₂ of the second wave plate is set Wavelength plate. The wavelength λ ₁ is a 405 nm wavelength band ranging from 395 to 420 nm, the wavelength λ ₂ is a 660 nm wavelength band ranging from 640 to 680 nm, and the wavelength λ ₃ is a 785 nm wavelength ranging from 765 to 805 nm. The wavelength selective wave plate according to claim 1, wherein the wavelength selective wave plate is a band. The wavelength λ ₁ is a 450 nm wavelength band ranging from 420 to 480 nm, the wavelength λ ₂ is a 533 nm wavelength band ranging from 500 to 560 nm, and the wavelength λ ₃ is a 645 nm wavelength ranging from 610 to 670 nm. The wavelength selective wave plate according to claim 1, wherein the wavelength selective wave plate is a band. Ordinary on a transparent substrate refractive index n _o, irregularities made of a birefringent material layer having birefringence to be extraordinary refractive index n _e is formed, the in the recess of the birefringent material layer n _o and the n a polarization diffraction element on which an optical material equal to _e is formed;
A wavelength selective diffraction element comprising at least one of the wavelength selective wavelength plates according to claim 1.   The wavelength selective diffraction element according to claim 4, wherein the uneven cross section of the birefringent material layer has a Fresnel lens shape. At least one light source that emits light of three different wavelengths;
An objective lens for condensing the light emitted from the light source on an optical recording medium;
A photodetector for detecting light reflected from the optical recording medium;
Transmits light from the light source toward the optical recording medium and reflects light from the optical recording medium toward the photodetector, or reflects light from the light source toward the optical recording medium and transmits the light. A polarizing beam splitter that transmits light from a recording medium toward the photodetector;
An optical head device comprising:
The light in which the wavelength selective diffraction element according to claim 4 or 5 is disposed in an optical path between the light source and the polarizing beam splitter, or in an optical path between the polarizing beam splitter and the photodetector. Head device.

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