System for coupling a multi-core optical fiber with at least one single-core optical fiber, and corresponding coupling method

Abstract

The invention relates to a coupling system between a multi-core optical fiber (MCF) and an assembly of at least one single-core optical fiber (SCF), each single-core fiber of said assembly being coupled to a distinct core of the multi-core fiber, said coupling system comprising: - a graded-index lens (GRIN-L), positioned between said multi-core fiber and said at least one single-core fiber; - a first section of graded-index fiber coupled to the end of the multi-core fiber to form a first microlensed end (GRIN-MCF); - for each single-core fiber, a second section of graded-index fiber coupled to the end of said single-core fiber to form a second microlensed end (GRIN-SCF); said first and second microlensed ends being coupled on either side of said lens with a radial offset of one relative to the other defined as a function of a core parameter of the multi-core fiber. Figure 1.

Description

Title of the invention: System for coupling a multi-core optical fiber with at least one single-core optical fiber, and corresponding coupling method Technical field of the invention

[0001] The invention relates to the field of optical data transmission, and more particularly that of the interconnection between a multi-core optical fiber and at least one single-core optical fiber. State of the prior art

[0002] Telecommunications networks require large transmission capacities. Thus, multi-core optical fibers can be preferred to single-core optical fibers, since they allow simultaneous transmission of several optical signals in a more compact manner. Interconnection solutions between multi-core fibers and single-core fibers are therefore used to transfer optical signals from one to the other and vice versa.

[0003] Thus, many interconnection methods exist. For example, the article “Compact Multi-core Fiber Fan-in / out Using GRIN Lens and Microlens Array”, Arao H. et al., 2014, OECC / ACOFT, hereinafter referenced as “Arao”, proposes a method of interconnection between a multi-core optical fiber and a single-core optical fiber array. Thus, in the Arao article, a discrete microlens array is used following a gradient index lens and a glass spacer, in order to redirect the optical beams from the multi-core fiber, towards the single-core optical fiber array. However, the use of a glass spacer in order to increase the beam spacing is a very non-compact solution. In addition, the use of microlenses involves alignment problems, lack of compactness and therefore difficulties in implementing the system.

[0004] Consequently, it is necessary to propose a compact and simple to implement solution, in particular making it easier to align the different elements of the system. Summary of the invention

[0005] The present invention meets this need by proposing a coupling system between a multi-core optical fiber and a set of at least one single-core optical fiber, each single-core fiber of said set being coupled to a separate core of the multi-core fiber, said coupling system comprising: - a gradient index lens, positioned between said multi-core fiber and said at least one single-core fiber; - a first section of graded index fiber coupled to the end of the multi-core fiber to form a first micro-lensed end; - for each single-core fiber of said assembly, a second section of graded index fiber coupled to the end of said single-core fiber to form a second micro-lensed end;

[0006] said first and second micro-lensed ends being coupled on either side of said lens with a radial offset of one relative to the other defined as a function of a core parameter of the multi-core fiber.

[0007] Thus, such a coupling system makes it easier to align the various elements. In particular, the use of sections of graded index fibers having a diameter similar to the diameter of the multi / single-core fibers allows for definitive fixing by welding of the graded index fibers to the ends of the multi / single-core fibers. Aligning the elements of such a system is therefore simpler than with a micro-lens array as described in the prior art.

[0008] In addition, such a coupling system is compact since it does not require the use of a glass spacer or a micro-lens array. Indeed, the gradient index fiber sections make it possible to gain in compactness by acting as both a lens / collimator, allowing precise control of the angle and diameter of each of the beams in order to guide them correctly from one optical fiber to another.

[0009] According to a particular characteristic of the invention, the radial offset of each single-core fiber is defined by a distance h, the distance h being defined as follows:

[0010] / , = ex^

[0011] with the following optical parameters:

[0012] said core parameter, defined as the radial distance between the central axis of the multi-core fiber and the axis of the core of the multi-core fiber to which said single-core fiber is coupled;

[0013] z / i the refractive index at the center of said first section of graded index fiber;

[0014] the quadratic coefficient of said first section of gradient index fiber;

[0015] n2 the refractive index at the center of said gradient index lens;

[0016] S2 the quadratic coefficient of said gradient index lens.

[0017] Thus, the distribution of the single-core fibers, and therefore the distance between them, is optimized in order to obtain a compact coupling system. In addition, the distance h must be greater than a threshold value chosen according to the diameter of the single-core fibers.

[0018] According to a particular characteristic of the invention, the length / ] of the first graded index fiber section of said multi-core fiber is inversely proportional to its quadratic coefficient; the length of the graded index lens is inversely proportional to its quadratic coefficient ^2; and the length / 3 of the graded index fiber section of said at least one single-core fiber, is inversely proportional to its quadratic coefficient #3.

[0019] Thus, the length of the graded index fiber sections is adapted in order to correctly transmit the optical beams from one optical fiber to another. These graded index fiber sections have a role similar to that of a lens / collimator, and make it possible to orient and control the beams to correctly guide them from one optical fiber to another.

[0020] According to one embodiment of the invention, the cores of the multi-core optical fiber and the set of at least one single-core optical fiber are of the single-mode type.

[0021] Thus, the coupling system can be used in the context of signal transmission over long distances. Indeed, single-mode optical fibers make it possible to limit noise and losses, and are therefore preferentially used over long distances.

[0022] According to another embodiment, the cores of the multi-core optical fiber and the set of at least one single-core optical fiber are of the multimode type, and transmit spatially multiplexed signals.

[0023] Thus, the coupling system can be used in the context of the simultaneous transmission of several optical signals, often over shorter distances than for single-mode fibers. Indeed, multimode optical fibers transmit spatially multiplexed signals composed of a plurality of signals, reflected at different angles to form a plurality of modes (modal multiplexing). The transmitted data rate is therefore increased.

