EP2174171A1

EP2174171A1 - Hybrid microstructured optical fibre for guidance by means of photonic forbidden bands and total internal reflection optimised for non-linear applications

Info

Publication number: EP2174171A1
Application number: EP08837519A
Authority: EP
Inventors: Yves Quiquempois; Géraud Bouwmans; Mathias Perrin; Aurélie Betourne; Marc Douay; Karen Delplace; Antoine Le Rouge; Laurent Bigot
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite de Lille 1 Sciences et Technologies
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite de Lille 1 Sciences et Technologies
Priority date: 2007-07-26
Filing date: 2008-07-25
Publication date: 2010-04-14
Also published as: EP2020611A1; US20100226614A1; WO2009047404A1; JP2010534860A; US8442372B2

Abstract

The microstructured optical fibre comprises a core (4) surrounded by a sheath (1) comprising a base material having a refraction index (ni) and a plurality of at least two different types of inclusion: a first type of inclusion (2) having a refraction index n2 (n2 >n1), and a second type of inclusion (3) having a refraction index n3 (n3<n1). The inclusions (2, 3) are arranged and dimensioned in such a way as to ensure guidance, by total internal reflection (RTI), of a fundamental mode of the light, centred on a wavelength λRTι, and of a fundamental mode of the light in the first photonic forbidden band (BG1), centred on a wavelength λBG1, which is different to that λRτι of the fundamental mode guided by total internal reflection (RTI).

Description

HYBRID MICROSTRUCTURE OPTICAL FIBER FOR PERMITTED PHOTONIC BAND GUIDING

TOTAL INTERNAL REFLECTION AND OPTIMIZED FOR

NON-LINEAR APPLICATIONS

Technical area

The present invention relates to a hybrid microstructured optical fiber for guiding light by both a photonic bandgap mechanism and a total internal reflection mechanism. This hybrid microstructured optical fiber has a new transverse section, which makes it particularly suitable for non-linear applications, such as, for example, but not exclusively, frequency doubling (w + w -> 2w), frequency trimming (w + w + w -> 3w), the generation of twin photons (2w -> w + w), the generation of triplet photons (3w -> w + w + w), etc ... This optical fiber can advantageously be used in many technical fields, such as, for example, high-power "all-fiber" lasers, the securing of telecommunications networks by quantum cryptography, the generation of new frequencies by non-linear effects in various technical fields, and for example in the field of telecommunications or in the biomedical field.

Prior art

Recent research in the field of optical fibers has led to the development of a new type of so-called microstructured fibers, also known as photonic crystal fibers (PCF) or photonic bandgap fibers. fibers "or" PBGF ").

In general, these microstructured fibers comprise a core ("core") surrounded by a sheath ("cladding"), which is made of a base material ("background material") most often silica, and which comprises a transverse network of longitudinal inclusions having refractive indices different from the refractive index of the base material of the sheath.

The refractive index of the base material of the optical fiber (eg silica) varies with wavelength (dispersion). Unlike index-jump optical fibers, the microstructured fibers advantageously make it possible, by virtue of the presence of said inclusions of different refractive index, to modify this dispersion considerably.

A first type of inclusions used consists of inclusions having a refractive index higher than the refractive index of the base material. For example, but not exclusively, when the base material is silica, a first type of inclusions of higher refractive index may consist of germanium oxide (GeO ₂ ) doped silica inclusions.

A second type of inclusions used consists of inclusions having a refractive index lower than the refractive index of the base material. For example, but not exclusively, when the base material is silica, a second type of inclusions of lower refractive index may consist of cavities or holes filled with a fluid (gas or liquid), and the more often filled with air.

Depending on the case, the sheath may comprise only inclusions of high refractive index, or only inclusions of low refractive index, or a mixture of the two types of inclusions.

As the case may be, the inclusions may be arranged in the form of a periodic network or may be randomly distributed, or distributed in a configuration comprising both periodic and non-periodic regions.

Depending on their transverse structure, the microstructured optical fibers allow a propagation of the light along the longitudinal axis of the fiber, by a total internal reflection guide or a guide by photonic forbidden bands.

In 2003 Jesper Laegsgaard and Anders Bjarklev described in the publication "Doped photonic bandgap fibers for short-wavelength no ϋnear devices ", Vo! 28, No. 10, p 783 (2003), OPTICS LETTERS, a microstructured optical fiber having germanium oxide doped silica pads and air holes, and having a visible dispersion zero. The microstructured optical fiber described in this publication allows light guidance only in photonic band gaps, and does not allow light guidance by total internal reflection, due to the presence of the six central air holes.

In 2006, Arismar Cerqueira et al described in the publication entitled "Hybrid photonic crystal fiber", Vol 14, No. 2, p. 926 (2006), OPTiCS EXPRESS, a microstructured optical fiber that includes air holes and contact pins. germanium-doped silica inserted into an undoped silica base material, and whose transverse structure allows the light to be guided both by a photonic band mechanism prohibited in a first direction, and by total internal reflection in a second direction perpendicular to said first direction. In this fiber structure, and for the same wavelength, the mode is guided both by photonic forbidden bands and by total reflection.

