Multi-core optical fiber

Abstract

A multi-core optical fiber (10) for transmission is provided with a plurality of cores (11a-11d) and a cladding (12) surrounding each of the cores (11a-11d), and the propagation loss of measurement light having a wavelength other than the communication wavelength band differs between the core (11a) and the other cores (11b-11d).

Description

Technical Field

[0001] The present invention relates to a multi-core optical fiber. Background Art

[0002] To increase the transmission capacity of optical fiber communication devices, it is known to use a multi-core optical fiber having multiple cores as waveguides surrounded by a single cladding, and to transmit multiple signals using light propagating through each core. Patent Document 1 below describes such a multi-core optical fiber.

[0003] In the multi-core optical fiber disclosed in Patent Document 1 below, propagation loss differs between a specific core and the other cores among a plurality of cores, and therefore, core alignment can be performed based on this difference in propagation loss.

[0004] Patent Document 1: Japanese Patent Application Laid-Open No. 2014-048645 Summary of the Invention

[0005] (1) Technical issues to be resolved

[0006] In optical communications, it is desirable to reduce variations in the propagation loss of light propagating through various cores. To meet this demand, the multi-core optical fiber described in Patent Document 1 above considers reducing the difference in propagation loss between a specific core and other cores. However, reducing the difference in propagation loss makes it difficult to identify a specific core.

[0007] Therefore, an object of the present invention is to provide a multi-core optical fiber that can easily identify a specific core even if the variation in propagation loss of light having a wavelength in a communication band in the core is small.

[0008] (2) Technical solution

[0009] A first aspect of the present invention is a multi-core optical fiber comprising: a plurality of cores; and a cladding surrounding each core, wherein propagation loss of measurement light having a wavelength outside a communication band is different between a specific core and the other cores.

[0010] In Scheme 1, as described above, the propagation loss of measurement light with wavelengths outside the communication band differs between a specific core and other cores. Therefore, according to Scheme 1, even if the propagation loss of light with wavelengths in the communication band within a core varies slightly, the specific core can be easily identified by using the aforementioned measurement light to measure the propagation loss of each core. Examples of methods for measuring propagation loss include measurements using OTDR (Optical Time Domain Reflectometer), OFDR (Optical Frequency Domain Reflectometry), and OLTS (Optical Loss Test Sets). In particular, when using OTDR and OFDR, propagation loss can be measured simply by injecting light from one end of a multi-core optical fiber, making measurement easy.

[0011] A second aspect of the present invention is the multi-core optical fiber according to the first aspect, wherein the difference between the propagation loss of the measurement light in the specific core and the propagation loss in the other cores is 0.1 dB / km or more.

[0012] The difference in propagation loss between cores in a typical multi-core optical fiber tends to be less than 0.1 dB / km. Therefore, according to the second solution, a specific core can be easily identified.

[0013] A third aspect of the present invention is the multi-core optical fiber according to the first or second aspect, characterized in that the wavelength of the measuring light is greater than or equal to 1360 nm and less than 1460 nm.

[0014] It is known that when the wavelength of light propagating through a core is in the so-called E-band, that is, between 1360 nm and less than 1460 nm, the optical loss due to hydroxyl groups contained in the core increases, sometimes referred to as OH loss. According to Scheme 3, by increasing the hydroxyl content in a specific core relative to the hydroxyl content in other cores, the propagation loss of the measurement light in the specific core can be made greater than the propagation loss in the other cores. Therefore, the difference between the propagation loss of the measurement light in the specific core and the propagation loss in the other cores can be made greater than the difference between the propagation loss of light with a wavelength in the communication band in the specific core and the propagation loss in the other cores.

[0015] A fourth aspect of the present invention is the multi-core optical fiber according to any one of aspects 1 to 3, characterized in that the wavelength of the measuring light is shorter than the communication band.

[0016] The shorter the wavelength of light propagating through the cores, the more the inter-core crosstalk of that light tends to be suppressed. Therefore, according to Scheme 4, when using the aforementioned measurement light to investigate the propagation loss of each core, inter-core crosstalk of the measurement light propagating through the cores can be suppressed, compared to a case where the communication bands are the same and the wavelength of the measurement light is longer than the communication band. Therefore, Scheme 4 makes it easier to identify a specific core.

