Sham-Tsong Shiue

Thermal stresses in carbon-coated optical fibers at low temperature

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, Youngs modulus, and Poissons 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 Youngs modulus, thermal expansion coefficient, and Poissons ratio of the carbon coating should be decreased. Finally, an optimal selection of the carbon coating for optical fiber is discussed.

Thermal stresses in double‐coated optical fibers at low temperature

The thermal stresses in double‐coated optical fibers at low temperature have been analyzed. The lateral pressure and normal stresses in the glass fiber, primary coating, and secondary coating have been derived. The thermal stresses in the optical fiber are affected by the temperature drop, material properties of the primary and secondary coatings, and their thickness. It is possible to select a suitably polymeric coating to produce minimum lateral pressure in the glass fiber. In order to minimize the thermally induced bending loss, it was found that if the thickness of the polymeric coatings increases, the Young’s modulus of primary coating should increase, but the Young’s modulus of the secondary coating should decrease.

Thermal stresses in tightly jacketed double‐coated optical fibers at low temperature

The thermal stresses in tightly jacketed double‐coated optical fibers at low temperature have been analyzed by the thermoelastic approach. The lateral pressure and normal stresses in the glass fiber, primary coating, secondary coating, and jacket have been derived. The thermal stresses in the optical fibers are affected by the temperature difference, material properties of polymeric coatings, and their thicknesses. It is possible to select the suitable polymeric coatings to produce a minimum lateral pressure in the glass fiber. The microbending loss is dominated by the lateral pressure in the glass fiber. To minimize such a microbending loss, the Poisson’s ratio of the primary and secondary coatings, and the Young’s modulus of the jacket should be increased. On the other hand, the Young’s modulus of the primary coating should be decreased. However, the thickness of the primary coating, and the thickness and Young’s modulus of the secondary coating exist the optimum values. The glass transition temperature...

Thermally induced stress voids in hermetically carbon-coated optical fibers with different coating thickness

The thermally induced stress voids in carbon-coated optical fibers are investigated. Ten samples of carbon-coated optical fibers with different coating thickness are prepared using the plasma-enhanced chemical vapor deposition method. The carbon structure exhibits a larger amount of graphite-like phase and a smaller amount of disordered diamond-like phase. After these fibers are immersed in the liquid nitrogen for 1 day, thermally induced stress voids propagated along the fiber axial direction are found. It is believed that stress voids are caused by the thermally induced maximum tangential stresses. The stress voids slightly decrease with increasing the coating thickness in the beginning, and then increase. The carbon-coated optical fiber with a coating thickness of approximately 255 nm induces the minimum number of stress voids. Nevertheless, the stress voids become larger and even in-line cracks when the coating thickness is equal to or larger than 800 nm.

Thermal stresses in metal-coated optical fibers

The thermal stresses in metal-coated optical fibers are analyzed. Several thermal stresses are important in a metal-coated optical fiber. First, the interfacial radial stress would produce microbending loss. Secondly, the thermally induced axial force in the glass fiber would induce buckling of the fiber and also results in an increase of bending loss. Thirdly, when the interfacial shear stress is larger than its shear strength, the metal coating would be delaminated from the glass fiber. Finally, when the normal stress in the metal coating is larger than its tensile strength, the metal coating would be broken. These thermal stresses could be minimized by appropriately selecting physical properties of the metal coating and its thickness. To minimize these thermal stresses, the Young’s modulus, thermal expansion coefficient, and Poisson’s ratio of the metal coating should be decreased. On the other hand, the thickness of the metal coating has an optimal value. Based on the strength consideration, an optima...

The elastic interaction between screw dislocations and cracks emanating from an elliptic hole

Based on the solution of an internal crack, we have investigated the elastic interaction of a screw dislocation and cracks emanating from an elliptic hole by using a conformal mapping technique. We have derived the stress field, the image force on the dislocation, the stress intensity factor at the crack tip, and the crack extension force. From the image force, we find the unstable equilibrium positions of dislocation. It is possible to form a plastic zone from the region containing the unstable equilibrium position by collecting dislocations. By extending the concept of the unstable equilibrium position, we also develop the dislocation emission criterion. It is found that the critical applied stress for dislocation emission is dependent on the geometry of the crack. In addition, we also find that the elliptic hole prefers to emanate double cracks. Finally, it is worthwhile to mention that the mapping function cannot be arbitrarily chosen.

Effect of coating thickness on thermal stresses in tungsten-coated optical fibers

This study investigates the effect of coating thickness on the thermal stresses in tungsten-coated optical fibers. Theoretical results indicate that the maximum normal stress in the tungsten coating decreases with increasing coating thickness. However, the maximum shear stress at the interface of the glass fiber and tungsten coating increases. Eight samples of tungsten-coated optical fibers with coating thicknesses of 58, 75, 101, 128, 158, 383, 557, and 1013 nm, respectively, are immersed in liquid nitrogen for one day. Experimental results show that thermal stresses will either break or delaminate the tungsten coating. The crack density decreases with increased coating thickness, while the delaminated area of tungsten coating increases. The theoretical results can explain the break and delamination of the tungsten coating in the optical fiber. To minimize the break and delamination of the tungsten coatings in the optical fibers, the optimal thickness of the tungsten coating is about 158 nm.

Axial strain‐induced microbending losses in double‐coated optical fibers

The microbending losses induced by axial strain in double‐coated optical fibers have been analyzed. The microbending loss is dominated by the lateral pressure in the glass fiber, which is proportional to the axial strain and is affected by the material properties of primary coating and secondary coating and their thicknesses. To minimize the microbending losses induced by axial strain, it is found that if the lateral pressure is compressive, the thickness and Poisson’s ratio of primary coating should be increased; but the Young’s modulus of primary coating should be decreased. On the other hand, the thickness, Young’s modulus, and Poisson’s ratio of secondary coating should be decreased.

Thermal stresses in hermetically double-coated optical fibers

The thermal stresses in hermetically double-coated optical fibers are analyzed. Several thermal stresses and forces are important in a hermetically double-coated optical fiber. First, the shear stress at the interface of the glass fiber and primary coating will make hermetical coatings be delaminated from the glass fiber. Second, the compressive radial stress at the interface of the glass fiber and primary coating would produce microbending loss. Third, the compressive axial force in the glass fiber would induce the buckling of the fiber and also results in an increase of bending loss. Finally, the normal stresses in the hermetical coatings will make the hermetical coatings be broken. To minimize these thermal stresses and forces, the thermal expansion coefficient of the primary coating and the Poisson’s ratios of the primary coating and secondary coating should be decreased. The thermal stresses in the hermetically double-coated optical fibers are compared to those in the hermetically single-coated optic...

The spring constant in the buckling of tightly jacketed double-coated optical fibers

The primary coating in real optical fiber applications is a soft polymer with a low Young’s modulus that is used as a strain buffer to minimize microbending loss. The secondary coating and jacket are hard polymers with higher Young’s moduli and sufficient thicknesses to sustain the mechanical force. The buckling of optical fibers causes additional transmission loss, thereby necessitating that it be minimized. The optical fiber’s spring constant is a critical factor in the buckling phenomenon. Moreover, a higher value of the spring constant corresponds to a higher resistance to buckling. A closed form solution of the spring constant in the buckling of tightly jacketed double-coated optical fibers is presented. The effects of polymeric coatings on the spring constant are also considered. To obtain a higher value of the spring constant and adhere to optical fiber applications, the thickness of the primary coating was decreased, and the Poisson’s ratio of the primary coating and the Young’s moduli and Poisson...