A clear quantum-confined Stark shift is reported in a 150 quantum-well GaInAs/InP pin diode grown by AP-MOCVD. 4.1 dB modulation (2.6:1 contrast ratio, 4.9 dB insertion loss) is achieved in perpendicular (mesa) geometry at 1.59/μm wavelength in an electric field of ~1.3×105 V cm−1. Growth is on a semi-insulating rather than a p+-substrate, to eliminate the effects of free-carrier absorption.