Cladding mode efficient filtering method for microstructure optical fiber

Abstract

The invention provides a cladding mode efficient filtering method for microstructure optical fibers, and relates to the technical field of optical fibers, and the method comprises the steps: taking two microstructure optical fibers which are different in structure and have a diameter difference value of a fiber core fundamental mode field within a diameter difference value range, and carrying out the non-oblique-angle cutting of the end faces of the two microstructure optical fibers through a cutting knife; and aligning the axial directions of the two sections of cut micro-structure optical fibers, and splicing at the cutting end surface to obtain a spliced micro-structure optical fiber. The method has the beneficial effects that two sections of microstructure optical fibers with different structures but similar fiber core fundamental mode field diameters are selected, oblique-angle-free cutting is carried out, splicing (welding, butt joint or coupler coupling) is carried out at the cutting end face, and it is ensured that splicing points are axially coaxial but cladding waveguide structures have differences. At a continuous connection point, because the cladding microstructures (such as air hole arrangement, number, shape and the like) of the two sections of optical fibers are discontinuous, the continuity of the cladding waveguide is damaged. Therefore, cladding mode energy is efficiently filtered or scattered, and the strength of the cladding mode energy is greatly weakened.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber technology, and in particular to a method for efficiently filtering out the cladding mode of a microstructured optical fiber. Background Art

[0002] Microstructure optical fiber (MOF) exhibits excellent optical properties due to its unique photonic structure (such as periodically arranged air holes) and diverse light-guiding mechanisms (such as solid-core photonic crystal fiber, photonic bandgap fiber, and antiresonant fiber). The synergistic effects of the core, cladding, and jacket regions make it widely used in lasers, communication systems, and other fields. Efficiently coupling optical signals into microstructured optical fibers is a key step in realizing their application. Common methods include lens free-space coupling, connector butt coupling, and pigtail fusion coupling. However, regardless of the coupling method used, there is a common mode field mismatch between the light field output by the light source and the fundamental mode light field actually supported by the core of the microstructured optical fiber. This mismatch causes a portion of the incident light to inevitably couple into the cladding microstructure region of the optical fiber, forming a cladding mode.

[0003] While these cladding modes themselves may not have high transmission losses, they propagate in parallel with the signal light (fundamental mode) transmitted in the fiber core. The presence of these cladding modes can have significant negative consequences: on the one hand, they may carry some energy and propagate through the fiber, causing signal energy dispersion; on the other hand, and more critically, these cladding modes can act as noise sources, crosstalking with the fiber core fundamental mode signal or generating nonlinear and scattering effects, significantly degrading the signal-to-noise ratio (SNR) of the transmitted signal and impacting system performance.

[0004] Currently, in the application of microstructured optical fibers, especially in the field of splicing technology between different types of microstructured optical fibers (including fusion splicing, butt splicing, and coupler coupling), technological development mainly focuses on reducing splicing losses, ensuring mechanical strength, and maintaining mode matching (mainly referring to the core fundamental mode). However, existing splicing technologies generally have a major technical flaw: they completely ignore the need to effectively filter out the existing cladding mode at the splicing point.

[0005] Existing splicing processes and methods do not actively utilize or design splicing structures to introduce high-loss mechanisms for cladding modes. Consequently, cladding modes excited in the fiber before the splice typically continue to transmit with low loss to the next section of fiber after passing the splice. The resulting noise interference persists throughout the link and can accumulate, unable to be effectively suppressed or eliminated through the splicing process. This has become a key bottleneck limiting the further performance improvement of microstructured fiber systems, particularly those involving cascades of multiple MOFs. Summary of the Invention

[0006] In view of the problems existing in the prior art, the present invention provides a method for efficiently filtering the cladding mode of a microstructured optical fiber, comprising:

[0007] Step S1, taking two sections of microstructured optical fibers with different structures and a diameter difference of the core fundamental mode field within a preset diameter difference range, and performing non-bevel cutting on the end faces of the two sections of the microstructured optical fibers using a cleaver;

[0008] Step S2: After aligning the axial directions of the two cut sections of the microstructured optical fiber, splicing them at the cut end faces to obtain a spliced ​​microstructured optical fiber.

