JP2021189349A - Optical low-pass filter - Google Patents

Abstract

To provide a thinner optical low-pass filter.SOLUTION: An optical low-pass filter 100 has a two-layer structure of a lithium niobate plate 10 and an infrared absorption layer 20. The lithium niobate plate 10 is formed of lithium niobate as a double-refractive material, and the separation amount between a normal light beam and an abnormal light beam by double refraction is in the range of 0.39 μm and 8.13 μm, both inclusive. The infrared absorption layer 20 is held in contact with the lithium niobate plate 10.SELECTED DRAWING: Figure 2

Description

The present invention relates to an optical low pass filter.

In smartphones and the like, an image sensor 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 grid pattern. Therefore, it is known that when the spatial frequency of the image to be imaged is higher than the sampling frequency of the light receiving element array, a false signal such as moire is generated. In order to prevent such a false signal, an optical low-pass filter is generally inserted in front of the image sensor.

As such an optical low-pass filter, four-point separation type optics in which a phase plate or a birefringence plate is sandwiched between two birefringence plates and an infrared absorber is held between the one birefringence plate and the phase plate. A low-pass filter has been proposed (Patent Document 1). According to this structure, it is said that a lightweight optical low-pass filter having an infrared absorption function can be obtained.

Japanese Unexamined Patent Publication No. 2006-208470

However, in the optical low-pass filter as described above, the birefringent plate is made of quartz, and there is a limit to reducing the thickness. Considering the current situation where smartphones and the like are expected to be further miniaturized and thinned, a thinner optical low-pass filter is desired.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a thinner optical low-pass filter.

The optical low-pass filter according to the first aspect of the present invention comprises a birefringent layer made of lithium niobate and having a separation amount of 0.39 μm or more and 8.13 μm or less between normal light rays and abnormal light rays. It has an infrared absorbing layer that is held in contact with the birefringent layer. This makes it possible to reduce the thickness of the optical low-pass filter by reducing the thickness of the birefringent layer.

The optical low-pass filter according to the second aspect of the present invention is the above-mentioned optical low-pass filter, in which the birefringence layer has an optical axis tilt angle of 45 ° with respect to incident light and a thickness of 0.01 mm or more and 0. It is desirable that it is configured to be .21 mm or less. This makes it possible to reduce the thickness of the optical low-pass filter by reducing the thickness of the birefringent layer.

The optical low-pass filter according to the third aspect of the present invention is the above-mentioned optical low-pass filter, in which the birefringence layer has a thickness of 0.133 mm and the inclination angle of the optical axis with respect to the incident light is 1 ° or more and 45 ° or less. Alternatively, it is desirable that the temperature is 45 ° or more and 89 ° or less. This makes it possible to reduce the thickness of the optical low-pass filter by reducing the thickness of the birefringent layer.

The optical low-pass filter according to the fourth aspect of the present invention is the above-mentioned optical low-pass filter, in which the infrared absorbing layer is prepared by applying a material in which a dye that absorbs infrared rays is dispersed in a solvent to the birefringence layer. It is desirable that it is formed by curing the coating film. As a result, the infrared absorbing layer can be easily formed, and the manufacturing cost and the lead time can be reduced.

According to the present invention, it is possible to provide a thinner optical low-pass filter.

It is a figure which shows typically the structure of the camera mounted on the mobile device. It is sectional drawing which shows typically the structure of the optical low-pass filter which concerns on Embodiment 1. FIG. It is a figure which shows the design value of the BG optical low-pass filter composed of BG which is the comparative example 1, and the LN optical low-pass filter composed of LN. It is a figure which shows the inclination angle θ which is the angle formed by the incident surface and the optical axis of a material. It is a figure which shows the relationship between the tilt angle of the optical axis and the separation amount in LN and crystal. It is a figure which shows the relationship between the thickness of LN and crystal, and the separation amount when the inclination angle θ of an optical axis is 45 °. It is a figure which shows the MTF (Modulation Transfer Function) curve of the optical low-pass filter 100 which concerns on Embodiment 1. FIG. It is a figure which shows the absorption characteristic of an infrared absorption layer and general blue glass (thickness 0.21mm, comparative example).

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In each drawing, the same elements are designated by the same reference numerals, and duplicate explanations are omitted as necessary.

Embodiment 1
The optical low-pass filter according to the first embodiment will be described. FIG. 1 schematically shows the configuration of a camera mounted on a mobile device. In a camera mounted on a general portable device such as a smartphone, an optical system 2 composed of one or more lenses, an optical low-pass filter 1, and an image pickup element 3 such as COMS are arranged in order from the side of the object to be imaged. In the following, the direction from the image pickup object to the image pickup device 3 will be referred to as the X direction for convenience. As can be seen from FIG. 1, in order to reduce the thickness of the camera, it is desirable that the thickness of the optical low-pass filter 1 can be made as thin as possible. Therefore, in the present embodiment, an optical low-pass filter having a configuration capable of being thinner than a general optical low-pass filter is provided.

Hereinafter, the optical low-pass filter 100 according to the present embodiment will be described. FIG. 2 is a cross-sectional view schematically showing the configuration of the optical low-pass filter 100 according to the first embodiment. The optical low-pass filter 100 has an LN plate 10 which is a birefringent layer of a plate-shaped member made of lithium niobate (hereinafter referred to as LN), and an infrared absorbing layer 20 is held on the LN plate 10. Has been done.

The pixel pitch of an image sensor mounted on a smartphone or a digital camera is generally 1.0 to 8.4 μm. Further, considering the optical limit, the optical low-pass filter is required to correspond to a pixel pitch of 0.8 μm. Therefore, the optical low-pass filter 100 according to the present embodiment can correspond to an image pickup device having a pixel pitch of 0.8 to 8.4 μm by setting the thickness of the LN plate 10 to a suitable value.

The refractive index of LN is, for example, 2.273, which is larger than the refractive index of 1.564 of bluegrass (hereinafter referred to as BG) and the refractive index of 1.55 of quartz. Therefore, by using LN, a low-pass filter thinner than BG or crystal can be configured. Hereinafter, a specific description will be given.

For example, when mounting an optical low-pass filter on a smartphone, it is necessary to insert the optical low-pass filter in the space specified by the specified specifications. That is, it is required to configure an optical low-pass filter so as to keep the optical path length between the end surface (X + side surface) 2A of the lens constituting the optical system 2 and the image pickup surface 3A of the image pickup device 3 constant. Hereinafter, a case where the bluegrass (hereinafter referred to as BG) used in the configuration of the optical low-pass filter is replaced with LN will be described.

FIG. 3 shows the design values of the BG optical low-pass filter using BG, which is Comparative Example 1, and the LN optical low-pass filter using LN. In this example, the refractive index of BG is 1.564 and the refractive index of LN is 2.273. Further, according to the specifications, the distance d1 between the surface 1A on the X + side of the optical low-pass filter and the image pickup surface 3A of the image sensor 3 is fixed at 0.3 mm, and the lens faces the surface 1B on the X- side of the optical low-pass filter. It is assumed that the distance from the end surface 2A of the image sensor 3 to the image pickup surface 3A of the image sensor 3 is fixed at 0.91 mm.

Assuming that the thickness t of the BG optical low-pass filter according to Comparative Example 1 is 0.21 mm, the optical path length thereof is 0.328 mm. At this time, the distance d2 between the BG optical low-pass filter and the end face 2A of the lens is 0.4 mm. In this case, the optical path length from the end surface 2A of the lens to the image pickup surface 3A of the image pickup device 3 is 1.028 mm.

Therefore, when the BG optical low-pass filter is replaced with the LN optical low-pass filter under this condition, the optical path lengths from the X-side surface 1B of the optical low-pass filter to the image pickup surface 3A of the image pickup element 3 are substantially equal. It is required to construct an LN optical low-pass filter. In this example, if the thickness of the LN optical low-pass filter like the LN plate 10 is 0.133 mm, the optical path length is 0.302 mm. On the other hand, the distance between the LN optical low-pass filter and the end face 2A of the lens is 0.477 mm. In this case, the optical path length from the end surface 2A of the lens to the image pickup surface 3A of the image pickup element 3 is 1.079 mm, so that it is possible to realize an optical path length almost the same as when a BG optical low-pass filter is used.

