Application Briefs

As warfare becomes more asymmetric, civilians and other non-combatants become a larger percentage of the casualties, along with unintended property damage. The military, of course, hopes to avoid these types of casualties and destruction. With advancing technologies that enable more precision from their weapons, they also need better pointing and targeting capabilities, while remaining covert. Improved targeting technologies that allow detection and identification at longer standoff distances from the designators also are needed. For instance, lasers are excellent at precision pointing, but it is important that others be able to covertly image the scene as well.

Figure 1. Quantum Efficiency chart illustrates the response curves for silicon, InGaAs, visible-InGaAs, and night vision tube detectors in the visible and shortwave infrared wavelengths. Key laser wavelengths also are noted.
To address these targeting challenges, the military has deployed lasers that allow them to not only designate the target where munitions should hit, but to use these same lasers to measure the distance to the target, illuminate the surrounding area, or point out to others something of interest. Visualizing where the lasers are pointing, tracking moving targets, and minimizing collateral damage require imaging systems that see the active lasers used in the field. Room-temperature indium gallium arsenide (InGaAs) cameras give users this capability in day or night conditions.

Figure 2. SWIR (InGaAs) imaging at night of 1.06-μm laser, 4 km away.
Most laser-guided munitions are directed by lasers with a wavelength of 1.06 μm. These lasers are very powerful and they can be used to point at objects many miles away. The distance is limited largely by how accurately the user can see what he is designating. This includes the laser spot, the target, and the objects around the target. Currently, most systems use an indium antimonide (InSb) detector array to image the spot. These InSb systems are thinned to allow response down to the 1.0 μm laser wavelength, which is far below the normal InSb peak sensitivity range (between 3 and 5 μm). That range is used for its main application as a mid-wave IR thermal detector.

InSb cameras allow the infrared laser to be seen and they provide situational awareness around the laser spot due to the thermal emissions of the scene. The downside of these systems is that the detector needs significant cooling (down to 77K) and their sensitivity to 1.06-μm lasers is poor, due to 70% and room-temperature operation. They enable imaging laser spots at a greater standoff distance with a much lighter system (Figure 2).

Lasers are not only used to guide munitions to their target, but also can provide the warfighter with information on the target and its surroundings. Laser range finders allow the user to determine the distance to the target. These lasers now use an approximate 1.5-μm wavelength. This wavelength is considered to be “eye-safe” because the energy does not focus on the eye’s retina, and the optical power needed to blind someone hit by the laser is very high. These lasers are invisible to night vision goggles (NVGs) as well as to the eye, thereby making them suitably covert. The advantage is that the target is unaware they are being marked by the laser; the downside is that the warfighter also has trouble knowing if he is correctly aimed at the target. Because InGaAs is also very sensitive to the eye-safe lasers, the SWIR imaging InGaAs cameras are being deployed so warfighters can verify that their targeting system is still boresighted correctly, even if the system has been banged up in the field.

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