A technique is proposed for the extraction of precise values of field-dependent absorption coefficient alpha and refractive index n from photocurrent and transmittance measurements of optical modulator structures. The technique uses approximate results of alpha and n extracted from a simplified device as the initial input into an iterative procedure that utilizes the consistency between alpha and n to obtain successively better estimates of these parameters. The technique was applied to results that were measured experimentally, and we verified the accuracy by using synthetic data. Errors caused by measurement inaccuracy are also investigated. It is shown that the absorption coefficient has a modest sensitivity whereas the refractive index is insensitive to these errors. (C) 1999 Optical Society of America. OCIS codes: 120.4530, 230.5590, 230.2090, 230.4110, 190.5970, 310.6860.