HomeSciencePhysicsAir Waveguide from “Donut” Laser Beams

# Air Waveguide from “Donut” Laser Beams

&bullet; Physics 16, 11

A waveguide sculpted in air with lasers transmits mild over a distance of almost 50 meters, which is 60 occasions farther than earlier air-waveguide schemes.

Conventional optical waveguides reminiscent of optical fibers and planar waveguides encompass a core surrounded by a cladding with a decrease index of refraction. Light is effectively confined within the core by whole inner reflection on the core-cladding boundary. Optical fibers can transport mild over 100s of kilometers, however there are purposes—reminiscent of high-power transmission and atmospheric monitoring—the place the usage of fibers turns into impractical. Sending mild instantly by means of air just isn’t an choice, as diffraction results trigger the beam to unfold out. A possible answer is to “sculpt” waveguides within the air with laser pulses that produce a low-density cladding round a central core of unperturbed air. Using a brand new methodology with donut-shaped beams, Andrew Goffin from the University of Maryland, College Park, and colleagues have created a 45-m-long waveguide in air [1], reaching 60 occasions farther than the report they beforehand established for an air waveguide. The achievement might allow supply of high-power laser pulses to distant targets, opening up a variety of purposes reminiscent of distant sensing, lightning management (see Research News: A Laser-Based “Lightning Rod”), and microwave guiding.

The precept of an air waveguide is to fireplace a femtosecond laser pulse that opens a short-lived channel within the air by means of which a subsequent “probe” pulse can cross [2]. The first pulse generates the mandatory refractive index distinction between the core and cladding by heating the air constituents (reminiscent of N2, O2, and noble gases). The heated air expands in such a means that the density of the cladding turns into decrease than that of the encircling air. The ensuing air waveguide can final a couple of milliseconds—sufficient time to ship the probe sign by means of it.

But one may marvel, why is the primary laser pulse in a position to propagate forward of the probe with out spreading out? The reply lies in a nonlinear course of, referred to as filamentation, that outcomes from the steadiness of two competing results in air: a self-focusing impact induced by so-called Kerr nonlinearity and a defocusing impact induced by the formation of a plasma created by the heart beat itself [3]. Filamentation can keep a laser subject narrowly confined over a distance a lot better than that allowed by diffraction underneath linear propagation circumstances. However, a laser filament can have a width no bigger than

$\sim$

200

$𝜇\text{m}$

and a peak depth no better than

$\sim$

1014 W/cm2, finally limiting the typical energy within the filament core. For this purpose, the filaments generated by femtosecond laser pulses are usually not an efficient means—by themselves—for delivering excessive energy. But when used to generate an air waveguide, the filaments can create a path for high-power mild beams.

Goffin and colleagues first demonstrated the precept of air waveguiding in 2014. In that earlier experiment, the workforce handed a crimson laser beam by means of a four-segment masks to create 4 laser filaments in a sq. sample. These filaments fashioned a “light fence” that confined mild inside its core. The researchers used this air waveguide to transmit a 110-mJ pulse of inexperienced mild over a

$\sim$

70-cm size in air (see Viewpoint: A Waveguide Made of Hot Air).

The workforce has now achieved a powerful extension of this earlier work. The comparatively quick size of the group’s first air waveguide was because of the restricted variety of filaments within the mild fence, which restricted the width of the waveguide and the power of the density distinction between core and cladding. To improve the variety of filaments, one may naively think about utilizing a masks with extra segments to seed a bigger variety of filaments. However, in follow, it’s tough to make sure that the segments produce beam lobes with equal power and regionally easy part fronts. As an alternate, the authors use a donut-shaped beam or, technically talking, a easy Laguerre-Gaussian LG01 mode. They generate this mode utilizing a spiral part plate that concentrates the laser mild into a hoop with a diameter of some millimeters (Fig. 1). The concentrated mild initiates random filamentation in a uniform distribution across the donut ring. Using a bigger beam robotically produces extra filaments—supplied that the native laser fluence stays fixed—which implies the ensuing cladding covers the entire waveguide circumference.

In a hallway subsequent to their laboratory, the authors demonstrated air waveguiding over a distance of 45 m. Their waveguide generator was a 300-fs laser pulse with a wavelength of 800 nm and whole power of 120 mJ. This pulse was imprinted with the LG01 donut mode, inflicting it to type roughly 30 filaments round a hoop with diameter of 5.6 mm. Through the ensuing waveguide, the researchers transmitted a 7-ns probe pulse with a wavelength of 532 nm and a complete power of 1 mJ. A detector recorded the quantity of transmitted mild at varied distances, displaying that the sunshine delivered was round 20% better with the waveguide than with out (Fig. 2). The researchers additionally confirmed that the air waveguide had a protracted lifetime of tens of milliseconds.

This waveguiding scheme, nonetheless, has a couple of drawbacks, together with a comparatively excessive propagation loss, a poor mode profile of the guided beam, and a excessive power consumption within the formation of the air waveguide. To enhance the scheme’s efficiency, researchers might want to develop extra subtle light-sculpture methods. If the preliminary donut beam could be made extra uniform, then the a number of filaments ought to type in a extra deterministic means, and this could result in air waveguides which might be extra steady and reproducible.

In the long run, the authors envisage air waveguides that may ship high-power mild over distances of a kilometer or extra. They estimate that reaching kilometer-scale transmission would require a high-energy (as much as 2 J) LG01 pulse supporting ring protection of ~40–80 filaments. Such air waveguiding opens the door to many sensible purposes requiring environment friendly laser-energy supply to distant places within the ambiance. One instance is detecting fuel pollution by thrilling them with UV mild carried by means of the ambiance by an air waveguide [4, 5]. The emitted mild from the excited pollution may then be analyzed spectroscopically. An analogous scheme may remotely detect radioactive supplies [6]. Another attainable utility—which was just lately demonstrated—is lightning safety by means of the technology of a plasma channel that might information lightning to the bottom.

## References

1. A. Goffin et al., “Optical guiding in 50-meter-scale air waveguides,” Phys. Rev. X 13, 011006 (2023).
2. N. Jhajj et al., “Demonstration of long-lived high-power optical waveguides in air,” Phys. Rev. X 4, 011027 (2014).
3. A. Couairon and A. Mysyrowicz, “Femtosecond filamentation in transparent media,” Phys. Rep. 441, 47 (2007).
4. J. Kasparian et al., “White-light filaments for atmospheric analysis,” Science 301, 61 (2003).
5. J. Yao et al., “High-brightness switchable multiwavelength remote laser in air,” Phys. Rev. A 84, 051802 (2011).
6. R. M. Schwartz et al., “Remote detection of radioactive material using mid-IR laser–driven electron avalanche,” Sci. Adv. 5, aav6804 (2019).

Ya Cheng acquired his BS diploma from Fudan University in 1993 and PhD diploma from the Shanghai Institute of Optics and Fine Mechanics in 1998. He is at the moment director of the State Key Laboratory of Precision Spectroscopy on the East China Normal University. His analysis focuses on ultrafast nonlinear optics and photonic integration based mostly on skinny movie lithium niobate. He is an Optica Fellow and Fellow of the Institute of Physics, UK.

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