[0024] According to yet another embodiment, the cores of the multi-core optical fiber and the assembly of at least one single-core optical fiber support WDM (Wavelength Division Multiplexing) type transmissions, and therefore transmit wavelength multiplexed signals.

[0025] Thus, the coupling system can be used in the context of the simultaneous transmission of several optical signals, often over shorter distances than for single-mode fibers. Indeed, like multimode fibers, WDM transmissions make it possible to increase the data transmission rate on an optical fiber, in comparison with a single-mode optical fiber comprising only a single signal, since the wavelength division multiplexed signals are composed of a plurality of signals each having a distinct wavelength.

[0026] Alternatively, the coupling system comprises at least two different types of single-core fibers and multi-core fiber cores among single-mode type, single-mode type multimode and the type supporting WDM transmission, i.e. compatible coupling for all wavelengths from 1260 nm to 1675 nm.

[0027] Thus, the coupling system may implement single-core fibers and multi-core fiber cores of different types. For example, a multi-core fiber may comprise a single-mode core and a multi-mode core, each of these cores being respectively associated with a single-mode fiber and a multi-mode fiber within the coupling system.

[0028] According to a particular embodiment, said first and second micro-lensed ends are directly coupled at the input and output of said gradient index lens.

[0029] Thus, the coupling system is simple and compact, with easy implementation, in particular such a system makes it easier to align the different elements composing it.

[0030] According to another particular embodiment, at least one of said first and second micro-lensed ends is coupled to the input and / or output of said gradient index lens via an optical component belonging to the group comprising: an optical blade; an optical filter; an anti-reflective component; an optical isolator.

[0031] Thus, the system can have additional elements, making it possible to obtain a more elaborate system with additional optical functions, the system remaining compact and simple to implement.

[0032] According to a variant of the invention, the end of the multi-core fiber and the corresponding first micro-lensed end are included in an independent ferrule; each single-core fiber end of said assembly and the corresponding second micro-lensed end are included in an independent ferrule; and the gradient index lens is independent.

[0033] According to another variant of the invention, the end of the multi-core fiber, the corresponding first micro-lensed end and the gradient index lens are included in the same ferrule, on the one hand; and each end of the single-core fiber of said assembly and the corresponding second micro-lensed end are included in an independent ferrule, on the other hand.

[0034] According to yet another variant of the invention, the end of the multi-core fiber and the corresponding first micro-lensed end are included in a first independent ferrule on the one hand; and the assembly of at least one single-core fiber end, each corresponding second micro-lensed end, and the lens at

[0035]

[0036]

[0037]

[0038]

[0039]

[0040]

[0041] index gradients are included in a second ferrule, on the other hand. Thus, different coupling modes can be used for implementing the coupling system. Different blocks of elements can thus be interconnected via independent ferrules, these ferrules forming separate blocks of elements (independent blocks) and allowing a removable mechanical assembly between these different blocks. The term "independent" specifies the separate nature of the different blocks of elements, as well as the removable nature of the possible connection between these blocks. The invention also relates to a method for coupling a multi-core optical fiber and a set of at least one single-core optical fiber, characterized in that it comprises: - welding a first section of graded index fiber to the end of said multi-core fiber to form a first micro-lensed end; - for each single-core fiber of said assembly, welding a second section of graded index fiber at the end of said single-core fiber to form a second micro-lensed end; - mechanical assembly of said first micro-lensed end of said multi-core fiber with a first interface of a gradient index lens; - mechanical assembly of said second micro-lensed end of said single-core fiber with a second interface of said gradient index lens, with a radial offset relative to the first micro-lensed end, defined as a function of a core parameter of the multi-core fiber. Thus, such a coupling process allows for simple assembly of the system where alignment of the different elements is facilitated. In particular, the use of graded index fiber sections with a diameter similar to the diameter of the multi / single-core fibers allows for definitive fixing by welding. The system thus obtained is compact and simple to implement. Presentation of figures Other aims, characteristics and advantages of the technique will appear on reading the following description given for illustrative and non-limiting purposes only, and which refers to the appended figures, in which: [Fig.l] represents a coupling system according to an exemplary embodiment of the invention. [Fig.2] represents a coupling system according to another exemplary embodiment of the invention. [Fig.3] represents a cross section and a longitudinal section of a multi-core optical fiber according to an exemplary embodiment.

[0042] [Fig.4] represents a cross section and a longitudinal section of a single-core optical fiber according to an exemplary embodiment.

[0043] [Fig.5] represents the coupling of a multi-core optical fiber with a section of graded index fiber, according to an exemplary embodiment.

[0044] [Fig.6] represents the coupling of a single-core optical fiber with a section of graded index fiber, according to an exemplary embodiment.

[0045] [Fig.7] represents the coupling of a multi-core optical fiber and a section of graded index fiber with a graded index lens, according to an exemplary embodiment.

[0046] [Fig.8] is a diagram representing the steps of the coupling method according to one embodiment of the invention.

[0047] [Fig.9] represents a cross-section and a longitudinal section of a multi-core optical fiber according to a first particular example of embodiment, in which the multi-core optical fiber comprises 4 cores.

[0048] [Fig. 10] represents the coupling of a multi-core optical fiber with a section of graded index fiber, according to the first particular example of embodiment.

[0049] [Fig. 11] represents the refractive index profiles of the different elements of the coupling system as a function of the distance from the center of each element, according to the first particular example of embodiment.

[0050] [Fig. 12] represents a coupling system, according to the first particular example of embodiment.

[0051] [Fig. 13] represents a cross-section and a longitudinal section of a multi-core optical fiber according to a second particular exemplary embodiment, in which the multi-core optical fiber comprises five cores, including a central core.

[0052] [Fig. 14] represents the coupling of a multi-core optical fiber with a section of graded index fiber, according to the second particular example of embodiment.

