JP5697157B2 - Core expansion single mode optical fiber and optical transmission system - Google Patents

Description

  The present invention relates to a single mode optical fiber and an optical transmission system using the single mode optical fiber.

  In recent high-speed and large-capacity optical transmission systems, a wavelength division multiplexing (WDM) transmission system is widely used. In WDM transmission, it is important to reduce optical nonlinearity in an optical fiber, and various optical fibers having an expanded effective cross-sectional area in a used wavelength band have been developed and put into practical use in order to achieve this purpose. In addition, a distributed Raman amplification transmission system for the purpose of suppressing optical nonlinearity by reducing signal light intensity or improving signal-to-noise ratio characteristics is also used for general purposes. When WDM transmission in the wavelength 1550 nm band is performed using the distributed Raman amplification transmission method, single mode operability at 1460 nm or more, which is the excitation wavelength of Raman amplification, is maintained, and effective area in the 1550 nm band is maintained. It is important to expand.

However, in increasing the effective area of a single-mode optical fiber, it is common to make the refractive index distribution multi-layered and perform more precise refractive index distribution control, which also increases the difficulty in manufacturing the optical fiber. was there. For example, in Non-Patent Document 1, by using an optical fiber having a refractive index change of two to four layers in the radial direction from the core center, the bending loss at a wavelength of 1550 nm is suppressed to 1 dB / 100 turn or less, while A technique for expanding the effective area at a wavelength to about 100 μm ² is disclosed.

  The nonlinearity of the optical fiber can also be realized by reducing the nonlinear refractive index. However, when the refractive index distribution is multilayered, it is generally necessary to obtain a change in the relative refractive index difference of ± 0.5% or more, and the nonlinear refractive index increases as the number of additives such as germanium and fluorine increases. There was a problem.

Furthermore, there is a trade-off relationship between shortening the effective cutoff wavelength and increasing the effective cross-sectional area in a single-mode optical fiber. When the effective cut-off wavelength is 1460 nm or less and the effective cross-sectional area is increased, the wavelength is 1460 nm. Therefore, there is a problem that the practical effective area on the long wavelength side is limited to about 100 μm ² . For example, in Table 2 of Non-Patent Document 2, in a general-purpose cutoff shift fiber having an effective cutoff wavelength of 1530 nm or more, the mode field diameter (MFD) at a wavelength of 1550 nm may be in the range of 9.5 to 13.0 μm. The effective area Aeff at the center value (11.25 μm) of the MFD is approximately 100 μm ² because Aeff = π × (mode field radius) ² .

P. Nouchi, et al., "Maximum effective area for non-zero dispersion-shifted fiber," in Proc. Of OFC'98, ThK3, (1998). ITU-T Recommendation G.654, "Characters of a cut-off shifted, single-mode optical fiber and cable," (2010).

  As described above, the wavelength division multiplexing (WDM) transmission method is widely used in high-speed and large-capacity optical transmission systems. Furthermore, in WDM transmission, since it is important to reduce optical nonlinearity in an optical fiber, various optical fibers having an expanded effective cross-sectional area in the used wavelength band have been developed. In addition, a distributed Raman amplification transmission system for the purpose of suppressing optical nonlinearity by reducing signal light intensity or improving signal-to-noise ratio characteristics is also used for general purposes. Therefore, when performing WDM transmission in the communication wavelength 1550 nm band using these using the distributed Raman amplification transmission method, single mode operability at 1460 nm or more, which is the excitation wavelength of Raman amplification, is maintained, and 1550 nm. It is important to increase the effective area of the belt.

However, in the conventional technology, in order to increase the effective area of the single mode optical fiber, it is necessary to control the refractive index distribution by multilayering the refractive index distribution. there were. Further, since shortening the effective cutoff wavelength and increasing the effective cross-sectional area in a single mode optical fiber are in a trade-off relationship, when the cut-off wavelength is set to 1460 nm or less and the effective cross-sectional area is increased, the wavelength from 1460 nm is increased. There is also a problem that a practical effective area on the long wavelength side is limited to about 100 μm ² .

The present invention has been made in view of the background as described above, and the object of the present invention is to maintain a cutoff wavelength characteristic of a wavelength of 1460 nm or less without performing a multilayered refractive index distribution and to have a wavelength of 1460 nm or more. It is an object of the present invention to provide a core-enlarged single-mode optical fiber suitable for WDM transmission and distributed Raman amplification transmission, in which the effective cross-sectional area is expanded to 100 μm ² or more.

