JP4913675B2 - Fiber optic cable - Google Patents

Description

  The present invention relates to a tape assembly type optical fiber cable in which a plurality of optical fiber ribbons, in which a plurality of optical fibers are arranged in parallel and integrated, are mounted, and in particular, light with reduced polarization mode dispersion characteristics in each optical fiber. It relates to fiber cables.

  As shown in FIG. 1, a plurality of optical fiber tape cores, in which a plurality of optical fibers (fiber optic strands) are arranged in parallel and integrated, are assembled, and a tape assembly type optical fiber cable that is mounted is an optical fiber cable. Since multi-core simultaneous connection is possible and connection efficiency is excellent, it is currently widely used from the optical transmission path of the backbone relay system to the optical transmission path of the optical access system.

  In optical fiber communication, one of the dominant factors of signal degradation when performing high-speed and long-distance transmission is polarization mode dispersion (PMD: Polarization mode dispersion) of an optical fiber cable constituting a transmission path. PMD is a phenomenon caused by a group delay difference generated between two orthogonal polarization modes of signal light propagating in an optical fiber. Such a group delay difference is known to be due to birefringence (given by the difference in refractive index between two orthogonal axes) of the core of the optical fiber.

  In the manufacturing process of the above-described tape assembly type optical fiber cable, the main factors that the optical fiber has birefringence are as follows: (1) When the optical fiber is manufactured from the glass base material, the cross-sectional shape of the core of the optical fiber (2) caused by non-uniform stress applied to the optical fiber in the tape core due to the residual residual stress of the coating resin used in manufacturing the tape core. For example, it is caused by a load stress caused by bending, pulling, or twisting applied to the optical fiber when the tape core is mounted in the optical fiber cable.

Moreover, as shown in Patent Document 1, by adding a twist to the optical fiber in the optical fiber cable at a predetermined twist rate (given by the reciprocal of the twist pitch of one direction of the tape core wire in the cable) It is known that PMD is reduced. This is because the birefringence is reduced by twisting the optical fiber and averaging between the orthogonal linearly polarized light beams of the optical fiber.
JP 2002-122762 “Optical fiber cable” A. Galtarossa, CG Someda, A. Tommasini, BA Schrefler, G. Zavarise, M. Schiano, "Stress Distribution in Optical-fiber ribbons", IEEE Photonics Technology Letters, Vol. 9, No. 3, pp.354-356 , 1997 RE Schuh, ESR Sikora, NG Walker, AS Siddiqui, LM Gleeson, and DHO Bebbington, "Theoretical analysis and measurement of effects of fiber twist on polarisation mode dispersion of optical fibers", Electronics Letters, Vol. 31, No. 20, 1995 , pp.1772-1773

  However, although PMD is reduced by twisting an optical fiber at a predetermined twist rate, it is known that PMD increases conversely when the twist rate exceeds a certain value. This is because the birefringence between the clockwise circularly polarized light and the counterclockwise circularly polarized light increases conversely when the twist rate becomes a certain value or more.

  In general, it is difficult to control the birefringence of each optical fiber constituting the tape core wire mounted on the optical fiber cable, which varies within a certain range. This is because, as shown in Non-Patent Document 1, since the tape core wire is obtained by arranging a plurality of optical fibers in parallel and coating them, the cured residual stress due to the coating resin is applied between the optical fibers at both ends and the optical fibers between them. A major factor is the difference in the way they join.

  For this reason, in a structure in which a twist is applied to a tape core wire at a predetermined twist rate in an optical fiber cable using a conventional method, depending on the birefringence of the optical fiber constituting the tape core wire, PMD There are optical fibers in which the PMD is sufficiently reduced, and conversely, there are optical fibers in which the PMD increases, and the maximum value of PMD of an optical fiber cable in which several tens to thousands of optical fibers are mounted is large. There was a problem of becoming.

An optical fiber cable according to the present invention that can solve the above-mentioned problems is a collection of a plurality of optical fiber tape cores in which a plurality of optical fibers are arranged in parallel and integrated, and each is added with a unidirectional twist. In the optical fiber cable, the maximum value of the intrinsic birefringence of the tape core in the untwisted state of all the optical fibers constituting the plurality of optical fiber ribbons is B1, and the minimum is B2 (the unit is rad). / M, both of which are positive real numbers), the twist pitch P of the unidirectional twist (unit is m, positive real number)

It is characterized by being set to.

Alternatively, in an optical fiber cable in which a plurality of optical fiber ribbons integrated with a plurality of optical fibers arranged in parallel and integrated with each other and twisted in one direction are respectively mounted, the twist pitch of the one-way twist is set. When P (unit is m, positive real number), as the plurality of optical fiber ribbons , the intrinsic birefringence value of the ribbon in an untwisted state is

An optical fiber ribbon that is composed only of optical fibers that fall within an arbitrary maximum value B1 and minimum value B2 satisfying the relationship (unit: rad / m, both of which are positive real numbers) is selected. .

