Multicore fiber

Abstract

A multicore fiber (10) is provided with a plurality of cores (11a)-(11d) and a cladding (12) that surrounds of the cores (11a)-(11d). The propagation loss of measurement light differs by 0.01 dB / km or more between a specific core (11a) and the other cores (11b)-(11d), the measurement light having a wavelength that is not higher a wavelength at which light in a mode that is by one order higher than a mode used in a communication wavelength band can propagate.

Description

Multicore Fiber

[0001] The present invention relates to a multicore fiber.

[0002] In order to increase the transmission capacity of optical fiber communication devices, a technology is known in which a multicore fiber in which the outer peripheries of multiple cores are surrounded by a single cladding is used, and signals are transmitted by light propagating through each core. Such a multicore fiber is described in Patent Document 1 listed below.

[0003] In a multicore fiber, even when the cores are arranged in a rotationally symmetrical manner, there are cases where rotational alignment is performed by distinguishing between the cores. In the multicore fiber of Patent Document 1 listed below, a specific core among the multiple cores has a different propagation loss from the other cores. Patent Document 1 describes that it is possible to distinguish between the cores and perform alignment based on this difference in propagation loss.

[0004] JP 2014-048645 A

[0005] In optical communications, from the viewpoint of ensuring uniformity of communications, there is a demand for reducing the variation in propagation loss of light propagating through each core. To meet this demand, it is conceivable to reduce the difference in propagation loss of light between a specific core and other cores in the multicore fiber of Patent Document 1. However, reducing the difference in propagation loss makes it difficult to identify a specific core.

[0006] Therefore, an object of the present invention is to provide a multicore fiber that can distinguish a specific core from other cores even when the difference in propagation loss of light of wavelengths in the communication wavelength band between the cores is small.

[0007] A first aspect of the present invention is a multicore fiber comprising a plurality of cores and cladding surrounding each of the cores, wherein the propagation loss of measurement light having a wavelength equal to or shorter than a wavelength at which light in a mode that is one order higher than the mode of light in the communication wavelength band can propagate differs by 0.01 dB / km or more between a specific core and the other cores.

[0008] According to Aspect 1, even when the variation in propagation loss of light in the communication wavelength band between cores is small, a specific core can be easily identified by examining the propagation loss of each core using the above-mentioned measurement light. In Aspect 1, the measurement light propagating through the core may include light in the same mode as the mode of light in the communication wavelength band propagating through the core, and light in a higher-order mode than the mode of the light. Light in higher-order modes tends to easily leak out of the core and is therefore more likely to be lost. Therefore, in Aspect 1, the difference between the propagation loss of the measurement light in a specific core and the propagation loss in other cores is easily made larger than the difference between the propagation loss of light in the communication wavelength band in a specific core and the propagation loss in other cores, making loss measurement easier. Furthermore, in Aspect 1, the difference between the propagation loss of the specific core and the propagation loss of other cores is 0.01 dB / km or more, making measurement using a measurement device easy, and even when the difference in propagation loss of light in the communication wavelength band is small, a specific core can be identified from other cores by measuring the loss of the measurement light.

[0009] A second aspect of the present invention is the multi-core fiber of the first aspect, wherein a difference between the propagation loss of the measurement light in the specific core and the propagation loss in the other cores is larger than a difference between the propagation loss of light having a wavelength in the communication wavelength band in the specific core and the propagation loss in the other cores.

[0010] According to the second aspect, it is possible to reduce the difference in loss between a specific core and other cores in the communication wavelength band, and to suppress variations between cores in optical communication.

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

[0012] In a typical multicore fiber, the difference in propagation loss between cores for light in the communication wavelength band tends to be less than 0.1 dB / km. On the other hand, according to Aspect 3, the difference in propagation loss of the measurement light is 0.1 dB / km or more, so that a specific core can be more easily identified.

[0013] A fourth aspect of the present invention is the multicore fiber according to any one of the first to third aspects, characterized in that the wavelength at which light in a mode that is one order higher than the mode of light in the communication wavelength band can propagate is less than 1260 nm.

[0014] In this case, the communication wavelength can be set to 1260 nm or more, which is the wavelength used for general communication.

[0015] A fifth aspect of the present invention is the multi-core fiber according to the fourth aspect, wherein the wavelength of the measurement light is not less than 800 nm and not more than 950 nm.

[0016] In multimode fiber communications, the communication wavelength may be 800 nm or more and 950 nm or less. Therefore, general-purpose measurement devices exist for measuring light in this wavelength band. Therefore, according to aspect 5, a specific core can be identified by measuring the propagation loss using an existing measurement device.

[0017] A sixth aspect of the present invention is the multi-core fiber according to the fourth aspect, wherein the wavelength of the measurement light is equal to or greater than 360 nm and less than 830 nm.

[0018] Light with a wavelength of 360 nm or more and less than 830 nm is visible light, and this light appears darker as its energy decreases. Therefore, according to this aspect, when measurement light is incident on a core, a specific core can be identified by visually checking the brightness of the emitted light.

[0019] A seventh aspect of the present invention is the multi-core fiber according to any one of the first to sixth aspects, characterized in that the specific core is doped with a larger amount of dopant that absorbs light having the wavelength of the measurement light than the other cores.

[0020] With this configuration, a difference in propagation loss may be created between a specific core and other cores.

[0021] An eighth aspect of the present invention is the multi-core fiber according to any one of the first to seventh aspects, characterized in that an absorbing portion that absorbs light having the wavelength of the measurement light is provided in the vicinity of the specific core.

[0022] A ninth aspect of the present invention is the multi-core fiber according to the eighth aspect, wherein the absorbing portion is an absorbing layer surrounding the specific core.

[0023] The configurations of Aspects 7 and 8 may be used to differentiate the propagation loss between a specific core and other cores. In particular, in the present invention, as described above, the wavelength of the measurement light is equal to or shorter than the wavelength at which light in a mode one order higher than the mode of light in the communication wavelength band can propagate, and the measurement light tends to leak out of the core more easily than light with a communication wavelength. Therefore, according to this aspect, the loss of the measurement light can be easily increased while suppressing the loss of light in the communication wavelength band.

[0024] A tenth aspect of the present invention is the multicore fiber according to any one of Aspects 1 to 4, characterized in that the specific core has more structural defects that cause loss of light of the wavelength of the measurement light than the other cores.

[0025] With this configuration, a difference in propagation loss may be created between a specific core and other cores.

[0026] As described above, according to the present invention, a multicore fiber is provided that can distinguish a specific core from other cores even when the difference in propagation loss of light with wavelengths in the communication wavelength band between the cores is small.

