  In this paper we describe a novel, graphical method, allowing the fast
computation of field operator transformations for linear lossless optical
devices in Quantum Optics (QO). The advantage of this method grows with the
complexity of the considered optical setup. As case studies we examine the
field operator transformations for the beam splitter (BS), the Mach-Zehnder
interferometer (MZI) and the double MZI. We consider the simple case with
monochromatic input light, as well as extensions to the non-monochromatic case.
