Mode Coupling Between Substrate Integrated Waveguide and Coplanar Waveguide for Traveling-Wave Electrooptical Modulator

A traveling-wave electrooptical modulator that makes use of a mode coupling mechanism between a substrate integrated waveguide (SIW) and coplanar waveguide (CPW) is proposed for millimeter-wave (mmW) radio-over-fiber applications. In this structure, SIWs are added at the input/output ports of conventional modulators from which mode conversion and signal transfer take place between lossy CPW traveling-wave electrodes and low-loss mmW SIW terminals. Such a gradual coupling from SIW to CPW in the modulation path creates a nonuniform field overlap integral and gives the ability of using the modulator at higher frequency. A laser micromachining technique for a transparent LiNbO₃ substrate and a titanium in-diffusion technique for achieving a good optical field confinement are used to fabricate the SIW and optical waveguide, respectively. The bending structure of the SIW, fabrication of pad resistors, and broadband transitions are considered in the proposed modulator. 7-GHz bandwidth was measured for the proposed modulator at 60 GHz.