Patent Application
20230348314
The present invention relates to novel compositions of oxide glasses that are photosensitive and transparent in the visible and the infrared. More particularly, the present invention relates to glasses that are photosensitive and transparent for wavelengths between 400 nm and 800 nm in the visible spectral range and between 800 nm and 8000 nm in the infrared range.
The present invention relates also to a method for writing structures of modulation of refractive index in the volume of such a transparent and photosensitive glass by femtosecond laser beam irradiation. In particular, the method is suitable for producing three-dimensional structures of modulation of refractive index to form a Bragg grating.
A Bragg grating describes, generally, a periodic modulation of optical refractive index produced in a transparent material in order to filter the incident light. The Bragg grating reflects the incident light at a particular wavelength, called Bragg wavelength and transmits the other wavelengths of the spectrum. The efficiency of the spectral response of a Bragg grating depends partly on the following parameters: the pitch of the refractive index modulation or the pitch of the grating Λ (or the spatial frequency f = 1/Λ), the length of the grating and Δn the amplitude of the refractive index modulation, as well as the index modulation profile constituting each modulation period. It is thus possible to optimize the diffraction efficiency of the grating by adjusting the different parameters, notably the modulation period and the modulation amplitude.
A Bragg grating can be produced in guided configuration, in the core of an optical fiber, or in free space, in the bulk of a substrate. In the second configuration, it is a bulk Bragg grating which is an essential optical component used notably for the wavelength stabilization of lasers on the one hand, and also for the spectral filtering in high-resolution spectroscopy on the other hand.
One conventional means of obtaining a Bragg grating in a transparent material consists in submitting photosensitive transparent material to a lighting with spatial profile of sinusoidal type obtained by the interference of two beams at the wavelength of sensitivity of the material, in order to modulate the refractive index provoked by a variation of the distribution of charges within the glass. The grating is then stabilized and made permanent by curing techniques.
Germanium-doped silicate material is known for producing the optical fibers in telecommunications. However, the amplitude of the variation of optical refractive index induced by UV insulation is limited more often than not to some 10-5.
It is also known practice to use a novel, so-called photothermoreactive (PTR) material to produce a bulk Bragg grating. This is a glass composed of a mixture of oxides, silica, zinc and aluminum and doped with photosensitive silver ions, fluorine and cerium. The index variations are obtained according to a photothermic process based on the precipitation of dielectric microcrystals inside the glass, once the latter has been exposed to a UV radiation and thermally treated beyond the glass transition temperature. This material can be set in the form of a thin and easily polishable blade because of its composition and its glassy nature. The glass obtained is transparent in the visible and offers a transmission range between 0.3 and 3 microns. However, this material is not very well suited for optical applications demanding a spectral light transmission window beyond 3 microns. Moreover, the notion of transparency has to be modulated according to the applications targeted: as well as being transparent in the infrared, the existing losses do not allow realistic applications in terms of laser sources beyond 2 µm.
To obtain an efficient bulk Bragg grating, it is essential to be able to work in a wide spectral band in order to be able to cover the high-energy optical applications. It must have a perfect control of the periodicity of the grating with great spatial resolution. Moreover, the index modulation must be as high as possible, generally greater than some 10-3.
The emergence of femtosecond laser sources has made it possible to develop direct 3D laser writing technologies in transparent materials such as glass. However, no direct laser writing technique has been demonstrated satisfactorily to allow the direction inscription of optical refractive index modulation structures of submicron dimension in depth in a transparent glass beyond 3 microns.
There is therefore a need for novel glasses that are transparent both in the visible and infrared ranges up to 8 microns, to be able to be incorporated in high-energy optical applications. Another object of the present invention is to propose a photosensitive transparent glass that has a composition suitable for allowing a bulk photo-structuring by a short and ultrashort pulsed laser beam, in order to be able to produce three-dimensional structures of high optical refractive index modulation, generally greater than some 10-3, with a submicronic spatial resolution, and with great repeatability.
