FR3002934A1 - METHOD FOR MANUFACTURING A SEGMENTED MONOLITHIC GLASS OPTICAL STRUCTURE - Google Patents

Abstract

The present invention relates to a method for producing a monolithic segmented optical glass structure comprising at least the steps of - (1) having a glass substrate, - (2) structuring in 3 dimensions by laser ablation, in particular by femtosecond laser ablation, all or part of the outer surface of at least one of the faces of said glass substrate, - (3) planarizing the external surface of the microprisms forming the relief generated in (2) and freed of the ablation residues, with a precursor composition of a sol-gel material, said composition being deposited with a layer thickness adjusted to reduce the peak-to-valley gap of said outer surface, - (4) impose at least said coated surface of said layer a heat treatment conducive to the formation and densification of a layer of a solgel material.

Description

The present invention relates to methods for preparing segmented optical structures and more particularly to Fresnel lenses. A Fresnel lens consists of a lenticular central part generally surrounded by structures forming concentric rings (spherical or aspherical lens) or linear shapes (cylindrical lens). A Fresnel lens may have a smaller thickness compared to a conventional convergent lens and therefore advantageously a higher transmission optical coefficient with equal intercept surface. It also has a lower weight compared to a conventional convergent lens.

The optical quality of a Fresnel lens depends, among other things, on the shape of the Fresnel zones, the surface condition, the material and the vertical flanks separating two consecutive zones. Fresnel lenses are now highly valued in the photovoltaic field.

However, high-concentration photovoltaic modules that take advantage of these Fresnel lenses have a yield of about 26% with cells whose efficiency is close to 35 ° A. The losses are mainly attributed to the focusing imperfections of the Fresnel lenses used in most systems. A feasible approach to increase the efficiency of these photovoltaic modules would therefore be to significantly minimize or even overcome these imperfections. Moreover, it is clear that by increasing the concentration of sunlight, on a smaller area of photovoltaic cells required for the development of a solar panel, the final cost of the module will be advantageously reduced. . Most commonly, conventional Fresnel lenses are formed from a polymeric film (eg silicone or polymethylmethacrylate PMMA). The Fresnel structures are obtained by hot printing this film and are then placed on the inside of the tempered glass plates closing the module. The most common failures observed with these lenses are essentially yellowing, deformation (generating defocusing), cracking and delamination of this polymer film. In order to improve the effectiveness of these lenses, it has already been considered to optimize the polymer material to make it more efficient. However, another approach would be to manufacture optical structures only in glass. Glass is indeed a particularly interesting material with regard to its intrinsic properties, namely improved optical properties compared to a polymer film as described above, as well as stability, reliability, longer service life. for a lower maintenance cost.

Unfortunately, the manufacture of all-glass optical structures raises production problems directly related to the physical nature of the material. As the glass is amorphous, hard and non-ductile, it can not be machined using conventional machining means used in particular for metals. Admittedly, it is known, for example from Hansen et al. (CIRP Annals - Manufacturing Technology 60 (2011) 695-714), that a replication of the micro-and nanometric surface geometries of the glass, having a surface roughness conducive to optical applications, can be achieved by molding under a high temperature press , forging, or micro-injection molding. However, these methods have the disadvantage of obligatorily bringing the substrate beyond its glass transition temperature, often between 300 and 800 ° C. However, these temperatures are detrimental to the intrinsic mechanical properties of tempered glass. Moreover, the sudden contraction of the substrate during cooling at room temperature is an important source of imperfections in the structure obtained. In addition, the glass has, even in the liquid state beyond its transition temperature, a high viscosity that is not very compatible with the molding of micrometric patterns having obtuse and acute angles, such as in Fresnel lenses. Recent studies propose laser ablation to obtain such patterns. Tseng et al. For example, Optics and Lasers in Engination 45 (2007) 975-992 have fabricated on a glass substrate micrometric patterns such as microholes and furrows of varied geometry using an excimer laser. The advantages of this method consist in the low-thermal laser-material interaction and in a very localized ablation action, without damage to the untreated zones. However, the surfaces obtained by laser ablation have too much roughness to be compatible with an optical application. Finally, less conventional Fresnel lens fabrication methods have been proposed, for example using lithography (Lin et al., Electronic Letters 30 (1994) 448-449) or two photon photopolymerization (Guo et al. Optics Express 14 (2006) 2810-816). For obvious reasons, these methods are not suitable for the manufacture of a large lens or development on an industrial scale. There is therefore a need for a relatively simple method of implementation for manufacturing segmented optical glass structures, especially quenched, with increased optical efficiency.

