JP2011233208A - Wavelength selective wave plate, wavelength selective diffraction element and optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength selective wave plate having a function as follows: when three linearly polarized lights respectively belonging to different wavelength bands enter the wave plate, two linearly polarized lights exit the wave plate with their plane of polarization being substantially orthogonal to a plane of polarization of the remaining light.SOLUTION: A wavelength selective wave plate includes two wave plates, in which the optic axes of which are parallel to their plate planes and are also aligned in the thickness direction, are stacked so that their optic axes are crossed. The wave plate adjusts pretilt angles αand α, which are defined as respective angles between the direction of linear polarization of incident lights and the optic axis of a first wave plate and the optic axis of a second wave plate, and retardation values of the first wave plate Rd(λ), Rd(λ), Rd(λ) and retardation values of the second wave plate Rd(λ), Rd(λ), Rd(λ), where λ, λand λare the respective wavelengths of three 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, and CD, a light source such as a semiconductor laser having a different wavelength is used for each optical disc of different standards, and an information recording surface of the optical disc is formed by an objective lens having a different numerical aperture NA. The light emitted from the light source is condensed, and the reflected light is branched by a beam splitter and received by a photodetector, whereby it is converted into an electrical signal to record and 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. A photodetector that receives signal light reflected by a DVD and signal light reflected by a CD in a common single package is also 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 is conceivable to use as a wavelength-selective diffraction element that generates desired diffracted light only for light of a specific wavelength and 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, by arranging the three-wavelength light in the return path as a hologram diffraction element in the common optical path, the number of light receiving surfaces and the area of the light receiving surfaces of the photodetector can be reduced, 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, and as a result, 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 has desired diffraction efficiency for at least one wavelength Furthermore, the present invention provides a wavelength selective diffraction element having a small wavelength dependency of diffraction efficiency and a high light utilization efficiency, and a display device such as an optical head device and a laser projector using the same.

The present invention provides a straight line of 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 the polarization state of the polarized light, the wavelength selection wavelength plate includes a first wavelength plate and a second wavelength plate in order from the light incident side, and the first wavelength The plate and the second wave plate have optical axes parallel to the wave plate surfaces and aligned in the thickness direction, and the light of the wavelength λ ₁ incident on the first wave plate, the wavelength λ _{2 and} the light of wavelength λ ₃ are linearly polarized light in the same direction, and the slow axis of the first wave plate and the second light are based on the polarization direction of the incident linearly polarized light. A combination with the slow axis of the wave plate, or the fast axis of the first wave plate and the second wave plate Each angle, which consist of a combination of fast axis, the pre-twist angle alpha _{1 [°],} when the alpha _{2 [°],} the wavelength lambda ₁ of the light emitted from the wavelength selection wavelength plate, the wavelength lambda ₂ light and ellipticity of the wavelength lambda ₃ of the light is 0.5 or less any further, when giving a first direction, a second direction perpendicular to said first direction, said first direction On the other hand, the angle formed by the polarization direction of the component with the largest vibration of light of any one wavelength is in the range of −26 to 26 [°], and among the total components of the light emitted at this wavelength, 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 to 26. Of all the components of light emitted at these wavelengths in the range of [°]. Wherein as the ratio of the respective light components in the second direction is 80% or more, the alpha _1, wherein the alpha _2, the first retardation value Rd ₁ and the second wave plate retardation value of the wavelength plate A wavelength selective wave plate in which Rd ₂ is set is provided.

Further, the retardation value of the first wave plate for the light of wavelength λ ₁ is Rd ₁ (λ ₁ ), and the retardation value of the second wave plate for the light of wavelength λ ₁ is Rd ₂ (λ ₁ ). Then, the value of Rd ₁ (λ ₁ ) is a substantially integer multiple of 2 or more of the value of λ ₁ , and the value of Rd ₂ (λ ₁ ) is a substantially integer multiple of 0.5 of the value of λ ₁ A wavelength selective wave plate is provided.

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 520 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 beam splitter that transmits light from the optical recording medium toward the photodetector, and an optical head device provided in an optical path between the light source and the beam splitter and / or the beam splitter. Provided is an optical head device in which the above-described wavelength selective diffraction element is disposed in an optical path to the photodetector.

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. Moreover, the wavelength selective diffraction element which arrange | positions the polarization | polarized-light diffraction element from which the diffraction efficiency differs with the polarization direction of the light which radiate | emitted the wavelength selection wavelength plate with a wavelength selection wavelength plate can be provided. 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. Explanatory drawing of the Poincare sphere showing a polarization state. The graph which shows the wavelength ((LAMBDA)) dependence of the polarization transmittance of the wavelength selection wavelength plate which concerns on a design example. The graph which shows the wavelength ((LAMBDA)) dependence of the polarization | polarized-light transmittance of the wavelength selection wavelength plate which concerns on a comparative example. 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 a design example. Explanatory drawing which shows the polarization state of each wavelength which permeate | transmits a wavelength selection wavelength plate by the track | orbit on a Poincare sphere. 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 Examples 1-6, and the plane schematic diagram which shows the relationship between the polarization direction of the linearly polarized light which injects, and an optical axis. 5 is a graph showing the wavelength dependence of ellipticity and azimuth angle according to the wavelength selection wavelength plate of Example 1. 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. In the wavelength selection wavelength plate according to the wavelength selection diffraction element of Example 6, the pretwist angle α ₁ can be obtained by the thickness (d ₁ ) of the first wavelength plate and the thickness (d ₂ ) of the second wavelength plate. The figure which shows distribution of a value. In the wavelength selective wave plate according to the wavelength selective diffraction element of Example 6, the pretwist angle α ₂ can be obtained by the thickness (d ₁ ) of the first wave plate and the thickness (d ₂ ) of the second wave plate. The figure which shows distribution of a value. 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 7. 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 8. 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 9.

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 wavelengths of light emitted in the -Y plane, the light of the 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 in the thickness direction. It is composed of a plurality of wave plates made of uniform birefringent material layers. The retardation value and optical axis direction of each wave plate are different and set within a predetermined range, so that desired characteristics can be obtained and the optical characteristics are large with respect to wavelength fluctuations of incident light. It is possible to obtain stable optical characteristics that do not change, that is, have 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 wave plate 23 is composed of two or more wave plates made of a birefringent material parallel to the plane of the wavelength selection wave plate 23 and having optical axes aligned in the thickness direction, so that these optical axes intersect. It is composed of overlapping. Specifically, the first wave plate 21 and the second wave plate 22 are provided, and parameters such as the thickness of each wave plate and the angle between the incident light and the optical axis are set as will be described later. Desired optical characteristics are obtained. Moreover, although the method of aligning the optical axis of the wave plate in the thickness direction varies depending on the birefringent material used, a known method can be used.

The first wave plate 21 and the second wave plate 22 constituting the wavelength selection wave plate 23 are stretched by stretching a birefringent crystal such as quartz or LiNbO ₃ or an organic material such as polycarbonate, acrylic, or polyester. A birefringent film having optical axes aligned in the direction can be used. In addition, a wave plate having a liquid crystal layer made of liquid crystal or polymer liquid crystal, a wave plate having structural birefringence, a wave plate formed by oblique deposition, a photonic crystal, or the like can be used. .

