JP2019053118A - Optical system and imaging device - Google Patents

Abstract

To improve diffraction efficiency and realize a compact optical system equipped with flare-free satisfactory image-forming performance by a simple configuration.SOLUTION: Provided is an optical system comprising, in order from object side to image side, a first optical element and a second optical element coupled with the first optical element, wherein the optical system includes a diffraction optical part formed on the joint surface between the first optical element and the second optical element, one of the first optical element and the second optical element is formed with a first material having low refraction index and high dispersion properties and the other is formed with a second material having relative high refraction index and low dispersion properties, the image-side surface of the second optical element is formed in shape of a convex surface, and the relationship 0.7<f2/f<1.3 is satisfied, where f represents the focal distance of the entire optical system and f2 represents the focal distance of the convex surface.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a small imaging optical system.

In recent years, imaging devices are mounted on various electronic devices. As an optical system used in such an image pickup apparatus, a low-cost and small-sized one with high resolution is required. As means for realizing such an optical system, an example of an optical system in which a plurality of lenses are joined together is disclosed.
For example, Patent Document 1 discloses an optical system that improves chromatic aberration correction and imaging performance with the following configuration. A first optical element having a diffractive surface, a second optical element bonded to the first optical element via the diffractive surface, and a third optical element bonded to the second optical element on the surface opposite to the diffractive surface; , An optical system composed of a fourth optical element bonded to the first optical element on the surface opposite to the diffraction surface.
Further, Patent Document 2 discloses an optical system having an optical element in which flare generated in the diffraction section is reduced and chromatic aberration is corrected by the following configuration. It is an optical element in which a diffraction grating part in which a plurality of diffraction gratings are laminated and a refractive part made of a solid material having a refractive action are in close contact, and optical power and dispersion are set.
In addition, as a small and low-cost optical system, a wafer is formed on a wafer (lens substrate) using a resin material, a plurality of wafers are stacked, bonded, and then cut into individual optical systems. A technique called level optics is known. A plurality of optical elements are tightly bonded to one lens substrate of such an optical system. For example, Patent Document 3 discloses an optical system that is small and has high image quality with the following configuration. An optical system having a first element having positive power on the object side of the lens substrate, a second element having a diffractive surface on the image side of the lens substrate, and a third element having negative power on the image side of the second element. It is.

Japanese Patent No. 4817076 Japanese Patent No. 5366673 Japanese Patent No. 4387368

However, in the optical system described in Patent Document 1, two optical elements are used to form a diffractive surface, and a third optical element and a fourth optical element are arranged in order to use a refraction effect. The refracting surface and the diffractive surface are formed by different optical elements.
Moreover, since the optical element described in Patent Document 2 is a cemented lens used for a part of the entire optical system, it is difficult to use such an optical element as a single optical system.
Further, in the optical system described in Patent Document 3, flare generated on the diffractive surface is not removed, the diffraction efficiency is low, and there is a concern about deterioration in image quality.

  The present invention has been made in view of the circumstances as described above, and an object thereof is to improve a diffraction efficiency and realize a small optical system having a good imaging performance with less flare with a simple configuration. .

The first aspect of the present invention is:
In an optical system having a first optical element and a second optical element bonded to the first optical element in order from the object side to the image side,
A diffractive optical part formed on a joint surface between the first optical element and the second optical element;
Either one of the first optical element and the second optical element is formed of a first material having a relatively low refractive index and a high dispersion, and the other is a second material having a relatively high refractive index and a low dispersion. Formed of material,
The image side surface of the second optical element is formed as a convex surface,
When the focal length of the entire optical system is f and the focal length of the convex surface is f2,
0.7 <f2 / f <1.3
An optical system characterized by satisfying the above is provided.

The second aspect of the present invention is:
The optical system described above;
An image sensor for receiving an image formed by the optical system;
There is provided an imaging device characterized by comprising:

  According to the present invention, it is possible to improve the diffraction efficiency and realize a small optical system having a good imaging performance with less flare with a simple configuration.

