In the present paper a coupled cavities filter with potential applications in photonic crystal microcircuits is studied. The
analysis is based on "coupled mode theory in time" which is an approximate method that allows relatively simple
derivations of optimal design parameters. Coupled cavities appear everywhere in optical circuits. In many cases they
induce parasitic effects like big reflections back to the source. In other situations, if carefully designed, a device
consisting of two micro cavities can act as a bandpass filter.