JPH09127322A - Diffraction optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively prevent a flare, a ghost, etc., from being generated by easily manufacturing the diffraction optical element and reducing the wavelength dependency of its diffraction efficiency. SOLUTION: This diffraction optical element has 1st, 2nd, and 3rd areas 11, 12, and 13 which are laminated in order very closely or closely, a 1st relief pattern 21 which is formed on the border surface between the 1st and 2nd areas 11 and 12, and a 2nd relief pattern 22 which is formed on the border surface between the 2nd and 3rd areas 12 and 13. Then the 1st, 2nd, and 3rd areas 11, 12, and 13 are formed of mutually different materials which are substantially transparent to the wavelength of light in use and the 1st and 2nd relief patterns 21 and 22 have substantially equal pitch distributions and mutually different groove depths d1 and d2 and are so arranged that their corresponding positions are close to each other.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diffractive optical element used with a plurality of wavelengths or band lights.

[0002]

2. Description of the Related Art A diffractive optical element, for example, a diffractive optical element (diffractive lens) configured to have a lens function is known to have features which are not present in conventional refracting lenses, as shown below. There is. Since the aspherical wave can be easily generated, the aberration can be effectively corrected. Since it has virtually no thickness, it has a high degree of freedom in design,
It is possible to realize a compact optical system. Since the amount corresponding to the Abbe number in the refracting lens has a negative value in the diffractive lens, chromatic aberration can be effectively corrected by combining with the refracting element.

For improving the performance of an optical system by utilizing the characteristics of such a diffractive lens, for example, Bina
ry Optics Technology; The Theory and Design of Mul
ti-Level Diffractive Optical Element, Gary J. Swans
on, Technical Report 854, MIT Lincoln Laboratory, A
It is described in detail in ugust 1989.

As described above, the diffractive optical element has many useful features that conventional refracting elements do not have, but on the other hand, since the diffraction efficiency depends on the wavelength, the following principle is required. There's a problem. For example, a diffractive optical element applied to an optical system is often used as a lens element, but in such an application, the presence of a plurality of diffracted lights (a plurality of focal points) is generally not preferable. Therefore, in a conventional diffractive optical element (specifically, a diffractive lens), generally, as shown in FIG.
Further, a relief pattern 2 having a sawtooth-shaped cross section (blazed) is formed to concentrate energy on diffracted light of a specific order.

However, as shown in FIG. 19, when the cross section is processed into a sawtooth shape, the wavelength at which energy can be concentrated, that is, the wavelength at which the diffraction efficiency is maximized is different depending on the groove depth, so that the band light having a wavelength width is It becomes impossible to concentrate energy on the diffracted light of a specific order. This phenomenon is not a problem in the case of an optical system that uses monochromatic light, such as a laser, but in an optical system that uses white light, such as a camera, the diffraction efficiency of light of a specific wavelength If is optimized, there is a problem that the diffraction efficiency is reduced at other wavelengths.

FIG. 20 shows a diffractive optical element having the sectional shape shown in FIG.
Relief pattern 2 is 1 at wavelength λ = 510 nm.
It shows the relationship between the first-order diffraction efficiency and the wavelength when the groove is formed so that the second-order diffraction efficiency is 100%.
As is clear from FIG. 20, in the range from λ = 400 nm to λ = 700 nm, which can be generally regarded as the visible wavelength range, the diffraction efficiency decreases as the wavelength deviates from the optimized wavelength λ = 510 nm, and particularly in the short wavelength range, the decrease is remarkable. Become.
Such a decrease in diffraction efficiency is not limited to the problem that the spectral transmittance is simply decreased, and diffracted light of an unnecessary order is generated at a wavelength where the groove depth is not optimized. Therefore, when such a diffractive optical element is applied to an optical system that uses band light, for example, an imaging optical system that uses white light, flare and ghosts occur, and the performance of the optical system deteriorates.

Here, the relief pattern having a sawtooth-shaped cross section shown in FIG. 19 can be expressed by a phase shift function φ (x) as shown in FIG. This φ (x) is a function that characterizes the wavefront modulation action of the relief pattern, and its shape is a periodic function corresponding to the cross-sectional shape of the relief pattern. The m-th order diffraction efficiency η _m of the relief pattern represented by this phase shift function φ (x) can be obtained by using its swing width (hereinafter referred to as phase amplitude) a.

(Equation 3) Given by

In the equation (1), the phase amplitude a is defined as follows: the refractive index of air is 1, the refractive index of the substrate on which the relief pattern is formed is n, the groove depth is d, and the wavelength of the light used is λ.

(Equation 4) Is the amount defined by. Here, at the wavelength λ ₀ , the groove depth d optimized so that the m ₀ -order diffraction efficiency is 100%
₀ is

(Equation 5) Therefore, the phase amplitude a at this time is

(Equation 6) Becomes

The expression (4) means that the phase amplitude a depends on the wavelength with respect to a certain groove depth d ₀ , and the wavelength dependence of the phase amplitude a makes it clear from the expression (1). Thus, the wavelength dependence of the diffraction efficiency is caused. For example, the wavelength dependence of the diffraction efficiency shown in FIG. 20 is also the result of such a phenomenon.

[0010]

The present applicant has studied in detail the mechanism of wavelength dependence of diffraction efficiency as described above,
A new type of relief type diffractive optical element with reduced wavelength dependence of diffraction efficiency has already been proposed (Japanese Patent Application No. 7-22).
0754). As shown in FIG. 22, the diffractive optical element proposed here has a high refractive index and low dispersion optical material 10a.
And two types of optical materials different from the low refractive index and high dispersion optical material 10b are combined, and the relief pattern 20 is formed on the boundary surface between the two different types of optical materials 10a and 10b.

Here, in the case where the cross-sectional shape of the relief pattern 20 is a saw-tooth wave shape as shown in FIG. 22, and the groove depth is optimized so that the m ₀ -order diffraction efficiency is 100% at the wavelength λ _0. The phase amplitude a (λ) of

(Equation 7) Given by However, n ₁ (λ) represents the refractive index of the optical material 10 a having a high refractive index and low dispersion, and n ₂ (λ) represents the refractive index of the optical material 10 b having a low refractive index and high dispersion.

