# An Eight-Channel C-Band Demux Based on Multicore Photonic Crystal Fiber
## 2 Materials and Methods  
Figure 1a-c show the full refractive index profile structure of the demux PCF design on the XZ plane (y = 0 cm), XY plane (z = 0 cm), and XY plane (z = 5 cm), respectively. In these figures, the background material is pure silica, and it is marked in a light blue color; the air-hole areas are marked in a purple color, the LiNbO\({}_{3}\) areas are marked in a red color, and the Si\({}_{3}\)N\({}_{4}\) areas are marked in a yellow color. The geometrical parameters of the PCF structure are d, \(\Lambda\) (pitch), and z. In Figure 1a,d represents the hole diameter of the air-holes, pitch represents the distance between two air-holes, and z represents the light propagation axis. Figure 1: _Cont._  
The principle work of the PCF demux device is based on light confinement inside the channel (core) and controlling the light coupling length size between two closer ports (cores). These effects can be obtained by replacing some air-hole regions with a high-index material along the fiber length. Furthermore, the coupling length value can be shifted by changing the operating wavelength. Thus, the suitable coupling length value between two closer ports can be found by optimized the PCF geometrical parameters (z, \(\Lambda\), d) and the materials' layer locations over the light propagation axis (z). In our design, the light-guiding mechanism is based on the bandgap and the MTIR, which means that light can be coupled between two closer ports that have the same refractive index value (light blue color areas in Figure 1a), and a strong light confinement can be obtained by a port that has a high index value (red and yellow areas in Figure 1a). It can be noticed from Figure 1a that the areas in which light can be coupled between closer silica (light blue color) ports (cores) are located as follows: port 1 and port 8 over the Z-axis range of 0-2.03 cm (L\({}_{\text{main}}\)), port 8 and port 4 over the Z-axis ranges of 2.03-2.1 cm and 3.445-3.93 cm (L\({}_{\text{right}}\),3), port 1 and port 5 over the Z-axis range of 2.5-2.545 cm and 4-4.545 cm (L\({}_{\text{left},3}\)), port 4 and port 2 over Z-axis range of 2.03-3.01 cm (L\({}_{\text{right},1}\)), port 5 and port 7 over the Z-axis range of 2.5-3.46 cm (L\({}_{\text{left},1}\)), port 2 and port 6 over the Z-axis range of 3.01-3.44 cm (L\({}_{\text{right},2}\)), port 3 and port 7 over the Z-axis range of 3.47-3.995 cm (L\({}_{\text{left},2}\)). In addition, it can be noticed that each port at the output has a light confinement area, as shown in Figure 1a. Figure 1b,c show the input port at z = 0 (port 1) and eight output ports at z = 5 cm (red and yellow colors). Table 1 shows the refractive index material values for the C-band range [26; 27; 28], and it is important to mention that the Si\({}_{3}\)N\({}_{4}\) and LiNbO\({}_{3}\) materials have a very low absorption in this range. Another advance to use these materials is that light cannot be coupled between closer Si\({}_{3}\)N\({}_{4}\) and LiNbO\({}_{3}\) layers, because they have a small difference in the index value, as shown in Table 1.  
The Si\({}_{3}\)N\({}_{4}\) and the LiNbO\({}_{3}\) layers were used to enable a strong light confinement inside the port that ensures that light cannot be coupled to other closer ports. In the proposed design, there are 11 layers that function as follows: eight output lines (four LiNbO\({}_{3}\) layers for ports 2, 3, 5, and 8, and four Si\({}_{3}\)N\({}_{4}\) layers for ports 1, 4, 6, and 7) and three selected lines in a similar way to the classical definitions of the digital 1 \(\times\) 8 demultiplexer.