Laser Combiner a Compact Solution for Wearable Displays

A laser beam combiner no larger than a grain of rice can enable laser-scanning displays in wearable computer systems.

A team from the University of Fukui developed the new device. It could offer immediate, hands-free access to information via gadgets such as smart glasses, which overlay computer-generated visual data on the user’s field of vision.

The beam combiner comprises three optical waveguides for RGB light inputs, as well as three directional couplers. The couplers can switch and exchange the light beams from one to the other, the researchers said, allowing for combined output light from the center waveguide.

Schematic illustration of the RGB laser beam combiner. Images courtesy of SPIE.

Green light enters the device from the center waveguide, which is straight. Two curved waveguides to either side of the center admit blue and red light. The sequence of the three directional couplers was deemed crucial, as it was the only configuration that obtained high output efficiency.

The separate RGB light signals joined together at the center output waveguide, with the average output powers reaching as high as 96 percent of the input. That output power showed no polarization dependence when the researchers used square cross-sectioned waveguides. Additionally, the laser combiner is “robust with respect to wavelength deviation in the laser sources,” according to the researchers.

They noted that the wavelength dependence of the output power for each RGB light remained above 85 percent for wavelength deviations of ±10 nm. This proved that the system has a high tolerance for wavelength variation.

Light propagation in the RGB laser beam combiner.

The team originally created a prototype of the laser combiner using microfabrication on a silicon substrate, finding that, at 0.06 × 7.8 mm, it is one order of magnitude smaller than conventional mirror-type devices.

The combiner could be more efficient than LCDs and laser image displays for compact wearable projectors and other such displays.

LCDs can be challenging, according to the researchers, because focusing an image onto a user’s eyes typically requires an added, accurate lens system.

Laser image displays, which contain both a multiplexer and a beam scanner, can emit RGB light and do not require lens focusing because an image can be obtained by scanning the laser beam. However, realizing such a compact display is difficult, the researchers said, because combining the RGB laser beams requires a complicated lens and mirror display system.

Other potential applications for the compact laser combiner include augmented-reality wearable computers.

The Fukui team next plans to attempt extremely compact optical engines for laser projection using a DC-type combiner with RGB semiconductor laser chips on a silicon substrate platform.

The research was published online by SPIE Newsroom (doi: 10.1117/2.1201410.005641).

For more information, visit www.u-fukui.ac.jp.