JP2007171370A - Optical element and imaging apparatus having the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element having less changes in a separation width of exiting beams with respect to wavelengths and providing a preferable optical low-pass effect. <P>SOLUTION: The optical element includes a plurality of optically anisotropic materials in which an incident beam is divided into polarized beams having the polarization directions orthogonal to each other at exiting the element with a desired separation width. The plurality of materials include at least two materials having different dispersions, wherein a first material of the two divides an incident beam into two beams in a predetermined direction, and a second material divides the incident beam into two beams having a component in the predetermined direction into a different direction from the predetermined direction. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical element that splits an incident light beam into a plurality of light beams by utilizing optical anisotropy.

  In addition, the present invention is suitable as an optical low-pass filter in an imaging apparatus such as a digital camera or a video camera using a solid-state imaging device such as a CCD or MOS.

  An imaging apparatus such as a digital camera or a video camera using a two-dimensional solid-state imaging device such as a CCD or MOS obtains image information by sampling a subject image for each pixel pitch.

  In this imaging apparatus, when a subject having a frequency component higher than the spatial frequency of the pixel pitch is photographed, the high frequency component is turned back to a low frequency and detected as image noise along with the image information.

  In addition, in an image pickup apparatus using a single-plate color image pickup device, when a subject having a high spatial frequency high frequency component is photographed, false color noise determined by the arrangement of color filters arranged in front of the pixels is generated. And detected together with the image information.

  Such noise components are very difficult to remove after being converted into electrical signals. Therefore, conventionally, the surplus signal (noise signal) is removed from the optical image before being formed on the image sensor surface. In many imaging apparatuses, this excess signal is removed using an optical low-pass filter.

  The optical low-pass filter serves to remove an optical signal having a high spatial frequency from the image information. The most typical method among them is to separate an incident light beam into a plurality of light beams using an optical anisotropic medium such as quartz.

  That is, it is a method using a light beam separation action. Here, the anisotropic medium refers to a medium having a different refractive index depending on the direction of polarized light, and has an optical axis in a direction in which the normal velocities of polarized lights whose vibration directions are orthogonal to each other coincide. A medium having a different refractive index depending on the vibration direction of the light beam in only one of the three directions, such as quartz, is called a uniaxial crystal, and the direction of the different refractive index is the optical axis. When this anisotropic medium is cut out in parallel (as a parallel plate), and circularly polarized light is incident on the optical axis inclined at a predetermined angle from the normal to the incident / exit plane, the anisotropy in the direction in which the optical axis is inclined Indicates. As a result, the incident light beam is separated into an ordinary ray and an extraordinary ray. At this time, the separation width of the ordinary ray and the extraordinary ray is determined by each refractive index of the anisotropic medium, the thickness in the light traveling direction, and the angle between the optical axis and the incident surface normal.

  Conventionally, there has been proposed an optical element that separates an incident light beam into a plurality of light beams by utilizing the light beam separation action of quartz (Patent Documents 1 to 4).

  In Patent Documents 1 and 2, a filter on a stripe is assumed as a color filter. The false color signal generated when the spatial frequency of the subject is synchronized with the color filter is reduced. As a means for this, a configuration is disclosed in which a parallel plate having birefringence such as quartz is used to separate a light beam into an ordinary light beam and an extraordinary light beam, and an object image is formed on the imaging surface.

  In particular, Patent Document 1 discloses a configuration in which a single crystal of crystal is cut out and used so that its optical axis forms an angle of approximately 45 degrees with respect to an input / output surface of a parallel plate.

  In Patent Documents 3 and 4, for example, a Bayer color filter is assumed as the color filter. Then, by combining a plurality of birefringent plates, the light beam is separated into an ordinary light beam and an extraordinary light beam, and a subject image is formed on the imaging surface. This discloses a configuration that effectively reduces the generation of false resolution signals and false color signals generated by high-frequency components of the subject.

  Furthermore, an optical element having a light beam separating action of an anisotropic medium having anisotropy stronger than that of quartz has been proposed (Patent Documents 5 and 6).

  In Patent Document 5, a material having stronger anisotropy than quartz is used, so that the separation width of the light flux per unit thickness is increased and the mechanical thickness is reduced. However, in order to reduce the separation width of the light beam, the thickness becomes too thin and the processing becomes difficult, and the mechanical strength also becomes weak.

