Waveguide design for wearable RGB scanning projector

December 14, 2020 // By Nick Flaherty
Waveguide design for wearable RGB scanning projector
A scanning laser module measuring 8×4×3 mm at the University of Fukui in Japan can be used to project a high resolution image into the eye

Researchers at the University of Fukui in Japan have created a high resolution miniature projector for wearables using a new waveguide design.

The team integrated a compact RGB laser module measuring only 8×4×3 mm with a microelectromechanical (MEMS) mirror. The direction in which the MEMS mirror reflects light from the laser module can be controlled electronically, making it possible to project high-quality 2D images through laser scanning over the projected area.

The laser beam scanning module can project 1280 × 720 colour video onto the retina by tuning the MEMS driving frequency. These characteristics, together with its small size and its potentially low battery consumption, make the scanning projector a promising device for wearable displays.

The working with silicon manufacturer Seiren KST, the team expects to commercialise the module within a year. Seiren KST specialises in thick thermal oxide layers up to 25um on silicon substrates for optical devices.

One aspect of making the laser module was combining the light beams from three independent laser sources to obtain the RGB output. To achieve this, the team used a waveguide-type combiner, where each of the three waveguides receives light from each of the primary colours.

Although only the centre green waveguide is connected to the actual optical output, the blue and red light travelling on adjacent dead-end waveguides are passed to the centre waveguide through strategically placed couplers.

"The measured efficiency of the combiner was as high as 97%, which represents a loss of only 0.13 dB," said Akira Nakao, assistant professor at the University of Fukui, lead author of the study. "The outputs from the individual RGB lasers end up perfectly aligned thanks to the nature of the waveguide-type combiner."

Using an achromatic lens achieves a circular focused beam while also providing the option to use other lenses to produce collimated beams with larger diameters.

Further tuning will be required to make it possible

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