JP5118107B2 - Hole structure optical fiber - Google Patents

Description

  The present invention relates to a hole-structured optical fiber having a plurality of holes in the cross section of the fiber.

  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. Since the nonlinear effect becomes more prominent as the optical power density in the optical fiber is higher, an increase in the effective area of the optical fiber is effective in suppressing the nonlinear effect.

  Therefore, as shown in Non-Patent Document 1, a single mode optical fiber having an enlarged effective area has been developed and used in a long-distance transmission system such as a core network or a submarine system. In addition, since the conventional optical fiber structure with a controlled refractive index distribution has a limit on the expansion of the effective cross-sectional area, recently, studies are being made to increase the core using a photonic crystal fiber having a hole structure in the fiber. . 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 conventional photonic crystal fiber, and FIG. 11B shows an equivalent refractive index distribution of the photonic crystal fiber.

  As shown in FIG. 11A, the conventional photonic crystal fiber 1 is made of a uniform material such as quartz glass, and has a plurality of holes 2 in the cross section of the optical fiber 1. The air holes 2 have a uniform size (diameter) in the longitudinal direction of the optical fiber 1 and a uniform period and size (diameter) in the cross section of the optical fiber 1. In FIG. 11A, d is the diameter of the hole 2 and Λ is the center interval (inter-hole interval) between adjacent holes 2.

The equivalent refractive index profile of the conventional photonic crystal fiber 1 is a step type as shown in FIG. For example, when satisfying the cutoff wavelength and bending loss of ITU-T G.656, which is a standardization recommendation, as shown in Non-Patent Document 2, the effective cross-sectional area of the conventional photonic crystal fiber 1 is a maximum of 157 μm ^{2 at a} wavelength of 1550 nm ^2. It is.

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 μm2 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. L. Montmorillon et al, "All-solid G.652.D with ultra low bend losses down to 5 mm bend radius," the Proceedings of OFC2009, OTuL3, Mar. 2009.

However, there is a trade-off between single mode operation, reduction of bending loss, and increase of effective area, for example, single mode in the low loss wavelength band (1460-1625nm) commonly used in long-distance optical transmission. In order to realize operation and practical bending loss characteristics, from Non-Patent Documents 2 and 3, the effective cross-sectional area of the conventional single mode fiber and the conventional photonic crystal fiber is limited to about 160 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 compensating for a single mode operation and maintaining a practical bending loss characteristic. It is an issue to provide.

The hole-structured optical fiber of the present invention that solves the above problems is a hole-structured optical fiber having a plurality of holes in the cross section of the fiber.
A core region, a first cladding region surrounding the core region, and a second cladding region surrounding the first cladding region;
Each of the first cladding region and the second cladding region has a plurality of holes having different diameters,
The hole diameter in the first cladding region is larger than the hole diameter in the second cladding region,
The refractive index in the core region is higher than the effective refractive index of the first cladding region and the second cladding region, and the effective refractive index of the first cladding region is the effective refraction of the second cladding region. Lower than rate,
An interval Λ between adjacent holes in the first cladding region and the second cladding region is 17.1 μm or less,
A ratio d / Λ of a gap Λ between adjacent holes in the second cladding region and a hole diameter d in the second cladding region is smaller than 0.57,
The effective area of light at a wavelength of 1550 nm is 160 μm ² or more,
It operates in a single mode at a light wavelength of 1460 to 1625 nm, and the bending loss at a bending radius of 30 mm is 0.5 dB or less per 100 turns,
It is characterized by.

  According to the hole-structured optical fiber of the present invention, by having the above-described characteristics, it is possible to compensate for single mode operation and maintain a practical bending loss characteristic, while having a larger effective cutoff than conventional optical fibers. Since the area can be realized, the input power limitation in the optical transmission system is relaxed, the management of the optical transmission system is further increased, the management is simplified by increasing the relay interval, the cost of the optical transmission system is reduced, and the optical transmission system There is an effect of improving the degree of design freedom.

