## 3 Results and Discussion
With an increase in frequency, the voltage amplitudes of microwave signals slowly decreased, which was caused by the decrease in the gain of the PD. In addition, slight amplitudes fluctuations at different frequencies were observed with same trend, which originated from the power spectrum fluctuation of the mode-locked laser. Furthermore, as shown in Figure 2, the microwave pulse duration is mainly determined by the interference area in the time domain. Considering the increased time of the PD and the nonlinear effect of the DCF, the pulse duration is on the order of ten picoseconds to hundreds of nanoseconds and can be designed by controlling the length of the DCF. Owing to the fact that the maximum difference of the pulse duration at different frequencies was only 600 ps, the pulse duration of the microwave waveform was regarded as approximately 24 ns in our experiment. Figure 4: (**a**) The measured electrical spectrum of pulsed microwave signal at 10.6 GHz. (**b**) A partial enlargement. (**c**) The linewidth and (**d**) the single-sideband phase noise of microwave frequency line at 10.592 GHz. Furthermore, in order to verify the frequency stability of the generated microwave signals, the pulse trains were measured at different frequencies. Limited by the calculation capabilities of computers, these trains that contained about 220 pulses were analyzed using short-time Fourier transform (STFT) and are shown in Figure 6a. The results showed that a good consistency of microwave frequencies was maintained in each pulse train. Since it is mainly affected by the optical path difference in the unbalanced single-arm interferometer, the frequency stability of the microwave pulse train could be further improved for a long time by enhancing the structural stability of the GDS. Additionally, some special waveforms, such as triangular-, rectangle- and square-shaped, can be decomposed into a series of sine waveforms via Fourier series analysis, which means that the generated microwave pulses with a single frequency should have the ability to synthesize complex waveform pulse. Therefore, the synthesis of a near-triangular-shaped microwave pulse was carried out in our experiment, being achieved by the superpositioning of two sinusoidal pulses with a frequency ratio of 1:3 and an amplitude ratio of 9:1. The temporal waveform of the near-triangular-shaped microwave signal with frequency of about 4.5 GHz is shown in Figure 6b. Through fast Fourier transform (FFT) analysis, the frequency component and power difference were obtained and are shown in Figure 6c. They are consistent with our design parameters. However, it is worth mentioning that a slight distortion of the synthesized signal (in Figure 6b), caused by the multiple frequency lines near 4.5 GHz and 13.5 GHz, was discovered. This kind of distortion could be eliminated by using an electrical bandpass filter to extract the single frequency line as a spectrally pure microwave, which we will implement in future research. Figure 5: (**a**) The overall waveforms of measured microwave pulse at different microwave frequencies. (**b**) A partial enlargement. Figure 6: (**a**) STFT analysis of the measured microwave pulse train at different frequencies. (**b**) The temporal waveform and (**c**) FFT analysis of the near-triangular-shaped microwave pulse signal.