High-Speed Directional Coupler Modulator in AlGaAs with Capacitively Loaded Electrode Structure

An electrode structure for the directional coupler modulator based on the capacitive-loaded design was designed, as shown in Figure 1, where the RF signal line and ground plane are placed on either side of optical waveguides and connected to the electrodes on top of the ridges through 50 Jim wide air-bridges. The impedance matching, and tapers, are designed using the constant impedance in coplanar waveguide. As the RF signal is applied to one end of the electrode, it travels through the side electrode which has the capacitance Cd, and because it is connected to the electrode on top of the ridge, additional lump capacitance Cd’ will be added to the structure, as shown in Figures 2a and 2b.