Multi-core fiber coupler based on single-clad optical fibers, and taper ratio design method therefor

Abstract

A multi-core fiber coupler based on single-clad optical fibers (1), and a taper ratio design method therefor. Each single-clad optical fiber (1) comprises a fiber core (13), a cladding (12), and a coating layer (11), arranged in sequence from inside to outside. The multi-core fiber coupler based on single-clad optical fibers (1) comprises single-clad optical fibers (1) and a sleeve (2). Several single-clad optical fibers (1) are nested within the sleeve (2), and the total number of the single-clad optical fibers (1) is equal to the total number of fiber cores in a target multi-core optical fiber (3). One end of the sleeve (2) is tapered, and the other end is not tapered. A mode field diameter of each single-clad optical fiber (1) at the non-tapered end of the sleeve (2) matches a mode field diameter of a single fiber core of the target multi-core optical fiber (3). The single-clad optical fibers (1) and the multi-core fiber coupler have the advantages of low fabrication difficulty, low cost, and the capability of achieving low-loss connection with multi-core optical fibers and standard single-mode optical fibers.

Description

Multi-core fiber couplers based on single-clad fiber and their tapered ratio design method Technical Field

[0001] This invention relates to the field of optical fiber technology, and more specifically, to a multi-core optical fiber coupler based on a single-clad optical fiber and a tapered ratio design method thereof. Background Technology

[0002] With the rapid popularization and development of internet technology, people's lifestyles and work patterns have been fundamentally changed, greatly promoting the transformation and development of the economy and society. At the same time, this has also led to a continuous increase in people's demand for data traffic. The current backbone network, based on single-mode fiber, is approaching its maximum transmission capacity limit and cannot meet the upcoming demands of the Internet of Things.

[0003] Space division multiplexing (SDM) technology based on multi-core optical fibers, which utilizes multiple fiber cores within a single cladding structure to provide new spatial multiplexing dimensions and exponentially increase the capacity of communication systems, has become a research hotspot. In the application of multi-core optical fibers, multi-core fiber couplers capable of providing low-loss connections between standard single-mode and multi-core fibers are essential for compatibility with existing communication systems based on standard single-mode fibers. Therefore, the design and fabrication of multi-core fiber couplers have become one of the key technologies for promoting multi-core optical fibers.

[0004] Currently, the main methods for fabricating all-fiber multi-core fiber couplers include micro-hole fabrication, fiber bundle tapering by etching, and multi-clad fiber tapering.

[0005] The micro-hole fabrication method involves inserting etched standard single-mode and multi-core optical fibers into cylindrical sleeves that have been mechanically drilled or laser-drilled. Alignment is achieved using an alignment platform, followed by adhesive application for fixation. However, due to limitations in etching and drilling precision, the coupler suffers significant losses.

[0006] The etched fiber taper method involves inserting a standard single-mode fiber, after etching, into a sleeve, tapering it, cutting and polishing it, and then connecting it to a multi-core fiber to fabricate a coupler. Etching the fiber can easily lead to dimensional deviations and increased brittleness, resulting in higher insertion loss and increased fiber breakage.

[0007] Multi-clad fiber tapering involves tapering an optical fiber with multiple cladding layers. By optimizing the refractive index and dimensions of different cladding layers, the loss of multi-core fiber couplers can be effectively reduced. However, due to the multi-cladding structure of the fiber, interference effects are introduced during tapering, limiting the spectral range of transmittable light, and the transmission loss fluctuates significantly with wavelength.

[0008] In summary, current methods for fabricating various multi-core fiber couplers have issues in terms of etching accuracy, fiber geometric alignment accuracy, processing difficulty, cost, and loss.

[0009] Summary of the Invention

[0010] To overcome the shortcomings of existing technologies, such as high fabrication difficulty, high fabrication cost, and high connection loss of multi-core fiber couplers, this invention provides a multi-core fiber coupler based on a single-clad fiber that is easy to fabricate, low in fabrication cost, and has low connection loss.

