JP2012211964A - Multi-core fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multi-core fiber capable of suppressing a deterioration of crosstalk between specific cores even in a case where the multi-core fiber is non-linearly installed.SOLUTION: A multi-core fiber 1 includes: a plurality of cores 11 and 12; and cladding layers 30 for surrounding an outer peripheral surface of each of the plurality of cores 11 and 12. A difference between propagation constants of at least a pair of cores 11 and 12 adjacent to each other out of the plurality of cores 11 and 12 varies along a longitudinal direction. An increase in crosstalk between specific cores can be suppressed by thus setting the plurality of cores 11 and 12 even in a case where the multi-core fiber 1 is installed in a curved state.

Description

  The present invention relates to a multi-core fiber that can suppress deterioration in crosstalk between specific cores even when installed non-linearly.

  An optical fiber used in a currently popular optical fiber communication system has a structure in which the outer periphery of one core is surrounded by a clad, and information is transmitted by propagation of an optical signal in the core. Is done. In recent years, with the spread of optical fiber communication systems, the amount of information transmitted has increased dramatically. With such an increase in the amount of information transmitted, in optical fiber communication systems, large numbers of long-distance optical communications are performed by using a large number of optical fibers such as tens to hundreds. .

  In order to reduce the number of optical fibers in such an optical fiber communication system, a plurality of signals are transmitted by light propagating through each core using a multi-core fiber in which the outer circumferences of the plurality of cores are surrounded by one clad. It has been known.

  Non-Patent Document 1 below describes an example of such a multi-core fiber. In this multi-core fiber, a plurality of cores are arranged in one clad. However, as pointed out in Non-Patent Document 1, in a multi-core fiber, optical signals propagating through the cores may interfere with each other, and noise may be superimposed on the optical signals propagating through the cores. is there. Therefore, Non-Patent Document 1 shows that the relative refractive index difference of each core with respect to the cladding is changed as one method for reducing the crosstalk between the cores.

  Patent Document 1 below describes a multi-core fiber including a plurality of cores having different propagation constants by making a difference in refractive index, a core diameter, and the like different from each other.

  Further, Patent Document 2 described below describes a multi-core fiber including a plurality of cores having different core diameters.

Masanori KOSHIBA “Heterogeneous multi-core fibers: proposal and design principal”, IEICE Electronics Express, Vol. 2, 98-103

International Publication No. 2010/0388863 US Pat. No. 5,519,801

  As described in Non-Patent Document 1, the cores adjacent to each other can be obtained by changing the relative refractive index difference with respect to the cladding of each core, or by making the diameters of the cores different from each other as in Patent Documents 1 and 2. The propagation constant (waveguide condition) of the optical signal is different, and the crosstalk is certainly reduced. However, the optical fiber may be installed not only in a straight line but also in a non-linear manner. For example, when installed so as to draw an arc of a certain diameter, one of the adjacent cores is inside the arc, and the other is outside the arc, so that the propagation constant of each light of the adjacent cores May match. And depending on the state of installation of the optical fiber, the state where the propagation constants of the lights of the cores adjacent to each other may continue for a long time. As described above, when the light propagation constants of the adjacent cores coincide with each other for a long distance, there is a problem in that crosstalk between the cores easily occurs. In other words, even if the optical fiber is installed non-linearly, even if the propagation constants of light in the cores adjacent to each other are changed by changing the relative refractive index difference with respect to the cladding of the cores adjacent to each other, Crosstalk between specific cores may easily occur.

  Therefore, an object of the present invention is to provide a multi-core fiber that can suppress the deterioration of crosstalk between specific cores even when installed non-linearly.

  The multi-core fiber of the present invention includes a plurality of cores and a clad surrounding an outer peripheral surface of each of the cores, and a difference in propagation constant between at least one pair of cores adjacent to each other among the plurality of cores is long. It changes along the direction.

  A set of cores of such a multi-core fiber is arranged in a non-linear manner, even if a place where propagation constants coincide in at least one set of cores adjacent to each other occurs. In the core, the difference in mode field diameter varies along the longitudinal direction, so that the propagation constants of the respective cores do not match in a long section. Therefore, even when the multi-core fiber of the present invention is installed in a spiral or curved state, it is possible to suppress the deterioration of the crosstalk between a pair of adjacent cores.

  Further, it is preferable that the difference in mode field diameter of each light propagating through at least one pair of cores adjacent to each other varies along the longitudinal direction.

  The difference in the mode field diameter of light propagating through a pair of adjacent cores changes along the longitudinal direction. This means that the difference in the propagation constant between a pair of adjacent cores changes along the longitudinal direction. It is none other than being. Therefore, in such a multi-core fiber, it is possible to easily check that the difference between a pair of adjacent propagation constants changes along the longitudinal direction by checking the change in the mode field diameter.

  In the at least one pair of cores adjacent to each other, it is preferable that a difference in relative refractive index difference with respect to the cladding changes along the longitudinal direction.

  With such a configuration, a difference in propagation constant and a difference in mode field diameter between adjacent cores can be easily changed in the longitudinal direction. In addition, since the change in crosstalk with respect to the amount of change in the refractive index is very large, it is possible to prevent the crosstalk from deteriorating by minutely changing the refractive index.

  Thus, when the difference in relative refractive index difference with respect to the cladding changes along the longitudinal direction, the difference in relative refractive index difference between the at least one pair of adjacent cores is the difference in relative refractive index difference with respect to the cladding. Is preferably larger than the amount of change along the longitudinal direction.

  By setting the relative refractive index difference in this way, crosstalk can be further reduced when the bending radius of the multi-core fiber is large.

  Alternatively, when the difference in relative refractive index difference with respect to the cladding changes along the longitudinal direction as described above, the difference in relative refractive index difference between the at least one pair of adjacent cores is equal to the relative refractive index with respect to the cladding. The difference is preferably smaller than the amount of change that varies along the longitudinal direction.

  By setting the refractive index difference in this way, the crosstalk value can be stabilized even when the bending diameter applied to the fiber changes.

  Moreover, it is preferable that the difference in diameter of the at least one pair of cores adjacent to each other changes along the longitudinal direction.

  Even with such a configuration, the difference in propagation constant and the difference in mode field diameter between adjacent cores can be easily changed in the longitudinal direction. Further, the diameter of the core can be determined by changing the outer diameter of the glass rod serving as the core, and the fluctuation of the outer diameter can be adjusted after the manufacturing of the glass rod serving as the core.

  Moreover, it is preferable that the difference in the propagation constant changes at a constant interval along the longitudinal direction.

  By adopting such a configuration, the propagation constants of the cores adjacent to each other may be changed at regular intervals so that the difference in mode field diameter varies at regular intervals. Accordingly, when the multi-core fiber having such a configuration is industrially produced, the multi-core fiber can be manufactured under conditions that are easy to manage.

  Alternatively, it is preferable that the difference between the propagation constants changes at irregular intervals along the longitudinal direction.

  With such a configuration, in the fiber preform used to manufacture the multi-core fiber, the diameter and refractive index of the glass rod as the core can be changed at random intervals, and the multi-core fiber can be easily manufactured. Can do. In addition, when the difference in propagation constants changes at irregular intervals along the longitudinal direction in this way, the multi-core fiber is used by being wound in a regular shape such as being spirally wound. In this case, it is possible to prevent the crosstalk from deteriorating regularly.

  More preferably, at least one of the at least one pair of adjacent cores is arranged in a spiral shape so as to rotate around the central axis of the cladding.

  The position of the spirally arranged core in the cross section perpendicular to the longitudinal direction of the fiber varies along the longitudinal direction of the fiber. Therefore, even if such a multi-core fiber is installed non-linearly and a section where the light propagation constants of the spirally arranged core and other cores coincide with each other occurs, Since the positional relationship between this specific core and other cores changes, the light propagation constant between these cores does not match in a long section. Therefore, even when the multi-core fiber is installed in a curved state, it is possible to further suppress the deterioration of the crosstalk between adjacent cores. In the above-described pair of adjacent cores, one core may be arranged in a spiral shape, and the other core may be arranged in parallel with or parallel to the central axis of the clad. May be arranged.

  In this case, the pitch of the spirally arranged cores rotating around the central axis of the clad may be a constant interval or an irregular interval.

