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As illustrated in the diagram, laser beams are applied to two points equidistant
from the grating disc’s center of revolution. One diffraction beam is positive first order (+1) and the other is negative first order (-1). For each 1 pitch that the grating disc revolves, the ±1 diffraction light will change each phase by ±2π. Reflecting the ±1 diffraction light into respective mirrors and then reapplying it to the grating disc changes the phase by ±4π. In this way, each time the grating disc revolves 1 pitch, the brightness interference signals for 4 cycles can be obtained, making highly accurate angle sensing possible.
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