A thick, bimodal segment of specific length and height between two single mode sections of a planar waveguide can serve as an integrated optical interferometer. It is realized by etching a wide strip form a guiding film. A vertically guided, laterally unguided beam of light is then made to traverse the strip perpendicularly. For a wide range of materials the structure can be dimensioned such that it shows the proper behavior of an interferometer: depending on the phase gain of the two modes in the thick region, the guided light interferes either almost completely destructively at the transition to the output segment, i.e. the power is radiated away into the substrate and cover regions, or constructively, i.e. most of the power passes the device. We believe that for certain applications structures of this kind can be a simple substitute for instruments like Mach-Zehnder interferometers or directional couplers. This is illustrated by two numerically simulated examples: A polarizer constructed from silicon based waveguides, which offers 30 dB polarization discrimination and 0.1 dB insertion loss with a total length of only 10 micrometers, and a proposal for an integrated magneto optic isolator experiment, where the freedom in the lateral direction can be exploited for a proper tuning of the device.