# Real-Time Eye Diagram Monitoring for Optical Signals Based on Optical Sampling
## 3 Results and Discussion  
A proof-of-concept experiment was performed based on the setup shown in Figure 1. The MZM (CETC-40) has a 3-dB bandwidth of about 40 GHz. The PD (Kang Guan KG-PD-10G) has a 3-dB bandwidth of around 10 GHz and a photoresponsivity of 0.8 A/W. The ADC has a sampling rate of 1.5 Gsps, a 10-bit quantization resolution, and a bandwidth of 4 GHz. A pulse having a 1.5-GHz repetition rate and a duty ratio of 1:8 is generated from an EPG (Anritsu MU183021A). When a continuous lightwave with an optical power of 0 dBm is injected into the MZM, the sampling pulse after photodetection is measured in Figures 5a and 5b by a real-time oscilloscope (Teledyne Lecroy SDA 820 Zi-B). The broadened sampling pulse is measured using the same method in Figure 5b. To observe the pulse width and the period of the optical pulses, the measured signals in Figures 5a and 5b are re-plotted in Figures 5c and 5d, respectively, with the recording time ranging from 0 to 4 ns. It is seen that the sampling pulse width is broadened from 83.5 to 227 ps, which matches the trace-and-hold capability of the ADC. The period of the optical pulse stays the same at 666.7 ps, corresponding to the sampling rate of 1.5 GHz. By synchronizing the EPG and the ADC with a 1.5-GHz clock, the ADC trace-and-hold window is located at the flat top of the broadened sampled pulse. Then, the broadened sampled pulse is quantized with high quantizing efficiency by the ADC with a 1.5-GHz repetition rate. Considering the 1.5-GHz sampling frequency and the ~500,000 recording points for the eye diagram recovery, the recording time for the eye diagram is about 350 us, which means real-time eye diagram monitoring can be implemented. Figure 6 shows the measured eye diagrams and time-domain parameters, such as period, time jitter, \(Q\) value, and BER, of the optical signal generated from a commercial optical transmitter (HUAWEI-25 GHz-40 km). When the optical signals under test have different powers of \(-8\), \(-14\), and \(-20\) dBm, the eye diagrams and the corresponding time-domain parameters are measured in Figures 6a, 6b and 6c, respectively. It is seen that the periods of the optical signals with different powers are measured the same as 401.8 ps, corresponding to the same rate of about 2.488 GHz. The time jitter histograms are also extracted according to the measured eye diagrams. As can be seen, the root means square (RMS) jitters are measured to be 8.4, 9.1, and 30.5 ps when the optical signals have different powers of \(-8\), \(-14\), and \(-20\) dBm. Furthermore, the Q values and the BER are also obtained according to the measured eye diagrams and Equations (2) and (3). When the optical signals have different powers of \(-8\), \(-14\), and \(-20\) dBm, the \(Q\) values are measured to be 19.1, 12.8, and 4.1, corresponding to different BERs of 1.3 \(\times\)\(10^{-81}\), 1.4 \(\times\)\(10^{-37}\), and 2.4 \(\times\)\(10^{-5}\), respectively. In addition, the measured results by the proposed eye diagram monitoring systems are verified by the measured eye diagram in Figure 6d from a commercial real-time oscilloscope. It is seen that the period of the optical signal is measured to be 401.8 ps, the same as that measured by the proposed eye diagram monitoring system. Figure 5: The measured optical pulse train by a real-time oscilloscope. (**a**) The measured pulse train after the MZM with a recording time of 200 ns; (**b**) The measured broadened pulse train before the PD with a recording time of 200 ns; (**c**) The measured pulse train after the MZM with the recording time range from 0 to 4 ns; (**d**) The measured broadened pulse train before the PD with the recording time range from 0 to 4 ns.