The microstructured fibers described in the aforementioned publications can not be used for nonlinear applications, such as, for example, frequency doubling (w + w -> 2w), frequency trimming (w + w + w -> 3w) , the generation of twin photons (2w -> w + w), the generation of triplet photons (3w -> w + w + w), etc. To date, in these nonlinear optical applications, crystals are used nonlinear. These nonlinear crystals have several disadvantages. In such crystals, the optical wave is not guided, so that the nonlinear interaction lengths are of the order of a few centimeters against a few meters for the optical fibers. In addition the insertion of massive components implementing these non-linear crystals inevitably leads to losses and alignment problems, and therefore to fragile and expensive devices. Another study published by A. Efimov, AJ. Tayior, FG Omenetto, JC Knight, WJ. Wadsworth and P. StJ. Russel in the publication "Phase matched third harmonies generation in microstructured fibers", October 6, 2003, Vol11 No 20 p 2567 - OPTICS EXPRESS, describes a solution for obtaining a frequency tripling with a phase index agreement. The solution is to achieve a phase agreement between the fundamental mode propagates by total internal reflection in the microstructured optical fiber and higher order transverse modes, especially third harmonic. However, the results obtained in this study are not really satisfactory, because the transverse structure of the third harmonic mode has multiple lobes, which is detrimental to the performance and exploitation of this solution.

Objectives of the invention

An object of the invention is to propose a new microstructured optical fiber which can advantageously be used as a replacement for nonlinear crystals in nonlinear optical applications such as, for example, the generation of twin photons (2w - ^ w + w with w = 2πc / λ, where c is the speed of light in vacuum, or more generally Wi -> W ₂ + W ₃ ), the generation of triplet photons (3w -> w + w + w or more generally w- | - > W ₂ + W ₃ + W ₄ ), or such as for example the generation of frequencies by nonlinear effect, and in particular the mixture with 3 waves (wi + w ₂ -> W ₃ ) or more particularly the doubling of frequency (w + w -> 2w), the 4-wave mixture (W ₁ + W ₂ + W ₃ -> W ₄ , W ₁ + W ₂ -> W ₃ + W ₄ ) or more particularly the frequency tripling (w + w + w -> 3w).

Summary of the invention

This objective is achieved by the microstructured optical fiber defined in claim 1.

This microstructured optical fiber of the invention has the following technical characteristics, known and described in the aforementioned publication "Doped photonic bandgap fibers for short- waveIeπgth non linear devices », Vo! 28, N ⁰ IO, p 783 (2003), OPTICS LETTERS, Jesper and Anders Laegsgaard Bjarklev. It comprises a core surrounded by a sheath, the sheath comprising a base material of refractive index (ni) and a plurality of inclusions of at least two different types, a first type of refractive index inclusions ( n2) greater than the refractive index (ni) of the sheath material, and a second type of inclusions with index (n3) less than the refractive index (ni) of the base material, said fiber permitting a Photonic bandgap guidance of a fundamental mode of light

Characteristically according to the invention, the inclusions of the sheath are arranged and dimensioned so as to allow a total internal reflection guidance (RTI) of a fundamental mode of light, centered on a wavelength λ _R τι, and guiding a fundamental mode of light in the first photonic forbidden band (BG1), centered on a wavelength λ _BG i which is less than that ATRTI of the fundamental mode guided by total internal reflection (RTI).

The microstructured optical fiber of the invention may also include other additional and optional technical features mentioned in the appended claims.

Brief description of the drawings

The characteristics and advantages of the invention will appear more clearly and in a more complete manner on reading the following detailed description of a preferred variant embodiment of a microstructured optical fiber of the invention, which detailed description is given in As a nonlimiting and non-exhaustive example of the invention, and with reference to the accompanying drawings in which:

FIG. 1 is a cross-sectional view of a microstructured optical fiber according to a first embodiment of the invention;

Figure 2 is an enlargement of a region of the cross-section of Figure 1; FIG. 3 is a curve showing the variation of the refractive index difference (Δ) of the pure silica at two wavelengths λ / 2 and λ, for pump wavelength λ between 0.3 μm and 3 μm, the refractive index of the silica according to Sellmeier's law; FIG. 4 is a curve representing the variation of the refractive index difference (Δ) of the pure silica at two wavelengths λ / 3 and λ, for pump wavelengths λ between 0.3 μm and 3 μm, the refractive index of the silica according to Sellmeier's law; FIG. 5 is a diagram of the photonic forbidden bands of an optical fiber comprising the same network of inclusions of high refractive index as the fiber of FIG. 1, but without defects at the center and without inclusion of a weak index (holes of air), the index of the silica being assumed to be constant and equal to 1.45; FIG. 6 is a diagram of photonic forbidden bands of an optical fiber comprising the same network of inclusions as the fiber of FIG. 1, but without defects in the center, the ratio d ₃ / Λ of the inclusions of index n ₃ of a network of 0.04, the index of the silica being assumed to be constant and equal to 1.45; FIG. 7 is a diagram of photonic forbidden bands of an optical fiber comprising the same network of inclusions as the fiber of FIG. 1, but without defect in the center, the ratio d ₃ / Λ of inclusions of index n ₃ being 0.1518, the index of the silica being assumed to be constant and equal to 1.45; FIG. 8 is a diagram showing the effective index variation, as a function of the wavelength (in μm), of the fundamental modes in the first photonic forbidden band (BG1) and in the total internal reflection band (RTI). of a microstructured optical fiber according to FIG. 1, and characterized by a pitch Λ of inclusions network equal to 0.798 μm and by a ratio d ₃ / Λ of inclusions of index n ₃ equal to 0.1518, the index of the silica varies according to Sellmeier's law; FIG. 8 is a diagram showing the effective index variation, as a function of the wavelength (in μm), of the fundamental modes in the first photonic forbidden band (BG1) and in the total internal reflection band (RTI) of a microstructured optical fiber according to FIG. 1, and characterized by a pitch Λ of inclusions network equal to 1.386 μm and by a ratio d ₃ / Λ of inclusions with index n ₃ equal to 0.1518, the index silica varying according to Sellmeier's law; FIG. 10 is a diagram showing the effective index variation, as a function of the wavelength (in μm), of the fundamental modes in the first photonic forbidden band (BG1) and in the total internal reflection band (RTI). of a microstructured optical fiber according to FIG. 1, and characterized by a pitch Λ of an inclusions network equal to 1.785 μm, and by a ratio d ₃ / Λ of inclusions of index n ₃ equal to 0.1518, the silica index varying according to Sellmeier's law; FIG. 11 is a diagram showing the effective index variation, as a function of the wavelength (in μm), of the fundamental modes in the first photonic forbidden band (BG1) and in the total internal reflection band (RTI) of a microstructured optical fiber according to FIG. 1, and characterized by a pitch Λ of inclusions network equal to 0.585μm and by a ratio d ₃ / Λ of inclusions with index n ₃ equal to 0.1518, the index of the silica varies according to Sellmeier's law;