(A) is the schematic showing an example of the cross-section of the hole structure optical fiber which concerns on Embodiment 1 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 showing the bending loss of a fundamental mode regarding the hole structure optical fiber of this invention. It is a characteristic view which shows the confinement loss of the 1st higher order mode regarding the hole structure optical fiber of this invention. It is a characteristic view showing the hole structure for satisfying the bending loss and the cut-off wavelength recommended in ITU-T G.656 relating to the hole structure optical fiber of the present invention, and (a) shows the second cladding region. Represents the case where the ratio d / Λ between the gaps Λ between adjacent holes and the hole diameter d in the second cladding region is 0.40, (b) represents the case where d / Λ is 0.43, and (c) represents d This represents a case where / Λ is 0.45, and (d) represents a case where d / Λ is 0.50. It is a characteristic view showing the space | interval Λ between air holes for satisfying the bending loss and the cut-off wavelength recommended in ITU-T G.656 relating to the air hole structure optical fiber of the present invention. It is a characteristic view showing d / Λ for satisfying the bending loss and cut-off wavelength recommended in ITU-T G.656 relating to the hole-structured optical fiber of the present invention. (A) is a characteristic diagram showing an example of the effective cross-sectional area related to the hole structure optical fiber of the present invention, an example of the bending loss and the first higher-order mode confinement loss related to the hole structure optical fiber of the present invention. FIG. (A) is a characteristic diagram showing an example of the effective cross-sectional area and the confinement loss of the first higher-order mode related to the hole structure optical fiber of the present invention, and (b) is a bending related to the hole structure optical fiber of the present invention. It is a characteristic view showing an example of a loss. It is the schematic showing an example of the cross-section of the hole structure optical fiber which concerns on Example 2 of Embodiment of this invention. It is a characteristic view showing an example of an effective cross-sectional area, a bending loss, and the confinement loss of a 1st higher order mode regarding the hole structure optical fiber of this invention. (A) is the schematic which shows the cross-section of the conventional photonic crystal fiber (hole structure optical fiber), (b) is a figure which shows the refractive index distribution of the said photonic crystal fiber.

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

<Embodiment 1>
FIG. 1A shows an outline of a cross-sectional structure of a hole structure optical fiber according to Embodiment 1 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 structure optical fiber 11 of the first embodiment is made of a single material that does not contain impurities such as pure quartz, and a plurality of holes are formed in the cross section of the optical fiber 11. 15. The hole-structured optical fiber 11 has a core region 12, a first cladding region 13 that surrounds the core region 12, and a second cladding region 14 that surrounds the first cladding region 13. The holes 15 are formed along the longitudinal direction of the hole-structured optical fiber 11 and have a uniform size in the longitudinal direction (the holes 15 in the first cladding region 13 have a constant diameter d and the second cladding region 14 The holes 15 have a constant diameter d ₁ ). In addition, the holes 15 in the first cladding region 13 and the second cladding region 14 are formed at a uniform period in the cross section of the hole-structured optical fiber 1, and the interval between the centers of adjacent holes 15 (vacancy). (Inter-hole spacing) Λ is constant.

  The core region 12 is located in the central portion of the hole-structured optical fiber 1 and is configured by a region without the holes 15. In the case of the illustrated example, in the first cladding region 13, a plurality of holes 15 are arranged in a regular hexagonal shape with the core region 12 as a center, thereby forming a single hole layer 13 </ b> A. In the second cladding region 14, a plurality of holes 15 having a diameter d centering on the core region 12 are arranged in a regular hexagonal shape, thereby forming two layers of hole layers 14 </ b> A and 14 </ b> B.

The first cladding region 13 surrounding the core region 12 and the second cladding region 14 surrounding the first cladding region 13 are each composed of a plurality of holes 15 having different hole diameters d ₁ and d. Further, the diameter d _{1 of the} hole 12 in the first cladding region 13 is larger than the diameter d of the hole 12 in the second cladding region 14.

