# Measurement of Ultra-High Speed by Optical Multistage Cascade Frequency Reduction Technology
## 2 Principle
### Simulation Verification  
Using OptiSystem 15.0 software, the optical path shown in Figure 2 is designed and optimized in further detail. The optical path structure is established in the software. The optical path is the same as the two-stage optical cascade frequency reduction optical path introduced in the principle analysis, and it is designed based on the premise that the data acquisition system (oscilloscope) has 8 GHz bandwidth frequency response. According to the principle of two-stage optical cascade frequency reduction analyzed above, the central wavelengths of three lasers are set at intervals with the bandwidth of data recording system as the scale, and the linewidth and power of lasers are defined, as shown in Table 1.  
In the simulation, the circulator isolation has a great influence on the interference signal quality, so it is focused on its evaluation. Because the isolation between ports 1 and 3 of the circulator is not ideal (infinite), a limited amount of light energy output by the main laser will directly leak into the photodetector through the path of ports 1-3. The beam light and the signal light will be mixed to generate an additional \"ghosting\" signal, which will affect the extraction of spectral features. Under the current process conditions, the typical isolation between ports 1 and 3 of the circulator is 55-60 dB. When 55 dB is selected as the measured value, the gain of the photoelectric amplification module is controlled in the range of 15-30 dB (with 10 gain points taken at equal intervals), and the leakage optical power curve input to the photoelectric conversion module is obtained, as shown in Figure 3.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Name** & **Wavelength/nm** & **Line Width/kHz** & **Power/mW** &
\begin{tabular}{c} **Relative Intensity** \\ **Noise/dB/Hz** \\ \end{tabular} \\ \hline Main Laser & 1550.192 & 100 & 500 & \(-\)130 \\ Reference Laser 1 & 1550.128 & 100 & 50 & \(-\)130 \\ Reference Laser 2 & 1550.000 & 100 & 50 & \(-\)130 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameter setting of each laser in simulation.  
Figure 3: Leakage optical power input to photoelectric conversion module under different gains.  
It can be seen from the simulation results that the maximum leakage optical power is about 360 nW. By setting a virtual RF spectrometer at the back end of the photoelectric conversion module, the signal spectrum generated by mixing leakage light with reference lasers 1 and 2 can be analyzed, and the results are shown in Figure 4. Figure 4a shows the frequency spectrum of the mixed signal generated by the leakage light and the output light of reference laser 1. It can be seen that there is a characteristic peak at 7.96 GHz, which is virtually consistent with the frequency difference (8 GHz) between the two lasers. Figure 4b shows the frequency spectrum of the mixed signal generated by the leakage light and the output light of reference laser 2. It can be seen that there is a characteristic peak at 23.90 GHz, which is approximately consistent with the frequency difference (24 GHz) between the two lasers.  
In order to additionally evaluate the quality of interference signal, a tuned laser is set at the probe end of the optical path, and the frequency change of its output light (signal light) simulates the Doppler frequency shift caused by the velocity change in the measured target. In the simulation, the tuning laser power is set to 50 uW, and the frequency sweeps 10 points in turn. Through the virtual oscilloscope set at the back end of the photodetector, the virtual interference signal can be obtained by simulation.