One subject of the present invention therefore relates to transparent glasses based on oxides of silica, of phosphate or of germanium containing photosensitive silver ions suitable for bulk inscription of a structure by a femtosecond laser beam.
The transparent glass according to the present invention comprises at least 99% to 100%, by weight, with respect to the total weight of the material, of a composition of the following formula (I):
The glass according to the present invention comprises at least 99%, by weight, with respect to the total weight of the material, of a composition of the following formula (II):
in which
In a particular embodiment, the oxide chosen to form the glassy matrix is an oxide of germanium. The compositions according to this embodiment will be called germanates.
The glass according to the present invention comprises at least 99%, by weight, with respect to the total weight of the material, of a composition of the following formula (III):
in which
According to an embodiment of the invention, the glass further comprises halogenated compounds (fluoride, chloride, bromide) whose function is to modulate the photosensitivity or to facilitate the shaping and the purifying of the glass.
According to an embodiment of the invention, the glass further comprises dopants supplementing the composition of the formula (I), (II) or (III) to reach the 100% weight. According to the invention, the dopants are chosen from among the following metallic ions: Ag+, Au3+, Cu+.
According to an embodiment of the invention, the glass as defined above exhibits a transmission greater than 90% in a range comprised between 400 nm and 8000 nm.
Another subject of the present invention relates to a method for writing a three-dimensional structure of variation of refractive index by a femtosecond laser beam in a photosensitive transparent oxide glass comprising silver ions as defined above, the method comprising:
According to an embodiment of the invention, the variation of refractive index Δn is a positive variation of at least greater than 10-3.
The features set out in the following paragraphs can, optionally, be implemented. They can be implemented independently of one another or in combination with one another:
According to an embodiment of the invention, the structure produced is formed by at least one plane of variation of refractive index, the thickness of said plane being less than 200 nm, substantially equal to 80 nm.
According to another embodiment of the invention, the structure produced is a periodic structure comprising a plurality of planes of variation of refractive index to form a bulk Bragg grating, with a grating pitch Λ between 200 nm and 1.5 µm.
According to another aspect of the invention, also proposed is a bulk Bragg grating comprising a grating of planes of variation of refractive index, the variation of refractive index being greater than 10-3, the thickness of each plane being less than 200 nm, preferably substantially equal to 80 nm, the pitch of the grating between 200 nm and 1.5 µm.
Other features, details and advantages of the invention will become apparent on reading the following detailed description, and on analyzing the attached drawings, in which:
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For greater clarity, the elements identical or similar are identified by identical reference symbols throughout the figures.
In the context of the present disclosure, “glass” is understood to mean an amorphous inorganic solid, exhibiting the glass transition phenomenon. Glass is obtained by cooling from a liquid phase.
In the context of the present disclosure, “transparent” is understood to mean a material that can be seen through. The transparency of a material is specified by measurements of transmission of a light beam. A material is considered transparent for a given wavelength when its transmittance is greater than or equal to 90% excluding Fresnel reflection.
In the present description, the terms “material” or “materials” designate the transparent glasses of the present invention.
In the context of the present disclosure, the numbers x, a, b, c and d relating to the reference composition of the formula 1 represent molar proportions. Furthermore, in the present invention, when a number is indicated comprised between two values, the limits indicated are included in the range of values. Thus, “x lies between 25 and 35” is understood to mean that x is comprised between 25 and 35, 25 and 35 being included.
In the context of the present disclosure, “femtosecond laser” is understood to mean a laser which delivers pulses of a duration comprised between a few femtoseconds and a few hundreds of femtoseconds.