  At this time, the equivalent refractive index distribution in the hole-structured optical fiber 11 of the present invention is a W-type distribution as shown in FIG. That is, the refractive index in the core region 12 is higher than the effective refractive index of the first cladding region 13 and the second cladding region 14, and the effective refractive index of the first cladding region 13 is effective in the second cladding region 14. Lower than the refractive index.

  In conventional optical fibers that do not use holes, it is known that W-type optical fibers can achieve a larger effective area when maintaining the same cutoff wavelength and bending loss characteristics as compared to step-type optical fibers. Yes. Compared with the same step type, 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. Therefore, by using a structure like the hole-structured optical fiber 11 of the present invention, a larger effective area than the conventional optical fiber can be realized. The hole-structured optical fiber 11 of the present invention is composed of a single material in which the core region 12 is not added with impurities such as pure quartz, and does not form holes in the core region 12, so A low-loss optical fiber can be realized with little influence of scattering.

  In FIG. 1, the number of hole layers in the first cladding region 13 is one, and the number of hole layers in the second cladding region 14 is two. The same effect can be obtained even with the number of layers. In FIG. 1, the hole arrangement in the first cladding region 13 and the second cladding region 14 is a regular hexagonal shape, but the same effect can be obtained by other hole arrangements such as other regular polygonal shapes or annular shapes. It is done.

FIG. 2 shows an example of the bending loss of the fundamental mode at the wavelength of 1450 nm in the hole-structured optical fiber 11 of the present invention. In FIG. 2, the vertical axis represents the bending loss, and the horizontal axis represents the ratio d ₁ / Λ of the diameter d _{1 of the} holes 15 in the first cladding region 13 and the interval Λ between the holes.

Here, the ratio d / Λ = 0.43 of the diameter d of the holes 15 in the second cladding region 14 and the interval Λ between the holes is 0.43, and the bending radius of the hole-structured optical fiber 11 is 30 mm. The solid line, broken line, and dotted line in FIG. 2 represent the bending loss when Λ = 18, 16, and 14 μm, respectively. 2 that the bending loss decreases as the hole diameter d ₁ in the first cladding region 13 increases. Here, it is known from Non-Patent Document 4 that in a hole-structured optical fiber having a relatively large effective cross-sectional area, the bending loss increases as the wavelength becomes shorter. Therefore, under the condition of FIG. 2, a bending loss smaller than the value in FIG. 2 is obtained at a light wavelength of 1450 nm or more. As an example, considering the condition that the bending loss is 0.5 dB / 100 or less at a light wavelength of 1450 nm to 1625 nm, from FIG. 2, when Λ = 14 μm or more, d ₁ / Λ is set to 0.69 or more. It turns out that it is satisfactory.

FIG. 3 shows an example of the confinement loss of the first higher-order mode at the light wavelength of 1450 nm in the hole-structure optical fiber 11 of the present invention. In FIG. 3, the vertical axis represents the confinement loss of the first higher-order mode, and the horizontal axis represents d ₁ / Λ.

Here again, d / Λ = 0.43 as in FIG. The solid line, the broken line, and the dotted line in FIG. 3 represent the confinement loss of the first higher-order mode when Λ = 18, 16, and 14 μm, as in FIG. From FIG. 3, the confinement loss of the first higher-order mode has the same tendency as the bending loss of the fundamental mode, and the confinement of the first higher-order mode increases as the diameter d _{1 of the} hole 15 in the first cladding region 13 increases. It can be seen that the loss is reduced. Here, it is known that the confinement loss becomes larger toward the longer wavelength side. Further, from Non-Patent Document 3, if the confinement loss of the first higher-order mode is 0.8 dB / m or more, the higher-order mode cannot be propagated. It is known that single mode operation can be obtained at a wavelength. Therefore, if the confinement loss of the first higher-order mode is 0.8 dB / m or more at a predetermined wavelength, single mode operation can be obtained in the wavelength band of the predetermined wavelength or more. As an example, when considering the condition that the cut-off wavelength is 1450 nm or less (single-mode operation at a wavelength of 1450 nm or more), it is understood that the above condition is satisfied by setting d ₁ / Λ to 0.75 or less when Λ = 14 μm or more. .

