JP2009294417A - Polarized beam splitter, polarization conversion element and image display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polarized beam splitter made small-sized and thin while attaining high diffraction efficiency over a wide wavelength range to improve the use efficiency of light by utilizing a diffraction optical element, and to provide a polarization conversion element and an image display device. <P>SOLUTION: The polarized beam splitter has a first optical element component 11 comprising an isotropic optical material and having a first diffraction optical surface, and a second optical element component 12 adjacent to the first optical element component 11 and comprising a birefringent optical material and having a second diffraction optical surface, wherein when the refractive index of the isotropic optical material is expressed by N1, the refractive index of the birefringent optical material to an ordinary beam is expressed by No and the refractive index of that to an extraordinary beam is expressed by Ne and the refractive index nearer to the refractive index N1 in the refractive indexes No and Ne is expressed by N2n, a condition of the relation of ¾N1-N2n¾<0.01 and ¾Ne-No¾>0.05 is satisfied. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a polarization separation element, a polarization conversion element, and an image display device using a diffractive optical element.

  A projection apparatus (projector) that modulates light from a light source with a liquid crystal panel and enlarges and projects it on a screen by a projection lens is disclosed (for example, see Patent Document 1).

Japanese Patent Laid-Open No. 1-182877

  In a light valve that displays an image by light modulation of specific polarization, such as a liquid crystal panel, only the specific polarization works effectively, and a polarization component orthogonal to the specific polarization causes deterioration such as a decrease in contrast. For this reason, a polarizer is inserted in the front stage (and rear stage) of the liquid crystal panel to control the polarization state. However, since the illumination light emitted from the light source is often a non-polarized beam (natural light), when a polarized light component in only one specific direction is selected by the polarizer, about half of the light is lost.

  By the way, in recent years, in the use of an image display device such as a projector, there is a high demand for a bright projection image that can be recognized without making the room too dark, and thus it is important to improve the light utilization efficiency of the liquid crystal light valve. In addition, projectors are required to be portable for home use and business presentations, and accordingly, the polarization separation element and the polarization conversion element to be mounted are expected to be reduced in size and thickness.

  The present invention has been made in view of such problems, and provides a polarization separation element that can achieve downsizing and thinning while improving the light utilization efficiency, and a polarization conversion element having the polarization separation element. For the purpose.

  In order to achieve such an object, according to a first aspect illustrating the present invention, a first optical element element having a first diffractive optical surface made of an isotropic optical material, and the first optical element A second optical element element having a second diffractive optical surface made of a birefringent optical material adjacent to the element, wherein the refractive index of the isotropic optical material is N1, and the birefringent optics When the refractive index with respect to the ordinary ray of the material is No and the refractive index with respect to the extraordinary ray is Ne, and the value closer to the refractive index N1 among these refractive indexes No and Ne is N2n, the following expression | N1-N2n | < Provided is a polarization separation element that satisfies the conditions of 0.01 and | Ne-No |> 0.05.

  Further, according to the second aspect exemplifying the present invention, the polarization separation element according to the first aspect and the half-wave plate that rotates the polarization plane of the transmitted light by 90 °, which are arranged in order from the light source side. There is provided a polarization conversion element characterized by comprising:

  According to a third aspect of the present invention, the polarization conversion element according to the second aspect and an image generation optical system that modulates light emitted from the polarization conversion element to generate image light are provided. An image display device is provided.

  As described above, according to the present invention, by using a diffractive optical element, a high diffraction efficiency is obtained over a wide wavelength range, and the light utilization efficiency is improved, and a reduction in size and thickness is achieved. A polarization separation element and a polarization conversion element having the polarization separation element can be provided.

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

  Conventionally, various attempts have been made to incorporate a diffractive optical surface into an optical system such as a pickup lens for an optical disk, in order to achieve high performance and miniaturization that cannot be achieved by a refractive optical system or a reflective optical system. However, in a single-layer type optical diffractive element having such a diffractive optical surface, there is a problem that flare occurs due to light in a wavelength region deviated from the design wavelength, and the image quality and imaging performance are impaired. It was limited to use in a single wavelength such as a laser light source or a narrow wavelength range.

  Therefore, in recent years, a diffractive optical element called a multilayer type (or laminated type) has been proposed. This type of diffractive optical element has, for example, a diffractive optical surface (relief pattern) formed in a sawtooth shape, and a plurality of optical element elements having different refractive indexes and dispersions are laminated in a separated or closely contacted manner. Thus, high diffraction efficiency is maintained in almost the entire desired wide wavelength range (for example, visible light range). That is, the wavelength characteristic of the diffraction efficiency is good.

Here, the structure of the multi-layer diffractive optical element will be described. Generally, as shown in FIGS. 5A and 5B, the first optical element element 111 made of the first material and the refractive element are refracted. The second optical element element 112 is made of a second material having a different rate and dispersion value, and sawtooth diffraction gratings 111a and 112a are formed on the opposing surfaces of the respective optical element elements. Then, the grating height (groove height) h1 of the first optical element element 111 is determined to be a predetermined value so as to satisfy the achromatic condition for specific two wavelengths, and the second optical element element 112 The grid height h2 is determined to another predetermined value. As a result, the diffraction efficiency is 1.0 for two specific wavelengths, and a considerably high diffraction efficiency can be obtained for other wavelengths. Thus, by making the diffractive optical element a multilayer type, the diffractive optical element can be applied to almost all wavelengths. The diffraction efficiency is a ratio η between the intensity I _{0 of} light incident on the diffractive optical element and the intensity I ₁ of first-order diffracted light included in the light transmitted through the diffractive optical element in the transmission type diffractive optical element. (= I ₁ / I ₀ ).

