Method for making a diffractive optical element with rotational symmetry

Abstract

This method for producing a rotationally symmetric diffractive optical element comprises the following steps: - Placing a diopter (11) on a platform (9) in a physical vapor deposition system (5) for thin films; - Positioning a mask (3) opposite the diopter (11), the mask (3) comprising openings forming a deposition zone; - Rotating the mask (3) relative to the diopter (11); - Initiating the deposition of a thin film of predetermined radial thickness onto the diopter (11) by passing a vapor (27) of a material (13) through the deposition zone of the mask (3). Figure for the abbreviation: Fig. 1

Description

Title of the invention: Method for making a diffractive optical element with rotational symmetry technical field

[0001] The present invention relates to the production of a diffractive optical element by the use of a thin film deposition technique.

[0002] In particular, the present invention relates to the realization of a diffractive optical element with rotational symmetry. Previous techniques

[0003] Among optical components, phase plates and diffractive optical elements ("Diffractive Optical Elements" in Anglo-Saxon terms) can be considered similar in certain aspects.

[0004] Indeed, these optical components make it possible to add a phase to an electromagnetic wave passing through them, a phase shift that can create interference and thus highlight a diffraction phenomenon.

[0005] However, the realization of such diffractive optical elements is complex.

[0006] Diffractive optical elements can be designed by machining, by holography, or by structuring their surface in the form of a metasurface.

[0007] However, the dimensions of metasurfaces are limited and their manufacturing cost is high. Moreover, they are produced in such a way as to form a network, in other words a discontinuity, reinforcing their complex diffractive nature, which is difficult to control precisely.

[0008] Regarding holograms, they are not feasible in the wavelength ranges of the middle infrared, and in the far infrared, in other words MWIR and LWIR.

[0009] Machining remains possible, particularly on an aspheric-diffractive diopter, but this solution remains complex and imprecise for the production of a phase plate or diffractive optical element on a plane diopter. Description of the invention

[0010] The present invention therefore aims to overcome the aforementioned drawbacks and to provide a method for manufacturing a diffractive optical element quickly and at a lower cost, particularly on a sample of the plane diopter type.

[0011] The present invention relates to a method for producing a diffractive optical element with rotational symmetry, the method comprising the following steps:

[0012] - Placement of a diopter on a platform within an enclosure of a physical deposition system in vapor phase of thin films, the tray being configured to perform an epicycloidal movement relative to the enclosure during deposition;

[0013] - Positioning of a mask centered opposite the diopter, the mask comprising openings forming a deposit area;

[0014] - Rotation of the mask relative to the diopter along an axis of rotation passing through the center of the mask and the diopter;

[0015] - Starting a deposit on the diopter of a thin film of radial thickness predetermined by the passage of a vapor from a material through the mask's deposition zone onto the interface; and

[0016] - Stopping the rotation of the mask relative to the diopter and stopping the deposition of the layer thin once the radial thickness of said thin layer has reached the predetermined radial thickness.

[0017] Thus, the rotating mask during thin film deposition allows control of the exposure time of the diopter to the vapor of material that will be deposited on the diopter as a thin film. By additive manufacturing, a variable thickness of thin film on the diopter can therefore be produced along a radius of said diopter extending from its center, so as to form a diffractive optical element, consisting of the diopter and the thin film, with rotational symmetry.

[0018] In a particular embodiment, the diopter is an aspherical lens or a flat plate.

[0019] Advantageously, the mask reproduces the shape of the diopter opposite said diopter.

[0020] Advantageously, the mask is positioned at a distance between 0.1 millimeters and 2 centimeters from the diopter and preferably between 1 millimeter and 2 centimeters from the diopter.

[0021] In one embodiment, the step of rotating the mask relative to the diopter is configured so that at least one hundred rotations of the mask relative to the diopter are carried out during the deposition of the thin film, the rotation being carried out at a constant speed.

[0022] Advantageously, the mask deposition zone has a shape configured so that, once rotated and once the thin film deposition has started, it simulates a radial exposure time of the diopter to the vapor of the material so as to deposit the predetermined radial thickness of thin film on said diopter, the predetermined radial thickness being calculated so as to induce a phase shift modulo 2ir between the bare diopter of the thin film and a part of the diopter covered with the thin film.

[0023] Advantageously, the optical index of the thin film material is the same as the optical index of the diopter, preferably the thin film material being identical to the diopter material.

[0024] In a particular embodiment, the deposition of the thin film on the diopter is carried out under normal incidence on the diopter.

[0025] In another embodiment, the deposition of the thin film on the diopter is carried out under incidence with the oblique angle deposition technology.

