## 5 Conclusions  
This paper addressed the challenge of an SOI MRR's temperature dependence, compromising the performance of an MRR-enabled mmW OSSB RoF transmitter. The varying ambient or in-system-induced temperature changes the relative alignment of the laser (carrier) and the MRR's resonant wavelengths. Therefore, we identified the need for a closed-loop control system to stabilize the transmitter by means of wavelength interlocking. We presented the underlying theory and proposed a concept for the CLC system. Based on this concept, we designed and manufactured several prototype devices constructing or assisting the CLC system. We established the experimental setup to quantitatively evaluate  
\begin{table}
\begin{tabular}{c|c c c c c} \hline \hline
**Parameter** & **Relies on** & **Insensitive to** & **Allows for** & **Resonance** & **Analog** \\
**Reference []** & **Complicated** & **Laser Wavelength** & **Closed-Loop** & **Alignment** & **Position** \\
**Reference []** & **PIC Design** & **Variations** & **Operation** & **Precision** & **Solution** \\ \hline  & \multirow{2}{*}{TO effect} & \multirow{2}{*}{PN junction} & Explicit & Implicit & & \\  & & & feedback & feedback & & \\ \hline \multirow{2}{*}{This paper} & & \multirow{2}{*}{X} & \multirow{2}{*}{DP} & \textless{4 pm @ 1564 nm} & \multirow{2}{*}{X} \\  & & & & & \textless{491 MHz} & \\ \hline
[25] & X & & & & T & \textless{8000 MHz} & N/R \\ \hline
[26] & X & & X & DP & N/R & X \\ \hline
[27] & X & & X & DP & **\textless{0.04 K}** & X \\ \hline
[29] & X & & X & DP & **\textless{10 pm @ 1555 nm}** & \\  & & & & & \textless{1241 MHz} & \\ \hline
[30] & X & & X & DP & N/R & X \\ \hline
[31] & & X & X & TP & N/R & \\ \hline
[32] & X & & X & BER & N/R & \\ \hline
[33] & X & & X & DP & N/R & X \\ \hline
[36] & X & & X & SHL & \textless{15 pm @ 1544 nm} & \\  & & & & \textless{1861 MHz} & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Key performance parameters of the proposed wavelength locking system in comparison with related work.  
the performance of the proposed system. Using our CLC system, we successfully locked the carrier wavelength of the TEC-driven DFB laser with an unavoidable wavelength dither of less than 0.004 nm (~491 MHz). Considering the latter and the parameters of our system (such as fiber length, RF-modulation frequency range, and MRR characteristics) we managed to keep the power-penalty deterioration under 2 dB. While this paper focused on proving the stabilization concept, it should be noted that for a dedicated application the MRR might need to be carefully selected or designed, given the actual parameters of the RoF link under consideration (i.e., intended channel bandwidth, required link budget, etc.). Therefore, we expect that a purposefully designed MRR should offer better performance, which should be investigated in future research.  
Conceptualization, K.V.B. and M.V.; methodology, K.V.B.; software, K.V.B.; validation, B.B., A.D. and K.V.B.; formal analysis, K.V.B.; investigation, K.V.B. and A.D.; resources, B.B. and A.D.; data curation, K.V.B.; writing--original draft preparation, K.V.B. and A.D.; writing--review and editing, B.B. and A.D.; visualization, K.V.B.; supervision, B.B.; project administration, K.V.B., A.D. and B.B.; funding acquisition, B.B. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Slovenian Research and Innovation Agency under grants J2-3048, J2-50072 and research core funds P2-0415 and P2-0246.  
Not applicable.  
Data Availability Statement: Data are contained within the article.  
We thank [PERSON] from Photonics Research Group of Ghent University for providing silicon PICs with MRRs.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.