TAMPERE, Finland, Feb. 15, 2016 — A technique for producing ghost-imaging in the time domain offers promise for the dynamic imaging of ultrafast waveforms with applications in communications, remote sensing and ultrafast spectroscopy.
Researchers from the Tampere University of Technology and the University of Eastern Finland have demonstrated how ultrafast pulses that carry information over durations <1 billionth of a second can be detected without actually “seeing" those pulses directly. The method correlates in time the intensity of two light beams, neither of which independently carries information about the signal.
The conventional approach to decode information carried by ultrafast optical signals that propagate in optical fibers employs fast detectors that convert the temporal intensity variations of a light beam into a radio-frequency electrical signal. This technique is at the core of ultrafast optical communications, enabling the transmission of information at speeds exceeding several billion bits per second.
Schematic of an experimental setup for performing ghost-imaging in the time domain. Courtesy of the Optics Laboratory, Tampere University of Technology.
"Ghost imaging is an all-new physical imaging method that enables us to image a target by correlating two beams of light, neither of which contains image information,” said Eastern Finland professor Ari Friberg. “One beam sees the object and measures the overall output, while the other is in no contact with the object although its space distribution is measured. When these measured intensities are correlated, the image of the target magically appears, like a ghost.”
In their experiments, the key element was to use a laser source with random intensity fluctuations at a time scale of a picosecond, a feature which is generally highly detrimental for the standard transmission of information. By correlating these fluctuations with the total power of the modulated signal, it was possible to reconstruct a perfect copy of the ultrafast signal.
"Even more fascinating", says Professor Genty, "is the fact that the technique is completely insensitive to distortion that the signal may experience due to dispersion, nonlinearity, or attenuation, for example,” said Tampere professor Goëry Genty.
To demonstrate this inherent property associated with ghost imaging, the researchers scrambled the information carried by the optical pulses using a multimode fiber whose large dispersion spread the individual pulses in time to the extent that they overlapped, thus preventing the information from being faithfully retrieved with a conventional fast detector. When the ghost imaging approach was used, the team showed that a perfect replica of the original signal could be obtained and the information could be recovered.
This video shows how the ghost image (signal-to-noise ratio) improves as a function of the number of realizations. Courtesy of the Optics Laboratory, Tampere University of Technology.
The method is scalable, the resarchers said, and could be can be integrated on-chip.
The research was published in Nature Photonics (doi: 10.1038/nphoton.2015.274).