Patent Publication Number: US-10331011-B2

Title: Optical device with photon flipping

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This applications is the U.S. National Stage of PCT/EP2015/057658, filed Apr. 9,2015, which in turn claims priority to French Patent Application No. 1453176 filed Apr. 10, 2014, the entire contents of all applications are incorporated herein by reference in their entireties. 
     TECHNICAL FIELD OF THE INVENTION 
     The technical field of the invention is that of optical devices with photon flipping. An aspect of the present invention relates to an optical device with photon flipping for converting a flux of incident light into a quasi-monochromatic light beam. The present invention notably finds application in the field of photovoltaic quantum solar concentrators. 
     TECHNOLOGICAL BACKGROUND OF THE INVENTION 
     Photonic crystal optical fibres, also called “microstructured fibres” or “photonic forbidden band fibres”, use a light confinement mechanism based on the periodicity of their index structure. Two types of microstructured fibres may principally be distinguished:
         high index core fibres, in which a cladding including a periodic arrangement of low index inclusions surrounds a defect, that is to say an absence of inclusion, which serves as core,   low index core fibres, in which a cladding including a periodic arrangement of high index inclusions surrounds a defect, that is to say an absence of inclusion, which serves as core.       

     In the first case of high index core fibres, it is possible to assimilate the index of the microstructured cladding with an average index that is less than the index of the core. The confinement mechanism may thus be assimilated with the guiding mechanism in the dielectric waveguide. 
     In the second case of low index core fibres, the periodic index structure of the microstructured cladding leads to the formation of forbidden energy bands: thus, certain wavelengths cannot propagate in the microstructured cladding. Consequently, if a defect is introduced into the structure that can support a mode of which the effective index is lower than that of the core and is located in the forbidden band, light will be confined in this defect. In other words, the photonic crystal microstructure of the cladding will behave as a reflective mirror for the photons of which the directions of propagation are transversal with respect to the axis of the fibre and of which the wavelength lies within the forbidden band. 
     Several types of microstructured fibres exist:
         In the case of a microstructured fibre of Bragg fibre type, the photonic crystal is obtained by the alternation of concentric layers of quarter wave type of two materials of different indices, for example polymethyl methacrylate PMMA on the one hand and polystyrene on the other hand.   In the case of a microstructured fibre of “Holey fibre” type, tubular holes parallel to the axis of the fibre, filled with air, that is to say of optical index equal to 1, surround the core of the fibre, whereas the remainder of the matrix of the fibre is formed of a material, for example polymer, of optical index greater than 1. The holes produce a forbidden band and may be arranged according to a hexagonal “honeycomb” network, according to a square network or according to one or more concentric rings around the core.       

     Furthermore, fluorescent concentrators are notably developed in the field of solar energy research in order to improve the efficiency of solar cells. A fluorescent concentrator is typically made of PMMA and contains a fluorescent dye. The fluorescent dye has an absorption spectral band and an emission spectral band. When a photon, of wavelength λ a  lying within the absorption spectral band of the dye, reaches the concentrator and encounters a molecule of dye, it is absorbed by said molecule then re-emitted at a wavelength lying within the emission spectral band of the dye. The emission wavelength λ e  is greater than the absorption wavelength λ a , which reflects the fact that the re-emitted photon has lower energy than the photon initially absorbed. 
     The field of optical devices with photon flipping is based on the two fields which have been mentioned, that is to say photonic crystal optical fibres and fluorescent concentrators. An optical device with photon flipping thus typically comprises a core area, a flip area including a fluorescent flip dye and a microstructured cladding area. 
     The microstructured cladding area has an allowed spectral band and a forbidden spectral band. Photons of wavelength lying within the allowed spectral band can propagate in the cladding area, whereas photons of wavelength lying within the forbidden spectral band cannot propagate in the cladding area. 
     The fluorescent dye is chosen so that its absorption spectral band overlaps at least partially the allowed spectral band of the microstructured cladding area, and so that its emission spectral band overlaps at least partially the forbidden spectral band of the microstructured cladding area. 
     An optical device with photon flipping thus makes it possible to convert an incident light flux, captured laterally by the microstructured cladding area, into a quasi-monochromatic and anisotropic beam propagating in the core area. Nevertheless, optical devices with photon flipping of the prior art only have low conversion efficiency, less than 10% for thicknesses of light guide greater than a millimetre. 
     SUMMARY OF THE INVENTION 
     The present invention makes it possible to resolve this drawback, by proposing an optical device with photon flipping having better conversion efficiency. 
     An aspect of the invention thus relates to an optical device with photon flipping for converting an incident light flux into a quasi-monochromatic light beam, the optical device comprising:
         a cladding area including a photonic crystal microstructure, the photonic crystal microstructure having an allowed spectral band and a forbidden spectral band,   a flip area including a fluorescent flip dye having:
           a fluorescence absorption spectral band overlapping at least partially the allowed spectral band of the photonic crystal microstructure, and   a fluorescence emission spectral band overlapping at least partially the forbidden spectral band of the photonic crystal microstructure,   
           a core area arranged to enable the propagation of a monochromatic light beam having a wavelength lying within the forbidden spectral band of the photonic crystal microstructure, the photonic crystal microstructure surrounding the core area;   the core area having a thickness E 1  less than or equal to five times the wavelength of the maximum fluorescence emission of the fluorescent flip dye.       

     In the present description, “quasi-monochromatic light beam” is taken to mean a beam of light having a spectral band of which the full width at half maximum is less than or equal to the minimum between the full width at half maximum of the emission spectral band of the fluorescent dye and the full width at half maximum of the forbidden spectral band of the photonic crystal microstructure. As an example, the full width at half maximum of the spectral band of the quasi-monochromatic light beam is generally less than 100 nm, and often less than 50 nm. 
     “The photonic crystal microstructure surrounds the core area” is taken to mean the fact that the photonic crystal microstructure partially or completely surrounds the core area. 
     “Photon flipping” is taken to mean the fact that a luminous beam of which the photons propagate substantially at least along a straight reference line is obtained from a multidirectional luminous flux, of which the photons propagate along multiple directions of propagation in space. An optical device with photon flipping thus enables the change of direction, or in other words the flipping, of at least a part of the photons of an initially multidirectional luminous flux. 
     Thanks to the invention, the cladding area including a photonic crystal microstructure, designated “microstructured cladding area”, is used in order to capture an incident light flux. The allowed spectral band of the microstructured cladding area overlaps at least partially the spectrum of the incident light flux. The photons of the incident light flux of which the wavelength lies within the allowed spectral band of the microstructured cladding area can thus propagate through the microstructured cladding area. The fluorescent dye of the flip area is chosen in order that its fluorescence absorption spectral band overlaps at least partially the allowed spectral band of the microstructured cladding area, and that its fluorescence emission spectral band overlaps at least partially the forbidden spectral band of the microstructured cladding area. Thus, photons of which the wavelength lies within both the allowed spectral band of the microstructured cladding area and the fluorescence absorption spectral band of the fluorescent dye can propagate in the microstructured cladding area and reach the flip area, to be absorbed by the fluorescent dye then be re-emitted by the fluorescent dye at a wavelength lying within both the fluorescence emission spectral band of the fluorescent dye and the forbidden spectral band of the microstructured cladding area. The fact that the core area has a thickness E 1  less than or equal to five times the wavelength of the maximum fluorescence emission of the fluorescent flip dye advantageously makes it possible to create a quantum effect at the heart of the phenomenon of photon flipping, and thus to propose an optical device with photon flipping with improved conversion efficiency. 
     This quantum effect has in fact for origin the anisotropic inhibition of spontaneous de-excitation in near field of the fluorescent dye, due to electromagnetic interaction of the photonic crystal with the wave function of the photons re-emitted by the fluorescent dye. Said quantum effect characterises the amplitude of the quantum interaction between the photonic crystal of the cladding area and fluorescent de-excitation of the fluorescent dye. The intensity of this quantum effect is strongly influenced by the diameter of the central core of the fibre. The diameter of the central core of the fibre is thus chosen less than, and advantageously five times less than, the value of the wavelength of the emitted photons, because said quantum effect only takes place in near field. Obtaining said quantum effect is moreover all the more favoured the higher the quality factor of the waveguide formed by the core of the fibre. A high quality factor of the waveguide formed by the core of the fibre in fact favours the Purcell effect. The Purcell effect exalts fluorescence while amplifying the fluorescence quantum yield of the dye, while favouring radiative transitions to the detriment of non-radiative transitions. To do so, the photonic crystal is advantageously the most efficient possible: it involves on the one hand radially increasing the volume of the photonic crystal so that the photons can encounter a large number of patterns within said photonic crystal, and on the other hand that the characteristic dimensions of said photonic crystal are very little dispersed—the dispersion of said characteristic dimensions is advantageously less than or equal to 5%. 
     Apart from the characteristics that have been mentioned in the preceding paragraph, the optical device with photon flipping according to the invention may have one or more additional characteristics among the following, considered individually or according to any technically possible combinations thereof:
         The allowed spectral band of the microstructured cladding area is advantageously the widest possible. Thus, the allowed spectral band of the microstructured cladding area overlaps the largest possible part of the spectrum of the incident light flux.   The allowed spectral band of the microstructured cladding area is advantageously comprised within the fluorescence absorption spectral band of the flip dye. Thus, photons of wavelength lying within the allowed spectral band of the microstructured cladding area are also of wavelength lying within the fluorescence absorption spectral band of the flip dye. In other words, all the photons able to propagate in the microstructured cladding area are also capable of being absorbed by the flip dye. Generally speaking, the overlap between the allowed spectral band of the microstructured cladding area and the fluorescence absorption spectral band of the flip dye is advantageously maximum.   The fluorescence emission spectral band of the flip dye is advantageously comprised in the forbidden spectral band of the cladding area. Thus, photons of wavelength lying within the fluorescence emission spectral band of the flip dye are also of wavelength lying within the forbidden spectral band of the cladding area. In other words, all the photons emitted by the flip dye are capable of being guided by the core area. Generally speaking, the overlap between the emission fluorescence spectral band of the flip dye and the forbidden spectral band of the cladding area is advantageously maximum.   The thickness E 1  of the core area is advantageously less than or equal to three times the wavelength of the maximum fluorescence emission of the fluorescent flip dye. This thus contributes to favouring the quantum effect behind the phenomenon of photon flipping in order to propose an optical device with photon flipping with optimised conversion efficiency. The core area is preferentially of dielectric waveguide type.   The flip area has a thickness E 2  such that:       