  From FIG. 2 and FIG. 3, the reduction of bending loss and the realization of single mode operation are in a trade-off relationship, but a structure satisfying a predetermined cutoff wavelength and bending loss can be obtained by appropriately designing the structural parameters. .

4A to 4D show structural conditions for the hole-structured optical fiber 11 of the present invention that satisfy the bending loss and cutoff wavelength conditions recommended by ITU-T G.656. 4A to 4D, the vertical axis is d ₁ / Λ, and the horizontal axis is Λ.

Here, the conditions recommended by ITU-T G.656 are a bending loss of 0.5 dB / 100 or less at a bending radius of 30 mm and a cutoff wavelength of 1450 nm or less at a wavelength of 1460 to 1625 nm. 4A to 4D show cases where d / Λ is 0.40, 0.43, 0.45, and 0.50, respectively. Also, the solid line and the broken line in FIGS. 4A to 4D respectively indicate d ₁ / Λ where the confinement loss of the first higher-order mode at the light wavelength of 1450 nm is 1 dB / m, and at the light wavelength of 1450 nm. The bending loss of the fundamental mode is d ₁ / Λ which is 0.5 dB / 100 turns at a bending radius of 30 mm. Therefore, in each d / Λ, the ITU-T G.656 condition can be satisfied in the hatched portion surrounded by the solid line and the broken line. In particular, since the effective area is proportional to the area of the circle inscribed in the holes 15 of the first layer (hole layer 13A) (area of the core region 12), the first layer (hole layer from the core center of the core region 12). 13A), the larger the distance to the hole 15, that is, the larger the Λ, the larger the effective area. Therefore, the effective area can be maximized while satisfying a predetermined condition at the intersection of the solid line and the broken line, which is the structure having the largest Λ in the shaded portion in FIGS. FIG. 4 also shows that a predetermined condition cannot be satisfied at Λ larger than the intersection of the solid line and the broken line.

  FIG. 5 shows an inter-hole spacing Λ that satisfies the conditions of bending loss and cut-off wavelength recommended in ITU-T G.656 relating to the hole-structured optical fiber 11 of the present invention. In FIG. 5, the vertical axis is Λ and the horizontal axis is d / Λ.

  Here, the solid line in FIG. 5 corresponds to Λ at the intersection of the solid line and the broken line in FIG. 4, and the above bending loss and cutoff wavelength conditions can be satisfied when Λ is equal to or smaller than the solid line. From FIG. 5, it is preferable that Λ is 17.1 μm or less because the bending loss and cutoff wavelength conditions recommended by ITU-T G.656 can be satisfied.

FIG. 6 shows d ₁ / Λ for the hole-structured optical fiber of the present invention that satisfies the bending loss and cutoff wavelength conditions recommended by ITU-T G.656. In FIG. 6, the vertical axis is d ₁ / Λ, and the horizontal axis is d / Λ.

Here, the solid line in FIG. 6 corresponds to d ₁ / Λ at the intersection of the solid line and the broken line in FIG. 4, and the above bending loss and cutoff wavelength conditions can be satisfied when d ₁ / Λ is equal to or less than the solid line. 6 is a straight line in which the hole diameters d1 and d of the first cladding region 13 and the second cladding region 14 are equal, that is, d ₁ = d. From FIG. 6, the maximum value of d ₁ / Λ required to satisfy the above condition decreases as d / Λ increases. On the other hand, in order for the equivalent refractive index profile in the holey structure optical fiber 111 of the present invention to be W-shaped, it is necessary that d ₁ > d. Therefore, from FIG. 6, by setting d / Λ to 0.57 or less, the bending loss and cutoff wavelength conditions recommended by ITU-T G.656 can be satisfied, and a W-type equivalent refractive index profile can be obtained. preferable.