FIG. 12 is a diagram showing the effective index variation, as a function of the wavelength (in μm), of the fundamental modes in the first photonic forbidden band (BG1) and in the total internal reflection band (RTI). of a microstructured optical fiber according to FIG. 1, and characterized by a pitch Λ of inclusions network equal to 0.92 μm and by a ratio d ₃ / Λ of inclusions of index n ₃ equal to 0.1518, the index the silica varies according to the law of Selimeier;

FIG. 13 is a curve representing the evolution (in μm) of the grating pitch Λ of an optical fiber according to FIG. 1, as a function of a pump length λ _RT ι between 0.6 μm and 2 μm, for obtain a doubling of frequency from this pump wavelength, the index of the silica varying according to the Selimeier law;

FIG. 14 is a curve representing the evolution (in μm) of the grating pitch Λ of an optical fiber according to FIG. 1, as a function of a pump length λ _R τι between 1 μm and 2 μm, to obtain a frequency tripling from this pump wavelength, the index of the silica varying according to the Selimeier law;

FIG. 15 is a cross-sectional view of a microstructured optical fiber according to a second variant embodiment of the invention;

- Figure 16 is an enlargement of a region of the cross section of Figure 15.

FIG. 17 is a cross-sectional view of a microstructured optical fiber according to a third embodiment of the invention;

FIG. 18 is a diagram showing the effective index variation, as a function of the wavelength (in μm), of the fundamental modes in the first photonic forbidden band (BG1) and in the total internal reflection band (RTI). ) of a microstructured optical fiber according to FIG. 17, and characterized by a pitch Λ of inclusions network equal to 1.79 μm and by a ratio ds / Λ of inclusions with index n ₃ equal to 0.4, the index silica varying according to Selimeier's law;

FIG. 19 is a diagram showing the effective index variation, as a function of the wavelength (in μm), of the fundamental modes in the first photonic forbidden band (BG1) and in the total internal reflection band (RTI). ) an optical fiber microstructured according to FIG. 17, and characterized by a pitch Λ of inclusions network equal to 2.06 μm and by a ratio d ₃ / Λ of inclusions of index n ₃ equal to 0.4, the index of the silica varying according to the Sellmeier law; FIG. 20 is a diagram showing the effective index variation, as a function of the wavelength (in μm), of the fundamental modes in the first photonic forbidden band (BG1) and in the total internal reflection band (RTI). of a microstructured optical fiber according to FIG. 17, and characterized by a pitch Λ of inclusions network equal to 2.67μm and by a ratio d ₃ / Λ of inclusions with index n ₃ equal to 0.4, the index silica varying according to Sellmeier's law; FIG. 21 is a curve representing the evolution (in μm) of the grating pitch Λ of an optical fiber according to FIG. 17, as a function of a pump length between 0.8 μm and 3 μm, to obtain a doubling of frequency from this pump wavelength, the silica index varying according to Sellmeier's law; Figure 22 a cross-sectional view of a microstructured optical fiber according to a fourth embodiment of the invention; FIG. 23 is a diagram showing the effective index variation, as a function of the wavelength (in μm), of the fundamental modes in the first photonic forbidden band (BG1) and in the total internal reflection band (RTI) of a microstructured optical fiber according to FIG. 22, and characterized by a pitch Λ of inclusions network equal to 1, 22 μm and by a ratio d ₃ / Λ of inclusions of index n ₃ equal to 0.32, the index silica varying according to Sellmeier's law; FIG. 24 is a diagram showing the effective index variation, as a function of the wavelength (in μm), of the fundamental modes in the first photonic forbidden band (BG 1) and in the total internal reflection band (RT!) of a microstructured optical fiber according to FIG. 22, and characterized by an inclusion network pitch Λ of 1.42 μm and by a ratio d ^ of inclusions of index n ₃ equal to 0.32, the silica index varying according to Sellmeier's law;

FIG. 25 is a diagram showing the effective index variation, as a function of wavelength (in μm), of the fundamental modes in the first photonic forbidden band (BG1) and in the total internal reflection band (RTI). ) of a microstructured optical fiber according to FIG. 22, and characterized by a pitch Λ of inclusions network equal to 1.86 μm and by a ratio d ₃ / Λ of inclusions of index n ₃ equal to 0.32, the silica index varying according to Sellmeier's law,

FIG. 26 is a curve representing the evolution (in μm) of the grating pitch Λ of an optical fiber according to FIG. 22, as a function of a pump length between 0.8 μm and 2.5 μm, to obtain a frequency doubling from this pump wavelength, the silica index varying according to Sellmeier's law;

detailed description

Referring to Figure 1, a preferred embodiment of the hybrid microstructured optical fiber of the invention comprises a sheath 1 which is made of a base material ("background") of refractive index (ni), in which is realized a periodic network of inclusions 2, 3 which extend over the entire length of the fiber. In the center of the fiber, this network of inclusions 2, 3 has a central defect 4 (absence of inclusions) forming the core of the optical fiber.

