Structured light
Does twisted laser light survive a trip across a city?
Open access · cc by · source: Europe PMC
Twisted light beams sent across a city kept their visible intensity shape but lost the phase information that communications and quantum links rely on, especially for higher twist.
Study at a glance
- Design
- Other — Outdoor optical link experiment: OAM modes generated by an SLM were sent across a 1.6 km urban path and analysed with a mode sorter, compared with turbulence models and simulations.
- N
- No participant N; nine OAM modes (l = 0 to plus/minus 4) plus mode superpositions transmitted over one link.
- Population
- Laser beams carrying orbital angular momentum propagated through the atmosphere above a city
- Outcome
- OAM cross-talk spectrum, average received OAM, beam wander, turbulence strength, and preservation of intensity patterns
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Key findings
For the untwisted beam and the lowest twist, the measured leakage matched the thin-phase turbulence model, but for higher twist the average received twist was lower than what was sent, which the models do not predict. Simulations indicated this is explained by a high-order vortex breaking into several single vortices, amplified by the finite receiver aperture. Superpositions of opposite twists kept their petal-shaped intensity, confirming earlier long-distance reports, yet their phase was strongly distorted, and tracking a two-vortex beam showed the distortion changed faster than a tenth of a second, pointing to the moving atmosphere.
Methodology
The researchers generated laser beams carrying orbital angular momentum with a hologram and sent them over a 1.6 km open-air link in a German city. At the receiver a mode sorter split the light by twist value so they could measure how much power leaked into other modes. They also tracked beam wander, estimated turbulence strength, compared results with standard Kolmogorov turbulence models, and simulated the effect of vortices splitting apart.
Limitations
This is a single link under particular weather and alignment conditions, with no automatic tip-tilt correction, so the size of the effects may not transfer to other paths. The source of vortex splitting is only partly separated: static optical flaws, back-reflections and turbulence near the transmitter could all contribute. The receiver aperture was about half the beam diameter for the highest mode, which itself shapes the result, and the proposed mitigation (very pure mode generation plus adaptive optics) was not tested.
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