Mode converter based on side-cast D-type optical fiber

Abstract

The invention relates to the field of optical fibers, and particularly discloses a mode converter based on a side-polished D-type optical fiber, which comprises a conventional D-type optical fiber, a flat side-polished surface is formed by removing a cladding layer of a side part of a common optical fiber through side polishing, and a fiber core structure is kept unchanged; the phase matching D-type optical fiber forms a flat side polishing surface through side surface polishing and grinding, and realizes phase matching through structure control; the side faces of the two optical fibers are polished and attached to carry out mode coupling conversion, a traditional fused biconical taper welding technology is not needed, the structural continuity of optical fiber cores is kept, excessive optical power loss caused by biconical taper is avoided, and insertion loss is remarkably reduced; an accurate phase matching design is utilized, so that only a specific target mode participates in coupling conversion. Compared with a multi-mode fiber bundle or strong fusion coupling method, excitation of a non-target mode is reduced, the purity of an output mode is high, mode crosstalk is obviously reduced, and the requirement for isolation between channels in mode multiplexing communication can be met.

Description

Technical Field

[0001] The invention belongs to the field of optical fibers, in particular to a mode converter based on side-throw D-type optical fibers. Background Art

[0002] There are multiple transmission modes when light is transmitted in optical fiber. Different modes correspond to different distribution forms of electromagnetic fields in optical fiber. Traditional optical fiber communication systems generally use single-mode optical fiber, which only supports fundamental mode (such as LP 01 In order to improve the capacity of optical fiber communication, high-order intrinsic modes (such as high-order linear polarization modes LP) are used to transmit the data. n,1 Orbital angular momentum mode (OAM) n,1 ) Parallel transmission of multiple independent data channels has become a research hotspot. This puts new demands on mode converters, namely the need to achieve efficient and low-loss conversion between the fundamental mode and higher-order modes.

[0003] In the existing technology, a variety of fiber mode conversion schemes have been proposed, but they generally have shortcomings. For example, the scheme of using multiple few-mode optical fibers to fuse and taper to form a fiber bundle can only achieve the conversion of a single high-order mode, and the mode coupling region is short, the structure is complex, the manufacturing difficulty is high, and the conversion efficiency and mode purity are limited. In addition, the taper process is prone to introduce large insertion loss, resulting in performance degradation. There are also technologies that achieve mode conversion through special multi-core optical fibers or optical fiber end-face microstructure processing, but there are problems such as complex processing, high cost, large additional loss, and poor mode purity, which makes it difficult to meet the low-loss and high-purity conversion requirements. Although traditional fused-tapered fiber coupling technology is widely used in mode converters, it also has obvious limitations. The fused-tapering process is prone to cause core deformation and mode mismatch, resulting in increased insertion loss, and it is easy to cause non-selective mode crosstalk in multi-mode or few-mode optical fibers, making it difficult to achieve efficient, single-target mode conversion. These problems seriously restrict the application prospects of fused-tapered mode converters in mode multiplexing communication systems. Therefore, there is an urgent need for a new type of optical fiber mode converter with a simple structure, easy integration, low insertion loss and high mode purity, which can break through the bottleneck of traditional technology and meet the needs of modern optical communication systems for efficient and high-quality mode conversion. Summary of the Invention

[0004] To address the aforementioned technical issues, the present invention provides a mode converter based on side-throw D-type optical fiber, designed to achieve efficient conversion between different modes within an optical fiber. This mode converter aims to overcome the shortcomings of existing technologies, significantly reducing insertion loss and crosstalk during mode conversion, improving mode conversion efficiency and output mode purity, and meeting the low-loss, high-capacity transmission requirements of optical fiber communications and mode multiplexing applications.

[0005] A mode converter based on side-throw D-type optical fiber, comprising:

[0006] Conventional D-type optical fiber, through side polishing to remove part of the side cladding to form a flat side polishing surface, while the core structure remains unchanged;

[0007] Phase-matching D-type optical fiber, with a flat side polished surface and phase matching achieved through structural control;

[0008] The side-cast planes of the conventional D-type optical fiber and the phase-matched D-type optical fiber are closely fitted to form a coupling region, thereby realizing the conversion of optical fiber modes between the two optical fibers.