[0011] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0012] A single-clad optical fiber, the single-clad optical fiber comprising a core, a cladding and a coating layer arranged sequentially from the inside to the outside.

[0013] The present invention also proposes a multi-core fiber coupler based on a single-clad fiber, comprising a single-clad fiber and a sleeve;

[0014] The sleeve contains several single-clad optical fibers, and the total number of the single-clad optical fibers is equal to the total number of fiber cores in the target multi-core optical fiber.

[0015] One end of the sleeve is tapered, and the other end is not tapered; the tapered end of the sleeve is used to connect to the target multi-core optical fiber; each single-clad optical fiber in the untapered end of the sleeve is used to connect to a standard single-mode optical fiber, and the mode field diameter of the single-clad optical fiber in the untapered end of the sleeve matches the mode field diameter of a single core of the target multi-core optical fiber.

[0016] This invention also proposes a tapered ratio design method based on multi-core fiber couplers, which applies the above-mentioned multi-core fiber coupler based on single-clad fiber and includes the following steps:

[0017] The parameters of the single-clad fiber are adjusted according to the mode field diameter A of a single core in the target multi-core fiber, so that the difference between the mode field diameter B and the mode field diameter A of the single-clad fiber in the untapered end of the sleeve is not greater than a preset threshold.

[0018] Let the initial value of the tapering ratio r of the multi-core fiber coupler be 1. Using simulation software or numerical calculation methods, with the tapering ratio r set as a decreasing variable, calculate the mode field diameter C of the single-clad fiber in the tapered end of the sleeve.

[0019] When the tapered ratio r is not 1 and the mold field diameter C is equal to the mold field diameter B, the calculation stops, and the size of the tapered ratio r at this time is recorded as R, thus obtaining the optimal range of the tapered ratio r [R, 1].

[0020] This invention also proposes a design method based on multi-core fiber couplers, including the following steps:

[0021] Based on the core spacing Λ of the target multi-core optical fiber, the cladding diameter d2 of the single-clad optical fiber in the untapered end of the sleeve is calculated. The expression for calculating the cladding diameter d2 includes: d2 = Λ / r

[0022] In the formula, r represents the taper ratio of the multi-core fiber coupler;

[0023] After determining the size of the cladding diameter d2, according to the fiber core arrangement scheme of the target multi-core fiber, the size of the outer circle diameter D1 under the tight arrangement of the single-cladding fiber is set as the optimal inner diameter of the sleeve.

[0024] The minimum outer diameter D2 of the sleeve is calculated based on the cladding diameter d3 of the target multi-core optical fiber. The expression for the minimum outer diameter D2 includes: D2 = d3 / r.

[0025] Compared with the prior art, the beneficial effects of the technical solution of the present invention are:

[0026] The single-clad optical fiber comprises a core, a cladding, and a coating layer arranged sequentially from the inside out. Its structure is simple, and compared to double-clad optical fiber, its fabrication difficulty and cost are lower. The total number of single-clad optical fibers in the multi-core fiber coupler based on the single-clad optical fiber is equal to the total number of cores in the target multi-core optical fiber. Furthermore, the mode field diameter of the single-clad optical fiber at the untapered end of the sleeve matches the mode field diameter of a single core of the target multi-core optical fiber, effectively reducing the connection loss between the multi-core fiber coupler and the target multi-core optical fiber. Attached Figure Description

[0027] Figure 1 is a schematic diagram of the structure of the single-clad optical fiber proposed in Example 1;

[0028] Figure 2 is a schematic diagram of the structure of the multi-core fiber coupler based on single-clad fiber proposed in Example 1;

[0029] Figure 3 is a schematic diagram of the small-scale tapered puller proposed in Example 1;

[0030] Figure 4 shows the refractive index distribution of the single-clad fiber proposed in Example 1;

[0031] Figure 5 is a schematic diagram of the structure of the multi-core optical fiber proposed in Example 1;

[0032] Figure 6 is a schematic diagram of the changes in mode field diameter and loss during the small-scale tapering process of the single-clad fiber proposed in Example 3.

