WO2024022871A1

WO2024022871A1 - Optical fibre comprising a bragg grating

Info

Publication number: WO2024022871A1
Application number: PCT/EP2023/069806
Authority: WO
Inventors: Romain COTILLARD; Nicolas Roussel; Guillaume Laffont
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2022-07-29
Filing date: 2023-07-17
Publication date: 2024-02-01
Also published as: FR3138531A1

Abstract

The invention relates to an optical fibre comprising a Bragg grating (20), wherein: - each pattern (Mi) of the Bragg grating extends mainly in a plane, referred to as the "pattern plane", perpendicular to a longitudinal axis of the optical fibre; - the reflection power spectrum of the Bragg grating has discernible harmonics having an order greater than 100 in the range of wavelengths from 600 nm to 2000 nm; - each pattern consists of a plurality of bubbles (Bj; B1j, B2j) arranged side by side in the pattern plane; and - the area of the orthogonal projection of all of the bubbles of the pattern on the pattern plane is less than 50% of the area of the cross-section of the core (10) of the optical fibre.

Description

Optical fiber comprising a Bragg grating

[1] The invention relates to an optical fiber comprising a Bragg grating as well as a method of manufacturing this optical fiber.

[2] Such optical fibers are for example used as a transducer to measure temperature or stress or any other optical application.

[3] It is emphasized that a Bragg grating should not be confused with a juxtaposition, along an optical fiber, of Fabry-Perrot cavities. Indeed, the spectral characteristics of an optical fiber comprising such a juxtaposition of Fabry-Perrot cavities depend on the lengths of each Fabry-Perrot cavity as well as the reflectivity of the diopters located at each end of each Fabry-Perrot cavity. Unlike a Bragg grating, the diopters are not spaced from each other at a constant pitch to form a periodic structure.

[4] An optical fiber comprising a very high order Bragg grating and its manufacturing process are described in the following article: Pengtao Luo et Al: “Femtosecond laser plane-by-plane inscribed ultrahigh-order fiber Bragg grating and its application in multi-wavelength fiber lasers”, Optic letter, 06/15/2022. This article is subsequently designated by the reference “LUO2022”.

[5] In this text, "very high order" refers to the fact that the reflected power spectrum of the Bragg grating has discernible harmonics of order higher than N in the field of optics, where N is an integer greater than 100 and, preferably, greater than 500 or 1000. In other words, in the reflection power spectrum of a very high order Bragg grating, there exist harmonics of order k, greater than N, which each correspond to a power peak distinct from the peaks corresponding to the harmonics of orders k-1 and k+1. This peak of order k is also greater than the noise. This peak of order k is located at the wavelength À _k defined by the following relation (1): À _k = 2*n _e *A/k, where:

- k is an integer equal to the order of the harmonic,

- n _e is the effective index of the optical fiber,

- A is the step of the Bragg grating, and - the symbol “*” designates the scalar multiplication operation.

[6] This peak of order k is in the field of optics, that is to say that its wavelength À _k is included in the range of wavelengths usually used in optics. In this text, the field of optics refers to the wavelength range which extends from 600 nm to 10000 nm and, frequently, from 600 nm to 2000 nm.

[7] The reflected power spectrum is the power spectrum of the optical signal reflected by the Bragg grating. In this text, unless otherwise indicated, the term “power spectrum” designates the power spectrum in reflection.

[8] A peak in the reflection power spectrum corresponds to an absorption line in the transmission power spectrum of the same Bragg grating.

[9] In the LUO2022 paper, each Bragg grating pattern consists of a single oblong bubble that occupies the entire cross-section of the optical fiber core. More precisely, the section of this bubble in a plane which contains the longitudinal axis of the optical fiber in the shape of an ellipse. This pattern is formed in the core of the optical fiber by a pulse from a femtosecond laser. Furthermore, in the LUO2022 article the pitch of the formed Bragg grating is taken equal to 100 pm, which makes it possible to obtain discernible harmonics of order k greater than one hundred in the field of optics.

[10] There are other known methods of making Bragg gratings that use pulses of ultraviolet radiation or CO ₂ lasers. It is emphasized that these other known processes do not make it possible to manufacture very high order Bragg gratings. In general, Bragg gratings made by these other known methods exhibit only discernible harmonics of order less than twenty. It seems that this comes from the fact that the variations in the refractive index in the optical fiber obtained by implementing these other known processes are much less clear than those obtained using a femtosecond laser.

