ES2458271B2 - Holographic filter and holographic filter synthesis method - Google Patents

Abstract

Holographic filter and holographic filter synthesis method # Holographic filter and its synthesis method, based on a photopolymerizable glass that comprises a porous vitreous matrix in which an organic species and a species of high refractive index are introduced. The limitations in thickness of this type of photomaterials are overcome by an adequate selection of the amount of species of high refractive index in the sample, and by a control of the environmental conditions during the synthesis.

Description

Holographic filter and holographic filter synthesis method

Field of the invention

The present invention relates to the field of holographic filters, and more specifically, to a holographic filter synthesized in a photopolymerizable glass, to a method of synthesis thereof, and to its use as a bandpass filter and band suppression filter.

BACKGROUND OF THE INVENTION

A holographic filter is any optical device implemented on a holographic photomaterial whose refractive index can be locally modulated according to a three-dimensional pattern of light, allowing to act on the phase of the incident wave fronts on said photomaterial. Said modulation of refractive index (Lln) occurs due to physical, chemical, or physical-chemical changes in the material during a recording stage, in which the photomaterial is exposed to an interferential pattern generated by two coherent light beams.

Depending on the characteristics of the light beams used during recording, it is possible to implement holographic filters with very diverse properties and applications. For example, in recent decades the use of holographic filters has been extended for applications as varied as microscopy, pattern recognition, beam transformation, chemical spectra identification, and spectral filtering. In particular, one of the most frequent applications is the band suppressor filters (in English, notch filters), for example for suppression of the Rayleigh rejection line in Raman spectroscopy. These filters typically require a very narrow suppression band, with a very high extinction rate in said suppression band.

There are several requirements that must be fulfilled by a holographic photomaterial so that it allows to implement holographic filters of good quality. First, it is necessary to achieve a high modulation of refractive index, also called dynamic range, thus allowing to achieve high diffraction efficiencies.

..

Likewise, it is recommended that the index modulation be obtained as a result of a short exposure and intensity, that is, that the material present a high sensitivity, in order to reduce the effect on the instability filter in the recording assembly. of the same.

On the other hand, it is essential that the photomaterial present a good optical quality, with low dispersion (in English, scattering). There are several sources of scattering, among which we can highlight the quality of the surface, the dispersion generated by traversing the volume of the material, and the dispersion or noise recorded along with the holographic filter during the exposure process of the photomaterial.

Also, for use in applications with high power lasers (pulsed or continuous), it is necessary a low absorption, and a high threshold of laser damage.

Finally, it is necessary that the holographic photomaterial allows the synthesis of samples of high thickness, without in the process generating fractures or deterioration of the samples. The maximum thickness achievable with a photomaterial is an important factor, since it limits the volume in which the interference pattern that defines the holographic filter can be designed. For example, in the case of bandpass or band suppression filters, the bandwidth is inversely proportional to the thickness of the filter, so it is necessary to have high-thickness photomaterials to be able to implement very narrow filters.

A wide variety of holographic photomaterials are known in the state of the art, such as photopolymers, organic and inorganic photorefractives, dichromate gelatins, silver halides, photoresins, etc. Among them, the most promising results have been obtained in media based on photopolymers, such as those presented by US 4 994 347 A, US 4 970 129 A and US 3 993 485 A; And those based on porous glass that include light-curing materials, such as those presented by ES 2141 652 A1 and EP 0184856 A2.

However, all of them present limitations in the maximum index modulation, in the optical quality of the filters that can be implemented, in the maximum achievable thickness, and / or in the threshold of laser damage of the material. In addition, the vast majority require either very high exposure times, or multiple pulses of shorter duration. This results in a reduction in the quality of the filter, either due to variations in the recording conditions during the exposure time (vibrations, variations in the beam generated by the laser source, etc.), or due to changes in the surface of the holographic material produced before the end of the recording, which are transmitted in the form of noise or parasitic networks to the interior of the material by the recording beams that affect the photomaterial since said modifications are made until the end of the recording process.

