Modulation of midinfrared beam by photoinduced intersubband absorption in undoped Si/SiGe quantum wells is investigated at normal incidence. Optical pumping of interband transitions is used to photocreate carriers in the wells. The modulated transmission of the sample is measured by Fourier transform spectroscopy. Resonant dispersion associated with intersubband transitions as well as interference effects in the quantum well stack are clearly evidenced. It is shown that the phase modulation due to refractive index variations may compensate for amplitude modulation due to absorption. Measurements are well fitted by a phenomenological model. It is also shown that the modulation depth and the wavelength modulation profile both depend on the interband excitation wavelength. This result is attributed to the different nature of photoinduced transitions according to the excitation wavelength. ©1995 American Institute of Physics.