A quantum well electroabsorption waveguide modulator utilizing a field-induced refractive index change was designed and fabricated. The on/off ratios of the device were investigated as a function of wavelength over the spectral range of 850 nm to 910 nm for the various reverse biases. The field-induced refractive index variations associated with quantum-confined Stark effect was theoretically obtained based on the measured on/off ratios. The resulting maximum refractive index change(</TEX>${DELTA}n) of ~7.5 { imes} 10^{-4}$</TEX> at -8 V was estimated.