JP5843366B2 - Optical fiber - Google Patents

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 transmission capacity, mode multiplex transmission in which a multimode is used for a transmission optical fiber and signals are transmitted in parallel using a plurality of propagation modes has been studied (for example, see Non-Patent Document 3). .

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.
In addition, in order to realize mode multiplexing transmission using N signals, the optical fiber needs to have at least N or more propagation modes, and the existing multimode fiber has several tens of propagation modes. A number mode fiber in which the number of modes is controlled is required.
In addition, the effective area of each mode should be large in order to reduce the nonlinear effect generated in the optical fiber.

但し、ｎ_ｃｏｒｅおよびｎ_ｃｌａｄはそれぞれコアおよびクラッドの屈折率である。 For example, assuming a step type as shown in FIG. 1 for the refractive index distribution of the optical fiber, calculate the design range where the operating wavelength band is 1530-1625 nm, the bending loss is 0.5 dB / 100 turn or less, and the number of propagation modes is 2. The result is shown in FIG. The relative refractive index difference Δ is obtained by the equation (1).

Here, n _core and n _clad are the refractive indexes of the core and the clad, respectively.

  The condition that the third mode does not propagate is that the bending loss at the bending radius of 140 mm of the LP21 mode, which is the third propagation mode 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.

From FIG. 2, the maximum core radius obtained is a = 8.6 μm, and the relative refractive index difference is Δ = 0.324%. When the effective area of the LP01 mode in the above structure is calculated using the finite element method, it becomes 180 μm ² , and this value is the maximum effective area.

  In order to increase the effective area, a photonic crystal fiber (PCF) described in Non-Patent Document 1 has been studied. FIG. 3 shows a cross-sectional structure of the PCF. The PCF is an optical fiber having a plurality of uniform holes in the longitudinal direction in the optical fiber, and the holes are arranged in a triangular lattice shape with the core region as the center. Note that the gap between holes is Λ, and the hole diameter is d.

In the PCF, when the number of layers of the holes to be arranged is four, the used wavelength band is 1530 to 1625 nm, the bending loss is 0.5 dB / 100 turn or less, and the number of propagation modes is 2, the normalized hole diameter d FIG. 4 shows the maximum effective area A _eff obtained for / Λ. From FIG. 4, it is necessary to set 14 <Λ <16 μm and 0.65 <d / Λ <0.66 in order to set the desired bending loss and the number of modes to 2 and increase the effective area A _eff. Recognize. However, it can be seen that the effective area A _eff obtained cannot significantly improve the effective area obtained with the conventional step index fiber.

T. T. et al. Matsui, et al. , "Applicability of Photonic Crystal Fiber With Uniform Air-Hole Structure to High-Speed and Wide-Band Transmission OverTownConjugation" 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 160μm2 and loss of 0.175dB / km and using large-Aeff holey fibers enabling transmission over 600nm bandwidth", the Proceedings of OFC2008, OThR1 , Feb. 2008. Nobutomo Hanzawa, Kunimasa Saitoh, Taiji Sakamoto, Takashi Matsui, Shigeru Tomita, Masanori Koshiba, "Demonstration of mode-division multiplexing transmission over 10 km two-mode fiber with mode coupler", OFC2011, paper OWA4 (2011) Y. Katsyuyama, M .; Tokuda, N .; Uchida, and M.M. Nakahara, “New method for measuring the V-value of a single-mode optical fiber”, Electron. Lett. , Vol. 12, pp. 669-670, Dec. 1976.

  In order to reduce the non-linear phenomenon that occurs in the optical fiber, it is necessary to further increase the effective area, but there has been a problem that it is difficult to significantly increase the effective area even if a conventional photonic crystal fiber is used. .

  The present invention realizes a multimode fiber having a larger effective area by providing a multimode optical fiber having a plurality of holes.