There is also a need to have such a method for making segmented optical glass structures, especially quenched, large, compatible with an implementation on an industrial scale. There is also a need for a method for making segmented optical glass structures that do not require temperatures above the glass transition temperature of the substrate. The present invention aims to meet this need by proposing a manufacturing method for considerably reducing the roughness of the surface of a Fresnel lens obtained by structuring a glass substrate by laser ablation. Thus, according to a first of its aspects, the present invention relates to a method of manufacturing a monolithic segmented optical glass structure comprising at least the steps of - (1) providing a glass substrate, - (2) structuring in 3 dimensions by laser ablation, in particular by femtosecond laser ablation, all or part of the external surface of at least one of the faces of said glass substrate, - (3) planarizing the external surface of the microprisms forming the relief generated in (2 ) and freed of the ablation residues, with a precursor composition of a sol-gel material, said composition being deposited with a layer thickness adjusted to reduce the peak-to-valley gap of said outer surface, - (4) impose at least on said surface coated with said layer a heat treatment conducive to the formation and densification of a layer of a sol-gel material. For the purposes of the invention, the term "peak-to-valley gap", also referred to as the "peak-to-valley" difference in Anglo-Saxon, refers to the difference measured between the deepest hollow and the peak. the highest level of relief in 3 dimensions, created during laser ablation, necessary for the realization of microprisms.

Thus according to the invention, the peak-to-hollow gap is measured on the surface of the microprisms. The peak-to-valley gap can be measured by AFM or tactile profilometer. Against all expectations, the inventors have found that glass is an effective material for developing monolithic segmented optical structures in the image of Fresnel lenses with significantly improved efficiency provided that it is transformed according to the aforementioned method. Thus, as opposed to a polymeric film lens, the optical structures obtained according to the invention are advantageously monolithic. As a result, they are in no way subject to the phenomenon of detachment likely to occur at the level of a lens formed by the application of a polymer film to a glass substrate. Moreover, glass is not very sensitive to UV, unlike polymer films. This results in a longer stability over time in this respect. Similarly, the monolithic glass optical structures obtained according to the invention are also not affected by temperature variations. According to another of its aspects, the present invention also relates to a monolithic segmented optical glass structure having a profitability or efficiency greater than 40%, especially greater than 50%, in particular greater than 60%, or even greater than 70%. , and more particularly having at least one surface obtained by the process as defined above. PROCESS FOR PREPARING ACCORDING TO THE INVENTION

As stated above, the process requires, as starting material, a glass substrate whose transformation according to the invention will lead to the expected optical structure. This substrate is a monolithic glass substrate. In general, this glass can be chosen from soda-lime-silica glasses, ceramic glasses, borosilicate glasses or even lead glasses. In the context of the present invention, it is preferred the use of a borosilicate glass, particularly interesting in view of its low coefficient of expansion. More particularly, it is a toughened glass, particularly interesting with regard to mechanical properties reinforced in terms of resistance to external aggression. As an example of a tempered glass, it can be called tempered glass marketed under the name "Low IRON FLOAT GLASS USED FOR SILICONE-ON-GLASS LENS PARQUETS" of the company Reflexite. The refractive index gradient of the glass at ambient temperature and pressure can range from 1.4 to 1.9 and is preferably about 1.5.

The substrate may be of any shape, in particular circular or polygonal, for example square or rectangular. The size of the substrate, in particular its diameter, may be between a few mm and several tens of cm, for example between 1 mm and 100 cm.

By "substrate size", one must understand the largest dimension of the substrate. The size of the substrate generally depends on the application for which the segmented optical structure according to the invention is intended. As suggested above, the method according to the invention is particularly advantageous insofar as it lends itself to the production of large-area Fresnel lens. The thickness of the substrate may, for example, be between a few tens of microns and several tens of mm. The face of the substrate dedicated to be shaped according to the invention to carry the microprisms is advantageously flat. Alternatively, the face of the substrate may be non-planar, for example curved convex. As mentioned above, the shaping of microprisms is done by Laser.