[0027] Fig. 1 is a diagram showing a cross section perpendicular to the longitudinal direction of a multicore fiber according to a first embodiment of the present invention; Fig. 2 is a diagram showing the configuration of an apparatus for measuring a multicore fiber; Fig. 3 is a diagram showing optical characteristics of a multicore fiber; Fig. 4 is a diagram showing a cross section perpendicular to the longitudinal direction of a multicore fiber according to a second embodiment of the present invention; Fig. 5 is a diagram showing a cross section perpendicular to the longitudinal direction of a multicore fiber in a modified example of the second embodiment;

[0028] Preferred embodiments of the multicore fiber according to the present invention will be described in detail below with reference to the drawings. The embodiments exemplified below are intended 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 embodiments within the scope of the claims without departing from the spirit thereof. Note that, for ease of understanding, the scales of the drawings may differ from the scales described in the following description.

[0029] First Embodiment Fig. 1 is a diagram showing a cross section perpendicular to the longitudinal direction of a multicore fiber according to this embodiment. A multicore fiber 10 of this embodiment is an uncoupled or weakly coupled multicore fiber for communication. As shown in Fig. 1, the multicore fiber 10 includes four cores 11a to 11d, claddings 12 that tightly surround the outer circumferential surfaces of each of the cores 11a to 11d, and a coating layer 13 that coats the outer circumferential surface of the cladding 12. Note that the cladding 12 is a single cladding that commonly surrounds the outer circumferential surfaces of each of the cores 11a to 11d, and is therefore also called a common cladding.

[0030] In this embodiment, the outer shape of the cross section perpendicular to the longitudinal direction of the cladding 12 is generally circular, and the cores 11a to 11d are arranged at positions that are generally four-fold rotationally symmetric about the center of the cladding 12. Therefore, the cores 11a to 11d are arranged at the vertices of a square. Note that the number of cores may be any number other than four as long as it is plural. The core arrangement may also be other than the above, and the cores 11a to 11d may be arranged, for example, at the vertices of an isosceles trapezoid.

[0031] The refractive index of each of the cores 11a to 11d is higher than the refractive index of the cladding 12. The cores 11a to 11d are made of silica glass doped with a dopant such as germanium that increases the refractive index, and the cladding 12 is made of silica glass that does not have any dopant added. Alternatively, the cores 11a to 11d may be made of silica glass that does not have any dopant added other than the dopants listed below, and the cladding 12 may be made of silica glass doped with a dopant such as fluorine that decreases the refractive index. Furthermore, when the cores 11a to 11d and the cladding 12 are doped with a dopant, the dopant added to the cores 11a to 11d and the cladding 12 is not limited to the above, and other dopants may also be added.

[0032] The coating layer 13 is made of a resin such as an ultraviolet curable resin.

[0033] The multicore fiber 10 of the present embodiment communicates in a communication wavelength band of a predetermined wavelength, for example, 1260 nm or more, and is any one of the so-called O band (1260 nm or more and less than 1360 nm), the so-called E band (1360 nm or more and less than 1460 nm), the so-called S band (1460 nm or more and less than 1530 nm), the so-called C band (1530 nm or more and less than 1565 nm), and the so-called L band (1565 nm or more and less than 1625 nm).

[0034] The cutoff wavelength of the multicore fiber 10 is equal to or shorter than the communication wavelength band, and in the above case is, for example, 1260 nm. The cutoff wavelength here refers to the shortest wavelength at which light in a mode that is one order higher than the mode of light in the communication wavelength band does not propagate. Therefore, when light with a wavelength shorter than the cutoff wavelength propagates through the multicore fiber 10, light in a mode that is one order higher than the mode of light in the communication wavelength band can propagate. For example, when the multicore fiber 10 performs single-mode communication, light in a mode equal to or higher than the LP11 mode can propagate in a wavelength band shorter than the cutoff wavelength.

[0035] Here, the core 11a is referred to as a specific core, and the cores 11b to 11d are referred to as other cores. In the multicore fiber 10 of this embodiment, when light of a predetermined wavelength less than the cutoff wavelength propagates through each of the cores 11a to 11d, the loss of the light differs by 0.01 dB / km or more between the specific core 11a and the other cores 11b to 11d. Furthermore, when light of the predetermined wavelength propagates through each of the cores 11a to 11d, the loss of the light differs preferably by 0.1 dB / km or more between the specific core 11a and the other cores 11b to 11d, more preferably by 0.3 dB / km or more, and even more preferably by 0.5 dB / km or more. Furthermore, the difference in loss of light of the predetermined wavelength between the other cores 11b to 11d is preferably less than 0.01 dB / km, and is preferably approximately equal to each other. In this embodiment, the loss of light of the predetermined wavelength in the specific core 11a is greater than that of light of the predetermined wavelength in the other cores 11b to 11d.

[0036] Furthermore, when light in the communication wavelength band propagates through each of the cores 11a to 11d, it is preferable that the difference in loss of the light between the specific core 11a and the other cores 11b to 11d is less than 0.01 dB / km. In other words, it is preferable that in the multicore fiber 10, the difference in propagation loss of light with a predetermined wavelength less than the cutoff wavelength between the specific core 11a and the other cores 11b to 11d is larger than the difference in propagation loss of light with a wavelength in the communication wavelength band between the specific core 11a and the other cores 11b to 11d. Therefore, when light of the above-mentioned specified wavelength propagates through each of the cores 11a to 11d, if the loss of the light differs by 0.1 dB / km or more between a specific core 11a and the other cores 11b to 11d, it is preferable that when light in the communication wavelength band propagates through each of the cores 11a to 11d, the difference in loss of the light between a specific core 11a and the other cores 11b to 11d be less than 0.1 dB / km.

[0037] When the communication wavelength band is 1260 nm or more, the predetermined wavelength is preferably, for example, 800 nm or more and 950 nm or less, or 360 nm or more and less than 830 nm. In this embodiment, a dopant that absorbs light of a predetermined wavelength less than the cutoff wavelength is added to the specific core 11a in a larger amount than the other cores 11b to 11d. For example, the specific core 11a is doped with the dopant, while the other cores 11b to 11d are not doped with the dopant. The dopant has a higher light absorption rate at the predetermined wavelength than the communication wavelength band.

