Multi-core optical fiber

Abstract

The MCF according to one embodiment simultaneously achieves excellent economic rationality and high compatibility in short-distance optical transmission. The MCF includes a plurality of core parts, a common cladding and a resin coating. Each core part includes a core, an inner cladding and a groove layer. At least four core parts arranged in a straight line have substantially the same relative refractive index difference between the core and the inner cladding. The refractive index distribution of the first core part and the second core part adjacent to each other among the four core parts has such a shape, wherein the refractive index of the inner cladding is offset relative to the refractive index of the common cladding, so that the magnitude relationship of the refractive index between the inner cladding and the common cladding is opposite.

Description

Technical Field

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

[0002] In recent years, as an optical transmission medium for long-distance communication, research on a multi-core optical fiber (hereinafter referred to as "MCF") including a plurality of cores and a common cladding surrounding each of the plurality of cores has been vigorously advanced.

[0003] For example, non-patent document 1 (T. Matsui et al., “Design of 125μm cladding multi-core fiber with full-band compatibility to conventional single-mode fiber,” in Eur. Conf. Opt. Commun. (ECOC), Valencia, 2015, p. We. 1.4.5) discloses an MCF in which four groove-assisted structured cores (cores compatible with the 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 optical fiber cross section are contained in a cladding having an outer diameter of 125μm. Non-patent document 1 discloses that the MCF achieves optical characteristics compatible with optical fibers defined in the ITU-T G.652 standard (hereinafter referred to as "G.652 optical fibers") within the wavelength range from the O band to the L band (1260 nm to 1625 nm).

[0004] Non-patent document 2 (T. Hayashi et al., "125-μm-Cladding 8-Core Multi-Core Fiber Realizing Ultra-High-Density Cable Suitable for O-Band Short-Reach Optical Interconnects," in Opt. Fiber Commun. Conf. (OFC), Los Angeles, 2015, p. Th5C.6) discloses an MCF in which eight G.652-compatible cores (having a groove-assisted structure) are arranged in a ring and thus contained in a cladding having an outer diameter of 125 μm. Non-patent document 2 discloses that the MCF achieves optical properties that are compatible with G.652 optical fiber only in the O band (above 1260 nm and below 1360 nm). In the wavelength range from C band to L band (1530 nm or more and 1625 nm or less), the light in the core may be coupled to the coating or leak to the coating, thereby increasing the transmission loss. Therefore, in the wavelength range from C band to L band, the MCF disclosed in non-patent document 2 is not compatible with G.652 optical fiber.

[0005] Non-patent document 3 (DL Butler et al., "Multicore optical fiber and connectors for short reach, high density links," in IEEE Photon. Conf. (IPC), Burlingame, CA, USA, 2012, pp. 878-879) discloses a quad-core optical fiber (MCF) that includes four cores arranged in a line in an annular cladding. Non-patent document 3 does not disclose the outer diameter of the cladding, the structure or characteristics of each core, but proposes that the outer diameter of the cladding of the quad-core optical fiber is greater than the standard outer diameter of 125 μm. From the non-patent document 3, Figure 1 and Figure 3 The comparison between them shows that Figure 1 The core pitch of the optical fiber 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 (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) discloses a two-core optical fiber (MCF) having two step-type refractive index cores contained in a 125 μm outer diameter cladding. In this two-core optical fiber, in the optical fiber cross section, the two cores are arranged at the same distance from the center of the cladding. In addition, each core has a mode field diameter (hereinafter referred to as "MFD") of 8.1 μm at a wavelength of 1310 nm and a cut-off wavelength of 1160 nm. Non-patent document 4 discloses that such a core arrangement can sufficiently suppress transmission loss and crosstalk (hereinafter referred to as "XT") at each wavelength of 1310 nm, 1490 nm, and 1550 nm. Non-patent document 4 further discloses that by installing two-core optical fibers on an LC simplex connector, the number of cores equivalent to that of an LC duplex connector can be achieved through the LC simplex connector.

[0007] 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) discloses a four-core fiber (MCF) in which four step-type refractive index cores are contained in a cladding having an outer diameter of 125 μm. The four-core fiber is a strongly coupled MCF in which four cores are closely arranged in a line with each other. That is, the optical fiber according to non-patent document 5 is an optical fiber that uses all four cores as one waveguide to guide waves in a propagation mode, and therefore cannot suppress crosstalk between the cores.

[0008] 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, "125μm 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) and Non-patent document 8 (Y.Sasaki et al., "Crosstalk-managed heterogeneous single-mode 32-core Each of the non-patent documents 6 to 8 discloses a heterogeneous core MCF. The design (or prototype production) is performed under the following conditions: the effective cross-sectional area (effective area) or MFD is made equal between the heterogeneous cores, and the cutoff wavelength is set to a certain value or less for all the cores. However, the dispersion is ignored. Unfortunately, however, it is difficult to make the dispersion characteristics equal between the heterogeneous cores using the core structures disclosed in non-patent documents 6 to 8. Summary of the invention

[0009] As a result of the investigation conducted on the traditional MCF, the inventors of the present invention found the following problems. That is, as disclosed in the above non-patent documents 1 to 8, even though vigorous research has been conducted in recent years, MCF has not yet been commercialized as a communication transmission medium. This may be because the traditional MCF cannot achieve both economic rationality and wide compatibility at the same time. Specifically, it is difficult for the traditional MCF to simultaneously meet the following conditions (1) to (3). In addition, when it is assumed that short-distance interconnection as a transmission line connected between electronic circuits or between a semiconductor chip and an electronic circuit can achieve data communication, it is expected to develop a MCF that meets the following condition (4) included in the ITU-T G.652 standard and meets the following conditions (1) to (3).

[0010] (1) The outer diameter of the cladding is 125 μm, or the coating diameter is 250 μm or less.

[0011] (2) There are four or more cores whose centers are located on a straight line in the cross section of the optical fiber.

[0012] (3) There exists an MFD and cutoff wavelength that are compatible with a standard single-mode optical fiber (hereinafter referred to as "SMF") conforming to the ITU-T G.652 standard, at least in the O-band (the 1310 nm wavelength band commonly used in short-distance optical transmission, particularly the wavelength band above 1260 nm and below 1360 nm).

[0013] (4) The zero dispersion wavelength falls within a 24 nm wide wavelength band (preferably 1300 nm or more and 1324 nm or less).

[0014] Regarding the above condition (1), in order to achieve an MCF that suppresses the XT between cores, it is generally necessary to suppress the overlap of the electric field distribution of the modes between the cores. For this purpose, it is necessary to (a) have sufficient core spacing between each other, (b) fully confine the light within the core, or (c) satisfy both conditions. In order to have sufficient core spacing between each other, in the case of many cores, it is necessary to increase the outer diameter of the cladding, or in the case where the outer diameter of the cladding is predetermined, it is necessary to limit the number of cores contained in the cladding to a certain number or less. In order to enhance the light confinement within the core, the MFD of each core must be reduced (to below the ITU-T G.652 standard). For this reason, in order to suppress the leakage loss on the outermost core, multiple cores with an MFD compatible with G.652 optical fiber are arranged in a cladding with a limited standard 125μm outer diameter, which will reduce the number of cores that may be included.

[0015] The above condition (2) is based on the following requirement: while suppressing the degradation of optical characteristics, a one-dimensional array core arrangement that can realize light in / out on the MCF inexpensively is used, and a large number of cores are included in a cladding with a standard outer diameter of 125 μm. In addition, in order to suppress the degradation of optical characteristics, the interval between adjacent cores and the distance from the outermost core to the coating need to be maintained above a certain level.