As described above, when mounting an optical low-pass filter on an optical device such as a smartphone, it can be understood that the thickness can be reduced by configuring the optical low-pass filter with an LN from the viewpoint of the optical path length.

In particular, with the progress of development of devices such as smartphones in the future, it is expected that the overall thickness of the devices will be further reduced. In this case, the optical low-pass filter mounted on such a device is required to be further thinned. In response to such a demand, it is difficult for quartz to cope with thinning due to the viewpoint of optical path length and the reason described below, and LN is advantageous for thinning.

Although the optical low-pass filter using LN can be made thinner from the viewpoint of the optical path length, it is necessary to design it according to the pixel pitch of the target optical element. Hereinafter, focusing on the amount of light separation due to birefringence, the amount of light separation between the LN and the crystal (that is, the distance between the optical axes of the normal light ray and the abnormal light ray) will be compared.

FIG. 4 shows an inclination angle θ which is an angle formed by the incident surface and the optical axis of the material. In FIG. 4, of the incident light L, the polarization component perpendicular to the paper surface (referred to as vertical polarization) is indicated by a black dot, and the polarization component parallel to the paper surface (referred to as horizontal polarization) is indicated by a double-headed arrow. The incident light L is incident on the paper surface perpendicularly to the incident surface of the material M. 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 enters the material M, travels straight ahead, and emits light from the material M (so-called normal light ray OD). On the other hand, for example, horizontal polarized light is incident on the material M, then refracted, travels in a direction different from the normal light ray, and is emitted from the emission surface of the material M in a direction parallel to the normal light ray (so-called abnormal light ray EX). .. The separation amount (also referred to as separation width) refers to the distance between the emission positions of vertically polarized light (normal light rays) and horizontally polarized light (abnormal light rays).

The thickness of the material M t, the refractive index of the material M for the normal light n _od, when the refractive index of the material M and n _ex for the extraordinary ray, the separation amount D in this case, be represented by the following formula Are known.

It is known that the separation amount D shown above is maximized when the tilt angle θ of the optical axis is 45 °, that is, the separation amount when the tilt angle is (45−α) ° is tilted. The angle is equal to the amount of separation at (45 + α) °.

FIG. 5 shows the relationship between the tilt angle of the optical axis and the separation amount in LN and crystal. Here, the thickness of the LN is 0.133 mm and the thickness of the crystal is 0.21 mm, as described above. Both LN and crystal have the maximum separation amount when the tilt angle of the optical axis is 45 °. Further, it can be understood that the separation amount of LN is larger than the separation amount in the crystal regardless of the magnitude of the tilt angle of the optical axis.

Further, since the separation amount of quartz is 1.29 μm when the thickness is 0.21 mm and the tilt angle θ of the optical axis is 45 °, it can be applied to an optical low-pass filter of an image pickup element having a pixel pitch of 1 μm. be. However, since it is not possible to increase the separation amount without increasing the thickness of the crystal, it can be seen that the crystal is not suitable in principle for further thinning the optical low-pass filter.

FIG. 6 shows the relationship between the thickness of the LN and the crystal and the separation amount when the tilt angle θ of the optical axis is 45 °. As shown in FIG. 6, in LN, the separation amount is 1.55 μm when the thickness is 0.04 mm, and it can be applied as an optical low-pass filter of an image pickup device having a pixel pitch of 1 μm. On the other hand, in order to realize the same amount of separation using quartz, a thickness of 0.21 μm is required as described above, so that it can be understood that LN is advantageous for reducing the thickness of the optical low-pass filter.

Further, in the LN, the separation amount can be adjusted in the range of 0.39 to 8.13 by adjusting the tilt angle θ of the optical axis, so that the pixel pitch described above is 0.8 to 8.4 μm. It can be understood that it can be sufficiently applied to an image sensor.

On the other hand, when a crystal is used, it is difficult to apply it to an image pickup device having a pixel pitch of more than 1.6 μm because the separation amount cannot be further increased without increasing the thickness. Therefore, by using LN, it is possible to greatly expand the range of pixel pitches that can be supported as compared with quartz.

