About the thickness of a hair and the length of a few millimeters, quantum cascade lasers long have been a laboratory curiosity that only scientists could understand and operate. But recent advances in power efficiency, design and high temperature functionality have pushed this class of semiconductor lasers closer to real-world utility. One day they may provide the military with directional infrared countermeasures, target illumination, chemical warfare sensing and search-and-rescue capabilities, technologists say.
Quantum cascade lasers, as the name infers, employ quantum mechanics principles to convert electrical power into optical energy. Though these devices are fabricated in the same way as their traditional diode counterparts, with thousands of thin layers of materials, quantum cascade lasers produce energy in a different way.
Semiconductor lasers typically require electrons and “holes” to generate light energy. Quantum cascade lasers need only the electrons, which gain atom-like properties when confined into very narrow spaces.
Atoms have specific energy levels to which they can be excited. When they relax, they “fall” to a lower level and emit a photon. Electrons in the laser act the same way. Lining up multiple energy levels in a descending “staircase” causes electrons to cascade continuously and produce a photon at every step.
The material composition in a traditional semiconductor laser determines the energy wavelength that is produced. For example, a carbon dioxide laser emits light at approximately 10.6 microns.
In quantum cascade lasers, the wavelength is dictated not by material but by the size of the energy level positions, or “belts.” Technologists can control the wavelength by increasing or decreasing the size of the belts.
“Now I don’t have to go to exotic materials,” says Kumar Patel, founder and president of Pranalytica Inc. based in Santa Monica, Calif.
Most semiconductor lasers must be cryogenically cooled in order to function. Even when cooled, they generate only milliwatts of power, which renders them inefficient for practical use. “That’s why this field remained fallow for many, many years,” says Patel.
The high-power quantum cascade lasers produced by Pranalytica can generate watts of power and run on continuous operation at room temperature. “They’re different from any other semiconductor laser that ever existed before,” Patel says. “Because I can tailor the energy level positions, I can make the system operate at room temperature.”
But quantum cascade lasers have had a problem: They typically don’t work very efficiently. “If you wanted one watt of power output, the laser would consume 50 watts,” explains Patel.
Under a Defense Advanced Research Projects Agency program, the company sought to produce 20 milliwatts of continuous wavelength and power with an initial goal of attaining 40 percent wall plug efficiency. The team achieved 3 watts of power output with 15 percent efficiency. It may not have hit the target, but Patel says the laser is sufficient for practical applications, such as protecting airplanes from shoulder-fired missiles.
Heat-seeking sensors on such missiles can lock onto the gasses discharged by airplane engines. If a laser could shoot the same wavelength at the sensor, it would disable the guidance system and send the missile veering off-course. Lasers that have attempted the feat previously could not produce the wavelength required to blind the sensor. The quantum cascade laser developed by Patel’s company can.
In a demonstration in February, the laser burned a small hole through a business card placed in close proximity. The same technology fired over longer distances will disable the seeker on shoulder-fired missiles, Patel says. Several defense contractors are testing the laser in their developing countermeasure systems, he adds.
The laser also has utility for infrared illumination. Current target illuminators have short ranges because a substantial amount of the light disperses as it travels through the air. Another problem is that the laser’s wavelengths can be seen by enemies using commercial viewers.
Those cameras cannot detect the quantum cascade laser emission, so the company has produced a battery-operated laser that can illuminate a target up to 2 miles away. Only NATO allies and U.S. forces have technologies that can see this kind of light, Patel points out.
The same laser technology also can be incorporated into a beacon to help the military locate downed pilots and troops. “Having a quantum cascade laser as a source increases the range so significantly that it becomes a viable device for search and rescue,” says Patel. “If you’re at 4.6 microns or longer, you can see [it] from space.
There are no natural sources of radiation at that wavelength.”