A method and circuit is disclosed wherein two waveguides coupled to an optical resonant ring resonator are further coupled such that beams propagating therein interfere with one another to provide a desired output response. In one embodiment, multiplexed channels of light can be demultiplexed by the device described heretofore, or alternatively, the phase relationship between these two beams can be altered prior to their being combined to provide, for example, a linearised output response useful in applications such as wavelength locking. By varying the reflectivity of the FSR and the coupling ratios and/or by varying the phase relationship between the two beams, a variety of desired output responses can be realised. By providing suitable coupling ratios and providing optical path lengths of the two waveguides which are identical between the ring resonator and the couplers coupling the two waveguides as well as having the couplers placed 180 degrees opposite to each other, an interleaver/de-interleaver function is realised. In other embodiments, different forms of optical cavity resonators (etalons) are used instead of the ring resonator.