JP2007226051A - Optical cable - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical cable capable of surely suppressing a PMD (Polarization Mode Dispersion) caused in coated optical fibers due to various factors. <P>SOLUTION: A plurality of coated optical fibers 26, each twisted around its axis, are integrated using resin 27 to form a multiple fiber unit 25 which is an optical fiber unit. The multiple fiber unit 25, without being twisted back, is housed in the spiral groove 24 of a slot 22, so that the unit 25 is housed in the twisted state. Then, the coated optical fibers 26 within the multiple fiber unit 25 are twisted in the direction reverse to the twisting direction of the multiple fiber unit 25. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical cable containing an optical fiber unit in which a plurality of optical fiber cores are integrated with a resin.

  Conventionally, an optical cable containing an optical fiber unit in which a plurality of optical fiber cores are integrated with resin has been used. As such an optical cable, an optical fiber tape core wire in which a plurality of optical fiber core wires are integrated with a resin in a spacer formed by forming a spiral groove portion on the outer periphery of a long body having a substantially circular cross section. A stack of a plurality of layers 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 expressed as ps / km ^1/2. 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 due to optical rotation in the polarization direction due to twisting distortion. 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 capable of solving the above-described problems is an optical fiber unit obtained by integrating a plurality of optical fiber core wires with a resin in a state in which the optical fiber is twisted about each axis, and the plurality of optical fiber units are An optical cable assembled by twisting, wherein the optical fiber unit is twisted in accordance with the twisting direction and accommodated in the optical cable, and the twisting direction of the optical fiber core wire in the optical fiber unit And the twisting direction of the said optical fiber unit is reverse, It is characterized by the above-mentioned.

  The optical cable which concerns on this invention WHEREIN: It is preferable that the twist pitch of the said optical fiber core wire in the said optical fiber unit and the twist pitch of the said optical fiber unit are equal.

Further, in the optical cable according to the present invention, the optical fiber unit is housed in the groove portion of a spacer formed with a spiral groove portion on the outer periphery of a long body having a substantially circular cross section. It is preferably twisted at a twist pitch equal to the pitch and twisted in accordance with the twist direction in the groove. That is, a so-called tape slot type optical cable is preferable.
Alternatively, in the optical cable according to the present invention, the plurality of optical fiber units may be bundled and twisted inside the optical cable. That is, a so-called slotless optical cable may be used.

  According to the optical cable of the present invention, the optical fiber core wire originally has the optical fiber unit by twisting the optical fiber unit in a direction opposite to the twisting direction of the optical fiber core wire twisted in advance in the optical fiber unit. In addition, it is possible to average the orthogonal polarization mode dispersion, and to suppress the occurrence of a difference in propagation speed of circularly polarized light and to reduce polarization mode dispersion by twisting. 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 a phenomenon A in which 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 (shown in FIG. 1). Curve A), and the other is a phenomenon B (curve shown in FIG. 1) in which PMD is negatively increased due to optical rotation in the polarization direction caused by twisting, causing a group delay difference between clockwise circularly polarized light and counterclockwise circularly polarized light. B).
This phenomenon B cannot be ignored in an optical cable housed so that twisting is applied to the optical fiber core wire.

As a result of intensive studies by the present inventor, a major cause of increasing PMD in an optical cable containing an optical fiber unit in which a plurality of optical fiber cores are integrated with resin is that the optical fiber core wire is, for example, a tape core wire. It has been found that this is anisotropy of stress generated by an external force generated by integrating and unitizing, and in order to reduce this, it is necessary to twist the optical fiber unit.
That is, in order to reduce the PMD of the optical fiber core, it is considered ideal to twist the optical fiber unit without causing mechanical twisting distortion in the optical fiber core itself.

(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. 2 is a cross-sectional view of the optical cable showing the structure of the optical cable according to the present embodiment.
As shown in FIG. 2, 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.

The spacer 22 has a plurality of spirally formed groove portions 24 formed on the outer peripheral surface thereof, and these groove portions 24 are formed in a spiral shape in one direction along the longitudinal direction. The groove portion 24 of the spacer 22 accommodates a multi-core unit 25 that is an optical fiber unit. The multi-core unit 25 is a unit in which a plurality (eight in this example) of optical fiber core wires 26 are bundled and integrated with a resin 27. A thermoplastic resin is preferably used as the resin 27, and the multi-fiber unit 25 is formed by extruding the resin 27 around the bundled optical fiber core wires 26.
The spacer 22 has a presser winding 28 wound around the outer periphery thereof, and the outer periphery thereof is covered with an outer cover 29.