  Therefore, the angle θ formed by the surface normal of the optical element and the optical axis is set within the range of 10 ° <θ <30 ° and 60 ° <θ <80 ° by shifting from the angle 45 ° at which the separation width of the luminous flux is maximum. ing.

  In Patent Document 6, an optical element obtained by cutting a single crystal of lithium niobate in parallel is used.

  The normal of the main surface cut out of the single crystal rotates ± 3 degrees around the z-axis from the position rotated 46.1 ± 20 degrees or 133.9 ± 20 degrees from the y-axis with respect to the x-axis of the crystal. It is in the range. Since lithium niobate has a larger refractive index difference than quartz, the mechanical thickness can be reduced if the separation width is the same.

  On the other hand, optical elements that two-dimensionally separate incident light beams have been proposed for two-dimensional pixel arrays (Patent Documents 7 and 8).

  In Patent Document 7, two substrates having optical anisotropy are used for a two-dimensional arrangement of pixels. The first base material separates the light flux in a direction of 45 degrees from the long side direction of the optical element, and the second base material in a direction of 90 degrees from the long side direction. Since this optical element can be formed by using only two substrates having optical anisotropy, it is advantageous for miniaturization. The light beam incident on this optical element is separated and emitted in the direction of each vertex of a parallelogram with one diagonal being 45 degrees.

  Patent Document 8 discloses an optical element in which substrates having different refractive indexes are combined.

  In general, when a material having weak anisotropy is used, the thickness is too thick to obtain a predetermined amount of light beam separation width.

  On the other hand, when a material having strong anisotropy is used, when the separation width of the light beam is reduced, the material is too thin. When the thickness is less than the processing limit, it cannot be manufactured.

In Patent Document 8, a material having a weak anisotropy in which the separation direction is reversed by 180 degrees is combined with a material having a strong anisotropy, thereby making the thickness of the entire optical element appropriate and facilitating manufacture. Yes.
Japanese Utility Model Publication No. 47-18688 Japanese Utility Model Publication No. 47-1889 JP 59-75222 A JP-A-60-164719 JP 2001-147404 A JP 2002-107540 A JP 2001-209008 A JP 2003-329979 A

  In an optical element that separates a light beam into a plurality of parts using an optically anisotropic medium, the light beam is separated by three parameters: a refractive index difference between an ordinary ray and an extraordinary ray, a thickness, and an angle of an optical axis. A width is required.

  FIG. 15 is an image diagram when the light beam is separated into two. In FIG. 15, 21 is a parallel plate substrate.

  Further, t is the thickness of the substrate 21, d is the separation width of both polarizations, and Si is the direction of the pointing vector of the incident light beam. Srs is the direction of the s-polarized pointing vector in the substrate 21, and Srp is the direction of the p-polarized pointing vector in the substrate 21.

  In general, it is assumed that a base material of a uniaxial crystal is formed so that the angle formed by the normal line of the incident surface and the optical axis is θ. At this time, the angle φ formed by the traveling direction of the ordinary ray and the extraordinary ray when the circularly polarized light is perpendicularly incident on the substrate is expressed by the following equation.

Where n _o is the ordinary index of refraction of the substrate 21, n _e is the refractive index of the extraordinary ray of the substrate 21. Both polarized light beams emitted from the base material 21 travel vertically since the wave vectors are stored. That is, the separation width d of both polarized light is as follows when the thickness of the substrate 21 is t.

It is represented by

  Optical materials have wavelength dispersion. In a material having optical anisotropy, the refractive power no of ordinary rays and the refractive index ne of extraordinary rays are usually different and have dispersion. Refractive indexes no and ne vary with wavelength. For this reason, depending on the wavelength, the separation width d in equation (2) varies. When such a base material is used as an optical low-pass filter disposed on the entire surface of the imaging device, for example, the separation width of the light flux differs with respect to the wavelength. For this reason, the resolution changes depending on the color. Since this is directly attributable to the image, it can cause image degradation.

  An object of the present invention is to provide an optical element that has a small difference in the separation width of light flux with respect to wavelength and that can obtain a good optical low-pass effect in a wide wavelength region when used in an imaging apparatus having a solid-state imaging element.

