DE102005036633A1 - A method for manufacturing an achromatic diffraction grating with increased efficiency has a triangular rectangle of one part separated and attached to the triangular substrate part - Google Patents

Abstract

The first grating part (14a,14b) is separated at the median point of the smallest side and the hypotenuse and the resulting triangular rectangle (14b) positioned (34) on the substrate material (12) leaving a gap such that the light path (36) through the grating is unchanged.

Description

BACKGROUND THE INVENTION

1 , Field of the invention

The The invention relates to an achromatic diffractive optical element (DOE) according to the preamble of claim 1. Under an achromatic DOE is an element whose diffraction efficiency is over a larger wavelength range is largely wavelength independent. The invention further relates to a method for designing a such achromatic diffractive optical element.

2. Description of the prior art

diffractive optical elements (DOEs), e.g. as diffractive lenses or linear diffraction gratings are used, contain structures, which is produced either by structuring a substrate or a layer which is applied to a substrate. For the most part so far used diffractive optical elements consist of the structures all made of the same material.

The Structures of diffractive optical elements can be different Have profiles. Diffractive optical elements with binary, sinusoidal, blazed, arbitrarily trapezoidal and arbitrary binary rounded profiles.

in the In general, diffractive optical elements diffract light into a variety of diffraction orders. To achieve high efficiency in one chosen Diffraction order structures are used with blazed profiles.

this includes one understands an asymmetrical triangular profile with inclined Blaze flank and (generally) vertical anti-blaze flank. Often become surfaces, which are arranged inclined to a reference plane, by a staircase-like Profile approximated, which is also called a multilevel profile. The number k of Steps is for manufacturing reasons in most cases a power of 2, so k = 2, 4, 8, 16, 32, ....

If the structures consist of a dielectric material of refractive index n, h is obtained when selecting the tread depth h = λ 0 / (n - 1) (1) at the operating wavelength λ ₀ neglecting rigorous effects and at normal incidence of light in the first diffraction order a diffraction efficiency of 100 , If one wants to achieve a diffraction efficiency of 100% in the mth diffraction order, the profile depth h must be increased by a factor of m.

However, if one deviates from the working wavelength λ ₀ , the diffraction efficiency is reduced, ie part of the light is directed into adjacent orders, which can lead to undesirable scattered light.

From the scalar theory follows that the efficiency η of the diffraction order as a function of the wavelength λ by η (λ) = sinc 2 (1 - λ / λ 0 ) (2) is given with sinc (x) = sin (πx) / (πx). (3)

Assuming, for example, that the diffractive optical element is designed for a working wavelength λ ₀ = 550 nm, at the lower limit of the visual spectrum at 400 nm only a diffraction efficiency η of about 61% and at the upper limit is obtained 700 nm, a diffraction efficiency η of about 86%.

However, equation (2) is still not completely correct since the dispersion n (λ) present in real materials is not taken into account. If these are included in the calculation, the following relationship for the diffraction efficiency is obtained: η (λ) = sinc 2 (1 - (λ 0 / λ) · (n (λ) -1) / (n (λ) -1)) (4)

It is possible to exploit the material dispersion in order to keep the diffraction efficiency η at a high level over a certain wavelength range, eg from 400 nm to 700 nm, ideally at 100%. This would mean that in equation (4) the argument of the sinc function must be zero, indicating the condition n (λ) = (λ / λ 0 ) · (N (λ 0 ) - 1) + 1 (5) leads.

This means that n (λ) is one with the wavelength λ linear growing function, what should be but not the case with real materials with normal dispersion is. Where n (λ) a monotonically decreasing function.

From the US 5,764,502 A For example, a diffractive optical element is known in which structures made of a material with the dispersion n ₁ (λ) are operated not against air but against a second material with a different dispersion n ₂ (λ). Thus, condition (5) becomes Δn (λ) = (λ / λ 0 ) · .DELTA.n (λ 0 ) (6)

This means only the difference between the two refractive indices Δn = n1-n2 is a linear function the wavelength λ must be. This is for Real materials can not be exactly fulfilled, but you can still Find material combinations that are very close to this condition.

One Disadvantage here is that the higher refractive index Material a lower dispersion than the low-refractive material must have, which limits the material combinations in question very much, as a higher Refractive index usually with a stronger one Dispersion goes along.

From the US 6 122 104 A For example, a diffractive optical element is known in which this difficulty is avoided. In the diffractive optical element described there is an additional air wedge between the two materials. This represents an additional degree of freedom in the design of the diffractive optical element, so that a very good achromatization can be achieved practically for any combination of materials.

