Increased sensitivity and parametric discrimination using higher order modes of etched-core fiber Bragg grating sensors

The sensitivity of an etched-core fiber Bragg grating sensor to a change of the surrounding medium index can be significantly increased by measuring the higher order modes. The sensitivity of the third-order mode is increased by at least a factor of six as compared to that of the fundamental mode for a fiber etched down to a core diameter of 5 mu m. The change of the Bragg wavelength for the higher order modes is the same as that for the fundamental mode in a situation when only the physical properties of the fiber, such as the core index and the grating period, are altered, which is not the case when the index of the surrounding medium is changed.