JP2007328096A - Diffraction optical element, manufacturing method thereof and optical module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffraction optical element which permits highly flexible design and has high efficiency of diffraction over a wide wavelength region. <P>SOLUTION: The diffraction optical element 201 has a relief pattern formed on a surface of thereof, wherein a fine periodic structure 203 having a pitch shorter than the minimum wavelength of working wavelengths is formed on the relief pattern 202, and by changing parameters such as pitch, width and height (or depth) of the fine periodic structure 203, the wavelength dependency of effective refractive index in the fine periodic structure 203 is adjusted. As a result, the diffraction optical element 201 having the higher efficiency of diffraction on desired plural wavelengths or in a desired wavelength range is provided and the diffraction optical element designed more flexibly is provided. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a diffractive optical element and an optical module that can be applied to various optical devices such as an imaging optical system, a projection optical system, and an image processing apparatus. More specifically, the present invention relates to a plurality of wavelengths or specific wavelengths. The present invention relates to a diffractive optical element used in a range, in particular, a diffractive optical element having high diffraction efficiency over the entire visible light range, a method for manufacturing the diffractive optical element, and an optical module having the diffractive optical element.

  Conventionally, lenses, prisms, and the like existed as elements utilizing light refraction, but diffractive optical elements can achieve these functions with a flat plate structure, and thus can be reduced in size and weight. It is also possible to make an element having a function that could not be realized by a conventional optical element using refraction, such as beam branching or intensity distribution conversion. As a diffractive optical element having these functions, for example, it is achieved by a structure 13 in which an uneven shape is carved on the surface of a transparent material 12 as shown in FIG. ).

  However, the surface relief type diffractive optical element 11 shown in FIG. 16 is designed with a single wavelength, and as shown in FIG. 17, the diffraction efficiency decreases when used for light having a wavelength different from the design wavelength. However, there has been a problem that the light use efficiency is reduced, or the imaging optical system becomes flare and deteriorates the image quality.

This will be described below using mathematical formulas.
The maximum value φ of the phase modulation amount of light passing through the element is
φ = (n−1) d / λ Equation (2)
It is represented by
Here, n is the refractive index of the material, and d is the maximum depth of the relief structure. Using this, the m-th order diffraction efficiency η _m is expressed by the following equation (3).

Therefore, it can be seen that due to the wavelength dependence of φ, the diffraction efficiency decreases at a wavelength other than the design wavelength as shown in FIG.
A diffractive optical element that solves this problem and can be used in a wide wavelength range is proposed in Patent Document 1 (Japanese Patent No. 3717555).

In the prior art described in Patent Document 1, as shown in FIG. 18, on the first optical material 22 (refractive index n ₁ ) having the surface relief type structure 23, the second different in refractive index and wavelength dispersion. It is shown that the diffractive optical element 21 having high efficiency in a wide wavelength region can be obtained by laminating the optical material 24 (refractive index n ₂ ). In this structure, the maximum value φ of the phase modulation amount is expressed by the following equation (4).

According to this, if the design wavelength is λ ₀ and an arbitrary wavelength other than the design wavelength to be used is λ, the phase modulation amount φ (λ) at λ is expressed by the following equation (5).

In order to reduce the wavelength dependency of the phase modulation amount φ (λ) as compared with the surface relief type diffractive optical element, it can be achieved by selecting a refractive index material that satisfies the following equation (6).
However, λ ₁ >λ> λ ₂ : λ ₁ and λ ₂ are the lower and upper light wavelengths of the used wavelength, respectively.

  In recent years, a diffractive optical element having a fine periodic structure whose grating period is shorter than a used wavelength (hereinafter referred to as a sub-wavelength structure) has been proposed in Patent Document 2 (Japanese Patent No. 3547665). Such a subwavelength structure is also usually used at a single wavelength, but Patent Document 3 (Japanese Patent Laid-Open No. 2001-343512) describes means for obtaining high light utilization efficiency in a wide wavelength range. In the prior art described in Patent Document 3, it is shown that a diffractive optical element having high efficiency in a wide wavelength region can be obtained by embedding a different material in the sub-wavelength structure.

Japanese Patent No. 3717555 Japanese Patent No. 3547665 JP 2001-343512 A JP 2001-272505 A Digital diffractive optics Maruzen p.360-363 Sov.Phys.JETP 2 466- (1956) J.Opt.Soc.Am.A 15 (6) 1577-1587 (1998) Appl.Opt. 33,863-867 (1994) Opt.Eng. 19, 297-305 (1980)

  In order to obtain high diffraction efficiency in a wide wavelength range as in the above-described conventional technology, a method using two or more different materials has been adopted. In such a method, in order to obtain high diffraction efficiency in a wide wavelength range, it is necessary to strictly optimize the dispersion characteristics of a plurality of materials, and it is difficult to find such conditions from existing materials. there were.

  The present invention has been made in view of the above circumstances, and a first problem is to solve the problems of the prior art, enable a design with a higher degree of freedom, and have a higher diffraction efficiency in a wide wavelength range. It is to provide an optical element.

