JP2005084485A - Diffraction optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffraction optical element that can be manufactured highly precisely and can control a transmission phase in a shape nearer to continuity than the conventional. <P>SOLUTION: A transparent substrate 2 has unevenness on the upper face as shown in the figure, and has one projection 3 and one recess 4 at a constant pitch q. A q value is shorter than light wavelength in which an optical system using the diffraction optical element 1 is handled. As shown in the figure, the width of the projection 3 is the largest in the leftmost end cycle, the width of the projection 3 is slightly narrower than it in the next cycle, and the width of the projection 3 is gradually narrow up to five cycles. The average reflective index of the pattern structure of the different cycle can be changed by gradually changing the width a of the projection 3 in a pattern structure of a different cycle. Consequently, the diffraction optical element has characteristics near to the diffraction optical element having a sawtooth wave shape. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a diffractive optical element capable of giving a phase difference to transmitted light according to a portion through which light is transmitted.

It is known that a diffractive optical element, for example, a diffractive optical element (diffractive lens) configured so as to have a lens action has characteristics not found in conventional refractive lenses, as will be described below.
(1) Since an aspheric wave can be easily generated, aberration can be corrected effectively.
(2) Since there is substantially no thickness, the design freedom is high and a compact optical system can be realized.
(3) Since the amount corresponding to the Abbe number in the refractive lens is a negative value in the diffractive lens, the chromatic aberration can be effectively corrected by the combination with the refractive element.

  For example, Binary Optics Technology; The Theory and Design of Multi-Level Diffractive Optical Element, Gary J. Swanson, Technical Report 854, MIT. It is described in detail in Lincoln Laboratory, August 1989.

  As described above, the diffractive optical element has many useful features that the conventional refractive element does not have, but on the other hand, since the diffraction efficiency depends on the wavelength, there are the following fundamental problems. . For example, a diffractive optical element applied to an optical system is often used as a lens element. However, in such applications, it is generally not preferable that a plurality of diffracted lights (a plurality of focal points) exist. Therefore, in a conventional diffractive optical element (specifically, a diffractive lens), a relief pattern having a sawtooth waveform (blazed) in cross section is generally formed on a transparent substrate at a wavelength to be used. The energy is concentrated on the diffracted light.

  An example of such a diffractive optical element is shown in FIG. This type of element uses the difference in refractive index between the substrate medium and the atmosphere to give an optical path length difference to the light transmitted through each part of the element. The optical path length difference of the light transmitted through the element is At the same time as the phase difference, because of the blazed shape, the rate at which incident light is converted into diffracted light of a specific order (for example, first order) becomes very high.

Since the required phase difference is within 2π, the height of the unevenness is within about λ / (n−1), where n is the refractive index of the substrate and λ is the wavelength of the light of interest. In the case of an element operating at a wavelength in the vicinity of visible light, this value is about 1 μm, mechanical processing is difficult, and the concavo-convex structure is produced by an etching process.
JP 2001-318217 A Binary Optics Technology; The Theory and Design of Multi-Level Diffractive Optical Element, Gary J. Swanson, Technical Report 854, MIT Lincoln Laboratory, August 1989.

  However, when a phase-type diffraction element is to be manufactured by such a method, it is difficult to continuously control the height of the unevenness on the surface. Therefore, in many cases, even when continuous height control is required, a method of discrete manufacture is employed. For example, when an optical element as shown in FIG. 5 is manufactured, a pattern having a plurality of depths is provided by patterning using a plurality of masks, and as shown in FIG. A good cross-sectional shape.

  That is, initially, a resist film is applied to the surface of a transparent substrate 11 (quartz or the like) having parallel front and back surfaces, the resist is exposed and developed with a mask having a pattern such as A, and the remaining resist is transparent as a protective layer. The substrate 11 is etched. Subsequently, a resist film is applied again on the surface of the transparent substrate, the resist is exposed and developed with a mask having a pattern such as B, and the transparent substrate 11 is etched using the remaining resist as a protective layer. Thereby, a diffractive optical element having a cross-sectional shape as shown in FIG. 6 is obtained. This type of element is called a binary type element.

