Advanced multifunctional detectors for laser-gated imaging applications

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.