     
       
         
           
             
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              with λ E  the wavelength of the maximum fluorescence emission of the fluorescent flip dye and n the refractive index of the flip area. This thus contributes to favouring the quantum effect behind the phenomenon of photon flipping in order to propose an optical device with photon flipping with optimised conversion efficiency. Advantageously, the thickness E 2  of the flip area is chosen such that: 
           
         
       
    
     
       
         
           
             
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              The flip area is situated in at least one part of the cladding area. Thus, the photon flipping area is advantageously distinct from the area of propagation in the core area. Consequently, the fluorescent flip dye is located in the cladding area and does not lead to any attenuation in the core area. This notably enables the core area to be air. 
             The flip area is advantageously situated in at least one part of the core area. Thus, the coupling of photons propagating in the cladding area with fundamental modes propagating in the core area is favoured. It then involves choosing for the fluorescent flip dye a sufficiently high optical density, at the absorption wavelength, so that photon flipping is efficient, while being sufficiently low, at the emission wavelength, to enable good propagation of the quasi-monochromatic beam. 
             Advantageously, the flip area is situated in at least one central part of the core area. In this way, high coupling is obtained between the electric dipolar moment of the radiative transition of the fluorescence emission photons and the fundamental modes of the core area of the device according to an aspect of the invention. Said coupling is all the greater the more said electrical dipolar moment is oriented perpendicularly to the axis of propagation of the quasi-monochromatic beam in the core area. 
             The flip area is advantageously situated in at least one part of the cladding area and in at least one part of the core area. Thus, coupling of the fluorescence emission photons with the fundamental modes propagating in the core area is facilitated, while making it possible to limit the optical density in the core area, said optical density being capable of penalising the propagation of photons in the core area. 
             The optical device with photon flipping has optical fibre geometry, the optical device with photon flipping extending along a reference axis and having symmetry of revolution around said reference axis. 
              In this case, the core area of the optical device with photon flipping is substantially cylindrical and has a lateral surface of revolution. “The core area is surrounded by the photonic crystal microstructure of the cladding area” is taken to mean the fact that the microstructured cladding area is in contact with the lateral surface of revolution of the core area. Still according to this case, a thickness is measured along a radial direction with respect to the reference axis. 
              Such a device with optical fibre geometry advantageously makes it possible to facilitate the formation of a multi-core device, using for example several different fluorescent flip dyes. Moreover, such optical fibre geometry enables the formation of a hollow core area, using air as propagating medium. Finally, very precise manufacturing techniques are available for optical fibres, making it possible to obtain very small geometric dispersions. 
             Alternatively, the optical device with photon flipping has optical film geometry, the optical device with photon flipping extending along a reference plane and having symmetry with respect to the reference plane. 
              In this case, the core area is a layer extending parallel to the reference plane, whereas the microstructured cladding area has first and second regions extending parallel to the reference plane. “The core area is surrounded by the photonic crystal microstructure of the cladding area” is taken to mean the fact that the core area is comprised between the first and second regions of the cladding area. In other words, the first and second regions of the cladding area extend in contact with the core area, respectively on either side of the core area. Still according to this case, a thickness is measured along a direction perpendicular to the reference plane. 
              Such a device with optical film geometry advantageously offers a larger collection surface for the photons of the incident light flux than in the case of optical fibre geometry. Furthermore, existing techniques of production by lamination make it possible to produce directly and rapidly large collection surfaces. It should all the same be noted that to obtain collection surfaces of nanometric dimensions by lamination, several lamination steps are generally required. 
             The optical device with photon flipping advantageously comprises a first conversion area situated around the flip area, the first conversion area including a first fluorescent conversion dye having a fluorescence emission spectral band overlapping at least partially the fluorescence absorption spectral band of the fluorescent flip dye. Thus, this advantageously enables an increase in the quantity of incident photons, from the incident light flux, that the flip dye of the flip area is going to be able to absorb. Advantageously, the sum of the absorption spectral bands of the first fluorescent conversion dye and the fluorescent flip dye covers the widest possible range of the solar spectrum. The overlap between the fluorescence emission spectral band of the flip dye and the fluorescence absorption spectral band of the flip dye is advantageously maximum. 
           
         
       
    
     Another aspect of the invention relates to a multi-core optical device with photon flipping comprising:
         a core area comprising a plurality of optical devices with photon flipping according to an aspect of the invention, and   a cladding area surrounding the core area.       

     Thus, this contributes advantageously to optimising the collection of incident photons. A multi-core optical device with photon flipping has in fact an optimised total photon collecting surface. The total photon collecting surface of the multi-core optical device with photon flipping is the sum of the photon collecting surfaces of each optical device with photon flipping. The photon collecting surface of an optical device with photon flipping is the surface of the core area of said optical device with photon flipping. 
     Another aspect of the invention relates to an electricity production device comprising:
         at least one optical device with photon flipping according to an aspect of the invention, and   at least one photovoltaic cell arranged to be able to capture the quasi-monochromatic light beam emitted at the output of the at least one optical device with photon flipping according to an aspect of the invention.       

     Such an electricity production device advantageously makes it possible to reduce significantly, typically of the order of 100 times, the surface of the photovoltaic semiconductor of the at least one photovoltaic cell used, compared to an electricity production device using a device with photon flipping according to the prior art. In fact, the optical device with photon flipping according to an aspect of the invention, which is used in said electricity production device, makes it possible to obtain a light beam on the one hand more intense and on the other hand quasi-monochromatic at the wavelength corresponding to the maximum quantum efficiency of the photovoltaic semiconductor. 
     Such an electricity production device also makes it possible to dispense with any system for following the source of the incident light flux, that is to say typically the sun. In fact, an optical device with photon flipping according to an aspect of the invention operates efficiently, independently of the incident angle of the incident light flux with respect to the optical device with photon flipping. 
     Another aspect of the invention relates to a catalysis device comprising:
         at least one optical device with photon flipping according to an aspect of the invention, and   at least one catalytic cell arranged to be able to capture the quasi-monochromatic light beam emitted at the output of the at least one device with photon flipping.       

     Another aspect of the invention relates to a laser emission device comprising:
         at least one optical device with photon flipping according to an aspect of the invention, and   at least one laser arranged to be able to capture the quasi-monochromatic light beam emitted at the output of the at least one device with photon flipping.       