SUMMARY THE INVENTION

task The invention is to provide a method with which in a simple way from the known achromatic diffractive optical elements structurally of which completely different modifications let generate. In particular, these modifications should be made with regard to their optical properties and / or their manufacturability with comparable or even superior to known elements be.

• a) konzeptionelles Bereitstellen zweier geblazeter Strukturen, die
• – bezüglicher einer Bezugsebene übereinander angeordnet sind,
• – in einer Querrichtung die gleiche Ausdehnung haben und
• – jeweils ein im wesentlichen dreieckiges Profil mit jeweils einer zu der Bezugsebene parallelen und sich in der Querrichtung erstreckenden Grundseite, einer zu der Grundseite geneigten Blazeflanke und einer zu der Grundseite senkrechten Anti-Blazeflanke haben, wobei die Anti-Blazeflanken der beiden Strukturen einander gegenüber liegen und
• – unterschiedliche Brechzahlen haben;
• b) konzeptionelles Entfernen eines Teils einer der Strukturen, dessen Ausdehnung in der Querrichtung kleiner ist als die Ausdehnung der Strukturen in der Querrichtung;
• c) konzeptionelle Verlagerung des entfernten Teils in einer Richtung senkrecht zu der Bezugsebene.

An inventive method for designing an achromatic diffractive optical element comprises the following steps:

• a) conceptually providing two blazed structures, the
• - are arranged one above the other with respect to a reference plane,
• - have the same extension in a transverse direction and
• - Each have a substantially triangular profile, each having a parallel to the reference plane and extending in the transverse direction base side, a blazed to the base side Blazeflanke and an anti-Blaze flank perpendicular to the base, the anti-blaze flanks of the two structures face each other and
• - have different refractive indices;
• b) conceptual removal of a portion of one of the structures whose extent in the transverse direction is smaller than the extent of the structures in the transverse direction;
• c) conceptual displacement of the removed part in a direction perpendicular to the reference plane.

This method can be used to design different achromatic diffractive optical elements. Thus, the achromatic diffractive optical element may have a plurality of identically structured sections, which are arranged in a transverse direction one behind the other. The sections each have at least two superimposed structures of materials with different refractive indices. At least one structure has an extension along the transverse direction which is smaller than the extent of the respective Ab cut along the transverse direction.

• a) eine erste Unterstruktur, die aus einem ersten Material mit einer ersten Brechzahl besteht,
• b) eine zweite Unterstruktur, die aus einem zweiten Material mit einer zweiten Brechzahl besteht,

wobei sich die erste Brechzahl von der zweiten Brechzahl unterscheidet und die zweite Struktur ein Material mit der ersten Brechzahl enthält.Another achromatic diffractive optical element that can be developed by the method includes a plurality of similarly structured portions arranged one behind the other in a transverse direction and each having first and second structures superimposed and separated by a fluid-filled or evacuated space. The first structure comprises:

• a) a first substructure consisting of a first material having a first refractive index,
• b) a second substructure consisting of a second material having a second refractive index,

wherein the first refractive index differs from the second refractive index and the second structure contains a material having the first refractive index.

SHORT DESCRIPTION OF THE DRAWINGS

Further Features and advantages of the invention will become apparent from the following Description of an embodiment based on the drawing. Show:

1a a diffractive optical element according to the prior art in a cross section;

1b the diffractive optical element from the 1 after the conceptual separation of a part of a diffraction structure;

1c the separated diffractive optical element after a conceptual shear;

2 a graph in which the dependence of the diffraction efficiency of the wavelength for in the 1c shown diffractive optical element is applied;

3 to 11 various embodiments of diffractive optical elements, which are designed according to the inventive method;

12a another diffractive optical element according to the prior art in a cross section;

12b the diffractive optical element from the 12a after the conceptual separation of a part of a diffraction structure and vertical relocation;

13 the diffractive optical element from the 12b but with an additional gas-filled gap;

14a another diffractive optical element according to the prior art in a cross section with indicated sectional planes;

14b the diffractive optical element from the 14a after the conceptual separation and vertical relocation;

15 the diffractive optical element from the 14b after further vertical relocation;

16 one of the 1c corresponding representation of another embodiment.

DESCRIPTION PREFERRED EMBODIMENTS

In the 1a is a section 10 of a diffractive optical element according to the prior art shown in a cross section. The entire diffractive optical element is created by juxtaposing several such sections 10 along a transverse direction indicated by x. Remains the width P of the sections 10 along the transverse direction x constant, the width P corresponds to the grating period. Often, however, the width P is varied to achieve, for example, a lensing effect.

