Design of an Optical Head with Two Phase-Shifted Interference Signals for Direction Detection of Small Displacement in an Absolute Surface Encoder

This paper presents the design and construction of a new optical head with two phase-shifted interference signals in an absolute surface encoder by using a mode-locked femtosecond laser. A series of discrete absolute positions of the scale grating is obtained from a series of peak wavelengths of the spectrum of the +1st- or -1st-order diffracted beam. The two beams at a specific wavelength λ i interfere with each other to generate an incremental interference signal for high-resolution displacement measurement over a small interpolation range around the corresponding discrete absolute position x i . In the previous design of the optical head, the two beams were guided by optical fibers into a fiber coupler for the interference. This fiber optics design was simple and stable but could not identify the moving direction of small displacement within each interpolation range because only one interferential signal could be generated. The aim of this study is to develop a new design of the optical head, where two interference signals with a phase difference of π/2 are generated. For this purpose, free-space optics, instead of fiber optics, is adopted in the new optical head. Experiments are conducted to confirm the generation of the two phase-shifted interference signals. A Lissajous figure is plotted to verify the phase difference between the two signals.