This paper proposes a new modulation scheme using high-order harmonic information to solve the so-called ambiguity problem of interferometry. To start with, we build a fiber Fabry-Perot interferometer to serve as a displacement sensor, which has two operation modes-coarse and fine modes. Integrating the afore-developed sensor, a piezoelectric actuator, and a scheduled proportional-integral/adaptive-sliding controller, we construct a dual-stage nanopositioning system. The experimental results

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... w that the proposed system has the capability to extend the positioning range beyond the limit of the wavelength while keeping the naturally high resolution, ±5 nm, of interferometry.