  In addition, “pitch” in the present specification means the number of rotations of the spiral core per unit length in the longitudinal direction of the fiber. Thus, for example, if the spiral core makes one revolution around the central axis of the cladding per meter of fiber, the pitch of this core is 1 turn / m.

  In this case, it is preferable that the pitch interval of the cores arranged in a spiral shape rotates around the central axis of the cladding is a non-integer multiple of the interval at which the difference in mode field diameter changes. .

  That is, the period of change in the mode field diameter of the core is a non-integer multiple of the period of twisting of the core. In this way, when the multicore fibers are arranged non-linearly by making the pitch interval at which the core rotates helically and the interval at which the difference in mode field diameter changes, the spiral core is By rotating, the light propagation constant between a pair of cores does not match in a long section, and the difference in mode field diameter changes, so that the light propagation constant between a pair of cores is a long section. Inconsistent effects occur in each individual location. Therefore, it is possible to prevent the light propagation constants between a pair of cores from matching in a long section. For this reason, it can further suppress that the crosstalk of specific cores deteriorates, and the dispersion | variation in crosstalk can be converged.

  It is preferable that the spiral core is arranged so as to repeat a clockwise rotation and a counterclockwise rotation around the central axis of the cladding.

  Since the core is spirally arranged so as to repeat the right rotation and the left rotation, the fiber can be manufactured while twisting the right rotation and the left rotation alternately, so that the core rotates in one direction. Compared with an optical fiber having a structure having only an optical fiber, it is possible to provide a multi-core fiber having an improved spiral core and an inexpensive spiral core.

  In this specification, the number of rotations in the case where the core rotates spirally by repeating clockwise and counterclockwise rotation around the central axis of the clad is the number of clockwise rotations, and This means the number of rotations in the case of adding both rotations of the left rotation as positive. Thus, for example, the spiral core rotates 0.5 turns to the right around the central axis of the cladding in the 0.5 m section of the fiber, and this time 0.5 turns to the left in the subsequent 0.5 m section. In this case, the number of rotations per 1 m is 0.5 + 0.5 = 1 rotation, and the core pitch in this case is 1 turn / m.

  As described above, according to the present invention, there is provided a multi-core fiber that can suppress deterioration of crosstalk between specific cores even when installed in a non-linear manner.

It is a figure which shows the mode of the multi-core fiber which concerns on 1st Embodiment of this invention. It is a figure which shows the mode of a multi-core fiber in case the multi-core fiber of FIG. 1 is bent. It is a figure which shows the mode of the multi-core fiber which concerns on 2nd Embodiment of this invention. It is a figure which shows the modification of a multi-core fiber. It is a figure which shows the modification of a multi-core fiber. It is a figure which shows the bending direction of a multi-core fiber. It is a figure which shows the mode of the change of the crosstalk when the bending diameter of the multi-core fiber of the comparative example 1 is changed. It is a figure which shows the maximum value of the crosstalk at the time of changing the bending radius of the multi-core fiber of the comparative example 1, the minimum value, and an average value. It is a figure which shows the mode of the change of the crosstalk when the bending diameter of the multi-core fiber of the reference example 1 is changed. It is a figure which shows the maximum value of crosstalk when the bending radius of the multi-core fiber of the reference example 1 is changed, the minimum value, and the average value. It is a figure which shows the mode of the change of the crosstalk when the bending diameter of the multi-core fiber of the reference example 2 is changed. It is a figure which shows the maximum value of crosstalk when the bending radius of the multi-core fiber of the reference example 2 is changed, the minimum value, and the average value. It is a figure which shows the mode of the change of the crosstalk when the bending diameter of the multi-core fiber of Example 1 is changed. It is a figure which shows the maximum value of crosstalk when the bending radius of the multi-core fiber of Example 1 is changed, the minimum value, and the average value. Example 2 It is a figure which shows the mode of the change of the crosstalk when the bending diameter of the multi-core fiber of the comparative example 1 is changed. It is a figure which shows the maximum value of crosstalk when the bending radius of the multi-core fiber of Example 2 is changed, the minimum value, and the average value. It is a figure which shows the mode of the change of the crosstalk when the bending diameter of the multi-core fiber of Example 3 is changed. It is a figure which shows the maximum value of crosstalk when the bending radius of the multi-core fiber of Example 3 is changed, the minimum value, and the average value. It is a figure which shows the mode of the change of the crosstalk when the bending diameter of the multi-core fiber of Example 4 is changed. It is a figure which shows the maximum value of crosstalk when the bending radius of the multi-core fiber of Example 4 is changed, the minimum value, and the average value. It is a figure which shows the mode of the change of the crosstalk when changing the bending diameter of the multi-core fiber of Example 5. FIG. It is a figure which shows the maximum value of crosstalk when the bending radius of the multi-core fiber of Example 5 is changed, the minimum value, and the average value. It is a figure which shows the mode of the change of the crosstalk when the bending diameter of the multi-core fiber of Example 6 is changed. It is a figure which shows the maximum value of crosstalk when the bending radius of the multi-core fiber of Example 6 is changed, the minimum value, and the average value. It is a figure which shows the mode of the change of the crosstalk when the bending diameter of the multi-core fiber of Example 7 is changed. It is a figure which shows the maximum value of crosstalk when the bending radius of the multi-core fiber of Example 7 is changed, the minimum value, and the average value. It is a figure which shows the mode of the change of the crosstalk when the bending diameter of the multi-core fiber of Example 8 is changed.

  Hereinafter, preferred embodiments of a multi-core fiber according to the present invention will be described in detail with reference to the drawings. For ease of understanding, the scale described in each drawing may be different from the scale described in the following description.

(First embodiment)
FIG. 1 is a diagram showing a state of a multi-core fiber according to an embodiment of the present invention. Specifically, FIG. 1 (A) is a diagram showing a structure in a cross section perpendicular to the longitudinal direction of the multi-core fiber, FIG. 1B is a diagram showing a refractive index distribution along the line BB passing through the center of the multi-core fiber 1 of FIG. FIG. 1B shows the refractive index distribution when the multi-core fiber is linear.

  As shown in FIG. 1A, the multi-core fiber 1 of the present embodiment surrounds the plurality of cores 11 and 12, and the plurality of cores 11 and 12 as a whole, and fills between the cores 11 and 12, A clad 30 surrounding the outer peripheral surface of each core 11, 12, an inner protective layer 31 covering the outer peripheral surface of the clad 30, and an outer protective layer 32 covering the outer peripheral surface of the inner protective layer 31 are provided.

  In the present embodiment, one core 11 is arranged along the central axis of the clad 30, and a plurality of cores 12 are arranged around the one core 11 at equal intervals. The plurality of cores 12 arranged around the core 11 are arranged in parallel to the central axis of the clad 30. In the present embodiment, as shown in FIG. 1A, the number of cores is seven as a whole, one core 11 is arranged at the center, and the other six cores 12 are arranged on the outer periphery of the cladding 30. Are arranged along. Thus, the central core 11 and the respective outer peripheral cores 12 are arranged in a triangular lattice pattern. Therefore, the distances between the centers of the cores 11 and 12 are equal to each other. The plurality of cores 11 and 12 arranged in this way are symmetric with respect to the central axis of the clad 30. That is, when the multi-core fiber 1 is rotated by a predetermined angle around the central axis of the clad 30, the position of each core 12 on the outer peripheral side after the rotation is the same as the position of the other core 12 on the outer peripheral side before the rotation. Become. Further, the core 11 arranged at the center does not move even when the multi-core fiber 1 is rotated around the central axis. As described above, by arranging the cores 11 and 12 at positions that are symmetric with respect to the central axis of the clad 30, the optical properties due to the arrangement of the cores 11 and 12 can be made uniform.

  The size of each member constituting the multi-core fiber 1 is not particularly limited, but the diameter of the clad 30 is, for example, 140 μm, and the outer diameter of the inner protective layer 31 is, for example, 205 μm. The outer diameter of the outer protective layer 32 is, for example, 265 μm. Further, the distance between the centers of the cores 11 and 12 is not particularly limited, but is, for example, 39.2 μm.