  In addition, by satisfying the predetermined condition, as shown in FIG. 5B, the lattice height h1 of the first optical element element 111 and the lattice height h2 of the second optical element element 112 are matched. It becomes possible to achieve a contact multilayer type diffractive optical element. In this close-contact multi-layer type diffractive optical element, the error sensitivity (tolerance) of the grating height is less than that of the separated multi-layer type shown in FIG. ) Is easy to manufacture, and has advantages such as excellent productivity, high mass productivity, and favorable cost reduction of optical products.

  Further, either one of the first optical element element 111 and the second optical element element 112 is first precisely formed, and then the other optical element element is poured into a UV curable resin and molded. You can also In this case, there is an advantage that the previously formed lattice can be used as a mold, and the lattice to be formed later can be precisely formed, and the eccentricity of the two does not occur at all.

  Two optical element elements constituting such a contact multilayer diffractive optical element are such that one optical element element is a material having a relatively high refractive index and low dispersion and the other optical element element is relatively low. Although it is essential to be made of a material having a high refractive index dispersion, either material may be disposed on the object side (light incident side). However, in order to reduce the manufacturing error sensitivity to a desired level, it is preferable that the refractive index difference between the two optical element elements in the d-line is 0.45 or less. Moreover, it is still more preferable if the refractive index difference of two optical element elements is 0.2 or less.

  The polarization separation element of the present invention and the polarization conversion element having the same obtain the high diffraction efficiency over a wide wavelength region by applying the properties of the multilayered diffractive optical element as described above, and the light utilization efficiency. While achieving this, it is possible to achieve downsizing and thinning.

  First, an image display apparatus (projector) equipped with a polarization conversion element (including a polarization separation element) according to the present embodiment will be described with reference to FIG. The projector includes a light source unit 20, an illumination optical system, an image generation optical system, and a projection lens 60.

  The light source unit 20 includes a light source 22 that emits white light, and a reflector 24 that reflects illumination light from the light source 22 toward the illumination optical system. Illumination light from the light source unit 20 enters the illumination optical system along the illumination optical axis 26.

  The illumination optical system includes a polarization conversion element 32 and a relay lens 34. As shown in FIG. 2, the polarization conversion element 32 includes a first fly-eye lens 32a, a polarization separation element 10, a half-wave plate 32b, and a second fly-eye lens 32c arranged in order from the light source side. Have The first and second fly-eye lenses 32a and 32c have a plurality of small lenses arranged on a surface perpendicular to the illumination optical axis 26, and uniformize and emit the incident light. Although described in detail later, the polarization separation element 10 separates the illumination light from the light source 22 into two linear components (P-polarized light and S-polarized light) whose polarization planes are orthogonal to each other. The half-wave plate 32b rotates the polarization plane of one linearly polarized light separated by the polarization separating element 10 by 90 degrees so that the polarization planes of the two linearly polarized lights are the same. The relay lens 34 relays the illumination light emitted from the second fly-eye lens 32c to the image generation optical system. In FIG. 2, either Ne or No of the optical member of the second optical element element 12 is a smaller value than that of the first optical element element 11. Conversely, when either Ne or No of the optical member of the second optical element element 12 is larger than the first optical element element 11, the diffracted light (broken line) travels upward in the figure. To do.

  The image generation optical system includes reflecting mirrors 40, 42, 44, dichroic mirrors 46, 48, three transmissive liquid crystal panels 50 R, 50 G, 50 B, and a cross dichroic prism 52. The illumination light homogenized by the illumination optical system is incident on the dichroic mirror 46 after the optical path is bent by the reflecting mirror 28. The dichroic mirror 46 transmits blue light (B light) included in white light and reflects red light (R light) and green light (G light) to separate the B light. The separated B light is reflected by the reflecting mirror 40 and enters the liquid crystal panel 50B. The R light and G light reflected by the dichroic mirror 46 enter a dichroic mirror 48. The dichroic mirror 48 transmits R light, reflects G light, and separates R light and G light. The G light enters the liquid crystal panel 50G as it is, and the R light is reflected by the reflecting mirrors 42 and 44 and enters the liquid crystal panel 50R.

  In the liquid crystal panels 50R, 50G, and 50B, image information is given to the incident R light, G light, and B light, respectively. Each light beam transmitted through the liquid crystal panels 50R, 50G, and 50B enters the cross dichroic prism 52. The cross dichroic prism 52 is configured by combining four right angle prisms, and has two types of dichroic surfaces orthogonal to each other. The dichroic surface emits a light beam incident from the side surface of the cross dichroic prism 52 in a direction different from the incident direction according to the polarization characteristics and wavelength. The dichroic surface reflects the R light reflecting surface 52a that reflects the R light and the B light. B light reflecting surface 52b.

  The R light transmitted through the liquid crystal panel 50R is reflected in the direction perpendicular to the projection optical system by the R light reflecting surface 52a. The G light transmitted through the liquid crystal panel 50G passes through the R light reflecting surface 52a and the B light reflecting surface 52b, travels straight, and enters the projection optical system. The B light transmitted through the liquid crystal panel 50B is reflected in the direction perpendicular to the projection optical system by the B light reflecting surface 52b. As a result, R, G, and B color image lights are combined to generate color image light. The combined image light is incident on the projection lens 60 along the projection optical axis 56, is enlarged, and is projected on the screen 70.

  Although the transmissive type is used as the liquid crystal panels 50R, 50G, and 50B, the present invention is not limited to this, and a reflective type liquid crystal panel may be used.