[0016] Condition (3) is an optical property suitable for short-distance transmission. For long-distance transmission, it is necessary to achieve low loss, high dispersion and large A in the C band (above 1530nm and below 1565nm). eff At the same time, crosstalk is suppressed with the shortest possible core spacing. Therefore, the cable cut-off wavelength is set to be significantly greater than 1260nm (for example, more than 1300nm, more than 1400nm, or slightly less than 1530nm). However, such optical fiber is not suitable for short-distance transmission using the O band (more than 1260nm and less than 1360) that can achieve low dispersion in typical quartz glass optical fibers. In addition, the optical fiber as described above is different from the dispersion-shifted optical fiber and does not necessarily have low dispersion in the C band, so it has large dispersion in the operating band in single mode.

[0017] Regarding the above condition (4), it is known that XT can be effectively suppressed when there is a sufficiently large effective refractive index difference between adjacent cores. However, in order to have a sufficiently large effective refractive index difference between adjacent cores, it is necessary to have a large refractive index profile difference between the cores. Therefore, it is considered difficult to achieve an MCF that obtains a sufficiently large effective refractive index difference between cores while maintaining optical properties compatible with standard SMFs in terms of MFD, cutoff wavelength, and dispersion.

[0018] The present disclosure has been made to solve the problems as described above, and the present invention aims to provide an MCF having a structure for simultaneously achieving excellent economic rationality and high compatibility in short-distance optical transmission.

[0019] The disclosed multi-core optical fiber (MCF) of the present invention 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 includes: a core extending along the central axis; an inner cladding surrounding the periphery of the core; and a groove layer surrounding the outer peripheral surface of the inner cladding. The common cladding surrounds the outer peripheral surface of the groove 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. In addition, the disclosed multi-core optical fiber of the present invention includes a linear array group consisting of at least four core portions, each of the four core portions having a core center located on a straight line defined on a cross section of the MCF, the cross section being orthogonal to the central axis. Each of the four core sections constituting the linear array group has a refractive index distribution in which at least the relative refractive index difference between the core and the inner cladding is equal to the relative refractive index difference between the core and the inner cladding of the other core sections. In addition, the four core sections include a first core section and a second core section adjacent to each other with an interval Λ in μm between the centers of the respective cores, and the relative refractive index difference between the first core section and the second core section relative to the refractive index of the common cladding differs by more than 0.001Λ in %. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a diagram showing a cross-sectional structure of an MCF according to a first embodiment of the disclosure of the present invention;

[0021] Figure 2 is a diagram showing a cross-sectional structure and arrangement relationship between a first core portion and a second core portion adjacent to each other;

[0022] Figure 3 It is along the MCF Figure 1 The refractive index distribution of the straight line shown in;

[0023] Figure 4 is a diagram showing a cross-sectional structure of an MCF according to a second embodiment of the disclosure of the present invention;

[0024] Figure 5 is a diagram showing a cross-sectional structure of an MCF according to a third embodiment of the disclosure of the present invention;

[0025] Figure 6 is a table showing structural parameters of a plurality of prepared samples;

[0026] Figure 7 It is shown Figure 6 a table of the measurement results for each sample shown in ; and

[0027] Figure 8 It is a graph showing the relationship between the fiber bending radius [μm] and the cumulative breakage probability (the breakage probability calculated assuming 1 turn bending for 10 years) for a plurality of samples having different fiber diameters (outer diameter of a common cladding). DETAILED DESCRIPTION

[0028] First, in order to confirm the structural conditions required for the disclosed multi-core optical fiber (MCF) of the present invention, the MCF described in the above-mentioned non-patent documents 1 to 8 will be further investigated in detail. As described above, MCF has been actively studied in recent years, but because it is difficult to simultaneously satisfy economic rationality and wide compatibility, MCF has not yet been commercialized as a transmission medium for communication. In order to improve the economic rationality of MCF, it is desired that the outer diameter of the cladding of the MCF is 125 μm, which is equal to the outer diameter of the cladding of a conventional standardized optical fiber, and the number of cores is desired to be at least four or more.

[0029] Each of the above-mentioned non-patent documents 1 and 2 proposes an MCF, wherein, in order to realize an MCF containing more cores while maintaining optical properties suitable for communication, multiple cores are contained in a cladding with an outer diameter of 125 μm in a state of two-dimensional arrangement in the cross section of the optical fiber. However, the MCF including the cores arranged two-dimensionally as described above requires a three-dimensional core array conversion device for fan-in / fan-out when connected to the SMF. A "three-dimensional core array conversion device" is a device in which the cores are three-dimensionally wired, and is a device that converts the MCF core arrangement into a core array such as a corresponding SMF array or SMF bundle. An optical transceiver utilizes a semiconductor substrate of an optical integrated circuit in a silicon photonic device (silicon photonics), etc. As an example, in the case of performing light in / out (edge ​​coupling) from an edge of a semiconductor substrate parallel to the substrate plane in multiple channels, it is necessary to perform light in / out of these channels from a core arranged one-dimensionally at the edge of the substrate to a core arranged two-dimensionally of the MCF, and in this case, a three-dimensional core array conversion device is also required. In addition, a three-dimensional core array conversion device is also required to connect MCFs with different core arrays. Unfortunately, however, such three-dimensional core array conversion devices are difficult to mass produce and are currently very expensive, thus becoming an obstacle to the practical use of MCFs.

[0030] The above-mentioned non-patent document 3 discloses a MCF in which all four cores are arranged in a line. The use of this MCF will facilitate the coupling of the edge with the silicon photonic substrate. Specifically, the MCF has a cladding outer diameter much larger than 125 μm, thereby achieving both the suppression of the inter-core XT in each core and good optical properties. Unfortunately, however, the optical fiber whose cladding outer diameter greatly exceeds the standard outer diameter (125 μm) will increase the manufacturing cost of the optical fiber itself. In addition, it will be difficult to use products with standard sizes as connecting components (connector ferrules or V-groove substrates for arrays). This will also increase the cost of these connecting components, thereby causing the economic rationality of the MCF to be impaired.

[0031] The above-mentioned non-patent document 4 discloses an MCF in which the outer diameter of the cladding is a standard 125 μm and all the cores are arranged in a line. Unfortunately, however, the number of cores is only two, resulting in a low benefit (low economic rationality) in increasing the number of spatial channels per optical fiber brought about by using MCF.

[0032] The above-mentioned non-patent document 5 discloses an MCF in which the outer diameter of the cladding is a standard 125 μm and all four cores are arranged in a line. Unfortunately, however, in this MCF, the cores are strongly coupled to each other, resulting in extremely high inter-core XT. That is, this MCF cannot allow each core to be used as an independent spatial channel. When the four cores are regarded as one waveguide, three spatial modes called supermodes are independently used as waveguides, in which it will not be easy to create a device for inputting and outputting signals to each supermode separately. Therefore, the MCF of non-patent document 5 also undermines the economic rationality of the optical fiber transmission system using MCF.

[0033] Each of non-patent documents 6 to 8 discloses a heterogeneous core MCF in which the refractive index distribution is different in each core. By adopting a heterogeneous core structure, a large propagation constant (or effective refractive index) difference will appear between the cores, thereby suppressing the phase matching between the cores (suppressing XT). On the other hand, it is not easy to make the effective refractive index difference between the cores large enough to suppress XT. That is, various optical properties will have differences between the cores, resulting in differences in the transmission quality of signal light between the cores. Note that the MCF of each of non-patent documents 6 to 8 is mainly focused on the application of long-distance optical transmission, so the effective cross-sectional area (determined by the electric field distribution of the propagating light) as an important parameter in long-distance optical transmission is equalized, and for all cores, the cutoff wavelength is set to a fixed value or less, thereby ensuring single-mode operation in the wavelength band used. On the other hand, dispersion is not considered in long-distance optical transmission (because it can be compensated by digital signal processing). The disclosed core structure is considered to make it difficult to equalize the dispersion characteristics between heterogeneous cores.