  However, since it is difficult to make the height stage sufficiently fine, an element having an approximate cross-sectional shape as shown in FIG. 6 is a phase type compared with an element having an ideal shape as shown in FIG. The performance as a diffraction element is reduced. Furthermore, in a type of element having irregularities with a scale larger than the wavelength on the surface, a diffractive effect due to the irregularities on the surface is generated, so that unnecessary diffracted light is inevitable. For example, consider a grating that has a pattern pitch of 2.4 μm with respect to light having a wavelength of 500 nm and is blazed by first-order diffracted light. In this case, when light enters the element from the air, the diffraction efficiency for the first-order diffracted light is about 78% in the ideal shape as shown in FIG. When the shape is such as 6, the diffraction efficiency is reduced to about 64% (however, the calculation was performed assuming that the incident light is not polarized).

  Japanese Patent Laid-Open No. 2001-318217 (Patent Document 1) proposes a method of providing an optical path length difference by changing the effective refractive index of an optical element as a countermeasure against the difficulty of manufacture. Here, a widely known property that diffracted light (other than the 0th order) is not generated in a periodic structure with a scale less than or equal to the wavelength of the target light is used. In this case, however, it is difficult to finely control the surface structure because a unit pattern in which the portions having the same pitch are continuous is used. Considering the limitation on the size of microfabrication, it is practically impossible to produce a diffraction element whose phase changes on a scale of about a wavelength by this method.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a diffractive optical element that can be manufactured with high accuracy and that can control the transmission phase to be more continuous than in the past with fewer steps.

  A first means for solving the problem is a diffractive optical element having a function of modulating a wavefront of transmitted light, and is one-dimensional or two-dimensional in a direction perpendicular to a direction in which the light passes. The pattern structure has a period equal to or less than the wavelength of the light, and in the one-cycle pattern structure, the refractive index is perpendicular to the direction in which the light passes. And the average refractive index in the one-cycle pattern structure gradually changes between the pattern structures. (Claim 1)

  In this means, since the unit period of the pattern structure is equal to or less than the wavelength of the target light, diffracted light other than the 0th order is hardly generated for the light. Further, in a one-cycle pattern structure, the refractive index changes in a direction perpendicular to the direction in which the light passes. The average refractive index in the pattern structure of one cycle is an average of the refractive indexes in the pattern structure region.

That is, in the xyz orthogonal coordinate system in which the traveling direction of incident light is taken as the z-axis, the average refractive index n _eff is expressed by the following equation.

ここに、ｎ（x,y,z）は、座標(x,y,z)における屈折率であり、積分範囲は、ｘ−ｙ方向にパターン領域のｘ−ｙ方向区分範囲内、ｚ方向に回折光学素子の最大厚さ部分であり、Ｓはパターン領域のｘ−ｙ方向区分範囲の面積、Δｚは回折光学素子の最大厚さである。

Here, n (x, y, z) is the refractive index at the coordinates (x, y, z), and the integration range is in the xy direction segmentation range of the pattern region in the xy direction, and in the z direction. The maximum thickness portion of the diffractive optical element, S is the area of the xy direction section range of the pattern region, and Δz is the maximum thickness of the diffractive optical element.

  In this means, the refractive index is changed in the direction perpendicular to the direction in which light passes within the one-cycle pattern structure, whereby the average refractive index in the one-cycle pattern structure is The pattern structure is gradually changed. As a result, even if the thickness of the diffractive optical element is not gradually changed as in the prior art, the phase of the light transmitted through the element gradually changes according to the transmission position. It can have characteristics as an element.

  As a result, as will be described later in the section of the embodiment, even if the thickness of the diffractive optical element is not necessarily changed depending on the location, the same effect can be obtained. As compared with the prior art, the phase difference between adjacent periodic pattern structures can be made finer, and a diffractive optical element having smooth characteristics can be obtained. Such a diffractive optical element can be used as a prism, a diffraction grating, or a lens (including a Fresnel lens) by setting an appropriate phase difference between adjacent periodic pattern structures.

  The second means for solving the problem is a diffractive optical element having a function of modulating the wavefront of transmitted light, and is one-dimensional or two-dimensional in a direction perpendicular to the direction in which the light passes. The pattern structure has a period equal to or less than the wavelength of the light, and in the one-cycle pattern structure, the refractive index is perpendicular to the direction in which the light passes. And the average refractive index in the one-cycle pattern structure periodically changes with a period in units of the one-cycle pattern structure. (Claim 2).

  This means basically has the same structure as the first means, but the average refractive index in one period of the pattern structure is a period with a plurality of one period of the pattern structure as a unit, and is periodic. It is characterized by changing. As an example of this means, it is possible to have the same action as a diffractive optical element provided with a blaze that enhances only predetermined-order diffracted light as shown in FIG. In this case, in this means, the phase difference of the light transmitted through the adjacent pattern structure can be reduced as compared with the conventional binary type diffractive optical element as shown in FIG. Characteristics close to the pattern as shown in FIG.

