Multi-mode multicore optical fiber, and design method of multi-mode multicore optical fiber

Abstract

To provide a multi-mode multicore optical fiber capable of reducing a group delay spread (GDS) and suppressing an increase in signal processing load, and a design method of the multi-mode multicore optical fiber.SOLUTION: A multi-mode multicore optical fiber includes: a core area having at least a first core, a second core, and a third core transmitting two or more plurality of optical wave modes; and a clad region which has a refractive-index below a refractive-index of a core area and is arranged to include the core area. The first core and the second core have different core structures. An effective refractive-index difference between adjacent different optical wave modes between the first core and the second core is 0. On a cross-section of the multi-mode multicore optical fiber, a third core is disposed at a position of a regular triangle having a line connecting the first core and the second core where a movement from an apex not on one side to one side in the vertical direction of the one side occurs just for a prescribed shift amount.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a few-mode multi-core optical fiber and a method for designing a few-mode multi-core optical fiber. [Background technology]

[0002] In optical fiber communication systems, the transmission capacity is limited by nonlinear effects and fiber fuses that occur in the optical fiber. Non-Patent Documents 1 to 3 disclose, as spatial multiplexing technologies that alleviate these limitations, parallel transmission using a multicore fiber having multiple cores in one optical fiber, and mode multiplexing transmission using a multimode fiber in which multiple propagation modes exist in the core.

[0003] Non-Patent Document 2 discloses a MIMO technique capable of compensating for crosstalk at the receiving end.

[0004] Non-Patent Document 4 discloses that in order to ensure sufficient transmission quality in an optical communication system, the power penalty should be kept to 1 dB or less, and the crosstalk between cores, which causes degradation of signal quality in transmission using a multicore fiber, should be kept to -26 dB or less.

[0005] Non-Patent Document 5 discloses that in a single-mode multicore fiber in which each core has a structure that propagates a single mode, the core structure and core spacing are adjusted so as to induce random coupling between modes.

[0006] Non-Patent Document 6 discloses a structure that induces random coupling between the same LP modes in adjacent cores in a few-mode multi-core optical fiber designed to propagate multiple modes in each core. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] H. Takara et al., "1.01-Pb / s (12 SDM / 222 WDM / 456 Gb / s) Crosstalk-managed Transmission with 91.4-b / s / Hz Aggregate Spectral Efficiency," in ECOC2012, paper Th.3.C.1 (2012) [Non-Patent Document 2] T. Sakamoto et al., "Differential Mode Delay Managed Transmission Line for WDM-MIMO System Using Multi-Step Index Fiber," J. Lightwave Technol. vol. 30, pp. 2783-2787 (2012). [Non-Patent Document 3] Y. Sasaki et al., “Large-effective-area uncoupled few-mode multi-core fiber,” ECOC2012, paper Tu.1.F.3 (2012). [Non-Patent Document 4] T. Ohara et al., “Over-1000-Channel Ultradense WDM TransmissionWith Supercontinuum Multicarrier Source,” IEEE J. Lightw. Technol., vol. 24, pp.2311-2317 (2006) [Non-Patent Document 5] T. Sakamoto, T. Mori, M. Wada, T. Yamamoto, F. Yamamoto, and K. Nakajima, "Fiber Twisting- and Bending-Induced Adiabatic / Nonadiabatic Super-Mode Transition in Coupled Multicore Fiber," J. Lightwave Technol. 34, 1228-1237 (2016). [Non-Patent Document 6] T. Sakamoto, T. Mori, M. Wada, T. Yamamoto, F. Yamamoto and K. Nakajima, "Coupled Few-Mode Multicore Fiber With Low Differential Mode Delay Characteristics," J. Lightwave Technol. 35, 1222-1227 (2017). Summary of the Invention [Problem to be solved by the invention]

[0008] According to the MIMO technology disclosed in Non-Patent Document 2, even if the core distance is small and the crosstalk is larger than -26 dB, the power penalty can be reduced to less than 1 dB by signal processing. Therefore, the spatial utilization efficiency can be improved. However, if the group delay spread (GDS) caused by the differential group delay (DMD) between multiple signal lights generated in the transmission path is large, the impulse response width of the transmission path becomes large, leading to an increase in the signal processing load.

[0009] The technology disclosed in Non-Patent Document 5 can make the GDS larger in proportion to the square root of the distance by inducing random coupling between modes in a single-mode multicore fiber, in which each core structure propagates a single mode. As a result, the GDS can be significantly reduced in long-distance transmission (100 km or more).