In general, the inclusions 2 are characterized by a refractive index n2 which is greater than the refractive index and the material of the sheath. The inclusions 3 are characterized by a refractive index n3 which is lower than the refractive index and the base material of the sheath 1. In the variants illustrated in the figures, the core 4 of the fiber is made of the same material as the base material of the sheath 1, and thus has a refractive index n4 identical to the refractive index and the sheath. . This is however not limiting of the invention; in another variant, the core 4 could have a refractive index n4 different from the refractive index and the base material of the sheath. In all cases, it will be ensured that the refractive index n2 of the inclusions 2 is preferably greater than the index of refraction n4 of the core 4. When the core 4 is made of a homogeneous material, its refractive index n4 corresponds to the refractive index of said material. If the core 4 is non-homogeneous, the refractive index of the heart will be a mean index. Thus, in the present text, the term "index of refraction n4" denotes the refractive index of the material constituting the core 4 in the case of a homogeneous core, or the average refractive index of the core 4 in the case of a non-homogeneous heart 4.

More particularly, the sheath 1 is for example silica. The inclusions 2 consist of longitudinal plots of silica doped with germanium oxide (GeO ₂ ). Inclusions 3 are longitudinal air holes.

The invention is not limited to this particular type of inclusions. For example, and non-exhaustively, the inclusions 2 of higher refractive index could consist of phosphorus doped silica, or glasses of tellurium, chalcogenide, ... The air holes 3 can be replaced by longitudinal cavities filled with a fluid (liquid or gas different from air) having a refractive index lower than the index of refraction (ni) of the base material of the sheath 1 (such as, for example, water, ethanol). The air holes 3 may be replaced by longitudinal solid pads made of a material having a refractive index lower than the refractive index and the base material of the sheath 1, for example fluorine-doped silica, Titanium oxide doped silica (TiO ₂ ). Although for the implementation of the invention, silica is a material particularly suitable for producing the sheath 1, the invention is however not limited to this material alone. For example, the base material of the cladding of the optical fiber could consist of chalcogenide glasses, tellurium glasses or any other glass having negligible absorption in the spectral regions considered by the invention.

The invention is not limited to the implementation of only two different types of inclusions 2 and 3, but in other embodiments, an optical fiber according to the invention may comprise more than two different types of embodiments. inclusions.

In Figures 1 and 2, the longitudinal inclusions 2 have a circular cross section of diameter d ₂ , and the longitudinal inclusions 3 have a circular cross section of diameter d ₃ . The invention is not limited to these particular forms, the inclusions 2 and 3 may have in cross section forms totally different from those illustrated in Figure 1. In addition, the inclusions 2 (or 3) of the same index of refraction are not necessarily identical and may have different cross-sectional dimensions and / or shapes.

In the particular example illustrated in Figure 1, and without limitation of the invention, the inclusion network 2,3 is periodic and is more particularly honeycomb type. With reference to FIG. 2, this periodic network is characterized by a network pitch Λ, also commonly referred to as "pitch", and corresponding in this example to the distance between the centers of two adjacent inclusions 2.

More particularly in this network of honeycomb inclusions, the inclusions 2 are arranged in the form of concentric hexagonal rings (three crowns in the illustrated example), and each inclusion 2 is surrounded by six inclusions 3 regularly distributed over its periphery, with the exception of inclusions 2 of the central crown. The core 4 of the optical fiber (default in the periodic network) is obtained by removing in the center of this periodic network a central inclusion 2 and its six peripheral inclusions 3.

It will be noted that the inclusions of the central ring surrounding the core 4 consist of inclusions 2 of high refractive index n2, all the inclusions 3 of low refractive index n3 being located outside this central ring by report to the heart. Thus, the light that propagates in the core 4 sees inclusions of refractive index greater than the refractive index n4 of the core 4, which contributes to obtaining a total internal reflection guidance. Conversely, if in such a periodic network, the central ring surrounding the core was constituted by inclusions 3 of low refractive index n3 (n3 <n4), such a mode of propagation by total internal reflection would be difficult.

In the example of Figure 1, the core 4 is homogeneous and made of the same material as the base material of the sheath. In the context of the invention, it is possible to introduce into the core 4 defects that may be non-periodic and of very small size relative to the pitch Λ of the grating, such as micrometric air holes, provided that the refractive index n2 of the inclusions 2 of the central ring surrounding the core 4 is greater than the refractive index n4 of the core 4.

Although the implementation of a periodic network of inclusions, in particular of the honeycomb type, is particularly suitable for producing a microstructured optical fiber of the invention, the invention is not limited to this type of configuration of the inclusions, but more generally extends to any distribution of inclusions in cross section (periodic distribution, non-periodic, or comprising both periodic regions and non-periodic regions) making it possible to obtain the technical characteristics of the claim 1.

In the remainder of the description, and for the sake of clarity and In a simplification of the disclosure of the invention, the technical considerations apply to a microstructured optical fiber having the cross-section of FIGS. 1 and 2, with a sheath 1 made of silica, inclusions 2 made of silica doped with germanium oxide (GeO ₂ ), and inclusions 3 constituted by air holes. It is, however, within the immediate scope and in the ordinary skills of a person skilled in the art to adapt and to transpose these technical considerations to produce an optical fiber of the invention having a different transverse structure.

