Expansion of Applicability of Multiple Cabling Technology

3.1 Inner pipe (IP29)

The maximum number of fibers in one underground optical cable is 1000. Thus, to accommodate 3000 fibers in a conduit, we must install two additional inner pipes that can accommodate two 1000-core cables in a conduit that already contains an existing 1000-core cable. The new method must accommodate a large existing first cable and the additional inner pipes must have a small outer diameter and thin wall to accommodate the additional 1000-core cables and still fit in the conduit (Fig. 3). Moreover, to increase the installation length of the inner pipe, we must reduce both the pipe’s weight and friction coefficient. Fortunately, a thinner wall satisfies the former requirement and a smaller diameter satisfies the latter.

Fig. 3. Handling a large existing cable.

Our new inner pipe (IP29) will be used in place of conventional inner pipe (IP30) in the case where two additional inner pipes are installed. Various construction experiments designed to measure construction tension led to the following dimensions being chosen for IP29: outer diameter of 33 mm, wall thickness of 2 mm, inner diameter of 29 mm, and unit weight of 0.18 kg/m (representing reductions of 3 mm in outer diameter, 1 mm in wall thickness, and 0.1 kg/m in unit weight compared with IP30). The dimensions of IP29 let us accommodate a large existing cable and enable two inner pipes to be installed and later removed simultaneously as usual. Moreover, IP29 supports a longer installation length. When the existing cable has a diameter of less than 24 mm, up to 250 m of twin IP29s can be installed in a straight conduit.

3.2 Gripping part and duct spacer for IP29

We changed the shape of the gripping part and the duct spacer. These developments are based on the concept of accommodating an existing cable having a maximum outer diameter of 31 mm and two IP29 inner pipes. Because IP29 has a thin wall, the conventional holding method would cause the pipe to buckle and deform. Therefore, we changed the holding method from a fingernail-type grip to corrugations over the entire inner surface (Fig. 4). This method holds the inner pipe without deforming it.

Fig. 4. New gripping part for IP29.

As shown in Fig. 2, the conventional twin-IP30 method only works with a small existing cable (max.: 200-core). To enable our new twin-IP29 method to work with a large existing cable (e.g., 1000-core), we had to design a new duct spacer with a larger U-shaped aperture. The holes for the inner pipes and the reduced amount of material around the U-shaped aperture mean that the duct spacer has less structural material, which consequently reduces its rigidity. Therefore, we increased the length of the rib to provide greater strength (Fig. 5). This allows us to standardize on one duct spacer that can be used for either the conventional or new inner pipes, which will reduce costs.

Fig. 5. New duct spacer.

3.3 New optical cable

Because we must consider inner pipe flatness when a pipe is installed in a conduit, the diameter of the new cable designed to go in the new inner pipe (IP29) should be considered in terms of the inner diameter of the flattened IP29. The outer diameter of the new 1000-core cable was designed based on this requirement: the maximum value is 24 mm (Fig. 6). The structures of the conventional and new 1000-core optical cables are compared in Fig. 7. Conventional 1000-core optical cable has a diameter of 29 mm. To get a smaller outer diameter, we needed to design a new thinner slotted rod to accommodate the optical fiber tape. Moreover, this rod’s diameter had to be reduced in order to reduce the ditch size, depth, and intervals (Fig. 8) without affecting fiber extraction work etc. while guaranteeing that the cable characteristics are at least as good as those of the conventional cable. The new small-diameter 1000-core cable incorporates the following technologies.

• The tape thickness was designed to reduce the slot depth (in accordance with the required specifications).
• Bend-resistant fibers are used to ensure that the ribbon does not bend as a result of its tight accommodation in the ditch.
• The ditch intervals and slot size are optimized for the smaller ditch size.

Using these technologies, we achieved an outer cable diameter of 23 mm using the same 13-slot structure as in conventional cable.

Fig. 6. Calculation of cable outer diameter.

Fig. 7. Cable structures.

Fig. 8. Construction of small-diameter cable.