JP2007108424A - Optical cable - Google Patents

Abstract

An optical cable in which PMD generated in an optical fiber core wire due to various factors is reliably suppressed is provided.
A two-core optical fiber ribbon 25 in which two optical fiber cores 26 are integrated is housed in a groove 24 of a spacer 22 having a unidirectionally twisted spiral groove 24 around it. The optical fiber ribbon 25 is twisted by periodically changing the twist direction in the longitudinal direction. The optical fiber ribbon 25 is fixed to the spacer 22 at a position where the twisting direction is reversed and at an interval equal to or larger than the spiral pitch of the groove 24.
[Selection] Figure 9

Description

  The present invention relates to an optical cable in which an optical fiber core wire is housed in a groove portion of a spacer formed by forming a one-way twisted spiral groove portion on the outer periphery.

  Conventionally, a plurality of optical fiber core wires are collectively laid as an optical cable. As such an optical cable, one having a cable core in which a plurality of optical fiber core wires are housed in a spiral groove formed around a spacer is known.

One of the causes of signal degradation when performing high-speed and long-distance transmission with an optical fiber is polarization mode dispersion (PMD). This PMD is caused by a group delay difference between two orthogonal polarization modes of an optical signal propagating in an optical fiber. The difference in signal transmission speed between two orthogonal polarizations is psec / km ^{1 /} It is evaluated in units of ² .

  The anisotropy of the optical fiber that causes PMD is caused by the elliptical shape of the core of the optical fiber or birefringence due to various stresses. Specifically, the anisotropy of the glass base material that is the base material of the optical fiber. Distortion caused by thermal history, variation in heating temperature and cooling temperature in the circumferential direction when drawing an optical fiber from a glass base material, deviation between the axis of the optical fiber and the center of the core in the state of an optical cable, and the cross-sectional shape of the core Is caused by an ellipse or the like.

  For this reason, in an optical cable in which a plurality of optical fiber cores are housed in the spiral groove of the spacer, in order to reduce PMD of the optical fiber core wire, In particular, it is known to increase the twist rate (twist amount per unit length), to average the orthogonal polarization mode dispersion of the optical fiber originally possessed, and to reduce PMD (for example, , See Patent Document 1).

JP 2002-122762 A

  However, the PMD of the optical fiber does not decrease if the twist rate is simply increased, and when the twist rate exceeds a certain twist rate, the PMD starts increasing again. This is because a group delay difference occurs between right-handed circularly polarized light and left-handed circularly polarized light by optical rotation in the polarization direction caused by twisting. For this reason, in order to produce a low PMD optical cable, it is necessary to suppress an increase in PMD due to this phenomenon.

  An object of the present invention is to provide an optical cable in which PMD generated in an optical fiber due to various factors is reliably suppressed.

  An optical cable according to the present invention that can solve the above-described problems is an optical cable including a cable core in which an optical fiber core wire is accommodated in the groove portion of a spacer having a one-way twisted spiral groove portion on the outer periphery. The optical fiber core is housed in the groove in a twisted state in which the twisting direction is periodically changed in the longitudinal direction.

  In the optical cable according to the present invention, it is preferable that the optical fiber core wire periodically has a portion where the integral value of the twist angle in the longitudinal direction becomes zero in the longitudinal direction.

In the optical cable according to the present invention, the polarization mode dispersion in a state before the optical fiber core is housed in the groove is 1.0 ps / km ^1/2 or less, and the optical fiber core Preferably, the twist angle amplitude α is 40 ° or more, and the twist angle amplitude α and the inversion pitch Λ in the twist direction satisfy the following relationship.
Λ ≦ 1.3 × (α−24) ^1/2

Further, in the optical cable according to the present invention, the polarization mode dispersion in a state before the optical fiber core is housed in the groove is 0.5 ps / km ^1/2 or less, and the optical fiber core Preferably, the twist angle amplitude α is 50 ° or more, and the twist angle amplitude α and the inversion pitch Λ in the twist direction satisfy the following relationship.
Λ ≦ 2.0 × (α−24) ^1/2

  Moreover, the optical cable which concerns on this invention WHEREIN: It is preferable that the inversion pitch of the twist direction of the said optical fiber core wire is more than the helical pitch of the said groove part. Furthermore, it is preferable that the optical fiber core wire is fixed to the spacer at a position where the twisting direction is reversed.

