The present disclosure relates to a multi-core optical fiber and a design method thereof.
In an optical fiber communication system, a transmission capacity is limited by a non-linear effect generated in an optical fiber and a fiber fuse. In order to alleviate these limitations, spatial multiplexing techniques such as parallel transmission (see, for example, Non Patent Literature 1) using a multi-core optical fiber in which one optical fiber includes a plurality of cores, mode multiplexing transmission (see, for example, Non Patent Literature 2) using a multi-mode fiber in which a plurality of propagation modes exist in a core, and a few-mode multi-core optical fiber (see, for example, Non Patent Literature 3) in which multi-core and mode multiplexing are combined have been studied.
In transmission using a multi-core optical fiber, signal quality deteriorates when crosstalk between cores occurs, and thus it is necessary to separate the cores from each other by a certain distance or more in order to suppress the crosstalk. In general, in order to ensure sufficient transmission quality in an optical communication system, it is desirable to set a power penalty to 1 dB or less, and for this purpose, the crosstalk needs to be −26 dB or less as described in Non Patent Literatures 1 and 4. Such a fiber is referred to as an uncoupled multi-core optical fiber.
Meanwhile, when the distance between the cores is increased in order to sufficiently reduce the inter-core crosstalk, there is a problem that a distance between the core arranged on the outer side and a cladding boundary is decreased in a case of a constant cladding outer diameter, and a bending loss is thus increased (see, for example, Non Patent Literature 5).
In addition, in order to reduce the bending loss of each core, a trench-assisted structure in which a low refractive index region surrounds the periphery of each core as described in Non Patent Literature 6 has been studied for refractive index distribution of each core, however, as described in Non Patent Literature 6, there is a problem that a cutoff wavelength of the core arranged on the center side is increased due to an influence of the low refractive index region of the core arranged in the periphery.
Meanwhile, as described in Non Patent Literature 7, a fiber type called a coupled multi-core optical fiber having whose inter-core crosstalk is large has been studied, and when a multiple-input multiple-output (MIMO) technology is used, it is possible to compensate for the crosstalk at a reception side, and it is possible to reduce the distance between the cores and to make the power penalty less than 1 dB by signal processing even when the crosstalk is −26 dB or more, thereby improving space utilization efficiency.
Also in such a coupled multi-core optical fiber, in order to reduce the bending loss described above, the distance between the core arranged on the outer side and the cladding boundary needs to be a predetermined value or more. In addition, when a core refractive index having trench-type refractive index distribution is adopted in order to reduce the bending loss, a higher-order mode guiding the central core is propagated by the low refractive index region of the peripheral core, and the same problem that the cutoff wavelength is increased as that of the uncoupled multi-core optical fiber described above occurs.
Therefore, in order to solve the above problems, an object of the present invention is to provide a multi-core optical fiber that can prevent an increase in bending loss even when a distance between a peripheral core and a cladding boundary is decreased, and can improve a bending loss characteristic in a state where an influence on a cutoff wavelength and a mode field diameter is small, and a design method thereof.
In order to achieve the above object, a multi-core optical fiber according to the present invention has a structure including a ring-shaped low refractive index region surrounding a plurality of cores.
Specifically, the multi-core optical fiber according to the present invention includes a ring-shaped common trench having a refractive index lower than a refractive index of a cladding and surrounding all cores in a cross section, in which a minimum value E (μm) from a boundary between the core and the cladding to an inner diameter of the common trench satisfies Formula C1:
[Formula C1]
E>−0.35542−35.576Δ−−93.643Δ−−86.407Δ−3 (C1)
where Δ− is a relative index difference (%) of the common trench with respect to the cladding.
Further, the common trench of the multi-core optical fiber according to the present invention has an inner diameter C (μm) and a ring width W (μm) that satisfy a trench volume x of Formula C2:
where x (μm2%) is a product of an area of the common trench in the cross section of the multi-core optical fiber and an absolute value Δ− of the relative index difference of the common trench,
αB0 is a desired bending loss (dB/100 turns) of the core,
λcc0 is a desired cutoff wavelength (nm) of the core,
αB is a bending loss (dB/100 turns) of the core in a case where there is no common trench, and
λcc is a desired cutoff wavelength (nm) of the core in a case where there is no common trench.
Further, a design method according to the present invention is a design method of a multi-core optical fiber, in which
the multi-core optical fiber includes a ring-shaped common trench having a refractive index lower than a refractive index of a cladding and surrounding all cores in a cross section, and a minimum value E (μm) from a boundary between the core and the cladding to an inner diameter of the common trench is designed to satisfy Formula C1.
In addition, in the design method according to the present invention, an inner diameter C (μm) and a ring width W (μm) of the common trench are designed to satisfy a trench volume x of Formula C2.
As the multi-core optical fiber has the above-described structure, the distance E between the peripheral core and the cladding boundary can be reduced, and a larger number of cores can be arranged in a multi-core optical fiber having a smaller cladding diameter or for a predetermined cladding diameter (for example, 125 μm).
By providing the common trench which is the low refractive index region, an effect of improving the bending loss characteristic is obtained without making the cutoff wavelength of the core present in the central region longer than the cutoff wavelength of the peripheral core. In addition, by providing the common trench which is the low refractive index region, an effect of reducing the bending loss is obtained without affecting the mode field diameter.
