Minimization of low-temperature microbending losses in initially curved tightly jacketed double-coated optical fibres

Thermally induced microbending losses in initially curved tightly jacketed double-coated optical fibres at low temperature are investigated. The initial deflections are described by a more general formula. The deflections in an initially curved fibre increase gradually with increasing thermally induced compressive axial force, and result in an increase of microbending loss. In order to minimize such a microbending loss, the Young's modulus and Poisson's ratio of the primary coating should be increased. The best value of the Poisson's ratio of the primary coating is 0.5. On the other hand, the thickness of the primary coating, the Young's modulus and effective thermal expansion coefficient of the secondary coating, and the thickness, Young's modulus and effective thermal expansion coefficient of the jacket should be decreased. These results are compared to those of double-coated optical fibres. Thermally induced lateral pressure in the glass fibre can also produce microbending loss. When the effects of axial force and lateral pressure on the microbending losses are both considered, the optimum selection of polymeric coatings is to let the lateral pressure be less than zero, and the magnitude of lateral pressure and the ratio of the final-initial deflection be as small as possible. When the material properties of polymeric coatings and their thicknesses are selected, there are two ways to achieve the optimum design.