Low-cost optical fiber refractive-index sensor based on core diameter mismatch

A simple, compact, and low-cost optical fiber refractive-index (RI) sensor is reported. It consists of a multimode fiber in which a short section of standard single-mode fiber (SMF) is inserted. Owing to the core diameter mismatch, the cladding of the SMF guides light. This makes the device sensitive to the external RI. The maximum resolution of the sensor is about 7/spl times/10/sup -5/. The fabrication of the sensor only requires cleaving and fusion splicing; moreover, the device can operate at different wavelengths, which makes it attractive for diverse applications.