CN113759562A - Optical low-pass filter - Google Patents

Abstract

A thinner optical low-pass filter is provided. The optical low-pass filter has a two-layer structure including a lithium niobate plate and an infrared ray absorption layer. The lithium niobate plate is made of lithium niobate which is a material having birefringence, and is configured such that the amount of separation between ordinary rays and extraordinary rays due to birefringence is 0.39 μm or more and 8.13 μm or less. The infrared absorbing layer is held in contact with the lithium niobate plate.

Description

Optical low-pass filter

Technical Field

The present invention relates to an optical low-pass filter.

Background

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

As such an optical low-pass filter, a four-point separation type optical low-pass filter is proposed in which a phase plate or a birefringent plate is sandwiched between two birefringent plates and an infrared absorber is held between one birefringent plate and the phase plate (japanese patent laid-open No. 2006-208470). According to this configuration, a lightweight optical low-pass filter having an infrared absorption function can be formed.

Disclosure of Invention

However, in the optical low-pass filter described above, the birefringent plate is made of crystal, and there is a limitation in reducing the thickness. In view of the current situation where further miniaturization and thinning of smartphones and the like are expected, thinner optical low-pass filters are 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.

An optical low-pass filter according to claim 1 of the present invention includes: a birefringent layer made of lithium niobate and configured such that a separation amount of normal light and abnormal light is 0.39 μm or more and 8.13 μm or less; the infrared absorbing layer is held in contact with the birefringent layer. Thus, the optical low-pass filter can be made thin by reducing the thickness of the birefringent layer.

In the optical low-pass filter according to claim 2 of the present invention, preferably, in the optical low-pass filter, the birefringent layer is configured such that an inclination angle of the optical axis with respect to the incident light is 45 ° and a thickness is 0.01mm or more and 0.21mm or less. Thus, the optical low-pass filter can be made thin by reducing the thickness of the birefringent layer.

In the optical low-pass filter according to claim 3 of the present invention, preferably, in the optical low-pass filter, the birefringent layer has a thickness of 0.133mm, and an inclination angle of an optical axis with respect to incident light is 1 ° or more and 45 ° or less or 45 ° or more and 89 ° or less. Thus, the optical low-pass filter can be made thin by reducing the thickness of the birefringent layer.

In the optical low-pass filter according to claim 4 of the present invention, it is preferable that the infrared absorbing layer is formed by applying a material in which an infrared absorbing dye is dispersed in a solvent to the birefringent layer and then curing the coating film. This makes it possible to easily form the infrared absorbing layer, and to reduce the manufacturing cost and the delivery date.

According to the present invention, a thinner optical low-pass filter can be provided.

The above and other objects, features and advantages of the present disclosure will be more fully understood from the detailed description given below and the accompanying drawings, which are given by way of illustration only, and thus should not be taken as limiting the present disclosure.

Drawings

Fig. 1 is a diagram schematically showing a configuration of a camera mounted on a mobile device.

Fig. 2 is a cross-sectional view schematically showing the structure of the optical low-pass filter according to embodiment 1.

Fig. 3 is a graph showing design values of a BG optical low-pass filter configured by BG and an LN optical low-pass filter configured by LN of comparative example 1.

Fig. 4 is a diagram showing a tilt angle θ, which is an angle formed by the incident surface and the optical axis of the material.

Fig. 5 is a diagram showing a relationship between the tilt angle and the separation amount of the optical axes of LN and crystal.

Fig. 6 is a diagram showing the relationship between the thickness and the separation amount of LN and crystal when the inclination angle θ of the optical axis is 45 °.

Fig. 7 is a diagram showing an MTF (Modulation Transfer Function) curve of the optical low-pass filter 100 according to embodiment 1.

Fig. 8 is a graph showing absorption characteristics of an infrared absorbing layer and a general blue glass (thickness 0.21mm, comparative example).

Detailed Description

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted as necessary.

Embodiment mode 1

An optical low-pass filter according to embodiment 1 will be described. Fig. 1 schematically shows a 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 including one or more lenses, an optical low-pass filter 1, and an imaging element 3 such as a cmos are arranged in this order from the side of an object to be imaged. In addition, hereinafter, for convenience, a direction from the photographic subject toward the image pickup element 3 is referred to as an X direction. As is clear from fig. 1, in order to reduce the thickness of the camera, it is desirable to make the thickness of the optical low-pass filter 1 as thin as possible. Therefore, the present embodiment provides an optical low-pass filter having a structure that can be made thinner than a general optical low-pass filter.

