CH712016B1 - A method of manufacturing an optical element for a timepiece. - Google Patents

Abstract

The invention relates to a method of manufacturing an optical element (110, 122) intended to be integrated into a glass (100) of a timepiece. One such method comprises the steps of providing a substrate (90) in a translucent or transparent material; to modify the structure of at least one zone of the substrate (90) by a laser in order to make said zone more selective for chemical etching and to carry out chemical etching of said zone in order to form a geometric structuring, and / or to modify the structure of at least one zone of the substrate by a laser in order to modify the refractive index of said zone. The invention also relates to a one-piece crystal (100) and a timepiece, in particular a watch, comprising such an optical element (110, 122).

Description

Technical area

The present invention relates to a method of manufacturing an optical element for a timepiece, in particular a watch. The present invention also relates to a crystal of a timepiece comprising such an optical element. By crystal is meant not only the element commonly called thus and which, integrated into the case of a watch, makes it possible to see the displays through, but also any other crystal such as a crystal used inside a watch, such as a dial or a similar display element or participating in the reading of information given by the mechanism of a watch. Such an optical element can be formed on its own, or in combination with other optical elements, such as an optical system modifying the path and / or the properties of light in the optical element.

State of the art

For the manufacture of optical elements, the methods used are limited. Essentially, machining and polishing processes are used (see for example US6406769). In this way, however, optical elements can only be produced by modifying the relief of the surface of the ice, and thus obtain only simple optical elements, by forming a single diopter or else by forming a lens whose the two diopters form the two opposite faces of the ice.

For certain materials, shaping is carried out by ultraviolet ray lithography (see for example US2628160), which greatly limits the application to specific classes of glasses, often doped with elements impacting their optical appearance.

[0004] Furthermore, the refractive index of the crystal cannot be modified, which greatly limits the optical systems used in watchmaking. To partially overcome this drawback, it is also possible to manufacture separately an optical system or part of the complex optical system and to add it, for example by driving, to the watch crystal. However, this solution remains limited to certain geometries of optical systems and certain arrangements of optical systems in the mirror. Furthermore, this solution has many drawbacks, including manufacturing made more complex, a modified appearance and a seal made more fragile.

Brief summary of the invention

An object of the present invention is to provide a solution making it possible to manufacture a timepiece crystal with at least one optical element, free from the limitations of known solutions.

Another object of the invention is to provide a method of manufacturing an optical element intended to be integrated into a timepiece crystal. Indeed, the invention aims to produce a one-piece lens including one or more optical element (s) originating from the same material, or more precisely from the same substrate as the lens.

Another object of the invention is to make it possible to produce complex optical systems.

Another object of the invention is to make it possible to produce one or more optical element (s) arranged either on at least one of the faces of the mirror, or within the mirror without extend to one of the faces of the ice, i.e. both on at least one of the faces of the ice and inside the ice. Such an optical element may in particular include simple lenses, such as for example for magnification, or else lenticular arrays, such as for example those used to achieve various optical effects.

Another object of the invention is to make it possible to produce one or more optical element (s) whose refractive index is different from the rest of the material of the crystal.

Another object of the invention is to make it possible to produce one or more optical element (s) having a portion opening out at least on one of the faces of the crystal and / or of the substrate and having a relief contributing to the modification of the light path.

Another object of the invention is to provide a method of manufacturing an optical element that is easy to implement, whether for small series or large series.

According to the invention, these goals are achieved in particular by means of a method of manufacturing an optical element intended to be integrated into a timepiece crystal, comprising the following steps: providing a substrate in a material translucent or transparent; and modifying the structure of at least one area of the substrate with a laser in order to make said area more selective for chemical etching, and performing chemical etching of said area in order to form a geometric pattern; and / or modify the structure of at least one zone of the substrate with a laser in order to modify the refractive index of said zone.

It is understood that the laser beam modifies the structure of the material of an area of the substrate, in a way (either by increasing the reactivity to chemical etching or by modifying the refractive index of the material of this area) which creates or helps to create an area in which the light path is different from the light path before this laser treatment.

[0014] Thus, the optical element may result from the modification of the optical properties of the exposed zone of the substrate and / or of the optical properties of the resulting geometry after ablation of the exposed zone, if necessary.

