An active optical intensity interferometry scheme enables synthetic aperture imaging from over a kilometer away

Intensity interferometry is a promising technique that enables the precise measurement of spatial properties (i.e., distances, shapes and light properties) by probing fluctuations in the intensity (i.e., brightness) of light, as opposed to the exact timing and phase of light waves probed by amplitude (phase) interferometry. Intensity interferometry could overcome some of the limitations of amplitude interferometry, as it is less sensitive to atmospheric factors and optical imperfections.

Despite their promise for the precise reconstruction of images, intensity interferometry techniques rely on thermal light, which spans across various wavelengths, contains only a few photons in each measurable light packet and is prone to rapid beam divergence. This has so far limited their ability to collect high-resolution images over long distances.

Researchers at the University of Science and Technology of China and other institutes recently introduced a new active intensity interferometry approach that enables the collection of high-definition images from across distances of over 1 km. Their paper, published in Physical Review Letters, demonstrates the use of this technique for the optical imaging of millimeter-scale objects from 1.36 km away.

“Our recent paper draws its inspiration from the pioneering work of Hanbury Brown and Twiss in the 1950s, who first demonstrated intensity interferometry by observing Sirius, the brightest star in the night sky,” Qiang Zhang, co-author of the study, told Tech Xplore.

“The intensity interferometry is insensitive to atmospheric turbulence and telescope optical defects, which has unique advantages in long-baseline optical synthetic aperture imaging with extremely high angular resolution. However, traditional intensity interferometry is mainly limited to passive observations of bright astronomical sources.”

As part of their study, Zhang and his colleagues set out to further enhance the performance of intensity interferometry techniques, leveraging recent advancements in the development of LiDAR technology. More specifically, they combined intensity interferometry with active illumination and found that this allowed them to image distant objects with high resolution.

“Our proposed active intensity interferometry approach relies on two main components: an active illumination system and the receiving system,” explained Zhang. “The active illumination system is made up of an array of laser beams. As these multiple lasers travel through different atmospheric turbulence paths, their phases become randomized and independent—mimicking the thermal light that originates from stars, which we call ‘pseudo-thermal’ illumination.”

The second component of the team’s proposed interferometry method is the so-called receiving system. This system is an intensity interferometer with a tunable baseline, designed to collect sufficient target spatial frequency information.

“For the pseudo-thermally illuminated target, we measure the tiny fluctuations in intensity at two separate detectors and correlate them over time,” said Zhang. “This gives us information about the spatial structure of the target—the modulus of its Fourier transform—which we can then computationally reconstruct into a high-resolution image. This is the essence of optical synthetic aperture imaging using intensity correlations.”

The active intensity interferometry technique developed by this team of researchers has various advantages over previously proposed approaches. Most notably, it can attain state-of-the-art imaging resolution in realistic atmospheric environments at kilometer-scale.

To assess the potential of their approach, Zhang and his colleagues built both the active illumination and the receiving system they designed. They then used these systems to conduct imaging experiments aimed at capturing 2D double-slit and letter targets, test patterns or objects commonly used to test the resolution optical techniques.

“We successfully achieved millimeter-level resolution at 1.36 kilometers,” said Zhang. “The experimental results in the PRL paper clearly demonstrate the high-resolution imaging capability of our approach.”

Overall, the results gathered by this team of researchers highlight the promise of their approach, showing that their multi-laser emission array-based system addresses the crucial limitations of pseudo-thermal light sources employed by other existing intensity interferometers. In fact, the pseudothermal light sources used by many other imaging systems have large divergence angles and thus struggle to provide effective pseudo-thermal illumination over long distances.

“We also demonstrated both theoretically and experimentally that increasing the number of laser emitters in the active illumination system makes the pseudo-thermal illumination behave more closely resemble ideal thermal illumination,” said Zhang. “This is interesting because illumination closer to ideal thermal light directly translates to higher signal-to-noise ratios in intensity interferometry, thus improving imaging quality.”

The recent work by Zhang and his colleagues could open new possibilities for the precise imaging of small objects across long distances. Specifically, it outlines a promising path for the future advancement and refinement of intensity interferometers.

“By continuing to develop and scale up multi-laser illumination systems like the one presented in our paper, future active intensity interferometers can be made even more robust, achieve higher performance, and ultimately become more practical and widely applicable for various long-range, high-resolution imaging tasks,” added Zhang.

“In our next studies, we plan to extend our approach to high-resolution imaging of more complex objects and even three-dimensional objects. We will also focus on developing practical intensity interferometers that can be applied to specific fields such as astronomical imaging.”

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