Directionally coupled mode group demultiplexer for graded-index few-mode fiber

Abstract

The present invention discloses a directional coupling mode group demultiplexer for graded refractive index few-mode optical fiber, comprising four demultiplexing zones, the first of which is LP 31a LP 31b LP 12a LP 12b The mode group demultiplexing area uses an orthogonal two-mode directional coupling structure consisting of a graded-index few-mode fiber core and two identical two-mode fiber cores. The two-mode fiber core is located in the same horizontal and vertical directions as the few-mode fiber core. The LP in the two-mode fiber core is 11a LP 11b Modes and LP in the Core of Few-Mode Fibers 31a LP 31b LP 12a LP 12b Mode group phase matching; the second is LP 21a LP 21b LP 02 Mode group demultiplexing area, using a similar structure to demultiplex the mode group; the third is LP 11a LP 11b Mode group demultiplexing area, using a two-mode fiber matched with the few-mode fiber to demultiplex the mode group; the fourth is LP 01 The mode group demultiplexing area uses a single-mode optical fiber matched with the few-mode optical fiber to demultiplex the mode group. The present invention can demultiplex the mode group.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber passive device design, in particular to a directional coupling mode group demultiplexer for graded refractive index few-mode optical fiber. Background Art

[0002] Mode division multiplexing (MDM) technology in few-mode fiber (FMF) has been widely researched because it can significantly increase single-fiber transmission capacity. For MDM coherent optical transmission systems using graded-index FMF, multiple-input multiple-output (MIMO) digital signal processing (DSP) technology is required at the receiver to address mode crosstalk. Current commercial single-mode fiber communication systems lack sophisticated MIMO DSP capabilities in their transceivers, significantly limiting the practical application of MDM technology. By using mode group multiplexing / demultiplexing in GIFMF, intensity modulation and direct detection optical transmission can be achieved without MIMO DSP, thereby promoting the commercialization of FMF technology.

[0003] The core refractive index distribution of graded-index few-mode fiber makes the modes degenerate, and the modes with very close propagation constants are divided into a mode group. For example, there are four LP mode groups in a graded-index few-mode fiber that supports 10 modes. The first one is LP 01 Mode group, the second one is LP 11a LP 11b Mode Group, Third LP 21a LP 21b LP 02 Mode group, the fourth one is LP 31a LP 31b LP 12a LP 12b Mode groups. When different modes are transmitted over few-mode fiber, crosstalk between modes within a mode group is severe, while weak coupling conditions exist between mode groups. By treating all mode members of the same mode group as a single channel, some modal channels are lost, but the use of MIMO DSP can be avoided. In this case, the transmitter transmits only one mode within the mode group per channel. After transmission over few-mode fiber, the energy of this mode will couple to other modes within the mode group. Therefore, the receiver must simultaneously demultiplex the optical power of all modes within the mode group to ensure stable reception.

[0004] There are two main existing methods for mode group demultiplexing. The first uses photonic lanterns. Although these offer low loss, they suffer from significant mode crosstalk, making long-distance multi-mode group multiplexing difficult without a MIMO DSP. The second approach relies on multi-plane optical converters. This approach offers lower mode crosstalk and supports a greater number of mode groups, but its implementation is complex and expensive. Summary of the Invention

[0005] The present invention provides a directional coupling mode group demultiplexer for graded-index few-mode fiber to solve the problems of the above-mentioned prior art. The technical solution is as follows:

[0006] A directional coupling mode group demultiplexer for a graded-index few-mode fiber is provided. The demultiplexer comprises four mode group demultiplexing sections, which respectively demultiplex the input four mode groups consisting of ten modes, and then cascade the demultiplexing structures of each mode group. The four mode groups are: LP 31a LP 31b LP 12a LP 12b Mode group; LP 21a LP 21b LP 02 Mode group; LP 11a LP 11b Mode group; LP 01 Pattern Group;

[0007] The first demultiplexing area is LP 31a LP 31b LP 12a LP 12b The mode group demultiplexing area uses an orthogonal two-mode directional coupling structure, which consists of a graded refractive index few-mode fiber core and two identical two-mode fiber cores. The two two-mode fiber cores are located in the same horizontal and vertical directions as the few-mode fiber core. The LP in the two-mode fiber core is 11a LP 11b Modes and LP in the Core of Few-Mode Fibers 31a LP 31b LP 12a LP 12b Mode group phase matching;

