JP2014002297A - Multi-mode optical fiber and method of designing multi-mode optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multi-mode optical fiber having a bending loss characteristic of no more than 0.5 dB/100 turn at a bending radius of 30 mm.SOLUTION: A multi-mode optical fiber of the present invention has a W-shaped refractive index distribution and includes a core section 11, a trench section 12 which is a low refractive index region surrounding the core section 11, and a cladding section 13 surrounding the trench section 12. A radius (a) of the core section 11, a radius (b) of the border between the trench section 12 and the cladding section 13, and a relative refractive index difference (Δ) of the core section 11 with respect to the cladding section 13 satisfy either: a≤10.2 μm, 1<b/a<1.91, 0.275%≤Δ; or a≤6.3 μm, 1.5≤b/a≤3.00, 0.76%≤Δ.

Description

  The present invention relates to a technology for expanding transmission capacity using a multimode optical fiber.

  In optical fiber communication systems, non-linear effects and fiber fuses that occur in optical fibers are problematic, and transmission capacity is limited. In order to alleviate these restrictions, it is necessary to reduce the density of light guided to the optical fiber. As shown in Non-Patent Documents 1 and 2, a large core fiber has been studied.

  However, bending loss reduction, single mode operation area expansion, and effective area expansion are in a trade-off relationship with each other, and there is a problem that the amount of effective area expansion under certain conditions is limited. In view of this, for the purpose of expanding the transmission capacity, multi-mode transmission using a multimode transmission optical fiber and transmitting signals in parallel using a plurality of propagation modes is being studied.

  In a mode multiplex transmission system, each transmitted signal propagates through a different propagation mode, so all modes must achieve the desired bending loss. For example, ITU-T G.I. The recommended bending loss at 656 is 0.5 dB / 100 turn or less at a bending radius of 30 mm.

Further, in order to realize mode multiplexing transmission using N signals, the optical fiber needs to have at least N or more propagation modes.
The condition that the mode does not propagate is that the bending loss at a bending radius of 140 mm in the wavelength band used is 1 dB / m or more. This condition is based on the assumption that a bending radius of 140 mm is used to measure the cutoff wavelength as described in Non-Patent Document 4 and that the loss does not propagate at 1 dB / m or more as described in Non-Patent Document 1. Is based.

T.A. Matsui, et al. , “Applicability of Photonic Crystal Fiber With Uniform Air-Hole Structure to High-Speed and Wide-Band Transmission OverTownConventionmm. Lightwave Technol. 27, 5410-5416, 2009. K. Mukasa, K .; Imamura, R.A. Sugizaki and T.A. Yagi, "Comparisons of merits on wide-band transmission systems between using extremely improved solid SMFs with Aeff of 160 μm2 and loss of 0.175dB / km and using large-Aeff holey fibers enabling transmission over 600nm bandwidth", the Proceedings of OFC2008, OThR1, Feb. 2008. N. Hanzawa, Kunimasa Saitoh, Taiji Sakamoto, Takashi Matsui, Shigeru Tomita, Masanori Koshiba, "Demonstration of mode-division multiplexing transmission over 10km two-mode fiber with mode coupler", OFC2011, paper OWA4 (2011) Y. Katsyuyama, M .; Tokuda, N.A. Uchida, and M.M. Nakahara, “New method for measuring V-value of a single-mode optical fiber”, Electron. Lett. , Vol. 12, pp. 669-670, Dec. 1976. Okamoto, “Basics of Optical Waveguide”, Corona

  An object of the present invention is to provide a multimode optical fiber having a bending loss characteristic of 0.5 dB / 100 turn or less at a bending radius of 30 mm.

A multimode optical fiber according to the present invention is a multimode having a W-type refractive index distribution including a core portion, a low refractive index region surrounding the core portion, and a cladding portion surrounding the low refractive index region. In the optical fiber, the radius of the core portion is represented as a, the radius of the boundary between the low refractive index region and the cladding portion is represented as b, and the relative refractive index difference of the core portion with respect to the cladding portion is represented by Δ ₁ when expressed, a ≦ 10.2μm, 1 satisfy <b / a <1.91,0.275% ≦ Δ 1.