This optical element belongs to an optical system. Such an optical system can be simple, in particular if it only comprises said optical element, or more complex, in particular in the presence of several juxtaposed optical elements. In the latter case, it is possible to have an association of optical elements, one or more of which arose from the modification of the refractive index in one or more adjoining zones, and / or including one or more others (in particular which lead to the surface of the substrate) result from a chemical etching creating a relief with depressions obtained by chemical etching and removal of material from the area treated by laser.

The term "translucent or transparent" material is understood to mean a material which allows all or part of the light rays to pass having a wavelength visible to the human eye as well as the wavelength (s) of the laser used for modify the structure.

[0017] The invention also relates to a one-piece watch piece crystal comprising at least one optical element obtained by the manufacturing method according to the present invention.

The present invention also relates to a timepiece, in particular a watch, comprising a one-piece crystal comprising at least one optical element obtained by the manufacturing process according to the present invention.

This solution has the particular advantage over the prior art of making it possible to modify in any zone of the substrate, at the surface, at the core, both at the surface and at the core, according to any geometry. (desired shape and dimensions) the structure of the material constituting this zone to contribute to the formation of an optical element in this zone.

Preferably, a laser is used, the pulse duration of which is between 1 femtosecond and 1000 femtoseconds.

According to a first aspect of the invention, there is provided a method of manufacturing an optical element intended to be integrated into a timepiece crystal, comprising the following steps: providing a substrate in a translucent or transparent material ; modifying the structure of at least one zone opening onto one of the faces of the substrate with a laser in order to make said zone more selective for chemical etching; and chemical etching of said zone in order to form a geometric structuring contributing to the production of said optical element.

In this case it is understood that the optical element is selectively manufactured by removing material in said zone.

In one embodiment, said zone opens onto one of the faces of the substrate, in the case where the modification of the structure makes said zone more selective for chemical etching and where the method comprises a step of chemical etching of said zone in order to form a geometric structuring contributing to the production of said optical element. Thus, said optical element is selectively manufactured.

According to a second aspect of the invention, there is provided a method of manufacturing an optical element intended to be integrated into a timepiece crystal, comprising the following steps: providing a substrate in a translucent or transparent material ; and modifying the structure of at least one zone of the substrate by a laser in order to modify the refractive index of said zone.

In this case it is understood that the optical element is selectively manufactured by differentiating the refractive index between said zone and the rest of the substrate. More precisely, said zone has a refractive index modified by the laser radiation and all around said zone, the material has an unmodified refractive index which is equal to that of the material constituting the starting substrate.

In one embodiment, the first aspect of the invention is implemented in a first area of the substrate and the second aspect of the invention is implemented in a second area of the substrate which is different from the first zoned. In this case, at least one optical element is formed in a first region of the substrate by removal of material in said first region of the substrate and at least one optical element is formed in a second region of the substrate by modification of the refractive index.

In another embodiment, the second aspect of the invention is implemented in a second area of the substrate and the first aspect of the invention is implemented in a first area of the substrate which is at least partially included in the first zone. In this case, at least one optical element, which is located in a location common to the first zone and to the second zone, is formed both by material removal and by modification of the refractive index.

Advantageously, the material of the substrate is one of the following materials: glass, mineral glass, corundum, quartz, sapphire, synthetic ruby, polycrystalline ruby, silica, a glass ceramic, a ceramic and a polymer.

Brief description of the figures

Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which: Figure 1 schematically illustrates an enlarged and partial sectional view of a glass with an optical element in progress obtaining, according to a first embodiment of the invention; FIGS. 2A and 2B illustrate a top view of two variants of a lens with an optical element in the process of being obtained, according to the first embodiment of the invention; FIG. 3 is a partial enlarged sectional view of a lens with an optical element, according to a second embodiment of the invention; FIG. 4 is an enlarged sectional view of a glass with two optical elements, according to a third embodiment of the invention; FIG. 5 is an enlarged sectional view of a glass with two optical elements, according to a variant of the third embodiment of the invention; FIG. 6 illustrates in partial section a glass with two variants of an optical element, according to a fourth embodiment of the invention; FIG. 7 illustrates in partial section a glass with a series of optical elements forming a complex optical system, according to a fifth embodiment of the invention; FIG. 8 illustrates in partial section a glass of the prior art with an antireflection layer; and FIG. 9 illustrates in partial section a glass according to a sixth embodiment of the invention having an anti-reflective layer.

Example (s) of embodiment of the invention

Figures 1, 2A and 2B illustrate a first embodiment of a glass 100 in which one implements the manufacturing method according to the first aspect to form as an optical element a polarizing layer. To this end, the manufacturing method according to the invention is used to form a geometric structuring in the form of stripes 101 which will make it possible to orient the polarizing particles of a layer (not shown) which is deposited on the upper face of the crystal. 100 covering the stripes 101.

