JP7516241B2 - Fiber optic cable - Google Patents

Description

The present invention relates to an optical fiber cable that can suppress an increase in transmission loss, particularly at high temperatures.

In recent years, with the spread of the Internet, FTTH (Fiber To The Home), which realizes high-speed communication services by directly installing optical fiber into ordinary homes, has rapidly expanded. In general, the optical fiber cables used for FTTH contain an assembly of optical fibers to handle large-volume data communication.

One such optical fiber cable is one in which intermittent ribbon cores are twisted together to form an optical fiber unit, which is then twisted further to form a cable core, and then covered with an outer jacket containing an embedded tension member (Patent Document 1).

JP 2019-109400 A

In this way, in a typical optical fiber cable, optical fiber cores are twisted together to form an optical fiber unit, and multiple optical fiber units are aligned and twisted together concentrically, forming layers from the inside to the outside. In addition, a pressure wrap is wrapped around the outside of the optical fiber unit in the outermost layer, and an outer jacket is extruded around the outer circumference.

When such an optical fiber cable is installed in an outdoor environment, the optical fiber cable is stretched at high temperatures and contracted at low temperatures due to the linear expansion of the components of the optical fiber cable. In this case, the linear expansion coefficient of the outer jacket, which is generally a composite of an outer jacket made of plastic such as polyethylene and a tension member made of steel wire or FRP, is greater than that of the optical fiber made of glass and ultraviolet curing resin. For this reason, the expansion and contraction caused by temperature changes differ between the outer jacket and the optical fiber, and the optical fiber is subjected to tensile strain in a high-temperature environment and compressive strain in a low-temperature environment.

FIG. 5 is a conceptual diagram showing a state in which a plurality of optical fiber units 100 are twisted together. In the illustrated example, the optical fiber units 100 are twisted together at a twisting pitch p _0. FIG. 6(a) is a diagram showing a state in which the twisted optical fiber units are unfolded. As illustrated, if the layer core diameter of the optical fiber unit (the central diameter of the layer around which the optical fiber unit is wound in the cross section) is d ₀ , the length of the optical fiber unit per pitch is represented by l _0. In this case, the length l ₀ of the optical fiber unit relative to the pitch p ₀ is defined as the twisting ratio. In other words, the twisting ratio (%) is defined as ((l ₀ /p ₀ )-1)×100.

When an optical fiber cable having an optical fiber unit in such a state is placed in a low-temperature environment, the jacket shrinks as described above. Fig. 6(b) is a development view showing a state in which the jacket shrinks and the pitch changes from _p0 to _p1 . In this case, the optical fiber unit (internal optical fiber) is compressed, but the optical fiber bends in a wavy manner, thereby releasing the compressive strain. In this case, if the space inside the jacket is narrow, the optical fiber cannot be bent uniformly, and the optical fiber is bent with a locally small bending diameter, which causes an increase in macrobend loss. In contrast, by ensuring a certain amount of space inside the jacket, the increase in transmission loss at low temperatures can be reduced.

On the other hand, when an optical fiber cable is placed in a high-temperature environment, the jacket expands as described above. Fig. 7(a) is a development view showing a state in which the jacket expands and the pitch changes from _p0 to _p2 . In this case, the optical fiber unit (internal optical fiber) is pulled. In this case, as shown in Fig. 7(b), the layer core diameter of the optical fiber unit becomes smaller (i.e., the layer core diameter is tightened from _d0 to _d1 ), and the tensile strain of the optical fiber is released. However, if the layer core diameter changes significantly, the optical fiber receives lateral pressure from the neighboring optical fiber or bundle tape, which may cause an increase in microbend loss.

The present invention was made in consideration of these problems, and aims to provide an optical fiber cable that can suppress increases in transmission loss, especially at high temperatures.

In order to achieve the above-mentioned object, the present invention provides an optical fiber cable comprising a plurality of optical fiber cores, wherein a plurality of the optical fiber cores are twisted together to form an optical fiber unit, and wherein a plurality of the optical fiber units are twisted together in a plurality of layers, wherein the twisting ratio of the optical fiber units in each layer at room temperature is greater than a standard twisting ratio calculated from the twisting pitch and layer core diameter of the optical fiber units in each layer, and wherein the difference between the twisting ratio of the optical fiber units in each layer and the standard twisting ratio in each layer increases from the outer layer to the inner layer .

