Optical fibers are not usable for outside installation purposes if they are not protected properly. The loose tube is one of the most widespread methods to protect optical fibers from environmental stresses and climatic changes. Optical fibers are protected by the secondary coating. The secondary coating is nothing but further protection or secondary protection over bare fiber.
The secondary coating is also known as a buffering process. Optical fibers are protected by many different methods. The most common is by putting the fibers in a loose tube, by applying a tight coating, or by ribbonization. Here we will discuss the loose tube process.
The loose tube making process is very important in the sense that there are many practical difficulties associated with it. The first one is the control of excess fiber length inside the loose tube. Called EFL by the industry, the excess fiber length has a vital role to play in the process of loose tube fiber cable stranding and also on the temperature characteristics of the cable.
The bare optical fiber used for telecommunication purpose complying with one of the recommendations of ITU T G.651, G.652A, G.652B, G.652C, G.652D, G.653, G.654, G.655A, G.655B, G.655C, G.655D, G.655E, G.656, G.657A, G.657B will be having a nominal diameter of 250micro meters. Optical fibers are colored to distinguish before using in bundles.
The loose tube contains a plastic protection cover called a tube. The fibers are safe inside the tube usually filled with jelly. The current practice to reduce installation time where water penetration threats are minimal, a fully dry cable design, free from jelly inside the tube is also employed. The dry loose tube can contain water blocking threads or polymer powder filled inside along with fibers.
Material for making a loose tube, among plastics are PBT – Polybutylene Terephthalate, PP – Polypropylene, PC – Polycarbonates, Nylon-12. PBT is the widely used plastic for the loose tube. PBT has a very low thermal coefficient of expansion values while retaining the required tensile strength and elongation properties. Nylon is also a superior material, but it is comparatively much costlier than PBT. It is estimated that PBT is around 3 times costlier than Nylon-12. Polycarbonates are used for dry tube design.
Loose tubes can accommodate fiber counts from 1 to 72 as the case may be, but up to 12 numbers of fibers are an industry practice and generally accepted norm both from the process point of view and from an installation convenience point of view. The thickness of the tube depends on the crush resistance the cable will experience. Also, the thickness varies according to the outer diameter of the tube.
For a 2 fiber loose tube, 1.7mm outer diameter and 1.1mm inner diameter is suitable. Similarly, a 4 fibers tube can be of 1.8mm outer diameter and 1.2mm inner diameter, 6 fibers can be put into 1.9mm outer diameter and 1.3mm inner diameter tube. 8 fiber tubes are with 2.0mm outer diameter and 1.35mm inner diameter. Currently, many manufacturers use 2.2mm outer diameter and 1.4mm inner diameter tube for 12 fibers. The old practice was to use a tube with 2.5mm outer diameter and 1.7mm inner diameter. This gave a good clearance for 12F bundles.
Loose tubes are designed in a variety of ways. One common way is to put the loose colored fibers into the tube. The fibers are safe inside the jelly injected into the tube during the tubing process. Water blocking yarn or powder can be placed inside the tube. A typical diagram of a 12F loose tube is shown below:
Loose tubes can accommodate fiber bundles also. This increases the fiber count capacity of the loose tube. Fibers are made into bundles of 12 or 24 before they enter into the tube in the loose tube line. The bundles are tied together with one or two binder yarns of different colors. In that way, up to 6 bundles or 8 bundles can be inserted into a loose tube. In most cases, manufacturers are limited to put two bundles into a tube to make a 24F loose tube. A sample diagram of 48 bundled fiber loose tube with 4 bundles of 12 fibers is shown below:
The loose tube can accommodate ribbon fibers too. Ribbonized fibers are placed into the relatively bigger size loose tube. This way, if we put 6 numbers of 12F ribbon, we can manufacture 72F loose tube. The BSNL, the state-owned telecom operator in India designs High count ribbon cables using this technology. BSNL design for 576F high count metal-free optical fiber cables is designed with 72F loose tubes. These 72F loose tubes are manufactured by putting 6 numbers of 12F ribbons. Each ribbon is printed for identification. A typical diagram of a 72F loose tube with 6 numbers of 12F ribbons is shown below: