Theoretical optimisation of Electrooptical Phase Modulation in InGaAsP Quantumwells.

The superior performance of light modulators based on the quantum confined Stark effect has been accepted for some time. Many experimental and theoretical data were obtained for GaAs-AlGaAs QW and SL. In the 1.55μm optical window, which is of particular interest for optical communication purposes, InGaAs and InGaAsP QW give better performances. For the ternary material refractive index data were published1 and the use of InGaAs and InGaAsP QW's in waveguide modulators2 and DBR lasers3 was demonstrated. Recently the advantage of using quaternary quantum wells for electrooptic phase modulation at 1.55μm was stressed4. The main point of interest is the additional degree of freedom, namely the InGaAsP composition. The problem of finding the optimal composition together with the best possible QW size for maximal phase modulation, within certain restrictions for the applied field, is treated on a theoretical basis in this paper. The modeling of the electric field dependent absorption in QW’s has already obtained a lot of attention5. Refractive index changes under applied field are much more difficult to model accurately due to the long energy range of the changes induced by the field. Recently Yamamoto et al.6 presented some theoretical results in a 30nm InGaAsP QW. They neglect exciton effects and only consider contributions of heavy holes. Both approximations work quite well in the large well limit, but are less accurate for smaller QW's.