1 is a schematic diagram illustrating a main part of an imaging apparatus according to Embodiment 1. FIG. Schematic which shows the principal part of the optical system of Example 1. FIG. The figure which shows the diffraction grating formed between the 1st optical element of Example 1, and the 2nd optical element The figure which shows the 1st-order diffraction efficiency with respect to the wavelength in a normal diffraction grating as a comparative example The figure which shows the 1st-order diffraction efficiency with respect to the wavelength in the diffraction grating of Example 1. Various aberration diagrams of the optical system according to Example 1 Diagram for explaining a method of manufacturing an optical system using the wafer level optics technology Schematic which shows the principal part of the optical system of Example 2. Various aberration diagrams of the optical system according to Example 2 Schematic which shows the principal part of the optical system of Example 3. FIG. Various aberration diagrams of the optical system according to Example 3

The present invention relates to an optical system of an imaging apparatus, and more particularly to an extremely small optical system that can be manufactured using a technique called wafer level optics. This type of optical system is called a wafer level lens, and an image pickup apparatus using the wafer level lens as an image pickup optical system is also called a wafer level camera. The optical system and the image pickup apparatus according to the present invention can be applied to general digital cameras and digital video cameras, but can be preferably applied to cameras for incorporation in various electronic devices because of their small size and low cost. Examples of the electronic device include a mobile phone, a smartphone, a tablet terminal, a personal computer, a game machine, and a wearable terminal.
Preferred embodiments of the present invention will be described below with reference to the drawings. Each drawing may be drawn on a different scale for convenience. Moreover, in each drawing, the same reference number is attached | subjected about the same member and the overlapping description is abbreviate | omitted.

[Example 1]
Example 1 will be described below.
FIG. 1 is a schematic diagram illustrating a main part in a YZ section including an optical axis AX in an imaging apparatus 1 including an optical system 10 according to the present embodiment.
The image pickup apparatus 1 according to the present embodiment includes an optical system 10 as an image pickup optical system, an image pickup device 2 including an image pickup surface (light receiving surface) disposed at a position of an image plane IMG of the optical system 10, a cable 3, and a processing unit 4. Is provided.
In the imaging apparatus 1, the optical system 10 collects a light beam from a subject (object) (not shown) on the left side of FIG. 1 and forms an image of the subject on the imaging surface of the imaging device 2. The image sensor 2 photoelectrically converts an object image formed by the optical system 10 and outputs an electrical signal. The processing unit 4 processes the electrical signal from the image sensor 2 transmitted via the cable 3 and acquires image data of the subject. As the imaging device 2, a solid-state imaging device (photoelectric conversion device) such as a CCD sensor or a CMOS sensor can be employed.

FIG. 2 is a schematic diagram illustrating a main part in a YZ section including the optical axis AX of the optical system 10.
The optical system 10 according to this embodiment includes an aperture stop STO, a third optical element L3, an optical substrate (hereinafter referred to as a substrate) SUB, a first optical element L1, a second optical element, and the like. It has an optical element L2 and a fourth optical element CG.
The substrate SUB is a parallel plate member. As will be described later, the optical system 10 of the present embodiment is manufactured by wafer level optics in which a number of optical elements are formed on a wafer (parallel plate) using a resin material, and individual optical elements are cut out and used. A part of the wafer becomes the substrate SUB.

The third optical element L3 is bonded to the surface SUBa on the object side of the substrate SUB. The third optical element L3 is a plano-concave lens in which the object-side surface L3a is concave and the image-side surface L3b is flat, and has negative power. The first optical element L1 is bonded to the image-side surface SUBb of the substrate SUB, and the second optical element L2 is bonded to the image-side surface L1b of the first optical element L1.
In this embodiment, the diffraction grating 5 serving as a diffractive optical section is formed on the junction surface (boundary surface) between the image-side surface L1b of the first optical element L1 and the object-side surface L2a of the second optical element L2. Is formed.
The object side surface L1a of the first optical element L1 is a flat surface. The image-side surface L2b of the second optical element L2 has a convex surface L2b1 formed so as to be convex toward the image side, and has positive power. The fourth optical element CG is an optical filter such as an IR cut filter.
In FIG. 2, a light beam from an object (not shown) enters the aperture stop STO, and at this time, the light beam width is limited. The light beam that has passed through the aperture of the aperture stop STO is transmitted in the order of the third optical element L3, the substrate SUB, the first optical element L1, the second optical element L2, and the fourth optical element CG to form an image plane IMG.

