Chipmakers could soon have a new tool to increase the scope and performance of integrated photonic chips — a sound-mediated way to control and steer light. A research team at the University of Twente (UT) developed a scalable, efficient, integrated platform for Stimulated Brillouin Scattering (SBS) using thin-film lithium niobate (TFLN). In the optical material lithium niobate (LN), acoustic waves can be steered by the direction of light and integrated in photonics technology. Moreover, TFLN is an established platform for light-based chips. To date, leveraging SBS for integrated photonics has not been practical. Acoustic waves have also presented a challenge, due to their tendency to propagate in all directions and disperse energy. With SBS added to their toolkit, optical engineers will be able to incorporate sub-hertz linewidth lasers, ultraselective filters, and many other high-performance components into their photonic circuits. The researchers harnessed strong SBS in their TFLN platform by exploiting the anisotropy of the material. The significant SBS gain made TFLN suitable for developing a Brillouin photonics engine capable of diverse functions. Artistic rendering of the Stimulated Brillouin Scattering (SBS) process. Researchers developed a scalable, efficient, integrated platform for SBS on thin-film lithium niobate (TFLN). Courtesy of the University of Twente. Working with a research group at the City University of Hong Kong, the UT team, led by professor David Marpaung, fabricated an on-chip Brillouin amplifier and a laser — two key components in a photonic integrated circuit — in the TFLN platform. The researchers also created a multifunctional Brillouin microwave photonic processor capable of filtering an incoming signal. “Integrated Brillouin photonics is very fertile ground, both scientifically and commercially, and our work takes it from the lab to the fab,” Marpaung said. The researchers identified two distinct SBS processes on the TFLN platform — one driven by surface acoustic waves with a 20-megahertz (MHz) linewidth, and the other driven by bulk acoustic waves with a linewidth exceeding 4 gigahertz (GHz). The narrowband internal net gain amplifier demonstrated by the researchers overcame propagation losses within a 10-centimeter (cm) spiral waveguide. A stimulated Brillouin laser, generated in TFLN by incorporating SBS gain into a high-quality racetrack resonator, achieved a tuning range of greater than 20 nm and supported the generation of high-purity radio frequency (RF) signals with a linewidth of 9 hertz (Hz). A programmable, multifunctional, integrated Brillouin microwave photonic processor, also developed by the team, is capable of notch filtering, bandpass filtering, and true time delay. By using SBS to control the positive feedback loop between light waves passing through a medium and the sound waves generated in the material’s crystal lattice, the researchers developed a new way to transport and process information. “After electrons in electronics and photons in integrated photonics, think of the phonon-mediated interactions as a third way to shape, redirect, or process signals,” Marpaung said. Linking SBS with advanced TFLN technologies could unlock new paradigms for integrated Brillouin photonics. Marpaung has already started to explore some practical applications. “SBS can drastically reduce the dimensions of atomic clocks, since SBS allows for miniaturization of the ultraprecise and stable lasers required by these devices,” he said. “Chip-scale lasers will enable cost-effective integration of atomic clocks in satellites and unmanned aerial vehicles (drones). Thanks to precise on-board timekeeping, these devices wouldn’t have to rely on GPS for navigation.” Integrating Brillouin photonics in TFLN will also allow for ultraprecise filtering of unwanted signals, Marpaung said. “Integration with high-speed modulators will lead to higher performance, smaller size, and lower cost,” he said. “These filters can be used for mitigation of unwanted interference and jamming, which is important for 6G radios and GPS navigation.” The research was published in Science Advances (www.doi.org/10.1126/sciadv.adv4022).