In order to overcome the limitations associated with the modulation of refractive index and optical quality of the photomaterial, WO 03/077033 A2 presents a type of holographic photomaterial called photopolymerizable glass, synthesized by sol-gel techniques. The photopolymerizable glasses comprise a silicon-based matrix in which monomeric species and a photoinitiator are incorporated. The holographic recording in these light-curing glasses is based on their exposure to an interferential pattern generated by the superposition of two mutually coherent laser beams whose emission wavelength is capable of activating the photoinitiator, which in turn induces the polymerization of the monomer in the illuminated areas of the material. By increasing the length of the polymer chain, it loses mobility in the glass matrix, maintaining its position permanently, even after the exposure process ends. Also, the polymerization of the monomer causes the concentration of the same in the illuminated areas to decrease, which generates a diffusion of the remaining monomer from the areas with less intensity of the interference pattern towards the zones with greater intensity.

Subsequently, a second generation of light-curing glasses was able to overcome these results by incorporating a High Refractive Index Species (HRIS) into the material, specifically zirconium isopropoxide Zr (Oipr). 4 'accompanied by methacrylic acid to avoid hydrolysis (F. Del Monte, Ó Martínez-Matos, JA Rodrigo, ML Calvo and P. Cheben, "A volume holographic sol-gel material with large enhancement of dynamic range by incorporating high refractive index species ", Advanced Materials Volume 18, page 2014, 2006.) During the exposure of the holographic material, the concentration of HRIS is also permanently modified by the interference pattern, thus allowing to reach index modulation values of up to 10-2 , the highest reported to date, along with the increase in fJ.n, the relationship between the incorporation of particles based on in zirconium to optical films and the consequent increase in the sensitivity of the material and its gelation point.

The synthesis of the photopolymerizable glass with HRIS involves the hydrolysis of the zirconium and silicon alkoxides ({Si, Zr} (OR) n, with R an alkyl group), followed by a rapid polycondensation reaction. The use of this photomaterial for the recording of holographic diffraction gratings of transmission volume has demonstrated great versatility for different uses and experiments, maintaining high performance for a wide range of spatial frequencies and recording and reading conditions.

However, the inclusion of HRIS in the photomaterial generates an increase in the rigidity of the samples, resulting in fractures both during light exposure and in the absence thereof, these fractures being more common the greater the thickness of the sample synthesized. In particular, for the composition and synthesis method disclosed so far, it is only possible to synthesize holographic glasses with a maximum thickness of 120 microns, which limits the implementable applications in said glasses. For example, for bandpass or band suppression filters, this maximum thickness limit limits how narrow the bandwidth of the filter can become.

Therefore, there is a need in the art for holographic filters implemented in a photomaterial that allows to synthesize supports of a high thickness, presenting at the same time a high modulation of refractive index (ie, high dynamic range), high sensitivity and responsiveness to the recording beams, a high laser damage threshold, and high optical quality and low noise.

Description of the invention

The present invention solves the problems described above by means of a holographic filter implemented in a photopolymerizable glass synthesized by sol gel techniques, which incorporates a high refractive index species in a proportion that allows the synthesis of samples of high thickness, maintaining a high responsiveness and Dynamic range.

In a first aspect of the invention, a holographic filter implemented in a photopolymerizable glass synthesized by a sol-gel technique is presented, in which a solution with the components of the photopolymerizable glass undergoes a gelling process until an organic porous vitreous matrix is formed. inorganic In this matrix the following elements are incorporated:

-

At least one polymerizable organic species comprising a material whose refractive index changes locally upon exposure to actinic radiation. Note that throughout the document, actinic radiation means any electromagnetic radiation capable of causing chemical changes in the photomaterial, regardless of its nature or wavelength.

-

At least one photoinitiator that initiates the polymerization of the organic species upon receiving said actinic radiation.