In the present embodiment, the diameters d ₁ and d _{2 of the} adjacent cores 11 and 12 are made different from each other. The diameter d _{1 of the} core 11 disposed at the center is, for example, 8.0 μm, and the diameter d _{2 of the} core 12 disposed on the outer peripheral side is, for example, relative to the diameter of the core 11 disposed at the center. The cores 12 that are different from each other by −5% to 5% and arranged on the outer peripheral sides adjacent to each other have, for example, diameters d ₂ that are different from each other by −5% to 5%. Even if the diameters d ₁ and d _{2 of the} cores 11 and 12 adjacent to each other are slightly different from each other, the diameter of each of the cores 11 and 12 is almost the same as the light propagating through the cores 11 and 12. Although the optical characteristics are substantially the same, the diameters d ₁ and d _{2 of the} adjacent cores 11 and 12 are physically slightly different from each other, so that the crosstalk of the adjacent cores 11 and 12 is slightly different. Can be suppressed. Thus, it is preferable that the difference between the diameters of the adjacent cores 11 and 12 is within 10% of the diameter from the viewpoint of equalizing the optical characteristics of the respective cores while suppressing crosstalk. Further, even if the cores adjacent to each other have the same diameter, crosstalk can be suppressed by the effect of characteristic fluctuation in the longitudinal direction of the cores described later.

Further, the diameter d _{1 of the} core 11 disposed at the center of the clad 30 does not change with respect to the longitudinal direction of the multi-core fiber 1 and is constant. On the other hand, the diameter d ₂ of each core 12 arranged on the outer peripheral side of the clad 30 changes along the longitudinal direction of the multicore fiber 1. Therefore, the difference in diameter (d ₁ -d ₂ ) between the core 11 arranged at the center and each core 12 arranged on the outer peripheral side so as to be adjacent to the core 11 arranged at the center is longitudinal. It is changing along the direction. For example, the diameter _{d 2} of each of the core 12, the core 11 along the longitudinal direction, and varies between 7.6Myuemu～7.9Myuemu, a core 11 disposed in the center, which is arranged in the center The diameter difference (d ₁ -d ₂ ) between each core 12 arranged on the outer peripheral side so as to be adjacent to each other varies in the range of 0.1 μm to 0.4 μm along the longitudinal direction. . The interval of this change is not particularly limited, but it is sufficient if there is a change of one cycle or more in the range of the length used as the cable. When the cable is used with a span of several tens of kilometers like a submarine cable, a stable effect can be obtained at an interval of 1 time / km. Moreover, since it is assumed that the cable used in the access system is used in a span of several tens of meters to several hundreds of meters, it is stable if there is a change in the order of 0.1 times / m to 1 time / m. An effect is obtained. Moreover, it is desirable to give a period of about 1 time / m to 10 times / m for wiring applications in the order of several meters such as wiring in a station. For this reason, as a period of a change, it is desirable that it is the range of 0.001 times / m-10 times / m.

Furthermore diameter d ₂ of the outer peripheral side of the core 12 along the longitudinal direction, rather than changes in the same manner with respect to each other with the core 12 with each other, have changed different from each other. For example, if the diameter of the specific core 12 on the outer peripheral side changes at a predetermined cycle along the longitudinal direction, the diameters of the other cores 12 adjacent to the core 12 are along the longitudinal direction. The specific core 12 changes in a period different from the period in which the diameter of the core 12 changes.

Further, as shown in FIG. 1B, in this embodiment, the refractive index n of the core 11 disposed at the center is n ₁ and the refractive index n of each core 12 disposed on the outer peripheral side. ₂ is higher than the refractive index n ₃ of the clad 30, and the refractive index n ₂ of each core 12 disposed on the outer peripheral side is such that the refractive index of the core 11 disposed at the center is n _It is higher than ₁ . The refractive indexes of the adjacent cores 11 and 12 are approximately −10% to 10% in terms of the relative refractive index difference with respect to the clad 30, and the optical characteristics of the respective cores are equivalent while suppressing crosstalk. From the viewpoint of making it.

Further, the refractive index n ₁ of the core 11 disposed at the center of the clad 30 does not vary with respect to the longitudinal direction of the multi-core fiber 1 and is constant. On the other hand, the refractive index n ₂ of each core 12 arranged on the outer peripheral side of the clad 30 changes along the longitudinal direction of the multi-core fiber 1. Accordingly, the refractive index of the core 11 disposed in the center and each core 12 disposed on the outer peripheral side so as to be adjacent to the core 11 disposed in the center is not particularly limited. The difference in refractive index varies along the longitudinal direction within a range of about −10% to 10% of the relative refractive index difference of the core 11. The change interval is sufficient if there is a change of one cycle or more in the length range used as the cable. When the cable is used with a span of several tens of kilometers like a submarine cable, a stable effect can be obtained at an interval of 1 time / km. Moreover, since it is assumed that the cable used in the access system is used in a span of several tens of meters to several hundreds of meters, it is stable if there is a change in the order of 0.1 times / m to 1 time / m. An effect is obtained. Moreover, it is desirable to give a period of about 1 time / m to 10 times / m for wiring applications in the order of several meters such as wiring in a station. For this reason, as a period of a change, it is desirable that it is the range of 0.001 times / m-10 times / m.

Moreover the refractive index n ₂ of the outer peripheral side of the core 12 along the longitudinal direction, rather than changes in the same manner with respect to each other with the core 12 with each other, have changed different from each other. For example, if the refractive index of a specific core 12 on the outer peripheral side changes in a predetermined cycle along the longitudinal direction, the refractive index of another core 12 adjacent to the core 12 is along the longitudinal direction. Thus, the refractive index of the specific core 12 changes at a different period from the period at which it changes. Therefore, in the core 12 on the outer peripheral side, the cores 12 adjacent to each other also have places having the same refractive index along the longitudinal direction, but have different refractive indexes in many places.

The refractive index n ₂ of the outer peripheral side of the core 12 may be varied at fixed intervals, it may vary at irregular intervals.

  In FIG. 1B, the refractive indexes of the inner protective layer 31 and the outer protective layer 32 are omitted.

The propagation constant of light propagating through the core of the optical fiber is defined by the relative refractive index difference Δ with respect to the refractive index of the cladding based on the refractive index of the core and the diameter of the core. Here, when the i = 1, 2, the relative refractive index difference delta _i with respect to the cladding 30 of the core 11, 12 having a refractive index of _{n i} is defined by the following equation. N ₃ represents the refractive index of the cladding 30.

As described above, since the refractive index n ₁ of the core 11 is constant with respect to the longitudinal direction of the multi-core fiber 1, the relative refractive index difference Δ _{1 with} respect to the cladding 30 of the core 11 varies along the longitudinal direction. For example, it is set to 0.40%. On the other hand, since the refractive index n ₂ of each core 12 changes along the longitudinal direction of the multi-core fiber 1 as described above, the relative refractive index difference Δ _{2 with} respect to the cladding 30 of each core 12 is equal to the multi-core fiber. 1 along the longitudinal direction, for example, between 0.38% and 0.42% along the longitudinal direction. Therefore, the cores 11 and 12 adjacent to each other have a difference in relative refractive index difference (Δ ₂ −Δ ₁ ) with respect to the cladding that varies along the longitudinal direction. For example, as described above, the relative refractive index difference between the cores 11. When Δ ₁ is 0.40% and the relative refractive index difference Δ _{2 of} the core 12 changes between 0.38% and 0.42% along the longitudinal direction, the adjacent cores 11 12, the difference in relative refractive index difference (Δ ₂ −Δ ₁ ) varies from −0.02% to 0.02% along the longitudinal direction. When the relative refractive index difference of the core 11 is used as a reference, the amount of change in the relative refractive index difference is -5% to 5%.

Since the core 11 has a constant diameter d ₁ and a relative refractive index difference Δ ₁ with respect to the cladding 30 as described above, the mode field diameter MFD ₁ of light propagating through the core 11 is constant. On the other hand, since each core 12 adjacent to the core 11 as described above has a diameter d ₂ and a relative refractive index difference Δ ₂ with respect to the clad 30 change along the longitudinal direction, a mode field of light propagating through the core 12. The diameter MFD ₂ varies along the longitudinal direction of the multi-core fiber 1. Therefore, the difference in mode field diameter (MFD ₁ -MFD ₂ ) of the light propagating through the core 11 and the core 12 varies along the longitudinal direction of the multicore fiber 1.