The optical fiber of the present invention is an optical fiber having two propagation modes , LP ₀₁ mode and LP ₁₁ mode,
A plurality of uniform holes in the longitudinal direction in the optical fiber, the holes are arranged in a triangular lattice shape around the core region ,
A core region having a size corresponding to a region obtained by removing seven holes located in the center of the triangular lattice arrangement;
When the number of layers of the holes to be arranged is N, the space between the holes is Λ (μm), and the diameter of the holes is d (μm), the value of d / Λ is given by meet the,
In both of the two propagation modes, the bending loss with respect to the bending of R30 mm is 0.5 dB / 100 turn or less .
(Equation 2)
(A ₁₁ + B ₁₁ Λ + C ₁₁ Λ ² ) <d / Λ <(A ₂₁ + B ₂₁ Λ + C ₂₁ Λ ² ) (2)
A ₁₁ = 0.32 + 0.15105N−0.02724N ² + 0.00137N ³
B ₁₁ = 0.2162−0.13547N + 0.018337N ² −0.00082N ³
C ₁₁ = −0.01949 + 0.01066N−0.00124N ² + 0.00005N ³
A ₂₁ = 0.244 + 0.23487N−0.06006N ² + 0.00428N ³
B ₂₁ = 0.17839−0.13719N + 0.02704N ² −0.0017N ³
C ₂₁ = −0.015959 + 0.01045N−0.00193N ² + 0.000116667N ³

  When the number N of holes is four, the gap interval Λ is greater than 5.88 μm and less than 7.3 μm, and the value of d / Λ is greater than 0.30 and less than 0.32.

  When the number N of the holes is five, the gap interval Λ is more than 5.72 μm and less than 7.55 μm, and the value of d / Λ is more than 0.263 and less than 0.297.

  When the number N of the holes is 6, the gap interval Λ is more than 5.69 μm and less than 7.35 μm, and the value of d / Λ is more than 0.245 and less than 0.287.

  When the number N of the holes is seven, the gap interval Λ is more than 5.65 μm and less than 7.0 μm, and the value of d / Λ is more than 0.238 and less than 0.282.

According to the present invention, it is possible to realize a bending loss of 0.5 dB / 100 turn or less and an effective area of 180 μm ² or more in a multi-mode optical fiber for mode multiplexing transmission having a used wavelength band of 1530 to 1625 nm and a propagation mode number of 2. It is possible to suppress nonlinear effects in the fiber.

The refractive index profile of a step type optical fiber is shown. In the step type optical fiber, the propagation mode is set to 2, and a and Δ regions for satisfying a desired bending loss are shown. An example of sectional drawing of a photonic crystal fiber is shown. In the photonic crystal fiber, the effective area is calculated when the wavelength band used is 1530 to 1625 nm, the bending loss is 0.5 dB / 100 turn or less, and the number of propagation modes is 2. An example of the photonic crystal fiber which concerns on Embodiment 1 is shown. The calculation result of the bending loss in R140mm of 2nd higher mode when d / Λ = 0.32 of the photonic crystal fiber is shown. The calculation result of the bending loss in R30mm of a 1st higher-order mode when d / Λ = 0.32 of a photonic crystal fiber is shown. In the photonic crystal fiber in which the number of layers of holes is 4, the design range for setting the operating wavelength band to 1530 to 1625 nm, the bending loss to 0.5 dB / 100 turn or less, and the number of propagation modes to 2 is shown. In the photonic crystal fiber in which the number of layers of holes is 5, a design range for setting the operating wavelength band to 1530 to 1625 nm, the bending loss to 0.5 dB / 100 turn or less, and the number of propagation modes to 2 is shown. In the photonic crystal fiber having the number of hole layers of 6, the design range for setting the use wavelength band to 1530 to 1625 nm, the bending loss to 0.5 dB / 100 turn or less, and the number of propagation modes to 2 is shown. In the photonic crystal fiber having the number of hole layers of 7, the design range for setting the operating wavelength band to 1530 to 1625 nm, the bending loss to 0.5 dB / 100 turn or less, and the number of propagation modes to 2 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.

