EoW July 2008

new developments in fibre optics

Measurement at the top of the draw tower LIS-G instruments (Laser Interferometric Sensors for Glass) from Cersa-MCI are widely used in optical fibre manufacturing for bare diameter measurement and fine airline and glass defect detection.

The metrology performance of the LIS-G makes spinning measurement at the top of the draw tower possible using the Non Circularity (NC) of the fibre, even down to 0.05µm of NC. Cersa-MCI’s LIS-G5 offers absolute in-line diameter measurement within ±0.15µm, highly stable, with no need for periodic calibration; repeatability and stability within ±0.004µm at 50kHz band pass, and independent fibre vibration measurement. LIS-G instruments give an exact view in real time of the dimensional characteristics of the fibre, allowing users to perfect the drawing process, to maintain the fibre at high diameter stability, and to certify the whole production characteristics.

At high drawing speed it is challenging to produce fibres of less than 0.05µm of NC. However, with a typical NC of 0.1µm at 125µm, the instrument shows clearly the spinning influence on the diameter fluctuation. As the circumference of the fibre section is not perfectly round, the LIS-G diameter fluctuation shows a ‘one turn signature’. The diameter is recorded at an appropriate time resolution over several spinning periods and then, using signal computing, the ‘signature’ can be recognised. To do that, Cersa-MCI has developed its own correlation function (EATP) to show spinning frequency, peak-to-peak non-circularity amplitude and the spinning frequency profile, even though the spinning frequency changes continuously. The ‘one turn signature’ might be 2 peaks (oval), 3 peaks (triangle) or even 4 peaks (square) due to the NC shape. Graph 2 ❍ ❍ : A modelling of a constant signal amplitude, constant period, constant frequency, showing a slight shape (width) difference between peaks, one and two

Graph 1 ❍ ❍ : A modelling of a critical ‘triangle’ shape, with a large amplitude fluctuation and frequency change, with over diameter modulation to test signal computing

Graph 3 ❍ ❍ : The results on a typical real signal recorded by the LIS-G on a draw tower and its display on FiberWin PC software. NC = 0.07µm , 2 peaks per turn (Signature). This is close to the measurementlimits

Graph 4 ❍ ❍ : The results on a typical real signal recorded by the LIS-G on a draw tower and its displayonFiberWinPCsoftware. NC=0.24µm ,2 peaks(almost3)perturn

Graph 5 ❍ ❍ : Shows the spinning frequency profile (Spinning evolution) as well as its FFT. The FFT peak shows a 2 Hz frequency, which means one hertz of profile period. The peaks are alternatively directions

To validate the signal computing, Cersa-MCI simulated specific limit signals in shapes and amplitudes. Then the in line experimentations gave excellent results. Today more than 20 draw towers benefit from this extended feature. Cersa-MCI – France Fax : +33 4 42 02 79 79 Email : sale@cersa-mci.com Website : www.cersa-mci.com

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EuroWire – July 2008