[0009] Preferably, the phase matching is achieved through structural control in any of the following ways:

[0010] The optical fiber is tapered and then side-polished to keep the core structure intact;

[0011] The mode propagation constant is adjusted by removing part of the fiber core through side-throwing depth control.

[0012] Preferably, the mode conversion satisfies:

[0013] The incident light is LP n，1 / OAM n，1 When the mode is input from the phase-matched D-type fiber, the output light is LP n+m，1 / OAM n+m，1 Mode is output from conventional D-type fiber; or the incident light is LP n+m，1 / OAM n+m，1 When the mode is input from a conventional D-type optical fiber, the output light is LP n，1 / OAM n，1 The mode is output from the phase-matched D-type fiber;

[0014] Where n is the mode order and m is a natural number.

[0015] Preferably, the conventional D-type optical fiber and the phase-matched D-type optical fiber are single-mode optical fibers, few-mode optical fibers or multi-mode optical fibers, and the two are the same or different types of optical fibers.

[0016] Preferably, the tapering process causes the core mode field radius and the effective refractive index of the phase-matched D-type optical fiber to gradually change along the coupling direction, so that the target mode at a specific position satisfies the propagation constant matching with the conventional D-type optical fiber.

[0017] Preferably, the side polishing depth control is achieved by differential polishing:

[0018] Conventional D-type optical fiber retains a thin cladding layer outside the core after polishing;

[0019] The polishing depth of the phase-matched D-type optical fiber is extended to remove part of the core structure, so that the propagation constants of the target modes of the two optical fibers are matched.

[0020] Preferably, the tight fit is achieved by fixing with optical glue or packaging with a mechanical clamp, and the distance between the two optical fiber cores is smaller than the evanescent field coupling threshold.

[0021] Preferably, the mode conversion includes:

[0022] Cylindrical vector mode, linear polarization mode and orbital angular momentum mode;

[0023] Conversion between the fundamental mode and higher-order modes, or conversion between higher-order modes across m orders.

[0024] Preferably, the method further comprises coating a refractive index adjustable medium layer on the side polishing surface for dynamically adjusting the phase matching condition.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1. Low insertion loss: The two optical fibers are polished and bonded together by side surfaces for mode coupling conversion. The traditional fusion taper splicing process is not required. The structural continuity of the optical fiber core is maintained, avoiding excessive optical power loss caused by taper splicing, and significantly reducing insertion loss.

[0027] 2. High Mode Selectivity and Low Crosstalk: Utilizing precise phase matching, only the target mode is coupled and converted. Compared to multimode fiber bundles or strong fusion coupling, this method reduces the excitation of non-target modes, resulting in high output mode purity and significantly reduced mode crosstalk, meeting the inter-channel isolation requirements of mode-multiplexing communications.

[0028] 3. Simple Structure and Easy Integration: This invention utilizes an all-fiber structure, primarily composed of two sections of side-cast D-type optical fiber, resulting in a compact and compact design. The converter can be directly integrated into existing fiber systems via optical connectors, making it easy to integrate into optical communication equipment. Furthermore, it eliminates the need for complex optical components or expensive micro-nanofabrication, reducing manufacturing costs.

[0029] 4. Compatible with multiple mode conversions: By selecting different types of phase-matched optical fibers, the mode converter of the present invention can be used for multiple mode conversions, including but not limited to LP 0，1 Fundamental mode to higher-order LP n，1 Mode conversion, high-order LP n，1 Mode and LP n+m，1 conversion between modes, etc. This has broad application prospects for future optical fiber communications and mode control technologies that use multiple modes for multiplexing. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a schematic diagram of the principle of the mode converter based on the side-throw D-type optical fiber of the present invention, schematically showing the working principle of mode conversion achieved by coupling two side-throw optical fibers.

[0031] Figure 2 This is a schematic diagram of the tapered phase matching structure of the mode converter of the present invention, showing the structural form of tapering the optical fiber in the coupling region to achieve phase matching.

[0032] Figure 3 for Figure 2 The schematic diagram of the optical fiber cross section of the tapered phase matching structure shown shows the positional relationship between the two optical fibers in the coupling region cross section as well as the core and cladding distribution.

[0033] Figure 4 This is a schematic diagram of the polishing phase matching structure of the mode converter of the present invention, showing the structural form of achieving phase matching by selecting the side polishing depth and optical fiber parameters.

