CN114488375A

CN114488375A - Optical low-pass filter, manufacturing method of optical low-pass filter, optical equipment and unmanned aerial vehicle

Info

Publication number: CN114488375A
Application number: CN202111269820.7A
Authority: CN
Inventors: 门间文彰; 小泉康平; 山本明; 木村隆治
Original assignee: Jiemufu Zhejiang Photoelectric Co ltd; New World Ltd; Japan Steel Works Ltd
Current assignee: Jiemufu Zhejiang Photoelectric Co ltd; New World Ltd; Japan Steel Works Ltd
Priority date: 2020-11-13
Filing date: 2021-10-29
Publication date: 2022-05-13
Also published as: JP2022078399A

Abstract

An optical low-pass filter according to an aspect of the present invention includes: a birefringent layer formed using lithium niobate, and configured to separate incident light, which is linearly polarized light, into ordinary light and extraordinary light; a depolarization layer formed using SRF, which converts linearly polarized light passing through the birefringent layer into circularly polarized light; and a birefringent layer formed using lithium niobate, and separating light passing through the depolarizing layer into ordinary light and extraordinary light.

Description

Optical low-pass filter, manufacturing method of optical low-pass filter, optical equipment and unmanned aerial vehicle

Technical Field

The invention relates to an optical low-pass filter, a manufacturing method of the optical low-pass filter, optical equipment and an unmanned aerial vehicle.

Background

In a smartphone or the like, an imaging element such as a CMOS image sensor is used to capture an image formed by a lens. Such an image pickup device has a bayer array in which light receiving elements are arranged in a lattice. Therefore, it is known that when the spatial frequency of the captured image is higher than the sampling frequency of the array of light receiving elements, a false signal such as moire is generated. In order to prevent such a spurious signal, an optical low-pass filter is generally inserted in the vicinity of the imaging element.

Japanese patent application laid-open No. 2006-208470 discloses a technique relating to a lightweight optical low-pass filter having an infrared absorption function. The optical low-pass filter disclosed in jp 2006-208470 a is a 4-point separation type optical low-pass filter, and has a structure in which a phase plate or a birefringent plate is sandwiched between 2 birefringent plates, and an infrared absorber is held between 1 birefringent plate and the phase plate.

Disclosure of Invention

However, in the optical low-pass filter described above, the birefringent plate is made of crystal, and there is a limit to the reduction in thickness. In view of the current situation of waiting for further miniaturization and thinning of smartphones and the like, thinner optical low-pass filters are desired.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a thin optical low-pass filter, a method of manufacturing the optical low-pass filter, an optical apparatus, and an unmanned aerial vehicle.

An optical low-pass filter according to an aspect of the present invention includes: a 1 st birefringent layer formed using lithium niobate, and separating incident light, which is linearly polarized light, into ordinary light and extraordinary light; a depolarization layer formed using SRF (Super Retardation Film) and converting linearly polarized light passing through the 1 st birefringent layer into circularly polarized light; and a 2 nd birefringent layer formed using lithium niobate, and separating light passing through the depolarizing layer into ordinary light and extraordinary light.

An optical apparatus according to an aspect of the present invention includes the above optical low-pass filter and an imaging element.

The unmanned aerial vehicle provided by the invention is provided with the optical equipment.

A method for manufacturing an optical low-pass filter according to an aspect of the present invention includes the steps of: a step of preparing a 1 st birefringent layer, wherein the 1 st birefringent layer is formed using lithium niobate, and separates incident light that is linearly polarized light into ordinary light and extraordinary light; a step of preparing a depolarization layer formed using an SRF (Super Retardation Film) for converting linearly polarized light passing through the 1 st birefringent layer into circularly polarized light; preparing a 2 nd birefringent layer, wherein the 2 nd birefringent layer is formed by using lithium niobate, and separates the light passing through the depolarizing layer into ordinary light and extraordinary light; and a step of bonding the 1 st birefringent layer, the depolarizing layer, and the 2 nd birefringent layer.

According to the present invention, a thin optical low-pass filter, a method for manufacturing an optical low-pass filter, an optical device, and an unmanned aerial vehicle can be provided.

The foregoing and other objects, features and advantages of the present disclosure will be apparent from the following detailed description and the accompanying drawings, which are illustrative only, and it is to be understood that the disclosure is not limited thereto.

Drawings

Fig. 1 is a sectional view for explaining an example of the configuration of an optical low-pass filter according to embodiment 1.

Fig. 2 is a diagram for explaining the optical properties of the birefringent layer.

Fig. 3 is a diagram for explaining an example of dot image separation generated by the optical low-pass filter of embodiment 1.

Fig. 4 is a table for explaining a configuration example of the optical low-pass filter according to embodiment 1.

Fig. 5 is a cross-sectional view for explaining an example of the configuration of the optical low-pass filter according to embodiment 2.

Fig. 6 is a flowchart for explaining a method of manufacturing an optical low-pass filter according to embodiment 2.

Fig. 7 is a sectional view for explaining an example of the configuration of the optical low-pass filter according to embodiment 3.