[0053] [Fig. 15] represents a coupling system, according to the second particular example of embodiment.

[0054] [Fig. 16] represents a coupling system comprising additional optical functions, according to an exemplary embodiment.

[0055] [Fig. 17] represents a coupling system comprising different interconnected blocks, according to a first exemplary embodiment.

[0056] [Fig. 18] represents a coupling system comprising different interconnected blocks, according to a second exemplary embodiment.

[0057] [Fig. 19] represents a coupling system comprising different interconnected blocks, according to a third exemplary embodiment. Detailed description of the invention 1. General principle

[0058] The present invention relates to a compact and simple to implement coupling technique between a multi-core optical fiber and at least one single-core optical fiber.

[0059] More particularly, as illustrated by [Fig.l], the coupling technique according to the invention implements multi-core (MCF) and single-core (SCF) fibers, each of these optical fibers having a section of graded index fiber at its end, forming a micro-lensed end (GRIN-MCF, GRIN-SCF). In addition, a graded index lens (GRIN-L) is interposed between the micro-lensed end of the multi-core fiber on the one hand, and the micro-lensed ends of the single-core fibers on the other hand.

[0060] The multi-core and single-core fibers implemented according to the technique of the invention may be of the multimode and / or single-mode type and / or capable of supporting WDM type transmission. In the case of single-mode fibers, the propagation of the optical beams within the multi-core and single-core fibers may be carried out according to an axial mode (zero angle of incidence at the entrance to the fiber core).

[0061] According to a first embodiment, the coupling system can be implemented to transmit optical beams from at least one single-core optical fiber to the cores of a multi-core optical fiber.

[0062] According to a second embodiment, the coupling system can be implemented to transmit optical beams from the cores of a multi-core optical fiber to at least one single-core optical fiber. This second embodiment is used as a description support hereinafter, it being understood that the technique according to the invention is not limited thereto.

[0063] Thus, the coupling system according to the invention implements a multi-core optical fiber comprising n cores, n being a natural integer greater than or equal to 2, on the one hand; and m single-core optical fibers, m being a natural integer greater than or equal to 1, and less than or equal to n, on the other hand. Thus, in relation to [Fig.2], the optical beam at the input of the single-core optical fiber j (SCFj) comes from the core i (CMCF i) of said multi-core optical fiber, with i = j and i & {1, ..., n} and i {1, ..., m}.

[0064] According to a particularly advantageous embodiment, each core of the multi-core fiber corresponds to a single-core fiber, i.e. n = m.

[0065] LL Multi-core optical fiber (MCF)

[0066] On the one hand, the coupling system according to the invention therefore implements a multi-core optical fiber. This multi-core optical fiber comprises at least two cores. As previously stated, the cores of the multi-core fiber may be single-mode and / or multi-mode and / or capable of supporting WDM type transmission.

[0067] As illustrated by [Fig.2] and [Fig.3], the multi-core fiber (MCF) has a diameter dMCF. This diameter dMCF is typically between 80 and 225pm, for example 125pm. The diameter of the optical beam within the core CMcf4 of the multi-core fiber is œMCF-î, Finally, a distance e> can be defined between the center of the multi-core fiber (corresponding to an axis O0), and the center of the core CMcf4 of the multi-core fiber (corresponding to an axis OMCfJ-

[0068] The O0 axis corresponding to the center of the multi-core optical fiber defines the main optical axis of the system. The OMcf4 axis is therefore parallel to the main axis of the system, and spaced by a distance ei from this main axis.

[0069] The coupling system according to the invention makes it possible to transmit the optical beam from the core CMcf4 of the multi-core optical fiber to the core of the single-core optical fiber SCF-j, it being understood that the beam can also be transmitted in the opposite direction.

[0070] According to a first embodiment illustrated by [Fig.9], the multi-core optical fiber (MCF) can comprise 4 cores (Cmcfi, CMcf 2, CMcf 3, CMcf 4) - According to this example, the center of each of these cores is spaced by a distance e; (eb e4) relative to the main axis of the system (O0). More precisely, [Fig.9] represents on the one hand, a cross-section (AA) of a multi-core optical fiber, with a uniform distribution of the 4 cores around the main optical axis of the system (O0); and on the other hand, [Fig.9] represents a longitudinal section (BB) of this optical fiber, passing through the main optical axis of the system, at the level of the cores Cmcfi, and CMCf 4 - The plane of this longitudinal section BB therefore comprises the axes O0, OMCf 1 and OmCF-4 -

[0071] According to a second exemplary embodiment illustrated by [Fig. 13], the multi-core optical fiber (MCF) may comprise a central core. More particularly, according to [Fig. 13], the multi-core optical fiber may comprise five cores (CMcf 1, CMcf-2, CMcf-3, CMCF 4, CMCf-5). According to this example, the center of each of these cores is spaced by a distance e; (eb e4) from the main axis of the system (O0), with a zero distance for the core CMCf 5. The core CMCf 5 is therefore positioned at the center of the multi-core fiber. More precisely, [Fig. 13] represents on the one hand, a cross-section (AA) of a multi-core optical fiber, with a uniform distribution of four cores around the main optical axis of the system (O0) and a central core; and on the other hand, [Fig.13] represents a longitudinal section (BB) of this optical fiber, passing through the main optical axis of the system, at the level of the cores CMCf 1, CMCf4 and CMCf 5 - The plane of this longitudinal section BB therefore includes the axes O0, OMcf 1, OMcf 4 and O mcf 5, with the axes O0 and OMcf 5 merged.