FIG. 3 is a schematic cross-sectional view showing the diffraction grating 5 formed on the joint surface between the first optical element L1 and the second optical element L2. In FIG. 3, for convenience of explanation, a diagram deformed with respect to the thickness direction (z direction) of the diffraction grating 5 is used. The structure of the diffraction grating 5 is not particularly limited, and may be, for example, a blazed structure or a binary structure that approximates the blazed structure to a staircase shape.
As shown in FIG. 3, the diffraction grating 5 is formed on a joint surface between the image-side surface L1b of the first optical element L1 and the object-side surface L2a of the second optical element L2. When chromatic aberration correction is performed only on the refracting surface, materials having different Abbe numbers are used for the positive lens and the negative lens, and chromatic aberration correction is performed based on the Abbe number difference. However, in the case of wafer level optics, since the materials that can be used are limited in manufacturing, it is difficult to make an appropriate Abbe number difference. On the other hand, when a diffraction grating is used, there is an advantage that is not limited to the selection of materials in terms of chromatic aberration correction.

In the diffraction surface on which the diffraction grating is formed, the direction of chromatic aberration with respect to a light beam having a certain reference wavelength is opposite to the direction of the refracting surface having a positive power. Color correction is possible. The Abbe number of the diffractive surface is −3.45, and chromatic aberration can be sufficiently corrected even with a slight power of the diffractive surface.
In the present embodiment, a convex surface L2b1 is provided on the image side surface L2b of the second optical element L2, and positive power is generated. A diffraction grating 5 is formed on the joint surface between the first optical element L1 and the second optical element L2. This diffraction grating 5 can correct chromatic aberration generated on the convex surface L2b1 having a positive power as described above.

In a lens system having a diffraction grating, when the light beam in the wavelength range to be used is concentrated on the diffracted light with respect to a specific one order and a specific wavelength (hereinafter also referred to as a set wavelength), diffraction of other orders The light intensity is low, and the diffraction efficiency is high for a specific order.
FIG. 4 is a diagram showing the first-order diffraction efficiency with respect to the wavelength in a normal diffraction grating as a comparative example.
As shown in FIG. 4, the diffraction grating of the comparative example has high diffraction efficiency near the design wavelength, but the diffraction efficiency decreases as the distance from the design wavelength increases.

In the present embodiment, as a configuration for improving the diffraction efficiency, a low refractive index and high dispersion material (hereinafter referred to as a first material) having a relatively low refractive index and a high dispersion is used as the material of the closely attached optical element forming the diffraction grating 5. In addition, a relatively high refractive index and low dispersion high refractive index and low dispersion material (hereinafter referred to as a second material) is used. At this time, any one of the first optical element L1 and the second optical element L2 may be formed of the first material and the other is formed of the second material.
When the refractive index of the first material is N _1λ , the refractive index of the second material is N _2λ , the height of the diffraction grating is h, and the diffraction order is m, h = mλ / (N _2λ −N _1λ ) is all When satisfied with the wavelength, the wavelength dependence of the diffraction efficiency disappears. From this equation, it is necessary to increase the difference in refractive index between the two materials at longer wavelengths, but this is achieved by using a low refractive index, high dispersion material for the first material and a high refractive index, low dispersion material for the second material. And diffraction efficiency can be improved.
FIG. 5 is a graph showing the first-order diffraction efficiency with respect to the wavelength in the diffraction grating of this example.
As can be seen by comparing the first-order diffraction efficiencies shown in FIG. 4 and FIG. 5, in the optical system having the diffraction grating of this embodiment, the diffraction efficiency is improved and an optical system with less flare can be realized. .

