(151a) Instabilities in the Fabrication of Polymer Optical Fibers

Polymer optical fibers (POFs) are of current interest as high-bandwidth (or high-speed) data-transmission media for short distance communication as in local area networks or home networks. In spite of their higher attenuation (or loss of optical power) than glass optical fibers (GOFs), POFs are perceived to be more suitable for these applications due to their flexibility and durability. These properties make it possible for POFs to have a much larger core diameter in the order of 1 mm compared to 5 ~ 10µµm of single-mode GOFs. In order for a POF to have high-bandwidth capability, it should have a special structure in that the refractive index of the fiber should vary smoothly in the radial direction. Such a fiber with a near-parabolic refractive index profile is called a graded-index polymer optical fiber (GI-POF). GI-POFs can be made by several different methods that may be classified into two main categories; coextrusion fiber spinning and thermal drawing of preforms. In all these fabrication methods, some kind of instabilities are encountered in that the fiber radius may fluctuate and/or the cylindricity of the fiber is lost depending on the processing conditions and the material properties. Besides draw resonance that is well known since mid 1960s as sustained periodic oscillation of radius, a long wave fluctuation of radius is found to be associated with a characteristic time of a fabrication process. Non-cylindrical shapes of the fiber cross-section, that are quite regular as elliptical, triangular or pentagonal, seem to be associated with the material properties especially the molecular weight of the polymer.