In the section 10 are the profiles of a first blazed diffraction structure 12 and a second blazed diffraction structure 14 recognizable. Between the two diffraction structures 12 . 14 there is a gap 16 which is filled or evacuated with a gas such as air. The two diffraction structures 12 . 14 can also be arranged slightly offset from each other in the transverse direction x, as described in US 2002/0012170 A1.

The profile of the first diffraction structure 12 has the shape of a right triangle. The longer side of this triangle forms a base 18 which runs parallel to a reference plane of the diffractive optical element and can be applied to a substrate, for example. At a Blaze angle α inclined to the base 18 runs a blaze edge 20 which forms the hypotenuse of the triangle. The shorter side of the triangle runs perpendicular to the reference plane and acts as an anti-blaze edge 22 designated.

The second diffraction structure 14 is structured in the same way. The basic page is there with 24 with the blaze flank 26 and the anti-blaze edge with 28 designated.

A diffraction efficiency η close to 100% for two different wavelengths λ ₁ , λ ₂ with λ ₁ <λ ₂ is obtained when diffracted into the first diffraction order if the profile depths h ₁ , h _{2 of} the two diffraction structures 12 respectively. 14 the condition H 1 · (N 1 - 1) / λ 1 - H 2 · (N 2 - 1) / λ 2 = 1 (7) suffice. Here, n ₁ , n ₂ denote the refractive indices of the material from which the first diffraction structure 12 or the second diffraction structure 14 consists.

Such a diffractive optical element is referred to as achromatic, since the diffraction efficiency for the two wavelengths λ ₁ , λ _{2 is} close to 100% and decreases only relatively slowly in the vicinity of these two wavelengths. Since as a production parameter both the profile depths h ₁ , h ₂ and the refractive indices n ₁ , n _{2 of} the diffraction structures 12 respectively. 14 are freely selectable within certain limits; can be achieved within wide limits arbitrary wavelengths λ ₁ , λ ₂ very high diffraction efficiencies η.

In the following table, some combinations of materials, profile depths h _i and wavelengths λ _i are given which satisfy condition (7).

The quantities n _d and ν _d denote the refractive index or the Abbe number for the respective glasses, which is a measure of the dispersion. For the numbers 4 to 7, these sizes of the EP 1 394 574 A be removed.

The following is based on the 1b and 1c a method is described, starting from that in the 1a As shown and known per se diffractive optical element achromatic diffractive optical elements can develop their diffraction efficiency is similar achromatic, but at least partially easier to produce and sometimes also have additional interesting optical properties that prove to be beneficial in certain applications.

In this method, in a first step, one part is conceptually separated from one of the two diffraction structures. In the in the 1a to 1c shown embodiment, this is the second diffraction structure 14 along a horizontal, ie parallel to the reference plane, cutting plane 30 one with 14b designated part separated. The cutting plane 30 is placed so that the separated part in the transverse direction x has a length P / 2.

Like in the 1b is recognizable, the separated part 14b now conceptually exclusively vertically, ie along the z-direction, so far shifted down to the first diffraction structure arranged underneath 12 strikes. In the 1b is this shift by an arrow 33 indicated.

In a third step, the separated part 14b now sheared along the z-direction around its left vertex as shown in the 1c with an arrow 34 is indicated. As a result of the shear is the abge parted 14b now without gaps on the Blaze flank 20 the first diffraction structure 12 at. The gap 16 is in this way no longer trapezoidal in profile, but receives the shape of an irregular polyhedron.

The shear of the severed part 14b ensures that a ray of light, which in the 1a shown section 10 passing through in a certain place, in the in the 1c shown variation experiences the same phase difference. Is exemplary in the 1a such a light path with a dashed line 36 indicated. As you can easily see, has been due to the vertical displacement of the part 14b and the subsequent shear of the geometric light path 36 not changed in the different materials.

The modification, the subject of the in the 1c shown embodiment is thus in functional terms with in the 1a equivalent to the arrangement shown. By shifting the separated part 14b but you get at the remainder 14a the second diffraction structure 14 a flat surface 37 , The opposite inclined surface 38 of the separated part 14b is weaker than the Blaze flank 26 the Indian 1a shown arrangement. This can bring advantages in certain manufacturing processes.

