ABSTRACT
A passive interferometry phase detection method is presented, which emulates the closed-loop high gain approach technique. This method comprises an all-digital closed-loop observer based on the variable structure and sliding modes nonlinear control theory, able to demodulate the phase of an open-loop feedback-free interferometer hardware. A proof-of-concept experiment is conducted by measuring complex displacements (module and angle) generated by a piezoelectric actuator. The results show that this method simplifies the optical hardware while providing features such as direct detection of the optical phase, increase of the dynamic range, high robustness, and dismissing of the feedback phase modulator and reset circuits.
ABSTRACT
This work presents a novel nonlinear control system designed for interferometry based on variable structure control and sliding modes. This approach can fully compensate the nonlinear behavior of the interferometer and lead to high accuracy control for large disturbances, featuring low cost, ease of implementation and high robustness, without a reset circuit (when compared with a linear control system). A deep stability analysis was accomplished and the global asymptotic stability of the system was proved. The results showed that the nonlinear control is able to keep the interferometer in the quadrature point and suppress signal fading for arbitrary signals, sinusoidal signals, or zero input signal, even under strong external disturbances. The system showed itself suitable to characterize a multi-axis piezoelectric flextentional actuator, which displacements that are much smaller than half wavelength. The high robustness allows the system to be embedded and to operate in harsh environments as factories, bringing the interferometry outside the laboratory.
