A novel two-mode fiber-optic interferometer based on HE_(11)-TE_(01) modal interference utilizing a liquid-crystal-clad fiber modal filter

摘要

A novel scheme of two-mode fiber-optic interferometer based on interference between HE_(11) and TE_(01) modes of a circular symmetric fiber is reported. This interferometer exploits a modal filter made of a piece of liquid-crystal-clad fiber, to select one of the second-order modes TE_(01) and filter the other TM_(01) and HE_(21) modes. Thus, two spatial modes, fundamental mode HE_(11) and second-order mode TE_(01), serve as the two arms of the interferometer to exhibit their phase modulation produced within a sensing region, on the output intensity pattern. A polarizer (analyzer) is employed at the output end of the fiber to achieve a two-lobe radiation pattern. Theoretical and experimental studies on the operating characteristics of such a two-mode fiber-optic interferometer are presented. The variation of the two-lobe pattern has a predictable dependence on the phase retardation between the two modes and can easily obtain an optimized visibility by rotating the polarizer (analyzer). The application of this interferometer for temperature sensing between 20 and 100 deg C is demonstrated. The measurement results show a phase shift sensitivity of 0.9 rad/m deg C, comparable to that of the existing two-mode fiber interferometer using an elliptical-core fiber.