  A third means for solving the problem is the first means or the second means, wherein the pattern structure has a mechanical periodic structure arranged at equal intervals in the pattern structure. It is formed by changing the ratio (claim 3).

  In this means, by changing the material composition ratio in a pattern structure having mechanical periodic structures arranged at equal intervals, the direction perpendicular to the direction in which light passes within the pattern structure of one period. In addition, a pattern structure is formed in which the refractive index is changed and the average refractive index in the one-cycle pattern structure is gradually changed between the pattern structures. If two or more substances (including air) having different refractive indices are used as the substance, the refractive index changes in a direction perpendicular to the direction of light passing through in a one-cycle pattern structure. A structure can be created, and in this case, the average refractive index can be gradually changed between the pattern structures by gradually changing the constituent ratio of the substance.

  A fourth means for solving the above-mentioned problem is the third means, wherein the pattern structure is formed by providing an uneven structure on a substrate surface that transmits light. (Claim 4).

  The average refractive index can be changed by providing irregularities on the surface of the substrate that transmits light and adjusting the ratio of the concave and convex portions per unit area in the direction perpendicular to the direction in which the light passes. Thus, for example, in the case of one dimension, one concave portion and one convex portion are provided at regular intervals, and the width of the convex portion is gradually increased or decreased for each adjacent regular interval region, or the regular intervals. The third means can be realized by periodically changing the width of the convex portion with a width of a predetermined number of times as a cycle.

  A fifth means for solving the above problem is the third means, wherein the pattern structure is formed by providing an uneven structure at an interface between substrates that transmit light and have different refractive indexes. (Claim 5).

  The technical idea of this means is the same as that of the fourth means. However, in this means, a pattern structure is formed by providing a concavo-convex structure at the interface between substrates having different refractive indexes. Yes. Such a thing has the same effect as said 4th means.

  A sixth means for solving the above-mentioned problem is the third means, wherein the pattern structure is provided with a concavo-convex structure on a substrate surface that transmits light, and the concave portion has a refractive index different from that of the substrate. It is formed by disposing a substance (claim 6).

  The technical idea of this means is the same as that of the fifth means, but in this means, an uneven structure is provided on the surface of the substrate that transmits light, and a substance having a refractive index different from that of the substrate is disposed in the recess. By doing (for example, embedding), a pattern structure is formed. Such a thing has the same effect as said 5th means.

A seventh means for solving the problem is any one of the first means to the sixth means, wherein the pattern structure is configured to transmit light transmitted through the optical element between adjacent pattern structures. is intended to enable the phase difference [Delta] [phi ₁₂ is characterized in that it is constructed such that [pi / 2 or less (claim 7).
Here, the effective phase difference Δφ ₁₂ is obtained when the average refractive indexes of two adjacent pattern structures are n _1eff and n _2eff , respectively.

で表される量である。

It is the quantity represented by.

Here, n _2eff and n _1eff are amounts corresponding to the average refractive index n _eff in each pattern structure. Therefore, the effective phase difference Δφ ₁₂ corresponds to the phase difference of light transmitted through two adjacent pattern structures.

  In the first to sixth means, it is possible to construct a wavefront that is not approximately continuous by increasing the effective phase difference length between adjacent pattern structures. Since the change is not smooth, unnecessary diffracted waves increase.

  For example, when the grating blazed to the first-order diffracted light as shown in FIG. 5 is made of a material having a refractive index of 1.5, if the element shape is perfect, the diffraction efficiency is excluded except for the reflection component of 4% at the limit where the pitch is increased. Asymptotically approach 96%. However, in the case of the stepped structure as shown in FIG. 6, it reaches 91% at 8 steps (phase difference π / 4) and only 78% at 4 steps (phase difference π / 2), and the remaining intensity. Becomes unnecessary diffraction light. The situation is the same also in the element of the present invention. Increasing the difference in phase of the transmitted wave between adjacent pattern structures leads to generation of extra diffracted light, which degrades the performance as a diffraction element. Bring.

  In order to suppress the generation of unnecessary diffracted light and to configure the device with sufficient performance, it is important that the modulation amount per unit periodic structure is reduced to about π / 2 or less in terms of the phase of transmitted light. It is. Therefore, in this means, the effective phase difference length between adjacent pattern structures represented by the equation (1) is limited to π / 2 or less.