[0072] 1.2. Single-core optical fiber (fSCF)

[0073] On the other hand, the coupling system implements at least one single-core optical fiber. As previously stated, the core of the single-core fiber can be

[0074]

[0075]

[0076]

[0077]

[0078]

[0079]

[0080]

[0081]

[0082]

[0083]

[0084] single-mode, multi-mode or capable of supporting WDM-type transmission. As illustrated by [Fig.2] and [Fig.4], the single-core fiber (SCFj) has a diameter dscF-j- This diameter dscF-j is typically between 80 and 225pm, more particularly between 80 and 125pm, for example 125pm. The single-core fiber comprises a core Cscp j- The diameter of the optical beam within the core is ^SCF-j. Finally, single-core fibers may have a radial offset relative to the multi-core optical fiber, depending on a core parameter of the multi-core optical fiber. More particularly, this radial offset can be defined as a function of eî, by a distance hj defined between the center of the single-core fiber (corresponding to an axis OScf j), and the principal optical axis of the system. The axis Oscfî is therefore parallel to the principal axis of the system, and spaced by a distance hj from this principal axis. More precisely, this distance hj can be defined by the following formula: hj - x 7^ with nor the refractive index at the center of the graded-index fiber section coupled to the multi-core fiber; and the quadratic coefficient of the graded index fiber section coupled to the multi-core fiber; n2 the refractive index at the center of the gradient index lens; and S2 the quadratic coefficient of the gradient index lens. This distance hj makes it possible to optimize the placement of single-core optical fibers, thus obtaining a compact coupling system. In addition, this distance is chosen to be greater than a predefined threshold value depending on the diameter of the single-core fibers. 1.3. Graded index fiber (GRIN-MCF) section at the end of the multi-core fiber As illustrated in [Fig.5], the end of the multi-core optical fiber (MCF) is coupled to a cylindrical-shaped graded-index fiber section, resulting in a first micro-lensed end (GRIN-MCF). Thus, this graded-index fiber section has a lensing and collimation function to control the radius and angle of the transmitted optical beam. The diameter of the graded index fiber section is d grin-mcf- This diameter dgrin-mcf can be between 80 and 400pm, and more particularly between 80 and 250pm. According to an exemplary embodiment, the diameter doRIN^MCF is advantageously similar to the diameter of the multi-core fiber dMCF. However, a tolerance of a few hundred micrometers can be applied between dGRi^MCF and dMCF. Indeed, it is possible to weld (join) fibers of different diameters with a tolerance of the order of a few 100 pm. More specifically, the following tolerance can be applied: dGRIN.MCF = ^mcf — ^OQjum. For example, a multi-core optical fiber, having a diameter dMCF equal to 125 pm, can be coupled to a section of graded index fiber having a diameter dGRIN_MCF equal to 250 pm. Or according to another example, illustrated by the examples in [Fig. 10] and [Fig. 14], a multi-core optical fiber, having a diameter dMCF equal to 125 pm, can be coupled to a section of graded index fiber having a diameter dGRIN_McF equal to 125 pm.

[0085] This coupling can thus be carried out by welding due to the dimensional similarity of the diameters of these two elements.

[0086] In addition, the graded index fiber section has a central axis 0Grin mcf-According to a particular embodiment, this axis OGrin mcf is aligned with the main optical axis of the system O0, with a tolerance of <± Ipm. Thus, according to a particular embodiment illustrated by [Fig.5], this central axis OGrin_Mcf is coincident with the main axis of the system O0.

[0087] The graded index fiber section (GRIN-MCF) may be composed of organic or mineral glass. The material used may be doped to obtain a specific refractive index profile. According to an exemplary embodiment, this profile decreases from the center of the section toward its circumference. Thus, the refractive index at the center of the graded index fiber section is According to an exemplary embodiment in which the section is made of pure silica, this index ni is greater than the refractive index of silica nM, with ^5 / =1.457 at 632.8nm (for a HeNe laser). In this case, the refractive index profile may be obtained by doping the silica with one or more elements making it possible to increase or decrease the refractive index of the silica (Ge, P, Al, Cl, Ti, Br, B, F).

[0088] The quadratic coefficient of the index gradient section is This quadratic coefficient S{ has values ​​typically between 0.5 and 10 mm '.

[0089] The length of the gradient index section is This length 1} is between 100pm and 10mm, for example equal to 500pm. This length / ] defined as a function of the quadratic coefficient of the section *i. More particularly, this length is defined by the formula:

[0090] =

[0091] As illustrated in [Fig.2], the diameter of the optical beam at the interface between the graded index fiber section of the multi-core optical fiber and the graded index lens is This diameter can be defined by the following formula (with 2 the wavelength of the transmitted beam):

[0092] =---to---

[0093] As illustrated by [Fig.5], the angle of the optical beam, coming from (or towards) the CMCF core i, at the output (or input) of the graded index fiber section is &t. This angle can be defined by the following formula:

[0094] 0i = eixgl

[0095] Thus, the graded index fiber section allows the replacement of prior art micro-lens assemblies. This section has a lensing and collimation function in order to orient and control the transmitted optical beam. All of the aforementioned parameters are co-dependent and can be adapted to obtain the desired dimensions and orientations, and thus optimize the transmission of the optical beam. In addition, due to the dimensions of this graded index fiber section, it can be directly welded to the multi-core optical fiber. The welding thus makes it possible to definitively fix a specific alignment of the elements. This therefore makes it possible to limit alignment errors and to facilitate the implementation of the coupling system, while obtaining a compact assembly.

[0096] 1.4. Graded index fiber section (GRIN-SCF) at the end of each fiber single-core

[0097] As illustrated in [Fig.6], the end of a single-core optical fiber (SCFj) is coupled to a cylindrical-shaped graded-index fiber section, resulting in a second micro-lensed end (GRIN-SCFj). Thus, this graded-index fiber section has a lensing and collimation function to control the radius and angle of the transmitted optical beam.

