A coupling structure and method of a counter resonant hollow core fiber and a single mode fiber

Abstract

This invention discloses a coupling structure and method for an anti-resonant hollow fiber and a single-mode fiber, belonging to the field of fiber coupling technology. The technical solution is as follows: a coupling structure for an anti-resonant hollow fiber and a single-mode fiber, characterized by comprising a broadband light source, a first single-mode fiber, an anti-resonant hollow fiber, a tapered coreless fiber, a tapered single-mode fiber, a second single-mode fiber, and a spectrometer. The beneficial effect of this invention is that by adjusting the parameters of the tapered coreless fiber, the light emitted from the anti-resonant hollow fiber is focused into the tapered single-mode fiber, thereby achieving mode field diameter matching between the single-mode fiber and the anti-resonant hollow fiber. Simulation and experimental results show that, compared to the traditional coupling of a tapered single-mode fiber and an anti-resonant hollow fiber, fusing a tapered coreless fiber at the tip of the tapered single-mode fiber significantly improves the coupling efficiency.

Description

Technical Field

[0001] This invention relates to the field of optical fiber coupling technology, and in particular to a coupling structure and coupling method for an anti-resonant hollow optical fiber and a single-mode optical fiber. Background Technology

[0002] The rapid development of hollow-core fiber (HCF) has made it the best alternative to existing solid-core fiber (SCF) in many applications. Compared to solid-core SCF, HCF possesses unique optical properties such as low optical loss, low nonlinearity, low glass mode overlap, high optical damage threshold, and wide transmission bandwidth. These significant characteristics make it ideal for various applications, such as optical communication, high-power laser transmission, interferometric sensing and measurement, and gas lasers. In these applications, the fine and complex vacuum guidance of HCF greatly expands the transmission mechanism of traditional solid-core SCF. In particular, recent breakthroughs in ultra-low loss of HCF clearly demonstrate its urgent need for long-range applications in communication and lasers; in fact, its first commercial deployment has already been reported. A key to the widespread adoption of HCF in these applications is its ability to effectively connect with existing components, including light sources and solid-core SCF-based devices. However, existing low-loss transmission systems are based on single-mode fiber (SMF), therefore ensuring low-loss coupling between HCF and existing SMF-based systems is crucial.

[0003] Achieving high-performance, ultra-low-loss interconnection between hollow-core fiber HCF and single-mode fiber SMF requires consideration of two aspects. First, the mode field diameters (MFDs) of both fibers must be matched, which is the most crucial factor determining coupling efficiency. Second, the 3.5% Fresnel back reflection at the fiber glass-air interface must be suppressed. Many studies have already achieved direct or free-space coupling of light to hollow-core fiber HCF via lenses (and graded-index multimode fiber GIF), achieving considerably high coupling efficiencies. However, these free-space coupling structures are complex and difficult to apply flexibly in fiber optic systems; ideally, an all-fiber approach is preferred to reduce coupling loss. The most direct method is to use a single-mode fiber SMF with a large mode field area (MFD) that matches the MFD of the hollow-core fiber HCF. However, in most applications, the single-mode fiber SMF mode has a different MFD than the hollow-core fiber HCF mode. Ensuring MFD matching between commercially available single-mode fiber SMFs such as SMF-28 and various hollow-core fiber HCFs requires a method for adjusting the MFD. Tapered optical fibers not only allow for flexible adjustment of the mode field diameter (MFD), but also naturally possess the advantage of suppressing Fresnel back reflections. However, the upper limit of the mode field diameter (MFD) of currently commercially available tapered single-mode fiber (TSMF) is around 25 μm, making it difficult to adapt to large-core hollow fiber (HCF).

[0004] How to solve the above-mentioned technical problems is the challenge facing this invention. Summary of the Invention

[0005] To address the aforementioned issues, this invention proposes a coupling structure and method for anti-resonant hollow fiber (ARHCF) and single-mode fiber (SMF). It presents a novel method for high-efficiency coupling of tapered single-mode fiber (TSMF) and tapered coreless fiber (TNCF) with anti-resonant hollow fiber (ARHCF), overcoming the limitation of tapered fibers being difficult to adapt to large-core hollow fibers. This improves the application of tapered technology in low-loss coupling of single-mode fiber (SMF) with hollow fiber (HCF).

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A coupling structure between an anti-resonant hollow fiber and a single-mode fiber includes a broadband light source BBS, a single-mode fiber SMF-1, an anti-resonant hollow fiber AHCF, a tapered coreless fiber TNCF, a tapered single-mode fiber TSMF, a single-mode fiber SMF-2, and a spectrometer OSA.