  In the multimode optical fiber according to the present invention, the b / a may be 1.50 ≦ b / a ≦ 1.75.

A multimode optical fiber according to the present invention is a multimode having a W-type refractive index distribution including a core portion, a low refractive index region surrounding the core portion, and a cladding portion surrounding the low refractive index region. In the optical fiber, the radius of the core portion is represented as a, the radius of the boundary between the low refractive index region and the cladding portion is represented as b, and the relative refractive index difference of the core portion with respect to the cladding portion is represented by Δ ₁ when expressed, a ≦ 6.3μm, satisfy 1.5 ≦ b / a ≦ 3.00,0.76% ≦ Δ 1.

  In the multimode optical fiber according to the present invention, the b / a may be 2.00 ≦ b / a ≦ 2.50.

The multimode optical fiber design method according to the present invention has a W-type refractive index distribution including a core portion, a low refractive index region surrounding the core portion, and a cladding portion surrounding the low refractive index region. A design method of a multimode optical fiber, wherein a radius of the core portion is represented by a, a radius of a boundary between the low refractive index region and the cladding portion is represented by b, and a relative refractive index of the core portion with respect to the cladding portion. when representing the difference delta ₁ and, a ≦ 10.2 .mu.m and 1 <b / a <1.91 selects a combination of a and b / a satisfying, optical signals of wavelength 1530~1625nm the multimode optical fiber the when incident to determine the value of such delta ₁ as 0.275% ≦ delta is ₁ and the number of modes is two.

  In the multimode optical fiber design method according to the present invention, the b / a may be 1.50 ≦ b / a ≦ 1.75.

The multimode optical fiber design method according to the present invention has a W-type refractive index distribution including a core portion, a low refractive index region surrounding the core portion, and a cladding portion surrounding the low refractive index region. A design method of a multimode optical fiber, wherein a radius of the core portion is represented by a, a radius of a boundary between the low refractive index region and the cladding portion is represented by b, and a relative refractive index of the core portion with respect to the cladding portion. when representing the difference delta ₁ and selects a combination of a and b / a satisfying a ≦ 6.3 [mu] m and 1.5 ≦ b / a ≦ 3.00, the wavelength 1530~1625nm the multimode optical fiber A value of Δ ₁ is determined such that 0.76% ≦ Δ ₁ and the number of modes is 3 when an optical signal is incident.

  In the multimode optical fiber design method according to the present invention, the b / a may be 2.00 ≦ b / a ≦ 2.50.

  According to the present invention, it is possible to provide a multimode optical fiber having a bending loss characteristic at a bending radius of 30 mm of 0.5 dB / 100 turn or less.

1 shows a W-type gradient index optical fiber according to the present invention. The propagation mode of the W-shaped refractive index profile optical fiber is limited to 2, shows the largest a, a delta ₁ satisfying the desired bending loss. The propagation mode of the W-type gradient index optical fiber is limited to 2, and the maximum A _eff satisfying the desired bending loss is shown. When b / a = 1.50 and Δ ₂ = −0.4%, the propagation mode is limited to 2, and a combination of a and Δ ₁ satisfying a desired bending loss is shown. Limit the propagation modes of the W-shaped refractive index profile optical fiber 3 shows the largest a, a delta ₁ satisfying the desired bending loss. The propagation mode of the W-type gradient index optical fiber is limited to 3, and the maximum A _eff that satisfies the desired bending loss is shown. When b / a = 2.25 and Δ ₂ = −0.5%, the propagation mode is limited to 3, and a combination of a and Δ ₁ satisfying a desired bending loss is shown.

  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. In the present specification and drawings, the same reference numerals denote the same components.

FIG. 1 shows the refractive index distribution of the proposed optical fiber. The proposed optical fiber includes a core part 11, a so-called W-type refractive index distribution, which includes a core part 11, a trench part 12 that is a low refractive index region surrounding the core part 11, and a cladding part 13 surrounding the trench part 12. Is a multimode optical fiber. Radius of the core portion 11 is represented as a, represents a relative refractive index difference of the core portion 11 the cladding portion 13 delta ₁ and represents the radius of the boundary of the trench portion 12 and the clad portion 13 is b, the trench portion and the cladding portion 13 Expressing relative refractive index difference of 12 and delta _2, delta ₁ or delta ₂ the following equation (2), the formula (3).

