Wavelength separation element and optical element

Abstract

An objective of the present invention is to provide: a wavelength separation element that suppresses the spread of diffracted light even when a diffraction element having a large diffraction angle is used; and an optical element using said wavelength separation element. The present invention solves the abovementioned problem by using said wavelength separation element, which comprises a diffraction element, an optically anisotropic layer, and an adhesive layer or an alignment film layer positioned between the diffraction element and the optically anisotropic layer. The absolute value of an angle α, which is formed by a line normal to the plane of the diffraction element and the direction of regular reflection or regular transmission of incident light, and the absolute value of a diffraction angle β of the diffraction element satisfy the expression "|α|±|β| ≥ 20°." The film thickness distribution of the entirety of the wavelength separation element is within ±3.0% of the average film thickness, and the surface roughness (Ra) thereof is 30 nm or less. In the expression, the positive sign (+) is used if the direction of diffracted light is different from the direction toward the direction of regular reflection or the direction of regular transmission with reference to the line normal to the plane of the diffraction element, and the negative sign (-) is used if the former direction is the same as the latter direction.

Description

Wavelength separation element and optical element

[0001] The present invention relates to the field of optical communication technology, and more particularly to a wavelength separation element and an optical element using the wavelength separation element.

[0002] In optical transmission networks, in order to accommodate expansion of network capacity, there is a demand for improved performance of optical wavelength selective switches (WSSs) used in wavelength division multiplexing communications, as well as for an increase in the number of usable WSSs through miniaturization of the devices.

[0003] In order to separate the wavelengths of the incident light and distribute them in the appropriate directions, a light direction control device, typically a combination of a wavelength separation element and LCOS (Liquid Crystal on Silicon), is used within the device. Such a light direction control device is required to input the light of each wavelength into the optical fiber on the output side with as little loss as possible.

[0004] Japanese Patent Application Laid-Open No. 2016-103021

[0005] In recent years, with the advancement of device performance and miniaturization, wavelength separation elements with particularly large diffraction angles are often used. In such wavelength separation elements, if the diffracted light is spread by the diffraction element itself or the optical member to which the diffraction element is bonded, some light does not enter the optical fiber, resulting in energy loss. As a result of extensive research by the present inventors, it was found that the spread of the diffracted light is caused by the smoothness of the diffraction element itself and the optical member to which the diffraction element is bonded.

[0006] An object of the present invention is to provide a wavelength separation element that can suppress the spread of diffracted light even when a diffraction element having a large diffraction angle is used, and an optical element that uses this wavelength separation element.

[0007] The present inventors have found that the above problems can be solved by the following configuration.

[0008] [1] A wavelength separation element having at least a diffraction element, an optically anisotropic layer, and an adhesive layer or an alignment film layer therebetween, wherein the diffraction element is a diffraction element in which the absolute value of the angle α (|α|) between the normal to the diffraction element plane and the specular reflection or specular transmission direction of incident light, and the absolute value of the diffraction angle β (|β|) of the diffraction element satisfy formula (1), and the thickness distribution of the entire wavelength separation element is within ±3.0% of the average thickness, and the surface roughness (Ra) is 30 nm or less. ||α|±|β||≧20° ... formula (1) Here, the positive (+) sign indicates that the direction of the diffracted light is different from the direction of the specular reflection or specular transmission direction from the normal to the diffraction element plane, and the negative (-) sign indicates that the direction is the same. [2] The wavelength separation element according to [1], wherein the diffraction element is a liquid crystal diffraction element. [3] The wavelength separation element according to [1] or [2], wherein the diffraction element is a stack of multiple liquid crystal diffraction elements. [4] A wavelength separation element according to any one of [1] to [3], wherein the optically anisotropic layer is made of a liquid crystal compound. [5] A wavelength separation element according to any one of [1] to [4], wherein the optically anisotropic layer is made of a liquid crystal compound and is a λ / 4 plate. [6] An optical element having the wavelength separation element according to any one of [1] to [5] and a photorefractive element.

[0009] According to the present invention, it is possible to provide a wavelength separation element that suppresses the spread of diffracted light and has a large diffraction angle, and an optical element that uses this wavelength separation element.

[0010] Fig. 1 is a diagram conceptually showing the configuration of the present invention. Fig. 2 is a diagram for explaining a diffraction element. Fig. 3 is a diagram for explaining a diffraction element. Fig. 4 is a diagram for explaining a diffraction element. Fig. 5 is a diagram for explaining a diffraction element. Fig. 6 is a diagram for explaining a method for measuring diffracted light in an example. Fig. 7 is a conceptual diagram for explaining a liquid crystal diffraction element.

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A wavelength splitting element and an optical element according to the present invention will be described in detail below with reference to preferred embodiments shown in the accompanying drawings.

[0012] In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0013] FIG. 1 conceptually shows an example of an optical element of the present invention using an example of a wavelength separation element of the present invention. The optical element shown in FIG. 1 has a diffraction element 10, an adhesive layer 11, an optically anisotropic layer 12, an adhesive layer 13, and a photorefractive element 14. In this optical element, the diffraction element 10, the adhesive layer 11, and the optically anisotropic layer 12 constitute the wavelength separation element of the present invention. In the wavelength separation element of the present invention, the adhesive layer 11 provided between the diffraction element 10 and the optically anisotropic layer 12 may be an alignment film layer. Similarly, in the optical element of the present invention, the adhesive layer 13 provided between the optically anisotropic layer 12 and the photorefractive element 14 may also be an alignment film layer.

[0014] Here, in the wavelength separation element of the present invention, the absolute value of the angle α (|α|) between the normal to the plane (surface) of the diffraction element and the direction of specular reflection or specular transmission of incident light, and the absolute value of the diffraction angle β (|β|) by the diffraction element satisfy "||α|±|β||≧20°." In this formula, the positive value (+) indicates that the direction of the diffracted light is different from the direction from the normal to the plane of the diffraction element toward the specular reflection or specular transmission direction, and the negative value (-) indicates that the direction is the same. Furthermore, in the wavelength separation element of the present invention, the film thickness distribution throughout the wavelength separation element is within ±3.0% of the average film thickness, and the surface roughness Ra is 30 nm or less. By having such a configuration, the wavelength separation element of the present invention realizes a wavelength separation element with a large diffraction angle and suppressed spread of the diffracted light. This point will be described in more detail later.

