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International audience; Large-scale spatiotemporal photonic reservoir computer (RC) systems offer remarkable solutions for massively parallel processing of a wide variety of hard real-world tasks. In such systems, neural networks are created by either optical or electronic coupling. Here, we investigate the impact of the optical coherence on the performance of large-scale spatiotemporal photonic RCs by comparing a coherent (optical coupling between the reservoir nodes) and incoherent (digital coupling between the reservoir nodes) RC systems. Although the coherent configuration offers significant reduction on the computational load compared to the incoherent architecture, for image and video classification benchmark tasks, it is found that the incoherent RC configuration outperforms the coherent configuration. Moreover, the incoherent configuration is found to exhibit a larger memory capacity than the coherent scheme. Our results pave the way towards the optimization of implementation of large-scale RC systems.

In previous works, it has been shown that reservoir computing (RC) systems using a laser subject to a delayed optical feedback and stabilized by an injected signal may be highly sensitive to the feedback phase. In this Letter, we show that a RC system using a single quantum cascade laser subject to a delayed optical feedback but without injection is robust to the feedback phase for a large range of values of the parameters., SCOPUS: ar.j, info:eu-repo/semantics/published

International audience; In this paper, we investigate the possibility of using optical injection to efficiently suppress the time-delay (TD) signatures of chaotic signals in a large experimentally accessible parameter range of semiconductor ring lasers (SRLs). We also study how this optical injection can improve the signal bandwidths. The injection signal is obtained from a master SRL with either optical self-or cross-feedback. For optical self-feedback configurations, it is found that the suppression of TD signatures is similar to what has been found in injected Fabry-Perot semiconductor lasers, i.e., narrow range of parameters with respect to the detuning and injection strengths. For cross-feedback configurations, however, the TD signatures can be suppressed in a wide range of parameters meanwhile the bandwidths are significantly improved for the same range of parameters. This is particularly useful for the security in chaos-based communications and also for generating random bits with improved properties.

In this brief, we numerically demonstrate a photonic delay-based reservoir computing system, which processes, in parallel, two independent computational tasks even when the two tasks have unrelated input streams. Our approach is based on a single-longitudinal mode semiconductor ring laser (SRL) with optical feedback. The SRL emits in two directional optical modes. Each directional mode processes one individual task to mitigate possible crosstalk. We illustrate the feasibility of our scheme by analyzing the performance on two benchmark tasks: 1) chaotic time series prediction and 2) nonlinear channel equalization. We identify some feedback configurations for which the results for simultaneous prediction/classification indicate a good performance, but with slight degradation (as compared with the performance obtained for single task processing) due to nonlinear and linear interactions between the two directional modes of the laser. In these configurations, the system performs well on both tasks for a broad range of the parameters.

Optoelectronic oscillators based on whispering-gallery mode resonators (WGMRs) are efficient oscillators for ultra-stable microwave generation, in which the energy can be stored in an ultra-high $Q$ -factor resonator instead of the usual long delay line. In this paper, we derive a stochastic model for the calculation of the phase noise performance in these systems considering three different configurations, namely, the case where the energy is stored in high- $Q$ disk-shaped WGMR, the case where energy is stored in an optical delay-line only, and the case for which the energy is stored simultaneously by the resonator and the delay line. Our investigations explain how the WGMR can be optimally used for spurious peak rejection in optoelectronic oscillators. Our experimental measurements are found to be in excellent agreement with analytical and simulation results.

Reservoir computing (RC) systems are computational tools for information processing that can be fully implemented in optics. Here, we experimentally and numerically show that an optically pumped laser subject to optical delayed feedback can yield similar results to those obtained for electrically pumped lasers. Unlike with previous implementations, the input data are injected at a time interval that is much larger than the time-delay feedback. These data are directly coupled to the feedback light beam. Our results illustrate possible new avenues for RC implementations for prediction tasks., SCOPUS: ar.j, info:eu-repo/semantics/published

International audience; We consider an optoelectronic oscillator for ultra-pure microwave generation with two nonlinearly-coupled delay loops in which the output of one of the loops modulates the other. We introduce a suitable dynamical model and study the phase noise performance. Tuning the parameters of the second loop allows us to reduce the detrimental effect of the multiplicative phase noise and damp delay-induced spurious peaks. Also, nonlinear dynamics allows for stable microwave emission with larger amplitude than a single loop system.

