A multimode optical fiber with uniform light field distribution and a preparation method thereof

Abstract

The present application relates to a kind of multimode optical fiber of light field distribution uniformity and its preparation method, the optical fiber includes core layer and cladding and resin coating wrapped in the outer circumferential surface of cladding, it is characterized in that in the outer circumferential surface of optical fiber, microbend sleeve is tightly sleeved, the inner hole of microbend sleeve is provided with multiple protrusions, the protrusions apply multiple compressive stress to the outer circumferential surface of optical fiber, form multidimensional stress gradient field, promote the transfer of transmission light from low-order mode to high-order mode.The present application is simple in structure and strong in compatibility, by setting and adjusting microbend structure parameter, according to specific application scene and working wavelength, mode coupling intensity and efficiency can be flexibly controlled, under the premise that optical fiber loss is not significantly increased, the controllable energy redistribution between low-order mode and high-order mode of optical signal is realized, the mode energy distribution uniformity is significantly improved, can meet the application demand of high stability optical branching and energy distribution.

Description

Technical Field

[0001] This invention relates to a multimode optical fiber with uniform optical field distribution and its fabrication method, belonging to the field of optical fiber communication and optical transmission technology. Background Art Multimode fiber has become the main transmission medium for short-distance optical communication networks due to its lower system cost and higher data transmission rate, and is widely used in data center interconnection, local area networks, high-performance computing centers and vehicle-mounted optical communication systems.

[0002] In multimode fiber, optical signals are transmitted simultaneously through multiple modes. However, due to differences in the propagation characteristics of different modes and the influence of external environmental factors (such as temperature changes and mechanical vibrations), the energy distribution among modes in multimode fiber often exhibits instability, leading to uneven distribution of the output optical field. This phenomenon is particularly pronounced in passive optical devices such as multimode optical splitters and couplers, severely affecting the stability and accuracy of optical power allocation. To improve the mode energy distribution in multimode fiber, methods such as mode scramblers or special core designs are commonly used. However, these solutions have many limitations: mode scramblers have complex structures and cause significant insertion loss; special core designs are costly, have complex fabrication processes, poor compatibility with standard multimode fiber, and are difficult to deploy directly in existing systems. Particularly prominent is the excessively high proportion of low-order mode energy in conventional multimode fiber, resulting in concentrated and unevenly distributed energy at the center of the output spot. This characteristic constitutes a technical bottleneck in applications requiring uniform power allocation (such as multi-path optical splitting, optical sensing, and laser energy transmission). Currently, there is a lack of a simple, compatible, passive, and low-loss solution that can effectively control the energy distribution between modes while maintaining the basic transmission performance of multimode fiber. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a multimode optical fiber with uniform optical field distribution and its preparation method, which not only promotes energy transfer from low-order modes to high-order modes and achieves uniformity of output optical field distribution, but also maintains compatibility with standard multimode optical fibers.

[0004] The multimode optical fiber technology solution adopted by the present invention to solve the above-mentioned problems is as follows: it includes a core layer, a cladding layer, and a resin coating covering the outer peripheral surface of the cladding layer. The feature is that a micro-bent sleeve is tightly fitted on the outer periphery of the optical fiber. The inner hole of the micro-bent sleeve is provided with multiple protrusions. The protrusions apply multiple local compressive stresses to the outer peripheral surface of the optical fiber, forming a multi-dimensional stress gradient field, which promotes the transmission of light from a low-order mode to a high-order mode.

[0005] According to the above scheme, the micro-bent sleeve includes a sleeve body, with multiple protrusions spaced apart in the inner hole of the sleeve body. The axial length of a single micro-bent sleeve is 10~500mm, and (multiple micro-bent sleeves) are spaced apart or continuously and tightly fitted onto the optical fiber.

[0006] According to the above scheme, the protrusion is one or more of the following: dot-shaped protrusion, block-shaped protrusion, and strip-shaped protrusion.

[0007] According to the above scheme, the radial protrusion height of the protrusion relative to the inner wall of the micro-bent sleeve is 3~500μm, and the center distance between adjacent protrusions is 5~5000μm.

