FPGA based wavefront correction for measurements through optical disturbances

Particle Image Velocimetry (PIV) is a well-established camera-based method to analyze flow fields, which offers a high-resolution and low measurement uncertainty. Distortions in the optical path between the measurement volume and the system can degrade the measurement. An example is the measurement of the internal flow in a water droplet with a fluctuating interface. The deflection of the light beams distorts the PIV image. To correct the deflection we implemented a closed loop system, where the distortion is sampled with a guide star and then measured with a Hartmann Shack wavefront sensor. An in-house Field Programmable Gate Array (FPGA)-based signal processing technique controls a fast deformable membrane mirror (Alpao DM69) to correct low-order Zernike distortions. The adaptive laser system has a control rate of 1600 Hz and is able to compensate disturbances of up to 200 Hz.