[0033] Figure 7 is a schematic diagram showing the changes in coupling loss when the cladding diameter of the single-clad optical fiber proposed in Example 3 is taken for different values;

[0034] Among them, 1-single-clad fiber bundle, 11-single-clad fiber coating layer, 12-single-clad fiber cladding, 13-single-clad fiber core, 2-sleeve, 3-target multi-core fiber, 31-multi-core fiber coating layer, 32-multi-core fiber cladding, 33-multi-core fiber core. Detailed Implementation

[0035] The accompanying drawings are for illustrative purposes only and should not be construed as limiting the scope of this embodiment.

[0036] To better illustrate this embodiment, some parts in the accompanying drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions;

[0037] It will be understood by those skilled in the art that certain well-known structures and their descriptions may be omitted in the accompanying drawings.

[0038] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0039] Example 1

[0040] This embodiment proposes a single-clad optical fiber. Figure 1 is a schematic diagram of the structure of the single-clad optical fiber proposed in this embodiment.

[0041] The single-clad optical fiber 1 includes a core 13, a cladding 12, and a coating layer 11 arranged sequentially from the inside to the outside.

[0042] In this embodiment, Figure 1 is a schematic diagram of the structure of the single-clad optical fiber proposed in this embodiment; as shown in Figure 1, the preparation process of the single-clad optical fiber with one layer of cladding is simple, does not require etching or other treatments, has good scalability, and a high yield.

[0043] In an optional embodiment, the refractive index at the interface between the core 13 and the cladding 12 changes abruptly, and the refractive index at the interface between the cladding 12 and the coating layer 11 changes abruptly. The refractive index of the cladding 12 is less than that of the core 13, and the refractive index of the coating layer 11 is less than that of the cladding 12.

[0044] In an optional embodiment, the refractive index of the fiber core 13 is either step-type or gradient-type.

[0045] In practice, the single-clad optical fiber includes a core, a cladding, and a coating layer arranged sequentially from the inside out. The structure is simple and, compared to double-clad optical fiber, it is easier and cheaper to manufacture.

[0046] This embodiment also proposes a multi-core fiber coupler based on a single-clad fiber. Figure 2 is a schematic diagram of the structure of the multi-core fiber coupler based on a single-clad fiber in this embodiment.

[0047] The multi-core fiber coupler based on a single-clad fiber proposed in this embodiment includes a single-clad fiber 1 and a sleeve 2.

[0048] The sleeve 2 is nested with several single-clad optical fibers 1, and the total number of single-clad optical fibers 1 is equal to the total number of fiber cores in the target multi-core optical fiber 3.

[0049] One end of the sleeve 2 is tapered, and the other end is not tapered; the tapered end of the sleeve 2 is used to connect to the target multi-core optical fiber 3; each single-clad optical fiber 1 in the untapered end of the sleeve 2 is used to connect to a standard single-mode optical fiber, and the mode field diameter of the single-clad optical fiber 1 in the untapered end of the sleeve 2 matches the mode field diameter of a single core of the target multi-core optical fiber 3.

[0050] In the specific implementation process, the total number of single-clad fibers in the multi-core fiber coupler is equal to the total number of fibers in the target multi-core fiber, and the mode field diameter of the single-clad fiber in the untapered end of the sleeve matches the mode field diameter of a single fiber core of the target multi-core fiber, which can effectively reduce the connection loss between the multi-core fiber coupler and the target multi-core fiber.

[0051] In an optional embodiment, the fiber bundle composed of several single-clad optical fibers 1 is nested into the sleeve 2 according to the core arrangement of the target multi-core optical fiber 3.

[0052] One end of the sleeve 2, which contains several single-clad optical fibers 1, is tapered and the flat area of ​​the tapered sleeve 2 is cut off before it is connected to the target multi-core optical fiber 3.

[0053] In an optional embodiment, the expression for the taper ratio r of the multi-core fiber coupler includes: r = R2 / R1

[0054] In the formula, r represents the tapering ratio of the multi-core fiber coupler; R2 represents the core diameter of the single-clad fiber 13 in the tapered end of the sleeve 2; R1 represents the core diameter of the single-clad fiber 13 in the untapered end of the sleeve 2.

