Contents I. Introduction
Optical modulators are enabling components for a wide range of applications such as fiber-optic transmission, radio-over-fiber, and instrumentation. All these applications require low-cost, low drive voltage external modulators with very wide bandwidth. This work reports a technology based on compound semiconductors and substrate removal processing to deliver these requirements. Although compound semiconductors have relatively small electrooptic coefficients, they have high index of refraction and very small dielectric constant dispersion up to optical frequencies. Furthermore, high-quality epitaxial growth and advanced processing techniques enable flexibility in designing and fabricating novel designs. Low dielectric constant dispersion requires microwave velocity slowing for velocity matching. This can be achieved using capacitavely loaded slow wave electrodes. The substrate removal technique allows placing metal electrodes on both sides of an epitaxial layer which provides the required capacitive loading without increasing microwave loss. This also enables the fabrication of an ideal push—pull modulator and keeps the drive voltage low. We have demonstrated such bulk GaAs—AlGaAs modulators with a drive voltage of 3.7 V-cm [1]. In this letter, we report optimized modulator designs with a drive voltage as low as 5 V-cm and bandwidths as high as 40 GHz.
