A new in-line Sagnac fiber optic transformers (FOT) based on all digital closed-loop detection is introduced. With the core of digital signal processor (DSP)

the square wave scheme is used for biasing modulation and the digital ramp feedback is adopted for closed-loop detection. The coefficient of the factor for the closed-loop transformer is analyzed. The test results show the design viability with the system percentage current error less than 0.5% in the range of -40 ℃～+60 ℃ and phase error less than 30′.