  Here, the polarization separation element 10 will be described. As shown in FIG. 3, the polarization separation element 10 includes a first optical element element 11 having a first diffractive optical surface made of an isotropic optical material, and birefringence adjacent to the first optical element element 11. A second optical element element 12 having a second diffractive optical surface made of the optical material, and the first optical element element 11 and the second optical element element 12 are arranged to face each other, and are of a multilayer type This diffractive optical element is configured. Although not shown in the drawing, the birefringent optical material constituting the second optical element element 12 is sealed with a third optical material. In this way, by using a multilayer diffractive optical element for the polarization separating element 10, chromatic aberration is well corrected over a wide wavelength range, sufficiently high diffraction efficiency is ensured, and it is small, lightweight, thin, and Excellent optical performance can be achieved. In other words, the polarization separation element 10 of the present embodiment uses the characteristics of the multi-layer diffractive optical element to give refractive power to the grating forming layer on one side to increase the degree of freedom of aberration correction, and to achieve excellent optical performance. Performance has been achieved.

  The birefringent optical material constituting the second optical element element 12 has a refractive index No for ordinary light and a refractive index Ne for extraordinary light. Accordingly, the diffraction effect exerted by the grating interface (diffractive optical surface) 13 with the isotropic optical material constituting the first optical element element 11 is also different between ordinary rays and extraordinary rays. In this polarized light separating element 10, the refractive index of the optical material related to the second optical element element 12 with respect to either the ordinary ray or the extraordinary ray is almost the same as the refractive index of the optical material related to the first optical element element 11. The material is selected to be

  For example, when the refractive index N1 of the optical material related to the first optical element element 11 with respect to the ordinary light beam and the refractive index No of the optical material related to the second optical element element 12 are the same, the ordinary light beam is not subjected to the diffraction action. Therefore, the extraordinary ray is deflected by receiving the diffraction action at the grating interface 13. On the contrary, when the refractive index N1 of the optical material related to the first optical element element 11 with respect to the extraordinary ray and the refractive index Ne of the optical material related to the second optical element element 12 are the same, the extraordinary ray undergoes a diffraction action. Without passing through the grating interface 13, the ordinary ray is deflected by receiving the diffraction action at the grating interface 13. In this way, the polarization separation element 10 separates the incident illumination light into two linearly polarized light whose polarization planes are orthogonal to each other by disposing the first optical element element 11 and the second optical element element 12 so as to face each other. can do.

  Based on the above configuration, the polarization separation element 10 of the present embodiment is configured such that the refractive index of the isotropic optical material is N1, the refractive index of the birefringent optical material with respect to the ordinary ray is No, and the refractive index with respect to the extraordinary ray is When Ne is set and N2n is a value closer to the refractive index N1 among the refractive indexes No and Ne, the conditions of the following expressions (1) and (2) are satisfied.

| N1-N2n | <0.01 (1)
| Ne-No |> 0.05 (2)

  The above conditional expression (1) is the lattice interface 13 between the first optical element element 11 and the second optical element element 12, and is closer to the refractive index N1 (small refractive index difference) in the second optical element element 12. This is a requirement for the corresponding polarized light to travel substantially straight. If the upper limit value of the conditional expression (1) is exceeded, the transmitted light has a declination at the lattice interface 13, and illumination light is emitted from the optical system or optical element arranged behind the polarization separation element 10 (projection side). This is not preferable because it causes a loss of light amount. In addition, so-called Fresnel reflection occurs, and the loss of light quantity due to this reflection increases, which is not preferable because the transmittance as an illumination system is lowered. In order to fully exhibit the effect of conditional expression (1), the upper limit value is preferably set to 0.005.

  The conditional expression (2) defines the birefringence amount (= | Ne−No |) that forms the second optical element element 12. Satisfying the conditional expression (2) makes it possible to increase the diffraction angle of the polarization separating element 10 and shorten the total length of the polarization conversion element 32 without increasing the number of half-wave plates 32b. it can. Further, the fine processing of the half-wave plate 32b becomes unnecessary, and the production cost can be suppressed. If the lower limit value of conditional expression (2) is not reached, not only the polarization conversion element 32 will be enlarged, but also light loss caused by vignetting due to chipping of the edge portion of the thin film or dimensional error of the half-wave plate 32b will occur. There is also a possibility of becoming large, which is not preferable. In order to sufficiently exhibit the effect of conditional expression (2), it is preferable to set the lower limit value to 0.1.

  In order to obtain a more excellent polarization separation element 10 and polarization conversion element 32, it is preferable that conditional expressions (1) and (2) are simultaneously satisfied.

  In the present embodiment, the refractive index for the ordinary ray of the birefringent optical material constituting the second optical element element 12 is No and the refractive index for the extraordinary ray is Ne, and among these refractive indexes No and Ne, When the value far from the refractive index N1 of the isotropic optical material composing the first optical element element 11 is N2f, it is preferable to satisfy the condition of the following formula (3).

  0.02 <| N1-N2f | <0.45 (3)

  Conditional expression (3) defines an appropriate range for the refractive index difference between the optical material N1 constituting the first optical element element 11 and the refractive index N2f of the optical material constituting the second optical element element 12. Is. If the upper limit of conditional expression (3) is exceeded, the refractive index difference becomes too large, and there is a disadvantage that the sensitivity to the manufacturing error of the grating at the grating interface 13 becomes too large. On the other hand, if the lower limit value of conditional expression (3) is not reached, the grating height required for the grating interface 13 becomes too large, which is disadvantageous in production, or a shadow is generated by a step portion of the grating, resulting in diffraction efficiency of blazed light. Or stray light due to scattering and reflection of light incident on the wall surface of the grating is greatly generated, which may cause deterioration in image quality. In order to secure the effect of the conditional expression (3), it is preferable to set the upper limit value to 0.20. In order to secure the effect of conditional expression (3), it is preferable to set the lower limit of conditional expression (3) to 0.03.