Figures 3 and 4 have been calculated considering that the refractive index of silica follows Sellmeier's law (IH Malitson, "Interspecimen Comparison of the Refractive Index of Fused Silica", JOSA 55, 1205 (1965)). These FIGS. 3 and 4 show that with a homogeneous material of index n i, the refractive index differential (Δ) is never zero, but goes through a minimum. It follows that the refractive indices n _λ and n _{\ /} z at the two wavelengths λ and λ / 2 are never equal, and that the refractive indices n _\ and r \ _{\ /} 3 at the two lengths of waves λ and λ / 3 are never equal. Thus, with a homogeneous material of index ni, it is not possible to have a phase index agreement between two wavelengths [(FIG. 3 - λ and λ / 2); (Figure 4 - λ and λ / 3)].

In the optical fiber of the invention, the inclusions 2 and 3 are dimensioned and arranged in such a way that said fiber makes it possible to guide a fundamental (quasi-Gaussian) mode of the light by photonic forbidden bands, and to guide a fundamental mode (quasi-Gaussian type) of the light by total internal reflection, according to the same direction of propagation (longitudinal direction of the fiber), the fundamental mode guided in the first photonic forbidden band (BG1) and the mode fundamental guided by total internal reflection being respectively centered on different wavelengths λβGi and λRτι.

In addition, according to an optional and additional characteristic, the optical fiber of the invention allows a phase index agreement at these two wavelengths (λ _R γi, λβ _d ) between the fundamental mode guided in the first forbidden band (BG1) and the fundamental mode guided by total internal reflection, and in particular at two lengths (ΛRTI, ABGI) double or triple.

In an optical fiber having the particular structure of FIG. 1, the appearance of these two modes of light guidance depends in particular on the diameter (d3) of the inclusions of index n3. This will be better understood in the light of Figures 5 to 12 which will now be detailed.

The curves of FIGS. 5 to 7 were obtained using the MIT Photonic-bands (MPB) software. This software uses the plane wave development method to calculate the structure of photonic band gaps in a periodic structure. The index of the silica is assumed to be constant and equal to 1.45 for the diagrams of FIGS. 5 to 7.

Ref: Steven G. Johnson and JD Joannopoulos, "Block-iterative frequency-domain methods for Maxwell's equations in a planewave basis (http://www.opticsexpress.org/abstract.cfm?URl=OPEX~8-3-173) Optics Express 8 (3) 173-190 (2001).

The results of the curves of FIGS. 8 to 12 were obtained by a finite element vector calculation method implemented using the COMSOL multiphysics ™ software (version 3.3a). This method of calculation consists in solving the Helmholtz equation in a transverse section of the fiber for a given wavelength, and in deducing from it the effective index of the fundamental mode as a function of the wavelength.

In these calculations, for inclusions 3 of low index (air holes), the refractive index of these inclusions was equal to 1. For inclusions 2 of high index, the calculations were performed by modeling the refractive index of each inclusion 2 of high index by the following parabolic law: + nl

Where r is the distance from the center of the inclusion, R is the radius of inclusion, Δn is the refractive index difference between the high-index inclusions 2, and that of the ni-silica. It is further considered that neither follows Sellmeier's law (IH Malitson, "Interspecimen Comparison of the Refractive Index of Fused Silica", JOSA 55, 1205 (1965)). For the calculations, Δn was a constant of 32.10 ^"3 .

The curves of FIGS. 5 to 12 have in addition been calculated for optical fibers whose inclusion network has the following geometrical characteristics (common to FIGS. 5 to 12): d2 / Λ = 0.725, where:

d2 is the diameter of the inclusions 2,

- Λ is the distance between two centers of inclusions 2 (not of the network). Figures 5 to 7 - Influence of the d3 / Λ ratio on the appearance of the total internal reflection guidance mode

The diagrams of photonic forbidden bands of FIGS. 5 to 7 have been calculated for an optical fiber comprising the transverse structure of FIG. 1, but without central defects 4 in the periodic network of inclusions. In addition to these FIGS. 5 to 7, in addition to the photonic forbidden bands (BG1, BG2, BG3,...), There is shown (curve referenced nfsm) the variation of the average effective index of the cladding of the optical fiber (FIG. that is to say, the average index that would have the homogeneous material corresponding to the microstructured material of the sheath, which is characterized by a periodic grating of inclusions of refractive index n3 and n2) as a function of the length of d normalized wave (λ / Λ). The calculation is made assuming further that the silica index is constant and assumed equal to 1.45.

More particularly, for Figure 5, the calculations were made for a network comprising only inclusions 2 of high index (germanium doped silica) and having no air holes 3. The nfsm curve remains above 1.45 (refractive index of pure silica), which demonstrates that it is not possible with such a fiber to guide a fundamental mode of light by total internal reflection.

For FIG. 6, the calculations were made for a network comprising both high index inclusions 2 (germanium doped silica) and low index inclusions 3 (air holes), and for a diameter ratio d ₃ inclusions of index n3 on the pitch Λ equal to 0.04. The nfsm curve remains above 1.45 (refractive index of pure silica), which demonstrates that it is not possible with such a fiber to guide a fundamental mode of light by total internal reflection, the radius inclusions of index n3 being too weak to significantly lower the average refractive index nfsm.

