Multicore Fiber

Abstract

To induce a multi-core fiber, which can have different cores induced to couple together between identical LP modes and between different LP modes, to have random coupling between modes at random at a radius of bending free of an increase in loss accompanying a decrease in radius of bending.SOLUTION: There is a multi-core fiber that has, in a section of the optical fiber, a plurality of core regions having refractive indexes larger than the refractive index of a clad region, in which the core regions have a ring type refractive index distribution where an inside refractive index is lower than an outside refractive index, respectively, and also guide a plurality of propagation modes at an in-use wavelength, respectively, and identical and different modes propagated in the respective core regions are coupled together between cores.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a multicore fiber that propagates light in multiple modes in each core. [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. To alleviate these limitations, spatial multiplexing technologies are being considered, such as parallel transmission using multi-core fiber (hereinafter sometimes referred to as MCF (Multi-Core Fiber)) that has multiple cores in a single optical fiber, and mode-multiplexed transmission using multi-mode fiber in which multiple propagation modes exist within the core.

[0003] In transmission using a multicore fiber, crosstalk between cores deteriorates signal quality, so the cores must be spaced apart at a certain distance to suppress crosstalk. Generally, to ensure sufficient transmission quality in optical communication systems, it is desirable to keep the power penalty below 1 dB, and to achieve this, crosstalk must be kept below -26 dB, as described in Non-Patent Documents 1 and 4.

[0004] On the other hand, MIMO (Multiple Input Multiple Output) technology can compensate for crosstalk at the receiving end, shortening the inter-core distance and reducing the power penalty to less than 1 dB through signal processing even when crosstalk is -26 dB or greater, thereby improving spatial efficiency. However, when MIMO technology is applied, if the group delay spread (hereinafter sometimes abbreviated as GDS) caused by the differential group delay (hereinafter sometimes abbreviated as DMD) between multiple signal lights generated in the transmission path is large, the impulse response width of the transmission path increases, leading to increased signal processing.

[0005] In single-mode MCFs, in which each core has a structure that propagates a single mode, as described in Non-Patent Document 5, coupled single-mode MCFs have been investigated in which the core structure and core spacing are adjusted to induce random coupling between modes.

[0006] Generally, even in homogeneous-core single-mode MCFs, the structure of each core varies slightly due to manufacturing errors, and the group velocities of the modes propagating through each core are different. Therefore, DMD will not be zero even when designed with a homogeneous core structure. However, by inducing random coupling between modes, GDS increases in proportion to the square root of the distance, making it possible to significantly reduce GDS, especially in long-distance transmissions (100 km or more).

[0007] On the other hand, few-mode MCFs, which are designed so that multiple modes propagate in each core, can realize a large number of spatial channels in a limited optical fiber cross section and are expected to be used as fibers for high-density spatial multiplexing, and attempts have been made to induce random coupling between modes in this fiber structure as well (e.g., Non-Patent Document 6). However, in the literature, the core structure is designed so that the LP01 and LP11 modes propagate in each core, and random coupling between the same LP modes in adjacent cores is observed, but coupling between different LP modes does not occur.

[0008] 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 profile of the optical fiber, but precise control of the refractive index profile is required, and it is difficult to make the DMD 0 due to manufacturing errors. In particular, it is extremely difficult to make the DMD 0 over the entire communication wavelength band.

[0009] Furthermore, no technology has been reported to date that can induce random coupling between different LP modes within the same core. The GDS in few-mode MCF increases in proportion to the distance, and the increased signal processing load associated with the increased GDS has been an issue in long-distance transmission. [Prior art documents]

Non-licensed literature

[0010]

Non-licensed literature 1

Non-licensed Document 4

Non-licensed Document 5

[0011] Therefore, few-mode multicore fibers have been studied, which are designed so that modes propagating through each core can couple with the same and different modes of adjacent cores by adjusting the core structure and core spacing to desired values (see Non-Patent Document 7 for details). However, because coupling does not occur unless the bending radius is equal to or smaller than a desired value, an increase in loss as the bending radius decreases is an issue.

[0012] The present disclosure aims to induce random coupling between modes at a bending radius that does not cause an increase in loss with a decrease in bending radius in a multicore fiber that is capable of inducing random coupling between the same LP mode and between different LP modes between different cores. [Means for solving the problem]

[0013] The present disclosure relates to a few-mode multi-core fiber in which two or more cores capable of propagating multiple modes are arranged on the cross section of the optical fiber, and by making the refractive index profile of the cores ring-shaped and appropriately designing the number of cores and the core spacing, random coupling between the same LP mode and different LP modes can be induced between different cores with a larger bending radius than in a step-type multi-core fiber.

