Single-Mode Optical Fiber Technology III. Double-Crucible Drawing Technology and Compound Glass Fibers
From the very beginning of optical fiber studies it has been evident that the optical fiber will modify the electrical communication network either radically or not at all. The large volume needed is preferably produced in a continuous production process. A second, and often more important, advantage of a continuous process is that the product is more uniform. Within this philosophy in 1974 Koizumi et al.1 took a considerable step forward when they introduced the double-crucible system for continuous production of compound glass fibers. Because of the promising results obtained with the silica-CVD-based methods at about the same time, the double-crucible technique has been studied in greater detail in only a few centers: Nippon Sheet Glass, Philips Research Laboratories and, most intensively, by the British Post Office. Initially each of these centers concentrated for graded-index fibers mainly on proprietary glass systems. These were chosen with the particular aim of obtaining a fast enough diffusion rate during the time when the core and cladding glasses are in high-temperature contact in the double crucible.
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