[0026] The present invention also relates to an implementation assembly of the process as defined above, comprising a mask and a physical vapor phase deposition system for thin films, said system comprising an enclosure housing a platform on which to place the diopter, the material configured to be vaporized, a first arm supporting the platform, a second arm configured to support the mask, said second arm being movable in rotation about the axis of rotation passing through the center of the mask, the first arm being supported by a third arm connected to the enclosure, the first arm and the third arm being movable in rotation so that the platform is configured to perform an epicycloidal movement relative to the enclosure. Brief description of the drawings

[0027] Other objects, features and advantages of the invention will become apparent from the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings in which:

[0028] [Fig.1] is a schematic view of an assembly enabling the implementation of a method for producing a diffractive optical element with rotational symmetry according to the invention;

[0029] [Fig.2] is a schematic representation of an epicycloidal motion;

[0030] [Fig.3] is a schematic view of the steps in the process of making a diffractive optical element with rotational symmetry according to the invention;

[0031] [Fig.4] is a schematic representation of an example of a mask used in the process according to [Fig.3];

[0032] [Fig.5] is a graph illustrating the phase function corresponding to the mask according to [Fig.4];

[0033] [Fig.6] is a schematic representation of another example of a mask used in the process according to [Fig. 3]; and

[0034] [Fig.7] is a graph illustrating the phase function corresponding to the mask according to [Fig.6].

[0035] Detailed description of at least one embodiment

[0036] Figure [Fig.1] schematically represents an assembly 1 comprising a mask 3, preferably having a disk shape, and a physical vapor phase deposition system 5 for thin films enabling the realization of a diffractive optical element with rotational symmetry.

[0037] The thin film physical vapor deposition system 5 comprises an enclosure 7 housing a tray 9.

[0038] The platform 9 is designed to accommodate a sample onto which a thin film will be deposited. In particular, the platform 9 is configured to accommodate a sample such as a diopter 11 onto which a thin film will be deposited in order to create a diffractive optical element. The diopter 11 is also preferably in the form of a disk.

[0039] The diopter 11 is fixed to the plate 9 by at least one fixing element, for example a mechanical element, or adhesive, or exerting a depression between said diopter 11 and the plate 9.

[0040] The system 5 further includes a material 13 configured to be vaporized, for example by sputtering, or by electron beam evaporation, or any other technology.

[0041] Advantageously, the optical index of the material 13 intended to be vaporized and to form the thin film is the same as the optical index of the diopter 11, so as to avoid interference between the material 13 and the diopter 11.

[0042] Preferably, the material 13 of the thin film is identical to the material of the diopter 11. It may be, for example, germanium or zinc sulfide.

[0043] The system 5 comprises a first arm 15 supporting the platform 9, a second arm 17 configured to support the mask 3 opposite the platform 9, said second arm 17 being movable in rotation about an axis of rotation 19 passing through the center of the mask 3. The axis of rotation 19 preferably passes through the center of the platform 9, said center of the platform 9 being configured to accommodate the center of the diopter 11.

[0044] The first arm 15 is connected and supported by a third arm 21 connected to the enclosure 7, the first arm 15 and the third arm 21 being mobile in rotation so that the platform 9 is configured to be able to perform an epicycloidal movement relative to the enclosure 7.

[0045] For example, the first arm 15 and the third arm 21 are configured to each enter into rotation about their own axis of rotation.

[0046] Figure 2 shows an example of epicyclic motion of the plate 9 relative to the enclosure 7. Figure 2 illustrates a first revolutional motion 23, for example performed by the third rotating arm 21, and a second rotational motion 25, for example performed by the first arm 15. The first Revolutionary motion 23 and the second rotational motion 23 together form the epicycloidal motion.

[0047] The epicycloidal movement allows a uniform deposition of a thin layer without defects related to the internal environment of the enclosure 7.

[0048] During a thin film deposition according to the invention, the plate 9 is thus in motion in an epicycloidal motion relative to the enclosure 7, while the mask 3 is in rotational motion relative to the plate 9, and therefore to the diopter 11.

[0049] As illustrated, the thin film is deposited on the interface 11 at normal incidence on the interface 11; in other words, the surface of the interface 11 intended to receive the thin film is orthogonal to the average direction of the vapor 27 of the material 13 during deposition. The deposition cone formed by the vapor 27 of the material 13 may nevertheless have an opening of up to 30°, preferably only up to 15°. In this case, the deposition velocity of the vapor 27 on the interface 11, induced by the shape of the mask 3, does not have a real effect on the refractive index of the deposited thin film.

[0050] In another embodiment, the thin film can be deposited onto the interface 11 at an angle of incidence, particularly using oblique angle deposition (OAD). In this embodiment, the surface of the interface 11 intended to receive the thin film is oriented at 10° to the average direction of the vapor 27 of the material 13 during deposition. In this case, the deposition rate induced by the shape of the mask 3 influences the atomic organization of the thin film being formed, and thus affects the refractive index of the final thin film, and a potential variation of said refractive index within said thin film.

