JP2010217472A - Hole structure optical fiber and optical transmission system using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hole structure optical fiber that has a larger effective cross section than a conventional optical fiber while guaranteeing single-mode operation and maintaining a standard for bending loss, and to provide an optical transmission system using the same. <P>SOLUTION: The hole structure optical fiber 11 has a structure configured such that the optical fiber has a plurality of holes 12 with uniform diameters along the length of the optical fiber and the plurality of holes 12 are arrayed in a plurality of layers around a core region to form a plurality of hole layers 14A, 14B, and 14C, the diameter d1 of holes 12 in the innermost hole layer 14C among the plurality of hole layers 14A, 14B, and 14C being larger than the diameter (d) of holes 12 in the outermost hole layer 14C among the plurality of hole layers 14A, 14B, and 14C. The optical transmission system includes: an optical transmission line including the hole structure optical fiber; an optical signal regeneration unit having a DCF provided to the optical transmission line; an optical transmission unit; and an optical receiving unit. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a hole structure optical fiber and an optical transmission system using the same.

  In the optical transmission system, when the optical power is increased, the transmission quality is deteriorated due to the nonlinear effect, so that the input optical power is limited, which hinders an increase in the transmission capacity and an increase in the relay interval. Here, since the occurrence of the nonlinear effect is inversely proportional to the optical power density in the optical fiber, an increase in the effective area of the optical fiber is effective for suppressing the nonlinear effect.

  Therefore, as shown in Non-Patent Document 1, a single mode optical fiber having an enlarged effective cross-sectional area has been developed and is mainly used in a submarine transmission system. In addition, the conventional optical fiber structure with a controlled refractive index distribution has a limit in the expansion of the effective area, and recently, the study of increasing the core using a photonic crystal fiber having a hole structure in the optical fiber has progressed. Yes. For example, Non-Patent Document 2 achieves a larger effective cross-sectional area than Non-Patent Document 1 while having a single-mode operating region and bending loss characteristics equivalent to those of a conventional single-mode fiber.

  FIG. 11A shows a cross-sectional structure of a photonic crystal fiber having a hole structure in a conventional optical fiber (hole structure optical fiber), and FIG. 11B shows an equivalent of the photonic crystal fiber. Refractive index distribution is shown.

As shown in FIG. 11A, a conventional photonic crystal fiber 1 has a uniform size in the longitudinal direction of an optical fiber and a uniform size in an optical fiber made of a uniform material such as quartz glass. A plurality of holes 2 having a period and size are provided. In FIG. 11A, d is the diameter of the hole 2, and Λ is the center interval between adjacent holes 2 among the plurality of holes 2.
The equivalent refractive index profile of this conventional photonic crystal fiber is a step type as shown in FIG. For example, G.I. is a standardization recommendation for a conventional 1.31 μm band zero-dispersion single mode fiber. When the cutoff wavelength and bending loss characteristics of 652 are satisfied, as shown in Non-Patent Document 2, the effective area of the conventional photonic crystal fiber is a maximum of 133 μm ² at a light wavelength of 1.55 μm.

T. Kato et al, “Ultra-low nonlinearity low-loss pure silica core fiber for long-haul WDM transmission,” Electronic Letters, vol. 35, no. 19, pp. 1615-1617, Sep. 1999. Matsui et al., “Examination of effective area expansion of photonic crystal fiber,” 2008 Society Conference of IEICE, p. 275, Sep. 2008. K. Mukasa et al, “Comparisons of merits on wide-band transmission systems between using extremely improved solid SMFs with Aeff of 160 ・ m2 and loss of 0.175 dB / km and using large-Aeff holey fibers enabling transmission over 600 nm bandwidth,” the Proceedings of OFC2008, OthR1, Feb. 2008. T. Sorensen et al, “Macro-bending loss properties of photonic crystal fiber,” Electronics Letters, vol. 37, no. 5, pp. 387-289, Mar. 2001.

However, for example, when maintaining a single-mode operating region and bending loss characteristics equivalent to those of a conventional 1.31 μm band zero-dispersion single-mode fiber, non-patent document 2 shows a photonic crystal fiber having a uniform hole structure. Thus, the effective sectional area is 133 μm ² .