[0015] In the wavelength separation element shown in FIG. 1, the diffraction element 10 receives incident light I ０ is diffracted, and the diffracted light I ａ (wavelength a) and diffracted light I ｂ (wavelength b). The optically anisotropic layer 12 separates the incident light I ０ The optical refraction element 14 is provided to change the polarization state of the incident light I into a state suitable for diffraction by the diffraction element 10. In the wavelength separation element, the diffraction element 10 and the optically anisotropic layer 12 are bonded together by an adhesive layer 11. In the optical element shown in FIG. ０The control of the incident angle of the diffraction element 10 and the diffracted light I diffracted by the diffraction element 10 ａ and diffracted light I ｂ In the optical element, the photorefractive element 14 is attached to the optically anisotropic layer 12 by the adhesive layer 13.

[0016] In the optical element shown in FIG. 1, incident light I is a mixture of light of wavelength a and light of wavelength b. ０ For example, the incident light I is incident on one surface of the optical refractive element 14 (for example, a prism) from the normal direction. ０ The incident light I is transmitted through the photorefractive element 14 and the adhesive layer 13 and enters the optically anisotropic layer 12. ０ The incident light I is converted into circularly polarized light, for example, and passes through the optically anisotropic layer 12, then passes through the adhesive layer 11, and enters the diffraction element 10. In the optical element (wavelength separation element) shown in FIG. 1, the diffraction element 10 is, for example, a reflective diffraction element. ０ is reflected and diffracted by the diffraction element 10, and the diffraction angle varies depending on the wavelength a and wavelength b of the light. ａ and diffracted light I ｂ The diffracted light I is separated according to wavelength. ａ and diffracted light I ｂ The diffracted light I is transmitted through the adhesive layer 11 and enters the optically anisotropic layer 12. ａ and diffracted light I ｂ The diffracted light I is converted into linearly polarized light, for example, and then passes through the optically anisotropic layer 12, the adhesive layer 13, and enters the photorefractive element 14. ａ and diffracted light I ｂ The light beams of wavelength a are transmitted through the optical refractive element 14 with their propagation directions controlled by the refraction of the optical refractive element 14, and are divided into light beams of wavelength a (diffracted light beams I ａ ) and light of wavelength b (diffracted light I ｂ) and is output from the optical element. The light of wavelength a and the light of wavelength b output from the optical element are incident, for example, on optical fibers that make up an optical transmission network. Here, both the light of wavelength a and the light of wavelength b have been wavelength-separated by the wavelength separation element of the present invention, and the spread of the light is suppressed. Therefore, both the light of wavelength a and the light of wavelength b are properly input into the optical fiber without any loss and are transmitted through the optical transmission network.

[0017] (Diffraction Element) In the wavelength separation element of the present invention, the diffraction element 10 can be any of various known diffraction elements, including, but not limited to, liquid crystal diffraction elements, surface relief elements having a fine concavo-convex pattern, and volume holograms having a refractive index distribution within the element. Among these, liquid crystal diffraction elements are preferred. In particular, liquid crystal diffraction elements formed using a composition containing a liquid crystal compound and having a predetermined liquid crystal orientation pattern in which the orientation of the optical axis derived from the liquid crystal compound continuously rotates toward one direction in the plane are preferred. Furthermore, liquid crystal diffraction elements having a so-called twist structure (torsion structure) in which the orientation of the liquid crystal compound continuously changes from one interface side to the other interface side in the thickness direction, and a cholesteric structure with a short twist structure period, are preferred from the viewpoints of high efficiency of diffracted light and polarization maintenance. In the wavelength separation element of the present invention, the diffraction element may be a reflective diffraction element or a transmissive diffraction element.

[0018] FIG. 7 conceptually illustrates the above-described liquid crystal orientation pattern in a liquid crystal diffraction element. FIG. 7 shows the surface of the alignment film side of the liquid crystal layer constituting the liquid crystal diffraction element. As described above, this liquid crystal orientation pattern is a liquid crystal orientation pattern in which the orientation of the optical axis 40A derived from the liquid crystal compound 40 changes while continuously rotating in one direction within the plane of the liquid crystal layer. The optical axis 40A derived from the liquid crystal compound 40 is the slow axis of the liquid crystal compound 40. In FIG. 7, a rod-shaped liquid crystal compound is exemplified as the liquid crystal compound 40, so the optical axis 40A is aligned along the long axis direction of the liquid crystal compound. As shown in FIG. 7, in a liquid crystal layer having this liquid crystal orientation pattern, the liquid crystal compound 40 is two-dimensionally aligned in the orthogonal X and Y directions within the plane.

[0019] In the liquid crystal layer shown in the figure, the orientation of the optical axis 40A changes while continuously rotating toward the X direction. That is, the angle between the optical axis 40A of the liquid crystal compound 40 aligned in the X direction and the X direction changes sequentially from θ to θ+180° or θ-180° along the X direction. On the other hand, the orientation of the optical axis 40A is the same in the Y direction, which is orthogonal to the X direction. In the X direction, the length over which the orientation of the optical axis 40A rotates 180°, that is, the distance Λ over which the angle between the optical axis 40A and the X direction changes from θ to θ+180° or θ-180° along the X direction, is one period in the diffractive structure of the liquid crystal diffraction element. In other words, when the orientation of the optical axis 40A at a certain position in the X direction is 0°, the distance Λ over which the orientation of the optical axis 40A continuously changes from 0° to 180° is one period in the diffractive structure of the liquid crystal diffraction element. In the present invention, one period in the diffractive structure of the liquid crystal diffraction element is basically constant, and the distance Λ of this one period corresponds to the in-plane pitch a, which will be described later, in the liquid crystal diffraction element.