The laser used to structure the substrate is advantageously a laser with ultra-short pulses, whose duration can be of the order of femtosecond, nanosecond or picosecond, and preferably is of the order of femtosecond. The usual sources of laser to obtain ultra-short pulses include an Nd: Yag laser at a wavelength of approximately 266, 355, 532 or 1060 nm or a Ti: sapphire laser at a wavelength of about 400 or 800 nm.

The pulses can be emitted at a power of a few mW to several tens of W, for example from 5 mW to 50 W, and preferably less than 2.5 W, at a frequency of a few Hz to several tens of kHz, or even of the order of MHz, for example from 5 Hz to 50 kHz, and preferably about 5 kHz.

The laser beam may be of the Gaussian type, for example of constant profile ("top-hat") or linearly varying, circular, polygonal or elliptical, and preferably has a beam diameter of 0.5 to 40 iam, in particular about 20 i.tm. Advantageously, the depth etched by ablation according to the laser parameters applied such as the power, speed and scanning mode, beam shaping, number of passage, is validated generally beforehand by a calibration. For obvious reasons, the realization of this preliminary calibration is clearly within the skill of the person skilled in the art. The structuring of the outer surface of at least one face of the glass substrate by laser ablation is intended to engrave a segmented optical structure thereon. The profile of this structure can be close to the theoretical profile of a lenticular structure such as the Fresnel lens. This adjustment to the theoretical profile of a lenticular structure is also clearly within the expertise of the skilled person. The microprisms preferably have a height of between 10 micrometers and 300 micrometers, more preferably between 100 and 250 micrometers. To perform the structuring in 3 dimensions to obtain a monolithic segmented optical glass structure, the laser beam can be mobile or stationary. When the laser beam is mobile, the substrate may be movable or stationary, and is preferably stationary. According to one variant, the laser and the substrate are both mobile. When the laser beam is stationary, the substrate is advantageously mobile.

According to a variant, the structuring is carried out using a mobile laser beam, preferably following rectilinear trajectories, or moving along concentric circles while said glass substrate is stationary. According to this variant, the laser beam can be moved linearly by crossing the passages at certain angles (for example 0, 36, 72, 108 and 144 °) while the substrate is stationary, in order to obtain a smoothest machined surface and According to another variant, the structuring can also be performed using a stationary laser beam, the glass substrate being movable and preferably being rotated about its central point.

When the laser beam is immobile, the glass substrate can also be moved along rectilinear paths by crossing the passages at certain angles (for example 0, 36, 72, 108 and 144 °). The beam of a stationary laser can be focused by an x20 lens. The structuring of the surface can be performed by one or more lasers per substrate. Advantageously, a laser is used by segmented optical structure to be etched. When only one structure is considered, the structuring can be performed by a single laser, for example with a mobile laser and a rotating substrate. When several optical structures are simultaneously considered on a substrate, the structuring can be performed by at least one mobile laser structure to be engraved on a stationary substrate. The space between the lines scanned by the laser beam may range from 0.1 to 30 μm and is preferably about 15 μm. The scanning speed of the laser beam or the displacement of the substrate may be constant or varied, and is preferably constant. This speed can range from a few mm / s to several m / s, for example from 5 mm / s to 5 m / s, and is preferably about 75 mm / s.

At the end of this step, the outer surface treated is thus provided with a three-dimensional etching. However, there may remain ablation residues in and around the ablation area that it is desirable to discard before any subsequent treatment of the 3-D structured or etched surface.

The method according to the present invention thus advantageously comprises an operation to rid the surface of these ablation residues, if present. According to this operation, the surface can be advantageously freed of the ablation residues without altering the protective properties of the glass surface, in particular the part of the glass which had not been structured during step (2).

Thus, according to a particular embodiment, the etched surface obtained at the end of step (2) is, prior to step (3), freed of the ablation residues by acid treatment. Preferably, this acidic treatment comprises at least bringing said at least one surface obtained at the end of step (2) into contact with a solution of hydrofluoric acid. In particular, the hydrofluoric acid solution can have a concentration between 10 and 60%, preferably between 15 and 50%, and can be brought into contact with said surface obtained at the end of step (2) during 1 at 60 min, preferably for 3 to 40 min, at a temperature between 20 and 50 ° C, especially about 30 ° C. In general, the surface treated in (2), or even freed of the ablation residues, has a "peak-to-hollow gap" of the order of 0.1 to 4 μm, preferably 1 i.tm, and in particular 2 iam, represented by inequalities of surfaces present on the external surface of the microprisms created. As specified above, the obtaining of an optical structure according to the invention 25 requires subsequent to the etching step (2) and, where appropriate, subsequent to the step described above aimed at cleaning the surface if necessary. treated of any ablation residues, a planarization step.