[0034] It is currently extremely difficult to equalize not only the effective cross-sectional area and MFD determined by the electric field distribution of the propagating light, but also the dispersion between heterogeneous cores. In addition, it is impractical to obtain different propagation constants between cores while sufficiently enhancing the light confinement within the core to suppress the XT between cores.

[0035] The inter-core (or inter-mode) power coupling coefficient per fiber length can be obtained as follows. As a premise, the MCF has two cores (modes) and the transmission loss in the MCF is negligible. The power coupling coefficient between one core (mode) m and another core (mode) n is h, and the light intensity in one core (mode) m is I m , the light intensity in the other core (mode) n is I n , and the position in the fiber length direction is z. Under these conditions, the relationship expressed by the following formula (1) holds:

[0036]

[0037] When light is incident on only one core (mode) m at one end of the MCF with sufficiently small crosstalk or power coupling between the two cores, and when the output light intensity from one core (mode) m at the other end of the MCF is I1, the output light intensity from the other core (mode) n is I2, and the MCF length is L Z When , the power coupling coefficient h between one core (mode) m and another core (mode) n can be obtained by the following formula (2):

[0038]

[0039] In particular, assuming application to short-distance interconnection, it is expected to perform optical transmission with low power consumption by minimizing compensation for signal waveform distortion caused by signal processing. At this time, it is expected that the dispersion (absolute value) is sufficiently suppressed in the wavelength band used and the variation between cores is small. The ITU-T G.652 standard stipulates that the zero dispersion wavelength should be in the range of 1300nm or more and 1324nm or less to reduce the absolute value of dispersion in the 1310nm band (1260nm or more and 1360nm or less), which is generally used for short-distance optical transmission. This leads to the structural conditions:

[0040] (1) A heterogeneous core MCF capable of sufficiently suppressing the propagation constant difference between cores XT; and

[0041] (2) At a wavelength of 1310 nm for short-distance optical transmission, the MFD of all cores falls within the range of 8.6±0.6 μm to 9.5±0.6 μm, the optical cable cutoff wavelength is below 1260 nm, and the zero dispersion wavelength falls within a 24 nm wide wavelength band (more preferably, within the range of above 1300 nm and below 1324 nm).

[0042] Unfortunately, however, no MCF has been disclosed to date that meets the above specifications, and achieving this is clearly difficult as is known in the literature as background art to those skilled in the art.

[0043] [Description of Disclosed Embodiments of the Invention]

[0044] Based on the above detailed investigation results, the disclosure of the present invention proposes an MCF suitable for short-distance optical transmission, wherein many cores, that is, four or more cores (including at least four cores arranged in a line in the cross section of the optical fiber) are contained in a cladding having an outer diameter of 124μm or more and 181μm or less. Note that the MFC disclosed in the present invention focuses on short-distance optical transmission using the O-band (1260nm or more and 1360nm or less), and therefore allows for degradation of optical characteristics that only occur in long-distance optical transmission using the C-band, etc. In addition, the MCF disclosed in the present invention allows adjacent cores to have different refractive index distributions, thereby achieving optical characteristics that are fully suitable for signal transmission using short-distance optical transmission using the O-band.

[0045] More specifically, the disclosed MCF examples of the present invention achieve the desired characteristics of the above-mentioned MFD, cutoff wavelength and dispersion. In addition, the MCF according to one example has the following configuration: having a standard cladding outer diameter of 125±1 μm and four or more cores arranged in a line in the optical fiber cross section, so that the MCF is suitable for short-distance optical transmission within the wavelength of the 1310 nm band.

[0046] In addition, another example of the disclosed MCF according to the present invention also achieves the above-mentioned desired characteristics of MFD, cutoff wavelength and dispersion. In addition, the MCF according to another example has a standard cladding outer diameter of less than 180 μm, and 12 or more cores are arranged in a square lattice in the cross section of the optical fiber, so that the MCF is suitable for short-distance optical transmission at a wavelength of more than 1260 nm and less than 1625 nm.

[0047] Hereinafter, details of disclosed embodiments of the present invention will be listed and described separately.

[0048] (1) A multi-core optical fiber (MCF) according to one aspect of the disclosure of the present invention 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 includes: a core extending along the central axis; an inner cladding surrounding the outer periphery of the core; and a groove layer surrounding the outer peripheral surface of the inner cladding. In this way, each of the plurality of core portions has a groove-assisted structure with a good light confinement effect. Considering the matching with the outer diameter of the cladding of the standard SMF, the common cladding surrounds the outer peripheral surface of the groove 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. In addition, the disclosed multi-core optical fiber of the present invention includes a linear array group consisting of at least four core portions, each of the four core portions having a core center located on a straight line defined on a cross section of the MCF, the cross section being orthogonal to the central axis. Each of the four core parts constituting the linear array group has a refractive index distribution in which at least the relative refractive index difference between the core and the inner cladding is equal to the relative refractive index difference between the core and the inner cladding of the other core parts. In addition, the four core parts include a first core part and a second core part adjacent to each other and having an interval Λ[μm] between the centers of the respective cores, and the relative refractive index difference of the first core part and the second core part relative to the refractive index of the common cladding differs by more than 0.001Λ[%]. That is, the first core part and the second core part adjacent to each other have mutually different structures. In addition, as an example, in the case where the linear array group includes four core parts, the core part corresponding to the above-mentioned first core part and the core part corresponding to the above-mentioned second core part are arranged alternately along a straight line. Here, "a refractive index distribution in which the relative refractive index difference between the core and the inner cladding is equal" refers to a refractive index distribution in which the difference in the relative refractive index difference between the core and the inner cladding is less than 0.02%.

[0049] (2) As an aspect of the disclosure of the present invention, it is preferred that the MCF has equal optical characteristics for all core parts within a 1310 nm wavelength band (1260 nm to 1360 nm). Specifically, it is preferred to have a structure in which each of at least four core parts has an MFD of 8.0 μm or more and 10.1 μm or less at a wavelength of 1310 nm, has an optical cable cutoff wavelength of less than 1260 nm, and has a transmission loss of less than 0.5 dB / km in a wavelength band of 1310 nm. The transmission loss in the 1310 nm wavelength band is more preferably less than 0.4 dB / km. In this case, the leakage loss LL to the resin coating is sufficiently low near a wavelength of 1310 nm. With respect to the variation in the zero dispersion wavelength of the four core parts, the difference between the maximum zero dispersion wavelength and the minimum zero dispersion wavelength is preferably less than 24 nm. In addition, in a state where 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 (core-to-core XT) between the cores of the first core portion and the cores of the second core portion 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.

[0050] (3) As one aspect of the present invention, as a structural parameter of the first core portion, the relative refractive index differences of the core, inner cladding and trench layer with respect to the refractive index of the common cladding are set to Δ co,1 [%], Δ ic,1 [%] and Δ t,1 [%]. In contrast, as the structural parameters of the second core portion, the relative refractive index differences of the core, inner cladding and trench layer with respect to the refractive index of the common cladding are set to Δ co,2 [%], Δ ic,2 [%] and Δ t,2 [%]. At this time, it is preferred that the first core portion and the second core portion satisfy the following relationship:

[0051] Δ co,1 >Δ ic,1 >Δ t,1

[0052] Δ co,2 >Δ ic,2 >Δ t,2

[0053] 0.32%≤Δ co,1 -Δ ic,1 ≤0.40%

[0054] 0.32%≤Δ co,2 -Δ ic,2 ≤0.40%

[0055] Δ t,1 ≤0%

[0056] Δ t,2 <0%

[0057] Δ ic,1 ≤0.10%

[0058] -0.10%≤Δ ic,2 .