[0020] (Reflective Diffraction Element) In the wavelength separation element of the present invention, there are no limitations on the reflective diffraction element, and various known reflective diffraction elements can be used. In the case of a reflective diffraction element, the in-plane pitch a [nm], i.e., the distance of one period in the diffraction structure of the diffraction element, is determined from the following equation for first-order diffracted light. In a reflective diffraction element, the shorter the in-plane pitch a, i.e., the shorter the period in the diffraction structure, the larger the diffraction angle of light by the diffraction element. In the case of a reflective liquid crystal diffraction element, the optical alignment film can be subjected to appropriate interference exposure based on this. n × a × (sin β + sin α) = λ. Here, in the case of a liquid crystal diffraction element, as described above, the in-plane pitch a is the distance over which the liquid crystal compound continuously changes in plane from 0° to 180°. Furthermore, n is the ambient refractive index on the incident light side in contact with the diffraction element, λ is the wavelength [nm] of the incident light, α is the angle between the light incident on the diffraction element and the normal to the surface of the diffraction element, and β is the angle between the reflected diffracted light by the diffraction element and the normal to the surface of the diffraction element.

[0021] A reflective liquid crystal diffraction element is a cholesteric liquid crystal layer having the above-mentioned liquid crystal orientation pattern. That is, a reflective liquid crystal diffraction element must have a cholesteric orientation in which the liquid crystal compound is helically twisted and oriented in the thickness direction. In other words, a reflective liquid crystal diffraction element must have a structure in which a cholesteric liquid crystal phase is fixed. Furthermore, in the cholesteric liquid crystal layer, the film thickness d [nm] of the liquid crystal layer can be appropriately adjusted according to the required efficiency. For example, to improve the light utilization efficiency, the thickness of the liquid crystal layer can be adjusted to be 7 times or more the helical pitch of the cholesteric orientation. That is, the cholesteric liquid crystal layer has this helical pitch repeated multiple times, and it is preferable that the helical pitch be repeated 7 times or more. As is well known, a cholesteric liquid crystal layer selectively reflects light in a specific wavelength range depending on the length of the helical pitch of the cholesteric orientation. Specifically, the longer the helical pitch of a cholesteric liquid crystal layer, the more selectively it reflects light of longer wavelengths. Therefore, the helical pitch of the cholesteric liquid crystal layer can be appropriately determined depending on the wavelength used (wavelength of incident light). Specifically, the helical pitch of the cholesteric orientation is the thickness between the helical twisted orientation of the liquid crystal compound in the thickness direction of the cholesteric liquid crystal layer, where the orientation of the liquid crystal compound changes from 0 ° to 360 ° in the thickness direction. Such reflective liquid crystal diffraction elements include, for example, the liquid crystal diffraction elements described in JP 2023-160888 A and WO 2021-256422 A. Various known reflective liquid crystal diffraction elements are available.

[0022] (Transmissive Diffraction Element) On the other hand, the diffraction element may be a transmissive diffraction element. There are no limitations on the transmissive diffraction element, and various known transmissive diffraction elements can be used. In the case of a transmissive diffraction element, the in-plane pitch a [nm], i.e., one period in the diffraction structure of the diffraction element, is determined from the following equation for first-order diffracted light. Even in a transmissive diffraction element, the shorter the in-plane pitch a, i.e., one period in the diffraction structure, the larger the diffraction angle of light by the diffraction element. In the case of a liquid crystal diffraction element, the optical alignment film can be subjected to appropriate interference exposure based on this. n × a × (sin β - sin α) = λ. As described above, in the case of a liquid crystal diffraction element, the in-plane pitch a is the distance over which the liquid crystal compound continuously changes in plane from 0° to 180°. Furthermore, n is the ambient refractive index on the incident light side in contact with the diffraction element, λ is the wavelength [nm] of the incident light, α is the angle between the light incident on the diffraction element and the normal to the surface of the diffraction element, and β is the angle between the transmitted diffracted light and the normal to the surface of the diffraction element.

[0023] The transmissive liquid crystal diffraction element has a liquid crystal layer having the liquid crystal orientation pattern described above. In addition, in the case of a transmissive liquid crystal diffraction element, the liquid crystal compound forming the liquid crystal layer and the film thickness of the liquid crystal layer are appropriately selected so that Δnλ × d is λ / 2, which is represented by the product of the refractive index anisotropy Δnλ at the wavelength λ [nm] of the optically anisotropic layer described later and the film thickness d [nm] of the liquid crystal layer. Such a transmissive liquid crystal diffraction element, for example, the liquid crystal diffraction elements described in WO 2021 / 256413 and JP 2023-160888 A, various known transmissive liquid crystal diffraction elements, can be used.

[0024] (Diffraction Element with Large Diffraction Angle) In the present invention, a diffraction element with a large diffraction angle refers to a diffraction element with a large angle between the specular reflection direction or specular transmission direction of incident light and the diffracted light by the diffraction element, as conceptually shown in FIGS. 2 to 5 . Specifically, the absolute value of the angle α between the normal to the diffraction element plane and the specular reflection direction or specular transmission direction of the incident light, and the absolute value of the diffraction angle β of the diffraction element, satisfy the following formula (1): ||α|±|β||≧20°...formula (1). Here, the specular reflection direction refers to the direction of light that is reflected in a direction different from the incident light at the same angle as the angle between the incident light and the normal to the diffraction element plane. The specular transmission direction refers to the direction of light that transmits through a material in a direction parallel to the incident light. Furthermore, ||α|±|β|| is positive (+) when the direction of the diffracted light is different from the direction toward the specular reflection direction or specular transmission direction from the normal to the diffraction element plane, and negative (-) when the direction is the same. In other words, equation (1) indicates that the diffraction angle increases as the direction of diffracted light deviates from the direction of specular reflection or specular transmission. When using a diffraction element with such a large diffraction angle, the film thickness distribution and surface roughness Ra have a significant effect on the spreading and separation of diffracted light.

[0025] FIG. 2 shows the diffracted light I dif The direction of the (dotted line) is the normal to the plane (surface) of the diffraction element, and the incident light I in In this case, as shown in FIG. 2, the normal (dashed line) to the plane (surface) of the diffraction element and the incident light I in the absolute value |α| of the angle α between the direction of specular reflection of the diffracted light I dif and the absolute value |β| of the diffraction angle β, ||α|+|β||, is 20° or more.

