# The Low-Loss Spot Size Converter for Alignment with Cleaved Single Mode Fiber
## 4 Conclusions  
In this paper, a new design of low-loss SSC based on the integration of the ion-implanted GRIN waveguide and the 2-D inversely-tapered waveguide has been proposed and demonstrated. The SSC could be used for low-loss coupling with cleaved SMF. For the light beam with a MFD of 8 \(\upmu\)m, the TE or TM coupling loss is 0.29 dB or 0.27 dB. For the light beam with a MFD of 10 \(\upmu\)m, the TE or TM coupling loss is 0.58 dB or 0.56 dB. The device also demonstrated polarization insensitivity with a PDL of less than 0.25 dB. The  
\begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**References**} & \multirow{2}{*}{**MFD (\(\upmu\)m)**} & \multirow{2}{*}{**Width of Taper Tip (nm)**} & \multicolumn{2}{c}{**Coupling Loss (dB)**} & **Simulated or** \\ \cline{3-6}  & & & **TE Mode** & **TM Mode** & **Experimental** \\ \hline
[3] & 8 & 70 & 2 & 2 & Experimental \\ \hline
[5] & 6/10 & 150 & 0.42/0.75 & NA & Simulated \\ \hline
[8] & 8 & 200 & 3.5 & 3.7 & Simulated \\ \hline
[9] & 10.5 & 100 & 1.5 & 2.1 & Experimental \\ \hline
[10] & 9\(\sim\)10 & No Inversely Taper & 2.6 \(\pm\) 0.3 & 2.6 \(\pm\) 0.3 & Experimental \\ \hline This Work & 8/10 & 200 & 0.29/0.58 & 0.27/0.56 & Simulated \\ \hline \end{tabular}
\end{table}
Table 1: Comparison of coupling loss for different SSCs aligned with the cleaved SMF.  
Figure 9: The 1-dB alignment tolerance (**a**) in the vertical direction (\(Z\)-axis); (**b**) in the horizontal direction (\(Y\)-axis).  
1-dB alignment tolerance is \(\sim\)2.0 \(\upmu\)m for the vertical direction or horizontal direction under TE or TM mode illuminations, which provides a large tolerance for the alignment with the cleaved SMF. For the 2-D inversely-tapered waveguide, the coupling loss could still be less than 1 dB when the refractive index difference between Cladding 1 and Cladding 2 is up to 0.015, which shows an acceptable tolerance for the device fabrication.  
Comparing with the high-cost electron beam lithography technique, the manufacturing technology employed in our designed SSC could involve grayscale technology for micro-optical device fabrication and multiple ion implantation for customized refractive index waveguide fabrication, both of which are relatively mature at this moment. Moreover, the tapered waveguide tip width of 200 nm could allow the SSC to be fabricated in current commercial silicon photonics fabs. Therefore, we believe that this SSC could be manufactured at a lower cost. In the future, we will use the above-mentioned technologies to fabricate the real SSC, and the coupling loss will be characterized experimentally.  
Conceptualization, J.Z. and N.Y.; methodology, J.Z. and N.Y.; software, J.Z. and Z.W.; validation, J.Z. and Z.W.; formal analysis, J.Z.; investigation, J.Z.; resources, J.Z.; data curation, J.Z. and Z.W.; writing--original draft preparation, J.Z.; writing--review and editing, J.Z. and N.Y.; visualization, J.Z.; supervision, N.Y.; project administration, F.F. and Y.S.; funding acquisition, F.P., Y.S. and N.Y. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key Research and Development Program of China (grant number 2019 YFB1802901); and the Natural Science Foundation of China (NSFC) (grant number 62175143).  
Not applicable.  
We also thank [PERSON] for providing guidance on writing.  
The authors declare no conflict of interest.