In the equation (5), the refractive indexes n ₁ and n ₂ of the two kinds of optical materials are n ₁ (λ)> n ₂ (λ) over the wavelength band used, as shown in FIG. Then, the refractive index difference appearing in the numerator increases as the wavelength λ increases, and the change of λ in the denominator is canceled. Therefore, the wavelength change of the phase amplitude can be suppressed to be small as compared with the case represented by the expression (4), and as a result, the wavelength change of the diffraction efficiency can be suppressed to a small value.

However, the relationship between the refractive index and the dispersion (wavelength dispersion of the refractive index) of an actually existing optical material tends to increase as the refractive index increases. It is not easy to find a combination of materials. For example, there are many kinds of optical materials used for visible light, but basically, dispersion increases as the refractive index increases. Most of the optical materials used in the visible band light are so-called optical glass, but when optical glass is selected as each of the two types of optical materials, the workability is poor. It is not easy to form a relief pattern. Further, in consideration of easiness of manufacturing, at least one of the two types of optical materials may be a plastic optical material that can be easily processed. However, since there are few types of plastic optical materials, an optical material having a sufficient effect can be obtained. Material combinations are greatly limited. In particular, it is not easy to improve the wavelength dependence of the diffraction efficiency with a combination of plastic optical materials.

The present invention has been made by paying attention to the above-mentioned problems, and can be manufactured easily, and appropriately reduce the wavelength dependence of the diffraction efficiency to effectively prevent the occurrence of flare, ghost and the like. An object of the present invention is to provide a diffractive optical element configured.

[0015]

In order to achieve the above object, a diffractive optical element according to a first aspect of the present invention comprises first, second and third regions, which are stacked in close contact or in close proximity and sequentially.
A first relief pattern formed on a boundary surface between the first and second areas and a second relief pattern formed on a boundary surface between the second and third areas; The second and third regions are composed of different materials that are substantially transparent at the wavelength of light used, and the first and second relief patterns have substantially equal pitch distributions and different groove depths. And corresponding portions thereof are arranged in proximity to each other.

Further, the diffractive optical element according to the second invention is formed on the boundary surface between the first, second and third regions and the first, second and third regions which are sequentially stacked closely or closely. And a second relief pattern formed on a boundary surface between the second and third regions, wherein the first region is made of a material that reflects light to be used, The second and third regions are formed of different materials that are substantially transparent at the wavelength of light used, and the first and second relief patterns have different pitches with substantially equal pitch distribution. It is characterized in that it has a depth and its corresponding parts are arranged close to each other.

Further, the diffractive optical element according to the third invention.
Is a first, second and
And the third area and the boundary surface between the first and second areas
The formed first relief pattern and the second and the second relief patterns.
The second relief pattern formed on the boundary surface of the area 3
And said first, second and third regions are used
Construct with different materials that are substantially transparent at the wavelength of light.
And the first and second relief patterns are
With the qualitatively equal pitch distribution, the corresponding parts are placed close to each other.
The first, second and third regions, and
Let n be the refractive index of each material.₁(λ), n_Two(λ) and
And n_Three(λ) and the first and second relief putters
The groove depth of each₁ And d _Two, Their ratio
Is α = d_Two/ d₁ As

ΔN (λ) = {n ₁ (λ) −n ₂ (λ)} + α {n
₂ (λ) −n ₃ (λ)} where λ is the wavelength of light,

[Formula 9] | ΔN (λ ₂ ) |> | ΔN (λ ₁ ) |>0; λ ₂ > λ ₁ where λ _{1 is the} wavelength at the short wavelength end of the wavelength range of the light used λ _{2 is} the light used It is characterized by satisfying the wavelength at the long wavelength end of the wavelength range of.

[0018]

1 is a conceptual view of a diffractive optical element according to a first invention, and schematically shows a part of a cross section. This diffractive optical element includes a first region 11, a second region 12 and a third region 13, which are sequentially stacked, and a first region 11.
The first relief pattern 21 is formed on the boundary surface between the area 11 and the second area 12, and the second relief pattern 22 is formed on the boundary surface between the second area 12 and the third area 13. The first, second and third regions 11,
12 and 13 are composed of different materials which are substantially transparent in the wavelength band of the light used. Here, the refractive index of the first region 11 is n ₁ (λ), the refractive index of the second region 12 is n ₂ (λ), and the refractive index of the third region 13 is n.
Let ₃ (λ).

Further, the first and second relief patterns 21 and 22 are formed in a saw-tooth wave cross section having an equal pitch distribution, and the corresponding portions are opposed to each other. Here, the groove depth of the first relief pattern 21 is d ₁ ,
The groove depth of the relief pattern 22 is d ₂ , and the interval between the top of the first relief pattern 21 and the bottom of the second relief pattern 22 is d ₃ .

In the structure shown in FIG. 1, the light incident on the diffractive optical element is the first and second relief patterns 21 and 2.
2 will undergo phase modulation respectively. In this case, the phase amplitude a ₁ (λ) of the first relief pattern 21
Is

(Equation 10) And the phase amplitude a of the second relief pattern 22 becomes
₂ (λ) is

[Equation 11] Becomes

Here, the first and second relief patterns 2
Assuming that the structure made up of 1 and 22 is integrated and light incident on the diffractive optical element is modulated at substantially the same time, the phase amplitude a (λ) that characterizes the phase shift action is

(Equation 12) Can be expressed as

Furthermore, if the groove depth at this time is optimized so that the m ₀ -order diffraction efficiency becomes 100% at the wavelength λ ₀ , from the condition of a (λ ₀ ) = m ₀ ,

(Equation 13)Becomes However, α is the first relief as follows.
Groove depth d of pattern 21 ₁And the second relief pattern
22 groove depth d_TwoIt is an amount defined by the ratio with.

[Equation 14]

As described above, the phase amplitude a (λ) of the diffractive optical element according to the first aspect of the invention is, as shown in the equation (8), the phase amplitude a ₁ (λ) of the first relief pattern 21, Second
Is given by the sum of the phase amplitude a ₂ (λ) of the relief pattern 22 and the wavelength dependence characteristic depends on the parameter α defined by the equation (10). Where the parameter α
Is a parameter that can be arbitrarily determined irrespective of the optimization of the diffraction efficiency at the specific wavelength λ ₀ , as expressed by the equation (9).