[11] The LUO2022 optical fiber presents a power spectrum comprising a succession of very close and very fine peaks in a wavelength range of interest of at least 100 nm width in the field of optics. Furthermore, the heights of these peaks are substantially the same over this range of at least 100 nm width because each of these peaks corresponds to a very high order harmonic. In this text, “very close together” means that the distance between two consecutive peaks is less than or equal to 10 nm. “very fine” means that the width at half height of each peak is less than 3 nm. Such a succession of peaks is here called “a comb of peaks” or simply “a comb”. An example of such a comb is shown in Figure 3 of the article LUO2022.

[12] Furthermore, it has also been known, for a longer time, to manufacture, using a femtosecond laser, a Bragg grating formed from a succession of disjointed bubbles arranged one behind the other along the longitudinal axis of an optical fiber and separated from each other at a regular pitch. An example of such a manufacturing process is for example disclosed in application CN211603608U. However, it seems that before the publication of the LUO2022 paper, no one had noticed that this manufacturing process could be exploited to create very high order Bragg gratings. Indeed, in all these publications, the step of the Bragg grating is chosen for:

- that wavelength À _B of the fundamental resonant frequency f _B is in the field of optics, or

- that only the first harmonics of order less than twenty are in the field of optics.

[13] Thus, the pitch of these Bragg gratings are systematically less than 50 pm or 20 pm. Under these conditions, the Bragg grating produced is not a very high order Bragg grating. Indeed, in this case, even if harmonics of order k greater than one hundred are discernible in the power spectrum, the wavelength À _k of these harmonics is not in the domain of optics. In other words, the wavelengths À _k of harmonics of order k greater than one hundred are all less than 600 nm.

[14] The state of the art is also known from: CA2512327A1, US2019/064432A1, FR3082954A1 and the following articles:

- Jens U Thomas et Al: “Selective fiber Bragg grating mode”, Proceedings of SPIE Conference Frontiers in Ultrafast Optics: Biomedical, Scientific and Industrial Application, 01/24/2010, pages 75890J-1 - 75890J-9, and

- Sascha Liehr et Al: “Femtosecond Laser Structure of Polymer Optical Fibers for Backscatter Sensing”, Journal of Lightwave Technology, IEEE, USA, vol. 31, n°9, 1/05/2013, pages 1418-1425.

[15] Despite its many advantages, the optical fiber in the LUO2022 article has high insertion losses. In other words, a substantial part of the energy of the incident optical signal is neither reflected by the Bragg grating nor transmitted through the Bragg grating.

[16] The invention aims to remedy this drawback by proposing an optical fiber having the same advantages as that disclosed in the LUO2022 article while having lower insertion losses.

[17] Its subject therefore is an optical fiber conforming to claim 1.

[18] The invention also relates to a process for manufacturing this optical fiber.

[19] The invention will be better understood on reading the description which follows, given solely by way of non-limiting example and made with reference to the drawings in which:

- Figure 1 is a schematic illustration, partial and in longitudinal section, of an optical fiber;

- Figure 2 is a schematic illustration, in cross section, of the optical fiber of Figure 1;

- Figure 3 is a graph representing a portion of the power spectrum of the optical fiber of Figure 1;

- Figure 4 is a flowchart of a manufacturing process for the optical fiber of Figure 1;

- Figures 5 and 6 are schematic illustrations, in cross section, respectively, of a first and a second variants of the optical fiber of Figure 1.

[20] In these figures, the same references are used to designate the same elements. In the remainder of this description, the characteristics and functions well known to those skilled in the art are not described in detail.

[21] Chapter I: Examples of embodiments

[22] Figure 1 represents an optical fiber 2 which extends along a longitudinal axis 4 parallel to a direction Z of an orthogonal coordinate system XYZ. In this text, the figures are oriented in relation to this XYZ reference. For example, the Z direction is horizontal and the Y direction is vertical.

[23] To simplify Figure 1, only a portion of the optical fiber 2 is shown. The optical fiber 2 is designed to guide an optical signal along the axis 4. The wavelength À _s of the guided optical signal is in the optical domain. For example, in this embodiment, the wavelength À _s is equal to 1550 nm.

[24] Optical fiber 2 is a single-mode optical fiber also known by the acronym SMF (“Single Mode Fiber”).

