Mode exchanger and mode exchange method

Abstract

To provide a mode converter and a mode conversion method that are suitable for large-capacity communication and can also suppress an increase in GDS.SOLUTION: A mode converter 301 according to the present invention includes a multi-core optical fiber that exchanges optical power between desired modes. The multi-core optical fiber has a core spacing Λ serving as a super-mode waveguide region, and is given a twist period Λtwist corresponding to between the desired modes. The mode converter 301 is arranged in the middle of a transmission line 50 of an optical transmission system (at a connection point between a transmission line 50-1 and a transmission line 50-2).SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] This disclosure relates to a mode switcher and a mode switching method having the function of exchanging optical power between propagating modes in a mode-multiplexing transmission system using multicore or multimode optical fibers. [Background technology]

[0002] In optical fiber communication systems, nonlinear effects and fiber fuses that occur in the optical fiber present problems that limit the increase in transmission capacity. To alleviate these limitations, it is necessary to reduce the density of light guided in the optical fiber, and large-core fibers are being considered, as shown in Non-Patent Document 1.

[0003] However, there is a trade-off between reducing bending loss, expanding the single-mode operating region, and increasing the effective area, and there is a problem that there is a limit to the amount of increase in the effective area under certain conditions. Therefore, a mode-multiplexed transmission system that uses a multimode fiber as the transmission fiber and performs parallel transmission using multiple propagating modes is being studied as a technology to achieve a dramatic increase in capacity (see, for example, Non-Patent Document 2).

[0004] Furthermore, optical MIMO transmission has been proposed, which compensates for intermode coupling occurring in the transmission path through MIMO signal processing at the receiving end. In addition, since a large intermode group delay difference increases the load on MIMO signal processing, low intermode group delay difference (DMD) fibers are being investigated (see Non-Patent Documents 3 and 4 for details).

[0005] On the other hand, even with multi-core optical fiber (MCF), MIMO 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 (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 increases, leading to increased signal processing.

[0006] In single-mode multi-core optical fibers in which each core has a structure that propagates a single mode, as described in Non-Patent Document 5, a coupled single-mode MCF in which the core structure and core spacing are adjusted to induce random coupling between modes has been studied.

[0007] 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). [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] T. Matsui, et al., “Applicability of Photonic Crystal Fiber With Uniform Air-Hole Structure to High-Speed ​​and Wide-Band Transmission Over Conventional Telecommunication Bands,” J. Lightwave Technol. 27, 5410-5416, 2009. [Non-patent document 2] N. Hanzawa et al., “Demonstration of mode-division multiplexing transmission over 10 km two-mode fiber with mode coupler,” OFC2011, paper OWA4 (2011) [Non-patent document 3] R. Ryf et al., “Mode-division multiplexing over 96 km of few-mode fiber using coherent 6 × 6 MIMO processing,” J. Lightw. Technol., vol. 30, pp. 521-531 (2012). [Non-patent document 4] T. Mori et al., “Few-mode fibers supporting more than two LP modes for mode-division-multiplexed transmission with MIMO DSP,” J. Lightw. Technol., vol. 32, pp. 2468-2479 (2014). [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. Fujisawa et al., “Group delay spread analysis of coupled-multicore fibers: A comparison between weak and tight bending conditions,” Opt. Commun., vol. 393, no. 9, pp. 232-237, 2017. Summary of the Invention [Problem to be solved by the invention]

[0009] However, as described in Non-Patent Document 5, if the core spacing is made too small, the effective refractive index difference between propagation modes increases, reducing the amount of coupling between modes in the fiber, preventing random coupling, and increasing GDS. Thus, there is a preferred range of core spacing for multicore optical fibers to obtain random coupling. On the other hand, in order to achieve higher capacity communication with a limited fiber cross-sectional area, it is desirable to increase the core density (i.e., reduce the core spacing).

[0010] Thus, multicore optical fibers, which are suitable for high-capacity communication due to their high core density (small core spacing), have the challenge of difficulty in suppressing the increase in GDS due to the core spacing. Therefore, the present invention aims to provide a mode converter and mode conversion method that are suitable for high-capacity communication and can also suppress the increase in GDS in order to solve the above problem. [Means for solving the problem]

[0011] In order to achieve the above object, a mode converter according to the present invention includes a multi-core fiber with a narrow core spacing, and imparts an appropriate twist to the multi-core optical fiber according to the effective refractive index difference between modes.

