Quadrature Phase Shift Interferometer (QPSI) decoding algorithms and error analysis

摘要

A He-Ne laser based Quadrature Phase Shift Interferometer (QPSI) has been developed for the topographic measurement of ultra-smooth surfaces, such as those used as magnetic recording disks. The design uses the polarization property of the light to create two independent interference signals, which are phase shifted by 90 degrees with respect to one another. Because the phase angle is the argument of a sine and cosine function, wrapping of the phase occurs, i.e., the interference amplitude is a periodic function of the phase. An unwrapping or decoding algorithm has been developed using the maximum/minimum intensity method. Finding accurate maximum and minimum values of the intensity signals is the key to minimizing the decoding error. An approach we have developed and describe here provides a more reliable method for finding the values of maximum and minimum intensity of the interference signals in order to create accurate intensity envelopes, which are required for the phase unwrapping algorithm. The decoding error of the algorithms has been evaluated with the synthetic waveforms, which are computer generated to simulate the interference signals from a disk surface with introduced amplitude modulation and phase angle error.