Tuning the response of long-period fiber gratings for chemical sensing applications

摘要

In recent years, the use of long-period gratings (LPGs) as fiber optic chemical sensors has been proposed by several authors. Such implementations take advantage of the changes in the LPG transmittance characteristics with ambient refractive index and may make use of a polymer coating to enhance chemical selectivity and sensitivity While technically feasible, these designs are subject to fairly rigid constraints related to the optical characteristics of the fiber and grating, as well as the thickness and refractive index of the chemically selective polymer.. Compromises in design may lead to sub-optimal sensor performance in terms dynamic range, sensitivity, linearity, stability and response time. In this work, LPG sensor designs based on one-, two- and three-layer geometries are explored, where the outer layer is the chemically selective polymer and the properties of the other layers (thickness, refractive index) can be adjusted. It is demonstrated through calculations based on a hybrid mode model that the use of more than one layer greatly enhances the flexibility of sensor design and allows the response characteristics to be tuned for optimal performance. A case study is used to illustrate how the same sensor can be optimized for several factors, including linearity, range, sensitivity, and stability.