High-bandwidth velocity-matched optical directional coupler modulator in A1GaAs/GaAs

We report a design of a very high frequency A1GaAs/GaAs optical modulator using a dielectric loading velocity matching technique. The optical modulator is a directional coupler which is covered with a thin coating of Ta₂0₅ to match the velocities of the lightwave and the if modulating signal to within 2%. The optical and rf characteristics of the modulator are analyzed using the effective index and finite difference methods. The small signal 3 dB bandwidth for the modulator is predicted to exceed 45 GHz and 100 GHz for biasing at the null point and in the linear region respectively, with a maximum modulation voltage of 5V at 830nm wavelength.