[0038] When the predetermined wavelength is 800 nm or more and 950 nm or less, examples of the dopant include vanadium and chromium. Among these, vanadium is preferred from the viewpoint of suppressing dopant-induced optical loss in the communication wavelength band of 1260 nm or more and increasing dopant-induced optical loss at the predetermined wavelength. To achieve a difference of 0.01 dB / km or more in optical loss at the predetermined wavelength between the specific core 11a and the other cores 11b to 11d, when the dopant is vanadium, for example, the specific core 11a is doped with the dopant at a concentration of 0.01 ppb or more, while the cores 11b to 11d are not doped with the dopant. Similarly, when the dopant is chromium, the specific core 11a is doped with the dopant at a concentration of 0.01 ppb or more, while the cores 11b to 11d are not doped with the dopant. Furthermore, in order to cause the difference in loss of light at the predetermined wavelength between the specific core 11a and the other cores 11b to 11d to be 0.1 dB / km or more, if the dopant is vanadium, for example, the specific core 11a is doped with the dopant at a concentration of 0.1 ppb or more, while the cores 11b to 11d are not doped with the dopant. Similarly, if the dopant is chromium, for example, the specific core 11a is doped with the dopant at a concentration of 0.1 ppb or more, while the cores 11b to 11d are not doped with the dopant. Furthermore, in order to cause the difference in loss of light at the predetermined wavelength between the specific core 11a and the other cores 11b to 11d to be 0.3 dB / km or more, if the dopant is vanadium, for example, the specific core 11a is doped with the dopant at a concentration of 0.3 ppb or more, while the cores 11b to 11d are not doped with the dopant. Similarly, when the dopant is chromium, for example, the specific core 11a is doped with the dopant at a concentration of 0.3 ppb or more, while the cores 11b to 11d are not doped with the dopant. Furthermore, in order to make the loss of light at the predetermined wavelength between the specific core 11a and the other cores 11b to 11d differ by 0.5 dB / km or more, when the dopant is vanadium, for example, the specific core 11a is doped with the dopant at a concentration of 0.5 ppb or more, while the cores 11b to 11d are not doped with the dopant.Similarly, if the dopant is chromium, for example, the dopant is added to a specific core 11a at a concentration of 0.5 ppb or more, while the dopant is not added to the cores 11b to 11d.

[0039] When the predetermined wavelength is 360 nm or more and less than 830 nm, examples of the dopant include nickel, chromium, cobalt, iron, manganese, copper, and vanadium. Among these, cobalt is preferable from the viewpoint of suppressing dopant-induced loss in the communication wavelength band and increasing dopant-induced loss at the predetermined wavelength. In order to make the loss of light at the predetermined wavelength between the specific core 11a and the other cores 11b to 11d differ by 0.01 dB / km or more, when the dopant is nickel, for example, the specific core 11a is doped with the dopant at a concentration of 0.005 ppb or more, while the cores 11b to 11d are not doped with the dopant. Similarly, when the dopant is cobalt, for example, the dopant is added to a specific core 11a at a concentration of 0.0025 ppb or more, and the dopant is not added to the cores 11b to 11d. When the dopant is chromium, for example, the dopant is added to a specific core 11a at a concentration of 0.0033 ppb or more, and the dopant is not added to the cores 11b to 11d. When the dopant is iron, for example, the dopant is added to a specific core 11a at a concentration of 0.025 ppb or more, and the dopant is added to the cores 11b to 11d. When the dopant is manganese, for example, the dopant is added to a specific core 11a at a concentration of 0.05 ppb or more, and the dopant is not added to cores 11b to 11d; when the dopant is copper, for example, the dopant is added to a specific core 11a at a concentration of 0.05 ppb or more, and the dopant is not added to cores 11b to 11d; when the dopant is vanadium, for example, the dopant is added to a specific core 11a at a concentration of 0.033 ppb or more, and the dopant is not added to cores 11b to 11d. Furthermore, in order to make the loss of light of the above-mentioned specified wavelength between the specific core 11a and the other cores 11b to 11d differ by 0.1 dB / km or more, if the dopant is nickel, for example, the dopant is added to the specific core 11a at a concentration of 0.05 ppb or more, and the dopant is not added to the cores 11b to 11d.Similarly, when the dopant is cobalt, for example, the dopant is added to a specific core 11a at a concentration of 0.025 ppb or more, and the dopant is not added to the cores 11b to 11d. When the dopant is chromium, for example, the dopant is added to a specific core 11a at a concentration of 0.033 ppb or more, and the dopant is not added to the cores 11b to 11d. When the dopant is iron, for example, the dopant is added to a specific core 11a at a concentration of 0.25 ppb or more, and the dopant is added to the cores 11b to 11d. When the dopant is manganese, for example, the dopant is added to a specific core 11a at a concentration of 0.5 ppb or more, and the dopant is not added to cores 11b to 11d; when the dopant is copper, for example, the dopant is added to a specific core 11a at a concentration of 0.5 ppb or more, and the dopant is not added to cores 11b to 11d; when the dopant is vanadium, for example, the dopant is added to a specific core 11a at a concentration of 0.33 ppb or more, and the dopant is not added to cores 11b to 11d. Furthermore, in order to make the loss of light of the above-mentioned specified wavelength between the specific core 11a and the other cores 11b to 11d differ by 0.3 dB / km or more, if the dopant is nickel, for example, the dopant is added to the specific core 11a at a concentration of 0.15 ppb or more, and the dopant is not added to the cores 11b to 11d.Similarly, when the dopant is cobalt, for example, the dopant is added to a specific core 11a at a concentration of 0.075 ppb or more, and the dopant is not added to the cores 11b to 11d. When the dopant is chromium, for example, the dopant is added to a specific core 11a at a concentration of 0.1 ppb or more, and the dopant is not added to the cores 11b to 11d. When the dopant is iron, for example, the dopant is added to a specific core 11a at a concentration of 0.75 ppb or more, and the dopant is added to the cores 11b to 11d. When the dopant is manganese, for example, the dopant is added to a specific core 11a at a concentration of 1.5 ppb or more, and the dopant is not added to cores 11b to 11d; when the dopant is copper, for example, the dopant is added to a specific core 11a at a concentration of 1.5 ppb or more, and the dopant is not added to cores 11b to 11d; and when the dopant is vanadium, for example, the dopant is added to a specific core 11a at a concentration of 0.1 ppb or more, and the dopant is not added to cores 11b to 11d. Furthermore, in order to make the loss of light of the above-mentioned specified wavelength between the specific core 11a and the other cores 11b to 11d differ by 0.5 dB / km or more, if the dopant is nickel, for example, the dopant is added to the specific core 11a at a concentration of 0.25 ppb or more, and the dopant is not added to the cores 11b to 11d.Similarly, when the dopant is cobalt, for example, the dopant is added to a specific core 11a at a concentration of 0.13 ppb or more, and the dopant is not added to the cores 11b to 11d. When the dopant is chromium, for example, the dopant is added to a specific core 11a at a concentration of 0.17 ppb or more, and the dopant is not added to the cores 11b to 11d. When the dopant is iron, for example, the dopant is added to a specific core 11a at a concentration of 1.3 ppb or more, and the dopant is not added to the cores 11b to 11d. When the dopant is manganese, for example, the dopant is added to a specific core 11a at a concentration of 2.5 ppb or more, and the dopant is not added to cores 11b to 11d; when the dopant is copper, for example, the dopant is added to a specific core 11a at a concentration of 2.5 ppb or more, and the dopant is not added to cores 11b to 11d; when the dopant is vanadium, for example, the dopant is added to a specific core 11a at a concentration of 0.17 ppb or more, and the dopant is not added to cores 11b to 11d.

