TFLN-based RGB multiplexer for energy-efficient laser beam scanning

(Press-News.org) As technology advances, photonic systems are gaining ground over traditional electronics, using light to transmit and process information more efficiently. One such optical system is laser beam scanning (LBS), where laser beams are rapidly steered to scan, sense, or display information. This technology is used in applications ranging from barcode scanners at grocery stores to laser projectors in light shows. To process a wider range of signals or enable full-color output, these systems utilize multiplexers that merge the red, green, and blue (RGB) laser beams into a single beam.

Traditionally, this was achieved by directly modulating each laser, turning them on and off to control the output. However, this approach is relatively slow and energy intensive. A recent study by researchers at the TDK Corporation (Japan) reports the development of a faster and more energy-efficient RGB multiplexer based on thin-film lithium niobate (TFLN). The work is published in Advanced Photonics Nexus.

Lithium niobate is a versatile material widely used in photonics due to its excellent electro-optic, nonlinear-optic, and acousto-optic properties. TFLN is widely used in infrared optical modulators and is increasingly valued for its ability to guide visible light. Unlike conventional systems, the TFLN-based approach uses electric fields to control how light propagates and combines, enabling higher modulation speeds and lower power consumption.

Sharing the team’s motivation for the study, corresponding author Atsushi Shimura notes, “A TFLN-based RGB multiplexer is essential for LBS with lower power consumption and higher resolution; however, this had never been demonstrated, and the RGB multiplexer has been limited to glass-based photonic integrated circuit.”

The multiplexer, measuring just 2.3 millimeters in length, was created using a physical vapor deposition (“sputter”) technique to deposit the LN film, followed by etching to form waveguides that 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 carefully designed to guide light efficiently, with a trapezoidal cross-section that helped reduce signal loss. By adjusting the lengths of the combining sections, the researchers were able to fine-tune the performance for each color. When evaluated, the RGB combiner successfully 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 (green + blue), magenta (red + blue), and yellow (red + green), and even white light, by combining all three primary colors. Such precise color control is essential for LBS-based displays.

While the results are encouraging, the study also highlights some important challenges that 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 (blue light), 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 remarks.

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 notes.

For details, see the original Gold Open Access article by A. Shimura et al., “Visible light red, green, and blue multiplexer by sputter-deposited thin-film lithium niobate,” Adv. Photon. Nexus 4(5), 056001 (2025), doi: 10.1117/1.APN.4.5.056001.

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