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 the case of such ideal transmitted light, when using light of a component with a specific polarization direction, there is no component light with a polarization direction orthogonal thereto, so there is no component that loses the amount of light, and a polarization diffraction element to be described later , 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 of transmission through 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 orthogonal to each other are given in a plane orthogonal to the optical axis of the light emitted from the wavelength selection wavelength plate 23, and incident light of wavelength λ ₁ and wavelength λ ₂ of the light and the wavelength lambda ₃ of the light, when one of the light emitting wavelength selective wave plate 23 relative to the first direction, the ΔΨ is in the range of -26~26 [°], use 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. In this way, one of the three wavelengths of light has a utilization efficiency η of 80% or more in the first direction, and the other two have a utilization efficiency η of 80% in the second direction. The conditions of each wave plate constituting the wavelength selection wave plate 23 having the above characteristics are set.

As described above, the wavelength selection wavelength plate 23 sets the parameters of the first wavelength plate 21 and the second wavelength plate 22 so as to obtain a desired polarization state with respect to the incident light of each wavelength. Specifically, the principle of the design using the Poincare sphere will be described. Here, the retardation value of the first wave plate 21 at the wavelength λ _R near the wavelength of the incident light is Rd ₁ (λ _R ), the retardation value of the second wave plate 22 is Rd ₂ (λ _R ), and the wavelength When the phase difference of the first wave plate 21 and the phase difference of the second wave plate 22 with respect to the light of λ _R are respectively Φ ₁ and Φ ₂ ,
_{Φ 1 = Rd 1 (λ R} ) / λ R = (m 1/2) λ R ··· (1a)
_{Φ 2 = Rd 2 (λ R} ) / λ R = (m 2/2) λ R ··· (1b)
(Where m ₁ and m ₂ are natural numbers),
It shall have the characteristic that holds.

At this time, the polarization state of the light transmitted through the wavelength selection wave plate 23 is represented by the Stokes parameter S, and the conditions of Rd ₁ (λ _R ) and Rd ₂ (λ _R ) are obtained by solving the Mueller matrix described later. it can. The Stokes parameter S can be represented by a normal (S ₀ , S ₁ , S ₂ , S ₃ ) four-dimensional vector, where S ₀ is the light intensity, and S ₁ is 0 ° with respect to the X-axis direction, for example. The intensity of the electric field oscillating in the direction, S ₂ means the intensity of the electric field oscillating in the 45 ° direction, and S ₃ means the intensity of circularly polarized light. Hereinafter, the Stokes parameter S will be described as a three-dimensional vector (S ₁ , S ₂ , S ₃ ) with the light intensity S ₀ omitted.

Here, the Stoke parameter S _a of the incident light using the Stokes parameter in the completely polarized state is S _a = (S ₁ , S ₂ , S ₃ ), and this light is the first wave plate 21, second light It is assumed that the light travels in the Z direction in the order of the wave plate 22 and enters. FIG. 2B is a schematic diagram showing a plane of the wavelength selection wavelength plate 23, where the polarization direction 24 of linearly polarized light incident on the wavelength selection wavelength plate 23 is the X-axis direction, and the first wavelength is based on this. The angle formed with the optical axis 25 of the plate 21 is α ₁ [°], and the angle formed with the optical axis 25 of the second wavelength plate 22 is α ₂ [°]. The concept of the sign of the angle is positive (+) counterclockwise and negative (−) clockwise with reference to the polarization direction 24 of the incident linearly polarized light as viewed from the light incident surface. The optical axis 25 of the first wave plate 21 and the optical axis 26 of the second wave plate are combinations of slow axes with each other or fast axes with each other. When there is no, it shall be a combination of slow axes. Α ₁ and α ₂ are also referred to as pretilt angles.

Further, the number of incident light beams having different wavelengths is set to n (an integer of n ≧ 2), k is any one integer between 1 and n, and the light of wavelength λ _k is the first wave plate 21. in case the phase difference generated was [Phi _1k, Stokes parameters _S kb ₌ representing the polarization state of light exiting the first wave plate _{21 (S 1kb, S 2kb,} S 3kb) has the following formula (2) Can be expressed as

Here, when light of wavelength λ ₁ (k = 1) is incident, the absolute value of the difference between the ordinary light refractive index n _o1 (λ ₁ ) and the extraordinary light refractive index n _e1 (λ ₁ ) of the first wave plate. Assuming that the refractive index anisotropy represented by Δn ₁ (λ ₁ ) and the thickness (gap) of the first wave plate 21 is d ₁ ,
Δn ₁ (λ ₁ ) × d ₁ = p × λ ₁ (3)
(Where p ≧ 2 is an integer),
And so as to adjust the thickness d _1. Further, when light of wavelength λ ₂ is incident, the Stokes parameter of the light emitted from the first wave plate 21 can be expressed as k = 2 in the above equation (2). Therefore, in this case, the phase difference can be expressed by [Phi _12.

FIG. 4 shows a Poincare sphere, where the latitude of polarization on the Poincare sphere when an arbitrary polarization state A is given is θ _2bS1 , and the polarization of polarization on the Poincare sphere is θ _2bS3 . Then, these can be represented by the following formulas (4a) and (4b). Here, for example, among “2bS1” of the angle θ _2bS1 , “2” represents the “k” -th wavelength, “b” means emitted light (from the first wave plate), and “S1” S Shall be related to the ₁ axis. In the equation _{(4a), S 12b} components of _{S 1} axis in A any _{point, S 22b} are components of the _{S 2} axis at A an arbitrary point and _{S 22b} are components of the _{S 2} axis at A an arbitrary point, Represents.

However, the _sign of θ _2bS1 is negative when S _22b <0. Similarly, θ _3bS1 and θ _3bS3 can be obtained using light with a wavelength λ ₃ which is a wavelength different from both the wavelengths λ ₁ and λ ₂ by using a Poincare sphere.

Next, a change in the polarization state of light transmitted through the second wave plate 22 will be described. When light having the wavelength λ ₁ is incident on the second wave plate 22, the refractive index difference represented by the absolute value of the difference between the ordinary light refractive index n _o2 (λ ₁ ) and the extraordinary light refractive index n _e2 (λ ₁ ). When the directivity is Δn ₂ (λ ₁ ) and the thickness (gap) of the second wave plate 22 is d ₂ ,
Δn ₂ (λ ₁ ) × d ₂ = {(2q + 1) / 2} × λ ₁ (5)
(Where q ≧ 0 is an integer),
And so as to adjust the thickness d _2.

Here, with respect to light having a wavelength lambda _k, when a phase difference generated by the second wave plate 22 and [Phi _2k, the phase difference of the wavelength lambda ₂ of the optical phase difference [Phi ₂₂ and the wavelength lambda ₃ of the light [Phi ₂₃ ,
Φ ₂₂ ± θ _2bS3 ≒ iπ (6a)
_{Φ 23 ± θ 3bS3 ≒ jπ ···} (6b)
(Where i and j are integers),
The filled, further, the wavelength lambda ₁ of the light, the wavelength lambda ₂ of the light and the wavelength lambda ₃ of the light, one of the polarization state of the light wavelength, be a different from the polarization state of light of other wavelengths Thus, the wavelength selectivity of the polarization state of the emitted light can be given. At this time, in particular, by adjusting the wavelength dependency of the refractive index anisotropy Δn (λ), that is, the chromatic dispersion characteristic, the above characteristic can be adjusted.

