This might be an interesting topic of discussion to a few engineers engaged in fiber optic cable design. We don’t intend to provide any mathematical formula used in the fiber optic cable design in this post, but this may show you the trend in optical fiber cable design around the world.
Optical fiber cables with loose tubes are dominant in the European market. Loose tube technology is sophisticated compared to ribbon technology. Loose tubes need excess fiber length inside them in order to withstand temperature and mechanical effects. Excess fiber length or in short EFL is the key to loose tube technology. Controlling the excess fiber length in a loose tube is a skillful job. The theoretical calculation can guide the engineers to set the pay-off and take-up tensions, water trough temperatures, etc, but the practical manufacturing conditions are important factors to decide the excess fiber length. These may be different from factory to factory and needs optimization. This optimization requires analytical skills.
The inner diameter and therefore outer diameter of the loose tube was depending on the control of excess fiber length. Fiber cable manufacturing machine makers have come up with solutions to control excess fiber length in a loose tube. The outer diameter of a loose tube containing 12 fibers was around 3.0mm and the inner diameter was 2.0mm some years back. It was brought down to 2.5mm and 1.7mm respectively in an attempt to reduce the cable diameter and cable cost. For many years, 2.5mm was the standard loose tube size in many parts of the world. The last 10 years record shows that many cable manufacturers dared to experiment towards lower size loose tubes. Due to these efforts, 12 fibers loose tube size was brought down to around 2.2mm.
The development of microduct cables encouraged fiber optic cable manufacturers to further experiment with the lower sizes of loose tubes. In microduct cables, 12 fibers are put into a loose tube having an outer diameter of approximately 1.5mm and the inner diameter of approximately 1.1mm. Such a smaller size has been achievable with the support of machine suppliers. What is required is a small capstan in a loose tube line between the extruder and wheel capstan to control the excess fiber length.
Reduced size fiber optic cables become practically possible with the help of a small capstan or any other device to control the excess fiber length. The mechanical performances of the small size cables will not be equal to that of the big size cables. Smaller size cables require lesser force to install and therefore the required pulling strength will also be less. The changes towards lower sizes save material, manufacturing, and installation cost.
East Asian markets where ribbon technology is dominant followed basically the concept of reduced sizes of fiber optic cables. For example in Japan, NTT has driven research in fiber optic cable manufacturing facilities to use underground ducts to accommodate a maximum number of cables. The reduced slotted core diameter and development of thin ribbons made it possible to achieve smaller sizes for ribbon slotted core cables also. With NTT’s installation techniques a duct having 75mm outer diameter can accommodate 3 ribbon cables of 1000 fibers. This means 3000 optical fibers in a 75mm duct!
Recent trends show the development of smaller size cables around the world. If you have not still decided to develop smaller size cables, it is not too late. Smaller size cables will present severe competition in the tenders, where conventional cable makers will face threats. Responsible cable design engineers must put their efforts towards change in the design to reduce the cable cost.