  Therefore, in view of the above circumstances, the present invention is a hole-structured optical fiber that can realize a larger effective area than a conventional optical fiber while guaranteeing single-mode operation and maintaining a standard of bending loss. It is another object of the present invention to provide an optical transmission system using the same.

The hole-structured optical fiber of the first invention that solves the above-mentioned problems has a plurality of holes having a uniform diameter in the longitudinal direction of the optical fiber in the optical fiber,
The plurality of holes are arranged in a plurality of layers around the core region to form a plurality of hole layers,
The hole diameter in the innermost hole layer of the plurality of hole layers is larger than the hole diameter of the outermost hole layer in the plurality of hole layers. .

Further, the hole structure optical fiber of the second invention is the hole structure optical fiber of the first invention.
An interval between centers of adjacent holes among the plurality of holes is 12.8 μm or more,
And the diameter of the hole in the innermost hole layer is 1.37 times or more than the diameter of the hole in the outermost hole layer.

Further, the hole structure optical fiber of the third invention is the hole structure optical fiber of the first or second invention,
The effective area at a wavelength of 1.55 μm is 133 μm ² or more,
A single mode operation is performed at a light wavelength of 1.26 to 1.625 μm, and a bending loss at a bending radius of 30 mm is 0.1 dB or less per 100 turns.

An optical transmission system according to a third aspect of the present invention is an optical transmission line including the hole-structured optical fiber according to any one of the first to third aspects, and the sign of the hole-structured optical fiber is reversed at a light wavelength of 1550 nm. An optical transmission system comprising an optical signal regeneration unit having a dispersion compensating optical fiber having chromatic dispersion characteristics;
An optical transmitter for inputting an optical signal to the optical transmission system;
An optical receiver for receiving an optical signal transmitted by the optical transmission system;
It is provided with.

  Since the hole structure optical fiber of the present invention can realize a larger effective area than that of the conventional optical fiber, the input power limitation in the optical transmission system is relaxed, and the transmission system is further increased in capacity and increased in the relay interval. This makes it possible to simplify management, reduce system costs, and improve the degree of freedom in designing transmission systems.

(A) is the schematic which shows the cross-section of the hole structure optical fiber which concerns on the embodiment of this invention, (b) is a figure which shows the refractive index distribution of the said hole structure optical fiber. It is a characteristic view which shows the confinement loss of the 1st higher mode of the hole structure optical fiber which concerns on the example of embodiment of this invention. It is a characteristic view which shows the bending loss of the hole structure optical fiber of this invention. It is a characteristic view which shows the effective cross-sectional area of the hole structure optical fiber of this invention. It is a characteristic view showing the structure for implement | achieving reduction of the bending loss of the hole structure optical fiber of this invention, and expansion of an effective area. It is a characteristic view which shows the hole structure optical fiber effective area and bending loss of this invention. It is. It is a characteristic view which shows the wavelength dispersion of the hole structure optical fiber of this invention. (A) is the schematic which shows the 1st structural example of the optical transmission system using the hole structure optical fiber of this invention, (b) is the 2nd of the optical transmission system using the hole structure optical fiber of this invention. FIG. 4C is a schematic diagram showing a third configuration example of an optical transmission system using the hole structure optical fiber of the present invention. FIG. 5 is a characteristic diagram showing a DCF that performs chromatic dispersion compensation of a hole-structured optical fiber according to an embodiment of the present invention, and the chromatic dispersion of an optical transmission line that is compensated using the DCF. It is. (A) is the schematic which shows the cross-section of DCF which performs wavelength dispersion compensation of the hole structure optical fiber of this invention, (b) is a figure which shows the refractive index distribution of said DCF. (A) is the schematic which shows the cross-section of the conventional hole structure optical fiber, (b) is a figure which shows the refractive index distribution of the said hole structure optical fiber.

  Embodiments of the present invention will be described below in detail with reference to the drawings.