In the context of the present disclosure, “repetition rate” is understood to mean the number of laser pulses per second. When the delay between two successive pulses is shorter than the thermal relaxation time of the glass, there is thermal accumulation and the temperature of the material at the point of impact of the beam increases progressively. This thermal charge induces a zone of physical-chemical modification around the irradiation point, in order to inscribe a structure of variation of refractive index. It should be noted that the thermal accumulation is weak in the present method, with a temperature rise much lower than the glass transition temperature. That means that there is no melting/annealing of the glass under laser irradiation, nor any significant modifications of the glassy matrix: there is only a photo-activation of the mobility of the silver ions, with the pulse-after-pulse creation of a local variation of index supported by the spatial distribution of new silver species created in the process.
In the context of the present disclosure, “focusing zone” is understood to mean a zone of interaction resulting from the impact of the spot of the laser beam in a focal plane situated at a depth in the glass.
In the context of the present disclosure, “inscription of a structure in bulk in a glass” is understood to mean an inscription of a structure of local variation or modulation of optical refractive index at a depth of the glass induced by impacts of the laser beam, linked with the result of the photochemistry induced on the silver elements but without modifying the structure of the glassy matrix.
In the context of the present disclosure, “submicronic resolution” is understood to mean a spatial resolution between 5 nm and 1 µm, preferably between 5 and 500 nm.
In the context of the present application, “sub-diffraction” is understood to mean a resolution lower than the optical resolution limited by the diffraction of the light at the wavelength considered.
The drawings and the description hereinbelow contain, for the most part, elements of a certain nature. They will therefore not only serve to give a better understanding of the present invention, but will also contribute to the definition thereof, where appropriate.
The glasses are produced according to a conventional glassmaking method associated with a choice of the compositions of formula (I) of the present invention.
The manufacturing method comprises the following successive steps:
In a last step, the glass is cut to a given thickness, to a thickness of 1 mm for example. This thickness can be adapted to greater thicknesses according to the requirements, notably for the production of bulk Bragg gratings, the height of which can be several mm, then optically polished on two parallel faces for the phase of structuring by a femtosecond laser beam.
The starting oxides and their possible precursors are in conventional commercial powder form. The oxide precursors can be in a carbonate form. For example, a precursor of Na2O can be Na2CO3 and that of K2O can be in the form of K2CO3. In this case, the mixture then undergoes a decarbonation treatment in order to eliminate the CO2 in order to obtain the oxide of the composition.
The glass according to the present invention, which is photosensitive and transparent, comprises a composition of the following formula (I):
In the formula (I) above, the oxides Oxy1 represent the glass-forming oxides.
According to the invention, the oxides of silicon, of germanium or of phosphate are associated with oxides of gallium. The two oxides represent the two essential components of the materials of the present invention.
In the materials according to the present invention, contrary to the materials of the prior art, the materials according to the present invention comprise a significant Na2O and BaO content. The addition of the oxides Oxy3 makes it possible to contribute to the mobility of the silver ions and to confer particular properties of inscription and of reinscription of structures of variation of refractive index by a laser beam of femtosecond pulse duration. The oxides Oxy2 make it possible to reduce the melting temperature and to minimize the problems of crystallization.
In an embodiment, the material of the present invention further comprises silver ions to confer the property of photosensitivity of the material. This feature is essential to the direct structuring induced by femtosecond laser of photoluminescent patterns resulting from a nonlinear phenomenon provoked by the multiphotonic absorption of the material which makes it possible to form silver aggregates. In particular, the materials of the present invention favor the formation of silver aggregates linked to the interaction of silver ions with the femtosecond laser with high repetition rate and with a local spatial distribution of these aggregates, allowing the inscription of structures of variation of refractive index. According to the present invention, by shrewdly associating ions such as Na2O and BaO with the silver ions, the applicants have found that it is possible to re-inscribe a structure of variation of refractive index in a zone that has already undergone an irradiation.
The materials of the present invention are also transparent in the visible range and in the infrared range. This feature is necessary to allow the use of these materials to produce optical components such as bulk Bragg gratings that are effective for the visible, between 400 nm and 800 nm and the infrared between 800 and 8000 nm.