-

At least one species of high refractive index, with a refractive index greater than the average refractive index of the vitreous matrix. In particular, said high refractive index species preferably contains a zirconium isopropoxide, preferably in a compound together with methacrylic acid. The high refractive index species accounts for a percentage of the total mass of the photopolymerizable glass between 5% and 15%, making it possible to obtain light-curing glasses with thicknesses above 250 microns. More preferably, the mass percentage of the high refractive index species is between 10% and 12%. Also preferably, the thickness of the photopolymerizable glass is equal to or greater than 500 microns. Note that the mass percentages of the high refractive index species refer to the totality of said high index species, that is, in one of the preferred cases, to the compound formed by zirconium isopropoxide and methacrylic acid, which is why the mass of the species of high index of refraction would be the sum of the mass of both components.

Preferably, the photopolymerizable glass is deposited on a mostly flat support, transparent to the recording and reading wavelength of the holographic filter, and with side walls that allow the deposition of a greater quantity of the liquid solution undergoing the gelation process. of the sol-gel technique. Said gelation begins when the support is sealed, preventing the components evaporated during gelation from leaving the support, and that air is introduced into it. After this first stage in which the support remains closed, and which has a typical duration around 24 hours, a second stage of gelation begins in which the sealing is gradually reduced by the progressive realization of holes in the seal, thus allowing to increase the air flow that is introduced into the support. This process is preferably carried out at a temperature between 35 ° C and 45 ° C. The total gel time of the samples depends on the thickness of the samples and the gelation temperature, with typical values around two weeks for samples of 500 microns in thickness at 40 ° C.

Note that higher temperatures result in the creation of pores of very different sizes in the vitreous matrix, which generates internal stresses that can result in fractures during the holographic filter recording process, or during the simple storage of the photopolymerizable glass. Also, lower temperatures result

In gelling times that are too high, to the point of preventing the matrix from gelling correctly.

Preferably, the recording of the filter is performed by a single pulse of light, lasting less than or equal to one second. Said pulse of light is divided into two coherent beams 15 which interfere in the volume of the photopolymerizable glass. Thanks to the high sensitivity, responsiveness, thickness, and dynamic range of the photopolymerizable glasses synthesized according to the present invention, a pulse of short duration and low power is sufficient to form diffraction gratings with diffraction efficiencies close to 100%. In this case, most of the diffraction network is not formed during the actinic exposure, but afterwards, due to a diffusion process in the dark through which there is a migration of the monomer and the species of high refractive index from the areas that have been poorly illuminated, to the highly illuminated areas, in which polymerization has begun to take place. The monomer diffused in this way also becomes part of the polymerization,

Thus increasing the refractive index modulation of the photopolymerizable glass.

As a result of the recording by a single pulse of limited duration, holographic filters of high quality and low noise are achieved. This is because, when using longer pulses, or multiple pulses, the recording conditions may vary, for example due to vibrations, or instabilities in the recording beams. All these factors modify the original interference pattern, reducing the quality of the filter. Also, during the formation of the network, deformations on the surface of any holographic support are generated. The scattering introduced by said deformations can be markedly reduced by applying an index compensating liquid, but if the deformations are present during the exposure (for example, if

occur as a consequence of the first pulse of a series of multiple pulses), the recording beams suffer the scaftering produced by the surface of the photomaterial, and transmit it permanently to the filter.

Two of the preferred options for the holographic filters implemented according to the invention are bandpass filters and band suppression filters (in English, "notch" filters), implemented by flat phase diffraction volumetric networks, operating well in transmission, good in reflection. These types of filters benefit directly from the increased thickness of the photopolymerizable glass, since their bandwidth is inversely proportional to the thickness of the photomaterial on which they are implemented. However, any other type of holographic filter implemented within the framework of the protection of the present invention (for example, filters for pattern recognition, beam transformation, etc.) is not discarded.

