New and simple method for selective mode group excitation in graded-index optical fibres

A modified monomode-fibre offset technique as well as a new and very simple method for selective mode excitation in multimode graded-index fibres is presented. It is shown that an inclined plane wave launch followed by a mandrel-wrap mode filter is a powerful way to excite a definite number of mode groups in graded-index fibres. Besides its extreme simplicity this method yields improved selectivity. Experimental results are presented.