阅读说明：本技术 一种用于制备模分复用器的异质多芯光纤及其制备方法 (Heterogeneous multi-core optical fiber for preparing mode division multiplexer and preparation method thereof ) 是由 苑立波 王东辉 夏启 于 2021-10-12 设计创作，主要内容包括：本发明提供的是一种用于制备模分复用器的异质多芯光纤及其制备方法,其特征是：该光纤具有多个单模纤芯,纤芯的折射率、直径或折射率剖面不同,使其纤芯中基模传播常数不同,纤芯呈圆对称分布,纤芯之间设计有气孔或小芯径辅助纤芯用来调节光纤拉锥过程的超模相位差,将该光纤插入低折射率毛细管中后,可拉制成用于涡旋模式复用的光子灯笼。本发明可用于涡旋模式与高斯光束之间的转换,可广泛用于基于涡旋模式复用的光纤通信系统中。(The invention provides a heterogeneous multi-core fiber for preparing a mode division multiplexer and a preparation method thereof, which are characterized in that: the optical fiber is provided with a plurality of single-mode fiber cores, the refractive indexes, the diameters or the refractive index profiles of the fiber cores are different, so that the propagation constants of fundamental modes in the fiber cores are different, the fiber cores are circularly and symmetrically distributed, air holes or small-core-diameter auxiliary fiber cores are designed among the fiber cores and are used for adjusting the supermode phase difference in the tapering process of the optical fiber, and the optical fiber can be drawn into a photon lantern for vortex mode multiplexing after being inserted into a capillary with a low refractive index. The invention can be used for conversion between vortex mode and Gaussian beam, and can be widely used in optical fiber communication system based on vortex mode multiplexing.)

1. A heterogeneous multi-core fiber for preparing a mode division multiplexer and a preparation method thereof are characterized in that: the heterogeneous multi-core fiber comprises a plurality of single-mode fiber cores, the fiber cores are distributed in a circular symmetry mode, the refractive indexes, the diameters or the refractive index profiles of the fiber cores are different, so that the propagation constants of the fundamental modes in the fiber cores are different, the fiber is inserted into a capillary with low refractive index and then tapered, the Gaussian fundamental modes of the heterogeneous multi-core fiber cores can be converted into vortex modes by selecting parameters such as proper taper length, the fundamental modes of the different fiber cores are converted into vortex modes of different orders, and the two modes have a one-to-one correspondence relationship.

2. A heterogeneous multicore optical fiber for fabricating a mode division multiplexer according to claim 1, wherein: the heterogeneous multi-core fiber is in a spiral structure or a non-spiral straight structure.

3. A heterogeneous multicore optical fiber for fabricating a mode division multiplexer according to claim 1, wherein: the number of fiber cores of the heterogeneous multi-core optical fiber is N, N is an integer, and N is more than or equal to 3.

4. A heterogeneous multicore optical fiber for fabricating a mode division multiplexer according to claim 1, wherein: the refractive index profile of the fiber core in the heterogeneous multi-core fiber is in a step type, a parabolic type or a Gaussian type.

5. A heterogeneous multicore optical fiber for fabricating a mode division multiplexer according to claim 1, wherein: the cladding structure of the heterogeneous multi-core fiber is a double cladding, and the mode field area and numerical aperture of the inner cladding of the double cladding are matched with those of the output few-mode fiber after the inner cladding of the double cladding is shrunk in the tapered region.

6. A heterogeneous multicore optical fiber for fabricating a mode division multiplexer according to claim 1, wherein: air holes or small-core-diameter auxiliary fiber cores are designed among the fiber cores of the heterogeneous multi-core fiber, so that the phase difference value of the symmetric supermode and the anti-symmetric supermode in the mode conversion tapered region is controlled.

7. A heterogeneous multi-core fiber for preparing a mode division multiplexer and a preparation method thereof are characterized in that:

step 1: designing corresponding number of optical fiber preforms with different fiber cores according to the mode multiplexing number of the mode division multiplexer, wherein the cladding refractive indexes of the preforms are required to be equal, and the preforms corresponding to the degenerate mode are completely the same;

step 2: and tapering a plurality of different optical fiber preforms to prepare an intermediate, wherein the radius of the intermediate is determined according to the rod assembling condition.

