Reduced Gravity Zblan Optical Fiber

AbstraqtTwo optical fiber pullers have been designed forpulling ZBLAN optical fiber in reduced gravity.One fiber puller was designed, built and flown onboard NASA's KC135 reduced gravity aircraft. A"second fiber puller has been designed for use onboard the International Space Station.IntroductionMid infrared fiber optics have many promisingapplications. These include high capacitywavelength multiplexed fiber optic devices,remoting of infrared focal planes, infrared laserdevices, long-length fiber optic sensor systems andnonlinear optical systems. Heavy metal fluorideglasses have shown the most promise to date. Themost stable heavY metal fluoride with respect tocrystallization appears to be those in the ZrF4-BaF2-LaF3-AIF3-NaF family, commonly known as._"ZBLAN" glasses. 2Intrinsic and extrinsic processes limit lightpropagation_at low powers in ZBLAN. a Intrinsicpr(_perties include band gap absorption, Rayleighscatter and multiphonon absorption. Extrinsicprocesses include impurities such as rare earth andtra_sition metal ions and crystallites formed duringpreform processing and fiber pulling. Thethebretical loss coefficient for ZBLAN is 0.002dB/km at 2 microns. Achieving this lower limit ishampered by both intrinsic and extrinsic processes.All of the intrinsic processes and extrinsicimpurities can be controlled through processing ofthe initial raw materials and in preparation of theglass preform. The devitrification of ZBLAN is dueto a narrow working range and low viscosity at thepulling temperature. 4 These two factors make thisglass unstable and prone to crystallization.Microgravity processing offers the potential tominimize these losses in ZBLAN glasses, s It alsooffers the potential of minimizing phaseseparation and crystallization during glass formingsteps, s Fluoride glass synthesis has not beenattempted in microgravity to date due to thecorrosive nature of the process, sCanadian work indicated the enhancedcrystallization of certain ZBLAN formulationsunder 2g and no evidence of crystallization in 0gusing a T-33 aircraft, s'6Research b/y the authorsof this paper found similar results. It is felt bythe authors that an approach of processingpreforms in microgravity to get rid of crystallitesfollowed by microgravity fiber pulling will affordoptimum optical fiber. This paper describes fiberpulling of ZBLAN.Experimental and ResultsTo try and understand the nuances of pullingfiber in reduced gravity a fiber pulling apparatus(FPA) was designed and built for use on NASA'sKC135 microgravity aircraft. A schematic of thisapparatus is shown in Figure 1. In operation, apreform is located at the top of the apparatus, h,platinum sting is fed from the take-up reel to thebottom of the preform. The preform tip iS heatedto just below the glass transition temperature, thesting touches the tip and fiber is drawn downwardonto the take-up reel. During the drawingoperation, the fiber passes through a polymercoater where a thin layer of polymee is applied tothe fiber surface and is subsequently cured by UVradiation. The polymer coating, serves to protectthe fiber from surface scratches and dirt whichwould lower fiber strength. The preform is fed by amotor which insures a continuous supply of glassfor drawing.1