Effects of gravity on processing heavy metal fluoride fibers
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Effects of gravity on processing heavy metal fluoride fibers Dennis S. Tucker Space Sciences Laboratory, ES75, Marshall Space Flight Center, Alabama 35812
Gary L. Workman and Guy A. Smith Center for Automation and Robotics, University of Alabama in Huntsville, Huntsville, Alabama 35899 (Received 30 October 1996; accepted 24 March 1997)
The effects of gravity on the crystal nucleation of heavy metal fluoride fibers have been studied in preliminary experiments utilizing NASA’s KC-135 reduced gravity aircraft and a microgravity sounding rocket flight. Commercially produced fibers were heated to the crystallization temperature in normal and reduced gravity. The fibers processed in normal gravity showed complete crystallization while the fibers processed in reduced gravity did not show signs of crystallization.
Heavy metal fluoride glasses have been of interest to researchers for over 20 years. One class that shows the most promise as an optical fiber is that of the ZrF4 – BaF2 –LaF3 –AlF3 –NaF class normally referred to as ZBLAN glass.1 ZBLAN fiber optics have many promising applications. These include fiber amplifiers, fiber optics gyroscopes, lasers for cutting, drilling and surgery, nuclear radiation resistant links, nonlinear optical systems, and ultralong repeaterless, transcontinental and transoceanic links.2 Although ZBLAN is the most resistant class of the heavy metal fluoride glasses to crystallization, nucleation and growth during preform production and fiber formation is still a problem. It is felt that the devitrification of ZBLAN is due to a narrow working range and low viscosity at the pulling temperature.3 These two factors make this glass unstable and prone to crystallization. It was suggested that microgravity processing has the potential to minimize nucleation and growth.4 Previous communications4,5 have indicated crystallization of certain tailored ZBLAN glasses is inhibited in a reduced gravity environment. ZBLAN fiber for this study was obtained from commercial vendors.6,7 One major difference in these fibers and the ZBLAN used in the previous studies4,5 is that the commercial fiber is produced with a cladding in which hafnium replaces zirconium. This is done to ensure optimum optical transmission characteristics. The sources supplied fiber in the form of two meter lengths with diameters of 300 and 530 mm. Fiber diameters measures both the core fiber and cladding. Both fibers had protective polymer coatings which were removed chemically and cut into 25 mm lengths. Individual fibers were placed in Heraeus Suprasil quartz ampoules and sealed under 10–4 to 10–5 Torr vacuum. Fiber processing furnaces were constructed for experiments on both the KC-135 aircraft and the sounding rocket. The deJ. Mater. Res., Vol. 12, No. 9, Sep 1997
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tails of each furnace construction have been previously reported.8,9 Figure 1 provides a diagram of the fiber and ampoule configuration. During the low-g period the fiber
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