[0008] One end of the single-mode fiber SMF is connected to the broadband light source BBS via an FC / APC connector as an input, and the other end of the single-mode fiber SMF is connected to the input end of the anti-resonant hollow fiber ARHCF.

[0009] The tapered single-mode fiber TSMF and the tapered coreless fiber TNCF are fused together and then inserted into the structural anti-resonant hollow fiber ARHCF.

[0010] The output end of the tapered single-mode fiber TSMF is connected to one end of the single-mode fiber SMF II, and the other end of the single-mode fiber SMF II is connected to the spectrometer OSA via an FC / APC connector as the output.

[0011] The outer cladding, inner cladding, and core diameters of the anti-resonant hollow fiber AHCF are 125 μm, 77.7 μm, and 46.7 μm, respectively, and the diameter and wall thickness of the capillary are 15.5 μm and 1.5 μm, respectively.

[0012] The cladding and core diameters of the single-mode fiber SMF2 are 125 μm and 8 μm, respectively, and the initial diameter of the coreless fiber NCF is 125 μm.

[0013] A coupling method between an anti-resonant hollow fiber and a single-mode fiber, using the aforementioned coupling structure, includes the following steps:

[0014] The first step is to place the single-mode fiber SMF2 and the coreless fiber NCF on the hydrogen-oxygen tapering machine one after the other. After setting the corresponding tapering program, tape the single-mode fiber SMF2 and the coreless fiber NCF to the required length.

[0015] The waist diameter of tapered single-mode fiber (TSMF) and tapered coreless fiber (TNCF) is controlled by changing the length of the tapered fiber.

[0016] The second step is to use a fiber optic cleaver to cut the taut, tapered single-mode fiber TSMF from the middle of the tapered region.

[0017] Then, after marking the required lengths at both ends of the waist of the tapered coreless fiber TNCF, cut it from one end using a fiber cleaver.

[0018] The third step is to place the tapered single-mode fiber TSMF and the tapered coreless fiber TNCF in the fiber optic fusion splicer (FURUKAWAS178C), splice them with a small amount of discharge, and then cut them from the mark made in the second step.

[0019] The fourth step is to place the fabricated tapered structure and hollow fiber HCF on a precision displacement platform, and after aligning them with a stepper motor, slowly insert the tapered structure into the hollow fiber HCF.

[0020] In the first step, the flow rates of oxygen and hydrogen were 8.0 SCCM and 110.0 SCCM, respectively, and the flow rate was 1 ml / min at 1 Pa and 25 °C.

[0021] In the first step, the flame scanning speed is 2 mm / s and the flame scanning length is 2 mm.

[0022] Working principle of the invention:

[0023] As the fiber tapers throughout the process, the cladding and core dimensions gradually decrease, causing a corresponding change in light transmission within the tapered fiber. Initially, the light is well confined within the core, resulting in a small mode field diameter (MFD). However, as the core size decreases to a certain extent, the light can no longer be confined within the core, and the MFD actually increases. As the taper continues and the cladding shrinks further, the MFD decreases along with the cladding. Figure 3 As shown in (a), the mode field diameter (MFD) undergoes a process of increasing and then decreasing as the fiber tapers, indicating that the MFD can be adjusted by setting the waist diameter of the taper. In actual tapering processes, the fiber diameter is generally controlled by the tapering length, but there is no fixed linear relationship between the two. To improve the accuracy of the simulation, the waist diameter corresponding to different tapering lengths was measured, and a curve showing the relationship between the two under the set taper program was fitted, as shown in (a). Figure 3 As shown in (b).

[0024] from Figure 3 As shown in (a), the maximum mode field diameter (MFD) achievable by the tapered single-mode fiber TSMF with a waist diameter of 45 μm is only about 24.2 μm. This is still different from the mode field diameter (MFD) of the fundamental mode of the anti-resonant hollow-core fiber ARHCF, which is about 35 μm. Furthermore, it can be predicted that as the core size of the anti-resonant hollow-core fiber ARHCF increases, the coupling efficiency between the tapered single-mode fiber TSMF and the anti-resonant hollow-core fiber ARHCF will decrease. Therefore, a section of tapered coreless fiber TNCF is fused before the tapered single-mode fiber TSMF to match the mode field diameter (MFD) of the output light from the anti-resonant hollow-core fiber ARHCF with that of the tapered single-mode fiber TSMF.