  The second problem of the present invention is to provide a diffractive optical element that is easier to manufacture and has a high diffraction efficiency in a wide wavelength range, and the third problem is only for specific polarized light. A fourth problem is to provide a diffractive optical element having a high diffraction efficiency in a wide wavelength range with respect to non-polarized light. is there.

By the way, a normal optical element has a loss due to Fresnel reflection caused by a difference in refractive index between an incident surface and an exit surface on the surface. Fresnel reflection occurs on each surface of the element, causing a decrease in light utilization efficiency, flare due to generation of stray light, and the like. Therefore, in order to reduce the influence of surface reflection, a single-layer antireflection film or a multilayer antireflection film in which a plurality of dielectric films are stacked is generally used. However, it is not easy to prevent reflection with such a film in a wide wavelength range and a wide incident angle, and in order to achieve this, high-precision film control is necessary and the cost is high. .
Accordingly, a fifth problem of the present invention is to solve such a problem and provide a highly efficient diffractive optical element in which surface reflection is reduced in a wide wavelength range.

It is known that a diffractive lens has a negative Abbe number, so that it can efficiently correct chromatic aberration when used in combination with a refractive lens. This is described, for example, in Non-Patent Document 1 (Digital Diffraction Optics Maruzen p. 360-363).
However, a normal diffractive lens has good diffraction efficiency at a single wavelength and low diffraction efficiency at other wavelengths, causing flare, uneven color, and reduced light utilization efficiency.
Accordingly, a sixth problem of the present invention is to solve such a problem and provide a diffractive optical element (for example, a diffractive lens) having high diffraction efficiency in a wide wavelength region.

  The seventh object of the present invention is to provide a diffractive optical element having a beam branching function that can be used in a wide wavelength range, and the eighth problem is a beam deflection function that can be used in a wide wavelength range. The ninth problem is to provide a diffractive optical element having a beam shaping function that can be used in a wide wavelength range, and the tenth problem is polarization dependence. Is to provide a diffractive optical element having a low diffraction rate and a high diffraction efficiency in a wide wavelength range, and an eleventh problem is to provide a diffractive optical element that is easy to design and has a high diffraction efficiency in a wide wavelength range. .

All of the conventional diffractive optical elements that can be used in a wide wavelength range described in Patent Document 1 and Patent Document 3 described above are manufactured using a plurality of materials. However, the use of a plurality of materials is time-consuming and expensive due to material costs and complicated manufacturing methods.
Accordingly, a twelfth problem of the present invention is to provide a diffractive optical element that can be used in a wide wavelength range and is low in cost and excellent in productivity.

The thirteenth problem of the present invention is to provide a diffractive optical element having a higher function, and the fourteenth problem is a diffractive optical element with higher durability or diffractive optical with higher light utilization efficiency. It is to provide a higher-performance diffractive optical element such as an element.
A fifteenth problem of the present invention is to provide a method for manufacturing a diffractive optical element excellent in mass productivity.
Furthermore, a sixteenth problem of the present invention is to provide an optical module that is highly functional and that is small and light.

In order to solve the above problems, the present invention adopts the following means.
According to a first means of the present invention, in the diffractive optical element having a relief pattern formed on the surface, a fine periodic structure having a pitch shorter than the minimum wavelength of the wavelength to be used is formed on the relief pattern, The refractive index n ₁ of the relief pattern portion and the effective refractive index n _eff of the fine periodic structure portion satisfy the following expression (1).

（ただし、λ₁＜λ＜λ₂：λ₁，λ₂はそれぞれ使用する波長の最小波長および最大波長）

(Where λ ₁ <λ <λ _{2, where} λ ₁ and λ ₂ are the minimum and maximum wavelengths used, respectively)

According to a second means of the present invention, in the diffractive optical element of the first means, the pitch and width of the fine periodic structure are substantially equal over the entire range.
According to a third means of the present invention, in the diffractive optical element of the first or second means, the fine periodic structure is formed so as to have substantially the same height in the element plane. .

According to a fourth means of the present invention, in the diffractive optical element of any one of the first to third means, the fine periodic structure is a one-dimensional grating periodic structure.
According to a fifth means of the present invention, in the diffractive optical element of any one of the first to third means, the fine periodic structure is a two-dimensional array structure having a columnar shape or a hole. It is characterized by.
Further, a sixth means of the present invention is the diffractive optical element of any one of the first to fifth means, wherein the fine periodic structure is thinner on the surface side.

According to a seventh means of the present invention, in the diffractive optical element of any one of the first to sixth means, the relief pattern has a concentric structure.
According to an eighth means of the present invention, in the diffractive optical element of any one of the first to sixth means, the relief pattern has a one-dimensional or two-dimensional periodic structure.
The ninth means of the present invention is the diffractive optical element of any one of the first to sixth means, wherein the relief pattern has a sawtooth structure.
Still further, according to a tenth means of the present invention, in the diffractive optical element of any one of the first to sixth means, the relief pattern is a structure designed to shape the beam into a predetermined shape. It is characterized by.