[0051] The different implementation steps of the process for making a diffractive optical element with rotational symmetry according to the invention are schematically represented in [Fig.3], said process being implemented for example using the assembly 1 described above and one embodiment of which is schematically illustrated in [Fig.1].

[0052] Firstly, a step 29 is carried out of placing the diopter 11 on the platform 9 in the enclosure 7 of the system 5 for physical vapor phase deposition of thin films.

[0053] As mentioned previously, the platform 9 is configured to perform an epicycloidal movement relative to the enclosure 7.

[0054] Then, a step 31 is performed of positioning the mask 3 centered opposite the diopter 11. The mask 3 and the diopter 11 have substantially the same shape and similar dimensions. Preferably, the mask 3 and the diopter 11 are disk-shaped, so that their respective centers are aligned along the same axis corresponding to the axis of rotation 19 of the mask 3.

[0055] As illustrated in [Fig. 4], the mask 3 comprises openings 33 forming a deposition zone 33, the remainder of the mask 3 being opaque. Vapor 27 from the material 13 can thus pass through the deposition zone 33 of the mask 3 and be blocked by the opacity of the remainder of the mask 3. The mask 3 is made of metal. The mask 3 can also be called a shadow mask or stencil.

[0056] In particular embodiments, the diopter 11 can be a spherical lens, an aspherical lens or preferably a flat plate.

[0057] Advantageously, the mask 3 reproduces the shape, in particular the transverse shape, of the diopter 11. For example, if the diopter 11 is a flat plate, the mask is flat. Conversely, if the diopter 11 is a spherical lens, the mask 3 is also partially spherical with the same radius of curvature.

[0058] Advantageously, the mask 3 is positioned at a distance between 0.1 millimeter and 10 centimeters from the diopter 11, preferably between 0.1 millimeter and 2 centimeters, even more preferably between 1 millimeter and 2 centimeters, thus close enough so that the patterns of the deposition zone 33 of the mask are found on the diopter 11 at the time of vapor deposition 27.

[0059] Next, a step 35 is carried out of rotating the mask 3 relative to the diopter 11 along its axis of rotation 19 passing through the center of the mask 3 and the diopter 11.

[0060] This step 35, which involves rotating the mask 3 relative to the interface 11, is configured so that at least one hundred rotations of the mask 3 relative to the interface 11 are performed during the deposition of the thin film, with the rotation occurring at a constant speed. This ensures temporal smoothing of the deposition, as a low number of rotations, such as fewer than one hundred, would not result in a deposit with reliable physical characteristics.

[0061] Next, step 37 is carried out to start the deposition on the diopter 11 of a thin layer of a predetermined radial thickness by passing the vapor 27 of material 13 through the deposition zone 33 of the rotating mask 3 onto the diopter 11.

[0062] At the same time, the platform 9 performs an epicycloidal movement relative to the enclosure 7 while the mask 3 remains positioned opposite the diopter 11.

[0063] With this technology, the thickness of the thin film deposited on the diopter 11 is fixed at a fixed distance from the center of the diopter 11 but can vary according to the distance to said center of the diopter 11 and according to the desired final result.

[0064] Finally, a step 39 is carried out to stop the rotation of the mask 3 with respect to the diopter 11 and to stop the deposition of the thin film once the radial thickness of said thin film has reached the predetermined radial thickness.

[0065] Radial thickness is understood to mean the thickness profile along a radius of the diopter 11.

[0066] Furthermore, thanks to the deposition zone 33 of the mask 3, the shape of which is specifically designed to carry out a deposition of a thin film of particular radial thickness on the diopter 11, the mask 3 makes it possible, during the deposition, to simulate a radial exposure time of the diopter 11 to the vapor 27 of the material 13 so as to deposit the predetermined and desired radial thickness of thin film on said diopter 11.

[0067] It will be noted that the predetermined radial thickness is calculated so as to induce a phase shift modulo 2ir between the bare diopter 11 of the thin film and a part of the diopter 11 covered with the thin film.

[0068] In other words, the addition of the thin film in certain places on the diopter 11 makes it possible to achieve a phase shift between 0 and 2ir compared to an absence of thin film.

[0069] Let f_rad(r) be the function radially defining the phase that we wish to add or subtract from electromagnetic radiation passing through the diopter 11 at a distance r from the axis of rotation 19. We note that f_rad(r) is between 0 and 2ir.

[0070] According to the propagation of electromagnetic fields, it can be estimated that 2ir = Xref, with Xref the reference wavelength for the use of the diopter 11.