[0014] Specifically, the multicore fiber of the present disclosure has: The optical fiber has, in its cross section, a plurality of core regions each having a refractive index greater than that of the cladding region; each of the core regions has a ring-shaped refractive index profile in which the refractive index of the inner portion is lower than the refractive index of the outer portion; each of the core regions guides a plurality of propagation modes at a wavelength used; The modes propagating through each core region are characterized by coupling between same and different modes between cores. [Effects of the Invention]

[0015] According to the present disclosure, in a multicore fiber that can induce random coupling between the same LP mode and between different LP modes between different cores by making the refractive index profile of the cores ring-shaped, it is possible to induce random coupling between modes at a bending radius that does not cause an increase in loss due to a decrease in the bending radius. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is an example of a cross-sectional configuration of a multi-core fiber according to an embodiment. [Figure 2] 4 is an example of characteristics of a multicore fiber according to an embodiment. [Figure 3] 1 is an example of group delay spread (GDS) of a multicore fiber according to an embodiment. [Figure 4] 10 is an example of the effective refractive index when the multi-core fiber according to the embodiment is bent. [Figure 5] 10 is an example of the bending radius dependency of the GDS of the multi-core fiber according to the embodiment. [Figure 6] 1 is an example of group delay spread (GDS) of the multi-core fiber according to the embodiment when the bending radius is 80 mm. [Figure 7] 1 is an example of group delay spread (GDS) of the multi-core fiber according to the embodiment when the bending radius is 80 mm. [Figure 8] 1 is an example of core spacing to obtain coupling to the number of cores. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0018] (Example 1) Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. FIG. 1 shows a cross-sectional view of a multicore fiber having four cores as an example of the present disclosure. The multicore fiber of this embodiment has four cores 10 arranged in a square lattice pattern with a core spacing Λ. Each core 10 guides multiple propagation modes at the wavelength used. The propagation modes of each core 10 are arbitrary, but this embodiment shows an example in which each core 10 guides the LP01 mode and the LP11 mode.

[0019] Each core 10 has a core region 11 with a core radius of a and a refractive index of n1. Between the core region 11 and the cladding region 20 of n2, n1 > n2 holds, and the relative refractive index difference is Δ. Inside the core region 11, each core 10 has a low refractive index region 12 with a radius of d and a relative refractive index difference of Δ or less, giving it a ring-shaped refractive index profile. In the present disclosure, this core 10 structure is referred to as a ring-core type.

[0020] Here, the condition n1>n2 can be realized by using pure silica glass or silica glass doped with impurities that increase the refractive index, such as germanium (Ge), aluminum (Al), or phosphorus (P), or impurities that decrease the refractive index, such as fluorine (F) or boron (B), as the material for each region, including the core region 11, the low refractive index region 12, and the cladding region 20.

[0021] In the figure, the low refractive index region 12 in the center of the core 10 is shown to have the same refractive index n2 as the cladding region 20, but the refractive index of the low refractive index region 12 only needs to be lower than the refractive index n1 of the core region 11 and does not need to be the same as the refractive index n2 of the cladding region 20.

[0022] With reference to Figure 2, we will compare the characteristics with the conventional step type (d=0). Figure 2 uses the structure of Non-Patent Document 6, where a=7.5 μm and Δ=0.3%, as the reference, and also shows the case where Δ=0.5%, taking into account the change in the light confinement effect due to the use of the ring core type, and shows the change in the effective refractive index of each mode relative to d / a. Note that the result when d / a=0 is a step type refractive index distribution is shown.

[0023] Increasing d / a increases the effective refractive index n eff decreases, and when d / a = 0.72, the effective refractive index of the step-type reference structure (a = 7.5 μm, Δ = 0.3%) and the LP11 mode (a = 7.5 μm, Δ = 0.5%) match. In other words, this means that the optical properties such as bending loss are roughly the same, and the effective refractive index difference Δn between the LP01 mode and the LP11 mode in that structure R is the step-type Δn S =17.8×10 -4 is significantly smaller than 7.2 × 10 -4 In order to obtain coupling between modes, it is necessary to realize a low effective refractive index difference, and it can be said that the ring-core type is more effective in increasing coupling between modes than the step type.