[0008] The second demultiplexing area is LP 21a LP 21b LP 02The mode group demultiplexing area uses an orthogonal two-mode directional coupling structure, which consists of a graded refractive index few-mode fiber core and two identical two-mode fiber cores. The two two-mode fiber cores are located in the same horizontal and vertical directions as the few-mode fiber core. The LP in the two-mode fiber core is 11a LP 11b Modes and LP in the Core of Few-Mode Fibers 21a LP 21b LP 02 Mode group phase matching;

[0009] The third demultiplexing area is LP 11a LP 11b Mode group demultiplexing area uses a two-mode fiber matched with a graded-index few-mode fiber for LP 11a LP 11b Demultiplexing of pattern groups;

[0010] The fourth demultiplexing area is LP 01 Mode group demultiplexing area uses a single mode fiber matched with a graded refractive index few mode fiber to perform LP 01 Demultiplexing of pattern groups.

[0011] Optionally, the first demultiplexing zone specifically converts the LP in the few-mode fiber 31a LP 31b LP 12a LP 12b Each mode in the mode group is coupled to the LP in two two-mode fibers in a directional coupling manner. 11a and LP 11b Finally, the power in the two two-mode fibers is added together through a combiner to achieve demultiplexing of the mode groups.

[0012] The second demultiplexing zone specifically converts the LP in the few-mode fiber 21a LP 21b LP 02 Each mode in the mode group is coupled to the LP in two two-mode fibers in a directional coupling manner. 11a and LP 11b Finally, the power in the two two-mode optical fibers is added through a combiner to achieve demultiplexing of the mode group.

[0013] Optionally, the implementation principle of the first demultiplexing zone is:

[0014] LP 31a LP 12a Mode can only be used with LP in horizontal dual-mode fiber 11a mode and LP in vertical two-mode fiber 11a Mode coupling, LP31b LP 12b Mode can only be used with LP in horizontal dual-mode fiber 11b mode and LP in vertical two-mode fiber 11b Mode coupling, so LP 31a LP 12a Mode and LP 31b LP 12b The coupled behaviors of the modes are independent and can be described separately:

[0015] For LPs of arbitrary size and phase 31a LP 12a The mode can be divided into two parts: equal in size and phase and equal in size and opposite in phase. The demultiplexing is as follows:

[0016] When the magnitude is equal, the phase is the same LP 31a LP 12a When the mode is injected into the core of the few-mode fiber, the LP obtained by horizontal demultiplexing of the two 11a The modes are also equal in size and phase, and can be superimposed for demultiplexing. However, the LP obtained by demultiplexing the two in the vertical direction is 11a The modes are equal in size but opposite in phase. After superposition, they cancel each other out and cannot be demultiplexed. Therefore, LP 31a LP 12a The parts with equal size and phase can only be demultiplexed to LP in horizontal two-mode fiber. 11a model;

[0017] When the LP is equal in size and opposite in phase 31a LP 12a When the mode is injected into the core of the few-mode fiber, the LP obtained by demultiplexing in the horizontal direction 11a The modes are equal in size but opposite in phase. After superposition, they cancel each other out in anti-phase and cannot be demultiplexed. However, the LP obtained by demultiplexing in the two-mode fiber in the vertical direction is 11a The modes are of equal size and phase, so demultiplexing can be accomplished;

[0018] In this way, LP 31a LP 12a The two parts of the mode can be demultiplexed to the LP in the horizontal and vertical two-mode fibers respectively using an orthogonal two-mode directional coupling structure consisting of a graded-index few-mode fiber and two identical two-mode fibers. 11a In mode;

[0019] LP 31b LP 12b The two parts of the mode can also be demultiplexed into the LP in the horizontal and vertical two-mode optical fibers using the same structure. 11b mode.