According to an exemplary embodiment of the invention, the glass is a silver-doped phosphate-gallium glass in which the composition is formulated according to the following relationship (II):
in which
An example of glass prepared according to the composition (II) will be presented hereinbelow.
According to another exemplary embodiment of the invention, the glass is a silver-doped germanium-gallium glass in which the composition is formulated according to the following relationship (III):
in which
An example of glass produced according to the composition (III) will be described hereinbelow.
For the exemplary embodiments of structures of variation of refractive index presented hereinbelow, the femtosecond laser used is a t-Pulse 500 laser (marketed by Amplitude Systems). The maximum power is 2.6 W.
The femtosecond laser emits a laser beam having a wavelength lying between 1000 nm and 1100 nm. The wavelength of the laser is chosen so as to be at least two times greater than the cutoff wavelength of the glass of the present invention, a wavelength from which the glass absorbs the light. For the exemplary embodiments, the wavelength can be chosen close to 1030 nm. The emission wavelength of the sapphire-titanium around 800 nm would also be suitable.
The laser is a femtosecond laser. However, the invention can be implemented provided that the pulse duration is less than 1 picosecond, preferably lying between 0.5 ps and 500 fs.
The method for writing structures comprises a configuration in which the chosen repetition rate is between 10 kHz and 100 MHz. While most of the demonstrations of activation of photochemistry of silver have been performed around 10 MHz, observations at 80 MHz, based on a laser/glass interaction from a sapphire-titanium oscillator have already been performed. In fact, this range of repetition rate makes it possible to favor the formation of aggregates and stabilize them.
The parameters of the laser beam such as the repetition rate, the number of pulses and the irradiance, are adapted and controlled to irradiate the glass of the present invention so as to be able to inscribe and re-inscribe three-dimensional structures of variation of optical refractive index at a given depth of the glass without modifying the crystalline structure of the glass. For that, the device further comprises an acousto-optical modulator 102 (AOM) placed at the output of the laser source, on the trajectory of the laser beam. By adjusting the amplitude, the duration and the period of the modulation voltage, it is possible to set the irradiance (power of the beam per unit of surface area), the number and the repetition rate of the pulses of the laser beam passing through the modulator.
The device comprises a microscope lens 103 which makes it possible to focus the material at a determined depth in the bulk of the glass. The numerical aperture of the microscope lies between 0.4 and 1.57 in the case of oil-immersion lenses of very strong numerical aperture. A trade-off in the numerical aperture can be envisaged according to the thickness of the bulk Bragg grating to be produced, according to the refractive index of the glassy matrix, even also the period targeted for the Bragg wavelength targeted for an effective first-order resonance: ideally, to obtain ideal periodicities and therefore optimal efficiencies, it will be recalled that the size D should preferentially be greater than the period targeted, while however taking care to obtain the greatest possible index modulations. The structures have been created in the bulk, typically at a depth of 160 µm under the surface of the sample, the productions having been done with lenses in air and in oil, with numerical apertures of 0.75 and 1.3, respectively. Thus, the structures can be formed at different depths under the surface of the glass. In the exemplary embodiments described hereinbelow, the microscope lens in air focuses the laser beam with a numerical aperture of 0.75, which corresponds to a focal spot of the order of 1.5 µm in diameter leading to index modifications at a distance D ranging from 1.6 to 1.8 µm, typically. In the case of the lens in oil used (NA = 1.3), beam diameters and therefore distances D ranging from 600 nm to 800 nm have been obtained, typically. Focusings with NA < 0.7 are often to be proscribed because they can be accompanied by additional non-linear self-focusing processes, leading to possible distortions of the focus and thus energy deposition that is less well controlled and less well localized spatially. The laser beam is focused at 160 µm under the surface of the glass.
Moreover, the device can comprise a fluorescence and phase contrast microscope to respectively visualize the distribution of the silver aggregates which emits fluorescence and the modification of refractive index in the structured zones of the sample after irradiation according to the method of the present invention.