Also, in order to make the polarization state of the light of wavelength λ ₁ out of the light emitted from the second wave plate 22 orthogonal to the polarization state of the light of wavelength λ ₂ and / or the polarization state of the light of wavelength λ ₃ The pretwist angle α ₂ [°], which is an angle formed by the polarization direction of the linearly polarized light incident on the first wave plate 21 and the optical axis of the second wave plate 22, is
45+ (θ _3bS1 / 4) −10 ≦ α ₂ ≦ 45 + (θ _3bS1 / 4) +10
... (7a)
45+ (θ _2bS1 / 4) −10 ≦ α ₂ ≦ 45 + (θ _2bS1 / 4) +10
... (7b)
−45+ (θ _3bS1 / 4) −10 ≦ α ₂ ≦ −45 + (θ _3bS1 / 4) +10
... (7c)
−45+ (θ _2bS1 / 4) −10 ≦ α ₂ ≦ −45 + (θ _2bS1 / 4) +10
... (7d)
Satisfy any of the following.

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 values, the value of utilization efficiency η does not change.

Further, even when the signs of α ₁ and α ₂ are changed with respect to the original values, the utilization efficiency η does not change. Furthermore, if each of the values of α ₁ and α ₂ is a value (angle) obtained by adding or subtracting 90 ° to the original value, the optical axes have been described as slow axes. Therefore, utilization efficiency η does not change.

  In the above description, the first wave plate 21 and the second wave plate 22 are described in the order in which light is incident. However, when the polarization direction of the incident light is used as a reference, the optical axis of the first wave plate 21 and Are arranged in the same manner and the angle formed by the optical axis of the second wave plate 22 is the same, the order in which light is incident is reversed (the order of the second wave plate 22 and the first wave plate 21). Even in this case, wavelength selectivity having similar optical characteristics is generated. Note that the first wave plate 21 and the second wave plate 22 are made of different materials even if they are made of the same material as long as they satisfy the wavelength dispersion characteristics of the refractive index of the incident light. May be. If they are made of the same material, for example, even if deformation due to temperature change occurs, large distortion or the like does not occur between these wave plates, which is preferable in terms of structure.

Next, the relationship between the wavelength of incident light, the thickness of the first wave plate 21 and the thickness of the second wave plate 22, and the wavelength dispersion characteristic of the refractive index of the intrinsic birefringent material is generalized. Think about it. 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 The refractive index anisotropy Δn (λ), Δn (λ) for the light of each wavelength, where n _o (λ _A ) is the ordinary refractive index of the wave plate for the light of and n _e (λ _A ) is the refractive index of the extraordinary light. _A )
_{Δn (λ) = | n e} (λ) -n o (λ) | ··· (8a)
Δn (λ _A ) = | n _e (λ _A ) −n _o (λ _A ) | (8b)
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 } (9)
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 (10a)
d ₂ (λ _A ) = Δn (λ _A ) · d ₂ / λ _A (10b)
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.

At this time, d ₁ (λ _A ) and d ₂ (λ _A ) of the wavelength selection wavelength plate 23 composed of the first wave plate 21 and the second wave plate 22 are shown in design example 1 in Table 1 below. As shown, the pre-twist angles α ₁ and α ₂ were set to ₂ , ₁ and ₂ , respectively. As Comparative Example 1, conditions for the case where the second wave plate 22 is not provided are also shown.

  Under the conditions in Table 1, the polarization direction of linearly polarized light incident on the wavelength selection wave plate 23 is 0 ° (X-axis direction in FIG. 2), and the transmission polarization direction of linearly polarized light is 0 ° on the output side. Consider the transmittance characteristics when a polarizer is installed. FIG. 5 shows the polarization transmittance Tp [%] with respect to Λ defined by Equation (9) under the conditions of Design Example 1. The light intensity of the linearly polarized light in the 0 ° direction incident on the wavelength selection wave plate 23 is 100%.

From FIG. 5, the band where the polarization transmittance Tp is near 100% and the band near 0% represent a state in which the outgoing polarization is orthogonal. That is, when the polarization direction of the linearly polarized light incident on the wavelength selection wavelength plate 23 is the X axis direction, the polarization direction of the emitted light is substantially the X axis direction when the polarization transmittance Tp is near 100%. When the polarization transmittance Tp is around 0%, the polarization direction of the emitted light is substantially the Y-axis direction. Therefore, when, for example, two lights out of a plurality of wavelength bands incident on the wavelength selection wavelength plate 23 are emitted as linearly polarized lights orthogonal to each other, the wavelength bands of the respective lights have a polarization transmittance Tp. By adjusting d ₁ and d ₂ so as to be a wavelength band that functions as a half-wave plate that becomes 0% and a wavelength band that functions as a wavelength plate (λ plate) where Tp becomes 100%, an arbitrary value can be obtained. It can be set as the wavelength selection wavelength plate which functions selectively with respect to a wavelength range.

  In FIG. 5, the value of Λ at which the polarization transmittance Tp is about 100% and the value of Λ at which the polarization transmittance Tp is about 0% are obtained. When considering the combination of the wavelengths of the three light beams incident on the wavelength selection waveplate, one of the three wavelengths is set to have a Tp of about 100% and the other two have a combination of about 0% Tp. Alternatively, the two may be designed to have a combination of about 100% Tp and the other about about 0% Tp. When considering the relationship between the polarization transmittance Tp and the above utilization efficiency η, it is preferable that the utilization efficiency η is 80% or more as described above. Therefore, Tp = 100% is an ideal wavelength band. It is preferable that Tp ≧ 80% for light and Tp ≦ 20% can be realized for light in a wavelength band where Tp = 0% is ideal.

  Further, it is more preferable that Tp ≧ 90% for light in an ideal wavelength band where Tp = 100% and Tp ≦ 10% can be realized for light in an ideal wavelength band where Tp = 0%. Further, it is more preferable that Tp ≧ 95% with respect to light in a wavelength band where Tp = 100% is ideal, and Tp ≦ 5% can be realized with respect to light in a wavelength band where Tp = 0% is ideal. In this way, it is preferable to design the utilization efficiency η to be 80% or more in the subsequent design examples with respect to the combination of light of three wavelengths incident on the wavelength selection wavelength plate.

As a specific design method, for example, the retardation value Rd ₁ (λ ₁ ) of the first wave plate 21 is s ₁ · λ ₁ and the retardation of the second wave plate 22 with respect to light of wavelength λ _1. The value Rd ₂ (λ ₁ ) is set to s ₂ · λ ₁ (integers of s ₁ , s ₂ ≧ 1), and each of the first wave plate 21 and the second wave plate 22 is an integral multiple of λ ₁ The center value of the design is determined under the condition of the wavelength plate. Then, for the entire Λ, the calculation is repeated so that the use efficiency η is 80% or more, that is, the range of Λ in which Tp ≧ 80% and Tp ≦ 20% is increased. Each design condition of the first wave plate 21 and the second wave plate 22 can be obtained by obtaining a desired Tp.

  FIG. 6 shows the polarization transmittance Tp [%] with respect to Λ defined by Equation (9) under the conditions of Comparative Example 1. Comparing FIG. 5 with FIG. 6, FIG. 5 under the conditions of design example 1 shows that the band where the polarization transmittance Tp is near 100% and the band near 0% are widened, and the band of the transient region between them is narrowed. I understand that. On the other hand, in FIG. 6 under the conditions of Comparative Example 1, the band where the polarization transmittance Tp is near 100% and the band near 0% are narrowed, and the arbitrary wavelength selection is lower than that in Design Example 1. That is, compared with the design example 1, in the comparative example 1, it is not possible to obtain many combinations of light having a wavelength of Tp = 0% and light having a wavelength of Tp = 100%. If Tp is set to a desired value (0% or 100%), Tp of the light of the other wavelength may be at an intermediate level. Thus, it can be seen that the wavelength range that functions as the wavelength selection wavelength plate in the design example 1 is widened, and the wavelength-dependent characteristics are greatly improved compared to the comparative example 1.