なお、コア部１１の屈折率、トレンチ部１２の屈折率、クラッド部１３の屈折率をそれぞれｎ_ｃｏｒｅ、ｎ_{ｔｒｅｎｃｈ}、ｎ_ｃｌａｄとしている。

The refractive index of the core part 11, the refractive index of the trench part 12, and the refractive index of the cladding part 13 are n _core , n _trench , and n _clad , respectively.

(Embodiment 1)
In the present embodiment, a multimode optical fiber and a design method thereof when there are two propagation modes will be described. FIG. 2 and FIG. 3 show the calculation range using the finite element method in which the design range in which the number of propagation modes is 2 under the condition that the used wavelength band is 1530 to 1625 nm and the bending loss is 0.5 dB / 100 turn or less.

Figure 2 is the b / a = from 1 to 2.25, it shows a maximum of a and minimum delta ₁ meeting the above specifications. The four lines in the figure show the calculation results when Δ ₂ = −0.3, −0.35, −0.4, and −0.45%. From the figure, it can be seen that a ≦ 10.2 μm and Δ ₁ ≧ 0.275%.

Here, the lower limit of a is, for example, 8.25 μm from FIG. Considering the condition that the effective area of the LP01 mode is equal to or greater than the effective area of the conventional single mode fiber, the lower limit of a is preferably 5.0 μm. The upper limit of the delta _1, for example, from FIG. 2, is 0.32. In consideration of the condition where the mode number at a wavelength 1530~1625nm is 2, the upper limit of the delta ₁ is preferably 0.60%.

FIG. 3 shows the maximum effective area A _eff at b / a = 1 to 2.25. The maximum A _eff obtained for a step-type optical fiber when a = b is 180 μm ² . On the other hand, the multimode optical fiber according to the present embodiment having a W-type refractive index profile achieves a larger A _eff than the step-type optical fiber in the range of 1 <b / a <1.91. It can be seen that A _eff increases in the range of 50 ≦ b / a ≦ 1.75.

In the multimode optical fiber design method according to the present embodiment, a combination of a and b / a satisfying a ≦ 10.2 μm and 1 <b / a <1.91 is selected, and the wavelengths of 1530 to 1625 nm are selected for the multimode optical fiber. The value of Δ ₁ is determined such that 0.275% ≦ Δ ₁ and the number of modes is 2 when an optical signal of.

When b / a = 1.5 and Δ ₂ = −0.40%, the propagation mode is limited to 2, and a combination of a and Δ ₁ that can obtain a desired bending loss is shown in FIG. Show. Dashed line is a delta ₁ which LP11 mode bending loss wavelength 1625nm is 0.5dB / 100turn. In the optical fiber, since the bending loss of light having a shorter wavelength than that of the long wavelength is small, if the bending loss condition is satisfied at 1625 nm, the bending loss condition is satisfied even at wavelengths shorter than that. Further, the bending loss of the LP01 mode since is smaller than the bending loss of the LP11 mode, the delta ₁ the dashed line is a large area, so that meet the bending loss is LP01 and LP11 modes in the following wavelength range 1625 nm.

The solid line is Δ ₁ at which the bending loss of the LP21 mode at 1530 nm is 1 dB / m. The LP21 mode is the third mode. That is, the propagation mode in the wavelength range of more than 1530nm in the region delta ₁ the solid line is small can limit the LP01 and LP11 modes, the number of modes is two. The bending loss and the number of propagation modes can be obtained by numerical calculation, and the result of FIG. 4 is a result of calculation in a region where a = 8.0 to 10.5 μm and Δ ₁ = 0.25 to 0.45%. is there.