The optical low-pass filter 100 according to the present embodiment will be described below. Fig. 2 is a cross-sectional view schematically showing the structure of the optical low-pass filter 100 according to embodiment 1. The optical low-pass filter 100 includes an LN plate 10 as a birefringent layer, the LN plate 10 is a plate-shaped member made of Lithium Niobate (hereinafter, referred to as LN), and the infrared absorbing layer 20 is held on the LN plate 10.

The pixel pitch of an imaging device mounted in a smartphone or a digital camera is usually 1.0 μm to 8.4 μm. In addition, in consideration of optical restrictions, an 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 be applied to an imaging element having a pixel pitch of 0.8 μm to 8.4 μm by setting the thickness of the LN plate 10 to an appropriate value.

LN has a refractive index of 2.273, for example, which is larger than the refractive index of blue glass (hereinafter referred to as BG) of 1.564, and the refractive index of crystal of 1.55. Therefore, by using LN, a thinner low-pass filter can be configured compared to BG and quartz. The following description will be specifically made.

For example, when an optical low-pass filter is mounted on a smartphone, it is necessary to insert the optical low-pass filter into a space defined by a certain specification. That is, the optical low-pass filter is required to be configured so that the optical path length from the end face (surface on the X + side) 2A of the lens configuring the optical system 2 to the imaging surface 3A of the imaging element 3 is kept constant. Hereinafter, a case where a blue glass (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 design values of a BG optical low-pass filter using BG and an LN optical low-pass filter using LN of comparative example 1. In this example, BG has a refractive index of 1.564 and LN has a refractive index of 2.273. Further, according to the specification, a distance d1 from the surface 1A on the X + side of the optical low-pass filter to the imaging surface 3A of the imaging element 3 is fixed to 0.3mm, and a distance from the end surface 2A of the lens facing the surface 1B on the X-side of the optical low-pass filter to the imaging surface 3A of the imaging element 3 is fixed to 0.91 mm.

When the thickness t of the BG optical low-pass filter according to comparative example 1 is 0.21mm, the optical path length 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 imaging surface 3A of the imaging element 3 is 1.028 mm.

Therefore, when the BG optical low-pass filter is replaced with the LN optical low-pass filter under such conditions, the LN optical low-pass filter is required to be configured such that the optical path length from the X-side surface 1B of the optical low-pass filter to the imaging surface 3A of the imaging element 3 is substantially equal. In this example, when the thickness of the LN optical low-pass filter is 0.133mm as in the LN plate 10, the optical path length thereof 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, since the optical path length from the end surface 2A of the lens to the imaging surface 3A of the imaging element 3 is 1.079mm, it is possible to achieve an optical path length almost the same as that in the case of using the BG optical low-pass filter.

As described above, when an optical low-pass filter is mounted 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 LN in terms of the optical path length.

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

From the viewpoint of optical path length, an optical low-pass filter using LN can be thinned, but a design matching the pixel pitch of the optical element as the target is required. Hereinafter, focusing on the amount of light separation due to birefringence, the amounts of light separation in the crystal and LN (i.e., the distance between the optical axes of the normal light ray and the abnormal light ray) are compared.

Fig. 4 shows the angle of incidence of the incident surface with respect to the optical axis of the material, i.e., the tilt angle θ. In fig. 4, a polarized light component perpendicular to the paper surface (referred to as vertical polarized light) in the incident light L is shown by a black dot, and a polarized light component parallel to the paper surface (referred to as horizontal polarized light) is shown by a double arrow. The incident light L is incident perpendicularly to the incident surface of the material M from 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 enters the material M, travels straight as it is, and exits the material M (so-called normal light OD). On the other hand, for example, the horizontally polarized light enters the material M, is refracted, travels in a direction different from the normal light, and exits from the exit surface of the material M in a direction parallel to the normal light (so-called abnormal light EX). The separation amount (also referred to as a separation width) refers to a distance between the emission positions of vertically polarized light (normal light) and horizontally polarized light (abnormal light).

If the thickness of the material M is t, the refractive index of the material M relative to normal light is n_odThe refractive index of the material M with respect to the extraordinary ray is set to n_exIt was found that the separation amount D at this time is expressed by the following formula.

It is understood that the above-described separation amount D is maximum when the inclination angle θ of the optical axis is 45 °, that is, the separation amount at the inclination angle of (45- α) ° is equal to the separation amount at the inclination angle of (45+ α) °.

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

Further, since the crystal has a separation amount of 1.29 μm when the thickness is 0.21mm and the inclination angle θ of the optical axis is 45 °, the crystal can be used as an optical low-pass filter of an imaging device having a pixel pitch of 1 μm to several μm. However, since the separation amount cannot be further increased without increasing the thickness of the crystal, the crystal is not suitable for further thinning of the optical low-pass filter in principle.