[0009] Preferably, the connection method in step S2 includes direct fiber connection, fiber fusion splicing, or coupling two sections of optical fiber via a coupler, so as to keep the transmission light from one section of the microstructured optical fiber into another section of the microstructured optical fiber.

[0010] Preferably, the types of the two sections of microstructured optical fibers include at least one of solid-core photonic crystal fibers, hollow-core / solid-core antiresonant fibers, and hollow-core / solid-core photonic bandgap fibers, and at most two of the types.

[0011] Preferably, the two sections of microstructured optical fibers are microstructured optical fibers of the same type but different structures.

[0012] Preferably, the two sections of microstructured optical fibers are microstructured optical fibers of different types.

[0013] Preferably, after step S2, the method further includes:

[0014] Then, at least one section of microstructured optical fiber is selected, and its end face is cut at a non-bevel angle with a cutting knife, and then the end face of the subsequent microstructured optical fiber is sequentially cascaded and spliced.

[0015] Preferably, the microstructured optical fibers at both ends of each splice are microstructured optical fibers with different structures and matching mode fields of the fiber core fundamental modes.

[0016] Preferably, the diameter difference range is ±7%.

[0017] The above technical solution has the following advantages or beneficial effects: by selecting two sections of microstructured optical fiber with different structures but similar core fundamental mode field diameters, performing non-bevel cleavage, and then splicing them at the cleaved end faces (fusion splicing, butt-jointing, or coupler coupling), the axial coaxiality at the splice point is ensured, but the cladding waveguide structure is different. At the splice point, the cladding microstructure of the two sections (such as the arrangement, number, and shape of the air holes) is discontinuous, which destroys the continuity of the cladding waveguide. As a result, the cladding mode energy previously transmitted from the optical fiber is efficiently filtered out or scattered, significantly weakening its intensity. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1FIG1 is a flow chart of a method for efficiently filtering the cladding mode of a microstructured optical fiber in a preferred embodiment of the present invention;

[0019] Figure 2 FIG1 is a schematic diagram of the cross-sectional structure of two sections of hollow-core antiresonant optical fibers with different structures and similar core fundamental mode fields in a preferred embodiment of the present invention;

[0020] Figure 3 for Figure 2 Schematic diagrams of the core fundamental mode field and cladding mode field of hollow-core antiresonant optical fibers with different structures and similar core fundamental mode fields are provided;

[0021] Figure 4 A schematic diagram of the splicing operation of microstructured optical fibers with different structures and similar core fundamental mode fields provided by an embodiment of the present invention;

[0022] Figure 5 The mode scattering conditions of different microstructure optical fibers after splicing are obtained through finite element simulation provided in the embodiment of the present invention;

[0023] Figure 6 A schematic diagram of cascading and connecting optical fibers with different microstructures provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0024] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. The present invention is not limited to this embodiment, and other embodiments may also fall within the scope of the present invention as long as they conform to the gist of the present invention.

[0025] In a preferred embodiment of the present invention, based on the above problems existing in the prior art, a method for efficiently filtering the cladding mode of a microstructured optical fiber is provided. Figure 1 Shown, including:

[0026] Step S1, taking two sections of microstructured optical fibers with different structures and a diameter difference of the core fundamental mode field within a preset diameter difference range, and performing non-bevel cutting on the end faces of the two sections of microstructured optical fibers respectively with a cleaver;

[0027] Step S2: After aligning the axial directions of the two cut sections of microstructured optical fiber, splicing them at the cut end faces to obtain a spliced ​​microstructured optical fiber.

[0028] Preferably, the diameter difference range is ±7%.

[0029] Specifically, in this embodiment, a microstructured optical fiber sample is prepared after splicing; light is coupled into the optical fiber from one open end of the optical fiber sample. Since the cladding waveguide structure of the microstructured optical fiber is different at the splicing point, the continuity of the cladding waveguide is destroyed, and the cladding mode is efficiently filtered out in the mode field output from the other open end of the optical fiber sample.

