TDK lab chip sets laser colours by voltage, could cut power 75%

Smart glasses have a display problem that their size makes hard to solve. Laser beam scanning avoids a fixed grid of pixels: a mirror sweeps a laser across the field of view and builds the image point by point. To sharpen that image without enlarging the optics, colour has to switch faster, and current systems struggle to change laser colour quickly and efficiently. A team led by Yasuhiro Takagi at TDK Corporation has tried a different way to control it.

Conventional systems set the brightness of each colour by changing the electrical current feeding its laser diode. That is slow: direct current modulation has a narrower bandwidth than external optical modulation, which caps how fast colours can change and, in turn, the resolution that can be reached. Laser diodes also have to exceed a threshold current before they emit enough light, which adds to power consumption. Takagi's chip leaves the lasers alone. Voltages applied to a thin film of lithium niobate, a material whose optical properties can be controlled electrically, set the intensity of the red, green and blue light before the three are merged. Each colour passes through a Mach–Zehnder interferometer, a structure that splits light and recombines it so that a voltage determines how much gets through, and a multiplexer made of lithium niobate waveguides brings the colours into a single beam.

In experiments, the researchers controlled the colour channels through voltage, and combining them produced full-colour output. The device was also integrated with a MEMS scanning mirror, a tiny mechanical mirror, to demonstrate its potential in a real laser scanning system. The chip reached a modulation bandwidth of 10 gigahertz, which the researchers say is sufficient for 4K resolution and higher. They also report that overall power consumption could fall by more than 75% compared with the conventional approach. Because the photonic circuit controls light intensity, the neutral-density filters that dim excess laser output are no longer needed.

For a wearer, the promise is a sharper image from a display that draws less power, in a frame that stays small. The first beneficiaries would be the makers of smart glasses, who could integrate fewer optical components to reach that result. The sources give no timeline, and this is a laboratory demonstration: the team's next step is reducing the optical losses that occur as light crosses the lithium niobate waveguides and as the colours are combined. Lower losses, the researchers argue, would strengthen the case for putting this kind of circuit into compact wearable displays.