  The optical element of the present invention is an optical element that separates incident light in accordance with polarized light, and has two base materials with different dispersions. Of the two base materials, the first base material is incident light. Are separated in a predetermined direction, and the second base material is characterized in that the incident light is separated in a direction having a component in the predetermined direction and in a direction different from the predetermined direction.

  According to the present invention, it is possible to obtain an optical element that has a small difference in the separation width of light flux with respect to the wavelength and can obtain an excellent optical low-pass effect in a wide wavelength region when used in an imaging apparatus having a solid-state imaging element.

  FIG. 1 is a cross-sectional view of a main part of a single-lens reflex camera (imaging device) having an optical element according to Embodiment 1 of the present invention.

  In the figure, reference numeral 101 denotes a photographic optical system as a replaceable photographic lens, and reference numeral 102 denotes a rotating mirror (quick return (QR) mirror). Reference numeral 103 denotes a focal plane shutter that mechanically limits the exposure time. Reference numeral 104 denotes an optical element according to the present invention, which is arranged in the optical path on the image plane side of the photographing optical system 101 and is composed of a plurality of substrates having optical anisotropy. Reference numeral 105 denotes an image pickup device (comprising a solid-state image pickup device such as a CCD or CMOS) as an image pickup means, which is disposed on a planned focal plane of the image pickup optical system 101. Reference numeral 106 denotes a focusing screen on which a subject image is formed. Reference numeral 107 denotes a finder optical system, which includes a penta roof prism 108 and an eyepiece 109 as image inverting means for inverting the subject image formed on the focusing screen 106.

  The optical element 104 and the solid-state imaging element 105 constitute an imaging unit.

  In this embodiment, the subject image formed by the imaging optical system 101 is formed on the focusing screen 106 when the rotating mirror 102 is disposed in the optical path. The subject image formed on the focusing screen 106 is observed as a finder image by the finder optical system 107. When the photographing operation is started by a release switch (not shown), the rotating mirror 102 is retracted out of the optical path of the photographing optical system 101.

  Then, the focus plane shutter 103 is opened on the image pickup surface of the image pickup element 105 via the optical element 4. An imaging signal is output from the imaging element 105 and stored in a memory (not shown).

  The optical element 104 of this embodiment is disposed between a solid-state imaging device 105 such as a CCD or MOS and the imaging optical system 101, and an incident light beam is divided into a plurality of light beams with a predetermined width corresponding to the pixel pitch of the solid-state imaging device 105. It is used as an optical low-pass filter for separation.

  The optical element 104 of this embodiment has a plurality of optically anisotropic base materials that separate incident light beams with polarized light beams orthogonal to each other at a desired width when emitted.

  Further, the optical element 104 of the present embodiment is configured by combining a plurality of base materials having different separations so that the difference in the separation width of the light beam with respect to the wavelength is reduced.

  As a result, the incident light beam is separated into a plurality of light beams with a desired separation width in a wide wavelength range.

  Further, if the optical element 104 of this embodiment is configured by combining base materials so that the light beam is simply separated in the same direction or the opposite direction, there is a possibility that the birefringence amount becomes an unrealistic thickness. There is.

  Therefore, when combining a plurality of base materials, the directions of separation are slightly tilted without being completely aligned. Thereby, it is configured so that the separation width as a whole optical element becomes a desired width with a combination of realistic thicknesses.

  Here, the base material is a parallel plate made of a single crystal of quartz or lithium niobate, a member having structural anisotropy made of a structure shorter than the wavelength of incident light flux, or a film having optical anisotropy. This refers to the resin.

  In particular, when the incident light beam is separated into a plurality of light beams and emitted by the base material, the separation of the base material varies depending on the wavelength of the incident light beam, so that the separation width of the emitted light beam varies depending on the wavelength. Say.

  Next, the configuration of each example of the optical element of the present invention will be described.

FIG. 2A is a schematic view of the essential portions of Embodiment 1 of the optical element of the present invention. FIG. 2 (B) is similar to FIG.
It is a schematic diagram of the A direction of (A). In FIG. 2, 1a, 1aH, and 1aT are orthogonal projections of the optical axis of the first substrate 11 and the respective surfaces of the optical axis. Similarly, 2a, 2aH, and 2aT are orthogonal projections of the optical axis of the second substrate 12 and the respective surfaces of the optical axis.

