Eight influencing factors and control methods of loose casing length of optical fiber

The excess length of the communication optical fiber is the key to the process control in the production process of the plastic sleeve process. The so-called excess length is the percentage per thousand of the difference between the length of the optical fiber and the length of the loose tube and the length of the loose tube:

Regarding the method of generating excess length of plastic sleeve in production, here is the introduction of the widely used temperature difference excess length control. The various factors that affect the excess length are listed below:

① Pay-off tension.

②Traction tension and take-up tension of the loose tube, the smaller the tension, the easier it is for the PBT tube to cool and shrink in the second section of water tank, thus increasing the excess length.

③Water temperature difference, the greater the temperature difference, the greater the excess length.

④ The number of turns and diameter of the traction wheel will have a certain impact on the size of the excess length.

⑤The faster the speed of the production line, the greater the excess length.

⑥ The viscosity of the ointment for optical fiber, the greater the viscosity under a certain shear force, the greater the excess length.

⑦ Whether the configuration of the needle tube is reasonable or not is also crucial to the control of the excess length.

⑧The length and speed of the SZ strand.

Among the above eight factors, especially the water temperature difference has the greatest influence on the residual length. In actual production, the other seven factors are basically stabilized and the water temperature difference is adjusted to change the excess length. Considering that the bending radius of the optical fiber should not be too small, the gap between the inner diameter of the sleeve and the optical fiber should be reasonably designed. If the tube diameter is small and the excess length is large, the bending radius of the optical fiber is too small. Although there is a good stress-strain curve, it will affect the low temperature performance and the service life of the optical cable.

Since the excess length control is so important for the sheathing and even the entire optical cable, the accurate measurement of the excess length is quite critical. The determination of the excess length value can not only guide the molding process, but also provide a basis for the determination of the structure of the optical cable and the production of the subsequent process. The excess length of the optical fiber in the finished optical cable can be basically determined by the tensile strain test of the optical cable, that is, the excess length of the optical fiber can be obtained more accurately through the difference between the cable strain and the fiber strain. However, it is more difficult to determine the excess length of the plastic sleeve. This is because the ambient temperature is constant during the production of the plastic sleeve, but it is generally a natural temperature in the cable forming process, and the temperature changes greatly, which makes the casing after the plastic sleeve enter the cable. There is a temperature difference, and there will be a certain force on the sleeve during production in the subsequent process, which will cause a certain change in the excess length of the optical fiber. The optical fiber is distributed in an approximate helical shape in the sleeve, and its length is not easy to measure directly, and the length difference with respect to the sleeve is even more difficult to determine. At present, most optical cable manufacturers use the following measurement methods:

1. For the casing that has just been produced, place it at a room temperature of about 20°C for about one hour, and wait for the casing to fully cool.

2. Measure a 4-6m casing at a distance of about 40m from the outer end of the casing, and cut off both ends at the same time with a blade.

3. Pull out the fiber in the sleeve and measure the length.

4. Calculate the excess length value according to the excess length formula. During testing, both the ferrule length and the fiber length must be accurately measured.

There are several points in the excess length test that are very important. First, when measuring a shorter casing, the difference is very small and difficult to determine. In order to reduce the measurement error, the measurement length should generally be above 4m. The second is that when cutting the casing, the casing should be in a free state and not be affected by external forces in the longitudinal direction. Both ends are cut quickly and simultaneously, the cutting knife should be perpendicular to the casing, and the knife edge should be sharp. If the cutting is not simultaneous and rapid, the excess length will change due to the looseness of the optical fiber in the sleeve. If the knife edge is not vertical, it will cause the casing to be inclined, and it is difficult to determine the length value. The knife edge is not sharp, which will make the section of the casing uneven. The third is the length measurement of the casing and optical fiber. Since the casings we cut are generally not straight, the casing must be straightened for measurement.