[0020] Optionally, the implementation principle of the second demultiplexing zone is:

[0021] For LPs 21b Demultiplexing of the mode:

[0022] LP 21b The coupling behavior of the mode is either horizontal or vertical, and the LP obtained by demultiplexing 11 Mode and LP 21a With LP 02 LP obtained by mode demultiplexing 11 The modes are different degenerate components, so LP 21b The coupled behavior of the modes does not affect the LP 21a With LP 02 coupled behavior of modes;

[0023] For LPs 21a With LP 02 Demultiplexing of the mode:

[0024] When the magnitude is equal, the phase is the same LP 21a With LP 02 When the mode is injected into the core of the few-mode fiber, the LP obtained by horizontal demultiplexing of the two 11a The modes are also equal in size and phase, and can be superimposed for demultiplexing. However, the LP obtained by demultiplexing the two in the vertical direction is 11a The modes are equal in size but opposite in phase. After superposition, they cancel each other out and cannot be demultiplexed. Therefore, LP 21a With LP 02 The parts with equal size and phase can only be demultiplexed to LP in horizontal two-mode fiber. 11a model;

[0025] When the LP is equal in size and opposite in phase 21a With LP 02 When the mode is injected into the core of the few-mode fiber, the LP obtained by demultiplexing in the horizontal direction 11a The modes are equal in size but opposite in phase. After superposition, they cancel each other out and cannot be demultiplexed. However, the LP in the vertical two-mode fiber 11a The modes are of equal size and phase, so demultiplexing can be accomplished;

[0026] In this way, LP 21a With LP 02 The two parts of the mode can be demultiplexed into the horizontal and vertical two-mode optical fibers respectively using an orthogonal two-mode directional coupling structure consisting of a graded-index few-mode optical fiber and two identical two-mode optical fibers;

[0027] LP 21bThe two parts of the mode can also be demultiplexed into the LP in the horizontal and vertical two-mode optical fibers using the same structure. 11b mode.

[0028] Optionally, the coupling efficiency and crosstalk characteristics of the demultiplexer are affected by the distance between the core of the graded-index few-mode fiber and the core of the two-mode fiber. The smaller the distance between the cores, the higher the coupling efficiency, but at the same time the greater the crosstalk.

[0029] The optimal coupling length of the demultiplexer has different values ​​at different wavelengths;

[0030] According to the specific parameters and wavelength of the selected graded-index few-mode fiber, the corresponding two-mode fiber parameters, the optimal inter-core distance between the few-mode fiber core and the two-mode fiber core, and the optimal coupling length are determined.

[0031] The beneficial effects brought about by the technical solution provided by the present invention include at least:

[0032] 1) The present invention can achieve mode group demultiplexing on a graded-index few-mode fiber without multiple-input multiple-output digital signal processing, thereby solving the problem of strong mutual crosstalk between modes and promoting the commercialization of few-mode fiber technology.

[0033] 2) The present invention uses an all-fiber waveguide structure to achieve mode group demultiplexing on a graded-index few-mode fiber, which is simpler and less costly than previous mode group demultiplexing solutions.

[0034] 3) The orthogonal two-mode directional coupling structure designed in the present invention can use two two-mode optical fiber waveguides to achieve demultiplexing of the LP mode group, effectively solving the problem of instability of demultiplexing due to mutual coupling of modes in the mode group.

[0035] 4) The present invention has good coupling efficiency and crosstalk characteristics in the entire C-band. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0037] Figure 1 This is a structural diagram of a directional coupling type cascaded four-mode group demultiplexer for graded-index few-mode optical fiber provided by an embodiment of the present invention;

[0038] Figure 2 1 is a structural diagram and a cross-sectional diagram of an orthogonal two-mode directional coupling solution provided by an embodiment of the present invention;

[0039] Figure 3 The LP provided by the embodiment of the present invention 21a LP 21b LP 02 Schematic diagram of mode horizontal and vertical demultiplexing;

[0040] Figure 4 It is LP 21 and LP 02 Schematic diagram of mode demultiplexing using a two-mode optical fiber. DETAILED DESCRIPTION

[0041] In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0042] like Figure 1 As shown, the embodiment of the present invention provides a directional coupling type cascaded four-mode group demultiplexer for graded refractive index few-mode fiber, the demultiplexer includes four mode group demultiplexing sections, which respectively demultiplex the four mode groups consisting of ten modes input, and then cascade the demultiplexing structures of each mode group. The four mode groups are: LP 31a LP 31b LP 12a LP 12b Mode group; LP 21a LP 21b LP 02 Mode group; LP 11a LP 11b Mode group; LP 01 Pattern Group;