In the region where Δ ₁ is smaller than the solid line, the propagation mode can be made smaller than 3, and in the region where Δ ₁ is larger than the broken line, the LP11 mode which is the second mode propagates, and 0.5 dB / 100 turn at a bending radius of 30 mm. This is the area that becomes: That is, the number of modes is limited to 2, and the combination of a and Δ ₁ that can satisfy all the desired bending losses in the propagation modes is a region where Δ ₁ is smaller than the solid line and Δ ₁ is larger than the broken line. Can be realized.

From the above results, the propagation mode is set to a ≦ 10.2 μm, Δ ₁ ≧ 0.275%, 1 <b / a <1.91, Δ ₂ = −0.3 to −0.45%. By limiting to 2, it is possible to obtain A _eff larger than the step type optical fiber while satisfying a desired bending loss. Thereby, the effective cross-sectional area of the optical fiber can be enlarged, and the nonlinear effect in the fiber can be suppressed.

(Embodiment 2)
In the present embodiment, a multimode optical fiber and its design method when there are three propagation modes will be described. FIG. 5 and FIG. 6 show the calculation range using the finite element method in which the design range in which the number of propagation modes is 3 under the condition that the used wavelength band is 1530 to 1565 nm and the bending loss is 0.5 dB / 100 turn or less.

Figure 5 is the b / a = 1.5 to 3.00, it shows a maximum of a and minimum delta ₁ meeting the above specifications. Note that the three lines in the figure show the calculation results when Δ ₂ = −0.45, −0.5, and −0.55%. From the figure, it can be seen that a ≦ 6.3 μm and Δ ₁ ≧ 0.76%.

FIG. 6 shows the maximum effective area A _eff at b / a = 1.5 to 3.00. It can be seen that an A _{eff of} 50 μm ² or more can be realized even in a three-mode region that cannot be obtained in the case of a step-type optical fiber. In particular, it can be seen that A _eff increases in the range of 2.00 ≦ b / a ≦ 2.50, and the effective area in each mode can be maximized when b / a = 2.25.

Here, the lower limit of a is, for example, 5.2 μm from FIG. Considering the condition that the effective area of the LP01 mode is equal to or greater than the effective area of the conventional single mode fiber, the lower limit of a is preferably 5.0 μm. The upper limit of the delta _1, for example, from 5, is 1.075%. In consideration of the condition that the number of modes is 3 at wavelengths of 1530 to 1565 nm, the upper limit of Δ ₁ is preferably 1.5%.

In the multimode optical fiber design method according to the present embodiment, a combination of a and b / a satisfying a ≦ 6.3 μm and 1.5 ≦ b / a ≦ 3.00 is selected, and the wavelength 1530 is set to the multimode optical fiber. The value of Δ ₁ is determined such that 0.76% ≦ Δ ₁ and the number of modes is 3 when an optical signal of ˜1565 nm is incident.

When b / a = 2.25 and Δ ₂ = −0.50%, the propagation mode is limited to 3, and a combination of a and Δ ₁ capable of obtaining a desired bending loss is shown in FIG. Show. The broken line is Δ ₁ at which the bending loss of the LP21 mode with a wavelength of 1565 nm is 0.5 dB / 100 turn. In the optical fiber, since the bending loss of light having a shorter wavelength than that of the long wavelength is small, if the bending loss condition is satisfied at 1565 nm, the bending loss condition is satisfied even at wavelengths shorter than that. Further, since the LP01, LP11 mode bending loss is smaller than the bending loss of the LP21 mode, the delta ₁ the dashed line is a large area becomes in a wavelength band 1565 nm LP01, LP11 mode is bending loss satisfy the.

The solid line is a delta ₁ which bending loss LP02 mode wavelength 1530nm is 1 dB / m. The LP02 mode is the fourth mode. That is, the propagation mode in the wavelength range of more than 1530nm in the region delta ₁ the solid line is small can limit the LP01, LP11, LP21 mode, the number of modes is three. The bending loss and the number of propagation modes can be obtained by numerical calculation, and the result of FIG. 7 is a result of calculation in a region where a = 5.0 μm to 6.5 μm and Δ ₁ = 0.7 to 1.2%. is there.

