COHERENT COUPLING OF RADIATION MODES IN MACH-ZEHNDER ELECTROOPTIC MODULATORS

In this letter the effects of the radiation modes in Mach-Zehnder GaAs electrooptic modulators are studied using the standard beam propagation method (BPM) [1]. The radiation mode contribution to the energy transfer in the passive structure is studied as well as the performances of the active device under modulation. We found that for compact weakly guiding structures a good geometrical design, that takes into account the propagation of the radiation modes, is not only effective in loss reduction but also important for improving the design of the active structure. A 20% change of the modulator bias voltage is predicted with a shortening of the interaction length from 5 to 4.6 mm. This result is physically understood when considering the coherent coupling of the radiation modes in the structure.