Acqubit has been developing GaInP GmAPD photon counting arrays using a back-side illuminated homojunction InGaP mesa design. Both single element and 32 x 32 arrays GaInP GmAPDs have been designed, fabricated, and tested. We observed low dark current density of 0.32 uA/cm2 before breakdown and a dark count rate of ~ 10 kHz at 3.5 V overbias at room temperature for a 50-micron device. Furthermore, we obtained a quantum efficiency (QE) of about 55% at 520 nm without a bottom reflector. The combination of mesa structure and removal of the substrate greatly suppresses pixel crosstalk while maintaining a high QE and photon detection efficiency (PDE). We have built focal plane arrays using 32x32 GaInP arrays integrated with read-out integrated circuits (ROICs). Furthermore, we removed the GaAs substrate for backside illumination and demonstrated good QE. We integrated SiO2 micro lens arrays for improved optical fill factor.

Multi-pixel short-wave infra-red (SWIR) Geiger-mode avalanche photodiode (GmAPD) light detection and ranging (LIDAR) cameras have enabled unprecedented sensing capabilities in commercial and government platforms. Ball Aerospace develops and produces state-of-the-art SWIR synchronous and asynchronous single photon counting GmAPD arrays for variety of active and passive imaging applications. This work demonstrates the advantage of asynchronous arrays which is based on individual pixel continuous photon sampling functionality and the underlying read-out integrated circuit (ROIC). The benefit is based on user-selectable pixel hold-off (reset) time (tunable from ~250 ns to beyond ~5 µs) that determines aggregate pixel availability and noise performance. Single photon sensitivity and precise sub-nanosecond photon arrival timing for each pixel in the array enables advanced active and passive systems with unparalleled engagement ranges as well as novel imaging methods.

The image formation process for airborne lidar systems utilizes physical sensor models in order to create three-dimensional imagery from range, scanning, position and attitude sensor measurements. To estimate surface locations from data collected with Geiger-mode lidar systems, typically range measurements are combined from many laser pulses, from many camera pixels, over multiple viewing geometries and multiple collection times. Sharp image formation requires well-calibrated camera and pointing models. The calibration method presented here utilizes aerial data collections spanning the full sensor field of regard to iteratively refine the system model parameters while minimizing relative imagery misplacement. Step one of the calibration adjusts parameters that only affect image quality and evaluates image sharpness using, for example, a Sobel sharpness filter. Step two adjusts parameters affecting image placement. This calibration method utilizes the vendor-supplied transform code inside the parameter optimization algorithm and is therefore independent from the lidar configuration and the physical sensor model. Utilizing the vendor-supplied transform ensures that the resulting calibration parameters are interpreted correctly. To reduce computational burden in step two, the input data from thousands of laser pulses is condensed into a single effective pulse that is passed through the transform step. This method has been used to calibrate four different Geiger-mode lidar systems and has enabled sharp imagery over the full field of regard prior to incorporating ground control points or performing registration with adjacent imagery. We present representative results for the Cuchillo lidar system and discuss execution time requirements and performance limitations.