We have investigated the bound-to-continuum absorption of InGaAs quantum dots as a function of n-doping. We show that the combination of multipass waveguide geometry, large number of quantum dot layers, and spectrally broad absorption leads to significant spectral oscillations on the absorption spectra. The oscillations result from the standing wave pattern caused by the interference of optical beams incident and totally reflected at the sample/air interface. The spectral modulations on the absorption spectra should not be attributed to resonant intersublevel transitions.