The interpretation of online measurements of optical fibre diameter

The requirements for optical fibre diameter measurement for feedback control during drawing and for quality assurance in precision applications are different. The signal from a 1000-Hz scanning laser gauge measuring 125-mu m nominal diameter fibre was examined to see if this instrument could service both applications. Static measurements showed that sensor noise was largely unaffected by the measurement position of the fibre, the sampling rate or external electrical /acoustic noise. In each case the sensor noise was essentially normally distributed with a standard deviation of 0.13 mu m. Offline fibre measurements (using a video grey-scale analyser) showed significant diameter variations on a 40-mm length scale. The distribution of this diameter variation was consistent with that from the online laser gauge. It was concluded that the current default option (scanning at 1000 Hz and using window-based averaging to give an effective sampling rate of 8 Hz) is adequate if the objective is to ensure a nominal average diameter on a long-length scale via feedback control. However, such an approach will prove inadequate for quality assurance in emerging precision applications. The latter objective will require a combination of a faster scanning rate with more sophisticated signal filtering to provide a means for monitoring short-scale diameter variations while simultaneously providing a signal for use by a controller more responsive than simple feedback. Such monitoring is also a necessary prerequisite for quantifying the extent of flow instabilities in the -vicinity of the drawing fibre as a function of furnace design.