Results of a detailed theoretical analysis on the detection of phase-modulated optical signals with an arbitrary modulation index (Delta >< 1) and modulation frequency (omega/omega(0) >< 1, where omega(0) is the characteristic cutoff frequency) by adaptive photodetectors that are based on photorefractive crystals and the nonsteady- state photo-electromotive force are presented. Besides general analytical results, numerical calculations of the output signal form and the amplitude and the phase of its first and second harmonics are given. The main predictions of the theoretical analysis are illustrated by the experimental data obtained for a high-sensitivity cubic photorefractive Bi(12)SiO(20) crystal at the wavelength (lambda = 488 nm) of an argon-ion laser.