All-optical modulators have been fabricated, based on the infrared photoinduced absorption produced within an optical waveguide upon visible light illumination. The modulation data are analyzed by means of simulation software based on a numerical mode solver. It is found that the modulation depth increases for pump illumination energy closer to the energy gap of the guiding material, while illumination at varying intensity shows a sublinear dependence of the photoinduced absorption. The results are discussed in terms of occupation statistics of gap states. It is shown that the major contribution to the photoinduced signal derives from the modulation of the occupation of tail states under illumination. Modeling of the phenomenon allows one to predict the behavior of a given device and opens the way to practical applications.