We present a novel method to extract the optical absorption coefficient of a semiconductor electroabsorption modulator (EAM) with antireflection coated facets. Only the transmission and photocurrent data are needed with this method. The method allows to obtain the total optical coupling loss, internal quantum efficiency, and optical absorption coefficients when input optical power is low. We also developed a method to analyze optical absorption coefficient with the saturation effect. By using those methods, we investigated the dependence of the transfer curves on the input optical power in a multiple-quantum-well EAM. The optical loss saturation effect and the increase of device temperature are the dominant mechanisms for the dependence of transfer curves on the input optical power in the low bias region and the high bias region, respectively.