Optical Spectrum Analysis of Chaotic Synchronization in a Bidirectional Coupled Semiconductor Laser System
In the present work two semiconductor lasers (SL) were optically coupled into a master—slave synchronization scheme. The master system was a SL operated under external optical feedback conditions in the low-frequency fluctuations (LFF) chaotic regime, whi
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Abstract In the present work two semiconductor lasers (SL) were optically coupled into a master—slave synchronization scheme. The master system was a SL operated under external optical feedback conditions in the low-frequency fluctuations (LFF) chaotic regime, while the slave system was a solitary laser with free emission. Chaotic synchronization regimes and optical spectrum behavior of the coupled system have been experimentally analyzed in relation to the emission spectra of the two independent laser systems. The results allow to better understand the mechanisms that contribute to the synchronization regimes stability and optical spectrum formation of the coupled system.
1 Introduction An external-cavity semiconductor laser (ECSL) system with optical feedback is typically used to generate chaotic laser emission in LFF regime [1]. The characteristics of the chaotic dynamics, like LFF frequencies, can be changed by applying a controlled variation to an accessible parameter of the ECSL system. The chaotic behavior has applications in the optical secure communications by synchronizing two such systems—one, named “master”, and the other “slave”. An encoded digital message attached to the chaotic optical carrier can be recovered by coupling the chaotic transmitter with a similar ECSL or solitary laser receiver [2]. The coupling between the two systems can be accomplished in unidirectional or bidirectional fashion, leading to different synchronization regimes [3]. The quality of recovered message depends on the time shift necessary for the receiver dynamics to synchronize with the transmitter [4], and on the similarity of the modal power
I.R. Andrei () G.V. Popescu C.M. Ticos M.L. Pascu National Institute for Laser, Plasma and Radiation Physics, P. O. Box MG 36, 077125 Magurele, Romania e-mail: [email protected] S.G. Stavrinides et al. (eds.), Chaos and Complex Systems, DOI 10.1007/978-3-642-33914-1 60, © Springer-Verlag Berlin Heidelberg 2013
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distribution of the emission spectra of the two lasers [2]. The time shift can be estimated as the time of flight necessary for the light to travel the optical path between the two semiconductor lasers. Based on the time shift between slave and master dynamics, three different synchronization regimes can be obtained, such as lag (LS), zero lag (ZLS)—when there are no delay times between the two dynamics—and anticipated (AS) [3].
2 Experimental Setup In this paper, the bidirectional coupling of an ECSL system with a solitary laser used as master and slave has been studied experimentally. The experimental setup is shown in Fig. 1 [5]. The two continuous wave SLs (Mitsubishi, ML101J8), were operated near the laser threshold currents, (emission at 661 nm; laser beam power about 2 mW). The optical feedback was obtained from a mirror placed at two different distances from the laser. The feedback intensity could be adjusted by controlling a neutral density filter with variable transmittance. The level of feedback intensity was adjusted until
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