As described above, in the design example 1, the pre-twist angles α ₁ and α ₂ are 16 ° and −43 °, 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. In the case of making the light emitted from the wavelength selection wavelength plate 23 orthogonal to each other in any combination of wavelength bands, the pretilt angle α is set with α ₁ and α ₂ of the design example 1 as a reference. ₁ ′, α ₂ ′,
α ₁ ′ = α ₁ + θ (11a)
α ₂ ′ = α ₂ + k · θ (11b)
(K is a coefficient and θ is an angle [°]). FIG. 7 shows the polarization transmittance Tp with respect to Λ defined by the equation (9) when k = 1.5 is fixed and θ = 0, 5, 10, 15, 20, 25 [°]. [%].

  As shown in FIG. 7, the polarization transmittance Tp in a specific wavelength band changes by changing the value of θ. Thus, the combination of wavelength bands for wavelength selection can be controlled by adjusting the pre-twist angle. Also, the range that k can take is the range of 1 to 2, and when changing from 1 to 2, the control range of the band is widened, but the change of Tp with respect to the change of Λ becomes gentle. When the combination is a wavelength band close to each other or a wavelength band relatively far away, the value of k may be set according to the target specification. Further, the range that θ can take is also established in the range of 0 to 20 ° and in the range of 0 to −20 °, and is generally about −20 ° to + 20 °.

  The following shows an example of the range that each parameter value can take when the design example 1 shown above is set as an initial value, the light of the three target wavelength bands is actually designed and optimized. The assumed combination of three wavelengths is 405 nm, 660 nm, 790 nm, or around 460 nm, 530 nm, and 660 nm. As an optimization method, it is possible to easily change each parameter individually so that the polarization transmittance approaches a desired value.

Here, W (λ) is a parameter depending on the wavelength λ,
W (λ) = Δn (λ) · d / λ (12)
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. Further, for example, using the equation (12), 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 the parameter of the first wave plate 22 for the light with the wavelength λ _a based on the above equation (12).

Here, the magnitude relationship between the three wavelengths of interest is considered as λ _a <λ _b <λ _c . [Delta] W _ba, consider [Delta] W _cb as a variable, pre-twist angle alpha ₁ of the first wave plate 21, the pre-twist angle alpha ₂ of the second wave plate 22, _{W 1} of the first wave plate in the optical wavelength lambda _a When the condition of W ₂ (λ _a ) of the second wave plate in the light of (λ _a ) and wavelength λ _a was calculated, the following results were obtained.

As will be described later, when considering the three wavelengths as a combination of wavelengths for an optical head device and a combination of wavelengths for a projection display device, in consideration of the birefringent material to be used, the practical range is as follows: In many cases, it may be sufficient to design it. Here, α ₁ , α ₂ , W ₁ (λ _a ) and W ₂ (λ _a ) are used as parameters, and
α ₁ = 20 °,
α ₁ −α ₂ = 55 °,
W ₁ (λ _a ) = 2.0,
W ₁ (λ _a ) / W ₂ (λ _a ) = 2.0,
The optimum value can be easily obtained by designing the wavelength selection wave plate 23 with the parameter as the center value.

Further, as a range of changing the parameters of the above, the value of [Delta] [alpha] ₁ changing relative alpha ₁ in the range of -10 ° ~ + 10 °, ( α 1 -α 2) Δ is changed with respect to (alpha ₁ -Α ₂ ) is in the range of −10 ° to + 10 °, ΔW ₁ (λ _a ) is changed with respect to W ₁ (λ _a ) is in the range of −0.15 to +0.15, and , {W ₁ (λ _a ) / W ₂ (λ _a )}, the value of Δ {W ₁ (λ _a ) / W ₂ (λ _a )} is in the range of −0.15 to +0.15. Should be assumed.

Next, let us consider a case where the magnitude relationship between the three wavelengths of interest satisfies λ _b <λ _a <λ _c . When α ₁ , α ₂ , W ₁ (λ _a ), and W ₂ (λ _a ) are calculated, 0.25 <W ₁ (λ _b ) / W ₁ (λ _a ) ≦ 0.75 If in the practical range as above,
α ₁ = 20 °,
α ₁ −α ₂ = 55 °,
W ₁ (λ _a ) = 1.9,
W ₁ (λ _a ) / W ₂ (λ _a ) = 2.0,
In many cases, it is sufficient to design.

Or
W ₁ (λ _a ) = 2.2,
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 above, the value of [Delta] [alpha] ₁ changing relative alpha ₁ in the range of -10 ° ~ + 10 °, is to delta (alpha changes with respect to (α ₁ -α _{2) 1} −α ₂ ) is in the range of −10 ° to + 10 °, and ΔW ₁ (λ _a ) that is changed with respect to W ₁ (λ _a ) is in the range of −0.2 to +0.2. Then, the value of Δ {W ₁ (λ _a ) / W ₂ (λ _a )} to be changed with respect to {W ₁ (λ _a ) / W ₂ (λ _a )} is −0.15 to +0.15. The range should be assumed.

In the case of 0.75 <W ₁ (λ _b ) / W ₁ (λ _a ) <1.25, in the practical range as described above,
α ₁ = 40 °,
α ₁ −α ₂ = 45 °,
W ₁ (λ _a ) = 2.0,
2.0 <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 above, the value of [Delta] [alpha] ₁ changing relative alpha ₁ in the range of -10 ° ~ + 10 °, is to delta (alpha changes with respect to (α ₁ -α _{2) 1} −α ₂ ) ranges from −10 ° to + 10 °, and the value of ΔW ₁ (λ _a ) that changes with respect to W ₁ (λ _a ) ranges from −0.15 to +0.15. The range should be assumed.

Next, let us consider a case where the magnitude relationship between three target wavelengths is λ _b <λ _c <λ _a . Then, α ₁ , α ₂ , W ₁ (λ _a ), and W ₂ (λ _a ) are obtained by calculation. When W ₁ (λ _b ) / W ₁ (λ _a ) ≦ 0.5, So that in a practical range,
α ₁ = 20 °,
α ₁ −α ₂ = 55 °,
W ₁ (λ _a ) = 2.0,
W ₁ (λ _a ) / W ₂ (λ _a ) = 2.0,
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 above, the value of [Delta] [alpha] ₁ changing relative alpha ₁ in the range of -10 ° ~ + 10 °, is to delta (alpha changes with respect to (α ₁ -α _{2) 1} −α ₂ ) is in the range of −10 ° to + 10 °, and ΔW ₁ (λ _a ) that is changed with respect to W ₁ (λ _a ) is in the range of −0.15 to +0.15, Then, the value of Δ {W ₁ (λ _a ) / W ₂ (λ _a )} to be changed with respect to {W ₁ (λ _a ) / W ₂ (λ _a )} is −0.15 to +0.15. The range should be assumed.

Further, when W ₁ (λ _b ) / W ₁ (λ _a )> 0.5, in the practical range as described above,
α ₁ = 30 °,
α ₁ −α ₂ = 37 °,
W ₁ (λ _a ) = 1.7 or 2.7
W ₂ (λ _a ) = 1.0
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 above, the value of [Delta] [alpha] ₁ changing relative alpha ₁ in the range of -10 ° ~ + 10 °, is to delta (alpha changes with respect to (α ₁ -α _{2) 1} −α ₂ ) is in the range of −10 ° to + 10 °, and ΔW ₁ (λ _a ) that is changed with respect to W ₁ (λ _a ) is in the range of −0.15 to +0.15, Then, the value of Δ {W ₁ (λ _a ) / W ₂ (λ _a )} to be changed with respect to {W ₁ (λ _a ) / W ₂ (λ _a )} is −0.15 to +0.15. It is sufficient to assume a range.