And step 3: and inserting the intermediate into the fluorine-doped capillary with low refractive index, gradually reducing the guided mode propagation constant of the intermediate from the outside to the inner fiber core, filling a proper number of coreless rods, and heating and tapering the combined rods to obtain the heterogeneous multi-core fiber.

And 4, step 4: according to the using scene, the heterogeneous multi-core fiber can be processed into the spiral heterogeneous multi-core fiber again.

Technical Field

The invention relates to a heterogeneous multi-core optical fiber for preparing a mode division multiplexer and a preparation method thereof, belonging to the technical field of optical fiber design.

Background

With the gradual commercialization of the fifth-generation mobile communication technology and the continuous development of the smart city of the internet of things, the traditional communication system based on the single-mode optical fiber cannot meet the increasing requirements for communication bandwidth and channel capacity. Researchers have developed various communication methods such as wavelength division multiplexing, polarization multiplexing, and space division multiplexing to expand channel capacity. How to achieve channel multiplexing proportionally is a common concern in the industry today.

Light waves can carry angular momentum in addition to momentum. The Angular momentum of the photons is generated by the rotation of the beam after spatial transmission, and the rotation of the polarization vector generates the Spin Angular Momentum (SAM); the wavefront rotation of light produces Orbital Angular Momentum (OAM). The spin angular momentum of light corresponds to the polarization state of light, while the orbital angular momentum of light corresponds to the spatial mode of light. The orbital angular momentum multiplexing does not depend on wavelength or polarization state, which shows that OAM multiplexing can be compatible with a wavelength division multiplexing system and a polarization multiplexing system, and has great application potential.

At present, various researchers in various countries propose various schemes for generating vortex modes carrying orbital angular momentum, which are mainly divided into two types, namely free space generation and generation in optical fibers. In free space, researchers often use analog-to-digital converters, such as spiral phase plates, phase holograms, metamaterials, cylindrical lens pairs, q-slides, and the like, which can only convert gaussian beams into certain order vortex beams, and cannot generate multi-order vortex beams simultaneously using the same device. The optical fiber method uses fiber gratings, spiral fiber gratings, fiber couplers, fiber end surface micromachining and other methods, which either use devices or have a certain degree of wavelength sensitivity or cannot simultaneously generate multi-order vortex beams.

The photon lantern is a waveguide device which is emerging in recent ten years, can realize the function of mode low-loss coupling between a single-mode fiber and a multimode fiber, and is an ideal fiber communication mode division multiplexing device. The photonic lantern connects a single multimode waveguide with a plurality of single mode waveguides, and is generally prepared by constraining a plurality of heterogeneous single mode fibers by a low-refractive-index capillary sleeve to be fused and tapered. The photon lantern is a reciprocal device, and can realize the function of a mode multiplexer for converting a basic mode of an optical fiber into a specific high-order mode and also can realize the function of an optical fiber mode demultiplexer for demodulating and coupling the high-order mode to a corresponding single-mode port. The performance of the device of the photon lantern is insensitive to the optical wavelength, so that the device has a huge application prospect in an optical fiber communication system, the optical fiber mode multiplexing represented by the device can be simultaneously carried out with the traditional time division multiplexing, wavelength division multiplexing, space division multiplexing and polarization multiplexing, the communication bandwidth and the channel number are expanded in a multiple mode, and the device is an important component of a 5G communication technology. How to transform the traditional photon lantern into the vortex optical mode conversion device required by the orbital angular momentum system is an important innovation point of the invention patent.

In order to solve the above problems, a patent with publication number 201810966528.2 proposes an OAM mode multiplexing device based on a photonic lantern, a manufacturing method and a multiplexing method, in which an input single mode is converted into an optical fiber vector mode using a conventional fusion tapering method of combining different single mode optical fibers, and a vortex beam is obtained by winding an output few-mode optical fiber to a mode polarization controller. The invention patent needs to use a mode polarization controller for mechanical adjustment, and the stability is not high enough. Because the positions of different fiber cores of the multi-core optical fiber are different during bending, the fiber cores are affected differently, and the method cannot be applied to a multi-core space division multiplexing vortex optical transmission system.