  However, the directionality from the incident side to the emission side is indicated by arrows. FIG. 2A shows the relationship between the positions of the first base material 11 and the second base material 12 and the angles of the optical axes 1a and 2a in the thickness direction.

  FIG. 2B shows the concept of the optical element 10 as seen from the incident direction. The angle formed by the orthogonal projections 1aH and 2aH of the optical axes 1a and 2a of the two substrates viewed from the incident direction is A. However, the angle A is an angle formed when the start points of the respective arrows are combined.

  The two base materials 11 and 12 constituting the optical element 10 of Example 1 have optical anisotropy.

  Then, the light beam incident on the optical element 10 becomes an orthogonal polarized light beam, which is separated and emitted with a desired width. The optical element 10 includes at least two base materials 11 and 12 having an action of separating incident light beams, and the dispersion of the two base materials 11 and 12 is different.

  Here, the first base material 11 separates the incident light beam into two light beams in a predetermined direction (X direction), and the second base material 12 has a component in the predetermined direction and a direction different from the predetermined direction. Are separated into two light beams.

  The optical element 10 of Example 1 uses a plurality (two) of birefringent plates, and further appropriately combines materials having different dispersions. Thereby, the change of the separation width of the light beam with respect to the wavelength of the entire optical element 10 is suppressed.

  In addition, the base materials are appropriately combined so that the thickness does not have a problem in mechanical strength and processing difficulty by combining the separation directions of the light beams.

  FIG. 3 is an explanatory diagram when the optical element 10 according to the first embodiment is arranged in the imaging apparatus having the solid-state imaging element shown in FIG. In FIG. 3, the first base material 11 in which the orthogonal projection of the optical axis of the single crystal onto the incident surface faces the long side direction of the solid-state imaging device, and the orthogonal projection of the optical axis from the long-side direction of the solid-state imaging device. The second substrate 12 facing 45 degrees is included.

  FIG. 4 is an explanatory diagram showing how the light beam is separated by the optical element 10. First, the incident light beam is separated into outgoing light beams 41 and 42 which are polarized perpendicularly to each other in the first base material 11 of FIG. Similarly, in the second substrate 12, the incident light beam 41 is separated into outgoing light beams 41 and 43 that are polarized perpendicularly to each other.

  Further, the incident light beam 42 is separated into outgoing light beams 42 and 44.

  For example, when lithium niobate is used as the substrate, the refractive index with respect to the wavelength is shown in FIG. Lithium niobate is a crystal called a negative crystal whose ordinary ray refractive index no is larger than the extraordinary ray refractive index ne. The ordinary ray refractive index no and the extraordinary ray refractive index ne have different dispersions, and have positive dispersion in which the refractive index increases as the wavelength becomes shorter. The separation width of the incident light beam per unit thickness when the lithium niobate is a parallel plate is shown in FIG. The separation width of the luminous flux per unit thickness on the vertical axis is tanφ in the equation (1). It can be seen that the shorter the wavelength, the larger the separation of the light beam becomes an optical substrate.

  When this is used for both the base materials 11 and 12 in FIG. 3, the optical element has a different light beam separation width with respect to the wavelength. This is schematically shown in FIG. In FIG. 7, dotted lines, broken lines, and alternate long and short dash lines represent light beams having different wavelengths λ1, λ2, and λ3, respectively. The incident light beam is separated into the outgoing light beam 71 and the outgoing light beam group 72 by the first substrate 11. The light beam 71 has no change with respect to the wavelength since light incident perpendicularly is directly emitted.

  However, since the emitted light beam group 72 is emitted in a form that satisfies the expression (2), the angle changes, and the width to be separated differs depending on the wavelength. Further, in the second substrate 12, the incident light beam 71 is separated into the outgoing light beam 71 and the outgoing light beam group 73, and the incident light beam group 72 is separated into the outgoing light beam group 72 and the outgoing light beam group 74. In the light beam group 73, a change in the separation width with respect to the wavelength is observed as in the separation of the emitted light beam 71 and the emitted light beam 72 on the first base material 11. The separation of the incident light beam group 72 also shows a change with respect to the wavelength in the direction of the outgoing light beam group 74. When these are included, the change with respect to the wavelength of the outgoing light beam group 74 becomes very large. When such an optical element is disposed, for example, in front of the solid-state image sensor (on the light incident side) and functions as an optical low-pass filter, a difference in separation width due to wavelength directly affects an image.

