Augmented multimodal spatial optical-fibre modes

Abstract

A device for inserting a plurality of optical beams into a single-mode optical fibre, a guiding structure of which is composed of a core with a first refractive index, a cladding with a second refractive index, and a coating with a third refractive index. The device includes an optical mixer configured to insert, into the single-mode optical fibre, the plurality of optical beams, at least one of which has a distribution of its radial and angular electromagnetic amplitude with a maximum amplitude peak in the cladding.

Claims

1. An injecting method comprising: injecting a plurality of optical beams into an output optical fiber, which has a guiding structure composed of a core with a first refractive index, a cladding with a second refractive index, and a sheath with a third refractive index, wherein the injecting comprises: obtaining the plurality of optical beams, and injecting the plurality optical beams by mixing the plurality of optical beams into the output optical fiber, wherein the core and the cladding play a role of a multimodal guiding core, and the sheath plays a role as cladding for the output optical fiber for at least certain spatial modes, and wherein at least one of the optical beams undergoes a conversion in which a respective multimodal spatial mode is applied prior to the injection into the output optical fiber, generating for the respective optical beam a distribution of its radial and angular electromagnetic amplitude with a maximum amplitude peak in the cladding of the output fiber.

2. The injecting method as claimed in claim 1, comprising, prior to the obtaining of the plurality of optical beams, extracting the optical beams from a plurality of optical fibers.

3. The injecting method as claimed in claim 2, wherein the optical fibers from which the optical beams which are extracted are single-mode optical fibers.

4. The injecting method as claimed in claim 2, wherein at least one of the optical fibers from which the optical beams are extracted is a few-mode fiber.

5. The injecting method as claimed in either claim 2, wherein at least one of the optical fibers from which the optical beams are extracted is a multimode fiber.

6. The injecting method as claimed in claim 1, wherein the output optical fiber is a single-mode fiber.

7. The injecting method as claimed in claim 1, wherein the output optical fiber is a few-mode fiber.

8. The injecting method as claimed in claim 1, wherein the output optical fiber is a multimode fiber.

9. A device for injecting a plurality of optical beams into an output optical fiber, which has a guiding structure composed of a core with a first refractive index, a cladding with a second refractive index, and a sheath with a third refractive index, the device comprising: an optical mixer configured to inject into the output optical fiber the plurality of optical beams wherein the core and the cladding play a role of a multimodal guiding core, and the sheath plays a role as cladding for the output optical fiber for at least certain spatial modes, a plurality of optical receivers configured to receive optical beams from a plurality of optical fibers, upstream of the optical mixer, and at least one spatial-mode converter configured to modify a distribution of a radial and angular electromagnetic amplitude of an optical beam, a first of the optical receivers and the optical mixer, and thereby generate the distribution with a maximum amplitude peak in the cladding of the output fiber.

10. The device for injecting a plurality of optical beams as claimed in claim 9, wherein the output optical fiber is a single-mode fiber.

11. The device for injecting a plurality of optical beams as claimed in claim 9, wherein the output optical fiber is a few-mode fiber.

12. The device for injecting a plurality of optical beams as claimed in claim 9, wherein the output optical fiber is a multimode fiber.

Description

4. PRESENTATION OF THE FIGURES

(1) Other advantages and features of the invention will become more clearly apparent on reading the following description of one particular embodiment of the invention, which embodiment is given simply by way of non-limiting, illustrative example, and the appended drawings, in which:

(2) FIG. 1 shows a cross-sectional view of an example of a single-mode fiber used as light-guiding structure by the method according to the invention;

(3) FIGS. 2a to 2k illustrate, for various spatial modes, two-dimensional distributions of the intensity of the optical signal over the cross section of the single-mode fiber, as a function of a radial and angular electromagnetic amplitude;

(4) FIG. 3 shows one example of a device for injecting a plurality of optical beams into a single-mode optical fiber, according to one aspect of the invention;

(5) FIG. 4 shows another example of a device for injecting a plurality of optical beams into a single-mode optical fiber, according to one aspect of the invention.

5. DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT OF THE INVENTION

(6) In the rest of the description, examples of a plurality of embodiments of the invention based on a single-mode output optical fiber are presented, but the invention is also applicable to other types of output optical fiber, such as few-mode fibers or multimode fibers.

