  We demonstrate an asymmetric dual-loop feedback scheme to suppress external
cavity side-modes induced in self-mode-locked quantum-dash lasers with
conventional single and dual-loop feedback. In this letter, we achieved optimal
suppression of spurious tones by optimizing the length of second delay time. We
observed that asymmetric dual-loop feedback, with large (~8x) disparity in
cavity lengths, eliminates all external-cavity side-modes and produces flat RF
spectra close to the main peak with low timing jitter compared to single-loop
feedback. Significant reduction in RF linewidth and reduced timing jitter was
also observed as a function of increased second feedback delay time. The
experimental results based on this feedback configuration validate predictions
of recently published numerical simulations. This interesting asymmetric
dual-loop feedback scheme provides simplest, efficient and cost effective
stabilization of side-band free optoelectronic oscillators based on mode-locked
lasers.
