Thermal stresses in carbon-coated optical fibers at low temperature
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Thermal stresses in carbon-coated optical fibers at low temperature Sham-Tsong Shiue and Wen-Hao Lee Department of Materials Science, Feng Chia University, Taichung, Taiwan, Republic of China (Received 23 July 1996; accepted 19 February 1997)
The thermal stresses in carbon-coated optical fibers at low temperature have been analyzed. The thermally induced lateral pressure in the glass fiber would produce microbending loss. In order to minimize such a microbending loss, the thickness, Young’s modulus, and Poisson’s ratio of the carbon coating should be decreased. On the other hand, the maximum thermal stress is the tangential stress in the carbon coating that occurs at the interface of the carbon coating and glass fiber. It was experimentally observed that if the maximum thermal stress is larger than the tensile strength of the carbon coating, the carbon coating will be broken along the axial direction. In order to minimize such a maximum thermal stress, the thickness of the carbon coating should be increased, but Young’s modulus, thermal expansion coefficient, and Poisson’s ratio of the carbon coating should be decreased. Finally, an optimal selection of the carbon coating for optical fiber is discussed.
I. INTRODUCTION
Optical fibers with low transmission loss and wide bandwidth have been developed, and there have been many practical transmission systems using these optical fibers. Long-term stability is an important requirement for optical transmission, so an optical fiber must maintain stable performance in the most severe conditions. The conventional optical fiber usually consists of the silica glass fiber coated by two or three layers of polymeric coatings. It has been found that transmission losses of optical fibers increase at low temperature. Many researchers1–7 studied this problem and found that it is caused by the stresses that occur because of the thermal expansion mismatches of the polymeric coating materials and the glass fiber. On the other hand, the use of carbon coatings on silica glass fibers can greatly improve fiber reliability by preventing both mechanical fatigue and increased hydrogen-induced losses.8–11 The coating speed of carbon on fiber has been increased to a rate sufficient to supply low-cost fiber for commercial networks.12 These excellent advantages have been widely reported and carbon-coated optical fibers are expected to be a future key technology for optical transmission lines. However, the thermal stress-induced problems in carbon-coated optical fibers have not been addressed. In this article, the thermal stresses in carbon-coated optical fibers at low temperature are investigated. The effects of the material properties of the carbon coating (including Young’s modulus, thermal expansion coefficient, and Poisson’s ratio) and its thickness on the thermal stresses are considered. Additionally, the lateral pressure-induced J. Mater. Res., Vol. 12, No. 9, Sep 1997
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