[0034] Figure 5 for Figure 4 The schematic diagram of the optical fiber cross section of the polished phase matching structure shows the relative position relationship of the two optical fibers in the cross section of the coupling region under the structure.

[0035] In the figure: 1. Phase-matched D-type fiber; 2. Conventional D-type fiber. DETAILED DESCRIPTION

[0036] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0037] like Figures 1 to 5 As shown:

[0038] Principle Overview: First refer to Figure 1 The overall structure of the mode converter includes a conventional D-type optical fiber 2 and a phase-matched D-type optical fiber 1. After side polishing, the flat side polished surfaces of the two are tightly fitted together to form a coupling region. Figure 1 In the figure, the upper optical fiber is phase-matched D-type optical fiber 1, and the lower optical fiber is conventional D-type optical fiber 2; the black arrow indicates that the optical signal enters the core of the phase-matched D-type optical fiber 1 from the left and propagates along it. When the optical signal enters the coupling region where the two optical fibers are side-cast together, a part of the optical field is gradually transferred from the upper optical fiber to the lower optical fiber through the evanescent field coupling of the side-cast plane. At the end of the coupling region, most of the optical power has been transferred to the conventional D-type optical fiber 2 and propagates in the optical fiber to the right end output in a set mode. For example, the input optical fiber is LP 1，1 or OAM 1，1 After conversion, LP is obtained in the output fiber 2，1 or OAM 2，1This principle diagram shows that the present invention achieves efficient conversion between specific modes through dual-fiber side-throw coupling.

[0039] Specifically, a mode converter based on side-polished D-type optical fiber is described. This mode converter comprises a conventional D-type optical fiber 2 and a phase-matched D-type optical fiber 1. Both fibers are side-polished to form flat surfaces. The side-polished surfaces of the conventional D-type optical fiber 2 and the phase-matched D-type optical fiber 1 are precisely bonded together to form an optical coupling region. Within the coupling region, modes in the optical fibers are transferred from one fiber to the other via evanescent field coupling, achieving mode conversion.

[0040] Conventional D-type fiber 2 can be a few-mode fiber or multimode fiber supporting multiple modes; phase-matched D-type fiber 1 can be a few-mode fiber or multimode fiber supporting multiple modes. After side-polishing, the distance between the core and the side-polishing surface of the two fibers is strictly controlled to adjust the effective refractive index of each light-guiding mode. When the two polished surfaces are aligned, the input mode in conventional D-type fiber 2 and the target mode in phase-matched D-type fiber 1 are designed to meet the phase matching condition, that is, the propagation constants of the two are equal or very close, thus achieving efficient directional coupling in the coupling region. After the optical signal passes through a certain length of the coupling region, the main power is transferred from the first fiber to the second fiber, achieving conversion from the input mode to the target mode, and output in the desired mode.

[0041] Furthermore, in order to achieve the required phase matching, the present invention provides two implementation structures:

[0042] 1. Tapered phase matching structure: The phase matching fiber is tapered in the coupling region so that its core mode field radius and effective refractive index gradually change along the coupling direction. By precisely controlling the dimensions of the tapered fiber, the mode propagation constant of the phase matching D-type fiber 1 at a certain position is equal to the target mode propagation constant of the conventional D-type fiber 2, forming efficient coupling at this position. Figure 2 This gradient structure ensures gradual mode coupling and reduces unnecessary loss and reflection.

[0043] 2. Polishing phase matching structure: Instead of tapering the optical fiber, phase matching is achieved by adjusting the side polishing depth or selecting specific optical fiber parameters, such as Figure 4As shown. For example, a phase-matching D-type fiber 1 with a specific structure can be selected. Its core diameter, refractive index profile, and other parameters are designed to ensure that the intrinsic propagation constant of its high-order modes is close to that of the fundamental mode in a conventional D-type fiber 2. Simultaneously, by controlling the side-throw depth of the two fibers—that is, the residual cladding thickness relative to the core plane—the effective refractive index of each mode can be further fine-tuned. When the two fibers are bonded together, phase matching conditions are met without the need for additional tapering, thereby achieving efficient mode coupling and conversion. This relatively simple structure facilitates precise control.

