Controlling the modal content coupled into an optical fiber can be desirable in many situations, e.g. for adjusting the sensitivity of the guided field distribution to external perturbations<sup>1</sup>. For this purpose we used a monolithic setup of a phase plate at a fiber input facet to excite selectively higher order modes, which theoretically can provide a mode purity of more than 99%. We investigated the capabilities of this approach by complete modal decomposition of the fiber output signals, considering the achievable mode purity with respect to several possible imperfections of the setup. The experiments are compared with detailed numerical simulations and show a high agreement. Additionally a comparison with a well known setup with free space phase plates<sup>2–4</sup> was undertaken. This showed the monolithic setup to be energetically twice as efficient.