     The invention and its different applications will be better understood on reading the description that follows and by examining the figures that accompany it. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The figures are presented for indicative purposes and in no way limit the invention. 
         FIG. 1  schematically represents a perspective view of an optical device with photon flipping according to a first variant of a first embodiment of the invention. 
         FIG. 2  shows a diagram of the allowed propagation frequencies ω as a function of the wave vector k z , in a photonic crystal microstructure of a cladding area of an optical device with photon flipping according to an embodiment of the invention. 
         FIG. 3 a    schematically represents a sectional view of the optical device with photon flipping of  FIG. 1 . 
         FIG. 3 b    schematically represents a sectional view of an optical device with photon flipping according to a second variant of the first embodiment of the invention. 
         FIG. 3 c    schematically represents a sectional view of an optical device with photon flipping according to a third variant of the first embodiment of the invention. 
         FIG. 4 a    schematically represents an optical device with photon flipping of “Holey fibre” type, of hexagonal section, and according to the third variant of the first embodiment of the invention. 
         FIG. 4 b    schematically represents an optical device with photon flipping of “Holey fibre” type, of hexagonal section, and according to the first variant of the first embodiment of the invention. 
         FIG. 4 c    schematically represents an optical device with photon flipping of Bragg fibre type and according to the third variant of the first embodiment of the invention. 
         FIG. 4 d    schematically represents a sectional view of an optical device with photon flipping of “Holey fibre” type, of circular section, and according to the third variant of the first embodiment of the invention. 
         FIG. 5 a    schematically represents a sectional view of an optical device with photon flipping according to the first variant of the first embodiment of the invention and comprising a first cascade of fluorescent dyes. 
         FIG. 5 b    schematically represents a sectional view of an optical device with photon flipping according to the first variant of the first embodiment of the invention and comprising a second cascade of fluorescent dyes. 
         FIG. 6 a    schematically represents a sectional view of an optical device with photon flipping according to a first variant of a second embodiment of the invention. 
         FIG. 6 b    schematically represents a sectional view of an optical device with photon flipping according to a second variant of the second embodiment of the invention. 
         FIG. 6 c    schematically represents a sectional view of an optical device with photon flipping according to a third variant of the second embodiment of the invention. 
         FIGS. 7 a  and 7 b    show first and second examples of multi-core fibres comprising a plurality of optical devices with photon flipping according to the first embodiment of the invention. 
         FIG. 8  shows a third example of a multi-core fibre having a core area comprising a plurality of optical devices with photon flipping according to the first embodiment of the invention. 
         FIGS. 9 a  and 9 b    show first and second examples of multi-core fibres comprising a cascade of fluorescent conversion dyes. 
         FIG. 10 a    schematically illustrates a first example of an optical solar concentrator comprising an optical device with photon flipping according to an embodiment of the invention. 
         FIG. 10 b    schematically illustrates a second example of an optical solar concentrator comprising an optical device with photon flipping according to an embodiment of the invention. 
         FIG. 10 c    schematically illustrates a third example of an optical solar concentrator comprising an optical device with photon flipping according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT OF THE INVENTION 
     Unless stated otherwise, a same element appearing in the different figures has a single reference. 
     In the present description, the terms “microstructure”, “photonic crystal” and “photonic crystal microstructure” are employed indiscriminately. 
     In the present description, the terms “dye”, “fluorescent dye” and “fluorescence dye” are employed indiscriminately. 
     In the present description, the terms “fluorescence absorption spectral band” and “absorption spectral band” are employed indiscriminately. In the same way, the terms “fluorescence emission spectral band” and “emission spectral band” are employed indiscriminately. 
       FIG. 1  shows schematically the operating principle of an optical device with photon flipping  10  according to a first variant of a first embodiment of the invention. The optical device with photon flipping  10  according to this first embodiment has optical fibre geometry: the optical device with photon flipping  10  extends along a reference axis A and has symmetry of revolution around said reference axis A. An optical device with photon flipping according to a second embodiment of the invention, having optical film geometry, will be described hereafter. 
     The optical fibre with photon flipping  10 , designated “FORP fibre” comprises:
         a core area  11 ,   a flip area  12  comprising a fluorescent flip dye, and   a microstructured cladding area  13  comprising a photonic crystal, the microstructured cladding area surrounding the core area  11 .       

     According to the first variant of the first embodiment of the invention, the flip area  12  is located in at least one part of the microstructured cladding area  13 , said at least one part being in contact with the core area  11  and surrounding the core area  11 . 
       FIG. 1  schematically shows a flux of incident photons Ph_i. The flux of incident photons Ph_i is typically polychromatic and multidirectional; it may for example be solar radiation. In the case where the flux of incident photons Ph_i is polychromatic, it has by definition a wide spectrum. As an indication, the solar spectrum extends at ground level between 280 nm and 4000 nm. More particularly, in the remainder of this description, the case of a polychromatic flux of incident photons Ph_i is considered; nevertheless, said flux of incident photons Ph_i may absolutely be monochromatic or quasi-monochromatic. Similarly, more particularly in the remainder of this description, the case is considered of a multidirectional flux of incident photons Ph_i, but said flux of incident photons Ph_i may absolutely be unidirectional. The photonic crystal of the microstructured cladding area  13  has an allowed spectral band and a forbidden spectral band. The incident photons Ph_i of which the wavelength lies within the allowed spectral band of the photonic crystal can propagate in the microstructured cladding area  13 . The allowed spectral band of the photonic crystal is advantageously as wide as possible. Thus, this enables the largest possible part of the incident photons Ph_i to propagate in the microstructured cladding area  13 . 
     The fluorescent flip dye of the flip area  12  has a fluorescence absorption spectral band and a fluorescence emission spectral band. The fluorescence emission spectral band is spectrally shifted to higher wavelengths with respect to the fluorescence absorption spectral band: this is the Stoke&#39;s shift. The flip dye is thus able to absorb a photon of wavelength lying within its fluorescence absorption spectral band, then re-emit it at a higher wavelength lying within its fluorescence emission spectral band. The re-emitted photon is of higher wavelength, thus of lower energy, than the photon initially absorbed. 
     The flip area  12  is situated in such a way that incident photons Ph_i propagating in the microstructured cladding area  13  can reach the flip area  12 . At the same time, the absorption spectral band of the flip dye overlaps at least partially the allowed spectral band of the microstructured cladding area  13 . Thus, incident photons Ph_i propagating in the microstructured cladding area  13  and of which the wavelength lies within the absorption spectral band of the flip dye may be absorbed by the flip dye, then re-emitted at a wavelength lying within the emission spectral band of said flip dye. 
     The fluorescence emission spectral band overlaps at least partially the forbidden spectral band of the microstructured cladding area  13 . At the same time, the flip area  12  is situated in such a way that photons re-emitted by the flip dye can reach the core area  11 . The core area  11  is moreover arranged to enable the propagation of photons of wavelength lying within at least one part of the forbidden spectral band of the microstructured cladding  13 . Thus, at least one part of the photons re-emitted by the flip dye of the flip area  12  may be guided in the core area  11 : they are output photons Ph_s, represented in  FIG. 1 . 
       FIG. 2  shows a diagram of the allowed propagation frequencies ω as a function of the wave vector k z , in the photonic crystal microstructure of the cladding area of an optical device with photon flipping according to an embodiment of the invention. Hereafter, the particular example of a FORP fibre  10  according to the first embodiment of the invention will be taken. The wave vector k z  is a vector perpendicular to the wave front of a monochromatic wave, which indicates the direction of propagation of said monochromatic wave. The diagram of  FIG. 2  thus shows:
         the allowed spectral band B_aut of the photonic crystal microstructure of the microstructured cladding area  13  of the FORP fibre  10 , and   the forbidden spectral band B_int of the photonic crystal microstructure of the microstructured cladding area  13  of the FORP fibre  10 .       

     Photon propagation modes exist in the allowed spectral band B_aut. On the contrary, no photon propagation mode exists in the forbidden spectral band B_int. 
       FIG. 2  also shows:
         the absorption spectrum S_abs of the flip dye of the flip area  12  of the FORP fibre  10 , and   the emission spectrum S_em of the flip dye of the flip area  12  of the FORP fibre  10 .       

     The diagram of  FIG. 2  thus shows that:
         the absorption spectrum S_abs of the flip dye partially overlaps the allowed spectral band B_aut. In the particular case of  FIG. 2 , the absorption spectrum S_abs of the flip dye is comprised in the allowed spectral band B_aut. The absorption of photons by the flip dye is thus isotropic,   the emission spectrum S_em of the flip dye completely overlaps the forbidden spectral band B_int. In the particular case of  FIG. 2 , the forbidden spectral band B_int is comprised in the emission spectrum S_em of the flip dye. The emission of photons by the flip dye is thus anisotropic. In fact, photons emitted by the flip dye and of wavelength lying within the forbidden spectral band B_int cannot propagate in the microstructured cladding area  13  that surrounds the core area  11 . The photons emitted by the flip dye thus can only propagate in the core area  11 .       

     Advantageously, the forbidden spectral band B_int is the most omnidirectional possible at the wavelength of the photons re-emitted by the flip dye. In other words, for a given frequency, the range of values of the wave vector k z  prohibiting the propagation of said re-emitted photons must be as large as possible. Let β be the angle of emission of photons re-emitted by the fluorescent flip dye, § being measured with respect to the axis A of the FORP fibre  10 . This emission angle β is expressed by the formula: 
             β   =       cos     -   1       ⁡     (       c   *     k   z         2   *   π   *   ϑ   *     n   c         )             
with c the speed of propagation of light, v the frequency of the re-emitted photons and n c  the optical index in the core area of the FORP fibre.
 
