NFAD Arrays for Single Photon Optical Communications at 1.5 um

For this program, we propose to develop large pixel-count single photon counting detector arrays suitable for deployment in spacecraft terminal receivers supporting long-range laser communication systems at 1.5 um. To surmount the present obstacles to higher photon counting rate -- as well as the complexity of back-end circuitry required -- in using conventional single photon avalanche diodes (SPADs), we will leverage initial success in monolithically integrating "negative feedback" elements with state-of-the-art SPADs to beneficially modify the device avalanche dynamics. This approach can achieve extremely consistent passive quenching, and appropriate implementations can lead to rather small avalanches (e.g., ~10^4 – 10^5 carriers), for which reduced carrier trapping provides lower afterpulsing that will no longer limit the photon counting rate. When correctly implemented, this "negative feedback" avalanche diode (NFAD) design is also extremely simple to operate: with just a fixed dc bias voltage, the NFAD will autonomously execute the entire arm, avalanche, quench, and re-arm cycle and generate an output pulse every time an avalanche event is induced. Phase I of this program will be focused on specific pixel-level design advancements related to the reduction of afterpulsing and timing jitter. Along with pixel-level goals, we will also fabricate and characterize test structures to define design and process innovations that guarantee high pixel yield and uniformity on large-scale NFAD arrays. The proof-of-feasibility tasks defined in Phase I will position us to demonstrate space-qualifiable large pixel-count (e.g., 80 x 80) NFAD arrays during a Phase II effort. Design and performance goals have been defined to meet anticipated lasercomm requirements for future space missions.