Multi-core optical fiber

Abstract

To provide an MCF (multi-core optical fiber) provided with a structure for simultaneously achieving excellent economic rationality and high interchangeability in short distance optical transmission.SOLUTION: An MCF comprises a plurality of core parts, a common cladding, and a resin covering. Each core part has a core, an inner cladding, and a trench layer. The at least four core parts each disposed on a straight line have a relative refractive index difference between the core and the inner cladding, substantially coincident with each other. Each refractive index profile of the first and second core parts adjacent to each other among the four core parts has such a shape that a refractive index of the inner cladding is offset relative to a refractive index of the common cladding so that a magnitude relationship between the refractive indices of the inner cladding and the common cladding is reversed.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to multi-core optical fibers. [Background technology]

[0002] In recent years, research has been actively conducted on multi-core optical fibers (hereinafter referred to as "MCFs") that have multiple cores and a common cladding surrounding each of the multiple cores as an optical transmission medium for long-distance communications.

[0003] For example, Non-Patent Document 1 discloses an MCF in which four trench-assisted cores (cores compatible with cores specified in the ITU-T G.652 standard: hereinafter referred to as "G.652-compatible cores") arranged to form a square lattice in the fiber cross section are embedded in a cladding with an outer diameter of 125 μm. Non-Patent Document 1 discloses that this MCF achieves optical characteristics compatible with fibers specified in the ITU-T G.652 standard (hereinafter referred to as "G.652 fiber") in the wavelength range from O-band to L-band (1260 nm or more and 1625 nm or less).

[0004] Non-Patent Document 2 discloses an MCF in which eight G.652-compatible cores (having a trench-assisted structure) are embedded in a cladding with an outer diameter of 125 μm, with the cores arranged in a circular ring shape. Non-Patent Document 2 discloses that this MCF achieves optical characteristics compatible with G.652 fiber only in the O-band (1260 nm to 1360 nm). In the wavelength range from the C-band to the L-band (1530 nm to 1625 nm), the core light may couple with the coating or leak into the coating, resulting in high transmission loss. Therefore, the MCF disclosed in Non-Patent Document 2 is not compatible with G.652 fiber in the wavelength range from the C-band to the L-band.

[0005] Non-Patent Document 3 discloses a four-core fiber (MCF) that has four cores arranged in a row embedded in a circular cladding. Non-Patent Document 3 does not disclose the structure or characteristics of each core, nor the outer diameter of the cladding, but it suggests that the outer diameter of the cladding of the four-core fiber is larger than the standard outer diameter of 125 μm. Furthermore, by comparing Fig. 1 and Fig. 3 in Non-Patent Document 3, the core pitch of the fiber in Fig. 1 is estimated to be 50 μm, and the outer diameter of the cladding is also estimated to be around 200 μm.

[0006] Non-Patent Document 4 discloses a two-core fiber (MCF) with two step-index cores embedded in a cladding with an outer diameter of 125 μm. In this two-core fiber, the two cores are arranged equidistant from the center of the cladding in the fiber cross section. Each core has a mode field diameter (MFD) of 8.1 μm at a wavelength of 1310 nm and a cutoff wavelength of 1160 nm. Non-Patent Document 4 discloses that this core arrangement can sufficiently suppress transmission loss and crosstalk (XT) at wavelengths of 1310 nm, 1490 nm, and 1550 nm. Non-Patent Document 4 also discloses that by incorporating this two-core fiber into an LC simplex connector, the number of cores equivalent to that of an LC duplex connector can be achieved with the LC simplex connector.

[0007] Non-Patent Document 5 discloses a four-core fiber (MCF) in which four step-index cores are embedded in a cladding with an outer diameter of 125 μm. This four-core fiber is a strongly coupled MCF in which the four cores are closely arranged in a line. In other words, Non-Patent Document 5 describes a fiber that guides a propagation mode in which all four cores form a single waveguide, and it is not possible to suppress crosstalk between the cores.

[0008] Non-Patent Documents 6 to 8 each disclose heterogeneous-core MCFs. They are designed (or prototyped) under the condition that the effective area (or MFD) between the heterogeneous cores is uniform, and the cutoff wavelength is equal to or less than a certain value for all cores. However, chromatic dispersion is ignored. However, with the core structures disclosed in Non-Patent Documents 6 to 8, it is difficult to uniform the chromatic dispersion characteristics between the heterogeneous cores. [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] T. Matsui et al., “Design of 125μm claddingmulti-core fiber with full-band compatibility to conventional single-modefiber,” in Eur. Conf. Opt. Commun. (ECOC), Valencia, 2015, p. We.1.4.5. [Non-patent document 2] T. Hayashi et al., “125-μm-Clading 8-CoreMulti-Core Fiber Realizing Ultra-High-Density Cable Suitable for O-BandShort-Reach Optical Interconnects,” in Opt. Fiber Commun. Conf. (OFC), LosAngeles, 2015, p. Th5C.6. [Non-patent document 3] DL Butler et al., “Multicore optical fiber and connectors forshort reach, high density links,” in IEEE Photon. Conf. (IPC), Burlingame, CA,USA, 2012, pp. 878-879. [Non-patent document 4] Y. Geng et al., “High-speed, bi-directional dual-core fiber transmission system for high-density, short-reach optical interconnects,” in Proc. SPIE, San Francisco, 2015, vol. 9390, Next-Generation Optical Networks for Data Centers and Short-Reach Links II, p. 939009. [Non-Patent Document 5] Y. Kokubun, T. Komo, K. Takenaga, S. Tanigawa, and S. Matsuo, “Selective mode excitation and discrimination of four-core homogeneous coupled multi-core fiber,” Optics express, vol. 19, no. 26, pp. B905-B914, 2011. [Non-Patent Document 6] Y. Amma et al., “High-density Multicore Fiber with Heterogeneous Core Arrangement,” in Opt. Fiber Commun. Conf. (OFC), Los Angeles, 2015, p. Th4C.4. [Non-Patent Document 7] T. Gonda, K. Imamura, R. Sugizaki, Y. Kawaguchi, and T. Tsuritani, “125 μm 5-core fibre with heterogeneous design suitable for migration from single-core system to multi-core system,” in Eur. Conf. Opt. Commun. (ECOC), Dusseldorf, 2016, pp. 547-549. [Non-Patent Document 8] Y. Sasaki et al., “Crosstalk-managed heterogeneous single-mode32-core fiber,” in Eur. Conf. Opt. Commun. (ECOC), 2016, pp. 550-552. Summary of the Invention [Problem to be solved by the invention]

[0010] The inventors have examined conventional MCFs and discovered the following problems.

[0011] That is, as disclosed in the above-mentioned Non-Patent Documents 1 to 8, MCF has been actively researched in recent years, but has not yet been commercialized as a communication transmission medium. This is thought to be because conventional MCF has not been able to simultaneously achieve both economic rationality and wide compatibility. Specifically, conventional MCF cannot simultaneously satisfy the following conditions (1) to (3). Furthermore, considering short-distance interconnects as transmission paths that connect electronic circuits or between semiconductor chips and electronic circuits and enable data communication, it is desirable to develop MCF that satisfies not only the following conditions (1) to (3) but also the following condition (4) included in the ITU-T G.652 standard. (1) The clad outer diameter is 125 μm, or the coating diameter is 250 μm or less. (2) The fiber has four or more cores whose centers are located on a single straight line in the cross section. (3) At least in the O-band (the 1310 nm wavelength band frequently used for short-distance optical transmission, specifically the wavelength band between 1260 nm and 1360 nm), the MFD and cutoff wavelength must be compatible with the ITU-T G.652 standard for general-purpose single-mode optical fiber (hereinafter referred to as "SMF"). (4) The zero-dispersion wavelength is within a 24 nm wavelength band (preferably 1300 nm or more and 1324 nm or less).

