Testing geometrical properties of AR waveguide stacks and plano-optical elements

08 Sep 2023

With the OptiSurf^® product portfolio TRIOPTICS has many years of experience in non-contact measurement of center thicknesses of single lenses and plano optics as well as for measurement of air gaps in optical systems.

The new OptiSurf^® PRO AR transfers the long-proven OptiSurf^® technology to a new range of applications. As an AR/VR measurement device it is now used for the accurate and reliable characterization of the geometrical properties of single waveguides and other plano-optical elements as well as stacks of waveguides. The unique system from TRIOPTICS accurately measures the tilt of the individual elements of stacked waveguides relative to each other.

A significant challenge in manufacturing high-quality AR waveguides is that stacked waveguides, consisting of two or three elements, are highly sensitive to the tilt of individual elements relative to each other. The tilt leads to color aberration defects, e.g. visible as “rainbows” at edges in the generated image. These color aberrations need to be either calibrated and corrected in the final product in an elaborate process or, - if the misalignment is too large – need to be discarded, lowering the yield of the manufacturing process.

The OptiSurf^® PRO AR accurately measures this tilt. The specific focus of the interest is the tilt of the single elements of a waveguide relative to each other or to a mechanical reference. An important application is the measurement of the total thickness variation (TTV) over the area of the waveguide as too much thickness variation will severely reduce the contrast (MTF) of the generated image. The OptiSurf^® PRO AR provides accurate measurement data about e.g. TTV, gravitational sag, flatness and bow of single waveguides or plano-optical elements, as well as the alignment (e. g. tilt and air gap distance) of waveguides in a stack. This is done with the highest accuracy in a non-destructive measurement process.

Regardless of whether it is used in manufacturing, research, or quality control of waveguide stacks or optical elements, the system enables measurements of individual center thickness and air gaps between components in a stack with sub-micron precision. The tilt which causes unwanted color defects is measured in the finished waveguide. With the corresponding data, a quality selection can be made in production or production processes can be improved.

The device also supports automatic pass/fail decisions for QC in mass production: here, measurement positions and criteria for the automatic pass/fail assessment of the samples are freely configurable by the user. Possible pass/fail criteria are e.g. air gap between sub-waveguides, relative tilt angles and bow of sub-waveguides. Using this feature, low-quality waveguides can be discarded and are not passed on to the next processing stage, which means that they fail before they become part of the end product.

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