Dr. Lilia Mashal Profile

Dr. Lilia Mashal

at Vrije Univ Brussel

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Publications (4)

Proceedings Article | 2 May 2014 Paper

Semiconductor ring lasers with delayed optical feedback: low-frequency fluctuations

Guy Van der Sande, Lilia Mashal, Romain Modeste Nguimdo, Miguel Cornelles-Soriano, Jan Danckaert, Guy Verschaffelt

Proceedings Volume 9134, 91341Y (2014) https://doi.org/10.1117/12.2052002

KEYWORDS: Semiconductor lasers, Semiconductors, Laser optics, Waveguides, Backscatter, Chaos, Bistability, Integrated optics, Feedback loops

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Semiconductor lasers subject to external feedback are known to exhibit a wide variety of dynamical regimes desired for some applications such as chaos cryptography, random bit generation, and reservoir computing. Low-frequency fluctuations is one of the most frequently encountered regimes. It is characterized by a fast drop in laser intensity followed by a gradual recovery. The duration of this recovery process is irregular and of the order of hundred nanoseconds. The average time between dropouts is much larger than the laser system characteristic time-scales. Semiconductor ring lasers are currently the focus of a rapidly thriving research activity due to their unique feature of directional bistability. They can be employed in systems for all-optical switching, gating, wavelength-conversion functions, and all-optical memories. Semiconductor ring lasers do not require cleaved facets or gratings for optical feedback and are thus particularly suited for monolithic integration. We experimentally and numerically address the issue of low-frequency fluctuations considering a semiconductor ring laser in a feedback configuration where only one directional mode is re-injected into the same directional mode, a so-called single self-feedback. We have observed that the system is very sensitive to the feedback strength and the injection current. In particular, the power dropouts are more regular when the pump current is increased and become less frequent when the feedback strength is increased. In addition, we find two different recovery processes after the power dropouts of the low-frequency fluctuations. The recovery can either occur via pulses or in a stepwise manner. Since low-frequency fluctuations are not specific to semiconductor ring lasers, we expect these recovery processes to appear also in VCSELs and edge-emitting lasers under similar feedback conditions. The numerical simulations also capture these different behaviors, where the representation in the phase space of the carriers versus the round trip phase difference gives additional insight into these phenomena. This proceedings paper gives a short overview of the results of L. Mashal et al. [L. Mashal et al., IEEE J. Quantum. Electron. 49, 790, 2013].

Proceedings Article | 15 May 2012 Paper

Semiconductor ring lasers as optical neurons

W. Coomans, L. Gelens, L. Mashal, S. Beri, G. Van der Sande, J. Danckaert, G. Verschaffelt

Proceedings Volume 8432, 84321I (2012) https://doi.org/10.1117/12.922269

KEYWORDS: Semiconductor lasers, Laser optics, Semiconductors, Neurons, Waveguides, Neural networks, Integrated optics, Numerical simulations, Continuous wave operation

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Proceedings Article | 15 May 2012 Paper

Experimental and numerical study of square wave oscillations due to asymmetric optical feedback in semiconductor ring lasers

Lilia Mashal, Guy Van der Sande, Lendert Gelens, Stefano Beri, Thomas Erneux, Jan Danckaert, Guy Verschaffelt

Proceedings Volume 8432, 84321R (2012) https://doi.org/10.1117/12.921953

KEYWORDS: Semiconductor lasers, Semiconductors, Optical fibers, Continuous wave operation, Backscatter, Waveguides, Polarization, Optical semiconductors, Stochastic processes

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Proceedings Article | 28 April 2010 Paper

Study of excitability in semiconductor ring lasers: theory and experiment

Lilia Mashal, Stefano Beri, Lendert Gelens, Guy Van der Sande, Gabor Mezosi, Marc Sorel, Jan Danckaert, Guy Verschaffelt

Proceedings Volume 7720, 772028 (2010) https://doi.org/10.1117/12.854741

KEYWORDS: Semiconductor lasers, Waveguides, Semiconductors, Stochastic processes, Optical fibers, Biomedical optics, Bistable lasers, Laser optics, Photodiodes

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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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