Preferably, the geometric structuring comprises nanometric stripes101. Indeed, stripes having nanometric dimensions, at least for the width and depth of the stripes 101, are suitable for the formation of a polarizing layer. These scratches 101 constitute hollows, in the form of indentations, notches or grooves dug in the face of the starting substrate 90 thanks to the structural modification carried out by the laser in each point of material to be removed in order to hollow out the scratches by chemical etching.

Preferably, the scratches 101 of the ice 100 according to the invention are produced by the “Selective Laser-induced Etching” (SLE) or “In-Volume Selective Laser-induced Etching” (ISLE) technique.

To this end, the scratches 101 are obtained as follows: a) a disc-shaped substrate 90 is provided, obtained by machining or polishing, intended to form the ice; b) a laser is provided with a pulse duration which can range from femtosecond (10 <-15> second) to picosecond (10 <-12> second); c) the structure of the substrate 90 is modified throughout the volume of material to be removed to form the stripes 101; d) a liquid chemical agent is provided which allows the material of the volume of the substrate whose structure has been modified by the laser in the previous step, to be dissolved more rapidly than the other areas of material whose structure has not not changed; e) the substrate 90 with the modified structure volume is immersed in a bath composed of the liquid chemical agent, the substrate 90 is maintained for a predetermined time in the bath so that all of the material of the structural volume amended is dissolved; and f) the part 100 thus formed is taken out of the bath, and it is washed in order to remove all traces of the chemical agent and thus stop the chemical reaction between the liquid chemical agent and the material of the substrate.

The glass 100 shown in FIG. 1 is then obtained directly with an upper face having stripes 101.

The modification of the structure of the substrate 90 in step c) is only possible by using a material for the substrate 90 which is transparent to the laser. In practice, with the focal point of the laser, point to point, the entire volume of the substrate 90 intended to form the scratches is traversed, including a portion of the face of the substrate or close to the surface of the substrate. A modification of the structure of the material is thus achieved by absorption with several photons, which requires a particularly high energy density.

In step d), the local modification of the structure by the laser makes it possible to choose a liquid chemical agent which is more reactive in the volume of modified structure than in the other unmodified areas.

Thus, preferably, the scratches 101 of the glass 100 according to the invention are obtained by a method of selective chemical attack, implemented after a preliminary step of modifying the structure of a three-dimensional zone of the substrate in front of be engraved.

Next, the method of manufacturing the optical element further comprises an additional step of depositing a polarizing layer on the geometric structuring, namely on the stripes 101.

This polarizing layer (not shown) preferably comprises dichroic compounds, at least part of the dichroic compounds being molecules or fragments of molecules with planar structures and forming a crystal lattice in the polarizing layer.

Advantageously, as seen in Figure 2A and in Figure 2B, the stripes 101 are distributed in groups of stripes substantially parallel to each other.

In general, the use of the ISLE technique makes it possible to control very precisely the orientation of the stripes 101 and therefore, following to control the polarizing effect by setting the polarizing effect with great definition.

In addition, thanks to the manufacturing method according to the first aspect of the invention, it is possible to form scratches on any type of material. In fact, traditionally, the scratches are obtained by satin-finishing the surface over a few nanometers, with a ceramic powder, the hardness of which is insufficient to treat a sapphire substrate.

Also, thanks to the manufacturing method according to the first aspect of the invention, it is possible to constitute in a timepiece crystal a multitude of different optical elements, and in particular birefringent optical elements, polarizing ...

FIG. 3 illustrates a second embodiment of an ice 100 in which the manufacturing method of the invention according to the first aspect is implemented to form, as an optical element, a diopter 102 on the surface of the substrate 90 forming the ice 100. For this purpose, the manufacturing process is used to form a geometric structure in the form of selective removal of material.

In the example shown in Figure 3, the geometric structuring comprises a non-plane diopter 102 produced on the lower face 100a of the glass 100. It 'acts in this case of a convex diopter 102. It will be understood that other types of non-planar diopter can be produced very easily by virtue of the manufacturing method according to the invention.

In an embodiment not shown, the geometric structuring comprises reliefs forming graphic signs and symbols. Thus, for example, these reliefs represent a logo and / or a number, a letter, a symbol so that by assembling these graphic signs it is possible to include information relating to the timepiece, the series of pieces of watchmaking, or other references. In particular, we can make the geometric structuring forming these reliefs on the side face 100b of the glass 100, forming the edge (see Figures 4 to 6).

