## 4 Key Parameters Desired to Optimize External Cavity Induced Side-Bands  
Measured experimental results demonstrates that side-band suppression depends on the three key parameters: lengths of both feedback loops, coupling strengths in each cavity, and optical delay phase settings attached within each loop. A theoretical analysis was conducted on the dual-loop optoelectronic oscillator and it was found that characteristics and performance depend on parameters including coupling strength ratio and delay phase setting [38]. The parameters mentioned above are discussed below. Loop-Length: The suppression of external cavity induced frequency-fluctuations depends on the length of second feedback delay-time relative to first. By simply choosing the second loop length equal to half the length of the first loop, suppress first harmonics of frequency while second harmonics remained unsuppressed. When the second delay time was resonant with the third harmonics of the first loop, two harmonics suppressed while third remain unsuppressed and appeared as modal overlap [34]. In [34], dual-loop feedback with large (8\(\times\)) disparity in loop lengths was introduced gives significant suppression in noise-oscillations and produces flat RF spectra close to the main peak. Therefore, for better suppression of external cavity side-bands, a precise length of the second delay time is highly desirable. Most recently, the suppression of noise-induced modulations in oscillatory systems using an additional feedback term was theoretically investigated [37]. Power split ratio: Dual-loop optical feedback with the power split ratio has a significant influence on the suppression of external cavity induced side-bands. The different coupling strengths via single- and dual-loop optical feedback influence the RF linewidth of our device under investigation [36]. Measured experimental results in Section 3.1 demonstrates that side-band suppression depends on the power split ratio through each external feedback loop. Better side-mode suppression can be achieved when equal coupling strength (\(\sim\!-22\) dB:\(\sim\!-22\) dB) passes through both feedback loops relative to other chosen combinations of feedback ratios and delay settings (\(\sim\!-23.29\) dB(c):\(\sim\!-28.06\) dB and \(\sim\!-23.29\) dB:\(\sim\!-28.06\) dB(c)). Unbalanced (different percentage of power split ratio) symmetric dual-loop was studied to achieve optimum stabilization in SML QDash laser [36]. It was also observed that balanced (equal percentage of feedback strength) symmetric dual-loop optical feedback leads QDash MLLs towards destabilization [32]. However, in this contribution, we observed that better side-band suppression was achieved under a balanced feedback ratio through asymmetric (loop-I: 160 m and loop-II: 80 m) dual-loop optical feedback configuration. The first-factor influence side-band suppression due to overlap of frequency spacing of modes of loop-I with that of loop-II, at the 3 dB coupler. The second factor that caused the suppression is the coupling strength through each external feedback cavity. Delay phase-tuning: Similar to loop lengths and power split ratio, the optical delay phase setting is another key parameter by tuning which the characteristics and efficiency of the SMLLs could be efficiently controlled. Most recently, a comprehensive analysis based on different optical delay phase settings and power split ratio is presented [36]. Including timing stability, optical delay phase settings improve external cavity induced side-band suppression. Measured results indicate that upon tuning of ODL-I, the side-mode \begin{table}
\begin{tabular}{c c c} \hline \hline
**Coupling Strength** & **SMSR (i)** & **SMSR (0–84 ps)** \\ \hline \(-23.29\) (c):\(-28.06\) dB & 13 dB & \textgreater{}1–13 dB \\ \(-23.29\):\(-28.06\) (c) dB & 22 dB & \textgreater{}6–22 dB \\ \(-22\):\(-22\) dB & 33 dB & \textgreater{}13–33 dB \\ \hline \hline \end{tabular}
\end{table}