[0012] Regarding condition (1), in general, to realize an MCF with reduced inter-core XT, it is necessary to suppress the overlap of the electric field distributions of the modes between the cores. To achieve this, (a) sufficient core spacing, (b) sufficient core confinement, or (c) both are required. To achieve sufficient core spacing, the cladding diameter must be large when the number of cores is large, or, if the cladding diameter is fixed, the number of cores contained in the cladding must be limited to a certain number or less. To strengthen the core confinement, the MFD of each core must be small (compared to the ITU-T G.652 standard). Therefore, if multiple cores with MFDs compatible with G.652 fiber are arranged within the limited standard cladding diameter of 125 μm while suppressing the leakage loss of the outermost core, the number of cores that can be incorporated is limited.

[0013] The above condition (2) is based on the requirement that a one-dimensional core arrangement that enables inexpensive input and output of light to the MCF be achieved, and that as many cores as possible be embedded in a cladding with a standard outer diameter of 125 μm while suppressing degradation of optical properties. In order to suppress degradation of optical properties, it is necessary to maintain a certain minimum spacing between adjacent cores and a certain minimum distance from the outermost core to the cladding.

[0014] The above condition (3) is an optical characteristic suitable for short-distance transmission. For long-distance transmission, low loss in the C band (1530 nm to 1565 nm), high chromatic dispersion, and large A eff While achieving this, it is necessary to suppress crosstalk with as short a core spacing as possible, and therefore the cable cutoff wavelength is significantly greater than 1260 nm (for example, 1300 nm or greater, 1400 nm or greater, or slightly less than 1530 nm). However, such optical fibers are not suitable for short-distance transmission using the O-band (1260 nm to 1360 nm), where low chromatic dispersion can be achieved with ordinary silica glass optical fibers. In addition, such optical fibers do not have low chromatic dispersion in the C-band like dispersion-shifted fibers, and therefore have large chromatic dispersion in the single-mode operating band.

[0015] Regarding condition (4), it is known that XT can be effectively suppressed by ensuring a sufficient difference in the effective refractive index between adjacent cores. However, achieving a sufficient difference in the effective refractive index between adjacent cores requires a large difference in the refractive index profile between the cores. For this reason, it was thought that it would be difficult to achieve an MCF that achieves a sufficient inter-core effective refractive index difference while maintaining optical properties compatible with conventional SMF in terms of MFD, cutoff wavelength, and chromatic dispersion.

[0016] The present disclosure has been made to solve the above-mentioned problems, and aims to provide an MCF having a structure that simultaneously achieves excellent economic rationality and high compatibility in short-distance optical transmission. [Means for solving the problem]

[0017] The multi-core optical fiber (MCF) of the present disclosure includes a plurality of core portions extending along the central axis of the MCF, a common cladding, and a resin coating. Each of the plurality of core portions has a core extending along the central axis, an inner cladding surrounding the outer periphery of the core, and a trench layer surrounding the outer peripheral surface of the inner cladding. The common cladding surrounds the outer peripheral surface of the trench layer of each of the plurality of core portions and has an outer diameter of 124 μm or more and 181 μm or less. The resin coating surrounds the outer peripheral surface of the common cladding and has an outer diameter of 195 μm or more and 250 μm or less. The multi-core optical fiber of the present disclosure also includes a linear array group of at least four core portions, each of whose core centers is located on a line defined on a cross section of the MCF that is orthogonal to the central axis. Each of the four core portions constituting the linear array group has a refractive index profile in which the relative refractive index difference between the core and the inner cladding is at least equal to one another. Furthermore, among the four core sections, the distance between the centers of each core is Λ [μm], and the difference in relative refractive index difference between the cores of the adjacent first core section and second core section, based on the refractive index of the common cladding, is 0.001Λ [%] or more. [Effects of the Invention]

[0018] The MCF of the present disclosure can simultaneously achieve excellent economic efficiency and high compatibility in short-distance optical transmission. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a diagram showing a cross-sectional structure of an MCF according to a first embodiment of the present disclosure. [Figure 2] FIG. 2 is a diagram for explaining the cross-sectional structures and arrangement of adjacent first core portions. [Figure 3] FIG. 3 shows the refractive index profile of the MCF along the line L0 in FIG. [Figure 4] FIG. 4 is a diagram showing a cross-sectional structure of an MCF according to the second embodiment of the present disclosure. [Figure 5] FIG. 5 is a diagram showing a cross-sectional structure of an MCF according to a third embodiment of the present disclosure. [Figure 6] FIG. 6 is a table showing the structural parameters of the prepared samples. [Figure 7] FIG. 7 is a table showing the measurement results of each sample shown in FIG. [Figure 8] Figure 8 is a graph showing the relationship between the fiber bending radius [μm] and the cumulative fracture probability (the fracture probability calculated assuming a one-turn bend after 10 years) for multiple samples with different fiber diameters (outer diameters of the common cladding). DETAILED DESCRIPTION OF THE INVENTION

[0020] First, in order to confirm the structural conditions necessary for the MCF (multi-core optical fiber) of the present disclosure, the MCFs described in the above Non-Patent Documents 1 to 8 will be examined in more detail. As mentioned above, MCFs have been actively researched in recent years, but they have not yet been commercialized as a transmission medium for communications because they cannot simultaneously achieve economic rationality and wide compatibility. In order to improve the economic rationality of MCFs, it is desirable that the cladding outer diameter of the MCF be 125 μm, which is the same as that of conventional standardized optical fibers, and that the number of cores be at least four.

[0021] In each of the above-mentioned Non-Patent Documents 1 and 2, an MCF is proposed in which multiple cores are embedded in a cladding with an outer diameter of 125 μm, with the cores arranged two-dimensionally in the fiber cross section, in order to realize an MCF with a larger number of built-in cores while maintaining optical properties suitable for communications. However, an MCF with such two-dimensional core arrangement requires a three-dimensional core arrangement conversion device for fan-in and fan-out when connecting to an SMF. A "three-dimensional core arrangement conversion device" is a device in which cores are wired three-dimensionally and converts the core arrangement of the MCF into a corresponding core arrangement such as an SMF array or SMF bundle. Optical transceivers use semiconductor substrates for silicon photonics optical integrated circuits. For example, when performing optical input / output (edge ​​coupling) of multiple channels from the edge of a semiconductor substrate parallel to the substrate plane, it is necessary to input / output light of these channels from cores arranged one-dimensionally at the edge of the substrate to cores arranged two-dimensionally in an MCF. In this case, a three-dimensional core arrangement conversion device is also required. A three-dimensional core arrangement conversion device is also required to connect MCFs with different core arrangements. However, such three-dimensional core array conversion devices are difficult to mass-produce, are currently very expensive, and are an obstacle to the practical application of MCFs.

[0022] Furthermore, Non-Patent Document 3, cited above, discloses an MCF in which all four cores are aligned. Using this MCF also facilitates edge coupling with a silicon photonics substrate. Specifically, this MCF has a cladding outer diameter significantly larger than 125 μm, thereby achieving both suppression of XT between cores and excellent optical properties for each individual core. However, optical fibers with cladding outer diameters significantly exceeding the standard outer diameter (125 μm) increase the manufacturing costs of the optical fiber itself. Furthermore, it becomes impossible to use standard-sized products for connecting components (such as connector ferrules and V-groove substrates for arraying). This increases the costs of these connecting components, again making the MCF uneconomical.

[0023] Non-Patent Document 4 above discloses an MCF with a standard cladding outer diameter of 125 μm and all cores arranged in a row. However, the number of cores is only two, and there is little benefit in increasing the number of spatial channels per optical fiber by using an MCF (it is not economically rational).

[0024] The above-mentioned Non-Patent Document 5 discloses an MCF with a standard cladding outer diameter of 125 μm and four cores all arranged in a row. However, in this MCF, the cores are strongly coupled, resulting in extremely high inter-core XT. In other words, with this MCF, each core cannot be used as an independent spatial channel. If the four cores are considered as one waveguide, four spatial modes called supermodes are independently guided. However, it is not easy to create a device that inputs and outputs signals to each supermode individually. Therefore, the MCF in the above-mentioned Non-Patent Document 5 also undermines the economic viability of optical fiber transmission systems using MCFs.

