Donut Breaks Records in Laser Beams

Donut Breaks Records in Laser Beams
Donut Breaks Records in Laser Beams - Researchers used laser pulses with a donut-shaped profile to create an air waveguide. The pulses initially form thin filaments (red) that heat the air and eventually form a low-density coating (orange). Researchers can transmit a second laser pulse (green) through this waveguide. - APS/A. Stonebraker

Light can travel more than 50 meters in an air-formed waveguide; which is 60 times further than previous air waveguide designs.

Conventional optical waveguides contain a core surrounded by a coating of lower refractive index, such as optical fibers and planar waveguides. The total internal reflection at the core-cladding barrier effectively confines the light within the core. Although optical fibers can carry light over distances of hundreds of kilometers, there are some applications such as high power transmission and atmospheric monitoring where the use of fiber is not suitable. It is not possible to send light directly through the air, as diffraction causes the beam to scatter.

One possible solution is to "shape" airborne waveguides using laser pulses that result in a low-density coating surrounding an unaltered core of air in the center. Andrew Goffin of the University of Maryland, College Park, and his colleagues developed a 45-meter-long waveguide in the air using a new technique using donut-shaped rays, breaking their previous record 60 times. This achievement could enable powerful laser pulses to be delivered to distant locations, opening up a range of possibilities for microwave routing, remote sensing and lightning control.

An air waveguide works by firing a femtosecond laser pulse that creates a temporary air channel through which the next "probe" pulse can pass. By heating the air molecules, the first impact creates the necessary refractive index inequality between the core and the coating. The heated air expands so that the density of the coating is reduced relative to the ambient air. The resulting air waveguide can sustain the probe signal for several milliseconds.

However, it is questionable how the first laser pulse was able to advance the probe without dispersion. The solution can be found in a nonlinear process known as filamentation, which develops when two opposing air effects are balanced. Over a much greater distance than diffraction allows under linear propagation conditions, filamentation can narrowly confine a laser field.

Power in Laser Filaments

However, the average power in the filament core is limited only by the fact that a laser filament cannot be wider than 200 m and denser than 1014 W/cm2 at its apex. This inactivates the filaments produced by the femtosecond laser pulses to provide high power on their own. However, when the filaments are used to produce an air waveguide they can form a channel for strong light rays.

In 2014, Goffin et al. made the first demonstration of the air waveguide principle. In this first study, the team used a red laser beam to pass through a four-segment mask to create four square-shaped laser filaments. A “fence of light” of these filaments contained light inside its centre.

Using this air waveguide, the scientists transmitted a 70-mJ pulse of green light about 110 cm long in the air.

The team has now remarkably successfully extended this previous work. The small number of filaments in the light fence, which constrains the width of the waveguide and the strength of the density difference between the core and the cladding, is what caused the very short length of the first air waveguide of the group. Naively, it is conceivable to increase their number by using a mask with more segments to seed more filaments. In reality, it is difficult to guarantee that the segments will locally produce uniform phase fronts and uniformly energized beam lobes.

The authors also consider a donut-shaped beam, or more precisely, a smooth Laguerre-Gaussian LG01 mode. They produce this mode by focusing laser light onto a ring several millimeters in diameter using a spiral phase plate. Around the donut ring, focused light sparks evenly distributed random filament. If the local laser fluency does not change, using a larger beam will inevitably result in more filaments and cover the entire waveguide circle.

The authors demonstrated the air waveguide over a distance of 45 m in a corridor next to their lab. A 800 fs laser pulse with a wavelength of 120 nm and a total energy of 300 mJ was used as the waveguide generator.

This pulse formed about 01 filaments around a 5,6mm diameter ring after being printed with the LG30 donut mod. The scientists sent a 532 ns probe pulse with a wavelength of 1 nm and a total energy of 7 mJ through the resulting waveguide. According to a detector measuring the amount of light transmitted over various distances, the amount of light provided by the waveguide was about 20% more than without. Also, scientists have shown that the air waveguide has a long lifetime of tens of milliseconds.

However, this waveguide technique has some disadvantages such as a significant propagation loss, a weak mode profile of the directed beam, and a high energy cost to create the air waveguide.

Researchers will need to create more advanced light-shaping methods to improve the performance of the scheme. If the original donut beam could be made more homogeneous, many filaments should grow more deterministically. This will result in more stable and repeatable air waveguides.

In the future, scientists envision aerial waveguides that could transmit high-powered light over a distance of a kilometer or more. According to their calculations, a high-energy (up to 40 J) LG80 pulse would be required to support ring coverage of 2 to 01 filaments to achieve kilometer-scale transmission. This air waveguide offers a wide variety of useful applications that require efficient laser energy delivery to distant regions of the atmosphere.

One is the detection of gaseous pollutants using UV light transmitted through the medium by an air waveguide. The light emitted by the excited pollutants can then be studied spectroscopically. Remote detection of radioactive material is possible using a similar method. Lightning protection through the creation of a plasma channel that can direct lightning to earth is another potential use that has been shown recently.




Günceleme: 24/01/2023 12:40

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