Parallel mode demultiplexer of double-clad ring core fiber and preparation method

Abstract

The invention relates to a parallel mode demultiplexer of a double-clad ring core optical fiber, which comprises a few-mode optical fiber and a plurality of single-mode optical fibers, the few-mode optical fiber is the double-clad ring core optical fiber few-mode optical fiber supporting N modes, and the few-mode optical fiber is arranged in the middle of the plurality of single-mode optical fibers, the single-mode optical fibers are arranged on the periphery of the few-mode optical fiber in the angular direction of the few-mode optical fiber, the single-mode optical fibers are attached to the outer wall of the few-mode optical fiber, and the initial core spacing between each single-mode optical fiber and the few-mode optical fiber is the sum of the cladding radius of the single-mode optical fiber and the cladding radius of the few-mode optical fiber. And the intervals between the adjacent single-mode optical fibers are the same. According to the parallel mode demultiplexer of the double-cladding ring core optical fiber, the ring core structure enables the mode refractive index difference in the optical fiber to be larger than 10 <-3 >, the inter-mode crosstalk loss is reduced, meanwhile, when a fundamental mode and a high-order mode are mutually converted through the double-cladding structure, the excellent heat insulation characteristic mode insertion loss is kept to be lower than 0.70 dB, and the conversion efficiency is larger than 85.0%.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber communication, and in particular to a parallel mode demultiplexer of a double-clad ring-core optical fiber and a preparation method thereof. Background Art

[0002] With the development of Internet technology, emerging technologies such as the Internet of Things, artificial intelligence, and big data that require massive amounts of computing data have emerged. At the same time, the sharp increase in people's demand for communication capacity has also promoted the replacement and development of communication technology. Since 1980, single-mode fiber communication technologies such as time division multiplexing (TDM), wavelength division multiplexing (WDM), and polarization division multiplexing (PDM) have emerged, driving the capacity of the transmission network to grow at a rate of about 45% per year. However, with the development of single-mode fiber communication to this day, the fiber communication system based on single-mode fiber has approached the Shannon capacity limit, that is, the capacity limit of 100Tb / s. In order to meet this challenge, scholars have made a lot of efforts and research. Among them, mode division multiplexing (MDM) technology based on few-mode fiber (FMF) or multimode fiber (MMF) is the main implementation form of the next generation of fiber communication systems. Mode division multiplexing technology refers to a technology that allows multiple orthogonal modes with different paths and mode field distributions and carrying different information to propagate together in the same multimode optical waveguide. If an optical fiber has N modes, then there are N independent and non-interfering transmission information channels, and the system capacity increases N times accordingly. Mode excitation, mode conversion, mode multiplexing / demultiplexing, etc. can be realized in the mode division multiplexing communication transmission system.

[0003] In the mode division multiplexing system, the high-order modes existing in the multimode optical fiber are fully utilized. However, how to excite and utilize the high-order modes is a key issue. It is converted from the fundamental mode through a mode converter. Therefore, in various mode multiplexing technologies, the mode converter is a key device for realizing mode conversion, specific mode excitation and specific mode shielding. According to the working principle and method, it is mainly divided into spatial matching type, silicon-based waveguide type and all-fiber type. Among them, the all-fiber mode converter is a mode conversion device made of few-mode fiber as a waveguide and supplemented by other devices or technologies. Light propagates only in the optical fiber, there is no free space optical element, the device has a small size, a simple structure and is easy to connect with optical fiber lines and other optical devices, with higher stability and feasibility. The all-fiber mode converter mainly uses the phase matching principle to realize the mutual conversion between the fundamental mode and the high-order mode. The representative structures include long-period fiber grating (LPFG), photon lantern (PL), mode selective coupler (MSC), etc. The working bandwidth of the long-period fiber grating is narrow, and the insertion loss will increase with the increase of the mode order. Mode selective couplers and photon lanterns have wide working bandwidth and low loss, but the preparation of photon lanterns is relatively complicated, and mode selective couplers face the problem of high mode crosstalk due to non-adiabatic transmission of high-order modes and small difference in effective refractive index of modes. Scholars from various countries have conducted a lot of research on all-fiber mode converters.

[0004] U.S. Patent US20240159966A1 proposes a mode selective coupler for double-clad optical fiber. The coupler is composed of two double-clad few-mode optical fibers with similar parameters. In order to output a high-purity mode, more precise numerical calculations are required in advance. However, when the optical fiber is actually applied, the higher the mode order, the smaller the effective refractive index difference of the mode in the core, and greater mode crosstalk will occur in actual application.