[0098] The diameter of the graded index fiber section is dGRiN.$cp.j. This diameter dGRIN-SCF-j can be between 80 and 400 pm, and more particularly between 80 and 250 pm. According to an exemplary embodiment, the diameter dcRiN-scF-i is advantageously similar to the diameter of the single-core fiber d^cp-j- However, a tolerance of a few hundred micrometers can be applied between dcRi^cF-j and dgcF-j- Indeed, it is possible to weld / assemble fibers of different diameters with a tolerance of the order of a few 100 pm. More particularly, the following tolerance can be applied: dcRiN-scF-i = dscF-j ± ~00 pm. For example, a multi-core optical fiber, having a diameter dscF-j equal to 125pm, can be coupled to a section of graded index fiber having a diameter dcRiNSCF-j equal to 250pm. Or according to another example, illustrated by the examples of [Fig.12] and [Fig.15], a multi-core optical fiber, having a diameter dscF-j equal to 125pm, can be coupled to a section of graded index fiber having a diameter dcRiN-scF-j equal to 125pm.

[0099] This coupling can thus be carried out by welding due to the dimensional similarity of the diameters of these two elements.

[0100] In addition, the graded index fiber section has a central axis OGR1N scij-According to a particular embodiment, this axis OGR1N sci j is aligned with the axis of the single-core fiber OSGir with a tolerance of <± Ipm. Thus, according to a particular embodiment illustrated by [Fig.6], this central axis OGRIX sci j is coincident with the axis of the single-core fiber OSGFj-

[0101] The graded index fiber section (GRIN-SCFj) may be composed of organic or mineral glass. The material used may be doped to obtain a specific refractive index profile. According to an exemplary embodiment, this profile decreases from the center of the section toward its circumference. Thus, the refractive index at the center of the graded index fiber section is According to an exemplary embodiment in which the section is made of pure silica, this index n3-j is greater than the refractive index of silica nsi, with ^=1.457 at 632.8nm (for a HeNe laser). In this case, the refractive index profile may be obtained by doping the silica with one or more elements making it possible to increase or decrease the refractive index of the silica (Ge, P, Al, Cl, Ti, Br, B, F).

[0102] The quadratic coefficient of the gradient index section is ^3, / . This quadratic coefficient #3 has values ​​typically between 0.5 and 10 mm '.

[0103] The length of the graded index fiber section (GRIN-SCFj) is l^j. This length ly; is between 100pm and 10mm, for example equal to 500pm. This length is defined as a function of the quadratic coefficient of the section &3-j. More particularly, this length is defined by the formula:

[0104] / 3, / =2¾

[0105] As illustrated in [Fig.2], the diameter of the optical beam at the interface between the graded index fiber section of the multi-core optical fiber and the graded index lens is 0?2. This diameter can be defined by the following formula (with 2 being the wavelength of the transmitted beam):

[0106] =----d----

[0107] with ^'2 the beam diameter at the interface between the GRIN-SCFj graded index fiber section and the graded index lens (GRIN-L).

[0108] According to the example illustrated by [Fig.6], the angle of the optical beam, coming from (or in the direction of) the single-core optical fiber SCFj, at the output (or input) of the graded index fiber section is zero.

[0109] As previously indicated in relation to the single-core optical fiber, the graded index fiber section associated with a single-core fiber has a radial offset relative to the multi-core optical fiber and the associated graded index fiber section, depending on a core parameter of the multi-core fiber. More particularly, this radial offset can be defined by the distance hj between the center of the single-core fiber and / or the center of the associated graded index fiber section (corresponding respectively to an axis OScfj and / or an OGrin scfj) on the one hand, and the principal optical axis of the system O0 on the other hand.

[0110] Thus, the graded index fiber section allows the replacement of prior art micro-lens assemblies. This section has a lensing and collimation function in order to orient and control the transmitted optical beam. All of the aforementioned parameters are co-dependent and can be adapted to obtain the desired dimensions and orientations, and thus optimize the transmission of the optical beam. In addition, due to the dimensions of this graded index fiber section, it can be directly welded to the multi-core optical fiber. The welding thus makes it possible to definitively fix a specific alignment of the elements. This therefore makes it possible to limit alignment errors and to facilitate the implementation of the coupling system, while obtaining an extremely compact assembly. [YES] 1.5. Gradient Index Lens (GRIN-L)

[0112] As illustrated by [Fig.7], the graded index fiber section (GRIN-MCF) previously coupled on the one hand to the multi-core fiber (MCF) is coupled on the other hand to the graded index lens (GRIN-L). More particularly, but not exclusively, the graded index lens is of overall cylindrical shape and has two connection interfaces (L1, L2). A first interface (L1) is therefore coupled to the graded index fiber section of the multi-core fiber (GRIN-MCF), while a second interface (L2) is intended to be coupled to the graded index fiber sections of the single-core fibers (GRIN-SCFj).

[0113] These couplings can be made by removable mechanical assembly, by gluing, or any other suitable assembly.

[0114] The diameter of the graded index fiber section is dGRIN_L. This diameter dgrin-l can be between 125pm and 4mm, typically equal to 1mm.

[0115] The alignment of the lens with the other elements of the system is limited by any defects present on the edges of the lens. More precisely, the gradient index lens has a central axis OGrin l- According to one embodiment, this axis OGrin l is aligned with the main axis of the system O0, with a tolerance of 3 pm, or with a tolerance of I pm. Thus, according to a particular embodiment illustrated by [Fig.7], this central axis OGR1N.L coincides with the main axis of the system O0.

[0116] The gradient index lens (GRIN-L) may be composed of organic or mineral glass. The material used may be doped to obtain a specific refractive index profile. According to an exemplary embodiment, this profile decreases from the center of the lens towards its circumference. Thus, the refractive index at the center of the lens at index gradient is ^2. According to an exemplary embodiment in which the lens is made of pure silica, this index is higher than the refractive index of silica nsi, with nsi = 1.457 at 632.8nm (for a HeNe laser). In this case, the refractive index profile can be obtained by doping the silica with one or more elements making it possible to increase or decrease the refractive index of the silica (Ge, P, Al, Cl, Ti, Br, B, F).