[0025] Compared with existing technologies, the advantages of this invention are as follows: Currently, only fusion splicing of short GIFs in all-fiber low-loss optical coupling systems promises to achieve high-efficiency coupling between single-mode fiber (SMF) and large-core diameter anti-resonant hollow fiber (ARHCF). To further expand the coupling applications of SMF and AHCF, this invention proposes a new method to improve the coupling efficiency of SMF and AHCF by fusion splicing tapered single-mode fiber (TSMF) and tapered coreless fiber (TNCF). The output optical field of AHCF, after being focused by TNCF, can achieve a higher degree of mode field diameter (MFD) matching with that of TSMF. This invention relates to fiber coupling technology, particularly based on NCF, tapered fiber, anti-resonant hollow AHCF, MFD, and coupling efficiency. Its advantages are mainly reflected in the following aspects:

[0026] (1) Improved coupling efficiency: This scheme significantly improves coupling efficiency by fusion splicing tapered single-mode fiber (TSMF) and tapered coreless fiber (TNCF) with anti-resonant hollow fiber (ARHCF). Compared with the traditional coupling of tapered single-mode fiber and anti-resonant hollow fiber, the coupling efficiency is significantly improved after splicing tapered coreless fiber to the tip of tapered single-mode fiber.

[0027] (2) Mode field diameter (MFD) matching: By adjusting the parameters of the tapered coreless fiber TNCF, the light emitted from the hollow fiber is focused into the tapered single-mode fiber TSMF, thereby achieving mode field diameter (MFD) matching and improving coupling efficiency.

[0028] (3) Applicable to large-core hollow fiber: The structure proposed in this scheme, which splices tapered single-mode fiber (TSMF) to tapered coreless fiber (TNCF), is suitable for matching with most large-core hollow fiber (HCF). Compared with traditional tapered fibers, it greatly improves the upper limit of coupling efficiency with large-core hollow fiber. Furthermore, the larger the core diameter of the anti-resonant hollow fiber (ARHCF), the more significant the improvement in coupling efficiency will be due to this method of connecting the mode field adaptation structure between the tapered single-mode fiber (TSMF) and the anti-resonant hollow fiber (ARHCF).

[0029] (4) Reduced preparation difficulty and cost: The length error generated when cutting tapered coreless fiber TNCF is almost not required during the preparation process, and expensive high-precision equipment is not needed, thus reducing the preparation difficulty and cost.

[0030] (5) Application of all-fiber systems: This work lays the foundation for realizing low-loss optical coupling all-fiber systems and brings new opportunities for the effective interconnection of large-core hollow fiber HCF with existing fiber systems. It is of great significance for realizing the matching of single-mode fiber SMF and large-core anti-resonant hollow fiber ARHCF in all-fiber systems.

[0031] (6) Effective interconnection of existing optical fiber systems: This solution brings new opportunities for high-efficiency interconnection of large-core hollow fiber HCF with existing optical fiber systems, and expands the coupling application of single-mode fiber SMF and large-core anti-resonant hollow fiber ARHCF.

[0032] (7) Reduce coupling loss: Ensure low-loss coupling between hollow fiber HCF and existing single-mode fiber SMF-based systems, which is crucial for remote applications of hollow fiber in communication and lasers.

[0033] (8) Improved System Compatibility: The novel method proposed in this scheme, which involves fusion splicing tapered single-mode fiber (TSMF) and tapered coreless fiber (TNCF) with anti-resonant hollow fiber (ARHCF) for high-efficiency coupling, overcomes the limitation of tapered fiber being difficult to adapt to large-core hollow fiber. This improves the application of tapering technology in low-loss coupling between single-mode fiber (SMF) and hollow fiber (ARHCF), thus enhancing system compatibility. More importantly, this innovative method opens up a new path for matching single-mode fiber (SMF) with large-core anti-resonant hollow fiber (ARHCF) in all-fiber structures, which is of great significance for achieving low-loss optical coupling in all-fiber systems. Attached Figure Description

[0034] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0035] Figure 1 This is a schematic diagram of the coupling structure in an embodiment of the present invention.

[0036] Figure 2 This is a simulation diagram of the MFD and fundamental mode spot as the length of the ARHCF varies according to an embodiment of the present invention.

[0037] Figure 3 In the figure, (a) shows the MFD for TSMF with different diameters, and (b) shows the relationship between the length and diameter of the tapered section of TSMF.

[0038] Figure 4 These are the variations of the ARHCF fundamental mode light entering the MFD within the NCF of different diameters according to embodiments of the present invention: (a) 125 μm, (b) 100 μm, (c) 80 μm, (d) 60 μm, (e) 45 μm and (f) 40 μm.