[0005] Chinese patent CN108051890A proposes a high-efficiency, low-loss all-fiber fused mode selective coupler. The multiplexer uses a specially made single-mode optical fiber, which enables the fundamental mode and the target mode in the few-mode optical fiber to meet phase matching at the same wavelength without pre-stretching, thereby achieving mode conversion. However, when converting high-order modes, due to its non-adiabatic transmission, non-target mode conversion will occur, resulting in greater mode crosstalk.

[0006] Chinese patent CN116699761A proposes a mode multiplexer / demultiplexer based on D-type optical fiber. The optical fiber has its side cladding removed, and the core structure is kept unchanged. Its cross section is similar to that of a D-type optical fiber. Two D-type optical fibers are bonded to their side planes to form a mode selection coupler for mode conversion, and the multiplexer / demultiplexer is a series connection of several D-type optical fiber couplers. Ideally, the optical fiber prepared by this side polishing method can reduce the core spacing between the two optical fibers and reduce the insertion loss. However, it is not easy to achieve in practical applications. The loss caused by mechanical wear is large, and the cascaded multiplexer / demultiplexer device is large in size, which is not convenient for practical use.

[0007] Chinese patent CN112965170A proposes a method for preparing an optical fiber mode selective coupler using a glass sleeve, wherein the glass sleeve needs to be specially prepared to match the size of the mode selective coupler, and the experimental environment requirements during the preparation process are strict, the operation is complicated, and the production cost is high, which is not suitable for commercial mass production.

[0008] In summary, the multiplexers / demultiplexers based on mode selective couplers are mostly cascaded, that is, two optical fibers are bonded together to form a mode selective coupler, and then N mode selective couplers are connected in series end to end to form N mode multiplexers / demultiplexers. However, considering the influencing factors in real applications, the multiplexers / demultiplexers made based on this method are not only large in size and inconvenient to move, but also have low mode conversion efficiency. Summary of the invention

[0009] To this end, the present invention provides a parallel mode demultiplexer for a double-clad ring-core optical fiber and a preparation method thereof, which can realize simultaneous multiplexing / demultiplexing of N modes and has the advantages of high mode conversion efficiency, low loss, shorter coupling length, and small size.

[0010] In order to solve the above technical problems, the present invention provides a parallel mode demultiplexer of a double-clad ring-core fiber, comprising: a few-mode fiber and a plurality of single-mode fibers, wherein the few-mode fiber is a double-clad ring-core fiber that supports N modes, and the few-mode fiber is arranged in the middle of the plurality of single-mode fibers, and the plurality of single-mode fibers are arranged around the few-mode fiber along the angular direction of the few-mode fiber, and the plurality of single-mode fibers are attached to the outer wall of the few-mode fiber, and the initial core spacing of each single-mode fiber and the few-mode fiber is the sum of the cladding radius of the single-mode fiber and the outer cladding radius of the few-mode fiber, and the spacing between adjacent single-mode fibers is the same. The parallel mode demultiplexer of the double-clad ring-core fiber of the present invention has a ring core structure that makes the mode refractive index difference in the optical fiber greater than 10 -3, reducing the inter-mode crosstalk loss. At the same time, the double-cladding structure can suppress the diffusion of the high-order mode field when the fundamental mode and the high-order mode are converted to each other, maintain excellent adiabatic properties, and improve the mode conversion efficiency. As a mode demultiplexer, the insertion loss is less than 0.70dB, and the mode conversion efficiency is greater than 85.0%, which can be widely used in various space-division multiplexing communication transmission systems.

[0011] In one embodiment of the present invention, the few-mode optical fiber includes a central core, an inner annular core, an outer annular core, an inner cladding and an outer cladding, which are arranged in sequence from the inside to the outside; wherein, in the 1550.0nm band, the relative refractive index difference between the refractive index n1 of the central core and the refractive index n5 of the outer cladding is 0.00280-0.00960, the relative refractive index difference between the refractive index n2 of the inner annular core and the refractive index n5 of the outer cladding is 0.00550-0.01430, the relative refractive index difference between the refractive index n3 of the outer annular core and the refractive index n5 of the outer cladding is 0.00410-0.01090, and the relative refractive index difference between the refractive index n4 of the inner cladding and the refractive index n5 of the outer cladding is 0.00070-0.00280.

[0012] In one embodiment of the present invention, at 1550.0 nm, the refractive index of the inner annular core>the refractive index of the outer annular core>the refractive index of the central core, and the refractive index of the inner cladding>the refractive index of the outer cladding.

[0013] In one embodiment of the present invention, the central core diameter d1 is 2.0μm to 10.0μm, the inner ring diameter of the inner annular core is consistent with the central core diameter, the outer ring diameter d2 of the inner annular core is 4.0μm to 18.0μm, the inner ring diameter of the outer annular core is consistent with the outer ring diameter of the inner annular core, the outer ring diameter d3 of the outer annular core is 6.0μm to 30.0μm, the inner cladding diameter d4 is 30.0μm to 80.0μm, and the outer cladding diameter d5 is 80.0μm to 125.0μm.

