We show that extremely high quality InGaAsP/InP superlattices grown by chemical beam epitaxy meet the stringent requirements for optical modulator application in the wavelength range from 1 to 1.6 &mgr;m. Double crystal x‐ray diffraction from multiple quantum well samples of 20 to 100 periods show that the satellite peaks have widths comparable to or narrower than the substrate peak width, which indicates very reproducible thickness and composition control along the growth direction. For optical modulation, ap–i–nwaveguide structure consisting of 20 periods 90/90 A˚ InGaAsP/InP exhibits very sharp excitonic feature and large quantum confined Stark effect near 1.5 &mgr;m. A room temperature exciton shift of about 25 meV has been measured for a bias voltage of 100 kV/cm in the photocurrent spectra.