[0010] According to the technology disclosed in Non-Patent Document 6, in a few-mode multi-core optical fiber designed to propagate multiple modes in each core, the core structure is designed so that the LP01 mode and the LP11 mode propagate in each core. According to Non-Patent Document 6, random coupling between the same LP modes of adjacent cores has been observed, but coupling between different LP modes has not occurred.

[0011] As described in Non-Patent Document 2, the DMD between different LP modes propagating through the same core can be reduced by controlling the refractive index distribution of the optical fiber. However, precise control of the refractive index distribution is required, and it is difficult to make the DMD zero due to manufacturing errors. In particular, it is extremely difficult to make the DMD between different LP modes propagating through the same core zero across the entire communication wavelength band.

[0012] In addition, no technology has been reported to induce random coupling between different LP modes in the same core. Therefore, the GDS in a few-mode multi-core optical fiber increases in proportion to the distance, and in long-distance transmission, the signal processing load increases with the increase in GDS.

[0013] The present disclosure has been made in view of the above problems, and has an object to provide a few-mode multi-core optical fiber and a design method for the few-mode multi-core optical fiber that can reduce group delay spread (GDS) and suppress an increase in the signal processing load. [Means for solving the problem]

[0014] In order to solve the above-mentioned problems, a few-mode multi-core optical fiber according to an embodiment of the present disclosure includes a core region having at least a first core, a second core, and a third core, each of which propagates two or more lightwave modes, and a cladding region having a refractive index lower than that of the core region and arranged to encompass the core region. The first core and the second core have different core structures. An effective refractive index difference between adjacent different lightwave modes between the first core and the second core is zero. In a cross section of the few-mode multi-core optical fiber, a third core is arranged at a position shifted by a predetermined amount from a vertex that is not on one side of an equilateral triangle having a line connecting the first core and the second core as one side. The predetermined shift amount is determined so that the coupling coefficient between the first core and the second core is 10 times or more larger than the coupling coefficient between the first core and the second core when the third core is arranged at the vertex.

[0015] A design method of a few-mode multi-core optical fiber according to an aspect of the present disclosure relates to a few-mode multi-core optical fiber including a core region having at least a first core, a second core, and a third core, each of which propagates two or more lightwave modes, and a cladding region having a refractive index lower than that of the core region and arranged to encompass the core region. The first core and the second core have different core structures. An effective refractive index difference between adjacent different lightwave modes between the first core and the second core is set to 0. In a cross section of the few-mode multi-core optical fiber, a third core is arranged at a position shifted by a predetermined amount from a vertex not on one side of an equilateral triangle having a line connecting the first core and the second core as one side. The predetermined shift amount is determined so that the coupling coefficient between the first core and the second core is 10 times or more as compared with the coupling coefficient between the first core and the second core when the third core is arranged at the vertex. Effect of the Invention

[0016] According to the present disclosure, it is possible to provide a few-mode multi-core optical fiber and a design method for a few-mode multi-core optical fiber that can reduce group delay spread (GDS) and suppress an increase in the signal processing load. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a cross-sectional view showing the structure of a few-mode multi-core optical fiber according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 is a schematic diagram showing the distribution of effective refractive index in a few-mode multi-core optical fiber. [Diagram 3] FIG. 3 is a schematic diagram showing the shift amount. [Figure 4] FIG. 4 is a diagram showing the relationship between the shift amount and the coupling coefficient. [Figure 5A] FIG. 5A is a diagram showing a first example of an arrangement of cores in a few-mode multi-core optical fiber according to an embodiment of the present disclosure. [Figure 5B]FIG. 5B is a diagram showing a second example of a core arrangement in a few-mode multi-core optical fiber according to an embodiment of the present disclosure. [Figure 6A] FIG. 6A is a diagram showing a third example of a core arrangement in a few-mode multi-core optical fiber according to an embodiment of the present disclosure. [Figure 6B] FIG. 6B is a diagram showing a fourth example of a core arrangement in a few-mode multi-core optical fiber according to an embodiment of the present disclosure. [Figure 6C] FIG. 6C is a diagram showing a fifth example of a core arrangement in a few-mode multi-core optical fiber according to an embodiment of the present disclosure. [Figure 6D] FIG. 6D is a diagram showing a sixth example of a core arrangement in a few-mode multi-core optical fiber according to an embodiment of the present disclosure. [Figure 7] FIG. 7 is a diagram showing the relationship between the distance between adjacent cores and crosstalk of the same mode between adjacent cores. [Figure 8] FIG. 8 is a diagram showing impulse response shapes when crosstalk between adjacent cores is changed. [Figure 9] FIG. 9 is a diagram showing the relationship between crosstalk and correlation coefficient. [Figure 10] FIG. 10 is a diagram showing the relationship between crosstalk and standard deviation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Next, an embodiment of the present disclosure will be described in detail with reference to the drawings. In the description, the same components are designated by the same reference numerals and duplicated description will be omitted.