In this optical cable 20, the multi-core unit 25 is twisted without changing the relative direction with respect to the groove 24 of the spacer 22 (that is, in a state without twisting) and accommodated in the groove 24. The multi-core unit 25 is twisted at a twist pitch equal to the spiral pitch of the groove portion 24 in accordance with the twisting direction of the groove portion 24 of the spacer 22.
The multi-core unit 25 is integrated with a resin 27 in a state where the optical fiber core wires 26 are twisted about the respective axes. The twisting direction of each optical fiber core 26 in the multi-core unit 25 and the twisting direction of the multi-fiber unit 25 twisted in accordance with the twisting direction of the groove 24 are opposite to each other. .
For example, in the optical cable 20 in which the multi-core unit 25 is twisted to the left with a pitch of 500 mm, the optical fiber core wire 26 in the multi-fiber unit 25 is given a right-hand twist with a pitch of 500 mm.

FIG. 3 shows changes in the direction of the groove 24, the multi-core unit 25, and the optical fiber core 26 in the longitudinal direction of the optical cable 20.
As the position of the optical cable 20 in the longitudinal direction changes, the direction Da of the groove portion 24 of the spacer 22 is gradually inclined toward the twisting direction in the order of (a), (b), and (c) in FIG.
When the groove portion 24 of the spacer 22 is tilted, the orientation Db of the multi-core unit 25 that is accommodated in the groove portion 24 without being twisted and twisted together with the groove portion 24 is also the same as the orientation Da of the groove portion 24. In FIG. 3, it gradually tilts in the twisting direction in the order of (a), (b), and (c).
Here, the optical fiber core wire 26 of the multi-core unit 25 is twisted at a twist pitch equal to the twist pitch of the multi-core unit 25 in the direction opposite to the twist direction of the multi-core unit 25 around the axis. Therefore, as shown in FIGS. 3A, 3 </ b> B, and 3 </ b> C, the optical fiber core 26 is in a state in which the direction Dc is always directed in the same direction.

  As described above, in this optical cable 20, the multi-core unit 25 is housed in a twisted state. However, the optical fiber core 26 that is twisted in the direction opposite to the twist direction in the multi-fiber unit 25. Is stored in a state without twisting stress in the longitudinal direction by canceling out the twisting amount.

  Here, FIG. 4 shows an optical fiber core wire in which the group delay difference between two orthogonal polarization modes of an optical signal propagating through the optical fiber without twisting of the multi-fiber unit is 0.5 ps / km. Shows the calculation result of PMD change with respect to twist rate (twist rotation number per length) when subjected to twist.

  The solid line in the graph of FIG. 4 indicates that the optical fiber core wire 26 is twisted in advance in the direction opposite to the twisting direction of the multi-core unit 25 so that there is no relative twist and no twist stress. 4 shows the PMD value of the optical fiber core 26 in the form, and the broken line in the graph of FIG. 4 shows, for example, a multi-fiber unit in which the optical fiber core wire is not twisted in a state where the groove is not twisted back The PMD value of the optical fiber core wire when the optical fiber core wire is twisted in accordance with the twisting direction of the groove and twisting stress is applied by housing is shown.

FIG. 5 shows an optical fiber core in which the group delay difference between two orthogonal polarization modes of an optical signal propagating in an optical fiber without twisting of the multi-fiber unit is 0.1 ps / km. The calculation result of the PMD change with respect to the twist rate when it receives twist is shown.
In addition, the solid line and the broken line in the graph of FIG. 5 show changes in PMD under the same conditions as the solid line and the broken line shown in FIG.

As shown in FIGS. 4 and 5, the present embodiment in which the twisting stress of the optical fiber core 26 is eliminated by pre-twisting the optical fiber core 26 in a direction opposite to the twist direction of the multi-core unit 25. In the structure, PMD decreases as the twist rate increases, and PMD does not increase thereafter. On the other hand, when there is a twisting stress in the optical fiber core wire, there exists an optimum twisting condition in which PMD once decreases and becomes the lowest, but when the twisting ratio becomes higher than that, PMD gradually increases. Resulting in.
Further, as shown in FIG. 5, when the PMD ability value is low, the twisting stress of the optical fiber core wire 26 is preliminarily twisted in the direction opposite to the twist direction of the multi-core unit 25. In the structure of this embodiment that eliminates the above, PMD hardly appears due to the low twist rate. From this, it can be seen that, in particular, in the case of the optical fiber core wire 26 having a low PMD ability value, it is possible to realize the PMD reduction by forming the cable without generating twisting stress.

In addition, since the PMD ability value varies and the PMD varies depending on the storage state of the unit, etc., all the optical fiber cores in the unit are twisted according to the optimal twisting condition under which the PMD temporarily decreases. That is extremely difficult. In particular, when the unit is a tape core, the PMD value tends to be different between the optical fiber core disposed on the outside and the optical fiber core disposed on the inner side. It is difficult to set the core wire 26 as the optimum twist condition. For example, the optimum twist rate condition of FIG. 4 and the optimum twist rate condition of FIG. 5 are different. When such optical fiber cores are included in the same unit, they are common to all the optical fiber cores. There is no optimal twist condition.
As a result, even when there is a variation in the PMD ability value in the unit, the optical fiber core wire 26 is pre-twisted in the direction opposite to the twisting direction of the multi-core unit 25 to cancel the twisting stress, and the twist rate By increasing the value, PMD can be favorably reduced.

Next, examples of other embodiments according to the optical cable of the present invention will be described.
FIG. 6 is a cross-sectional view of the optical cable showing the structure of the optical cable.
As shown in FIG. 6, the optical cable 30 stores a plurality of tape stacks 36 in which a plurality of optical fiber ribbons 35 that are optical fiber units are stacked. The optical fiber ribbon 35 is formed by arranging a plurality (four in this example) of optical fibers 26 (see FIG. 7) in parallel and extruding a resin such as a thermoplastic resin around them. It is.

In the optical cable 30 having this structure, the tape stack 36 is accommodated in the groove portion 34 without being twisted with respect to the groove portion 34 formed in a one-way twisted spiral shape of the spacer 32, thereby the tape stack 36. Are twisted at a twist pitch equal to the spiral pitch of the groove portion 24 in accordance with the twisting direction of the groove portion 34 of the spacer 32.
Moreover, in the optical fiber tape core wire 35 of the tape stack 36, each optical fiber core wire 26 is integrated by the resin 27 in the state twisted around each axis | shaft. Then, the twisting direction of each optical fiber core wire 26 in the optical fiber tape core wire 35 of the tape stack 36 is opposite to the twist direction of the tape stack 36 twisted in accordance with the twist direction of the groove 34. It is considered to be a direction.

FIG. 7 shows changes in the direction of the groove 34, the optical fiber ribbon 35, and the optical fiber 26 in the longitudinal direction of the optical cable 30. In FIG. 7, for convenience, only the lowermost optical fiber tape core wire 35 constituting the tape stack 36 is illustrated.
As the position of the optical cable 30 in the longitudinal direction changes, the direction Da of the groove portion 34 of the slot 32 is gradually inclined toward the twisting direction in the order of (a), (b), and (c) in FIG.
When the groove portion 34 of the spacer 32 is inclined, the direction Db of the optical fiber tape core 35 that is accommodated in the groove portion 34 without being twisted and twisted together with the groove portion 34 is also different from the direction Da of the groove portion 34. Similarly, it inclines gradually toward the twist direction in the order of (a), (b), and (c) in FIG.

Here, the optical fiber core wire 26 of the optical fiber tape core wire 35 has a twist equal to the twist pitch of the optical fiber tape core wire 35 in the direction opposite to the twist direction of the optical fiber tape core wire 35 around the axis. Since it is twisted at a pitch, as shown in FIGS. 7A, 7 </ b> B, and 7 </ b> C, the optical fiber core 26 is in a state in which the direction Dc is always directed in the same direction.
Thereby, in this optical cable 30, the optical fiber tape core wire 35 is housed in a twisted state, but the optical fiber core twisted in the direction opposite to the twisting direction in the optical fiber tape core wire 35 is stored. The wire 26 is accommodated in a state where there is no twisting stress in the longitudinal direction by canceling out the twisting amount. Therefore, the PMD of the optical fiber core 26 can be reduced.

  In the above embodiment, the multi-core unit 25 includes the spacers 22 and 32 in which the spiral groove portions 24 and 34 are formed, and the optical fiber core wire 26 is unitized in the groove portions 24 and 34 of the spacers 22 and 32. Or although the tape slot type optical cable which accommodated the optical fiber tape core wire 35 was mentioned as an example and demonstrated, the accommodation form of an optical fiber core wire is not restricted to this example.

FIG. 8 shows a slotless optical cable 40 in which a tape stack 45 is housed in the center. The tape stack 45 is twisted at a predetermined pitch along the longitudinal direction. In the optical cable 40, the optical fiber core wire is twisted in the direction opposite to the twisting direction of the optical fiber tape core wire 35 in the optical fiber tape core wire 35 constituting the tape stack 45.
Further, in this optical cable 40, a yarn 41 of cushioning material is disposed around the tape stack 45, the tape stack 45 is twisted together with the twist of the yarn 41, and four nylon threads 42 are wound around the yarn. The cable core is configured, and the outer peripheral side thereof is covered with a jacket 43. In the outer jacket 43, two tension members 44 are provided along the longitudinal direction.

FIG. 9 shows a slotless optical cable 50 having a tension member 51 at the center and accommodating the multi-core unit 25 together with the yarn 41 around the tension member 51. The structure of the optical cable 50 shown in FIG. 9 is also called a microduct cable. In the optical cable 50, the optical fiber core wire 26 is twisted in the multi-fiber unit 25 in the direction opposite to the twisting direction of the multi-fiber unit 25.
The optical cable 50 includes a multicore unit 25 bundled together with four yarns 42 made of nylon together with yarns 41 provided on the outer periphery of the optical cable 50. The outer periphery of the optical cable 50 is covered with a jacket 43. Covered. The outer jacket 43 is provided with two tension members 44 along the longitudinal direction.

  Also in the case of the slotless type optical cables 40 and 50 shown in FIGS. 8 and 9, the optical fiber core 26 is previously twisted in the direction opposite to the twisting direction of the optical fiber tape core 35 or the multi-fiber unit 25. Since the twisting stress is relatively eliminated, the PMD of the optical fiber core 26 can be reduced.

PMD _Q of various optical cables in which optical fiber cores were stored in various storage states was measured.
PMD _Q indicates an effective PMD value measured by connecting a plurality of optical cables, and the recommended standard value of IEC is PMD _Q ≦ 0.2 ps / km ^1/2 .

Example 1
(Optical cable type)
A 100-fiber slot-type optical cable (see FIG. 6) in which a tape stack 36 composed of optical fiber ribbons 35 is housed in a groove 34 of a spacer 32 was used.

(Storage form and measurement results)
In the above optical cable, the tape stack 36 was accommodated without being turned back into the groove portion 24 left-twisted at a pitch of 500 mm. As a comparative example, an optical cable having a structure in which the optical fiber core wire 26 is not twisted in the optical fiber tape core wire 35 is taken as an example 1, and as an embodiment according to the present invention, the light twisted to the left together with the spacer 32 is used. An optical cable in which the optical fiber core 26 is twisted rightward at a pitch of 500 mm in the optical fiber tape core 35 in the opposite direction to the fiber tape core 35 was taken as Example 2.
Table 1 shows the unit storage configuration and PMD _Q measurement results for the optical cables of Example 1 of the comparative example and Example 2 of the example.

As can be seen from Table 1, in the optical cable of Example 2, the optical fiber core wire 26 is pre-twisted in the direction opposite to the twist direction of the optical fiber tape core wire 35, and the sum of the twist amounts is set to 0, It was possible to realize a low PMD so that the optical fiber core wire 26 was free of twisting stress and met the IEC standards. That is, it was found that the optical cable can withstand a large capacity such as wavelength multiplexing communication.
In the above example, the twisting pitch is set to 500 mm. However, by further increasing the twisting rate by setting the twisting pitch to, for example, 250 mm, PMD _Q can be further reduced.

(Example 2)
(Optical cable type)
A 40-fiber slotless optical cable (see FIG. 8) that is housed around a tape stack 45 composed of optical fiber ribbons 35 was used.

(Storage form and measurement results)
In the above optical cable, the tape stack 45 was accommodated by left twisting at pitches of 500 mm and 250 mm. As a comparative example, an optical cable having a structure in which the optical fiber core wire 26 is not twisted in the optical fiber tape core wire 35 is taken as examples 3 and 4, and as an embodiment according to the present invention, the left twisted light is used. Examples 5 and 6 are optical cables in which the optical fiber core 26 is twisted rightward at a pitch of 500 mm and 250 mm within the optical fiber tape core 35 in the opposite direction to the fiber tape core 35.
Table 2 shows the unit storage configuration and PMD _Q measurement results for the optical cables of Comparative Examples 3 and 4 and Examples 5 and 6.

