Multi-core optical fiber

Abstract

A MCF according to the present disclosure has a structure that prevents quality deterioration of an optical transmission signal. The MCF includes cores, a common cladding layer, and a coating. Any of the cores has a coating leakage loss of 0.01 dB / km or more at a wavelength in a wavelength range of 850 nm to 1700 nm. The coating includes a leak light propagation suppressing cladding layer having a first optical property or a second optical property with respect to light in a wavelength range of 850 nm to 1700 nm or 1260 nm to 1625 nm. The first optical property is defined by an absorbance per 1 μm thickness as an attenuation index with respect to light of 0.1 dB or more. The second optical property is defined by a product of an absorbance per 1 μm thickness and a thickness of 0.1 dB or more.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a multi-core optical fiber (hereinafter referred to as "MCF").

[0002] This application claims priority to Japanese Patent Application No. 2020-049135 filed on March 19, 2020, the entire contents of which are incorporated herein by reference. BACKGROUND

[0003] In recent years, various studies have been made on MCFs applicable to transmission lines as shown in the following Non-Patent Documents 1 to 4. However, the cause of signal quality degradation in a wavelength range in which light leakage to the coated side becomes significant has not been sufficiently studied.

[0004] (Patent Document 1) WO2016 / 157639

[0005] (Non-Patent Document 1) B. Zhu et al., "Seven-core multicore fiber transmissions for passive optical network," OPTICS EXPRESS, Vol. 18, No. 11, May 24, 2010, pp. 11117-11122

[0006] (Non-Patent Document 2) Tetsuya HAYASHI, "Multi-Core Fiber for High-Capacity Spatially-Multiplexed Transmission," August 2013, [online], [retrieved on February 21, 2020], Internet

[0007] (Non-Patent Document 3) Tetsuya HAYASHI et al., "125-μm-Cladding Eight-Core Multi-Core Fiber Realizing Ultra-High-Density Cable Suitable for O-Band Short-Reach Optical Interconnects," JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 34, No. 1, January 1, 2016, pp. 85-92

[0008] ​(Non-Patent Literature 4) M. Travagnin, "BER Penalty Induced by Coherent MPI Noise in FTTH Optical Links," JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 31, No. 18, September 15, 2013, pp. 3021-3031 SUMMARY

[0009] In one aspect, a multi-core fiber (MCF) according to an embodiment of the present disclosure includes a plurality of cores, a common cladding, and a coating. Each of the plurality of cores extends along a central axis of the MCF. The common cladding surrounds each of the plurality of cores, and a refractive index of the common cladding is smaller than a refractive index of each of the plurality of cores. The central axis of the MCF passes through a center of a cross section of the common cladding perpendicular to the central axis. The coating surrounds the common cladding. Further, a leakage loss of the coating at a wavelength in a wavelength range of 850 nm to 1700 nm of any of the plurality of cores is 0.01 dB / km or more. The coating includes a leaky light propagation suppressing cladding layer. The leaky light propagation suppressing cladding layer has a first optical property or a second optical property with respect to light in a wavelength range of 850 nm to 1700 nm or 1260 nm to 1625 nm of wavelength. In this context, the first optical property is defined by "absorbance per 1 μm thickness" as an attenuation index with respect to the above light being 0.1 dB or more. The second optical property is defined by a product of "absorbance per 1 μm thickness" and "thickness" being 0.1 dB or more. BRIEF DESCRIPTION OF DRAWINGS

[0010] Figure 1 FIG. 1 is a graph showing a cross-sectional structure of an MCF according to a first embodiment of the present disclosure and a refractive index profile common to all embodiments of the present disclosure;

[0011] Figure 2 FIG. 2 is a graph showing a first cross-sectional structure and a second cross-sectional structure of an MCF according to a second embodiment of the present disclosure;

[0012] Figure 3 FIG. 3 is a graph showing a first cross-sectional structure and a second cross-sectional structure of an MCF according to a third embodiment of the present disclosure;

[0013] Figure 4 FIG. 4 is a graph showing a cross-sectional structure of an MCF according to a fourth embodiment of the present disclosure;

[0014] Figure 5 FIG. 5 is a graph showing a cross-sectional structure of an MCF according to a comparative example prepared as a measurement sample and a second cross-sectional structure of an MCF according to the second embodiment; and

[0015] Figure 6 A graph showing loss spectra of each core (core 1 to core 8) in the MCF according to the comparative example and loss spectra of each core (core 1 to core 8) in the MCF according to the second embodiment. DETAILED DESCRIPTION

[0016] [Problems to be Solved by the Invention]

[0017] The present inventors have found the following problem through research on the technology in the related art. That is, in a wavelength band in which light leakage to the cladding side becomes significant, the MCF in the related art shows deterioration of signal quality, which cannot be explained by transmission loss (decrease in light intensity) due to light leakage. According to the finding of the present inventors, the deterioration of signal quality is caused by interference between "light propagating in the core" and "light components recoupled to the core from among light components leaked from the core to the cladding".

