EuroWire January 2018

Technical Article

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

process, the temperature should be measured not only at the hot end, but also at the cold end. After the fibre has received the coating and has gone through the UV curing process, a further laser gauge head measures again the diameter of the optical fibre. After the coating, the diameter usually measures about 250µm. The coating diameter gauge can also provide an online measurement of the eccentricity of the coating with respect to the cladding, the so-called concentricity evaluation (CCE) measurement. The principle is based on the evaluation of the symmetry of the intensity signal captured by the gauge. Perfect symmetry means perfect concentricity and leads to a minimum measurement result. Increased asymmetry leads to an increase of the measurement value, so closed loop control integration is possible. For other scenarios, tolerance levels for warning and alarm outputs can be arbitrarily defined. In addition to diameter, tension, airline, CCE and temperature measurement, lump detectors are used for a continuous quality control in drawing towers. After the coating and at the end of the drawing process, a three-axis testing device reliably detects lumps with a length of 500µm.

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. Before the acrylate layers are applied onto the fibre during the coating process, the exact measurement of the optical fibre temperature is important. In order to achieve an optimal bonding of the coating and the fibre, the temperature of the fibre should be stable and between 40°C and 75°C. Manufacturers use the inert gas helium for cooling. The amount of helium flow that is used is calculated by means of experience. Often manufacturers use a higher helium flow than necessary to ensure that the fibre is not too hot for the coating. However, helium is a very expensive gas and, therefore, manufacturers of optical fibres want to reduce the usage to a minimum. At this position, a gauge head measures precisely the temperature of the optical fibre after cooling. With the information about the fibre temperature, the optical fibre manufacturer can control the exact amount of helium flow that is needed. To ensure an optimum optical fibre temperature and thus highest process stability during the entire fibre drawing

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. measurement devices can only detect the outer contour Conventional diameter

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January 2018