  FIG. 1A shows a cross-sectional structure of a hole structure optical fiber according to an embodiment of the present invention, and FIG. 1B shows a refractive index distribution of the hole structure optical fiber.

As shown in FIG. 1A, the hole-structured optical fiber 11 of the present embodiment has a plurality of holes 12 in an optical fiber made of a uniform material such as quartz glass. A plurality of (three layers in the illustrated example) hole layers 14A, 14B, and 14C are formed by arranging the holes 12 in a plurality of layers centering around the core region 13 at the center of the optical fiber. Any plurality of holes 12, the holes 12 in the longitudinal direction uniform size of the diameter (the direction perpendicular to the plane of FIG. 1) (Soraanaso 14A of the optical fiber is constant in diameter d ₁ The holes 12 of the hole layers 14B and 14B have a constant diameter d). The plurality of holes 12 are formed with a uniform period, and the center-to-center spacing Λ of adjacent holes 12 among the plurality of holes 12 is constant.

In the hole-structured optical fiber 11 according to the present embodiment, the diameter d _{1 of the} hole 12 in the innermost hole layer 14A among the plurality of hole layers 14A, 14B, 14C has a plurality of holes. It is larger than the diameter d of the hole 12 in the outermost hole layer 14C of the layers 14A, 14B, 14C.

  Therefore, the equivalent refractive index distribution of the hole structure optical fiber 11 is a W-type distribution as shown in FIG.

In a conventional optical fiber that does not use holes, a W-type optical fiber can achieve a larger effective area when compared to a step-type optical fiber when maintaining the same cutoff wavelength and bending loss characteristics. Are known. Compared with the same step-type distribution, from Non-Patent Document 2, an optical fiber using a hole structure can realize a larger effective area than an optical fiber not using a hole structure.
Therefore, the hole structure optical fiber 11 of the present embodiment can realize an effective area larger than that of a conventional optical fiber by adopting an optical fiber structure having a hole structure having a W-shaped distribution. In addition, since the core region 13 is made of a single material and no holes are formed in the core region 13 in the hole-structure optical fiber 11 of the present embodiment, scattering of impurities and hole boundary surfaces is performed. Therefore, a low-loss optical fiber can be realized.

In FIG. 1, the number of pore layers is three. However, the number of pore layers is not limited to this, and the number of pore layers may be two or four or more.
In FIG. 1, only the diameter d _{1 of the} hole 12 in the innermost hole layer 14A is made larger than the diameter d of the hole 12 in the outermost hole layer 14C. In the hole-structured optical fiber having three or more hole layers, for example, the innermost (first layer) hole layer and the adjacent (second layer) hole layer are two layers. A structure in which the diameters of the holes of the plurality of layers are larger than the diameters of the holes of the outermost hole layer, such as the diameters of the holes.

FIG. 2 shows the confinement loss characteristic of the first higher-order mode obtained by numerical calculation related to the hole structure optical fiber of the present invention. In FIG. 2, the horizontal axis is the ratio d ₁ / d between the hole diameter d ₁ in the innermost hole layer and the hole diameter d in the outermost hole layer, and the vertical axis is the confinement loss. It is.

Here, regarding the hole structure of the hole structure optical fiber of the present invention, the number N of hole layers is three, and the ratio between the hole diameter d in the outermost hole layer and the center interval Λ of adjacent holes. d / Λ was set to 0.5.
Further, since the confinement loss becomes smaller as the wavelength becomes shorter, the wavelength λ of light is set to 1.26 μm where the confinement loss becomes the smallest in the communication wavelength band of 1.26 to 1.625 μm.

From FIG. 2, the pore structure optical fiber of the present invention, most of the inner air hole layer as to increase the diameter d ₁ of the holes (i.e., the larger the d ₁ / d), the first higher-order mode It can be seen that the confinement loss increases. According to Non-Patent Document 3, if the first higher-order mode is 0.8 dB / m or more, it can be considered that a single mode operation is performed. For example, in FIG. 2, when Λ = 14 μm, if d ₁ / d is set to 1.09 or more, a confinement loss of 1 dB / m or more is obtained, and a single mode in a communication wavelength band (1.26-1.625 μm) is obtained. Operation can be guaranteed.

