The continued growth of wireless and cellular data traffic relies heavily on light waves. Microwave photonics is the field of technology that is dedicated to the distri­bution and processing of electrical infor­mation signals using optical means. Compared with tradi­tional solutions based on electronics alone, microwave photonic systems can handle massive amounts of data. Therefore, microwave photonics has become increa­singly important as part of 5G cellular networks and beyond. A primary task of microwave photonics is the reali­zation of narrow­band filters: the selection of specific data, at specific frequencies, out of immense volumes that are carried over light.

Many microwave photonic systems are built of discrete, separate components and long optical fiber paths. However, the cost, size, power consump­tion and production volume require­ments of advanced networks call for a new generation of microwave photonic systems that are realized on a chip. Inte­grated microwave photonic filters, particularly in silicon, are highly sought after. There is, however, a fundamental challenge: Narrowband filters require that signals are delayed for comparatively long durations as part of their processing. “Since the speed of light is so fast,” says Avi Zadok from Bar-Ilan University, “we run out of chip space before the necessary delays are accommodated. The required delays may reach over 100 nano­seconds. Such delays may appear to be short consi­dering daily experience, however the optical paths that support them are over ten meters long! We cannot possibly fit such long paths as part of a silicon chip. Even if we could somehow fold over that many meters in a certain layout, the extent of optical power losses to go along with it would be prohi­bitive.”

These long delays require a different type of wave, one that travels much more slowly. In their study, Zadok and his team from the Faculty of Engineering and Institute of Nano­technology and Advanced Materials at Bar-Ilan University, and colla­borators from the Hebrew University of Jerusalem and Tower Semi­conductors, suggest a solution. They brought together light and ultra­sonic waves to realize ultra-narrow filters of microwave signals, in silicon integrated circuits. The concept allows large freedom for filters design. Bar-Ilan Univer­sity doctoral student Moshe Katzman explains: “We've learned how to convert the information of interest from the form of light waves to ultra­sonic, surface acoustic waves, and then back to optics. The surface acoustic waves travel at a speed that is 100,000 slower. We can accommo­date the delays that we need as part of our silicon chip, within less than a millimeter, and with losses that are very reasonable.”

Acoustic waves have served for the processing of information for sixty years, however their chip-level inte­gration alongside light waves has proven tricky. Moshe Katzman continues: “Over the last decade we have seen landmark demons­trations of how light and ultra­sound waves can be brought together on a chip device, to make up excellent microwave photonic filters. However, the platforms used were more specialized. Part of the appeal of the solution is in its simplicity. The fabri­cation of devices is based on routine protocols of silicon waveguides. We are not doing anything fancy here.” The realized filters are very narrow­band: the spectral width of the filters passbands is only 5 MHz.

In order to realize narrowband filters, the infor­mation-carrying surface acoustic waves is imprinted upon the output light wave multiple times. Doctoral student Maayan Priel says: “The acoustic signal crosses the light path up to 12 times, depending on choice of layout. Each such event imprints a replica of our signal of interest on the optical wave. Due to the slow acoustic speed, these events are separated by long delays. Their overall summation is what makes the filters work.” As part of their research, the team reports complete control over each replica, towards the reali­zation of arbitrary filter responses. Maayan Priel concludes: “The freedom to design the response of the filters is making the most out of the integrated, micro­wave-photonic platform.” (Source: Bar Ilan U.)

Reference: M. Katzmann et al.: Surface acoustic microwave photonic filters in standard silicon-on-insulator, Optica 8, 697 (2021); DOI: 10.1364/OPTICA.421050

Link: Institute for Nano-Technology and Advanced Materials, Bar-Ilan University, Ramat-Gan, Israel