Dynamic focus control in a coherent lidar using a fast MEMS mirror

This paper demonstrates the use of a variable focus MEMS mirror in a frequency-modulated continuous-wave (FMCW) lidar system. The mirror can reveal short range targets that are typically hidden because the lidar was out of focus, and has demonstrated the ability to increase the signal level by over 20 dB for close-range targets. We have also demonstrated the ability to dynamically change the focus of the lidar during each sweep of the galvonometer, thus allowing the resolution of targets at different ranges during each sweep. The electrostatically driven 4 mm diameter mirror has a transition time of less than 1 ms and a focal length that can range from infinity to 56 mm which corresponds to wavefront Zernike fringe defocus coefficient ranging from 0 to 18 μm.