Coupling structure and coupling method of anti-resonance hollow-core optical fiber and single-mode optical fiber

Abstract

The invention discloses a coupling structure and a coupling method of an anti-resonance hollow-core optical fiber and a single-mode optical fiber, and belongs to the technical field of optical fiber coupling. According to the technical scheme, the coupling structure of the anti-resonance hollow-core optical fiber and the single-mode optical fiber is characterized by comprising a broadband light source, a first single-mode optical fiber, the anti-resonance hollow-core optical fiber, a conical coreless optical fiber, a conical single-mode optical fiber, a second single-mode optical fiber and a spectrograph. The device has the advantages that light emitted by the anti-resonance hollow-core optical fiber is focused into the conical single-mode optical fiber by adjusting parameters of the conical coreless optical fiber, and therefore mode field diameter matching of the single-mode optical fiber and the anti-resonance hollow-core optical fiber is achieved. Simulation and experiment results show that compared with traditional coupling of the conical single-mode fiber and the anti-resonance hollow-core fiber, the coupling efficiency is remarkably improved after the conical coreless fiber is welded to the tip of the conical single-mode fiber.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber coupling, and in particular to a coupling structure and a coupling method of an anti-resonance hollow core optical fiber and a single-mode optical fiber. Background Art

[0002] The rapid development of hollow core fiber (HCF) makes it the best choice to replace the existing solid core fiber (SCF) in many applications. Compared with solid core fiber (SCF), hollow core fiber (HCF) has unique optical properties such as low optical loss, low nonlinear effect, low glass mode overlap, high optical damage threshold and wide transmission bandwidth. These remarkable properties make it very suitable for various applications such as optical communication, high power laser transmission, interferometric sensing measurement and gas laser. In these applications, the fine and complex vacuum guidance of hollow core fiber (HCF) greatly expands the transmission mechanism of traditional solid core fiber (SCF). In particular, the recent breakthrough of hollow core fiber (HCF) in ultra-low loss clearly shows that hollow core fiber (HCF) is urgently needed for remote applications in communication and laser. In fact, its first commercial deployment has now been reported. The key to the popularization of hollow core fiber (HCF) in the above applications is the ability to effectively connect with existing components, including light sources and devices based on solid core fiber (SCF). However, the existing low-loss transmission systems are all based on single-mode fiber (SMF), so it is crucial to ensure low-loss coupling between hollow core fiber (HCF) and existing single-mode fiber (SMF)-based systems.

[0003] Two aspects must be considered to achieve high-performance, ultra-low-loss interconnection between hollow-core fiber HCF and single-mode fiber SMF. First, the mode field diameters MFD of the two must match, which is the most important factor determining the coupling efficiency. Second, the 3.5% Fresnel back reflection at the fiber glass-air interface must be suppressed. At present, many studies have achieved the coupling of light to the hollow-core fiber HCF through free space directly or through a lens (there is also a graded refractive index multimode fiber GIF), and have achieved a fairly high coupling efficiency. However, the above free-space coupling structure systems are complex and difficult to be flexibly applied in optical fiber systems. It is best to reduce the coupling loss in the form of all-fiber. The most direct method is to use a single-mode fiber SMF that matches the mode field diameter MFD of the hollow-core fiber HCF, that is, a thermal expansion core fiber with a large mode field area. However, in most applications, the single-mode fiber SMF mode has a different mode field diameter MFD from the hollow-core fiber HCF mode. In particular, to ensure that the commercial single-mode fiber SMF such as SMF-28 matches the mode field diameter MFD of various hollow-core fibers HCF, a mode field diameter MFD adjustment method is required. Tapered optical fiber can not only flexibly adjust the mode field diameter MFD, but also naturally has the advantage of suppressing Fresnel back reflection. However, the upper limit of the mode field diameter MFD of the current commercial tapered single-mode optical fiber TSMF is about 25μm, which is difficult to adapt to the large core diameter hollow core optical fiber HCF.

[0004] How to solve the above technical problems is the subject faced by the present invention. Summary of the invention

[0005] In order to solve the above problems, the present invention proposes a coupling structure and coupling method of an antiresonant hollow-core fiber ARHCF and a single-mode fiber SMF, and proposes a new method for efficiently coupling a tapered single-mode fiber TSMF and a tapered coreless fiber TNCF with an antiresonant hollow-core fiber ARHCF after fusion splicing, which makes up for the defect that the tapered fiber is difficult to adapt to a large-core hollow-core fiber, and improves the application of the tapering technology in the field of low-loss coupling of the single-mode fiber SMF with the hollow-core fiber HCF.