[0055] The optimal range of the tapering ratio r of the multi-core fiber coupler includes [R, 1]. When the tapering ratio r is 1, it is assumed that neither end of the sleeve 2 has been tapered. When the tapering ratio r is R, the mode field diameter of the single-clad fiber 1 in the untapered end of the sleeve 2 is equal to the mode field diameter of the single-clad fiber 1 in the tapered end of the sleeve 2.

[0056] In an optional embodiment, the expression for the diameter of the cladding 12 of the single-clad optical fiber 1 in the untapered end of the sleeve 2 includes: d2 = Λ / r

[0057] In the formula, d2 represents the diameter of the cladding 12 of the single-clad optical fiber 1 in the untapered end of the sleeve 2; Λ represents the core spacing of the target multi-core optical fiber 3; and r represents the taper ratio of the multi-core optical fiber coupler.

[0058] In an optional embodiment, the expression for the minimum outer diameter of the sleeve 2 includes: D2 = d3 / r

[0059] In the formula, D2 represents the minimum outer diameter of the sleeve 2; d3 represents the cladding diameter of the target multi-core optical fiber 3; and r represents the taper ratio of the multi-core optical fiber coupler.

[0060] Optionally, the sleeve 2 is a hollow sleeve.

[0061] Optionally, the connection method between the single-clad optical fiber 1 and the target multi-core optical fiber 3 includes fusion splicing or bonding with adhesive; optionally, when using fusion splicing, fusion splicing is achieved by a fusion splicer, the heat source of which is electric arc discharge, carbon dioxide laser or graphite.

[0062] Optionally, the diameter of the cladding 12 is less than 100 micrometers.

[0063] Optionally, the diameter of the fiber core 13 is 4 to 15 micrometers, and the relative refractive index difference with respect to the cladding 12 is 0.003 to 0.007.

[0064] As an example, Figure 3 is a schematic diagram of a small-scale tapered fiber proposed in this embodiment; Figure 4 is a refractive index distribution diagram of a single-clad fiber proposed in this embodiment, wherein the refractive index of the fiber core 13 in Figure 4 exhibits a step-type distribution; Figure 5 is a schematic diagram of the structure of a multi-core fiber proposed in this embodiment, wherein 31 represents the multi-core fiber coating layer, 32 represents the multi-core fiber cladding, and 33 represents the multi-core fiber core.

[0065] The optimal range of the tapering ratio r is considered as a small-ratio tapering, as shown in Figure 3. Taking advantage of the characteristic that the mode field diameter of the optical fiber changes gradually under a small-ratio tapering, that is, the change in the mode field diameter of a single-clad optical fiber is very small under a small-ratio tapering condition, this makes the mode field diameter of the optical fiber after tapering close to the mode field diameter of a single core of a multi-core optical fiber, thus resulting in very low loss. It is particularly important to note that under small-scale tapering conditions, light from a single-clad fiber is still confined to the original core for transmission. Furthermore, single-clad fibers have a simple single-core / single-clad structure (possessing only a core, cladding, and mechanical cladding). In contrast, existing technologies use double-clad fibers, which have a core, a first outer cladding, a recessed outer cladding, and a mechanical cladding. This significantly simplifies fiber fabrication and reduces costs. During tapering, the double-clad fiber actually corresponds to the large-scale tapering shown in Figure 3 (corresponding to the mode field diffusion region). That is, as the fiber bundle taperes, the core size, the first outer cladding, and the recessed cladding size decrease simultaneously until the original core structure can no longer support light transmission, and light leaks from the core into the first outer cladding. At this point, due to the presence of the recessed outer cladding, the first outer cladding / recessed outer cladding forms an optical waveguide transmission structure. Therefore, in the actual design process, it is often necessary to adjust the refractive index difference between the core / first cladding and the first cladding / depressed cladding to make the mode field diameter of the optical fiber close to the mode field diameter of a single core of the target multi-core optical fiber.