[0026] FIG. 3 shows the diffracted light I dif The direction of the (dotted line) is the normal to the plane (surface) of the diffraction element, and the incident light I in In this case, as shown in FIG. 3, the normal (dashed line) to the plane (surface) of the diffraction element and the incident light I in the absolute value |α| of the angle α between the direction of specular reflection of the diffracted light Idif and the absolute value |β| of the diffraction angle β, ||α|-|β||, is 20° or more.

[0027] FIG. 4 shows the diffraction light I dif The direction of the (dotted line) is the normal to the plane (surface) of the diffraction element, and the incident light I in In this case, as shown in FIG. 4, the normal (dashed line) to the plane (surface) of the diffraction element and the incident light I in The absolute value of the angle α between the direction of regular transmission of the diffracted light I dif and the absolute value |β| of the diffraction angle β, ||α|+|β||, is 20° or more.

[0028] FIG. 5 shows the diffraction light I dif The direction of the (dotted line) is the normal to the plane (surface) of the diffraction element, and the incident light I in 5, the normal (dashed line) to the plane (surface) of the diffraction element and the incident light I in and the absolute value |α| of the angle α between the direction of regular transmission of the diffracted light I dif and the absolute value |β| of the diffraction angle β, ||α|-|β||, is 20° or more.

[0029] (Formation of in-plane alignment pattern) There are no particular limitations on the formation of the in-plane alignment pattern required for diffraction, and known methods can be used. As an example, interference exposure using circularly polarized light may be used, as in the exposure apparatus described in FIG. 3 of WO 2021 / 256413. Specifically, in the case of a liquid crystal diffraction element, a film that will become a photo-alignment film containing a photo-alignment material is formed on the surface of an appropriately selected support. This photo-alignment film is subjected to interference exposure with circularly polarized light having an opposite rotation direction using the exposure apparatus described in FIG. 3 of WO 2021 / 256413, thereby forming an alignment pattern corresponding to the liquid crystal alignment pattern on the photo-alignment film. Thereafter, for example, a composition containing a liquid crystal compound is applied to the photo-alignment film on which the alignment pattern has been formed, dried (heated), and the composition is cured by ultraviolet (UV) irradiation or the like, as needed, to form a liquid crystal layer. As a result, a liquid crystal layer diffraction element having a liquid crystal alignment pattern in which the orientation of the optical axis derived from the liquid crystal compound changes while continuously rotating in one direction in the plane, as shown in FIG. 7, can be formed. To form an orientation pattern with the in-plane pitch required to obtain a diffraction angle that separates light according to the wavelength of the incident light, the optical elements of the exposure device can be installed so that the absolute value of the angle of incidence of each interference exposure is the same angle relative to the normal direction of the photo-alignment film surface. For example, in the case of the exposure device described in FIG. 3 of WO 2021 / 256413, the angle at which two circularly polarized light beams with opposite rotation directions that interfere and expose the photo-alignment film intersect is adjusted. By adjusting this angle, the length of the in-plane pitch can be adjusted, and in the case of a liquid crystal diffraction element, the in-plane pitch (the distance Λ of one period) of the liquid crystal orientation pattern can be adjusted. As mentioned above, in a diffraction element, the smaller the in-plane pitch, the larger the diffraction angle of the diffracted light. In addition, in a diffraction element, generally, the longer the wavelength of light, the larger the diffraction angle of the diffracted light.

[0030] (Formation of twist structure or cholesteric orientation) As mentioned above, it is preferable that the transmissive liquid crystal diffraction element has a twist structure in which the liquid crystal compound is helically twisted in the thickness direction. The twist angle (torsion angle) of this liquid crystal compound is preferably 360° or less. Furthermore, the reflective liquid crystal diffraction element has a cholesteric liquid crystal layer in which the liquid crystal compound is cholesterically oriented, repeating a helical pitch in which the liquid crystal compound is helically twisted 360° in the thickness direction multiple times. To obtain the desired twist structure and cholesteric orientation in the thickness direction, for example, the amount of chiral agent added to the liquid crystal composition for forming the liquid crystal layer described in WO 2021 / 256413 can be appropriately adjusted.

[0031] Regarding the support and the photo-alignment film, reference can be made to International Publication No. 2021 / 256413. However, when the wavelength separation element of the present invention is used for optical communication, it is necessary to consider that the wavelength of light used for optical communication is often in the infrared wavelength range.

[0032] The diffraction element may be a laminate of multiple liquid crystal diffraction elements. For example, if the liquid crystal diffraction element is a reflective diffraction element using a cholesteric liquid crystal layer having the above-mentioned liquid crystal orientation pattern, the diffraction element may be a laminate of multiple liquid crystal diffraction elements having cholesteric liquid crystal layers that selectively reflect different wavelength ranges. In addition, as a diffraction element of a laminate of multiple liquid crystal diffraction elements, a laminate of a reflective liquid crystal diffraction element with a right-handed cholesteric liquid crystal layer and a reflective liquid crystal diffraction element with a left-handed cholesteric liquid crystal layer, a laminate of reflective diffraction elements with different in-plane pitches, a laminate of elements that combine both, etc. can also be used.

[0033] (Optically Anisotropic Layer) In the wavelength separation element of the present invention, an optically anisotropic layer 12 is bonded to the diffraction element 10 via an adhesive layer 11 (alignment layer). The optically anisotropic layer 12 is not particularly limited, and various types can be used. A preferred example is an optically anisotropic layer 12 that adjusts the polarization state of light transmitted through the optically anisotropic layer 12 to a desired polarization state, i.e., an optically anisotropic layer 12 that adjusts the polarization state of light incident on the diffraction element 10 to a desired polarization state. The use of such an optically anisotropic layer 12 is preferable because it makes it less likely for the polarization state to change due to reflection, refraction, etc. The material for forming the optically anisotropic layer 12 is not limited, and known materials such as polymers, liquid crystal compounds, and inorganic substances can be used. In particular, when the diffraction element is a liquid crystal diffraction element, an optically anisotropic layer using the same type of liquid crystal compound as the liquid crystal diffraction element is more preferred in terms of reducing the refractive index difference.