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

[0007] A coupling structure of an anti-resonant hollow core fiber and a single-mode fiber, comprising a broadband light source BBS, a single-mode fiber SMF 1, an anti-resonant hollow core fiber ARHCF, 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 optical fiber SMF is connected to a broadband light source BBS via an FC / APC connector as input, and the other end of the single-mode optical fiber SMF is connected to the input end of the anti-resonance hollow core optical fiber ARHCF;

[0009] The tapered single-mode optical fiber TSMF is fused with the tapered coreless optical fiber TNCF and then inserted into the structural anti-resonance hollow-core optical fiber ARHCF.

[0010] The output end of the tapered single-mode optical fiber TSMF is connected to one end of the single-mode optical fiber SMF2, and the other end of the single-mode optical fiber SMF2 is connected to the spectrometer OSA through an FC / APC connector as an output.

[0011] The outer cladding, inner cladding and core diameters of the antiresonant hollow core optical fiber ARHCF 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 optical fiber SMF2 are 125 μm and 8 μm respectively, and the initial diameter of the coreless optical fiber NCF is 125 μm.

[0013] A method for coupling an antiresonant hollow core optical fiber to a single-mode optical fiber, using the coupling structure of the antiresonant hollow core optical fiber to the single-mode optical fiber, comprises the following steps:

[0014] In the first step, the single-mode optical fiber SMF 2 and the coreless optical fiber NCF are placed on the hydrogen-oxygen taper machine in sequence, and after setting the corresponding taper program, the single-mode optical fiber SMF 2 and the coreless optical fiber NCF are tapered 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 cleaver to cut the tapered single-mode optical fiber TSMF in a straight state in the middle of the tapered area;

[0017] Then, after marking the required length at both ends of the waist of the tapered coreless optical fiber TNCF, cut it from one end with a fiber cleaver;

[0018] In the third step, the tapered single-mode optical fiber TSMF and the tapered coreless optical fiber TNCF are placed in a fiber fusion splicer (FURUKAWAS178C), fused with a small discharge amount, and then cut from the mark made in the second step;

[0019] In the fourth step, the manufactured tapered structure and hollow-core optical fiber HCF are placed on a precision displacement platform, and the tapered structure is slowly inserted into the hollow-core optical fiber HCF after aligning the stepper motor.

[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 scanning speed of the flame is 2 mm / s, and the scanning length of the flame is 2 mm.

[0022] Working principle of the present invention:

[0023] During the entire tapering process, the cladding and core size of the optical fiber gradually decrease, and the transmission of light in the tapered optical fiber will also change accordingly. In the initial stage, the light is well confined in the core, and the mode field diameter MFD is very small. However, after the core size is reduced to a certain extent, the light can no longer be confined in the core, and the mode field diameter MFD becomes larger. As the tapering progresses, after the cladding is reduced to a certain extent, the mode field diameter MFD will decrease along with the cladding. Figure 3 As shown in (a), the mode field diameter MFD changes from small to large and then decreases as the taper is pulled, which shows that the mode field diameter MFD can be adjusted by setting the tapered waist diameter. In the actual taper process, the fiber diameter is generally controlled by the taper length, but there is no fixed linear relationship between the two. In order to ensure the accuracy of the simulation, the waist diameter corresponding to different taper lengths is measured, and the relationship curve between the two under the set taper program is fitted as shown below. Figure 3 (b) as shown.

[0024] from Figure 3 It can be seen from (a) that when the waist diameter of the tapered single-mode fiber TSMF is 45μm, the maximum mode field diameter MFD that can be achieved is only about 24.2μm. This is still different from the mode field diameter MFD of the antiresonant hollow core fiber ARHCF fundamental mode of about 35μm. And it can be predicted that as the core of the antiresonant hollow core fiber ARHCF increases, the coupling efficiency of the tapered single-mode fiber TSMF and the antiresonant hollow core fiber ARHCF will become lower and lower. For this reason, a section of tapered coreless fiber TNCF is fused in front of the tapered single-mode fiber TSMF to match the mode field diameter MFD of the antiresonant hollow core fiber ARHCF output light with the mode field diameter MFD of the tapered single-mode fiber TSMF.