In a second aspect of the invention, there is presented a method of synthesis of a holographic filter comprising a first step of synthesis by sol-gel technique of a photopolymerizable glass, and a second step of recording the holographic filter in the synthesized photopolymerizable glass.

In the synthesis step of the photopolymerizable glass, an organic-inorganic porous vitreous matrix is generated by the sol-gel technique, in which at least one photoinitiator, at least one polymerizable organic species are incorporated, and at least one species with a high index of refraction. Said at least one species of high refractive index presents a refractive index is higher than the average refractive index of the vitreous matrix. Also, the organic species comprises a material whose refractive index changes locally according to an interference pattern of an actinic radiation of filter recording. The high refractive index species comprises zirconium isopropoxide, preferably accompanied by methacrylic acid to prevent its hydrolysis.

The high refractive index species accounts for a percentage of the total mass of the photopolymerizable glass of between 5% and 15%. This makes it possible to obtain photopolymerizable glasses with thicknesses above 250 microns. More preferably, the mass percentage of the species of high refractive index with respect to the total mass of the photopolymerizable glass is between 10% and 12%. Also preferably, the thickness of the photopolymerizable glass is equal to or greater than 500 microns.

The synthesis step of the photopolymerizable glass preferably comprises depositing the solution undergoing the sol gel process on mostly flat and transparent supports, which are initially sealed to prevent the exchange of gases between the support and the exterior. Next, gelation occurs in two stages. In a first stage, the sealing of the support is preserved. In the second stage, the sealing is progressively interrupted by progressive drilling. This process allows to avoid the appearance of fractures as a consequence of abrupt changes in the gelling conditions of the samples. More preferably, this gelling process is carried out at a controlled temperature between 35 ° C and 45 ° C. The typical times of gelation by this process are around two weeks for samples of 500 microns thick at 40 ° C, varying depending on the parameters of thickness and temperature.

The recording step of the holographic filters comprises exposing the photopolymerizable glasses to an actinic radiation formed by the interference of two mutually coherent light beams. Preferably, the two beams are the result of a single recording pulse whose duration is less than or equal to one second, thus achieving a remarkable improvement in optical quality with respect to other photomaterials known in the state of the art, according to the reasons mentioned in the description of the holographic filter of the invention.

Two of the preferred options of the recording step comprises recording flat diffraction gratings that act either as band-pass filters or as band-suppression filters, benefiting from the increased thickness of the photopolymerizable glasses.

With this holographic filter and its synthesis method, high thickness filters are achieved with great responsiveness, sensitivity, and optical quality, allowing the manufacture of optical devices with great freedom of design and low scaftering. These and other advantages of the invention will be apparent in light of the detailed description thereof.

Description of the figures

In order to help a better understanding of the characteristics of the invention according to a preferred example of practical realization thereof, and to complement this description, the following figure is included as an integral part thereof, the character of which is illustrative and non-limiting:

Figure 1 presents a schematic of the recording of a flat transmission diffraction network in a holographic photomaterial.

Figure 2 shows a schematic of the recording of a refractive reflection diffraction network in a holographic photomaterial.

Figure 3 presents an outline of the recording of a generic diffraction transmission network in a holographic photomaterial.

Detailed description of the invention

In this text, the term "comprises" and its derivations (such as "understanding", etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined can include more elements, stages, etc.

