JP2017009629A - Multicore optical fiber, method for manufacturing optical fiber, and method for manufacturing optical fiber cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multicore optical fiber in which two or more cores are arranged, and the impulse response width of a fiber is reduced by appropriately designing a core-to-core distance in accordance with the torsion cycle of a fiber and actively causing inter-mode coupling.SOLUTION: Provided is a multicore optical fiber 11 in which a plurality of cores having a single propagation mode are arranged at minimum core intervals inside a clad having a smaller refractive index than the refractive index of a core 12, the multicore optical fiber 11 causing a crosstalk to occur in an even mode and an odd mode of propagation by being twisted in the lengthwise direction. A minimum core interval D (μm) satisfies D (μm) = -3log (γ/π) + D0 - 0.3, where D0 (μm) represents a core interval in which a group delay difference between coupling modes is 30 ps/km to 500 ps/km inclusive, and γ (rad/m) represents the torsion angle of the multicore optical fiber 11.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a multi-core optical fiber, an optical fiber manufacturing method, and an optical fiber cable manufacturing method.

  In an optical fiber communication system, transmission capacity is limited by nonlinear effects and fiber fuses that occur in an optical fiber. In order to alleviate these restrictions, parallel transmission using a multi-core optical fiber having a plurality of cores in one optical fiber, mode multiplex transmission using a multi-mode fiber having a plurality of propagation modes in the core, etc. Spatial multiplexing techniques have been studied (see, for example, non-patent documents 1 to 3).

H. Takara et al. , "1.01-Pb / s (12 SDM / 222 WDM / 456 Gb / s) Cross-managed Transmission with 91.4-b / s / Hz Aggregate Spectral Efficiency," in ECOC2012, T. Paper. 3. C. 1 (2012) T.A. Sakamoto et al. , “Differential Mode Delay Managed Transmission Line for WDM-MIMO System Using Multi-Step Index Fiber,” J. et al. Lightwave Technol. vol. 30, pp. 2783-2787 (2012). Y. Sasaki et al. , “Large-effective-area uncoupled two-mode multi-core fiber,” ECOC2012, paper Tu. 1. F. 3 (2012). T.A. Ohara et al. "Over-1000-Channel Ultradense WDM Transmission With Supercontinuum Multicarrier Source," IEEE J. Lighttw. Technol. , Vol. 24, pp. 2311-2317 (2006) T.A. Sakamoto, T .; Mori, M.M. Wada, T .; Yamamoto and F.M. Yamamoto, “Coupled Multicore Fiber Design With Low Low Differential Mode Delay for High-Density Space Division Multiplexing,” J. Yamamoto, “Coupled Multicore Fiber Design With Low Low Differential Mode Delay for High-Density Space Division Multiplexing.” Lighttw. Technol. , Vol. 33, no. 6, pp. 1175-1181 (2015) T.A. Hayashi, T .; Taru, O .; Shimakawa, T .; Sasaki and E.I. Sasaoka, “Design and fabrication of ultra-low cross and low-loss multi-core fiber,” Opt. Express, vol. 19, pp. 16576-16592 (2011). W. K. Burns and A.M. F. Milton, “Analytical solution for mode coupling in optical wave guide branches,” J. Am. Quantum Electronics. Vol. 16, no. 4, p. 446 (1980). T.A. Sakamoto, T .; Mori, M.M. Wada, T .; Yamamoto, T .; Matsui, K .; Nakajima and F.M. Yamamoto, “Experimental and numerical evaluation of inter-differential differential delay of weakly-coupled multi-core fibre,” Op. Express 22, 31966-31976 (2014). L. Cherbi and B. Abderrahmane, “Spun Fibers for Compensation of PMD: Theory and Characterization,” INTEC Open Access Publisher, 2012. Sato et al., “Low PM Characteristics of SZ Cable Using Single-Core Branched Tape Core”, IEICE Technical Report, OFT 2006-45 (2006)

  In transmission using a multi-core optical fiber, signal quality deteriorates when crosstalk occurs between cores. Therefore, in order to suppress crosstalk, the cores must be separated by a certain distance or more. In general, in order to ensure sufficient transmission quality in an optical communication system, it is desirable that the power penalty be 1 dB or less. Therefore, in the related art, the crosstalk must be −26 dB or less (for example, non-patent literature). 1 or 4).