[0019] [1. Configuration of few-mode multi-core optical fiber] Fig. 1 is a cross-sectional view showing the structure of a few-mode multi-core optical fiber according to an embodiment of the present disclosure. Fig. 1 shows a cross-sectional view of a cross section perpendicular to the extending direction (direction of the central axis) of a few-mode multi-core optical fiber FB. The few-mode multi-core optical fiber FB includes a core region and a cladding region 30.

[0020] The core region has at least a first core 11, a second core 12, and a third core 13, each of which propagates two or more light wave modes. Here, the first core 11 and the second core 12 have different core structures (core radius and relative refractive index difference). Furthermore, the third core 13 may have the same core structure as either the first core 11 or the second core 12.

[0021] Alternatively, the third core 13 may have a core structure different from either the first core 11 or the second core 12. In that case, the effective refractive index of the third core 13 may be adjusted so that the effective refractive index of the Lth light wave mode propagating through the third core 13 has a value between the effective refractive index of the Lth light wave mode propagating through the first core 11 and the effective refractive index of the Lth light wave mode propagating through the second core 12.

[0022] Fig. 1 shows an example in which one each of the first core 11, the second core 12, and the third core 13 are arranged. The number and arrangement of cores in the few-mode multi-core optical fiber FB are not limited to the example shown in Fig. 1. For example, the few-mode multi-core optical fiber FB may have any number of cores equal to or greater than two. In addition, in the few-mode multi-core optical fiber FB, the multiple cores may be arranged in a circular ring shape, or in a triangular lattice or square lattice shape.

[0023] Light rays propagating in the few-mode multi-core optical fiber FB propagate by repeated total reflection in the core region (first core 11, second core 12, third core 13). Here, the inclination of the light rays is not allowed to be any arbitrary value, and only light rays having a specific angle can propagate. Such a form of light rays is called a light wave mode. The light wave modes that can propagate in the core region can be found, for example, by electromagnetic field analysis.

[0024] For example, the light wave mode includes a linearly polarized mode (LP mode). mlHere, "m" is the mode order that indicates the angular variation of the intensity distribution of the transverse electric field of a light beam propagating through the core region, and "l" is the mode order that indicates the radial variation of the intensity distribution of the transverse electric field of a light beam propagating through the core region.

[0025] In this embodiment, the core region has at least two lightwave modes. For example, the core region may have an "LP 01 " and "LP 11 The light wave mode of the core region is not limited to the examples given here.

[0026] The cladding region 30 has a refractive index lower than that of the core region, and is disposed so as to enclose the core region. That is, in a cross section perpendicular to the extending direction (direction of the central axis) of the few-mode multi-core optical fiber FB, the cladding region 30 encloses the first core 11, the second core 12, and the third core 13.

[0027] To realize the relationship between the refractive index of the core region and the refractive index of the cladding region 30, impurities that increase the refractive index, such as germanium (Ge), aluminum (Al), or phosphorus (P), may be added to the silica glass. Also, impurities that decrease the refractive index, such as fluorine (F) or boron (B), may be added to the silica glass.

[0028] For the purpose of the following description, the distance between first core 11 and second core 12 (the distance between the centers of the cores) is defined as "Λ".

[0029] [2. Distribution of effective refractive index] Fig. 2 is a schematic diagram showing the distribution of the effective refractive index in a few-mode multi-core optical fiber. In Fig. 2, the vertical axis indicates the effective refractive index, and the horizontal axis indicates the radial position. In Fig. 2, the upper part shows the effective refractive index when the few-mode multi-core optical fiber FB is not bent, and the lower part shows the effective refractive index when the few-mode multi-core optical fiber FB is not bent. For simplicity, only the first core 11 and the second core 12 are shown.

[0030] For example, the distribution of the effective refractive index in a cross section passing through the central axis of the few-mode multi-core optical fiber FB in FIG. 1 is shown in FIG. 2. For example, if the effective refractive index of the Kth light wave mode in the first core 11 is “n 1,K ", the effective refractive index of the (K+1)th light wave mode in the second core 12 is "n 2,K+1 ". The first core 11 and the second core 12 are arranged at positions that are point-symmetric with each other across the central axis of the few-mode multi-core optical fiber FB, and the bending radius of the few-mode multi-core optical fiber FB is "R".

[0031] When the few-mode multi-core optical fiber FB is bent, the effective refractive index of each light wave mode increases in the outer core with respect to the bending direction, and decreases in the inner core. For example, it can be considered equivalent to the effective refractive index distribution having a slope as shown in the lower part of Fig. 2. In Fig. 2, the first core 11 is the inner core with respect to the bending direction, and the second core 12 is the outer core with respect to the bending direction.

[0032] In this case, the effective refractive index of the K-th light wave mode in the first core 11 is expressed by the following formula 1.

number

[0033] Moreover, the effective refractive index of the (K+1)th light wave mode in the second core 12 is expressed by the following formula 2.

number

[0034] To couple different optical modes, the effective refractive index difference "Δn" between adjacent optical modes is an important parameter. The effective refractive index difference is expressed by the following formula 3, with the effective refractive index of one core as the reference.

number

[0035] Equation 3 can be obtained by calculating the absolute value of the difference between Equation 1 and Equation 2. Note that higher-order terms included in the difference are omitted in the calculation process because they are minute.

[0036] As is clear from Equation 3, the effective refractive index difference is determined by the distance "Λ" between the centers of the first core 11 and the second core 12 and the bending radius "R" of the few-mode multi-core optical fiber FB.

[0037] In the coupling of the light wave modes, if the formula 3 is satisfied between the K-th mode in the first core 11 and the (K+1)-th mode in the second core 12, the light wave mode “LP 01 " and "LP 11 In other words, the first core 11 and the second core 12 are connected in the lightwave mode “LP 21 " "LP 02 The same is true in the case where " is propagated" or in a structure in which higher-order modes are propagated.

[0038] The phase matching condition is said to be satisfied when the effective refractive index difference is 0. The phase matching condition is also used as a design item in optical devices such as general optical couplers and WDM couplers. However, in the case of a few-mode multi-core optical fiber FB as the subject of this disclosure, even if the phase matching condition is satisfied, the coupling coefficient becomes small when the distance between the cores is large, and sufficient coupling between light wave modes does not occur at the assumed fiber length.

[0039] The present disclosure is characterized in that the positions of the first core 11, the second core 12, and the third core 13 are adjusted to reduce group delay spread (GDS).

[0040] [3. Setting the shift amount] Fig. 3 is a schematic diagram showing the shift amount. In Fig. 3, the first core 11 and the second core 12 are respectively arranged at positions P1 and P2. The third core 13 is arranged at a position PM which is moved by a predetermined shift amount S from a vertex (position P3) not on one side of an equilateral triangle having a line connecting the first core 11 and the second core 12 as one side toward the one side in a direction perpendicular to the one side.

[0041] 4 is a diagram showing the relationship between the shift amount and the coupling coefficient. In FIG. 4, the lightwave mode “LP 01 " "LP 11 10 shows how the coupling coefficient between the signal .DELTA..times ...

[0042] Here, the calculation results of the coupling coefficient are shown assuming that the core radius of the first core 11 is 7.8 μm, the core radius of the second core 12 is 8.4 μm, the relative refractive index difference of the first core 11 is 0.38%, the relative refractive index difference of the second core 12 is 0.30%, and the distance "Λ" between the centers of the first core 11 and the second core 12 is 44 μm. Note that in the calculation, the effective refractive index difference shown in Equation 3 is set to 0 by setting the bending radius "R" to 70 mm.

[0043] 4, it can be seen that the coupling coefficient increases by one order of magnitude or more when the predetermined shift amount S is increased from 15 μm to 26 μm, for example. It can also be seen that when the predetermined shift amount S is further increased from 26 μm, the coupling coefficient further increases, and the coupling between the optical wave modes is promoted.

[0044] Thus, the point where the predetermined shift amount S is 26 μm is the inflection point where the coupling coefficient increases sharply. At a predetermined shift amount S larger than the inflection point, a significant effect occurs in which the coupling coefficient increases by 10 times or more. In other words, at a predetermined shift amount S larger than the inflection point, coupling between light wave modes is promoted.

[0045] Therefore, in order to obtain a significant effect, the condition that the predetermined shift amount S should satisfy is expressed by the following formula 4.

number

[0046] The lower limit of the predetermined shift amount S, 0.59, is obtained by 26 μm / 44 μm. The upper limit of the predetermined shift amount S is obtained under the condition that the first core 11, the third core 13, and the second core 12 are aligned on the same straight line.