     A spectral band is completely forbidden at a given frequency, if no propagation mode exists for |cos β| lying within the interval [0,1]. The photonic crystal is thus selected so that its forbidden spectral band is the most extended in wave vector k z  or in cos(β), that is to say in such a way that the solid angle Ω, corresponding to the entire anisotropic inhibition area of fluorescence de-excitation, is as large as possible. The anisotropic efficiency of the photonic crystal is given by the ratio 
     
       
         
           
             
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     The flip dye is thus selected so that its emission spectrum overlaps at least partially the forbidden spectral band of the photonic crystal microstructure. On the other hand, the quantum coupling between the fluorescent flip dye and the photonic crystal microstructure must be as high as possible in order to contribute to favouring the efficiency of the photon flipping phenomenon. This latter condition is in particular linked to the respective dimensions of the core area  11  and the flip area  12  of the FORP fibre, as will be explained hereafter in relation with  FIGS. 3 a , 3 b    and  3   c.    
       FIGS. 3 a , 3 b  and 3 c    are described jointly.
           FIG. 3 a    schematically represents a sectional view of the FORP fibre  10  of  FIG. 1 , according to the first variant described previously in which the flip area  12  is located in at least one part of the microstructured cladding area  13 .     FIG. 3 b    schematically shows a sectional view of a FORP fibre  10  according to a second variant of the first embodiment of the invention, in which the flip area  12  is located in at least one part of the core area  11  and in at least one part of the microstructured cladding area  13 .     FIG. 3 c    schematically represents a sectional view of a FORP fibre  10  according to a third variant of the first embodiment of the invention, in which the flip area  12  is located in at least one part of the core area  11 .       
       FIGS. 3 a , 3 b  and 3 c    show:
         a core area  11  having a thickness E 1 . In the case of the first embodiment particularly illustrated in  FIGS. 3 a , 3 b  and 3 c   , the core area  11  is a solid cylinder of circular section and of axis A. The thickness E 1  of the core area  11  is then the diameter of said circular section.   a microstructured cladding area  13  having a thickness E 3 . In the case of the first embodiment particularly illustrated in  FIGS. 3 a , 3 b  and 3 c   , the microstructured cladding area  13  is a hollow cylinder of first annular section and of axis A, in other words a tube, surrounding the core area  11 . The thickness E 3  is then the thickness of the first annular section, measured radially with respect to the axis A.   a flip area  12  having a thickness E 2 . In the case of the first embodiment particularly illustrated in  FIGS. 3 a , 3 b  and 3 c   , the flip area  12  is a hollow cylinder of second annular section and of axis A, in other words a tube. The thickness E 2  is then the thickness of the second annular section, measured radially with respect to the axis A.       
     According to the first variant of the first embodiment, the flip area  12  is a hollow cylinder of second annular section and of axis A, in other words a tube, which surrounds the core area  11 . The thickness E 2  of the flip area  12  is less than or equal to the thickness E 3  of the microstructured cladding area  13 . When the thickness E 2  of the flip area  12  is equal to the thickness E 3  of the microstructured cladding area  13 , the flip area  12  and the microstructured cladding area  13  are merged. According to the first variant of the first embodiment, the core area  11  may be either solid, that is to say filled with a solid material, for example polymer; or hollow, that is to say not filled with a solid material, and including for example air. 
     According to the second variant of the first embodiment, the flip area  12  is located at least in part in the core area  11  and at least in part in the microstructured cladding area  13 . The flip area  12  is then:
         either a hollow cylinder of second annular section and of axis A, in other words a tube, when the flip area  12  only overlaps a part of the core area  11 ;   or a solid cylinder of circular section and of axis A, when the flip area  12  completely overlaps the core area  11 .       

     The thickness E 2  of the flip area  12  is less than or equal to the sum of the radius of the core area  11  and the thickness E 3  of the microstructured cladding area  13 . In other words, the thickness E 2  of the flip area  12  is such that 
     
       
         
           
             
               E 
               ⁢ 
               
                   
               
               ⁢ 
               2 
             
             ≤ 
             
               
                 
                   E 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 2 
               
               + 
               
                 E 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 3 
               
             
           
         
       
     
     When the thickness E 2  of the flip area  12  is equal to the sum of the radius 
               E   ⁢           ⁢   1     2         
or the core area  11  and the thickness E 3  of the microstructured cladding area  13 , the flip area  12  entirely overlaps the microstructured area  13  and the core area  11 . According to the second variant of the first embodiment, the core area  11  is solid, that is to say filled with a solid material, for example polymer.
 
     According to the third variant of the first embodiment, the flip area  12  is located at least in part in the core area  11 . The flip area  12  is then:
         either a hollow cylinder of second annular section and of axis A, in other words a tube. In this case, the thickness E 2  of the flip area  12  is less than the radius t,?E1/2 of the core area  11 ;   or a solid cylinder of circular section and of axis A. In this case, the thickness E 2  of the flip area  12  is less than or equal to the diameter E 1  of the core area  11 . When the thickness E 2  of the flip area is equal to the diameter E 1  of the core area  11 , the flip area  12  entirely overlaps the core area  11 , that is to say that the flip area  12  and the core area  11  are merged.       

     According to the third variant of the first embodiment, the core area  11  is solid, that is to say filled with a solid material, for example polymer. 
     It may be noted that within a same FORP fibre  10 , the core area  11  may have several consecutive sections, and notably at least one first solid section, and at least one second hollow section. 
     Such a core area  11  having at least one first solid section and at least one second hollow section may be qualified as “semi-hollow”. 
     The geometry and the dimensions of the core area  11  and the flip area  12  on the one hand, and the properties of the flip dye and the photonic crystal on the other hand, are chosen in order to contribute to high quantum coupling between the flip dye and the photonic crystal, in the forbidden spectral band. Thus, in a FORP fibre  10  according to any of the variants of the first embodiment of the invention, the core area  11  has a thickness E 1  less than or equal to five times the wavelength of the maximum fluorescence emission of the flip dye. Advantageously, the thickness E 1  of the core area  11  is less than or equal to three times the wavelength of the maximum fluorescence emission of the flip dye. Moreover, the flip area  12  advantageously has a thickness E 2  such that: 
     
       
         
           
             
               E 
               ⁢ 
               
                   
               
               ⁢ 
               2 
             
             &lt; 
             
               
                 λ 
                 E 
               
               
                 4 
                 * 
                 n 
               
             
           
         
       
     
     with λ E  the wavelength of the maximum fluorescence emission of the flip dye of the flip area  12 , and n the refractive index of the flip area  12 . 
     The quantum coupling thus favoured ensures inhibition of fluorescence de-excitation outside of the axis of the FORP fibre  10  and forces fluorescence de-excitation in the axis of the FORP fibre  10 . The optimisation of this quantum coupling thus contributes to obtaining a flux of output photons Ph_s at the output of the FORP fibre  10  significantly more important than in the case of a conventional fibre. For a same beam of incident photons Ph_i, the conversion efficiency of a FORP fibre  10  according to the first embodiment of the invention is thus significantly greater than that of a conventional fibre: it is greater than 80% at the maximum, absorption wavelength of the fluorescent flip dye, whereas it is only of the order of 10% in devices of the prior art. 
     The fluorescent flip dye may comprise organic fluorescent molecules, or alternatively inorganic chromophores such as rare earths—for example complexes of lanthanides—or oxides, or instead quantum-dots—for example lead sulphide PbS, with a maximum absorption at 650 nm and a maximum emission at 850 nm. In the case of the use of quantum-dots, the latter are typically diluted in a polymer. 
     The fluorescent flip dye may for example be selected from among the following list:
         rhodamine 6G, having an absorption maximum at 529 nm and an emission maximum at 551 nm;   rhodamine 123, having an absorption maximum at 550 nm and an emission maximum at 560 nm;   rhodamine 101, having an absorption maximum at 568 nm and an emission maximum at 600 nm;   styril 15, having an absorption maximum at 651 nm and an emission maximum at 880 nm;   coumarin 30, having an absorption maximum at 407 nm and an emission maximum at 478 nm   coumarin 6, having an absorption maximum at 459 nm and an emission maximum at 502 nm   fluorescein, having an absorption maximum at 500 nm and an emission maximum at 540 nm;   rhodamine B, having an absorption maximum at 542 nm and an emission maximum at 565 nm;   sulforhodamine 101, having an absorption maximum at 576 nm and an emission maximum at 591 nm;   cresyl violet, having an absorption maximum at 603 nm and an emission maximum at 623 nm;   rhodamine 800, having an absorption maximum at 682 nm and an emission maximum at 800 nm;   oxazine 170, having an absorption maximum at 613 nm and an emission maximum at 640 nm.       

