Frequency response of current modulation induced polarization switching in VCSELs

Abstract

We present an experimental and rate-equation based theoretical study of the current-driven polarization modulation properties of VCSELs. In such lasers a high-contrast polarization flip is often observed at a particular value of the pump current. When modulating the current around the polarization switching value, we measure the critical modulation amplitude necessary to force synchronized back-and-forward polarization flips, as a function of the modulation frequency. This yields the polarization modulation frequency response. For a proton-implanted VCSEL the shape of the measured response curve is characterized by time constants that are very long compared with the usual time scales of laser dynamics (such as photon and carrier lifetimes), and compatible with the measured thermal relaxation time. Indeed, both the polarization modulation and the thermal frequency response curves show a cut-off frequency of about 90kHz, independent of the particular value of the switching current. In the frequency response curve of an air-post VCSEL one clearly sees remnants of the thermal influence on the switching. However, one cannot say that a thermal cut-off inhibits polarization switching above a certain modulation frequency. Notwithstanding the difference in impact of thermal effects depending on the type of device under study, our results indicate that it is necessary to incorporate a temperature-dependent variable in realistic models describing the dynamical polarization properties of VCSELs.