[0047] [4. Example of core placement] Next, core arrangements having a predetermined shift amount S that makes the coupling coefficient 10 times or more are shown. Fig. 5A is a diagram showing a first example of a core arrangement in a few-mode multi-core optical fiber according to an embodiment of the present disclosure. Fig. 5B is a diagram showing a second example of a core arrangement in a few-mode multi-core optical fiber according to an embodiment of the present disclosure.

[0048] In FIG. 5A, six cores are arranged in a circular shape, and in FIG. 5B, eight cores are arranged in a circular shape.

[0049] 5A and 5B, each core can correspond to any one of the first core 11, the second core 12, and the third core 13. An example of a core arrangement is shown in which the first core 11 is "H" and the second core 12 is "L", and further, the third core 13 has the same structure as the second core 12. It can be seen that when any two cores consisting of a combination of "H" and "L" are selected, a third core 13 that satisfies the above-mentioned formula 4 exists.

[0050] Figure 6A is a diagram showing a third example of a core arrangement in a few-mode multi-core optical fiber according to an embodiment of the present disclosure. Figure 6B is a diagram showing a fourth example of a core arrangement in a few-mode multi-core optical fiber according to an embodiment of the present disclosure. Figure 6C is a diagram showing a fifth example of a core arrangement in a few-mode multi-core optical fiber according to an embodiment of the present disclosure. Figure 6D is a diagram showing a sixth example of a core arrangement in a few-mode multi-core optical fiber according to an embodiment of the present disclosure.

[0051] 6A, five cores are arranged. The central core corresponds to third core 13, and the cores on either side of this core correspond to first core 11 and second core 12. Therefore, it can be seen that first core 11, second core 12, and third core 13 are aligned on the same straight line, satisfying the above-mentioned mathematical formula 4.

[0052] In Fig. 6B, seven cores are arranged. The central core corresponds to the third core 13. It can be seen that when any two cores consisting of a combination of "H" and "L" are selected, a third core 13 that satisfies the above-mentioned formula 4 exists.

[0053] In Fig. 6C, seven cores are arranged. The central core corresponds to third core 13, and the cores on either side of that core correspond to first core 11 and second core 12. Therefore, it can be seen that first core 11, second core 12, and third core 13 are aligned on the same straight line, satisfying the above-mentioned mathematical formula 4.

[0054] In FIG. 6D, 12 cores are arranged, and it can be seen that the above-mentioned formula 4 is satisfied for three cores arranged on the same straight line.

[0055] In addition, even in the case of a multi-layer triangular lattice arrangement such as 19 cores arranged in a triangular lattice pattern, all cores except for the outermost core which corresponds only to the first core 11 correspond to any one of the first core 11, the second core 12, and the third core 13, and it can be seen that the above-mentioned mathematical formula 4 is satisfied.

[0056] [5. Crosstalk between cores] Whether or not the phase matching condition for the effective refractive index difference defined by the above-mentioned Equation 3 to be 0 is satisfied depends on the bending radius applied to the few-mode multi-core optical fiber FB.

[0057] According to the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) optical fiber recommendations, the standard for bending loss of general optical fibers is set at a bending radius of 30 mm or more, and losses increase at bending radii smaller than this. In addition, according to the cutoff wavelength measurement method described in the ITU-T recommendations, it is widely known that the bending radius condition during testing is an effective bending radius of 140 mm for optical fibers.

[0058] Therefore, it is reasonable to set the bending radius of the few-mode multi-core optical fiber FB to 30 to 140 mm.

[0059] So far, we have discussed the coupling conditions between different modes between the first core 11 and the second core 12, which are separated by the non-adjacent core interval "Λ". On the other hand, it is necessary for coupling between the same mode to occur in adjacent cores (for example, the first core 11 and the third core 13, or the second core 12 and the third core 13), so the conditions are described below. Figure 7 is a diagram showing the relationship between the distance between adjacent cores and crosstalk of the same mode between adjacent cores. Figure 7 shows the results of calculations of the relationship between the distance between adjacent cores (the distance between the centers of adjacent cores) and crosstalk between adjacent cores in the fundamental mode when the core radius of the adjacent cores is 7.5 μm and the relative refractive index difference is 0.30%.

