JP2014010266A - Multi-core fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multi-core fiber enabling a communication system that transmits a large amount of data using a simple communication system.SOLUTION: In a multi-core fiber 1, the outer circumferential surfaces of a plurality of cores 11 and 12 are surrounded by a clad 20 and each of the cores 11 and 12 propagates an optical signal having a wavelength of 1,530 nm to 1,625 nm. Each of the cores 11 and 12 can propagate light of a plurality of modes. A mode differential delay between the individual mode lights in each of the cores 11 and 12 is 1,000 picoseconds/km or more, and crosstalk of light of the highest order mode in the cores 11 and 12 which are adjacent to each other is -30 dB or less per 100 km in the wavelength range of optical signals.

Description

  The present invention relates to a multicore fiber in which each core transmits an optical signal in multimode.

  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 such a multi-core fiber. In this multi-core fiber, a plurality of cores are arranged in one clad. However, it is known that in multi-core fibers, crosstalk in which optical signals propagating through the cores interfere with each other is likely to occur. When optical communication is performed using a multi-core fiber in which such crosstalk occurs, the signal receiving side uses techniques such as MIMO (Multiple-Input and Multiple-Output), respectively, in a state where crosstalk has occurred. When a crosstalk does not occur from a plurality of signals emitted from the cores, signals emitted from the respective cores are extracted.

  In addition to using the above multi-core fiber as means for simultaneously transmitting a plurality of optical signals through one optical fiber, light of a plurality of modes is propagated through one core of one optical fiber, and each mode is transmitted. There is known communication for transmitting a signal by superimposing a signal on the light. Non-Patent Document 2 below describes means for performing communication using such a multimode.

S. Matsuo et al. “Large-effective-area ten-core fiber with cladding diameter of about 200 μm” Opt. Lett. , Vol36, 4626-4628, (2011). M.M. Salsi. "Mode Division Multiplexed Transmission with a weakly-coupled Few-Mode Fiber" OFC / NFOEC'12, OTU2C5 (2012)

  As described above, according to communication using a multi-core fiber and multi-mode communication in which a signal is superimposed on light of each mode, a large number of signals can be transmitted through one optical fiber. However, there is a demand for transmitting more optical signals through one optical fiber.

  In order to meet such a demand, it is conceivable that light of a plurality of modes is propagated in each core of the multimode fiber and communication using the multimode is performed in each core of the multicore fiber. However, as described above, it is known that the multi-core fiber is likely to cause crosstalk, and further, there may be inter-mode coupling in which light of different modes propagating through one core is coupled. When performing optical communication using a multi-core fiber that causes such crosstalk and inter-mode coupling, and using techniques such as MIMO as described above, when cross-talk and inter-mode coupling do not occur, respectively. The signal emanating from the core must be extracted.

  However, the use of advanced techniques such as MIMO complicates the communication system. Therefore, a communication system that suppresses crosstalk and coupling between modes and sends a large amount of data using a simple communication system using a technique such as simple MIMO is desired.

  Accordingly, an object of the present invention is to provide a multi-core fiber that can realize a communication system that sends a large amount of data using a simple communication system.

  In order to solve the above problems, the present invention provides a multi-core fiber in which outer peripheral surfaces of a plurality of cores are surrounded by a clad, and each of the cores propagates an optical signal having a wavelength of 1530 nm to 1625 nm. The mode group delay difference between the light of each mode in each of the cores is 1000 picoseconds / km or more, and the light of the highest order mode between adjacent cores. The crosstalk is -30 dB or less per 100 km in the wavelength range of the optical signal.

  According to such a multi-core fiber, the time difference per 1 km of light of each mode propagating through each core is 1000 picoseconds or more. By propagation of light in this way, it is possible to suppress the occurrence of mode coupling in each core. Furthermore, the crosstalk of light in a mode other than the light of the highest order mode between adjacent cores is suppressed to be smaller than the crosstalk of the light of the highest order mode. Therefore, according to the multi-core fiber having the above-described configuration, at least the crosstalk is suppressed to be less than −30 dB per 100 km in an optical signal having a wavelength propagating through the core of 1530 nm to 1625 nm. Therefore, according to such a multi-core fiber, crosstalk between cores is reduced and mode coupling between light in each mode is suppressed. Therefore, a signal superimposed on each light propagating through each core is transmitted. It can be easily separated. Therefore, by using such a multi-core fiber, a communication system that sends a large amount of data using a simpler communication system can be realized.