[0040] As long as the specific core 11a and the other cores 11b to 11d have different optical losses at a predetermined wavelength, the other cores 11b to 11d may be doped with less dopant than the specific core 11a, thereby adjusting the concentration of the dopant added to the cores 11a to 11d. A combination of multiple dopants may also be added. In this case, the concentration of each dopant is appropriately adjusted depending on the type and ratio of the combined dopants. The type of dopant is not limited to the above example, and the predetermined wavelength is not limited to 800 nm or more and 950 nm or more and 360 nm or more and less than 830 nm, as long as it is a wavelength less than the cutoff wavelength.

[0041] To add a dopant to the cores 11a to 11d as described above, the dopant is added to the core rods when producing the core rods that will become the cores 11a to 11d in the preform for producing the multicore fiber 10. Note that if the dopant is not added to the cores 11b to 11d, the dopant is not added to the core rods that will become these cores, and the dopant that absorbs light of a predetermined wavelength is added to the core rod that will become the core 11a. For example, when the core rod is produced using soot by a vapor phase axial deposition (VAD) method, an outside vapor deposition (OVD) method, a modified chemical vapor deposition (MCVD) method, a plasma activated chemical vapor deposition (PCVD) method, or the like, the dopant may be sprayed together with the soot to produce a porous glass body, and the glass body may be sintered. Alternatively, the porous glass body on which the soot has been deposited may be immersed in a solution in which the dopant has been dissolved, so that the pores are impregnated with the dopant, and then the glass body may be sintered. Note that the method for adding the dopant to the cores 11a to 11d is not limited to the above.

[0042] Next, the measurement of the multi-core fiber 10 will be described.

[0043] Fig. 2 is a diagram showing the configuration of an apparatus for measuring the multicore fiber 10. As shown in Fig. 2, in this embodiment, a fan-in / fan-out device 20 is connected to the multicore fiber 10, and a measurement apparatus 30 is connected to the fan-in / fan-out device 20.

[0044] The fan-in / fan-out device 20 mainly comprises optical fibers 21a to 21d, the number of which is the same as the number of cores 11a to 11d of the multicore fiber 10, and a pitch converter 22. The optical fibers 21a to 21d are single-core fibers of the same configuration, and are capable of propagating light of the wavelengths propagated through the cores 11a to 11d. The pitch converter 22 has waveguides 22a to 22d, the number of which is the same as the number of cores 11a to 11d of the multicore fiber 10, and one end of each of the waveguides 22a to 22d is optically connected to one of the cores 11a to 11d individually, and the other end is optically connected to a core of the optical fiber 21a to 21d. The waveguides 22a to 22d have the same configuration as each other, and are capable of propagating light of the wavelengths propagated through the cores 11a to 11d. The pitch conversion unit 22 is, for example, a bridge fiber whose cross-sectional structure is similar to that of the multicore fiber 10 and whose diameter decreases from the optical fibers 21a to 21d toward the multicore fiber 10. In this case, the waveguides 22a to 22d are cores, and the cores of the optical fibers 21a to 21d are individually connected to the cores at the end face of the bridge fiber on the optical fibers 21a to 21d side, and the cores 11a to 11d of the multicore fiber 10 are individually connected to the cores at the multicore fiber 10 side of the bridge fiber. Note that the pitch conversion unit 22 may have a different configuration from the above, as long as the cores of the optical fibers 21a to 21d are individually optically connected to the cores 11a to 11d of the multicore fiber 10. Below, a case will be described in which the optical fibers 21a to 21d are optically connected to the waveguides 22a to 22d in order, and the waveguides 22a to 22d are optically connected to the cores 11a to 11d of the multicore fiber 10 in order.

[0045] Examples of the measuring device 30 include an OTDR (Optical Time Domain Reflectometer), an OFDR (Optical Frequency Domain Reflectometry), and an OLTS (Optical Loss Test Sets). The measuring device 30 is optically connected to the optical fibers 21a to 21d and can select one of the optical fibers 21a to 21d and emit measurement light to the selected optical fiber. The wavelength of this measurement light is the predetermined wavelength. A case will be described in which the measuring device 30 is an OTDR. The measuring device 30, which is an OTDR, emits measurement light of the predetermined wavelength to the selected optical fiber. For example, it emits measurement light to the optical fiber 21a. The emitted measurement light passes through the optical fiber 21a and the waveguide 22a of the pitch converter 22 and enters the core 11a. In the core 11a, Rayleigh scattering of power corresponding to the power of the propagating light occurs, and the light passes from the core 11a through the waveguide 22a and the optical fiber 21a and enters the measuring device 30. Therefore, the optical loss of the core 11a can be measured by the measuring device 30. Furthermore, the measuring device 30 sequentially emits the measurement light of the predetermined wavelength to the optical fibers 21b to 21d, and measures the optical losses of the cores 11b to 11d.

[0046] FIG. 3 is a diagram showing the optical characteristics of the multicore fiber 10. In FIG. 3, a symbol indicating the corresponding portion is written on the waveform at each portion. In FIG. 3, the waveform WFa indicates the relative power of the light emitted from the optical fiber 21a to the measurement device 30 due to the measurement light incident on the optical fiber 21a. Similarly, the waveforms WFb to WFd indicate the relative power of each light emitted from the optical fibers 21b to 21d to the measurement device 30 due to the measurement light incident on the optical fibers 21b to 21d, respectively. In this embodiment, the waveforms WFb to WFd are the same as each other, and therefore are shown as a single waveform. Furthermore, the optical fibers 21a to 21d have the same configuration as each other, and the waveguides 22a to 22d have the same configuration as each other. Therefore, the portion of the waveform WFa corresponding to the optical fiber 21a and the waveguide 22a and the portions of the waveforms WFb to WFd corresponding to the optical fibers 21b to 21d and the waveguides 22b to 22d essentially overlap each other. Therefore, the portion of the waveform WFa corresponding to the end of the core 11a on the waveguide 22a side and the portions of the waveforms WFb to WFd corresponding to the end of the cores 11b to 11d on the waveguides 22a to 22d side are essentially overlapping. However, to avoid confusion due to overlapping, the waveform WFa and the waveforms WFb to WFd are shown shifted vertically in FIG. 3 . As described above, the loss of the measurement light between the specific core 11a and the other cores 11b to 11d differs by 0.01 dB / km or more. In this embodiment, the loss of the measurement light between the specific core 11a is greater than the loss of the measurement light between the other cores 11b to 11d by 0.01 dB / km or more. Therefore, the slope of the waveform WFa in the core 11a is greater in the negative direction than the slope of the waveforms WFb to WFd in the cores 11b to 11d. The power of light emitted from the optical fiber 21a due to the measurement light propagating through the core 11a of the multicore fiber 10 is attenuated more along the longitudinal direction of the multicore fiber 10 than the power of light emitted from the optical fibers 21b to 21d due to the measurement light propagating through the cores 11b to 11d. In other words, it is shown that the loss of measurement light in the core 11a of the multicore fiber 10 is larger than that in the other cores 11b to 11d.

