Multispot launching for single-mode excitation in multimode fibers for high-speed applications.

Several launching methods have been employed in multimode fibers (MMFs) to reduce the intermodal dispersion. Selective excitation of the fundamental mode alone in an MMF can eliminate intermodal dispersion completely, provided the mode coupling along the fiber is minimum. In this paper, we present the design of a two-core fiber taper that results in the generation of a mode-field diameter-matched twin-spot beam, which can effectively excite only the fundamental mode (LP01) in an MMF. We also propose a new launching method, viz. dual twin-spot launching, which excites the LP01 mode alone in an MMF for a wide range of input beam spot sizes as compared to the other existing methods. The design of a four-core fiber taper for generating the proposed dual twin-spot beam is also presented. Misalignment tolerance of the dual twin-spot beam launching is compared with mode-field diameter-matched center launch (MFDM-CL) and mode-field diameter-matched twin-spot launch (MFDM-TSL). We show that the dual twin-spot launching method has higher alignment tolerances.