[0014] In one embodiment of the present invention, at a wavelength of 1550 nm, the single-mode optical fiber includes a core, a groove and a cladding from the inside to the outside, and the refractive index of the core is n Na The refractive index n of the cladding c The relative refractive index difference is 0.00380 to 0.01100, and the refractive index n of the groove is b The refractive index n of the cladding c The relative refractive index difference is -0.00070 to -0.00630.

[0015] In one embodiment of the present invention, at 1550.0 nm, the refractive index of the core>the refractive index of the cladding>the refractive index of the trench.

[0016] In one embodiment of the present invention, the diameter a of the core is 4.2 μm to 12.0 μm, the diameter c of the cladding is 80.0 μm to 125.0 μm, and the outer ring diameter b of the groove is 18.0 μm to 40.0 μm.

[0017] The present invention provides a method for preparing a parallel mode demultiplexer of a double-clad ring-core optical fiber, comprising the following steps:

[0018] Step 1: The few-mode fiber is placed in the middle position, and several single-mode fibers are arranged in sequence at equal intervals in the angular direction of the few-mode fiber. One few-mode fiber and several single-mode fibers are placed parallel and attached, wherein the initial core spacing between each single-mode fiber and the few-mode fiber is the sum of the cladding radius of the single-mode fiber and the outer cladding radius of the few-mode fiber, and the spacing between every two adjacent single-mode fibers is the same;

[0019] Step 2: Place a fiber bundle formed by several single-mode optical fibers surrounding a few-mode optical fiber in parallel on a taper platform, heat and melt the few-mode optical fiber and the fiber bundle formed by several single-mode optical fibers at the same axial position, and fix the two ends of the fiber bundle with a clamp to stretch them outward by the same length. The fiber bundle after taper presents a tapered structure. The parts with unchanged diameters at both ends of the fiber bundle are the initial fiber size, and the tapered areas at both ends of the fiber bundle are the stretching areas. The waist of the fiber bundle with the smallest and unchanged diameter in the middle is the heating area, also called the coupling area. According to the relationship curve between the effective refractive index of the mode in the few-mode optical fiber core and the cladding diameter, determine the phase matching point of each mode in the few-mode optical fiber core and the corresponding fundamental mode in the several single-mode optical fiber cores;

[0020] Step 3: In order to make the stretched length of the few-mode fiber consistent with that of the corresponding single-mode fibers, all phase matching points are selected to be at the same taper diameter;

[0021] Step 4: The few-mode fiber and several single-mode fibers are simultaneously stretched by 5.0 mm to 10.0 mm to meet the taper size at the selected phase matching point. At this time, the fundamental mode and high-order mode in the few-mode fiber and the fundamental mode in the corresponding several single-mode fibers undergo mode conversion in the waist region of the tapered fiber bundle.

[0022] In one embodiment of the present invention, when used as a demultiplexer, one end of a fiber bundle formed by a few-mode fiber and several single-mode fibers is used as an input end, and the few-mode fiber is connected to the multiplexed signal transmitted by the transmission fiber. The multiplexed signal undergoes mode conversion to the fundamental mode in the corresponding single-mode fiber core in the coupling region of the tapered fiber bundle. Finally, at the other end of the demultiplexer, several single-mode fibers output the multiplexed N signals respectively, which is the demultiplexing process.

[0023] In one embodiment of the present invention, when used as a multiplexer, several single-mode optical fibers are used as input ends and are respectively connected to excitation. The N input signals undergo mode conversion to the modes in the corresponding double-clad ring-core few-mode optical fiber cores in the coupling region of the tapered optical fiber bundle. Finally, at the other end of the multiplexer, the N signals are simultaneously output from the double-clad ring-core few-mode optical fiber and enter the transmission optical fiber, which is the multiplexing process.

[0024] The above technical solution of the present invention has the following beneficial effects compared with the prior art:

[0025] 1. The structure of the parallel mode multiplexer / demultiplexer in the present invention includes a few-mode optical fiber and a plurality of single-mode optical fibers. Under the premise of using the same number of optical fibers, compared with the (de)multiplexer composed of cascaded N mode selective couplers, the volume size of the present invention is reduced by N times, and after stretching the same length, N modes can be multiplexed / demultiplexed at the same time, thereby improving the transmission efficiency;

[0026] 2. The double-clad ring-core few-mode optical fiber of the present invention has a ring-core structure that can increase the effective refractive index difference between the optical fiber modes, thereby reducing the optical fiber mode crosstalk. In addition, when performing mode conversion from high-order mode to fundamental mode, the inner cladding acts as a buffer layer to act as an insulator, thereby suppressing the mode field diffusion of the high-order mode and reducing the mode crosstalk.

