Scientists in South Africa and France have shown they can send structured light in the form of optical skyrmions across several hundred meters of turbulent air with very little distortion (Sci. Adv., doi: 10.1126/sciadv.aee2671). Assuming that suitably fast detectors can be built, they reckon the new topological scheme could significantly increase the bandwidth of free-space optics.

Enabling beam tailoring

Structured light promises to boost data transmission rates in optical communications by shaping the spatial profile of laser beams to add extra degrees of freedom to encoding schemes. One prominent example is that of orbital angular momentum (OAM), which is generated when a beam’s wavefront spirals around its axis of propagation. Data can be encoded by imparting different degrees of twist to the beam.

However, the need to precisely tailor beams’ amplitude, phase and polarization structure makes such techniques vulnerable to the distorting effects of many real-world environments. One such environment is the atmosphere, where turbulence caused by variations in temperature and pressure alters air’s refractive index. These distortions can be compensated for using adaptive optics or machine learning, for example, but such fixes add complexity and cost to data transmission.

An alternative approach is to use the physics of the atmosphere to work out how to structure light such that the degrees of freedom used to encode data remain intact even in turbulent conditions. However, this usually involves having real-time knowledge of the atmosphere, which is hard to come by. In the latest work, Andrew Forbes and colleagues at the University of the Witwatersrand, South Africa, and the University of Bordeaux, France, describe how to preserve transmitted data without such knowledge. They use particle-like topologies of light known as optical skyrmions, which they experimentally demonstrated can resist atmospheric turbulence.

Flying skyrmions

When a laser propagates through a turbulent medium, this number should be preserved even when the field itself is scrambled.

The idea is to interfere two light beams with different amounts of OAM mixed in with polarization. The spatial variation of the combined beam’s polarization forms a skyrmionic topology characterized by N—an integer number revealing how many times each polarization state is found in the light field. When a laser propagates through a turbulent medium, this number should be preserved even when the field itself is scrambled. In other words, while the atmosphere distorts the field it doesn’t change the fact that an N=1 field retains just one copy of each polarization state and an N=2 field has twice as many copies.

The researchers demonstrated their scheme by directing a green laser at a spatial light modulator to generate beams with varying amounts of OAM and then combined two such beams in a modified Mach-Zehnder interferometer. Conducting the experiment on the Braamfontein campus of the University of the Witwatersrand in the center of Johannesburg, they sent the resulting skyrmions across 270 meters of open space between the engineering and mathematics buildings. On the far side they used a polarization-sensitive camera and 50:50 beam splitter to measure three pairs of orthogonal polarization intensities in each arriving skyrmion.

They did so first by making all six measurements within the channel’s coherence time—the timescale over which atmospheric temperature and pressure fluctuations generally occur (typically tens to hundreds of milliseconds). After carrying out the experiment inside to obtain essentially undistorted results, they proceeded to test the setup with different atmospheric conditions—first in the morning when the air was quite cool and calm, then at midday when turbulence was at about its peak, and finally later in the afternoon when turbulence had died down somewhat.

In the morning, they found that the arriving skyrmions’ intensity profile was similar to that seen indoors and their polarization retained a cylindrical symmetry, while at midday, and to a lesser extent in the afternoon, both intensity and polarization showed major shifts in their spatial distribution. However, in all cases the researchers found that the skyrmion number remained intact.

They also carried out measurements averaged over timescales exceeding the coherence time, entering a regime where problems of decoherence and depolarization emerge. Again, they found that the skyrmion number remained unaltered as each light packet traveled from one end of the turbulent channel to the other—despite the beams’ degree of polarization dropping by nearly 40%. This, they say, demonstrates the scheme’s potential utility for quantum as well as classical communication, given the extended periods needed for quantum measurements.

Sending data

Finally, they demonstrated how the technique could be used to encode data by equating three skyrmion numbers to three different colors, which they used to build up the pixels in a map of South Africa’s nine provinces. They found they could recover the map at the far end of their optical link at all times of day, achieving a fidelity of 98.7% in the morning and 98.6% in the afternoon. The fidelity dropped to 86.9% at noon, but this resulted in some discoloration rather than corruption of the image as a whole.

Forbes and colleagues point out that the spatial light modulator they used to generate skyrmions and the camera they employed to measure them are both quite slow. They believe it should be fairly straightforward to up the modulation rate by using a Mach-Zehnder interferometer for that job as well. But they say it will be harder to significantly boost detection rates given the absence to date of devices that can efficiently separate skyrmion states. “These devices would be key enablers for bringing skyrmion-based communication to data rates competitive with existing structured-light schemes,” they wrote.