[0066] In the proposed solution, as shown in Figure 4, with a small-scale tapering, the light is confined in the fiber core. It is only necessary for the initial mode field diameter of the single-clad fiber to be close to the single-core mode field diameter of the multi-core fiber. By utilizing the characteristic of the slowly varying mode field of the small-scale tapered fiber, a low-loss connection can be achieved after the fiber bundle is tapered with a small scale.

[0067] Preferably, the refractive index of the single-clad fiber is selected with the same parameters as that of the multi-core fiber core; thus, there is no difference in refractive index between the two at the connection interface of the fiber bundle / multi-core fiber, which will not cause back reflection, which is beneficial to reduce signal interference in high-speed communication systems.

[0068] In existing multi-core fiber couplers corresponding to double-clad optical fibers, there is a difference in refractive index between the tapered fiber bundle and the multi-core fiber, which can cause power back reflection, thus hindering the transmission of communication systems.

[0069] In summary, this application utilizes the characteristic of a gradually varying mode field under small-scale tapered fiber conditions, allowing its core size and core / cladding refractive index to be consistent with those of multi-core fibers. Therefore, when connected to multi-core fibers, it exhibits lower connection loss and higher return loss. Furthermore, by utilizing the characteristic of a gradually varying mode field under small-scale tapered fiber conditions, this application can limit the coverage of the field and prevent field leakage outwards, thus resulting in lower inter-core crosstalk after connection with multi-core fibers.

[0070] Example 2

[0071] This embodiment proposes a tapered ratio design method based on a multi-core fiber coupler, applying the multi-core fiber coupler based on a single-clad fiber described in Embodiment 1.

[0072] The tapered ratio design method based on multi-core fiber couplers includes the following steps:

[0073] S1: Adjust the parameters of the single-clad fiber 1 according to the mode field diameter A of a single fiber core in the target multi-core fiber 3, so that the difference between the mode field diameter B and the mode field diameter A of the single-clad fiber 1 in the untapered end of the sleeve 2 is less than a preset threshold.

[0074] S2: Let the initial value of the tapering ratio r of the multi-core fiber coupler be 1. Using simulation software or numerical calculation methods, with the tapering ratio r set as a decreasing variable, calculate the mode field diameter C of the single-clad fiber 1 in the tapered end of the sleeve 2.

[0075] S3: When the tapered ratio r is not 1 and the mold field diameter C is equal to the mold field diameter B, stop the calculation and record the size of the tapered ratio r at this time as R, and obtain the optimal value range of the tapered ratio r [R, 1].

[0076] As an example, based on the mode field diameter A of a single core in the target multi-core fiber 3, the core diameter d1 and relative refractive index difference parameters of the single-clad fiber 1 are designed so that the difference between the mode field diameter B and the mode field diameter A of the single-clad fiber 1 is less than a preset threshold. The smaller the difference between the mode field diameter B and the mode field diameter A of the single-clad fiber 1, the smaller the coupling loss of the multi-core fiber coupler.

[0077] This embodiment also proposes a design method based on a multi-core fiber coupler, applying the multi-core fiber coupler based on a single-clad fiber described in Embodiment 1.

[0078] The design method based on multi-core fiber couplers includes the following steps:

[0079] S1: Based on the core spacing Λ of the target multi-core optical fiber 3, calculate the cladding diameter d2 of the single-clad optical fiber 1 in the untapered end of the sleeve 2. The expression for calculating the cladding diameter d2 includes: d2=Λ / r

[0080] In the formula, r represents the taper ratio of the multi-core fiber coupler;

[0081] S2: After determining the size of the diameter d2 of the cladding 12, according to the fiber core arrangement scheme of the target multi-core fiber 3, the size of the outer circle diameter D1 of the single-cladding fiber 1 under tight arrangement is set as the optimal inner diameter of the sleeve 2.

[0082] S3: Calculate the minimum outer diameter D2 of the sleeve 2 based on the cladding diameter d3 of the target multi-core optical fiber 3. The expression for the minimum outer diameter D2 includes: D2 = d3 / r.

