High-Accuracy Diffractometer for Augmented Reality Waveguide Characterization

Описание: AWE USA 2022, XR Enablement - High-Accuracy Diffractometer for Augmented Reality Waveguide Characterization

Due to the demanding optical architectures of diffractive waveguide gratings used for augmented reality applications, the gratings must be manufactured to extremely high accuracies, or the image quality will suffer. The grating period has to match the designs within tens of picometers, and the tolerances for the relative orientation of the gratings are in the arcsecond range. Both the production masters and the replicated gratings need to be characterized nondestructively, and the grating areas scanned to ensure uniformity. The measurement system should work for surface relief and volume holographic gratings in various material systems. We describe a Littrow diffractometer that can perform this challenging task. A narrow-band and highly stable laser source is used to illuminate a spot on the sample. Mechanical stages with high-accuracy encoders rotate and tilt the sample, until the laser beam is diffracted back to the the laser. This so-called Littrow condition is detected through a feedback loop with a beam splitter and a machine vision camera. The grating period and relative orientation can then be calculated from the stage orientation data. With the system properly constructed, and custom software algorithms performing an optimized measurement sequence, it is possible to reach repeatability in the picometer and arcsecond range for the grating period and relative orientation, respectively. By carefully calibrating the stages or by using golden samples, absolute accuracy for the grating period can also reach picometer range.

Speaker:
Janne Simonen - Team Lead, Optics at OptoFidelity

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