  This is greatly influenced by anisotropic wavelength dispersion.

  As a base material having a larger anisotropic dispersion than lithium niobate, there is a structure in which gratings finer than the wavelength of an incident light beam are periodically arranged. It is known that this structure has characteristics as a uniform medium because the incident light beam cannot recognize the structure itself. One example is a base material 81 having a structure in which lattices are periodically arranged one-dimensionally as shown in FIG.

  In FIG. 8, reference numeral 81 denotes a base material having dispersion. Within this structure, anisotropy occurs in optical characteristics according to the structure anisotropy as a whole. The anisotropy of such a periodic structure is larger than that of a normal crystal material.

Now, TiO ₂ is adopted as the periodic structure lattice medium. The pitch of the lattice structure is 80 nm, and the occupation ratio of TiO ₂ is 50%. This constitutes one base material. Then the refractive index at a wavelength of 530nm than the effective refractive index method is about ordinary refractive index _n o = 1.8252, the extraordinary refractive index n _e = 1.3209. Each refractive index _n o with respect to the wavelength, a n _e shown in FIG.

  FIG. 10 shows the separation width of the normal incident light beam with respect to the wavelength when the angle θ formed by the optical axis and the normal to the incident surface is 45 degrees. As is apparent from FIG. 9, the dispersion of the ordinary light refractive index no is very large compared to the extraordinary light refractive index ne. Thereby, since the anisotropic wavelength dispersion is very large, as shown in FIG. 10, the dispersion with respect to the wavelength becomes a very large base material.

  By effectively combining the base materials having different wavelength dispersions, it is possible to reduce changes with respect to the wavelength of the outgoing light beam group 74 shown in FIG.

For example, there is quartz as a base material having different dispersion. Each refractive index _n o with respect to the wavelength of the crystal, the n _e shown in FIG. 11. Quartz is a positive crystal whose ordinary light refractive index no is smaller than the extraordinary light refractive index ne. FIG. 12 shows the separation width of the normal incident light beam per unit thickness when the angle formed by the optical axis of the crystal and the normal to the incident surface is 45 degrees. Quartz has a smaller change in the separation width with respect to the wavelength than lithium niobate, but the separation width increases as the wavelength becomes shorter, similar to lithium niobate.

  In order to combine such substrates so that the change with respect to the wavelength is small, it is necessary to slightly change the mode of separation.

  For example, as shown in FIG. 4, consider an optical element that separates an incident light beam into each vertex of a parallelogram. When lithium niobate is used for the first base material 11 and quartz is used for the second base material 12, an optical element constituting it is shown in FIG. Compared to FIG. 3, the optical axis of the first base material 11 is reversed. FIG. 14 shows an image diagram of separation of a light beam perpendicularly incident on this optical element.

  The incident light beam 141 is separated into the outgoing light beam 141 and the outgoing light beam group 142 by the first substrate 11. The light beam 141 has no change with respect to the wavelength because light incident vertically is directly emitted.

  However, since the outgoing light beam group 142 is emitted in a form that satisfies the equation (2), the separation width differs depending on the wavelength. Further, in the second substrate 12, the incident light beam 141 is separated into the outgoing light beam 141 and the outgoing light beam group 143, and the incident light beam group 142 is separated into the outgoing light beam group 142 and the outgoing light beam group 144. In the outgoing light beam group 143, a change in the separation width with respect to the wavelength is observed as in the separation of the outgoing light beams 141 and 142 on the first base material 11. The separation of the incident light beam groups 141 and 142 also causes a change with respect to the wavelength in the direction of the outgoing light beam groups 143 and 144 by the second substrate 12.

  However, the difference from FIG. 7 is that base materials having different dispersions are used for both base materials, and further, they are effectively combined, and the combination is changed without changing the final separation direction. When combining substrates having positive dispersions such as quartz and lithium niobate, the direction of separation by the second substrate 12 is angular with respect to the direction of separation by the first substrate 11. By combining in the direction of cancellation, the change in separation width with respect to the wavelength in that direction can be suppressed. When FIG. 7 is compared with FIG. 14, the difference in the arrival position with respect to the wavelength of the outgoing light beam group 74 and the outgoing light beam group 144 which are finally separated twice is obvious, and the change to the wavelength can be suppressed by adopting the configuration of FIG. Can do.