FIG. 4 illustrates a third embodiment of an ice 100 in which the manufacturing method according to the second aspect of the invention is implemented to form an optical fiber 110 as an optical element. Indeed, the manufacturing process is used to form a geometric pattern in the form of modification of the refractive index of the substrate.

More specifically, there is a substrate 90 made of a material with a refractive index n3. In this case, the modification of the structure carries out a first modification of the refractive index of a first zone (with a modified refractive index n1) and a second modification of the refractive index of a second zone (with a modified refractive index n2) surrounding said first zone. The modified refractive index is lower in the second zone than in the first zone (n2 <n1). The first zone forms the core 111 of an optical fiber 110 and the second zone forms the cladding 112 of the optical fiber 110.

Thus, in a first step, the structural modification is carried out by the laser in a first zone intended to form the core 111 of the optical fiber 110, and having a refractive index n1. Preferably, the core 111 of the optical fiber has a circular section. Then, in a second step, the structural modification is carried out by the laser in a second zone intended to form the cladding 112 of the optical fiber 110, and having a refractive index n2 different from n1. Preferably, n1> n2. Preferably, the cladding 112 of the optical fiber has an annular section.

The order of the first step and the second step can be reversed during the manufacture of the optical fiber 110.

The path of the light in the optical fiber 110, within the core 111 of the optical fiber, is indicated by the reference sign 20.

In Figure 4, the glass 100 is shown with two optical fibers 110 rectilinear and parallel to each other extending from one edge to the other of the glass, opening onto the side face 100b forming the edge.

Preferably, at least one end of the optical fiber opens onto one face of the substrate 90. In FIG. 4, for each optical fiber 110, the two ends of the optical fiber 110 open onto one side of the substrate, in particular on the side face 100b.

The use of laser structuring makes it possible to control very precisely the dimensions and the refractive index of each part of the optical fiber, namely the core 111 and the cladding 112, and makes it possible to carry out, in situ in ice 100, optical fibers having all possible orientations, sizes and geometries.

Thus, in the case of the variant of the third embodiment of FIG. 5, curved optical fibers 110 'are produced by means of the manufacturing method according to the invention. Two curved optical fibers 110 ′ have been placed one above the other in the glass 100 of FIG. 5. In the upper part of FIG. 5, the curved optical fiber 110 ′ has two ends which open out onto the glass. side face 100b. In the lower part of FIG. 5, the curved optical fiber 110 ′ has a smaller radius of curvature and has two ends which do not open onto any face of the glass 100.

In general, in the case of the variant of the third embodiment, it is possible to produce, by virtue of the manufacturing method according to the invention, an optical fiber having a non-rectilinear direction.

Furthermore, machining the optical fiber by structuring material directly in the glass makes it possible to achieve very small radii and complex geometries for the optical fibers.

For example, such rectilinear optical fibers 110 or curved 110 'can be used alone, or in a bundle to allow light guidance to present hidden elements, display elements in unconventional places (corners of a square watch, ice edge).

FIG. 6 illustrates a fourth embodiment of an ice 100 in which the manufacturing method according to the invention is implemented both according to the first aspect and according to the second aspect. By implementing the manufacturing process according to the first aspect, a diopter 122 is formed as an optical element on the surface of the crystal 100, in particular on the lateral face 100b forming the edge of the crystal 100.

To this end, in order to conform the diopter 122 by removing material from the substrate 90, another modification of the structure of the substrate 90 must be made which makes a third zone, not shown, located at the end of the optical fiber 110 by extending the diopter 122, which is more selective for chemical etching, in order to selectively manufacture said diopter 122 as an optical element. For this, said manufacturing method according to the invention further comprises a step of chemical etching of said third zone in order to achieve a geometric structuring delimiting a non-planar diopter 122.

In addition, by the implementation of the manufacturing method according to the second aspect is formed as an optical element an optical fiber 110 with a core 111 having a first refractive index n1, surrounded by a sheath 112 having a second refractive index n2. As in the case of the third embodiment of FIG. 4, the optical fiber 110 is rectilinear and its two ends of the fiber open onto the lateral face 100b of the glass 100. Furthermore, in the case shown in FIG. 6, said diopter 122 is convex and together with the end of optical fiber 110 constitutes a converging lens forming a magnifying glass 123. Furthermore, said magnifying glass 123 extends the end of the core 111 of optical fiber 110 to form an optical system 110 ".

