Radio frequency (RF) signal control at sub-nanosecond timescales has been demonstrated on a chip-scale optical device, a discovery that could pave the way for improved wireless communication systems. This is a schematic illustration of the fast control of RF signals. Courtesy of University of Sydney. To reduce time delay and improve the functionality of RF photonic signal processing, researchers at the University of Sydney took an optical tuning approach, controlling and switching RF time delay using integrated optical ring resonators with a fast tuning speed. Their technique relies on the interference between a data signal and a reference signal to synthesize larger phase shifts in the microwave domain, resulting in RF signal group delay enhancement. In experiments, the technique was shown to enable flexible switching between advancement and delay of RF signals and allow for fast tunability up to a gigahertz (GHz) tuning speed, solely by controlling the optical power. By optically varying the control signal at GHz speeds, the time delay of the RF signal can be amplified and switched at the same speed. The research results point to the feasibility of wideband operation and compatibility with existing schemes based on dispersive photonic devices. Rapid and continuous tunability of time delay is a crucial functionality for RF photonic signal processing systems. The lack of the high tuning speed in current RF techniques, in applications ranging from personal communications to defense, is driving the need to develop solutions on a compact optical platform. Previous solutions using optical tuning have typically been limited by the low tuning speed (on the order of milliseconds) provided by on-chip heaters, and have consumed a lot of power. “To circumvent these problems, we developed a simple technique based on optical control with response time faster than one nanosecond: a billionth of a second — this is a million times faster than thermal heating,” said researcher Yang Liu. “Nowadays, there are 10 billion mobile devices connected to the wireless network (reported by Cisco last year) and all require bandwidth and capacity,” said researcher Yang Liu. “By creating very fast tunable delay lines on chip, one eventually can provide broader bandwidth instantaneously to more users. “To reduce power consumption and maximize reception range for future mobile communications, RF signals need to achieve directional and fast distributions to different cellular users from information centers, instead of spreading signal energy in all directions,” Liu said. Researchers David Marpaung, Benjamin Eggleton, Yang Liu and Amol Choudhary pointing at a thumbnail-size chip being evaluated in the broadband microwave testbed, inside the Sydney Nanoscience Hub. Courtesy of University of Sydney. Professor Benjamin Eggleton said the technology would not only be important for building more efficient radar but would also improve online communication for individual users. “Such a system will be crucial not only to safeguard our defense capabilities, it will also help foster the so-called wireless revolution — where more and more devices are connected to the wireless network. This includes the Internet of Things, fifth-generation communications, and smart home and smart cities,” said Eggleton. “We are currently working on the more advanced silicon devices that are highly integrated and can be used in small mobile devices,” he added. The optical tuning achieved on an integrated photonic chip could open the way toward ultrafast and reconfigurable on-chip RF systems that do not require complex design and that are compact, energy-efficient, flexible and compatible with existing RF functionalities. These features could be beneficial for applications in RF signal processors, multi-tap RF filters and phased array antennas. The research was published in Optica, a publication of the Optical Society of America. (doi: 10.1364/OPTICA.4.000418).