  Specifically, when the second optical element element 12 is formed of a birefringent optical material, the polarization direction determined by the refractive index N2f is a direction parallel to the paper surface (defined as P-polarized light) in FIG. Further, the direction orthogonal to the polarization direction is a direction orthogonal to the paper surface in FIG. 2 (defined as S-polarized light). That is, the optical axis is 45 degrees with respect to both polarization directions. The present invention is a polarization separation element 10 configured by combining a “dichroic” diffractive optical element having a refractive index different depending on the polarization direction and a contact multilayer diffractive optical element.

  In order to obtain an excellent polarization separation element 10 and polarization conversion element 32, it is preferable that the conditional expressions (1), (2), and (3) are simultaneously satisfied.

  In the present embodiment, the pitch of the first optical element element 11 and the pitch of the second optical element element 12 are substantially equal, the pitch of the first optical element element 11 is p, and the d-line (design main wavelength) When the wavelength (587.6 nm) is λ, it is preferable to satisfy the condition of the following formula (4).

  p / λ> 10.0 (4)

  Conditional expression (4) defines an appropriate grating pitch size p by converting it to a wavelength size λ. As a general property of diffraction, if the pitch of the grating becomes too fine, high-order diffracted light is generated and the light utilization efficiency is lowered. In order to avoid this, conditional expression (4) is defined in this embodiment. Since the diffraction phenomenon occurs according to the wavelength, the conditional expression (4) considers the size p of the grating pitch normalized by the wavelength λ. When lower than the lower limit value of the conditional expression (4), the diffraction efficiency of the first-order light is reduced to about 60% or less and deviates from the practical level. Therefore, the value of the diffraction efficiency of the first-order light is not preferable. According to the extended scalar theory, the approximate value is about 56%, and according to the electromagnetic field analysis calculation (FDTD method or the like), more accurate calculation is possible). Conditional expression (4) is also a condition for preventing a significant polarization characteristic from being added to the diffracted light. If the lower limit of conditional expression (4) is not reached, the diffracted light has a desired polarization. Since polarized light of a component different from the component is mixed in, it is not preferable because the amount of light of the desired polarization component is likely to decrease and the contrast is decreased due to the generation of stray light. In order to fully exhibit the effect of conditional expression (4), it is preferable to set the lower limit to 20.0.

  In the present embodiment, the first diffractive optical surface and the second diffractive optical surface are arranged so as to be in contact with each other, the grating height of the first optical element element 11 is h, and the first optical element When the pitch of the element 11 is p, it is preferable to satisfy the condition of the following formula (5).

  0.01 <h / p <3.0 (5)

  The conditional expression (5) defines an appropriate relationship between the grating height h and the pitch size p. When the upper limit of conditional expression (5) is exceeded, the grating pitch p becomes too small, the diffraction angle increases, the chromatic dispersion of the diffracting surface increases, and the illumination area on the irradiated surface is colored. Will occur. As a result, a disadvantage arises in which the illumination area must be narrowed and used. Further, not only is it difficult to process, but the diffraction efficiency is lowered, and there is a disadvantage that the image quality is deteriorated due to unnecessary flare light. Thus, it is important to set the minimum lattice pitch p appropriately. On the other hand, if the lower limit value of conditional expression (5) is not reached, the grating pitch p becomes too large, the diffraction angle becomes large, and the total length of the polarization separation element 10 and the polarization conversion element 32 having this increases. Will occur. In order to fully exhibit the effect of conditional expression (5), the upper limit value is preferably set to 1.0. In order to fully exhibit the effect of the conditional expression (5), it is preferable to set the lower limit to 0.05.

  In this embodiment, in the isotropic optical material, the refractive index N1 is a refractive index with respect to the d-line, a refractive index with respect to the C-line is nC1, a refractive index with respect to the F-line is nF1, and the compound In the refractive optical material, the refractive index N2f is a refractive index for the d-line, a refractive index for the C-line is nC2, a refractive index for the F-line is nF2, and the isotropic optical material and the birefringence When the difference in main dispersion with the optical material is Δ (NF−NC) = {(NF1−NC1) − (NF2−NC2)}, it is preferable that the condition of the following formula (6) is satisfied.

  -50.0 <| N1-N2f | / Δ (NF-NC) <− 1.0 (6)

  The conditional expression (6) is expressed as follows: the refractive index difference | N1-N2f | with respect to the d-line (at the lattice interface 13) in the optical material constituting the first optical element element 11 and the second optical element element 12, It defines an appropriate relationship with the difference Δ (NF−NC) of the variance NF−NC. Conditional expression (6) is an important condition for obtaining a high diffraction efficiency over a wide wavelength region in the multi-contact diffractive optical element as in the present embodiment. If it deviates from the range of the conditional expression (6), high diffraction efficiency over the entire use wavelength range cannot be obtained, and there is a disadvantage that the light use efficiency is lowered. In order to fully exhibit the effect of the conditional expression (6), it is preferable to set the lower limit to −45.0.

  In the present embodiment, when the diffraction efficiency at the d-line is Ed, the diffraction efficiency at the g-line is Eg, and the diffraction efficiency at the C-line is EC, the condition of the following expression (7) is satisfied. It is preferable.

