Dynamic focusing using a MEMS mirror in a coherent lidar

This paper describes the use of dynamic focus control in a coherent lidar and compares the theoretical and experimental results. The system uses a MEMS variable-focus mirror, which has an electrostatically driven deformable membrane with a diameter of 4 mm and a manufacturer's published 10%-to-90% rise time of less than 200 µs. The device permits fast, intra-scan adjustment of the downrange beam focus from 2.2 m to infinity for a lidar with a 25 mm diameter exit pupil. Experimentally, the variable-focus mirror improved the signal strength by more than 20 dB for objects within the first Rayleigh distance, in good agreement with theory. The fast response time allows the system to dynamically adjust the focus to different distances within a single sweep of a raster scan, in order to match the system focus on-the-fly to targets at different distances. Three-dimensional point clouds show that this technique allows some targets that are below the noise level when using a fixed focus to become visible when the focus is changed dynamically.