Testing an optical force-clamp

Here a DNA-molecule is being stretched between two optically trapped polystyrene micron-sized beads. We're using an FPGA-based real-time controller for steering the upper trap. It's programmed with a PI-loop which aims to keep the force acting on the lower bead constant. Around 10s into the video we switch on the feedback-loop and we see the actual force on the bead rise to the set-point.