Cladding-mode resonance of a double-cladding fiber at a near modal cut-off wavelength for RI sensing

Abstract

A fiber-optic refractive index (RI) sensor was proposed and demonstrated by using a double-cladding fiber (DCF). The DCF consists of a core, inner cladding and outer cladding. The RI of the core and outer cladding is higher than that of the inner cladding. Through evanescent wave coupling, the cladding mode of the DCF can be excited by the core mode resonantly at the phase-matching wavelength. By splicing a section of the DCF into a standard single mode fiber (SMF), a SMF–DCF–SMF structure was constructed to detect the resonant band-rejection filter spectrum. Based on the coupled mode theory, we found that the phase-matching wavelength is at a near modal cut-off wavelength. Under this condition, the cladding mode is more sensitive to the variation of the ambient RI. A resonant wavelength shift of 65 nm was achieved by changing the ambient RI from 1 to 1.4525.