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For a gaussian beam (which could come from a single mode fiber) injected into a single mode fiber, calculates the coupling ratio as a function of the beam waist radius, the fiber optic mode radius, the transmission coefficient at the entrance of the fiber, the wavelength, and the eventual fiber misalignments. It is assumed that without misalignments, the fiber optic entrance is centered on the propagation axis and is coinciding with the beam waist.