88 research outputs found
Stochasticity,complexity and synchronization in semiconductor lasers
The purpose of this Thesis is study the dynamical behavior of semiconductor
lasers with optical feedback, as well as analyze the synchronization of this kind of
systems under different coupling arquitectures. This study has been done from an
experimental point of view, but in some cases we have used numerical models in
order to verify and/or extend the experimental results.
A semiconductor laser in absence of any optical feedback emits light at constant
power. If one wants to induce dynamics in the laser, a good strategy is to introduce
an external cavity able to reflect the emitted light back into the laser. Due to this
feedback, the laser can show a large variety of dynamical behaviors. In this Thesis
we will focus mainly in a dynamical regime known as low frequency fluctuations
regime (LFF). The LFF regime takes place when the pump current of the laser is
close to its threshold current and the feedback strength is sufficiently large, and it
consists in sudden intensity dropouts arising at irregular times, followed by a gradual
and stepwise recovery.
During this Thesis, we have characterized in detail the dynamical behavior of
the time between intensity dropouts for a semiconductor laser with feedback, by
using different statistical techniques based on information theory concepts. We
have quantified the probability of appearance of certain patterns within the temporal
series, as well as its degree of complexity. As a result of these studies, we can conclude
that the dynamics of a semiconductor laser with optical feedback is stochastic for
pump current values close to the laser threshold. On the other hand, for larger pump
currents the dynamics is basically deterministic (chaotic). Numerical simulations
have shown a good qualitative and quantitive agreement with the experimental
results.
During this Thesis we have also studied the ability of semiconductor lasers to
synchronize under different coupling architectures. First, we have characterized the
leader-laggard dynamics showed by two semiconductor lasers bidirectionally coupled
operating at the LFF regime, with a method that takes into account the number
of forbidden patterns that appears in the temporal series. We have quantified the
degree of stochasticity of the system as a function of the pump current of both lasers.
A second coupling architecture studied here, consists in two lasers unidirectionally
coupled via two paths. In this case, we have analyzed how the synchronization is
affected under different values of the coupling strength of both paths, as well as
the potential of this system (or rather, the lack thereof) to be used in chaotic
communications.
Finally we have characterized the synchronization at zero lag for two lasers coupled
bidirectionally via a passive relay. In particular, we have studied the desynchronization
events and their statistics for different pump currents.
The experimental results obtained in this Thesis give a global perspective of the
dynamical statistical properties of semiconductor laser dynamics, both isolated or
coupled to other lasers, which contributes to a better understanding of this kind of
dynamical systems.Lâobjectiu dâaquesta Tesi ´es lâestudi de la din`amica de l`asers de semiconductor
amb retroalimentaci´o `optica, aix´ý com lâan`alisis de la sincronitzaci´o dâaquest tipus
de sistemes sota diferents arquitectures dâacoblament. Aquest estudi sâha fet sempre
des dâun punt de vista b`asicament experimental, tot i que en alguns casos hem
utilitzat models num`erics per tal de verificar i/o ampliar els resultats experimentals.
Un l`aser de semiconductor en abs`encia de retroalimentaci´o `optica o altres perturbacions
externes, emet llum a una intensitat pr`acticament constant. Aix´ý doncs, si el
que es vol ´es indu¨ýr din`amica en el l`aser, una bona estrat`egia ´es introdu¨ýr una cavitat
externa capa¸c de reflexar la llum cap al l`aser. Un cop la llum ´es reinjectada, els l`asers
de semiconductor poden mostrar una gran varietat de comportaments din`amics. En
aquesta tesis ens centrarem principalment en un r`egim din`amic anomenat r`egim de
fluctuacions de baixa frequ`encia (LFF en les seves sigles en angl`es). El r`egim dâLFF
es d´ona quan el corrent dâinjecci´o del l`aser es troba a prop del seu corrent llindar
i la intensitat de la retroalimentaci´o ´es suficientment gran, i est`a caracteritzat per
sobtades caigudes de la intensitat a temps irregulars, seguides per una recuperaci´o
gradual i escalonada.