  Each of the cores may propagate only LP01 mode light and LP11 mode light.

  Propagating only LP01 mode light and LP11 mode light means that light that can propagate a signal in optical communication is only LP01 mode light and LP11 mode light. It does not mean that there is no light at all. A simpler communication system can be realized by propagating light in such a small mode.

  Further, it is preferable that the cores adjacent to each other propagate light of the same mode, and the light of the highest mode in the cores adjacent to each other propagates at different speeds.

  Since the light of the same mode, which is the light of the highest order mode propagating through the cores adjacent to each other, propagates at different speeds, the crosstalk of the light of the highest order mode can be further suppressed. Therefore, according to such a configuration, crosstalk can be further suppressed.

A first cladding surrounding an outer peripheral surface of each of the cores; a second cladding surrounding the outer peripheral surface of the first cladding and surrounding the outer peripheral surface by the cladding;
Further comprising a refractive index of each of the core and n _1, the refractive index of the first clad and n _2, the refractive index of the second clad and n _3, the refractive index of the cladding and n ₄ If
n ₁ > n ₂ > n ₃
n ₁ > n ₄
n ₃ <n ₄
It is preferable to satisfy all of the above.

When the core, the first cladding, and the second cladding are viewed as core elements, the core is surrounded by a first cladding having a refractive index n ₂ smaller than the refractive index n _{1 of the} core, and the first cladding is the first because it is surrounded by a second cladding having a refractive index n ₃ smaller than the refractive index n ₂ of the cladding, when looking at each core element in terms of refractive index, each core element has a trench structure Yes. In the multi-core fiber having such a structure, the light propagating through the core is strongly confined by the core because the refractive index n ₃ of the second cladding is lower than the refractive index n ₂ of the first cladding. Leakage outside the core element is suppressed. For this reason, according to such a multi-core fiber, crosstalk between cores can be further reduced.

  As described above, according to the present invention, there is provided a multicore fiber that can realize a communication system that sends a large amount of data using a simple communication system.

It is a figure which shows the mode of the multi-core fiber which concerns on 1st Embodiment of this invention. An example of a region to be satisfied by a core radius for propagating only LP01 mode light and LP11 mode light and a relative refractive index difference of the core with respect to the cladding, and an example of an effective core area of the LP01 mode light FIG. An example of a region that should be satisfied by the radius of the core for propagating only the LP01 mode light and the LP11 mode light and the relative refractive index difference of the core with respect to the cladding, and an example of the effective core area of the LP11 mode light FIG. It is a figure which shows the relationship between the radius of the core of the delay difference between modes of the light of LP01 mode, and the light of LP11 mode, and the relative refractive index difference of the core with respect to a clad. It is a figure which shows the relationship between the crosstalk of the light of each mode which propagates mutually adjacent core, and the distance between cores. 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 relationship between the crosstalk of the light of wavelength 1550nm, and the light of wavelength 1625nm, and the distance between cores.

  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 structure in a cross section perpendicular to the longitudinal direction of a multi-core fiber according to the present embodiment.

  As shown in FIG. 1, the multi-core fiber 1 of the present embodiment surrounds a plurality of cores 11, 12 and the entire cores 11, 12 and fills the space between the cores 11, 12. 11 and 12 including a clad 20 that surrounds the outer peripheral surface of the clad 20 without any gap, an inner protective layer 31 that covers the outer peripheral surface of the clad 20, and an outer protective layer 32 that covers the outer peripheral surface of the inner protective layer 31. Reference numerals 11 and 12 denote multi-core fibers that propagate optical signals having wavelengths of 1550 nm to 1625 nm.

  The core 11 and the core 12 are adjacent to each other, and the cores 11 and the cores 12 are not adjacent to each other. FIG. 1 shows an example in which there are two cores 11 and 12 respectively. Accordingly, in FIG. 1, the cores 11 are located diagonally to each other, the cores 12 are located diagonally to each other, and the cores 11 and 12 are arranged so as to draw a square as a whole.

  First, the conditions of the cores 11 and 12 will be described.

  The refractive indexes of the cores 11 and 12 are higher than the refractive index of the clad 20. In the present embodiment, each of the cores 11 and 12 is configured to propagate only LP01 mode light and LP11 mode light with respect to light having a wavelength of 1550 nm to 1625 nm. In this case, each of the cores 11 and 12 of the multi-core fiber 1 performs communication by superimposing a signal only on the LP01 mode light and the LP11 mode light, but there is no other mode light propagating as noise. It doesn't mean that.