Fig. 8 is a flowchart for explaining a method of manufacturing an optical low-pass filter according to embodiment 3.

Fig. 9 is a view showing a state of polarization of linearly polarized light after passing through a depolarizing layer.

Fig. 10 is a diagram schematically showing a point image of light passing through the optical low-pass filter.

Fig. 11 is a sectional view for explaining a digital camera mounted with the optical low-pass filter of the present invention.

Fig. 12 is a front view for explaining an unmanned aerial vehicle including an optical device mounted with the optical low-pass filter of the present invention.

Detailed Description

< inventors' study on depolarizing layer >

Before describing the embodiments, the inventors of the present invention will describe the application of SRF (Super Retardation Film) as a depolarizing layer.

In the conventional design, an 1/4 wave plate is used as the depolarizing layer, and light linearly polarized at a wavelength of around 600nm, which is, for example, the approximate center of the visible light region, is shifted from ordinary light by 1/4 wavelengths, thereby realizing circularly polarized light. The present inventors have conducted studies to apply SRFs (for example, a phase difference of 10000nm can be achieved with a thickness of 0.08 mm) that can make the phase difference between the ordinary light and the extraordinary light very large to the depolarization layer.

Fig. 9 is a diagram showing the state of polarization of linearly polarized light after passing through the depolarizing layer, with the horizontal axis representing the wavelength of light and the vertical axis representing the state of polarization. In fig. 9, the case of using 1/4 wave plates as the depolarization layer and the case of using SRF are compared. The polarization state is defined by a formula of (sin (phase difference × circumferential ratio/wavelength)) 2, and in the graph, the center of the vertical axis (polarization state) is circularly polarized light, and the uppermost portion and the lowermost portion of the vertical axis are linearly polarized light. As can be seen from fig. 9, the SRF has a shorter period of change in polarization state with respect to wavelength than the 1/4 wave plate. That is, it is known that linearly polarized light in a plurality of wavelength ranges can be converted into circularly polarized light by using the depolarizing layer SRF.

Fig. 10 is a diagram schematically showing a point image of light passing through an optical low-pass filter in which a depolarizing layer is disposed so as to be sandwiched by 2 birefringent layers in the optical axis direction. The case where the 1/4 wave plate was used for the depolarizing layer and the case where the SRF was used were compared with each other for white light, yellow light, green light, and bluish light. In fig. 10, the image of 4 dots shows the intensity of light in light color, and the intensity of light is stronger when the color is darker. As is clear from the imaging of the 4-point image in fig. 10, the use of SRF enables polarization conversion (conversion from linearly polarized light to circularly polarized light) more efficiently than the use of 1/4 wave plates as the depolarization layer. That is, it is found that the use of SRF is more effective for the square 4-point separation of light than the use of 1/4 wave plates for the depolarizing layer. As described above, according to the prior studies of the inventors, it was confirmed that the SRF can be applied to the depolarization layer of the optical low-pass filter.

< embodiment mode 1 >

Embodiments of the present invention will be described below with reference to the drawings.

Fig. 1 is a sectional view for explaining an example of the configuration of an optical low-pass filter according to embodiment 1. As shown in fig. 1, the optical low-pass filter 1 of the present embodiment has a structure in which a birefringent layer 11, a depolarizing layer 12, and a birefringent layer 13 are arranged along the optical axis direction. The optical low-pass filter 1 of the present embodiment may include at least the birefringent layer 11, the depolarizing layer 12, and the birefringent layer 13, or may further include other layers.

The birefringent layer 11 is formed using Lithium Niobate (hereinafter, also referred to as LN) and is a layer that separates incident light, which is linearly polarized light, into ordinary light and extraordinary light. The depolarization layer 12 is formed using SRF (Super Retardation Film), and is a layer that converts linearly polarized light passing through the birefringent layer 11 into circularly polarized light. The birefringent layer 13 is formed of LN and is a layer that separates light passing through the depolarizing layer 12 into ordinary light and extraordinary light. Hereinafter, the optical function of the optical low-pass filter 1 according to the present embodiment will be described in detail.

Fig. 2 is a diagram for explaining the optical properties of the

birefringent layers

11, 13. In fig. 2, a polarized light component perpendicular to the paper surface (referred to as vertical polarized light) of the incident light L is indicated by a black dot, and a polarized light component parallel to the paper surface (referred to as horizontal polarized light) is indicated by a double-headed arrow. The incident light L enters the incident surface of the material M (corresponding to the birefringent layers 11, 13) perpendicularly from above the paper surface. At this time, the optical axis AX of the material M forms an inclination angle θ with respect to the incident surface. Since the material M has birefringence, for example, vertically polarized light still travels straight after entering the material M and exits the material M (so-called ordinary light OD). On the other hand, for example, the horizontally polarized light is refracted after entering the material M, travels in a direction different from the ordinary ray, and is emitted from the emission surface of the material M in a direction parallel to the ordinary ray (so-called extraordinary ray EX). In the present embodiment, the separation amount d (also referred to as a separation width) is a distance between emission positions of vertically polarized light (ordinary light) and horizontally polarized light (extraordinary light).

