# Real-Time Eye Diagram Monitoring for Optical Signals Based on Optical Sampling
## 4 Conclusions  
In summary, a real-time eye diagram monitoring method for optical signals was proposed and experimentally demonstrated based on gated on-off optical sampling. In the system, the optical signals under test were directly sampled by an optical sampling pulse train, achieved in an electrical pulse-modulated MZM. The sampled optical signals were then broadened and detected by a PD. A low-speed electrical ADC then quantified the detected electrical signals. The quantified data was then used to achieve the eye diagram based on the concept of infinitesimal calculus. According to the measured eye diagram, more time-domain parameters such as period, time jitter, \(Q\) value, and bit error rate (BER) were obtained for the optical signals under test. Compared to previous studies on eye diagram monitoring, the proposed eye diagram monitoring method has the advantages of higher sensitivity, a larger dynamic range, and faster eye diagram update time due to the broadband MZM, the high-dynamic ADC, and the highly repetition-rate optical sampling pulse. An experimental demonstration was implemented. A real-time of about 350-\(\upmu\)s eye \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Eye Diagram Methods** & **Reference** & **Sampling Rate** & **Sensitivity** & **Q Value** & **Time Jitter** \\ \hline \hline \multirow{2}{*}{Gated on-off optical sampling} & This work & 1.5 GHz & \(-\)22 dBm & 4.3 to 20.4 & 4.3 to 49.8 ps \\ \hline \multirow{2}{*}{Linear optical sampling} & [8] & 10 MHz & \(-\)20 dBm & n/a & n/a \\  & [9] & 10 MHz & \(-\)12.2 dBm & n/a & n/a \\  & [10] (Simulation) & n/a & 16 dBm & 5 to 20 & n/a \\ \hline \multirow{8}{*}{Nonlinear optical sampling} & [11] & 100 MHz & \(>\)1 dBm & n/a & n/a \\  & [12] & 497 MHz & \(-\)1 dBm & n/a & n/a \\  & [13] & 100 MHz & 10 dBm & n/a & n/a \\  & [14] & 100 MHz & \(>\)1 dBm & n/a & 0.16 ps \\  & [15] & 100 MHz & 24 dBm & n/a & n/a \\  & [16] & 200 MHz & 12 dBm & n/a & n/a \\  & [17] & 4 kHz & 5 dBm & n/a & n/a \\  & [18] & 100 MHz & 12 dBm & 6 to 13 & n/a \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Performance comparison of various eye diagram monitoring methods. Figure 7: The measured eye diagrams and time jitter histograms when the optical signals under test from an optical transmitter (HUAWEI-2.5 GHz–80 km) have different powers of (**a**) \(-\)10 dBm, (**b**) \(-\)17 dBm, and (**c**) \(-\)22 dBm. (**d**) The measured eye diagram by the commercial real-time oscilloscope. diagram monitoring and a dynamic range of about 12 dB was achieved for a 2.5-Gb/s optical signal. The results were also verified by a commercial real-time oscilloscope. The successful demonstration of the eye diagram monitoring method based on optical sampling provides a simple and real-time way to realize the eye diagram monitoring and time-domain parameters extraction of an optical signal, which is of fundamental importance for optical communications and cloud computations. Conceptualization, Z.F. and Q.Q. ; methodology, J.S. ; software, T.H. ; validation, Z.F., J.S. and Q.Q. ; formal analysis, T.H. ; investigation, T.H. ; resources, Z.F. and Q.Q. ; data curation, T.H. ; writing--original draft preparation, T.H. and J.S. ; writing--review and editing, Z.F. and Q.Q. ; supervision, Z.F. and Q.Q. ; project administration, Z.F. and Q.Q. ; funding acquisition, Z.F. and Q.Q. All authors have read and agreed to the published version of the manuscript. This research was funded by the support of the National Natural Science Foundation of China (No. 62201120, No. 61971110), the Research Foundation (No. Y030212059003044), and the National Natural Science Foundation of Sichuan Province. Not applicable. 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. The authors would like to acknowledge the support of the National Natural Science Foundation of China, the Research Foundation, and the National Natural Science Foundation of Sichuan Province.