# 16-Channel Wavelength Division Multiplexers Based on Subwavelength Grating
## 3 Results and Discussion  
We designed and simulated a 16-channel wavelength WDM based on SWG, and the channel interval iwas 4 nm. We only needed to adjust the period to meet the wavelength shift, and \(\Delta\lambda/\Delta\)pitch hardly changed within the 40 nm bandwidth, which means that we could use the 3 dB bandwidth of a single channel. The period of the perturbation grating \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Wb** & **Ws** & **Wa** & **Gap** & **Pitch** & **Duty** & **N** \\  & **(nm)** & **(nm)** & **(nm)** & **(nm)** & **(nm)** & **Cycle** & **N** \\ \hline
1310 nm & 650 & 350 & 100 & 650 & 234 & 0.5 & 855 \\
1550 nm & 600 & 400 & 100 & 700 & 321 & 0.5 & 623 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: O-band and C-band central wavelength SWG parameters. Figure 3: **(a)** The loss of WDM SWG in O–band with different parameters when transmitting C–band. **(b)** O–band coupling light field diagram. The light field map of the C-band (almost lossless output from the through to the end) when WDM is performed in the O–band. can be fine-tuned, and the corresponding filter channel can also be designed according to the needs of the specific scene. We simulated the designed structure in FDTD Solutions with a PML boundary condition. The incident light wave was a TE0 broad-band source with wavelength from 1290 nm to 1570 nm. The light wave of different channels propagated in the main waveguide and coupled to the branch output successively. Monitors were placed at the output of each channel collecting the transmission of the coupled light for evaluating the performance of the designed device. Table 2 illustrates the respective parameter settings of O-band and C-band. When the cycle changed by 1.5 nm, the wavelength shifted by 4 nm. Using the above parameters, we simulated the 16-channel WDM of the O-band and C-band. The simulation results are shown in Figure 4.  
Figure 4 shows the simulation results of designed 16-channel WDM. The wavelength demultiplexing of the channel was completed in the 1535-1565 nm bandwidth, and the loss was lower than 0.5 dB, with crosstalk less than 17 dB; The wavelength demultiplexing of the channel was completed in the 1290-1330 nm bandwidth, and the loss was lower than 0.5 dB, crosstalk less than 20 dB. It could be observed that in each channel, there was a slight decrease in loss before the center wavelength. This is caused by the reflection around the Bragg wavelength. To avoid possible crosstalk, we will tackle this problem in our next work. We also simulated fabrication-error-induced performance deterioration. Considering the structure was based on SWG perturbation and fabrication error usually takes on a different duty cycle value, the C4 duty cycles \(\times\) period of 159.7 nm (original) and 159.7 \(\pm\) 10 nm were simulated and compared, as shown in Figure 5. It can be seen in the figure that the fabrication error mainly resulted in a central wavelength shift while the cross-talk change was minor. The central wavelength was 1547 nm when the duty cycles \(\times\) period was 159.7 nm, and the central wavelength shift was \(\sim\)1 nm when the duty cycle \(\times\) period changed from 159.7 \(-\) 10 nm to 159.7 \(+\) 10 nm. In other words, for 20 nm duty cycles \(\times\) period device fabrication error, the central wavelength shift was \(\sim\)1 nm. Moreover, as the structure was based on SWG perturbation, the duty cycle fabrication errors for each channel were similar in the same device. This indicates that the central \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \(\Delta\lambda\) (nm) & \(\Delta\)pitch (nm) & **Loss (dB)** & **CT (dB)** & **Band (nm)** \\ \hline O-band & 5 & 1.5 & 0.5 & 20 & 45 \\ C-band & 5 & 1.5 & 0.5 & 17 & 40 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: O- and C-band wave decomposition multiplexing parameters.