###### Abstract  
We experimentally demonstrated a power split ratio and optical delay phase dependent dual-loop optical feedback to investigate the suppression of frequency-fluctuations induced due to delayed optical feedback. The device under investigation is self-mode-locked (SML) two-section quantum-dash (QDash) laser operating at \(\sim\)21 GHz and emitting at \(\sim\)1.55 \(\upmu\)m. The effect of two selective combinations of power split ratios (Loop-I: \(-\)23.29 dB and Loop-II: \(-\)28.06 dB, and Loop-I and Loop-II: \(-\)22 dB) and two optical delay phase settings ((i) stronger cavity set to integer resonance and fine-tuning the weaker cavity, (ii) weaker cavity set to integer resonance and fine-tuning of stronger cavity) on the suppression of cavity side-bands have been studied. Measured experimental results demonstrate that delayed optical feedback induced frequency-fluctuations can be effectively suppressed on integer resonance as well as on full delay range tuning (0-84 ps) by adjusting coupling strength \(-\)22 dB through Loop-I and Loop-II, respectively. Our findings suggest that power split ratio and delays phase-dependent dual-loop optical feedback can be used to maximize the performance of semiconductor mode-locked lasers. s +
Footnote †: journal: Medical  
cal telecommunications [14, 15], coherent wavelength division multiplexing (CoWDM) [16], arbitrary waveform generation [17, 18], all-optical signal processing [19], data center applications [20] and millimeter-wave generation [21]. In 1980, [PERSON] and [PERSON] reported some aspects of the dynamics of a semiconductor lasers exposed to external optical feedback [22, 23]. Delayed optical feedback enhances stabilizing effects of SMLLs, thereby reduced the RF linewidth and corresponding integrated timing jitter [24, 25, 26]. Beside improving stabilization, delayed optical feedback induces cavity side-bands [27]. These cavity side-bands can be effectively suppressed by simply adding a second feedback loop (dual-loops optical feedback configurations) [28, 29, 30]. Dual-loops configuration can be considered as a simple, rugged and low-cost approach to eliminate frequency-fluctuations without optical/electrical conversion [31, 32, 33, 34, 35, 36]. However, dual-loop optical feedback due to the resonance of the modes of second delay time to that of first, limits optimal suppression of cavity induces side-bands [33]. Therefore, the prediction of precise delay times required to effectively suppress side-bands including modal overlaps (interference of modes of second loop wit first loop) is highly desirable. Most recently, we demonstrate an asymmetric dual-loop feedback method to suppress cavity induced side-bands due to delayed optical single and dual-loop feedback [34] and results were compared with recently published numerical simulation [37] which show excellent agreement. Besides the pulse train stability of two-section SML QDash lasers was investigated by optimizing delay phase settings and power split ratio through dual-loops feedback. It was observed that symmetric dual-loop with different coupling strengths and fine-tuning of weaker cavity: far more effective than symmetric dual-loop with equal power splitting, as it reduce the timing jitter over a wider delay range as well as on integer resonance. Based on this study it was revealed that optical delay phase and power split ratio are determined to be crucial parameters in dual-loop feedback configurations that control the stability and performance of QDash SMLLs [32, 36]. Recently, based on the open-loop response method, a theoretical analysis was performed on the characteristics of an electrically combined dual-loop optoelectronic oscillator. It was observed that the performance of OEOs including phase-noise, side-mode suppression, and loop-efficiency can be improved by controlling the two key parameters: optical delay phase settings and power split ratio [38]. In [34], the influence of the equal power split ratio and different cavity lengths on the side-mode suppression was studied.