Directional couplers are also OGW devices that have been investigated for use as EM field sensors (7). In the interaction region, the two waveguides lie close enough together that the evanescent field of the lightwave in one guide couples into the other guide. In such a coupled-mode system, the energy in one lightguide can be switched to the other guide by changing the relative propagation constants in the channels with the electro-optic effect. If the fabrication parameters are chosen correctly, the transfer function given in the literature (7) for the directional coupler can be written in the form given in Table 1.