Fig. 6 shows LN when the inclination angle θ of the optical axis is 45 °, and the relationship between the thickness and the separation amount of the crystal. As shown in fig. 6, in LN, the separation amount at a thickness of 0.04mm is 1.55 μm, and can be used as an optical low-pass filter of an imaging element having a pixel pitch of 1 μm to several μm. On the other hand, in order to achieve the same separation amount using quartz, it is known that LN is advantageous for thinning the optical low-pass filter because the thickness of 0.21 μm is necessary as described above.

In addition, in LN, since the separation amount can be adjusted in the range of 0.39 to 8.13 by adjusting the inclination angle θ of the optical axis, it can be understood that the LN is sufficiently applicable to the above-described imaging element having the pixel pitch of 0.8 μm to 8.4 μm.

On the other hand, when crystal is used, the separation amount cannot be further increased without increasing the thickness, and therefore, it is difficult to apply the crystal to an imaging element having a pixel pitch exceeding 1.6 μm. Therefore, the range of pixel pitches that can be handled can be greatly expanded by using LN as compared with crystal.

Next, a comparison between the optical characteristics of a normal 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 embodiment 1. In fig. 7, the horizontal axis is spatial frequency and the vertical axis is contrast, and shows MTF curves of a light ray along a path that reaches the imaging element 3 through the lens center of the optical system 2 and a light ray that passes at positions that are 0.3F and 0.7F away from the lens center in the tangential direction (TAN) and the Sagittal (Sagittal) direction (SAG), where F is image height. In fig. 7, + represents the optical low-pass filter (OLPF) separation direction, and-represents 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 than a normal low-pass filter.

In addition, even when the LN plate 10 is formed by bonding two plates, the entire thickness can be made smaller than 0.133mm, which is more advantageous for thinning.

The infrared absorption layer 20 can be formed, for example, by: a fluid raw material (for example, SA7 manufactured by japan catalyst corporation) is applied to the LN plate 10 by spin coating, and then baked at a predetermined temperature for a predetermined time. When a commercially available normal infrared absorber is used, the thickness of the infrared absorbing layer 20 is, for example, about 5 μm, but the total thickness is 0.138mm even when the sum of the thickness and the thickness of the LN plate 10 is added, and the thickness can be made thinner than that of a normal low-pass filter.

The absorption characteristics of the infrared absorption layer 20 were examined. Fig. 8 shows absorption characteristics of the infrared absorption layer 20 and a general blue glass (thickness 0.21mm, 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, as in the case of the general blue glass.

Therefore, according to the present configuration, it is possible to provide a thinner optical low-pass filter that can appropriately absorb infrared rays and that can be applied to a portable device such as a smartphone.

In this embodiment, an LN plate is used as the birefringent layer. LN plates can be formed by cutting a circular plate out of an ingot of LN, and polishing it to a desired thickness. It is known that the ingot growth cycle of LN is shorter than that of a crystal which is a material of a normal birefringent plate. In the case of industrially producing crystals of crystal, a crystal growth cycle of several months is generally required, whereas the ingot of LN requires only about several days for drawing. Therefore, in the case of LN, the delivery time required for material arrangement can be greatly shortened, and as a result, the delivery time of the entire optical low-pass filter manufacturing can be shortened and the manufacturing cost can be reduced.

Other embodiments

The present invention is not limited to the above-described embodiments, and can be modified as appropriate without departing from the scope of the invention. For example, although a configuration in which the infrared absorbing layer is formed by coating has been described, it goes without saying that the coating method is not limited to spin coating. It goes without saying that various film forming methods such as vapor deposition and sputtering can be applied if the infrared absorbing layer of the desired low pass filter pixel pitch can be formed.

It is needless to say that 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 a camera or the like.

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

From the disclosure thus described, it will be obvious that the embodiments of the disclosure may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure, 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 (4)

1. An optical low-pass filter includes:

a birefringent layer made of lithium niobate and configured such that a separation amount of the ordinary ray and the extraordinary ray is 0.39 μm or more and 8.13 μm or less; and

an infrared absorbing layer in contact with the birefringent layer to be held.

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

the birefringent layer is configured such that the optical axis is inclined at an angle of 45 DEG with respect to the incident light and has a thickness of 0.01mm to 0.21 mm.

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

the birefringent layer is configured such that the thickness is 0.133mm and the inclination angle of the optical axis with respect to the incident light is 1 DEG or more and 45 DEG or less or 45 DEG or more and 89 DEG or less.

4. Optical low-pass filter according to claim 1 or 2,

the infrared ray absorption layer is formed by applying a material in which an infrared ray absorbing pigment is dispersed in a solvent to the birefringent layer and then curing the applied film.

Images