[0018] The present disclosure is made to solve the above problem. An object of the present disclosure is to provide an MCF having a structure that prevents deterioration of quality of an optical transmission signal due to a reflected component from the cladding.

[0019] [Advantageous Effects of Invention]

[0020] The MCF according to the present disclosure prevents deterioration of quality of an optical transmission signal.

[0021] [Description of Embodiments of the Present Disclosure]

[0022] The following description is a result of research related to the above Non-Patent Literature 1 to 4.

[0023] In the MCF disclosed in Non-Patent Literature 1, as can be seen from Figure 1 It can be seen that the transmission loss of the outer peripheral core (the outermost core in the cross section of the MCF perpendicular to the central axis) starts to increase from around 1450 nm, and vibrations similar to an interference pattern are generated in the wavelength range of 1450 nm to 1500 nm.

[0024] In the MCF disclosed in Non-Patent Literature 2, as can be seen from Figure 2 .20, the transmission loss of the outer peripheral core starts to increase from around 1400 nm, and an amplitude similar to an interference pattern is generated in the wavelength range of 1400 nm to 1700 nm. This is caused by interference between "light propagating in the core" and "light components recoupled to the core from among light components leaked from the core to the cladding" (see Figure 2 .30).

[0025] Non-Patent Literature 3 discloses an MCF including eight cores arranged in a ring within a cladding having an outer diameter of 125 μm. Furthermore,Figure 6 (a) shows the loss spectrum of each core. From Figure 6 (a) it can be seen that the leakage loss to the cladding is suppressed in the O-band (wavelength range from 1260 nm to 1360 nm), but the leakage loss becomes significant in a wavelength band longer than the O-band. In other words, in a wavelength band longer than the O-band, an interference pattern resulting from interference between "light propagating in the core" and "light component recoupled to the core among the light component leaked from the core to the cladding" becomes apparent.

[0026] Non-patent 4 discloses that multi-path interference (MPI) degrades the quality of optical transmission signals.

[0027] A multi-core fiber (MCF) according to the present disclosure has a leak light propagation suppression cladding layer in a cladding provided on an outer peripheral surface of a bare optical fiber or on a surface layer. This structure makes it possible to suppress the intensity loss of the light component leaked from each core to the cladding, and to reduce the intensity of the leak light component recoupled to each core by reflection to each core on the cladding surface. Thus, it is possible to prevent degradation in the quality of optical transmission signals due to interference between "light propagating in the core" and "light component recoupled to the core among the light component leaked from the core to the cladding". First, details of embodiments of the present disclosure will be individually listed and described.

[0028] (1) A MCF according to one aspect of the present disclosure includes a plurality of cores, a common cladding, and a cladding. Each of the plurality of cores extends along a central axis of the MCF. The common cladding surrounds each of the plurality of cores, and a refractive index of the common cladding is smaller than a refractive index of each of the plurality of cores. The central axis of the MCF passes through a center of a cross section of the common cladding perpendicular to the central axis. The cladding surrounds the common cladding. Further, a cladding leakage loss of any of the plurality of cores at a wavelength in a wavelength range from 850 nm to 1700 nm is 0.01 dB / km or more. The cladding includes a leak light propagation suppression cladding layer. The leak light propagation suppression cladding layer has a first optical property or a second optical property with respect to light in a wavelength range from 850 nm to 1700 nm or from 1260 nm to 1625 nm. Herein, the first optical property is defined by "absorbance per 1 μm thickness" as an attenuation index with respect to light in the above wavelength range being 0.1 dB or more. The second optical property is defined by a product of "absorbance per 1 μm thickness" and "thickness" being 0.1 dB or more.

[0029] In the plurality of cores, a difference between a transmission loss between the core center (cross-sectional center) closest to the cladding and a transmission loss between the core center farthest from the cladding can be 0.01 dB / km or more in a wavelength range of 850 nm to 1700 nm. Even with such macrobend loss, the MCF according to the present disclosure does not generate an interference pattern.

[0030] Here, the "thickness" of a certain layer is given by the length from the inner peripheral surface to the outer peripheral surface of the certain layer along the radial direction perpendicular to the central axis (or by the difference between the inner diameter and the outer diameter). Further, the cladding leakage loss of any of the plurality of cores can be measured by, for example, the method disclosed in the above-described Patent Literature 1. In particular, the "cladding leakage loss" refers to a transmission loss due to a decrease in intensity of light guided through the core, which is caused by leakage of the light guided through the core to the cladding having a higher refractive index than the core. When the cladding leakage loss is too large to be ignored, light of high intensity leaks into the cladding and propagates for a certain distance in the cladding. In general, the "absorbance" of a resin or the like (absorption and scattering of the material itself) is a value obtained by preparing a "sheet" of the resin having a predetermined thickness and multiplying the common logarithm of the light transmittance by -1. Here, the absorbance is proportional to the thickness of the sheet (the propagation length of light in the resin).

[0031] (2) As one aspect of the present disclosure, both the plurality of cores and the common cladding are preferably composed of quartz glass. Further, as one aspect of the present disclosure, the cladding is preferably composed of an ultraviolet-curable resin. The reason for this is that, as part of the manufacturing process of a typical quartz glass-based optical fiber, a resin including a light leakage propagation non-suppression cladding layer can be provided on the surface of the bare optical fiber after drawing (the outer peripheral surface of the common cladding).

