# Coupling of Modes in Step-Index Plastic Optical Fibers by Using D-Shape Technique
## 4 Experimental Results
Table 6 presents the variance in output light power distribution for the ESKA CK30 POF across different sections: 15 and 20 m without a D shape and 5 and 10 m with a D shape incorporated. The variance data acquired were utilized in Equation (2) to derive the mode coupling coefficient values, as detailed in Table 7, which presents the coefficients for the POF ESKA CK30. Figure 8 illustrates the resultant light power distribution across distinct segments of the POF. By comparing the theoretical and experimental \(Z\)s for the CK-30 POF, one can see a different condition from the SK-40. The CK-30 is a higher attenuation POF and will be used to validate the presented analysis. \begin{table}
\begin{tabular}{c c c} \hline  & **Stretch of the POF under** & **The Variance in the Launch** \\  & **Investigation** & **Beam Distribution \(\sigma\)2** \\ \hline \multirow{2}{*}{POF without D shape} & \(Z_{1}=15\) m & \(\sigma_{1}^{2}=0.14397\) \\ \cline{2-3}  & \(Z_{2}=20\) m & \(\sigma_{2}^{2}=0.13229\) \\ \hline \multirow{2}{*}{POF with D Shape} & \(Z_{1}=5\) m & \(\sigma_{1}^{2}=0.1197\) \\ \cline{2-3}  & \(Z_{2}=10\) m & \(\sigma_{2}^{2}=0.14946\) \\ \hline \end{tabular}
\end{table} 
Table 6: The variance in the launch beam distribution, \(\sigma_{2}\), for POF ESKA CK30, with and without a D shape. \begin{table}
\begin{tabular}{c c} \hline Mode coupling coefficient without the use of a D shape & \(1.168\times 10^{-3}\) (rad\({}^{2}\)/m) \\ \hline Mode coupling coefficient with the use of a D shape & \(2.976\times 10^{-3}\) (rad\({}^{2}\)/m) \\ \hline \end{tabular}
\end{table} 
Table 7: Mode coupling coefficient, D, values with and without D-shaped application in POF ESKA CK30. The series of figures from Figure 8a-g clearly demonstrate how the angular light power distribution changes with the extension of the OPF's length (z), distinctly showing that the SSD is achieved at a length of 20 m, as specifically highlighted in Figure 8f. After acquiring angular light power distribution patterns within different segments of the OPF up to the identification of the SSD, the subsequent step involved implementing the D-shape process in particular segments where the absence of SSD had been observed in  
Figure 8: Output light power distribution of the ESKA CK30 OPF for various OPF input angles and z lengths, obtained experimentally without using D-shape (\(\hat{\pi}=+-20^{\circ}\), \(\lozenge=+-15^{\circ}\), \(\Delta=+-10^{\circ}\), o = \(+-5^{\circ}\), and \(*=0^{\circ}\)). the previous measurements conducted without the D-shape process. The dimensions of the device to create a D shape are shown in Figure 4(b2). Using the data from the output light power distribution captured by the CCD, the variance in the optical beam injected into two sections of the POF was determined, following the same procedure as previously. As can be seen, the SSD was not obtained with a 10 m stretch of the POF in Figure 8d; after the insertion of the D shape, the SSD was observed. Figure 9 shows the CK30 POF with a 10 m stretch using the D-shape, showing complete convergence of the angular light power distribution for different entry angles into the POF. In contrast, in Figure 10, with the same POF with a 5 m stretch, we observe that convergence of the angular light power distribution does not occur ultimately, which indicates that the value of Zs is greater than 5 m and less than 10 m.  
The calculated Zs value for the ESKA CK30 POF with the D shaped insert was 7.56 m. Experimentation using a 5 m long POF was carried out to validate the agreement with the experimental results. Figure 10 shows the modal behavior of the ESKA CK30 POF with the D shape. The 5 m section did not exhibit complete convergence of the output light power distribution, even after introducing the D shape, aligning with the previously calculated Zs value. From Figures 9 and 10, it is possible to conclude that SSD is between 5 and 10 m.  
Table 8 shows a comparative analysis of the reduction in Zs obtained by applying the D-shape technique to the POF.