[0025] Compared with the prior art, the beneficial effect of the present invention is that: currently, in the all-fiber low-loss optical coupling system, only the fusion-splicing short GIF is expected to achieve high-efficiency coupling of single-mode fiber SMF and large-core anti-resonant hollow-core fiber ARHCF. In order to further expand the coupling application of single-mode fiber SMF and large-core anti-resonant hollow-core fiber ARHCF, this scheme proposes a new method of fusing a tapered single-mode fiber TSMF and a tapered coreless fiber TNCF to improve the coupling efficiency of single-mode fiber SMF and large-core anti-resonant hollow-core fiber ARHCF. The output light field of the anti-resonant hollow-core fiber ARHCF can be focused by the tapered coreless fiber TNCF to a higher degree to match the mode field diameter MFD of the tapered single-mode fiber TSMF. It involves optical fiber coupling technology, especially based on coreless fiber NCF, tapered optical fiber, anti-resonant hollow-core ARHCF, mode field diameter MFD and coupling efficiency. Its advantages are mainly reflected in the following aspects:

[0026] (1) Improved coupling efficiency: This solution significantly improves the coupling efficiency by fusing the tapered single-mode fiber TSMF and the tapered coreless fiber TNCF and coupling them with the antiresonant hollow-core fiber ARHCF. Compared with the traditional coupling of tapered single-mode fiber and antiresonant hollow-core fiber, the coupling efficiency is significantly improved after the tapered coreless fiber is fused at the tip of the 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 core fiber is focused into the tapered single-mode fiber (TSMF), achieving mode field diameter (MFD) matching and thus improving the coupling efficiency.

[0028] (3) Applicable to large-core hollow-core fibers: The structure of the tapered single-mode fiber TSMF spliced ​​with the tapered coreless fiber TNCF proposed in this scheme is suitable for matching with most large-core hollow-core fibers HCF. Compared with traditional tapered fibers, it greatly improves the upper limit of coupling efficiency with large-core hollow-core fibers. In addition, the larger the core diameter of the antiresonant hollow-core fiber ARHCF, the more obvious the improvement of coupling efficiency by connecting the mode field adapter structure between the tapered single-mode fiber TSMF and the antiresonant hollow-core fiber ARHCF.

[0029] (4) Reduce the difficulty and cost of preparation: During the preparation process, there is almost no need to consider the length error caused by cutting the tapered coreless optical fiber TNCF, and no expensive high-precision equipment is required, which reduces the difficulty and cost of preparation.

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

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

[0032] (7) Reduce coupling loss: Ensure low-loss coupling between the hollow-core fiber HCF and the existing single-mode fiber SMF system, which is crucial for the long-range application of hollow-core fiber in communications and lasers.

[0033] (8) Improve system compatibility: This solution proposes a new method of efficiently coupling the tapered single-mode fiber TSMF and the tapered coreless fiber TNCF with the anti-resonant hollow-core fiber ARHCF after fusion splicing, which makes up for the defect that the tapered fiber is difficult to adapt to the large-core hollow-core fiber, improves the application of the tapered technology in the field of low-loss coupling between the single-mode fiber SMF and the hollow-core fiber HCF, and improves the compatibility of the system. More importantly, this innovative method opens up a new path for the matching of the single-mode fiber SMF and the large-core anti-resonant hollow-core fiber ARHCF in the all-fiber structure, which is of great significance for realizing an all-fiber system with low-loss optical coupling. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0035] Figure 1 Schematic diagram of a coupling structure according to an embodiment of the present invention.

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

[0037] Figure 3 (a) shows the MFD of TSMF with different diameters, and (b) shows the relationship between different taper lengths and diameters of TSMF.

[0038] Figure 4 Graph showing the change in MFD when the ARHCF fundamental mode light of an embodiment of the present invention enters NCFs of different diameters, (a) 125 μm, (b) 100 μm, (c) 80 μm, (d) 60 μm, (e) 45 μm and (f) 40 μm.

[0039] Figure 5 Schematic diagram of the steps for manufacturing a tapered coupling structure according to an embodiment of the present invention.