8 pages, 15 figures., We analytically and numerically study the effect of an additional feedback in the semiconductor laser used to pump optoelectronic delay devices. We show that this additional feedback renders the system into chaotic regime for a broader parameter range and also induces a stronger chaotic behavior. We study the synchronization of this system as function of the parameter mismatch and show its capability for encoded message transmission., We gratefully thank Vasile Tronciu for helping us with the DDE-BIFTOOL program. As well, financial support from MICINN, Spain, and Feder under Project TEC2006-1009/MIC (PhoDECC) and Project FIS2004-00953 (CONOCE2) and by the EC Project PICASSO (IST-2005-34551) is acknowledged. R.M.Nguimdo also acknoledges the fellowship BES-2007-14627 under the FPI program of MICINN (Spanish Goverment).

Optical implementations of reservoir computing systems are very promising because of their high processing speeds and the possibility to process several tasks in parallel. These systems can be implemented using semiconductor lasers subject to optical delayed feedback and optical injection. While the amount of the feedback/injection can be easily controlled, it is much more difficult to control the optical feedback/injection phase. We present extensive numerical investigations of the influence of the feedback/injection phases on laser-based reservoir computing systems with feedback. We show that a change in the phase can lead to a strong reduction in the reservoir computing system performance. We introduce a new readout layer design that -at least for some tasks- reduces this sensitivity to changes in the phase. It consists in optimizing the readout weights from a coherent combination of the reservoir's readout signal and its delayed version rather than only from the reservoir's readout signal as is usually done.

Optoelectronic oscillators (OEOs) are microwave photonic systems intended to generate ultra-pure radio-frequency signals for aerospace and communication engineering applications. Typically, a long optical fiber delay line is inserted in the feedback loop as an energy storage element intended to improve the phase noise performance. An ultra-high Q whispering-gallery mode resonator can be inserted as well in order to filter-out the spurious ring-cavity peaks arising in the radio-frequency spectrum. However, dynamical instabilities induced by a relatively large delay have never been analyzed for these OEOs. In this paper, we systematically investigate the stability of the generated microwaves as a function of the effective OEO loop gain and delay. We find that close enough to the oscillation threshold, the generated microwaves are unconditionally stable, but as the gain is increased, their stability becomes dependent on the ratio between the time delay and the overall photon lifetime of the resonator. Our results allow an optimal design for these OEOs, leading to both enhanced spectral stability and phase noise performance., SCOPUS: ar.j, info:eu-repo/semantics/published

Semiconductor lasers subject to delayed optical feedback have recently shown great potential in solving computationally hard tasks. By optically implementing a neuro-inspired computational scheme, called reservoir computing, based on the transient response to optical data injection, high processing speeds have been demonstrated. While previous efforts have focused on signal bandwidths limited by the semiconductor laser's relaxation oscillation frequency, we demonstrate numerically that the much faster phase response makes significantly higher processing speeds attainable. Moreover, this also leads to shorter external cavity lengths facilitating future on-chip implementations. We numerically benchmark our system on a chaotic time-series prediction task considering two different feedback configurations. The results show that a prediction error below 4% can be obtained when the data is processed at 0.25 GSamples/s. In addition, our insight into the phase dynamics of optical injection in a semiconductor laser also provides a clear understanding of the system performance at different pump current levels, even below solitary laser threshold. Considering spontaneous emission noise and noise in the readout layer, we obtain good prediction performance at fast processing speeds for realistic values of the noise strength.

