## 6 Conclusions  
In this paper, reference wavefront source (RWS) based on silicon nitride (SiN) waveguide in point diffraction interferometer (PDI) is firstly introduced. Then, maximum coupling efficiency, 3 dB fabrication tolerance, 3 dB alignment tolerance and polarization-dependence loss of the Y-branch coupler and grating coupler are compared to improve the power of wavefront. Simulation results show that waveguide RWS can generate a reference wavefront whose root-mean-square (RMS) value is less than 0.03 nm and numerical aperture (NA) is over 0.5, owing to high refractive index difference between SiN core and silicon dioxide (SiO\({}_{2}\)) cladding. Using Y-branch coupler or grating coupler is an effective way to improve the power of wavefront. Compared with Y-branch coupler, grating coupler is the more recommended fiber-to-waveguide coupler because its maximum coupling efficiency (\(\eta_{\rm g}=0.35\)) is greater. Moreover, etching process and alignment process of grating coupler are easier. Y-branch coupler is a better choice if the coating technology cannot meet the requirement (fabrication tolerance is \(\pm 0.06\)\(\upmu\)m). Finally, polarization of the mode in the single-mode fiber (SMF) must be strictly controlled no matter for grating coupler or Y-branch coupler. This paper improves the power of the wavefront by selecting appropriate fiber-to-waveguide couplers for waveguide RWS. PDI based on the power improved waveguide RWS is expected to be used in many measurement fields.  
Conceptualization, Y.Y., J.B. and Y.C.; methodology, Y.C.; software, Y.C.; validation, Y.C., J.B. and C.W.; formal analysis, Y.C.; investigation, Y.X.; resources, J.B.; data curation, C.W.; writing--original draft preparation, Y.C.; writing--review and editing, Y.Y.; visualization, J.B.; supervision, Y.Y.; project administration, J.B.; funding acquisition, J.B. All authors have read and agreed to the published version of the manuscript.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \(W\) (\(\upmu\)m) & \(G\) (\(\upmu\)m) & \(\boldsymbol{d}\boldsymbol{x}\) (\(\upmu\)m) & \(\boldsymbol{dy}\) (\(\upmu\)m) & \(\boldsymbol{L_{\rm b}}\) (\(\upmu\)m) & \(\eta_{\rm y}\) & \(\eta_{\rm P}\) \\ \hline
0.10 & 0.86 & & & & \\ \(-0.10\)\(-\)0.05 & \(-0.70\)\(-\)1.40 & \(\sqrt{dx^{2}+dy^{2}}\leq 1.10\) & 10 & 0.24 & 0.11 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Characteristics of the Y-branch coupler.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \(\Lambda\) (\(\upmu\)m) & \(d\) & \(\boldsymbol{q}\) (\(\upmu\)m) & \(\boldsymbol{p}\) (\(\upmu\)m) & \(\boldsymbol{b}\) (\(\upmu\)m) & \(\boldsymbol{\theta}\) (\(\upmu\)m) \\ \hline
0.40 & 0.476 & 0.22 & 0.20 & & \\ \(-0.02\)\(-\)0.02 & \(-0.12\)\(-\)0.12 & \(-0.05\)\(-\)0.05 & \(-0.05\)\(-\)0.05 & 6 & 20 \\ \hline \(z\) & \(y\) (\(\upmu\)m) & \(h\) (\(\upmu\)m) & \(L_{\rm t}\) (\(\upmu\)m) & \(\eta_{\rm g}\) & \(\eta_{\rm p}\)\({}^{\prime}\) \\ \hline
3.4 & 3.2 & 2.52 & & \\ \(-1.8\)\(-\)3.4 & \(-1.2\)\(-\)\(+12.0\) & \(-0.06\)\(-\)0.06 & 300 & 0.35 & 0.30 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Characteristics of the grating coupler.  
**Funding:** This research was supported by National Natural Science Foundation of China (NSFC, grant numbers 61875173, 61627825).  
**Conflicts of Interest:** The authors declare no conflict of interest.