[0059] More preferably, the following relationship is satisfied:

[0060] Δ ic,2 <0%<Δ ic,1 .

[0061] In addition, the following are arrangement conditions for the first core portion and the second core portion to ensure suppression of the inter-core XT and suppression of the leakage loss LL to the resin coating side. That is, under the condition that Λ [μm] is defined as the interval between the core center of the first core portion and the core center of the second core portion, it is preferred that the first core portion and the second core portion satisfy the following relationship:

[0062] 0.001Λ≤Δ co,1 -Δ co,2

[0063] 0.001Λ≤Δ ic,1 -Δ ic,2 .

[0064] In addition, as an aspect of the present disclosure, the first core portion and the second core portion may satisfy the following relationship:

[0065] 0.002Λ≤Δ co,1 -Δ co,2

[0066] 0.002Λ≤Δ ic,1 -Δ ic,2 .

[0067] Between adjacent first and second core portions, their respective groove layers may contact (or may be connected to) each other.

[0068] (4) As one aspect of the present disclosure, the first core portion and the second core portion may satisfy the following relationship:

[0069] Δ t,1 ≤-0.5%

[0070] Δ t,2 ≤-0.5%

[0071] 0.34≤a1 / b1≤0.42

[0072] 0.34≤a2 / b2≤0.42.

[0073] Here, as the structural parameters of the first core portion, the outer diameters of the core, the inner cladding layer, and the trench layer are set to 2a1 [μm], 2b1 [μm], and 2c1 [μm], respectively, and

[0074] As structural parameters of the second core portion, the outer diameters of the core, the inner cladding layer, and the trench layer are set to 2a2 [μm], 2b2 [μm], and 2c2 [μm], respectively.

[0075] (5) As one aspect of the disclosure of the present invention, it is preferred that the MCF has a zero dispersion wavelength of 1300 nm or more and 1324 nm or less. As one aspect of the disclosure of the present invention, it is preferred that the MFC has a zero dispersion wavelength of 0.092 ps / (nm 2 ·km) or less. As one aspect of the disclosure of the present invention, it is preferred that the MCF has a bending loss of 0.25 dB / turn or less at a wavelength of 1310 nm under a bend with a curvature radius of 5 mm or more, or under a bend with a bending radius of 3 mm or more. In addition, as one aspect of the disclosure of the present invention, it is preferred that the common cladding has an outer diameter of 124 μm or more and 126 μm or less. At this time, it is preferred that the interval Λ [μm] between the core center of the first core portion and the core center of the second core portion satisfies the condition:

[0076] 22.5μm≤Λ≤27.8μm.

[0077] In addition, the interval Λ[μm] can satisfy the condition:

[0078] 23μm≤Λ≤25μm.

[0079] (6) As one aspect of the disclosure of the present invention, the shortest distance between the outer peripheral surface of the groove layer of the first core portion and the outer peripheral surface of the groove layer of the second core portion is set to the interval w [μm], or the value given by the expression "Λ-(c1+c2)" is defined as the interval w [μm]. At this time, it is preferred that the interval w [μm] and the interval Λ [μm] satisfy the following relationship:

[0080] 0μm≤w≤2.49μm

[0081] 0.0133w 3 -0.129w 2 +0.885w+22.5≤Λ≤-1.46w+27.8.

[0082] Furthermore, the deviation d [μm] between the midpoint of the first line segment connecting the outer peripheral surfaces of the groove 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 given by the following formula (3):

[0083]

[0084] At this time, it is preferable that the deviation d [μm] satisfies the following relationship:

[0085] d≤-(0.104w+0.324)Λ 2 +(5.721w+19.220)Λ-(79.360w+271.139)

[0086] d≤-0.246Λ-0.501w+6.471

[0087] d≥0.439Λ+0.501w-12.539.

[0088] Note that, in the positional relationship between the first core portion and the second core portion, in the case where the midpoint of the first line segment is closer to the first core portion than the midpoint of the second line segment, the deviation d [μm] will be a negative value.

[0089] (7) As one aspect of the disclosure of the present invention, it is preferred that, on the cross section of the MCF, the plurality of core portions including the above-mentioned linear array group are arranged so that the positions of the core centers are linearly symmetrical with respect to the reference straight line passing through the intersection between the central axis (the central axis of the MCF) and the cross section as the axis of symmetry. As one aspect of the disclosure of the present invention, it is permitted that, on the cross section of the MCF, the plurality of core portions including the above-mentioned linear array group are arranged so that the positions of the respective core centers have double or more rotational symmetry with respect to the intersection between the central axis (the central axis of the MCF) and the cross section as the center of rotation. As one aspect of the disclosure of the present invention, the MCF may include a mark having a refractive index different from that of the common cladding. In this case, the mark is arranged at a position that eliminates the symmetry (linear symmetry, rotational symmetry, etc.) of the core center positions in the plurality of core portions. The presence of such a mark enables distinction between the first and second core portions. In addition, as one aspect of the disclosure of the present invention, the outer radius (c1) of the groove layer in the first core portion may be different from the outer radius (c2) of the groove layer in the second core portion. Also in this case, the refractive index distribution of the MCF along the array direction of the four core portions loses the symmetry (linear symmetry, rotational symmetry, etc.) of the refractive index distribution in the cross section of the MCF, which enables distinction between the first and second core portions.

[0090] As described above, each aspect listed in [Details of the disclosed embodiments of the present invention] is applicable to all remaining aspects or to all combinations of these remaining aspects.

[0091] [Details of the disclosed embodiment of the present invention]

[0092] The specific examples of the disclosed multi-core optical fiber (MCF) according to the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the disclosure of the present invention is not limited to these examples, but is indicated by the scope of the claims, and is intended to include the meaning equivalent to the claims and all modifications within the scope. The same reference numerals are given to the same components, and repeated descriptions will be omitted.

[0093] (First embodiment)

[0094] Figure 1 is a diagram showing a cross-sectional structure of an MCF according to a first embodiment of the disclosure of the present invention. Figure 2 is shown as Figure 1 FIG. 1 is a diagram showing the cross-sectional structure and arrangement relationship of a first core section 100A and a second core section 100B which are part of four core sections (linear array group G0) and are adjacent to each other. Figure 3 It is along the MCF Figure 1 The refractive index distribution of the straight line L0 in . Note that Figure 3 The refractive index level LV of the common cladding 200 is shown clad .

[0095] exist Figure 1 In the example shown, the MCF 10A of the first embodiment includes: four core parts extending along the central axis AX of the MCF 10A; a common cladding 200 surrounding each of the four core parts; and a resin coating 300 surrounding the outer peripheral surface of the common cladding 200. The four core parts have the following configuration: including two types of core parts (a first core part 100A and a second core part 100B) with different structures, wherein these first core parts 100A and the second core parts 100B are alternately arranged along a straight line L0. That is, the core center AX1 of the first core part 100A and the core center AX2 of the second core part 100B are both located on the straight line L0. In this way, at least four core parts (a first core part 100A and a second core part 100B) including the core centers AX1 and AX2 arranged on the straight line L0 constitute a linear array group G0.