[0034] Specifically, when the diffraction element is a liquid crystal diffraction element, a retardation layer, particularly a λ / 4 wavelength plate, is preferably used as the optically anisotropic layer 12. When the diffraction element 10 is a liquid crystal diffraction element, circularly polarized light can be incident thereon to efficiently diffract the incident light. In particular, a reflective liquid crystal diffraction element (cholesteric liquid crystal layer) can efficiently diffract and reflect the incident light by incident circularly polarized light with a predetermined rotation direction. Therefore, when the diffraction element 10 is a liquid crystal diffraction element, a retardation layer, particularly a λ / 4 wavelength plate, is used as the optically anisotropic layer 12, and by converting the light incident on the diffraction element 10 into circularly polarized light, the diffraction element 10 can efficiently diffract the incident light and separate the wavelengths. There are no limitations on the λ / 4 wavelength plate, and any known λ / 4 wavelength plate can be used. Examples include a stretched polycarbonate film, a stretched norbornene-based polymer film, a transparent film containing and oriented inorganic particles having birefringence such as strontium carbonate, a thin film obtained by obliquely depositing an inorganic dielectric on a support, a film in which a polymerizable liquid crystal compound is uniaxially oriented and fixed in orientation, and a film in which a liquid crystal compound is uniaxially oriented and fixed in orientation, etc. Among these, for the reasons mentioned above, a film in which a liquid crystal compound is uniaxially oriented and fixed in orientation is preferably used.

[0035] (Adhesive Layer) The wavelength separation element of the present invention is constructed by bonding a diffraction element 10 and an optically anisotropic layer 12 together with an adhesive layer 11. Furthermore, a light refraction element 14, which will be described later, is bonded to the optically anisotropic layer 12 with an adhesive layer 13. Note that the following description will be given using the adhesive layer 11 as a representative example, but the same applies to the adhesive layer 13. There are no limitations on the adhesive layer 11 (adhesive layer 13), and various known adhesives can be used. Examples include a UV adhesive and an adhesive layer using SiOx. Furthermore, a pressure-sensitive adhesive can also be used for the adhesive layer 11.

[0036] Any commercially available adhesive can be used as the adhesive. Examples include epoxy resin adhesives and acrylic resin adhesives. A pressure-sensitive adhesive can also be used for the adhesive layer 11. From the viewpoint of reducing the surface roughness Ra of the wavelength separation element, the pressure-sensitive adhesive and adhesive can be selected to have an appropriate viscoelasticity or thickness so as to embed the surface irregularities of the layer to be adhered. From the viewpoint of embedding the surface irregularities, the pressure-sensitive adhesive and adhesive preferably have a viscosity of 50 cP or more. Furthermore, the thickness is preferably greater than the height of the surface irregularities.

[0037] The adhesive layer 11 (adhesive and pressure-sensitive adhesive) used to bond each layer preferably has a small difference in refractive index with adjacent layers in order to reduce unnecessary reflection and prevent a decrease in the degree of polarization of transmitted and reflected light. Specifically, the difference in refractive index between the adhesive layer 11 and adjacent layers is preferably 0.05 or less, and more preferably 0.01 or less. The refractive index of the pressure-sensitive adhesive and adhesive can be adjusted, for example, by adding and mixing fine particles. Examples of fine particles to be added include titanium oxide fine particles and zirconia fine particles.

[0038] Furthermore, bonding using an adhesive layer 11 using SiOx can be performed by the following steps (1) to (3): (1) The layers to be laminated are bonded to a temporary support made of a glass substrate. (2) An SiOx layer having a thickness of 100 nm or less is formed on both the surface of the layer to be laminated and the surface of the layer to be laminated by vapor deposition or the like. Vapor deposition can be performed using, for example, a vapor deposition device (model number ULEYES) manufactured by ULVAC, Inc., using SiOx powder as the vapor deposition source. Here, when bonding to a photorefractive element, the SiOx layer may be formed on the surface of the photorefractive element made of glass. (3) After the formed SiOx layers are bonded together, the temporary support is peeled off.

[0039] The wavelength separation element of the present invention can be fabricated by the following method, as an example. Note that the following example uses a liquid crystal diffraction element as the diffraction element, but wavelength separation elements using other diffraction elements can also be fabricated in accordance with the following method. First, an alignment film is formed on the surface of an appropriately selected support as described above, and a diffraction element 10 (liquid crystal diffraction element) is formed on this alignment film. This results in a laminate having the support, alignment film, and diffraction element 10. The alignment film and liquid crystal diffraction element may be formed by a known method appropriate to the forming materials, such as the method described above. On the other hand, an alignment film is formed on the surface of an appropriately selected support, and an optically anisotropic layer 12 made of, for example, a liquid crystal compound is formed on this alignment film. This results in a laminate having the support, alignment film, and optically anisotropic layer 12. The alignment film and optically anisotropic layer 12 may be formed by a known method appropriate to the forming materials, such as the method described above. Next, an adhesive to form the adhesive layer 11 is applied to the formed diffraction element 10 and / or optically anisotropic layer 12, and the two laminates are laminated together, with the diffraction element 10 and the optically anisotropic layer 12 facing each other. The adhesive is then cured by a method appropriate for the adhesive, such as ultraviolet irradiation or heating, to bond the diffraction element 10 and the optically anisotropic layer 12 together via the adhesive layer 11. After the diffraction element 10 and the optically anisotropic layer 12 are bonded together, the alignment film and support are peeled off from the diffraction element 10, and the alignment film and support are peeled off from the optically anisotropic layer 12. In this way, the wavelength separation element of the present invention, which includes the diffraction element 10, the adhesive layer 11, and the optically anisotropic layer 12, is produced.

[0040] (Support) As described above, the diffraction element 10 (liquid crystal diffraction element) and the optically anisotropic layer 12 are formed using a support, as an example. In this case, it is preferable that the diffraction element 10 and the optically anisotropic layer 12 are formed on a support with high rigidity. This is because forming the diffraction element 10 and the optically anisotropic layer 12 on a support with high rigidity can increase the smoothness of the laminated layers. The support with high rigidity is not particularly limited, but examples include a glass plate and a hard-coated film. There is also no limitation on the elastic modulus (Young's modulus) of the support, but it is preferably 5 GPa or more, more preferably 25 GPa or more, and even more preferably 50 GPa or more.