[0039] Figure 5 A schematic diagram illustrating the fabrication steps of the tapered coupling structure according to an embodiment of the present invention.

[0040] Figure 6 In the middle, (a) and (b) are the optical field diagrams of the TSMF and the power of the monitor before and after fusion splicing the TNCF, respectively. (c) and (d) are the far-field distribution diagrams of the SMF output before and after fusion splicing the TNCF, respectively.

[0041] Figure 7 These are the mode field diagrams of SMF, ARHCF, TNCF after focusing, and TSMF with a waist diameter of 45μm in the embodiments of the present invention.

[0042] Figure 8 In the figure, (a) shows the coupling efficiency of TSMF with different diameters and ARHCF, (b) shows the coupling efficiency of TSMF with a diameter of 45 μm after being fused with TNCF with different diameters and ARHCF, and (c) shows the transmission spectrum of TSMF before and after being fused with 45 μm TNCF and coupled with ARHCF.

[0043] The attached figures are labeled as follows: 1. Broadband light source; 2. Single-mode fiber one; 3. Anti-resonant hollow fiber; 4. Tapered coreless fiber; 5. Tapered single-mode fiber; 6. Single-mode fiber two; 7. Spectrometer. Detailed Implementation

[0044] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0045] This invention provides a coupling structure and method for anti-resonant hollow fiber and single-mode fiber, including...

[0046] A coupling structure between an anti-resonant hollow fiber and a single-mode fiber includes a broadband light source BBS1, a single-mode fiber SMF-2, an anti-resonant hollow fiber ARHCF3, a tapered coreless fiber TNCF4, a tapered single-mode fiber TSMF5, a single-mode fiber SMF-6, and a spectrometer OSA7.

[0047] like Figure 1 As shown, one end of the single-mode fiber SMF-2 is connected to the broadband light source BBS1 via an FC / APC connector as the input, and the other end of the single-mode fiber SMF-2 is connected to the input end of the anti-resonant hollow fiber ARHCF3.

[0048] After being fused together, tapered single-mode fiber TSMF5 and tapered coreless fiber TNCF4 are inserted into structural anti-resonant hollow fiber ARHCF3.

[0049] The output end of the tapered single-mode fiber TSMF5 is connected to one end of the single-mode fiber SMF26, and the other end of the single-mode fiber SMF26 is connected to the spectrometer OSA7 via an FC / APC connector as the output.

[0050] The outer cladding, inner cladding, and core diameters of the anti-resonant hollow fiber ARHCF3 are 125 μm, 77.7 μm, and 46.7 μm, respectively, while the capillary diameter and wall thickness are 15.5 μm and 1.5 μm, respectively.

[0051] The cladding and core diameters of the single-mode fiber SMF26 are 125 μm and 8 μm, respectively, while the initial diameter of the coreless fiber NCF is 125 μm.

[0052] A coupling method between an anti-resonant hollow fiber and a single-mode fiber, using a coupling structure between the anti-resonant hollow fiber and the single-mode fiber, includes the following steps:

[0053] The first step is to place the single-mode fiber SMF26 and the coreless fiber NCF on the hydrogen-oxygen tapering machine one after the other. After setting the corresponding tapering program, tape the single-mode fiber SMF26 and the coreless fiber NCF to the required length.

[0054] The waist diameter of tapered single-mode fiber TSMF5 and tapered coreless fiber TNCF4 is controlled by changing the length of the tapered fiber.

[0055] The second step is to use a fiber optic cleaver to cut the taut, tapered single-mode fiber TSMF5 from the middle of the tapered region.

[0056] Then, after marking the required lengths at both ends of the waist of the tapered coreless fiber TNCF4, cut it from one end using a fiber cleaver.

[0057] The third step is to place the tapered single-mode fiber TSMF5 and the tapered coreless fiber TNCF4 in the fiber optic fusion splicer (FURUKAWAS178C), splice them with a small amount of discharge, and then cut them from the mark made in the second step.

[0058] The fourth step is to place the fabricated tapered structure and hollow fiber HCF on a precision displacement platform, and after aligning them with a stepper motor, slowly insert the tapered structure into the hollow fiber HCF.

[0059] In the first step, the flow rates of oxygen and hydrogen were 8.0 SCCM and 110.0 SCCM, respectively, and the flow rate was 1 ml / min at 1 Pa and 25 °C.

[0060] In the first step, the flame scanning speed is 2 mm / s and the flame scanning length is 2 mm.

