We have determined the electron intersubband energy spacing in In0.4Ga0.60As/GaAs self-organized quantum dots from analysis of the temperature-dependent optical modulation response of single-mode lasers in which four dot layers from the gain region. The 3 dB modulation bandwidth increases from 7 GHz at 300 K to >20 GHz at 80 K. An intersubband energy spacing of 60 meV is measured. This value is in excellent agreement with the theoretically calculated value of 57.5 meV and verifies that intersubband relaxation in quantum dots occurs via electron-hole scattering.