The photopolymerizable glasses on which the holographic filters of the invention are engraved are synthesized by means of a sol-gel technique in which a liquid solution is deposited on transparent supports, with side walls to allow a greater quantity of solution to be accommodated. The solution is then allowed to gel in a controlled environment. In a preferred embodiment of the invention, the silicon-based solution is generated by acid hydrolysis of 3-glycidoxypropyltrimethoxysilane (GPTMS) and tetraethylorthosilicate (TEOS). The molar ratio between GPTMS and TEOS is selected to minimize the shrinkage of the samples before actinic exposure. To the mixture of TEOS and GL YMO is added ethyleneglycolyphenylene acrylate (POEA) which acts as an organic monomeric species. When the mixture is homogenized, a solution of hydrochloric acid is added, and after ten minutes of mixing, the photoinitiator. In this particular case, the photoinitiator is Irgacure-784 (Bis (.eta.5-2,4-cilcopentadieno-1-yl) -bis (2,6-difluoro-3- (1 H -pyrrol-1-yl) -phenyl) ) titanium), which is introduced in solid state, without light exposure to the photosensitivity wavelengths. Finally, the species of high refractive index, composed of a solution of zirconium isopropoxide and methacrylic acid, is added. The resulting solution is filtered through a filter with a pore size of 0.2 IJm.

After the deposition of the solution in the supports, said supports are sealed with wax paper, and introduced in an oven with a stable temperature of 40 ° C. The gelation begins therefore with the sealed sample. After 24 hours, the sealing is interrupted by making a small hole in the wax paper. The number of holes increases progressively on subsequent days, gradually increasing the flow of air to the photopolymerizable glass.

The light-curing glasses synthesized by this method present a high optical quality, with a negligible scaftering, and with a great transparency once the photoinitiator is consumed, allowing to implement holographic filters of great optical quality, low noise, and capable of supporting the incidence of light beams. high power light due to its low absorption.

Once the organic-inorganic matrix of the photopolymerizable glass is formed, the filter is recorded. Figure 1 shows a diagram of the process of recording a diffraction network (3) in a holographic support (1). In this case, it is a flat diffraction network that acts as a diffraction bandpass filter and band suppression filter in transmission, but the process is extendable to any other index modulation pattern required for the implementation of the corresponding holographic filter. On the holographic support (1) there are two coherent recording beams (2, 2 ') that generate a light pattern of interference in the volume of the holographic support (1). This pattern gives rise to the local activation of the photoinitiator, which in turn produces the polymerization of the monomeric species and the migration thereof, as explained above. As a consequence of this joint process of polymerization and migration, the diffraction network (3) is generated, which is permanently recorded in the holographic support (3).

Once the diffraction network (3) has been recorded, if it is struck with one of the recording beams (2), two beams are generated, one transmitted (4), and one diffracted (4 '), whose powers and phase profiles will depend on the characteristics of the network. Likewise, if we incise with a beam with an arbitrary angle and wavelength (5), the corresponding transmitted (6) and diffracted (6 ') beams will be generated, with the response of the diffraction grating (3) for that angle and wavelength.

Figure 2 shows another example of diffraction grating (3) in a holographic support (1). In this case, the recording beams (2, 2 ') impinge on opposite faces of the photomaterial, creating a flat diffraction network in reflection. In the same way as in the previous case, if you hit with one of the recording beams (2), two beams are generated, one transmitted (4), and one diffracted (4 '), with the difference that in this In this case, the diffracted beam (4 ') is generated in the semi-plane of reflection instead of transmission. The same structure is maintained for a beam with arbitrary wavelength and angle (5), and its transmitted (6) and diffracted (6 ') beams.

Note that both in the case of the reflection and transmission hologram, the diffraction network is not limited to periodic plane structures, but can take any arbitrary form, as exemplified in Figure 3, in which the network of diffraction (3) of the holographic support (1) has an arbitrary structure generated by the interference of the two recording beams (2, 2 ') in the volume of the holographic support. The power and phase profile of the transmitted (4), and diffracted (4 ') beams generated by a beam of the same wavelength as the recording beam (2), depends on the characteristics of the network. Likewise, if we impinge with, the same will be generated with the corresponding transmitted (6) and diffracted (6 ') beams generated by a beam with an arbitrary angle and wavelength (5).