[0043] The first demultiplexing area is LP 31a LP 31b LP 12a LP 12b The mode group demultiplexing area uses an orthogonal two-mode directional coupling structure, which consists of a graded refractive index few-mode fiber core and two identical two-mode fiber cores. The two two-mode fiber cores are located in the same horizontal and vertical directions as the few-mode fiber core. The LP in the two-mode fiber core is 11a LP 11b Modes and LP in the Core of Few-Mode Fibers 31a LP 31b LP 12a LP 12b Mode group phase matching;

[0044] The second demultiplexing area is LP 21a LP 21b LP02 The mode group demultiplexing area uses an orthogonal two-mode directional coupling structure, which consists of a graded refractive index few-mode fiber core and two identical two-mode fiber cores. The two two-mode fiber cores are located in the same horizontal and vertical directions as the few-mode fiber core. The LP in the two-mode fiber core is 11a LP 11b Modes and LP in the Core of Few-Mode Fibers 21a LP 21b LP 02 Mode group phase matching;

[0045] The third demultiplexing area is LP 11a LP 11b Mode group demultiplexing area uses a two-mode fiber matched with a graded-index few-mode fiber for LP 11a LP 11b Demultiplexing of pattern groups;

[0046] The fourth demultiplexing area is LP 01 Mode group demultiplexing area uses a single mode fiber matched with a graded refractive index few mode fiber to perform LP 01 Demultiplexing of pattern groups.

[0047] Optionally, the first demultiplexing zone specifically converts the LP in the few-mode fiber 31a LP 31b LP 12a LP 12b Each mode in the mode group is coupled to the LP in two two-mode fibers in a directional coupling manner. 11a and LP 11b Finally, the power in the two two-mode fibers is added together through a combiner to achieve demultiplexing of the mode groups.

[0048] The second demultiplexing zone specifically converts the LP in the few-mode fiber 21a LP 21b LP 02 Each mode in the mode group is coupled to the LP in two two-mode fibers in a directional coupling manner. 11a and LP 11b Finally, the power in the two two-mode optical fibers is added through a combiner to achieve demultiplexing of the mode group.

[0049] Optionally, the implementation principle of the first demultiplexing zone is:

[0050] LP 31a LP 12a Mode can only be used with LP in horizontal dual-mode fiber 11a mode and LP in vertical two-mode fiber 11aMode coupling, LP 31b LP 12b Mode can only be used with LP in horizontal dual-mode fiber 11b mode and LP in vertical two-mode fiber 11b Mode coupling, so LP 31a LP 12a Mode and LP 31b LP 12b The coupled behaviors of the modes are independent and can be described separately:

[0051] For LPs of arbitrary size and phase 31a LP 12a The mode can be divided into two parts: equal in size and phase and equal in size and opposite in phase. The demultiplexing is as follows:

[0052] When the magnitude is equal, the phase is the same LP 31a LP 12a When the mode is injected into the core of the few-mode fiber, the LP obtained by horizontal demultiplexing of the two 11a The modes are also equal in size and phase, and can be superimposed for demultiplexing. However, the LP obtained by demultiplexing the two in the vertical direction is 11a The modes are equal in size but opposite in phase. After superposition, they cancel each other out and cannot be demultiplexed. Therefore, LP 31a LP 12a The parts with equal size and phase can only be demultiplexed to LP in horizontal two-mode fiber. 11a model;

[0053] When the LP is equal in size and opposite in phase 31a LP 12a When the mode is injected into the core of the few-mode fiber, the LP obtained by demultiplexing in the horizontal direction 11a The modes are equal in size but opposite in phase. After superposition, they cancel each other out and cannot be demultiplexed. However, the LP in the vertical two-mode fiber 11a The modes are of equal size and phase, so demultiplexing can be accomplished;

[0054] In this way, LP 31a LP 12a The two parts of the mode can be demultiplexed to the LP in the horizontal and vertical two-mode fibers respectively using an orthogonal two-mode directional coupling structure consisting of a graded-index few-mode fiber and two identical two-mode fibers. 11a In mode;

[0055] LP 31b LP 12b The two parts of the mode can also be demultiplexed into the LP in the horizontal and vertical two-mode optical fibers using the same structure. 11b mode.

