Experimental study of feedback-induced dynamics in semiconductor lasers: from symbolic analysis to subwavelength position sensing

Author: Andres Aragoneses
Title: Experimental study of feedback-induced dynamics in semiconductor lasers: from symbolic analysis to subwavelength position sensing
Directors: M. C. Torrent and Cristina Masoller
Presentation date: June 2014
Link to text: http://www.tdx.cat/handle/10803/277569

Abstract: The aim of this thesis is the study of the dynamics induced by optical feedback in semiconductor lasers. This study aims, on the one hand, to improve our knowledge of stocahstic complex systems, and on the other hand, to use complex dynamics of semiconductor lasers to develop a protocol for subwavelength position sensing. The intensity of the light emitted by a semiconductor laser is stable, besides fluctuations due to spontaneous emission noise. When the light of the laser is reflected and part re-enters into the laser, the laser intensity can become unstable, displaying a broad range of dynamical behaviors. One of the dynamical regimes present in lasers with optical feedback is the low frequency fluctuations (LFF). This dynamics is characterized by sharp drops in the laser intensity (to almost switch the laser off), followed by gradual recoveries. The time intervals between two consecutive drops is irregular. The first part of this Thesis is focused on this dynamic regime, and a detailed experimental study has been performed to characterize it. A symbolic time series analysis has been used, based on the comparison of successive time intervals between dropouts. The dynamics of a semiconductor laser with feedback is governed by nonlinear light-matter interaction in the active medium of the laser, quantum noise due to spontaneous emission and time-delayed feedback. Therefore, the dropouts in the LFF regime can be noise-induced or triggered by deterministic processes. In this Thesis symbolic ordinal analysis has been used to statisticlly distinguish dropouts that can be noise-induced from those that have signatures of a deterministic origin. In this Thesis, the symbolic dynamics in the LFF regime has also been studied, and serial correlations have been found among several consecutive dropouts. It has been found a hierarchical and clustered structure of the symbolic dynamics. Moreover, a minimal iterative model has been found that, despite its simplicity, describes successfully the correlations found in the experiments. Because of the importance of external forcing in dynamical systems, the effect of current modulation on the symbolic dynamics of the LFFs has been studied. These experiments have allowed to characterize the effect of the modulation in the symbolic dynamics. The clusters of ordinal patterns formed without forcing remain under external periodic forcing. The minimal model has been verified, as it reproduces satisfactorily the symbolic dynamics of the experimental data. Also, in this Thesis a technique has been developed to detect displacements of two independent objects at subwavelength resolution (λ/160). With this purpose, a setup has been developed with a semiconductor laser with dual feedback. In addition to the high resolution, this protocol offers the advantage of sensing two objects by just measuring one variable.