[0061] When discussing the coupling of anti-resonant hollow fiber (ARHCF) and single-mode fiber (SMF), the matching of the mode field diameter (MFD) of the stable fundamental mode in AHCF and the MFD of the single-mode fiber (SMF) is typically considered. When directly connected, catastrophic coupling loss occurs due to the mismatch in MFDs. Therefore, single-mode fiber (SMF) taper technology can be used to increase the MFD of the SMF, thereby reducing the gap with the MFD of AHCF. However, currently, the commercially available 8 / 125μm single-mode fiber (SMF) has an MFD of only about 23μm after tapering, which is significantly smaller than the approximately 35μm MFD of the fundamental mode in AHCF. This upper limit of the MFD of tapered single-mode fiber (TSMF) severely restricts its application in high-efficiency, low-loss coupling between large-mode-field hollow fiber (HCF) and single-mode fiber (SMF).

[0062] To address this issue, tapered single-mode fiber (TSMF) and tapered coreless fiber (TNCF) were fused together. The output optical field of the anti-resonant hollow fiber (ARHCF), after being focused by the tapered coreless fiber (TNCF), can better match the tapered single-mode fiber (TSMF) with its smaller mode field diameter MFD. The coupling experimental structure diagram in this paper is shown below. Figure 1As shown, 1 is a broadband light source (BBS), 2 is a single-mode fiber 1, one end of which is connected to the broadband light source 1 via an FC / APC connector as an input, 3 is an anti-resonant hollow-core fiber (ARHCF), 4 is a tapered coreless fiber (TNCF), 5 is a tapered single-mode fiber (TSMF), which is fused with 4 and inserted into the anti-resonant hollow-core fiber (ARHCF) in structure 3, 6 is a single-mode fiber 2, one end of which is connected to the spectrometer 7 via an FC / APC connector as an output, and 7 is the spectrometer OSA. The outer cladding, inner cladding, and core diameters of the anti-resonant hollow-core fiber (ARHCF) are 125 μm, 77.7 μm, and 46.7 μm, respectively; the capillary diameter and wall thickness are 15.5 μm and 1.5 μm, respectively; the cladding and core diameters of the SMF are 125 μm and 8 μm, respectively; and the initial diameter of the NCF is 125 μm. Ideally, the input light, after passing through a certain length of anti-resonant hollow-core fiber (ARHCF), will form a fundamental mode output light with a mode field diameter (MFD) of approximately 35 μm. This output light, after being focused by a tapered coreless fiber (TNCF), can perfectly match the mode field diameter (MFD) of a tapered single-mode fiber (TSMF) of approximately 23 μm. Finally, it smoothly transitions through the tapered single-mode fiber (TSMF) to the single-mode fiber (SMF), achieving near-lossless transmission. Therefore, the structure of this scheme will be discussed in three parts: the anti-resonant hollow-core fiber (ARHCF), the tapered single-mode fiber (TSMF), and the tapered coreless fiber (TNCF). The coupling efficiency will ultimately be calculated by measuring the fundamental mode output power (P1) of the anti-resonant hollow-core fiber (ARHCF) and the output power (P2) of the single-mode fiber (SMF).

[0063] Figure 2 This represents the optical field MFD (Mean Dispersion) during the entire process of the optical field propagating within a 0.8m antiresonant hollow-core fiber (ARHCF) and coupling into a tapered fiber. It can be seen that within the AHCF, the attenuation rate of higher-order modes is much higher than that of the fundamental mode; only the fundamental mode remains stable during long-distance transmission. However, after passing through the 0.8m AHCF, the higher-order modes are largely leaked, and the optical field tends to stabilize. Figure 2 After the optical field stabilizes, the MFD of the fundamental mode spot of the anti-resonant hollow fiber ARHCF is approximately 35 μm.

[0064] The working principle of tapered single-mode fiber (TSMF) before and after splicing tapered coreless fiber (TNCF) is analyzed below. During the tapering process, as the cladding and core dimensions of the fiber gradually decrease, the light transmission within the tapered fiber changes accordingly. Initially, the light is well confined within the core, and the mode field diameter (MFD) is very small. However, as the core size decreases to a certain extent, the light can no longer be confined within the core, and the MFD actually increases. As tapering continues, and the cladding decreases to a certain extent, the MFD decreases along with the cladding. Figure 3As shown in (a), the mode field diameter (MFD) undergoes a process of increasing and then decreasing as the fiber tapers, indicating that the MFD can be adjusted by setting the waist diameter of the taper. In actual tapering processes, the fiber diameter is generally controlled by the tapering length, but there is no fixed linear relationship between the two. To improve the accuracy of the simulation, the waist diameter corresponding to different tapering lengths was measured, and a curve showing the relationship between the two under the set taper program was fitted, as shown in (a). Figure 3 As shown in (b).