  According to the optical cable of the present invention, by twisting the optical fiber core, the speed difference between the orthogonal polarization modes originally possessed by the optical fiber core is averaged, and the optical fiber core By periodically changing the twist direction in the longitudinal direction, it is possible to compensate for the group delay difference of the circularly polarized light in the longitudinal direction and to reduce the polarization mode dispersion due to the twist. That is, it is possible to reliably suppress polarization mode dispersion that occurs in the optical fiber core due to various causes.

Hereinafter, an example of an embodiment of an optical cable according to the present invention will be described with reference to the drawings.
(Principle of PMD reduction effect by twisting)
FIG. 1 is a graph showing PMD generated when an optical fiber core wire is twisted.
As shown in FIG. 1, when the optical fiber core is twisted, two phenomena related to changes in PMD occur in the optical fiber core. One of these phenomena is phenomenon A in which the PMD is lowered by averaging the speed difference between the two modes by averaging the orthogonal polarization modes due to the rotation of the fiber anisotropic direction by twisting (FIG. 1). Curve B), and the other is a phenomenon B (shown in FIG. 1) in which the optical rotation in the polarization direction is caused by twisting, a group delay difference is generated between clockwise circularly polarized light and counterclockwise circularly polarized light, and PMD is negatively increased. Curve B).

In general, PMD does not have positive and negative values like chromatic dispersion, so that it is difficult to perform dispersion management transmission. This is because coupling between orthogonal polarization modes occurs due to an external force or slight twist, and the polarization state is randomized.
However, dispersion between circularly polarized modes due to twisting distortion has no mode coupling and has positive and negative values with respect to the twisting direction. Therefore, in the housed state in which the twist rate that gives the effect of phenomenon A sufficiently is given to the optical fiber core wire, management transmission of polarization mode dispersion is possible by alternately swinging the twist direction.

  FIG. 2A shows the change in the twist angle of the optical fiber in the longitudinal direction of the optical cable, and FIG. 2B shows the change in the PMD in the longitudinal direction accompanying the change in the twist angle of the optical fiber. Show. By changing the twisting angle (twisting amount) of the optical fiber core wire in the longitudinal direction positively and negatively with the amplitude of the twisting angle at which the effect of phenomenon A is sufficiently obtained, as shown in FIG. That is, by periodically reversing the twisting direction so that there are portions where the integral value of the twisting angle in the longitudinal direction becomes zero (L1, L2 in FIG. 2) periodically in the longitudinal direction, As shown in FIG. 2B, transmission can be performed while compensating for PMD. And the PMD reduction effect by the phenomenon A is continuously exhibited over a longitudinal direction.

Here, a tape core wire in which a plurality of optical fiber core wires are integrated with resin is twisted with a twist angle amplitude α and a twist reversal pitch Λ, as shown in FIG. The calculation results of PMD with respect to the twisting amplitude and pitch of the tape core wire at this time are shown as contour graphs in FIGS. Note that the PMD ability value (PMD before being stored in the groove of the optical cable spacer, that is, PMD without being twisted) used for the calculation of FIG. 4 is 1.0 ps. / Km ^1/2 , and the PMD ability value of the core wire used in the calculation of FIG. 5 is 0.5 ps / km ^1/2 .
As shown in FIGS. 4 and 5, it can be seen that PMD is lowered by twisting the tape core in both directions. Further, it is possible to further reduce PMD by increasing the amplitude α of the twist angle when the tape core wire is twisted in both directions and decreasing the inversion pitch Λ, that is, increasing the twist rate. I understand that.

  In addition, the original PMD ability value of the optical fiber core wire varies. In addition, there is variation in the external force applied to the optical fiber core wire even within the tape core wire. Therefore, in order to guarantee PMD after cable formation, it is considered that the design should be such that the maximum value can be guaranteed in the distribution of variations in the actual value of the optical fiber core wire.

If the maximum value of the PMD actual value of the optical fiber is 1.0 ps ^{/ miles 1/2,} to ensure the suggested reference value PMDq <0.5ps ^{/ km 1/2} in IEC (International Organization for Standardization) is The cable may be configured to be in a range below the solid line C shown in FIG. That is, α ≧ 40 ° and Λ ≦ 1.3 × (α−24) ^1/2 may be satisfied. Here, Λ is the reverse pitch in the twist direction, and α is the twist angle amplitude.
The PMDq indicates an effective PMD value measured by connecting a plurality of optical cables, and is normally statistically estimated from the distribution of PMD values of the target optical cable.