For FIG. 7, the calculations were made for a network comprising both high index inclusions 2 (germanium doped silica) and low index inclusions 3 (air holes), and for a diameter ratio inclusions of refractive index n3 on the network pitch Λ equal to 0.1518. The nfsm curve passes below 1.45 (refractive index of pure silica), which demonstrates that it is possible with such a fiber to guide a fundamental mode of light, not only by prohibited soundtracks, but also by total internal reflection, the ratio O ₃ ZA inclusions of refractive index n3 is sufficiently large, and in particular large enough that the average effective index (nfsm) of the sheath 1 is less than the refractive index and of the base material of the sheath 1, at least for a range wavelength greater than the wavelength λs _G i of the fundamental mode guided in the first photonic forbidden band (BG1). ^r 8 - 12 - phase index agreement

The evolution of the effective index (n) of the fundamental mode in the first forbidden photonic band (BG1) and in the reflection band internal total (RT!) for an optical fiber according to FIGS. 1 and 2 (with inclusions 2 in germanium doped silica and inclusion 3 formed by air holes) has been calculated for different values of the pitch (Λ) of the network of inclusions. The results of these calculations correspond to the curves of FIGS. 8 to 12. The refractive index does not change according to the Sellmeier law. The values of the pitch (Λ) of the inclusion network were as follows:

Figure 8: Λ = 0.798μm Figure 9: Λ = 1.386μm Figure 10: Λ = 1.785μm Figure 11: Λ = 0.588μm Figure 12: Λ = 0.92μm

In these FIGS. 8 to 12, the referenced left curve (Mode BG1) represents the effective index variation (n), as a function of the wavelength (in μm), of the fundamental mode (of the quasi-Gaussian type). guided in the first photonic band gap (BG1); the right-hand referenced curve (RTI mode) represents the effective index variation (n), as a function of the wavelength (in μm), of the fundamental mode (of quasi-Gaussian type) guided in the heart by total reflection internal (RTI).

In this microstructured optical fiber of the invention, inclusions 2 of high refractive index are responsible for the appearance of a guiding mechanism along the longitudinal axis of the fiber of a fundamental mode of light in forbidden bands. photonics, and inclusions 3 of low refractive index are responsible for the appearance of a guiding mechanism along the longitudinal axis of the fiber of a fundamental mode of light by total internal reflection.

The curves of FIGS. 8 to 12 show that in each case it is possible with this microstructured optical fiber to have an equality of the effective indices (n) of the fundamental mode:

(i) at two different wavelengths ΛBGI and λ _RT ι, and

(ii) for a value of the effective index (n) that is less than the index This results in a phase index agreement at the wavelengths ABGI and ΛRTI between respectively the fundamental mode guided in the first forbidden band (BG1) and the fundamental mode guided by the present invention. total internal reflection. It will further be noted in FIGS. 8 to 12 that the difference between said wavelengths AB _G I and _t λ _R τι is such that the wavelength λ _R γι is located beyond the range of length d fundamental mode wave guided in the first forbidden band (BG1).

This phase index agreement allows to use the optical fiber for a mixture at at least three waves, granted in phase, with at least two of these waves having respective wavelengths Λ _BG I _e t ARTI. It is thus possible advantageously to use the optical fiber to generate, with a high optical efficiency, a fundamental mode centered on a wavelength ABGI from a fundamental mode centered on a wavelength λ _R τι of the upper pump. Of course, in the case, for example, of frequency doubling (respectively of frequency tripling), in a manner known per se by those skilled in the art, it will be ensured that the optical fiber also satisfies a susceptibility χ ⁽²⁾ (respectively χ ⁽³⁾ ), which can be obtained by a so-called "poling" method, as taught for example in: PG Kazansky, L. Dong and P. Russell "High second order nonlinearities in poled silicate fiber", Vol. . 19, No 10, p 701 (1994) Optics Letters.

More particularly in the examples of FIGS. 8 to 12, the phase index matching can advantageously be obtained for a wave mixture with frequency doubling or tripling at the following wavelengths:

Figure 8: ΛBGI = 0.4μm and λ _R τι = 0.8μm (frequency doubling) Figure 9: λ _BG i = 0.59μm and λ _RT [= 1, 18μm (frequency doubling) Figure 10: λ _BG i = 0.75μm and λ _RT ι = 1, 5μm (frequency doubling) Figure 11: A _B GI = 0.354μm and λ _RT ι = 1, 064μm (frequency tripling) Figure 12: λ _B ci = 0.5μm and λ _RT i = 1, 5μm (frequency tripling). The examples of Figures 8 to 12 are not limiting and exhaustive of the invention. More generally, the wavelengths (AR-Π; ΛBGI) corresponding to a phase index agreement may be an integer multiple of one another (AR-Π = k Λ _B GI, k being a integer greater than or equal to 2).

The curves of FIGS. 13 and 14 show the evolution of the pitch Λ of the inclusion network 2, 3 as a function of the pump wavelength ARTI, in order to obtain respectively a frequency doubling or a frequency tripling, starting from this ARTI pump wavelength. These curves of FIGS. 13 and 14 show that it is also possible to obtain a phase index matching with frequency doubling or frequency trimming for other values of the pitch Λ of the inclusion network 2, 3. the same is true with respect to the frequency tripling, or more generally the generation of a frequency which is an integer multiple of the pump frequency.

By way of example, FIGS. 15 and 16 show another variant embodiment of an optical fiber of the invention in which the inclusions with index (n3) have a cross section in the shape of an equilateral triangle of width (s), inscribed in a circle (fictitious) of diameter d3.

The invention is not limited to a microstructured optical fiber as described with reference to the appended figures and which could be used only for frequency doubling or frequency tripling by non-linear effects. More generally, the inclusions (2, 3) of the cladding of the microstructured optical fiber of the invention can be arranged to allow mixing with (m) waves by non-linear effects, m being an integer greater than or equal to 3, the inclusions of the cladding of the optical fiber of the invention being such that the fundamental mode guided in the first forbidden band (BG1) and the fundamental mode guided by total internal reflection (RTI) cumulatively respect the conditions (a) and (b) ) below: m being an integer greater than or equal to 3, and ΠAI being the value of the effective index of the fundamental mode propagating in the optical fiber at the wavelength λj _.