[0032] (3) As one aspect of the present disclosure, the cladding preferably has a first structure and a second structure. The first structure is composed entirely of a plurality of light leakage propagation suppression cladding layers. The second structure is composed of a light leakage propagation suppression cladding layer and a light leakage propagation non-suppression cladding layer having a third optical characteristic defined by an absorbance per 1 pm thickness of 0.005 dB or less with respect to light in the above-described wavelength range. This configuration makes it possible to suppress deterioration of the transmission loss of light propagating in each core in a waveguide mode even when a part of the electric field of the light propagating in each core in a waveguide mode penetrates the light leakage propagation non-suppression cladding layer.

[0033] (4) As one aspect of the present disclosure, in a case where the coating includes a leak light propagation suppression coating layer and a leak light propagation non-suppression coating layer, the leak light propagation non-suppression coating layer is preferably provided on the outer peripheral surface of the common cladding layer, and the leak light propagation suppression coating layer is preferably provided on the outer peripheral surface of the leak light propagation non-suppression coating layer. Further, as one aspect of the present disclosure, the Young's modulus of the leak light propagation non-suppression coating layer is preferably 2 MPa or less or 1 MPa or less. Further, the Young's modulus of the leak light propagation suppression coating layer is preferably 0.8 GPa or more, 1 GPa or more, or 2 GPa or more. In this case, an effect of sufficiently suppressing microbends applied from the common cladding layer to the inside of the MCF due to lateral pressure against the MCF can be obtained.

[0034] (5) As one aspect of the present disclosure, the leak light propagation non-suppression coating layer can include a first leak light propagation non-suppression coating layer and a second leak light propagation non-suppression coating layer. In this case, the first leak light propagation non-suppression coating layer is preferably provided on the outer peripheral surface of the common cladding layer, and the second leak light propagation non-suppression coating layer is preferably provided on the outer peripheral surface of the first leak light propagation non-suppression coating layer. Further, the leak light propagation suppression coating layer is preferably provided on the outer peripheral surface of the second leak light propagation non-suppression coating layer. As one aspect of the present disclosure, the Young's modulus of the first leak light propagation non-suppression coating layer is preferably 2 MPa or less or 1 MPa or less. The Young's modulus of the second leak light propagation non-suppression coating layer is preferably 0.8 GPa or more, 1 GPa or more, or 2 GPa or more. This configuration also shows a prospect of suppressing microbends applied from the common cladding layer to the inside of the MCF due to lateral pressure against the MCF.

[0035] (6) As one aspect of the present disclosure, the thickness of the leak light propagation suppression coating layer is preferably 1 μm or more and 10 μm or less, or 1 μm or more and 5 μm or less. Further, as one aspect of the present disclosure, preferably, the leak light propagation suppression coating layer exhibits a predetermined color by causing reflection or scattering of a predetermined spectrum in response to irradiation of white light in the visible light region. In this case, an operator can recognize the leak light propagation suppression coating layer, which makes it easier to handle the MCF.

[0036] (7) According to one aspect of the present disclosure, the leak light propagation suppression coating layer can be composed of carbon, and the leak light propagation suppression coating layer can be located on the outer peripheral surface of the common cladding layer in a state of being in contact with the outer peripheral surface of the common cladding layer. In this case, as one aspect of the present disclosure, the thickness of the leak light propagation suppression coating layer is preferably 10 nm or more and 1 μm or less. The thickness of the above range can simultaneously achieve suppression of leak light propagation and flexibility of the optical fiber.

[0037] (8) On the other hand, as one aspect of the present disclosure, the leakage light propagation suppressing coating layer can be composed of a metal, and the leakage light propagation suppressing coating layer can be located on the outer peripheral surface of the common cladding layer in a state of being in contact with the outer peripheral surface of the common cladding layer. In this case, as one aspect of the present disclosure, the thickness of the leakage light propagation suppressing coating layer is preferably 1 μm or more and 30 μm or less. The thickness of the above range can achieve both the suppression of the leakage light propagation and the flexibility of the optical fiber.

[0038] (9) As one aspect of the present disclosure, in a configuration in which the leakage light propagation suppressing coating layer is directly provided on the outer peripheral surface of the common cladding layer, the outer peripheral surface of the leakage light propagation suppressing coating layer can be covered with a coating layer composed of a resin. In this case, the coating composed of a resin serves as a physical protection layer for the leakage light propagation suppressing coating layer.

[0039] As described above, the aspects listed in [Description of Embodiments of the Present Disclosure] can be applied to other aspects or any combination of these aspects.

[0040] [Details of Embodiments of the Present Disclosure]

[0041] A specific example of the MCF according to the present disclosure will be described in detail with reference to the accompanying drawings. Note that the present disclosure is not limited to the following example, but is represented by the claims, and is intended to include matters equivalent to the claims and all modifications within the scope of the claims. In the description of the drawings, note that the same reference numerals will be used to denote the same elements, and redundant description will be omitted.

