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Optical component and sub-system maker Princeton Lightwave Inc (PLI) of Cranbury, NJ, USA has been awarded a two-year $3.5m contract by the US Defense Advanced Research Projects Agency (DARPA) to develop focal plane arrays (FPAs) with single-photon sensitivity for use in three-dimensional (3D) imaging systems operating at a wavelength of 1.06 microns.

The FPA modules will use InP-based Geiger-mode avalanche photodiodes (GmAPDs) to achieve single-photon sensitivity and perform time-of-flight ranging measurements on a per-pixel basis. The modules are intended for use as the optical engines at the core of 3D imaging flash ladar systems (such as those demonstrated in DARPA’s Jigsaw program).

The key component in the 3D imaging FPA is an InGaAsP/InP GmAPD photodiode array (PDA) optimized for detection of single photons at 1.06 microns. PLI’s commercially deployed single-element GmAPD detector is based on a highly reliable planar-passivated, diffused-junction photodiode structure. This has demonstrated the highest performance to date using this structure for critical single-photon detection parameters such as dark count rate, photon detection efficiency, and timing jitter, the firm claims.

The GmAPD, or single-photon avalanche diode (SPAD), is an avalanche photodiode structure that, when operated above its breakdown voltage, can generate a macroscopic current pulse in response to the absorption of just a single photon. Its operation and readout requires a specialized readout integrated circuit (ROIC) designed for 3D imaging applications. On a per-pixel basis, this ROIC senses the GmAPD output current pulse corresponding to the absorption of a single photon and assigns a time stamp indicating the time-of-flight between the launching of a short-duration ranging laser pulse and the photon detection event. The per-pixel time-of-flight information is translated to distance, as in conventional ladar measurements, and provides the third spatial dimension to complement the two-dimensional image provided by pixel location in the detector array. PLI says that it will hybridize the GmAPD PDAs and ROICs using flip-chip bonding, and a high optical fill factor will be achieved using an array of microlenses mated to the back-illuminated PDA chip.

“We’ve been developing single-photon counting technology for several years, and 3D imaging is an excellent application for it since we can leverage our expertise in both semiconductor device design and module packaging,” says chief technology officer Mark Itzler (principal investigator for the program). “By the time we complete this two-year development program, we expect to see product-scale demand for these sensors to provide 3D imaging capability in a variety of defense systems.”

Since single photon detection in a GmAPD provides a macroscopic current pulse that can be sensed using digital thresholding circuitry, GmAPD FPA technology (which was initially demonstrated by MIT Lincoln Laboratory under DARPA sponsorship of the Jigsaw program) provides a direct and (noiseless) ‘photons-to- bits’ conversion process. Among the benefits of single-photon sensitivity is the ability to obtain 3D image data using low-power pulsed sources and the collection of 3D images even in situations involving very large source attenuation. In particular, the Jigsaw program demonstrated the feasibility of using GmAPD FPA technology to create 3D images of objects obscured by forest canopy and camouflage netting.

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