Air-based 'waveguides' could help with remote sensing as well as laser beam weapons:
Tubular laser beams can create what amount to fiber-optic cables made of thin air, researchers report in a study to appear in Physical Review X.
Laser-heated air can efficiently transmit light signals without the need to lay fiber-optic cables. These air-based "waveguides" can also provide pathways for ultrahigh-energy lasers that don't propagate well through air on their own, building on earlier efforts that have been touted as a route to laser beam weapons (SN: 3/5/14).
The latest experiments, says physicist Howard Milchberg of the University of Maryland in College Park, beat out his team's prior waveguide attempts by more than 60 times, extending the length of a waveguide to nearly 50 meters.
The researchers ran a laser in "doughnut mode," where the beam has a hollowed-out core along its length, resembling a stack of luminous doughnuts. The beam creates a hot, tubular air layer wrapped around cooler, denser air. This creates something akin to a fiber-optic cable made of air: Instead of a transparent core covered in plastic cladding that guides light in conventional fiber optics, the dense air acts as the core, while the hot air around it serves as cladding.
Stepping up the waveguide length to kilometers, Milchberg says, is primarily a matter of boosting the doughnut-mode laser power. "The only issue is we don't have a kilometer" to safely fire a laser that far, he says. It may take a deal with one of the U.S. national laboratories to find space to run a longer version of the experiment.
Despite the potential military applications, Milchberg says he and his team are focused more on scientific uses, such as extending the range of systems that reveal the chemical makeup of materials by firing lasers at them and measuring the light they give off. Air-based waveguides could send emitted light back to remotely located systems, instead of needing to get up close to analyze a sample.
Journal Reference:
Physical Review X - Accepted Paper: Optical guiding in 50-meter-scale air waveguides, (DOI: https://journals.aps.org/prx/accepted/8707dK4dIb91a60bb6df4e56bdc44a53b2267be80)
(Score: 2) by Beryllium Sphere (r) on Wednesday January 11, @12:37AM (1 child)
I take it there is some reason folded optics with mirrors won't work for this? Is the equipment too heavy to load into an SUV and take to a desert someplace? It would have to be one with no visitors given the safety protocols for high output lasers.
(Score: 0) by Anonymous Coward on Wednesday January 11, @02:39AM
They need to go to a large outside range because this is probably what happened the last time they tried to turn up the laser power that much [youtu.be]. :)
Besides the setup and alignment hassles if they used a folded path with a lot of mirrors, you typically try to use as few mirrors in a high power laser system as possible because you can get very large power densities. Each mirror surface you lose a bit in power and beam quality because of scatter off of the roughness of the mirror surface, absorption in the mirror and the little bit you lose because your reflectances aren't 100%. You also get polarization changes off of each mirror reflection. Then when your laser beam exits the laser in its high power density form, you don't want to hit a mirror with it because that might be too much power in such a small spot that even with very high reflecting mirror surfaces, the amount that doesn't reflect can heat up and damage your mirror. For really high power beams, they pass them around inside a laser with as large of a beam diameter as they can to keep the power density low, then tighten up the beam in the last part of the optics.
The answer to your other question is most likely yes, the setup is too large to put in a vehicle. Their optics are set up on optical tables. Perhaps they can squeeze a lot of it down to a single table, but that's the kind of thing you'd probably need to set up in the back of a trailer or on a flatbed truck.
By the way, here's their paper on arXiv [arxiv.org].
(Score: 2, Funny) by Anonymous Coward on Wednesday January 11, @12:39AM (1 child)
How do we fix this to the sharks?
(Score: 2) by EvilSS on Wednesday January 11, @08:50AM
(Score: 4, Insightful) by hendrikboom on Wednesday January 11, @03:20AM (2 children)
WOuldn't convection destabilize the air constituting the waveguide?
(Score: 4, Funny) by krishnoid on Wednesday January 11, @04:38AM
Naah, the laser excitation will produce coherence in the air's thermal distribution as well. Sure, that sounds reasonable :-)
(Score: 4, Informative) by Anonymous Coward on Wednesday January 11, @04:46AM
According to their paper, the waveguide is stable on the order of a millisecond.