[0056] Alternatively, as Figure 3 As shown, the implementation principle of the second demultiplexing zone is:

[0057] For LPs 21b Demultiplexing of the mode:

[0058] LP 21b The coupling behavior of the mode is either horizontal or vertical, and the LP obtained by demultiplexing 11 Mode and LP 21a With LP 02 LP obtained by mode demultiplexing 11 The modes are different degenerate components, so LP 21b The coupled behavior of the modes does not affect the LP 21a With LP 02 coupled behavior of modes;

[0059] For LPs 21a With LP 02 Demultiplexing of the mode:

[0060] When the magnitude is equal, the phase is the same LP 21a With LP 02 When the mode is injected into the core of the few-mode fiber, the LP obtained by horizontal demultiplexing of the two 11a The modes are also equal in size and phase, and can be superimposed for demultiplexing. However, the LP obtained by demultiplexing the two in the vertical direction is 11a The modes are equal in size but opposite in phase. After superposition, they cancel each other out and cannot be demultiplexed. Therefore, LP 21a With LP 02 The parts with equal size and phase can only be demultiplexed to LP in horizontal two-mode fiber. 11a model;

[0061] When the LP is equal in size and opposite in phase 21a With LP 02 When the mode is injected into the core of the few-mode fiber, the LP obtained by demultiplexing in the horizontal direction 11a The modes are equal in size but opposite in phase. After superposition, they cancel each other out in anti-phase and cannot be demultiplexed. However, the LP obtained by demultiplexing in the two-mode fiber in the vertical direction is 11a The modes are of equal size and phase, so demultiplexing can be accomplished;

[0062] In this way, LP 21a With LP 02 The two parts of the mode can be demultiplexed into the horizontal and vertical two-mode optical fibers respectively using an orthogonal two-mode directional coupling structure consisting of a graded-index few-mode optical fiber and two identical two-mode optical fibers;

[0063] LP21b The two parts of the mode can also be demultiplexed into the LP in the horizontal and vertical two-mode optical fibers using the same structure. 11b mode.

[0064] like Figure 4 As shown, for LP 21a LP 21b LP 02 Mode group, when only one two-mode fiber is used to demultiplex it, the positive phase LP 02 Mode with Inverting LP 21a LP obtained by mode demultiplexing 11a The modes are equal in magnitude but opposite in phase. When superimposed, they cancel each other out, making demultiplexing impossible. Because the modes rotate randomly during transmission, resulting in uncontrollable phases, this mode group cannot be stably demultiplexed using only a single two-mode fiber. However, as described above, using two two-mode fibers provided by embodiments of the present invention allows for stable demultiplexing of this mode group.

[0065] Furthermore, for LP 21a With LP 02 Mode demultiplexing: The coupling behavior is described by the following coupled wave equation:

[0066]

[0067] Where z is the coupling length, β is the propagation constant of the LP mode, and It is LP in few-mode fiber 21a Mode and LP 02 The complex amplitude of the mode, It is the LP in the horizontal two-mode optical fiber 11a The complex amplitude of the mode, It is the LP in the vertical two-mode optical fiber 11b The complex amplitude of the mode, and LP 21a Model and LP 11a Mode, LP 02 Model and LP 11a mode coupling coefficient;

[0068] Solving the equations, we can obtain the complex amplitudes of the mode fields in the two two-mode optical fibers:

[0069]

[0070]

[0071] Among them, C1 and C2 are coefficients determined by the initial input;

[0072] LP 21a With LP 02 The total coupled power of the mode is:

[0073]

[0074] Among them, P 21a (0) and P 02 (0) is LP 21a Model and LP 02 The initial injection power of the mode, the coupling coefficient in the orthogonal two-mode qualitative coupling structure and is very close and can be approximated by κ a Instead, we get:

[0075]

[0076] It can be seen that: LP 21a and LP 02 The total coupled power of the mode is only affected by the coupling length z, LP 21a and LP 02 The effect of the total initial injection power of the mode, LP 21a and LP 02 The initial phase and power ratio of the mode have no effect on the results, so the appropriate structural parameters are selected to make LP 21b Mode coupling power, LP 21a and LP 02 When the total coupling power of each mode at a certain coupling length is the maximum, the total demultiplexing power of the second demultiplexing zone is the maximum, thereby achieving efficient demultiplexing.

