Directional coupling type mode group demultiplexer for graded-index few-mode optical fiber

Abstract

The invention discloses a directional coupling type mode group demultiplexer for a graded-index few-mode optical fiber, which comprises four demultiplexing regions, the first demultiplexing region is an LP31a, LP31b, LP12a and LP12b mode group demultiplexing region, an orthogonal two-mode directional coupling structure consisting of a graded-index few-mode optical fiber core and two same two-mode optical fiber cores is used, and the second demultiplexing region is an LP31a, LP31b, LP12a and LP12b mode group demultiplexing region. The two-mode optical fiber core is located in the horizontal direction and the vertical direction which are the same in distance with the few-mode optical fiber core. The LP11a mode and the LP11b mode in the two-mode optical fiber core are matched with the LP31a mode group, the LP31b mode group, the LP12a mode group and the LP12b mode group in the few-mode optical fiber core in phase. The second region is an LP21a, LP21b and LP02 mode group demultiplexing region, and the mode group is demultiplexed by using a similar structure; the third region is an LP11a and LP11b mode group demultiplexing region, and demultiplexing of the mode group is carried out by using a two-mode optical fiber matched with a few-mode optical fiber; and the fourth region is an LP01 mode group demultiplexing region, and demultiplexing of the mode group is carried out by using a single-mode optical fiber matched with the few-mode optical fiber. According to the invention, the mode group can be demultiplexed.

Description

Technical Field

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

[0002] Mode division multiplexing technology in few-mode optical fiber can greatly increase the transmission capacity of a single fiber, and therefore has been widely studied. For mode division multiplexing coherent optical transmission systems using graded-index few-mode optical fiber, multiple-input multiple-output (MIMO) digital signal processing (DSP) technology is required at the receiving end to handle mode crosstalk. The transceiver equipment of current commercial single-mode optical fiber communication systems does not have the capability of complex MIMO DSP, which greatly limits the practical application of mode division multiplexing technology. Intensity modulation and direct detection optical transmission without MIMO DSP can be achieved by using mode group multiplexing / demultiplexing in graded-index few-mode optical fiber, thereby promoting the commercialization of few-mode optical fiber 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 group. When different modes are transmitted in few-mode fiber, the crosstalk between the modes in the mode group is serious, while the weak coupling condition is satisfied between the mode groups. Although some mode channels are lost by treating all mode members of the same mode group as the same channel, the use of MIMO DSP can be avoided. In this case, each channel at the transmitter only transmits one mode in the mode group. After transmission in the few-mode fiber, the energy of the mode will be coupled to other modes in the mode group. Therefore, the receiver needs to demultiplex the optical power of all modes in the mode group at the same time to ensure the stability of reception.

[0004] There are two main existing methods for mode group demultiplexing. The first method is to use photon lanterns. Although photon lanterns have low loss, they have large mode crosstalk, making it difficult to achieve long-distance multi-mode group multiplexing transmission without MIMO DSP. The second solution is based on multi-plane optical converters. This method has lower mode crosstalk and can support more mode groups, but the implementation scheme is complex and the preparation cost is high. Summary of the invention

[0005] The present invention provides a directional coupling mode group demultiplexer for a gradient refractive index few-mode optical fiber, which is used to solve the problems existing in the above-mentioned prior art. The technical solution is as follows:

[0006] A directional coupling mode group demultiplexer for a gradient refractive index few-mode optical fiber is provided. The demultiplexer includes four mode group demultiplexing sections, which respectively demultiplex 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 Mode 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 is composed of a gradient 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 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 is composed of a gradient 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 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 The mode group demultiplexing area uses a two-mode fiber matched with a graded-index few-mode fiber to perform LP 11a LP 11b Demultiplexing of pattern groups;

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

[0011] Optionally, the first demultiplexing zone specifically demultiplexes the LP 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 through a combiner to achieve demultiplexing of the mode group;

[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 The mode can only be LP in horizontal two-mode fiber 11a mode and LP in vertical two-mode fiber 11a Mode coupling, LP31b LP 12b The mode can only be LP in horizontal two-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 any 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 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 same phase can only be demultiplexed to LP in horizontal two-mode fiber. 11a model;

[0017] When the LPs are equal in magnitude 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 equal in 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 optical fibers respectively using an orthogonal two-mode directional coupling structure consisting of a gradient refractive index few-mode optical fiber and two identical two-mode optical 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 fiber respectively using the same structure. 11b mode.

