Entanglement in quantum-optical bilinear devices

We examine the entanglement arising from the bilinear coupling of two nearl y single-mode optical beams. We consider both the case of active and passiv e linear devices. The two modes are initially prepared in a pair of uncorre lated pure Gaussian states, and the degree of entanglement is analytically evaluated in terms of the excess information entropy at the output. A gener al formula is obtained and relevant cases are analysed in some detail.