Long Waveguides from Thin Air

Fiber optic cables can effectively transmit streams of data, but gravity, distance and power limitations sometimes make such physical waveguides impractical. Yet when sent directly through free space, light spreads and its intensity suffers.  Enter the concept of air waveguides, which potentially might allow distant projection of high-peak- and high-average-power laser beams in the atmosphere.

Researchers led by scientists at the University of Maryland (UMD), USA, now say they have created a 45-m-long waveguide in air—60 times farther than the record they previously set (Phys. Rev. X, doi: 10.1103/PhysRevX.13.011006). The feat could lay the groundwork for yet-longer waveguides and enable numerous possible applications, including pollution detection, long-distance communication, laser weapons and lightning protection.

Filament “fences”

The UMD-led team’s technique, first demonstrated in 2014, uses femtosecond laser pulses to sculpt a channel in the air that confines the light of a secondary probe laser. This waveguide is created by the process of filamentation—a nonlinear effect in which sufficiently powerful laser light self-focuses until it collapses into a narrow beam, or filament.

Although filaments have a limited average power and thus aren’t themselves an effective means of transmitting high-power laser light, the filamentation of the pulses heats the air, creating a plasma and leaving behind a path of low-density air. These low-density “holes” have a lower refractive index than the air around them, so when they are arranged into multiple low-density tunnels that merge and surround a denser core of air, they act like the cladding on a fiber optic cable.

In the 2014 work, the team arranged four laser filaments in a square shape to produce a low-density “fence” around a center of unperturbed air, then successfully guided pulses a distance of 70 cm through this waveguide. Extending the distance is tough, however, as the limited number of filaments restricts the width of the waveguide. Simply adding more filaments makes it difficult to ensure that the lobes have equal energy and locally smooth phase fronts.

Lasers in the hall

In the new work, the authors addressed this issue by employing a smooth Laguerre-Gaussian LG₀₁ mode, generated using a spiral phase plate that concentrates the laser light into a donut shape. The number of filaments automatically scales with beam size, and they naturally distribute themselves around a ring, ensuring good circumferential coverage of the generated cladding and significantly increasing the potential range of the waveguide.

When the time came to put their new arrangement to the test, the researchers had a problem: they needed more space, and moving the laser wasn’t practical. Luckily, their lab is adjacent to a hallway. In an unusual step (and with significant preparation), they cut a hole in the wall and turned the hall into an extension of their lab, working overnight when the building was empty of students.

“It was a really unique experience,” noted lead author Andrew Goffin in a press release. “There's a lot of work that goes into shooting lasers outside the lab that you don't have to deal with when you're in the lab—like putting up curtains for eye safety.”

Using the new method, the researchers demonstrated air waveguiding over the whole length of the hallway—45 m, or 60 times longer than their previous record. These optical-waveguide structures are long-lived, lasting tens of milliseconds, providing more than enough time for the secondary probe laser pulse to pass through. Detectors showed that at the end of the path, the waveguide had maintained about 20% of the light that would have otherwise been lost. Experiments done in the lab with a shorter, 8-m waveguide delivered about 60% of the light to the target.

One kilometer and beyond

The researchers believe that their method is scalable, and their calculations suggest that they are not yet near the theoretical limit of the technique. They plan to conduct further experiments to improve their air waveguides, including making the donut ring more uniform to increase length and guiding efficiencies.

“Reaching the 50-m scale for air waveguides literally blazes the path for even longer waveguides and many applications,” senior author Howard Milchberg, UMD, USA, said in a press release. “Based on new lasers we are soon to get, we have the recipe to extend our guides to one kilometer and beyond.”