Light coupling between a singlemode- multimode-singlemode (SMS) fiber structure and a long period fiber grating

We propose a novel optical coupling technique based on evanescent fieldcoupling between a singlemode- multimode-singlemode (SMS) fiber structureand a long period fiber grating (LPFG). By parallel placement of the two fibersections in close proximity to each other, the excited multi-cladding modesfrom the SMS fiber section can be selectively coupled to the guided mode inthe LPFG, and vice versa. A theoretical analysis based for such a structureis undertaken and the simulated results are verified by experiments demonstratinga maximum coupling efficiency of up to 1.66\% (which could be improved to 27.5\%in theory) over a broadband resonance (42 nm with a 3 dB bandwidth).