  As described above, according to the present invention, it is possible to provide a diffractive optical element that can be manufactured with high accuracy and can control the transmission phase to be more continuous than in the past with fewer steps.

  Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings. FIG. 1A is a diagram showing a cross-sectional shape of a diffractive optical element which is a first example of an embodiment of the present invention. The diffractive optical element 1 has a predetermined width in a direction perpendicular to the paper surface, and a cross section cut along a plane parallel to the paper surface has a shape as shown in FIG. The light enters from above (or below) the paper surface and is transmitted below (or above) the paper surface. In the following, in FIG. 1, the vertical direction of the paper surface is the z-axis direction, the horizontal direction is the x-axis direction, and the direction perpendicular to the paper surface is the y-axis direction.

  As shown in the figure, the transparent substrate 2 has irregularities on its upper surface, and has one convex portion 3 and one concave portion 4 in a certain pitch q. That is, in this embodiment, the pattern of the concavo-convex structure is formed with a period of q. The value of q is shorter than the wavelength of light handled by the optical system in which the diffractive optical element 1 is used. As shown in the figure, the width of the convex portion 3 is the largest within the leftmost period, and the width of the convex portion 3 is slightly narrower within the next period, and the convex portion 3 is up to the fifth period. The width is gradually reduced.

Assuming that the thickness of the transparent substrate in the convex portion 3 is d ₁ , the thickness of the transparent substrate in the concave portion 4 is d ₂ , the width of the convex portion 3 in one period is a, and the refractive index of the transparent substrate is n. The average refractive index n _eff within the period is
n _eff = n {d ₁ * a + d ₂ * (q−a)} / (q * d ₁ )
It is represented by Therefore, the average refractive index of different pattern structures can be changed by gradually changing the width a of the convex portions 3 in the different pattern structures.

  In the diffractive optical element shown in FIG. 1A, the width of the convex portion 3 becomes narrower as it goes to the right in the figure for five periods, and is the same as the width of the first period in the sixth period. Has this repeating pattern. That is, in FIG. 1A, the pattern is repeated at a pitch p (= 5q). It will be apparent that such a structure is optically equivalent to the structure shown in FIG.

  That is, in the diffractive optical element as shown in FIG. 1A, 1 / m (m is an integer of 2 or more) of the structural period p for acting as a diffractive optical element (in this case, a diffraction grating) is actually The pitch q of the structure is such that at least one convex portion 3 and at least one concave portion 4 are formed in one pitch q. When attention is paid to the pitch q of the actual structure, the average refractive index gradually changes between adjacent structures, and this is repeated at the pitch p.

  As an example, when p = mq, the width of the recess 4 is changed by 0 / (m-1) p / m by p / m, and after (m-1) p / m, it is set to 0 again. If it repeats in this way, what has the characteristic close | similar to the diffractive optical element which has the shape of a sawtooth wave as shown in FIG. 5 can be obtained. In this case, if the depth of the concave portion 4 is λ / (n−1) corresponding to the phase difference 2π, it operates as a blazed grating in which the intensity concentrates on the first-order diffracted light. In other words, the structure periodically changes at the pitch p, and it can be seen that the structure is modulated at the pitch q. This is the same in the conventional example shown in FIG. 6, but in the present embodiment, this is realized not by changing the thickness of the substrate but by changing the width of the unevenness.

  When comparing the embodiment shown in FIG. 1 (a) with the conventional example shown in FIG. 6, in the conventional example, a binary type diffractive optical element is realized by controlling the thickness of the transparent substrate. On the other hand, in the embodiment shown in FIG. 1A, the thickness of the transparent substrate and the depth of the concave portion are made constant, and the width of the concave portion and the convex portion is gradually changed, so that the binary type The diffractive optical element having the same function as the diffractive optical element is formed.

  As described above, it is difficult to control the thickness of the transparent substrate, but it is easy to control the width of the concave and convex portions of the diffraction substrate by using a lithography technique (such as electron beam drawing). Therefore, the width can be easily controlled with an accuracy of about 100 nm. Therefore, the pitch of the phase difference can be made finer and the accuracy of the phase difference can be improved as compared with the conventional binary type diffractive optical element.

