Shijun Xiao, Changyuan Yu
This paper investigates the impact of optical path walk-off on coherent efficiency and signal power in scanning coaxial coherent LiDAR systems. In such systems, a rotating scanner is used to steer the transmitted laser beam, introducing small angular changes during the time delay between transmission and reception of the reflected signal from the target. Because the receiving path shares the same axis as the transmitting path, these angular changes cause the return signal path, after passing through the scanner, to deviate from the local oscillator (LO) path used for coherent detection. This misalignment—referred to as optical path walk-off—can significantly degrade both coherent efficiency and signal power, particularly at long ranges. The coherent efficiency is modeled using Siegman’s target-plane formalism, which incorporates a backpropagated LO (BPLO) beam and the transmitted beam. Analytical expressions based on Gaussian beam theory are derived to describe its dependence on scanner angular speed and target range. To mitigate the walk-off effect, two methods for introducing slight non-coaxial alignment between the BPLO and transmitted beams are proposed and analyzed. Both methods are shown to enhance signal power at long ranges and reduce the negative impact of walk-off. This work provides a theoretical framework for optimizing coherent efficiency and signal power in coaxial coherent LiDAR systems.