Stars twinkle at night. Street surfaces shimmer on sunny afternoons. Rainbows glisten after rain showers.
These optical phenomena are all forms of refraction, a law of physics that helps eyesight, delights children and inspires poetry. But refraction also poses challenges for researchers trying to extend the range and accuracy of lasers, something that would help with military applications.
Texas A&M University engineers are developing a unique solution, one drawn from a method astronomers use to better see the stars.
The term “laser” is short for “light amplification by stimulated emission of radiation.” A laser beam is an extremely directional beam of energy. Lasers are used everywhere from surgical wards to checkout lines.
The military deploys lasers to identify targets and is developing them to defend against incoming missiles, artillery shells and nautical threats. But a laser directed at anything far away is problematic. One of the main reasons gets back to that basic law of physics.
Refraction is the bending of light as it passes through substances of different density. Water has greater density than air, which helps explain why sunlight splits into a rainbow of colors after a shower. Hot air is less dense than cool air, which is why a hot road surface shimmers and mirages occur.
Lasers in the atmosphere face refraction from everyday variations — clouds, rain, heating and cooling. On a battlefield, refraction gets worse — thermal plumes from fires, flying debris, swirling winds, etc.
The Army will be able to put more laser heat on a target … even very far away.”
– Dr. Chris Limbach, Assistant Professor
Texas A&M is working on a system to counteract laser refraction by using something called “adaptive optics.” Astronomers developed adaptive optics to clear the clutter out of telescopic images. Sophisticated, changeable mirrors correct for distortions in the atmosphere, sharpening the images until observers can see fine details of faint objects.
Adaptive optics requires a reference point, which can be a nearby star. The reference is used to measure the blurring caused by atmospheric conditions so that mirrors can make corrections. Because stars are not always nearby, astronomers bounce lasers off of sodium atoms high in the atmosphere. Almost the entire sky can now be observed through earth-based telescopes, thanks to these “laser guide stars.”
Texas A&M researchers are trying to create a laser guide star system for long-distance lasers. Dr. Chris Limbach, an assistant professor in the Department of Aerospace Engineering, explained how it would work.
An ultraviolet laser sends out light with pulses shorter than nearly anyone could imagine: a few femtoseconds. A femtosecond is one quadrillionth of a second, or one millionth of one billionth of a second. When focused out in the air, this incredibly fast pulse splits oxygen molecules into free atoms. A second fast pulse a few nanoseconds later excites the atoms and causes an infrared laser beam to come back to the point of origin. Similar “backward lasing” has been achieved in air from nitrogen.
Limbach and other researchers will analyze the returning beam to see if it can be used to correct for refraction as lasers race through the sky toward targets farther away. Otherwise, laser beams diffuse, losing power and precision.
“With adaptive optics integrated into combat systems, the Army will be able to put more laser heat on a target … even from very far away. That’s much more effective,” Limbach said.