  (Eg + EC) / (2 × Ed)> 0.80 (7)

  The conditional expression (7) defines an appropriate range for the balance of diffraction efficiency with respect to the used light having a wide wavelength range. When the lower limit of conditional expression (7) is not reached, the diffraction efficiency is at least at one of the g-line having a relatively short wavelength and the C-line having a long wavelength with respect to the d-line that is the dominant wavelength. If it is too low, the diffraction flare becomes large and the image quality is impaired. That is, light having a wavelength or angle of view other than the blazed light becomes unnecessary diffracted light, and the occurrence of flare increases, so that good image quality cannot be obtained. In order to secure the effect of conditional expression (7), it is preferable to set the upper limit value to 0.98. In order to secure the effect of the conditional expression (7), it is preferable to set the lower limit value to 0.82.

  In the present embodiment, the first optical element element 11 and the second optical element element 12 are in close contact with each other at the same grating height, the pitches are substantially equal, and the grating height of the first optical element element 11 is h. When the pitch of the first optical element element 11 is p, it is preferable that the conditions of the following expressions (8) and (9) are satisfied.

p ≧ 10 μm (8)
h ≦ 30 μm (9)

  By satisfying the conditional expressions (8) and (9), the pitch p is loose and the grating height h is kept low, and the angular characteristics of the diffraction efficiency (the degree of decrease in diffraction efficiency with respect to the change in the incident angle of incident light). ) And the usable angle range is widened. That is, it is preferable because flare due to unnecessary diffracted light can be reduced. In addition, since the pitch is loose and the grating height is kept low, the processing of the diffraction grating is facilitated, such as easy processing accuracy, and thus the manufacturing cost can be reduced. However, if the lower limit value of conditional expression (8) is not reached, the pitch becomes too narrow and it becomes difficult to process, and the diffraction efficiency is lowered and the image quality due to unnecessary flare light is impaired. In order to ensure the effect of the conditional expression (8), the lower limit value is preferably set to 30 μm.

  On the other hand, if the upper limit value of conditional expression (9) is exceeded, the grating height h becomes too high, the loss during light transmission increases, and the angular characteristics may deteriorate. In order to ensure the effect of the conditional expression (9), the upper limit value is preferably set to 25 μm.

  Furthermore, in the present embodiment, it is preferable that the following conditions are satisfied.

  ΔNd (difference between the above-mentioned refractive indexes N1 and N2f) is the difference in refractive index at the d-line between the first optical element element 11 (isotropic optical material) and the second optical element element 12 (birefringent optical material). And the Abbe number difference Δνd between the first optical element element 11 and the second optical element element 12 with respect to the d-line, it is preferable that the condition of the following expression (10) is satisfied.

  0.001 <Δνd / ΔNd <0.01 (10)

  In order to obtain high diffraction efficiency over a predetermined wavelength range, the above conditional expression (10) indicates that a high refractive index and low dispersion material and a low refractive index and high dispersion material constituting a multilayer diffractive optical element It defines the appropriate relationship. If the upper limit value of the conditional expression (10) is exceeded, high diffraction efficiency over a wide wavelength range cannot be obtained, and light with a wavelength or angle of view other than blazed becomes unnecessary diffracted light, The occurrence of flare increases, and good image quality cannot be obtained. On the other hand, if the lower limit value of conditional expression (10) is not reached, high diffraction efficiency over a wide wavelength range cannot be obtained. In order to secure the effect of conditional expression (10), it is preferable to set the upper limit to 0.015. In order to secure the effect of conditional expression (10), it is preferable to set the lower limit to 0.002.

  In the present embodiment, when the focal length of the single microlens constituting the fly-eye lenses 32a and 32c used in combination with the polarization separating element 10 is f and the maximum incident diameter of the microlens is D. It is preferable that the condition of the following formula (11) is satisfied.

  f / D> 1.5 (11)

  Conditional expression (11) defines an appropriate ratio between the focal length f and the maximum incident diameter D of the microlenses constituting the fly-eye lenses 32a and 32c. If the lower limit of conditional expression (11) is not reached, aberrations due to microlenses are generated, and inconveniences such as loss of light quantity due to vignetting of light rays are likely to occur. Further, when applied to the first fly-eye lens 32a, if the lower limit value of the conditional expression (11) is not reached, the incident angle to the polarization separation element 10 provided on the rear side (projection side) becomes too large, and the transmission characteristics. The reflection characteristics deteriorate and high polarization conversion efficiency cannot be obtained, which is not preferable. In order to secure the effect of conditional expression (11), it is preferable to set the lower limit to −2.0.

  Moreover, in this embodiment, when the retardation of the 1st optical element element 11 which consists of isotropic optical materials is set to R (nm / cm), it is preferable to satisfy the conditions of following Formula (12).

  R <50 (12)

  If the upper limit value of the conditional expression (12) is exceeded, the linearly polarized light passing through the polarization separating element 10 becomes elliptically polarized light, and high polarization conversion efficiency cannot be obtained, which is not preferable. In order to secure the effect of the conditional expression (12), it is preferable to set the upper limit value to 20.

  Further, in the present embodiment, it is preferable that the condition of the following expression (13) is satisfied when the total thickness of the polarization separation element 10 is L and the thickness Lb of the second optical element element 12 (see FIG. 3). ).