Next, the light of wavelength λ _1, the light of wavelength λ _{2 and} the light of wavelength λ ₃ are converted into light of wavelengths corresponding to the optical head device, specifically wavelength λ ₁ = 405 nm, wavelength λ ₂ = 660 nm, and wavelength, respectively. Consider λ ₃ = 785 nm. Each of the first wave plate 21 and the second wave plate 22 for orthogonal polarization states of the wavelength lambda ₁ of the light, and a polarization state of the polarization state and wavelength lambda ₃ of the light of the wavelength lambda ₂ of light Parameters shall be set. For each parameter, the case of λ _a <λ _b <λ _c was used. A wavelength range of 395 nm to 420 nm is defined as a 405 nm wavelength band, a wavelength range of 640 nm to 680 nm is defined as a 660 nm wavelength band, and a wavelength range of 765 nm to 805 nm is defined as a 785 nm wavelength band.

At this time, with respect to the wavelength lambda ₁ of the light, the retardation value of the first wave plate 21 Rd ₁ (lambda ₁₎ is 2 [lambda] ₁ (= 810 nm), and the retardation value of the second wave plate Rd ₂ (lambda ₁ ) Is set to about λ ₁ (≈406 nm) to obtain a wavelength selective wave plate 23 having wavelength selectivity. At this time, as a material for forming the first wave plate 21 and the second wave plate 22, a material having general birefringence can be used, and refractive index anisotropy Δn with respect to light having a wavelength of 405 nm. The dispersion ratio defined by the ratio of refractive index anisotropy Δn (660) to light having a wavelength of 660 nm with respect to (405) can be realized, for example, in a material in the range of 1.0 to 1.3.

Note that the retardation value Rd ₁ (λ ₁ ) of the first wave plate 21 is 2λ ₁ with respect to light of wavelength λ ₁ , but Rd ₁ (λ ₁ ) is an integer multiple of 3 or more of λ _1. Almost the same value may be used. Furthermore, although the retardation value Rd ₂ (λ ₁ ) of the second wave plate is set to about λ ₁ with respect to the light with the wavelength λ ₁ , the retardation value Rd ₂ (λ ₁ ) is approximately the same as an integer multiple of 0.5 of λ _1. Good.

Next, a change in the polarization state until light of wavelength λ ₁ , light of wavelength λ ₂ , and light of wavelength λ ₃ enters and exits the wavelength selection wavelength plate 23 will be specifically described. Figure 8 is a Poincare sphere that represents the change in the polarization state of light of each wavelength, 8 (a) is the wavelength lambda ₁ of light, FIG. 8 (b) the wavelength lambda ₂ of the light and Figure 8, (c) is for light having a wavelength lambda _3. The light of any wavelength incident on the wavelength selection wave plate 23 is a linearly polarized light having an S _1- axis component, that is, a vibration direction in the X-axis direction.

FIGS. 8A, 8B, and 8C schematically show the locus on the Poincare sphere until the polarization state of the light emitted from the wavelength selection wavelength plate 23 is reached. . The light having the wavelength λ ₁ (= 405 nm) passes through the locus 41a on the Poincare sphere in the first wavelength plate 21, and then exits through the locus 42a on the Poincare sphere in the second wavelength plate 22. As shown in FIG. 8A, the light is emitted as linearly polarized light incident on the first wave plate 21 in the X-axis direction.

The light having the wavelength λ ₂ (= 660 nm) passes through the locus 41b on the Poincare sphere in the first wavelength plate 21, and then exits through the locus 42b on the Poincare sphere in the second wavelength plate 22. FIG. As shown in (b), the light is emitted as linearly polarized light in the Y-axis direction orthogonal to the linearly polarized light in the X-axis direction incident on the first wave plate 21. The light having the wavelength λ ₃ (= 785 nm) also passes through the locus 41c on the Poincare sphere in the first wavelength plate 21 and the locus 42c on the Poincare sphere in the second wavelength plate 22 as shown in FIG. 8C. The light is emitted as linearly polarized light in the Y-axis direction.

In this way, each wavelength of light has a locus on the Poincare sphere. Among these, the first wavelength plate 21 functions as a 2λ plate that generates a phase difference of 2λ ₁ , especially for light of the wavelength λ _1. Furthermore, since the second wave plate 22 functions as a λ plate that generates a phase difference of λ ₁ , the second wave plate 22 follows a locus that returns to the same position as the incident light. Further, as shown in FIG. 8, the light having the wavelength λ ₁ has a longer moving distance on the Poincare sphere than the locus of the light having the wavelength λ _{2 and} the light having the wavelength λ ₃ . This is arranged so that the light beam having the wavelength λ ₁ moves back when it passes through the first wave plate 21 and returns to the original position by the locus that moves when it passes through the second wave plate 22. This is because the polarization direction is orthogonal to the polarization direction of the light of wavelength λ ₁ without having a large wavelength dependence on the light of wavelength λ _{2 and} the light of wavelength λ ₃ .

Further, although the wavelength λ ₁ is 405 nm, the polarization state of the light emitted from the wavelength selection wavelength plate 23 does not change greatly even for light having any wavelength in the 405 nm wavelength band, which is the wavelength band of 395 to 420 nm. Stabilize. Similarly, light of any wavelength in the 660 nm wavelength band having the wavelength λ ₂ of 640 to 680 nm and light of any wavelength of the 785 nm wavelength band having the wavelength λ ₃ of 765 to 805 nm. The polarization state of the light emitted from the wavelength selection wavelength plate 23 is not greatly changed and is stabilized.

In the above description, the polarization direction of the light having the wavelength λ ₁ emitted from the wavelength selection wavelength plate 23, the polarization direction of the light having the wavelength λ ₂ exiting the wavelength selection wavelength plate 23, and the polarization direction of the light having the wavelength λ ₃ are made. Although the angle is set to 90 °, it is not limited to this. The angle formed by the polarization direction of light incident on the wavelength selection wave plate 23 and the optical axis of the first wave plate 21, and the angle formed by the optical axis of the first wave plate 21 and the optical axis of the second wave plate 22. Is adjusted, for example, with respect to the light of wavelength λ ₁ , the light of wavelength λ ₂ , and the light of wavelength λ ₃ incident in the same polarization direction, the polarization direction of the light of wavelength λ ₁ emitted from the wavelength selection wavelength plate 23 And a design that can arbitrarily give a difference between the polarization directions of the light of wavelength λ _{2 and} the light of wavelength λ ₃ emitted from the wavelength selection wavelength plate 23. Even in such a case, the dispersion characteristic of the refractive index of the material constituting the first wave plate 21 and the second wave plate 22 is utilized. Also good.

Further, the light having the wavelength λ _1, the light having the wavelength λ ₂ , and the light having the wavelength λ ₃ have been specifically exemplified as the wavelengths used in the optical head device, but the present invention is not limited thereto. For example, in addition to this, a 450 nm wavelength band that becomes Blue (420 to 480 nm), a 533 nm wavelength band that becomes Green (520 to 560 nm), and a Red (red) that is used for a projection display device (projector) using three colors of laser light. A wavelength band of 645 nm that is 610 to 670 nm) may be designed as these wavelength λ ₁ , wavelength λ _2, and wavelength λ ₃ , respectively.

