## 5 Conclusions  
A proof-of-concept of deep integration was demonstrated between fiber-optic communication using 2-PolSK and distributed fiber-optic vibration sensing based on forward transmission. No hardware changes are necessary to add distributed sensing functionality to the communication system. Sensing can be conducted without occupying additional communication bandwidth, due to the non-intrusive extraction of vibration information from received data. Data transmission and multi-point vibration sensing were characterized for signal recovery and crosstalk impact. It was observed that the \(Q_{dB}\) parameter of communication decreased (\(-4.3\) dB) in the presence of vibrations, though the sensing function merely analyzes what is already present in the received communication signal, and thus does not affect the quality of transmission. The sensing performance also declined slightly (\(-14\%\) in positioning accuracy and \(-10\%\) in LoD) when data transmission was enabled. This is attributed to the low-frequency components of the square-wave modulation overlapping with the low-frequency vibration signals. This work demonstrated a sensitivity of 0.4141 \(\upmu\)V/\(\upmu\)\(\upmu\)\(\upmu\) with a limit of detection (LoD) of 563 p\(\upmu\)/Hz\({}^{1/2}\)@100 Hz. The single-span sensing distance can reach up to 121 km (no optical amplification) with a positioning accuracy as small as 874 m. The transmission rate is 300 Mb/s, the \(Q_{dB}\) is 16.78 dB, and the corresponding BER is 5.202 \(\times\) 10\({}^{-12}\). For demonstration purposes, the tested vibration frequency range is between 100 Hz and 200 Hz. This work helps pave the way for high-speed fiber-optic communications with built-in distributed monitoring abilities.  
Figure 9: Communication–sensing crosstalk analysis. (**a**) No data transmission and vibration; (**b**) data transmission only; (**c**) vibration only; (**d**) data transmission with vibration. The vibration amplitude applied was 26.7 \(\upmu\)\(\upmu\). Red line: minimum to maximum value.  
**Author Contributions:** Conceptualization, M.C.; methodology, X.R. and S.D.; software, M.C.; validation, M.C., R.Z. and H.L.; formal analysis, X.R.; investigation, M.C. and J.L.; resources, G.Y.C.; data curation, M.C.; writing--original draft preparation, M.C.; writing--review and editing, M.C. and G.Y.C.; visualization, M.C., X.R., G.L. and J.L.; supervision, G.Y.C. and Y.W.; project administration, Y.W.; funding acquisition, G.Y.C. and Y.W. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The Ministry of Science and Technology of the People's Republic of China (2022 YFE0111400), the research start-up fund for high-end talent at Shenzhen University, the Scientific Instrument Development Project of Shenzhen University (2023 YQ027), National Natural Science Foundation of China (62275172), and the Shenzhen Science and Technology Program (Shenzhen Key Laboratory of Ultrafast Laser Micro/Nano Manufacturing ZDSYS20220606100405013).  
**Data Availability Statement:** Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.  
**Conflicts of Interest:** The authors declare no conflicts of interest.