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Semiconductor Ring Lasers (SRLs) are a novel class of semiconductor lasers whose active cavity is characterized
by a circular geometry. SRLs have attracted attention due to the possibility of monolithical integration of
thousands of them on the same chip in a cheap and reliable way. SRLs are interesting for applications that
rely on the presence of two counter-propagating modes inside the optical cavity. For instance, fully symmetric
coupled SRLs have been proposed as candidates for the realisation of small and fast all-optical memories. At
the same time, a wealth of nonlinear and stochastic dynamics have been predicted and observed in symmetric
SRLs which is a consequence of the underlying Z2-symmetry of the device. However, unavoidable fabrication
defects, material roughness and chip-cleaving break the device symmetry in an uncontrolled and unpredictable
way, which may result in a deterioration of the device's performance in applications such as all-optical signal-processing.
Despite their importance, the effects of symmetry breaking in SRLs remain unaddressed. In this
contribution we investigate theoretically and experimentally the stochastic dynamics of SRLs with weakly broken
Z2-symmetry . We show how the symmetry of an SRL can be experimentally manipulated using the reflection
from a cleaved facet of a multi-mode optical fibre and a control electrode on the bus waveguide. The experiments
are performed on an InP-based multi-quantum well SRL operating in single-longitudinal mode regime. The power
at the CCW output is collected using a fast photodiode connected to an oscilloscope with a sampling rate of 4.0
ns. For a not-too-weak symmetry breaking, we reveal that SRLs become excitable and therefore can emit large,
deterministic power bursts as a response to stochastic fluctuations. The origin of excitability is explained by
investigating the topology of the invariant manifolds of an asymptotic two-dimensional phase space model with
broken Z2-invariance. The results of the experiments confirm the prediction of the theory.
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