Two optical systems 110 "associating an optical fiber 110 and a magnifying glass 123 are thus produced in the glass of FIG. 6. In particular, as shown in the optical system 110" of the upper part of FIG. 6, said magnifier 123 has a refractive index n1 identical to that of the first zone forming the core 111 of the optical fiber 110. Regarding the other optical system 110 "of the lower part of FIG. 6, said magnifying glass 123 has a refractive index n2 identical to that of the second zone forming the cladding 112 of the optical fiber 110.

In this fourth embodiment of an ice 100, we create a lens (magnifying glass 123) integrated (monoblock) to a waveguide fiber 110 having a chosen refractive index, which may be that of the heart of the fiber 111 (refractive index n1) or of the cladding of the fiber 112 (refractive index n2). This has the advantage of combining the magnifying effect and the guiding of the light on a solid element and having no assembly play due to its one-piece nature.

According to a fifth embodiment of the manufacturing method according to the invention, implementing the second aspect, the modification of the structure produces a modification of the refractive index of said zone relative to the rest of the substrate, said zone delimiting a lens.

In a first variant, the modification of the structure produces a first modification of the refractive index of a first zone forming a first lens and of a second zone forming a second lens having the same optical axis as the first. lens and being contiguous with the first lens. Here, a first lens and a second lens are formed having the same refractive index, different from that of the substrate 90.

In a second variant, the modification of the structure produces a first modification of the refractive index of a first zone forming a first lens and a second modification of the refractive index of a second zone forming a second lens having the same optical axis as the first lens and being contiguous with the first lens. In this second variant of the fifth embodiment, a first lens and a second lens having a different refractive index between them and which is different from that of the substrate 90 are formed.

By way of example to illustrate this fifth embodiment, FIG. 7 shows an ice 100 in which has been manufactured directly in the material of the substrate 90 a complex optical system 130 comprising a series of lenses 131, 132, 133 and 134 aligned and contiguous with each other, having the same optical axis. In this example, the two lenses 131 and 134 forming the ends of the complex optical system 130 are planar convex lenses, the planar face being turned in the same direction for the lens 131 and for the lens 134 (in this case upwards in figure 7). The convex face of the lens 134 is flush with the lower face 100a of the crystal 100. A biconvex lens 132 is disposed under the upper end lens 131 and a third planar convex lens 133 is disposed between the biconvex lens 132 and the lens d. lower end 134.

In this way, we obtain a complex optical system 130 monobloc, manufactured with precision, which guarantees in particular the alignment of the lenses 131, 132, 133 and 134, and which is not fragile since it is integrated in the material of the substrate 90.

Such complex optical systems 130 can be manufactured in a simple manner to produce a magnifying glass with a complex or compound geometry, making it possible in particular to correct geometric and / or chromatic aberrations of simpler optical systems, such as a magnifying glass.

We turn to Figures 8 and 9 showing a sixth embodiment of an ice implementing the manufacturing method according to the second aspect. In this case, the modification of the structure brings about a modification of the refractive index of a zone relative to the substrate 90, said zone delimiting an antireflection layer.

Traditionally, as shown in FIG. 8, one or more antireflection layer (s) are produced by the successive deposition of thin layers having different refractive indices. Thus, the layer 140 of thickness e has the refractive index n and is placed on a substrate 90. The incident light 20 is thus reflected along the ray 20 ′ reflected on the surface of the layer 140 and along the ray 20 ″. reflected by the transition surface between layer 140 and substrate 90.

In Figure 9, there is obtained a layer 140 of constant thickness obtained by the manufacturing process according to the second aspect of the invention, which is located slightly below the upper face 100a. According to a variant not shown, the layer 140 is located directly on the surface of the ice 100 and delimits the upper face 100a. The modification of the refractive index of the substrate 90 (for example made of sapphire or organic glass) by laser irradiation directly into the material makes it possible to produce the antireflection layer 140 very simply and with great dimensional precision. manufacturing by dispensing with complicated depositions process to master

Reference numbers used in figures

90 Substrate 100 Glass 100a Upper or lower face 100b Side face 101 Scratches 102 Non-plane dioptre 110 Straight optical fiber 110 'Curved optical fiber 110 "Optical fiber with magnifying glass 111 Core of the optical fiber 112 Cladding of the optical fiber 122 Dioptre non-plane 123 Magnifier 130 Complex optical system 131 Convex plane lens 132 Biconvex lens 133 Convex plane lens 134 Convex plane lens 140 Anti-reflective coating