  According to the present invention, the one added to the optical fiber ribbon so that the maximum B1 and the minimum B2 of the birefringence of all the optical fibers constituting the optical fiber ribbon are equal and small B cable. Any maximum value B1 and minimum that sets the twist pitch P of the direction twist, or the Bcable in which the birefringence values at the twist pitch P of the unidirectional twist applied to the optical fiber ribbon are both equal and small. By selecting an optical fiber ribbon that is composed only of optical fibers that fall within the value B2, the birefringence x of all optical fibers in the optical fiber cable is always x ≦ Bcable. An optical fiber cable with a small variation in PMD and a small maximum value can be realized, and an optical fiber cable using such an optical fiber cable can be used. By building sending passage, it is possible to alleviate the transmission restriction by PMD, it is possible to realize a higher speed, long-distance optical fiber communication.

  Hereinafter, the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a typical structure of an optical fiber cable used in the present invention. FIG. 1 (A) is a sectional view of a structure using a slot rod, and FIG. 1 (B) is a sectional view of a slotless structure. .

  That is, in the structure of FIG. 1 (A), an optical fiber ribbon 2 in which a plurality of optical fibers are arranged in parallel and integrated in a groove of a slot rod 1 having one or more grooves (slots) provided on the surface. A plurality of layers are stacked and stored, and a press-wrapping tape 3 and a jacket 4 are provided around the periphery. In the structure of FIG. 1B, a plurality of optical fiber ribbons 2 in which a plurality of optical fibers are arranged in parallel and integrated are laminated (here, laminated in both vertical and horizontal directions), and the periphery thereof is filled with a filler 5. It is held and fixed, and a press-wrapping tape 3 and a jacket 4 are provided around it. In FIGS. 1A and 1B, reference numeral 6 denotes a tensile body.

  Here, in the optical fiber cable shown in FIG. 1 (A), the groove of the slot rod 1 is formed in a spiral shape, thereby applying a unidirectional twist to the optical fiber ribbon 2 housed in the groove. It is like this. Further, in the optical fiber cable shown in FIG. 1B, the laminated optical fiber ribbons 2 themselves are held and fixed by the filler 5 in a state in which a twist is applied in one direction.

<First Embodiment>
As a result of measuring the birefringence of each optical fiber constituting the five tape cores before being mounted on the optical fiber cable (20 fibers in total because four optical fibers are included per one tape core), the maximum value is obtained. Was 3.14 rad / m, and the minimum value was 0.24 rad / m.

  When the above-mentioned tape core is mounted on an optical fiber cable and twisted in one direction, the maximum and minimum birefringence (hereinafter referred to as the maximum and minimum intrinsic birefringence of the tape core, FIG. 2 shows the calculation results of the change with respect to the twist pitch (P) of B1 and B2).

  In general, the magnitude Bt of the birefringence of the optical fiber when the optical fiber is twisted is given by the following equation as shown in Non-Patent Document 2.

  Here, B is the magnitude of the birefringence of the optical fiber before twisting, T is the twist rate (reciprocal of twist pitch P; T = 1 / P), Bω = dB / dω, Bc = gT (g is rotation) Coefficient), Bcω = dBc / dω.

  At this time, assuming that the wavelength dependence of the photoelastic effect of the optical fiber can be ignored, it can be expressed as Bω = B / ω and Bcω = T × dg / dω. Further, considering that a general silica-based optical fiber is used at a communication wavelength of 1550 nm band, g = 0.16, ω = 2π × v / (1550 × 10 ^ −9) (v is in a vacuum) Of light = 3.0 × 10 ^ 8), and ω / g × dg / dω = 0.09.

  Using the above relationship, an optical fiber having the maximum value B1 and the minimum value B2 (unit: rad / m) of the intrinsic birefringence of the tape core wire is mounted on the optical fiber cable, and a predetermined twist pitch P (unit: m) When a unidirectional twist is applied, in order for the optical fiber to have an equally small birefringence Bt, it is obtained by substituting the maximum value B1 and the minimum value B2 into B in the above equation (1). Since the required Bt needs to be equal, the relationship of the following equation can be derived.

  As can be seen from the change in the maximum birefringence value B1 in FIG. 2, when the twist pitch is shortened, the birefringence becomes 0 at a certain twist rate, and increases again in a region where the twist pitch is short. This is because the birefringence between right-handed circularly polarized light and left-handed circularly polarized light becomes conversely larger when a certain twist rate is exceeded. The twist pitch at which the birefringence is 0 differs depending on the intrinsic birefringence of the tape core wire.

  Therefore, by grasping the change in birefringence when twist is applied only to the optical fiber having the maximum value B1 and the minimum value B2 of the intrinsic birefringence of the tape core wire, the birefringence is obtained. It can be seen that the other optical fibers in which x is in the range of B2 ≦ x ≦ B1 always fall within the range indicated by hatching in FIG.

  Further, here, the intersection of the two curves given by the change of the maximum value B1 and the minimum value B2 of the intrinsic birefringence of the tape core, that is, the twist that gives the above-mentioned equally small birefringence Bt (hereinafter referred to as Bcable). It can be seen that at a pitch of about 460 mm, it has a small birefringence maximum of about 0.12 rad / m.