Further, as the lower PMD, to ensure the recommended reference value PMDq <0.2ps ^{/ km 1/2} of the IEC, the maximum value of the PMD ability value is an optical fiber is 0.5 ps ^{/ miles 1/2} The cable may be configured so as to be in a range below the solid line D shown in FIG. That is, α ≧ 50 ° and Λ ≦ 2.0 × (α−24) ^1/2 may be satisfied.

(Embodiment)
Next, an example of an embodiment of an optical cable according to the present invention will be specifically described with reference to the drawings.
FIG. 6 is a cross-sectional view of the optical cable showing the structure of the optical cable of the present embodiment, and FIG. 7 is a side view showing only the spacer of the optical cable.

As shown in FIG. 6, the optical cable 20 has a cable core 21. The cable core 21 has a spacer 22 having a tension member 23 made of a steel wire or the like provided at the center. As shown in FIG. 7, the spacer 22 includes a plurality of groove portions 24 formed in a spiral shape in one direction on the outer peripheral surface, and a plurality of optical fiber ribbons 25 are formed in the groove portions 24. It is stored. As an example, the optical fiber ribbon 25 is composed of two ribbons, and two optical fibers 26 are arranged in parallel and integrated with a resin.
The spacer 22 has a presser winding 27 wound around the outer periphery thereof, and the outer periphery thereof is covered with an outer cover 28.

Next, with respect to the form of housing the optical fiber core 26 in the optical cable 20 having the above structure, the case where the optical fiber ribbon 25 integrated with the two optical fiber cores 26 is housed as described above will be described as an example. To do.
As described above, the spacer 22 constituting the cable core 21 of the optical cable 20 has the groove 24 formed in a spiral shape, and the direction of the spiral is one direction of left-handed twist.
Two optical fiber ribbons 25 are accommodated in the grooves 24 of the unidirectional (HL) twist spacer 22, and the optical fiber ribbons 25 are periodically twisted in the longitudinal direction. It has been changed and twisted.

The optical fiber ribbon 25 has a maximum PMD ability value of 1.0 ps / km ^1/2 , a twist angle amplitude α ≧ 40 °, and a twist direction reversal pitch Λ ≦ 1. 3 × (α−24) ^1/2 is satisfied.
Further, the maximum value of the PMD ability value of the optical fiber core 26 is set to 0.5 ps / km ^1/2 , the twist angle amplitude α ≧ 50 °, and the twisting direction inversion pitch Λ ≦ 2. More preferably, 0 × (α−24) ^1/2 is satisfied.
In addition, the optical fiber tape core wire 25 has an interval (reverse pitch) between positions where the twisting direction is reversed to be equal to or greater than the helical pitch of the groove portion 24 of the spacer 22. A line 25 is fixed to the spacer 22.

Here, FIG. 8 shows the twist angle of the optical fiber ribbon 25 and the twist of the groove 24 of the spacer 22. In the example shown in FIG. 8, the fixing pitch of the optical fiber tape core 25 to the spacer 22 is made substantially equal to the helical pitch of the groove 24 and the resin fiber 30 such as an adhesive is used at the reverse position of the optical fiber tape 25. The optical fiber ribbon 25 is fixed to the spacer 22.
And the twist of the optical fiber tape core wire 25 is reliably maintained by fixing the position where the twist direction of the optical fiber tape core wire 25 is reversed in this way.

  As a method of fixing the optical fiber ribbon 25 to the spacer 22, in addition to the method of filling the groove 24 with a resin 30 such as an adhesive and fixing the outer sheath 28, the outer cover 28 enters the groove 24. A method of making the gap portion in the cut groove portion 24 small and fixing, or the shape of the groove portion 24 is formed in accordance with the twisted posture of the optical fiber tape core wire 25, and further the twist of the optical fiber tape core wire 25 in the longitudinal direction. There is a method of maintaining the posture of the optical fiber ribbon 25 by forming it so as to be synchronized with the rotation.

  Further, as described above, the fixed pitch of the optical fiber ribbon 25 to the spacer 22 is preferably substantially the same as or greater than the helical pitch of the groove 24, for example, more than the helical pitch of the groove 24. If it is small, the optical fiber tape core wire 25 cannot move in the groove 24, so that distortion occurs due to bending of the optical cable 20.