[0042] (First Embodiment)

[0043] Figure 1 A diagram showing the cross-sectional structure of a multi-core fiber (MCF) according to the first embodiment of the present disclosure and the refractive index profile common to all embodiments of the present disclosure is shown.

[0044] Figure 1The upper part of FIG. 1 shows a cross-sectional structure of the MCF 10 according to the first embodiment, perpendicular to the central axis AX. The MCF 10 includes bare optical fibers 100 and a coating provided on the outer peripheral surface of the bare optical fibers 100. The bare optical fibers 100 include eight core portions 101 that surround the central axis AX at equal intervals and are equidistant from the central axis AX. In a region surrounded by the eight core portions 101, a marker 105 for checking the positions of the core portions 101 is provided. The core portions 101 and the marker 105 extend along the central axis AX. An inner cladding layer 102 is provided on the outer peripheral surface of each core portion 101, a trench 103 is provided on the outer peripheral surface of each inner cladding layer 102, and a common cladding layer 104 is provided on the outer peripheral surface of the trench 103. Each core portion 101, each inner cladding layer 102, each trench 103, and the common cladding layer 104 are preferably composed of quartz glass. Furthermore, the coating provided on the outer peripheral surface of the bare optical fibers 100 is preferably composed of an ultraviolet-curable resin. In the spinning of the optical fiber, a resin solution coated on the outer peripheral surface of the common cladding layer 104 is irradiated with ultraviolet rays to facilitate the manufacture of a coated optical fiber (MCF 10) having good manufacturability.

[0045] Figure 1 “Rc” in the upper part of FIG. 1 indicates the length from the cross-sectional center of the MCF 10 (the cross-sectional center through which the central axis AX passes, hereinafter referred to simply as the “center”) to the cross-sectional center (hereinafter referred to simply as the “center”) of each core portion 101. “OCT” indicates the shortest length from the center of each core portion 101 to the interface between the common cladding layer 104 and the coating. “CD” indicates the outer diameter of the bare optical fibers 100 (corresponding to the outer diameter of the common cladding layer 104).

[0046] Figure 1 The lower part of FIG. 1 shows the refractive index distribution around each core portion 101 in the MCF 10. This refractive index distribution shows the relative refractive index difference (using the refractive index of the common cladding layer 104 as a reference) at a position having a length r from the center of each core portion 101 (a length in the radial direction of each core portion 101, hereinafter referred to as the “core radius”). Figure 1 The length a indicates the radius of each core portion 101. The length b indicates the length from the center of each core portion 101 to the outer peripheral surface of each inner cladding layer 102 (the outer radius of each inner cladding layer 102). The length c indicates the length from the center of each core portion 101 to the outer peripheral surface of each trench 103 (the outer radius of each trench 103). As shown by the refractive index distribution of FIG. 1, around each core portion 101, the refractive index of each inner cladding layer 102 is lower than the refractive index of each core portion 101. The refractive index of each trench 103 is lower than the refractive index of each inner cladding layer 102. The refractive index of the common cladding layer 104 is higher than the refractive index of each trench 103 and lower than the refractive index of each core portion 101 and the refractive index of each inner cladding layer 102. Figure 1 Figure 1 ​The refractive index distribution in the lower portion of the core 101 is an example in which the common cladding layer 104 is composed of pure silica. However, the material composing the common cladding layer 104 can be quartz glass containing impurities (refractive index adjustors).

[0047] The cladding leakage loss at a wavelength in the wavelength range of 850 nm to 1700 nm of any of the eight cores 101 is 0.01 dB / km or more (for example, the cladding leakage loss can be measured by the method described in the above-described Patent Literature 1). Among the eight cores 101, the difference in transmission loss between the core with the shortest core-cladding distance and the core with the longest core-cladding distance can be 0.01 dB / km or more in the wavelength range of 850 nm to 1700 nm. Even with such macrobend loss, the MCF 10 according to the present disclosure does not produce an interference pattern.

[0048] In the first embodiment, the cladding of the MCF 10 is composed of a light leakage propagation suppression cladding layer 110A containing a resin (UV protection effect resin). The light leakage propagation suppression cladding layer 110A has a first optical property or a second optical property with respect to light in the wavelength range of 850 nm to 1700 nm or 1260 nm to 1625 nm in wavelength. The first optical property is defined by "absorbance per 1 pm thickness" or an attenuation index of 0.1 dB or more with respect to light in the above-described wavelength range. The second optical property is defined by the product of "absorbance per 1 pm thickness" and "thickness" being 0.1 dB or more.

[0049] Note that "absorbance", also known as optical density, is a dimensionless quantity in spectroscopy that expresses how much the intensity of light decreases when it passes through a specified object. In spectrochemistry, the absorbance A λ at a wavelength λ is defined as A λ = -log 10 (I / I O ). As one example, "absorbance per 1 pm thickness" of a substance with a thickness of T (unit: pm) is given by -(log 10 (I / I O )) / T. In other words, the natural logarithm of the ratio of the incident light intensity I O to the transmitted light intensity I (transmittance) gives these equations, and both equations use a negative sign because the value should be positive when light is absorbed. The transmittance decays exponentially with respect to the optical path length, whereas the absorbance is expressed in logarithm and is proportional to the optical path length. For example, in the case where the thickness of an object with a transmittance of 0.1 (absorbance of 1) is three times, the transmittance is 0.1 3 = 0.001, and the absorbance is simply three times.

