Comparative analysis of the performance limits of Franz-Keldysh effect and quantum-confined Stark effect electroabsorption waveguide modulators

The author systematically analyses and compares the performance limits of quantum-confined Stark effect (QCSE) and Franz-Keldysh effect (FKE) electroabsorption waveguide modulators, in terms of insertion loss, contrast ratio, drive power and bandwidth (or bit-rate). The author first derives the universal material figures of merit for electroabsorption modulators which form the basis for comparison. In addition to the magnitude of electroabsorption Delta alpha , the critical material parameters are Delta alpha / alpha /sub 0/ and Delta alpha /F/sup 2/ where alpha /sub 0/ is the onstate residual absorption, and F is the applied electric field. The author proposes a waveguide design which will meet the insertion loss and contrast ratio requirements while minimising the available power/bandwidth (P/sub ac// Delta f) ratio. The author shows that, while satisfying the same insertion loss requirement, a QCSE modulator employing the optimum quantum well structure can have, in principle, an order of magnitude better performance in terms of P/sub ac// Delta f than one based on FKE. Correspondingly, the Delta alpha /F/sup 2/ is an order of magnitude larger in QCSE than in FKE, principally because of the much larger absorption change that is possible with QCSE.