The idea of Integrated Quantum Optics using nonlinear optical waveguide substrates is to combine active optical elements such as frequency converters, electro- and acousto-optical devices with passive elements (directional couplers, tapers, beam splitters, junctions etc.). Of course a sophisticated device design will monolithically integrate a number of such functionalities within a single optical waveguide chip, a so called integrated optical circuit (IOC).
As an example Fig. 1 shows a simple device, which contains a parametric down- converter (periodically poled area) and a directional coupler (S-bend WDM) in order to separate the generated photons spatially. It also consists of high quality end-face coatings to improve the overall conversion efficiency and to block the pump light. Thus, this basic building block for quantum networks contains three functionalities .