[0077] Similarly, for the first LP 31a LP 31b LP 12a LP 12b In the mode group demultiplexing area, efficient demultiplexing can also be achieved by using the orthogonal two-mode directional coupling structure of the embodiment of the present invention.

[0078] The specific principles of the third and fourth demultiplexing zones are prior art and will not be described in detail here.

[0079] Optionally, the coupling efficiency and crosstalk characteristics of the demultiplexer are affected by the distance between the core of the graded-index few-mode fiber and the core of the two-mode fiber. The smaller the distance between the cores, the higher the coupling efficiency, but at the same time the greater the crosstalk.

[0080] The optimal coupling length of the demultiplexer has different values ​​at different wavelengths;

[0081] According to the specific parameters and wavelength of the selected graded-index few-mode fiber, the corresponding two-mode fiber parameters, the optimal inter-core distance between the few-mode fiber core and the two-mode fiber core, and the optimal coupling length are determined.

[0082] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A directional coupling mode group demultiplexer for graded-index few-mode fiber, characterized in that: The demultiplexer includes four mode group demultiplexing areas, which demultiplex the input four mode groups consisting of ten modes, and then cascade the demultiplexing structures of each mode group. The four mode groups are: LP 31a LP 31b LP 12a LP 12b Mode group; LP 21a LP 21b LP 02 Mode group; LP 11a LP 11b Mode group; LP 01 Pattern Group; The first demultiplexing area is LP 31a LP 31b LP 12a LP 12b The mode group demultiplexing area uses an orthogonal two-mode directional coupling structure, which consists of a graded refractive index few-mode fiber core and two identical two-mode fiber cores. The two two-mode fiber cores are located in the same horizontal and vertical directions as the few-mode fiber core. The LP in the two-mode fiber core is 11a LP 11b Modes and LP in the Core of Few-Mode Fibers 31a LP 31b LP 12a LP 12b Mode group phase matching; The second demultiplexing area is LP 21a LP 21b LP 02 The mode group demultiplexing area uses an orthogonal two-mode directional coupling structure, which consists of a graded refractive index few-mode fiber core and two identical two-mode fiber cores. The two two-mode fiber cores are located in the same horizontal and vertical directions as the few-mode fiber core. The LP in the two-mode fiber core is 11a LP 11b Modes and LP in the Core of Few-Mode Fibers 21a LP 21b LP 02 Mode group phase matching; The third demultiplexing area is LP 11a LP 11b Mode group demultiplexing area uses a two-mode fiber matched with a graded-index few-mode fiber for LP 11a LP 11b Demultiplexing of pattern groups; The fourth demultiplexing area is LP 01 Mode group demultiplexing area uses a single mode fiber matched with a graded refractive index few mode fiber to perform LP 01 Demultiplexing of pattern groups.

2. The directional coupling mode group demultiplexer according to claim 1, wherein: The first demultiplexing zone specifically converts the LP in the few-mode fiber 31a LP 31b LP 12a ,,LP 12b Each mode in the mode group is coupled to the LP in two two-mode fibers in a directional coupling manner. 11a and LP 11b Finally, the power in the two two-mode fibers is added together through a combiner to achieve demultiplexing of the mode groups. The second demultiplexing zone specifically converts the LP in the few-mode fiber 21a LP 21b LP 02 Each mode in the mode group is coupled to the LP in two two-mode fibers in a directional coupling manner. 11a and LP 11b Finally, the power in the two two-mode optical fibers is added through a combiner to achieve demultiplexing of the mode group.