According to an eleventh means of the present invention, in the diffractive optical element having a relief pattern formed on the surface, a fine periodic structure having a pitch shorter than the minimum wavelength of the wavelength to be used is two-dimensionally arranged at an equal pitch on the relief pattern. It is characterized by being formed periodically.
According to a twelfth means of the present invention, in the diffractive optical element of any one of the first to eleventh means, the diffractive optical element is formed of two or more kinds of materials.
Further, a thirteenth means of the present invention is characterized in that in the diffractive optical element of any one of the first to eleventh means, the diffractive optical element is formed of a single material.

According to a fourteenth means of the present invention, in the diffractive optical element of any one of the first to thirteenth means, at least one surface is a curved surface.
According to a fifteenth means of the present invention, in the diffractive optical element of any one of the first to fourteenth means, a thin film layer is formed on at least one surface.

  According to a sixteenth means of the present invention, in the production method for producing a diffractive optical element according to any one of the first to fifteenth means, the height of the diffractive optical element is spatially different when producing the structure of the diffractive optical element. The method includes a step of transferring a shape of a mold having a structure having a one-dimensional or two-dimensional periodic structure as a mold.

  A seventeenth means of the present invention is an optical module, comprising the diffractive optical element of any one of the first to fifteenth means, at least one lens, and a light receiving element. .

  In the prior art described above, it is difficult to match the dispersion characteristics of a plurality of materials. However, in the diffractive optical element of the first means, the pitch, width, and height (or depth) of the fine periodic structure are difficult. It is possible to adjust the wavelength dependence of the effective refractive index in a fine periodic structure by changing parameters such as the above, and by satisfying the above equation (1), diffraction is performed at a desired multiple wavelengths or wavelength range. A diffractive optical element with higher efficiency can be realized, and a diffractive optical element that can be designed more freely can be provided.

  In the diffractive optical element of the second means, in addition to the configuration and effects of the first means, the pitch and width of the fine periodic structure are substantially constant, so that the structure can be made easier to manufacture. It is.

In the diffractive optical element of the third means, in addition to the configuration and effect of the first or second means, it is possible to make the structure easier to manufacture by making the height substantially equal within the element surface. . This is due to the following reasons.
In order to fabricate the structure of the diffractive optical element of the present invention, since the heights of the convex portions are equal, only the concave portions may be fabricated by controlling the depth. Therefore, in order to produce the shape of the present invention by lithography, a resist applied flatly on a substrate was developed by irradiating an electron beam whose intensity was adjusted according to the depth at a portion where a recess was desired to be formed. Etching may be performed later. Alternatively, when the structure of the present invention is manufactured by transfer, the structure to be a mold is a structure having the same bottom surface and different height. Such a mold structure is easy to manufacture for structures having different bottoms and heights.

  In the diffractive optical element of the fourth means, in addition to the configuration and effect of any one of the first to third means, the fine periodic structure is a one-dimensional grating periodic structure. In the structure, the effective refractive index varies depending on the polarization direction. As a result, a diffractive optical element structure optimized for a certain polarization state does not have high diffraction efficiency for different polarized light, and thus a polarization-dependent diffractive optical element having different efficiency for polarized light is possible.

  In the diffractive optical element of the fifth means, in addition to the configuration and effect of any one of the first to third means, the fine periodic structure is a two-dimensional array structure of columns or holes. However, in the two-dimensional array periodic structure, there is no difference in the effective refractive index due to polarization. For this reason, it is possible to realize a broadband and high-efficiency diffractive optical element that can be used for light of any polarization state.

  In the diffractive optical element of the sixth means, in addition to the configuration and effect of any one of the first to fifth means, the fine periodic structure has a narrower surface side, so that it has a wider wavelength range. Thus, the surface reflection can be reduced well. Thereby, a diffractive optical element with higher light utilization efficiency can be obtained.

In the diffractive optical element of the seventh means, in addition to the configuration and effect of any one of the first to sixth means, the relief pattern has a concentric structure, so that it can function as a diffractive lens. it can.
A diffractive lens has a negative Abbe number, and it is known that chromatic aberration can be efficiently corrected by using it in combination with a refractive lens. It is an element having high diffraction efficiency in a wide wavelength region, and can be used as a diffraction lens with little flare, color unevenness, and reduction in light utilization efficiency.

In the diffractive optical element of the eighth means, in addition to the configuration and effect of any one of the first to sixth means, the relief pattern has a one-dimensional or two-dimensional periodic structure, so It can function as an efficient beam splitter.
In the diffractive optical element of the ninth means, in addition to the configuration and effect of any one of the first to sixth means, the relief pattern has a sawtooth structure, so that it has a wide bandwidth and high efficiency. It can function as a grating. Accordingly, it is possible to perform spectroscopy using a transmissive element, and it is also possible to reduce the size of the spectroscopic optical system.

In the diffractive optical element of the tenth means, in addition to the structure and effect of any one of the first to sixth means, the relief pattern has a structure designed to shape the beam into a predetermined shape. Thus, it becomes possible to shape light having a specific wavelength range or a plurality of wavelengths into an arbitrary shape with high light utilization efficiency.
Such a structure is particularly effective when homogenizing white light into a so-called top-hat beam pattern. Moreover, although the shift | offset | difference of a homogeneous area | region arises with a wavelength, the area | region homogenized in the wide wavelength range can be obtained in the center part.

