Investigation of tuning characteristics of electrically tunable long-period gratings with a precise four-layer model

In this paper, an investigation of the tuning characteristics of electrically tunable long-period gratings (LPGs) is presented. A precise four-layer model is used to quantitatively analyze the tuning potential of the gratings, and experimental data are provided to support the analysis. The four-layer model includes a silica core layer with an inscribed LPG, a thin silica cladding layer (∼ 40 μm), an ultrathin (∼ 50 nm) high refractive index indium - tin dioxide (ITO) inner electrode layer, and a tunable electrooptic (E-O) polymer layer. It has been found that the inner electrode layer, made of high refractive index ITO, can be modeled as a high refractive index overlay and causes the forward-propagating modes in the thin silica cladding to reorganize as the ambient refractive index changes. This reorganization effect can lead to a significant increase (ten plus fold) in the tuning range of LPG tunable filters. Moreover, the required specifications of the tunable polymer layer are quantitatively analyzed. Finally, the required characteristics of the E-O polymer is realized by using a nanocomposite ferroelectric relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer.