Laser beam scanning, used in applications ranging from laser projectors to barcode scanners in groceries, uses multiplexers which merge RGB laser beams into a single beam to process a wider range of signals. Traditionally, this is done by directly modulating each laser, turning them on and off to control the output. However, this approach is relatively slow and energy intensive. Researchers at the TDK Corporation developed a faster and more energy-efficient RGB multiplexer based on thin-film lithium niobate (TFLN). Lithium niobate is a material widely used in photonics due to its electro-optic, nonlinear-optic, and acousto-optic properties, and TFLN is widely used in infrared optical modulators and easily guides visible light. The RGB multiplexer with sputter-deposited thin-film lithium niobate successfully generates all colors, indicating promising performance as a light source for displays such as laser beam scanning. Courtesy of Atsushi Shimura. “A TFLN-based RGB multiplexer is essential for [laser beam scanning] with lower power consumption and higher resolution; however, this has never been demonstrated, and the RGB multiplexer has been limited to glass-based photonic integrated circuits,” said corresponding author Atsushi Shimura. The multiplexer, measuring just 2.3 mm in length, was created using a physical vapor deposition (“sputter”) technique to deposit the LN film, followed by etching to create the waveguides to direct the laser light. With this approach, fabrication avoids the complex bonding process typically required with bulk lithium niobate, resulting in scalable and cost-effective route to mass-producing compact light-based circuits. The structure of the waveguides was designed with a trapezoidal cross-section to reduce signal loss. The lengths of the sections were also adjusted to reduce signal loss. The combiner combined red (638 nm), green (520 nm), and blue (473 nm) laser beams through carefully designed waveguides. By adjusting the intensity of each beam, the researchers were able to generate mixed colors such as cyan, magenta, and yellow, and even white light, by combining all three primary colors. Such precise color control is essential for LBS-based displays. Although the work showed promising results, it also highlighted important challenges that will need to be addressed moving forward. A key issue is the lower crystal quality of sputter-deposited TFLN compared to bulk lithium niobate, which affects performance at shorter wavelengths. For example, at 473 nm, the measured optical loss was between 7 and 10 dB, significantly higher than the simulated value of 3.1 dB. This loss was mainly caused by surface roughness in the waveguides, which scatters light and reduces overall efficiency. “Optimizing fabrication processes to produce smoother surfaces is a key step toward realizing TFLN’s potential in visible-light photonics and applications,” Shimura said. Despite these limitations, the results lay a foundation for developing scalable, faster, and more energy-efficient multiplexers for future visible-light LBS systems. “This work demonstrates the feasibility of a passive RGB multiplexer as a first step toward developing active photonic integrated circuits,” Shimura said. The research was published in Advanced Photonics Nexus (www.doi.org/10.1117/1.APN.4.5.056001).