When the thickness of the material M is represented by t, the refractive index of the material M with respect to ordinary rays is represented by no, and the refractive index of the material M with respect to extraordinary rays is represented by ne, the separation amount d at this time is represented by the following equation.

[ number 1 ]

It is known that the separation amount d is maximum when the inclination angle θ of the optical axis is 45 °. That is, the separation amount at the tilt angle of (45-. alpha.). degree is equal to the separation amount at the tilt angle of (45+ alpha.). degree.

The optical low-pass filter 1 of the present embodiment is configured to separate point images of incident light entering the optical low-pass filter 1. The image of the point image separation formed by the optical low-pass filter 1 is determined by the optical properties of the

birefringent layers

11 and 13. In the present embodiment, the light separation direction of the birefringent layer 11 and the light separation direction of the birefringent layer 13 are shifted by 90 °. Here, the light separation direction indicates a direction in which extraordinary light is separated from ordinary light when incident light entering the birefringent layer is separated into the ordinary light and the extraordinary light. In the example shown in fig. 2, the light separation direction is the right direction of the paper.

Fig. 3 is a diagram for explaining an example of the point image separation generated by the optical low-pass filter of the present embodiment. Fig. 3 shows, as an example, the dot image separation in the case where the depolarization layer 12 imparts a phase difference of λ/4 to 2 orthogonal polarization components, and the light separation direction of the birefringent layer 11 is 90 ° and the light separation direction of the birefringent layer 13 is 0 °.

Since the light separation direction of the birefringent layer 11 is 90 °, when the incident light (orthogonal 2 linearly polarized lights) enters the birefringent layer 11 shown in fig. 1, the incident light is separated into the ordinary light 61 and the extraordinary light 62 as shown in fig. 3. Then, the linearly polarized light having passed through the birefringent layer 11 enters the depolarizing layer 12, and is converted from the linearly polarized light into circularly polarized light. That is, since the depolarizing layer 12 has a property of shifting one phase of incident linearly polarized light by λ/4 (so-called λ/4 plate), the linearly polarized light passing through the birefringent layer 11 is incident on the depolarizing layer 12 and converted into circularly polarized light. In other words, the depolarization layer 12 functions as a layer for eliminating polarization of linearly polarized light. Then, the light passing through the depolarizing layer 12 enters the birefringent layer 13. Since the light separation direction of the birefringent layer 13 is 0 °, as shown in fig. 3, the light incident into the birefringent layer 13 is light-separated into

ordinary rays

61, 62 and

extraordinary rays

63, 64.

As described above in the study of the depolarization layer by the present inventors, the phase difference of approximately 10000nm can be achieved when SRF is used for the depolarization layer 12, and thus the cycle of linearly polarized light converted into circularly polarized light with respect to the wavelength is shortened.

For the reasons described above, by using the optical low-pass filter 1 of the present embodiment, the incident light can be normally separated into 4 images.

Fig. 4 is a table for explaining a configuration example of the optical low-pass filter according to the present embodiment. By combining the birefringent layer 11, the depolarizing layer 12, and the birefringent layer 13 having the optical properties shown in fig. 4, the dot image separation shown in fig. 4 can be obtained. In fig. 4, θ 1 corresponds to the light separation direction. In addition, θ 2 corresponds to the inclination angle θ of the optical axis in fig. 2.

Fig. 4 shows 2 configuration examples (1) and (2) as an example. In configuration example (1) of fig. 4, a configuration example in which incident light is normally separated into directions parallel to the 0 ° direction and the 90 ° direction is shown. In addition, in the configuration example (2), the incident light is normally separated into the directions parallel to the 45 ° direction and the 135 ° direction.

In the configuration example shown in fig. 4, θ 2 of the

birefringent layers

11 and 13 is set to 45 °, but the value of θ 2 may be set to 1 ° or more and 45 ° or less or 45 ° or more and 89 ° or less. That is, the value of θ 2 may be changed according to the optical design of the optical low-pass filter. Note that, the content of the maximum separation amount d when θ 2 is 45 ° is as described above.

The value of θ 1 of the

birefringent layers

11 and 13 may be appropriately changed according to the optical design of the optical low-pass filter. The rotation angle of the depolarizing layer 12 can also be changed as appropriate in accordance with the optical design of the optical low-pass filter. For example, the rotation angle of the depolarizing layer 12 can be set to 45 °, 90 °, 225 °, 270 °. Note that since the depolarization layer 12 is a layer for eliminating polarization, θ 2 has a value of 0 ° (i.e., perpendicular to the optical axis).

In the optical low-pass filter 1 of the present embodiment described above, the

birefringent layers

11 and 13 are formed using lithium niobate. Since lithium niobate has a larger refractive index than crystal, the separation amount d per unit thickness can be increased. Therefore, the thickness of the

birefringent layers

11 and 13 can be reduced as compared with the case where the birefringent layers are formed using crystal.