[0024] Next, we confirm through calculations that the ring-core type multicore fiber increases inter-mode coupling. Figure 3 shows the calculated group delay spread (GDS) after 10 km propagation for a multicore fiber with four cores arranged in a square lattice pattern, where the bending radius over the entire length of the optical fiber is 80 mm. Note that the calculation was performed using principal mode analysis (see, for example, Non-Patent Document 8). The bending radius R is set to 80 mm, as discussed in Non-Patent Document 5.

[0025] For comparison, the calculation results for the step-index multicore fiber are also shown, where the change in GDS is calculated relative to the core spacing Λ. The results show that the ring-core multicore fiber can achieve significantly lower GDS characteristics than the conventional step-index multicore fiber.

[0026] Here, the GDS increases specifically with the core spacing once the core spacing exceeds 24 μm, and at 25 μm it becomes nearly step-type. Therefore, at core spacings greater than 24 μm, the inter-mode coupling between cores decreases as the core spacing widens. Furthermore, the lower limit of the core spacing that achieves the same GDS as that obtained at a core spacing of 24 μm is 20 μm. Therefore, when the number of cores is four, a few-mode multicore fiber with inter-mode coupling can be achieved by setting the core spacing between 20 μm and 24 μm.

[0027] The reason for the decrease in coupling when the core spacing is reduced is as follows. FIG. 4 shows the effective refractive index structure of each mode when the optical fiber is not bent or when it is bent. For simplicity, the effective refractive index structure of the core 11 is shown here. I and 11 O Although the structure will be described as a step type, it is the same for a ring core type. I and 11 O This example shows an example in which the LP01 mode and the LP11 mode propagate in the same fiber. When the optical fiber is not bent, the effective refractive index of the same LP mode between adjacent cores is equal, and the effective refractive index between LP modes is Δn eff0 Let's say.

[0028] The effective refractive index of each mode when the optical fiber is bent is the refractive index of the outer core 11 O The refractive index of the inner core 11 I The refractive index of the core 11 on the right side is reduced, and the refractive index of the core 11 on the right side is reduced. O In the case of the two-core structure shown in Figure 4, the effective refractive index is given by, where R is the bending radius. (Number 1) n eff (1+Λ / (2R)) (1) Left Core 11 I The effective refractive index of (Number 2) n eff (1-Λ / (2R)) (2) This becomes: where n eff indicates the effective refractive index of the LP01 or LP11 mode.

[0029] To allow coupling between different LP modes, the effective refractive index Δn between adjacent different LP modes must be eff is an important parameter. Let n be the effective refractive index of different adjacent LP modes. eff_K and n eff_K+1 When the effective refractive index of one core is used as the reference, (Number 3) Δn eff =|n eff_K - n eff_K+1 (1+Λ1 / R)| (3) It is determined by the core spacing Λ and the bending radius R as shown below.

[0030] As the core spacing Λ is reduced, the effective refractive index difference Δn eff As Λ increases, there is also a lower limit to the core spacing Λ in order to achieve the mode coupling condition of the present disclosure.

[0031] Figure 5 shows the bending radius dependence of GDS when the number of cores is four, the core spacing is 24 μm, and the transmission distance is 10 km. The same figure shows the calculation results of the distance dependence of GDS when the bending radius is 50 mm, the smallest in the figure, for the step-type fiber, and 100 mm, the largest, for the ring-core type. It is known that when coupling does not occur or random coupling is not achieved, GDS increases in proportion to distance (see Non-Patent Document 6). For the step-type fiber, even with the smallest bending radius of 50 mm in the figure, GDS increases proportionally up to 100 km, indicating that random coupling is not achieved. On the other hand, for the ring-core type multicore fiber, even with a bending radius of 100 mm, the slope of GDS with respect to distance decreases at transmission distances of 0.1 km or more. This captures the phenomenon where GDS increases in proportion to the square root of distance due to random coupling. We have confirmed that the core structures and core spacings shown below fall within the region where GDS is proportional to the square root of distance.

[0032] Figure 6 shows the calculated GDS after 10 km propagation for a multi-core fiber with six cores arranged in a circular ring shape and a bending radius of 80 mm over the entire length of the optical fiber. As with the result for a four-core fiber, the ring-core multi-core fiber is able to obtain GDS characteristics that are significantly lower than those of conventional step-index multi-core fibers.