3. The directional coupling mode group demultiplexer according to claim 1, wherein: The implementation principle of the first demultiplexing zone is: LP 31a LP 12a The mode can only be used with the LP in the horizontal dual-mode fiber 11a mode and LP in vertical two-mode fiber 11a Mode coupling, LP 31b LP 12b The mode can only be used with the LP in the horizontal dual-mode fiber 11b mode and LP in vertical two-mode fiber 11b Mode coupling, so LP 31a LP 12a Mode and LP 31b LP 12b The coupled behaviors of the modes are independent and can be described separately: For LPs of arbitrary size and phase 31a LP 12a The mode can be divided into two parts: equal in size and phase and equal in size and opposite in phase. The demultiplexing is as follows: When the magnitude is equal, the phase is the same LP 31a LP 12a When the mode is injected into the core of the few-mode fiber, the LP obtained by horizontal demultiplexing of the two 11a The modes are also equal in size and phase, and can be superimposed for demultiplexing. However, the LP obtained by demultiplexing the two in the vertical direction is 11a The modes are equal in size but opposite in phase. After superposition, they cancel each other out and cannot be demultiplexed. Therefore, LP 31a LP 12a The parts with equal size and phase can only be demultiplexed to LP in horizontal two-mode fiber. 11a model; When the LP is equal in size and opposite in phase 31a LP 12a When the mode is injected into the core of the few-mode fiber, the LP obtained by demultiplexing in the horizontal direction 11a The modes are equal in size but opposite in phase. After superposition, they cancel each other out and cannot be demultiplexed. However, the LP in the vertical two-mode fiber 11a The modes are of equal size and phase, so demultiplexing can be accomplished; In this way, LP 31a LP 12a The two parts of the mode can be demultiplexed to the LP in the horizontal and vertical two-mode fibers respectively using an orthogonal two-mode directional coupling structure consisting of a graded-index few-mode fiber and two identical two-mode fibers. 11a In mode; LP 31b LP 12b The two parts of the mode can also be demultiplexed into the LP in the horizontal and vertical two-mode optical fibers using the same structure. 11b mode.

4. The directional coupling mode group demultiplexer according to claim 1, wherein: The implementation principle of the second demultiplexing zone is: For LPs 21b Demultiplexing of the mode: LP 21b The coupling behavior of the mode is either horizontal or vertical, and the LP obtained by demultiplexing 11 Mode and LP 21a With LP 02 LP obtained by mode demultiplexing 11 The modes are different degenerate components, so LP 21b The coupled behavior of the modes does not affect the LP 21a With LP 02 coupled behavior of modes; For LPs 21a With LP 02 Demultiplexing of the mode: When the magnitude is equal, the phase is the same LP 21a With LP 02 When the mode is injected into the core of the few-mode fiber, the LP obtained by horizontal demultiplexing of the two 11a The modes are also equal in size and phase, and can be superimposed for demultiplexing. However, the LP obtained by demultiplexing the two in the vertical direction is 11a The modes are equal in size but opposite in phase. After superposition, they cancel each other out and cannot be demultiplexed. Therefore, LP 21a With LP 02 The parts with equal size and phase can only be demultiplexed to LP in horizontal two-mode fiber. 11a model; When the LP is equal in size and opposite in phase 21a With LP 02 When the mode is injected into the core of the few-mode fiber, the LP obtained by demultiplexing in the horizontal direction 11a The modes are equal in size but opposite in phase. After superposition, they cancel each other out in phase and cannot be demultiplexed. However, the LP obtained by demultiplexing in the two-mode fiber in the vertical direction is 11a The modes are of equal size and phase, so demultiplexing can be accomplished; In this way, LP 21a With LP 02 The two parts of the mode can be demultiplexed into the horizontal and vertical two-mode optical fibers respectively using an orthogonal two-mode directional coupling structure consisting of a graded-index few-mode optical fiber and two identical two-mode optical fibers; LP 21b The two parts of the mode can also be demultiplexed into the LP in the horizontal and vertical two-mode optical fibers using the same structure. 11b mode.

5. The directional coupling mode group demultiplexer according to claim 1, wherein: The coupling efficiency and crosstalk characteristics of the demultiplexer are affected by the distance between the core of the graded-index few-mode fiber and the core of the two-mode fiber. The smaller the distance between the cores, the higher the coupling efficiency, but at the same time the crosstalk is also greater. The optimal coupling length of the demultiplexer has different values ​​at different wavelengths; According to the specific parameters and wavelength of the selected graded-index few-mode fiber, the corresponding two-mode fiber parameters, the optimal inter-core distance between the few-mode fiber core and the two-mode fiber core, and the optimal coupling length are determined.

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