# Measurement of Ultra-High Speed by Optical Multistage Cascade Frequency Reduction Technology
## 3 Discussion and Results  
### Simulation  
Virtual oscilloscopes 1 and 2 record signals generated by mixing signal light with reference lasers 1 and 2, respectively. Figure 5 shows the signals captured in two virtual  
Figure 4: Frequency spectrum of mixed signal generated by leakage light and output light of (**a**) reference laser 1 and (**b**) reference laser 2.  
oscilloscopes when the tuned laser wavelength is 1550.192 nm. Clear interference fringe signals can be seen from the virtual oscilloscope, in which the period of the signal in the first oscilloscope is 0.129 ns, and the corresponding signal frequency is 7.74 GHz, which is \(-0.06\) GHz different from the design value of 8 GHz, as shown in Figure 5a. The period of the signal in the second oscilloscope is 0.046 ns, and the corresponding signal frequency is 21.7 GHz, which is \(-2.3\) GHz different from the design value of 24 GHz, as shown in Figure 5b. Through the virtual radio frequency spectrometer set at the back end of the photoelectric conversion module, the spectrum of interference signals can be further analyzed. Virtual RF spectrometers 1 and 2, respectively, record the frequency spectrum of the signal generated by mixing the signal light with tunable lasers 1 and 2, which correspond to the first-stage frequency conversion spectrum and the second-stage frequency conversion spectrum, respectively. Figure 6a,b show the first-stage frequency conversion spectrum recorded by virtual RF spectrometer 1 when the tuned laser wavelengths are 1550.176 nm and 1550.048 nm, respectively. Three characteristic peaks can be observed in both spectra: (1) The first characteristic peak is located at about 8 GHz (point B in Figure 6a and point C in Figure 6b), which is formed by interference between leakage light and output light of reference laser 1, and its position does not alter during frequency conversion, which is a fundamental frequency signal. (2) The second characteristic peak is formed by interference between the signal light and the output light of reference laser 1, and it corresponds to point A in Figure 6a,b. The peak has higher intensity than other peaks and is a normal signal (frequency conversion signal) that can be expected to be obtained. (3) The third characteristic peak is formed by the interference of signal light and leakage light, corresponding to point C in Figure 6a and point B in Figure 6b, and the position of the peak moves with the shift of light wavelength, which is a ghost signal and is linear. Figure 6c shows the second-stage frequency conversion spectrum recorded by virtual RF spectrometer 2 when the tuned laser wavelengths are 1550.048 nm, respectively. The spectrum of \"three characteristic peaks\" is similar to those in Figure 6a,b, in which point A is the characteristic peak of fundamental frequency signal, which is 23.92 GHz; and point B is the characteristic peak of ghost signal, which is 17.97 GHz; and point C is the characteristic peak of frequency conversion signal, which is 5.96 GHz. Figure 5: Interference signals in (**a**) virtual oscilloscope 1 and (**b**) virtual oscilloscope 2. By reading the frequency spectrum of each sweep frequency point and drawing the intensity map, the two-dimensional frequency spectrum of the frequency conversion signal can be generated. The wavelength of the tuned laser is 10 points at equal intervals between 1550.256 nm and 1549.936 nm, and the two-dimensional spectrum diagram drawn is shown in Figure 7. Figure 7a is generated by the spectrum collected by virtual RF spectrometer 1, which is the first-stage frequency conversion spectrum. Figure 7b is generated by the spectrum collected by virtual RF spectrometer 2, which is the second-stage frequency conversion spectrum. Figure 6: (**a**) First-stage frequency conversion spectrum (tuned laser wavelength is 1550.176 nm); (**b**) first-stage frequency conversion spectrum (tuned laser wavelength is 1550.048 nm); (**c**) frequency spectrum of the second stage (tuned laser wavelength is 1550.048 nm).