We have developed two kinds of fiber optic gyro prototypes, an all-fiber gyro and an optic integrated gyro, which will be used for AHRS. The dimension of a typical gyro is (Phi) 75 mm X 25 mm with 200 m PM fiber. By means of the phase traced technique, the dynamic range is over +/- 500 degree(s)/s with 10-3 - 10-4 scale factor linearity. The gyro's noise equivalent to rate is 0.5 degree(s)/h - 1 degree(s)/h (1 (sigma) ). In order to study some characteristic which is relative to the state of polarization (SOP), we used the Jones matrix to establish a model of SOP of FOG, in which the fiber loop is represented by a `two-way linear phase delayer.' As a result of deduction, the output signal of FOG is a function of the delay value and direction of birefringence axes of the fiber. If we send the depolarized light into the fiber loop, the output intensity will be independent of the phase delay and direction of optic axes. By using the model of SOP, we also deduced that the depolarization is conditioned by cos(2 (beta) ) equals 0, where (beta) is an angle of birefringence axes between two fibers in a depolarizer.