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 the 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 of 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.
Highlights
Further increase in feedback ratio to -22 dB resulted in significant reduction in RF linewidth and timing jitter for single and symmetric dual-loop (SDL) configuration subject to both balanced and unbalanced feedback ratios
We investigated the effectiveness of single-loop and SDL optical feedback as means of robust stabilization of self-mode-locked quantum dash (QDash) lasers operating at 21 GHz pulse repetition rate and emitting at 1550 nm wavelength
We demonstrated that unbalanced SDL feedback provides best stability, maintaining stable RF spectra with narrow linewidth and low timing jitter over a range of delay detuning 80 ps, which means it would be insensitive to temperature, vibration and other common environmental variations
Summary
Quantum nanostructure based mode-locked semiconductor lasers are of increasing interest for many applications as frequency comb sources in data centers [1], optical clock recovery [2] and high capacity coherent terabit communication systems [3]. While picosecond pulses from these lasers have been demonstrated routinely, these pulses have significant chirp and poor timing jitter The latter is usually determined by measuring the linewidth of the repetition-rate peak in the RF intensity fluctuation spectrum. Several techniques such as single-loop external optical feedback [4,5,6,7,8], coupled optoelectronic oscillators (OEOs) [9,10,11], hybrid mode-locking [12], injection-locking [13,14,15] and dual-loop feedback [16,17,18,19,20] have been proposed and demonstrated to improve this key parameter of mode-locked lasers (MLLs). Our proposed unbalanced SDL scheme provides an effective regime of resonant feedback parameters much wider than single-loop and balanced SDL feedback, making it ideal for practical applications
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