For example, as described above, when the core 11 has a diameter d ₁ of 8.0 μm and a relative refractive index difference Δ ₁ of 0.40%, when light having a wavelength of 1310 nm propagates through the core 11, The mode field diameter MFD ₁ of light is 8.5 μm. When light having a wavelength of 1550 nm propagates through the core 11, the mode field diameter MFD ₁ of light is 9.6 μm. In addition, for example, as described above, each core 12 has a diameter d ₂ that varies between 7.6 μm and 7.9 μm along the longitudinal direction, and the relative refractive index difference Δ ₂ is equal to the longitudinal direction. If varies between 0.38% ～0.42% along, when light of wavelength 1310nm core 12 propagates, the mode field diameter MFD ₂ of this light, 8.2Myuemu～8.6Myuemu When the light having a wavelength of 1550 nm propagates through the core 12, the mode field diameter MFD ₂ of the light changes between 9.3 μm and 9.7 μm. In these cases, when light having a wavelength of 1310 nm propagates to the respective cores 11 and 12 of the multi-core fiber 1, the mode field diameter MFD ₁ of the light propagating through the core 11 and the mode field diameter MFD of the light propagating through the respective cores. the difference between the _{2 (MFD 2} -MFD ₁₎ along the longitudinal direction, varies between -0.3Myuemu～0.1Myuemu. Similarly, when light having a wavelength of 1550 nm propagates to each of the cores 11 and 12, the difference between the mode field diameter MFD ₁ of the light propagating through the core 11 and the mode field diameter MFD ₂ of the light propagating through the respective core is Along the longitudinal direction, it varies between −0.3 μm and 0.1 μm.

The difference between the mode field diameter MFD ₁ of the light propagating through the core 11 and the mode field diameter MFD ₂ of the light propagating through each core is such that the refractive index n ₂ of the core 12 on the outer peripheral side changes at a constant interval. If the refractive index n ₂ of the core 12 on the outer peripheral side changes at an irregular interval, it changes at an irregular interval.

In addition, in the case where the diameter d ₂ and the refractive index n ₂ of the respective cores 12 on the outer peripheral side do not change in the same manner between the respective cores 12 along the longitudinal direction, but are different from each other. The mode field diameter MFD ₂ of the light propagating through the respective cores 12 on the outer peripheral side changes differently.

  In such a multi-core fiber, examples of the material of the cores 11 and 12 include silica glass to which a dopant such as germanium for increasing the refractive index is added. As a material of the clad 30, any dopant is added. A silica glass that is not present can be mentioned. Furthermore, examples of the material for the inner protective layer 31 and the outer protective layer 32 include an ultraviolet curable resin. When the material of the core 12 is silica glass to which a dopant such as germanium is added, the amount of the dopant added to the core 12 varies along the longitudinal direction of the multi-core fiber 1.

  In the present embodiment, each of the cores 11 and 12 propagates light in a single mode.

  Next, the operation of the multicore fiber 1 of the present embodiment will be described.

  The change in the refractive index when the optical fiber is bent can be obtained, for example, by an equivalent refractive index method. That is, the refractive index when the optical fiber is bent can be obtained equivalently by multiplying the refractive index distribution of the linear optical fiber by (1+ (r / R) cos θ). Here, (r, θ) is a polar coordinate in a plane perpendicular to the longitudinal direction of the optical fiber, θ = 0 is an outer direction in a state where the optical fiber is bent, and r is from the center of the optical fiber. Distance. R represents the radius of curvature of the optical fiber.

  Here, it is considered that the multi-core fiber 1 is bent in the direction along the line BB in FIG. In this case, the BB line is a line including θ = 0. FIG. 2 is a diagram illustrating a state of the multicore fiber 1 when the multicore fiber 1 is bent as described above. Specifically, FIG. 2A is a diagram showing a structure in a cross section perpendicular to the longitudinal direction of the multi-core fiber 1, and FIG. 2B is an equivalent refraction taken along line BB in FIG. It is a figure which shows distribution of a rate. In FIG. 2B, for easy understanding, the refractive index distribution when the multi-core fiber 1 shown in FIG. 1B is linear is indicated by a broken line. As shown in FIG. 2, when the multi-core fiber 1 is bent along the line BB, the center of the multi-core fiber 1 becomes the origin, the direction of the bend on the line BB becomes θ = 0, and BB The direction inside the bend on the line is θ = 180 degrees. In this case, from the above equation, the equivalent refractive index of the outer core or cladding of the multicore fiber 1 is increased, and the equivalent refractive index of the inner core or cladding of the multicore fiber 1 is decreased.

  Accordingly, when the outer core 12 has a higher refractive index than the center core 11 as described above, the effective refractive index of the center core 11 is determined by setting the bending radius of curvature of the multi-core fiber 1 to a specific size. Of the cores 12 on the outer peripheral side, the effective refractive index of the core located on the bending inner side of the multi-core fiber 1 may coincide. Although not specifically described with reference to the drawings, unlike the above description, when the central core 11 has a higher refractive index than the outer core 12, the effective refractive index of the central core 11 and Of the cores 12 on the outer peripheral side, the effective refractive index of the core located outside the bend of the multi-core fiber 1 may coincide with the effective refractive index of the specific core 12.

  In the above description, for the sake of easy understanding, the effective refractive index has been described in the same way as the refractive index of the actual glass. However, in practice, the spread in the cross section of the guided light is included. The parameter to be determined. The effective refractive index has a one-to-one relationship with the propagation constant.

  As described above, since the effective refractive index of the central core 11 and the effective refractive index of the outer core 12 are changed by bending, the diameters of the central core 11 and the outer core 12 are set as in the present embodiment. Even when they are changed, the propagation constants of a specific core 12 in the central core 11 and the outer core 12 may coincide.

However, in the multi-core fiber 1 of the present embodiment, as described above, the difference between the mode field diameter MFD ₁ of the light propagating through the central core 11 and the mode field diameter MFD ₂ of the light propagating through the outer core 12. Varies along the longitudinal direction of the multi-core fiber 1. That is, the difference between the propagation constant of the central core 11 and the propagation constant of the outer core 12 changes along the longitudinal direction. For this reason, even when the propagation constants of the specific core 12 in the central core 11 and the core 12 on the outer peripheral side coincide with each other by bending the multi-core fiber 1 as described above, such a state is maintained in the multi-core fiber 1. It is not maintained in the long section. Therefore, according to the multi-core fiber 1 of the present embodiment, even when installed in a spiral or curved state, the crosstalk between the core 11 disposed in the center adjacent to each other and the core 12 on the outer peripheral side is deteriorated. Can be suppressed.

Further, the difference between the mode field diameter MFD ₁ of the light propagating through the core 11 and the mode field diameter MFD ₂ of the light propagating through each core may change at a constant interval, or at an irregular interval. It may change. When the difference between the mode field diameter MFD ₁ of the light propagating through the core 11 and the mode field diameter MFD ₂ of the light propagating through the respective cores changes at irregular intervals, the multi-core fiber 1 is spirally wound. It is possible to prevent the crosstalk from deteriorating regularly when bent and installed in a regular shape such as being used.

Furthermore, when the mode field diameter MFD ₂ of the light propagating through the respective cores 12 on the outer peripheral side changes differently, it is possible to prevent the crosstalk between the respective cores 12 on the outer peripheral side from deteriorating. it can.

In the present embodiment, the diameter d ₁ of the central core 11 is constant, the diameter d ₂ of each core 12 on the outer peripheral side changes along the longitudinal direction of the multi-core fiber 1, and further, the central core a refractive index n ₁ of 11 is constant, that the refractive index n ₂ of each of the outer peripheral side of the core 12 varies along the longitudinal direction of the multicore fiber 1, the mode field diameter MFD of the light propagating in the core 11 of the central ₁ and the mode field diameter MFD ₂ of the light propagating through the outer core 12 change along the longitudinal direction of the multi-core fiber 1. However, the present embodiment is not limited to such a configuration. For example, as long as the difference between the mode field diameter MFD ₁ and the mode field diameter MFD ₂ changes along the longitudinal direction of the multicore fiber 1, The diameter d ₁ and the refractive index n ₁ may change along the longitudinal direction of the multi-core fiber 1, and the mode field diameter MFD _{1 of} light propagating through the core 11 may change along the longitudinal direction of the multi-core fiber 1. Further, at least one parameter of the diameter d ₁ and the refractive index n _{1 of} the core 11 and the diameter d ₂ and the refractive index n ₂ of each core 12 varies along the longitudinal direction of the multi-core fiber 1. Thus, the difference between the mode field diameter MFD ₁ and the mode field diameter MFD ₂ may change along the longitudinal direction of the multi-core fiber 1.