  FIGS. 2 and 3 show an LP01 mode light and an LP11 mode in an optical fiber having a so-called step type refractive index distribution in which the refractive index in the core does not change in the radial direction and the refractive index is uniform in the core. It is a figure which shows an example of the area | region where the radius of the core for propagating only the light of and the relative refractive index difference of the core with respect to a clad satisfy | fill. 2 and 3, the horizontal axis indicates the radius of the core, and the vertical axis indicates the relative refractive index difference (core Δ) of the core with respect to the cladding.

  In this example, as long as the optical fiber satisfies the conditions of any region as long as it is the region shown in FIGS.

In addition, a line designated as LP11 _Limit indicates a limit that allows LP11 mode light to be propagated when light having a bending radius of 30 mm and a wavelength of 1625 nm is propagated. In this case, LP11 mode light can be propagated. As described above, the multi-core fiber 1 of this embodiment propagates only LP01 mode light and LP11 mode light with respect to light having a wavelength of 1550 nm to 1625 nm. Therefore, LP11 mode light must propagate. By the way, the most severe bending radius in practical use of an optical fiber may be set to 30 mm. Further, in the above wavelength range, the wavelength having a large bending loss is 1625 nm. Therefore, as in this example, by setting the region where the LP11 mode light propagates at the wavelength with the largest bending loss at the most severe bending radius of the optical fiber, if the optical fiber falls within this region, LP11 mode light can be propagated under normal use conditions.

Further, a line designated as LP21 _Limit indicates a limit capable of propagating LP21 mode light, and an LP21 mode light is propagated as long as the optical fiber satisfies a condition that is an upper region from this line. . Since the multi-core fiber 1 of the present embodiment propagates only LP01 mode light and LP11 mode light with respect to light having a wavelength of 1550 nm to 1625 nm as described above, LP21 mode light should not propagate. . By the way, in a single mode fiber, when used in a state where the bending radius is 140 mm, the wavelength at which the loss per meter of LP11 mode light is 1 dB is regarded as the wavelength at which LP11 mode light cannot propagate, In general, this wavelength is called a cable cutoff wavelength. This concept can also be applied to other modes of light. When an optical fiber is used with a bending radius of 140 mm, the wavelength at which the loss per meter of light in the LP21 mode, which is one higher order than the LP11 mode, is 1 dB. It is possible to set a limit wavelength at which LP21 mode light cannot be propagated. In FIG. 2 and FIG. 3, the line designated as LP21 _Limit is a line where the loss per meter of light in the LP21 mode, which is one higher order than the LP11 mode, is 1 dB when the optical fiber is used with a bending radius of 140 mm. It is. For this reason, if it is an optical fiber which satisfy | fills the conditions used as the area | region below a line made into LP21 _Limit , it can be said that the light of LP21 mode cannot be propagated.

Therefore, in the example of FIGS. 2 and 3, if the optical fiber has a core that satisfies the condition that the region is located above the line designated LP11 _Limit and below the line designated LP21 _Limit , the optical fiber of LP01 mode is used. Only light and LP11 mode light propagate.

As described above, since the cores 11 and 12 of the multi-core fiber 1 are supposed to propagate only LP01 mode light and LP11 mode light, the respective cores 11 and 12 of the multi-core fiber 1 are step-type refracting. when a core having a rate, the relative refractive index difference of the core 11, 12 to the radius and the clad 20 of each core 11, 12, the upper side than the line which is _{LP11 Limit} of FIGS. 2, _{3, LP21 Limit} The condition that the region is lower than the line is satisfied.

Further, in FIG. 2, as an example showing the distribution of the effective core area A _eff of the LP01 mode light, the distribution of the effective core area A _eff of the LP01 mode light at the wavelength of 1550 nm is from 90 μm ² to 210 μm ^2. are indicated by dashed lines, FIG. 3, as an example showing the distribution of the effective core area _{a eff} of light LP01 mode, the LP11 light distribution of the effective core area _{a eff} of modes at a wavelength of 1550 nm, 140 .mu.m ² to 300 μm ² are indicated by broken lines. As shown in FIGS. 2 and 3, in order to increase the effective core cross-sectional line A _eff of the LP01 mode light and the LP11 mode light, the cores 11 and 12 are made to be less than the line _defined as LP11 _Limit. in the _upper, in the area below the line it is _{LP21 Limit,} in between the lines that is lines and _{LP21 Limit} that is _{LP11 Limit,} and a line is a line and the _{LP21 Limit} that is _{LP11 Limit} It can be seen that the condition close to the intersecting point should be satisfied.

