  Using exact three-dimensional vectorial simulations of radiation coupling
into uncoated dielectric fiber probes, we calculate amplitude transfer
functions for conical single-mode fiber tips at the light wavelength of 633 nm.
The coupling efficiency of glass fiber tips is determined in a wide range of
spatial frequencies of the incident radiation for opening angles varying from
30 to 120 degrees. The resolution in near-field imaging with these tips is
considered for field distributions limited in both direct and spatial-frequency
space. The characteristics of the transfer functions describing the relation
between probed optical fields and near-field images are analyzed in detail. The
importance of utilizing a perfectly sharp tip is also examined.
