Fig. 1. Optical sampling oscilloscope prototype. (a) Schematic diagram; (b) internal structure diagram; (c) physical appearance diagram

Fig. 2. Schematic diagram of asynchronous delayed optical sampling software synchronous eye diagram reconstruction^[1-2]

Fig. 3. Accurate synchronization algorithm based on ZoomFFT. (a) Eye diagram of 9.18 Gbit/s OOK signal measured by 50 GSa/s high-performance oscilloscope; (b) distribution of 7168 downsampling data points

Fig. 4. Schematic diagram of ZoomFFT software synchronization

Fig. 5. Software coarse synchronization. (a) Amplitude spectrum of the Fourier transform of yi; (b) eye diagram of downsampling data points recovered from equivalent sampling time for coarse synchronization

Fig. 6. Fine synchronization signal processing based on ZoomFFT transformation. (a) Fine amplitude spectrum obtained by ZoomFFT transformation of yi; (b) signal eye diagram based on ZoomFFT fine synchronization

Fig. 7. Structure of Optical PAM4 signal transmitter

Fig. 8. Comparison of oversampling and downsampling eye diagrams, the symbol rate of PAM4 signal is 6.259 GBaud. (a) Sampling rate of 50 GSa/s; (b) downsampling

Fig. 9. Comparison of oversampling and downsampling eye diagrams, the symbol rate of PAM4 signal is 9.696 GBaud. (a) Sampling rate of 50 GSa/s; (b) downsampling

Fig. 10. Structure of optical QPSK signal transmitter

Fig. 11. Eye diagrams of optical QPSK signals with different symbol rates. (a) 10 Gbaud; (b) 20 Gbaud

Fig. 12. Eye diagrams and constellation diagrams of QPSK signals with different symbol rates measured by optical sampling oscilloscope prototype. (a) 10 Gbaud; (b) 20 Gbaud

Fig. 13. Schematic of effect of background noise on optical sampling oscilloscope

Fig. 14. Variation curve of Q value with input optical power