Vertical Cavity Surface Emitting Lasers (VCSELs) are expected to become some of the most popular laser diodes in the near future. Their general characteristics, performance, fabrication aspects and applications are reviewed in other chapters of this book (Blum 1999, Ebeling 1999). A peculiarity of these lasers, as compared to conventional edge emitting laser diodes, is that, due to their different geometry, the polarisation of the laser light is not well stabilised. This becomes a problem for applications which require a well stabilised polarisation and it has motivated the proposal of a number of methods to fix the polarisation state of the emitted light. These methods include introducing polarisation sensitive DBR mirrors (Ser et al. 1995), geometrical or stress-induced anisotropies (Mukaihara et al. 1993) or engineering of the semiconductor material or the growth process (Chavez-Pirson et al. 1993, Sun et al. . 1975) to favour the gain of one of the two independent polarisation directions. A different attitude is, instead of suppressing the vector degree of freedom associated with the polarisation of light, to learn how to control and use it in possible applications based on the polarisation state, such as optical switching (Nishikawa et al. 1995, Kawaguchi and Kawakami 1977), information processing or storage, etc. This requires the understanding of the basic physical mechanisms that control the polarisation of laser light. Such understanding should make it possible to follow the theme of this book: "From quantum physics to smart devices". In these lectures I review, from a laser physics point of view, a macroscopic model of VCSELs which incorporates those basic mechanisms.
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