  In the diffractive optical element of the eleventh means, the fine periodic structure having a pitch shorter than the minimum wavelength of the wavelength to be used is formed on the relief pattern in a two-dimensional periodic shape at an equal pitch, thereby making it easier. It is possible to provide a diffractive optical element having a structure that can be manufactured and having high diffraction efficiency in a wide wavelength range. In addition, since the diffractive optical element of the present invention has no polarization dependency, it is non-polarized light such as sunlight, light emitted from a lamp, light emitted from a light emitting diode (LED), and is high with respect to light having a width in a wavelength region. It can be used with light utilization efficiency. That is, it can be used for various optical devices such as a camera, a projection device, and a spectroscopic device. By using such a diffractive optical element inside the optical device, it is possible to reduce the weight and size of the device.

In the diffractive optical element of the twelfth means, in addition to the configuration and effect of any one of the first to eleventh means, the diffractive optical element is formed of two or more types of materials, thereby increasing the degree of freedom in design. A more efficient diffractive optical element can be realized.
The diffractive optical element of the thirteenth means can be manufactured by transfer from a mold by being composed of a single material in addition to the structure and effect of any one of the first to eleventh means. Thus, a diffractive optical element having excellent mass productivity can be realized.

  In the diffractive optical element of the fourteenth means, in addition to the structure and effect of any one of the first to thirteenth means, at least one surface is a curved surface, so that in addition to the function as a diffractive optical element, a refractive lens It becomes possible to add the function as. In the diffractive optical element of the present invention, for example, a diffractive lens having a high diffraction efficiency in a wide wavelength region and a refractive lens can be provided as a single configuration. As a result, the number of lenses in the optical system can be reduced, and further reduction in size and weight can be achieved.

  In the diffractive optical element of the fifteenth means, in addition to the configuration and effect of any one of the first to fourteenth means, a thin film layer is formed on at least one surface, so that durability and reliability are further improved. An excellent diffractive optical element can be obtained. Alternatively, an optical function by a multilayer film can be added, and a higher-performance diffractive optical element can be obtained.

  In the diffractive optical element manufacturing method of the sixteenth means, when the structure of the diffractive optical element of any one of the first to fifteenth means is manufactured, the one-dimensional or two-dimensional height is spatially different. By using a structure having a periodic structure as a mold and transferring the shape of the mold, diffractive optical elements having high diffraction efficiency in a wide wavelength range can be easily produced in large quantities at low cost. Is possible.

  The optical module of the seventeenth means includes a diffractive optical element of any one of the first to fifteenth means, at least one lens, and a light receiving element, so that various cameras and spectroscopes can be used. It is possible to provide a compact and light-weight optical module that can be used in such optical equipment.

  Hereinafter, the configuration, operation, and effects of the present invention will be described in detail based on the illustrated embodiments. In addition, the Example of illustration is represented typically and does not represent an exact dimension.

[Example 1]
First, examples of the diffractive optical element according to the first to fourth, ninth, twelfth, and thirteenth means will be described.
The characteristics of a fine periodic structure having a wavelength shorter than that used in the diffractive optical element of the present invention (hereinafter referred to as a sub-wavelength structure) will be described using a one-dimensional periodic structure as shown in FIG. FIG. 1 is a cross-sectional view schematically showing a cross section of a one-dimensional periodic structure 102 of a diffractive optical element 101 according to the present invention. Usually, when light enters the periodic structure 102 as shown in FIG. 1, diffracted light is generated. The diffraction angle θ of the diffracted light is expressed by the following equation (7) where λ is the wavelength of the incident light and p is the pitch of the periodic structure 102. Here, m represents m-th order diffracted light.
sinθ = mλ / p Equation (7)

From equation (7), it can be seen that when the pitch p of the periodic structure 102 is shorter than the wavelength λ, the diffraction angle does not have an actual solution, that is, does not diffract. At this time, it is known that the periodic structure 102 can be expressed as a homogeneous medium having an effective refractive index n _eff with respect to incident light. For example, Non-Patent Document 2 (Sov. Phys. JETP 2 466- (1956)) shows an approximate solution of the effective refractive index n _eff . According to this, the effective refractive index n _eff has a wavelength dependency, and the wavelength dependency of the effective refractive index changes by changing the pitch p and the duty of the sub-wavelength structure.

Here, as an example, regarding the glass manufactured by SCHOTT: LASF31 (main refractive index nd = 1.88, Abbe number νd = 41), the effective refractive index n _eff of the periodic structure of FIG. 1 is shown in FIG. As each parameter, the calculation was performed in the wavelength range of 400 nm to 700 nm with the pitch p = 300 nm and the width a of the periodic structure = 180 nm. The broken line in the graph of FIG. 2 represents the original refractive index of the material, and the solid line represents the effective refractive index of the periodic structure in TM polarized light (component in which the vibration direction of the electric field is perpendicular to the periodic direction). Thus, the effective refractive index has a dispersion characteristic different from the original refractive index of the material.