[0065] from Figure 3 As shown in (a), the maximum mode field diameter (MFD) achievable by the tapered single-mode fiber TSMF with a waist diameter of 45 μm is only about 24.2 μm. This is still different from the mode field diameter (MFD) of the fundamental mode of the anti-resonant hollow-core fiber ARHCF, which is about 35 μm. Furthermore, it can be predicted that as the core size of the anti-resonant hollow-core fiber ARHCF increases, the coupling efficiency between the tapered single-mode fiber TSMF and the anti-resonant hollow-core fiber ARHCF will decrease. Therefore, a section of tapered coreless fiber TNCF is fused before the tapered single-mode fiber TSMF to match the mode field diameter (MFD) of the output light from the anti-resonant hollow-core fiber ARHCF with that of the tapered single-mode fiber TSMF.

[0066] In the experiment, a very short segment from the middle of a tapered coreless fiber (TNCF) was fused with a tapered single-mode fiber (TSMF). Because the diameter of the middle segment of the TNCF changes slowly, the simulation approximated the corresponding waist diameter of the tapered coreless fiber (TNCF) with a coreless fiber (NCF) of the same diameter. Simulation results show that the self-focusing characteristics of NCFs with different diameters vary significantly. Figure 4 As can be seen, coreless fiber (NCF) maintains stable self-focusing characteristics at any diameter, and the spot size is nearly identical within the self-focusing range and at each self-focusing point. For example... Figure 4As shown in (e), when the coreless fiber NCF has a diameter of 45 μm, the mode field diameter (MFD) changes slowly at its self-focusing point and remains essentially constant within a 100 μm range near the self-focusing point. Furthermore, it can be seen from the rectangular frame that the MFD of the spot at the self-focusing point in each cycle is nearly identical. This means that the structure of splicing a tapered coreless fiber TNCF between a tapered single-mode fiber TSMF and an anti-resonant hollow fiber ARHCF requires almost no consideration of length errors caused by cutting during fabrication. Recent methods involving precise cutting of GIF (accurate to the micrometer level, at least 5 μm) to adjust the MFD of GIF and ARHCF to be approximately equal are difficult to fabricate even with extremely precise and expensive equipment; often, even a small length error can cause catastrophic coupling loss. In contrast, the ultra-long error tolerance and stable periodic self-focusing characteristics of the structure presented in this scheme make length selection more flexible and easier to fabricate, thus possessing significant practical value and market application potential.

[0067] like Figure 5 In the fabrication process, firstly, single-mode fiber (SMF) and coreless fiber (NCF) are placed sequentially on a hydrogen-oxygen tapering machine. After setting the appropriate tapering program, the SMF and NCF are tapered to the required length. The waist diameter of the tapered SMF and NCF is controlled by changing the length of the tapered fiber. The flow rates of oxygen and hydrogen are 8.0 SCCM and 110.0 SCCM, respectively (1 ml / min at 1 Pa and 25°C). The flame scanning speed is 2 mm / s, and the flame scanning length is 2 mm. Secondly, the taut tapered SMF is cut from the middle of the tapered region using a fiber cleaver. Then, after marking the required lengths at both ends of the waist of the tapered NCF, it is cut from one end using a fiber cleaver. The third step involves placing the tapered single-mode fiber (TSMF) and the tapered coreless fiber (TNCF) in a fiber fusion splicer (FURUKAWAS178C), splicing them with a small discharge, and then cutting them at the marked point from the second step. Next, the fabricated tapered structure and the hollow fiber (HCF) are placed on a precision displacement platform, and after alignment using a stepper motor, the tapered structure is slowly inserted into the hollow fiber (HCF).

