Oscillatory disturbance in force calibration of optical tweezers

In the calibration of the optical trap stiffness, it is found that there appears an attenuating oscillation as an oscillatory disturbance added tothe trapped bead movement, when the scanner is driven by a triangular waveinput. An equivalent oscillator model is put forward to explain themechanism of the oscillatory disturbance. Both the measurements andcalculations show that the attenuating oscillation comes from theoscillation of the scanner and the triangular wave drive causes thisadditional oscillation of the scanner. Furthermore, the analysis indicatesthat the oscillatory disturbance will become stronger, when the stiffness ofthe trap increases or the natural frequency of the scanner decreases. Weadopt another driving way, i.e. a sinusoidal wave input is used instead ofthe triangular wave input. Our experiment has verified that in this case theoscillatory disturbance is eliminated completely.