[0060] In the region where the distance between adjacent cores is 36 μm or more, the amount of coupling between the same LP modes becomes very small (for example, -20 dB / km or less), and therefore it is impossible to realize a few-mode multi-core optical fiber FB in which all LP modes are coupled.

[0061] In addition, for higher-order modes, the amount of crosstalk between adjacent cores is larger than that of the fundamental mode, so it is sufficient to confirm here that the crosstalk of the fundamental mode, which is the worst case condition, is equal to or greater than the desired value (for example, see the documents "ITU-T Recommendation G.652 Characteristics of a single-mode optical fibre and cable" and "ITU-T Recommendation G.650.1 Definitions and test method for linear, deterministic attributes of single-mode fibre and cable").

[0062] [6. Impulse response shape] Fig. 8 shows the impulse response shape when the crosstalk between adjacent cores is changed. Considering that the repeater section sandwiched between optical amplifiers is generally 40 km or longer, we calculated the impulse response shape when the inter-core crosstalk is changed when the transmission distance is 40 km.

[0063] The differential group delay (DMD) between cores is assumed to be 1ns / km. A pulse width that is sufficiently small compared to the impulse response is input, and the output waveform shows the sum of the output light from all cores. Here, the peak waveforms at both ends when the crosstalk is -20dB / km are the same as the input pulse width.

[0064] When the crosstalk is -20 dB / km, there are pulses with high intensity at both ends, and their width is 40 ns, which is the same as the cumulative DMD (1 ns / km x 40 km).When the crosstalk is -10 dB / km, the pulse intensity at both ends is reduced, but the impulse response width is the same as the cumulative DMD.

[0065] On the other hand, when the crosstalk is -5 dB / km or more, the impulse response shape is Gaussian. It is well known that when the inter-core coupling is strong, the impulse response shape is Gaussian. When the crosstalk is -3 dB / km, the shape is also Gaussian, but the width is smaller.

[0066] According to the literature, "R. Ryf, NK Fontaine, B. Guan, R.-J. Essiambre, S. Randel, AH Gnauck, S. Chandrasekhar, A. Adamiecki, G.Raybon, B, Ercan, RP Scott, SJ Ben Yoo, T. Hayashi, T. Nagashima, and T. Sasaki, "1705-km transmission over coupled-core fibre supporting 6 spatial modes," ECOC, paper PD. 3.2 (2014)," when the impulse response shape is Gaussian, the impulse response width is proportional to the square root of the distance.

[0067] Compared to uncoupled fibers, in which the impulse response width is proportional to the distance, the impulse response width is proportional to the square root of the distance, so the impulse response width can be reduced, especially in long-distance transmission. Therefore, we calculated the crosstalk with a Gaussian impulse response width.

[0068] Fig. 9 is a diagram showing the relationship between crosstalk and correlation coefficient. Fig. 9 shows the result of calculating the correlation coefficient when the impulse response shape is fitted with a Gaussian waveform. Fig. 9 shows that when the crosstalk is -10 dB / km or more, the impulse response has a shape with a correlation coefficient of 95% or more with an ideal Gaussian waveform.

[0069] Figure 10 shows the relationship between crosstalk and standard deviation. Figure 10 shows the relationship with the standard deviation obtained by Gaussian fitting of the impulse response waveform. From Figure 10, it can be seen that the larger the crosstalk, the smaller the impulse response width becomes.

[0070] From these results, if crosstalk greater than -10 dB / km is obtained between cores, the impulse response width can be significantly reduced. Therefore, the distance between adjacent cores "Λ0" should be designed so that crosstalk of -10 dB / km or more is obtained. Λ0 is the distance between adjacent cores (for example, between the first core 11 and the third core 13, or between the second core 12 and the third core 13). The amount of crosstalk "XT" between adjacent cores can be calculated by the following formula 5.

number

[0071] Therefore, the condition for the adjacent core distance “Λ0” to obtain crosstalk of −10 dB / km or more is expressed by the following Equation 6.

number

[0072] Here, κ L , β L are the coupling coefficient and the propagation constant of the L-th lightwave mode between adjacent cores (for example, between the first core 11 and the third core 13, or between the second core 12 and the third core 13), respectively. L It is possible to calculate the effective refractive index and electric field distribution of each mode (see the literature "Fundamentals of Optical Waveguides by Okamoto, Corona Publishing, ISBN 4-339-00602-5"), and in multicore fibers it is used to calculate the amount of crosstalk between cores.

