We propose an optical arrangement of a modulator of laser radiation based on nonlinear ring interferometer (NRI). In this instrument the information microwave signal affects the initially isotropic nonlinear (Kerr) medium changing its optical length. A dynamic model of the nonlinear phase change in the nonlinear medium is developed under certain approximations. Series of bifurcation diagrams and modulation characteristics, demonstrating the capabilities of the modulator, are presented.