Stiffer optical tweezers through real-time feedback control

Using real-time re-programmable signal processing we connect acousto-optic steering and back-focal-plane interferometric position detection in optical tweezers to create a fast feedback controlled instrument. When trapping 3 mu m latex beads in water we find that proportional-gain position-clamping increases the effective lateral trap stiffness similar to 13-fold. A theoretical power spectrum for bead fluctuations during position-clamped trapping is derived and agrees with the experimental data. The loop delay, similar to 19 mu s in our experiment, limits the maximum achievable effective trap stiffness. (c) 2008 American Institute of Physics.