FIG. 3 shows bending loss characteristics obtained by numerical calculation related to the hole-structured optical fiber of the present invention. In FIG. 3, the horizontal axis is the ratio d ₁ / d between the hole diameter d ₁ in the innermost hole layer and the hole diameter d in the outermost hole layer, and the vertical axis is the bending loss. is there.

Here, the hole structure of the hole structure optical fiber of the present invention is the same as that shown in FIG.
Further, according to Non-Patent Document 4, in a hole structure having a relatively large effective area, bending loss increases as the wavelength becomes shorter. Therefore, the wavelength λ of light is set to 1.26 μm, which is the shortest wavelength in the communication wavelength band (1.26 to 1.625 μm).

FIG. 3 shows that the hole-structure optical fiber of the present invention can reduce bending loss as d ₁ / d is increased. Here, an alternate long and short dash line in FIG. This is a recommendation of a bending loss at 652, and represents 0.1 dB / 100 winding at a bending radius of 30 mm.
From FIG. 2 and FIG. Since d ₁ / d that satisfies the bending loss of 652 is sufficiently larger than d ₁ / d that compensates for single mode operation, a single mode operation can be achieved simultaneously by using a d ₁ / d structure that satisfies the bending loss condition. Can be guaranteed.

FIG. 4 shows the characteristics of the effective area obtained by numerical calculation related to the hole structure optical fiber of the present invention. In FIG. 4, the horizontal axis represents the ratio d ₁ / d between the hole diameter d ₁ in the innermost hole layer and the hole diameter d in the outermost hole layer, and the vertical axis represents the effective area. It is.

In this case, the hole structure of the hole structure optical fiber of the present invention is the same as that shown in FIGS.
The wavelength λ of light is 1.55 μm generally used in the optical transmission system.

FIG. 4 shows that the effective area of the hole-structured optical fiber of the present invention decreases monotonously as d ₁ / d is increased.

FIG. 5 shows a design example obtained by numerical calculation related to the hole structure optical fiber of the present invention. In FIG. 5, the horizontal axis is the distance Λ between adjacent holes, and the vertical axis is the hole diameter d ₁ in the innermost hole layer and the hole diameter d in the outermost hole layer. The ratio d ₁ / d.

The solid line and the broken line in FIG. A structure that realizes a bending radius of 30 mm, 0.1 dB / 100 windings, which is a recommendation for a bending loss of 652, and 133 μm ² that is the upper limit of the effective area of a conventional photonic crystal fiber is shown.
As shown in FIG. 5, since the bending loss becomes smaller as d ₁ / d becomes larger, the bending loss needs to be above the solid line, and the effective area increases as d ₁ / d becomes smaller. Need to be on the side. That is, the effective area at a light wavelength of 1.55 μm is 133 μm ² or more, single mode operation is performed at a light wavelength of 1.26 to 1.625 μm, and the bending loss at a bending radius of 30 mm is 0.1 dB per 100 turns. It is necessary that:
Therefore, in order to achieve the desired properties, the center-to-center spacing of adjacent holes Λ is above 12.8, it is preferred d ₁ / d is 1.37 or more.

For reference, FIG. 5 shows that the confinement loss in the first higher-order mode is 1 dB / m at a light wavelength of 1.26 μm or more, and a single mode in the communication wavelength band (1.26-1.625 μm). The structure which becomes an operation | movement is shown with the dotted line.
From FIG. 5, it can be confirmed that the structure that operates in the single mode in the communication wavelength band (1.26-1.625 μm) includes a structure that satisfies the bending loss condition. It can be confirmed that one-mode operation can be guaranteed.

  FIG. 6 shows an example of the hole structure optical fiber designed according to FIG. 5 in the hole structure optical fiber of the present invention. In FIG. 6, the horizontal axis represents the wavelength of light, and the vertical axis represents the effective area on the left side and the bending loss on the right side.