[0030] In this embodiment, by selecting two sections of microstructured optical fibers with different structures but similar core fundamental mode mode field diameters (it is stipulated here that the fundamental mode field diameters ω1 and ω2 of the core fundamental mode field of the two sections of microstructured optical fibers are considered to be similar if the difference is within ±7%, and assuming that the mode field is close to the Gaussian light field, the coupling efficiency is 0.01% under the mode field matching condition. ), perform non-bevel cutting and perform splicing (fusion splicing, docking or coupler coupling) at the cut end face to ensure that the axes are coaxial at the splicing point but there are differences in the cladding waveguide structure. At the splicing point, due to the discontinuity of the cladding microstructure of the two sections of optical fiber (such as the arrangement, number, shape, etc. of the air holes), the continuity of the cladding waveguide is destroyed. This discontinuity introduces extremely high scattering loss to the mode (cladding mode) transmitted in the cladding. Thereby, the cladding mode energy previously transmitted from the optical fiber is efficiently filtered out or scattered, greatly weakening its intensity. This directly solves the core problem of the background technology that the cladding mode is continuously transmitted and generates crosstalk with the core signal, reducing the signal-to-noise ratio (SNR), and achieves the technical effect of efficiently filtering out the cladding mode and significantly suppressing noise interference.

[0031] In this embodiment, when selecting optical fibers, it is particularly required that the core fundamental mode mode field diameters of the two sections of optical fibers are similar, and axial coaxial alignment is ensured during splicing. The matching of the core fundamental mode mode field diameters and the careful axial alignment enable the fundamental mode optical signal transmitted in the core to achieve high-efficiency, low-loss coupling when passing through the splicing point (the goals pursued by the existing splicing technology in the background art are still met here). Therefore, this filtering method specifically targets harmful cladding modes for high-loss processing, while causing almost no additional loss or interference to the useful core fundamental mode signal. This ensures that the main body of signal transmission (fundamental mode) is not affected, and is the key basis for improving the signal-to-noise ratio (SNR) of the final output signal. It can ensure low-loss transmission of the core fundamental mode and maintain signal quality.

[0032] The present invention's efficient cladding mode filtering method cleverly exploits the inherent cladding waveguide discontinuity at the splice point of microstructured optical fibers with different structures (generally considered a defect to be avoided). By precisely controlling the core fundamental mode mode field matching and axial alignment, this discontinuity is transformed into a mechanism for highly efficient and selective cladding mode filtering. This method directly addresses and resolves the key drawback identified in the background art: existing splicing techniques completely ignore cladding mode filtering, resulting in persistent cladding mode noise and degraded signal-to-noise ratio. This goal is achieved using conventional, low-cost processes with virtually no loss of the primary signal (core fundamental mode).

[0033] In a preferred embodiment of the present invention, the splicing method in step S2 includes direct fiber connection, fiber fusion splicing, or coupling two sections of optical fiber via a coupler to keep the transmitted light from one section of microstructured optical fiber into another section of microstructured optical fiber.

[0034] Specifically, the core operation of this embodiment is based on conventional microstructured optical fiber splicing processes (no bevel cutting, axial alignment, fusion / butt splicing, or coupler coupling), eliminating the need for additional complex filter components (such as long fiber attenuation sections, mode-selective couplers, or specialized gratings). This fully utilizes existing, mature optical fiber processing technologies (cleaving, fusion splicing, etc.), making the implementation of cladding mode filtering extremely simple and cost-effective.

[0035] In a preferred embodiment of the present invention, the two sections of microstructured optical fibers include at least one of solid core photonic crystal fibers, hollow core / solid core antiresonant fibers, and hollow core / solid core photonic bandgap fibers, and at most two of the following.

[0036] Furthermore, in this embodiment, the two sections of microstructured optical fibers are microstructured optical fibers of the same type but different structures.

