Glass laser test board can withstand lasers designed to melt steel  (Source: GTRI)
System uses special glass that can withstand laser energy

Lasers have moved from fiction into reality, and the military is nearing completion of many weapon systems using directed energy beams. Some of these weapons will be in the field in the coming years to protect troops and civilians from things like mortars, rockets, and missiles.

One of the problems with testing lasers for military use has been finding a way to easily evaluate the efficiency of the laser. A group of researchers at the Georgia Tech Research Institute (GTRI) has developed a system to accurately measure the power of a laser. These sorts of tests are crucial before laser weapons can be used in the field.

The system designed by the researchers is able to measure the power and spatial energy of an energy distribution at the same time. The system directs the laser beam onto a special glass target board that is able to withstand the intensity of the laser and not affect the beam as it passes through.

GTRI senior researcher David Roberts said, "The high-energy laser beam delivers its energy to a small spot on the target -- only a couple inches in diameter -- but the intensity is strong enough to melt steel. Our goal was to develop a method for determining how many watts of energy were hitting that area and how the energy distribution changed over time so that the lasers can be optimized."

The special glass used in the testing system chosen by GTRI is made by a company called OptiGrate. The glass is handmade from a sodium-zinc-aluminum-silicate material that is doped with silver, cerium, and fluorine. The team of researchers changed the characteristics of the glass so that the target board the glass is part of could withstand laser damage and degradation. OptiGrate also had to produce a new mold to create the glass used by the team because the researchers needed glass 4-inches by 4-inches, which was four times larger than other pieces of the glass previously produced by OptiGrate.

Roberts said, "This glass is unique in that it is transparent, but also photosensitive like film so you can record holograms and other optical structures in the glass, then 'develop' them in a furnace."

The team secured the glass target board between a test target and the laser. The laser beam irradiance profile on the glass target is measured by a remote camera and the images captured can them be analyzed to provide a contour map of the laser beam that shows the lasers power density in watts per square inch at every location the beam hits the glass target.

Roberts said, "We can also simultaneously collect power measurements as a function of time with no extra equipment. Previously, measuring the total energy delivered by the laser required a ball calorimeter and temperature measurements had to be collected as the laser heated the interior of the ball. Now we can measure the total energy along with the total power and power density anywhere inside the beam more than one hundred times per second."

Prototype testing boards were delivered to the Air Force Laser Effects Test Facility at Kirtland Air Force base in May. At the test facility, the glass targets were used to test a 50-kilowat fiber laser and measured power density as high as 10,000 watts per square centimeter without any damage to the test device. 

Laser weapons are being tested for use by multiple branches of the military. Raytheon tested its laser weapon system in July and scored four kills with it. The Air Force also has a fleet of aircraft with lasers on board that are near combat ready.

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