[0008] According to the above scheme, the micro-bent sleeve is tightly fitted onto the cladding of the optical fiber, and the convexity acts on the outer peripheral surface of the cladding.

[0009] According to the above scheme, the micro-bent sleeve is tightly fitted onto the resin coating of the optical fiber, the protrusion acts on the outer peripheral surface of the resin coating, and the resin coating is an outer resin coating or an inner resin coating.

[0010] According to the above scheme, the cladding diameter of the optical fiber is 125±1μm, the core layer has a step-type or graded refractive index distribution, and the core diameter is 50μm, 62.5μm or 105μm.

[0011] According to the above scheme, the sleeve body of the micro-bent sleeve is a closed sleeve structure or a C-shaped sleeve structure with a longitudinal opening, and the longitudinal opening width of the C-shaped sleeve structure is 10~200μm.

[0012] According to the above scheme, the strip-shaped protrusion is annular, spiral, or corrugated, and the strip-shaped protrusion is a continuous strip-shaped protrusion structure or an intermittent (discontinuous) strip-shaped protrusion structure. The cross-section of the strip-shaped protrusion is V-shaped, U-shaped, trapezoidal, rectangular, or semi-circular.

[0013] According to the above scheme, the dot-shaped protrusions are regularly or randomly distributed particles or micro-ridge structures, and the material of the dot-shaped protrusions is one or more of the following: corundum, silicon carbide, silicon dioxide, metal materials, metal oxides, glass, and polymer materials.

[0014] According to the above scheme, the micro-bent sleeve is composed of one or more of ultraviolet curable resin, thermoplastic polymer, thermosetting resin, elastomer material, and metal-polymer composite material, and the outer diameter of the micro-bent sleeve is 200~3000μm.

[0015] The technical solution of the multimode optical fiber fabrication method of the present invention includes the following steps: Select multimode fiber, identify the microbend section, remove the resin coating of the microbend section to expose the fiber cladding, or retain part or all of the resin coating of the microbend section. Fabrication of a micro-bent sleeve preform with a raised inner hole. The micro-bend tube preform is precisely fitted onto the predetermined micro-bend section of the optical fiber, so that the protruding structure of the micro-bend tube forms a predetermined contact relationship with the outer peripheral surface of the optical fiber. The micro-bent sleeve is tightly wrapped and fixed to the micro-bent section of the optical fiber through a compression process, so that the protruding structure applies multiple compressive stresses to the outer peripheral surface of the optical fiber, forming a multi-dimensional stress gradient field. Finally, the prepared optical fiber was subjected to mode distribution testing and parameter optimization to ensure that the preset mode coupling efficiency was achieved.

[0016] According to the above scheme, the tightening process for C-shaped sleeve structure micro-bent sleeve with longitudinal opening directly aligns the opening with the optical fiber and engages it with the outer peripheral surface of the optical fiber through elastic deformation.

[0017] According to the above scheme, the shrinking process uses heat shrinking, ultraviolet curing, chemical cross-linking, mechanical compression or bonding to tightly wrap and fix the micro-bent sleeve to the micro-bent section of the optical fiber.

[0018] The beneficial effects of this invention are as follows: 1. It has a simple structure and strong compatibility, and can be directly implemented on the basis of standard multimode fiber without changing the architecture of the existing optical fiber communication system; 2. It adopts a passive working mode and realizes mode energy redistribution through a controllable microbending structure, without the need for external energy input, which improves system reliability and reduces overall power consumption; 3. By setting and adjusting the parameters of the microbending structure, the mode coupling strength and efficiency can be flexibly controlled according to the specific application scenario and working wavelength. Under the premise of not significantly increasing fiber loss, it realizes controllable energy redistribution of optical signals between low-order and high-order modes, significantly improves the uniformity of mode energy distribution, and can meet the application requirements of high-stability optical splitting and energy distribution; 4. The fabrication method is simple and can provide a variety of implementation schemes, including short-length sleeves, segmented sleeves, C-shaped open sleeves, etc., which are convenient for application and promotion; 5. It has a wide range of applications and is suitable for various optical devices and systems such as vehicle communication networks, multimode fiber sensing systems, laser energy transmission, biomedical imaging, and optical splitting devices. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a radial cross-section structure according to an embodiment of the present invention.

