The phase-modulated optical signal produced by light propagation through stressed transparent media is detected using the non-steady-state photoelectromotive force technique. The mechanical system, including the glass plate and piezoelectric transducer, demonstrates resonant behavior in the vicinity of 100 kHz. The measured distribution of the optical phase is a bell-shaped surface for these frequencies. The simulation of the piezo-optic response shows qualitative coincidence with the experimental data and provides maps of the stress field. The characterization of the β-Ga2O3 adaptive detector is performed for the light wavelength λ=532 nm.