[0096] The common cladding 200 has an outer diameter of not less than 124 μm and not more than 181 μm. In addition, in order to suppress the increase in leakage loss LL from each core portion to the resin coating 300, the outer cladding thickness (OCT) is set so as to determine the appropriate position of the linear array group G0 in the cross section of the common cladding 200. In this specification, "OCT" is the shortest distance from the core center of the core portion farthest from the central axis AX to the outer peripheral surface of the common cladding 200. The resin coating provided on the periphery of the common cladding 200 has an outer diameter of not less than 195 μm and not more than 250 μm. Note that the resin coating 300 does not have to be composed of a single layer. Figure 1 In the example of , the resin coating 300 includes: an inner coating 310 surrounding the outer peripheral surface of the common cladding 200; and an outer coating 320 surrounding the outer peripheral surface of the inner coating 310. Considering the match with the outer diameter of the cladding of the standard SMF, it is desirable that the outer diameter of the common cladding 200 is 124μm to 126μm. At this time, it may be desirable to reduce the outer diameter of the resin coating 300 to a minimum of 195μm, because this range can be achieved without compromising optical characteristics and productivity. Similarly, considering the match with the outer diameter of the resin coating of the standard SMF, it is further desirable that the outer diameter of the resin coating 300 is approximately 250μm. At this time, it is desirable that the outer diameter of the common cladding 200 is set to a maximum value of 179μm to 181μm, because this range can be achieved without compromising optical characteristics and productivity.

[0097] Figure 2 Shown as Figure 1 An example of the cross-sectional structure and arrangement relationship between the first core portion 100A and the second core portion 100B which are part of the four core portions shown in FIG. Figure 2 As shown, the first core portion 100A has a groove-assisted structure. The groove-assisted structure includes: a core 110A having an outer diameter 2a1 and including a core center AX1; an inner cladding 120A having an outer diameter 2b1 and having a refractive index lower than the refractive index of the core 110A; and a groove layer 130A having an outer diameter 2c1 and having a refractive index lower than the refractive index of the inner cladding 120A. In contrast, the second core portion 100B has a groove-assisted structure. The groove-assisted structure includes: a core 110B having an outer diameter 2a2 and including a core center AX2; an inner cladding 120B having an outer diameter 2b2 and having a refractive index lower than the refractive index of the core 110B; and a groove layer 130B having an outer diameter 2c2 and having a refractive index lower than the refractive index 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. In addition, these adjacent trench layers 130A and 130B may overlap each other.

[0098] In addition, if Figure 2 As shown, the positional relationship between the first core portion 100A and the second core portion 100B adjacent to each other is determined by the shortest distance w [μm] between the groove layers 130A and 130B and the interval Λ [μm], which is the center-to-center distance between the core center AX1 and the core center AX2. Specifically, in the case where the common cladding 200 has an outer diameter of 124 μm or more and 126 μm or less, the interval Λ [μm] satisfies one of the following conditions:

[0099] 22.5μm≤Λ≤27.8μm

[0100] or

[0101] 23μm≤Λ≤25μm.

[0102] Furthermore, in the case where the interval w [μm] is defined as the shortest distance between the outer peripheral surface of the groove layer 130A of the first core portion 100A and the outer peripheral surface of the groove layer 130B of the second core portion 100B or is defined as a value given by the expression “Λ-(c1+c2)”, the interval w [μm] and the interval Λ [μm] satisfy the following relationship:

[0103] 0μm≤w≤2.49μm

[0104] 0.0133w 3 -0.129w 2 +0.885w+22.5≤Λ≤-1.46w+27.8.

[0105] In addition, the first line segment is a line segment connecting the outer peripheral surfaces of the groove layers 130A and 130B of the first and second core portions 100A and 100B at the shortest distance, and the second line segment is a line segment connecting the core centers AX1 and AX2 of the first and second core portions 100A and 100B, and the deviation d [μm] between the midpoint of the first line segment and the midpoint of the second line segment is given by the above formula (3), and the deviation d [μm] satisfies the following relationship:

[0106] d≤-(0.104w+0.324)Λ 2 +(5.721w+19.220)Λ-(79.360w+271.139)

[0107] d≤-0.246Λ-0.501w+6.471

[0108] d≥0.439Λ+0.501w-12.539.

[0109] Note that, in the positional relationship between the first core portion and the second core portion, in the case where the midpoint of the first line segment is closer to the first core portion than the midpoint of the second line segment, the deviation d [μm] will be a negative value.

[0110] Figure 3 It is shown that in the cross section of the MCF 10A according to the first embodiment, along the straight line L0 (see Figure 1 and Figure 2 ), in particular, the refractive index distribution of the linear array group G0 in which the first core portion 100A (including the core center AX1) and the second core portion 100B (including the core center AX2) are alternately arranged. Figure 3 As shown, the outer diameters of the core 110A, the inner cladding 120A, and the 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 relative to the refractive index of the common cladding 200) of the core 110A, the inner cladding 120A, and the trench layer 130A constituting the first core portion 100A is Figure 3 The refractive index level LV of the common cladding 200 is shown in clad ) are Δ co,1 [%], Δ ic,1 [%] and Δ t,1 [%]. The outer diameters of the core 110B, the inner cladding 120B, and the trench layer 130B constituting the second core portion 100B are 2a2 [μm], 2b2 [μm], and 2c2 [μm], respectively. In addition, the relative refractive index differences (relative refractive index difference Δ with respect to the refractive index of the common cladding 200) of the core 110B, the inner cladding 120B, and the trench layer 130B constituting the second core portion 100B are Δ co,2 [%], Δ ic,2 [%] and Δ t,2 [%].

[0111] In this specification, the relative refractive index difference Δ% of a certain glass region (e.g., cores 110A, 110B, etc.) having a refractive index n1 relative to the refractive index n0 of the common cladding 200 is given by the following expression:

[0112] Δ=100(n1 2 -n0 2 ) / 2n0 2 .

[0113] Therefore, the relative refractive index difference of glass regions having a higher refractive index than the common cladding 200 will be positive, while the relative refractive index difference of glass regions having a lower refractive index than the common cladding 200 will be negative. In addition, the relative refractive index difference between any glass regions other than the common cladding 200 should be given by the absolute value of the difference between the relative refractive index difference of one glass region with respect to the refractive index of the common cladding 200 and the relative refractive index difference of another glass region with respect to the refractive index of the common cladding 200.

[0114] like Figure 3 As shown, the refractive index distribution of each of the four core sections constituting the linear array group G0 is designed so that at least the relative refractive index difference (Δ co,1 -Δ ic,1 ) is substantially matched to the relative refractive index difference (Δ co,2 -Δ ic,2 ). In particular, the refractive index distribution of the first core portion 100A including the core center AX1 is designed so that the refractive index of the inner cladding 120A is higher than the refractive index of the common cladding 200. In contrast, the refractive index distribution of the second core portion 100B including the core center AX2 is designed 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 opposite to that of the first core portion 100A). The first core portion 100A and the second core portion 100B having such different refractive index distributions are alternately arranged along the straight line L0, realizing an offset structure of the refractive index distribution.

[0115] More specifically, comparing the structure of the refractive index distribution of the first core portion 100A with the structure of the refractive index distribution of the second core portion 100B, the first core portion 100A and the second core portion 100B in the present embodiment satisfy the following relationship:

[0116] Δ co,1 >Δ ic,1 >Δ t,1

[0117] Δ co,2 >Δ ic,2 >Δ t,2

[0118] 0.32%≤Δ co,1 -Δ ic,1 ≤0.40%

[0119] 0.32%≤Δ co,2 -Δ ic,2 ≤0.40%

[0120] Δt,1 ≤0%

[0121] Δ t,2 <0%

[0122] Δ ic,1 ≤0.10%

[0123] -0.10%≤Δ ic,2 .

[0124] More preferably, the following relationship is satisfied:

[0125] Δ ic,2 <0%<Δ ic,1 .