[0012] Specifically, the mode converter according to the present invention is a mode converter equipped with a multicore optical fiber for exchanging optical power between desired modes, The multi-core optical fiber comprises: The core spacing is a supermode waveguide region, and A corresponding twist period is given between the desired modes. It is characterized by:

[0013] Furthermore, the mode conversion method according to the present invention is a mode conversion method that involves passing light from an optical transmission path through a multicore optical fiber and exchanging optical power between desired modes in the optical transmission path, The multi-core optical fiber comprises: The core spacing is a supermode waveguide region, and A corresponding twist period is given between the desired modes. It is characterized by:

[0014] Even in coupled multicore optical fibers with narrow core spacing and where random coupling between modes cannot be obtained, applying appropriate twisting facilitates the exchange of optical power between arbitrary modes. Therefore, a multicore optical fiber can be obtained that satisfies the conflicting requirements of narrowing the core spacing for high-capacity communication and suppressing the increase in GDS. By utilizing this multicore optical fiber, a mode converter and mode conversion method suitable for high-capacity communication and capable of suppressing the increase in GDS can be provided.

[0015] For example, the twist period is Λtwist=λ / Δneff The present invention is characterized in that it is defined as follows. where Λtwist (mm) is the twist period, λ (mm) is the wavelength of the propagating light, and Δneff is the effective refractive index difference between the desired modes.

[0016] The above inventions can be combined as much as possible. [Effects of the Invention]

[0017] The present invention can provide a mode converter and a mode conversion method that are suitable for large-capacity communications and can also suppress increases in GDS. [Brief explanation of the drawings]

[0018] [Figure 1] 1A and 1B are diagrams illustrating the cross-sectional structure of a coupled multi-core fiber. [Figure 2] FIG. 10 is a diagram illustrating the calculation results of the group delay spread (GDS) in a coupled multicore fiber with a three-core structure. [Figure 3] FIG. 10 is a diagram illustrating the relationship between the core spacing, the number of cores, and the GDS. [Figure 4] 10A and 10B are diagrams illustrating calculation results of the effective refractive index and the maximum differential group delay between modes of a four-core multi-core optical fiber. [Figure 5] 1A and 1B are diagrams illustrating the electric field distribution of each mode in a four-core multi-core optical fiber. [Figure 6] 1 is a diagram illustrating an optical transmission system in which a mode converter according to the present invention is arranged. [Figure 7] FIG. 10 is a diagram illustrating the results of calculating GDS when the transmission line 50 is 1 km long in an optical transmission system in which a mode converter according to the present invention is arranged. [Figure 8] FIG. 10 is a diagram illustrating the relationship between the core spacing Λ and the inter-mode beat length Λ beat in an optical fiber bent at a bending radius R of 1000 mm. [Figure 9] FIG. 10 is a diagram illustrating the calculation results of the change in intensity of each mode with respect to the propagation length in a multi-core optical fiber with a four-core structure. [Figure 10] 1A and 1B are diagrams illustrating a method for designing a multimode optical fiber of a mode converter according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] The following description of the preferred embodiments of the present invention will be given with reference to the accompanying drawings. The preferred embodiments described below are examples of the present invention, and the present invention is not limited to the preferred embodiments. In this specification and the drawings, components having the same reference numerals are intended to represent the same components.

[0020] Figure 1 is a diagram explaining the cross-sectional structure of a coupled multicore fiber. Examples of the number of cores shown are 2, 3, 4, and 6, where the core radius is a and the core spacing (center-to-center spacing of adjacent cores) is Λ. Figure 2 is a diagram explaining the calculation results of the group delay spread (GDS) in a coupled multicore fiber with a three-core structure in which the cores are arranged in a circular ring shape.

