WCA March 2018

❍ ❍ The measuring values are clearly visualised at the monitor of a processor system

The gauge head calculates the tension from the vibration of the fibre with fast Fourier transformation (FFT). The single values of the fibre position are graphically visualised by a processor system in the form of a scatter plot and are available by Ethernet. The scatter plot shows the distribution of the short-term variations of the position of the fibre. The measuring process ensures a measuring accuracy for the diameter of 0.05µm at a repeatability of 0.02µm. 2,500 measurements per second, with high single value precision and a short exposure time of 1.2µs, ensure permanent accuracy. At the hot end of the drawing process (measuring range 500°C to 1,500°C) there is usually also a gauge head installed for measuring the temperature of the optical fibre. This provides an important process parameter for controlling the furnace. A second gauge head based on laser technology measures the cold diameter of the optical fibre and the spinning after cooling and before coating. Control is done either by the hot or cold gauge head. Additional devices also detect airlines in the optical fibre and give information about the temperature of the optical fibre at the cold end. There is a risk during production of the preform that air is trapped and which, when drawn, becomes stretched air pockets, so called airlines. These airlines damage the quality of the optical fibre and, therefore, must be detected. Conventional diameter measurement devices can only detect the outer contour of the fibre but not the airlines inside. This is why Sikora has developed a gauge head which detects airlines in the fibre by taking 700 measurements per second.

In addition, quality plays an important role as drawn optical fibres are further processed to optical cables before they are installed, for example, in a subsea cable. This requires the compliance with optical fibre specifications to perfectly fit into the final application (subsea cable) and to ensure durability. Today, quality control of optical fibres takes place in the drawing tower by using innovative measuring and control technologies at different production stages. At the beginning of the production of an optical fibre is the preform. The preform is manufactured by means of chemical vapour deposition and is available as a glass rod with a diameter of 30–200mm. In the first step of fibre production, the preform is heated up in an inductive furnace to a temperature of 2,100°C. A new technology developed by Sikora, based on millimetre waves, can not only help to measure the diameter and the position of the remaining preform above the furnace, but also has the potential to distinguish differently doped glasses. From the melted glass, the optical fibre is drawn. At this point, the fibre typically has a diameter of 125µm. After cooling down, the optical fibre is coated with an acrylate layer that is hardened under UV light. The coating protects the optical fibre from mechanical damage and allows the fibre to be bent. For further processing, the fibres are wound onto reels. Typically, a first gauge head based on laser technology is installed below the draw furnace to measure the diameter and position of the uncoated fibre.

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Wire & Cable ASIA – March/April 2018