High Contrast Reflection Electro-Absorption Modulator With Zero Phase Change

Abstract

Vertical cavity quantum-well optical modulators utilizing the quantum-confined Stark effect (QCSE) are being increasingly investigated for use in optical communication and computing.1 Reflection modulators based upon large absorption changes in quantum wells have shown high contrast ratios2 while exhibiting lower parasitic phase modulation than conventional waveguide modulators.3 Since quantum wells are capable of large absorption changes (Δα), they also exhibit large refractive index changes (Δn).4-5 Understanding the role of quantum well excitons and phase is crucial for high speed optical switching since such switching will be critically limited by Δn induced pulse broadening. The ratio of phase shift to absorption is typically reported through the chirp parameter where a low chirp parameter is desirable for low-dispersion amplitude modulation. Here we describe how parasitic Δn can be eliminated in a novel, optimized structure thus yielding a pure reflection modulator with a zero chirp parameter. At the operating wavelength, such a device would exhibit a high reflectivity change and zero phase change when switched between two bias points. We have designed and developed such devices by analyzing excitonic lineshapes, numerically simulating our structures including quantum well absorption and dispersion effects, and creating a flexible modulator design which allows for optimization after growth. We note that our theory also applies to eliminating Δn in waveguide modulators which rely upon the QCSE.