[0117] The quadratic coefficient of the gradient index section is S2. This quadratic coefficient S2 has values ​​typically between 0.5 and 10 mm '.

[0118] The length of the gradient index fiber section (GRIN-L) is l2. This length Z2 is between 500pm and 10mm, typically of the order of a millimeter. This length l2 is defined as a function of the quadratic coefficient of the lens ë2. More particularly, this length is defined by the formula: [0H9] 12=2¾

[0120] Thus, this gradient index lens also has a lensing and collimation function in order to control the radius and angle of the transmitted optical beam. All of the aforementioned parameters are co-dependent and can be adapted to obtain the desired dimensions and orientations, and thus optimize the transmission of the optical beam. 2. Coupling process

[0121] The coupling method according to the invention can implement different types of assemblies such as welding, removable mechanical assembly, mechanical assembly by gluing, etc.

[0122] More particularly, the method comprises welding the gradient index sections (GRIN-MCF, GRIN-SCF) to the corresponding multi-core (MCF) and single-core (SCF) fibers. Thus, as illustrated by [Fig.8] the coupling method according to the invention comprises: - a step of welding (S-MCF) a first section of graded index fiber at the end of said multi-core fiber to form a first micro-lensed end; - for each single-core fiber (SCF) of said assembly, a step of welding (S-SCF) a second section of graded index fiber (GRIN-SCF) at the end of said single-core fiber to form a second micro-lensed end;

[0123] Since the graded index fiber sections preferably have a diameter similar to that of the corresponding optical fibers, welding is thus facilitated, and this makes it possible to create micro-lensed ends that are correctly aligned and permanently fixed to the multi-core and single-core optical fibers.

[0124] In addition, each of the micro-lensed ends thus formed can be mechanically assembled to the gradient index lens (GRIN-L), for example, by gluing, via a removable mechanical connection such as ferrules, etc. More particularly, as illustrated by [Fig.8], the coupling method according to the invention also comprises: - a mechanical assembly step (AS-MCF / L) of the first micro-lensed end of the multi-core fiber with a first interface of a gradient index lens; - a mechanical assembly step (AS-SCF / L) of the second micro-lensed end of the single-core fiber with a second interface of said gradient index lens.

[0125] Thus, such a coupling method allows for simple assembly where alignment of the different elements is facilitated. In particular, the use of sections of graded index fibers having a diameter similar to the diameter of the multi / single-core fibers allows for definitive fixing by welding. The system thus obtained is compact and simple to implement.

[0126] It is understood that the elements described in relation to the system can be applied to the method and vice versa. More particularly, the radial offset of a single-core fiber (and the corresponding micro-lensed end) relative to the multi-core fiber (and the corresponding micro-lensed end) depends on a core parameter of the multi-core fiber. More precisely, the optical axis of the assembled single-core fiber advantageously has an offset of a distance h with the main optical axis of the system, the distance h being defined as a function of the distance e between the main optical axis of the system and the optical axis of the core of the multi-core optical fiber whose beam is transmitted to the assembled single-core optical fiber. This distance h is greater than a threshold value chosen as a function of the diameter of the single-core fibers, typically a minimum threshold value facilitating the coupling of the single-core fibers on the GRIN-L lens. 3. Examples of embodiments

[0127] 3.1. Example 1

[0128] A first particular example of a coupling system according to the invention is represented by [Fig.9], [Fig.10], [Fig.11] and [Fig.12]. According to this first example, the multi-core optical fiber has four cores (CMCf-i, CMCf-2, CMCf 3, CMCf 4), and the system comprises four single-core fibers (SCFi, SCF2, SCF3, SCF4), the transmission of the optical signal being carried out from the multi-core fiber to the single-core fibers.

[0129] According to this example, the optical beam from the CMCf 1 core is intended to be transmitted to the single-core fiber SCFi, the beam from the CMCf 2 core is intended to be transmitted to the SCF2 fiber, the beam from the CMCf-3 core is intended to be transmitted to the SCF3 fiber, the beam from the CMCf-4 core is intended to be transmitted to the SCF4 fiber,

[0130] Furthermore, the center of each of these cores is spaced by a distance e; (eb e4) from the main axis of the system (O0). More precisely, [Fig.9] represents on the one hand, a cross-section (AA) of a multi-core optical fiber, with a uniform distribution of the four cores around the main optical axis of the system (O0); and on the other hand, [Fig.9] represents a longitudinal section (BB) of this optical fiber, passing through the main optical axis of the system, at the level of the cores CMCf i, and CMCf 4- The plane of this longitudinal section BB therefore includes the axes O0, OMCf i and OmCF-4-

[0131] In the present example, the wavelength 2 chosen is equal to 380 pm. The diameter of all the optical fibers and the graded index fiber sections is equal to 125 pm. In addition, each graded index fiber section is perfectly aligned with the optical fiber to which it is coupled.

[0132] Concerning the multi-core fiber (MCF), the center of each of its cores is at a distance e=10pm from the center of the multi-core fiber (corresponding to the principal optical axis of the system O0). In addition, the diameter of the optical beam a'o is equal to 5pm at the output of the multi-core fiber.

[0133] Regarding the graded index fiber section of the multi-core fiber (GRIN-MCF), as illustrated in [Fig.10] and [Fig.12], the refractive index at the center of the section is equal to 1.472. The quadratic coefficient of the section is equal to 4.3mm '. The length / ] of the section is therefore determined as follows: / j = 7777 ~ 365 pm. In addition, the diameter of the beams at the output of the graded index fiber section is determined as follows: —____£___= 38 mm- And finally, as indicated by [Fig. 10], the angles 0] and of the beams coming from the cores CMcf i and CMCf 4, at the exit of the section, measured with respect to the principal optical axis of the system O0, are equal to Gj = ex = 0.043 rad.