[0020] Figure 2 This is a schematic diagram of an axial cross-sectional structure according to an embodiment of the present invention.

[0021] Figure 3 This is a comparison diagram of the output ring luminous flux distribution of one embodiment of the present invention and a comparative example.

[0022] Figure 4 This is a schematic diagram of the axial cross-sectional structure of another embodiment of the present invention. Detailed Implementation

[0023] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0024] Example 1: Example 1 of the present invention is as follows Figure 1 , Figure 2 As shown, the device includes an optical fiber and a micro-bent sleeve. The optical fiber is a graded-index multimode optical fiber conforming to the IEC 60793-2-10 standard, comprising a core layer 1, a cladding layer 2, and a resin coating 3 covering the outer periphery of the cladding layer. The core layer diameter is 50±2.5μm, the cladding layer diameter is 125±1μm, and the resin coating diameter is 245±10μm. The micro-bent sleeve 4 is tightly fitted onto the resin coating of the optical fiber. The inner hole of the micro-bent sleeve has multiple protrusions 5, which act on the outer periphery of the resin coating. The micro-bent sleeve is made of thermoplastic polyurethane (TPU) material, with an outer diameter of 500±10μm and an axial length of 150mm, facilitating insertion and installation from the end of the optical fiber. When a 450mm working section is required, three micro-bent sleeves of the same specification are arranged sequentially along the optical fiber axis, with a spacing of 20mm between adjacent sleeves. The inner wall surface of the micro-bent sleeve has corundum-material protruding particles with an average particle diameter of 46μm. When an optical signal is transmitted in an optical fiber, the micro-bent sleeve applies non-uniform mechanical stress to the surface of the optical fiber through the inner protrusions, forming a multi-dimensional stress gradient field. This disrupts the stable transmission conditions of low-order modes and causes optical energy to transfer from low-order modes to high-order modes.

[0025] A multidimensional stress gradient field refers to a composite stress distribution formed by the combined effect of non-uniform compressive stresses applied by multiple protrusions in different directions such as the optical fiber axis, radial direction, and angular direction. This asymmetric, multi-directional stress disturbance can effectively disrupt the orthogonality of transmission modes in the optical fiber, thereby promoting energy coupling between different modes, especially transferring the energy of low-order modes originally concentrated at the core center to high-order modes far from the center, thus achieving homogenization of the output optical field.

[0026] Comparative Example 1 uses standard GI50 / 125 graded-index multimode fiber of the same specifications and length as in this embodiment, without any micro-bend sleeve structure.

[0027] Test results show that at a wavelength of 850 nm, when the optical signal passes through a 450 mm long micro-bent structure region, the proportion of the encircled flux (EF) at the output end within the 4.5 μm radius region is significantly reduced (e.g., ...). Figure 3 As shown in the figure, the mode bandwidth was increased by 38% from 23% in Comparative Example 1 to 9%, with an additional loss of only 0.15dB.

[0028] Example 2: This example provides a micro-bend cladding multimode optical fiber, such as... Figure 3 As shown, the substrate fiber is a graded-index multimode fiber conforming to the IEC 60793-2-10 standard, with a core diameter of 50±2.5μm, a cladding diameter of 125±1μm, and an outer coating diameter of 250±15μm. Micro-bent sleeves are directly fitted onto the outside of the substrate fiber cladding, i.e., the outer coating is pre-removed from a 1500mm long area (consisting of three 500mm long micro-bent sleeves connected in series). The micro-bent sleeves are made of UV-cured acrylate resin material, with an outer diameter of 260±5μm and an inner diameter of 130±2μm.