[0126] Furthermore, the first core portion 100A and the second core portion 100B satisfy the following relationship:

[0127] 0.001Λ≤Δ co,1 -Δ co,2

[0128] 0.001Λ≤Δ ic,1 -Δ ic,2

[0129] or

[0130] 0.002Λ≤Δ co,1 -Δ co,2

[0131] 0.002Λ≤Δ ic,1 -Δ ic,2 .

[0132] Furthermore, it is preferred that the first core portion 100A and the second core portion 100B satisfy the following relationship:

[0133] Δ t,1 ≤-0.5%

[0134] Δ t,2 ≤-0.5%

[0135] 0.34≤a1 / b1≤0.42

[0136] 0.34≤a2 / b2≤0.42.

[0137] (Second embodiment)

[0138] Figure 4 : is a diagram showing a cross-sectional structure of an MCF according to a second embodiment of the disclosure of the present invention. Figure 4The illustrated MCF 10B according to the second embodiment is different from the MCF 10A according to the above-described first embodiment in the number and array structure of core portions provided in a common cladding 200 , but can obtain technical effects similar to those of the MCF 10A according to the first embodiment.

[0139] Figure 4 The MCF 10B of the second embodiment shown in comprises: 12 core parts; a common cladding 200 surrounding each of the 12 core parts; and a resin coating 300 surrounding the outer peripheral surface of the common cladding 200. The resin coating 300 comprises: an inner coating 310 surrounding the outer peripheral surface of the common cladding 200; and an outer coating 320 surrounding the outer peripheral surface of the inner coating 310. Note that the resin coating 300 can be composed of a single layer. The 12 core parts include a group corresponding to the above-mentioned first core part 100A and a group corresponding to the above-mentioned second core part 100B, and are arranged to form a square lattice. In the second embodiment, on the cross section (orthogonal to the plane of the central axis AX) of the MCF 10B, the 12 core parts are arranged along four straight lines L1, L2, L3 and L4. Straight lines L1 and L2 are parallel to each other, and straight lines L3 and L4 are also parallel to each other. The two straight lines L3 and L4 are orthogonal to the two straight lines L1 and L2.

[0140] On straight line L1, the four core parts constituting linear array group G1 are arranged so that the first core part 100A and the second core part 100B are arranged alternately. On straight line L2, the four core parts constituting linear array group G2 are arranged so that the first core part 100A and the second core part 100B are arranged alternately. On straight line L3, the four core parts constituting linear array group G3 are arranged so that the first core part 100A and the second core part 100B are arranged alternately. On straight line L4, the four core parts constituting linear array group G4 are arranged so that the first core part 100A and the second core part 100B are arranged alternately.

[0141] exist Figure 4In the example shown in, 12 core parts including four linear array groups G1 to G4 are arranged so that the positions of the respective core centers AX1 and AX2 are linearly symmetrical relative to the reference straight line as the axis of symmetry intersecting the central axis AX of the MCF 10B. In addition, the 12 core parts are arranged so that the positions of the respective core centers AX1 and AX2 have double or more rotational symmetry around the central axis AX of the MCF 10B as the rotation center. In such a core part arrangement with linear symmetry or rotational symmetry, it is difficult to distinguish between the first core 100A and the second core 100B. In order to solve this problem, in the second embodiment, a mark 400 having a refractive index different from the refractive index of the common cladding 200 is provided. The mark 400 is arranged at a position that eliminates the symmetry of the arrangement of the core centers AX1 and AX2 in the 12 core parts. The mark 400 enables to distinguish between the first core part 100A and the second core part 100B.

[0142] (Third Embodiment)

[0143] Figure 5 : is a diagram showing a cross-sectional structure of an MCF according to a third embodiment of the disclosure of the present invention. Figure 5 The illustrated MCF 10C according to the third embodiment is different from the MCF 10A according to the above-described first embodiment in the number and array structure of core portions provided in a common cladding 200 , but can obtain technical effects similar to those of the MCF 10A according to the first embodiment.

[0144] Figure 5 The MCF 10C of the third embodiment shown in the figure includes: eight core parts; a common cladding 200 surrounding each of the eight core parts; and a resin coating 300 surrounding the outer peripheral surface of the common cladding 200. The resin coating 300 includes: an inner coating 310 surrounding the outer peripheral surface of the common cladding 200; and an outer coating 320 surrounding the outer peripheral surface of the inner coating 310. Note that the resin coating 300 can be composed of a single layer. The eight core parts include a group corresponding to the above-mentioned first core part 100A and a group corresponding to the above-mentioned second core part 100B. In the third embodiment, on the cross section of the MCF 10C (a plane orthogonal to the central axis AX), the four core parts are arranged along a straight line L0 (a straight line intersecting the central axis AX of the MCF 10C). The array of the core parts on the straight line L0 is similar to the array in the above-mentioned first embodiment. That is, on the straight line L0, the four core sections constituting the linear array group G0 are arranged so that the first core sections 100A and the second core sections 100B are alternately arranged.

[0145] In addition, Figure 5In the example shown in , the eight core portions including the linear array group G0 are arranged so that the positions of the respective core centers AX1 and AX2 are line-symmetrical with respect to the reference axis intersecting the central axis AX of the MCF 10B as the axis of symmetry. In addition, the eight core portions are arranged so that the positions of the respective core centers AX1 and AX2 have double or more rotational symmetry around the central axis AX of the MCF 10C as the center of rotation. Note that in the third embodiment, the markers 400 can be arranged similarly to the case of the above-described second embodiment.

[0146] (Measurement results)

[0147] Figure 6 is a table showing structural parameters of a plurality of samples prepared for measurement. Figure 7 It is shown Figure 6 The measurement results for each sample are shown in the table.

[0148] The prepared sample #1 is similar to the first embodiment ( Figure 1 ) includes a linear array group G0 (an array element in which four core portions are arranged in one line (one column)), in which the outer diameter of the common cladding 200 (hereinafter referred to as "cladding outer diameter") is 125 μm. Figure 1 ), sample #2 is a sample including the linear array group G0 and has a cladding outer diameter of 125 μm. Figure 1 ), sample #3 is a sample including the linear array group G0 and having a cladding outer diameter of 125 μm. Sample #4 is similar to the second embodiment ( Figure 4 ) includes a sample of four linear array groups G1 to G4 (12 core portions are arranged to form an array component in a square lattice shape), in which the cladding outer diameter is 180 μm.

[0149] Figure 6 The structural parameters of the first core portion 100A of each of the samples #1 to #4 are shown, that is, the refractive index (at Figure 3 The refractive index level LV of the common cladding 200 is shown in clad ), the relative refractive index difference Δ of the core 110A co,1 [%], relative refractive index difference Δ of the inner cladding 120A ic,1 [%], relative refractive index difference Δ of the groove layer 130A t,1 [%], the radius a1 [μm] of the core 110A, the outer radius b1 [μm] of the inner cladding 120A, and the outer radius c1 [μm] of the trench layer 130A. Figure 6Further shown are the structural parameters of the second core portion 100B of each of the samples #1 to #4, namely, the relative refractive index difference Δ co,2 [%], relative refractive index difference Δ of the inner cladding 120B ic,2 [%], relative refractive index difference Δ of the groove layer 130B t,2 [%], the radius a2 [μm] of the core 110B, the outer radius b2 [μm] of the inner cladding 120B, and the outer radius c2 [μm] of the trench layer 130B. Figure 6 In the table in , “OCT” is the shortest distance from the core center of the core portion farthest from the central axis AX of the MCF to the outer peripheral surface of the common cladding 200 .