  Such a multi-core fiber 1 can be manufactured as follows.

  First, a core glass member to be the core 11 and the core 12 and a clad glass member to be the clad 30 or a part of the clad 30 are prepared. The core glass member to be the core 11 has a constant diameter and a constant refractive index in the longitudinal direction, and the core glass member to be the core 12 has a diameter and a refractive index that change in the longitudinal direction. . The diameter and the refractive index of the glass member for the core are the changes in the diameter and the refractive index of the core 12 on the outer peripheral side of the multicore fiber 1 when the optical fiber preform is spun into the multicore fiber 1 as described later. It is changed according to the change of Next, by arranging the core glass member to be the prepared core 11 and core 12 in the clad glass member and collapsing, the arrangement in the cross section is shown in FIG. A fiber preform substantially similar to the clad 30 is produced. The fiber base material is heated, melted and spun to form a multi-core fiber 1. Alternatively, the prepared core glass member serving as the core 11 and the core 12 may be disposed in the clad glass member, and these glass members may be spun while being core-wrapped.

(Second Embodiment)
Next, a second embodiment of the present invention will be described in detail with reference to FIG. In addition, about the component which is the same as that of 1st Embodiment, or an equivalent component, the overlapping description is abbreviate | omitted except the case where it attaches | subjects the same referential mark and demonstrates especially. FIG. 3 is a diagram showing a multi-core fiber 2 according to the second embodiment of the present invention. In FIG. 3, for easy understanding, the clad 30 is indicated by a broken line, the inner protective layer 31 and the outer protective layer 32 are omitted, and the scale in the longitudinal direction and the radial direction of the multicore fiber 1 is the same as that of the actual multicore fiber. It is changed and described.

  In the present embodiment, as shown in FIG. 2, the core 11 at the center is arranged along the central axis of the cladding 30 as in the first embodiment, but the core 12 on the outer peripheral side is the cladding 30. Is different from the multi-core fiber 1 of the first embodiment in that it is arranged in a spiral shape so as to rotate in the same direction. Note that the diameter and refractive index of the central core 11 and the outer core 12 in the multi-core fiber 2 of the present embodiment are the same as the diameter and refractive index of the central core 11 and outer core 12 of the multi-core fiber 1 in the first embodiment. The rate is similar. As shown in FIG. 2, the core 12 on the outer peripheral side is arranged so as to rotate clockwise along the direction of the arrow A. That is, in the multi-core fiber 2, the core 12 is solidified in a state of being formed in a spiral shape as described above, and the core 12 is permanently twisted.

  In FIG. 2, the spiral outer core 12 has the same pitch around the central axis of the clad 30. That is, the number of rotations of the core 12 per unit length of the multicore fiber 2 is constant in any section in the longitudinal direction of the multicore fiber 2.

  However, in the multi-core fiber 2, the spiral core 12 may have a section in which the pitch that rotates around the central axis of the cladding 30 changes. That is, for example, the rotation pitch of the spiral core 12 may be 1 turn / m in a predetermined section, and may be 0.5 turns / m in another section. You may make it rotate with another pitch in the area. Furthermore, the pitch of the spiral core 12 may always change.

  Although not particularly illustrated, the spiral core 12 may repeat the clockwise rotation and the counterclockwise rotation around the central axis of the clad 30. That is, the spiral core 12 may be arranged so as to rotate right in a predetermined section, and may be arranged to rotate left in a section adjacent to the predetermined section. Further, in this case, the core 12 may be arranged in parallel with the core 11 without rotating around the axial center of the clad 30 between the section in which the core 12 rotates to the right and the section in which it rotates to the left. Furthermore, the length of each section in which the spiral core 12 rotates to the right and the length of each section to rotate to the left may not be constant.

  The core 12 preferably rotates around the central axis of the clad 30 at an average pitch of 1 turn / m or more, and more preferably at an average pitch of 4 turns / m or more. As described above, when the core 12 is rotating in the right direction and the left direction, this pitch is obtained by adding both the right rotation number and the left rotation number as positive. Calculate the pitch. For example, the spiral core 12 rotates 0.5 times around the central axis of the clad 30 in the 0.5 m section of the multi-core fiber 2 to the right, and in the subsequent 0.5 m section, this time 0. In the case of 5 rotations, the number of rotations per 1 m is 0.5 + 0.5 = 1, and the rotation pitch of the core 12 in this case is 1 rotation / m.

Further, in the multi-core fiber 2, the pitch interval at which the outer peripheral core 12 arranged in a spiral rotates around the central axis of the clad 30 is such that the mode field diameter MFD ₁ of the core 11 and the core 12 are Although it may be an integer multiple of the interval at which the difference in mode field diameter MFD ₂ changes, it is preferably a non-integer multiple.

  Next, the operation of the multi-core fiber 2 of the present embodiment will be described.

  Here, the description will be given with reference to FIG. 2 again. In FIG. 2, the multi-core fiber 2 is shown in parentheses. As in the first embodiment, it is considered that the multicore fiber 2 is bent in a direction along the line BB passing through the center of the multicore fiber 2 in FIG. As shown in FIG. 3, when the multi-core fiber 2 is bent along the line BB, the center of the multi-core fiber 2 becomes the origin, the direction of the bend on the line BB becomes θ = 0, and BB The inside of the bend on the line is θ = 180 degrees. In this case, as described in the first embodiment, the equivalent refractive index of the core or cladding inside the bend is lowered.

  Accordingly, as described in the first embodiment, the effective refractive index of the central core 11 may coincide with the effective refractive index of the specific core 12 on the outer peripheral side.

However, in the multi-core fiber 2 of the present embodiment, the position of the core 12 arranged in a spiral shape in a cross section perpendicular to the longitudinal direction of the multi-core fiber 2 changes along the longitudinal direction of the multi-core fiber 2. Accordingly, the multi-core fiber is installed with being bent at a specific radius of curvature in this way, and as shown in FIG. 2, the refractive index of the center core 11 and the inner side of the outer periphery side core 12 in the bending direction. Even if there is a section where the effective refractive index of one core 12 located at the same position occurs, a specific core on the outer peripheral side whose effective refractive index matches that of the central core 11 along the longitudinal direction of the multi-core fiber 2 Since the positional relationship between the core 12 and the central core 11 changes, the effective refractive indexes of the specific core 12 and the central core 11 do not match in a long section. Similarly to the multi-core fiber 1 of the first embodiment, the difference between the mode field diameter MFD _{1 of} light propagating through the central core 11 and the mode field diameter MFD _{2 of} light propagating through the outer core 12 is the multi-core. Since it changes along the longitudinal direction of the fiber 1, the state in which the propagation constants of the specific core 12 in the central core 11 and the outer core 12 coincide with each other is not maintained in a long section. Therefore, according to the multicore fiber 2 of the present embodiment, it is possible to further suppress the deterioration of the crosstalk between the specific core 12 and the central core 11 than the multicore fiber 1 of the first embodiment. As described above, according to the multi-core fiber 2 of the present embodiment, it is possible to suppress the deterioration of the crosstalk between specific cores even when installed in a curved state. Can be suppressed.

Further, as described above, the pitch interval at which the outer peripheral core 12 arranged in a spiral rotates around the central axis of the clad 30 is such that the mode field diameter MFD ₁ of the core 11 and the mode of the core 12 are the same. It is preferably a non-integer multiple of the interval at which the difference in field diameter MFD ₂ changes. In this case, when the multi-core fiber 2 is arranged in a non-linear manner by setting the pitch at which the core 12 rotates and the interval at which the difference between the mode field diameter MFD ₁ and the mode field diameter MFD ₂ changes. When the spiral core 12 rotates, the propagation constant between the center core 11 and the outer core 12 does not match in a long section, and the difference between the mode field diameter MFD ₁ and the mode field diameter MFD ₂ As a result of the change, the effect that the light propagation constants of the core 11 and the core 12 do not match in a long section occurs in each individual place. Therefore, it is possible to prevent the light propagation constants of the central core 11 and the outer peripheral core 12 from matching in a shorter section. For this reason, it is possible to further suppress the deterioration of crosstalk between specific cores, and to converge variations in crosstalk.

