  This thesis is mainly devoted to the study of the quantum properties of
optical parametric oscillators (OPOs), which are nowadays the sources of the
highest-quality quantum-correlated light, apart from fundamental tools in the
classical-optics realm, allowing for the conversion of laser light into
virtually all regions of the optical spectrum. Regarding its content, the
thesis might seem a bit unusual, because two thirds of it are devoted to a
self-contained (though dense) introduction to quantum optics, including the
quantum physics of harmonic oscillators, the quantization of the
electromagnetic field in an open optical cavity and the detection of its output
light, as well as the derivation of the basic model and known properties of
OPOs. Hence, all the original results of the thesis are contained in the last
third, were it is proven that all OPOs can be understood as multi-mode devices
whose quantum properties can be explained in terms of three basic phenomena:
bifurcation squeezing, spontaneous symmetry breaking, and pump clamping, which
are introduced through simple, yet realistic examples.
