A multi-channel orbital angular momentum mode all-fiber coupler

Abstract

The present application relates to a kind of multi-channel track angular momentum mode all-fiber coupler, and the fiber coupler structure is made of high refractive ring core and surrounding 4 round cores.By reasonably accurate design fiber structure size parameter, LP 01 Mode in different round core channels can excite different high-order modes in ring core, and after a transmission distance, the phase difference of high-order odd-even supermode can satisfy the condition of π / 2, without additional phase modulator or other optical processing means, finally four different high-order OAM modes, including OAM 41 , OAM 31 , OAM 21 And OAM 11 Mode can be directly generated in ring core.Compared with the currently reported OAM mode excitation method, the fiber coupler structure is compact and small, without additional optical device, and the excitation method is simple and effective, and the fiber has potential application value in all-fiber OAM space division multiplexing communication system.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fibers, and particularly relates to a full-fiber multi-channel coupler capable of exciting four different orbital angular momentum modes. BACKGROUND

[0002] Orbital angular momentum (OAM) beams have become a research hotspot in the fields of spatial light and optical fiber communication in recent years due to their unique optical characteristics. Different topological charge OAM beams are orthogonal to each other and can carry different optical information for transmission as independent channels.

[0003] With the development of OAM optical fiber communication towards full fiber, miniaturization and integration, a simple and effective OAM mode excitation and control method has become one of the most critical scientific problems in the field of optical fiber OAM communication. The currently reported OAM mode excitation methods include spiral optical fibers or gratings [Photonics Research, 2020, 8 (8): 1278-1288; Optics Express, 2020, 28 (18): 27044-27051], polarization couplers [Optical Fiber Technology, 2021, 64: 102543], spatial light modulators and the like. These methods have relatively complex devices, low coupling efficiency and large volume, which are not conducive to the integration and miniaturization of the optical fiber OAM communication system. In addition, there are various methods for adjusting and changing the mode phase by using the birefringence characteristics of microstructured optical fibers or polarization controllers to realize the conversion between intrinsic vector modes and OAM modes. This kind of method is simple and effective, but it cannot excite high-order OAM modes and can only change the phase difference between intrinsic odd and even modes to realize the excitation of the same order OAM mode.

[0004] It can be seen that there is currently a lack of compact, simple and effective full-fiber excitation methods in the field of optical fiber OAM communication that can directly convert ordinary LP 01 modes into different high-order OAM modes. SUMMARY

[0005] The present application designs a multi-channel orbital angular momentum mode full-fiber coupler to solve the problems existing in the field of optical fiber OAM mode excitation. By reasonably and accurately designing the fiber structure size parameters, the LP 01 modes in different circular core channels can excite different high-order modes in the ring core, and after a certain transmission distance, the phase difference between the high-order odd and even supermodes can satisfy the condition of π / 2. Without additional phase modulators or other optical processing means, four different high-order OAM modes, including OAM 41 , OAM 31OAM 21 and OAM 11 This coupler has potential application value in OAM fiber optic communication systems.

[0006] The all-fiber coupler of this invention consists of a cladding, four circular cores, and a central ring core. The cladding material is pure silicon dioxide, while the circular and ring cores are made of doped silicon dioxide. An air hole structure is located in the center of the ring core. The inner diameter of the ring core is r0, and its outer diameter is r1. The radius of each of the four circular cores is r2. The distances from the center of the four circular cores to the fiber center are R1, R2, R3, and R4, respectively. The refractive indices of the four circular cores are n1, n2, n3, and n4, respectively, and the refractive index of the ring core is n5. LP 01 The fundamental mode can enter the optical fiber through four identical circular core channels. After transmitting a certain distance, four different OAM modes will be directly excited and generated within the central ring core. 41 OAM 31 OAM 21 and OAM 11 ).

[0007] The OAM mode excitation principle of this invention is as follows: When specific size, wavelength, and material structure conditions are met, the LP input in the core... 01 The fundamental mode couples with the higher-order vector modes in the ring core, generating two pairs of odd-even supermodes. When the transmission distance meets the mode coupling length, the supermode superposition excites higher-order vector odd-even modes. When the phase accumulated by the higher-order vector odd-even modes at that transmission distance exactly satisfies a π / 2 phase difference, a higher-order OAM mode is generated. Similarly, when the fiber structure parameters meet specific conditions, the LP inputs in the four ring cores will... 01 The base mode will directly generate four different OAM higher-order modes in the middle ring core under a certain transmission length.