[0068] The stable fundamental mode light field output after passing through single-mode fiber (SMF) and 0.8m anti-resonant hollow fiber (ARHCF) is saved as a separate light field file and used as the input light source for subsequent simulations to ensure the accuracy of the simulation. Although Figure 4(f) The 40μm diameter coreless fiber NCF can focus the mode field diameter (MFD) of the outgoing light from the anti-resonant hollow fiber ARHCF to approximately 24μm, achieving perfect mode field matching with the tapered single-mode fiber TSMF, which has an MFD of 24.2μm. However, the diameter of the outgoing light spot from the anti-resonant hollow fiber ARHCF is approximately 46μm, and leakage loss will occur directly after passing through the coreless fiber NCF with a diameter that is too small. Furthermore, considering the axial offset during fiber splicing and the ease of the experiment, a 45μm diameter coreless fiber NCF becomes the optimal choice. Finally, the fundamental mode light field stably transmitted through the anti-resonant hollow fiber ARHCF, after passing through the tapered coreless fiber TNCF, achieves better mode field diameter (MFD) matching with the tapered single-mode fiber TSMF, as shown below. Figure 6 As shown. Figure 6 The input light source is the fundamental mode light field output from the anti-resonant hollow fiber AHCF. The high-energy lines show the values ​​of the fiber core power monitor, and the low-energy lines show the values ​​of the cladding power monitor. Figure 6 (a) is the optical field diagram of a tapered single-mode fiber TSMF. Figure 6 (b) compared to Figure 6 (a) An additional section of tapered coreless fiber TNCF was fused together. It can be seen that after fusion splicing the tapered coreless fiber TNCF, the energy coupled into the fiber core increases significantly, while the energy coupled into the cladding decreases significantly, greatly improving the coupling efficiency of anti-resonant hollow fiber ARHCF and single-mode fiber SMF. Figure 6 (c) and Figure 6 (d) These are the far-field distribution diagrams of the single-mode fiber SMF before and after splicing tapered coreless fiber TNCF. The diagrams clearly show that splicing tapered coreless fiber TNCF reduces the cladding energy, increases the core energy, and significantly improves the coupling effect. Figure 7 These are the mode field diagrams of single-mode fiber (SMF), anti-resonant hollow fiber (ARHCF), focused by tapered coreless fiber (TNCF), and tapered single-mode fiber (TSMF).

[0069] First, the coupling efficiency of anti-resonant hollow fiber (ARHCF) and tapered single-mode fiber (TSMF) was compared. Simulations and comparative experiments were conducted in the waist diameter range of 10 to 125 μm. Simulation results show that at a wavelength of 1550 nm, the coupling efficiency of tapered single-mode fiber (TSMF) reaches its highest level of 84.51% when the waist diameter is 45 μm, and the transmission loss is reduced by 3.85 dB compared to single-mode fiber (SMF) with a waist diameter of 125 μm. Figure 8 As shown in (a). In addition, the coupling efficiency and the mode field diameter (MFD) show slightly different trends with the waist diameter in the simulation. This error may be caused by the fluctuation of the mode field diameter (MFD) due to different mesh accuracy settings in the simulation. Figure 8(c) shows the transmission spectrum from the spectrometer. The experimental results show that at a wavelength of 1550 nm, the transmission loss of the tapered fiber with a waist diameter of 45 μm is reduced by 3.73 dB compared to the 125 μm single-mode fiber (SMF). Furthermore, the trend of the experimental results in the waist diameter range of 10 to 125 μm is basically consistent with the simulation results.

[0070] A tapered single-mode fiber TSMF with a waist diameter of 45 μm was selected. After connecting tapered coreless fibers TNCF of different diameters, the coupling efficiency of the anti-resonant hollow fiber AHCF and the tapered single-mode fiber TSMF at a wavelength of 1550 nm is as follows: Figure 8 As shown in (b), it can be seen that the coupling efficiency is highest at 93.5% when the waist diameter of the tapered coreless fiber TNCF is 43μm, which is 4.28dB lower than the transmission loss of the 125μm single-mode fiber SMF directly connected to the hollow-core fiber HCF. However, when the waist diameter of the tapered coreless fiber TNCF is less than 43μm, optical field leakage occurs at the connection point due to the 46μm spot diameter of the anti-resonant hollow-core fiber ARHCF, resulting in a sharp decrease in coupling efficiency. When the waist diameter of the tapered coreless fiber TNCF is greater than 43μm, the mode field diameter MFD is very similar to that of the tapered single-mode fiber TSMF in the range of 43μm to 45μm, and the coupling efficiency remains basically unchanged. Experimental results show that... Figure 8 In (c), at a wavelength of 1550 nm, the transmission loss of tapered single-mode fiber TSMF and tapered coreless fiber TNCF with a waist diameter of 45 μm is reduced by 4.13 dB compared with that of single-mode fiber SMF with a diameter of 125 μm connected to anti-resonant hollow fiber ARHCF.