  On the other hand, if MIMO technology is used, it is possible to compensate for crosstalk at the receiving end, reduce the inter-core distance, and reduce the power penalty to less than 1 dB by signal processing even if the crosstalk is -26 dB or more. It is possible to improve the space use efficiency.

  However, when applying the MIMO technology, if the group delay difference (DMD) between a plurality of signal lights generated in the transmission path is large, the impulse response width of the transmission path is increased, resulting in an increase in signal processing. In general, in the related art, DMD between a plurality of modes propagating through the same core can be reduced by controlling the refractive index distribution of the optical fiber (for example, see Non-Patent Document 2).

  On the other hand, regarding the relationship between the crosstalk amount between the cores and the DMD, it is known that the DMD increases as the inter-core distance decreases in the related art. In other words, even if crosstalk between cores is allowed, there is a lower limit of the distance between cores in order not to increase DMD, and it has been found that there is a limit in improving space utilization efficiency (for example, non-patent) Reference 5).

  In order to solve the above-mentioned problems, the present invention is an optical fiber in which two or more cores are arranged, and the distance between the cores is appropriately designed according to the twisting period of the fiber, and the coupling between the modes is positively performed. The purpose of this is to reduce the impulse response width of the fiber.

  In order to achieve the above object, in the present invention, in a multi-core optical fiber having a structure in which a plurality of cores are twisted, the rotation angle (twist angle) of the twist per unit length of the fiber is γ [rad / m]. The distance D [μm] between the core centers is set so as to satisfy a predetermined calculation.

Specifically, the multi-core optical fiber according to the present invention is:
A multi-core optical fiber having a plurality of cores arranged with a minimum core spacing D in a clad having a refractive index smaller than that of the core;
The core is a structure having a single propagation mode;
Having a group delay difference between propagating n-order and n ± 1st-order coupling modes of 30 ps / km or more and 500 ps / km or less, D0 (μm);
The minimum core distance D satisfies the formula (C1) with respect to the twist angle γ (rad / m) of the multi-core optical fiber.
(Number C1)
D (μm) = − 3 log (γ / π) + D0−0.3 (C1)

Specifically, the multi-core optical fiber according to the present invention is:
A multi-core optical fiber having a plurality of cores arranged with a minimum core spacing D in a clad having a refractive index smaller than that of the core;
The core has a structure having a plurality of propagation modes;
A group delay difference between the n-order and n ± 1st coupled modes of coupled modes composed of the same LP mode has D0 (μm) that is 30 ps / km or more and 500 ps / km or less,
With respect to the twist angle γ (rad / m) of the multi-core optical fiber, the minimum core interval D satisfies the formula (C2).
(Number C2)
D (μm) = − 3 log (γ / π) + D0−0.3 (C2)

In the multi-core optical fiber according to the present invention,
The core is
It may have a graded type refractive index distribution in which the group delay difference between LPs calculated by the core alone is 600 ps / km or less.

In the multi-core optical fiber according to the present invention,
The core is
Waveguide 2LP mode,
The number of cores may be 6 or 7, and the distance between adjacent cores may be 20 or more and 26 μm or less.

Specifically, the manufacturing method of the optical fiber according to the present invention is:
When the multi-core optical fiber preform is heated and melted for drawing, the multi-core optical fiber preform is drawn while applying torsional rotation according to a predetermined twist angle γ (rad / m), The multi-core optical fiber described above is manufactured.

Specifically, the manufacturing method of the optical fiber according to the present invention is:
When drawing the multi-core optical fiber preform by heating and melting, the multi-core optical fiber preform and the multi-core light after drawing are twisted according to a predetermined twist angle γ (rad / m). The multi-core optical fiber described above is manufactured by drawing while simultaneously applying to the fiber.