[0083] In this optional embodiment, the change in mode field diameter of a single-clad fiber (small clad diameter fiber) 1 under tapering is calculated using simulation software or numerical calculation methods. The range of tapering ratio [R, 1] is recorded, where 1 represents the single-clad fiber 1 before tapering, and the mode field diameter at this time is B*1 = B. As tapering progresses, the clad diameter and core diameter of the single-clad fiber 1 decrease proportionally, and the mode field diameter B will undergo a process of first decreasing and then increasing. The tapering ratio when the mode field diameter increases to B is recorded as R. R takes the value [0, 1]. Utilizing the characteristic that the mode field diameter of an optical fiber changes slowly within a small tapering range [1, R], when the tapering ratio r of the device is included in [R, 1], a low-loss connection between the single-clad fiber 1 and the multi-core fiber 3 can be achieved after tapering.

[0084] Based on the core spacing Λ of the multi-core fiber 3, the diameter d2 of the cladding 12 of the single-clad fiber 1 can be obtained using the formula d2=Λ / r. Based on the obtained diameter d2 parameter of the cladding 12 of the single-clad fiber 1, and according to the core arrangement scheme of the multi-core fiber 3, the outer circle diameter D1 of the single-clad fiber 1 under tight arrangement is determined to be the optimal inner diameter of the sleeve 2. Based on the cladding diameter d3 of the multi-core fiber 3, the minimum outer diameter D2 of the sleeve 2 can be obtained using the formula D2=d3 / r.

[0085] This embodiment also proposes a method for fabricating a multi-core fiber coupler. Applying the multi-core fiber coupler based on a single-clad fiber described in Embodiment 1, the method includes the following steps:

[0086] S1: After the fiber bundle containing several single-clad optical fibers 1 is nested into the sleeve 2 according to the fiber core arrangement of the target multi-core optical fiber 3, the sleeve 2 is tapered.

[0087] S2: After cutting the flat area of ​​the tapered sleeve 2, connect the sleeve 2 to the target multi-core optical fiber 3;

[0088] S3: Connect each single-clad fiber 1 in the untapered end of the sleeve 2 to a standard single-mode fiber.

[0089] This embodiment proposes a computer device, including a memory and a processor. The memory stores computer-readable instructions. When the computer-readable instructions are executed by the processor, the processor performs the steps of the tapered ratio design method, design method, or fabrication method of the multi-core fiber coupler described in Embodiment 1.

[0090] Example 3

[0091] This embodiment presents a specific implementation example of the tapered ratio design method, design method, and fabrication method based on the multi-core fiber coupler.

[0092] Figure 6 is a schematic diagram showing the change of the mode field diameter of the single-clad fiber proposed in this embodiment as a function of the tapering ratio; Figure 7 is a schematic diagram showing the change of coupling loss when the cladding diameter of the single-clad fiber proposed in this embodiment takes different values.

[0093] In the specific implementation process, a corresponding number of single-clad optical fibers 1 are prepared according to the number of cores of multi-core optical fiber 3. These fibers are inserted into a tapered sleeve 2 and tapered at a small ratio. After tapering, the flat area of ​​the device is cut and connected to the multi-core optical fiber 3. This overcomes the technical problems of connecting multi-core optical fibers with single-mode optical fibers. By utilizing the gradual change of the mode field under the condition of small-ratio tapering, the connection loss, return loss and inter-core crosstalk of the multi-core optical fiber coupler can be effectively reduced.

[0094] The tapered sleeve 2 is a glass tube, and the multi-core fiber 3 is a 4-core fiber. The parameters of the 4-core fiber are: cladding diameter d3 of 125 micrometers, core spacing Λ of 42 micrometers, and mode field diameter A at 1550 nm of 9.5 micrometers. To demonstrate the universality of the proposed method, for the parameters of the single-clad fiber 1, a single-mode fiber conforming to the G652.D fiber standard is selected, with a core diameter 13 of 9.5 micrometers and a relative refractive index difference of 0.0042. The calculated mode field diameter of this fiber is 10.2 micrometers.

