# Fluorinated Polymers for Photonics--From Optical Waveguides to Polymer-Clad Glass Optical Fibers
## 2 Fluorinated Polymers for Passive Optical Waveguides
### Composite Waveguiding Materials Containing SiO\({}_{2}\) and MgF\({}_{2}\) Nanoparticles
The disadvantage of this route is that the co-polymer cannot be separated and must be used directly from the reaction mixture. In addition, this solution shows only a short shelf time of ca. 1 month, with gelation occurring afterwards due to further sol-gel reaction. M-line measurement exhibited a TOC value of \(-(0.95\)-\(1.15)\times 10^{-4}\) K\({}^{-1}\), and ellipsometry exhibited a TOC value of ca. \(-1.25\times 10^{-4}\) K\({}^{-1}\) (a steep fall in the refractive index at the beginning was attributed to the loss of residual water) (Figure 14). In this case, the polymer can be prepared only with silica NPs inside, and there is no basis polymer for comparison. The impact of NPs in situ on the TOC is not clear. However, we did not observe the glass transition of the polymer up to 200 \({}^{\circ}\)C. An additional cross-linking via Si-O-Si linkages in situ could be a plausible explanation for this effect. Examples of optical propagation loss measurements are shown in Figure 15 in comparison to the starting polymer. Generally, the optical propagation losses in nanocomposite waveguides are comparable with those of the unmodified polymers (Figure 16). The trifluoroacetic acid-modified MgF\({}_{2}\) NPs described above were also introduced into the polymer solution. This resulted in the formation of relatively good-quality films with a thickness of a few micrometers. Unfortunately, some visible scattering appeared after UV curing of the nanocomposite films (apparently, agglomeration of NPs due to polymer network building). It was still possible to measure the refractive index and TOC (but not  
Figure 6:4 Temperature dependence of \(n\) (TOCs of corresponding materials are shown as numbers on the plots) for a polymer with SiO\({}_{2}\) NPs in a side chain obtained by in situ sol–gel reaction: **left**—m-line spectroscopy; **right**—ellipsometry. Figure 6:5 Examples of optical propagation loss measurements at 1547 nm for an oxirane-containing polymer and its nanocomposite with SiO\({}_{2}\) NPs (red lines are exponential fits): **left**—unfilled polymer, **right**—nanocomposite. optical propagation losses) using the method of m-line spectroscopy (Figure 10, right). It was demonstrated that the introduction of MgF\({}_{2}\) NPs decreases the refractive index and the TOC. The optical parameter results for the studied polymer and polymer composite materials are collated in Figure 6:6 \begin{table}
\begin{tabular}{l c c} \hline \hline
**Polymer Material** & \
\begin{tabular}{c} **Propagation Loss at** \\ **1547 nm, dB\(\cdot\)cm\({}^{-1}\)** \\ \end{tabular} &
\begin{tabular}{c} **TOC, \(\times\)10\({}^{-4}\) K\({}^{-1}\)** \\ \end{tabular} \\ \hline Monomer materials based on acrylates, core (\(n\) = 1.48 and 1.5) & 0.3–0.6 & \(-\)(1.3–1.5) \\ \hline Monomer materials based on acrylates, cladding (\(n\) = 1.45) & 0.4–0.5 & \(-\)(1.3–1.5) \\ \hline Glycidyl-containing polymer, core (\(n\) = 1.48 and 1.5) & 0.5–0.7 & \(-\)(0.65–0.85) \\ \hline Glycidyl-containing polymer, cladding (\(n\) = 1.45) & 0.6–0.8 & \(-\)(0.8–1.0) \\ \hline Monomer mixtures + SiO\({}_{2}\) NP dispersions in solvents & 0.2–1.0 & \(-\)(0.6–1.0) \\ \hline Monomer mixtures + SiO\({}_{2}\) NP dispersions in acrylates & 0.3–0.4 & \(-\)(0.8–1.5) \\ \hline Monomer mixtures + MgF\({}_{2}\) NPs & 1.7–2.5 & \(-\)0.9 \\ \hline Glycidyl-containing polymer + MgF\({}_{2}\) NPs & - & \(-\)1.0 \\ \hline Glycidyl-containing polymer + SiO\({}_{2}\) NP dispersions in solvents & 0.5–1.0 & \(-\)(0.55–1.25) \\ \hline Glycidyl-containing polymer containing SiO\({}_{2}\) NPs in chain & - & \(-\)(1.1–1.3) \\ \hline Glycidyl-containing polymer containing SiO\({}_{2}\) NPs formed in situ & - & \(-\)(0.95–1.15) \\ \hline Reference material, ChemOptics [25] & 0.35 & \(-\)(1.5–2.2) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Optical parameters of polymer materials developed for core and cladding of optical waveguides.