[0134] Regarding the gradient index lens (GRIN-L), as illustrated in [Fig. 12], the lens diameter d2 is about 1mm. The refractive index at the center of the lens is equal to / 7f The quadratic coefficient S2 of the lens is equal to 0.6mm '. The section length is therefore determined as follows l2 = 777 — 2618 / zm. In addition, the diameter of the beams at the output of the gradient index fiber section is determined as follows: . , _ / _ to Finally, the angle of the ~ ~ «0 x 77777 - 03.8 pm b optical beam relative to the principal optical axis at the exit of the lens is zero.

[0135] Regarding the graded index fiber sections of the single-core fibers (GRIN-SCFj, GRIN-SCF2, GRIN-SCF3, GRIN-SCF4), as illustrated by the [Fig. 12], the refractive index at the center of section n3 is equal to 1.6. The diameter of the optical beam at the exit of the section (and at the entrance of the single-core fiber) is equal to = o?0 = 5 gm. Finally, the distance between the optical axis of the system and the center of each of the single-core fibers is equal to / v fi — txr)iX„ = 71

[0136] Concerning the single-core optical fibers (SCFi, SCF2, SCF3, SCF4) these are focused on the sections of graded index fiber to which they are coupled, and are therefore also offset by a distance h — 11 / dm relative to the main axis of the system O0.

[0137] In the case of a multi-core optical fiber according to this first example, the optical parameters are chosen so that the distance h meets the following inequality: . In this case: h > 62.5 pm.

[0138] Finally, in the case of this first example, the gradient index fiber sections and the gradient index lens used have specific refractive index profiles. Thus, [Fig. 11] illustrates the evolution of the refractive index as a function of the distance from the center of each of these elements. More precisely, considering — dGR1N_MCFj 2, the refractive index of the graded index fiber section of the multi-core fiber has a maximum value at its center equal to n\ then a decreasing value down to nsi for a center distance of 7?1 or - 7?1. Considering A2 = dG[ilN.Ll 2, the refractive index of the graded index lens has a maximum value at its center equal to then a decreasing value down to nsi for a center distance of R2 or -R2. Finally, considering Æ3 = dGR[N_SCF / 2, the refractive index of the graded index fiber section of a single-core fiber has a maximum value at its center equal to '6, then a decreasing value down to for a center distance of 7?3 or -R3.

[0139] 3.2. Example 2

[0140] A second particular example of a coupling system according to the invention is represented by [Fig.13], [Fig.14] and [Fig.15].

[0141] According to this example, a multi-core optical fiber may have a central core (C mcf-s)> and peripheral cores (CMCF-i, Cmcf_2, Cmcf_3, Cmcf_4).

[0142] By applying the technique previously described, the beam coming from the central core of the multi-core fiber is not deflected, to be transmitted to a single-core optical fiber. Indeed, since ¢5 = 0, then: #5 = ex = 0 and h = 0.

[0143] There is therefore no offset of the single-core optical fiber to which the beam from the central core of the multi-core fiber is transmitted. The SCF5 single-core optical fiber is centered on the same main axis as the multi-core optical fiber.

[0144] More precisely, [Fig. 13] represents on the one hand, a cross section (AA) of a multi-core optical fiber, with a uniform distribution of four cores around the main optical axis of the system (O0) and a central core; and on the other hand, [Fig. 13] represents a longitudinal section (BB) of this optical fiber, passing through the main optical axis of the system, at the level of the cores CMcf i, CMcf4 and CMcf 5- The plane of this longitudinal section BB therefore includes the axes O0, OMcf i, OMcf 4 and O mcf 5, with the axes O0 and OMcf 5 merged. 4. Supplements

[0145] 4.1. Additional optical functions

[0146] Additional optical functions may be added to the system, they may be implemented by the addition of at least one of the following optical components: an optical blade; an optical filter; an anti-reflective component; an optical isolator.

[0147] These components may be arranged between two elements of the coupling system, in particular, between the graded index lens and one of the graded index fiber sections of the multi-core fiber and / or a single-core fiber.

[0148] According to an embodiment illustrated by [Fig. 16], an additional component having a specific optical function is added between the graded index lens and the graded index fiber sections associated with single-core fibers. More particularly, each of the illustrated sections (GRIN-SCFi, GRIN-SCF4) is separated from the lens (GRIN-L) by a separate optical component (11, 14). However, these additional components can also be identical for all the graded index fiber sections, or can be added only at the end of some of them.

[0149] Thus, according to a particular embodiment, at least one of the graded index fiber sections of the multi-core fiber or of a single-core fiber is coupled to the input and / or output of said graded index lens via an additional optical component.

[0150] Furthermore, in order to add such an additional element, it is possible to play on the choice of the parameters of the index gradient, the external diameter and the length of the section of fiber with index gradient.

[0151] These additional components can be installed within the system using different methods, for example the so-called “V-groove” method (or V-shaped grooves), or a method using fibers mounted in cylindrical ferrules with an alignment sleeve. The “V-groove” method uses V-shaped grooves in which the fibers are placed. elements to be aligned and assembled, the antlers facilitating alignment; while the "sleeve" method uses a sleeve into which ferrules are inserted in order to align the fibers (or other elements) present in these ferrules. These methods can also be used for the alignment of other elements of the coupling system.

[0152] Furthermore, the addition of such additional optical components makes it possible to obtain a more elaborate and complete system with additional optical functions, the system remaining compact and simple to implement.

[0153] 4.2. Implementation with ferrule type connectors

[0154] The coupling system according to the invention can be implemented via the use of specific connectors such as ferrules, making it possible to form blocks of elements to be interconnected.