It is desirable that the difference between the twist rate of the optical fiber unit in the innermost layer and the reference twist rate of the innermost layer be 0.04% or more.

It is desirable that the difference between the twist rate of the optical fiber unit in the outermost layer and the reference twist rate of the outermost layer be 0.015% or less.

According to the present invention, by setting the actual twisting ratio of the optical fiber unit higher than the reference twisting ratio calculated from the twisting pitch and layer core diameter of the optical fiber unit in each layer in advance, it is possible to follow the increase in the twisting pitch of the optical fiber unit even at high temperatures. This makes it possible to suppress the increase in loss due to the increase in tensile strain of the optical fiber unit and the increase in lateral pressure due to tightening.

In addition, by increasing the difference between the twist rate of the optical fiber unit and the reference twist rate in each layer from the outer layer to the inner layer, it is effective in dealing with the increased loss of the optical fiber unit in the inner layer, which is a more severe condition.

In particular, by making the difference between the twist rate of the optical fiber unit in the innermost layer and the reference twist rate of the innermost layer 0.04% or more, it is possible to more reliably suppress the increase in loss.

In addition, since the outermost circumference is held down by the outer sheath, increasing the twist rate of the outermost circumference may increase transmission loss due to lateral pressure from the outer sheath, but this effect can be suppressed by keeping the difference between the twist rate of the optical fiber unit in the outermost layer and the reference twist rate of the outermost layer to 0.015% or less.

The present invention provides an optical fiber cable that can suppress increases in transmission loss, especially at high temperatures.

FIG. FIG. 2 is a cross-sectional view of the optical fiber cable 1, showing the dimensions of each part. FIG. 4A is a diagram showing the twisted state of the optical fiber unit 5, and FIG. 4B is a development view of the optical fiber unit. 13 is a diagram showing the relationship between layer core diameter and ideal twisting rate (reference twisting rate) in the case of a 750 mm pitch. FIG. 2 is a diagram showing a twisted state of the optical fiber unit 100. FIG. 4A is a development view of an optical fiber unit, and FIG. 4B is a development view of the optical fiber unit at a low temperature. 1A is a development view of the optical fiber unit at a high temperature, and FIG. 1B is a development view of the optical fiber unit in a tightened state.

The following describes an embodiment of the present invention with reference to the drawings. Fig. 1 is a cross-sectional view of an optical fiber cable 1. The optical fiber cable 1 is a slotless type cable that does not use slots, and is mainly composed of a tension member 9, an outer jacket 13, a cable core 15, etc.

The cable core 15 is formed by twisting together a number of optical fiber units 5. The optical fiber unit 5 is formed, for example, by twisting together a number of optical fiber cores 3. That is, the optical fiber cable 1 is made up of a number of optical fiber cores 3. The optical fiber cores 3 are, for example, intermittently bonded optical fiber ribbon cores that are bonded intermittently in the longitudinal direction.

A pressure wrap 7 is provided on the outer periphery of the cable core 15. The pressure wrap 7 is a tape-like member or nonwoven fabric, and is arranged, for example, by being attached vertically so as to cover the entire outer periphery of the cable core 15. In other words, the longitudinal direction of the pressure wrap 7 is approximately aligned with the axial direction of the optical fiber cable 1, and the width direction of the pressure wrap 7 is aligned with the circumferential direction of the optical fiber cable 1, so that the pressure wrap 7 is attached vertically to the outer periphery of the cable core 15.

In a cross-sectional view perpendicular to the longitudinal direction of the optical fiber cable 1, tension members 9 are provided on both sides of the cable core 15 (presser winding 7). That is, a pair of tension members 9 are provided in positions facing each other with the cable core 15 in between. In addition, tear cords 11 are provided in a direction approximately perpendicular to the facing direction of the tension members 9, facing each other with the cable core 15 (presser winding 7) in between.

An outer sheath 13 is provided on the outer periphery of the cable core 15. The tension member 9 and the tear cord 11 are embedded in the outer sheath 13. In other words, the outer sheath 13 is provided so as to cover the cable core 15, the tension member 9, etc. The outer shape of the outer sheath 13 is approximately circular. The outer sheath 13 is, for example, a polyolefin-based resin. Note that the tension member 9, the tear cord 11, and the outer sheath 13 may be collectively referred to as the outer sheath portion.

