A laser interferometer encoder with two micromachined gratings generating phase-shifted quadrature

This paper presents a laser interferometer encoder which generates a quadrature signal by using two gratings with a phase difference. The phase difference comes from an 86 nm step difference between the gratings. The step is about 1/8 of the wavelength of the laser. Thus, two diffracted optical signals generated from the two gratings have about 90° phase difference from each other. The two gratings are fabricated on a quartz wafer with two different periods, 4 and 6 µm, to avoid the overlapping of the two diffraction signals in detection. Simulation results using propagation of an angular spectrum of the laser source through the gratings are compared with experiment results for displacement measurement. The comparison of both results shows very close correlation. It also confirms that the resolution of the developed sensor is 1/8 of the wavelength of the HeNe laser by using the quadrature signals