According to the configuration shown in FIG. 1, the first and the first
The second and third regions 11, 12 and 13 are different from each other.
Is formed of a material having a refractive index difference Δn₁(λ) and
Δn _Two(λ) will show different wavelength dependence
Then, by changing the setting of parameter α, equation (9)
Of the phase amplitude a (λ) of
You can

Therefore, according to the first invention,
By optimally setting the groove depth ratio of the second relief pattern, that is, the parameter α, it is possible to keep only the diffraction efficiency at the specific wavelength λ ₀ optimal and independently of it, only the wavelength dependence of the diffraction efficiency is preferable. You can control it. Generally, if the groove depths of the two relief patterns are different from each other, that is, if α ≠ 1, the wavelength dependence of the diffraction efficiency can be optimally set. By properly combining the materials forming the first, second, and third regions, it is possible to optimally set the wavelength dependence of the diffraction efficiency with α = 1.

FIG. 2 is a conceptual diagram of a diffractive optical element according to the second invention. This diffractive optical element includes a first region 14, a second region 15 and a third region which are sequentially stacked.
Area 16, the first relief pattern 23 formed on the boundary surface between the first area 14 and the second area 15, and the second
And a second relief pattern 24 formed on the boundary surface between the region 15 and the third region 16. First area 14
Is made of a material that reflects the light used, and the second and third regions 15 and 16 are made of different materials that are substantially transparent in the wavelength band of the light used.

Here, the refractive index of the second region 15 is n.
₂ (λ) and the refractive index of the third region 16 is n ₃ (λ). In addition, the first and second relief patterns 23 and 24
Are formed to have a sawtooth-shaped cross section, as in the case of FIG. 1, so that they have the same pitch distribution and the corresponding portions face each other. Here, as in the case of FIG. 1, the groove depth of the first relief pattern 23 is d ₁ , the groove depth of the second relief pattern 24 is d ₂ , the top of the first relief pattern 23, The distance from the bottom of the relief pattern 24 of 2 is d
_Set to ₃ .

In the configuration shown in FIG. 2, the third region 16
Light incident on the diffractive optical element from the side undergoes phase modulation by the second and first relief patterns 24 and 23, respectively. Here, the first relief pattern 23
Faces the first region 14 made of a reflective material, so that the light incident on the element is reflected by the first relief pattern 23. Therefore, the diffractive optical element having such a configuration functions as a reflective diffraction grating as a whole.

Here, as in the case of FIG. 1, the first and second
Assuming that the structure composed of the relief patterns 23 and 24 of 1 is integrated, and the lights incident on the diffractive optical element are modulated substantially at the same time, the m ₀ -order diffraction efficiency is 10 at the wavelength λ ₀ .
When the groove depths of the relief patterns 23 and 24 are set so as to be 0%, the phase amplitude is

(Equation 15) Can be expressed as The equation (11) corresponds to the equation (9) representing the phase amplitude of the configuration shown in FIG.
Some parameters alpha, a groove depth d ₁ of the first relief pattern 23, by the groove depth d ₂ of the second relief pattern 24, is defined by equation (10).

The above equation (11) is consistent with the equation (9) representing the phase amplitude of the configuration shown in FIG. 1 in which the refractive index of the first region 14 where light does not enter is set to zero. That is, also in the case of the reflection type diffraction grating having the configuration shown in FIG. 2, its phase amplitude is expressed in a format including an arbitrary parameter α, as in the case of the configuration shown in FIG. Therefore, also in the diffractive optical element having the configuration shown in FIG. 2, by optimizing the arbitrary parameter α, the diffraction efficiency at the specific wavelength λ ₀ is optimized as in the case of the diffractive optical element having the configuration shown in FIG. It is possible to preferably control only the wavelength dependence of the diffraction efficiency independently of the above.

In the first aspect of the invention, in order to reduce the wavelength dependence of the diffraction efficiency, the wavelength dependence of the phase amplitude a (λ) shown in the equation (8) or the equation (9) is made smaller. There is a need. For example, in equation (9), it is the numerator that determines the wavelength dependence of the phase amplitude a (λ).
Two refractive index difference terms, Δn ₁ (λ) and αΔn ₂ (λ),
It is λ in the denominator. Therefore, in order to further reduce this wavelength dependence, the absolute value ΔN (λ) of the sum of the terms of the two refractive index differences is

When ΔN (λ) = | Δn ₁ (λ) + αΔn ₂ (λ) | (12), ΔN (λ) increases as the wavelength λ increases, as in the third invention. Thus, it is effective to optimally set the combination of materials and the ratio of the groove depth.

In this way, the wavelength-dependent effects of the numerator and denominator of formula (9) cancel each other out, so that it is possible to realize a diffractive optical element in which the wavelength dependence of the diffraction efficiency is further reduced. The expression (9) is an expression defined for the configuration of the first aspect of the invention. By setting n ₁ (λ) = 0, the expression (11) is obtained. The description regarding the formulas similarly holds for the second invention.

By the way, in many cases, the absolute value of the refractive index difference Δn (λ) in the case of combining existing optical materials decreases as the wavelength λ increases. That is, a wavelength dependence opposite to the desired characteristic is often generated. This is because many existing optical materials are distributed in the direction from high refractive index and high dispersion to low refractive index and low dispersion. In such a case, in the above-mentioned first invention and second invention, 2
It is effective to set the algebraic sign of the groove depth ratio α so that the two refractive index difference terms cancel each other out, in that the wavelength dependence opposite to the desired characteristic is canceled. As a result, even when a high-refractive-index, high-dispersion material and a low-refractive-index, low-dispersion material, which are suitable in terms of ease of material selection, are combined, the wavelength dependence opposite to the desired characteristics is canceled out, so The wavelength dependence of efficiency can be further reduced.