[0095] Next, the simulation calculation of the mode field diameter of a single-clad fiber as a function of the tapering ratio is shown in Figure 6. The dotted line represents the change in mode field diameter during the tapering process of the single-clad fiber. The mode field diameter first decreases and then increases as the tapering ratio decreases. When the tapering ratio is 0.7, the mode field diameter returns to its value before tapering. In this process, the change in mode field diameter is only 3.03%. Its coupling loss with the 9.5-micron mode field diameter of a single multi-core fiber is less than 0.03 dB.

[0096] When the taper ratio r is within the range of [0.7, 1], any taper ratio can achieve low-loss connection between the tapered small-clad fiber and the core of a multi-core fiber. Figure 7 shows the coupling loss variation corresponding to different values ​​of the cladding diameter of the single-clad fiber. We select two special points for illustration. When the taper ratio P1 is 0.84, the difference in mode field diameter between the two is the smallest, and the coupling loss is less than 0.01dB. According to the formula d2 = Λ / r, the cladding diameter of the small-clad fiber should be 50 micrometers. When we select point P2, the taper ratio is 0.7, the difference in mode field diameter between the two is the largest, and the coupling loss also reaches its maximum, but it is still less than 0.03dB. According to the formula d2 = Λ / r, the cladding diameter of the small-clad fiber should be 60 micrometers.

[0097] When the cladding diameter of a single-clad fiber is set to 60 micrometers, the corresponding outer circle diameter after arranging 4-core fibers is 145 micrometers, meaning the optimal inner diameter of sleeve 2 is 145 micrometers. Based on the cladding diameter d3 of the multi-core fiber 3 being 125 micrometers, the minimum outer diameter D2 of sleeve 2 can be calculated using the formula D2 = d3 / r, which gives a minimum outer diameter D2 of 178 micrometers.

[0098] After determining the parameters of all materials, the fiber bundle containing four single-clad optical fibers is nested into the sleeve 2 according to the core arrangement of the four-core optical fibers and tapered at a ratio of 0.7. After cutting the flat area of ​​the tapered sleeve, it is connected to the multi-core optical fiber. The fiber bundle of untapered single-clad optical fibers is then connected to the standard single-mode optical fiber.

[0099] The same or similar labels correspond to the same or similar parts;

[0100] The terms used to describe positional relationships in the accompanying drawings are for illustrative purposes only and should not be construed as limiting the scope of this embodiment.

[0101] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A single-clad optical fiber, characterized in that, The single-clad optical fiber (1) includes a core (13), a cladding (12), and a coating layer (11) arranged sequentially from the inside to the outside.

2. The multi-core fiber coupler based on single-clad fiber according to claim 1, characterized in that, The refractive index at the interface between the core (13) and the cladding (12) changes abruptly, and the refractive index at the interface between the cladding (12) and the coating (11) changes abruptly. The refractive index of the cladding (12) is less than that of the core (13), and the refractive index of the coating (11) is less than that of the cladding (12).

3. The multi-core fiber coupler based on single-clad fiber according to claim 1, characterized in that, The refractive index of the fiber core (13) is either step-type or gradually changing.

4. A multi-core fiber coupler based on a single-clad fiber, using the single-clad fiber described in any one of claims 1 to 4, characterized in that, It includes a single-clad optical fiber (1) and a sheath (2); The sleeve (2) is nested with several single-clad optical fibers (1), and the total number of the single-clad optical fibers (1) is equal to the total number of fiber cores in the target multi-core optical fiber (3). One end of the sleeve (2) is tapered, and the other end is not tapered; the tapered end of the sleeve (2) is used to connect to the target multi-core optical fiber (3); each single-clad optical fiber (1) in the untapered end of the sleeve (2) is used to connect to the standard single-mode optical fiber, and the mode field diameter of the single-clad optical fiber (1) in the untapered end of the sleeve (2) matches the mode field diameter of a single core of the target multi-core optical fiber (3).

