Using Broad-Area VCSEL Arrays for High Performance Multi-Beam LiDAR

In order to improve road safety and mobility, successful implementation in vehicles of advanced driver assistance systems (ADAS) applications, and ultimately autonomous driving (AD), is essential. Fundamental to the progress towards AD is the development of sensors able to perceive the vehicle’s surroundings such that it can navigate through traffic without any human input. One of such sensors is a LiDAR (light detection and ranging) used to obtain an accurate and reliable localization of objects in the vehicle’s environment. Typically, a LiDAR system sends out a laser beam pulse which is reflected back by an object in the environment and again captured by the system. From the time difference between the transmitted and received signal, the distance to that object can be calculated. XenomatiX is a high-tech company providing automotive vision solutions for ADAS and AD applications based on their true solid state multi-beam LiDAR patented concept. This thesis is the result of a cooperation between XenomatiX and the Applied Physics (APHY) research group of the Vrije Universiteit Brussel (VUB), with the aim to further improve the multi-beam LiDAR systems.

The focus of this thesis is on the laser illumination side of the XenomatiX LiDAR system which uses a vertical-cavity surface-emitting laser (VCSEL) array as laser source. The goal is to get a better understanding of the dynamics of the VCSEL array and its individual emitters throughout the transmitted pulse and its impact on the time-of-flight measurement. Firstly, we examine how the transverse irradiance distribution of a VCSEL emitter’s near field and the VCSEL array’s far field fluctuate during a pulse through the use of a time-gated camera. Furthermore, we track the spatial extent of the VCSEL emitter’s near field and the VCSEL array’s far field throughout the pulse using the maximal thermal hazard method. Additionally, we expand this laser beam width estimation method to determine the spot size of a laser beam that extents to only a few pixels on a sensor, which is comparable to the multi-beam images captured by the XenomatiX LiDAR system. Secondly, we experimentally investigate the switch-on and -off characteristics of the VCSEL emitters across the VCSEL array. From the xperimental results, we link the spatial and temporal dynamics of the VCSEL array to the noise contributions observed in time-of-flight measurements of the XenomatiX LiDAR system. Thirdly, based on the gained insights a new illumination concept is proposed and studied with the goal to control the intrinsic dynamic behavior of the multi-beam pattern projected onto the scene and to improve the time of-flight measurements. Finally, we investigate how the optical phenomenon of speckle influences time-of-flight measurements and experimentally demonstrate that speckle is an important noise contributor. A speckle reduction method, and thus a noise reduction method for the time-of-flight measurements, is implemented and studied in the XenomatiX LiDAR system.