(7) The principle of the proposed method and device is to reuse the structure of the output fiber to propagate more modes than in the prior art, each mode forming a distinct transmission channel. To this end, and going against the prejudices of those skilled in the art, the assembly composed of the core, of the cladding and of the sheath of the output fiber is used as guiding structure.

(8) FIG. 1 shows a cross-sectional view of an example of a single-mode fiber used as light-guiding structure by the method according to the invention.

(9) The SMF structure is circular with: a core C of about 8 μm diameter with a glass of refractive index irC of about 1.442 at 1300 nm a cladding G of 125 μm diameter with a glass of refractive index irG of about 1.440 at 1300 nm a sheath E of 250 μm diameter with a polymer of refractive index irE between 1.25 and 1.55 at 980 nm.

(10) The principle is therefore to use the two glass structures, namely the core and the cladding, as the new multimodal guiding core. The sheath now plays a role as new cladding for at least certain of the spatial modes.

(11) FIGS. 2a to 2k illustrate, for various spatial modes, two-dimensional distributions of the intensity of the optical signal over the cross section of the SMF, as a function of a radial and angular electromagnetic amplitude.

(12) These distributions were obtained by mathematical simulation, for a guiding structure with dimensions and refractive indices such as described with reference to FIG. 1, but other dimensions and refractive indices are envisionable.

(13) For each of FIGS. 2a to 2k, the graph on the left-hand side shows the radial distribution of the light, in other words its amplitude as a function of distance from the center of the fiber. The image on the right-hand side shows the resulting distribution of light in a two-dimensional cross section of the optical SMF. The angular distribution is the number of “spots” per ring; this may be seen in the image on the right-hand side and is not shown in a graph.

(14) The first propagation mode, which is illustrated in FIG. 2a, is the natural propagation mode of the single-mode fiber, i.e. the mode named LP.sub.0,1, the intensity peak of which is concentrated in the core of the fiber.

(15) FIG. 2b illustrates the LP.sub.1,1 mode, in which 2 maximum peaks in light intensity are present in the cladding, on either side of the core.

(16) FIG. 2c illustrates the LP.sub.2,1 mode, in which 4 maximum peaks in light intensity are present in the cladding, on either side of the core.

(17) FIG. 2d illustrates the LP.sub.3,1 mode, in which 6 maximum peaks in light intensity are present in the cladding, on either side of the core.

(18) FIG. 2e illustrates the LP.sub.0,2 mode, in which 2 maximum peaks in light intensity are present in the cladding, in the form of 2 rings around the core.

(19) FIG. 2f illustrates the LP.sub.1,2 mode, in which 2 times 2 maximum peaks in light intensity are present in the cladding, on either side of the core.

(20) FIG. 2g illustrates the LP.sub.3,2 mode, in which 2 times 6 maximum peaks in light intensity are present in the cladding, distributed in 2 rings around the core.

(21) FIG. 2h illustrates the LP.sub.0,3 mode, in which 3 maximum peaks in light intensity are present in the cladding, in the form of 3 rings around the core.

(22) FIG. 2i illustrates the LP.sub.1,3 mode, in which 3 times 2 maximum peaks in light intensity are present in the cladding, on either side of the core.

(23) FIG. 2j illustrates the LP.sub.3,3 mode, in which 3 times 6 maximum peaks in light intensity are present in the cladding, distributed in 3 rings around the core.

(24) FIG. 2k illustrates the LP.sub.6,3 mode, in which “times 12 maximum peaks in light intensity are present in the cladding, distributed in 3 rings around the core.

(25) It may therefore be seen that the guiding structure according to the invention is highly multimode, with, for each mode, specific propagation characteristics. The table below collates these characteristics for the LP.sub.x,y modes of FIGS. 2a to 2d, giving their propagation constant (beta), their effective index (n.sub.eff) for a wavelength of 1310 nm, their effective area (A.sub.eff), and their cut-off wavelength (lambda):

(26) TABLE-US-00001 x y beta n.sub.eff A.sub.eff lambda 0 1 5.84237/μm 1.441254 268.5 μm.sup.2 1 1 5.83836/μm 1.440265 386.3 μm.sup.2 79355.21 nm 2 1 5.83676/μm 1.439871 5941.8 μm.sup.2 50125.58 nm 3 1 5.83647/μm 1.439800 5698.0 μm.sup.2 37400.37 nm

(27) FIG. 3 shows an example of a device for injecting a plurality of optical beams into a single-mode optical fiber, according to one aspect of the invention.

