Single-Photon-Sensitive Imagers

Single-photon-sensitive imagers are able to register the arrival of each individual photon at a detector. A familiar example of this is a Geiger counter, which detects gamma-ray photons from a radioactive source and produces an audible click in response to each. The same principle can be applied to building a camera that works at wavelengths in the ultraviolet, visible, and near infrared. Each pixel of such a camera detects individual photons and digitally counts them or records their times of arrival. The benefit of building a photon-counting camera is that it does not degrade the photon signal with additional noise.

The noise performance of imagers deserves further discussion. An electronic camera collects light during some exposure time. In the conventional approach, the light absorbed in the pixel produces a photocurrent that accumulates to form a packet of charge. The charge packets from the pixels are sensed using analog amplifier circuits, and the resulting output voltages are digitized either on or off chip. The charge- sensing process introduces electronic noise, known as readout noise, which gets worse when the amplifiers are operated at high speed to support short exposure times. Readout noise limits the quality of the images, especially under low-light conditions.

Image acquired by Lincoln Laboratory ALIRT Flash LADAR systemFigure 1. Three-dimensional image acquired by the Lincoln Laboratory Airborne Ladar Imaging Research Testbed (ALIRT) flash LADAR system. The colors indicate relative heights above ground; red = highest; blue = lowest.


In a single-photon-sensitive pixel, on the other hand, each absorbed photon produces an electronic pulse, which is converted to digital data within the pixel. Reading out these data introduces no noise or other corruption of the information. The camera can extract information from each photon and produce useful imagery when light is scarce or when images need to be acquired very frequently in order to "freeze" a rapidly changing scene.

Lincoln Laboratory flash LADARMovie of actual data taken at Huntsville base with Lincoln Laboratory's flash LADAR system and technology showing the foliage penetration capability of LADAR. By changing the range gate, data from decreasing heights can be excluded, revealing ultimately what is at ground level below the tree canopy. View movie

The Advanced Imager Technology Group has pioneered the development of arrays of solid-state Geiger-mode avalanche photodiodes (GMAPDs). A Geiger-mode avalanche photodiode is a semiconductor-based light detector that produces a digital logic signal in response to the absorption of a single photon. Several applications of this technology have been addressed, and cameras have been demonstrated by Lincoln Laboratory researchers. Passive photon-counting imaging cameras have been built for night vision and other low-light imaging applications.

In flash-laser-radar (LADAR) cameras, a short flash of light is produced to illuminate a scene, and each pixel measures the arrival time of the photons that are reflected back to the camera, thereby measuring the distance to each point in the scene to produce an image with three-dimensional information which can be very useful for analysis, such as the one shown above left. Also, an array of photon-counting pixels can be very useful in applications not related to imaging. Researchers at the Laboratory have demonstrated their use to detect laser communication signals that have been reduced to faint power levels by transmission over long distances.


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