## 3 Experiment and Results
When the 1/2 subharmonic microwave signal failed to be applied to PM, the spur suppression ratio was only 27 dB with a large 3-dB bandwidth of 12.5 MHz, as shown in Figure 8. The stability of the generated microwave signals between the OEO with subharmonic microwave modulation and the OEO without subharmonic microwave modulation were illustrated in Figure 9. It could be seen that the generated microwave signal of the OEO with subharmonic microwave modulation showed high stability for more than 30 min. Thus, the frequency stability, SMSR and spectral purity of the generated signal were effectively improved by subharmonic microwave modulation. Figure 6: Experimental results of the 15 GHz microwave signal generated by three different kinds of OEOs when the span of ESA is 2 MHz. Figure 7: Experimental results of the 15 GHz microwave signal generated by dual-loop OEO when the span of ESA is 1 GHz. As can be seen in Figure 10, we made a comparison between the three kinds of OEOs to further analyze the signal's phase noise at 15 GHz. The generated 15 GHz microwave signal's phase noise in the open-loop system was \(-103.9\) dBc/Hz at 10 kHz frequency offset, whereas those of the single- and dual-loop systems were \(-108.7\) and \(-117.6\) dBc/Hz, respectively. The phase noise of the dual-loop structure was 13.7 dB lower than that in the open-loop system and 8.9 dB lower than that in the single-loop OEO. According to the experimental results shown in Figure 10, the phase noise performance was greatly optimized because of the dual loop with feedback. The phase noises of the microwave signals with the frequencies of 14, 15 and 16 GHz were also probed, as can be seen in Figure 11. The phase noises of the proposed dual-loop OEO were \(-114.8\), \(-117.6\) and \(-119.5\) dBc/Hz at 10 kHz frequency offset for different frequencies. At the same time, the phase noise was lower than \(-81\) dBc/Hz when the frequency offset was 1 kHz. Figure 8: Experimental results of the generated 15.31 GHz signal without subharmonic microwave modulation when the span of ESA is 1 GHz. Figure 9: The frequency stability of the generated microwave signals for more than 30 min. According to the nonlinear dynamic theory of semiconductor lasers, an FP-LD was used as the SL. A DFB laser and a tunable laser source (NKT E15) were individually used as ML1 and ML2. Then, a dual-frequency dual-loop OEO was realized. When the frequencies of 1/2 or 1/3 subharmonic microwave signals applied to the PM were tuned to 5.5, 5.75 and 6 GHz, the wavelength and output power of ML1 and ML2 were fixed. The wavelength difference between ML1 and ML2 was approximately 1 nm so that the frequency difference between the two lasers could be much greater than the bandwidth of the BPD. The output wavelength and power of the free-running FP-LD were fixed by controlling the laser driver. The optical power of light injected into the SL could be tuned by adjusting the PCs in the OEO link. As can be seen in Figure 12, when the frequency of the subharmonic microwave signal was 6 GHz, the two frequencies of the microwave signals generated from the proposed dual-frequency dual-loop OEO could be tuned up to 12 and 18 GHz. They were also restricted by the maximum frequencies of the amplifier, electro-optic modulator and photodetector. Figure 2:0 Measured phase noise of the microwave signals with different frequencies generated by the dual-loop OEO. Figure 2:1 Measured phase noise of the 15 GHz microwave signals generated by three types of structures.