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

[0041] Figure 7 It is the mode field diagram of SMF, ARHCF, TSMF with a waist diameter of 45 μm after focusing through TNCF in the embodiment of the present invention.

[0042] Figure 8 (a) is the coupling efficiency between TSMFs of different diameters and ARHCF, (b) is the coupling efficiency between TSMF with a diameter of 45 μm and ARHCF after being fused with TNCFs of different diameters, and (c) is the transmission spectrum experimental diagram of TSMF coupling with ARHCF before and after being fused with 45 μm TNCF.

[0043] Among them, the figures are marked as: 1. broadband light source; 2. single-mode optical fiber 1; 3. anti-resonant hollow-core optical fiber; 4. tapered coreless optical fiber; 5. tapered single-mode optical fiber; 6. single-mode optical fiber 2; 7. spectrometer. DETAILED DESCRIPTION

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

[0045] The present invention provides a coupling structure and a coupling method for an anti-resonant hollow core optical fiber and a single-mode optical fiber, comprising:

[0046] A coupling structure of an anti-resonant hollow core fiber and a single-mode fiber, comprising a broadband light source BBS1, a single-mode fiber SMF-2, an anti-resonant hollow core 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 optical fiber SMF-2 is connected to the broadband light source BBS1 through an FC / APC connector as input, and the other end of the single-mode optical fiber SMF-2 is connected to the input end of the anti-resonance hollow core optical fiber ARHCF3;

[0048] The tapered single-mode optical fiber TSMF5 is fused with the tapered coreless optical fiber TNCF4 and then inserted into the structural anti-resonant hollow-core optical fiber ARHCF3.

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

[0050] The outer cladding, inner cladding and core diameters of the antiresonant hollow core fiber ARHCF3 are 125 μm, 77.7 μm and 46.7 μm, respectively, and 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 optical fiber SMF26 are 125 μm and 8 μm respectively, and the initial diameter of the coreless optical fiber NCF is 125 μm.

[0052] A method for coupling an antiresonant hollow core optical fiber with a single-mode optical fiber, using a coupling structure of the antiresonant hollow core optical fiber with the single-mode optical fiber, comprises the following steps:

[0053] In the first step, the single-mode optical fiber SMF26 and the coreless optical fiber NCF are placed on the hydrogen-oxygen taper machine in sequence. After setting the corresponding taper program, the single-mode optical fiber SMF26 and the coreless optical fiber NCF are tapered to the required length;

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

[0055] The second step is to use a fiber cleaver to cut the tapered single-mode optical fiber TSMF5 in a straight state in the middle of the tapered area;

[0056] Then, after marking the required length at both ends of the waist of the tapered coreless optical fiber TNCF4, cut it from one end with a fiber cleaver;

[0057] In the third step, the tapered single-mode optical fiber TSMF5 and the tapered coreless optical fiber TNCF4 are placed in a fiber fusion splicer (FURUKAWAS178C), fused with a small discharge amount, and then cut from the mark made in the second step;

[0058] In the fourth step, the manufactured tapered structure and hollow-core optical fiber HCF are placed on a precision displacement platform, and the tapered structure is slowly inserted into the hollow-core optical fiber HCF after aligning the stepper motor.

[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 scanning speed of the flame is 2 mm / s, and the scanning length of the flame is 2 mm.

[0061] When discussing the coupling between antiresonant hollow core fiber ARHCF and single-mode fiber SMF, usually only the matching of the mode field diameter MFD of the stable fundamental mode mode field in the antiresonant hollow core fiber ARHCF and the mode field diameter MFD of the single-mode fiber SMF is considered. When the two are directly connected, catastrophic coupling loss will occur due to the mismatch of the mode field diameter MFD. Therefore, the mode field diameter MFD of the single-mode fiber SMF can be improved through the tapering technology of the single-mode fiber SMF to reduce the gap with the mode field diameter MFD of the antiresonant hollow core fiber ARHCF. However, the mode field diameter MFD of the single-mode fiber SMF with a size of 8 / 125μm commonly used in the commercial market is only about 23μm after tapering, which is still far from the mode field diameter MFD of the antiresonant hollow core fiber ARHCF fundamental mode of about 35μm. The upper limit of the mode field diameter MFD of the tapered single-mode fiber TSMF seriously limits the application of tapered fiber in the high-efficiency and low-loss coupling of large-mode field hollow core fiber HCF and single-mode fiber SMF.