  In the above description, the case where the effective refractive index of the specific core 12 on the outer peripheral side coincides with the effective refractive index of the central core 11 has been described. Even when the rates match, the effective refractive indexes of the respective cores 12 do not match in the long section. Therefore, it is possible to prevent the crosstalk between the cores 12 from deteriorating.

  Such a multi-core fiber 2 is manufactured as follows.

  First, a fiber preform is produced in the same manner as in the first embodiment. In the same manner as in the first embodiment, the fiber base material is heated and melted and spun to form a multi-core fiber, and the multi-core fiber is covered with the inner protective layer 31 and the outer protective layer 32. At this time, the multi-core fiber immediately after spinning is rotated around the axis. By rotating the multi-core fiber around the axis in this way, the rotational force at the axis center is transmitted to the semi-finished product of the multi-core fiber before being spun from the fiber preform and solidified, as shown in FIG. Further, the outer peripheral core 12 is formed in a spiral shape so as to rotate around the central axis of the clad 30. In other words, permanent twist is imparted to the outer core 12.

  By manufacturing multi-core fibers with spirally arranged cores in this way, for example, fiber tapes and cables, laying operations, and suppression of continuous and permanent crosstalk deterioration under actual conditions are suppressed. can do. Further, from the viewpoint of mechanical reliability, it is desirable to previously arrange the cores of the multicore fiber in a spiral shape in this way.

  In order to rotate the multi-core fiber immediately after spinning around the axis, for example, the turn pulley with which the spun multi-core fiber first contacts may be rotated around the axis of the multi-core fiber. By rotating the turn pulley in one direction, the core 12 has a shape that rotates around the central axis of the clad 30 in one direction. Further, by rotating the turn pulley without rotating in one direction, the core 12 is rotated around the central axis of the clad 30 alternately in the right direction and the left direction.

  As mentioned above, although this invention was demonstrated to the example for embodiment, this invention is not limited to these.

  For example, in the above embodiment, the number of cores is seven, but the number of cores is not particularly limited as long as there are a plurality of cores. Further, for example, in FIGS. 1 and 3, the outer peripheral side core 12 may be one, or may be seven or more. Further, the core 11 along the central axis of the clad 30 is not an essential requirement as long as the number of the entire core is plural. For example, three cores may be arranged in the clad 30 so as to form an equilateral triangle, and the mode field diameter MFD of the light propagating through the adjacent cores may change so as to be different from each other along the longitudinal direction. Accordingly, these cores may be arranged spirally with respect to each other. Further, in the multi-core fiber of the present invention, the plurality of cores do not need to be symmetric with respect to the central axis of the clad 30. For example, the plurality of cores are arranged in a grid with 5 rows × 4 columns. It may be a thing.

  As described above, the multi-core fiber of the present invention includes a plurality of cores, and among these cores, the difference in the mode field diameter of each light propagating through at least one pair of adjacent cores varies along the longitudinal direction. As long as it does, it can change suitably according to a use application. FIG. 4 is a view showing a modification of such a multi-core fiber.

  FIG. 4A shows a multi-core fiber in which one core 11 is arranged at the center, four cores 12 are arranged so as to surround the core, and the center-to-center distance between the core 11 and the core 12 is equal. FIG. 4B, six cores 13 are provided on the outer peripheral side of the outer peripheral core 12 in the multi-core fiber 1 of the above-described embodiment shown in FIG. 1A, and each of the cores 11 to 13 is triangular. It is a figure which shows the example arranged in the grid | lattice form and arrange | positioned at the star shape as a whole. FIG. 4C shows a structure in which twelve cores 13 are provided at equal intervals on the outer peripheral side of the core 12 on the outer peripheral side in the multi-core fiber 1 of the embodiment shown in FIG. It is a figure which shows the example by which the core of this book is arranged in the shape of a triangular lattice, and is closely packed.

  FIG. 5A is a diagram showing an example in which the respective cores 11 and 12 in the multi-core fiber 1 of the embodiment shown in FIG. 1A are surrounded by a plurality of holes 14. In this case, by the action of the respective holes 14, light is more strongly confined in the cores 11 and 12, and crosstalk between adjacent cores can be further suppressed. Further, FIG. 5B shows that each of the cores 11 and 12 in the multi-core fiber 1 of the embodiment shown in FIG. 1A is surrounded by a first clad 15 having a refractive index similar to that of the clad 30. 3 is a diagram showing an example in which a first cladding 15 is surrounded by a second cladding 16 having a refractive index lower than that of a cladding 30. FIG. That is, the core element formed by the core 11 (12), the first clad 15 and the second clad 16 is a so-called trench type, so that the light is more strongly confined in the cores 11 and 12, and the adjacent cores Crosstalk can be further suppressed.

  Hereinafter, the content of the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

(Comparative Example 1)
A simulation was performed assuming a multi-core fiber in which one core is arranged at the center, six cores are arranged on the outer peripheral side so as to surround the core, and the distance between the centers of the cores is equal. In this multi-core fiber, each core is assumed to be aligned or parallel to the central axis of the multi-core fiber.

  In this multi-core fiber, the diameter of the center core is 7.734 μm, the diameter of the outer core is 7.734 μm, the center-to-center distance L of each core is 35.0 μm, The outer diameter was 140 μm. Further, the relative refractive index difference Δ with respect to the cladding of the central core was set to 0.40%, and the relative refractive index differences with respect to the cladding of the outer peripheral core were made equal to each other and higher than the central core by 0.01%.

  Changes in crosstalk between the core at the center and the core on the outer peripheral side when the multi-core fiber was gradually bent by propagating light having a wavelength of 1.55 μm to each core of the multi-core fiber were examined. FIG. 6 is a diagram illustrating a bending direction of the multi-core fiber. In this example, the multi-core fiber is bent along the X axis. Specifically, first, as shown in FIG. 6, the multicore fiber is rotated so that one core on the outer peripheral side of the multicore fiber rotates clockwise by about 15 degrees from the Y axis perpendicular to the X axis that is the bending direction. Was rotated around the axis. Then, the portion located on the origin of the multi-core fiber is fixed, and a part of the multi-core fiber extending in the longitudinal direction perpendicular to the X-axis and Y-axis is bent in the negative direction of the X-axis with reference to the fixed portion. I let you. The result is shown in FIG. In FIG. 7, the wavy line indicates the crosstalk value of each core, and the solid line indicates the average value. Furthermore, the multicore fiber was rotated about the axis while maintaining the bending direction of the multicore fiber, and the maximum value, the minimum value, and the average value of the crosstalk were examined at each bending diameter. The result is shown in FIG. In FIG. 8, the wavy lines indicate the maximum value and the minimum value, respectively, and the solid lines indicate the average value. Moreover, the crosstalk shown in FIGS. 7 and 8 is a value after propagating light for 100 m.

  As shown in FIGS. 7 and 8, the multi-core fiber of this comparative example had poor crosstalk when the specific core of the outer peripheral side core, particularly when the bending diameter was about 700 mm.

(Reference Example 1)
A multi-core fiber under the same conditions as the multi-core fiber of Comparative Example 1 was assumed except that the outer peripheral core was spirally rotated around the central axis of the clad. In this multi-core fiber, the twisting pitch was set to 0.01 times / m.

  Then, when the same light as in Comparative Example 1 is propagated to the respective cores and the multicore fiber is gradually bent in the same manner as in Comparative Example 1, the change in crosstalk between the central core and the outer peripheral core is shown. Examined. The result is shown in FIG. In FIG. 9, the wavy line indicates the crosstalk value of each core, and the solid line indicates the average value. Furthermore, the multicore fiber was rotated about the axis while maintaining the bending direction of the multicore fiber, and the maximum value, the minimum value, and the average value of the crosstalk were examined at each bending diameter. The result is shown in FIG. In FIG. 10, the wavy line indicates the maximum value and the minimum value, respectively, and the solid line indicates the average value. Moreover, the crosstalk shown in FIGS. 9 and 10 is a value after light has propagated 100 m.