       FIG. 4 a    schematically represents a FORP fibre  10  of hexagonal section and of “Holey fibre” type, according to the third variant of the first embodiment of the invention. The microstructured cladding area  13  is formed of a material, for example polymer, of optical index greater than 1 and comprising a plurality of holes  14 . The holes  14  are typically filled with air, of optical index equal to 1. In the example represented in  FIG. 4 a   , the plurality of holes  14  is arranged according to a hexagonal “honeycomb” network. The plurality of holes  14  is the microstructure of the cladding area  13 . The core area  11  is surrounded by the plurality of holes  14 . The quality of the photonic crystal is directly linked to its thickness and to the number of patterns seen by the photons. The core area  11  is formed of a material, for example polymer, in which is implanted the fluorescent dye, for example by doping. The flip area  12  is thus in the core area  11 . 
       FIG. 4 b    schematically represents a FORP fibre  10  of hexagonal section and of “Holey fibre” type, according to the first variant of the first embodiment of the invention. The flip area  12  comprising the fluorescent dye is located this time in the microstructured cladding area  13 , around the core area  11 . In the particular example represented in  FIG. 4 b   , the core area  11  is hollow. The fluorescent dye is typically implanted in the material of the microstructured cladding area  13  by doping. 
       FIG. 4 c    schematically represents a FORP fibre  10  of circular section and of Bragg fibre type, according to the third variant of the first embodiment of the invention. The microstructured cladding area  13  is formed of an alternation of first  15  and second  16  concentric layers of quarter wave type of two materials of different indices. The first and second layers  15  and  16  are typically made of polymer, for example selected from among the following list:
         polymethyl methacrylate PMMA;   polystyrene,   polycarbonate,   polyacrylate.       

     Titanium dioxide powder may potentially be added to the polymer used, which has the advantage of increasing its refractive index while allowing it to absorb UV rays. 
     Thus, it is possible for example to use polymethyl methacrylate PMMA for the first layers  15  and polystyrene or polycarbonate for the second layers  16 . As in the example represented in  FIG. 4 a   , the core area  11  is formed of a material, for example polymer, in which is implanted the fluorescent dye, for example by doping. The flip area  2  is thus in the core area  11 . 
       FIG. 4 d    schematically represents a FORP fibre  10  of circular section and of “Holey fibre” type, according to the third variant of the first embodiment of the invention. The plurality of holes  14  of the microstructured cladding area  13  is this time arranged according to a ring around the core area  11 . As in the examples represented in  FIGS. 4 a  and 4 c   , the core area  11  is formed of a material, for example polymer, in which is implanted the fluorescent dye, for example by doping. The flip area  12  is thus in the core area  11 . 
       FIG. 5 a    schematically represents a sectional view of an optical device with photon flipping according to the first variant of the first embodiment of the invention, that is to say a FORP fibre  10 , which comprises a first conversion area C 1  including a first fluorescent conversion dye. 
     The first conversion area C 1  is a hollow cylinder of annular section and of axis A, in other words a tube. The first fluorescent conversion dye is typically implanted in the first conversion area C 1  by doping. In the example represented in  FIG. 5 a   , the first conversion area C 1  surrounds the microstructured cladding area  13 . Alternatively, the first conversion area C 1  may be located at least partially in the microstructured cladding area  13 . 
     The principle of this first conversion area C 1  is the following:
         The first conversion dye of the first conversion area C 1  has an absorption spectral band overlapping at least partially the spectrum of the flux of incident photons Ph_i, and an emission spectral band overlapping at least partially the absorption spectral band of the flip dye of the flip area  12 . Since the flux of incident photons Ph_i is typically solar radiation, the first conversion dye will advantageously have an absorption spectral band covering the widest possible range of the solar spectrum. The overlap between the absorption spectral band of the first conversion dye and the spectrum of the flux of incident photons Ph_i is advantageously maximum.   The part of the incident photons Ph_i of which the wavelength lies within the absorption spectral band of the first conversion dye may be absorbed by said first conversion dye.   The first conversion dye is then capable of re-emitting by fluorescence de-excitation at least one part of the absorbed photons, at least one part of the re-emitted photons having a wavelength lying within the absorption spectral band of the fluorescent flip dye. The overlap between the emission spectral band of the first conversion dye and the absorption spectral band of the flip dye is advantageously maximum.   Thus, the flip dye of the flip area  12  is capable of absorbing:
           on the one hand, the part of the incident photons Ph_i of which the wavelength lies within the absorption spectral band of the flip dye,   on the other hand, at least one part of the photons re-emitted by the first conversion dye of the first conversion area C 1 .   
               

     The first conversion dye of the first conversion area C 1  thus advantageously makes it possible to increase the quantity of incident photons Ph_i, from an incident light flux, which the flip dye of the flip area  12  is going to be able to absorb. It is said that the first fluorescent conversion dye and the fluorescent flip dye form a cascade of fluorescent dyes. 
     The first conversion area C 1  has been described in the particular case of a FORP fibre  10  according to the first variant of the first embodiment, but it is naturally compatible with any other variant of the first embodiment, and notably with the second and third variants of the first embodiment. 
     The example has been described above of a cascade of fluorescent conversion dyes comprising, apart from the flip dye, a single conversion dye, but it is naturally possible to use a large number of conversion dyes.  FIG. 5 b    schematically represents a sectional view of an optical device with photon flipping according to the first variant of the first embodiment of the invention, that is to say a FORP fibre  10 , which comprises:
         the first conversion area C 1  including the first fluorescent conversion dye,   a second conversion area C 2  including a second fluorescent conversion dye,   a third conversion area C 3  including a third fluorescent conversion dye,   a fourth conversion area C 4  including a fourth fluorescent conversion dye.       

     The second, third and fourth conversion areas d C 2 , C 3  and C 4  are, in the same way as the first conversion area C 1 , hollow cylinders of annular section and of axis A, in other words tubes. The second, third and fourth conversion dyes are typically implanted respectively in the second, third and fourth conversion areas C 2 , C 3  and C 4  by doping. The second conversion area C 2  surrounds the first conversion area C 1 ; the third conversion area C 3  surrounds the second conversion area C 2  and the fourth conversion area C 4  surrounds the third conversion area C 3 . In the example represented in  FIG. 5 b   , the first, second, third and fourth conversion areas C 1 , C 2 , C 3  and C 4  surround the microstructured cladding area  13 . Alternatively, the microstructured cladding area  13  may comprise at least one part of the set formed by the first, second, third and fourth conversion areas C 1 , C 2 , C 3  and C 4 . The first, second, third and fourth conversion dyes form with the flip dye a cascade of fluorescent dyes of which the operation is the following:
         The fourth conversion dye of the fourth conversion area C 4  has an absorption spectral band overlapping at least partially the spectrum of the flux of incident photons Ph_i. The part of the incident photons Ph_i of which the wavelength lies within the absorption spectral band of the fourth conversion dye may be absorbed by said fourth conversion dye, then re-emitted at a wavelength lying within the emission spectral band of the fourth conversion dye. The overlap between the absorption spectral band of the fourth conversion dye and the spectrum of the flux of incident photons Ph_i is advantageously maximum.   The third conversion dye of the third conversion area C 3  is chosen so that its absorption spectral band overlaps at least partially the emission spectral band of the fourth conversion dye. Thus, the third conversion dye is capable of absorbing:
           on the one hand, the part of incident photons Ph_i of which the wavelength lies within the absorption spectral band of the third conversion dye,   and, on the other hand, the photons re-emitted by the fourth conversion dye.   
               

     The third conversion dye can then re-emit by fluorescence de-excitation at least one part of the absorbed photons, at a higher wavelength lying within its fluorescence emission band. The overlap between the emission spectral band of the fourth conversion dye and the absorption spectral band of the third conversion dye is advantageously maximum.
         On the same principle, the second conversion dye of the second conversion area C 2  is chosen in order that its absorption spectral band overlaps at least partially the emission spectral band of the third conversion dye. The second conversion dye is then capable of absorbing not only the part of the incident photons Ph_i of which the wavelength lies within its absorption spectral band, but also the photons re-emitted by the third conversion dye. The overlap between the emission spectral band of the third conversion dye and the absorption spectral band of the second conversion dye is advantageously maximum.   Finally, in the example of  FIG. 5 b   , the first conversion dye of the first conversion area C 1  is this time chosen so that its absorption spectral band overlaps at least partially the emission spectral band of the second conversion dye, and that its emission spectral band overlaps at least partially with the absorption spectral band of the fluorescent flip dye of the flip area  12 . The first conversion dye can then absorb the part of the photons of the incident flux of which the wavelength lies within its absorption spectral band, as well as the photons re-emitted by fluorescence de-excitation by the second dye. The first conversion dye can then re-emit by fluorescence de-excitation at least one part of the absorbed photons, at least one part of the re-emitted photons having a wavelength lying within the absorption spectral band of the fluorescent flip dye. The overlap between the emission spectral band of the second conversion dye and the absorption spectral band of the first conversion dye is advantageously maximum. Similarly, the overlap between the emission spectral band of the first conversion dye and the absorption spectral band of the flip dye is advantageously maximum.       