Durant aquesta Tesi, hem caracteritzat de forma detallada el comportament din`amic
de la distribuci´o dels temps entre les caigudes dâintensitat dâun l`aser de semiconductor
amb retroalimentaci´o `optica, utilitzant diferents m`etodes estad´ýstics basats
en conceptes de teoria de la informaci´o. En particular, hem quantificat la probabilitat
dâaparici´o de certs patrons dins les s`eries temporals, aix´ý com el grau de
complexitat dâaquestes. Durant aquest estudi hem observat que la din`amica dâun
l`aser de semiconductor amb retroalimentaci´o es estoc`astica per valors del corrent
dâinjecci´o propers al corrent llindar del l`aser. Dâaltra banda, per a valors m´es grans
del corrent dâinjecci´o la din`amica ´es m´es determinista (ca`otica). Les simulacions
num`eriques realitzades han mostrat un acord qualitatiu i quantitatiu amb els resultats
experimentals.
Durant aquesta Tesi tamb´e hem estudiat la sincronitzaci´o entre l`asers de semiconductor.
Hem analitzat diferents arquitectures dâacoblament. Primer hem caracteritzat
la din`amica leader-laggard que presenten dos l`asers de semiconductor acoblats
bidireccionalment operant en r`egim de LFFs, amb un m`etode que t´e en compte el
nombre de patrons prohibits que apareixen en la s`erie temporal. Hem quantificat
el grau dâestocasticitat del sistema en funci´o del nivell de bombeig al qual est`an
sotmesos els dos l`asers.
La seg¨uent arquitectura dâacoblament que hem estudiat consisteix en dos l`asers
acoblats unidireccionalment a trav´es de dos camins. En aquest cas hem analitzat
com es veu afectada la sincronitzaci´o sota diferents valors de lâacoblament dels dos
camins, aix´ý com el potencial dâaquest esquema experimental per realitzar comunicacions
ca`otiques.
Per ´ultim hem caracteritzat la sincronitzaci´o a retard zero per dos l`asers acoblats
bidireccionalment, a on els dos l`asers tenen la seva pr`opia realimentaci´o `optica. En
particular, hem estudiat els events de desincronitzaci´o i la seva estad´ýstica per a
diferents corrents dâinjecci´o.
Els resultats experimentals obtinguts en aquesta Tesi, ofereixen una prespectiva
global de les propietats estad´ýstiques de la din`amica de l`asers de semiconductor,
tant a¨ýllats com acoblats a altres l`asers, que contribueixen a entendre millor aquests
sistemes din`amics
Experimental and Numerical Study of Locking of Low-Frequency Fluctuations of a Semiconductor Laser with Optical Feedback
We study the output of a semiconductor laser with optical feedback operated in the lowfrequency fluctuations (LFFs) regime and subject to weak sinusoidal current modulation. In the LFF regime, the laser intensity exhibits abrupt drops, after which it recovers gradually. Without modulation, the drops occur at irregular times, while, with weak modulation, they can lock to the external modulation and they can occur, depending on the parameters, every two or every three modulation cycles. Here, we characterize experimentally the locking regions and use the wellknown LangâKobayashi model to simulate the intensity dynamics. We analyze the effects of several parameters and find that the simulations are in good qualitative agreement with the experimental observations.This research was funded by the Spanish Ministerio de Ciencia, Innovacion y Universidades (PGC2018-099443-B-I00) and the ICREA ACADEMIA program of Generalitat de Catalunya.Postprint (published version
Time crystal dynamics in a weakly modulated stochastic time delayed system
Time crystal oscillations in interacting, periodically driven many-particle systems are highly regular oscillations that persist for long periods of time, are robust to perturbations, and whose frequency difers from the frequency of the driving signal. Making use of underlying similarities of spatiallyextended systems and time-delayed systems (TDSs), we present an experimental demonstration of time-crystal-like behavior in a stochastic, weakly modulated TDS. We consider a semiconductor laser near threshold with delayed feedback, whose output intensity shows abrupt spikes at irregular times. When the laser current is driven with a small-amplitude periodic signal we show that the interaction of delayed feedback and modulation can generate long-range regularity in the timing of the spikes, which lock to the modulation and, despite the presence of noise, remain in phase over thousands of modulation cycles. With pulsed modulation we fnd harmonic and subharmonic locking, while with sinusoidal modulation, we fnd only subharmonic locking, which is a characteristic feature of timecrystal behavior.We thank JaeUk Kim and Salvatore Torquato for useful discussions. Tis work was funded by the Spanish Ministerio de Ciencia, Innovacion y Universidades (PGC2018-099443-B-I00) and the ICREA ACADEMIA program of Generalitat de Catalunya.Postprint (published version