For shorter widths P, however, due to the higher number of interfaces that can interfere with the propagation of light, the diffraction efficiency may be slightly lower than with diffractive optical elements in which the diffraction structures are formed in a conventional manner. This effect is however at larger widths P of the sections 10 , eg for P> 100λ, negligible.

The 2 shows the diffraction efficiency η (λ) of the 1c shown embodiment of the material combination PMMA / polycarbonate, which is indicated in the first line of the above table. It can be seen that the diffraction efficiency for the wavelength λ ₁ = 420 nm and the wavelength λ ₂ = 625 nm is about 100% and only drops by less than 2% in between.

It is understood that all surfaces of the in the 1c shown embodiment, with respect to the base page 18 inclined, for reasons of ease of manufacture can be approximated stepwise, as is known per se in the prior art.

In the following, further embodiments of modifications based on the 3 to 15 described that can be designed by using the method described above. In some cases, the method steps described above are repeated several times in succession, which can lead to quite complicated rearrangements. Identical or corresponding parts are designated by reference numerals, the order 100 are increased compared to the respective preceding embodiment.

In the in the 3 the embodiment shown is the sectional plane 130 not like in the 1 horizontally but inclined to the reference plane has been placed. In this way he gets with 114a The remaining part of the second diffraction structure has a symmetrical profile. The severed, with 114b designated part has in profile the shape of an irregular triangle. The first diffraction structure 112 stays unchanged.

Starting from the in the 1a shown arrangement can also be above the cutting plane 30 lying part, which has a rectangular profile, conceptually be moved vertically. This is in the 4 shown. By shear along the z-direction receives the separated part 214b in profile the shape of a parallelogram. The one below the cut-out part 214b located region with triangular profile has been moved vertically upward, whereby the remaining part of the second diffraction structure structures only from two blazed part 214a . 214a ' whose width is only P / 2.

The above the cutting plane 30 but separated part with a rectangular profile can also be moved further down, so that a shear is not required. Such an embodiment is in the 5 shown. Such a displacement of the separated part results in that also from the first diffraction structure 312 conceptually a part 312b must be separated and moved vertically upwards. However, the effect of the first diffraction structure does not change with such a shift.

That in the 6 embodiment shown goes from the in the 5 shown embodiment, by a section with a rectangular profile of the first diffraction structure 312a above the diffraction structure 314a is relocated. This shift turns the first diffraction structure into three substructures 412a . 412b and 412c divided up.

The substructure 412b Can also be used as a kind of coating on the substructure 414a be applied. Such a coating is in the in 7 shown embodiment with 512b designated.

That in the 8th embodiment shown goes from the in the 4 shown embodiment, if you from there with 212 said first diffraction structure cuts out an area of rectangular profile, displaces it vertically upwards and shears it like a coating 612b on the in the 8th With 614a designated diffraction structure is applied. In this embodiment, the structures 614a . 614a ' on the one hand and the structures 612a . 612a ' on the other hand period-blazed blaze profiles, which in one half of the period with structures from the other material 612b respectively. 614b are coated.

Be the blazed profiles at the in the 8th shown assembly summarized on one side of the space and the non-blazed profiles on the other side, this results in the 9 shown embodiment. The substructures 712b . 714b above the gap 716 Both have a rectangular profile of width P / 2, resulting in an overall combination of a Blaze profile and a binary profile, which results through the gap 716 are separated from each other.

That in the 10 embodiment shown goes from the in the 3 shown embodiment, when the left half of the structure 114a displaced vertically downward above the gap and against a portion of the first diffraction structure 112 exchanges. Above the gap 716 then you get blazed structures 812b . 814a ' with different profile height and with different refractive indices.

Of course, it is possible to apply the vertical cuts elsewhere than the section center. The 11 shows an embodiment in which the cuts are placed so that the below the gap 916 structure has an overall rectangular profile, but of two substructures 912a . 914b is composed of different refractive index.

In all the embodiments described above, the position and the thickness of the gap 16 can be varied or replaced by a solid material.

The 12a shows in a to the 1a ajar representation another diffractive optical element, in which between blazed diffraction structures 1012 . 1014 no gap remains, which is filled with a gas or a solid material. Here, too, can be achieved by conceptual vertical displacement of one or more herausgetrennter parts of a novel structure.

In the in the 12b the embodiment shown is in the 12a With 1012b designated, in profile rectangular part separated and vertically upwards verla siege. The substructures originally present there have been shifted downwards accordingly.

Possibly. the entire section can be divided again horizontally, so that between two substructures 1112 ' . 1114 ' with rectangular profile and blazed substructures 1112a . 1112a ' and 1114a . 1114a ' again a gap 1116 arises, as in the 13 is shown. This embodiment is characterized by a particularly simple manufacturability.