  FIGS. 7A and 7B show optical characteristics in an example of a structural design with an enlarged effective cross-sectional area related to the hole-structured optical fiber 11 of the present invention. In FIG. 7A, the vertical axis is the effective area, the horizontal axis is the wavelength of light, the vertical axis in FIG. 7B is the bending loss and the first higher-order mode confinement loss, and the horizontal axis is the wavelength of light. is there.

Here, Λ = 17 μm, d ₁ /Λ=0.74, and d / Λ = 0.43. FIG. 7A shows the effective area. From FIG. 7A, the effective cross-sectional area at the wavelength of 1550 nm of the hole-structured optical fiber 11 designed here was 212 μm ² . A typical 1.31 μm zero-dispersion single mode optical fiber (SMF) has an effective area of about 80 μm ² at a wavelength of 1550 nm, and the hole-structure optical fiber 11 of the present invention has an effective area of about three times. It has been. In addition, the effective area of about 1.5 times is obtained even when compared with the maximum value of 160 μm ² which is the maximum value of the conventional photonic crystal fiber. FIG. 7B shows the bending loss and the confinement loss of the first higher-order mode. The solid line and the broken line in FIG. 7B correspond to the bending loss at the bending radius of 30 mm and the confinement loss in the first higher-order mode, respectively. The bending loss was 0.5 dB / 100 or less at a light wavelength of 1460 nm or more, and the confinement loss of the first higher-order mode was 0.8 dB / m or more at a light wavelength of 1250 nm or more. Therefore, the hole-structure optical fiber 11 designed here has a bending loss of 0.5 dB / 100 or less at a light wavelength of 1460 to 1625 nm and a cutoff wavelength of 1450 nm or less, which is recommended by ITU-T G.656. It can be confirmed that the bending loss and cutoff wavelength conditions can be satisfied.

  FIGS. 8A and 8B show optical characteristics in an example of a structural design with a reduced bending loss related to the hole-structured optical fiber of the present invention. In FIG. 8A, the vertical axis represents the effective cross-sectional area and the first higher-order mode confinement loss, the horizontal axis represents the wavelength of light, the vertical axis in FIG. 8B represents the bending loss, and the vertical axis represents the bending radius.

Here, Λ = 11 μm, d ₁ /Λ=0.76, and d / Λ = 0.43. FIG. 8A shows the effective area and the confinement loss of the first higher-order mode. From FIG. 8A, the effective area of light at a wavelength of 1550 nm is 88 μm ² , which is almost the same as SMF. In addition, from FIG. 8A, the confinement loss of the first higher-order mode is 0.8 dB / m or more at a light wavelength of 1300 nm or more, and is single at light wavelengths 1310, 1490, and 1550 nm, which are general communication wavelengths. Mode operation is obtained. Here, the mode field diameter (MFD) at a wavelength of 1310 nm of light calculated from the obtained electric field distribution using the FFP moment was 9.1 μm. In ITU-T G.652, which is a standardization recommendation corresponding to SMF, the MFD at a light wavelength of 1310 nm is 8.8 to 9.5 μm, and the hole-structured optical fiber 11 designed here satisfies the MFD of the recommendation. I can confirm. FIG. 8B shows the bending loss characteristics at a light wavelength of 1550 nm. FIG. 8B shows that the bending loss can be reduced to about 0.001 dB / winding at a bending radius of 5 mm. For example, according to Non-Patent Document 5, by using an optical fiber having a trench type refractive index profile, a bending radius of 5 mm is achieved while satisfying the MFD conditions in ITU-T G.652 at a single mode operation and a light wavelength of 1310 nm. A bending loss of 0.1 dB / winding is obtained. It can be seen that the hole-structure optical fiber 11 of the present invention can realize a bending loss that is two orders of magnitude smaller while obtaining the equivalent MFD as compared with the trench type. At this time, since it has an MFD equivalent to SMF, the connection loss can be reduced, and since a very low bending loss can be obtained, the handling property of the optical fiber related to construction and operation can be improved, which is preferable.