In the present invention, it has been found that the wavelength dependence of diffraction efficiency can be improved by utilizing such characteristics of the sub-wavelength structure. That is, the relief pattern has been formed on the surface of the material, in the diffractive optical element that is sub-wavelength structure is formed on the relief pattern, the effective refractive index n _eff in the refractive index n ₁ and the sub-wavelength structure of the relief pattern portion The wavelength dependency of the diffraction efficiency can be improved by the structure satisfying the following formula (1). Further, the wavelength dependence of the effective refractive index in the sub-wavelength structure can be adjusted by changing parameters such as the pitch p, width a, and height (or depth) d of the sub-wavelength structure.

（ただし、λ₁＜λ＜λ₂：λ₁，λ₂はそれぞれ使用する波長の最小波長および最大波長）

(Where λ ₁ <λ <λ _{2, where} λ ₁ and λ ₂ are the minimum and maximum wavelengths used, respectively)

Next, an example of a specific periodic structure is shown below using FIG.
As a material, the design was performed by using glass made by SCHOTT: LASF31 (nd = 1.88, νd = 41) as an example. When the sub-wavelength structure portion has the same pitch p = 300 nm as described above and the width of the periodic structure a = 180 nm, the wavelength dependency of the effective refractive index is as shown in FIG. FIG. 3 shows a diffractive optical element 201 having a structure in which the sub-wavelength structure 203 having the pitch p and the width a and the stepped surface relief structure 202 are combined. In such a structure, incident light can be concentrated only on a specific order of diffracted light, that is, incident light can be deflected in a specific direction.

FIG. 4 shows a theoretical calculation value of the diffraction efficiency of the primary light with respect to the TM polarized light having the structure shown in FIG. In the graph of FIG. 4, the solid line represents the diffraction efficiency of the structure shown in FIG. 3, and the dotted line represents the diffraction efficiency of the surface relief structure optimized without the conventional subwavelength structure. Thus, by adding the sub-wavelength structure 203 to the stepped surface relief structure 202, a diffractive optical element 201 having a wide band and high diffraction efficiency can be obtained.
At this time, the diffraction efficiency is not optimized in the TE polarized light (polarized component orthogonal to the TM polarized light), and only the TM polarized light has a high diffraction efficiency in a wide wavelength region. Thus, a diffractive optical element having high diffraction efficiency for only specific polarized light can be realized.

Here, although the relief structure part and the subwavelength structure part were designed using the same material, it is also possible to use different materials. By using different materials, the degree of design freedom increases, so that it is possible to design a more efficient diffractive optical element. In addition, a structure formed of different materials can be manufactured by the following method, for example.
After the relief structure is formed on the surface of the quartz substrate by etching, the relief structure is filled with a photocurable resin and planarized. Thereafter, exposure is performed with a pattern having a fine periodic structure to cure the photocurable resin, and then the unexposed portion is removed. As a result, a structure in which the relief structure is made of quartz and the sub-wavelength structure is made of a photocurable resin is produced.

  As an advantage of using the same material for such a structure using two types of materials, it can be easily manufactured. Since it is made of a single material, a pattern can be formed by one transfer or by lithography, and the diffractive optical element is excellent in mass productivity.

  In this way, it is possible to use the same material or different materials depending on the application. In addition to the glass material, a polymer resin or a metal oxide can be used as the material.

Such a subwavelength structure is not necessarily formed on the entire surface with the same pitch and width, and the pitch and / or the width may be modulated.
Further, such a relief structure or sub-wavelength structure does not need to be formed on the entire surface of the element, and may be formed in a part of the effective light beam range. Alternatively, such a structure may be formed on both sides of the element.

In FIG. 3, the step-shaped relief structure 202 is shown. However, a relief structure 302 having a continuous shape such as a sawtooth, which is generally used, like the diffractive optical element 301 shown in FIG. The sub-wavelength structure 303 may be provided on the relief structure 302. Further, like the diffractive optical element 401 shown in FIG. 6, the relief structure 402 may be a structure in which the unevenness is not a periodic structure.
3 and 5, the sub-wavelength structures 203 and 303 are shown to have the same height on the element, but a structure in which the height of the entire element is spatially different may be used.

[Example 2]
Next, examples of the diffractive optical element according to the fifth and eleventh means will be described.
FIG. 7A shows an overall image of an embodiment of the diffractive optical element 501 using a sub-wavelength structure having a two-dimensional period, and FIG. 7B shows a cross-sectional view thereof. Here, the structure is such that a two-dimensional periodic circular hole array 502 having a fine pitch is arranged with spatially different depths. In such a two-dimensional periodic sub-wavelength structure as well, the effective refractive index shows wavelength dependence as in the one-dimensional structure of Example 1 described above (J. Opt. Soc. Am. A 15 (6) 1577-1587 (1998)).
Therefore, even in such a structure, a diffractive optical element capable of obtaining high diffraction efficiency in a wide wavelength region can be provided.