Dawson et al. (US 4,417,790) have suggested for simple ophthalmic lenses, from a usual manufacture and having surface roughness not suitable for optical applications, to treat the rough surface with a varnish finish to overcome the usual polishing for eliminate asperities. This method uses a layer thickness of the order 10 * Ra, where Ra is the arithmetic roughness, and therefore requires to completely cover the surface for the purpose of smoothing it. According to the invention, the planarization is to be distinguished from such a leveling step. It aims to significantly reduce the inequalities that may be generated during the laser engraving operation on the surface of the relief or the microprisms created during step (2). For obvious reasons, this step (3) must not alter the 3-dimensional relief that was precisely generated during the 3D structuring and which must be preserved to guarantee an optical efficiency to the expected monolithic segmented structure.

This planarization of the structured surface in step (2) is carried out according to the invention by forming a film of a sol-gel material on the surface of the asperities present in the microprisms constituting the etching or the 3D pattern of the face. engraved with the monolithic glass substrate. This film is made as detailed below, from a precursor composition of a sol-gel material. The production of a sol-gel material from such a composition is clearly within the skill of those skilled in the art. He knows that the principle of the sol-gel process, which makes it possible to obtain sol-gel materials such as glassy materials, ceramics and organo-mineral hybrids, is based on the use of a succession of hydrolysis-condensation reactions with moderate temperature to prepare oxide networks. The reaction is initiated by hydrolysis of the metal species allowing the formation of M-OH groups.

Thus, according to one embodiment, the precursor composition of a sol-gel material is a prehydrolysed metal oxide solution. This prehydrolysed metal oxide solution may be in advance of step (3) prepared from metal salts in aqueous solution or from metal alkoxides in organic solution. In one aspect, the prehydrolysed metal oxide solution may contain prehydrolysed silica. Those skilled in the art can adjust the conditions of the prehydrolysis to prepare the precursor composition of a sol-gel material to achieve a desired viscosity.

According to a preferred variant, the surface treated in step (2) and freed of the ablation residues undergoes, prior to its contact with the precursor composition of a sol-gel material, a surface activation conducive to the generation hydroxyl functions. This activation is intended to enhance the penetration and attachment of the sol-gel precursor in the porosity of the structured surface. This improvement can in particular be obtained via the generation of surface hydroxyl functions. This activation can be carried out by exposing said treated surface in (2) and freed from the ablation residues to UV irradiation, a plasma torch, an acidic and oxidative solution, and preferably a sodium hydroxide solution.

These pretreatments also make it possible to eliminate any organic matter physisorbed on the surface. As specified above, the formation of a film of a sol-gel material is made from a precursor composition of a sol-gel material which is applied in the form of a layer whose thickness is precisely adjusted to obtain the desired level of planarization according to the invention on the external surface of the microprisms constituting the relief.

To do this, said precursor composition of a sol-gel material may advantageously be deposited during step (3) with a layer thickness adjusted to reduce the peak-to-hollow gap of the external surface of the microprisms forming the relief by a factor of at least 4, preferably at least 10, and in particular at least 20.

The deposition of the precursor composition of a sol-gel material can be carried out by known methods, such as drawing or "spray-coating". In one embodiment, the precursor composition layer of a sol-gel material may be formed by drawing, in step (3), at the surface of the glass substrate.