[0062] In order to solve this problem, the tapered single-mode fiber TSMF and the tapered coreless fiber TNCF are fused. After the output light field of the antiresonant hollow-core fiber ARHCF is focused by the tapered coreless fiber TNCF, it can better match the tapered single-mode fiber TSMF with a smaller mode field diameter MFD. The coupling experiment structure diagram in this paper is shown in the figure. 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 through an FC / APC connector as input, 3 is an antiresonant hollow-core fiber ARHCF, 4 is a tapered coreless fiber TNCF, 5 is a tapered single-mode fiber TSMF, which is inserted into the antiresonant hollow-core fiber ARHCF of structure 3 after being fused with 4, 6 is a single-mode fiber 2, one end of which is connected to the spectrometer 7 through an FC / APC connector as output, and 7 is a spectrometer OSA. The outer cladding, inner cladding and core diameters of the antiresonant hollow-core fiber ARHCF are 125μm, 77.7μm and 46.7μm, respectively, the diameter and wall thickness of the capillary are 15.5μm and 1.5μm, respectively, the cladding and core diameters of the SMF are 125μm and 8μm, respectively, and the initial diameter of the NCF is 125μm. Ideally, the input light will form a fundamental mode output light with a mode field diameter MFD of about 35μm after passing through a certain length of antiresonant hollow core fiber ARHCF, and then after being focused by the tapered coreless fiber TNCF, it can perfectly match the mode field diameter MFD of the tapered single-mode fiber TSMF of about 23μm. Finally, it will smoothly transition to the single-mode fiber SMF through the tapered single-mode fiber TSMF to achieve almost lossless transmission. Therefore, the structure of this scheme will be divided into three parts for discussion: antiresonant hollow core fiber ARHCF, tapered single-mode fiber TSMF, and tapered coreless fiber TNCF. Finally, the coupling efficiency will be calculated by measuring the fundamental mode output power P1 of the antiresonant hollow core fiber ARHCF and the output power P2 of the single-mode fiber SMF.

[0063] Figure 2 It is the MFD of the light field during the whole process of the light field propagating in the antiresonant hollow core fiber ARHCF with a length of 0.8m and then coupling into the tapered fiber. It can be seen that the attenuation rate of the higher-order mode in the antiresonant hollow core fiber ARHCF is much higher than that of the fundamental mode, and only the fundamental mode can exist stably during long-distance transmission. After passing through the 0.8m antiresonant hollow core fiber ARHCF, the higher-order mode is basically leaked and the light field tends to be stable. Figure 2 After the light field is stabilized, the MFD of the fundamental mode spot of the antiresonant hollow core fiber ARHCF is about 35μm.

[0064] The following is an analysis of the working principle of the tapered single-mode optical fiber TSMF before and after fusion splicing the tapered coreless optical fiber TNCF. During the entire tapering process, the cladding and core size of the optical fiber gradually decrease, and the transmission of light in the tapered optical fiber will also change accordingly. In the initial stage, the light is well confined in the core, and the mode field diameter MFD is very small. However, after the core size is reduced to a certain extent, the light can no longer be confined in the core, and the mode field diameter MFD becomes larger. As the tapering progresses, after the cladding is reduced to a certain extent, the mode field diameter MFD will decrease along with the cladding. Figure 3As shown in (a), the mode field diameter MFD changes from small to large and then decreases as the taper is pulled, which shows that the mode field diameter MFD can be adjusted by setting the tapered waist diameter. In the actual taper process, the fiber diameter is generally controlled by the taper length, but there is no fixed linear relationship between the two. In order to ensure the accuracy of the simulation, the waist diameter corresponding to different taper lengths is measured, and the relationship curve between the two under the set taper program is fitted as shown below. Figure 3 (b) as shown.

[0065] from Figure 3 It can be seen from (a) that when the waist diameter of the tapered single-mode fiber TSMF is 45μm, the maximum mode field diameter MFD that can be achieved is only about 24.2μm. This is still different from the mode field diameter MFD of the antiresonant hollow core fiber ARHCF fundamental mode of about 35μm. And it can be predicted that as the core of the antiresonant hollow core fiber ARHCF increases, the coupling efficiency of the tapered single-mode fiber TSMF and the antiresonant hollow core fiber ARHCF will become lower and lower. For this reason, a section of tapered coreless fiber TNCF is fused in front of the tapered single-mode fiber TSMF to match the mode field diameter MFD of the antiresonant hollow core fiber ARHCF output light with the mode field diameter MFD of the tapered single-mode fiber TSMF.

