Modal interferometric sensor based in a birefringent boron-doped microstructured fiber

Abstract

Microstructured optical fibers (MOFs) have been widely studied owing to their potential for obtaining novel transmission, nonlinear and sensing characteristics. Sensing applications of MOFs cover various types of devices for measurements of different physical and specific chemical compounds in gases and liquids employing evanescent field techniques. Such fibers can also be used as active and passive elements in fiber-optic polarimetric and interferometric sensors. We present an in-line fiber modal interferometer fabricated in boron-doped highly birefringent microstructured fiber. The boron-doped region located in the middle of the core decreases the effective index of the fundamental mode and facilitates coupling between the fundamental and the first order mode. The coupling regions have the form of fiber narrowings fabricated using CO2 laser and are distant by a few millimeters. The spectral intensity at the sensor output is modulated only by intermodal interference produced by a short piece of fiber between the two coupling points. Moreover, as the fiber is highly birefringence, each pair of polarization modes produces its own intermodal fringes, which results in the contrast modulation of the overall interference signal observed at the fiber output, and provides an additional degree of freedom to measure simultaneously a pair of measurands.