  Next, as an example of the optical cable 20, the optical cable 20 in which the spiral pitch of the groove portion 24 of the spacer 22 is 500 mm, the inversion period Λ of the optical fiber ribbon 25 is 500 mm, and the twisting amplitude α is 360 °. explain.

  FIG. 9 is a schematic cross-sectional view showing a change in twisting of the optical fiber ribbon 25 in the groove 24 of the spacer 22, and FIG. 10 shows the angle of the groove 24 of the spacer 22 and the angle of the optical fiber ribbon 25. It is a graph which shows the change in the longitudinal direction.

  9, the optical fiber tape core wire 25 is also left-handed with respect to the left-twisted groove portion 24, and as shown in FIG. 10, the inversion period is a spacer. 22 is equal to the helical pitch of the grooves 24, and the twisting amplitude of the optical fiber ribbon 25 is 360 °. Therefore, the optical fiber ribbon 25 is accommodated without being twisted with respect to the groove 24. Then, the twisting direction of the optical fiber ribbon 25 is reversed from left to right at the position i).

9, the twisting direction of the optical fiber ribbon 25 is twisted to the right as opposed to the groove portion 24 of the spacer 22 as shown in FIG. 10. Therefore, the orientation of the optical fiber tape core wire 25 is two rotations relative to the groove portion 24 regardless of the orientation of the groove portion 24 of the spacer 22 (the optical fiber tape is relative to the groove portion 24 rotated once to the left). The core 25 is rotated once to the right). Then, the twisting direction of the optical fiber ribbon 25 is reversed from right to left at the position a).
Thus, the optical fiber tape core wire 25 is fixed to the spacer 22 by the resin 30 at the positions a) and i) where the twisting direction of the optical fiber tape core wire 25 is reversed.

  The twisted optical fiber ribbon 25 has an integrated value of the twist angle in the longitudinal direction of the optical fiber ribbon 25 at the position a), and a) to i). The group delay difference between the clockwise circularly polarized light and the counterclockwise circularly polarized light is compensated every time the twisting of ~ a) is performed.

  In order to obtain the optical cable 20 in which the optical fiber ribbon 25 is housed as described above, a supply-winding rotary type gathering machine that winds while storing the optical fiber ribbon 25 in the groove 24 of the spacer 22 is used. At 500 mm in the sections a) to h), the core twisting device for twisting the optical fiber ribbon 25 is fixed without rotating, and in the sections i) to p), the spacer 22 advances by 500 mm. In between, the optical fiber ribbon 25 is twisted by the core twisting device so that it rotates twice in the right direction, and this movement is performed alternately and continuously.

  At this time, it is preferable that the supply bobbin for supplying the optical fiber ribbon 25 is also rotated in the right direction so that the twist of the optical fiber ribbon 25 does not accumulate. Note that the rotation of the bobbin does not necessarily have to be synchronized with the movement of the core wire twisting device, and may be rotated at an average pace of 500 mm, which is easier to control and simplifies the equipment. It is advantageous for cost reduction.

  In the above example, the twist inversion period Λ of the optical fiber ribbon 25 and the helical pitch of the groove 24 of the spacer 22 are made equal, and the twist amplitude α of the optical fiber ribbon 25 is 360 °. Although the case has been shown, in general, the core twisting device is controlled as follows.

  The cord twist angular velocity is ωf = 2π · v / P ± α · v / Λ (rad / min). However, v is a linear velocity (m / min). Here, the positive and negative signs in the above equation are negative when the twist direction of the core wire is the same as the twist direction of the groove 24 of the spacer 22, and positive when the direction is opposite, and the speed is periodically changed. The twisting speed of the core wire supply bobbin is constantly rotated in one direction at ωb = 2π · v / P.

As described above, according to the optical cable according to the above-described embodiment, the optical fiber core wire 26 is originally orthogonal by twisting the optical fiber core wire 26 by periodically changing the twisting direction in the longitudinal direction. The polarization mode dispersion can be averaged, and the group delay difference of the circularly polarized light can be compensated in the longitudinal direction to reduce the polarization mode dispersion by twisting.
That is, polarization mode dispersion generated in the optical fiber core 26 due to various causes can be reliably suppressed.

  In the above example, for convenience of explanation, the optical cable 20 containing the two optical fiber ribbons 25 integrated with the two optical fibers 26 has been described as an example. The form is not limited to the two-fiber optical fiber ribbon 25.