  In the core-enlarged single mode optical fiber of the present invention, the diameter is set to 125 ± 1 μm and the refractive index is uniform, and the diameter is set to 2a and the relative refractive index difference with respect to the cladding is Δ. 7 core parts set, and the seven core parts are one core part arranged at the center of the clad part, and six core parts around the one core part Are arranged in a hexagonal close-packed structure with an interval Λ, the diameter 2a of the core portion is 3 to 4 μm, the normalized frequency of the core portion is 0.7 to 0.9, and the normalized inter-core distance Λ / 2a is The range is set to 1.2 to 1.8.

According to the core expansion single mode optical fiber of the present invention having the above-described configuration, by appropriately arranging seven cores having a simple step-type refractive index distribution, without performing multilayering of the refractive index distribution, Since an effective cutoff wavelength characteristic of a wavelength of 1460 nm or less and an effective cross-sectional area of 100 μm ² or more at a wavelength of 1460 nm or more can be realized, a single mode operation at a wavelength of 1460 nm or more is maintained and an effective cross-sectional area at a wavelength of 1460 nm or more is maintained. Can be expanded more easily than in the past, so that the optical non-linearity in the optical fiber in WDM transmission and distributed Raman amplification transmission using the wavelength 1550 nm band can be reduced, and the transmission characteristics of the optical transmission system can be improved. Play.

In addition, since the core-enlarged single-type optical fiber of the present invention can be configured in a range where the normalized frequency is small, that is, in a range where the relative refractive index difference is small, an increase in the nonlinear refractive index due to the addition of germanium or fluorine can be reduced. There are also effects such as.

Conceptual diagram showing the configuration of an optical communication system using a core-enlarged single-mode optical fiber Conceptual diagram showing the cross-sectional structure of an expanded core single-mode optical fiber Diagram showing the structural conditions of a core-enlarged single-mode optical fiber as a function of normalized frequency and normalized inter-core distance Conceptual diagram showing a cross-sectional structure that realizes improved bending loss characteristics in a core-enlarged single-mode optical fiber Conceptual diagram showing a cross-sectional structure that realizes improved bending loss characteristics in a core-enlarged single-mode optical fiber

  Below, one embodiment of the core expansion single mode optical fiber of the present invention is described using a drawing.

  FIG. 1 is a conceptual diagram showing a configuration of an optical transmission system using a core expansion single mode optical fiber 10 according to an embodiment of the present invention. An optical transmission system using a core-enlarged single mode optical fiber according to an embodiment of the present invention operates in a single mode at a wavelength of 1460 nm or more with a transmission unit (Tx) 20 that generates and transmits an optical signal in a wavelength of 1550 nm. The core expansion single mode optical fiber 10 and a receiving unit (Rx) 30 that receives and demodulates signal light from the core expansion single mode optical fiber 10 are configured.

  The transmission unit 20 and the reception unit 30 include a 1460 nm band excitation light source for distributed Raman amplification, and a coupling element that couples the excitation light from the light source to the core expansion single mode optical fiber 10. It doesn't matter. Further, the optical transmission line constituted by the core expansion single mode optical fiber 10 can be extended by inserting an erbium-doped optical amplifier or the like in the middle thereof.

  In the first embodiment, a method for realizing the core expansion single mode optical fiber 10 described above will be described with reference to the drawings.

  FIG. 2 is a conceptual diagram showing a cross-sectional structure of the core-enlarged single mode optical fiber 10A of the present embodiment. The core-enlarged single-mode optical fiber of the present example has a clad part 101 having a uniform refractive index and a diameter D set to 125 ± 1 μm, a diameter set to 2a (radius = a), and a cladding 7 core portions 102 having a relative refractive index difference with respect to the portion 101 set to Δ, and the seven core portions 102 are arranged in a hexagonal close-packed structure at intervals Λ at the center of the clad portion 101. Yes. That is, the seven core portions 102 are arranged in a hexagonal close-packed structure with a spacing Λ around one core portion 102 disposed at the center of the clad portion 101 and around the one core portion 102. It is installed.

  FIG. 3 is a drawing showing the structural conditions of the core expansion single mode optical fiber 10 of this embodiment as a function of the normalized inter-core distance Λ / 2a and the normalized frequency V. Here, the normalized frequency V is defined by the following equation (1) using the diameter 2a and relative refractive index difference Δ of the core portion 102 and the refractive index n1 of the core portion 102. In equation (1), λ is the wavelength of the optical signal propagating through the optical fiber 10A.

