Fiber-optic evanescent wave sensor with a segmented structure

A segmented structure optic fiber sensor is proposed and demonstrated; it is based on evanescent wave absorption theory. The waveguide modes of the segmented structure are simulated and analyzed by using the beam propagation method, which shows the high-order modes are excited repeatedly at the first transition of each segmented region. The effect of the number of segments and the core diameters on the sensitivity of the sensors is investigated experimentally. The sensitivity of the sensors is tested by using different concentrations of methylene blue solution. The experimental results show that our sensor's sensitivity is ∼0.0476  L/g, which is two times higher than that of the conventional single straight sensor, with the same core diameters and length of the sensing region, and the absorbance is linearly proportional to the number of segments, while being inversely proportional to the residue core diameter of the etched segments within the concentration range from 2 to 14  g/L. These results are consistent with theoretical models and simulation analysis. The proposed sensor not only has high sensitivity, but it is also robust due to the large core diameter of the segmented region, which is suitable for materials spectrum measurements.