In addition according to the invention, among the wavelengths λj complying with the conditions (a) and (b), at least one wavelength (λ _R τι) belongs to the fundamental mode guided by total internal reflection (RTI). , and at least one wavelength (ΛB _G I) belongs to the fundamental mode guided in the first forbidden band (BG 1).

The conditions (a) and (b) described above respectively correspond to the photon energy conservation condition and the wave vector conservation condition, with the following sign conventions: wavelength is considered negative when the photon is annihilated and positive when the photon is created by the wave mixing mechanism.

For example, in the particular case of a three-wave mixture with creation of a photon at W ₃ (m = 3 and Wi + W ₂ -> w ₃ ), the conditions (a) and (b) above become:

For example, in the particular case of a four-wave mixture with creation of a photon at W ₄ (m = 4 and Wi + W ₂ + W ₃ -> w 4), the conditions (a) and (b) above become :

+ - 1 = 0 _Λ o

FIGS. 17 and 22 show two other embodiments of a fiber of the invention comprising inclusions 2, 3 forming a periodic pitch network Λ.

In the variant of FIG. 17, inclusions 2 and 3 are identical in size and shape and differ only in their respective refractive indices n2 and n3. In a manner similar to the variants of FIGS. 1 and 15, the core 4 is surrounded by a hexagonal central crown constituted by inclusions 2 of high refractive index n2, the inclusions 3 of reliable refractive index n3 being all located at outside this central crown. The inclusions 2 of high refractive index n2 are, for example, inclusions of germanium doped silica and the inclusions 3 of low refractive index n3 are, for example, air holes.

The curves of FIGS. 18 to 20 show the evolution of the effective refractive index (n) of the fundamental mode in the first forbidden photonic band (BG1) and guided by total internal reflection (RTI) for an optical fiber according to FIG. 17 (with inclusions 2 germanium doped silica and inclusion 3 formed by air holes). These curves were obtained in the same manner as the curves of Figures 8 to 12 and with the following parameters: d2 / d3 = Λ / Λ = 0.4 .DELTA.N = 32.10- ³ Figure 18: Λ = 1, 79μm Figure 19 : Λ = 2.06μm Figure 20: Λ = 2.67μm

The curves of FIGS. 18 to 21 show that for different steps Λ of the grating, it is possible to produce a three-phase degenerate wave mixture, with two of these waves, which have double wavelengths λ _R τι and λ _B Gi [λRτι = 2.λβGi], and which belong respectively to the fundamental mode guided by total internal reflection (RTI) and the fundamental mode guided in the first photonic forbidden band (BG 1).

In the variant of Figure 22, the inclusions 2 and 3 are of different sizes and form a square periodic network. More particularly, the core 4 is surrounded by a square central crown consisting of inclusions 2 of high refractive index n2, the inclusions 3 of low refractive index n3 being all located outside this central crown. The inclusions 2 of high refractive index n2 are, for example, inclusions of germanium doped silica and the inclusions 3 of low refractive index n3 are, for example, air holes.

The curves of FIGS. 23 to 25 show the evolution of the effective refractive index (n) of the fundamental mode in the first forbidden photonic band (BG1) and guided by total internal reflection (RTI) for an optical fiber according to FIG. 22 (with inclusions 2 germanium doped silica and inclusion 3 formed by air holes). These curves were obtained in the same way as for the curves of FIGS. 8 to 12 and with the following parameters: d2 / Λ = 0.72; d3 / Λ = 0.32 Δn = 30.10 ^"3 Figure 23: Λ = 1, 22μm Figure 24: Λ = 1, 42μm Figure 25: Λ = 1, 86μm

The curves of FIGS. 22 to 26 show that for different steps Λ of the grating, it is possible to achieve a three-wave mixing tuned in phase, with two of these waves, which have double wavelengths λ _RT i and λβd [ λRτι = 2.λ _B Gi], which respectively belong to the fundamental mode guided by total internal reflection (RTI) and to the fundamental mode guided in the first photonic forbidden band (BG1). The invention is not limited to the particular embodiments described with reference to the appended figures. Other embodiments within the scope of those skilled in the art and covered by the appended claims may be envisaged, without departing from the scope of the invention.

Claims

REVENDtCATiQNS

A microstructured optical fiber comprising a core (4) surrounded by a sheath (1), the sheath (1) comprising a base material of refractive index (ni) and a plurality of inclusions of at least two different types , a first type of inclusions (2) having a refractive index (n2) greater than the refractive index (ni) of the base material, and a second type of inclusions (3) having a refractive index (n3) ) less than the refractive index (ni) of the base material, said fiber allowing a photonic bandgap guidance of a fundamental mode of light, characterized in that the inclusions (2, 3) of the sheath are arranged and dimensioned to allow internal total reflection guidance (RTI) of a fundamental mode of light, centered on a wavelength λκr _\ , and guidance of a fundamental mode of light in the first band gap photonics (BG 1), centered on a wavelength λβd which is less than that λRTi of the fundamental mode guided by total internal reflection (RTI).

2. microstructured optical fiber according to claim 2, characterized in that the average effective index (nfsm) of the sheath (1) is less than the refractive index (ni) of the base material of the sheath (1) to less for a longer wavelength range

3. microstructured optical fiber according to one of claims 1 or 2, characterized in that the heart (4) is surrounded by a central ring consisting of the first type of inclusions (2), all inclusions (3) of the second type being located outside this central crown.