[0155] [Fig.17], [Fig.18] and [Fig.19] illustrate three different configurations that can be implemented to allow the interconnection of different blocks of system elements.

[0156] Thus, according to a first example, [Fig. 17] illustrates a first configuration in which: the end of the multi-core fiber (MCF) and the corresponding graded index fiber section (GRIN-MCF) are included in an independent ferrule (21); each single-core fiber end (SCFj) and the corresponding graded-index fiber section (GRIN-SCFj) are included in an independent ferrule (22j); and the gradient index lens (GRIN-L) is independent.

[0157] According to a second example, [Fig. 18] illustrates a second configuration in which: the end of the multi-core fiber (MCF), the corresponding graded index fiber section (GRIN-MCF) and the graded index lens (GRIN-L) are included in the same ferrule (23), on the one hand; each single-core fiber end (SCFj) and the corresponding graded index fiber section (GRIN-SCFj) are included in an independent ferrule (24j), on the other hand.

[0158] Finally, [Fig. 19] illustrates a third configuration in which: the end of the multi-core fiber (MCF) and the corresponding graded index fiber section (GRIN-MCF) are included in a first independent ferrule (25) on the one hand; the end of the single-core fibers (SCFj), each corresponding graded-index fiber section (GRIN-SCFj), and the graded-index lens (GRIN-L) are included in a second ferrule (26), on the other hand.

[0159] Thus, the use of such connectors makes it possible to facilitate the implementation of the coupling system, by facilitating the alignment of the elements and their interconnection.

Claims

Claims

1. A system for coupling between a multi-core optical fiber (MCF) and a set of at least one single-core optical fiber (SCF), each single-core fiber of said set being coupled to a separate core of the multi-core fiber, said coupling system comprising: - a gradient index lens (GRIN-L), positioned between said multi-core fiber and said at least one single-core fiber; the system being characterized in that it comprises: - a first section of gradient index fiber coupled to the end of the multi-core fiber to form a first micro-lensed end (GRIN-MCF); - for each single-core fiber of said set, a second section of gradient index fiber coupled to the end of said single-core fiber to form a second micro-lensed end (GRIN-SCF);said first and second micro-lensed ends being coupled on either side of said lens with a radial offset of one relative to the other defined as a function of a core parameter of the multi-core fiber.;

2. Coupling system according to claim 1, characterized in that the radial offset of each single-core fiber is defined by a distance h, the distance h being defined as follows: II - £ X with the following optical parameters: e said core parameter, defined as the radial distance between the central axis of the multi-core fiber (MCF) and the axis of the core of the multi-core fiber to which said single-core fiber is coupled; the refractive index at the center of said first graded index fiber section (GRIN-MCF); the quadratic coefficient of said first graded index fiber section (GRIN-MCF); the refractive index at the center of said graded index lens (GRIN-L); S2 the quadratic coefficient of said graded index lens (GRIN-L).

3. Coupling system according to claim 1, characterized in that: - the length l\ of the first section of gradient index fiber (GRIN-MCF) of said multi-core fiber is inversely proportional to its quadratic coefficient 8 j; - the length / 2 of the gradient index lens (GRIN-L) is inversely proportional to its quadratic coefficient S2; - the length / 3 of the gradient index fiber section (GRIN-SCF) of said at least one single-core fiber, is inversely proportional to its quadratic coefficient ^3.

4. Coupling system according to claim 1, characterized in that the cores of the multi-core optical fiber and the set of at least one single-core optical fiber are of the single-mode type.

5. Coupling system according to claim 1, characterized in that said first and second micro-lensed ends are directly coupled to the input and output of said gradient index lens.

6. Coupling system according to claim 1, characterized in that at least one of said first and second micro-lensed ends is coupled to the input and / or output of said gradient index lens via an optical component belonging to the group comprising: - an optical blade; - an optical filter; - an anti-reflection component; - an optical isolator.

7. Coupling system according to claim 1 characterized in that: - the end of the multi-core fiber (MCF) and the corresponding first micro-lensed end (GRIN-MCF) are included in an independent ferrule; - each end of the single-core fiber (SCF) of said assembly and the corresponding second micro-lensed end (GRIN-SCF) are included in an independent ferrule; and - the gradient index lens (GRIN-L) is independent.

8. Coupling system according to claim 1 characterized in that: - the end of the multi-core fiber (MCF), the corresponding first micro-lensed end (GRIN-MCF) and the gradient index lens (GRIN-L) are included in the same ferrule, on the one hand; - each end of the single-core fiber (SCF) of said assembly and the corresponding second micro-lensed end (GRIN-SCF) are included in an independent ferrule, on the other hand.

9. Coupling system according to claim 1 characterized in that: - the end of the multi-core fiber (MCF) and the first end corresponding micro-lens (GRIN-MCF) are included in a first independent ferrule on the one hand; - the assembly of at least one single-core fiber end (SCF), each corresponding second micro-lensed end (GRIN-SCF), and the gradient index lens (GRIN-L) are included in a second ferrule, on the other hand.

10. Method for coupling a multi-core optical fiber (MCF) and a set of at least one single-core optical fiber (SCF), characterized in that it comprises: - welding (S-MCF) a first section of graded index fiber (GRIN-MCF) at the end of said multi-core fiber (MCF) to form a first micro-lensed end; - for each single-core fiber (SCF) of said assembly, welding (S-SCF) of a second section of graded index fiber (GRIN-SCF) at the end of said single-core fiber to form a second micro-lensed end; - mechanical assembly (AS-MCF / L) of said first micro-lensed end of said multi-core fiber with a first interface (Ll) of a gradient index lens; - mechanical assembly (AS-SCF / L) of said second micro-lensed end of said single-core fiber with a second interface (L2) of said gradient index lens, with a radial offset relative to the first micro-lensed end, defined as a function of a core parameter of the multi-core fiber.

Images