# Coupled Dual-Loop Optoelectronic Oscillator Based on Stimulated Brillouin Scattering
## 3 Experiment and Results  
An experiment based on the setup shown in Figure 1a was performed. A distributed feedback laser diode (DFB-LD) acted as the signal laser (SL). The SL can emit continuous-wave light with 1549.920 nm, and the power is less than 20 dBm. A tunable laser (NKT Photonics Koheras AdjustiK) served as the pump laser (PL). The PL can emit continuous-wave light range from 1549.380 to 1550.475 nm. The tuning step of the PL was 0.001 nm and the power of the PL was less than 17 dBm. The DOMZM and MZM had 3-dB bandwidth of 20 GHz. The length of SMF was 1 km and the HNLF was 3 km. The PD1 had a 3-dB bandwidth of 18 GHz. The PD2 had a 3-dB bandwidth of 40 GHz. The EA1 and EA2 had 3-dB bandwidth of 18 GHz and gain of 30 dB. To verify the performance of the proposed TOEO scheme, we did detailed research on the microwave power spectrum, phase noise and stability. As shown in Figure 1a, when the Loop1 was connected and the Loop2 was disconnected in the blue box, there was a single loop SBS-based TOEO, which was able to freely run. The generated 10 GHz microwave power spectrum of the single loop SBS-based OEO with 3 km Loop1 is shown in Figure 2; the SMSR was 3 dB and mode interval was 63.3 kHz. When Loop1 and Loop2 were both connected, there is a coupled dual-loop SBS-based TOEO, which combines the CDL and SBS. The generated 10 GHz microwave power spectrum of the coupled SBS-based TOEO with 3 km Loop1 and 1 km Loop2 is shown in Figure 3; the SMSR was 60 dB and mode interval was 63.3 kHz. Compared with the single loop SBS-based TOEO, the SMSR of the coupled dual-loop SBS-based TOEO had been improved by 57 dB. The measured 10 GHz microwave power spectrum of the coupled SBS-based TOEO with span of 100 MHz is shown in Figure 4. The 10 GHz signal generated almost realized single mode operation. The frequency tunability of the coupled SBS-based TOEO was then investigated by tuning the pump wave length of the pump laser. Figure 5 shows the superimposed spectrum of the generated signal with tunable frequency range from 2 to 18 GHz with a tuning step of 1 GHz. To evaluate the phase noise performance, a comparison between the single loop SBS-based TOEO and the coupled dual-loop SBS-based TOEO was made. As shown in Figure 6, the 10 GHz microwave signal phase noise of the single loop SBS-based TOEO was \(-\)79.8 dBc/Hz at 10 kHz frequency offset. The 10 GHz microwave signal phase noise of the coupled dual-loop SBS-based TOEO was \(-\)95.2 dBc/Hz at 10 kHz frequency offset, which was 15.4 dB lower than that of the single loop SBS-based TOEO. The phase noise performance is sufficiently improved due to the use of the coupled dual-loop. As shown  
Figure 3: The generated 10 GHz microwave power spectrum of the coupled dual-loop SBS-based TOEO. Figure 2: The generated 10 GHz microwave power spectrum of the single loop SBS-based TOEO. in Figure 7, when the oscillation frequency of the coupled dual-loop SBS-based TOEO was chosen as 5, 10, or 15 GHz, the phase noise was as low as \(-95\) dBc/Hz at 10 kHz frequency offset. Figure 4: The generated 10 GHz microwave power spectrum of the coupled dual-loop SBS-based TOEO with span of 100 MHz. Figure 5: The generated signal spectrum with tunable frequency range from 2 GHz to 18 GHz. Figure 6: The 10 GHz microwave signal phase noise comparison between single loop SBS-based TOEO and the coupled dual-loop SBS-based TOEO. The stability of the coupled dual-loop SBS-based TOEO was also investigated. Within the time duration of 30 min, the electrical spectrum of the generated signals were measured every 1 min. Figure 8 shows that the frequency drift at 10 GHz was within 0.3 ppm and the power drifts at 10 GHz was within 0.2 dB during 30 min. It can be seen from above experimental results that the coupled dual-loop plays an important role in the performance improvement of SBS-based TOEO. The coupled dual-loop SBS-based TOEO shows great improvement in side mode, phase noise and stability over the single-loop SBS-based TOEO.