# Coupling of Modes in Step-Index Plastic Optical Fibers by Using D-Shape Technique
## 6 Conclusions  
This study investigated the characteristics of the output light power distribution of the step-index POF, with and without a D shape. Optical propagation analysis along the POF was simulated and conducted experimentally. The simulation analysis was based on the Gloge equation solution using a simulation tool developed by the authors. The experimental analysis used ESKA SK40 POF with a diameter of 1 mm. The optical  
Figure 12: Comparative Evaluation of Bit Error Rate (BER) at a 250 Mbps Transmission Rate over a 10-m CK30 POF. (**a**) depicts the BER measurement without the D-shape insert, while (**b**) demonstrates the BER measurement with the D-shape insert, highlighting the impact of the insert on transmission quality.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Length**} & \multicolumn{2}{c}{**BER without D**} & \multirow{2}{*}{**BER with D Shape**} & \multirow{2}{*}{**Ratio**} \\  & **Shape** & & \\ \hline
1 & 7.6 \(\times\) 10\({}^{-7}\) & 3.7 \(\times\) 10\({}^{-10}\) & 2054 \\ \hline
3 & 4.57 \(\times\) 10\({}^{-7}\) & 6.8 \(\times\) 10\({}^{-8}\) & 6.7 \\ \hline
10 & 9.38 \(\times\) 10\({}^{-6}\) & 2.0 \(\times\) 10\({}^{-8}\) & 469 \\ \hline
20 & 1.2 \(\times\) 10\({}^{-7}\) & 8.8 \(\times\) 10\({}^{-8}\) & 1.36 \\ \hline
30 & 1.03 \(\times\) 10\({}^{-6}\) & 4.8 \(\times\) 10\({}^{-7}\) & 2.5 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Comparative analysis of BER with and without the insertion of the format D in the POF.  
propagation analysis results were applied to a general POF, such as the ESKA CK30 (0.75 mm diameter), which had an excellent agreement for simulation and measured data. The results confirm that the optical propagation analysis conducted is robust. A modal coupling investigation was conducted, which considered the D shaped fiber as a solution to filter high-order modes without the need for additional components or more complex setups.  
These findings suggest that the D shape effectively reduces the mode stabilization length and, by extension, minimizes mode dispersion effects. Such a reduction facilitates more efficient data transmission in embedded systems that employ POFs as an internal data bus medium. This advance in understanding the properties of D shape-modified POFs paves the way for significant improvements in optical communication systems' reliability and performance, particularly in applications where signal integrity and reliability are critical, as mentioned in Section 1, where the involved lengths are short.  
This study also opens up the possibility of using POFs with a D shape in different devices to replace short-distance electrical connections, such as computers, robots, and others.  
Conceptualization, C.M.F.S. and G.M.P.; data curation, C.M.F.S., G.M.P., J.P., and L.A.R.R.; formal analysis, C.M.F.S. and G.M.P.; funding acquisition, C.M.F.S.; investigation, C.M.F.S., G.M.P., J.P., and L.A.R.R.; methodology, C.M.F.S. and G.M.P.; project administration, C.M.F.S.; resources, C.M.F.S. and G.M.P.; software, C.M.F.S.; supervision, C.M.F.S.; validation, C.M.F.S. and L.A.R.R.; visualization, C.M.F.S. and G.M.P.; writing--original draft, C.M.F.S. and G.M.P.; writing--review and editing, C.M.F.S. and G.M.P. All authors have read and agreed to the published version of the manuscript. This research received no external funding.  
Not applicable.  
The raw data supporting the conclusions of this article will be made available by the authors upon request.  
The authors would like to thank [PERSON] and [PERSON] for making support devices for the plastic fibers (POFs), and INB (Industrias Nucleares do Brasil) and UNIFOA (Centro Universitario de Volta Redonda-RJ) for their support.  
The authors declare no conflicts of interest.