Optimization of quantum well electroabsorption waveguide modulators.

摘要

Electro-optic waveguide modulators employing III-V compound semiconductor quantum wells were investigated and developed. These modulators are based on the Quantum Confined Stark Effect (QCSE). This effect includes electroabsorption and electrorefraction. For electroabsorptive waveguide modulators, a theoretical design algorithm was developed for optimizing the total performance in terms of four figures of merit: contrast ratio (CR), modulated optical throughput, drive voltage and bandwidth. The approach is to maximize the bandwidth/drive voltage ratio while satisfying a given CR and optical throughput. An experimental evaluation of devices based on the theoretical design was made using, for this purpose, the InGaAs/InAlAs quantum well material system. The modulator design embodies a novel approach using a large-core, multimode, passive waveguide. This is a simple and robust approach that reduces the coupling loss from a conventional single-mode input fiber without compromising other performance requirements. The MQW is embedded in a thin intrinsic active layer within the passive waveguide. The waveguide therefore has a small optical filling factor. The figures of merit for the material design of the quantum well structure were identified to be {dollar}Deltaalpha{dollar}/F and {dollar}Deltaalpha{dollar}/{dollar}alphasb{lcub}rm o{rcub}{dollar}, where {dollar}Deltaalpha{dollar} is the absorption change and F is the applied electric field. Experimental methods were developed for measuring the coupling efficiency, the optical filling factor ({dollar}gamma{dollar}) and the absorption coefficient ({dollar}alphasb{lcub}rm o{rcub}{dollar}) of the waveguide modulator, and for characterizing the material figures of merit. The experimental measurements support the underlying assumptions used in the theoretical treatment, and show that the passive, small-{dollar}gamma{dollar} waveguide design significantly improves the modulated optical throughput of the modulator compared with the conventional large-{dollar}gamma{dollar} design reported in the literature.