[0047] As described above, in the multicore fiber 10 of this embodiment, the propagation loss of measurement light having a wavelength equal to or shorter than the wavelength at which light in a mode that is one order higher than the mode of light in the communication wavelength band can propagate differs by 0.01 dB / km or more between the specific core 11a and the other cores 11b to 11d. Therefore, even if the variation in the propagation loss of light in the communication wavelength band between the cores is small, the specific core 11a can be easily identified by examining the propagation loss of each core using the above measurement light.

[0048] In this embodiment, the specific core 11 a is doped with a dopant that absorbs the measurement light of the predetermined wavelength. Therefore, the specific core 11 a can be identified without providing the multicore fiber 10 with any special structure other than the cores 11 a to 11 d.

[0049] Conversely, a dopant that absorbs light of the predetermined wavelength may be added to the other cores 11b to 11d. That is, the loss of light of the predetermined wavelength in the other cores 11b to 11d may be 0.01 dB / km or more greater than the loss in the specific core 11a. In this case, in the description of the dopant addition concentration, the specific core 11a may be read as the other cores 11b to 11d. In this case, the cores 11b to 11d may be considered as the specific cores and the core 11a as the other cores, and it may be understood that the loss of light of the predetermined wavelength in the specific cores 11b to 11d is 0.01 dB / km or more greater than the loss in the other core 11a.

[0050] Second Embodiment Next, a second embodiment of the present invention will be described. Note that components identical or equivalent to those in the first embodiment will be assigned the same reference numerals and will not be described again unless otherwise specified.

[0051] Fig. 4 is a diagram showing a cross section perpendicular to the longitudinal direction of the multi-core fiber according to this embodiment. As shown in Fig. 4, the multi-core fiber 10 according to this embodiment differs from the multi-core fiber 10 according to the first embodiment in that an absorbing portion that absorbs light of a predetermined wavelength is provided near a specific core 11 a.

[0052] In this embodiment, the absorbing portion is an absorbing layer 15a surrounding a specific core 11a. The absorbing layer 15a is a layer capable of absorbing at least a portion of the light leaking out from the core 11a. The cladding 12 is located between the absorbing layer 15a and the core 11a. The absorbing layer 15a in this embodiment is a layer made of glass doped with a dopant that absorbs light of a predetermined wavelength. The absorbing layer 15a is a layer that absorbs at least a portion of the light leaking out from the core 11a. Therefore, the absorbing layer 15a is located at a position where it overlaps with the light of the predetermined wavelength that propagates through the core 11a and leaks out from the core 11a. This predetermined wavelength is the same as the predetermined wavelength described in the first embodiment. The thickness of the absorbing layer 15a is, for example, 0.5 μm or more and 10 μm or less.

[0053] In this embodiment, the difference between the propagation loss of light of a predetermined wavelength propagating through the specific core 11a and the propagation loss of light of a predetermined wavelength propagating through the other cores 11b to 11d is 0.01 dB / km or more, preferably 0.1 dB / km or more, more preferably 0.3 dB / km or more, and even more preferably 0.5 dB / km or more. As in the first embodiment, this embodiment will be described using an example in which the propagation loss of light of a predetermined wavelength propagating through the specific core 11a is greater than the propagation loss of light of a predetermined wavelength propagating through the other cores 11b to 11d.

[0054] When the highest-order mode light propagating through the core 11a at a predetermined wavelength is normalized by the intensity of the light of the mode at the portion of the light that overlaps with the absorbing layer 15a, the difference in loss of the desired light is b (dB / km), and the amount of light absorption per unit concentration determined for each dopant is c (dB / (km·ppb)), the necessary doping concentration d (ppb) of the dopant added to the absorbing layer 15a can be expressed by the following formula. Note that when the absorbing layer is provided only around the core 11a, as in this embodiment, the difference in loss b of the desired light is the amount of light absorbed by the absorbing layer 15a. d=b / (a×c)

[0055] The normalized intensity a can be calculated as follows. The normalized intensity a can be calculated based on the distance from the core 11a to the absorbing layer 15a, the thickness of the absorbing layer 15a, the wavelength, etc. Specifically, first, the intensity distribution of the light of the highest-order mode propagating through the core 11a is obtained from the distribution of the relative refractive index difference of the core 11a with respect to the cladding 12 and the wavelength of the light. Next, this intensity distribution is integrated within the region of the absorbing layer 15a to obtain the intensity of the light overlapping the absorbing layer 15a. Then, this intensity distribution is integrated to infinity to obtain the intensity of the light of the highest-order mode. Finally, the intensity of the light overlapping the absorbing layer 15a is normalized by the intensity of the light of the highest-order mode, thereby obtaining the normalized intensity a. Here, the intensity distribution of the light of the highest-order mode can be calculated using an existing calculation method such as the finite element method. For example, if the distance from the core 11a to the absorption layer 15a is 14 μm, the thickness of the absorption layer 15a is 6 μm, and the predetermined wavelength is 400 nm, then the above a is 0.00001.

[0056] When the predetermined wavelength is 800 nm or more and 950 nm or less, the type of dopant added to the absorption layer 15a is the same as the dopant exemplified in the first embodiment. In this wavelength band, the light absorption amount c is 3 dB / (km·ppb) for vanadium and 1 dB / (km·ppb) for chromium. Furthermore, when the predetermined wavelength is 360 nm or more and less than 830 nm, the type of dopant added to the absorption layer 15a is the same as the dopant exemplified in the first embodiment. In this wavelength band, the light absorption amount c is 2 dB / (km·ppb) for nickel, 4 dB / (km·ppb) for cobalt, 3 dB / (km·ppb) for chromium, 0.4 dB / (km·ppb) for iron, 0.2 dB / (km·ppb) for manganese, and 0.2 dB / (km·ppb) for copper.