[0027] 3. Compared with the standard single-mode optical fiber, the single-mode optical fiber of the present invention adds a negative refractive index groove between the cladding and the core, which can achieve better matching with the double-clad ring-core optical fiber;

[0028] 4. When the traditional few-mode fiber or ring-core fiber is used to prepare the mode (de)multiplexer, the stretching length is 5.0mm to 40.0mm. The mode (de)multiplexer composed of double-clad ring-core few-mode fiber and adapted single-mode fiber with negative groove has a stretching length of 5.0mm to 10.0mm, which is four times shorter, reducing the preparation process time;

[0029] 5. The mode conversion efficiency of the mode multiplexer / demultiplexer designed in the present invention is greater than 85.0% at a wavelength of 1550.0 nm.

[0030] 6. The present invention can be widely applied to various types of analog-division multiplexing or space-division multiplexing transmission systems, and can multiplex or demultiplex multiple signals at the same time, thereby reducing the complexity of system configuration and improving signal processing efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific embodiments of the present invention in conjunction with the accompanying drawings, wherein

[0032] Figure 1 It is a side view of the parallel mode demultiplexer of the double-clad ring-core optical fiber of the present invention;

[0033] Figure 2 It is a cross-sectional schematic diagram of a parallel mode demultiplexer of a double-clad ring-core optical fiber of the present invention;

[0034] Figure 3 is a cross-sectional view of the few-mode optical fiber of the present invention;

[0035] Figure 4 is the refractive index profile of the few-mode optical fiber of the present invention;

[0036] Figure 5 is a cross section of a single-mode optical fiber of the present invention;

[0037] Figure 6 is the refractive index distribution diagram of the single-mode optical fiber of the present invention;

[0038] Figure 7 is a curve showing the relationship between the mode effective refractive index and the cladding diameter of the few-mode optical fiber of the present invention;

[0039] Figure 8 It is a schematic diagram of a parallel mode demultiplexer of a double-clad ring-core optical fiber of the present invention;

[0040] Fig. 9 It is a schematic diagram of a parallel mode multiplexer of a double-clad ring-core optical fiber of the present invention.

[0041] Description of the accompanying drawings in the specification: few-mode optical fiber 1, 11-central core, 12-inner ring core, 13-outer ring core, 14-inner cladding, 15-outer cladding, single-mode optical fiber 1 2, single-mode optical fiber 2 3, single-mode optical fiber 3 4, single-mode optical fiber 4 5, single-mode optical fiber 5 6, single-mode optical fiber 6 7, d1-central core diameter, n1-central core refractive index, d2-inner ring core diameter, n2-inner ring core refractive index, d3-outer ring core diameter, n3-outer ring core refractive index, d4-inner cladding diameter, n4-inner cladding refractive index, d5-outer cladding diameter, n5-outer cladding refractive index, N1-core, N2-groove, N3-cladding, a-core diameter, n Na - core refractive index of single-mode fiber N, b - outer diameter of trench, n b - Trench refractive index, c-cladding diameter, n c - Cladding refractive index. DETAILED DESCRIPTION

[0042] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0043] like Figure 1 As shown, the parallel mode demultiplexer of the double-clad ring-core optical fiber of the present invention comprises a few-mode optical fiber 1 with six linear polarization modes, a single-mode optical fiber 1 2, a single-mode optical fiber 2 3, a single-mode optical fiber 3 4, a single-mode optical fiber 4 5, a single-mode optical fiber 5 6 and a single-mode optical fiber 6 7; wherein the single-mode optical fiber 1 2, the single-mode optical fiber 2 3, the single-mode optical fiber 3 4, the single-mode optical fiber 4 5, the single-mode optical fiber 5 6 and the single-mode optical fiber 6 7 have the same structure, and the outer walls of the single-mode optical fiber 1 2, the single-mode optical fiber 2 3, the single-mode optical fiber 3 4, the single-mode optical fiber 4 5, the single-mode optical fiber 5 6 and the single-mode optical fiber 6 7 are arranged in sequence around the few-mode optical fiber 1 at equal intervals in the angular direction; the few-mode optical fiber 1 supporting 6 modes is specifically a double-clad ring-core structure, such as Figure 2 As shown, from inside to outside, it includes a central core 11, an inner ring core 12, an outer ring core 13, an inner cladding 14 and an outer cladding 15. At a wavelength of 1550.0 nm, in order to satisfy the requirement that there are 6 modes in the few-mode fiber 1, the normalized frequency V value ranges from 5.5 to 6.0.