In the optical low-pass filter 1 of the present embodiment, the depolarization layer 12 is formed using an SRF (Super Retardation Film). When the depolarization layer 12 is formed using SRF as described above, the thickness of the depolarization layer 12 can be reduced as compared with the case where the depolarization layer is formed using crystal. That is, SRF is a film having a large in-plane retardation value, and the depolarization layer 12 can be made thinner than when crystal is used. For example, the SRF can be constructed using a polyethylene terephthalate film. For example, COSMOSHIN (registered trademark) spun by tokyo can be used as the SRF.

For example, the refractive index of crystal is no 1.5443 or ne 1.5534, and the refractive index of LN is no 2.2997 or ne 2.2142 (both calculated at a wavelength of 589.3 nm). In this case, in order to obtain a square separation characteristic of 5 μm using the optical low-pass filter having the configuration shown in fig. 1, it is necessary to set the thicknesses of the

birefringent layers

11 and 13 to 0.132mm, respectively, and the thickness of the depolarizing layer 12 to 0.08 mm. The inclination angle θ 2 of the

birefringent layers

11 and 13 is 45 °. When the thickness of the adhesive layer for bonding the layers is set to 0.015mm, the thickness of the optical low-pass filter (the present invention) is 0.374 mm.

On the other hand, when an optical low-pass filter having the same optical characteristics is formed using quartz, it is necessary to set the thickness of each of the

birefringent layers

11 and 13 to 0.85mm and the thickness of the depolarizing layer 12 to 0.215 mm. The inclination angle θ 2 of the

birefringent layers

11 and 13 is 45 °. When the thickness of the adhesive layer for bonding the respective layers was set to 0.015mm, the thickness of the optical low-pass filter (comparative example) was 1.945 mm.

Thus, the thickness of the optical low-pass filter of the comparative example was 1.945mm, whereas the thickness of the optical low-pass filter of the present invention was 0.374 mm. Therefore, the thickness of the optical low-pass filter can be reduced by 1.571mm (the thickness can be reduced by about 81%) as compared with the comparative example.

As an example of the design value, in the optical low-pass filter of the present embodiment, for example, in order to obtain a square separation characteristic of 5.88 μm or more and 8.40 μm, the thickness of the birefringent layer 11 is 0.15mm or more and 0.22mm or less, the thickness of the depolarizing layer 12 is 0.05mm or more and 0.08mm or less, and the thickness of the birefringent layer 13 is 0.15mm or more and 0.22mm or less. In order to obtain a square separation characteristic of 4.18 μm or more and 5.97 μm, the thickness of the birefringent layer 11 is 0.11mm or more and 0.16mm or less, the thickness of the depolarizing layer 12 is 0.05mm or more and 0.08mm or less, and the thickness of the birefringent layer 13 is 0.11mm or more and 0.16mm or less. In the optical low-pass filter of the present embodiment, parameters such as the thicknesses of the birefringent layer 11, the depolarizing layer 12, and the birefringent layer 13 may be appropriately changed to obtain predetermined optical characteristics.

Next, a method for manufacturing the optical low-pass filter of the present embodiment will be described.

In manufacturing the optical low-pass filter of the present embodiment, the birefringent layer 11 is prepared, which is formed using lithium niobate and separates incident light, which is linearly polarized light, into ordinary light and extraordinary light. Further, a depolarizing layer 12, which is formed using SRF and converts linearly polarized light passing through the birefringent layer 11 into circularly polarized light, is prepared. Further, a birefringent layer 13, which is formed using lithium niobate and separates light passing through the depolarizing layer 12 into ordinary light and extraordinary light, is prepared. The prepared birefringent layer 11, depolarizing layer 12, and birefringent layer 13 are bonded together, whereby the optical low-pass filter of the present embodiment can be manufactured. A specific method for manufacturing the optical low-pass filter is described in detail in

embodiments

2 and 3.

As described above, in the invention of the present embodiment, the

birefringent layers

11 and 13 are formed using lithium niobate, and the depolarizing layer 12 is formed using SRF. Therefore, the optical low-pass filter can be made thinner than the optical low-pass filter formed using crystal as in the comparative example.

< embodiment 2 >

Next, embodiment 2 of the present invention will be explained. In embodiment 2, a specific configuration example of an optical low-pass filter will be described. The same components as those of the optical low-pass filter 1 described in embodiment 1 are denoted by the same reference numerals, and redundant description thereof is omitted.

Fig. 5 is a cross-sectional view for explaining a configuration example of the optical low-pass filter of the present embodiment. As shown in fig. 5, the optical low-pass filter 2 of the present embodiment has a structure in which an optical glass 22, a birefringent layer 11, an infrared absorption layer 21(IR cut layer), a depolarization layer 12, and a birefringent layer 13 are arranged along the optical axis direction. These materials constituting the optical low-pass filter 2 are bonded using an adhesive.

In the present embodiment, the

birefringent layers

11 and 13 are formed using lithium niobate, and the depolarizing layer 12 is formed using SRF. In the present embodiment, the light separation direction of the birefringent layer 11 is set to 90 °, and the light separation direction of the birefringent layer 13 is set to 0 °. In this case, the rotation angle of the depolarizing layer 12 is 45 °.