2 and 3, for example, an optical fiber having a core that propagates only LP01 mode light and LP11 mode light has a step-type refractive index distribution, and the core Δ is 0.45%. An optical fiber having a core with a core radius a of 6.47 μm can be mentioned. In this case, the effective core area A _eff of the LP01 mode light is about 110 μm ² from FIG. ² , and the effective core area A _eff of the LP11 mode light is about 180 μm ² from FIG.

FIG. 4 is a diagram illustrating the inter-mode delay difference DMGD between the LP01 mode light and the LP11 mode light in the region illustrated in FIGS. 2 and 3. In FIG. 4, the inter-mode delay difference DMGD with the inter-mode delay difference DMGD of 0 to 4000 picoseconds / km is indicated by a broken line. As shown in FIG. 4, in an optical fiber having a step-type refractive index profile as described above, if the region is above the line that is LP11 _Limit and below the line that is LP21 _Limit , The inter-mode delay difference DMGD between the LP01 mode light and the LP11 mode light is greater than 1000 picoseconds / km. Further, in order to increase as possible inter-mode delay difference DMGD the light of LP01 mode light and LP11 _modes, in between the lines that is lines and _{LP21 Limit} that is _{LP11 Limit,} increased by the core Δ can be What should I do. As described above, if the core Δ is 0.45%, the core radius a is 6.47 μm, and the optical fiber has a step type refractive index core, the inter-mode delay difference DMGD is 2500 picoseconds. / Km or so. Therefore, if each of the cores 11 and 12 of the multi-core fiber 1 in FIG. 1 satisfies such conditions, for example, each of the cores 11 and 12 propagates only LP01 mode light and LP11 mode light. The inter-mode delay difference DMGD is at least 1000 picoseconds / km or more.

  Next, the relationship between the core 11 and the core 12 will be described.

  In this embodiment, each of the core 11 and the core 12 propagates only the LP01 mode light and the LP11 mode light as described above, and the inter-mode delay difference DMGD satisfies at least 1000 picoseconds / km or more. The crosstalk of the highest order mode light between adjacent cores is set to −30 dB or less at 100 km. Therefore, in this embodiment, the crosstalk between the LP11 mode light propagating through the core 11 and the LP11 mode light propagating through the core 12 is set to −30 dB or less at 100 km.

FIG. 5 shows that the cores 11 and 12 are step-type refractive indexes, the core Δ is 0.45%, and the core radius a is 6.47 μm, as described above. It is a figure which shows the crosstalk of the light of each mode which propagates a core. Generally, the highest order mode light propagating through each core has the largest effective core area A _{eff, and} therefore tends to have the largest crosstalk. In this embodiment, since the light of the highest order mode is the light of the LP11 mode, the crosstalk of the light of the LP11 mode of the LP11 of the cores 11 and 12 should be −30 dB or less at 100 km. FIG. 5 shows that in order to achieve such crosstalk, it is sufficient that the refractive indexes of the cores 11 and 12 are equal to each other, and the distance between the centers of the cores 11 and 12 (inter-core distance) is 52 μm or more. In this case, the crosstalk of the LP01 mode light propagating through the cores 11 and 12, and the crosstalk of the LP01 mode light and the LP11 mode light is smaller than −30 dB at 100 km.

  As described above, according to the multicore fiber 1 according to the present embodiment, the time difference per 1 km of the light of each mode propagating through the respective cores 11 and 12 is set to 1000 picoseconds or more. Therefore, it is possible to suppress the mode coupling between the cores 11 and 12. Furthermore, since the light of the highest order mode of the cores 11 and 12 adjacent to each other is set to −30 dB or less per 100 km, the crosstalk of the light of other modes becomes smaller than −30 dB. Therefore, when optical communication is performed using the multi-core fiber 1 of the present embodiment, crosstalk between the cores 11 and 12 is reduced, and mode coupling between light in each mode is suppressed, so that signal reception is performed. On the side, the signals superimposed on the respective lights propagating through the respective cores 11 and 12 can be easily separated. Therefore, by using the multi-core fiber 1 of the present embodiment, a communication system that sends a large amount of data using a simpler communication system can be realized.

