Securing noise-adaptive selection of interference signal by nonlinear detection.

In an interferometer, the interference fringe signal is typically linearly detected and modulated to a specific frequency region. A nonlinear detector yields the interference fringes of the fundamental and high-harmonic waves in different frequency regions. We analyze the formation of nonlinearly detected interference fringes. We demonstrate, for the first time, that the interference fringes of high-harmonic waves can also be used to determine the position of zero optical path length difference between reference and object mirrors. This noise-adaptive selectability of the interference fringe signal is guaranteed by the fact that the positions of the peak envelopes of the interference fringes for the fundamental and high-harmonic waves are matched at zero optical path length. The proposed method is verified using experimental data. This technique can be applied not only to surface profiling but also to length measurements.