Next, the cable core 15 will be described in detail. As described above, the optical fiber unit 5 is formed by twisting together a plurality of optical fiber cores 3. The cable core 15 is also formed by twisting together a plurality of optical fiber units 5. At this time, the plurality of optical fiber units 5 are twisted together in multiple layers.

Figure 2 is a conceptual diagram of a cable core 15 formed in multiple layers. In the illustrated example, the cable core 15 has a four-layer structure consisting of a first layer 17a, a second layer 17b, a third layer 17c, and a fourth layer 17d. Note that, as long as the cable core 15 is formed in multiple layers, the number of layers of the cable core 15 is not particularly limited.

In each layer, multiple optical fiber units 5 are twisted together. At this time, the boundaries between each layer are divided into approximately circular sections, and the center line of each divided layer is the core diameter of each layer. In the illustrated example, the core diameter of the first layer 17a is D1, the core diameter of the second layer 17b is D2, the core diameter of the third layer 17c is D3, and the core diameter of the fourth layer 17d is D4. That is, the core diameter of each layer is calculated as (outer diameter + inner diameter) / 2 of each layer (however, the inner diameter of the first layer 17a, which is the innermost layer, is 0). In addition, the thickness of each layer is approximately the same as the diameter of the optical fiber unit (however, the outer diameter of the innermost layer is approximately the same as the diameter of the optical fiber unit x 2).

Fig. 3(a) is a diagram showing the twisted state of the optical fiber unit 5, and Fig. 3(b) is a development view of the optical fiber unit 5. In the illustrated example, the twist pitch of the optical fiber unit 5 is P, and the layer core diameter is D. In this case, in an ideal state, the length of the optical fiber unit 5 in one pitch is the length _L0 of the hypotenuse of a right triangle with a base P and a height πD. The twisting ratio in this case is twisting ratio (%) = (( _L0 /P)-1) x 100 = (√( ^P2 + ^π2 · ^D2 )/P-1) x 100. That is, the ideal twisting ratio is simply calculated for each layer.

Figure 4 shows, as an example, the relationship between the layer core diameter and the ideal twisting ratio when the pitch is 750 mm. Here, the ideal twisting ratio is the twisting ratio when the twisting pitch and twisting diameter (layer core diameter) are constant and there is no variation, and the wire is wound without any slack, and is hereinafter referred to as the reference twisting ratio.

As shown in Fig. 4, for example, the standard twisting ratio is about 0.014% when the layer core diameter is 4 mm, about 0.008% when the layer core diameter is 3 mm, and about 0.0035% when the layer core diameter is 2 mm. Here, in the present invention, the actual twisting ratio of the optical fiber units 5 in each layer of the cable core is greater than the standard twisting ratio calculated from the twist pitch P and layer core diameter D of the optical fiber units in each layer. Specifically, as shown by the dotted line A in Fig. 3(b), the length of the optical fiber units 5 is set longer than the length _L0 of the hypotenuse of the right triangle, and a slight slack is formed in the optical fiber units 5.

As mentioned above, when the jacket expands at high temperatures, the pitch P increases, and a tensile force is applied to the optical fiber unit 5. To alleviate this tensile force, the layer core diameter of the optical fiber unit 5 tends to become smaller (see Figures 7(a) and 7(b)).

In response to this, by forming slack in the optical fiber unit 5 in advance at room temperature and setting the actual twist ratio higher than the reference twist ratio, it is possible to suppress an increase in the tensile force applied to the optical fiber unit 5 even if the pitch P increases at high temperatures. Therefore, even a slight change in the layer core diameter can alleviate the tensile strain.

As shown in Figure 4, when the layer core diameter is 4 mm, a change in layer core diameter of 1 mm reduces the standard twisting ratio by approximately 0.006%, but when the layer core diameter is 3 mm, a change in layer core diameter of 1 mm reduces the standard twisting ratio by only approximately 0.0045%. In other words, if the pitch is constant, when the layer core diameter is large (i.e., on the outer layer side), even a slight change in the layer core diameter can reduce the standard twisting ratio (i.e., the required length of the optical fiber unit), but when the layer core diameter is small (i.e., on the inner layer side), the change in the layer core diameter only reduces the standard twisting ratio (i.e., the required length of the optical fiber unit).

In other words, the tensile strain of the optical fiber unit on the outer layer side is alleviated by the optical fiber unit moving slightly toward the center of the cable core 15 to reduce the layer core diameter. On the other hand, the amount of tensile strain alleviated of the optical fiber unit on the inner layer side is small even if the optical fiber unit moves slightly toward the center of the cable core 15 to reduce the layer core diameter. For this reason, the tensile strain is not alleviated unless the optical fiber unit moves significantly toward the center of the cable core 15.

In other words, for the optical fiber unit on the outer layer side, even if the difference between the actual twisting ratio and the reference twisting ratio is small, the tensile strain is alleviated by a slight change in the layer core diameter of the optical fiber unit. In contrast, for the optical fiber unit on the inner layer side, if the difference between the actual twisting ratio and the reference twisting ratio is small, the tensile strain is not alleviated by a slight change in the layer core diameter of the optical fiber unit. For this reason, it is desirable to increase the difference between the actual twisting ratio and the reference twisting ratio for the optical fiber unit on the inner layer side.

In this way, it is desirable to increase the difference between the actual twist rate and the reference twist rate as the layer core diameter decreases. In other words, it is desirable to configure the difference between the twist rate of the optical fiber unit in each layer and the reference twist rate in each layer to increase from the outer layer to the inner layer.

It is also desirable that the difference between the twisting rate of the optical fiber unit in the innermost layer and the standard twisting rate of the innermost layer is 0.04% or more, and more preferably, the difference between the twisting rate of the optical fiber unit in the innermost layer and the standard twisting rate of the innermost layer is 0.09% or more. If the difference between the twisting rate of the optical fiber unit in the innermost layer and the standard twisting rate of the innermost layer is too small, the effect of suppressing loss increase is small.

On the other hand, if the difference between the actual twist rate and the reference twist rate is increased, the amount of slack in the optical fiber unit from the reference state increases. Normally, this slack is absorbed by slack in the longitudinal direction and slack toward the adjacent outer layer. However, since the outer layer side of the optical fiber unit in the outermost layer is the outer jacket 13, slack is suppressed by the outer jacket 13, which causes increased loss especially at low temperatures. For this reason, it is desirable for the difference between the actual twist rate and the reference twist rate of the outermost layer to be small.

For this reason, it is desirable that the difference between the twisting rate of the optical fiber unit of the outermost layer and the standard twisting rate of the outermost layer be 0.015% or less, and more preferably, the difference between the twisting rate of the optical fiber unit of the outermost layer and the standard twisting rate of the outermost layer be 0.010% or less. If the difference between the twisting rate of the optical fiber unit of the outermost layer and the standard twisting rate of the outermost layer is large, the lateral pressure due to the jacket increases, and the transmission loss increases.

As described above, according to this embodiment, by making the twisting ratio of the optical fiber units 5 in each layer larger than the reference twisting ratio of each layer, it is possible to suppress an increase in transmission loss, especially at high temperatures. In this case, by configuring the difference between the twisting ratio of the optical fiber units in each layer and the reference twisting ratio of each layer to increase from the outer layer to the inner layer, an appropriate amount of slack can be ensured in each layer.

Various types of optical fiber cables were created, and the increase in transmission loss at high and low temperatures was confirmed. First, 12 optical fibers with a diameter of 200 μm were intermittently bonded together to create an intermittent ribbon core wire with 12 cores. Then, 12 of the resulting intermittent ribbon core wires were twisted together and wrapped with a 2 mm wide plastic tape to create an optical fiber unit with 144 cores.

A total of 48 optical fiber units with 144 cores were twisted together in 4 layers in the following order from the inner layer: 2-9-15-22. The twist pitch of the optical fiber units in each layer was 750 mm. When twisting the optical fiber units together, the twisting ratio of each layer was adjusted by changing the balance of the supply tension of each optical fiber unit. At the area where the 48 optical fiber units were supplied, a water-absorbent nonwoven fabric was attached vertically, rolled up with a forming jig, and then nylon pressing thread was wrapped around it to create a cable core with 6,912 cores.

The cable core thus created, a tension member made of φ2.0 mm glass fiber reinforced resin, and a ripping string for ripping the jacket were sheathed into a cylindrical shape with jacket material to create an optical fiber cable. The jacket material was LLDPE. The outer diameter of the optical fiber cable was 29 mm, and the inner diameter of the jacket was 22 mm.

For each manufactured optical fiber cable, 20 m was run, the jacket was removed, each optical fiber unit was taken out, and the length was measured to determine the difference from the cable length of 20 m, and the average value for each layer was calculated. The twisting ratio was calculated as (√(P ² +π ² ·D ² )/P-1)×100(%), where P is the twist pitch of the optical fiber unit and D is the layer core diameter of the unit.

The standard values for the twisting rate are as follows: the outer diameter of the optical fiber unit is 2.85 mm, the core diameter of the first layer is 2.85 mm, the core diameter of the second layer is 2.85 + 2.85 x 2 = 8.55 mm, the core diameter of the third layer is 8.55 + 2.85 x 2 = 14.25 mm, and the core diameter of the outermost layer is 14.25 + 2.85 x 2 = 19.95 mm.

The temperature characteristics of each optical fiber cable were measured. The optical fiber cable was wound around a drum with a diameter of 1800 mm and the drum was placed in a large thermostatic chamber, the temperature inside the chamber was changed, the transmission loss was measured at a wavelength of 1550 nm using an OTDR, and the difference from the measured value at room temperature was calculated. The evaluation results of the twisting ratio and temperature characteristics of the transmission loss for each optical fiber unit are shown in Table 1.

In Examples 1 to 4, the twisting ratio of each layer was large compared to the reference value, and the maximum loss increase at high temperatures (70°C) and low temperatures (-30°C) was kept to 0.15 dB/km or less. In contrast, the comparative example, which was manufactured to approximately the reference value, had a maximum loss increase at high temperatures (70°C) of more than 0.15 dB/km.

More specifically, Examples 1 to 3 were configured so that the difference between the twist rate of the optical fiber units in each layer and the reference value increased from the outer layer to the inner layer. In contrast, Example 4 increased the twist rate by approximately 0.05% overall compared to the reference value. In Example 4, the loss increase at high and low temperatures was suppressed to 0.15 dB/km or less, but the loss increase at low temperature (-30 degrees) was higher than in the comparative example, exceeding 0.10 dB/km.

In Example 3, the twist rate was increased with a gradient of 0.001-0.046% from the innermost layer to the outermost layer. In Example 3, the loss increase at high temperatures was reduced compared to the comparative example, but could not be reduced below 0.10 dB/km. However, because the difference from the standard twist rate of the outermost layer was 0.001%, less than 0.01%, the loss increase at low temperatures was only slightly higher than in the comparative example.

In Example 2, the difference from the standard twist rate of the innermost layer was set to 0.09% or more, so the loss increase at high temperatures was suppressed to 0.10 dB/km or less. The difference from the standard twist rate of the outermost layer was also 0.01% or less, so the loss increase at low temperatures was only slightly higher than in the comparative example.

In Example 1, the difference in the standard twist rate of the innermost layer was set to 0.1% or more, which enabled the loss increase at high temperatures to be suppressed even more than in Example 2. On the other hand, since the difference from the standard twist rate of the outermost layer exceeded 0.01%, the loss increase at low temperatures was slightly higher than in Example 2, but since the difference from the standard twist rate of the outermost layer was 0.015% or less, the loss increase at low temperatures could be suppressed to 0.10 dB/km or less.

Although the embodiment of the present invention has been described above with reference to the attached drawings, the technical scope of the present invention is not limited to the above-mentioned embodiment. It is clear that a person skilled in the art can come up with various modified or revised examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

1... Optical fiber cable 3... Optical fiber core 5... Optical fiber unit 7... Pressure winding 9... Tension member 11... Tear string 13... Outer jacket 15... Cable core 17a... First layer 17b... Second layer 17c... Third layer 17d... Fourth layer 100... Optical fiber unit

Claims (3)

An optical fiber cable consisting of a plurality of optical fiber cores,
A plurality of the optical fiber cores are twisted together to form an optical fiber unit, and a plurality of the optical fiber units are twisted together in a plurality of layers,
the twisting ratio of the optical fiber unit in each layer at room temperature is greater than a reference twisting ratio calculated from the twist pitch and layer core diameter of the optical fiber unit in each layer;
1. An optical fiber cable, comprising : an optical fiber unit having a twist ratio of 100% to 150% by weight; a twist ratio of 100% to 150% by weight; a twist ratio of 100% to 150% by weight; 2. The optical fiber cable according to claim 1 , wherein the difference between the twist ratio of the optical fiber unit in the innermost layer and the reference twist ratio of the innermost layer is 0.04% or more. 3. The optical fiber cable according to claim 1 , wherein a difference between a twist ratio of the optical fiber unit in the outermost layer and a reference twist ratio of the outermost layer is 0.015% or less.

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