[0017] Scheme 5 of the present invention is a multi-core optical fiber according to any one of Schemes 1 to 4, characterized in that the difference between the propagation loss of the measurement light in the specific fiber core and the propagation loss in the other fiber cores is greater than the difference between the propagation loss of light with a wavelength in the communication band in the specific fiber core and the propagation loss in the other fiber cores.

[0018] According to solution 5, a specific fiber core can be identified more easily.

[0019] Scheme 6 of the present invention is based on any multi-core optical fiber in Schemes 1 to 5, characterized in that the wavelength of the measuring light is: less than the wavelength at which light of a mode one order higher than the mode of light of the wavelength of the communication band propagating in the fiber core can propagate.

[0020] According to Scheme 6, the measurement light propagating through the core includes light of the same mode as the light of the communication wavelength band propagating through the core, and light of a higher-order mode than that mode. The higher-order mode of light is, the more likely it is to escape from the core through which it propagates, and is therefore more susceptible to the influence of propagation loss factors located at the periphery of the core or outside the core. Examples of propagation loss factors include elements added to the periphery of the core or cladding that easily absorb the measurement light, irregularities at the boundary between the core and cladding, and markers located near the core. Examples of influences from markers include the influence of impurities added to the markers and the influence of irregularities at the interface between the marker and cladding. The higher-order mode light that escapes from the core is affected by these propagation loss factors, and the loss of the higher-order mode light increases due to the influence of these propagation loss factors. Therefore, according to the sixth aspect, for example, by utilizing these propagation loss factors to make the propagation loss of light in a mode higher than the mode of light with a wavelength in the communication band greater in a specific core than in other cores, the propagation loss of the measurement light in the specific core can be made greater than the propagation loss in other cores. Consequently, the difference between the propagation loss of the measurement light in the specific core and the propagation loss in other cores can be made greater than the difference between the propagation loss of light with a wavelength in the communication band in the specific core and the propagation loss in other cores.

[0021] A seventh aspect of the present invention is the multi-core optical fiber according to any one of aspects 1, 2, and 4 to 6, characterized in that the wavelength of the measuring light is not less than 800 nm and not more than 950 nm.

[0022] The wavelength of light used in multimode optical fiber communication is sometimes between 800nm and 950nm, and existing OTDRs and OLTSs utilize light in this wavelength range. Therefore, according to Solution 7, for example, a specific fiber core can be identified by measuring propagation loss using an existing OTDR or OLTS.

[0023] Aspect 8 of the present invention is the multi-core optical fiber according to any one of aspects 1, 2, and 4 to 6, characterized in that the wavelength of the measuring light is greater than or equal to 360 nm and less than 830 nm.

[0024] Light with a wavelength of 360 nm or longer and less than 830 nm is visible light, which appears dimmer as its energy decreases. Therefore, according to Solution 8, for example, a specific core can be identified by visually observing the brightness of the return light when the aforementioned measurement light is injected into each core from one end of a multi-core optical fiber.

[0025] A ninth aspect of the present invention is the multi-core optical fiber according to any one of the first to eighth aspects, characterized in that a marker is provided, wherein the marker is located closer to a specific core than to the other cores.

[0026] As light propagates through a fiber core, some of it escapes from the core. As described above, in Solution 9, the marker is located closest to a specific core. Therefore, the light emanating from that specific core is susceptible to the marker's influence, and the loss of the measurement light propagating through that specific core can be increased compared to the loss of the measurement light propagating through other cores. Therefore, according to Solution 9, even if the cores of a multicore optical fiber are made of the same material, the propagation loss of the measurement light can be made different between that specific core and the other cores.

[0027] (3) Beneficial effects

[0028] As described above, according to the present invention, it is possible to provide a multi-core optical fiber that can easily identify a specific core even if the variation in propagation loss of light having a wavelength in the communication band in the core is small. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a conceptual diagram showing an optical communication device including a multi-core optical fiber according to an embodiment of the present invention.

