I. Introduction
External optical modulators of InP-based materials, for application in optical-fiber communication, have drawn much attention over the past years due to the possibility of easy integration and low chirp [1]–[6]. The InGaAs–InAlAs multiple quantum well (MQW) system can be grown on InP substrates and it presents a lower valence band offset than that of the InGaAs–InP one. This last property is desirable to avoid saturation effects due to a long hole escape time from the quantum wells (QWs). Low-driving voltage, polarization insensitivity, low losses, and negative chirp are crucial requirements for optical telecommunication systems. InGaAs–InAlAs amplitude modulator structures, which are ready to satisfy one of these requirements, were already obtained [1], [7]. However, difficulties in achieving all the above-mentioned requirements simultaneously have already been pointed out [8]. Therefore, if the InGaAs–InAlAs system is expected to be used in the next multigigabit long-haul fiber transmission systems, the MQW structure containing these materials should be further optimized.