The key objective of our project is to advance photon-counting detectors for NASA exoplanet missions. The proposed technology provides zero read noise, ultra-high dynamic range, and ideal linearity over the relevant flux range of interest. It could be the best realizable detector for a planet finding spectrograph, and it would have outstanding properties for the wavefront sensor and imaging focal plane.
The benefit of this device is that it will dramatically improve sensitivity beyond what is capable with non-photon counting detectors, thereby increasing science return for a fixed mission life; for low-light-level cases, such as spectroscopy, the proposed device will reduce the necessary exposure time for detecting planetary features by 50-80%. The proposed device always operates in photon counting mode and is thus not susceptible to excess noise factor that afflicts other technologies. It continues operating with shot-noise limited performance up to extremely high flux levels, >10^6 photons/second/pixel, and delivers signal-to-noise ratios on the order of thousands for higher fluxes. Its performance is expected to be maintained at a high level throughout mission lifetime in the presence of the expected radiation dose.
The detector has a Geiger-Mode Avalanche Photodiode (GM-APD) per pixel, each individually bump-bonded to a silicon readout circuit. Each pixel independently registers the arrival of a photon and can be reset and ready for another photon within 100 ns. The pixel has built-in circuitry for independently counting photo-generated events. The readout circuit is multiplexed to read out the photon arrival events. The signal chain is inherently digital, allowing for noiseless transmission over long distances.
We propose to advance photon-counting detectors for exoplanet missions using a two- pronged approach. First, we will advance 256x256 detectors based on existing diode designs from TRL 4 to 5 through a full testing program, including the exposure of several detectors to high energy radiation and follow-up testing. Second, we will advance 256x256 detectors with higher fill factor from TRL 3 to 4 through new design to minimize dark current and a full laboratory testing program. Both types of detectors use a readout integrated circuit that has already been designed and is now being fabricated. All testing will be performed in relevant environments that simulate an exoplanet space mission. Our team has significant experience in all aspects of the proposed program. Lincoln Laboratory pioneered GM-APD arrays for LIDAR applications and is now collaborating with Rochester Imaging Detector Laboratory in developing devices for imaging applications.
Technology Development for Exoplanet Missions