## 3 Results and Discussion  
Semiconductor lasers are extremely sensitive to external optical feedback [22] and this technique has been proven to be an effective means of stabilizing SMLLs. Like single-loop optical feedback, dual-loop optical feedback configurations have a significant effect on the stabilization of SMLLs [28; 31; 32; 33; 34; 35; 36]. The major advantage of adding a feedback cavity to single-loop feedback provides additional control on parameters including optical delay line, variable optical attenuator and polarization controller. These parameters have a significant influence on the stability of QDash MLLs including suppression noise-induced oscillations subject to delayed optical feedback. Effect of Coupling Strengths and Delay Phase Settings on the Suppression of Cavity Induced Frequency-Fluctuations  
High performance of SMLLs including low timing jitter and narrow RF linewidth can be achieved using long fiber spool in a feedback loop. With low phase-noise, such feedback configuration induces narrowly spaced noise-oscillations close to the fundamental mode-locked frequency. Frequency spacing around the target frequency depends on the length of external feedback loops. In the following, we discussed the effects of two chosen feedback strengths (Loop-I: \(-\)23.29 dB and Loop-II: \(-\)28.06 dB, Loop-I and Loop-II: \(-\)22 dB) and two optical delay phase settings (weaker cavity set to integer resonance, fine-tuning of stronger cavity and stronger cavity set to integer resonance, fine-tuning of weaker cavity) on the suppression of external cavity induced frequency-fluctuations. 1.1 Loop-I: \(\sim\)\(-\)23.29 dB and Loop-II: \(\sim\)\(-\)28.06 dB (Fine-Tuning of Stronger Cavity and Set Weaker Cavity to Fully Resonance)  
First, higher coupling strength was fixed through loop-I (\(\sim\)\(-\)23.29 dB) and lower feedback ratio through loop-II (\(\sim\)\(-\)28.06 dB). The ODL-II attached with loop-II was adjusted to fully resonance and ODL-I(c) was fine-tuned over maximum available delay range, 0-84 ps. Measured side-band suppression versus microscopic fine-tunable delay (0-84 ps) is shown in Figure 3 (black squares). A maximum 13 dB suppression in first-order side-band was achieved when the mode of loop-I coincides precisely with that of loop-II (at optical delay phase setting 28 ps). With this selected combination of feedback ratio, the SMSR varies in between 0 dB to 13 dB from delay range (0-28 ps). However, when the delay was tuned from 28 to 49 ps, no SMSR was achieved because the laser was unstable between these delay ranges and no significant suppression in cavity side-band occurred. Figure 2: Schematic of the experimental arrangement for dual-loop configurations. _Acronyms_—OC: Optical circulator; SOA: Semiconductor Optical Amplifier; ISO: Optical isolator; PD: Photodiode; RF Amp. : RF amplifier; ODL: Optical delay line; Att: Variable optical attenuator; PC: Polarization controller; ESA: Electric spectrum analyzer; OSA: Optical spectrum analyzer; SMF: Single mode fibre; PM: Power Meter; QDash MLL: Quantum dash mode-locked laser. 3.1.2. Loop-I: \(\sim\)\(-\)23.29 dB and Loop-II: \(\sim\)\(-\)28.06 dB (Fine-Tuning of Weaker Cavity and Set Stronger Cavity to Fully Resonance)  
The SMSR was further investigated for feedback coupling ratio \(\sim\)\(-\)23.29 dB through loop-I and \(\sim\)\(-\)28.06 dB through loop-II. Besides ODL-I was adjusted to integer resonance and ODL-II was fine-tuned. Measured side-band suppression versus microscopic fine-tunable delay (0-84 ps) is shown in Figure 3 (blue squares). It can be seen that under integer resonance a maximum 22 dB side-mode suppression was achieved (at optical delay phase setting \(\sim\)7 ps) which was \(\sim\)2\(\times\) higher than the previous combination. Furthermore, on maximum available delay range (0-84 ps), a better side-band suppression was observed.