  A solid line 501 in the figure indicates a structural condition in which the cutoff wavelength is 1460 nm, and a cutoff wavelength of 1460 nm or less can be realized in a region on the left side of the solid line 501. A solid line 502, a solid line 503, and a solid line 504 indicate the confinement loss, and the calculation results of the confinement loss when the diameter 2a of the core portion 102 is 2 μm, 3 μm, and 4 μm, respectively. It becomes possible to reduce the confinement loss to 0.01 dB / km or less in the region below (right) the solid lines 502, 503, and 504. Therefore, by setting the normalized inter-core distance and the normalized frequency in the region surrounded by the solid line 501 and the solid lines 502, 503, and 504 shown in FIG. 3, the cutoff wavelength characteristic of 1460 nm or less and the confinement loss of 0.01 dB / km or less are set. The characteristics can be realized at the same time. That is, it is possible to realize a good single mode operation in a wavelength band of 1460 nm or more.

When the standardized inter-core distance Λ / 2a is 1, adjacent core portions 102 are in contact with each other, and manufacturing difficulty increases. For this reason, the standardized inter-core distance Λ / 2a is preferably set to about 1.2 or more. Here, when the cross-sectional area of the disposed core portion 102 is the smallest, that is, the diameter 2a of the core portion 102 is 2 μm, the normalized frequency is 0.58, and the normalized inter-core distance is 1. Focusing on the case of 2, the effective area at a wavelength of 1460 nm is about 90 μm ² . When the diameter 2a of the core portion 102 is expanded to 3 μm, the normalized frequency is 0.72, and the distance between the normalized cores is 1.2, the effective area at the wavelength of 1460 nm is expanded to 130 μm ² . Further, when the diameter 2a of the core portion 102 is 4 μm, the normalized frequency is 0.86, and the distance between the normalized cores is 1.2, the effective area at the wavelength of 1460 nm can be expanded to about 175 μm ² , More preferable characteristics can be realized.

Therefore, in the core expansion single mode optical fiber 10A shown in FIG. 2 of this embodiment, the diameter 2a of the core portion 102 is 3 to 4 μm, the normalized frequency is 0.7 to 0.9, and the normalized inter-core distance is By setting in the range of 1.2 to 2.5, it has an effective cut-off wavelength characteristic of wavelength 1460 nm or less, and the effective area at wavelength 1460 nm or more is 100 μm ² or more. It becomes possible to realize the transmission property and the effective cross-sectional area characteristic of 100 μm ² or more at a wavelength of 1460 nm or more. Here, when the normalized frequency range 0.7 to 0.9 is realized in the range 3 to 4 μm of the diameter 2a of the core portion 102, the relative refractive index difference Δ of the core portion 102 is less than 0.3%. In addition, an increase in the nonlinear refractive index due to the addition of germanium or the like can be suppressed.

  In the second embodiment, a technique for improving the bending loss characteristic of the core expansion single mode optical fiber 10A described in the first embodiment without deteriorating the transmission characteristic will be described.

  4 and 5 are conceptual diagrams of two types of core expansion single-mode optical fibers 10B and 10C having a cross-sectional structure with improved bending loss characteristics in the second embodiment.

As shown in FIG. 4, one type of core expansion single mode optical fiber 10B in the second embodiment is arranged so as to circumscribe a circumference having a distance of x μm from the center of the cladding portion 101, and the cladding. An annular low refractive index region 103 having a refractive index lower than that of the portion 101 and a thickness of z ₁ μm is provided. Further, as shown in FIG. 5, the other type of core-enlarged single mode optical fiber 10C is arranged at equal intervals so as to circumscribe a circumference having a distance of x μm from the center of the clad 101 and has a diameter. It has 10 or more hole regions 104 of z ₂ μm.

Although so as to obtain the same effect as the low refractive index region 103 shown in FIG. 4 by providing a plurality of holes area 104 in the embodiment shown in FIG. 5, the low refractive index region 103 by vacancy region 104 In order to obtain the same effect, it is preferable to provide ten or more hole regions 104. In addition, the inside of the hole region 104 exhibits a low refractive index equivalent to that of the low refractive index region 103 by being filled with air, but the gas filling the hole region 104 is not limited to air and has a low refractive index. Any gas other than air may be used as long as it has a low refractive index equivalent to that of the region 103.

The thus configured core expansion single mode optical fibers 10B and 10C can confine the electric field distribution of propagating light in the low refractive index region 103 or the cladding region inside the plurality of hole regions 104. Thus, it is possible to reduce an increase in transmission loss accompanying bending. Here, if the distance x from the center of the clad 101 is too small, the transmission characteristic of the propagation light is deteriorated, and if it is too large, it is difficult to obtain a sufficient confinement effect. Therefore, the distance x is preferably set to 1.0 to 1.5 times the mode field diameter (MFD). Here, the effective area A can be described by a relational expression of A = πW ² using the mode field radius W.