The convex surface L2b1 of the image-side surface L2b of the second optical element L2 further satisfies the following formula (1), thereby causing the second optical element L2 to bear much of the power of the entire optical system, and coma aberration. Astigmatism can be corrected well. Here, in the optical system 10 according to the present embodiment, when the focal length of the convex surface L2b1 of the second optical element L2 is f2 (mm) and the focal length of the entire optical system is f (mm), the expression (1 ) Is satisfied. Specifically, in this embodiment, f2 = 2.282 mm and f = 2.289 mm, and f2 / f in equation (1) is 0.997.
0.7 <f2 / f <1.3 (1)
When f2 / f is 1.3 or more, the power of the convex surface L2b1 of the second optical element L2 becomes weak, and the object-side surface L3a of the third optical element L3 needs to have positive optical power. There is a concern that this may be disadvantageous in correcting aberrations. On the other hand, when f2 / f is 0.7 or less, there is a concern that the power of the convex surface L2b1 of the second optical element L2 becomes too strong, and it is difficult to correct astigmatism generated on this surface.
Furthermore, it is more preferable to satisfy the following formula (1a).
0.75 <f2 / f <1.2 (1a)

Here, the substrate SUB arranged to be bonded to the object-side surface L1a of the first optical element L1 is formed of a material having an Abbe number larger than that of the second material used to form the diffraction grating 5. It is preferable that it is.
This is because the present embodiment assumes that the optical system is manufactured by wafer level optics in order to achieve a small and low cost optical system.
Since the substrate SUB optically hardly affects the image performance, it is desirable to use a material generally called white glass or an inexpensive material such as quartz glass. Such a material has a larger Abbe number compared to the second material. In this embodiment, white glass is used for the substrate SUB, and its Abbe number is 62.6. The Abbe number of the second material is 38.6, which satisfies the above condition.

Further, it is preferable that the first optical element L1 is formed of the first material and the second optical element L2 is formed of the second material.
When the second material is used as the material of the second optical element L2, the occurrence of the Petzval image plane (field curvature) can be suppressed. Therefore, by using the second material for the second optical element L2 and the first material for the first optical element L1, it is possible to simultaneously improve the diffraction efficiency and suppress the occurrence of the Petzval image plane. Become.

Further, when the distance from the aperture stop STO to the diffraction grating 5 on the optical axis is D1 (mm) and the distance from the aperture stop STO to the image plane IMG is D (mm), the following equation (2) holds: preferable.
D1 ≦ D / 2 (2)
Expression (2) sets an interval from the aperture stop STO to the diffraction grating 5 in order to appropriately correct chromatic aberration. The distance from the optical axis when the axial ray is incident on the diffraction grating 5 is h, and the distance from the optical axis when the principal ray is incident on the diffraction grating 5 is h bar (indicated by a horizontal bar above h), The power of the diffraction grating 5 is Φ, and the Abbe number of the diffraction grating 5 is ν. At this time, the longitudinal chromatic aberration coefficient L and the lateral chromatic aberration coefficient T can be expressed by the following expressions L and T as follows.

At this time, h has a large value at the position of the aperture stop STO and decreases as it approaches the image plane IMG, while h bar has a value of 0 at the position of the aperture stop STO and increases as it approaches the image plane IMG. Therefore, it is preferable to satisfy Expression 2 so as to contribute in a balanced manner to the longitudinal chromatic aberration coefficient L and the magnification chromatic aberration coefficient T.
When D1 exceeds D / 2, the principal ray height becomes too high from the optical axis compared to the axial ray height, and there is a concern that it is difficult to balance axial chromatic aberration and lateral chromatic aberration. In this embodiment, since D1 = 1.501 and D / 2 = 2.037, the expression (2) is satisfied.