(Embodiment 1)
FIG. 5 shows a cross-sectional structure of the optical fiber according to the present embodiment. The optical fiber according to the present embodiment has a plurality of holes that are uniform in the longitudinal direction in the optical fiber, and the holes are optical fibers that are arranged in a triangular lattice pattern around the core region. The number of layers of the vacancies is four, and has a core region having a size corresponding to a region obtained by removing seven vacancies located at the center of the triangular lattice arrangement.

In the optical fiber according to the present embodiment, the bending loss for the R140 mm bending of the LP ₂₁ mode which is the second higher order mode when the number of holes is 4 and d / Λ = 0.32 is calculated. Is shown in FIG. In order to set the number of modes to 2, since the loss of the LP ₂₁ mode must be 1 dB / m or more in the wavelength range of 1530 to 1625 nm, it can be understood that Λ <5.9 μm.

Similarly, FIG. 7 shows the calculated bending loss for R30 mm bending in the LP11 mode, which is the first higher-order mode when d / Λ = 0.32. It can be seen that 4.8 μm <Λ <8 μm is required so that the bending loss with respect to the bending of the LP ₁₁ mode R30 mm is 0.5 dB / 100 turn or less in the wavelength range of 1530 to 1625 nm. Incidentally, the bending loss of the fundamental mode, smaller than the bending loss of the _{LP 11} mode, if the bending loss of _{LP 11} mode and 0.5dB / 100turn less, two propagation modes are both 0.5dB / 100turn less That is guaranteed.

  Therefore, it can be seen that 4.8 μm <Λ <5.9 μm is required in order to set the used wavelength band to 1530 to 1625 nm, the bending loss to 0.5 dB / 100 turn or less, and the propagation mode number to 2.

Similarly, FIG. 8 shows a structure calculated by changing d / Λ and satisfying the condition. The shaded area is a design area that satisfies the condition that the number of propagation modes is 2, and the broken line displayed in parallel with A _eff = 100 μm ₂ indicates the effective cross-sectional area of the LP01 mode. (A ₁₁ + B ₁₁ Λ + C ₁₁ Λ ² ) in equation (2) when the broken line displayed along LP ₁₁ is N = 4, and the broken line displayed along LP ₂₁ is N = 4 (A ₂₁ + B ₂₁ Λ + C ₂₁ Λ ² ) in Equation ( ² ) is shown.

In order to exceed the maximum effective area of 180 μm ² obtained with the step index fiber, the value of d / Λ satisfies the equation (2), and 5.88 μm <Λ <7.3 μm, 0.30 <d / It is necessary that Λ <0.32. For this reason, the value of d / Λ satisfies the formula (2), 5.88 μm <Λ <7.3 μm, and 0.30 <d / Λ <0.32, so that the used wavelength band is 1530 to 1625 nm. The bending loss can be 0.5 dB / 100 turn or less and the number of propagation modes can be 2.

For example, the structure of Λ = 6.9 μm and d / Λ = 0.31 satisfies the condition that the bending loss is 0.5 dB / 100 turn or less and the number of propagation modes is 2. As calculated using the finite element method, the effective area _{A eff} obtained at this time was the 346Myuemu ² at 241μm ^2, _{LP 11} mode _{LP 01} mode. Therefore, by adopting the structure of Λ = 6.9 μm and d / Λ = 0.31, it is possible to obtain a value larger than the step index and the maximum effective area 180 μm ² obtained with the conventional photonic crystal fiber. Recognize. The calculation result of the mode confinement loss at this time is 0.036 dB / km in the LP ₀₁ mode and 3.22 dB / km in the LP ₁₁ mode.

(Embodiment 2)
FIG. 9 shows the calculated structure of the cross-sectional structure of the optical fiber shown in FIG. 5 that satisfies the conditions when the number of holes is five. The shaded area is a design area where the bending loss is 0.5 dB / 100 turn or less and the number of propagation modes is 2, and the broken line displayed in parallel with A _eff = 100 μm ₂ is the effective LP01 mode. The cross-sectional area is shown. The broken line displayed along LP ₁₁ represents (A ₁₁ + B ₁₁ Λ + C ₁₁ Λ ² ) in Equation (2) when N = 5, and the broken line displayed along LP ₂₁ represents N = 5. (A ₂₁ + B ₂₁ Λ + C ₂₁ Λ ² ) in Equation ( ² ) is shown.

