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Polarization and Intensity Noise in Vertical-Cavity Surface-Emitting Lasers

Mulet, Josep (Directors M. San Miguel and C.R. Mirasso )
Memoria d'Investigacio UIB , (2001)

The important advances in the laser field and quantum electronics, carried out in the last decades, have lead to a variety of everyday life applications for these devices. Some examples include: optical communication systems, the explosive growth of Internet applications, CD players, laser pointers, laser printers, as well as medical and industrial applications. The development of semiconductor lasers and the techniques to improve their performance have provided devices very attractive for applications, specially in optical communication systems. The design of new structures as well as the implementation of novel materials have lead to semiconductor lasers with rather peculiar performance. Vertical-cavity surface-emitting lasers, hereafter VCSELs, have sparked the interest of many scientific group, covering from the most applied point of view to that of the fundamental physics. It is well known that VCSELs present a number of advantages with respect to the conventional edge-emitting semiconductor lasers, although they may display instabilities associated with the polarization and transverse degrees of freedom. In many applications, it is crucial to achieve devices emitting in a well stabilized and controlled polarization and transverse mode. This fact motivates the study, characterization and control of polarization and transverse dynamics in VCSELs. In this work, we investigate the impact of polarization components fluctuation in the characteristics of the laser emission. These fluctuations arise from spontaneous emission processes governed by the laws of quantum mechanics. The features of these fluctuations can be regarded as fingerprints of the involved physical mechanisms, thus providing a natural interpretation of the experimental results when comparing with theory. This report is organized in four chapters as follows. In chapter 1, we sketch some fundamental concepts required to introduce the laser physics. We introduced them sequentially within an historical perspective. With the help of these concepts, it is then possible to outline the working principles of VCSELs. In chapter 2, we focus on modeling issues, and particularly, on the spontaneous emission noise sources, polarization properties, and spin relaxation processes. We introduce the spin flip model (SFM) that provides a theoretical framework to describe the polarization dynamics in VCSELs. At the end of chapter 2, we introduce the linearized SFM, being the starting point to study polarization fluctuations. In chapter 3, the main part of this work, we discuss within a semiclassical framework, the fluctuations of the polarization components. In order to understand the behavior of fluctuations, it is first necessary to introduce some concepts that arise from the linear stability analysis of the SFM. Concepts like non-linear anisotropies, effective birefringence, and regimes of operation are sequentially introduced. Thereafter, we analyze the power spectra, that reflect the magnitude of fluctuations at different frequencies, of the linear and circular components of the electric field. A complete understanding of the polarization dynamics is possible by introducing the Poincar´e sphere representation. The later is a projection of the dynamics onto a spherical coordinate system described by two polarization angles. The power spectra of the polarization angles fluctuations are also shown in several regions of operation. We also give evidences of the role of some relevant parameters that affect the polarization dynamics and its fluctuations. At the end of chapter 3, we describe the correlation, that has been experimentally observed, among different polarization components. Finally in chapter 4, we present the conclusions as well as some perspectives related to modeling issues.