  0.05 <Lb / L <0.5 (13)

  Conditional expression (13) defines an appropriate ratio between the total thickness L of the polarization separation element 10 and the thickness Lb of the second optical element element 12. If the upper limit value of the conditional expression (13) is exceeded, the thickness Lb of the second optical element element 12 becomes too large, and not only the light transmittance decreases due to absorption by the birefringent material, but also the remaining retardation. This is not preferable because the extinction ratio deteriorates. On the other hand, if the lower limit value of conditional expression (13) is not reached, the thickness Lb of the second optical element element 12 becomes too small, and manufacturing problems that make it difficult to manufacture the second optical element element 12 are likely to occur. Absent. In addition, about several times the order of the used wavelength is the lower limit at which a predetermined diffracted wave can be generated.

  In the present embodiment, as described above, the first optical element element 11 located on the light source side is made of an isotropic optical material, and the second optical element element 12 located on the projection side is a birefringent optical material. It is made of a material (see FIG. 2). In this configuration, when the illumination light from the light source 22 passes through the isotropic optical material first, there is no retardation, so that the polarization state does not change in the material, and polarization separation is performed at the diffraction interface 13. Is done. In particular, when natural light (non-polarized light) is emitted from the light source 22, it is convenient as a “polarized light separating element” because it is divided into S-polarized light and P-polarized light by 50%.

  However, in this embodiment, the reverse optical combination, that is, the first optical element element 11 located on the light source side is made of a birefringent optical material, and the second optical element element 12 located on the projection side is isotropic. It is also possible to constitute the optical material. However, when the illumination light from the light source 22 passes through the birefringent optical material first, there is retardation, so that the polarization state changes in the material, and a predetermined polarization separation characteristic cannot be obtained. For example, the oblique incident light with respect to the diffraction interface 13 has a polarization component bias, and the linearly polarized light becomes elliptically polarized light. Further, when the birefringent member becomes thick, elliptically polarized light is likely to be generated, which causes problems such as illumination unevenness.

  Furthermore, in this embodiment, it is more preferable to satisfy the following conditions.

  In the present embodiment, the first and second optical element elements 11 and 12 are reduced in order to reduce scattering by the walls (steps) of the first and second optical element elements 11 and 12 and a decrease in diffraction efficiency of blazed light. It is preferable to incline and tilt the wall toward the entrance pupil of the optical system after passing through the illumination optical system. More specifically, it is preferable to give a gradient following the principal ray. As a result, the first and second optical element elements 11 and 12 can be manufactured by resin shaping using a mold, so that mass productivity can be improved and costs can be reduced.

  In the present embodiment, it is preferable that the optical material constituting the first optical element element 11 is made of a resin material having a specific gravity of 2.0 or less in order to reduce the size and weight. Since the specific gravity of the resin is smaller than that of glass or the like, it is very effective for reducing the weight of the optical system. In order to fully exhibit the effect, the specific gravity is preferably 1.6 or less. Furthermore, the optical material constituting the first and second diffraction gratings 11 and 12 preferably has a refractive surface with positive refractive power at the interface with air, and this refractive surface is preferably an aspherical surface. Thereby, size reduction and weight reduction can be promoted.

  In this embodiment, the birefringent material constituting the second optical element element 12 is not limited to the nematic liquid crystal shown in the examples described later, but is a uniaxial crystal and a birefringent material. Any liquid crystal or crystal that exhibits the same effect may be used (for example, crystal or calcite).

  In the present embodiment, the isotropic material constituting the first optical element element 11 is not limited to the optical glass shown in the examples described later, and may be a UV curable resin, a thermoplastic resin, or the like. . However, in the case of using a UV curable resin, it is preferable to use a material having a material viscosity (uncured product viscosity) of at least 40 in order to maintain good moldability and ensure excellent mass productivity. If the viscosity of the material is 40 or less, the resin tends to flow during molding, which makes it difficult to mold a precise shape, which is not preferable.

  In the present embodiment, the surface roughness at the grating interface 13 between the first optical element element 11 and the second optical element element 12 is designed with an RMS value in order to reduce scattering (diffraction) flare at the grating interface 13. It is preferably 1/100 or less of the reference wavelength.

  In the present embodiment, when the polarization separating element 10 is configured, the first and second fly-eye lenses 32a and 32c and the half-wave plate 32b may be arranged in a one-dimensional arrangement. preferable. However, a two-dimensional array may be used.

  Furthermore, in the present embodiment, it is preferable that the first fly-eye lens 32a and the second fly-eye lens 32c have the same shape because mass productivity is improved and cost reduction is achieved.

  An optical system composed of a plurality of components obtained by incorporating the polarization separation element 10 or the polarization conversion element 32 of the present invention as described above does not depart from the scope of the present invention. The same applies to an optical system obtained by incorporating a gradient index lens, a crystal material lens, or the like.

(First embodiment)
In the polarization separating element 10 in this embodiment, the isotropic optical material constituting the first optical element element 11 is SLAM3 manufactured by OHARA INC., And the birefringent optical material constituting the second optical element element 12 is used. Was made into 5CB liquid crystal (OPTICAL ENGINEERING, Vol.32, No.8, P.1775-1780 (1993)). Table 1 below shows optical data of the polarization separation element 10 in the first example and values corresponding to the conditional expressions (1) to (13). In the optical data, ng, nF, ne, nd, and nC are the refractive index for g-line (wavelength 435.835 nm), the refractive index for F-line (wavelength 486.133 nm), the refractive index for e-line (wavelength 546.074 nm), d. The refractive index for the line (wavelength 587.562 nm) and the refractive index for the C line (wavelength 656.273 nm) are shown. The above description is the same in other embodiments, and the description thereof is omitted. The refractive index at a wavelength (g line, F line, e line, C line) other than the d line of the birefringent optical material (5CB liquid crystal) can also be calculated from the above paper.

