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Abstract
With conventional semiconductor lasers undergoing external optical feedback, a chaotic output is typically observed even for moderate levels of the feedback strength. In this paper we examine single mode quantum dot lasers under strong optical feedback conditions and show that an entirely new dynamical regime is found consisting of spontaneous mode-locking via a resonance between the relaxation oscillation frequency and the external cavity repetition rate. Experimental observations are supported by detailed numerical simulations of rate equations appropriate for this laser type. The phenomenon constitutes an entirely new mode-locking mechanism in semiconductor lasers.
Highlights
Mode-locked lasers and their associated frequency combs have been at the centre of many important advances in technology and fundamental science in recent years
In [18] a long cavity model was introduced accounting well for the observed features in multimode quantum dot (QD) experiments. We show that this model can be further improved by taking into account two distinct delays corresponding to the short laser chip cavity and the long external cavity
We show that the pulse train results from a spontaneous mode-locking amongst external cavity modes (ECMs) which in turn results from a resonance between the relaxation oscillation frequency (ROF) and the external cavity repetition rate
Summary
Mode-locked lasers and their associated frequency combs have been at the centre of many important advances in technology and fundamental science in recent years. This paper uncovers a new mechanism leading to mode-locking in semiconductor lasers It takes the form of a resonant modelocking arising via a locking phenomenon between the two most important time scales in a semiconductor laser: the cavity round-trip frequency and the relaxation oscillation frequency (ROF). While the famous Lang-Kobayashi rate equations [17] reproduce the observations for conventional semiconductors very well, they are unsuitable for the QD case This is mainly because high feedback levels and long external cavities are required for the observations of dynamical instabilities. The locking mechanism arises via rational ratios of the ROF and the external cavity repetition rate This is reminiscent of mode-locking via the devil’s staircase [19,20,21] and we may speak of a winding number in this context as the ratio of the two frequencies. Because of the varying basin of attraction sizes some traces are far more likely to be observed experimentally
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