[25] Optical fiber 2 comprises:

- a core 10 in which the optical signal guided by this fiber 2 propagates,

- an optical sheath 12 made of a material whose refractive index makes it possible to maintain the optical signal inside the core 10 by reflection at the interface between the core 10 and this sheath 12, and

- a mechanical sheath, typically made of polymer, and which covers the sheath 12.

[26] To simplify Figure 1, the mechanical sheath of optical fiber 2 has not been shown.

[27] Fiber 2 includes a very high order Bragg grating 20. Here, the array 20 is designed to obtain a comb of peaks over a wavelength range of interest containing the wavelength λ _s and whose width is greater than 100 nm. For example, here this range extends from 1500 nm to 1600 nm.

[28] In addition, network 20 is designed so that this comb is formed by the harmonics of network 20 of order close to 1024.

[29] For this purpose, the network 20 is composed of a succession of patterns Mi arranged one behind the other along axis 4. The index i is the order number of the pattern in the Z direction. The index i of the first leftmost pattern in array 20 is equal to 1 and the index i of the last rightmost pattern in array 20 is equal to p. p is equal to the number of patterns Mi of the network 20. In Figure 1, only the first two and the last two patterns of the network 20 have been represented. The presence of intermediate patterns located between patterns M ₂ and M _p .i is represented by black points on axis 4.

[30] The number p of patterns is greater than or equal to three and, preferably, greater than or equal to ten. Indeed, it has been observed that the larger the number p, the more the width at half-height of each peak decreases. Here, the number p is also chosen sufficiently small so that the length of the network 20 remains small, that is to say less than 1 meter and, preferably, less than 10 cm. The length of the network 20 is equal to the distance between the patterns Mi and M _p measured along axis 4. Typically, the number p is less than 200 or 100. [31] The step A between two immediately consecutive patterns Mi and M _i+i in the direction Z is constant whatever the index i. The step A is therefore equal to the distance, along axis 4, which separates two immediately consecutive patterns Mj and M _{i +i} .

[32] Here, step A is calculated so that the wavelength À _c of the harmonic of order k _c closest to the center of the comb to be created is equal to or very close to the wavelength À _s . Here, the order k _c of the harmonic closest to the wavelength À _s is chosen equal to 1024.

[33] For this, the step A is between 0.9*[k _c *À _s /(2*n _e )] and 1, 1 *[k _c *À _s /(2*n _e )] and , preferably, between 0.98*[k _c *À _s /(2*n _e )] and 1.02*[k _c *À _s /(2*n _e )], where:

- n _e is the effective index of optical fiber 2, and

- the symbol “*” designates the scalar multiplication operation.

[34] The effective propagation index n _e is also known as the “mode phase constant”. It is defined by the following relationship: n _g = n _eff - Àdn _eff /dÀ, where n _g is the group index and À is the wavelength of the optical signal guided by the optical fiber 2. The effective index propagation of an optical fiber depends on the dimensions of the core of this optical fiber and the materials forming this core and the optical cladding of this optical fiber. It can be determined experimentally or by numerical simulation.

[35] Here, optical fiber 2 is made from an optical fiber marketed under the reference SMF-28 by the company Corning®. The index n _e of this optical fiber is equal to approximately 1.4676. Under these conditions, the term k _c *À _s /(2*n _e ) is equal to approximately 540.8 pm. Here, step A is chosen equal to 540.8 pm. With the choice of this value for step A, only the harmonics of order between 806 and 2684 are in the optical domain. In particular, the wavelength À _B of the fundamental frequency of the network 20 is not in the field of optics.

[36] For this value of step A and so that the length L ₂₀ of the network 20 is less than 10 cm, the number p of patterns is chosen less than 185. Here, p is chosen equal to 120, so that the length L ₂₀ of network 20 is approximately equal to 65 mm.

[37] The Mj motifs are all structurally identical to each other and differ from each other only in their position along axis 4. Thus, subsequently, only the Mj motif is described in detail. This pattern Mj extends mainly in a plane Pi perpendicular to axis 4. This plane Pi is therefore parallel to the directions X and Y. In Figure 1, only the planes Pi, P ₂ , P _p .i and P _p in which extend respectively, the patterns Mi, M ₂ , M _P -I and M _p are represented. [38] Figure 2 shows the pattern Mi in more detail. In Figure 2, only the cross section of the core 10 is shown.