[0044] Example 1: Tapered phase matching structure converter. Figure 2 and Figure 3 In this embodiment, the mode converter adopts a tapered phase matching structure to achieve OAM 3，1 Mode to OAM 4，1 Specifically, a section of ring-core fiber is selected as the conventional D-type fiber 2, which supports OAM 1，1 Mode to OAM 4，1 Mode transmission; another section of the same ring-core fiber is selected as phase-matched D-type fiber 1. First, the phase-matched D-type fiber 1 undergoes a tapering process: heating the fiber and applying tension in its coupling region creates a tapered transition region, where the core and cladding diameters gradually taper along the coupling direction. The tapering process requires strict control of both the degree and length of the taper to ensure a gradual expansion of the core mode field and avoid losses caused by abrupt changes. After tapering, the phase-matched D-type fiber 1 will have a propagation constant that matches that of the conventional D-type fiber 2 at a certain point in the transition region. Next, the midsection of the two fibers undergoes side polishing: The fibers are secured in a polishing fixture and polished along the fiber axis for a distance until a flat side polished surface is formed, leaving only the very thin remaining cladding thickness from the core. A monitoring system ensures precise control of the polishing depth, allowing the evanescent field in this region to pass through the core periphery.

[0045] After completing the above pretreatment, the side-polished surfaces of the two optical fibers are precisely aligned face to face and fitted together to form a close-contact coupling interface. Use a fixture to fix the positions of the two optical fibers and keep them parallel and aligned to ensure that the relative positions of the fiber cores achieve the best coupling conditions. Figure 3 The cross-section of the coupling region shows the structure, where the cores of the upper and lower optical fibers are very close to each other. Based on this, the two optical fibers are firmly bonded together by curing optical adhesive or using other packaging methods.

[0046] Once assembled, the entire mode converter forms a coupling structure with a tapered transition. When input light enters the left core of phase-matched D-type fiber 1 and propagates to the coupling region, the light energy is gradually coupled from core 1 to core 2, as the tapered transition region satisfies the phase matching conditions with the specific high-order modes in conventional D-type fiber 2. Because the tapered structure provides a gradual mode evolution process, the entire conversion process is smooth and efficient, with extremely low insertion loss.

[0047] Example 2: Polishing the Phase Matching Structure Converter Figure 4 and Figure 5 In this embodiment, the mode converter adopts a polished phase matching structure to achieve OAM 3，1 Mode to OAM 4，1 Specifically, a section of ring-core fiber is selected as the conventional D-type fiber 2, which supports OAM 1，1 Mode to OAM 4，1 Mode transmission; another section of the same ring-core fiber is selected as the phase-matched D-type fiber 1. First, similar to Example 1, the side surfaces of the two optical fibers are polished at appropriate locations to form a flat side polished surface of sufficient length. The polishing depth is selected according to the type of optical fiber, so that the remaining cladding thickness of the conventional optical fiber core from the plane and the thickness of the phase-matched optical fiber core from the plane reach a specific ratio, thereby adjusting the effective refractive index of each guided mode of the two optical fibers. For example, the ring-core fiber can be polished until only about 1μm of cladding remains outside the core, while the phase-matched fiber can be polished until the 1.5μm core structure is destroyed. Through such differentiated polishing, the originally different mode propagation constants in the two optical fibers gradually become consistent.

[0048] After polishing, the conventional D-type optical fiber 2 is aligned with the side polished surface of the phase-matched D-type optical fiber 1. Figure 5 A schematic cross-section diagram of the bonded fibers is shown. At this point, the two fibers maintain their original diameters within the coupling region, with no tapering. A high-precision three-dimensional fine-tuning fixture adjusts the fiber positions, ensuring the two cores are as close as possible and at the appropriate relative height within the cross section, maximizing coupling.

[0049] In this polished phase-matching structure, when input light enters the coupling region along the core of phase-matched D-type fiber 1, the light field is efficiently coupled from fiber 1 to fiber 2, as the mode propagation constants of the two fibers are matched through polishing depth and core parameter design. The entire coupling process is similar to that of Example 1, also completed stepwise along the coupling length. By controlling the length of the coupling region, by the time the light reaches the end of the coupling region, most, or even nearly all, of the power has been transferred to the conventional D-type fiber 2. Because this embodiment avoids the tapering process, the structure is simpler, the manufacturing process is more easily controlled, and it can still achieve low-loss and efficient mode conversion.

