Local Birefringence in Unidirectionally Spun Fibers

The application of a spinning step in the drawing process is known to improve optical-fiber polarization mode dispersion. For a more comprehensive understanding of the mechanism through which spinning modifies fiber performances, in this paper, the authors investigate the effect of unidirectional spinning on local fiber birefringence in terms of the effectiveness of spin-function transfer and reduction of the local intrinsic birefringence under different drawing conditions. The actual frozen-in spin is experimentally recovered by means of a cut-back procedure. Different from the case of a periodic spinning, a unidirectional spinning correctly reproduces the nominally imparted spin rate in agreement with theoretical modelizations of the transfer effectiveness of the spinning process. A theoretical explanation for the experimental evidence, recently proved by tomographic stress measurements, of spinning affecting fiber linear intrinsic birefringence is provided. In particular, the interaction of drawing parameters and spinning process in defining stress development into fibers is considered. To validate the proposed model, further tomographic reconstructions of stress profiles in fiber spun at different rates and drawing speed were carried out. Besides, corresponding stress-induced birefringence values were estimated and compared with those recovered by the cut-back technique. Variations of spun fiber beatlength values with respect to the unspun case, as obtained from both measurement techniques, are in good agreement, providing a further reliable confirmation that an improvement of the beatlength may proceed as a consequence of the applied spin