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Historical Overview
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. 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 [1]. Since nearly the invention of heat-seeking missile technology, engineers have been simultaneously developing countermeasures to trick the sensor into missing its target. At first, these countermeasures were relatively crude in design, consisting of flares that the pilot could deploy when under attack. 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 [2]. These early devices were essentially larger versions of the Globar used in modern infrared (IR) spectrometers. 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. 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 [2].
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. 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. In 1997, D. B. Meeker, working for the United States Navy, patented one of the earliest known aerial laser IR countermeasures, designed for use onboard an aircraft [3].
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. 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. This article will go on to explain why the mid-IR spectral region is so vital for heat-seeking missiles, and it will further elaborate on the importance of QCLs with examples of modern commercially available devices.
Quantum Cascade Lasers
While, technically, QCLs are a subset of semiconductor lasers, in practice, they are fundamentally different enough to warrant their unique classification. Unlike classical semiconductor lasers which rely on single electron-hole recombination to generate photons, QCLs utilize a large number of thin active sections, which, when combined, create a series of stepwise electric potentials across the device, with a sloping potential gradient, which in turn results in the electron having to cascade down through the potential wells releasing a photon at each step. Figure 1 below illustrates the differences between the band structure of traditional laser diodes and QCLs.
As a result, the wavelength is not limited by the intrinsic energy gap of the material and can produce wavelengths in the mid-IR range. Furthermore, the cascade effect creates an enhancement factor that allows for QCLs to provide several orders of magnitude more power at a given wavelength than traditional semiconductor lasers. For a more detailed review of QCL technology, the reader is encouraged to read “An Overview on Quantum Cascade Lasers: Origins and Development,” by Raúl Pecharromán-Gallego [4].
Detection Wavelength
The basic functionality of a heat-seeking missile is quite similar to that of a night vision imaging system; in that it uses an IR imaging device, such as an InSb focal plane array, to lock on to the aircraft by tracking its blackbody emissions. The main difference between these two technologies is the wavelength range over which they operate; most of the objects on earth have a temperature ranging from 275 K to 300 K, which correlates to a peak black body emission wavelength of roughly 10 microns. By contrast, a typical turbojet engine has an exhaust gas temperature ranging from 1200 K to 1900 K [1], corresponding to a peak black body emission wavelength ranging from 2.4 microns to 1.5 microns. Unfortunately, in this spectra region, there is too great of overlap with the sun’s emission spectrum to get a clear contrast between the engine’s exhaust and the solar radiation.
Furthermore, as shown in figure 2 above, there is only a discreet number of spectral bands in the mid-IR region where the atmosphere is not highly absorptive. As a result of these considerations, most heat-seeking missiles rely on detectors operating in the 4-micron to 5-micron range, which is long enough to avoid the solar radiation and simultaneously short enough to avoid the thermal emissions from the earth. Since the missile’s guidance system is designed to track thermal radiation in this region accurately, D. B. Meeker’s 1997 patent application specifically called out a dual laser approach utilizing 4-micron and 5-micron laser wavelengths [3]. With this understanding, it should now be self-evident why QCL technology is so essential for this application.
State of the Art High Power QCLs
When choosing the right QCL for IR countermeasures, there are a few major points to consider. Most commercially available QCLs are designed for spectroscopic sensing applications, which require single longitudinal mode (SLM) operation. While these lasers have excellent spectral characteristics, their output powers are often limited to a few milliwatts. Therefore, these lasers are far from ideal for tricking heatseeking missiles, where the laser needs to be powerful enough to overcome the thermal emissions from the jet engine. This brings us to our next consideration, Quasi Continuous-Wave (QCW) operation of QCL’s. The QCW output of theses lasers generate less heat within the system and allows for a much better wall plug efficiency than that of continuous wave (CW) systems. This is because in QCW operation, the laser is only turned on for some percentage of the time, depending on the percentage of duty cycle set, greatly reducing overall power consumption. Additionally, QCW repetition rates are very high (in the hundreds of kHz range), which cannot be detected by the missile’s sensors. The final major consideration to mention is beam quality. Lasers for countermeasure applications need to be single spatial mode to allow for optimal collimation and beam steering.
One source for QCLs which meet these demands for power, beneficial QCW characteristics, and beam quality is the PowerMir series, designed and manufactured by mirSense in Orsay, France. They offer highpowered lasers at 4.0-micron, 4.6-micron, 4.8-micron, and 9-micron, each of which corresponds to atmospheric transmission windows, as shown in figure 2. Based on a rugged monolithic Fabry-Perot laser design, these lasers operate in the quasi-CW regime, with typical pulse widths of a few tens of nanoseconds and pulse repletion rates in the MHz range. The 4.0-micron, 4.6-micron, and 4.8-micron options are all ideal for IR countermeasures not just because of their wavelengths, but also because they are all capable of producing over 1 watt of average power (>1.5 W at 4.8-microns) with a TEM00 output beam with an M2 < 1.5.
On top of the superb optical characteristics of these lasers, the PowerMir series of QCLs from mirSense comes in a compact 9pin HHL package, shown in Figure 3 above, which includes a built-in thermoelectric cooler (TEC). The entire laser package is only 1.75 x 1.25 x 0.75 inches, weighing only 70 grams, making it ideal for integration into aerial combat vehicles where size and weight are of the utmost importance. In addition to the lasers, mirSense also offers a 24 VDC OEM laser driver for ease of integration. These drivers provide TEC control, frequency modulation, and external TTL triggering, in addition to basic ON/OFF functionality. Designed to be extremely lightweight and compact, they measure only 1.75 x 4.33 x 1.00 inches and weigh just 120 grams. The form factor, high average power, and wall-plug efficiency of these QCLs all combine to make the PowerMir series ideally suited for countermeasure and other defense
applications.
About RPMC Lasers
RPMC Lasers Inc (Founded in 1996) is the leading laser distributor in North America. We are an OEM supplier working with the technology leading laser manufacturers from the US and Europe. RPMC supports the Industrial, Medical, Military, and Scientific markets. RPMC offers diode lasers, laser modules, solid-state lasers and amplifiers, ultra-short pulse lasers, microchip lasers, and fiber lasers and amplifiers. In addition, we offer a wide range of custom solid-state lasers and laser diode subsystems.
References
[1] C. Kopp, “Heat-Seeking Missile Guidance,” Australian Aviation, (1982, 2005).[2] R. McDaniel, “Coating Technology Enables Effective Missile Countermeasures,” Aerospace & Defense
Technology, June 2019.
[3] D. B. Meeker, “IR Projector Countermeasure System,” United States Patent Number 5,703.314, December 30, 1997.
[4] R. Pecharromán-Gallego, “An Overview on Quantum Cascade Lasers: Origins and Development,” Quantum Cascade Lasers, April 2017.