Simple fiber optic refractive index sensor based on evanescent higher order modes

Abstract

This paper presents an optical fiber refractive index sensor based on the evanescent higher order modes. Its structure and principle are quite simple. The sensor is composed of two segments of optical fibers that are spliced together. An ordinary multimode fiber with a core diameter of 50 μm is used to input the light. The functions of a second multimode fiber with a core diameter of 200 μm are twofold. In the region of the splice, a section of the cladding a few centimetre long is removed by an electrical discharge. This part works as a sensing element, and the rest of the fiber is used to output the light. Once the light travels though the input fiber and crosses the splice to enter the second fiber, numerous modes both guided and leaky are generated due to the abrupt increase of the core diameter. The evanescent light fields of these guided modes are sensitive to changes in the refractive index of the material surrounding the fiber cladding. The evanescent field change directly causes a change in the output light intensity. The developed sensor is compact in size, simple to fabricate, promising in performance, and has a high potential for practical applications.