The optical glass 22 is disposed on the outermost surface of the optical low-pass filter 2 and has a function of protecting the optical low-pass filter 2. The optical glass 22 can be made of glass having a predetermined thickness (for example, about 0.32 mm). The surface 31 of the optical glass is coated with a UV-IR cut-off layer. The UV-IR cut layer is a layer that cuts Ultraviolet (UV) and Infrared (IR) rays, and TiO can be laminated by evaporation, for example₂、SiO₂And the like.

The birefringent layer (LN90 °)11 was formed using lithium niobate. On the surface 32 of the birefringent layer (LN90 °)11 on the optical glass 22 side and the surface 33 of the infrared absorbing layer 21 side, matching layers for matching the refractive index of the adhesive with the refractive index of the birefringent layer (LN90 °)11 are formed, respectively. That is, since lithium niobate constituting the birefringent layer (LN90 °)11 has a high refractive index, a matching layer for matching the refractive indices of the adhesive on both surfaces of the birefringent layer (LN90 °)11 and the birefringent layer (LN90 °)11 is provided therebetween. The matching layer can be formed by evaporation of, for example, La_xTi_yO_z、SiO₂And the like.

The infrared absorption layer 21 is a layer that absorbs infrared rays included in incident light (layer that blocks infrared rays), and can be formed using, for example, infrared blocking glass. The thickness of the infrared absorption layer 21 is not particularly limited, and for example, infrared cut glass of about 0.3mm can be used.

The depolarizing layer 12 is constructed using SRF. SRF is a film having a large in-plane retardation value. For example, the SRF can be constructed using a polyethylene terephthalate film. For example, COSMOSHIN (registered trademark) spun by tokyo can be used as the SRF.

The birefringent layer (LN0 °)13 is formed using lithium niobate. A matching layer for matching the refractive index of the adhesive with the refractive index of the birefringent layer (LN0 °)13 is formed on the surface 35 of the birefringent layer (LN0 °)13 on the depolarizing layer 12 side. That is, since lithium niobate constituting the birefringent layer (LN0 °)13 has a high refractive index, a refractive index for refracting the birefringent layer (LN0 °)13 is provided between the adhesive on the depolarizing layer 12 side of the birefringent layer (LN0 °)13 and the birefringent layer (LN0 °)13A matching layer for rate matching. The matching layer can be formed by evaporation of, for example, La_xTi_yO_z、SiO₂And the like. Further, an antireflection film (AR coating) is formed on the outer surface 36 of the birefringent layer (LN0 °) 13. The antireflection film can be formed by vapor deposition of, for example, SiO₂、Ti₂O₅、Al₂O₃And the like.

The optical glass 22, the birefringent layer (LN90 °)11, the infrared absorption layer (IR cut layer) 21, the depolarizing layer (SRF)12, and the birefringent layer (LN0 °)13 are bonded by applying an adhesive to each of the interfaces 32 to 35. For example, an adhesive containing an epoxy resin can be used as the adhesive.

Next, a method for manufacturing the optical low-pass filter of the present embodiment will be described. Fig. 6 is a flowchart for explaining a method of manufacturing the optical low-pass filter according to the present embodiment. When manufacturing the optical low-pass filter of the present embodiment, first, matching layers are formed on both surfaces of the birefringent layer (LN90 °)11 (step S1). The matching layer can be formed by depositing La on both surfaces of the birefringent layer (LN90 DEG) 11_xTi_yO_z、SiO₂And the like. The vapor deposition can be performed by Electron Beam (EB) vapor deposition or the like.

Next, a matching layer is formed on one surface 35 of the birefringent layer (LN0 °)13, and an antireflection film is formed on the other surface 36 (step S2). The matching layer can be formed by depositing La on one surface 35 of the birefringent layer (LN90 DEG) 11 by vapor deposition_xTi_yO_z、SiO₂And the like. The antireflection film can be formed by depositing SiO on the other surface 36 of the birefringent layer (LN90 DEG) 11₂、Ti₂O₅、Al₂O₃And the like.

Next, a UV-IR cut layer is formed on the surface 31 of the optical glass 22 (step S3). The UV-IR cut-off layer can be formed by evaporation of, for example, TiO₂、SiO₂And the like.

Next, an adhesive is applied to the interfaces 32 to 35 of the optical glass 22, the birefringent layer (LN90 °)11, the infrared absorbing layer (IR cut layer) 21, the depolarizing layer (SRF)12, and the birefringent layer (LN0 °)13, and these components are bonded together (step S4). For example, the optical glass 22, the birefringent layer (LN90 °)11, the infrared absorbing layer (IR cut-off layer) 21, the depolarizing layer (SRF)12, and the birefringent layer (LN0 °)13 can be bonded by applying an adhesive to each of the interfaces 32 to 35, spreading the adhesive using a spin coater, and then drying the adhesive. For example, an adhesive containing an epoxy resin can be used as the adhesive. For example, the spin coater can be set to a rotation speed of 3500rpm and a rotation time of 30 seconds.

