Optimum coupling efficiency is reached when the beam diameter of the incoming beam equals one of the beam diameters listed in the table. For an ideal Gaussian beam (M²=1), the convergence of the focussed beam equals the acceptance angle θ of the fiber while the laser spot on the fiber facet corresponds to the mode field diameter accepted by the single mode fiber.
Except for an 8% loss from Fresnel reflection during entrance into and exit from the fiber, an ideal Gaussian beam is transported completely.
For fiber numerical aperture NA values not listed in the table, the optimum focal length of the laser beam coupler at a given beam diameter is given by:
ƒ' = 0,61 · Øbeam / NA
The factor 0.61 caters for the differently defined diameters of a Gaussian beam (13.5% level) and numerical aperture (5% level). A mismatch is inefficient. For a focal length choosen to large the laser spot is larger than the accepting mode field and, conversely, for a focal length too small the convergence of the focussed beam is larger than the fiber acceptance angle θ.