IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 39, NO. 10, OCTOBER 2003 1177 Frequency Response of Polarization Switching in Vertical-Cavity Surface-Emitting Lasers Guy Verschaffelt, Member, IEEE, Jan Albert, Bob Nagler, Michael Peeters, Jan Danckaert, Sylvain Barbay, Giovanni Giacomelli, and Francesco Marin Abstract—We present an experimental study of the current- driven polarization modulation properties of VCSELs. In some VCSELs, abrupt polarization switching (PS) between two polar- ization modes is observed at a particular value of the pump current. We investigate the dynamics and the associated dominating time scales of PS as these features are strongly linked with the under- lying physical mechanism causing the PS. To this end, we measure both for gain- and index-guided VCSELs the critical modulation amplitude necessary to steadily force PS back and forth across the PS point as a function of the modulation frequency. This yields the current-driven polarization modulation frequency response, which we compare with the thermal frequency response of the studied de- vices. The dynamic behavior turns out to be strikingly different for the different VCSEL types. Thermal effects only play a minor role in the PS in our index-guided VCSELs, while they really seem to lie at the origin of PS in the gain-guided VCSELs. By implementing this in a rate-equation based theoretical model, we are able to ex- plain the peculiarities of the measured response curves and to re- produce the experimental findings. Index Terms—Amplitude modulation, numerical analysis, op- tical polarization, stochastic systems, surface-emitting lasers. I. INTRODUCTION D URING the last decade, vertical-cavity surface-emitting lasers (VCSELs) have evolved from laboratory curiosities to successful optical components used in a wide variety of appli- cations. Nevertheless, not all of their properties are completely understood. One of these properties is the polarization state of the emitted light. VCSELs usually emit linearly polarized light in one of two orthogonal linear polarization modes (PMs). These PMs are predominantly oriented along two specific crystallo- graphic directions and they have an optical frequency difference Manuscript received January 9, 2003; revised June 9, 2003. The work of J. Danckaert, G. Verschaffelt, and B. Nagler was supported by the Fund for Sci- entific Research–Flanders (FWO). The work of J. Albert was supported by the Institute for Science and Technology–Flanders (IWT). This work was supported by the Belgian Office for Scientific, Technical, and Cultural affairs through the framework of the Interuniversity Attraction Pole program, by the Concerted Re- search Action “Photonics in Computing” and the Research Council (OZR) of the Vrije Universiteit Brussels, and by the European COST (#268). The collab- oration between the groups in Brussels and Florence was made possible through the European RTN network VISTA (Contract HPRN-CT-2000-00034). G. Verschaffelt, J. Albert, B. Nagler, M. Peeters, and J. Danckaert are with the Department of Applied Physics and Photonics (TW—TONA), Vrije Universiteit Brussels, 1050 Brussels, Belgium (e-mail: guy.verschaffelt@vub.ac.be). S. Barbay was with Istituto Nazionale di Ottica Applicata and INFM, 50125 Firenze, Italy. He is now with LPN, 92225 Bagneux, France. G. Giacomelli is with Istituto Nazionale di Ottica Applicata and INFM, 50125 Firenze, Italy. F. Marin is with the Department of Physics, University Firenze and INFM and LENS, 50019 Sesto, Italy. Digital Object Identifier 10.1109/JQE.2003.817241 in the range of 1 to 40 GHz [1], [2]. In some VCSELs, abrupt polarization switching (PS) between the PMs is observed when the injected current is changed. While uncontrolled PS is highly undesirable in most applications, controlled current-induced PS might be interesting as an alternative switching mechanism. Over the past several years, a number of experimental [3]–[9] and theoretical [10]–[13] publications have aimed at under- standing this phenomenon. Roughly speaking, the different proposed mechanisms can be divided into two categories: those invoking slow (lattice) thermal mechanisms [3], [5], [7] and those relying on other, faster mechanisms [4], [11], [12]. These models are, to a large extent, equivalent at the level of steady-state characteristics, but are much less so when looking at the dynamical properties of the PS. Therefore, we investigate in this work the dynamics and the associated dominating time scales of PS in VCSELs. Our dynamical characterization is based on the measurement of the current-driven polarization modulation frequency response. This means in short that we have determined the minimum modulation amplitude needed to steadily force PS back and forth across the PS point, as a function of the modulation frequency. This dynamical analysis represents a critical test for the models describing the polar- ization features of VCSELs. This is not only important from a physical point of view, but also of great practical importance if one wishes to exploit the VCSEL’s polarization properties for applications [14], [15], where fast and controlled PS is required. In order to understand and describe the PS dynamics, we use a nearly degenerate two-mode intensity rate-equation model. Thermal effects are included in this model by making the gain difference between the two PMs temperature dependent. We reduce this model and obtain an accurate theoretical description of the PS dynamics, which we have verified to be consistent with our numerical simulations. We have carried out the dynamical characterization on two different types of VCSELs. First, we have studied proton-im- planted (PI) gain-guided GaAs–AlGaAs VCSELs from VIXEL Corporation, operating around 850 nm with a threshold of about 7 mA. As these are commercial devices we have no positive in- formation about their structure. From the literature [16], how- ever, we guess that the device structure contains 3 GaAs QWs of 8-nm thickness centered in a 1 cavity with a 29.5 pair of n-doped bottom distributed Bragg reflectors (DBRs) and a 19 pair of p-doped top DBRs. The cavity diameter is 8 m. The PS in these lasers occurs in the single-transverse mode regime from longer to shorter wavelength for increasing current (i.e., type-II PS) while being operated under continuous wave (CW) conditions on the high-frequency side of the gain maximum [6], 0018-9197/03$17.00 © 2003 IEEE