Extended Wavelength InGaAs
Standard InGaAs has a long wavelength cutoff of 1.68 μm. Many applications require the detection of light with longer wavelengths. An important example is the ability to measure moisture content in agricultural products by measuring water absorption at 1.9 μm. Another example is LIDAR (light detection and ranging) used in airplanes to detect clear air turbulence. LIDAR systems often use lasers that emit light with a wavelength of 2.05 μm. InxGa1-xAs, with a longer cutoff, is called “extended wavelength InGaAs.”
How is this created? Simply adding a little more InAs to the mix increases the lattice constant of the thin film, which causes a mismatch with the substrate, reducing the quality of the thin film. Figure 2 shows the quantum efficiency of standard InGaAs in blue, together with the quantum efficiencies of two extended wavelength alloys X=0.74 (green) and X=0.82 (red). The spectral response of silicon also is shown.
InGaAs in IR Imaging
InGaAs allows imaging in the SWIR, without requiring a cooling system to improve the signal-to-noise ratio. This allows for smaller, less expensive cameras that operate at lower power than most other IR imagers. Because the InGaAs cameras have non-uniformity corrections (NUCs) that last for the lifetime of the cameras, they are simple to operate and do not need new NUCs in the field.
The cameras and imagers are radiometric, so laser beam profiling and other quantitative measurements can be readily conducted with this imaging technology. InGaAs is a highly manufacturable material that allows high-volume production and low-cost cameras.
When imaging objects above 120 °C, the solid-state InGaAs shortwave infrared imaging cameras offer a distinct advantage over other IR detectors containing mid- and long-wavelength materials such as mercury cadmium telluride (HgCdTe) and indium antimonide (InSb). This thermal imaging benefit is put to use in hot processes like glassmaking and molten metal production. The InGaAs cameras can be used to image through plain glass windows in a variety of manufacturing industries. The imagers use standard glass optics or lenses, avoiding the challenge of other infrared cameras that require special optics or optical assemblies made of expensive materials such as germanium, sapphire, or silicon. InGaAs imagers do not require cooling, making them less expensive to run than thermal imagers (InSb, HgCdTe, or QWIPs) that require either liquid nitrogen or multi-stage thermoelectric coolers. InGaAs imagers also can image at high frame rates, which is a remarkable advantage over the uncooled bolometer technology.
Applications for InGaAs Imagers
There are many applications for InGaAs imagers, from military to commercial. In the military, the imagers are being used for imaging at night. They are capable of imaging light that is seen by current night vision technology, as well as light that cannot be detected by the current tube technology (see Figure 3). This permits the imagers to see “invisible” lasers that are currently being used on the battlefield. This digital imaging technology allows users to fuse multiple wavelength bands of information to help in the detection and identification of objects in the field.
In the commercial sector, InGaAs detector arrays are used in diverse applications, from imaging hot processes such as glassmaking, to monitoring the light on fiber-optic cable for telecommunication applications, to monitoring the quality of metals during molten metal processing. In industrial processes that use machine vision, InGaAs imagers provide the ability to image phenomenology that cannot be detected easily or at all in the visible wavelength bands.
In the pharmaceutical industry, InGaAs imagers are now being used to detect liquid fill-levels in opaque bottles, instead of using traditional weighing methods. The food industry is using InGaAs imagers as an affordable alternative to silicon-based detectors to reveal the level of bruising in fruits and vegetables, as shown in Figure 4.
SWIR imaging provides a unique solution for non-destructive identification of materials, their composition, coatings, and other characteristics without the need for expensive and exotic optical materials.
This article was written by Dr. Marshall J. Cohen, President and CEO of Sensors Unlimited, Inc., Princeton, NJ.