Next, a comparison between the optical characteristics of a general optical low-pass filter and the optical characteristics of the optical low-pass filter 100 according to the present embodiment will be described. FIG. 7 shows an MTF (Modulation Transfer Function) curve of the optical low-pass filter 100 according to the first embodiment. In FIG. 7, the horizontal axis is the spatial frequency and the vertical axis is the contrast, and the light rays following the path from the lens center of the optical system 3 to the image pickup element 3 and the tangential direction (TAN) and sagittal direction from the lens center. (SAG) shows the MTF curves of light rays passing through positions separated by 0.3F and 0.7F (where F is the image height). Further, in FIG. 7, + indicates the OLPF separation direction, and − indicates the direction opposite to the OLPF separation direction. As shown in FIG. 7, it can be understood that the optical low-pass filter 100 can more effectively reduce the contrast on the high frequency side as compared with a general low-pass filter.

Further, when the LN plate 10 is formed by laminating two plates, the total thickness can be made smaller than 0.133 mm, which is more advantageous due to the thinning.

The infrared absorption layer 20 is formed by, for example, applying a fluid raw material (for example, SA7 manufactured by Nippon Shokubai Co., Ltd.) to the LN plate 10 using a spin coat, and then baking at a predetermined temperature and time. Can be done. The thickness of the infrared absorbing layer 20 is, for example, about 5 μm if a general infrared absorber on the market is used, but the total thickness of the infrared absorbing layer 20 is 0.138 mm even when combined with the LN plate 10, which is general. It is possible to make it thinner than a low-pass filter.

The absorption characteristics of the infrared absorption layer 20 will be examined. FIG. 8 shows the absorption characteristics of the infrared absorption layer 20 and general blue glass (thickness 0.21 mm, comparative example). It can be understood that the infrared absorption layer 20 has good characteristics in the infrared region on the wavelength side longer than 700 μm, like general blue glass.

Therefore, according to this configuration, it is possible to provide a thinner optical low-pass filter that can suitably absorb infrared rays and is applicable to mobile devices such as smartphones.

Further, in the present embodiment, an LN plate is used as the birefringence layer. The LN plate can be formed by cutting out a disk from an LN ingot, polishing it, and processing it to a desired thickness. LN ingots are known to have a shorter growth period than quartz crystals, which are a common material for birefringent plates. Industrial production of quartz crystals generally requires a crystal growth period of several months, whereas pull-up production of LN ingots takes only a few days. Therefore, in the case of LN, the lead time required for material procurement can be significantly shortened, and as a result, the overall lead time for manufacturing the optical low-pass filter can be shortened and the manufacturing cost can be reduced.

Other Embodiments The present invention is not limited to the above embodiments, and can be appropriately modified without departing from the spirit. For example, the infrared absorbing layer has been described as being formed by coating, but it goes without saying that the coating method is not limited to spin coating. Needless to say, if an infrared absorbing layer having a desired low-pass filter pixel pitch can be formed, various film forming methods such as a thin film deposition method and a sputtering method can be applied.

Needless to say, the optical low-pass filter according to the above-described embodiment can be used in combination with various optical devices or optical elements other than cameras and the like.

Further, although the LN plate 10 has been described as a single-layer LN plate, depending on the application, a plurality of two or more LN layers may be bonded to form an LN plate.

1 Optical low-pass filter 1A, 1B surface 2 Optical system 2A End surface 3 Imaging element 3A Imaging surface 10 LN plate 20 Infrared absorption layer 1000 Optical low-pass filter

Claims (4)

A birefringent layer made of lithium niobate and configured so that the amount of separation between normal light rays and abnormal light rays is 0.39 μm or more and 8.13 μm or less.
An infrared absorbing layer that is in contact with and held in contact with the birefringent layer.
Optical low-pass filter. The birefringence layer is configured such that the inclination angle of the optical axis with respect to the incident light is 45 ° and the thickness is 0.01 mm or more and 0.21 mm or less.
The optical low-pass filter according to claim 1. The birefringence layer has a thickness of 0.133 mm and is configured such that the inclination angle of the optical axis with respect to the incident light is 1 ° or more and 45 ° or less or 45 ° or more and 89 ° or less.
The optical low-pass filter according to claim 1. The infrared absorbing layer is formed by applying a material in which a dye that absorbs infrared rays is dispersed in a solvent to the birefringent layer, and then curing the coating film.
The optical low-pass filter according to any one of claims 1 to 3.

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