A comparative study of the temperature dependence of the two-wave mixing gain is performed at three wavelengths (i.e., 1.06, 1.32, and 1.54 mu m) for the fringe spacing values Lambda = 1.6 mu m and 5 mu m. With the latter the gain sign changes at low temperature for lambda = 1.06 and lambda = 1.54 mu m, but not for lambda = 1.32 mu m. The gain measurements are performed both with and without an external electric field. The theoretical model must fit all the experiments. In our case the energy level of the second defect is found to be near the valence band. We demonstrate that the agreement between the theory and the experiments performed at room temperature is improved by incorporating the indirect transitions (involving the excited state of iron) into the two-defect model. The result of this approach is presented as the extended two-defect model. (C) 1997 Optical Society of America.