Refractive Index Sensing by Using Mechanically Induced Long-Period Grating

In this paper, a refractive index sensor using a mechanically induced long-period grating is proposed and experimentally demonstrated. The sensing element is based on a composite optical waveguide, which is made of a single-mode fiber, a Teflon-cannula, and the medium pending to test. The research shows that the resonant wavelength of the high- order mode coupling is sensitive to the refractive index of the liquid. The pressure applied on the experimental setup should keep constant in the refractive index measurements for different liquids, where the mode coupling is in the undercoupling state. The sensitivity of the sensor for LP₁₄ mode resonance is about 2.78 × 10^-4 refractive index unit in the refractive index range from 1.33 to 1.43.