An anomalous photovoltaic film formed by simultaneous oblique sputter deposition of CdTe and CdS was integrated on a LiNbO₃ optical waveguide modulator which works as a Mach-Zehnder interferometric modulator (MZM). Light irradiation with a low power (1 mW) and a low cost laser source with a wavelength of 830 nm induced the photovoltage around 5V which is as high as the half-wave voltage of the MZM. This photovoltage was used to control the signal light in the waveguide. Since the addition of the photoconductive CdS decreased the film resistance, the rise time of the photovoltage became much shorter than that of the conventional anomalous photovoltaic film formed by the oblique deposition of CdTe. An auto-stabilizing system was constructed, which compensates the drift of the optical working point of the MZM by negatively feeding back the drift value as the light intensity irradiated on the film. This all optical feed-back control system successfully attained the auto-stabilization of the modulator. This system enables a light modulation of a high signal noise ratio, since it does not use the electric wire causing an electromagnetic noise to transmit the feedback voltage. When the system is used to stabilize a system for measuring an electromagnetic field, it enables the high accurate measurement without disturbing the electromagnetic field to be measured.