A fiber-optic sinusoidal phase modulating (SPM) interferometer was proposed for the acquisition and reconstruction of three-dimensional (3-D) surface profile. Sinusoidal phase modulation was induced by controlling the injection current of light source. The surface profile was constructed on the basis of fringe projection. Fringe patterns are vulnerable to external disturbances such as mechanical vibration and temperature fluctuation, which cause phase drift in the interference signal and decrease measuring accuracy. A closed-loop feedback phase compensation system was built. In the subsystem, the initial phase of the interference signal, which was caused by the initial optical path difference between interference arms, could be demodulated using phase generated carrier (PGC) method and counted out using coordinated rotation digital computer (CORDIC) , then a compensation voltage was generated for the PZT driver. The bias value of external disturbances superimposed on fringe patterns could be reduced to about 50 mrad, and the phase stability for interference fringes was less than 6 mrad. The feasibility for real-time profile measurement has been verified.
A novel fiber-optic interferometer fringe projector with the sinusoidal phase-modulating method is presented. The system utilizes the integrating bucket method to detect the desired phase or the displacement and a CMOS image sensor to detect four frames obtained by integration of the time-varying intensity in an interference image during the four quarters of the modulation period. Since this technique with the method modulating the injection current of the piezoelectric transducer (PZT), measurement accuracy is not affected by an intensity modulation that usually appears in the current modulation. The system also utilizes the Fresnel reflection signal to adjust the phase-modulation coefficient z to eliminate the disturbance of initial phase ψ0 . The experimental results for surface profiles of a convex hull show that the sinusoidal phase modulating interferometer proposed here confirms its applicability to practical application.
A fiber-optic sinusoidal phase-modulating (SPM) interferometer for fringe projection is presented. The system is based on the SPM technique and makes use of the Mach–Zehnder interferometer structure and Young’s double pinhole interference principle to achieve interference fringe projection. A Michelson interferometer, which contains the detection of Fresnel reflection on its fiber end face and interference at one input port of a 3 dB coupler, is utilized to achieve feedback precise control of the fringe phase, which is sensitive to phase drifting produced by the nature of the fiber. The phase diversity for the closed-loop SPM system can be real-time measured with a precision of 3 mrad. External disturbances mainly caused by temperature fluctuations can be reduced to 57 mrad for the fringe map. The experimental results have shown the usefulness of the system.