[0071] Thus, expressed in wavelength, the phase function f is written:

[0072] -, whose value is between 0 and 1. J-*-

[0073] If we wish to translate this phase function into a thickness of material with index n, we obtain, in micrometers:

[0074] f _ f with Xref in um. Jum 1 — n

[0075] Thus, with a properly formed mask 3, / _x(r) = 1 and for one rotation of the mask 3, the material 13 is continuously deposited as a thin film on the diopter 11 at a distance r from the axis of rotation 19.

[0076] Conversely, for / _x(r) = 0 and for one rotation of the mask 3, the material 13 is not deposited on the diopter 11 at the distance r from the axis of rotation 19.

[0077] Finally, for / _x(r) = 0.5 and for one rotation of the mask 3, the material 13 is deposited on the diopter 11 at a distance r from the axis of rotation 19 for only half of the rotation.

[0078] Figures 4 and 5 show an example of mask 3 associated with its phase function profile y_x(r) = r 2.

[0079] It should be noted that several different masks 3 can be made for the same phase function.

[0080] Figures 6 and 7 show an example of mask 3 associated with its phase function profile / _x(r) = 2r-. In this situation, the theoretical phase function / _X exceeds 1 to reach a maximum of 2. The phase function f_m, which is used to form The mask 3 illustrated on [Fig.6] is drawn modulo 1, in other words modulo 2ir according to the phase function f_rad expressed in radians.

Claims

Demands

1. A method for making a diffractive optical element with rotational symmetry, characterized in that it comprises the following steps: - Placing (step 29) a diopter (11) on a platform (9) in an enclosure (7) of a physical vapor deposition system (5) for thin films, the platform (9) being configured to perform an epicyclic movement relative to the enclosure (7) during deposition; - Positioning (step 31) a mask (3) centered opposite the diopter (11), the mask (3) comprising openings forming a deposition zone (33); - Rotating (step 35) the mask (3) relative to the diopter (11) about an axis of rotation (19) passing through the center of the mask (3) and the diopter (11); - Starting (step 37) a deposit on the diopter (11) of a thin layer of a predetermined radial thickness by passing a vapor (27) of a material (13) through the deposition zone (33) of the mask (3) onto the diopter (11);and - Stopping (step 39) the rotation of the mask (3) relative to the diopter (11) and stopping the deposition of the thin film once the radial thickness of said thin film has reached the predetermined radial thickness.;

2. Method according to claim 1, wherein the diopter (11) is an aspherical lens or a plane plate.

3. A method according to any one of claims 1 and 2, wherein the mask (3) reproduces the shape of the diopter (11) opposite said diopter (11).

4. A method according to any one of claims 1 to 3, wherein the mask (3) is positioned at a distance between 0.1 millimeters and 2 centimeters from the diopter (11) and preferably between 1 millimeter and 2 centimeters from the diopter (11).

5. A method according to any one of claims 1 to 4, wherein the step (35) of rotating the mask (3) relative to the diopter (11) is configured so that at least one hundred rotations of the mask (3) relative to the diopter (11) are carried out during the deposition of the thin film, the rotation being carried out at a constant speed.

6. A method according to any one of claims 1 to 5, wherein the deposition zone (33) of the mask (3) has a shape configured to, once rotated and once the thin film deposition has started, simulate a radial exposure time of the diopter (11) to the vapor (27) of the material (13) so as to deposit the predetermined radial thickness of thin film on said diopter (11), the predetermined radial thickness being calculated so as to induce a phase shift modulus 2ir between the diopter (11) bare of the thin film and a part of the diopter (11) covered with the thin film.

7. A method according to any one of claims 1 to 6, wherein the optical index of the thin film material (13) is the same as the optical index of the diopter (11), preferably the thin film material (13) being identical to the diopter material (11).

8. A method according to any one of claims 1 to 7, wherein the deposition of the thin film on the diopter (11) is carried out under normal incidence on the diopter (11).

9. A method according to any one of claims 1 to 7, wherein the deposition on the diopter (11) of the thin film is carried out under incidence with the oblique angle deposition technology.

10. Assembly (1) for implementing the method according to any one of claims 1 to 9, comprising a mask (3) and a physical vapor deposition system (5) for thin films, said system (5) comprising an enclosure (7) housing a platform (9) on which to place the diopter (11), the material (13) configured to be vaporized, a first arm (15) supporting the platform (9), a second arm (17) configured to support the mask (3), said second arm (17) being rotatable about the axis of rotation (19) passing through the center of the mask (3), the first arm (15) being supported by a third arm (21) connected to the enclosure (7), the first arm (15) and the third arm (21) being rotatable so that the platform (9) is configured to perform an epicyclic movement relative to the enclosure (7).

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