Here, regarding the hole structure of the hole structure optical fiber of the present invention, the interval Λ between adjacent holes is 18 μm, the hole diameter d in the outermost hole layer and the interval Λ between adjacent holes Λ. D / Λ is 0.5, and the ratio d ₁ / d of the hole diameter d ₁ in the innermost hole layer and the hole diameter d in the outermost hole layer is 1.52. did.

The solid line and the broken line in FIG. 6 represent the effective cross-sectional area and the bending loss at a bending radius of 30 mm, respectively.
6, the bending loss of the hole-structured optical fiber of the present invention is 0.1 dB / 100 or less in the entire communication wavelength band (1.26-1.625 μm). It can be confirmed that the recommendation of 652 is satisfied. The effective cross-sectional area of the hole-structure optical fiber of the present invention is 230 μm ² at a light wavelength of 1.55 μm, which is compared with a general 1.31 μm-band zero-dispersion single mode fiber (effective cross-sectional area 80 μm ² ). The effective cross-sectional area is about 3 times that of a photonic crystal fiber having a uniform hole.

  FIG. 7 shows an example of chromatic dispersion characteristics related to the hole-structured optical fiber of the present invention. In FIG. 7, the horizontal axis represents the wavelength of light, and the vertical axis represents chromatic dispersion.

  Here, the hole structure of the hole structure optical fiber of the present invention is the same as that of FIG.

The solid line and broken line in FIG. 7 represent the chromatic dispersion of the hole structure optical fiber of the present invention and the material dispersion of pure quartz, respectively.
From FIG. 7, it can be confirmed that the hole-structured optical fiber of the present invention has characteristics close to those of pure quartz material dispersion. This is because in the hole structure optical fiber of the present invention, the light wave is strongly confined in the core region made of pure quartz, and the influence of waveguide dispersion is reduced. Since chromatic dispersion in an optical fiber causes deterioration of transmission characteristics in an optical transmission system, it is necessary to compensate for chromatic dispersion in the optical transmission system.

  8 (a), 8 (b) and 8 (c) show a configuration example of an optical transmission system using the hole structure optical fiber of the present invention.

  Each of the optical transmission systems according to the embodiments of the present invention shown in these drawings is provided in the optical transmission path 22 including the hole-structured optical fiber 11 (shown by a solid line) of the present invention, and the optical transmission path 22. An optical signal regeneration unit 24 having a dispersion compensating optical fiber (DCF) 23 (shown by a broken line) having a chromatic dispersion characteristic whose sign is opposite to that of the hole-structured optical fiber 11 at a light wavelength of 1550 nm; The optical transmission unit 22 includes an optical transmission unit for inputting an optical signal and an optical reception unit 26 for receiving the optical signal transmitted through the optical transmission channel 22.

  Therefore, in this optical transmission system, the chromatic dispersion in the optical transmission line 22 (hole structure optical fiber 11) is compensated by the optical signal regeneration unit 24 (DCF 23).

  In the optical transmission line 22 constituted by the hole-structured optical fiber 11 and the optical signal regeneration unit 24 (DCF 23), the arrangement of the optical signal regeneration unit 24 (DCF 23) is dispersed as shown in FIG. Even after the compensating optical fiber 22 (optical signal output side), as shown in FIG. 8B, before the dispersion compensating optical fiber 22 (optical signal input side), as shown in FIG. It may be between the dispersion compensating optical fiber 11 on the signal input side and the dispersion compensating optical fiber 11 on the rear side (optical signal output side).

  The DCF is widely used because it has excellent connectivity with an optical transmission line and has a wider wavelength band for performing chromatic dispersion compensation than other systems. Therefore, by appropriately designing the fiber structure of the DCF, the chromatic dispersion of the hole structure optical fiber of the present invention can be compensated in a wide wavelength range, and high-speed optical transmission can be realized in a wide wavelength range. .

  FIG. 9 shows an example of DCF dispersion characteristics for compensating the chromatic dispersion of the hole-structured optical fiber of the present invention. In FIG. 9, the horizontal axis is the wavelength of light, and the vertical axis is the chromatic dispersion before compensation and the right is chromatic dispersion after compensation. FIG. 10A shows a cross-sectional structure of a DCF that performs wavelength dispersion compensation of the hole-structure optical fiber of the present invention, and FIG. 10B shows a refractive index distribution of the DCF.