[0037] Furthermore, in this embodiment, the two sections of microstructured optical fibers are of different types.

[0038] Specifically, the microstructured optical fiber in this embodiment may be a solid-core photonic crystal fiber, a hollow-core / solid-core antiresonant optical fiber, or a hollow-core / solid-core photonic bandgap optical fiber.

[0039] The two selected sections of different microstructured optical fibers can be a combination of the above-mentioned different types of microstructured optical fibers, such as solid-core photonic crystal fiber and hollow-core / solid-core antiresonant fiber; or they can be a combination of microstructured optical fibers of the same type but different structures, such as five-hole hollow-core antiresonant fiber and six-hole hollow-core antiresonant fiber.

[0040] In a preferred embodiment of the present invention, after step S2, the following steps are further included:

[0041] Then, at least one section of microstructured optical fiber is selected, and its end face is cut at a non-bevel angle with a cutting knife, and then the end face of the subsequent microstructured optical fiber is sequentially cascaded and spliced.

[0042] In a preferred embodiment of the present invention, the microstructured optical fibers at both ends of each splice are microstructured optical fibers with different structures and similar mode fields of the core fundamental modes.

[0043] Furthermore, two examples in the experimental process are used to specifically illustrate the technical principle of the solution for efficient cladding filtering based on splicing of hollow-core anti-resonant optical fibers with different structures.

[0044] The first embodiment includes the following steps:

[0045] Step 1: Take two hollow-core anti-resonance fibers 1 and 2 with different structures and similar mode field diameters of the core fundamental mode. The cross-sectional structures of the hollow-core anti-resonance fibers are as follows: Figure 2As shown in the figure, the cross-sectional hollow-core antiresonant fiber domain is divided into the core region 11 / 21, the cladding region 12 / 22 and the jacket region 13 / 23 from the inside to the outside; the mode field of the core fundamental mode of the two sections of hollow-core antiresonant fiber and the cladding mode field are shown in the figure. Figure 3 As shown in, where Figure 3 (a) and (b) are the core fundamental mode fields of hollow-core antiresonant fibers 1 and 2, respectively. Figure 3 (c) and (d) are the cladding mode fields of hollow-core antiresonant fibers 1 and 2, respectively.

[0046] Step 2: Cut two sections of hollow-core anti-resonant optical fiber without an angle, and then splice the two sections of anti-resonant optical fiber at the cut end faces, keeping the two sections of optical fiber axially aligned at the splicing point, and prepare samples of different hollow-core anti-resonant optical fiber splices. Figure 4 As shown, 4 and 5 represent the connected end faces of the two optical fibers, respectively, and 6 and 7 represent the open end faces of the two optical fibers, through which light can be coupled into the optical fibers.

[0047] Step 3: Take an open end face of the hollow-core anti-resonance fiber sample after the splicing operation ( Figure 4 6 or 7) in the figure is used as the light field coupling end face to couple the light into the connected optical fiber sample, and the cladding mode components in the output light field at the other open end of the optical fiber sample will be efficiently filtered out.

[0048] Figure 5 The degree of cladding mode suppression in hollow-core antiresonant fibers after splicing, as simulated using the finite element method, is presented. Specifically, at the splice point, the cladding mode transmitted in one microstructured fiber segment can be considered to be scattered onto the cladding mode series of the other fiber segment. The total efficiency of the scattering onto the cladding mode series of the other fiber segment can be equivalent to the degree of cladding mode suppression. The lower the total scattering efficiency, the higher the degree of cladding mode suppression. The scattering efficiency can be calculated using the mode field overlap integral in optical waveguide theory.

[0049]

[0050] In the above formula, Eff scattering Represents the total scattering efficiency. The degree of mode suppression can be defined by the total scattering efficiency as Suppression = -10·lg(Eff scattering ), in the above formula, A represents the exit end face at the connecting surface, and B represents the incident end face at the connecting surface. represents the normalized electric field intensity of the cladding mode at the output end face A, represents the normalized magnetic field intensity of the cladding mode m at the incident end face B. Simulation results show that the cladding mode can be suppressed by about 30 dB after the splicing operation.