  Further, as described above, the respective cores 11 and 12 of the multi-core fiber 1 of the present embodiment propagate only the LP01 mode light and the LP11 mode light. Therefore, a simpler communication system can be realized.

  In the present embodiment, only the LP01 mode light and the LP11 mode light are propagated, but higher order light than the LP11 mode may be propagated. Even in this case, the cores 11 and 12 are configured so that the crosstalk of the light of the highest order mode propagating through the cores 11 and 12 is −30 dB or less per 100 km.

2 and 3, the multi-core fiber 1 has a uniform refractive index in the radial direction of the cores 11 and 12, and has a step-type refractive index distribution. However, each of the cores 11 and 12 may have a higher refractive index in a region closer to the central axis of each of the cores 11 and 12. In the case of a multi-core fiber having such a refractive index distribution, the LP01 mode light and the LP11 mode light are located above the line designated as LP11 _Limit and below the line designated as LP21 _Limit . The inter-mode delay difference DMGD is not necessarily greater than 1000 picoseconds / km. In this case, the inter-mode delay difference DMGD between the LP01 mode light and the LP11 mode light is 1000 picoseconds / km above the LP11 _Limit line and below the LP21 _Limit line. It is necessary to construct a multi-core fiber under conditions of a larger region.

  The core 11 and the core 12 may have different refractive indexes or different diameters, and may have different light propagation conditions. In this case, the distance between the cores 11 and 12 adjacent to each other can be further reduced while the crosstalk of the highest order mode light is −30 dB or less per 100 km. Therefore, the diameter of the multi-core fiber 1 can be reduced.

  Further, the propagation conditions of the core 11 and the core 12 are changed from each other so that the adjacent cores 11 and 12 propagate the light of the same mode, and the light of the highest order mode of each core propagates at a different speed. Can be configured. By configuring in this way, it is possible to further suppress the crosstalk of the highest order mode light of the cores 11 and 12 adjacent to each other. As such cores 11 and 12, for example, when the cores 11 and 12 propagate only LP01 mode light and LP11 mode light, one of the cores 11 and 12 has a radius of 6.1 μm and the core Δ is For example, the core is 0.55%, the other is a core having a radius of 8.0 μm and a core Δ of 0.365%. According to such a combination, the light in the LP11 mode, which is the highest order mode of the cores 11 and 12 adjacent to each other, can be varied by 1000 picoseconds / km or more.

(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 in particular.

  FIG. 6 is a view showing a structure in a cross section perpendicular to the longitudinal direction of the multi-core fiber according to the second embodiment of the present invention.

  As shown in FIG. 6, the multi-core fiber 2 of the present embodiment includes a first cladding 13 that surrounds the outer peripheral surfaces of the cores 11 and 12 without a gap, and an outer peripheral surface that surrounds the outer peripheral surface of the first cladding 13 without a gap. It differs from the multi-core fiber 1 of 1st Embodiment in having the 2nd clad 14 enclosed by 20 without gap. Here, the core 11, the first cladding 13 surrounding the core 11, and the second cladding 14 surrounding the first cladding 13 are used as the first core element 11 a, and the core 12, the first cladding 13 surrounding the core 12, The second cladding 14 surrounding the first cladding 13 is defined as a second core element 12a. In the present embodiment, the outer diameters of the first claddings 13 are equal to each other, and the outer diameters of the second claddings 14 are equal to each other. Accordingly, the thicknesses of the first clads 13 are equal to each other, and the thicknesses of the second clads 14 are equal to each other.

Further, when the refractive index of the core 11 is n _1-1 and the refractive index of the core 12 is n _1-2 , the refractive index n ₂ of the first cladding 13 is the refractive index n _{1-1 of the} cores 11 and 12. , N _1-2 , and the refractive index n ₃ of the second cladding 14 is further lower than the refractive index n ₂ of the first cladding 13. In addition, the refractive index n ₄ of the clad 20 is lower than the refractive indexes n _1-1 and n _1-2 of the cores 11 and 12 and higher than the refractive index n ₃ of the second clad 14. In other words, the respective refractive indices n _{1-1 to} n ₄ are
_{n 1-1>} n 2> n ₃
n _1-2 > n ₂ > n ₃
n _1-1 > n ₄
n _1-2 > n ₄
n ₃ <n ₄
All of these are satisfied. Therefore, when each core element 11a, 12a is viewed from the viewpoint of refractive index, each core element 11a, 12a has a trench structure.

