Towards Uncooled Thermal Imaging Focal Plane Devices with NETD near 1mK
Abstract
A resonant-cavity pyrometer technology is proposed to reduce the NETD of uncooled LWIR detectors to near 1mK. In this system, a micromechanical detector absorbs L WIR light on its top surface, which is made of a thin absorbing material such as carbon. This heats the device and induces a thermomechanical transducer, such as a thermal bimorph, to deflect the plate. The bottom of the plate has a thin reflective metal, which serves as part of the top mirror of an underlying short wavelength optical cavity. The detector plate is separated by a vacuum gap from the rest of the cavity. The thermal deflection of the plate changes the spacing between the plate and the rest of the cavity, causing the resonant wavelength of the cavity to shift. The resulting reflectivity changes can be read using low power lasers that illuminate many pixels or an entire array. The reflected light can be read by a position-sensitive detectors, CCD arrays, or similar devices. It will be shown that such a device can reach an uncooled NETO near 1mK. While many aspects of the proposed technology are unique, it still builds on foundations from prior art, including the use of thermally isolated plates, the integration of thermal transducers such as bimorphs, and the separation of the wavelength ranges of absorption and readout. Many other technologies that also use these elements have limitations that prevent extremely high uncooled sensitivity. For example, microbolometers already operate near their theoretical limits, and lower NETO devices are difficult to design for video frame rates. Much of this limitation is due to the need for electrical conductors on the microbolometer supports. Optomechanical infrared detectors, such as cantilever thermal bimorphs or thermo-optic detectors had stress uniformity issues, large pixel masses, or excessively complex readout. The proposed devices must approach theoretical performance limits dictated by temperature and thermomechanical fluctuations. In order to better understand and apply these limits, the program will begin with a study of the thermomechanical noise of micromachined structures. The frequency, temperature, and Q dependence of thermomechanical position fluctuations will be characterized, with a theory developed to include variations in the thermomechanical properties across single devices. The temperature dependence of these effects are of particular interest because if the thermal dependence of the noise itself could be read as a signal, there would be no need for thermomechanical transducers such as bimorphs. Even if this were impossible, different thermomechanical effects may produce different performance limits. Detection processes to distinguish signals with SNR < 1 will be explored. In tandem with the above studies, a theory of device performance will be developed that includes mechanics, noise sources, optical readout, and bimorph transduction. In the first I 8 months of the program, single-pixel resonant-cavity pyrometers will be microfabricated and tested, with small arrays to follow in the latter half of the program. It is expected that the results will shed light not only on this particular architecture but on the possibilities for reaching NETO - lmK for the entire class of optomechanical thermal detectors. To date, the primary application of uncooled detection has been in camera imaging, and the past three decades have seen an emphasis on putting more pixels on target, making ever larger and denser focal plane arrays with only incidental effort to decrease the NETO of uncooled systems. There is a great need to find an alternative technology that can increase uncooled sensitivity without unacceptable losses of speed or SW AP. The resonant pyrometry concept proposed here will help achieve these goals. It has the fundamentally low noise of optomechanics without the excessive size and thermal mass of prior detector architectures using optical readouts.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 14, 2019
- Source ID
- W911NF1810272
Entities
People
- Joseph Talghader
Organizations
- Army Contracting Command
- United States Army
- University of Minnesota