# Coupling of Modes in Step-Index Plastic Optical Fibers by Using D-Shape Technique
## 3 Material and Methods, Components, and Experimental Setup
Figure 4a illustrates the device used to grind the POF and create the D-shaped configuration. When using the device in Figure 4(b1), there is a reduction in the POF diameter in the D-shaped region from 1 mm to 0.75 mm and from 0.75 mm to 0.50 mm with the device in Figure 4(b2), depending on the POF used. Figure 4c illustrates the D-shape after the sanding process. The length and depth of the groove determine the carachteristics of the D-shape in the POF. When analyzing the light losses of the polishing methods, it was found that using a lower grit sandpaper, which results in a less rough surface, reduced light scattering, increasing the amount of light transmitted through the D-shape. On the other hand, using rougher grits, such as P400, reduced the initial POF transmitted signal. In summary, the key to optimal D-shaped filter operation lies in striking a perfect balance between surface roughness and transmission losses due to polishing. Analyses from prior research suggest that this equilibrium is attainable by employing P600 grit sandpaper for polishing the D-shape [14]. The ESKA SK40 POF, whose specifications are detailed in Table 2, was the fiber chosen for this study. This fiber was selected because of its exemplary performance as a commercial component designed for light transmission. Upon evaluating the initial results, the decision was made to extend the application of the outlined methodology to another type of fiber, specifically the POF ESKA CK30, whose characteristics are presented in Table 3. \begin{table}
\begin{tabular}{c c} \hline \hline Core material & Polymethyl–Methacrylate Resin \\ \hline Cladding material & Fluorinated Polymer \\ \hline Core refractive index & 1.49 \\ \hline Refractive index profile & Step Index \\ \hline Numerical Aperture & 0.5 \\ \hline Core diameter & 980 \(\upmu\)m \\ \hline Cladding diameter & 1000 \(\upmu\)m (1 mm) \\ \hline Transmission loss at 650 nm & 150 dB/km \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Features of POF ESKA SK40. Figure 4: The device for creating the D-shape in POFs. (**a**) The D-shaped insertion device, (**b1**) a side view of the device for creating the D-shape in a 1 mm diameter POF, and (**b2**) a side view of the device for creating the D-shape in a 0.75 mm diameter POF. (**c**) A POF with a D-shape. (**d**) A POF accommodated in the steel device. A helium-neon (He-Ne) laser at 632 nm was used as the light source, and a manual rotation stage (MRS) was used to control the angle of light injection into the POF, and at the output of the POF, a CCD connected to a computer registered an FFP by software was used. Figure 5a shows the block diagram of the setup, and Figure 5b shows the setup in the laboratory. \begin{table}
\begin{tabular}{c c} \hline Core material & Polymethyl–Methacrylate Resin \\ \hline Cladding material & Fluorinated Polymer \\ \hline Core refractive index & 1.49 \\ \hline Refractive index profile & Step Index \\ \hline Numerical Aperture & 0.5 \\ \hline Core diameter & 735 \(\upmu\)m \\ \hline Cladding diameter & 750 \(\upmu\) (0.75 mm) \\ \hline Transmission loss at 650 nm & 200 dB/km \\ \hline \end{tabular}
\end{table} 
Table 3: Features of POF ESKA CK30. Figure 5: (**a**) A block diagram of the setup for measuring angular light power distribution using MRS, and (**b**) an image of the setup in the laboratory. According to Figure 5, the He-Ne laser was positioned so its light could be coupled to the OPF under evaluation. Using the MRS, the injection angles were selected as 0, 5, 10, 15, and 20 degrees. The aim of varying the length of the fiber under test is to obtain a more comprehensive set of data that allows the distribution of angular light power and the state of SSD to be evaluated. As already mentioned, various POF z lengths were used for the evaluation.