Specifically, the manufacturing method of the optical fiber cable according to the present invention is:
Twist rotation according to a predetermined twist angle γ (rad / m) is applied to each of the plurality of optical fiber cores, and the plurality of optical fiber cores to which the twist rotation is applied is cabled.

  The above inventions can be combined as much as possible.

  According to the present invention, an optical fiber in which two or more cores are arranged, the inter-core distance is appropriately designed according to the twisting period of the fiber, and the coupling between modes is positively induced. The impulse response width of the fiber can be reduced.

An example of the schematic of the cross-section of the optical fiber of this invention is shown. An example of the schematic of the fiber to which twist and bending were provided is shown. An example of a change in propagation constant in a two-core fiber to which twisting and bending are given will be shown. An example of a twist angle and a coupling coefficient is shown. An example of the dependence of the even / odd mode electric field amplitude on the distance between the cores when θ changes from 0 ° to 180 ° is shown. An example of the power change of the even mode in a different core structure is shown. An example of the relationship between the distance between cores in different core structures and DMD is shown. An example of the upper and lower limits of the twist angle, the inter-core distance where the even mode and the odd mode are combined at a = 0.45 μm and Δ = 0.35% is shown. An example of the measurement result of the distance dependence of the impulse response width of the prototype 2-core fiber is shown. An example of a refractive index distribution that reduces DMD between LP modes in the same core is shown. An example of a refractive index distribution that reduces DMD between LP modes in the same core is shown. An example of the α dependence of DMD in a graded fiber with a = 9 μm and Δ = 0.4% is shown. An example of the calculation result of electric field distribution is shown. An example of the core radius dependence of the mode field radius in a step type fiber with Δ = 0.35% is shown. An example of DMD and core radius in a graded fiber with Δ = 0.4% and α = 3.12 is shown. An example of calculation of coupling coefficient between coupling modes based on the same LP mode in a fiber in which seven graded core structures having a core radius of 8 μm, a refractive index difference of 0.5%, and α = 3.15 are arranged. Indicates. An example of a multi-core in which seven cores having a core radius of 8 μm, a refractive index difference of 0.5%, and α = 3.15 are arranged in a hexagonal close-packed structure is shown. An example of the combination of the core structure in FIG. 6 is shown. An example of the manufacturing process of the multi-core optical fiber which concerns on this embodiment is shown. An example of the manufacturing process of the multi-core optical fiber which concerns on this embodiment is shown.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited to embodiment shown below. These embodiments are merely examples, and the present invention can be implemented in various modifications and improvements based on the knowledge of those skilled in the art. In the present specification and drawings, the same reference numerals denote the same components.

(Embodiment 1)
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view of a clad 13 and a multi-core optical fiber 11 having two cores 12. A core region having a refractive index of n1 and a clad region of n2 exist and n1> n2.

  In the structure of FIG. 1, the condition of n1> n2 is that the material of each region is pure quartz glass, germanium (Ge), aluminum (Al), phosphorus (P) or other impurities that increase the refractive index, fluorine (F) It can be realized by using quartz glass to which impurities such as boron (B) are added to reduce the refractive index. The distance between the cores is D.

  In the multi-core optical fiber 11, different signal lights can be propagated to each core 12 and transmitted in parallel. However, when the distance between the cores 12 is short, the signal lights propagating through the different cores 12 crosstalk and cause signal degradation. It becomes a factor. In the related art, in order to suppress crosstalk that causes signal deterioration between the cores 12, for example, the distance between the cores 12 is set to about 37 μm (for example, see Non-Patent Document 1).

  On the other hand, crosstalk between the cores 12 can be compensated by installing a MIMO equalizer in the receiver (for example, see Non-Patent Document 2). That is, the distance between the cores 12 can be further reduced. However, in the related art, by reducing the distance between the cores, the DMD between the coupled modes increases, and the signal processing load in the MIMO equalizer increases (for example, see Non-Patent Document 5).

FIG. 2 shows a state where the optical fiber is bent while being twisted. In the figure, an example of a two-core fiber is shown, the bending radius is R, the propagation direction of the optical fiber is z, and the rotation angle of the optical fiber is θ. The propagation constant of the coupled mode is obtained by the following equation (1).

