Design of an all-fiber multimode-interference-based multimode optical beam generator: theory and experiment

TLDR: In this paper, an all-fiber multimode interference method for achieving pre-determined free-space propagation of multimode optical beams exiting from the fiber end-facet.

Abstract: We show an all-fiber multimode interference method for achieving pre-determined free-space propagation of multimode optical beams exiting from the fiber end-facet. This method provides independent control over the relative amplitudes and the relative phase difference between the LP0,1 and LP0,2 modes of the output few-mode fiber, ensuring predictable free-space propagation behavior. We also show that the LP0,1 beam suffers greater unpredictability in the free-space propagation as compared to the LP0,2 beam if the phase difference between the two modes is not controlled. The negative effect of mode-mixing can be avoided if both the modes are in-phase at the fiber end-facet. We also demonstrate significant features in the free-space propagation behavior for different optical beams, justifying the theory.