[0041] (Lamination via Alignment Film) As described above, in the wavelength separation element of the present invention, an alignment film layer may be used instead of the adhesive layer 11. There are no limitations on the alignment film layer, and known alignment films such as the above-mentioned photo-alignment film can be used depending on the material of the liquid crystal layer to be formed on the alignment film. When an alignment film is used instead of the adhesive layer 11, the diffraction element 10 and the optically anisotropic layer 12 are adhered via the alignment film. In other words, when forming layers, the next layer is formed on the already formed layer via the alignment film, thereby eliminating the need for an adhesive layer.

[0042] For example, after preparing a laminate having a support, an alignment film, and an optically anisotropic layer 12 as described above, an alignment film may be formed on the surface of the optically anisotropic layer, and a liquid crystal diffraction element may be formed on this alignment film as the diffraction element 10. Alternatively, after preparing a laminate having a support, an alignment film, and a diffraction element 10 as described above, an alignment film may be formed on the surface of the diffraction element 10, and an optically anisotropic layer 12 made of a liquid crystal compound may be formed on this alignment film.

[0043] The same applies to the adhesive layer 13 in the optical element of the present invention in the above respects. That is, in the optical element shown in Fig. 1 described later, an alignment film may be formed on the surface of the photorefractive element 14, and an optically anisotropic layer 12 made of a liquid crystal compound may be formed on this alignment film.

[0044] The optical element of the present invention comprises such a wavelength separation element of the present invention and a photorefractive element. The optical element of the present invention shown in Figure 1 comprises a wavelength separation element of the present invention having a diffraction element 10, an adhesive layer 11, and an optically anisotropic layer 12, and a photorefractive element 14. In this optical element, the optically anisotropic layer 12 and the photorefractive element 14 are bonded together by an adhesive layer 13.

[0045] (Photorefractive element) The photorefractive element 14 is used to control the angle of incidence of light incident on the diffraction element 10 and to control the direction of diffracted light. There are no limitations on the photorefractive element 14, and various known members capable of refracting light can be used. Suitable examples of the photorefractive element 14 include lenses and prisms.

[0046] (Film thickness distribution and surface roughness Ra of the entire wavelength separation element) The wavelength separation element of the present invention has a film thickness distribution of the entire wavelength separation element within ±3.0% of the average film thickness, and a surface roughness (Ra) of 30 nm or less. In the present invention, the entire wavelength separation element is a laminate of a diffraction element, an optically anisotropic layer, and an adhesive layer (or alignment film layer) therebetween. That is, in the present invention, the entire wavelength separation element is from the surface of the diffraction element (the side opposite the adhesive layer) to the surface of the optically anisotropic layer (the side opposite the adhesive layer). In addition, in the present invention, the surface roughness Ra is the surface roughness Ra of the diffraction element 10. By having such a configuration, the wavelength separation element of the present invention realizes a large diffraction angle using a diffraction element that satisfies the above-mentioned formula (1), and realizes a wavelength separation element that suppresses the spread of diffracted light despite having such a large diffraction angle.

[0047] As is well known, a diffraction element generally has a larger diffraction angle for light with a longer wavelength. Therefore, when light containing multiple wavelengths is incident on a diffraction element, the element functions as a wavelength separation element, separating the light according to its wavelength. However, according to the inventors' research, a diffraction element with a large diffraction angle, particularly a wavelength separation element that is a laminate including a diffraction element, may cause the diffracted light to spread. In severe cases, light of the same wavelength may be separated. The inventors have extensively investigated the cause of this problem. As a result, they have found that the cause of the spread of diffracted light is the film thickness distribution of the entire wavelength separation element and the surface roughness Ra of the wavelength separation element. In other words, when the film thickness distribution of the entire wavelength separation element and the surface roughness Ra of the wavelength separation element are large, the diffracted light spreads and even separates. Here, in a wavelength separation element, a large waviness, such as a film thickness distribution, can function similarly to a lens with a large radius of curvature (gentle refraction). Therefore, a large film thickness distribution of a wavelength separation element can cause the diffracted light to spread. Furthermore, in a wavelength separation element, the surface roughness Ra, which corresponds to minute surface irregularities, can itself function similarly to a diffraction element. Therefore, if the surface roughness Ra is large, this can cause diffracted light of a single wavelength to be separated. In the following description, "separation of diffracted light" refers to the separation of diffracted light of a single wavelength.

[0048] The present invention was made possible by obtaining such findings, and aims to achieve a wavelength separation element having a film thickness distribution within ±3.0% of the average film thickness and a surface roughness Ra of 30 nm or less. The wavelength separation element of the present invention has such a configuration, thereby realizing a wavelength separation element that suppresses the spread of diffracted light despite having a large diffraction angle. If the film thickness distribution of the entire wavelength separation element exceeds ±3.0% of the average film thickness, the spread of diffracted light cannot be sufficiently suppressed, resulting in problems such as the diffracted light not properly entering the desired position on a light-receiving element such as an LCOS or optical fiber, or only partially entering the desired position. Furthermore, if the surface roughness Ra of the wavelength separation element exceeds 30 nm, similar to the above, the spread of diffracted light cannot be sufficiently suppressed, resulting in problems such as the diffracted light not properly entering the desired position on a light-receiving element such as an LCOS or optical fiber, or only partially entering the desired position. In the wavelength separation element of the present invention, the film thickness distribution of the entire wavelength separation element is preferably within ±2.5% of the average film thickness, and more preferably within ±2.0% of the average film thickness. In the wavelength splitting element of the present invention, the surface roughness Ra is preferably 25 nm or less, and more preferably 20 nm or less.

[0049] In the wavelength separation element of the present invention, the smaller the film thickness distribution and surface roughness Ra of the entire wavelength separation element, the better, and there is no lower limit. However, considering productivity, production cost, yield, and coating accuracy of the device, it is preferable that the film thickness distribution of the entire wavelength separation element in the wavelength separation element of the present invention is an average value of the average film thickness ±0.5% or more. For the same reason, it is also preferable that the surface roughness Ra of the wavelength separation element of the present invention is 1 nm or more.