The 14a shows a profile of a portion of a diffractive optical element, which differs from that in the 1 shown profile for the purpose of better visibility only by the greater profile height of the diffraction structures 1212 . 1224 and the smaller space 1216 between its diffraction structures 1212 . 1224 different.

Dashed lines indicate cut surfaces that define the profile of the diffraction structures 1212 . 1214 divide into rectangular blocks and right triangles. The right-angled triangles border directly on the gap 1216 at. In a first step, the substructures with triangular profile are now omitted and the remaining blocks with a rectangular profile are rearranged vertically. The rearrangement takes place such that the larger blocks come to lie down, while the smaller blocks are stacked up. A lateral shift of blocks is not allowed.

The 14b shows the resulting arrangement, if one of the in the 14a shown subdivision of the diffraction structures 1212 . 1224 emanates. The section as a whole has a blazed profile, each with a base side parallel to a reference plane and extending in the transverse direction 1218 , a Blaze flank inclined to the base 1220 and an anti-blaze flank perpendicular to the base 1222 ,

The in the 14b shown arrangement is in terms of diffraction efficiency with in the 14a shown known species arrangement largely equivalent. Differences result from omitting the triangular profile substructures. The finer the subdivision into blocks with rectangular profile, the better the approximation in the 14b shown arrangement to that in the 14a shown profile.

It is simpler to manufacture, if in the 14b blocks further rearranged and thereby possibly further subdivided, as in the 15 is shown. In this way, larger contiguous areas of the same material, which may have manufacturing advantages.

Produced can be the diffractive optical elements described above methods known per se, e.g. structured coating processes, lithographic methods or the MSG method of Schott.

The 16 shows a further embodiment, which consists of in the 1a shown arrangement by separation of a part 1314b shows its width along the transverse direction x of the width P of the section 1310 equivalent. The cut surface is placed here so that it has the anti-blaze edge 28 the second diffraction structure 14 bisected and runs along the edge where the blaze flank 26 and the base page 24 the second diffraction structure 14 adjoin one another. The second diffraction structure 14 remaining structure 1314a thus remains blazed, but with half blaze angle.

Claims (24)