[0057] As described above, b is 0.01 dB / km or more, preferably 0.1 dB / km or more, more preferably 0.3 dB / km or more, and even more preferably 0.5 dB / km or more. Therefore, the absorption layer 15 a can be configured by adjusting the doping concentration d in accordance with b.

[0058] The absorption layer 15a can be provided as described above as follows. A preform for manufacturing the multi-core fiber 10 can be manufactured by inserting a plurality of core-coated rods, in which core rods that become the cores 11a to 11d are coated with cladding glass layers that become part of the cladding, into a cladding rod that becomes another part of the cladding and has through holes through which each core-coated rod can be inserted. In this manufacturing process, a dopant is attached to the outer peripheral surface of the cladding glass layer of the core-coated rod that includes the core rod that becomes the core 11a. This attachment is performed, for example, by immersing the core-coated rod in a solution in which the dopant is dissolved. Alternatively, in the manufacturing process, the dopant is attached to the inner peripheral surface of a through hole in the cladding rod into which the core-coated rod that includes the core rod that becomes the core 11a is inserted. This attachment is performed, for example, by passing a solution in which the dopant is dissolved through the through hole. In the above case, the concentration of the dopant added to the absorption layer 15a can be adjusted by adjusting the concentration of the solution. Furthermore, the amount of dopant adhered may be adjusted by roughening the outer peripheral surface of the core-coated rod or the inner peripheral surface of the through hole of the clad rod. The outer peripheral surface of the core-coated rod may be roughened by, for example, sandblasting. The inner peripheral surface of the through hole of the clad rod may be roughened by, for example, increasing the abrasive grains of a drill tool when drilling the through hole. Alternatively, the core-coated rod may be covered with a glass tube to which a dopant has been added, and the core-coated rod together with the glass tube may be inserted into the through hole of the clad rod. Furthermore, a multi-core fiber having an absorption layer 15a may be manufactured by other known methods.

[0059] (Modification) Next, a modification of the present embodiment will be described. Note that configurations that are the same as or equivalent to those described above will be assigned the same reference numerals and duplicated descriptions will be omitted unless otherwise specified.

[0060] FIG. 5 is a diagram showing a cross section perpendicular to the longitudinal direction of a multicore fiber according to this modification. As shown in FIG. 5, the multicore fiber 10 of this modification differs from the above embodiment in the configuration of the absorbing section that absorbs light of the predetermined wavelength. The absorbing section 16a is a portion that can absorb at least a portion of the light leaking from the core 11a. The absorbing section 16a of this embodiment differs from the absorbing layer 15a of the above embodiment in that it is provided near a specific core 11a and has a circular cross-sectional shape. The size of the absorbing section 16a is, for example, smaller than the cores 11a to 11d, e.g., two-thirds the diameter of the cores 11a to 11d. The absorbing section 16a is provided at a position such that the layout of the absorbing section 16a and the cores 11a to 11d combined is asymmetrical with respect to a line passing through the center of the cladding 12. Therefore, the absorbing section 16a can also function as a marker. Note that the size and shape of the absorbing section 16a are not limited to those described above.

[0061] In this modification as well, the propagation loss of light of a predetermined wavelength propagating through the specific core 11a differs from the propagation loss of light of a predetermined wavelength propagating through the other cores 11b to 11d by 0.01 dB / km or more, preferably by 0.1 dB / km or more, more preferably by 0.3 dB / km or more, and even more preferably by 0.5 dB / km or more. In this modification as well, an example will be described in which the propagation loss of light of a predetermined wavelength propagating through the specific core 11a is greater than the propagation loss of light of a predetermined wavelength propagating through the other cores 11b to 11d.

[0062] In the above embodiment, the absorbing layer 15a has an annular shape in a cross section perpendicular to the longitudinal direction of the multicore fiber 10. In contrast, in this modification, the absorbing portion 16a has a circular shape. The normalized intensity a described in the above embodiment can be calculated based on the distance from the core 11a to the absorbing portion 16a, the diameter of the absorbing portion 16a, the wavelength, etc., taking into account the shape of the absorbing portion 16a. Specifically, first, the intensity distribution of the highest-order mode light propagating through the core 11a is obtained from the distribution of the relative refractive index difference of the core 11a with respect to the cladding 12 and the wavelength of the light. Next, the intensity distribution is integrated within the region of the absorbing portion 16a to obtain the intensity of the light overlapping with the absorbing portion 16a. Then, the intensity distribution is integrated to infinity to obtain the intensity of the light in the highest-order mode. Finally, the normalized intensity a can be obtained by normalizing the intensity of the light overlapping with the absorbing portion 16a by the intensity of the light in the highest-order mode.

[0063] If a can be found in this way, c is the same as above, and therefore the absorbing portion 16a can be formed by adjusting the additive concentration d in accordance with b.

[0064] The absorbing portion 16a can be provided as described above as follows. A plurality of core-coated rods are prepared, in which core rods that become the cores 11a to 11d are coated with a clad glass layer that becomes part of the clad; an absorbing portion-coated rod in which an absorbing portion rod that becomes the absorbing portion 16a is coated with a clad glass layer that becomes part of the clad; and a clad rod that becomes another part of the clad and has a plurality of through holes. Next, each of the core-coated rods and the absorbing portion-coated rod is inserted into the through holes of the clad rod. A preform that becomes the multi-core fiber 10 is thus prepared, and the preform is drawn to form the multi-core fiber 10. The absorbing portion rod is doped with the dopant that absorbs light of a predetermined wavelength. The method of doping the dopant into the absorbing portion rod is, for example, the same as the method of doping the dopant into the cores 11a to 11d described in the first embodiment. In this case, the concentration of the dopant is appropriately adjusted so that the loss of light of the wavelength differs by 0.01 dB / km or more between the specific core 11a and the other cores 11b to 11d. It is also possible to produce multiple absorbing rods that become the absorbing section 16a by appropriately changing the dopant concentration, and to produce absorbing rods in which the loss of light at the above wavelength differs by 0.01 dB / km or more between a specific core 11a and the other cores 11b to 11d.

[0065] The measurement of the multi-core fiber 10 in this embodiment including the modified examples is performed in the same manner as the measurement of the multi-core fiber 10 in the first embodiment, for example.

[0066] As described above, the multi-core fiber 10 of this embodiment is provided with an absorbing layer 15a or absorbing portion 16a that absorbs light of the wavelength of the measurement light near the specific core 11a. With this configuration, the difference in propagation loss of the measurement light between the specific core 11a and the other cores 11b to 11d may be 0.01 dB / km or more. In particular, as described above, the specific wavelength, which is the wavelength of the measurement light, is equal to or shorter than the wavelength at which light of a mode one order higher than the mode of light in the communication wavelength band can propagate. Therefore, light of the specific wavelength tends to leak out of the core 11a more easily than light of a communication wavelength. Therefore, the leaked light is more easily absorbed by the absorbing layer 15a or absorbing portion 16a. Therefore, according to this embodiment and its modified examples, the loss of the measurement light of the specific wavelength can be easily increased.

