Visible to SWIR Fast eAPDs for Panchromatic FTS Instrument

This proposal focuses on advancing sensors used for air quality forecasting by observing ozone and trace gases via imaging spectrometers such as the Panchromatic Fourier Transform Spectrometer (PanFTS) instrument. Solar reflected radiances of these trace molecules vary over 2 orders of magnitude between 350 nm to 2500 nm wavelength range, and over high and low albedo scene variations. Typically, two or more imaging spectrometers are required to do these measurements. An imaging spectrometer generally has either large dynamic range or small flux sensitivity, but not both. To reduce system Size, Weight and Power (SWaP) and cost, we propose a large dynamic range Focal Plane Array (FPA) with an in-flight programmable 1 – 300x gain, capable of observing combined ozone, water vapor, CO, NO2, methane, and N2O in one imaging spectrometer. We propose to hybridize a high dynamic range 16-bit Digital ROIC (DROIC) with an electron avalanche photodiode (eAPD) FPA. The eAPD feature enables us to adjust the gain and effective least significant bit (LSB) continuously over 2+ orders of magnitudes, vs the binary high and low gain settings (~10x) of a traditional FPA. This eAPD technology was invented for Astrophysics Science and we will extend this technology to Earth Science applications and our wavelength range. The eAPD gain is in-flight programmable, which enables a broadband imaging Fourier transform spectrometer to adjust its flux sensitivity over the UV-to-SWIR wavelength range when measuring different species. The detector proposed is a significant advancement in the development of eAPD FPAs for JPL's PanFTS instrument, where the FPA needs to run at 1 kHz frame rate. This task leverages existing eAPD FPAs used on other NASA programs. However, the existing eAPD FPA's cut-on wavelength starts at 800 nm, due to the CdTe buffer layer through which photons have to transduce prior to reaching the HgCdTe absorber layer. This task extends eAPD cut-on wavelength into visible and/or UV wavelength range, which requires removal of the GaAs substrate and the CdTe buffer layer. The most challenging portion of the project is the development of a passivation layer at the illuminating surface that transmits photons between visible and/or UV wavelength range to 2500 nm, while minimizing surface recombination velocity at the surface to maintain low dark current at 120 K. Our baseline is to demonstrate an eAPD FPA with 500 nm cut-on wavelength, with a goal of extending the cut-on wavelength into the ultraviolet (UV) for the ozone measurement. The FPA will employ a proven 640 x 480 DROIC with 20-micron pixel pitch fabricated in a 65 nm foundry process. The photocurrent from the detector charges the input capacitance of the front-end of the DROIC pixel (~ 1fF) until the voltage trips a comparator which produces a pulse and resets the input. The front-end is configured to accommodate both n-on-p and p-on-n detector polarities and can be switched on command. The pulse corresponds to an LSB that can be varied from 1000 to 2300 e-. These pulses trigger a 16-bit ripple counter that can be globally set for up/down operation which is useful for some operating modes. Integration time is set by stopping the counters at the specified interval, not by resetting the detector capacitance as in an analog readout. The first year of the program includes the detector design, material growth of eAPD wafers, processing of detector arrays, characterization of test arrays and test hardware prep for the digital FPAs. The second year will be the hybridization of eAPD detector arrays to digital ROICs, integration and assembly of the digital FPAs into the prototype instrument followed by test. Entry level for this technology is TRL2; exit level TRL at the end of the program after two years will be TRL4. Upon completion, this effort will enable smaller, lighter instruments with lower power consumption capable of addressing Atmospheric Composition focus area [1].

Accelerate mission development and reduce risk by developing critical components and subsystems for advanced instruments and observing systems

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