Laser diode arrays for expanded mine detection capability

ABSTRACT A tactical unmanned aerial vehicle-size illumination system for enhanced mine detection capabilities has been designed, developed, integrated, and tested at the Coastal Systems Station. Airborne test flights were performed from June 12, 2001 to February 1, 2002. The Airborne Laser Diode Array Illuminator uses a single-wavelength compact laser diode array stack to provide illumination and is coupled with a pair of intensified CCD video cameras. The cameras were outfitted with various lenses and polarization filters to determine the benefits of each of the configurations. The first airborne demonstration of a laser diode illumination system is described and its effectiveness to perform nighttime mine detection operations is shown. Keywords: Laser diode array, Active illumination, Nighttime reconnaissance 1. INTRODUCTION The Airborne Littoral Reconnaissance Technologies (ALRT) project at Coastal Systems Station (CSS), under direction from the Office of Naval Research (ONR), is continuing development and investigation of the Airborne Laser Diode Array Illuminator (ALDAI) prototype system, leveraged from the Joint Mine Detection Technology project, to fill the current capability gap of unmanned aerial vehicle (UAV) compatible nighttime reconnaissance, mine, and obstacle detection. This deficiency has been identified by ONR as a critical area in need of solutions. Passive electro-optical technologies lack low-light and nighttime capabilities, tending to rely on sunlight. This severely restricts their operational window. This restriction can be addressed with the addition of active illuminators. However, many illuminators are frequently not suitable for UAV deployment. They are too large, require extensive cooling, unreachable input power, or may not be powerful enough to allow for a reasonable UAV altitude. The ALDAI presents a solution to these problems. The ALDAI testbed was designed, developed, and integrated into a UAV surrogate by the CSS ALRT project team. The ALDAI system contains a compact, efficient, laser light source that transmits a fixed diverging beam and is designed to be capable of deployment in a UAV-sized platform. The system uses two single-wavelength laser diode array stacks together with a pair of intensified CCD video cameras. The cameras can be outfitted with various polarizers and filters for polarization cuing along with spectral differentiation should other wavelength diode lasers be added. From the first flight demonstration on June 12, 2001, the testbed provided measures of operational conditions, detection performance, system functional capability, and system reliability, allowing the assessment of the ALDAI systems ability to provide enhanced detection of mines, minefields, and other trafficability issues in an airborne scenario. All aspects of performance are subordinate to sufficient ground illumination. This is primarily a function of laser power and atmospheric conditions. A baseline comparison was made by qualifying how this system compares to one operated under passive nighttime illumination sources in the visual to near infrared, especially as it relates to the moon, natural and artificial skyglow. For given laser characteristics, the two primary factors that reduce light transmission are aerosol scattering and altitude. The amount of backscatter from aerosols was compared for different polarization images. This led to specific polarizer suggestions for maximum transmission. The upper ranges of operational altitude for the ALDAI system were investigated during these tests. They can be limited by illumination power, scattering, or camera characteristics such as sensitivity, QE, and ground resolution.