Princeton Lightwave Inc. (PLI), a maker of optical semiconductor components and subsystems, has received a two-year $3.5-million contract from DARPA to develop focal plane arrays (FPAs) with single-photon sensitivity for use in three-dimensional imaging systems at 1.06 µm. The modules are intended for use as the optical engines at the core of 3-D imaging flash ladar systems such as those demonstrated in DARPA’s Jigsaw 3-D imaging laser radar program. PLI will develop FPA modules that 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 GmAPD, also commonly known as a 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 a single photon. The operation of the PDA requires a specialized readout integrated circuit (ROIC) designed for the 3-D imaging application. On a per-pixel basis, the ROIC senses the GmAPD output current pulse corresponding to the absorption of a single photon and assigns a time stamp indicating the interval between the launch 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 2-D image provided by pixel location in the detector array. PLI said 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. Mark Itzler, PLI’s CTO and principal investigator for the program, said, “We’ve been developing single-photon-counting technology for several years, and 3-D imaging is an excellent application for it since we can leverage our expertise in both semiconductor device design and module packaging. By the time we complete this two-year development program, we expect to see product-scale demand for these sensors to provide 3-D imaging capability in a variety of defense systems.” The GmAPD FPA technology, initially demonstrated by MIT Lincoln Laboratory under DARPA sponsorship of the Jigsaw program, has a number of highly desirable features, PLI said. Since single photon detection in a GmAPD provides a macroscopic current pulse that can be sensed using digital thresholding circuitry, the device technology provides a direct -- and noiseless -- “photons-to- bits” conversion process. Among the benefits of single-photon sensitivity is the ability to obtain 3-D image data using low-power pulsed sources and the collection of 3-D images even in situations involving very large source attenuation. In particular, the Jigsaw program demonstrated the feasibility of using the GmAPD FPA technology to create 3-D images of objects obscured by forest canopy and camouflage netting, PLI said.For more inforjmation, visit: www.princetonlightwave.com