Here, the difference in the algebraic sign of the groove depth ratio α is
It corresponds to the inversion of the relief pattern unevenness. That is,
In the definition of α shown in the equation (10), α is positive, as shown in FIG. 3, corresponding to the peaks and peaks (valleys and valleys) of the first and second relief patterns. If, that is, 2
This is the case where the relief patterns of the two reliefs correspond to each other. On the contrary, α becomes negative when the peaks and valleys (valleys and peaks) of the first and second relief patterns correspond to each other, that is, when the two are as shown in FIG. This is the case where the relief pattern unevenness is reversed and corresponding.

In the first and second inventions, the third regions 13 and 16 may be an atmosphere of the environment in which the regions contact. For example, in normal use, the environment is air, so the third areas 13 and 1
It is also possible to configure 6 with air. In this case, the difference in the refractive index with the transparent material to be combined can be increased, so that the required groove depth of the relief pattern can be reduced, and thus a high-performance diffractive optical element can be realized. .

Incidentally, the diffractive optical element is generally classified into a thick type and a thin type, but it is a band light having a wavelength width, and particularly when it is applied to an imaging optical system, the incident angle dependence and the wavelength dependence are relatively large. It is preferable to have a small thickness. Here, as a parameter that characterizes the thickness of the diffractive optical element,

[Equation 17] The Q value given by is well known, and in general, Q <1
At that time, the diffractive optical element is classified into a thin type. However, in the equation (13), λ is the wavelength, T is the pitch of the periodic structure, D is the depth of the periodic structure, and n ₀ is the average refractive index of the periodic structure. Therefore, it is preferable that the diffractive optical element according to the present invention is also configured to satisfy Q <1.

From the equation (13), the parameter Q indicating the thickness of the diffractive optical element depends on the wavelength λ, but is generally used in order to maintain the uniformity of the diffraction efficiency over the entire wavelength range used. Regarding the center wavelength of the band light, the thin condition, Q <1 may be satisfied. Therefore,
Also in the diffractive optical element according to the present invention, it is preferable that the central wavelength of the band light used is set to satisfy Q <1. For example, when the diffractive optical element according to the present invention is applied to an optical system used in visible band light,
The center wavelength can be set in the range of approximately 480 nm to 550 nm. However, considering that the parameter Q depends on the wavelength λ, thin conditions in the entire wavelength range,
It goes without saying that it is desirable to satisfy Q <1.

Further, according to the study by the present inventor, Q
It was confirmed that the relief grating at <0.1 better represents the thin nature. Therefore, also in the diffractive optical element according to the present invention, it is more preferable to configure the periodic structure so that Q <0.1.

Here, in the diffractive optical element having the structure shown in FIG. 1, the depth D of the periodic structure and the average refractive index n ₀ are

(Equation 18) Given by In the diffractive optical element having the configuration shown in FIG. 2, the depth D of the periodic structure and the average refractive index n ₀ are

[Equation 19] Given by

The diffractive optical element according to the present invention is highly effective when the wavelength range used has a width of a predetermined amount or more. Here, in an ordinary relief grating in which the diffraction efficiency is optimized at an arbitrary wavelength λ, the width of the wavelength change in which the change in the diffraction efficiency is negligible is about ± 5% of λ as a guide, so the diffraction according to the present invention The optical element is effective when using band light having a wavelength width of ± 5% or more with respect to an arbitrary center wavelength λ.

As described above, the present invention is applied to the first, second and third aspects.
However, the present invention is not limited to this, and an adhesive layer is provided at the boundary of each of the above regions, and the same effect can be obtained even if the regions are arranged close to each other. be able to.

The diffractive optical element according to the present invention can be applied to all optical devices that use a plurality of wavelengths or band lights. Among them, it is particularly effective when applied to an optical device having an imaging optical system.

FIG. 17 shows an example of the application, and is a conceptual diagram when the diffractive optical element according to the present invention is applied to an image pickup apparatus, for example, a taking lens of a camera. In FIG. 17, an image pickup optical system 60 has a refracting lens 51 and a diffractive lens 41 according to the present invention, and is configured to form an image of an object on an image pickup element 61. Here, since the diffractive lens 41 according to the present invention can obtain high diffraction efficiency in the entire visible band light, for example, flare and ghost can be effectively prevented when a color image is taken. .

FIG. 18 shows another application example of the diffractive optical element according to the present invention, which is a concept applied to an optical device including an observation optical system, for example, a finder of a camera or an eyepiece of a microscope. It is a figure. In FIG. 18, an objective lens 53 forms a magnified real image of an object, and an eyepiece optical system 62 having a refraction lens 52 and a diffractive lens 42 according to the present invention further magnifies the real image and projects it on the retina of an observer. Is configured to. Also in this case, the same effect as in the case of the imaging device shown in FIG. 17 can be obtained.

[0045]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 3 shows a first embodiment of the present invention. In this example, a transmission type diffractive lens is shown, and an optical glass La made by OHARA is used as the first region 101.
L14 (nd = 1.6968, νd = 55.5) is used as the second region 102 for the ultraviolet curable resin (nd = 1.5).
2, νd = 52) and polycarbonate (nd = 1.58, νd = 30.5) are used as the third region 103, and these are sequentially laminated. In addition, the first region 10
1 and the second area 102, and the second area 1
02 and the third region 103, the first relief pattern 201 and the second relief pattern 201 having the same pitch distribution are provided on the boundary surface.
The relief patterns 202 are formed so that the tops of the first relief patterns 201 and the bottoms of the second relief patterns 202 are in contact with each other.

First and second relief patterns 201,2
In No. 02, each pitch arrangement is optimized so as to have a predetermined lens action, each cross section has a sawtooth wave shape, and the wavelength λ =
The groove depth is optimized so that the first-order diffraction efficiency is maximized at 550 nm. In this embodiment, the groove depth d ₁ of the first relief pattern 201 is d ₁ = 7.90 μm,
Of the groove depth d ₂ of the relief pattern 202, d ₂ = 1
Assuming 3.74 μm, the parameter α defined by the above equation (10) is α≈1.74. Also facing 2
Both of the end faces 301 and 302 are flat, and an antireflection coating is applied on each end face.