  In the embodiment shown in FIG. 7, the cylindrical fine holes are arranged in a two-dimensional array vertically and horizontally, but the two-dimensional array may be arranged in a hexagonal close-packed manner, or in other ways. The holes do not need to be circular, and can take various shapes such as an elliptical shape and a rectangular shape. Alternatively, the periodic structure does not need to be a hole, and may be a pillar-shaped periodic structure 602 having spatially different heights, such as a diffractive optical element 601 shown in FIG.

[Example 3]
Next, examples of the diffractive optical element according to the sixth means will be described.
The diffractive optical element of this example is shown in FIG. The diffractive optical element 201 has a structure in which the sub-wavelength structure 203 and the stepped surface relief structure 202 are combined as in FIG. 3, but the tip is gradually narrowed at the tip of the sub-wavelength structure 203. 204 is provided. By adopting such a structure, it becomes possible to reduce Fresnel reflection on the element surface with respect to light in a wide wavelength range. More desirably, in the sub-wavelength structure portion 203, the structure 204 is gradually narrowed above the highest point of the relief structure 202.

Such a structure can be produced, for example, by the following method.
After applying a resist on the quartz substrate, exposure is performed while changing the intensity of the electron beam. When the resist is developed, a structure having fine irregularities according to the intensity of the irradiated electron beam can be obtained. Using this as a mask, dry etching is performed to transfer the unevenness of the resist onto quartz. At this time, it is possible to form a shape with a thin tip by optimizing the dry etching conditions. In this manner, a structure 204 having a gradually narrowed tip can be produced at the tip of the sub-wavelength structure 203 as shown in FIG. Note that a method of manufacturing a structure having a weight shape with a pointed end by etching is disclosed in, for example, Japanese Patent Application Laid-Open No. 2001-272505.

[Example 4]
Next, examples of the diffractive optical element according to the seventh and fourteenth means will be described.
As the diffractive optical element 701 of the present embodiment, as shown in FIG. 10A, the relief structure 702 has a concentric so-called diffractive lens shape. The relief structure may be quantized and may have a stepped shape.
Such a diffractive lens can efficiently correct chromatic aberration by combining it with a normally used refractive lens. Further, the refractive lens and the diffractive lens of the present embodiment are integrated, and the surface opposite to the surface having the diffractive lens shape has a curved surface 704 as shown in FIG. Then, the curved surface 704 can function as a refractive lens (lens surface), and it is not necessary to provide a separate lens, and the number of elements can be reduced.

[Example 5]
Next, examples of the diffractive optical element according to the eighth means will be described.
A diffractive optical element 801 of this example is shown in FIG. In this diffractive optical element 801, the relief structure 802 also has a periodic structure, and a sub-wavelength structure 803 is provided thereon. With such a structure, incident light can be branched into a plurality of lights. In the diffractive optical element 801 having the relief structure 802 as shown in FIG. 11, the light can be branched into three orders of zero-order light, first-order light, and −1st-order light, and the height of the relief structure 802 is set optimally. By doing so, it is possible to make the three strengths the same. Further, by making the relief structure 802 a two-dimensional periodic structure, light can be divided two-dimensionally.

[Example 6]
Next, examples of the diffractive optical element according to the tenth means will be described.
In the diffractive optical element of this example, the relief pattern has a structure as shown in FIG. FIG. 12A shows a so-called computer-generated hologram (CGH) pattern, in which the shade of color represents the depth of the relief pattern. In the diffractive optical element in which the sub-wavelength structure is formed on the relief pattern having the shape of FIG. 12A, when white parallel light is incident, the shape as shown in FIG. 12B at each wavelength at a specific distance. Will be played. Thus, by using a hologram pattern as a relief pattern, a diffractive optical element capable of shaping a beam into an arbitrary shape can be manufactured.

As a method for calculating the shape of the hologram pattern, it is possible to use an iterative Fourier transform method or a simulated annealing method.
These calculation methods are described in detail in Non-Patent Document 4 (Appl. Opt. 33,863-867 (1994)) and Non-Patent Document 5 (Opt. Eng. 19, 297-305 (1980)), respectively.
The iterative Fourier transform method is more preferable because a hologram having a large number of pixels can be calculated in a short time compared to the simulated annealing method.

[Example 7]
Next, examples of the diffractive optical element according to the fifteenth means will be described.
As shown in FIG. 13A, the diffractive optical element 901 of this embodiment has a structure in which a thin film 903 is formed on a surface on which a fine subwavelength structure 902 is formed. The sub-wavelength structure 902 is easily damaged because it has a relatively high aspect ratio. On the other hand, as shown in FIG. 13A, the thin film 903 formed on the sub-wavelength structure 902 can serve as a protective film, and can improve reliability and durability. Is possible. The thin film 903 may have any composition, but it is necessary to have a transmittance with respect to the wavelength range used. Alternatively, not only a protective film but also an antireflection film can be formed of a dielectric or resin. Alternatively, by using a multilayer film 904 as shown in FIG. 12B instead of a single layer film, it is possible to add a function of a bandpass filter, a function of polarization separation, and the like. It can be set as an element. The single layer film 903 and the multilayer film 904 can be added to the surface where the structure is not formed, whereby the diffractive optical element can be highly functionalized.

  As described above, the embodiments of the diffractive optical element according to the present invention have been described. However, the sub-wavelength structures having the structures shown in these embodiments all have a structure that satisfies the above-described expression (1). is there.