Moreover, it is preferable to have a concave-shaped part formed concavely toward the object side between the aperture stop STO and the image-side surface L1b of the first optical element L1.
By providing such a concave-shaped portion, negative power can be provided, so that an effect of correcting field curvature can be obtained. In the present embodiment, the object-side surface L3a of the third optical element L3 has a concave shape portion L3a1 that is concave toward the object side.

Further, when the Abbe number at the d-line of the second optical element L2 is ν2, the Abbe number at the d-line of the diffraction grating is νdoe, and the focal length of the diffraction grating is fdoe, the following expression (3) is satisfied. It is preferable.
0.4 ≦ | fdoe × νdoe | / (f2 × ν2) ≦ 1.2 (3)
Expression (3) is for appropriately setting the optical power and dispersion of the second optical element L2 and the diffraction grating 5 and correcting chromatic aberration satisfactorily.
If | fdoe × νdoe | / (f2 × ν2) exceeds 1.2, there is a concern that the power of the diffraction grating 5 becomes weak and chromatic aberration generated on the convex surface L2b1 of the second optical element L2 cannot be corrected. On the other hand, if | fdoe × νdoe | / (f2 × ν2) is less than 0.4, the power of the diffraction grating becomes too strong, so there is a concern that chromatic aberration will be excessively corrected and image performance will deteriorate.
In this embodiment, since fdoe = 2.39, νdoe = −3.45, and ν2 = 38.6, equation (3) is 0.798, which satisfies the above condition. Here, the second optical element L2 is preferably formed of the second material, but is not limited thereto, and may be formed of the first material.

The convex surface L2b1 of the second optical element L2 is preferably an aspherical surface.
By making this surface an aspherical surface, coma and astigmatism off-axis aberrations can be corrected more satisfactorily.
Further, when the refractive index of the second material is Nh, it is preferable to satisfy the following formula (4).
Nh> 1.55 (4)
On the surface using a material having a high refractive index, the occurrence of the Petzval image surface can be suppressed. In the optical system of the present embodiment, most of the optical power is generated on the convex surface of the second optical element L2 as in Expression (1), and therefore, a material in a range satisfying Expression (4) should be used for this optical element. Thus, the field curvature can be corrected more effectively. In this embodiment, Nh = 1.618, which satisfies the above condition. Here, the second optical element L2 may be formed from the second material, but the present invention is not limited to this, and the first optical element L1 may be formed from the second material.

<Numerical Example 1>
Next, Numerical Example 1 corresponding to the present embodiment will be shown.
In the numerical examples, the surface numbers are in the order of optical surfaces counted from the object surface to the image surface. * Means a surface having a diffractive optical part.

ここで、ｚは非球面形状の光軸方向のサグ量（ｍｍ）、ｃは光軸ＡＸ上における曲率（１／ｍｍ）、ｋは円錐係数、ｈは光軸ＡＸからの半径方向の間隔（ｍｍ）、Ａ，Ｂ，Ｃ，・・・の夫々は４次項，６次項，８次項，・・・の非球面係数、である。なお、この非球面式において、第１項はベース球面のサグ量を示しており、このベース球面の曲率半径はＲ＝１／ｃである。また、第２項以降の項は、ベース球面上に付与される非球面成分のサグ量を示している。「Ｅ−Ｘ」は「１０^-Ｘ」を、「Ｅ＋Ｘ」は「１０^Ｘ」を意味する。 Each of the aspherical optical surfaces in this embodiment has a rotationally symmetric shape about the optical axis, and is represented by the following general aspherical expression.

Here, z is the sag amount (mm) of the aspherical surface in the optical axis direction, c is the curvature (1 / mm) on the optical axis AX, k is the conic coefficient, and h is the radial distance from the optical axis AX ( mm), A, B, C,... are aspherical coefficients of the fourth, sixth, eighth,. In this aspherical formula, the first term indicates the sag amount of the base spherical surface, and the radius of curvature of the base spherical surface is R = 1 / c. The second and subsequent terms indicate the sag amount of the aspherical component provided on the base spherical surface. “E−X” means “10 ^−X ”, and “E + X” means “10 ^X ”.