As shown in FIG. 10A, the structure of the DCF 23 is a W-type distribution generally used in DCF, the diameter 2a ₁ of the center core 31 is 2 μm, the diameter 2a of the depressed layer 32 is 10 μm, and the center core 31 The relative refractive index difference Δ with respect to the pure quartz layer 33 was 1.88%, and the relative refractive index difference Δ ₁ of the depressed layer 32 with respect to the pure quartz layer 33 was −0.3%.

  The solid line and the dotted line in FIG. 9 are the chromatic dispersion of the hole structure optical fiber of the present invention shown in FIG. 7 and the chromatic dispersion of the DCF designed to compensate for the chromatic dispersion of the hole structure optical fiber. . Further, the broken line in FIG. 9 represents the chromatic dispersion after compensating the chromatic dispersion of the hole-structured optical fiber of the present invention with the designed DCF.

Here, the ratio between the length of the hole structure optical fiber of the present invention and the DCF was set so that the chromatic dispersion after compensation becomes 0 at the wavelength of 1550 nm of light.
From FIG. 9, it can be confirmed that the chromatic dispersion of the designed DCF has a value opposite to that of the hole-structured optical fiber of the present invention and a reverse slope at a light wavelength of 1300 to 1600 nm. In addition, the chromatic dispersion at the wavelength of 1550 nm of the hole-structure optical fiber of the present invention is 25.3 ps / nm · km, whereas the chromatic dispersion after compensation is 0 to −1 ps / nm at the wavelength of 1490 nm or more of the light.・ It can be confirmed that it is made in km.
From FIG. 9, the hole-structure optical fiber according to the present invention uses a general DCF structure to reduce chromatic dispersion in a wide wavelength band and realize high-speed transmission. Although an example of the structure of the DCF is shown here, chromatic dispersion compensation is performed in a wider wavelength range by optimizing the structure of the DCF in accordance with the wavelength dispersion characteristics of the hole structure optical fiber to be compensated. It is also possible.

  The hole structure optical fiber of the present invention can be used as a transmission medium in an optical transmission system.

DESCRIPTION OF SYMBOLS 11 Hole structure optical fiber 12 Hole 13 Core area | region 14A, 14B, 14C Hole layer 22 Optical transmission line 23 Dispersion compensation optical fiber (DCF)
24 Optical signal regeneration unit 25 Transmission unit 26 Reception unit 31 Center core 32 Depressed layer 33 Pure quartz layer

Claims (4)

In the optical fiber, having a plurality of holes having a uniform diameter in the longitudinal direction of the optical fiber,
The plurality of holes are arranged in a plurality of layers around the core region to form a plurality of hole layers,
The hole diameter in the innermost hole layer of the plurality of hole layers is larger than the hole diameter in the outermost hole layer of the plurality of hole layers. Hole structure optical fiber. The hole-structured optical fiber according to claim 1,
An interval between centers of adjacent holes among the plurality of holes is 12.8 μm or more,
The hole structure optical fiber is characterized in that the hole diameter in the innermost hole layer is 1.37 times or more than the hole diameter in the outermost hole layer. The hole-structure optical fiber according to claim 1 or 2,
The effective area at a wavelength of 1.55 μm is 133 μm ² or more,
A hole-structured optical fiber that operates in a single mode at a light wavelength of 1.26 to 1.625 μm and has a bending loss of 0.1 dB or less per 100 turns at a bending radius of 30 mm. An optical transmission line including the hole-structured optical fiber according to any one of claims 1 to 3,
An optical signal regeneration unit including a dispersion compensating optical fiber provided in the optical transmission line and having a wavelength dispersion characteristic having a sign opposite to that of the hole structure optical fiber at a light wavelength of 1550 nm;
An optical transmitter for inputting an optical signal to the optical transmission path;
An optical receiver for receiving an optical signal transmitted by the optical transmission system;
An optical transmission system comprising:

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