Date: Dec 15, 2009 Author: Joan M. Zimmermann Source: MDA (
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by Joan M. Zimmermann/jzimmermann@nttc.edu
Voxtel, Inc. (Beaverton, OR), has developed a near-infrared (NIR) detector with single-photon sensitivity that can be used to improve many applications, from better eye-safe lasers to speeding up tomorrow's optical computers.
Voxtel addresses the high noise that can hamper photodetectors, without requiring complex optical filtering or ultralow superconductor temperatures, in part by using a new class of avalanche photodetector (APD) in the design scheme: a carrier multiplication device (CMD). The company's key innovation is its unique patented detector structure that enables greatly reduced noise and improved gain, leading to improved performance and lower-cost systems with smaller optics that require less laser power.
In real-life applications, APDs are often used in light-sensing systems due to their low cost, small package, and ease of employment. But due to their noise and limited gain, APDs provide only limited improvements in sensor sensitivity. To achieve single photon sensitivity, users must operate APDs in the so-called Geiger mode, above the breakdown voltage of the APD — a digital mode that only can detect the presence of an optical signal, but not its magnitude. The loss of signal amplitude severely limits the scene reconstruction capabilities of Geiger-mode operation. Geiger-mode devices are also hampered by "range blooming", a period after detecting an optical signal in which the APD pixel is blind, due to the dead period caused by trapped signal charge in the APD.
Voxtel's CMD sensor improves on the performance of APD technology in terms of amplification magnitude, as well as the gain and its associated amplification noise. And, because it does not have afterpulsing, it is able to detect objects only centimeters apart at distances of tens of kilometers.
Focal-plane array CMDs have been fabricated with independent integrated circuits placed at each pixel, which can detect optical pulses as low as a single photon and detect their time of arrival to within 250 picoseconds. Voxtel's innovation is thus highly suited for improving the efficacy of many types of imaging, such as high-resolution, three-dimensional (3-D) imaging ladar systems on unmanned airborne platforms, to identify military ground vehicles hiding under camouflage or foliage.
Voxtel's CMD sensor emerged from a 2006 MDA SBIR Phase II contract to develop a new sensor for the Airborne Laser (ABL) program, while seeking a way to have the laser system detect and correct for atmospheric distortion. Such correction is made possible by detecting the returns from laser sources propagated through the atmosphere, which are often as low as a single photon. This effort reduced the noise approximately 40-fold compared to commercial APDs, and increased the available gain from approximately 30 to 8,000. The result is photon-counting at gigahertz rates, allowing ladar systems to be capable of higher-definition 3-D imaging.
Voxtel's single-photon detectors could make both optical sensors and communication systems more efficient by using photon-counting receivers to reduce or eliminate the complex filtering that is normally required to reduce background noise. These detectors also can increase the sensitivity and efficiency of systems with active sensors that use lasers for illumination. Compared with competing technologies such as supercooled, superconductor-based sensors, the APD-based sensor is lower in cost, less complex, and smaller in footprint. Though excellent low-noise, single-photon-detection performance has been reported for superconducting devices, their low-temperature requirements (colder than --269°C) limit their practical use. APDs operate in a much more user-friendly range, above --40°C.
Voxtel's innovation can improve imaging in robotic eye designs, the sensitivity of autonomous vehicle and cruise control sensors, and the resolution of imagers that ensure quality control systems in manufacturing warehouses. It could also enhance thermal imaging for industrial security, the precision of topography measurements, and the speed of laser communications systems. The technology also holds the potential for use in the nanotechnology realm, quantum communications, quantum computing, and laser communications for gigabit Internet interfaces. There are many other applications that could benefit from Voxtel's innovations, among them DNA sequencing, which has already been greatly streamlined by the use of optical methods; optical coherence tomography, an emerging technology that can image tissues and cells without ionizing radiation; and the fluorescence imaging used in cellular and molecular markers.
Voxtel is working with automotive manufacturers of laser radar to develop eye-safe alternatives to existing products. In addition, the company is working with telecommunications system integrators to develop the new APD technology for use in fiber-optic transceivers. By increasing the sensitivity of the detector in such transceivers, fewer repeaters would be required per optical transmission line, according to Voxtel's chief, George Williams. Therefore, the use of Voxtel's technology could significantly reduce costs and increase reliability. The company also is looking to use its technology in the emerging fields of quantum communications, where information is recorded and transmitted by single photons.