[0067] In this embodiment, conversely to the above, an absorption layer or an absorption portion that absorbs light of the wavelength of the measurement light may be provided near the other cores 11b to 11d. That is, the loss of light of a predetermined wavelength in the other cores 11b to 11d may be 0.01 dB / km or more greater than the loss in the specific core 11a. In this case, it may be understood that the cores 11b to 11d are the specific cores and the core 11a is the other core, and that the loss of light of a predetermined wavelength in the specific cores 11b to 11d is 0.01 dB / km or more greater than the loss in the other core 11a.

[0068] Third Embodiment Next, a third embodiment of the present invention will be described. Note that components identical or equivalent to those in the first embodiment will be assigned the same reference numerals and will not be described again unless otherwise specified.

[0069] The cross section perpendicular to the longitudinal direction of the multicore fiber according to this embodiment is similar to that of the multicore fiber 10 according to the first embodiment shown in Fig. 1. The multicore fiber 10 according to this embodiment differs from the first embodiment in that a specific core 11a has more structural defects that cause loss of light of a predetermined wavelength less than the cutoff wavelength than the other cores 11b to 11d.

[0070] This structural defect is a defect such as breaking of a bond in the ideal structure of glass represented by ≡Si—O—Si≡.

[0071] An example of a structural defect is a state of excess oxygen called POL (Peroxy Linkage), represented by ≡Si-O-O-Si≡. When POL is present, light in the 177 nm and 326 nm wavelength bands is absorbed. Therefore, in this example, the predetermined wavelength is at least one of the 177 nm and 326 nm bands.

[0072] To generate POL in the core, a core rod is fabricated using soot in an oxygen-excess state, and a preform for a multicore fiber is drawn using the core rod. Therefore, when fabricating the core rod that will become the core 11a, soot is deposited in an oxygen-excess state to the extent that POL is sufficiently generated. That is, the oxygen concentration is appropriately adjusted so that the loss of light at the above wavelength differs by 0.01 dB / km or more between the specific core 11a and the other cores 11b to 11d. Multiple core rods that will become the core 11a may be fabricated by appropriately changing the oxygen concentration, and an absorbing portion rod may be fabricated in which the loss of light at the above wavelength differs by 0.01 dB / km or more between the specific core 11a and the other cores 11b to 11d.

[0073] Furthermore, structural defects include oxygen deficient centers (ODCs), which are represented by ≡Si-Si≡ and are in a state of oxygen deficiency. When ODCs are present, light in the 163 nm wavelength band is absorbed. Therefore, in this example, the predetermined wavelength is the 163 nm band.

[0074] To generate ODC in the core, a core rod is fabricated using soot in an oxygen-deficient state, and a preform for a multicore fiber is drawn using the core rod. Therefore, when fabricating the core rod that will become the core 11a, soot is deposited in an oxygen-deficient state to the extent that ODC is sufficiently generated. That is, by appropriately adjusting the oxygen concentration, the degree of oxygen deficiency is adjusted so that the loss of light at the above wavelength differs by 0.01 dB / km or more between the specific core 11a and the other cores 11b to 11d. Multiple core rods that will become the core 11a may be fabricated by appropriately changing the oxygen concentration, and an absorbing portion rod may be fabricated in which the loss of light at the above wavelength differs by 0.01 dB / km or more between the specific core 11a and the other cores 11b to 11d.

[0075] Examples of structural defects include NBOHC (Non-Brigding Oxygen Hole Center), represented by ≡Si-O·, in which the bond between one Si atom and one O atom is broken from an ideal glass, resulting in an unpaired electron on the O atom, and E' (E prime center), represented by ≡Si·, in which the bond between the Si atom and the O atom is broken, resulting in an unpaired electron on the Si atom separated from the O atom. When NBHOC is present, light in the 258 nm or 620 nm wavelength band is absorbed. Furthermore, when E' is present, light in the 214 nm band is absorbed. Therefore, in this example, the predetermined wavelength is either the 258 nm band, the 620 nm band, or the 214 nm band. NBOHC and E' generally occur simultaneously.

[0076] To generate NBOHC and E', glass is irradiated with gamma rays. Therefore, when manufacturing the multicore fiber 10, the core rod that will become the core 11a is irradiated with gamma rays, while the core rods that will become the cores 11b to 11d are not irradiated with gamma rays. Therefore, the core rod that will become the core 11a is irradiated with gamma rays to an extent that NBOHC and E' are sufficiently generated as described above. In other words, the irradiation amount of gamma rays is adjusted so that the loss of light at the above wavelength differs by 0.01 dB / km or more between the specific core 11a and the other cores 11b to 11d. Multiple core rods that will become the core 11a may be manufactured by appropriately changing the irradiation amount of gamma rays, and an absorbing portion rod may be manufactured in which the loss of light at the above wavelength differs by 0.01 dB / km or more between the specific core 11a and the other cores 11b to 11d.

[0077] In order to generate E' in the core, a preform having a core rod including a structural defect of POL may be drawn. Therefore, when manufacturing the multicore fiber 10, a structural defect of POL is generated in the core rod that will become the core 11a as described above. A preform having such a core rod is drawn. Therefore, a structural defect of POL is generated in the core rod that will become the core 11a to an extent that E' is sufficiently generated as described above. That is, the oxygen concentration is appropriately adjusted so that the loss due to E' of light of the above wavelength differs by 0.01 dB / km or more between the specific core 11a and the other cores 11b to 11d. Multiple core rods that will become the core 11a may be produced by appropriately changing the oxygen concentration, and an absorbing portion rod may be produced in which the loss of light of the above wavelength differs by 0.01 dB / km or more between the specific core 11a and the other cores 11b to 11d. When a base material having a core rod with structural defects in the POL is drawn, not all of the POLs become E', but a certain percentage of the POLs become E'.

[0078] In addition, as a structural defect, STH １２ An example of such a structure is called a self-trapped hole (SHT) and is represented by ≡Si-O.-Si≡, where O has one unpaired electron. In this structure, the ".," which indicates the unpaired electron, is written above the "O." １２ When present, light in the 475 nm wavelength band is absorbed.