[0050] Further explanation: The mode converter of the present invention can be modified and adjusted in various ways according to actual needs. For example, different types of phase-matched fibers can be selected based on the specific mode combination to be converted. For example, step-index few-mode fibers can be used to achieve LP mode conversion. In some cases, both fibers can be tapered to optimize coupling efficiency. Alternatively, while ensuring phase matching, the mode coupling characteristics can be adjusted by changing the ambient refractive index, for example, by coating the side-polished surface with a dielectric layer of a specific refractive index. All of these variations fall within the scope of protection of the present invention.

[0051] In summary, the present invention achieves efficient mode conversion without the need for fused tapers by cleverly utilizing side-thrown D-type fiber splicing and coupling. Its compact structure and excellent performance make it widely applicable to mode division multiplexing systems in fiber-optic communications and other optical fields requiring interconversion between different mode light fields. The above description of the embodiments is intended to facilitate understanding of the principles of the present invention. All equivalent substitutions and modifications made within the spirit of the present invention and the scope of the appended claims are intended to be encompassed within the scope of protection of the present invention.

[0052] The embodiments of the present invention are provided for the purpose of illustration and description. Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limitations on the present invention. Any changes, modifications, replacements and variations of the above embodiments by ordinary technicians in this field within the scope of the present invention should be included in the scope of protection of the present invention.

Claims

1. A mode converter based on side-throw D-type optical fiber, characterized in that: include: Conventional D-type optical fiber (2), wherein a portion of the side cladding is removed by side polishing to form a flat side polished surface, and the core structure remains unchanged; Phase matching D-type optical fiber (1), which is formed into a flat side polished surface by side polishing and achieves phase matching through structural control; The conventional D-type optical fiber (2) and the side-cast plane of the phase-matching D-type optical fiber (1) are closely fitted to form a coupling region, thereby realizing the conversion of optical fiber modes between the two optical fibers.

2. A mode converter based on side-throw D-type optical fiber according to claim 1, characterized in that: The phase matching is achieved through structural control in any of the following ways: The optical fiber is tapered and then side-polished to keep the core structure intact; The mode propagation constant is adjusted by removing part of the fiber core through side-throwing depth control.

3. A mode converter based on side-throw D-type optical fiber according to claim 1, characterized in that: The mode conversion satisfies: The incident light is LP n，1 / OAM n，1 When the mode is input from the phase-matched D-type fiber (1), the output light is LP n+m，1 / OAM n+m，1 Mode is output from the conventional D-type optical fiber (2); or the incident light is LP n+m，1 / OAM n+m， When 1 mode is input from a conventional D-type optical fiber (2), the output light is LP n，1 / OAM n，1 The mode is output from the phase-matched D-type optical fiber (1); Where n is the mode order and m is a natural number.

4. A mode converter based on side-throw D-type optical fiber according to claim 1, characterized in that: The conventional D-type optical fiber (2) and the phase-matched D-type optical fiber (1) are single-mode optical fibers, few-mode optical fibers or multi-mode optical fibers, and the two are optical fibers of the same or different types.

5. A mode converter based on side-throw D-type optical fiber as claimed in claim 2, characterized in that: The tapering process causes the core mode field radius and the effective refractive index of the phase-matching D-type optical fiber (1) to gradually change along the coupling direction, and satisfies the propagation constant matching with the target mode of the conventional D-type optical fiber (2) at a specific position.

6. A mode converter based on side-throw D-type optical fiber as claimed in claim 2, characterized in that: The side polishing depth control is achieved through differentiated polishing: Conventional D-type optical fiber (2) retains a thin cladding layer outside the core after polishing; The polishing depth of the phase-matching D-type optical fiber (1) is extended to remove part of the core structure, so that the propagation constants of the target modes of the two optical fibers are matched.

7. A mode converter based on side-throw D-type optical fiber according to claim 1, characterized in that: The tight fit is achieved by fixing with optical glue or packaging with a mechanical clamp, and the distance between the two optical fiber cores is smaller than the evanescent field coupling threshold.

8. The mode converter based on side-throw D-type optical fiber according to claim 1, characterized in that: The mode conversion includes: Cylindrical vector mode, linear polarization mode and orbital angular momentum mode; Conversion between the fundamental mode and higher-order modes, or conversion between higher-order modes across m orders.

9. The mode converter based on side-throw D-type optical fiber according to claim 1, characterized in that: The method also includes coating a refractive index tunable medium layer on the side polishing surface for dynamically adjusting the phase matching conditions.

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