## 2 Results and Discussion
### Physical Properties and Fabrication Process of PFGI-POF  
#### 2.1.1 Material Properties and Fabrication Process  
Thermoplastic PMMA (Polymethylmethacrylate) has been the first material used for POF fabrication. Known under the acronym Plexiglass(r), the PMMA is an organic compound based on a polymerized material with an amorphous structure and a glass transition temperature (T\({}_{\mathrm{g}}\)) close to 100 \(\mathrm{\SIUnitSymbolCelsius}\). The Plexiglass(r) is composed of several MMA monomers, each of them showing 8 C-H bonds as described in Figure 2(a). The 6 th and 5 th harmonic waves (occurring at 627 nm and 736 nm respectively) of the MMA monomer are responsible for the high level of attenuation of the PMMA based POF, especially from the visible to the infra-red spectral ranges (110 dB/km at 650 nm). In order to decrease the intrinsic absorption of the PMMA based POF resulting particularly from the vibrational overtones, the idea is to perform the partial or complete substitution of the hydrogen compound by heavy atoms like deuterium (also called heavy hydrogen \({}^{2}\)H, a stable isotope of hydrogen having twice the atomic mass of the hydrogen atom) or fluorine atoms. Even if the use of heavy hydrogen induces a significant improvement of the fiber attenuation [5] over the visible spectrum (one order of magnitude), the deuterium based POF are very sensitive to the water vapor in the ambient air which is absorbed by the core material leading to an increase of the attenuation (In fact, the deuterium is progressively replaced by hydrogen atoms resulting from the water vapor contamination inside the fiber). By replacing the hydrogen atoms with fluorine atoms inside the fiber core, it is possible to reject the absorption bands (C-F bonds) of the used material to the infra-red spectra, far away from the telecommunication window (850 nm-1,550 nm). According to the available [6] polymer materials (polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether (PFA), _etc._), the minimum fiber absorption has been obtained by Asahi Glass Company (AGC) from Japan with the cyclic transparent optical polymer (CYTOP(r)), which contains only C-F bonds as shown in Figure 2(b). The CYTOP(r) material is an amorphous fluoropolymer having a T\({}_{\mathrm{g}}\) close to 108 \(\mathrm{\SIUnitSymbolCelsius}\) and where the graded index variation inside the fiber core is realized by copolymerization of two monomers and by a doping process (interfacial-gel polymerization method). Nevertheless, even if the fiber attenuation decreases to 15 dB/km at 1,300 nm (Figure 3) with the use of the CYTOP(r) material, the calculated attenuation [6] threshold is not reached mainly due to the extrinsic losses induced by the fabrication process (impurities owing to the gas, material crystallization  
Figure 2: Molecular geometry and bonding of the Plexiglas® (**a**) and the CYTOP® (**b**) materials. (scattering centers, contamination and so on). Several manufacturing processes have been reported to make the fabrication of the graded index POF:  
* the interfacial gel polymerization technique;
* the centrifuging process and the combined copolymerization/rotating methods;
* the combined extrusion of the core and the cladding materials. [PERSON] _et al._[8-10] from the Keio University (Japan) have developed the interfacial gel polymerization method to realize the preform (Figure 4). A mixture composed of two kinds of monomers (a classical one and a dopant) has been inserted in a tube (diameter equivalent to the preform diameter) and heated at 80 \(\,\mathrm{\SIUnitSymbolCelsius}\) to be preliminary liquefied. During this process, the formation of a gel layer is done in the inner wall of the tube (polymer gel phase) and the smaller size monomer diffuses from the edge of the preform to the center in order to form the graded index profile. The graded index profile is correlated to the dopant distribution inside the preform.