As can be seen from Table 2, in the optical cables of Examples 5 and 6, the optical fiber core wire 26 is pre-twisted in a direction opposite to the twisting direction of the optical fiber tape core wire 35, and the sum of the twist amounts is set to zero. As a result, it was possible to achieve a low PMD so that the optical fiber core wire 26 was free of twisting stress and met the IEC standards. That is, it was found that the optical cable can withstand a large capacity such as wavelength multiplexing communication. In Examples 3 and 4 which are comparative examples, the twisting amount of the optical fiber core wire is larger in Example 4 in which the twist pitch of the optical fiber tape core wire is smaller (that is, the twist rate is larger). Although PMD _Q is large, in Examples 5 and 6 which are examples according to the present invention, Example 6 in which the twisting pitch of the optical fiber tape core wire 35 is smaller (that is, the twist rate is larger) is more twisted. Since the twist rate of the optical fiber core 26 is increased in the state without the rotational stress, the PMD _Q can be further reduced.

(Example 3)
(Optical cable type)
A 40-fiber slotless optical cable (see FIG. 9) in which five multi-fiber units 25 are twisted and housed around the tension member 51 was used.

(Storage form and measurement results)
In the above optical cable, the multi-core unit 25 was stored in a left-handed twist while being twisted to the left with a pitch of 500 mm and 250 mm. As a comparative example, an optical cable having a structure in which the optical fiber core wire 26 is not twisted in the multi-core unit 25 is taken as Examples 7 and 8, and as an embodiment according to the present invention, a multi-core unit twisted to the left is used. Example 9 and Example 10 are optical cables in which the optical fiber core 26 is twisted rightward at a pitch of 500 mm and 250 mm in the multi-fiber unit 25 in the opposite direction to 25.
Table 3 shows the unit storage form and PMD _Q measurement results for the optical cables of Comparative Examples 7 and 8 and Examples 9 and 10.

As can be seen from Table 3, in the optical cables of Examples 9 and 10, the optical fiber core wire 26 is pre-twisted in the direction opposite to the twisting direction of the multi-core unit 25, and the sum of the twist amounts is set to zero. Thus, the PMD can be reduced so that the optical fiber core wire 26 is free of twisting stress and satisfies the IEC standard. That is, it was found that the optical cable can withstand a large capacity such as wavelength multiplexing communication. In Comparative Examples 7 and 8, PMD _Q is larger in Example 8 in which the twisting pitch of the multi-fiber unit is smaller (that is, the twist rate is larger) because the twisting amount of the optical fiber core wire is larger. However, in Examples 9 and 10, which are examples according to the present invention, Example 6 in which the twisting pitch of the multi-core unit 25 is smaller (that is, the twist rate is larger) is in a state without twisting stress. Since the twist rate of the optical fiber core 26 is increased, PMD _Q can be further reduced.

It is a graph which shows PMD which arises when an optical fiber core wire is twisted. It is sectional drawing which shows the example of the structure of the optical cable which concerns on this invention. It is a schematic diagram which shows the change of direction of a groove part, a multi-fiber unit, and an optical fiber core wire. It is a graph which shows the relationship between the twist of the optical fiber core line accommodated in the optical cable, and PMD. It is a graph which shows the relationship between the twist of the optical fiber core line accommodated in the optical cable, and PMD. It is sectional drawing which shows the structure of the optical cable which concerns on other embodiment. It is a schematic diagram which shows the change of direction of a groove part, an optical fiber tape core wire, and an optical fiber core wire. It is sectional drawing of the optical cable which shows the other structure of an optical cable. It is sectional drawing of the optical cable which shows the other structure of an optical cable.

Explanation of symbols

20, 30, 40, 50 Optical cable 22, 32 Spacer 24, 34 Groove 25 Multi-fiber unit (optical fiber unit)
26 Optical fiber core 27 Resin 35 Optical fiber tape core (optical fiber unit)
36, 45 tape stack

Claims (4)

An optical cable in which a plurality of optical fiber cores are twisted around each axis and integrated with a resin to form an optical fiber unit, and a plurality of the optical fiber units are twisted and assembled,
The optical fiber unit is accommodated in the optical cable by being twisted according to its twist direction,
An optical cable characterized in that the twisting direction of the optical fiber core in the optical fiber unit is opposite to the twisting direction of the optical fiber unit. The optical cable according to claim 1,
An optical cable, wherein a twist pitch of the optical fiber core wire in the optical fiber unit is equal to a twist pitch of the optical fiber unit. The optical cable according to claim 1 or 2,
The optical fiber unit is twisted at a twist pitch equal to the spiral pitch of the groove portion by being accommodated in the groove portion of the spacer formed by forming a spiral groove portion on the outer periphery of the long body having a substantially circular cross section. And a tape slot type optical cable which is twisted in the groove portion in accordance with its twisting direction. The optical cable according to claim 1 or 2,
A slotless optical cable, wherein the plurality of optical fiber units are bundled and twisted inside an optical cable.

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