[0150] Figure 7 It is shown Figure 6 0 is a table of measurement results for each of samples #1 to #4 shown in . Figure 7 The table lists the cable cutoff wavelength λ of each of the first core section 100A and the second core section 100B. cc [μm], zero dispersion wavelength λ0 [μm], dispersion slope (zero dispersion slope) S0 [ps / (nm] at wavelength λ0 2 ·km)] and leakage loss LL [dB / km]. In addition, "inter-core XT" in the table represents the crosstalk between the first core portion 100A and the second core portion 100B adjacent to each other. MFD is a value at a wavelength of 1310nm. In the leakage loss LL and inter-core XT, the values ​​at a wavelength of 1310nm are shown for samples #1 to #3, and the value at a wavelength of 1625nm is shown for sample #4. Inter-core XT represents the value at a fiber bending radius of 0.14m.

[0151] In the standard SMF with low transmission loss introduced in recent years, a transmission loss of about 0.3dB / km is obtained at a wavelength of 1310nm. Based on this fact, in order to obtain a transmission loss of 0.4dB / km or less at a wavelength of 1310nm in the MCF, it is preferred that the leakage loss LL to the coating at a wavelength of 1310nm is 0.1dB / km or less. The transmission loss of the standard SMF at a wavelength of 1310nm is greater than 0.32dB / km and less than 0.35dB / km. Therefore, in order to obtain a transmission loss of 0.4dB / km or less at a wavelength of 1310nm in the MCF, the leakage loss LL to the coating at a wavelength of 1310nm is preferably less than 0.08dB / km, and more preferably less than 0.05dB / km. From Figure 7 It can be seen that this embodiment meets this requirement specification.

[0152] from Figure 7 It can be seen from the measurement results shown in that when the first core section 100A and the second core section 100B meet the above structural conditions, the MCF 10A to 10C according to the above first to third embodiments can obtain the following preferred optical characteristics: 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 a wavelength band of 1260 nm or more and 1360 nm or less. In addition, in the zero dispersion wavelength of the four core sections constituting the linear array group G0 (or the 12 core sections constituting the linear array groups G1 to G4), the difference (wavelength width) between the maximum zero dispersion wavelength and the minimum zero dispersion wavelength can be adjusted to be 24 nm or less. In a state where the MCF 10A to 10C is bent with a bending radius of 7 cm or more and 14 cm or less, or with a bending radius of 14 cm or more and 20 cm or less, the XT between the core 110A in the first core portion 100A and the core 110B in the second core portion 100B will be less than 0.01 / km. The zero dispersion wavelength of the MCF 10A to 10C can be set to a range of 1300 nm or more and 1324 nm or less. In addition, the dispersion slope of the MCF 10A to 10C at such a zero dispersion wavelength is 0.092 ps / (nm 2 Furthermore, the bending loss of the MCFs 10A to 10C at a wavelength of 1310 nm can be adjusted to 0.25 dB / turn or less under bending with a curvature radius of 5 mm or more (preferably 3 mm or more).

[0153] Next, the basis for the appropriate ranges of optical characteristics in the disclosed MCFs 10A to 10C of the present invention will be described. Figure 8 This is a graph showing the relationship between the fiber bending radius [μm] and the cumulative breakage probability (the breakage probability calculated assuming 1 turn of bending for 10 years) for a plurality of samples having different fiber diameters (outer diameter of the common cladding). Figure 8 , curve G810 shows the measured value of the sample having a cladding outer diameter of 250 μm (corresponding to the outer diameter of the common cladding 200 of the optical fiber diameter), curve G820 shows the measured value of the sample having a cladding outer diameter of 225 μm, curve G830 shows the measured value of the sample having a cladding outer diameter of 200 μm, curve G840 shows the measured value of the sample having a cladding outer diameter of 175 μm, curve G850 shows the measured value of the sample having a cladding outer diameter of 150 μm, and curve G860 shows the measured value of the sample having a cladding outer diameter of 125 μm.

[0154] Setting the outer diameter of the cladding to 125±1μm enables the use of the same peripheral components as those used in standard SMF, such as conventional connector ferrules. Setting the outer diameter of the cladding to 125±1μm or more and 180±1μm or less enables coating with a resin coating having the same outer diameter (about 250μm) as conventional standard SMF, thereby enabling the use of conventional optical cable technology. This is preferred in terms of reducing the cost of the transmission system. In addition, as Figure 8 As shown in FIG. 1 , although the probability of breakage when the optical fiber is bent increases as the curvature radius of the optical fiber bend decreases, the threshold curvature radius at which the breakage probability rises sharply will also increase as the outer diameter of the cladding increases. Preferably, the outer diameter of the cladding is 125±1 μm or less, because this setting can suppress the threshold curvature radius to less than 5 mm.

[0155] When the transmission loss at a wavelength of 1310 nm is 0.5 dB / km, preferably 0.4 dB / km, there is no significant increase in the transmission loss for a single-core SMF conforming to the currently widely used ITU-T G.652 and G.657 standards.

[0156] In addition, assuming that the transmission loss of the core that is not affected by the leakage loss LL is the core intrinsic transmission loss, a core intrinsic transmission loss of about 0.3dB / km at a wavelength of 1310nm can be obtained using a pure quartz core. Using a GeO2-doped core, a core intrinsic transmission loss of more than 0.32dB / km and less than 0.35dB / km can be obtained. Therefore, in the disclosed MCF of the present invention, the "transmission loss" obtained by adding the "leakage loss LL" to the "core intrinsic transmission loss" at a wavelength of 1310nm is preferably 0.5dB / km, and more preferably 0.4dB / km or less. Even more preferably, the "transmission loss" is less than 0.4dB / km over the entire range of the O band.

[0157] Setting the MFD at a wavelength of 1310 nm to 8.0 μm or more or 10.1 μm or less can reduce the connection loss with a system or component based on a conventional standard SMF. In addition, even if a certain degree of axis deviation occurs, the connection loss can be suppressed to a small value.

[0158] At a wavelength of 1310 nm, the bending loss when the radius of curvature R is 5 mm or more or 3 mm or more is preferably 0.25 dB / turn or less. In this case, even when an optical fiber with a small radius of curvature is adopted in the use of optical interconnect wiring for high-performance computing or data centers, a significant increase in transmission loss can be suppressed. Setting the bending loss at a radius of curvature R of 3 mm at a wavelength of 1310 nm to 0.10 dB / turn or less allows the transmission loss to be suppressed even under more severe conditions.

[0159] Setting the zero dispersion wavelength to 1300 nm or more and 1324 nm or less makes it possible to suppress dispersion in the O band used for short-distance optical transmission. This makes it possible to suppress the cost (price and power consumption cost) of dispersion compensation in a transceiver.

[0160] Setting the optical cable cutoff wavelength to 1260 nm or less can ensure single-mode operation in the O-band for short-distance optical transmission. More preferably, the 2m cutoff wavelength is set to 1260 nm or less.

[0161] The XT between adjacent cores at a wavelength of 1310 nm is set to 0.001 / km or less, so that when the signal light is transmitted in the reverse propagation direction between the adjacent cores, the noise caused by the XT between the cores can be sufficiently suppressed. In addition, the XT between adjacent cores is set to 0.001 / km or less, so that even when the signal light is transmitted in the same propagation direction in all the cores, the noise caused by the XT between the cores can be sufficiently suppressed.

[0162] As described above, in the core with the shortest distance from the core center to the resin coating among four or more cores, the transmission loss at a wavelength of 1550nm or 1625nm is greater than 0.4dB / km. It is intentionally set to a large value that does not meet the ITU-TG.652 or G.657 standards so that a smaller OCT can be achieved. In this case, since the outermost core can be brought close to the coating, more cores can be wrapped in a cladding with an outer diameter of about 125μm in a state of being arranged in a row. This also makes it possible to wrap a core with a larger MFD in a cladding with an outer diameter of about 125μm. In addition, this expands the manufacturing design margin of the core-to-core XT and the spacing Λ (the center-to-center distance between adjacent cores).