In the region where Δ ₁ is smaller than the solid line, the propagation mode can be made smaller than 4. In the region where Δ ₁ is larger than the broken line, the LP21 mode which is the third mode propagates, and 0.5 dB / 100 turn at a bending radius of 30 mm. This is the area that becomes: In other words, the number of modes is limited to 3 and the combination of a and Δ ₁ that can satisfy all the desired bending losses in the propagation modes is a region where Δ ₁ is smaller than the solid line and Δ ₁ is larger than the broken line. realizable.

From the above results, propagation is achieved by setting a ≦ 6.3 μm, Δ ₁ ≧ 0.76%, b / a = 1.5 to 3.00, and Δ ₂ = −0.45 to −0.55%. By limiting the mode to 3, an optical fiber satisfying a desired bending loss can be realized. In a mode multiplex transmission system, the greater the number of modes, the greater the number of mode multiplexes. By realizing a three-mode fiber, the transmission capacity can be expanded as compared with the case of using a two-mode fiber.

  The present invention can realize large-capacity / long-distance communication by suppressing nonlinear phenomena in optical fibers or using modes.

11: Core portion 12: Trench portion 13: Clad portion

Claims (8)

A multi-mode optical fiber having a W-type refractive index distribution including a core portion, a low refractive index region surrounding the core portion, and a cladding portion surrounding the low refractive index region,
The radius of the core portion is represented as a, the low refractive index region and the radius of the boundary of the cladding portion represents is b, when the relative refractive index difference of the core portion to the cladding portion was expressed as delta _1,
a ≦ 10.2μm, 1 <b / a < multimode optical fiber that satisfies 1.91,0.275% ≦ Δ _1. 2. The multimode optical fiber according to claim 1, wherein the b / a is 1.50 ≦ b / a ≦ 1.75. A multi-mode optical fiber having a W-type refractive index distribution including a core portion, a low refractive index region surrounding the core portion, and a cladding portion surrounding the low refractive index region,
The radius of the core portion is represented as a, the low refractive index region and the radius of the boundary of the cladding portion represents is b, when the relative refractive index difference of the core portion to the cladding portion was expressed as delta _1,
a ≦ 6.3μm, 1.5 ≦ b / a ≦ 3.00,0.76% ≦ Δ multimode optical fiber that satisfies _1. The multi-mode optical fiber according to claim 3, wherein the b / a is 2.00 ≦ b / a ≦ 2.50. A design method of a multi-mode optical fiber having a W-type refractive index distribution composed of a core portion, a low refractive index region surrounding the core portion, and a cladding portion surrounding the low refractive index region,
The radius of the core portion is represented as a, the low refractive index region and the radius of the boundary of the cladding portion represents is b, when the relative refractive index difference of the core portion to the cladding portion was expressed as delta _1,
When a combination of a and b / a satisfying a ≦ 10.2 μm and 1 <b / a <1.91 is selected and an optical signal having a wavelength of 1530 to 1625 nm is incident on the multimode optical fiber, 0.275 A method of designing a multimode optical fiber, wherein the value of Δ ₁ is determined such that% ≦ Δ ₁ and the number of modes is 2. 6. The multimode optical fiber design method according to claim 5, wherein the b / a is 1.50 ≦ b / a ≦ 1.75. A design method of a multi-mode optical fiber having a W-type refractive index distribution composed of a core portion, a low refractive index region surrounding the core portion, and a cladding portion surrounding the low refractive index region,
The radius of the core portion is represented as a, the low refractive index region and the radius of the boundary of the cladding portion represents is b, when the relative refractive index difference of the core portion to the cladding portion was expressed as delta _1,
When a combination of a and b / a satisfying a ≦ 6.3 μm and 1.5 ≦ b / a ≦ 3.00 is selected and an optical signal having a wavelength of 1530 to 1565 nm is incident on the multimode optical fiber, 0 A method of designing a multimode optical fiber, wherein a value of Δ ₁ is determined such that .76% ≦ Δ ₁ and the number of modes is 3. The multimode optical fiber design method according to claim 7, wherein the b / a satisfies 2.00 ≦ b / a ≦ 2.50.

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