[0044] like Figure 3 As shown, in order to make the mode effective refractive index difference > 10 -3 The relative refractive index difference between the refractive index n1 of the central core 11 and the refractive index n5 of the outer cladding 15 is 0.00280-0.00960, the relative refractive index difference between the refractive index n2 of the inner annular core 12 and the refractive index n5 of the outer cladding 15 is 0.00550-0.01430, the relative refractive index difference between the refractive index n3 of the outer annular core 13 and the refractive index n5 of the outer cladding 15 is 0.00410-0.01090, and the relative refractive index difference between the refractive index n4 of the inner cladding 14 and the refractive index n5 of the outer cladding 15 is 0.00070-0.00280.

[0045] The central core 11, the inner annular core 12 and the outer annular core 13 constitute the core region, the inner cladding 14 and the outer cladding 15 constitute the cladding, and the refractive index of the core region is generally greater than the refractive index of the cladding, wherein the refractive index of the inner annular core > the refractive index of the outer annular core > the refractive index of the central core, and the refractive index of the inner cladding > the refractive index of the outer cladding.

[0046] The central core 11, the inner annular core 12, the outer annular core 13, and the inner cladding 14 are made of germanium-doped or erbium-doped-silicon dioxide materials with different concentrations. Different doping concentrations will result in different refractive indices. The outer cladding 15 is made of pure silicon dioxide material.

[0047] The diameter d1 of the central fiber core 11 is 2.0μm to 10.0μm, the inner ring diameter of the inner annular fiber core 12 is consistent with the central fiber core diameter, the outer ring diameter d2 of the inner annular fiber core 12 is 4.0μm to 18.0μm, the inner ring diameter of the outer annular fiber core 13 is consistent with the outer ring diameter of the inner annular fiber core, the outer ring diameter d3 of the outer annular fiber core 13 is 6.0μm to 30.0μm, the diameter d4 of the inner cladding 14 is 30.0μm to 80.0μm, and the diameter d5 of the outer cladding 15 is 80.0μm to 125.0μm.

[0048] In the past, the common preparation of mode selective couplers required the pre-tapering of few-mode fiber or single-mode fiber, and then the secondary tapering to achieve phase matching. Multiple tapers will increase the additional loss of the fiber. In order to reduce this loss and simplify the preparation process of the mode selective coupler, the size of the single-mode fiber mentioned in the present invention is based on the relationship curve of the effective refractive index of the double-clad ring-core few-mode fiber mode versus the cladding diameter proposed in the present invention (such as Figure 5 ) determines the size and refractive index of the fiber core.

[0049] The mode effective refractive index when the outer cladding diameter of the few-mode fiber 1 is 35.0μm is selected as the phase matching point, the single-mode fiber 1 2 corresponds to the parameters of the few-mode fiber 1 in the LP01 mode, the single-mode fiber 2 3 corresponds to the parameters of the few-mode fiber 1 in the LP11 mode, the single-mode fiber 3 4 corresponds to the parameters of the few-mode fiber 1 in the LP21 mode, the single-mode fiber 4 5 corresponds to the parameters of the few-mode fiber 1 in the LP02 mode, the single-mode fiber 5 6 corresponds to the parameters of the few-mode fiber 1 in the LP31 mode, and the single-mode fiber 6 7 corresponds to the parameters of the few-mode fiber 1 in the LP31 mode. When tapering, the few-mode fiber and the single-mode fiber become thinner in proportion, so the original physical size of each single-mode fiber can be calculated proportionally according to the diameter after tapering.

[0050] like Figure 5 As shown in the figure, the single-mode optical fiber is characterized by including a core, a groove and a cladding from the inside to the outside. The physical dimensions of the six single-mode optical fibers are the same, as shown in the figure. Figure 6 As shown, the diameter a of the core N1 is 4.2μm to 12.0μm, the outer ring diameter b of the groove N2 is 18.0μm to 40.0μm, and the diameter c of the cladding N3 is 80.0μm to 125.0μm, wherein N represents single-mode optical fiber N, and N is 2, 3, 4, 5, 6, 7.

[0051] At a wavelength of 1550.0 nm, the refractive index n of the core Na The refractive index n of the cladding c The relative refractive index difference is 0.00380 to 0.01100, and the refractive index n of the groove is b The refractive index n of the cladding cThe relative refractive index difference is -0.00070 to -0.00630. The core refractive index> cladding refractive index is greater than the trench refractive index.

[0052] The core refractive indexes of the six single-mode optical fibers are different. The six cores use germanium-doped or erbium-doped-silica materials with different concentrations to match the effective refractive index of the corresponding mode in the few-mode optical fiber core. The grooves use fluorine-doped or boron-doped-silica materials, and the cladding uses pure silica materials.