[0051] The second embodiment includes the following steps:

[0052] Step 1. After performing the same operations as in Example 1, the hollow-core anti-resonant fiber sample obtained after the splicing operation is taken out, and another section of hollow-core anti-resonant fiber is taken, and the same operations as in Example 1 are performed again to ensure that the structures of the two sections of hollow-core anti-resonant fiber at the splicing end face are different and the mode field diameters of the core fundamental mode are close, thereby obtaining a hollow-core anti-resonant fiber sample with a newly added splicing point (i.e., the splicing of two sections of microstructured optical fibers is changed to the sequential splicing of three sections of microstructured optical fibers).

[0053] Step 2: Figure 6 As shown, the operation in step 1 is repeated n times, where n is a natural number, to obtain a hollow-core antiresonant fiber sample with n+2 cascaded splicing points. The hollow-core antiresonant fibers on both sides of each splicing point of the sample meet the conditions of different structures and close fundamental mode field diameters of the fiber cores.

[0054] Step 3: Using an open end face of the hollow-core anti-resonant fiber sample after the cascade splicing operation ( Figure 6 8 or 9) in the figure is used as the light field coupling end face to couple the light into the connected optical fiber sample, and the cladding mode components in the output light field at the other open end of the optical fiber sample will be efficiently filtered out.

[0055] The cascaded continuous splicing method in this embodiment has good scalability. By cascading multiple splicing points that meet the "different structures and similar mode fields" conditions on a single optical fiber, more efficient and thorough cladding mode filtering can be achieved, meeting the higher requirements of signal-to-noise ratio in different application scenarios, with very limited cost increase.

[0056] The above are only preferred embodiments of the present invention and do not limit the implementation mode and protection scope of the present invention. For those skilled in the art, it should be aware that all solutions obtained by equivalent substitutions and obvious changes made using the contents of this specification and illustrations should be included in the protection scope of the present invention.

Claims

1. A method for efficiently filtering out the cladding mode of a microstructured optical fiber, characterized in that: include: Step S1, taking two sections of microstructured optical fibers with different structures and a diameter difference of the core fundamental mode field within a preset diameter difference range, and performing non-bevel cutting on the end faces of the two sections of the microstructured optical fibers using a cleaver; Step S2: After aligning the axial directions of the two cut sections of the microstructured optical fiber, splicing them at the cut end faces to obtain a spliced ​​microstructured optical fiber.

2. The cladding mode efficient filtering method according to claim 1, characterized in that: The connection method in step S2 includes direct fiber connection, fiber fusion connection, or coupling two sections of optical fiber via a coupler, so as to keep the transmission light from one section of the microstructured optical fiber to the other section of the microstructured optical fiber.

3. The cladding mode efficient filtering method according to claim 1, characterized in that: The two sections of microstructured optical fibers include at least one of solid-core photonic crystal fibers, hollow-core / solid-core antiresonant fibers, and hollow-core / solid-core photonic bandgap fibers, and at most two of the above.

4. The cladding mode efficient filtering method according to claim 3, characterized in that: The two sections of microstructured optical fibers are of the same type but different structures.

5. The cladding mode efficient filtering method according to claim 3, characterized in that: The two sections of microstructured optical fibers are of different types.

6. The cladding mode efficient filtering method according to claim 1, characterized in that: After step S2, the method further includes: Then, at least one section of microstructured optical fiber is selected, and its end face is cut at a non-bevel angle with a cutting knife, and then the end face of the subsequent microstructured optical fiber is sequentially cascaded and spliced.

7. The cladding mode efficient filtering method according to claim 6, characterized in that: The microstructured optical fibers at both ends of each splice are microstructured optical fibers with different structures and matching mode fields of the fiber core fundamental modes.

8. The cladding mode efficient filtering method according to claim 1, characterized in that: The diameter difference range is ±7%.

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