[0030] Figure 2 Yes Figure 1 A diagram showing a cross section perpendicular to the longitudinal direction of the multi-core optical fiber for transmission shown.

[0031] Figure 3 It is a diagram showing a cross section perpendicular to the longitudinal direction of a single-core optical fiber.

[0032] Figure 4 Graphs showing optical characteristics of the optical communication device according to this embodiment.

[0033] Figure 5 It is a diagram showing a cross section perpendicular to the longitudinal direction of a multi-core optical fiber for transmission according to a modified example. DETAILED DESCRIPTION

[0034] Below, preferred embodiments of the multi-core optical fiber of the present invention are described in detail with reference to the accompanying drawings. The following exemplary embodiments are provided to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention can be modified and improved from the exemplary embodiments without departing from its main purpose. Furthermore, for ease of understanding, the scales of the various figures may differ from those described in the following description.

[0035] Figure 1 The multicore fiber of this embodiment is a transmission multicore fiber 10 , and the optical communication device 1 includes the transmission multicore fiber 10 and a fan-in / fan-out device 20 as main components.

[0036] Figure 2 Yes Figure 1 FIG2 is a diagram showing a cross section perpendicular to the longitudinal direction of a transmission multi-core optical fiber 10. The transmission multi-core optical fiber 10 of this embodiment is an uncoupled multi-core optical fiber having four cores 11a to 11d, a cladding 12 surrounding the outer circumference of each core 11a to 11d without a gap, and a coating 13 covering the outer circumference of the cladding 12. The number of cores is not limited as long as it is multiple.

[0037] In this embodiment, the cross-section of the cladding 12 perpendicular to the longitudinal direction has a substantially circular shape, and the cores 11a to 11d are arranged at positions that are substantially four-fold rotationally symmetrical about the center of the cladding 12. The arrangement of the cores is not limited.

[0038] The refractive index of each core 11a-11d is higher than that of the cladding 12. In this embodiment, the relative refractive index differences of each core 11a-11d with respect to the cladding 12 are the same. The cores 11a-11d are composed of, for example, silica glass doped with a dopant such as germanium that increases the refractive index, while the cladding 12 is composed of, for example, undoped silica glass. Alternatively, the cores 11a-11d may be composed of undoped silica glass, while the cladding 12 may be composed of silica glass doped with a dopant such as fluorine that lowers the refractive index.

[0039] In this embodiment, the hydroxyl content of the core 11a is greater than that of the cores 11b to 11d. One method of increasing the hydroxyl content of the core 11a relative to the cores 11b to 11d is, for example, to shorten the dehydration process for the core rod that will become the core 11a compared to the dehydration process for the core rods that will become the cores 11b to 11d during the manufacturing process of the base material for the multi-core optical fiber 10 for transmission.

[0040] The coating layer 13 is made of a resin such as an ultraviolet curable resin.

[0041] Next, the fan-in / fan-out device 20 will be described. A fan-in / fan-out device is sometimes referred to as a FIFO, and hereinafter, the fan-in / fan-out device 20 is sometimes referred to as a FIFO 20.

[0042] like Figure 1 As shown, the FIFO 20 of this embodiment mainly includes four single-core optical fibers 30a to 30d and a waveguide component 40. The number of single-core optical fibers 30a to 30d only needs to be the same as the number of cores 11a to 11d of the transmission multi-core optical fiber 10, and may vary depending on the number of cores 11a to 11d.

[0043] The structures of the single-core optical fibers 30a to 30d are identical to each other. Therefore, the single-core optical fiber 30a will be described below, and the other single-core optical fibers 30b to 30d will be denoted by the same reference numerals unless otherwise specified, and repeated description will be omitted.

[0044] Figure 3 The single-core optical fiber 30a of this embodiment includes a core 31 , a cladding 32 seamlessly surrounding the outer circumference of the core 31 , and a cladding 33 covering the outer circumference of the cladding 32 .

[0045] In this embodiment, the cross-section of the cladding 32 perpendicular to the longitudinal direction is substantially circular, and the core 31 is arranged at the center of the cladding 32. The diameter of the core 31 is substantially the same as the diameters of the cores 11a to 11d of the multi-core optical fiber 10 for transmission.

