Interferometric Sizing of Single‐Axis Birefringent Glass Fibers

This paper deals with the on‐line sizing of small diameter glass fibers (i.e. d<30 μm) produced for textiles and reinforcement applications. Two models based on the Lorenz‐Mie Theory are introduced to predict the basic light scattering properties and the response of a phase Doppler interferometer (PDI) to the sizing of infinite glass fibers. Among other parameters, these models take into account particular effects such as the fiber's single‐axis birefringence and the fiber's refractive index dependence on its cooling rate (i.e. diameter). Both effects have a weak influence on the mean response of the PDI but a strong influence on the resonance structures of its phase‐diameter relationship. Two optical set‐ups were selected from a numerical optimization procedure and tested experimentally. Experimental results are presented demonstrating the validity of the models and the ability of the developed PDI set‐ups to study some features of the fiber drawing‐process: fluctuations of the fiber diameter when the nozzle is submitted to a convective perturbation and, when the fiber take‐up velocity is modulated, the detection of hollow fibers.