Investigation of single-mode–multimode–single-mode and single-mode–tapered-multimode–single-mode fiber structures and their application for refractive index sensing

All-fiber in-line single-mode–multimode–single-mode (SMS) and single-mode–tapered-multimode–single-mode (STMS) fiber structures are investigated. A wide-angle beam propagation method in cylindrical coordinates is developed and employed for numerical simulations of the light propagation performance of such fiber devices. The effect of strong mode interference on the performance of the devices is studied and verified numerically; results indicate that the proposed STMS structure can be exploited for measuring a broad refractive index range with reasonable high resolution, compared with the conventional SMS structure.