The stray light of gratings lowers the image contrast of augmented reality display devices based on lightguide gratings. We propose to reduce the stray light of gratings fabricated with the scanning exposure method in a Lloyd's mirror interferometer setup. The photoresist-coated substrate is moved in the longitudinal direction parallel to the exposure interference fringes during exposure to average out the laser speckle-induced noise. A phase locking module is designed to compensate for the unwanted lateral displacement caused by the straightness error and vibration of the translation stage. The stability and accuracy of phase locking in the Lloyd's mirror interferometer are analyzed with control system theory. Low-stray-light lightguide gratings were fabricated successfully. The stray light level was decreased by more than 50%, and the imaging contrast was increased from 65% to 85%.
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