[0025] Each of the above Non-Patent Documents 6 to 8 discloses heterogeneous-core MCFs in which each core has a different refractive index profile. The heterogeneous-core structure results in a large difference in the propagation constant (or effective refractive index) between the cores, suppressing phase matching between the cores (reducing XT). However, it is not easy to increase the difference in effective refractive index between the cores to a degree that is effective in suppressing XT. This results in differences in various optical properties between the cores, which in turn leads to differences in the transmission quality of signal light between the cores. The MCFs described in Non-Patent Documents 6 to 8 are primarily intended for long-distance optical transmission. Therefore, they ensure single-mode operation in the wavelength band used by matching the effective cross-sectional area (determined by the electric field distribution of the propagating light), which is a very important parameter in long-distance optical transmission, and by keeping the cutoff wavelength below a certain value for all cores. Meanwhile, chromatic dispersion is ignored in long-distance optical transmission (because it can be compensated for by digital signal processing). It is considered impossible to match the chromatic dispersion characteristics between heterogeneous cores with the disclosed core structures.

[0026] At present, it is extremely difficult to align the effective cross section and MFD, which are determined by the electric field distribution of the propagating light, between different cores, and also to align the chromatic dispersion between the different cores.In addition, it has been impossible to achieve a difference in the propagation constant between cores while sufficiently strengthening the light confinement within the cores to suppress inter-core XT.

[0027] The power coupling coefficient between cores (or modes) per fiber length can be calculated as follows. As a premise, we assume that the MCF has two cores (modes) and the transmission loss within the MCF can be ignored. Let h be the power coupling coefficient between one core (mode) m and the other core (mode) n, and I be the optical intensity in one core (mode) m. m , the optical intensity in the other core (mode) n is I n , and the position in the longitudinal direction of the fiber is z. In this case, the following equation (1):

number

number

[0028] In particular, when considering application to short-distance interconnects, it is desirable to perform optical transmission with low power consumption by minimizing compensation for signal waveform distortion caused by signal processing. In this case, it is desirable that chromatic dispersion (absolute value) is sufficiently suppressed in the wavelength band used, and that there is little variation between cores. In order to minimize the absolute value of chromatic dispersion in the 1310 nm wavelength band (1260 nm to 1360 nm), which is often used for short-distance optical transmission, the ITU-T G.652 standard requires that the zero-dispersion wavelength be in the range of 1300 to 1324 nm. From this, (1) It is a heterogeneous core MCF with a propagation constant difference between the cores that can sufficiently suppress XT. (2) At the wavelength of 1310 nm used for short-distance optical transmission, the MFD of all cores must be within the range of 8.6 ± 0.6 to 9.5 ± 0.6 μm, the cable cutoff wavelength must be 1260 nm or less, and the zero-dispersion wavelength must be within a 24 nm wavelength band (preferably within the range of 1300 nm to 1324 nm). However, an MCF that satisfies the above specifications has not been known until now, and it is common technical knowledge for those skilled in the art that it is difficult to achieve, as is clear from prior art documents.

[0029] [Description of the embodiments of the present disclosure] Based on the results of the detailed investigations described above, this disclosure proposes an MCF suitable for short-distance optical transmission, which has a large number of cores (four or more, including at least four cores arranged in a row in the fiber cross section) embedded in a cladding with an outer diameter of 124 μm to 181 μm. The MCF of this disclosure is intended for short-distance optical transmission using the O-band (1260 nm to 1360 nm), and tolerates the degradation of optical characteristics that occurs only in long-distance optical transmission using the C-band, etc. On the other hand, the MCF of this disclosure achieves optical characteristics well suited to short-distance optical transmission using the O-band for signal transmission by differentiating adjacent cores with different refractive index profiles.

[0030] More specifically, an example of an MCF according to the present disclosure achieves the required characteristics of the MFD, cutoff wavelength, and chromatic dispersion described above. Furthermore, the MCF according to the example has a standard cladding outer diameter of 125±1 μm, has four or more cores arranged in a row in the fiber cross section, and is an MCF suitable for short-distance optical transmission in the 1310 nm wavelength band.

[0031] Other examples of MCFs disclosed herein also achieve the required characteristics of MFD, cutoff wavelength, and chromatic dispersion. Furthermore, the MCFs according to the other examples have a cladding outer diameter of a standard 180 μm or less, 12 or more cores arranged in a square lattice pattern in the fiber cross section, and are suitable for short-distance optical transmission at wavelengths of 1260 nm or more and 1625 nm or less.

[0032] The contents of the embodiments of the present disclosure will be individually listed and described below. (1) In one embodiment, a multi-core optical fiber (MCF) according to the present disclosure includes a plurality of cores extending along the central axis of the MCF, a common cladding, and a resin coating. Each of the cores includes a core extending along the central axis, an inner cladding surrounding the outer periphery of the core, and a trench layer surrounding the outer surface of the inner cladding. Thus, each of the cores has a trench-assisted structure with a high optical confinement effect. The common cladding surrounds the outer surface of the trench layer of each of the cores and has an outer diameter of 124 μm or more and 181 μm or less, taking into consideration matching with the outer diameter of the cladding of a conventional SMF. The resin coating surrounds the outer surface of the common cladding and has an outer diameter of 195 μm or more and 250 μm or less. The multi-core optical fiber according to the present disclosure also includes a linear array group of at least four cores, each of whose core centers is located on a line defined on a cross section of the MCF that is perpendicular to the central axis. The four core portions constituting the linear array group each have a refractive index profile in which the relative refractive index difference between the core and the inner cladding is at least equal. Furthermore, among the four core portions, the spacing between the core centers is Λ [μm], and the difference in the relative refractive index difference between the cores of adjacent first and second core portions, based on the refractive index of the common cladding, differs by 0.001Λ [%] or more. In other words, adjacent first and second core portions have different structures. Also, as an example, when the linear array group is composed of four core portions, core portions corresponding to the above-mentioned first core portion and core portions corresponding to the above-mentioned second core portion are alternately arranged along the above-mentioned line. Here, "refractive index profile in which the relative refractive index difference between the core and the inner cladding is equal" means a refractive index profile in which the difference in the relative refractive index difference between the core and the inner cladding is 0.02% or less.

[0033] (2) In one embodiment of the present disclosure, the MCF preferably has consistent optical characteristics for all cores in the 1310-nm wavelength band (1260 nm to 1360 nm). Specifically, each of the at least four cores preferably has an MFD of 8.0 μm to 10.1 μm at 1310 nm, a cable cutoff wavelength of 1260 nm or less, and a transmission loss of 0.5 dB / km or less in the 1310-nm wavelength band. The transmission loss in the 1310-nm wavelength band is more preferably 0.4 dB / km or less. In this case, the leakage loss LL to the resin coating is also sufficiently small near the 1310-nm wavelength. Regarding the variation in the zero-dispersion wavelengths of the four cores, the difference between the maximum and minimum zero-dispersion wavelengths is preferably 24 nm or less. Furthermore, when the MCF is bent at a radius of 7 cm to 14 cm or at a radius of 14 cm to 20 cm, the crosstalk (inter-core XT) between the cores in the first core section and the cores in the second core section is preferably 0.01 / km or less. Such an XT value is sufficiently low as the XT between adjacent cores in short-distance optical transmission.