  An optical cable 20a shown in FIG. 11 has a plurality of single optical fiber cores 26 accommodated in a groove 24, and the plurality of optical fiber cores 26 are periodically twisted in the left-right direction. . When the single optical fiber core 26 is housed in the groove 24, the single optical fiber cores 26 may be twisted together.

  In addition, the optical cable 20b shown in FIG. 12 has a groove formed by stacking a plurality of (for example, four) optical fiber cores 26 stacked in parallel and laminated with optical fiber tape cores 31 integrated in a tape shape with resin. The stack is housed in 24 and the stack is periodically twisted in the left-right direction.

Further, the optical cable 20c shown in FIG. 13 is accommodated in the groove portion 24 as a multi-core unit 33 in which a plurality of (for example, eight) optical fiber cores 26 are bundled and integrated by resin. Twisted periodically in the direction.
In any of the housing forms shown in these optical cables 20a, 20b, and 20c, the optical fiber cores 26 are periodically inverted and twisted, so that the group delay difference of circularly polarized light is increased in the longitudinal direction. By compensating in the direction, PMD due to twisting can be reduced well.

  The PMD of various optical cables in which optical fiber core wires were housed in various housing forms in the groove portions of the unidirectionally twisted spacers was measured.

Example 1
(Optical cable type)
As shown in FIG. 6, a two-core optical fiber ribbon-contained optical cable in which two optical fiber ribbons 25 are accommodated in the groove portion 24 of the spacer 22 was used.

(Storage structure and measurement results)
In the above optical cable, as an example corresponding to the present invention, optical cables having structures 1 to 8 having different twist reversal pitch Λ and twist angle amplitude α are prepared, and as a comparative example, the conventional optical fiber does not invert the twist. A type of optical cable was prepared.
Table 1 shows specific storage configurations and polarization mode dispersion measurement results for optical cables of structures 1 to 8 and the comparative example.
Note that the spiral pitch of the groove portions 24 of the spacer 22 was all 500 mm, and in the comparative example, the optical fiber ribbon was twisted in one direction in the longitudinal direction according to the groove portion (that is, without twisting back).

  As can be seen from Table 1, in the optical cables having structures 1 to 8 to which the present invention in which the twisting directions of the two optical fiber ribbons 25 are periodically reversed are applied, PMDq is suppressed and the groove portion It turned out that it becomes smaller than the comparative example twisted in one direction according to the helical pitch of.

Further, PMDq satisfies the IEC recommended standard value of 0.5 ps / km ^1/2 or less in the optical cables having the structures 1 to 4, and PMDq is another recommended standard value of 0.2 IEC in the optical cables of the structures 5 to 8. / Km ^1/2 or less is satisfied.

(Example 2)
(Optical cable type)
As shown in FIG. 11, a single-core optical fiber housing type optical cable in which a plurality of single-core optical fibers 26 are housed in the groove 24 of the spacer 22 was used.
(Storage structure and measurement results)
In the above optical cable, as an example corresponding to the present invention, optical cables having structures 1 to 8 having different twist reversal pitch Λ and twist angle amplitude α are prepared, and as a comparative example, the conventional optical fiber does not invert the twist. A type of optical cable was prepared.
Table 2 shows specific storage configurations and polarization mode dispersion measurement results for the optical cables of structures 1 to 8 and the comparative example.
Note that the spiral pitch of the groove portions 24 of the spacers 22 is all 500 mm. In the comparative example, the optical fiber core wire is twisted in one direction in the longitudinal direction according to the groove portions.

  As can be seen from Table 2, in the optical cables having the structures 1 to 8 to which the present invention in which the twist direction of the single-core optical fiber 26 is periodically reversed is applied, PMDq is suppressed and the groove portion It turned out that it becomes smaller than the comparative example twisted in one direction according to the helical pitch.

In the optical cables having the structures 1 to 4, PMDq satisfies the IEC recommended standard value of 0.5 ps / km ^1/2 or less, and in the optical cables having the structures 5 to 8, PMDq is another recommended standard value of 0.2 ps of IEC. / Km ^1/2 or less is satisfied.

(Example 3)
(Optical cable type)
As shown in FIG. 12, a stack housing type optical cable in which a stack in which a plurality of optical fiber ribbons 32 are stacked in the groove portion 24 of the spacer 22 is housed is used.