[Example 8]
Next, an embodiment of a method for producing a diffractive optical element according to the sixteenth means will be described with reference to FIG.
As shown in FIG. 14A, a mold 1101 formed of metal or the like is used, a transfer material 1102 made of resin or the like is poured into the mold 1101, and the transfer material 1102 is cured, and then released from the mold 1101. Thus, the diffractive optical element 1103 can be easily obtained. This method can be reused and transferred many times as long as the mold 1101 can be made once, so that it is excellent in mass productivity and is a low-cost production method.
As the transfer material, a polymer material such as a thermoplastic resin or a photo-curable resin, a sol / gel material which is a mixed organic / inorganic material, a low melting point glass, or the like can be used. The curing method is selected to be optimal for the material, but usually either the thermal imprint method or the photoimprint method is used. In the thermal imprinting method, a thermoplastic material is used, and after being brought into close contact with the mold at a high temperature, the temperature is lowered to release the mold. In the photoimprint method, a photocurable resin or a sol / gel material is used, and after the material is poured into a mold, the material is cured mainly by irradiating ultraviolet rays, and then the mold release is performed.

  As a mold manufacturing method, a method as shown in FIG. In this method, exposure is performed by applying a resist material 1202 on a substrate 1201 and irradiating an electron beam 1203 while changing the intensity depending on the location. When the resist is developed after the exposure, an uneven resist 1204 is formed according to the intensity of the irradiated electron beam. A metal mold 1206 is obtained by depositing a metal film 1205 on this, performing electroforming, and then releasing the substrate 1201 and the resist 1204. If this metal mold 1206 is used as the mold 1101 in FIG. 14A, the diffractive optical element 1103 can be easily obtained as described above. Note that the above-described mold manufacturing method is an example, and the present invention is not limited to this method.

[Example 9]
Next, an example of the optical module according to the seventeenth means will be described.
The diffractive optical element of the embodiment as described above can be used as an optical module having a high function, a small size, and a light weight when combined with other optical elements. As described above, by combining the refractive lens and the diffractive optical element of the present invention (for example, a diffractive lens), an optical module capable of efficiently reducing chromatic aberration can be obtained.

  Here, an optical module 5 shown in FIG. 15 will be described as an example of the optical module. In FIG. 15, the diffractive optical element 1 has a sub-wavelength structure formed in a relief structure having a deflection function, and is a diffractive optical element having high diffraction efficiency over the entire visible light range. A lens 2 is installed immediately after the diffractive optical element 1 having this structure, and a light receiving element (for example, a line sensor) 3 is disposed at a condensing point. Thereby, the incident light 4 having a plurality of wavelengths is dispersed with high utilization efficiency, and is condensed on the line sensor 3 at different positions for each wavelength. As described above, it is possible to realize the optical module 5 having high light use efficiency in a wide wavelength range, small in size, and having a spectral function.

15 is an example of an optical module, and is not limited to this configuration. By appropriately combining the diffractive optical element described in Examples 1 to 7, at least one lens, and a light receiving element, An optical module having various functions can be configured.
In addition, the diffractive optical element described in Embodiments 1 to 7 and an optical module using the diffractive optical element can be applied to various optical devices such as an imaging optical system, a projection optical system, and an image processing apparatus.

It is sectional drawing which represented typically the cross section of the one-dimensional periodic structure (subwavelength structure) of the diffractive optical element which concerns on this invention. It is a graph which shows the example of the wavelength dependence of the effective refractive index in the subwavelength structure of the diffractive optical element shown in FIG. 1 is a schematic cross-sectional view of a main part of a diffractive optical element showing one embodiment of the present invention. It is a graph which shows the wavelength dependence of the theoretical calculation value of the diffraction efficiency in the structure of the diffractive optical element shown in FIG. It is a schematic principal part sectional drawing of the diffractive optical element which shows another Example of this invention. It is a schematic principal part perspective view which shows typically another form of the relief structure of the diffractive optical element which concerns on this invention. It is a figure which shows another Example of this invention, Comprising: (a) is a schematic principal part perspective view of a diffractive optical element, (b) is a schematic principal part sectional drawing of a diffractive optical element. It is a schematic principal part perspective view of the diffractive optical element which shows another Example of this invention. It is a schematic principal part sectional drawing of the diffractive optical element which shows another Example of this invention. FIG. 6 is a diagram showing still another embodiment of the present invention, in which (a) is a schematic cross-sectional view of a diffractive optical element that functions as a diffractive lens, and (b) is a combination of a refracting lens and the diffractive lens of (a) FIG. It is a schematic principal part sectional drawing of the diffractive optical element which shows another Example of this invention. FIG. 6 is a diagram showing still another embodiment of the present invention, in which (a) represents the depth of the relief structure composed of a computer-generated hologram pattern in gradation, and (b) is represented by (a). It is the figure which showed the pattern of the light reproduced when light of a single wavelength is allowed to pass through the relief structure. FIG. 6 is a diagram showing still another embodiment of the present invention, in which (a) is a schematic cross-sectional view of a principal part of a diffractive optical element in which a single-layer film is provided on a sub-wavelength structure, and (b) is an upper view of the sub-wavelength structure. 1 is a schematic cross-sectional view of a principal part of a diffractive optical element provided with a multilayer film. It is a figure which shows an example of the manufacturing method of the diffractive optical element of this invention, Comprising: (a) is a figure which shows the procedure of the manufacturing process which transfers the structure of a type | mold to a transfer material, (b) is a manufacturing process of (a). It is a figure which shows the procedure of the production process of the type | mold used. It is a schematic sectional drawing of the optical module which shows another Example of this invention. It is a schematic principal part sectional drawing which shows an example of the structure of the conventional diffractive optical element. It is a graph which shows the wavelength dependence of the diffraction efficiency in the conventional diffractive optical element shown in FIG. It is a schematic principal part sectional drawing which shows another example of the structure of the conventional diffractive optical element.