(28) The device 100 comprises, at least, a mixer 110 that accepts as input a plurality of optical beams, and produces as output a resulting optical beam OSM-SMF that is injected into an output single-mode optical fiber. One particularity of the mixer is that at least one optical beam received as input is multimodal, i.e. at least one of these optical beams has a distribution of its radial and angular electromagnetic amplitude with a maximum amplitude peak that is positioned in the cladding of the output optical SMF, and not in the core thereof.

(29) The mixer 110 mixes the optical beams received as input without modifying their spatial distribution and injects the optical beam OSM-SMF resulting from the mixing into the optical SMF.

(30) The device 100 may consist of just the mixer 110, but, advantageously, it may also comprise optical receivers R and light-profile converters 101, 102, 103.

(31) Optical beams OS1-SMF, OS2-SMF, OS3-SMF are for example obtained by the optical receivers R from single-mode fibers.

(32) The light-profile converters are optical components configured to modify the radial and angular distribution of an optical beam, which respectively receive one of the extracted optical beams OS1-SMF, OS2-SMF, OS3-SMF with a view to respectively converting them into the beams OS1-LP0,1, OS2-LPx,y, 053-LPn,m. Such converters 101, 102, 103 are known and are for example multi-plane light converters (MPLC), which are based on a modification of optical phase.

(33) An MPLC converts a single-mode input beam, i.e. a beam the energy of which is concentrated at the center of the optical medium, into a multi-mode output beam, the energy of which is distributed between a plurality of locations in the optical medium. Each distribution is characterized by a radial and angular amplitude denoted LP.sub.x,y, such as those illustrated in FIGS. 2b to 2k, where x indicates the number of separate energy peaks in a semicircle around the center, and y indicates the number of rings containing energy peaks.

(34) FIG. 4 shows an example of a device for injecting a plurality of optical beams into a single-mode optical fiber, according to another aspect of the invention.

(35) The device 120 differs from the device 100 in that it may deliver, to the mixer 110, certain beams, such as for example the beam OS1-SMF and the beam OS3-FMF, without modifying them with respect to their state on input into the device, and certain other beams, such as for example the beam OS2-SMF converted into the beam OS2-LPx,y, after having converted them.

(36) The beam OS1-SMF is received by a receiver R in the device, from a (single-mode) SMF, and is already in an LP.sub.0,1 spatial mode. This beam is therefore delivered without conversion to the mixer 110.

(37) The beam OS3-FMF is received by a receiver R in the device from a (few-mode) FMF, and is already in an LP.sub.n,m spatial mode. This beam is therefore delivered without conversion to the mixer 110.

(38) It will be understood that many configurations are envisionable for the device 120. It may accept as input either solely SMF, as the device 100, or indeed any combination of SMF, FMF and MCF.

(39) The examples of devices illustrated in FIGS. 3 and 4 are based on delivery of 3 optical beams to the mixer as input, but any number of input optical beams, whether higher or lower than 3, are envisionable.

(40) Likewise, the injecting device according to the invention may employ a single MCF that delivers a plurality of input beams, for example single-mode and/or multi-mode beams, or a combination of these 2 types of beams, that the mixer 110 injects, optionally after conversion, into an SMF.

(41) By virtue of the injecting device 100 or 120 according to the invention, the number of data-transmission channels that the output SMF is able to transport is equal to the sum of the modes present in the beams converted, or provided as input for beams that are already multimodal. The data to be transmitted modulate one or more wavelengths with different polarizations of an optical beam. All of these beams therefore form a multi-mode transmission channel composed of “n” beams and “l” wavelengths in “p” polarization states. The components used to perform this optoelectronic modulation are for example external Mach-Zehnder modulators or directly modulated laser sources.

(42) The output optical SMF used by way of example in the figures may be replaced by a few-mode fiber (FMF) or by a multimode fiber (MCF). By virtue of the invention, it is therefore possible to increase the transmission capacity of an optical fiber instead of replacing it, whether it be an SMF, FMF or MCF.