[0079] STH １２ In order to generate the STH, the glass is irradiated with gamma rays. Therefore, when manufacturing the multi-core fiber 10, the core rod that will become the core 11a is irradiated with gamma rays, but the core rods that will become the cores 11b to 11d are not irradiated with gamma rays. Therefore, the core rod that will become the core 11a is irradiated with the STH as described above. １２ In other words, the amount of gamma rays irradiated is adjusted to a degree that the STH of the light of the above wavelength is sufficiently generated between the specific core 11a and the other cores 11b to 11d. １２The difference in loss due to the irradiation of gamma rays is set to 0.01 dB / km or more. By appropriately changing the irradiation amount of gamma rays, a plurality of core rods that become the core 11a may be produced, and an absorbing rod may be produced in which the difference in loss of light of the above wavelength between a specific core 11a and the other cores 11b to 11d is 0.01 dB / km or more. When the core rod is irradiated with gamma rays, the STH １２ , NBOHC, and E′ are produced, respectively.

[0080] Another structural defect is called a peroxy radical (POR), represented by ≡Si-O-O., in which the bond between one Si atom and one O atom of the POR is broken, resulting in an unpaired electron on the O atom. When a POR is present, light in the 163 nm, 258 nm, 265 nm, and 630 nm wavelength bands is absorbed. Therefore, in this example, the predetermined wavelengths are the 163 nm, 258 nm, 265 nm, and 630 nm bands.

[0081] In order to generate POR in the core, a preform having a core rod containing a POL structural defect is drawn. Therefore, when manufacturing the multicore fiber 10, a POL structural defect is generated in the core rod that will become the core 11a as described above. A preform having such a core rod is drawn. Therefore, a POL structural defect is generated in the core rod that will become the core 11a to the extent that a POL is sufficiently generated as described above. That is, the oxygen concentration when producing the core rod is appropriately adjusted so that the POR loss of light of the above wavelength differs by 0.01 dB / km or more between the specific core 11a and the other cores 11b to 11d. Multiple core rods that will become the core 11a may be produced by appropriately changing the oxygen concentration, and an absorbing portion rod may be produced in which the POR loss of light of the above wavelength differs by 0.01 dB / km or more between the specific core 11a and the other cores 11b to 11d. When a base material having a core rod with structural defects of POL is drawn, not all POL become POR, but a certain percentage of POL become POR.

[0082] The measurement of the multi-core fiber 10 in this embodiment is performed, for example, in the same manner as the measurement of the multi-core fiber 10 in the first embodiment. The wavelength of the measurement light in this case is the predetermined wavelength exemplified above.

[0083] As described above, in the multicore fiber 10 of this embodiment, the specific core 11a has more structural defects that cause loss of light of the wavelength of the measurement light than the other cores. With this configuration, the difference in propagation loss of the measurement light between the specific core 11a and the other cores 11b to 11d may be 0.01 dB / km or more.

[0084] In this embodiment, conversely to the above, the other cores 11b to 11d may have more structural defects than the specific core 11a. That is, the loss of light of a predetermined wavelength in the other cores 11b to 11d may be 0.01 dB / km or more greater than the loss in the specific core 11a. In this case, cores 11b to 11d may be considered as specific cores and core 11a as other cores, and it may be understood that the loss of light of a predetermined wavelength in the specific cores 11b to 11d is 0.01 dB / km or more greater than the loss in the other core 11a.

[0085] Although the present invention has been described above using the above-described embodiments as examples, the present invention is not limited thereto. In the above-described embodiments, an example in which there is one specific core 11a has been shown. However, the number of specific cores may be a plurality of specific cores, which is less than the total number of cores. Even in this case, if there are a plurality of other cores, the difference in loss of measurement light having a predetermined wavelength between the other cores is preferably smaller than the difference in loss of measurement light between the specific core and the other cores, preferably less than 0.01 dB / km, and more preferably approximately equal to each other. Furthermore, by using a configuration different from the configuration exemplified above, the propagation loss of measurement light at a wavelength equal to or shorter than the wavelength at which light in a mode one order higher than the mode of light in the communication wavelength band can propagate may be made to differ by 0.01 dB / km or more between the specific core and the other cores.

[0086] In the first and second embodiments, the predetermined wavelength, which is the wavelength of the measurement light, is not limited to the wavelengths exemplified above. Furthermore, the communication wavelength may be less than 1260 nm, for example, the communication wavelength may be in the so-called T band, which is 1000 nm or more and less than 1260 nm, and the wavelength of the measurement light may be one of the wavelengths exemplified above that is less than the cutoff wavelength.

[0087] Furthermore, the measurement of the multicore fiber is not limited to the above-exemplified method. For example, a fan-out device having a configuration similar to the fan-in / fan-out device 20 may be optically connected to the end of the multicore fiber 10 opposite to the end optically connected to the fan-in / fan-out device 20. Then, light may be sequentially input into the optical fibers 21 a to 21 d, and the power of the light emitted from the optical fiber of the fan-out device via the multicore fiber may be measured, thereby determining the propagation loss of the cores 11 a to 11 d in the multicore fiber 10.

[0088] The above embodiments may also be combined.

[0089] As described above, according to the present invention, a multi-core fiber that can distinguish a specific core from other cores is provided even when the difference in propagation loss of light of wavelengths in the communication wavelength band between the cores is small, and is expected to be used in fields such as optical fiber communications.

Claims

1. A multicore fiber comprising a plurality of cores and a cladding surrounding the plurality of cores, wherein the propagation loss of measurement light of a wavelength equal to or shorter than a wavelength at which light in a mode one order higher than the mode of light in the communication wavelength band can propagate differs by 0.01 dB / km or more between a specific core and other cores.

2. The multicore 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 larger than the difference between the propagation loss of light of a wavelength in the communication wavelength band in the specific core and the propagation loss in the other cores.

3. A multi-core fiber according to claim 1 or 2, 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 0.1 dB / km or more.

4. A multicore fiber according to any one of claims 1 to 3, characterized in that the wavelength at which light in a mode that is one order higher than the mode of light in the communication wavelength band can propagate is less than 1260 nm.

5. The multi-core fiber according to claim 4, wherein the wavelength of the measurement light is 800 nm or more and 950 nm or less.

6. The multi-core fiber according to claim 4, wherein the wavelength of the measurement light is not less than 360 nm and less than 830 nm.

7. A multicore fiber according to any one of claims 1 to 6, characterized in that the specific core is doped with a larger amount of dopant that absorbs light of the wavelength of the measurement light than the other cores.

8. A multi-core fiber according to any one of claims 1 to 6, characterized in that an absorbing section that absorbs light of the wavelength of the measurement light is provided in the vicinity of the specific core.

9. The multi-core fiber according to claim 8, wherein the absorbing portion is an absorbing layer surrounding the specific core.

10. A multicore fiber according to any one of claims 1 to 4, characterized in that the specific core has more structural defects that cause loss of light of the wavelength of the measurement light than the other cores.

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