Quantifying the synchronization of the spikes emitted by coupled lasers
Synchronization phenomena is ubiquitous in nature, and in spite of having been studied for decades, it still attracts a lot of attention as is still challenging to detect and quantify, directly from the analysis of noisy signals. Semiconductor lasers are ideal for performing experiments because they are stochastic, nonlinear, and inexpensive and display different synchronization regimes that can be controlled by tuning the lasersâ parameters. Here, we analyze experiments done with two mutually optically coupled lasers. Due to the delay in the coupling (due to the finite time the light takes to travel between the lasers), the lasers synchronize with a lag: the intensity time traces show well-defined spikes, and a spike in the intensity of one laser may occur shortly before (or shortly after) a spike in the intensity of the other laser. Measures that quantify the degree of synchronization of the lasers from the analysis of the intensity signals do not fully quantify the synchronicity of the spikes because they also take into account the synchronization of fast irregular fluctuations that occur between spikes. By analyzing only the coincidence of the spike times, we show that event synchronization measures quantify spike synchronization remarkably well. We show that these measures allow us to quantify the degree of synchronization and, also, to identify the leading laser and the lagging one.C.M. acknowledges the support of Ministerio de Ciencia, InnovaciĂłn y Universidades (No. PID2021-123994NB-C21) and InstituciĂł Catalana de Recerca i Estudis Avançats (Academia).Peer ReviewedPostprint (author's final draft
Dynamics of a semiconductor laser with feedback and modulation: experiments and model comparison
Š 2222 Optica Publishing Group. Users may use, reuse, and build upon the article, or use the article for text or data mining, so long as such uses are for non-commercial purposes and appropriate attribution is maintained. All other rights are reserved.We study experimentally and numerically the dynamics of a semiconductor laser near threshold, subject to optical feedback and sinusoidal current modulation. The laser operates in the low frequency fluctuation (LFF) regime where, without modulation, the intensity shows sudden spikes at irregular times. Under particular modulation conditions the spikes lock to the modulation and their timing becomes highly regular. While the modulated LFF dynamics has received a lot of attention, an in-depth comparison with the predictions of the Lang-Kobayashi (LK) model has not yet been performed. Here we use the LK model to simulate the laser dynamics and use the Fano factor to quantify the regularity of the timing of the spikes. The Fano factor is calculated by counting the number of spikes in successive segments of the intensity time-series and keeps information about temporal order in the spike sequence that is lost when the analysis is based on the distribution of inter-spike intervals. Here we compare the spike timing regularity in experimental and in simulated spike sequences as a function of the modulation amplitude and frequency and find a good qualitative agreement. We find that in both experiments and simulation for appropriate conditions the spike timing can be highly regular, as revealed by very small values of the Fano factor.Institució Catalana de Recerca i Estudis Avançats (Academia); Ministerio de Ciencia, Innovación y Universidades (PGC2018-099443-B-I00).Peer ReviewedPostprint (published version
Vertical Azimuth Display simulator for wind-Doppler lidar error assessment
(c) 2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works.This works presents a simplified Vertical Azimuth Display (VAD) motion simulator for off-shore wind lidars. The simulator is rooted to the case of a conically-scanning lidar (e.g., the Zephyr lidar), where the wind speed vector is retrieved from the Line-of-Sight velocities over one scan period. The methodological part addresses the geometrical foundations of the simulator and how the lidar attitude is assimilated in matrix form. The discussion part considers the case of time-invariant, horizontally-homogeneous wind under two motional cases of the lidar, static and dynamic. Cases examples are parameterized by Horizontal Wind Speed, Wind Direction and tilt amplitude.Postprint (author's final draft
Comparing the dynamics of periodically forced lasers and neurons