[0071] Currently, in all-fiber low-loss optical coupling systems, only fusion splicing of short GIFs (short girders) holds promise for achieving ultra-high-efficiency coupling between single-mode fiber (SMF) and anti-resonant hollow fiber (ARHCF). To further expand the coupling applications of SMF and AHCF, this paper proposes a novel insertion coupling method that fusion splices tapered single-mode fiber (TSMF) and tapered coreless fiber (TNCF) to improve the coupling efficiency of SMF and AHCF. A coupling model was established using the beam propagation method. Simulation results show that by approximately focusing the mode field diameter (MFD) of the AHCF output optical field through the tapered coreless fiber (TNCF) to the size of the mode field diameter (MFD) of the tapered single-mode fiber (TSMF), the coupling efficiency between the tapered single-mode fiber (TSMF) and AHCF can be further increased from 84.51% to 93.5%. The proposed structure of fusion splicing tapered single-mode fiber (TSMF) with tapered coreless fiber (TNCF) is suitable for matching with most anti-resonant hollow fiber (ARHCF) types. Compared to traditional tapered optical fibers, this method significantly improves the upper limit of coupling efficiency with antiresonant hollow-core fiber (ARHCF). Furthermore, the larger the core diameter of the AHCF, the more significant the improvement in coupling efficiency becomes due to this method of connecting a mode field adaptation structure between tapered single-mode fiber (TSMF) and AHCF. In addition, this structure requires almost no consideration of length errors caused by cutting tapered coreless fiber (TNCF) during fabrication, eliminating the need for expensive high-precision equipment. This work lays the foundation for realizing low-loss optical coupling all-fiber systems and brings new opportunities for high-efficiency interconnection of large-core AHCF with existing fiber optic systems.

[0072] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A coupling method between an anti-resonant hollow fiber and a single-mode fiber, characterized in that, Includes the following steps: First, place the single-mode fiber 2 (6) and the coreless fiber on the hydrogen-oxygen tapering machine. After setting the corresponding tapering program, tape the single-mode fiber 2 (6) and the coreless fiber to the required length. The waist diameter of the tapered single-mode fiber (5) and the tapered coreless fiber (4) is controlled by changing the length of the tapered fiber; The second step is to use a fiber optic cleaver to cut the taut tapered single-mode fiber (5) from the middle of the tapered region. Then, after marking the required lengths at both ends of the waist of the tapered coreless optical fiber (4), cut it from one end with an optical fiber cleaver. The third step is to place the tapered single-mode fiber (5) and the tapered coreless fiber (4) in the fiber fusion splicer, fusion splice them with a small discharge, and then cut them from the mark made in the second step. The fourth step is to place the fabricated tapered structure and hollow fiber on a precision displacement platform, and after aligning them with a stepper motor, slowly insert the tapered structure into the hollow fiber. The coupling structure between anti-resonant hollow fiber and single-mode fiber includes a broadband light source (1), single-mode fiber one (2), anti-resonant hollow fiber (3), tapered coreless fiber (4), tapered single-mode fiber (5), single-mode fiber two (6), and spectrometer (7). One end of the single-mode fiber (2) is connected to the broadband light source (1) via an FC / APC connector as an input, and the other end of the single-mode fiber (2) is connected to the input end of the anti-resonant hollow fiber (3). The tapered single-mode fiber (5) and the tapered coreless fiber (4) are fused together and then inserted into the anti-resonant hollow fiber (3).

2. The coupling method between anti-resonant hollow fiber and single-mode fiber according to claim 1, characterized in that: The output end of the tapered single-mode fiber (5) is connected to one end of the single-mode fiber II (6), and the other end of the single-mode fiber II (6) is connected to the spectrometer (7) via an FC / APC connector as an output.

3. The coupling method between anti-resonant hollow fiber and single-mode fiber according to claim 2, characterized in that: The outer cladding, inner cladding and core diameters of the anti-resonant hollow fiber (3) are 125µm, 77.7µm and 46.7µm, respectively, and the diameter and wall thickness of the capillary are 15.5µm and 1.5µm, respectively. The cladding and core diameters of the single-mode fiber 2 (6) are 125µm and 8µm, respectively, and the initial diameter of the coreless fiber is 125µm.

4. The coupling method between anti-resonant hollow fiber and single-mode fiber according to claim 1, characterized in that: In the first step, the flow rates of oxygen and hydrogen are 8.0 SCCM and 110.0 SCCM, respectively, and the flow rate is 1 ml / min at 1 Pa and 25°C.

5. The coupling method between anti-resonant hollow fiber and single-mode fiber according to claim 1, characterized in that: In the first step, the flame scanning speed is 2 mm / s and the flame scanning length is 2 mm.

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