The 201910359407.6 patent proposes a method of making an orbital angular momentum photon lantern by placing a fiber bundle inside a low refractive index glass sleeve and controlling the taper length to achieve an OAM mode. The invention patent cannot control the high-order vortex beam and the +/-1 order vortex beam to have pi/2 phase difference at the same time, and only the +/-1 order vortex beam and the 0 order vortex beam can be obtained. Because the optical fibers inserted into the sleeve cannot be increased greatly, the invention patent cannot be expanded to multi-core space division multiplexing vortex optical communication.

The patent with publication number 202010207437.8 proposes a photon lantern type degenerate module multiplexer-demultiplexer and a transmission method, the photon lantern designed by the invention patent can only generate optical fiber vector beams and cannot generate vortex beams.

The invention discloses a heterogeneous multi-core optical fiber for preparing a mode division multiplexer and a preparation method thereof. The device can be used for preparing an orbital angular momentum photon lantern capable of realizing mode division multiplexing, has the advantages of low crosstalk and low insertion loss, and can be widely applied to a multi-core few-mode multiplexing optical fiber communication system based on orbital angular momentum

Disclosure of the invention

The invention aims to provide a heterogeneous multi-core optical fiber for preparing a mode division multiplexer and a preparation method thereof.

The purpose of the invention is realized as follows: the heterogeneous multi-core fiber comprises a plurality of single-mode fiber cores, wherein the fiber cores are circularly and symmetrically distributed, and the refractive indexes, the diameters or the refractive index profiles of the fiber cores are different, so that the propagation constants of fundamental modes in the fiber cores are different. The optical fiber is inserted into a capillary with low refractive index and then tapered, and proper parameters such as taper length and the like are selected to convert the Gaussian fundamental mode of the heterogeneous multi-core optical fiber core into a vortex mode, convert the fundamental modes of different fiber cores into vortex modes of different orders, and have one-to-one correspondence relationship.

The heterogeneous multi-core fiber is in a spiral structure or a non-spiral straight structure.

The number of fiber cores of the heterogeneous multi-core optical fiber is N, N is an integer, and N is more than or equal to 3.

The refractive index profile of the fiber core in the heterogeneous multi-core fiber is in a step type, a parabolic type or a Gaussian type. The heterogeneous multi-core optical fiber has no fiber core isolation channel, and the performance of the device can be reduced due to the introduction of the channel.

The cladding structure of the heterogeneous multi-core fiber is a double cladding, and the mode field area and numerical aperture of the inner cladding of the double cladding are matched with those of the output few-mode fiber after the inner cladding of the double cladding is shrunk in the tapered region.

Air holes or small-core-diameter auxiliary fiber cores are designed among the fiber cores of the heterogeneous multi-core fiber, so that the phase difference value of the symmetric supermode and the anti-symmetric supermode in the mode conversion tapered region is controlled.

A heterogeneous multi-core fiber for preparing a mode division multiplexer and a preparation method thereof are characterized in that:

step 1: designing corresponding number of optical fiber preforms with different fiber cores according to the mode multiplexing number of the mode division multiplexer, wherein the cladding refractive indexes of the preforms are required to be equal, and the preforms corresponding to the degenerate mode are completely the same;

step 2: and tapering a plurality of different optical fiber preforms to prepare an intermediate, wherein the radius of the intermediate is determined according to the rod assembling condition.

And step 3: and inserting the intermediate into the fluorine-doped capillary with low refractive index, gradually reducing the guided mode propagation constant of the intermediate from the outside to the inner fiber core, filling a proper number of coreless rods, and heating and tapering the combined rods to obtain the heterogeneous multi-core fiber.

And 4, step 4: according to the using scene, the heterogeneous multi-core fiber can be processed into the spiral heterogeneous multi-core fiber again.

The heterogeneous multi-core optical fiber is inserted into the fluorine-doped capillary with low refractive index, then the end of the cone region is connected with the six-mode optical fiber through optical fiber tapering and adjustment of the length and the shape of the cone region, so that a photon lantern for orbital angular momentum can be obtained, and in order to realize the connection of the heterogeneous multi-core optical fiber and the common single-mode optical fiber, a specially-made fan-in fan-out device is generally needed. The manner in which a heterogeneous multicore fiber is used in an orbital angular momentum mode division multiplexing system is now described with reference to fig. 2-3.

The eigenmodes of the few-mode fiber are vector modes or scalar modes, while the vortex modes in the fiber can be combined from the vector modes or scalar modes, the following formula is an expression between the vortex modes and the fiber order modes,

OAM in the formula represents a high-order vortex beam mode with orbital angular momentum in the optical fiber, the topological charge number and the order of the mode are determined by a first subscript of the expression, a second subscript represents the number of nodes in the radial direction of the mode, and a mode superscript represents the polarization state of the mode. HE, EH, TE and TM at the right end of the formula are vector modes of the optical fiber, even and odd marks on the right end of the formula represent the symmetry of the modes, and subscripts are defined as vortex modes. The imaginary symbol i in the formula represents the phase difference of pi/2 between the modes. This expression illustrates that the vector mode and the vortex mode in the fiber are switchable to each other.

The heterogeneous multicore fiber comprises a plurality of different fiber cores, and the guided modes of the single fiber cores are Gaussian fundamental modes, but in the invention patent, the supermode characteristic of the multicore fiber needs to be considered integrally. At most two fiber cores in the heterogeneous multi-core fiber have the same structural parameters, fundamental modes of the two fiber cores are mutually coupled to form a supermode with energy distributed in the two fiber cores, the energy distribution is still in a Gaussian shape of the fundamental modes, wave front phases in the two identical fiber cores are different, if the fundamental modes in the two fiber cores have the same phase, the supermode is called a symmetric supermode, and the supermode with the phase difference of pi is called an anti-symmetric supermode. The symmetric supermode and the antisymmetric supermode are in a nearly degenerate state, and the effective refractive indexes of the supermode and the antisymmetric supermode are very close to each other. If the symmetric supermode and the anti-symmetric supermode with equal power exist at the same time, the phase between the symmetric supermode and the anti-symmetric supermode determines the optical field distribution in the multi-core optical fiber, if the phase between the two supermodes is the same or the phase difference is pi, the Gaussian fundamental mode of a certain core is respectively excited, the Gaussian fundamental mode can be led out to a single-mode optical fiber by the fan-in fan-out device, and if the phase difference between the two supermodes is not 0 or pi, the two optical cores in the multi-core optical fiber can both generate Gaussian fundamental modes with different power, and cannot be output to the same single-mode optical fiber through the fan-in fan-out device.

After the heterogeneous multi-core optical fiber is inserted into the low-refractive-index sleeve and tapered, the guided supermode can evolve into a vector or scalar mode of the few-mode optical fiber end gradually. In this process, the tapered structure must satisfy adiabatic transformation, i.e., in an optical waveguide whose shape parameters and refractive index profile change slowly, a certain mode at the input end can be converted to a certain same-order mode at the output end without loss. The whole tapering region satisfies adiabatic coupling conditions as shown below

The subscripts j and l in the formula represent a guided fundamental mode and other modes respectively, β is a transmission constant of a local mode, Ψ is a normalized electromagnetic field distribution of the local mode, k is 2 π/λ is a wave number of an electromagnetic wave, z is an axial coordinate of a tapered structure, ρ is a shrinkage ratio of a cladding, n is a refractive index distribution function of a tapered region, and A is a cross section of the tapered structure. The formula defines a judgment condition related to the length of the tapering and the shape expression rho (z), and the judgment condition can measure the theoretical performance of the tapering area. When the tapered region meets the adiabatic coupling condition, the Gaussian fundamental mode of a certain fiber core in the heterogeneous multi-core fiber can be divided into a symmetric supermode and an anti-symmetric supermode which are respectively converted into two degenerate scalar modes of the fiber, and the specific corresponding relation is related to the structural design of the fiber.

