By John McHale
WASHINGTON - Researchers at the Naval Research Laboratory (NRL) Optical Science division in Washington developed a new type of semiconductor laser for military laser applications that operates at room temperature and emits light in the "mid-wave infrared" spectral region at wavelengths between 3 and 5 microns.
The laser`s ability to operate at room temperature enables users to operate it with less power than other comparable lasers, which makes the new device a promising candidate to counter infrared guided missiles fired at combat aircraft.
"Over half of all U.S. aircraft losses in combat are caused by heat-seeking missiles," says Jerry Meyer at NRL`s Optical Science division. "The semiconductor lasers we have developed are of great importance to the military as a way to meet this threat."
Meyer says no other lightweight high-power light sources exist at these wavelengths that operate at room temperature.
While most lasers operate most efficiently in cold temperatures, Meyer says, having a laser that operates at room temperature saves power by eliminating the need to cool the device, he says.
The NRL research team used "wavefunction engineering" that has enabled them to design and model complex layered quantum-well structures, Meyer says. All of the structures are based on the antimonide family of III-V semiconductors from which experts from NRL and at industry laboratories grow quantum wells.
Potential military applications for this laser include chemical sensing for leak detection, monitoring atmospheric pollution, chemical weapons, or drugs. Other applications involve protecting military aircraft from heat-seeking missiles, laser surgery, laser radar, and infrared scene projection.
One example of this is the "W" laser - named W after the shape of the quantum well - in which two indium arsenide electron quantum wells and aluminum antimonide quantum barriers surround a gallium indium antimonide hole quantum well on both sides. This makes the most of the optical gain while suppressing the power losses inside the device, Meyer explains.
The deeper the well, the less chance that electrons will escape, he says. The more the electrons the more efficient the laser; it is similar to trying to pop all the kernels in a bag of popcorn, he says.
An NRL-developed optically pumped W laser was the first interband infrared laser to operate at room temperature. A W laser also holds the record for maximum continuous-wave operating temperature (-53 degrees Celsius) for all III-V mid-infrared lasers, NRL officials claim.
NRL researchers are also developing the interband cascade laser, which produces a cascade of photons as electrons emit an additional photon at each step. NRL team members report they have operated a 3.5-micron pulsed interband cascade laser with a W active region to temperatures as high as 13 degrees Celsius. This, they say, is nearly room temperature and is more than 60 degrees warmer than the best temperature for any early interband III-V laser emitting at such a long wavelength.
Room temperature operation will be important if mid-infrared lasers are to find extensive use in a broad range of environments.
A third structure recently designed and tested at NRL is the first II-V mid-infrared vertical-cavity surface emitting laser, known as VCSEL. This optically pumped laser with a W active region operated nearly to room temperature - 7 degrees Celsius - for pulsed operation.
In VCSELs, light emits from the top of the device rather than the side, and the active volume can be extremely small. For this reason the continuous-wave pumping threshold - power at which the laser could be turned on - for the NRL VCSEL at low temperatures was only 4 milliwatts, which is smaller than any earlier mid-infrared semiconductor laser, NRL researchers claim.
