Self-calibrated photonic chip developed

Australian researchers have found a way to create an advanced photonic integrated circuit that builds bridges between so-called data superhighways, which is set to revolutionise the connectivity of current optical chips and replace bulky 3D optics with a wafer-thin slice of silicon. Their breakthrough, which was three years in the making, has been described in the journal Nature Photonics.

Whether it’s turning on a TV or keeping a satellite on course, photonics (the science of light) is transforming the way we live. Photonic chips can transform the processing capability of bulky, bench-sized utilities onto fingernail-sized chips.

“As we integrate more and more pieces of bench-sized equipment onto fingernail-sized chips, it becomes more and more difficult to get them all working together to achieve the speed and function they did when they were bigger,” said study co-author Dr Andy Boes, from The University of Adelaide. “We overcame this challenge by creating a chip that was clever enough to calibrate itself so all the components could act at the speed they needed to in unison.”

The project’s lead investigator, Monash University’s Professor Arthur Lowery, said the work complements Dr Bill Corcoran’s 2020 invention of a new optical microcomb chip that can squeeze three times the traffic of the entire nbn through a single optical fibre, regarded as the world’s fastest internet speed from a single fingernail-sized chip. The optical microcomb chip built multiple lanes of the superhighway; now the self-calibrating chip has created the on and off ramps and bridges that connect them all and allow greater movement of data.

Internet-reliant technologies like self-driving cars, remote-controlled mining equipment and medical equipment will require even faster and increased bandwidth in the future. Bandwidth increase is not just about improving the optical fibres the internet travels through — it’s about providing compact switches of many colours, going many directions, so data can be sent down many channels at once.

“Self-calibration is significant because it makes tunable photonic integrated circuits useful in the real world,” Lowery said. “Applications include optical communications systems that switch signals to destinations based on their colour, very fast computations of similarity (correlators), scientific instrumentation for chemical or biological analysis, and even astronomy.

“Electronics saw similar improvements in the stability of radio filters using digital techniques that led to many mobiles being able to share the same chunk of spectrum: our optical chips have similar architectures, but can operate on signals with terahertz bandwidths.”

Photonic circuits are able to manipulate and route optical channels of information, but they can also provide some computational ability; for example, searching for patterns. Pattern searching is fundamental to many applications: medical diagnosis, autonomous vehicles, internet security, threat identification and search algorithms.

Rapid and reliable reprogramming of the chips enables new search tasks to be programmed speedily and accurately. However, this manufacturing needs to be precise to the degree of a tiny wavelength of light (nanometres), which is currently difficult and extremely expensive. Self-calibration overcomes this problem.

A key challenge of the research was integrating all the optical functions onto a device that could be ‘plugged in’ to existing infrastructure. The solution, said Lowery, was to “calibrate the chips after manufacturing, to tune them up in effect by using an on-chip reference, rather than by using external equipment”.

“We use the beauty of causality — effect following cause — which dictates that the optical delays of the paths through the chip can be uniquely deduced from the intensity versus wavelength, which is far easier to measure than precise time delays,” Lowery said. “We have added a strong reference path to our chip and calibrated it. This gives us all the settings required to ‘dial up’ a desired switching function or spectral response.”

The method is a critical step to make photonic chips practically useful. Rather than searching for a setting, akin to tuning in an old radio, the researchers could tune the chip in one step, enabling the quick and reliable switch of data streams from one destination to another.

“Our photonic technology is now sufficiently advanced so that truly complex systems can be integrated on a single chip,” said study co-author Professor Arnan Mitchell, from RMIT’s Integrated Photonics and Applications Centre (InPAC). “The idea that a device can have an on-chip reference system that allows all its components to work as one is a technological breakthrough that will allow us to address bottleneck internet issues by rapidly reconfiguring the optical networks that carry our internet to get data where it’s needed the most.”

The development has the ability to speed up the global advancement of artificial intelligence and offers real-world applications such as safer driverless cars capable of instantly interpreting their surroundings; smaller switches for reconfiguring optical networks; enabling AI to more rapidly diagnose medical conditions; and making natural language processing even faster for apps such as Google Homes, Alexa and Siri. Reliable tuning of photonic chips also opens up applications such as optical correlators, which can almost instantaneously find patterns of data in data streams, such as images — something the group has also been working on.

Image credit: Monash University/RMIT.

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