[0046] The refractive index of the core 31 is higher than that of the cladding 32. The core 31 is composed of, for example, silica glass to which a dopant is added to increase the refractive index, while the cladding 32 is composed of, for example, silica glass to which no dopant is added. Alternatively, the core 31 may be composed of silica glass to which no dopant is added, while the cladding 32 may be composed of silica glass to which a dopant is added to lower the refractive index.

[0047] The coating layer 33 is made of a resin such as an ultraviolet curable resin.

[0048] The waveguide component 40 optically couples the cores 31 of the single-core optical fibers 30a to 30d to the cores 11a to 11d of the transmission multi-core optical fiber 10. In this embodiment, the waveguide component 40 includes a multi-core optical fiber 50 and a pitch conversion portion 60.

[0049] The multi-core optical fiber 50 of this embodiment has the same structure as the multi-core optical fiber 10 for transmission, and includes four cores 11a to 11d, a cladding 12, and a coating layer 13. One end of the multi-core optical fiber 50 is connected to one end of the multi-core optical fiber 10 for transmission, so that each core 11a to 11d of the multi-core optical fiber 50 is individually optically coupled to the cores 11a to 11d of the multi-core optical fiber 10 for transmission.

[0050] The pitch conversion section 60 of this embodiment is a waveguide substrate having four waveguides 61a to 61d extending from one end to the other. The arrangement of the waveguides 61a to 61d at one end of the pitch conversion section 60 corresponds to the arrangement of the cores 11a to 11d of the multi-core optical fiber 50. The other end of the multi-core optical fiber 50 is connected to one end of the pitch conversion section 60, with each core 11a to 11d of the multi-core optical fiber 50 individually bonded to one end of these waveguides 61a to 61d. The waveguides 61a to 61d at the other end of the pitch conversion section 60 are arranged in a straight line, with the distance between the waveguides 61a to 61d at the other end being greater than the distance between the waveguides 61a to 61d at the one end. One end of the single-core optical fiber 30a is connected to the other end of the pitch conversion section 60, bonded to the waveguide 61a bonded to the core 11a of the multi-core optical fiber 50. One end of the single-core optical fiber 30b is connected to the other end of the pitch conversion section 60 by being coupled to a waveguide 61b coupled to the core 11b of the multi-core optical fiber 50. One end of the single-core optical fiber 30c is connected to the other end of the pitch conversion section 60 by being coupled to a waveguide 61c coupled to the core 11c of the multi-core optical fiber 50. One end of the single-core optical fiber 30d is connected to the other end of the pitch conversion section 60 by being coupled to a waveguide 61d coupled to the core 11d of the multi-core optical fiber 50. In this way, the cores 31 of each of the single-core optical fibers 30a to 30d are individually optically coupled to the cores 11a to 11d of the transmission multi-core optical fiber 10 via the waveguide component 40.

[0051] In addition, the waveguide component 40 only needs to optically couple the cores 31 of each single-core optical fiber 30a~30d and the cores 11a~11d of the transmission multi-core optical fiber 10 individually, without any restrictions. For example, the configuration of the waveguides 61a~61d at the other end of the spacing conversion part 60 can also be a configuration arranged at predetermined intervals on a predetermined circumference. The waveguide component 40 can also be composed of only the spacing conversion part 60, and the spacing conversion part 60 can also be a spatial optical system composed of multiple lenses. In addition, the spacing conversion part 60 can also be formed as a whole with multiple single-core optical fibers 30a~30d. Such a spacing conversion part 60 can be formed, for example, by extending the end of a fiber bundle formed by bundling multiple single-core optical fibers 30a~30d.

[0052] Figure 4 is a diagram showing the optical characteristics of the optical communication device 1 of this embodiment, and is a diagram showing the optical characteristics of the optical communication device 1 of this embodiment. Figure 1 The waveforms measured by the OTDR 70 are shown in FIG. The measurement light of the OTDR 70 is incident on the end of each single-core optical fiber 30a-30d in the FIFO 20, opposite the waveguide component 40. The measurement light has a wavelength outside the communication band, and the wavelength of the measurement light is the so-called E-band, which is 1360 nm to less than 1460 nm. Furthermore, the wavelength of the measurement light is shorter than the communication band. In this embodiment, the communication band is the so-called C-band, which is 1530 nm to less than 1565 nm, and the so-called L-band, which is 1565 nm to less than 1625 nm. Figure 4 , a waveform WFa when the measuring light is incident on the single-core optical fiber 30 a and a waveform WFb when the measuring light is incident on the single-core optical fiber 30 b are shown side by side in the vertical direction.