The sample 10 is disposed on a high-precision plate 105 that is motorized in translation in all three directions with a precision of the order of 30 nm, in order to ensure the correct positioning of the laser beam in the glass. The sample is disposed such that the incident radiation of the beam is preferably at normal incidence on the sample. As
The emergence of femtosecond laser sources has made it possible to develop 3D direct laser writing technologies in transparent dielectric materials. However, to date, no inscription technology has been proposed for bulk inscribing in a silver-doped oxide glass to induce a positive variation of optical refractive index.
The applicants have surprisingly found that, by controlling the parameters of the laser beam, namely the irradiance, the number of pulses or the relative speed of movement between the beam and the sample and the repetition rate of the pulses, and by choosing glasses with suitable oxide compositions, that it is possible to produce, locally in the bulk of these photosensitive silver-doped oxide glasses, a photochemical phenomenon which induces a positive variation of refractive index of the glass in a peripheral zone around the irradiation point. The applicants further show that, by controlling the parameters of the laser beam, it is also possible to erase the refractive index generated in a preceding irradiation in a portion of this zone of variation of refractive index, by making the portion of this zone coincide with an intense zone of the laser beam (not necessarily the center of the beam) where the intensity is sufficiently high on this portion to induce a photodissociation of silver aggregates accumulated around the irradiation point, which causes the variation of index generated by the distribution of silver aggregates, which are then photodissociated, to be erased. Likewise, the applicants show that it is possible to re-inscribe a zone of variation of refractive index in a zone that has already undergone an erasure of variation of optical index. In other words, the parameters of the laser beam are controlled so as to always maintain, in a zone of the glass having undergone an irradiation, a reservoir of silver ions that is sufficient to ensure a reinscription, that is to say to be able once again to generate an accumulation of silver aggregates in a peripheral zone around the irradiation point.
By virtue of this inscription and reinscription process, and by controlling the parameters that are the irradiance, the repetition rate of the pulses, the number of pulses and the relative speed of movement between the sample and the laser beam, and the positioning between two successive irradiation points, the applicants show that it is possible to produce a grating of planes of variation of refractive index. By producing a series of planes of variation of index, and by ensuring an overlapping of these planes, it is then possible to optimize the geometrical dimension of the index modulation zones and therefore propose the production of a bulk Bragg grating.
With reference to
In a first phase of the interaction of the laser during a femtosecond laser pulse, the glass is photoexcited by nonlinear absorption. This is reflected by the generation of a gas of quasi-free electrons which are rapidly trapped by the Ag+ ions to form Ag0 atoms. The nonlinear nature of the interaction confines the distribution of the Ag0 atoms in a zone slightly smaller than the diameter of the laser beam, represented by a dotted line circle in
In a second phase, in the case where the characteristic thermal diffusion time is greater than the time interval between two laser pulses which lies between 10 µs and 12.5 ns (corresponding to laser repetition rates of 10 kHz to 80 MHz), the temperature of the glass increases locally during the successive deposition of the pulses and generates a scattering of the Agmx+ metal species from the center (greatly concentrated) to the periphery (weakly concentrated). This migration is represented by the arrows in
In the examples presented below, the glass comprises only silver ions. In other embodiments, the metal aggregates are aggregates of gold or of copper. In another embodiment, the material comprises ions of different natures such as gold, copper or silver in different or equal quantities.
The next pulse has the effect of destroying the silver aggregates by a process of photodissociation in the central region of the volume of interaction where the intensity is greater than an intensity that is sufficient to degrade the silver aggregates previously inscribed. Simultaneously, this new pulse regenerates free electrons which are once again trapped to form aggregates on the peripheral zone only.