[39] Each pattern Mj reflects part of the incident optical signal. Another part of the incident optical signal passes through the pattern Mj. Finally, each pattern Mj diffuses part of the energy of the incident optical signal which is then neither reflected nor transmitted through this pattern Mj. It is this energy diffused by each pattern Mj which creates the insertion losses caused by the presence of the network 20 in the core 10 of the optical fiber 2. To minimize these insertion losses, here, the surface S _Mi of the cross section of the pattern Mj occupies less than half of the surface Sw of the cross section of the heart 10. The surface S _Mi is equal to the surface of the orthogonal projection of the pattern Mj on the plane Pi. The surface S is equal to the area of the cross section of the core 10. Typically, the area Sw is constant along the entire length of the optical fiber 2.

[40] Preferably, the surface S _Mi is less than 0.1*Sw or 0.05*Sw or 0.01*Sw. Here, the surface S _M is less than 0.05*Sw.

[41] To obtain sufficient reflectivity of the pattern Mi to limit the number p of patterns and therefore to limit the length L ₂ o of the network 20, the surface S _Mi is greater than 0.016 pm ² , that is to say greater twice the surface of the orthogonal projection of a spherical bubble of 100 nm in diameter on the plane Pi. In this embodiment, the surface S _Mi is greater than or equal to 0.032 pm ² .

[42] To this end, the pattern Mi is made up of several bubbles Bj. The index j is an identifier which makes it possible to uniquely identify the bubble Bj among all the other bubbles of the same pattern Mi. The index j is here an integer between 1 and q, where q is equal to the number of bubbles Bj of the pattern Mi. The number q is greater than or equal to two or four. Here the number q is equal to six.

[43] In this embodiment, all bubbles Bj are structurally identical to each other. Only their positions in the Pi plane make it possible to distinguish them from each other.

[44] Each bubble Bj creates a significant variation in the refractive index of the core 10 in the direction of propagation of the optical signal. For this, the difference between the refractive index n _r w of the core 10 and the refractive index n® of the bubble Bj is greater than 0.3 or 0.4. Here, the interior of each bubble is empty or practically empty which corresponds to a difference between the indices n _r w and n _r B greater than or equal to 0.4. [45] Furthermore, so that the variation in refractive index is abrupt, the diameter D of each bubble B is less than 200 nm and, preferably, less than 100 nm. Generally, the diameter D, is also greater than 10 nm or 50 nm.

[46] Each bubble B is mainly spherical. Thus, the diameter Dj of the bubble Bj is equal to the diameter of the sphere of smallest volume which entirely contains the bubble Bj. Here, this diameter Dj is less than 100 nm.

[47] The center of each bubble Bj is contained in the plane Pi.

[48] In this embodiment, the bubbles Bj are disjoint, that is to say they do not overlap and they are not fluidly connected to each other.

[49] The pattern Mj is centered on axis 4. For this, the bubbles Bj are arranged next to each other so that the barycenter of the pattern Mj is located less than 100 nm from axis 4 and the center of at least one of the bubbles Bj is located less than 100 nm from axis 4.

[50] In this first embodiment, the barycenter of the pattern Mi is located on axis 4. In addition, the pattern Mi is symmetrical with respect to axis 4.

[51] The centers of the bubbles Bj are located one behind the other on an axis Ai which intersects axis 4 and which belongs to the plane Pi. The pattern Mi therefore includes a line of disjoint bubbles. In this case, the arrangement of the disjointed bubbles forms what is called a “dotted line” in this text. Here, the axis Ai is parallel to the direction Y. In this embodiment, the bubbles B ₃ and B ₄ are located, respectively, above and below the axis 4. The centers of the bubbles B ₃ and B ₄ are less than 100 nm from axis 4.

[52] The distance between two immediately consecutive bubbles Bj, B _j+i along the axis Ai is constant. In other words, whatever the pair of bubbles Bj, B _j+i immediately consecutive along the axis Ai, the distance which separates the centers of these two bubbles is the same.

[53] Figure 3 represents the power spectrum of optical fiber 2 between 1545 nm and 1555 nm. The reflectivity of the peaks of the comb obtained reached -21 dBm.

[54] Figure 4 represents a process for manufacturing optical fiber 2. This process begins with a step 50 of supplying an optical fiber whose core 10 is initially devoid of Bragg grating. For example, the optical fiber supplied is the optical fiber marketed under the reference SMF-28 by the company Corning®. [55] Here, the mechanical sheath of this optical fiber is transparent to the pulses of a femtosecond laser so that it is not necessary to remove this mechanical sheath at the locations where the Mi patterns must be produced.