[0050] (Second Embodiment)

[0051] Figure 2 FIGS. 1 and 2 are diagrams showing a first cross-sectional structure and a second cross-sectional structure of a multi-core fiber (MCF) according to a first embodiment of the present disclosure.

[0052] The coated structures of the MCFs 20A and 20B according to the second embodiment are different from the MCF 10 according to the first embodiment shown in the upper portion of Figure 1 On the other hand, both of the MCFs 20A and 20B according to the second embodiment include bare optical fibers (quartz-based optical fibers) whose cross-sectional structures are the same as that of the bare optical fiber 100 included in the MCF 10 according to the first embodiment.

[0053] In the MCF 20A according to the second embodiment shown in the upper portion of Figure 2 In the MCF 20A according to the second embodiment shown in the upper portion of

[0054] In the MCF 20B according to the second embodiment shown in the lower portion of Figure 2In the MCF 20B according to the second embodiment shown in the lower part of FIG. 1, the coating includes a first leaky-light-propagation non-suppressing coating layer 120A provided on the outer peripheral surface of the common cladding layer 104 and a second leaky-light-propagation non-suppressing coating layer 120B provided on the outer peripheral surface of the first leaky-light-propagation non-suppressing coating layer 120A. The leaky-light-propagation suppressing coating layer 110B is provided on the outer peripheral surface of the second leaky-light-propagation non-suppressing coating layer 120B. This configuration also makes it possible to suppress the deterioration of the transmission loss of the light propagating in the core portions 101 in the waveguide mode even when a part of the electric field of the light propagating in the waveguide mode in the core portions 101 penetrates the leaky-light-propagation non-suppressing coating layers 120A and 120B. Further, in order to obtain the effect of suppressing the microbends applied from the common cladding layer 104 to the inside of the MCF 20B due to lateral pressure on the MCF 20B, the Young's modulus of the first leaky-light-propagation non-suppressing coating layer 120A is preferably 2 MPa or less, and more preferably 1 MPa or less. On the other hand, the Young's modulus of the second leaky-light-propagation non-suppressing coating layer 120B is preferably 0.8 GPa or more, more preferably 1 GPa or more, and further preferably 2 GPa or more. Here, the thickness of the leaky-light-propagation suppressing coating layer 110B is preferably 1 μm or more and 10 μm or less, and more preferably 1 μm or more and 5 μm or less.

[0055] In both the MCFs 20A and 20B according to the second embodiment, the entire coating provided on the outer peripheral surface of the common cladding layer 104 is composed of a resin.

[0056] In the MCFs 20A and 20B according to the second embodiment, the coating is preferably composed of an ultraviolet-curable resin. In the spinning of optical fibers, the resin solution coated on the outer peripheral surface of the common cladding layer 104 is irradiated with ultraviolet rays to facilitate the manufacture of a coated optical fiber (MCF 20A and 20B) having good manufacturability.

[0057] Further, in the configuration in which the leaky-light-propagation suppressing coating layer 110A is provided outside the leaky-light-propagation non-suppressing coating layer 120, or in the configuration in which the leaky-light-propagation suppressing coating layer 110B is provided outside the first leaky-light-propagation non-suppressing coating layer 120A and the second leaky-light-propagation non-suppressing coating layer 120B, as in the MCFs 20A and 20B according to the second embodiment, both the leaky-light-propagation suppressing coating layer 110A and the leaky-light-propagation suppressing coating layer 110B preferably exhibit a predetermined color by reflecting or scattering a predetermined spectrum in response to the irradiation of white light in the visible light region. Specifically, the leaky-light-propagation suppressing coating layer 110A and the leaky-light-propagation suppressing coating layer 110B preferably exhibit a predetermined color such as red, blue, yellow, green, black, white, gray, orange, and pink. Thus, the operator can recognize the leaky-light-propagation suppressing coating layers 110A and 110B, which makes it easier to handle the MCFs 20A and 20B.

[0058] (third embodiment)

[0059] Figure 3 A drawing showing a first cross-sectional structure and a second cross-sectional structure of a multi-core fiber (MCF) according to a third embodiment of the present disclosure.

[0060] The coating structures of the MCFs 30A and 30B according to the third embodiment are different from Figure 1 the MCF 10 according to the first embodiment and the MCFs 20A and 20B according to the second embodiment shown in the upper portion of FIG. 1. On the other hand, both of the MCFs 30A and 30B according to the third embodiment include bare optical fibers (quartz-based optical fibers) having the same cross-sectional structure as that of the bare optical fiber 100 included in the MCF 10 according to the first embodiment.

