We describe a technique for nulling the optical phase change induced by a fiber interferometric sensor. This scheme utilizes a triangular waveform to modulate an integrated optic phase modulator. The output signal of the interferometer is then mixed with a switching pulse train with proper pulse width and space. The switching pulse trains are generated from a triangular waveform and three comparators with different polarity. The operating principle is demonstrated by using a fiber optic gyroscope (FOG). A sensor system with wide dynamic range and linear scale factor has been successfully implemented by using this technique.