[0050] Furthermore, in a wavelength separation element having a diffraction element, an optically anisotropic layer, and an adhesive layer (alignment layer) therebetween, if the film thickness distribution and surface roughness Ra of each component constituting the wavelength separation element are large, the diffracted light may be broadened and separated, as described above. Considering this point, in the wavelength separation element of the present invention, it is preferable that the diffraction element, adhesive layer (alignment layer), and optically anisotropic layer also have small film thickness distributions and surface roughness Ra. Specifically, in the wavelength separation element of the present invention, it is preferable that each of these components has a film thickness distribution within ±2.0% of the average film thickness and a surface roughness Ra of 20 nm or less. By satisfying these conditions, the wavelength separation element of the present invention is preferable in that it can more effectively suppress the broadening of diffracted light, more effectively suppress the separation of diffracted light, and accurately direct diffracted light to the desired position of a light-receiving element such as an LCOS or optical fiber. In addition, in the present invention, the film thickness distribution of each component constituting the wavelength separation element is more preferably within ±1.5% of the average film thickness, and even more preferably within ±1.0% of the average film thickness. In the present invention, the surface roughness Ra of each member constituting the wavelength splitting element is more preferably 15 nm or less, and even more preferably 10 nm or less.

[0051] In a wavelength separation element having a diffraction element, an optically anisotropic layer, and an adhesive layer (alignment layer) therebetween, there is no limitation on the method for producing a wavelength separation element whose film thickness distribution and surface roughness Ra satisfy the above-mentioned conditions, and various methods can be used. As an example, a method is exemplified in which a support having high surface smoothness and high rigidity (elastic modulus), such as a glass plate or a film with a hard coat layer, is used as a support for forming a diffraction element, particularly a liquid crystal diffraction element, and an optically anisotropic layer, particularly an optically anisotropic layer made of a liquid crystal compound. As will be shown in the examples later, when forming a liquid crystal diffraction element and an optically anisotropic layer made of a liquid crystal compound, if a general resin film such as a PET (polyethylene terephthalate) film is used as the support for forming at least one of them, the overall film thickness distribution and / or surface roughness Ra tend to be large. In contrast, by using a glass plate or the like as a support for forming a liquid crystal diffraction element and an optically anisotropic layer made of a liquid crystal compound, a wavelength separation element having the above-mentioned film thickness distribution and surface roughness Ra can be stably produced.

[0052] The order of the steps for stacking each component is not particularly limited and can be selected arbitrarily. Here, if another layer is stacked on a layer with a large film thickness distribution and surface roughness, these may be further amplified. Therefore, from the viewpoint of reducing the film thickness distribution and surface roughness Ra of the entire wavelength separation element, it is preferable to stack the layers starting from the layer with the smallest film thickness distribution and surface roughness Ra. Furthermore, in addition to the film thickness distribution and surface roughness Ra, the order of stacking can also be selected from the viewpoint of quality evaluation, manufacturing yield, cost reduction, etc.

[0053] Furthermore, if dust adheres to the surfaces of the components during the production of the diffraction element and the optically anisotropic layer, etc., and during bonding of these components, even if highly smooth components are used, the film thickness distribution and surface roughness Ra of the entire wavelength separation element may deteriorate. Therefore, in order to prevent dust from adhering to these components, a clean room, a static electricity removal fan, an air spray, etc. may be used as appropriate during the production of each component and bonding of the components.

[0054] The film thickness of the wavelength separation element can be measured, for example, by observing the cross sections of the entire wavelength separation element and each of its constituent layers using an optical microscope and a scanning electron microscope (SEM). Specifically, the cross section of the entire wavelength separation element is imaged using an SEM or the like to obtain image data for multiple cross sections (e.g., five cross sections) at different locations. The positions of the upper and lower end surfaces at multiple positions in each image data are extracted from the multiple image data measured at different locations on the cross sections, allowing the film thickness distribution and average film thickness to be calculated. Furthermore, the surface roughness Ra (arithmetic mean roughness) can be measured using a commercially available measuring instrument, such as the non-contact surface / layer cross-sectional shape measurement system VertScan (manufactured by Ryoka Systems Co., Ltd.).

[0055] The features of the present invention will be explained in more detail below with reference to examples. The materials, reagents, amounts used, amounts of substances, ratios, treatment details, and treatment procedures shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.

[0056] (Formation of Liquid Crystal Diffraction Element) The following composition 1 was prepared as a liquid crystal composition for forming a liquid crystal diffraction element.

[0057] Composition 1 ----------------------------------- Rod-like liquid crystal compound L-1 100.00 parts by mass Photopolymerization initiator (KAYACURE DETX-S, manufactured by Nippon Kayaku) 1.00 part by mass Chiral agent Ch-1 1.70 parts by mass Methyl ethyl ketone 142.06 parts by mass

[0058] Rod-shaped liquid crystal compound L-1 (containing the following structure in the mass ratio shown on the right)

[0059] Chiral agent Ch-1

[0060] Next, a composition containing a photo-alignment material (manufactured by DIC Corporation, SD-1) was applied to a glass support (manufactured by Cornings, EagleXG) and dried. This coating film was subjected to interference exposure using the exposure apparatus shown in FIG. 3 of WO 2021 / 256413 to form a photo-alignment film. Then, the above-mentioned composition 1 was applied to the photo-alignment film, dried, and UV-cured under a nitrogen atmosphere to form a reflective liquid crystal diffraction element.

[0061] (Formation of Optically Anisotropic Layer (Example 1)) The following composition 2 was prepared as a liquid crystal composition for forming an optically anisotropic layer.

[0062] Composition 2 ----------------------------------- Rod-like liquid crystal compound L-1 100.00 parts by mass Photopolymerization initiator (KAYACURE DETX-S, manufactured by Nippon Kayaku) 1.00 part by mass Surfactant-1 0.20 parts by mass Methyl ethyl ketone 142.06 parts by mass ---------------------------------------------------

[0063] Surfactant-1

[0064] Next, a composition containing a photo-alignment material was applied to a glass support (Cornings, Eagle XG, elastic modulus 77 GPa) and dried. This coating film was exposed to polarized UV to form a photo-alignment film. Composition 2 was applied to the formed photo-alignment film and dried. Composition 2 was then UV-cured under a nitrogen atmosphere to form an optically anisotropic layer.