By using the method described above, the optical low-pass filter of the present embodiment can be manufactured. The sequence of steps S1 to S3 is not limited to the above sequence, and may be any sequence.

In the optical low-pass filter 2 of the present embodiment described above, the

birefringent layers

11 and 13 are formed using lithium niobate, and the depolarizing layer 12 is formed using SRF. Therefore, the optical low-pass filter can be made thinner than an optical low-pass filter formed using crystal.

< embodiment 3 >

Next, embodiment 3 of the present invention will be explained. In embodiment 3, a specific configuration example of the optical low-pass filter will be described. The same components as those of the optical low-pass filters 1 and 2 described in embodiments 1 and 2 are denoted by the same reference numerals, and redundant description thereof is omitted.

Fig. 7 is a cross-sectional view for explaining a configuration example of the optical low-pass filter of the present embodiment. As shown in fig. 7, the optical low-pass filter 3 of the present embodiment has a structure in which an optical glass 22, a birefringent layer 13, an infrared absorption layer 21(IR cut layer), a depolarization layer 12, and a birefringent layer 11 are arranged along the optical axis direction. The materials constituting the optical low-pass filter 3 are bonded using an adhesive.

In the present embodiment, the

birefringent layers

The optical low-pass filter 3 of the present embodiment is different from the optical low-pass filter 2 of embodiment 2 in that the position of the birefringent layer (LN90 °)11 is opposite to the position of the birefringent layer (LN0 °) 13.

The optical glass 22 is disposed on the outermost surface of the optical low-pass filter 3, and has a function of adjusting the optical length and a function of protecting the optical low-pass filter 3. As the optical glass 22, glass having a predetermined thickness (for example, about 0.32 mm) can be used. The surface 41 of the optical glass is coated with a UV-IR cut-off layer. The UV-IR cut layer is a layer that cuts Ultraviolet (UV) and Infrared (IR) rays, and can be formed by depositing TiO, for example₂、SiO₂And the like.

The birefringent layer (LN0 °)13 is formed using lithium niobate. Matching layers for matching the refractive index of the adhesive to the refractive index of the birefringent layer (LN0 °)13 are formed on the surface 42 of the birefringent layer (LN0 °)13 on the optical glass 22 side and the surface 43 of the infrared absorbing layer 21 side, respectively. That is, since lithium niobate constituting the birefringent layer (LN0 °)13 has a high refractive index, a matching layer for matching the refractive indices of the adhesive on both surfaces of the birefringent layer (LN0 °)13 and the birefringent layer (LN0 °)13 is provided therebetween. The matching layer can be formed by evaporation of, for example, La_xTi_yO_z、SiO₂And the like.

In the present embodiment, the surface 44 of the depolarizing layer 12 on the infrared absorption layer 21 side and the surface 45 of the birefringent layer (LN90 °)11 side are coated with urethane acrylate based materials, respectively. By applying the urethane acrylate material to the

surfaces

44 and 45 of the depolarizing layer 12 in this manner, bubbles or clouding on the

surfaces

44 and 45 of the depolarizing layer 12 can be suppressed.

That is, when the surface of the depolarization layer 12 is not coated, radicals having a carbon center may be generated on the surface of the depolarization layer 12 due to the influence of heat, light, or the like. Then, the radicals at the carbon center are oxidized to generate hydrogen, and bubbles may be generated on the surface of the depolarization layer 12. In addition, this may make the depolarizing layer 12 cloudy. In order to solve such a problem, in the present embodiment, a urethane acrylate material is applied to the

surfaces

44 and 45 of the depolarizing layer 12. By applying the urethane acrylate material to the

surfaces

44 and 45 of the depolarizing layer 12 can be suppressed.

The birefringent layer (LN90 °)11 was formed using lithium niobate. A matching layer for matching the refractive index of the adhesive to the refractive index of the birefringent layer (LN90 °)11 is formed on the surface 45 of the birefringent layer (LN90 °)11 on the depolarizing layer 12 side. That is, since lithium niobate constituting the birefringent layer (LN90 °)11 has a high refractive index, a matching layer for matching the refractive indices of the adhesive on the depolarizing layer 12 side of the birefringent layer (LN90 °)11 and the birefringent layer (LN90 °)11 is provided therebetween. The matching layer can be formed by evaporation of, for example, La_xTi_yO_z、SiO₂And the like. An antireflection film (AR plating) is formed on the outer surface 46 of the birefringent layer 13. The antireflection film can be formed by vapor deposition of, for example, SiO₂、Ti₂O₅、Al₂O₃And the like.

The optical glass 22, the birefringent layer (LN0 °)13, the infrared absorbing layer (IR cut layer) 21, the depolarizing layer (SRF)12, and the birefringent layer (LN90 °)11 are bonded together by applying an adhesive to each of the interfaces 42 to 45. For example, an adhesive containing an epoxy resin can be used as the adhesive.

