## 4 Experiment Results and Discussion
### Integration Analysis  
The mutual impact between communication and sensing signals is studied by observing and comparing the system performance under different operational states. Figure 9 reveals the outcome from four types of experiments, to systematically compare the system performance when either the EOM (communication) or PZT (vibration sensing) is active. Figure 9a shows the system noise in the absence of both high-frequency intensity modulation (PolSK) and low-frequency intensity modulation (vibration). The noise is mainly attributed to the random changes in the SOP caused by fluctuations in the ambient environment. Figure 9b shows the PolSK modulation without vibration perturbations, which reveal an extinction ratio of 5.79 dB, a \(Q_{dB}\) of 16.78 dB, and a BER of 5.202 \(\times\) 10\({}^{-12}\). Figure 9c shows the detection of sinusoidal vibration signals without PolSK modulation, with an LoD of 564 \(\mathrm{p}\varepsilon/\mathrm{Hz}^{1/2}\) and a positioning accuracy (STD) of 766 m. Finally, Figure 9d features the existence of both PolSK modulation and vibration signals, which gives rise to an extinction ratio of 6.02 dB (trough of vibration envelope) or 7.23 dB (peak of vibration envelope), a \(Q_{dB}\) of 12.46 dB, and a BER of 1.359 \(\times\) 10\({}^{-5}\). The sensing performance features an LoD of 621 \(\mathrm{p}\varepsilon/\mathrm{Hz}^{1/2}\) and a positioning accuracy (STD) of 874 m.  
It is clear that communication has a degree of impact on the sensing of vibrations, through comparing LoD (OFF 564 \(\mathrm{p}\varepsilon/\mathrm{Hz}^{1/2}\) vs. ON 621 \(\mathrm{p}\varepsilon/\mathrm{Hz}^{1/2}\)) and STD (OFF 766 m vs. ON 874 m) between Figure 9c and Figure 9d. As for the impact of vibrations on communications, by comparing the extinction ratio (OFF 5.79 dB vs. ON 6.02-7.23 dB), \(Q_{dB}\) (OFF 16.78 dB vs. ON 12.46 dB), and BER (OFF 5.202 \(\times\) 10\({}^{-12}\) vs. ON 1.359 \(\times\) 10\({}^{-5}\)) between Figure 9b and Figure 9d, it is clear that external vibrations deteriorate the data transmission quality. BER deterioration can affect data integrity, throughput and efficiency, error correction overhead, user experience, system design complexity, reliability and availability, and regulatory compliance. To regulate BER within acceptable limits, engineers often employ a variety of techniques, including using stronger error correction coding, higher SNR through better hardware design, adaptive modulation schemes, and robust synchronization between the transmitter and receiver. It needs to be stressed that communication signals usually already have vibration information encoded as noise, and the addition of the distributed sensing ability does not add extra noise. Furthermore, the sensing function is non-intrusive, as it does not change hardware nor occupy additional communication bandwidth.  
Figure 8: Eye diagram of PolSK communication–sensing system under the influence of vibrations.  
Although the bit rate (300 Mb/s) demonstrated in this proof-of-concept study is low compared to commercial fiber-optic communication systems, the viability of deep integration between polarization-based communication and sensing was verified. The transmission rate can be massively increased via additional modulation formats with wavelength- and space-division multiplexing.