Kenneth L. Walker

High rate MCVD

Abstract We have fabricated optical fibers using the MCVD process at a deposition rate of 0.9 grams/minute for doped silica glasses. This result has been achieved by reacting chlorides flowing at 8 grams/minute in a large diameter thin wall silica support tube using a broad hot zone torch. The efficiency of particle collection has been enhanced by water cooling the support tube downstream from the torch thereby shortening the soot trajectory to the inside wall surface and increasing the particle collection as predicted from previous studies of the thermophoretic forces. The broad hot zone is necessary for both the chloride reaction at these high flow rates and the sintering of the resulting thick particulate layers. The deposition efficiency for SiO 2 remains on the order of 45% or lower resulting in a considerable quantity of undeposited soot. This is effectively removed using a modified “dust catcher” avoiding this potential limitation to the number of deposit passes possible. A further study of the interdependence of torch traverse speed, deposition temperature, dopant incorporation and deposit taper has resulted in a multimode gradient index GeO 2 P 2 O 5 SiO 2 core, B 2 O 3 P 2 O 5 SiO 2 barrier layer fiber deposited at an average rate of 0.9 grams/minute with a minimum loss of 2dB/km at 1.3 microns and a 1.39 micron OH peak of 4.5 dB/km.

Hermetically Coated Optical Fibers: Hydrogen Permeation And Fatigue Properties

Hydrogen permeation rates have been measured for fibers coated with a CVD applied amorphous carbon hermetic barrier. The H2 permeation was characterized by measuring optical loss increases at elevated temperatures at both high and low hydrogen pressures. Very low predicted values for the rates of hydrogen permeation over a wide range of operating temperatures were found, with an experimentally determined activation energy of 23.7 kcal/mole. In-situ measurements of loss increases upon exposure to hydrogen have shown the existence of a lag time where no hydrogen reaches the core of the fiber. Fiber strengths for the carbon coated hermetic fibers are typically 500-600 ksi. Fatigue properties are markedly improved by the presence of this type of hermetic coating. Dynamic fatigue results show extremely high n values on the order of 350-1000. Static fatigue tests in 21 ° C air and 50% relative humidity give n values of about 200. This is a great improvement over the fatigue results for non-hermetic fibers which exhibit n values of approximately 20.

Deposition of Hermetic Carbon Coatings on Silica Fibers

Carbon coated optical fibers have recently been shown to have excellent resistance to both static fatigue and hydrogen induced losses. The deposition technique used to form the carbon coating strongly affects the coatings ability to resist these degradation mechanisms. The system developed by AT&T utilizes an atmospheric CVD chamber in which a hydrocarbon has is pyrolyzed on the fiber surface. The heat retained in the fiber from the fiber forming process is used to drive the reaction, and high draw speeds are typically used to attain the ˜900°C temperature required to deposit the hermetic form of the carbon coating. Deposition rates of ˜1μm/sec are required to produce the ˜500 A coating.