Laser event distribution and timing circuit design constraints in direct TOF LiDAR applications

Abstract

Advances in laser diode technology enable the generation of eye-safe laser pulses with short pulse duration and high peak power. This opens up new opportunities for Light Detection and Ranging (LiDAR)-systems based on the direct time-of-flight (dTOF) principle because their range performance is mainly limited by the requirement of eye-safe laser pulse energy. Another limiting factor for dTOF LiDAR is the sensitivity to background noise. Shorter pulse width enables better parasitic light suppression inside the LiDAR system for improved performance in high background flux scenarios. With the improvements caused by using short laser pulses, new challenges emerge. Shorter pulse duration and limited achievable timing resolution of time discrimination circuits inside of dTOF detectors lead to histogram data distributions in which the laser originated time stamps can only fill few time bins. The time stamp histogram of the detected and clocked laser photons shows a sharp exponential decline. The slope is strongly dependent on the occurring laser event rate inside the system. In an extreme case, all laser generated events fall into one time bin. Because of the coarse discrete arrangement of those laser generated events, a need for new algorithmic approaches arises. This work illustrates the dependency between the occurring laser photon rate in the system and its distribution inside the measurement data. Influence of the time discrimination circuit's time bin width is discussed with regards to resulting histogram shapes.