Next, the function of the polarization diffraction element 12 will be described. In FIG. 2 (a), the wavelength lambda ₁ of light and the wavelength lambda ₂ of light as an example, and the light of the wavelength lambda _3, and proceeds from the first wave plate 21 as the light in the X direction of the linear polarized light in the Z direction, by the wavelength selective wave plate 23, light of the wavelength lambda ₁ is light in the X direction of the linear polarization of light, the wavelength lambda ₂ of the light and the wavelength lambda ₃ enters the polarization diffraction element 12 is a light linearly polarized in the Y-direction Think about the case. Here, the polarization diffraction element 12 linearly transmits light having a wavelength λ ₁ that is incident as linearly polarized light in the X direction, and changes the wavelength λ ₁ that is incident into linearly polarized light in the Y direction by the wavelength selection wavelength plate 23. The light of wavelength _{2 and} the light of wavelength λ ₃ have a 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 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 that are incident on each other 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. Further, 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. The structure which adjusted the longitudinal direction may be sufficient.

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)
This embodiment is an optical head device in which a wavelength selective diffraction element is arranged 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. 9 is 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 that have passed through the polarization beam splitter 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 and becomes linearly polarized light orthogonal to the forwardly polarized light when passing through the quarter wavelength plate 108, and is reflected by the reflection mirror 107. , Reflected by the collimator lens 106 and 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 light of the three wavelengths causes the concave lens function to be generated only for the light of the 785 nm wavelength band for the CD by the wavelength selective diffraction element 50 so that all of the photodetectors 112 reach the same position, and the return path The focal length of the light is adjusted. In FIG. 9, the trajectory of light in the 405 nm wavelength band for BD and the light in the 660 nm wavelength band for DVD is indicated by a solid line, and the trajectory of light in the 785 nm wavelength band for CD is indicated by a dotted line.

  Next, the wavelength selective diffraction element 50 will be specifically described. FIG. 10A 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 described in the embodiment of the wavelength selection wavelength plate, the wavelength selection wavelength plate 51 uses a configuration in which a wavelength plate having optical axes aligned in the thickness direction is laminated in two layers with the optical axes crossing each other. . The polarization diffraction element 52 includes a birefringent material layer 54 a formed on a transparent substrate 53 as a diffraction grating 54 having a Fresnel lens cross section, and a transparent material 55 formed 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.

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. 10B is a diagram for explaining the shape of the Fresnel lens that generates the concave lens action, and the curve α in FIG. 10B indicates the difference in the amount of change in the phase received by the light after passing through the lens. Is shown. Here, in the case of the function of the concave lens, the distribution of the difference in the amount of change in phase is expressed by Expression (13).
φ (r) = a ₁ r ² + a ₂ r ⁴ + a ₃ r ⁶ + a ₄ r ⁸ +... (13)
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. 10C 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 transparent material 55 refractive index _{n s} of (lambda ₃₎ is not limited to the case to match the ordinary refractive index _{n o} of the birefringent material layer (lambda _3), and an extraordinary refractive index _{n e} (λ ₃₎ match, the fast axis direction 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. 11 is a schematic diagram of a compatible optical head device 200 that records and reproduces optical discs of the respective standards 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 light in the return path of each wavelength band reflected from the optical disk 208a or 208b becomes linearly polarized light orthogonal to the forward linear polarization when reciprocating the quarter-wave plate 206a or 206b, and reaches the polarization beam splitter 202 in the same optical path as the forward path. move on. 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. 12A 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 described in the embodiment of the wavelength selection wavelength plate, the wavelength selection wavelength plates 61 and 62 have a configuration in which the wavelength plates with the optical axes aligned in the thickness direction are stacked in two layers with the optical axes intersecting each other. Is used.

For example, the optical head device 200 uses the one-beam method only for light in the 405 nm wavelength band, and uses the three-beam method for generating three beams by diffracting light in the 660 nm wavelength band and light in the 785 nm wavelength band. Considered as a wavelength selective diffraction element 60 adapted to the case. 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 Z direction and transmitting straightly only with respect to linearly polarized light in the Y direction, the polarization diffraction element 12 has light of 660 nm wavelength band and 785 nm wavelength band. For light, three 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 linearly polarized 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 in a specific wavelength band.

  As described above, by aligning the polarization direction of the linearly polarized light of each wavelength that has passed through the wavelength selective diffraction element 60, the forward light and the backward light of each wavelength incident on the polarization beam splitter 202 of the optical head device 200. Therefore, the optical head device can be downsized. 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. 12B is a schematic cross-sectional view showing the configuration of the wavelength selective diffraction element 70, 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.

  The wavelength selection wave plates 71a and 71b have a configuration in which, as described in the embodiment of the wavelength selection wave plate, the wave plates whose optical axes are aligned in the thickness direction are laminated in two layers with the optical axes crossing each other. Is used. In FIG. 12B, 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 pitches and different diffraction angles for light having a wavelength to be diffracted. May be designed to be different. Here, the slow axis directions of the polarization diffraction elements 72a and 72b are the Y direction, the transparent material 75 has a refractive index equivalent to the ordinary light refractive index of the birefringent material layer 74a, and the transparent material 78 is a birefringent material. The layer 77a has a refractive index equivalent to the ordinary light refractive index.

Here, the demultiplexing function described as the first function of the wavelength selective diffraction element 70 will be described. The light of each wavelength reflected by the polarization beam splitter 202 of the optical head device 200 enters 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 uses light in each wavelength band in common, the signal light from each optical disk can be accurately reached. The apparatus 200 can be downsized.

  In FIGS. 12A and 12B, 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. 13 is a schematic diagram 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. 14 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 configuration in which a wavelength plate having optical axes aligned in the thickness direction is laminated in two layers with the optical axes crossing each other. . In FIG. 14, 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. 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. 14 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. 14, the wavelength selective diffraction 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 light in each wavelength band possessed by each wavelength selective wavelength plate. 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)
Up to now, 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 display device. FIG. 15 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 (520 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 the light in these three wavelength bands 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 light.

  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 lights 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 wave plate 305 converts linearly polarized light perpendicular to the direction of the linearly polarized light incident on the red light and the green light, that is, converts it into linearly polarized light in the X direction. Transmit without changing the polarization state. As a result, the three colors of light incident on the collimator lens 306 become 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.

(Example based on wavelength selective wave plate and wavelength selective diffraction element)
The present embodiment is based on the configuration of the wavelength selection wave plate 20. FIG. 16A shows the configuration of the wavelength selection wavelength plate 400 based on the first embodiment, and an example of the manufacturing method and design will be described.

  A polymer liquid crystal layer 403a in which a polymer liquid crystal is aligned in the alignment processing direction of the alignment film 402a on the transparent substrate 401a on which the alignment film 402a subjected to the horizontal alignment process is formed on one side, and has a thickness of about 8.6 μm. Form. The formed polymer liquid crystal layer 403a has 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 0.075 for light with a wavelength of 785 nm.

  Next, in a similar manufacturing method, a polymer liquid crystal layer 403b made of the same polymer liquid crystal material and having a film thickness of about 4.3 μm is formed on the transparent substrate 401b on which the alignment film 402b subjected to the horizontal alignment treatment is formed on one side. To do. Note that the first wave plate 21 and the second wave plate 22 of the wavelength selection wave plate 20 in FIG. 2 correspond to the polymer liquid crystal layers 403a and 403b in FIG. 16A, respectively.

