Commercial Spinoffs from Military R&D Programs

Tuesday, December 01 2009

Single-Photon Detection Improves Laser Range-Finding and Communications

A near-infrared (NIR) detector with single-photon sensitivity can be used to improve many applications, from better eye-safe lasers to speeding up tomorrow’s optical computers.

Voxtel’s (Beaverton, OR) sensor emerged from a 2006 Missile Defense Agency (MDA) Small Business Innovation Research (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 avalanche photodetectors (APDs), and increased the available gain from approximately 30 to 8,000.

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.

How it Works

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 APD in the design scheme: a carrier multiplication device (CMD). The company’s key innovation is its 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 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 Geigermode 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.