The absorption due to transitions between valence and conduction subbands in doping superlattices is calculated analytically and numerically. Results for various materials and design parameters of the superlattice configuration are presented for illustration. It is found that with an appropriate choice of design parameters the absorption coefficient exhibits a pronounced steplike structure for photon energies less than the band gap of the host material. The absorption of a given specimen is strongly tunable by external fields, by carrier injection, and by the generation of carriers due to the absorption process itself. At fixed photon energy, large-amplitude oscillations of the absorption coefficient as a function of both the external electric field and the induced carrier concentration are predicted. The limitations of our approach, and consequences of our results for interesting applications, are discussed briefly.