The present invention can provide the multi-core optical fiber that can prevent an increase in bending loss even when the distance between the peripheral core and the cladding boundary is decreased, and can improve the bending loss characteristic in a state where the influence on the cutoff wavelength and the mode field diameter is small, and the design method thereof.
Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. Note that components having the same reference signs in the present specification and the drawings indicate the same components. Unless otherwise specified, the term “bending loss” means a loss when a bending radius is 30 mm.
Note that the present invention is also applicable to a multi-core structure other than the 4-core structure illustrated in
In general, in designing the multi-core optical fiber, the bending loss is increased and a propagation loss is increased when the core is arranged on the outer side of the cladding, and thus, it is necessary to design an appropriate cladding thickness. Referring to
On the other hand, since the multi-core optical fiber 51 of the present embodiment includes the common trench, the required cladding thickness can be 41 μm and the bending loss can be 10−3 dB/km. That is, the common trench is provided, which makes it possible to design the multi-core optical fiber having a smaller cladding diameter than that of the conventional multi-core optical fiber, and to increase the thickness of the cladding, and as a result, mechanical reliability is improved. In addition, in a case where the cladding diameter is the same as that of the conventional multi-core optical fiber, the common trench is provided, which makes it possible to increase a core density and arrange more cores than those in the conventional multi-core optical fiber.
In designing the common trench, C, W, and Δ− are parameters, but in designing an optical characteristic, a trench volume x, which is a product of an area of the common trench and Δ−, is used as a parameter.
It can be seen from the result that even when C, W, and Δ− are changed, the bending loss characteristic is substantially constant as long as the trench volume x is constant. That is, an effect of improving the bending loss by the common trench can be estimated using the trench volume x.
[Formula 1]
ΔαB=0.043771x−1.3385×10−5x2 (When x<1250 μm2%)
ΔαB=18.114+0.012932x (When x≥1250 μm2%) (1)
(the above formula is indicated by a solid line in
The improvement of the bending loss characteristic by the provision of the common trench acts not only on a propagating fundamental mode but also on a higher-order mode. In a case of a design in which each core is operated in a single mode, as a condition, a higher-order mode does not propagate in a desired communication wavelength band, but it is necessary to perform designing in consideration of an increase in cutoff wavelength due to the provision of the common trench.
[Formula 2]
Δλ=0.10453x−0.00010793x2 (2)
Note that the above formula is indicated by a solid line in
In addition to the bending loss and the cutoff wavelength, a mode field diameter (MFD) is also an important parameter related to a connection loss and a nonlinear characteristic in the optical fiber communication. Therefore, a change in MFD due to the common trench is calculated.
Here, “E” is defined as a core-common trench distance indicating a minimum value between a boundary of the core and an inner diameter boundary of the common trench. “E” when the relative index difference Δ− of the common trench is changed is indicated by a solid line in
That is,
[Formula 3]
E>−0.35542−35.576Δ−−93.643Δ−2−86.407Δ−3 (3)
then, the change in MFD due to the provision of the common trench can be 1% or less, and the change in MFD can be almost ignored in terms of transmission characteristics.
Next, dependence of the effect of the common trench on the core structure is confirmed.
For example, in a case of implementing optical characteristics conforming to the G.652 standard, the cutoff wavelength is increased by the common trench, and thus, it is necessary to consider a core structure having a high bending loss instead of providing the common trench to the core structure having the cutoff wavelength of 1260 nm. Here, it is sufficient to consider three types of structures in each of a design (L5) having the smallest MFD and a design (L6) having a large MFD under three bending loss design conditions (L2 to L4). The effect of improving the bending loss by the provision of the common trench is calculated for the six types of core structures of points (P1 to P6) illustrated in
(a) The bending loss improvement amount Das is changed depending on the core structure.
(b) In the core structure having the same bending loss characteristic, the bending loss improvement amount ΔαB is substantially the same regardless of the MFD.
(c) A curve shape indicating a relationship between the trench volume x and the bending loss improvement amount ΔαB is substantially the same in any core structure, and only an intercept is changed.
[Formula 4]
b=45.3−0.56αB (4)
That is, the bending loss improvement amount Das due to the provision of the common trench is obtained by adding an intercept of Formula (4) to Formula (1), and can be estimated by using:
[Formula 5]
ΔαB=0.043771x−1.3385×10−5x2+b (When x<1250 μm2%)
ΔαB=18.114+0.012932x+b (When x≥1250 μm2%)
b=45.3−0.56αB (5)
That is, in a case where the structure of the core has the bending loss αB and the cutoff wavelength of λcc when it is assumed that the core exists alone, in order to obtain a desired target bending loss value αB0 and the cutoff wavelength×cc0, it is sufficient that the trench volume x is set to satisfy:
For example, in order to obtain characteristics equivalent to those of the general SMF, it is sufficient that αB0<0.1 dB/100 turns and λcc0<1260 nm when the bending radius is 30 mm.
The present invention can be used as a transmission medium in an optical transmission system.
Number | Date | Country | Kind |
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2019-134908 | Jul 2019 | JP | national |
Filing Document | Filing Date | Country | Kind |
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PCT/JP2020/028205 | 7/21/2020 | WO |