・・・（５） The phase function of the diffraction grating is expressed by the following formula (5). In the following equation, Φ is a phase function of a diffraction grating with a reference wavelength λ0, Cn is a phase function coefficient, and h is a height from the optical axis. In this embodiment, the phase function coefficient Cn is described with the reference wavelength λ0 set to 587.6 nm.

... (5)

Each data of Numerical Example 1 corresponding to the present example is shown below.

Table 1 shows surface data of each optical surface in the optical system 10. In Table 1, r is a radius of curvature (mm), d is a surface interval (mm), nd is a refractive index with respect to d-line, and νd is d. Represents the Abbe number for the line. However, the surface interval d is positive when going to the image side along the optical path and negative when going to the object side.
As shown in Table 1, in the optical system 10 according to this example, the object side surface L1a (surface number 2) of the first optical element L1 and the image side surface L2b (surface number 6) of the second optical element L2. ) Is an aspherical surface. The boundary surface (surface number 5) between the first optical element L1 and the second optical element L2 is a diffraction surface on which the diffraction grating 5 is formed.
Table 2 shows various data of the optical system 10, and Fno represents an aperture value (F value). As shown in Table 2, the optical system 10 according to the present example has characteristics of Fno = 2.80 and an angle of view of 60 ° (± 30 °) while having a small length of 4.07 mm. ing.
Table 3 shows the values of Expression (1), Expression (3), and Expression (4) in the optical system 10 according to the present example.

6A to 6C are graphs showing various aberrations of the optical system 10 according to the present example.
In the spherical aberration diagram shown in FIG. 6A, aberrations relating to 850 nm, 656 nm, 588 nm, and 486 nm are shown.
In the astigmatism diagram shown in FIG. 6B, m and s represent a meridional image plane and a sagittal image plane, respectively.
As is apparent from FIG. 5, various aberrations are favorably corrected at 400 to 850 nm. In particular, it is desirable for the surveillance camera to correct aberrations from the visible range to the near-infrared range for nighttime monitoring, and the present optical system can meet this demand.

7A and 7B are views for explaining a method of manufacturing an optical system using the wafer level optics technique of this embodiment.
In wafer level optics, a semiconductor process or a resin molding technique is used to form a large number of optical elements on both surfaces of a wafer made of a glass material, and then cut to produce a large number of cemented lenses. Of the cemented lens created by cutting, the wafer portion becomes the substrate SUB.
FIG. 7A shows a schematic perspective view of a wafer on which a large number of optical elements are formed as viewed from above, and FIG. 7B shows an enlarged view of the wafer on which a large number of optical elements are formed as viewed from the side.

As shown in FIG. 7A, first, the third optical element L3 is formed on the wafer W. The third optical element L3 is formed by disposing a curable resin material on the wafer W and pressing a mold for the third optical element.
Similarly, on the surface of the wafer W opposite to the surface on which the third optical element L3 is formed, the first optical element L1 and the second optical element L2 are replaced with the first optical element L1 and the second optical element. It produces using the type | mold in order of L2. Curable resin materials include photo-curing and thermo-curing types, but the cost can be reduced by mounting the optical system together with electronic components such as IC chips by reflow treatment. It is desirable to use a certain photo-curing type.
Thereafter, by cutting the wafer into individual lens units, hundreds to tens of thousands of cemented lenses can be manufactured. The size of one cemented lens is on the order of several millimeters. Finally, the cemented lens, the aperture stop, and the image sensor are assembled to obtain an image pickup apparatus.

  As described above, according to the configuration of the optical system 10 of this embodiment, it is possible to improve the diffraction efficiency and realize a small optical system having a good imaging performance with less flare with a simple configuration. . In addition, by using such an optical system, it is possible to manufacture a small and high-quality image pickup apparatus at low cost.

