Nonlinear absorption spectra of bulk GaAs and 299-, 152-, and 76-angstrom GaAs/AlGaAs multiple-quantum-well (MQW) samples have been studied. Nonlinear index spectra were calculated by Kramers-Kronig transformations of the absorption spectra. It was concluded that the principal nonlinear refractive mechanisms for optical bistability in bulk GaAs etalons are band filling and reduction of the Coulomb enhancement of continuum states; exciton saturation and bandgap renormalization occur at lower carrier densities, but their contributions are of opposite sign and almost cancel. Exciton saturation nonlinearities dominate for bistability in MQW samples with <100-angstrom wells. Optical bistability was found in a strip-loaded waveguide formed by reactive ion etching; the guiding layer consisted of sixty periods of 100-angstrom Al0.28 Ga0.72As. The power for bistability was about the same in the etalon and the waveguide. The 100-fold increase in length of the nonlinear material from the etalon to the waveguide requires greater detuning to keep the absorption acceptably low, resulting in lower carrier density and a weaker nonlinear refractive effect per unit length.