A quadrature phase tracker for open-loop fiber-optic gyroscopes

A phase detection system for open-loop fiber-optic gyroscopes which is able to determine the Sagnac phase over several interference fringes is presented. It employs two quadrature signals of the Sagnac phase, as well as an electronic feedback signal that is a first-order estimate of the phase. Linearity is achieved by means of a trigonometric transformation, several of which have been identified, and a first-order control system that generates the feedback signal. The dynamic properties such as ramp and step response of the phase tracker have been studied. It is shown that the introduction of a simple feedforward loop completely eliminates the velocity error which is inherent in the first-order feedback system. For the particular loop gain used in the experimental system, a ramp of 280×10³ rad/s could be tracked without detectable velocity error. The introduction of feedforward improved the risetime for a step input of 0.86 rad from 2.4 μs to 550 ns