In this embodiment, said glass substrate can be immersed, preferably vertically, in a vessel containing a precursor composition of a sol-gel material and can be extracted therefrom, the drawing speed and the viscosity of said composition. being adjusted to deposit a thickness of the precursor composition of a sol-gel material, adjusted to reduce the peak-to-hollow gap of the treated surface in (2) and cleared of the ablation residues. For example, the speed of the print may be of the order of 5 cm / min when the viscosity of the precursor composition of a sol-gel material is less than 50 centipoise. In particular, the draft can be performed vertically, forming an angle of 80 to 1000 relative to the surface of the precursor composition of a sol-gel material, and is preferably carried out in the normal direction relative to the surface. The formation of the layer of sol-gel material requires the application of a heat treatment to densify the layer of the precursor composition deposited in step (3) and evaporate the residual solvents and residual precursors. According to a particular embodiment, the heat treatment of step (4) is conducted at a temperature of at least 100 ° C and in particular at about 200 ° C, for at least 1 h and in particular for about 2 hours. This heat treatment can be carried out by known means, for example by thermal annealing in an oven or oven.

The layer of the sol-gel material thus densified advantageously has a thickness sufficient to smooth in part the surface roughness microprisms of the relief created in step (2), typically of the order of 1 to 2 microns. Preferably, the refractive index of said layer of densified sol-gel material is lower than that of the glass on which it is deposited. The thickness of the layer of the densified sol-gel material resulting from step (4) is advantageously less than the peak-to-hollow gap characterizing the reliefs generated in step (2). Advantageously, this thickness makes it possible to obtain a peak-to-hollow gap after step (4) of less than 300 nm, in particular at 200 nm and better still at 100 nm.

The thickness of the layer of the sol-gel material deposited and densified can be measured by methods known to those skilled in the art, for example afm or tactile profilometer. Operating Conditions The steps of laser structuring and possible acid treatment can take place in an enclosure whose temperature is generally adjusted to a value ranging from 3 ° C to 50 ° C. The steps of laser structuring, possible acid treatment, eventual surface activation, planarization and heat treatment may, for example, take place in an enclosure in which there is a pressure of between atmospheric pressure and several tens of bars. The steps of laser structuring, possible acid treatment and eventual surface activation can take place in a chamber in which a relative humidity of between 10% and 70%, preferably between 20 and 55%, prevails.

The planarization step may take place in an enclosure in which a relative humidity of between 10% and 40% prevails.

By "relative humidity", it is necessary to understand the ratio of the partial pressure of water vapor present in the enclosure, said enclosure having a temperature and a given pressure, on the saturation vapor pressure of water at said temperature and pressure. of the enclosure.

Advantageously, at each of the steps of the method according to the invention, the surface temperature of the substrate does not exceed the glass transition temperature of the substrate, so that the process does not damage the surface mechanical properties when the substrate is glass tempered.

SEGMENTALLY OPTICAL STRUCTURE OBTAINED ACCORDING TO THE INVENTION The geometry of a variant of the segmented optical structure obtainable according to the invention is shown schematically in FIG. 1. The monolithic segmented optical glass structure obtained according to the invention can exhibit a size of at least 10 mm and especially several tens of centimeters, for example about 26 or 34 cm. By "size of the segmented optical structure" is meant the dimension of the largest structure, for example the diameter for a circular structure. It may have one or more focal point (s), and preferably has a focal point. The focal length (s) may range from a few centimeters to several tens or even hundreds of centimeters, for example from 5 cm to 500 cm, and is / are preferably about 240 mm for a 160 mm lens. of diameter. The focal length (s) may be in particular about 1.5 times the size of the corresponding segmented optical structure. The monolithic segmented optical glass structure manufactured by the method of the present invention may consist of a lenticular central portion surrounded by concentric annular segments. In particular, the monolithic segmented optical glass structure may be a lenticular structure, and is preferably a Fresnel lens.

When the structure corresponds to the structure of a Fresnel lens, these concentric annular segments are called "Fresnel zones". The monolithic optical segmented glass structure obtained according to the invention advantageously has a profitability greater than 40%, especially greater than 50%, in particular greater than 60% or even at least 70%. Description of the Figures The invention will be better understood on reading the following detailed description, non-limiting examples of implementation thereof, and on examining the appended drawing, in which: FIG. 1 schematically represents a variant of the segmented optical structure seen from the side; FIGS. 2A, 2B and 2C as well as 3A, 3B and 3C represent photographs of the Fresnel lenses obtained in Examples 1 and 2, respectively.