When the refractive indexes of the core 11 and the core 12 are equal to each other and the refractive index is n ₁ , the above formula is
n ₁ > n ₂ > n ₃
n ₁ > n ₄
n ₃ <n ₄
Can be rewritten.

As described above, the refractive index n ₃ of the second cladding 14 is made smaller than the refractive index n ₂ of the first cladding 13 and the refractive index n _{4 of the} cladding 20, so that the light confinement effect on the cores 11 and 12 is obtained. It becomes large and it can prevent that the light which propagates the cores 11 and 12 leaks from each core element 11a, 12a. The second clad 14 and clad 20 having a low refractive index serve as a barrier, and crosstalk between the adjacent cores 11 and 12 can be further prevented.

  Accordingly, the cores 11 and 12 of the present embodiment have the same configuration as the cores 11 and 12 of the first embodiment, and the crosstalk between the cores 11 and 12 of the present embodiment and the cores 11 and 12 of the first embodiment. If the crosstalk is the same, the distance between the cores of the first cores 11 and 12 of the present embodiment can be made smaller than the distance between the cores of the cores 11 and 12 of the first embodiment. Therefore, according to the multi-core fiber of this embodiment, the optical fiber can be thinned.

  Also in this embodiment, each core of the multi-core fiber 2 may propagate higher-order light than the LP11 mode. Further, the cores 11 and 12 of the multi-core fiber 2 may have a higher refractive index toward the center axis of each core. However, even in these cases, in the multi-core fiber 2, the mode group delay difference between the light beams of the respective modes in the respective cores 11 and 12 is 1000 picoseconds / km or more, and the adjacent cores 11 and 12 are mutually adjacent. The crosstalk of light of the highest order mode between each other is set to −30 dB or less per 100 km.

  The core 11 and the core 12 may have different refractive indexes or different diameters, and may have different light propagation conditions. Further, the thickness of the first cladding 13 and the thickness of the second cladding may be different between the core elements 11a and 12a, and the refractive index of the first cladding 13 and the refractive index of the second cladding may be different. .

  Further, the cores 11 and 12 adjacent to each other may propagate light of the same mode, and the light of the highest order mode of each core may propagate at different speeds.

  Further, in each core element of the present embodiment, the second cladding 14 is formed of a material having a lower refractive index than the first cladding 13 and the cladding 20 by surrounding the outer peripheral surface of the first cladding 13 without a gap. However, the configuration of the second cladding 14 is not limited to this. For example, a plurality of holes may be formed in a region to be the second cladding 14, and a region other than the holes in this region may be filled with the same material as the cladding 20. That is, the outer peripheral surface of the first cladding 13 may be surrounded by a plurality of holes, and a region surrounding the outer peripheral surface of the first cladding 13 including the plurality of openings may be regarded as the second cladding. In this case, the size and number of holes are adjusted so that the average refractive index of the second cladding is smaller than that of the first cladding 13 and the cladding 20.

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

  For example, in the first and second embodiments, the number of cores is four, but the number of cores may be four or more.

  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.

Example 1
The multi-core fiber shown in FIG. 1 was produced. This multi-core fiber is an optical fiber having a step-type refractive index distribution in which each core has a uniform refractive index in the radial direction, the core Δ is 0.45%, the core radius is 6.47 μm, The distance between them was 52 μm. With this configuration, each core propagates only the light in the LP01 mode and the light in the LP11 mode in calculation when propagating light having a wavelength of 1550 nm. Further, in the case of propagating light having a wavelength of 1550 nm and light having a wavelength of 1625 μm, crosstalk of LP11 mode light of cores adjacent to each other was obtained by calculation. The result is shown by a solid line in FIG.

The effective core area A _eff of this optical fiber is 110 μm ² for light having a wavelength of 1550 nm, which is the same as the effective core area obtained by calculation. Next, light having a wavelength of 1550 nm was propagated, and crosstalk between adjacent cores was measured. The result is shown in FIG. As shown in FIG. 7, the crosstalk between adjacent cores was about −42 dB per 100 km, which was smaller than −30 dB obtained by calculation. Further, the mode group delay difference DMGD was measured by propagating light of the same wavelength, and was found to be 3000 picoseconds / km.

  Next, light having a wavelength of 1625 nm was propagated through the multi-core fiber, and crosstalk between cores adjacent to each other was measured. The result is shown in FIG. As shown in FIG. 7, the crosstalk between adjacent cores was about −32 dB per 100 km.