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

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

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

[0023] For LP 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 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 same phase can only be demultiplexed to LP in horizontal two-mode fiber. 11a model;

[0025] When the LPs are equal in magnitude 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 two-mode fiber in the vertical direction 11a The modes are equal in 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 fiber respectively 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 optical fiber and the core of the two-mode optical 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 optical fiber, the parameters of the corresponding two-mode optical fiber, the optimal inter-core distance between the few-mode optical fiber core and the two-mode optical 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 realize mode group demultiplexing on a graded-index few-mode fiber without multiple-input multiple-output digital signal processing, solve the problem of strong mutual crosstalk between modes, and thus promote 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 gradient refractive 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 It is a structural diagram of a directional coupling type cascaded four-mode group demultiplexer for a graded refractive index few-mode optical fiber provided by an embodiment of the present invention;

[0038] Figure 2 It 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 more clear, 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 gradient 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 Mode 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 is composed of a gradient 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 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 is composed of a gradient 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 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 The mode group demultiplexing area uses a two-mode fiber matched with a graded-index few-mode fiber to perform LP 11a LP 11b Demultiplexing of pattern groups;

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

[0047] Optionally, the first demultiplexing zone specifically demultiplexes the LP 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 through a combiner to achieve demultiplexing of the mode group;

[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 The mode can only be LP in horizontal two-mode fiber 11a mode and LP in vertical two-mode fiber 11aMode coupling, LP 31b LP 12b The mode can only be LP in horizontal two-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 any 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 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 same phase can only be demultiplexed to LP in horizontal two-mode fiber. 11a model;

[0053] When the LPs are equal in magnitude 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 two-mode fiber in the vertical direction 11a The modes are equal in 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 optical fibers respectively using an orthogonal two-mode directional coupling structure consisting of a gradient refractive index few-mode optical fiber and two identical two-mode optical 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 fiber respectively using the same structure. 11b mode.

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

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

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

[0059] For LP 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 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 same phase can only be demultiplexed to LP in horizontal two-mode fiber. 11a model;

[0061] When the LPs are equal in magnitude 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 equal in 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 fiber respectively 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 normal phase LP 02 Mode with Inverting LP 21a LP obtained by mode demultiplexing 11a The modes are equal in size but opposite in phase, and after superposition, they cancel each other out in anti-phase and cannot be demultiplexed. Since the modes rotate randomly during transmission, the phase cannot be controlled, so this mode group cannot be stably demultiplexed when only one two-mode optical fiber is used. However, as described above, this mode group can be stably demultiplexed using two two-mode optical fibers provided by the embodiment of the present invention.

[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 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 Mode and LP 11a Mode, LP 02 Mode and LP 11a The coupling coefficient of the mode;

[0068] Solving the equations, we 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] Where P 21a (0) and P 02 (0) is LP 21a Mode 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 Substituting, 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 on the mode, LP 21a and LP 02 The initial phase and power ratio of the mode have no effect on the result, so the appropriate structural parameters are selected to make LP 21b Mode coupled power, LP 21a and LP 02 When the total coupling power of each mode under 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 optical fiber and the core of the two-mode optical 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 optical fiber, the parameters of the corresponding two-mode optical fiber, the optimal inter-core distance between the few-mode optical fiber core and the two-mode optical 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 principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A directional coupling mode group demultiplexer for a graded-index few-mode optical fiber, characterized in that: The demultiplexer includes 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 Mode 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 is composed of a gradient 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 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 is composed of a gradient 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 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 The mode group demultiplexing area uses a two-mode fiber matched with a graded-index few-mode fiber to perform LP 11a LP 11b Demultiplexing of pattern groups; The fourth demultiplexing area is LP 01 The mode group demultiplexing area uses a single-mode fiber matched with a graded-index few-mode fiber to perform LP 01 Demultiplexing of pattern groups.

2. The method according to claim 1, characterized in that 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 through a combiner to achieve demultiplexing of the mode group; 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 method according to claim 1, characterized in that The implementation principle of the first demultiplexing zone is: LP 31a LP 12a The mode can only be LP in horizontal two-mode fiber 11a mode and LP in vertical two-mode fiber 11a Mode coupling, LP 31b LP 12b The mode can only be LP in horizontal two-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 any size and phase 31a LP 12a The mode can be divided into two parts: equal in size and same in 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 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 LPs are equal in magnitude 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 two-mode fiber in the vertical direction 11a The modes are equal in 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 optical fibers respectively using an orthogonal two-mode directional coupling structure consisting of a gradient refractive index few-mode optical fiber and two identical two-mode optical 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 fiber respectively using the same structure. 11b mode.

4. The method according to claim 1, characterized in that: The implementation principle of the second demultiplexing zone is: For LP 21b Demultiplexing of the mode: LP 21b The coupling behavior of the mode is either horizontal or vertical. The demultiplexed LP 11 Mode and LP 21a With LP 02 LP obtained by mode demultiplexing 11 modes are different degenerate components, so LP 21b The coupled behavior of the modes does not affect the LP 21a With LP 02 The coupled behavior of modes; For LP 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 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 same phase can only be demultiplexed to LP in horizontal two-mode fiber. 11a model; When the LPs are equal in magnitude 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 equal in 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 fiber respectively using the same structure. 11b mode.

5. The method according to claim 1, characterized in that The coupling efficiency and crosstalk characteristics of the demultiplexer are affected by the distance between the core of the graded refractive 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. 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 optical fiber, the parameters of the corresponding two-mode optical fiber, the optimal inter-core distance between the few-mode optical fiber core and the two-mode optical fiber core, and the optimal coupling length are determined.

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