Claims (26)

1. A method of manufacturing an optical element intended to be integrated into a glass (100) of a timepiece, comprising the following steps: providing a substrate (90) in a translucent or transparent material; modifying the structure of at least one area of the substrate (90) with a laser in order to make said area more selective for chemical etching, and performing chemical etching of said area in order to form a geometric pattern; and / or modify the structure of at least one zone of the substrate (90) by a laser in order to modify the refractive index of said zone. 2. The manufacturing method according to claim 1, wherein the duration of the laser pulses is between 1 femtosecond and 1000 femtoseconds. 3. The manufacturing method according to claim 1, wherein said zone opens onto one of the faces of the substrate (90), and in the case where modification of the structure makes said zone more selective for chemical etching and where the method comprises a step of chemical etching of said zone in order to form a geometric structure. 4. The method of claim 3, wherein the geometric structuring comprises nanometric stripes (101). 5. The method of claim 4, further comprising an additional step of depositing a polarizing layer on the geometric structuring. 6. The method of claim 5, wherein said polarizing layer comprises dichroic compounds, at least some of the dichroic compounds being molecules or molecule fragments with planar structures and forming a crystal lattice in the polarizing layer. 7. The manufacturing method according to claim 4, wherein the stripes (101) are distributed in groups of stripes substantially parallel to each other. 8. The manufacturing method according to claim 3, wherein the geometric structuring comprises at least one non-plane diopter (102). 9. The manufacturing method according to claim 3, wherein the geometric structuring comprises reliefs forming graphic signs and symbols. 10. The manufacturing method according to claim 1 wherein the modification of the surface structure takes the form of a lenticular network. 11. The manufacturing method according to claim 1, wherein the modification of the structure brings about a modification of the refractive index of said zone relative to the substrate (90), said zone delimiting an antireflection layer (140). 12. The manufacturing method according to claim 1, wherein the modification of the structure carries out a first modification of the refractive index of a first zone and a second modification of the refractive index of a second zone surrounding said first zone. zone, the modified refractive index being lower in the second zone than in the first zone, said first zone forming the core (111) of an optical fiber (110) and said second zone forming the cladding (112) of said optical fiber (110). 13. The method of claim 12, wherein at least one end of the optical fiber (110) opens onto one face of the substrate (90). 14. The method of claim 13, wherein another modification of the structure makes a third zone, located at the end of the optical fiber (110), more selective to chemical etching, said method further comprising an etching step. chemical of said third zone in order to achieve a geometric structuring delimiting a non-planar diopter (122). 15. The method of claim 14, wherein said diopter is convex and forms with the end of the optical fiber (110) a converging lens forming a magnifying glass (123). 16. The method of claim 15, wherein said magnifying glass (123) extends the end of the core (111) of the optical fiber (110). 17. The method of claim 15, wherein said magnifying glass (123) has a refractive index identical to that of the first zone forming the core (111) of the optical fiber (110). 18. The method of claim 15, wherein said magnifying glass (123) has a refractive index identical to that of the second zone forming the cladding (112) of the optical fiber (110). 19. Method according to one of claims 12 to 18, wherein said optical fiber (110) has a non-rectilinear direction. 20. The manufacturing method according to claim 1, wherein the modification of the structure effects a modification of the refractive index of said zone relative to the rest of the substrate (90), said zone delimiting a lens (131, 132, 133). , 134). 21. The method of claim 20, wherein the modification of the structure carries out a first modification of the refractive index of a first zone forming a first lens and of a second zone forming a second lens having the same optical axis as. the first lens and being contiguous with the first lens. 22. The method of claim 20, wherein the modification of the structure achieves a first modification of the refractive index of a first zone forming a first lens and a second modification of the refractive index of a second zone forming. a second lens having the same optical axis as the first lens and being contiguous to the first lens. 23. Method according to one of claims 1 to 22, wherein the material of the substrate (90) is one of the following materials: glass, mineral glass, corundum, quartz, sapphire, synthetic ruby, polycrystalline ruby, silica, a ceramic and a polymer. 24. One-piece watch piece crystal (100) comprising an optical element obtained according to a manufacturing process according to one of claims 1 to 23. 25. Timepiece comprising a crystal (100) according to claim 24. 26. Timepiece according to claim 25, said timepiece being a watch.