## 4 Conclusions  
In conclusion, we introduce a novel four-channel coherent optical chaotic secure communication (COCSC) system that integrates four simultaneous photonic reservoir computers with a coherent demodulation apparatus. This system utilizes a QD-spin-VCSEL as the driving laser, while an autonomous QD-spin-VCSEL acts as the reservoir laser. Individually, the four encoded messages, QPSK, 4 QAM, 8 QAM and 16 QAM are modulated onto four distinct polarization components derived from the ground state (GS) and excited state (ES) emissions in the drive QD-spin-VCSEL. Moreover, we construct four concurrent photonic reservoirs using the polarization components originating from the GS and ES of the reservoir QD-spin-VCSEL. Our system achieves a four-channel COCSC system with a capacity of 4 \(\times\) 100 GHz through chaos synchronization founded on these four parallel photonic reservoirs. Within this arrangement, we ensure robust wideband chaos synchronization between corresponding polarization components of the driving and reservoir lasers. This precise synchronization allows for the accurate decoding of the four distinct messages (QPSK, 4 QAM, 8 QAM and 16 QAM), each masked within different chaotic polarization components. The decoded messages are then demodulated via correlation techniques and further processed using various digital signal processing methodologies, successfully reconstructing the four separate baseband signals encapsulated within the QPSK, 4 QAM, 8 QAM and 16 QAM formats. Through detailed analysis with eye diagrams, bit error rates, and temporal trajectories of the coherently demodulated baseband signals, we observe that each baseband-signal set is impececably recovered, evidenced by large eye openings in the eye diagrams and a bit error rate that approaches zero for each baseband-signal set. This innovative approach, which harnesses the power of reservoir computing based on a QD spin-VCSEL, paves the way towards advancing multi-channel coherent optical chaotic communications with enhanced security features.  
Conceptualization, D.Z. and T.W.; methodology, D.Z. and T.W.; software, T.W., Q.W. and Y.C.; formal analysis, T.W., Y.C., Q.W., C.Q., Y.W., H.Z. and J.X.; writing--original draft preparation, T.W.; writing--review and editing, D.Z.; and visualization, T.W. All authors have read and agreed to the published version of the manuscript.  
Figure 10: Temporal trajectories and eye-diagrams of the original baseband signals \(b_{1}\)–\(b_{4}\) and their respectively retrieved baseband signals \(b_{1}^{{}^{\prime}}\)–\(b_{4}^{{}^{\prime}}\). Here, (**a1**,**a2**) the temporal trajectories of the \(b_{1}\) and \(b_{1}^{{}^{\prime}}\), respectively, and (**b1**,**b2**) their respectively eye-diagrams. (**a3**,**a4**) the temporal trajectories of the \(b_{2}\) and \(b_{2}^{{}^{\prime}}\), respectively, and (**b3**,**b4**) their corresponding eye-diagrams; (**a5**,**a6**) the temporal trajectories of the \(b_{3}\) and \(b_{3}^{{}^{\prime}}\), respectively, and (**b5**,**b6**) their respectively eye-diagrams; and (**a7**,**a8**) the temporal trajectories of the \(b_{4}\) and \(b_{4}^{{}^{\prime}}\), respectively, and (**b7**,**b8**) their corresponding eye-diagrams.  
**Funding:** This research was funded by the National Natural Science Foundation of China (62075168); the Special Project in Key Elds of Guangdong Universities: the New Generation of Communication Technology (2020 ZDZX3052); the Guang dong Basic and Applied Basic Research Foundation (2023A1515010726); the Innovation Team Project of Colleges and Universities in Guangdong Province (2023 KCXTD041, 2021 KCXTD051); and the Major Projects of Guangdong Education Department for Foundation Research and Applied Research (2017 KZDX086).  
**Institutional Review Board Statement:** No applicable.  
**Informed Consent Statement:** No applicable.  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.  
**Conflicts of Interest:** The authors declare no conflicts of interest.