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EXPERIMENTAL EVALUATION OF PHASE DIGITIZING NONLINEARITY CORRECTION IN HETERODYNE INTERFEROMETRY

INTRODUCTION Frequency leakage in polarization-dependent heterodyne interferometers results in nonlinearity, or periodic error, in the phase digitized result. Phase digitizing is the process of converting the interferometer signal, obtained from the interference of the reference and measurement beams, into a digital representation of phase. In a previous approach [1-5], the periodic error is removed in a two-step process including phase digitizing, and second, first order periodic error (i.e., a cyclic error with one cycle per wavelength of optical path change) removal from phase digitized data. The second step is repeated for second order periodic error (with two cycles per wavelength of optical path change). This paper describes experimental results for a method where the first order periodic error is removed in the phase digitizing process in a single step. The accuracy of this measurement is not sensitive to target velocity, provided a minimum velocity threshold is at least temporarily exceeded. Because the latency requirement is much reduced from the two-step process, the achievable accuracy in measurement and compensation is improved.