[0034] (3) In one embodiment of the present disclosure, the structural parameters of the first core section are the outer diameters of the core, inner cladding, and trench layer of 2a1 [μm], 2b1 [μm], and 2c1 [μm], respectively. Furthermore, as additional structural parameters of the first core section, the relative refractive index differences of the core, inner cladding, and trench layer based on the refractive index of the common cladding are Δ co,1 [%], Δ ic,1 [%], Δ t,1 [%]. On the other hand, as structural parameters of the second core portion, the outer diameters of the core, inner cladding, and trench layer are set to 2a2 [μm], 2b2 [μm], and 2c2 [μm], respectively. Furthermore, as additional structural parameters of the second core portion, the relative refractive index differences of the core, inner cladding, and trench layer based on the refractive index of the common cladding are set to Δ co,2 [%], Δ ic,2 [%], Δ t,2 [%]. At this time, the first core part and the second core part are Δ co,1 > Δ ic,1 > Δt,1 Δ co,2 > Δ ic,2 > Δ t,2 0.32% ≦ Δ co,1 - Δ ic,1 ≦ 0.40% 0.32% ≦ Δ co,2 - Δ ic,2 ≦ 0.40% Δ t,1 ≦ 0% Δ t,2 < 0% Δ ic,1 ≦ 0.10% -0.10% ≦ Δ ic,2 It is preferable that the following relationship be satisfied. More preferably, Δ ic,2 < 0% < Δ ic,1 In addition, to ensure the suppression of the inter-core XT and the leakage loss LL to the resin coating side, the arrangement conditions of the first core section and the second core section are shown. That is, when the distance between the core center of the first core section and the core center of the second core section is Λ [μm], the first core section and the second core section are 0.001Λ ≦ Δ co,1 - Δ co,2 0.001Λ ≦ Δ ic,1 - Δ ic,2 In one embodiment of the present disclosure, the first core portion and the second core portion preferably satisfy the following relationship: 0.002Λ ≦ Δ co,1 - Δ co,2 0.002Λ ≦ Δ ic,1 - Δ ic,2 The trench layers between the adjacent first core portion and second core portion may be in contact with each other (may be connected to each other).

[0035] (4) In one aspect of the present disclosure, the first core portion and the second core portion are Δ t,1 ≦ -0.5% Δt,2 ≦ -0.5% 0.34≦ a1 / b1≦ 0.42 0.34≦ a2 / b2≦ 0.42 may satisfy the following relationship.

[0036] (5) In one embodiment of the present disclosure, the MCF preferably has a zero-dispersion wavelength of 1300 nm or more and 1324 nm or less. In one embodiment of the present disclosure, the MCF preferably has a zero-dispersion wavelength of 0.092 ps / (nm 2 It is preferable that the MCF has a dispersion slope of 0.25 dB / turn or less (km). As an aspect of the present disclosure, the optical characteristics of the MCF bent at a curvature radius of 5 mm or more, or as an optical characteristic of the MCF bent at a curvature radius of 3 mm or more, the MCF preferably has a bending loss of 0.25 dB / turn or less at a wavelength of 1310 nm. Furthermore, as an aspect of the present disclosure, it is preferable that the common cladding has an outer diameter of 124 μm or more and 126 μm or less. In this case, the distance Λ [μm] between the core centers of the first core portion and the second core portion is 22.5μm≦ Λ ≦ 27.8μm Furthermore, it is preferable that the interval Λ [μm] satisfies the following condition: 23μm≦Λ ≦ 25μm The following conditions may be satisfied.

[0037] (6) In one aspect of the present disclosure, the shortest distance between the outer peripheral surface of the trench layer of the first core portion and the outer peripheral surface of the trench layer of the second core portion is defined as the interval w [μm], or the value given by the formula "Λ-(c1+c2)" is defined as the interval w [μm]. In this case, the interval w [μm] and the interval Λ [μm] are expressed as follows: 0μm≦ w ≦ 2.49μm 0.0133w 3 -0.129w 2 +0.885w+22.5 ≦ Λ ≦ -1.46w+27.8

[0038] It is preferable that the following relationship be satisfied: The amount of deviation d [μm] between the midpoint of the first line segment connecting the outer peripheral surfaces of the trench layers of the first and second core portions at the shortest distance and the midpoint of the second line segment connecting the core centers of the first and second core portions is expressed by the following formula (3):

number

[0039] (7) As an aspect of the present disclosure, on the cross section of the MCF, the multiple core sections including the linear array group described above are preferably arranged so that the positions of their respective core centers are line-symmetric with respect to a reference line passing through the intersection of the central axis (the central axis of the MCF) and the cross section. As an aspect of this embodiment, on the cross section of the MCF, the multiple core sections including the linear array group described above may be arranged so that the positions of their respective core centers have two-fold or more rotational symmetry with respect to the intersection of the central axis (the central axis of the MCF) and the cross section. As an aspect of the present disclosure, the MCF may have a marker having a refractive index different from that of the common cladding. In this case, the marker is arranged at a position that breaks the symmetry (line symmetry, rotational symmetry, etc.) of the core center positions of the multiple core sections. The presence of such a marker makes it possible to distinguish between the first and second core sections. Furthermore, as an aspect of the present disclosure, the outer radius (c1) of the trench layer in the first core section may be different from the outer radius (c2) of the trench layer in the second core section. In this case, too, the refractive index profile of the MCF along the arrangement direction of the four core portions loses the symmetry (linear symmetry, rotational symmetry, etc.) of the refractive index profile on the cross section of the MCF, making it possible to distinguish between the first and second core portions.

[0040] As described above, each aspect listed in the [Description of Embodiments of the Present Disclosure] section can be applied to all of the remaining aspects individually or to all combinations of these remaining aspects.

[0041] [Details of the embodiments of the present disclosure] Specific examples of MCFs (multi-core optical fibers) according to the present disclosure will be described in detail below with reference to the accompanying drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In addition, in the description of the drawings, identical elements are assigned the same reference numerals, and duplicate explanations will be omitted.

[0042] (First embodiment) Fig. 1 is a diagram showing the cross-sectional structure of an MCF according to a first embodiment of the present disclosure. Fig. 2 is a diagram for explaining the cross-sectional structure and arrangement relationship of first core portion 100A and second core portion 100B, which are part of the four core portions (linear array group G0) shown in Fig. 1 and are adjacent to each other. Fig. 3 is a refractive index profile of the MCF along the line L0 in Fig. 1. Note that in Fig. 3, the refractive index level LV of the common cladding 200 is clad is shown.

[0043] In the example shown in FIG. 1 , the MCF 10A of the first embodiment includes four core portions each extending along the central axis AX of the MCF 10A, a common clad 200 surrounding each of the four core portions, and a resin coating 300 surrounding the outer circumferential surface of the common clad 200. The four core portions are composed of two types of core portions (first core portion 100A and second core portion 100B) with different structures, and these first core portions 100A and second core portions 100B are alternately arranged along a straight line L0. That is, the core center AX1 of the first core portion 100A and the core center AX2 of the second core portion 100B are both located on the straight line L0. In this way, a linear array group G0 is configured by at least four core portions (first core portion 100A, second core portion 100B) whose core centers AX1 and AX2 are arranged on the straight line L0.

[0044] The common cladding 200 has an outer diameter of 124 μm or more and 181 μm or less. To suppress an increase in leakage loss LL from each core to the resin cladding 300, an outer cladding thickness (OCT) is set to determine the appropriate position of the linear array group G0 within the cross section of the common cladding 200. In this specification, "OCT" refers to the shortest distance from the center of the core farthest from the central axis AX to the outer surface of the common cladding 200. The resin cladding provided on the outer periphery of the common cladding 200 has an outer diameter of 195 μm or more and 250 μm or less. However, the resin cladding 300 does not need to be composed of a single layer. In the example of FIG. 1 , the resin cladding 300 is composed of an inner cladding 310 surrounding the outer surface of the common cladding 200 and an outer cladding 320 surrounding the outer surface of the inner cladding 310. Considering matching with the cladding outer diameter of a general-purpose SMF, the outer diameter of the common cladding 200 is preferably 124 to 126 μm. In this case, it is desirable that the outer diameter of the resin coating 300 be a minimum of 195 μm, as this can be achieved without impairing optical properties or manufacturability. Also, considering matching with the resin coating outer diameter of a general-purpose SMF, it is desirable that the outer diameter of the resin coating 300 be approximately 250 μm. In this case, it is desirable that the outer diameter of the common cladding 200 be a maximum of 179 to 181 μm, as this can be achieved without impairing optical properties or manufacturability.