[Example 2]
Example 2 will be described below. In the present embodiment, the different components from the first embodiment will be described, and the description of the same components as those in the first embodiment will be omitted.
FIG. 8 is a schematic diagram illustrating a main part in a YZ section including the optical axis AX of the optical system 20 of the present embodiment.
The optical system 20 according to the present embodiment includes an aperture stop STO, a first optical element L1, a second optical element L2, and a fourth optical element CG that limit the beam width in order from the object side to the image side. In the optical system 20, the first optical element L1 and the second optical element L2 are bonded to each other, and the diffraction grating 5 is formed on the bonding surface.
The aperture stop STO is formed of a light shielding film made of a metal film such as chromium or a dielectric multilayer film on the object-side surface L1a of the first optical element L1.

The object-side surface L1a of the first optical element L1 has a concave-shaped portion L1a1 formed in a concave shape toward the object side. The image side surface L1b of the first optical element L1 is a joint surface with the object side surface L2a of the second optical element L2.
The first optical element L1 has negative power. When the first optical element L1 is manufactured using a parallel plate-shaped wafer made of a glass material, it is desirable that the concave-shaped portion L1a1 is formed using a gray scale lithography process, and the diffraction grating is manufactured using a lithography process. By using the lithography technique in this way, manufacturing errors can be reduced and manufacturing can be performed with high accuracy.
The image side surface L2b of the second optical element L2 has a convex surface L2b1. The second optical element L2 has a positive power. The fourth optical element CG is an optical filter such as an IR cut filter.
In FIG. 8, a light beam from an object (not shown) enters the aperture stop STO, and at this time, the light beam width is limited. The light beam that has passed through the aperture of the aperture stop STO passes through the first optical element L1, the second optical element L2, and the fourth optical element CG in this order to form an image plane IMG.

<Numerical Example 2>
Similar to the first embodiment, Tables 4 to 6 show data of the second numerical embodiment corresponding to the present embodiment.

As shown in Table 4, in the optical system 20 according to this example, the object-side surface L1a (surface number 2) of the first optical element L1 and the image-side surface L2b (surface number 4) of the second optical element L2. ) Is an aspherical surface. The boundary surface (surface number 3) between the first optical element L1 and the second optical element L2 is a diffraction surface on which the diffraction grating 5 is formed.
As shown in Table 5, the optical system 20 according to the present example has characteristics of Fno = 2.80 and an angle of view of 60 ° (± 30 °) while having a small overall length of 3.65 mm. ing.
Table 6 shows values of Expression (1), Expression (3), and Expression (4) in the optical system 20 according to the present example. As can be seen from Table 6, the optical system 20 also satisfies all the expressions (1), (3), and (4). Further, since D1 = 0.929 and D / 2 = 1.824, the expression (2) is also satisfied.

9A to 9C are various aberration diagrams of the optical system 20 according to the present example. As is apparent from FIGS. 9A to 9C, various aberrations are favorably corrected in the range of 400 to 850 nm also by the configuration of the optical system 20 of the present example.
In the present embodiment, the object-side surface L1a of the first optical element L1 has the concave shape portion L1a1, but the present invention is not limited to this, and the surface L1a is a flat surface. May be.

[Example 3]
Example 3 will be described below. In the present embodiment, the different components from the first embodiment will be described, and the description of the same components as those in the first embodiment will be omitted.
FIG. 10 is a schematic diagram illustrating a main part in a YZ section including the optical axis AX of the optical system 30 of the present embodiment.
The optical system 30 according to the present embodiment includes an aperture stop STO, a substrate SUB, a first optical element L1, a second optical element L2, and a fourth optical element CG that limit the beam width in order from the object side to the image side.
In the optical system 30, the substrate SUB and the first optical element L1 are bonded, the first optical element L1 and the second optical element L2 are bonded, and the diffraction grating 5 is bonded to the bonding surface of the first optical element L1 and the second optical element L2. Is formed.
The aperture stop STO is formed of a light shielding film made of a metal film such as chromium or a dielectric multilayer film on the surface SUBa on the object side of the substrate SUB.

