Photoreflectance measurements of multilayered AlxGa1-xAs/GaAs semiconductor structure in the regime of high modulating laser powers are reported. The main photoreflectance feature observed at low powers at about 1.4 eV is related to Franz-Keldysh oscillations caused by electric field at the AlxGa1-xAs/GaAs interface. The value of electric field deduced from these oscillations is in agreement with that determined from calculation using the accepted value of the AlxGa1-xAs/GaAs band offset and the doping level in the GaAs layer. On increasing the power of the modulating laser, the spectrum changes its character and after decomposition reveals a feature at the energy corresponding to GaAs bandgap. The intensity of this line increases with the modulating laser power, moreover the line is in phase with the Franz-Keldysh oscillations. These observations are consistent with the assignment of the line to the photoreflectance at the underlying GaAs buffer/GaAs substrate interface. This finding underlines the altered character of the photoreflectance spectra at high light intensities, compared to the standard low intensity regime.