COMPARATIVE-ANALYSIS OF THE PERFORMANCE LIMITS OF FRANZ-KELDYSH EFFECT AND QUANTUM-CONFINED STARK-EFFECT ELECTROABSORPTION WAVE-GUIDE 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 eiectroabsorption modulators which form the basis for comparison. In addition to the magnitude of electroabsorption Delta alpha, the critical material parameters are Delta alpha/alpha(0), and Delta alpha/F-2, where alpha(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-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-ac/Delta f than one based on FKE. Correspondingly, the Delta alpha/F-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.