Explanation of symbols

1: Diffractive optical element having a deflection function 2: Lens 3: Light receiving element (line sensor)
4: Light having a plurality of wavelengths 5: Optical module 101: Diffractive optical element 102: Fine periodic structure (sub-wavelength structure)
201: diffractive optical element 202: stepped surface relief structure 203: sub-wavelength structure 204: structure in which the tip is gradually narrowed 301: diffractive optical element 302: serrated relief structure 303: sub-wavelength structure 401: diffractive optical element 402: Relief structure where irregularities are not periodic 501: Diffractive optical element 502: Circular hole array 601: Diffractive optical element 602: Periodic structure of pillar shape 701: Diffractive optical element having a diffractive lens shape 702: Concentric relief structure 703 : Sub-wavelength structure 704: Curved surface (lens surface)
801: Diffractive optical element 802: Relief structure of periodic structure 803: Sub-wavelength structure 901: Diffractive optical element 902: Sub-wavelength structure 903: Single-layer thin film (single-layer film)
904: Multilayer film 1101: Mold 1102: Transfer material 1103: Diffraction optical element 1201: Substrate 1202: Resist material 1203: Electron beam 1204: Resist 1205: Metal film 1206: Metal mold

Claims (17)

In a diffractive optical element having a relief pattern formed on the surface,
A fine periodic structure having a pitch shorter than the minimum wavelength of the wavelength to be used is formed on the relief pattern, and the refractive index n ₁ of the relief pattern portion and the effective refractive index n _eff in the fine periodic structure portion are A diffractive optical element satisfying the following formula (1):

_{(Where, λ 1 <λ <λ 2} : λ 1, minimum wavelength and a maximum wavelength of the wavelength to be used lambda _2, respectively) The diffractive optical element according to claim 1,
A diffractive optical element characterized in that the pitch and width of the fine periodic structure are substantially equal over the entire range. The diffractive optical element according to claim 1 or 2,
The diffractive optical element, wherein the fine periodic structure is formed to have substantially the same height in the element plane. The diffractive optical element according to any one of claims 1 to 3,
The diffractive optical element, wherein the fine periodic structure is a one-dimensional grating periodic structure. The diffractive optical element according to any one of claims 1 to 3,
2. The diffractive optical element according to claim 1, wherein the fine periodic structure is a columnar or vacant two-dimensional array periodic structure. The diffractive optical element according to any one of claims 1 to 5,
A diffractive optical element characterized in that the fine periodic structure has a thinner surface side. The diffractive optical element according to any one of claims 1 to 6,
The diffractive optical element, wherein the relief pattern has a concentric structure. The diffractive optical element according to any one of claims 1 to 6,
The diffractive optical element, wherein the relief pattern has a one-dimensional or two-dimensional periodic structure. The diffractive optical element according to any one of claims 1 to 6,
2. The diffractive optical element according to claim 1, wherein the relief pattern has a sawtooth structure. The diffractive optical element according to any one of claims 1 to 6,
The diffractive optical element, wherein the relief pattern has a structure designed to shape a beam into a predetermined shape. In a diffractive optical element having a relief pattern formed on the surface,
A diffractive optical element, wherein a fine periodic structure having a pitch shorter than a minimum wavelength of wavelengths to be used is formed on the relief pattern in a two-dimensional periodic shape at an equal pitch. The diffractive optical element according to any one of claims 1 to 11,
A diffractive optical element characterized by being formed of two or more kinds of materials. The diffractive optical element according to any one of claims 1 to 11,
A diffractive optical element characterized by being formed of a single material. The diffractive optical element according to any one of claims 1 to 13,
A diffractive optical element, wherein at least one surface is a curved surface. The diffractive optical element according to any one of claims 1 to 14,
A diffractive optical element, wherein a thin film layer is formed on at least one surface. In the manufacturing method which manufactures the diffractive optical element of any one of Claims 1 thru | or 15,
The manufacturing of the structure of the diffractive optical element includes a step of transferring the shape of the mold using a structure having a one-dimensional or two-dimensional periodic structure with spatially different heights as a mold. A method for producing a diffractive optical element. An optical module comprising the diffractive optical element according to any one of claims 1 to 15, at least one lens, and a light receiving element.

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