## 2 Results and Discussion
### Physical Properties and Fabrication Process of PFGI-POF
#### 2.1.4 Evaluation of the Fiber Bandwidth  
Contrary to the single mode fiber (SMF), the frequency response of the multimode fiber strongly depends on the spatial distribution of the power within the fiber and therefore on the modal launching conditions. This spatial distribution is correlated to the number of propagation mode inside the fiber core: the intermodal dispersion is the main limitation of the fiber bandwidth. Depending on the index profile and the wavelength, mode groups propagate at different velocities and the difference in propagation time between the fastest and the slowest modes groups is known as the Differential Mode Delays. Even if the PFGI-POF have been optimized to reduce these delays (graded index profile), the DMD should be minimized in order to provide a high bandwidth/length product suitable for high speed communication.  
Figure 9 shows the DMD charts of the four PFGI-POF and Table 2 summarizes the calculated DMD values. The DMD measurements [17] have been done with a test setup allowing the variation of the launch conditions (transversal sweep with a small size spot in front of the fiber core).  
Figure 9: DMD scanning of the four PFGI-POF at 850 nm (length: 200 m). (**a**) GigaPOF62-SR (**b**) Lucina (**c**) GigaPOF120-SR (**d**) GigaPOF50-SR.  
The measured DMD are well correlated to the fiber bandwidth measurement (Figure 10) and demonstrate the relative insensitivity of the large core PFGI-POF modal bandwidth to an offset launch injection.  
The _\"Do it Yourself\"_ concept is fully justified as a large offset launch does not influence the modal bandwidth of the PFGI-POF. From a material point of view, the CYTOP(r) perfluorinated material exhibits a promising dispersion leading to high bandwidth capacities as depicted in Figure 11.  
To confirm the high potential of the PFGI-POF to a misalignment, optical losses measurements (Figure 12), as a function of a transversal offset launch, are then carried out between two PFGI-POF (10 m length) having the same core diameter in order to effectively demonstrate the huge tolerance of such plastic fiber to an offset coupling. 3 dB optical losses is obtained for lateral offset coupling close to 56 \(\upmu\)m (r/a \(\sim\) 0.9) indicating that such a fiber is appropriate for a low cost system requiring a low level of maintenance.  
\begin{table}
\begin{tabular}{|c|c|c|c|c|} \hline
**PMMF Type** & _GigaPOF-120 SR_ & _Lucina_ & _GigaPOF-62 SR_ & _GigaPOF-50 SR_ \\ \hline Measured DMD (ps/m) & 2.7 & 3.97 & 3.15 & 3.68 \\ \hline \end{tabular}
\end{table}
Table 2: Measured DMD as a function of the fiber type.  
Figure 10: Optical Fiber modal bandwidth measurement as a function of the transverse offset launch. The measurement has been performed in a restricted mode launch condition (200 m).  
We have shown that the four presented PFGI-POF exhibit some interesting properties: acceptable fiber attenuation, high fiber bandwidth, core diameter and NA suitable to be compatible with commercially available devices such as 850 nm VCSEL and large active area (\(\#\)70 \(\upmu\)m) photodetectors. Moreover, the PFGI-POF remains a very challenging solution for home office/small office networks due to their ease of connection. No expensive tools are required to provide a clean facet of the fiber, contrary to the glass fiber world. Clip-on connectors have been developed by Nexans and Chromis Fiberoptics to be fixed on the external coating of the PFGI-POF which favor the _Do it Yourself_ concept. Nevertheless, the bandwidth potentialities of the PFGI-POF do not actually achieve the glass fiber bandwidth capabilities. The polymer fiber attenuation needs also to be improved in order to increase the link budget and to be competitive against the glass fiber.