[56] Then, a step 52 of forming the pattern Mj in the core 10 of the optical fiber provided is repeated at each location where such a pattern Mj must be formed.

[57] During step 52, each bubble B is created by a single pulse from the femtosecond laser. More precisely, during an operation 54 of creating a bubble B j, the beam of the femtosecond laser is focused on the center of the bubble B, to then create a pulse of a duration less than 500 fs or 250 fs is emitted and irradiates the point of the heart 10 where the center of the bubble B should be located. Bubble B is then created in heart 10.

[58] Then, the optical fiber is moved relative to the femtosecond laser so that the beam of the femtosecond laser is now focused on the center of the next bubble B _j+i to be created, then operation 54 is repeated.

[59] In this embodiment, the bubbles B are therefore created one after the other.

[60] The values of the different parameters of a femtosecond laser to create a bubble such as bubble B, depend on the characteristics of the optical fiber provided as well as the characteristics of the femtosecond laser used. The adjustment of these different parameters to create the B bubbles, previously characterized, is part of the skills of those skilled in the art. For example, by way of illustration, the reader can consult application CN211603608U on this subject which describes in detail an example of an installation making it possible to form bubbles such as bubbles B, in the core of an optical fiber. Here, the following parameters were used to manufacture optical fiber 2:

- the central wavelength of the femtosecond laser pulse is equal to 512 nm,

- the duration of each pulse of the femtosecond laser is equal to 160 fs, and

- the power of each pulse of the femtosecond laser is equal to 45 nJ.

[61] Figure 5 represents a pattern M2j which can be used instead of the pattern Mj to produce a very high order Bragg grating. Pattern M2j is identical to pattern Mj except that bubbles B overlap. There therefore exists between each pair of immediately consecutive bubbles B j, Bj+i, an overlapping zone Z _c . For this, the distance between the centers of each pair of bubbles Bj, Bj+i immediately consecutive is chosen less than Dj. Typically, this distance is between 0.05*Dj and 0.9*Dj or between 0.5*Dj and 0.9*Dj. In this embodiment, since the bubbles Bj overlap, they are fluidly connected to each other.

[62] Figure 6 represents a pattern M3j which can be used instead of the pattern Mi to produce a very high order Bragg grating. The M3j pattern has a first and a second dotted line. The first dotted line has four disjoint bubbles BT to B1 ₄ aligned one behind the other along an axis AT belonging to the plane Pi. The second dotted line has four disjoint bubbles B2i to B2 ₄ aligned one behind the other along an axis A2i belonging to the plane Pi. Here, the axes AT and A2j are both parallel to the direction Y. In addition, the axes AT and A2i are symmetrical to each other with respect to the axis 4. For example, the shortest distance between axis AT and axis 4 is between 50 nm and 100 nm.

[63] Bubbles B1 j and B2j are each identical to bubble Bj and arranged along their respective axes A1 j and A2j so that the pattern M3i is centered on axis 4.

[64] Chapter: Variants:

[65] Variants of the Bragg grating:

[66] The order k _c of the harmonic which is at the center of the comb to be produced is here greater than 100 and, preferably, chosen greater than 500 or 1000. This order k _c can also be chosen greater than 2000 or 4000 or 10000. Theoretically, there is no upper limit for this order k _c . However, it follows from relation (1) that the greater the order k _c , the greater the step A of the Bragg grating and therefore the Bragg grating is longer. In practice, it is therefore the maximum desired length for the Bragg grating which imposes an upper limit for the order k _c . Here, this maximum length is set at 1 m.

[67] Likewise, the minimum value of step A is greater than 20 pm and, typically, greater than 50 pm so that very high order harmonics are included in the field of optics. Theoretically, there is no maximum value for step A. In fact, whatever the value retained for step A, it is possible to find a value for the order k _c which makes it possible to place the length of _c wave between 600 nm and 2000 nm. However, the larger the step A, the longer the Bragg grating. In practice, it is therefore also the maximum length desired for the Bragg grating which imposes an upper limit for the value of step A. [68] The wavelength range of interest may be wider than 100 nm. For example, the width of this wavelength range of interest is, alternatively, greater than 200 nm or 300 nm. There is no upper limit for this wavelength range of interest except that it must be located in the optical domain. For example, by applying the teaching given in Chapter I, it is possible to obtain combs centered on the wavelengths commonly used in optics such as, in particular, the wavelength of 800 nm, 1000nm, 1300nm or 1500nm.

