Many technologies ranging from GPS, mobile networks, satellite navigation and radar use radio signals. However, the radio spectrum is overcrowded, making efficient and reliable filtering of radio signals from the radio spectrum crucial. A new technique not only makes radio signals very pure, but also allows them to be very accurately distinguished from other signals.
The technique was developed by researchers at the University of Twente, who share their findings in Nature Photonics. For years, researchers have been trying to create radio signals on chips via interactions between light and sound. It has long been known that this is possible, but the signal has so far been too weak in practice.
However, research by the University of Twente in collaboration with McMaster University in Canada now shows that a thin layer of glass can generate surface acoustic waves, which the researchers liken to 'mini-earthquakes'. These waves make the interaction between light and sound as much as 200 times stronger.
Passing only desired frequencies
The discovery is important because the radio spectrum is overcrowded. We therefore use radio frequency filters to filter radio signals from this spectrum. These filters let only the desired frequencies through and block other frequencies.
During a phone call, for example, radio frequency filters ensure that the conversation sounds clear. The filters are also used for various other technologies, ranging from radar and satellite networks to 6G.
From silicon nitride to tellurium oxide
Previous filters used silicon nitride, which is already widely made for optical applications. However, to make the sharpest signals, light and sound had to interact strongly in the chip material. In silicon nitride, that interaction has always been weak. Previous attempts to improve that used exotic, unstable materials or fragile structures that do not hold up outside the lab.
The new technique uses a different material: tellurium oxide. This material has long been used in commercial modulators. The researchers applied a thin layer to a standard silicon nitride chip. This layer generates a special type of sound wave, which the researchers say resembles a small earthquake and moves along the surface of the chip. As a result, the sound couples much more strongly with the laser light in the chip than was previously possible. This enhanced the interaction between light and sound more than 200 times.
Various benefits
This stronger interaction offers several advantages. For instance, it provides a Brillouin sound amplifier that actually works. Previous chips weakened signals as they passed through the chip, while the new chip actually amplifies signals. The researchers speak of "an important missing component for practical applications".
In addition, the stronger interaction produces very pure radio signals. With a resonator of less than half a millimetre, the chip generates a radio tone so stable, flexible and accurate that it can compete with systems that used to require equipment the size of a steering wheel, according to the researchers. The new filter can isolate one radio channel from a spectrum with thousands of channels. In the process, the filter can be shifted over a range of nine gigahertz.
The researchers point out that it is possible to apply the tellurium oxide layer only where it is needed, allowing it to be combined well with other technologies that already exist on silicon nitride chips. These include amplifiers, lasers and sensors.
The publication in Nature Photonics can be found here.