As an example, with respect to a diffraction grating that uses a transparent substrate of n = 1.5 and is used for light of λ = 500 nm, a non-polarized light is incident on the conventional example and the type shown in FIG. Table 1 shows the results obtained by actually measuring the diffraction efficiency of the first-order diffracted light when light is vertically incident on the substrate from the air. Two pitches of 1.2 μm and 2.4 μm were taken as the pitch p, and the pitch q was set to 0.3 μm. Therefore, m corresponding to the pitch p is 4 and 8, respectively. In Table 1, the uneven type (ideal) has a shape as shown in FIG. 5, and the uneven type (4-stage BOE: Binary Optical Element) has a shape as shown in FIG. is there.
(Table 1)

  As can be seen from Table 1, when the pitch p is as small as 1.2 μm, the diffractive optical element of the present invention is more efficient than a normal type element (ideal). This is presumably because in the case of a conventional type element, the generation of unnecessary diffracted light due to the structure in the height direction such as a protrusion is large, so the performance is considerably low. When the pitch p is as small as 1.2 μm, the phase difference can be changed only in four stages, so the deviation from the continuous ideal change is large, and therefore the absolute value of the diffraction efficiency is not large. In comparison, the element according to the invention is more efficient than the normal type element (ideal). However, in practice, it is considered that it is difficult to maintain the function as an element when the phase difference between adjacent pseudo-periodic structures q exceeds π / 2, which is the value in this example.

  When the pitch p is 2.4 μm, if the shape is ideally manufactured, the uneven type is more efficient, but the element according to the present invention is more efficient than the BOE having a lower number of steps. In the case of the BOE type, if the number of stages is increased, the efficiency should be improved. However, in addition to increasing the number of processes during the process, problems such as alignment errors may occur, which may improve the actual performance. There is a limit. On the other hand, in the device of the present invention, even if m is increased, the number of process steps is not increased. As described above, it can be seen from the results in Table 1 that this element operates as a blazed grating and, depending on the parameters, has higher performance than a normal one.

  FIG. 1B shows a cross-sectional shape of a diffractive optical element according to the second embodiment of the present invention. In this embodiment, the diffractive optical element is configured by combining transparent substrates 5 and 6 having different refractive indexes. In the example shown in the figure, the transparent substrate 5 has the same shape as the transparent substrate 2 shown in FIG. 1A, and the transparent substrate 6 is in close contact with the transparent substrate 5. It will not be necessary to explain that the diffractive optical element 1 having such a structure is equivalent to the diffractive optical element shown in FIG. However, the characteristics are reversed by reversing the refractive indexes of the transparent substrate 5 and the transparent substrate 6.

  FIG. 2 shows a perspective view of a diffractive optical element according to the third embodiment of the present invention. In the diffractive optical element 7, holes 9 are formed in the transparent substrate 8 with a period q in the x direction, and the diameter of the holes 9 decreases toward the left side of the figure. The holes 9 are arranged at a predetermined pitch in the y direction in the figure, but the diameters of the holes 9 are the same at the same position in the x direction. Light enters from the z direction in the figure.

  With such a configuration, the effective refractive index of the diffractive optical element 7 can be decreased for each pitch q as it goes to the right in the figure. Therefore, the diffractive optical element 7 of the third embodiment has a pseudo function as a prism.

  FIG. 3 shows a perspective view of a diffractive optical element according to the fourth embodiment of the present invention. In the diffractive optical element 7, a hole 9 is formed in the transparent substrate 8 with a period q in the x direction, and the diameter of the hole 9 decreases in three steps toward the left side of the figure, and this is a period of 3q. It is repeating. The holes 9 are arranged at a predetermined pitch in the y direction in the figure, but the diameters of the holes 9 are the same at the same position in the x direction. Light enters from the z direction in the figure.

  By adopting such a configuration, the effective refractive index of the diffractive optical element 7 can be increased in three steps toward the left in the figure for every pitch q, and this can be repeated at the pitch 3q. This acts as a diffraction grating with a blaze concept. In the figure, for the sake of explanation, the diameter of the hole 9 is changed over three steps, but since q is equal to or less than the wavelength of light, the number of steps can actually be increased.

  In the embodiment shown in FIGS. 2 and 3, the hole 9 is made hollow, but the same effect can be obtained even if a material having a refractive index different from that of the transparent substrate 8 is filled therein. In this case, it goes without saying that the characteristics are reversed due to the magnitude relationship between the refractive index of the transparent substrate and the refractive index of the filling material.