Neuromorphic photonics is a new paradigm for ultra-fast neuro-inspired optical computing that canrevolutionize information processing and artificial intelligence systems. To implement practicalphotonic neural networks is crucial to identify low-cost energy-efficient laser systems that can mimicneuronal activity. Here we study experimentally the spiking dynamics of a semiconductor laser withoptical feedback under periodic modulation of the pump current, and compare with the dynamics of aneuron that is simulated with the stochastic FitzHughâNagumo model, with an applied periodicsignal whose waveform is the same as that used to modulate the laser current. Sinusoidal and pulse-down waveforms are tested. Wefind that the laser response and the neuronal response to the periodicforcing, quantified in terms of the variation of the spike rate with the amplitude and with the frequencyof the forcing signal, is qualitatively similar. We also compare the laser and neuron dynamics usingsymbolic time series analysis. The characterization of the statistical properties of the relative timing ofthe spikes in terms of ordinal patterns unveils similarities, and also some differences. Our resultsindicate that semiconductor lasers with optical feedback can be used as low-cost, energy-efficientphotonic neurons, the building blocks of all-optical signal processing systems; however, the length ofthe external cavity prevents optical feedback on the chip.Peer ReviewedPostprint (published version
Abrupt transition from low-coherence to high-coherence radiation in a semiconductor laser with optical feedback
Semiconductor lasers are very sensitive to optical feedback. Although it is well known that coherent feedback lowers the threshold of the laser, the characteristics of the transition from low-coherence radiationâdominated by spontaneous emissionâbelow threshold to high-coherence radiationâdominated by stimulated emissionâabove threshold have not yet been investigated. Here we show experimentally that, in contrast to the transition that occurs in the solitary laser, in the laser with feedback the transition to high-coherence emission can occur abruptly. We use the speckle technique to show that the transition varies from smooth to abrupt as the amount of light fed back to the laser increases.Peer ReviewedPostprint (published version
Experimental study of spatial and temporal coherence in a laser diode with optical feedback
Š 2023 Optica Publishing Group. Users may use, reuse, and build upon the article, or use the article for text or data mining, so long as such uses are for non-commercial purposes and appropriate attribution is maintained. All other rights are reserved.Optical feedback can reduce the linewidth of a semiconductor laser by several orders of magnitude, but it can also cause line broadening. Although these effects on the temporal coherence of the laser are well known, a good understanding of the effects of feedback on the spatial coherence is still lacking. Here we present an experimental technique that allows discriminating the effects of feedback on temporal and spatial coherence of the laser beam. We analyze the output of a commercial edge-emitting laser diode, comparing the contrast of speckle images recorded using a multimode (MM) or single mode (SM) fiber and an optical diffuser, and also, comparing the optical spectra at the end of the MM or SM fiber. Optical spectra reveal feedback-induced line broadening, while speckle analyses reveal reduced spatial coherence due to feedback-excited spatial modes. These modes reduce the speckle contrast (SC) up to 50% when speckle images are recorded using the MM fiber, but do not affect the SC when the images are recorded using the SM fiber and diffuser, because the spatial modes that are excited by the feedback are filtered out by the SM fiber. This technique is generic and can be used to discriminate spatial and temporal coherence of other types of lasers and under other operating conditions that can induce a chaotic output.InstituciĂł Catalana de Recerca i Estudis Avançats (Academia); Agència de GestiĂł dâAjuts Universitaris i de Recerca (2021 SGR 00606, FI Scholarship); Ministerio de Ciencia, InnovaciĂłn y Universidades (PID2021-123994NBC21).Peer ReviewedPostprint (published version
Distinguishing signatures of determinism and stochasticity in spiking complex systems
Acknowledgements: This work was supported in part by grant FA8655-12-1-2140 from EOARD US, grant FIS2009-13360 from the Spanish MCI and grant 2009 SGR 1168 from the Generalitat de Catalunya. C. Masoller acknowledges partial support from the ICREA Academia programme. N. Rubido acknowledges the Scottish University Physics Alliance.Peer reviewedPublisher PD
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