[0066] In the experiment, a very short section of the tapered coreless fiber TNCF was selected from the middle and fused with the tapered single-mode fiber TSMF. Since the diameter of the tapered coreless fiber TNCF in the middle section changes slowly, the tapered coreless fiber TNCF with the same diameter was used to replace the tapered coreless fiber TNCF with the corresponding waist diameter in the simulation. The simulation results show that the self-focusing characteristics of coreless fibers NCF with different diameters are very different. Figure 4 It can be seen from the figure that the coreless fiber NCF can maintain stable self-focusing characteristics at any diameter, and the spot size within the self-focusing range and at each self-focusing point is almost the same. Figure 4As shown in (e), when the diameter of the coreless fiber NCF is 45μm, the mode field diameter MFD changes slowly at its self-focusing point, and its size remains basically unchanged within a range of 100μm near the self-focusing point. In addition, it can be seen from the rectangular box that the size of the mode field diameter MFD of the spot at the self-focusing point of each period is almost the same. This means that in the preparation process of the structure of splicing a tapered coreless fiber TNCF between a tapered single-mode fiber TSMF and an antiresonant hollow-core fiber ARHCF, there is almost no need to consider the length error caused by cutting. In recent years, the method of precisely cutting GIF (accurate to the micron level, at least 5μm) to adjust the mode field diameter MFD of GIF and antiresonant hollow-core fiber ARHCF to be approximately equal is difficult to prepare even with extremely precise and expensive equipment, and often a small length error will cause catastrophic coupling loss. In contrast, the ultra-long error tolerance range and stable periodic self-focusing characteristics of the structure of this scheme make it more flexible and easy to prepare when selecting the length, thus having great practical value and market application potential.

[0067] like Figure 5 In the production process, firstly, the single-mode optical fiber SMF and the coreless optical fiber NCF are placed on the hydrogen-oxygen taper machine in turn. After setting the corresponding taper program, the single-mode optical fiber SMF and the coreless optical fiber NCF are tapered to the required length. The waist diameter of the tapered single-mode optical fiber TSMF and the coreless optical fiber NCF is controlled by changing the length of the tapered optical fiber. Among them, the flow rates of oxygen and hydrogen are 8.0SCCM and 110.0SCCM respectively (the flow rate is 1ml / min at 1Pa and 25℃). The scanning speed of the flame is 2mm / s, and the scanning length of the flame is 2mm. The second step is to use a fiber cleaver to cut the tapered single-mode optical fiber TSMF in the straight state from the middle of the tapered area. Then, after marking the required length at both ends of the waist of the tapered coreless optical fiber TNCF, use a fiber cleaver to cut from one end. In the third step, the tapered single-mode fiber TSMF and the tapered coreless fiber TNCF are placed in a fiber fusion splicer (FURUKAWAS178C), fused with a small discharge amount, and then cut from the mark made in the second step. Then the produced tapered structure and hollow-core fiber HCF are placed on a precision displacement platform, and the tapered structure is slowly inserted into the hollow-core fiber HCF after the stepper motor is operated to align.