[0053] like Figure 4 As shown, in this embodiment, the inclination of the portion of waveform WFa corresponding to the transmission multi-core optical fiber 10 and the multi-core optical fiber 50 is greater than the inclination of the portion of waveform WFb corresponding to the transmission multi-core optical fiber 10 and the multi-core optical fiber 50. In other words, the propagation loss of the measurement light in the core 11a of the transmission multi-core optical fiber 10 and the multi-core optical fiber 50 is greater than the propagation loss in the core 11b of the transmission multi-core optical fiber 10 and the multi-core optical fiber 50. It is known that when the wavelength of light propagating in the core is in the E band, the light loss increases due to the hydroxyl groups contained in the core, which is sometimes referred to as OH loss. In this embodiment, due to OH loss, the propagation loss of the measurement light in the core 11a of the transmission multi-core optical fiber 10 and the multi-core optical fiber 50 is greater than the propagation loss in the core 11b of the transmission multi-core optical fiber 10 and the multi-core optical fiber 50.

[0054] Although illustrations are omitted, the waveforms when measuring light is injected into single-core optical fiber 30c and the waveform when measuring light is injected into single-core optical fiber 30d are substantially identical to waveform WFb. Therefore, the propagation loss of the measuring light differs between core 11a and the other cores 11b-11d, with the propagation loss of the measuring light in core 11a being greater than the propagation loss of the measuring light in the other cores 11b-11d. Furthermore, the waveforms when light in the communication band is injected into each of single-core optical fibers 30a-30d are substantially identical to waveform WFb. Therefore, the difference between the propagation loss of the measuring light in core 11a and the propagation loss in the other cores 11b-11d is greater than the difference between the propagation loss of light with a wavelength in the communication band in core 11a and the propagation loss in the other cores 11b-11d.

[0055] As described above, in the transmission multi-core optical fiber 10, which is the multi-core optical fiber of this embodiment, the propagation loss of measurement light at wavelengths outside the communication band differs between core 11a and the other cores 11b to 11d. Therefore, according to the transmission multi-core optical fiber 10 of this embodiment, even if the variation in the propagation loss of light at wavelengths in the communication band among cores 11a to 11d is small, core 11a can be easily identified by inspecting the propagation loss of each of cores 11a to 11d using the aforementioned measurement light.

[0056] In this embodiment, the difference between the propagation loss of the measurement light in core 11a and the propagation loss in the other cores 11b to 11d is greater than the difference between the propagation loss of light with a wavelength in the communication band in core 11a and the propagation loss in the other cores 11b to 11d. Therefore, the transmission multi-core optical fiber 10 of this embodiment makes it easier to identify core 11a.

[0057] Furthermore, the method for measuring propagation loss is not limited to measurement using an OTDR 70; for example, measurement using an OFDR, OLTS, or the like is also possible. When using an OTDR or OFDR, propagation loss can be measured simply by injecting light from one end of the transmission multi-core optical fiber 10, making measurement easy. Furthermore, since an operator is not required at the other end of the transmission multi-core optical fiber 10, costs can be reduced.

[0058] In this embodiment, the wavelength of the measurement light is shorter than the communication band. The shorter the wavelength of the light propagating through the cores, the more its inter-core crosstalk tends to be suppressed. Therefore, when using the aforementioned measurement light to investigate the propagation loss of each core 11a-11d, the inter-core crosstalk of the measurement light propagating through the cores 11a-11d can be suppressed, compared to a case where the communication bands are the same and the wavelength of the measurement light is longer than the communication band. Therefore, the multi-core transmission optical fiber 10 of this embodiment makes it easier to identify the core 11a.