In order to exceed the maximum effective area of 180 μm ² obtained with the step index fiber, the value of d / Λ satisfies the equation (2), and 5.72 μm <Λ <7.55 μm, 0.263 <d / It is necessary that Λ <0.297. For this reason, the value of d / Λ satisfies the formula (2), and is set to 5.72 μm <Λ <7.55 μm and 0.263 <d / Λ <0.297, so that the used wavelength band is 1530 to 1625 nm. The bending loss can be 0.5 dB / 100 turn or less and the number of propagation modes can be 2.

For example, the structure of Λ = 7.4 μm and d / Λ = 0.29 satisfies the condition that the bending loss is 0.5 dB / 100 turn or less and the number of propagation modes is 2. As calculated using the finite element method, the effective area _{A eff} obtained at this time was the 412Myuemu ² at 284μm ^2, _{LP 11} mode _{LP 01} mode. Therefore, by adopting the structure of Λ = 7.4 μm and d / Λ = 0.29, a value larger than the step index and the maximum effective area of 180 μm ² obtained with the conventional photonic crystal fiber can be obtained. Recognize.

Further, the calculation result of the confinement loss of the mode at this time is 6.3 × 10 ⁻⁴ dB / km in the LP ₀₁ mode and 0.16 dB / km in the LP ₁₁ mode. Therefore, the propagation loss can be reduced by increasing the number of vacancies to five layers compared to the case of four layers.

(Embodiment 3)
FIG. 10 shows the calculated structure of the cross-sectional structure of the optical fiber shown in FIG. 5 that satisfies the conditions when the number of holes is six. The shaded area is a design area that satisfies the condition that the bending loss is 0.5 dB / 100 turn or less and the number of propagation modes is 2, and the broken line displayed in parallel with A _eff = 100 μm ₂ is the effective LP01 mode. The cross-sectional area is shown. (A ₁₁ + B ₁₁ Λ + C ₁₁ Λ ² ) in equation (2) when the broken line displayed along LP ₁₁ is N = 6, and the broken line displayed along LP ₂₁ is N = 6 (A ₂₁ + B ₂₁ Λ + C ₂₁ Λ ² ) in Equation ( ² ) is shown.

In order to exceed the maximum effective area of 180 μm ² obtained with the step index fiber, the value of d / Λ satisfies the equation (2), and 5.69 μm <Λ <7.35 μm, 0.245 <d / It is necessary that Λ <0.287. For this reason, the value of d / Λ satisfies the formula (2), and 5.69 μm <Λ <7.35 μm and 0.245 <d / Λ <0.287, so that the used wavelength band 1530-1625 nm. The bending loss can be 0.5 dB / 100 turn or less and the number of propagation modes can be 2.

For example, the structure of Λ = 7.1 μm and d / Λ = 0.28 satisfies the condition that the bending loss is 0.5 dB / 100 turn or less and the number of propagation modes is 2. As calculated using the finite element method, the effective area _{A eff} obtained at this time was the 386Myuemu ² at 265μm ^2, _{LP 11} mode _{LP 01} mode. Therefore, it can be seen that values larger than the step index and the maximum effective area of 180 μm ² obtained with the conventional photonic crystal fiber can be obtained.

The calculation result of the mode confinement loss at this time is 3 × 10 ⁻⁵ dB / km in the LP ₀₁ mode and 0.0107 dB / km in the LP ₁₁ mode. Therefore, propagation loss can be reduced by increasing the number of vacancies to 6 layers compared to the case of 5 layers.

(Embodiment 4)
In the cross-sectional structure of the optical fiber shown in FIG. 5, FIG. 11 shows the calculated structure satisfying the condition when the number of holes is seven. The shaded area is a design area where the bending loss is 0.5 dB / 100 turn or less and the number of propagation modes is 2, and the broken line displayed in parallel with A _eff = 100 μm ₂ is the effective LP01 mode. The cross-sectional area is shown. The broken line displayed along LP ₁₁ represents (A ₁₁ + B ₁₁ Λ + C ₁₁ Λ ² ) in Equation (2) when N = 7, and the broken line displayed along LP ₂₁ represents N = 7. (A ₂₁ + B ₂₁ Λ + C ₂₁ Λ ² ) in Equation ( ² ) is shown.

