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  • Holograms lighten the payload of unmanned aerial vehicles

Photonics Spectra
Jul 2010
Lynn Savage, lynn.savage@photonics.com

ARLINGTON, Va. – US Air Force scientists have devised a way to make un-manned aerial vehicles (UAVs) better eyes in the sky, besides making them lighter and more efficient.

Also known as drones or pilotless aircraft, UAVs increasingly are being used for reconnaissance missions by both military and commercial operators. The aircraft can acquire intelligence on enemy combatants, locate terrorism suspects, measure instances of extreme weather, and inspect remote oil, gas and power lines, among many other uses.

In remote sensing and imaging operations, however, UAVs must peer through varying atmospheric conditions, most of which degrade the results. Adaptive optics systems, which are known chiefly for improving celestial images acquired by ground-based telescopes that also are subject to atmospheric distortions, would help, except that they are too bulky and heavy for practical use aboard UAVs.

Adaptive optics systems work by discerning errors in the wavefronts of light that reflects off imaging targets. Aside from the optics, which include a deformable mirror that uses tiny actuators to correct wavefront errors before the light is passed on to a CCD, these systems require a high-power computer to operate at high speeds. A significant portion of the weight and volume of an adaptive optics system belongs to the computer.

Dr. Geoff Andersen of the US Air Force’s Laser and Optics Research Center in Colorado Springs, Colo., and his colleague, Dr. Kent Miller of the Air Force Office of Scientific Research in Arlington, tackled the problem by stripping out the computer. To perform the intensive calculations required, they replaced the computer with a holographic wavefront sensor technology upon which is recorded the individual responses of each actuator on the deformable mirror.


Using a holographic adaptive optics system permits computer-free operation of unmanned aerial vehicles for surveillance missions. Courtesy of the US Air Force Office of Scientific Research. 


When the hologram containing the actuator response functions is used during imaging, wavefront errors are measured as intensity outputs from an array of fast photodetectors, then sent directly to the mirror’s controller. This makes the system an all-optical closed loop that requires no external electronic computation.

“Adding the holographic wavefront sensor technology improves speed and functionality while also decreasing mass, volume and complexity,” Andersen said. “By removing the computer, we can make the entire system smaller and lightweight and over 100 times faster.”

The system, dubbed HALOS (Holographic Adaptive Laser Optics System), has been tested as a proof of concept. According to Andersen, he and Miller are now in the process of testing full autonomous operation of the technology. They will also be trying it with more sensitive detectors.

“The idea of incorporating adaptive optics into UAV surveillance systems is now a real possibility,” he said. “This is not possible with current technologies.”


GLOSSARY
actuator
Mechanical device intended for the translation (rotational and linear) using high precision control from electronically operated circuits. See linear actuator; rotary actuator.
adaptive optics
Optical components or assemblies whose performance is monitored and controlled so as to compensate for aberrations, static or dynamic perturbations such as thermal, mechanical and acoustical disturbances, or to adapt to changing conditions, needs or missions. The most familiar example is the "rubber mirror,'' whose surface shape, and thus reflective qualities, can be controlled by electromechanical means. See also active optics; phase conjugation.
light
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
mirror
A smooth, highly polished surface, for reflecting light, that may be plane or curved if wanting to focus and or magnify the image formed by the mirror. The actual reflecting surface is usually a thin coating of silver or aluminum on glass.
photodetector
A device used to sense incident radiation.
wavefront
In considering a field of electromagnetic energy emanating from a source, the wavefront is a surface connecting all field points that are equidistant from the source.
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