Skip to main content
Optics and photonics

Optics and photonics

Photonic system is very good at locating radio transmitters

31 Mar 2022
Photonic RF source locator
Bench-top model: the optical setup of the Grenoble analogue photonic correlator, which can be used to locate a radio transmitter. (Courtesy: Hugues Guillet de Chatellus/Université Grenoble Alpes-CNRS)

A new analogue photonic platform that can rapidly identify the locations of radio-frequency sources has been unveiled by Guillaume Bourdarot, Jean-Philippe Berger and Hugues Guillet de Chatellus at Université Grenoble Alpes–CNRS in France. Their device works by cross-correlating the signals detected by a pair of antennas and operates over a wide bandwidth. The system is low-cost and uses off-the-shelf components. Potential applications include radio astronomy and telecommunications.

Cross-correlation is a useful technique for measuring the similarity between two or more signals. When signals originating from the same source are detected by two spatially separated antennas, their similarity can be calculated as a function of the relative time delay that occurs if one source is closer to one antenna than the other. This this enables a cross-correlation system to identify the location of the emitting source.

The cross-correlation of radio-frequency signals can be done using either digital or analogue techniques – but each of these has its own intrinsic limitations. In a digital correlators signals must fist be digitized and this makes it difficult to analyse real-time signals with bandwidths exceeding a few 100 MHz. This limit does not apply to analogue systems, but they have different constraints associated with the limited performance of their radio-frequency components.

Promising alternative

Photonic devices use light to process information and have emerged as a promising alternative to the analogue processing of wideband radio-frequency signals. These systems take advantage of the huge bandwidth of the optical spectrum, as well as the availability of cheap, high-performing optical components. In its study, the Grenoble team used these advantages to develop a new correlator architecture, based on a simple photonic platform.

Featuring no moving parts, the team’s system uses a pair of frequency-shifting fibre optic loops to up-convert radio-frequency signals into optical frequencies. It can simultaneously calculate the cross-correlation function for 200 time-delay intervals – which is many more than previous photonic systems. This means that the technique can be used to locate time-dependent signals.

Furthermore, the platform’s time-delay step can be adjusted over several orders of magnitude: ranging from nanoseconds down to just picoseconds. This means that radio frequencies ranging from megahertz to several gigahertz can be processed.