THERMALLY-INDUCED STRESSES IN AN OPTICAL-GLASS FIBER SOLDERED INTO A FERRULE

An analytical model is developed for the evaluation of the thermally induced stresses in the midportion of an optical glass fiber soldered into a ferrule. The purpose of the analysis is to select the appropriate solder material and the geometry of the solder joint. We show that the ferrule has the major effect on the stresses in the glass and the solder, and that, for conventional solders, low expansion ferrules (Invar) result in high-tensile radial and tangential stresses in both the glass and the solder. On the other hand, high-expansion ferrules (aluminum, nickel) result in excessive compressive stresses and therefore should also be avoided. However, ferrule materials of moderate expansion (Kovar) lead to low stresses and are recommended. We show also that high-modulus solders, such as gold-tin, result in substantially higher stresses than low modulus solders, such as silver-tin, and that thinner solder layers lead to lower stresses in the glass, but to higher tangential and axial stresses in the solder. We conclude that the appropriate solder material and the thickness of the solder ring can be established, based on the developed stress model, depending on the material and thickness of the ferrule, and the characteristics of the solder itself.