The use of the postprocessing method consisting of bitwise Exclusive-OR and least significant bits extraction to generate random bit sequences typically requires two distinct chaotic outputs. While the two signals are, in general, generated using two separated devices, e.g. two Fabry-Perot lasers, a single semiconductor ring laser can be used as an alternative due to its circular symmetry which facilitates lasing in two counterpropagating mode directions. We consider a chaotic semiconductor ring laser and investigate both numerically and experimentally its char- acteristics for fast random bit generation. In particular, we show that by sampling each directional mode's output signal using a 8-bit analog-digital converter and through Exclusive-OR operation applied to the two resulting signals (after throwing away 4 most significant bits), we can achieve fast random bit-streams with a bit rate 4 × 10 = 40 Gbit/s, passing the statisti- cal randomness tests. To optimize the system performance, we also study the dependence of randomness on the main system parameters and on noise.

OCIS codes:250.0250,060.0060,140.0140,190.0190), We consider an electro-optic phase chaos system with two feedback loops organized in a parallel configuration such that the dynamics of one of the loops remains internal. We show that this configuration intrinsically conceals in the transmitted variable the internal delay times, which are critical for decoding. The scheme also allows for the inclusion, in a very efficient way, of a digital key generated as a long pseudorandom binary sequence. A single digital key can operate both in the internal and transmitted variables leading to a large sensitivity of the synchronization to a key-mismatch. The combination of intrinsic delay time concealment and digital key selectivity provides the basis for a large enhancement of the confidentiality in chaos-based communications. © 2012 OSA, The authors thank L. Larger for valuable discussions. Financial support from MINECO, Spain, and Feder under Projects FIS2007-60327 (FISICOS) and TEC2009-14101 (DeCoDicA) and by the EC Project PHOCUS (FP7-ICT-2009-C-240763) is acknowledged. R.M.N. acknowledges fellowship BES-2007-14627 under the FPI program of MINECO. Also, he acknowledges the Research Foundation Flanders (FWO) for project support and fellowships, and the Belgian Interuniversity Attraction Pole Network photonics@be.

PACS numbers: 42.15.Eq, 05.45.Jn, 05.45.Tp, 05.45.Vx, We introduce a scheme that integrates a digital key in a phase-chaos electro-optical delay system for optical chaos communications. A pseudorandom binary sequence (PRBS) is mixed within the chaotic dynamics in a way that a mutual concealment is performed; e.g., the time delay is hidden by the binary sequence, and the PRBS is also masked by the chaos. In addition to bridging the gap between algorithmic symmetric key cryptography and chaos-based analog encoding, the proposed approach is intended to benefit from the complex algebra mixing between a (pseudorandom) Boolean variable, and another continuous time (chaotic) variable. The scheme also provides a large flexibility allowing for easy reconfigurations to communicate securely at a high bit rate between different systems., Financial support from MICINN, Spain, and Feder under Projects TEC2006-10009 (PhoDeCC), FIS2007- 60327 (FISICOS) and TEC2009-14101 (DeCoDicA) and by EC Project PHOCUS (FP7-ICT-2009-C-240763) is acknowledged. R. M. N. also acknowledges the fellowship BES-2007-14627 under the FPI program of MICINN.

We investigate theoretically and experimentally the detrimental e ect of ber dispersion on the synchroniza- tion of an optoelectronic phase chaos cryptosystem. We evaluate the root-mean square synchronization error and the cancellation spectra between the emitter and the re- ceiver in order to characterize the quality of the optical ber communication link. These two indicators explicitly show in temporal and spectral domain how ber dispersion does negatively a ect the phase chaos cancellation at the re- ceiver stage. We demonstrate that the dispersion manage- ment techniques used in conventional optical ber networks, such as dispersion-compensating modules/ bers or disper- sion shifted bers, are also e cient to strongly reduce the detrimental e ects of ber propagation in phase chaos com- munications. This compatibility therefore opens the way to a successful integration of more than 10-Gb/s phase chaos communications systems in existing networks, even when the ber link spans over more than 100 km.

In this paper, we experimentally and numerically study the dynamics of semiconductor ring lasers that are subjected to long delay and moderate self-feedback. Through varying the pump current or the feedback strength or both, we study the appearance and parameter dependence of low frequency fluctuations in these systems. In particular, we observe different routes to the building up of the initial power amplitude. © 2013 IEEE.