     Generally speaking, the sum of the absorption spectral bands of the fluorescent conversion dye(s), as well as the fluorescent flip dye, forming the absorption spectral band of the cascade of fluorescent dyes, covers the widest possible range of the solar spectrum. It may be noted that the solar spectrum extends at ground level from between 280 nm and 4000 nm. In the same way as for the fluorescent flip dye, the fluorescent conversion dye(s) may for example be: rhodamine 6G, rhodamine 123, rhodamine 101, styril 15, coumarin 30, coumarin 6, fluorescein, rhodamine B, sulforhodamine 101, cresyl violet, rhodamine 800 or oxazine 170. For a better understanding of the operation of a cascade of fluorescent dyes, the particular case according to which each fluorescent conversion dye is located in a particular conversion area has been described above. Nevertheless, several fluorescent conversion dyes may absolutely be mixed in a single and same conversion area, without modifying the cascade operating principle which has just been described. An example of interesting embodiment is thus that according to which a single conversion area comprises all of the fluorescent conversion dyes. 
     Advantageously, a FORP fibre  10  according to the first embodiment of the invention may also comprise:
         an antireflective layer and/or   an anti-UV protective layer and/or   a protective layer against mechanical aggressions.       

     The antireflective layer preferentially surrounds the microstructured cladding area  13  or, if need be, the outermost conversion area of the cascade of fluorescent dyes. The anti-UV protective layer preferentially surrounds, if need be, the antireflective layer. The protective layer against mechanical aggressions preferentially surrounds, if need be, the anti-UV protective layer. 
     The anti-UV protective layer is capable of absorbing ultraviolet photons. In an optional manner, the anti-UV protective layer may advantageously be fluorescent and absorb photons of wavelength lying within the interval [280 nm-400 nm] to re-emit them at wavelengths greater than 400 nm. In fact, photons of wavelength less than 400 nm are capable of degrading the optical properties of the polymer materials typically used in the production of FORP fibres  10 , as well as contributing to the photolysis and to the destruction of the fluorescent flip dye and/or, if appropriate, at least one fluorescent conversion dye, particularly if it is an organic fluorescent dye. The antireflective protective layer could for example be made of PMMA, ethylene vinyl acetate EVA or polyvinyl butyral PVB. To enable fluorescence of the antireflective protective layer, it is possible for example to use titanium dioxide, beta-carboline, 7-hydroxy-4-methylcoumarin, POPOP (also called “1,4-bis(5-phenyloxazol-2-yl)benzene”), bis-MSB (also called “1,4-bis(2-methylstyryl)benzol”), beta barium borate BBO, p-terphenyl or biphenyl. An antireflective and anti-UV layer could advantageously be used instead of the antireflective layer and the anti-UV protective layer. 
     The protective layer against mechanical aggressions may be made of PVB or EVA. 
     The description will now be made, in relation with  FIGS. 6 a , 6 b  and 6 c   , of an optical device with photon flipping  30  according to the second embodiment of the invention. The optical device with photon flipping  30  according to this second embodiment has optical film geometry with FORP photon flipping: the optical device with photon flipping  30  extends along a reference plane P and has symmetry with respect to said reference plane P. 
       FIGS. 6 a , 6 b  and 6 c    are described jointly.  FIGS. 6 a , 6 b  and 6 c    show a FORP film  30  including:
         a core area  31  extending along the reference plane P, the core area  31  being for example formed of a polymer material,   a flip area  32  including a fluorescent flip dye and extending parallel to the reference plane P,   a microstructured cladding area  33  including a first region  33 - 1  and a second region  33 - 2  arranged on either side of the core area  31 . The first and second regions  33 - 1  and  33 - 2  of the microstructured cladding area  33  may for example each comprise a Bragg stack, that is to say an alternation of quarter wave layers of two materials of different indices, for example polymethyl methacrylate PMMA on the one hand and polystyrene on the other hand.       

     The core area  31  has a thickness E 1  measured perpendicularly to the reference plane P. The first and second regions  33 - 1  and  33 - 2  of the microstructured cladding area  33  each have a thickness E 3  measured perpendicularly to the reference plane P. 
       FIG. 6 a    schematically represents a sectional view, along a sectional plane perpendicular to the reference plane P, of a FORP film  30  according to a first variant of the second embodiment. The first variant of the second embodiment is analogous to the first variant of the first embodiment, described previously in relation with  FIG. 3 a   . Thus, according to the first variant of the second embodiment, the flip area  32  is located in at least one part of the microstructured cladding area  33 . The flip area  32  thus has:
         a first region  32 - 1  situated in at least one part of the first region  33 - 1  of the microstructured cladding area  33  and in contact with the core area  31 , and   a second region  32 - 2  situated in at least one part of the second region  33 - 2  of the microstructured cladding area  33  and in contact with the core area  31 .       

     The first and second regions  32 - 1  and  32 - 2  of the flip area  32  each have a thickness E 2 , measured perpendicularly to the reference plane P, which is less than or equal to the thickness E 3  of the first and second regions  33 - 1  and  33 - 2  of the microstructured cladding area  33 . When the thickness E 2  of the first and second regions  32 - 1  and  32 - 2  of the flip area  32  is equal to the thickness E 3  of the first and second regions  33 - 1  and  33 - 2  of the microstructured cladding area  33 , the flip area  32  is merged with the microstructured cladding area  33 . 
       FIG. 6 b    schematically represents a sectional view, along a sectional plane perpendicular to the reference plane P, of a FORP film  30  according to a second variant of the second embodiment. The second variant of the second embodiment is analogous to the second variant of the first embodiment, described previously in relation with  FIG. 3 b   . Thus, according to the second variant of the second embodiment, the flip area  32  is located in at least one part of the microstructured cladding area  33  and in at least one part of the core area  31 . The flip area  32  thus has:
         a first region  32 - 1  situated in at least one part of the first region  33 - 1  of the microstructured cladding area  33  and in at least one part of the core area  31 , and   a second region  32 - 2  situated in at least one part of the second region  33 - 2  of the microstructured cladding area  33  and in at least one part of the core area  31 .       

     The first and second regions  32 - 1  and  32 - 2  of the flip area  32  each have a thickness E 2 , measured perpendicularly to the reference plane P, such that: 
     
       
         
           
             
               E 
               ⁢ 
               
                   
               
               ⁢ 
               2 
             
             ≤ 
             
               
                 
                   E 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 2 
               
               + 
               
                 E 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 3 
               
             
           
         
       
     
     When the thickness E 2  of the first and second regions  32 - 1  and  32 - 2  of the flip area  32  is such that 
                 E   ⁢           ⁢   2     =         E   ⁢           ⁢   1     2     +     E   ⁢           ⁢   3         ,         
the flip area  32  completely covers the microstructured cladding area  33  and the core area  31 .
 
       FIG. 6 c    schematically represents a sectional view, along a sectional plane perpendicular to the reference plane P, of a FORP film  30  according to a third variant of the second embodiment. The third variant of the second embodiment is analogous to the third variant of the first embodiment, described previously in relation with  FIG. 3 c   . Thus, according to the third variant of the second embodiment, the flip area  32  is located in at least one part of the core area  31 . The flip area  32  may thus have a single region, situated in at least one part of the core area  31  and having a thickness E 2 , measured perpendicularly to the reference plane P, which is less than or equal to the thickness E 1  of the core area  31 . When the thickness E 2  of the flip area  32  is equal to the thickness E 1  of the core area, the flip area  32  is merged with the core area  31 . 
     In an alternative not represented in  FIG. 6 c   , the flip area  32  may have:
         a first region  32 - 1  situated in a part of the core area  31  and in contact with the first region  33 - 1  of the microstructured cladding area  33 , and   a second region  32 - 2  situated in a part of the core area  31  and in contact with the second region  33 - 2  of the microstructured cladding area  33 .       

     According to this alternative, the first and second regions  32 - 1  and  32 - 2  of the flip area  32  each have a thickness E 2 , measured perpendicularly to the reference plane P, which is such that: 
     
       
         
           
             
               E 
               ⁢ 
               
                   
               
               ⁢ 
               2 
             
             &lt; 
             
               
                 
                   E 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 2 
               
               . 
             
           
         
       
     
     It may be noted according to the first, second and third variants of the second embodiment, the core area  31  is solid, that is to say filled with a solid material, for example polymer. 
     In the same way as for the FORP fibre  10  according to the first embodiment described previously, in a FORP film  30  according to the second embodiment, the geometry and the dimensions of the core area  31  and the flip area  32  on the one hand, and the properties of the flip dye and the photonic crystal on the other hand, are chosen in order to contribute to high quantum coupling between the fluorescent flip dye and the photonic crystal, in the forbidden spectral band. Thus, in a FORP film  30  according to any of the variants of the second embodiment of the invention, the thickness E 1  of the core area  31  is less than or equal to five times the wavelength of the maximum fluorescence emission of the flip dye. Advantageously, the thickness E 1  of the core area  11  is less than or equal to three times the wavelength of the maximum fluorescence emission of the flip dye. Moreover, the thickness E 2  of the flip area  32  or, if need be, the first and second regions  32 - 1  and  32 - 2  of the flip area  32  is advantageously such that: 
               E   ⁢           ⁢   2     &lt;       λ   E       4   *   n             
with λ E  the wavelength of the maximum fluorescence emission of the flip dye of the flip area  32 , and n the refractive index of the flip area  32 .
 
     A FORP film  30  according to any of the variants of the second embodiment of the invention may advantageously comprise one or more additional characteristics among the following, considered individually or according to any technically possible combinations thereof:
         the FORP film  30  comprises at least one conversion area including a conversion dye, said conversion dye forming a cascade of fluorescent dyes with the flip dye of the flip area  32 . The operating principle of such a cascade of fluorescent dyes has been described previously in the case of the first embodiment, in relation with  FIGS. 5 a  and 5 b   . The conversion area of the FORP film  30  has first and second regions which extend parallel to the reference plane P, on either side of the first and second regions  33 - 1  and  33 - 2  of the microstructured cladding area  33 , or alternatively in at least one part of the first and second regions  33 - 1  and  33 - 2  of the microstructured cladding area  33 .   The FORP film  30  comprises an antireflective layer as described previously in the case of the first embodiment, preferentially surrounding the microstructured cladding area  33  or, if need be, the outermost conversion area of the cascade of fluorescent dyes. In the particular case of the second embodiment, the antireflective layer has first and second regions that extend parallel to the reference plane P, on either side of the first and second regions  33 - 1  and  33 - 2  of the microstructured cladding area  33  or, if need be, on either side of the first and second regions of the outermost conversion area of the cascade of fluorescent dyes.   The FORP film  30  comprises an anti-UV protective layer as described previously in the case of the first embodiment, surrounding preferentially, if need be, the antireflective layer. In the particular case of the second embodiment, the anti-UV protective layer has first and second regions that extend parallel to the reference plane P, on either side, if need be, of the first and second regions of the antireflective layer.   The FORP film  30  comprises a protective layer against mechanical aggressions as described previously in the case of the first embodiment, preferentially surrounding, if need be, the anti-UV protective layer. In the particular case of the second embodiment, the protective layer against mechanical aggressions has first and second regions that extend parallel to the reference plane P, on either side, if need be, of the first and second regions of the anti-UV protective layer.       

     In order to increase the collection of incident photons, it is advantageously possible to use several optical devices with photon flipping, for example according to the first or the second embodiment that have been described, to form a multi-core optical device with photon flipping. In particular:
         several FORP fibres  10  according to any of the variants of the first embodiment may be used to form a multi-core FORP fibre,   several FORP films  30  according to any of the variants of the second embodiment may be used to form a multi-core FORP film. A multi-core FORP film thus comprises a stack of a plurality of FORP films  30 .       

       FIGS. 7 a  and 7 b    show first and second examples of multi-core FORP fibres  20 , comprising a plurality of FORP fibres  10  according to the first embodiment of the invention. 
       FIG. 7 a    shows a first example of a multi-core FORP fibre  20 , of which the core area  21  comprises a plurality of FORP fibres  10 . According to this first example, the core area  21  of the multi-core FORP fibre  20  comprises seven FORP fibres  10  of “Holey fibre” type, described in particular in  FIG. 4 d   , arranged according to a hexagonal network. The multi-core FORP fibre  20  comprises a cladding area  23  that surrounds the plurality of FORP fibres  10  of the core area  21 . 
       FIG. 7 b    shows a second example of multi-core FORP fibre  20 , of which the core area  21  comprises seven FORP fibres  10  of Bragg fibre type, described in particular in  FIG. 4 c    and arranged according to a hexagonal network. A multi-core FORP fibre  20  has an efficiency greater than the simple sum of the efficiencies of each FORP fibre  10 . Such a multi-core FORP fibre device  20  thus has more extensive structural coherent effects than in a FORP fibre  10 , which is reflected notably by a stronger confinement and a higher quality factor. 
     In particular in  FIGS. 7 a  and 7 b    are illustrated examples of multi-core FORP fibres  20 , of which the core areas  21  each comprise seven fibres FORP  10 . Nevertheless, the core area  21  of a multi-core FORP fibre  20  could naturally comprise a quantity of FORP fibres  10  less than or greater than seven. In particular, a multi-core FORP fibre  20  complying with an aspect of the invention could advantageously have a core area  21  comprising a number of FORP fibres  10  of the order of a thousand, or a million. 
       FIG. 8  shows a third example of a multi-core FORP fibre  20 , comprising a cladding area  23  surrounding a core area  21  comprising nine FORP fibres  10  according to the first embodiment. Each of said nine FORP fibres  10  is, in the particular example represented in  FIG. 7 , of “Holey fibre” type, more particularly described previously in relation with  FIG. 4 a   . The particular advantage of this third example of multi-core FORP fibre  20  resides in the fact that several different fluorescent flip dyes are used in the respective flip areas of the FORP fibres  10 . Said fluorescent flip dyes are advantageously chosen in order that the sum of their respective absorption spectral bands covers the widest possible spectral width, while overlapping as little as possible. Thus, a multi-core FORP fibre  20  according to this third example advantageously has increased efficiency for capturing incident photons Ph_i. 
     In the particular example represented in  FIG. 8 , there is thus:
         first, second and third FORP fibres  10 - 1 ,  10 - 2  and  10 - 3  of which the flip area comprises a first flip dye,   fourth and fifth FORP fibres  10 - 4  and  10 - 5  of which the flip area comprises a second flip dye, distinct from the first flip dye,   sixth and seventh FORP fibres  10 - 6  and  10 - 7  of which flip area comprises a third flip dye, distinct from the first and second flip dyes,   eighth and ninth FORP fibres  10 - 8  and  10 - 9  of which flip area comprises a fourth flip dye, distinct from the first, second and third flip dyes.       

     A multi-core optical device with photon flipping, such as a multi-core FORP fibre or a multi-core FORP film, may advantageously comprise one or more additional characteristics among the following, considered individually or according to any technically possible combinations thereof:
         the optical device with photon flipping comprises at least one conversion area including a conversion dye, said conversion dye forming a cascade of fluorescent dyes with the flip dye of each FORP fibre  10  or FORP film  30  of said multi-core device. The operating principle of such a cascade of fluorescent dyes has been described previously. The conversion area preferentially surrounds the cladding area of the multi-core device, or is located alternatively in at least one part of the cladding area of the multi-core device.   the multi-core optical device with photon flipping comprises an antireflective layer as described previously, preferentially surrounding the cladding area or, if need be, the outermost conversion area of the cascade of fluorescent dyes.   the multi-core optical device with photon flipping comprises an anti-UV protective layer as described previously, preferentially surrounding, if need be, the antireflective layer.   the multi-core optical device with photon flipping comprises a protective layer against mechanical aggressions as described previously, preferentially surrounding, if need be, the anti-UV protective layer.       

       FIGS. 9 a  and 9 b   , described jointly, show respectively a first example of a multi-core FORP fibre  20 , as described in particular in  FIG. 7 a   , and a second example of a multi-core fibre FORP  20 , as described in particular in  FIG. 7 b   , further comprising:
         a first conversion area  22  including a first fluorescent conversion dye,   a second conversion area  23  including a second fluorescent conversion dye,   a third conversion area  24  including a third fluorescent conversion dye,   a fourth conversion area  25  including a fourth fluorescent conversion dye,   an antireflective and anti-UV protective layer  26 .       
     The fourth conversion area  25  surrounds the third conversion area  24 . The third conversion area  24  surrounds the second conversion area  23 . The second conversion area  23  surrounds the first conversion area  22 . All of the conversion areas  22 ,  23 ,  24  and  25  may either surround the cladding area  23 , or be located partially or completely in the cladding area  23 . The antireflective and anti-UV protective layer  26  surrounds the fourth conversion area  25 . In the case where the fourth conversion area  25  is in the cladding area  23 , the antireflective and anti-UV protective layer  26  surrounds the cladding area  23 . 
     An order of magnitude of the dimensions of a multi-core FORP fibre  20  may for example be the following:
         diameter of the core area  21 , including the plurality of FORP fibres  10 : of the order of 400 μm,   thickness of the cladding area  23 , including potentially one or more conversion areas of a cascade of fluorescent conversion dyes: of the order of 50 μm, i.e. an accumulated diameter for the core area and the cladding area  23  of the order of 500 μm,   thickness of the potential protective antireflective and anti-UV layer: of the order of 30 μm, i.e. an accumulated diameter for the core area, the cladding area  23  and the antireflective and anti-UV protective layer of the order of 560 μm,   thickness of the potential protective layer against mechanical aggressions: of the order of 20 μm, i.e. a cumulated diameter for the core area, the cladding area  23 , the antireflective and anti-UV protective layer and the protective layer against mechanical aggressions of the order of 600 μm.       

