Now that it has been close to a quarter-century since the invention of the QCL, they are the source of choice for modern IR countermeasure systems. It is by no coincidence that this patent application coincides with the invention of the Quantum Cascade Laser (QCL) at Bell Labs in the mid-1990s, which is now capable of producing light anywhere between 4 microns and 12 microns. Meeker, working for the United States Navy, patented one of the earliest known aerial laser IR countermeasures, designed for use onboard an aircraft. As laser technology improved, it eventually became viable to replace the Globar with an IR laser source, reducing the weight and complexity of the system. Unlike flares, which can only be deployed once and required the pilot to deploy them once targeted actively, these so-called "hot sources" could remain active during the entire mission, significantly improving the effectiveness of the countermeasure. Coupled with a rotating, mechanical shutter that would modulate the thermal output, these sources make it impossible for the heat-seeking missile to lock on to the target. These hot sources consisted of a mass of silicon carbide, which, when current passes through, heats up to the point where it emits large quantities of IR radiation from roughly 4 microns to 15 microns in wavelength. These early devices were essentially larger versions of the Globar used in modern infrared (IR) spectrometers. As the technology evolved, flares were starting to be phased out, and "heat-seeking missile countermeasures based on hot sources" began to be phased in. At first, these countermeasures were relatively crude in design, consisting of flares that the pilot could deploy when under attack. Since nearly the invention of heat-seeking missile technology, engineers have been simultaneously developing countermeasures to trick the sensor into missing its target. The USAF first introduced heat-seeking technology in the 1950s, with the acquisition of the Falcon (AIM-4) and Sidewinder (AIM-9) missiles, and by the outbreak of the Vietnam War, heat-seeking missiles had become a staple of aerial combat. Heat-seeking missiles, also known as air-to-air guided missiles, have been a part of the United States Air Force's (USAF) arsenal for nearly 70 years.
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