  We experimentally investigate the RF linewidth and timing jitter over a wide
range of delay tuning in a self-mode-locked two-section quantum dash lasers
emitting at ~ 1.55 micron and operating at ~ 21 GHz repetition rate subject to
single and dual optical feedback into gain section. Various feedback conditions
are investigated and optimum levels determined for narrowest linewidth and
reduced timing jitter for both single and dual loop configurations. We
demonstrate that dual loop feedback, with the shorter feedback cavity tuned to
be fully resonant, followed by fine tuning of the phase of the longer feedback
cavity, gives stable narrow RF spectra across the widest delay range, unlike
single loop feedback. In addition, for dual loop configurations, under fully
resonant conditions, integrated timing jitter is reduced from 3.9 ps to 295 fs
[10 kHz-100 MHz], the RF linewidth narrows from 100 kHz to < 1 kHz, with more
than 30 dB fundamental side-mode suppression. We show that dual loop optical
feedback with separate fine tuning of both external cavities is far superior to
single loop feedback, with increased system tolerance against phase delay
mismatch, making it a robust and cost-effective technique for developing
practical, reliable and low-noise mode-locked lasers, optoelectronic
oscillators and pulsed photonic circuits.