This sequencing of physical-chemical phenomena and the succession of the pulses lead to a progressive, pulse-after-pulse accumulation of aggregates located in the peripheral zone of the laser beam, that is to say at the point where the laser intensity and the temperature of the glass are sufficiently low to prevent the photodissociation. The result thereof is a variation of refractive index in this peripheral zone generated by an annular spatial distribution of the aggregates in the direct laser inscription process in the case of an inscription around the fixed irradiation point. As
By controlling the parameters of the laser beam, namely the irradiance, the number of pulses and the pulse repetition rate, the irradiation by femtosecond laser in the oxide glass of the present invention induces a variation of refractive index in the annular zone around the point of irradiation of the beam.
The laser beam acts as an optical brush which makes it possible to induce in 3D a variation of optical refractive index on the peripheral zone and erase same at its center.
It is therefore possible to produce bulk 3D structures in the glass, by moving the sample in the two directions X and Y by means of the translation plate with nanometric precision.
With reference to
Since the intensity of the laser beam has a Gaussian profile, the result thereof is that the highest energy zone allowing a multiphotonic absorption is located in a central zone of each irradiation point where the phenomenon of photodissociation occurs when silver species already inscribed are located in a zone of strong irradiation. During the translation of the glass sample in the plane, the central zone of the laser beam passes once again substantially over the front edge of the ring previously inscribed. The aggregates formed on the front edge of the beam of the irradiation point referenced j are exposed by the beam of the next irradiation point referenced j+1 (diagram which is not to scale for reasons of clarity because the distance between the points j and j+1 is very small compared to the size of the diameter). The front edge of the ring referenced j is then progressively erased and the latter advances as the laser beam advances. It should be noted that there is no inscription on the back edge of the beam for physical-chemical dynamic reasons internal to the glass during the irradiation while moving. Thus, the result thereof is a writing process only on the edge of the passage of the laser beam, thus forming two parallel planes of variation of refractive index 16, 17 as represented in
According to an embodiment, to write two planes of variation of refractive index on either side of the line of passage of the laser beam in the glass, the method comprises the following steps:
In the examples of glasses presented hereinbelow, a variation of optical refractive index lying between 10-2 and 10-3 is extracted in the two planes. This variation is induced by an accumulation of aggregates in this zone, and with the local increase of polarizability linked to the creation of these new molecular silver species. The translation on the axis X thus leads to the inscription of two planes of variation of optical index. The two planes are parallel to the axis of translation of the sample X. The distance between the two planes is substantially equal to the diameter of the laser beam, generally between 0.5 µm and 3 µm. The thickness of each plane is less than 200 nm, even approximately 80 nm.
The method of laser inscription in the oxide glasses of the present invention makes it possible to produce, on each passage of the laser beam, the creation of two planes of variation of optical index in the bulk of the glass, by controlling the irradiation parameters of the beam. Thus, a laser beam passage in the glass makes it possible to form two planes exhibiting a variation of refractive index. This method based solely on the photochemistry of the silver ions and of the co-mobile ions makes it possible to achieve submicronic dimensions which are limited little by the focusing of the laser beam and therefore by the spatial extension of the point of irradiation and of energy deposition by multiphotonic absorption. This method therefore allies both a deposition by nonlinear optical process and a photochemistry whose characteristic dimensions are very much smaller than the characteristic lengths of energy deposition on the one hand and of thermal diffusion on the other hand, making it possible to obtain highly contrasting internal dimensions (Δn of some 10-3) while having transverse dimensions to a mesoscopic scale (less than 200 nm, even up to 80 nm thick).
With reference to
As in the case of the inscription of the double-plane, the final inscription of each laser beam passage is also conditioned by the distance Δy between two successive passages. When the distance Δy between two laser beam passages is greater than the distance between the two planes which correspond substantially to the diameter of the irradiation point (Δy > D/2), the passages of the laser beam are not superposed and make it possible to inscribe, on each passage, two planes of variation of optical refractive index on either side of the line of passage of the laser.