(Storage structure and measurement results)
In the above optical cable, as an example corresponding to the present invention, optical cables having structures 1 to 8 having different twist reversal pitch Λ and twist angle amplitude α are prepared, and as a comparative example, the conventional optical fiber does not invert the twist. A type of optical cable was prepared.
Table 3 shows specific storage structures and polarization mode dispersion measurement results for the optical cables of structures 1 to 8 and the comparative example.
In addition, the spiral pitch of the groove part 24 of the spacer 22 was all 500 mm, and in the comparative example, the optical fiber tape core wire 32 was twisted in one direction in the longitudinal direction according to the groove part.

  As can be seen from Table 3, in the optical cables having the structures 1 to 8 to which the present invention in which the twisting direction of the stack 32 is periodically reversed is applied, the average PMDq and the maximum value PMD of the polarization mode dispersion are suppressed. It was found that it was smaller than the comparative example that was twisted in one direction according to the spiral pitch of the groove.

In the optical cables having the structures 1 to 4, PMDq satisfies the IEC recommended standard value of 0.5 ps / km ^1/2 or less, and in the optical cables having the structures 5 to 8, PMDq is another recommended standard value of 0.2 ps of IEC. / Km ^1/2 or less is satisfied.

It is a graph which shows PMD which arises when an optical fiber core wire is twisted. (A) is a graph which shows the angle of the optical fiber core wire in an optical cable longitudinal direction, (b) is a graph which shows the longitudinal direction change of PMD accompanying the angle of an optical fiber core wire. It is a graph which shows the twisted state of a tape core wire. It is a figure which shows the calculation result of PMD with respect to the amplitude and pitch of twist in the tape core wire which received twist as a contour line. It is a figure which shows the calculation result of PMD with respect to the amplitude and pitch of twist in the tape core wire which received twist as a contour line. It is sectional drawing of the optical cable which shows the structure of the optical cable which concerns on this invention. It is a side view which shows the spacer of an optical cable. It is a figure which shows the twist angle of the optical fiber ribbon, and the twist of the groove part of a spacer. It is a cross-sectional schematic diagram which shows the change of the twist of the optical fiber tape core wire in the groove part of a slot. It is a graph which shows the change in the longitudinal direction of the angle of the slot part of a slot, and the angle of an optical fiber tape cable core. It is sectional drawing of the optical cable which shows the other structure of the optical cable which concerns on this invention. It is sectional drawing of the optical cable which shows the other structure of the optical cable which concerns on this invention. It is sectional drawing of the optical cable which shows the other structure of the optical cable which concerns on this invention.

Explanation of symbols

20, 20a, 20b, 20c Optical cable 21, 21a, 21b, 21c Cable core 22 Spacer 24 Groove 26 Optical fiber core wire 25, 32 Optical fiber tape core wire

Claims (6)

An optical cable comprising a cable core in which an optical fiber core wire is housed in the groove portion of the spacer formed by applying a unidirectional twisted spiral groove portion on the outer periphery,
The optical fiber, wherein the optical fiber core is housed in the groove in a twisted state in which the twisting direction is periodically changed in the longitudinal direction. The optical cable according to claim 1,
The optical fiber is an optical cable characterized in that a portion where the integral value of the twist angle in the longitudinal direction is zero periodically exists in the longitudinal direction. The optical cable according to claim 1 or 2,
Polarization mode dispersion in a state before the optical fiber core is housed in the groove is 1.0 ps / km ^1/2 or less,
The twist angle of the optical fiber is 40 ° or more,
An optical cable characterized in that the twist angle amplitude α and the twist direction reversal pitch Λ satisfy the following relationship:
Λ ≦ 1.3 × (α−24) ^1/2 The optical cable according to claim 1 or 2,
Polarization mode dispersion in a state before the optical fiber core is housed in the groove is 0.5 ps / km ^1/2 or less,
The twist angle of the optical fiber is 50 ° or more,
An optical cable characterized in that the twist angle amplitude α and the twist direction reversal pitch Λ satisfy the following relationship:
Λ ≦ 2.0 × (α−24) ^1/2 An optical cable according to any one of claims 1 to 4,
An optical cable, wherein an inversion pitch in a twisting direction of the optical fiber core wire is equal to or greater than a helical pitch of the groove. An optical cable according to any one of claims 1 to 5,
The optical fiber, wherein the optical fiber core wire is fixed to the spacer at a position where the twisting direction is reversed.

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