A team at the University of Grenoble Alpes-CNRS has created a radio-frequency (RF) correlator, based on a photonic platform, that is suitable for analog wideband RF signal processing and that enables the real-time calculation of RF signal correlation across a broadband. Correlation of RF signals is a requirement for many applications, including imaging, information processing, and sensing and detection. The analog, photonic-based lag correlator outperforms conventional analog and digital techniques, according the the research team. The analog photonic correlator calculates the location of the signal’s source by computing the cross-correlation function of two signals emitted from one source and detected by two antennas. It relies on the multiplexing of the relative delay between the input signals for its calculations. This eliminates the need for point-by-point measurements of the cross-correlation function and enables the entire correlation function to be calculated in parallel. The correlation is computed in real time, without any truncation of the input signal. Researchers developed an analog photonic correlator that can be used to locate an object transmitting a radio signal. The new correlator could be useful for locating cellphones, signal jammers, or a variety of tracking tags. The optical setup used for the research is pictured. Courtesy of Hugues Guillet de Chatellus/ University of Grenoble Alpes-CNRS. “The photonic architecture we developed uses no moving parts and enables real-time signal processing,” researcher Hugues Guillet de Chatellus said. The photonic correlator allows real-time calculation of the cross-correlation function of two input signals for about 200 values of relative delay simultaneously. “This is much higher than any photonic technique has been able to accomplish so far,” Guillet de Chatellus said. Like a photonic processor, the correlator uses fiber optic components to turn RF signals into optical signals. Once the cross-correlation function is calculated, a detection and processing chain enables it to be converted into a digital format. A frequency shifting loop generates and manipulates time-shifted replicas for an input signal. The photonic architecture features optical circuitry that is based on off-the-shelf telecom components, making it is simpler than conventional analog or digital wideband correlators. Need for a broadband laser source is avoided, except for a low-linewidth single-frequency laser. Unlike conventional correlators, the analog, photonic-based lag correlator requires only a single detector, which is reconfigurable. The time-delay step can be adjusted from a few nanoseconds down to a few picoseconds, enabling the processing of signals with megahertz to multigigahertz bandwidth. “Many of today’s radio signals have large bandwidths because they carry a great deal of information,” Guillet de Chatellus said. “Our photonic approach offers a simple method for correlating signals with bandwidths of up to a few GHz, a larger bandwidth than is available from commercial approaches based on purely digital techniques.” After testing the device using high-power simple signals, the researchers tested it with more complex signals, and then with signals propagating through free space and received by a pair of antennas. The researchers demonstrated localization of RF transmitters by time difference of arrival and obtained a precision close to 10 ps for a 100-ms integration time, which indicates that the system could locate an emitter with a precision of about 3 mm. Because it is faster than other methods and works with a range of RF signals, the analog photonic correlator could be useful for locating cellphones, signal jammers, or tracking tags. “Real-time processing helps ensure there isn’t any downtime, which is critical for defense applications, for example,” Guillet de Chatellus said. The analog photonic correlator could also be used in astronomy to cross-correlate signals coming from several telescopes to create high-resolution images. The research team plans to use two remote antennas to collect signals emitted from the sun at around 10 GHz. The team will cross-correlate the signals using the photonic correlator to create an image of the sun at radio wavelength. If this experiment is successful, the analog photonic correlator could be used in astronomy facilities, such as the Very Large Telescope Interferometer in Chile, for correlating signals in the infrared wavelength using heterodyne interferometry. Heterodyne interferometry has been used for radio-interferometry but has been limited to narrow correlation bandwidths. The researchers are also determining if the photonic correlator can be used to correlate three signals, which would enable 3D localization of transmitters by triangulation. They also plan to miniaturize and fully integrate the mechanism. The research was published in Optica (www.doi.org/10.1364/OPTICA.442906).