[0008] This invention proposes a multi-channel orbital angular momentum mode all-fiber coupler, which has the following advantages:

[0009] 1. This coupler requires no additional optical processing or phase modulator; the four circular cores are respectively input to LP. 01 The fundamental mode can directly generate four different higher-order OAM modes in the intermediate ring core. Compared with currently reported OAM mode excitation methods, this fiber coupler has a compact structure, requires no additional optical devices, and has a simple and effective excitation method. It can simultaneously excite four different higher-order OAM modes in a single ring core channel. This fiber has potential application value in all-fiber OAM space division multiplexing communication systems.

[0010] 2. The four circular cores of the coupler are the same size. The silica-doped material can better match and be compatible with ordinary single-mode optical fiber. The structure of the high-refractive-index ring core in the center also helps to match with common OAM transmission high-refractive-index ring core optical fiber, reducing coupling loss.

[0011] 3. By properly adjusting the fiber structure, different wavelengths and different types of OAM modes can be excited. It is not limited to the parameter characteristics provided by the specific implementation method. The coupler is flexible and controllable, and the excitation is simple and effective. Attached Figure Description

[0012] Figure 1 A schematic diagram of the cross-section of a multi-channel orbital angular momentum mode all-fiber coupler;

[0013] Figure 2 Effective refractive index and mode field diagram of four supermodes when the core of the fiber coupler ring and one of the circular cores resonate;

[0014] Figure 3 OAM generated under different transmission lengths 41 Model field diagram, phase diagram, and purity;

[0015] Figure 4 OAM generated under different transmission lengths 31 Model field diagram, phase diagram, and purity;

[0016] Figure 5 OAM generated under different transmission lengths 21 Model field diagram, phase diagram, and purity;

[0017] Figure 6 OAM generated under different transmission lengths 11 Model field diagram, phase diagram, and purity. Detailed Implementation

[0018] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0019] A multi-channel orbital angular momentum mode all-fiber coupler, with a cross-sectional structure as shown in the figure. Figure 1 As shown, the optical fiber center consists of a large air hole with radius r0 and a high-refractive-index ring core, surrounded by four circular cores. The cladding material is pure silicon dioxide, while the circular cores and ring core are made of doped silicon dioxide. The inner diameter of the ring core is r0, and its outer diameter is r1. The radius of each of the four circular cores is r2. The distances from the center of the optical fiber to the four circular cores are R1, R2, R3, and R4, respectively. The refractive indices of the four circular cores are n1, n2, n3, and n4, respectively, and the refractive index of the ring core is n5.

[0020] The cladding material of the fiber coupler is selected as pure silicon dioxide (assuming the refractive index of pure silicon dioxide is n0). The inner diameter of the ring core is r0 = 6 μm, the outer diameter is r1 = 8 μm, the radius of the four circular cores is r2 = 4 μm, the distances between the four circular cores and the center of the fiber are R1 = 15 μm, R2 = 16 μm, R3 = 15.5 μm, and R4 = 15 μm, respectively, the refractive indices of the four circular cores are n1 = n0 + 0.024, n2 = n0 + 0.02898, n3 = n0 + 0.0303, and n4 = n0 + 0.0269, respectively, and the refractive index of the ring core is n5 = n0 + 0.05.

[0021] Under these parameters, the OAM mode excitation characteristics of the fiber coupler are as follows: Figures 2 to 6 As shown.

[0022] Figure 2 The diagram shows the effective refractive index and mode field of two pairs of odd and even supermodes caused by coupling when the ring core and one of the circular cores of the fiber coupler resonate. Under these structural parameters, the HE in the right circular core... 11 Odd / even pattern will be related to the central ring core HE 31 Even and odd modes couple, and according to coupled-mode theory, this coupling generates two pairs of supermodes (labeled ABCD in the diagram). At this point, the coupling wavelength for both even and odd modes is 1550.6 nm. At this coupling wavelength, HE... 11 with HE 31 The maximum energy transfer between odd and even modes occurs at the coupling length. When the transmission distance is an odd multiple of the coupling length, the two pairs of odd and even supermodes will superimpose and generate HE within the ring core. 31 Odd / Even pattern; meanwhile, HE 31 The phase difference between odd and even modes also gradually accumulates with the transmission length. If the phase difference condition of π / 2 is met at this point, OAM will be triggered. 21 This fiber optic coupler utilizes a specially and ingeniously designed size and structure to achieve four circular core HE... 11 The conversion of the odd / even mode to four different OAM modes in the ring core is excited.