[0073] In addition, if the bending radius “R” of the few-mode multi-core optical fiber FB appearing in Equation 6 is set to 140 mm, which is the effective bending radius of the few-mode multi-core optical fiber FB described above, the crosstalk value under the worst condition can be calculated.

[0074] However, in order to ensure that adjacent cores do not come into contact with each other, the following formula 7 must be satisfied: where "a" is the average core radius between adjacent cores.

number

[0075] As described in Non-Patent Document 5, there is a phenomenon in which the impulse response width increases when the cores are close to each other. Empirically, therefore, it is necessary to satisfy the following Equation 8, which is a stricter condition than Equation 7.

number

[0076] From the above, by ensuring that the adjacent core distance “Λ0” and the non-adjacent core distance “Λ” satisfy the desired conditions and that the positional relationship between the first core 11, the second core 12, and the third core 13 meets the conditions of the present disclosure, an optical fiber in which all cores and modes are coupled can be realized.

[0077] [Effects of the embodiment] As described above in detail, the few-mode multi-core optical fiber and the design method of the few-mode multi-core optical fiber according to the present embodiment relate to a few-mode multi-core optical fiber comprising a core region having at least a first core, a second core, and a third core, each of which propagates two or more light wave modes, and a cladding region having a refractive index lower than that of the core region and arranged to encompass the core region.

[0078] The first core and the second core have different core structures. The effective refractive index difference between adjacent different light wave modes between the first core and the second core is set to 0. In the cross section of the few-mode multi-core optical fiber, a third core is disposed at a position shifted a predetermined amount from a vertex that is not on one side of an equilateral triangle having a line connecting the first core and the second core as one side.

[0079] The predetermined shift amount is determined so that the coupling coefficient between the first core and the second core is 10 times or more larger than the coupling coefficient between the first core and the second core when the third core is placed at the vertex.

[0080] This provides a few-mode multi-core optical fiber and a design method for a few-mode multi-core optical fiber that can reduce group delay spread (GDS) and suppress an increase in signal processing load. In particular, it is possible to induce random coupling between different LP modes in the same core, thereby reducing GDS. Furthermore, it is possible to reduce the impulse response width and suppress an increase in signal processing load.

[0081] In the few-mode multi-core optical fiber and the design method of the few-mode multi-core optical fiber according to the present embodiment, the effective refractive index difference Δn is expressed by

number

[0082] Here, the number of light wave modes propagating through the first core and the second core is N (N is an integer of 2 or more), the bending radius of the few-mode multi-core optical fiber is R, and the distance between the first core and the second core is Λ. In addition, the effective refractive index of the Kth light wave mode propagating through the first core is n K , the effective refractive index of the (K+1)th light wave mode propagating in the second core is n K+1 (Here, K is an integer greater than or equal to 1 and less than N).

[0083] This satisfies the phase matching condition within the range of the bending radius of the optical fiber expected in the usage environment, reduces group delay spread (GDS), and suppresses an increase in the signal processing load.

[0084] Furthermore, in the few-mode multi-core optical fiber and the design method of the few-mode multi-core optical fiber according to the present embodiment, a predetermined shift amount is defined as S,

number

[0085] This has the significant effect of increasing the coupling coefficient between optical wave modes by a factor of 10 or more, i.e., it is possible to promote coupling between optical wave modes.

[0086] In the few-mode multi-core optical fiber and the design method of the few-mode multi-core optical fiber according to the present embodiment, the first core and the third core, or the second core and the third core are considered as adjacent cores, and the inter-core distance, the coupling coefficient of the L-th lightwave mode, and the propagation constant of the adjacent cores are respectively defined as Λ, κ L , β L As,

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[0087] This achieves crosstalk between cores of greater than -10 dB / km, significantly reducing the impulse response width.

[0088] Furthermore, in the few-mode multi-core optical fiber and the design method of the few-mode multi-core optical fiber according to the present embodiment, the average core radius of adjacent cores is a,

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[0089] This allows the cores to be spaced apart, which in turn makes it possible to suppress the phenomenon of an increased impulse response width caused by the arrangement of cores close to each other.

[0090] In the few-mode multi-core optical fiber and the design method thereof according to the present embodiment, the bend radius of the few-mode multi-core optical fiber may be in the range of 30 to 140 mm, thereby satisfying the bending loss standard of the optical fiber according to the Optical Fiber Recommendation of the International Telecommunication Union Telecommunication Standardization Sector (ITU-T).

