Steering light with sound: a game changer for integrated photonics

Imagine having to find your way with only a compass and the stars and being handed a GPS system. This is what David Marpaung and colleagues have just done for the integrated-photonics community. Through their discovery of steering light with sound, the University of Twente (UT) researchers have made available a powerful new tool to expand the scope and performance of light-based chips, unlocking a wide range of new applications, including – but certainly not limited to – ultraprecise lasers that can be used for GPS-less navigation.

Detailed in the latest issue of Science Advances, Marpaung has essentially molded the precision and versatility of a well-known physical phenomenon called stimulated Brillouin scattering (SBS) into a form that’s ready for mass manufacturing. With SBS added to their toolkit, engineers will be able to incorporate sub-hertz linewidth lasers, ultraselective filters and many other components with unparalleled performance into their photonic circuits.

These chips are sure to make waves. “Integrated Brillouin photonics is very fertile ground, both scientifically and commercially, and our work takes it from the lab to the fab,” says Marpaung. “Our work seamlessly combines with existing, more mature integrated photonics platforms. I’m sure it will become mainstream in the not-too-distant future.”

Marpaung has already tested the waters by talking to a world-leading foundry, which has acknowledged the unique prospects of the platform.

Not practical – until now

For the telecom industry, Brillouin scattering is usually a nuisance. In an optical fiber, the interaction of light with the glass creates periodic changes in the medium’s density and refractive index, scattering the light and limiting the power that can be transferred from A to B.

But Brillouin scattering can also be used for good. In SBS, the scattering effect is encouraged. By carefully controlling the positive feedback loop between light waves passing through a medium and the sound waves (called phonons) that are generated in the material’s crystal lattice as a result of the interaction with light, a new way of transporting and processing information emerges. “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 explains.

Until recently, however, leveraging SBS hasn’t been very practical. “There have been many proof-of-concept demonstrations, but, for a variety of reasons, these face critical challenges in terms of practical deployment and scalability,” says Kaixuan Ye, a PhD student in Marpaung’s group and first author of the Science Advances paper. For example, the use of a thermally unstable material and complex fabrication procedures is unlikely to pique the interest of industry.

Another major roadblock has been an intrinsic of property of acoustic waves. Much like ripples spreading across the surface of the sea, acoustic waves tend to propagate in all directions, dispersing their energy. Marpaung, Ye and colleagues found that in the optical material lithium niobate, the acoustic waves can be steered by the direction of light, essentially taming it for insertion into integrated photonics technology.

The power of SBS

In bulk form, lithium niobate has a long history in telecommunications applications, but in recent years, thin films of it have been coming into view for light-based chips. Thin-film lithium niobate (TFLN), compared to other photonics platforms, stands out for its low optical loss, versatility and efficiency. SBS functionality will further enhance TFLN’s scope. “From now on, engineers will have the option of letting SBS-based components handle certain functions, which is very advantageous because of their superior characteristics,” Ye points out. “Ye has cracked the problem. This will be our legacy to the field, a real game changer,” Marpaung adds.

To provide a taste of the new functionality that’s available, Marpaung’s group collaborated with the research group of Cheng Wang of the City University of Hong Kong. Wang’s PhD student Hanke Feng fabricated an on-chip Brillouin amplifier and laser in the TFLN platform – two key components in any photonic integrated circuit.

Feng also created a more intricate component, a multifunctional Brillouin microwave photonic processor capable of modulating an incoming signal in different ways. The device aptly illustrates one of the key advantages of SBS functionality: tunability. Simply by controlling the light-sound interaction, the same light-guiding structures can perform different actions. In other integrated-photonics platforms, that would usually require a redesign of the component.

These demonstrations open the door to real-world applications, which Marpaung has already started to explore. “SBS can drastically reduce the dimensions of atomic clocks, since it allows for miniaturization of the ultraprecise and stable lasers required by these devices. Chip-scale lasers will enable cost-effective integration of atomic clocks in satellites and unmanned aerial vehicles. Thanks to precise on-board timekeeping, these devices wouldn’t have to rely on GPS for navigation,” he explains.

“Our work also allows for ultraprecise filtering of unwanted signals. Integration with high-speed modulators will lead to higher performance, smaller size and lower cost. These filters can be used for mitigation of unwanted interference and jamming, which is important for 6G radios and GPS/navigation.”

This article was written in close collaboration with the University of Twente. Top image credit: University of Twente