Achip-scale optical device, developed by a team from the University of Sydney’s Australian Institute for Nanoscale Science and Technology (AINST), achieves radio frequency signal control at sub-nanosecond time scales. The photonics breakthrough has the potential to provide broader bandwidth instantaneously to more users.

David Marpaung, Benjamin Eggleton, Yang Liu, and Amol Choudhary inside the AINST headquarters, the Sydney Nanoscience Hub, pointing at a thumbnail-size chip being evaluated in the broadband microwave testbed. (Credit: University of Sydney)

Researchers Dr. David Marpaung, along with Prof. Benjamin Eggleton and colleagues Yang Liu and Dr. Amol Choudhary, demonstrated how “gigahertz tuning” of optical elements supports a new — and perhaps “revolutionary” — approach to wireless communications.

Photonics & Imaging Technology: What is a “bandwidth bottleneck?”

Dr. David Marpaung: Bandwidth bottleneck is a term referring to the fast growth of global Internet traffic -- estimated to increase by 22% each year. The “bottleneck” causes the demand for bandwidth to rapidly exceed the supply. This can be understood by looking at the large number of devices connected to the Internet, which was estimated to reach 10 billion in 2016.

Currently there are several ways to relieve this bottleneck. One is to speed up mobile networks. This future mobile network, known also as 5G/fifth generation, can achieve higher capacity by implementing multiple electronically-steerable antennas as well as adaptive radio systems.

Our invention creates a revolutionary device that can be implemented in these systems: a broad-bandwidth, tunable radio frequency delay element that can be reconfigured very quickly. Such a time delay element is a key building block in steerable antennas.

P&IT: What is a tunable radio frequency delay element?

Marpaung: This element, also known as a tunable RF delay line, creates a time difference between input and output signal. By tuning the delay line, this time difference can be continuously changed. The uniqueness of our delay line is that this time difference can be changed really fast, of the order of a billionth of a second.

P&IT: What makes the achievement a “breakthrough?”

Marpaung: This technique solves the speed bottleneck existing in conventional radio frequency photonic delay time, which is limited by the slow thermal tuning response and heat dissipation. Prior to our work, the conventional way of tuning these chip delay lines is through thermal tuning, which is 1000x slower and consumes Watts of power. We completely remove the need of this power, hence creating an energy-efficient device.

P&IT: From a technology perspective, how do you provide broader bandwidth instantaneously?

Marpaung: Our device and technique show the potential to be a crucial part of an antenna system that creates multiple beams that can be configured very quickly, accessing more users in real time. By developing very fast (GHz order) tunable delay lines on chip, the direction of the beam can be reconfigured at nanosecond speed. This means that each beam carrying the information can be directed to each user to provide high capacity at any given time.

P&IT: What does the device look like?

Marpaung: The optical device at present consists of a silicon nitride ring resonator and a lithium niobate electro-optic modulator. The ring acts as a low-loss broadband optical delay line while the optical modulator acts as the delay tuning element, achieved through controlling the RF-modulated optical power going into the ring. We designed the key experiments to demonstrate the gigahertz tuning of the silicon nitride ring resonator optical delay element.

P&IT: What is most exciting to you about what the technology can accomplish?

Marpaung: The most exciting part of the technology is its universality. In principle, this technique can be used to activate any optical delay line element, not necessarily limited to the optical ring resonator in our demonstration.

In this way, one can optimize the properties of the delay line — the material platform, loss, or footprint, for example — and then activate it, or tune it very fast, using our technique. This allows us, in principle, to build a tunable delay line with all-optimized quality and very low power consumption, which was not possible before.

P&IT: What are you working on now?

Marpaung: As a next step, we are currently working on more advanced silicon devices where the modulator and the delay line element can be highly integrated in a single chip. The goal will be to implement the chips in small mobile devices.

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