[0068] The stable fundamental mode light field output after passing through the single-mode fiber SMF and the 0.8m antiresonant hollow core fiber ARHCF is saved as a light field file and used as the input light source for subsequent simulations to ensure the precision of the simulation. Figure 4(f) The coreless fiber NCF with a diameter of 40μm can focus the mode field diameter MFD of the output light of the antiresonant hollow core fiber ARHCF to about 24μm, so as to achieve perfect mode field matching with the tapered single-mode fiber TSMF with a mode field diameter MFD of 24.2μm. However, the diameter of the output light spot of the antiresonant hollow core fiber ARHCF is about 46μm, and leakage loss will directly occur when passing through the coreless fiber NCF with a diameter that is too small. In addition, considering the axial offset of the fiber docking process and the difficulty of the experiment, the coreless fiber NCF with a diameter of 45μm becomes the most preferred. Finally, the fundamental mode light field stably transmitted by the antiresonant hollow core fiber ARHCF passes through the tapered coreless fiber TNCF, achieving a better mode field diameter MFD match with the tapered single-mode fiber TSMF, as shown Figure 6 shown. Figure 6 The input light source in the middle is the fundamental mode light field output by the antiresonant hollow core fiber ARHCF. The high energy line shows the value of the core power monitor, and the low energy line shows the value of the cladding power monitor. Figure 6 (a) is the light field diagram of the tapered single-mode fiber TSMF. Figure 6 (b) Compared to Figure 6 (a) A section of tapered coreless fiber TNCF is fused. It can be seen that after the tapered coreless fiber TNCF is fused, the energy coupled into the core increases significantly, and the energy coupled into the cladding decreases significantly, which greatly improves the coupling efficiency of the antiresonant hollow core fiber ARHCF and the single-mode fiber SMF. Figure 6 (c) and Figure 6 (d) are the far-field distribution diagrams of the single-mode fiber SMF output before and after the tapered coreless fiber TNCF is fused. It can be seen more intuitively from the figure that after the tapered coreless fiber TNCF is spliced, the cladding energy decreases, the core energy increases, and the coupling effect is significantly improved. Figure 7 They are the mode field diagrams of single-mode fiber SMF, antiresonant hollow-core fiber ARHCF, after focusing through tapered coreless fiber TNCF, and tapered single-mode fiber TSMF.

[0069] First, the coupling efficiency of the antiresonant hollow core fiber ARHCF and the tapered single mode fiber TSMF was compared. Simulation and comparative experiments were performed in the waist diameter range of 10 to 125μm. The simulation results show that at a wavelength of 1550nm, the coupling efficiency of the tapered single mode fiber TSMF reaches a maximum of 84.51% when the waist diameter is 45μm, which is 3.85dB lower than the transmission loss of the 125μm single mode fiber SMF. Figure 8 In addition, the coupling efficiency and the mode field diameter MFD in the simulation have slightly different trends with the waist diameter. This error may be caused by the fluctuation of the mode field diameter MFD caused by the different grid accuracy settings in the simulation. Figure 8(c) is the transmission spectrum on the spectrometer. The experimental results show that at a wavelength of 1550nm, the transmission loss of the tapered fiber with a waist diameter of 45μm is 3.73dB lower than that of the single-mode fiber SMF with a waist diameter of 125μm. In the waist diameter range of 10 to 125μm, the variation trend of the experimental results 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 to tapered coreless fibers TNCF with different diameters, the coupling efficiency of the antiresonant hollow-core fiber ARHCF 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 when the waist diameter of the tapered coreless fiber TNCF is 43μm, the coupling efficiency is as high as 93.5%, which is 4.28dB lower than the transmission loss of the 125μm single-mode fiber SMF directly connected to the hollow-core fiber HCF. When the waist diameter of the tapered coreless fiber TNCF is less than 43μm, since the spot diameter of the antiresonant hollow-core fiber ARHCF is 46μm, light field leakage will occur at the connection point, and the coupling efficiency will drop sharply. When the waist diameter of the tapered coreless fiber TNCF is greater than 43μm, the difference in the mode field diameter MFD with the tapered single-mode fiber TSMF is very small in the range of 43μm to 45μm, and the coupling efficiency remains basically unchanged. The experimental results show that in Figure 8 In (c), at a wavelength of 1550nm, when the waist diameters of the tapered single-mode fiber TSMF and the tapered coreless fiber TNCF are both 45μm, the transmission loss of the antiresonant hollow-core fiber ARHCF connected to the single-mode fiber SMF with a diameter of 125μm is reduced by 4.13dB.