[0059] In this embodiment, the wavelength of the measurement light is in the so-called E-band, that is, between 1360 nm and less than 1460 nm. According to this embodiment, by increasing the hydroxyl content in core 11a compared to the other cores 11b-11d, as described above, the propagation loss of the measurement light in core 11a can be made greater than the propagation loss in the other cores 11b-11d. Therefore, the difference between the propagation loss of the measurement light in core 11a and the propagation loss in the other cores 11b-11d can be made greater than the difference between the propagation loss of light in core 11a and the propagation loss in the other cores 11b-11d for light with a wavelength in the communication band. Furthermore, the wavelength of light used in optical communications is sometimes in the E-band, and existing OTDRs, OFDRs, OLTSs, etc. utilize light in this band. Therefore, according to this embodiment, even existing OTDRs, OFDRs, OLTSs, etc. can measure propagation loss. Consequently, core 11a can be easily identified.

[0060] Furthermore, the difference between the propagation loss of the measured light in core 11a and the propagation loss in the other cores 11b to 11d is preferably 0.1 dB / km or greater. This configuration allows even measurement equipment with a large minimum measurable propagation loss value (e.g., a general-purpose measurement equipment) to measure the propagation loss difference, making it easy to identify core 11a. In particular, in the case of short multi-core optical fibers, the measured propagation loss may have large errors due to the small propagation loss. However, this configuration allows even short transmission multi-core optical fibers 10 to suppress errors and easily identify core 11a. Furthermore, the propagation loss difference between the cores in a typical multi-core optical fiber is often less than 0.1 dB / km. The propagation loss difference of the light mentioned above between the other cores 11 to 11d is preferably less than 0.1 dB / km. This configuration makes it easy to identify core 11a. Furthermore, the difference between the typical propagation loss value of a typical optical fiber and the upper limit of the propagation loss value specified in the optical fiber standard is often less than 0.1 dB / km. Therefore, if the propagation loss difference is 0.1 dB / km or greater, a difference of more than one digit from the above difference indicates that the propagation loss difference is so high that it deviates from the standard specifications for optical fibers commonly used for communications, making it easier and more reliable to identify core 11a. Furthermore, from these perspectives, the above difference is more preferably 0.5 dB / km or greater, and even more preferably 1.0 dB / km or greater. Furthermore, it is preferable that the difference between the propagation loss of light with a wavelength in the communications band in core 11a and the propagation loss in the other cores 11b to 11d be less than 0.1 dB / km.

[0061] The present invention has been described above using the aforementioned embodiment as an example, but the present invention is not limited thereto. It suffices that the propagation loss of the measurement light having a wavelength outside the communication band differs between core 11a and the other cores 11b to 11d. For example, the propagation loss of the measurement light in core 11a may be smaller than the propagation loss of the measurement light in the other cores 11b to 11d.

