  We further elaborate the theory of quantum fluctuations in vertical-cavity
surface-emitting lasers (VCSELs), developed in Ref. \cite{Hermier02}. In
particular, we introduce the quantum Stokes parameters to describe the quantum
self- and cross-correlations between two polarization components of the
electromagnetic field generated by this type of lasers. We calculate
analytically the fluctuation spectra of these parameters and discuss
experiments in which they can be measured. We demonstrate that in certain
situations VCSELs can exhibit polarization squeezing over some range of
spectral frequencies. This polarization squeezing has its origin in
sub-Poissonian pumping statistics of the active laser medium.
