JP2020046623A - Multimode multicore optical fiber - Google Patents

Abstract

To provide a multimode multicore optical fiber capable of reducing group delay spread (GDS).SOLUTION: The multimode multicore optical fiber can reduce a GDS by random coupling between modes, but can hardly induces the random coupling between the modes by the same core. Consequently, the multimode multicore optical fiber induces a random coupling between modes between adjacent cores; the random coupling between the modes between the adjacent cores can be promoted by controlling an effective refractive index difference Δneff and a coupling coefficient κ between the modes; κ can be controlled by a distance Λ between the cores, and Δneff can be controlled by Λ and a bending radius Rb (mainly Rb); and namely, the multimode multicore optical fiber controls Λ and Rb so that the random coupling between the modes between the adjacent cores is the induced Δneff and κ to reduce the GDS.SELECTED DRAWING: Figure 5

Description

  The present disclosure relates to a multi-core optical fiber in which each core propagates light in a plurality of propagation modes.

  In an optical fiber communication system, the transmission capacity is limited by a nonlinear effect or a fiber fuse generated in the optical fiber. In order to alleviate these restrictions, space transmission such as parallel transmission using a multi-core fiber having a plurality of cores in one optical fiber and mode multiplexing transmission using a multi-mode fiber in which a plurality of propagation modes exist in a core is described. Multiplexing techniques are being studied (for example, see Non-Patent Documents 1-3).

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 Spectrum Efficiency”, in EcoC 2012, In EcoC 2012. 3. C. 1 (2012) T. Sakamoto et al. , "Differential Mode Delay Managed Transmission Line for WDM-MIMO System Using Multi-Step Index Fiber", J. Amer. Lightwave Technology. vol. 30, pp. 2783-2787 (2012). Y. Sasaki et al. , “Large-effective-area uncoupled four-mode multi-core fiber”, ECOC2012, paper Tu. 1. F. 3 (2012). T. Ohara et al. , "Over-1000-Channel Ultradense WDM Transmission With Supercontinuum Multicarrier Source", IEEE J. Org. Lightw. Technol. , Vol. 24, pp. 2311-2317 (2006) T. Sakamoto, T .; Mori, M .; Wada, T.W. Yamamoto, F .; Yamamoto, and K. Nakajima, "Fiber Twisting- and Bending-Induced Adiabatic / Nonadiabatic Super-Mode Transition in Coupled Multicore Fiber", J. Lightwave Technology. 34, 1228-1237 (2016). Okamoto, Basics of Optical Waveguides, Corona ISBN 4-339-00602-5

  In transmission using a multi-core fiber, signal quality deteriorates when crosstalk occurs between cores. Therefore, cores must be separated by a certain distance or more in order to suppress crosstalk. In general, in order to ensure sufficient transmission quality in an optical communication system, it is desirable to set the power penalty to 1 dB or less. For that purpose, as described in Non-Patent Document 1 or 4, the crosstalk must be -26 dB or less. Must.

  On the other hand, if MIMO technology is used, it is possible to compensate for crosstalk at the receiving end, reduce the distance between cores, and reduce the power penalty to less than 1 dB by signal processing even if the crosstalk is -26 dB or more. And space utilization efficiency can be improved. However, when the MIMO technology is applied, if the group delay spread (GDS) caused by the group delay difference (DMD) between a plurality of signal lights generated in the transmission path is large, the impulse response width of the transmission path increases, This leads to an increase in signal processing.

  In a single-mode multi-core fiber in which the structure of each core propagates a single mode, as described in Non-Patent Document 5, the core structure and the core interval are adjusted so as to induce random coupling between supermodes. A combined single-mode MCF has been studied.

  In general, even in the same-core single-mode MCF, the structure of each core is slightly different due to a manufacturing error, and the group velocity of the mode propagating in each core is different. However, by inducing random coupling between the modes, the GDS becomes larger in proportion to the square root of the distance, and the GDS is largely reduced mainly in long-distance transmission (100 km or more). It is possible to

  On the other hand, a multi-mode MCF designed to propagate a plurality of modes in each core can realize a large number of spatial channels in a limited optical fiber cross section, and is expected as a fiber for high-density spatial multiplexing. In the present fiber structure, attempts have been made to induce random coupling between modes (for example, Non-Patent Document 6). However, in the literature, the core structure is designed so that the LP01 mode and the LP11 mode propagate in each core, and random coupling is observed between the same LP modes between adjacent cores, but coupling between different LP modes is performed. Has not occurred. Here, as described in Non-Patent Document 2, the DMD between different LP modes propagating in the same core can be reduced by controlling the refractive index distribution of the optical fiber. Control is required, and it is difficult to set DMD to 0 due to manufacturing errors. In particular, it is extremely difficult to set DMD = 0 over the entire communication wavelength band.

