Understanding the physics of coherent LiDAR.

Coherent LiDAR (Light Detecting And Ranging) is a promising 3D imaging technology that provides significant advantages over more traditional LiDAR systems. In addition to being immune to ambient light, it directly measures the velocity of moving objects by sensing Doppler shift of light, and can achieve exceptional depth accuracies. The goal of this manuscript is to explain the basic physics of coherent LiDAR with rigorous derivations from first principles. We first discuss the sensitivity of coherent detection, and derive the number of photons needed to robustly detect a LiDAR return. We then turn our attention to the collection efficiency of coherent LiDAR, and show that signal strength is strongly dependent upon how well the laser beams are focused.