🔬 optics

Outdoor 100-m-scale air waveguides

This paper reports the first outdoor demonstration of 100-meter-scale air waveguides generated by ultrashort-pulsed laser filamentation, establishing a new distance record and characterizing their performance under various real-world atmospheric conditions including turbulence and crosswinds.

Original authors: Andrew Goffin, Gregory Babic, Andrew DeRusha, Maksim Livshits, Howard Milchberg, Patrick Skrodzki

Published 2026-08-12

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CC BY 4.0

Original authors: Andrew Goffin, Gregory Babic, Andrew DeRusha, Maksim Livshits, Howard Milchberg, Patrick Skrodzki

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine trying to shine a flashlight across a vast, foggy field. Normally, the beam spreads out, gets dim, and eventually disappears into the darkness. This is because light naturally wants to scatter, a problem made worse by wind, heat waves, and dust in the air. Scientists have long dreamed of a way to keep that beam tight and bright over huge distances, almost like creating an invisible, hollow tube of air that acts as a highway for light. This idea is called an "air waveguide."

To build this invisible tube, researchers use a special kind of laser that fires incredibly fast, tiny bursts of energy. When these bursts hit the air, they don't just pass through; they heat up the air so quickly that it creates a ring-shaped tunnel. Think of it like a donut made of hot, thin air. Because hot air is less dense than cold air, light travels faster through the center of this "donut" than through the hot ring surrounding it. This difference tricks the light into staying trapped in the middle, bouncing along the tunnel instead of spreading out. While this has been done successfully inside quiet labs, the big question was: could this invisible tube survive the messy, windy, and unpredictable real world outside?

This paper tells the story of a team that took this experiment out of the lab and into the wild. They set up their equipment at a field site in Los Alamos, New Mexico, and successfully created an air waveguide that stretched for a record-breaking 100 meters (about the length of a football field). This is a significant jump from the previous indoor record of 42 meters. The team didn't just build the tube; they tested how long it lasted and how well it worked while battling real-world weather, including wind speeds up to 2 m/s, humidity changes, and atmospheric turbulence.

Here is what they found. The "invisible tube" worked! They sent a green laser beam through the tunnel and saw it stay focused and bright even at the far end, whereas without the tube, the beam would have scattered and faded away. In fact, the signal they received at 100 meters was 1.5 times stronger than if they hadn't used the waveguide. This is a big deal because it proves that this technology can work outside, not just in a controlled room.

However, the experiment also revealed the tube's biggest weakness: the wind. The air tunnel isn't a solid pipe; it's a ghostly structure made of heat. When the wind blows, it pushes this hot ring sideways, like a leaf drifting off a path. The researchers found that even a gentle breeze of 2 m/s could push the tunnel out of alignment with the laser beam, causing the guiding effect to fade after about 3 milliseconds (three-thousandths of a second). In their simulations, they confirmed that the faster the wind blows, the sooner the tunnel drifts away. Interestingly, they found that other factors like humidity or how "bumpy" the air is (turbulence) didn't ruin the guide as much as the wind did.

The team also looked at how long the tunnel could last. In a perfect, windless world, the heat from the laser would keep the tunnel open for about 100 milliseconds. But in the real world, the wind cuts that time down to just 2 to 4 milliseconds. Despite this short lifespan, the results are promising. The authors suggest that if we use lasers that fire thousands of times per second (a "kHz-scale" rate), we could constantly rebuild the tunnel faster than the wind can blow it away. This could lead to a "quasi-steady-state" guide, a continuous, invisible highway for light that could survive in challenging outdoor environments.

So, while the wind is currently the boss of how long these light highways last, this paper proves that the concept works in the real world. It suggests that with faster lasers, we might soon be able to send light signals over kilometers of open air, opening doors for better remote sensing, clearer communications, and new ways to direct energy, all without needing a physical cable.

Technical Summary: Outdoor 100-m-scale Air Waveguides

Problem Statement
Optical power densities required for standoff spectroscopy, remote sensing, directed energy, and free-space optical communications are fundamentally limited by diffraction and adverse atmospheric conditions, including turbulence, fog, and wind. While air waveguides (AWGs) generated by ultrashort-pulsed laser filamentation offer a potential solution to overcome beam diffraction and the inverse square signal falloff, previous demonstrations were restricted to indoor environments with a maximum record length of 42 meters. A critical gap existed in understanding how these waveguides perform in realistic outdoor conditions, particularly regarding the impact of wind, turbulence, and other environmental variables on waveguide lifetime and guiding efficiency.

