Probabilistic analysis of linear mode vs. Geiger mode APD FPAs for advanced LADAR enabled interceptors

To meet evolving ballistic missile threats, advanced seekers will include a multi-modal imaging capability in which a passive single- or multi-band infrared focal plane array (FPA) shares a common aperture with an active laser radar (LADAR) receiver - likely, a photon-counting LADAR receiver that can resolve photon times of arrival with sub-nanosecond resolution. The overall success of such a system will depend upon its photon detection efficiency and sensitivity to upset by spurious detection events. In the past, to perform photon counting functions, it has generally been necessary to operate near infrared (NIR) avalanche photodiode (APD) FPAs in Geiger Mode. Linear Mode APDs could not provide enough proportional gain with sufficiently low noise to make the photocurrent from a single photon detectible using existing amplifier technology. However, recent improvements in APDs, sub-micron CMOS technology, and concomitant amplifier designs, have made Linear Mode single-photon-counting APDs (SPADs) possible. We analyze the potential benefits of a LADAR receiver based on Linear Mode SPADs, which include: 1) the ability to obtain range information from more than one object in a pixel's instantaneous-field-of-view (IFOV), 2) a lower false alarm rate, 3) the ability to detect targets behind debris, 4) an advantage in the endgame, when stronger reflected signals allow dark current rejection via thresholding, and 5) the ability to record signal intensity, which can be used to increase kill efficiency. As expected, multiple laser shots of the same scene improves the target detection probability.