[0062] Furthermore, there are no restrictions on the communication band or the wavelength of the measurement light. For example, when the wavelength of light propagating through the core is greater than 400 nm and less than 600 nm, the optical loss tends to increase in proportion to the core's nickel, chromium, cobalt, iron, and copper content. When the wavelength of light propagating through the core is greater than 600 nm and less than 800 nm, the optical loss tends to increase in proportion to the core's nickel, chromium, cobalt, and iron content. When the wavelength of light propagating through the core is greater than 800 nm and less than 900 nm, the optical loss tends to increase in proportion to the core's chromium content. When the wavelength of light propagating through the core is greater than 900 nm and less than 1000 nm, the optical loss tends to increase in proportion to the core's chromium and hydroxyl content. When the wavelength of light propagating through the core is within the so-called T band, i.e., greater than 1000 nm and less than 1260 nm, the optical loss tends to increase in proportion to the core's nickel, cobalt, and hydroxyl content. When the wavelength of light propagating through the core is in the so-called O-band, i.e., 1260 nm to less than 1360 nm, the optical loss tends to increase in proportion to the core's nickel and cobalt content. When the wavelength of light propagating through the core is in the E-band, the optical loss tends to increase in proportion to the core's nickel, cobalt, and hydroxyl content. When the wavelength of light propagating through the core is in the so-called S-band, i.e., 1460 nm to less than 1530 nm; the so-called C-band, i.e., 1530 nm to less than 1565 nm; the so-called L-band, i.e., 1565 nm to less than 1625 nm; and the so-called U-band, i.e., 1625 nm to less than 1675 nm, the optical loss tends to increase in proportion to the core's nickel and cobalt content. When the wavelength of light propagating through the core is 1675 nm or greater, the optical loss tends to increase in proportion to the core's hydroxyl, cobalt, and nickel content. Therefore, for example, the above-mentioned structure can be formed by adjusting the content of the above-mentioned substances in the core 11a of the transmission multi-core optical fiber 10. For example, in order to make the difference between the propagation loss of light in the core 11a and the propagation loss of light in the other cores 11b to 11d larger than the difference between the propagation losses of light in the other cores 11b to 11d, for example, substantially the same amount of the same dopant can be added to the other cores 11b to 11d, while at least one of the type and amount of the dopant added to the core 11a is different from the type and amount of the dopant added to the other cores 11b to 11d.Therefore, for example, the communication band may be set to at least one of the C-band, L-band, O-band and S-band, and the bands other than the communication band may be set to at least one of the T-band, E-band, U-band, ultraviolet band with a wavelength of 10 nm or more and less than 360 nm, visible light band with a wavelength of 360 nm or more and less than 830 nm, and infrared region with a wavelength of 830 nm or more and 2.5 μm or less excluding the C-band, L-band, O-band and S-band.

[0063] The transmission multi-core optical fiber 10 may further include a member such as a marker that is different from the cores 11 a to 11 d , and a modification example of this will be described. Figure 5 1 and 2 are diagrams showing a cross section perpendicular to the longitudinal direction of a multi-core optical fiber 10 for transmission according to a modified example. Figure 5 The multi-core optical fiber 10 for transmission of the modified example shown has a mark 15. The distance between the core 11a and the mark 15 is smaller than the distance between the other cores 11b to 11d and the mark 15. Therefore, according to this modified example, the light transmitted from the core 11a of the multi-core optical fiber 50, which is a part of the coupled waveguide 21a, is easily affected by the mark 15. As the influence of the mark 15 on the light transmitted from the core 11a, the influence of impurities added to the mark 15, the influence of the unevenness of the interface between the mark 15 and the cladding 12, etc. can be listed. As the impurities added to the mark 15, moisture, the above-mentioned dopants that increase the loss of light, dopants such as germanium, iron, and boron can be listed. As a method for increasing the unevenness of the interface, for example, when the base material of the multi-core optical fiber 50 is produced by the perforation method, the surface roughness of the outer peripheral surface of the marker rod that becomes the mark 15 or the surface roughness of the surface of the hole for inserting the marker rod on the cladding rod that becomes the cladding 12 is made rougher. The light emitted from core 11a spreads to mark 15, increasing the loss of that light due to the aforementioned influence. Therefore, the loss of the measurement light propagating through core 11a can be made greater than the loss of the measurement light propagating through the other cores 11b-11d. Therefore, according to this variation, even if the materials constituting the cores 11a-11d are the same, the propagation loss of the measurement light can be made different between core 11a and the other cores 11b-11d. Furthermore, core 11a can be visually identified based on mark 15.

[0064] Alternatively, the wavelength of the measurement light can be equal to or less than the wavelength at which light of a mode one order higher than the mode of light with a wavelength in the communication band propagating through cores 11a-11d can propagate. In this case, the measurement light propagating through cores 11a-11d includes light of the same mode as the mode of light with a wavelength in the communication band propagating through cores 11a-11d, as well as light of higher-order modes. Light in higher-order modes tends to be more susceptible to propagation loss factors located at or outside the core periphery. Examples of propagation loss factors include elements added to the core periphery or cladding that easily absorb the measurement light, irregularities at the core-cladding boundary, and markers located near the core. Examples of influences from markers include the effects of impurities added to the markers and irregularities at the marker-cladding interface. Light in higher-order modes that escape the core is affected by these propagation loss factors, resulting in increased loss of light in these higher-order modes. Therefore, according to the above configuration, for example, by utilizing these propagation loss factors to make the propagation loss of light in a higher-order mode than the mode of light with a wavelength in the communication band greater in core 11a than in the other cores 11b-11d, the propagation loss of the measurement light in core 11a can be made greater than the propagation loss in the other cores 11b-11d. Consequently, the difference between the propagation loss of the measurement light in core 11a and the propagation loss in the other cores 11b-11d can be made greater than the difference between the propagation loss of light with a wavelength in the communication band and the propagation loss in the other cores 11b-11d. In this case, the mode of light with a wavelength in the communication band propagating in cores 11a-11d can also be single-mode. Furthermore, when the distance between the outer peripheral surface of the cladding and the core is small, the amount of light in the higher-order mode leaks increases, resulting in greater optical loss. Therefore, by adjusting the position of the core 11 a , the propagation loss of light in a mode higher than the mode of light with a wavelength in the communication band can be made larger in the core 11 a than in the other cores 11 b to 11 d .