FIG. 4 shows the wavelength-dependent characteristics of Δn ₁ given by the equation (6) and Δn ₂ given by the equation (7) in the visible wavelength region in the diffractive lens according to this example. Is. As is clear from FIG.
The refractive index difference Δn ₁ between LaL14 (first region 101) and ultraviolet curable resin (second region 102) is positive in the visible wavelength band because the refractive index of LaL14 is larger than that of the ultraviolet curable resin. Further, since the Abbe number νd of LaL14 and the ultraviolet curable resin are relatively close to each other, there is a tendency that the Abbe number νd slightly decreases as the wavelength λ increases. In contrast, the refractive index difference Δn ₂ between the ultraviolet curable resin (second region 102) and the polycarbonate (third region 103).
Has a negative refractive index in the visible wavelength band because the refractive index of polycarbonate is higher than that of UV curable resin.
In addition, there is a tendency that the wavelength greatly increases as the wavelength λ increases.

In this embodiment, Δn ₁ and Δn of FIG.
_{As is} clear from the wavelength-dependent characteristic of ₂ , since a positive value of α such that the absolute value of αΔn ₂ does not exceed the magnitude of Δn ₁ is selected, the refractive index difference defined by equation (12) is Term N
The wavelength dependence of (λ) increases as the wavelength λ increases. Therefore, λ that appears in the denominator of equation (9) is
The term N (λ) of the difference in refractive index is well offset, whereby the wavelength dependence of the phase amplitude is reduced, and the wavelength dependence of the diffraction efficiency is reduced.

FIG. 5 shows a comparison of the wavelength dependence characteristics of the phase amplitude between the diffractive lens of this example and the conventional diffractive lens. In FIG. 5, the solid line shows the wavelength-dependent characteristic of the phase amplitude of the diffractive lens according to this embodiment. Also,
The broken line shows the wavelength dependence characteristic of the phase amplitude of the conventional diffractive lens, and is the case where the blazed pattern is formed on the LaL 14 substrate so that the first-order diffraction efficiency is maximized at the wavelength λ = 510 nm. As is clear from FIG.
It can be seen that the wavelength dependence of the phase amplitude is satisfactorily reduced by optimizing the value of α.

FIG. 6 shows the wavelength dependence characteristic of the diffraction efficiency corresponding to the wavelength dependence characteristic of the phase amplitude shown in FIG.
A solid line and a broken line represent the same thing as the case of FIG. FIG.
As is apparent from the above, according to the diffractive lens of this example, the wavelength dependence of the diffraction efficiency is corrected very well as compared with the conventional one.

As described above, according to the diffractive lens of this embodiment, a high diffraction efficiency can be obtained in the entire visible band, so that flare and ghost can be effectively prevented when the visible band is used. . Therefore, for example, it can be suitably applied to an image pickup optical system such as a camera.

As shown in FIGS. 5 and 6,
In this embodiment, the wavelength optimized so that the first-order diffraction efficiency is 100% is set differently from the conventional example. This is because the optimized wavelength is generally set to balance the diffraction efficiency in the wavelength range used. That is, in the case of this embodiment and the case of the conventional example,
This is because the optimum wavelength for balancing the wavelength dependence of the diffraction efficiency is different. For example, in FIG. 5, the optimized wavelength of the conventional example is 510 nm, but it is 550 nm in this embodiment.

In this embodiment, the wavelength dependence of the diffraction efficiency is more effectively reduced on the short wavelength side of the wavelength band used. Therefore, the wavelength optimized to maximize the diffraction efficiency is preferably set on the longer wavelength side as compared with the conventional case. Specifically, it is desirable to set the optimized wavelength within a wavelength range within ± 10% of the wavelength width to be used with respect to the intermediate wavelength of the wavelength band to be used. here,
The wavelength width to be used is usually 400 nm to 700 nm in the case of an image forming optical system used for visible light.

Further, according to this embodiment, since the second region 102 is made of the ultraviolet curable resin,
The second relief patterns 201 and 202 are separately formed in the first and third regions 101 and 103, respectively, and then these are bonded together via an ultraviolet curable resin. A diffractive lens with reduced dependence can be manufactured. Therefore,
There is also an effect that the cost can be reduced.

This effect is greatest when the second region 102 is made of an ultraviolet curable resin,
More generally, the same effect can be obtained by forming the second region 102 with a plastic material.

Further, the first and second relief patterns 20
When the 1, 202 are bonded together, the moire fringes generated by the first and second relief patterns 201, 202 can be used for the alignment. That is, by aligning the first and second relief patterns 201 and 202 so that the moire fringes are completely eliminated, the corresponding portions can be opposed to each other.

FIG. 7 shows a modification of the first embodiment. This diffractive lens has two end faces 30 facing outward.
Among the three and 304, one end surface 303 is formed into a curved surface having a positive refractive power, and the other end surface 304 is formed into a curved surface having a negative refractive power, and other configurations are similar to those of the first embodiment. . The end faces 303 and 304 are each provided with an antireflection coating.

According to this diffractive lens, since it has both the power due to the diffractive action and the power due to the refraction action, it is possible to realize a lens element having a large power as a whole. Further, since the wavelength dispersion (Abbe number) of the power due to the diffracting power and the power due to the refracting action have opposite signs, the chromatic aberration can be realized by canceling the chromatic dispersion. In particular, as shown in FIG. 7, the end face 30 facing outwards.
By reversing the sign of the refractive power of 3,304, 2
It is possible to realize an achromatic single lens whose spectrum is corrected up to the next spectrum.

FIG. 8 shows a second embodiment of the present invention. This embodiment shows a transmission type diffractive lens, and the first region 104 is made of Asahi Glass fluorinated resin Cytop (nd = 1.34149, νd = 93.8).
As the second region 105, and the ultraviolet curable resin (nd =
1.52, νd = 51.8) as a third region 106 made of polycarbonate (nd = 1.58, νd = 30.
5) is used, respectively, and these are sequentially laminated. Further, on the boundary surface between the first area 104 and the second area 105 and the boundary surface between the second area 105 and the third area 106,
First relief pattern 20 having equal pitch distribution
3 and the second relief pattern 204, the top of the first relief pattern 203 and the second relief pattern 2
It is formed so as to be in contact with the bottom portion of 04.