  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. The wavelength selection wave plate 407 is formed.

First, the optical characteristics of the manufactured wavelength selection wavelength plate 407 are examined. When the polarization direction 410 of the incident linearly polarized light shown in FIG. 16B is used as a reference, the angle α ₁ formed by the alignment direction corresponding to the slow axis 411 of the polymer liquid crystal layer 403a that is the first wavelength plate is + 18 °, the angle alpha _2, which forms an alignment direction corresponding to the slow axis 412 of the liquid crystal polymer layer 403b is a second wave plate is -44 °, and so as to adjust the polarization direction. Further, an example under the conditions of each of the above parameters is set as Example 1, and is summarized and shown in Table 2. Since the polymer liquid crystal layers 403a and 403b have optical axes aligned in the thickness direction, the parameters β ₁ and β ₂ relating to the twist angle are each 0 [°].

  Under the conditions of Example 1, the optical characteristics were calculated when the wavelength of the incident light was changed without changing the polarization direction of the incident linearly polarized light. FIG. 17 shows the calculation of the continuous wavelength (λ) dependency of the ellipticity κ and the azimuth angle Ψ of the light emitted from the wavelength selection wavelength plate 407 at this time. The azimuth angle Ψ is an angle formed by the polarization direction of the outgoing linearly polarized light or the major axis direction of the outgoing elliptically polarized light with respect to the polarization direction of the incident linearly polarized light. Similarly, it is plus (+) counterclockwise and minus (−) clockwise relative to the direction of linearly polarized light incident as seen from the light incident surface.

  From this result, the light in the 405 nm wavelength band has almost zero ellipticity κ and azimuth angle ψ, and the polarization direction of the outgoing light and the polarization direction of the incident linearly polarized light are almost the same. The light in the 660 nm wavelength band and the light in the 785 nm wavelength band are emitted with an ellipticity κ of approximately 0 but an azimuth angle Ψ of approximately 90 °. Therefore, the light emitted in these wavelength bands can obtain linearly polarized light having an azimuth angle Ψ of about 90 °, that is, linearly polarized light substantially orthogonal to the polarization direction of the incident linearly polarized light. .

  FIG. 18 shows the light use efficiency η with respect to light in each wavelength band (λ). FIG. 18A corresponds to the ratio (component) of the light intensity at the azimuth angle Ψ = 0 [°], which is the polarization direction of the light used, of the light intensity emitted from the wavelength selection wavelength plate 407 in the 405 nm wavelength band. This shows the utilization efficiency η. 18 (b) and 18 (c) show the azimuth angle Ψ which is the polarization direction of the light to be used out of the light intensity emitted from the wavelength selection wavelength plate 407 in the light in the 660 nm wavelength band and the light in the 785 nm wavelength band, respectively. = Utilization efficiency η corresponding to the ratio (component) of light intensity at 90 °. In this case, the wavelength dependence is small and high utilization efficiency can be obtained.

Here, an example in which 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 azimuth angle with respect to light with a wavelength of 785 nm is Ψ ₃ [°]. Table 3 summarizes the results of 1. Table 3 also shows the positional relationship with the optical axis of the polarization diffraction element 408 (diffraction grating 405), which will be described later, in accordance with the azimuth angle of the light emitted from the wavelength selection wavelength plate 407.

  Next, a method for manufacturing the wavelength selective diffraction element 400 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 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.

The slow axis of the polymer liquid crystal layer 403a and the longitudinal direction of the diffraction grating 405 (the slow axis direction of the diffraction grating 405) are 44 ° on the side of the diffraction grating 405 and the opposite side of the alignment substrate 402b of the transparent substrate 401b. Make them face each other. That is, when viewed from the transparent substrate 401a side of the wavelength selection wavelength plate 407, the longitudinal direction (slow axis) of the diffraction grating 405 is positioned in the direction of 18 ° clockwise with respect to the slow axis of the polymer liquid crystal layer 403a. Like that. Then, using a transparent adhesive 406 the ordinary refractive index of the polymer liquid crystal (n _o) substantially equal uniform refractive index to form a diffraction grating 405 fills the concave portion of the diffraction grating 405, wavelengths including polarizing diffraction element 408 The selective diffraction element 400 is formed.

As shown in FIG. 16 (b), when a laser beam that is 405 nm linearly polarized light having a polarization direction 410 is incident on the manufactured wavelength selection wavelength plate 407, the zero-order diffraction efficiency (= straight transmission) η ₀ (405 ) Is 0.5% or less, while ± 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.

In Example 2 and Example 3, Example 1 the thickness of the polymer liquid crystal layer 403a as shown in Table 2 using the same manufacturing process as d ₁ and the polymer of the liquid crystal layer 403b thickness d _2, and Optical characteristics similar to those of Example 1 were calculated under conditions adjusted by changing the arrangement so as to satisfy α ₁ and α ₂ .

In Example 2, according to the conditions shown in Table 2, as shown in Table 3, the azimuth angles Ψ ₁ and Ψ ₂ for light with a wavelength of 405 nm and light with a wavelength of 660 nm are 84 °, and the azimuth angle with respect to light with a wavelength of 785 nm. Ψ ₃ is −6 [°], and the angles formed by ψ ₁ and ψ ₃ and ψ ₂ and ψ ₃ can be orthogonal. Further, by making the wavelength selective diffraction element 400 with the slow axis that is the orientation direction of the polarization diffraction element 408 (diffraction grating 405) being set to −6 [°], incident light in the 405 nm wavelength band and 660 nm wavelength band Thus, it is possible to obtain the wavelength selective diffraction element 400 that allows the light to travel straightly with high efficiency and diffract the incident light with a wavelength of 785 nm with high efficiency.

  19 (a), 19 (b), and 19 (c) show the 405 nm wavelength band, the 660 nm wavelength band, and the 785 nm wavelength band for the wavelength selective diffraction element 400 manufactured based on the conditions of Example 2. FIG. The light utilization efficiency η with respect to light is shown. In this case, the wavelength dependency is small and high utilization efficiency η can be obtained.

In Example 3, according to the conditions shown in Table 2, as shown in Table 3, the azimuth angles Ψ ₁ and Ψ ₃ with respect to light with a wavelength of 405 nm and light with a wavelength of 785 nm are 124 [°], and azimuth angles with respect to light with a wavelength of 660 nm. Ψ ₂ is 34 [°], and the angles formed by ψ ₁ and ψ ₂ and ψ ₂ and ψ ₃ can be made orthogonal. Further, by making the wavelength selective diffraction element 400 with the slow axis that is the orientation direction of the polarization diffraction element 408 (diffraction grating 405) being set to 34 [°], incident light in the 405 nm wavelength band and 785 nm wavelength band It is possible to obtain the wavelength selective diffractive element 400 that transmits the light straightly with high efficiency and diffracts incident light with a wavelength band of 660 nm with high efficiency.

  20 (a), 20 (b), and 20 (c) show the 405 nm wavelength band, the 660 nm wavelength band, and the 785 nm wavelength band for the wavelength selective diffraction element 400 manufactured based on the conditions of Example 3. FIG. The light utilization efficiency η with respect to light is shown. In this case, the wavelength dependency is small and high utilization efficiency η can be obtained.

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. This is effective when using.

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. 21A, FIG. 21B, 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. 22A, FIG. 22B, 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.

Example 6 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. 23A, FIG. 23B, 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.