Methodology
The authors conducted the first outdoor demonstration of air waveguiding at Los Alamos National Laboratory over a 100-meter range. The experimental setup utilized a Ti:sapphire laser system generating 210 mJ pulses at a central wavelength of 800 nm with a 1.6 ps duration (FWHM) and a 10 Hz repetition rate. The laser beam was shaped into a near-LG01 "donut" mode using a spiral phase plate and down-collimated to an 8 mm ring diameter, creating a linear Rayleigh range of approximately 130 m.

Filamentation occurred around the circumference of this ring, generating an array of filaments that heated the air. Through thermal diffusion, these individual density depressions merged to form a low-density annular cladding surrounding a relatively unperturbed central core, analogous to a graded-index optical fiber. To measure guiding performance, a 5-mW, 520 nm continuous-wave probe laser was injected into the waveguide.

Diagnostics included:

Burn paper to visualize filament core arrangements along the path.
An off-axis camera and beam block to image guided vs. unguided probe beams.
A photodiode to measure time-dependent transmission.
A scintillometer and hot-wire anemometer to monitor atmospheric turbulence (refractive index structure parameter $C_n^2$ ), humidity, pressure, and crosswind speeds.

Experiments were conducted over several days in August and September 2025, covering various times of day to capture a wide range of environmental conditions, including crosswind speeds up to ~2 m/s and $C_n^2$ values up to ~2.5×10⁻¹⁴ m⁻²/³.

Key Contributions and Results

Record Guiding Length: The study successfully demonstrated outdoor air waveguiding over a record distance of ~100 meters, surpassing the previous indoor record of 42 meters.
Environmental Correlation: The authors correlated waveguide performance with real atmospheric data. They found that while filament survival decreases with distance (typically 3–4 filaments surviving at 100 m compared to an estimated ~11 at the source), the remaining structure is sufficient to sustain guiding.
Signal Enhancement: At 100 meters, the waveguide provided a signal enhancement ( $\epsilon$ ) of approximately 1.5 in the core region and increased the signal-to-noise ratio (SNR) by ~2 dB compared to unguided propagation, even in bright sunlight.
Wind as the Primary Limiting Factor: The research identified crosswind speed as the most deleterious environmental factor. While thermal diffusion limits the waveguide lifetime to ~100 ms in still air, crosswinds cause the waveguide to drift transversely off the optical axis.
- Under wind speeds of ~2 m/s, the effective waveguide lifetime was reduced to < 3 ms.
- On days with lower wind speeds (< 0.4 m/s), the waveguide remained functional for up to 5 ms.
Simulation and Modeling: Accompanying propagation simulations confirmed that wind advection displaces the density hole profile, reducing guiding efficiency as delay increases. The simulations predicted that the waveguide lifetime scales inversely with wind speed ( $\Delta t_{life} \propto 1/v_{wind}$ ).
Turbulence Impact: The study concluded that typical outdoor turbulence levels ( $C_n^2 < 10^{-14}$ m⁻²/³) have a minimal impact on guiding performance over 100-meter scales, with simulations showing less than 1% variance in signal enhancement even at significantly higher turbulence levels.

Significance and Claims
The paper claims that this work paves the way for the deployment of quasi-continuous air waveguiding in challenging outdoor environments. The authors assert that while wind reduces the lifetime of single-shot waveguides, it does not scale with distance in a way that precludes long-range applications. Specifically, they propose that using high-repetition-rate (kHz-scale) filament drivers could replenish the waveguide faster than the wind drifts it, enabling quasi-steady-state guiding.

The authors modestly conclude that their results demonstrate the feasibility of kilometer-scale outdoor waveguides for remote sensing, optical communications, and directed energy applications, provided that high-repetition-rate lasers are employed to mitigate wind-induced drift. They emphasize that the transition from indoor to outdoor settings has been achieved with minimal loss of fidelity, validating the scalability of the technology.

Technical Summary: Outdoor 100-m-scale Air Waveguides

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