[0053] Whether it is a mode multiplexer or a demultiplexer, the key working principle is phase matching. The propagation constant β and the mode effective refractive index n eff It can be calculated by formula (1):

[0054]

[0055] In order to achieve mode selective excitation, the optical fiber needs to be stretched to a certain length so that the optical fiber reaches a certain diameter. The stretching length is generally an integer multiple of the coupling length. c The calculation of is shown in formula (2):

[0056]

[0057]

[0058] L = n × L c , n=1,2,3,…(4);

[0059] where κ is the coupling coefficient, δ is the difference between the propagation constants of the two modes in the waveguide, and L is the stretch length.

[0060] The steps for preparing a parallel six-mode multiplexer / demultiplexer of a double-clad ring-core optical fiber are as follows:

[0061] Step 1: Place the double-clad ring-core fiber supporting 6 modes in the middle, and arrange 6 single-mode fibers in the angular direction of the few-mode fibers at equal intervals, and place 6+1 fibers in parallel. The initial core spacing of each single-mode fiber and the few-mode fiber is the sum of the cladding radius of the single-mode fiber and the outer cladding radius of the few-mode fiber, and the spacing between every two adjacent single-mode fibers is the same;

[0062] Step 2: Place the fiber bundle formed by 6 single-mode optical fibers surrounding a few-mode optical fiber in parallel on the taper platform. Heat and melt the fiber bundle at the same axial position, and use clamps to fix both ends and stretch them outward to the same length. According to the relationship curve between the effective refractive index of the double-clad ring-core few-mode optical fiber mode and the taper diameter, determine the phase matching point of each mode in the double-clad ring-core few-mode optical fiber core and the fundamental mode in the corresponding 6 single-mode optical fibers;

[0063] Step 3: The selection of phase matching points is relatively flexible. In order to make all optical fibers stretched to the same length, all phase matching points are selected to be at the same taper diameter.

[0064] Step 4: Stretch 6 single-mode optical fibers and one few-mode optical fiber simultaneously by 5.0 mm to 10.0 mm to meet the taper size at the selected phase matching point. At this time, the fundamental mode and high-order mode in the few-mode optical fiber and the fundamental mode in the corresponding 6 single-mode optical fibers undergo mode conversion in the waist region of the tapered optical fiber bundle.

[0065] The fiber bundle consisting of the few-mode fiber and several single-mode fibers is melt-stretched in the axial region until the taper diameter at the matching point is met, and the mode multiplexer / demultiplexer is prepared.

[0066] A parallel mode demultiplexer / demultiplexer using a double-clad ring core optical fiber, when used as a demultiplexer, such as Figure 8 As shown, one end is used as the input end, and the double-clad ring-core few-mode fiber is connected to the multiplexed signal transmitted by the adapted transmission fiber. The multiplexed signal undergoes mode conversion to the fundamental mode in the corresponding single-mode fiber core in the coupling area of ​​the tapered fiber bundle. Finally, at the other end of the demultiplexer, 6 single-mode optical fibers output the multiplexed 6 signals respectively, which is the demultiplexing process.

[0067] Furthermore, when used as a multiplexer, such as Fig. 9 As shown, one end is used as the input end, and 6 single-mode optical fibers are respectively connected to the excitation as the input end. The 6 input signals undergo mode conversion to the modes in the corresponding double-clad ring-core few-mode optical fiber cores in the coupling region of the tapered optical fiber bundle. Finally, at the other end of the multiplexer, the 6 signals are simultaneously output from the double-clad ring-core few-mode optical fiber and enter the transmission optical fiber, which is the multiplexing process.

[0068] The essence of the parallel mode demultiplexer of the double-clad ring-core fiber of the present invention is the combination of mode selection couplers. Among them, the principle of the mode selection coupler is that when two optical fibers at a certain distance have the same or similar effective refractive index of the core mode, the two will resonate, that is, the exchange of mode energy. The point where the effective refractive index of the mode is equal or similar is called the phase matching point. The characteristics of the parallel mode multiplexer / demultiplexer mentioned in the present invention are that the double-clad ring-core fiber few-mode fiber supporting N modes is placed in the middle position, and N single-mode optical fibers are arranged in sequence at equal intervals in the angular direction of the few-mode optical fiber, and N+1 optical fibers are placed in parallel and fit. Among them, the initial core spacing of each single-mode optical fiber and the few-mode optical fiber is the sum of the cladding radius of the single-mode optical fiber and the outer cladding radius of the few-mode optical fiber, and every two adjacent single-mode optical fibers are spaced the same. The optical fiber bundle formed by N single-mode optical fibers surrounding a few-mode optical fiber is placed in parallel on the taper platform. The optical fiber bundle is heated and melted at the same position in the axial direction, and the two ends are fixed with a clamp to stretch the same length outward. The fiber bundle after tapering presents a tapered structure. The parts with unchanged diameters at both ends maintain the initial fiber size. The tapered areas at both ends are stretching areas. The waist with the smallest diameter and unchanged in the middle is the heating area, also called the coupling area. The fiber bundle is melt-tapered to a taper diameter that satisfies the selected phase matching point. At this time, the fundamental mode and high-order mode in the few-mode fiber and the fundamental mode in the corresponding N single-mode fibers undergo mode conversion in the waist area of ​​the tapered fiber bundle. Different from the mode multiplexer / demultiplexer constructed with cascaded mode selective couplers, the present invention can realize the simultaneous multiplexing / demultiplexing of N target modes at the same stretching length.