[0045] Fig. 2 shows an example of the cross-sectional structure and positional relationship of a first core portion 100A and a second core portion 100B that are adjacent to each other and are part of the four core portions shown in Fig. 1. As shown in Fig. 2, the first core portion 100A has a trench-assisted structure, and the trench-assisted structure is composed of a core 110A that has an outer diameter 2a1 and includes a core center AX1, an inner cladding 120A that has an outer diameter 2b1 and a refractive index lower than that of the core 110A, and a trench layer 130A that has an outer diameter 2c1 and a refractive index lower than that of the inner cladding 120A. Meanwhile, the second core portion 100B also has a trench-assisted structure, and this trench-assisted structure is composed of a core 110B having an outer diameter 2a2 and including a core center AX2, an inner cladding 120B having an outer diameter 2b2 and a refractive index lower than that of the core 110B, and a trench layer 130B having an outer diameter 2c2 and a refractive index lower than that of the inner cladding 120B. The outer radius (c1) of the trench layer 130A in the first core portion 100A may be different from the outer radius (c2) of the trench layer 130B in the second core portion 100B. Furthermore, these adjacent trench layers 130A and 130B may overlap.

[0046] 2, the positional relationship between the adjacent first core portion 100A and second core portion 100B is determined by the shortest distance w [μm] between the trench layer 130A and the trench layer 130B and the interval Λ [μm], which is the center-to-center distance between the core center AX1 and the core center AX2. Specifically, when the common cladding 200 has an outer diameter of 124 μm or more and 126 μm or less, the interval Λ [μm] is 22.5μm≦ Λ ≦ 27.8μm or 23μm≦Λ ≦ 25μm Meet the following conditions.

[0047] Furthermore, when the shortest distance between the outer circumferential surface of the trench layer 130A in the first core portion 100A and the outer circumferential surface of the trench layer 130B in the second core portion 100B is defined as the interval w [μm], or when the value given by the formula "Λ-(c1+c2)" is defined as the interval w [μm], the interval w [μm] and the interval Λ [μm] are expressed as follows: 0μm≦ w ≦ 2.49μm 0.0133w 3 -0.129w 2 +0.885w+22.5 ≦ Λ ≦ -1.46w+27.8 In addition, when the amount of deviation d [μm] between the midpoint of the first line segment connecting the outer circumferential surfaces of the trench layers 130A, 130B of the first and second core portions 100A, 100B at the shortest distance and the midpoint of the second line segment connecting the core centers AX1, AX2 of the first and second core portions 100A, 100B is given by the above formula (3), the amount of deviation d [μm] is d ≦ -(0.104w+0.324)Λ 2 +(5.721w+19.220)Λ-(79.360w+271.139) d ≦ -0.246Λ-0.501w+6.471 d ≧0.439Λ+0.501w-12.539 In addition, in the positional relationship between the first core part and the second core part, if the midpoint of the first line segment is located closer to the first core part than the midpoint of the second line segment, the deviation amount d [μm] will be a negative value.

[0048] FIG. 3 shows a refractive index profile along a straight line L0 (see FIGS. 1 and 2) of the MCF 10A in a cross section of the MCF 10A according to the first embodiment, particularly a refractive index profile of a straight line array group G0 in which first core portions 100A (including core centers AX1) and second core portions 100B (including core centers AX2) are alternately arranged. As shown in FIG. 3, the outer diameters of the cores 110A, inner cladding 120A, and trench layer 130A constituting the first core portion 100A are 2a1 [μm], 2b1 [μm], and 2c1 [μm], respectively. In addition, the relative refractive index difference (relative refractive index difference Δ based on the refractive index of the common cladding 200) of the cores 110A, inner cladding 120A, and trench layer 130A constituting the first core portion 100A is 2a1 [μm], 2b1 [μm], and 2c1 [μm]. clad are shown) are Δ co,1 [%], Δ ic,1 [%], Δ t,1 [%]. The outer diameters of the core 110B, inner cladding 120B, and trench layer 130B constituting the second core portion 100B are 2a2 [μm], 2b2 [μm], and 2c2 [μm], respectively. The relative refractive index differences (relative refractive index differences Δ based on the refractive index of the common cladding 200) of the core 110B, inner cladding 120B, and trench layer 130B constituting the second core portion 100B are Δ co,2 [%], Δ ic,2 [%], Δ t,2 [%].

[0049] In this specification, the relative refractive index difference Δ [%] of any glass region (e.g., cores 110A, 110B, etc.) having a refractive index n1 with respect to the refractive index n0 of the common cladding 200 is expressed as follows: Δ=100(n1 2 -n0 2 ) / 2n0 2 The relative refractive index difference between any two glass regions other than the common cladding 200 is given by the following formula: Therefore, the relative refractive index difference between glass regions having a higher refractive index than the refractive index of the common cladding 200 is a positive value, and the relative refractive index difference between glass regions having a lower refractive index than the refractive index of the common cladding 200 is a negative value. The relative refractive index difference between any two glass regions other than the common cladding 200 is given by the absolute value of the difference between the relative refractive index difference of one glass region based on the refractive index of the common cladding 200 and the relative refractive index difference of the other glass region based on the refractive index of the common cladding 200.

[0050] As shown in FIG. 3, the refractive index profile of each of the four core portions constituting the linear array group G0 is such that at least the relative refractive index difference (Δ co,1 -Δ ic,1 ) and the relative refractive index difference (Δ co,2 -Δ ic,2 ) are designed to substantially match. In particular, the refractive index profile of the first core portion 100A including the core center AX1 is set so that the refractive index of the inner cladding 120A is higher than the refractive index of the common cladding 200. On the other hand, the refractive index profile of the second core portion 100B including the core center AX2 is set so that the refractive index of the inner cladding 120B is lower than the refractive index of the common cladding 200 (the magnitude relationship is reversed from that of the first core portion 100A). The first core portion 100A and the second core portion 100B having such different refractive index profiles are alternately arranged along the straight line L0, thereby realizing an offset structure of the refractive index profiles.

[0051] More specifically, when comparing the structure of the refractive index profile of the first core portion 100A with the structure of the refractive index profile of the second core portion 100B, in this embodiment, the first core portion 100A and the second core portion 100B have the following characteristics: Δ co,1 > Δ ic,1 > Δ t,1 Δ co,2 > Δ ic,2 > Δ t,2 0.32% ≦ Δ co,1 - Δ ic,1≦ 0.40% 0.32% ≦ Δ co,2 - Δ ic,2 ≦ 0.40% Δ t,1 ≦ 0% Δ t,2 < 0% Δ ic,1 ≦ 0.10% -0.10% ≦ Δ ic,2 More preferably, the following relationship is satisfied: Δ ic,2 < 0% < Δ ic,1 In addition, the first core part 100A and the second core part 100B are 0.001Λ ≦ Δ co,1 - Δ co,2 0.001Λ ≦ Δ ic,1 - Δ ic,2 or 0.002Λ ≦ Δ co,1 - Δ co,2 0.002Λ ≦ Δ ic,1 - Δ ic,2 It fulfills a relationship.

[0052] Furthermore, the first core portion 100A and the second core portion 100B are Δ t,1 ≦ -0.5% Δ t,2 ≦ -0.5% 0.34≦ a1 / b1≦ 0.42 0.34≦ a2 / b2≦ 0.42 It is preferable that the following relationship is satisfied:

[0053] (Second embodiment) Fig. 4 is a diagram showing the cross-sectional structure of an MCF according to a second embodiment of the present disclosure. Note that the MCF 10B according to the second embodiment shown in Fig. 4 differs from the MCF 10A according to the first embodiment in the number and arrangement of cores provided in the common clad 200, but provides the same technical effects as the MCF 10A according to the first embodiment.

