We measure the modal absorption coefficient of the InGaN quantum wells (QWs) in the absorber section of (Al,In)GaN multisection laser diodes as a function of bias voltage and photon energy using optical gain-spectroscopy. In the examined laser diodes, the modal absorption at the laser wavelength of 430 nm has a maximum of 270 cm(-1) at low negative bias and decreases with increasing negative bias. We explain this behavior by comparing the measurements to absorption coefficients calculated from a band-edge profile simulation. The decrease of the absorption at large negative bias is caused by a shift in the transition energies in the quantum wells due to the quantum confined Stark effect. (C) 2010 American Institute of Physics. [doi:10.1063/1.3514232]