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ELLIOTT RITTENBERG, LEN KAMLET AND ARNOLD ADAMS, IRCAMERAS LLC

Indium gallium arsenide (InGaAs) sensors are the most common choice for imaging in the SWIR from 900 to 1700 nm due to their high quantum efficiency, ability to operate at or near room temperature, and relatively low production and packaging costs. “Visible InGaAs” sensors, with a response from 400 to 1700 nm, have also proliferated in recent years. InGaAs sensors, however, have been limited to an upper wavelength cutoff at 1700 nm due to difficulties in achieving uniform pixel-to-pixel response in two-dimensional s at longer wavelengths above 1700 nm.

Unfortunately, this leaves these sensors blind in the wavelength region from 1700 to 2500 nm, which contains significant information for both military and civilian applications. Additionally, InGaAs sensors are often subject to image lag or retention when used in very low-light-level applications requiring high-gain states in the sensor.

With advancements in the production of InSb focal planes, infrared cameras today can be equipped with FPAs providing a wide broadband spectral response unparalleled by other production sensors. Advances in the passivation of InSb FPAs eliminate surface states that retain charge when the detector is exposed to higher energy photons, thus removing the possibility of after-images and image lag, which has been an issue for both InSb and InGaAs sensors.

Typical spectral response of InSb FPA with antireflective coating for <1 to >5 μm. Courtesy of IRCameras.  http://www.photonics.com/images/Web/Articles/2016/3/7/Detectors_Spectral.jpg

Typically, the quantum efficiency of InSb focal planes prior to the application of an antireflection (AR) coating is about 60% from UV wavelengths to approximately 5000 nm. Improved AR coating processes increase the quantum efficiency to greater than 90% over the wavelength range from 1 to >5 µm. More specialized coatings are available that extend the lower wavelength cut to 400 nm and below, resulting in a single sensor that can be utilized for a broad range of imaging applications.

Finally, the use of digital readout integrated circuits for InSb FPAs results in performance improvements over more traditional analog-based FPAs. Digital InSb FPAs require lower power, eliminate pixel crosstalk, are resistant to blooming and simplify downstream electronics by digitizing data within the FPA instead of requiring an analog-to-digital converter external to the FPA.

InSb sensors with the capabilities

described above are available today in formats ranging from quarter video graphics array (VGA) (320 × 256) to high definition (1280 × 1024), with even higher-resolution (2520 × 2048 and beyond) FPAs just over the horizon.

Cooling InSb sensors

InSb sensors require cryogenic cooling to become photoconductive. Typically, InSb sensors are cooled to about 77 K, though for some applications, including imagers intended for SWIR applications, further cooling of the sensor to less than 70 K is desirable to reduce noise due to dark current.

To cool InSb sensors to the cryogenic temperatures necessary, the focal planes can be integrated into either liquid nitrogen (LN₂)-cooled Dewar assemblies, or closed-cycle integrated Dewar cooler assemblies (IDCAs). There are, of course, advantages and drawbacks to each approach.

Cut-away view of LN₂-cooled Dewar configured with filter wheel. Courtesy of IRCameras.   http://www.photonics.com/images/Web/Articles/2016/3/7/Detectors_Dewar.jpg

InSb cameras based on a LN₂-cooled Dewar assembly offer the ultimate flexibility for users who may wish to modify the system as application requirements change, or to demonstrate a proof-of-concept before acquiring a closed-cycle cooled system that cannot easily be reconfigured. With an LN₂-cooled system, it is possible for the user to easily change cold filters, apertures (ƒ-number) and even the optical back working distance to accommodate a variety of optics, or to define and test the interface to spectrometers and other instruments into which a camera may be integrated. It is also easy to replace FPAs in an LN₂-cooled camera to allow upgrades to different format sensors or even different sensor materials.

LN₂-cooled systems are also vibration-free, which can be an important consideration when used as an OEM component in a highly sensitive metrology product such as an interferometer. LN₂-cooled cameras may also be equipped with motorized multi-position cold filter wheels, allowing the use of spectral filters cooled to cryogenic temperatures to limit their thermal emission and prevent the filter’s emissions from affecting measurements.

Closed-cycle Stirling coolers provide a method for cooling InSb and other cryogenic cameras designed for continuous operation. These devices remove heat from the detector through the expansion and contraction of helium gas, and are intended for environments where the use of LN₂ is not feasible, or for applications where the sensor temperature must be driven below 63 K. Until recently, most cameras so configured incorporated a rotary Stirling cooler, for which manufacturers would historically claim a limited mean time to failure (MTTF) on the order of 8,000 hours. Actual experience, however, has demonstrated this figure to be optimistic. Stirling cooler lifetime is impacted by operating conditions — continuous use in high ambient temperatures and repeated power cycling of the cooler tend to negatively impact MTTF.

An important consideration for IR cameras based on closed-cycle Stirling coolers, whether rotary or linear, is that once the system is built, it is difficult to change its configuration. The detector, cold shield, cold finger and cold filter are housed in a metal Dewar that is welded closed, and must be cut open in order to make changes or effect repairs. Great care must be taken when cutting open a closed-cycle Dewar to ensure that damage is not inflicted on any of the internal components. Also, with the exception of highly specialized custom built systems, a typical closed-cycle IDCA does not support the use of a cold filter wheel assembly due to the increased cooling capacity required by the cold wheel and increased Dewar volume. This limits the spectral response of the system to that defined by the combination of the sensor and cold filter installed in the Dewar. The result is a purpose-built, continuously operable product that can be used either as a stand-alone product, or as an OEM component for integration into an end-user system.

Meet the authors

Elliott Rittenberg serves as vice president of sales and marketing at IRCameras LLC in Santa Barbara, Calif.; email: elliott@ircameras.com. Len Kamlet, Ph.D., is product development manager at IRCameras, and has been working exclusively with infrared technology for 14 years; email: len.kamlet@ircameras.com. Arnold Adams, Ph.D., is chief technical officer at IRCameras; email: arn.adams@ircameras.com.