A method of designing an achromatic diffractive optical element comprising the steps of: a) conceptually providing two blazed structures ( 12 . 14 ; 1012 . 1014 ), which - are arranged one above the other with respect to a reference plane, - have the same extent (P) in a transverse direction (x) and - each have a substantially triangular profile, each with a base side parallel to the reference plane and extending in the transverse direction ( 18 . 24 ), a Blaze flank inclined to the base ( 20 . 26 ) and an anti-Blaze flank perpendicular to the base ( 22 . 28 ), the anti-blaze flanks ( 22 . 28 ) of the two structures ( 12 . 14 ; 1012 . 1014 ) face each other, and - have different refractive indices; characterized by the following further steps: b) conceptual removal of a part ( 14b ; 114b ; 214b ; 314b ; 412b . 414b ; 512b . 514b ; 612b . 614b ; 712b . 714a '; 812b . 814b ; 912b . 914b ; 1012b ; 1112b ) one of the structures ( 14 ) whose extension in the transverse direction (x) is smaller than the extent of the structures ( 12 . 14 ; 1012 . 1014 ) in the transverse direction; c) Conceptual displacement of the removed part ( 14b ; 1314b ) in a direction (z) perpendicular to the reference plane. Method according to claim 1, characterized in that that at least an inclined surface, which arises when removing a part in step b), is approximated stepwise. Method according to claim 1 or 2, characterized in that in step b) for removing the part ( 14b ; 114b ; 214b ; 314b ; 412b . 414b ; 512b . 514b ; 612b . 614b ; 712b . 714a '; 812b . 814b ; 912b . 914b ; 1012b ; 1112b ) at least a flat section is performed. Method according to one of the preceding claims, characterized in that the removed part ( 14b ; 114b ; 512b ; 612b . 614b ; 812b . 814b ; 912b . 914b ) before or after the displacement in step c) is additionally sheared conceptually perpendicular to the reference plane. Method according to one of the preceding claims, characterized in that the removed part ( 14b ; 114b ; 814b ; 912b . 914b ) has a triangular profile. Method according to one of Claims 1 to 4, characterized in that the removed part ( 214b ; 314b ; 412b . 414b ; 512b . 514b ; 612b . 614b ; 712b . 714a '; 812b . 1012b ; 1112b ) a parallelogram-shaped Profile has. Method according to Claim 6, characterized in that the removed part ( 214b ; 314b ; 412b . 414b ; 512b . 514b ; 612b . 614b ; 712b . 714a '; 812b . 1012b ; 1112b ) in profile has two perpendicular to the reference plane extending sides. Method according to Claim 7, characterized in that the removed part has a rectangular profile ( 314b ; 412b . 414b ; 514b ; 712b ; 1012b ; 1112b ) Has. Method according to one of the preceding claims, characterized in that the extension of the removed part ( 214b ; 314b ; 412b . 414b ; 512b . 514b ; 612b . 614b ; 712b . 714a '; 812b . 1012b ; 1112b ) in the transverse direction (x) equal to half the extent (P) of the structures ( 12 . 14 ; 1012 . 1014 ) in the transverse direction (x). Method according to one of the preceding claims, characterized in that between the two structures ( 12 . 14 ) a gap ( 16 ; 116 ; 716 ; 816 ; 1116 ; 1216 ), which is filled with a gaseous, liquid or solid medium or contains a vacuum. Method according to claim 10, characterized in that that this Medium air is. Method according to one of Claims 1 to 9, characterized in that the two structures ( 1012 . 1014 ) Blazeflanken have, which adjoin one another gap-free. Achromatic diffractive optical element, with several identically constructed sections ( 10 ) which a) are arranged one behind the other in a transverse direction (x) and b) each have at least two superimposed structures of materials with different refractive indices, characterized in that at least one structure ( 14b ; 114b ; 214b ; 314b ; 412b . 414b ; 512b . 514b ; 612b . 614b ; 712b . 714a '; 812b . 814b ; 912b . 914b ; 1012b ; 1112b ) has an extension along the transverse direction (x) which is smaller than the extent (P) of the respective section ( 10 ) along the transverse direction (x). Element according to claim 13, characterized in that the at least one structure ( 14b ; 114b ; 814b ; 912b . 914b ) has a triangular profile. Element according to claim 13, characterized in that the at least one structure ( 214b ; 314b ; 412b . 414b ; 512b . 514b ; 612b . 614b ; 712b . 714a '; 812b . 1012b ; 1112b ) has a parallelogram-shaped profile. Element according to claim 15, characterized in that the at least one structure ( 214b ; 314b ; 412b . 414b ; 512b . 514b ; 612b . 614b ; 712b . 714a '; 812b . 1012b ; 1112b ) in profile has two perpendicular to a base of the element extending sides. Element according to claim 16, characterized in that the at least one structure ( 314b ; 412b . 414b ; 514b ; 712b ; 1012b ; 1112b ) has a rectangular profile. Element according to one of Claims 13 to 17, characterized in that the at least one structure ( 14b ; 114b ; 214b ; 314b ; 412b . 414b ; 512b . 514b ; 612b . 614b ; 712b . 714a '; 812b . 814b ; 1012b ; 1112b ) has an extension in the transverse direction which is equal to half the extent of the respective section in the transverse direction. Element according to one of Claims 13 to 18, characterized in that the sections ( 10 ) are arranged periodically with a period P along the transverse direction (x) one behind the other. Element according to one of claims 13 to 19, characterized that the Period P changes along the transverse direction (x). An element according to any one of claims 13 to 20, characterized in that each section comprises at least three structures, two of which are defined by a fluid-filled gap ( 16 ; 116 ; 716 ; 816 ; 1116 ; 1216 ) are separated from each other. Element according to one of claims 13 to 20, characterized in that the structures are arranged in a section such that the section as a whole has the profile of a right-angled triangle, each having a base side parallel to a reference plane and extending in the transverse direction (US Pat. 1218 ), an inclined to the base side inclined edge ( 1220 ) and a vertical side to the base side ( 1222 ) Has. Achromatic diffractive optical element, having a plurality of identically structured sections, which a) are arranged one behind the other in a transverse direction (x) and b) each have a first structure ( 12 . 14b ) and a second structure ( 14a ) arranged one above the other and through a fluid-filled gap ( 16 ), characterized in that the first structure comprises: c) a first substructure ( 12 ), which consists of a first material having a first refractive index, d) a second substructure ( 14b ), which consists of a second material having a second refractive index, e) wherein the first refractive index differs from the second refractive index, and f) the second structure (FIG. 14a ) contains a material having the first refractive index. Element according to claim 23, characterized that the second substructure has a non-parallelogram-shaped profile.