When the spacing Δy is less than the distance between the two planes (Δy < D/2), the central zone of the laser beam passes once again over one of the planes previously inscribed which is erased by photodissociation effect.
In order to be able to re-inscribe a plane of variation of optical refractive index in a zone previously inscribed and erased, that is to say by partially superposing a laser beam passage over the preceding passage, the laser irradiation produced comprising both the pulse intensity and the cumulative number of pulses at each point must be adapted so as to maintain a reservoir of silver ions that is sufficient to allow a reinscription and/or to ensure a photodissociation in terms of silver species that are sufficiently remobilizable upon the next passage.
The method of the present invention, by virtue of a combination of the appropriate parameters, namely the lateral spacing between two laser beam passages, the irradiance and the number of pulses, makes it possible to produce a grating of planes of variation of optical refractive index of a dimension less than 200 nm, even up to 80 nm, with a grating pitch lying between 200 nm and 1.5 µm (which corresponds to the diameter of the focused beam here). Structures having a double line of variation of index can also be produced for greater periods.
The examples which follow are intended to illustrate in more detail the present invention, but are in no way limiting. In particular, the methods described hereinbelow are laboratory methods, which can easily be adapted by the person skilled in the art to an industrial scale.
Example 1 relates to a series of silver-doped germanium-gallium-barium-potassium glasses comprising a composition of formula (III). The glass is prepared from gallium oxide, germanium oxide, barium carbonate and silver nitrate.
The glass is prepared according to a conventional melting-tempering method from reagents of high purity. The powders of the reagents are weighed and are introduced into a platinum crucible to be raised to melting point between 1350 and 1400° C. for 15 or so hours. This melting time is adapted to guarantee a uniform dispersion of the Ag+ ion on the atomic scale in order to obtain glasses that are optically adapted to receive femtosecond laser irradiation points. The mixture, in the molten liquid state in the crucible, is subjected to a water tempering in order to set the mixture while ensuring the uniformity of the mixture. The mixture is then subjected to a thermal annealing, at a temperature of 30° C. below the melting point Tg for 4 hours. In a final step, the sample is cut to 1 mm thickness then optically polished on two parallel faces.
Table 1 gives the experimental compositions by molar mass of a series of silver-doped germanium-gallium glasses, varying the BaO content.
The glass transition temperatures Tg have been measured. By replacing potassium with barium, a significant reduction of the glass transition temperature of approximately 15° C. is shown.
In
The device illustrated in
A 50 × 50 µm2 “speed-irradiance” irradiation matrix was produced in the BGGK glass at a depth of 160 µm under infrared femtosecond laser with an irradiance ranging from 6.3 to 8.9 TW.cm2 and a speed of movement of the plates ranging from 50 to 1100 µm.s-1. With constant irradiance, the more the speed increases, the less great the energy dose becomes.
The image (a) of
Thus, to inscribe structures of variation of refractive index in a BGGK glass, the applicants have revealed optimal ranges for the inscription:
It can be seen in the image (a) of
The applicants observe a spatial overlay between the profile of fluorescence intensity and the profile of index variation for the two inscribed structures, which reflects the fact that the index variation is supported by the accumulation of new silver molecular species (the silver aggregates): the increase of the index then results from the local increase of silver elements but above all by the increased polarizability of these silver molecular species.
A Bragg grating consists of a periodic modulation of the refractive index of the material. The Bragg gratings obtained according to the known methods in conventional glasses are generally effective in infrared range just into the red (650 nm), but cannot be used in the entire visible range without using higher orders of diffraction then causing effectiveness to drop. The Bragg gratings that are effective in the visible to the first order of diffraction were produced by using a UV laser but reducing the spatial selectivity conferred by a 3D laser inscription.