The object-side surface L1a of the first optical element L1 is a flat surface, and the image-side surface L1b is a joint surface with the object-side surface L2a of the second optical element L2. The image side surface L2b of the second optical element L2 has a convex surface L2b1. The second optical element L2 has a positive power. The fourth optical element CG is an optical filter such as an IR cut filter.
In FIG. 10, a light beam from an object (not shown) enters the aperture stop STO, and at this time, the light beam width is limited. The light beam that has passed through the aperture of the aperture stop STO is transmitted through the substrate SUB, the first optical element L1, the second optical element L2, and the fourth optical element CG in this order to form an image plane IMG.

<Numerical Example 3>
Similarly to Example 1, Tables 7 to 9 show data of Numerical Example 3 corresponding to the present Example.

As shown in Table 7, in the optical system 30 according to the present example, the image-side surface L2b (surface number 5) of the second optical element L2 is an aspherical surface. In addition, the diffraction surface 5 is formed on the boundary surface (surface number 4) between the first optical element L1 and the second optical element L2.
As shown in Table 8, the optical system 30 according to the present example has characteristics of Fno = 2.80 and an angle of view of 60 ° (± 30 °) even though the total length is a small configuration of 2.24 mm. ing.
Table 9 shows values of Expression (1), Expression (3), and Expression (4) in the optical system 30 according to the present example. As can be seen from Table 9, the optical system 30 also satisfies all the expressions (1), (3), and (4).
Further, since D1 = 0.978 and D / 2 = 1.118, the expression (2) is also satisfied.

  11A to 11C are aberration diagrams of the optical system 30 according to the present example. As is apparent from FIGS. 11A to 11C, various aberrations are favorably corrected at 400 to 850 nm also by the configuration of the optical system 30 of the present example.

  The preferred embodiments of the present invention have been described above, but the present invention is not limited to these embodiments, and various combinations, modifications, and changes are possible within the scope of the gist. For example, in the present embodiment, a small optical system in which the wavelength range of 400 to 850 nm is corrected is shown. .

  L1 ... first optical element, L2 ... second optical element, L2b1 ... convex surface, 5 ... diffraction grating, 10 ... optical system

Claims (10)

In an optical system having a first optical element and a second optical element bonded to the first optical element in order from the object side to the image side,
A diffractive optical part formed on a joint surface between the first optical element and the second optical element;
Either one of the first optical element and the second optical element is formed of a first material having a relatively low refractive index and a high dispersion, and the other is a second material having a relatively high refractive index and a low dispersion. Formed of material,
The image side surface of the second optical element is formed as a convex surface,
When the focal length of the entire optical system is f and the focal length of the convex surface is f2,
0.7 <f2 / f <1.3
An optical system characterized by satisfying 2. The optical system according to claim 1, wherein an optical substrate made of a glass material and having an Abbe number larger than that of the second material is disposed so as to be bonded to an object side surface of the first optical element. . The optical system according to claim 1, wherein the first optical element is formed of the first material, and the second optical element is formed of the second material. 4. The optical system according to claim 1, further comprising an aperture stop disposed on the object side of the first optical element. 5. When the distance from the aperture stop to the diffractive optical part is D1, and the distance from the aperture stop to the image plane is D,
D1 ≤ D / 2
The optical system according to claim 4, wherein: 6. The optical system according to claim 4, further comprising a concave-shaped portion formed concavely toward the object side between the image-side surface of the first optical element and the aperture stop. . When the Abbe number at the d-line of the second optical element is ν2, the Abbe number at the d-line of the diffractive optical unit is νdoe, and the focal length of the diffractive optical unit is fdoe,
0.4 ≦ | fdoe × νdoe | / (f2 × ν2) ≦ 1.2
The optical system according to claim 1, wherein: The optical system according to claim 1, wherein the convex surface is an aspherical surface. When the refractive index of the second material is Nh,
Nh> 1.55
The optical system according to claim 1, wherein: The optical system according to any one of claims 1 to 9,
An image sensor for receiving an image formed by the optical system;
An imaging device comprising:

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