First and second relief patterns 203, 2
04 is each pitch arrangement so as to have a predetermined lens action.
Is optimized, and each section has a sawtooth wave shape,
First-order diffraction at wavelength λ = 550 nm
The groove depth is optimized for maximum efficiency. I
Therefore, the groove depth d of the first relief pattern 203 ₁
And the groove depth d of the second relief pattern 204_TwoAnd each other
It has the opposite sign. In this embodiment, the first
Groove depth d of the groove pattern 203₁And d₁= -9.20
μm, groove depth d of the second relief pattern 204_TwoTo
d_Two= 17.84 μm and defined by the above equation (10).
The parameter α is set as α≈−1.94. Also outside
Two end faces 305 and 306 facing each other are flat
As a result, an antireflection coat is applied on each end face.

FIG. 9 shows the wavelength dependent characteristics of Δn ₁ given by the equation (6) and Δn ₂ given by the equation (7) in the diffractive lens according to this example. As is apparent from FIG. 9, the refractive index difference Δn ₁ between Cytop (first region 104) and the ultraviolet curable resin (second region 105), and the ultraviolet curable resin (second region 10).
The refractive index difference Δn ₂ between 5) and the polycarbonate (third region 106) has a negative value in the visible wavelength band due to the above-described magnitude relation of the refractive index. Moreover, since the combination of these two materials has a relationship of high refractive index and high dispersion-low refractive index and low dispersion, both Δn ₁ and Δn ₂ decrease in magnitude (absolute value) as the wavelength increases. To do.

In this embodiment, since the magnitude (absolute value) of αΔn ₂ does not exceed the magnitude (absolute value) of Δn ₁ , a negative value α is selected and set, so that (12) The wavelength dependence of the term N (λ) of the refractive index difference defined by the formula increases as the wavelength λ increases. Therefore, λ appearing in the denominator of the equation (9) is well canceled by this refractive index difference term N (λ), which reduces the wavelength dependence of the phase amplitude,
The wavelength dependence of the diffraction efficiency is reduced.

FIG. 10 shows a comparison of the wavelength dependence characteristics of the diffraction efficiency between the diffractive lens of this example and a conventional diffractive lens. In FIG. 10, the solid line shows the case of the diffractive lens according to this example, and the broken line shows the case of the conventional diffractive lens (optimized wavelength λ = 510 nm) in which a blazed pattern is formed on a Cytop substrate. As is apparent from FIG. 10, according to the diffractive lens of this example, the wavelength dependence of the diffraction efficiency is corrected very well as compared with the conventional one.

As described above, according to this embodiment, since high diffraction efficiency can be obtained in the entire visible band light, the problem of flare and ghost hardly occurs when used in the visible band light. It can be suitably applied to an image pickup optical system such as a camera. Moreover, since the first and third regions 104 and 106 are made of the plastic material, the first and second relief patterns 203 and 2 are formed.
04 can be formed very easily, and in particular, since the second region 105 is made of an ultraviolet curing resin, the first and second reliefs formed in the first and third regions 104 and 106 can be formed. The patterns 203 and 204 can be easily attached to each other, whereby a diffractive optical element in which the wavelength dependence of diffraction efficiency is reduced can be easily manufactured.

FIG. 11 shows a third embodiment of the present invention. This embodiment shows a double-focus diffractive lens, in which first, second and third regions 101,
102 and 103 are made of the same material as in the first embodiment. That is, the optical glass LaL14 made by OHARA (nd = 1.6968, νd =
55.5), the second region 102 is exposed to the ultraviolet curable resin (n
d = 1.52, νd = 52), and the third region 103 is made of polycarbonate (nd = 1.58, νd = 30.5). In addition, the first area 101 and the second area 102
And the second area 102 and the third area 10
The first relief pattern 205 and the second relief pattern 2 having the same pitch distribution on the boundary surface with
06 are formed so that the top of the first relief pattern 205 and the bottom of the second relief pattern 206 are in contact with each other.

First and second relief patterns 205, 2
No. 06 optimizes each pitch arrangement so as to have a predetermined lens action, and makes each cross section a rectangular shape having the same unevenness ratio so that the ± 1st-order diffraction efficiency is maximized at the wavelength λ = 600 nm. Optimize groove depth. In this example,
The groove depth d ₁ of the first relief pattern 205 is represented by d ₁ =
With the groove depth d ₂ of 4.02 μm and the second relief pattern 206 set to d ₂ = 7.03 μm, the parameter α defined by the above equation (10) is set to α≈1.75. Also,
The two outer facing end faces 307 and 308 are both flat and have an antireflection coating on each end face.

In this embodiment, the phase amplitude corresponding to the above equation (9) when the ± 1st order diffraction efficiency is maximum is

(Equation 20) And the m-th order diffraction efficiency η _{m at} that time is

(Equation 21) Given by The phase amplitude represented by the above equation (18) is
Since the phase amplitude represented by the equation (9) is only multiplied by the coefficient 1/2, the operation is exactly the same as in the case of the first embodiment.
The wavelength dependence of the phase amplitude can be reduced.

FIG. 12 shows the wavelength dependence of the ± 1st-order diffraction efficiency of the diffractive lens of this example.
It shows a result of applying the equation to the equation (19). FIG.
In 2, the solid line shows the case of the diffractive lens according to this embodiment, and the broken line shows the case of the conventional diffractive lens (optimized wavelength λ = 510 nm) in which a rectangular phase grating is formed on the LaL14 material substrate. As is apparent from FIG. 12, according to this example, the wavelength dependence of the diffraction efficiency is corrected well as compared with the conventional example. As described above, according to this embodiment, since the wavelength dependence of the diffraction efficiency can be reduced in the entire visible band light, it can be suitably applied to the double focus optical system used in the visible band light.

In the above, in the first to third embodiments, the case where the first and second relief patterns have a sawtooth wave cross section and the case where the cross section has a rectangular cross section have been described, but the cross sectional shapes of the first and second relief patterns are described. Is not limited to the above example, the present invention can be effectively applied to various cross-sectional shapes, and similar effects can be obtained.