[0033] Here, the GDS increases specifically with the core spacing once the core spacing exceeds 25 μm, and at 26 μm it becomes nearly step-type. This suggests that the wider core spacing at core spacings exceeding 25 μm reduces the inter-core mode coupling. Furthermore, the lower limit of the core spacing that achieves the same GDS as that obtained at a core spacing of 25 μm is 19 μm. Therefore, when the number of cores is 6, a few-mode multicore fiber with inter-mode coupling can be achieved by setting the core spacing between 19 μm and 25 μm.

[0034] Figure 7 shows the calculated GDS after 10 km propagation for a multi-core fiber with eight cores arranged in a circular ring shape and a bending radius of 80 mm over the entire length of the optical fiber. As with the result for a four-core fiber, the ring-core multi-core fiber is able to obtain GDS characteristics that are significantly lower than those of conventional step-index multi-core fibers.

[0035] Here, since the GDS increases specifically with the core spacing when the core spacing is 28 μm or more and becomes nearly step-type when the core spacing is 30 μm, it is thought that the inter-mode coupling between cores decreases as the core spacing widens when the core spacing exceeds 27.5 μm. Furthermore, the lower limit of the core spacing that achieves the same GDS as that obtained at a core spacing of 27.5 μm is 17 μm. Therefore, when the number of cores is 8, a few-mode multicore fiber with inter-mode coupling can be realized by setting the core spacing between 17 μm and 27.5 μm.

[0036] Figure 8 summarizes the above results and shows the upper and lower limits of the core spacing to obtain coupling for each number of cores. In the calculation, only the cases of 4, 6, and 8 cores are shown, but by using this data, the upper and lower limits of the core spacing can be obtained for other numbers of cores as well. If the number of cores is x, the upper limit Λ1 and the lower limit Λ2 of the core spacing are (Number 4) Λ1=0.1875x 2 -1.375x+26.5 (4) Λ2=-0.125x 2 +0.75x+19 (5) where x is an integer.

[0037] As described above, by adopting a ring-core type refractive index profile, the multicore fiber of the present disclosure can generate coupling between the same and different modes between different cores 10 even at a general bending radius that does not cause a decrease in bending radius, such as a bending radius of 80 mm or more to 100 mm. Therefore, the present disclosure can reduce an increase in loss that accompanies a decrease in bending radius in a multicore fiber that can induce random coupling between the same LP mode and different LP modes between different cores.

[0038] Furthermore, the multicore fiber of the present disclosure allows more cores to be arranged in a smaller area, thereby improving the core multiplicity and increasing the transmission capacity.Furthermore, the multicore fiber of the present disclosure has a small group delay spread after signal propagation, thereby reducing the calculation load in MIMO processing for compensating for inter-mode crosstalk at the receiving end. [Industrial Applicability]

[0039] The present disclosure can be used as a transmission medium in an optical transmission system. [Explanation of symbols]

[0040] 10: Core 11: Core Area 12: Low refractive index region 20: Cladding region

Claims

1. The optical fiber has, in its cross section, a plurality of core regions each having a refractive index greater than that of the cladding region; each of the core regions has a ring-shaped refractive index profile in which the refractive index of the inner portion is lower than the refractive index of the outer portion; each of the core regions guides a plurality of propagation modes at a wavelength used; A multicore fiber characterized in that modes propagating through each core region are coupled between the same and different modes between cores.

2. The mode propagating through the core region is an LP01 mode and an LP11 mode. The multicore fiber according to claim 1 .

3. In a cross section of the optical fiber, the four core regions are arranged in a square lattice pattern, The distance between the centers of two adjacent core regions is 20 μm or more and 24 μm or less. The multicore fiber according to claim 1 or 2.

4. In a cross section of the optical fiber, six of the core regions are arranged in an annular shape, The distance between the centers of two adjacent core regions is 19 μm or more and 25 μm or less. The multicore fiber according to claim 1 or 2.

5. In a cross section of the optical fiber, eight of the core regions are arranged in a circular ring shape, The distance between the centers of two adjacent core regions is 17 μm or more and 27.5 μm or less. The multicore fiber according to claim 1 or 2.

6. When the number of cores is x, the upper limit Λ of the distance between the centers of two adjacent core regions is １ and the lower bound Λ ２ is defined by the following formula: The multicore fiber according to any one of claims 1 to 5. L １ =0.1875x ２ -1.375x+26.5 L ２ =-0.125+ ２ +0.75x+19 Here, x is an integer.

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