Flash LiDAR

Flash LiDAR is a laser-based 3D imaging system in which a large area is illuminated by each laser pulse and a focal plane array (FPA) is used to simultaneously detect light from thousands of adjacent directions. Mapping and 2-D/3-D imaging are examples of applications for such systems. Flash LiDAR is especially advantageous when compared to scanning lidar, when the camera, scene, or both are moving since the entire scene is illuminated at the same time. With scanning LiDAR, motion can cause “jitter” from the lapse in time as the laser rasters over the scene. In flash LiDAR, the entire field of view is illuminated with a wide diverging laser beam in a single pulse. This is in contrast to conventional scanning LiDAR, which uses a collimated laser beam that illuminates a single point at a time, and the beam is raster-scanned to illuminate the field of view point-by-point.

In the 1990s, some researchers put forward the concept of non-scanning LiDAR, which belongs to 3D imaging LiDAR. As Figure 1 shows, flash LiDAR uses a camera-like working mode. The photosensitive element is different from an ordinary camera. Each pixel can record photon flight time information. The emitted array laser irradiates upon the target, and the target scatters from the incident light. The object has three-dimensional space properties so that the light irradiated to different parts of the object has different flight times. It is detected by the focal plane detector array and the output is a “three-dimensional” image of depth information.

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Figure 1 Schematic of the Flash LiDAR system

As shown in Figure 2, Flash LiDAR has also undergone a miniaturization development process. Its dimension space has occupied from the initial compartment to the desk, and to the current centimeter-level, all benefited from the development of compact laser arrays and detector arrays.

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Figure 2 Miniaturization evolution of Flash LiDAR

In 2010, the camera technology of 3D Flash LiDAR was reported, and the application of multi-moving target detection and tracking confirmed the feasibility of the concept. NASA has researched and reported that Flash LiDAR in the form of 3D imaging can be used for the automatic landing of spacecraft on planetary bodies. The system architecture is shown in Figure 3. Flash LiDAR has a 256×256 pixel detector and a scanning rate of 30 Hz, carrying out related tests at a distance of 1 km in a simulated scene. In addition, in the field of surveying and mapping, to detect different terrains, forests, and other environments, a Flash LiDAR with a wavelength of 1064 nm, a 20° field of view, and a frame rate of 55 Hz is applied in the experiment.

Figure 3 Configuration of Flash LiDAR

The performance of Flash LiDAR is mainly determined by the focal plane detector array. The focal plane detector array can use a PIN photoelectric detector, to the front of which a lens unit is added, meanwhile, a high-performance readout circuit can be used to achieve short-distance detection, the system is shown in Figure 10. For long-distance detection needs, the current high-performance detector—avalanche photodetector (APD), which meets the needs, has high detection sensitivity, and can realize single-photon detection. An APD-based array detector is featured with long-distance single-frame imaging and it is easy to miniaturize.

APD detectors have two working modes: linear and Geiger: linear mode avalanche photodiode (LMAPD) and Geiger mode avalanche photodiode (GMAPD). The Lincoln laboratory has conducted a series of researches on the array detectors that can be applied to LiDAR. Imaging speed of 5000～10000 times per second and a distance resolution of 15 cm can be realized in a 32×32 GMAPD working in the visible light band integrated with a 500 MHz timing circuit. In the case of GMAPD with a 256×256 array scale shown in Figure 11, the detector is monolithically mixed and integrated into the CMOS readout circuit, to realize passive photon-counting imaging. After 30 ms of multi-frame superimposition, a clearer image can be realized at 3.5km. Raytheon Company reported that the HgCdTe-material LMAPD array is used for 3D LiDAR detection and reaches a 256×256 array size, and it has verified that single-photon counting in linear mode can provide real-time and long-distance detection.

The range resolution and angle resolution of Flash LiDAR are directly limited by the performance of the detector. In order to achieve long-distance detection, the focal plane array detector requires an expensive avalanche photodetector (APD). Detectors with a larger area and higher performance are difficult to obtain. In order to solve the problem of the limited resolution of Flash LiDAR, a research team at KAIST in South Korea proposed the use of polarization modulating Pockels cell (PMPC) and array micro-polarization charge-coupled device. Because the polarization state of the laser changes with the distance, the MCCD can detect the polarization state of the echo and calculate the distance. The MCCD with 1024×1024 array elements can make up for the shortcomings of the large-area APD array and can obtain a higher angular resolution. The experiment obtained an angular resolution of 0.12 mrad and a distance resolution of 5.2 mm within a 16 m range.

Flash LiDAR allows for 3D imaging because of the camera’s ability to emit a larger flash and sense the spatial relationships and dimensions of the area of interest with the returned energy. This allows for more accurate imaging because the captured frames do not need to be stitched together, and the system is not sensitive to platform motion. This results in less distortion.

Neuvition S2 flash LiDAR

S2 is the first flash LiDAR product in the Neuvition LiDAR family! S2 is applicable to these scenarios: AGV, smart factory and warehouse sorting and handling, production line inspection, low-speed autonomous cleaning car, etc.

Reference:

Research progress in solid-state LiDAR – Chen Jingye

Wikipedia