Here, βa and βb are propagation constants when the respective cores 12 exist alone in a state where the fiber is bent, and δβ is obtained by the following equation (2).

Here, κ is a mode coupling coefficient between modes having βa and βb. The mode coupling coefficient is the same as that calculated in the design of the uncoupled multi-core optical fiber according to the related art, and can be easily calculated by the electric field distribution and the refractive index distribution (see Non-Patent Document 6, for example). ). Now, βa and βb can be obtained from Equation (3) below.

  Here, in this multi-core optical fiber 11, it is assumed that the cores 12 have the same refractive index distribution, and have the same propagation constant β0 when there is no bending. FIG. 3 shows the result of calculating the propagation constant of the coupled mode (even mode and odd mode) propagating in the fiber using the above formula.

  Even-mode and odd-mode propagation constants do not intersect at any angle, but in the same type of core 12, the propagation constant difference Δβ becomes small at θ = 90 °, and coupling between the modes is likely to occur.

The coupling between modes can be handled in the same manner as the coupling between modes in the waveguide according to the related art, and can be obtained from the following formula (4) (for example, see Non-Patent Document 7). Here, Ae or Ao is an even-mode or odd-mode electric field amplitude, and γ is a twist angle (rotation angle of twist per unit length, unit is rad / m).

Here, η shown in the following equation (5) is the electric field of the mode between the minute rotation angles Δθ when the rotation angle of twist with respect to the minute propagation distance Δz of the multi-core optical fiber 11 is the minute rotation angle Δθ. It is expressed by the overlap integral of the distribution.

  By numerically calculating Equation (5) based on the minute propagation distance Δz, the distance between the cores 12 that actively generates coupling between modes with respect to the twist angle of the fiber can be obtained. Specifically, using the relationship of Δθ = γ × Δz, η is obtained by dividing the calculation result of equation (5) by Δθ and substituted into equation (4). Here, x and y in Expression (5) represent orthogonal coordinates in the fiber cross section.

FIG. 4 shows the calculated power coupling coefficient | ηΔθ | ² with respect to the rotation angle. Although the distance D between the cores 12 is 15, 20, 25, and 40 μm, it can be seen that the power coupling coefficient takes the maximum value when θ = 90 °. Next, FIG. 5 shows the calculation of the electric field amplitude of each mode when only the even mode is excited at the time of incidence and the fiber is rotated 180 °.

  The bending radius is 80 mm. When the distance between the cores 12 is large, the incident even mode is converted into an odd mode at θ = 90 °. Similar mode conversion occurs at θ = 270 °, and the calculation confirms that the converted odd mode is converted back to the even mode at θ = 270 °.

  In this case, in the related art, the average group delay of each mode that changes in the rotation of θ = 0 to 360 ° is the same in all propagation paths in an ideal homogeneous core structure (for example, Non-Patent Document 8). ,reference.). However, it has been found that a delay difference of several hundreds ps / km is generated at the receiving end due to a core variation caused by a manufacturing error.

  In the present application, when the rotation is performed from θ = 0 ° to 360 °, conversion between modes occurs at θ = 90 ° and 270 °, but when the original mode is completely restored at 360 °, mode coupling is performed. If the power is partially or completely transferred to another mode, it is assumed that mode coupling has occurred.

  On the other hand, when the distance between the cores 12 is small, the light incident in the even mode is not converted to the odd mode at any angle. That is, the incident even mode remains even even after 360 ° rotation, and mode coupling does not occur. However, in the related art, the impulse response width is widened by DMD between coupled modes (for example, see Non-Patent Document 5).

  In summary, in FIG. 5, in the region where Ae is either 0 or 1, mode coupling does not occur when rotated 360 °, and in the region where Ae is not 0 or 1, mode coupling occurs. .

  In other words, by designing with D = 20 to 25 μm, when the fiber is rotated from 0 ° to 360 °, crosstalk occurs between the even mode and the odd mode, and the group delay difference and the coupling mode due to the manufacturing error described above. The effect of suppressing the impulse response spread due to DMD in the meantime is obtained.