[0065] Comparative Example 1 An optically anisotropic layer was formed in the same manner as in Example 1, except that a PET film (manufactured by Fujifilm, film thickness 50 μm) was used instead of a glass plate as the support on which the optically anisotropic layer was formed.

[0066] Comparative Example 2 An optically anisotropic layer was formed in the same manner as in Example 1, except that the optically anisotropic layer (Composition 2) was dried while being exposed to strong air.

[0067] (Comparative Example 3) An optically anisotropic layer was formed in the same manner as in Example 1, except that a PET film (manufactured by Fujifilm, film thickness 50 μm) was used instead of a glass plate as a support for forming the optically anisotropic layer, and the optically anisotropic layer (composition 2) was dried while being exposed to strong air.

[0068] (Fabrication of Wavelength Splitter) The liquid crystal diffraction element and each optically anisotropic layer formed as described above were bonded together with a UV adhesive (NOA87, manufactured by Norland Corp.) After the UV adhesive was cured with UV light, the supports were peeled off to form a wavelength splitter.

[0069] (Preparation of Wavelength Separation Element of Example 1) A UV adhesive (NOA87, manufactured by Norland) was applied to a liquid crystal diffraction element layer formed using a glass support. Then, an optically anisotropic layer formed using a glass support was pressed against the layer so that the optically anisotropic layer side was facing the UV adhesive side, and UV irradiation was performed to bond the layers together. Then, the photo-alignment film was dissolved in pure water, and the bonding layer between the liquid crystal diffraction element and the optically anisotropic layer was peeled off from the support, thereby preparing the wavelength separation element of Example 1. Therefore, this wavelength separation element has a configuration in which a liquid crystal diffraction element, an adhesive layer, and an optically anisotropic layer are stacked in this order.

[0070] (Preparation of Wavelength Separation Element of Comparative Example 1) A wavelength separation element of Comparative Example 1 was prepared in the same manner as in Example 1, except that an optically anisotropic layer formed using a support made of a PET film was used.

[0071] (Preparation of wavelength separation element of Comparative Example 2) A wavelength separation element of Comparative Example 2 was prepared in the same manner as in Example 1, except that a glass support was used and an optically anisotropic layer was formed by drying under strong airflow.

[0072] (Preparation of wavelength separation element of Comparative Example 3) A wavelength separation element of Comparative Example 3 was prepared in the same manner as in Example 1, except that a support made of a PET film was used and an optically anisotropic layer was formed by drying under strong airflow.

[0073] (Evaluation of film thickness distribution and surface roughness Ra) The surface roughness Ra and film thickness distribution of the fabricated wavelength separation element were measured. The surface roughness Ra was measured using a VertScan (manufactured by Ryoka Systems Co., Ltd.). The surface roughness Ra was measured on the surface of the diffraction element. The film thickness distribution was calculated from five SEM images taken by cutting a portion of the fabricated wavelength separation element and capturing the cross section with an SEM. The positions of the upper and lower interfaces of the SEM images were profiled using image processing, and the film thickness at each position was calculated from the position information (100 data per image) and magnification, and the average of these was taken as the average film thickness (average film thickness). The film thickness distribution was calculated using the following formula: Film thickness distribution on the + side: 100 x (maximum film thickness - average film thickness) / (average film thickness) Film thickness distribution on the - side: 100 x (minimum film thickness - average film thickness) / (average film thickness)

[0074] (Evaluation of spread of diffracted light) As conceptually shown in Figure 6, an infrared laser (FOLS-02, 1550 nm laser, laser diameter 10 mm, manufactured by Sawaki Kobo Co., Ltd.) was incident on the surface of the fabricated wavelength separation element on the diffraction grating 20 side, and the beam diameter of the diffracted light was measured as the spread of the diffracted light using a beam profiler (BP209IR1 / M, manufactured by Thorlabs). Note that in Figure 6, only the diffraction element 20 is shown to simplify the drawing.

[0075] Incident light I in The incident angle of the infrared laser (solid line) was set to 45°. in The absolute value of the angle α (|α|) between the direction of specular reflection (two-dot chain line) of the diffracted light I dif The absolute value (|β|) of the diffraction angle β of the wavelength separation element I (dashed line) was 50.3°. dif was different from the direction of the specular reflection relative to the normal to the plane of the diffractive element. Therefore, in this example, ||α|+|β||=|45°+50.3°|=95.3°.

[0076] The film thickness distribution, surface roughness, and the corresponding spread of diffracted light are shown in Table 1 below. It was confirmed that the laser diameter did not change in Example 1, whereas the laser diameter spread in the comparative example. This clearly demonstrates the effect of the present invention.

[0077]

[0078] 10, 20 Diffraction element 11 Adhesive layer 12 Optically anisotropic layer 13 Adhesive layer 14 Photorefractive element

Claims

1. A wavelength separation element having at least a diffraction element, an optically anisotropic layer, and an adhesive layer or an alignment film layer therebetween, wherein the absolute value of the angle α (|α|) between the normal to the diffraction element plane and the specular reflection direction or specular transmission direction of incident light, and the absolute value of the diffraction angle β (|β|) of the diffraction element satisfy formula (1), and the thickness distribution of the entire wavelength separation element is within ±3.0% of the average thickness, and the surface roughness (Ra) is 30 nm or less. ||α|±|β||≧20° ... formula (1) Here, the case where the direction of the diffracted light is different from the direction toward the specular reflection direction or specular transmission direction from the normal to the diffraction element plane is positive (+), and the case where the direction is the same is negative (-).

2. The wavelength separation element according to claim 1, wherein said diffractive element is a liquid crystal diffractive element.

3. The wavelength separation element according to claim 1, wherein the diffraction element is a stack of a plurality of liquid crystal diffraction elements.

4. The wavelength separation element according to claim 2, wherein said optically anisotropic layer is made of a liquid crystal compound.

5. The wavelength separation element according to claim 1, wherein said optically anisotropic layer is made of a liquid crystal compound and is a λ / 4 plate.

6. An optical element comprising the wavelength separation element according to any one of claims 1 to 5 and a photorefractive element.

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