5. The multi-core fiber coupler based on a single-clad fiber according to claim 4, characterized in that, The fiber bundle composed of several single-clad optical fibers (1) is nested into the sleeve (2) according to the core arrangement of the target multi-core optical fiber (3); One end of the sleeve (2) containing several single-clad optical fibers (1) is tapered and the flat area of ​​the tapered sleeve (2) is cut off before it is connected to the target multi-core optical fiber (3).

6. The multi-core fiber coupler based on a single-clad fiber according to claim 4, characterized in that, The expression for the taper ratio r of the multi-core fiber coupler includes: r = R2 / R1 In the formula, r represents the tapering ratio of the multi-core fiber coupler; R2 represents the core (13) diameter of the single-clad fiber (1) in the tapered end of the sleeve (2); R1 represents the core (13) diameter of the single-clad fiber (1) in the untapered end of the sleeve (2). The optimal range of the tapering ratio r of the multi-core fiber coupler includes [R, 1], where R ≠ 1. When the tapering ratio r is 1, it is assumed that neither end of the sleeve (2) has been tapered. When the tapering ratio r is R, the mode field diameter of the single-clad fiber (1) in the untapered end of the sleeve (2) is equal to the mode field diameter of the single-clad fiber (1) in the tapered end of the sleeve (2).

7. The multi-core fiber coupler based on a single-clad fiber according to any one of claims 4 to 6, characterized in that, The expression for the diameter of the cladding (12) of the single-clad optical fiber (1) in the untapered end of the sleeve (2) includes: d2=Λ / r In the formula, d2 represents the cladding (12) diameter of the single-clad optical fiber (1) in the untapered end of the sleeve (2); Λ represents the core spacing of the target multi-core optical fiber (3); and r represents the taper ratio of the multi-core optical fiber coupler.

8. The multi-core fiber coupler based on a single-clad fiber according to any one of claims 4 to 6, characterized in that, The expression for the minimum outer diameter of the sleeve (2) includes: D2 = d3 / r In the formula, D2 represents the minimum outer diameter of the sleeve (2); d3 represents the cladding diameter of the target multi-core optical fiber (3); and r represents the tapering ratio of the multi-core optical fiber coupler.

9. A tapered ratio design method based on multi-core fiber couplers, using the multi-core fiber coupler based on single-clad fiber as described in any one of claims 4 to 8, characterized in that, Includes the following steps: The parameters of the single-clad fiber (1) are adjusted according to the mode field diameter A of a single fiber core in the target multi-core fiber (3) so that the difference between the mode field diameter B and the mode field diameter A of the single-clad fiber (1) in the untapered end of the sleeve (2) is not greater than a preset threshold. Let the initial value of the tapering ratio r of the multi-core fiber coupler be 1. Using simulation software or numerical calculation methods, with the tapering ratio r set as a decreasing variable, calculate the mode field diameter C of the single-clad fiber (1) in the tapered end of the sleeve (2). When the tapered ratio r is not 1 and the mold field diameter C is equal to the mold field diameter B, the calculation stops, and the size of the tapered ratio r at this time is recorded as R, thus obtaining the optimal range of the tapered ratio r [R, 1].

10. A design method based on a multi-core fiber coupler, using the multi-core fiber coupler based on a single-clad fiber as described in any one of claims 4 to 8, characterized in that, Includes the following steps: Based on the core spacing Λ of the target multi-core optical fiber (3), the cladding (12) diameter d2 of the single-clad optical fiber (1) in the untapered end of the sleeve (2) is calculated. The expressions include: d2=Λ / r In the formula, r represents the taper ratio of the multi-core fiber coupler; After determining the size of the cladding (12) diameter d2, according to the fiber core arrangement scheme of the target multi-core fiber (3), the size of the outer circle diameter D1 of the single-cladding fiber (1) under the tight arrangement is set as the optimal inner diameter of the sleeve (2); The minimum outer diameter D2 of the sleeve (2) is calculated based on the cladding diameter d3 of the target multi-core optical fiber (3). The expression for the minimum outer diameter D2 includes: D2 = d3 / r.

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