Tightly focused beam propagation in the waist region

By simulation of focusing using the rigorous solution in 2D-case it is shown that the second spatial moment of intensity distribution does not characterize propagation at the waist region if the waist diameter is close to the wavelength. The effectiveness of beam-fiber coupling is defined by the size of focal spot and depth of focusing. Increasing of the numerical aperture for typical model beam does not proportionally decrease the width of the focal spot but increase the magnitude and weight of sidelobes and probability of aberrations that dramatically decrease efficiency of coupling. The on-axis intensity is found to be proportional to the sinus of divergence angle calculated as generalized value.