  In this way, by adopting a configuration in which the change in the separation width of the light beam with respect to each wavelength is reversed, it is possible to suppress a change in the separation wavelength of the light beam in the final optical element. However, if the same material is used for both the base materials 11 and 12 in FIG. 13, the desired separation width cannot be completely corrected in consideration of the wavelength characteristics. This can solve both problems by effectively combining substrates having different dispersions.

  As described above, in an optical element having an action of separating incident light beams for each orthogonally polarized light, a change in separation width with respect to wavelength can be suppressed by effectively combining base materials having different dispersions.

  As an aspect of the optical element 10 of Example 1, there exists the following structure.

  The first base material has an ordinary ray refractive index no larger than the extraordinary ray refractive index ne. The second substrate has an ordinary ray refractive index no larger than the extraordinary ray refractive index ne. The angle A formed by orthogonal projections of the optical axes of the first and second substrates on the incident surface is larger than 90 degrees.

  The first base material has an ordinary ray refractive index no larger than the extraordinary ray refractive index ne. The second substrate has an ordinary ray refractive index no smaller than the extraordinary ray refractive index ne. The angle A formed by orthogonal projections of the optical axes of the first and second substrates on the incident surface is smaller than 90 degrees.

  The first base material has an ordinary ray refractive index no smaller than the extraordinary ray refractive index ne. The second substrate has an ordinary ray refractive index no smaller than the extraordinary ray refractive index ne. The angle A formed by orthogonal projections of the optical axes of the first and second substrates on the incident surface is larger than 90 degrees.

  O At least one of the first substrate and the second substrate has a structure smaller than the wavelength of the incident light beam and has optical anisotropy.

  O At least one of the first substrate and the second substrate is made of a resin having optical anisotropy.

  Next, specific examples will be described.

  Example 2 is an optical element that separates an incident light beam into each vertex of a parallelogram as shown in FIG. As design values, the separation width of the outgoing light beam 41 and the outgoing light beam 42, the separation width of the outgoing light beam 41 and the outgoing light beam 43 are 5 μm, and the angle θ formed by the light beams 42, 41 and 43 is 45 degrees. Quartz is used for the first base material 11 constituting the optical element, and lithium niobate is used for the second base material 12. Each of the first and second substrates is a parallel flat plate cut out with an angle formed by the optical axis and the incident surface normal being 45 degrees. This is the angle of the optical axis that maximizes the separation width of the vertically incident light beam in quartz or lithium niobate. As shown in FIG. 6, in order to obtain a beam separation width of 5 μm at a wavelength of 550 nm using lithium niobate, the thickness may be 0.13 mm. As shown in FIG. 12, in order to obtain a light beam separation width of 7 μm at a wavelength of 550 nm using quartz, the thickness may be 0.85 mm. As shown in FIG. 13, the first base material 11 separates the light flux in the long side direction, and the second base material 12 separates the first base material 11 from the separation direction of the first base material 11. Separate in the direction of 135 degrees.

  First, the separation width when the incident light beam is separated into the outgoing light beams 141 and 142 by the wavelength dispersion of the crystal has a difference of 0.22 μm at a wavelength of 400 to 700 nm.

  Further, due to the wavelength dispersion of lithium niobate, the separation width when the incident light beam is separated into the outgoing light beams 141 and 143 has a difference of 1.23 μm at a wavelength of 400 to 700 nm. The amount of deviation of the luminous flux with respect to the wavelength of the outgoing luminous flux group 144 combining this is 0.65 μm in the x direction and 0.86 μm in the y direction.

  That is, the difference between the wavelength 400 nm and the wavelength 700 nm in the outgoing light beam group 144 is 1.08 μm.

  When only lithium niobate is used for the base material and combined in the manner shown in FIG. 7, the difference between the wavelengths of 400 nm and 700 nm in the outgoing light beam group 74 is 2.10 μm. From this, the change of the separation width with respect to the wavelength is approximately halved in the combination of materials and the separation mode.

  The third embodiment is an optical element that separates an incident light beam into each vertex of a parallelogram as in the second embodiment.

