The Physics Behind LiDAR Technology - Lasers and LiDAR Wavelength

LiDAR stands for Light Detection and Ranging. It uses optical laser and detection technology to create a 3D map of static or moving objects and the surrounding environment. Laser energy launched from the LiDAR reflects off from various objects in the scene, and this is collected by an optical detector. Photons detected by the detector are converted into electrons and processed and filtered to produce a point cloud (an x-y-z representation of the scene). In some types of LiDAR, relative axial velocity between the LiDAR and surrounding objects are also directly measured.

An important aspect of a LiDAR system is the operating wavelength. LiDARs operate either at the 8XX-9XX nm or the 13XX-15XX nm wavelength. The former uses GaAs lasers and Silicon detectors, whereas the latter uses InP-InGaAs for the laser and detector. A comparison of the impact of wavelength is provided in Table 1 below:

Apart from the wavelength, two other aspects of the LiDAR system are important:

1. The type of scanning used to address the Field of View (FoV):  Either flash illumination (entire FoV addressed simultaneously) or scanning (sequential adressing of points in the FoV) is deployed.  Scanning can be achieved via opto-mechanical or MEMS (Micro-electro-mechanical) actuators, electronic or optical scanning (the last 2 are solid state techniques with no moving parts).  Some architectures scan along the vertical and horizontal FOV, whereas some use scanning in one direction and flash illumination in the other,
2. The operating physics:.
   1. Time of Flight (ToF) LiDAR is a direct detection technique where the time lapse between launch of a pulsed laser and its detection is used to compute the depth information.  It uses short pulse fiber or diode lasers, with nanosecond pulse widths.
   2. Frequency Modulated Continuous Wave (FMCW) LiDAR uses CW laser energy of a coherent laser beam. The reflected energy is mixed with a portion of the outgoing pulse to extract range and relative velocity information. 
   3. Detection Modality: ToF LiDAR uses linear APDs (Avalanche Photodiodes) which are biased below the breakdown voltage or SPADs (Single Photon Sensitive Avalanche Detectors) which operate above the breakdown voltage or the so called “Geiger Mode”.