[0021] Here, the refractive index profile of the core is step type, the core radius a is 4.5 μm, the relative refractive index difference is 0.35%, and the GDS was calculated when the core spacing Λ was changed. The analytical method and parameters described in Non-Patent Document 6 were used to calculate the GDS. Specifically, the bending radius of the optical fiber was 140 mm, the twisting speed was 0.5πrad / m, and the standard deviation of the twisting speed σ γ is set to 0.1 rad / m, and the wavelength is set to 1550 nm. The propagation distance is set to 1 km. However, as described in Non-Patent Document 6, it is not realistic to assume that the refractive index profile of each core of a multicore fiber is ideally the same, considering the errors that occur in the manufacturing of optical fiber. Therefore, in this calculation, the deviation σ of the relative refractive index difference between the cores is set to Δ It is set to 0.001%.

[0022] As shown in Figure 2, the coupling state between modes can be classified into three main types. In the region where the core spacing is large, the structure is classified as a so-called uncoupled multicore fiber, and the GDS spreads in proportion to the distance due to inter-core skew caused by structural deviations in each core. This is defined as the weak coupling region, and the GDS spreads in proportion to the group velocity difference between each propagation mode when each core is considered to have an independent structure.

[0023] On the other hand, in the region where the core spacing is 25-30 μm, random coupling occurs between modes as designed in Non-Patent Documents 5 and 6, and the GDS exhibits a characteristic that increases in proportion to the square root of the distance. This region is defined as the random coupling region, and the GDS waveform exhibits a Gaussian waveform.

[0024] If the core spacing is further reduced, modes called supermodes that guide across multiple cores propagate, and as the core spacing becomes smaller, the difference in effective refractive index between them increases, suppressing coupling between supermodes. As a result, the GDS increases in proportion to the group delay difference between modes, just like in few-mode fibers. This is defined as the supermode guiding region.

[0025] As shown in Figure 2, as the core spacing is narrowed, the GDS reaches its minimum value in the range of 25-30 μm, and increases as the spacing is narrowed further.

[0026] The above-mentioned trends are not limited to the number of cores and show similar trends. Figure 3 shows the calculations of GDS when the core spacing is set to 18 μm or 25 μm and the number of cores is changed from 2 to 7. The calculation conditions are the same as those described in Figure 2. The cores are also arranged in a circular pattern.

[0027] It can be seen from Figure 3 that the GDS of a multi-core optical fiber with a core spacing of 25 μm is smaller than that of a multi-core optical fiber with a core spacing of 18 μm regardless of the core number. From this result, it can be said that for multi-core optical fibers with any core number, the GDS shows a Gaussian waveform when the core spacing is 25 μm, and random coupling between modes is obtained, whereas the amount of coupling between modes decreases when the core spacing is 18 μm or less.

[0028] (Embodiment 1) This embodiment discloses an optical communication system that uses, as a transmission line, a coupled multi-core fiber (for example, a core spacing of 18 μm or less) with a supermode waveguide region where the core spacing is small and random coupling between modes is not obtained. This optical communication system is characterized in that power between arbitrary modes is exchanged by the multi-core optical fiber of the transmission line or a multi-core optical fiber for mode exchange inserted in the middle of the transmission line.

[0029] 4A and 4B are diagrams illustrating the effective refractive index (FIG. 4A) and the calculation results of the maximum inter-mode group delay difference (FIG. 4B) of the multi-core optical fiber having a four-core structure arranged in an annular shape disclosed in this embodiment. Here, the refractive index profile of the core is a step type, the core radius is 4.5 μm, and the relative refractive index difference is 0.35%. The wavelength used in the calculation is 1550 nm.

[0030] The number of modes propagating through an optical fiber is four, since there are four cores. Figure 4(A) shows that as the core spacing becomes narrower, the effective refractive index difference between modes increases. This means that as the core spacing becomes narrower, the coupling between modes weakens. Figure 4(B) also shows that as the core spacing becomes narrower, the group delay characteristic (GDS) between modes increases. For example, when the core spacing is 18 μm, random coupling between modes cannot be obtained, and the GDS is 460 ps, ​​the same value as the group delay difference between modes.

[0031] 5 is a diagram for explaining a calculation example of the electric field distribution of a propagating mode. In a coupled multi-core fiber (generally with a core spacing of 25 μm or less) in which the inter-core distance is smaller than that of an uncoupled multi-core fiber (generally with a core spacing of 35 μm or more), the propagating mode has an electric field distribution spanning multiple cores.