In any case, the possibility of recording the filter with a single pulse of short duration, forming the diffraction index modulation pattern by diffusion of the monomer and the species of high refractive index in the absence of actinic exposure, is a remarkable advantage over all photomaterials known in the state of the art. By reducing the recording time, instabilities and variations in recording conditions are not transmitted to the recorded filter, nor do surface deformations generate scattering transmitted permanently to the filter by successive recording pulses.

In an application example, band suppression filters with a bandwidth of 0.3 nm and a suppression ratio of -27.5 dB have been synthesized by a flat network with a spatial frequency of 2800 lines / mm and 510 microns in thickness. However, more selective filters can be generated by increasing the thickness of the samples and the spatial frequency of the network. Also, the large dynamic range and resolution of the present photomaterial allows the free design of holographic filters within a wide range of parameters, and with various applications, such as microscopy, pattern recognition, beam transformation, chemical spectra identification, and spectral filtering.

MODE OF CARRYING OUT THE INVENTION

The present invention is illustrated by the following examples, which are not limiting of its scope.

Example 1:

In a particular example of synthesis, the amounts of the different elements are the following: 240 mg of GPTMS, 37.5 mg of TEOS, 166.5 mg of POEA, 0.037 mg of H20

+ 0.8 mg of HCI, 4.4 mg of Irgacure, 30.13 mg of MA, and 30 mg of zirconium isopropoxide; considering a circular support of 35 mm in diameter and a thickness of 500 microns. One of the main advantages of the invention is the controllable thickness of the photopolymerizable glasses, since it suffices to modify the quantity of deposited solution, maintaining the proportions of the different components of the same, to obtain samples of different thickness.

Next, the solution is gelled following the procedure described. In total, the typical gel time for a 500 micron thick sample at 40 ° C is 14 days. Once the silicon-based matrix of the photopolymerizable glass is gelled, the filter is recorded using the techniques and mechanisms described above. In particular, for a photopolymerizable glass of 500 microns in thickness synthesized according to the method of the invention, it is possible to record a diffraction grating with an efficiency close to 100% with a single recording pulse of 1 second in length and an intensity of 3 mW / cm2. However, this power and duration will depend on the amount of photoinitiator and monomer in the sample, as well as the characteristics of the filter that you want to obtain. The photopolymerizable glasses synthesized according to the method of the invention have a large dynamic range, with greater modulations of the refractive index being possible with a simple increase in the recording power.

Example 2: In an application example, band suppression filters with a bandwidth of 0.3 nm and a suppression ratio of -27.5 dB were synthesized by a flat network with a spatial frequency of 2800 lines / mm and 510 microns of thickness. However, more selective filters can be generated by increasing the thickness of the samples and the

5 spatial frequency of the network. Also, the large dynamic range and resolution of the present photomaterial allows the free design of holographic filters within a wide range of parameters, and with various applications, such as microscopy, pattern recognition, beam transformation, chemical spectra identification, and spectral filtering.

In view of this description and figures, the person skilled in the art will be able to understand that the invention has been described according to some preferred embodiments thereof, but that multiple variations can be introduced in said preferred embodiments, without

15 leave the object of the invention as it has been claimed.

Nº application26 / 03 / 2015F.OEPM26 / 03 / 2015F.Efectiva

Claims (18)

1.

2.

3.

Four.

5. Holographic filter comprising a volume-phase diffraction grating permanently etched on a photopolymerizable glass synthesized using the sol-gel technique, and comprising the photopolymerizable glass prior to the recording of the holographic filter: at least

-

an organic-inorganic porous vitreous matrix;