Next, a method for manufacturing the optical low-pass filter of the present embodiment will be described. Fig. 8 is a flowchart for explaining a method of manufacturing the optical low-pass filter according to the present embodiment. In manufacturing the optical low-pass filter of the present embodiment, first, a birefringent layer is formed on a birefringent layerBoth surfaces of (LN0 °)13 form matching layers (step S11). The matching layer can be formed by depositing La on both surfaces of the birefringent layer (LN0 DEG) 13_xTi_yO_z、SiO₂And the like. The vapor deposition can be performed by Electron Beam (EB) vapor deposition or the like.

Next, a matching layer is formed on one surface 45 of the birefringent layer (LN90 °)11, and an antireflection film is formed on the other surface 46 (step S12). The matching layer can be formed by depositing La on one surface 45 of the birefringent layer (LN90 DEG) 11 by evaporation_xTi_yO_z、SiO₂And the like. The antireflection film can be formed by depositing SiO on the other surface 46 of the birefringent layer (LN90 DEG) 11₂、Ti₂O₅、Al₂O₃And the like.

Next, a urethane acrylate material was coated on each of the surface 44 of the depolarizing layer 12 on the infrared absorption layer 21 side and the surface 45 of the birefringent layer (LN90 °)11 side (step S13). The thickness of the coating layer in this case is preferably 1 μm or more.

Next, adhesives were applied to the interfaces 42 to 45 of the optical glass 22, the birefringent layer (LN0 °)13, the infrared absorbing layer (IR cut layer) 21, the depolarizing layer (SRF)12, and the birefringent layer (LN90 °)11, and these components were bonded together (step S14). For example, the optical glass 22, the birefringent layer (LN0 °)13, the infrared absorbing layer (IR cut-off layer) 21, the depolarizing layer (SRF)12, and the birefringent layer (LN90 °)11 can be bonded by applying an adhesive to each of the interfaces 42 to 45, spreading the adhesive using a spin coater, and then drying the adhesive. For example, an adhesive containing an epoxy resin can be used as the adhesive. For example, the spin coater can be set to a rotation speed of 3500rpm and a rotation time of 30 seconds.

Next, a UV-IR cut layer is formed on the surface 31 of the optical glass 22 (step S15). The UV-IR cut-off layer can be formed by evaporation of, for example, TiO₂、SiO₂And the like. Here, in the present embodiment, the UV-IR cut layer is formed after the members are bonded in step S14. For example, when the UV-IR cut layer is formed on the surface of the optical glass 22 alone (step S3) as in the manufacturing method described in embodiment 2 (see fig. 6), the optical glass 22 alone is thinSince the UV-IR cut layer composed of a plurality of layers is formed on the surface, the optical glass 22 may be warped singly. In this case, when the members are bonded, the optical glass 22 alone is warped, and there is a problem that the thickness unevenness of the optical low-pass filter increases.

In contrast, in the present embodiment, the UV-IR cut layer is formed after the members are bonded in step S14. Therefore, all the members after bonding can be warped to avoid warping of the optical glass 22 alone, and surface accuracy can be ensured. In addition, the thickness unevenness of the optical low-pass filter can be reduced.

In this case, the vapor deposition temperature needs to be lowered due to the heat resistance of the adhesive. Therefore, in this embodiment, it is preferable that the Deposition of the UV-IR cut layer uses an IAD (Ion Assisted Deposition) method. By using the IAD method, the deposition temperature can be reduced (to about 130 ℃).

By using the method described above, the optical low-pass filter 3 of the present embodiment can be manufactured. The sequence of steps S11 to S13 is not limited to the above sequence, and may be any sequence.

In the optical low-pass filter 3 of the present embodiment described above, the

birefringent layers

In the optical low-pass filter 3 according to the present embodiment, the depolarizing layer 12 and the birefringent layer (LN90 °)11 are disposed adjacent to each other. The reason for this is as follows.

As described above, in the present embodiment, the UV-IR cut layer is formed after the members are bonded in step S14. In the case of forming the UV-IR cut layer, the temperature of the bonded member is raised, but at this time, thermal stress is generated between the layers due to the difference in the warping direction (difference in thermal expansion) of the layers, and the adhesive may peel off due to the thermal stress. Further, components other than the adhesive such as air and water may enter a portion where the adhesive is peeled off, and this portion may become an optical defect.

However, since an imaging element used in a digital camera or the like has a rectangular shape having a long side and a short side, an optical low-pass filter also has a rectangular shape having a long side and a short side. Here, the warping directions of the

birefringent layers

11 and 13 and the depolarizing layer (SRF)12 depend on the optical axis direction. Specifically, the birefringent layer (LN0 °)13 has a property of being easily warped in the longitudinal direction, and the birefringent layer (LN90 °)11 and the depolarizing layer (SRF)12 have a property of being easily warped in the short-side direction. The longitudinal direction of the optical low-pass filter is set to a direction corresponding to the light separation direction of 0 °.

