# Microwave Photonic Frequency Conversion Based on a Wavelength Swept Laser
## 3 Experimental Results and Discussion
Figure 6: **(a)** The optical spectrum of the WSL; (**b**) the time domain signal of the WSL beating signal after the PD (the inset is the zoom-in of the pulse shape from 0.349 ns to 0.362 ns). the function of down-conversion. Then, the chirp rate was switched to be positive and the length of the SMF was tuned. The up-conversion electrical spectrum is shown in Figure 8.  
As the length of the SMF increased from 0 km to 8.11 km, the center frequency of the up-conversion spectrum moved from 5 GHz to 5.577 GHz, and there was also a certain distortion in the spectrum when the length of the SMF was long. The amplitudes of the right side-lobes were lower than those of the left side-lobes, and the amplitude difference became larger when the SMF was longer. The distortion of the spectrum may have been caused by the jitters on the rectangular pulse waveform, as shown in Figure 6b. By switching the sign of the WSL chirp rate \(r\) to a positive value, the experiment realized the conversion of the frequency converter from the down-conversion function to the up-conversion function, which proved the effectiveness of the method. The comparison of the down-conversion and the up-conversion frequencies between the simulations and the experiment results is shown in Table 1.  
Figure 8: **(a–i)** The electrical spectrum of the up conversion when the SMF was 0 km, 1.04 km, 2.07 km, 3.11 km, 4.17 km, 5 km, 6.04 km, 7.07 km and 8.11 km. Figure 7: **(a)** The waveform of the pulse when the length of SMF was 0 km and 1.04 km; **(b)** the electrical spectrum of the frequency conversion system. We can see that the experiment results of the down-conversion and up-conversion for the 5 GHz input signal consist well with the simulations. The up-conversion performance of the proposed system for a higher input frequency is shown in Figure 9.  
The up-conversion spectra for the 8 GHz and 16 GHz signals were similar to the spectra for the 5 GHz signal. The spectrum distortion was more serious when the frequency was higher. For higher input frequencies, the spectrum separation between the different conversion efficiencies was higher. The bandwidth of the main-lobe remained the same when the input frequency or the length of the SMF was different, for it only depended on the pulse width of the WSL. Thus, if we use a WSL with a high duty cycle, the bandwidth of the conversion spectrum will be further suppressed. The tunability results of the up-conversion system for the 5 GHz, 8 GHz and 16 GHZ input signals are summarized in Table 2.  
Through the above experiments, we can see that the microwave photonic frequency converter was based on the WSL up-conversion and down-conversion. The tuning of the conversion frequency was achieved by adjusting the chirp rate \(r\) of the WSL and the dispersion of the DCE, and the switching of the up-down conversion function of the frequency converter system was achieved by changing the sign of the WSL chirp rate. The above experiments also proved the feasibility of the program. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Conversion** & **Input Frequency** & \
\begin{tabular}{c} **Output Frequency** \\ **(Simulation)** \\ \end{tabular} &
\begin{tabular}{c} **Output Frequency** \\ **(Experiment)** \\ \end{tabular} \\ \hline Down & 5 GHz & 4.935 GHz & 4.936 GHz \\ Up & 5 GHz & 5.067 GHz & 5.067 GHz \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The comparison of the down-conversion and up-conversion frequencies between the simulations and the experiment results. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**SMF Length** & \begin{tabular}{c} **Up-Conversion for 5** \\ **GHz Input** \\ \end{tabular} & \
\begin{tabular}{c} **Up-Conversion for 8** \\ **GHz Input** \\ \end{tabular} &
\begin{tabular}{c} **Up-Conversion for 16** \\ **GHz Input** \\ \end{tabular} \\ \hline
1.04 km & 5.067 GHz & 8.094 GHz & 16.208 GHz \\
3.11 km & 5.207 GHz & 8.369 GHz & 16.629 GHz \\ \hline \hline \end{tabular}
\end{table}