In 2019, the French-German Research Institute of Saint-Louis (ISL) is celebrating its 60th anniversary, but the roots of the institute are going back to the early end of the WWII when a team of German scientists led by Prof. Schardin from the Air Force Technical Academy (Technische Akademie der Luftwaffe) in Berlin-Gatow came in the Upper Rhine to work for the French Army. There, the Prof. Schardin was working in the field of ballistics and was world-wide renowned for his works in high-speed physics. In his early years, as a permanent assistant to the eminent German ballistics Professor Carl Cranz, he developed the famous Cranz-Schardin camera, a revolutionary electro-optical high-speed cinematography method using electric sparks or flash x-rays for illumination and able to work at frame rates over 106 images/second. This technique brought huge improvements in the comprehension of ballistic phenomenon and moreover in high-speed physics.
Since 1945, the LRSL, renamed ISL in 1959, maintained a leading position in the domain of high-speed phenomenon. In the beginning of the 60's, the invention of the laser was a true revolution that permits the emergence of new techniques like holography and interferometric holography. With the introduction of semiconductor lasers, ISL has a leading position on range-gated active imaging and deploys a significant research effort in a new emerging domain: computational imaging which includes scientific thematic such as see around the corner, compressed sensing or imaging with multiple scattered photons.
Recent world events have highlighted that the proliferation of UAVs is bringing with it a new and rapidly increasing threat for national defense and security agencies. Whilst many of the reported UAV incidents seem to indicate that there was no terrorist intent behind them, it is not unreasonable to assume that it may not be long before UAV platforms are regularly employed by terrorists or other criminal organizations. The flight characteristics of many of these mini- and micro-platforms present challenges for current systems which have been optimized over time to defend against the traditional air-breathing airborne platforms. A lot of programs to identify cost-effective measures for the detection, classification, tracking and neutralization have begun in the recent past. In this paper, lSL shows how the performance of a UAV detection and tracking concept based on acousto-optical technology can be powerfully increased through active imaging.
A good measure for the probability to detect a sniper telescopic sight is the effective bi-directional laser retro-reflection cross section. This angular (bi-directional) property of an optical device can be measured and can be used for a fist estimation of its probability to be detected by an active imaging. In the present paper, the authors give examples for resolved and non-resolved sensing of a telescopic sight under mono-static and bi-static conditions. As a result of these measurements, the resolved sensing under mono-static conditions shows the highest signal response in a wide angular range.
Homogeneous and speckle-free laser illumination devices are the key technology for high resolution active imaging and range-gated viewing systems. At ISL, a waveguide technology was developed to effectively reduce laser speckle of solid-state laser sources by a spatial or angular diversity approach, respectively. Further, a distant scene is illuminated with a homogeneous rectangular top-hat profile. In the present publication we give a theoretical description of the waveguide laser illumination devices and present results from ray tracing simulations and experimental investigation.
We present simple illumination devices with built-in speckle reduction using the spatial diversity approach. These devices are based on a waveguide homogenization technique with a transmission efficiency of up to 95%. Even for single-laser pulse illumination from a solid-state laser source a reduction of the speckle contrast by a factor of 4.5 was demonstrated. In detail, we present two different illumination devices based on either a solid-state laser source or an array of semiconductor laser diodes. These illumination devices are used for range-gated imaging and active polarimetry with speckle-free and homogeneous illumination.
One coherent processor and one incoherent processor, both including an active contour optical implementation were constructed and are presented. The coherent processor consists of a complete optical target tracking processor combining a Joint Transform Correlator with an optical implementation of a segmentation method based on active contours or "snakes". The incoherent processor is an optoelectronic multichannel processor that is able to segment an object in a real image. The process is based on an active contour algorithm that has been transposed to optics in order to accelerate image processing. The correlator, in its multichannel version, speeds up the overall frame rate of the optoelectronic processor. Experimental results for both processors are presented.
A complete target tracking processor including a segmentation method based on active contours or "snakes" has been optically implemented and its behavior has been evaluated. The purpose of this segmentation method is to darken the background in order to avoid its disruptive influence on tracking. The optical processor includes a Joint Transform correlator (JTC) with improved performance since with the proposed method the target will appear with a maximum of details in the reference. The complete processor is very complex and depends on the technology of several photonic devices. It consists of three nematic liquid crystal spatial light modulators, a preprocessor using an optical high pass filter, two different channels separated by the polarization direction of the light, a holographic edge enhancement filter, an electromechanical iris diaphragm, a camera and several photodetectors. The complete optical tracking processor must exhibit a negligible response time compared to a tracking cycle. To meet this requirement, research was conducted into two directions -- the algorithms and the components. A new snake optimization criterion is introduced and two different tracking algorithms are compared. The parameters of each component are studied and optimized, and their influences on the performance of the optical processor in terms of speed and tracking accuracy are analyzed. A complete system level demonstration of target tracking is presented as a conclusion.
Holographic interferometry and speckle interferometry are currently used in the field of non-destructive testing. More particularly, they permit to detect defects in structures by analysis of their behavior under stresses. In this work we have studied the possibility of using these techniques for the detection of cracks in reinforced concrete structures. We present some results with classical double-pulse holography and with the double reference technique. We also present some results with speckle interferometry on CCD camera.
Buried mine detection is a very complex problem. Many types of mines contain few metallic parts, which complicates the detection with classical electromagnetic systems. In this work we have studied the application of holographic interferometry techniques to the visualization of seismic wave propagation. Perturbations of the wave propagation can reveal the presence of a buried object. At first, classical holographic interferometry is tested to follow the propagation of the seismic wave. To make the holographic acquisition system more portable, we have also tested the speckle interferometry technique. Results concerning both methods are shown and discussed.
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