Further, as an example of this example, consider a case where an error range in manufacturing is taken into consideration when the value of the condition of Example 5 shown in Table 4 is used as a reference. At this time, the results of calculating α ₁ and α ₂ using the thickness d ₁ and the thickness d ₂ as parameters (variables) are shown in FIGS. 24 and 25, respectively. Figure 24 is a distribution diagram showing the angle alpha ₁ can take the first thickness d ₁ and a second condition of the thickness d ₂ of the wave plate wave plate, for example, alpha ₁ is 34 to Under the conditions of d ₁ and d ₂ corresponding to the region of 35 [°], an optimal solution is included in α ₁ within a range of 34 to 35 [°]. Similarly, FIG. 25 is a distribution diagram showing angles that α ₂ can take in the conditions of the thickness d ₁ of the first wave plate and the thickness d ₂ of the second wave plate, for example, α ₂ is In the conditions of d ₁ and d ₂ corresponding to the range of −26 to −24 [°], α ₂ includes an optimal solution within the range of −26 to −24 [°].

These results are given by the following empirical formulas by giving coefficients A to L:
_{X = A + Bcos (C (} d 1 -D)) + Ecos (F (d 2 -G))
+ Hcos (I (d ₁ −J)) cos (K (d ₂ −L)) (14)
Is shown in Table 6. That is, when the formula left X is alpha ₁ (14), provided alpha ₁ row of coefficients A~L respectively when alpha _2, gives the coefficient A~L of alpha ₂ rows, respectively.

Note that the error ranges of α ₁ and α ₂ when d ₁ and d ₂ are determined are preferably in the range of −20 to +20 [°] in the distributions of FIGS. 24 and 25, and −10 to +10. [°] is more preferable, and −5 to +5 [°] is more preferable.

Further, as an example based on the wavelength selective diffraction element, in addition to the combination of the wavelength 405 nm, the wavelength 660 nm, and the wavelength 785 nm, the first wavelength plate constituting the wavelength selective wavelength plate 407 for the combination of the wavelength 450 nm, the wavelength 533 nm, and the wavelength 645 nm. The conditions of the polymer liquid crystal layer 403a and the polymer liquid crystal layer 403b as the second wave plate are given as Examples 7 to 9, and the conditions are summarized in Table 7. The manufacturing method and the materials used are the same as those in Example 1, except that the thicknesses d ₁ and d ₂ and the pretwist angles α ₁ and α ₂ of the polymer liquid crystal layers 403a and 403b are changed.

Here, in each wavelength selection wavelength plate 407 manufactured under the conditions of Table 7, the azimuth angle with respect to light with a wavelength of 450 nm is Ψ ₁ [°], the azimuth angle with respect to light with a wavelength of 533 nm is Ψ ₂ [°], and the wavelength is 645 nm. Table 8 summarizes the results of each example when the azimuth angle with respect to light is Ψ ₃ [°].

In Example 7, according to the conditions shown in Table 7, as shown in Table 8, the azimuth angle Ψ ₁ for light with a wavelength of 450 nm is 0 °, the azimuth angles Ψ for light with a wavelength of 533 nm and light with a wavelength of 645 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. In addition, the wavelength selective diffraction element 400 is manufactured by adjusting the slow axis that is the orientation direction of the polarization diffraction element 408 (diffraction grating 405) to 0 [°], so that the incident light is diffracted and incident. Thus, it is possible to obtain the wavelength selective diffractive element 400 that transmits the 533 nm light and 645 nm light in a straight line with high efficiency.

Similarly, the angles formed by Ψ ₁ and Ψ ₂ and Ψ ₂ and Ψ ₃ can be orthogonalized according to the conditions of Example 8 shown in Table 7, and Ψ ₁ and Ψ ₃ 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. 26 to FIG. 28 show transmission or diffraction utilization efficiencies in light of each wavelength band of the wavelength selective diffraction elements manufactured under the conditions of Examples 7 to 9, respectively. From these results, the light utilization efficiency in the 450 nm wavelength band (440 nm to 470 nm), the 533 nm wavelength band (520 nm to 550 nm), and the 645 nm wavelength band (630 nm to 660 nm) all show high values.

1, 2, 3, 23, 51, 61, 62, 71a, 71b, 81, 303, 305, 407 Wavelength selection wave plate 20, 50, 60, 70, 80, 400 Wavelength selection diffraction element 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 24, 410 Polarization direction of incident linearly polarized light 21 First wave plate 22 Second wave plate 25, 411 First Optical axis 26, 412 of wave plate Optical axis 41a, 41b, 41c, 42a, 42b, 42c of second wave plate Trajectory 100, 200, 250 on the Poincare sphere 101a, 10b, 101c, 201, 301R, 301G, 301B Light sources 102a, 102b, 102c Grating elements 103, 104, 204 Dichroic prisms 105, 202, 302, 304 Polarizing beam splitters 106, 203, 306 Collimating lenses 107, 205 Mirrors 108, 206a, 206b 1/4 wavelength plates 109, 207a, 207b Objective lenses 110, 208a, 208b Optical discs 111, 209 Cylindrical lenses 112, 210 Optical detector 300 Display system optical system 402a, 402b Alignment films 403a, 403b High Molecular liquid crystal layer 404, 406 Adhesive

Claims (7)

Of linearly polarized light having at least one wavelength among three kinds of wavelengths λ ₁ , wavelength λ ₂ , and wavelength λ ₃ (λ ₁ <λ ₂ <λ ₃ ) having predetermined different bands, In the wavelength selection waveplate that changes the polarization state,
The wavelength selection wavelength plate is provided with a first wavelength plate and a second wavelength plate in order from the light incident side,
The first wave plate and the second wave plate have optical axes parallel to each wave plate surface and aligned in the thickness direction,
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 are linearly polarized light in the same direction, and the polarization of the incident linearly polarized light. With reference to the direction, the combination of the slow axis of the first wave plate and the slow axis of the second wave plate, or the fast axis of the first wave plate and the second wave plate When the pre-twist angles α ₁ [°] and α ₂ [°] are angles formed in combination with the fast axis,
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,
Furthermore, when a first direction and a second direction orthogonal to the first direction are given, the polarization direction of the component having the largest vibration of light of any one wavelength with respect to the first direction Is in the range of −26 to 26 [°], and the ratio of the light component in the first direction out of all the components of light emitted at this wavelength is 80% or more, and the second The angle formed by the polarization direction of the component with the largest vibration of the light of the other two wavelengths with respect to the direction is in the range of −26 to 26 °, and all the components of the light emitted at these wavelengths Of the second direction, the α ₁ , the α ₂ , the retardation value Rd ₁ of the first wave plate, and the second wave plate so that the ratio of the respective light components in the second direction is 80% or more. A wavelength selective wave plate in which a retardation value Rd ₂ is set. When the retardation value of the first wave plate for the light of the wavelength λ ₁ is Rd ₁ (λ ₁ ), and the retardation value of the second wave plate for the light of the wavelength λ ₁ is Rd ₂ (λ ₁ ). ,
Rd ₁ along with the value of (lambda ₁₎ is substantially an integral multiple of 2 or more values of lambda _1, claim 1 value of Rd ₂ (lambda ₁₎ is 0.5 approximately an integral multiple of the lambda ₁ value The wavelength selective wave plate described in 1. 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 520 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 in which an optical material having a refractive index 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 5, 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 wavelength selective diffraction element according to claim 5 or 6 is arranged in an optical path between said light source and said polarizing beam splitter and / or in an optical path between said polarizing beam splitter and said photodetector. Optical head device.

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