Title: Stabilization of self-mode-locked quantum dash lasers by symmetric dual-loop optical feedback,
Abstract: We report experimental studies of the influence of symmetric dual-loop
optical feedback on the RF linewidth and timing jitter of self-mode-locked
two-section quantum dash lasers emitting at 1550 nm. Various feedback schemes
were investigated and optimum levels determined for narrowest RF linewidth and
low timing jitter, for single-loop and symmetric dual-loop feedback. Two
symmetric dual-loop configurations, with balanced and unbalanced feedback
ratios, were studied. We demonstrate that unbalanced symmetric dual loop
feedback, with the inner cavity resonant and fine delay tuning of the outer
loop, gives narrowest RF linewidth and reduced timing jitter over a wide range
of delay, unlike single and balanced symmetric dual-loop configurations. This
configuration with feedback lengths 80 and 140 m narrows the RF linewidth by
4-67x and 10-100x, respectively, across the widest delay range, compared to
free-running. For symmetric dual-loop feedback, the influence of different
power split ratios through the feedback loops was determined. Our results show
that symmetric dual-loop feedback is markedly more effective than single-loop
feedback in reducing RF linewidth and timing jitter, and is much less sensitive
to delay phase, making this technique ideal for applications where robustness
and alignment tolerance are essential.