4. microstructured optical fiber according to one of claims 1 to 3, characterized in that the refractive index (n2) inclusions (2) is greater than the refractive index (n4) of the core (4).

5. microstructured optical fiber according to one of claims 1 to 4, characterized in that the core (4) of the fiber comprises the same basic material as the sheath (1).

6. microstructured optical fiber according to one of claims 1 to 5, characterized in that the refractive index (n4) of the core (4) is equal to the refractive index (ni) of the base material of the sheath

(1) -

7. microstructured optical fiber according to claim 6, characterized in that the core (4) of the fiber is constituted by the same material as the base material of the sheath (1).

8. microstructured optical fiber according to one of claims 1 to 7, characterized in that the inclusions (2,3) are arranged in the form of a periodic network characterized by a constant pitch (Λ).

9. microstructured optical fiber according to claim 8, characterized in that the dimension of the inclusions (3) of the second type and the pitch (Λ) of the inclusion network are chosen such that the average effective index (nfsm) of the sheath (1) is less than the refractive index (ni) of the base material of the sheath (1) at least for a wavelength range greater than the wavelength

10. microstructured optical fiber according to one of claims 1 to 9, characterized in that the inclusions (2, 3) of the sheath are arranged and sized to allow a mixture of m waves (m> 3) tuned in phase , with at least two of these waves having different wavelengths (λRTI, λ _B Gi), and respectively belonging to the fundamental mode guided by total internal reflection (RTI) and to the fundamental mode guided in the first photonic forbidden band (BG1 ).

11. microstructured optical fiber according to one of claims 1 to 10, characterized in that the fundamental mode guided in the first bandgap (BG1) and the fundamental mode guided by total internal reflection (RTI) cumulatively respect the following conditions (a) and (b):

(b) Σ ^ = 0

m being an integer greater than or equal to 3, and nλi being the value of the effective index of the fundamental mode propagating in the optical fiber at the wavelength λj, and in that among the wavelengths λj respecting the conditions (a) and (b), at least one wavelength (λ _R τι) belongs to the fundamental mode guided by total internal reflection (RTI), and at least one wavelength (ΛBGI) belongs to the guided fundamental mode in the first bandgap (BG1).

12. microstructured optical fiber according to one of claims 1 to 11, characterized in that the inclusions (2, 3) of the sheath are arranged and dimensioned so that for at least two different wavelengths λ _RT ι and λ _B Gi respectively belonging to the fundamental mode guided by total internal reflection (RTI) and to the fundamental mode guided in the first photonic forbidden band (BG1), the effective refractive indices of the fiber (n _λRT ι; n _λ B _G i) at these two wavelengths λ _RT | and λ _BG i are equal to and lower than the index of refraction (ni) of the sheath base material.

13. microstructured optical fiber according to one of claims 1 to 12, characterized in that the two wavelengths λ _RT ι and λ _BG i comply with the following condition: λ _RT ι = k.λ _B Gi, k being a integer greater than or equal to 2.

14. microstructured optical fiber according to claim 13, characterized in that the two wavelengths λ _R τι and Λ _B GI comply with the following condition: λ _R τι = 2λ _BG -ι.

15. microstructured optical fiber according to claim 13, characterized in that the two wavelengths ΛRTI and ΛB _G I comply with the following condition: ΛRTI = 3ΛBGI-

16. microstructured optical fiber according to one of claims 1 to 15, characterized in that the wavelength ΛRTI fundamental mode guided by total internal reflection (RTI) is located outside the range of wavelength mode fundamental guided in the first bandgap (BG1).

17. microstructured optical fiber according to one of claims 1 to 16, characterized in that the network of inclusions (2,3) is hexagonal type honeycomb.

18. microstructured optical fiber according to one of claims 1 to 16, characterized in that the network of inclusions (2,3) forms a square.

19. microstructured optical fiber according to one of claims 1 to 18, characterized in that the inclusion network (2,3) comprises at least two concentric rings formed by inclusions (2) of the first type, each inclusion. (2) of the first type being surrounded by several inclusions (3) of the second type, with the exception of inclusions (2) of the first type of the central crown.

20. microstructured optical fiber according to one of claims 1 to

19, characterized in that the base material of the sheath is silica and the inclusions (3) of the second type comprise air holes.

21. microstructured optical fiber according to one of claims 1 to

20, characterized in that the base material of the sheath is silica and the inclusions (2) of the first type comprise doped silica inclusions.

22. microstructured optical fiber according to claim 21, characterized in that the inclusions (2) of the first type comprise germanium doped silica or silica inclusions. phosphorus doped.

23. Microstructured optical fiber according to one of claims 1 to

22, characterized in that the base material of the sheath is silica and the inclusions (2) of the first comprise inclusions of tellurium glass or chalcogenide glass.

24. Microstructured optical fiber according to one of claims 1 to

23, characterized in that the inclusions (2) of the first type comprise inclusions containing a liquid or gas having a refractive index (n2) greater than the refractive index (ni) of the base material of the sheath (1) .

25. microstructured optical fiber according to one of claims 1 to 24, characterized in that the inclusions (3) of the second type comprise inclusions containing a liquid or gas having a refractive index (n3) less than the refractive index. (ni) the base material of the sheath (1).

26. Use of a microstructured optical fiber according to one of claims 1 to 25 for the generation of frequencies by non-linear effects, and in particular for one of the following applications: generation of twin photons, generation of triplet photons, generations multiple frequencies, in particular frequency doubling, frequency trimming.