[69] Variants of the pattern:

[70] In another embodiment, the pattern Mi is not centered on axis 4.

[71] The bubbles B of a pattern can be arranged non-symmetrically with respect to axis 4.

[72] Alternatively, the distance between two immediately consecutive bubbles Bj, B _j+i along the axis Ai is not constant and varies as a function of the index j.

[73] Many shapes are possible for the patterns. For example, instead of having one dotted line, the pattern has several dotted lines, for example, parallel to each other in the plane of the pattern. The pattern can also take a shape other than the shape of a dotted line. For example, as a variant, the pattern includes bubbles whose centers are located on each of the vertices of a polygon with at least three vertices. For example, the polygon is a triangle or a rectangle.

[74] Variants of the manufacturing process:

[75] Alternatively, if the mechanical cladding of the optical fiber is not transparent or semi-transparent with respect to the pulses of the femtosecond laser used, the mechanical cladding is removed at the location where each pattern must be manufactured of the Bragg grating before applying the pulses of the femtosecond laser.

[76] Other values are possible for the femtosecond laser parameters to create bubbles such as Bj bubbles. In particular, the central wavelength of the femtosecond laser pulse can be chosen between 233 nm and 1300 nm . The pulse duration can be chosen between 10 fs and 500 fs. The pulse power can be chosen between 10 nJ and 250 nJ.

[77] In another embodiment, the femtosecond laser pulse is simultaneously focused on different points in the cross section of the heart of the optical fiber to simultaneously form several bubbles with a single pulse of the femtosecond laser. For this, a spatial light modulator, better known by the acronym SLM (“Spacial Light Modulator) is, for example, used.

[78] To guarantee that a dotted line passes right through the cross section of the core 10, the formation of bubbles arranged along the axis Ai can begin in the sheath 12 and end in the sheath 12.

[79] In another embodiment, when forming a pattern, the laser power is varied to obtain bubbles of different diameters. In this case, the bubbles which constitute the same Bragg grating pattern do not all have the same diameter.

[80] It is also emphasized that the optical fiber 2 can be manufactured using a manufacturing process other than those using a femtosecond laser if this other process makes it possible to obtain, in the core 10, variations in the refractive index as clear as those obtained using a femtosecond laser.

[81] Other variants:

[82] Optical fibers other than SMF-28 fiber can be used. For example, the optical fiber may be a multimode optical fiber or MMF (Multi-Mode Fiber).

[83] It is not necessary for the core of the optical fiber to consist of specific doping. Thus, the manufacturing process described can be implemented with optical fibers whose core is made of germanosilicates, pure silica, aluminosilicates doped with rare earths or sapphire.

[84] Everything that has been described previously in the particular case where the wavelength À _c of the peak of order k _c is between 600 nm and 2000 nm also applies to the case where the wavelength À _c is between 2000 nm and 10000 nm and, in particular, in the case where the wavelength À _c is included in the infrared range. When the wavelength À _c is in the infrared range, the core of the optical fiber is for example made of chalcogenide glass.

[85] Chapter III: Advantages of the embodiments described:

[86] The different embodiments of the optical fiber described have the same advantages as the optical fiber of article LUO2022 while having lower insertion losses. Indeed, the use of several bubbles in each pattern makes it possible to obtain a pattern smaller than that used in the LUO2022 article and therefore substantially reduce insertion losses. In addition, the fact of using several bubbles makes it possible to obtain a sufficiently reflective pattern to reduce the number of patterns and therefore to maintain the compactness of the Bragg grating while limiting insertion losses.

[87] The fact that the bubbles which constitute a pattern of the Bragg grating are all disjoint reduces the diffusion losses at the level of each pattern and therefore the insertion losses. Indeed, when the bubbles overlap, the areas of overlap between several bubbles are subjected to several successive pulses of the femtosecond laser. It was observed that an area of the core of the optical fiber which is subjected to several pulses of the femtosecond laser, deteriorates. This degradation increases diffusion losses. Conversely, when the bubbles are disjoint, such overlapping zones do not exist, which limits diffusion losses.

[88] The fact that the surface of the orthogonal projection of the pattern on the plane of the pattern is less than 5% of the surface of the cross section of the optical fiber, makes it possible to obtain a Bragg grating whose diffusion losses are very weak, especially compared to the Bragg grating of the LUO2022 article.

[89] The fact that the pattern is solely made up of a line of disjointed bubbles makes it possible to both increase the reflectivity of the pattern while limiting the increase in its diffusion losses. In addition, such a pattern remains simple to make.

[90] The fact that the pattern is made up of several lines of disjointed bubbles makes it possible to increase the reflectivity of this pattern.

[91] The fact that the number of patterns of the Bragg grating is greater than or equal to ten makes it possible to limit the width of each of the lines of the power spectrum between 600 nm and 2000 nm.

[92] The fact that the pattern is centered on axis 4 of optical fiber 2 makes it possible to increase the interactions between the pattern and the propagating optical signal. This therefore makes it possible, with equal performance, to reduce the number of patterns and therefore to obtain a Bragg grating that is shorter than if the pattern was not centered on axis 4.

Claims

1. Optical fiber comprising:

- a core (10) which extends along a longitudinal axis (4) of the optical fiber and inside which an optical signal guided by the optical fiber is able to propagate along the longitudinal axis optical fiber,

- a Bragg grating (20) produced in the heart of the optical fiber, this Bragg grating comprising at least three identical patterns (Mi; M2j; M3j) aligned one behind the other along the longitudinal axis of the fiber optical, this Bragg grating having the following characteristics:

- each pattern extends mainly in a plane, called “pattern plane”, perpendicular to the longitudinal axis of the optical fiber, and

- the power spectrum in reflection of the Bragg grating presents discernible harmonics of order greater than one hundred in the wavelength range from 600 nm to 10,000 nm, characterized in that:

- each pattern is made up of several bubbles (B, ; B1 j, B2j) arranged next to each other in the plane of the pattern, and

- the area of the orthogonal projection of all the bubbles of the pattern on the plane of the pattern is less than 50% of the cross-sectional area of the core of the optical fiber.

2. Optical fiber according to claim 1, in which all the bubbles of the same pattern are disjoint.

3. Optical fiber according to any one of the preceding claims, in which the area of the orthogonal projection of all the bubbles of the pattern on the plane of the pattern is less than 5% of the area of the cross section of the core of the optical fiber .

4. Optical fiber according to claim 3, in which the pattern consists of a single line of disjoint bubbles.

5. Optical fiber according to any one of claims 2 to 3, in which the pattern consists of several lines of disjointed bubbles.

6. Optical fiber according to any one of the preceding claims, in which the number of patterns (Mi; M2j; M3j) is greater than or equal to ten.

7. Optical fiber according to any one of the preceding claims, in which:

- the center of at least one bubble of the pattern is located less than 100 nm from the longitudinal axis of the optical fiber, and

- the barycenter of the pattern (Mj; M2j; M3i) formed by these bubbles is located less than 100 nm from the longitudinal axis of the optical fiber.

8. Optical fiber according to any one of the preceding claims, in which:

- the patterns (Mj; 2j; M3j) of the Bragg grating are spaced from each other by a constant step greater than or equal to 20 pm, and

- the difference between the refractive index of the core of the optical fiber and the refractive index of each pattern of the Bragg grating is greater than 0.3.

9. Optical fiber according to any one of the preceding claims, in which each bubble is mainly spherical and the centers of all these bubbles which constitute the same pattern of the Bragg grating are all contained in the plane (P i) of the pattern.

10. Optical fiber according to any one of the preceding claims, in which the reflected power spectrum of the Bragg grating has discernible harmonics of order greater than one hundred in the wavelength range from 600 nm to 2000 nm .

11. Optical fiber according to any one of the preceding claims, in which the optical fiber is a single-mode optical fiber.

12. Method of manufacturing an optical fiber according to any one of the preceding claims, in which, for each location where a pattern of the Bragg grating must be formed, the method comprises a step (52) of forming this pattern using a femtosecond laser, characterized in that each step (52) of forming a pattern comprises at least one operation (54) of creating one or more bubbles in the core of the optical fiber by irradiation from the heart of the optical fiber using a femtosecond laser pulse so as to form, at the location of this pattern, several bubbles arranged next to each other in the plane of the pattern, the dimensions of these bubbles formed being such that the area of the orthogonal projection of all the bubbles of the pattern on the plane of the pattern is less than 50% of the cross-sectional area of the core of the optical fiber.