[0163] The dispersion slope at the zero dispersion wavelength is set to 0.092 ps / (nm 2 ·km) or less, so that signal waveform distortion caused by high-order dispersion can be suppressed during high-speed optical transmission. In addition, when using a wide wavelength range in the O-band for wavelength division multiplexing transmission, the maximum value of the dispersion in the used band can also be suppressed (suppressing signal waveform distortion).

[0164] The disclosed MCF of the present invention is preferably an all-solid optical fiber without holes in the cladding. In the case of an MCF with holes, foreign matter such as water may enter the holes and affect the transmission characteristics of the core. In order to suppress this influence, it is necessary to carry out a process of closing the holes on the end face of the optical fiber. In addition, the presence of the holes increases the cost of optical fiber connection and attaching the connector to the optical fiber. In addition, the presence of the holes also increases the manufacturing cost because the air pressure in the holes needs to be finely controlled during the optical fiber drawing process (the hole diameter needs to be controlled so that the optical properties of the core reach the desired value).

[0165] As described above, according to the disclosed MCF of the present invention, excellent economic rationality and high compatibility can be simultaneously achieved in short-distance optical transmission.

Claims

1. A multi-core optical fiber, comprising: a plurality of core portions extending along a central axis, each of the plurality of core portions comprising a core extending along the central axis, an inner cladding surrounding an outer periphery of the core, and a trench layer surrounding an outer peripheral surface of the inner cladding; a common cladding layer surrounding an outer peripheral surface of the groove layer of each of the plurality of core portions and having an outer diameter of 124 μm or more and 181 μm or less; as well as a resin coating having an outer diameter of 195 μm or more and 250 μm or less while surrounding the outer peripheral surface of the common cladding; wherein the multi-core optical fiber comprises a linear array group consisting of at least four core portions, each of the four core portions having a core center located on a straight line defined on a cross section of the multi-core optical fiber, the cross section being orthogonal to the central axis, Each of the four core sections constituting the linear array group has a refractive index profile in which at least a relative refractive index difference between the core and the inner cladding is equal to a relative refractive index difference between the core and the inner cladding of the other core sections, and The four core portions include a first core portion and a second core portion adjacent to each other with a spacing Λ in μm between the core centers, the first core portion and the second core portion having a relative refractive index difference with respect to the refractive index of the common cladding that differs by more than 0.001Λ in %, and An outer radius of the groove layer in the first core portion is different from an outer radius of the groove layer in the second core portion.

2. The multi-core optical fiber according to claim 1, in, Each of the four 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, has an optical cable cutoff wavelength of 1260 nm or less, and has a transmission loss of 0.5 dB / km or less in a wavelength band of 1260 nm or more and 1360 nm or less, The difference between the maximum zero dispersion wavelength and the minimum zero dispersion wavelength among the zero dispersion wavelengths of the four core portions is 24 nm or less, and When the multi-core optical fiber is bent with a bending radius of 7 cm to 14 cm or 14 cm to 20 cm, crosstalk between the core of the first core portion and the core of the second core portion is 0.01 / km or less.

3. The multi-core optical fiber according to claim 1, in, The first core portion and the second core portion satisfy the following relationship: D co,1 >D ic,1 >D t,1 D co,2 >D ic,2 >D t,2 0.32%≤Δ co,1 -D ic,1 ≤0.40% 0.32%≤Δ co,2 -D ic,2 ≤0.40% D t,1 ≤0% D t,2 <0% D ic,1 ≤0.10% -0.10%≤Δ ic,2 , As the structural parameter of the first core portion, Δ co,1 is defined as the relative refractive index difference of the core with respect to the refractive index of the common cladding, expressed in %. ic,1 is defined as the relative refractive index difference of the inner cladding relative to the refractive index of the common cladding and is expressed in %. t,1 is defined as the relative refractive index difference of the trench layer with respect to the refractive index of the common cladding layer, and As the structural parameter of the second core portion, Δ co,2 is defined as the relative refractive index difference of the core with respect to the refractive index of the common cladding, expressed in %. ic,2 is defined as the relative refractive index difference of the inner cladding relative to the refractive index of the common cladding and is expressed in %. t,2 is defined as the relative refractive index difference of the trench layer with respect to the refractive index of the common cladding layer.

4. The multi-core optical fiber according to claim 3, in, The first core portion and the second core portion satisfy the following relationship: D t,1 ≤-0.5% D t,2 ≤-0.5% 0.34≤a1 / b1≤0.42 0.34≤a2 / b2≤0.42, As structural parameters of the first core portion, 2a1 in μm is defined as the outer diameter of the core, 2b1 in μm is defined as the outer diameter of the inner cladding, and 2c1 in μm is defined as the outer diameter of the trench layer, and As structural parameters of the second core portion, 2a2 in μm is defined as the outer diameter of the core, 2b2 in μm is defined as the outer diameter of the inner cladding, and 2c2 in μm is defined as the outer diameter of the trench layer.

5. The multi-core optical fiber according to claim 1, in, The multi-core optical fiber has a zero dispersion wavelength of 1300 nm or more and 1324 nm or less.

6. The multi-core optical fiber according to claim 5, in, At the zero dispersion wavelength, the multi-core optical fiber has a wavelength of 0.092 ps / (nm 2 ·km) or less dispersion slope.

7. The multi-core optical fiber according to claim 1, in, The multi-core optical fiber has a bending loss of 0.25 dB / turn or less at a wavelength of 1310 nm under bending with a curvature radius of 5 mm or more.

8. The multi-core optical fiber according to claim 3, in, The common cladding has an outer diameter of 124 μm or more and 126 μm or less, and The interval Λ in μm satisfies the following condition: 22.5μm≤Λ≤27.8μm.

9. The multi-core optical fiber according to claim 8, in, The interval w in μm is defined as the shortest distance between the outer peripheral surface of the groove layer of the first core portion and the outer peripheral surface of the groove layer of the second core portion, or is defined as a value given by the following expression: Λ-(c1+c2), The interval w in μm and the interval Λ in μm satisfy the following relationship: 0μm≤w≤2.49μm 0.0133w 3 -0.129w 2 +0.885w+22.5≤Λ≤-1.46w+27.8, and The deviation d in μm between the midpoint of the first line segment and the midpoint of the second line segment is given by the following formula (1): The first line segment is a line segment connecting the outer peripheral surface of the groove layer of the first core portion and the outer peripheral surface of the groove layer of the second core portion at the shortest distance, The second line segment is a line segment connecting the core center of the first core portion and the core center of the second core portion, and The deviation d in μm satisfies the following relationship: 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.

10. The multi-core optical fiber according to claim 1, in, On the cross section of the multi-core optical fiber, the plurality of core portions comprising the linear array group are arranged so that the positions of the centers of the respective core portions are linearly symmetrical with respect to a reference straight line passing through an intersection between the central axis and the cross section as an axis of symmetry.

11. The multi-core optical fiber according to claim 1, in, On the cross section of the multi-core optical fiber, the multiple core parts including the linear array group are arranged so that the positions of the centers of the respective cores have two or more rotational symmetry relative to the intersection point between the central axis and the cross section as the rotation center.

12. The multi-core optical fiber according to claim 10 or 11, in, The multi-core optical fiber includes a mark having a refractive index different from the refractive index of the common cladding, and the mark is arranged at a position that eliminates symmetry of the arrangement of the core centers among the plurality of core portions.

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