[0061] In both of the MCFs 30A and 30B according to the third embodiment, a leak light propagation suppression coating layer 110C composed of carbon (nonmetal) or metal is provided on the outer peripheral surface of the bare optical fiber 100 (the outer peripheral surface of the common cladding 104). In particular, in the MCF 30A according to the third embodiment shown in the upper portion of FIG. 3, a carbon coating serving as the leak light propagation suppression coating layer 110C is provided on the outer peripheral surface of the bare optical fiber 100. Figure 3 In the MCF 30A according to the third embodiment shown in the upper portion of FIG. 3, a carbon coating serving as the leak light propagation suppression coating layer 110C is provided on the outer peripheral surface of the bare optical fiber 100. As in the MCF 30A, when the leak light propagation suppression coating layer 110C composed of carbon (nonmetal) is employed, the thickness of the leak light propagation suppression coating layer 110C is preferably 10 nm or more and 1 μm or less. On the other hand, in the MCF 30B according to the third embodiment shown in the lower portion of FIG. 3, a metal coating serving as the leak light propagation suppression coating layer 110C is provided on the outer peripheral surface of the bare optical fiber 100. As in the MCF 30B, when the leak light propagation suppression coating layer 110C composed of metal is employed, the thickness of the leak light propagation suppression coating layer 110C is preferably 1 μm or more and 30 μm or less. In the MCF 30B, a leak light propagation non-suppression coating layer (coating layer composed of resin) 120 for protecting the leak light propagation suppression coating layer 110C is provided on the outer peripheral surface of the leak light propagation suppression coating layer 110C. The leak light propagation non-suppression coating layer can be provided on the carbon coating. Figure 3 In the MCF 30B according to the third embodiment shown in the lower portion of FIG. 3, a metal coating serving as the leak light propagation suppression coating layer 110C is provided on the outer peripheral surface of the bare optical fiber 100. As in the MCF 30B, when the leak light propagation suppression coating layer 110C composed of metal is employed, the thickness of the leak light propagation suppression coating layer 110C is preferably 1 μm or more and 30 μm or less. In the MCF 30B, a leak light propagation non-suppression coating layer (coating layer composed of resin) 120 for protecting the leak light propagation suppression coating layer 110C is provided on the outer peripheral surface of the leak light propagation suppression coating layer 110C. The leak light propagation non-suppression coating layer can be provided on the carbon coating.

[0062] In this way, in the case where the leak light propagation suppression coating layer 110C is composed of carbon or metal, the propagation of leak light can be prevented within the above thickness range, and the flexibility of the optical fiber can be achieved.

[0063] (fourth embodiment)

[0064] Figure 4 A drawing showing a cross-sectional structure of a multi-core fiber (MCF) according to a fourth embodiment of the present disclosure.

[0065] The coated structure of the MCF 40 according to the fourth embodiment is different from Figure 1 The MCF 10 according to the first embodiment and the MCFs 20A and 20B according to the second embodiment, and the MCFs 30A and 30B according to the third embodiment shown in the upper part of the drawing. On the other hand, the MCF 40 according to the fourth embodiment includes bare optical fibers (quartz-based optical fibers) whose cross-sectional structures are the same as that of the bare optical fiber 100 included in the MCF 10 according to the first embodiment.

[0066] On the outer peripheral surface of the bare optical fiber 100 (on the outer peripheral surface of the common cladding 104), the MCF 40 according to the fourth embodiment has a coated structure in which the structures in the second and third embodiments are combined. In other words, the coating of the MCF 40 according to the fourth embodiment includes the light leakage propagation suppressing coating layer 110C (third embodiment) on the outer peripheral surface of the common cladding 104, the light leakage propagation non-suppressing coating layer 120 on the outer peripheral surface of the light leakage propagation suppressing coating layer 110C, and the light leakage propagation suppressing coating layer 110A (second embodiment) on the outer peripheral surface of the light leakage propagation non-suppressing coating layer 120.

[0067] Figure 4 The light leakage propagation suppressing coating layer 110C shown in the drawing is composed of carbon or metal. When the light leakage propagation suppressing coating layer 110C composed of carbon is provided on the outer peripheral surface of the common cladding 104, the thickness of the light leakage propagation suppressing coating layer 110C is preferably 10 nm or more and 1 μm or less. On the other hand, when the light leakage propagation suppressing coating layer 110C composed of metal is provided on the outer peripheral surface of the common cladding 104, the thickness of the light leakage propagation suppressing coating layer 110C is preferably 1 μm or more and 30 μm or less. Further, Figure 4 The light leakage propagation suppressing coating layer 110A shown in the drawing is provided on the outer peripheral surface of the light leakage propagation non-suppressing coating layer 120 (a resin layer provided on the outer peripheral surface of the light leakage propagation suppressing coating layer 110C) and is composed of a resin.

[0068] In the MCF 40 according to the fourth embodiment shown in the drawing, Figure 4 In the MCF 40 according to the fourth embodiment shown in the drawing, the coating provided on the outer peripheral surface of the bare optical fiber 100 includes the light leakage propagation suppressing coating layer 110A and the light leakage propagation suppressing coating layer 110C that are different from each other, and the light leakage propagation non-suppressing coating layer 120 is interposed between the light leakage propagation suppressing coating layer 110A and the light leakage propagation suppressing coating layer 110C. Therefore, the propagation of light leakage can be more reliably suppressed, and the flexibility of the optical fiber can be ensured.