(Comparative Example 1)
A multi-core fiber similar to that of Example 1 was prepared except that the inter-core distance was 44 μm. Similarly to the multi-core fiber of the first embodiment, this multi-core fiber propagates only the light in the LP01 mode and the light in the LP11 mode in calculation when propagating light having a wavelength of 1550 nm. In addition, when light having a wavelength of 1550 nm and light having a wavelength of 1625 μm are propagated through this multi-core fiber, the crosstalk of LP11 mode light of cores adjacent to each other is considered to be worse than that of the first embodiment.

The effective core area A _eff of this optical fiber is 110 μm ² for light having a wavelength of 1550 nm, which is the same as the effective core area obtained by calculation. Next, light having a wavelength of 1550 nm was propagated, and crosstalk between adjacent cores was measured. The result is shown in FIG. As shown in FIG. 7, the crosstalk between adjacent cores was about -11 dB per 100 km, which was larger than -30 dB. Further, the mode group delay difference DMGD was measured by propagating light of the same wavelength, and was found to be 3000 picoseconds / km.

  Next, light having a wavelength of 1625 nm was propagated through the multi-core fiber, and crosstalk between cores adjacent to each other was measured. The result is shown in FIG. As shown in FIG. 7, the crosstalk between adjacent cores was about -1 dB per 100 km.

  It is considered that the crosstalk of the cores adjacent to each other has the worst crosstalk of light in the highest order mode. Therefore, in order to reduce the crosstalk between the cores adjacent to each other to −30 dB or less per 100 km, it is considered that the crosstalk of the light in the highest order mode should be −30 dB or less per 100 km. In the multi-core fiber of Example 1, it was confirmed that the crosstalk of the adjacent cores in the LP11 mode was −30 dB per 100 km, so that the crosstalk of the adjacent cores was less than −30 dB per 100 km. . Furthermore, in the multi-core fiber of Comparative Example 1, it can be confirmed that the crosstalk between adjacent cores is greater than −30 dB by increasing the crosstalk of LP11 mode light between adjacent cores to greater than −30 dB per 100 km. It was. Therefore, when designing the multi-core fiber, it was confirmed that the light of the highest order mode propagating through the cores adjacent to each other should be −30 dB or less per 100 km.

  As in this embodiment, if multi-core fibers in which the crosstalk of light in the highest order mode between adjacent cores is −30 dB or less per 100 km at the wavelength at which each core propagates are used, It is considered that a communication system that sends a large amount of data using a simple communication system can be realized on the condition that the mode group delay difference between the light beams in each mode is 1000 picoseconds / km or more.

  As described above, according to the present invention, a multi-core fiber capable of realizing a communication system that sends a large amount of data using a simple communication system is provided and can be used in the field of data communication and the like. .

DESCRIPTION OF SYMBOLS 1, 2 ... Multi-core fiber 11, 12 ... Core 11a, 12a ... Core element 13 ... 1st clad 14 ... 2nd clad 20 ... Cladding 31 ... Inner protective layer 32 ... Outer protective layer

Claims (4)

A multi-core fiber in which outer peripheral surfaces of a plurality of cores are surrounded by a clad, and each of the cores propagates an optical signal having a wavelength of 1530 nm to 1625 nm,
Each of the cores is capable of propagating a plurality of modes of light,
The mode group delay difference between the light of each mode in each of the cores is 1000 picoseconds / km or more,
The multi-core fiber is characterized in that the crosstalk of light of the highest order mode between adjacent cores is −30 dB or less per 100 km in the wavelength range of the optical signal. The multi-core fiber according to claim 1, wherein each of the cores propagates only LP01 mode light and LP11 mode light. The multi-core fiber according to claim 1, wherein the adjacent cores propagate light of the same mode, and light of the highest order mode in the adjacent cores propagates at different speeds. . A first cladding surrounding an outer peripheral surface of each of the cores;
A second clad surrounding the outer peripheral surface of the first clad and the outer peripheral surface being surrounded by the clad;
Further comprising
If the refractive index of each of the core and n _1, the refractive index of the first clad and n _2, the refractive index of the second clad and n _3, the refractive index of the cladding and n _4,
n ₁ > n ₂ > n ₃
n ₁ > n ₄
n ₃ <n ₄
The multicore fiber according to any one of claims 1 to 3, wherein all of the above are satisfied.

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