<Embodiment 2>
FIG. 9 is a schematic view showing an example of a cross-sectional structure of a hole-structure optical fiber according to Embodiment 2 of the present invention.

  As shown in FIG. 9, the hole-structure optical fiber 11 of the second embodiment is arranged outside the second cladding region 14 in the structure of the hole-structure optical fiber 11 of FIG. In this structure, a ring layer 16 made of a material (such as quartz doped with impurities such as fluorine) having a lower refractive index than the material of the core region 12 (such as pure quartz) is provided. The other structure of the hole structure optical fiber 11 is the same as that of the hole structure optical fiber 11 shown in FIG. 2, and a detailed description thereof is omitted here. In the hole-structure optical fiber 11 of FIG. 2, the number of hole layers in the second cladding region 14 is two (hole layers 14A, 14B), whereas in the hole-structure optical fiber 11, holes are formed. The number of layers is one (hole layer 14A).

  In the hole-structure optical fiber 11 of the second embodiment, the confinement of the light wave can be strengthened by providing the ring layer 16 having a lower refractive index than that of the core region 12 outside the second cladding region 14. At this time, a sufficient confinement effect can be obtained even when the number of hole layers in the first cladding region 13 and / or the second cladding region 14 is small, and the outer diameter of the hole-structured optical fiber 11 can be made thinner. ,preferable.

  FIGS. 10A and 10B show an example of optical characteristics related to the hole-structured optical fiber 11 of the present invention. 10A, the vertical axis represents the effective area, the horizontal axis represents the wavelength of light, the vertical axis represents the bending loss and the first higher-order mode confinement loss, and the horizontal axis represents the light wavelength. is there.

Here, Λ = 17 μm, d ₁ /Λ=0.74, d / Λ = 0.43, the number of hole layers in the first cladding region 13 and the second cladding region 14 is one, and the core is outside the second cladding region 14. A ring layer 16 having a refractive index 0.1% lower than that of the region 12 by a relative refractive index difference Δ is disposed. FIG. 10A shows the effective area. From FIG. 10A, the effective area of light at a wavelength of 1550 nm is 212 μm ² , and an effective area equivalent to that in FIG. 7 is obtained. FIG. 10B shows the bending loss and the confinement loss of the first higher-order mode. From Fig. 10 (b), the bending loss of 0.5 dB / 100 or less at the light wavelength of 1500 nm or more and the confinement loss of the first higher-order mode of 0.8 dB / m or more at the wavelength of 1530 nm or more are obtained. Single mode operation and relatively low bending loss were obtained at the wavelengths of light used from 1530 to 1565 nm.

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

DESCRIPTION OF SYMBOLS 11 Hole structure optical fiber 12 Core area | region 13 1st cladding area | region 13A Hole layer 14 2nd cladding area | region 14A, 14B Hole layer 15 Hole 16 Ring layer

Claims (1)

In a holey structure optical fiber having a plurality of holes in the cross section of the fiber,
A core region, a first cladding region surrounding the core region, and a second cladding region surrounding the first cladding region;
Each of the first cladding region and the second cladding region has a plurality of holes having different diameters,
The hole diameter in the first cladding region is larger than the hole diameter in the second cladding region,
The refractive index in the core region is higher than the effective refractive index of the first cladding region and the second cladding region, and the effective refractive index of the first cladding region is the effective refraction of the second cladding region. Lower than rate,
An interval Λ between adjacent holes in the first cladding region and the second cladding region is 17.1 μm or less,
A ratio d / Λ of a gap Λ between adjacent holes in the second cladding region and a hole diameter d in the second cladding region is smaller than 0.57,
The effective area of light at a wavelength of 1550 nm is 160 μm ² or more,
It operates in a single mode at a light wavelength of 1460 to 1625 nm, and the bending loss at a bending radius of 30 mm is 0.5 dB or less per 100 turns,
Pore structure optical fiber characterized.

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