FIG. 13 shows a fourth embodiment of the present invention. This embodiment shows a transmission type diffractive lens in which an acrylic resin (nd = nd) is used as the first region 107.
1.49, νd = 57.7) is used as the second region 108 for polycarbonate (nd = 1.58, νd = 30.
5) is used, respectively, and these are sequentially laminated. The third region is an atmosphere in which the diffractive lens is placed, and is practically air. A first relief pattern 207 and a second relief pattern 208 having the same pitch distribution are provided on the boundary surface between the first area 107 and the second area 108 and the boundary surface between the second area 108 and the air. The first
Of the relief pattern 207 and the bottom of the second relief pattern 208 are in contact with each other.

First and second relief patterns 207, 2
No. 08 optimizes each pitch arrangement so as to have a condensing function, and each cross section has a sawtooth wave shape, and the wavelength λ = 550.
The groove depth is optimized so as to maximize the first-order diffraction efficiency in nm. In this embodiment, the groove depth d ₁ of the first relief pattern 207 is d ₁ = 15.16 μm, and the groove depth d ₂ of the second relief pattern 208 is d ₂ = 3.3.
4 μm, the parameter α defined by the above equation (10)
Is set to α≈0.22.

In this embodiment, the third region is the atmosphere in which this diffractive lens is placed, which is air in practice, and its refractive index is 1. Therefore, the third region is diffracted by the same action as in the above-mentioned embodiment. The wavelength dependence of efficiency can be corrected. Furthermore, in this embodiment, since the refractive index of the third region is low, the refractive index difference Δn ₂ between the second region 108 and the third region has a sufficiently large value, and the groove depth of the second relief pattern 208 is small. The depth d ₂ can be made relatively shallow. Thereby, the diffractive lens can be made thinner, and the pitch of the relief pattern can be made smaller.

FIG. 14 shows a comparison of the wavelength dependence characteristics of the diffraction efficiency between the diffractive lens of this example and a conventional diffractive lens. In FIG. 14, the solid line shows the case of the diffractive lens according to this embodiment, and the broken line shows the case of the conventional diffractive lens (optimized wavelength λ = 510 nm) in which a blazed pattern is formed on an acrylic substrate. As is clear from FIG. 14, also in the case of this embodiment, the wavelength dependence of the diffraction efficiency can be corrected very well as compared with the conventional one.

FIG. 15 shows a fifth embodiment of the present invention. In this embodiment, a reflection type diffraction grating is shown. Metal aluminum (Al) is used as the first region 111 and polycarbonate (nd = 1.5) is used as the second region 112.
8, vd = 30.5) and acrylic resin (nd = 1.49, vd = 57.7) as the third region 113, and these are sequentially laminated. In addition, the first area 11
1 and the boundary surface between the second area 112 and the second area 1
The first relief pattern 211 and the second relief pattern 211 having the same pitch distribution are provided on the boundary surface between the second region 113 and the third region 113.
Of the first relief pattern 211 so that the top of the first relief pattern 211 and the bottom of the second relief pattern 212 bite into each other, that is, the distance d ₃ between the first and second relief patterns 211 and 212 is It is formed so as to have a negative value.

First and second relief patterns 211 and 211
In No. 12, each pitch is made constant, each cross section is formed in a sawtooth wave shape, and the groove depth is optimized so that the first-order diffraction efficiency at the wavelength λ = 550 nm is maximized. In this embodiment, the groove depth d ₁ of the first relief pattern 211 is
If d ₁ = 0.53 μm and the groove depth d ₂ of the second relief pattern 212 is d ₂ = 6.04 μm, the above (1
The parameter α defined by the equation (0) is α≈11.40.

In this diffraction grating, the first region 11
Since 1 is made of a reflective material made of metallic aluminum, the light incident from the incident surface 311 is reflected by the first relief pattern 211. Therefore, the diffraction grating functions as a reflection type diffraction grating. Further, the wavelength dependence of the diffraction efficiency in this diffraction grating can be explained by the above equation (11). That is, (1
Since the expression (1) can be regarded as a special case where the refractive index of the first region is set to zero in the expression (9) representing the phase amplitude of the transmission type diffractive lens described so far, the action of this embodiment is also essential. The operation is the same as that of the above-mentioned transmission type diffractive lens.

According to this embodiment, by setting α to the optimum value, the wavelength dependence of the refractive index of the polycarbonate forming the second region 112 can be made equal to that of the polycarbonate.
Can be suitably corrected by the wavelength dependence of the refractive index difference from the acrylic resin forming the region 113. As a result, the wavelength dependence of the phase amplitude expressed by the equation (11) is reduced, and further the wavelength dependence of the diffraction efficiency is reduced.

FIG. 16 shows a comparison of the wavelength dependence characteristics of the diffraction efficiency between the reflection type diffraction grating according to this embodiment and the conventional reflection type diffraction grating. In FIG. 16, the solid line shows the case of the reflection type diffraction grating according to this embodiment, and the broken line shows the case of the conventional reflection type blazed grating (optimized wavelength λ = 510 nm). As is clear from FIG. 16, in the case of this embodiment, the wavelength dependence of the diffraction efficiency can be corrected very well as compared with the conventional one.

Additional Notes 1. The diffractive optical element according to claim 1, wherein the first,
Refractive index of materials forming the second and third regions, respectively
, N₁(λ), n_Two(λ) and n_Three(λ) and the first
And the groove depth of the second relief pattern,
d₁ And d _Two, Their ratio is α = d_Two/ d₁ As

[Formula 22] ΔN (λ) = {n ₁ (λ) −n ₂ (λ)} + α
{N ₂ (λ) −n ₃ (λ)} where λ is the wavelength of light,

[Expression 23] | ΔN (λ ₂ ) |> | ΔN (λ ₁ ) |>0; λ ₂ > λ ₁ where λ _{1 is the} wavelength at the short wavelength end of the wavelength range of the used light λ _{2 is the} used light A diffractive optical element characterized by satisfying the wavelength at the long wavelength end of the wavelength range of. 2. The diffractive optical element according to claim 1, 2, or 3, wherein the first relief pattern has a phase shift action and the second relief pattern has a phase shift action that cancels each other algebraically. A diffractive optical element, wherein a groove depth ratio α of the pattern and the second relief pattern is set. However, α = d ₂ / d
₁ ; d ₁ and d ₂ are the groove depths of the first and second relief patterns, respectively. 3. The diffractive optical element according to claim 1, 2 or 3, wherein the third region is an atmosphere of an environment in which the region is in contact. 4. The diffractive optical element according to claim 1, 2 or 3, wherein an average refractive index n ₀ , a thickness D, and a relief pattern of a composite relief structure region composed of the first relief pattern and the second relief pattern. When the minimum pitch of is T,

(Equation 24) A diffractive optical element that satisfies: Where λ ₀
Is the center wavelength of the band light used. 5. The diffractive optical element according to claim 1, 2 or 3, wherein the wavelength λ ₁ at the short wavelength end and the wavelength λ ₂ at the long wavelength end of the band light used are

(25) A diffractive optical element characterized by satisfying λ ₂ −λ ₁ > 0.05λ ₀ . Where λ ₀
Is

(Equation 26) It is an intermediate wavelength between λ ₁ and λ ₂ defined in. 6. An optical device comprising the diffractive optical element according to claim 1.