[0069] It should be noted that the light leakage propagation suppression coating 110A, 110B, and 110C made of carbon or metal, which are composed of resin and used in the first to fourth embodiments, all possess a first optical characteristic or a second optical characteristic relative to light in the wavelength range of 850 nm to 1700 nm or 1260 nm to 1625 nm. The first optical characteristic is defined as "absorbance per 1 μm thickness" or an attenuation index of 0.1 dB or more relative to light in the aforementioned wavelength range. The second optical characteristic is defined as the product of "absorbance per 1 μm thickness" and "thickness" being 0.1 dB or more.

[0070] (Measurement Results)

[0071] Figure 5 The diagram shows the cross-sectional structure of the MCF 500 prepared as a test specimen according to the comparative example and the second cross-sectional structure of the MCF 20B according to the second embodiment.

[0072] Figure 6 The diagram illustrates the loss spectra of each core 101 (core 1 to core 8) in the MCF 500 according to the comparative example and the loss spectra of each core 101 (core 1 to core 8) in the MCF 20B according to the second embodiment. As a technical effect of the leakage propagation suppression coating, the inventors conducted experiments and confirmed the effect of suppressing interference between "light propagating in the core" and "light components that are recoupled to the core in the light components leaking from the core to the coating".

[0073] MCF 500 prepared according to the comparative example ( Figure 5 The upper part) and MCF 20B according to the second embodiment ( Figure 5 The lower part of each fiber (MCF 500 and MCF 20B) consists of bare optical fibers with the same cross-sectional structure. In other words, both MCF 500 and MCF 20B are obtained by drawing the same fiber matrix, and the only difference is the coating structure.

[0074] In the bare optical fiber 100, using the refractive index of the common cladding 104 as a reference (e.g., pure silica level), the relative refractive index difference of each core 101 is 0.35 ± 0.05% (0.30% to 0.40%), the relative refractive index difference of each inner cladding 102 is 0.05 ± 0.05% (0% to 0.1%), and the relative refractive index difference of each trench 103 is -0.70 ± 0.05% (-0.75% to -0.65%). The outer diameter 2a of each core 101 is 6.4 ± 0.5 μm (5.9 μm to 6.9 μm). The ratio a / b of the outer diameter 2a of each core 101 to the outer diameter (inner diameter of each trench) 2b of each inner cladding 102 is 0.4 ± 0.05 (0.35 to 0.45). The ratio b / c of ​​the outer diameter 2b of each inner cladding 102 to the outer diameter 2c of each groove 103 is 0.6±0.1 (between 0.5 and 0.7).

[0075] The outer diameter CD of the common cladding 104 is 125±15μm (between 110μm and 140μm). The length Rc from the center of the common cladding 104 (the center of the section intersecting the central axis AX) to the center of each core 101 is 40.5±1.5μm (between 39.0μm and 42μm). The shortest length OCT from the center of each core 101 (the center of the section) to the interface between the common cladding 104 and the coating is 22±2μm (between 20μm and 24μm).

[0076] Based on the coverage of the comparative example MCF 500 ( Figure 5 The upper part of the MCF 500 includes a first light leakage propagation non-suppression coating layer 120A disposed on the outer peripheral surface of the common cladding layer 104 and a second light leakage propagation non-suppression coating layer 120B disposed on the outer peripheral surface of the first light leakage propagation non-suppression coating layer 120A. The MCF 500 according to the comparative example does not include a light leakage propagation suppression coating layer. The outer diameter of the coating including the first light leakage propagation non-suppression coating layer 120A and the second light leakage propagation non-suppression coating layer 120B is 250 ± 15 μm (235 μm or more, 265 μm or less). The absorbance per 1 μm thickness of the first light leakage propagation non-suppression coating layer 120A and the second light leakage propagation non-suppression coating layer 120B is 0.003 dB / μm or less in the wavelength range of 850 nm to 1700 nm, and 0.0015 dB / μm or less in the wavelength range of 850 nm to 1600 nm.

[0077] On the other hand, the coverage of MCF 20B according to the second embodiment ( Figure 5The lower part includes a first light leakage propagation non-suppression coating layer 120A located on the outer peripheral surface of the common cladding layer 104, a second light leakage propagation non-suppression coating layer 120B located on the outer peripheral surface of the first light leakage propagation non-suppression coating layer 120A, and a light leakage propagation suppression coating layer 110B located on the outer peripheral surface of the second light leakage propagation non-suppression coating layer 120B. In the MCF 20B according to the second embodiment, the first light leakage propagation non-suppression coating layer 120A and the second light leakage propagation non-suppression coating layer 120B have the same structure as the MCF 500 according to the comparative example.