In order to exceed the maximum effective area of 180 μm ² obtained with the step index fiber, the value of d / Λ satisfies the equation (2), and 5.65 μm <Λ <7.0 μm, 0.238 <d / It is necessary that Λ <0.282. For this reason, the value of d / Λ satisfies the equation (2), and 5.65 μm <Λ <7.0 μm and 0.238 <d / Λ <0.282, so that the used wavelength band 1530 to 1625 nm. The bending loss can be 0.5 dB / 100 turn or less and the number of propagation modes can be 2.

For example, the structure of Λ = 7.1 μm and d / Λ = 0.28 satisfies the condition that the bending loss is 0.5 dB / 100 turn or less and the number of propagation modes is 2. As calculated using the finite element method, the effective area _{A eff} obtained at this time was the 365Myuemu ² at 251μm ^2, _{LP 11} mode _{LP 01} mode. Therefore, it can be seen that values larger than the step index and the maximum effective area of 180 μm ² obtained with the conventional photonic crystal fiber can be obtained.

The calculation result of the confinement loss of the mode at this time is 2 × 10 ⁻⁵ dB / km in the LP ₀₁ mode and 4 × 10 ⁻⁴ dB / km in the LP ₁₁ mode. Therefore, the propagation loss can be reduced by increasing the number of holes to 7 as compared with the case of 6 layers.

  The present invention can be applied to the information communication industry.

Δ: relative refractive index difference a: core radius d: hole diameter Λ: hole interval

Claims (5)

An optical fiber having two propagation modes, LP ₀₁ mode and LP ₁₁ mode,
A plurality of uniform holes in the longitudinal direction in the optical fiber, the holes are arranged in a triangular lattice shape around the core region ,
A core region having a size corresponding to a region obtained by removing seven holes located in the center of the triangular lattice arrangement;
When the number of layers of the holes to be arranged is N, the space between the holes is Λ (μm), and the diameter of the holes is d (μm), the value of d / Λ is given by meet the,
An optical fiber characterized in that a bending loss with respect to the bending of R30 mm is 0.5 dB / 100 turn or less in both of the two propagation modes .
(A ₁₁ + B ₁₁ Λ + C ₁₁ Λ ² ) <d / Λ <(A ₂₁ + B ₂₁ Λ + C ₂₁ Λ ² )
A ₁₁ = 0.32 + 0.15105N−0.02724N ² + 0.00137N ³
B ₁₁ = 0.2162−0.13547N + 0.018337N ² −0.00082N ³
C ₁₁ = −0.01949 + 0.01066N−0.00124N ² + 0.00005N ³
A ₂₁ = 0.244 + 0.23487N−0.06006N ² + 0.00428N ³
B ₂₁ = 0.17839−0.13719N + 0.02704N ² −0.0017N ³
C ₂₁ = −0.015959 + 0.01045N−0.00193N ² + 0.000116667N ³ The number N of the holes is 4 layers,
The gap interval Λ is greater than 5.88 μm and less than 7.3 μm,
2. The optical fiber according to claim 1, wherein the value of d / Λ is greater than 0.30 and less than 0.32. The number N of holes is 5 layers,
The gap interval Λ is greater than 5.72 μm and less than 7.55 μm,
2. The optical fiber according to claim 1, wherein the value of d / Λ is greater than 0.263 and less than 0.297. The number N of the holes is 6 layers,
The gap interval Λ is greater than 5.69 μm and less than 7.35 μm,
The optical fiber according to claim 1, wherein the value of d / Λ is more than 0.245 and less than 0.287. The number of holes N is 7 layers,
The gap interval Λ is greater than 5.65 μm and less than 7.0 μm,
The optical fiber according to claim 1, wherein the value of d / Λ is more than 0.238 and less than 0.282.

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