# 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 top route exhibits the direct blending of NPs into the polymer matrix in solution; the middle route is the direct blending of NPs into the monomer mixture, followed by photochemical radical co-polymerization on the substrate; and the bottom route is the co-polymerization of double-bond-modified NPs together with additional acrylate/vinyl monomers. Therefore, their application can increase the optical propagation losses due to a high concentration of C-H bonds. The experiments with NPs dispersed in solvents (MEK-ST from Nissan and Nanopol C764 and C784 from Evonik) were carried out [2]. Both core (_n_ = 1.48, 1.50) and cladding (_n_ = 1.45) mixtures were formulated successfully by varying the concentration of SiO\({}_{2}\) NPs from 10 to 40 wt%. We also varied the monomer composition to achieve the required refractive index. Most of these mixtures produced layers on the substrates of sufficient optical quality and thickness to obtain a complete optical characterization. The UV-cured polymeric materials exhibited a relatively low TOC of \(-\)(0.6-1.0) \(\times\) 10\({}^{-4}\) K\({}^{-1}\) depending on the composition. Compared to the original acrylate mixture (TOC = \(-\)(1.3-1.5) \(\times\) 10\({}^{-4}\) K\({}^{-1}\)) and the commercial materials form ChemOptics (\(-\)(1.5-2.2) \(\times\) 10\({}^{-4}\) K\({}^{-1}\)) [28], these values were significantly lower. Exemplary measurements of some materials with low TOCs developed by us are shown in Figure 7.  
Thus, the effect of TOC reduction caused by the introduction of an inorganic material, SiO\({}_{2}\) (Nanopol dispersions), is also clearly demonstrated. Also, the glass transition temperature, \(T_{\rm g}\), is clearly manifested as a kick in the _n_-_T_ plot. The NPs increased the glass transition temperature by only 20 C in this case (Figure 7, right). The best samples were comparable with the commercial material from ChemOptics (0.35 dB\(\cdot\)cm\({}^{-1}\)[25]) or Ormocer-type materials from Microresist (0.5-0.7 dB\(\cdot\)cm\({}^{-1}\) at 1547 nm [24; 28]). The values of optical propagation losses varied from 0.2 to 1.0 dB\(\cdot\)cm\({}^{-1}\) at 1547 nm. The examples of the optical propagation loss measurements for the nanocomposites are shown in Figure 8. In the case of SiO\({}_{2}\) NP dispersions in organic solvents, we were able to prepare both core and cladding nanomaterials. The results based on SiO\({}_{2}\) NP dispersions in acrylate monomers (Nanocryls and Nanobyk) were less conclusive [2]. We also investigated SiO\({}_{2}\) NP monomer formulations with NP concentrations of 20-30 wt%. In this case, a simultaneous increase in the inorganic content and the concentration of C-H bonds in the formulation (addition of non-fluorinated acrylate from the dispersion medium of NPs) should happen. The TOC values were \(-\)(0.8-1.5) \(\times\) 10\({}^{-4}\) K\({}^{-1}\), thus exhibiting a no clear difference from the materials without  
Figure 7: Temperature dependence of \(n\) (numbers on the plots are TOCs determined as slopes of linear dependencies): **left**—core acrylate mixture containing 30 wt% of SiO\({}_{2}\) NPs (from Nanopol C784, \(n\) = 1.48) and base core acrylate mixture (with \(n\) \(\approx\) 1.49) used to prepare nanocomposite mixture in red; **right**——core acrylate mixture containing 25 wt% of SiO\({}_{2}\) NPs (from Nissan MEK-ST, \(n\) = 1.5) and base core acrylate mixture (with \(n\) \(\approx\) 1.49) used to prepare nanocomposite mixture in red. NPs. However, despite the additional contribution from non-fluorinated acrylates to optical propagation losses (due to an increase in the concentration of C-H bonds), the best samples exhibited low optical propagation losses of 0.3-0.4 dB-cm\({}^{-1}\) at 1547 nm. The fact that such polymers could be used in optical applications under harsh environmental conditions is particularly important for optical glass fibers but also has an implication for optical microchips on the polymer platform. An application in polymer-clad fibers will be considered in Section 3, below.