[0054] The MCF 10B according to the second embodiment shown in FIG. 4 includes 12 cores, a common cladding 200 surrounding each of the 12 cores, and a resin coating 300 surrounding the outer circumferential surface of the common cladding 200. The resin coating 300 is composed of an inner coating 310 surrounding the outer circumferential surface of the common cladding 200 and an outer coating 320 surrounding the outer circumferential surface of the inner coating 310. However, the resin coating 300 may be composed of a single layer. The 12 cores are divided into groups corresponding to the first cores 100A and the second cores 100B, and are arranged to form a square lattice. In addition, in this second embodiment, the 12 cores are arranged along four straight lines L1, L2, L3, and L4 on the cross section (plane perpendicular to the central axis AX) of the MCF 10B. The lines L1 and L2 are parallel to each other, and the lines L3 and L4 are also parallel to each other. Furthermore, the two straight lines L3 and L4 are perpendicular to the two straight lines L1 and L2.

[0055] On the straight line L1, the four core parts that make up the linear array group G1 are arranged such that the first core parts 100A and the second core parts 100B are alternately arranged. On the straight line L2, the four core parts that make up the linear array group G2 are arranged such that the first core parts 100A and the second core parts 100B are alternately arranged. On the straight line L3, the four core parts that make up the linear array group G3 are arranged such that the first core parts 100A and the second core parts 100B are alternately arranged. On the straight line L4, the four core parts that make up the linear array group G4 are arranged such that the first core parts 100A and the second core parts 100B are alternately arranged.

[0056] In the example shown in FIG. 4 , the twelve core portions including the four linear array groups G1 to G4 are arranged such that the positions of their respective core centers AX1 and AX2 are line-symmetric with respect to a reference line intersecting the central axis AX of the MCF 10B. The twelve core portions are also arranged such that the positions of their respective core centers AX1 and AX2 have two-fold or more rotational symmetry with respect to the central axis AX of the MCF 10B. A core portion arrangement with such line symmetry or rotational symmetry makes it difficult to distinguish between the first core portion 100A and the second core portion 100B. Therefore, in the second embodiment, a marker 400 having a refractive index different from that of the common cladding 200 is provided. The marker 400 is arranged at a position that disrupts the symmetry of the arrangement of the core centers AX1 and AX2 of the twelve core portions. The marker 400 enables the first core portion 100A and the second core portion 100B to be distinguished from each other.

[0057] (Third embodiment) Fig. 5 is a diagram showing the cross-sectional structure of an MCF according to a third embodiment of the present disclosure. Note that the MCF 10C according to the third embodiment shown in Fig. 5 differs from the MCF 10A according to the first embodiment in the number and arrangement of cores provided in the common clad 200, but provides the same technical effects as the MCF 10A according to the first embodiment.

[0058] The MCF 10C according to the third embodiment shown in FIG. 5 includes eight cores, a common cladding 200 surrounding each of the eight cores, and a resin coating 300 surrounding the outer circumferential surface of the common cladding 200. The resin coating 300 is composed of an inner coating 310 surrounding the outer circumferential surface of the common cladding 200 and an outer coating 320 surrounding the outer circumferential surface of the inner coating 310. However, the resin coating 300 may be composed of a single layer. The eight cores are composed of a group corresponding to the first core 100A and a group corresponding to the second core 100B. In addition, in the third embodiment, four cores are arranged along a straight line L0 (a straight line intersecting the central axis AX) on a cross section (a plane perpendicular to the central axis AX) of the MCF 10C. The arrangement of the cores on the straight line L0 is the same as in the first embodiment. That is, the four core portions constituting the linear array group G0 are arranged on the straight line L0 such that the first core portions 100A and the second core portions 100B are arranged alternately.

[0059] 5, the eight cores comprising the linear array group G0 are arranged such that the positions of their respective core centers AX1 and AX2 are line-symmetric with respect to a reference line intersecting the central axis AX of the MCF 10C. The eight cores are also arranged such that the positions of their respective core centers AX1 and AX2 have two-fold or more rotational symmetry with respect to the central axis AX of the MCF 10B. In the third embodiment, a marker 400 may be arranged in the same manner as in the second embodiment.

[0060] (Measurement results) Fig. 6 is a table showing the structural parameters of multiple samples prepared for measurement, and Fig. 7 is a table showing the measurement results of each sample shown in Fig. 6.

[0061] The prepared sample #1 is a sample including a linear array group G0 (arrangement elements in which four cores are arranged in a row (series)) similar to the first embodiment (FIG. 1), and the outer diameter of the common clad 200 (hereinafter simply referred to as "clad outer diameter") is 125 μm. Sample #2 is also a sample including a linear array group G0 similar to the first embodiment (FIG. 1), and the clad outer diameter is 125 μm. Sample #3 is also a sample including a linear array group G0 similar to the first embodiment (FIG. 1), and the clad outer diameter is 125 μm. Sample #4 is a sample including four linear array groups G1 to G4 (arrangement elements in which 12 cores are arranged to form a square lattice) similar to the second embodiment (FIG. 4), and the clad outer diameter is 180 μm.

[0062] 6 shows the refractive index of the common cladding 200 (the refractive index level LV of the common cladding 200 in FIG. 3) as a structural parameter of the first core portion 100A in each of the samples #1 to #4. clad The relative refractive index difference Δ co,1 [%], relative refractive index difference of inner cladding 120A Δ ic,1 [%], relative refractive index difference Δ of trench layer 130A t,1 6 shows the relative refractive index difference Δ of the core 110B with the refractive index of the common cladding 200 as a reference value as the structural parameters of the second core portion 100B in each of the samples #1 to #4. co,2 [%], relative refractive index difference of inner cladding 120B Δ ic,2 [%], the relative refractive index difference Δ of the trench layer 130B t,2 6, the distance from the center of the core furthest from the central axis AX of the MCF to the outer surface of the common cladding 200 is shown.

[0063] Fig. 7 is a table showing the measurement results for each of the samples #1 to #4 shown in Fig. 6. The table in Fig. 7 shows the cable cutoff wavelength λ of each of the first core portion 100A and the second core portion 100B. cc [μm], zero dispersion wavelength λ0 [μm], dispersion slope at λ0 (zero dispersion slope) S0 [ps / (nm 2 ·km), MFD [μm], and leakage loss LL [dB / km] are shown. In the table, "inter-core XT" refers to the crosstalk between the adjacent first core portion 100A and second core portion 100B. Note that MFD is a value at a wavelength of 1310 nm. For samples #1 to #3, the leakage loss LL and inter-core XT are shown as values ​​at a wavelength of 1310 nm, and for sample #4, the value is shown as a value at a wavelength of 1625 nm. Inter-core XT is a value at a fiber bending radius of 0.14 m.

[0064] The low-loss, general-purpose SMFs that have been introduced in recent years have achieved a transmission loss of approximately 0.3 dB / km at 1310 nm. Therefore, to achieve a transmission loss of 0.4 dB / km or less at 1310 nm in an MCF, the leakage loss (LL) to the coating at 1310 nm is preferably 0.1 dB / km or less. Since the transmission loss of a typical general-purpose SMF is between 0.32 dB / km and 0.35 dB / km at 1310 nm, to achieve a transmission loss of 0.4 dB / km or less at 1310 nm in an MCF, the leakage loss (LL) to the coating at 1310 nm is preferably 0.08 dB / km or less, and more preferably 0.05 dB / km or less. As can be seen from Figure 7, this required specification is met by this embodiment.

