Feedback Control for Optical Tweezers Instruments: Position Clamping and Detection Laser Intensity Stabilization

Position clamping in optical tweezers allows us to use less laser power at a cer-tain trapping stiffness which decreases the damage caused by the trapping laser.With intensity stabilized detection lasers smaller step sizes should be visible.High intensity laser light in optical tweezers can damage biomolecules understudy. We employ digital feedback control of the trap position to increasethe trap stiffness 13-fold[1]. We use predictive control to further increase, by~30%, the effective lateral trap stiffness. This predictive approach provideshigher control gain values than simple proportional control.The trap is formed with a CW laser into an inverted microscope. The trap ismoved with two orthogonal acousto-optical deflectors (AOD). A field-pro-grammable gate array (FPGA) -board calculates the trap position and controlsthe AODs accordingly. Bead position is measured with two detection lasersusing back-focal plane interferometry[2] with position sensitive detectors.Molecular motors use step sizes as small as 0.34 nm[3]. To detect sub-nanome-ter movement we use our FPGA-controller to intensity stabilize the detectionlasers[4]. The detection beams are coupled into polarization maintaining sin-gle-mode fibers. Part of the light emerging from the fiber is reflected onto a pho-todiode. The FPGA measures the intensity on the photodiode and controls thedriving current of the detection laser diode. This setup pushes the low fre-quency region of the bright-noise spectrum (below 2 Hz a drop of ~1 dB forintensity signal) closer to the dark-noise limit of our detection electronics