  Further, a technique for inducing random coupling between different LP modes in the same core has not been reported so far, and GDS in several mode MCF increases in proportion to distance, and in long-distance transmission, The problem is that the signal processing load increases as the GDS increases.

  Therefore, an object of the present invention is to provide a multi-mode multi-core optical fiber with reduced GDS.

  In order to solve the above-mentioned problem, the several-mode multi-core optical fiber according to the present invention appropriately designs a core structure and a core interval to induce random coupling between the same LP mode and between different LP modes between adjacent cores. By doing so, the impulse response width of the fiber was reduced.

Specifically, the number mode multi-core optical fiber according to the present invention is a number mode multi-core optical fiber that can propagate a plurality of propagation modes in the wavelength region used by each core,
The bending radius Rb and the interval Λ between the adjacent cores are:
Between the adjacent cores, the relationship between the effective refractive index difference Δn _eff between the L-order propagation mode and the L + 1-order propagation mode and the coupling coefficient κ between the L-order propagation mode and the L + 1-order propagation mode is several C1. Is a value that satisfies

Although GDS can be reduced by random coupling between modes, it is difficult to induce random coupling between modes in the same core. Therefore, the number-mode multicore optical fiber induces random coupling between modes between adjacent cores. Random coupling between modes between adjacent cores can be promoted by adjusting Δn _eff and κ. Then, κ can be adjusted by the inter-core distance Δ, and Δn _eff can be adjusted by the inter-core distance Λ and the bending radius Rb (mainly Rb). That is, in the number-mode multicore optical fiber, the inter-core distance Λ and the bending radius Rb are adjusted so that Δn _eff and κ at which random coupling between modes is induced between adjacent cores, thereby reducing GDS. Therefore, the present invention can provide a few-mode multi-core optical fiber with reduced GDS.

  For example, the several-mode multicore optical fiber according to the present invention is characterized in that the L-order propagation mode is LP01 mode, the L + 1-order propagation mode is LP11 mode, and the bending radius Rb is 280 mm or less.

  Further, the number mode multi-core optical fiber according to the present invention is characterized in that the number of the cores is three, the cores are arranged in a triangular lattice in cross section, and the interval Λ between the cores is 18 μm or less.

  Further, the number mode multi-core optical fiber according to the present invention is characterized in that the number of the cores is four, the cores are arranged in a square lattice shape in cross section, and the interval の between the cores is 18 μm or less.

  The present invention can provide a few-mode multicore optical fiber with reduced GDS.

FIG. 1 is a cross-sectional view illustrating a multimode multicore optical fiber according to the present invention. FIG. 3 is a diagram illustrating a change in the effective refractive index when the number mode multi-core optical fiber according to the present invention is bent. FIG. 1 is a cross-sectional view illustrating a multimode multicore optical fiber according to the present invention. FIG. 4 is a diagram illustrating the relationship between Δn _eff and standardized GDS of the multimode multicore optical fiber according to the present invention. FIG. 5 is a diagram illustrating ranges of Δn _eff and κ at which the normalized GDS is 0.5 or less in the multimode multi-core optical fiber according to the present invention. FIG. 1 is a cross-sectional view illustrating a multimode multicore optical fiber according to the present invention. 5 is a table showing calculation results of Λ, κ, Δneff, and normalized GDS for explaining characteristics of the multimode multi-core optical fiber according to the present invention. FIG. 1 is a diagram illustrating an optical transmission system using a several-mode multi-core optical fiber according to the present invention as a mode coupler.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiment described below is an example of the present invention, and the present invention is not limited to the following embodiment. In the specification and the drawings, components having the same reference numerals indicate the same components.

(Embodiment 1)
FIG. 1 is a cross-sectional view of a multi-core optical fiber having two cores. The relative refractive index difference between the cores is Δ, the core radius is r, and the core region having the refractive index n1 of the core and the cladding region of n2 satisfy n1> n2, and the core interval is 間隔.