     Several examples of optical solar concentrators comprising at least one optical device with photon flipping, such as a FORP fibre  10  according to the first embodiment or a FORP film  30  according to the second embodiment will now be described. 
     Generally speaking, an optical solar concentrator includes:
         at least one optical device with photon flipping, that is to say for example at least one FORP fibre  10  or at least one FORP film  30 ;   at least one photovoltaic cell Ce.       

     An optical solar concentrator may advantageously comprise one or more additional characteristics among the following, considered individually or according to any technically possible combinations thereof:
         at least one means of guiding the photons emitted by the at least one device with photon flipping to the at least one photovoltaic cell Ce,   a reflective film, for example made of aluminised polymer. The reflective film advantageously makes it possible to increase the collection efficiency of incident photons. In fact, in the case where an incident photon has passed through the at least one device with photon flipping without interacting with a fluorescent flip dye, this incident photon may be reflected by the reflective film and thus acquire a new possibility of being captured, converted and guided by said at least one device with photon flipping,   a protective screen, advantageously making it possible to protect the optical solar concentrator against mechanical aggressions and/or against ultraviolet radiation,   a frame, for example made of polycarbonate or of epoxy resin. The frame advantageously makes it possible to increase the robustness of the optical solar concentrator, and to facilitate the mechanical implantation of the different elements of the optical solar concentrator during its manufacture.       

       FIG. 10 a    schematically illustrates a first example of an optical solar concentrator  40  comprising a FORP fibre  10 . The optical solar concentrator  40  further comprises first and second photovoltaic cells Ce arranged on either side of the FORP fibre  10 . 
       FIG. 10 a    shows a source S of polychromatic and multidirectional light; this source S is typically the sun. The source S emits a polychromatic radiation R, of which the spectrum is for example comprised between 400 nm and 800 nm. This radiation R is represented in  FIG. 10 a    for a given angle of incidence. The FORP fibre  10  captures laterally at least one part of the polychromatic and multidirectional radiation R in its microstructured cladding area  13 . The FORP fibre  10  converts in its flip area  12  at least one part of the captured radiation R into a quasi-monochromatic and unidirectional flux F, along the axis of the FORP optical fibre. Finally, the FORP fibre  10  guides the flux F in its core area  11 . First and second photovoltaic cells Ce are arranged on either of the FORP fibre  10  so as to capture the flux F emitted at the output of the FORP fibre  10 . The photovoltaic cells Ce can thus ensure a conversion of the luminous flux F into electricity. 
       FIG. 10 b    schematically illustrates a second example of an optical solar concentrator  40  comprising a plurality of FORP fibres  10 , forming a first carpet T 1  of first FORP fibres  10 . The first carpet T 1  extends parallel to a first plane P 1 . The first FORP fibres  10  extend along a first axis A 1 . The optical solar concentrator of  FIG. 10 b    further comprises first and second sets of photovoltaic cells Ce, forming first and second photovoltaic lines L 1  and L 2 . The first and second photovoltaic lines L 1  and L 2  are arranged on either side of the first carpet T 1 , perpendicularly to the first axis A 1 , so as to be able to capture then to convert into electricity the luminous flux emitted by the first FORP fibres  10 . 
       FIG. 10 c    schematically illustrates an exploded view of a third example of an optical solar concentrator  40  comprising first and second carpets T 1  and T 2  of FORP fibres  10 . More precisely, the optical solar concentrator  40  comprises a frame  43  extending along the first plane P 1 , and a reflective film  41  extending onto the frame  43 . 
     The first carpet T 1  of first FORP fibres  10  extends onto the reflective film  41  and a second carpet T 2  of second FORP fibres  10  extends onto the first carpet T 1 . The first FORP fibres  10  of the first carpet T 1  extend parallel to the first axis A 1 . The second FORP fibres  10  of the second carpet T 2  extend parallel to a second axis A 2  substantially perpendicular to the first axis A 1 . 
     The optical solar concentrator  40  according to the third example illustrated in  FIG. 10 c    comprises first, second, third and fourth photovoltaic lines L 1 , L 2 , L 3  and L 4 , each photovoltaic line including at least one photovoltaic cell Ce. 
     The first and second photovoltaic lines L 1  and L 2  are intended to receive photons emitted by the first FORP fibres  10  of the first carpet T 1 . The first and second photovoltaic lines L 1  and L 2  are advantageously arranged on either side of the first carpet T 1 , along an axis substantially perpendicular to the first axis A 1 . The first and second lines T 1  and T 2  may for example extend onto the frame  43 . First and second prisms P 1  and P 2  could be used in order to contribute to guiding the photons emitted by the first FORP fibres of the first carpet T 1 , respectively to the first and second photovoltaic lines L 1  and L 2 . 
     In an analogous manner, the third and fourth photovoltaic lines L 3  and L 4  are intended to receive photons emitted by the second FORP fibres  10  of the second carpet T 2 . The third and fourth photovoltaic lines L 3  and L 4  are advantageously arranged on either side of the second carpet T 2 , along an axis substantially perpendicular to the second axis A 2 . The third and fourth lines T 3  and T 4  may for example extend onto the frame  43 . Third and fourth prisms P 3  and P 4  could be used in order to contribute to guiding photons emitted by the second FORP fibres of the second carpet T 2 , respectively to the third and fourth photovoltaic lines L 3  and L 4 . Finally, a protective screen  42  may advantageously extend onto the second carpet T 2 . 
     In the example that has just been described, each of the first and second carpets T 1  and T 2  extends parallel to the first plane P 1 . Alternatively, the first and second carpets T 1  and T 2  of FORP fibres may respectively extend along a curved plane. 
     The particular case has been described where the first FORP fibres of the first carpet T 1  are arranged substantially perpendicularly to the second FORP fibres of the second carpet T 2 . Nevertheless, the angle between the axis of the first FORP fibres of the first carpet T 1  on the one hand, and the axis of the second FORP fibres of the second carpet T 2  on the other hand, could more generally be comprised between 0° and 360°. 
     It is also entirely possible to use at least one multi-core fibre  20  in an optical solar concentrator  40 , and for example at least one carpet of multi-core fibres  20 . Similarly, it is also possible to use at least one multi-core film in an optical solar concentrator  40 . 
     The photovoltaic cells Ce are chosen so that their range of maximum photovoltaic quantum effect efficiency coincides with the wavelength of the photons emitted at the output of the optical device with photon flipping. 
     The photovoltaic cells Ce may typically be made from a semiconductor material. The photovoltaic cells Ce are then chosen so that the wavelength of the forbidden spectral band of said semiconductor material is different to the wavelength of the photons emitted at the output of the optical device with photon flipping. 
     The case is considered where the optical solar concentrator  40  comprises several optical devices with photon flipping, comprising respectively different fluorescent flip dyes and wavelength ranges. In this case, for each wavelength range, a different type of photovoltaic cell Ce could naturally be used, for example made from different semiconductor materials, chosen to optimise the capture of photons lying within said wavelength range. 
     Alternatively, the photovoltaic cells may be multi-junction semiconductor cells. Thus, several beams of photons of different wavelengths may be captured efficiently by a single type of photovoltaic cell. This possibility is notably interesting in the case of an optical solar concentrator  40  comprising several FORP fibres  10  or several FORP films  30  emitting in different wavelength ranges, and/or at least one multi-core fibre  20  or at least one multi-core film emitting in different wavelength ranges. 
     Other devices may be envisaged, which implement at least one optical device with photon flipping according to an embodiment of the invention. Thus, an optical device with photon flipping according to an embodiment of the invention may advantageously be used to concentrate an incident light flux, for example solar radiation, on the following devices:
         a catalytic cell, for example a hydrogenation by catalysis cell, notably in order to synthesise fuels such as hydrogen, or hydrocarbons such as alkanes or alcohols;   a laser rod, in order to pump it.