[0032] 6 is a diagram illustrating an optical transmission system in which the mode converter of this embodiment is arranged. In this optical transmission system, a mode converter 301 of this embodiment is arranged midway along the transmission line 50 (at the connection point between the transmission lines 50-1 and 50-2). The mode converter 301 includes a multi-core optical fiber that exchanges optical power between desired modes, and the multi-core optical fiber is characterized by having a core spacing Λ that forms a supermode waveguide region and being given a twist period Λtwist that corresponds to the desired modes.

[0033] The transmission line 50 is, for example, a coupled multi-core optical fiber. In this case, the coupled multi-core optical fiber of the transmission line 50 and the multi-core optical fiber of the mode converter 301 can be of the same type (a configuration in which the mode converter 301 is arranged midway through the transmission line 50 can be formed by adding a twist to a part of one coupled multi-core optical fiber). Furthermore, the transmission line 50 may be a few-mode fiber. In this case, in order to input a mode propagating through the few-mode fiber of the transmission line 50 into the multi-core optical fiber of the mode converter 301, a mode multiplexer / demultiplexer and a fan-in / fan-out for the multi-core optical fiber are appropriately arranged between the few-mode fiber and the multi-core optical fiber.

[0034] Fig. 7 is a diagram illustrating the calculation results of GDS (GDS when the twisting speed (period length of one rotation) is changed with the twist section L constant) when the transmission line 50 is 1 km in the optical transmission system of Fig. 6. The transmission line 50 is a coupled multi-core optical fiber, and both the transmission lines 50-1 and 50-2 are 500 m long. The multi-core optical fiber of the mode converter 301 has a four-core structure arranged on a ring (here, core spacing Λ = 18 μm), and is twisted while gently bending (here, bending radius R = 1100 mm). Here, the period length of one rotation of the multi-core optical fiber is Λtwist (mm), and the length of the multi-core optical fiber is L = 67 mm. The calculation results show that the GDS is reduced at a specific twist period (here, Λtwist = 45 mm).

[0035] The twist period length Λtwist that can reduce GDS is determined by the difference in effective refractive index between modes. Figure 8 illustrates the relationship between the core spacing Λ and the beat length Λbeat between modes in an optical fiber bent with a bending radius R = 1000 mm. The wavelength λ is 1550 nm. The beat length Λbeat can be calculated by the following equation, where neff is the average effective refractive index in the mode propagation direction. [Formula 1] Λbeat = λ / Δneff λ is the wavelength of the propagating light, and Δneff is the effective refractive index difference between modes.

[0036] In the calculated 4-core fiber, the second and third modes have nearly identical effective refractive indices. Therefore, the combinations of effective refractive index differences Δneff between modes are: between the first and second / third modes (dotted line), between the second / third and fourth modes (dashed line), and between the first and fourth modes (solid line). For example, when the core spacing Λ = 18 μm, the beat length corresponding to the effective refractive index difference between the first and second / third modes, and between the second / third and fourth modes, is approximately 40 mm.

[0037] Beat length is a phase matching condition widely used in long-period gratings that perform mode conversion in optical fibers. Generally, in optical fiber gratings, mode conversion is possible by periodically modulating the refractive index of the core (alternating between high and low refractive index regions in the longitudinal direction of the optical fiber) and setting the period to satisfy Equation 1.

[0038] In other words, as shown in Figure 8, by adding a twist period Λtwist that matches the beat length corresponding to the core spacing, power is exchanged between modes at a specific fiber length, and GDS can be reduced. The relationship between the twist period Λtwist and the beat length is [Formula 2] Λtwist≒Λbeat=λ / Δneff However, Λbeat is a value determined from the effective refractive index averaged in the propagation direction, and Λtwist and the beat length do not necessarily coincide.

[0039] Figure 9 illustrates the calculation results of the intensity change of each mode with respect to propagation length in the aforementioned 4-core multicore optical fiber. From the figure, the first mode is first converted to the second and third modes, and then to the fourth mode. These changes are periodic with respect to propagation length, and by setting an appropriate optical fiber length, the exchange of optical power between the first and fourth modes is achieved. In other words, from results like those in Figure 9, it is possible to find the fiber length L that generates the exchange between the desired modes.