  Here, the condition of n1> n2 is that the material of each region is made of pure quartz glass, impurities that increase the refractive index such as germanium (Ge), aluminum (Al), phosphorus (P), fluorine (F), and boron. This can be realized by using quartz glass to which an impurity for reducing the refractive index such as (B) is added.

FIG. 2 shows a profile of the effective refractive index of each mode in a state where the optical fiber is not bent or a state where the optical fiber is bent. FIG. 2 shows an example in which the LP01 mode and the LP11 mode propagate to each core. In a state in which the optical fiber is not bent, the effective refractive index _{n EffLP11} the LP01 mode effective index _{n EffLP01} and LP11 mode is the same in any of the core (FIG. 2 (A)). Here, an effective refractive index difference between LP modes between adjacent cores is _defined as Δn _eff0 .

In a state where the optical fiber is bent, it can be regarded as equivalent to a refractive index distribution having a gradient such that the refractive index of the outer core increases with respect to the bending direction and the refractive index of the inner core decreases (FIG. 2). (B)). FIG. 2 (B) shows a state where it is bent to the left as viewed from the cross section. Therefore, the effective refractive index of each mode also increases in the outer core and decreases in the inner core. That is, if the bending radius is R and n _eff is n _effLP01 or n _effLP11 ,
The effective refractive index of the right core is
n _eff (1 + Λ / (2R)) (1)
The effective index of the left core is
n _eff (1-Λ / (2R)) (2)
Becomes

In order to cause coupling between different LP modes between adjacent cores, the effective refractive index difference Δn _eff between the LP modes is an important parameter. In the case of FIG.
The effective refractive indices of the _LP01 mode and the LP11 mode in the right core are n ^R _effLP01 and n ^R _effLP11 , respectively.
The effective refractive indices of the _LP01 mode and the LP11 mode in the left core are n ^L _{eff LP01} and n ^L _eff LP11, respectively.
Then
Δn _eff = n ^R _effLP11 −n ^L _effLP01
Or Δn _eff = n ^R _effLP01 −n ^L _effLP11
It is.
Then, when Δn _eff approaches zero (the effective refractive indices of both modes are close to each other), coupling is induced between both modes.

Here, using the equations (1) and (2), the effective refractive index difference Δn _eff is generally expressed based on the effective refractive index of one core as follows.
Δn _eff = | n _{eff M} -n _{eff M + 1} (1 + Λ / R _b ) | (3)
That is, the effective refractive index difference is determined by the core interval Λ and the bending radius Rb. Note that n _{eff M} is the effective refractive index of the M-th propagation mode, and the equation (3) can be applied even between the LP01 mode and the LP11 mode.

  FIG. 3 is a cross-sectional view of an optical fiber having three cores and having the cores arranged in a triangular lattice. The core spacing, core radius, and core relative refractive index difference are the same as those of the optical fiber of FIG. Each core has a step-type refractive index distribution, r = 7.5 μm, and Δ = 0.40%. In the optical fiber of FIG. 3, the LP01 and LP11 modes propagate in the wavelength band of 1530 to 1565 nm in each core.

FIG. 4 shows the result of calculating the relationship between Δn _eff and GDS in the optical fiber shown in FIG. The numerical results of this calculation are shown in FIG. Note that GDS indicates a value normalized by dividing by the value of DMD (normalized GDS).

This calculation result shows three types of data of the inter-mode coupling coefficients κ = 140, 260, and 690 m ⁻¹ . The inter-mode coupling coefficient κ is related to the core interval Λ. As the core interval Λ becomes smaller, κ increases. Each κ has a core spacing Λ of 19.5, 18, and 15.5 μm.
In addition, the calculation shows the results of changing the bending radius Rb in three types of 80, 140, and 280 mm while keeping the core structure constant. The bending radius Rb is related to Δn _eff , and the smaller the bending radius Rb, the smaller the Δn _eff .

The following can be seen from FIG.
(I) If the value of Δn _eff is constant, increasing κ (decreasing the inter-core distance Λ) reduces the normalized GDS.
(Ii) When the bending radius Rb is reduced, the normalized GDS is reduced.

That is, κ and Δn _eff change according to the core interval Λ and the bending radius Rb, and the normalized GDS characteristics change. In particular, by realizing a large κ and a small Δn _eff (equivalent to reducing the inter-core distance Λ and reducing the bending radius Rb), the normalized GDS can be reduced.