-

at least one organic species polymerizable in said vitreous matrix,

the at least one organic species comprising a material whose index of refraction changes locally upon exposure to actinic radiation; - at least one species with a high refractive index, the refractive index of which is greater than the average refractive index of the glass matrix; -and at least one photoinitiator; characterized in that the species with a high refractive index represents a percentage of the total mass of the photopolymerizable glass comprised between 10% and 12%, and that the photopolymerizable glass has a thickness greater than or equal to 250 microns. Holographic filter according to the preceding claim, characterized in that the photopolymerizable glass has a thickness greater than or equal to 500 microns Holographic filter according to any of the preceding claims, characterized in that the high refractive index species is a compound comprising zirconium isopropoxide. Holographic filter according to claim 3, characterized in that the high refractive index species is a compound formed by zirconium isopropoxide and methacrylic acid. Holographic filter according to any of the preceding claims, characterized in that the holographic filter comprises a mostly flat transparent support for the photopolymerizable glass, said flat transparent support also comprising a side wall, and that the photopolymerizable glass is synthesized by means of a first gelling stage in which the transparent support is sealed, and a second stage in which the seal of the support is progressively removed by perforations 6. 7.

8.

9.

10.

successive, increasing the air flow to the light-curing glass. Holographic filter according to claim 5, characterized in that the photopolymerizable glass is synthesized by gelling at a controlled temperature between 35 ° C and 45 ° C. Holographic filter according to any of the preceding claims, characterized in that the holographic filter is engraved on the photopolymerizable glass by means of a single recording pulse with a shorter duration. or equal to one second, said pulse being divided into two coherent beams that interfere with the photopolymerizable glass. Holographic filter according to any of the preceding claims, characterized in that the phase diffraction grating is a planar grating adapted to act as a band pass spectral filter. Holographic filter according to any of the preceding claims, characterized in that the phase diffraction grating is a planar grating adapted to act as a band suppression spectral filter. Synthesis method of a holographic filter comprising: synthesizing by sol-gel techniques a photopolymerizable glass comprising an organic-inorganic porous glassy matrix; being incorporated into said glassy matrix at least one photoinitiator, at least one polymerizable organic species, and at least one species with a high refractive index, the refractive index of which is greater than the average refractive index of the glassy matrix; the at least one organic species comprising a material whose local refractive index changes upon exposure to actinic radiation; etching the holographic filter permanently on the synthesized photopolymerizable glass by actinic radiation of at least two interfering coherent light beams in the volume of the photopolymerizable glass; characterized in that the high refractive index species represents a percentage of the total mass of the photopolymerizable glass comprised between 10% and 12%, and that the photopolymerizable glass has a thickness greater than or equal to 250 microns. Synthesis method according to claim 10 characterized in that the glass

eleven.

12.

13.

14.

fifteen.

16.

17.

18.

light-curing has a thickness greater than or equal to 500 microns. Synthesis method according to any of claims 10 and 11, characterized in that the high refractive index species is a compound comprising zirconium isopropoxide. Synthesis method according to claim 12, characterized in that the high refractive index species is a compound formed by zirconium isopropoxide and methacrylic acid. Synthesis method according to any of claims 10 to 13, characterized in that the step of synthesizing the photopolymerizable glass using sol-gel techniques comprises in turn:

-

depositing a starting solution of the sol-gel technique on a mostly flat support, said mostly flat support also comprising a side wall;

-

seal the mostly flat support; -gelling the starting solution in a first gelling stage in which the support remains sealed;

-

progressively pierce the seal of the support during a second gelling stage, in which an air flow to the photopolymerizable glass is progressively increased.

Synthesis method according to claim 14, characterized in that the first gelling stage and the second gelling stage are carried out at a controlled temperature between 35 ° C and 45 ° C. Synthesis method according to any of claims 10 to 15, characterized in that the step of recording the holographic filter comprises a single recording pulse with a duration less than or equal to one second, said pulse being divided into two coherent beams that interfere with the photopoly glass. i merizable. Synthesis method according to any of claims 10 to 16, characterized in that the step of recording the holographic filter comprises recording a planar phase diffraction grating adapted to act as a band pass spectral filter. Synthesis method according to any of claims 10 to 16 characterized in that the step of etching the holographic filter comprises etching a planar phase diffraction grating adapted to act as a band suppression spectral filter.