In a newly developed lidar system, researchers at Kyoto University integrated 3D flash and beam-scanning laser light sources to create a 3D, nonmechanical lidar system. The system can measure the distance of poorly reflective objects in the field of view (FOV) and automatically track the motion of these objects — a capability not found in conventional lidar and one that will enable autonomous vehicles to drive more safely, the researchers said. Most lidar systems rely on moving parts such as motors to scan the laser beam, making them bulky, expensive, and unreliable. Flash lidar, a nonmechanical lidar system that simultaneously illuminates and evaluates the distances of objects in the FOV with a single beam, cannot measure the distances of poorly reflective objects due to the miniscule amount of light these objects reflect. Flash systems also tend to be large because of the external lenses and optical elements needed to create the flash beam. “With our lidar system, robots and vehicles will be able to reliably and safely navigate dynamic environments without losing sight of poorly reflective objects such as black metallic cars,” professor Susumu Noda said. To power the lidar system, the researchers developed dually modulated photonic-crystal lasers (DM-PCSELs). The DM-PCSEL light source features both a flash source — one that can illuminate a wide, 30° × 30° FOV — and a beam-scanning source that can provide spot illumination with 100 narrow laser beams. In the DM-PCSELs, a singularity point of the photonic crystal, which lies outside of the light-cone, is used for laser oscillation. Light is diffracted into the light-cone in the desired directions by modulating the positions and sizes of the photonic crystal lattice points simultaneously. Since the researchers’ DM-PCSEL light source is chip-based, it could eventually enable the development of an on-chip, all-solid-state 3D lidar system. For the beam-scanning laser source, an array of DM-PCSELs is used to scan the beam over the desired range of emission angles. For on-chip flash illumination, the researchers broaden the beam by modulating the DM-PCSELs. Researchers developed a nonmechanical 3D lidar system, which is the size of a business card (seen in front of the system on the left). The system uses dually modulated surface-emitting photonic-crystal lasers (DM-PCSELs) as flash and beam-scanning sources. Courtesy of Susumu Noda/Kyoto University. Beyond the system achieving both flash and scanning illumination, it does so with without any moving parts or bulky external optical elements, such as lenses and diffractive optical elements, Noda said. To implement the DM-PCSEL in the lidar system, the researchers developed a driving circuit for each of the beam-scanning and flash-illumination DM-PCSELs. They also installed a time-of-flight (ToF) camera to perform distance measurements and developed software that enables the system to automatically track the motion of poorly reflective objects using beam-scanning illumination. The electronics for the system, including the driving circuit, the ToF camera module, and control units are all miniaturized. “The lasers, ToF camera, and all associated components required to operate the system were assembled in a compact manner, resulting in a total system footprint that is smaller than a business card,” Noda said. The researchers used the system to measure the distances of poorly reflective objects placed on a table in a laboratory setting. They also showed that the system can automatically recognize poorly reflective objects and track their movement using selective illumination. The team plans to demonstrate the system in practical applications, such as the autonomous movement of robots and vehicles. It also wants to see if replacing the ToF camera with a more optically sensitive single-photon avalanche photodiode array would allow the measurement of objects across even longer distances. The research was published in Optica (www.doi.org/10.1364/OPTICA.472327).
