# O-Band Grating Couplers Using Silicon Nitride Structures
## 4 Conclusions  
A new design of a GC based on Si\({}_{3}\)N\({}_{4}\) structures for efficient light coupling in the O-band spectrum has been demonstrated. The proposed GC design, utilizing a uniformly grooved teeth rectangular configuration, offers improvements in terms of cost-effectiveness and ease of fabrication, compared to other GC models. Through numerical investigations using the FDTD method, the optimal key values of the GC have been found to fulfill the Bragg condition for increasing the Si\({}_{3}\)N\({}_{4}\) waveguide TE mode. The optimal grating period \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Si\({}_{3}\)N\({}_{4}\) GC Technology**} & \multirow{2}{*}{**Coupling Efficiency (\%)**} & \multirow{2}{*}{**Wavelength (\(\mu\)m)**} & \multicolumn{2}{c}{**Complexity of Fabrication (Yes/No)**} & \multicolumn{1}{c}{**Fabrication Cost**} \\  & & & & **(High/Low)** \\ \hline GC with bottom grating & \multirow{2}{*}{27} & \multirow{2}{*}{1.55} & \multirow{2}{*}{yes} & \multirow{2}{*}{high} \\ reflector [43] & & & & \\ \hline GC based on butt-coupled [44] & 38 & 1.54 & & yes & high \\ \hline GC between waveguide and & \multirow{2}{*}{22} & \multirow{2}{*}{1.56} & \multirow{2}{*}{no} & \multirow{2}{*}{low} \\ fiber [45] & & & & \\ \hline GC combined inverse taper & \multirow{2}{*}{42} & \multirow{2}{*}{1.56} & \multirow{2}{*}{yes} & \multirow{2}{*}{high} \\ using [46] & & & & \\ \hline GC with bottom reflectors [47] & 56 & 1.57 & & high \\ \hline In this work & 28 & 1.31 & & low \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparative analysis of Si\({}_{3}\)N\({}_{4}\) Insulator-Based Chip-to-Fiber GC. Figure 11: BR calculations using FDTD method: (**a**) FDTD simulation setup, (**b**) normalized BR as a function of the optical path (cT, where c is the speed of light and T is time). (\(\Lambda\)) was determined to be 0.84 \(\upmu\)m, ensuring robust performance even with slight deviations. The thicknesses of the SiO\({}_{2}\) and Si\({}_{3}\)N\({}_{4}\) layers were carefully chosen with a thickness of 1 \(\upmu\)m for SiO\({}_{2}\) and 0.76 \(\upmu\)m for Si\({}_{3}\)N\({}_{4}\), and the etch depth was set to be 0.36 \(\upmu\)m with a tolerance range of \(\pm\)0.05 \(\upmu\)m. The coupling angle (\(\theta\)) from the SMF was optimized at 7.5 degrees, and the distance between the SMF and GC was determined to be 2.6 \(\upmu\)m, resulting in an excellent coupling efficiency of 28% between the GC and SMF at a wavelength of 1310 nm. Also, through simulation, the electric field distribution over the SMF has been studied and shows strong light confinement inside the SMF core, which helps us to maintain the optical energy at the SMF output for testing signals. In addition, two areas for testing measuring with high efficiency have been found, which can be useful to deal with mechanical errors in the optical setup. The proposed GC device can support the O-band spectrum with a high coupling efficiency of over 24%, and can easily handle the laser drift effect. The analysis of the proposed Si\({}_{3}\)N\({}_{4}\)-based GC with other designs and technologies highlighted its advantages in terms of cost-effectiveness, ease of fabrication, and performance. Based on the findings of the BR analysis, 31.5% of the input light is reflected into the Si substrate layer. This issue can be handled by putting a bottom mirror before the Si layer. However, this solution will increase the fabrication cost and complexity. Also, only a minimum of 0.015% BR power is coming back into the input waveguide, which can be easily handled by adding an adiabatic Si\({}_{3}\)N\({}_{4}\) waveguide taper. By eliminating the need for a waveguide converter from Si\({}_{3}\)N\({}_{4}\) to Si, our design minimizes power losses and BR, reducing the footprint size of the photonic chip and enabling better performances. Hence, the proposed Si\({}_{3}\)N\({}_{4}\)-based GC design offers a promising solution for efficient light coupling in the O-band spectrum.