A two mode-fiber interferometer which makes use of two lowest order spatial modes was investigated for sensor application. The use of two-mode fibers allows the development of several very usefull sensors that are difficult to realize using conventional single-mode fibers optic technology.
First, we examined the polarization and modal characteristics of a two-mode fiber with an elliptical core. Propagation constants of the two spatial modes in the fiber was measured by prism output coupling technique. We obtained the modal beat-length($L_B$) of 160 ㎛. The two-mode fiber we used had a cutoff frequency close to but grater than 632.8 nm.
Next we made a modal interferometer with the two-mode fiber and measured differential phase shift between the two-modes induced by strain and temperature change. We investigated two different kinds of interference signals which are interference between the two spatial modes and between two polarization modes. Experimental results showed that the two spatial modes experienced 2χ differential phase shift for the elongation of 770 ㎛ and 1500 ~ 3500 ㎛ for the two polarization states, and the two polarization modes ($LP_01$) have beatlength of 65 ㎛. Differential phase shift per unit length between the two modes induced by temperature change were 1.674 rad/m℃ and 1.848 rad/m℃ for the two polarization states, and 5.15 rad/m℃ between the two polarization modes ($LP_01$).