A finite difference method is used to analyze the behavior of photorefractive InP:Fe at high fringe contrast under externally applied field. The evolution with increasing fringe contrastmof the electron‐hole resonance, which occurs in the presence of a continuous field, is studied. Asmincreases, the field tends to concentrate in a small zone and reaches very large local values. On the other hand, the resonance loses height and widens. For illuminations closed to the resonance, the two‐wave mixing gain presents the usual aspect of a decreasing function ofm. However, for other illuminations, it can be larger at a small pump‐to‐probe ratio than at a large one. When an ac field technique is used, nonlinearities drastically reduce the two‐wave mixing gain, even at small fringe contrast. Asmincreases, the space‐charge field tends to take a square shape. Even for rather smallmvalues (0.1), the gain presents a maximum near the dc field resonant illumination, which is not predicted by the linear theory. Finally, when drift is the dominant process for the grating formation, the applied field limits the amplitude of the space‐charge field, independently of an optimized dopant concentration which allows high gain in the smallmapproximation. ©1995 American Institute of Physics.