[0065] The wavelength of the measurement light may also be between 800 nm and 950 nm. The wavelength of light used in multimode optical fiber communications is sometimes between 800 nm and 950 nm, and existing OTDRs and OLTSs utilize light in this wavelength range. Therefore, with this configuration, for example, by measuring propagation loss using an existing OTDR or OLTS, it is possible to identify the fiber core 11a.

[0066] The wavelength of the measurement light can also be in the so-called U-band, that is, between 1625 nm and less than 1675 nm. The wavelength of light used in multimode optical fiber communications is sometimes in the U-band, and existing OTDRs and OLTSs utilize light in this band. Therefore, with this configuration, even existing OTDRs and OLTSs can measure propagation loss, for example. This makes it easier to identify the fiber core 11a.

[0067] The wavelength of the measurement light may be greater than or equal to 360 nm and less than or equal to 830 nm. Light with a wavelength greater than or equal to 360 nm and less than or equal to 830 nm is visible light, which appears dimmer as its energy decreases. Therefore, with this configuration, for example, core 11a can be identified by visually observing the brightness of the return light when the measurement light is injected from one end of the transmission multi-core optical fiber 10 toward each of the cores 11a to 11d. Furthermore, core 11a can be identified even without using a device for measuring propagation loss.

[0068] As described above, the present invention provides a multi-core optical fiber that can easily identify a specific core even when the propagation loss of light with a wavelength in the communication band in the core varies slightly, and is expected to be used in fields such as optical fiber communications.

Claims

1. A multi-core optical fiber, characterized in that: have: multiple fiber cores; and The cladding surrounding each fiber core, The propagation loss of the measurement light having a wavelength outside the communication band is different between the specific core and the other cores.

2. The multi-core optical fiber according to claim 1, wherein The difference between the propagation loss of the measurement light in the specific core and the propagation loss in the other cores is 0.1 dB / km or more.

3. The multi-core optical fiber according to claim 1 or 2, characterized in that The wavelength of the measuring light is greater than or equal to 1360 nm and less than or equal to 1460 nm.

4. The multi-core optical fiber according to any one of claims 1 to 3, characterized in that The wavelength of the measuring light is shorter than the communication band.

5. The multi-core optical fiber according to any one of claims 1 to 4, characterized in that The difference between the propagation loss of the measurement light in the specific core and the propagation loss in the other cores is greater than the difference between the propagation loss of light with a wavelength in the communication band in the specific core and the propagation loss in the other cores.

6. The multi-core optical fiber according to any one of claims 1 to 5, characterized in that The wavelength of the measuring light is equal to or less than a wavelength at which light of a mode having an order one higher than a mode of light having a wavelength in the communication band propagating in the core can propagate.

7. The multi-core optical fiber according to any one of claims 1, 2, and 4 to 6, characterized in that The wavelength of the measuring light is 800 nm or more and 950 nm or less.

8. The multi-core optical fiber according to any one of claims 1, 2, and 4 to 6, wherein: The wavelength of the measuring light is 360 nm or more and less than 830 nm.

9. The multi-core optical fiber according to any one of claims 1 to 8, characterized in that A mark is provided, wherein the distance between the mark and a specific core is smaller than the distance between the mark and the other cores.

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