Optical internal transitions relevant to experiments based on the optical detection of far-infrared resonances are computed for the biexciton in a quantum dot in the strong-confinement regime. Valence sub-band-mixing effects are taken into account in second-order perturbation theory. The probability of transition from the ground state is concentrated in a few states with relatively high excitation energies. Level collisions with oscillator strength transfer are observed as the magnetic field is raised.