[0091] Furthermore, in the few-mode multi-core optical fiber and the design method for a few-mode multi-core optical fiber according to the present embodiment, the third core may have the same core structure as the first core or the second core. Also, the effective refractive index of the Lth light wave mode propagating through the third core may be a value between the effective refractive index of the Lth light wave mode propagating through the first core and the effective refractive index of the Lth light wave mode propagating through the second core. This can promote coupling between the light wave modes among the first core, the second core, and the third core.

[0092] Although the contents of the present disclosure have been described above along with the embodiments, the present disclosure is not limited to these descriptions, and various modifications and improvements are possible, which are obvious to those skilled in the art. The descriptions and drawings forming a part of this disclosure should not be understood as limiting the present disclosure. Various alternative embodiments, examples, and operating techniques will be apparent to those skilled in the art from this disclosure.

[0093] Of course, the present disclosure includes various embodiments not described herein. Therefore, the technical scope of the present disclosure is defined only by the invention-specifying matters related to the scope of the claims appropriate from the above description. [Industrial Applicability]

[0094] According to the present disclosure, it is possible to provide a few-mode multi-core optical fiber capable of reducing group delay spread (GDS) and suppressing an increase in the signal processing load, and the few-mode multi-core optical fiber can be used as a transmission medium in an optical transmission system. [Explanation of symbols]

[0095] 11 First Core 12 Second Core 13 Third Core 30 Cladding Region FB few-mode multi-core optical fiber P1~P3,PM position

Claims

1. a core region including at least a first core, a second core, and a third core, each of which propagates two or more lightwave modes; a cladding region having a refractive index less than the refractive index of the core region and disposed to encompass the core region; A few-mode multi-core optical fiber comprising: the first core and the second core have different core structures; an effective refractive index difference between adjacent different light wave modes between the first core and the second core is 0; In a cross section of the few-mode multi-core optical fiber, the third core is disposed at a position of an equilateral triangle having a side defined by a line connecting the first core and the second core, the third core being shifted from a vertex not on the side toward the side in a direction perpendicular to the side by a predetermined shift amount, a predetermined shift amount is determined so that a coupling coefficient between the first core and the second core is 10 times or more compared to a coupling coefficient between the first core and the second core when the third core is arranged at the apex.

2. The number of light wave modes propagating through each of the first core and the second core is N (N is an integer of 2 or more), The bending radius of the few-mode multi-core optical fiber is R, The distance between the first core and the second core is Λ, The effective refractive index of the Kth light wave mode propagating through the first core is n １，Ｋ The effective refractive index of the (K+1)th light wave mode propagating through the second core is n ２，Ｋ＋１ (where K is an integer equal to or greater than 1 and less than N), The effective refractive index difference Δn is [0010] The few-mode multi-core optical fiber according to claim 1 ,

3. The predetermined shift amount is S, [0025] The few-mode multi-core optical fiber according to claim 2 ,

4. The first core and the third core, or the second core and the third core are considered as adjacent cores, and the distance between the adjacent cores, the coupling coefficient of the L-th lightwave mode, and the propagation constant are respectively defined as Λ ０ , κ Ｌ , β Ｌ As, [0030] The few-mode multi-core optical fiber according to claim 2 ,

5. The average core radius of the adjacent cores is a, [0045] The few-mode multi-core optical fiber according to claim 4 ,

6. 2. The few-mode multi-core optical fiber according to claim 1, wherein a bending radius of the few-mode multi-core optical fiber is in a range of 30 to 140 mm.

7. The third core has the same core structure as the first core or the second core, or 7. The few-mode multi-core optical fiber according to claim 1, wherein an effective refractive index of an L-th light wave mode propagating through the third core is a value between an effective refractive index of an L-th light wave mode propagating through the first core and an effective refractive index of an L-th light wave mode propagating through the second core.

8. a core region including at least a first core, a second core, and a third core, each of which propagates two or more lightwave modes; a cladding region having a refractive index less than the refractive index of the core region and disposed to encompass the core region; A method for designing a few-mode multi-core optical fiber comprising: the first core and the second core have different core structures; an effective refractive index difference between adjacent different light wave modes between the first core and the second core is set to 0; In a cross section of the few-mode multi-core optical fiber, the third core is disposed at a position of an equilateral triangle having a side defined by a line connecting the first core and the second core, the third core being shifted from a vertex not on the side toward the side in a direction perpendicular to the side by a predetermined shift amount; a design method for a few-mode multi-core optical fiber, determining the specified shift amount so that a coupling coefficient between the first core and the second core is 10 times or more compared to a coupling coefficient between the first core and the second core when the third core is arranged at the vertex.

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