JANUARY 7, 2008--Princeton Lightwave (PLI; Cranbury, NJ, USA; www.princetonlightwave.com), a manufacturer of high-performance optical semiconductor components and subsystems, has been awarded a two-year $3.5M contract for the development of focal-plane arrays (FPAs) with single-photon sensitivity for use in three-dimensional (3-D) imaging systems at 1.06 μm. For this program, sponsored by the US Defense Advanced Research Projects Agency (DARPA), PLI will develop FPA modules that employ InP-based Geiger-mode avalanche photodiodes (GmAPDs) to achieve single-photon sensitivity and perform time-of-flight ranging measurements on a per-pixel basis. These 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 program.
The key component that enables the end performance of the 3-D imaging FPA is an InGaAsP/InP GmAPD photodiode array (PDA) optimized for detection of single photons at 1.06 μm. PLI is leveraging its best-in-class GmAPD device technology for the design and fabrication of this PDA. The company's commercially deployed single-element GmAPD detector is based on a highly reliable planar-passivated, diffused-junction photodiode structure that has demonstrated the highest performance to date utilizing this structure for critical single photon detection parameters such as dark count rate, photon detection efficiency, and timing jitter.
The GmAPD, also commonly referred to 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 just a single photon. The operation and readout of this PDA requires a specialized readout integrated circuit (ROIC) designed for the 3-D imaging application. 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 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 are very excited to be a key contributor to the development of 3-D imaging systems," says Mark Itzler, PLI CTO and principal investigator for the program. "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. Since single-photon detection in a GmAPD provides a macroscopic current pulse that can be sensed using digital thresholding circuitry, this device technology provides a direct and noiseless "photonstobits" 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.
