Interference between the modes of an optical fiber generates specific mode (intensity) patterns which get modulated by disturbances in the optical fiber system. Mode-mode interference has been analyzed from first principles and a model based on differential phase modulation presented. Mode-mode interference effects such as intensity modulation of the mode patterns are directly related to differential phase modulation between modes which arises due to the difference between the propagation constants of the constituent modes. Practical implementation of modal methods involves selective launching of modes and processing of the output pattern to demodulate the information.

Axial strain has been chosen as the modulating mechanism in experiments designed to quantify mode-mode interference effects. Quasi-statically varying strain as well as vibrational strain was used to study 'dc' and 'ac' mechanisms. Specific mode combinations have been excited and their radiation patterns identified. Mode pattern changes have been described. Experimental observations and results correlate very well with analysis.