  Optical scanning is a prevalent technique for optical neural interfaces where
light delivery with high spatial and temporal precision is desired. However,
due to the sequential nature of point-scanning techniques, the settling time of
optical modulators is a major bottleneck for throughput and limits
random-access targeting capabilities. While fast lateral scanners exist,
commercially available varifocal elements are constrained to >3ms settling
times, limiting the speed of the overall system to hundreds of Hz. Faster
focusing methods exist but cannot combine fast operation and dwelling
capability with electrical and optical efficiency. Here, we present a varifocal
mirror comprised of an array of piston-motion MEMS micromirrors and a custom
driver ASIC, offering fast operation with dwelling capability while maintaining
high diffraction efficiency. The ASIC features a reconfigurable nonlinear DAC
to simultaneously compensate for the built-in nonlinearity of electrostatic
actuators and the global process variations in MEMS mirrors. Experimental
results demonstrate a wide continuous sweeping range that spans 22 distinctly
resolvable depth planes with refresh rates greater than 12 kHz.
