# Microwave Photonic Frequency Conversion Based on a Wavelength Swept Laser
## 3 Experimental Results and Discussion  
Based on the above simulations and discussions, we performed corresponding experiments to verify the performance of the frequency conversion system. Figure 5 shows the experimental setup of the proposed microwave photonic frequency conversion system. We built a wavelength swept laser based on positive and negative dispersion as the optical source [24] in the lab. The positive and negative dispersion components in the cavity were two chirped fiber Bragg gratings (CFBGs, Proximion) with opposite dispersion (\(\pm 1321\) ps/nm). After a polarization controller (PC) was used, the output of the WSL was launched into an intensity modulator (PHOTLINE, MX-LN-40, 40 GHz bandwidth). A microwave source (Hitting HMC-T2240) was used to provide microwave signal. An SMF (YOFC G625) array with a switchable length was used as the DCE. The modulated optical signal was launched into an SMF array as the switchable dispersion device. The dispersed signal was finally detected by a  
Figure 4: (**a**) The trend of \(\kappa\) as \(L\) and \(r\) changed; (**b**) the trend of \(\omega_{r}\) as \(\Delta\omega\) and \(r\) changed. Figure 3: (**a**) The down-conversion simulation spectra as \(\tau\) arose; (**b**) the up-conversion simulation spectra as \(\tau\) arose (the blue lines are the discrete longitudinal modes; the green curves are the envelopes of the _Sinc_ function). PD (Discoverysemi, DSC20H, 50 GHz bandwidth). The beating signals from the PD were measured using a high-speed oscilloscope (Tektronix, DSA72004B: the bandwidth was 36 GHz and the maximum sample rate was 80 GS/s) and an electrical spectrum analyzer (ESA, Keysight, N9020B, Santa Rosa, CA, USA). Figure 6a shows the optical spectrum of the WSL, measured by an optical spectrum analyzer (OSA, YOKOGAWA, AQ6370C, Tokyo, Japan). The center wavelength and bandwidth of the WSL were 1553 nm and 4.02 nm, respectively. Figure 6b shows the time domain WSL pulse train and the detailed waveform that was directly detected by the PD and the oscilloscope. The chirp rate of the WSL was \(\pm\) 99.6 GHz/ns and the repetition frequency was 6.7 MHz. The waveform of a single pulse was approximately rectangular, with a pulse width of about 5.284 ns, as shown in the insert figure. Due to the limitations of current laboratory conditions, the duty cycle was calculated to be 3.5%, which was much lower than the simulated single mode condition. In future works, an MLL with a large duty cycle needs to be researched in order to meet the requirements of the previous part of the system simulation. Figure 7 shows the time domain pulse waveform and the electrical spectrum of the frequency down-conversion system (\(r=-99.6\) GHz/ns), with a different SMF length when the input frequency was 5 GHz. After the modulation step, the pulse waveform was shaped by the microwave signal, as shown in Figure 7a. The pulse waveform when \(L\) was 1.04 km was expanded, compared with when \(L\) was 0 km. Due to the optical loss of the fiber and the expanding effect, the peak voltage of the pulse declined. The electrical spectrum of the frequency conversion system was discrete and the envelope had a main-lobe and several side-lobes, as shown in Figure 7b, which accorded well with the simulations. The center frequency of the down-converted spectrum was 4.936 GHz, and the conversion coefficient was calculated to be 0.987, which also accorded well with (7). It can be seen from Figure 7 that the input signal with a frequency of 5 GHz was successfully down-converted to 4.936 GHz, and the experimental results were in line with the expected results. At the same time, the experiment also proved the effectiveness of the proposed WSL-based microwave photonic frequency converter with  
Figure 5: The experimental setup of the proposed microwave photonic frequency conversion system (the red curve is the optical signal; the black curve is the electrical signal).