[0069] After the parallel mode demultiplexer based on double-clad ring-core fiber is successfully prepared, it needs to be packaged and stored. The specific operations are as follows:

[0070] 1. Place the prepared multiplexer / demultiplexer coupling area in the quartz groove, drip a small amount of curing glue at both ends, and then cure it with ultraviolet light;

[0071] 2. After the first step, place the above components into the aluminum groove, cover with an aluminum plate, and finally tighten with screws.

[0072] The coupling efficiency and loss of the multiplexer / demultiplexer at a wavelength of 1550nm are shown in Table 1.

[0073] <![CDATA[LP 01 ]]> <![CDATA[LP 11 ]]> <![CDATA[LP 21 ]]> <![CDATA[LP 02 ]]> <![CDATA[LP 31 ]]> <![CDATA[LP 12 ]]> Coupling efficiency (%) 99.80 98.62 95.22 85.88 90.25 87.60 Insertion loss(dB) 0.0087 0.0604 0.2127 0.6611 0.4455 0.5750

[0074] Table 1

[0075] As a mode multiplexer / demultiplexer, the present invention can multiplex / demultiplex N modes simultaneously without the need for prefabricated parts, and greatly reduces the size and volume of the device. The insertion loss is less than 0.70 dB, and the mode conversion efficiency is greater than 85.0%. It can be widely used in various space-division multiplexing communication transmission systems.

[0076] Obviously, the above embodiments are merely examples for clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the invention.

Claims

1. A parallel mode demultiplexer for double-clad ring-core optical fiber, characterized in that: include: A few-mode optical fiber and several single-mode optical fibers, wherein the few-mode optical fiber is a double-clad ring-core optical fiber few-mode optical fiber supporting N modes, and the few-mode optical fiber is arranged in the middle position of the several single-mode optical fibers, the several single-mode optical fibers are arranged around the few-mode optical fiber along the angular direction of the few-mode optical fiber, and the several single-mode optical fibers are attached to the outer wall of the few-mode optical fiber, the initial core spacing of each of the single-mode optical fiber and the few-mode optical fiber is the sum of the cladding radius of the single-mode optical fiber and the outer cladding radius of the few-mode optical fiber, and the intervals between adjacent single-mode optical fibers are the same.

2. The parallel mode demultiplexer of the double-clad ring-core optical fiber according to claim 1, characterized in that: The few-mode optical fiber comprises a central core, an inner ring core, an outer ring core, an inner cladding and an outer cladding arranged in sequence from the inside to the outside; wherein, in the 1550.0nm band, the relative refractive index difference between the refractive index n1 of the central core and the refractive index n5 of the outer cladding is 0.00280-0.00960, the relative refractive index difference between the refractive index n2 of the inner ring core and the refractive index n5 of the outer cladding is 0.00550-0.01430, and the refractive index of the outer ring core is The relative refractive index difference between n3 and the refractive index n5 of the outer cladding is 0.00410-0.01090, the relative refractive index difference between the refractive index n4 of the inner cladding and the refractive index n5 of the outer cladding is 0.00070-0.00280; in the 1550.0nm band, the refractive index of the inner annular core>the refractive index of the outer annular core>the refractive index of the central core, the refractive index of the inner cladding>the refractive index of the outer cladding, and the effective refractive index difference between the modes of the few-mode optical fiber is greater than 10 -3 .

3. The parallel mode demultiplexer of the double-clad ring-core optical fiber according to claim 2, characterized in that: The central core diameter d1 is 2.0μm to 10.0μm, the inner ring diameter of the inner annular core is consistent with the central core diameter, the outer ring diameter d2 of the inner annular core is 4.0μm to 18.0μm, the inner ring diameter of the outer annular core is consistent with the outer ring diameter of the inner annular core, the outer ring diameter d3 of the outer annular core is 6.0μm to 30.0μm, the inner cladding diameter d4 is 30.0μm to 80.0μm, and the outer cladding diameter d5 is 80.0μm to 125.0μm.

