1. Consider the design of an optical fibre. The fibre has a core refractive index of 1.47. The radius a and the relative refractive index difference A to be selected to make this a single mode fibre. (1) Sketch the design characteristic of the refractive index difference A versus core radius for the fibre representing the conditions for single mode propagation at a wavelength of 1.3 um. (1) Determine whether this fibre remains single-mode at a transmission wavelength of 0.85 um when its core radius 1s 4.5 pm