Modulation of mid infrared beam by photo induced intersubband absorption in undoped SiGe?Si quantum wells was investigated at normal incidence. An Ar +laser is used as optcal pumping of interband transitions to photocreate carriers in the wells. The modulated transmission of the sample was measured by a step scan Fourier transform spectroscopy. Resonant dispersion associated with intersubband transitions and mutiple reflections effect in the multi quantum wells were clearly evidenced. Theoretical and experimental results show that the phase modulation due to refractive index variations may compensate for amplitude modulation due to absorption.