[0029] The inner side of the micro-bent sleeve is provided with a spiral or wavy protrusion structure, with a protrusion height of 40 μm, a protrusion width of 80 μm, a protrusion period of 180 μm, and a U-shaped protrusion cross-section. This combination of parameters can generate a periodic micro-bending stress field on the cladding surface without causing microcracks.

[0030] To prevent the exposed cladding from weakening at the raised contact points due to environmental humidity or subsequent operations, a low-refractive-index flexible silicone sealant is applied to the outside of the cladding before installation. After installation, it is cured under ultraviolet light to form a buffer protective layer. This protective layer has a lower refractive index than the cladding, which not only fixes the sleeve position but also isolates moisture and disperses localized point stress.

[0031] Tested at a wavelength of 1300nm, when the optical signal passes through a 1500mm long micro-bend structure region and is input to a 1×8 optical splitter, the output power fluctuation is reduced from ±1.2dB of the original fiber to ±0.4dB, which significantly improves the splitting uniformity, and the total insertion loss increases by no more than 0.3dB.

[0032] This embodiment specifically verifies the impact of the protrusion width parameter on performance. While keeping the protrusion height constant at 40 μm and the period constant at 180 μm, comparative tests were conducted by adjusting the protrusion width. The results show that when the protrusion width is within the range of 65-105 μm, the mode energy distribution at 1300 nm wavelength is the most uniform; when the width is less than 65 μm, the stress is too concentrated, leading to a decrease in fiber mechanical reliability; when the width is greater than 105 μm, the stress gradient is insufficient, and the mode coupling efficiency is significantly reduced.

[0033] Example 3: This example provides a method for fabricating a micro-bent tube multimode optical fiber with a randomly distributed semi-circular protrusion structure: 1. Select graded-index multimode fiber (GI50 / 125) conforming to IEC 60793-2-10 standard, and remove the middle 2000mm length. The outer coating of the temperature zone (consisting of 5 zones 400 mm long) was cleaned with anhydrous ethanol; 2. A thermosetting epoxy resin sleeve with an inner diameter of 255 μm and an outer diameter of 450 μm was manufactured using precision mold injection molding. Its inner surface... It has randomly distributed semi-circular protrusions, with protrusion height ranging from 3μm to 200μm, protrusion width ranging from 5μm to 150μm, and protrusion spacing ranging from 10μm to 1900μm; 3. After softening the micro-bent sleeve by heating it to 120℃, place it onto the treated optical fiber area; 4. Slowly cool while applying a radial pressure of 0.1 MPa to ensure the micro-bent sleeve tightly adheres to the optical fiber; 5. Curing at 60℃ for 2 hours completes the fabrication of micro-bent cladding multimode optical fiber.

[0034] Tests show that in 980nm laser transmission applications, the power fluctuation of this optical fiber is reduced from ±1.5dB to ±0.4dB, significantly improving the uniformity of optical energy distribution. Furthermore, its performance remains stable after 10,000 bending tests, demonstrating the mechanical reliability of this invention.

[0035] This embodiment specifically verifies the impact of the protrusion aspect ratio on performance. Experimental data shows that when the protrusion aspect ratio is between 0.6 and 4.0, the mode coupling efficiency at different wavelengths can achieve good results, with additional loss less than 0.25 dB; when the aspect ratio is less than 0.6, the stress is too concentrated, and the transmission loss increases significantly; when the aspect ratio is greater than 4.0, the stress gradient is insufficient, and the mode coupling efficiency decreases significantly.

[0036] Example 4: This example provides a multimode optical fiber with an extremely small microbend structure, with the same substrate fiber parameters as in Example 1. Spherical protrusions made of silica are distributed inside the microbend sleeve, with an average particle diameter of 8 μm (near the lower limit of the parameter range of 3 μm to 500 μm), an adjacent particle spacing of 15 μm, and a total distribution area length of 50 mm (composed of a single 50 mm long microbend sleeve). The microbend sleeve is made of highly elastic silicone rubber material with an outer diameter of 300 μm and is fixed to the outside of the substrate fiber coating by UV curing. Tests show that at a wavelength of 850 nm, this structure increases the radius of the output spot's 86% ring luminous flux (EF) energy share from 11.5 μm to 15.3 μm, significantly improving the higher-order mode energy share, with an additional loss of 0.08 dB, proving the feasibility of this invention at the lower limit of the parameter range.