[0040] (Embodiment 2) In this embodiment, a design method for the multicore optical fiber provided in the mode converter 301 is described. Figure 10 is a flowchart illustrating this design method. This design method is Select a multi-core optical fiber with a core spacing that results in the supermode waveguide region (Step S01). Calculate the torsional period corresponding to the modes to be swapped (step S02), and Step S03: Find the torsional interval L from the change in intensity with respect to the propagation length of each mode that is about to undergo mode exchange. Do the following.

[0041] Here, the twist period is Λtwist=λ / Δneff The present invention is characterized in that it is defined as follows. where Λtwist (mm) is the twist period, λ (mm) is the wavelength of the propagating light, and Δneff is the effective refractive index difference between the desired modes.

[0042] (Other embodiments) (1) Core sequence In the first embodiment, a mode converter including a multi-core optical fiber in which cores are arranged in a circular ring shape has been described. However, the core arrangement of the multi-core optical fiber is not limited to a circular ring shape. Even for any core arrangement such as a lattice arrangement or a hexagonal close-packed arrangement, the beat length Λbeat can be calculated using Equation 1, and the twist period Λtwist to be applied to the multi-core optical fiber can be obtained using Equation 2. Then, the length L to be applied to the twist is obtained from the calculation results of the intensity change of each mode with respect to the propagation length, as shown in Fig. 9.

[0043] (2) Bending radius of multicore optical fiber In the first embodiment, two bending radii, 1000 mm and 1100 mm, of the multi-core optical fiber included in the mode converter are exemplified. However, the bending radius of the multi-core optical fiber is not limited to these examples. The bending radius may be 30 mm or more. When the bending radius changes, the relative refractive index difference Δneff changes, and Λbeat changes as shown in Equation 1 (for example, when the bending radius becomes smaller, Δneff increases, and Λbeat becomes shorter). That is, Λbeat according to the bending radius of the multi-core optical fiber included in the mode converter is calculated, and the twist period Λtwist to be applied to the multi-core optical fiber can be obtained by Equation 2. Then, the length L to be twisted is obtained from the calculation result of the intensity change of each mode with respect to the propagation length, as shown in Fig. 9.

[0044] (3) The number of mode converters to be installed in the optical transmission system FIG. 6 illustrates an optical transmission system equipped with one mode converter 301. However, the optical transmission system may be equipped with multiple mode converters 301. As illustrated in FIG. 8, multimode optical fibers with two types of twist (approximately 20 mm and approximately 40 mm) are required to switch all modes (switch between the first and second / third modes, between the second / third modes and the fourth mode, and between the first and fourth modes). Therefore, to perform all mode switching in an optical transmission system, a mode switcher equipped with a multi-core optical fiber with a 20 mm twist and a mode switcher equipped with a multi-core optical fiber with a 40 mm twist are required, and the two mode switchers are arranged in series between the transmission lines. In other words, the optical transmission system is equipped with the number of mode switchers corresponding to the modes to be switched.

[0045] (effect) The mode converter according to the present invention can promote mode coupling from a specific mode to another specific mode by twisting a multi-core optical fiber with a core spacing that forms a supermode waveguide region with a twist period obtained from the beat length and placing the twisted multi-core optical fiber in an optical transmission line. The mode converter according to the present invention reduces the group delay spread after signal propagation, thereby reducing the computational load in MIMO processing to compensate for inter-mode crosstalk at the receiving end. It also averages out characteristic differences, such as loss differences between modes, that occur in the transmission path, thereby improving signal quality at the receiving end. [Industrial Applicability]

[0046] The present invention can realize large-capacity, long-distance communication in a mode-division multiplexing transmission system using a multi-core or multi-mode optical fiber. [Explanation of symbols]

[0047] 50, 50-1, 50-2: Transmission line 301: Mode converter

Claims

[Claim 1] 1. A mode switching method for passing light of an optical transmission line through a multi-core optical fiber and switching optical power between desired modes in the optical transmission line, comprising: The aforementioned multicore optical fiber is The core spacing is a supermode waveguide region. a corresponding twist period is provided between the desired modes; and The aforementioned torsional period is, Λtwist=λ / Δneff It is determined by A mode switching method characterized by the following. where Λtwist (mm) is the twist period, λ (mm) is the wavelength of the propagating light, and Δneff is the effective refractive index difference between the desired modes.

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