Stabilization of self-mode-locked quantum dash lasers by symmetric dual-loop optical feedback

TL;DR

This study demonstrates that symmetric dual-loop optical feedback significantly improves the RF linewidth and timing jitter of self-mode-locked quantum dash lasers, offering enhanced robustness and tunability over single-loop configurations.

Contribution

The paper introduces an optimized symmetric dual-loop feedback scheme that outperforms single-loop feedback in stabilizing quantum dash lasers, with detailed analysis of feedback ratios and delay tuning.

Findings

Unbalanced symmetric dual-loop feedback yields the narrowest RF linewidth and lowest timing jitter.

Feedback lengths of 80 and 140 meters achieve 4-67x and 10-100x linewidth narrowing.

Symmetric dual-loop feedback is more effective and robust than single-loop 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.