[0071] At present, only the fusion short GIF in the all-fiber low-loss optical coupling system is expected to achieve ultra-high efficiency coupling between single-mode fiber SMF and anti-resonant hollow-core fiber ARHCF. In order to further expand the coupling application of single-mode fiber SMF and anti-resonant hollow-core fiber ARHCF, this paper proposes a new plug-in coupling method that fuses the tapered single-mode fiber TSMF and the tapered coreless fiber TNCF to improve the coupling efficiency of single-mode fiber SMF and anti-resonant hollow-core fiber ARHCF. The coupling model was established using the beam propagation method. The simulation results show that by focusing the mode field diameter MFD of the output light field of the anti-resonant hollow-core fiber ARHCF through the tapered coreless fiber TNCF to the mode field diameter MFD of the tapered single-mode fiber TSMF, the coupling efficiency of the tapered single-mode fiber TSMF and the anti-resonant hollow-core fiber ARHCF can be further improved from 84.51% to 93.5%. The structure of the tapered single-mode fiber TSMF fused with the tapered coreless fiber TNCF proposed in this scheme is suitable for matching with most anti-resonant hollow-core fibers ARHCF. Compared with traditional tapered optical fibers, the upper limit of the coupling efficiency with the antiresonant hollow core fiber ARHCF is greatly improved. And the larger the core diameter of the antiresonant hollow core fiber ARHCF, the more obvious the improvement of the coupling efficiency by connecting the mode field adapter structure between the tapered single-mode fiber TSMF and the antiresonant hollow core fiber ARHCF. In addition, during the preparation process of this structure, there is almost no need to consider the length error caused by cutting the tapered coreless fiber TNCF, and no expensive high-precision equipment is required. This work lays the foundation for the realization of an all-fiber system with low-loss optical coupling, and brings new opportunities for the efficient interconnection of large-core antiresonant hollow core fiber ARHCF with existing optical fiber systems.

[0072] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A coupling structure of an antiresonant hollow core optical fiber and a single mode optical fiber, characterized in that: It includes a broadband light source (1), a single-mode optical fiber 1 (2), an anti-resonant hollow-core optical fiber (3), a tapered coreless optical fiber (4), a tapered single-mode optical fiber (5), a single-mode optical fiber 2 (6), and a spectrometer (7); One end of the single-mode optical fiber (2) is connected to a broadband light source (1) via an FC / APC connector as an input, and the other end of the single-mode optical fiber (2) is connected to the input end of the anti-resonant hollow core optical fiber (3); The tapered single-mode optical fiber (5) is fused with the tapered coreless optical fiber (4) and then inserted into the structural anti-resonance hollow-core optical fiber (3).

2. The coupling structure of antiresonant hollow core optical fiber and single mode optical fiber according to claim 1, characterized in that: The output end of the tapered single-mode optical fiber (5) is connected to one end of the second single-mode optical fiber (6), and the other end of the second single-mode optical fiber (6) is connected to a spectrometer (7) via an FC / APC connector as an output.

3. The coupling structure of antiresonant hollow core optical fiber and single mode optical fiber according to claim 2, characterized in that: The outer cladding, inner cladding and core diameters of the antiresonant hollow core optical fiber (3) 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; The cladding and core diameters of the single-mode optical fiber 2 (6) are 125 μm and 8 μm respectively, and the initial diameter of the coreless optical fiber is 125 μm.

4. A method for coupling an antiresonant hollow core optical fiber to a single mode optical fiber, characterized in that: The coupling structure of the antiresonant hollow core optical fiber and the single mode optical fiber according to any one of claims 1 to 3 comprises the following steps: In the first step, the single-mode optical fiber 2 (6) and the coreless optical fiber are placed on the hydrogen-oxygen taper machine in sequence, and after setting the corresponding taper program, the single-mode optical fiber 2 (6) and the coreless optical fiber are tapered to the required length; The waist diameters of the tapered single-mode optical fiber (5) and the tapered coreless optical fiber (4) are controlled by changing the length of the tapered optical fiber; The second step is to cut the tapered single-mode optical fiber (5) in a straightened state in the middle of the tapered region using an optical fiber cutter; Then, after marking the required lengths at both ends of the waist of the tapered coreless optical fiber (4), a fiber cleaver is used to cut the fiber from one end; The third step is to place the tapered single-mode optical fiber (5) and the tapered coreless optical fiber (4) in an optical fiber fusion splicer, fusion splice them with a small discharge amount, and then cut them from the mark made in the second step; The fourth step is to place the manufactured tapered structure and hollow-core optical fiber on a precision displacement platform, operate the stepper motor to align them, and then slowly insert the tapered structure into the hollow-core optical fiber.

5. The method for coupling an antiresonant hollow core optical fiber to a single mode optical fiber according to claim 4, characterized in that: 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.

6. The method for coupling an antiresonant hollow core optical fiber to a single mode optical fiber according to claim 4, characterized in that: In the first step, the scanning speed of the flame is 2 mm / s, and the scanning length of the flame is 2 mm.

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