Consider a case in which lithium niobate is used for the first base material 11 and a base material using optical anisotropy of a structure composed of a grating finer than the wavelength as shown in FIG. 8 is used for the second base material 12. As for the structure of the second base material 12, assuming that TiO ₂ is used as the medium of the periodic structure (lattice), the pitch of the structure is 80 nm, and the occupation ratio of TiO ₂ is 50%. Each is a parallel flat plate cut out at an angle of 45 degrees between the optical axis and the incident surface normal. As shown in FIG. 6, in order to obtain a light separation width of 5 μm at a wavelength of 550 using lithium niobate, the thickness may be 0.13 mm. As shown in FIG. 10, in order to obtain a separated light flux width of 5 μm at a wavelength of 550 nm having a structure made of a grating finer than the wavelength, the thickness may be 16.2 μm.

  As shown in FIG. 13, the first base material 11 separates the light beam in the long side direction, and the second base material 12 is 135 degrees from the separation direction of the first base material 11. Separate in the direction of.

  First, due to wavelength dispersion when lithium niobate is used, the separation width of the outgoing light beam 141 and the outgoing light beam 142 has a difference of 1.23 μm at a wavelength of 400 to 700 nm.

  Further, due to the wavelength dispersion having a structure finer than the wavelength of light, the separation width of the outgoing light beam 141 and the outgoing light beam 143 is a wavelength of 400 to 700 nm, and a difference of 1.30 μm appears. The amount of deviation of the luminous flux with respect to the wavelength of the outgoing luminous flux group 144 combined with this is 0.31 μm in the x direction and 0.92 μm in the y direction.

  That is, the difference between the positions of wavelengths 400 nm and 700 nm in the outgoing light beam group 144 is 0.97 μm.

  When only lithium niobate is used for the base material and combined in the manner shown in FIG. 7, the difference between the wavelength of 400 nm and 700 nm in the outgoing light beam group 74 is 2.40 μm. From this, the change of the separation width with respect to the wavelength is approximately halved in the combination of materials and the separation mode.

  Example 4 is an optical element using a crystal and lithium niobate and having no wavelength dependency in the x-direction as shown in FIG. The angle θ formed by the light beams 42, 41, 43 is 45 degrees. Each of the two substrates is a parallel plate obtained by cutting an angle formed by the optical axis and the normal to the incident surface at 45 degrees.

  From FIG. 12, when quartz is used, the amount of change in the separation width of the luminous flux per unit thickness of wavelength 400 to 700 nm is 0.00025. As shown in FIG. 6, when lithium niobate is used, the amount of change in the separation width per unit thickness at a wavelength of 400 to 700 nm is 0.00955.

  In order to optimally combine the two, quartz is used for the first substrate 11 and lithium niobate is used for the second substrate 12 in FIG. In order to eliminate the wavelength dependency of the outgoing light beam group 144 in the x direction, the thickness of the crystal and lithium niobate may be set to a ratio of 26: 1. That is, when the thickness of lithium niobate is 0.1 mm, the thickness of the crystal is 2.7 mm, and the x-direction of the outgoing light beam group 144 is completely matched as shown in FIG. The separation width of this optical element is 15.9 μm in the x direction and 3.9 μm in the 45 ° direction from the x direction.

  Since this optical element has a very large difference in the separation width of the light beam depending on the direction, it is preferable to adjust the angle of the optical axis of lithium niobate to lower the normal incident separation width or to combine with other materials.

Example 5 uses lithium niobate and a structure made of a grating finer than the wavelength of the incident light beam so that the incident light beam does not have wavelength dependency in the x direction like the outgoing light beam 144 as shown in FIG. This is an optical element. The angle θ formed by the light beams 42, 41, 43 is 45 degrees. A base material having a structure composed of a grating finer than the wavelength of the incident light beam assumes that the periodic structure (lattice) medium is TiO ₂ and has a structure pitch of 80 nm and a TiO ₂ occupation ratio of 50%.

  Each is a parallel plate cut out at an angle of 45 degrees between the optical axis and the incident surface normal.

  As shown in FIG. 6, when lithium niobate is used, the amount of change in the separation width of the luminous flux per unit thickness of wavelength 400 to 700 nm is 0.00955. From FIG. 10, when a structure finer than the wavelength of the incident light beam is used, the amount of change in the separation width of the light beam per unit thickness of wavelength 400 to 700 nm is 0.08063. In order to optimally combine the two, lithium niobate is used for the first base material 11 in FIG. 13 and a structure finer than the wavelength of the incident light beam is used for the second base material 12. In order to eliminate the wavelength dependency of the outgoing light beam group 144 in the x direction, the thickness of the lithium niobate and the structure may be set to a ratio of 9: 1.