4. The parallel mode demultiplexer of the double-clad ring-core optical fiber according to claim 2, characterized in that: The central fiber core, inner annular fiber core, outer annular fiber core and inner cladding are made of germanium-doped or erbium-doped silicon dioxide materials with different concentrations, different doping concentrations and different refractive indices, and the outer cladding is made of pure silicon dioxide material.

5. The parallel mode demultiplexer of double-clad ring-core optical fiber according to claim 1, characterized in that: In the 1550nm band, the single-mode optical fiber includes a core, a groove and a cladding from the inside to the outside, and the refractive index of the core is n Na The refractive index n of the cladding c The relative refractive index difference is 0.00380 to 0.01100, and the refractive index n of the groove is b The refractive index n of the cladding c The relative refractive index difference is -0.00070 to -0.00630; in the 1550.0 nm band, the refractive index of the core is greater than the refractive index of the cladding and greater than the refractive index of the groove.

6. The parallel mode demultiplexer of double-clad ring-core optical fiber according to claim 5, characterized in that: The diameter a of the core is 4.2 μm to 12.0 μm, the diameter c of the cladding is 80.0 μm to 125.0 μm, and the outer ring diameter b of the groove is 18.0 μm to 40.0 μm.

7. The parallel mode demultiplexer of double-clad ring-core optical fiber according to claim 6, characterized in that: The cores of the single-mode optical fibers have different refractive indices. The cores are made of germanium-doped or erbium-doped-silicon dioxide materials with different concentrations to adapt to the effective refractive index of the corresponding few-mode optical fiber mode. The grooves are made of negatively doped materials, and the cladding is made of pure silicon dioxide material.

8. A method for preparing a parallel mode demultiplexer of a double-clad ring-core optical fiber, characterized in that: The steps include: Step 1: The few-mode fiber is placed in the middle position, and several single-mode fibers are arranged in sequence at equal intervals in the angular direction of the few-mode fiber. One few-mode fiber and several single-mode fibers are placed parallel and attached, wherein the initial core spacing between each single-mode fiber and the few-mode fiber is the sum of the cladding radius of the single-mode fiber and the outer cladding radius of the few-mode fiber, and the spacing between every two adjacent single-mode fibers is the same; Step 2: Place a fiber bundle formed by several single-mode optical fibers surrounding a few-mode optical fiber in parallel on a taper platform, heat and melt the few-mode optical fiber and the fiber bundle formed by several single-mode optical fibers at the same axial position, and fix the two ends of the fiber bundle with a clamp to stretch them outward by the same length. The fiber bundle after taper presents a tapered structure. The parts with unchanged diameters at both ends of the fiber bundle are the initial fiber size, and the tapered areas at both ends of the fiber bundle are the stretching areas. The waist of the fiber bundle with the smallest and unchanged diameter in the middle is the heating area, also called the coupling area. According to the relationship curve between the effective refractive index of the mode in the few-mode optical fiber core and the cladding diameter, determine the phase matching point of each mode in the few-mode optical fiber core and the corresponding fundamental mode in the several single-mode optical fiber cores; Step 3: In order to make the stretched length of the few-mode fiber consistent with that of the corresponding single-mode fibers, all phase matching points are selected to be at the same taper diameter; Step 4: The few-mode fiber and several single-mode fibers are simultaneously stretched by 5.0 mm to 10.0 mm to meet the taper size at the selected phase matching point. At this time, the fundamental mode and high-order mode in the few-mode fiber and the fundamental mode in the corresponding several single-mode fibers undergo mode conversion in the waist region of the tapered fiber bundle.

9. The method for preparing a parallel mode demultiplexer of a double-clad ring-core optical fiber according to claim 8, characterized in that: When used as a demultiplexer, one end of the fiber bundle formed by the few-mode fiber and several single-mode fibers is used as the input end. The few-mode fiber is connected to the multiplexed signal transmitted by the transmission fiber. The multiplexed signal undergoes mode conversion to the fundamental mode in the corresponding single-mode fiber core in the coupling area of ​​the tapered fiber bundle. Finally, at the other end of the demultiplexer, several single-mode fibers output the multiplexed N signals respectively, which is the demultiplexing process.

10. The method for preparing a parallel mode demultiplexer of a double-clad ring-core optical fiber according to claim 8, characterized in that: When used as a multiplexer, several single-mode optical fibers are used as input ends and are connected to the excitation respectively. The N input signals undergo mode conversion to the modes in the corresponding double-clad ring-core few-mode optical fiber cores in the coupling area of ​​the tapered optical fiber bundle. Finally, at the other end of the multiplexer, the N signals are simultaneously output from the double-clad ring-core few-mode optical fiber and enter the transmission optical fiber, which is the multiplexing process.

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