  FIG. 6 shows an example in which an even mode is excited on the incident side, and the amplitude of the even mode at θ = 180 ° is calculated with a different core structure. The core structure is as shown in FIG. 18, where γ = 0.8π and R = 140 mm.

  In any core structure, the distance D0 between the cores 12 that causes significant coupling of the even mode electric field amplitude smaller than 1 and larger than 0 is different. However, in each core structure, when DMD between coupling modes is calculated, It was found to be in the range of 30 to 500 ps / km. The relationship between the distance between the cores 12 and the DMD is as shown in FIG.

FIG. 8 shows the calculation of the upper and lower limits of the distance D between the cores 12 where the significant coupling between the modes occurs with respect to γ. The core structure has a core radius of 4.5 μm and a relative refractive index difference = 0.35%. For γ (rad / m), the approximate curve of equation (6) below is a solid line in the figure.

  It is in good agreement with the change of D with respect to γ, and by using this equation, the distance between the cores 12 that causes coupling between modes can be designed. The unit of D0 is μm, and the calculation of D0 with which the DMD between coupled modes is 30 to 500 ps / km is as follows (see Non-Patent Document 5, for example).

  D0 varies depending on the refractive index distribution of each core, and a specific calculation method is to perform numerical calculation (for example, electromagnetic field analysis using a finite element method) for a cross-sectional structure of an optical fiber including a plurality of cores. Then, DMD which is the difference between the calculated even mode and odd mode group velocity is obtained, and the inter-core distance at which DMD is 30 to 500 ps / km can be determined as D0.

Further, for example, a matrix of Expression (7) shown below is constructed from the propagation constant βi of two adjacent core structures and the mode coupling coefficient κ obtained from the mode coupling theory, and obtained by diagonalizing this matrix. Equation (9) based on the diagonal matrix of Equation (8) shown below is DMD.

Similarly, D0 can determine the inter-core distance at which DMD is 30 to 500 ps / km. Note that ω is an angular frequency (see Non-Patent Document 5, for example).

  In the case where the number of cores is larger than 2, for example, if the number of cores is x, x number of coupling modes will occur, but significant coupling is shown in the example of θ = 90 ° in FIG. Since it occurs between the two cores 12 having such a positional relationship, the calculation described so far may be applied to the specific two cores, and the multi-core optical fiber 11 having x cores 12 can be easily applied. .

  FIG. 9 shows the impulse response spread in the case of a two-core fiber having two cores 12 with a core radius of 4.5 μm and a relative refractive index difference of 0.35%, and the distance D between the cores 12 is 20, 25, and 40 μm. The measurement results are shown. Here, a 10 dB down width of the impulse response is shown.

  It can be seen that the impulse response spread increases in proportion to the distance at D = 40 μm, and increases in proportion to the square root of the distance at D = 20, 25 μm. When mode coupling does not occur, it is well known that the impulse response width increases in proportion to the DMD and transmission distance, and that no mode coupling occurs at D = 40 μm is consistent with the calculation.

  Further, the characteristic that the impulse response spread is proportional to the square root of the distance is equivalent to the polarization mode dispersion, and the modes are coupled in the fiber. Also in the calculation, since the mode coupling occurs at D = 20 to 25 μm, the validity of the calculation can be confirmed.

  In addition, it can be seen that by actively causing mode coupling in the fiber, the impulse response width can be particularly reduced in long-distance transmission (several tens of kilometers or more), and the load of MIMO processing at the receiving end can be reduced.

(Embodiment 2)
When the mode propagating for each core 12 is 2 or more, in addition to the small DMD between the same LP modes between the cores 12, the basic mode and higher-order mode DMD propagating through the same core 12 must also be small. First, it is necessary to use a graded index (GI) type or staircase type refractive index distribution as shown in FIGS.

For example, FIG. 12 shows the calculated change in the group delay difference DMD between the LP01-LP11 modes at the wavelength of 1550 nm with respect to the α value of GI when the core radius a = 9 μm and the relative refractive index difference Δ = 0.4%. . Regarding the relationship between the α value and the refractive index distribution, n (r) is the refractive index at the position r in the radial direction from the center, and n ₁ is the refractive index of the core center. The distribution satisfies the equation (10).