[0023] Figure 3 OAM generated under different transmission lengths 41 The mode field diagram, phase diagram, and purity of the model. When HE 11 When the odd / even mode is input from the lower circular core, at the coupling wavelength of 1550.8 nm, HE 11 Odd / even pattern will be related to the central ring core HE 51 Odd-even mode coupling occurs, and after a certain transmission length (967mm), the intermediate ring core will be directly excited to generate OAM. 41 The model has a purity of up to 99.07%.

[0024] Figure 4Mode field pattern, phase pattern and purity of the OAM mode excited by different transmission lengths 31 When HE 11 Odd-even modes are input from the left side circular core, at the coupling wavelength 1550.4nm, HE 11 Odd-even modes will be coupled with the center ring core HE 41 Odd-even modes occur coupling, after a certain transmission length (463mm or 464mm), the middle ring core will directly excite to generate OAM 31 Mode, and the purity is 98.63%.

[0025] Figure 5 Mode field pattern, phase pattern and purity of the OAM mode excited by different transmission lengths 21 When HE 11 Odd-even modes are input from the right side circular core, at the coupling wavelength 1550.6nm, HE 11 Odd-even modes will be coupled with the center ring core HE 31 Odd-even modes occur coupling, after a certain transmission length (285mm or 286mm), the middle ring core will directly excite to generate OAM 21 Mode, and the purity is 97.55%.

[0026] Figure 6 Mode field pattern, phase pattern and purity of the OAM mode excited by different transmission lengths 11 When HE 11 Odd-even modes are input from the upper circular core, at the coupling wavelength 1550.6nm, HE 11 Odd-even modes will be coupled with the center ring core HE 21 Odd-even modes occur coupling, after a certain transmission length (874mm), the middle ring core will directly excite to generate OAM 11 Mode, and the purity is 98.84%.

[0027] In addition, the optical fiber coupler structure is flexible and adjustable, by setting reasonable structure parameters, the coupling wavelength, transmission length and the type of excited OAM mode can be adjusted, and the optical fiber coupler has potential application value in the field of all-fiber OAM space division multiplexing communication system.

[0028] The part not described in detail in the application is the common knowledge of those skilled in the art.

Claims

1. A multi-channel orbital angular momentum mode all-fiber coupler, characterized in that: The coupler is composed of a cladding, a plurality of high refractive index circular cores and a middle high refractive index annular core; common LP 01 The fundamental mode is directly input from one of the circular cores, and after a certain transmission length, the OAM mode can be directly generated in the annular core; different circular core input LP 01 The fundamental mode can generate different OAM modes in the annular core at the same time.

2. A multi-channel orbital angular momentum mode all-fiber coupler according to claim 1, characterized in that: The coupler cladding material is pure silica, the high refractive index ring core and the plurality of circular cores are made of doped silica, and the high refractive index ring core is provided with an air hole in the center.

3. A multi-channel orbital angular momentum mode all-fiber coupler according to claim 1, characterized in that: The ring core is in the center of the optical fiber, and the plurality of circular cores are arranged around the ring core.

4. The multi-channel orbital angular momentum mode all-fiber coupler of claim 1, wherein: The number of the circular cores is four, the inner diameter of the high refractive index ring core is r0, the outer diameter is r1, the radius of the four circular cores is r2, the distances between the four circular cores and the center of the optical fiber are R1, R2, R3 and R4 respectively, the refractive indexes of the four circular cores are n1, n2, n3 and n4 respectively, and the refractive index of the ring core is n5.

5. A multi-channel orbital angular momentum mode all-fiber coupler according to claim 4, characterized in that: The inner diameter r0 of the ring core is 6 μm, the outer diameter r1 is 8 μm, the radius r2 of the four circular cores is 4 μm, the distances between the four circular cores and the center of the optical fiber are R1 = 15 μm, R2 = 16 μm, R3 = 15.5 μm and R4 = 15 μm respectively.

6. A multi-channel orbital angular momentum mode all-fiber coupler according to claim 4, characterized in that: The refractive indexes of the four circular cores are n1 = n0 + 0.024, n2 = n0 + 0.02898, n3 = n0 + 0.0303 and n4 = n0 + 0.0269 respectively, and the refractive index of the ring core is n5 = n0 + 0.05, wherein n0 is the refractive index of pure silica.

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