The photographs at 2A and 3A show the lens as obtained at the end of step b) (acid treatment), in front of a non-recognizable pattern through the translucent lens; photographs 2B and 3B show the same installation with the lens as obtained at the end of step f), the pattern being clearly visible; and photographs 2C and 3C show the on-screen focusing of a white light beam of the lens as obtained at the end of step f). FIGS. 4A to 4D schematically represent the surface state of the microprisms created by the laser structuring, comprising a magnification of the surface, at the different stages of the method according to the invention; FIG. 4A gives an account of the surface state microprisms as obtained at the end of step (1), the frosted surface having a peak-to-hollow gap 1 and having ablation residues 2; FIG. 4B shows the surface state of the microprisms as obtained at the end of step (2), the surface of which has been freed of the ablation residues, before the eventual activation of the surface; FIG. 4C shows the surface state of the microprisms as obtained at the end of step (2), whose surface 3 has been activated; FIG. 4D shows the surface state of the microprisms as obtained at the end of step (3), planarized with a sol-gel material 4, the peak-to-small gap 1 being reduced.

Preparation of a Fresnel Lens Example 1: BK7 Glass Substrate a) Structuring the Substrate by Laser Ablation A substrate is available in the form of a BK7 type glass plate marketed by the acm company. dimensions 5x5 cm2 and thickness 1.2 mm.

Laser ablation is performed with a femtosecond laser of the type Ti: Sa (Bright Raymax Lasers, at a wavelength of 800 nm, a maximum power of 2.5 W and a pulse frequency of 5 kHz) with a Gaussian beam (20 μm beam diameter) for a Fresnel pattern having a diameter of 10 mm, a constant height of 100 μm, and a focal length of 17 mm (corresponding to a number of Fresnel zones of 10). During ablation, this laser is moved in concentric circles with a scanner whose working area is 34x34mm, and the substrate is held fixed. The space between the lines scanned by the scanner is 15 μm. The peak-to-hollow gap present on the external surface of the microprisms of the generated relief is evaluated at about 2.2 μm measured by tactile profilometer. b) Acid treatment to rid the ablation residues The substrate obtained at the end of step a) is immersed in a 40% (v / v) hydrofluoric acid bath at 30 ° C. for 3 minutes with stirring by ultrasound. c) Surface Activation The substrate obtained at the end of step b) is immersed in a solution of sodium hydroxide (0.7 g of sodium hydroxide diluted in 100 ml of ethanol and 75 ml of water) in order to create OH bonds on the surface of the glass and to increase the wettability of the substrate and homogeneity of the deposit of Sol-Gel solution on the surface. d) Synthesis of the precursor composition of the Sol-Gel material A precursor solution of the Sol-Gel material is prepared by (i) pre-hydrolysis for 1 hour at room temperature with stirring of a solution containing: - 4 ml of Tetramethylorthosilicate, 16 mL of Trimethoxymethylsilane, 1.32 mL of hydrochloric acid (0.6 M), (ii) addition of 1.23 mL of H2O to the solution from (i), and (iii) aging at room temperature under stirring for 72 hours. The viscosity of the precursor composition of the Sol-Gel material is less than 50 centipoise. e) Planarization of the lens The surface-activated lens obtained after step c) is immersed in a tank containing the solution prepared in step d) and the precursor composition of Sol-Gel is deposited by vertical pulling of the tank substrate at a speed of 5 cm / min. f) Thermal treatment A thermal annealing of 200 ° C. for 2 hours was applied to the structured and planarized lens resulting from step e) in order to densify the deposited layer and evaporate the solvents and precursors remaining present in the material.

The peak-to-hollow gap on the outer surface of the microprisms in the relief is evaluated at 0.29 μm, measured by tactile profilometer. It was therefore reduced by a factor of 2.2 / 0.29 = 7.5. Example 2: Tempered Glass Substrate The procedure described above for Example 1 was repeated replacing the BK7 type glass substrate with a substrate in the form of a tempered glass plate marketed under the name " Low IRON FLOAT GLASS USED FOR SILICONEON-GLASS PARQUET LENS "from the company Reflexite of dimensions 5x5 cm2 and thickness 6 mm.

The peak-to-hollow deviation measured by tactile profilometer is evaluated at 2.2 μm after the laser structuring step a) and before step b), and at 0.29 μm after the treatment step f). thermal. It was therefore reduced by a factor of 2.2 / 0.29 = 7.5. Example 3 Characterization of the Optical Properties The Fresnel lenses obtained in Examples 1 and 2 were first visually characterized. Photographs of these two lenses are shown in Figures 2A-2C (Example 1) and 3A-3C (Example 2). The surface of the non-planarized lens is diffusing and conceals a pattern at the back of the lens (Photographs 2A and 3A). A projected shadow of the lens appears on the screen placed behind the lens.

