Strained-layer superlattice thermal camera improves on QWIP technology

Improved quantum-well infrared photodetector (QWIP) imager is 10 times more sensitive over a broader spectral band.

Strained-layer superlattice thermal camera improves on QWIP technology
Strained-layer superlattice thermal camera improves on QWIP technology
The image on the right shows the improved resolution of a strained-layer superlattice (SLS) detector array as compared to that of a quantum-well infrared photodetector (QWIP) array at left. (Credits: NASA)

An advanced, highly compact thermal camera that traces its heritage to a camera now flying on NASA's Landsat 8 satellite has been mounted in a corner of the upcoming Robotic Refueling Mission 3 (RRM3) payload that will soon be received by the International Space Station (ISS).

From that corner position, the new camera, called the Compact Thermal Imager (CTI), will image and take videos of Earth’s surface below, once the SpaceX Dragon resupply vehicle delivers the payload to the orbiting outpost in November of this year (2018). CTI's task will be to image and measure fires, ice sheets, glaciers, and snow surface temperatures. CTI will also measure the transfer of water from soil and plants into the atmosphere.

Strained-layer superlattice
CTI's enabling technology is a relatively new photodetector technology known as strained-layer superlattice (SLS). In addition to being very small (40 x 15 cm), SLS consumes little power, operates at liquid-nitrogen temperatures, is easily fabricated in a high-technology environment, and is inexpensive "almost to the point of being disposable," says Murzy Jhabvala, a detector engineer at NASA's Goddard Space Flight Center (Greenbelt), Maryland. Jhabvala collaborated with his industry partner QmagiQ (Nashua, NH) to develop the SLS detector assembly.

The detector technology is also quickly and easily customized for different applications, he adds. The Goddard Detector Development Laboratory, for example, recently fabricated a 1024 x 1024 pixel SLS array and plans to increase its size to 2048 x 2048 pixels in the very near future.

The demonstration’s goal, Jhabvala says, is to raise SLS's technology-readiness level to nine (TRL-9), meaning that it has flown in space and has demonstrated that it operates well under the extreme environmental conditions found in space.

QWIP-based technology
SLS is based on the quantum-well infrared photodetector (QWIP) technology that Jhabvala and his government and industry collaborators spent more than two decades refining. The QWIP detectors are now operating on Landsat 8 and will be flying on the upcoming Landsat 9 Thermal Infrared Sensor Instrument, which Goddard scientists built to monitor the ebb and flow of land-surface levels and the health of vegetation.

Like its QWIP predecessor, SLS is a large-format detector. The arrays are fabricated on a semiconductor wafer. The wafer’s surface consists of hundreds of alternating, very thin layers of differing epitaxially grown materials.

Compared with the QWIP predecessor, SLS detectors are 10 times more sensitive and operate over a broader IR spectral range and at substantially warmer temperatures — 70 K for the SLS array compared with 42 K for the QWIP array. The increase in operating temperature will have multiple positive effects on future missions, says Jhabvala.


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