(Table 1)
[Optical data]
ng nF ne nd nC
Isotropic material (SLAM3) 1.735870 1.724790 1.720560 1.717000 1.712530
Birefringent material (5CB) 1.555500 1.543200 1.534500 1.530500 1.525800
[Conditional expression values]
N1 = 1.71700
N2n = 1.7171 (= No)
N2f = 1.5305 (= Ne)
p = 30
λ = 0.5876
h = 3.158
Δ (NF-NC) =-0.00514
Eg = 72.6
EC = 96.7
Ed = 100.0
Δνd = 27.9943
ΔNd = 0.1865
f = 15
D = 5
R = 20
Lb = 2
L = 5
Conditional expression (1) | N1-N2n | = 0.0001
Conditional expression (2) | Ne-No | = 0.1866
Conditional expression (3) | N1-N2f | = 0.1865
Conditional expression (4) p / λ = 51.055
Conditional expression (5) h / p = 5.374
Conditional Expression (6) | N1-N2f | / Δ (NF-NC) = − 36.28
Conditional expression (7) (Eg + EC) / (2 × Ed) = 0.8465
Conditional expression (8) p = 30
Conditional expression (9) h = 3.158
Conditional expression (10) Δνd / ΔNd = 0.00666
Conditional expression (11) f / D = 3
Conditional expression (12) R = 20
Conditional expression (13) Lb / L = 0.4

  In the present embodiment, it is understood that all the conditional expressions (1) to (13) are satisfied. The curve shown in FIG. 4A shows the distribution of diffraction efficiency when the polarization separation element 10 of this embodiment is set to have a diffraction efficiency of 100% at the d-line, that is, an isotropic optical material (SLAM3 ) And a birefringent optical material (5CB liquid crystal). As can be seen from the figure, in this example, a high diffraction efficiency (diffracted light intensity) of 0.73 or more could be obtained in the wavelength region from the g-line to the C-line.

(Second embodiment)
In the polarization separating element 10 in this embodiment, the isotropic optical material constituting the first optical element element 11 is SLAL8 manufactured by OHARA INC., And the birefringent optical material constituting the second optical element element 12 is used. Was a 5CB liquid crystal. Table 2 below shows optical data of the polarization separation element 10 in the second embodiment and values corresponding to the conditional expressions (1) to (13).

(Table 2)
[Optical data]
ng nF ne nd nC
Isotropic material (SLAL8) 1.729430 1.722210 1.716150 1.713000 1.708970
Birefringent material (5CB) 1.555500 1.543200 1.534500 1.530500 1.525800
[Conditional expression values]
N1 = 1.71300
N2n = 1.7171 (= No)
N2f = 1.5305 (= Ne)
p = 15
λ = 0.5876
h = 3.220
Δ (NF−NC) = − 0.00041
Eg = 76.0
EC = 96.7
Ed = 100.0
Δνd = 23.3634
ΔNd = 0.1825
f = 20
D = 10
R = 20
Lb = 2
L = 5
Conditional expression (1) | N1-N2n | = 0.0041
Conditional expression (2) | Ne-No | = 0.1866
Conditional expression (3) | N1-N2f | = 0.1825
Conditional expression (4) p / λ = 25.523
Conditional expression (5) h / p = 5.480
Conditional expression (6) | N1-N2f | / Δ (NF-NC) = − 43.87
Conditional expression (7) (Eg + EC) / (2 × Ed) = 0.863
Conditional expression (8) p = 15
Conditional expression (9) h = 3.220
Conditional expression (10) Δνd / ΔNd = 0.00781
Conditional expression (11) f / D = 2
Conditional expression (12) R = 20
Conditional expression (13) Lb / L = 0.4

  In the present embodiment, it is understood that all the conditional expressions (1) to (13) are satisfied. The curve shown in FIG. 4B shows the distribution of diffraction efficiency when the polarization separation element 10 of this embodiment is set so that the diffraction efficiency at the d-line is 100%, that is, an isotropic optical material (SLAL8). ) And a birefringent optical material (5CB liquid crystal). As can be seen from the figure, in this example, a high diffraction efficiency (diffracted light intensity) of 0.76 or more could be obtained in the wavelength region from the g-line to the C-line.

(Third embodiment)
In the polarized light separating element 10 in the present embodiment, the isotropic optical material constituting the first optical element element 11 is an optical glass SBAL12 manufactured by OHARA INC., And the birefringence constituting the second optical element element 12 is used. The optical material was LQC liquid crystal (Nagaoka University of Technology paper (refer to pages 265 to 266 of the 2005 Japanese Liquid Crystal Society discussion)). Table 3 below shows optical data of the polarization separation element 10 in the third example and values corresponding to the conditional expressions (1) to (13). Note that the refractive index of the birefringent optical material (LQC liquid crystal) at wavelengths other than the d-line (g-line, F-line, e-line, C-line) can also be estimated from the above paper.