Besides adiabatic conversion conditions, the phase relation between modes in the tapered region also determines the conversion efficiency from a certain core guided mode to vortex light beams in the heterogeneous multi-core optical fiber. And inserting the heterogeneous multi-core fiber into the low-refractive-index sleeve, and determining the conversion process from the input mode to the output mode in the whole tapering process by a local coupling mode equation. In this process, the vortex beam at the output end can be decomposed into even components of vector mode and odd mode with pi/2 phase difference. The evolution processes and results of the two modes in the cone are slightly different, generally speaking, the even mode of the optical fiber can be obtained by the symmetric supermode evolution of the heterogeneous multi-core optical fiber end, and the odd mode of the optical fiber can be obtained by the antisymmetric supermode evolution of the heterogeneous multi-core optical fiber. If non-0 or non-pi phase difference exists between the two evolved optical fiber vector modes, a non-single state vortex mode can be simultaneously excited, and the one-to-one correspondence relationship between the Gaussian mode and the vortex mode in a single input fiber core cannot be formed.

The heterogeneous multi-core fiber in the spiral form can also control the phase difference value between the supermodes, but the phase difference is only relevant to the mode conversion tapering area and is not relevant to the non-tapering area.

In the whole mode conversion tapering region, a single-core Gaussian fundamental mode at the input end of a heterogeneous multi-core optical fiber can be decomposed into an equipower superposition state of a symmetric supermode and an anti-symmetric supermode, the equipower superposition state and the anti-symmetric supermode can be respectively evolved into an even mode of the optical fiber and an odd mode with a pi/2 phase difference value, and the even mode and the odd mode are combined and assembled into a vortex mode with orbital angular momentum.

The mode phase transformation brought by mode evolution in the mode conversion tapered region is mainly divided into two parts, one part is called dynamic phase and is determined by the propagation constant of each section eigenmode in the tapered region along the tapered length and the integral result of a shape expression rho (z). The other part of the phase can be called geometric phase, which is determined by the energy distribution evolution process of each mode in the tapering region, and is independent of the tapering length of the tapering region. By calculating the geometric phase and the dynamic phase, the optimal length and shape of the mode conversion tapered region can be obtained, so that the symmetric supermode and the anti-symmetric supermode can obtain the pi/2 accumulated phase difference in the whole tapered region evolution process. Therefore, at the tail end of the mode conversion tapered region, the phase difference between the odd mode and the even mode evolved from the supermode is pi/2, vortex modes with corresponding orders can be formed, and the whole device obtains the one-to-one corresponding conversion relation between a certain fiber core Gaussian fundamental mode and an output vortex mode of the heterogeneous multi-core fiber.

According to the relevant theory, the mode accumulated phase shifts obtained in the mode conversion tapering region in the supermode evolution process corresponding to +/-1 order vortex and +/-2 order vortex are different, and in order to enable the +/-1 order vortex and the +/-2 order vortex to reach the same pi/2 phase difference at the same time, a targeted optical fiber design is required, the same pi/2 phase difference cannot be obtained at the same time by simply controlling the taper length or changing the taper of the tapering region, and the characteristic is a decisive factor for restricting the vortex photon lantern from expanding to a higher order. According to the graph of the change of the propagation constant of each eigenmode in the mode conversion tapering region of the attached drawing along with the shrinkage rate, the propagation constant curves of the symmetric supermode and the anti-symmetric supermode of the + -1 order eddy optical rotation obtained by evolution are respectively a second curve and a third curve from top to bottom, and a certain amount of propagation constant difference exists between the two curves in the middle section of the tapering region, and the term is the kinetic phase difference value of the two curves. The propagation constant curves of the symmetric supermode and the anti-symmetric supermode which represent the + -2 order vortex rotation are respectively the fourth curve and the fifth curve from top to bottom, and the kinetic phase difference value of the two curves is different from that of the + -1 order vortex rotation. This indicates that the dynamic phase obtained by the + -2 order vortex light and the + -1 order vortex light in this process are not the same, and simply increasing the taper length of the fiber or changing the taper of the taper region does not make the two the same. In contrast, the present invention designs several structures of heterogeneous multicore fibers to assist in achieving equal accumulated phase difference of different order modes in the taper region, for example, methods of adding air holes in the same fiber core of the heterogeneous multicore fiber, independently controlling the core pitch, adding a fiber core with a small core diameter between the core and the core, and controlling the distance between the central fiber core and the edge fiber core. By the method, the phase difference obtained by vortex rotation with different orders in the cone region can be accurately adjusted, the vortex photon lantern with the vortex order larger than 2 is supported by the method to be possible to realize, and the common photon lantern cannot ensure that a plurality of vortex modes can be excited simultaneously, namely the one-to-one corresponding relation between the input Gaussian mode and the output vortex state cannot be established.