  As shown in FIGS. 9 and 10, the multi-core fiber of this reference example is more specific than the multi-core fiber of Comparative Example 1 because the core on the outer peripheral side spirally rotates around the central axis of the cladding. As a result, it was reduced that the crosstalk of the core of the core deteriorated at a specific bending diameter.

(Reference Example 2)
A multi-core fiber under the same conditions as the multi-core fiber of Reference Example 1 was assumed except that the twisting pitch was 1 turn / m.

  Then, when the same light as in Comparative Example 1 is propagated to the respective cores and the multicore fiber is gradually bent in the same manner as in Comparative Example 1, the change in crosstalk between the central core and the outer peripheral core is shown. Examined. The result is shown in FIG. In FIG. 11, the wavy line indicates the crosstalk value of each core, and the solid line indicates the average value. Furthermore, the multicore fiber was rotated about the axis while maintaining the bending direction of the multicore fiber, and the maximum value, the minimum value, and the average value of the crosstalk were examined at each bending diameter. The result is shown in FIG. In FIG. 12, the wavy line indicates the maximum value and the minimum value, respectively, and the solid line indicates the average value. Moreover, the crosstalk shown in FIGS. 11 and 12 is a value after light has propagated 100 m.

  As shown in FIGS. 11 and 12, the multi-core fiber of this reference example is more specific than the multi-core fiber of Comparative Example 1 because the core on the outer peripheral side rotates spirally around the central axis of the cladding. As a result, the deterioration of the crosstalk of the core at a specific bending diameter was reduced, and the variation of each crosstalk was more concentrated than in Reference Example 1.

Example 1
The same condition as the multi-core fiber of Comparative Example 1 except that the relative refractive index difference of the core on the outer peripheral side changes sinusoidally at 3 times / m along the longitudinal direction and the rate of change is 0.005%. Multicore fiber was assumed. Thus, the relative refractive index difference of the core on the outer peripheral side changes along the longitudinal direction, so that the mode field diameter MFD of light propagating through the core disposed at the center of the cladding and the core disposed on the outer peripheral side. The difference from the mode field diameter MFD of the light propagating through the optical fiber changes along the longitudinal direction of the multi-core fiber.

  Then, when the same light as in Comparative Example 1 is propagated to the respective cores and the multicore fiber is gradually bent in the same manner as in Comparative Example 1, the change in crosstalk between the central core and the outer peripheral core is shown. Examined. The result is shown in FIG. In FIG. 13, the wavy line indicates the crosstalk value of each core, and the solid line indicates the average value. Furthermore, the multicore fiber was rotated about the axis while maintaining the bending direction of the multicore fiber, and the maximum value, the minimum value, and the average value of the crosstalk were examined at each bending diameter. The result is shown in FIG. In FIG. 14, the wavy lines indicate the maximum value and the minimum value, respectively, and the solid lines indicate the average value. Further, the crosstalk shown in FIGS. 13 and 14 is a value after light has propagated 100 m.

  As shown in FIGS. 13 and 14, the multi-core fiber of the present example is more specific in crosstalk of a specific core than the multi-core fiber of Comparative Example 1 by changing the relative refractive index difference of the core on the outer peripheral side. As a result, the worsening of the bending diameter was reduced.

(Example 2)
A multi-core fiber under the same conditions as the multi-core fiber of Example 1 was assumed except that the change rate of the relative refractive index difference of the core on the outer peripheral side was 0.01%.

  Then, when the same light as in Comparative Example 1 is propagated to the respective cores and the multicore fiber is gradually bent in the same manner as in Comparative Example 1, the change in crosstalk between the central core and the outer peripheral core is shown. Examined. The result is shown in FIG. In FIG. 15, the wavy line indicates the crosstalk value of each core, and the solid line indicates the average value. Furthermore, the multicore fiber was rotated about the axis while maintaining the bending direction of the multicore fiber, and the maximum value, the minimum value, and the average value of the crosstalk were examined at each bending diameter. The result is shown in FIG. In FIG. 16, the wavy lines indicate the maximum value and the minimum value, respectively, and the solid lines indicate the average value. Further, the crosstalk shown in FIGS. 15 and 16 is a value after light is propagated 100 m.

  As shown in FIGS. 15 and 16, the multi-core fiber of this example is more specific than the multi-core fiber of Example 1 because the relative refractive index difference of the core on the outer peripheral side changes more than that of Example 1. The result was a further reduction in the worsening of the core crosstalk at a particular bend diameter.

(Example 3)
A multi-core fiber having the same conditions as the multi-core fiber of Example 1 was assumed except that the outer peripheral core was spirally rotated around the central axis of the clad. In this multi-core fiber, the twisting pitch was 3 times / m.

  Then, when the same light as in Comparative Example 1 is propagated to the respective cores and the multicore fiber is gradually bent in the same manner as in Comparative Example 1, the change in crosstalk between the central core and the outer peripheral core is shown. Examined. The result is shown in FIG. In FIG. 17, the wavy line indicates the crosstalk of each core, and the solid line indicates the average value. Furthermore, the multicore fiber was rotated about the axis while maintaining the bending direction of the multicore fiber, and the maximum value, the minimum value, and the average value of the crosstalk were examined at each bending diameter. The result is shown in FIG. In FIG. 18, the wavy lines indicate the maximum value and the minimum value, respectively, and the solid lines indicate the average value. Moreover, the crosstalk shown in FIGS. 17 and 18 is a value after light has propagated 100 m.

  As shown in FIGS. 17 and 18, the multi-core fiber of the present example has a specific core crossing structure as compared with Example 1 because the core on the outer peripheral side rotates spirally around the central axis of the cladding. As a result, the worsening of the talk at a specific bending diameter was further reduced, resulting in a more concentrated variation in crosstalk than in the first embodiment.

Example 4
A multi-core fiber under the same conditions as the multi-core fiber of Example 2 was assumed, except that the outer peripheral core was spirally rotated around the central axis of the cladding. In this multi-core fiber, the twisting pitch was 3 times / m.

  Then, when the same light as in Comparative Example 1 is propagated to the respective cores and the multicore fiber is gradually bent in the same manner as in Comparative Example 1, the change in crosstalk between the central core and the outer peripheral core is shown. Examined. The result is shown in FIG. In FIG. 19, the wavy line indicates the crosstalk value of each core, and the solid line indicates the average value. Furthermore, the multicore fiber was rotated about the axis while maintaining the bending direction of the multicore fiber, and the maximum value, the minimum value, and the average value of the crosstalk were examined at each bending diameter. The result is shown in FIG. In FIG. 20, the wavy lines indicate the maximum value and the minimum value, respectively, and the solid lines indicate the average value. Further, the crosstalk shown in FIGS. 19 and 20 is a value after light has propagated 100 m.

  As shown in FIGS. 19 and 20, the multi-core fiber of the present example has a specific core crossing structure as compared with Example 2 because the core on the outer peripheral side rotates spirally around the central axis of the cladding. It was further reduced that the talk deteriorated at a specific bending diameter, and each crosstalk variation was more concentrated than in Example 2.

(Example 5)
The relative refractive index difference of the core on the outer peripheral side changes at 2 times / m along the longitudinal direction, and the twist pitch is 1 time / m, (the period of change in the relative refractive index difference of the core) / (core A multi-core fiber under the same conditions as the multi-core fiber of Example 4 was assumed except that the twisting period was set to 2.00.

  Then, when the same light as in Comparative Example 1 is propagated to the respective cores and the multicore fiber is gradually bent in the same manner as in Comparative Example 1, the change in crosstalk between the central core and the outer peripheral core is shown. Examined. The result is shown in FIG. In FIG. 21, the wavy line indicates the crosstalk value of each core, and the solid line indicates the average value. Furthermore, the multicore fiber was rotated about the axis while maintaining the bending direction of the multicore fiber, and the maximum value, the minimum value, and the average value of the crosstalk were examined at each bending diameter. The result is shown in FIG. In FIG. 22, the wavy line indicates the maximum value and the minimum value, respectively, and the solid line indicates the average value. Further, the crosstalk shown in FIGS. 21 and 22 is a value after light is propagated 100 m.

