We present an analysis of conditioned launch in a gradient- index multimode fiber (MMF). Selective mode excitation is obtained by nonaxial evanescent coupling between two highly asymmetric side- polished fibers: a single-mode fiber and an MMF with an exposed core. An index-matching slab improves the coupling efficiency and selectivity. The simulation shows that about 40% of modes, only high-order modes, are excited in the MMF. When compared to an overfilled launch, the bandwidth of a perturbed index fiber is enhanced more than twofold for large parameter variations.
Laurent Vaissie, Eric G. Johnson, "Selective mode excitation by non-axial evanescent coupling for bandwidth enhancement of multimode fiber links," Opt. Eng. 41(8) (1 August 2002)