(Table 3)
[Optical data]
ng nF ne nd nC
Isotropic material (SBAL12) 1.551220 1.546270 1.542120 1.539960 1.537190
Birefringent material (LQC) 1.482000 1.475000 1.470000 1.468000 1.464000
[Conditional expression values]
N1 = 1.53996
N2n = 1.535 (= Ne)
N2f = 1.468 (= No)
p = 80
λ = 0.5876
h = 7.927
Δ (NF-NC) =-0.00192
Eg = 79.8
EC = 95.7
Ed = 99.7
Δνd = 16.922
ΔNd = 0.07198
f = 30
D = 10
R = 20
Lb = 1
L = 2
Conditional expression (1) | N1-N2n | = 0.00496
Conditional expression (2) | Ne-No | = 0.067
Conditional expression (3) | N1-N2f | = 0.07196
Conditional expression (4) p / λ = 136.15
Conditional expression (5) h / p = 13.490
Conditional Expression (6) | N1-N2f | / Δ (NF-NC) = − 37.48
Conditional expression (7) (Eg + EC) / (2 × Ed) = 0.8801
Conditional expression (8) p = 80
Conditional expression (9) h = 7.927
Conditional expression (10) Δνd / ΔNd = 0.00425
Conditional expression (11) f / D = 3
Conditional expression (12) R = 20
Conditional expression (13) Lb / L = 0.5

  In the present embodiment, it is understood that all the conditional expressions (1) to (13) are satisfied. The curve shown in FIG. 4C is a diffraction efficiency distribution when the diffraction efficiency at the d-line is set to 100% in the polarization separation element 10 of this embodiment, that is, an isotropic optical material (SBAL12). ) And a birefringent optical material (LQC liquid crystal). As can be seen from the figure, in this example, a high diffraction efficiency (diffracted light intensity) of 0.80 or more could be obtained in the wavelength region from the g-line to the C-line.

  As described above, in order to make the present invention easy to understand, the configuration requirements of the embodiment have been described, but it goes without saying that the present invention is not limited to this.

It is a figure which shows the cross-sectional structure of the projector which has the polarization separation element and polarization conversion element which concern on this embodiment. It is a figure which shows the cross-sectional structure of the polarization conversion element which concerns on this embodiment. It is a figure which shows the cross-sectional structure of the polarization beam splitter which concerns on this embodiment. (A) in 1st Example, (b) in 2nd Example, (c) is a figure which shows the diffraction efficiency with respect to each wavelength of the polarization separation element in 3rd Example. It is a schematic cross-sectional view of a multilayer diffractive optical element, (a) is a schematic cross-sectional view of a separated multi-layer diffractive optical element, and (b) is a schematic cross-sectional view of a contact multilayer diffractive optical element. is there.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Polarization separation element 11 1st optical element element 12 2nd optical element element 13 Grating interface (diffractive optical surface)
32 Polarization Conversion Element 32a First Fly Eye Lens 32b Half Wave Plate 32c Second Fly Eye Lens

Claims (12)

A first optical element element having a first diffractive optical surface made of an isotropic optical material, and a second optical element element having a second diffractive optical surface made of a birefringent optical material adjacent to the first optical element element. An optical element element,
The refractive index of the isotropic optical material is N1,
When the refractive index for ordinary light of the birefringent optical material is No and the refractive index for extraordinary light is Ne, and the value closer to the refractive index N1 among these refractive indexes No and Ne is N2n, | N1-N2n | <0.01
| Ne-No |> 0.05
A polarization separation element satisfying the following conditions: When the refractive index with respect to ordinary light of the birefringent optical material is No, the refractive index with respect to extraordinary light is Ne, and the value farther from the refractive index N1 among these refractive indexes No and Ne is N2f. 0.02 <| N1-N2f | <0.45
The polarization separation element according to claim 1, wherein the following condition is satisfied. The pitch of the first optical element element and the pitch of the second optical element element are substantially equal,
When the pitch of the first optical element element is p and the wavelength of the d-line is λ, the following formula is given: p / λ> 10.0
The polarization separating element according to claim 1, wherein the following condition is satisfied. The first diffractive optical surface and the second diffractive optical surface are disposed so as to contact each other;
When the grating height of the first optical element element is h and the pitch of the first optical element element is p, the following expression 0.01 <h / p <3.0
The polarization separation element according to claim 1, wherein the following condition is satisfied. In the isotropic optical material, the refractive index N1 is a refractive index for the d-line, a refractive index for the C-line is nC1, and a refractive index for the F-line is nF1.
In the birefringent optical material, the refractive index N2f is a refractive index for the d-line, a refractive index for the C-line is nC2, and a refractive index for the F-line is nF2.
When the difference in main dispersion between the isotropic optical material and the birefringent optical material is Δ (NF−NC) = {(NF1−NC1) − (NF2−NC2)}, the following formula − 50.0 <| N1-N2f | / Δ (NF-NC) <− 1.0
The polarization separation element according to claim 2, wherein the following condition is satisfied. When the diffraction efficiency at the d line is Ed, the diffraction efficiency at the g line is Eg, and the diffraction efficiency at the C line is EC, the following equation (Eg + EC) / (2 × Ed)> 0.80
The polarization separation element according to claim 1, wherein the following condition is satisfied. The pitch of the first optical element element and the pitch of the second optical element element are substantially equal,
When the pitch of the first optical element element is p (μm), the following formula p ≧ 10
The polarization separation element according to claim 1, wherein the following condition is satisfied. The first diffractive optical surface and the second diffractive optical surface are disposed so as to contact each other;
When the grating height of the first optical element element is h (μm), the following formula h ≦ 30
The polarization separation element according to claim 1, wherein the following condition is satisfied. Arranged in order from the light source side,
The polarization separation element according to any one of claims 1 to 8,
A polarization conversion element, comprising: a half-wave plate that rotates and converts a polarization plane of transmitted light by 90 °.   The polarization conversion element according to claim 9, further comprising a first fly-eye lens disposed on a light source side of the polarization separation element.   11. The polarization conversion element according to claim 9, further comprising a second fly-eye lens disposed on the projection side of the half-wave plate. The polarization conversion element according to any one of claims 9 to 11,
An image display apparatus comprising: an image generation optical system that modulates light emitted from the polarization conversion element to generate image light.

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