  Therefore, in the first embodiment of the present invention, when the maximum birefringence B1 and the minimum value B2 of the intrinsic birefringence of the tape core are obtained, the twist pitch P of the unidirectional twist is obtained by the above equation (2). Set to a value. Thereby, the variation in birefringence of all the optical fibers in the optical fiber cable can be reduced, and the maximum birefringence can be reduced.

  FIG. 3 shows a PMD measurement result of each optical fiber in an optical fiber cable to which the present invention is applied and a PMD measurement result of each optical fiber in an optical fiber cable to which the present invention is not applied. The axis is shown as a comparison of appearance frequency. All optical fiber cables have the same twist pitch of 500 mm, which is a value close to the twist pitch of about 460 mm to which the B cable is given. In the optical fiber cable to which the present invention is not applied, the intrinsic birefringence of the tape core wire is not grasped. Therefore, it can be seen that the PMD varies greatly when mounted on the optical fiber cable. On the other hand, it can be seen that the optical fiber cable to which the present invention is applied has a small PMD and a small variation.

  As described above, by using the present invention, variation in birefringence of each optical fiber in the optical fiber cable can be reduced, and maximum birefringence can be reduced, thereby reducing PMD. An optical fiber cable can be realized.

<Second Embodiment>
FIG. 4 shows a combination of the maximum value B1 and the minimum value B2 of the intrinsic birefringence of the tape core wire to which the B cable is given in the optical fiber cable in which the tape core wire is mounted at a twist pitch of 500 mm. Here, the combination of the maximum value B1 and the minimum value B2 of the intrinsic birefringence of the tape core wire is the twist pitch P (unit is m), and the maximum value B1 and the minimum value B2 (in units of the intrinsic birefringence of the tape core wire). rad / m, both of which are positive real numbers), a range that satisfies the relationship of the following equation.

  From the arbitrary point on the solid line in FIG. 4, the maximum value B1 and the minimum value B2 of the intrinsic birefringence of the tape core wire can be calculated. For example, in FIG. 4, point A is (B1, B2) = (2.8, 0.5), and the tape core wire to be mounted on the optical fiber cable is selected from the range of 0.5 ≦ x ≦ 2.8. There is a need to. As the point A in FIG. 4 approaches the straight line B1 = B2, the widths of the maximum and minimum values B1 and B2 of the intrinsic birefringence of the tape core are reduced, and the tape core to be mounted on the optical fiber cable is selected. Therefore, it is preferable to select an arbitrary point on the solid line in FIG. 4 away from the straight line B1 = B2.

  In the second embodiment of the present invention, as described above, based on the maximum value B1 and the minimum value B2 of the intrinsic birefringence of the tape core wire given from any one point on the solid line in FIG. The tape core wire to be mounted on the fiber cable is selected. In other words, when the twist pitch of the unidirectional twist applied by the optical fiber cable is P, the value of the birefringence satisfies the relationship of the above formula (3). By selecting an optical fiber ribbon that consists only of optical fibers that fall between the maximum value B1 and the minimum value B2, variation in the birefringence of the optical fiber in the optical fiber cable can be reduced, and maximum The birefringence can be reduced, and an optical fiber cable with reduced PMD can be realized.

  The above formulas (2) and (3) are based on the premise that the optical fiber constituting the tape core is a general silica-based optical fiber. However, the communication wavelength is not limited to the 1550 nm band, but the 1310 nm band. , 850 nm band, etc. can be used as well.

Sectional drawing which shows the typical structure of the optical fiber cable used for this invention The figure showing the calculation result of the change with respect to the twist pitch of the maximum value B1 and the minimum value B2 of the intrinsic birefringence of a tape core wire The figure which shows distribution of PMD in the optical fiber cable which applied this invention, and the optical fiber cable which did not apply this invention The figure which shows the combination of the maximum value B1 and the minimum value B2 of the intrinsic birefringence of the tape core wire to which Bcable is given in the optical fiber cable of twist pitch 500mm

Explanation of symbols

  1: slot rod, 2: optical fiber ribbon, 3: wound tape, 4: jacket, 5: filler, 6: tensile body.

Claims (2)

In an optical fiber cable in which a plurality of optical fiber ribbons, in which a plurality of optical fibers are arranged in parallel and integrated, are assembled by adding a twist in one direction to each,
When the maximum value of the intrinsic birefringence of the optical fiber in the untwisted state of all optical fibers constituting the plurality of optical fiber ribbons is B1 and the minimum value is B2, the twist of the one direction Twist pitch P

An optical fiber cable characterized in that In an optical fiber cable in which a plurality of optical fiber ribbons, in which a plurality of optical fibers are arranged in parallel and integrated, are assembled by adding a twist in one direction to each,
When the twist pitch of the unidirectional twist is P, the value of the intrinsic birefringence of the tape core wire in the untwisted state as the plurality of optical fiber tape core wires is

An optical fiber cable comprising: an optical fiber ribbon composed only of an optical fiber that falls between an arbitrary maximum value B1 and a minimum value B2 satisfying the above relationship.

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