[0037] Example 5: This example provides a multimode optical fiber with a large-size microbending structure. The substrate fiber is a 62.5 / 125μm graded-index multimode fiber. Cubic protrusions made of silicon carbide are distributed inside the microbending sleeve. The protrusion height is 480μm, the protrusion width is 450μm, and the center-to-center distance between adjacent protrusions is 4800μm (within the range of 5μm to 5000μm). The total length of the distribution area is 4500mm (composed of nine 500mm long microbending sleeves connected in series). The microbending sleeve is made of high-hardness thermoplastic polyester material with an outer diameter of 2950μm and is fixed to the outside of the substrate fiber coating by mechanical compression. Tests show that at a wavelength of 1300nm, this structure improves the output uniformity of a 1×16 optical splitter from ±1.8dB to ±0.5dB while maintaining a total loss of <0.25dB, demonstrating the technical feasibility of this invention at the upper limit of the parameter range.

[0038] Example 6: The micro-bend sleeve in Example 1 is replaced with a C-shaped opening. The micro-bend sleeve has a longitudinal opening with a width of 80μm, which can be directly snapped onto the installed optical fiber without needing to be inserted from the end of the optical fiber.

[0039] Installation tests show that the C-shaped opening micro-bend sleeve can be installed within 5 seconds. After 1000 bending tests (bending radius 15mm), the bonding strength between the C-shaped opening micro-bend sleeve and the optical fiber decreased by no more than 5%, demonstrating the mechanical reliability of the structure. Furthermore, the non-circular, asymmetrical, non-uniform mechanical stress generated by the C-shaped opening structure further disrupts the orthogonality of the transmission modes, effectively improving mode coupling efficiency.

[0040] Example 7: To verify the parameter boundary effect, this example uses standard GI50 / 125 multimode fiber, with a rectangular protrusion structure (height 100μm, width 200μm, period 1000μm) and length 1000mm set inside the micro-bend sleeve. Test results under these parameters show that at 850nm wavelength, the energy percentage at the center of the output spot decreases from 23% to 12%, the mode bandwidth increases by 45%, and the additional loss is 0.12dB. When the protrusion height approaches the parameter boundary (3μm or 500μm), the additional loss increases to 0.35dB or the mode coupling efficiency decreases by 30%.

[0041] This invention is applicable not only to 50 / 125μm multimode optical fiber, but also to other specifications of multimode optical fiber such as 62.5 / 125μm and 105 / 125μm, as well as various applications in the visible to infrared band (400nm to 1600nm).

Claims

1. A multimode optical fiber with uniform optical field distribution, comprising a core layer, a cladding layer, and a resin coating covering the outer periphery of the cladding layer, characterized in that... A micro-bend sleeve is tightly fitted around the outer periphery of the optical fiber. The inner hole of the micro-bend sleeve is provided with multiple protrusions. The protrusions apply multiple local compressive stresses to the outer periphery of the optical fiber, forming a multi-dimensional stress gradient field, which causes the transmitted light to transfer from a low-order mode to a high-order mode.

2. The multimode optical fiber with uniform optical field distribution according to claim 1, characterized in that... The micro-bent sleeve includes a sleeve body with multiple protrusions spaced apart in the inner hole of the sleeve body.

3. The multimode optical fiber with uniform optical field distribution according to claim 1 or 2, characterized in that... The individual axial length of the micro-bent sleeve is 10~500mm, and it is tightly fitted onto the optical fiber at intervals or continuously.

4. The multimode optical fiber with uniform optical field distribution according to claim 1 or 2, characterized in that... The protrusions are one or more of the following: dot-shaped protrusions, block-shaped protrusions, and strip-shaped protrusions.