From FIG. 4, when κ = 140 m ⁻¹ (core interval Λ = 19.5 μm), the normalized GDS does not become 0.5 or less even when Δn _eff is reduced (the bending radius Rb is reduced), but κ = When it is 260 m ⁻¹ or more (core interval Λ = 18.0 μm or less), it can be seen that Δn _{eff at} which the normalized GDS becomes 0.5 or less exists.

  Note that κ is a mode coupling coefficient between the LP01 mode and the LP11 mode between adjacent cores, and can be calculated from the effective refractive index and electric field distribution of each mode as described in Non-Patent Document 6. In a multi-core fiber, it is a parameter generally used for calculating the amount of crosstalk between cores.

Here, when the normalized GDS is reduced by half with respect to the DMD, significant mode coupling occurs, and it can be said that the normalized GDS reduction effect is obtained by random coupling. Therefore, the relationship between κ and Δn _eff at which the normalized GDS becomes 0.5 was calculated. Note that Δn _{eff at} which the normalized GDS becomes 0.5 is a result obtained by approximation using a linear function based on the data in FIG.

FIG. 5 shows the calculation results. In a region where Δn _eff is small or κ is large with respect to the plot position, the normalized GDS becomes 0.5 or less. In a region where κ is less than 140 m ⁻¹ , Δn _eff required for random coupling changes sharply, but in a region where κ is 140 m ⁻¹ or more, the amount of change is small. The change can be approximated by equation (4) and is shown as a solid line in FIG. In the lower right area of the solid line, the normalized GDS can be set to 0.5 or less.

That is, the optical fiber of the present embodiment is a multi-mode multi-core optical fiber that can propagate a plurality of propagation modes in the wavelength region used by each core,
The bending radius Rb and the interval Λ between the adjacent cores are:
The relationship between the effective refractive index difference Δn _eff between the L-order propagation mode and the L + 1-order propagation mode and the inter-mode coupling coefficient κ between the L-order propagation mode and the L + 1-order between the adjacent cores is _expressed by Equation (4). Is a value that satisfies the following region.

  Here, the calculation in FIG. 5 is performed under the condition that the bending radius Rb is 280 mm or less, and this condition is applicable when the bending radius is 280 mm or less. Generally, the allowable bending radius of a single mode fiber is 30 mm, and the bending radius used when measuring a cutoff wavelength is 140 mm. Therefore, it is considered that a sufficiently practical bending radius can be covered by setting the bending radius to 280 mm or less under a cable and a laying condition in a general communication optical fiber network.

(Embodiment 2)
FIG. 6 shows a cross-sectional structure of a four-core fiber. As in the first embodiment, the relative refractive index difference of the core is Δ, the core radius is r, and the core interval is Λ. In the optical fiber of this embodiment, r = 7.5 μm, Δ = 0.3%, and Λ = 18 μm. When the bending radius Rb was 150 mm, it was confirmed by calculation that the normalized GDS was 0.253. Here, Δn _eff obtained under this condition is 0.00161 and κ is 219 m ⁻¹ , which indicates that the condition of Expression (4) is satisfied.

  That is, the range in which the normalized GDS defined by the equation (4) is 0.5 or less is not limited to the three-core structure, and is also applied to the case where the number of cores is different. When the core interval Λ is smaller than 18 μm, κ increases and the normalized GDS becomes smaller. Therefore, the normalized GDS can be set to 0.5 or less in a core interval region of 18 μm or less.

(Embodiment 3)
The optical fibers described in the first and second embodiments can be arranged in the optical transmission system as a mode coupler. FIG. 8 is a diagram illustrating an optical transmission system 100 including the optical fiber as the mode coupler 15.

  The optical fiber transmission system 100 transmits and receives signals using MIMO technology. Specifically, the optical fiber transmission system 100 includes a transmitter 21, a mode multiplexer 22, a multi-core multiplexer 23, an optical fiber 10, a mode coupler 15, a multi-core splitter 31, a mode It includes a duplexer 32, a receiver 33, and an equalizer. In the present embodiment, the number of the mode multiplexers 22 and the mode demultiplexers 32 is D, the number of the transmitters 21 is D × B, the number of the receivers 33 is D × C, and the optical fiber 10 has the first embodiment. The multi-mode multi-core optical fiber described in (2) or (3) is used.

  The signals emitted from the D × B transmitters 21 are converted by the mode multiplexer 22 into B kinds of modes for each of the B transmitters and combined. Next, each light is coupled to D cores by the multi-core multiplexer 23, and propagates through different cores in the multi-core multi-mode fiber.

