Smart glasses need compact and power-efficient displays capable of producing detailed images without consuming much energy. One approach is laser beam scanning: instead of a fixed pixel matrix, such a display directs a laser across the field of view and builds the picture point by point.
To increase resolution, colors can be switched faster using a fast-moving mirror without increasing the size of the optics. However, existing systems are limited in the speed and efficiency of laser color control.
Typically, red, green and blue beams are combined into one for a full-color image. The brightness of each color is adjusted by changing the electric current through the laser diode. This method has a limited bandwidth - the range of frequencies in which the signal can change without distortion - so colors switch more slowly, which reduces the achievable resolution. In addition, laser diodes require current above a threshold, and this increases power consumption.
Researchers from TDK Corporation led by Yasuhiro Takagi proposed a photonic chip that controls light with voltage rather than by continuously changing current. The work was published in the journal Advanced Photonics Nexus. In the chip, voltages regulate the brightness of red, green and blue before the beams are combined into a single output beam.
The device is made from a thin film of lithium niobate - a material whose optical properties can be changed electrically. The chip contains three Mach-Zehnder interferometers, one for each primary color. These structures split and recombine light so that an applied voltage controls the intensity of the radiation passing through the channel. A multiplexer on lithium niobate waveguides then combines the three colors into a single beam.
In experiments, the researchers controlled individual color channels via voltage on the chip. Combining the channels made it possible to obtain a full-color output. The device was also combined with a microelectromechanical system (MEMS) micromirror - a miniature movable mirror - demonstrating the potential for creating full-color images in a laser scanning system.
Speed became the key parameter. The chip achieved a modulation bandwidth of 10 gigahertz, which, according to the authors, is sufficient for 4K resolution and above. Abandoning direct modulation of laser diodes removes one of the resolution limitations of traditional laser displays.
Voltage control also substantially reduced power consumption: according to the researchers' estimate, total energy consumption was cut by more than 75% compared with the conventional approach. Because brightness is regulated by the photonic circuit rather than by maintaining high laser currents, there is no need for neutral density filters to attenuate excess radiation.
Together, these changes address several related challenges of miniaturizing laser scanning displays: switching speed, power consumption and the number of optical components.
The authors conclude that the thin-film lithium niobate scheme improves the feasibility of combining high resolution with the small size required for smart glasses. The demonstrated 10 GHz bandwidth and full-color generation provide experimental confirmation that external voltage control can enable ultracompact laser scanning displays.
Further improvements will focus on reducing optical losses as light passes through the lithium niobate waveguides and when the three colors are combined. Lower losses could increase the overall efficiency of the system and strengthen the case for integrating such photonic circuits into compact wearable displays.