# Fluorinated Polymers for Photonics--From Optical Waveguides to Polymer-Clad Glass Optical Fibers
## 2 Fluorinated Polymers for Passive Optical Waveguides
### Glycidyl-Containing Polymer Materials  
It has already been discussed above that the presence of C-F bonds leads to a decrease in optical losses and refractive indices. However, the presence of other C-halogen bonds (C-Cl or C-Br) also leads to a decrease in optical propagation losses and an increase in the refractive index. By using different halogenated derivatives, it is possible to tune the refractive indices of polymer layers, which is especially important for the core layer. Here, we used the co-polymer based on acrylate; fluorinated, chlorinated and brominated acrylates; or other compounds containing double bonds (for example, styrene-type compounds).  
Figure 1: Measurement of optical propagation losses by transmitted and scattered light intensity: image of measurement at 633 nm, schematic of measurement and measurement at 1547 nm for core material with \(n=1.5\) of acrylate monomer mixture based on Ebecryl 150 oligomer (0.6 dB\(\cdot\)cm\({}^{-1}\)) (green line represents exponential fit).  
Figure 2: Measurements of TOCs of core and cladding materials with m-line spectroscopy at 1547 nm.  
Changing the concentrations of F, Cl and Br allowed a reduction in optical losses and tuning of the refractive indices for core (\(n\) = 1.48 or 1.5) and cladding polymers (\(n\) = 1.45). Adjustment to the required layer thickness (10 \(\upmu\)m for cladding and 3 \(\upmu\)m for cores) was also possible by changing the concentration of polymers in the solution. To cross-link polymers in layers and to make the polymer layers patternable either by direct photolithography or by reactive etching (Schemes 3 and 4), glycidyl methacrylate or acrylate was used as one of the co-monomers in radical co-polymerization. The oxirane group in the polymer chain is typically used in negative photoresists and allows direct structuring of such polymers via photolithography. To decrease optical losses, the content of glycidyl comonomers must be kept to a minimum, while the photochemical reaction of this group leads not only to cross-linking of the polymer chain, rendering the polymer insoluble, but also to the formation of O-H groups. These groups increase the optical propagation losses. Some examples of useful co-monomers are collated in Scheme 6.  
The scheme of polymer fabrication (radical co-polymerization) and application for the manufacture of waveguide layers (UV-curing in the layers) is shown in Scheme 7.  
The TOC values (0.65-0.85 \(\times\) 10\({}^{-4}\) K\({}^{-1}\) for cores and 0.8-1.0 \(\times\) 10\({}^{-4}\) K\({}^{-1}\) for cladding) were measured (see example in Figure 3).  
Optical propagation losses of 0.5-0.7 dB-cm\({}^{-1}\) at 1547 nm (core) and 0.6-0.8 dB-cm\({}^{-1}\) (cladding) were achieved. Some examples of loss measurements are shown in Figure 4.  
By using different monomers, such as styrene or 4-chlorostyrene, and reducing the glycidyl acrylate content to 5 wt.%, it was possible to reduce the optical propagation losses (Figure 5).  
Examples of photolithographic processed core materials are shown in Figure 6.  
Figure 4: Measurement of propagation optical losses for core \(n=1.5\) ((**a**) 0.6 dB\(\cdot\)cm\({}^{-1}\)), core \(n=1.48\) ((**b**) 0.5 dB\(\cdot\)cm\({}^{-1}\)) and cladding \(n=1.45\) ((**c**) 0.6 dB\(\cdot\)cm\({}^{-1}\)) polymers prepared according to Scheme 7 using m-line spectroscopy at 1547 nm (green lines represent exponential fits).  
Figure 3: Measurements of TOC for core polymer obtained according to Scheme 7 with m-line spectroscopy at 1547 nm.