A typical six-mode vortex field mode division multiplexer made using heterogeneous multicore fibers can convert each of the gaussian fundamental modes in the six cores to a corresponding vortex mode. The optical fiber comprises a heterogeneous multi-core fiber, wherein a 0-order vortex light beam is converted from a fiber core fundamental mode with the largest fundamental mode propagation constant in the heterogeneous multi-core fiber, a +/-1-order vortex light beam is converted from two same fiber core fundamental modes with the second largest fundamental mode propagation constant in the heterogeneous multi-core fiber, a +/-2-order vortex light beam is converted from two same fiber core fundamental modes with the third largest fundamental mode propagation constant in the heterogeneous multi-core fiber, and a radial 1-order light beam of the 0-order vortex light beam is converted from a fiber core fundamental mode with the smallest fundamental mode propagation constant in the heterogeneous multi-core fiber.

The invention discloses a heterogeneous multi-core optical fiber for preparing a mode division multiplexer and a preparation method thereof. The device can be used for preparing the orbital angular momentum photon lantern for realizing the mode division multiplexing, has the advantages of low crosstalk and low insertion loss, and can be widely applied to a multi-core few-mode multiplexing optical fiber communication system based on the orbital angular momentum.

Drawings

Fig. 1 is a structural and end view of a heterogeneous multi-core fiber for fabricating a mode division multiplexer, which includes both a straight fiber and a spiral fiber.

Fig. 2 is an orbital angular momentum photon lantern prepared based on heterogeneous multi-core fibers. The figure comprises heterogeneous multi-core fibers (1), low-refractive-index sleeves (2), a mode conversion tapered region (3), few-mode fibers (4) and end faces of devices: the refractive index profile (5) of the heterogeneous multi-core fiber, the refractive index profile (6) of the tail end of the mode conversion tapered region and the refractive index profile (7) of the output few-mode fiber.

FIG. 3 is a completed orbital angular momentum photon lantern based on the preparation of a spiral heterogeneous multi-core fiber.

Fig. 4 is a schematic end view of a plurality of heterogeneous multi-core fibers, (a) heterogeneous six-core fibers, (b) heterogeneous three-core fibers, (c) double-clad heterogeneous six-core fibers, (d) double-clad heterogeneous three-core fibers, (e) double-clad heterogeneous five-core fibers, (f) double-clad heterogeneous ten-core fibers.

FIG. 5 is a schematic diagram of the transition of the superposition of the hetero-multicore fiber supermode and the Gaussian fundamental mode of a single core. As can be seen from the figure, the single Gaussian fundamental mode in two identical cores in the heterogeneous multicore fiber can be composed of a symmetric supermode (the two cores have the same phase) and an anti-symmetric supermode (the two cores have opposite phases). As shown in the figure, when two supermodes are directly aliased, the supermode is equivalent to a Gaussian fundamental mode of a certain fiber core; when the antisymmetric supermode is subjected to 180-degree phase shift, the aliasing of the two supermodes is equivalent to a Gaussian fundamental mode in the other same fiber core.

FIG. 6 is a graph of the propagation constants of the eigenmodes of the super-modes at each cross-section in the mode-switched tapered region. The curves are from top to bottom respectively a mode corresponding to 0 order vortex light, a symmetric supermode related to +/-1 order vortex, an anti-symmetric supermode related to +/-1 order vortex, a symmetric supermode related to +/-2 order vortex, an anti-symmetric supermode related to +/-2 order vortex and a mode corresponding to radial 1 order light beam of the 0 order vortex light beam.

