Improved zero-order fringe positioning algorithms in white light interference based atomic force microscopy

Abstract In white light interference based atomic force microscopy (WLIAFM), the vertical displacement of the probe is obtained by zero-order fringe positioning on the probe cantilever, so the accuracy of zero-order fringe positioning will affect directly that of the WLIAFM. However, due to non-uniform distribution of light intensity and photoelectric noises, accurate zero-order fringe positioning becomes a problem. In this paper, two algorithms are proposed to improve the zero-order fringe positioning accuracy. In the first algorithm which is called improved maximum algorithm, multi-row maximum positions of the interference fringes are obtained and error theory is applied to eliminate erroneous maximum positions, then the average of remaining maximum positions is used as the zero-order fringe position. Another is called phase evaluation algorithm, in which wavelet transform is applied to eliminate effects from disturbances mentioned above and Hilbert transform is used for phase evaluation to obtain the zero-order fringe position. The practicability and accuracy of the two algorithms have been verified by series of experiments. The experiment results indicate that both two algorithms are suitable in this condition and the phase evaluation algorithm has higher accuracy while the improved maximum algorithm has higher processing speed.