[0080]

According to the present invention, by controlling the groove depth ratio of the first and second relief patterns,
That is, by setting the groove depth ratio of the two relief patterns to be optimally different, the wavelength dependence of the phase amplitude can be controlled independently of the optical characteristics of the substrate material, etc. The sex can be optimized according to the purpose. Therefore, there are no restrictions on the combination of materials that can reduce the wavelength dependence of the diffraction efficiency.
By combining materials that are easy to manufacture, the wavelength dependence of the diffraction efficiency can be effectively reduced and the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of a diffractive optical element according to a first invention.

FIG. 2 is a conceptual diagram of a diffractive optical element according to a second invention.

FIG. 3 is a sectional view showing a first embodiment of the present invention.

FIG. 4 is a diagram showing wavelength dependence characteristics of a refractive index difference between successive regions in the diffractive lens of the first example.

FIG. 5 is a diagram showing a comparison of wavelength dependence characteristics of phase amplitude between the diffractive lens according to the first example and the conventional diffractive lens.

6 is a diagram showing the wavelength dependence characteristic of the diffraction efficiency corresponding to the wavelength dependence characteristic of the phase amplitude shown in FIG.

FIG. 7 is a sectional view showing a modification of the first embodiment.

FIG. 8 is a sectional view showing a second embodiment of the present invention.

FIG. 9 is a diagram showing wavelength-dependent characteristics of the refractive index difference between successive regions in the diffractive lens of the second example.

FIG. 10 is a diagram showing a comparison of wavelength dependence characteristics of diffraction efficiency between a diffractive lens according to a second example and a conventional diffractive lens.

FIG. 11 is a sectional view showing a third embodiment of the present invention.

FIG. 12 is a diagram showing wavelength-dependent characteristics of ± first-order diffraction efficiency in the diffraction lens of the third example.

FIG. 13 is a sectional view showing a fourth embodiment of the present invention.

FIG. 14 is a diagram showing a comparison of wavelength dependence characteristics of diffraction efficiency between a diffractive lens according to a fourth example and a conventional diffractive lens.

FIG. 15 is a sectional view showing a fifth embodiment of the present invention.

FIG. 16 is a diagram showing a comparison of wavelength dependence characteristics of diffraction efficiency between a reflection type diffraction grating according to a fifth example and a conventional reflection type diffraction grating.

FIG. 17 is a diagram showing an application example of the diffractive optical element according to the present invention.

FIG. 18 is a diagram similarly showing another application example.

FIG. 19 is a sectional view showing a conventional diffractive optical element.

20 is a schematic view of the conventional diffractive optical element shown in FIG.
It is a figure which shows an example of the wavelength dependence characteristic of the secondary diffraction efficiency.

21 is a diagram showing a phase shift function φ (x) of the relief pattern having a sawtooth-shaped cross section shown in FIG.

FIG. 22 is a sectional view showing a basic configuration of a diffractive optical element previously proposed by the applicant.

23 is a diagram showing an example of wavelength-dependent characteristics of refractive indexes of two kinds of optical materials in the diffractive optical element shown in FIG.

[Explanation of symbols]

11, 14, 101, 104, 107, 111 First area 12, 15, 102, 105, 108, 112 Second area 13, 16, 103, 106, 113 Third area 21, 23, 201, 203 , 205, 207, 211
First relief pattern 22, 24, 202, 204, 206, 208, 212
Second relief pattern

Claims (3)

[Claims] 1. A first, a second and a third region which are sequentially stacked closely or closely to each other, a first relief pattern which is formed on a boundary surface of the first and the second region, and the second And a second relief pattern formed on the boundary surface of the third region, wherein the first, second and third regions are made of different materials which are substantially transparent at the wavelength of the light used. The first and second relief patterns have substantially equal pitch distributions, different groove depths, and corresponding portions thereof are arranged close to each other. element. 2. A first, a second and a third region, which are sequentially stacked closely or closely to each other, a first relief pattern formed on a boundary surface between the first and the second regions, and the second And a second relief pattern formed on a boundary surface of the third region, the first region is made of a material that reflects light to be used, and the second and third regions are used. The first and second relief patterns have substantially equal pitch distributions, different groove depths, and corresponding portions thereof. A diffractive optical element characterized in that the two are arranged close to each other. 3. A first layer, which is sequentially laminated in close or close proximity
The first, second, and third regions and the first layer formed on the boundary surface between the first and second regions.
A leaf pattern and a second pattern formed on the boundary surface between the second and third regions.
And a leaf pattern, wherein the first, second and third regions are wavelengths of light to be used.
And the first and second relief patterns are substantially equal to each other.
Corresponding parts are arranged close to each other with a new pitch distribution, and each of the first, second, and third regions is configured.
The refractive index of the material₁(λ), n_Two(λ) and n
_Three(λ) and each groove of the first and second relief patterns
Depth, d₁ And d _Two, Their ratio is α = d_Two/ d₁ 
As follows: ΔN (λ) = {n₁(λ) −n_Two(λ)} + α {n
_Two(λ) −n_Three(λ)} where λ is the wavelength of the light, |_Two ) |> | ΔN (λ₁) |> 0; λ_Two > Λ₁ Where λ₁: Wavelength at the short wavelength end of the wavelength range of the light used λ_Two : A diffractive optical element characterized by satisfying the wavelength at the long wavelength end of the wavelength range of the light used.