Abstract: A method and circuit is disclosed wherein two beams exiting opposite ends of a resonant optical cavity, such as a Fabry-Perot (FP) etalon for example, are provided via waveguides or via free space directed by mirrors to a combining region where the beams can 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 linearized output response useful in applications such as wavelength locking. By varying the reflectivity of the optical cavity reflectors and/or by varying the phase relationship between the two beams exiting the optical cavity, a variety of desired output responses can be realized.