Design and sea trial results of a 48‐fiber submarine optical fiber cable

A new 48‐fiber submarine optical fiber cable and the technology for a multifiber joint box have been developed to provide compatibility with the number of fibers in land cables. Submarine optical fiber cables require durable construction to cross distances as long as tens of kilometers and to withstand high water pressure. In particular, they must be sufficiently waterproof to prevent moisture penetration into the fiber area in the event of cable damage. Simply increasing the number of fibers in a cable, however, degrades the forementioned characteristics, and the following technology has been developed to allow for fiber expansion without harmful effects. A new pressure‐resistant pipe configuration has been constructed which does not require an inner pressure‐resistant pipe and which can house the increased number of fibers, specifically, a 48‐fiber unit assembly containing four 12‐fiber units. A cable diameter equivalent to that of conventional 12‐fiber cables, a unit length of 30 km, and a loss due to laying of 2 mdB/km have been achieved. For the joint box, a mass fusion splicing technique has been developed in which four fibers are spliced at one time, thereby achieving an average fusion‐splicing loss of 0.13 dB for DSF fibers. This mass fusion splicing can be performed on a small scale in a short period of time on board a repair ship. The foregoing technology has enabled the construction of the first 48‐fiber submarine optical fiber cable in the world. In addition, as a result of strain distribution measurements performed during a sea trial using Brillouin OTDA, the residual elongation train in optical fibers due to new cable construction, laying, and recovery was found to be under 0.1 percent.