## 3 Experiment and Results  
An experiment based on the proposed scheme as shown in Figure 1 was displayed to verify the proposed approach. First, the performance of the dual-loop OEO with a single frequency was demonstrated. The experimental setup was shown in Figure 3. The ML in the OEO loop was a tunable laser source (NKT E15). The power of light generated by the ML was set as 13 dBm, and the wavelength was set as 1549.505 nm. The optical signal from the ML was transmitted to a PM (EOSPACE) with 3-dB bandwidth of 40 GHz to be modulated. Then, the modulated optical signals were injected into the DFB laser with a free-running power of 10.5 dBm through the PC and CIR. Then, the optical signals from the SL were modulated by the DOMZM with 3-dB bandwidth of 20 GHz to construct the OEO loop. In the OEO cavity, the fiber length of the long-loop was approximately 10 km when using a spool of SME, and that of the short-loop was approximately 10 m. The BPD (U2T) in the OEO loop with a 3-dB bandwidth of 43 GHz was utilized to realize the optical-to-electrical conversion as well as the Vernier effect to improve SMSR. The EA with an operational bandwidth from 0.3 to 18 GHz and a gain of more than 34 dB was used for signal amplification. The EBPF with an operational bandwidth from 10 to 18 GHz and an insertion loss of less than 2 dB was utilized to filter the oscillation signal. Finally, the generated signals were analyzed using the ESA and SSA. The frequencies of the generated microwave signals could be modified by changing the detuning frequency \(\Delta f\) when \(R\) was fixed. In the experiment, \(\Delta f\) was adjusted by changing the free-running wavelength of the SL when the wavelength of the light output from the ML was fixed. The free-running wavelength of the SL could be changed from 1549.580 to 1549.615 nm by adjusting the semiconductor laser driver. In this way, the detuning frequency \(\Delta f\) could be tuned from 9.375 to 13.75 GHz. To demonstrate the performance of the proposed OEO structure, we studied the generated microwave signal's spectrum, SMSR and phase noise. As shown in Figure 4, when the detuning frequency \(\Delta f\) was modified from 9.375 to 13.75 GHz and the frequency of the 1/2 subharmonic microwave signal was adjusted from 6.25 to 8.75 GHz, the generated signal would be tuned from 12.5 to 17.5 GHz. In addition, the zoom-in spectra of the generated 14, 15 and 16 GHz microwave signals are displayed in Figure 5, at the time when the span of ESA was set as 4 GHz. It is important to note that the tunable range of the generated signal was mainly restricted by the operating frequencies of the amplifier, electro-optic modulator and photodetector. Thus,  
Figure 2: Operating principle of the optically injected semiconductor laser. part of this paper. When the generated signal's frequency was 15 GHz, the subharmonic microwave signal's frequency was 7.5 GHz, with the phase noise of \(-116\) dBc/Hz at 10 kHz frequency offset and \(-87\) dBc/Hz at 1 kHz frequency offset. The SMSR of the signals generated by three different kinds of structures were investigated. The generated signal spectra with a span of 2 MHz were shown in Figure 6. The open-loop structure was constituted when there was no feedback in the dual-loop OEO. The SMSR of the open-loop system was 47 dB. The fiber length in the single-loop structure was approximately 10 m, which comprised the pigtail fibers of the optic-electronic devices employed in the system. The SMSR was 50 dB in the single-loop OEO, which was 3 dB lower than that in the open-loop system. The SMSR of the proposed dual-loop OEO was 55 dB, demonstrating an increase of 5 dB and 8 dB compared with that of the single-loop and open-loop system, respectively. Furthermore, the experimental result of the measured 15 GHz microwave power spectrum with a span of 1 GHz, as shown in Figure 7, had a spur suppression ratio of 68 dB and a 3-dB bandwidth of \(<\)100 kHz. The generated 15 GHz signal could almost be considered as a single-mode microwave signal.