Therefore, in the core expansion single mode optical fibers 10B and 10C of the present embodiment in which the effective cross-sectional area is about 100 to 180 μm ² , the mode field radius W is about 5 to 8 μm, and the distance from the center of the cladding portion 101 is increased. The distance x is preferably set to about 10 to 24 μm. Further, when the annular low refractive index region 103 is set, a sufficient confinement effect can be obtained if the relative refractive index difference of the low refractive index region 103 with respect to the cladding portion 101 is approximately −0.2% or less. Become. Similarly, when the hole region 104 is set, a sufficient confinement effect can be obtained if the relative refractive index difference of the hole region 104 with respect to the cladding portion 101 is approximately −0.2% or less. Furthermore, the thickness z ₁ of the low-refractive index region 103 and the diameter z ₂ of the hole region 104 may be approximately equal to or larger than the wavelength, that is, 1 μm or larger. However, if z ₁ and z ₂ are too large, the confinement effect for higher-order modes is also increased. Therefore, z ₁ and z ₂ are preferably about 5 μm or less.

As described above, according to the core expansion single mode optical fibers 10A, 10B, 10C and the optical transmission system of this embodiment, the single mode transmission property at the wavelength of 1460 nm or more is maintained and the effective interruption at the wavelength of 1460 nm or more is maintained. By expanding the area to 100 μm ² or more, it is possible to improve transmission characteristics in WDM transmission and distributed Raman amplification transmission using the wavelength 1550 nm band.

According to the core-enlarged single mode optical fiber of the present invention, since it has an effective cutoff wavelength characteristic of a wavelength of 1460 nm or less and an effective cross-sectional area of a wavelength of 1460 nm or more is 100 μm ² or more, a single mode at a wavelength of 1460 nm or more is obtained. Since the effective area at the wavelength of 1460 nm or more is maintained as compared with the conventional one, the optical nonlinearity in the optical fiber in the WDM transmission using the wavelength 1550 nm band and the distributed Raman amplification transmission is reduced, and the optical transmission is performed. The transmission characteristics of the system can be improved

DESCRIPTION OF SYMBOLS 10,10A, 10B, 10C ... Core expansion single mode optical fiber, 20 ... Transmission part (Tx), 30 ... Reception part (Rx), 101 ... Cladding part, 102 ... Core part, 103 ... Low refractive index area, 104 ... hole area.

Claims (4)

A clad portion having a diameter of 125 ± 1 μm and a uniform refractive index, and seven core portions having a diameter of 2a and a relative refractive index difference with respect to the clad portion set to Δ; Have
The seven core parts are arranged in a hexagonal close-packed structure with a center interval Λ around one core part arranged at the center of the clad part, and around the one core part,
The diameter 2a of the core part is set to 3 to 4 μm, the normalized frequency of the core part is set to 0.7 to 0.9, and the normalized inter-core distance Λ / 2a is set to a range of 1.2 to 1.8, respectively .
Having an effective cutoff wavelength characteristic of a wavelength of 1460 nm or less,
An expanded core single-mode optical fiber characterized by having an effective area of 100 μm ² or more at a wavelength of 1460 nm or more and a confinement loss of 0.01 dB / km or less . The core expanded single mode optical fiber of claim 1,
An annular low-refractive- index region having a refractive index lower than that of the cladding part and having a thickness set to z ₁ is arranged so as to circumscribe a circumference having a distance x from the center of the cladding part; ,
The distance x is set to 1.0 to 1.5 times the mode field diameter, the relative refractive index difference of the low refractive index region with respect to the cladding portion is set to −0.2% or less, and the low refractive index The core expansion single mode optical fiber, wherein the thickness z ₁ of the region is set to ₁ to 5 μm. The core expanded single mode optical fiber of claim 1,
It is arranged at equal intervals so as to circumscribe the circumference where the distance from the center of the cladding part is x, and has 10 or more hole regions each having a diameter set to z ₂ ,
The distance x is set to 1.0 to 1.5 times the mode field diameter, the relative refractive index difference of the hole region with respect to the cladding part is set to −0.2% or less, A diameter-enlarged single-mode optical fiber having a core z ₂ set to ₁ to 5 μm. A transmitter that generates and transmits signal light in a wavelength band of 1550 nm;
A core expansion single mode optical fiber that propagates signal light from the transmitter; and
An optical transmission system including a receiving unit that receives and demodulates signal light from the core-enlarged single-mode optical fiber,
The core expansion single mode optical fiber in any one of Claims 1 thru | or 3 is used as said core expansion single mode optical fiber. The optical transmission system characterized by the above-mentioned.

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