  At the output end of the optical fiber 10, the multi-core splitter 31 splits the signal into the D port, and the split D-type signal is split into C ports by the mode splitter 32 and received by the receiver. Is done. After that, M signals from the plurality of receivers 33 are input to the equalizer 34 installed at the subsequent stage, and the signal deterioration received by the optical fiber 10 is compensated.

  Note that D is a value equal to or less than the number of cores of the optical fiber 10, and in the present embodiment, D = 2, 3, or 4. B is equal to or less than the number of modes propagating per core 11 of the optical fiber 10, and in this embodiment, B = 2. C is a value equal to or greater than B, and M is a value equal to or greater than 2 and equal to or less than D.

  Note that an FIR filter can be used for the equalizer 34. The FIR filter can also compensate for mode dispersion, chromatic dispersion, and polarization mode dispersion. In addition, not only the light propagating through the same core 11-1 or 11-2, but also the signal propagating through different cores 11-1 or 11-2 are input to the equalizer 34 so that different cores 11-1 and 11-2 are transmitted. Crosstalk between signals propagating in 1 or 11-2 can also be compensated.

The signal transmitted from the transmitter a as the n-th symbol is x _a (n), the signal received from the receiver b as the n-th symbol is y _b (n), and x _a (n) after passing through the FIR filter. Let the restored signal be u _a (n). In this case, when x _a (n) = u _a (n), transmission can be performed without error.

The FIR filter can compensate for linear distortion generated in the optical fiber 10, and can be generated in the optical fiber 10 by appropriately setting the amount of delay and the coefficient of the tap, and can be transmitted from the transmitter 21. Signal degradation due to interference, mode dispersion, chromatic dispersion, and polarization mode dispersion can be compensated. The tap coefficient w _b (i) for correctly restoring the received signal in the receiver 33 can be obtained by using an adaptive equalization algorithm. A known training symbol is added to the transmission signal transmitted from the transmitter 21 in addition to the data portion.

  The signal obtained when the received signal passes through the FIR filter must match the transmitted signal. When the training signal is used, the transmission symbol and the restored symbol can be compared, and the tap coefficient is controlled using an adaptive algorithm so that the restoration error is reduced. After the coefficients are determined using all the training symbols, the subsequent data part is restored by the FIR filter using the determined tap coefficients. In addition, a Least Mean Square (LMS) algorithm and a Recursive least square (RLS) algorithm can be used for the adaptive equalization algorithm.

  A mode coupler 15 is connected to the optical fiber 10 and performs mode coupling between adjacent cores of the optical fiber 10. When the mode coupler 15 performs the mode coupling, the DMD in the optical fiber 10 can be reduced, and the load of the MIMO processing by the equalizer 34 can be reduced.

(The invention's effect)
INDUSTRIAL APPLICABILITY The optical fiber of the present invention can arrange many cores in a smaller area, so that the multiplicity of cores is improved and the transmission capacity is expanded. Further, since the optical fiber of the present invention has a small group delay spread after signal propagation, it has the effect of reducing the calculation load in MIMO processing for compensating for inter-mode crosstalk at the receiving end.

10: Optical fiber 15: Mode coupler 21: Transmitter 22: Mode multiplexer 23: Multi-core multiplexer 31: Multi-core splitter 32: Mode splitter 33: Receiver 34: Equalizer

Claims (4)

A multi-mode multi-core optical fiber that can propagate a plurality of propagation modes in the wavelength region used by each core,
The bending radius Rb and the interval Λ between the adjacent cores are:
Between the adjacent cores, the relationship between the effective refractive index difference Δn _eff between the L-order propagation mode and the L + 1-order propagation mode and the coupling coefficient κ between the L-order propagation mode and the L + 1-order propagation mode is several C1. A number mode multi-core optical fiber, characterized by satisfying the following conditions.

  2. The multi-mode multi-core optical fiber according to claim 1, wherein the L-order propagation mode is an LP01 mode, the L + 1-order propagation mode is an LP11 mode, and a bending radius Rb is 280 mm or less. 3. The number of the cores is 3, and the cores are arranged in a triangular lattice in cross section;
The number mode multi-core optical fiber according to claim 1 or 2, wherein the core spacing Λ is 18 µm or less. The number of the cores is 4, and the cores are arranged in a square lattice shape in cross section,
The number mode multi-core optical fiber according to claim 1 or 2, wherein the core spacing Λ is 18 µm or less.

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