  Moreover, the finite element method is used for the calculation. In this structure, the LP01 mode and the LP11 mode propagate, and by changing α, the DMD between the LP01 and LP11 modes can be controlled positively and negatively, and by appropriately controlling α, the DMD can be reduced. In the case of the multi-core optical fiber 11 in which the structure of the core 12 guides a plurality of propagation modes, each mode has a coupled mode based on the LP mode calculated by the core 12 alone.

  For example, in the case of the multi-core optical fiber 11 in which seven cores 12 for guiding two LP modes are arranged, seven coupling modes based on the LP01 mode, seven coupling modes based on the LP11a mode, and LP11b Seven coupling modes based on the mode are generated. The electric field distribution of each mode is as shown in FIG. 13, and the electric field distribution of the coupling mode based on each LP mode is also shown in FIG.

  Even in the case of using a core structure in which a plurality of modes propagate, in the coupling between modes based on the same LP mode as in the first embodiment, the DMD between the coupling modes is in the range of 30 to 500 ps / km. What is necessary is just to adjust the distance between the cores 12 so that it may become. Modes based on different LP modes are not coupled because of a large propagation constant difference. However, as shown in FIG. 12, DMD between LP modes is achieved by making each core 12 have a graded refractive index profile. Can be reduced.

  FIG. 14 shows the result of calculating the change in mode field diameter (MFD) of the fundamental mode with respect to the core radius of the step index fiber when the relative refractive index difference Δ = 0.35%. Currently, ITU-T Fiber Recommendation G. 652, the tolerance of MFD is defined as ± 0.4 μm, and when the radius a = 4.5 μm of a general single mode optical fiber is used as a reference, the core radius changes at least within a range of ± 0.6 μm or less. I understand that

  FIG. 15 shows the calculated DMD change when the core radius changes in the GI core structure with the relative refractive index difference Δ = 0.4% and α = 3.12. Assuming the fluctuation range of ± 0.6 μm of a as described above, it can be seen that a DMD of a maximum of 600 ps / km can occur in absolute value.

  FIG. 16 shows a calculation result of ηΔθ with respect to the distance between the cores 12 in a multi-core in which seven cores 12 having a core radius of 8 μm, a refractive index difference of 0.5%, and α = 3.15 are arranged in a hexagonal close-packed structure. The core arrangement is as shown in FIG. The multi-core optical fiber 11 according to the present embodiment may be a multi-core in which six cores 12 are arranged.

  In the multi-mode multi-core design, it is necessary to set the distance between the cores 12 at which crosstalk occurs between the coupled modes composed of the LP modes and the coupled modes composed of the LP modes. By setting the distance between the cores 12 to 20 to 26 μm, it is possible to design to suppress the impulse response width.

(Embodiment 3)
As shown in FIG. 5, by increasing the twisting cycle, the distance between the cores 12 can be reduced and the spatial multiplexing density can be further improved, as can be seen from the equation (6). In general, even in a single mode fiber according to related art, for the purpose of reducing polarization mode dispersion, as shown in FIG. 19, the fiber is twisted when the fiber is manufactured.

  A preform 14 shown in FIG. 19 is a base material used when producing the multi-core optical fiber 11 according to the present embodiment. The preform 14 rotates at a period of the twist angle γ (rad / m) described above. The rotating preform 14 is heated and melted by the heater 15 and drawn. The multi-core optical fiber 11 according to the present embodiment is manufactured in a twisted state and is wound by the optical fiber winder 17.

  Therefore, in the manufacture of a coupled multi-core optical fiber in which the distance between the cores 12 of the present invention is smaller than that of the uncoupled multi-core optical fiber according to the related art, the distance between the cores 12 can be further increased by manufacturing while twisting the fiber. Can realize a small fiber. As a method of twisting the fiber at the time of manufacturing the fiber, a method according to related art can be used (for example, refer to Non-Patent Document 9).

