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

An analytical model is developed for the evaluation of the thermally i nduced stresses in the midportion of an optical glass fiber soldered i nto a ferrule. The purpose of the analysis is to select the appropriat e solder material and the geometry of the solder joint. We show that t he ferrule has the major effect on the stresses in the glass and the s older, and that, for conventional solders, low expansion ferrules (Inv ar) result in high-tensile radial and tangential stresses in both the glass and the solder. On the other hand, high-expansion ferrules (alum inum, nickel) result in excessive compressive stresses and therefore s hould also be avoided. However, ferrule materials of moderate expansio n (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 thinn er solder layers lead to lower stresses in the glass, but to higher ta ngential and axial stresses in the solder. We conclude that the approp riate solder material and the thickness of the solder ring can be esta blished, based on the developed stress model, depending on the materia l and thickness of the ferrule, and the characteristics of the solder itself.