  That is, when the thickness of the structure is 0.01 mm, the thickness of the crystal is 0.09 mm, and the x-direction of the outgoing light beam group 144 is completely matched as shown in FIG. The separation width of this optical element is 3.9 μm in the x direction and 3.1 μm in the 45 ° direction from the x direction.

  As described above, according to each embodiment, by appropriately setting the relationship between the dispersion characteristics of the two base materials, the optical axis, and the normal of the incident surface, an optical that suppresses the change in the separation width of the light beam with respect to the incident angle. An element is obtained. Furthermore, an image pickup unit having the same and an image pickup apparatus having the same and high optical performance can be obtained.

Schematic diagram of main parts of an imaging apparatus having an optical element of the present invention Conceptual diagram of an optical element having an action of separating an incident light beam by using a substrate having optical anisotropy Image diagram of an optical element having an effect of separating a light beam two-dimensionally using optical anisotropy Explanatory drawing of the state of light beam separation of the optical element shown in FIG. Illustration of each refractive index with respect to the wavelength of lithium niobate Explanatory drawing of separation width of perpendicular incident light flux per unit thickness with respect to wavelength of parallel plate cut out at 45 degree angle formed between optical axis of lithium niobate and normal of incidence plane Schematic diagram of outgoing light flux for each wavelength, assuming that the optical element material shown in FIG. 3 has chromatic dispersion Schematic diagram of fine periodic structure Explanatory drawing of each refractive index with respect to the wavelength of the fine periodic structure shown in FIG. Explanatory drawing of the separation width of perpendicular incident light per unit thickness with respect to the wavelength of the fine periodic structure shown in FIG. Illustration of each refractive index with respect to the wavelength of quartz Explanatory drawing of separation width of perpendicular incident light flux per unit thickness with respect to wavelength of parallel plate cut out at 45 degree angle formed by optical axis of crystal and incident surface normal Image diagram when two types of base materials having positive dispersion are used as the base material of the optical element having the function of separating the light beam two-dimensionally using optical anisotropy. Schematic diagram of outgoing light flux for each wavelength of optical element shown in FIG. Image of a parallel plate that has the effect of separating the light beam using optical anisotropy

Explanation of symbols

DESCRIPTION OF SYMBOLS 104 Optical element 10 Optical element 11 1st base material 12 2nd base material 1a, 2a Optical axis 1aH, 1aT, 2aH, 2aT Projection of an optical axis 41-44 Light beam 71-74 Light beam 141-144 Light beam 21 Parallel plate

Claims (8)

  An optical element that separates incident light according to polarization, and has two base materials with different dispersions. Among the two base materials, the first base material separates incident light in a predetermined direction, An optical element characterized in that the second substrate separates incident light in a direction having a component in the predetermined direction and in a direction different from the predetermined direction.   The first base material has an ordinary ray refractive index larger than the extraordinary ray refractive index, and the second substrate has an ordinary ray refractive index larger than the extraordinary ray refractive index. 2. The optical element according to claim 1, wherein an angle formed by orthogonal projections of the optical axis of the base material on the incident surface is larger than 90 degrees.   The first substrate has an ordinary ray refractive index larger than the extraordinary ray refractive index, and the second substrate has an ordinary ray refractive index smaller than the extraordinary ray refractive index. The optical element according to claim 1, wherein an angle formed by orthogonal projections of the optical axis of the base material on the incident surface is smaller than 90 degrees.     The first substrate has an ordinary ray refractive index smaller than the extraordinary ray refractive index, and the second substrate has an ordinary ray refractive index smaller than the extraordinary ray refractive index. 2. The optical element according to claim 1, wherein an angle formed by orthogonal projections of the optical axis of the base material on the incident surface is larger than 90 degrees.   The at least one base material of the first base material or the second base material has a structure smaller than the wavelength of incident light and has optical anisotropy. The optical element described.   2. The optical element according to claim 1, wherein at least one of the first base and the second base is made of a resin having optical anisotropy.   7. An imaging unit comprising: the optical element according to claim 1; and a solid-state imaging element arranged on a light emitting side of the optical element.   An imaging apparatus comprising the imaging unit according to claim 7.