In view of this, in the optical low-pass filter 3 of the present embodiment, as shown in fig. 7, a birefringent layer (LN90 °)11 and a depolarizing layer (SRF)12, which are likely to warp in the short-side direction, are disposed adjacent to each other. When such a configuration is adopted, the thermal stress acting between the birefringent layer (LN90 °)11 and the depolarizing layer (SRF)12 can be reduced as compared with the optical low-pass filter 2 of embodiment 2. This can suppress the occurrence of peeling of the adhesive between the birefringent layer (LN90 °)11 and the depolarizing layer (SRF) 12.

< other embodiments >

The optical low-pass filter of the present invention described above can be used for various optical devices, mobile devices such as smart phones, electronic devices, and the like. A digital camera is an example of an optical device including the optical low-pass filter and the image pickup device of the present invention. Fig. 11 is a sectional view for explaining a digital camera mounted with the optical low-pass filter of the present invention.

As shown in fig. 11, the digital camera 100 includes a lens group 102, an optical low-pass filter 103, and an imaging element 104 inside a housing 101. For example, the imaging element 104 is a CMOS image sensor or a CCD image sensor. The optical low-pass filter 103 is disposed adjacent to the imaging element 104. Incident light entering the digital camera 100 passes through the lens group 102, passes through the optical low-pass filter 103, and reaches the image pickup element 104. In the present embodiment, incident light is square-split after passing through the optical low-pass filter 103. The light square-split by the optical low-pass filter 103 is guided to the image pickup device 104.

The optical device having the optical low-pass filter of the present invention mounted thereon can be mounted on various electronic devices, vehicles, aircrafts, and the like. Fig. 12 is a front view for explaining an unmanned aerial vehicle including an optical device mounted with the optical low-pass filter of the present invention. As shown in fig. 12, the drone 200 includes a main body portion 201 and a plurality of

propellers

202, 203. Note that, although fig. 12 is a front view, the number of propellers illustrated is 2, the unmanned aerial vehicle 200 has a structure in which 4 propellers are arranged along a diagonal line. The drone 200 is mounted with an optical device (camera) 204. Therefore, when the unmanned aerial vehicle 200 flies while the plurality of

propellers

202, 203 are rotated, the scenery around the unmanned aerial vehicle 200 can be photographed using the optical device (camera) 204.

From the disclosure set forth above, it should be apparent that the embodiments of the disclosure can be modified in various ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

1. An optical low-pass filter, comprising:

a 1 st birefringent layer formed using lithium niobate, and separating incident light, which is linearly polarized light, into ordinary light and extraordinary light;

a depolarization layer formed using SRF (Super Retardation Film) and converting linearly polarized light passing through the 1 st birefringent layer into circularly polarized light; and

and a 2 nd birefringent layer formed using lithium niobate, and separating light passing through the depolarizing layer into ordinary light and extraordinary light.

2. An optical low-pass filter according to claim 1,

the optical axes of the 1 st birefringent layer and the 2 nd birefringent layer are inclined at an angle of 45 ° with respect to the incident light.

3. An optical low-pass filter according to claim 1,

the light separation direction of the 1 st birefringent layer is shifted by 90 ° from the light separation direction of the 2 nd birefringent layer.

4. An optical low-pass filter according to claim 3,

the light separation direction of the 1 st birefringent layer is 0,

the light separation direction of the 2 nd birefringent layer is 90 °.

5. An optical low-pass filter according to claim 4,

the thickness of the 1 st birefringent layer is 0.11mm to 0.22mm,

the thickness of the depolarization layer is more than 0.05mm and less than 0.08mm,

the thickness of the 2 nd birefringent layer is 0.11mm to 0.22 mm.

6. An optical low-pass filter according to claim 5,

the depolarizing layer and the 2 nd birefringent layer are disposed adjacent to each other.

7. An optical low-pass filter according to any one of claims 1 to 6,

the optical film further comprises an infrared absorbing layer disposed on a surface of the depolarizing layer opposite to the surface on which the 2 nd birefringent layer is disposed.

8. An optical low-pass filter according to any one of claims 1 to 6,

the surface of the depolarization layer is coated by polyurethane acrylate materials.

9. An optical low-pass filter according to any one of claims 1 to 6,

the 1 st birefringent layer, the depolarizing layer, and the 2 nd birefringent layer are bonded using an adhesive containing an epoxy resin.

10. An optical device, comprising:

an optical low-pass filter according to any one of claims 1 to 6; and

and a shooting element.

11. An unmanned aerial vehicle is characterized in that the unmanned aerial vehicle,

an optical device according to claim 10.

12. A method for manufacturing an optical low-pass filter, comprising the steps of:

a step of preparing a 1 st birefringent layer, wherein the 1 st birefringent layer is formed by using lithium niobate, and separates incident light which is linearly polarized light into ordinary light and extraordinary light;

a step of preparing a depolarization layer formed using an SRF (Super Retardation Film) for converting linearly polarized light passing through the 1 st birefringent layer into circularly polarized light;

a step of preparing a 2 nd birefringent layer, wherein the 2 nd birefringent layer is formed by lithium niobate, and separates the light passing through the depolarizing layer into an ordinary ray and an extraordinary ray; and

and a step of bonding the 1 st birefringent layer, the depolarizing layer, and the 2 nd birefringent layer.