  As shown in FIG. 20, when the preform 14 is heated and melted for drawing, the rotation with the period of the twist angle γ (rad / m) described above is rotated in accordance with the preform 14 and the drawn multi-core optical fiber 11. The lines may be drawn while being simultaneously applied. In addition, the multi-core optical fiber 11 that has been drawn without rotating the preform 14 may be rotated according to the period of the twist angle γ (rad / m). The drawn multi-core optical fiber 11 according to the present embodiment is manufactured in a twisted state and is wound up by the optical fiber winder 17. Note that the rotation is not limited to a fixed direction, and a forward rotation / reverse rotation may be repeated so that the twist angle is 0 to 360 °.

(Embodiment 4)
As described in the third embodiment, the same effect can be obtained by providing the cable for housing the optical fiber with a structure for twisting the fiber, in addition to the method for twisting the fiber during manufacture. For example, the twist generated by housing the tape core wire according to the related technology in the SZ cable can be similarly applied to the fiber of the present invention (for example, Non-Patent Document 10, reference.).

  Since many cores 12 can be arranged in a smaller area by the optical fiber of the present invention, the multiplicity of the cores 12 is improved, and the transmission capacity is increased. Since the group delay difference between the propagating modes is small, there is an effect that the calculation load in the MIMO processing for compensating for the crosstalk between modes at the receiving end is reduced.

  Since the distance between the cores 12 can be made smaller than the multi-core optical fiber according to the related art, the clad thickness from the core 12 to the end of the clad 13 can be increased and the loss can be reduced when compared with fibers having the same clad diameter. Play.

  The present invention can be applied to a transmission medium in an optical transmission system.

11: Multi-core optical fiber 12: Core 13: Clad 14: Preform 15: Heater 16: Coating device 17: Optical fiber winder

Claims (7)

A multi-core optical fiber having a plurality of cores arranged with a minimum core spacing D in a clad having a refractive index smaller than that of the core;
The core is a structure having a single propagation mode;
Having a group delay difference between propagating n-order and n ± 1st-order coupling modes of 30 ps / km or more and 500 ps / km or less, D0 (μm);
The multicore optical fiber, wherein the minimum core interval D satisfies the formula (C1) with respect to the twist angle γ (rad / m) of the multicore optical fiber.
(Number C1)
D (μm) = − 3 log (γ / π) + D0−0.3 (C1) A multi-core optical fiber having a plurality of cores arranged with a minimum core spacing D in a clad having a refractive index smaller than that of the core;
The core has a structure having a plurality of propagation modes;
A group delay difference between the n-order and n ± 1st coupled modes of coupled modes composed of the same LP mode has D0 (μm) that is 30 ps / km or more and 500 ps / km or less,
The multi-core optical fiber, wherein the minimum core interval D satisfies the formula (C2) with respect to the twist angle γ (rad / m) of the multi-core optical fiber.
(Number C2)
D (μm) = − 3 log (γ / π) + D0−0.3 (C2) The core is
3. The multi-core optical fiber according to claim 2, wherein the multi-core optical fiber has a graded refractive index distribution in which a group delay difference between LPs calculated by a single core is 600 ps / km or less. The core is
Waveguide 2LP mode,
The multi-core optical fiber according to claim 3, wherein the number of cores is 6 or 7, and the distance between adjacent cores is 20 to 26 μm. When drawing by heating and melting the preform for multi-core optical fiber, drawing while applying torsion rotation according to a predetermined twist angle γ (rad / m) to the preform for multi-core optical fiber, An optical fiber manufacturing method for manufacturing the multi-core optical fiber according to claim 1. When drawing the multi-core optical fiber preform by heating and melting, the multi-core optical fiber preform and the multi-core light after drawing are twisted according to a predetermined twist angle γ (rad / m). A method for producing an optical fiber, wherein the multi-core optical fiber according to any one of claims 1 to 4 is produced by drawing the fiber while simultaneously applying it to the fiber. Applying twisting rotation corresponding to a predetermined twist angle γ (rad / m) to each of a plurality of optical fiber cores, and cable forming the plurality of optical fiber core wires to which twisting rotation is applied. An optical fiber cable manufacturing method.

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