This paper reports the performance of a long range 3D imaging system operating at a wavelength of 1550nm incorporating a Geiger mode 32x32 array InGaAs/InP camera. A cross-correlation technique were used to mitigate range aliasing and therefore enable the measurement of the absolute range to single or multiple surfaces within the instantaneous field of view of each pixel in the 2D array. The system uses a fibre amplified laser source operating at an average pulse repetition rate of 125kHz with pulse energies of 2.4μJ per pulse. Measurements of the absolute range to remote manmade Lambertian surfaces and foliage at ranges up to 10km with range accuracy of better than 4cm are reported. The simultaneous imaging and measurement of the absolute range of two remote manmade Lambertian surfaces separated by >1km is also presented.
Active systems, using a near-infrared pulse laser and a fast, gated detector, are now adopted for most long range imaging
applications. This concept is often called laser-gated imaging (LGI) or burst-illumination LIDAR (BIL). The SELEX
solid state detector is based on an array of HgCdTe avalanche photodiodes, and a custom-designed CMOS multiplexer to
perform the fast gating and photon signal capture. This paper describes two recent developments. The first is aimed at
reducing the size, weight, power and cost of steerable platforms which often have to contain a large number of electrooptic
tools such as lasers, range finders, BIL, thermal imaging and visible cameras. A dual-mode infrared detector has
been developed with the aim of shrinking the system to one camera. The detector can be switched to operate as a passive
thermal imager, a laser-gated imager or a solar flux imager. The detector produces a sensitivity in the MW thermal band
of 16-18mK and a sensitivity in the BIL mode as low as 10 photons rms, in other words close to the performance of
dedicated imagers. A second development was to extend the current BIL capability to 3D. In complex scenes, with camouflage and concealment, the ability to generate 3D images provides a signal-to-clutter advantage. Also in airborne applications, especially, it is useful to have 3D information to provide agile, feedback control of the range gating in a dynamic environment. This report describes the development of the 3D detector and camera, and the results of field trials
using a prototype system.
The rapid pace of development in the field of long-range imaging is illustrated by two new detector technologies for passive and active imaging. Active systems, using a near-infrared pulse laser and a fast, gated detector, are now adopted for most long range imaging applications. This concept is often called burst-illumination LIDAR or BIL. The SELEX solid state detector is based on two major components: an array of HgCdTe avalanche photodiodes, and a custom-designed CMOS multiplexer to perform the fast gating and photon signal capture. These hybrid arrays produce sensitivities as low as 10 photons rms, due largely to very high, almost noise-free avalanche gain in the HgCdTe diodes. The sensitivity, dynamic range and image quality is now such that the camera performance is usually limited by coherence and scintillation effects in the scene. With this strong sensor capability, it has been possible to launch the next generation of multiplexers to satisfy systems of the future. For instance, most laser-gated imaging systems use a suite of passive infrared and visible cameras to complement the BIL channel. It is highly advantageous to combine these functions into one electro-optic system, leading to a simpler, smaller, lower power and lower cost system. The key technical steps are to find solutions for the difficult multifunctional detector and the dual-wavelength optic. A detector has been developed to image passively in the medium and short wavebands, and actively in BIL mode. The performance of the detector and optic is described. Another major systems enhancement is to be able to generate 3D images, particularly in complex scenes, to further improve background clutter rejection and provide agile, feedback control of the range gating in a dynamic environment. Here the detector senses the range, as well as the laser pulse intensity, on a pixel-by-pixel basis, providing depth context for each laser pulse. A prototype detector has been successfully demonstrated and shown to provide good quality 2D and 3D data for each laser pulse.
Laser based imaging systems are becoming common in a number of applications. Many of these systems rely on
scanning the laser and receiver over the scene to construct an image. A single pulse laser gated imaging system
employing a two dimensional focal plane array sensor has been developed by SELEX Sensors and Airborne Systems
Ltd (SELEX S&AS). The system has been deployed on field trials to gather data in order to assess the suitability of the
technology for a number of applications. The test system has been used to characterise and optimise subsystem and
system level performance, to assess the effects of atmospheric phenomena on system performance, and to gather field
data in various applications and scenarios. Recent system and subsystems enhancements to the Laser Gated Imaging, or
Burst Illumination LADAR (BIL), test rig are described. Changes to the laser subsystem, the sensor subsystem and the
system level integration aspects of the rig resulting from field trials experience and on going system development are
discussed. The recent developments for the system control, data and image processing suites are also addressed.
Operational observations, images and results from recent field trials, including operation in an airborne environment, are
presented and discussed.
BAE SYSTEMS has developed a laser-illuminated, gated imaging system for long range target identification which has generated bright images at ranges in excess of 10km from modest laser energies. The system is based on a short pulsewidth laser and a custom detector for sensing the return pulse. The source is a Nd YAG laser converted by an optical parametric oscillator (OPO) to 1571nm and producing 20ns pulses at 15Hz. The detector (named SWIFT) is a 320x256 array of HgCdTe photodiodes operating with high avalanche gain to achieve sensitivities as low as 10 photon rms. A custom silicon multiplexer performs the signal injection and temporal gating function, and adds additional electronic gain. Trials show that the current detectors have gate edges equivalent to 1.5m in range and complete extinction of signals outside of the gated range. The detector is encapsulated in an integrated-detector-cooler-assembly and utilises standard productionised thermal imaging electronics to perform non-uniformity correction and grey scale images. Imaging trials using the camera have shown little excess noise, crosstalk or non-uniformity due to the use of avalanching in the HgCdTe photodiodes up to gains of over 100. The images have shown high spatial resolution arising from the use of solid state focal plane array technology. The imagery, collected both in the laboratory and in field trials, has been used to explore the phenomenology unique to laser-illuminated targets and to verify system models.