FIG. 7 is a diagram of the evolution process of guided fundamental modes in each core of a heterogeneous multi-core fiber in a mode conversion tapered region. The Gaussian guided mode of each fiber core in the heterogeneous multi-core fiber at the left end of the tapering region is gradually evolved into each order vortex mode at the right end, and the process is reciprocal. The right end of the figure shows the mode field distribution and the phase distribution of each order vortex beam.

FIG. 8 is a graph comparing the output mode of the orbital angular momentum photon lantern prepared based on heterogeneous multi-core fiber with the standard fiber vortex mode. The vertical pictures are standard vortex modes in the few-mode optical fiber, the horizontal pictures are patterns output by vortex photon lanterns after single-mode optical fiber injection, and data in the patterns are integration results between two groups of modes. The data on the diagonal line of the graph represents the loss of the vortex mode in the mode conversion tapering region, and the data on the off-diagonal line represents the signal crosstalk of the vortex mode in the mode conversion tapering region. The purity of the output vortex mode was greater than 95%. The data unit in the figure is dB.

FIG. 9 is a schematic diagram of a heterogeneous six-core optical fiber intermediate group rod, wherein the gray scale of the image represents the refractive index of the material, and the deeper the color is, the higher the refractive index is.

FIG. 10 is a schematic diagram of the positions of the inter-core air holes and the auxiliary core in the hetero-six-core optical fiber. Design parameters that can be used for the adjustment are: the center-to-edge core distance P, the edge-to-edge core spacing D, and the edge-to-edge core angular distance θ, also demonstrate the positions of the inter-core voids and the auxiliary fiber core.

Detailed Description

The invention is further illustrated below with reference to specific examples.

Example 1: a design rule for preparing heterogeneous multi-core optical fibers of a mode division multiplexer and a preparation method of an orbital angular momentum photon lantern are disclosed.

The rear end matching output few-mode optical fiber uses a six-mode optical fiber with the core diameter of 18.5um and the numerical aperture of 0.12. It can accommodate a variety of vortex beams with orbital angular momenta of + -2, + -1, 0. The fiber core quantity of heterogeneous multicore optic fibre is 6, and the core footpath of each fiber core is 11um, 9um, 9um, 8um, 8um, 6.5um, and typical core interval is 40 um. The cladding index is 1.444 and the core cladding numerical aperture is 0.12. The low index sleeve has a refractive index of 1.4398, and the inner diameter of the sleeve is equal to the outer diameter of the heterogeneous multicore fiber and is 125 um.

And inserting the heterogeneous multi-core optical fiber into the low-refractive-index sleeve to perform adiabatic tapering, so as to obtain the mode conversion tapered region. The shape and length of the tapered cone can be determined by simulation, and the specific simulation method is as follows, numerical simulation software is used for respectively calculating phase values of output ports under the lengths of 4cm, 6cm and 8cm of the tapered cone area, linear fitting is carried out on the phase values, the slope term of a fitting curve determines the dynamic phase, and the constant term determines the geometric phase of the structure. The length or the shape of the cone area is adjusted, aiming at vortex light beams with different orders, air holes between cores are designed in a targeted mode, the core spacing is adjusted, the small fiber core is inserted, and the distance between the fiber core and the center is adjusted, so that the vortex light beams with orbital angular momentum of +/-2 and +/-1 and 0 can have phase shift of (N +0.5) pi, and a one-to-one corresponding relation is established between Gaussian light beams in a single fiber core and input vortex states. A typical cone length is 4.2cm, and the cone is a linear cone.

And (3) connecting the heterogeneous multi-core optical fibers with the fan-in fan-out device matched with the heterogeneous multi-core optical fibers, so that the energy of each single-mode optical fiber can be input into each fiber core of the heterogeneous multi-core optical fibers without crosstalk. At the moment, a conversion model between each input optical fiber fundamental mode and a vortex mode loaded with different orbital angular momentum is established, and each fiber core excites different vortex light beams respectively.