[0078] Furthermore, in accordance with the second implementation scheme of MCF 20B ( Figure 5 In the lower part of the coating, the thickness of the light leakage propagation suppression coating 110B is 3 ± 2 μm (more than 1 μm and less than 5 μm). The absorbance per 1 μm of thickness of the light leakage propagation suppression coating 110B is 0.1 dB / μm or more across the entire wavelength range of 850 nm to 1700 nm. The product of the absorbance per 1 μm of thickness and the thickness is 0.3 ± 0.2 dB or more (more than 0.1 dB or more, or more than 0.5 dB or more).

[0079] Compare Figure 6 The loss spectra of each core 101 (core 1 to core 8) shown in the upper part (according to the comparative example MCF 500) and Figure 6 The loss spectra of each core 101 (core 1 to core 8) shown in the lower part (MCF 20B according to the second embodiment) of the two MCFs are similar in that, in the wavelength range of 1300 nm to 1400 nm, the transmission loss tends to increase with increasing wavelength due to leakage to the coating. However, in the long wavelength region of the wavelength range of 1300 nm to 1400 nm, the transmission loss spectrum of the MCF 500 according to the comparative example shows a significant interference pattern (amplitude). In other words, in the MCF 500 according to the comparative example, in the long wavelength region of the wavelength range of 1300 nm to 1400 nm, there is strong interference between "light propagating in the core" and "light components that are recoupled to the core from the light components leaking from the core to the coating".

[0080] On the other hand, it can be seen that even in the long wavelength region of the 1300nm to 1400nm wavelength range, the MCF 20B according to the second embodiment is able to suppress the interference between "light propagating in the core" and "the light component recoupled to the core in the light component leaking from the core to the cover". Therefore, when the transmission line uses any of the MCFs (MCF 10, MCF 20A, MCF 20B, MCF 30A, MCF 30B and MCF 40) according to the embodiments that suppress the interference between "light propagating in the core" and "the light component recoupled to the core in the light component leaking from the core to the cover", even in the band where the cover leakage loss is significantly increased (in Figure 6 In the example shown (wavelength range of 1400nm to 1600nm), although transmission loss increases, optical signals can still be transmitted without signal quality degradation due to interference.

Claims

1. A multi-core optical fiber, comprising: Multiple cores extending along the central axis; A common cladding surrounding each of the plurality of cores, wherein the refractive index of the common cladding is less than the refractive index of each of the plurality of cores; and The covering surrounding the common cladding, The coating leakage loss of any one of the plurality of cores in the wavelength range of 850 nm to 1700 nm is greater than 0.01 dB / km, and The coating includes a light leakage suppression coating layer having a first optical characteristic or a second optical characteristic relative to light in the wavelength range of 850 nm to 1700 nm or 1260 nm to 1625 nm. The first optical characteristic is defined by an absorbance of 0.1 dB or more per 1 μm thickness as an attenuation index relative to the light, and the second optical characteristic is defined by a product of the absorbance per 1 μm thickness and the thickness being 0.1 dB or more. The coating also includes a non-suppressive coating layer for light leakage propagation with a third optical property, defined by an absorbance of less than 0.005 dB per 1 μm thickness relative to the light. The light leakage propagation non-suppression coating layer includes a first light leakage propagation non-suppression coating layer disposed on the outer peripheral surface of the common cladding layer and a second light leakage propagation non-suppression coating layer disposed on the outer peripheral surface of the first light leakage propagation non-suppression coating layer, and The light leakage propagation suppression coating is disposed on the outer peripheral surface of the second light leakage propagation non-suppression coating.

2. The multi-core optical fiber according to claim 1, Both the plurality of cores and the common cladding are made of quartz glass.

3. The multi-core optical fiber according to claim 1 or 2, The coating is made of a UV-curable resin.

4. The multi-core optical fiber according to claim 1, The Young's modulus of the first non-suppressive light leakage propagation coating layer is below 2 MPa. The Young's modulus of the second non-suppressive light leakage propagation coating is above 0.8 GPa, and The Young's modulus of the light leakage propagation suppression coating is above 0.8 GPa.

5. The multi-core optical fiber according to claim 4, The Young's modulus of the first light leakage propagation non-suppressive coating is below 1 MPa.

6. The multi-core optical fiber according to claim 4, The Young's modulus of the second light leakage propagation non-suppressive coating is above 1 GPa.

7. The multi-core optical fiber according to claim 4, The Young's modulus of the second non-suppressive light leakage propagation coating is above 2 GPa.

8. The multi-core optical fiber according to claim 4, The Young's modulus of the light leakage propagation suppression coating is above 1 GPa.

9. The multi-core optical fiber according to claim 4, The Young's modulus of the light leakage propagation suppression coating is above 2 GPa.

10. The multi-core optical fiber according to claim 1 or 4, The thickness of the light leakage propagation suppression coating is between 1 μm and 10 μm.

11. The multi-core optical fiber according to claim 10, The thickness of the light leakage propagation suppression coating is more than 1 μm and less than 5 μm.

12. The multi-core optical fiber according to any one of claims 1 to 11, The light leakage propagation suppression coating exhibits a predetermined color by reflecting or scattering a predetermined spectrum in response to irradiation with white light in the visible light region.

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