The applicants have demonstrated in the present disclosure that it is possible to inscribe and re-inscribe, line-by-line, a periodic structure of variation of refractive index in a silver-doped BGGK glass by shrewdly choosing the composition of the oxides constituting the glass, namely the molar mass of the oxides of gallium, of the oxides of germanium, of the oxides of barium, and of the silver ions, and by choosing the irradiation parameters that are the irradiance, the relative speed of movement of the beam and the spacing between two beam passages.
The example 2 relates to a photosensitive glass comprising a composition according to the relationship (II) produced from gallium oxide, sodium carbonate, phosphoric acid and silver nitrate. Once the precursors have been weighed, they are placed in a beaker to become a solid which is then ground. The powders are introduced into a platinum crucible to be raised to melting point at 1400° C. for 24 hours. This melting time is adapted to guarantee the stabilization and the uniform dispersion on the atomic scale of the Ag+ ions in order to obtain glasses that are optically adapted to receive reproducible femtosecond laser irradiation points. The mixture, in the molten liquid state in the crucible, is subjected to a water tempering in order to set the mixture while ensuring the uniformity of the mixture. The mixture is then subjected to a thermal annealing, at a temperature of 30° C. below the melting point Tg for 4 hours. In a final step, the sample is cut to 1 mm thickness and 150 µm, then optically polished on two parallel faces.
Table 2 gives the composition by molar mass of the various constituents of this glass. The silver content is set at 2 mol%. The ratio [O]/[P] = 4.3 reveals an orthophosphate glass. This glass has a low glass transition temperature of 368° C. and almost 50% NaO2 element. Such a composition allows a strongly photosensitive and chemically durable.
The GPN glass was subjected to an ultraviolet nanosecond laser irradiation. The emission spectrum obtained for an excitation wavelength at 355 nm shows that the GPN glass has a wide band in the visible range centered toward 550, revealing the majority presence of silver aggregates.
The refractive index n of the glass is 1.541 at 589 nm. The density ρ is 3.08 g.cm-3.
This glass exhibits a transparency in the infrared up to approximately 3.2-3.3 µm, the limitation of which is associated with the vibration energies of the phosphate clusters giving rise to various absorptions from 3 µm. In the ultraviolets they exhibit an absorption edge between 250 nm and 350 nm linked to the presence of silver ions in this glass.
The device of
The GPN glass blade is irradiated by laser pulses focused at a depth of 160 µm under the surface of the glass by virtue of the microscope lens of 0.75 numerical aperture and a 20x enlargement. The irradiation pulses have a wavelength of 1030 nm, with a pulse duration of 390 fs, with a repetition rate of 9.1 MHz and a maximum power of 2.6 W. To produce the structures of variation of refractive index shown in
The applicants show that it is possible to inscribe and re-inscribe structures of positive variation of refractive index in the GPN glass comprising sodium ions which are co-mobile with the silver. The applicants show that it is possible to inscribe, progressively line-by-line, to form a periodic structure of planes of variation of refractive index of thickness less than 200 nm, even of the order of 80 nm, with a submicronic periodicity controlled by laser inscription with lateral movements Δy < D. By virtue of the combination of the nanometric dimension of the structure and a small periodicity, it is possible to produce Bragg gratings that act in the visible to the first order of diffraction.
The oxide glass of the present invention offers a benefit and numerous advantages in the photonic domain for the production of optical components such as bulk Bragg gratings, Bragg grating in a waveguide or in the core of an optical fiber. By virtue of the specific glassy composition of the different oxides of the present invention, the glasses exhibit, on the one hand, a strong photosensitivity and, on the other hand, a property of reinscription due to the presence of the ions which are co-mobile with the silver ions. Furthermore, the glass exhibits a spectral range of transmission that is widened compared to the standard glasses in the infrared range. The glass of the invention is particularly suited for a femtosecond laser beam-assisted inscription to fabricate a Bragg grating with lines of variation of nanometric dimension and submicronic grating pitches which can be configured according to the requirements of the applications.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR20 09534 | Sep 2020 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2021/051598 | 9/20/2021 | WO |