  As shown in FIGS. 21 and 22, the multi-core fiber of this example has (the period of change in the relative refractive index difference of the core) just double the (period of twist of the core), and has a large bending diameter. Focusing was slightly disturbed in the region (region close to a straight line).

(Example 6)
A multi-core fiber under the same conditions as the multi-core fiber of Example 5 was assumed except that (period of change in relative refractive index difference of core) / (period of twist of core) = 2.01.

  Then, when the same light as in Comparative Example 1 is propagated to the respective cores and the multicore fiber is gradually bent in the same manner as in Comparative Example 1, the change in crosstalk between the central core and the outer peripheral core is shown. Examined. The result is shown in FIG. In FIG. 23, the wavy line indicates the crosstalk of each core, and the solid line indicates the average value. Furthermore, the multicore fiber was rotated about the axis while maintaining the bending direction of the multicore fiber, and the maximum value, the minimum value, and the average value of the crosstalk were examined at each bending diameter. The results are shown in FIG. In FIG. 24, the wavy line indicates the maximum value and the minimum value, respectively, and the solid line indicates the average value. Moreover, the crosstalk shown in FIGS. 23 and 24 is a value after light has propagated 100 m.

  As shown in FIG. 23 and FIG. 24, the multi-core fiber of the present example has a cross period (change period of the relative refractive index difference of the core) slightly larger than an integer multiple of the (core twist period). However, the result was more focused than in Example 5.

(Example 7)
A multi-core fiber under the same conditions as the multi-core fiber of Example 5 was assumed except that (period of change in relative refractive index difference of core) / (period of twist of core) = 2.02.

  Then, when the same light as in Comparative Example 1 is propagated to the respective cores and the multicore fiber is gradually bent in the same manner as in Comparative Example 1, the change in crosstalk between the central core and the outer peripheral core is shown. Examined. The result is shown in FIG. In FIG. 25, the wavy line indicates the crosstalk of each core, and the solid line indicates the average value. Furthermore, the multicore fiber was rotated about the axis while maintaining the bending direction of the multicore fiber, and the maximum value, the minimum value, and the average value of the crosstalk were examined at each bending diameter. The result is shown in FIG. In FIG. 26, the wavy line indicates the maximum value and the minimum value, respectively, and the solid line indicates the average value. Moreover, the crosstalk shown in FIG. 25 and FIG. 26 is a value after propagating light for 100 m.

  As shown in FIG. 25 and FIG. 26, the multi-core fiber of this example has a larger (period of change in relative refractive index difference of the core) / (period of twist of the core) than that of Example 6, As a result, the crosstalk was more concentrated than in Example 6.

(Example 8)
Next, in the core on the outer peripheral side, the relative refractive index difference changes in a sinusoidal shape at 0.6 times / m along the longitudinal direction, the rate of change is 0.02%, and a twist of a cycle of 1 time / m. A multi-core fiber having the same conditions as the multi-core fiber of Comparative Example 1 was assumed except that the above conditions were satisfied. Further, the difference in relative refractive index difference between the central core and the outer core is 0.0%, 0.0025%, 0.005%, 0.01%, 0.02%, 0.025%, 0 0.03%, 0.04%, and 0.05%. The reference relative refractive index difference is 0.4% (the relative refractive index difference with respect to the central core cladding is 0.4%), so the difference in relative refractive index difference is 0.05%. This means that 0.45% is set as the relative refractive index difference of the core on the side.

  Then, when the same light as in Comparative Example 1 is propagated to the respective cores and the multicore fiber is gradually bent in the same manner as in Comparative Example 1, the change in crosstalk between the central core and the outer peripheral core is shown. Further, the average crosstalk of all the cores on the outer peripheral side in the difference in the relative refractive index difference was examined. The result is shown in FIG. Note that the crosstalk shown in FIG. 27 is a value after light has propagated 100 m.

  As shown in FIG. 27, the amount of change (0.02%) in the longitudinal direction of the relative refractive index difference of the core on the outer peripheral side is the relative refractive index difference of the central core and the relative refractive index difference of the outer core. Multi-core fiber (difference in relative refractive index difference between the central core and the outer core is 0.0%, 0.0025%, 0.005%, 0.01%, Assuming that 0.02% multi-core fiber) is group A, in group A multi-core fiber, the amount of change in crosstalk can be reduced over a very wide range of bending diameters from 100 mm to 10,000 mm.

  Further, as shown in FIG. 27, the amount of change (0.02%) in the relative refractive index difference of the outer peripheral core along the longitudinal direction is such that the relative refractive index difference of the central core and the relative refractive index of the outer core. Group B (difference in the relative refractive index difference between the central core and the outer core is 0.025%, 0.03%, 0.04%, 0.05%), in the group B multi-core fiber, the crosstalk was further improved in the region where the bending diameter was large. Further, as in the multi-core fiber of Comparative Example 1 shown in FIG. 7, there is no rapid deterioration of crosstalk at a specific bending diameter that exists when there is no characteristic variation in the longitudinal direction.

  As described above, according to the multi-core fiber in which the difference in mode field diameter of each light propagating through at least one pair of cores adjacent to each other as in the present invention varies along the longitudinal direction, it is installed non-linearly. It has been found that even in the case of the problem, it is possible to suppress the deterioration of the crosstalk between specific cores.

  Furthermore, in the multi-core fiber of the present invention, at least one core of at least one pair of cores adjacent to each other is arranged in a spiral shape so as to rotate around the central axis of the cladding, thereby further crossing the specific core. It has been found that the talk is reduced from worsening at a particular bend diameter and the crosstalk variation is focused. In this case, it was found that if the period of the change in the mode field diameter of the core is a non-integer multiple of the period of twisting of the core, the crosstalk is more focused.

  Further, the difference in the relative refractive index difference between at least one pair of cores adjacent to each other is larger in the region where the difference in the relative refractive index difference relative to the cladding is larger than the amount of change along the longitudinal direction. It was found that crosstalk was improved.

  As described above, according to the present invention, there is provided a multi-core fiber that can suppress deterioration of crosstalk between specific cores even when installed non-linearly.

DESCRIPTION OF SYMBOLS 1, 2 ... Multi-core fiber 11, 12 ... Core 14 ... Hole 15 ... 1st clad 16 ... 2nd clad 30 ... Cladding 31 ... Inner protective layer 32 ...・ Outer protective layer

Claims (11)

With multiple cores,
A clad surrounding the outer peripheral surface of each of the cores;
With
A multi-core fiber, wherein a difference in propagation constants of at least one pair of cores adjacent to each other among the plurality of cores varies along a longitudinal direction.   The multi-core fiber according to claim 1, wherein a difference in mode field diameter of each light propagating through at least one pair of adjacent cores varies along the longitudinal direction.   3. The multi-core fiber according to claim 1, wherein a difference in a relative refractive index difference with respect to the clad varies in the longitudinal direction between at least one pair of cores adjacent to each other.   The difference in relative refractive index difference between the at least one pair of cores adjacent to each other is larger than an amount of change in which the difference in relative refractive index difference with respect to the cladding changes along the longitudinal direction. Multi-core fiber.   The difference in relative refractive index difference between at least one pair of cores adjacent to each other is smaller than an amount of change in which the difference in relative refractive index difference with respect to the cladding changes along the longitudinal direction. Multi-core fiber.   6. The multi-core fiber according to claim 1, wherein a difference in diameter of at least one pair of cores adjacent to each other varies along a longitudinal direction.   The multi-core fiber according to any one of claims 1 to 6, wherein the difference in the propagation constant changes at a constant interval along a longitudinal direction.   The multi-core fiber according to any one of claims 1 to 6, wherein the difference in propagation constant changes at irregular intervals along a longitudinal direction.   The at least one core in the at least one pair of cores adjacent to each other is arranged in a spiral shape so as to rotate around the central axis of the clad. The multi-core fiber described in 1.   10. The pitch interval of the cores arranged in a spiral shape rotating around the central axis of the clad is a non-integer multiple of the interval at which the difference in mode field diameter changes. The multi-core fiber described in 1.   11. The multi-core fiber according to claim 9, wherein the spiral core is arranged so as to repeat a clockwise rotation and a counterclockwise rotation around the central axis of the cladding.

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