On the other hand, the same pattern is clearly visible when it is behind the planarized lens (Photographs 2B and 3B). A focal point of the lens is present on the screen (photographs 2C and 3C). The planarized Fresnel lenses of Examples 1 and 2 were then characterized using a HeNe laser emitting at 633 nm. The collimated beam passes beforehand in a beam expander in order to illuminate the entire surface of the lens (diameter 10 mm). A detector is placed at the focal point of the lenses (17mm). From these results, the effectiveness of these lenses has been determined as the entire flux arriving on the detector with respect to the initial flux assuming that the detector integrates the specular and diffuse portion of the beam passing through the lens, and presents a value of 72% for example 1 and 78% for example 2. Unless otherwise stated, the expression "comprising / including a" should be understood as "comprising / including at least one". Unless otherwise stated, the expression "included (e) between. and ... "must be understood as included boundaries. Unless otherwise indicated, the expression "about" or "of the order of" indicates a degree of tolerance of ± 30%, better than ± 20%, and still more preferably ± 10% of a given value.

Claims (13)

REVENDICATIONS1. A method of manufacturing a monolithic segmented optical glass structure comprising at least the steps of (1) providing a glass substrate, (2) structuring in 3 dimensions by laser ablation, in particular by femtosecond laser ablation, all or part of of the outer surface of at least one of the faces of said glass substrate, (3) planarizing the outer surface of the microprisms forming the relief generated in (2) and freed of the ablation residues, with a precursor composition of a sol material -gel, said composition being deposited with a layer thickness adjusted to reduce the peak-to-valley gap of said outer surface, - (4) impose at least said coated surface of said layer a heat treatment conducive to the formation and densification of a layer of a sol-gel material. 2. Method according to the preceding claim wherein the structuring is carried out using a moving laser beam, preferably following rectilinear trajectories, or moving in concentric circles while said glass substrate is stationary. 3. Method according to claim 1, wherein structuring is carried out by means of a stationary laser beam, the glass substrate being movable and preferably being rotated about its central point. 4. Method according to any one of the preceding claims wherein, prior to the planarization of step (3), the surface obtained at the end of step (2) is freed from the ablation residues by a treatment. acid. 25 5. Method according to the preceding claim wherein the acid treatment comprises at least bringing said at least one surface obtained at the end of step (2) into contact with a solution of hydrofluoric acid. 6. Method according to the preceding claim wherein said hydrofluoric acid solution has a concentration between 10 and 60%, preferably between 15 and 50%, and is brought into contact with said surface obtained at the end of the step ( 2) for 1 to 60 min, preferably for 3 to 40 min, at a temperature between 20 and 50 ° C, especially about 30 ° C. 7. Method according to any one of the preceding claims wherein the surface treated in (2) and freed of the ablation residues undergoes, prior to its contact with the precursor composition of a sol-gel material, an activation of surface conducive to the generation of hydroxyl functions. 8. The method of claim 7 wherein said activation is carried out by exposing said treated surface in (2) and removed ablation residues to UV irradiation, a plasma torch, an acidic and oxidative solution, or a solution of soda, and preferably a sodium hydroxide solution. 9. A process according to any one of the preceding claims wherein the precursor composition of a sol-gel material is a prehydrolysed metal oxide solution. 10. A method according to any one of the preceding claims wherein the layer of precursor composition of a sol-gel material is formed by drawing, in step (3), on the surface of the glass substrate. 20 11. A method according to any one of the preceding claims wherein the heat treatment of step (4) is conducted at a temperature of at least 100 ° C and in particular at about 200 ° C, for at least 1h and in particular for about 2 hours. 12. A method according to any one of the preceding claims wherein the monolithic segmented optical glass structure is a lenticular structure. 13 "Method according to the preceding claim wherein the monolithic segmented optical glass structure is a Fresnel lens. A method according to any one of the preceding claims wherein the monolithic segmented optical glass structure has a size of at least 10 mm and in particular of about 26 or 34 cm. A monolithic segmented optical glass structure having at least one surface obtained by the method of any one of claims 1 to 14.