An apparatus for performing mode scrambling in a multimode optical fibre 1 comprises an electromechanical transducer 3 and a signal generator 9. A portion of the fibre 1, which is in the form of a loop 5, is arranged with its ends 6 fixed to the transducer 3, but with the remainder of the portion being left free to vibrate. The signal generator 9 drives the transducer 3 so as to form a succession of bends of differing bend radii in the portion of the fibre as a beam of electromagnetic radiation travels through the fibre, thereby "scrambling" the beam as it travels through the fibre.