  FIG. 4 shows an outline of a diffractive optical element according to the fifth embodiment of the present invention. (A) is a top view, (b) is AA sectional drawing in (a). In this embodiment, one convex portion and one concave portion are provided at a pitch q in the radial direction, and this is repeated at a pitch of p = mq (in the figure, m = 3). Within one pitch p, the width of the convex portion within the pitch q is gradually reduced toward the outer periphery. Thus, in the fifth embodiment, the diffractive optical element acts as a circular blazed diffraction grating.

  In the figure, for simplicity, m is constant at p = mq. For example, by changing m and changing the amount of change of the convex portion in the adjacent structure at the pitch q depending on the location, a Fresnel lens can be obtained. The optical characteristics can be provided.

  As described above, in the present invention, the amount of change in the width of the concave and convex portions between adjacent pitch structures and the width of the cycle in which these pitches are gathered are changed to change the prism, diffraction grating, and lens. Thus, it is possible to provide the characteristics of a general diffractive optical element. In addition, the thickness of each part of the diffractive optical element is not controlled as in the prior art, but the predetermined optical characteristics are provided by controlling the width of the convex part and the concave part. Processing can be performed, and an accurate diffractive optical element can be obtained by one lithography process.

  In the above description, the height of the convex portion and the depth of the concave portion of the diffractive optical element have been treated as being constant, but means for changing them stepwise may be used in combination. In this case, it is not necessary to finely control the height of the convex portion and the depth of the concave portion. For example, a sufficient effect can be obtained by changing in two steps.

It is a figure which shows the cross-sectional shape of the diffractive optical element which is the 1st example of embodiment of this invention, and a 2nd example. It is a perspective view of the diffractive optical element which is the 3rd Embodiment of this invention. It is a perspective view of the diffractive optical element which is the 4th Embodiment of this invention. It is a figure which shows the outline | summary of the diffractive optical element which is the 5th Embodiment of this invention. It is a figure which shows the example of the diffractive optical element which formed the relief pattern which made the cross-section sawtooth wave shape (blazed), and was made to concentrate energy on the diffracted light of a specific order. It is a figure for demonstrating a binary type diffractive optical element and its manufacturing method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Diffractive optical element, 2 ... Transparent substrate, 3 ... Convex part, 4 ... Concave part, 5 ... Transparent substrate, 6 ... Transparent substrate, 7 ... Diffractive optical element, 8 ... Transparent substrate, 9 ... Hole

Claims (7)

A diffractive optical element having a function of modulating a wavefront of transmitted light, the pattern structure having a one-dimensional or two-dimensional period in a direction perpendicular to a direction in which the light passes. Is less than the wavelength of the light, and within the one-cycle pattern structure, the refractive index changes in a direction perpendicular to the direction in which the light passes, and the one-cycle pattern A diffractive optical element, wherein an average refractive index in a structure gradually changes between the pattern structures. A diffractive optical element having a function of modulating a wavefront of transmitted light, the pattern structure having a one-dimensional or two-dimensional period in a direction perpendicular to a direction in which the light passes. Is less than the wavelength of the light, and within the one-cycle pattern structure, the refractive index changes in a direction perpendicular to the direction in which the light passes, and the one-cycle pattern The diffractive optical element according to claim 1, wherein an average refractive index in the structure periodically changes with a period of a plurality of pattern structures of one period as a unit. 3. The optical element according to claim 1, wherein the pattern structure is formed by changing a material composition ratio in a pattern structure having a mechanical periodic structure aligned at equal intervals. A diffractive optical element characterized by the above. 4. The diffractive optical element according to claim 3, wherein the pattern structure is formed by providing an uneven structure on a substrate surface that transmits light. 4. The diffractive optical element according to claim 3, wherein the pattern structure is formed by providing an uneven structure at an interface between substrates that transmit light and have different refractive indexes. 4. The optical element according to claim 3, wherein the pattern structure is formed by providing a concavo-convex structure on a surface of a substrate that transmits light, and disposing a substance having a refractive index different from that of the substrate in the concave portion. A diffractive optical element. An optical element according to claims 1 to any one of claims 6, wherein the pattern structure, between adjacent pattern structure, the effective phase difference [Delta] [phi ₁₂ of light passing through the optical element [pi / A diffractive optical element characterized by having a structure of 2 or less.
Here, the effective phase difference Δφ ₁₂ is obtained when the average refractive indexes of two adjacent pattern structures are n _1eff and n _2eff , respectively.

It is the quantity represented by.

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