[0065] 7, by the first core region 100A and the second core region 100B satisfying the above-described structural conditions, the MCFs 10A to 10C according to the first to third embodiments can achieve the following preferable optical characteristics: an MFD of 8.0 μm or more and 10.1 μm or less at a wavelength of 1310 nm, a cable cutoff wavelength of 1260 nm or less, and a transmission loss of 0.5 dB / km or less (preferably 0.4 dB / km or less) in the wavelength band of 1260 nm or more and 1360 nm or less. Furthermore, at the zero-dispersion wavelengths of the four core regions constituting the linear array group G0 (or the 12 core regions constituting the linear array groups G1 to G4), the difference (wavelength width) between the maximum and minimum zero-dispersion wavelengths can be adjusted to 24 nm or less. When the MCFs 10A to 10C are bent with a bending radius of 7 cm to 14 cm or 14 cm to 20 cm, the XT between the core 110A in the first core portion 100A and the core 110B in the second core portion 100B is 0.01 / km or less. The zero-dispersion wavelength of the MCFs 10A to 10C can be set within the range of 1300 nm to 1324 nm. Furthermore, the dispersion slope of the MCFs 10A to 10C at such a zero-dispersion wavelength is 0.092 ps / (nm 2 Furthermore, it is also possible to adjust the bending loss of the MCFs 10A to 10C bent at a curvature radius of 5 mm or more (preferably 3 mm or more) to 0.25 dB / turn or less at a wavelength of 1310 nm.

[0066] Next, the basis for the appropriate range of optical characteristics of MCFs 10A to 10C of the present disclosure will be described. Fig. 8 is a graph showing the relationship between the fiber bending radius [µm] and the cumulative fracture probability (the fracture probability calculated assuming 10 years of bending with one turn) for multiple samples with different fiber diameters (outer diameters of the common cladding). In Fig. 8, graph G810 shows the measured values ​​for a sample having a cladding outer diameter of 250 µm (the outer diameter of the common cladding 200 corresponding to the above-mentioned fiber diameter), graph G820 shows the measured values ​​for a sample having a cladding outer diameter of 225 µm, graph G830 shows the measured values ​​for a sample having a cladding outer diameter of 200 µm, graph G840 shows the measured values ​​for a sample having a cladding outer diameter of 175 µm, graph G850 shows the measured values ​​for a sample having a cladding outer diameter of 150 µm, and graph G860 shows the measured values ​​for a sample having a cladding outer diameter of 125 µm.

[0067] A cladding outer diameter of 125±1 μm allows the use of peripheral components such as connector ferrules that are the same as those used with conventional general-purpose SMFs. A cladding outer diameter of 125±1 μm or more and 180±1 μm or less allows the use of resin coatings with an outer diameter of approximately 250 μm equivalent to that of conventional general-purpose SMFs, allowing the use of conventional cabling techniques. This is desirable in terms of reducing the cost of transmission systems. Furthermore, as shown in Figure 8, the probability of fiber breakage when bending increases as the radius of curvature of the fiber decreases, but the threshold radius of curvature at which the probability of breakage rises sharply increases as the cladding outer diameter increases. A cladding outer diameter of 125±1 μm or smaller is desirable because it allows the threshold radius of curvature to be kept below 5 mm.

[0068] Since the transmission loss at a wavelength of 1310 nm is 0.5 dB / km, preferably 0.4 dB / km or less, there is no significant increase in transmission loss compared to single-core SMFs conforming to the currently widely used ITU-T G.652 and G.657 standards.

[0069] Furthermore, if the core's transmission loss is defined as the core's inherent transmission loss, then a pure silica core can achieve an inherent core transmission loss of approximately 0.3 dB / km at a wavelength of 1310 nm. A GeO-doped core can achieve an inherent core transmission loss of 0.32 dB / km or more and 0.35 dB / km or less. Therefore, in the MCF of the present disclosure, the "transmission loss" at a wavelength of 1310 nm, calculated by adding the "core's inherent transmission loss" and the "leakage loss LL," is preferably 0.5 dB / km or less, or even 0.4 dB / km or less. It is even more preferable that this "transmission loss" be 0.4 dB / km or less across the entire O-band.

[0070] With an MFD of 8.0 μm or more and 10.1 μm or less at a wavelength of 1310 nm, it is possible to reduce connection loss with systems and components that are based on conventional general-purpose SMF. In addition, connection loss can be kept small even if a certain degree of axial misalignment occurs.

[0071] At a wavelength of 1310 nm, it is preferable that the bending loss be 0.25 dB / turn or less when the radius of curvature R is 5 mm or more or 3 mm or more. In this case, when used as optical interconnect wiring in high-performance computing, data centers, etc., even if the fiber is bent with a small radius of curvature, a large increase in transmission loss can be suppressed. Furthermore, by having a bending loss of 0.10 dB / turn or less when the radius of curvature R is 3 mm at a wavelength of 1310 nm, it is possible to further suppress an increase in transmission loss even under more severe conditions.

[0072] By setting the zero-dispersion wavelength between 1300 nm and 1324 nm, it is possible to minimize chromatic dispersion in the O-band used for short-distance optical transmission, thereby reducing the cost (price and power consumption) of chromatic dispersion compensation in the transceiver.

[0073] A cable cutoff wavelength of 1260 nm or less ensures single-mode operation in the O band used for short-distance optical transmission. It is even more preferable that the 2 m cutoff wavelength is 1260 nm or less.

[0074] At a wavelength of 1310 nm, when XT between adjacent cores is 0.001 / km or less, noise caused by inter-core XT can be kept sufficiently small when transmitting signal light in counter-propagation directions between adjacent cores. Also, when XT between adjacent cores is 0.001 / km or less, noise caused by inter-core XT can be kept sufficiently small even when transmitting signal light in the same propagation direction in all cores.

[0075] As mentioned above, the core with the shortest distance from the core center to the resin coating among the four or more cores has a transmission loss of more than 0.4 dB / km at wavelengths of 1550 nm or 1625 nm. By deliberately setting a large value that does not comply with the ITU-T G.652 or G.657 standards, a smaller OCT can be achieved. In this case, the outermost core can be placed closer to the coating, allowing for more cores to be packed in series within a cladding with an outer diameter of approximately 125 μm. It also allows for cores with larger MFDs to be packed within a cladding with an outer diameter of approximately 125 μm. Furthermore, it also widens the manufacturing design margin for variations in the core-to-core XT and spacing Λ (the center-to-center distance between adjacent cores).

[0076] The chromatic dispersion slope at the zero-dispersion wavelength is 0.092 ps / (nm 2 ·km) or less, distortion of the signal waveform due to higher-order chromatic dispersion can be suppressed during high-speed optical transmission. Also, when using a wide wavelength range within the O-band for wavelength division multiplexing transmission, the maximum value of chromatic dispersion within the band used can be suppressed (distortion of the signal waveform can be suppressed).

[0077] The MCF of the present disclosure is preferably an all-solid-state fiber with no air holes in the cladding. In the case of an MCF with air holes, foreign matter such as water may enter the air holes, affecting the transmission characteristics of the core. To prevent this, the air holes must be sealed at the fiber end face, and the air holes increase the costs of fiber splicing and fiber connector attachment. Furthermore, since the air pressure inside the air holes must be precisely controlled during fiber drawing (the air hole diameter must be controlled to achieve the desired core optical characteristics), the air holes also increase manufacturing costs. [Explanation of symbols]

[0078] 10A, 10B, 10C...MCF, 100A...first core portion, 100B...second core portion, 110A, 110B...core, 120A, 120B...inner clad, 130A, 130B...trench layer, 200...common clad, 300...resin coating, 310...inner coating, 320...outer coating, 400...marker, G0, G1, G2, G3, G4...linear array group.

Claims

1. a plurality of core portions extending along a central axis, each of the core portions having a core extending along the central axis, an inner clad surrounding an outer periphery of the core, and a trench layer surrounding an outer periphery of the inner clad; a common clad surrounding an outer peripheral surface of the trench layer of each of the plurality of core portions; a resin coating surrounding an outer peripheral surface of the common clad; A multi-core optical fiber comprising: the plurality of core portions each have a refractive index profile in which the relative refractive index difference between at least the core and the inner cladding is equal, Among the plurality of core portions, the interval between the respective core centers is Λ [μm], and the difference in relative refractive index difference between the cores of the first core portion and the second core portion adjacent to each other, based on the refractive index of the common cladding, is 0.001 Λ [%] or more, an outer radius of the trench layer in the first core portion is different from an outer radius of the trench layer in the second core portion; Single-mode operation in O-band Multicore optical fiber.

2. a plurality of core portions extending along a central axis, each of the core portions having a core extending along the central axis, an inner clad surrounding an outer periphery of the core, and a trench layer surrounding an outer periphery of the inner clad; a common clad surrounding the outer peripheral surface of the trench layer of each of the plurality of core portions and having an outer diameter of 124 μm or more and 181 μm or less; a resin coating surrounding an outer peripheral surface of the common clad; A multi-core optical fiber comprising: a linear array group composed of at least four core portions, each having a core center positioned on a straight line defined on a cross section of the multi-core optical fiber and perpendicular to the central axis, the four core portions constituting the linear array group each have a refractive index profile in which the relative refractive index difference between at least the core and the inner cladding is equal, Among the four core portions, the interval between the respective core centers is Λ [μm], and the difference in relative refractive index difference between the cores of the first core portion and the second core portion adjacent to each other, based on the refractive index of the common cladding, is 0.001 Λ [%] or more, On the cross section of the multi-core optical fiber, the plurality of core portions including the linear array group are arranged such that the positions of the core centers are line-symmetric with respect to a reference line passing through an intersection of the central axis and the cross section as an axis of symmetry, an outer radius of the trench layer in the first core portion is different from an outer radius of the trench layer in the second core portion; Single-mode operation in O-band Multicore optical fiber.

3. a plurality of core portions extending along a central axis, each of the core portions having a core extending along the central axis, an inner clad surrounding an outer periphery of the core, and a trench layer surrounding an outer periphery of the inner clad; a common clad surrounding the outer peripheral surface of the trench layer of each of the plurality of core portions and having an outer diameter of 124 μm or more and 181 μm or less; a resin coating surrounding an outer peripheral surface of the common clad; A multi-core optical fiber comprising: a linear array group composed of at least four core portions, each having a core center positioned on a straight line defined on a cross section of the multi-core optical fiber and perpendicular to the central axis, the four core portions constituting the linear array group each have a refractive index profile in which the relative refractive index difference between at least the core and the inner cladding is equal, Among the four core portions, the interval between the respective core centers is Λ [μm], and the difference in relative refractive index difference between the cores of the first core portion and the second core portion adjacent to each other, based on the refractive index of the common cladding, is 0.001 Λ [%] or more, on the cross section of the multi-core optical fiber, the plurality of core portions including the linear array group are arranged such that positions of the core centers have two-fold or more rotational symmetry with respect to an intersection point between the central axis and the cross section as a rotation center, an outer radius of the trench layer in the first core portion is different from an outer radius of the trench layer in the second core portion; Single-mode operation in O-band Multicore optical fiber.

4. The resin coating is composed of a single layer. The multi-core optical fiber according to claim 1 .

5. the resin coating is composed of an inner coating surrounding the outer peripheral surface of the common clad and an outer coating surrounding the outer peripheral surface of the inner coating. The multi-core optical fiber according to claim 1 .

6. each of the plurality of core portions has a mode field diameter of 8.0 μm or more and 10.1 μm or less at a wavelength of 1310 nm, a cable cutoff wavelength of 1260 nm or less, and a transmission loss of 0.5 dB / km or less in a wavelength band of 1260 nm or more and 1360 nm or less; Among the zero-dispersion wavelengths of the plurality of core portions, a difference between a maximum zero-dispersion wavelength and a minimum zero-dispersion wavelength is 24 nm or less; when the multi-core optical fiber is bent at a bending radius of 7 cm or more and 14 cm or less, or at a bending radius of 14 cm or more and 20 cm or less, crosstalk between the cores in the first core portion and the cores in the second core portion is 0.01 / km or less. The multi-core optical fiber according to claim 1 .

7. As the structural parameters of the first core portion, the outer diameters of the core, the inner cladding, and the trench layer are each 2a １ [μm], 2b １ [μm] and 2c １ [μm], and the relative refractive index differences of the core, the inner cladding, and the trench layer based on the refractive index of the common cladding are Δ ｃｏ，１ [%], Δ ｉｃ，１ [%], Δ ｔ,１ [%]year, As the structural parameters of the second core portion, the outer diameters of the core, the inner cladding, and the trench layer are each 2a ２ [μm], 2b ２ [μm] and 2c ２ [μm], and the relative refractive index differences of the core, the inner cladding, and the trench layer based on the refractive index of the common cladding are Δ ｃｏ，２ [%], Δ ｉｃ，２ [%], Δ ｔ,２ When the ratio is expressed as [%], the first core portion and the second core portion are D ｃｏ，１ > D ｉｃ，１ > D ｔ，１ D ｃｏ，２ > D ｉｃ，２ > D ｔ，２ 0.32% ≦ D ｃｏ，１ - D ｉｃ，１ ≦ 0.40% 0.32% ≦ D ｃｏ，２ - D ｉｃ，２ ≦ 0.40% D ｔ,１ ≦ 0% D ｔ，２ < 0% D ｉｃ，１ ≦ 0.10% -0.10% ≦ D ｉｃ，２ fulfilling the relationship The multi-core optical fiber according to claim 1 .

8. The first core portion and the second core portion are D ｔ，１ ≦ -0.5% D ｔ，２ ≦ -0.5% 0.34≦ a １ / b １ ≦ 0.42 0.34≦ a ２ / b ２ ≦ 0.42 fulfilling the relationship The multi-core optical fiber according to claim 7.

9. A zero dispersion wavelength of 1300 nm or more and 1324 nm or less, The multi-core optical fiber according to claim 1 .

10. At the zero dispersion wavelength, it is 0.092 ps / (nm ２ -has a dispersion slope of less than or equal to 1.5 km, The multi-core optical fiber according to claim 9.

11. The optical characteristics of the multi-core optical fiber bent at a curvature radius of 5 mm or more include a bending loss of 0.25 dB / turn or less at a wavelength of 1310 nm. The multi-core optical fiber according to any one of claims 1 to 10.

12. the common clad has an outer diameter of 124 μm or more and 126 μm or less; The interval Λ [μm] is 22.5μm≦Λ≦27.8μm satisfy the following conditions, The multi-core optical fiber according to claim 7.

13. the common clad has an outer diameter of 124 μm or more and 126 μm or less; When the distance between the core center of the first core portion and the core center of the second core portion is Λ [μm], the distance Λ [μm] is 22.5μm≦Λ≦27.8μm satisfy the following conditions, The multi-core optical fiber according to any one of claims 8 to 11.

14. The shortest distance between the outer peripheral surface of the trench layer of the first core portion and the outer peripheral surface of the trench layer of the second core portion is defined as a distance w [μm], or the following formula: L- (c) １ +｡ ２ ) When the value given by the formula is the interval w [μm], The interval w [μm] and the interval Λ [μm] are 0μm≦w≦2.49μm 0.0133w ３ -0.129w ２ +0.885w+22.5 ≦ Λ ≦ -1.46w+27.8 Fulfilling the relationship, The deviation amount d [μm] between the midpoint of a first line segment connecting the outer peripheral surfaces of the trench layers of the first and second core portions at the shortest distance and the midpoint of a second line segment connecting the core centers of the first and second core portions is expressed by the following formula (1): [Equation 1] When the deviation d [μm] is given by d ≦ -(0.104w+0.324)Λ ２ +(5.721w+19.220)Λ-(79.360w+271.139) d ≦ −0.246Λ−0.501w+6.471 d ≧0.439Λ+0.501w-12.539 fulfilling the relationship The multi-core optical fiber according to claim 11 or 13.

15. a marker having a refractive index different from that of the common cladding, the marker being arranged at a position that disrupts the symmetry of the arrangement of the core centers in the plurality of core portions; The multi-core optical fiber according to claim 2 or 3.

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