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# Air Waveguide from “Donut” Laser Beams

&bullet; Physics 16, 11

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

Typical optical waveguides resembling optical fibers and planar waveguides include a core surrounded by a cladding with a decrease index of refraction. Gentle is effectively confined within the core by complete inside reflection on the core-cladding boundary. Optical fibers can transport mild over 100s of kilometers, however there are purposes—resembling high-power transmission and atmospheric monitoring—the place using fibers turns into impractical. Sending mild straight by way of air isn’t an possibility, 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. Utilizing a brand new technique with donut-shaped beams, Andrew Goffin from the College of Maryland, School Park, and colleagues have created a 45-m-long waveguide in air [1], reaching 60 instances farther than the file they beforehand established for an air waveguide. The achievement could allow supply of high-power laser pulses to distant targets, opening up a variety of purposes resembling distant sensing, lightning management (see Analysis Information: A Laser-Primarily 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 way of which a subsequent “probe” pulse can move [2]. The primary pulse generates the required refractive index distinction between the core and cladding by heating the air constituents (resembling N2, O2, and noble gases). The heated air expands in such a manner that the density of the cladding turns into decrease than that of the encompassing air. The ensuing air waveguide can final a number of milliseconds—sufficient time to ship the probe sign by way of it.

However 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 stability 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 preserve a laser subject narrowly confined over a distance a lot better than that allowed by diffraction below linear propagation situations. Nonetheless, 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 common energy within the filament core. For that reason, the filaments generated by femtosecond laser pulses will not be an efficient means—by themselves—for delivering excessive energy. However 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 group handed a purple laser beam by way of a four-segment masks to create 4 laser filaments in a sq. sample. These filaments shaped a “mild 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 from Sizzling Air).

The group has now achieved a powerful extension of this earlier work. The comparatively brief size of the group’s first air waveguide was as a result of restricted variety of filaments within the mild fence, which restricted the width of the waveguide and the energy of the density distinction between core and cladding. To extend the variety of filaments, one might naively think about utilizing a masks with extra segments to seed a bigger variety of filaments. Nonetheless, in apply, it’s tough to make sure that the segments produce beam lobes with equal vitality and domestically easy part fronts. As a substitute, 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. Utilizing a bigger beam mechanically 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 complete vitality 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. By the ensuing waveguide, the researchers transmitted a 7-ns probe pulse with a wavelength of 532 nm and a complete vitality of 1 mJ. A detector recorded the quantity of transmitted mild at numerous distances, exhibiting 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 an extended lifetime of tens of milliseconds.

This waveguiding scheme, nonetheless, has a number of drawbacks, together with a comparatively excessive propagation loss, a poor mode profile of the guided beam, and a excessive vitality 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 manner, and this could result in air waveguides which are extra steady and reproducible.

Sooner or later, the authors envisage air waveguides that may ship high-power mild over distances of a kilometer or extra. They estimate that attaining 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 environment. One instance is detecting fuel pollution by thrilling them with UV mild carried by way of the environment by an air waveguide [4, 5]. The emitted mild from the excited pollution might then be analyzed spectroscopically. An identical scheme might remotely detect radioactive supplies [6]. One other doable utility—which was just lately demonstrated—is lightning safety by way of the era of a plasma channel that would 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 clear media,” Phys. Rep. 441, 47 (2007).
4. J. Kasparian et al., “White-light filaments for atmospheric evaluation,” Science 301, 61 (2003).
5. J. Yao et al., “Excessive-brightness switchable multiwavelength distant laser in air,” Phys. Rev. A 84, 051802 (2011).
6. R. M. Schwartz et al., “Distant detection of radioactive materials utilizing mid-IR laser–pushed electron avalanche,” Sci. Adv. 5, aav6804 (2019).

## Concerning the Creator

Ya Cheng acquired his BS diploma from Fudan College in 1993 and PhD diploma from the Shanghai Institute of Optics and Superb Mechanics in 1998. He’s presently director of the State Key Laboratory of Precision Spectroscopy on the East China Regular College. His analysis focuses on ultrafast nonlinear optics and photonic integration primarily based on skinny movie lithium niobate. He’s an Optica Fellow and Fellow of the Institute of Physics, UK.

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