5. The multimode optical fiber with uniform optical field distribution according to claim 1 or 2, characterized in that... The radial protrusion height of the protrusion relative to the inner wall of the micro-bent sleeve is 3~500μm, and the center-to-center distance between adjacent protrusions is 5~5000μm.

6. The multimode optical fiber with uniform optical field distribution according to claim 1 or 2, characterized in that... The micro-bend sleeve is tightly fitted onto the cladding of the optical fiber, and the convexity acts on the outer peripheral surface of the cladding.

7. The multimode optical fiber with uniform optical field distribution according to claim 1 or 2, characterized in that... The micro-bent sleeve is tightly fitted onto the resin coating of the optical fiber, and the protrusion acts on the outer peripheral surface of the resin coating, which is either an outer resin coating or an inner resin coating.

8. The multimode optical fiber with uniform optical field distribution according to claim 1 or 2, characterized in that... The optical fiber has a cladding diameter of 125±1μm, and the core layer has a step- or graded refractive index distribution with a core diameter of 50μm, 62.5μm or 105μm.

9. The multimode optical fiber with uniform optical field distribution according to claim 1 or 2, characterized in that... The sleeve body of the micro-bent sleeve is a closed sleeve structure or a C-shaped sleeve structure with a longitudinal opening, wherein the longitudinal opening width of the C-shaped sleeve structure is 10~200μm.

10. The multimode optical fiber with uniform optical field distribution according to claim 4, characterized in that... The strip-shaped protrusions are annular, spiral, or corrugated, and are either continuous or spaced strip-shaped protrusions.

11. The multimode optical fiber with uniform optical field distribution according to claim 10, characterized in that... The cross-section of the strip-shaped protrusion is V-shaped, U-shaped, trapezoidal, rectangular, or semi-circular.

12. The multimode optical fiber with uniform optical field distribution according to claim 4, characterized in that... The dot-like protrusions are regularly or randomly distributed particles or micro-ridge structures.

13. The multimode optical fiber with uniform optical field distribution according to claim 12, characterized in that... The material of the dot-like protrusions is one or more of the following: corundum, silicon carbide, silicon dioxide, metallic materials, metal oxides, glass, and polymer materials.

14. The multimode optical fiber with uniform optical field distribution according to claim 1 or 2, characterized in that... The micro-bent sleeve is composed of one or more of ultraviolet-curable resin, thermoplastic polymer, thermosetting resin, elastomer material, and metal-polymer composite material, and the outer diameter of the micro-bent sleeve is 200~3000μm.

15. A method for fabricating a multimode optical fiber with uniform optical field distribution according to any one of claims 1-14, characterized in that: Select multimode fiber, identify the microbend section, remove the resin coating of the microbend section to expose the fiber cladding, or retain part or all of the resin coating of the microbend section. Fabrication of a micro-bent sleeve preform with a raised inner hole. The micro-bend tube preform is precisely fitted onto the predetermined micro-bend section of the optical fiber, so that the protruding structure of the micro-bend tube forms a predetermined contact relationship with the outer peripheral surface of the optical fiber. The micro-bent sleeve is tightly wrapped and fixed to the micro-bent section of the optical fiber through a compression process, so that the protruding structure applies multiple compressive stresses to the outer peripheral surface of the optical fiber, forming a multi-dimensional stress gradient field. Finally, the prepared optical fiber was subjected to mode distribution testing and parameter optimization to ensure that the preset mode coupling efficiency was achieved.

16. The method for fabricating a multimode optical fiber with uniform optical field distribution according to claim 15, characterized in that... The aforementioned shrinking process uses heat shrinking, UV curing, chemical cross-linking, mechanical compression, or bonding to tightly wrap and fix the micro-bent sleeve to the micro-bent section of the optical fiber.

17. The method for fabricating a multimode optical fiber with uniform optical field distribution according to claim 15, characterized in that... The aforementioned compression process involves directly aligning the opening of the C-shaped sleeve structure micro-bent sleeve with a longitudinal opening with the optical fiber, and then elastically deforming and engaging it onto the outer circumferential surface of the optical fiber.

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