Martin 19 th Coherent Laser Radar Conference CLRC 2018, June 18 – 21 1 Coherent Lidar for 3D-imaging through obscurants Aude Martin (a), Jérôme Bourderionnet (a), Luc Leviander (a), John F. Parsons (b), Mark Silver (b), Patrick Feneyrou (a) (a) Thales Research & Technology France, 1, Avenue Augustin Fresnel, 91767, Palaiseau, Cedex, France (b) Thales Ltd, 1 Linthouse Road, Glasgow, G51 4BZ, UK., Email: [email protected]Abstract: In the context of unmanned vehicles, eye-safe LiDARs able to measure simultaneously range and speed in degraded visual environments, are required. The ability of frequency modulated continuous wave (FMCW) coherent LiDAR at 1.55 μm to differentiate hard target from diffuse ones like clouds is used here to demonstrate 3D- imaging through obscurants. Detection experiments of a moving target up to 50 m using a compact FMCW LiDAR based on silicon photonics are also highlighted. The photonic integrated circuit, without any moving parts, allows emission and detection in 8 different collimated directions spread over the desired angle. Keywords: Coherent Laser Radar, Frequency Modulated Continuous Wave Lidar, Photonic Integrated Circuit. 1. Introduction Multidimensional imaging systems either use an array of detectors such as gated viewing cameras [1]; single photons array [2], or use a single pixel technology such as laser ranging with a beam scanning technology [3] or a single pixel camera [4]. In this work, we have used the FMCW LiDAR technology with a scanning head for imaging through obscurants and used a non-mechanical scanning system with a LiDAR on-chip to detect a hard target. The main idea for FMCW lidar is to obtain spatial resolution using a frequency modulation in a coherent detection scheme. Both local oscillator and emitted signal are frequency modulated. The backscattered signal is time-delayed due to the light propagation time from the lidar to the target and frequency shifted due to Doppler shift. Hence, the interference between local oscillator and backscattered light shows frequency plateaus, the average of which corresponds to the Doppler shift (speed measurement) and the difference of which corresponds to the range multiplied by the frequency modulation slope. As illustrated in Figure 1, this type of detection also enables FMCW LiDAR to distinguish a (relatively weak) static target signal from a much larger signal backscattered by a cloud. Since the cloud presents a significant thickness, the power spectral density of the backscattered light is spread over a large number of frequency bins. In contrast, the signature of a hard target consists of two very sharp peaks. By only selecting narrow peaks, the system can reduce false alarms and, hence, increase the signal processing speed. The signal processing used in the following experiments [5] also enables detection of multiple targets within the collimated beam. Figure 1. Experimental power spectral density of a target behind a cloud Mo10
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Martin 19th Coherent Laser Radar Conference
CLRC 2018, June 18 – 21 1
Coherent Lidar for 3D-imaging through obscurants
Aude Martin (a), Jérôme Bourderionnet (a), Luc Leviander (a),
John F. Parsons (b), Mark Silver (b), Patrick Feneyrou (a) (a) Thales Research & Technology France, 1, Avenue Augustin Fresnel, 91767, Palaiseau, Cedex, France