Less-mode optic fiber evanescent wave absorbing sensor: Parameter design for high sensitivity liquid detection

This paper describes a theoretical way to design a fiber-optic evanescent wave sensor made by a partly unclad commercial optical fibers. The effects of the sensorʹs main parameters, such as absorbing medium refractive index, diameter of the fiberʹs bare core and the launching conditions, are analyzed theoretically. The differences of the radiation-field intensity in each optical mode are considered. The sensing region was obtained by HF etching with a silicon support. The prototype sensing devices were tested and partly explained by the calculation results. The methylene blue dye of 3.5 × 10−7 mol/L concentration was detected with the length of the sensing region of 14 mm. The bromcresol green in de-ionized water was measured in the experiment with 6.0 × 10−7 mol/L concentration.