A very large electro-optic effect has been observed in relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer. By adding a small amount of zinc sulfide nanoparticles, the refractive index of the nanocomposite can be tuned between 1.4 and 1.5 while retaining a large electro-optic effect and high transparency. Tunable long-period fiber gratings have been fabricated with the polymer nanocomposite as the second cladding, and a resonance wavelength shift of over 50nm has been achieved when electric field changed by 30V∕μm. This magnitude of shift corresponds to pure refractive index change of Δn∕n≈0.4% in the nanocomposite.
A unique all-fiber tunable filter is based on the combination of a single resonant band long period grating (LPG) and an electro-optic polymer second cladding layer. The single resonant band LPG is fabricated by etching the cladding of a standard 125 µm thick fiber and using either ultraviolet (UV) illumination or electric arc discharge to write the grating. Once a single resonant band has been achieved, a polymer second cladding layer is applied to the LPG. The refractive index of the polymer cladding determines the resonant wavelength of the filter and is tuned by applying an external electric field. The grating fabrication method and type of polymer used for the second cladding affect filter performance, and both must be considered when designing an application specific all-fiber filter.
In this work, 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 is 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 ultra-thin (~ 50 nm) high refractive index indium-tin dioxide (ITO) inner electrode layer, and a tunable electro-optic polymer layer. It has been found that the inner electrode layer, made of high refractive index ITO, can be modeled as a high 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 (10 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 electro-optic polymer are realized by using a nano-composite of zinc sulfide and ferroelectric relaxor poly(vinylidene fluoride -trifluoroethylenechlorofluoroethylene) terpolymer.
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